Study On Steam Gasification Of Coal Using Blast Furnace Slag As Heat Carrier

With the rapid development of China's economy,the energy and environment issues became more and more prominent,and the energy conservation and emission reduction had become one of the major problems in our country.Blast furnace slag,a kind of by-product during the ironmaking process,carried a great deal of high grade waste heat,which was equivalent to 13%of consumption in the blast furnace process.However,there were several constraints on recovering blast furnace slag waste heat,eg.low thermal conductivity and viscosity changing dramatically,which were the main challenges that restricted the industrial application of waste heat recovered from high temperature slag.At present,the main method to manage the blast furnace slag was water quenched method,which did not only waste the water and pollute the environment,but also not utilize the waste heat of the blast furnace slag.Therefore,effectively solving the problem of utilizing the blast furnace slag waste heat was the key to realize the energy saving of iron and steel industry and relieving the energy crisis,which could help to achieve energy conservation and emissions reduction in China.On the other hand,in the coal utilization,a large of amount of CO₂,NOx,SO₂ and heavy metal was generated,which would do harm to environment seriously.So,developing and applying the clean and effective coal utilization technology was the great importance to the sustainable development of China.In this paper,an integrated system was established to produce syngas by coal/steam gasification reaction using blast furnace slag as heat carrier,so as to achieve the object of recovering waste heat efficiently and transforming coal resource cleanly.For developing this technical route,the fundamental researches were conducted in this paper as follows:?1?An integrated system was established to produce syngas by coal/steam gasification reaction using blast furnace slag as heat carrier.The high grade sensible heat of the blast furnace slag was converted to the chemical heat effectively by the strong endothermic coal gasification reaction?2?The thermodynamic analysis and optimal design were performed for the integrated system,which coupled multiple gasification reaction processes driven by the waste heat.The Gibbs free energy minimization through Lagrange multiplier method was used for gasification reaction by HSC Chemistry and ASPEN Plus software.Simulations were carried out to study the operation temperature,pressure,steam to coal ratio and blast furnace slag,based on the chemical equilibrium calculations.Meanwhile,the mass and energy balance of the integrated system were performed.The analysis indicated that blast furnace slag could act as not only heat carrier but also the reactant.The production of H₂ increased and that of CO₂ decreased with the slag addition.Temperature,pressure and steam to coal ratio were contributed to the coal gasification significantly.Therefore,the optimal operation conditions for gasification and waste heat recovery were achieved at 775?,steam to coal ratio of 2.0?3.0 and atmospheric pressure.Under these conditions,hydrogen concentration reached to 62.36%and the total gas production was 2.45 mol per mole of carbon in the coal.The recovery efficiency of slag waste heat reached up to 83.08%in this system.By thermodynamic analysis,not only was the boundary of coal gasification determined,but also the composition of syngas production was indicated.These also supplied the theory reference for process condition design of steam gasification of coal using blast furnace slag as heat carrier.?3?The characteristics and kinetics of the coal pyrolysis reaction using blast furnace slag as heat carrier were studied through temperature-programmed thermogravimetry.Experiments were carried out to study the effect of coal type,heating rate and slag to coal ratio on pyrolysis reaction,and a kinetic model of the pyrolysis reaction using slag as heat carrier was established the by Coats-Redfern method and Malek method.The analysis indicated that the coalification degree of the coals made the difference in the pyrolysis processes.The greater coalification degree of the coal was,the more closely the system was,and the lower reactivity of the coal reaction was with stable net structure.The heating rate had a positive.effect on the coal pyrolysis reaction.With the heating rate increasing,the volatile component was more easily separated out and formed the coal char.Meanwhile,the blast furnace slag had a significant effect on the coal pyrolysis reaction,but it had different effects on different coal types.The kinetics of coal pyrolysis reaction using blast furnace slag followed C3 model,and values obtained by the kinetic model agreed well with the experimental values.By kinetic analysis of the pyrolysis reaction,the pyrolysis process and mechanism of the coal using blast furnace slag as heat carrier were obtained,and the reaction rate of the pyrolysis was accurately predicted.?4?The characteristics and kinetics of the coal gasification processes using blast furnace slag as heat carrier were studied through isothermal thermogravimetry.Experiments were carried out to study the effect of temperature and coal to slag ratio on gasification reaction,and we established the kinetic model of the gasification reaction using slag as heat carrier.The analysis indicated that different types of coal could gasify smoothly and had a higher reaction rate by this method.Both the carbon conversion and the reaction rate increased with temperature increasing.However,the influence of temperature on gasification weakened gradually and the reaction rate kept stable when the temperature was higher than the melting temperature of the ash.The blast furnace slag had a catalytic effect on the coal gasification,and the carbon conversion and reaction rate enhanced,especial for the DZ low rank coal.Meanwhile,the mechanism functions of coal gasification using blast furnace slag as heat carrier were obtained by kinetic analysis.Take FS coal as example,the reaction model D1 had the best fit with the gasification when the coal to slag ratio was 1:0.The reaction model R2 was consistent with the gasification when the coal to slag ratios was 1:1 and 1:2.By kinetic analysis of the gasification reaction,the coal gasification process and mechanism using blast furnace slag as heat carrier were obtained and mastered much deep theory.It also helped to control the gasification reaction and improved the gasification efficiently.?5?A rotary viscometer was used to measure the viscosity of the molten blast furnace slag based on the rotating cylinder method.The object of the study was to explore the effects of the basicity,MgO and Al₂O₃ on the viscosity of the molten slag.What's more,the estimating viscosity model for the viscosity of molten blast furnace slag was established according to the Urbain model and experimental data of the slag viscosity.The analysis indicated that the viscosity of the molten slag decreased firstly and then increased with theCaO/SiO₂ ratio increasing,and the suitable CaO/SiO₂ ratio was 1.10?1.20.The viscosity increased with the content of MgO and Al₂O₃ increasing in the slag,which should be lower than 8.22%and 11.00%,respectively.In the model,the activation energy was calculated using the slag compositions based on the redefinition of the material types in the slag,and the temperature was described by the Weymann-Frenkel equation.The viscosity of the blast furnace slag system estimated by the proposed estimating viscosity model?modified Urbain model?fitted well with the experimental data and the mean deviation was only about 17.98%.By the investigation of viscosity properties and prediction model,the changing rules of viscosity with slag composition and temperature were obtained.It would provide the theoretical basis for subsequent experiments and applications.?6?The isothermal tests for steam gasification of coal using blast furnace slag as heat carrier were performed in a lab-scale continuous gasification system to investigate the status of gasification and syngas production.The effects of temperature,steam to coal ratio and coal type were studied,and the catalytic mechanism of coal gasification using slag as heat carrier was schematically proposed.The analysis indicated that the carbon conversion,gasification efficiency and syngas production were improved with the temperature increasing.However,the hydrogen production was not improved as the temperature increasing.The added steam enhanced the carbon conversion and hydrogen production in the gasification process.Nevertheless,the extra steam more than the necessary for gasification introduced would cause the decrease of the gasification efficiency.The optimal operation parameters for gasification in molten blast furnace slag were achieved at 1350? and steam to coal ratio of 1.5?2.0.Meanwhile,the problem of the low gasification efficiency was solved by using blast furnace slag as heat carrier,and the high grade waste heat of slag was used to produce hydrogen-enriched syngas with high calorific value effectively.Ultimately,the mechanism of the steam gasification of coal in blast furnace slag was established,so as to provide the theoretical instruction for the practical production.It has been showed that the high grade waste heat of the blast furnace slag was utilized effectively,and the gas production with high H₂ and CO content and high heating value was obtained by utilizing the coal gasification reaction using blast furnace slag as heat carrier.The investigations contributed to further modification and optimization of orientation gasification of coal with hot blast furnace slag for syngas production and heat recovery.