Fundamental Study On Innovative Process Of Low-Carbon Blast Furnace Ironmaking By Coke Oven Gas Injection

The steel industry is one of important pillar industries for the national economy, and the blast furnace-converter process plays a dominant role in the whole steel production in quite a long period of time in the future in China. It is similar in the worldwide range. In 2012,69.6% of global crude steel was produced by the blast furnace-converter process and 94% of global pig iron by blast furnace. However, among all the sectors of steel production, blast furnace is responsible for the biggest energy consumption and most CO2 emission, accounting for above 50% and 70% of the whole steel production system, respectively. Therefore, blast furnace ironmaking is critical to realize energy saving, low carbon emission and sustainable development of steel industry. However, after the developments in the past decades, the operation efficiency of blast furnace ironmaking has been close to the extremity limitation, and it is quite difficult to substantially achieve low carbon and ultra-high efficiency just by means of conventional technologies, such as ore beneficiation, burden distribution, high top pressure, pulverized coal injection, and so on. In order to further reduce energy consumption and CO2 emissions, and to realize low carbon ironmaking, ironmaking workers need to actively strengthen relevant theoretical and experimental researches on hydrogen metallurgy to achieve hydrogen enrichment medium used in blast furnace injection.Based on current international research focuses and future development direction of blast furnace, fundamental study on new low-carbon ironmaking process of blast furnace by injecting coke oven gas (COG) are put forward in this paper, includes the establishment of raceway mathematical model, the establishment of exergy analysis mathematical model, numerical simulation on blast furnace operation with coke oven gas injection, numerical simulation on optimizational effects of hot burden charging to coke oven gas injection, numerical simulation on optimizational effects of top gas recycling to coke oven gas injection and effect mechanism of coke oven gas injection on metallurgical properties of comprehensive burden. First, numerical analysis on innovatory processes of injecting coke oven gas into blast furnace through tuyere were performed in this study by means of blast furnace model on the premise of keeping theoretical flame temperature, bosh gas flow rate and hot metal temperature consistent with conventional operations of blast furnace. Second, experimental atmosphere acquired through numerical simulation was utilized to study effects of coke oven gas injection rate on reduction swelling property of pellet and softening and melting properties of comprehensive burden in the laboratory.Through this study, for a steel enterprises 2580 m3 blast furnace in Northeast China, conclusions can be drawn as follows:(1) On keeping original operations of blast furnace unchanged, coke oven gas is injected into blast furnace through tuyere directly. After that, the temperature of raceway is decreased, while bosh gas flow rate is increased. In order to make thermal conditions of raceway zone consistent throughout the cases examined here, this study put forward the thermal compensation measures by reducing blast flow rate and enhancing oxygen enrichment. Meanwhile, with 1 m3·s-1 increasing of coke oven gas injection, the blast flow rate is decreased by 94.95 m3·min-1 and oxygen enrichment is increased by 1.36%. With the increase of coke oven gas injection, the temperature level in the upper zone of blast furnace is decreased. Reducing gas concentrations are increased. CO utilization rate is enhanced while that of H2 is decreased. The proportion of H2 in indirect reduction gives a raised tendency, which accelerates the reduction rate of burdens. Due to the location of cohesive zone moving down, the thickness getting thinner and the pressure drop reducing obviously, the permeability of blast furnace gets better. For 11.89 m3·s-1 coke oven gas injection, the productivity of blast furnace is increased by 26.36%, and the utilization factor increases substantially. Coke rate, total reducing agent rate and carbon emission rate are decreased by 13.5%,4.1% and 17.5%, respectively. With the increase of coke oven gas injection, internal exergy loss of blast furnace is decreased while external exergy loss is increased. And total exergy loss is decreased. Compared with no coke oven gas injection, for 11.89 m3·s-1 coke oven gas injection, thermodynamic perfection degree and exergy efficiency are increased by 2.42% and 0.86% respectively. Chemical exergy of blast furnace top gas is increased by 366.85 MJ·t-1. So it's important to strengthen the recycling of top gas.(2) Aiming at the problem of insufficient heat supply in the upper zone of blast furnace with coke oven gas injection, optimization operation of hot burden charging to coke oven gas injection is put forward. In order to master the effect mechanism of hot burden charging on the operation indices of conventional blast furnace, numerical simulation on blast furnace operation with only hot burden charging is carried out firstly. Compared with conventional blast furnace operation, the ratio of ore to coke, solid burden charging rate and the yield of hot metal are increased with hot burden charging. Coke rate, fuel rate and carbon emission rate are all decreased at a certain degree. When the charging temperatures of both pellet and coke are 800 ? (PC800), wo/wc is enhanced by 4.79%, solid burden charging rate is increased by 7.55 kg·s-1, and hot metal productivity is increased by 6.38%. Coke rate, fuel rate and carbon emission rate are decreased by 13.4,22.1 and 19.25 kg·t-1, respectively. The solid phase temperature in the top zone is increased while that in the middle and lower zone temperature is decreased, especially near furnace wall. In addition, the concentrations of reducing gases are decreased and the average height of cohesive zone shifts downward. The flow rate of top gas is decreased while the temperature is increased. Heat taken away by top gas and energy consumption per ton hot metal are increased. For PC800, heat taken away by top gas is increased by 68.97%, and energy consumption of per ton hot metal is increased by 6.40%. Hot burden charging is adverse to energy utilization of blast furnace.(3) Based on the study of conventional blast furnace operation with only hot burden charging, numerical simulation on optimizational effects of hot burden charging to coke oven gas injection is carried out. Compared with conventional blast furnace operation and coke oven gas injection operation, the temperature in the upper zone increases for the new process, while in-furnace temperature has little improvement in the middle shaft area, and even decrease in the lower zone, in which the COI12-PC800 case with 12.00 m3·s-1 coke oven gas injection and hot charging of both coke and pellet 800 ? shows the most significant change. The permeability of blast furnace gets better, the location of cohesive zone tends to descend and becomes narrower. The reduction degree of iron bearing burden increases obviously in the region of furnace top, decreases at a certain degree in the shaft before reaching the cohesive zone and decreases sharply on the same furnace height, especially in the COI12-PC800 case. Ratio of ore to coke, solid burden charging rate and hot metal productivity are increased, coke rate, fuel rate and carbon emission are decreased, which are the most evident for COI12-PC800. Compared with COI12 operation, the former are increased by 3.42%,5.75% and 7.14% respectively, and the latter are decreased by 4.0%, 5.0% and 5.3% respectively. Total heat output per ton of hot metal are increased by 2.28%, 7.61% and 9.64% corresponding to COI12-P800, COI12-C800 and COI12-PC800 compared with COI12 case, and decreased by 16.73%,12.40% and 10.74% in comparison with conventional blast furnace. Therefore, coke oven gas injection together with hot burden charging operation not only improves the temperature level in the upper zone of blast furnace under the condition of coke oven gas injection but also solves the problem of the increase in total heat output with only hot burden charging, which is beneficial to energy utilization of blast furnace.(4) Aiming at the problem of low utilization rate of reducing gas in top gas with coke oven gas injection, optimization operation of top gas recycling to coke oven gas injection is put forth. Compared with conventional operation and COI 12 case, the average height of cohesive zone shifts downward for COI 12-TI, COI12-TI-SI and COI12-TI-PC800, and the temperature decreases in the region of shaft. Compared with COI 12-TI case, the temperature in furnace has a little increase for COI12-TI-SI case, and the temperature in the upper top zone improves more evident for COI12-TI-PC800 case. The concentrations of CO and H2 in furnace are all increased for the last three top gas recycling processes. Reducing atmosphere in furnace is strengthened, which accelerates the reduction rate of burdens. Compared with COI 12 case, the pig iron productivities are increased by 13.9%,15.8% and 16.2% respectively corresponding to COI12-TK COI12-TI-SI and COI12-TI-PC800. Coke rate are decreased by 4.7%,6.6% and 5.7% respectively, and fuel rate are decreased by 7.4%,9.1% and 8.7% respectively. Carbon emission rate are decreased by 42.2%,50.3% and 43.5% respectively. Energy consumption of blast furnace are decreased by 15.8%,14.8% and 3.8% respectively. The utilization rates of CO and H2 in top gas have different trends, and the CO utilization rate are increased by 6.95%,8.82% and 4.97% respectively corresponding to COI12-TI, COI12-TI-SI and COI12-TI-PC800. For COI12-TI case, H2 utilization rate decreases evidently. For COI12-TI-SI and COI12-TI-PC800 cases, H2 utilization rates are all improved in comparison with COI12-TI case, in which H2 utilization of hot charge operation is increased by 3.41% compared with COI 12 case. Thus, the operations of hot reducing gas injection through shaft or hot burden charging can improve utilization rate of reducing gas for blast furnace operation with coke oven gas injection.(5) Reduction swelling property of pellet and softening and melting properties of comprehensive burden were studied in the laboratory under the condition of hydrogen-rich gas injection. On keeping CO content unchanged, as H2 content in reducing gas rises from 2% to 18%, the recovery ratio of pellet increases from 39.85% to 69.58%, and the reduction swelling index of pellet decreases from 10.12% to 5.57%. The only increase of H2 content in reducing gas is conductive to accelerate reduction rate of pellet and to lower reduction swelling index of pellet. As injection rate of coke oven gas rises from 0 to 11.89 m3·s-1, the recovery ratio of pellet increases from 31.57% to 36.39%, and the reduction swelling index of pellet decreases from 10.71% to 9.54%. With the increasing of H2 content in reducing gas, the reduction swelling index of pellet tends to reduce. With the increasing of coke oven gas injection rate, softening start temperature (T4) of comprehensive burden decreases, softening finish temperature (T40) increases, and softening temperature range (T40-T4) becomes wider. With 1 m3·s-1 increasing of coke oven gas injection, softening temperature range increases 5.4?. Melting start temperature (Ts) increases evidently, and dripping temperature (TD) has slight increase. The melting range (TD-Ts) of cohesive zone tends to narrow, and the location has a little downward. With 1 m3·s-1 increasing of coke oven gas injection, melting range shrinks 4.62?. Softening-dripping characteristics S decreases sharply, and burden permeability improves evidently.