Environmental & Economic Impacts of Combusting Biogas in a Gas Turbine Engine
A gas turbine engine (also known as a combustion turbine engine) operates on the Brayton Cycle, which is the same thermodynamic cycle that a microturbine engine operates on, but the capacity of a gas turbine engine is larger, ranging from around 1 megawatt (1000 kilowatts) to 500 megawatts for a single engine. Gas turbine engines that are used to generate power from biogas typically range up to around 7.9 megawatts. The conversion efficiency of generating electricity using simple-cycle systems ranges from around 21-31%.
In California, several landfill gas and wastewater treatment projects employ biogas powered gas turbine engines, including Altamont landfill ( 2 x 3-megawatt combustion turbine engines), Fesno-Clovis water resource recovery plant ( 2 x combustion turbines of around 3.5 megawatts each), Calabasas landfill (3 x combustion turbine engines of around 4 megawatts each), Brea-Olinda Landfill (4 x 5.6-megawatt combustion turbine engines), Sunshine Gas Producers, LLC Sylmar ( 5 x 4.9-megawatt combustion turbine engines), Amaresco Chiquita Energy, Landfill Valencia (2 x 4.6-megawatt combustion turbine engines), and East Bay Municipal Utility District wastewater facility ( 1 x 4.5-megawatt gas turbine engine).
Efficiency of a Gas Turbine Engine
According to the EPA report, Evaluating the Air Quality, Climate & Economic Impacts of Biogas Management Technologies, the efficiency of converting biogas into electricity using gas turbine engine generators powered by biogas ranges from 21%-31% based on a higher heating value and ambient air temperature of 60 °F). Input energy ranges from around 20-87 million international British thermal units per hour (MMBtu/hour) for a 1,200-kilowatt gas turbine engine and 7,900-kilowatt gas turbine engine respectively (see table 9 on page 15 of the EPA report). Energy conversion efficiency and energy production decrease as ambient temperatures rise or site elevation increases.
Uncontrolled pollutant emissions stemming from biogas powered combustion turbine engines tend to range from between 15-25 parts per million (ppm) for nitrogen oxides, 25-50 ppm for carbon monoxide, and around 5 ppm for volatile organic compounds. Source tests for biogas powered gas turbine engines conducted in the San Joaquin Valley Air Pollution Control District, Bay Area Air Quality Management District, and South Coast Air Quality Management District in California showed that average nitrogen oxide emissions from gas turbine engines designed with lean pre-mix combustors (Dry Low-NOx turbine engines) were around 8ppm. Systems featuring selective catalytic reduction to control nitrogen oxide emissions averaged 2.9 ppm NOx (see Table 10 and Figure 11 & 12 on page 16 of the EPA report for a more detailed analysis of output-based emissions produced by biogas powered combustion turbine engines).
Greenhouse Gas Emissions
Emission factors for greenhouse gases are expected to be similar to those produced by microturbine engines, with an average greenhouse gas emission factor for methane of 0.167lbs CH4/MMBtu, while the average GHG emission factor for carbon dioxide is 191.3 lbs CO2/MMBtu, and the average nitrous oxide emission factor was 0.00026 lbs N2O/MMBtu.
Based on the emission factors above together with engine conversion efficiencies and the global warming potential (GWP100) of the greenhouse gas in question, the output-based greenhouse gas emissions (carbon dioxide equivalent) of methane for gas turbine engines is estimated to range from 92.3 lb CO2eq/MWh (for a 1200 kW operation) to 62.1 lb CO2eq/MWh (7900 kW operation). The greenhouse gas emissions of biogenic carbon dioxide is estimated to range from 3,110 lb CO2eq/MWh (for a 1200 kW operation) to 2,090 lb CO2eq/MWh (7900 kW operation). The greenhouse gas emissions of nitrous oxide is estimated to range from 1.24 lb CO2eq/MWh (for a 1200 kW operation) to 0.83 lb CO2eq/MWh (7900 kW operation).
Installation & Running Costs Associated with Gas Turbine Engines
Installation costs of combustion turbine engines range from $5,300/kW for a gas turbine engine of 1,200 kW in size to around $2,500/kW for a 7,900 kW gas turbine engine. Estimated levelized cost of energy ranges from $80/MegaWatt hour to $42/MegaWatt hour for the capacities reviewed.
Featured Image courtesy of MWM