Sustainable production of fuel bioethanol from switchgrass in Uruguay
Sustainability concerns due to long-term depletion of fossil fuels and climate change are responsible for a renewed interest on biofuels and biorefineries. Fuel bioethanol produced from lignocellulosic materials using modern technology could lead to high greenhouse gases (GHG) emissions savings. Bio...
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| Format: | doctoralThesis |
| Language: | English |
| Published: |
2018
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| Subjects: | |
| Online Access: | https://hdl.handle.net/20.500.12008/28054 |
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| Summary: | Sustainability concerns due to long-term depletion of fossil fuels and climate change are responsible for a renewed interest on biofuels and biorefineries. Fuel bioethanol produced from lignocellulosic materials using modern technology could lead to high greenhouse gases (GHG) emissions savings. Biorefineries integrate the production of materials, chemicals, fuels, and energy. This could maximize the value obtained from biomass and minimize environmental impacts. Switchgrass (Panicum virgatum L.) is considered a good source of biomass because of its high productivity, longevity, high efficiency in water and nutrient use, and low production cost. Although several works have studied bioethanol production from switchgrass, a complete analysis of techno-economic and environmental sustainability for the current technology and conditions in Uruguay is necessary to promote the sustainable national production of bioethanol. In this work, switchgrass was evaluated as a feedstock for the production of bioethanol in a biorefinery located in Uruguay using a liquid hot water (LHW) pretreatment. Material and energy use was determined for different scenarios and process configurations through process modeling. Material and energy results were used in a techno-economic model to analyze the effect of different parameters and configurations on the economics of the process. The minimum ethanol selling price (MESP) obtained for ethanol in a facility producing only ethanol and electricity was within the expected price range for advanced alcohol fuels and could compete with oil prices above 100 $/ barrel. Working on a biorefinery scenario producing furfural, acetic acid, and formic acid as high-value co-products, decreased the MESP. The MESP was sensitive to plant size and to switchgrass composition. Enzyme dosage, solids content, and hydrolysis and fermentation efficiencies are the operating parameters with higher impact on MESP, experimental information on how they are related (e.g. efficiency vs solids content) is necessary for more reliable assessments. Experimental assays were performed to evaluate the cellulose enzymatic hydrolysis of LHW pretreated switchgrass at high solids content. LHW pretreatment (200ºC, 5 min) proved to be a suitable alternative for a biorefinery approach. It was found that the washing of solids and initial pH had a significant effect on hydrolysis efficiency. The effect of solids content, enzyme dosage, and partial cellulase substitution by xylanase, were studied experimentally. Glucose concentration and hydrolysis efficiency were significantly affected by solids content and enzyme dosage. Very high glucose concentrations (189 g/L) were achieved. High hydrolysis efficiencies were found even for high solids content (>90% for 25% solids content) but only for high enzyme dosage (40-70 mgprotein/gglucan). Experimental results were combined with the process and techno-economic models. Maximizing glucose concentration or hydrolysis efficiency did not directly correlate to minimizing the MESP. Enzyme dosage and solids content had a significant effect on MESP and it was found that an enzyme dosage of 37 mgprotein/gglucan and a solids content of 21 %, minimized MESP. A life cycle assessment (LCA) was performed to evaluate GHG emissions and non-renewable fossil energy consumption associated with the production of fuel bioethanol in Uruguay using results from material and energy balances previously obtained. GHG emissions for bioethanol produced in all the scenarios analyzed were lower than the reference emissions for fossil fuel. The biorefinery scenario was better than the ethanol and electricity facility in terms of the environmental impacts and the biofuel produced there could meet GHG reduction requirements. All the factors analyzed (switchgrass composition, enzyme dosage, fermentation and hydrolysis efficiency and solids content) had a significant effect on the environmental performance of fuel bioethanol, enzyme use being the most significant factor. When compared with other works for Uruguay, the ethanol from the scenario with only electricity as co-product had a worst environmental performance than ethanol from sugarcane and sorghum grain. However, the ethanol from the biorefinery scenario performed better. Otherscenarios analyzed (e.g. low enzyme dosage) also had a good environmental performance. Optimal conditions for both economics and GHG emissions were found from models based on experimental data. These conditions (21 % solids w/w, 37 mgprotein/gglucan) had a good environmental performance (well to tank: -68 5 gCO2eq/MJethanol, GHG emissions) and good process economics (MESP of 0.84 $/L). Therefore, environmentally sustainable production of ethanol from switchgrass on a biorefinery located in Uruguay (in terms of GHG emissions and fossil energy use) could be possible with the technology and yields currently available. Economic sustainability for current technology and yields depends on oil prices and/or policies (carbon taxes). Scale-up of the experimental results obtained and appropriated industrial equipment are critical aspects of the technical feasibility. |
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