Energy from Waste (EfW)
Energy from waste (EfW) facilities combust waste under controlled conditions, to reduce its volume and hazardousness, and to generate electricity and/or heat. Electricity generated from a typical 100,000tpa facility would be roughly equivalent to the electricity usage of 10,000 houses and would be exported to the national grid. Any heat produced by the plant could be used in industrial or district heating schemes if appropriate to the need and the infrastructure can be developed. Plant capacities are regarded as small scale (~150,000 tpa or less), medium scale (~150 - 250,000 tpa) or large scale (> 250,000 tpa) based on available throughputs. CHP schemes are particularly suitable in urban locations where there is likely to be a heat demand within
a reasonable distance of the CHP plant.
Incinerators require control measures for stack emissions, flue gas cleaning equipment e.g. acid scrubbing plant, carbon injection system, electrostatic precipitators or fabric 'type' filters must be provided, depending on the type of control system employed. The equipment employed for the control of stack emissions can occupy much of the space required by the plant and these cleaning processes are now well established in the UK (and additional good practice can be demonstrated in EfW plant overseas, where emissions levels are tighter still) and form a significant proportion of the overall capital costs of the plant (30 – 60%).
There is also a requirement to deal with the residues of the combustion process. There are two principal solid residues from thermal treatment systems: the bottom ash, which is the solid remainder of the waste feedstock after processing; and the Flue Gas Treatment (or Air Pollution Control) residues from the air pollution control process.
Some of the residues from stack emission control process are classified as hazardous waste and may be difficult to dispose of.
Energy from Waste (incineration with energy recovery)
The majority of incinerators currently operating in the UK are moving grate energy from waste plants designed to handle large volumes of household wastes with no pre-treatment. EfW facilities typically have two or three combustion units, which range from 100 tpd to 3000 tpd. Modern waste to energy plants tend to be moving grate incinerators: the waste is slowly propelled through the furnace by a moving mechanical grate, waste continuously enters at one end and the ash is discharged at the other. The combustion process is also aided by gravity. As the waste descends it goes through three stages in the process: drying, combustion and burnout. Considerable attention has to be given to ensuring the necessary conditions for optimum combustion.
Schematic of Inputs and Outputs of a typical Energy from Waste facility process
 
Typical capacity: 60,000 tpa - 600,000 tpa (although typically 100,000 – 250,000tpa)
Land requirements: Typically 2.5 to 3.5 Ha
Capital costs: 100,000tpa plant ~ (See full report.)
200,000tpa plant ~ (See full report.)
400,000tpa plant ~ (See full report.)
Operating costs: 100,000tpa plant ~ (See full report.)
200,000tpa plant ~ (See full report.)
Staffing requirements: Staffing levels, including technical competence, management and administrative resources will required and will vary depending on the size of the facility. An estimated 30-55 persons are required to operate medium sized facilities of around 200,000 - 450,000 tpa
Strengths and Weaknesses of 'Mass Burn' Incineration Units
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Strengths
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Weaknesses
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Handle MSW waste with no pre-treatment required
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High capital costs. Since fixed costs are high the need for consistently high utilisation is paramount.
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State-of-the-art technology in global use including pollution control technology
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Negative public perception - NIMBY (stack emissions and lack of understanding of technology)
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Energy recovery including Combined Heat and Power (CHP) plants and opportunity for district heating programmes
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Residue quality and disposal, although bottom ash can be reused
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No long term liability
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Debate over measurement and long term health effects of dioxin emissions, it should be noted that controls issued throughout the 1990s and more recently with the Waste Incineration Directive have reduced dioxin emissions to well below that of other combustion processes.
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Proven and commercially available technology
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Minimum materials recovery, except for ferrous materials
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Reduces volume of waste by ~90%
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Minimum or guaranteed tonnage may be required by the operator to cover costs
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Other Issues
Energy from waste has a particularly poor public image and has been the subject of campaigns by environmental groups on the grounds of perceived health impacts from emissions to air. The waste incineration industry has reduced its emissions over the last ten years by a factor of 10 or more due to enhanced legislative environmental controls. It remains the most proven means of thermal treatment of waste to reduce, sanitise and recover energy from municipal type wastes. The energy recovery is not as efficient from such facilities compared to some more advanced thermal treatment processes, however if the heat is also utilised (via a district heating scheme) the process becomes far more efficient in terms of energy recovered.
Recycling of the bottom ash from EfW plant into construction applications reduces the quantity requiring landfill disposal to under 10% of the feedstock mass. Flue gas treatment residues from the emission control system either require further treatment or disposal to hazardous waste landfill. Some metals may also be extracted from the bottom ash for recycling, however this (and ash recycling) does not count towards recycling targets.
Electricity produced from EfW does not fall within the Renewables Obligation (and receive a preferential sale price), although the electricity does count towards the UK Renewable Energy targets.
Further Reading
The Environment Agency View: Click here to go to the EA web site article which supports Incineration when all the necessary criteria are met.
Waste Incineration Directive: "There are 7,000 waste incinerators in England and Wales, ranging from small farm incinerators to large operations that burn household waste. The incinerators burn waste at very high temperatures, which turns the waste to ashes. Waste incinerator operators have improved their environmental performance greatly in recent years. They used to be the largest producers of harmful substances called dioxins, but these days, they're among the lowest". More....from the EA. (NB: All incinerators will be WID compliant by the end of 2005.)
Pros and cons of the burning issue: The burning issues concerning incineration, mainly of municipal solid waste, are debated in this article with contributions from the energy from waste lobby and from environmentalists broadly opposed to this method of waste disposal.
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