Combustion technology developments in power generation in response to environmental challenges

Combustion system development in power generation is discussed ranging from the pre-environmental era in which the objectives were complete combustion with a minimum of excess air and the capability of scale up to increased b oiler unit performances, through the environmental era (1970-), in which re duction of combustion generated pollution was gaining increasing importance , to the present and near future in which a combination of clean combustion and high thermodynamic efficiency is considered to be necessary to satisfy demands for CO2 emissions mitigation. From the 1970s on, attention has increasingly turned towards emission contr ol technologies for the reduction of oxides of nitrogen and sulfur, the so- called acid rain precursors. By a better understanding of the NOx formation and destruction mechanisms in flames, it has become possible to reduce sig nificantly their emissions via combustion process modifications, e.g. by ma intaining sequentially fuel-rich and fuel-lean combustion zones in a burner flame or in the combustion chamber, or by injecting a hydrocarbon rich fue l into the NOx bearing combustion products of a primary fuel such as coal. Sulfur capture in the combustion process proved to be more difficult becaus e calcium sulfate, the reaction product of SO2 and additive lime, is unstab le at the high temperature of pulverized coal combustion. It is possible to retain sulfur by the application of fluidized combustion in which coal bur ns at much reduced combustion temperatures. Fluidized bed combustion is, ho wever, primarily intended for the utilization of low grade, low volatile co als in smaller capacity units, which leaves the task of sulfur capture for the majority of coal fired boilers to flue gas desulfurization. During the last decade, several new factors emerged which influenced the de velopment of combustion for power generation. CO2 emission control is gaini ng increasing acceptance as a result of the international greenhouse gas de bate. This is adding the task of raising the thermodynamic efficiency of th e power generating cycle to the existing demands for reduced pollutant emis sion. Reassessments of the long-term availability of natural gas, and the d evelopment of low NOx and highly efficient gas turbine-steam combined cycle s made this mode of power generation greatly attractive also for base load operation. However, the real prize and challenge of power generation R&D remains to be the development of highly efficient and clean coal-fired systems. The most promising of these include pulverized coal combustion in a supercritical s team boiler, pressurized fluid bed combustion without or with topping combu stion, air heater gas turbine-steam combined cycle, and integrated gasifica tion combined cycle. In the longer term, catalytic combustion in gas turbin es and coal gasification-fuel cell systems hold out promise for even lower emissions and higher thermodynamic cycle efficiency. The present state of t hese advanced power-generating cycles together with their potential for app lication in the near future is discussed, and the key role of combustion sc ience and technology as a guide in their continuing development highlighted . (C) 2000 Elsevier Science Ltd. All rights reserved.