Energy recovery in China from solid wastes by the moving grate and circulating fluidized bed technologies

Abstract

In recent years, the Chinese waste-to-energy (WTE) industry is growing at the rate of about thirty new plants each year. The municipal solid waste (MSW) fuel has a low heating value of 4–7 MJ/kg, in comparison to about 11 MJ/kg in U.S. and 8–11 MJ/kg in EU. Combustion of the low heating value fuel on a moving grate (MG), the dominant combustion technology worldwide, is difficult to control and measures have to be taken to remove some moisture prior to combustion. For this and other reasons, an alternative technology, the circulating fluid bed (CFB) has been implemented in China. This paper is a comparative study of the two technologies and was carried out by Columbia University and two senior engineers, representing the MG and CFB technologies of China. Data were derived from industrial operating plants and from the literature. The fuel to MG furnaces is as-received MSW, while the MSW to CFB reactors is pre-shredded using high-torque low-speed shredders. The availability of MG plants, over a 1-year period, is 90% + , while that of CFB facilities is 80% +. Also, the in-plant electricity consumption of MG plants is slightly lower than the consumption of CFB plants. The MG furnace is less compact, than that of a CFB combustion chamber, with a heat flux range from 0.5 to 0.6 MW/m² of grate surface area, while that of CFB furnace was about 1.7 MW/m² of furnace cross-section. The bottom ash in a MG process is typically wet-discharged and the recovery of metals is less efficient. A drawback of the CFB process is that the fly ash generated is 5–10% of the weight of MSW combusted, as compared to 1–3% for moving grate plants in China.