Article pubs.acs.org/est
Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Ultrafine Particle Emissions from Natural Gas, Biogas, and Biomethane Combustion Jian Xue,† Yin Li,† Joshua Peppers,† Chao Wan,‡ Norman Y. Kado,§ Peter G. Green,† Thomas M. Young,† and Michael J. Kleeman*,† †
Department of Civil and Environmental Engineering, University of California−Davis, Davis, California 95616, United States Atmospheric Science Graduate Group, University of California−Davis, Davis, California 95616, United States § Department of Environmental Toxicology, University of California−Davis, Davis, California 95616, United States Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on November 13, 2018 at 10:45:41 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Biogas and biomethane (=purified biogas) are major renewable fuels that play a pivotal role in the evolving global energy economy. Here, we measure ultrafine particle (UFP; Dp (particle diameter) < 100 nm) emissions from the combustion of biomethane and biogas produced from five different representative sources: two food waste digesters, two dairy waste digesters, and one landfill. Combustion exhaust for each of these sources is measured from one or more representative sectors including electricity generation, motor vehicles, and household use. Results show that UFP emissions are similar when using biomethane and natural gas with similar sulfur and siloxane content. Approximately 70% of UFPs emitted from water heaters and cooking stoves were semivolatile, but 30% of the UFPs were nonvolatile and did not evaporate even under extremely high dilution conditions. Photochemical aging of biomethane combustion exhaust and natural gas combustion exhaust produced similar amounts of secondary organic aerosol (SOA) formation. The results of the current study suggest that widespread adoption of biogas and biomethane as a substitute for natural gas will not significantly increase ambient concentrations of primary and secondary UFPs if advanced combustion technology is used and the sulfur and siloxane content is similar for biogas/biomethane and natural gas.
1. INTRODUCTION
CH4 combustion emits less particulate matter (PM) than other fuels (gasoline, diesel, coal, fuel oil, solid biomass, etc.), but all of the PM produced by CH4 combustion is in the ultrafine particle (UFP; diameter 0.05). These results generally correspond to the sulfur content of the fuel and suggest that rigorous sulfur pretreatment should be used to remove sulfur from biomethane to avoid increasing UFP emissions from natural gas home appliances. Previous studies found that the indoor nanoparticle concentrations produced by a domestic gas cooker varied from 103 to 106 # cm−3 based on the type of fuels, fuel consumption rates, and the primary air addition.13 The raw particle concentration measured in the current study using the cooking stove and water heater (103 to 105 # cm−3) fall into this range. 3.1.3. Engine-Generators. Engine-generators at each biogas combustion site were tested directly with locally produced biogas since it was impractical to retune the engines to operate using other fuels. Test results therefore represent differences in the composition of the local biogas and the technology employed by each of the engine-generators. Figures 1d and 2 indicate that UFPs measured from the four engine-generators can be divided into a higher emissions category and a lower emissions category. The PN measured in the exhaust of the facility #4 and the facility #5 enginegenerators were 331 and 210 times higher than the PN
Figure 2. Particle number and mass emission rates (ERPN and ERPM) measured at typical source tests DF (=13−50) using CNG, biomethane, or biogas. Data were corrected for background. Error bars represent two times the standard deviation.
them as molecular particle precursors. The larger nucleation mode particles are likely soot particles formed through physical coagulation of the smaller particles.10,32 The nucleation mode particles dominated the overall particle number concentration, while the accumulation mode particles accounted for ≥95% of the UFP mass. Concentrations of accumulation mode particles of >20 nm were significantly higher in the vehicle tests than in other sources. Previous studies have shown that accumulation mode particles from vehicles are a combination of motor oil and soot agglomerates formed through incomplete combustion of fuel hydrocarbons.33 CNG and biomethane are simple fuels composed mostly of methane that are not expected to contribute significantly to accumulation mode particles.13,32 It is likely that the accumulation mode particles measured in the CNG/biomethane vehicle exhaust tests originated from motor oil. The molecular particle precursors were not detected during the tests other than the automobile source because the instrument used in other tests (SMPS with long DMA) did not cover the smaller size bins. No significant variation was found among three tested fuels for either PSDs or ERPN/ERPM at a confidence level of 95% (Student’s t-test, unpaired), as shown by the overlapping error bars in Figures 1a and 2. This finding suggests that primary UFP number emission rates from automobiles powered by biomethane and natural gas are very similar. 3.1.2. Home Appliances. Home appliances were tested with pipeline CNG and biomethane produced at facility #1, facility #2, and facility #3. Figure 1b,c shows that UFP emissions from the home appliances generally had a unimodal PSD irrespective of fuel type, with a nucleation mode at particle diameter
