**5.2.2 Catalyst reduced activity**

The CRA is defined as the ratio between the activity of the catalyst in use and that of a conventional Ni catalyst (Xu and Froment, [19]) at typical feed conditions (temperature, pressure, and composition) [26]. The CRA is the key factor in determining the reforming reaction rate. For the rated case, the CRA is defined as 0.003 [7] in Table 5. Fig. 11 and Fig. 12 present the effect of the CRA on the performance of the heat exchange reformer.

The density is related to the pressure and the temperature, which are decided by the gas

decreases, so the density decreases along the flow direction while, in the hot gas passage, both the pressure and the temperature decrease. The ratio of pressure and temperature along the passage is increased, so the density of the hot gas increases along the flow

Two primary factors that affect the velocity are the mass flow rate and the density. Here, the mass flow rate is constant, and the velocity is mainly determined by the density. That is to say, the velocity increases in the cold fuel passage and decreases in the hot gas passage,

Specific heat capacity, thermal conductivity, and dynamic viscosity are primarily influenced by the temperature and the gas composition. This has been discussed by Todd and Young

In this section, some key parameters that affect the heat exchange reformer performance are investigated, such as the steam to carbon ratio (STC), catalyst reduced activity (CRA), and

In general, the STC must be greater than 2.0 to avoid carbon coking in the fuel lines, reformer, and fuel cell stack [25]. The effect of different STCs on the heat exchange reformer

Fig. 9 presents effect of STC on the methane and hydrogen distribution along the heat exchange reformer. In the internal reforming high temperature fuel cell, the endothermic reforming reaction will cause a great temperature gradient, which could decrease the life of the fuel cell stack due to excessive thermal stress. Therefore, too much remaining methane would be no good for the steady operation of the high temperature fuel cell. With the STC changing from 2:1 to 4:1, less methane remains at the exit (Fig. 9 (a)), while the hydrogen molar fraction at the exit is almost the same as at the entrance (Fig. 9 (b)). Therefore, a suitable and acceptable STC is essential for the internal reformation of high temperature fuel

The temperature distribution of cold fuel and hot gas is illustrated in Fig. 10. When the STC changes from 2:1 to 4:1, less methane is provided at the inlet, and less heat is needed for the steam reforming reaction. Meanwhile, a higher STC will result in a higher rate of the exothermic water gas-shift reaction, so the temperature curves of both the cold and hot

The CRA is defined as the ratio between the activity of the catalyst in use and that of a conventional Ni catalyst (Xu and Froment, [19]) at typical feed conditions (temperature, pressure, and composition) [26]. The CRA is the key factor in determining the reforming reaction rate. For the rated case, the CRA is defined as 0.003 [7] in Table 5. Fig. 11 and Fig. 12

present the effect of the CRA on the performance of the heat exchange reformer.

. In the cold fuel passage, the temperature increases and the pressure

state equation *P RT*

direction.

cells.

stream are higher.

**5.2.2 Catalyst reduced activity** 

following the trend of the density.

passage operating pressure.

**5.2.1 Steam to carbon ratio** 

is presented in Fig. 9 and Fig. 10.

[24] and Lijin WANG [22] for high temperature SOFCs.

**5.2 Analysis of the influence of some parameters** 

Fig. 9. STC effect on the methane (a) and hydrogen (b) molar fraction distributions.

(a)

(b)

Fig. 11. CRA effect on the methane (a) and hydrogen (b) molar fraction distributions.

Fig. 10. STC effect on the cold fuel (a) and hot gas (b) temperature distributions.

(a)

(b)

Fig. 10. STC effect on the cold fuel (a) and hot gas (b) temperature distributions.

Fig. 11. CRA effect on the methane (a) and hydrogen (b) molar fraction distributions.

The passage pressure often changes with the operation condition, even during malfunctions or damage. The effect of the cold passage outlet pressure on the heat exchange reformer is

(a)

(b)

Fig. 13. Cold passage outlet pressure effect on the methane (a) and hydrogen (b) molar

fraction distributions.

investigated in this section and illustrated in Fig. 13 and Fig. 14.

**5.2.3 Passage operating pressure** 

Fig. 12. CRA effect on the cold fuel (a) and hot gas (b) temperature distributions.

The influence on the methane and hydrogen molar fraction distribution along the heat exchange reformer is shown in Fig. 11. When the CRA changes from 0.0015 to 0.006, the rate of the methane reforming reaction increases, so more methane is consumed (Fig. 11 (a)) and more hydrogen is produced (Fig. 11 (b)). More heat is needed to satisfy the requirements of the high endothermic reaction, so the temperature curves of both the cold and hot stream are lower (Fig. 12).

#### **5.2.3 Passage operating pressure**

238 Heat Exchangers – Basics Design Applications

(a)

(b)

The influence on the methane and hydrogen molar fraction distribution along the heat exchange reformer is shown in Fig. 11. When the CRA changes from 0.0015 to 0.006, the rate of the methane reforming reaction increases, so more methane is consumed (Fig. 11 (a)) and more hydrogen is produced (Fig. 11 (b)). More heat is needed to satisfy the requirements of the high endothermic reaction, so the temperature curves of both the cold and hot stream

Fig. 12. CRA effect on the cold fuel (a) and hot gas (b) temperature distributions.

are lower (Fig. 12).

The passage pressure often changes with the operation condition, even during malfunctions or damage. The effect of the cold passage outlet pressure on the heat exchange reformer is investigated in this section and illustrated in Fig. 13 and Fig. 14.

Fig. 13. Cold passage outlet pressure effect on the methane (a) and hydrogen (b) molar fraction distributions.

In this section, the transient behaviours of the compact heat exchange reformer are investigated. Several step-change input parameters (such as inlet mass flow rate and inlet temperature of both the cold and hot stream) are imposed when the device has been

Fig. 15 illustrates the dynamic response of the temperatures at the cold and hot passage exits, when the cold fuel mass flow rate has a step increase of 10%. The cold passage exit temperature has a sudden decrease at the initial period due to the step input. Then, because of the great thermal inertia of the solid structure, the temperature decreases gradually. Therefore, the temperature at the cold passage exit decreases. Owing to a greater cold fuel mass flow rate, more heat is provided from the hot side, so the temperature at the hot

(a)

(b) Fig. 15. Dynamic response of the temperatures at the Cold (a) and hot (b) passage exits when

**5.3 Dynamic simulation result** 

passage exit has a gradual decrease.

cold fuel mass flow rate up by 10%.

operated for 500s.

Fig. 14. Cold passage outlet pressure effect on the cold fuel (a) and hot gas (b) temperature distributions.

The cold passage outlet pressure has little influence on the heat exchange reformer performance. When the passage pressure is elevated from 1E+5Pa to 4E+5Pa, less methane is consumed, less hydrogen is produced (Fig. 13), and less heat is needed for the methane steam reforming reaction, so the cold fuel and hot gas temperatures are higher (Fig. 14).

#### **5.3 Dynamic simulation result**

240 Heat Exchangers – Basics Design Applications

(a)

Fig. 14. Cold passage outlet pressure effect on the cold fuel (a) and hot gas (b) temperature

The cold passage outlet pressure has little influence on the heat exchange reformer performance. When the passage pressure is elevated from 1E+5Pa to 4E+5Pa, less methane is consumed, less hydrogen is produced (Fig. 13), and less heat is needed for the methane steam reforming reaction, so the cold fuel and hot gas temperatures are higher (Fig. 14).

distributions.

(b)

In this section, the transient behaviours of the compact heat exchange reformer are investigated. Several step-change input parameters (such as inlet mass flow rate and inlet temperature of both the cold and hot stream) are imposed when the device has been operated for 500s.

Fig. 15 illustrates the dynamic response of the temperatures at the cold and hot passage exits, when the cold fuel mass flow rate has a step increase of 10%. The cold passage exit temperature has a sudden decrease at the initial period due to the step input. Then, because of the great thermal inertia of the solid structure, the temperature decreases gradually. Therefore, the temperature at the cold passage exit decreases. Owing to a greater cold fuel mass flow rate, more heat is provided from the hot side, so the temperature at the hot passage exit has a gradual decrease.

Fig. 15. Dynamic response of the temperatures at the Cold (a) and hot (b) passage exits when cold fuel mass flow rate up by 10%.

(a)

(b)

Fig. 17. Dynamic response of the temperatures at the cold (a) and hot (b) passage exits when

the hot inlet temperature down to 1100K.

Fig. 16 shows the dynamic effect on methane, hydrogen, and the water molar fraction distribution when the cold fuel mass flow rate has a step increase of 10%. The methane and water molar fraction increase a little, while the hydrogen decreases a little. It can be shown that the molar fraction has a little change when the cold fuel inlet mass flow rate changes.

Fig. 17 presents the dynamic response of the cold fuel and hot gas temperatures when the hot gas inlet temperature decreases to 1100K from 1200K. The temperature at the cold passage exit is influenced by the thermal capacity of the solid structure, and decreases gradually. Owing to the decrease of the inlet temperature, the temperature at the hot gas passage exit also undergoes a decrease (Fig. 17 (b)). When the temperature of the cold stream decreases, the rate of the steam reforming reaction will be slower. Therefore, less fuel is reformed, which can be shown from the methane molar fraction distribution in Fig. 18 (a); less hydrogen is produced (Fig. 18 (b)) and more water remains (Fig. 18 (c)).

Fig. 16. Dynamic response of methane (a), hydrogen (b) and water (c) distributions when cold fuel mass flow rate up by 10%.

Fig. 16 shows the dynamic effect on methane, hydrogen, and the water molar fraction distribution when the cold fuel mass flow rate has a step increase of 10%. The methane and water molar fraction increase a little, while the hydrogen decreases a little. It can be shown that the molar fraction has a little change when the cold fuel inlet mass flow rate changes. Fig. 17 presents the dynamic response of the cold fuel and hot gas temperatures when the hot gas inlet temperature decreases to 1100K from 1200K. The temperature at the cold passage exit is influenced by the thermal capacity of the solid structure, and decreases gradually. Owing to the decrease of the inlet temperature, the temperature at the hot gas passage exit also undergoes a decrease (Fig. 17 (b)). When the temperature of the cold stream decreases, the rate of the steam reforming reaction will be slower. Therefore, less fuel is reformed, which can be shown from the methane molar fraction distribution in Fig. 18 (a);

(a) (b)

(c)

Fig. 16. Dynamic response of methane (a), hydrogen (b) and water (c) distributions when

cold fuel mass flow rate up by 10%.

less hydrogen is produced (Fig. 18 (b)) and more water remains (Fig. 18 (c)).

Fig. 17. Dynamic response of the temperatures at the cold (a) and hot (b) passage exits when the hot inlet temperature down to 1100K.

Based on all the dynamic performance figures from Fig. 15 to Fig. 18, the inertial delay time of this kind of heat exchange reformer is about 3000s. Such a substantial thermal inertia can seriously influence the whole fuel cell hybrid system transient performance and the design

A compact heat exchange reformer for high temperature fuel cell systems is presented in this paper. Based on the volume-resistance characteristic modeling technique, the distributed-lumped parameter method, and the modular modeling idea, a simulation model that is suited for quick and real time simulations is completed. The model can predict the key distribution characteristic parameters and the influence of some factors, such as the steam to carbon ratio, catalyst reduced activity, and passage pressure. The dynamic results

Both the model and modeling method will be useful and valuable for other heat exchange reformer designs and optimization; it can also provide a reference for the design of the

Financial support from the National Natural Science Foundation of China (NSFC) under the contract no. 50676061 and Shanghai Key Research Program from Science and Technology Committee of Shanghai Municipal under the contract No. 09DZ1200701 and 09DZ1200702 is

k parameters used in Table 1, or geometry parameter used in formula (8) (m)

*p* partial pressure of component i in the cold fuel passage (Pa)

indicate that this kind of heat exchange reformer has a great thermal inertia.

of the control system.

control system in the future.

**7. Acknowledgement** 

gratefully acknowledged

*C* molar concentration (molm-3) *Cp* specific heat capacity (kJkg-1K-1)

D*h* hydraulic diameter (m) *DEN* parameter used in Table 1 *f* fanning friction factor G mass flow rate (kgs-1) Gm mass velocity (kgm-2s-1)

K parameter used in Table 1

*L* heat exchanger length (m) *l* offset strip fin length (m) *M* molecular weight (kgmol-1)

*J* Colburn factor

*n* number

*P* pressure (Pa) Pr Prandtl number *R* gas constant (Jmol-1K-1) Re Reynolds number

**8. Nomenclature**  A area (m2)

**6. Conclusions** 

Fig. 18. Dynamic response of methane (a), hydrogen (b) and water (c) distributions when the hot inlet temperature down to 1100K.

Based on all the dynamic performance figures from Fig. 15 to Fig. 18, the inertial delay time of this kind of heat exchange reformer is about 3000s. Such a substantial thermal inertia can seriously influence the whole fuel cell hybrid system transient performance and the design of the control system.
