Concerning operational aspects in supercritical water gasification of kraft black liquor

Kraft Black Liquor (KBL) is an aqueous solution of lignin residues, hemicelluloses and inorganic chemicals produced during the pulping process. It is sometimes classified as waste even if this definition is wrong in most of the cases where the BL is reutilized (recovered) by using it as fuel in recovery boilers. In this process, the pulping chemicals are recovered and sent again to the cooking process of wood. BL has more than half of the energy content of the inlet wood. KBL is generally used as an energy source although recovery boilers are still quite thermally inefficient compared to coal or gas fired power producing ones. Among the various solutions, supercritical water gasification (SCWG) enables bio-fuels and value products to be obtained from KBL. The present study deals with the KBL SCWG by using a plug-flow reactor operating continuously at 250 bar pressure and variable temperatures in the range 500–700 °C. The objective was to identify the experimental conditions that lead to the development of operational problems and to document them accurately. In this study, real time visualization of online measurements has been conducted by means of the LabView software. In order to highlight the differences in behavior, tests were conducted on two different substrates, the sucrose and KBL. Results highlight how tests were carried out without problems at 500 °C resulting in a total gas yield of 18.31 g-mol/kgfeed, consisting mainly of H2 (7.87 g-mol/kgfeed), CO2 (5.84 g-mol/kgfeed), H2S (2.29 g-mol/kgfeed) and CH4 (1.96 g-mol/kgfeed). On the contrary, in the case of KBL SCWG the test was interrupted at 600 °C because of salt deposits and consequent clogging in the entrance of the reactor. The LabView software showed particularly rapid pressure drops which were not addressed to technical problems in the layout. However, the recording of the pressure drop was only the final symptom of this occurrence. The saline content in the investigated inlet KBL also suggested the need of a pre-treatment for the removal of the inlet salts in the case where a non-catalytic reactor would be used.