Gasification Kinetics Study Of Coal Char And Unburned Carbon In Slag

The study of coal gasification kinetics is the essential for the development of coal gasification technology. Gasification processes have been widely applied for coal utilization. Understanding the factors affecting the coal gasification and establishing proper kinetics model will be essential for designing an efficient gasifier and resolving the practical problems. The main purposes of this paper are investigating the changes of chemical and physical characteristics of coals during pyrolysis and gasification, studying the mechanism of coal char gasification under different reactive agents, establishing proper kinetics model and supplying valuable data for the development of coal gasification technology.Four types of Chinese coals and gasifier slag were chosen as experimental samples. The behavior of char-CO2 and char-H2O gasification and the factors affecting the char gasification reactivities were studied. The normal distribution model was established basing on the relationship between the gasification rate and residence time. Factors affecting the gasification of unburned carbon in gasifier slag have been investigated. Influences of melting slag/ash on char gasification were also studied. The main contents and results were summarized as follows:(1) Effects of pyrolysis on the pore structure of four Chinese coals were studied. Pyrolysis not only influences the specific surface area of chars, but also changes the shape of pores in coal particles. Adsorption/desorption isotherm indicated the possibility of existing slit pores, as the dominant forms of the pore structure for the original coal. The pore type was mainly composed of open cylindrical pore with one dead end for chars FP and bottle-shaped pores were dominating in chars SP. The melting ash very probably inhibited the development of inner porosities and surface morphology when coal particle was pyrolyzed in the slow heating rate at the range of AFT, while volatile matter plays more important roles on the formation of pore structure and surface morphology when pyrolyzed in rapid heating rate. Fractal theory was suitable to describe the pore structure and surface morphology of chars. Fractal dimension not only relates to pyrolysis temperature and heating rate, but also connects to the parent coal.(2) A normal distribution model was proposed to fit the kinetics data. Compared with the random pore model, results show that the normal distribution model can be used to describe the gasification rate changing with reaction time at different temperatures and pressures. The effects of CO, H2 and N2 on char-CO2 and char-H2O gasification reaction were investigated by TGA. nth model and Langmiur-Hinshelwood (L-H) model have been used to fit the reactivity data and kinetics parameters were obtained.(3) The reactivities of 15 coal chars of varying rank with CO2 and H2O have been determined to examine the effects of fixed carbon, ash and volatile matters on the gasification rate of coal chars. A universal correlation between the gasification reactivity of coal particles and coal types was presented. A general index,Rc=(Vm)/(FC)·B·100, which is used to indicate the gasification reactivity of coals with CO2 and H2O was present. Rc depends only on the proximate analysis and ash composition of coal and reflects the gasification behavior in a combined way, so that Rc is called the gasification index of coal.(4) The shrinking unreacted model is applicable for describing Shenfu char gasification. Influences of particle size on char gasification were investigated. At temperature 850-1000℃, the char-CO2 and char-H2O gasification are both controlled by chemical reaction. The apparent activation energies of char-CO2 and char-H2O gasification are 150-170 kJ/mol and 150-190 kJ/mol, respectively. The effects of pore diffusion on char gasification were analyzed at higher temperature range.(5) The catalysis of AM on coal gasification happened not only in gasification phase but also in pyrolysis phase. AM inhibited the progress of graphitization of the base carbon making the crystalline carbon structure more disorder and thus resulting in more reactive char. The kinetic model of catalytic gasification was established, which is suitable to describe the gasification rate changes in the char conversion for catalytic gasification of char.(6) CO2-gasification reactivity of unburned carbon in both coarse and fine slags was studied in a pressurized thermogravimetric analyzer (TGA) and compared with a char obtained from a drop tube furnace (DTF) at 1400℃from the same original coal. Unburned carbon in coarse slag has the highest gasification rate which is always higher than that of corresponding fine slag. The existence of minerals has catalysis on the CO2-gasification of unburned carbon in coarse slag, but inhibits CO2-gasification in fine slag. DTF char has the lowest reactivity, compared to unburned carbon in both coarse and fine slags. Higher ordering of carbon layers and lower content of catalytic mineral components may be the main reasons for the lower reactivity of the unburned carbon in fine slag than that of in coarse slag.(7) The gasification of the mixture of raw coal with corresponding ash/slag from entrained-flow gasifier was investigated in the range of ash/slag melting temperature (950-1380℃). The influences of coal ash and slag on coal gasification were investigated. It was found that gasification rate have more to do with the ash/slag melting characters of coal when gasified at relative higher temperatures.