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10 Evaluation of Opportunities for Converting Indigenous UK Wastes to Fuels and Energy AEA/ED45551/Issue 1 Combustion, or incineration is by far the most common method of extracting energy from solid waste in most European countries, in either untreated form or as a prepared fuel. Our conclusions following a review of the area are as follows; • Combustion is a well established technology and the main technical challenges have been addressed. Whatever the difficulties with the properties of the fuel engineers have developed robust solutions that have been proven to work commercially at most scales. As a result of this, combustion remains the default technology for heat and power against which all new, more innovative, options must be measured. • There seems to be a combustion solution for all waste fuels. • There remains however significant technical uncertainty surrounding the impact of the composition of the waste on the performance of the heat exchange surfaces, particularly fouling and corrosion. • Other barriers remain, such as the largely negative public perception of such sites in some countries, notably the UK. • There are countless examples of combustion practice worldwide on which to draw. The technologies available for conversion of waste to heat and power are more widespread than those for conversion to transport fuels, although work is ongoing on advanced conversion for liquid biofuels. Combustion gasification and pyrolysis all use heat to break down the structure of the feedstock so that the chemical energy can be released either as heat or a fuel product. The conditions in the reactor determine the products. When combustible waste enters a high temperature environment it will first dry and then decompose into volatile gas and char components. Gasification processes use a limited supply of oxidant; usually air, to maintain both combustion and reducing reactions in the same reactor. Most of the energy in the fuel is transferred into the calorific value (CV) of the gas leaving the reactor. This gas can be burned separately in a boiler, engine or gas turbine. Alternatively it can be converted by a synthesis reaction to methane for injection to the Natural Gas Grid or liquid fuels for transport. In a pyrolysis process there is no oxygen and the char and volatile gas remain largely unchanged. The energy in the waste is retained in the CV of the gas and char removed from the reactor. There are a number of perceived advantages that are often quoted by gasification and pyrolysis suppliers as solutions for perceived weaknesses in the current market leader – incineration. These claims with our comment are shown in the Table 1 below. Table 1 Claims for gasification/pyrolysis with comments Incineration Gasification/pyrolysis Comment Large (Regional) size and fuel demand suppresses recycling. Small volume of hazardous and toxic fly ash. Bulky residual ash can be used as a aggregate. Can be built economically at the smaller sizes that can be integrated into local schemes. This remains to be demonstrated, but is probably true. No dioxin found in fly ash. This seems to be valid. Some options give low volume compact ash. Valid. Some incinerators can give low volume slag like ash but at a very high energy cost for ash melting.
Evaluation of Opportunities for Converting Indigenous UK Wastes to Fuels and Energy AEA/ED45551/Issue 1 Incineration Gasification/pyrolysis Comment Poor electrical efficiency and lack of suitable heat loads gives poor resource use efficiency. Some options can give very high electrical efficiencies. Smaller sizes can match heat loads better. Particularly as these are often independent of the power supply. Electricity and or heat only. Some options can be used to generate gas or transport fuels. Very few demonstrations of high electrical efficiency. Lack of enthusiasm for CHP. Installations in wood waste sector now making progress. True at very large scale. Following our review our conclusions were as follows; • Gasification technologies are being developed but often encounter severe technical difficulties when attempting the step of using the gas for power generation in an engine or gas turbine. • The robust approach of firing the gas in a boiler is proving reliable and a worthwhile innovation. These units can be produced in smaller sizes than conventional incineration and can be matched to heat loads in CHP and process heating applications to give potentially the best GHG abatement potential. • Gasification as a pre-treatment is likely to become more popular in cofiring as generators seek to increase the proportion of biomass in coal fired utility boilers. • There are few commercial pyrolysis plant operating on waste. • The use of gasification at very large scale for the production of transport fuels or pipeline methane is attracting increasing attention and is potentially an efficient use of resources. Biological processes for waste include anaerobic digestion for the production of methane fuel gas and the hydrolysis and subsequent fermentation to ethanol fuels of the hemi-cellulose and cellulose fractions of lignocellulosic materials. Following our review of these technologies our conclusions were as follows; • Anaerobic digestion processes are technically viable and available for most wet waste feedstocks including sewage bio-solids. • Using a proportion of food waste is important to the economics of anaerobic digestion due to its high gas generation potential and gate fee. Manures are bulky and have poor gas potential. • Digestate disposal is becoming difficult due to constraints in nitrogen sensitive areas. • Composting is a lower cost alternative to AD but does not produce energy and can be regarded as a competitor to AD. It is popular with waste managers due to the cost and its status as a recovery, rather than a disposal process. Disposal of compost to land suffers from the same constraints as AD digestates. • Hydrolysis and subsequent fermentation to alcohol fuels of the hemicellulose and cellulose fractions of lignocellulosic materals is still at the research stage although there are plans for demonstration plants in the EU and USA. These are likely to be co-located with conventional grain ethanol facilities. • Feedstocks are limited at present to clean wood and agricultural residues. • We question whether it should be a priority for the UK at present where clean residues are relatively expensive and could potentially be used in more efficient ways. 1.3 Risks and barriers to deployment The risks and barriers that are faced when adopting novel technologies can be considerable. In the table below we first identify the main risks in deploying the technologies described above we then describe in more detail those that we have classed as high or medium. 11
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South East 327 - - 55% of MSW by 20
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Appendix 3 MBT Methods Specific in
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Figure 27 Orchid Environmental proc
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