Technology

Incineration involve directly combusting the MSW in an oxygen-rich environment, typically at temperatures between 700°C and 1,350°C. An exhaust gas composed primarily of CO₂ and water is produced, which flows through a boiler to produce steam to drive a steam turbine generator, producing electricity. Inorganic materials in the MSW are converted to bottom ash and fly ash^[1].

Mass burn is the incineration on a grate of bulk waste that has not undergone sorting, classification or size reduction of any type. The main advantage is that garbage is fed as it comes (except that very large pieces more than around 1m and bulky items have to be shredded) and it is said to be the most efficient solution for handling household waste. Sometimes it is combined with some upfront selective collection according to requirements of the local municipalities. With population size increase and the development of large urban centres the trend is going towards larger mass burn capacity plants. Mass burn is obviously not suitable for smaller volumes where instead alternative technologies could be employed^[2].

Figure 1 - Grate Incinerator Schematics^[3]

The waste is first dried on the grate and then burnt at a high temperature (850 to 950°C) accompanied with a supply of air. With a crane, the waste itself is emptied into an opening in the grate. The waste then moves towards the ash pit and it is then treated with water, cleaning the ash out. Air then flows through the waste, cooling the grate. Sometimes grates can also be cooled with water instead. Air gets blown through the boiler once more (but faster this time) to complete the burning of the flue gases to improve the mixing and excess of oxygen^[4].

References

1. Adapted from: Yuzhong Tan, 15^th Apr 2013, College of Engineering, University of California, Berkeley: Feasibility Study on Solid Waste to Energy - Technological Aspects.
2. CNIM personal communication.
3. Vermeulen, Isabel et al.: Sustainable waste processing in a grate furnace and in a fluidized bed incinerator: WtE, recycling and environmental concerns, WIT Transactions on State-of-the-art in Science and Engineering 84 (2014): 83-92.
4. Rachael Lew, 29^th Nov 2022, BioEnergyConsult, Moving Grate Incineration: The Most Common WTE Technology.