Scale-up and energy estimations of single- and two-stage vinasse anaerobic digestion systems for hydrogen and methane production

Abstract

This study evaluated the full-scale vinasse anaerobic digestion for industrial application. Twelve scenarios were simulated, considering single (methanogenic reactor) and two-stage systems (acidogenic + methanogenic reactors), raw and concentrated vinasse, thermophilic and mesophilic conditions for methanogenesis and usage of acidogenic or methanogenic reactor effluent to dilute vinasse. A single-stage anaerobic digestion system of 17,433 m³ necessary to treat all the raw vinasse produced by a mill with a grinding capacity of 3.3 million tons of sugarcane per season can generate a power of 11.1 MW with an energy yield of 4.6 MJ/kg CODremoved. Two-stage system scenarios where vinasse was diluted with acidogenic reactor effluent had worse energetic recoveries than the single-stage system. Nonetheless, the usage of the methanogenic reactor effluent to dilute vinasse and its own influent in a two-stage system achieved an energetic recovery at least 34% higher than that of the single-stage system. In the best two-stage scenario, the total working volume was 19,666 m³, combining thermophilic acidogenic reactors and mesophilic methanogenic reactors, with an energy yield of 7.0 MJ/kg CODremoved. The different combinations of temperatures allow for a safer and more economic operation. The proposed novel industrial configuration can be divided into two parallel acidogenic reactors of 1,800 m³ each, and those reactors are in series with eight parallel methanogenic reactors of 2,828 m³ each. The electricity produced by the full-scale system can supply an energy demand of 24 thousand inhabitants per harvest season (9.7 kWh per ton of sugarcane). This study presents a novel discussion for sugarcane industry decision makers, such as plant managers, process and project engineers, providing a trade-off between highest energy production, optimal COD removal and lowest reactor volume (CAPEX investment) that better fits their needs.