Mercury Reserves In Coal Combustion Residuals Inventory From Coal-fired Power Plants In China

Recently,coal combustion residuals（CCRs,including bottom ash,fly ash,and gypsum）from national coal-fired power plants（CFPPs）in China have increased year by year,the inventory of Hg reserves in CCR from national CFPPs in China could be used to evaluate the potential risk of solid Hg waste and help the government safely dispose of solid Hg waste at a national level.This study integrated both national sampling campaign and literature survey to summarize distribution proportion of Hg in CCR of 124 coal-fired power plants.The124 CFPP units were divided into 11 groups according to their types of boilers and APCDs.We used Crystal Ball method and Bootstrap method to simulate the distribution proportion of the type of CCR.The simulations of Crystal Ball method and Bootstrap method revealed that all the Hg distribution proportions in bottom ash and gypsum among the 11 APCD groups met the lognormal distribution.Hg distribution proportions in fly ash met the beta or Weibull distribution.Hg distribution proportions for stack emissions met the beta or lognormal distribution.The Hg distribution proportions in bottom ash,fly ash,gypsum,and stack gas was 0.03-0.75%,11.2%-93.8%,5.1-68.6%,and 3.1-53.9%,resepectively.We developed highly resolved inventories of Hg distributions in CCR and Hg emissions to air from national CFPPs in China using a mass distribution model.The model combined data from individual power plants,provincial Hg concentrations in coal,distribution proportions of Hg in CCR in various APCD combinations.The results illustrated that 2.2 tons（0.7-16.9 tons）,154.4 tons（98.5-277.8 tons）,and 56.1 tons（35.1-119.9 tons）of Hg were stored in bottom ash,fly ash,and gypsum from China’s CFPPs in 2018,respectively.Spatial distribution of Hg reserves in CCR illustrated strong regional heterogeneities.High Hg reserves in CCR were concentrated in East China and North China Plain.The utilization or deposition of fly ash and gypsum would cause secondary releases of Hg.As the most important utilization approach of fly ash,cement plants re-emitted 3.6 tons of Hg into the air and left 35.0 tons of Hg in the cement clinker.The utilization approaches of fly ash including the manufacture of lowend building materials,manufacture of concrete,and recycling process in paving/minebackfilling caused 20.0 tons,15.4 tons,and 4.6 tons of Hg immobilized in materials,respectively.42.0 tons of Hg flowed into various industries as gypsum was utilized,29.7 tons of Hg flowed into cement plants.10.1 tons of Hg flowed into construction industry and 3.1 tons of Hg was re-emitted into the air.The remaining Hg entered the other industry,such as agriculture.Atmospheric Hg emissions were estimated to be 56.8 tons（35.2-106.7 tons）using the mass distribution model,which were higher 28.9% than those estimated by an emission factor model.Under two methods,atmospheric mercury emissions in Hainan and Jiangsu were greatly different due to the mercury was re-emitted to atmosphere during the long-term storage of gypsum in APCD,which was underestimated in emission factor method.Additionally,we investigated the future trend in the reserves and gas emissions under various scenarios of future installation rate of the ULE technology.Scenario simulations informed that more Hg in flue gas and gypsum would be transported to fly ash with the increasing installation rate of the ultralow emission technology in future CFPPs,which further inform a new challenge regarding the disposal of solid Hg waste.However,as coal consumption declines,mercury in solid waste and atmosphere will gradually decline,which could reduce the risk of Hg pollution.The results can help the government initiate policies to safely dispose of solid Hg waste and develop treatment criteria from CFPPs,as required in the Minamata Convention on Mercury.