Clean catalytic combustion of low heat value fuels from gasification processes

Abstract

The conversion of biomass, coal and some waste materials by gasification offers an opportunity to utilise such fuel sources cleanly. Many of the aggressive species inherent in the fuel precursors can be retained in process, but one exception is nitrogen, which appears in the derived fuel gas generally as ammonia. Amounts of ammonia can be large, up to 3–4000 vppm from biomass [Combustion of Low Heating Value Gas in a Gas Turbine, Power Production from Biomass, Vol. II, Espoo, Finland, March 1995]. Heat values of the derived fuel gases depend on the process, but are typically one-tenth to one-half that of natural gas [Combustion of Low Heating Value Gas in a Gas Turbine, Power Production from Biomass, Vol. II, Espoo, Finland, March 1995; Catal. Today 27 (1996) 55; Fuel 71 (1995) 1363; Developments in Thermochemical Biomass Conversion, Blackie Academic and Professionals, London, 1997, pp. 817–827]. Although the derived gases can be water washed to give low contaminant levels (especially ammonia), typically 50 vppm; this creates a waste stream for disposal and represents a thermodynamic loss to the cycle. Efficient conversion of gaseous fuels to electrical power is accomplished in gas turbines, preferably in combined cycle mode, where thermal efficiencies can be greater than 65%. Simple open cycle, high pressure ratio machines can achieve efficiencies greater than 40% and form the basis for Integrated Gasification Combined Cycles. Primary issues for the gas turbine combustor when using gasification gases are: • Their large volumetric flows; • The fuel-bound nitrogen content represented by the ammonia fraction. Fuel-bound nitrogen conversion in flame combustors can be large, even with stoichiometry control of the combustion process, where reduction to molecular nitrogen can be achieved to limit nitrogen oxides emission. The lowest levels of conversion to NO x reported in turbulent flames are of order 20% of the input fuel-bound nitrogen content, and this will, in many cases, exceed the permitted range for nitrogen oxides emissions [Combustion of Low Heating Value Gas in a Gas Turbine, Power Production from Biomass, Vol. II, Espoo, Finland, March 1995; Ammonia formation and NO x conversions with various biomass and waste fuels at the Varnamo 18 MWth IGCC plant, in: Proceedings of the Fifth Conference on Progress in Thermochemical Biomass Conversion, Tyrol, Austria, September 2000, pp. 17–22]. The paper describes experiments aimed at using catalytic combustion with reaction-specific catalysts to reduce the ammonia conversion rates, and so enable high levels of ammonia to be accommodated in the fuel gas without the disadvantages of waste disposal and thermodynamic loss to the engine cycle.