Enhanced Ignition Process Of Anthracite/Coke Induced By Microwave

To accommodate renewable energy on the grid,coal-fired generators need to switch from base-load to peak shaving,which results in a significant increase in the number of start-ups over their entire lifecycle.If the traditional fuel ignition method is continued,a huge cost burden will be incurred.Whereas oil-free plasma ignition technology has enormous economic benefits.Nevertheless,plasma ignition still has technical difficulties with low-volatile coal(Vdaf < 20%),which accounts for 25% of China’s power coal.Traditionally,the energy of an arc plasma torch is concentrated,the single large "fire core" owns a limited area of action,and low-volatile coals can only be partially ignited near the arc.To expand the plasma action area,this project proposed to add a microwave action zone downstream of the arc plasma torch.In this microwave action zone,coke was added as a microwave absorbent to enhance microwave-induced heating and discharge,achieving in-situ pyrolysis and gasification of coal particles by thermal and non-thermal effects,and generating additional volatile matter.When the ignition temperature is reached,a relatively dispersed number of small "fire core" can be formed,achieving the ignition of lowvolatile-coal.Hence,this thesis examined the ignition and combustion characteristics of coke/low-volatile coal in a microwave field where anthracite is used as a representative material.The first objective of this thesis was to examine the microwave ignition combustion characteristics of anthracite/coke mixture in a fixed bed reactor to explore the intrinsic reaction characteristics.Firstly,the effect of electric discharge on microwave heating of coke/anthracite mixture was investigated;then this section analyzed the stages of microwave ignition combustion of the mixture on the basis of achieving rapid temperature rise;researched the effects of microwave power,oxygen concentration and gas flow rate on microwave ignition of the mixture and subsequent flame combustion;Lastly analyzed the gas reaction products and flame during combustion.According to the results,the microwave ignition and combustion process of coke/anthracite mixtures can be divided into three stages: microwave heating,local ignition,and flame burning accompanied with particle combustion;during the initial rapid rise in temperature of the mixture,microwave-induced coke discharge played an essential role;increasing microwave power,increasing oxygen concentration,and reducing total gas flow could accelerate the ignition process and prolong the duration of flame combustion;microwave discharge intensity was positively related to flame combustion range;modified coke could enhance microwave discharge,and the height of the flame increased from about 1 cm to about 7 cm at the moments of discharge.Additionally,for successful ignition,coke in the mixture must maintain a certain degree of aggregation.This thesis investigated the microwave-induced coke discharge process furtherly based on the dependence where the coke properties and aggregation degree solidly influenced the microwave ignition and combustion process of anthracite/coke mixture.This section investigated the influence of coke physical properties on microwave discharge stability,and conducted a correlation analysis between the intensity of the discharge and the concentration of combustion gas products.Results indicated that coke with large particles and a moderate degree of graphitization was easily induced stable microwave discharges;CO was the main flammable atmosphere that supported flame combustion,and its generation was directly related to microwave discharges.Furthermore,this section also found a synergistic effect existed between microwave discharge and flame combustion: aside from enhancing the CO generation,the discharge process also produced various active molecules and carbon fragments,such as O2*,CO*,C,and C2,which further accelerated the combustion of the flame;a flame,which is a weakly ionized plasma,was more conducive to electron diffusing and developing than air,leading to the development of centimeter-sized discharge plasmas.Based on the results of the fixed bed reactor,this thesis explored the effect of particle dynamic mixing on the microwave-coke interaction by increasing the vertical gas flow.First,the effects of dynamic mixing of coke particles on microwave heating,microwave absorption efficiency,and discharge were investigated;subsequently the causal relationship between sample temperature rise,discharge enhancement,and microwave absorption efficiency in the sample was analyzed,then the reasons for the rapid absorption of microwave energy during the mixing process of coke particles were also discussed;then the COMSOL software simulated the evolution of the electric field intensity in coke particles and their surrounding gas space when two coke particles approached to each other.Lastly,the effect of microwave combustion on coke during particle dynamic mixing was also investigated.Results showed that under low microwave power(50 W,100 W),as the gas flow increased,the temperature of coke particles decreasedgradually due to the enhanced heat dissipation;under high microwave power(200 W,300 W),the sample temperature and microwave discharge intensity suddenly increased,during the transition from static to dynamic mixing of coke particles,and the microwave absorption efficiency of sample could increase from 35% to 80%.However,the rise in temperature(25 to 700 °C)and the enhancement of microwave gas phase discharge were not the main factors contributing to the rapid increase in microwave absorption efficiency of coke during dynamic mixing.The COMSOL simulation results indicated that the intensity of the microwave electric field on the adjacent sides of the coke particles increased rapidly,up to two orders of magnitude,when two coke particles approach each other along the direction of the microwave electric field.Last but not least,the combustion characteristics of coke/anthracite dynamic mixing in microwave fields were examined.The effects of coke/coal mixing degree,microwave power,gas flow rate,coke/coal mixing ratio,and background gas on the microwave combustion process were investigated,particularly CO and NOx emissions.Results showed the direct pyrolysis of coke/coal particles by microwave discharge in a pure nitrogen atmosphere was limited,with peak CO concentrations below 1 vol% and peak CH4 concentrations below 4000 ppm,including volatiles in the gas products.The increase in microwave power(from 400 to 700 W)under an N2/O2 atmostphere did not significantly affect the peak concentration of CO in the gaseous product(about 7 vol%);however,it would notably increase the generation of NO(peak concentration increases from about 2000 ppm to 7500 ppm)and NO2(peak concentration increases from about 1000 ppm to 6000 ppm).And decreasing the flow of gas could increase the concentration of CO in the gas product: under the working conditions of 14.9 ml/s N2 and 3.8 ml/s O2,the peak concentration of CO could reach 14 vol%,and the lowest concentration after stabilization was approximately 8.9 vol%.Increasing the coal/coke blending ratio significantly delayed the sample temperature rise and reaction progress,and the peak concentration of CO would be postponed from approximately 120 seconds(coal/coke = 1:1)to about 250seconds(coal/coke = 3:1).Additionally,the microwave combustion reaction of coke/anthracite under the CO2/O2 atmosphere could reduce the NO concentration by one order of magnitude and the NO2 concentration by two orders of magnitude,while maintaining the same sample burnout rate.