Feedstock loss from drought is a major economic risk for biofuel producers

High cost of technology is seen as the primary barrier to full commercialization of cellulosic biofuels. There is broad expectation that once conversion technology breakthroughs occur, policy support is only needed to accelerate cost reductions through “learning by doing” effects. In this study, we show that droughts pose a significant economic risk to biofuel producers and consumers regardless of the rate at which technology costs fall. We model a future switchgrass derived cellulosic biorefinery industry in Kansas based on spatially resolute historic (1996–2005) weather data, representing a rainfall regime that could reflect drought events predicted to occur throughout the U.S. Midwest by climatologists (Karl et al. (2009) U.S. Global Change Research Program USA). We find that droughts reduced modeled biorefinery capacity factors, on average, by 47%, raising biofuel production costs by 35% between a modeled dry and wet year. Interestingly, we find that two logical strategies to plan for drought; (1) building large biorefineries to source feedstock from a larger area and, (2) Storing switchgrass in good production years for use in drought years; are not very effective in reducing drought risks. Our findings should be of particular concern to low carbon fuel policies like California's Low Carbon Fuel Standard and the U.S. Second Renewable Fuel Standards (RFS2) whose costs of compliance may be much higher than expected.     

Development and preliminary evaluation of an integrated field scale model for perennial bioenergy grass ecosystems in lowland areas

Computer models are useful tools for evaluating environmental and economic sustainability of proposed dedicated cellulosic grass ecosystems for biofuel production. This study developed an integrated, field scale, and process-based ecosystem model (DRAINMOD-GRASS) for simulating hydrological processes, soil carbon and nitrogen cycling, and plant growth in cropping systems for producing bioenergy grasses in lowland areas. We tested the model using measurements from three replicated switchgrass (Panicum virgatum) plots located in eastern North Carolina, USA. Results showed that the model accurately predicted 5-year (2009–2013) biomass yield. Predicted daily water table depth closely matched field measurements with Nash-Sutcliffe coefficient of 0.86. The model also accurately predicted temporal dynamics of daily soil moisture and temperature with Nash-Sutcliffe coefficients of 0.7 and 0.9, respectively. Predicted seasonal changes in net N mineralization and nitrification rates were comparable to field measurements in 2011 and 2012.     

Evaluation of production yield and thermal processing of switchgrass as a bio-energy crop for the Mediterranean region

Switchgrass, a warm-season perennial grass, could play an important role for Europe in supplying sustainably produced lignocellulosic biomass, as its establishment cost is low and its productivity high under low input conditions. The aim of this study was to evaluate the adaptability and biomass productivity of switchgrass under the Greek climatic conditions and furthermore its suitability for heat and electricity applications, by performing fuel analyses, pyrolysis and combustion tests. Three-year field trials with different irrigation and fertilization levels showed that each level of irrigation had a significant effect on dry matter yields, while nitrogen effect was not pronounced. Yields were similar in the second and third year and ranged from 15.4% to 24 tons/ha.Fuel characterization showed a high volatile content and calorific value, whereas low ash, sulphur, nitrogen and chlorine contents. Ashes were rich in Si, K and P and some micronutrients, such as Zn. The ash fusion temperatures of the stems, which were richer in alkali, were low for combustion processes, revealing slagging/fouling problems in boilers without crop pre-treatment. However, their ash content was very low. The thermochemical reactivity of the stems was higher than that of the leaves, especially in air. The greater amount of minerals in the leaves inhibited the reaction rates in either nitrogen or air atmospheres. A first-order parallel reactions model for pyrolysis and a power low model for combustion fitted the experimental results accurately and kinetic parameters were derived. Irrigation/fertilization treatment had a positive effect on the combustion performance of the stems.     

Characteristics and compositional change in round and square switchgrass bales stored in South Central Oklahoma

Storage studies were conducted in 2009 and 2010 on large round (1.83 m × 1.53 m) and square (1.22 m × 1.22 m × 2.44 m) switchgrass bales stored for 6 months. Round and square bales were stored outside under different conditions: tarped (on pallets, gravel and ground) and untarped (on pallets, gravel and ground). Round and square bales were also stored inside and served as a control treatment. During both years, outside tarped bales resisted moisture accumulation and thus dry matter losses were equivalent to bales stored inside. The average dry matter loss for the round and square bales stored inside was 0.6% compared to 0.9 and 2.8% for the tarped round and square bales stored outside, respectively. However, untarped round (11.3%) and square (32.7%) bales on an average had greater dry matter loss than the tarped round (0.9%) and square (2.8%) bales. The untarped square bales consistently had higher moisture contents than untarped round bales which resulted in greater dry matter loss in untarped square bales. Hemicellulose content was more severely affected than cellulose content during storage. In 2010, untarped square bales stored on gravel, ground and pallets lost 30%, 24% and 16% of hemicellulose content, respectively.     

Biomass yield and quality of 20 switchgrass populations in southern Iowa, USA

Renewable bioenergy could be supplied by high yielding grass crops, such as switchgrass (Panicum virgatum L.). Successful development of a bioenergy industry will depend on identifying cultivars with high yield potential and acceptable biofuel quality. The objective of this study was to evaluate 20 switchgrass populations in a field study planted in May 1997 in southern Iowa, USA. The populations included released cultivars and experimental germplasm of both upland and lowland ecotypes. Yield, plant height, stand, lodging, leaf:stem ratio, cell wall fiber, total plant nitrogen, and ash were determined on all entries between 1998 and 2001. Ultimate and proximate analyses together with chlorine and major oxide determinations were made on three cultivars in 2000 and 2001. Biomass yield was determined from a single autumn harvest each year. The lowland cultivars ‘Alamo’ and ‘Kanlow’ produced the most biomass, exceeding the production of the widely recommended upland cultivar ‘Cave-In-Rock’. Other traits differed among the cultivars, although the range was less than that for yield. The differences among years were substantially greater for the ultimate, proximate, and major oxide analyses than differences among cultivars. The highest yielding cultivars had low ash, slightly lower fiber concentrations, and moderate levels of important minerals, suggesting that excellent germplasm is available for biofuel production. The persistence of the lowland cultivars in southern Iowa may need more research because the winters during the experiment were mild.     

Investigation of rapid conversion of switchgrass in subcritical water

The reaction characteristics of switchgrass conversion in subcritical water were investigated using a batch reactor under conditions of rapid rising to 250–350 °C and pressure of 20 MPa, with reaction times varying from 1–300 s. The effects of temperature and reaction time on product distribution and yields of chemical products were investigated. High conversion of switchgrass (90 wt.% on dry biomass basis) can be obtained in less than 60 s under a relative lower reaction temperature of 350 °C, compared with that in a switchgrass flash pyrolysis process where switchgrass conversion achieves only 58.9–78.8 wt.% in temperature range of 450–550 °C. The yield of water solubles (WS) can reach 37 wt.% after reaction for 1 s at 250 °C. The increases in temperature and reaction time lead to increases of the biomass conversion and the yield of gas, while WS yield decreases by secondary decomposition reactions. Many lignin-derived compounds were identified by GC-MS analysis and could well be recovered in methanol solubles (MS). Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) analysis of methanol insolubles (MI) indicated that the lignocellulosic matrix could be significantly decomposed, and no char formation was observed, while many lignin structures were left in the MI products. These results provide important information for recovering value-added chemicals from energy crops and biomass waste.     

Switchgrass growth and morphological changes under established pine-grass agroforestry systems in the lower coastal plain of North Carolina,United States

Switchgrass (Panicum virgatum L.) intercropped with Loblolly pine (Pinus taeda L.) has been proposed as a potential biomass feedstock for biofuel production in the southeastern United States. This study investigated effects of treatments (intercropping vs. grass only) on biomass increment processes and morphological properties of switchgrass at two experimental plots (Lenoir1) located in the coastal plain of North Carolina. We also evaluated effects of trimming lower tree branches of pine trees on switchgrass growth at another watershed-scale site (Carteret7) in the same region. Results showed that biomass yield of intercropped switchgrass was reduced by adjacent trees and negatively affected by relative position of grass to trees at the 6th year after planting at Lenoir1. Relative grass-to-tree position was also found to be a significant (p < 0.001) factor affecting grass growth at Carteret7 site with tree age of 5 years old, which is irrespective to the trimming practice. Trimming lower tree branches did not significantly (p = 0.57) improve biomass yield of switchgrass at Carteret7. We also observed intercropped switchgrass typically had higher specific leaf area and grew taller compared to grass-only plots. Stem-to-leaf ratios of switchgrass were significantly (p = 0.02) affected by trees at Lenoir1, but not by trimming lower branches in Carteret7 and relative position of grass to trees at both study sites. Findings from this study are important for evaluating the viability of producing biofuel feedstocks using this proposed intercropping system in the southeastern United States.     

Combined ethanol and methane production from switchgrass (Panicum virgatum L.) impregnated with lime prior to steam explosion

Pretreatments are crucial to achieve efficient conversion of lignocellulosic biomass to soluble sugars. In this light, switchgrass was subjected to 13 pretreatments including steam explosion alone (195 °C for 5, 10 and 15 min) and after impregnation with the following catalysts: Ca(OH)₂ at low (0.4%) and high (0.7%) concentration; Ca(OH)₂ at high concentration and higher temperature (205 °C for 5, 10 and 15 min); H₂SO₄ (0.2% at 195 °C for 10 min) as reference acid catalyst before steam explosion. Enzymatic hydrolysis was carried out to assess pretreatment efficiency in both solid and liquid fraction. Thereafter, in selected pretreatments the solid fraction was subjected to simultaneous saccharification and fermentation (SSF), while the liquid fraction underwent anaerobic digestion (AD). Lignin removal was lowest (12%) and highest (35%) with steam alone and 0.7% lime, respectively. In general, higher cellulose degradation and lower hemicellulose hydrolysis were observed in this study compared to others, depending on lower biomass hydration during steam explosion. Mild lime addition (0.4% at 195 °C) enhanced ethanol in SSF (+28% than steam alone), while H₂SO₄ boosted methane in AD (+110%). However, methane represented a lesser component in combined energy yield (ethanol, methane and energy content of residual solid). Mild lime addition was also shown less aggressive and secured more residual solid after SSF, resulting in higher energy yield per unit raw biomass. Decreased water consumption, avoidance of toxic compounds in downstream effluents, and post process recovery of Ca(OH)₂ as CaCO₃ represent further advantages of pretreatments involving mild lime addition before steam explosion.     

Removal of inorganic constituents from pine barks and switchgrass

The ash content of biomass has some negative impacts on the efficient utilization of biomass for the production of value-added products. In this paper, the techniques of ash removal from Mountain Pine Beetle (MPB) killed pine bark and switchgrass were examined systematically. Size fractionation was found to be effective for ash removal not only for the heterogeneous switchgrass but also for the homogeneous pine bark. Inorganic constituents of untreated samples and treated samples showed that leaching by water and four dilute acid solutions was effective in removing most of ash from both biomass samples. The inorganic element distributions in ash of pine bark samples before and after the acid leaching showed that substantial reductions were achieved in different main elements, Al (66%), Ca (90%), Fe (66%), K (92%), Mg (98%), Na (100%) and S (25%). For switchgrass, most of the element concentrations were reduced by water leaching because of its high content of water-soluble metals in the ash. For the purpose of practical applications of the two types of biomass, especially the underutilized pine barks, different leaching methods should be evaluated to identify the most suitable method for the removal of inorganic constituents.     

Response surface studies that elucidate the role of infiltration conditions on Agrobacterium tumefaciens-mediated transient transgene expression in harvested switchgrass (Panicum virgatum)

Agrobacterium tumefaciens-mediated transient expression (agroinfiltration) experiments were performed in harvested switchgrass (Panicum virgatum) leaves to identify the effects of wounding by bead beating, surfactant concentration and vacuum application on in planta β-glucuronidase expression and leaf decay. Expression was scored based on a consistent pattern of visual observations of histochemical staining over the leaf surface as might be observed in stable gene expression in switchgrass leaves. Assays on extracts from leaves were also performed to measure expression levels; however, these assays showed low expression levels, which may have been due to low recombinant protein recovery and decomposition in the leaf. Bead beating was successful for wounding the plant surface, but did not improve the consistency of expression based on histochemical staining observations. Surfactant was necessary for improving contact between the leaf surface and Agrobacterium suspension and consistently improved expression when vacuum application level was low (25 kPa). Increasing vacuum application from 25 to 5 kPa improved expression only when surfactant concentration was low. When a suspension of A. tumefaciens containing 1000 ppm Break-Thru surfactant was added to harvested leaves and 25 kPa vacuum applied, a fairly uniform expression was visualized across the leaf surface within 2–3 days of incubation, suggesting that agroinfiltration is a rapid tool for examining expression of transgenes in switchgrass leaves.