Firing Technology for Fuel Cells
Technology OverviewThe 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.
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 AssembliesThe 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.
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.
Long-Range BenefitsAdditional 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.