Rice is cultivated in the large area of different parts of the world especially in Asia, and its straw is mainly burned by farmers. Rice straw contains cellulosic and hemicellulosic, which are suitable as a source for renewable fuel. The cellulose and hemicellulose of the rice straw are not easily accessible to biodegradation, that it is related to the compact structure, presence of high lignin and silica. In the present study, steam explosion pretreatment in different cycles (pressure, time and moisture) was used to modify the structure of rice straw. Finally, the effects of the pretreatment on the composition and structure of rice straw were evaluated by performing anaerobic batch digestion and comparing the accumulated methane yields.

3.1 steam explosion pretreatment conditions
The effects of main parameters pressure, time and moisture of the steam explosion pretreatment were examined on rice straw structure modification. Rice straw was pretreated in a steam explosion reactor at pressures 5-15 bar. Rice straw was kept at each pressure inside pressure vessel for 1-15 min. Rice straw was investigated in three levels of primary moisture (0-70%), resulting in a total of 36 pressure/time/moisture combinations. Typically pretreatment pressure and time are within the ranges of 5-40 bar, 160-280 °C, and several seconds to a few minutes, respectively. Most studies have examined a limited range of steam explosion parameters, also primary moisture parameter of feedstock has been poorly considered in steam explosion pretreatment. (Kobayashi et al., 2004) has studied Methane production from steam-exploded bamboo under various steaming times of 1, 3, 5, 10, and 20 min, at a steam pressure of 35 bar (243 °C). (Teghammar et al., 2010) has examined pretreatment of paper residuals for improved biogas. Steam explosion pretreatment conditions were varied 15 and 20 bar (190–220 °C) within 10 or 30 min. (Vivekanand et al., 2012) has studied Biogas production from the brown seaweed Saccharina latissima. Thawed SW was steam exploded at 130 and 160 °C for 10 min. Similarly, milled WS were subjected to the steam explosion at 210 °C for 10 min.

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3.2 Effects of Pretreatments on the Chemical Composition of Rice Straw
The unpretreated rice straw consisted of cellulose (23%), hemicellulose (14%), lignin (18.3%), and ash (5.3%). Results of Chemical Composition changes of pretreated rice straw are presented in Table 1. According to results with increasing pressure and retention time, the percentage of cellulose content increased in pretreated rice straw compared with that of the unpretreated. However, other components such as lignin, hemicellulose, and ash have been reduced significantly. The analysis demonstrated that in the cycle 10 bar and 10 min with blank moisture followed by 15 bar and 15 min, 15 bar and 10 min with moistures 35% and 70% respectively, resulted in more lignin and ash removal. This indicates that the pretreatment technique is capable of removing lignin and other components under high pressure.
Pressure is the most important parameter in steam explosion pretreatment compared to the other parameters such as reaction time and moisture ratio. During the steam explosion pretreatment, the hemicellulose is first degraded followed by the lignin when the temperature >150 °C. (Ma et al., 2014; Xiao et al., 2014). In addition to the removal of the hemicellulosic, the steam explosion induces important modifications in the structure of lignins. Lignin degrades in the temperature range of 120-200 °C and it divides into smaller particles and is separated from celluloses. (Fan et al., 2016). It damaged the cell wall of rice straw by disrupting the lignin structure.
The pretreatment in cycle 5 bar pressure with low retention time resulted in the almost same values of lignin, 23–24%, ash (43) and carbohydrate (48) as the unpretreated one. Over the same pretreatment time, the content of hemicellulose in the pretreated rice straw gradually decreased as the steam explosion pressure increased, as it did with increased retention time at a given steam explosion pressure. Compared to cellulose, the content of hemicellulose was generally reduced in the treated rice straw (Zhou et al., 2016).
The analysis revealed that pretreated rice straw contained 91.8% total solids and 60.9% volatile solids on a wet weight basis that is increased by different cycles of pretreatment. Table 2 shows the percentage of VS and TS of the pretreated rice straws. These results also show that increase pressure of the pretreatment led to the increase of VS content, indicating higher carbohydrate material contents in the pretreated materials (Dehghani et al., 2015).