| In this paper,the problem of low-nitrogen combustion treatment of crude-oil-tank off-gas in the process of crude oil extraction is studied.A large amount of combustible alkane gas will be volatilized in the storage tank after oil exploitation,which will bring about the emission of greenhouse gas and energy waste,while the treatment method of traditional free-space diffusion combustion will cause serious secondary pollution.To this end,this paper introduces the technology of double-layered porous medium combustion to realize the combustion treatment and energy utilization of the low-nitrogen crude-oil-tank off-gas.Based on this,a one-dimensional laminar flame model of crude-oil-tank off-gas/air premixed combustion was first established,and combined with GRI-Mech3.0 gas-phase kinetics mechanism,the premixed combustion characteristics of crude-oil-tank off-gas/air were investigated.Secondly,a two-dimensional porous media model of double-layer and non-equilibrium for premixed combustion of crude-oil-tank off-gas/air was established,and a detailed computational fluid dynamics simulation was carried out.The factors such as porous media material?equivalence ratio and inlet-velocities on important characteristics such as flame temperature?pollutant emissions and combustion stability were investigated.The main research conclusions are as follows:(1)In this paper,the basic combustion characteristics of the premixed-gas of crude-oil-tank off-gas/air are calculated,and the flame speed and adiabatic flame temperature at different operating conditions are obtained,as well as the ignition-delay time at different equivalence ratios.In addition,through ROP analysis supplemented by sensitivity analysis,it can be seen that factors such as equivalence ratio and different fractions have a certain influence on the main reaction path.(2)The reaction surface of the premixed-gas combustion in the double-layered porous medium is relatively stable,and the main reaction position is close to the interface of upstream and downstream and extends to the downstream area.The main reactants and oxygen fractions decreased sharply at the position where the reaction rate of H+O2<=>O+OH started to increase rapidly,and continued to participate in the reaction in the products-area.The production CO2 and H2O also start to be produced in large quantities at the same location.The temperature field of the gas-phase is parabolic,and the temperature of the solid in the upstream is generally higher than that of the gas,while opposite of the downstream,the preheating effect of the solid to the gas is very obvious.The content of NO components rises sharply at the flame front and is continuously generated in the products-area;the content of NO near the side wall is significantly lower than that at the axis.The fraction content of NO2 begins to be generated before the fast-reacting flame front,and is continuously converted into NO near the flame front.After the flue gas leaves the porous media area,the component content of NO2 increases significantly.The composition distribution of CO is similar to the concentration distribution of OH.CO begins to be produced in large quantities at the reaction front and is continuously consumed in the subsequent product area.(2)As far as the effect of porous media materials is concerned,the upstream material is SiC,which can effectively reduce CO emissions but is not conducive to reducing NOx,and the downstream material is Zr O2,which can significantly suppress NOx emissions.According to analysis,the peak flame temperature is not much different.The amount of NOx generated is obviously affected by the thickness of the flame/width of the chemical reaction surface.Through chemical kinetic analysis,this is related to the elementary reactions H+O2<=>O+OH and N+O2<=>NO+O,etc.The relative positions of the reactions are related,and a reasonable selection of material combinations can balance the emissions of the two main pollutants.(3)In addition,the upstream porous medium with low specific heat capacity and the downstream porous medium with high thermal conductivity are beneficial to broaden the lean limit and improve the combustion stability.In the combination with the best combustion stability,the stable combustion range of the crude-oil-tank off-gas/air is very wide.Within the stability range,the equivalence ratio is constant.Increasing the inlet-velocities will make the flame front move into downstream for a certain distance,but in general,the increase of the inlet-velocity of the premixed-gas is generally beneficial to the increase of the total temperature of the combustion chamber,which is mainly due to the increase of the input power,which makes the combustion generate more heat.(4)This paper also studies the pollutant emission characteristics of extreme lean-burn combustion.The inlet-velocities is constant,and the lower the equivalence ratio,the lower the mass concentration of NOx emitted at the outlet.This is easy to understand:the generation of thermal NOx is greatly affected by the combustion temperature.However,the NOx emission in the higher equivalence ratio is not necessarily higher.In the stability range,the faster inlet-velocitiy will significantly reduce the mass concentration of the outlet NOx emission,which is related to the residence time of the gas in the high temperature area. |
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