Scale-up of Ethanol Production From Sweet Sorghum Stems Using Advanced Solid-State Fermentation Technology

Compared with conventional submerged liquid fermentation (SLF), solid-state fermentation (SSF) can considerably reduce the process costs of substrate pretreatment and water usage, avoid the pollution problems resulted from the disposal of liquid residues. However, due to the inherent barriers of heat and mass transfer, SSF has not yet proved to be feasible in large-scale ethanol production. Accordingly, Tsinghua University had developed and operated a process of advanced solid-state fermentation (ASSF) at 127 m³ scale using fermentable sugars from sweet sorghum stems in Inner Mongolia, China, in 2010. The field trial achieved high ethanol productivity that were comparable to conventional SLF, indicating that mass ethanol production in SSF with lower costs and less pollution problems has the great potential for its industrial application. In this study, to further optimize the process of ASSF, an improved rotary-drum fermenter at 555 m³, the largest solid-state fermenter in the world so far, was constructed and deployed recently in Inner Mongolia, China, which was operated as continuous way for high process efficiency. During the process, the solid substrate and seed culture continue to be fed into the reactor and the fermented residues was conveyed immediately into distiller, making the whole process keep running without interval that improve the process efficiency to a great extent. Consequently, the continuous process achieved ~91% of theoretical ethanol yield that was parallel to 127 m³ scale in the batch fermentation. The retention time of solid substrate (fermentation time) was only less than 30 h. In the meantime, tech-economic analysis (TEA) from National Renewable Energy Laboratory (NREL) indicated that 16 tons of sweet sorghum stems (with 13.5 % sugar content and 70% moisture) can be used to produce 1 ton ethanol; the fermented residues can be used to feed 1 cattle; the fuel ethanol production cost is $615.4/ton (49cent/litter) and the sweet sorghum stem price is at $30/t. In this study, the results of continuous fermentation and distillation at 555 m³ scale are agree with the data from bench scale and pilot plant well. The TEA further demonstrates that ASSF is a cost-effective process that is highly feasible to be applied at industrial and commercial scale for fuel ethanol production, and eventually is able to improve the biofuel industry.