Steam gasification of digestate after anaerobic digestion and dark fermentation of lignocellulosic biomass to produce syngas with high hydrogen content

Abstract

Anaerobic conversion technologies are common for organic waste processing. However, the overaccumulation of digestate remaining after anaerobic conversion of biomass requires consideration of new economically viable ways of its disposal. In this work, the possibility of using steam gasification of digestates after anaerobic digestion (AD) and dark fermentation (DF) of lignocellulosic biomass, in order to obtain syngas with high hydrogen content, was investigated. Several digestates were used as feedstocks: (1) after AD of cow manure and mixture of cow manure and amaranth, and (2) from a dark fermentation (DF) reactor fed with by-products of cereals, bran, sunflower meal and rape cake, and operating at hydraulic retention time of 1 and 2.5 days. A methodology for determining the performance indicators of gasification is presented. The thermochemical conversion temperature was 1000 °C. The main indicators of the gasification process were determined at steam to digestate ratios from 0.45 to 0.65. Syngas obtained by steam gasification of digestates contained up to 62.7% H2 and up to 40.4% CO. The syngas from digestates after DF had high values of the molar ratio of H2 to CO, from 1.9 to 2.3, which makes it promising for conversion to liquid fuel by Fischer-Tropsch synthesis. The lower heating value of the syngas from all digestates was in the range from 12.1 to 12.4 MJ/m3. A combination of biological and thermochemical conversion can be used to generate the maximum amount of hydrogen from agro-industrial waste. This study contributes to the creation of a closed technological cycle for the processing of large-tonnage waste in the agro-industrial complex with the production of high value-added products.