High volumes of fuel are used in steel heating processes, therefore, practical advances that can dramatically reduce fuel and associated costs are of keen interest. Michigan Seamless Tube & Pipe (MST) achieved a 63% reduction in specific fuel consumption in its billet reheating operation after retrofitting the rotary-hearth furnace with “flameless” REBOX oxyfuel technology from Linde North America. The technical solution also greatly improved reheating capacity and eliminated the need for a walking-beam furnace.
The conversion process offers lessons for steel mills that hot work billets, ingots, slabs and other intermediate steel sections. The goal at MST was to increase billet reheating capacity while reducing fuel consumption and maintenance costs without sacrificing billet yield or quality.
MST, a division of the Optima Specialty Steel Group, is a precision seamless-tube manufacturer specializing in cold-drawn carbon, alloy and high-chrome mechanical and pressure tubing and pipe products. The mill produces approximately 39,000 tons of cold-drawn seamless carbon and alloy tubing and pipe per year.
MST’s process is similar to other seamless steel mills. First, solid steel bars are cut to length, and the round billets are then reheated in a rotary-hearth furnace. The billets are then rotary pierced in a Mannesmann-style piercing mill. The rotary furnace (originally built in the late 1940s) was equipped with a traditional air-fuel heating system and most recently updated in 2007. As MST increased the size of billets over the years – currently 2.875-4.75 inches in diameter – it installed a walking-beam furnace (WBF) in the early 1980s for preheating large-diameter material prior to charging in the rotary-hearth furnace.
The oxyfuel solution featured an improved heating profile (Fig. 3). The faster and optimized heating rate of the actual solution reduced heating time of the 4.75-inch billets by 50% compared to the previous air-fuel heating system, which required a WBF to preheat large-diameter billets. It also reduced energy consumption by 63%. The MST conversion included replacement of the air-fuel burners in the rotary-hearth furnace with REBOX® flameless oxyfuel burners, modification of the flue-gas system, a new stack and the installation of new baffles at the charge and discharge doors.
Implementing “Flameless” Oxyfuel
Review of MST’s existing rotary-hearth furnace resulted in a new combustion-system solution based on process experience, application knowledge and simulated process modeling. The elements of the REBOX flameless oxyfuel solution were as follows:
- A turnkey project, to be commissioned within 3.5 months from order receipt
- Replacement of existing air-fuel burners with flameless oxyfuel burners
- Implementation of a new combustion-control system for fuel and oxygen, including flow trains for each
- Installation of a liquid-oxygen tank and piping to the flow train and rotary-hearth furnace
- Modification of the existing flue-gas system, including the addition of an active damper, disconnection from the preceding walking-beam furnace and installation of an entirely new stand-alone exhaust stack
- Integration of the walking-beam furnace controls into a new control system
- Fulfillment of a set of performance guarantees and specific start-up guidelines
Short Turnkey Project Timeline and On-Time Installation
A 3.5-month project timeline was allotted for completion of the system design (including control system and programming), fabrication and assembly of flow trains, burners and flue-system components. Installation was completed during a 16-day outage in the summer of 2011. Refractory work was also performed on the furnace during this outage. The 13 flameless oxyfuel burners were easily installed in the positions left vacant by the air-fuel burners (Figs. 1A and 1B).
The flue-gas system was modified, including removal of the existing flue-gas duct connected to the WBF and installation of a new damper and new stack (Fig. 2). The previous flue-gas system and stack connected to the now-unfired WBF was idled.
Revised Heating Profile and Increased Heating Rates
The new, self-cooled, ceramic flameless oxyfuel burners were placed in the same positions as the previous air-fuel burners, and three heating control zones were created. Burners in two zones (1 and 3) include starter burners and UV sensors for flame supervision.
The proposed furnace and combustion-system changes were based on the revised heating profile (Fig. 3), which takes into account the properties of flameless oxyfuel heating. These include higher combustion efficiency than air-fuel combustion, better and more uniform heat-transfer characteristics and 70-80% less flue gases.
The heating time was reduced to eight minutes per inch of billet diameter (RHF residence time) from 16 minutes per inch for a larger billet diameter (Fig. 4).
The amount of scale (oxides) formed in open combustion furnaces can largely be attributed to the exposure time at temperatures above ~2000˚F (1092˚C). As shown in Figure 3, the time at these elevated temperatures for MST was reduced from 19 minutes (air-fuel system) to 11 minutes with the REBOX system. In day-to-day operation, however, this reduction in scale formation is not entirely realized due to furnace delays stemming from frequent tool changes in the piercing operation, especially when production levels are increased.
Improved Furnace Temperature Control
Temperature control of each zone is now easily managed with oxyfuel combustion and flue-gas volumes reduced by 70-80%. Successful operation with a significantly reduced volumetric flow of flue gas was accomplished through minor modifications inside the furnace and updates of the pressure monitoring and control system. Two baffles were installed – one each near the charge and discharge doors of the RHF – in order to maintain pressure control (and eliminate air infiltration), and an active damper was installed at the flue-gas exit. The pressure-control system was then redesigned. All thermocouples were repositioned in accordance with the new zone arrangement and control requirements.
A temperature controller and a programmable logic controller (PLC) with a human machine interface (HMI) are used as a stand-alone system. The system controls the oxygen-to-natural-gas ratio, the oxyfuel flow rate and the temperature of the RHF in each zone, as well as burner ignition sequences and safety interlocks (Fig. 5).
Fulfilled Performance Guarantee
As part of the project, tests were carried out to verify the performance versus guarantees after conversion of the RHF to REBOX flameless oxyfuel technology. These tests were conducted in August and December 2011, and both tests achieved the performance guarantees. Since the performance tests were conducted, MST continues to see positive results in the daily operation of the furnace. Specific accomplishments from the tests include:
- For the 2.875-inch billet size, the target increase on the heating throughput rate was 22%. In actuality, a 35% increase was realized on this size of material.
- For the 4.75-inch billet size, a reduction of 15% in heating throughput rate was anticipated. In performance tests, a 12.5% reduction was realized.
- Both heating-rate throughput guarantees were exceeded while meeting the total maximum specific fuel-consumption guarantee of 1.15 mmBtu/ton.
- Specific gross fuel consumption decreased by 63% (in Btu/ton units). The specific energy consumption (the fuel required to effectively heat billets) is now 1.15 mmBtu/ton for cold-charged material.
Improved Working Conditions and Reduced Maintenance
Today, MST operators report improved working conditions with the REBOX flameless oxyfuel compared to the prior air-fuel-fired system. The noise level is dramatically lower due to the removal of the electric air blowers used for the previous air-fuel system. Removal of the blowers has also eliminated associated maintenance. The RHF’s outer shell is cooler as a result of the installation, making it more comfortable for workers during maintenance and repair activities.
Walking-Beam Furnace Maintenance Cost Savings
The high efficiency of the new oxyfuel system enabled the WBF to be eliminated as part of the billet reheating system. In practice, MST still utilizes the unfired WBF to transport billets to the rotary-hearth furnace. Taking the WBF offline realized significant operational benefits:
- More than $200,000 in annual refractory maintenance savings
- Decreased maintenance on the mechanical components of the WBF
- Improved working conditions when maintenance or operational needs require personnel to be near the furnace
Open-Door Operation Challenges
Despite extensive engineering and preparatory work, a few challenges arose after the initial installation of the oxyfuel system. With the traditional air-fuel combustion system, the hearth operated with doors open at all times during production, and high volumes of gas escaped from both doors. With the lower flue-gas volumes associated with the oxyfuel operation, it was more difficult to maintain positive pressure. At lower production rates, a negative furnace pressure occurred that allowed air to infiltrate the furnace. This was a source of energy loss, temperature variations and an overall increase in scale formation on billets.
To correct this pressure-control issue, MST and Linde installed two baffle walls made of MAFTECTM fiber material (Fig. 6). The MAFTEC material was selected due to its high-temperature sustainability, low shrinkage rates (1.5% at 2912˚F), durability, ease of installation, flexibility, physical strength and erosion resistance. These traits were particularly important due to the proximity of the baffle walls in relation to the burner flame.
The installation was completed successfully in July 2011, and MST has been realizing the benefits of a flameless oxyfuel heating system ever since. The important results of the project are summarized as follows.
- Decreased specific fuel consumption by 63%
- Reduced heating time from 16 minutes per inch to 8 minutes per inch for large billet sizes
- Increased capacity for small billet sizes by 27%
- Safe project execution with project completion within 3.5 months of order receipt
“The full results of this work were presented at AISTech 2013, Pittsburgh, Pa., May 2013, and published in the Conference Proceedings.”
For more information about REBOX oxyfuel technology, contact Linde North America, LLC, 575 Mountain Ave., Murray Hill NJ 07974; tel: 908-771-1215; e-mail: www.lindemetallurgy.com. Author Tom Sleder is hot-mill manager for Michigan Seamless Tube LLC; South Lyon, Mich. Tony Palermo is metallurgy program manager and Grzegorz Moroz is Sr. engineer for Linde North America, LLC.
How “Flameless” Oxyfuel Systems Work
The term “flameless combustion” describes the visual aspect of this type of combustion. Another way to describe flameless combustion is that it is combustion that is “diluted” or “extended” in time and space because it is spread over a large volume. Such combustion has a lower and more uniform temperature than conventional combustion techniques. Furthermore, flameless oxyfuel combustion involves the replacement of air (as the source of the oxidant) with industrial-grade oxygen.
Flameless combustion results in more uniform heating of a volume due to its extended and low-peak-temperature nature. Flameless oxyfuel combustion also results in low levels of thermal NOx generation due to the lack of nitrogen and lower flame temperature.
At elevated temperatures, such as those required when reheating steel sections, the predominant mode of heat transfer is the radiative mode. At lower temperatures, convective and conductive heat-transfer modes have greater importance. Oxyfuel combustion leads to more efficient radiant heat transfer than air-fuel combustion because of its higher concentrations of carbon dioxide and water vapor in the gaseous products of combustion. It is also beneficial that the lack of nitrogen (a ballast) in industrial-grade oxygen (versus 78% in air) minimizes the sensible heat that is lost in the flue gas from a heating process. The flue-gas volumes are reduced by up to 80% with no more need for bulky flue-gas systems or recuperators. The nitrogen ballast also has to be transported by electrical blowers through oversized ducting. All of this has a negative impact on capital costs.
Linde’s REBOX® oxyfuel technology reduces specific fuel consumption in steel reheat furnaces. In addition to more efficient energy input and increased heating rate, it offers significant environmental and maintenance advantages, including reduced scaling, minimized NOx formation and reduced total flue-gas emissions.
The REBOX portfolio includes equipment and control systems for oxygen enrichment, oxygen lancing, oxyfuel boosting and 100% oxyfuel operation in reheat furnaces and annealing lines.