Integration of second generation biofuel production into existing industrial processes for short term commercial implementation

[摘要] ENGLISH ABSTRACT: Since the production of second-generation biofuel (SGB) from lignocellulosic plant biomass is only economically attractive if driven by government incentives, stakeholders are reluctant to commercialise the product despite its potential to mitigate global warming and socio-economic conditions. Integrating SGB processes with the facilities of biomass-based industries could reduce the production costs of SGB through pre-existing services and infrastructure. Integrating SGB, however, is technically viable only if available biomass residues are used effectively to co-produce fuel while maintaining the overall energy balance, financially viable only if it attracts private investment without governmental assistance and environmentally viable only if it reduces the carbon imprint of a fossil-intensive host industry.In this dissertation, novel scenarios for integrating SGB with the South African raw sugar and the pulp and paper industries (RSI and P&PI, respectively) were explored. Focus was on ethanol production based on fermenting hemicellulose substrates, potentially available in both industries, and on SGB production through gasification-synthesis processes in the contextual representatives of these industries. This was accomplished through flow-sheet analysis in Aspen Plus® using process simulations constructed from protocols in published literature and experimental data.In respect of RSI, integrating bioethanol production with electricity from sugarcane bagasse and harvesting residues were deemed both technically and economically viable and competitive against the exclusive generation of electricity. The necessity of Pinch Point Analysis was established through flow-sheet analysis, which had also shown the synergistic interaction of technologies in various processing stages, such as the variants in ethanol distillation technologies and heat and power production technologies. In respect of P&PI, represented by sulphite mills, ethanol production from spent sulphite liquor (SSL) pulping residue was deemed economically viable if the SSL fermentation substrate was concentrated. To attain net reduction of greenhouse gas emissions for the integrated ethanol-sulphite facility, it was essential to provide all process energy requirements from supplementary biomass sources rather than coal.In respect of RSI, integrating methanol or Fischer-Tropsch syncrude via gasification synthesis was deemed not feasible at the current state of efficiency with which sugar mills are operating. In respect of P&PI, combining synthetic fuel production with bioethanol production at a sulphite mill improved economic potential, since disposal costs were negated through the use of waste biomass for synthesis processes and the yield of valuable products was enhanced on a small scale. In respect of both RSI and P&PI, however, integrating gasification-synthesis processes required the statistical optimisations of flow sheets to arrive at the optimum operating parameters for competing technologies for syngas production. In these contexts, syngas production based on optimised allothermal gasification had lower costs than optimised autothermal gasification.To validate the process concepts developed in this thesis, it is firstly recommended that robust and recombinant microbial strains be readily available to ferment pentose-rich substrates, such as SSL and hemicellulose hydrolysates. Secondly, the effect of the chemical alteration of SSL on the recovery performance of process chemicals at sulphite mills should be examined and, thirdly, the catalytic gasification of biomass should be developed and demonstrated on pilot and pre-commercial scales.

[发布机构] Stellenbosch University