Energetics and water requirement of a commercial-scale sweet sorghum ethanol production

Abstract

This study quantifies the energy balance and water requirement for ethanol production from sweet sorghum. The energy balance assessment is important to verify if the system actually achieves a positive net energy balance, while inventory of water requirement provides primary approximation of the water economy of this alternative feedstock. The boundary of the assessment is from the production of the feedstock to the products’ end-use (cradle-to-grave). All the balances were based from a 30-M L yr-1 capacity commercial bioethanol plant that operates for 270 d yr-1. The net energy balance of the system was computed by accounting the total energy consumed by the materials and processes in the boundary equated with the total energy produced through the products – power and biofuel. From the assessment, it was verified that the production gains a net energy equivalent to 475,621,789.51MJ yr-1 or 15.85 MJ L-1 of ethanol produced. Since the assessment assumed that a new bioethanol facility will be put up, the analysis included the energy invested during this pre-operational period, termed as “energy debt.” Construction of the whole facility expended a total of 1,127,076,244.75MJ energy or 37.57 MJ L-1 ethanol. However, because the system gains a net energy, a payback period for the energy invested was computed by dividing the total energy debt by the net energy gain. It was deduced that energy debt can be offset or paid back within 2.37 years of operation. Meanwhile, the total water economy in the construction of the bioethanol plant amounts to 960,453.44 m3. Likewise, the whole operation consumed a total of 12,368,904,260.86 L for a year’s operation, which is equivalent to 412.30 L water L-1 ethanol produced, or 19.45 L MJ-1, or 24,541.48 L T-1 cane processed.