Combustion Characteristics And Organization Optimization Of Seimicoke And Bituminous Coal Blends

Cascade utilization of coal is one of the important ways for the clean and efficient utilization of coal.The by-product of the coal pyrolysis and gasification,semicoke,is used as a high-grade clean fuel to generate electricity,which is an important part of realizing the coal cascade utilization.To solve the difficulties in ignition,stable combustion,low burnout ratio and high NO_x emission in tangential boilers under large proportion of semicoke cocombustion,based on the analysis of existing combustion technologies,a new type of strong stabilized combustion fuel-rich/lean direct-flow burner is studied and its better combustion organization is optimized further in this paper.For the Shenmu semicoke and Shenhua bituminous coal blends（coal quality ratio 1:1）,a 0.5MW bias pulverized coal combustion simulator which can simulate ignition and combustion process of one bias pulverized coal straight-flow burner in an actual tangentially fired boiler is set up in this paper.Through the combination of laboratory thermal-state test and the numerical simulation,the nozzle structure of the novelty strongly stabilized combustion direct-flow burner is progressively optimized,and a strongly stabilized fuel-rich/lean combustion direct-flow burner applied for cocombustion of semicoke and bituminous coal blends is proposed.However,due to the larger blended ratio of semicoke,the fuel-rich/lean strongly stabilized combustion technology has encountered a challenge in the NO_x emission reduction.Therefore,a novel quasi MILD combustion technology combining stabilized combustion technology with Oxy-MILD（oxy-fuel and flameless oxidation）is proposed.The quasi MILD microscopic combustion and its NO_x emission characteristics of large proportion of semicoke and bituminous coal blends are studied by numerical simulation.Firstly,the height-width ratio of the primary air nozzle of the direct-flow burner is optimized,which lays a foundation for the subsequent key structural parameters.When the height-width ratio is as low as 1.65,the ignition delay is obvious,and the flame is too concentrated in the centerline of the furnace.When the height-width ratio is as high as3.22,it is easy to cause the deflection of the primary air jet and the high temperature flame sticking to the furnace wall because the jet rigidity is much weaker.However,at a moderate height-width ratio of 2.32,the flame stability,ignition and burnout characteristics are significantly improved.The NO_x production in the reduction zone is positively correlated with the oxygen concentration,but negatively correlated with the CO concentration and the temperature.The NO_x emission at the outlet of the main combustion zone decreases first and then increases with the increase of aspect ratio,while the burnout ratio increases first and then decreases.Based on the research results in this paper,the recommended height-width ratio of the direct-flow burner is 2.32.Based on the height-width ratio of 2.32,the influence of the triangular bluff body cone angle on the combustion characteristics of semicoke and bituminous coal blends is studied.The results show that with the increase of the cone angle of the bluff body from0°to 37°,the ignition is obviously advanced,the main reaction zone moves to the upstream of the furnace,the flame extends radially,indicating that the flame stability is enhanced.The overall average burnout rates of semicoke and bituminous coal in the furnace decrease slowly with the increase of the cone angle,but the burnout ratio at the outlet of the furnace has increased to 99%atα=25°due to the increased residence time of char particles in the furnace.The strong diffusion ability of oxygen molecules increases the proportion of consuming char particle by the oxidation reaction C+O₂→CO,while the overall lower reaction temperature reduces the proportion of consuming char particle by the gasification reaction C+CO₂→CO due to its endothermic property.The maximum value of homogeneous Damk（?）hler number decreases from 198.6 to 51.5 with the increase of bluff body cone angle,implying that the mixing rate of reactants and oxidants accelerates with the increase of the turbulence intensity.With the increase of cone angle,the proportion of reduction zone increases,and the NO_x emission at the furnace outlet decreases by approximately 27%.Based on the research results in this paper,the recommended bluff body cone angle of the direct-flow burner is 25°-29°.Based on the height-width ratio of 2.32 and the cone angle of the triangular bluff body of 25°,the bluff bodies in the fuel-rich pulverized coal jet are installed separately to develop a strongly stabilized combustion fuel-rich/lean burner and to expand the stabilized combustion and low-NO_x emission advantages of the fuel-rich/lean combustion technology.With the increase of the bluff body blockage ratio（BR）,the radiation heat transfer is enhanced,the flue gas entrainment rate（K_v）increases,and the temperature field tends to be more uniform.Under the conditions of triangular bluff body with larger blockage ratio,the delayed ignition times of bituminous coal and semicoke on the fuel-rich side are shortened,and then their average burnout times are reduced.The maximum values of the homogeneous Damk（?）hler number Da_t under all conditions are above 113,indicating that the fuel-rich/lean combustion under the addition of stabilized burning bluff bodies is determined by the traditional combustion mode controlled by the turbulence and diffusion regime.On the fuel-rich side,the“local MILD”combustion zone（Da_t<10&Ka>>1）expands with the increase of the bluff body blockage ratio.The heterogeneous combustion reaction of char particles is relatively gentle,and the heterogeneous Damk（?）hler number is small.It is believed that the heterogeneous reaction control of char particles is similar to that in the MILD combustion regime.When the bluff body is a triangular bluff body with a larger blockage ratio of 21%,the fuel-rich stream is more likely to establish a“heterogeneous MILD”combustion regime.The location of the minimum concentration of NO on the fuel-rich side is the same as that of the maximum value of the flue gas entrainment rate K_v.In the X-Y plane of the maximum K_v（Z=1 m）,there is a closer correlation between the local NO concentration and the heterogeneous Damk（?）hler numbers(Da,_O2,Da,_CO2)compared with the homogeneous Damk（?）hler number Da_t.Increasing BR can significantly reduce the maximum NO concentration on the fuel-rich side,thus reducing the NO_x emission at the furnace outlet.Secondly,based on the burner with an optimal triangular bluff body cone angle of25°,with the help of an idea of the classical MILD combustion,a quasi MILD combustion of semicoke and bituminous coal blends under a low preheating temperature is established numerically by increasing the velocity V_sec of secondary air jet.The results show that the jet velocity of 51.8 m/s becomes a critical point of conversion from the traditional combustion mode to the quasi MILD combustion mode due to the insufficient flue gas entrainment ability.With the increase of V_sec,the increased flue gas entrained ability is conducive to the dilution and diffusion of O₂ molecules,the main reaction zone shrinks significantly upstream,the temperature peak decreases,and the temperature field becomes more uniform.The char oxidation reaction C+O₂→CO is enhanced,while the char gasification reactions C+CO₂→CO and C+H₂O→CO+H₂ are both weakened.With the increase of V_sec,the peak of homogeneous Damk（?）hler number Da_t decreases,the minimum value of Karlovitz number Ka increases,and the semicoke and bituminous coal cocombustion gradually develops from the diffusion control of the traditional combustion to the diffusion/kinetic control of the quasi MILD combustion.When V_sec increases to158.6 m/s,the“local quasi MILD”combustion zone（1<Da_t<10）extends to the whole reaction zone and tends to the ideal quasi-MILD combustion state（Da_t≈1,Ka>>1）.The low flame peak value,low-oxygen dilution,strong flue gas entrainment,uniform temperature distribution and slow reaction rate under the high velocity jet conditions can inhibit the NO formation in the main reaction zone to a great extent.Finally,to further reduce the NO_xemission concentration,based on the quasi MILD combustion technology,aiming at the strongly stabilized combustion fuel-rich/lean burner with a blockage ratio of 21%,the quasi Air-MILD combustion and the quasi Oxy-MILD combustion of the semicoke and bituminous coal blends are established by increasing the jet velocity V_sec and the CO₂ dilution concentration.The results show that with the increase of V_sec,the high temperature zone shrinks and the peak flame temperature decreases by approximately 150 K.As the concentration of CO₂ increases from 0 vol.%to 79 vol.%,the quasi Oxy-MILD combustion is continuously enhanced,the enhanced radiation heat transfer and flue gas entrainment ability promote the temperature uniformity,and the peak temperature reduces by approximately 113 K.For the fuel-rich and lean jets,compared with the quasi Oxy-MILD combustion cases,the char ignition delay times and burnout times under the Air-MILD combustion cases are shorter.The combustion regimes of the quasi Air-MILD and the quasi Oxy-MILD are composed of small flame fragments,and their reactions are distributed in the flamelets in eddies regime（Da_t=0.1-1）,while the traditional fuel-rich/lean combustion flame is distributed in the reaction sheets regime（Da_t=10-100）.With the increase of V_sec,the fuel-lean side jet is more likely to form a heterogeneous quasi MILD combustion regime,while with the increase of CO₂ concentration,fuel-rich side jet is more likely to form a heterogeneous quasi MILD combustion.As the fuel-rich/lean combustion transferred from the traditional combustion mode to the quasi Air-MILD combustion mode,the NO_xconcentration at the furnace outlet significantly reduces,and the NO_x concentration further reduces under the enhanced quasi Oxy-MILD combustion.Considering the problem such as char burnout delay,the recommended CO₂ concentration is 59 vol%.After a deep optimization of the strongly stabilized combustion burner,the NO_x emission reduction of the quasi Oxy-MILD combustion technology reaches up to 30%.