Effects of Wet CO Oxidation on the Operation of Engines and Power Generators J. of the Braz. Soc. of Mech. Sci. & Eng. Copyright   2003 by ABCM October-December 2003, Vol. XXV, No. 4 / 341 F. de S. Costa, J. Cardoso and T. E. A. Villela Laboratório Associado de Combustão e Propulsão INPE, Caixa Postal 01 12630-000 Cachoeira Paulista, SP. Brazil fernando@lcp.inpe.br, jcardoso@lcp.inpe.br and tales@lcp.inpe.br C. A. G. Veras Departamento de Engenharia Mecânica Faculdade de Tecnologia, UNB Asa Norte 70910-900 Brasília, DF. Brazil gurgel@enm.unb.br Effects of Wet CO Oxidation on the Operation of Engines and Power Generators A simplified method is used to determine the optimum water content in the flue gases of charcoal gasifiers to be utilized as alternative fuels in the operation of engines and gas turbines for power generation. Computational models of plug flow reactors and well stirred reactors are employed to simulate the reaction and post-flame zones, adopting different chemical mechanisms. In the simulations reactants enter the reactors at 1000 K, 1 atm and equivalence ratio 0.25. It was observed that mixtures about 3% to 4% in volume of water vapor allow to obtain optimal operation characteristics, including high blowout limit, low ignition delay, maximum reaction zone temperature, high CO2 prodution and low thermal NO formation. It was observed that increasing water contents reduce significantly ignition times up to 3% in volume, while blowout mass flow rates increase continuously up to 6 % in volume, the maximum value considered. Formation of NO decreases continuously with humidity after the flame zone, while there are peaks of NO formation within the flame zone below 1% in volume. Higher water vapor content decreases the final temperatures below 1700 K, leading to a lower thermal efficiency. The method can be used to estimate optimum operational conditions with other input parameters. Keywords: Gasifier, carbon monoxide, power generator, engine, gas turbine Introduction Operation of engines and power-generators has been made by using the flue gases produced by charcoal gasifiers (Morais, 1981; Ismail and Morais, 1983). Carbon monoxide, CO, is a main component of the mixtures produced by charcoal gasifiers, which are employed as alternative fuel. It is well known that oxidation of CO in the presence of water occurs more rapidly than oxidation in dry conditions, since water acts as a catalyst and is not consumed during the reaction (Dryer and Glassmann, 1973; Lewis and Von Elbe, 1985; Glassmann, 1996). Therefore addition of small amounts of water can yield a better combustion efficiency in power- generators and engines, once CO appear in the combustion process. On the other hand, the presence of liquid water or water vapor can reduce the final temperatures, leading to lower thermodynamic efficiencies and can, also, affect ignition conditions, blowout characteristics and the mechanisms of pollutant formation. * A number of works has been published about water addition, in liquid or vapor form, in combustion processes. Dryer (1976) presented a review of concepts and applications related to the use of water in combustion systems, mentioning the use of water vapor in turbine operation during the 18 th century. Greeves et al. (1978) investigated the utilization of water emulsions in diesel engines to reduce fuel consumption, ignition delay and emissions of CO, NO and HC. Rightley and Williams (1997) analysed the effect of addition of H 2 or H 2 O on the propagation velocity of premixed flames of CO and O 2 . Recently, Bhargava et al. (2000) compared experimental results, obtained by injecting humid air in aeronautical turbines, with simplified computational models involving a series of perfectly stirred reactors. They conjectured that water vapor presence reduces the concentration of the O radical, decreasing the formation of thermal NO and N 2 O, while the larger concentration of OH decreases the amount of NO formed through the Fenimore mechanism, the so-called prompt-NO, which depends on the presence of CH, CN and HCN in the reaction zone. Presented at COBEM 2001 – 16th Brazilian Congress of Mechanical Engineering. 26-30 November 2001, Uberlândia, 2001, Uberlândia, MG. Brazil. Paper accepted August, 2003. Technical Editor: Atila P. Silva Freire. Thus, the objective of this work is to investigate the optimum water content in the flue gases of charcoal gasifiers (assumed as mainly composed of CO) to be used as alternative fuel in gas turbines and engines. A simplified method is used to determine the optimum water content, based on computational models of plug flow and well stirred reactors. Different chemical mechanisms are considered with the help of CHEMKIN 3.5 and MATLAB 5.0. Optimal operation characteristics include a high blowout limit, low ignition delay, maximum reaction zone temperature, high CO 2 prodution and low thermal NO formation. Perfectly stirred reactors are used to study the flame zone characteristics and plug flow reactors are used to estimate ignition delays and NO formation after the flame zone. Nomenclature A = pre-exponential factor, 1/s B = temperature exponent, dimensionless CV = control volume, m 3 c P,i = specific heat of species i, J/(kg K) h i = specific enthalpy of species i, J/kg 0 ,i f h = formation enthalpy of species i, J/kg k = reaction constant, units depend on reaction order M i = molecular weight of species i, kg/kg-mol m  = total mass flow rate, kg/s N = total number of species in the mixture, dimensionless P = pressure, Pa PFR = plug flow reactor PSR = perfectly stirred reactor R o = universal gas constant, J/kg-mol/K T = temperature, K t = time, s V = volume of reactor, m 3 v x = axial velocity, m/s i ω  = reaction rate of species i, kg-mol/m 3 /s X i = molar fraction of species i, dimensionless x = axial coordinate, m Y i = mass fractions of species i, dimensionless z = stoichiometric coefficient of water, dimensionless