Modeling of a Novel Catalytic Supercritical Water Gasification Power Plant Combined with Reversible Solid Oxide Cell
Authors: Recalde Mayra , Botta Giulia , Fernandes Alvaro , Woudstra Theo , Aravind P.V. 
Published: February 2020

Using catalytic supercritical water gasification (CSCWG) in generating energy from wet biomass (e.g sewage sludge, moisture content > 80%wt) is efficient and environmental friendly. However, one of the main challenges in using CSCWG is the low syngas heating value. Syngas for power and synthetic fuel production requires high-purity and high heating value. In this work, a novel system is proposed which increases the CSCWG syngas heating value and produce electricity using reversible solid oxide cell (ReSOC). Further, CSCWG-SOFC power plant is a thermally efficient system, SOFC releases heat, and CSCWG needs heat and both processes work at similar temperature. Eventually the plant can be used for syngas production, working in electrolyser mode powered by excess renewable electrical energy. This is expected to increase the financial returns on the investment.

The CSCWG-ReSOC presented in this work operates in two modes: CSCWG-solid oxide fuel cell (SOFC) for power production or CSCWG-solid oxide electrolyser cell (SOFC) for hydrogen and methane production. CSCWG-SOEC mode: electricity output from wind or solar energy, powers the electrolyser, the heat required for high temperature electrolysis come from by feeding the electrolyser with steam rich CSCWG gas. As methanation is an exothermic reaction, heat could potentially be added by promoting spontaneous methanation in the electrolyser. Combustion of small amounts of syngas can also provide a part of heat required for electrolysis and gasification processes. CSCWG-SOFC mode: heat required for endothermic reactions of supercritical water gasification is supplied by exothermal oxidation of fuel in the SOFC.

CSCWG (operating conditions 500 °C, 250 bar) convers almost 100% organic matter into gas phase. Ash contained in the biomass is recovered in a salt separation system and ZnO layer removes remaining sulfur compounds before reach the gasifier Boukis et. al 2017. The gas at equilibrium contains CH4, H2, CO2 and around 80% H2O at high pressure and temperature. Before the gas is fed to the SOFC, the excess steam is removed and pressure and temperature are changed to SOFC operating conditions. Besides, the SOFC can also be feed with fuel produced in the CSCWG-SOEC mode.

In contrast, methanation is sustained at high pressure. Subsequently steam rich gas at high pressure and temperature represents preheated electrolyser input. The pressure of the gas is adjusted at around 680°C and 20 bar. Then the high pressure SOEC at endothermal conditions generates additional hydrogen and methane which can be converted into synthetic fuel, or stored in tanks and converted into electricity by solid oxide fuel cells.

After designing and modelling the process in Aspen PlusTM, it is seen with the preliminary results that the CSCWG syngas LHV increases significantly. The energy efficiency of the CSCWG--SOEC is in the order of 60-70%. The effects of CSCWG and ReSOC pressure and temperature on the amount of hydrogen and methane production are analysed. Details of the hydrogen and methane yields, SOFC power production, stack efficiencies etc will be described in the complete paper.

The thermodynamic analysis of CSCWG for syngas production combined with ReSOC operation shows to be attractive for fuel and power production at reduced cost. Such a system might become even more attractive when is combined with an auxiliary methanation unit or if the system is applied to sanitation purposes, i.e., where the wet biomass is human waste.