125. There is a range of materials that can be converted into energy using a variety of processes and technologies. Energy crops are plant materials grown specifically for use as a fuel, for example short rotation coppicing of willow or poplar. Forestry residues include "brash" (the material from conventional timber extraction and tree thinning which would otherwise be left on the forest floor, and "whole tree comminution" i.e. the mechanical felling and chipping of whole small trees to produce wood fuel chips. Agricultural wastes include straw, chicken litter and farm slurries. Other wastes that can be converted into energy include sewage sludge and municipal solid waste.

126. A general distinction can be made between biomass fuels produced from plant material or animal wastes and waste to energy fuels produced from municipal and industrial wastes. The distinction is not clear cut and can lead to much debate. In particular, as noted in NPPG 6, waste combustion developments may not always be, following an assessment of the Best Practical Environmental Option, the most acceptable means of managing waste under the National Waste Strategy (NWS).

127. It is proposed to include within the Renewables Obligation (Scotland), only energy derived from the biodegradable element of waste, as long as the fuel stream is at least 98% biodegradable. The incineration of municipal waste will thus not be supported. However, in order to support the development of more advanced waste technologies, such as pyrolysis, gasification and anaerobic digestion, the biodegradable fraction of energy from municipal waste using these technologies will qualify for inclusion, as will energy derived from forestry biomass, energy crops and biodegradable agricultural residues by any process.

128. The nature of the particular fuel will determine the way that energy is recovered. Dry combustible fuels, such as those from forestry and agriculture, can be burned (combusted) to produce heat and/or power. Wet wastes, particularly from farm slurries, can be digested to produce a methane-rich biogas, which can then be burned as a fuel.

129. Increasing use is being made of advanced conversion technologies, such as gasification and pyrolysis systems, which offer superior efficiencies compared with combustion for power generation. Gasification is a thermo-chemical process in which biomass is heated in the absence of air, to produce a low-energy gas containing hydrogen, carbon dioxide and methane. The gas can be used as a fuel in a turbine or combustion engine to generate electricity. Fast pyrolysis is a high temperature process in which biomass is rapidly heated in the absence of oxygen. As a result it decomposes to generate mostly vapours and aerosols and some charcoal. After cooling and condensation, a dark brown mobile liquid is formed which has a heating value about half that of conventional fuel oil.

130. For both dry and wet biomass wastes, disposal can often be an issue - for example, poultry litter, straw or slurries. Using the biomass wastes as an energy resource not only provides an environmentally acceptable method of waste disposal thereby assisting traditional agricultural activity, but also gives economic benefits by providing a source of heat or power.

131. Combined heat and power (CHP) is becoming an increasingly attractive option for biomass plant, offering a reliable low-cost heat source for industrial or commercial use (such as a district heating system for a small community), together with electricity that can be sold to the local grid. Forest residues, industrial wood wastes and a range of agricultural wastes are often readily available as fuel for CHP plant. Energy crops, such as willow or poplar coppice are also becoming important.

132. Several biomass fuels, such as energy crops and forestry residues differ from other sources of renewable energy in that they are grown rather than harnessed. They trap carbon dioxide when growing and give it off when burned. However, they are regarded as "CO₂ neutral" as the carbon released on combustion is only that which was absorbed during growth (‘contemporary carbon’) - the gas is simply recycled. When burned, instead of fossil fuels, a net reduction in carbon emissions is achieved.

Energy Crops

133. Energy crops are important as a renewable energy technology as they can be grown to meet the needs of the market. They may be grown specifically for use as a fuel and can provide long-term secure resources. This sets them apart from other renewable resources that must be harnessed where they occur. They require very little input of herbicides and pesticides, and when established on agricultural land usually result in an increase in bio-diversity.

134. The most advanced energy crop for northern European conditions is coppiced willow, grown on a rotation of 2-4 years, and commonly referred to as Short Rotation Coppice, or SRC. The crop is established by planting up to 18,000 cuttings per hectare. After 1 year these are cut back close to the ground which causes them to form multiple shoots (i.e. to coppice). The crop is then allowed to grow for 2 to 4 years, after which time the fuel is harvested by cutting the stems close to the soil level. The cut stems again form multiple shoots which grow on for a further 2 to 4 years to become the next harvest. This cycle of harvest and re-growth can be repeated many times. The shoots can be harvested as chips, short billets or as whole stems of 25-50mm diameter and 3-4 metres length.

"Establishing Short Rotation Coppice" Forestry Commission Practice Note 7. 1999.

135. For short rotation coppice, the area of land required to support the fuel consumption for each MW of power generation at 20% efficiency would typically be of the order of 630 hectares. This might fall to less than 350 hectares at the higher conversion efficiency available from ‘gasification’. Agricultural land which is taken out of food production under the European Community Arable Areas Payments Scheme, and is therefore ‘set aside’ for at least five years, is considered to have particular potential for short rotation coppice. Opportunities may also exist on derelict land undergoing restoration.

Power station size (MW_e net output)

1.5

6.0

30.0

Annual Wood Consumption (green tonnes)

Plant efficiency of 20%

22,000

86,000

432,000

Plant efficiency of 35%

12,000

49,000

247,000

Wood Fuel from Conventional Forestry

Integrated harvesting is the mechanical extraction and processing of whole trees in a single operation. The tree is separated into stem wood and wood fuel products on site. This leaves clear ground which can be replanted. This is considered to offer the significant long term potential for the cost-effective harvesting of fuel wood.

140. The use of wood for power generation can thus make use of materials which might otherwise be regarded as waste. The extraction of dead and over-mature trees and scrub for fuel can assist in the rejuvenation of derelict woodlands. Green wood, such as hedgerow cuttings which are currently burned at the roadside or dumped, might similarly become a useable product - rather than a waste - when a wood fuel market is established.

Environmental Issues Associated With Forestry Residues

141. There are a few environmental issues associated with the use of forestry residues as fuel. For example, forestry machinery can compact the soil and after a site has been cleared there is more water run-off, which could lead to soil erosion. However, this is a characteristic of forest clearing generally and does not arise specifically as a result of the activity associated with the collection of forestry residues for energy use. As such these are manageable by using good forestry practice.

142. By-products of wood fuel combustion include ash, limited quantities of gaseous nitrogen and sulphurous oxides, and, as noted above, carbon dioxide. Emissions of nitrogen and sulphurous oxides are negligible and significantly less than those of comparable fossil fuel stations, and can meet the relevant emission regulations. Flue gas is discharged from the plant via a chimney.

143. Large wood chip piles may produce liquids which could leach to watercourses, requiring a collection ditch around the storage area. Collection and control of run-off will probably be required in any case for larger plants, given the transport movements that such power plant will generate.

144. Wood ash is produced at a rate of around 1% of the total weight of the wood burned. If residues from forests are used, the inclusion of ‘tramp’ materials such as soil may increase this ash level to 3-4%. The ash may be usable in the manufacture of fertiliser.

Environmental Issues Associated With Agricultural Wastes

145. There are no significant environmental issues related to the use of straw as a fuel. For chicken litter, gaseous emissions from the combustion process can be reduced by state-of-the-art clean-up technology to meet statutory requirements at both EU and UK level. Transporting fuel to a power plant could increase traffic locally and will need to be taken into account in planning the facility. Ash produced as a combustion by-product can be recovered from the furnace (‘bottom ash’) and from the exhaust flue. It can then be used as a fertiliser.

146. Anaerobic digestion can be applied to sewage sludge and specific types of municipal solid waste to produce a biogas with a high methane content. The methane can be used to produce heat, electricity, or a combination of the two. The process has the benefit of using waste substances which are otherwise difficult to dispose of in an environmentally acceptable manner. It also maximises energy efficiency, requires the minimum pre-treatment of the gas and uses simple, well proven technology. There are no emissions requirements for anaerobic digestion plant, but there are requirements regarding the handling of the potentially explosive methane produced by the process.

147. New power and heat schemes in the UK must meet very high environmental and quality standards before they can be approved for development. Gaseous emissions from the combustion process are reduced by using state-of-the-art gas cleaning technologies to ensure compliance with both UK and European Union emission standards. For example, a facility for a wood fuel plant will require authorisation from either the local authority or SEPA under Part 1 of the Environmental Protection Act 1990. The smaller units will be covered by a local authority authorisation, with the larger units requiring an authorisation from SEPA when emissions to air, water and land will be considered.

148. A power plant using biomass or waste as fuel is an industrial development that may well be sited in a rural area. This can bring the advantage to the community of secure, skilled jobs in what are often economically depressed areas. Inevitably, though, there are some impacts due to increased traffic flows and the operation of the plant.

149. Many plants will be small and may be easily incorporated into existing agricultural buildings. They may not therefore require specific planning permission if they are ancillary to the main use of the site. However, heat and power generation plants will require planning permission.

150. Planning authorities will wish to consider the following issues :

• visual intrusion, particularly of the chimney;

• noise from engines, boilers, handling equipment and traffic;

• the local ecology; and

• traffic resulting from the transport of the wood fuel to the site and subsequent removal of by-products/wastes.

151. A conventional generating station will require a supply of water for steam production and cooling and, where water supplies present a problem, air cooling can be employed. Where gasification is used, water requirements will differ from those associated with conventional generating techniques. Gasification plants will also require cooling water systems but these will be smaller than in conventional plant, as the conversion efficiencies to electricity are higher. For conventional plant there is also a need for high quality feed water requiring the installation of on-site water treatment equipment. Gasification plants may not require treated water.

152. An estimated 12 million tonnes of waste is currently generated each year in Scotland. More than 95% of this waste is disposed of in landfill sites. Although this proportion will fall in the long term as a result of changes in waste management practices with, for example, increasing recycling, incineration and gasification, landfill is likely to contribute to waste disposal for some time to come.

153. Organic waste materials such as food, paper and garden wastes decompose in landfill sites to produce landfill gas, a mixture of methane and carbon dioxide in approximately equal quantities together with a wide range of minor components. Methane is the main product of the later phases of landfill gas generation. Using landfill gas provides energy from a source which would otherwise be wasted if it were just flared off or vented to the atmosphere.

154. Using landfill gas brings a number of environmental benefits. It originates from a renewable resource - waste. It encourages the comprehensive collection and management of landfill methane, a potent greenhouse gas, which might otherwise be released into the atmosphere. Combustion of landfill gas reduces its potency as a pollutant and a possible safety hazard. Landfill gas contains only contemporary carbon, therefore when used as a substitute for fossil fuels, it reduces net carbon dioxide emissions to the atmosphere while producing power and heat.

The Gas Source

155. Most landfill sites containing biodegradable organic matter will produce landfill gas through a complex process of microbial decomposition. The period of time over which landfill gas as actively produced will vary according to local conditions. Under favourable conditions, substantial gas generation from a large landfill site would probably be completed in 25-30 years. However, many factors control the decomposition process, including the proportion and nature of the organic material in the waste, moisture content, temperature, acidity, and the design and management of the site. These in turn affect the amount and decomposition of gas produced. Many landfill sites are already equipped with landfill gas collection and control systems designed to prevent the lateral migration of the gas into neighbouring land.

Gas Collection and Management

156. The gas is piped to an extraction plant on the edge of the landfill site. The plant will typically comprise :

• gas conditioning equipment;

• extraction pumps;

• a flare stack;

• pipework and valves; and

• control and monitoring equipment.

157. Gas is drawn from the waste from vertical and/or horizontal wells, each of which is monitored and regulated. It is then conveyed to the extraction plant, usually in polyethylene pipes placed underground. Landfill gas comes out of a landfill site warm and saturated with moisture. As it cools in the extraction pipework, liquid condenses out. The pipework is therefore laid at a gradient and incorporates condensate traps to prevent this liquid from hindering the gas flow. The type of gas conditioning equipment required depends on the use to which the gas will be put.

158. The extraction plant is normally built on a concrete slab in a fenced compound. The plant should be suitably bunded to ensure that there is no uncontrolled leakage of liquid effluent to ground or surface waters. A flare stack is required to control the gas at the landfill site even if there is no energy recovery.

Electricity Generation

159. Landfill gas can be used to generate electricity by fuelling various sorts of heat engine, such as large spark ignition engines which operate like a car’s petrol engine, dual fuel engine or gas turbines. These technologies are well established, although the use of landfill gas as a fuel for them has only been extensively demonstrated since the mid 1980’s.

160. Electricity generation plants tend to be located at or near the landfill site to minimise the need to pipe the gas across country. The generation equipment is usually integral with the gas extraction plant, in a compound typically 25m x 25m in size.

161. The degree of shelter required depends on the type of equipment installed. The gas extraction pumps and conditioning equipment might be in the open air, under an open-sided roof, or in a building along with the generator. Some generators are supplied in weatherproof prefabricated containers (3m high, 2.5m wide and 10m long) which are fixed onto a concrete plinth. Transformers, switchgear, control panels and instrumentation are housed away from any gas handling plant in separate rooms or buildings.

162. In addition to planning permission for the use of the land, a landfill site requires a waste management licence under Part II of the Environmental Protection Act 1990. The licence will normally cover the operational aspects of a site during its active life but may also need to include conditions relating to landfill gas generation long after landfilling has ceased and the site has been partially or completely restored. The waste regulation authority is also responsible for monitoring a site once it has closed and for issuing a completion certificate. Both these responsibilities are long-term considerations which may run in parallel with landfill generation.

163. Gas collection and storage equipment is not prescribed in the Prescribed Processes and Substances Regulations and will not therefore require an Integrated Pollution Control authorisation under Part I of the Environment Protection Act 1990.

164. The storage on-site of hazardous substances not connected with the waste disposal operation may require Hazardous Substance Consent under the Town and Country Planning (Hazardous Substances) (Scotland) Regulations 1993. Guidance is contained in SOEnD Circulars 5/1993 and 6/1993.

165. Standards for emissions from single engines of less than 20 MW thermal energy input or multiple engines of less than 50MW aggregated thermal energy input, such as those used in most landfill gas utilisation schemes, are not currently specified in the Environmental Protection (Prescribed Processes and Substances) Regulations. As such, the emissions from typical landfill gas plant are not currently regulated.

166. Liquid effluent derived from gas drying and treatment process will normally be treated and passed to the local foul sewage system, or returned to the landfill. Consent will be required under Section 24 of the Sewerage (Scotland) Act 1968 for discharges to the sewer system, and under Section 32(1)(a)(iii) of the Control of Pollution Act 1974 for discharges onto land.

Environmental Effects

167. In determining applications to generate electricity from landfill gas, planning authorities may wish to consider the following :

• safety matters associated with the handling, transporting and burning of gas;

• noise from the mechanical equipment;

• exhaust emissions to the atmosphere;

• visual intrusion, particularly the equipment and gas flare; and

• effluent and residue control.

Siting

168. Landfill gas plant should be located away from housing and other sensitive land uses, for reasons of safety and amenity. In practice this will rarely be difficult to achieve, given the large scale of landfill sites and the fact that they are often situated away from human settlement.

Landscape Impact

169. If the generation scheme is located amid large workings in which mineral extraction and waste disposal are continuing adjacent to a completed landfill, the visual impact of the development may be relatively insignificant. Alternatively, if extraction and landfill works have ended and the site is undergoing restoration, the planning authority should consider any appropriate mitigating measures to ameliorate any visual intrusion caused by the plant.