Environmental assessment of energy-driven biorefineries: the case of the coffee cut-stems (CCS) in Colombia

Abstract

Coffee cut-stems (CCS) are typically left in the field after coffee harvesting as fertilizer or used partially for cooking and drying of coffee grains. However, the energy contained in this residue is not completely exploited. For this reason, different applications for CCS have been considered to obtain innovative products. This work aims to evaluate the environmental impact of energy generation through two biorefinery systems using CCS as feedstock. The life cycle assessment (LCA) considers a cradle-to-gate approach, beginning at the seed germination and ending at the production of ethanol, electricity, and low-pressure steam. Inventory data of the coffee production are collected from national reports. Mass and energy balances are calculated using an integrated methodological approach comprising the conceptual design and optimization of the CCS biorefineries. CCS production is one of the hotspots in both evaluated biorefineries due to the use of high amounts of fertilizers, contributing to most of the environmental impact categories. From the two assessed biorefinery configurations, the system with the lowest environmental benefits was the one that considered the generation of electricity and steam. Factors such as the high emissions of exhaust gases (mainly composed of CO2) and the use of water for steam generation were the main contributors. The alternative solution I (AS-I) has the lowest environmental impact in comparison with the base case. From the sensitivity analyses, the use of energy allocation approach provided better performance than the system expansion approach. However, the selection of one approach over the other highly depends on the evaluated impact category based on the uncertainty analysis. The production of CCS has the highest contribution to the overall environmental impact of the evaluated biorefineries, and thus, we need available information of the coffee crop production including the production of co-products, such as CCS. We present a detailed inventory of the production of CCS in Colombia as an important contribution for further research in the area of coffee-based biorefineries. Based on our inventory, the production of bioenergy (electricity and steam) for a coffee-based biorefinery seems to provide the best environmental performance in comparison to the production of biofuels (ethanol).