Life Cycle Analysis of Bioethanol Production from Sugarcane Molasses and Sweet Sorghum Stalk Juice in Kenya

Abstract

Biofuels development has received increased attention in recent times in the hope that they are cleaner and cheaper fossil fuel supplement towards mitigating climate change, expanding the fuel energy resource mix and fostering rural development. This has led to increased production and utilization of biofuels worldwide. In order to ensure sustainable bioethanol production in Kenya, information on its contribution to the energy security, economical development and environmental burden of the country is required. Thus, the main objective of this study was to evaluate the energy balances, production and environmental costs, and environmental impacts in the production of bioethanol from sugarcane molasses and sweet sorghum stalk juice in Kenya, from a life cycle perspective. In this study, Chain Management by Life Cycle Assessment (CMLCA) software was used to perform inventory analysis and impacts assessment for each of the bioethanol systems. The inventory analysis quantified all the emissions for each of the bioethanol systems, Fossil energy and renewable energy inputs of each bioethanol system were determined from which the energy balances were calculated. Production costs of each bioethanol system were determined from the costs of farm inputs, industrial chemicals and hiring of farm machinery. Environmental costs of each bioethanol system were determined using the Environmental Priority Strategies (EPS) model. In both the sugarcane molasses and the sweet sorghum stalk juice bioethanol systems, more than 85% of the total energy consumption was renewable energy. The calculated values for net renewable energy value (NREV) were 19.75 and 19.68 MJ per litre of bioethanol for the sugarcane molasses and the sweet sorghum stalk juice bioethanol systems, respectively. The calculated values for net energy ratio (NER) for the sugarcane molasses and the sweet sorghum stalk juice bioethanol systems were 14.62 and 13.60 respectively, for every litre of bioethanol. The high positive values of NREV and NER obtained indicated that there was less non-renewable energy input in the production of bioethanol in each case. The net energy value (NEV) of the sugarcane molasses-based bioethanol and the sweet sorghum stalk juice-based bioethanol were evaluated to be 3.88 MJ and 11.12 MJ per litre of bioethanol, respectively. The positive NEV values indicate that the energy required to produce bioethanol in both bioethanol systems is less than the energy content of bioethanol. The net greenhouse gas (GHG) emissions of the sugarcane molasses-based bioethanol and the sweet sorghum stalk juice-based bioethanol were estimated to be 270.88 and 424.19 gCO2eq per litre of bioethanol, respectively. Cultivation was found to produce the highest proportion of the total GHG emissions in both bioethanol systems. Low GHG emissions were reported in this study mainly due use of biomass (bagasse and vinasse) as the source of energy (steam and electricity). Similarly, low values of acidification potential (AP) and photochemical ozone creation potential (POCP) were obtained in this study attributable to no biomass burning prior to harvesting and no use of coal as a source of energy. Lower values of human toxicity potential (HTP) were also obtained in this study attributable to no biomass burning prior to harvesting associated in emissions of heavy metals and particulates. Low ecotoxicity potential (EP) values were obtained in both bioethanol systems, attributable to use of lower amounts of fertilizers in cultivation. The largest cost component of the total cost for both bioethanol systems was found to be in cultivation stage which was more than 75% of the total cost in each case. Further, the study found that more than 80% of the total external environmental costs in both bioethanol systems were due to fossil oil fuel use. Emissions due the use of agrochemicals (fertilizers, herbicides and pesticides) and fossil fuels (diesel and gasoline) during farming of the biocrops contribute greatly to the environmental impacts considered. Energy balances of the sugarcane molasses bioethanol system and the sweet sorghum stalk juice bioethanol system indicate low fossil energy use.