Two of the biggest problems that the world is facing are a dwindling supply of fossil fuels and global warming. To alleviate the problems associated with petroleum-based liquid transportation fuels, technology has been developed for commercial production of ethanol via fermentation of sugarcane juice and corn-based starch. Technologies for the production of ethanol from more abundant and renewable feedstock such as lignocellulosic biomass (in short, biomass) have been developed but have not reached the commercial implementation. Whereas the production of ethanol from sugarcane juice and corn-based starch is relatively simple, bioconversion of biomass is more complex. Before biomass can be hydrolyzed with commercial enzymes to release fermentable sugars for use in ethanol fermentation, it must go through a pretreatment step to remove some of the barriers imparted by lignin, thus opening up the fiber structure for enhancement of enzymatic activity [1
]. Techno-economic analysis (TEA) results of biomass bioconversion unequivocally indicated that pretreatment chemicals are the largest operating cost and also one of the largest cost components of the overall process [2
]. Therefore, in future biorefineries, the use of inexpensive and renewable reagents for biomass pretreatment is highly desirable.
Green liquor, which is an intermediate liquid stream containing Na2
S as the two major components plus other impurities in smaller quantities generated in a kraft pulp mill, can be used for pretreatment of both woody biomass [5
] and herbaceous biomass [7
]. Recently, it was demonstrated that near theoretical saccharification of sweet sorghum bagasse (SSB) pretreated with a simulated green liquor could be obtained after enzymatic hydrolysis [9
]. In this study, it was shown that the contribution of Na2
S to the efficiency of the pretreatment process was minimal. In other words, it was shown that a highly efficient pretreatment of SSB could possibly be achieved with a Na2
solution without Na2
S under certain conditions. It has been demonstrated that a Na2
solution could be produced by absorption of CO2
from an ethanol fermenter in a NaOH solution and subsequently used for dual purposes, i.e., for pH control and to provide the required carbonate in the fermentative production of succinic acid [10
]. Carbon dioxide is the major co-product of ethanol fermentation and also is one of the greenhouse gases that contribute to the increase in temperature of the atmosphere. Several countries, which include the United States, have developed energy and climate policies that give incentives to the capture and sequestration of CO2
from existing biorefineries. This practice could be valued under low-carbon fuels policy, biofuels mandates, supportive carbon capture and sequestration (CCS) policy, and other climate policy instruments [11
Considerable efforts have been made in the United States and other countries to develop feedstocks other than corn and sugar cane for ethanol production. Among these, sweet sorghum has attracted considerable interest because of its many good characteristics such as rapid growth and high sugar accumulation, high biomass production potential, wide adaptability, drought resistance, lodging tolerance, and salinity resistance. The ability to withstand severe drought conditions and its high water usage efficiency make sweet sorghum a good ethanol feedstock suitable for cultivation in arid regions such as the southern US and many areas in Africa and Asia [12
]. Sweet sorghum juice (SSJ), which can be extracted from the stalks with the same equipment used for extraction of sugarcane juice, contains high levels of sugars that can be readily fermented to ethanol. The residual biomass, i.e., sweet sorghum bagasse (SSB), can also be used as a potential feedstock for ethanol production.
In the present investigation, the technical feasibility of capturing the CO2 produced in SSJ ethanol production by absorption in a NaOH solution and use of the resultant Na2CO3 solution for pretreatment of SSB is demonstrated. The potential use of the pretreated SSB together with SSJ for additional ethanol production also is investigated.
In the present study, a process has been developed for the production of ethanol from SSJ and SSB together in a single fermenter. The CO2 produced in SSJ fermentation was captured in a NaOH solution and the resultant Na2CO3 solution was used for pretreatment of the SSB to facilitate high-efficiency enzymatic hydrolysis of the pretreated material. In theory, the whole sorghum plant can be cut into small pieces and used for ethanol fermentation. However, from a process engineering point of view, the use of the whole plant suffers from several disadvantages. These disadvantages include: (1) Difficulty in maintaining uniform temperature profiles in a heterogeneous system to achieve the optimum metabolic activity of the yeast; (2) restricted access of the sugars in the unextracted juice to the yeast; and (3) significant quantities of sugars in the juice may be destroyed during the pretreatment process, which is required for high-efficiency enzymatic hydrolysis of the fibers. In the proposed process, instead of escaping to the atmosphere, the CO2 produced in the SSJ fermentation was captured and fixed as Na2CO3. The use of the resultant Na2CO3 for pretreatment of the SSB undoubtedly will generate a waste stream containing residual NaOH, Na2CO3, and lignin. This waste stream is characteristically similar to the waste waters generated in a Kraft paper mill but will have much lower strengths because the reagent used is Na2CO3, which will result in much lower lignin solubilization compared to strong NaOH solutions typically used in Kraft mills. Treatment of the waste stream generated in the proposed process, therefore, is expected to be much less expensive than paper mill wastewater treatment. Techno-economic analysis (TEA) is needed to verify this point. A TEA, however, is not within the scope of the present study.
The results shown in Table 1
indicated that 5 M NaOH solution was quite efficient for CO2
absorption. The absorption efficiency was determined to be 92.0%. With the apparatus used in the study, 63.0% of the CO2
produced from the ethanol fermenter was removed as Na2
. Since the absorption efficiency was quite high at 92.0%, it is anticipated that most of the rest of the CO2
produced probably could be removed in a second absorption column using 5 M NaOH solution. The feasibility of using the resultant Na2
solution for SSB pretreatment also was demonstrated. Under the conditions used for the pretreatment, 37.7% of lignin was removed. On the other hand, the pretreatment resulted in no loss of glucan and only 8.1% loss of xylan. The loss of arabinan was higher at 24.7%. However, arabinan normally is only a minor source of fermentable sugar due to its low content in most biomass feedstocks. The arabinan content in the SSB was only 2.3 wt% (Table 2
). The loss of arabinan, therefore, will not be a strong negative factor on the subsequent bioconversion of the pretreated biomass for production of fuels and value-added chemicals. In a previous study using simulated green liquor for pretreatment of SSB, under the conditions optimized for production of fermentable sugars by enzymatic hydrolysis, the retentions of glucan and the combined xylan + arabinan + galactan were 91.8 wt% and 76.7 wt%, respectively [9
]. The results obtained in the present study, therefore, compared quite favorably to those obtained with synthetic Na2
solution used in the present study actually was not pure Na2
but rather a mixture of 2.3 M Na2
and 0.4 M NaOH (1.6% w
NaOH). In the study by Cao et al. [15
], SSB was pretreated with 2% w
NaOH for 1 h at 121 °C, which was significantly lower than the temperature used in the present study. These investigators observed only 6.6% loss of glucan but 67.8% loss of hemicellulose. In their enzymatic hydrolysis using 2 wt% solid loading, which was lower than the solid loading used in the present study, 72.1% theoretical yield of glucose was observed. All these results clearly demonstrate the advantage of using the 2.3 M Na2
solution obtained by absorption of ethanol fermentation-derived CO2
in a 5 M NaOH solution compared to the use of pure NaOH for pretreatment of SSB.
The results of the ethanol fermentation using the pretreated SSB in SSJ (Table 3
) clearly demonstrated the high efficiency of the pretreatment using the Na2
solution obtained by absorption of the ethanol fermentation co-product CO2
in 5 M NaOH solution. As shown in Table 3
, the ethanol yield from glucan in the pretreated SSB was 81.7% of the theoretical value. Since there was no loss of glucan during the pretreatment process, this value also is true for glucan in the original SSB. The ethanol yield was calculated based on glucan because the yeast strain used in this study was only capable of metabolizing glucose but not the C5 sugars (xylose and arabinose). The yield of xylose was relatively low at 56.9% of the theoretical value based on the xylan content of the pretreated SSB. A xylose yield value of about 70% or higher probably will be more desirable. However, it should be noted that the fermentation was a simultaneous saccharification and fermentation (SSF) process. In other words, the hydrolysis of the pretreated SSB proceeded in the presence of ethanol, which has been shown to be a potential inhibitor of cellulases and hemicellulases [16
]. Higher xylose yield can possibly be achieved in a modified process that includes a biomass hydrolysis stage at the optimum temperatures of the cellulolytic enzymes (50–55 °C) for 24 h followed by lowering the temperature to 30–32 °C and inoculation with a yeast culture to start the SSF process. It is likely that higher ethanol yield from glucan can also be achieved in this modified process.