The control of particulate matter (PM) emissions from coal combustion becomes an urgent work due to their adverse effects on human health. Coal blending is a promising option for submicron particulate (PM1) reduction. This study addressed the effects of coal blending on the formation and properties of particulate matter in combustion process. Coal blends from lignite and bituminous coal,with different blend ratios (9:1,7:3,5:5,3:7 and 1:9),were combusted in a drop tube furnace. The mass size distribution,concentration,elemental composition and morphology of the particulate matter generated under O2/N2 and O2/CO2 conditions were characterized. Particulate matter was collected by a low pressure impactor (LPI),which aerodynamically segregated particulates into thirteen fractions with sizes ranging from 0.03 to 9.8 μm. The results showed that coal blending reduced PM1 generation,compared with the calculated average values from the combustion of constituent coals. This indicated that the mineral interactions had a great effect on PM1 reduction. The blend ratio also played an important role in the suppression of PM1 genera-tion. In this experimental study,PM1 generation suffered a maximum suppression at the blend ratio of 7:3. The O2/CO2 atmos-phere affected the formation and properties of the PM1 during coal blends combustion. Compared with the O2/N2 combustion,the interaction of minerals was weakened under O2/CO2 combustion,thus the suppression of PM1 generation decreased after coal blending. Compared with the calculated values,the concentrations and percentages of Ca,Fe in PM1 decreased,but the concentra-tions of Ca,Fe,Si and Al in coarse particulates (PM10+) increased after coal blends combustion. The interactions between the aluminosilicates in the bituminous coal and volatile elements Ca,Fe in the lignite were thought to contribute to the suppression of PM1 generation during the combustion of coal blends.
ZHOU Ke XU MingHou YU DunXi WEN Chang ZHAN ZhongHua YAO Hong
Combustion experiments for three coals of different ranks were conducted in an electrically-heated drop tube furnace. The size distributions of major elements in the residual ash particles (>0.4μm) such as Al, Si, S, P, Na, Mg, K, Ca and Fe were investigated. The experimental results showed that the con-centrations of Al and Si in the residual ash particles decreased with decreasing particle size, while the concentrations of S and P increased with decreasing particle size. No consistent size distributions were obtained for Na, Mg, K, Ca and Fe. The established deposition model accounting for trace element dis-tributions was demonstrated to be applicable to some major elements as well. The modeling results indicated that the size distributions of the refractory elements, Al and Si, were mainly influenced by the deposition of vaporized elements on particle surfaces. A dominant fraction of S and P vaporized during coal combustion. Their size distributions were determined by surface condensation, reaction or adsorption. The partitioning mechanisms of Na, Mg, K, Ca and Fe were more complex.
Nanoparticles are thought to induce more severe health impacts than larger particles. The nanoparticles from coal-fired boilers are classified into three size fractions with a 13-stage low pressure impactor. Their physicochemical properties are characterized by the high-resolution field emission scanning electron microscope and X-ray fluorescence spectrometer (XRF). The results show that coal-derived nanoparticles mainly consist of individual primary particles of 20―150 nm and their aggregates. Inor-ganic nanoparticles primarily contain ash-forming elements and their aggregates have a dense struc-ture. Organic nanoparticles are dominated by the element carbon and their aggregates have a loose structure. Nanoparticles from the same boiler have a similar composition and are primarily composed of sulfur, refractory elements and alkali/alkaline elements. Some transition and heavy metals are also detected. For different boilers, greater differences are observed in the production of the nanoparticles and their composition, possibly due to the use of low-NOx burners. Coal-derived nanoparticles have a small size, large specific surface area and complicated chemical composition, and thus are potentially more harmful to human health.
YU DunXi XU MingHou YAO Hong LIU XiaoWei ZHOU Ke WEN Chang LI Lin
