Safety Aspects and Practical Constraints in the Combustion of Odorous Gases in Black Liquor Recovery Boilers

摘要

Non-condensable gases (NCGs) formed during kraft pulping process are generally divided into strong and weak gases. Typical processes where NCGs are formed include the digester, evaporation train, pulp washing, and methanol handling. These gases are called strong or concentrated non-condensable gases (CNCG) or low volume high concentration gases (LVHC). Non-condensable gases collected from tanks, basins, drains, and other sources in contact with ambient air are called weak or diluted non-condensable gases (DNCG) or high volume low concentration gases (FIVLC). For environmental and economic reasons both diluted and concentrated gases are today collected and destroyed either in a dedicated NCG incinerator, lime kiln, power boiler, or in the black liquor recovery boiler. In modern kraft pulp mills, combustion of NCGs in the recovery boiler has become a standard procedure, especially in eucalyptus-based kraft pulp mills. However, combustion of NCGs in a recovery boiler is not a straightforward process due to numerous safety issues and several incidents have been recorded at the mills. Therefore, safety should be the top priority when designing new or repairing existing NCG systems. Another constraint in the combustion of NCGs in the recovery boiler is the possible effects on the furnace chemistry and thus on dust composition and gaseous emissions. Since NCGs contain both ammonia (NH3) and sulfur-laden compounds, their effects on nitrogen oxide (NO_x) and sulfur dioxide (SO2) emissions have to be taken into account already in the design phase. However, if all the safety aspects have been taken into account in the design of the NCG systems and all the other constraints have been thoroughly considered, combustion of both concentrated and diluted gases in the recovery boiler can be done safely and without increased environmental loading. This paper summarizes the most important safety aspects in the safe combustion of NCGs in the recovery boiler furnace, as well as practical constraints to be considered in the design of the systems. Examples from operating boilers are included to highlight the importance of safety. The effect of NCGs on furnace chemistry, emissions, and dust behavior is also reviewed.