Massachusetts Institute of Technology

Abstract

In this chapter, technology options for solid biomass and waste fuel utilisation in large-scale combustion plants are presented with the focus on fuel flexible technologies allowing multi-fuel operation. Fuel flexibility (the ability to use various fuels) protects power plant investment from changes in competitiveness between different fuels, which can be caused, for example, by changes in legislation. In multi-fuel operation (burning various fuels at the same time), synergies between different types of fuels can be exploited allowing higher generation efficiency also from lower-quality fuels.The three main combustion technologies for solid fuels are grate, fluidised-bed and pulverised-fuel combustion. Grate boilers are adaptable to various fuels but are quite sensitive to changes in fuel quality and have limited multi-fuel operation capability. Fluidised-bed boilers provide the best fuel flexibility. With circulating fluidised-bed (CFB) boilers, biomass/waste fuels can be co-fired with coal on 0–100% share. Various technologies have been developed to enable co-firing biomass with coal in pulverised coal (PC) boilers as the enormous capacity of existing PC boilers offers huge potential for increasing biomass utilisation and economic benefits compared to new stand-alone biomass/waste plants. Capital costs are reduced markedly, and the high electrical efficiency of large PC boilers and the favourable properties of coal ash can be exploited. Three different routes can be taken to utilise co-firing in PC boilers: (1) direct co-firing, (2) indirect co-firing, and (3) parallel co-firing. In addition, a PC boiler can be converted into a bubbling fluidised-bed (BFB) boiler with attractive economics.