In 2008, 25% of the DRI production was from the coal-based processes. [33]
Despite not being a big percentage, the share of coal-based processes in DRI production is gradually increasing. This may be attributed to the high global reserves of coal which exceeds the natural gas as shown in figure 2.12.
12Figure 2.12 Global energy reserves
Moreover, experts are sure that on the long run coal will continue to be less expensive and its price will be more stable than other forms of energy. [23]
In coal-based processes, rotary kilns are used as the reducing reactor. The main differences in the individual processes are related to the control system especially for temperature. [30]
2.6.1 General Process Description of Rotary Kiln Technologies
In all coal-based DR processes, lump ore or pellets (or both) together with coarse fraction of non-coking coal are fed to the inlet end of the rotary kiln.
The size ranges of lump ore, pellets, and non-coking coal are respectively 4-20 mm, 9-20 mm, and 6-20 mm. This coal is referred to as co-current coal, and it acts as a reducing agent, and as a major heat supplier to the kiln. A finer
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fraction of coal (-6 mm) is also injected from the discharge end of the kiln using primary air as the carrier gas. This coal is called countercurrent coal, and it helps in completing the reduction, and supplying heat.
A fluxing material like limestone or dolomite should also be added in order to control the sulfur pick up by the reduced materials from the coal ash. The flux is mainly in fine form (-4 mm), and is added with the countercurrent coal.
In these processes, optimizing the temperature of the bed charge is crucial.
At the inlet end, the temperature should be high enough so that the reduction reactions proceed rapidly. On the other hand, the temperature should be low enough to prevent the fusion of the coal ash. This is achieved by conserving a balance between the solid-bed temperature, and the temperature in the atmosphere above the bed (normally at least 100-150oC higher). [30] This is mainly achieved by burning combustibles released from the bed using secondary air. The latter is blown by fans through burner tube space uniformly along the length of the kiln. [34]
The product from the kiln is mainly a mixture of DRI and char. The product's temperature is about 950-1000oC, and it is cooled in an indirectly water-cooled rotary cooler to about 120oC. After that, the DRI is separated from the coal char using magnetic separators, and finally screening is performed. The separated char is mainly recycled as a feed material.
Waste gases leaving the kiln at the inlet end pass through a dust chamber and a post-combustion chamber, before being cooled and cleaned in electrostatic precipitators, scrubbers, or bag filters. Alternatively, the clean kiln gases can be used in waste heat boilers to utilize the sensible heat in producing steam. [5] Figure 2.13 shows a schematic representation of DRI production in rotary kilns. [30]
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13 Figure 2.13 A schematic representation of DRI production in rotary kilns
2.6.2 Encountered Reactions during coal-based DR Processes
The main reactions that take place within the rotary kiln are the frequent reduction reactions.
CO + Fe2O3 CO2 + 2 FeO (1)
CO + FeO CO2 + Fe (2) Reaction 2 takes place in the last 30% of the kiln's length.
The carbon monoxide results from combustion of coal in the presence of controlled amounts of air
C + 0.5 O2 CO (3)
The produced carbon dioxide resulting from the reduction reacts quickly with the carbon present in coal to produce carbon monoxide according to the famous boudouard reaction
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C + CO2 2 CO (4)
This cycle continues to maintain the reducing conditions prevailing in the kiln. Moreover, coal pyrolysis takes place inside the kiln, where volatiles tend to evolve till about 600oC. However, it is to be noted that most of these volatiles don't have real contribution to the actual process of reduction. Part of these volatiles is being combusted by the secondary air injected to the kiln.
This combustion transfers heat to the charge directly by radiation, and also by conduction from the kiln lining. [16]
2.6.3 Comparison between Different Rotary Kiln DR Processes in Commercial Use
The coal-based DR processes are similar to great extent. The main industrially applied processes are SL/RN, Codir, DRC, Jindal, and SIIL. The main differences between them are in the tolerable size of raw materials, and energy consumption. However, it worth noting that SL/RN process is the mother of all the other coal-based DR processes, and it is the most widely applied. [30]
2.6.4 Advantages of Rotary Kiln Processes
Rotary Kiln can effectively mix the solid charge as it undergoes simultaneous heating and reduction. Intimate mixing of the charge helps in diluting CO2 formed around the iron ore particles, and this helps the reduction reactions to proceed.
Since large freeboard space is available above the solid charge in any kiln, the gas phase can tolerate the presence of heavily dust-laden gases. In gas-based processes, generation of dust can lead to channeling.
Rotary kilns are commercially proven, and there is a lot of operating experiences with it especially in cement industry.
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The temperature of iron oxide reduction is much lower than that of blast furnace (1000oC against 1300-1600oC). As a result, less energy is required for reduction. [30]
2.6.5 Disadvantages of Rotary Kiln Processes
The productivity is very low compared to shaft furnaces in gas-based DR processes. In the latter, yield is up to 5 times more than the rotary kilns for the same inner volume. Thus, for large capacity plants, multiple rotary kilns are needed.
The reactor rotates at 0.4-0.5 rpm which makes it difficult to incorporate process control and quality control systems. Moreover, the engineering of such kilns is difficult.
The fact of cooling the product in order to perform magnetic separation is a huge source of energy losses. Thus, these processes exhibit very low energy efficiency.
Because of the repeated fall and rise of the charge during rotation, the solids undergo size degradation. Thus, the coarser particles tend to float on the top of the charge, and the fines tend to settle at the bottom, and thereby increasing the tendency of adhering to refractory lining. The latter gives rise to ring formation. Once rings are formed, uniform movement of the charge becomes difficult, and shutdown of the kiln becomes a must. [30]