Effects of limestone on polycyclic aromatic hydrocarbon emissions during coal atmospheric fluidized bed combustion

The aim of this paper is to investigate the effects that the incorporation of limestone to the feed may have on polyaromatic hydrocarbon (PAH) emissio ns during coal atmospheric fluidized bed combustion (AFBC). The study was p erformed at a combustion temperature of 850 degreesC, an oxygen excess perc entage of 10%, and a total air velocity of 0.24 m/s (around twice the minim um fluidization velocity of the heaviest solid) in a laboratory scale pilot plant with a fluidized bed reactor (7 cm i.d., 76 cm height). The emission s of 16 PAH's (listed by the US Environmental Protection Agency as priority pollutants) were analyzed considering their feeding and their fluidized be d nature. The samples analyzed were taken from a cyclone, a condenser, a Te flon filter (1-micron pore size), and XAD-2 resin. After sonication extract ion with dimethylformamide (DMF) as the solvent the 16 PAH's were analyzed by fluorescence spectroscopy (FS) in the synchronous mode. The results obta ined show that not all the PAH's could be detected and that the highest PAH content was emitted when a limestone bed was used. Wih regard to the PAH g as/solid partitioning emissions, most of them were concentrated on the soli d phase when limestone coal blend was added to the reactor, while most of t he emissions were produced in the gas phase when only coal was fed. To asse ss a possible Ca catalytic role, additional combustion experiments were per formed through feeding coal (without limestone) to the bed obtained from co al-limestone blend feed. Finally, the influence of the calcite porosity and the Ca presence on PAH formation, emission, and distribution are also repo rted and discussed.