Research Of Hydrothermal Dewatering Process Effect On Low Rank Coal Property And Heavy Metal Elements Transfer

Coal is China's most important primary energy.With the growing economy demands for energy,large-scale utilization of low-rank coals becomes more imperative.However,low-rank coals have disadvatanges like high moisture content,low heat value,making it siganificant to remove moisture from the low-rank coals and improve coal quality.Hydrothermal dewatering(HTD)is an effective method to remove moisture.As the water is removed as liquid,the latent heat of vaporisation is saved and high efficiency is achieved.HTD process is likely to change coal property.Trace elements mercury(Hg),arsenic(As)and selenium(Se)are highly volatile so that they are hardly captured in the flue gas,which may lead to serious air pollution.So researchers spent much of their time on how to solve this problem.It was reported that HTD can both decrease the moisture of coal and remove hazardous trace elements in coal.So this research was designed to verify the removal capability of both water and trace elements(Hg,As,Se)by HTD.Coal samples selected in this research was three coal from Inner Monglia.Firstly influence of HTD on basic coal physical and chemical properties was investigated,including proximate/ultimate analysis of coal,pore change,IR spectrum and pyrolysis characteristic analysis.From the results of proximate and ultimate analysis,it could be found that moisture and volatile in coal decrease,heat value and coal rank increase after HTD.For pore structure analysis,moisture removal processes lead to pore collapse,making parameters such as specific area,average pore diameter and pore volume decrease.However,with the growing of HTD temperature,these paremeters start to rise,which could be explained by volatile loss process producing new pores.For IR spectrum analysis,oxygen function groups and aliphatic hydrocarbon groups decrease while aromatics hydrocarbon groups increase,indicating that coal rank improved during HTD process.Pyrolysis experiment results showed that pyrolysis characteristic of upgraded coals changed.Due to the loss of volatile during HTD,weight loss of pyrolysis decreased and activation energy increased.Next occurrence modes of trace elements in coal were studied through the sequential chemical extraction procedure(SCEP)method.In this paper four fractions were specified,that was,exchange species fraction,carbonate fraction,sulfide fraction and residual fraction.According to the exctration results,Hg in BR coal and YM coal were pyrite associated(sulfide fraction),while Hg in PZ coal was in residual fraction;As in three coals were primarily pyrite associated,then some were in arsenate mineral;Se was most in residual fraction and sulfide fraction.Considering organic fraction was likely to exist in the residual fraction,Se could concentratet in organic fraction and sulfide fraction.Floating experiment results showed different mass distribution for different coal.The largest mass portion of YM coal occurred in density section<1.3 and 1.3?1.4g/cm3,While the largest mass portion of PZ coal occurred in density section 1.3?1.4 and 1.4?1.5 g/cm3.Trace elements were found to concentrate on high density section,especially on>1.8 g/cm3.It indicated that a large portion of trace elements were associated with high density inorganic minerals.On the base of study above,influence of HTD on trace elements transfer were invwstigated.With the comparsion of trace elements concentrations in dry basis coal samples before and after HTD process,it can be found that all the elements were removed by HTD over different extents,that was,Se was the most easiest,Hg was next,and As was hardest.Further mass balance analysis showed almost 37%of total Se,31%of total Hg and 19%of total As in raw coals were removed when the temperature was 300?.Trace elements output were all in solid and liquid phase.From the SCEP results,it can be observed that organic fraction reduced most among four fractions during HTD process.XRD results showed HTD have little effect on major minerals in coal,but it may get rid of minor minerals.Comparing trace elements removal efficiency of pyrolysis and HTD process,it was found that,for removal of Hg,pyrolysis behavior plays a key role in the HTD process;for removal of As and Se,it was not..Finally the mechanism of HTD effect on trace elements was discussed.Removal of trace elements by HTD principally was the outcome of pyrolysis behavior and good solubility of slightly solubility substance in coal.Thermodynamic equilibrium calculations by Factsage were performed to predict the distribution of trace elements during combustion and gasification.The variable paremeters considered in this paper were atmosphere,pressure and temperature for both combustion and gasification processes.Calculation results showed that atmosphere and pressure are low factors for both combustion and gasification,and temperature is the key factor to determine species generated.Compared to gasification,output of combustion concentrates in fewer species.Hg is in Hg(g)phase,most of As is in As4O6,and most of Se is in SeO2.