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When a co-firing or 100 percent repowering project is being contemplated, the effects on the boiler, in terms of combustion air requirements, flue gas rates, emissions and the balancing of heat duty between the radiant and convection sections, need to be examined in detail. However, certain evaluations can be made during a preliminary or feasibility study, prior to performing the rigorous modeling that should be done before the project engineering is complete.
Flame temperatures for producer gas can be quite high when the producer gas is delivered to the boiler hot and the combustion air is preheated. A producer gas having a higher heating value of only 175 Btu/scf, delivered to a boiler at 700°F, with 10 percent excess combustion air preheated to 500°F will have an adiabatic flame temperature in excess of 3,200°F. This temperature is well in excess of the limits that will be imposed by thermal NOX considerations.
Flue gas rates
The flue gas rates experienced
This is only 2 percent higher than the 915 lbs of flue gas per million Btu experienced with coal.
Gasification, as an intermediate step included in the co-firing of biomass or the repowering of boilers with biomass, has the potential to greatly reduce issues normally associated with direct co-combustion of biomass and coal. However, the gasification technology applied needs to include filtration and removal of the ash from the producer gas prior to combustion, in order to obtain these benefits. Specifically, removal of alkali metals and chlorine should be part of the gasification technology. Additionally, an engineering assessment of specific equipment capabilities, fan capacities, boiler layouts, boiler metallurgies, etc., should be done early in the co-firing or repowering project to determine the effects of changes to the system on performance, efficiency and capacity. If the producer gas is delivered hot, effects on flame temperature and flue gas rates should be minimal.
Editor’s note: This paper, PWR2010-27360, was printed with permission from ASME and was edited from its original format. To purchase this paper in its original format or find more information, visit the ASME Digital Store at www.asme.org.
T. J. Paskach, Ph.D. is the director of Business Development for Frontline Bioenergy in Ames, Iowa. Paskach received his Ph.D. in Chemical Engineering from Iowa State University in 2002. He received the Research Excellence Award in 2002 and the Professional Progress in Engineering Award in 2005 from Iowa State University. You may contact him at email@example.com.
John P. Reardon co-founded Frontline BioEnergy, LLC in 2003. He has a bachelor’s degree in Mechanical Engineering from the University of New Mexico, and a master’s degree in Chemical Engineering from the University of New Mexico. You may contact him at firstname.lastname@example.org.
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