Incineration is a simple technology that involves passing waste over reciprocating or inclined roller grates, which allow air to be blown both through and over the top of the waste. The organic component of the waste is oxidised into carbon dioxide and water. The unburnt ash or slag is cooled in water and disposed of. Flue gas contains water, combustion gases, oxygen and nitrogen. The process generates heat, which is used for to generate steam or hot water for local heating and/or as electricity.
Incineration is not a very efficient way to generate power, as most of the energy is lost as heat and in flue gas. There is also significant potential for air pollution with flue gases containing particulates, dust, NOx, acid gases, dioxins, furans, polyaromatic hydrocarbons and heavy metals. Reducing the emission of these materials into the atmosphere adds significantly to the cost of this technology.
Additionally, incineration has been criticised for discouraging recycling due to it being more economically viable to simply combust all the waste than spend effort and resources in sorting and reprocessing recyclable materials. These factors and the risk of pollution have led to decreased social acceptance of this form of waste disposal.
Incineration is a mature technology, long used all over the world and still widely used in Europe, among other places. In Australia the last solid waste incinerator, the Waverley Woollahra facility in south Sydney closed in 1997. At the time, community and government concern over stack emissions and the availability of relatively low cost landfilling made continuation of waste incineration a politically unpalatable option. Since then, acceptance of other forms of AWT has all but eliminated mass burn incineration as a viable waste processing option.
Pyrolysis and Gasification59
Pyrolysis and gasification, like incineration (combustion), are forms of thermal treatment that convert waste into energy-rich fuels by heating waste under controlled conditions in contrast to incineration where waste is fully converted into energy and ash. The processes deliberately limit the conversion to energy and ash so that combustion does not take place directly. Rather, waste is converted into valuable intermediates that can be further processed for materials recycling or energy recovery. These technologies promise to extract more energy from waste than is possible with traditional incineration, and to do it more cleanly.
Pyrolysis and gasification plants can deliver about 1MW/hr of electricity for every tonne of waste and can be twice as efficient as incineration, depending on the waste used. The basic technology is not new and generally, specific processes have been developed and optimised for feedstocks of varying properties and quantities such as for waste tyres, sewage sludge or mixed municipal waste.
Both methods potentially produce fewer environmental emissions and higher levels of energy than incineration although the technology is largely unproven. Most pyrolysis and gasification systems developed to date have not been fully demonstrated for mixed (unsegregated) municipal or commercial waste but several proprietary processes have been developed in recent years. More than 150 companies around the world now marketing systems based on pyrolysis and gasification for waste treatment. Some examples of proprietary technologies include WasteGen, TwinRec, GEM Thermal Cracking and Thermal Convertor among others. A gasification plant that uses municipal waste to generate electricity became fully operational on the Isle of Wight (UK) in March. It is operated by the Norwegian firm Energos, which already has four plants in Norway and one in Germany. The British plant apparently generates electricity at a rate of 2.3MW per hour for sale to the national grid.
Pyrolysis refers to the thermal degradation of waste in the absence of air, that is, waste is cooked to about 800C without oxygen. The waste falls apart, separating into a compact residue (char), pyrolysis oil and syngas, which can be turned into transport fuels or can be burnt for heat, electricity or both.
A full-scale version of the WasteGen process has been operating in Germany since 1987. In this facility materials and energy are recovered from the incoming waste stream in a conventional materials segregation process, followed by a pyrolysis gas production process using a steel kiln rotating inside an insulated jacket clad in metal. The gas is subsequently burnt in either a gas turbine or a burning chamber to raise steam to drive a steam turbine. The materials segregation is designed to remove the unsuitable material, the material for composting and the recyclable materials both mechanically and if required, by manual selection. The resulting char product is not used and is sent to landfill. The dust and fly ash recovered in a baghouse gas cleaning system is considered hazardous and must be disposed of appropriately.60
Australian company Crucible Carbon is developing its own pyrolysis unit that aims to generate about three megawatts of electricity from 24,000 dry tonnes per annum of municipal solid waste derived-biomass61.
Gasification is similar to pyrolysis but uses a small amount of air in the heating process. Hydrocarbons are broken down into a syngas by carefully controlling the amount of oxygen present. Gasification technology appears to be subject to significant research and commercial effort and investment, perhaps due to the possible synergies with sequestration of clean char in soil.
The advantages gasification has over incineration include:
Flue gas cleaning can be performed on the syngas instead of flue gasses after combustion of which there are much larger volumes;
Electric power can be generated in engines and gas turbines, which are cheaper and more efficient than the steam cycle used in incineration.
A significant amount of energy is required to process the waste and clean the gas and this offsets to a significant extent the high efficiency of converting syngas to electric power. Although several waste gasification processes have been proposed, the few that have been built and tested processing real waste are doing so using fossil fuels. As an example, a plant in Chiba, Japan, has been operating since 2000, but has yet to produce any documented positive net energy.
Some examples of those operating in the gasification area include the Indian-based Ankur Scientific Energy Technologies. This company made 120 gasifiers in 2005 and has them operating in Italy, Germany, Russia, Vietnam, Sri Lanka among other countries. Its wood-fuelled gasifiers have electrical power outputs ranging from 3 kW to 850 kW. A 200 kW demonstration system is being established in Pokeno, New Zealand.63,64
The SilvaGas process developed by Future Energy Resources Corporation is being used by Biomass Gas & Electric (BG&E) in the US to produce energy from wood-waste. This process has been designed specifically for biomass, unlike other gasification processes which are based on coal gasification designs. This particular project is projected to be in commercial operation in June 2011.65,66
Commercial gasification plants using the Twin Rec process have been operating in Japan (six plants) while thermal converter technology has been used in commercial plants in Japan and Europe. GEM Thermal Cracking Technology has been running in pilot plants in Europe and the UK and pilot testing in the US has taken place using briquettes of dewatered sewage sludge, with coal as a binder.67
This technology works by passing relatively high voltage, high current electricity between two electrodes, spaced apart, creating an electrical arc. Inert gas or air under pressure is passed through the arc into a sealed container of waste material. Temperatures more than 13,800°C are reached in the arc column. At these temperatures most types of waste are broken into basic elemental gases and solid waste (slag). The device in which this takes place is called a plasma converter.
A good example of a full-scale commercial plant is the Canadian Plasco Conversion System.69 The GasPlasma process, a hybrid plasma and gasification system, is in operation in a pilot plant in the UK.70
Depending on the input waste (plastics tend to be high in hydrogen and carbon), gas from the plasma containment can be removed as syngas, and may be refined into various fuels at a later stage.
One of the disadvantages of trying to use biomass, particularly food waste, as an energy source is that the high moisture content of the waste requires a significant amount of energy to remove before combustion can begin. However, a new radiation technology under development in Brazil removes water from biomass using electromagnetic radiation. This is done without having to carbonize the wood, as might occur in a high temperature furnace. The process also energizes the biomass with higher calorific power.71
The electromagnetic dryer does not need high temperatures to work but uses simple molecular agitation so that water is removed but not the inner hydrocarbons of the wood. The process avoids the production of residual ashes in thermoelectric boilers and reduces air pollution.