Understanding The Mechanism For Ash Deposition During The Combustion Of Zhundong Coal With A High Calcium

Zhundong Coalfield located in Xinjiang was a huge open-pit coal mine newly discovered with a storage capacity of hundreds of billion tons in China.The reserves were predicted as 390 billion tons,and it was the largest fully-packed coal field in China.So Zhundong coalfield would become a very important energy connection and strategic reserve area.Zhundong coal as a good power coal could cause severe slagging and fouling during combustion in boiler.This limited the unilities of Zhundong coal.Compared with other coal,the content of alkali and alkaline earth metals in Zhundong coal was high,and the phase transition temperature of these materials was low.The gaseous alkali and alkaline earth metals above a certain temperature of the flue gas reached the metal heating surfaces and condensated on it,causing ash deposition.The content of Calcium in Zhundong coal was rich.During the combustion,it was easy to form low-temperature eutectic with iron,silicon and other minerals in the ash.The low-temperature eutectic up to the melting state could precipitate,resulting severe slagging and fouling.Therefore,the effect of calcium on the formation of sedimentary layer burning Zhundong coal was studied by means of combination of experiment and calculation in this paper.The aim was to reveal the mechanism of slagging and fouling for Zhundong coal with high calcium from macroscopic and microscopic perspectives.The main conclusions were as follows.The migration characteristics of calcium was studied by the experimental during the combustion of Zhundong coal.The result showed that the presence of calcium-containing minerals in Zhundong raw coal mainly includes organic calcium in the form of calcium carboxylate,calcium sulfate as the main component of gypsum and anhydrite,calcium carbonate salts such as calcite and dolomite,and some mica minerals.The change of the coal particle size has little effect on the precipitation rate of the calcium during the combustion of Zhundong coal.With the increase of combustion temperature,the precipitation rate of calcium in Zhundong coal can increase significantly.It indicates that the combustion temperature has an important influence on the migration characteristics of calcium during the combustion of Zhundong coal.With the increase of oxygen content,the precipitation rate of calcium in Zhundong coal can gradually increase.The analysis of morphology and mineral composition for ash deposition in actual power plant boilers was studied.The result showed that along the direction of the flue gas flow,that ash deposits from R-WW,R-PR FR,FS and PS are composed of loosely distributed small balls and clinker,while deposits from D-PS and R-PS are made up of large body structures.The ash deposits can be classified into silicates and sulfates based on mineral analysis.The deposits from D-PS and R-PS contain more silicates and they are found to have low alkali acid ratio,low fouling index,high viscosity and high AFT,causing the formation of a hard slag.While the deposits from R-WW,R-PR,FR,FS and PS contain more sulfates and they have high alkali acid ratio,high fouling index,low viscosity and low AFT,causing the formation of fouling.The classification method of ash deposition along the flue gas flow direction was well verified by means of experiments of drop furnace and the analysis of thermodynamic equilibrium,and it was found that temperature is an important factor for the effect the different sediment samples.Finally,the deposition characteristics of Ca salt in the formation of initial layer for ash deposition were studied by the calculation of quantum chemistry,molecular dynamics and kinetic Monte Carlo.The behavior of Ca and Na particles adsorption on iron oxide film was studied by quantum chemical method to reveal the adhesion characteristics of alkali metal and alkaline earth metal on the metal heating surface.The calculation results showed that the surface energy ofα-Fe₂O₃（110）as the most stable crystal face is the lowest.The binding energies of Na and Ca on the surface ofα-Fe₂O₃（110）are-12.44 e V and-13.73e V,respectively.And the amount of the charge transfer for Ca atoms on the surface ofα-Fe₂O₃（110）is significantly higher than that of Na.The orbital hybridization of Na on the surface ofα-Fe₂O₃（110）is significantly weaker than that of Ca.This indicated that Ca is more easily adsorbed on the surface of the metal oxide film.It is verified that compared to Na,Ca plays a more important role t in the formation of the initial layer of ash deposition.Considering the structural defects of the actual crystal,the adhesion characteristics of Ca particles on the defect surface ofα-Fe₂O₃（110）were further investigated.The results showed that the binding energy of Ca on the surface of iron and oxygen defectsα-Fe₂O₃（110）is-15.13 e V and-4.16 e V,respectively.And the amount of the charge transfer for Ca on the iron-defect surface is significantly higher than that on the oxygen-defect surface.The orbital hybridization of Ca on iron-defect surface is stronger than that on the oxygen-defect surface.It indicates that the adsorption characteristics of Ca on the iron-defect surface is higher,and the iron-defect surface can be prone to cause the formation of actual initial layer.The actual deposition process is greatly affected by the temperature,and the porosity of the oxide film is high.Therefore,the temperature and the pore size parameters of the oxide film were introduced.The adsorption and diffusion characteristics of the calcium-containing minerals on the surface of the metal oxide film were studied by the mean of molecular dynamics theory.The result showed that under the same temperature,the binding energy between Ca SO₄ and the surface of the oxide film is higher than these between Ca O,Ca Si O₃ and the surface of oxide film,while the binding energy between Ca O,Ca Si O₃ and the surface of the oxide film is not much different.The bond length between Ca SO₄ and the surface ofα-Fe₂O₃（110）can be significantly smaller than those between Ca O,Ca Si O₃ and the surface ofα-Fe₂O₃（110）.It indicated that the reaction between Ca SO₄ and the surface ofα-Fe₂O₃（110）is a chemistry progress.With the increase of temperature,the diffusion coefficients of three kinds of calcium-containing minerals gradually improve,and the diffusion coefficients of Ca O and Ca SO₄ are significantly higher than that of Ca Si O₃.The diffusion coefficient of Ca O can be least affected by temperature,while the diffusion coefficient of Ca SO₄ can be most affected.This indicates that the system composed of Ca SO₄ and oxide film is greatly influenced by the temperature,causing severe ash deposition under high temperature.With the increase of pore size,the diffusion coefficient between Ca O,Ca Si O₃ and Ca SO₄ and oxide film increases gradually.The calcium-containing minerals at the large pore size are more easily filled into the pores of the oxide film.This makes the initial layer stronger and exacerbates the phenomenon of ash deposition.The coating can increase the surface compactness and reduce the adhesion to the adsorbate.So it may effectively alleviate the problem of slagging and fouling on the heating surface of the boiler.And it is worthy of engineering application.The kinetic Monte Carlo method was used to study the surface coverage of metal oxide film and the surface morphology ofα-Fe₂O₃（110）under single component and various components at different temperatures.The main conclusions included that the surface coverage of Ca O,Ca SO₄ and Ca Si O₃ inα-Fe₂O₃（110）increases with the increase of temperature under the single component.Under the single component,the adsorption rate constant of Ca SO₄is obviously higher than that of Ca O and Ca Si O₃.The number of particles of Ca SO₄on the oxide film is more than that of Ca Si O₃and Ca O.Under high temperature,the number of particles of Ca O on the oxide film is more than that of Ca Si O_3.But under low temperature,the number of particles of Ca Si O₃ on the oxide film is more than that of Ca O.This indicates that Ca SO₄ is more likely to deposit on the surface of the oxide film.Under multi-component,the content of Ca SO₄ on the surface of oxide film is significantly higher than that of Ca Si O₃ and Ca O.The content of calcium-containing minerals on the surface of oxide film improve with the in the increase of time under the same temperature.The time of the coverage for a full layer of particles on the surface of the oxide film decreases with the improvement of temperature at the same time.Under the same condition,the degree of particle compaction on the surface of the oxide film under the multi-component is obviously higher than that under the single component.But the deposition rate of the calcium-containing mineral under the multi-component is lower than that under the single component.The deposition rate of calcium-containing minerals.However,the overall deposition phenomenon is aggravated under multiple components.It indicated the three components exist the interaction force,promoting the formation of deposit layer.The mechanism of the action of calcium-containing substances burning Zhundong coal from the macroscopic and microscopic perspectives was revealed in this paper.And it provided a theoretical foundation for guiding actual engineering and solving the problems of slagging and fouling for Zhundong coal.