Techno-economic analysis and environmental impact assessment of lignocellulosic lactic acid production

Abstract

Lignocellulosic Lactic acid (LA) production is considered for the diversification and value addition to an existing sugar industry by annexing a biorefinery using lignocellulosic residues, sugarcane bagasse and leaves (SCBL). In biorefinery, the hemicellulose (C5 sugars) and cellulose (C6 sugars), resulting from lignocellulosic matrix fractionation, can be converted into building block chemicals and fuels by biotechnological or chemical pathways. To co-produce LA with other fuels and chemicals from SCBL, it is important to determine whether LA production from C5 or C6 sugars should be preferred, based on economic viability and environmental impacts. In this paper we developed six process scenarios for LA production from either the hemicellulose liquid fraction and cellu-lignin solid fractions of SCBL after steam explosion pretreatment, based on 200tonne/day of SCBL feed. The lignocellulose fraction not used for LA production will be used for integrated production of other marketable products in a biorefinery. Scenarios S1, S3, & S5 use the hemicellulose liquid fraction as feed for LA production, while scenarios S2, S4, & S6 use the cellu-lignin solid fraction. Two scenarios, S1 & S2, use Ca(OH)2 as neutralizer in fermentation, resulting in co-production of gypsum as solid waste, scenarios S3 and S4 use an isolated acid-tolerant thermophilic bacteria, Bacillus coagulans isolates for fermentation, thus avoiding the need for neutralization, while Scenarios S5 and S6 use Mg(OH)2 as neutralizer in fermentation, with recycling of Mg(OH)2. LA was separated and purified from the fermentation broth by reactive distillation. The six process scenarios were modeled and simulated by Aspen Plus® with the resulting mass & energy balance and equipment-sizing used as inputs to economic analysis and environmental impact assessment. Standard life cycle assessment (LCA) method is used in SimaPro® for environmental impact quantification. Cellulose based processes have larger LA production rates than the hemicellulose based processes, for the same flow rate of SCBL, which impacts on their total capital investment (TCI), operating cost (OPEX) and revenues for the same process technology. Increases of 7.08%, 58.13%, 12.04% and 60% in TCI, revenue, OPEX and LA production rates, respectively were observed in moving from S1 to S2. Similar increases of 10.88%, 86.52%, 18.52% and 91.07% in TCI, revenue, OPEX and LA production respectively in moving from S3 to S4. An increment of 10.08%, 61.57%, 18.32% and 63.90% in TCI, revenue, OPEX and LA production respectively was also found in moving from S5 to S6. Scenarios S3 and S4 are found to have lower TCI and OPEX but their production rate is low as well making them economically less attractive with internal rate of return (IRR) of 9.13% and 15.23% respectively. While S1 and S2 have IRR of 13.11% and 19.4% respectively and yet higher TCI and OPEX. S5 and S6 have the highest IRR, 14.72% and 21.28% respectively with TCI and OPEX of way below their equivalent scenarios S1 and S2 respectively. From the environmental point of view S1 and S2 are found to be environmentally less friendly with higher environmental burdens in almost all impact categories considered. Gypsum free scenarios, S5 and S6, are found to be the most attractive processes from both economic and environmental points of view, with S6 being superior than S5.