**2.2 Methodology**

In this study, the fuel was reformed by coupling the arc energy from a spark discharge with the catalyst converter for partial oxidation reforming. The arc was the external energy source of the system to cause the formation of the plasma; and the arc was stretched by the kinetic energy induced by the tangential intake air flow which allowed for a more even discharge distribution. In addition, the production of hydrogen-rich gas was improved due to the use of catalysts. The product was collected in gas bags for analysis via gas chromatography.

Currently, the industries have several approaches for producing hydrogen; these approaches can be classified into three main categories, based on the energy sources: (1) Hydrogen production via hydrocarbon fuels; (2) hydrogen production via non-hydrocarbon fuels; and (3) hydrogen production via a combination of hydrocarbon and non-hydrocarbon fuels. Other classification for hydrogen production includes steam reforming (SR), partial oxidation reforming (POX), autothermal reforming (ATR), catalytic decomposition, and coal gasification. The reforming approaches and mechanisms for hydrogen production are shown in Fig. 4. This study generated hydrogen by the reformation of hydrocarbon fuels by POX and ATR.

Fig. 5 shows the variation of the reformate gas yields produced by methane under various O2/CH4 ratios at different reforming temperatures by theoretical calculation. It showed that the temperature of the reformate gas was crucial for the overall yield; higher reaction

reaction chamber Gas collector

Catalyst section

at the three points were used to define the actual temperature of the reformate gas. This temperature was also used to correspond with the theoretical calculation to verify the trend

In this study, the fuel was reformed by coupling the arc energy from a spark discharge with the catalyst converter for partial oxidation reforming. The arc was the external energy source of the system to cause the formation of the plasma; and the arc was stretched by the kinetic energy induced by the tangential intake air flow which allowed for a more even discharge distribution. In addition, the production of hydrogen-rich gas was improved due to the use of catalysts. The product was collected in gas bags for analysis via gas

Currently, the industries have several approaches for producing hydrogen; these approaches can be classified into three main categories, based on the energy sources: (1) Hydrogen production via hydrocarbon fuels; (2) hydrogen production via non-hydrocarbon fuels; and (3) hydrogen production via a combination of hydrocarbon and non-hydrocarbon fuels. Other classification for hydrogen production includes steam reforming (SR), partial oxidation reforming (POX), autothermal reforming (ATR), catalytic decomposition, and coal gasification. The reforming approaches and mechanisms for hydrogen production are shown in Fig. 4. This study generated hydrogen by the reformation of hydrocarbon fuels by

Fig. 5 shows the variation of the reformate gas yields produced by methane under various O2/CH4 ratios at different reforming temperatures by theoretical calculation. It showed that the temperature of the reformate gas was crucial for the overall yield; higher reaction

Fig. 3. The configuration of the reactor

Fuel Injector

Plasma reaction

zone Mid-section of

of the experimental results.

**2.2 Methodology** 

chromatography.

POX and ATR.

Fig. 4. A relationship chart of reforming mechanisms

Fig. 5. The variation of the reformate gas yields produced by methane under various O2/CH4 ratios at different reforming temperatures

temperatures and lower O2/CH4 ratios gave better H2 and CO yields. However, the appeal of this system was its portability, and thus external energy supply was not an option. Therefore, heat released during the oxidation reforming process was used to provide the high temperature required for the reactions. This approach reduced the need for external energy, and was also advantageous in minimizing the size of the converter.

Waste Heat Recycling for Fuel Reforming 365

In this study, the methane flow rate was in the range of 1 to 10 L/min, the O2/C (Oxygen/Carbon) ratio was between 0.5 and 1.0, S/C (H2O/Carbon) ratio was between 0 and 2, and the arc frequency was 200 Hz. Methane and air were supplied from tanks and were directed into the plasma reaction zone through the fuel nozzle via a tangential induction. The frequency of the arc was controlled by the signal generator, and was monitored on an oscilloscope. This study measured the voltage and current of the primary side (the low-voltage side), and monitored the arc frequency with an oscilloscope and a clamp tester. The voltage and current of the secondary side (high-voltage side) was measured with the oscilloscope and the clamp tester, in coordination with an automotive

Partial oxidation and autothermal reforming reactions were performed in this study. With an appropriate amount of oxygen, the heat released from the fuel oxidization dissociated carbon (C) and hydrogen (H) atoms from the fuel with water addition, and reformed the atoms into hydrogen gas (H2) and carbon monoxide (CO). The equations (1) - (4) below

CH z (O 3.76N ) 2z CO 2H (1 2z )C 3.76z N 4 02 2 0 2 0 0 2 (1)

CH z (O 3.76N ) (2 2z )H O CO (4 2z )H 3.76z N 4 0 2 2 0 2 2 0 2 02 (2)

CH z(O 3.76N ) x(aCO bCO dH eO fCH 42 2 2 22 4 2 2 gN ) hH O jC (3)

CH z(O 3.76N ) yH O x(aCO bCO dH eO fCH gN ) hH O jC 42 2 2 2 22 4 2 2 (4)

*O O O O CH air CH CH CH CH O CH*

0.21

(5)

4 4 44 2 4

*O n M QM Q C n m QM Q*

2 2 22 4

*z0* is the equilibrium constant in the theoretical reaction; *z*, *a*, *b, d*, *e*, *f*, *g*, *h*, *j* and *y* are the equilibrium constants in the actual reaction, and *x* is the proportionality constant, respectively. Several expressions are used to describe the ratio of fuel and air in chemical equilibrium, and can be mainly classified as air-fuel mass ratio, and oxygen-carbon molar ratio. This study expressed the reforming reaction processes using the oxygen-carbon molar ratio. The equation for the oxygen-carbon (O2/C) ratio related to volumetric flow rate is

*O*

2

*m*

*CH*

4

*M*

**2.3 Experimental parameters and the related calculations** 

diagnostic analyzer.

1. Theoretical reactions Partial oxidation reforming

Autothermal reforming

2. Actual reactions Partial oxidation reforming

Autothermal reforming

shown in equation (5).

2

represent the theoretical and actual reactions.

Fig. 6 shows the enthalpy of reaction under different O2/C ratios in methane reforming processes. The spots in the figure represent different reaction processes, and the processes are displayed sequentially as: the cracking process, the partial oxidation process, and the complete combustion. It could be observed that lower O2/C ratios were involved in endothermic processes, as lower O2/C ratios could not provide the required energy for the reformation processes. Therefore, methane of a higher O2/C ratio was used in this study for reforming reactions. However, O2/C could not be too high, or H2 and CO would be oxidized into H2O and CO2. This study contained the O2/C ratio between B and C, as shown in Fig. 6; methane reaction with O2/C ratios within this range provided the required energy for reforming, and simultaneously prevented a reduction in catalyst activity due to the formation of carbon deposition in fuel rich when the O2/C ratio was too low. Many possible reaction processes may happen for this study; the reactions listed in Table 2 are the possible routes, such as oxidation, shifting, and methanation, which could occur in this study.


Fig. 6. A comparison of the enthalpy of reaction under different O2/C ratios in methane reforming processes

Fig. 6 shows the enthalpy of reaction under different O2/C ratios in methane reforming processes. The spots in the figure represent different reaction processes, and the processes are displayed sequentially as: the cracking process, the partial oxidation process, and the complete combustion. It could be observed that lower O2/C ratios were involved in endothermic processes, as lower O2/C ratios could not provide the required energy for the reformation processes. Therefore, methane of a higher O2/C ratio was used in this study for reforming reactions. However, O2/C could not be too high, or H2 and CO would be oxidized into H2O and CO2. This study contained the O2/C ratio between B and C, as shown in Fig. 6; methane reaction with O2/C ratios within this range provided the required energy for reforming, and simultaneously prevented a reduction in catalyst activity due to the formation of carbon deposition in fuel rich when the O2/C ratio was too low. Many possible reaction processes may happen for this study; the reactions listed in Table 2 are the possible

routes, such as oxidation, shifting, and methanation, which could occur in this study.

Reaction Equation ΔHSTP

0.0 0.5 1.0 1.5 2.0

**/C ratio**

**O**

**D**

**C**

**B**

**=CO+2H2**

**+2H2**

**=CO+2H2**

**O**

**+2H2 O**

**O2**

Fig. 6. A comparison of the enthalpy of reaction under different O2/C ratios in methane

**Exothermic reaction**

**E**

**F**

**Endothermic reaction**


**Enthalpy of reaction (kJ/mole-CH4**

reforming processes

**)**

**A**

**A. CH4**

**B. CH4**

**C. CH4**

**D. CH4**

**E. CH4**

**F. CH4**

**=C+2H2**

**+0.5O2**

**+1.5O2**

**=CO2**

**=C+2H2**

**=CO2**

**+O2**

**+O2**

**+2O2**

Partial oxidation CH4 + 0.5O2 → CO + 2H2 -35.6 Water gas shifting CO + H2O → CO2 + H2 -41.2 Complete oxidation CH4 + 2O2 → CO2 + 2H2O -802.2 H2 oxidation H2 + 0.5O2 → H2O -241.8 CO oxidation CO + 0.5O2 → CO2 -283.0 CO methanation CO + 3H2 → CH4 + H2O -206.2 CO2 methanation CO2 + 4H2 → CH4 + 2H2O -165.0 Table 2. The reaction in the reforming processes and the corresponding enthalpy of reaction

(kJ/mol)

#### **2.3 Experimental parameters and the related calculations**

In this study, the methane flow rate was in the range of 1 to 10 L/min, the O2/C (Oxygen/Carbon) ratio was between 0.5 and 1.0, S/C (H2O/Carbon) ratio was between 0 and 2, and the arc frequency was 200 Hz. Methane and air were supplied from tanks and were directed into the plasma reaction zone through the fuel nozzle via a tangential induction. The frequency of the arc was controlled by the signal generator, and was monitored on an oscilloscope. This study measured the voltage and current of the primary side (the low-voltage side), and monitored the arc frequency with an oscilloscope and a clamp tester. The voltage and current of the secondary side (high-voltage side) was measured with the oscilloscope and the clamp tester, in coordination with an automotive diagnostic analyzer.

Partial oxidation and autothermal reforming reactions were performed in this study. With an appropriate amount of oxygen, the heat released from the fuel oxidization dissociated carbon (C) and hydrogen (H) atoms from the fuel with water addition, and reformed the atoms into hydrogen gas (H2) and carbon monoxide (CO). The equations (1) - (4) below represent the theoretical and actual reactions.

1. Theoretical reactions

Partial oxidation reforming

$$\rm CH\_4 + z\_0(O\_2 + 3.76N\_2) \to 2z\_0CO + 2H\_2 + (1 - 2z\_0)C + 3.76z\_0N\_2 \tag{1}$$

Autothermal reforming

$$\rm{CH}\_4 + \rm{z}\_0(\rm{O}\_2 + 3.76 \rm{N}\_2) + (2 - 2 \rm{z}\_0) \rm{H}\_2\rm{O} \rightarrow \rm{CO}\_2 + (4 - 2 \rm{z}\_0) \rm{H}\_2 + 3.76 \rm{z}\_0 \rm{N}\_2 \tag{2}$$

2. Actual reactions

Partial oxidation reforming

$$\rm{CH}\_4 + \rm{z(O}\_2 + \rm{3.76N}\_2) \rightarrow \rm{x(aCO + bCO\_2 + dH\_2 + eO\_2 + fCH\_4 + gN\_2) + hH\_2O + jC} \tag{3}$$

Autothermal reforming

$$\text{CH}\_4 + \text{z(O}\_2 + 3.76 \text{N}\_2) + \text{yH}\_2\text{O} \rightarrow \text{x(aCO} + \text{bCO}\_2 + \text{dH}\_2 + \text{eO}\_2 + \text{fCH}\_4 + \text{gN}\_2) + \text{hH}\_2\text{O} + \text{jC} \text{ (4)}$$

*z0* is the equilibrium constant in the theoretical reaction; *z*, *a*, *b, d*, *e*, *f*, *g*, *h*, *j* and *y* are the equilibrium constants in the actual reaction, and *x* is the proportionality constant, respectively. Several expressions are used to describe the ratio of fuel and air in chemical equilibrium, and can be mainly classified as air-fuel mass ratio, and oxygen-carbon molar ratio. This study expressed the reforming reaction processes using the oxygen-carbon molar ratio. The equation for the oxygen-carbon (O2/C) ratio related to volumetric flow rate is shown in equation (5).

$$\frac{\dot{m}\_{O\_2}}{\overline{C}} = \frac{\dot{m}\_{O\_2}}{\dot{m}\_{CH\_4}} = \frac{\frac{\dot{m}\_{O\_2}}{\overline{M}\_{O\_2}}}{\frac{\dot{m}\_{CH\_4}}{\overline{M}\_{CH\_4}}} = \frac{\rho\_{O\_2} \times Q\_{O\_2}}{\rho\_{CH\_4} \times Q\_{CH\_4}} \times \frac{M\_{CH\_4}}{M\_{O\_2}} = \frac{0.21 \times Q\_{air}}{Q\_{CH\_4}}\tag{5}$$

Waste Heat Recycling for Fuel Reforming 367

release, resulting in a lower H2+CO concentration. Therefore, higher O2/C ratios are required to generate greater H2+CO output concentrations at low input flow rates. In contrast, the oxidization effect is stronger at greater flow rates and higher O2/C ratios; consequently, H2 and CO would combine with excess O2 to form CO2 and H2O; the overall H2+CO output concentration also decreases due to the large quantity of dilute gas (N2). Further in this figure, it is observed that the differences between the theoretical calculation and the experimental results in H2+CO concentrations are obvious at low O2/C ratios. This is mainly because the low heat release of oxidation at lower O2/C ratios leads to a lower conversion efficiency. Therefore, when the O2/C ratio is low, waste heat recycling could be simultaneously used to improve the fuel conversion efficiency, and thus increase the overall reforming efficiency.

The effect of O2/C ratio on the enthalpy of reaction and energy loss will be discussed in the

*e f e i f i*

<sup>0</sup> , represent the number of kilogram-moles, enthalpy of formation and

**Square:1L/min Circle:2L/min Triangle:4L/min**

() () (6)

**Methane feeding rate**

**Diamond:6L/min Pentangle:8L/min Hexagon:10L/min**

following section. The change of enthalpy of reaction (Δ*H*) is denoted in equation (6).

*oduct ac t H nh h nh h* 0 0 Pr Re tan

Fig. 8 shows the effect of O2/C ratio on the enthalpy of reaction and the energy loss at different methane flow rates in the reforming process. Greater energy loss implies greater heat release in the reforming process. By comparing different O2/C ratios at the same fuel flow rate, it is found that more oxygen reacts with the fuel at higher O2/C ratios, therefore more heat is released. Consequently, excessive oxygen has a tendency to oxidize H2 and CO to become H2O and CO2, respectively, resulting in a greater loss of energy in the reaction

**0.5 0.6 0.7 0.8 0.9 1.0**

Fig. 8. The relationship between various O2/C ratios and the enthalpy of reaction and

**/C molar ratio**

**O2**

energy loss percentage at different methane flow rates

**Theoretical calculation**

10

15

20

25

30

**Energy loss percentage (%)**

35

40

45

50

55

enthalpy change of species *e* and *i* for the products and reactants, respectively.

Where *n h and h f*

**-450**

**-400**

**-350**

**-300**

**-250**

**Enthalpy of reaction (kJ/mole-CH4**

**)**

**-200**

**-150**

**-100**

**-50**

Where *n m and Q* , represent the molar, mass and volumetric flow rates of the species, respectively; *M* and *ρ* represent the molecular weight and density, respectively.

Generally, chemical equilibrium is described by two methods, the equilibrium constant and the minimum of Gibbs free energy. This study described the state of the reforming process by temperature and pressure. Gibbs free energy can easily achieve minimization; in addition, temperature and pressure are both the original variables of the Gibbs function; therefore, this study took the Gibbs free energy minimization for calculation. The calculation and analysis were performed using the commercialized HSC Chemistry software (©ChemSW Software, Inc., 2002). HSC software was designed to analyze many different types of chemical reactions and to perform equilibrium calculations. This study used the HSC system to find the equilibrium components, and the thermodynamic properties. The predicted results were then used as a reference to set the parameters and to validate the experimental results.

### **3. Results and discussion**

#### **3.1 The effect of O2/C ratio**

Fig. 7 shows the effect of O2/C ratio on H2+CO concentration at various fuel flow rates. H2+CO is generally known as syngas, it can be directed into an engine as a fuel, or be processed via a water gas shifting reaction or a gas separation method for fuel cell applications. The overall trend shows that the higher the input fuel flow rate, the greater the H2+CO output concentration; and within the tested parameter range, the output concentration gradually converges as the flow rate increases. Particularly, at a low flow rate and a low O2/C ratio, a great amount of the fuel cannot be converted due to a lower heat

Fig. 7. The influence of O2/C ratios on the output concentration of H2+CO at different methane flow rates

Where *n m and Q* , represent the molar, mass and volumetric flow rates of the species,

Generally, chemical equilibrium is described by two methods, the equilibrium constant and the minimum of Gibbs free energy. This study described the state of the reforming process by temperature and pressure. Gibbs free energy can easily achieve minimization; in addition, temperature and pressure are both the original variables of the Gibbs function; therefore, this study took the Gibbs free energy minimization for calculation. The calculation and analysis were performed using the commercialized HSC Chemistry software (©ChemSW Software, Inc., 2002). HSC software was designed to analyze many different types of chemical reactions and to perform equilibrium calculations. This study used the HSC system to find the equilibrium components, and the thermodynamic properties. The predicted results were then used as a reference to set the parameters and to validate the

Fig. 7 shows the effect of O2/C ratio on H2+CO concentration at various fuel flow rates. H2+CO is generally known as syngas, it can be directed into an engine as a fuel, or be processed via a water gas shifting reaction or a gas separation method for fuel cell applications. The overall trend shows that the higher the input fuel flow rate, the greater the H2+CO output concentration; and within the tested parameter range, the output concentration gradually converges as the flow rate increases. Particularly, at a low flow rate and a low O2/C ratio, a great amount of the fuel cannot be converted due to a lower heat

**Methane flow rate**

**1 L/min 2 L/min 4 L/min** **6 L/min 8 L/min 10 L/min**

**Equilibrium Calculation**

**0.5 0.6 0.7 0.8 0.9 1.0**

**/C ratio**

**O2**

Fig. 7. The influence of O2/C ratios on the output concentration of H2+CO at different

respectively; *M* and *ρ* represent the molecular weight and density, respectively.

experimental results.

**3. Results and discussion 3.1 The effect of O2/C ratio** 

methane flow rates

**H2+CO concentration (Vol.%)**

release, resulting in a lower H2+CO concentration. Therefore, higher O2/C ratios are required to generate greater H2+CO output concentrations at low input flow rates. In contrast, the oxidization effect is stronger at greater flow rates and higher O2/C ratios; consequently, H2 and CO would combine with excess O2 to form CO2 and H2O; the overall H2+CO output concentration also decreases due to the large quantity of dilute gas (N2). Further in this figure, it is observed that the differences between the theoretical calculation and the experimental results in H2+CO concentrations are obvious at low O2/C ratios. This is mainly because the low heat release of oxidation at lower O2/C ratios leads to a lower conversion efficiency. Therefore, when the O2/C ratio is low, waste heat recycling could be simultaneously used to improve the fuel conversion efficiency, and thus increase the overall reforming efficiency.

The effect of O2/C ratio on the enthalpy of reaction and energy loss will be discussed in the following section. The change of enthalpy of reaction (Δ*H*) is denoted in equation (6).

$$
\Delta H = \sum\_{\text{Product}} n\_e (\overline{h\_f^0} + \overline{\Delta h})\_e - \sum\_{\text{Re } act \, \text{tan } t} n\_i (\overline{h\_f^0} + \overline{\Delta h})\_i \tag{6}
$$

Where *n h and h f* <sup>0</sup> , represent the number of kilogram-moles, enthalpy of formation and enthalpy change of species *e* and *i* for the products and reactants, respectively.

Fig. 8 shows the effect of O2/C ratio on the enthalpy of reaction and the energy loss at different methane flow rates in the reforming process. Greater energy loss implies greater heat release in the reforming process. By comparing different O2/C ratios at the same fuel flow rate, it is found that more oxygen reacts with the fuel at higher O2/C ratios, therefore more heat is released. Consequently, excessive oxygen has a tendency to oxidize H2 and CO to become H2O and CO2, respectively, resulting in a greater loss of energy in the reaction

Fig. 8. The relationship between various O2/C ratios and the enthalpy of reaction and energy loss percentage at different methane flow rates

Waste Heat Recycling for Fuel Reforming 369

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

200 300 400 500 600 700 800 900 1000 1100

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

Fig. 10. The relationship between the reformate gas temperature and the selectivity of H2

**Square: 1L/min Circle: 2L/min Triangle: 4L/min**

**Methane flow rate**

**Diamond: 6L/min Pentangle: 8L/min Hexagon: 10L/min**

**Carbon monoxide selectivity (%)**

**C)**

*n moles of CnHm consumed* ( ) 100% (7)

*m moles of CnHm consumed* ( ) 100% (8)

*<sup>x</sup> moles of CxHy formed <sup>S</sup>*

*y moles of CxHy formed <sup>S</sup>*

*C base*

*H base*

oxidized to form the CO2 and H2O.

and CO at different methane flow rates

**Hydrogen 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 the fuel flow rate increases.
