Firing Technology for Fuel Cells

May 30, 2003
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A new furnace system can help reduce the cost of manufacturing solid oxide fuel cells by providing energy savings, productivity improvements and a high level of temperature uniformity.

Figure 1. The operating schematic of the new furnace.
Fuel cells promise to provide a source of environmentally friendly and efficient electrical energy for the future. However, the high cost of manufacturing these devices continues to limit their applications in today’s markets. For solid oxide fuel cells (SOFCs), the energy and productivity costs associated with firing the fuel cell stacks and anode and cathode materials pose some of the biggest barriers to reducing overall manufacturing costs.

A solid oxide fuel cell. Photo courtesy of Ceramic Fuel Cells Ltd., Noble Park, Victoria, Australia.
A new furnace system* has been developed specifically to assist manufacturers of SOFCs and component materials with their cost reduction programs (see Figure 1). The furnace’s excellent temperature uniformity and atmosphere control make it ideal for firing monolithic and multi-layer ceramic bodies, and it can also be used in the high-temperature processing of ceramic powders. It offers lower capital and operating costs compared to conventional kilns, and it also requires less plant floor space. The furnace is designed to improve firing quality, yield and reliability while reducing process atmosphere consumption, heating and cooling costs, labor, and consumables. With this new technology, SOFC manufacturers can bring their products closer than ever to large-scale commercial viability.

Technology Overview

The heart of the new furnace system—and the source of many of its advantages over traditional pusher-type tunnel, roller hearth and mesh-belt kilns—is the load carrier (see Figure 2). Stacked setters are supported on structural ceramic bars, which are carried by carrier blocks. The pushing force that moves the loads through the heating and cooling sections is applied to the carrier blocks, and the friction that resists the motion occurs outside of the refractory envelope, between the carrier blocks and the rail on which they slide.

Figure 2. The load and hearth assembly of the new furnace.
This design provides a number of direct benefits. For example, there is no massive pusher plate, which is usually the most thermal-shock-sensitive part of the load and a considerable thermal mass. Additionally, the system can process wide, low-profile loads, which yields improved efficiency in capital, floor space and heat loss. It also reduces gas and heat flow distances and removes impediments, which improves firing uniformity and reduces fuel consumption.

The system also provides many indirect advantages. Because the friction occurs outside of the refractory envelope rather than within the furnace, it is possible to use ceramic fiber insulation—a relatively low-cost, high-purity material. This also provides added flexibility, since a lower-mass kiln can reach new steady-state conditions relatively quickly compared to a brick-lined kiln. Additionally, reliability can be dramatically improved by eliminating the chance of a load crash caused by pusher plate fracture, load chatter or refractory failure.

Table 1 compares a conventional tunnel kiln to two of the new furnaces—one with the same production rate and one with the same tunnel length. The data for the new furnace system were calculated and/or extrapolated from existing equipment and analyses of proposed designs and included the following assumptions:

• The return conveyor is underneath the system.

• The production rate figures conservatively take into account the system’s ability to ramp and cool faster. In a production situation, the rates would probably be even faster due to the system’s high temperature gradient relaxation rates. (See the "Sidebar: Furnace Prototype" for more information about temperature gradient relaxation rates.)

• The capital cost is an estimate and depends on the construction materials.

Processing Fuel Cell Assemblies

The binder removal and sintering process for SOFCs requires uniform temperatures and exposure to process atmospheres.

Temperature differences are of two basic types: transient and steady state. Transient temperature gradients relax at a rate inversely proportional to the square of the distance the heat must travel. For loads that are heated from the top and bottom, this is half the load height. By cutting the load in half, the new furnace is able to heat four times as fast with an equivalent level of uniformity. Many thermal soak times are defined so that the time at temperature is long compared to the time it takes for the temperature gradients to relax. In these cases, the soak times can also be reduced. Additionally, unlike many existing firing ramps, which are limited by the thermal shock of the pusher plates, the load in the new furnace is carried by less massive and more thermal-shock-resistant structures. As a result, the new furnace can process lower-profile loads and still offer an enhanced return on investment at a substantially lower operating cost than a traditional pusher tunnel kiln.

Steady-state differences arise when parts of the load are exposed to a different temperature than other parts. In a continuous-push kiln, the process is steady state. However, the load “sees” a series of different environments at different times, and it is from the load’s point of view that temperature and atmosphere uniformity must be considered. In traditional kilns, steady-state temperature differences can be caused by the introduction of process atmosphere along the sidewalls while the heat flows from the top and bottom. At high gas flows, some of the heat required to heat the gas comes from one side of the load by radiation to the sidewall and by convection to the gas. The new furnace eliminates this effect by passing the process atmosphere up through the floor of the chamber. The gas is heated indirectly by radiation from the heating elements. The sidewalls, although not directly heated, use an increased thickness of insulation to minimize the exposure of the load to temperature differences caused by radiation.

Introducing the gas in this way also improves the uniformity of the products’ exposure to the gas, which enables the gas flow to be minimized while still meeting process requirements. The process atmosphere passes from bottom to top and along the axis of the furnace to controlled exhausts. Because the furnace’s transport system precisely controls the load’s position, the gas flow in the furnace chamber can be controlled by close-fitting zone baffles.

In a conventional pusher tunnel kiln, allowances must be made for expansion, creep, wandering loads, etc. These considerations often lead to designs with dead volumes and large bypass paths for the process atmosphere. The close geometries possible with the new furnace permit the elimination of these problems.

Figure 3. An integrated system for high production fuel cell component firing.

Long-Range Benefits

Additional long-range potential benefits can be realized by integrating all the firing steps—from binder removal to sintering and creep flattening—into one unit or an integrated system (see Figure 3). This type of system would also lend itself to automated loading and unloading, since assembly and disassembly of the load stack would only need to be performed once. As shown in Figure 3, the heat-treatment equipment stands outside the air-conditioned workspace, and the plant material flows across the ends of each furnace. This plant would have a production capacity greater than four times that of a 20-m-long, 200-mm-load-width pusher tunnel kiln with all its support furnaces and utilities. Additionally, the estimated plant floor space required is reduced by a factor of greater than five.

This new furace system has been termed "the first major paradigm shift" in furnace design and construction, and its proven design is supported by in-depth process and thermal engineering know-how. For manufacturers of SOFCs and other technical ceramics, the potential rewards for implementing this new technology can be dramatic.

Sidebar: Furnace Prototype

A prototype of the new furnace system, which is similar to the illustration shown below, is currently under development. The prototype will be used to demonstrate rapid binder removal and sintering, thermal uniformity and critical control elements, as well as to conduct sample firings of materials. Researchers will also use the prototype system to collect and analyze the data required to scale up the technology to optimized production volumes with maximized yields of in-specification ceramic components. To quantify the competitive enhancements that can be achieved by implementing the new system, a detailed economic analysis will also be prepared.

For more information:

For more information about the new furnace system, contact Harper International Corp., W. Drullard Ave., Lancaster, NY 14086; (716) 684-7400; fax (716) 684-7405; e-mail info@harperintl.com; or visit http://www.harperintl.com.

Reference:

*The Harper-Hearth Ceramic Sintering System, developed and patented by Harper International Corp., Lancaster, N.Y. (U.S. Patent No. 5,848,890)

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