Hemicellulose feedstocks

Cellulosic and hemicellulosic feedstocks may be converted into 100+ chemicals, among them drop-in products but also several novel chemicals. Aside from biofuels and their precursors, they may be classified as end products, platform chemicals, monomers, and chemical intermediates (leading to a specific end-product). Some of these are already produced commercially from carbohydrates whereas others are currently petroleum or natural gas derived. Most of the chemicals described here are not produced commercially to date. 

The C-5 sugar alcohols produced from the hydrolysis of hemicellulose are both xylitol and arabitol [6], Equivalence testing was performed with Ni/Re catalyst in the batch reactor to verily similar performance between xylitol and arabitol feedstocks. The operating conditions were 200°C and 8300kPa H2 using the procedure outlined in section Catalyst Screening section. 

Arabitol testing was performed to ensure that it behaved similarly to xylitol in this reaction to validate xylitol as a model compound. Testing was performed using Ni/Re catalyst from the initial batch screening. Shown in Table 1, the results from the two sugar alcohols were nearly equivalent. This gave some confidence that xylitol should be a valid model compound in the absence of actual hemicellulose derived feedstock. 

To break up cellulose/hemicellulose, it is treated physically (milling), with heat, and hydrolyzed (sulfuric acid + enzymes). Also in this case, improved (bio)cata-lytic hydrolysis processes for cellulose/hemicellulose are needed. The sugar can then serve as feedstock for standard fermentation plants. 

Agricultural residues (stem, leaves, etc.) currently left in the fields after harvesting are made of cellulose, hemicellulose, and lignin. They are not competing with the feedstock for the food industry. 

Most often, the rates for feedstock destruction in anaerobic digestion systems are based upon biogas production or reduction of total solids (TS) or volatile solids (VS) added to the system. Available data for analyses conducted on the specific polymers in the anaerobic digester feed are summarized in Table II. The information indicates a rapid rate of hydrolysis for hemicellulose and lipids. The rates and extent of cellulose degradation vary dramatically and are different with respect to the MSW feedstock based on the source and processing of the paper and cardboard products (42). Rates for protein hydrolysis are particularly difficult to accurately determine due the biotransformation of feed protein into microbial biomass, which is representative of protein in the effluent of the anaerobic digestion system. 

Hemicellulose (or polyose) is primarily composed of xylan, a branched polymer composed of five-carbon sugar, xylose. Typical polymerization degree of hemicellulose is 50 - 200, which is shorter than the cellulose molecules. The acid hydrolysis of hemicellulose, (C6H10O5)n, produces mainly xylose (C6H10O5), which can be converted to furfural, a chemical feedstock, or can be fermented to ethanol. 

The present utilization of carbohydrates as a feedstock for the chemical industry is modest, when considering their ready availability, low cost and huge potential [92], The bulk of the annually renewable carbohydrate biomass consists of polysaccharides, but their non-food utilization is still modest. The low-molecular-weight carbohydrates, that is, the constituent units of these polysaccharides, are potential raw materials for several commodity chemicals in fact, glucose (available from cornstarch, bagasse, molasses, wood), fructose (inulin), xylose (hemicelluloses) or the disaccharide sucrose (world production 140 Mtons year-1) are inexpensive and available on a scale of several ten thousands. 

Lignocellulosic biomass is a valuable and plentiful feedstock commodity and its high cellulose and hemicellulose content (about 80% of total) provides considerable potential for inexpensive sugars production. However, enzymatic deconstruction of these polysaccharides remains a costly prospect. Strides in cellulase cost reduction have been made, yet further improvements are needed to reach the goal of 0.10/gal of EtOH expected to enable this new industry. Strategies to reach this goal will combine reduction in the cost to produce the needed enzymes as well as efforts to increase enzyme efficiency (specific activity). As this work proceeds, the more easily attained achievements will be made first, and thus the overall difficulty increases with time. 

Hemicelluloses can be hydrolysed into their component sugars and used as a fermentation feedstock for the production of ethanol and other alcohols (e.g. butanol, arabitol, glycol and xylitol), organic acids (e.g. acetic acid), acetone and gases (e.g. methane and hydrogen). The wider monosaccharide profile offers opportunities to develop different products to those derived from glucose alone. 

Pentoses contained in hemicellulose are used to produce furfural, a useful industrial chemical, used as a solvent for resins and waxes and in petrochemical refining. It is also used as a feedstock for a range of aromatic substances (it has an almond odour) including preservatives, disinfectants and herbicides. Furfural can be converted to furfuryl alcohol and used to make resins for composite applications with fibreglass and other fibres. These are of interest in the aircraft component and automotive brake sectors. Furfural is commercially derived from acid hydrolysis of waste agricultural by-products, such as sugarcane bagasse, com cobs and cereal brans. Around 450 000 tonnes is produced by this method per year. 

Alfalfa (Medicago sativaL.) is a feedstock for the production of fuel, feed and other industrial materials. Alfalfa consists mainly of celluloses, hemicelluloses, lignin, pectin and proteins. Therefore, it is a potential feedstock for ethanol production and also other chemicals. 

The food, feed, and paper industries use hemicellulase to convert hemicellulose to useful products (Wong and Saddler, 1993). At the concentrations found in vegetable waste matter, hemicellulose has been viewed intermit-tendy as a potential enzyme substrate and as feedstock for commercial ethanol and furfural production. 

Table 15.8 presents the feedstock composition expressed in key components cellulose, hemicellulose (xylan, arabinan, mannan, galactan) and lignin. 

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