Kinetic model for supercritical water gasification of algae

Abstract

The article reports the first quantitative kinetics model for supercritical water gasification (SCWG) of real biomass (algae) that describes the formation of the individual gaseous products. The phenomenological model is based on a set of reaction pathways that includes two types of compounds being intermediate between the algal biomass and the final gaseous products. To best correlate the experimental gas yields obtained at 450, 500 and 550 °C, the model allowed one type of intermediate to react to gases more quickly than the other type of intermediate. The model parameters indicate that gas yields increase with temperature because higher temperatures favor production of the more easily gasified intermediate and the production of gas at the expense of char. The model can accurately predict the qualitative influence of the biomass loading and water density on the gas yields. Sensitivity analysis and reaction rate analysis indicate that steam reforming of intermediates is an important source of H(2), whereas direct decomposition of the intermediate species is the main source of CO, CO(2) and CH(4).

Methods of energy extraction from microalgal biomass: a review

“…The syngas yield increased with increasing temperature from 302-652 °C, in agreement with a recent model of the kinetics of SCWG that indicated that higher temperatures favour production of intermediates which are more easily gasified together with the production of gas at the expense of char (Guan et al 2012b). …”

“…Conventional biomass gasification processes require dry feedstock (Guan et al 2012a), but supercritical water gasification (SCWG) is an alternative gasification technology for the conversion of high moisture biomass and it is suggested that in well-engineered systems it can be net energy positive (Guan et al 2012b). Chakinala et al (2010) found that higher temperatures, low microalgal concentrations, and longer residence times improved the efficiency of SCWG of Chlorella vulgaris.…”

Supercritical Water Gasification of Biomass: A Literature and Technology Overview

“…It has been reported by many authors (such as [47,85,86]) that water gas shift and methanation reactions are assumed to be the main gas phase reactions which may result in an equilibrium state. Equations (7) and (8) show the water gas shift reaction and methanation reactions, respectively.…”

“…Sasaki et al [31,101] further studied the kinetics of cellulose and cellobiose conversion in sub-and supercritical water. Regarding modelling, Resende and Savage [86] developed a kinetic model for non-catalytic supercritical water gasification of cellulose and lignin at high temperatures, and Guan et al [85] proposed a reaction mechanism for the supercritical water gasification of a real biomass feedstock; microalgae. However, these kinetic models and parameters have limited applicability as:…”

“…Savage's group [85,86] performed sensitivity analyses on the reaction rates for non-catalytic supercritical water gasification of biomass processes. It can be concluded from the results that the main reactions which determine the hydrogen and methane formation are not the water gas shift and methanation reactions in the gas phase, but the reactions which directly result in the gas formation from the feedstock and/or intermediates.…”