Application Of Laser Diagnostic Techniques In Research Of Combustion Characteristics Of Coal And Its Gasification Gas

Staged-utilization are one of those important methods which can ensure the utilization of coal and biomass fuels with high efficiency and low pollution emission.In staged-utilization,gasification is one key process which is used to produce syngas from coal.The syngas can be used as raw materials in different type of chemical industry,and also it can be burned in gas turbine to generate electricity.Syngas produced from gasification can have quite different composition due to different fuels and process techniques.The main composition of syngas can be H₂,CO,CH₄,N₂,CO₂ and et.al.H₂ and CO is the main combustible composition of syngas,and they have quite different combustion characteristics from nature gas.Additionally,there is large amount of incombustible gas existing in syngas,such as N₂ and CO₂,which induces syngas containing low heat value.This causes a challenge to use syngas with high efficiency and low emission.The research to understande the combustion characteristics of syngas with varied composition and developing suitable technique for improving the stability of syngas combustion is important.Moreover,lots of alkali species is released during the gasification or combustion of coal and biomass fuels.The alkali species can cause severe corrosion of heating tube in furnace.It also can reduce the quality of syngas,since the alkali species in syngas can also corrode the blade of gas turbine.It becomes an important topic investigating the mechanism of the alkali release from burning solid fuels.In this work,different laser diagnostic techniques were used in the study on these points.First,the laminar burning velocity of syngas with varied composition were measured accurately.Laminar burning velocity is one of those important combustion characteristics.It was widely used in the development of combustion chemical kinetics.In this work,two typical methods were used in burning velocity measurement.They are heat flux method and OH-PLIF based Bunsen flame method.Different type of syngas was mimicked with varying the proportion of H₂,CO,N₂ and CO₂.The proportion of H₂ in combustible component was varied from 5%to 75%.The proportion of dilution gas,i.e.N₂ and CO₂ in syngas was changed from 0%to 50%.The experimental results show that the trend of the laminar burning velocity of syngas with different equivalence ratios is quite different from that of hydrocarbon gas,such as methane.For methane,the peak burning velocity has its equivalence ratio around 1.0,while syngas has its peak burning velocity at fuel rich side,which can be larger than the equivalence ratio of 2.0.H₂ can enhance the burning velocity significantly and diluent can reduce it.According to the analysis with simulation,it is found that these combustion characteristics of syngas were mainly controlled by H radical.There is a good linear relationship between laminar burning velocity of syngas and the peak concentration of H radical in the burning reaction zone of syngas.Similarly,the burning velocity of H₂/CO/O₂ with different dilution was also measured and analyzed.Empirical formula was obtained based on these experimental results and can be used in the estimation of the laminar burning velocity of different syngas.OH-PLIF was also applied in the investigation of the combustion characteristics of the turbulent flame of syngas with varied composition The premixed turbulent jet flames have low turbulent intensity,and they are wrinkled flames and corrugated flames.They was generated with different Reynolds number by a flat flame burner which contains a center jet tube.The single shot of OH-PLIF image can present the structure of the flame front clearly.These images were used in this study to calculate the turbulent flame speed of the syngas with varied composition and turbulent intensity.The trend of the varying of the turbulent flame speed with different proportion of hydrogen and diluent is similar to that of laminar flame speed of corresponding syngas.Higher proportion of hydrogen induces higher turbulent flame speed,higher concentration of OH and smaller flame.Dilution has the opposite effect.Increasing of Reynolds number also can increase turbulent flame speed and OH concentration It is found that the effect of the turbulence on the combustion of syngas is quite independent from the composition of syngas after the analysis of the ratio between turbulent flame speed and corresponding laminar flame speed,for the turbulent flames under low turbulent intensity.Meanwhile,the effect of the dilution on the combustion characteristics of methane/air turbulent flame was also conducted by OH-PLIF and CH₂O-PLIF.The flames also belonged to wrinkled flame.The increasing of dilution can reduce the concentration of OH and postpone the burning of fuel.The CO₂ diluent has more significant effect on OH concentration than N₂,but similar effect on the concentration of CH₂O in the preheat zone of flame was observed.Dilution doesn’t have much influence on the flame structure with given turbulent intensity.Additionally,the combustion enhancement with plasma was investigated.In this work,the effect of ozone on the combustion was studied,since ozone is one of those important plasma-generated species.The work mainly focused on the investigation of the effect of ozone on the produce of CH₂O in methane/air premixed flame using CH₂O-PLIF.The adding of ozone can increase the concentration of CH₂O significantly.The concentration of CH₂O in the flame with equivalence ratio of 1.4 increased by 50%with 4500ppm ozone adding.The enhancement rate of CH₂O with different equivalence ratio was obtained.The ex^erimental result was compared with the results from the simulation using the kinetic combining GRI-mech 3.0 and Konnov ozone sub-mechanism.It was proved that the kinetic has good prediction.Ozone mainly enhances the reactions in the preheat zone of the flame according to the analysis of simulation.At last,the quantitative measurement of atomic potassium in the plume of burning coal and biomass was achieved by TDLAS technique.It is found that the release characteristics of potassium was significantly affected by the property of fuels.Coal has its main release during the stage of char reaction due to its large amount of fixed carbon.Biomass contains lots of volatile and water soluble potassium which makes biomass have its main release during de-volatilization stage.Moreover,the effect of different hot ambient gas was also investigated.Comparing to combustion,gasification process weakens the reaction of volatile and char and reduces the potassium release.