Reheat Furnace Start-up Using Ceramic Coatings
As our economy continues to strengthen and steel production increases in the U.S., several of the mills that have been idled for years are beginning to come back on line. With the increase in production rates – up over 6.1% from last year – the demand for U.S. steel is on the rise.
The increase in steel demand has led to the start-up of several mills that have been permanently closed due to hard economic times and the rise in imported steel. One such mill is Georgetown Steel (aka, ArcelorMittal Georgetown Steel), which is now Liberty Steel, Georgetown after the acquisition of assets by Liberty House Group of the U.K.
The mill is capable of producing 750,000 tons of wire rod annually and is the primary wire-rod producer in the U.S. Georgetown Steel produces wire rod that is used in rubber tires (tire cord and tire bead), wire rope and pre-stressed concrete strand. It also supplies various industrial-quality wire-rod grades from low- to high-carbon content and several types of micro-alloy and medium-alloy steels.
In 2008 when the steel economy was at its peak and gas prices were too ($10 per million BTU), ArcelorMittal Georgetown Steel decided to invest in a means to reduce the fuel consumption associated with their reheat furnace. In addition to reducing fuel costs, Georgetown Steel was also interested in reducing their high cost of refractory maintenance.
The hot mill consists of one pusher-type reheat furnace capable of producing 750,000 tons annually. 2008 production rates were approximately 88 tons per hour and averaged 1.5 MM BTU/ton of steel produced with a total usage of 3,168 MM BTU per day. The furnace’s designed production rate of 120 tons per hour could not be attained due to heat loss from the badly spalled and cracked refractory lining as well as major BTU losses through the skids due to missing pipe refractory shapes.
The furnace lining consisted of 9 inches of plastic refractory and 3 inches of lightweight castable in the roof as well as 16 inches of plastic refractory and 3 inches of lightweight gunnite refractory on the sidewalls. The operating temperature was 2150°F (1177°C), and it was run approximately five days a week (225 days a year). On weekends the furnace was throttled back to approximately 1600°F (871°C).
With the badly consumed and aging refractory lining and under these current operating conditions, Georgetown Steel needed to address two major challenges:
- Excessive heat loss and shell temperatures on roof and sidewalls
- Minimal time and financial resources to completely reline the entire furnace
Facing these two constraints, Georgetown Steel looked to ceramic coating technology as an economical means to repair and prolong the life of their refractory lining as well as reduce heat losses and fuel consumption plaguing their operation.
In the summer of 2008, Georgetown Steel partnered with International Technical Ceramics (ITC) to apply their ceramic coating to the entire inside of the reheat furnace, roof, sidewalls, end walls, and both charge and discharge doors. This engineered approach consisted of spray coating the entire hot-face surface area with a proprietary high-temperature, energy-efficient ceramic coating.
Next, the entire hot-face surface area was veneered with ½-inch-thick ceramic-fiber blanket cut into 12-inch x 12-inch squares, saturated in the ceramic coating. Once the veneering process had air-dried for 24 hours, the entire surface was again sprayed with the ceramic coating, creating a complete monolithic installation. Figures 1-4 show the installation, the veneering process on the sidewalls and bullnose and the completed installation.
This veneering process allows ITC to completely seal off all badly spalled and cracked refractory where heat can penetrate through the lining to the shell and be lost to the outside atmosphere. The coatings themselves can only be applied mils thick, so the saturated ceramic fiber helps bridge the gaps and cracks in the refractory, which keeps heat inside the furnace.
The major benefit of the coatings is its ability to re-radiate approximately 90% of all the radiant energy or BTUs put out by the burners. At these elevated temperatures the major mode of heat transfer required for reheating is through radiation (T4) and if the refractory lining is in poor condition, heat loss through the lining can be very costly.
Q = Eh x d x (Th4 – Tc4)
Q = Re-radiated energy (BTU/hour-feet2)
Eh = Emissivity of hotter surface
d = Stefan-Boltzmann Constant
Th = Temperature of hotter surface
Tc = Temperature of colder surface
After six months of operation with the new ceramic-coated lining, ArcelorMittal decided to idle the mill due to declining economic conditions. During those six months that the furnace ran, however, the benefits attained through the use of the coatings were quite impressive. Georgetown Steel was able to reduce their fuel consumption and the amount of energy required for the process, increase throughput due to additional energy available and reduce the amount of scale being generated.
Before and after thermal images of the roof and sidewalls showed a significant reduction in lost heat. Initial calculations done on the sidewalls predicted a drop in shell temperature of 38°F, or approximately 17%. Based on thermal images, the actual drop in shell temperature was in the range of 100°F on average, or approximately 36%. Calculations done for the roof predicted a drop in cold-face temperature of approximately 60°F (20%). However, thermal images showed the actual drop in cold-face temperatures was approximately 85°F, or 25%. Initial BTU savings were estimated at 1.9 MM BTU per hour, but with the above reduced heat loss through the furnace refractories, the actual savings was 2.2 MM BTU per hour.
Upon start-up of the furnace after the installation, operators found that if they attempted to maintain the operating temperatures at pre-installation levels, they were beginning to melt the billets. The mill was able to drop normal furnace operating temperatures as indicated by their thermocouples from 2150°F to 1840°F as well as increase furnace throughput from 88 tons per hour to as much as 120 tons per hour. This is due to the ability of the ITC coatings to reflect radiant energy back into the furnace and maintain a much more even heat distribution.
With the billets now coming to temperature much more quickly in the heating zones, the burners in the soak zone were cut back while the speed of the billets through the furnace was increased. This reduction in operating temperatures led directly to additional fuel savings as well as reductions in NOx emissions.
With the increase in throughput and a decrease in soak-zone temperatures, ArcelorMittal gained an approximate 2% increase in yield due to substantial reductions in scale generation. Furnace operators indicated that the scale boxes that were previously emptied on a daily basis are now only being emptied on the weekends. This increased yield was calculated at an additional 9,000 tons of saleable product per year based on the mill operating at full capacity. Figures 5 and 6 show the veneered lining condition six months after installation.
A summary of the benefits provided by ArcelorMittal are:
- The combination of lower heat losses through the refractory, lower operating temperatures and increased furnace throughput has dropped the heat required per ton of steel produced from 1,500,000 BTU/ton to 800,000 BTU/ton. The plant ran five days a week, 50 weeks per year with a 90% operating availability, which is equivalent to 225 days per year.
- The previous production rate of 88 tons per hour at 1.5 MM BTU/ton gives a total usage of 3,168 MM BTU per day.
- The new production rate of 120 tons per hour at 0.8 MM BTU/ton gives a total usage of 2,304 MM BTU per day.
- With an approximate fuel cost of $10 per mm BTU, the annual savings in fuel would be approximately $1,950,000 per year.
- The increased production due to a 2% increase in yield based on the reduction in scale is approximately 9,000 additional tons per year. At $400/ton, this is an increase of $3,600,000 per year in additional steel.
With the furnace operating at full capacity, the total annual potential savings and increased production amounts to $5,550,000. Based on this figure and the total cost of coating installation at roughly $200,000, payback for this project was approximately 9 days.
Current Furnace Condition
In the spring of 2018, Liberty House Group finalized the acquisition of all assets of Georgetown Steel from ArcelorMittal. The mill sat idle for nearly two and a half years and went through two hurricanes and major flooding in the fall of 2015, yet the plant remained in relatively good condition.
During the second quarter of 2018, the plant was fully commissioned, the first heat was tapped and the reheat furnace produced its first billet. The current condition of the reheat furnace was operational but in need of refractory repair to both the roof and sidewalls. Overall, minimal damage to the furnace refractories and ceramic coating took place during the shutdown (Figs. 7-12).
Upon start-up of the furnace, Liberty Steel reviewed the benefits and saving potential of the coatings from the previous operation and has elected to replace all damaged areas and recoat the complete furnace. Although natural gas prices are lower today than in 2009 ($10.00 per MM BTU versus roughly $3.00 per MM BTU today), fuel-consumption savings will still be significant.
The reduction in fuel required to produce a ton of steel is expected to remain at approximately 46%, or 0.8 MM BTU/ton, based on a production rate of 120 tons per hour. Additionally, the reductions in scale will continue to generate an increase of roughly 2% additional yield per year. If the same yearly increase of 9,000 additional tons at today’s current cost for hot-rolled coil steel of $740.00 per net ton is assumed, revenue would be increased by $6,000,000.00.
Lastly, by recoating the furnace, Liberty Steel will continue to achieve lower heat losses through the refractory lining, thus increasing the furnace efficiency while increasing product quality, improving temperature uniformities and reducing the overall high cost of refractory maintenance required.
An efficient furnace lining is essential for reducing overall maintenance costs and ensuring that the hot mills’ reheat furnaces run smoothly without unwarranted revenue loss due to downtime. High-temperature, energy-efficient ceramic coatings for refractories – no longer “theoretical” technology – are being used successfully in furnace applications to reduce energy consumption, improve temperature uniformity, reduce maintenance and increase production while improving product quality.
The potential application of these coatings cuts across a wide spectrum of thermal-processing industries and types of equipment. Numerous installations have proven successful in steel plants, both in the melt shop and hot mill, across the globe. ITC offers a full line of high-temperature, energy-efficient ceramic coatings used on both refractory and metal substrates. These products are water-based, so no solvents are required for dilution or cleanup. On curing they are environmentally inert and do not require special handling or disposal.
International Technical Ceramics, LLC continues to look for new and challenging opportunities in the steel industry where coating technology has yet to be used. How can we help you?