Indirect emissions from waste and residue feedstocks: 10 case studies from the United States
Biofuels made from food crops are commonly associated with significant indirect greenhouse gas (GHG) emissions. By contrast, the remainders of food and forestry crops and other biogenic waste materials, known as by-products, wastes, and residues, often emit little carbon when used as feedstock for biofuels; indeed, they are often assigned zero upstream GHG emissions in traditional life-cycle assessments (LCA). Moreover, these secondary outputs, ranging from corn stalks and inedible rendering products to spoiled crops and sawdust, represent a small fraction of the economic value of supply chains, whether for agricultural, forestry or other products. And because wastes are a small part of the value chain, increased demand for them will not likely spur increased cultivation of the primary resource (crops, forest trees, cattle, and the like) and therefore will not increase GHG emissions associated with primary resource production. In sum, the characteristics of these marginal materials—they are low-carbon and unlikely to drive production of more primary product—make them attractive feedstocks for biofuels.
However, a closer look reveals that these non-food feedstocks may also spur further indirect emissions when diverted from other uses to produce biofuels. Those other uses require substitute inputs, which have their own greenhouse gas footprints. For example, inedible tallow diverted from soapmaking or livestock feed to make biofuel is no longer available to make soap and feed, which must then be made from other materials—whose associated GHG emissions may be greater than those of the low-carbon waste fat. In other words, the use of low-emissions secondary materials for biofuels may prompt the use of high-emissions materials elsewhere.
The shift of material usage across different industries can result in significant GHG emission impacts. Quantifying these impacts is somewhat akin to modeling land-use change; a shift in demand for one unit of feedstock is tracked across multiple, interacting supply chains. However, when conducting an indirect LCA for wastes, residues and by-products, current practice is to assess only the impacts of their displacement from existing uses, while not usually considering increased production of the feedstock material itself. This form of consequential LCA is known as displacement analysis. Although some displacement analyses for waste, residue, and by-product feedstocks have been conducted by researchers and environmental regulators over the past decade, the practice has not yet been widely adopted in biofuel LCA policies.
Here we develop a displacement emissions methodology and apply it to 10 waste, residue, and by-product biofuel feedstocks in the U.S. context. These materials are either current or potential biofuel feedstocks in the U.S. market and span numerous industry sectors. The figure below presents a summary of our calculations, measured in grams of carbon dioxide equivalent (g CO2e) per Megajoule (MJ) of biofuel. Both direct emissions (in blue) from fuel production and indirect emissions from feedstock displacement (in brown) are shown in comparison with an emissions baseline for fossil fuels (the dotted line). We also incorporate a sensitivity analysis to reflect uncertainty in our final results.
Our results highlight the effects that inclusion of displacement emissions have on the overall carbon intensity (CI) score of biofuels. While the feedstocks assessed here are generally considered wastes and residues, and therefore biofuels produced from them are typically thought to have very low GHG emissions, our analysis shows they all may have significant indirect emissions effects. Across our ten case studies, indirect emissions vary greatly, ranging between -49 gCO2e/MJ to 176 gCO2e/MJ. In one case, sawmill residue diesel, these impacts are so high as to render the biofuel pathway higher-GHG than that of fossil fuel. Outside the biofuels sector, sawmill residues are primarily used in heat and power production at lumbermills and as a low-cost material for the fiber products industry. Since sawmill residue’s likeliest substitutes (pulpwood and natural gas) are associated with significant upstream GHG emissions, diverting this feedstock toward biofuels production results in large indirect emissions impacts. Feedstocks can also have negative displacement effect emissions when a unit of material is diverted to biofuels. This is the case for manure and food waste biogas, the pathways with the lowest CI scores of all feedstocks analyzed. Both food waste and manure emit significant quantities of methane when they are left untreated in covered landfills, lagoons, or other disposal applications and require no material substitutes. Thus, capturing and upgrading methane gas for biofuel production results in a negative displacement effect.
Displacement analyses are one tool to help policymakers identify feedstocks with the highest risk of adverse environmental impact. Life-cycle emissions associated with biofuels consumption do not begin and end at the pump, so evaluating interacting market effects between feedstocks can inform regulators of their overall climate impact. A more thorough understanding of indirect emissions may change the prioritization of these feedstocks within a transport decarbonization strategy.