Effect of Phosphorous Transformation on the Reduction of PM₁₀ Formation During Co-Combustion of Coal and Sewage Sludge

Co-combustion of Municipal Sewage Sludge with coal will become increasingly widely used, regarded as an important incineration method with the high thermal efficiency, low emissions, low investment and operating costs. However, the presence of phosphorus in fine particle has gained increased attention due to its environmental adverse affection and deactivation of SCR DeNOx catalysts. Therefore, the behavior of phosphorus in fine particles during co-combustion of coal and sewage sludge was investigated in a 25 kW quasi one-dimensional down-fired pulverized coal combustor, where PM₁₀ was collected from the furnace centerline in the outlet of flue gas cooler by using a two-stage nitrogen-aspirated, water-cooling isokinetic sampling probe followed a 13-stage electric low pressure impactor. Then the formation mechanism of PM₁₀ was investigated by observing the different fractions of sewage sludge in the coal.

Similar to the coal combustion, the particle-size-distributions (PSD) of PM₁₀ mass concentration by co-combustion of sewage sludge with coal exhibit two distinct modes separated by a fraction of 0.157–0.263 μm, ultrafine mode and intermediate mode. With the sewage sludge blended sludge up to 15% (thermal ratio), the mass concentration of the total fly ash and PM₁₀₊ (Dp > 10 μm) vastly increased from 1,088 and 547 mg/Nm³ (during coal combustion) to 5,059 and 4,403 mg/Nm³. However, the mass concentration of fine particulates, such as PM₁, PM_2.5 and PM₁₀ was maintained at the emission level of coal combustion. When the fraction of sewage sludge less than 15%, the mass concentration of fine particle is higher than the emission during coal combustion, while the growth rate is only by the 3.6, 7.9 and 4.8% of the total concentration of fly ash (5% thermal).

The change of the PSD of mass concentration during co- combustion of sewage sludge and coal, mainly was caused by the interaction between Si、Al and Ca、Fe、K、Na、Mg, which lowed the fusing temperature of the aluminosilicate and silicate, then readily aggregated to form larger particles. Sulphate and phosphate process during the cooling of flue gas will reduced the mass concentration of fine particle, when the fraction of sewage sludge was 15% (thermal), the phosphate and P₂O₅ will dominantly increased in the ultrafine mode.