The steel industry provides a major foundation for North American manufacturing. Increased production of high-quality steel is essential to support investment in infrastructure and meet the needs of the commercial construction and automotive industries.
Globally, steel production is shifting from the developed countries to India and other BRIC nations. One key challenge that will affect the long-term competitiveness of the North American steel industry is achieving raw material security by decreasing its dependence on imports of metallurgical coke. As North American steel companies analyze the options for increasing coke-making capacity, factors including operating costs and energy efficiency will be reviewed as investment decisions are finalized.
Based on the level of automation and sophistication of the plant configuration, the capital cost of a heat-recovery coke plant is up to 25% lower than a byproduct plant of the same capacity. Heat-recovery coke plants are typically built with an electric power turbine to permit power generation from the waste heat recovered from the coking process. Since these plants operate at a negative pressure and incinerate all the process gas, emissions from the ovens are also reduced. In many of the heat-recovery coke-making processes, the coal is pressed into a cake. This step permits the production of metallurgical coke with lower-quality coal as feedstock.
Due to the lower capital cost, the ability to use lower-grade coals as raw material and electrical generation capabilities, the investment in heat-recovery coke making is gaining favor globally.
Recently, new products and installation techniques have been developed by Unifrax to extend refractory lining life and reduce heat loss in heat-recovery coke ovens.
Areas for Improvement
In a typical heat-recovery coke oven, 72% of the energy produced is used to heat gas, 17% is lost through the refractory or leaks, and 11% is used to convert the coal to coke. Approximately 60% of heat loss in the refractory occurs through the doors, with 30% lost in the waste-heat-recovery ducts. Other losses account for the remaining 10%.
One of the factors that increases the operating cost of the heat-recovery coke oven is the high level of maintenance required for the oven doors and waste-heat duct system. In addition to the direct cost associated with refractory repair, operators experience a hidden cost in the form of energy loss through failed door or duct surfaces. Every million BTUs lost through the door refractory can reduce power output by 117.2 kilowatt hours.
In order to reduce operating costs related to the refractory lining in oven doors and waste-heat ducts, Unifrax has been working with coke plants globally to make improvements in three areas: first, through the development of new materials designed to extend the service life and reduce heat loss in oven doors; second, through the development of installation techniques that permit hot repair of duct and door surfaces. Finally, redesign of the refractory lining reduces heat loss through the lining and extends the lining life.
New High-Performance Maintenance Materials and Techniques
In North America and Europe, the use of refractory ceramic-fiber ropes and braids gained wide use in conventional byproduct coke ovens in the 1970s. These products were used by coke-plant operators as alternatives to asbestos packing materials for door frames and stand pipes. RCF blankets, textiles and wet mixes have been applied in both byproduct and heat-recovery coke ovens over the last 40 years. Due their low density and temperature limit, however, the use of these materials was generally limited to seals, backup insulation and heat-shielding applications. In order to survive severe operating conditions inside the coking chamber and heat-recovery ducts, new products were developed by Unifrax Brazil.
Silplate® Mass is a ceramic coating material that was initially developed to reduce the thermal shrinkage of ceramic-fiber linings and to increase the mechanical strength of the linings’ surface. This material is manufactured in several temperature grades in order to match good economy to a wide range of operating temperatures. This family of products is made from a blend of high-purity refractory oxides, reinforcing fiber and inorganic binders.
In a laboratory test, a sample was prepared featuring 10-pcf high-purity modules on one exposed surface and Silplate Mass 1500 troweled to a thickness of ½ inch on the other. This test sample was then heated to a soaking temperature of 2730ºF (1500ºC) for 24 hours.
In addition to excellent thermal stability, Silplate Mass coatings achieve a strong, hard surface that protects the substrate from mechanical abuse and erosion from flue-gas velocity.
Due to its mechanical characteristics and high wet strength, Silplate Mass can be built up to 1 inch thick in surface layers. Based on the material’s thermal stability and low thermal conductivity, Silplate Mass has been applied over dense refractory bricks and castables as well as fiber in order to enhance the performance of the lining. It is easy and fast to apply and may be installed using hot gunning techniques.
Hot Repair Techniques
One major opportunity for reducing the operating cost in heat-recovery coke plants is through reducing maintenance of the coke-oven doors. When refractory problems – like other maintenance issues – are identified and corrected early in the operating cycle major repairs and catastrophic failures can be avoided. Due to back to back operating cycles, it is not practical to remove the doors in heat-recovery coke plants for routine inspection and preventive maintenance. A typical door set in a heat recovery oven is pulled off the cell for approximately 10 minutes in order to push the cell and recharge the oven with new coal. To replace missing refractory and patch cracks and shrinkage openings, Unifrax developed a hot gunning technique for oven door maintenance.
Door Lining Design
The refractory lining materials for a heat-recovery coke-oven door are typically selected based on the belief that normal operating temperatures are 2200-2300ºF (1205-1260ºC). Due to air infiltration and secondary combustion of the coal, actual peak temperatures may exceed this range. Based on the analysis of ash buildup on the doors, peak temperatures during the coking cycle can exceed 2700ºF. These high peak temperatures can result in thermal shrinkage of the door lining materials.
To construct a typical oven door lining, 2 inches of layered board is installed to provide an insulating backup layer. A 10-inch layer of low-cement 3000ºF (1650ºC)castable is formed over the board to complete the door lining. Stainless steel anchors extend from the casing through the backup insulation. The anchors terminate with a ceramic cup or “V” anchor to retain the refractory hot-face layer.
Over time, the castable layer can thermal shock and break away from the backup insulation, exposing the anchors. Once the anchors are exposed to 2200ºF (1205ºC) operating conditions, the metal begins to oxidize and creep. At this point, the refractory hot face fails and the backup insulation falls off the remaining section of the anchors.
Once the insulation is lost from the door lining, thermal expansion causes the door casting to warp and crack. This will accelerate the loss of the door seal and lead to more rapid failure of the door system.
The Unifrax door design combines the use of thermal-shock-resistant, 3000ºF castable at the perimeter of the door with thermally efficient ceramic-fiber modules on the main lining surface. To provide additional protection from thermal shrinkage, mechanical abuse and attack from ash, a layer of Silplate 1500 Mass is installed over the surface of the modules.
Installation begins by attaching metallic anchors for the castable and Anchor-Loc modules to the door casting. At this point, the Anchor-Loc modules are attached to the threaded fasteners under compression.
Using the modules and a removable form, the castable is poured to create a frame around the fiber lining.
The first new-design oven doors were put into service in March 2011. After six months of service, there was no evidence of shrinkage cracking on the module working lining or on the castable door frame. Note that the ash buildup on the door face is normal (Fig. 6).
Minor shrinkage cracks have started to develop on the door surface after one year of service. At this time, a coat of Silplate 1500 Mass was gunned on the hot surface as part of the preventive-maintenance campaign.
The thermal performance for the Unifrax coke-oven door lining was compared with the typical door lining used in the industry today. Cross sections for both designs are presented for reference in Figure 7.
Coking temperatures and cycle times vary based on a number of factors, including quality of the coal, oven design, operating practices and market demand. In order to compare the benefits provided by the Unifrax door design, the operating conditions were standardized based on input from several operators. For this comparison, a standard one-piece door with a working surface area of 85 square feet was selected. Operating conditions for the coking cycle were based on an average door temperature of 2550ºF (1400ºC) and a 48-hour coking cycle.
The economic benefit provided by the new lining was based on the market value of the incremental electricity produced in the co-generation station. For this analysis, the efficiency of the co-generation plant was set at 40% and the market value of electricity was assumed to be $0.085 per kilowatt hour (Fig. 8).
The economic analysis of the Unifrax coke-oven door system is summarized in Figure 9. In order to model the performance of a typical castable door design, service life was split into three periods to reflect the gradual deterioration of the refractory and increased heat loss over time. Based on our experience with the successful performance of the Unifrax door lining, the heat loss from this fiber-based design remained constant over the 24-month service life. The energy savings per hour was calculated by subtracting the heat loss of the Unifrax door lining from the estimated heat loss of the castable door design. Next, the potential generation of electrical power was determined by multiplying the energy saved per hour by the length of the cycle (48 hours) and the number of cycles per service period. An efficiency factor of 40% was applied to reflect the conversion of heat energy to electrical power.
Based on our calculations, each Unifrax door unit can save 130 megawatt hours of heat energy over a service life cycle of two years. When coupled to an electrical co-generation plant, this energy can yield an incremental $453 of electrical power per month based on a market price of $0.083 per kilowatt hour.
In order to attain a best-in-class position, a variety of goals must be met by the domestic steel industry. Securing sources of raw materials, reducing production cost, improving energy efficiency and implementing world-class technology are a few of the challenges that face the leaders of the steel industry.
Lower capital cost, improved coke quality and production of electric power make heat-recovery coke plants an interesting investment option for the industry. The development of new high-performance insulating products and improved installation techniques by Unifrax have the potential to increase energy savings and power production while reducing refractory maintenance and operating costs. IH
For more information: Contact Gary W. Deren - marketing manager, VFBU (Vacuum Forming Business Unit), Unifrax I LLC, 600 Riverwalk Parkway, Suite 120, Tonawanda , NY 14150; tel: 716-768-6500; fax: 716-768-6400; e-mail: email@example.com; web: www.unifrax.com. Silplate® is a registered trademark of Unifrax I LLC.