Study On Ash Deposition In Boiler Based On Modes Of Occurrence And Transformation Of Mineral Matters

All coals have a significant content of ash-forming inorganic material that cannot be economicallyremoved before combustion. Uncontrolled ash deposits on the heat transfer surfaces can interfere withoperation, and cause unplanned shut-downs or reduced output and efficiency. For the purpose to alleviate thoseabove ash related problems, this dissertation mainly contains the following parts:For Shenhua coal, of which the iron and calcium content are very high, and the AFTs are very low, thushas high ash deposition propensity, method of chemical leaching combined with ICP-AES and XRD is used todetermine the modes of occurrence of calcium and iron mineral matter and other aluminosilicates, the fuelproperty associated with high ash deposition propensity was determined.Model predicting the ash fusion temperatures using ash composition as oxides was built based onlaboratory bulk analysis. This part mainly focused on the discussion of advantages and disadvantages ofnonlinear neuron net(NN) and linear partial least square regression(PLSR) algorithms. Cross validation(CV)with variable importance for projection(VIP) and predicted residual sum of squares(PRESS) criteria are usedfor improving the feasibility and generalization of PLSR model, The results show that better model can bebuilt to associate the oxides content and the AFTs.Analysis and experiments were carried out on a case that ash deposited on the heat-insulationlayer(refractory) in one boiler burning coal water slurry with high AFTs(with IDT above 1400℃). Theenrichment of both iron and calcium leads to strong sintering propensity of the deposited particles, and thestrength developed by sintering provides forces to prevent deposits from shedding. The effect of coal waterslurry(CWS) reburning for NO_x reduction on ash deposition was also studied. The results show that due to thereburning of CWS, the local incident heat flux near the reburning burner is slightly decreased, which causeslonger oxidation history of char particles, thereby longer time is provided for the coalescence andagglomeration between included iron containing particles and other silicates; meanwhile, because of the localreducing atmosphere when reburning, the oxidation history of excluded pyrite discrete from coal particle isextended, so the duration time of molten Fe-S-O eutectic phase which has stronger deposition propensity thanthe final oxidation product hematite(Fe₂O₃) is prolonged, so that the deposition propensity of excluded pyriteproducts is higher. The incident radiation heat flux to the mineral particles is slightly lower near the reburningburner, which causes weaker fragmentation of the excluded calcite and pyrite particles, thus larger particlecontaining iron and calcium are formed, these larger particle are easily transported to the deposition surface byinertial impaction.Experiments were carried out on the pilot-scale combustion facility to investigate the depositionpropensity of Shenhua coal using probe measuring the heat flux in the furnace, a couple indices, viz. sheddingperiodicity of the deposits T_shed and the decaying amplitude of the heat flux A_q, were given to quantificationallyevaluate the deposition propensity of different coals; and, for the purpose of asssessing the relative depositionpropensity, another method was provided with the SiC probes inserting in the furnace with the gas flowdirection parallel with the probe axis, the deposition rate was defined and used as the cretia.Experiments was also conducted to investigate the changing propensity of thermal conductivity of bothfly ash and ash deposits fromr boiler with temperature increasing.Finally, experiments and discussion were made on one boiler burning residual oil which had sufferedfrom sever ash deposition problem. The results showed that in the initial particulate layer, the major elementalcontent, viz, V, S, Na, Ca, Fe are much higher than the outer layer, while Si and Al show the adversetendency. The major minerals in deposits at the water-wall zone is mullite, while for deposits from thesecondary superheater tube bank(at the furnace exit), besides mullite there are considerable cristobalite andmicrocline (potassium aluminosilicates). Some vanadium oxide or salt were detected for samples from primarysuperheater tube bank and the economizer zones. Deposition is mainly attribute to faujasite in the resudial oil.