Evaluation of Sorghum Biorefinery Concepts for Bioethanol Production

Abstract

In the field of sustainable biorefinery concepts, sorghum receives increasing attention as a raw material. Main advantages of various sorghum types are fast growth as well as efficient nutrient and water utilization. When considering the competition between food and energy crop production, sorghum could be part of a sustainable solution. Through convenient crop rotation, sorghum could be grown as a catch crop with good harvest results. This could constitute an advantage as opposed to classical crops like maize or wheat whose utilization as energy feedstock directly competes with food production. In this study, four process variants will be compared. First, a conventional bioenergy production which was considered as a benchmark scenario, and in addition three new variants of sorghum biorefinery concepts. The reference variant consisted of two conventional pathways. Here we considered a conventional biogas and a bioethanol plant with maize as a raw material. Because the necessary data for comparison was already available, this scenario served as a benchmark for the other process variants. The first new concept utilizes a grain sorghum type which exhibits rather high starch content. The grains and straw are separated during the harvest, and processed separately. While the straw is ensiled and fed into biogas production, the starch rich grains are used in a fermentation process with ethanol as the end product. The unfermented residue can be processed to DDSG, a valuable fodder for livestock or feed a biogas process to generate energy. A sugar rich sweet sorghum variant serves as raw material for the third scenario. In this case, the whole plant is stabilized after the harvest, and consecutively fed to a whole plant fermentation to produce ethanol. The residue could again be used for biogas production. The last process scenario we considered is similar to the previous one, with the distinction, that the sugar juice, which is mainly stored in the plant stem, is separated from the remaining plant prior to ethanol fermentation. Ethanol potentials for all crops were calculated according to the appropriate process scheme. It was found that some sorghum variants yield comparable ethanol potentials.