Water Management in Lignocellulosic Ethanol Production- a Case Study and Comparative Analysis from a Swedish Perspective

The project presented here has focused on studying the water balance in a wheat straw-based conceptual High Gravity (i.e. suspended solids in the bioreactors at above 20 %) lignocellulosic ethanol process using xylose-fermenting yeast, cultivated on the hydrolysate from the process. Based on an initial review of inhibitory substances in lignocellulosic ethanol production, different relevant inhibitors were selected to be included in the analysis of water flows in the process. Experimental analyses of compounds at different positions in the ethanol process were conducted, based on material extracted from the Biorefinery Demo Plant, in Ornskoldsvik, Sweden. The results from analyses were used in flowsheeting model development, which in turn was used in order to analyse the impact of recycling process streams in the conceptual ethanol process. The main result is a comparative analysis on energy efficiency and process economics between different recycling options for three different concepts (two High Gravity alternatives and one Low Gravity alternative with 10 % suspended solids). The results indicate the levels of inhibitory substances at different positions in the ethanol process, and connect this information with the opportunities for recycling and reducing water flow. It is shown that water is an important factor for the economic performance of the process, and that a higher solids content in the process gives better results due to lower investment costs. It is also shown that recycling process streams can have a strong effect on both energy performance (flash steam recycle) and economics (hydrolysate recycle).