Directional Control Of Mineral Matter Migration For Reducing Ultrafine PM Formation During Coal Combustion:Mechanism And Field Study

Ultrafine particulate matter（PM）is an important part of PM_2.5,which is enriched with hazardous components and more harmful;meanwhile it cannot be effectively removed by the common-used dust collectors.In this study,the partitioning characteristics of both volatile and refractory minerals into ultrafine PM were investigated based on lab-scale,pilot scale experiments as well as modeling.Then,the ability of inhibiting the formation of ultrafine PM and the corresponding mechanism of 8 mineral material were explored based on laboaratory experiments.Furthermore,a feasible additive was selected out and tested in a field study on a 1000 MW pulverized coal-fired power station unit,where the impacts of addtive addition on the formation of ultrafine PM in furnace and on the performace of the downstream electrostatic precipitator（ESP）under the actual operating conditions were determined.In the end,a series of field measurements were carried out in 8 coal-fired power stations to study the formation and emission characteristics of fine and ultrafine PM in the large scale power stations equipped with virous boilers and dust collectors.Pilot scale（3 MW）experiments burning blends of a high-Na-high-CI coal and a high-Si coal were carried out to determine the role of volativle mineral matter on the formation of ultrafine PM.And then further laboratory experiments on a drop tube furnace（DTF）combusting single coals with added gaseous HCl were performed to investigate the interactions between Na,S and Cl and their effects on the formation of ultrafine PM.Results showed that the volatile mineral species played an important role in the formation of ultrafine PM and the combined effects of increased Na and Cl in coal blends resulted in the increase of ultrafine PM yield.Sodium（Na）played the most important role,which controlled the portioning of Cl and S into ultrafine PM.The vaporization behavior of the refractory SiO₂ during the formation of ultrafine PM were studied based on both modeling and experiments.Firtsly,an improved model describing the vaporization behavior of SiO₂ in O₂/N₂/CO₂/H₂O atmospheres was established,and then checked via the synthetic char（with SiO₂ inclusions）combustion and PM collection experiments on the DTF.The predictions for the vaporization behavior of SiO₂ with the improved model agreed well with the experimental measurements.Results confirmed that SiO₂ vaporized as relative volatile intermediate products.Particle temperature played a key role in the vaporization of SiO₂.The increase in particle temperature and H₂ partial pressure due to the presence of H₂O offset the reducing effect of the decreased PCO/PCO₂ ratio（partial pressure ratio of CO and CO₂）and enhanced the vaporization of SiO₂.Searching for new effective additives to reduce the migration of core mineral matter into the ultrafine PM during the coal combustion was carried out in two experiments.At first,8 kinds of mineral were selected and experiments were carried out to determine the effects of each mineral additive on the formation of ultrafine PM.And then further combustion of sodium acetate（NaAc）and additives was conducted to explore the reduction mechanism of the additives.Kaolinite,montmorillonite,attapulgite and anatase could reduce the ultrafine PM formation.Anatase had an ultrafine PM removal efficiency of～39%under the experimental conditions,which was higher than that of kaolinite（～22%）.Meanwhile,kaolinite showed best performance in reducing PM_2.5.Additives such like anatase reduced the ultrafine PM emission mainly through capturing Na-contained vapour.Field experiments of adding kaolinite into the feed coal in a commercial 1000 MW utility boiler were performed to explore the feasibility of additive in reducing ultrafine PM under the real combustion conditions.Detailed information on the mass yield,size distribution,composition and morphology of the PM emitted from the furnace,PM removal performance of ESPs and properties of the ash were obtained.Adding kaolinite reduced the formation of PM0.3 and PM_2.5 significantly.Kaolinite addition reduced the concentration and the specific resistance,and thereby changed the fractioned removal efficiency of PM_0.3 and PM_2.5.Most importantly,the PM emission at the outlet of ESPs was reduced.Series of field measurements were performed to characterize the formation and emission of ultrafine PM and PM_2.5 from a 1000 MW ultrasupercritical（USC）utility boiler,two 135 MW circulating fluidized bed（CFB）power station units,ESPs and FFs respectively equipped in two 300 MW power station units that were equipped with identical utility boilers and SCR DeNOx units,two wet electrostatic precipitators（WESPs）at two 300 MW power station units.PM_2.5 emitted from the 1000 MW boiler exhibited a remarkable ultrafine modal peak.Moreover,the increasing use of 1000 MW USC utility boilers is likely to reduce the PM_2.5 emission,particularly when advanced dust removal devices are employed.PM_2.5 emitted from the coal combustion in CFB boilers was of uni-modal size distribution without obvious ultrafine modal peak and the only size peak located in the coarse mode.The CFB boilers had a higher PM_2.5 yield than the pulverized coal-fired boilers.The tested ESPs showed lower removal efficiency for PM in the size range of 0.1-1 μm and showed significant "penetration window".The PM_2.5 collection efficiency of FFs was 99.95%,which was only marginally（～0.32%）higher than that of the ESPs.The FFs presented lower capture efficiency for PM less than 2 μm.The studied WESPs exhibited a good performance on the removal of PM less than 2 μm meanwhile some new PM enriched in S and Na was produced in the WESP due to slurry droplet entrainment,which increased the emission of PM especially those larger than 2 μm.