This paper describes the successful development and application of industrial gas turbines using medium-Btu gaseous fuels, including those derived from biodegradation of organic matters found in sanitary landfills and liquid sewage. The effects on the gas turbine and its combustion system of burning these alternate fuels compared to burning high-Btu fuels, along with the gas turbine development required to use alternate fuels from the point of view of combustion process, control system, gas turbine durability, maintainability and safety, are discussed.

Experiments were performed in laboratory-and full-scale combustors to test the feasibility of meeting proposed EPA emission standards. It was found that by uniformly mixing gaseous fuel and primary zone air prior to combustion and burning fuel leanly (equivalence ratio <1.0), it was possible to meet the proposed emission standards in an industrial gas turbine. The characteristic narrow range of flame stability obtained with lean premix combustion necessitated the use of fuel staging or variable geometry to handle the operational range of the engine. Fuel staging was selected for its relative simplicity. Consequently, EPA proposed emission standards were met only over a narrow range covering the engine operation at and near the design point. Experiments on small scale models of various sizes operated with gaseous and liquid fuels showed that, contrary to expectation, NOx production from a lean premix combustion system is independent of the system pressure in the pressure range investigated (1 atm to 16 atm). The desirability of high combustor inlet temperature and pressure for premixing was indicated. Despite the complexities of premixing fuel and air, such a combustion system, in addition to meeting the proposed emission standards, offers advantages such as easing of combustor wall cooling problems, improved combustor exit temperature distribution, and freedom from exhaust and primary zone smoke.

An axisymmetric annular vortex combustor was tested atmospherically to determine the feasibility of meeting the 1977 proposed EPA emission standards for stationary gas turbines. Flame stabilization was achieved solely by swirl induced recirculation without the jet assisted recirculation of conventional practice. Buoyancy forces are used to enhance or diminish turbulent mixing. It was found that by uniformly mixing gaseous fuel and air prior to combustion and burning fuel lean it was possible to meet the proposed emission standards. The resultant narrow range of flame stability necessitated fuel staging for light-off and off-design operation. Fuel staging was achieved by stratified charge combustion (diffusion flame). Changes in combustor geometry were made and extensive traversing of the flame zone was carried out to measure concentrations of chemical species.

Development of a combustion system at the author’s Company was carried out at atmospheric pressure to achieve successful operation of a 5500-hp (Type H) gas turbine on gaseous fuels covering a wide range of Wobbe Index (WI) values from 48.04 MJ/nm3 (1220 Btu/scf) for natural gas to 18.90 MJ/nm3 (480 Btu/scf) for natural gas and carbon dioxide mixtures. Mixtures with the latter composition have direct applications for gas turbine engines burning fuels derived from sanitary landfills and liquid sewage. Wide-range operational capability offers the flexibility to fire gas turbines with either pipeline quality natural gas or medium-heating-value fuel, depending on availability. For such applications, if a fuel injector is designed to operate satisfactorily on natural gas fuel, it will have unacceptably high pressure drops when operating on fuels with WI values towards the lower end of the range. On the other hand, if the fuel injector is designed to operate properly on fuels with WI values on the lower end of the range, it will have unacceptably low pressure drops when operating on fuels with WI values towards the higher end of the range, which causes combustion-driven pressure oscillation feedback into the fuel-injection system. This feedback usually increases the amplitude of combustion-driven pressure oscillation and can cause significant damage to the gas turbine in a relatively short time. This paper describes the design and development work carried out to resolve these problems.