UC Davis

California Assembly Bill 32 (AB32) sets the goal to reduce greenhouse gas (GHG) emissions to a level 80% below 1990 levels by 2050. This deep decarbonization target requires major technology advancement and energy structure transition to a renewable and sustainable future. The environmental aspects of such a transition should be evaluated carefully before major investments in infrastructure are made. Biogas is a promising renewable energy resource in California that shares many similarities with natural gas but with the advantage of being carbon neutral since it generates energy from organic waste. However, biogas contains trace levels of numerous chemical compounds that depend on the feedstock and production process. The air quality implications of using biogas in different situations should be examined carefully before widespread adoption across California.

The second chapter of this thesis characterizes the chemical and biological composition of raw biogas produced at five facilities using different feedstocks. The toxicity of combusted biogas is tested under fresh and photo-chemically aged conditions. Results find no strong evidence of potential occupational health risk from the five California biogas sites. Results also show no obvious differences between the toxicity of different biogas combustion exhaust after atmospheric dilution and aging. The third chapter of this thesis examines the emissions from the combustion of upgraded biogas that has CO2 removed and CH4 concentrated to be qualified as renewable natural gas (RNG). A light-duty cargo van was tested with CNG and two RNG blends on a chassis dynamometer to compare the toxicity of the resulting exhaust. CNG vehicle engine exhaust showed a higher or similar level of various toxicity responses, and photochemical reactions did not seem to alter the observed trend. These preliminary results suggest that utilizing biogas for direct heat and electricity generation or as vehicle fuel after upgrading could be useful strategies to reduce carbon intensity without negatively impacting air quality or public health.

The fourth chapter of this thesis extends the scope by modeling air pollutant emissions from all California socio-economic sectors under different energy scenarios in the year 2050. To study the air quality implications of some key resources and technologies in the decarbonization transition, a total of six different scenarios were analyzed for various particulate and gaseous pollutant emissions. These scenarios include: 1) a business-as-usual future reference scenario "BAU", 2) a partial GHG reduction scenario that constrains only through 2030 with 40% reduction "CAP30", 3) a climate-friendly 80% GHG reduction scenario featuring deep penetration of advanced technologies and renewable energies "GHGAi", a same 80% GHG reduction scenario with the deployment of biomass carbon capture and sequestration technology "CCS", and two variation scenarios on GHGAi that examine the effect of using more natural gas in built environment "NGB" and power generation "NGT". Results show that major air quality benefits are expected from the GHGAi scenario, which includes aggressive decarbonization of electricity supply, electrification of most end-use appliances, improvement of appliances efficiency, and deployment of low-carbon transportation fuels and technologies. Bio-CCS technology holds promise as a shortcut to GHG mitigation and the utilization of natural gas bridges the transition from traditional to renewable energy systems, but neither of these technologies appear to be optimal from a future air quality management perspective. Adoption of biogas as an energy source plays a small but constructive role in the overall transition of California’s energy system towards a low carbon future.