The conversion of biomass to cellulosic ethanol is the most efficient and productive use of biomass to create a high-octane, environmentally friendly transportation fuel, according to a perspective paper published in the Journal of Chemical Technology & Biotechnology. Authors of the paper are Rathin Datta, cellulosic ethanol company Coskata’s chief scientific officer; Mark Maher, executive director, powertrain/vehicle integration, GM Powertrain; Coleman Jones, GM biofuels implementation manager; and Richard Brinker, Dean and Professor of Forestry and Wildlife Sciences, Auburn University.
The paper draws four primary conclusions: the conversion of biomass-to-ethanol has natural efficiencies; ethanol has a history of superior performance; today’s cars are ready for ethanol; and sufficient biomass exists to make an impact.
Natural efficiencies. Biomass feedstocks that are abundantly available in the US and worldwide contain a high quantity of oxygen (approximately 40% or higher). To utilize this biomass most efficiently, the conversion must include the entire feedstock, including the oxygen, the authors argue. Cellulosic ethanol is the alternative transportation fuel which retains most of the oxygen portion of the feedstock.
...based on the fundamentals of photosynthesis and simple laws of thermodynamics, the biomass feedstocks that are and will be readily available are highly oxygenated and are lignocellulosic materials. For liquid fuel or chemical feedstock production from this feedstock, the winning strategy is to produce a product that has proven and widespread use, with the highest yield using the entire feedstock—and that is ethanol.—Datta et al.
Ethanol’s compatibility with the oxygen in biomass and lower carbon requirements lead ethanol to have a higher yield and more BTUs of output per ton of incoming biomass than “drop in” alternatives such as butanol or hydrocarbons. The theoretical yield for ethanol, they note, is 51%; for n-butanol or iso-butanol, 41%; and for octane C8-hydrocarbon, 29.7%.
|Comparison of yields of ethanol vs other reduced products. Source: Datta et al. Click to enlarge.|
Conversion pathways. Much R&D has focused primarily on a biochemical pathway or a thermochemical pathway for producing ethanol from biomass, the authors note. The biochemical approach uses enzymes to convert pretreated lignocellulosic biomass materials into sugars, which can then be fermented into ethanol. The thermochemical approach gasifies a biomass feedstock to produce syngas, which is then converted into ethanol by a chemical reaction utilizing chemical catalysis.
Biochemical approaches are hampered by expensive pre-treatment requirements; the inability to ferment lignin (biomass contains 20 to 25% lignin); the complexity of biological conversion of cellulose to glucose; and limited feedstock flexibility.
Thermochemical approaches are hampered by lack of catalyst selectivity; thermodynamic inefficiency caused by combination of the exothermic reactions and need for specific H2:CO ratios; high pressure requirements (>1000 psig) increases the mechanical complexity and capital costs; and sensitivity to impurities.
The authors argue that Coskata’s approach, consisting of gasification, syngas fermentation, and separation, is attractive because of its feedstock flexibility; its ability to use all the feedstock; its low greenhouse gas profile; selective ethanol production; low operating costs; and low capital costs.
Emissions and engine efficiency. The authors note that numerous studies and reports have been made on the reductions of harmful emissions such as carbon monoxide, VOCs (volatile organic compounds), and sulfur oxides form the use of ethanol.
Ethanol’s high octane enables the use of higher compression ratios, particularly in dedicated ethanol vehicles. The high heat of vaporization produces a charge cooling effect, particularly in direct injection engines, that can again allow higher compression ratios. This effect is enhanced by the increased volume of fuel that is required to compensate for the lower energy content of ethanol, they note.
Even when a vehicle is not optimized to take advantage of some of ethanol’s attributes, the higher octane and faster flame propagation speeds for ethanol result in increased energy efficiency (miles per BTU of energy present in the fuel used) for high ethanol blends relative to gasoline.
On average, 2010 model year vehicles deliver a 2% improvement in energy efficiency with E85. There is significant variation within and between manufacturers; for example General Motors products delivered an average 3.19% improvement in energy efficiency. This is typically due to vehicle calibration variation and how well each vehicle can take advantage of the improved properties of E85.
These studies support the expectation that ethanol’s octane values and other attributes will increase combustion efficiency and lead to more efficient power output in high ethanol content blends such as E85. However, improvements beyond those shown are unlikely to be realized in the near future in the commercial world because of the long time it will take to transition from predominantly gasoline to predominantly ethanol as the liquid motor fuel.—Datta et al.
Impact. The authors cited the recent major studies conducted by the USDA, DOE and major National Laboratories which projected that large and sustainable biomass feedstock supplies are available and going to be available to produce ethanol efficiently in very large quantities of around 340 billion L (90 billion gallons) per year in the US.
The authors also summarized the experience gained over the past 70 years in the south-eastern USA to further support the fact that efficient and sustainable biomass supply can be developed and maintained to support much increased usage.
Rathin Datta, Mark A. Maher, Coleman Jones, and Richard W. Brinker (2011) Ethanol – the primary renewable liquid fuel. J. Chemical Technology and Biotechnology 86: 473-480 doi: 10.1002/jctb.2580