Thermal Treatment
MSW gasification
Apart from Municipal Solid Wastes (MSW) combustion, gasification is another thermal treatment option for MSW to recover energy from the organic portions of the wastes. The gasification process turns carbon-based substances into energy-rich fuel by heating under controlled conditions to produce a syngas which is the mixture of carbon monoxide and hydrogen.
Pretreatment of MSW is required to ensure better performance of the gasification process, this includes: separation of inorganic ingredients, and controlling the size and moisture of feedstock.
Gasification of MSW involves a number of processes: pyrolysis, oxidation, and reduction.
Pyrolysis process
In the pyrolysis process, organic substances are split into gaseous, liquid and solid fractions through a combination of thermal cracking and condensation reactions under an oxygen-free environment. The pyrolysis process occurs at a temperature between 400 and 600 degrees Celsius. The products of pyrolysis include:
- a gas stream containing primarily hydrogen, carbon monoxide, methane and other hydrocarbons;
- a tar and/or oil stream that is liquid at room temperatures;
- A char consisting of almost pure carbon and any inert material that may have entered the process.
Oxidation process (combustion process)
In the oxidation process, gaseous and liquid products derived from the pyrolysis process will be further oxidized by oxygen to produce carbon dioxide and carbon monoxide. The energy produced in the combustion process will be used to power the reduction (gasification) process.
Reduction process (gasification process)
In the reduction process, the char derived from the pyrolysis process will be furthered heated up in order to produce a syngas mainly composed of carbon monoxide and hydrogen. The gas can then be cleaned and burned in a gas engine or gas turbine to generate electricity.
