Pretreatments to enhance the digestibility of lignocellulosic biomass

Trends in biotechnological production of fuel ethanol from different feedstocks.

New transportation fuels are badly needed to reduce our heavy dependence on imported oil and to reduce the release of greenhouse gases that cause global climate change; cellulosic biomass is the only inexpensive resource that can be used for sustainable production of the large volumes of liquid fuels that our transportation sector has historically favored. Furthermore, biological conversion of cellulosic biomass can take advantage of the power of biotechnology to take huge strides toward making biofuels cost competitive. Ethanol production is particularly well suited to marrying this combination of need, resource, and technology. In fact, major advances have already been realized to competitively position cellulosic ethanol with corn ethanol. However, although biotechnology presents important opportunities to achieve very low costs, pretreatment of naturally resistant cellulosic materials is essential if we are to achieve high yields from biological operations; this operation is projected to be the single, most expensive processing step, representing about 20% of the total cost. In addition, pretreatment has pervasive impacts on all other major operations in the overall conversion scheme from choice of feedstock through to size reduction, hydrolysis, and fermentation, and on to product recovery, residue processing, and co-product potential. A number of different pretreatments involving biological, chemical, physical, and thermal approaches have been investigated over the years, but only those that employ chemicals currently offer the high yields and low costs vital to economic success. Among the most promising are pretreatments using dilute acid, sulfur dioxide, near-neutral pH control, ammonia expansion, aqueous ammonia, and lime, with significant differences among the sugar-release patterns. Although projected costs for these options are similar when applied to corn stover, a key need now is to dramatically improve our knowledge of these systems with the goal of advancing pretreatment to substantially reduce costs and to accelerate commercial applications. © 2007 Society of Chemical Industry and John Wiley & Sons, Ltd

Pretreatment: the key to unlocking low-cost cellulosic ethanol

http://www.biotek.lu.se/fileadmin/bioteknik/Utbildning/Kurser/KKKA05/ProgressETOHprocess.pdf

Progress in bioethanol processing

Poplar wood was treated with peracetic acid, KOH, and ball milling to produce 147 model lignocelluloses with a broad spectrum of lignin contents, acetyl contents, and crystallinity indices (CrIs), respectively. An empirical model was identified that describes the roles of these three properties in enzymatic hydrolysis. Lignin content and CrI have the greatest impact on biomass digestibility, whereas acetyl content has a minor impact. The digestibility of several lime-treated biomass samples agreed with the empirical model. Lime treatment removes all acetyl groups and a moderate amount of lignin and increases CrI slightly; lignin removal is the dominant benefit from lime treatment.

Fundamental factors affecting biomass enzymatic reactivity.

Xylan backbones in native plant cell walls are extensively acety-lated. Previously, no direct investigations as to their role in cellulolytic enzyme resistance have been done, though indirect results point to their importance. An in vitro deesterification of aspen wood and wheat straw has been completed using hydroxylamine solutions. Yields of 90% acetyl ester removal for both materials have been accomplished, with little disruption of other fractions (i.e., lignin). Apparently, as the xylan becomes increasingly deacetylated, it becomes 5–7 times more digestible. This renders the cellulose fraction more accessible, and 2–3 times more digestible. This effect levels off near an acetyl removal of 75%, where other resistances become limiting.

The role of ester groups in resistance of plant cell wall polysaccharides to enzymatic hydrolysis

Previous work in our laboratories has demonstrated the effectiveness of peracetic acid for improving enzymatic digestibility of lignocellulosic materials. The use of dilute alkali solutions as a pre-pretreatment prior to peracetic acid lignin oxidation increased carbohydrate hydrolysis yields in a synergistic as opposed to additive manner. Deacetylation of xylan is easily achieved using dilute alkali solutions under mild conditions. In this article, we evaluate the effectiveness of peracetic acid combined with an alkaline pre-pretreatment through simultaneous saccharification and cofermentation (SSCF) of pretreated hybrid poplar wood and sugar cane bagasse. Respective ethanol yields of 92.8 and 91.9% of theoretical are achieved using 6% NaOH/ 15% peracetic acid-pretreated substrates and recombinant Zymomonas mobilis CP4/pZB5. Reduction of acetyl groups of the lignocellulosic materials is demonstrated following alkaline pre-pretreatments. Such processing may be helpful in reducing peracetic acid requirements. The influence of deacetylation is more significant in combined pretreatments using lower peracetic acid loadings.

Simultaneous saccharification and cofermentation of peracetic acid-pretreated biomass.

Prehydrolysis with dilute acid and steam explosion constitute the most promising methods for improving enzymatic digestibility of biomass for ethanol production. Despite worldwide acceptance, these methods of pretreatment are quite expensive considering costs for the reactor, energy, and fractionation. Using peracetic acid is a lignin-oxidation pretreatment with low-energy input by which biomass can be treated in a silo-type system without need for expensive capitalization. Experimentally, ground hybrid poplar and sugar cane bagasse are placed in plastic bags and a peracetic acid solution is added to the biomass in different concentrations based on oven-dried biomass. The ratio of solution to biomass is 6:1 and a 7-d storage period at ambient temperature (20°C) has been used. As an auxiliary method, a series of pre-pretreatments using stoichiometric amounts of sodium hydroxide and ammonium hydroxide based on 4-methyl-glucuronic acid and acetyl content in the biomass are performed before addition of peracetic acid. The basic solutions are added to the biomass in a ratio of 14:1 solution to biomass, and mixed for 24 h at the same ambient temperature. Biomass is filtered and washed to a neutral pH before peracetic acid addition. The aforementioned procedures give high xylan content substrates as a function of the selectivity of peracetic acid for lignin oxidation and the mild conditions of the process. Consequently, xylanase/ β-glucosidase combinations were more effective than cellulase preparations in hydrolyzing these materials. The pretreatment efficiency was evaluated through enzymatic hydrolysis and simultaneous saccharification and cofermentation (SSCF) tests. Peracetic acid treatment improves enzymatic digestibility of hybrid poplar and sugar cane bagasse with no need of high temperatures. Alkaline treatments are helpful in reducing peracetic acid requirements in the pretreatment.

Alkaline and peracetic acid pretreatments of biomass for ethanol production

Optimizing peracetic acid pretreatment conditions for improved simultaneous saccharification and co-fermentation (SSCF) of sugar cane bagasse to ethanol fuel

Douglas-fir and red oak wood meal, cellulose, and an 0-acetyl-4-0-methylglucuronoxylan were exposed to finely divided iron powder under conditions favorable for rusting. Analyses of the wood meal and polysaccharides following exposure indicated that rusting iron causes a decomposition of all wood constituents. Cellulose was oxidized in the presence of rusting iron to form an oxycellulose which was predominantly reducing in character. Direct depolymerization of cellulose and xylan also occurred. The deterioration was favored by an acidic environment, contrary to earlier reports that the primary degradation mechanism is alkalidependent. An iron-catalyzed oxidation of wood constituents is theorized to occur as a result of free-radical production associated with ferrous ion oxidation in the presence of organic compounds. The free radicals produced lead to the formation of hydrogen peroxide which allows Fenton-type reactions to occur.

Herbert A. Schroeder

Papers

The role of ester groups in resistance of plant cell wall polysaccharides to enzymatic hydrolysis

Simultaneous saccharification and cofermentation of peracetic acid-pretreated biomass.

Alkaline and peracetic acid pretreatments of biomass for ethanol production

Optimizing peracetic acid pretreatment conditions for improved simultaneous saccharification and co-fermentation (SSCF) of sugar cane bagasse to ethanol fuel

Iron-Catalyzed Oxidation of Wood Carbohydrates*