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The Natural Gas Pathway to Sustainable Transportation

ImageIf you read the business section of a major U.S. newspaper with any regularity, chances are you’ve seen something about the potential for the shale gas boom to transform the transportation sector. Much of this excitement is derived from the fact that natural gas currently enjoys a three-fold advantage over traditional petroleum fuels: it burns cleaner than oil-based gasoline – both from an environmental and health standpoint; it is domestic for the US; and it is significantly less expensive. Not surprisingly then, most discussions of the shale gas revolution (by pro-industry sources at least) trend toward national security, energy independence, greenhouse gas and local health-related emissions, and wide-ranging economic benefits. Alternatively, the environmental community seems to have spent the majority of its time and financial resources fighting the controversial extraction technique that has allowed this domestic energy boom, “fracking.”

Something not, however, often discussed in the mainstream media are the longer-term benefits and implications of a large-scale transition toward the use of natural gas as a transportation fuel – either compressed or liquefied – as a result of the inherent chemical properties of the fuel, which is primarily methane or CH4. The same infrastructure (pipelines, compressors, refueling stations, etc.) and engine technology now being deployed as natural gas vehicle use expands, can also be used to unlock a vast and largely untapped resource — renewable natural gas (RNG) or biomethane.

A switch from diesel to CNG/LNG may result in reduced greenhouse emissions of 14%-23% on a tank-to-wheel (TTW) basis. Alternatively, the production and use of RNG, which is made from a variety of organic waste sources including landfills, wastewater facilities, farms, dairies or other agricultural operations, food processing facilities, restaurant and residential food waste, etc., requires no drilling and reduces lifecycle (well-to-wheels) greenhousee gas emissions by close to 88%, according to research by the California Air Resources Board (CARB).1

The biological process of anaerobic digestion (AD), which occurs throughout nature as organic materials are broken down by microorganisms in airless environments, was first developed in the laboratory in the 1930’s.2 However, since then a variety of commercial AD technologies have emerged (differentiated by tank design, temperature, type of bacteria, etc.), largely in response to the wide-range of available organic feedstocks – from liquid manure and wastewater sludge to dense, dry, food and agricultural waste. But regardless of feedstock or AD system (excluding landfills) the result is the same: a mixture of predominantly CH4 (50-55% typically) and CO2 (~40%) known as biogas, and compost-like biosolids that can be returned – to the land from which they originated – as soil amendments. Therefore, if the biogases are captured, upgraded (purified) and utilized as a transportation fuel and the biosolids are returned to nature, what we have is a fully sustainable, closed-loop renewable energy system.

Therefore, as natural gas vehicle use continues to expand, so too do opportunities to tap into vast quantities of organic waste to produce renewable natural gas, since RNG, like biodiesel, is a “drop-in” fuel in that it can be put to use without major changes to infrastructure. Like other renewable technologies, the costs associated with the production, transmission and use of RNG are largely dependent on project size and economies of scale. The major cost components, in addition to O&M costs, relate to the production and capture of biogas and purification from raw biogas to renewable natural gas. And although project economics may vary widely by scale, location and feedstock, one thing remains constant: the production and use of RNG represents one of the lowest-carbon commercially viable transportation fuel options available, yet also one of the least discussed.
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