Nitrogen oxide formation during black liquor char combustion and gasification

Nitrogen oxide emissions from the recovery boilers originate to a large part from the organically bound nitrogen in the black liquor: During devolatilization, some 70-80% of the liquor nitrogen Is released as gaseous nitrogen species, while the rest, 20-30% remains in the char residue. In this work, the fate of this char nitrogen during char oxidation is studied by burning or gasifying single liquor droplets in a laboratory furnace at well-defined conditions (700-1000 degrees C, 1-10% O2, 0-30% CO2) and by analyzing the nitrogen oxides formed. Most of the char nitrogen was oxidized to NO when the char was burned at high temperatures. However if the char was gasified by CO2 or burned at lower temperatures most of the char nitrogen remained in the smelt residue until all of the char carbon was consumed. However the nitrogen, in;the smelt was released if the smelt was allowed to oxidize. The results are summarized in a novel scheme showing the most important pathways by which the NO is formed during burning of black liquor in a recovery boiler This paper indicates that a significant part of the black liquor nitrogen will leave the recovery furnace along with the smelt. This smelt nitrogen is suggested to be the cause of the high amounts of ammonia and related nitrogen compounds recently reported to be found in the green liquor and in the dissolving tank vent stacks in several pulp milk.