Advantages of Mullite-Fiber Linings for High-Temperature Furnaces
This article was originally published on October 8, 2014.
Industrial furnaces that fire to temperatures of 2300°F (1260°C) and higher continuously pose serious challenges to insulating fiber and dense refractory furnace linings.
Industrial manufacturers that use high-temperature furnaces are focused on temperature uniformity, energy efficiency and low maintenance cost. It’s difficult to achieve these ends simultaneously using insulating fiber and dense refractory furnace linings.
High-Temperature Lining Options
The alumina-silica refractory ceramic fibers available on the market have very good insulating properties but have significant crystalline changes that occur at temperatures above 2000°F (1093°C). The silica in the fiber starts to react with oxygen and other airborne gases, such as N2 and CO, from the furnace atmosphere. This vitrifies the fibers, causing the furnace lining to shrink. This process is continuous with heat exposure, never stopping until the lining needs to be repaired or replaced. If temperatures of 2600°F (1427°C) are sustained for any appreciable time frame, this process is catastrophic, causing the fiber to basically melt.
The high-alumina brick and monolithic linings available yield very high strengths and much greater resistance from mechanical/chemical attack and abrasion than refractory ceramic fibers, but they have bad thermal-shock resistance and yield marginal insulating values due to their very high densities. With repetitive thermal cycling, dense refractory will crack and spall, which weakens the hot-face lining and subsequently requires complete replacement. Other issues with dense linings are turnaround time on initial heat-up after installation to remove water from the refractory. Also, furnace cycle times are much higher versus lightweight linings due to the high mass of refractory present in the furnace lining. The net effect is that either option – lightweight ceramic fiber or dense refractory linings – have serious challenges in yielding an energy-efficient, low-maintenance-cost furnace lining for the long term.
In the past 25 years, mullite fibers have been developed for long-term exposure to high-temperature environments and chemical resistance. These fibers are comprised of 72% alumina and 28% silica. The silica is tied up very securely in a mullite bond, which keeps it from openly reacting with oxygen and other airborne gases at temperatures up to 2912°F (1600°C). By resisting chemical attack from the atmosphere, the silica in the mullite does not rapidly create SiO2, which is what forms the glassy coating at much lower temperatures with ceramic fiber.
These molecular changes are what cause the fiber to shrink and why mullite has such low shrinkage up to 2912°F (Fig. 1). This is in stark contrast to refractory ceramic fiber, where molecular change begins at temperatures under 2000°F. Due to this extremely high temperature and chemical stability, mullite fibers provide several options for furnace linings versus refractory ceramic fiber, dense brick and monolithic linings.
Mullite fibers are extremely low in shot content and very consistent in fiber diameter at 5-7 microns, unlike ceramic fibers with diameters typically in the 2-4 micron range with high shot contents. These large-diameter, low-shot-content fibers are needled (much like ceramic fiber) into tightly woven, high-tensile-strength blankets. The blankets are then engineered into custom furnace modules and panelized systems for lining furnace walls. The combination of high-temperature stability up to 2912°F along with the ability to engineer the fibers into durable furnace linings allows the end user to capitalize on energy savings, fast furnace cycle times and low maintenance cost.
Until fairly recently, the majority of lining systems available for mullite fibers have been comprised of 100% pure mullite-based custom folded, stack bonded or panelized engineered systems. These linings have been engineered and installed in reheat furnaces for the iron and steel industry, technical ceramics, heat treating and various other applications requiring insulation protection in continuous firing conditions 2000°F and above.
Not all of the mullite-fiber products manufactured are able to stand up to the mechanical abuse the fibers experience during the conversion process, but the mullite fiber developed by Mitsubishi, named Maftec®, is able to handle typical conversion processing due to its large average fiber diameter of 5-7 microns, extremely low shot content and purity of the mullite fiber produced. Due to this mullite fiber’s high tensile strength, the Maftec material can be converted into 100% serpentine folded modules, stack bonded modules and panelized systems, and hanging and baffle walls without any issues. These systems have proven to yield extremely good thermal stability versus ceramic fiber, but they do come at a significant cost difference. The mullite-fiber systems are able to justify their higher price by decreasing fuel usage and heat loss at extreme firing temperatures due to being highly insulating and yielding very low shrinkage, which equates to lower long-term maintenance and lining replacement cost.
Since mullite fibers have been marketed to the high-temperature furnace industry, the installed cost has been the biggest stumbling block for most end users. Even with the future energy savings, maintenance cost reduction and capacity increases that can be expected, most end users have difficulty justifying the higher up-front cost of mullite fiber versus the other materials in the market.
Mullite-fiber lining costs are significantly higher than ceramic fiber. Versus dense refractory, this calculation becomes more complicated due to the magnitude of options available. Overall, 100% mullite-fiber linings cost much more than most conventional 70% alumina-based dense linings. Future savings will generally pay for the higher initial cost, but this can be a very tough sell for most end users due to limited funds available for furnace-lining maintenance.
Composite Insulation Module Systems
For this reason, composite insulation module systems are becoming more and more prevalent in the market. By replacing a large portion of the mullite fiber with HTZ 2600 ceramic fiber, the cost is decreased significantly. Original composite systems utilized an interlock between mullite and zirconia-based HTZ 2600 ceramic fiber. Some use high-compression methods between alternating layers of mullite and ceramic fibers, and yet others use refractory cement between layers of mullite and ceramic fiber. The biggest concern for these module systems is the ability to stand up to abuse in overhead furnace roof areas, where the joint between the mullite fiber and ceramic fiber comes together.
A module system has recently been developed by Temtek Solutions named the PuzzleJoint™, which uses a mechanical puzzle joint (Fig. 2) to adhere the mullite and ceramic fiber together. When combined with high compression, it makes it virtually impossible for the installed layers to come apart (Fig. 3). The composite modules give the end user options that yield 100% mullite fiber on the hot face with up to a 50% cost savings versus 100% mullite-fiber module systems. With these types of composite modules available, it makes mullite-fiber linings more accessible to manufacturers that may not have previously been able to justify the higher capital cost.
The energy savings when comparing mullite linings to dense refractory are significant due to the massive weight reduction in the furnace lining. Normally, the roofs of furnaces are the most expensive part of dense refractory linings because of the complex and expensive anchoring systems required to carry the extreme loads present with dense brick or monolithic high-alumina linings.
The dense refractory linings are around 165 pounds per cubic foot density, compared to mullite-fiber systems at 10 pounds per cubic foot. This extreme weight reduction equates to fuel savings, lower cold-face shell temperatures, lower heat loss, faster cycle times and increased furnace capacity.
Steel reheat-furnace roof conversions from dense refractory to 12-inch-thick mullite fiber have yielded outstanding results and illustrate the advantages of using mullite fibers for replacing dense refractory. Paybacks can be varied due to the fluctuation in gas cost, but if all variables are considered, the conversion to mullite fiber is a huge winner for end users in most cases. Figure 4 is a comparison for a reheat-furnace roof firing to 2525°F (1385°C) operating temperature with a dense monolithic castable and mullite-fiber roof side by side illustrating the significant increases in furnace efficiency.
The overall objective of this article is to convince the end users of high-temperature industrial furnaces to look at mullite fibers as a viable option for lining their furnaces. The initial capital cost tends to scare away a lot of operations and plant managers because of their focus on keeping operating budgets at the bare minimum.
Furnace operating costs need to be broken down by fuel usage per year converted into dollars, refractory maintenance cost, overall furnace cycling time, downtime due to repairs, temperature uniformity, emissions control and (last but not least) capacity throughput for the end product.
If all these variables are put into a comparison and the overall costs considered for mullite-fiber engineered systems versus dense refractory and ceramic-fiber linings, the mullite fiber will yield a very cost-effective furnace lining solution for the long term. The extremely good chemical resistance, mechanical strength and low shrinkage at temperatures up to 2912°F will provide end-use customers with a lining they can trust to give them years of solid performance. IH
For more information: Contact D. Scott Carter, Temtek Solutions Inc., Home of MSSI Refractory Products, 2 John Street, PO Box 398, McKees Rocks, PA 15136; tel: 888-265-2608 X20; fax: 412-771-3148; e-mail: firstname.lastname@example.org; web: www.temteksolutions.com