**5. Results and discussions**

In this section, due to the high cost of the complicated experiments, only simulation studies are employed on a counter-flow type heat exchange reformer. Section 5.1 provides the distributed characteristics of some important parameters, such as fuel species, temperature, and fluid properties (pressure, density, velocity, heat capacity, thermal conductivity and dynamic viscosity), under steady state conditions. Section 5.2 compares and analyzes the results under different input parameter conditions, such as steam to carbon ratio, catalyst reduced activity, and operating outlet pressure. In Section 5.3, the dynamic behaviours of the compact heat exchanger reformer are investigated.

## **5.1 Steady state result analysis**

For the rated condition, some related parameters are presented in Table 6, such as inlet temperature, mass flow rate, molar fraction, and outlet pressure.

Fig. 4 presents the fuel molar fraction along the heat exchange reformer length. The flow direction in the fuel channel is from 1.0 to 0 in the figures, so all the parameters in the fuel channel should be understood to proceed from 1.0 to 0. At the cold fuel passage inlet, the fluid only contains methane and water. The steam reforming reaction takes place on the surface of the catalyst along the flow direction. Therefore, the methane is gradually consumed. The methane and water concentration decreases along the flow direction. The concentration of produced hydrogen gradually increases. The methane steam reforming reaction has two simultaneous effects. The carbon monoxide molar fraction increases and the carbon dioxide molar fraction increases along the flow direction. At the exit, the flow composition is 4.24% of CH4, 45.35% of H2, 10.00% of CO, 3.84% of CO2, and 36.57% of H2O.

Fluid molar fraction 0.25CH4,0.75H2O (STC=3:1)

Fluid molar fraction 0.1CO2,0.2H2O,0.1O2,0.6N2

difference algorithms for both the front and end modules are treated independently.

At the same time, some simplifying conditions are used to solve the equations; for example, the heat flux of both the solid structure at inlet and outlet are considered to be zero. As a result, contrasted to the centre difference algorithm in the middle of the solid structure, the

In this section, due to the high cost of the complicated experiments, only simulation studies are employed on a counter-flow type heat exchange reformer. Section 5.1 provides the distributed characteristics of some important parameters, such as fuel species, temperature, and fluid properties (pressure, density, velocity, heat capacity, thermal conductivity and dynamic viscosity), under steady state conditions. Section 5.2 compares and analyzes the results under different input parameter conditions, such as steam to carbon ratio, catalyst reduced activity, and operating outlet pressure. In Section 5.3, the dynamic behaviours of

For the rated condition, some related parameters are presented in Table 6, such as inlet

Fig. 4 presents the fuel molar fraction along the heat exchange reformer length. The flow direction in the fuel channel is from 1.0 to 0 in the figures, so all the parameters in the fuel channel should be understood to proceed from 1.0 to 0. At the cold fuel passage inlet, the fluid only contains methane and water. The steam reforming reaction takes place on the surface of the catalyst along the flow direction. Therefore, the methane is gradually consumed. The methane and water concentration decreases along the flow direction. The concentration of produced hydrogen gradually increases. The methane steam reforming reaction has two simultaneous effects. The carbon monoxide molar fraction increases and the carbon dioxide molar fraction increases along the flow direction. At the exit, the flow composition is 4.24% of CH4, 45.35% of H2, 10.00% of CO, 3.84% of CO2, and 36.57% of

Simulation conditions

Inlet mass flow rate (kgs-1) 0.06 Inlet temperature (K) 898

Outlet pressure (Pa) 1.0E+5

Outlet pressure (Pa) 1.0E+5 Table 6. Key simulation parameters under the basic condition.

Inlet mass flow rate (kgs-1) 0.4 Inlet temperature (K) 1200

the compact heat exchanger reformer are investigated.

temperature, mass flow rate, molar fraction, and outlet pressure.

Cold fuel

Hot waste gas

**5. Results and discussions** 

**5.1 Steady state result analysis** 

H2O.

Fig. 4. Fuel molar fraction along the heat exchange reformer length.

The temperature profiles of the cold stream, hot stream, and solid structure along the heat exchange reformer length are presented in Fig. 5. Because of the high endothermic methane reforming reaction, the cold fuel temperature decreases a little at the entrance. Then, the cold fuel temperature increases along its flow direction due to the heat transfer from hot gas. The temperatures of the hot gas stream and the solid structure decrease along the heat exchange reformer length. It should be noted that the temperature curve is just the line between measured points, so it can't indicate the trend at both ends.

Fig. 5. Temperature distribution along the heat exchange reformer length.

Fig. 7. Cold fuel properties along the heat exchange reformer length.

Fig. 8. Hot gas properties along the heat exchange reformer length.

The pressure profiles in the cold fuel and hot gas passages are illustrated in Fig. 6. Owing to the friction of the passage, the pressure loss is about 0.08% in the cold fuel passage, and about 4.23% in the hot gas passage. The primary reason that the pressure loss is greater in the hot gas passage is that the mass flow rate in the hot gas passage is larger than that in the cold passage. Of course, the geometrical configuration is a key factor as well.

Fig. 6. Pressure distribution along the heat exchange reformer length.

The dimensionless fluid properties (such as: density, velocity, heat capacity, thermal conductivity, and dynamic viscosity) of the cold fuel and hot gas along the heat exchange reformer are illustrated in Fig. 7 and Fig. 8, respectively. The dimensionless properties are defined as the ratio of local values and corresponding inlet values, which can be calculated by the inlet conditions in the methods depicted in the reference [23]. Examples of this include situations where: the density is based on the gas state equation; the velocity is calculated by the mass flow rate, density and the channel cross area; the heat capacity of the multi-component gas mixture is related to the single component heat capacity and the corresponding molar fraction; the dynamic viscosity of the multi-component gas mixture is based on the Reichenberg's expression; the thermal conductivity of multi-component gas mixtures is based on Wassiljewa's expression and the Mason & Saxena modification.

The pressure profiles in the cold fuel and hot gas passages are illustrated in Fig. 6. Owing to the friction of the passage, the pressure loss is about 0.08% in the cold fuel passage, and about 4.23% in the hot gas passage. The primary reason that the pressure loss is greater in the hot gas passage is that the mass flow rate in the hot gas passage is larger than that in the

cold passage. Of course, the geometrical configuration is a key factor as well.

Fig. 6. Pressure distribution along the heat exchange reformer length.

The dimensionless fluid properties (such as: density, velocity, heat capacity, thermal conductivity, and dynamic viscosity) of the cold fuel and hot gas along the heat exchange reformer are illustrated in Fig. 7 and Fig. 8, respectively. The dimensionless properties are defined as the ratio of local values and corresponding inlet values, which can be calculated by the inlet conditions in the methods depicted in the reference [23]. Examples of this include situations where: the density is based on the gas state equation; the velocity is calculated by the mass flow rate, density and the channel cross area; the heat capacity of the multi-component gas mixture is related to the single component heat capacity and the corresponding molar fraction; the dynamic viscosity of the multi-component gas mixture is based on the Reichenberg's expression; the thermal conductivity of multi-component gas

mixtures is based on Wassiljewa's expression and the Mason & Saxena modification.

Fig. 7. Cold fuel properties along the heat exchange reformer length.

Fig. 8. Hot gas properties along the heat exchange reformer length.

(a)

(b)

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

The density is related to the pressure and the temperature, which are decided by the gas state equation *P RT* . In the cold fuel passage, the temperature increases and the pressure 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 direction.

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, following the trend of the density.

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