Preliminary Investigation into the Cold Plasma Powered Water Gas Shift Reaction – Experiments And Analysis

Abstract

Water gas shift reaction (CO + H2O ↔ H2 + CO2) is a CO oxidation reaction in the presence of steam resulting in the formation of H2 and CO2. Traditionally, water gas shift (WGS) reaction is achieved using multiple catalytic beds operated at different temperatures i.e., high-temperature followed by low-temperature shift reactors to maximize CO conversion into H2 in the presence of excess steam. Catalytic processes have certain limitations and there is a constant search for non-catalytic routes to achieve WGS reaction. Cold plasma is one such promising route and this work reports a preliminary study of cold plasma powered WGS reaction at ambient pressure and low temperatures without the use of any catalysts. Cold plasmas are known to produce high densities of reactive plasma species (electrons, ions, and radicals) at atmospheric pressure and low-temperature conditions. These species facilitate chemical reactions which otherwise are not possible in the absence of plasma at atmospheric conditions. In this study, an attempt is made to address the influence of various factors on the WGS reaction using cold plasma. Parametric study to capture the effect of steam to CO molar ratio, gas residence time and plasma discharge power on CO conversion to H2 is reported. The non-catalytic, cold plasma-based parametric analysis on WGS reaction using a pure CO – H2O stream being reported in the current work is a first of its kind intervention. Investigations begin with thermodynamic equilibrium calculations of the homogeneous phase WGS reaction using NASA CEAgui application based on free energy minimization technique. The equilibrium results are compared with literature reported results on catalytic and limited plasma WGS reaction process. As this provides a basis for addressing the behavior of the homogenous WGS reaction in a plasma environment deviating from the equilibrium conditions as reported in the literature. Thermodynamic calculations suggest that at a temperature below 500 K methanation is a predominant reaction whereas at temperatures above 500 K, WGS dominates, leading to increasing H2 concentration with a maximum H2 concentration of 48 mol % (dry basis) realized at 700 K and steam to CO molar ratio of 9. Increasing steam to CO molar ratio above 9 did not show any significant change in H2 concentration in the product gas. Experimental cold plasma investigations show that, at operating temperature of 423 K, steam to CO molar ratio of 20, 2600 ms gas residence time and plasma discharge power of 70 W, maximum CO conversion of 63 ± 4 % can be realized with an H2 concentration of 48 ± 2 mol % and CH¬4 concentration of 0 mol % in the product gas. Thermodynamic equilibrium calculations, however, suggest only 2 mol % H2 and 24 mol % CH4 at the same temperature conditions. In the range of parameters studied, the parametric investigations show that for plasma powered WGS reactions i) higher steam to CO molar ratio results in higher CO conversion and higher H2 concentration in the product gas; ii) increasing gas residence time increases CO conversion to H2 concentration in the product gas & iii) higher plasma powers increases CO conversion only at lower steam to CO molar ratios. Under pure homogeneous reaction conditions, stoichiometry suggests equal moles of CO2 and H2 be produced for every mole of CO reacting with H2O. Measurements, however, indicate higher H2 fraction as compared to a CO2 fraction in the product gas. The enhanced hydrogen generation compared to stoichiometric analysis is argued based on the presence of condensed phase carbon in the plasma reactor. Specific studies show that the CO2 formed contributes positively to form excess H2 in the product gas. In the plasma reactor, the CO2 formed dissociates to form condensed phase carbon which reacts with OH- radicals leading to the excess H2 formation in the product gas. Based on this, a possible reaction pathway, involving the formation of formic acid, for cold plasma powered WGS is hypothesized. Finally, the study suggests that the water gas shift reaction using cold plasma could be a possible solution with a future emphasis on optimizing the specific energy consumption to generate hydrogen.