Exergy and renewability analysis of liquid biofuels production routes

Abstract

Liquid biofuels can be produced from a variety of feedstocks and processes. Ethanol and biodiesel production processes based on conventional raw materials are already commercial, but subject to further improvement and optimization. Biofuels production processes using lignocellulosic feedstocks are still in the demonstration phase and require further R&D to increase their production efficiency. Exergy analysis is a primary tool to assess the efficiency and renewability of biofuels production processes from an integrated point of view. In this chapter, an exergy-based comparative analysis of four biofuels production routes are described and discussed. The selected feedstocks are glucose and sugarcane syrups, the fruit and flower stalk of banana tree and palm oil. For each production route, the effect of process variables on the exergy efficiency and the renewability exergy index (presented in Chap. 2) are determined allowing the identification of possible ways to optimize the production of such biofuels. According to the values of the renewability exergy index, ethanol production process using sucrose, amilaceous, or lignocellulosic material cannot be considered renewable, while biodiesel production from palm oil can be considering renewable. The main reason for these conclusions is due to the irreversibilities that take place along the energy conversion processes of these biofuels production routes. These unexpected conclusions highlight that although renewable raw materials are used as feedstocks, the biofuel itself cannot be considered renewable due especially to the exergy destruction of its production process.