Study On The Adsorption Mechanism Of Mineral Components On Arsenic And Lead During Coal Combustion

Toxic elements such as arsenic（As） and lead（Pb） will be released into the flue gas in the form of As₂O₃,PbO,and PbCl₂ during coal combustion.If gaseous and fine particulate emissions are formed,it will seriously endanger the ecological environment and human health.The mineral components have good adsorption properties for gas phase arsenic and lead,and are rich in raw materials,which have been widely studied.However,the mechanism of action of different mineral components on arsenic and lead is quite different,and there are complex interactions in the adsorption process.At present,there is no unified understanding of the mineral components to control arsenic and lead during coal combustion,and there is insufficient guidance for industrial applications.This paper is based on experiments,combined with quantum chemistry calculations,in-depth study of the interaction characteristics of mineral components and arsenic and lead and its mechanism,revealing the law of competitive adsorption of acid gases with arsenic and lead;further screening of mineral components and exploring the adsorption reaction to construct a coal blending combustion model that couples the adsorption characteristics of arsenic and lead by mineral components.Finally,a technical route based on coal blending to optimize the blending of mineral components to achieve the control of arsenic and lead emissions from coal is proposed.The main conclusions are as follows:（1）The adsorption characteristics and mechanism of different mineral components to arsenic and lead.The main minerals in coal were selected as sorbents.The adsorption capacity of the mineral sorbents was tested on the self-made arsenic and lead generation-adsorption device,and the reaction mechanism of minerals adsorbing arsenic and lead was revealed by quantum chemical calculations.The results show that CaO has a good arsenic adsorption effect.The order of arsenic adsorption capacity at 700°C is CaO>Fe₂O₃>Na₂O>MgO>Al₂O₃>SiO₂.The valence states of arsenic in adsorption products are As³⁺and As⁵⁺.The calculation results show that the exposed O and Fe atoms on the surface of the sorbents can combine with As atoms.The arsenic adsorption is chemical adsorption.Al₂O₃ and SiO₂have a good lead adsorption effect,the adsorption effect on PbO is obviously better than that of PbCl₂,and the valence state of lead in adsorption products is Pb²⁺.The exposed Si atoms,O atoms,and Al atoms on the surface of the sorbents are the active sites for lead adsorption.The adsorption of PbO and PbCl₂ is chemical adsorption.The adsorption of arsenic and lead by Al₂O₃ is complementary.The exposed O and Al atoms on the surface of Al₂O₃ are the active sites for arsenic and lead adsorption,respectively.（2）Analysis of thermodynamic characteristics of arsenic and lead adsorption by mineral components.The mineral components in coal have a better effect on controlling arsenic at 700°C,while the effect of controlling lead at 900°C is better.At 300-900°C,the arsenic adsorption capacity of CaO,MgO and Al₂O₃ increases with temperature,while when temperature rise to 900°C,the particles melted,the pore structure collapsed,the arsenic adsorption capacity of Fe₂O₃ and Na₂O decreases rapidly.When the adsorption temperature is lower than 500°C,the arsenic in products is mainly As³⁺,when the adsorption temperature is higher than 500°C,the As³⁺in products transforms to As⁵⁺,which will help the stability of adsorbed arsenic.When the temperature increases from 300°C to 900°C,the Gibbs free energy of As₂O₃ adsorption by CaO increases from -137.04 kJ/mol to -4.51 kJ/mol.At 700-1200°C,the lead adsorption capacity of Si/Al-based sorbents first increases and then decreases.The mixture of SiO₂ and Al₂O₃ has an interactive effect to promote lead adsorption.As the temperature increases from 300°C to 1500°C,the Gibbs free energy of PbO adsorption on SiO₂（001）increased from-312.36 kJ/mol to 32.42 kJ/mol,and that of PbCl₂adsorption increased from-75.73 kJ/mol to 30.53 kJ/mol.In addition,after calcination at 1300°C,the arsenic and lead adsorption capacity of sorbents decreased.（3）The influence mechanism of different components in flue gas on the adsorption behavior of arsenic and lead.The competitive adsorption mechanism of HCl/SO₂/CO₂,arsenic and lead by mineral sorbents was revealed,the release,and condensation characteristics of Na compounds in high-alkali coal was explored,and the effect of condensed NaCl on arsenic and lead adsorption was investigated.Acid gases have obvious inhibitory effect on arsenic adsorption,but hardly affect lead adsorption.The inhibitory strength of acid gases at 700°C is HCl>SO₂>CO₂.The release characteristics of Na compounds in coal are mainly affected by temperature,and at 600-800°C,Sodium is mainly released in the form of NaCl.When the temperature drops to 700°C,the Na compounds in the flue gas begin to condense.The condensed NaCl changes the surface melting characteristics,pore structure,and mass transfer process of the adsorbent process,inhibits the arsenic adsorption by Mg O,Fe₂O₃,and CaO below 700°C,but promotes the arsenic adsorption by Al₂O₃ and SiO₂.It promotes the lead adsorption by CaO,Fe₂O₃ and Mg O,but inhibits the lead adsorption by Al₂O₃ and SiO₂.（4）A technical scheme for the control of arsenic and lead emissions by the mine components of blending coal was proposed,and a coal blending model with coupled arsenic and lead control was constructed.Based on the characteristics of the above-mentioned mineral components to adsorb As₂O₃ and PbO,a coal blending blending model was constructed.An index that can quantitatively describe the release of heavy metals is proposed,and a model for controlling arsenic and lead in coal-fired flue gas by mineral components is constructed.Using this model can predict the concentration of arsenic and lead emissions from coal combustion,and can provide guidance for the reduction of arsenic and lead in flue gas during coal combustion.