Бурение грунтовых зондов, установка энергетических колодцев

Steam explosion, or steam pretreatment, is likely the most exten­sively studied and frequently applied physicochemical pretreat­ment method in cellulosic ethanol production. This pretreatment is called steam explosion because of the thinking that an explosive action on the fibers is needed for them to become accessible to hydrolyzing enzymes. This method is sometimes known as auto­hydrolysis as well [24]. Steam explosion is seen as one of the most cost effective processes for pretreatment of lignocellulosic biomass and this technology has been tested in laboratory reactors, pilot plants, and large-scale demonstration plants as well [5].

As described earlier, physical pretreatment or size reduction of biomass is the first step, then these smaller pieces are exposed to steam explosion pretreatment. The biomass is typically treated with high pressure steam at temperatures in the range 180-220°C and at operating pressures between 1 and 2.3 MPa. The usual reten­tion time range is 2-10 min [25]. However, temperatures and times as low as 160°C and 60 s, have also been tested with fine materi­als such as algae to maximize the recovery of thermolabile mol­ecules. The pressure is held for only a short period of time, and in this time a part of the more easily hydrolyzable hemicellulose depolymerizes and dissolves in the liquid phase, whereas lignin is transformed as a result of the high temperature. Partial hydrolysis of hemicellulose is thought to be mediated by the acetic acid gener­ated from hydrolysis of acetyl groups associated with hemicellu — lose and other acids released during steam explosion pretreatment. This may further catalyze the hemicellulose hydrolysis resulting in the release of xylose and some glucose monomers, therefore the term autohydrolysis is also used in some publications to describe steam explosion pretreatment technique [26, 1]. A higher degree of hemicellulose solubilization can be achieved by keeping at higher temperatures for a short time like 270°C, for 1 min. However, lower temperature and longer residence time, like 190°C for 10 min, have been shown to be more favorable because they avoid the formation of sugar degradation by products like furans and acids, which can inhibit the enzymes during the fermentation [26].

There are many literature examples on the application of steam explosion as the main pretreatment technique on various forms of biomass and some of these are: poplar wood [27], pinewood [28], eucalyptus grandis [29, 30], olive tree wood [31, 32], sugarcane bagasse [33], [34, 35], corn stover [36-38], wheat stover [39], rice straw [40, 41], sunflower stalks [42], lespedeza stalks [43], manda­rin peel [44], lemon peel [45] and eel grass [46].

These literature examples prove that steam explosion is an effective pretreatment for different kinds of biomass feedstocks. Furthermore, steam explosion can be used to pretreat large particle sizes, which can greatly reduce overall costs of the process [47]. Steam explosion can greatly improve sugar recovery efficiencies of various biomass forms and some literature examples are summarized in Table 5.2.

In a study on the effects of steam explosion on woody (poplar and eucalyptus) and herbaceous (Sorghum sp. bagasse, wheat straw, and Brassica carinata residue) biomass forms, the samples have been subjected to steam explosion at 210°C for 2-8 minutes. In this study, Ballesteros et al. reported that it was possible to reach simultaneous saccharification and fermentation (SSF) ethanol yields in the range of 61-72% of the maximum theoretical SSF yield, based on the glu­cose available in the pretreated materials in 72-82 h [27].

Asada et al. have recently reported [52] the effect of steam explo­sion pretreatment with ultra-high temperature on softwood bio­mass. In this experiment, Japanese cedar (Cryptomeria japonica) was used as the softwood biomass, and was subjected to pressure and temperature of 67 atm and 281 °C, respectively. These conditions are the highest pressure and temperature values reported for any steam explosion. In this study, chopped wood chips (2-4 cm in length and 1-3 cm in width) were exposed to the saturated steam for 1-10 min, and then a ball valve at the bottom of the reactor was suddenly opened to bring the reactor rapidly to atmospheric pres­sure. This sudden release in the pressure caused the steam explo­sion of the material. Asada and coworker claimed that 49.6 g of glucose or 17.4 g of ethanol could be obtained from 100 g Japanese cedar by using this ultra-high temperature-pressure steam explo­sion technique [52].

There are few recent studies on the use of delignification as a sec­ond pretreatment after an initial steam explosion pretreatment, as the lignin fraction is not removed in the common steam explosion. Rocha and coworkers have recently studied steam explosion pre­treatment followed by delignification and then enzymatic hydroly­sis, for the production of ethanol from sugarcane bagasse in a pilot plant-scale experiment [34, 53]. In this study steam explosions at 180, 190 and 200°C for 15 min were applied to sugarcane bagasse in a 2.5 m3 reactor. Then pretreated bagasse was delignificated by sodium hydroxide and was hydrolyzed with cellulases, or submitted directly to enzymatic hydrolysis after the steam explosion pretreat­ment. The pretreatments led to remarkable hemicellulose solubiliza­tion. The maximum hemicellulose solubilization yield of 92.7% was obtained for steam explosion pretreatment performed at 200°C.

All pretreatment conditions led to high hydrolysis conversion of cel­lulose, with the maximum of 80.0% achieved at 200°C. Delignification increased the enzymatic conversion from 58.8% in steam-exploded sample to 85.1% in the steam explosion + delignificated sample of the material pretreated at 180°C. However, for the material pretreated at 190°C, the improvement was less remarkable, while for the sample pretreated at 200°C the hydrolysis conversion decreased after the alkaline delignification treatment as shown in Table 5.3.

Earlier studies were carried out without any added acid or bases. However, recent steam explosion experiments have shown that

high xylose solubilities can be achieved by using acidic environ­ments during the steam explosion pretreatment. In one example, Tucker and coworkers at NREL studied the effects of tempera­ture and moisture on dilute-acid steam explosion pretreatment of corn stover and cellulase enzyme digestibility [54]. In this study, dilute H2SO4 pretreatment of corn stover was performed in a steam explosion reactor at 160°C, 180°C, and 190°C, approx 1 wt% H2SO4, and 70 s to 840 s residence times. The combined severity (Log10 [Rq] — pH), an expression relating pH, temperature, and residence time of pretreatment, ranged from 1.8 to 2.4. Tucker and cowork­ers reported that soluble xylose yields varied from 63 to 77% of theoretical from pretreatments of corn stover at 160 and 180°C. However, yields > 90% of theoretical were found with dilute acid pretreatments at 190°C [54]. Furthermore, simultaneous sacchari­fication and fermentation (SSF) of washed solids from corn stover pretreated at 190°C, and using an enzyme loading of 15 filter paper units (FPU)/g of cellulose, gave ethanol yields in excess of 85%. Some of their results showing the effects of pretreatment temper­ature on SSF ethanol yield from exhaustively washed, pretreated residues are shown in Figure 5.6. [54]. The highest SSF ethanol yield

of 92% was obtained for corn stover samples treated at 190°C for 90 s in the presences of 1.1% sulfuric acid, and these samples were subjected to SSF with enzyme loading of 25 FPU/g cellulose.

There are a number of attractive features in steam explosion, and the most important one is the fact that it does not need any chemi­cals, as pure steam is the reagent, and it is a green technology with no or minimum environmental cost. Also, it does not result in exces­sive dilution of the resulting sugars; and it requires lower energy input compared to some other methods. Steam explosion can be carried out in both batch and continuous systems. Batch reactors are cheap, versatile and simple to manage. They are usually used on lab scale to define the process and investigate the effects of the process on different feed stocks. The main disadvantages of steam explosion are incomplete destruction of lignin-carbohydrate matrix resulting in the risk of condensation and precipitation of soluble lignin components making the biomass less digestible. Another disadvantage is as the steam explosion happens in very high pres­sures, strong stainless steel vessels are required for the procedure, and this may be an important cost factor in building a large-scale operation. Additionally is the partial depolymerization of the xylan in hemicellulose and possible generation of fermentation inhibitors such as furfural at higher temperatures and the need to wash the hydrolyzate. This may decrease overall saccharification yields by 20-25% of initial dry matter due to removal of soluble sugars.

This technique is particularly effective for the pretreatment of hardwoods and agricultural residues, but less effective for soft­woods. In the case of softwoods, use of an acid catalyst with the steam pretreatment is especially important. Steam explosion tech­nique is nearing the commercialization phase and has been tested on a pilot scale at the NREL pilot plant in Golden, Colorado, USA, the SEKAB pilot plant in Sweden, the Italian steam explosion pro­gram at the Trisaia Centre in Southern Italy, and by a demonstra­tion-scale Iogen ethanol plant in Ottawa, Canada.

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