Review of Greenhouse Gas Life Cycle Emissions, Air Pollution Impacts and Economics of Biomass Production and Consumption in Scotland

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5. Environmental Impacts of Biomass Energy Options

5.1 Background

Air Quality and Associated Legislation

The utilistion of biomass for energy can affect air quality in a variety of ways. A large proportion of the total air pollutants of a bioenergy production chain are released during combustion of biomass or biomass-derived fuels. Emission levels of some of these pollutants, such as NOx and SOx depend heavily on the chemical composition of individual fuels while emission levels of other pollutants such as particulates ( PM) and carbon monoxide depend on the completeness of the combustion process. Air pollutants are also emitted at other stages of the fuel chain. Application of fertilisers during bioenergy crop production, for example, can lead to increased emissions of ammonia.

The emissions to atmosphere from combustion can cause adverse impact on human health and the environment at a local, national and transboundary scale. For example, Scotland has a high number of sensitive habitats. However, the pollutants associated with bioenergy are also associated with other combustion processes and the use of biomass can lead to an increase or decrease in emission; the relative contribution will depend on the type of fuel and combustion technology displaced by the biomass. Some of these pollutants ( e.g.PM, SO2, NOx) are regulated by British/European legislation.

For example, PM emissions from residential and industrial combustion sources are controlled under the Clean Air Act 1993 and, for larger combustion plant, Pollution Prevention and Control regulations. The Clean Air Act restricts smoke emissions from premises and includes powers to designate 'smoke control areas'. In these areas, primarily urban areas which had a concentration of industry and/or coal-fired dwellings, there are restrictions on release of smoke from industrial premises and only exempted fuels and appliances can be used. Exempted appliances have undergone type-approval emission tests to determine if they can operate within prescribed PM emission limits.

There are three EU ambient air quality directives that have been transposed into UK law that govern the levels of of these pollutants in ambient air:

  • 96/62/EC Council Directive of 27 September 1996 on ambient air quality assessment and management (the Ambient Air Framework Directive).
  • 1999/30/EC Council Directive of 22 April 1999 relating to limit values for sulphur dioxide, nitrogen dioxide, oxides of nitrogen, particulate matter and lead in ambient air (the First Daughter Directive).
  • 2000/69/EC Directive of the European Parliament and the Council of 16 Nov 2000 relating to limit values for benzene and carbon monoxide in ambient air (the Second Daughter Directive).

The National Air Quality Strategy for England, Scotland, Wales and Northern Ireland ( DETR 2000) allows the UK to comply with the EU Air Quality Daughter Directives. The air quality strategy pollutants and principle sources are detailed in Table 5.1 below :

Table 5.1: Air Quality Strategy Pollutants and Main Sources

Pollutant

Main source

Other key sources

nitrogen dioxide

Traffic

Industry (major combustion sources), aircraft

sulphur dioxide

Industry (major coal and oil combustion)

residential coal burning, shipping, rail

benzen

Traffic

Industry (petroleum refining), petrol stations, petrol storage

1,3 butadiene

Industry (petrochemical)

-

lead

Industry (metal industry and organic chemicals)

-

carbon monoxide

Traffic

-

particles ( PM10 )

Traffic

Industry, domestic solid fuel combustion, materials handling, aircraft

Note that Scotland has developed lower air quality objectives to be achieved by 2010 for benzene and PM10 than are in place in other parts of the UK.

Notwithstanding emissions arising from biomass processing or refining, Table 5.1 suggests that changes in emissions from increased biomass use are most likely to impact on those areas of air quality affected by traffic and combustion. Analysis of the first round of air quality management review and assessment reports for local authorities in England and Wales indicates that traffic sources are the main reason for declaring air quality management areas (75%), industrial and other sources may be direct or indirect contributory sources (17%) but were rarely the sole reason recorded for exceeding air quality objectives (<10%) (Leksmono et al. 2002). These data indicate that any increase in traffic-related emissions from use of biofuels could be of concern for local air quality management. Although the data suggest that emissions from biomass combustion in industry and for domestic heating are less likely than traffic to give rise to air quality issues, it should be noted that air quality objectives are now lower.

It is proposed to extend the air quality strategy pollutants to include PM2.5 with adoption of proposed EU air quality target of 25 µg.m -3 for 2010 and a reduction of exposure of 20% by 2020 (Defra 2006). The UK currently expects to be able to achieve the 2010 target at most locations under existing measures but there will be locations where the target could be exceeded. The UK considers the 20% reduction objective to be very demanding.

There is a UKPAH objective and this is also under review but, no change to the objective has been recommended although further work is proposed by Defra and the devolved administrations to assess costs and benefits of changing the objective.

Other relevant agreements for air pollutants include the UN protocol on heavy metals, the Stockholm convention on persistent organic pollutants and, the National Emission Ceiling Directive.

LCA

To ensure consistency with the chapter on greenhouse gas and energy balances (Chapter 4), this study reviewed the literature with information on life cycle air pollutant emissions as well as combustion emissions.

The characteristics of life cycle assessment ( LCA) studies have been fully discussed in Chapter 4. Many life cycle assessment studies present aggregated results of pollutant emissions according to their final environmental impact. Often the preferred categories are acidification and eutrophication, although sometimes ecotoxicity or photochemical ozone impacts are also presented. This involves the allocation of impact 'potentials' in much the same way CH4 and N 2O are assigned global warming potentials relative to CO2. Acidification potentials are calculated on the basis of SO2 equivalents while eutrophication potentials are presented as nitrate ( NO3-) or phosphate ( PO4-3) equivalents. Characterisation factors for these potentials are shown in Table 5.2. Whenever photochemical ozone potential is presented, it is usually expressed as C 2H 4 equivalents (Calzoni et al. 2000). As in Chapter 4, the LCA results presented for air pollutants and non- GHG environmental impacts are unique to the context of the studies they referred to and should not be assumed to be representative of Scottish conditions. Summary tables of all studies reviewed are presented in Annex 4.

Table 5.2: Characterisation Factors for Acidification and Eutrophication Potentials.

Pollutant substance

Acidification Potential
(g SO2-eq/g substance)

Eutrophication potential
(combined N and P g NO3-eq/g substance)

Sulphur dioxide ( SO2)

1

-

Nitrogen oxides ( NOx)

0.70

1.35

Ammonia ( NH3)

1.88

3.64

Hydrochloric acid ( HCl)

0.88

-

Phosphate ( PO4-3)

-

10.45

Source: Calzoni et al. 2000

Projection of Biomass Energy Impacts on Air Quality

The final section of the air quality assessment component of this review consists of a projection of changes in air pollutant emissions arising from biomass combustion for heat/electricity and also from transport biofuel combustion under four scenarios for 2020. Details of the projection work are provided in Section 5.5

Other Environmental Impacts

Besides air quality, bioenergy systems can also affect soil quality, water quality and biodiversity. The direction and magnitude of these impacts depend greatly on the land use system they replace. When perennial energy crop systems replace annual row crops, for example, the effects are usually positive, whereas replacement of forest systems will usually be accompanied by negative environmental impacts. Sections 5.8, 5.9 and 5.10 of this chapter briefly review these environmental impacts associated with bioenergy systems. These impacts are not reviewed as exhaustively in this report as air quality impacts.

5.2 Air Quality Impacts of Heat Production from Biomass

5.2.1 Review of Combustion Emission Factors

An emission factor 'is a representative value that attempts to relate the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant' ( USEPA 1995). Factors are typically expressed as the mass of pollutant released per unit of production or, for combustion processes, per mass of fuel burned (g kg -1). Other ways of expressing emission factors include relating the amount of pollutant released per unit of energy input (g GJ-1) or energy output (g kWh -1). Frequently researchers provide emission concentrations which can be volume (molar) concentrations, for example ppm. Other units commonly used include mass concentrations (typically mg m -3). However, concentration data are of little use unless they have been standardised to a stated reference oxygen concentration, moisture content, temperature and pressure - typically 6, 10, 11 or 13% oxygen and dry for solid fuel appliances. For mass concentrations the reference temperature and pressure need to be stated (typically 0ºC, 101.3 kPa). Concentrations may be standardised to an 'air free' condition. Occasionally concentrations are standardised to a reference CO2 content (12 or 13%). Such standardisation normalises the data to a particular excess air condition and allows meaningful comparison of the data.

A number of studies with data on combustion emissions of biomass technologies ranging from domestic heaters to large scale electricity plant were identified in this review. The range of technologies covered, however, made it difficult to identify combustion factors which could easily represent future developments in Scotland.

Residential/Institutional Biomass Combustion Units

Development of wood-fired residential boilers and stoves has been the focus of considerable activity over a number of years in Europe and Scandinavia. However, measurement data on modern combustion units is comparatively scarce. The last decade has seen an improvement in the quality of residential/institutional heating units, some with efficiencies over 90% (Rippengal 2005). Reinstating traditional fireplaces or use of wood logs in place of current solid fuels will occur to an extent but it is highly probable that most residential biomass combustion would require installation of automatic heating boilers and/or manual/automatic stoves generally replacing existing appliances. These appliances typically burn wood pellets or wood chips and produce less particulate and in particular, products of incomplete combustion emissions, than traditional fireplaces. Emissions of NMVOC, total organic carbon ( TOC) and particulate matter from modern wood boilers and pellet burners can be over 100 times lower than for old low-efficiency residential heating systems (Johansson et al. 2004).

Although many countries (including the UK) have type approval schemes for residential solid fuel appliances to demonstrate operation within smoke or particulate matter ( PM) emission standards, these data are not often available and cover a limited range of pollutants. The European Standard EN303 Part 5 (1999) includes PM, CO and TOC (referred to as OGC in the Standard) limits for such appliances. However, it should be noted that the EN303 Part 5 Standard is not a 'harmonised' Standard and its emission limits are not mandatory. Many countries have national requirements for PM emissions which differ from those in EN303 Part 5.

The UNECE/ EMEP taskforce on emission inventories and projections ( TFEIP) recently updated its methodology chapter on emission factors for residential combustion. This Corinair guidance is published to support inventory teams, provide a resource of default emission factors and facilitate consistent inventory reporting.

Following review of the literature survey, it was concluded that the emission factors within the Corinair handbook for advanced wood fired techniques smaller than 1 MWth are the most applicable to modern residential and institutional wood and biomass combustion appliances in Scotland. The Corinair default factors cover four appliance types :

  • Advanced stove
  • Pellet stove
  • Manual boiler
  • Automatic boiler

For the scenario modeling exercise (section 5.6), an aggregate default factor for all biomass combustion is proposed assuming that there would be little public acceptance of manual boilers, most fuel would be burned in automatic boilers (for heating) and no more than 10% of fuel consumed would be in the advanced or pellet stoves. A summary of the Corinair emission factors and the derived aggregate factor is provided in Table 5.3.

Table 5.3: Summary of Corinair Default Emission Factors for Advanced Wood Combustion Technologies.

Pollutants

Emission factors, g GJ-1

Advanced stove

Pellet stove

Manual boiler

Automatic boiler

Aggregate factor

SO2

30

30

30

30

30

NOX (as NO2 )

150

150

150

150

150

PM

450

130

250

80

122

PM10

400

120

230

70

108

PM2.5

400

120

230

70

108

CO

3000

500

3000

300

590

nm VOC

250

20

250

20

43

mg GJ-1

As

0.5

0.5

1

0.5

0.5

Cd

1

0.5

0.3

0.5

0.55

Cr

8

3

2

4

4.3

Cu

2

1

3

2

1.9

Hg

0.5

0.5

1

0.5

0.5

Ni

2

2

200

2

2

Pb

30

20

10

20

21

Se

0.5

-

-

-

0.05

Zn

80

80

5

80

80

PAH

400

50

150

40

77

ITEQ ng/ GJ

PCDD/F

300

50

300

50

75

%

Fuel use for aggregate factor, %

10

10

0

80

100

The aggregate wood combustion factor is compared with Corinair default factors for other fuels similar-sized technologies in Table 5.4

Table 5.4: Indicative Comparison of Emissions from Biomass and Fossil Fuels in Small Boilers

Pollutants

Emission factors, g GJ-1

Aggregate advanced wood combustion

Default gas

Default oil

Default coal

Default MSF

Aggregate advanced coal

<1 MW

<50kW

<1 MW

<50kW

<1 MW

<50 kW

<1 MW

<50 kW

<1 MW

<1 MW

SO2

30

0.5

0.5

140

140

900

900

500

500

495

NOX (as NO2 )

150

70

70

70

100

200

200

200

150

195

PM

122

0.5

NA

5

5

300

200

120

100

107

PM10

108

0.5

NA

3

3

260

170

100

80

93

PM2.5

108

0.5

NA

3

3

260

170

100

80

93

CO

590

30

30

40

40

4000

5

3000

1500

760

nm VOC

43

10

3

15

15

300

3

200

100

48

mg GJ-115

As

0.5

NA

NA

1

1

5

5

3

4

0.6

Cd

0.55

NA

NA

2

0.3

3

3

0.7

0.7

2

Cr

4.3

NA

NA

20

20

15

15

10

10

2

Cu

1.9

NA

NA

10

10

30

30

20

20

9

Hg

0.5

NA

NA

1

1

10

10

10

7

1

Ni

2

NA

NA

300

300

20

20

13

13

3

Pb

21

NA

NA

20

20

200

200

120

100

82

Se

0.05

NA

NA

NA

NA

2

2

1.5

1.5

0.7

Zn

80

NA

NA

10

10

300

300

200

160

110

PAH

77

NA

NA

30

26

710

320

150

90

96

ITEQ ng/ GJ

PCDD/F

75

NA

2

10

10

500

400

200

100

86

Source: CORINAIR Emission Inventory, Chapter B216

Table 5.4 highlights the potential change in emissions associated with replacing other fuels with modern biomass technology for smaller combustion units. These have been summarised in general terms in Table 5.5 below, similar impacts can also be expected for larger installations although the scale of change on larger plant will be reduced because of the greater use of emission control technology. Projections of actual changes in emission in Scotland based on the NAEI data for existing facilities are provided at Section 5.5.

Table 5.5: General Effects of Replacing Fossil Fuel by Modern Biomass Combustion Technologies

Pollutant

Advantage '+' or disadvantage '-' from change to modern biomass technology (wood) from fossil fuel

Gas

Oil

Coal

SO2

--

++

+++

NOX

-

-

+

PM/ PM10 / PM2.5

---

--

+

CO

-

-

+

nm VOC

-

-

+

Trace elements

--

+

+

PAH

--

-

+

PCDD/F

--

-

+

5.2.2 Review of Life Cycle Emissions

Very few life cycle assessments for heat production from biomass include air pollutant impacts. As is true with most LCA studies, the studies that have done are context-specific, so that it is difficult to readily compare results. Annex 3 lists LCA studies that have been carried out which include air pollutant balances for different biomass heat chains.

One of the few comparative LCA studies on heat systems was conducted by Gustavsson and Karlsson (2002). In this study, the impacts of different heating systems, including natural gas and oil-fired boilers, electricity based heat pumps and resistance heaters as well as wood chip and pellet boilers, were compared per MWh of heat consumed in detached houses in Sweden. The results of the study are presented in Figure 5.1 for total hydrocarbons ( THC), SOx and NOx emitted. The results show that whereas THC emissions from pellet boiler systems were found to be comparable with natural gas based electric systems, they are considerably lower than oil boiler systems, where THC is emitted predominantly in the oil production stage. THC emissions from wood boilers were found to be considerably higher, however, due to the specific end-use combustion conditions which are not reflective of modern efficient boilers currently available on the market. NOx emissions of wood fuel boilers found to be considerably greater than local gas or oil-fired boilers and greater than natural-gas based electric boilers. The heat pump system consistently resulted in the lowest comparative emissions, reflecting the low primary energy input into the system. SOx emissions for wood fuel local heating systems, although higher than the negligible emissions from natural gas boilers, were significantly lower than oil-based systems. The study also indicated that lifecycle NOx and SOx emissions from electric heaters driven by electricity generated by gasification of wood residues at stand-alone power plant were greater than those resulting from use of local wood and pellet boilers for equivalent reference units.

In a comprehensive study, Kaltschmitt and Reinhart (1997) performed life cycle assessments for a range of bioenergy options of relevance to Germany which included an unusually extensive list of local air pollutants ( SO2, NMHC, NH3, PM, HCl, formaldehyde, TCDD equivalents, benzol, benzopyrene). Among the bioenergy chains covered in the study were heat from wheat straw, heat from rape straw, heat from short rotation coppice (both willow and poplar) and heat from forestry residues (both beech wood and spruce wood). The results from the study showed that, for German conditions, substitution of heating oil based systems with bioenergy carriers resulted in reduced life cycle emissions of SO2 for poplar, willow and forestry residue based systems but increased emissions for rape straw systems with insignificant differences recorded for heat production from wheat straw. When all acidifying pollutants ( SOx, NOx, HCl, NH3) were considered, however, both straw chains and both short rotation coppice chains were at a disadvantage to their oil-based counterpart systems, with no significant change being observed for the forestry residue chains. This was largely attributed to life cycle NOx emissions, which were significantly greater for the biomass chains than for the fossil fuel chains. It should be stated that this work is now almost 10 years old and would require substantial modification to reflect Scottish conditions.

A more recent EU-wide project called ' Bioenergy for Europe: Which One Fits Best 'was carried out in the framework of the FAIR Project. This study examined the environmental impacts of several biomass energy production systems with heat, transport and electricity end uses. The average European

results of the heat systems are shown in Figure 5.2. The heat systems covered included district heat from miscanthus, district heat from short rotation coppice, residential heating from traditional firewood and district heat from straw. The study found that in all instances, biomass systems result in increased eutrophication impacts relative to fossil fuel systems, and in all district heat systems the biomass systems were at a disadvantage with regard to acidification impacts. The biomass systems did, however, sometimes show an advantage in photochemical smog production.

Figure 5.1: Illustrative Life Cycle Emissions for Biomass and Fossil Heat Systems (g/ MWh) (not necessarily representative of Scotland)

image of Figure 5.1: Illustrative Life Cycle Emissions for Biomass and Fossil Heat Systems (g/MWh) (not necessarily representative of Scotland)

Figure adapted from Gustavsson and Karlsson (2002). HP - heat pump, NGCC - Natural Gas Combined Cycle, BIGCC - Biomass Integrated Gasification Combined Cycle, NGB - Natural Gas Boiler, OB - Oil Boiler, WB - Wood boiler (old technology), PB - pellet boiler, RH - resistance heater. Study based on Swedish heat market conditions. Emissions in g MWh -1.

Acidification impacts of all four heat chains included in the study were similar, but eutrophication impacts of the cultivated biomass options ( SRC and miscanthus) were substantially greater than those based on residues (straw and wood fuel). The study also undertook analyses of ecotoxicity effects, but the uncertainty of these figures was so high that it is not wise to derive any conclusions from them.

A Note on Biogenic VOC Emissions

Volatile organic compounds ( VOCs) can be emitted by vegetation as well as through anthropogenic sources. Among the most important VOCs emitted from vegetation are isoprene and monoterpene. These compounds affect the concentrations of greenhouse gases such as methane and also precursors for tropospheric ozone, which has adverse impacts on human health and agricultural production (Purves et al. 2005). Ozone is formed through the reaction of VOCs with nitrous oxides ( NOx). Emissions from VOCs are overlooked in many LCA studies but are mentioned here for completeness.

5.3 Air Quality Impacts of Electricity/Large CHP Production from Biomass

5.3.1 Review of Combustion Emission Factors

Technology can influence emissions of air pollutants from large power plant considerably. Installation of flue gas desulphurization can dramatically reduce emissions of sulphur dioxide while a range of technological options are also available to reduce NOx emissions. Air staging or fuel staging technologies, for example, can reduce NOx emissions by 50-80%. If combined with secondary abatement measures such as selective catalytic reduction, reductions of up to 95% can be achieved (Nussbaumer 2003). The influence of fuel effect is therefore very small in relation to the influence of technological effect in determining emissions of air pollutants in biomass to electricity/ CHP plant. There are also technologies in place for abatement of particulates.

Figure 5.2 : Illustrative Life Cycle Balances for Environmental Impacts of Biomass Heat Chains (not necessarily representative of Scotland)

image of Figure 5.2: Illustrative Life Cycle Balances for Environmental Impacts of Biomass Heat Chains (not necessarily representative of Scotland)

Source: Calzoni et al. (2000)

Small and Medium-scale Plant

The literature review identified a number of emission surveys on medium-sized combustion units including units used for electricity generation. However, the relevance of many of these plant to Scotland is unclear. The Corinair default emission factors for wood combustion plant between 1 and 50 MWth provide a reliable set of emissions factors for this size range. In addition, the USEPA emission factors for PM are higher than published by Corinair which may indicate that the USEPA data are for older or smaller installations with poorer particulate abatement than would be found on new facilities. The Corinair and USEPA default emission factors are compared in Table 5.6. Although 50 MWth is a key size threshold for combustion plant (larger installations fall within the scope of IPPC regulations), the scenarios do not include this size of plant because most development is anticipated in facilities <1 MWth and larger plant.

Table 5.6: Summary of Emission Factors for Bioelectricity Generators (1-50 MWth)

Pollutant

Emission factors, g GJ-1

Corinair

USEPA

1-50 MWth

Dry wood

Bark & Wet wood

SO2

30

11

NOx

150

95

210

PM

50

159

129

PM10

40

145

116

PM2.5

40

86

69

CO

300

260

nm VOC

60

7.3

mg GJ-1

As

1

9.5

Cd

2

1.8

Cr

3

9.1

Cu

5

21

Hg

0.5

1.5

Ni

2

14

Pb

20

21

Se

-

1.2

Zn

80

180

PAH

40

-

ITEQ ng GJ-1

PCDD/F

200

-

Source: CORINAIR (2005), USEPA (2003)

Large Plant

Two plants with an electrical output of 40-50 MW are already under construction (Lockerbie) or in planning (Tullis Russell) in Scotland. Combustion plant installations which are larger than 50 MWth (typically larger than 15 MW electrical generation) fall within the scope of IPPC regulations. Advanced energy systems such as gasification are not considered for scenario development but these would also be regulated under IPPC.

The literature review identified a number of emission surveys on combustion units for electricity generation. However, several of the papers reviewed were for smaller plant than those envisaged as potential candidates and the bulk of the remaining data were USEPA default factors (2003).

For the scenario modeling, the Environment Agency IPPC fuel and power sector benchmark emission limits for application of Best Available Techniques ( BAT) to biomass combustion have been modified to emission factors (Table 5.7). Where emission data were not available for biomass then comparable data for fossil fuel are provided. It should be noted that actual emissions should be lower than the benchmark emission limits.

Table 5.7: Summary of Emission Factors for Large (Over 50 MWth) Bioelectricity Generators.

Pollutant

Emission factors, g GJ-1

Comment

PM

7.2

NOx

72

SO2

108

CO

9.0

VOC

1.8

Notional value - no limit applied

PAH

0.000022

BREF, Table 6.17 highest of coal/oil/orimulsion

PCDD

7.9E-13

BREF, Table 6.17 highest of coal/oil/orimulsion

HCl

36

N2O

43

Only applicable for fluid bed plant

NH3

1.8

Only applicable for plant with SCRNOX abatement

Source: Modified IPPC emission limits for Best Available Techniques

Co-firing

The literature search found little data on the impact of co-firing biomass on power station emissions. Recent studies by Laux et al. (2003) and for the DTI (Irons 2005) suggest that SO2 and NOx emissions are reduced, while particulate emissions may increase slightly but remain within permitted limits.

The sulphur content of woody biomass is lower than for coal and hence a reduction in SO2 emission at boilers can be expected. This is reported as being up to 10% ( DTI 2005) but for the modeling component of this report it was considered that a more realistic estimate will be 5%. None of the boilers at Longannet or Cockenzie are fitted with flue gas desulphurisation ( FGD) but, if FGD is installed at Longannet then the additional reduction in SO2 due to biomass substitution would be very small and for the purpose of the air quality impact scenarios should be considered as zero.

Many wood biomasses have a lower nitrogen and higher volatile matter content than coal and this is reported to lead to a reduction in NOX formation. Reductions of up to 10% have been reported but are dependent on the biomass and combustion system. Once again, it was considered that a realistic estimate of NOX reduction would be 5%. It must be emphasized, however, that future modification to combustion plant to meet LCPD (Large Combustion Plant Directive) NOX emission limits may improve NOX control such that any reduction due to biomass use may be insignificant.

Although emissions of other pollutants may be modified by use of biomass ( PM emission increases have been reported) no firm data are available and for the purposes of the scenario modeling work, these emissions were considered to remain unchanged.

5.3.2 Review of Life Cycle Emissions

There have been few LCA studies on electricity production from biomass that include information on non- GHG environmental impacts. Relative to the current US-electricity grid mix, Keoleian and Volk (2005) estimated that electricity generation from willow reduced total NOx emissions by 75-92%, SO2 by 70-95% and PM emissions by approximately 98%, although non-methane hydrocarbon ( NMHC) emissions were found to increase by 139-1089% depending on the electricity production technology. This must be qualified, however, by the fact that electricity production in the USA relies heavily on coal.

Other studies that have performed life cycle assessments for air pollutant emissions from biomass CHP and bioelectricity technologies are listed in Annex 3. Spitzley and Keoleian (2005) report overall acidification impacts of willow electricity generation to be in the same order as those from electricity generation from natural gas, although there is considerable improvement in relation to conventional coal-based systems.

Conversion efficiency plays a decisive part in determining air pollutant impacts of electricity production systems as does effective flue-gas cleaning. Lower efficiencies mean that more fuel is required to produce a given amount of electricity which is translated into increased emissions from the fuel production stage.

As with other systems, the individual details of the fuel chain are very important in the final environmental balances produced. For electricity production from forestry residues, for example, Malkki & Virtanen (2005) reported higher emissions for systems based on off-road chipping rather than road-side chipping. Similarly, systems based on the collection of green residues led to higher emissions per unit of power produced than systems based on brown residues. In a Swedish study of the impact of biomass fuel supply chains on air emissions, Hansson et al. (2003) found that the use of separate storage facilities increased air emissions by about 30% due to increased transportation and loading activities.

Co-firing

Keoleian and Volk (2005) performed a full life cycle assessment that included total air pollutants for co-firing of coal with willow (10%) and a recycled furniture residue/willow blend (10%). They estimated a decrease of approximately 10% in lifecycle SOx emissions and a decrease of about 6% in total PM emissions relative to 100% coal firing for both scenarios. There was a difference in NOx emissions between both scenarios, however, with the co-firing with willow resulting in a 5% decrease in emissions while co-firing with the residue blend resulted in a 10% increase in emissions, due to the high nitrogen content of the furniture residues.

Hartmann & Kaltschmitt (1999) conducted an LCA study of two co-firing systems (10% straw and 10% forestry residues) of relevance to Germany. The study found that both NOx and SOx emissions were significantly reduced in relation to the reference 100% coal scenario, with no significant change in NH3 emissions. Due to high chorine content, HCl emissions from co-firing with straw were greatly increased in relation to the reference scenario although emissions were reduced when co-firing with straw.

Comparison with Other Renewable and Non-renewable Electricity Sources

Different renewable technologies operate at a range of scales and finding a basis for comparison is not without problems. These differences can be overcome to a certain extent by using a functional unit related to the amount of energy generated as the basis for comparison. A recent review by Spitzley and Kooleian (2005) compared life cycle emissions of willow electricity systems with a range of other renewables and fossil systems on the basis of kilowatt hour of electricity generated. The results of the study, which were normalised by the authors to take account of different parameters such as service life of the technologies in question, are shown in Figure 5.3. The study did not include any forestry residue based systems, but only willow and poplar based biomass systems. In relation to other renewables, life cycle NOx emissions from willow and poplar systems were significantly increased as were life cycle NH3 emissions. Life cycle SOx emissions, although generally greater than those of other renewable systems such as solar, hydro and wind, were very small. The advantage relative to fossil fuels depends on the fossil fuel in question. Although life cycle SOx emissions of willow-based systems were muchy lower than for coal systems, they were found to be slightly higher, but generally in the same order as natural gas life cycle emissions. A similar trend is seen for NH3 emissions and NOx emissions, although with the advent of clean coal technologies, the difference in life cycle emissions in comparison to biomass systems may not be expressive. The results of the Spitzley and Kooleian analysis are echoed, at least qualitatively by other sources. For electricity generation systems in Sweden, Vatenfall (2005) reported reduced NOx emissions for biomass-fuelled CHP in relation to coal-fuelled and oil-fuelled CHP, but considerably greater life cycle emissions in relation to non-fuel based renewable technologies such as wind, hydro and solar as well as greater emissions than nuclear and natural gas IGCC system. Life cycle PM emissions in the Spitzley and Keoleain (2005) study were found to be extremely low for the willow and poplar chains, as with hydro and wind technologies. For renewable technologies not based on fuel, the construction stage dominates emissions, while the operational phase is generally dominant for fuel-based electricity generation systems.

Pehnt (2006) compared life cycle air quality impacts of a range of renewable energy technologies of relevance to Germany. Relative to the German electricity mix, the acidification impacts of different biomass technologies were found to vary, with electricity production from SRC having greater acidification impacts than the mix production while electricity from forestry residues had a lower impact. Relative to other renewables, the biomass chains invariably performed worse in both the acidification and eutrophication categories. There was not much difference, however, in terms of total life cycle SOx and PM emissions between biomass-based systems and other renewable technologies. The higher acidification impacts were largely due to the much higher NOx emissions, although NH3 emissions also contributed to acidification impacts in the energy crop systems, which were found to have higher acidification impacts than forest wood systems.

Figure 5.3: Illustrative Life Cycle Emissions of Several Air Pollutants for DifferentElectricity Generation Systems (not necessarily representative of Scotland)

(a)

image of Figure 5.3: Illustrative Life Cycle Emissions of Several Air Pollutants for Different Electricity Generation Systems

(b)

image of Figure 5.3: Illustrative Life Cycle Emissions of Several Air Pollutants for Different Electricity Generation Systems

(c)

image of Figure 5.3: Illustrative Life Cycle Emissions of Several Air Pollutants for Different Electricity Generation Systems

(d)

image of Figure 5.3: Illustrative Life Cycle Emissions of Several Air Pollutants for Different Electricity Generation Systems

Description of systems compared in the Spitzley and Keoleian study (2005) :

1) Solar: Building integrated photovoltaic module system of triple-junction thin film amorphous silicon photovoltaic cells deposited on stainless steel. Based on study by Keolian and Lewis (2003), complemented with data from the DEAMLCA database.

2) Large hydro: Based on Glen Canyon hydroelectric plant (1296 MW) on the Colorado River. Based on data from Pacca and Holvarth (2002) complemented with emission data from the DEAM database.

3) Wind: Two 25 MW windfarm in different location in the United States with different wind speeds. The technology used is 500 kw advanced airfoil designs with 40m hub height and 38m rotor diameter. Based on EPRI (1997) with emission data from the DEAM database.

4) Hybrid poplar: based on use in a 113 MW gasification combined cycle plant. Based on Mann and Spath (1997).

5) Willow: Three systems included in the study: a) high pressure direct heat gasification combined cycle (based on EPRI models), b) low pressure indirect heat gasification combined cycle (based on EPRI models), c) direct-fire in a stoker grate boiler (based on models developed by EPRI). Based on yields of 15 odt ha -1. Based largely on Heller et al. (2004).

6) Coal: Based on Spath et al. (1999). Average plant: Direct-fired pulverized coal boiler, 360 MW, 60% capacity factor. Advanced plant: New source performance standards plant described in Spath et al, 425 MW. Future plant: Low emission boiler system described in Spath et al. (1999).

7) Co-firing with willow: Based on 10% willow blend based on the retrofit of an existing coal fired boiler in New York

8) Natural gas: Based on a study by Mann and Spath (2000) for a 505 MW combined cycle power plant.

9) Nuclear: Based on a typical 1 GW Pressurized Water Reactor ( PWR) facility. Based largely on data from a WNA report (2006) with emissions data obtained from the DEAM database.

Note on Biogas Production Life Cycle Emissions

Comparatively little work has been done to quantify the environmental impacts of biogas production. A recent Swedish study by Borjesson and Berglund (2006) provides a notable exception. The study was comprehensive in that it compared a full range of biogas chains from a diverse set of raw materials including ley crops, straw, sugar beet residues, liquid manure, food industry waste and municipal organic waste and included both large-scale and farm-scale processes. In addition, transport end-uses of biogas were also considered. Besides greenhouse gas emissions, life cycle air pollutant balances were also calculated for carbon monoxide, NOx, SO2, non-methane hydrocarbons and particulates. Figure 5.4 summarises the results of the study. In general, biogas from liquid manure and food industry waste was found to perform better environmentally than biogas from ley crops and agricultural residues, entirely as a result of emissions in the biogas production stage of the life cycle. For biogas originating from the same raw materials, emissions of most pollutants, especially NOx, were greater for transport end uses than for heat and electricity end uses. A subsequent study comparing the impacts of biogas systems with various reference systems has been carried out by the same authors and is expected to be published shortly.

Figure 5.4: Comparison of Life Cycle Air Pollutant Emissions of Biogas Systems (g MJ-1)

imagr of Figure 5.4: Comparison of Life Cycle Air Pollutant Emissions of Biogas Systems (g MJ-1)

Based on Borjesson and Berglund (2006).

5.4 Air Quality Impacts of Transport Biofuel Production from Biomass

5.4.1 Review of Combustion Emission Factors

Increasingly stringent environmental legislation has meant that emissions of major transport pollutants have, on the whole, decreased in recent years. In petrol engines, the introduction of three-way catalyst systems with closed-loop lambda controls has enabled emission reductions of a factor of 20 for most air pollutants ( TNO 2004). With the introduction of stricter Euro 5 and 6 standards, these emissions will be reduced even further. Diesel engines have not achieved the same level of emission reduction as petrol engines and diesel still contributes significantly to total PM and NOx emissions. The development of diesel particulate filters and NOx-storage catalysts should mean that emissions will be reduced in the near future, however.

Interpreting published data on emissions from transport fuels including biofuels is not without problem. These depend on a range of factors, including fuel systems, drive cycles and blend specifications. It is also very easy for such data to become outdated, due to continuous advancements in engine and after-treatment technology. Recently, spark ignition direction injection ( SIDI) engines that operate under complex combustion strategies have been introduced in the market. Emission data on biofuels used in such engines is of such scarcity that no conclusions can be drawn about emissions ( TNO 2004). Diesel engines have increasingly shifted from indirect injection to being also completely direct injected. Such technological changes render much of the older data inapplicable to the current situation.

There are other important issues that must be borne in mind when analysing literature data on emission factors from transport fuel combustion. Variability is often quite high and studies that present only average values often do not provide an indication of the significance of the results. It is difficult to draw conclusions from a considerable volume of the existing data as much of it relates to American vehicles, which are not really representative of the UK fleet. An attempt has been made in this review to present only data which is relevant to Scotland.

Biodiesel

Light Duty Vehicles

The different engine configurations of light duty and heavy duty vehicles results in different emission performance. Table 5.8 summarises the results of some recent European studies on changes in biodiesel emissions for light duty vehicles relative to fossil diesel. A recent study by CONCAWE (2005) compared emissions from 5% RME from two passenger cars, both Euro III engines, with data collected over NEDC cycle. Although small changes were observed for NOx and PM emissions, the limits on the error bars produced in the report lie outside the very small changes observed. CONCAWE also found no significant differences for HC and CO emissions. Although there is much variability in the data, as demonstrated in Table 5.9, there seems to be a general trend in light duty vehicles towards slightly increased NOx emissions relative to fossil diesel, but decreased HC, PM and CO emissions ( TNO 2004). Formaldehyde and acetaldehyde emissions are highly variable.

Table 5.8: Illustrative Changes in Biodiesel Exhaust Emissions from Light Duty Vehicles Relative to Mineral Diesel.

Ref.

Fuel

Ref. fuel

Drive Cycle

NOx

HC

PM

CO

Aldeydes

Krahl et al. 2003

100% RME

Low S Diesel

ECE49

+ 6%

- 56%

+16%

-44%

-40% (total)

Aako 2000

30% RME

EN590 Diesel

FTP 75

-2 +/- 5%

-12.5%

-14 +/- 10%

+5.5%

Form: +25%
Ace: +6%

Aako 2000

30% UVOME

EN590 Diesel

FTP 75

-3 +/- 5%

-30%

-25 +/- 12%

-22%

Form: +12%
Ace: -14%

Concawe 2005

5% RME

Low S Diesel

NEDC

-2 +/-2.5%

-10 +/- 25%

-4.5 +/- 5%

-10 +/- 15%

-

Acronyms: RME - Rape Methyl Ester; UVOME - Used Vegetable Oil Methyl Ester; cat. - catalyst; Form. - Formaldehyde; Ace - Acetaldehyde;

Heavy Duty Vehicles ( HDVs)

A major review of the exhaust emissions of biodiesel from heavy duty vehicles was published in 2002 ( EPA 2002), but this was based largely on studies from the 1990's. Furthermore, many of the studies reviewed used soybean biodiesel.

The generic results of the study, without taking into consideration engine standards, were as follows:

[ NOx] = [ NOx] MO + 0.1[biodiesel] i.e. for 20% biodiesel there is a 2% increase in NOx
[ PM] = [ PM] MO - 0.5[biodiesel], i.e. for 20% biodiesel there is a 10% decrease in PM
[ HC] = [ HC] MO - 0.9[biodiesel], i.e. for 20% biodiesel there is a 18% decrease in HC
[ CO] = [ CO] MO - 0.5[biodiesel], i.e. for 20% biodiesel there is a 10% decrease in CO

Table 5.9 gives the % change in emissions reported by two European studies for heavy duty vehicles running on various biodiesel blends in relation to mineral diesel. These findings are broadly in line with the EPA study results, although it is clear that there is, again, much variability in the data.

Table 5.9: Changes in Pollutant Emissions from Biodiesel Combustion in Relation to Mineral Diesel

Ref.

Fuel

Ref. fuel

Standard

Drive Cycle

NOx

HC

PM

CO

Aako 2000

100% RME

EN 590 Diesel

Euro II w/ cat.

FTP 75

+12 +/- 3%

-47 +/- 5%

-82 +/-10%

-3 +/- 5%

Aako 2000

30% RME

EN 590 Diesel

Euro II w/ cat.

FTP 75

-3 +/- 4%

-2 +/- 8%

-32 +/- 7%

+6 +/- 5%

Aako 2000

30% UVOME

EN590 Diesel

Euro II w/cat.

FTP 75

-1 +/- 6%

-6 +/- 6%

-46 +/- 21%

-3 +/- 4%

Concawe 2005

5% RME

Low S Diesel

Euro III w/ cat.

NEDC

+1 +/- 12%

+10%

-10 +/-10%

+15 +/-20%

Vegetable Oil

Ricardo (2004) collected emission data from two EURO II cars running on vegetable oil, after they had had conversion kits fitted. Although no significant differences in NOx emissions were found relative to control, there were significant increases in HC and CO emissions as well as general increases in PM emissions. In all instances, however, variability was high. For the Passat, there were also detectable differences in PAH and formaldehyde emissions. The results of this study are summarised in Table 5.10. There is very little data available on emissions from pure vegetable oil. A recent review on the use of vegetable oil as a transport fuel was published by the GAVE Programme (Senter Novem 2005b). In this review, the Ricardo results are compared with the results of a Swiss study on converted Euro I and Euro III passenger cars (Folkcenter 2000) which arrived at completely different conclusions. Emissions in the EURO III vehicle were found to be higher than in the EURO I vehicle, which resulted in decreased emissions relative to mineral diesel. This could be a result of the engine fine-tuning to minimise emissions in the more modern car, which once altered to accommodate vegetable oil had a negative impact on emissions (Senter Novem 2005b). More studies are necessary to form a more conclusive picture of vehicle emissions from cars running on pure vegetable oil, however.

Table 5.10: Illustrative Combustion Emissions from Pure Vegetable Oil.

Ref.

Fuel

Ref. fuel

Standard

NO x

HC

PM

CO

Aldeydes

Ricardo 2004

Rapeseed oil (100%)

Diesel

EURO II

0 +/- 15%

+100% +/- 50%

+50% +/-50%

+300% +/-200%

+

Folkcenter 2000

Rapeseed oil (100%)

Diesel

EURO I

-36%

-41%

-42%

Folkcenter 2000

Rapeseed oil (100%)

Diesel

EURO III

+162%

+291%

-14%

+17%

Bioethanol

Perhaps the most relevant data for bioethanol emissions in the UK is from an AEA Technology (Reading et al. 2002) study carried out in 2002. The main conclusions of this study, which compared emissions from an E10 splash blended ethanol/gasoline mixture with petrol, are shown in Table 5.11. It is evident from the table that, as with biodiesel emissions, there is much variability associated with emissions from bioethanol, although there does appear to be a consistent decrease in PM emissions and considerable increase in acetaldehyde emissions.

Table 5.11: Illustrative Change in Emissions (%) of E10 Bioethanol Blend in Relation to Petrol.

Pollutant

Size

Error

FC

No consistent change

+/- 4%

NOx

No consistent change

+/- 50%

PM

-46%

+/- 13%

HC

No consistent change

+/- 20%

CO

-21%

+/- 24%

Acetaldehyde

+500%

+/- 300%

1,3 butadiene

+28%

+/- 24%

particle number emissions

-50%

+/- 25%

A recent review on bioethanol emissions was produced by TNO for the GAVE (Gaseous and Liquid Climate Neutral Energy Carriers) Programme (Senter Novem 2005a). In general terms, these results are in agreement with those from the earlier AEA Technology report, even though no numerical values were provided in the report.

Another recent report released through the GAVE Programme compared emissions of four different ethanol blends (E5, E10, E70, E85) for three flexifuel passenger cars ( EUROIV). The study was comprehensive insofar as tests were performed in a range of different drive cycles, including NEDC and Artemis urban, rural and highway cycles. No clear fuel effects were observed for HC and PM emissions, although NOx emissions in Artemis AU and AEU cycles were lower by as much as 70% for the E70 and E85 fuels for individual cars, relative to low blend biopetrol. This effect was masked, however, by the incredibly high variability in NOx emissions among individual cars. Other conclusions related to bioethanol emissions made by the study are as follows:

  • NEDC tests conducted at -7º generally showed higher emissions compared to those at 22ºC. Emissions of CO and HC were more than ten times higher at the lower temperature although there was no obvious difference in NOx emissions.
  • Emissions of CO and HC in the Artemis AH cycle were observed to be higher than AU and AEU cycles although NOx emissions were generally lowest in the AH cycle.
  • NEDC tests with engine pre-heater for the E70 blend resulted in considerably reduced emissions of CO, HC, PM and acetaldehyde (10-50% reductions.
  • NEDC tests showed a clear increase in acetaldehyde emissions for the higher fuel blends (E70 and E85).

The conclusions of the GAVE review (Senter Novem 2005a), quoted verbatim below, can be seen as representing the best available knowledge regarding air pollutant emissions from bioethanol:

Regulated pollutants

The use of ethanol in both petrol and diesel engines reduces PM emissions. For the other regulated pollutants ( CO, HC and NOX) less consistent results are reported. The effect of ethanol on these pollutants can be either positive, negative or negligible. In general low ethanol blends tend to decrease CO emissions as ethanol acts as an oxygenate. This effect, however, is most prominent in older vehicles without closed loop catalysts.

The variations in published results cannot be easily explained by ethanol content or major vehicle class (light-duty versus heavy-duty). It may in part be explained by variations in fuel composition as ethanol fuels from different origins and production processes may have ethanol contents ranging from 50% to 100% and may contain other components. Also variations in test procedures, and in engine types and emission control technology, will cause variations in results of emissions measurements.

A few studies have examined the potential of high ethanol blends to achieve future emissions standards with modified vehicles. These studies were generally successful, which suggests that attainment of future standards should be expected for high ethanol blends, provided that engine modifications are being made. In this respect, a potentially significant advantage is noted in using ethanol in spark ignition direct injection ( SIDI) engines because of a reduction in soot and PM formation.

Unregulated pollutants

With respect to unregulated emissions, experimental data consistently indicates increased aldehyde emissions with increasing ethanol content, and in particular substantial increases in acetaldehyde emissions, which are a by-product of ethanol combustion. In contrast the literature consistently reports a reduction in benzene and 1,3 butadiene emissions with increasing ethanol content.

5.4.2 Review of Life Cycle Emissions

Many lifecycle assessments of biofuels have been made, some of which are well to tank in that they include combustion emission factors, but most of which are well to wheel as they don't include combustion emissions. As discussed in the introduction to this chapter, the results of these studies can be presented in several forms, but more often than not they are presented in terms of different environmental impacts rather than as complete balances for different emissions.

A comprehensive review of transport biofuel LCA studies was carried out by IFEU (2004). The study concluded that in contrast to energy and GHG balances, very few studies consider life cycle air pollutant balances. Although the numerical values vary according to individual cases, the review found that acidification, eutrophication and ozone depletions impacts of biofuels from crops are ' qualitatively stable, being at a disadvantage for biofuels'. Results of biofuels from residues were found to be much more variable. The main findings of the report for bioethanol and biodiesel are shown in Tables 5.12 and 5.13 respectively.

Table 5.12: Summary of Qualitative Life Cycle Air Pollutant Impacts forBioethanol Production Systems Compared with Petrol.

Acidification ( SO2 eq.)

Eutrophication ( PO4 eq.)

Photosmog (C 2H 4 eq.)

Ozone Depletion (N 2O)

Wheat Bioethanol

-

-

+/-

-

Sugar beet bioethanol

-

-

+

-

Cellulose bioethanol

+/-

+/-

-

-

Potato bioethanol

-

-

+

-

(+ = advantage to biofuel, - = disadvantage to biofuel) Source: IFEU 2004

Table 5.13: Summary of Qualitative Life Cycle Air Pollutant Impacts for Biodiesel Production Systems Compared to Diesel

Acidification ( SO2 eq.)

Eutrophication ( PO4 eq.)

Photosmog (C 2H 4 eq.)

Ozone Depletion (N 2O)

RME

-

-

+

-

Tallow biodiesel*

+

N/A

+

+

Used Cooking Oil Biodiesel

N/A

N/A

N/A

N/A

Rapeseed Oil

+/-

-

+

-

(+ = advantage to biofuel, - = disadvantage to biofuel) Source: IFEU 2004
* Based only on one study

One of the more transparent biofuel LCA studies including non- GHG environmental effects was recently undertaken for the GAVE Programme ( TNO 2005). The study, which included LCA of both biodiesel from oilseed rape and bioethanol from wheat, was unique in that in that it involved a full range of stakeholders in the analysis so that the assumptions of the LCA study were ground-truthed for real-life conditions. The main assumptions made in the study are listed in Figure 5.5 and results were expressed on the basis of emissions per km travelled. The study was designed to best reflect the current Dutch biofuel market and can therefore not be taken as being representative of Scotland.

Figure 5.5: Illustrative LCA Environmental Impacts for Rapeseed Biodieselvs. Mineral Diesel and for Bioethanol from Wheat vs. Petrol (Not necessarily representative of Scotland).

(a) rapeseed biodiesel vs. mineral diesel

image of Figure 5.5: (a) rapeseed biodiesel vs. mineral diesel

* presented as scores relative to maximum value.

(b) wheat bioethanol vs petrol

image of Figure 5.5: (b) wheat bioethanol vs petrol

* presented as scores relative to maximum value

Source: Senter Novem 2005

Key Assumptions
Feedstocks: bioethanol: 100% wheat from western Europe biodiesel: 50% rapeseed from western Europe, 25% rapeseed from Eastern Europe, 25% world rapeseed
Co-products: Wheat and rapeseed straw: 1/3 left in field, 1/3 animal bedding, 1/3 electricity generation; bioethanol and biodiesel cake used as animal fodder; glycrine sold to pharmaceutical market; CO2 from ethanol product not consider co-product
Land reference: Set-aside Time horizon: three years
Allocation: economic Transport: sea
Conversion Plant: Biodiesel - 150,000 kt/yr, Bioethanol: 200 Ml/yr
End-use: maximum allowable blends used (5% RME biodiesel, 5% bioethanol)

In line with the findings of the IFEU review, the GAVE study estimated that in the acidification and eutrophication categories, bioethanol performed worse than gasoline by 33% and 100% respectively (Figure 5.5). This was due entirely to agricultural emissions during the feedstock production stage, where emissions of ammonia, NOx, SOx and phosphates occurred. Tailpipe emissions of these gases were found not to vary significantly between bioethanol and gasoline. Biodiesel was also found to compare unfavourably with mineral diesel with regards to acidification impacts (increase of 57%) and eutrophication impacts (increase of 500%). From these results, it is clear that transport biofuels produced from purpose-grown crops have negative impacts on water quality in relation to fossil fuel equivalents.

The study highlighted the high sensitivity of the results to fertiliser application and crop yields. Of the acidification impact of bioethanol, for example, 56% was attributed to ammonia emissions, 27% to NOx emissions and 17% to SOx emissions. Ammonia emissions are almost entirely due to fertiliser use while NOx emissions were split evenly between fertiliser production and tractor fuel consumption. Similarly, eutrophication impacts were found to be predominantly due to ammonia emissions from fertiliser application (61%) with lesser contributions from NOx (30%) and phosphates (5%). Photochemical oxidation (smog) due to PM emissions originates mainly from end-use vehicular combustion and there was no major difference in relation to the mineral gasoline chain. The larger fertiliser requirements per hectare for rapeseed for biodiesel production translated into the even greater acidification and eutrophication effects observed with this production chain (57% and 500% respectively). Recent work by Bernesson et al. (2006) confirms the importance of the production step in transport biofuel environmental balances, as scales of production plant were found to make little difference in acidification and eutrophication impacts in relation to the environmental burden of agricultural production.

5.5 Air Quality Impacts of Illustrative Bioenergy Scenarios for 2020

Based on the combustion emission factor data collected during the literature review, Netcen projected the change in air pollutant emissions that would occur for four different biomass/biofuel scenarios to 2020. Life cycle emissions were not considered in the projections. The scenarios were as follows:

Scenario number

Scenario 1 (Heat/Electricity)

1.7 million odt of wood available by 2020. Of this, 25% is used for residential heat purposes, 25% for heat in the commercial/institutional sector and 50% in the large CHP/electricity sector.

Scenario 2 (Heat/electricity)

1.7 million odt of wood available by 2020. Of this, 30% is co-fired in power stations, 35% is utilised in the large CHP/electricity sector. The remaining 35% is split evenly between the residential and commercial/institutional sectors.

Scenario 3 (Transport)

All diesel contains a blend of 5% bio-fuel and all petrol contains a blend of 5% bio-fuel

Scenario 4 (Transport)

5% of diesel cars and 5% of LGVs run solely on 100% biodiesel. Biopetrol blend as Scenario 3.

Heat and Electricity Scenarios

The figure of 1.7 million oven-dried tonnes of available wood fuel in 2020 is based on a combination of the 1.2 million odt derived from the FREDS Report (2005) and an estimated 500,000 odt of short-rotation coppice that could become available if 50,000 ha of land are cultivated with this crop with an average yield of 10 odt ha -1.

The UK's National Atmospheric Emissions Inventory ( NAEI) estimates that in 2002, approximately 0.032 million tonnes of wood was combusted by the residential sector in Scotland. If wood combustion in Scotland is predicted to grow in line with national UK forecasts ( DTI's UEP12) then under business as usual, this rises to 0.041, 0.047 and 0.053 million tonnes in 2010, 2015 and 2020 respectively. The NAEI currently assumes that no wood is consumed in the commercial/institutional and power station sectors and this will not change in the future.

In Scenarios 1 and 2 it has been assumed that the additional wood combustion for heat in the residential and commercial sectors replaces a fossil fuel mix of LPG and coal consumed in these sectors. For the CHP/electricity sectors it has been assumed that the emissions from wood combustion replace coal combustion in power stations. The change in emissions has been predicted for 2010, 2015 and 2020 and it has been assumed that the additional wood available will grow in a linear fashion from 2006 reaching its full potential in 2020. In each case, to give some idea of the significance of the change in emissions, the results have been compared against the projected UK emissions in these years. It was not possible to compare the results against projected emissions for Scotland, as these were not available.

The scenarios chosen were considered to reflect current developments in the biomass sector in Scotland and were based on recent reports (Rippengal 2005, SDC 2005, FREDS 2005). It is recognized that the target for residential heat systems may be overly optimistic, as the economics of residential heating systems is currently unfavourable. Nevertheless, residential systems have been included in suggested targets for biomass heat in Scotland for 2020 (Rippengal 2005) and development of appropriate grant schemes could make this option more attractive.

Likewise, the role of co-firing in the future of the bioenergy industry in Scotland is uncertain. As current legislation stands, co-firing will no longer attract ROCs beyond 2016 and therefore may not play any part in the use of biomass for energy generation purposes by 2020. To account for this uncertainty, two scenarios were assumed: 1) no co-firing, 2) 30% of biomass resource used for co-firing. The scenarios will obviously not reflect reality completely, as there are smaller CHP already in development ( e.g. Caithness) and these have not been taken into consideration explicitly in the scenarios. They do provide a simple approximation, however, of the main lines along which the biomass industry could develop in Scotland.

The results of emission changes for scenarios 1 and 2 are summarised in Tables 5. 14 and 5.15. Whether scenario 1 or 2 is most beneficial depends on the pollutant of concern. For example, the most beneficial for sulphur dioxide is scenario 1, whereas the most beneficial for NOx, is scenario 2. Both projections suggest an increase in PM2.5 emissions.

Table 5.14: Additional Annual Emissions Arising as a Result of Scenario1 (Ktonnes).

Year

SO2

NOx

PM10

PM2.5

NMVOC

Additional emissions

2010

-4.38

-0.23

-0.34

0.13

-0.32

Additional emissions

2015

-9.01

0.30

-0.58

0.23

-0.46

Additional emissions

2020

-13.65

0.48

-0.89

0.36

-0.77

% of UK baseline

2010

-0.90%

-0.02%

-0.25%

0.17%

-0.04%

% of UK baseline

2015

-2.27%

0.03%

-0.43%

0.32%

-0.05%

% of UK baseline

2020

-3.79%

0.05%

-0.63%

0.49%

-0.09%

Table 5.15: Additional Annual Emissions Arising as a Result of Scenario 2 (Ktonnes).

Year

SO2

NOx

PM10

PM2.5

NMVOC

Additional emissions

2010

-3.25

-0.25

-0.23

0.08

-0.17

Additional emissions

2015

-6.76

0.04

-0.38

0.14

-0.24

Additional emissions

2020

-10.04

0.17

-0.61

0.24

-0.49

% of UK baseline

2010

-0.67%

-0.02%

-0.17%

0.10%

-0.02%

% of UK baseline

2015

-1.70%

0.00%

-0.29%

0.20%

-0.03%

% of UK baseline

2020

-2.79%

0.02%

-0.43%

0.32%

-0.06%

Transport Biofuel Scenarios

One aspect of defining appropriate scenarios is the concentration of biofuel that might be put into the tanks of vehicles. The government has set a Renewable Transport Fuel Obligation ( RTFO) that 5% of our transport fuel should be from renewable sources by 2010. In addition, the current Government consultation on the RTFO indicates that ambition levels beyond 5% are being considered post-2010.

For a 5% RTFO two extreme scenarios can be envisaged:

  • Scenario 1: all fuel is a blend containing 5% bio-fuel added to fossil-fuel
  • Scenario 2: 95% of the fuel sold is derived only from fossil-fuels, and 5% is pure bio-fuel.

Whilst both these scenarios meet the 5% RTFO they lead to different emission characteristics, with scenario 2 leading to a number of technical challenges.

If there were a larger penetration/usage of biofuels then analogous scenarios to those described above again represent the extremes of how the fuel might be supplied.

Conventional petrol and diesel fuels are required to conform to EU-wide standards which specify certain physical and chemical parameters. Conventional petrol must conform to the specification of EN228 and conventional diesel to the specification of EN590, and it is generally a condition of engine manufacturers' warranties that fuel used in their engines meet these specifications. Testing of 5% blends by volume of both biodiesel and bioethanol has shown that addition of the fuels at this level maintains conformity to these standards. However, the majority of engine manufacturers have imposed a 5% 'technical limit' for the inclusion of biodiesel and bioethanol in conventional fuels, beyond which warranties on engines are no longer valid.

This 5% figure will change as the technical limit is altered through regulation and co-operation with vehicle manufacturers. In a recent technical note the SMMT noted that: The motor industry is in discussions with the oil industry and other stakeholders through the European Committee on Standardisation ( CEN) to develop future European standards that enable the use of higher percentage biofuel blends in all new vehicles (10 per cent blends - E10, B10). One aspect that will need to be addressed is the development of additive packages to tailor the physical and chemical properties of fuels that contain higher percentages of bio-fuel to help vehicles achieve their required durability. For the purposes of this study, 5% is assumed as an upper constraint on the supply of renewable transport fuels.

The results of the projections, based on the best available emission factors, are given in Tables 5.16 and 5.17. Vegetable oil was not considered to be a major contributor to the total transport fuel market in 2020 and not yet commercial transport fuels have not been included in the projections. Biodiesel from tallow, although currently produced in Scotland by Argent Energy, was not included in the projections as no reliable, recent emission data was obtained for biodiesel from this source.

Table 5.16: Additional Annual Emissions Arising as a Result of Scenario 3 (Ktonnes).

Year

NOx

PM10

Additional emissions

2010

0.1

-0.06

Additional emissions

2015

0.7

-0.05

Additional emissions

2020

0.9

-0.05

% of Scotland road transport emissions

2010

0.3%

-4.2%

% of Scotland road transport emissions

2015

2.6%

-4.5%

% of Scotland road transport emissions

2020

3.8%

-4.6%

Note: a negative number indicates an emission decrease

In the road transport sector, scenario 3 leads to an overall reduction in PM10 emissions. This is as a result of the 5% bio-diesel-fuel blend leading to a 4% reduction in PM10 emissions in Euro II, III and IV light duty vehicles and the 5% bio-petrol fuel blend leading to a 23% reduction in PM10 emissions. In scenario 4, where 5% of cars and light goods vehicles are dedicated to run on 100% bio-diesel this leads to an increase in PM10. Despite the expected NOx reduction in Euro II, III and IV light duty vehicles when running on a 5% bio-diesel blend, the overall NOx emissions increase under this scenario due to the large increase in NOx emitted from heavy goods vehicles.

Table 5.17: Additional Annual Emissions Arising as a Result of Scenario 4(Ktonnes).

Year

PM10

Additional emissions

2010

0.03

Additional emissions

2015

0.02

Additional emissions

2020

0.02

% of Scotland road transport emissions

2010

1.7%

% of Scotland road transport emissions

2015

1.9%

% of Scotland road transport emissions

2020

2.0%

Negative numbers indicate a decrease in emissions.

5.6 Uncertainty Considerations

Combustion Emissions

The overall uncertainty of the emissions projections is dependent on the uncertainties in several elements, including the uncertainty in activity statistics (the fuel burned) and emission factors. Other relevant factors include the reliability of activity data, particularly with respect to fuel allocations to the different sectors.

Default emission factors are rarely provided with a quantitative assessment of uncertainty. Papers or reports for individual measurements may provide a quantified uncertainty for a measurement but this is usually for a particular set of measurements and on a particular piece of equipment using a particular monitoring protocol.

Both USEPA and Corinair have adopted a qualitative description of uncertainty for emission factors which essentially provides an indication of whether the emission factor is appropriate or not. Quantitative estimates of uncertainty are needed by users; for example modellers can use uncertainty estimates to apply sensitivity analysis techniques to inventories and projections.

Bands of uncertainty have been developed by Corinair however these uncertainty ranges are limited, appear to vary according to pollutant and, are not available for all combustion processes (for example NO uncertainty ratings or figures are provided by Corinair for large industrial and non-industrial combustion plant. The USEPA provides qualitative ratings for default emission factors but does not quantify uncertainty.

For many pollutants in the NAEI, uncertainties primarily arise from emission factor uncertainties. Activity data is considered to be more reliable and better characterised. For projections there is the problem that the uncertainty in the activity data may outweigh the uncertainty in the emission factor however the main purpose of the projections in this work is to assess the range of change in emissions to atmosphere from changing to a higher biomass fuel mix. The absolute values and uncertainties of emission factors are of less significance.

It is not possible to give uncertainties for the pollutants assessed. However the NAEI publishes estimates of the uncertainties in the UK inventories (Table 5.18) and these are provided for guidance on the relative magnitude of the uncertainties likely to be associated with these projections. Note that these are the estimated uncertainties for the total UK inventories - uncertainty for smaller range of sources associated with biomass combustion may be higher.

Table 5.18: Estimated Uncertainties of Selected UK National Pollutant Inventories

Pollutant

Estimated uncertainty %

CO

+/- 20

PM10

-20 to +30

PM2.5

-20 to +30

SO2

+/- 3

NOX

+/- 8

NMVOC

+/- 20

HCl

+/- 20

BaP

-70 to +200

PCDD/f

-50 to +200

As

-50 to +100

Hg

-30 to +40

Source: NAEI

Life Cycle Emissions

Many of the issues highlighted in chapter 4 regarding uncertainties of greenhouse and energy gas balances also hold true for life cycle air pollutant balances. LCA air pollution impact data is context-specific. Emission of pollutants depends on a large range of parameters including transport type and distances ( e.g.NOx, CO, PM), fertilizer application rates (eg. NH3) and use of fossil fuel in processing activities. System boundaries, reference systems and allocation procedures can also affect results, in much the same way they affect carbon and energy balance results (chapter 4).

No LCA studies with pollutant emission data were found that were based on Scottish conditions. Given the site-specificity of these studies, all of the illustrative results presented in this chapter would need to be modified to be representative of Scotland.

5.7 Water Quality

Agricultural production of biomass feedstocks can have an impact on the quality of both surface and groundwater resources. Among the processes capable of compromising water quality are nitrate leaching, pesticide application and the release of acidifying gases such as ammonia.

Nitrate ( NO3) leaching refers to the movement of dissolved nitrates into deeper soil layers or leaving the bottom of the soil profile. Nitrate can also be lost in surface runoff. Several factors influence leaching rates besides fertiliser loads. Soil type plays an important role in nutrient leaching rates, for example, as the nitrogen leaching rate of coarse-textured clay soils is approximately twice that of fine-textured clay soils (Borjesson 1999). Rainfall levels also influence nitrate leaching rates.

Pesticides are also susceptible to leaching and can move into deeper soil layers, contaminating groundwater. This depends on the properties of individual pesticides, such as solubility and adsorption. Among the most common active pesticide substances encountered in groundwater are isoproturon and mecoprop (Turley et al. 2004).

Several pollutants released during the life cycle of biomass systems can have an acidifying effect on the environment (see air quality section), including SO2, NOx and HCl. In agricultural systems, ammonia is very important in the acidification and eutrophication of natural environments. A primary means of ammonia release to the atmosphere is through the process of ammonia volatilization where nitrogen is lost as ammonia to the atmosphere upon application of liquid manures and slurries. Ammonia reacts in the atmosphere to form ammonium which is removed by rain. NH3 compounds can be transported atmospherically over significant distances and can result in acidification of water and soil resources in areas detached from agricultural zones.

Water quality can also be compromised by soil erosion which deposits high sediment loads in nearby water courses and also leads to increased levels of nutrients such as phosphates.

5.7.1 Forestry and Agricultural Residues

Removal of forestry and agricultural residues does not involve additional application of fertilisers or pesticides and therefore is expected not to further contribute to soil water quality through increased nutrient inputs. Nevertheless, the removal of residues could leave soils more susceptible to erosion and result on an impact on water quality through increased sedimentation of water courses. There is some debate as to how important these losses may be in Scotland, however (Carling et al. 2001). There are Forestry and Water Guidelines which regulate harvesting and planting activities which will affect water course on state-sector land and private sector land covered by grants (these must meet UKWAS standards).

5.7.2 Dedicated Energy Crops

Reduced fertiliser and pesticide inputs in combination with reduced soil disturbance should lead to reduced nitrate leaching rates once arable lands have been converted to energy crop systems, although considerable leaching may occur in the establishment season of energy crops (Bullard et al. 2003). In the absence of fertiliser, however, leaching rates can be reduced significantly to approximately 3 kg N ha -1, as opposed to > 150 kg N ha -1 under typical arable crop conditions (Christian et al. 1997). Whereas conversion from annual crops to dedicated perennial bioenergy crops will result in a net decrease in fertilizer application, increased cultivation of grain-based biofuel crops would probably require a net increase in fertilizer use. The longer growing season, year-round soil cover and extensive root systems of perennial energy crops all help to reduce nitrate leaching in comparison to annual row crops. Consequently, dedicated energy crops will tend to have favourable impacts on water quality in comparison to conventional biofuel crops.

Weed control is required for short rotation coppice before cultivation, shortly after planting and following harvest (Forestry Commission 2002). Similarly, miscanthus only requires pesticide application during the early establishment phase to keep out competitors (Bullard & Metcalfe 2001). For most of the growing cycle, therefore, no additional pesticide is required, resulting in lower probability of contamination of groundwater sources through pesticide than with annual food crops.

The net impact of growing bioenergy crops on water quality depends upon the management practices used when growing the energy crops and the practices used on the land use that the energy crops replace (St Clair 2006).

Effects on Hydrology

A recent study for MAFF (2001) reviewed the effects of bioenergy crop systems on soil hydrology. The results of the study showed that both short rotation coppice and miscanthus grown in different agroclimatic zones in England consistently used more water than they might replace. This is due to both increases in surface run-off as well as decreases in deep percolation below the root zone. The relative influence of each process is determined by site-specific factors such as rainfall levels and soil water availability. In soils with large water availability, the large water requirements of energy crops can lead to reductions in water percolation below the root zone. When the soil's water carrying capacity is the limiting factor, water uptake by energy crops is driven mainly by rainfall levels. These effects on hydrology are more significant in drier areas, such as the east of England and are less significant in Scotland where rainfall levels are consistently high.

5.7.3 Transport Biofuel Feedstocks

Managing oilseed rape for biodiesel or cereals for bioethanol production offers little opportunity to reduce fertiliser and pesticide inputs compared to their management for food (Turley et al. 2004; St Clair 2006). Replacement of natural regeneration set-aside land with these crop alternatives will lead to increased inputs of pesticides and fertilisers and also to higher nitrate leaching levels. However, nitrate leaching rates are not determined by fertiliser rates alone, and typically set-aside has higher residual nitrogen levels which are subject to over winter loss (Turley et al. 2004). In general, cereals are more efficient in terms of fertiliser N use, compared to root crops and oilseed rape and consequently have lower nitrate leaching rates (Table 5.19). Oilseed rape may represent a higher risk of nitrate leaching relative to other arable crops, due to high levels of residual N left in the soil following harvest (Turley et al. 2004).

Table 5.19: Nitrate Leaching Loss from Arable Crops

Crop

Amount of NO3 N leached (kg ha -1 yr -1)

Potatoes

98

Oilseed rape

74

Sugar Beet

30

Cereals

30

Unfertilised grass

10

Source: Turley et al. 2004

Water quality can also be compromised by pesticide application. Table 5.20 provides information on the average weight of pesticide active substance applied to different transport biofuel crop options. Cereals typically require greater pesticide applications than oilseed rape, but both crops require substantially less than potatoes and more than natural regeneration set-aside.

Table 5.20: Total Weight of Active Substance Applied per Hectare of Crop Grown

Crop

Total weight of a.s. applied per ha (kg)

Winter wheat

4.37

Winter barley

3.8

Ware potatoes

14.86

Oilseed rape

2.37

Spring barley

1.51

Sugar beet

3.72

Natural regeneration set- aside

.93

Source: Turley et al. 2004.

5.7.4 Relevance to Scotland

The most likely scenario is that energy crop production will take place on set-aside land. Overall, major negative impacts on water quality would not be expected due to the very modest fertiliser and pesticides inputs these crops require. Of the biofuel feedstocks, oilseed rape is expected to be the major crop option for Scotland. When replacing set-aside land, the effect on water quality is expected to be negative due to higher nitrate leaching rates and pesticide inputs. Replacement of other break crops in grain rotation systems, however, would be less environmentally detrimental, although it is hard to predict with accuracy what changes in water quality would be. Detailed computer models to simulate nitrate leaching are available and the impact of energy crops on nitrate leaching could be assessed in the future.

5.8 Soils

Biomass feedstock production impacts soils mainly through effects on soil erosion and soil fertility. Erosion can reduce crop yields directly by physically damaging plant crops or indirectly through decreased soil fertility resulting from organic matter losses. Severe erosion is relatively uncommon in the UK and ultimately erosion risk is determined by such factors as soil type, slope, land use and management practice (Brierley et al. 2005, Turley et al. 2004). Frequent tillage, for example, can increase the likelihood of erosion. Soil erosion can lead to reduced organic matter content which in turn results in decreased soil fertility.

The impacts of biomass feedstocks on soils can be negative or positive, depending on the crop and on the land use system being displaced. Besides impacts on soil erosion and fertility, some biomass feedstocks can also be used to remediate soils contaminated with high heavy metal loads.

5.8.1 Forestry and Agricultural Residues

Incorporation of crop residues into soil improves soil fertility and excessive removal could have detrimental consequences on soil quality. There is evidence, however, that up to 50% of agricultural residues can be removed without any major effects on fertility ( IEA 2004).

Removal of forestry residues from forest floors can also impact on soil quality. The extent of this impact depends largely on the mode and intensity of harvesting as well as on the soil type (Brierly et al. 2004). Peatland soils, for example, face a higher risk of damage than podzolic soils or shallow gley soils. The utilisation of brash mats is extremely important during residue harvesting and their removal can greatly increase erosion and promote nutrient depletion (Brierley et al. 2004). Without brash mats, the heavy harvesting machinery used can also result in increased soil compaction.

A recent study for the Department of Trade and Industry (Brierly et al. 2004), modelled the suitability of different woodland sites in the UK for extraction of forest residues, based on a set of different environmental criteria, including impacts on soil fertility, nutrient leaching, soil compaction and erosion. Although the researchers advocated a precautionary analysis of the results as the modelling procedure was based on simplistic classification systems, the final results indicated that there are only limited opportunities for forest residue extraction in Scotland's upland soils. This was due to high compaction of Scotland's wet peaty soils and in the west of Scotland, high acidification impacts.

5.8.2 Dedicated Energy Crops

Conventional agricultural practices involving intensive soil tillage make the soil susceptible to erosion and lead to decreased soil fertility. Energy crops can reduce erosion risk by providing a higher ground cover than annual crops. In this respect, the benefits of energy grasses are usually greater than short-rotation coppice, as they achieve complete ground cover over a short period of time (Bullard et al. 2003). It must be noted, however, that the establishment phase of energy crops will require soil disturbance which could lead to higher initial erosion rates, which fall rapidly once ground cover is increased.

Use of willow crops as windbreaks in areas of high wind erosion could potentially result in increased yields of surrounding food crops (Borjesson 1999). Similarly, energy crops could be used as vegetation zones to protect against water erosion, provided soil coverage is high (Borjesson 1999).

Bioremediation Potential

Short-rotation coppice crops can be used to mitigate the effects of soil contamination, a process called bioremediation. This is commonly used to neutralize the effects of high heavy metal concentrations, for example. Willow and poplar have high cadmium and zinc uptake rates and can thus be used to remediate heavily contaminated land (Britt et al. 2002).

This property of SRC crops also provides advantages for the disposal of sewage sludge, slurries and other wastes, as the crops are able to take up the often high metal contents of these substances. Willow varieties vary in their uptake potential and tolerance to heavy metals. There is also some evidence that miscanthus can have a positive effect on heavy metal remediation, although high levels of heavy metal can reduce plant productivity (Britt et al. 2002). It must be stressed that the application of sewage and agricultural wastes to willow can be accompanied by some unfavourable consequences, including the possibility of heavy metal bioaccumulation in the food chain and contamination with pathogens which may present a risk to wildlife. For a thorough examination of the bioremediation potential of energy crops, the review by Britt et al. (2002) for DEFRA is recommended. There is also potential for SRC willow to be used to treat landfill leachates, a practice which is already being carried out in Sweden (Borjesson 1999).

5.8.3 Transport Biofuel Feedstocks

The frequent tillage of annual crops such as oilseed rape or wheat results in a higher soil erosion risk than cultivation of energy crops. Evans (2002) devised a classification for the erosion risk posed by individual crop types in which the percentage of observed channel erosion was expressed as a fraction of the percentage of arable land cover of the crop for England and Wales. Results from this analysis are shown in Table 5.21. As seen in the table, the overall erosion risk of winter cereals and oilseeds is relatively small in comparison to root crops such as potatoes and sugar beet, although the ultimate erosion risk is heavily influenced by topography and soil type.

Table 5.21: Index of Channel Erosion of Possible Transport Biofuel Crops.

Crop

% erosion occurrence/ % arable area

Sugar beet

4.05

Potatoes

3.28

Spring cereals

.83

Winter cereals

.69

Winter oilseed rape

.29

Source: Turley et al. (2004), based on Evans (2002).

Oil seed crops, if they replace other arable crops, will yield little benefit for soil structure and may have negative impacts if they replace long term setaside.

5.8.4 Relevance to Scotland

Very little information exists on erosion rates resulting from agricultural practices in Scotland. Lilly (1999) estimated that 53.4% of Scottish soils face a moderate risk of erosion while a further 32.1% face a high erosion risk. This analysis did not include vegetation cover as a parameter, however. Nevertheless, the high erosion and compaction risks associated with many Scottish soils renders them unsuitable for residue extraction. Conversion of set-aside to production of oilseed rape under conventional tillage would most likely increase soil erosion risk. Conversion to energy crop systems, however, would require less cultivation and result in greater year-round soil coverage with no great risks of erosion. Further studies are necessary, however, to provide a more informed analysis of the possible impacts of the expansion of biomass feedstock production on soil quality in Scotland.

5.9 Biodiversity

As with effects on soil, the effects of energy systems on biodiversity depend on the land use transition that occurs and the intensity of cultivation. Conversion of native vegetation to agricultural use will undoubtedly have a negative impact on biodiversity, but when the conversion is from set-aside to cropland or degraded area to cropland, impacts on biodiversity can be favourable (Christian et al. 1997).

5.9.1 Forestry and Agricultural Residues

Although there are very few studies in this area, the available evidence suggests that removal of forest residues can have an adverse effect on local biodiversity. Bengsson et al. (1998) compared the invertebrate fauna of two sites in Sweden, one where residues had been removed and another where they were maintained on-site, and found that the removal of residues during whole-tree harvesting greatly reduced populations of spiders and other predatory insects (30-60% reduction).

Brash removal may also deprive small vertebrates, invertebrates and fungi of important habitat and food resources, resulting in decreased biodiversity (Brierly et al. 2004). The local depletion of nutrients caused by brash removal may also affect biodiversity in more indirect ways. It could, for example, result in egg-shell thinning which has been attributed to reduced nesting success of European birds in recent decades (Green et al. 1998).

Positive impacts would include restructuring of woodlands through increased thinning which can benefit wildlife.

5.9.2 Energy Crops

The patchwork of different age-stands created by short rotation coppice plantations can provide a heterogeneous network of habitats that can provide biodiversity benefits for different animal groups, including phytophagous insects, songbirds (especially warblers) and pheasants. Ground flora plays a key role in the resulting biodiversity of SRC willow plantations and the application of pesticides can counteract any positive biodiversity effects obtained by the patchwork of different-aged stands created (Britt et al. 2002). Other management practices such as application of sewage sludge are believed to influence biodiversity, but there are very few studies to support these suggestions. Addition of organic wastes could even indirectly promote biodiversity through addition of nutrients that would promote growth of ground flora (Britt et al. 2002). Soil biodiversity is also increased in relation to annual row crops, as shifts to perennial energy crop systems have been shown to improve the diversity of soil fauna such as earthworms, wood lice and carbides in general (Borjesson 1999).

Short rotation coppice is likely to be more beneficial than energy grasses such as miscanthus and reed canary grass, as these are non-native grasses and the shade produced by them is likely to exclude other flora (Turley et al. 2004). Relative to annual crop systems, however, energy grass plantations can be beneficial for populations of certain bird species (Murray et al. 2003). Nevertheless, large areas of energy grasses without the presence of other habitats are unlikely to bolster local biodiversity.

5.9.3 Transport Biofuel Feedstocks

Replacement of natural regeneration set-aside with oilseed rape or cereals would have a detrimental impact on some farmland birds, although some species that may use oilseed rape as a food source in summer would benefit (Turley et al. 2004). Replacement of set-aside for winter oilseed rape would also reduce the availability of stubble that many birds depend on during the winter season. Some of these detrimental impacts on biodiversity could be mitigated, however, by positive management practices such as the maintenance of field margins.

The relatively high requirement for pesticides in cereals and oilseed rape is also likely to have impacts on local biodiversity, due to impacts on non-target species and indirect effects on weeds and invertebrates, which provide food for several bird species. Populations of farmland birds in Britain have been in stark decline over the last three decades and this could be largely linked to the decline in winter stubble (Turley et al. 2004). Many species, however, are capable of nesting on oilseed rape crops including skylarks, yellow wagtails and sedge warblers. Impacts on biodiversity are also complicated by timing of management, which may favour breeding habits of some birds over others, for example.

A note on Imported Biofuels

There has been much concern of late over the replacement of native ecosystems in Africa, Asia and Latin America for biofuel crop production. Prime examples of this include the displacement of native Indonesian rainforest with oil palm plantation and the replacement of native forest and savanna ecosystems in Brazil with soybean plantations (Pearce 2005; Galbraith 2005). This concern should serve as an additional motivation for the strengthening of a biofuel industry based on home-grown crops.

5.9.4 Relevance to Scotland

In Scotland, growth of energy crops is likely to occur on set-aside land. In this case, further studies are necessary to ascertain what the effects of expansion of bioenergy crop production would be on biodiversity. Replacement of set-aside with oilseed rape is likely not to yield the same biodiversity benefits as short rotation coppice, but replacement of other break crops in arable rotations is likely to reduce negative impacts on biodiversity.

5.10 Environmental Impact Conclusions and Recommendations

Combustion Emissions

  • Substantial gaps in reliable emission data exist for biomass combustion for energy. .
  • Substantial gaps in reliable emission data exist for transport emissions associated with biofuel use.
  • Key areas for a better understanding of bioenergy combustion emissions include include PM2.5, PAH, VOC, ultrafine and trace element emissions.
  • Combustion emissions for heat/power/ CHP are heavily influenced by the conversion technology used, with the conversion technology often having a more significant role than the fuels themselves. In relation to coal, SOx and NOx emissions are generally reduced. Co-firing with 10% biomass can decrease SOx and NOx emissions by 5-10%.
  • Changes in air pollutant emissions resulting from the uptake of 1.7 million tonnes of woodfuel for energy in Scotland in 2020 were projected for two biomass scenarios: 1) 50% small-scale heat and 50% large CHP and 2) 30% cofiring, 35% large CHP and 35% small-scale heat. Results showed that this would lead to reductions in SOx equivalent to 2.8-3.8% of the UK baseline, increases in NOx of 0.02-0.05% of the UK baseline and a decrease in total PM of between 0.43 - 0.63% of the UK baseline.
  • The projections suggest that biomass combustion for energy will benefit SO2 and PM emissions in Scotland although NOX emissions would be slightly increased. An increase in PM2.5 emissions merits further investigation as an air quality objective is proposed for this pollutant.
  • The range of combustion emissions reported from transport biofuels is wide and there is much uncertainty associated with these estimates. The general trend in light duty vehicles is that in relation to fossil diesel, HC, PM and CO emissions are decreased while there tend to be increases in NOx emissions. For low-blend bioethanol, there appear to be no major significant changes in emissions of NOx and HC, although emissions of PM are found to be significantly decreased, while acetaldehyde emissions are greatly increased.
  • Changes in air pollutant emissions arising from uptake of biofuels under two different scenarios to 2020 were projected: 1) all vehicles run on 5% biofuel, 2) all diesel vehicles run on 100% biodiesel and all petrol vehicles on 5% ethanol. The first scenario would result in an increase in NOx emissions of 3.8% relative to Scotland's current road transport emissions and decrease in PM emissions of 4.6% relative to Scotland's current PM emissions, whereas the second scenario would lead to increases in PM emissions by 2.0% relative to Scotland's current total PM emissions. This is due to the disproportionate influence of heavy duty vehicles in this scenario.
  • The increase in road traffic NOX and PM emissions is significant for local air quality management and indicates further work is needed to quantify impacts on air quality.

Life Cycle Air Pollutant Emissions

  • Compared to greenhouse gas and energy balances, there are very few LCA studies that include air pollutant balances, and none were identified that were directly representative of Scottish conditions. Scottish-specific studies need to be undertaken.
  • The LCA studies reviewed suggest that life cycle emissions of major pollutants such as NOx, SOx and PM from biomass energy production are higher than for other renewable technologies and higher than gas, although reduced in relation to coal.
  • Studies that report summed life cycle non- GHG environmental impacts (acidification/eutrophication) generally report that bioenergy systems compare unfavourably with other renewable technologies and in many cases unfavourably with fossil fuel (gas or oil) based systems, although there is significant improvement in relation to coal. High NOx emissions from biomass systems are largely responsible for this trend.
  • Life cycle non- GHG impacts from transport biofuels produced from dedicated crops are consistently greater than those of reference fossil-fuel based systems. Fertiliser NH3 emissions have a considerable bearing on this.

Soil quality, water quality and biodiversity

  • The impacts of biomass energy systems on soil quality, water quality and biodiversity depend both on the biomass energy system in question and on the land use being replaced. As a general rule, fertilizer-intensive systems based on frequent tillage ( e.g. oilseed rape) are likely to have more negative impacts than those requiring less fertiliser input and providing more ground cover ( e.g. short rotation coppice). When set-aside land is replaced by oilseed rape, the effects on air quality, water quality and soil structure are likely to be unfavourable.

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