4.8 Article

Chemical and Toxicological Properties of Emissions from a Light-Duty Compressed Natural Gas Vehicle Fueled with Renewable Natural Gas

Journal

ENVIRONMENTAL SCIENCE & TECHNOLOGY
Volume 55, Issue 5, Pages 2820-2830

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/acs.est.0c04962

Keywords

Biomethane; CNG; RNG; Vehicle emissions; Toxicity

Funding

  1. California Air Resources Board (CARB) [13-418, 17-ISD004]
  2. California Energy Commission (CEC) [500-13-006]
  3. National Institute of Environmental Health Sciences of the National Institutes of Health (NIH) [R01 ES029126]

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The study found that the toxicity of renewable natural gas (RNG) exhaust is comparable or lower than fossil compressed natural gas (CNG) exhaust, with lower concentrations of potentially toxic chemical constituents in the exhaust. This suggests that adopting RNG may be an effective strategy to reduce the carbon intensity of transportation fuels.
Biogas consisting primarily of methane (CH4) and carbon dioxide (CO2) can be upgraded to a transportation fuel referred to as renewable natural gas (RNG) by removing CO2 and other impurities. RNG has energy content comparable to fossil compressed natural gas (CNG) but with lower life-cycle greenhouse gas (GHG) emissions. In this study, a light-duty cargo van was tested with CNG and two RNG blends on a chassis dynamometer in order to compare the toxicity of the resulting exhaust. Tests for reactive oxygen species (ROS), biomarker expressions (CYP1A1, IL8, COX-2), and mutagenicity (Ames) show that RNG exhaust has toxicity that is comparable or lower than CNG exhaust. Statistical analysis reveals associations between toxicity and tailpipe emissions of benzene, dibenzofuran, and dihydroperoxide dimethyl hexane (the last identification is considered tentative/uncertain). Further gas-phase toxicity may be associated with tailpipe emissions of formaldehyde, dimethyl sulfide, propene, and methyl ketene. CNG exhaust contained higher concentrations of these potentially toxic chemical constituents than RNG exhaust in all of the current tests. Photochemical aging of the vehicle exhaust did not alter these trends. These preliminary results suggest that RNG adoption may be a useful strategy to reduce the carbon intensity of transportation fuels without increasing the toxicity of the vehicle exhaust.

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