13 - EXTERNAL COMBUSTION ENGINES AND USE OF BIOMASS FUELS

Abstract

This chapter discusses the external combustion engines and use of biomass fuels. The choice between an open cycle design, which requires a continuous supply of fresh water to pump into the boiler, and the closed cycle design, where water circulates in the system, is, in practice, connected to the choice of steam data and is largely governed by the efficiency requirements. The availability of suitable water for the boiler can also be an important factor in this choice. The simplest steam power plant would have an open cycle, a maximum pressure of 10 bars, and no superheating of the steam. The efficiency of such a plant would be fairly low, that is, hardly above 5%. Higher efficiency can be obtained by raising the boiler pressure to 15 bars and superheating the steam somewhat to 280°C. This would give an efficiency of up to 7%–8%. Moreover, superheating to 350°C makes it possible to reach an efficiency of about 10%. The same simple treatment of the boiler feed water is used for pressures of up to 40 bar. For small power outputs, however, the efficiency of the machine as such can be made higher for a steam engine than for a steam turbine and there is also less economic incentive to choose advanced steam data.