Biofuel co-products as livestock feed - Opportunities and challenges

16Biofuel co-products as livestock feed – Opportunities and challengesethanol supply was 120 PJ from EU cereals and sugar beet,with imports of sugar cane bio-ethanol.BIOFUEL PROCESSESDifferent crop products are used to make biofuels, usingalternative technologies:• Sugar and starch fermentation to bio-ethanol.• Vegetable oil transesterification or hydrogenation tobiodiesel.• Anaerobic digestion to biomethane.• Hydrolysis of lignocellulosic feeds followed by fermentationto bio-ethanol.• Gasification of lignocellulosic feeds followed by biodieselsynthesis.Starch in cereal crops and sugar in crops such as sugarcane and sugar beet are converted to bio-ethanol usingfermentation, leaving the remaining DDGS from cereals andpulp from sugar beet for use as animal feed. Vegetable oilsare extracted from oilseed crops, such as rapeseed and soybean,and converted to biodiesel using transesterificationor hydrogenation processes, leaving the remaining oilseedcakes or meals for use as animal feed. Transesterificationof vegetable oils uses methanol and gives aglycerine coproduct.While various work has been done to show thatglycerine can be used as an animal feed, it is unlikely thatit will be used to any significant extent. This is becausepurification of crude glycerine would probably be neededto eliminate the risk from associated methanol and becausethere are alternative higher value markets for glycerol:upgrading for pharmaceutical use, manufacture of chemicalsand in the EU there are incentives to use crude glycerolfor renewable power generation.Anaerobic digestion is able to utilize a large range offeedstocks to produce biogas, which can be used to generateheat and power, or purified to make biomethane. Thebiomethane can be fed into the gas grid or used as a biofuel.Anaerobic digestion decomposes the starch, sugar, oiland protein in the feedstock to produce methane, while theremaining components, including phosphate and potashand the nitrogen from the protein fraction, are returned toland as digestate, or decomposed in an aerobic oxidationunit.Lignocellulosic feedstocks such as wheat straw, maizestover and wood need more aggressive processing to accesscomponents for biofuels. One option is to use hydrolysisof the feedstocks to extract the sugar, for fermentation toethanol. The other option is gasification of the feedstockto hydrogen and carbon monoxide, followed by synthesisprocesses to produce methanol, ethanol, dimethyl ether ormiddle distillate using Fischer-Tropsch synthesis and hydrocracking.The remaining components are used as fuel todrive the process. These processes all require a large capitalinvestment. Only the sugar-, starch- and oilseed-basedprocesses (so called “First-generation” processes) normallyprovide animal feed as a co-product.Biofuel production process efficienciesA comparison of biofuel recovery and energy efficiencylosses for different biofuel processes (Ingledew 1993; Adenet al., 2002; FNR, 2009; Nexant, 2007) is shown in Table 1.The biofuel component is the component in the feedstockthat is used to make the biofuel, and different technologiesare used. The extraction efficiency is the proportion of theavailable feedstock component that is extracted or utilizedfor the biofuel process. For example, in anaerobic digestion,lignin and cellulose can not all be utilized in the process.The biofuel selectivity is the proportion of the biofuelcomponent that is converted to biofuel, while the rest isconverted to other by-products. The crop energy efficiencyis the proportion of energy in the crop or feedstock thatis converted to useful energy products such as biofuel oranimal feed.Although some potential biofuel is lost in fermentationand transesterification technologies due to inefficienciesin vegetable oil extraction and in fermentation, there is aloss of only 1 percent to 2.5 percent in the overall energyTABLE 1Biofuel recovery and energy efficiency losses for different biofuel processesParameterCereals and sugarcropsBiofuel feedstockOil seeds Green maize Maize stover and straw WoodComponent Starch and sugar Vegetable oil Carbon Carbohydrate and sugar CarbonComponent extraction Hydrolysis Extraction Anaerobic digestion Hydrolysis GasificationtechnologyComponent extractionefficiency98.5% 80%–96% 81% 58% 100%Biofuel productiontechnologyFermentation Trans esterification Anaerobic digestion Fermentation Fischer-TropschprocessProduct Ethanol Fatty acid methyl ester Methane Ethanol Middle distillateComponent selectivity 93% 95% 75% 82% 74%Stoichiometric energyconversion efficiency95.9% 99.7% 90% 96% 69%Crop energy efficiency 97.5% 99.9% 55% 46% 51%