| With the sustained and rapid development of economy in China, the energy and environmental problem became more and more serious; one of the difficulties in our country is the energy conservation and emission reduction. The resources and energy consumption of iron-steel enterprises is large, especially energy consumption in iron-making process is about 60% of total energy consumption. Though, many technologies such as CDQ and TRT were used, iron-making have achieved tremendous improvements in its energy efficiency. But the energy of molten blast furnace (BF) slag has only been used limitedly so far.Thus in order to resolve these problems, in this paper, starting from the energy conservation and emission reduction in china, waste heat recovery in metal manufacturing industry and energy situation in china, giving consideration to the development of coal gasification, centering on the waste heat recovery of molten BF(blast furnace) slag, CO2 recycling and the sustainable development of metal manufacturing industry, we proposed the new technical route of recovering waste heat of molten BF slag and reducing carbon emission, and carried out relative basic research. The main research results as follows.(1)A new system of recovering waste heat of molten BF slag and reducing carbon emission was proposed. This system consists of melting gasifier, the rotary cup atomizer, the solid granules heat carrier gasifier and waste heat boiler. It can recover the waste heat of molten slag and slag granules, and the temperature of solid slag granules from boiler is below 473K. In this process, CO2-rich smoke was used as gasification agent. It not only recovers the waste heat of molten BF slag but also realizes the recycling of CO2.(2)Based on the first law of thermodynamic and second law of thermodynamic, using enthalpy-exergy compass, the thermodynamic anslysis of the waste heat recovery system using different chemical reaction to reacover the sensible heat of molten BF slag was studied. The results show that the heat efficiency of thermal energy recovery methods is 76.9%, the exergy efficiency of recovery as steam is 34.2%; the heat efficiency of chemical energy recovery methods is 92.2%, the exergy efficiency is higher than 60%. The heat efficiency and exergy efficiency of chemical energy recovery methods are higher than that of thermol energy recovery methods. On the promise of making comprehensive consideration for the exergy efficiency of system, the exergy efficiency of melting gasifier and solid slag granules heat carrier gasifier, the reaction condition, the consumption of reactant and the amount of product, the C-CO2/H2O reaction are selected as the best reaction.(3)The kinetics of coal-CO2 gasification using molten BF slag as heat carrier was studied by the temperature-prohrammed thermogravimetry and isothermal thermogravimetry using thermal analyzer, and the kinetic model was bulited. The results show that when the gasification reaction temperatures near the ash fusion temperature, the carbon conversion and the peak value of reaction rate reach the maximate. With the increasing heating rate, the carbon conversion curve and reaction rate curve shift to high temperature, the peak value of reaction rate increases, the time required for the complete carbon conversion decreases. Molten BF slag act as good catalyst to coal gasification, it can speed up the reaction rate, decrease reaction temperature and shorten time for thr complete carbon conversion. The CaO has certain catalytic effect on gasification reaction; however, the catalysis of CaO weakens with increasing reaction temperature. MgO also plays a catalytic role in the gasification reaction when the MgO content in a certain range. Al2O3 depresses gasification reaction especially at high reaction temperature. SiO2 has no catalytic effect on gasification reaction. In meting gasifier, the addition of BF slag does not change the reaction mechanism. The mechanism function of coal gasification using molten BF slag as heat carrier is A1 model. Its activation energy is ranging from 50 to 90 kJ·mol-1. The kinetics model is shown as follows.(4) Coal gasification reactivity and operation parameter of the melting gasifier was studied by the small experiment system. The results show that the adaptability of coal gasification using molten BF slag as heat carrier on coal sample and granules is widely. It is benefit to the gaidication character of lean colas. The gasification character of lean coal (SN coal) is almost the same as high-quality coal (DT coal), and its heat value is about 10000 kJ·m-3. The content of carbon monoxide increased with the increasing temperature, whereas the content of hydrogen, the heat value, and gasification efficiency increased firstly and then decreased. The heat value and gasification efficiency reach the highest at 1623K. The CO content decreased with increasing CO2/C mole ratio. The H2 content increased firstly and then decreased, while The C/H ratio decreased firstly and then increased with the increasing ratio. When the CO2/C mole ratio is 1.02, the carbon conversion is about 0.95, the heat value and H2 content reach maximum value. When the injection rate is 0.14m3·h-1, the heat value, carbon conversion, CO content and H2 content reach maximum value. Thus we concluded that in this experiment the optimum CO2/C mole ratio, reaction tempreture and injection rate is 1.02,1623K and 0.14 m3-h-1, respectively.(5) The kinetics of coal-CO2 gasification using solid BF slag granules as heat carrier was studied by isothermal thermogravimetry using thermal analyzer, and the kinetic model was bulited. The results show that the addition of BF slag can decrease intial reaction temperature, and the temperature drop of lean coals is higher. With increasing reaction temperature, carbon conversion, the peak value of reaction rate, the intrinsic surface reaction rate and the reactivity index increase, the time for complete carbon conversion decreased. BF slag act as good catalyst to coal gasification, with increasing BF slag content, carbon conversion, the peak value of reaction rate, the intrinsic surface reaction rate and the reactivity index increase, the activation energy decrease. The carbon conversion and peak value of reaction rate increase with increasing alkalinity of BF slag. CaO has certain catalytic effect on gasification reaction; however, the catalysis of CaO weakens with increasing reaction temperature. MgO and Al2O3 also play a catalytic role in the gasification reaction when the content in a certain range. Al2O3 can depress gasification reaction especially at high reaction temperature. SiO2 has no catalytic effect on gasification reaction. The addition of solid BF slag can change the reaction mechnisum, which is different from gasification using molten BF slag as heat carrier. When the coal/slag=1:0, the reaction model A2 has the best fit on coal gasification; when the coal/slag=1:1,1:2and 1:3, the best fit on coal gasification using solid BF slag granules as heat carrier is the reaction model A2, A3 and A4, respectively. Both the activation energy and pre-exponential factor decrease with increasing BF slag content. The kinetics model is shown as follows.(6) Coal gasification reactivity and operation parameter of the solid heat carrier gasifier was studied by the small experiment system. The results show that reaction rate, CO content, H2 content and heat value increase with increasing temperature, however, when the reaction temperature is higher than a certain value, the fusion were taken on the surface of solid slag granules. On the promise of making comprehensive consideration, we conclude that the best reaction temperature is 1373K. And in this experience, the best injection rate is ranging from 0.25 to 0.30 m3·h-1. The gasification character of DT coal is better than that of FX coal and SN coal. But the differences of gasification character between FX coal and SN coal are little. And at appropriate conditions, the difference of heat value between lean coals and high-quilty coals is little. Thus we conclude that the adaptability of coal gasification using solid BF slag granules as heat carrier on coal sample and granularities is widely. It is benefit to the gaidication character of lean colas. It realizes not only the waste heat recovery of BF slag but also the utilization of lean coals.(7)Based on the mass balance, element balance, energy balance and chemical balance, the process calculation method of melting gasifier and solid slag granules heat carrier gasifier was proposed. The difference between calculating results and experimental results is little. In this system, the cold gas efficiency of melting gasifier and solid slag granules heat carrier gasifier is higher than 1. In melting gasifier, when waste heat is recovered, the waste heat recovery rate of BF slag is 98%, and the exergy efficiency is 86.42%; when waste heat is not recovery, the waste heat recovery rate of BF slag is 60%, and the exergy efficiency is 51.09%. In solid slag granules heat carrier gasifier, when waste heat is recovered, the waste heat recovery rate of BF slag is 98%, and the exergy efficiency is 85.62%; when waste heat is not recovery, the waste heat recovery rate of BF slag is 66%, and the exergy efficiency is 54.07%. |
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