Coke has been used in the steel industry for years. Coke ovens exist to supply coke for blast furnaces and to a smaller extent the foundry industry. Coke is used to provide Carbon and heat which chemically reduces iron ore to pig iron in blast furnaces. Coke also supports the materials in blast furnaces because of its strength and provides permeability for gases to penetrate the material bed due to shape.3 Coke is a major plays a major role in producing quality steel. Since coke has been used for a long time in the steel industry, there have been coke making challenges that have risen.
Coke making has been facing shutdowns which threaten to reduce production capacity in North America.5 The process of coke making has changed very little over its more than one hundred year history. There are natural aging coke plants that face shutdowns due to the tightening of environmental regulations. To comply with these regulations a considerable amount of money would be spent to fix the problems. In turn this would create higher production costs.3,4 Historically coke making is the largest source of emissions of all processes associated with raw material preparation in steel making. Some of the environmental problems faced include the leakage of environmentally unacceptable materials into the atmosphere. This includes dust, condensed tars, liquids, and gas which are mostly due to ageing coke ovens which have failing door seal closures.1,2,3 Gases also come off the material from intense heating which then creates air pollutants. These air pollutants are criteria air pollutants, hazardous air pollutants, and ammonia. Hazardous sludge is also associated with different recovery processes which then cause environmental problems. In the coke making process the finished material is doused with water which in turn the water then has to be disposed of as waste water.5 Table 1 shows the increase of various pollutants emitted by coke making processes from 1996 to 1999.
There are some issues that concern coke making. One such issue is the necessity of coal washing when using coking coal. Since the ash content is high this would significantly reduce the ash content in coking coal. Also there is Low Volatile High Rank (LVHR) Beneficiation. Of the thirty two billion tons of coking coal reserve, this accounts for 8.5 billion tons. These coals are not readily available to be used in the steel industry. After the beneficiation process, these coals can be used as a blending agent.1 Another challenge faced is the availability of coke due to a few factors. The prices of coke have risen over the years. China has had the highest production capacity in the mid 1990s. But the closure of many coking coal mines for safety issues and the drop in availability has strongly affected the price of coke. Freight transport also is a large factor in the price of coke to rise because the cost of freight transport has also increased.2 This leads to the exposure of vulnerability of supply for steel companies. Most coking coal mines are underground mines. Underground mining only yields forty percent of the total reserve which is not very efficient. The coke making industry is in dire need of extending the range of coals used in coke making.
Since these coke making challenges have risen, there also have been new developments in coke making technology. These different processes are found in many different countries such as the Netherlands, Japan, Australia, Germany, United States, Russia and India. One of the processes that the Japanese developed is called Scope 21 (super coke ovens for productivity and environmental enhancement).3 The goals of this process are energy savings of twenty percent and a productivity increase of three hundred percent. The Japanese came up with this process based on the 1970s price differentials between coking and steamed coals. Formed coke is a part of Scope 21. This is a continuous and enclosed process. This process is also more environmentally acceptable according to the Environmental Protection Agency (EPA). This is found true by the growing number of processes approved by the EPA. There are three parts of the process in making formed coke. The coal undergoes pretreatment, carbonization and then coke upgrading.3 Another new process emerging is non-recovery coke making.3 In this process the coke oven chambers are in a horizontal configuration and are operated under a negative pressure. Since the coke ovens operate under a negative pressure, door leaks are therefore non-existent. Also in this process volatile matter is recycled and combusted which then provides additional heating to the ovens. This process of non-recovery coke making is environmentally friendly because it kills off 99.9 percent of toxic hydrocarbon emissions. The steam that is produced from the heat is extracted and used for heating and a source of power.3
Examples of other new processes of coke making have occurred in other countries such as India. The coking coal reserves are only in the amount of thirty two billion tons which is only thirteen percent of the total coal reserve. Stamp charging has emerged as a process that has an advantage over other technologies. Stamp charging is a process that involves coal stamping (coal blending).1 In this process a machine stamps the coal into a stamping box to form coal into a thick, dense cake. Moisture is used to compact the coal into the cake-like form. From there the cake gets put into an oven where it is then blended with imported coals which produces a higher strength coke at an acceptable cost. Figure 1 and Figure 2 compare the coke strength and the yield of coke after it is blended.
Briquette blending and selective crushing is also a couple of new processes that have emerged.1 The preheating of coal charge is a helpful process because it reduces the prime coking coal requirement. Also the injection of high grade non-coking coal in blast furnaces is highly encouraged in such countries as India. It reduces the coke rate and coal based sponge iron plant. The COREX process which is process of production of hot metal utilizing non-coking coal has some advantages as well. It includes the production of hot metal without coking and sinter plants which make the process more cost efficient and it is an environmentally compatible process.6 Figure 3 displays the COREX process compared to a conventional blast furnace process.
A continuous coke making process is being developed at The Ukrainian State Research Institute for Carbochemistry. In this process a vertical shaft and a piston are used to force metallurgical coke blends through the heated zones of the system.3 New technology is also forming in the United States called A Calderon Cokemaking Technology. This involves continuously producing coke from metallurgical coal which leads to the cleaning and cracking of the gases under sealed conditions. Out of this process cleaned gases are recovered and used as a syngas.3
Coke making has had its challenges, but the continuous improvement of the process from various countries has helped alleviate those challenges. New coke making technologies help to make the process become more cost-effective, environmental friendly and produce stronger and longer lasting products. As the drive for better technology increases, it is determined that it is more likely the coke making process will improve and last for many years.
References
