Systematic cost evaluations of biological and thermochemical processes for ethanol production from biomass residues and industrial off-gases

•Compared ethanol bio-refineries using sugarcane, invasive alien plants and off-gas.•Cost-effectiveness determined via evaluation of minimum ethanol selling price.•Ethanol production at sugar-mill had lowest minimum selling price of 0.52 US$/l.•Production via hydrolysis (0.72 US$/l) more cost-effective than via gasification.•Low ethanol yield a disincentive for production in energy self-sufficiency scenario. Bioethanol, as a sustainable fuel or as a precursor for high-value chemicals, may be produced through a variety process technologies, depending on the feedstock available. Thus, these processes are comprehensively evaluated to identify the most cost-effective process for a given feedstock, as well as cost reduction measures. Process designs for first (1G), second (2G) and third (3G) generation ethanol production were simulated in Aspen Plus® for technical evaluations, followed by economic modelling to determine the minimum ethanol selling price (MESP). 1G-ethanol from A-molasses at sugar mills was most cost-effective (MESP = 0.52US$/l). While the 1G-2G option (using sugarcane lignocelluloses) produced 98% more ethanol (MESP = 0.62US$/l), costs of enzymes were inhibitive. For 2G-ethanol using invasive alien plants, hydrolysis-fermentation had the lower MESP (0.72 US$/l) compared to gasification-fermentation because the latter had comparatively high capital costs (i.e.215vs135 million US$). Energy self-sufficiency for 3G ethanol from off-gas decreased the ethanol yield (178 to 114 l/tonne) and increased the MESPs (0.61 to 0.87 US$/l) compared to the buy-in of the required electricity. Similarly, for the gasification-fermentation scenario, a MESP of 1.58US$/l was required to obtain energy self-sufficiency. Energy self-sufficiency therefore increases process costs, but simultaneously abates the potential use of fossil fuels. Thus, it is worth pursing renewable electricity to meet process energy needs to improve ethanol yields and lower production costs. Furthermore, fermentation processes should be tailored to tolerate lower water quality requirements.