Techno-economic and sustainability assessment of solar-aided lignocellulosic biorefineries

Abstract

The aim of this thesis is to conduct a techno-economic and sustainability assessment of solar-aided biorefineries in South Africa. Previous studies on solar-aided lignocellulosic biorefineries are limited to concept illustration and biofuel cost estimation. Thus, a comprehensive assessment of the technical performance, environmental impact and manufacturing costs of potential co-products including lignin and biochar has never been conducted. In addition, although South Africa generates about 16 million metric tonnes of corn residues per annum, and possesses one of the highest solar irradiances in the world, a techno-economic study for solar-aided lignocellulosic biorefineries in South Africa is presently lacking. In order to address the above-mentioned knowledge gaps, the conversion of corn stover into ethanol was simulated. Various scenarios in which solar energy could be added to the biorefinery to generate process steam and electricity were evaluated. Also, a novel gasification configuration allowing the integration of solar energy as a heat source for gasification chemical reactions was modelled and studied. Further, processes enabling the conversion of corn stover into methanol via gasification and CO2 hydrogenation were developed. The techno-economic and environmental performance of all modelled scenarios was then assessed. The results obtained revealed that by the incorporation of solar energy into a corn stover-to-ethanol biorefinery combined with the export of lignin as a co-product could enhance the biorefinery’s overall energy conversion efficiency from 34.3 to 53 – 77%. Moreover, for the standalone scenarios, the minimum ethanol selling price (MESP) was 0.61 USD/litre (25.88 USD/GJ), which increased to 43.17 – 68.84 USD/GJ in the solar-aided scenarios. Lignin's minimum selling price was found to vary between 1.63 USD/kg to 3.55 USD/kg depending on the mode of solar energy integration. The incorporation of solar energy combined with export of lignin resulted in an overall potential environmental impact (PEI/hr) 14 to 46% lower. Also, in the solar-aided scenarios, up to 2.06 km2 of land was required by the biorefinery.

In the corn stover-to-methanol biorefinery, the export of char was found to improve the overall conversion efficiency from 46.1 to 61.5%. Further, the introduction of a solar-aided gasifier enhanced net gasification efficiency by 56 to 87%. The minimum methanol selling price (MMSP) for the standalone and solar-aided configurations were respectively found to be 0.31 USD/litre (17.47 USD/GJ) and 0.50 USD/litre (27.88 USD/GJ). While the biochar minimum selling price was estimated to be 13.04 USD/GJ (0.37 USD/kg). The overall PEI/hr for the solar-aided biorefinery was found to be 36% lower that the standalone configuration. Also, the integration of solar energy required a total land area of 6.92 km2, which is substantial. Data generated in this study are expected to serve as a building block for the future implementation of the circular bioeconomy production concept. The study is also expected to be value-added to the literature by providing data that could be used as a reference point for the efficient integration of solar energy into lignocellulosic biorefineries.