What are the Environmental & Economic Impacts of Combusting Biogas in Reciprocating Engines

Environmental & Economic Impacts of Biogas Combustion in Piston/Reciprocating Engines

Reciprocating internal combustion engines, or piston engines, are used worldwide as a low-cost method of generating power. Reciprocating combustion engines have also been widely used to generate power from biogas. However, out of all the current biogas management technologies, these engines are responsible for the highest levels of exhaust emissions, particularly nitrogen oxides.

Reciprocating engines powered by biogas are typically natural gas engines that have been adapted to run on biogas. These engines operate on a 4-stroke, spark-ignition Otto cycle, and fall into one of two classes: rich-burn engines and lean-burn engines.

Rich-burn (or stoichiometric) reciprocating engines operate with an air-fuel ratio that is almost stoichiometric, meaning it takes in precisely the amount of air needed to burn all the fuel. Rich-burn engines tend to emit fewer hydrocarbons but more nitrogen oxides compared to lean-burn engines. Rich-burn engines are typically used in gasoline-fueled vehicles.

Lean-burn reciprocating engines on the other hand consume twice the amount of air required to completely burn the fuel. As a result, peak combustion temperatures are lower, which in turn results in lower nitrogen oxide emissions. However, emissions (hydrocarbons and volatile organic compounds) resulting from incomplete combustion can be higher than those produced by rich-burn engines and will require further catalytic reduction through urea injection (nitrogen oxides) or oxidation (carbon monoxide and VOCs). Lean-burn engines can be slightly more fuel-efficient than rich-burn engines.

Efficiency of Reciprocating Engines

According to the EPA report, Evaluating the Air Quality, Climate & Economic Impacts of Biogas Management Technologies, the efficiency of biogas-powered stationary engine generators varies from around 25% for 100-kilowatt units to 40% for units of 4-5 Megawatts.

Emissions from Reciprocating Engines

Criteria Pollutants

The authors of the above report reviewed more than 31 source tests for biogas powered reciprocating engines in California, most of which were located in the Bay Area Air Quality Management District and San Joaquin Valley Air Pollution Control District where nitrogen oxide emissions of 65-70 parts per million (ppm) were permitted. They also reviewed three years of source tests conducted on an engine permitted for nitrogen oxide emissions of 11 parts per million at a dairy located in the San Joaquin Valley Air Pollution Control District that employs selective catalytic reduction to reduce nitrogen oxide emissions. According to the report, the average nitrogen oxide emission factor for reciprocating engines permitted for nitrogen oxide emissions ranging between 60-70 parts per million was 0.128 lbs NOx/MMBtu (33 ppm), while the average emission factor data for reciprocating engines with selective catalytic reduction and CatOx emission control systems resulting in nitrogen oxide emissions of 11 ppm was 0.042 lbs NOx/MMBtu. Results from the engine source tests showed that the average carbon monoxide emission factor for reciprocating engines was 0.49 lbs CO/MMBtu, while average volatile organic carbon emissions were 0.04 lbs VOCs/MMBtu.

Green House Gas Emissions

The average greenhouse gas emission factor for methane with a methane destruction efficiency of 98% was 0.838 lbs CH4/MMBtu, while the average GHG emission factor for carbon dioxide (based on an assumption of stoichiometric combustion of biogas composed of 60% methane) was 191.3 lbs CO2/MMBtu, and the average nitrous oxide emission factor was 0.00192 lbs N2O/MMBtu.

Based on the emission factors above together with engine conversion efficiencies and the global warming potential of the greenhouse gas in question, the output-based greenhouse gas emissions (carbon dioxide equivalent) of methane for reciprocating engines is estimated to range from 408 lb CO2eq/MWh (for a 100 kW operation) to 261 lb CO2eq/MWh (3000 kW operation). The greenhouse gas emissions of biogenic carbon dioxide were estimated to range from 2,740 lb CO2eq/MWh (for a 100 kW operation) to 1,750 lb CO2eq/MWh (3000 kW operation). The greenhouse gas emissions of nitrous oxide were estimated to range from 8,2 lb CO2eq/MWh (for a 100 kW operation) to 5,2 lb CO2eq/MWh (3000 kW operation).

Costs

According to the EPA report, the installation costs for reciprocating engines is estimated to range from $4,114/kW for a 100 kW unit to around $2,289/kW for a 3,000 kW engine. The estimated levelized cost of energy ranges from $90/Mega Watt hour to $48/Mega Watt hour for the capacities reviewed.

Featured Image courtesy of MWM