Study On Influence Mechanism Of CO₂/H₂O On Zhundong Coal Pyrolysis And Combustion Characteristics Under Photothermal Fast Heating

It is imperative for carbon emission reduction to face the severe global warming problem.The low CO₂ emissions and flexible coal-fired power generation are of great significance to the new power system with new energy as the mainstay.Oxy-fuel combustion and supercritical CO₂ cycle technologies are of great application potential,where the concentration of CO₂ and H₂O in the reaction atmosphere is significantly different from that in traditional combustion conditions,which will inevitably affect coal pyrolysis and combustion process.Studying the influence mechanisms and kinetic characteristics of CO₂/H₂O on coal pyrolysis and combustion are of great significance for extending the basic theory of coal combustion,guiding the development of low CO₂ emissions for coal combustion technologies.Firstly,a concentrating photothermal thermogravimetric analyzer（CP-TGA）was developed.Based on the concentrating radiation heating,an active-passive combined temperature control method was applied,precise heating processes with the wide range of heating rates（5～1000℃/min）were realized.Zhundong coal with rich reserves in China was selected as the research object,the effect of the cooling rate on the structure characteristics and reactivity of the coal char was quantitatively analyzed,and the coal/char structural characterization method with fast cooling rate was proposed.Secondly,Zhundong coal pyrolysis experiments under different heating rates in the range of 5～1000℃/min were carried out on the CP-TGA.When the heating rate increased from 5℃/min to 1000℃/min,the maximum mass loss rate of Zhundong coal during pyrolysis increased linearly from 0.07%/s to 1.10%,and the average activation energy exponentially decreased from 235.41k J/mol to 28.37k J/mol.The increase of the heating rate promoted the conversion rate of functional groups and improved the aromatization degree.Then,the effect of CO₂ on Zhundong coal pyrolysis characteristics,kinetic parameters and structural evolutions under fast heating rate was studied.With the increase of CO₂,during the fast heating pyrolysis process,the maximum mass loss rate gradually decreased,and the mass loss curve shifted to the higher temperature zone.The Zhundong coal pyrolysis process conformed to the three-dimensional diffusion（D3）mechanism model based on the Jander equation,and the apparent activation energy was 34.52～36.22k J/mol.During coal pyrolysis process below 700℃,CO₂ inhibited the aromatization degree and decreased the conversion of functional groups and as well as the formation of stable free radicals,while the aromatization degree of char was improved and the coal pyrolysis process was promoted due to the char-CO₂ gasification reaction above 700℃.Next,the influence mechanism of H₂O on the Zhundong coal pyrolysis under fast heating rate were analyzed.H₂O promoted coal pyrolysis process,and the mass loss rate was increased with the peak position of the maximum mass loss rate shifted to the lower temperature region.The pyrolysis process of Zhundong coal under fast heating rate in H₂O all conformed to the D3 model.The activation energy of coal pyrolysis reaction decreased as the H₂O concentration increased.There was a competitive interaction between CO₂ and H₂O on Zhundong coal pyrolysis,and the competition between CO₂ and H₂O became more obvious with the increase of H₂O concentration.Finally,the effect of CO₂/H₂O on the combustion characteristics and structural evolution of Zhundong coal under fast heating rate was further studied.The maximum temperature and maximum mass loss rate decreased with the increase of CO₂ concentration,but increased with the increase of H₂O concentration.In the initial stage of coal combustion process,CO₂decreased the conversion rate of functional groups and reduced the degree of aromatization of coal/char,while H₂O shows a promoting effect.In the later stage of coal combustion process,the char-CO₂ and char-H₂O gasification reaction further enhanced the aromatization degree of char.Based on the experimental results during fast heating combustion process,the correlation models between the specific reaction rate and the Raman structure parameter of Zhundong coal/char in complex O₂/CO₂/H₂O atmosphere were established.