Energy crops for ethanol: a processing perspective

2008

Many crops have been proposed as feedstocks for the emerging cellulosic ethanol industry, but information is lacking about the relative importance of feedstock production and quality. We compared yield and sugar content for seven bioenergy cropping systems in south-central Wisconsin (ARL) and southwestern Michigan (KBS) during three growing seasons (2012 through 2014). The cropping systems were (i) continuous corn stover (Zea mays L.), (ii) switchgrass (Panicum virgatum L.), (iii) giant miscanthus (Miscanthus × giganteus Greef & Deuter ex Hodkinson & Renvoize), (iv) hybrid poplar (Populus nigra × P. maximowiczii A. Henry ‘NM6’), (v) native grass mix, (vi) early successional community, and (vii) restored prairie. A high-throughput pretreatment and fermentation assay showed corn stover with the highest sugar content (213 g glucose kg–1 [Glc] and 115 g xylose kg–1 [Xyl]) followed by the two monoculture perennial grass treatments (154 [Glc] and 88 [Xyl]) and then the herbaceous polycultures (135 [Glc] and 77 [Xyl]). Biomass production and sugar content were combined to calculate ethanol yields. Miscanthus had the highest per hectare ethanol yields (1957 l ha–1 yr–1 ARL, 2485 l ha–1 yr–1 KBS) followed by switchgrass (1091 l ha–1 yr–1 ARL, 1017 l ha–1 yr–1 KBS) and corn stover (1121 l ha–1 yr–1 ARL, 878 l ha–1 yr–1 KBS). Perennial grass cropping systems (i.e., switchgrass and miscanthus) had higher per hectare ethanol yields at both sites relative to diverse systems that included dicots. Despite feedstock differences in fermentable sugars, biomass production was the strongest driver of per hectare ethanol yield.

Proceedings of the National Academy of Sciences, 2011

Switchgrass is a leading dedicated bioenergy feedstock in the United States because it is a native, high-yielding, perennial prairie grass with a broad cultivation range and low agronomic input requirements. Biomass conversion research has developed processes for production of ethanol and other biofuels, but they remain costly primarily because of the intrinsic recalcitrance of biomass. We show here that genetic modification of switchgrass can produce phenotypically normal plants that have reduced thermal-chemical (≤180°C), enzymatic, and microbial recalcitrance. Down-regulation of the switchgrass caffeic acid O-methyltransferase gene decreases lignin content modestly, reduces the syringyl:guaiacyl lignin monomer ratio, improves forage quality, and, most importantly, increases the ethanol yield by up to 38% using conventional biomass fermentation processes. The down-regulated lines require less severe pretreatment and 300-400% lower cellulase dosages for equivalent product yields using simultaneous saccharification and fermentation with yeast. Furthermore, fermentation of diluted acid-pretreated transgenic switchgrass using Clostridium thermocellum with no added enzymes showed better product yields than obtained with unmodified switchgrass. Therefore, this apparent reduction in the recalcitrance of transgenic switchgrass has the potential to lower processing costs for biomass fermentation-derived fuels and chemicals significantly. Alternatively, such modified transgenic switchgrass lines should yield significantly more fermentation chemicals per hectare under identical process conditions. biofuel crop | cellulosic ethanol | lignin modification | Panicum virgatum L ignocellulosic biomass is an abundant, domestic, renewable feedstock source that can be converted to liquid transportation fuels and other chemicals by fermentation. Cellulosic ethanol is a promising near-term technological option to reduce transportation sector greenhouse gas emissions (1). Because lignocellulosic biomass is made up of the complex structures of cellulose, hemicellulose, and lignin, such feedstock is highly recalcitrant to bioconversion of its carbohydrates into ethanol compared with starch (2, 3). Current biomass fermentation processes for fuels and chemicals have a relatively high cost primarily because of this recalcitrance, which in turn has limited commercialization of biomass ethanol (4). To achieve sustainable energy production, it is necessary to overcome the chemical and structural properties of biomass that inhibit its deconstruction in dedicated bioenergy crops (5).

GCB Bioenergy

Increasing the diversity of lignocellulosic feedstocks accepted by a regional biorefinery has the potential to improve the environmental footprint of the facility; harvest, storage, and transportation logistics; and biorefinery economics. However, feedstocks can vary widely in terms of their biomass yields and quality characteristics (chemical composition, moisture content, etc.). To investigate how the diversity of potential biofuel cropping systems and feedstock supply might affect process and field-scale ethanol yields, we processed and experimentally quantified ethanol production from five different herbaceous feedstocks: two annuals (corn stover and energy sorghum) and three perennials (switchgrass, miscanthus, and mixed prairie). The feedstocks were pretreated using ammonia fiber expansion (AFEX), hydrolyzed at high solid loading (~17%-20% solids, depending on the feedstock), and fermented separately using microbes engineered to utilize xylose: yeast (Saccharomyces cerevisiae Y128) or bacteria (Zymomonas mobilis 8b). The field-scale ethanol yield from each feedstock was dependent on biomass quality and cropping system productivity; however, biomass yield had a greater influence on the ethanol yield for low-productivity crops, while

Agronomy Journal, 2012

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. C ellulosic refineries will require substantial amounts of biomass on a year-round basis and are expected to have higher capital costs than similar sized grain ethanol plants based on first-generation biomass refining technology (Wright and Brown, 2007). A reliable feedstock supply will be essential in maintaining stable operational costs. Further, cellulosic refineries will be required to convert biomass with potentially greater feedstock quality variability than existing corn (Zea mays L.) grain ethanol plants. Switchgrass is being developed as a biomass energy crop for the temperate regions of North America (Vogel and Mitchell, 2008). Temporal and spatial variation information across production years and fields for biomass yield and quality will be needed for establishing reliable feedstock supply areas for a cellulosic biorefinery. Information on field-scale spatial and temporal variation for biomass yield of switchgrass is becoming available (Schmer et al., 2010). Switchgrass biomass composition and theoretical ethanol production at the field-scale have thus far not been reported. Biomass conversion to transportation fuels by biochemical methods will be dependent on efficient cellulose and hemicellulose polymer hydrolysis to simple sugars and then conversion to oxygenated hydrocarbons (Himmel et al., 2007). First generation cellulosic biorefineries, using biochemical methods, will produce primarily ethanol by converting cellulose, hemicellulose, and noncell wall carbohydrates into simple sugars which are then fermented to ethanol by genetically engineered organisms (Lynd et al., 1991; Perlack et al., 2005). Lignin, an abundant phenolic polymer in cell walls, can be used for combined heat and power generation (Demirbas, 2001; Lynd and Wang, 2003; Sheehan et al., 2003). Biochemical methods involve a pretreatment to reduce cell wall recalcitrance and increase cell wall porosity, a saccharification process to hydrolyze complex polysaccharides to monosaccharides, and a fermentation process to convert monosaccharides to a biofuel (Stephanopoulos, 2007). Near-term commercialized efforts to convert lignocellulosic feedstocks to biofuels through biochemical methods will likely involve simultaneous saccharification and fermentation (SSF). Alternative conversion systems such as consolidated bioprocessing which combines the enzymatic production, hydrolysis, and fermentation process into one reactor, thus reducing capital costs and increasing biorefinery efficiency, are expected to be commercially available as well (Lynd et al., 2005). Cellulosic biomass conversion to biofuels via biochemical or thermochemical methods require more complex and ABstrAct Information on temporal and spatial variation in switchgrass (Panicum virgatum L.) biomass composition as it affects ethanol yield (L Mg-1) at a biorefinery and ethanol production (L ha-1) at the field-scale has previously not been available. Switchgrass biomass samples were collected from a regional, on-farm trial and biomass composition was determined using newly developed near-infrared reflectance spectroscopy (NIRS) prediction equations and theoretical ethanol yield (100% conversion efficiency) was calculated. Total hexose (cell wall polysaccharides and soluble sugars) concentration ranged from 342 to 398 g kg-1 while pentose (arabinose and xylose) concentration ranged from 216 to 245 g kg-1 across fields. Theoretical ethanol yield varied significantly by year and field, with 5 yr means ranging from 381 to 430 L Mg-1. Total theoretical ethanol production ranged from 1749 to 3691 L ha-1 across fields. Variability (coefficient of variation) within established switchgrass fields ranged from 1 to 4% for theoretical ethanol yield (L Mg-1) and 14 to 38% for theoretical ethanol production (L ha-1). Most fields showed a lack of spatial consistency across harvest years for theoretical ethanol yield or total theoretical ethanol production. Switchgrass biomass composition from farmer fields can be expected to have significant annual and field-to-field variation in a production region, and this variation will significantly affect ethanol or other liquid fuel yields per ton or hectare. Cellulosic biorefineries will need to consider this potential variation in biofuel yields when developing their business plans.

Biomass and Bioenergy, 2012

Optimizing cellulosic ethanol yield depends strongly on understanding the biological variation of feedstocks. Our objective was to study variation in capacity for producing fermentable sugars from straw of winter wheat cultivars with a high-throughput pretreatment and hydrolysis well-plate technique. This technique enabled us to estimate cultivarrelated and environmental correlations between sugar yield, chemical composition, agronomic qualities, and distribution of botanical plant parts of wheat straw cultivars. Straws from 20 cultivars were collected in duplicates on two sites in Denmark. Following hydrothermal pretreatment (180 C for 17.6 min) and co-hydrolysis, sugar release and sugar conversion were measured. Up to 26% difference in sugar release between cultivars was observed. Sugar release showed negative cultivar correlation with lignin and ash content, whereas sugar release showed positive cultivar correlation with content of carbohydrates and plant height. Accessibility to cellulose can impede the sugar conversion rate, and convertibility of each botanical fraction might be more important to overall sugar conversion than the relative proportions of botanical fractions. Our results suggest that selection of cultivars for improved biofuel feedstock of wheat straw is possible, because heritability of sugar release is 57% and there are few adverse correlations to other agronomic traits. ª b i o m a s s a n d b i o e n e r g y 3 7 ( 2 0 1 2 ) 2 2 1 e2 2 8 0961-9534/$ e see front matter ª

Biomass and Bioenergy, 2012

A feasible crop breeding strategy for straw utilisation in the ethanol industry requires an in-depth study of the interaction between the feedstock and the microbial agents during the production process. This study investigated the ethanol yield potential of the straw from nine wheat cultivars through consolidated bioprocessing by two strains of Fusarium oxysporum. Plant parameters assessed were plant lignin, cellulose and phenolic acid content; fungal parameters analysed were biomass and cellulase/xylanase activity. Wheat cultivars varied significantly in the amount of ethanol produced from straw by F. oxysporum. Ethanol yielded negatively correlated with the hemicellulose and lignin content of the straw, and the amount of the lignin phenolics syringic acid and coniferyl alcohol. Significant positive correlations were observed between ethanol yield and the activity of the fungal enzymes b-glucosidase and b-xylosidase. If crops such as wheat are to be used for multifunctional purposes, i.e. food and fuel, the assessment of lignocellulose composition and biodegradability must be assessed as part of our crop breeding programmes.

Increasing energy demand and rising petroleum prices necessitate development of alternative energy resources. Bioethanol, an important renewable transportation fuel, has been considered as one of the most promising alternatives to petroleum. The grass seed production industry in the Pacific Northwest produces about 2 Mt per year of grass straw as a coproduct. Various species of grasses, with yields of up to 5 tons per acre and containing up to 35% cellulose, are potential feedstocks for ethanol production. Three common grass seed crops, perennial ryegrass (Lolium per-enne L.), tall fescue (Festuca arundinacea Schreb) and bentgrass (Agrostis sp.), were investigated as feedstocks for ethanol production. Composition analysis was performed using standard NREL laboratory analytical procedures. The grasses were pre-treated at 10% solid loading at 180°C for 15 min using hot water, dilute acid, or dilute alkali. Scanning electron microscope (SEM) images of untreated and pretreated grasses were used to visually analyze structural changes in the biomass after pre-treatment. Both untreated and pretreated grass straw was enzymatically hydrolyzed at 50°C for 120 h. All pretreatments were effective in increasing enzymatic digestibility of pretreated straw compared to untreated samples. For all experimental conditions investigated, most of the hydrolysis (>80%) was completed in the first 48 h. No single pretreatment gave the maximum hydrolysis yield for all grasses. Potential maximum ethanol yields for tall fescue, perennial ryegrass, and bentgrass were 360, 297, and 276 L ton-1 of biomass, respectively.

PLoS ONE, 2014

The production of ethanol from pretreated plant biomass during fermentation is a strategy to mitigate climate change by substituting fossil fuels. However, biomass conversion is mainly limited by the recalcitrant nature of the plant cell wall. To overcome recalcitrance, the optimization of the plant cell wall for subsequent processing is a promising approach. Based on their phylogenetic proximity to existing and emerging energy crops, model plants have been proposed to study bioenergyrelated cell wall biochemistry. One example is Brachypodium distachyon, which has been considered as a general model plant for cell wall analysis in grasses. To test whether relative phylogenetic proximity would be sufficient to qualify as a model plant not only for cell wall composition but also for the complete process leading to bioethanol production, we compared the processing of leaf and stem biomass from the C 3 grasses B. distachyon and Triticum aestivum (wheat) with the C 4 grasses Zea mays (maize) and Miscanthus x giganteus, a perennial energy crop. Lambda scanning with a confocal laserscanning microscope allowed a rapid qualitative analysis of biomass saccharification. A maximum of 108-117 mg ethanol?g 21 dry biomass was yielded from thermo-chemically and enzymatically pretreated stem biomass of the tested plant species. Principal component analysis revealed that a relatively strong correlation between similarities in lignocellulosic ethanol production and phylogenetic relation was only given for stem and leaf biomass of the two tested C 4 grasses. Our results suggest that suitability of B. distachyon as a model plant for biomass conversion of energy crops has to be specifically tested based on applied processing parameters and biomass tissue type.

Bioenergy Research, 2016

Biofuels and bio-based products can be produced from a wide variety of herbaceous feedstocks. To supply enough biomass to meet the needs of a new bio-based economy, a suite of dedicated biomass species must be developed to accommodate a range of growing environments throughout the USA. Researchers from the US Department of Agriculture's Agricultural Research Service (USDA-ARS) and collaborators associated with the USDA Regional Biomass Research Centers have made major progress in understanding the genetics of switchgrass, sorghum, and other grass species and have begun to use this knowledge to develop new cultivars with high yields and appropriate traits for efficient conversion to bio-based products. Plant geneticists and breeders have discovered genes that reduce recalcitrance for biochemical conversion to ethanol and drop-in fuels. Progress has also been made in finding genes that improve production under biotic and abiotic stress from diseases, pests, and climatic variations.