Recovering energy from unrecycled waste is preferable to landfilling it, according to the government’s waste hierarchy.
There are a number of ways of recovering energy from waste. The most basic one is collection of the landfill gas which is given off as wastes degrade in landfills. After hydro power, landfill gas is the second largest contributor to renewable energy in the UK (contributing 23% in 2004).
Another method is to separate the combustible parts of the waste (such as paper, food and plastics) and make them into a solid fuel. Biodegradable waste (such as food ) can also be treated by anaerobic digestion to produce a biogas. This is the same process which is used to process sewage sludge.
The most widely used method of recovering energy from waste (EfW), and the one most people mean when they use the term, is the combustion (or incineration) of waste to recover the heat which is released. This heat can be used to provide steam or hot water to nearby industry or housing, or to generate electricity for the National Grid. Sometimes it is used for both, which is known as Combined Heat and Power (CHP).
While there are a number of different ways to recover energy from waste, not all are suited to deal with all wastes.
Landfill gas recovery
This uses a system of perforated pipes laid into the landfill site through which the gas given off as wastes rot is drawn. The collected gas can be used to run vehicles or to drive gas engines. Landfill gas is around 50% methane, which is a very potent greenhouse gas, so capturing it rather than allowing it to seep into the air is important. However the European Landfill Directive, which will dramatically reduce the amount of degradable waste being landfilled in the next ten years, will gradually have the effect of reducing the amount of gas generated.
Anaerobic Digestion (AD)
The UK Government is strongly supporting the use of AD for treating biodegradable waste, because it removes the material from landfill and reduces the potential for greenhouse gas emissions. AD is a biological process where the decomposition of waste is accelerated in an enclosed system, using micro-organisms to degrade the material. This produces a biogas which can be used as a fuel. There is also a solid residue from the process (known as digestate) which can be further processed to make it into compost, and a liquid residue.
AD is only suitable for treating biodegradable waste so materials such as glass, metals, stones and plastics must first be removed. There are around 6 AD plants handling food waste operating in the UK at present, and a large number planned.
Gasification is a partial combustion process where waste is heated but not fully burned. Usually the waste has to be shredded to a uniform size. The process produces a fuel gas suitable for use in power generation, and a solid residue for disposal to landfill. Typically gasification takes place at higher temperatures (1000 – 1200 degrees C) than municipal waste incineration (typically 850 degrees C) or pyrolysis (typically 500 degrees C). The process is similar to the one formerly used to produce town gas from coal.
Pyrolysis is often used in combination with gasification, and is a method of thermally treating waste to break it down, producing a gas. The process is similar to the method used in charcoal manufacture. The gas can be used to generate electricity, or it can be condensed to form a fuel oil, leaving a solid residue for disposal.
Pyrolysis and gasification are not suitable for treating mixed waste. There are no pyrolysis plants treating municipal waste in the world, although some plants process single wastes such as old tyres. There is an increasing number of gasification plants handling waste – maybe 30-40 – across the EU, and a few under construction or proposed in the UK.
In the most common system waste is burned on a moving grate, with air fed in above and below to ensure it burns thoroughly and, as a result, gives off fewer emissions. The hot gases given off are directed to a boiler to recover the heat. The ash which remains below the grate (known as bottom ash) is removed and can be used to make road bases or building blocks. It is not hazardous.
Once cleaned the gases are discharged up the chimney. The residues from the different processes used to clean the gases – known as air pollution control residues – together with any dust particles trapped in the chimney stack are kept separate and sent for disposal in a dedicated landfill.
They are treated as though they were hazardous, although typically the biggest hazard comes from their heavily alkaline nature because of the lime used in the gas cleaning process. Air pollution control equipment can cost more than half of the total cost of the EfW plant.
There are more than 400 waste incineration plants operating in the EU, and worldwide it is the most proven and advanced technology for dealing with mixed municipal waste which remains after recycling. Unlike other technologies, there is no need to pre-treat the waste and the plant can handle all wastes.
In the UK there are currently 18 waste incineration plants in operation, and a number of additional ones planned.
Public perception of incineration
At the beginning of the last century, incineration was used to reduce the volume of waste without first recovering recyclable materials or capturing the energy from the process, and with little or no control of the emissions. There are no longer any incineration facilities like that in the UK, because stricter rules have been introduced and old plants which could not meet new standards have been closed down.
Modern plants burn only the waste that remains after dry recyclables have been separated. The more materials that are separated for recycling and composting, the less residual waste there will be, but there will always be some as not everything can be re-used, recycled or composted.
Recovering energy from waste by incineration is widely used around the world, including in countries where there are high recycling rates showing that there is no conflict between recycling and EfW.
Controls on EfW
Before an EfW plant – using any technology – can begin operating, it must be approved by the Environment Agency and acquire a licence to demonstrate that it can meet the European Union’s strict rules on emissions in the Waste Incineration Directive.
The Environment Agency continues to monitor the plants and often modern plants have on-line access to their emissions data so that everyone can see whether it is meeting its limits.