Particulate Emissions from Char Bed Burning, 2004 International Chemical Recovery Conference

Field studies have demonstrated the presence of intermediate-sized particulate (ISP) in operating recovery boilers. These 1-100 µm particles may have a significant role in fouling of heat transfer surfaces, but the mechanism of ISP formation has not been defined. Kochesfahani’s 1998 results suggested that the char bed was the major source of combustion aerosols, but his calculations of sub-micron fume and ISP amounts were based on assumed collection efficiencies in series of wet impingers and chemical analysis of the impinger solutions.

For this work, a new char bed reactor was designed for direct, physical capture of both fume and ISP. Black liquor was added to the reactor and heated under nitrogen to form a char mass prior to introducing the combustion gas (4- 17% O2 in N2). Burning rates were determined by analysis of CO and CO2 in the flue gas and char surface temperature was measured by thermocouples and optical pyrometry. Larger particulate was collected in a cyclone and sub-micron fume was collected on a filter and back-up water impinger.

For char surface temperatures of 900-1200°C, total particulate matter generated in the new reactor was similar to the amounts determined in the original reactor (1-9% of initial black liquor solids mass); however, the maximum estimated contribution by the ISP was lower, 0.10-3.9% of liquor solids (1.5% average) versus the earlier studies 0.04-6.0% (2.4% average). Observation of collected particulate suggests that a significant amount of fume was included in those values and that the actual amount of ISP emitted during char bed burning in the new reactor was even lower, not exceeding 1% of the initial black liquor solids.