Бурение грунтовых зондов, установка энергетических колодцев

Hot water at high pressures is used in this pretreatment method; the pressure helps to maintain the water in the liquid state dur­ing the pretreatment. This technique is similar to steam pretreat­ment, but uses high temperature water, typically 120-260°C range instead of steam. Generally, hot compressed liquid water comes in contact with biomass for up to 15 min, and about 40-60% of the total biomass dissolves in the process, with 4-22% of the cellulose, 35-60% of the lignin, and most of the hemicellulose hydrolyzed to pentosans. As a result of removal of hemicellulose, cellulose becomes readily accessible to cellulases in the hydrolysis step. Water at high temperatures is believed to act as an acid, and together with acetyl groups within hemicelluloses are thought to catalyze exten­sive hydrolysis of hemicellulose to its component monosaccharides and smaller oligosaccharides, primarily xylose. The effectiveness of LHW pretreatment on cellulose digestibility is strongly related to the severity of the conditions. The result of severe pretreatment conditions is an accumulation of organic acids such as levulinic acid, acetic acid, and formic acid, which subsequently results in an acidic environment. The acidity in the media can cause degradation of monomeric sugars present in the liquid fraction to compounds such as 5-hydroxymethylfurfural (HMF), and furfural, which can inhibit the fermentation step [55]. A small concentration of mineral acid can be added making the water more acidic, and that would make the process similar to dilute acid pretreatment. However, cat­alytic degradation of sugars results in more undesirable side prod­ucts. During liquid hot water pretreatment the pH of the water is affected by temperature and degraded acids, therefore a base like sodium hydroxide can be added to maintain the pH above 5 and below 7 to minimize the formation of monosaccharides, and pH control can reduce further degradation of monosaccharides to fer­mentation inhibitors as well [55, 1].

A large volume of work has been conducted in testing liquid hot water pretreatment (LHW) on the laboratory — and pilot-plant scale on various forms of biomass, and unlike in the case of steam explo­sion the majority of literature examples are from applications in the treatment of non-wood forms of biomass, suggesting that hot water pretreatment is more suitable for soft biomass forms.

Some of the biomass samples tested are: sugarcane bagasse [56, 57], sorghum bagasse [57, 58], corn stover [59], rice straw [60, 61], rye straw [62], soybean straw [63], eucalyptus [64], [57], oil palm frond [65] and populus tomentosa [66].

The three types of liquid hot water reactor configurations that are known in liquid hot water pretreatment (LHW) technology are: (1) flow-through, (2) counter-current, and (3) co-currrent (Figure 5.7).

In a liquid hot water pretreatment flow-through reactor, hot water is passed over a stationary bed of lignocellulosic biomass which undergoes partial hydrolysis, and part of the lignocellulosic

(c)

biomass components dissolves and carries them out of the reactor. The counter-current pretreatment vessel is designed to move water and lignocellulose in opposite directions through the pretreatment reactor as shown in Figure 5.7b. In the co-current pretreatment design, slurry of biomass and water is heated to the desired tem­perature and held at the pretreatment conditions for a controlled
residence time inside a reactor loop before being cooled as shown in Figure 5.7c.

Liquid hot water treatment and steam treatments are similar techniques, however they act differently on the biomass. Leser and coworkers have compared the two techniques by employing simi­lar severity and using sugarcane bagasse as the biomass [67]. Solid concentration ranged from 1% to 3% for LHW pretreatment and was >50% for steam pretreatment. Reaction temperature and time ranged from 170 to 220°C and 2 to 46 min, respectively. Key performance met­rics included fiber reactivity, xylan recovery, and the extent to which pretreatment hydrolyzate inhibited glucose fermentation. Some of the results of their comparison study are shown in Table 5.4 [67].

In four cases of LHW, xylan recovery % decreased as the tem­perature increased from 170 to 220°C. At the same time, conversion by simultaneous saccharification and fermentation (SSF) increased from 32% to 93%. The highest conversion of 93% was achieved with LHW at 220°C for 2 min. Leser and coworkers concluded that these results are consistent with the notion that autohydrolysis plays an important, if not exclusive role in batch LHW pretreatment [67].

In another study, Ingrim et al. have compared liquid hot water treatment with a number of other pretreatment techniques like soda pulping process and ethanol organosolv pretreatment, using rye straw as the lignocellulosic material [62]. The organosolv pretreated rye straw was shown to require the lowest enzyme loading in order to achieve a complete saccharification of cellulose to glucose. At biomass loadings of up to 15% (w/w), cellulose conversion of LHW and organosolv pretreated lignocellulose was found to be almost equal. The soda pulping process showed lower carbohydrate and lignin recoveries compared to the other two processes. [62].

In 2013 Imman and coworkers published their results on auto­hydrolysis during the LHW pretreatment [68]. In this study, vari­ous tropical agricultural residues including sugarcane bagasse (BG), rice straw (RS), corn stover (CS), and empty palm fruit bunch (EPFB) were investigated. It was found that LHW pretreatment at 200°C for 5-20 min resulted in high levels of hemicellulose solubi­lization into the liquid phase and marked improvement on enzy­matic digestibility of the solid cellulose-enriched residues. The maximal yields of glucose and pentose were 409.8-482.7 mg/g and 81.1-174.0 mg/g of pretreated substrates, respectively. Comparative analysis based on severity factor showed varying susceptibility of biomass to LHW in the order of BG > RS > CS > EPFB.

Additionally, changes in biomass microstructures pretreated under the conditions for maximal sugar yields were analyzed using scanning electron microscopy (SEM). Scanning electron micro­graphs of native and pretreated biomass under the optimal condi­tions are shown in Figure 5.8 [68]. Comparison of the SEM images

of the native and pretreated biomass showed that microstructures of the agricultural residues are disrupted by pretreatment. Cavities and cracks in the plant cell wall were observed in the pretreated biomass as shown in Figure 5.8, which reflected the removal of hemicellulose and modification of the surface lignin. Furthermore, structural analysis revealed surface modification of the pretreated biomass along with an increase in crystallinity index. In addition, Imman and coworkers reported that, overall, 75.7-82.3% yield of glucose and 27.4-42.4% yield of pentose from the dried native bio­mass could be recovered in the pretreated solid residues, while 18.3-29.7% of pentoses were recovered in the liquid phase with dehydration byproduct concentration under the threshold for etha — nologens [68].

Similar to steam explosion, no chemicals are used in liquid hot water pretreatment, therefore, this is an environmentally friendly technique and the low cost of the solvent is also an advantage for large-scale application. Another major advantage in the LHW method is the operation at lower temperatures compared to steam explosion, minimizing the formation of degradation products. This eliminates the need for a final washing step or neutralization step. Then there are disadvantages also in the LHW pretreatment; the amount of solubilized product is higher, while the concentration of these products is lower compared to steam explosion or steam pretreatment [69]. Therefore, down-stream processing is also more energy demanding since large volumes of water are involved.

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