When the US ethanol industry was getting started in the late 1970s, its main attraction was its displacement of scarce and expensive petroleum. Ethanol substitutes for gasoline in blends of up to 85%, while relatively little of the energy used in growing corn and turning it into ethanol comes from petroleum. Some studies put oil's contribution to ethanol at 10% or less, including all farm and distribution fuels involved.
By the time the US Congress enacted the Energy Independence and Security Act of 2007, with its ambitious new RFS, ethanol and other biofuels were identified as solutions for mitigating the greenhouse gas (GHG) emissions from conventional fuels. Yet while the direct GHG reduction from using corn-derived ethanol can reach 30% or more of the emissions from gasoline, the indirect effects of its production, including the diversion of 43% of last year's drought-constrained US corn crop, send ripples across the globe.
In its latest report on the "Impacts, Adaptation, and Vulnerability" of climate change, the IPCC highlighted the influence of lower US corn exports on global food prices, and on land-use changes elsewhere that can have emissions consequences as importing countries clear land to grow corn. The IPCC also raised concerns about the growing water consumption of crop-based biofuels like corn ethanol, citing the potential for irrigation of biofuel crops to consume up to 9% of US freshwater by 2030.
What does all this have to do with natural gas? Many studies have examined the energy balance of corn ethanol since the 1970s. A 2008 study from the US Department of Agriculture, linked on the website of the Department of Energy's Alternative Fuel Data Center, provides a detailed breakdown of the energy inputs and outputs of a typical dry-mill ethanol plant. Depending on how "distillers dried grain" byproducts are accounted for, from 60% to 77% of the energy in each gallon of ethanol produced is attributable to fossil-fuel inputs. Of those, natural gas or inputs derived from gas, including fertilizer and electricity, make up more than 80%. That means that around half or more of the energy in a typical gallon of US ethanol comes from natural gas.
Effectively, ethanol is another pathway for turning natural gas--leveraged by sunlight--into transportation fuel. With the environmental impact of ethanol's agricultural inputs coming under renewed scrutiny, it seems reasonable to compare its emissions benefits to those from using gas directly as a motor fuel. This should be of particular interest in states like California, with its groundbreaking Low-Carbon Fuel Standard (LCFS). Attainment of the emissions targets in the LCFS depends on steady market gains by alternative fuels including ethanol, natural gas, hydrogen and electricity.
California's Air Resources Board, which is responsible for administering the LCFS, has evaluated the emissions benefits of many different fuel pathways, including compressed natural gas (CNG) and liquefied natural gas (LNG). It estimates the direct emissions from CNG and LNG at roughly the same level as average corn ethanol from sources in the Midwest. All three come in around 30% lower than the CA reformulated gasoline into which ethanol would be blended. Ethanol from locally sourced corn, or produced using biomass or other alternatives for process heat, has somewhat lower direct emissions, but that's before considering ethanol's indirect emissions that the state estimates at around 30% of gasoline's.
Factor in the much lower water consumption of natural gas compared to ethanol--from hundreds to thousands of times less lifecycle water use for the same energy content, even after accounting for hydraulic fracturing ("fracking")--and CNG and LNG offer reduced vehicle emissions with lower overall environmental impacts than conventional ethanol, particularly in water-stressed locations.
Biofuels such as ethanol enjoy wide political and popular support. They will also become more efficient and beneficial if developers are successful in transferring techniques for making them from non-food feedstocks from the laboratory to full-scale production. However, since around half the energy in a gallon of corn ethanol comes from natural gas anyway --accounting for over a billion cubic feet per day, or up to 2% of US natural gas consumption--CNG and LNG may provide a way to use the same gas to reduce transportation emissions by a similar amount, but with less use of other resources and potentially lower indirect environmental impacts. Perhaps sustainability, rather than renewability, should be the metric on which the EPA's fuel policies focus.