Acidified glycerol pretreatment for enhanced ethanol production from rice straw

Rice straw was pretreated using an industrial grade glycerol for ethanol production. The pretreatment was conducted at 130–210 °C for 1–24 h with 5% solid loading. The glucan content in the regenerated rice straw increased with increasing pretreatment temperature and time. The production of fermentable sugars initially increased as the pretreatment temperature and reaction time increased, but then decreased somewhat at the higher temperatures and with longer reaction duration. The highest amount of reducing sugar produced by the enzymatic hydrolysis was achieved at 190 °C for 10 h with 5% solid loading, optimal condition for the glycerol pretreatment of rice straw. Furthermore, it was observed that glycerol pretreatment with the addition of HCl improved the digestibility of fermentable sugars by 4–5 times that of untreated samples. Fermentation of hydrolysates resulted in an ethanol yield of 0.44 g/g sugar, corresponding to a theoretical yield of 84.3%. It was concluded that acidified glycerol is one of the good candidates of the organic solvent for the pretreatment of lignocellulosic biomass.     

Contents of various sources of glucose and fructose in rice straw, a potential feedstock for ethanol production in Japan

The major carbohydrates of rice straw samples were determined in order to evaluate the potential of using rice straw as a feedstock for ethanol production in Japan. Straw samples were harvested by cutting the plants at ground level when the grain was mature and immediately heating or chilling the samples. In all cases, significant amounts (62-303 g kg⁻¹) of soft carbohydrates defined as consisting of glucose, fructose, sucrose, starch and ??-1,3-1,4-glucan were detected, in addition to structural carbohydrates (cellulose and xylan). These results indicate that rice straw is a rich source of fermentable sugars from both soft carbohydrates and lignocellulosic portions of the cell wall.     

Evaluation of fuel ethanol production from aqueous ammonia-treated rice straw via simultaneous saccharification and fermentation

Rice straw (RS) has been considered a promising feedstock for ethanol production in Asia. However, the recalcitrance of biomass, particularly the presence of lignin, hinders the enzymatic saccharification of polysaccharides in RS and consequently decreases the ethanol yield. Here, we used aqueous ammonia pretreatment to remove lignin from RS (aRS). The reaction conditions were a solid:liquid ratio of 1:12, an ammonia concentration of 27% (w w⁻¹), room temperature, and a 2-week incubation. We evaluated enzymatic digestibility and the ethanol production yield. A 42% reduction in lignin content increased the glucan conversion of aRS to glucose from 20 to 71% using a combination of Cellic Ctec2 cellulases and Cellic Htec2 xylanases at enzyme loads of 15 FPU +100 XU g⁻¹ solid. Scanning electron microscopy highlighted the extensive removal of external fibres and increased porosity of aRS, which aided the accessibility of cellulose for enzymes. Using the same enzyme dosage and a solid load of 100 g L⁻¹, simultaneous saccharification and fermentation using a monoculture of Saccharomyces cerevisiae and co-culture with Candida tropicalis yielded ethanol concentrations of 22 and 25 g L⁻¹, corresponding to fermentation efficiencies of 96 and 86% fermentation, respectively. The volumetric ethanol productivities for these systems were 0.45 and 0.52 g L⁻¹ h⁻¹. However, the ethanol yield based on the theoretical glucose and xylose concentrations was lower for the co-culture (0.44 g g⁻¹) than the monoculture (0.49 g g⁻¹) due to the low xylose consumption. Further research should optimise fermentation variables or select/improve microbial strains capable of fermenting xylose to increase the overall ethanol production yield.     

Effect of nutrient supplementation on ethanol production in different strategies of saccharification and fermentation from acid pretreated rice straw

The effect of nutrient supplementation on ethanol production by recently selected thermotolerant yeast (Kluyveromyces marxianus NRRL Y-6860) was investigated in different strategies of saccharification and fermentation employing rice straw pretreated by dilute acid. Among the evaluated strategies, similar ethanol yields (Y_P/S ∼ 0.23 g g⁻¹) were obtained with or without nutrient addition. However, considering the whole process time, the strategy based on simultaneous saccharification and fermentation (SSF), without pre-hydrolysis, was assigned as the most suitable configuration due to the highest ethanol volumetric productivity (1.4 g L⁻¹ h⁻¹), about 2-fold higher in relation to the others. The impact of enzymatic preparation employed in this study was also evaluated on glucose fermentation in semi-synthetic medium. The enzymatic preparation affected both glucose consumption and ethanol production by K. marxianus NRRL Y-6860, but just in the absence of nutrients. Therefore, the enzyme type and loading should be carefully defined, not only by the capital costs involved, but also by the possibility of increasing the fermentation inhibitors.     

Potential alternatives of heat and power technology application using rice straw in Thailand

Rice straw could be used for heat and power with the current technologies available in Thailand. The cost of rice straw for power generation at 0.38–0.61 Baht/MJ_e (at rice straw price 930–1500 Baht/t) is not competitive with coal at 0.30 Baht/MJ_e but comparable with other biomass at 0.35–0.53 Baht/MJ_e. However, utilization of rice straw in industrial boilers is a more competitive and flexible option with two alternatives; (1) installing rice straw fired boilers instead of heavy oil fired or natural gas ones when selecting new boilers; and (2) fuel switching from coal to rice straw for existing boilers with cost saving of feedstock supply by 0.01 Baht/MJ_h. Based on its properties (Slagging index, R_s = 0.04; fouling index, R_f = 0.24), rice straw is not expected to have significant operating problems or different emissions compared with wheat straw and rice husk under similar operating conditions.     

Liquid phase reforming of rice straw for furfural production

In this study, rice straw was employed in liquid-phase reforming for furfural production. Rice straw was hydrolyzed with acid, ethanol, methyl-isopropyl ketone (MIPK), and water in an autoclave, and then reacted at 140 °C. Cellulose, hemicellulose, and lignin were extracted, and their contents were determined. The Taguchi method was employed to obtain the optimal operation conditions for xylose dehydration. The optimal operation conditions were modified again to reduce costs and then applied to rice straw hydrolysate dehydration.     

Understanding the reductions in US corn ethanol production costs: An experience curve approach

The US is currently the world's largest ethanol producer. An increasing percentage is used as transportation fuel, but debates continue on its costs competitiveness and energy balance. In this study, technological development of ethanol production and resulting cost reductions are investigated by using the experience curve approach, scrutinizing costs of dry grind ethanol production over the timeframe 1980–2005. Cost reductions are differentiated between feedstock (corn) production and industrial (ethanol) processing. Corn production costs in the US have declined by 62% over 30 years, down to 100$₂₀₀₅/tonne in 2005, while corn production volumes almost doubled since 1975. A progress ratio (PR) of 0.55 is calculated indicating a 45% cost decline over each doubling in cumulative production. Higher corn yields and increasing farm sizes are the most important drivers behind this cost decline. Industrial processing costs of ethanol have declined by 45% since 1983, to below 130$₂₀₀₅/m³ in 2005 (excluding costs for corn and capital), equivalent to a PR of 0.87. Total ethanol production costs (including capital and net corn costs) have declined approximately 60% from 800$₂₀₀₅/m³ in the early 1980s, to 300$₂₀₀₅/m³ in 2005. Higher ethanol yields, lower energy use and the replacement of beverage alcohol-based production technologies have mostly contributed to this substantial cost decline. In addition, the average size of dry grind ethanol plants increased by 235% since 1990. For the future it is estimated that solely due to technological learning, production costs of ethanol may decline 28–44%, though this excludes effects of the current rising corn and fossil fuel costs. It is also concluded that experience curves are a valuable tool to describe both past and potential future cost reductions in US corn-based ethanol production.     

Logistics cost analysis of rice straw pellets for feasible production capacity and spatial scale in heat utilization systems: A case study in Nanporo town,Hokkaido, Japan

Rice straw is a promising renewable energy source because it is abundantly available in Asia. This study conducted a case study of logistics cost analysis for rice straw pellets by considering all stages in the supply chain to define the main factors affecting the selling price of rice straw pellets: collection (job-commission or employment of part-time workers), transportation, storage (vinyl greenhouses or storage buildings with larger capacity), pelletizing, and delivery to users with biomass boilers. The selling price was found to be strongly dependent on the production capacity because the investment cost for the pellet production facility had a significant effect of economies of scale. A production capacity of larger than 1500 t y⁻¹ is required for rice straw pellets to compete with wood pellets and fossil fuels in the studied Japanese context if the subsidy rate for the investment is 50%, part-time workers conduct the collection, and rice straw is stored in the storage buildings. Our sensitivity analysis also showed an economically feasible spatial scale: for example, rice straw should be collected within a 20 km radius and the users should be within a 38 km radius when the production capacity is 1500 t y⁻¹. In addition, other critical factors related to the collection of rice straw from the paddy fields and transportation of rice straw rolls to storage were identified as planning factors to further reduce the total logistics cost of rice straw pellets.     

Review on fermentative biohydrogen production from water hyacinth,wheat straw and rice straw with focus on recent perspectives

Hydrogen (H₂) is often considered as the best option to store energy coming from renewable sources. Hydrogen production from lignocellulosic biomass via fermentation offers low cost and environmental friendly method in terms of energy balance and provides a sustainable pathway for utilization of huge amount of unused biomass. In this regard, special attention on potential of different lignocellulosic biomass is required. In this paper, the fermentative hydrogen production from three carbohydrates-rich biomass: water hyacinth, wheat straw and rice straw is comprehensively reviewed. In other point of view, usage of H₂ has a 10% growth annually that will reach to 8–10% of total energy in 2025. Furthermore, research on recent trends of fermentative hydrogen production is crucial and vital. However, the majority of the published researches in the last decade confirmed that some challenges exists which are the process optimization, effecting parameters and commercialization aspects.     

Pretreatment enhanced structural disruption,enzymatic hydrolysis,fermentative hydrogen production from rice straw

Rice straw (RS) is one of the major lignocellulosic wastes in the world and an abundant feedstock for producing biofuels and chemicals. However, RS is difficult to decompose. In this study, NaOH/urea and electrohydrolysis pretreated RS were used to enhance the structural disruption, enzymatic hydrolysis, and fermentative hydrogen production. Scanning electron microscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy analyses demonstrated that both NaOH/urea and electrohydrolysis pretreatments could effectively disrupt the lignin structure and increase the cellulose crystallinity of RS. Following pretreatment, RS was hydrolyzed by cellulase. After 96 h of enzymatic hydrolysis, NaOH/urea- and electrohydrolysis-pretreated RS produced 3.2- and 1.7-fold higher total reducing sugars than the unpretreated RS (232.95 ± 3.60 mg/g), respectively. Finally, the obtained RS hydrolysates were used for fermentative hydrogen production. NaOH/urea- and electrohydrolysis-pretreatment hydrolysates produced 125.0 and 163.0 mL H₂/g RS, respectively, which is much higher than the hydrogen yield of unpretreated hydrolysates.     

Biohydrogen production from rice straw hydrolyzate in a continuously external circulating bioreactor

The biohydrogen production from rice straw hydrolyzate in a continuously external circulating bioreactor (CECBR) was carried out in this study. The rice straw hydrolyzate was obtained by a concentrated sulphuric acid pretreatment. The original hydrolyzate concentration was 40–50 g total sugar/L. The feeding concentration of hydrolyzate was adjusted to 20 g total sugar/L by tap water. The working volume of CECBR was 300 mL with a height of 22.5 and a width of 7.5 cm respectively. The positions of external circulating ports were 13 and 5 cm high of CECBR with volumetric circulating rate of 9.6 L/min.     

Thermophilic dark fermentation of untreated rice straw using mixed cultures for hydrogen production

Biohydrogen production from untreated rice straw using different heat-treated sludge, initial cultivation pH, substrate concentration and particle size was evaluated at 55 °C. The peak hydrogen production yield of 24.8 mL/g TS was obtained with rice straw concentration 90 g TS/L, particle size <0.297 mm and heat-treated sludge S1 at pH 6.5 and 55 °C in batch test. Hydrogen production using sludge S1 resulted from acetate-type fermentation and was pH dependent. The maximum hydrogen production (P), production rate (R_m) and lag (λ) were 733 mL, 18 mL/h and 45 h respectively. Repeated-batch operation showed decreasing trend in hydrogen production probably due to overloading of substrate and its non-utilization. PCR-DGGE showed both hydrolytic and fermentative bacteria (Clostridium pasteurianum, Clostridium stercorarium and Thermoanaerobacterium saccharolyticum) in the repeated-batch reactor, which perhaps in association led to the microbial hydrolysis and fermentation of raw rice straw avoiding the pretreatment step.     

Effects of raw material particle size on the briquetting process of rice straw

Biomass feedstocks need to be milled or chopped into particles before briquetting, and the particle size has great effects on the energy consumption and product quality. In this study, the effects of the particle size on the rice straw briquetting process were investigated. The raw materials were milled or chopped into four different sized test materials. Experiments were carried out with an electronic universal testing machine and a self-designed single pellet unit on the basis of a simplex-centroid design. Several parameters, including briquetting time, energy consumption, maximum extrusion force, product compressive strength, and product density, were tested and recorded. The experimental data were processed by the methods of regression analysis and variance analysis. Finally, effects of raw material particle size on the briquetting process, energy consumption, maximum extrusion force, product compressive strength, and product density were obtained. Results showed that, compared with simple sized materials, mixed materials achieved lower energy consumption, higher product compressive strength, and higher product density.     

Sub- and supercritical liquefaction of rice straw in the presence of ethanol–water and 2-propanol–water mixture

The critical liquefaction of rice straw to bio-oil with sub- and supercritical mixtures (ethanol–water and 2-propanol–water mixture) was studied in a 1000 ml autoclave at 533–623 K, 6–18 MPa, respectively. The results showed that the maximum yield of bio-oil was 39.7% for the 2-propanol:water volume ratio of 5:5 at 573 K, while the higher heating value (HHV) of bio-oil increased with the reaction temperature and solvent volume ratio. The formation of low-boiling-point materials was reduced by a mixture. Using a mixture could inhibit the formation of residue and then promote the conversion of rice straw with the ratio of 1:9–5:5. The bio-oil was analyzed by GC/MS and Elemental Analyzer, while the morphological changes of residue were observed by a scanning electron microscope (SEM).     

A processing scheme for industrial ethanol production from straw

We describe a model for the industrial production of ethanol, using wheat straw as a raw material, for use in automobiles as a fuel mixed with gasoline. In particular for Greece, we have in mind that wheat straw production of the Thessalia Plain.The energy requirements of the factory are covered totally by burning the produced lignin. Particularly, the electric energy needs are covered by an electric generator which consumes part of the thermal energy produced by the combustion of lignin.The fermentable sugars are obtained from cellulose by hydrolysis, using an enzymatic system of cellulases which are produced from a cultivation of Trichoderma viride.The cellulose is separated from the lignin by heating the straw at 100°C, in water solution of NaOH 1%.We propose a fast fermentation method in order to reduce the cost, and the rejected/24 hr yeast from the Laval separators is processed for the production of a protein-containing animal food (8 kg of pressed yeast per 1001 of alcohol).We give the mass and energy balance and the cost, in current prices, of the produced ethanol, which is estimated to be 7.9 Dr ($0.19 U.S.) per liter. We also estimate that the plant cost will be about 20 million dollars for a production capacity of 100 thousand litres of ethanol per day.     

Hydrogen production by anaerobic co-digestion of rice straw and sewage sludge

In this study, the rich carbon content of rice straw and the high nitrogen content of sewage sludge make the straw a good potential substrate and the sludge a viable inoculum for biohydrogen production. Two treatment conditions for the sewage sludge (raw and heat-treated) were used in the present experiments. Batch test using a mixture of rice straw and sewage sludge were carried out to investigate the optimum carbon to nitrogen (C/N) ratio for effective biohydrogen production. The experimental results indicate that untreated sludge could be used as the inoculum for efficient hydrogen production when mixed with the appropriate proportion of rice straw. According to our results, biogas and hydrogen production in all raw sludge cases ramped up more quickly and also exhibited longer and higher hydrogen production in comparison with heat-treated cases. At the C/N ratio of 25 in untreated sludge, hydrogen production was 33% higher than heat-treated one. Additionally, under the same conditions, high and stable hydrogen content (58%) and the maximal hydrogen yield (0.74 mmol H₂/g-VS added straw) were obtained.     

The addition of hydrolyzed rice straw in xylose fermentation by Pichia stipitis to increase bioethanol production at the pilot-scale

The pretreatment of agricultural biomass by diluted acid is often employed to facilitate the release of monosaccharide for the subsequent enzyme hydrolysis for lignocellulosic ethanol production. However, furfural and hydroxymethylfurfural are usually generated and markedly decrease the yield of pentose fermentation during this pretreatment. In the present study, the enhancement of lignocellulosic ethanol production was successfully demonstrated at pilot scale with extra addition of hydrolyzed rice straw into pentose fermentation by Pichia stiptis. This way has resulted into the increase of P. stiptis cell mass was shown to play a positive role. The ethanol yield, 0.45 g_p/g_s, with the addition of hydrolyzed rice straw in hemicellulosic hydrolysate from plywood, bagasse and bamboo were increase 20–51% to demonstrate the applicability of this technology in a variety of lignocellulosic ethanol processes due to the efficient conversion of xylose.     

Evaluation of bio-hydrogen production using rice straw hydrolysate extracted by acid and alkali hydrolysis

This study was conducted to investigate the properties of hydrolysates obtained from acid and alkali hydrolysis and to evaluate the feasibility of employing them for bio-hydrogen production. High sugar concentrations of 16.8 g/L and 13.3 g/L were present in 0.5% and 1.0% H₂SO₄ hydrolysates, respectively. However, H₂SO₄ hydrolysis resulted in large amounts of short-chain fatty acids (SCFAs) and furan derivatives, which were removed by detoxification. In bio-hydrogen production, 1.0% H₂SO₄ hydrolysate showed a 55.6 mL of highest hydrogen production and 1.14 mol-H₂/mol-hexose equivalent_added of hydrogen yield. In control and 1.0% NaOH hydrolysate, 29.7 mL and 36.9 mL of hydrogen were produced, respectively. Interestingly, relatively high acetate and butyrate production resulted in lactate reduction. Also, NH₄OH hydrolysate produced less than 10 mL of hydrogen. Thus, these results indicate that hydrogen production and metabolite distribution can vary depending on the sugars and by-product composition in the hydrolysate.     

Hydrolysis of pretreated rice straw by an enzyme cocktail comprising acidic xylanase from Aspergillus sp. for bioethanol production

The most crucial enzyme involved in xylan hydrolysis is endoxylanase which cleaves the internal glycosidic bonds of xylan. The aim of this work was to study the production of extracellular xylanase by a locally isolated strain of Aspergillus sp. under solid-state fermentation (SSF) and to evaluate the potential of the enzyme in enzymatic hydrolysis of pretreated rice straw. Xylanase production reached maximum with incubation period (96 h), moisture level (80%), inoculum size (3 × 10⁶ spores/mL), pH (4.8), temperature (25 °C), carbon source (wheat bran) and nitrogen source (yeast extract). Under optimized conditions, xylanase production reached to 5059 IU/gds. Crude xylanase was used for supplementing the enzyme cocktail comprising cellulases (Zytex, India), β-glucosidase (In-house) and xylanase (In-house) for the saccharification of alkali-pretreated rice straw to get the maximum reducing sugar production. The cocktail containing the three enzymes resulted a maximum of 574.8 mg/g of total reducing sugars in comparison to 430.2 mg/g sugars by the cocktail without xylanase. These results proved that the crude xylanase preparation from Aspergillus sp. could be a potent candidate for the enzyme cocktail preparation for biomass hydrolysis in lignocellulosic bioethanol program.     

Low concentration of NaOH/Urea pretreated rice straw at low temperature for enhanced hydrogen production

In lignocellulose-to-hydrogen bioconversion, reducing the concentration of chemical agents in pretreatment is of great interest. In this study, rice straw (RS) pretreated at reduced NaOH and urea (NU) concentrations was evaluated. Results showed that the composition of RS exhibited excellent pretreatment performance at a reduced concentration of NU. When the concentration of NaOH was decreased to 3 wt% in combination with 6 wt% urea, the lignin was reduced by 59.52% with a cellulose and hemicellulose loss of less than 17%. Moreover, extending the pretreatment time at a low concentration of NU could effectively promote the biodegradability of RS. Upon fermentation by Thermoanaerobacterium thermosaccharolyticum M18 for H₂ production, the H₂ production increased up to 213.06 mL/g with a substrate treated by 3 wt% NaOH/6 wt% urea at low solid loading for 15 d, which was 16.31% higher than the counterpart subjected to a 7 wt% NaOH/12 wt% urea pretreatment. The present results suggest the NU pretreatment can be carried out at low concentrations to improve the conversion of RS into bio-H₂ production.