Abstract:
Torrefaction has been reported to reduce the costs associated with biomass logistics (i.e., storage, transportation), because the torrefaction process reduces the moisture content of the biomass while improving the energy density, hydrophobicity, and grindability. In the case of biofuel production via enzymatic hydrolysis of lignocellulose, these effects can benefit the overall supply chain, but it is necessary to determine how torrefaction later impacts end use of the biomass. Prior work has explored the impact of torrefaction on glucose yield of wheat straw, as well as the role of alkaline pretreatment in impacting yield. However, a need exists to investigate the manner in which torrefaction affects the yield of glucose from biomass. Also, information on the economic performance of the combination of alkaline pretreatment and torrefaction on wheat straw is not currently available in the literature.
The goal of this study is, therefore, to characterize a system for glucose production from torrefied wheat straw via technoeconomic analysis, after evaluating the impacts of torrefaction and alkaline pretreatment.
In the first phase of this study the interactions of torrefaction and alkaline pretreatment of wheat straw on glucose yields from enzymatic hydrolysis was investigated. The order of torrefaction relative to alkaline pretreatment, torrefaction severity, and alkaline concentration were varied, and both glucose yield and pH were measured. Results indicate that, across a range of torrefaction severities with temperatures of 200, 220, and 240°C and durations of 20, 40, and 60 minutes, torrefaction has a negative impact on glucose yield of wheat straw with or without alkaline pretreatment. Compared to raw biomass, the glucose yield decreased by 86% and 96%, when torrefaction was carried out for 60 min at 200 and 240°C, respectively. Conversely, alkaline pretreatment has a positive impact on glucose yield. Compared to torrefied only biomass, the glucose yield increased by 915%, and 136% for alkaline pretreated and torrefied biomass when torrefaction was subsequently carried out for 60 min at 200 and 240°C, respectively. Alkaline pretreatment after torrefaction results in higher glucose yield than alkaline pretreatment before torrefaction, or torrefaction alone. A decreasing trend in pH with increasing torrefaction severity occurs for unbuffered samples, suggesting that torrefaction-induced chemical changes causing the decrease in pH might be associated with the reduced glucose yield.
In the second phase of study, fibrous cellulose was used as a model material for studying the various physical, chemical, and structural changes that occur during torrefaction and alkaline pretreatment. This included TG-MS, FT-IR, NMR, and Surface Area measurements, as well as tensile strength and fluid uptake capacity tests. Torrefaction reduces the enzymatic hydrolysis of fibrous cellulose similar to its impact on lignocellulose. Treatment with alkaline solution lessens that negative impact, again similar to the performance of lignocellulose. Glucose yield drops from 955 mg/g of substrate for raw filter paper to 690 mg/g of substrate for the same filter paper sample torrefied at 200°C. Glucose yield of the torrefied sample increases to 808 and 933 mg/g of substrate with 1% and 10% alkaline treatment, respectively. The wet-tensile strength of cellulose filter paper increases with torrefaction. The uptake of oil, which is a non-polar molecule, is not impacted by torrefaction whereas the water uptake percentage is positively correlated to glucose yield, which suggests that water-substrate interaction may be a driver of the hydrolysis process. One of the major reactions during torrefaction was found to be chemical elimination of water. FTIR spectra suggest that torrefaction induces a slight disruption of the intrachain hydrogen bond network between adjacent glucan units. It is likely that cellulose’s ability to bond to water is affected in torrefied samples as evidenced by the change in the OH stretching region of cellulose in FTIR spectra. An inhibitory compound may also be responsible for some of the effects of torrefaction on enzymatic hydrolysis yield. The compound likely has a carbonyl group as there is emergence of a peak at 1720 cm-1 in the spectrum of torrefied cellulose which was originally not present in the raw cellulose. FTIR results and solution H-NMR results show evidence of the removal of a compound by alkaline pretreatment. As torrefaction severity increases, surface area decreases. Hence, decreased hydrolysis efficiency may be a combined function of surface area, surface composition and chemistry, and H-bonding network changes.
In the third phase of this study, Techno-Economic Analysis (TEA) was carried out to simulate commercial scale (700,833 dry Mg/year) ethanol production from wheat straw, either from raw and alkaline pretreated wheat straw (System R-AP), or from torrefied and alkaline pretreated wheat straw (System T-AP). A discrete-unit process model was developed using SuperPro software (SuperPro Designer, Intelligen, Inc.) simulating the production of bioethanol from raw or torrefied wheat straw. The system boundary for the TEA in this study encompasses all steps from acquisition of wheat straw (farm gate) to production of ethanol. Results indicate that Net Present Value (NPV) for the base case T-AP system is 37% lower than for the R-AP system. However, research indicates that utilizing torrefied biomass can be a profitable choice in areas where electricity prices exceed $0.14/kW-h or if the roundtrip distance to the biorefinery is =188 km from the wheat straw procurement area. Also, profitability of T-AP system can be improved by increasing the alkaline concentration for pretreatment. Increasing NaOH concentration (from 1% to 2%) increased the NPV for T-AP system by 2% whereas, it decreased the NPV by 45% for R-AP system.
These findings indicate that there is a scientific basis and a commercial potential to deploy a T-AP system under certain conditions. Further research regarding the development or discovery of a pretreatment chemical or technology that is more efficient in improving the water-substrate interaction of torrefied biomass would amplify the potential of torrefied biomass as a feedstock for bioethanol.
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