Supercritical water gasification of biomass : kinetic approach & process simulation

Thermochemical biomass conversion is a method to transform stored energy to higher energy density solid, liquid or gas fuels by chemically decomposing the high molecular weight biopolymers. Biomass derived green gas has been worked extensively with the aim of taking over the place of fossil based combustible gas, for people's self-sufficiency in energy consumption along the earth and preserving a sustainable environment. Biomass gasification at high temperature - low pressure conditions targets to produce a combustible gas, abundant of H2 (high calorific gas) and CO (reactant of further shift reactions). Stated technology is able to scale up to 600 MW energy production capacities for various biomass or residues but with different thermal efficiencies due to the external heat requirement. Among all other physical and chemical properties, moisture content of the biomass may result in thermal efficiency issues and parallel to this power production decreases. Supercritical Water Gasification (SCWG) allows the thermochemical conversion of wet biomass in hydrothermal media. High-pressure biomass slurry can be kept in aqueous media until the critical point of water. Right above near-critical temperature level (∼330 ℃) hydrogen bond number decreases and water molecule becomes more apolar, as a consequence water solubility increases allowing one-phase homogenous reactions; furthermore, ion concentration in the hydrothermal media increases, which favors the acid or base catalyzed reactions. Thermo dynamical benefits have aroused the curiosity of several science groups for the last 20 years, providing modeling and experiment studies on supercritical water gasification of biomass (SCWG). However, detailed kinetic modeling of the process for lignocellulosic biomass is missing, strongly needed for further technology improvements. In this work, review and evaluation of the biomass kinetic approach has been performed with the aim of proposing reaction network and rate parameters for three main biomass structural compounds; cellulose, hemicellulose and lignin. AspenPlus 8.3 engineering software package has been aided for the simulation of the tubular reactors, integrated with the compiled kinetic data from literature and proposed network. Validation results show that reactor simulation has a good accuracy of predicting gasification efficiency, carbon efficiency and gas yield for a range of residence time (15-47 s), pressure (20-30 MPa) and biochemical composition (mostly agricultural residues). Reactor simulation validations showed the possibility of inserting consecutive reactors to a simplified SCWG process simulation. Process scheme including 3 tubular flow reactors and auxiliary units has been designed; according to which, carbonmonoxide low product gas and high gasification efficiency (4,45 % , 92,24%, respectively) have been achieved. The results of this work show that kinetic model developed for lignocellulosic biomass, based on the literature data is able to predict gasification efficiency, intermediate compound composition, gas composition and thermal energy output for different operating conditions and reactor lengths.