The Study of Ignition and Reaction Kinetic Mechanism under Pressized Oxy-Fuel Conditions and Mechanism of CO Formation in Diluted Combustion

Author:Liu Yang

Supervisor:zou chun

Database:Doctor

Degree Year:2019

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Pages:171

Size:5221K

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The great number of CO2 emission from the burning of fossils fuels is one of the main reason for global warming.Pressurized oxy-fuel combustion is a new generation of oxy-fuel technology,and is a very promising technical means to achieve CO2 capture and storage.However,there is a lack of experimental data under pressurized oxy-fuel conditions,and the reaction kinetic of pressurized oxy-fuel combustion is still immature.Thus,it is of great practical value and scientific interest to study the ignition delay times and chemical kinetics under pressurized oxy-fuel conditions.Firstly,the ignition characteristics of CH4 under 0.8,1.75 and 10 atm were studied in the shock tube.The experimental results show that the effect of CO2 on the ignition delay time of methane is minimal at 0.8 atm,and the CO2 inhibits the ignition of methane significantly as pressure increase to 10 atm when compared with N2 atmosphere.Five mainstream chemical kinetic models(“Ranzi”,USC 2.0,GRI 3.0,Aramco 1.3 and FFCM-1)were evaluated using the present and previous experimental data.The result shows that none of the five models can fully predict the ignition delay times of methane under O2/CO2atmosphere(0.21O2+0.79CO2)at 0.8–10 atm.Therefore,an updated and optimized model OXYMECH-1 is based on Aramco 1.3,and is able to accurately predicte the ignition delay times of methane under O2/N2 atmosphere(0.21O2+0.79N2)and O2/CO2 atmosphere(0.21O2+0.79CO2)at 0.8,1.75 and 10 atm.In this study,the ignition delay times of C2H6 in O2/CO2 atmospheres were first measured at pressure of 0.8,2.0 and 10 bar,equivalent ratio of 0.5,1.0 and 2.0 using a shock tube.It is found that the effect of equivalent on the ignition delay times of ethant under O2/CO2 atmosphere gradually reduces as the pressure changes form 0.8 to 2.0 bar.At 10 bar,the effect of the equivalence ratio on the ignition delay times of ethane further weakens at high temperatures.In the low temperature range,the ignition delay times decrease with the increasing equivalence ratio.The three model(Aramco 2.0,“Ranzi”and OXYMECH-1)were evaluated using the present ignition data.It is found that none of the three models can fully predict the experimental results.The new model(OXYMECH-2)was carried out and based on OXYMECH-1.OXYMECH-2 can be applied to calculate the ignition delay times of methane and ethane under pressurized oxy-fuel conditions,and can also be used to calculate the ignition delay time of H2,CH4 and C2H6 under O2/Ar atmospheres.By comparing Aramco 2.0 to OXYMECH-2,the reason for good performance of OXYMECH-2 is that the scientifically and rationally chose the rate constant of reaction C2H6+HO2(?)C2H5+H2O2,H+O2(+M)(?)HO2(+M),CH3+HO2(?)CH3O+OH,C2H4+H(+M)(?)C2H5(+M)and H+O2(?)O+OH.The effect of physical and chemical properties of CO2 on the ignition of methane under pressurized oxy-fuel conditions were investgated uning OXYMECH-2.The physical effect of CO2 inhibit the ignition of methane,and is not sensitive to temperature in the pressure range of study.At 0.8 atm,the inhibition of CO2 on the ignition of methane is weakened with the increase of temperature,and it is minimal in high temperature.At 10 atm,the CO2strongly inhibit the ignition of methane,and this effect dose not change with temperature.At0.8 atm high temperature region,the chaperon effect of CO2 promotes the ignition of methane due to HCO+M(?)CO+H+M;at low temperature region,the chaperon effect of CO2inhibits the ignition of methane due to 2CH3(+M)(?)C2H6(+M).At 10 atm,the chaperon effect of CO2 inhibits the ignition of methane due to the strong inhibition of 2CH3(+M)(?)C2H6(+M).At 0.8–10 atm,the reactions with CO2 inhibits the ignition of methane due to the inhibition of CO+OH(?)CO2+H.For ethane pressurized oxy-fuel combustion,at 0.8 bar,the CO2 has a minimal effect on the ignition of ethane.At 10 bar,the addition of CO2 significant inhibit the ignition delay times of ethane.The physical properties of CO2 inhibit the ignition of ethane,and the effect of chemical properties of CO2 on the ignition of ethane strongly depends on pressure.At 0.8bar,the chaperon effect of CO2 promotes the ignition of ethane due to the promotion of C2H4+H(+M)(?)C2H5(+M);the reactions with CO2 inhibits the ignition of ethane due to the inhibition of CO+OH(?)CO2+H.At 10 bar,the chaperon effect of CO2 still inhibits the ignition of ethane due to the inhibition of 2CH3(+M)(?)C2H6(+M);the reactions with CO2inhibits ignition of ethane due to the inhibition of CO+OH(?)CO2+H and CH2(S)+CO2(?)CH2O+CO.Under dilution combustion without preheated air,there is still a high CO concentration zone inside the furnace.In the high concentration zone,the reaction O+CH3(?)H+CH2O and H+O2(?)OH+O is enhanced due to high concentration of O2.Therefore,the reaction OH+CH2O(?)HCO+H2O is promoted,and generates more HCO.The HCO is rapidly consumed by the reaction HCO+O2(?)HO2+CO,HCO+H2O(?)H+CO+H2O and HCO+M(?)H+CO+M,and generate a large amount of CO.In the central zone,the HO2concentration is very high,result in the path of CH3→CH3O→CH2O being enhanced.The CO2 formation path is CH4→CH3→CH3O→CH2O→HCO→CO→CO2 in central zone.In the recirculation zone,the majority of CO come from the diffusion and conversion of central and high concentration zones,and then most of it is consumed by OH+CO(?)H+CO2.Consequently,the CO emission is very low in furnace.