Mark T. Holtzapple

Fundamental Factors Affecting Biomass Enzymatic Reactivity

Poplar wood was treated with peracetic acid, KOH, and ball milling to produce 147 modellignocelluloses 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 removesallacetyl groups and a moderate amount of lignin and increases CrIslightly; lignin removal is the dominant benefit from lime treatment.

Comparative Sugar Recovery and Fermentation Data Following Pretreatment of Poplar Wood by Leading Technologies

Through a Biomass Refining Consortium for Applied Fundamentals and Innovation among Auburn University, Dartmouth College, Michigan State University, the National Renewable Energy Laboratory, Purdue University, Texas A&M University, the University of British Columbia, and the University of California at Riverside, leading pretreatment technologies based on ammonia fiber expansion, aqueous ammonia recycle, dilute sulfuric acid, lime, neutral pH, and sulfur dioxide were applied to a single source of poplar wood, and the remaining solids from each technology were hydrolyzed to sugars using the same enzymes. Identical analytical methods and a consistent material balance methodology were employed to develop comparative performance data for each combination of pretreatment and enzymes. Overall, compared to data with corn stover employed previously, the results showed that poplar was more recalcitrant to conversion to sugars and that sugar yields from the combined operations of pretreatment and enzymatic hydrolysis varied more among pretreatments. However, application of more severe pretreatment conditions gave good yields from sulfur dioxide and lime, and a recombinant yeast strain fermented the mixed stream of glucose and xylose sugars released by enzymatic hydrolysis of water washed solids from all pretreatments to ethanol with similarly high yields. An Agricultural and Industrial Advisory Board followed progress and helped steer the research to meet scientific and commercial needs.

Lime Pretreatment of Switchgrass

Lime (calcium hydroxide) was used as a pretreatment agent to enhance the enzymatic digestibility of switchgrass. After studying many conditions, the recommended pretreatment conditions are: time = 2 h, temperature = 100°C and 120°C, lime loading = 0.1 g Ca(OH)2/g dry biomass, water loading = 9 mL/g dry biomass. Studies on the effect of particle size indicate that there was little benefit of grinding below 20 mesh; even coarse particles (4–10 mesh) digested well. Using the recommended pretreatment conditions, the 3-d reducing sugar yield was five times that of untreated switchgrass, the 3-d total sugar (glucose + xylose) yield was seven times, the 3-d glucose yield was five times, and the 3-d xylose yield was 21 times. A material balance study showed that little glucan (approx 10%) was solubilized as a result of the lime pretreatment, whereas about 26% of xylan and 29% of lignin became solubilized.

The ammonia freeze explosion (AFEX) process - A practical lignocellulose pretreatment

The Ammonia Freeze Explosion (AFEX) process treats lignocellulose with high-pressure liquid ammonia, and then explosively releases the pressure. The combined chemical effect (cellulose decrystallization) and physical effect (increased accessible surface area) dramatically increase lignocellulose susceptibility to enzymatic attack. There are many adjustable parameters in the AFEX process: ammonia loading, water loading, temperature, time, blowdown pressure, and number of treatments. The effect of these parameters on enzymatic susceptibility was explored for three materials: Coastal bermudagrass, bagasse, and newspaper. Nearly quantitative sugar yields were demonstrated for Coastal bermudagrass and bagasse, using a very low enzyme loading (5 IU/g). Newspaper proved to be much more resistant to enzymatic hydrolysis.

Benefits from Tween during enzymic hydrolysis of corn stover.

Corn stover is a potential substrate for fermentation processes. Previous work with corn stover demonstrated that lime pretreatment rendered it digestible by cellulase; however, high sugar yields required very high enzyme loadings. Because cellulase is a significant cost in biomass conversion processes, the present study focused on improving the enzyme efficiency using Tween 20 and Tween 80; Tween 20 is slightly more effective than Tween 80. The recommended pretreatment conditions for the biomass remained unchanged regardless of whether Tween was added during the hydrolysis. The recommended Tween loading was 0.15 g Tween/g dry biomass. (The critical relationship was the Tween loading on the biomass, not the Tween concentration in solution.) The 72-h enzymic conversion of pretreated corn stover using 5 FPU cellulase/g dry biomass at 50 degrees C with Tween 20 as part of the medium was 0.85 g/g for cellulose, 0.66 g/g for xylan, and 0.75 for total polysaccharide; addition of Tween improved the cellulose, xylan, and total polysaccharide conversions by 42, 40, and 42%, respectively. Kinetic analyses showed that Tween improved the enzymic absorption constants, which increased the effective hydrolysis rate compared to hydrolysis without Tween. Furthermore, Tween prevented thermal deactivation of the enzymes, which allows for the kinetic advantage of higher temperature hydrolysis. Ultimate digestion studies showed higher conversions for samples containing Tween, indicating a substrate effect. It appears that Tween improves corn stover hydrolysis through three effects: enzyme stabilizer, lignocellulose disrupter, and enzyme effector. Copyright 1998 John Wiley & Sons, Inc.

Lime pretreatment of crop residues bagasse and wheat straw

Lime (calcium hydroxide) was used as a pretreatment agent to enhance the enzymatic digestibility of two common crop residues: bagasse and wheat straw. A systematic study of pretreatment conditions suggested that for short pretreatment times (1–3 h), high temperatures (85-135°C) were required to achieve high sugar yields, whereas for long pretreatment times (e.g., 24 h), low temperatures (50–65°C) were effective. The recommended lime loading is 0.1 g Ca(OH)2/g dry biomass. Water loading had little effect on the digestibility. Under the recommended conditions, the 3-d reducing sugar yield of the pretreated bagasse increased from 153 to 659 mg Eq glucose/g dry biomass, and that of the pretreated wheat straw increased from 65 to 650 mg Eq glucose/g dry biomass. A material balance study on bagasse showed that the biomass yield after lime pretreatment is 93.6%. No glucan or xylan was removed from bagasse by the pretreatment, whereas 14% of lignin became solubilized. A lime recovery study showed that 86% of added calcium was removed from the pretreated bagasse by ten washings and could be recovered by carbonating the wash water with CO2 at pH 9.5.

Oxidative lime pretreatment of high-lignin biomass: poplar wood and newspaper.

Lime (Ca[OH]2) and oxygen (O2) were used to enhance the enzymatic digestibility of two kinds of high-lignin biomass: poplar wood and newspaper. The recommended pretreatment conditions for poplar wood are 150°C, 6 h, 0.1 g of Ca(OH)2/g of dry biomass, 9 mL of water/g of dry biomass, 14.0 bar absolute oxygen, and a particle size of −10 mesh. Under these conditions, the 3-d reducing sugar yield of poplar wood using a cellulase loading of 5 filter paper units (FPU)/g of raw dry biomass increased from 62 to 565 mg of eq. glucose/g of raw dry biomass, and the 3-d total sugar (glucose + xylose) conversion increased from 6 to 77% of raw total sugars. At high cellulase loadings (e.g., 75 FPU/g of raw dry biomass), the 3-d total sugar conversion reached 97%. In a trial run with newspaper, using conditions of 140°C, 3 h, 0.3 g of Ca(OH)2/g of dry biomass, 16 mL of water/g of dry biomass, and 7.1 bar absolute oxygen, the 3-d reducing sugar yield using a cellulase loading of 5 FPU/g of raw dry biomass increased from 240 to 565 mg of eq. glucose/g of raw dry biomass. A material balance study on poplar wood shows that oxidative lime pretreatment solubilized 38% of total biomass, including 78% of lignin and 49% of xylan; no glucan was removed. Ash increased because calcium was incorporated into biomass during the pretreatment. After oxidative lime pretreatment, about 21% of added lime could be recovered by CO2 carbonation.

Biomass conversion to mixed alcohol fuels using the MixAlco process

The MixAlco process is a patented technology that converts any biodegradable material (e.g., sorted municipal solid waste, sewage sludge, industrial biosludge, manure, agricultural residues, energy crops) into mixed alcohol fuels containing predominantly 2-propanol, but also higher alcohols up to 7-tridecanol. The feed stock is treated with lime to increase its digestibility. then, it is fed to a fermentor in which a mixed culture of acid-forming microorganisms produces carboxylic acids. Calcium carbonate is added to the fermentor to neutralize the acids to their corresponding carboxylate salt. The dilute (−3%) carboxylate salts are concentrated to 19% using an amine solvent that selectively extracts water. Drying is completed using multi-effect evaporators. Finally, the dry salts are thermally converted to ketones which subsequently are hydrogenated to alcohols. All the steps in the MixAlco process have been proven at the laboratory scale. A techno-economic model of the process indicates that with the tipping fees available in New York (

Carboxylate Platform: The MixAlco Process Part 1: Comparison of Three Biomass Conversion Platforms

126/dry tonne), mixed alcohol fuels may be sold for

Pretreatment of lignocellulosic municipal solid waste by ammonia fiber explosion (AFEX)

0.04/L (