Alternative energy valorization routes of black liquor by stepwise supercritical water gasification: Effect of process parameters on hydrogen yield and energy efficiency

Supercritical water gasification is potentially suitable for wet biomass; however, commercializing this process requires comprehensive analysis of process conditions and addressing the plugging issue. The objectives of this study are to illustrate a step-wise injection method addressing solid deposition and plugging in the lab-scale tubular reactors and to determine the inter-dependent impacts of process conditions on the yields and reaction phenomena. In contrast to continuous feed, the step-wise injection method introduces solid deposition dispersed over the reactor, thus reducing the risk of reactor opening. However, the industrial application still requires a design enabling solid separation to prevent deposition. The investigated process conditions include stainless steel and Inconel reactors, temperatures of 600–750 °C, and short (133–162 s) versus long (300 s) residence times. Among these conditions, the Inconel reactor at 750 °C with long residence time provided the highest efficiency (87.8% cold gas efficiency) and hydrogen yield (24.92 mol/kg dry-ash-free, 0.646 mol/kg non-inert); also, the lowest char yield (25% carbon conversion to char) promoted gasification and suppressed repolymerization. When comparing various results from the techno-economic assessment viewpoint, the other considerable sets of process conditions include stainless steel reactor at 750 °C with short residence time and at 700 °C with long residence time. In addition, rather than dry or dry-ash-free basis, it is more informative to compare the yields from various studies in non-inert basis together with residence time and reactor material: implicitly assessing the comparison of investment and operation costs as well.