A Study of Greenhouse Gas Emissions and Performance of an Internal Combustion Engine Operating on Landfill Gas Blends as Fuel
Landfill gas (LFG) is an abundant resource naturally occurring from the decomposition of municipal solid waste. It can be generated in U.S. landfills at a rate of 24.2 to 59.1 GNm3 per year.1 LFG presents both a problem and solution with respect to global warming. Composed of carbon dioxide (CO2, 30-40%) and methane (CH4, 45-55%), as well as nitrogen (N2, 10-15%), oxygen (O2, 0-5%), and other trace gasses1, LFG that is permitted release into the atmosphere will contribute to the greenhouse gas effect due to the presence of CH4 and CO2. Standard practice at landfill facilities is to flare the gas, the products of that combustion having a lower global warming potential than the original gas.2 Without the utilization of the energy released during flaring (combustion), there presents a missed opportunity to use the LFG as a working fuel. Yet, direct use of LFG in internal combustion (IC) engines is hindered by low and varying energy density of the fuel. Thus, further processing of a portion of the LFG to produce syngas, composed of hydrogen (H2) and carbon monoxide (CO), increases the energy density, improves the flame stability, and decreases emissions such as nitrogen oxides (NOx), CO, and unburned hydrocarbons (UHCs).1 Due to the natural variability of the CO2/CH4 ratio of the LFG from temperature and waste composition fluctuations3, fuel blends at a range of CO2/CH4 ratios representative of field values must be evaluated, as do a range of syngas ratios with the LFG, as well as the CO/H2 ratio in the syngas itself. With a robust data matrix covering a wide band of realistic operating conditions, including engine load, three objectives were achieved. 1 - Characterization of the fuel blends to quantify emissions. 2 - Identification of the lower limit for engine operation (stall condition) for each fuel blend. 3 - Creation of a robust dataset whereby given the system inputs (LFG composition, maximum expected load), a recommended fuel blend can be referenced for suitable engine operation and desired emission targets. Experiments revealed that the variance of performance parameters, such as engine thermal efficiency and emissions, with applied electrical load was consistent with previous findings in the literature. In-cylinder temperature was not measured directly, however calculated adiabatic flame temperature indicated relative changes in combustion temperature. NOx concentration of the engine exhaust increased by 36% with increased adiabatic flame temperature from 2105K to 2210K. CO and UHC concentrations were found to be coupled by emissions analysis; as one increased, so did the other. CO and UHCs both increased at engine operation conditions that exhibit poor combustion, such as the fuel-starved condition immediately prior to stalling. The fuel-starved condition resulted in an 878% increase in UHCs compared to the idle condition and was accompanied by degradation in engine operability. Testing was extended to simulated LFG with the addition of syngas of up to 45% by volume. Iterations of engine testing were completed with simulated LFG of varying 0%-49% CO2 composition and with syngas of varying 47%-100% H2 composition. Data was compared across the test cases to identify operational limits of the engine and regions of performance interest.
combustion, landfill gas, syngas, emissions, Mechanical Engineering
City University of New York - City College
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