Bioethanol – status report on bioethanol production from wood and ...

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Lignin is a byproduct of this process, and can be used as a boiler fuel or processed into specialty chemicals. Hydrolysis and fermentation can be conducted simultaneously in one stage but simultaneous saccharification and fermentation (SSF) is yet to be implemented commercially, significant advances are being made in this area. Thermochemical conversion Thermochemical conversion transforms the lignocellulosic feedstock into carbon monoxide and hydrogen (syngas) by partial combustion (Fig. 2). These gases can be converted to liquid transportation fuels or commodity chemicals by catalytic or biological pathways. The biological process converts carbon monoxide to ethanol using a non-yeast fermentation microorganism (eg. Clostridium ljungdahlii). Alternatively, the syngas can be fed to a catalytic reactor where the carbon monoxide and water are combined via a metal-catalysed process to produce methanol, ethanol, other higher alcohols or liquid fuels (Fischer-Tropsch liquids). Gasification is important because lignin, which constitutes about 25 <strong>–</strong> 30% of cellulosic biomass, is also converted to syngas and subsequently converted to fuel. CURRENT STATE OF TECHNOLOGIES AND TECHNICAL CHALLENGES Biochemical Pretreatment - the usefulness of cellulose as a feedstock has been limited by its rigid structure and difficulty to breakdown into simple sugars. Costeffective pretreatments are needed to liberate the cellulose from the lignin/hemicellulose matrix and reduce its crystallinity. Pretreatments of increasing severity are needed as feedstock recalcitrance increases from nonwoods (agricultural residues) to hardwoods to softwoods. Many pretreatments are currently being explored, ranging in chemistries from very acidic to mildly alkaline, such as dilute acid, ammonia fibre expansion (AFEX), wet oxidation, solvent based pulping (i.e. organosolv) and steam explosion. The ideal pretreatment liberates hemicellulose, exposes the cellulose and allows the lignin to be separated and must also minimise the formation of degradation products that can inhibit the subsequent hydrolysis and fermentation processes. Lignin <strong>–</strong> As lignin is mainly responsible for lignocellulosic recalcitrance, particularly in softwoods, studies have shown its separation during pretreatment greatly enhances cellulose accessibility and enzyme effectiveness (6). Pretreatments that minimise lignin redeposition and condensation on the fibre surfaces are favoured. Separation of lignin and production of specialty lignin co-products also has the potential to improve the overall economics. Hemicellulose <strong>–</strong>Is composed primarily of 5 carbon sugars, these may be liberated during the pre-treatment process or require further treatment with hemi-cellulase enzymes. The C5 sugars may be fermented to ethanol or sold as a co-product. Hydrolysis - Cellulose is broken down into individual glucose units by cellulase enzymes, under mild conditions. Research is on-going to find reduce the costs of enzyme systems that produce high sugar yields at accelerated rates and without the formation of inhibitory byproducts. Currently, the per unit cost of enzymes is considered to be a deterrent to the commercial success of the biochemical pathway. Alternative strategies to reduce enzyme cost include the recycling of enzymes and the use of polymers to reduce the binding of enzymes to the substrate (7). Fig. 2 Schematic of a thermochemical cellulosic ethanol production process (5).
Fermentation - The hydrolysate contains both 5-carbon (pentose) and 6-carbon (hexose) sugars. The conversion of pentose sugars into ethanol is less efficient than conversion of hexose sugars. A system of mixed-sugar fermenting microorganisms is required to utilise the full range of sugars present and thus maximise the production of ethanol. Metabolic engineering is on-going to find low-cost, microorganisms capable of C5 and C6 sugar cofermentation that are also resistant to inhibitors (acetic acid, furfural) that may be present. Thermochemical Contamination <strong>–</strong> various components of the biomass feedstock can cause problems in the gasification and catalytic synthesis stages. Contaminants such as tars and inorganic components (halides, alkalis, ash) present in the syngas can deactivate the catalysts and must be removed prior to catalytic conversion. The formation of tars, and measures to deal with their removal, are significant challenges in biomass gasification. Advances in gas clean-up and catalyst preparations are also needed in order to make large-scale biomass to liquid facilities practical. ECONOMICS Both the biochemical and thermochemical pathways require sophisticated processing steps that have higher operating costs and need significant capital investment compared with grain-based ethanol processes. Based on the current state of technology, capital costs for biochemical cellulosic ethanol are estimated to be between US$4.03 and $5.60 per US gallon of annual capacity (8, 9). Operating costs are estimated to be between US$1.34 and $1.69 per US gallon, depending upon the assumptions made about feedstock costs, enzyme costs, and the kind of pretreatment to be employed (8,9). Projected capital costs for future plants employing anticipated improvements in biochemical conversion are estimated to be US$3.33- 4.44 per US gallon ethanol annual capacity with operating costs dropping to US$0.40-0.89 per US gallon of ethanol (10). The US Department of Energy (DOE) has determined that competitiveness with petroleum can be achieved at an ethanol production cost of US$1.07/US gallon (in 2002 dollars) and aims to achieve this goal by 2012 (5). This compares to the production cost of Brazilian sugarcane ethanol of US$0.81/gal. Lignocellulosic based ethanol has a further advantage over starch based ethanol in that the feed stock is not commodity based and subject to brokers and trader speculation on pricing. Lignocellulosic based feedstocks avoid the arbitrage that starch-based ethanol is subject to; project developers are better able to forecast and manage feedstock costs reducing project risk. Many starch-based ethanol producers are currently struggling to survive and many may go bankrupt due to the volatile swings in corn, wheat, sugar and ethanol prices. A recent study of the production of Fischer-Tropsch (FT) liquids from syngas has shown the thermochemical platform to be economically comparable to the biochemical platforms (8). CELLULOSIC ETHANOL PLANTS This section provides brief details on all publically announced bioethanol plants based on lignocellulosic feedstocks. The authors have endeavoured to be comprehensive but do not guarantee all facilities are included. Table 1 summarises all known facilities as of February 2009, and these are shown geographically on Fig. 4. Table 1 Lignocellulosic Facilities as at February 2009. Pilot 1 / Demonstration 2 Commercial 3 Biochemical 25 9 Thermo Chemical 5 3 Note: 1. Pilot Scale is R&D 2. Demonstration Scale is < 10 ML/yr 3. Commercial Scale is > 10 ML/yr United States Currently the US has a target of 136,260 million litres per year (ML/yr) of renewable fuels production by 2022. The US Renewable Fuels Standard calls for 60,560 million liters to come from lignocellulosic sources. The driving force for this requirement is primarily energy independence; however green house reduction is a significant motivating factor for the inclusion of lignocellulosic based fuel. Demonstration-scale cellulosic ethanol plants are under construction as part of the government’s goal to make cellulosic ethanol cost competitive by 2012. The plants cover a wide variety of feedstocks, conversion technologies and plant configurations to help identify viable technologies and processes for full-scale commercialization. All demonstration plants, which are sized at 10% of a commercial-scale biorefinery, are expected to be operational by 2012. Commercial-scale plants are in the planning stages. Demonstration and commercial plants include: • Abengoa - Abengoa is using a $76 million DOE grant to develop a 42 ML/yr of cellulosic ethanol integrated biorefinery, in Hugoton, Kansas. The biomass plant will be situated next to a conventional cereal-to-ethanol facility to share feedstocks, including wheat straw and corn stover. Both enzymatic hydrolysis and gasification will be
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