Coal char reactivity and structural evolution during combustion—Factors influencing blast furnace pulverized coal injection operation

A fresh char was prepared and reacted with oxygen under conditions similar to those prevailing in the raceway region of the blast furnace (BF) during pulverized coal injection (PCI), using a well-characterized drop-tube furnace (DTF). Char combustion under the present conditions was found to be controlled by the combination of pore diffusion and chemical reaction. Both the char density and size gradually decrease with burnoff, while the char surface area increases up to a burnoff of 40 to 50 pct due to the formation of a large amount of meso- and micropores, which were observed by high-resolution field-emission scanning electron microscopy (FESEM) and gas adsorption measurements. Despite the obvious increase in surface area, the char combustion reactivity decreases with burnoff. This is due to the loss of the intrinsic reactivity of char during combustion, as confirmed by fixed-bed (FB) measurements of fresh char and chars partly burnt in a DTF. The structural characterization by quantitative X-ray diffraction analysis (QXRDA) shows that the amorphous concentration (f _am ) of the char decreases during combustion, while the aromaticity (f _ar ) and the average crystallite size (L ₀₀₂) of the char increase. The char becomes more ordered during combustion, which is in accordance with the observations made using high-resolution transmission electron microscopy (HRTEM). The char structural ordering observed was found to be responsible for the loss of char intrinsic reactivity during combustion. Based on the QXRDA, a char structure model has also been suggested to explain the char structural evolution observed during combustion. The implications of char structural evolution for char combustion during a PCI operation are also discussed.     

用于高炉喷吹的低阶煤梯级转化半焦的燃烧性能

 采用固定床热解装置制备神木长焰煤与凤眼莲和冶金工业固体废弃物热解终温分别为450和550 ℃的共热解半焦;利用管式沉降炉模拟高炉喷吹条件研究共热解半焦的燃烧性能,并考察了热解终温和共热解物质对半焦燃烧性能的影响。研究表明：高炉瓦斯泥和冷轧氧化铁红的加入起到催化煤热解的作用,其中高炉瓦斯泥的催化热解效果最优,挥发分析出的增幅为18.4%;煤与冷轧氧化铁红在热解终温550 ℃时共热解半焦的燃烬度最佳为98%,与550 ℃时的原煤半焦的燃烬度相比,提高了22.5%;SEM扫描电镜结果显示,煤与冷轧氧化铁红共热解半焦与原煤半焦相比,半焦表面产生了更多的裂纹;除煤与轧钢氧化铁皮及煤与高炉瓦斯泥共热解半焦外,试验所制备的其他共热解半焦的各项性能均符合中国喷吹用煤指标。     

高炉喷吹混煤时碳与二氧化碳反应的探索

 通过采用热重及反应物岩相观察,研究不同煤种在不同配比下混煤残碳与CO2反应行为,由热重曲线可知：2种煤混合时,在900~1100℃,混煤实际与加权失重率相差不大,甚至实际失重率小于加权失重率;在1100~1200℃,混煤的实际失重率大于加权失重率,最大时相差10%。3种煤混合时,煤粉的实际失重率既与温度也与挥发分有关。得出结论：在1100~1200℃时,高炉喷吹混煤反应性更强,焦炭的保护作用更为突出,高炉内煤粉利用率更高。从煤粉碳素熔损反应后形态来看,高煤质煤粉残碳先于低煤质煤粉残碳反应完全。煤粉的挥发分含量不同时,其反应形态也不同：低挥发分的混煤中,烟煤的内孔反应和气泡产生不剧烈;而在高挥发分的混煤中,烟煤残碳反应现象较为剧烈。     

Reactivity and Conversion Behaviour of Brazilian and Imported Coals,Charcoal and Blends in view of their Injection into Blast Furnaces

For about 10 years the steel industry in Brazil has been using pulverized coal injection (PCI) technology in blast furnaces based on imported coals. In order to decrease the dependence on imported coals, Brazilian coal which has limited use due to high ash content is suggested to be mixed with imported coal and charcoal. The aim of this study is to examine the reactivity and combustion behaviour of the mentioned materials. The use of charcoal in the Brazilian steel industry reduces the CO₂ emissions, since it represents a renewable source of carbon. The reactivity of the coal, charcoal and mixtures is evaluated through TGA. To provide a useful insight into the practice of PCI in blast furnaces, experiments are carried out with a laboratory rig at RWTH Aachen University that simulates the behaviour of fines injected into the raceway. The results of this study are presented and discussed.     

不同热解半焦与高炉喷吹煤粉燃烧性能差异性

 研究不同半焦与喷吹煤粉的燃烧性能有利于半焦进行高炉喷吹、降低炼铁成本。通过改变热解温度、热解升温速率、热解保温时间和热解气氛在管式炉中制备了不同的半焦样品,通过工业分析和热重试验、结合比表面积、发热量和活化能变化对不同热解半焦和两种喷吹煤粉燃烧性能差异进行了分析。结果表明,不同热解条件下制备的半焦,其燃烧性能大多介于两种喷吹煤之间,热解温度是影响半焦反应性能最重要的因素,热解温度对活化能的影响呈现先减小后增大的趋势。不同热解温度制备的半焦孔隙结构均比喷吹煤发达,且累积孔容积和比表面积均呈现先增加后减少的趋势,孔隙特点与燃烧性能基本对应。550 ℃是制备高反应性半焦较适宜的热解温度。可以通过控制热解条件制备高反应性半焦,这有利于半焦在高炉喷吹中的应用。     

Thermogravimetric Analysis of Coal Char Combustion Kinetics

Four chars prepared from pulverized coals were subjected to non-isothermal and isothermal combustion tests in a thermogravimetric analysis(TGA)device.Three different test methods,i.e.,non-isothermal single heating rate(A),non-isothermal multiple heating rate(B),and isothermal test(C),were conducted to calculate the kinetic parameters of combustion of coal char.The results show that the combustion characteristics of bituminous coal char is better than that of anthracite char,and both increase of heating rate and increase of combustion temperature can obviously improve combustion characteristics of coal char.Activation energies of coal char combustion calculated by different methods are different,with activation energies calculated by methods A,B and C in the range of 103.12-153.77,93.87-119.26,and 46.48-76.68kJ/mol,respectively.By using different methods,activation energy of anthracite char is always higher than that of bituminous coal char.In non-isothermal tests,with increase of combustion temperature,the combustion process changed from kinetic control to diffusion control.For isothermal combustion,the combustion process was kinetically controlled at temperature lower than 580℃ for bituminous coal char and at temperature lower than 630℃ for anthracite char.     

CFD Modelling and Analysis of Pulverized Coal Injection in Blast Furnace: An Overview

In order to understand the complicated phenomena of pulverized coal injection (PCI) process in blast furnace (BF), several mathematical models have been developed by the UNSW and BSR cooperation. These models are featuring from coal combustion in a pilot‐scale test rig, to coal combustion in a real BF, and then to coal/coke combustion in a real BF, respectively. This paper reviews these PCI models in aspects of model developments and model applicability. The model development is firstly discussed in terms of model formulation, their new features and geometry/regions considered. The model applicability is then discussed in terms of main findings followed by the model evaluation on their advantages and limitations. It is indicated that the three PCI models are all able to describe PCI operation qualitatively. The model of coal/coke combustion in a real BF is more reliable for simulating in‐furnace phenomena of PCI operation qualitatively and quantitatively. Such model gives a more reliable burnout prediction over the raceway surface, which could better represent the amount of unburnt char entering the coke bed. These models are useful for understanding the flow‐thermo‐chemical behaviours and then optimising the PCI operation in practice.     

宝钢高炉炉尘中燃煤残炭颗粒的显微形态及含量分析

首先进行了单种煤等离子炉燃烧试验，分析了单种煤半焦颗粒的显微形态。在此基础上对宝钢高炉炉尘中燃煤残炭颗粒的存在状况进行了分析。分析结果表明：在宝钢高炉炉尘中存在燃煤残炭颗粒，且燃煤残炭颗粒的数量有随煤比增加的趋势。     

CHARACTERISTICS OF INDIAN NON-COKING COALS AND IRON ORE REDUCTION BY THEIR CHARS FOR DIRECTLY REDUCED IRON PRODUCTION

Studies on the chemical and physical properties (proximate analysis, sulphur content, reactivity, iron ore reduction potential, caking index, and ash fusion temperatures) of coals, procured from 16 different mines in Orissa, India, were undertaken for their judicial selection in Indian sponge iron plants. These coals were found to have low sulphur (range of 0.40–0.66%) and a moderate-to-high ash (range: 22–53%) contents. The results indicated that there were no caking characteristics in any of the coals except Basundhara. The majority of the studied coal ashes were found to have higher fusion temperatures (ST: 1349–1547°C; HT: 1500–1663°C; and FT: 1510–1701°C). An increase in the fixed carbon content in the coal char, in general, led to a decrease in its reactivity toward CO₂. The majority of the chars exhibited significantly higher reactivities (>4.0 cc of CO/g·sec). Further reduction studies in coal chars at 900°C indicated an increase in the degree of reduction of fired hematite iron ore pellets with an increase of char reactivity and reduction time. The authors recommend using the majority of the studied coals as such and some of them (Lakhanpur, Samleshwari, Orient OC–4, and Dhera coals) after blending or beneficiation.     

Combustion Property and Kinetic Modeling of Pulverized Coal Based on Non-isothermal Thermogravimetric Analysis

Non-isothermal combustion kinetics of two kinds of low volatile pulverized coals （HL coal and RU coal） were investigated by thermogravimetrie analysis. The results show that the combustibility of HL coal was better than that of RU coal, and with increasing heating rate, ignition and burnout characteristics of pulverized coal were improved. The volume model （VM）, the random pore model （RPM）, and the new model （NEWM） in which the whole combustion process is considered to be the overlapping process of volatile combustion and coal char combustion, were used to fit with the experimental data. The comparison of these three fitted results indicated that the combustion process of coal could be simulated by the NEWM with highest precision. When calculated by the NEWM, the activation energies of volatile combustion and coal char combustion are 130.5 and 95.7 kJ · mol^-1 for HL coal, respectively, while they are 114.5 and 147.6 kJ ·mol^-1 for RU coal, respectively.     

高炉喷吹煤粉燃烧性与反应性的研究与应用

为了研究高炉喷吹煤粉的冶金性能,采用热重分析法对6种无烟煤和1种烟煤进行燃烧性与反应性研究。结果表明,与无烟煤相比,烟煤在不同温度下的燃烧性和反应性均明显高于无烟煤。混煤的燃烧率实测值大于加权值,使用混煤可以发挥无烟煤和烟煤各自的优点,加快燃烧过程,提高混煤燃烧率。随着混煤中粒度小于74 μm煤粉所占比例的增加,燃烧率增大。在高煤比喷吹条件下,混煤煤粉粒度小于74 μm的比例控制在75%左右。结合无烟煤的燃烧性和反应性试验结果,建议喷吹煤粉采购中应尽可能多地采购无烟煤C资源,同时控制无烟煤E的采购量;高炉提煤比操作中应将无烟煤C确定为喷吹用无烟煤的首选煤种,以提升混煤燃烧率和发挥未燃煤粉保护焦炭的作用。     

高炉喷吹用秸秆炭性能表征

实施碳减排是中国全面执行《巴黎协定》的承诺,利用秸秆炭替代煤粉用于高炉喷吹可实现部分碳减排。采用热分析(TG、DTG、DSC)和角锥法对热解小麦秆炭、玉米秆炭、棉秆炭、稻杆炭、2种无烟煤和1种烟煤的燃烧特性和灰熔特性进行了研究,考虑到碱负荷对高炉顺行的影响,考察了棉杆炭可替代烟煤的比例。结果表明:秸秆炭较煤粉着火点温度、燃尽点温度低,燃烧速率快,释放热量高,燃烧过程稳定,综合燃烧性能好。但秸秆炭(除了棉秆炭)碱金属含量较高,灰熔点温度较低,易结渣,替代煤粉比例不宜较高;而棉杆炭由于CaO含量较高,其灰分软化温度大于1 200℃,灰融特性较好。在不影响高炉顺行的条件下,棉杆炭可以部分替代烟煤,其喷吹量为11.37kg/t,CO2减排量可达65.7kg/t。     

不同高炉煤粉的基础性能分析

煤粉是高炉冶炼的重要燃料,煤粉的基础性能对高炉的稳定顺行有很大影响。采用灰熔点测定仪测定了3座高炉喷吹煤粉的灰熔融特性,利用热重分析法(TGA)分析了这3种煤粉的燃烧性,并通过新型高炉喷煤模拟燃烧试验装置,模拟了3种煤粉在氧气高炉条件下的燃烧规律。同时,利用DAEM模型计算煤粉燃烧过程的活化能。结果表明,3种煤粉的灰熔融特性温度都不满足喷吹要求;A煤粉的燃烧性和燃烧率最差;A煤粉燃烧的表观活化能为59.16 kJ/mol,同时燃烧过程存在补偿效应。     

双枪喷煤时直吹管内煤粉燃烧的研究

高炉直吹管内煤粉燃烧是在有限空间内进行的,煤粉的燃烧受到限制。为了促进直吹管内煤粉的燃烧,通过二维模型研究了不同的煤粉性能和高炉操作条件对双枪喷煤时直吹管内煤粉燃烧率的影响。结果表明。煤粉的粒度越小,煤粉的燃烬率越高;煤粉的挥发分质量分数越高、热风的氧气质量分数越高,煤粉的燃烬率也越高。较高的热风温度有利于煤粉的燃烧.但当热风温度提高到1 200℃以上时,继续提高热风温度对煤粉燃烬率的影响较小。此外,还对直吹管内挥发分、温度以及各种气体成分的分布情况进行了对比分析,以便更好地理解上述操作条件对煤粉燃烧的影响。     

动力用劣质煤燃烧特性研究（上）

用热重分析法采用不同升温速率研究了六种煤样的ＴＧ，ＤＴＧ，ＤＴＡ及Ｔ曲线，用基辛格法计算得到不同煤燃烧反应的活化能，该值的大小与挥发分及灰分含量的比值的变化趋势基本相同。研究了同生矿物和后生矿物对煤着火和燃烧的不同影响。同时发现在用比表面分析得到的吸附等温曲线及比表面积、平均孔径和体积等数据与热重分析、矿物质分析数据间可建立合理的关联。提出评价煤燃烧反应特性应综合考虑燃烧反应前提条件－氧气与碳充分     

Gas-Particle Flow and Combustion Characteristics of Pulverized Coal Injection in Blast Furnace Raceway

The two-dimensional steady-state discrete phase mathematical model is developed to analyze gas-particle flow and combustion characteristics of coal particles,as well as components concentration and temperature distribution of coal gas in the process of pulverized coal injection of blast furnace raceway.The results show that a great deal of coal gas discharges on the top of raceway away from the tuyere,and the residence time of coal particles in the region of blowpipe and tuyere is 20 ms or so and 50 ms when it reaches raceway boundary.The pressure is the highest at the bottom of raceway and the maximal temperature is about 2423 K.The char combustion is mainly carried out in the raceway and the maximum of char burn-out rate attains 3×10-4 kg/s.     

高炉块状带焦炭强度模拟研究

 重点研究焦炭在高炉块状带内的抗压强度与温度和反应程度的关系。使用1台热模型对高炉块状带进行了模拟研究,1台可调气氛高温抗压试验机对所获焦炭试样进行了抗压强度检验,得到了焦炭在实际温度下的反应后抗压强度分布。在高炉间接还原区内,焦炭抗压强度随温度的升高而直线下降,在高温区内强度下将更为严重。焦炭失碳率是影响其强度的主要因素,在高煤比条件下应采用高反应性煤种并保留适量未燃煤,以保护焦炭强度。     

Influence of conveyance methods for pulverised coal injection in a blast furnace

Pulverised coal injection (PCI) is a widely adopted industry practice for reducing blast furnace coke rates. The conditions under which pulverised coal (PC) is injected and combusted, including the co-injection of natural gas (NG), can lead to complex combustion phenomena inside the blast furnace, which must be understood to provide improved furnace performance. This research examines computational simulations of the co-injection phenomena, as well as the industrial drivers behind the project. A wide-ranging parametric study was conducted utilising numerous variations in furnace operating conditions, as well as a new technique for the conveyance of PC. It was found that utilising NG as the carrier gas for PCI could increase coal burnout across the raceway region from about 71% to approximately 87% without altering the design of the tuyere/blowpipe region, with an increase to 96% possible if a shift to a dual lance design for NG injection is considered.     

Some Challenges and Opportunities in Blast Furnace Operations

Blast furnace iron making faces a lot of pressure to decrease use of metallurgical coke and carbon footprint. In many shops around the world in the meanwhile, the convectional blast furnace process is being run at near optimum efficiency for a given set of raw material conditions. Efforts are therefore being made to generate innovative ideas. Top gas recycling and nitrogen-less blowing seem to be attracting attention. Efforts are continuing for increased pulverized coal injection (PCI) to decrease coke consumption. Several furnaces experience pressure drop problems when trying to increase coal injection due to poor burden quality. The present paper attempts to analyze few of the issues related to top gas recycling in a conventional practice and those of pressure drop during PCI. Recycling of top gas even without removal of CO₂ seems to hold possibilities for enhanced PCI though with worse fuel rate. Managing layer structure and the reactivity of coke particles can be addressed in various ways to improve bed permeability.     

Dissolution rates of coals and graphite in Fe-C-S melts in direct ironmaking: Influence of melt carbon and sulfur on carbon dissolution

Carbon dissolution from graphite and coals was investigated by using a carburizer cover technique in an induction furnace. The intent of the study was to investigate the influence of factors governing the rate of carbon dissolution from carbonaceous materials, especially coals, into Fe-C-S melts. The factors studied were the initial melt carbon and sulfur concentrations and the wettability between carbonaceous materials and the melt. It was found that graphite dissolves markedly faster than coal. The rate of carbon dissolution from graphite could be decreased by increasing the sulfur in the melt. Also, poor wetting could retard the rate of carbon dissolution by reducing the surface area for mass transfer. Carbon dissolution from graphite is controlled by mass transfer in the liquid boundary layer adjacent to the solid/liquid interface. The rate of carbon dissolution from coal is more sensitive to the molten iron composition. A higher initial melt carbon and sulfur content retards the rate of carbon dissolution from coal more significantly than from graphite. However, the rate constant of coal char dissolution does not show a strong dependence on the wettability. Carbon dissolution from coals is most likely governed by a mixed-control mechanism that includes liquid-side mass transfer. The mechanisms underlying the influence of bath sulfur on carbon dissolution from graphite and coals are discussed.