Migration And Transformation Of Nitrogen And Formation Mechanism Of N₂ During Coal Pyrolysis And Combustion

The emission of NO_x during coal combustion contributes to acid rain and photochemical smog formation,which would have a negative impact on quality of atmospheric environment.As a result,minimizing NO_x levels is critical for improving the efficient utilization of coal.Pyrolysis of coal is a key step in the coal conversion process.NO_x production is directly affected by the transformation of coal nitrogen during the pyrolysis process.The formation mechanism of pyrolysis products HCN and NH₃ has been quite clear.N₂is the main nitrogen-containing substance in the pyrolysis process and will not be oxidized to atmospheric pollutant NO_x during combustion.By increasing the formation of N₂ during pyrolysis,the terminal emission of NO_x can be effectively reduced.However,the mechanism of N₂ formation remains unknown.The formation mechanism of NO_x during combustion has been clarified.While N₂ produced during combustion is by direct pyrolysis of char nitrogen or the char nitrogen being first oxidized to NO_x and then reduced to requires further investigation.Clarifying the formation path of N₂ during pyrolysis and combustion is helpful to develop the technologies to control NO_x emissions,which is of great significance for reducing NO_x emissions.In this dissertation,the migration of coal nitrogen was studied by nitrogen balance experiment and quantum chemical calculation methods,as well as the reaction mechanism of N₂ formation during pyrolysis and combustion was studied at the molecular level.The main research contents and findings are as follows:First,the effects of pyrolysis temperature（800～1100 ℃）and inherent minerals on nitrogen migration during coal pyrolysis were investigated by nitrogen balance experiment.Increase pyrolysis temperature can promote coal nitrogen migration from solid nitrogen to volatile nitrogen and the formation of N₂.With the increase of temperature,the volatile nitrogen of raw coal increased from 24.12 wt.%to 35.36 wt.%,and the N₂generation content increased from 4.97%to 12.16%.While the presence of minerals in coal inhibits the migration of coal nitrogen to volatile and the formation of N₂.The volatile nitrogen and N₂ production of demineralized coal are higher than that of raw coal.The volatile nitrogen of demineralized coal increased from 29.21 wt%to 40.43 wt.%,and the N₂generation content increased from 12.45%to 14.48%.The mechanism of N₂ formation during pyrolysis and combustion was studied by density functional theory.The zigzag dinitrogen carbonaceous model with seven aromatic rings was selected to investigate the effects of different nitrogen forms（pyrrole nitrogen and pyridine nitrogen）on the formation of N₂ by using quantum chemistry method.The form of nitrogen does not change the process of coal pyrolysis to N₂.However,different nitrogen forms result in different charge distribution and bond strength.The rate-determining step energy barrier of N₂ formation from pyrrole-pyridine dinitrogen and dipyridine nitrogen carbonaceous models are 552.74 k J/mol and 492.36 k J/mol.According to the kinetic calculation,the reaction rate of N₂formation from dipyridine nitrogen structure is faster.Indicating that the char model with dipyridine nitrogen was easier to generate N₂during pyrolysis.The rate-determining step energy barrier for N₂ generation during pyrolysis is higher than that for HCN and NH₃ generation（197.90～444.70 k J/mol）.It indicates that the structure of char decomposed to N₂ is more stable than that decomposed to HCN and NH₃.Next,the effects of different dinitrogen active sites on the formation of N₂ from armchair and zigzag structures were further investigated.The different dinitrogen active sites in the armchair and zigzag structures primarily alter the electronic properties of the carbonaceous surface,resulting in different energy barriers required for the cleavage of the C-N bond in the structure.In the two structures of different active sites of armchair structure,the energy difference required for the first N and second N atom stripping is 7.60 k J/mol and 12.10k J/mol.The energy difference of the first N and second N atom in the two structures of different active sites of zigzag structure is 30.02 k J/mol and 174.46 k J/mol.However,it has no effect on the primary pyrolysis properties of carbonaceous structure.Despite the fact that the N in the structure is at different active sites,the path from reactant to product is essentially the same.The energy barrier needed for the rate-determining step of the zigzag configuration to generate N₂ is determined to be substantially higher than that of the armchair structure by examining the data of the investigated configurations.In other words,the seven-ring zigzag structure with double nitrogen atoms is more stable and is more difficult to decompose,which is more consistent with the published experimental results,indicating that the seven-ring zigzag dinitrogen configuration is more accurate to simulate pyrolysis to N₂.Second,nitrogen balance experimental methods were used to investigate the effect of catalyst on nitrogen migration during coal pyrolysis.It was found that Fe and Ca can promote the migration of coal nitrogen to volatile nitrogen and make more coal nitrogen migrate to N₂.The catalytic effect of Fe is stronger than that of Ca:At 1000 ℃,Fe can increase the volatile nitrogen from 39.06 wt.%to 46.50 wt.%,the N₂ conversion rate from13.85 wt.%to 20.45 wt.%,and Ca can increase volatile nitrogen from 39.06 wt.%to 43.37wt.%,the N₂ conversion rate from 13.85 wt.%to 18.75 wt.%.Then,the effects of catalysts Fe and Ca on the N₂ formation pathway were studied by quantum chemical calculations.The catalyst Fe can promote the formation of N₂by shorting the reaction pathway of pyrolysis and lowering the rate-determining step energy barrier by 102.09 k J/mol.Ca prevents the formation of N₂ by raising the energy barrier of the rate-determining step by47.48 k J/mol.This may be because Ca does not stimulate N₂ production during pyrolysis from all of the structures.Finally,the effects of combustion temperature（800～1100 ℃）and catalyst type（Fe and Ca）on NO_x release during coal combustion were investigated.The high combustion temperature（≥900 ℃）makes demineralized char to have better reduction performance,so the reduction rate of NO_x is much higher than the formation rate of NO_x,and the amount of NO_x generated during the combustion of demineralized coal reaches its peak at 900 ℃.For the demineralized coal loaded with catalysts,the conversion of char-N to NO_x decreases monotonically with the increase of temperature,indicating that the catalytic effect of the catalyst on NO-char reduction is more significant than that on char-NO_x oxidation at high temperature.The catalytic effect of Fe and Ca is mainly reflected in:the catalyst has reduction performance at high temperature during combustion(Fe²⁺/α-Fe),which can reduce NO_x to N₂;the formation of metal nitrides can reduce the activation energy of the reaction which makes it easier to decompose into N₂.Fe and Ca have greater catalytic effect,and the catalytic effect of Fe is more visible.Then the formation pathways of N₂and NO during combustion were analyzed in detail by quantum chemical calculations.It is easier to generate N₂ from O₂directly adsorbed on the surface of the R2 structure at the C2 position.And it is easier to generate NO with a single O atom adsorbed at the N6 position.During combustion,N₂ is produced in six steps,and the energy barrier of the rate-determining step is 398.12 k J/mol.NO is formed in only three steps,and the rate-determining energy barrier is 372.15 k J/mol.It can be seen that it is easier to generate NO during combustion.The energy barrier required for the rate-determining step of direct generation of N₂ during combustion（398.12 k J/mol）is higher than that required for heterogeneous reduction of NO to N₂（49.54～282.62 k J/mol）,which demonstrates that the formation of N₂ in combustion process mainly comes from the heterogeneous reduction of NO,and the migration path from coal nitrogen to N₂ is:coal-N→NO→N₂.It shows that the catalytic effect of metal catalysts on N₂ formation during combustion is mainly reflected in the reduction of NO_x.