Бурение грунтовых зондов, установка энергетических колодцев

Detoxification of lignocellulosic hydrolyzate is an integral part of the cellulosic ethanol process, because during pretreatment and hydrolysis steps, a considerable amount of compounds that can inhibit the subsequent fermentation are formed in addition to the fermentable sugars. The detoxification step is applied to remove or reduce the concentrations of these undesirable compounds to tolerable levels before biological processes. In the separate hydro­lysis and fermentation (SHF) method, detoxification can be applied before exposure to cellulase enzymes, or in the case of direct acid hydrolysis type saccharification, detoxification is applied before the fermentation step. In the simultaneous saccharification and fer­mentation (SSF) technique detoxification is required before expo­sure to an enzyme cocktail affecting hydrolysis and fermentation. These inhibitory substances are produced in four different ways in pretreatments or exposure to acids [1,2].

1. As a result of hydrolysis of the extractive components, organic or sugar acids, saponification of esters to acids like acetic, formic, glucuronic, galacturonic acids, and solubilization of phenolic derivatives.

2. Degradation of soluble sugars to furfural, 5-hydroxy- methylfurfural (HMF) and further degradation of these substances to levulinic acid, formic and acetic acid.

3. Degradation of lignin to cinnamaldehyde, p-hydroxy- benzaldehyde, syringaldehyde, and related compounds.

4. Solubilization of metal ions from biomass.

Depending on the type of pretreatment and hydrolysis pro­cess employed, concentrations of these inhibitory substances can be varied in a wider range. Consequently, a variety of detoxifica­tion methods have been developed depending on the downstream requirements. As pointed out by Palmqvist and Hahn-Hagerdal, these methods cannot be directly compared because they vary in the neutralization degree of the inhibitors [3]. In addition, the fer­menting microorganisms have different tolerances to the inhibitors. The main features of the detoxification methods employed for lig — nocellulosic ethanol production and selected examples are summa­rized in Table 8. 1.

Detoxification methods can be divided into three groups: physi­cal, chemical and biochemical methods, as shown in Table 8.1. In the model processes developed at National Renewable Energy Laboratory (NREL), ionic exchange and adding excess of Ca(OH)2 or over-liming have been proposed as detoxification methods. The calcium hydroxide method is especially useful in the case of dilute — acid hydrolyzates, where furan aldehydes and phenolic compounds are formed and can be efficiently removed leading to great improve­ment in fermentability [9]. Detoxification with calcium hydroxide (over-liming) has shown better results than treatment with sodium or potassium hydroxide, but this difference has not been under­stood. Martinez and coworkers have performed experimental opti­mization of the amount of lime needed in the detoxification, which depends on the content of acids in each hydrolyzate [10]. In this study they developed a method for predicting the optimal addi­tion dosage based on the titration of hydrolyzate with standard­ized NaOH. Persson et al. indicate that the positive effects of alkali

treatment cannot be completely explained by the removal of inhibi­tors, and this method could have possible stimulatory effects on fermenting microorganisms as well [9].

In addition to the single-step methods, there are diverse and multi­step detoxification methods which include: neutralization with lime followed by the addition of activated carbon, and filtration for ace­tic acid removal; partial removal of acetic acid, furfural and soluble lignin by molecular sieves; vapor stripping for removal of volatile inhibitors, and; adsorption using activated carbon, addition of diato — mite, bentonite and zeolite after neutralization or over liming [11,6]. Biological detoxification is an attractive alternative to physical and chemical methods. Khiyami et al. have proposed a biofilm method [8], which uses a mixed culture of aerobic bacteria cells naturally immo­bilized on a plastic support. In this way, the biofilm-associated cells are more resistant to the toxic substances released during the biomass pretreatment. The use of extraction with supercritical fluids has also been tested as a detoxification method for wood hydrolyzates [12].

Most of the studies on the effects of toxic compounds on growth and ethanol production have been performed for common Baker’s yeast Saccharomyces cerevisiae and xylose-fermenting yeast. Palmqvist et al. carried out extensive experiments for assessing the effect of acetic acid, furfural and p-hydroxybenzoic acid on growth and ethanol productivity of S. cerevisiae and C. shehatae through full factorial design [13]. Oliva et al. performed a study of the effect of compounds released during the pretreatment of poplar biomass by steam explosion for the thermotolerant yeast Kluyveromyces marxianus, showing that growth is more affected than ethanol pro­duction, and this microorganism exhibits an important resistance to aromatic compounds [14,15]. Additionally, Zaldivar et al. have investigated recombinant microorganisms regarding their toler­ance capacity for fermenting lignocellulosic hydrolyzates contain­ing common inhibitors [16,17].

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