Biofuel co-products as livestock feed - Opportunities and challenges

Biofuels: their co-products and water impacts in the context of life-cycle analysis 493advantages of bio-ethanol. Emissions during the use phaseconstitute the bulk of GHG emissions for both maize andcellulosic ethanol (based on switchgrass).CO-PRODUCT ALLOCATION METHODOLOGIESAND IMPACTS ON LCA RESULTSBiofuel co-products introduce complexity into biofuel LCA.Wang, Huo and Arora (2011) explore six methods of allocatingenergy and emissions impacts among biofuel coproducts.Table 5 conveys the differences in methodologyamong these approaches and the advantages and drawbacksof each. Wang, Huo and Arora (2011) consideredthe pathways and the displaced products listed in Table 6.The life-cycle impacts of gasoline and diesel were includedin the analysis as baseline fuels, and Wang and co-workersallocated impacts among petroleum refinery co-products bytheir energy contents. Not every pathway was analysed withall co-product allocation methods. For example, electricity ismassless, so evaluation of the switchgrass-to-ethanol pathwaydid not include a mass-based allocation analysis.We present well-to-wheel (WTW) energy consumptionand GHG emissions results for the biofuel pathways in thisanalysis. Figure 12 illustrates total energy use for the productionof the two petroleum-fuel-based cases and for four biofuelpathways. The feedstock for biodiesel and renewablediesel is soybeans (Figure 5). All biofuel pathways consumemore energy than the petroleum-based fuels because, whenbiomass feedstocks undergo conversion to biofuels, a largeramount of energy is lost. When considering fossil energy usein Figure 13, however, biofuels consume less fossil energyin their life cycles than do petroleum-based fuels. This isbecause while energy in a petroleum feedstock is fossilenergy, energy in biomass is not fossil energy. Sometimes,energy debates on biofuels vs petroleum fuels centre ontotal energy. But the renewable energy in biofuels is notrelevant to energy issues such as energy resource deple-TABLE 5Co-product allocation methodologies in LCACode Method Description Benefits DrawbacksDDisplacementDetermine life-cycle impacts ofconventional products to bedisplaced by biofuel co-products.Account for the displacementof these impacts by the biofuelco-product in the biofuel LCA.Tends to represent actualeffects of creating multipleproducts.Must conduct LCAs for conventional, displacedproducts. May produce distorted results whena significant amount of biofuel co-product isproduced.M Mass-Based Allocate energy use and emissionsburdens by mass output shares.Straightforwardassumptions. Typically usedin consumer product LCAs.Problematic when co-products have differentuses (e.g. electricity vs fertilizer) or no mass(electricity).EEnergy-BasedAllocate energy use and emissionsburdens by energy output shares.Applicable when majorityof products are energy (e.g.fuels or electricity).Problematic when co-products have differentuses (such as nutrition for animal feed).$ MarketValueAllocate energy use and emissionsburdens by economic revenueshares of individual products.Normalizes all products toa common basis regardlessof use.Subject to price fluctuations, including those inthe future. Does not reflect physical processesconsuming energy and generating emissions.PProcess-PurposeEstimate energy use and emissionsburdens of individual processes in afacility, and assign to products.Straightforward when unitprocesses produce a singleproduct.Many processes have multiple product outputs.Requires detailed energy and emission data atprocess level for a facility. Energy and emissionsupstream of the facility still require use of otherallocation methods.HHybridAllocationCombine one or more of abovemethods.Obtain more preciseallocation of impacts.Notes: The code column refers to horizontal axis labels in Figures 12 to 14, q.v.Increases complexity of analysis. Createsinconsistency of allocation methods.TABLE 6Conventional products to be displaced by biofuel co-products for the displacement methodBiofuel pathway Co-products Displaced productsGHG Credit(g CO 2e/MJ biofuel)Maize to ethanol DGS Maize, soybean meal, urea 12Switchgrass to ethanol Electricity United States average electricity 19Soybeans to biodiesel Soybean meal Soybeans 24Glycerin Petroleum glycerin 0.46Soybeans to renewable diesel Soybean meal Soybeans 34 (1)Fuel gas Natural gas 0.38Heavy oil Residual oil 0.26Notes: (1) The GHG credit in grams per MJ of fuel of soybean meal for renewable diesel is larger than that for biodiesel because fuel yield inMJ per unit of soybean is smaller for renewable diesel than for biodiesel. Thus, normalization of soymeal credit to fuel production results inlarger credits for renewable diesel than for biofuels.