Abstract: During a clean-out period when black liquor supply lines to paper pulp mill recovery furnace fuel spray guns are circulated with purge water or when black liquor firing is interrupted due to low consistency solids, the removal and unauthorized insertion of such fuel spray guns is positively verified and prevented by swing gates and series condition switches respective to each gun port. In order for a liquor circulation pump to be restarted following an automatic shutdown, all gates must be closed across respective gun ports which simultaneously closes all gate switches.

Abstract: Gas-liquid contact reaction efficiency in a two stage heavy black liquor oxidation process is improved by co-current contact in the first stage and countercurrent contact in the second stage or vice versa. Such improved efficiency is sustained by preventing the obstruction of gas discharge nozzles.