Air Quality Implications of Using Biogas to Replace Natural Gas in California

Upgraded biogas or biomethane can be used as a renewable fuel in place of petroleum natural gas in California. Different feedstocks produce biogas and biomethane with different levels of trace impurities, affecting air pollution emissions. This project evaluated the air quality impacts of using biogas and biomethane fuels produced from different feedstocks in multiple end sectors, including commercial electricity generation (five locations), mobile sources (cargo van), and home appliances (stove and water heater). All combustion emissions were diluted to representative atmospheric concentrations under simulated day and night conditions before collection and analysis. The research team analyzed combustion exhaust samples for toxicity, chemical composition, and biological organisms.
The combustion exhaust from biomethane exhibited low levels of toxicity similar to natural gas exhaust. Toxicity differences were apparent between biomethane and natural gas in the first round of results from home appliances tests. DNA damage produced by exposure to combustion exhaust from a water heater and a cooking stove was slightly higher for biomethane than petroleum natural gas. Likewise, mutagenicity (the capacity to create mutations), related to possible carcinogenicity, produced by exposure to the exhaust from a cooking stove was slightly higher for biomethane than for petroleum natural gas. A second round of cooking stove tests with multiple samples confirmed the initial biomethane combustion results, but in that round, toxicity from natural gas emissions was measured at similarly high levels. The increased toxicity of the natural gas combustion exhaust may have been caused by changes in the natural gas composition between the first and second round of tests. The toxicity trends are related to the chemical composition of the combustion exhaust, but more thorough testing is needed to determine how the feedstock and production methods for biomethane should be enhanced to reduce toxicity. It may be possible to reduce toxicity by controlling more tightly the composition of both biogas and biomethane and natural gas.