Influence Of Sodium On Char Formation And Fine Particulates Control During Coal Combustion

Coal plays a dominant role in China’s primary energy. The combustion of coal inevitably generates massive particulate matter (PM) and CO2, and a significant part of PM2.5(particles with an aerodynamic diameter of <2.5μm) escapes to the atmosphere due to its high penetration to particle emission control devices, and poses serious threats to the environment and human health. Oxy-fuel (O2/CO2) combustion is regarded as one of the effective ways to reduce CO2emissions from coal combustion and mitigate climate warming. Considering the characteristics differences between CO2and N2, it is worthwhile to compare the char and PM2.5formation behaviors during conventional air combustion and oxy-fuel combustion, in order to acquire more directional information for modifying the existed air combustion plants to oxy-fuel combustion plants.The presence of sodium in coals may have a significant impact on the char characteristics and PM2.5formation; moreover, its chemical form may also affect the capability of kaolin to reduce PM2.5formation. Sodium in coals occurs as three main states: as soluble salts (e.g., NaCl), in an organic form (e.g., carboxylate) and as insoluble silicate or aluminosilicate (e.g., Na-Al-Si). Therefore, the present work aims to reveal the influence of sodium on char characteristics, PM2.5formation and reduction during coal combustion.First of all, the preparation of the chars from the leached and NaAc-loaded coal samples was carried out in a laboratory-scale drop tube furnace under N2and CO2atmospheres. The sodium contents in chars and their pore distributions and crystalline structures were further compared to determine the influencing factors on char reactivity. It is found that the effect of sodium on coal char formation is closely associated with the coal rank and atmosphere. Due to a weaker effect of sodium on bituminous coal-derived char characteristics, the reactivity of these chars formed under N2and CO2atmospheres correspondingly only have slight changes. In contrast, the effect of sodium on characteristics of lignite-derived chars becomes more obvious, and due to a stronger catalysis effect of sodium, the lignite-derived char formed under CO2atmosphere has a considerable reactivity. The microcrystalline structures of lignite-derived chars are closely related to the volatile yields during the char formation. Less organic substance is kept in the char with the higher volatile yield, which indicates that the char correspond to a higher value of order degree and have a more stable structure. Due to the catalysis effect, the presence of sodium in the coal has an impact on the evolution of the char crystalline structure, besides, the correlation between order degree of the microcrystalline structure and reactivity of the char will be further destroyed. In a specific temperature range, the reactivity of the char is controlled by the catalysis of sodium coupled with pore structure of the char, instead of its chemical microcrystalline structure.Then, the capability of sodium as carboxylate and chloride transforming into aluminosilicate and the mineral characteristics alteration after the interaction between kaolin and different chemical form sodium in O2/N2and O2/CO2atmospheres were investigated. Meanwhile, based on the description of these characteristics, the influence of the interaction between kaolin and sodium on PM2.5reduction was also further proposed. The conclusions are drawn as follows. The mineral with higher melting ability tends to coalesce. The sodium as carboxylate interacting with kaolin has a higher transformation into Na-Al-Si than that as chloride, and the correspondingly formed Na-Al-Si also tends to melt. Consequently, the mineral particles coalescence after the interaction between kaolin and NaAc is more obvious. The mineral melting goes against the pore structure. With the melting intensified, the BET surface area and pore volume both gradually decrease. Besides, the adverse influence of mineral melting on mesopores is most obvious. Atmosphere has a slight effect on the interaction between kaolin and the sodium as chloride. However, high concentration of CO2could reduce the release of sodium as carboxylate and transformation rate into metallic sodium, and the mineral particles coalescence become weaker in O2/CO2atmosphere than that in O2/N2atmosphere.Final work aims to reveal the influence of sodium species on PM2.5reduction by kaolin during coal combustion. A combustion experiment of a treated low-sodium coal mixed with sodium aluminosilicate was conducted in a lab-scale high temperature drop tube furnace to reveal the effect of mineral melting and coalescence on PM2.5reduction. Meanwhile, two typical Na-loaded coals (in which the sodium was loaded in the form of NaCl and sodium carboxylate, respectively) with kaolin added were also burnt under O2/N2and O2/CO2atmospheres to investigate the effect of interaction between kaolin and different chemical forms of sodium on PM2.5reduction. The results show that, the surface reaction between kaolin and vapor metals to reduce ultrafine mode particles formation is not the only pathway for coal-derived PM2.5reduction by adding kaolin. The aluminosilicate such as Na-Al-Si formed by the reaction is able to produce "liquidus" at high temperature and induce migration of fine particles toward coarse particles to reduce PM2.5formation. The ultrafine mode particles reduction by kaolin is higher during NaAc-loaded coal combustion than that during NaCl-loaded coal combustion. Moreover, it is also lower in O2/CO2combustion than that in O2/N2combustion. The mineral coalescence resulting from the melting of sodium aluminosilicate is an important factor influencing central mode particles reduction by kaolin. Besides, the central mode particles emission depends on coal properties and combustion atmosphere, and the difference in emission will cause different collision frequency between particles and additive, which also influences the central mode particles reduction by kaolin. With the joint actions of mineral coalescence and particle collision, the NaAc-loaded coal has a higher central mode particles reduction by kaolin than the NaCl-loaded coal, especially under the O2/N2combustion.