Implications on Feedstock Processing and Safety Issues for Semi-Batch Operations in Supercritical Water Gasification of Biomass

Abstract

Biomass with a large amount of moisture is well-suited to be processed by supercritical water gasification, SCWG. The precipitation of inorganics, together with char formation and re-polymerization, can cause reactor plugging and stop the process operations. When plugging occurs, sudden injections of relatively large mass quantities take place, influencing the mass flow dynamics significantly in the process. Reactor plugging is a phenomenon very well observed during SCWG of industrial feedstock, which hinders scale-up initiatives, and it is seldom studied with precision in the literature. The present study provides an accurate evaluation of continuous tubular reactor dynamics in the event of sudden injections of water. An interpretation of the results regarding water properties at supercritical conditions contributes to comprehending mass and heat transfer when plugging occurs. Experiments are then compared to SCWG of a biomass sample aiming to give key insights into heat transfer and fluid dynamics mechanisms that could help develop operational and control strategies to increase the reliability of SCWG. In addition, a simplified model is presented to assess the effect of material integrity on burst-event likelihood, which states that SCWG is safe to operate, at 250 bar and 610 °C, in tubular reactors made of 0.22 wall thickness-to-diameter ratio Inconel-625 with superficial microfractures smaller than 30 µm. We also suggest improvement opportunities for the safety of SCWG in continuous operation mode.

Analysis of operational issues in hydrothermal liquefaction and supercritical water gasification processes: a review

“…Besides high temperature and pressure, the sudden variation of these two conditions can provoke disastrous effects as observed in the lab-scale experiments [106,112]. The variations of temperature and pressure result from sudden mass injection into SCW conditions and partial plugging due to solid deposition in the reactor.…”

“…The burst pressure is calculated based on the ultimate tensile strength of the material and the ratio of outer diameter to the wall thickness [105]. However, the burst pressure is usually calculated for a flawless material and reduces with fatigue, corrosion, and fractures on the microstructure of the material [106]. As a rule of thumb, it is suggested to operate under the pressure one-fourth of the calculated burst pressure [107].…”

“…The variations of temperature and pressure result from sudden mass injection into SCW conditions and partial plugging due to solid deposition in the reactor. A sudden mass injection into the reactor at supercritical conditions results in a fast expansion, which stresses the reactor metal [106]. Moreover, the fast injection of mass could also provoke a contraction of the reactor walls, which would also concern the locking system, as demonstrated in Fig.…”

“…In addition, char is formed also throughout the rector due to the repolymerization of phenolics and aromatic intermediates at high temperatures and long residence time [39,50,53,122]. The solid deposition is an obstacle for SCWG processes due to introducing the risks of plugging at the reactor inlet and rupture of reactor wall [106,112]. In a continuous operation with an isothermal reactor, the feedstock reaches SCWG conditions immediately at the reactor inlet, thus precipitating salts and char at the reactor inlet.…”