Composition of Rice Straw
Like any other plant variety, chemical composition of rice straw depends on the species as well as seasonal factors and maturity of the plants [74-76]. The average composition of typical rice straw is shown in Table 3.8.
All the common pretreatment methods have been tested on rice straw, and there are a number of excellent laboratory-scale studies. This include dilute acid [77, 78], aqueous ammonia [79], lime [80, 81], alkali under microwave conditions [82], microwave [83], steam explosion [84], wet disk milling [85], hydrothermal [86], alkaline and hydrogen peroxide [87], ammonia and ionic liquid [88],
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Table 3.8 Average chemical composition of typical rice straw (wt% on dry basis) [74-76].
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two-stage ammonia, then aqueous acid [89], supercritical carbon dioxide [90], and fungal methods [91].
There are two excellent review articles in this area, the first on the production of cellulosic ethanol from rice straw [92] and the second on rice straw as a lignocellulosic resource: collection, processing, transportation, and environmental aspects [74]. Aqueous acid pretreatment and steam explosion are the most widely studied pretreatment techniques on rice straw, then there are several studies on using a combination of acid and steam explosion. One of the most recent and interesting example was reported by Chen et al. in 2011 [77]. In this study, optimum operational conditions for the first dilute acid hydrolysis step were determined to be 165°C for 2 min with 2% H2SO4, and the second steam explosion step was to be carried out at 180°C for 20 min; this gave the most favorable combination in terms of an integrated process. Furthermore, they found that rice straw pretreated by the dilute-acid/steam explosions had a higher xylose yield, a lower level of inhibitors in the hydroly — zate and a greater degree of enzymatic hydrolysis; this resulted in a 1.5-fold increase in the overall sugar yield when compared to the single-step, acid-catalyzed steam explosion [77].