Techno-economic analysis and life cycle assessment for production of biofuels from spent coffee grounds
Authors:   , Kisiga Wilberforce 
Published: September 2024

Spent Coffee Grounds (SCGs) are one of the most abundant agro-industrial residues generated from the coffee brewing industry and coffee espresso machines in restaurants, cafeterias, cafes and homes. It is believed that for every ton of coffee beans processed, 650 kg of SCG is left as solid residues. Coffee being the second traded commodity after petroleum, means that a lot of SCGs are generated annually and end up into landfills. Efforts are being made to turn this valuable waste into biofuels, however, most of these efforts end up at laboratory benches and few studies have focused on industrial scale production of biofuels from SCG. Six biomass-to-energy conversion technologies were compared from technical, economic and environmental perspectives: Fast pyrolysis, Hydrothermal Liquefaction (HTL), gasification, Anaerobic Digestion (AD), fermentation and biodiesel production. The processing technologies were selected because they are the most researched biomass-to-fuel conversion routes. Each of the processing routes was simulated in Aspen plus V11 using input data from literature. The mass and energy balances obtained from simulations were used to conduct Techno-Economic Analyses (TEAs) and Life Cycle Assessments (LCAs). TEA was conducted with help of Aspen Process Economic Analyzer (APEA) and Microsoft Excel spreadsheets whereas OpenLCA V1.11.0 software was employed for LCA. After the processing routes were successfully simulated, APEA was used to estimate the installed Cost of all Equipment (COE). The Capital Expenditure (CAPEX) required to build the biorefineries was then estimated basing on COE for each biorefinery. Then the Operating Expenses (OPEX) required for running the day-to-day operations of the plant were estimated as the sum of Variable Operating Expenses (VOC) and Fixed Operating Expenses (FOC). The revenues from the sales of finished products were estimated and used to calculate the gross profit. For the plant life of 25 years; using straight-line depreciation of 10% per year, discount rate of 12% and tax rate of 28%, the Discounted Cash Flow Analysis (DCFA) was used to calculate the economic indicators i.e. the Net Present Value (NPV), Profitability Index (PI), Internal Rate of Return (IRR) and Discounted Payback Period (DPBP). For LCA, the methodology outlined by the ISO 14040/44 framework was used. The method outlines four steps followed to conduct LCA i.e. goal and cope definition, Life Cycle Inventory (LCI), Life Cycle Impact Assessment (LCIA) and interpretation of results. The goal of this study was to identify the processing route with least environmental impacts and the cradle-to-gate system boundary was selected. LCI was conducted using the mass and energy balances obtained from Aspen plus simulation and the flows present in the Agribalyse Version 3 database, downloaded from OpenLCA nexus. LCIA was conducted using the ReCiPe 2016 Midpoint (H) and was also downloaded from OpenLCA nexus. Eight impact categories namely, global warming, fossil resource scarcity, particulate matter formation, terrestrial acidification, freshwater eutrophication, marine eutrophication, mineral resource scarcity and water consumption were selected. The results were analysed to identify the conversion route with less environmental effects. Results from the economic analysis showed that fast pyrolysis was the most economically profitable processing route with a NPV, PI, DPBP and IRR of 6.3 million USD, 1.85, 5.4 years and 37%, respectively. In the second position was biogas production with a NPV, PI, DPBP and IRR of 3.4 million USD, 1.65, 5.7 years and 34%, respectively. Gasification was in the third position with a NPV, PI, DPBP and IRR of 5.4 million USD, 1.48, 6.0 years and 32%, respectively. In the fourth position was biodiesel production with a NPV, PI, DPBP and IRR of 3.9 million USD, 0.86, 8.0 years and 24%, respectively. HTL was in the fifth position with a NPV, PI, DPBP and IRR of 0.68 million USD, 0.29, 13.0 years and 16%, respectively. Bioethanol production was not economically profitable as the revenues generated from sales of finished products were smaller than the operating expenses, thus no profit could be generated. Results from environmental impact assessment showed that fast pyrolysis was the most environmentally friendly processing route, followed by biogas production, biodiesel production, gasification, and bioethanol production, whereas HTL had the highest environmental impacts. Electricity consumption was the biggest contributor to the environmental impacts, making HTL, which was the highest electricity consuming processing route, to be the worst environmentally. However, biogas production was the least electricity consuming processing route but not the best environmentally due to large production of carbon dioxide and methane (biogas) from anaerobic digestion. The large production of carbon dioxide can be mitigated through using it to grow algae or in supercritical carbon dioxide extraction of lipids. However, the cost associated with additional unit processes can escalate the biogas production costs. These greenhouse gases were the biggest contributors of global warming, pushing biogas production to the second position after pyrolysis.Fast pyrolysis was proposed to be the best environmentally and economically feasible processing route for the production of biofuels from SCG.