What is (and is not) vital to advancing cellulosic ethanol

What is (and is not) vital to advancing cellulosic ethanol

(Parte 2 de 2)

Opinion TRENDS in Biotechnology Vol.25 No.4 155 w.sciencedirect.com with high yields vital to the economic viability. In addition, the choice of feedstock influences the selection of pretreatment and vice versa. Size reduction requirements for cellulosic biomass are determined by pretreatment heat and mass transfer considerations. The choices of enzymes to produce and their balance of activities are dictated by the unconvertedsugarpolymersand oligomers left in the solids after pretreatment. Pretreatment also releases natural inhibitorscontainedinthebiomassandgeneratesinhibitors through degradation, affecting the extent and cost of their removal and the associated yield losses before enzymatic hydrolysis and fermentation. Sugar and ethanol concentrations are influenced strongly by pretreatment water use, which, in turn, has important cost implications for fermentation and product recovery economics. In addition, pretreatment controls whether the proportion of sugars released in each stream presents challenges due to typical fermentativeorganismpreferenceforglucoseattheexpense of poor yields from other sugars. Beyond this, pretreatment could shorten the time required for enzymatic hydrolysis of anhydrous sugars left in the solids to a few days. Pretreatment even affects waste treatment loadings as well as the quantity and quality of the lignin-rich solids that can be burned toproduceprocess and exportedheatand electricity or used for making other products.

Advanced biological processing The other major costs in biological processing are for enzymes and their breakdown of polymeric carbohydrates leftinthepretreatedsolids.Despitesubstantialfundsbeing spent to reduce their costs, cellulases are still expensive to produceandtheiractionisslow.Thus,inadditiontomaking more reactive solids through pretreatment, enzymes with greaterspecificactivityareneededtoincreasereactionrates and achieve high conversions with much less enzyme. Consolidatedbioprocessing(CBP)usesthermophilicmicrobesto anaerobicallyproducecellulosomeenzymesthathavebetter cellulolytic activity than the typical fungal cellulases and ferment all of the sugars released into ethanol in the same vessel. This would achieve low costs when ethanol selectivity and concentrations for these thermophiles are improved

[36,37]. However, development of high-yield fermentative thermophiles that are matched to optimal cellulase operatingconditionswouldbeanimportantstepforward.Enzyme cocktailsthatcaneffectivelyreleasethehemicelluloseleftin pretreated solids are also important for achieving the high yields needed for large-scale competitiveness [38].F inally, although many advocate large-scale, continuous enzymatic hydrolysis and fermentation, limited relevant experience, data or design methodologies have been developed for cellulosic ethanol [36]. A better understanding of the factors that control the interactions of substrates and enzymes would be invaluable in identifying pathways to better systems [39].

Better feedstocks Because the first commercial plants will most probably use existing low-to-near-zero cost feedstocks, such as agricultural and forestry residues or paper sludge, processing costs present the major obstacle to the initial introductions of cellulosic ethanol [2]. Nonetheless, emergence of the industry would be accelerated by reliable data for existing feedstocks, including amounts, locations, compositions, variability, costs and storability. Improved feedstocks will be invaluable in the longer term, once a cellulosic industry is established. In particular, higher productivities will make greater impact possible from a given land area [2]. Plant modifications that facilitate conversion to sugars will also have great payouts, as will such traits as drought tolerance, reduced fertilizer demands, and greater carbohydrate content. Fast growing crops containing recoverable protein would be valuable for producing animal feed as well as, possibly, food and reduce potential conflicts between land use for food versus fuel [40].

Closing thoughts The message from the above is, simply, that limited funds must be focused where they can have the most impact. The most vital needs to realize the great benefits of cellulosic ethanol are to commercialize the technology now, and to fund aggressively research that targets advances in overcoming the recalcitrance of biomass, to achieve low-cost ethanol production. The diversion of substantial resources into more evaluations of process economics, energy balances, greenhouse gas impacts and other studies are not merited without proper motivation. Nor will market penetration benefit much from spending substantial fractions of the limited funds allocated to cellulosic ethanol research on developing organisms that can ferment all five sugars or for ethanol recovery. Rather, the immediate and most important quests are to develop effective policies to accelerate commercialization, improve our knowledge of cellulosic conversion systems to reduce risk and identify opportunities for advances, and support technology that has the potential for substantial cost reductions for breaking down cellulose and hemicellulose into sugars. Waiting for a miracle from some other still-to-be-discovered technology is an invitation to disaster.

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Table 1. Potential effects of pretreatment on biological processing of cellulosic biomass

Process step Potential effects

Biomass production Effectiveness of pretreatment Harvesting and/or storage Hardening and drying of feedstock

Size reduction Heat and mass transfer and energy inputs

Pretreatment Loss of sugars to degradation, maximum digestion yields

Enzyme production Choice of enzyme activities

Enzymatic hydrolysis Enzyme loadings, hydrolysis times, and concentration of sugars

Glucose fermentation Diauxic effects: preference for glucose rather than other sugars

Hydrolyzate conditioning Type of conditioning, loss of sugars

Hydrolyzate fermentation Sugar and ethanol concentrations, diauxic effects

Ethanol recovery Ethanol concentration, mineral fouling

Residue usage Heat content of solid residue, mineral concentrations, fouling

Waste treatment Loading and concentration of dissolved wastes

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