Mechanism Study On The Formation Of NOx Precursor During Coal Pyrolysis

Coal,as a typical fossil fuel,is a current major contributor to the global emission of nitrogen oxides(NOx).Functional forms of nitrogen in the coals have been determined by X-ray photoelectron spectroscopy.Within all of the N-containing compounds contained in coal,it was found that pyrrolic-type species were the predominant nitrogen source.The NOx formation process during coal utilization(combustion,gasification,pyrolysis)can be described as the thermal decomposition of N-containing model compounds.Pyrrole and its derivatives have been extensively studied and commonly selected as N-containing model compounds in coal.Research on the formation mechanism of NOx has attracted widespread attention of scholars both at home and abroad.It is believed that the key to determining NOx production is the formation of its precursors.The primary NO.v pollutant precursors formed from coal are HCN and NH3,and so the NOx formation process during coal utilisation can be described by the thermal decomposition of N-containing model compounds to NH3 and HCN.Therefore,N-model compounds were selected to explore the conventional and catalytic pyrolysis mechanisms of NO.v precursours by employing density functional theory(DFT)calculations.Within this research,we use DFT to investigate the formation mechanism of HCN as a NOx precursor during pyrolysis of pyrrole in the presence of hydrogen(H)radicals.Firstly,three different reaction positions for H attacking are compared.It was identified that H radical initially reacts with pyrrole at the location adjacent to N through a single elementary reaction step.Additionally,to examine the role of H radical in the pyrrole pyrolysis to form HCN,12 subsequent reaction pathways are theoretically investigated.It was found that one of the pathway(Pathway a-4)involving hydrogen transfer followed by carbon-carbon cleavage processes is the route with the lowest energy barrier of all of the mechanisms supposed,thus it plays an important role in the formation of HCN from the pyrrolic components of coal.These results further indicated that the H radicals significantly reduce the energy barrier of the pyrrole pyrolysis.In this study,the effects of H2O on pyrrole pyrolysis to form NOx precursors HCN and NH3 are investigated using the DFT method.The calculation results indicate that the presence of H2O can lead to the formation of both NH3 and HCN during pyrrole pyrolysis,while only HCN is formed in the absence of H2O.The initial interaction between pyrrole and H2O determines the N products.In addition,the DFT calculations clearly indicate that the formation of NH3 will be promoted by H2O.whereas the formation of HCN is inhibited.The existence of alkali metal ions,Na+and K+,during the coal utilisation process has a significant influence on the formation of NOx species.Within this research,the effect of Na+ and K+ on the formation mechanism of NOx during pyrrole pyrolysis are investigated using DFT.A hydrogen migration occurs from the meta-position C of pyrrole-N is transferred to the ortho-position C,and then pyrrole-N disconnected from the ortho-position C.which makes the ring opened.Lastly,in a concerted mechanism,a long carbon bond breaking between the migrating hydrogen and the carbon,nitrogen atoms·It was found that Na+ and K+ have a catalytic effect on the internal hydrogen transfer and ring-opening of pyrrole but have an inhibitory effect on internal hydrogen isomerization and concerted decomposition reaction.It was also found that those alkali metal ions(Na+and K+)have strong interactions with pyrrole and its derived compounds(HCN and propyne molecules),those interactions are much larger than the existing attractive interactions among HCN,propyne molecules and their complexes.Based on the proposed mechanism and combined with the previous research on the path of the non-catalytic pyrolysis of pyrrole to form HCN.Both Na+ and K+can reduce the activation energy of pyrrole pyrolysis to form HCN,which can promote the formation of HCN,and the catalytic ability of the two alkali metal ions is Na+>K+.Based on the DFT calculation,the effect of alkaline earth metal Ca2+ on the pyrrole pyrolysis to form HCN is also investigated.Compared with alkali metal Na influence in path of pyrrole pyrolysis to form HCN,Ca2+ is superior than Na+ in the internal hydrogen transfer reaction,molecular isomerization reaction and synergistic cracking.As a result,Ca2+ can greatly reduce the activation energy of pyrrole pyrolysis to generate NOx pollutant precursor HCN.Moreover,the influence of alkali metal Na and alkaline earth metal Ca2+ on the pyrrole pyrolysis reaction is Ca2+>Na+.In fact,the main reason for the different catalytic effects of Na+and Ca2+is that the two ions form different spatial interaction sites with the pyrrole derivatives.