Mercury Speciation In Coal And Transformation And Stabilization In Ultra-low Emissions Coal-Fired Units

As one of the poisonous trace heavy metal elements in coal,mercury(Hg)has been widely attracted around the world due to its high volatility and strong accumulation during the processes of coal conversion and utilization.The mercury speciation in different-ranked coals and its transformation in ultra-low emissions coal-fired units were thoroughly investigated based on the data collecting from coalfields and power plants together with laboratory test and theoretical analysis.Moreover,several methods were attempted to stabilize Hg in desulfurization slurry from an ultra-low emissions coal-fired unit.A combined experimental device was designed and built for exploring Hg release behavior from coal,mainly including two-staged temperature control,oxidation combustion and on-line monitor.Considering Hg is a trace element in coal,all experimental data in this dissertation are based on the mass balance of mercury in the overall system.The speciation and thermal stability of mercury was investigated in the coal samples with different ranks.The effect of coal rank on the distribution of Hg speciation was examined,and the dependence of Hg on organic sulfur and inorganic sulfur was explored.To better understand the speciation and transformation of mercury in raw coal and products from the coal washing process,the effect of inorganic minerals on Hg migration and transformation was investigated.Furthermore,the flue gas,raw coal,and combustion by-products were sampled under the stable operational conditions to obtain Hg migration and transformation in ultra-low emissions coal-fired power units,and the transformation and stabilization of Hg was explored for WFGD slurry.The main contents and conclusions are listed below:Hg speciation and its thermal stability in 14 samples from Ningwu Coalfield and Qinshui Coalfield in Shanxi,China,including peat,lignite,sub-bituminous coal,bituminous coal and anthracite,were investigated by using the combination of temperature-programmed decomposition method coupled with sequential chemical extraction and thermal analysis.The main Hg speciation can be divided into exchangeable mercury(F1),carbonate+sulfate+oxide bound mercury(F2),silicate+aluminosilicate bound mercury(F3),sulfide bound mercury(F4)and organic bound mercury(F5).The results show that F2,F4,and F5 are the main species of Hg in coal.accounting for more than 90%of the total mercury in each coal sample.Hg migration route in the coalification process is obtained from Hg speciation in different samples with different coal ranks.Generally,the proportion of F2 and F5 decreases with the degree of coalification,while the proportion of F4 increases significantly.Hg enriched in carbonates,sulfates,and oxides might migrate to sulfides with the transformation of minerals during the coalification process.The thermal stability of Hg speciation is ordered as F1<F5<F2<F4<F3.Meanwhile,the release of Hg is accompanied with the sulfur-containing gas during coal pyrolysis,which is heavily dependent on the rank of coal.Based on the Hg content and speciation in 59 different-ranked samples from Shentou Mine,Xuangang Mine and Antiabo Open Pit,and 12 gangue samples from the coal washing plants in Ningwu Coalfield,a two-step statistical analysis method for the relationship between mercury and sulfur was suggested by coupled Spearman rank correlation and Pearson linear correlation.The dependence of mercury on sulfur in coal was investigated,which was combined with both the degree of coalification and Hg speciation distribution in coal.It can be found that Hg enrichment degree is affected by both organic and inorganic minerals in coal.Generally,the organic sulfur has the biggest impact on Hg in peat,which becomes weak with increasing the coalfication degree from lignite to bituminous coal.The relationship between Hg and inorganic sulfur is depending on the contents of both Hg and ash within coal.As sulfate content is small in coal,Hg content related to sulfate is only found in peat or lignite.However,the linear correlation coefficients of total Hg content and pyrite sulfur in lignite,sub-bituminous and bituminous coal are 0.479,0.709 and 0.887,respectively,which indicates that the correlation between mercury and pyrite sulfur increases with the degree of coalification.The strong correlative relationship between organic bound mercury(F5)and organic sulfur only found in peat with a linear correlation coefficient of 0.812,which confirms that the total Hg content in peat is mainly determined by the content of organic sulfur.The linear correlation between inorganic bound mercury and ash content in lignite,sub-bituminous coal and bituminous coal gradually increased with the value of 0.502,0.757 and 0.837,respectively,which implies that both mercury and sulfur enrich in coal largely due to organic state migrating to inorganic state with the increase of coalification degree.The correlation between Hg and pyrite sulfur in coal increases with the increase of coalification degree.Furthermore,the correlation analysis between the Hg speciation and sulfur content confirms that Hg has a strong affinity with sulfur in coal and the feasibility of the sequential chemical extraction method for identifying Hg speciation.The washing process of raw coal is the first step of clean coal combustion and conversion.The transformation of Hg speciation is investigated in raw coal and products from 2 coal washing plants at Shentou Mine and Xuangang Mine,respectively,in Ningwu Coalfield,Hg migration was analyzed by considered the thermal release characteristics of sulfur in coal.It can be found that the redistribution of Hg speciation is the main reason for the difference in the Hg enrichment in coal washing products.The migration characteristics of Hg in coal is mainly controlled by inorganic minerals.The Hg content of gangue is 322.8%higher than that in raw coal,while the content of Hg in cleaned coal decreases to 40%with the removal efficiency of 56.4%.The Hg speciation in raw coal dominates the Hg distributions in different coal-washed products.56.6%of Hg associated with sulfides is concentrated in gangue,but the Hg associated with organic matters in cleaned coal and slime is higher than that in raw coal.The effects of pyrolysis temperature,reaction time and Hg speciation on the thermal release of mercury and sulfur from coal were conducted.The results show that the binding degree of Hg and its occurrence in coal are relatively weaker than that of sulfur.The removal rate of mercury is more than 50%at 500?,while the release rate of sulfur is only about 20%at 800?.The thermal release characteristics of Hg is also heavily related to its speciation in nature,which determines the Hg release behavior directly depends on the pyrolysis temperature of coal.The majority of organic bound Hg is released with the decomposition of organic matter when the temperature is lower than 300?,whilst sulfide bound Hg mainly releases between 400? and 600?.Hg is easy to release from cleaning coal and slime at relative low temperatures,which is largely due to high volatile content linked to organic mercury.Furthermore,the kinetics characteristics of mercury accompanied with sulfur is obtained during the coal pyrolysis.Currently,almost of all coal-fired power generation units in China have realized the ultra-low emissions for dust,SO2 and NOx.Hg content in coal,flue gas,bottom slag,fly ash and gypsum were on-line sampled from four 300 MW-leveled units in Shanxi Province and compare with two pulverized coal furnaces and two circulating fluidized bed ones under the stable operational conditions.The synergic control of Hg by the existed flue gas purification devices was further understanded through the migration and transformation of Hg in the ultra-low emissions units,and then the thermal stability of Hg in feed coal and combustion by-products was investigated.The field tests show that Hg content in bottom slag is extremely low and almost all of the Hg in coal released into the flue gas.With the decrease of flue gas temperature,the Hg concentration in flue gas shows a step-typed decrease from the inlet of precipitator,the inlet of wet desulfurization tower,and the inlet and outlet of wet electric dust removal.The fly ash of CFB unit has a stronger ability to capture mercury in flue gas,which makes the Hg concentration of flue gas is reduced to less than 1 ?g/m³ at the outlet of precipitator in front of desulfurization tower.However.the Hg concentration in flue gas of PC unit at the outlet of precipitator is about 6 ?g/m³.and is reduced to 1 ?g/m³ at the outlet of desulfurization tower.Therefore,the desulfurization tower shows a certain synergistic effect of mercury removal in PC units.After coal is burned in the furnace,Hg migrates into fly ash and desulfurization gypsum,resulting that the change of Hg speciation and thermal stability accordingly.Although F2.F4 and F5 are still the main Hg speciation in feed coal,the inorganic speciation of mercury(F2+F4)in feed coal of CFB unit is much higher than that of PC unit since the low calorific value coal with high ash content is generally used for CFB unit.The mercury in fly ash of the two types of units mainly exists as F2,which might consist with HgCl2,HgO and other inorganic mercury.In desulfurization gypsum,the majority Hg speciation is F2 together with a small amount of F4,which should correspond to HgSO₄,HgO,HgCl2 and HgS.In order to reutilize both wastewater and gypsum from the ultra-low emissions coal-fired unit,the migration and transformation of Hg was investigated by using the gas-liquid-solid slurry phase from a 330 PC unit,and three additives were added for preventing Hg release from desulphurization slurry.The gaseous Hg0 increases with the increase of slurry temperature,while Hg contents increase in both gas phase and gypsum medium with the increase of pH in the slurry.The concentration of Cl-or SO₄2-increase in slurry could inhibit the reduction of Hg2+ to Hg0 and increase Hg proportion in gypsum,but SO32-accumulation is beneficial to the Hg enrichment in gypsum and a part of Hg2+ reduced into HgO.The thermal release characteristics and environmental risk of mercury in gypsum and its mechanism were further explored.The effect of additive on the secondary Hg release was investigated from the desulfurization slurry.When an additive is added,Hg2+ is turned into the insoluble and makes more than 75%Hg transfer to gypsum,among of which DTCR-4 is the best one.Meanwhile,the additives used in this study can inhibit the re-emission of Hg0 reduced from Hg2+.The solidification effect of three additives on mercury in the slurry was compared,and the thermal release and environmental toxicity of mercury in solid gypsum were analyzed when the additive was added,and the change of thermal stability and environmental stability were discussed from the perspective of the Hg speciation in gypsum.Generally,the temperature of Hg thermal release is decreased in gypsum after the additive is introduced,which is mainly due to the stability difference between original gypsum and Hg(EDTA)2,[Hg-DTCR]n or HgS(black).The chemical stability of mercury in gypsum is improved,which is directly related to the contents of water-soluble mercury,acid-soluble mercury and oxidizable mercury in gypsum.The findings in this dissertation could provide a series of fundamental data and the corresponding operational suggestions for reducing mercury emissions in flue gas from the coal-fired unit and utilizing wastewater and solid gypsum as resources from the wet flue gas desulfurization tower.