Pretreatment of corn stover
In the aqueous-phase saccharification-fermentation route, all biomass forms require a pretreatment step as a preparation for the enzymatic saccharification step. There are numerous published studies on pretreatment of corn stover using a variety of pretreatment methods. These pretreatment methods include aqueous acid [18-20,3,21,22], ammonia [23-27], hydrothermal [28-30], steam [31-34], lime [35-37], microbial [38, 39], supercritical carbon dioxide [40], lactic acid and acetic acid [41], and ferrous sulfate [42].
The most commonly studied, as well as used, chemical pretreatment processes are based on dilute acid, lime, aqueous ammonia steeping followed by dilute acid hydrolysis, and sodium hydroxide. These four methods were recently evaluated to provide comparative performance data [18]. In this study a correlation was established between lignin removal and enzymatic digestibility of pretreated corn stover. Compared with the other three pretreatments, pretreatment of corn stover with 2% aqueous NaOH substantially increased the lignin removal and enhanced the accessibility and digestibility of cellulose. The hydrolysis yield of NaOH-pretreated corn stover reached 81.2% by 48 h at 8.0% substrate concentration and at a cel — lulase dosage of 20 FPU g-1 substrate. Chemical analysis showed that the enzymatic hydrolyzate from NaOH-pretreated corn stover contained higher content of fermentable sugars and less inhibitors, which is suitable for subsequent fermentation process to produce ethanol. A comparison of various pretreatments followed by enzymatic hydrolysis applied to corn stover and the resulting total sugar yields is shown in Table 3.5.
In the processing of corn stover, simultaneous saccharification and fermentation (SSF) is a viable alternative to the stepwise saccharification and fermentation process. In an interesting comparison study [43] these two different process configurations, simultaneous saccharification and fermentation (SSF) and separate hydrolysis and fermentation (SHF) have been compared at 8% water insoluble solids (WIS), regarding ethanol production from steam pretreated corn stover. The enzymatic loading in these experiments was 10 FPU/g WIS and the yeast concentration in SSF was 1 g/L (dry weight) of a Saccharomyces cerevisiae strain. In their experiments, the whole slurry from the pretreatment stage was used as it was diluted to 8% WIS with water and pH adjusted; SSF gave a 13% higher overall ethanol yield than SHF (72.4% versus 59.1% of the theoretical). The impact
of the inhibitory compounds in the liquid fraction of the pretreated slurry was shown to affect SSF and SHF in different ways. The overall ethanol yield (based on the untreated raw material) decreased when SSF was run in the absence of inhibitors compared to SSF with inhibitors present. On the contrary, the presence of inhibitors decreased the overall ethanol yield in the case of SHF. However, the SHF yield achieves in the absence of inhibitors was still lower than the SSF yield achieves with inhibitors present.
In 2010 Kazi et al. published [44] their techno-economic comparison of process technologies for ethanol production from corn stover. In this study, four pretreatment technologies (dilute acid, two-stage dilute acid, hot water, and ammonia fiber explosion or AFEX), and three downstream process variations (pervaporation, separate 5-carbon and 6-carbon sugars fermentation, and on-site enzyme production) were analyzed. Each of these scenarios was modeled and economic analysis was performed for an "nth plant" (a plant with the same technologies that have been employed in previous commercial plants) to estimate the total capital investment (TCI) and product value (PV). PV is the ethanol production cost, including a 10% return on investment. Sensitivity analysis has been performed to assess the impact of process variations and economic parameters on the PV. The dilute-acid pretreatment process has the lowest PV among all process scenarios, which is estimated to be $1.36/L of gasoline equivalent [LGE] ($5.13/gal of gasoline equivalent [GGE]). Sensitivity analysis showed that the PV is most sensitive to feedstock cost, enzyme cost, and installed equipment costs. A significant fraction of capital cost is related to producing heat and power from lignin in the biomass. This type of corn-stover-based cellulosic ethanol production has yet to be commercialized. Hence, a pioneer plant is expected to be more costly to build and operate than an nth plant. To assess the impact of technological maturity on pioneer plant cost, a cost growth analysis was also performed by Kazi et al. [44]. As expected, the estimated PV for the pioneer plant is substantially larger for the nth plant. The PV for the pioneer plant model with dilute-acid pretreatment was $2.30/LGE ($8.72/GGE) for the most probable scenario, and the estimated total investment was more than double the nth plant. This 2010 techno-economic analysis is somewhat discouraging to investors. However, steady advances in pretreatment, enzyme technologies, and ethanol processing methods are needed to make the PV of corn stover ethanol competitive with gasoline.