**1. Introduction**

220 Heat Exchangers – Basics Design Applications

[25] Bejan, "Convection Heat Transfer," first ed., John Wiley & Sons, New York, 1984. [26] S. Kakac, Y. Yener, "Convective Heat Transfer," second ed., CRC Press, Begell House,

[27] A.F. Mills, "Heat Transfer," second ed., Prentice Hall, Upper Saddle River, 1999.

Boca Raton, 1995.

High temperature fuel cell systems are an attractive emerging technology for stationary power generation, especially for the distributed generation [1]. Today, there are mainly two types of high temperature fuel cell systems, including the molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC), which are generally operated at high temperatures ranging from 823K to 1273K. Several advantages of this setup are listed in the references [2]. The main advantages of both fuel cells are related to what could be done with the waste heat and how they can be used to reform fuels, provide heat, and drive engines. Therefore, high temperature fuel cell systems can never be simply considered as fuel cells; instead, they must always be thought of as an integral part of a complete fuel processing and heat generating system [2].

Steam reforming is a well-established industrial fuel process for producing hydrogen or synthetic gas from natural gas, other hydrocarbon fuels, and alcohols [3]. In the high temperature fuel cell systems, the pre-reformer is usually needed for fuel processing. Due to the high endothermic reaction, a great amount of heat must be provided from the outside, such as waste heat from the fuel cell, catalyst combustion, etc.

High temperature heat exchangers are widely used in the high temperature fuel cell/gas turbine system, closed cycle gas turbine system, high temperature gas cooled reactors, and other thermal power systems. It is an effective method of improving the whole system efficiency [4]. Compact heat exchangers are generally characterized by extended surfaces with large surface area/volume ratios that are often configured in either plate-fin or tube-fin arrangements [5]. In a plate-fin exchanger, many augmented surface types are used: plainfins, wavy fins, offset strip fins, perforated fins, pin fins, and louvered fins. Offset strip fins, which have a high degree of surface compactness and feasible manufacturing, are very widely applied.

In general, the high temperature heat exchanger is used to preheat the air or fuel, while the pre-reformer is used to produce hydrogen rich fuel from methane or other hydrocarbons. Fig. 1 shows one of the fuel cell systems, which consists of a direct internal reforming solid oxide fuel cell (DIR-SOFC), a high temperature heat exchanger (HTHE), a low temperature heat exchanger (LTHE), a pre-reformer, a gas turbine, a generator, etc. In order to simplify the system, reduce the cost, and improve the fuel cell system's efficiency, it is suggested that

[9, 10]. A novel micro fuel processor for PEMFCs with heat generation by catalytic

All these previous works were mainly developed based on experiments, but the steady state and dynamic performance simulations have not been investigated in detail. The heat supplied for the methane steam reforming reaction has different sources, such as catalytic combustion [11, 12] and auto-thermal methane reforming reactions [10]. The purposes are mainly for the portable devices [9, 10] or the low temperature fuel cells [11-13]. Here, the waste heat from the high temperature fuel cell systems will be used as the heat resource in

This chapter aims to: design a compact heat exchange reformer for the high temperature fuel cell systems; develop a real time simulation model using the volume-resistance characteristic modeling technique; study the steady state distribution characteristics by considering local fluid properties such as pressure, velocity, density, heat capacity, thermal conductivity, dynamic viscosity, etc; discuss some factors that will affect the performance of the reformer during steady state operation under the same operating condition; and finally, investigate dynamic behavior under different input parameters including step-change

The configuration of the heat exchange reformer is similar to the compact heat exchanger. The only difference is that the catalyst is coated in the cold passage to make steam reforming

As shown in Fig. 3, the configuration of the offset strip fin heat exchanger is adopted here. The fin surface is broken into a number of smaller sections. Generally, each type of fin is characterized by its width *X*, height *Y*, thickness *t*, and length of the offset strip fin *l*. The detailed configuration can also be found in other references for the heat exchanger [14-18].

(a)

combustion was developed and characterized in South Korea [11-13].

the compact heat exchange reformer for the steam reforming reaction.

**2. Description of heat exchange reformer** 

conditions.

**2.1 Configuration** 

reactions take place.

a compact heat exchange reformer replace the heat exchanger and the pre-reformer. The new fuel cell system is illustrated in Fig. 2. The offset strip fin heat exchanger and prereformer are combined into the heat exchange reformer. In this device with the counter-flow type, the high temperature waste gas from the fuel cell flows in the hot passage, and the fuel flows in the cold passage. In particular, the Ni catalyst is coated on the fuel passage surface [6, 7]. When the fuel flows along the passage, the endothermic steam reforming reaction will take place using the heat transferring from the hot side.

Fig. 1. Schematic view of the traditional SOFC/GT hybrid system.

Fig. 2. Schematic view of the SOFC/GT hybrid system with novel concept heat exchange reformer.

Several kinds of compact heat exchange reformers have been investigated and designed in the past. In 2001, Kawasaki Heavy Industries in Japan developed a plate-fin heat-exchange reformer with highly dispersed catalyst [8]. A planar micro-channel concept was proposed by Pacific Northwest National Laboratories (PNNL), but this kind of micro-channel device is oriented toward the low to medium power range (20-500W) for man-portable applications [9, 10]. A novel micro fuel processor for PEMFCs with heat generation by catalytic combustion was developed and characterized in South Korea [11-13].

All these previous works were mainly developed based on experiments, but the steady state and dynamic performance simulations have not been investigated in detail. The heat supplied for the methane steam reforming reaction has different sources, such as catalytic combustion [11, 12] and auto-thermal methane reforming reactions [10]. The purposes are mainly for the portable devices [9, 10] or the low temperature fuel cells [11-13]. Here, the waste heat from the high temperature fuel cell systems will be used as the heat resource in the compact heat exchange reformer for the steam reforming reaction.

This chapter aims to: design a compact heat exchange reformer for the high temperature fuel cell systems; develop a real time simulation model using the volume-resistance characteristic modeling technique; study the steady state distribution characteristics by considering local fluid properties such as pressure, velocity, density, heat capacity, thermal conductivity, dynamic viscosity, etc; discuss some factors that will affect the performance of the reformer during steady state operation under the same operating condition; and finally, investigate dynamic behavior under different input parameters including step-change conditions.
