Evaluation of the energy efficiency of liquid biofuel synthesis using energy quality metrics

Abstract

Biofuel production is a rapidly growing industry that aims to eventually replace non-renewable fuels. A consequence of biofuel synthesis is the abundant production of waste biomass, or lignocellulosic material. The lignocellulose may be utilised for biofuel production in the second generation process, or converted to process energy. The efficacy of industrial-scale biofuel processes is conventionally measured in life-cycle assessment studies using energy balances in the form of the fossil energy ratio (FER) and the net energy ratio (NER). The application of these metrics, however, have considerable drawbacks. It was shown in this study that these traditional metrics provide reversed indications of "renewability" when cogeneration of biomass is considered. Process models were developed from literature to evaluate the production of bioethanol from sugarcane and biodiesel from soybean from a process energy-efficiency perspective. The utilisation of the waste- and by-products was assessed in various process scenarios. A set of grounded and robust metrics were employed to evaluate the efficiency of the processes, namely, the change in energy quality and the change in energy yield. The change in energy quality evaluates whether there has been an increase in the energy density from the feedstock to the product. The change in energy yield, on the other hand, quantifies the amount of energy transferred from the feedstock to the biofuel, where the energy consumption of the process is taken into account. The change in energy quality for bioethanol and biodiesel showed a 63 % and 107 % increase compared to the respective parent crops. Even with the improvement in energy density, the change in energy yield achieved is only 10 %–27 % for biodiesel and 28 % 36 % for bioethanol. Compared to the ideal route of upgradation possible for soybean and sugarcane, the maximum energy recovered from soybeans is less than 28 % of the ideal case, and from sugarcane, it is less than 46 %. For the soybean-biodiesel configurations explored, the change in energy yield increases by up to 161 % when the waste- and by-product streams are utilised for process energy. For the sugarcane-to-bioethanol process, the valorisation of the lignocelluose to bioethanol in the second generation process was also explored. The second generation process contributes marginally to the change in energy yield, where only a 3 % increase is seen compared to traditional first generation processes with cogeneration. From an energy perspective, the benefits of second generation bioethanol cannot be justified with regard to the additional infrastructure. The energy recovery of both processes suffer because the feedstocks, the sugarcane crop and the soybean crop, are predominantly composed of lignocellulose—straw, meals/hulls, or bagasse. The extractible intermediate feedstocks, saccharides from sugarcane and oil from soybeans, are only a fraction of the entire biomass. As a result, there are pronounced energy losses from the lignocelullosic by-products even when used for process energy generation or second generation biofuel production. This sentiment is also seen when comparing the end-use of the fuel and the feedstock: the biomass-to-fuel-to-engine cycle yields a lower change in energy yield compared to converting the biomass directly to electricity. There are pronounced inefficiencies in the naturally guided synthesis pathways that are difficult to overcome. Focus should be placed on utilising suitable feedstocks for biofuel production and industrialising fuel production pathways that maximise energy recovery from the entire parent biomass.