WTE TECHNOLOGY

There are three (3) main types of thermal conversion Waste-to-Energy (WTE) technologies; Gasification (including Plasma-arc Gasification), Combustion, and Pyrolysis (including High-Temperature Pyrolysis).

Combustion

Combustion of waste is achieved by heating waste in excess of oxygen. ‘Mass burn’ refers to a process that accepts raw or post recycling municipal solid waste (MSW) without any additional pre-treatment (e.g., no shredding or refuse-derived fuel (RDF) production). The process can best be explained by examining the combustion of waste in grate furnaces, where the fuel forms a bed on top of the grate, and the combustion air is injected through the grate. 

Combustion is a sequence of close-coupled physical and chemical reactions; initially, the waste dries. Then reactions occur as volatile compounds are heated and de-volatilised from the solid phase into the gaseous phase combusted. As this process occurs, ash comprised mainly of inorganic components is left behind. Combustion usually takes place with an excess of air (from below the grate as underfire air and via secondary and tertiary injection as overfire air), to ensure the proportion of fuel reacting with the oxygen is maximised.

Combustion of MSW is a very well-established technology with many hundreds of operational plants worldwide. Energy recovery is invariably in the form of a steam boiler which recovers heat from the hot flue gases to generate superheated steam. A steam turbine is used to generate electricity, except in cases where the steam can be used directly in a co-located industrial process for district heating or desalination. 

The net electrical efficiency of a WTE combustion plant generally varies from 15 – 25%, depending on the size of the plant and steam conditions. Efficiencies are relatively low compared to fossil fuel plants because of the lower calorific value of the fuel and limitations on steam temperatures to avoid excessive corrosion caused by acid gases and other compounds produced by the combustion of MSW. 

Thermal efficiencies above 30% are possible in combined heat & power configurations, where a proportion of the heat rejected to the atmosphere in plants that produce only electricity is recovered for process use. 

Mass burn combustion only converts the organic content of the MSW to energy. It leaves behind the inert content called ash, which comprises inorganic material mixed with post-combustion residues of ferrous/non-ferrous metals. The amount of ash varies with the demographics of the communities being served by the facility and the extent of recycling undertaken. However, typical thermal processing facilities produce ash in the range of 20% to 30% by weight of the total waste feed. However, since ash is relatively dense, on a volume basis, the waste is reduced in volume by about 90%. 

Depending on the regulatory framework and treatment process, ash from the combustion grate (bottom ash) can be treated and reused as construction material after further treatment, such as weathering (carbonation) or melting via plasma or slagging processes. Ash collected from the flue gases (including ash particulates arising from the boiler and air pollution control residues from bag filters) will contain hazardous compounds and generally requires pre-treatment, stabilisation, and careful disposal in fully engineered landfills.

The hot flue gases produced in the combustion furnace pass into a water tube boiler, where the energy is recovered via heat transfer to form superheated steam inside the tubes. The gases then pass through the Air Pollution Control (APC) system to be cleaned, where pollutants such as acid gases, oxides of nitrogen (Nox), heavy metals and dioxins/furans are removed before the cooled flue gas is emitted to the atmosphere via a chimney. Superheated steam is used within a steam turbine to generate electrical power. The combustion process generates two solid residues; Bottom Ash (IBA) and Air Pollution Control (APC) residues incorporating fly ash from the abatement equipment.