Effects of Limestone on Polycyclic Aromatic Hydrocarbon Emissions during Coal Atmospheric Fluidized Bed Combustion

Abstract

The aim of this paper is to investigate the effects that the incorporation of limestone to the feed may have on polyaromatic hydrocarbon (PAH) emissions during coal atmospheric fluidized bed combustion (AFBC). The study was performed at a combustion temperature of 850 °C, an oxygen excess percentage of 10%, and a total air velocity of 0.24 m/s (around twice the minimum 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 emissions of 16 PAH's (listed by the US Environmental Protection Agency as priority pollutants) were analyzed considering their feeding and their fluidized bed nature. The samples analyzed were taken from a cyclone, a condenser, a Teflon filter (1-micron pore size), and XAD-2 resin. After sonication extraction with dimethylformamide (DMF) as the solvent, the 16 PAH's were analyzed by fluorescence spectroscopy (FS) in the synchronous mode. The results obtained 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 gas/solid partitioning emissions, most of them were concentrated on the solid phase when limestone coal blend was added to the reactor, while most of the emissions were produced in the gas phase when only coal was fed. To assess a possible Ca catalytic role, additional combustion experiments were performed through feeding coal (without limestone) to the bed obtained from coal-limestone blend feed. Finally, the influence of the calcite porosity and the Ca presence on PAH formation, emission, and distribution are also reported and discussed.