Wear and seal furnace components family
The energy cost for operation of the indurating furnace is very important. Therefore, plant operation and maintenance department managers pay close attention to energy consumption.
The furnace must be energy efficient and a number of parts are designed precisely for sealing the furnace. They prevent heat losses. They also prevent undesired cold ambient air from entering the firing zones. In both situations, the furnace becomes less efficient. More energy will be required to fire the pellets. If it is not possible to compensate with a higher energy input, the grate speed will be reduced with a consequent reduction of the production.
Since the grate is moving, those sealing parts are in contact with moving parts and thus are subject to friction and wear. The wear progressively decreases the seal and will impact the productivity of the furnace.
Metal 7 is specialized in wear protection and developed protective coating solutions for sealing parts including its I-19 coating for the slide rails (also called windbox seal bars) and a coating for the drop bars. Both bar types are assembled on the two sides of the furnace and are in contact with the moving pallet cars.
At the last stage of the pelletizing process, the pellets have to be hardened in a large furnace, referred to as an induration furnace. The hardening is required for the handling and shipping of the pellets to the steel making operations.
The energy required for the firing of the pellets represents a lot of money : the main cost in the pelletizing operation for most of the plants. Therefore, the furnace should be well maintained to be as energy efficient as possible. Any leaks would reduce this efficiency and would also disturb the pellet firing process. It is essential to ensure the furnace sealing elements are kept in good condition at all times.
Deterioration of the sealing parts of the furnace causes losses of hot process gases in positive pressure zones and entry of cold air in negative pressure zones resulting in the decrease of the energy efficiency of the pellet induration process.
Plant operators would either feed more energy through the burners, usually in the form of fossil fuel, to maintain the uniformity of the firing. If the addition of more heat is not possible due to limitations of the furnace, they would need to reduce the speed of the travelling grate with a direct impact on the productivity of the plant.
Energy losses have impact on the quality of the pellets and productivity.
In the straight travelling grate process, the pallet cars move slowly inside the furnace. Moving parts present a real challenge for sealing the furnace. The high temperatures, particularly in the firing zones, poses an additional problem in the selection of the material. Lubrication is also required to reduce friction between the fixed and moving parts.
We find four main parts on the pallet cars which purpose is to ensure the sealing: the two spring bars located under the car and the two side seal bars mounted on the two sides of the cars.
1- The spring bars: They are located under the pallet car and mounted on springs. The springs allow the bars to slightly move up and down to follow the horizontal profile of the furnace structure. For sealing, the spring bars slide against steel rails assembled on the furnace windboxes structure (called slide rails or windbox seal bars).
2- The side seal bars: They are positioned on both sides of the car. They are fixed. To provide the sealing, they are in constant contact with the drop bars mounted on the furnace walls. The adjustment of the height with the movement of the cars on the rails comes from up and down movement of the drop bars. Slots at the attachment points of the drop bars.
In both cases, we are in presence of metal against metal friction, high temperatures and a very abrasive environment with all the ore dust. The steel bars wear progressively and lose their function, the sealing of the furnace.
The challenge was to develop solutions to improve the wear resistance of the steel sealing bars and maintain the sealing of the furnace over a much longer period.
The expertise of Metal 7 in surface engineering and coating was once again the key in the development of solutions for those sealing components.
In studying the deterioration patterns and the abrasion conditions, it was decided to conduct two distinct R&D projects, one for the spring bars/slide rails and one for the slide bars/drop bars. We found that it was likely the solution would be different for both applications. This assumption proved to be accurate.
A- Spring bars and slide rails (or windbox seal bars): The spring bars are mounted on the pallet cars. The pallet cars are removed from the furnace periodically for maintenance. The spring bars could easily be replaced during those maintenances. However, the slide rails are bolted on the furnace structure on the top of the windboxes. A cold shutdown is required to replace them. It is not an easy task to reach them in view of their location.
Therefore, it was decided to focus on the design of a protective coating for the fixed part, the slide rails. A number of coating options were considered but coating evaluation and laboratory tests demonstrated that the carbide base M7-I19 was the most cost-effective solution to improve the wear resistance of the slide rails.
The M7-I19 coating applied on the carbon steel bars possess the following properties:
- Lower friction than other wear resistant material used for the fabrication of the slide;
- Very high wear resistance. It is among the harder material handled by Metal 7. Laboratory abrasion test demonstrate a wear resistance 98 times superior than CHT400 (400 Brinell plates).
- Very high adherence to the steel substrate;
- As sprayed smooth and crack free surface. The coating on the rails does not even require grinding or polishing.
B- Side seal bars and drop bars: Discussions between Metal 7 and end users lead to the decision to focus on a solution for the drop bars. They have a shorter life and they cause particular concerns for the operation.
Among the coatings considered, the M7-A29 carbide base coating was the one selected for this particular application. Its surface has some similarities with the other coating applied on the slide rails (see above) but it is slightly denser. The adherence obtained is also higher knowing the drop bars are in constant movement up and down and are potentially subject to more mechanical shocks.
- Decrease in fossil energy consumption thanks to improved furnace airtightness;
- Increase in production capacity of the furnace;
- Users report savings in power consumption of the system driving the grate (smooth surface of the bars with lower coefficient of friction);
- High durability of parts which results in less downtime and increase in furnace availability;
- Fewer man hours to replace worn parts;
- Coated parts rapidly available from Metal 7;
- Excellent quality-price ratio;
- Decrease in green-house gas;
- Better furnace operation stability with consequent improvement in the quality of the pellets;
- Health and safety gains with lower hot gas and fine dust leakage around the furnace