Research On Prevention And Mechanism Of Slagging And Fouling During Zhundong Coal Combustion In A Circulating Fluidized Bed

Zhundong?ZD?coalfield,with a huge reverse,is the largest integrated coalfield in China up till now.Its large-scale development and utilization is meaningful for constructing Chinese energy system.However,severe slagging and fouling inevitably occur on heating surfaces of boilers during ZD coal combustion.How to utilize ZD coal safely and efficiently is an urgent problem to be solved at present.The object of this study is to experimentally and theoretically investigate the prevention and mechanism of slagging and fouling during ZD coal combustion in circulating fluidized bed?CFB?,in order to provide the theory basis and suggestions for the widely utilization of ZD coal.The main work and results include the following five parts:?1?The occurrence modes of alkali metals and its transformation characteristics during ZD coal combustion were investigated,and the results were compared with those obtained from Taer coal.The occurrence modes of alkali metals were acquired according to the sequential chemical extraction method.Na is the primary alkali metals in coals and exists mainly in the form of H₂O-soluble Na.The combustion experiments were conducted in an electrically heated tube furnace.The experimental results show that volatilized Na mainly come from H₂O-solube and NH₄Ac-soluble Na,while part of H₂O-solube Na will transform to HCl-soluble or none-soluble Na.The volatility of Na is closely related to the contents of Si and Al in coal.?2?The slagging and fouling of ZD coal at different combustion temperatures were investigated in a 30 kW CFB test system equipped with self-designed ash deposition probes.Through comprehensive analysis on deposited ash and fly ash,it is found that compacted slags in the furnace are triggered by the condensation of Na₂SO₄,and they are rich in low-melting compounds including Fe₂O₃ and CaSO₄.The deposited ash on the convection heating surface is relatively loose,and its formation has close relationships with the flow field of flue gas and particle size distribution of fly ash.Besides,the deposited ash will deteriorate the heat transfer of tubes.?3?The occurrence modes of alkali metals in the flue gas and the transformation characteristics of minerals in ash were theoretically investigated using Factsage software.Subsequently,the“temperature-dependent”ash deposition mechanism during ZD coal combustion in CFB were summarized in combination with the experimental results.?4?The influence of kaolin,SiO₂,and Al₂O₃ on the Na release and ash sintering temperature were experimentally investigated.According to the combustion experiments conducted in the electrically heated tube furnace,the order of Na capture efficiency for three additives is:kaolin>SiO₂>Al₂O₃;Ash sintering temperature was measured by the pressure-drop method.The experimental results show that SiO₂ can significantly increase the ash sintering temperature of ZD coal,whereas the influence of kaolin and Al₂O₃ are limited.The results can provide the theoretical basis for choosing kaolin as additives in the tests in Chapter 6.?5?The influence and mechanism of adding kaolin and co-combusting Shenhua?SH?coal on the slagging and fouling of ZD coal were investigated in the CFB test system.Kaolin can effectively suppress the volatilization of alkali metals,leading to the disappearance of compacted slags in the furnace.At the addition of 3%kaolin,the collection efficiency of ash deposition probe reaches the minimum value;SH coal has an important function on the dilution of alkali and alkali earth metals.At an addition ratio of more than 20%,the compacted slags practically disappear,while the ash deposition tendency in the convective backpass sharply decreases resulting from the increasement of particle size of fly ash.Among slagging and fouling indices based on ash composition,G and SiO₂/Al₂O₃ are comparatively consistent with the test results.