Ethanol Production From Rice Straw Pdf Programs

BackgroundRice straw and husk are globally significant sources of cellulose-rich biomass and there is great interest in converting them to bioethanol. However, rice husk is reportedly much more recalcitrant than rice straw and produces larger quantities of fermentation inhibitors. The aim of this study was to explore the underlying differences between rice straw and rice husk with reference to the composition of the pre-treatment liquors and their impacts on saccharification and fermentation. This has been carried out by developing quantitative NMR screening methods. ResultsAir-dried rice husk and rice straw from the same cultivar were used as substrates. Carbohydrate compositions were similar, whereas lignin contents differed significantly (husk: 35.3% w/w of raw material; straw 22.1% w/w of raw material). Substrates were hydrothermally pre-treated with high-pressure microwave processing across a wide range of severities.

25 compounds were identified from the liquors of both pre-treated rice husk and rice straw. However, the quantities of compounds differed between the two substrates. Fermentation inhibitors such as 5-HMF and 2-FA were highest in husk liquors, and formic acid was higher in straw liquors. At a pre-treatment severity of 3.65, twice as much ethanol was produced from rice straw (14.22% dry weight of substrate) compared with the yield from rice husk (7.55% dry weight of substrate). Above severities of 5, fermentation was inhibited in both straw and husk. In addition to inhibitors, high levels of cellulase-inhibiting xylo-oligomers and xylose were found and at much higher concentrations in rice husk liquor. At low severities, organic acids and related intracellular metabolites were released into the liquor.

ConclusionsRice husk recalcitrance to saccharification is probably due to the much higher levels of lignin and, from other studies, likely high levels of silica. Therefore, if highly polluting chemical pre-treatments and multi-step biorefining processes are to be avoided, rice husk may need to be improved through selective breeding strategies, although more careful control of pre-treatment may be sufficient to reduce the levels of fermentation inhibitors, e.g. Through steam explosion-induced volatilisation. For rice straw, pre-treating at severities of between 3.65 and 4.25 would give a glucose yield of between 37.5 and 40% (w/DW, dry weight of the substrate) close to the theoretical yield of 44.1% w/DW, and an insignificant yield of total inhibitors. The energy crisis and how to address it has been long debated, encompassing a wide range of topics from the economic implications of climate change and “peak oil” to the improvements in technologies for producing renewable or low carbon energy. Renewable and low carbon electrical energy is a rapidly developing sector involving nuclear, wind power or photovoltaic technologies ,.

However, the bulk of road vehicles require liquid fuels and this has led to global programmes for producing renewable biofuels that have the potential to be sustainable, and emit minimal levels of greenhouse gases ,.Of interest are second-generation biofuels such as cellulosic bioethanol. Cellulose is the most abundant source of glucose, and is found in lignocellulosic biomass and wastes including agricultural residues such as forestry residues and pulping wastes, cereal straws, and threshing husks, as well as food processing coproducts such as brewers spent grain ,. As Rajaram and Varma reported in 1990, there were about 2900 million tonnes of lignocellulosic waste from cereal crops, 160 million tonnes from pulse crops, 14 million tonnes from oilseed crops and 540 million tonnes from plantation crops.Rice is one of the most widely grown cereal crops, with enormous levels of production in Asian countries leading to an abundance of rice husk and rice straw lignocellulosic wastes.

The world annual production of rice husk has been reported as approximately 120 million tonnes. Kim and Dale reported that 667.59 million tonnes rice straw were at that time produced in Asia, and Binod and colleagues calculated that this could theoretically be converted into 281.72 billion litres of ethanol.However, converting the cellulose and other cell wall sugars to ethanol is highly challenging due to the protective biochemical and structural nature of the lignocellulose , which hampers the hydrolysis of the polysaccharides to fermentable monosaccharides ,. Generically, the conversion processes employed comprise four main steps: hydrothermal pre-treatment, enzymatic or chemical saccharification, fermentation and purification. The aim of pre-treatment is to separate the lignin from the cellulose, reduce the structural barriers created by hemicelluloses, reduce cellulosic crystallinity and thereby improve the accessibility of cellulose to cellulases ,. The fermentable sugars released can be latterly converted to products by microorganisms such as bacteria and yeasts ,. Finally, the product of interest can be recovered from the fermentation liquor, for example by distillation.

Each of all those steps has a range of options, and the different combinations of those four steps can cause various results.Previously, we systematically demonstrated that rice straw and rice husk exhibit very different propensities for enzymatic saccharification and fermentation behaviour in response to steam explosion pre-treatment. The aim of this study has been to evaluate in greater depth the differences in the composition of these lignocellulosic materials, and the changes that occur in them during hydrothermal pre-treatments relevant to their biorefining potential, with special reference to the release of potential fermentation inhibitors and related chemicals. This has been achieved by using enclosed hydrothermal pre-treatment conditions to avoid loss of volatile substances that might occur during steam explosion. Furthermore, by using variations of time and temperature, a much higher range of pre-treatment severities have been assessed. Conditions conducive to optimal simultaneous saccharification and fermentation have also been explored. Sugar and lignin analysis of air-dried rice husk and rice strawSugar compositions in both rice husk and rice straw comprised rhamnose, fucose, arabinose, xylose, mannose, galactose and glucose (Table ) and are in keeping with previous studies ,.

Uronic acid was not quantified. Cellulose-derived glucose was the most abundant sugar (38.7% in rice straw and 36.8% in rice husk) followed by hemicellulosic xylose (22.9% in rice straw and 19.7% in rice husk).

Lignin (corrected for ash; Table ) was much higher in rice husk (35% w/w) compared with straw (22.1% w/w). Enzymatic saccharification of pre-treated rice husk and rice strawEnzymatic saccharification of hydrothermally pre-treated rice husk and rice straw was performed in 15 ml volumes (5% w/v substrate) at 50 °C for 96 h. The results in Fig. Present the reducing sugar and free glucose yields as a function of pre-treatment severities. Overall, reducing sugar and glucose both increased with increasing severity. Consistent with the results of steam explosionenzymatic hydrolysis of hydrothermally pre-treated rice straw released much higher quantities of reducing sugars (maximum 66.1% at severity 4.27) and glucose (maximum 43.6% at severity 5.15) compared with rice husk (maximum 35.3% reducing sugar at severity 4.55 sugar and 16.3% glucose at severity 5.44).

In rice husk, reducing sugar yield grew steadily with increasing severity up to 4.5 then slowly decreased, whilst glucose yield continued to increase at above this severity. In rice straw, reducing sugar yield reached a peak at a severity of 4.3 and then decreased rapidly at higher pre-treatment severities.

In contrast to husk, the peak of glucose yield (at a severity of 4.8) was followed by a decrease in glucose yield at higher severities. Thus under similar conditions of pre-treatment and enzyme loading, significantly higher sugar and glucose yields were achieved from rice straw compared with rice husk. Simultaneous saccharification and fermentations (SSF)SSF was carried out at a lower temperature (25 °C) by simultaneously adding cellulase (Ctec-2) and a yeast strain ( Saccharomyces cerevisiae NCYC 2826) which ferments hexose sugars, but not pentoses.

Four pre-treatment severities spanning the range used above were selected from low to very high (1.57, 3.65, 5.35, and 5.45). The results (Table ) show that: (1) ethanol yields were significantly higher from RS compared with RH after pre-treatment at severities 1.57 and 3.65, indicating that yeast behaves differently on the different lignocellulose hydrolysates; (2) ethanol yields were very low in both RH and RS pre-treated at severities 5.15 and 5.45, which suggests that yeast behaviour was being suppressed. Previously , we showed that washing pre-treated (steam exploded) rice straw prior to SSF reduced such severity-related decline in SSF efficiency and concluded that this was due to the removal of fermentation inhibitors. The impact of these inhibitors appears to be predominantly on the fermentation step as indicated by the data in Fig.

Which shows that the saccharification of total pre-treated slurries occurs at all the severities. Chemical analysis of supernatants from pre-treated RH and RS by using nuclear magnetic resonance (NMR)A more comprehensive understanding of the range of breakdown and solubilised components created during pre-treatment of the RH and RS was achieved by analysing the liquors by NMR. The results showed that 25 different compounds were readily detectable and quantifiable.

The diagnostic spectral regions of the compounds for RH and RS samples pre-treated at severities 1.57, 3.65, 5.15 and 5.45 are shown in Fig. (see Additional file: Figure S1 for a higher magnification version of the spectra), scaled to address variation in concentration.

The quantities of these compounds, as affected by severity of pre-treatment are shown graphically in Figs.,. Acetaldehyde and acetaldehyde hydrate were quantified as one compound. 1H NMR spectra of 25 chemical compounds identified from the liquors of pre-treated rice husk and rice straw. Four severities (severities 1.57, 3.65, 5.15, 5.45) were selected as examples to present the identification method. The complete spectra were split into two main parts ( a, b), which were further divided into several fragments and scaled differently to indicate compounds produced at low level. The red lines show the chemical shift (-ppm) scale with chemical shifts of individual compounds indicated on the figure.

Associations of those compounds with severities and with each other have been presented with principal component analysis (PCA) and shown in Fig. Severities are shown by the vectors (arrows), whilst the chemical compounds released are shown as coloured circles. The components identified were categorised as: nine previously unidentified compounds (green circles, mostly positioned around low severity vectors, bottom left); 7 sugars (shown as yellow circles, positioned adjacent to moderate severity vectors); and 9 established fermentation inhibitors (shown as red circles, generally positioned to the right-hand side of Fig. Associated with the higher severity pre-treatment). Figure shows compounds created and/or released during low severity pre-treatments. Several of these are organic acids typically found in intermediary metabolism, namely pyruvic, succinic, fumaric and 2-oxoglutaric acids.

Ethanol Production From Rice Straw Pdf Programs Online

In addition, acetoin, glycolic acid and glycerol were detected. Succinate, fumarate and pyruvate were produced in higher quantities at higher severities, and particularly in PTRS.

Acetoin and glycolic acid increased consistently from low severities to high severities, but glycerol, pyruvic and 2-oxoglutarate started to decrease after reaching their peaks indicating degradation. Ethanol was produced in small quantities from both PTRH and PTRS (at higher severities). Betaine levels and trends differed between PTRH and PTRS, showing marked degradation at higher severities in PTRH.At moderate pre-treatment severities, sugars and oligosaccharides were released (Fig. ). These all showed similar trends in that the levels peaked at around a severity of 4.5 after which they decreased, presumably due to degradation (concomitant with the increase in fermentation inhibitors shown in Fig. ). Generally, rice husk released higher amounts of sugar compounds than rice straw at any given severity. The presence of galactose may reflect the hydrolysis of small quantities of pectic polymers in the cereal biomass, whilst the xylose, xylo-oligomers and arabinose are likely to be derived from xylans and arabinoxylan hemicelluloses.Compounds known to cause significant inhibition on saccharification or fermentation were released at higher severities and are shown in Fig.

In keeping with previous studies , most of the inhibitors increased with increasing severities. Hydroxy-methyl furfural (5-HMF), furfural (2-FA) and acetic acid were the most abundant inhibitors produced from both PTRH and PTRS. Complementing Wood et al. , considerably higher levels of all the inhibitors were produced from rice husk at the higher severities, consistent with the higher levels of sugar release and breakdown shown in Fig. However in the present study, the levels of 5-HMF, 2-FA and acetic acid produced at the much higher severities were very much greater than those reported by Wood et al. (confirmed by HPLC—results not shown). This may be due to two factors: firstly, in the previous study, the maximum pre-treatment severity was 4.8, whilst in this study the severity went to higher levels; secondly, it is very likely that considerable quantities of these volatile compounds were lost into the vented steam during the explosion process.

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Measurable amounts of formic acid, acetol, acetaldehyde and methanol were also produced significantly from pre-treated samples. Choline and levulinic acid were produced at much lower levels than the other inhibitors and were produced more from PTRS than PTRH.