Gasification converts hydrocarbon materials into a syngas at high temperature in the presence of oxygen in the form of air, steam, carbon dioxide or pure oxygen. It has been extensively used for coal gasification, but has also been extended as an energy technology for processing biomass, sewage sludge, municipal solid waste etcetera.
Waste management needs to involve the reduction of waste through public education, recycling where practically (and economically) possible, followed by high-efficiency thermal conversion of what remains.
Recently we have started to see gasification emerge as a key next generation technology in the treatment of residual wastes for the production of clean energy.
One could say that the increasing popularity of gasification is a result of a few perceived shortfalls of incineration, namely efficiency, flexibility and public perception. The uptake of incineration was partially due to the need to move away from our reliance on landfill, and the associated costs of landfill disposal increasing significantly in recent years.
Gasification has potential benefits over incineration, including efficiency, but also the flexibility in the way in which the energy is utilised. For instance, the syngas can either be combusted directly, used as a fuel in gas engines/ turbines, stored, or processed through catalytic processes to produce liquid fuels or chemicals.
The liquid fuels can be very versatile. The process of gasification produces syngas. However, it is the composition and quality, as well as what happens to this syngas, which are all of great interest.
An influence of this is the quality of the material which is put into the gasifier. In general a poor feedstock will yield poor quality outputs. As the recycling rate increases, the composition of the residual waste which would become the feedstock for gasification plants changes. Some of the changes are positive; for example, the theoretical decreases in food waste (and so the moisture content decreases too) within the residual stream as food waste collections are rolled out. However, increased recycling of plastics lowers
the overall calorific value of the waste.
Another notable change in residual wastes is the actual quantity. This plays to another advantage of gasification technologies – scale. While incineration typically is scaled for 150 000 tonne per year and over, gasification can operate at lower capacities (for example, the Energos facility on the Isle of Wight).
The pre-processing of waste is of key importance, to remove materials with a commodity value but of no value for energy recovery (i.e. metals) and to refine the fuel to a desired quality.
The recovery of materials from waste is becoming more common, through the development of more advanced MRFs and the development of mechanical biological treatment (MBT). MBT facilities, which produce refined fuels of much higher calorific values than unprocessed waste, could present a good synergy with gasification facilities.
Gasification could benefit from the changes in our waste, including the decrease in the quantity of residual waste. There will always be a residual waste stream, and considered processing options, recovering valuable materials in the process, could provide future gasification plants with high-quality feedstocks enabling the reliable production of energy or high-value liquid fuels and chemicals.
Dr Stuart Wagland is lecturer in Renewable Energy from Waste at Cranfield University, UK.
