Emission Characteristics of Continuous Combustion Systems of Vehicular Powerplants — Gas Turbine, Steam, Stirling
The low emission potential of continuous combustion systems has generated considerable interest in their application to vehicular powerplants. While the prime consideration for a vehicular combustion system is low exhaust emissions, emphasis must also be placed on the usual considerations of performance, reliability, durability and simplicity.
The General Motors Research Laboratories has had experience with three different types of continuous combustion powerplants; namely, gas turbines, steam engines and Stirling engines. Combustors for the steam and Stirling engines were patterned after the can-type burner of the regenerative gas turbine. The sizes and configurations of the burners varied markedly because of the differences in the heat release rates and the thermodynamic cycles of the powerplants which accounted for the widely-different operating ranges of inlet pressure, inlet temperature and fuel-air ratio of the three burners.
Unburned hydrocarbon (HC), carbon monoxide (CO) and nitric oxide (NO) emissions were measured from typical combustion systems of the three powerplants. Using these measurements, steady-state emission characteristics of the combustion systems were compared on the bases of exhaust gas concentration (ppm), emission index (gm/kg of fuel burned) and specific emission (gm/bhp-hr). While the emission index is exclusively an indicator of burner performance, the specific emission parameter reflects overall powerplant performance.
These test data indicated the major problem associated with each burner to be high emissions of oxides of nitrogen. The NOX emission indices of the regenerative gas turbine and Stirling engine burners were comparable, reflecting the strong influence of high inlet temperature. The NOX emission index of the steam engine burner was considerably lower due to operation at ambient inlet air temperature, but was still higher than required to meet the 1976 Federal standards. Emissions of unburned hydrocarbons and carbon monoxide from each burner were decidedly low, although operation of the steam engine burner at ambient inlet conditions necessitated considerable burner development to minimize these pollutant emissions. Exhaust smoke and aldehyde emissions of the burners have not constituted serious problems.
Emission control techniques were applied to each burner after reviewing the chemical kinetics of the pollutant species of concern. It was determined that two conflicting requirements must be satisfied for a low emission burner; namely, low temperature — short residence time for low NO emission and high temperature — long residence time for low HC and CO emissions.
The gas turbine type of burner was deemed to be more amenable to the control of NO emissions than the other two types of burners because of the availability of a large quantity of excess air that can be used for emission control and a reasonably-low outlet temperature that precludes continuing chemical reactions. Early quench and primary zone equivalence ratio modification have been effective means for reducing NO emissions from the conventional gas turbine burner. Theoretical studies reveal that other control techniques may be equally or more effective depending on the design and operating characteristics of the particular engine burner.
Vehicle mass emissions are influenced significantly not only by the design and operating conditions of the combustion system but also by the performance of the powerplant and the duty cycle of the vehicle. In seeking a vehicle that will meet the 1976 Federal mass emission requirements, it will be most advantageous to consider the combustion system and other major components of the powerplant as an integrated system. Since vehicle mass emissions (grams per mile) are dependent on the emission index of the burner as well as the fuel economy of the vehicle (miles per gallon of fuel burned), reductions in vehicle mass emissions can be obtained by either separate or preferably combined improvements in burner, engine and vehicle performances.
KeywordsNitric Oxide Mass Emission Primary Zone Stirling Engine Emission Index
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