Бурение грунтовых зондов, установка энергетических колодцев

Alkali pretreatments can be carried out using sodium, potassium, or calcium hydroxide as chemical reactant. Sodium hydroxide has received the most attention as it is less expensive than others, and there are extensive studies on using sodium hydroxide on various biomass forms [197-201]. Hydroxide bases are known to disrupt the lignin structure by breaking the linkages between lignin and other carbohydrate fractions in lignocellulosic biomass, thus mak­ing carbohydrates in the heterogeneous matrix more accessible [202-204]. Sodium hydroxide pretreatment is usually carried out by treating biomass with 1-12% aqueous sodium hydroxide, typi­cally at 50-120°C; the exposure time may vary from minutes to a few hours. Some of the biomass forms tested with aqueous sodium hydroxide pretreatments are corn stover [202], sunflower husks [203], rapeseed straw [197], cotton stalks [205, 206], coconut fiber [207], and spruce [204]. Some selected examples of aqueous NaOH pretreatment results are summarized in Table 5.9.

Sun and coworkers studied the influence of alkaline pretreat­ments on the cell wall components of wheat straw in detail. In par­ticular, treatments with increasing amounts of sodium hydroxide at a range of temperatures and exposure times have been investi­gated [210]. Optimal conditions for delignification and dissolution of hemicellulosic polysaccharides were found to be pretreatment with 1.5% sodium hydroxide for 144 h at 20°C. This resulted in the release of 60% and 80% of lignin and hemicellulose, respectively. Xylose was the major component produced from the hemicellu — lose fraction during the pretreatment, while glucose and galactose were formed in smaller amounts [210]. Recently, Zhao and cowork­ers showed the effectiveness of sodium hydroxide pretreatment for hardwoods, wheat straw, switchgrass, and softwoods with less than 26% lignin content. Sodium hydroxide pretreatment has also been shown to increase biogas production from corn stover by 37% compared to that of untreated corn stover [204].

A number of research groups have studied combinations of alka­line pretreatment with other pretreatment methods such as wet oxidation, steam explosion, ammonia fiber explosion, and ammo­nia recycled percolation. Adding oxidizing reagents to aqueous sodium hydroxide has attracted attention. In one example, Jeyanthi and coworkers used hydrogen peroxide as the oxidizer in NaOH pretreatment [207]. In this study, green coconut fiber was used as the lignocellulosic biomass; it was treated with sodium hydroxide and alkaline hydrogen peroxide and was subjected to microwave radiation. Pretreated solids were enzymatically hydrolyzed and were examined in a simultaneous saccharification and fermenta­tion (SSF) process. The results showed that raw materials subjected

to alkaline hydrogen peroxide pretreatment produces higher reduc­ing sugar and ethanol yields. In another application of modification of alkaline pretreatment, Mohsenzadeh et al. studied the pretreat­ment of softwood spruce and hardwood birch by NaOH/thiourea, NaOH/urea, NaOH/urea/thiourea, and NaOH/polyethylene gly­col (PEG) to improve ethanol production [211]. In this investigation, pretreatments were carried out at different temperatures between -15 and 80°C with NaOH/thiourea (7/5.5 wt%), NaOH/urea (7/12 wt%), NaOH/urea/thiourea (7/8/6.5 wt%), and NaOH/ PEG (7/1 wt%) aqueous solutions. The pretreated materials were then subjected to enzymatic hydrolysis for 72 h. The pretreatments by NaOH/thiourea at -15°C improved the hydrolysis yields of spruce from 11.7% to 57% of theoretical yield, and for birch from 23.1% to 83% of theoretical yield. The enzymatic hydrolysis and fermentation of these pretreated materials by NaOH/thiourea with baker s yeast resulted in 54.0% of theoretical yield compared with 10.9% for untreated spruce and 80.9% of theoretical yield compared with 12.9% for untreated birch [211].

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