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Fueling Low-NOx Engines with RNG Is the Quickest Path to Cleaner Air, Report Shows

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Buses running on near-zero NOx engines and RNG will emit far less GHG emissions, CO2, and PM than electric and fuel cell buses over the next 40 years.

July 18, 2016

Combining low-NOx engines with RNG may be the ticket to eliminating emissions from California’s public bus system, according to a new draft report from M.J. Bradley & Associates and Ramboll Environ.

The Los Angeles County Metropolitan Transportation Authority and the Advanced Transit Vehicle Consortium commissioned the Zero Emission Bus Options: Analysis of 2015–2055 Fleet Costs and Emissions report, expected to be final this summer. LACMTA, which operates an all-CNG fleet of more than 2,000 buses, must gauge how various low-emission technologies could help it meet CARB’s proposed Zero Emission Bus rule.


CNG buses will reduce more NOx emissions than any other alternative-fuel bus.

“The modeling indicates that the use of RNG and transition to LNOx buses will be more effective at reducing PM, total CO2, total GHGs, and total NOx from the LACMTA fleet than transition to either electric or fuel cell buses,” the draft report states.

Analysts compared four scenarios for the LACMTA fleet against its baseline of conventional CNG buses: CNG buses running RNG, CNG buses with low-NOx engines running RNG, an all-battery electric bus fleet, and an all-hydrogen fuel cell bus fleet. In the low-NOx plus RNG scenario, LACMTA would begin phasing in the ultralow-emission engines in 2018 and would have a complete fleet by 2028. LACMTA would adopt the alternative technologies over a period from 2025 through 2039.

Comparing Future Emissions

The report predicts the amount of NOx and particulate matter that various configurations of LACMTA’s bus fleet would emit within and outside of the South Coast Air Basin over the next 40 years, including tailpipe emissions and upstream emissions created by the buses’ production and delivery.

Low-NOx buses plus RNG would cut NOx emissions by 43 percent in basin and 82 percent out of basin, and PM pollution by 131 percent in basin and 602 percent out of basin, the draft report says. This combination also would reduce methane emissions by 17 percent, carbon dioxide emissions by 81 percent, and CO2-equivalent GHG emissions by 72 percent. The dramatic reduction of out-of-basin pollutants is due to the lower upstream emissions from RNG production compared with standard natural gas production.

The report projects that electric buses would reduce NOx emissions by 45 percent in basin and 51 percent out of basin, and PM emissions by 51 percent both in basin and out of basin. They would reduce methane emissions by 54 percent and both carbon dioxide and CO2-equivalent GHG emissions by 52 percent. The transit agency could reduce emissions slightly more by charging the buses en route as well as in the depot.

The analysis projects a lot less certainty—and fewer environmental benefits—for fuel cell buses. They would reduce NOx emissions by 1–40 percent in basin and 37–39 percent out of basin. They would cut methane emissions by 35 percent, carbon dioxide emissions by 19–41 percent, and CO2-equivalent GHG emissions by 21–42 percent.

The method for producing hydrogen at the depot drastically changes the level of PM emissions from fuel cell buses. Using electrolysis would cut PM emissions 39 percent in basin but actually increase emissions by 6 percent out of basin; using steam methane reforming would increase PM emissions 792 percent in basin but reduce them by 34 percent out of basin.

Comparing Transition CostsCNGVC_071816_LNOx_report3

The report shows that transitioning to low-NOx buses plus RNG is by far the least expensive option for LACMTA, which already owns 11 CNG stations. The combination would cost $273 million over the next 40 years. Switching to electric buses would cost the agency $3.8 billion to $6 billion, while adopting fuel cell buses would cost $5.8 billion to $7.6 billion, including infrastructure expenses.

Looking 40 years ahead, the report predicts that electric buses would have the lowest annual GHG emissions, but wouldn’t catch up with the net benefits of low-NOx buses until 2099. The total cumulative GHG emissions would be lower from low-NOx buses plus RNG through 2099 thanks to lower emissions between now and 2055, the report says. Fuel cell buses wouldn’t achieve similar results until 2358, even if they used electrolysis to produce the hydrogen, according to the report’s calculations.

The models are specific to LACMTA, but the findings could guide transit agencies across the state, and are especially relevant to those with existing NGV infrastructure.

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Photo ©Westport Innovations, used by permission

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