Study Of The Combustion Characteristics In Natural Gas Non-Catalytic Pratial Oxidation Reformer

In the non-catalytic partial oxidation(NC-POX)process of gaseous hydrocarbons such as natural gas,the combustion process in reformer has an important influence on the conversion efficiency of gaseous hydrocarbons and the service life of key equipments such as burner.This thesis takes the combustion process in natural gas NC-POX reformer as research object.The characteristics of CH4-O2 diffusion flame in normal temperature atmosphere,high temperature syngas atmosphere and bench scale reformer conditions are studied through both experimental and numerical methods,the features of turbulence-chemistry interactions in POX process are also studied.The main contents are given below.Appearances and stability behaviors of CH4-O2 inverse diffusion flame in room temperature atmosphere are studied.Transition velocities and rules of transition paths among three flame modes,namely partial premixed,attached and lifted inverse diffusion flame,are studied.The effects of burner lip thickness are studied and a method to calculate the critical nozzle lip thickness is proposed;when the burner lip thickness is larger than the critical thickness,the flame stability behavior manifests as "attachment-blowoff";when the burner lip thickness is smaller than the critical thickness,the flame stability behavior manifests as"attachment-liftoff-blowout" in suitable flow conditions.The effects of oxidant dilution and atmosphere are studied and results show that dilution of oxidant will reduce the stability of flame and change the flame stability behavior.In N2 atmosphere,the measured flame extinction limits are the same as in air,but only the "attachment-blowoff' stability behavior exists.Results of numerical simulation show that the instability of attached flame is related to the changes of eddy structures near the burner lip,while whether the flame will be lifted or extinguished depends on the flow field conditions downstream the eddies.A jet-in hot syngas coflow flame model is developed to simulate the combustion process in NC-POX reformers.Flame structures,combustion modes and effects of operating conditions are studied through numerical simulation.Results show that MILD combustion can be established when CH4 and O2 are in inverse diffusion configuration(IDC)in hot syngas coflow,the flame temperature is uniform with a peak value of 1797 K and the flame is stabilized by auto-ignition.The fully dilution of O2 before ignition in IDC is the key point in establishing MILD combustion in hot syngas environment and this conclusion show a technical direction to realize MILD combustion in POX reformers.Decreasing of jet velocity and increasing of coflow temperature,pressure,and preheating temperature will lead to the upward moving of reaction area and the increasing of flame peak temperature.In typical operating ranges of industrial reformers,changes of the above conditions have no obvious influence on combustion features and modes.The characteristics of CH4-O2 diffusion flames are studied in a bench scale NC-POX reformer and the effects of diffusion configuration,jet velocity and O2/CH4 mole fraction ratio are investigated.In normal diffusion configuration(NDC),a visible flame can be observed near the burner;in inverse diffusion configuration(IDC),no visible flame can be observed,the temperature is uniform with a peak value of 1446 K,MILD combustion was established in the furnace.Experimental results validate the simulation results of the jet-in hot syngas coflow flame.In IDC,no flame are formed when the jet velocity is larger than 40m/s and O2/CH4 mole fraction ratio is smaller than 1.39,Numerical simulations of the reformer are also performed and results show that predictions given by eddy dissipation concept(EDC)model agree well with the experimental results.The reflux ratio inside the furnace is in the range of 4-9,which indicates the fully dilution of reaction process.The fully dilution of oxygen by fuel and entrained gas before ignition is the key point of establishing MILD combustion in NC-POX reformers and the necessary condition for this process is to set a barrier stream outside the oxygen stream.Simulation results obtained by using fast-reaction based combustion models or global reaction mechanisms show a large discrepancy with the experimental results.A new characteristic chemical time scale identification method is proposed to study the turbulence-chemistry interactions in POX flames.In IDC,the chemical time scale is 10-5-10-4 s in combustion area and is 10-2s in reforming area.Compared with existing methods,results given by the new method agree better with the MILD combustion nature of the flame,which indicates that the new method is more suitable for the analysis of POX flame.The analysis of POX flame in NDC shows that the developed method can give satisfactory results in fast,mild and slow reaction regimes.Mild combustion regime exists in both IDC and NDC flames,so detailed turbulence-chemistry interactions must be considered in the modeling of POX combustion process.