Progress of petroleum coke combusting in circulating fluidized bed boilers—A review and future perspectives

Abstract

With the increase of solid wastes and higher requirements for environment protection, circulating fluidized bed (CFB) technology is playing a more and more important role in recycling and reusing solid wastes. The current thesis discusses the status quo of research and applications of CFB in petroleum coke combustion. All the problems of petcoke-fired CFB boilers such as emissions, erosion, agglomeration, ash behavior, ash marketing and operation problem are described in detail. The existing problems and orientations are pointed out for further study: from the perspective of stable ignition and pollutants emissions, the effect of combustion of mixed fuels with petroleum coke over coal is significantly better than that of pure coke as a single fuel, and there is an optimal mass ratio R of 1 and 3 ( R is the ratio of petcoke to coal); a certain proportion of limestone is necessary for petcoke-fired CFB boilers. Limestone cannot only decrease SO 2 and NO x emissions and reduce the serious erosion of SO 2 on heating surfaces, but also lighten the high temperature erosion caused by metallic elements (such as vanadium, nickel, etc.) through catching vanadium into ash. Comprehensively considering pollutants emissions and erosion factors, the optimal Ca/S molar ratio is 2.0–3.0; the NO x emission can be acceptable, despite there are many influence factors such as bed temperature, Ca/S molar ratio, the primary air fraction and excessive air coefficient; with petroleum coke, CO, the chlorides, fluorides, dioxin content in the flue gas are well under the limits since the chlorine and fluorine content in the petroleum coke are very low; the agglomeration occurs under the influence of the fluidized bed temperature, fluidizing speed, the concentration of materials in the flue gas and the wall temperature of heating surfaces; there are serious problems of deposition and sintering at tail convective heating surfaces after a long time operation because the fly ash has the characteristic of strong adhesiveness. The analysis results of physical and chemical characteristics of ash prove that there is almost no heavy metal secondary pollution in this kind of ash, and the bottom ash can be used as building material, waste stabilizer and soil modifier; the operation problems mainly lie in high carton content in fly ash and solid agglomerations which need to be solved. All the applications prove that there exist opportunities and difficulties synchronously when CFB boilers are used to cope with petroleum coke, and it is reasonable to say that CFB boiler will play a more important role in coping with petroleum coke in the future.