Ammonia-Based Pretreatments Method
There are few basic pretreatment techniques that use ammonia as the major component; the most popular ones are:
1. Ammonia fiber/freeze explosion (AFEX)
2. Ammonia recycle percolation (ARP)
3. Soaking aqueous ammonia (SAA)
5.1.1 Ammonia Fiber/Freeze Explosion (AFEX)
As a physicochemical process, ammonia fiber/freeze explosion (AFEX) is similar to steam explosion operating at high pressure, but it is conducted at relatively lower temperatures. In the ammonia fiber/freeze explosion (AFEX) pretreatment, biomass is treated with liquid anhydrous ammonia at moderate temperatures of about 60-100°C and high pressure of about 17-20 atmospheres for a short period of about five minutes. After this holding period, the vent valve is opened rapidly to explosively relieve the pressure. This rapid release in pressure causes evaporation of the ammonia, which is volatile at atmospheric pressure, and a concomitant sudden drop in temperature of the system occurs [70, 71]. In the first phase of process lignocellulosic biomass swells due to the chemical effect of ammonia under pressure, thus increasing the accessible surface area while decrystallizing cellulose. This process results in a phase change in the crystal structure of cellulose I to cellulose III [1]. Additionally, a small portion of hemicellulose is solubilized in the oligomeric form during ammonia fiber/freeze explosion (AFEX) pretreatment. The lignin distribution in biomass remains reasonably the same during ammonia fiber/freeze explosion (AFEX) pretreatment, but lignin structure is rigorously altered resulting in increased water-holding capacity and digestibility.
Ammonia fiber/freeze explosion (AFEX) technique is popular in the pretreatment of soft non-wood biomass forms, and some literature examples on these biomass forms are: corn stover [72-77], switchgrass [78, 75, 79], Bermuda grass [80, 81], and rice straw [82]. As evident from these recent publications, ammonia fiber/freeze explosion (AFEX) technique can significantly improve the saccharification rates of herbaceous plants, agricultural residues, and municipal solid waste. For example, roughly 90% of the theoretical amount of sugars could be hydrolyzed in ammonia fiber/freeze explosion (AFEX) pretreated Bermuda grass [80, 81].
Dale and coworkers have optimized the AFEX pretreatment conditions for corn stover [83]. The efficacy of the pretreatment was evaluated by measuring the ethanol yield produced after simultaneous saccharification fermentation (SSF) of the treated corn stover. The key SSF ethanol yield results of AFEX pretreated corn stover under different temperature and time conditions are shown in Table 5.5. According to this study, corn stover with 60% moisture content produced the highest ethanol yield of 96.3% when subjected to 5 min AFEX at 90°C, as shown in Table 5.5 [83].
The AFEX treatment has some unique, distinctive features and some advantages over other pretreatments, which are:
1. Nearly all of the ammonia can be recovered and reused, while the remaining ammonia serves as a nitrogen source for microbes in downstream processes.
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Table 5.5 Simultaneous saccharification fermentation (SSF) ethanol yield results of AFEX pretreated corn stover under different temperature and time conditions.
Cellulase loading: 15 FPU/g of glucan; all experiments are 168 hr of SSF; ammonia loading for all the runs: 1:1. |
2. There is no need for neutralization prior to the enzymatic hydrolysis of AFEX-treated biomass.
3. There is no wash stream in the process; dry matter recovery following the AFEX treatment is essentially 100%; AFEX is basically a dry to dry process. Treated biomass is stable for long periods and can be fed at very high solids loadings in enzymatic hydrolysis or fermentation process.
4. Cellulose and hemicellulose are well preserved in the AFEX process, with little or no degradation.
5. Enzymatic hydrolysis of AFEX-treated biomass produces clean sugar streams for subsequent fermentation process.