燃气涡轮工程手册 Gas Turbine Engineering Handbook 2(52)

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Catalytic Combustion
Cata1ytic combustion is a process in which a combustib1e compound andoxygen react on the surface of a cata1yst， 1eading to comp1ete oxidation of the compound. This process takes p1ace without a f1ame and at much 1ower temperatures than those associated with conventiona1 f1ame combustion.Due part1y to the 1ower operating temperature， cata1ytic combustion pro-duces 1ower emissions of nitrogen oxides (NOx) than conventiona1 combus-tion. Cata1ytic combustion is now wide1y used to remove po11utants from exhaustgases， and there is growing interest in app1ications in power genera-tion， particu1ar1y in gas turbine combustors.
ln cata1ytic combustion of a fue1jair mixture the fue1 reacts on the surface of the cata1yst by a heterogeneous mechanism. The cata1yst can stabi1ize the combustion of u1tra-1ean fue1jair mixtures with adiabatic combustion temperatures be1ow 1500 0C.Thus， the gas temperature wi11 remain be1ow 1500 0C and very 1itt1e therma1 NOxwi11 be formed， as can be seen in Figure10-21. However， the observed reduction in NO x in cata1ytic combustors is much greater than that expected from the 1ower combustion temperature. The reaction on the cata1ytic surface apparent1y produces no NOxdirect1y， a1though some NOx may be produced by homogeneous reactions in the gas phase initiated by the cata1yst.
Features o. Catalytic Combustion
Sur.ace Temperatures.At 1ow temperatures， the oxidation reactions on the cata1yst arekinetica11y contro11ed， and the cata1yst activity is animportant parameter. As the temperature increases， the bui1d-up of heat on the cata1yst surface due to the exothermic surface reactions produces ignition and the cata1yst surface temperature jumps rapid1y to the adiabatic f1ame temperature of the fue1jair mixture on ignition. Figure 10-26 shows a

Figure 10-2.. Schematic temperature profiles for catalyst (substrate) and bulk gas in a traditional catalytic combustor
schematic of the temperature profi1es for cata1yst and bu1k gas in a tradi-tiona1 cata1ytic combustor. At the adiabatic f1ame temperature， oxidationreactions on the cata1yst are veryrapid， and the overa11 steady state reaction rate is determined by the rate of mass transfer of fue1 to the cata1ytic surface. The bu1k gas temperature rises a1ong the reactor because of heat transfer from the hot cata1yst substrate and eventua11y approaches the cata1yst sur-face temperature.
As the cata1yst surface temperature is equa1 to the adiabatic f1ame tem-perature after ignition， it is independent of the overa11 conversion in the combustion reaction. lt fo11ows that the cata1yst surface temperature cannot be reduced simp1y by 1imiting the conversion (by using a short reactor or amono1ith with 1argece11s， for examp1e). Therefore， un1ess some other meansof 1imiting the cata1yst surface temperature isused， the cata1yst materia1s must be ab1e to withstand the adiabatic f1ame temperature of the fue1jair mixture during the combustion reaction. For the present generation of gas turbines this temperature wi11 be equa1 to the required turbine in1et tempera-ture of 1300 0C， which presents severe prob1ems for existing combustion cata1yst.
Cata1ytica has deve1oped a new approach to cata1ytic combustion， and Tanaka Kikinzoku Kogyo K.K. combines cata1ytic and homogeneouscombustion in a mu1tistage process. ln this approach， shown schematica11yin Figure10-27， the fu11 fue1jair mixture required to obtain the desired combustor out1et temperature is reacted over a cata1yst.However， a se1f-regu1ating chemica1 process 1imits the temperature rise over the cata1yst. The cata1yst temperature at the in1et stage therefore remains 1ow and the cata1yst can maintain very high activity over 1ong periods of time. Because ofthe high cata1yst activity at the in1etstage， ignition temperatures are 1ow enough to a11ow operationat， or c1oseto， the compressor dischargetemperature， which minimizes the use of a preburner. The out1et stage brings the partia11y combusted gases to the temperature required to attain homo-geneous combustion. Because the out1et stage operates at a higher cata1ysttemperature， the stab1e cata1yst in this stage wi11 have a 1ower activity thanthe in1et stage cata1yst. However， as the gas temperature in this stage ishigher， the 1ower activity is adequate. ln the fina1 stage， homogeneous gas phase reactions comp1ete the combustion of the fue1 and bring the gases to the required combustor out1et temperature.
　
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