The Environmental Appeal of Second-Generation Biofuels
Second-generation biofuels from cellulosic feedstocks that were until recently thought to be always “five years away,” are now being produced commercially. In 2014, about 18 million gallons were produced, which was substantially higher than the 432,000 gallons produced in 2013. Poet-DSM and Abengoa are two of the refineries that began production in 2014 using corn stover as a feedstock with capacities of 20 million gallons and 25 million gallons respectively. Additional refineries are currently under development and are being scaled up for commercial production.
The long-awaited emergence of commercial cellulosic biofuel production has occurred as U.S. consumption of ethanol has hit a blend wall that limits our capacity to blend ethanol with gasoline based on the existing fleet of conventional and flex-fuel vehicles and a downward plunge in oil prices from over $110 per barrel in mid-2014 to under $50 per barrel in early 2015.
A major driver of investment in cellulosic biofuel production has been the Renewable Fuel Standard (RFS) that mandated production of 16 billion gallons of biofuels from non-food cellulosic feedstocks by 2022. Limits on capacity to blend ethanol have led to uncertainty about the continued implementation of the RFS, while oil price volatility has the potential to jeopardize the viability of existing and future investments in cellulosic biofuel production.
Nevertheless, as we consider a possible future with second-generation biofuels, it is important to examine the alternative pathways to the production of second-generation biofuels and the features that would define a biofuel-based economy that is economically, environmentally, and socially sustainable. A key aspect for the economic sustainability of these biofuels in the medium to long term will be that they become competitive with the fossil fuels they seek to displace at free-market prices, without any government mandates, tax credits, or subsidies. This also implies that the price of biomass at which they are competitive should provide a positive net gain in returns to land to a farmer relative to the returns that could be earned from the existing use of that land.
To be considered environmentally sustainable, biofuels at a minimum should have a lower greenhouse gas intensity (measured by the carbon emissions per unit of energy equivalent to that of oil) than oil.
Additionally, the feedstocks used to produce these biofuels should improve soil quality, water quality, and biodiversity compared to existing use of land. Lastly, the production of biofuels is more likely to be socially sustainable if their competition with food/feed production is minimal.
Second-generation biofuels have the potential to meet 30 percent of U.S. transportation fuel needs, according to an assessment by the U.S. Department of Energy. The study estimated that the United States could produce 1 billion tons of biomass from crop and forest residues, dedicated energy crops, and forest biomass. They also have significant potential to lower greenhouse gas intensity of transportation fuel and provide other environmental benefits without the same requirements for diverting land from food and feed production as food crop-based biofuels. However, it is important to recognize that not all biofuels are the same. There are many choices for biomass for cellulosic biofuels and these can differ widely in their economic, land use, and environmental impacts. This brief describes alternative pathways for these biofuels and differences in their environmental implications. Subsequent briefs will continue this discussion of sustainable biofuels by examining the economic and environmental trade-offs they pose and the role that policy can play in guiding the development of a sustainable bioeconomy.
Yield and Land Requirements for Second-Generation Biofuels
There are many choices of biomass (feedstock) for producing second-generation biofuels, including crop residues from corn and wheat, numerous types of energy crops like miscanthus, switchgrass, energy cane, agave, and energy sorghum, and short-rotation woody crops like poplar and willow. These feedstocks vary considerably in their yields, input, and land requirements, carbon intensity, and other environmental impacts.
Crop residues are readily available without changing the use of the land. Key considerations in the case of using crop residues is the amount of residue that can be removed without adversely affecting soil organic matter and the amount of fertilizer application needed to replace the nutrients removed with the harvested residue.
Dedicated energy crops can be grown on land that may be currently idle or in a crop-pasture rotation. These crops are typically long-lived perennials with a lifespan of 10 to 20 years. They can be produced under rain-fed conditions and with low-input applications. Because these crops have deep root systems, it is generally expected that they can be grown productively on low-quality soils, thereby avoiding the need for diverting cropland from food and feed production.
High-yielding energy crops like miscanthus and energy cane can provide twice as many gallons of biofuel per acre of land as compared to corn ethanol. In a recent publication (Dwivedi et al., 2015) we compared the yields of corn stover, miscanthus and switchgrass and their effects on soil carbon levels. We simulated yields of miscanthus and switchgrass at three different locations (Illinois, Indiana and Alabama) under 30 years of weather conditions. We examined the yields of miscanthus and switchgrass on high- and low-quality soils, keeping all management practices the same. In the case of corn stover, we assumed corn residue harvest rates of 30 percent with conventional-till practices and 50 percent with no-till practices.
Harvested yield of corn stover is relatively low (0.8-1.6 tons per acre) and that of switchgrass was 6-7 tons per acre and of miscanthus was 8-11 tons per acre per year. We found that on average, yield of miscanthus and switchgrass grown on low-quality soil was about 10 percent lower compared to that on high-quality soil. Despite this, the cost of producing these crops on low-quality land could be 12 to 30 percent lower than that of producing them on high-quality soils after factoring in the lower-opportunity cost of converting low-quality land to producing these crops. The incentives for maximizing net returns to land can be expected to drive production of these energy crops on low-quality land, if available. However, the extent to which this low-quality marginal land is available for energy crop production remains to be determined. Farmers may also be unwilling to convert their low-quality land to energy crop production because it may be providing other amenity values or because it is spatially fragmented, making it logistically unviable. They may on the other hand be also willing to convert high-quality land to energy crops to diversify their crop portfolio and reduce its riskiness.
Greenhouse Gas Intensity of Second-Generation Biofuels
The greenhouse gas (GHG) emissions associated with each stage of production of second-generation biofuels can be estimated using life-cycle accounting methods. This involves accounting for the effects of collecting crop residues and growing energy crops on carbon accumulated in the soil as well as carbon emissions generated during the process of planting energy crops, harvesting and transporting biomass, and converting it to fuel in a biorefinery.
Additionally, if land is diverted from food/feed crop production to energy crops, it could contribute to raising the price of food/feed crops that could lead to conversion of forests and grasslands in the United States or the rest of the world to cropland. This indirect change in land use would lead to a loss in carbon sequestered in vegetation and soils and offset some of the savings from using biofuels to displace gasoline.
In the case of energy crops that are produced on low-quality land that is either in a crop-pasture rotation or is idle, the indirect effect of change in land use on carbon sequestered in land is expected to be negligible. Energy crops produced on cropland would displace food/feed crops and could lead to some indirect land use change. However, given the high yield of these crops per acre of land displaced, this indirect effect on carbon emissions is expected to be small and much lower than that of crop-based biofuels like corn ethanol.
The process of converting biomass to biofuel is much less carbon-intensive than the process of converting corn to ethanol. This is because it relies on using a part of the biomass for generating the energy needed for the biorefinery instead of using coal or natural gas. Additionally, electricity is generated as a co-product of the cellulosic biorefinery process and this can be sold on the grid to displace coal/natural gas based electricity and generate a carbon credit.
The life-cycle GHG intensity of cellulosic biofuels differs greatly across feedstocks. The GHG intensity of corn stover-based ethanol depends on the rate at which it is collected; high rates of corn stover harvesting can lower soil organic matter and require high levels of nutrient replacement. Our model simulations show that even with 30 percent removal rate with conventional tillage and 50 percent removal rate with no-till practices with corn-soy rotations or continuous corn production, there was a decline in soil carbon stocks. As a result, the life-cycle GHG savings with corn stover-based ethanol is lower than with ethanol from miscanthus and switchgrass. Compared to gasoline, the reduction in GHG intensity from miscanthus-based ethanol ranged between 130 to 156 percent, and that from switchgrass ranged between 97 to 135 percent. These energy crops therefore have negative carbon emissions and are sinks for carbon. The corresponding range for GHG savings with corn stover relative to gasoline was 57 to 95 percent. The threshold for savings in GHG intensity for biofuels classified as cellulosic biofuels under the RFS relative to gasoline is 60 percent and as such there could be situations where biofuels made from corn stover would not qualify as cellulosic biofuels under the RFS.
Other Environmental Impacts
Long-term experimental data also show that these perennial energy crops lower nitrate runoff from land compared to annual crops. Field trials showed that nitrate leaching with these perennial crops after they are established was less than 10 percent of that under a corn-corn-soybean rotation (Smith et al., 2012). Nitrate loadings in a watershed were estimated to decrease by 6 to 30 percent depending on the amount of land converted and the amount of nitrogen fertilizer applied to miscanthus (Ng et al., 2010). Studies also find that the removal of corn stover can reduce nitrogen and phosphorus loadings in a watershed but would increase soil erosion and sediment runoff (Gramig et al., 2013).
The production of non-native crops, such as miscanthus, raises concerns about invasiveness. The particular species (Miscanthus x giganteus) that is being considered for bioenergy has been shown to be sterile with a low risk for invasiveness. Large-scale production of energy crops also raises concerns about monocultures replacing a polyculture of native grasses and reducing biodiversity; these can be mitigated by restricting energy crop production to cultivated/grazed land where they would add diversity to the existing monocultures of corn and soybeans.
Second-generation biofuels can be expected to have multi-dimensional environmental impacts. These impacts will differ widely across feedstocks and locations, implying a need for designing appropriate management practices and policies to balance their benefits and costs. Current policies such as the RFS and the cellulosic biofuel production tax credit treat all cellulosic biofuels uniformly. They do not provide the incentives needed to produce feedstocks that have a smaller environmental footprint than others, particularly if they are also more expensive. Policies that reward biofuels based on specific performance metrics would provide the signals needed to induce production of more environmentally sustainable feedstocks in the long run.
Gramig, B.M., C. J. Reeling, R. Cibin, and I. Chaubey, “Environmental and Economic Trade-Offs in a Watershed When Using Corn Stover for Bioenergy,” Environmental Science & Technology, 2013, 47 (4), 1784–1791, DOI: 10.1021/es303459h
Ng, T.L, J. W. Eheart, X. Cai, and F. Miguez, “Modeling Miscanthus in the Soil and Water Assessment Tool (SWAT) to Simulate Its Water Quality Effects As a Bioenergy Crop,” Environmental Science & Technology, 2010, 44 (18), 7138–7144, DOI: 10.1021/es9039677
Smith, C.M. M.B. David, C.A. Mitchell, M.D. Masters, K.J. Anderson-Teixeira, C.J. Bernacchi, and E.H. DeLucia, “Reduced Nitrogen Losses after Conversion of Row Crop Agriculture to Perennial Biofuel Crops,” Journal of Environmental Quality, 2013, 42 (1), 219-228.
Dwivedi, P., W. Wang, T. Hudiburg, D. Jaiswal, W. Parton, S. Long, E. DeLucia, and M. Khanna, “Cost of Abating Greenhouse Gas Emissions with Cellulosic Ethanol, Environmental Science & Technology , 49 (4), 2512–2522, 2015, DOI: 10.1021/es5052588