Sodium Behavior In Zhundong Coal C Ombustion And The Effect Of Phosphorus-Rich Additives On Combustion And Ash Fusion Characteristics

Zhundong coal is rich in reserves,shallow buried and easy to exploit.However,due to its high sodium content,serious slagging and fusion problems occurred in the actual combustion of Zhundong coal,which affected the large-scale use of Zhundong coal.Aiming at the high sodium in Zhundong coal,this paper mainly studied the effects of ash model compounds on combustion performance and ash fusion characteristics of synthetic coal,the effects of phosphorus-rich additives on combustion performance and ash fusion characteristics of Zhundong coal,and gas phase transformation of sodium during the combustion of Zhundong coal.The basic physical and chemical properties of Zhundong,such as combustion behavior,emission and ash fusion characteristics were studied.The results showed that Zhundong coal was easy to ignite,had short combustion process and concentrated heat release.The main combustion stages were concentrated in 300??600?,accompanied by the release CO2,CO and H2 in large amounts.The obvious release of SO2 above 1000? proved that sulfate minerals can be produced in large quantities in ash.According to the XRD results of ashing of three Zhundong coals at 800?300??1200?,the essential causes for the easy fusion and serious slagging of Zhundong coal were obtained:The conversion of high melting point silicates,CaO and periclase into fusible or low melting point minerals,such as albite,nepheline,diopside,other feldspar and pyroxene minerals resulted in the decrease of the proportion of refractory minerals,the increase of fusible minerals and amorphous components,making the ash fusion temperature in low level.Taking synthetic coal as the research object,the effects of four kinds of ash components Na2O,Fe2O3,CaO and MgO,and coupling interaction of calcium and sodium,magnesium and sodium and iron and sodium on the combustion behavior and ash fusion characteristics of synthetic coal were studied by means of thermogravimetry-differential heat combination and ash fusion tests.The results showed that synthetic coal could recover the combustion and ash fusion characteristics of Zhundong coal,and reliable experimental results could be obtained;the increase of Na2O could improve the combustion performance of synthetic coal,while the increase of CaO,MgO would lead to the decline of comprehensive combustion performance.Fe2O3>3%,the comprehensive combustion characteristics of synthetic coal were higher than those with low Fe2O3 ratio.With the increase of Na2O,and CaO,MgO in the range of 0%?22.5%and 0%?1%respectively,quartz,corundum,periclase and hatrurite with high melting poin were transformed into fusible minerals such as cinoenstatite,hedenbergite,resulting in the decrease of ash fusibility;the melting temperature of synthetic coal ash increased slightly with the increase of Fe2O3;excess CaO and MgO could increase ash fusion temperature,especially CaO.With the decrease of CaO/Na2O,MgO/Na2O,Fe2O3/Na2O ratio,the ash fusibility decreased significantly under the coupling interaction of calcium and sodium,while the ash fusibility improved under the coupling interaction of iron and sodium.However,in the coupling interaction of magnesium and sodium,too high or too low Mg/Na ratio could improve the ash fusibility,and the other ratios would cause the decline of ash fusibility.Using the same experimental methods,the effects of municipal sludge,phosphorus-rich model compounds AIPO4,Ca3(PO4)2 and NH4H2PO4 on the combustion behavior and ash fusibility of Zhundong Coal were investigated,and the mechanism of phosphorus on increasing ash fusion temperature and fixing sodium was summarized.The results showed that all four phosphorus-rich additives could reduce the combustion performance;both of them could increase the ash fusion temperature of ash,and the degree of increase was as follows:Ca3(PO4)2>NH4H2PO4>municipal sludge>AIPO4;modification of sludge with aluminium could improve the melting resistance;when PO43VNa>2,AIPO4 would act as flux instead.The mechanism of phosphorus on increasing ash fusibility and fixing sodium was as follows:the existence of phosphorus had changed the mineral transformation mechanism of ash;phosphorus inhibited the formation of low melting silicate minerals and the occurrence of low temperature eutectic phenomenon,phosphorus leaded to refractory phosphates(AIPO4,Ca3(PO4)2,Ca2P2O7,Mg2P2O7,Na3Fe(PO4)2,Na2CaP2O7)and formed in the ash,improving the fusibility of ash and reduce the release of sodium.Combining of chemical kinetics calculation and mass spectrometry experiments,the release of sodium in combustion and the gas phase sulfation pathway of sodium were investigated.The results showed that sodium was released mainly in the form of NaCl and accompanied by a large amount of HC1 before 400“C,while sodium released in the form of Na atom gradually became the main form of sodium release above 400?.A large number of sodium,sulfur and iron were enriched in the bottom slag of Zhundong coal-fired boiler in which Na2S2O3?Na6Fe(SO4)2?Na2Ca(SO4)2 and other sodium-containing sulfate minerals exist.It was proved that the sulfation process of sodium was an important factor in the initial stage of slagging.The main sulfation pathway of sodium in gas was established.It was found that the formation of Na2SO4 was closely related to the precursors NaSO4,NaCl and NaOH,and NaCl played a key role in the formation of Na2SO4 as both the product and the reactant.