Experimental Study On Heterogeneous Ash Formation And Fouling Characteristics Of A High-alkali Coal Under Oxyfuel Combustion

Fossil fuel,especially the coal,will occupy the dominant position in the energy structure of China.Oxy-fuel combustion is an advanced CO₂ capture and storage technology for coal-fired boiler,and has received extensive attention in the recent years.The different atmosphere in oxy-fuel combustion makes the heat transfer of the boiler,the characteristics of coal combustion and the emission of the pollutants changed greatly compared with conventional air combustion.These changes will also affect the transformation behavior of minerals,and then lead to the change of the deposition characteristics in the boiler.However,there is still no agreement on the ash deposition trend under oxy-fuel combustion.Based on these risks,the heterogeneous characteristics of ash formation and ash deposition behavior of high-alkali coal under the oxy-fuel combustion was studied under different test rigs.Firstly,CCSEM was used to study the heterogeneous characteristics of minerals during the oxy-fuel combustion in a high temperature drop furnace.And the melting trend of ash is discussed in detail by studying the characteristic of size and composition distribution of calcium and iron in the minerals.The results indicated that compared with 20%O₂/N₂ condition,the coalescence and fragmentation of minerals was inhibited and the average size of the fine mineral particles was decreased in the oxy-fuel combustion with the same O₂ concentration;with the increasing of the O₂ concentration,the melting trend of ash was increasing.It also can be found that oxy-fuel combustion could inhibit the decomposition and fragmentation of calcite and pyrite,and decrease the concentration of calcium and iron in the small and medium particles.There were differences between the transformation of Ca-related minerals and Fe-related minerals when the O₂ concentration was increasing in the oxy-fuel combustion.For the Ca-related minerals,Higher O₂ concentration promoted the enrichment of Ca in the small and medium particles and the co-melting reaction of Ca-related minerals with other minerals,which led to higher trend of ash melting and deposition.For the Fe-related minerals,the content of Fe in the small and medium particle is increased partly with the increasing of O₂ concentration;but the main reaction of the pyrite was the oxide reaction to form the iron oxide.When the O₂ concentration was increasing in oxy-fuel combustion,the stronger reducing atmosphere and shorter existence time of reducing atmosphere resulted in the content of Fe-related sticky particles and the trend of deposition increasing firstly and then decreasing.Secondly,the effect of flue gas temperature,combustion atmosphere and the blended coal combustion on the deposition of coal with high sodium and calcium was conducted in high temperature drop tube furnace by using an ash deposit probe with water-cooling.It could be found that the ash deposition was sintering and the composition was mainly calcium sulfate when the flue gas temperature was higher than 1150? in the 20%O₂/N₂ condition;while the flue gas temperature was 900 ?,the deposition was obvious layered and the composition of deposition was alkali metal sulphates which made the deposition increased.The blended combustion and high concentration of CO₂ did not change the composition of the deposition;the tendency of slagging was alleviated after blending,but aggravated by the high concentration of CO₂.Thirdly,the deposition behavior of high sodium and high calcium in the combustion process on a 350MW power plant burned a high-alkali coal was studied by using ash deposition probe with air-cooling,and the deposition mechanism of high sodium and high calcium was obtained.It was investigated that the type of deposition on the surface of water wall was iron-phase melting minerals with low melting point;for platen superheater,high temperature superheater and high temperature reheater,the inner layer of the deposit was adhered tightly to the surface and exhibited as the typical deposition of the initial layer,and Na2SO4 deposition appeared on the windward surface obviously,as well as Ca-related sticky deposit particles.In the outer layer of the deposit,the main form of silicon was SiO₂ and co-melting aluminosilicate minerals with Na and Ca,while CaSO₄ is the main form of calcium.Heterogeneous condensation and thermophoresis of Na2SO4 in the inner layer of deposit and the enrichment of calcium in the outer layer of deposit was the main reason that caused the serious slagging and fouling on the heat exchangers.Finally,the characteristics of boiler and ash deposition under oxy-fuel combustion were studied on 3 MWth oxy-fuel combustion full scale facility.It indicated that the whole system could operate safely and stably under different conditions for a long time.It was found that the CO₂ concentration of dry flue gas could be more than 80%stably and the co-melting of minerals in oxy-fuel combustion enhanced the trend of ash deposition on the surface of the heat exchanger.