Process Economics Program Report 63A
Sulfur Dioxide Removal From Stack Gases
Published: August 1973
The emphasis of this supplemental report is somewhat different from that of our original report on SO2, removal from the stack gases of electric power plants (PEP Report No. 63, issued in 1970). There is now no talk in the industry of making money from the SO2; the objective is pollution control per se, of which removal of SO2 from stack gases is one aspect. The other pollutants of concern in stack gases are dust (flyash) and NOx. Since dust removal is interrelated with SO2 removal, the process evaluations in this report show costs for both.
Three new process designs are presented--double alkali, carbon sorption with reduction, and magnesia scrubbing. Information is updated for four of the processes covered in the previous report--lime or limestone scrubbing, dry catalytic oxidation, carbon sorption, and alkali sulfite absorption.
Aqueous scrubbing processes operate under cool, moist conditions. In some early attempts at SO2 control, the ground level SO2 concentration in the vicinity of the power plant actually increased because the cool gases sank instead of rising into the upper atmosphere. Another objection to cool and moist discharge is the visible plume, which is considered aesthetically displeasing. There is no general agreement on the minimum temperature for gases that are to be released to the stack. However, reheating to 50�F is included in our evaluations of wet processes. This is only illustrative, because dispersion depends on stack height, stack exit velocity, and meteorological conditions, besides gas temperature. Aside from the equipment required, each 40�F of reheat reduces by approximately 1% the thermal efficiency of the power plant. An induced draft fan drawing gas from the scrubber reheats the gas in proportion to its differential pressure.
The stack gas cleaning processes studied in this report cannot be applied to some currently operating plants because of lack of space. Continued operation of such plants may be dependent on use of a low sulfur fuel, which is either naturally low in sulfur or produced by desulfurization (PEP Report 47). For power plants not yet built, and for conventional power generation methods, competition is among burning of low sulfur fuels, burning of high sulfur fuels followed by stack gas cleaning, and nuclear power. Unconventional methods that are receiving considerable attention include advanced power cycles and combustion in a fluidized bed.
This report also briefly reviews ways of removing NOx.