Integration of solar energy into a superheated steam process stream for the pre-treatment of lignocellulosic biomass
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Abstract
The conversion of lignocellulosic wheat straw to bioethanol was found to bene5it from pre- treatment using superheated steam (SS). A 2-step pre-treatment method using pressurized hot water at 119 ̊C followed by SS at 220 ̊C yielded the best results, with the most pronounced effect on enzymatic sugar liberation occurring after 2 minutes of SS treatment. The net sugar conversion ef5iciencies were 46.4 and 53.5% for glucose and xylose, respectively. The glucose was recovered from the treated biomass, while the majority of the xylose was recovered from the supernatant of the hot water phase. With an energy demand of approximately 1998 kJ/kg of biomass, pre-treatment with SS consumes approximately 78% as much energy as steam explosion, and can produce ethanol at a comparable price, based on a preliminary economic assessment. Two technologies were examined for integration of solar energy into the SS process stream: evacuated tube solar collectors (ETSC) and parabolic trough solar collectors (PTSC). Utilization of ETSC was found not to be feasible for these applications due to low conversion ef5iciency (<12%) and progressive component failure at temperatures in excess of 175 ̊C. More promise was found with PTSC, which had higher conversion ef5iciencies of 36.4 and 55.8% at operating temperatures of 220 and 150 ̊C, respectively. Experimental results were based on numerical simulation, and indicate that on a large scale, a PTSC facility is capable of generating energy at a rate competitive with conventional energy sources (<$0.021/kWh), though the capital expenditures are substantial, if not prohibitive.