**5.2.1 Steam to carbon ratio**

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 is presented in Fig. 9 and Fig. 10.

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 cells.

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 stream are higher.
