Abstract
From the pulp and paper industry, black liquor is a process byproduct obtained from the digester discharge and is primarily composed of lignin, hemicellulose, cellulose residues, and water. Managing this byproduct is a critical aspect of the industry, and typically, this byproduct is destined for controlled disposal after treatment. As an alternative to using this byproduct for the generation of high-value-added products, the process of supercritical water gasification presents itself as a promising technology, enabling the conversion of black liquor into a stream of products predominantly composed of hydrogen. This work focuses on the thermodynamic study of the black liquor gasification process in supercritical water. To solve the combined phase and equilibrium problem, methodologies involving Gibbs energy minimization will be employed, formulated in the context of nonlinear programming simulating operational conditions of isothermal reactors. The optimization tool used is the GAMS® software version 23.9.5 with the assistance of the CONOPT4 solver. Initially, validation of the results obtained by the proposed modeling was performed against previously reported literature data, yielding satisfactory results with average relative errors less than 2.65% and 6.33% for H2 and CO2 formations, respectively. Thermodynamic approach results indicate that temperature increases tend to maximize hydrogen formation, an expected outcome considering that reactions with higher hydrogen formation indices are endothermic. In contrast to the temperature effect on hydrogen formation, increases in pressure and the amounts of black liquor in the process feed tend to minimize hydrogen formation. This can be justified by the fact that pressure increases disfavor product formation according to Le Chatelier's principle, and the increase in black liquor in the process feed implies lower black liquor/water ratios, thus unfavoring water displacement reactions and reducing hydrogen formation. In summary, higher hydrogen formation rates (67.33%) are achieved at elevated temperatures (800 °C), low pressures (220 bar), and low concentrations of black liquor in the process feed (0.60% wt), indicating a strong demand for water in facilitating this process when characterized for hydrogen formation.
