**3.3 The effect of the reformate gas temperature**

The following section discusses the effect of reformate gas temperature on the selectivity of H2 and CO. The definition of selectivity (*S*) is shown in equations (7) and (8), and can be classified as carbon-based (C-base) and hydrogen (H-base) selectivity.

process. In contrast, by comparing different fuel flow rates at the same O2/C ratio, it is found that increasing the fuel flow rate would lower the enthalpy of reaction per molar fuel. This is because the fuel oxidation releases more heat and the reaction temperature is relatively high; therefore, in this condition, the system does not rely on higher O2/C ratios to promote the conversion of fuel. As a result, the overall energy loss percentage decreases as

Conversion efficiencies are crucial indicators of reforming processes and can be used to indicate the reformation efficiency. Higher conversion efficiency implies that more fuel participates in the reactions, therefore more reformate products are produced. As shown in Fig. 9, the methane flow rates of this study are 2, 4, 6, 8, and 10 L/min, and the O2/C ratio is between 0.5 and 1.0. High fuel conversion efficiency is found at relatively high O2/C ratios and high methane flow rates. This is because at the same O2/C ratio, an increase in the flow rate of fuel would increase the heat release of oxidation; therefore the required energy for reaction is effectively obtained. It is also found that as the O2/C ratio increases, the increase in conversion efficiency gradually slows down because it nearly reaches the maximum

2 4 6 810

**Methane flow rate (L/min)**

Fig. 9. The effect of methane flow rate on methane conversion efficiency at different O2/C ratios

The following section discusses the effect of reformate gas temperature on the selectivity of H2 and CO. The definition of selectivity (*S*) is shown in equations (7) and (8), and can be

**O2**

**O2**

**O2**

**/C ratio=0.8**

**/C ratio=0.9**

**/C ratio=1.0**

the fuel flow rate increases.

**3.2 The effect of fuel supply rate** 

space velocity of the reforming system.

**O2**

**O2**

**O2**

**/C ratio=0.5**

**/C ratio=0.6**

**/C ratio=0.7**

30

**3.3 The effect of the reformate gas temperature** 

classified as carbon-based (C-base) and hydrogen (H-base) selectivity.

40

50

60

70

80

**Methane conversion efficiency (%)**

90

100

110

120

$$S\_{\text{C-base}} = \frac{\text{x}}{n} (\frac{\text{moles of CrHy formed}}{\text{moles of CrHm consumed}}) \times 100\% \tag{7}$$

$$S\_{H-base} = \frac{y}{m} (\frac{moles\ of\ CuHy\ formed}{moles\ of\ CuHm\ consumed}) \times 100\% \tag{8}$$

The selectivity is to demonstrate the form of the reformate gas existing after the reforming reactions. Briefly, hydrocarbon fuel reformation produces hydrogen-based and carbonbased products. The selectivity of hydrogen-based product may have the form of hydrogen (H2), water (H2O), or other hydrocarbons. Carbon-based products may exist as carbon monoxide (CO), carbon dioxide (CO2), and other hydrocarbons. Fig. 10 shows that as the temperature increases, H2 selectivity shows a quadratic increase, while CO selectivity has a logarithmic increase. Both H2 and CO have the greatest selectivity when the temperature is between 600 oC and 700 oC. In Fig. 11, the yields of H2 and CO are found to increase as the reformate gas temperature rises; the yield reaches the maximum value approximately between 750 and 800 oC, and is 77 % for H2 and 73 % for CO. As performing the calculation by using the best parameters, the experimental results are found to be close to the theoretical values; except that once the reformate gas temperature exceeds 800 oC, the experimental values diverge from the theoretical ones. Around this temperature, the inconsistency between the experimental results and the theoretical calculations is thought to result from the combustion of a portion of the methane at such a high temperature, and H2 and CO were oxidized to form the CO2 and H2O.

Fig. 10. The relationship between the reformate gas temperature and the selectivity of H2 and CO at different methane flow rates

Waste Heat Recycling for Fuel Reforming 371

0 5 10 15 20 25 30 35 40 45

**O2**

**/C ratio 0.5 0.6 0.7 0.8 0.9 1.0**

**CO2**

**C)**

**CO**

**H2**

**Tave.=780o**

**/C=0.8**

**Gas hourly space velocity /1000 (h-1)**

0 200 400 600 800 1000 1200

**Reformate gas temperature (o**

Fig. 13. Comparison of the theoretical calculations and the experimental results at the best

Fig. 12. The effect of space velocity on thermal efficiency at different methane flow rates and

**C Exp. Cal.**

0

parameter settings

**Reformate gas concentration (Vol.%)**

**H2**

**CO2**

**CH4**

**CH4**

 **(%) 28.3 28.6 CO (%) 13.2 13.9** 

**Methane flow rate: 10L/min, O2**

 **(%) 4.5 3.8**

 **(%) 0.73 0.0**

O2/C ratios

10

20

30

40

**Thermal efficiency (%)**

50

60

70

80

**Methane flow rate: 1, 2, 4, 6, 8,10 L/min**

Fig. 11. The relationship between the reformate gas temperature and the yield of H2 and CO at different methane flow rates

#### **3.4 The effect of space velocity**

Generally, space velocity is determined by the input mixture, the operating temperature, and the size of the catalyst. Space velocity mainly refers to the volumetric flow rate of input mixture per unit volume of catalyst bed, but it is also an indicator for the reforming capacity. Fig. 12 shows the effect of space velocity on thermal efficiency at different methane flow rates and O2/C ratios. The definition for thermal efficiency (*th* ) is as shown in equation (9).

$$
\eta\_{th} = \frac{\dot{m}\_{H\_2} LHV\_{H\_2} + \dot{m}\_{CO} LHV\_{CO}}{\dot{m}\_{fuel} LHV\_{fuel}} \times 100\,\%\tag{9}
$$

Where *m and LHV* represent the mass flow rate and lower heating value of the species, respectively.

It is apparent from Fig. 12 that as space velocity increases, thermal efficiency initially increases abruptly, and the rate of increase slows down soon after; the trend indicates that the parameters used in the system did not exceed the amount that the reforming system could treat. At best operating parameters (methane flow rate of 10 L/min and an O2/C ratio of 0.8), the greatest thermal efficiency of 72.25 % is obtained and the corresponding space velocity is approximately 35,000 h-1. The best operating parameters were used as the input for calculation by using a commercialized HSC program, and the calculated values were compared with the experimented results obtained from the best parameter setting of the reforming system. It can be observed from Fig. 13 that the difference in the reformate gas concentration between the theoretical and the experimental results is within 1 %.

**)**

**300 400 500 600 700 800 900 1000**

Fig. 11. The relationship between the reformate gas temperature and the yield of H2 and CO

Generally, space velocity is determined by the input mixture, the operating temperature, and the size of the catalyst. Space velocity mainly refers to the volumetric flow rate of input mixture per unit volume of catalyst bed, but it is also an indicator for the reforming capacity. Fig. 12 shows the effect of space velocity on thermal efficiency at different methane

*H H CO CO*

 100% 

*m LHV m LHV m LHV* 2 2

*fuel fuel*

Where *m and LHV* represent the mass flow rate and lower heating value of the species,

It is apparent from Fig. 12 that as space velocity increases, thermal efficiency initially increases abruptly, and the rate of increase slows down soon after; the trend indicates that the parameters used in the system did not exceed the amount that the reforming system could treat. At best operating parameters (methane flow rate of 10 L/min and an O2/C ratio of 0.8), the greatest thermal efficiency of 72.25 % is obtained and the corresponding space velocity is approximately 35,000 h-1. The best operating parameters were used as the input for calculation by using a commercialized HSC program, and the calculated values were compared with the experimented results obtained from the best parameter setting of the reforming system. It can be observed from Fig. 13 that the difference in the reformate gas

concentration between the theoretical and the experimental results is within 1 %.

flow rates and O2/C ratios. The definition for thermal efficiency (

*th*

**Reformate gas temperature (<sup>0</sup>**

 **Methane flow rate Square : 1 L/min Diamond : 6 L/min Circle : 2 L/min Pentange : 8 L/min Triangle : 4 L/min Star : 10 L/min**

**C)**

(9)

**0**

*th* ) is as shown in

**20**

**40**

**60**

**Carbon monoxide yield (%)**

**80**

**100**

**120**

**0**

at different methane flow rates

**3.4 The effect of space velocity** 

**10**

**20**

**30**

**40**

**Hydrogen yield (%)**

equation (9).

respectively.

**50**

**60**

**70**

**80**

**CH4**

**/C = 0.8**

**O2**

**Equlibrium calculation (CH4**

 **flow rate : 10 L/min**

**90**

Fig. 12. The effect of space velocity on thermal efficiency at different methane flow rates and O2/C ratios

Fig. 13. Comparison of the theoretical calculations and the experimental results at the best parameter settings

Waste Heat Recycling for Fuel Reforming 373

H2 yield (%)

A n/a 95.99 77.75 72.44 24.34 72.25 A + B 0.5 95.59 85.92 56.8 25.06 71.63 A + C n/a 98.16 80.3 77.88 23.23 75.71 A + D n/a 99.29 81.81 81.14 22.68 77.77 A+ B + D 0.5 98.71 86.26 70.28 23.27 76.6

Table 3. Comparison of plasma converter performance under different operating conditions

increase the amount of power generated if applied to fuel cells. From this table, the variations in hydrogen and carbon monoxide yields are significantly with the combination of water gas shifting reaction, although the methane conversion efficiency is reduced. This is because the system did not adopt an external heating source to maintain its working temperature, and water addition would induce regional endothermic reactions due to the latent heat of water vaporization. The regional endothermic reaction consequently lowers the reformate gas temperature of the system and results in a reduction in the methane conversion efficiency. Another possibility for the reduction of conversion efficiency is that

This section will focus on the comparison of the theoretical calculation of the chemical equilibrium for methane reformation. The variations between the experimental results and the calculated values are compared. A comprehensive comparison of the experimental results with the theoretical calculation of the dry analysis on the output concentration of H2+CO is shown in Fig. 15; the values were measured under all of the parameters of the original system, the autothermal reforming reactions, and the energy conservative method. Linear regression analysis reveals that the R2 value for the concentration of syngas (H2+CO) is as high as 0.9179, indicating that the values obtained via experiments and the values

The following section discusses the effect of plasma reforming parameters (PRP) on the overall reforming performance. The purpose is to compile all of the measured results of this study and to identify the correlations within the parameters. The reforming indicators, H2+CO concentration and thermal efficiency will be discussed. Fig. 16 shows the effect of the first plasma reforming parameters (PRP1) on the concentration of H2+CO. PRP1 is comprised of the methane molar flow rate, oxygen-carbon molar ratio, and the methane conversion efficiency; these reforming parameters are also closely associated with the concentration of H2+CO. The R2 value of the quadratic regression at different operating conditions can be as high as 0.9104. It is also shown in Fig. 16 that an increase of PRP1 signifies a greater methane flow rate, a more appropriate O2/C ratio, and a better methane conversion efficiency; and the concentration of H2+CO first increases as PRP1 increases, and then subsequently decreases. Thus, the data imply that there is an upper limit for the O2/C ratio in this system, and exceeding this limit would lead to a decrease in hydrogen

concentration due to the oxidization of H2+CO into H2O and CO2 by excess oxygen.

methanation leads to reverse reaction and increases the selectivity of methane.

CO yield (%)

Energy loss (%)

*ηth* (%)

Fuel Conv. (%)

A: Original system, B: ATR, C: Heat recycling, D: Heat insulation;

obtained from theoretical calculation are very close.

S/C ratio

Fuel flow rate: 10 L/min, O2/C ratio: 0.8

The reforming performance for all the parameters at different equivalent power of input fuel is compared, as shown in Fig. 14. The input power of fuel directed into the reaction chamber is shown on the horizontal axis, and the output power derived from the output flow rate of H2 and CO is shown on the vertical axis. The result of this study shows that when methane is used for reforming, the output power increases linearly with the input power. The linear regression result could also serve as a convenient reference for future parameter settings for reforming.

Fig. 14. Comparison of reforming performance at different input power
