World oil consumption has tended to increase even since the era of high prices begin following the oil crisis of 1972. Social concern regarding environmental problems has mounted daily. Controls have been placed on emission of pollutants into the ambient air in connection with production activities, not only for the concentration of pollutants, but also, as in Japan for example, for the total amount of pollutant emitted. In particular, steam power plants consuming large amounts of fossil fuels have taken countermeasures to prevent air pollution by using low sulfur fuel or by performing desulfurization of stack gases.