Overview of Solid Fuels Combustion Technologies

Abstract

AbstractThe study of coal and biomass combustion for power generation or heating processes is of extreme importance, if we are to conserve our sources of energy, while achieving the environmental goal efficiently and reliably in a world of increasing population and energy needs. This chapter provides a survey of the technologies that are either available or are being developed to enable all solid fuels to be used cleanly and with greater amenity. The physical and chemical properties of solid fuels, which influence the design and the performance of combustion processes, are discussed first. The conventional methods of coal combustion and the emerging clean coal technologies with higher thermodynamic efficiency and inherent emission control, such as fluidized bed combustion, supercritical pulverized‐coal combustion, low NOxburners, and near‐zero emission technologies are considered in detail. Currently available or under development biomass combustion technologies for industrial utilization, such as grate furnaces, underfeed stokers, fluidized bed systems, and dust combustion systems, are also described. Examples and experience from several countries around the world are presented.Pulverized‐coal combustion systems are the best‐proven technologies, accounting for well over 90% of world power plants. The average efficiency of larger subcritical plants is in the range 35–36%. Energy consumption is high and so are the emissions, if uncontrolled. Supercritical units achieve higher efficiencies, approaching 50%, and thus lower specific emissions. Unit sizes up to 1000 MWeexist. Long‐term programs in progress, to achieve efficiencies over 50%, involve materials developments, including the use of superalloys. Fluidized bed technologies have good fuel flexibility and lower emissions compared to conventional combustion systems. Bubbling fluidized bed combustion (BFBC) is used for small units up to 300 MW, having efficiencies around 30%. A key area for future development is extending the range of biomass and waste fired, improving the control of heavy metals, and materials of construction. Circulating fluidized‐bed combustion (CFBC) subcritical units in operation range in size from a few MW to 300 MWe; plant efficiencies are 38–40%. Larger supercritical boilers are under development and research on advanced alloys for heat exchangers is continuing. Pressurized fluidized‐bed combustion (PFBC) is a relatively new technology. Plant efficiencies of up to 44% and very low emission levels have been achieved. The use of a topping combustor, to increase the gas turbine inlet temperature, and flue gas cleanup, remain critical development areas.In situcontrol technologies for SOxand NOxemissions reduction achieve efficiencies between 30 and 80%. Technologies for near‐zero CO2emissions, including oxy‐fuel combustion and chemical looping combustion, are also under investigation.More than 95% of all biomass energy utilized today is obtained by direct combustion. Basic concepts include fixed bed, fluidized bed, and dust firing. Fixed bed furnaces are appropriate for biomass fuels with high moisture content, varying particle sizes, and high ash content. Typical plant capacities range between 20 and 50 MWewith related electrical efficiencies in the 25–30% range; however, larger plants up to 100 MW already exist. Moving grate boilers are the preferred technology. In recent years, the application of fluid‐bed combustion allows for efficient production of heat and electricity from biomass. On a scale of about 50–100 MWe, electrical efficiencies of 30–40% are possible.