**4.2.1 IRSOFC operating on biogas**

The main reactions involved in an IRSOFC running on biogas are listed in Table 7. Internal dry reforming of CH4 (reaction 1) proceeds on a Ni-based anode using CO2 inherently included in biogas without an external reformer. Then, the produced H2 and CO are electrochemically oxidized to produce electricity (reactions 2 and 3) (Shiratori et al., 2009a).


Table 7**.** Main reactions involved in IRSOFC running on biogas (Shiratori et al., 2009a).

were evaporated and mixed with H2O at 600 oC and then fed to the anode side together with

The C-H-O diagram (see Fig. 9) clearly shows that unless a generous amount of oxidant is added, biogas (CH4/CO2 = 1.5) and BDF will form coke on the anode material during SOFC operation. We added air and water to gaseous biogas and liquid BDF, respectively, to avoid

0

C

Carbon deposition region

20

Raw biogas (CH4/CO2 = 1.5)

Fig. 9. C-H-O ternary diagram showing the possibility of coking when the biofuels are fed

The main reactions involved in an IRSOFC running on biogas are listed in Table 7. Internal dry reforming of CH4 (reaction 1) proceeds on a Ni-based anode using CO2 inherently included in biogas without an external reformer. Then, the produced H2 and CO are electrochemically oxidized to produce electricity (reactions 2 and 3) (Shiratori et al., 2009a).

40

S/C = 3.5

60

1 CH4 + CO2 → 2CO + 2H2 Dry reforming

2 H2 + O2- → H2O + 2e- Electrochemical oxidation 3 CO + O2- → CO2 + 2e- Electrochemical oxidation Table 7**.** Main reactions involved in IRSOFC running on biogas (Shiratori et al., 2009a).

80

O 0 20 40 60 80 100

400

Air/Biogas = 1.0

100

0

20

40

60

600

800

1000 oC

80

100

Saturated 46.8 21.4 15.4 Unsaturated 53.2 78.6 84.6 Mono- 40.8 40.9 24.8 Di- 12.2 37.6 54.0 Tri- 0.26 0.19 5.75 Average structure C18.0H34.8O2 C18.7H35.0O2 C18.8H34.5O2 Table 6. Composition of saturated and unsaturated components in the tested BDFs (Shiratori

**Concentration / wt % BDFs Palm-BDF Jatropha-BDF Soybean-BDF** 

nitrogen carrier gas with the flow rate of 50 ml min-1.

carbon deposition from the thermodynamic point of view.

H

directly to SOFC (Sasaki & Teraoka, 2003).

**4.2. Long term test of** *Bio***-SOFCs 4.2.1 IRSOFC operating on biogas** 

100

Biodiesel S/C = 0

et al., 2011).

Only a few reports have provided the performance of IRSOFCs operating on biogas (Staniforth et al., 2000; Shiratori et al., 2008; Girona et al., 2009; Lanzini & Leone, 2010a), because carbon formation thermodynamically can take place on the anode material. Staniforth et al. (2000) has reported the results of direct-feeding of landfill biogas (general CH4-rich biogas). However, that was a short term experiment and not continuous feeding of as-produced real biogas. To avoid coking, pre-reforming of biogas is generally required (Van herle et al., 2004b). Recently, development of a new fermentation path producing H2 rich biogas (Leone et al., 2010) and highly active catalysts to assist biofuel reforming (Xuan et al., 2009; Yentekakis et al., 2006; Zhou et al., 2007) have been reported. Heretofore, we have succeeded in stable operation of an IRSOFC running on actual biogas produced in waste treatment center using Ni-ScSZ cermet as an anode material without any support catalysts.

Figure 10a shows the cell voltages of IRSOFC operating on biogas measured at 200 mA cm-2. Simulated biogas with the CH4/CO2 ratio of 1.5 led to a stable cell voltage above 0.8 V for 800 h. The degradation rate of only 0.4 %/1000 h proved that the biogas-fueled SOFC can be operated with an internal reforming mode. For the real biogas generated from the methane fermentation reactor, rather high voltage comparable to that obtained by simulated biogas was achieved for 1 month, although there is a voltage fluctuation. Monitoring of biogas composition simultaneously with the cell voltage (Fig. 10b) revealed that voltage fluctuation (a maximum of 50 mV level) appeared in synchronization with the fluctuation of CH4/CO2 ratio. An abrupt increase in CO2 concentration induced temporarily decreased cell voltage. The CH4/CO2 ratio in the real biogas fluctuated between 1.4 and 1.7 which corresponds to CH4 concentration range between 58 and 63 vol%. The biogas composition is influenced by many factors, for example, type of organic wastes, physical states and operational conditions of methane fermentation such as temperature and pH of the waste slurry.

Fig. 10. Performance of IRSOFC operating on biogas (Shiratori et al., 2009a); (a) the results measured at 800 oC using anode-supported button cells and (b) voltage fluctuation in synchronization with the fluctuation of biogas composition during the long term test (initial 200 h of (a)).

Fig. 11. Anode-supported cells after long term test shown in Fig. 10a using (a) simulated and (b) practical biogases.

From the thermodynamic point of view, a CH4/CO2 ratio of 1.5 should cause coking at 800 oC, however carbon formation did not occur on the anode support for simulated biogas even after 800 h as shown in Fig. 11a. Provided that a fuel cell current is applied, this fuel composition (see Fig. 9) close to the border of the carbon deposition region may not cause coking. As for the real biogas, severe coking occurred during the long term test (Fig. 11b). The trigger of the coking was trace H2S (< 0.5 ppm) in the real biogas which can cause deactivation of Ni catalyst for the dry reforming of methane (Sasaki et al., 2011; Shiratori et al., 2008). Acceleration of carbon deposition in the presence of trace H2S was also reported for SOFCs operated with partially-reformed CH4-based fuels (Yuki et al., 2009). Although a threshold of maximum H2S concentration tolerance for Ni-stabilized zirconia cermet anode must be provided in future work, total sulfur concentration should be reduced to less than 0.1 ppm level by employing an optimum desulfurization process.

#### **4.2.2 IRSOFC operating on biodiesel fuel (BDF)**

The main reactions inferred in the steam reforming of BDF are listed in Table 8 (Nahar, 2010). Steam reforming (reaction 1) produces H2 and CO, and pyrolysis (reaction 2) produces light hydrocarbons (CxHy) and coke as well as H2 and CO may occur as competing reactions. Both reactions are endothermic, more promoted at higher temperatures. Steam reforming is a heterogeneous reaction catalyzed by Ni, whereas pyrolysis, a non-catalytic gas phase reaction, tends to occur at water-lean regions or on the deactivated surface of the Ni-based anode. Excess H2O reacts not only with the light hydrocarbons (reaction 3) but also with the produced CO (reaction 4) to form further H2. Reactions 5 and 6 are exothermic hydrogenation reactions which consume H2 and produce CH4. Reactions 7 and 8 are endothermic gasification reactions of coke. While S/C = 3.5 is thermodynamically out of the carbon deposition region (see Fig. 9), contributions of reactions 5, 7 and 8 are not negligible once carbon is deposited on the anode surface.


Table 8. Main reactions involved in steam reforming of biodiesel fuel (Nahar, 2010).

Fig. 11. Anode-supported cells after long term test shown in Fig. 10a using (a) simulated and

From the thermodynamic point of view, a CH4/CO2 ratio of 1.5 should cause coking at 800 oC, however carbon formation did not occur on the anode support for simulated biogas even after 800 h as shown in Fig. 11a. Provided that a fuel cell current is applied, this fuel composition (see Fig. 9) close to the border of the carbon deposition region may not cause coking. As for the real biogas, severe coking occurred during the long term test (Fig. 11b). The trigger of the coking was trace H2S (< 0.5 ppm) in the real biogas which can cause deactivation of Ni catalyst for the dry reforming of methane (Sasaki et al., 2011; Shiratori et al., 2008). Acceleration of carbon deposition in the presence of trace H2S was also reported for SOFCs operated with partially-reformed CH4-based fuels (Yuki et al., 2009). Although a threshold of maximum H2S concentration tolerance for Ni-stabilized zirconia cermet anode must be provided in future work, total sulfur concentration should be reduced to less than

The main reactions inferred in the steam reforming of BDF are listed in Table 8 (Nahar, 2010). Steam reforming (reaction 1) produces H2 and CO, and pyrolysis (reaction 2) produces light hydrocarbons (CxHy) and coke as well as H2 and CO may occur as competing reactions. Both reactions are endothermic, more promoted at higher temperatures. Steam reforming is a heterogeneous reaction catalyzed by Ni, whereas pyrolysis, a non-catalytic gas phase reaction, tends to occur at water-lean regions or on the deactivated surface of the Ni-based anode. Excess H2O reacts not only with the light hydrocarbons (reaction 3) but also with the produced CO (reaction 4) to form further H2. Reactions 5 and 6 are exothermic hydrogenation reactions which consume H2 and produce CH4. Reactions 7 and 8 are endothermic gasification reactions of coke. While S/C = 3.5 is thermodynamically out of the carbon deposition region (see Fig. 9), contributions of reactions 5, 7 and 8 are not negligible

1 CnHmO2 + (n-2)H2O (n+m/2-2)H2 + nCO Steam reforming

3 CxHy + xH2O xCO + (x+y/2)H2 Steam reforming 4 CO + H2O H2 + CO2 Water-gas shift 5 C + 2H2 CH4 Hydrogenation 6 CO + 3H2 CH4 + H2O Hydrogenation 7 C + H2O CO + H2 Coke gasification 8 C + CO2 2CO Boudouard-reaction Table 8. Main reactions involved in steam reforming of biodiesel fuel (Nahar, 2010).

2 CnHmO2 gases (H2, CO, CxHy) + coke Pyrolysis

0.1 ppm level by employing an optimum desulfurization process.

**4.2.2 IRSOFC operating on biodiesel fuel (BDF)** 

once carbon is deposited on the anode surface.

(b) practical biogases.

Figure 12 shows the cell voltage of an IRSOFC operating on wet palm-BDF (S/C = 3.5) measured at 0.2 A cm-2. Stable operation with a degradation rate of about 0.1 mV h-1 (approx. 1.5 %/100 h) was recorded. Total ohmic resistance of the cell, *R*IR, total polarization resistance, *R*P and the internal resistance of the cell, *R*int (the sum of *R*IR and *R*P), measured under open circuit condition are also plotted in Fig. 12. *R*IR and *R*P increased linearly with operating time. The increasing rate of *R*P, which is associated with activation and concentration overvoltages, is 0.15 m cm2 h-1, smaller than that of *R*IR (0.21 m cm2 h-1), indicating that the gradual loss of electrical contact was the main reason of the degradation. In this durability test, distinct morphology change and coking were not detected inside of the porous Ni-ScSZ anode support (see Fig. 14).

The composition of the anode off-gas (reformate gas) of IRSOFC operating on wet palm-BDF (S/C = 3.5) at 800oC was monitored during the durability test. In Table 9, the composition of the anode off-gas and open circuit voltage (OCV) just before the durability test are listed together with the equilibrium gas composition estimated by HSC 5.1 software (Outokumpu Research Oy, Finland) and the theoretical electromotive force calculated by Nernst equation. Internal steam reforming of palm-BDF led to H2 rich reformate gas, indicating that syngas as well as electricity can be obtained directly from BDF using high temperature SOFC. Measured concentrations of H2, CO and CO2 were close to the calculated values. The initial OCV, 0.93 V, was close to the thermodynamic value of 0.94 V. The existence of methane and ethylene indicates that the internal steam reforming has not reached equilibrium. Especially, ethylene is well known as a precursor of carbon deposition (Yoon et al., 2008, 2009).

Fig. 12. The result of long term test of IRSOFC operating on wet palm-BDF (S/C = 3.5) at 800 oC, showing cell voltage, total ohmic resistance *R*IR, total polarization resistance *R*P and internal resistance of the cell *R*int during the test.


Table 9. Anode off-gas composition and OCV for an IRSOFC operating on palm-BDF operated at 800oC just before the durability test shown in Fig. 12.

Fig. 13. Time dependence of OCV, fuel conversion and anode off-gas composition under open-circuit condition during the long term test of an IRSOFC operating on wet palm-BDF (S/C = 3.5) at 800oC shown in Fig. 12.

Figure 13 shows the concentrations of the gaseous species in the anode off-gas, fuel conversion and OCV during the long term test of Fig. 12. Although OCV was stable over the test, H2 production rapidly degraded within the first 400 h. At least 800 h was necessary for the stabilization of the internal steam reforming of palm-BDF.

After stopping the supply of palm-BDF, cell temperature was decreased to room temperature under the thorough N2 purging of the anode compartment. FESEM images of the (a) surface and (b) cross section of the anode support after the long term test are shown in Fig. 14. Carbon deposition occurred only on the surface of the anode which is most susceptible to coking due to the highest concentration of long chain hydrocarbons or lowest S/C, whereas inside of the porous anode support there was no coke. The occurrence of electrochemical consumption of H2 leading to an increase in local S/C and direct electrochemical consumption of carbon may prevent coking inside of the anode. Carbon deposited on the anode surface can push up the current collector mesh attached to the anode surface resulting in the *R*IR increase shown in Fig. 12.

Fig. 14. FESEM images of (a) surface and (b) inside of the porous anode support after 800 h test of IRSOFC operating on wet palm-BDF (S/C = 3.5) at 0.2 A cm-2 and 800oC.

#### **4.3 Problems to be solved for the realization of** *Bio***-SOFCs**

The feasibility of an IRSOFC running on low-grade biofuels has been demonstrated in the previous research (Shiratori et al., 2010a, 2010b, 2011) using anode-supported button cells. However, as illustrated in Fig. 15, in the real SOFC system a strong temperature gradient along gas flow direction exists and can cause cell fracture.

OCV

OCV / V

0.0

Ni ScSZ

(a) (b)

10 m

0.2

0.4

0.6

0.8

1.0

Time / h 0 200 400 600 800

Figure 13 shows the concentrations of the gaseous species in the anode off-gas, fuel conversion and OCV during the long term test of Fig. 12. Although OCV was stable over the test, H2 production rapidly degraded within the first 400 h. At least 800 h was necessary for

After stopping the supply of palm-BDF, cell temperature was decreased to room temperature under the thorough N2 purging of the anode compartment. FESEM images of the (a) surface and (b) cross section of the anode support after the long term test are shown in Fig. 14. Carbon deposition occurred only on the surface of the anode which is most susceptible to coking due to the highest concentration of long chain hydrocarbons or lowest S/C, whereas inside of the porous anode support there was no coke. The occurrence of electrochemical consumption of H2 leading to an increase in local S/C and direct electrochemical consumption of carbon may prevent coking inside of the anode. Carbon deposited on the anode surface can push up the current collector mesh attached to the

Fig. 13. Time dependence of OCV, fuel conversion and anode off-gas composition under open-circuit condition during the long term test of an IRSOFC operating on wet palm-BDF

H2

CH CO2 <sup>4</sup> + C2H4

Fuel conversion

CO

0

the stabilization of the internal steam reforming of palm-BDF.

anode surface resulting in the *R*IR increase shown in Fig. 12.

Pt grid Carbon

**4.3 Problems to be solved for the realization of** *Bio***-SOFCs** 

along gas flow direction exists and can cause cell fracture.

500 m

test of IRSOFC operating on wet palm-BDF (S/C = 3.5) at 0.2 A cm-2 and 800oC.

Fig. 14. FESEM images of (a) surface and (b) inside of the porous anode support after 800 h

The feasibility of an IRSOFC running on low-grade biofuels has been demonstrated in the previous research (Shiratori et al., 2010a, 2010b, 2011) using anode-supported button cells. However, as illustrated in Fig. 15, in the real SOFC system a strong temperature gradient

20

40

Concentration, Fuel conversion / %

(S/C = 3.5) at 800oC shown in Fig. 12.

60

80

100

Fig. 15. Schematic view of *Bio*-SOFC and major problems to be solved.

The area near the fuel inlet is cooled down due to the strong endothermicity of reforming reactions (dry and steam reforming reactions of hydrocarbons), whereas cell temperature is gradually elevated toward the gas outlet by the exothermic electrochemical reactions. It is thermodynamically expected that impurity poisoning and carbon deposition are more significant at the cooled area.

#### **4.3.1 Impurity poisoning (a case study of biogas operation)**

Generally biofuels including biogas and BDFs contain several kinds of impurities such as sulfur compounds. According to thermochemical calculations (Haga et al., 2008), sulfur compounds exist as H2S at SOFC operational temperatures in equilibrium. Here, in order to investigate the influence of a fuel impurity, 1 ppm H2S was mixed with simulated biogas mixture (CH4/CO2 = 1.5) during the galvanostatic measurement under 200 mA cm-2. CO yield and selectivity were measured simultaneously with the electrochemical measurements. CO yield and selectivity are defined as

$$\text{CO yield} = v\_{\text{CO}} / \text{ (f}\_{\text{CH4}} + f\_{\text{CO2}}) \tag{6}$$

$$\text{CO selectionity} = v\_{\text{CO}} \,/\, \text{(}\upsilon\_{\text{CH4}} + \upsilon\_{\text{CO2}}\text{)}\tag{7}$$

where *v*CO is CO formation rate, and *f*CH4 and *f*CO2 are feeding rates of CH4 and CO2, respectively, and *v*CH4 and *v*CO2 are consumption rates of CH4 and CO2, respectively. *v*CO, *v*CH4 and *v*CO2 were estimated from the results of the exhaust gas analysis. The results of H2S poisoning test at 1000 oC are summarized in Fig. 16.

In this experiment, an electrolyte-supported cell was used to measure anodic overvoltage separately from cathodic overvoltage using a Pt reference electrode (Shiratori, 2008). The horizontal axis indicates the time after starting poisoning. As shown in Fig. 16a even if 1 ppm H2S was included in biogas, a cell voltage of about 1 V was stable during 20 h operation while voltage drop of about 100 mV (9 % of initial cell voltage) occurred in the first 1 h. The voltage drop was caused by increase in anodic overvoltage. Just after starting 1 ppm H2S poisoning, anodic overvoltage grew to be 3.6 times larger compared to the initial value, which would be due to sulfur surface coverage of Ni catalysts. On the other hand, the anode-side IR drop did not change by the H2S poisoning.

Fig. 16. Electrochemical and catalytic performance of Ni-ScSZ anode during 1 ppm H2S poisoning measured at 1000 oC under 200 mA cm-2; (a) cell voltage, anodic overvoltage (IR free) and anode-side IR drop and (b) reaction rates, CO yield and CO selectivity for internal dry reforming of methane. Simulated biogas mixture, CH4/CO2 = 1.5, was fed as a fuel (Shiratori et al., 2008).

The reaction rates of internal reforming, CO yield and CO selectivity during 1 ppm H2S poisoning test are plotted on Fig. 16b. Rapid deactivation of the reforming reaction was observed within 2 h after starting H2S poisoning resulting in about a 40 % decrease in reaction rates and CO yield. On the other hand, CO selectivity was less sensitive to H2S contamination (only 7 % decrease). After the initial deactivation of the catalytic activity, a quasi-stable state appeared. Fig. 17 schematically depicts the degradation mechanism of an IRSOFC operating on biogas under H2S contamination. Sulfur species chemisorbed on Ni catalyst not only deactivate reforming reaction but also block the triple phase boundary (TPB) which is the active reaction region for electrochemical oxidation of produced H2 and CO. Deactivation of reforming reaction causes deficiency of H2 and CO, and blockage of TPB sites causes an increase in local current density. These phenomena would appear as an initial increase in anodic overvoltage (or initial voltage drop). Cell voltage, CO yield and CO selectivity completely recovered within 4h after stopping H2S poisoning, indicating that H2S poisoning caused by adsorption of sulfur species is a reversible process and 1 ppm level H2S contamination is not fatal for the operation of *Bio*-SOFC at 1000 oC.

On the contrary, at 800 oC, 1 ppm H2S was more detrimental. Cell voltage and internal reforming rates kept on decreasing during 22 h of H2S poisoning. Voltage drop of about 170 mV (20 % of initial cell voltage) and 80 % decrease in the reaction rates occurred without the quasi-stable state as in the case of 1000 oC testing. The results summarized in Table 10 indicate that higher-grade desulfurization is required for lower operating temperatures, suggesting that the effect of impurity poisoning will become more detrimental at the cooled area in the cell (see Fig. 15).

operation while voltage drop of about 100 mV (9 % of initial cell voltage) occurred in the first 1 h. The voltage drop was caused by increase in anodic overvoltage. Just after starting 1 ppm H2S poisoning, anodic overvoltage grew to be 3.6 times larger compared to the initial value, which would be due to sulfur surface coverage of Ni catalysts. On the other hand, the

Voltage losses / V

CO yield, CO selectivity / %

1 ppm H2S

temp: 1000 oC current density: 200 mA cm-2 fuel: simulated biogas (CH4/CO2 = 1.5)

0 2 4 6 8 10 12 14 16 18 20

Time / h

CO selectivity CO yield H2 formation rate CO formation rate CH4 consumption rate CO2 consumption rate

Reaction rate / mol s-1 cm-2

0.00 0.05 0.10 0.15 0.20 0.25 0.30

(a) (b)

Fig. 16. Electrochemical and catalytic performance of Ni-ScSZ anode during 1 ppm H2S poisoning measured at 1000 oC under 200 mA cm-2; (a) cell voltage, anodic overvoltage (IR free) and anode-side IR drop and (b) reaction rates, CO yield and CO selectivity for internal dry reforming of methane. Simulated biogas mixture, CH4/CO2 = 1.5, was fed as a fuel

The reaction rates of internal reforming, CO yield and CO selectivity during 1 ppm H2S poisoning test are plotted on Fig. 16b. Rapid deactivation of the reforming reaction was observed within 2 h after starting H2S poisoning resulting in about a 40 % decrease in reaction rates and CO yield. On the other hand, CO selectivity was less sensitive to H2S contamination (only 7 % decrease). After the initial deactivation of the catalytic activity, a quasi-stable state appeared. Fig. 17 schematically depicts the degradation mechanism of an IRSOFC operating on biogas under H2S contamination. Sulfur species chemisorbed on Ni catalyst not only deactivate reforming reaction but also block the triple phase boundary (TPB) which is the active reaction region for electrochemical oxidation of produced H2 and CO. Deactivation of reforming reaction causes deficiency of H2 and CO, and blockage of TPB sites causes an increase in local current density. These phenomena would appear as an initial increase in anodic overvoltage (or initial voltage drop). Cell voltage, CO yield and CO selectivity completely recovered within 4h after stopping H2S poisoning, indicating that H2S poisoning caused by adsorption of sulfur species is a reversible process and 1 ppm level H2S contamination is not fatal for the operation of *Bio*-

On the contrary, at 800 oC, 1 ppm H2S was more detrimental. Cell voltage and internal reforming rates kept on decreasing during 22 h of H2S poisoning. Voltage drop of about 170 mV (20 % of initial cell voltage) and 80 % decrease in the reaction rates occurred without the quasi-stable state as in the case of 1000 oC testing. The results summarized in Table 10 indicate that higher-grade desulfurization is required for lower operating temperatures, suggesting that the effect of impurity poisoning will become more detrimental at the cooled

anode-side IR drop did not change by the H2S poisoning.

temp: 1000 oC current density: 200 mA cm-2 fuel: simulated biogas (CH4/CO2 = 1.5)

0 2 4 6 8 10 12 14 16 18 20

anodic overvoltage

anode-side IR drop

Time / h 1 ppm H2S

(Shiratori et al., 2008).

SOFC at 1000 oC.

area in the cell (see Fig. 15).

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Cell voltage / V

Fig. 17. Possible degradation mechanism of IRSOFC operating on biogas caused by H2S contamination of biogas (Shiratori et al., 2008).


Table 10. Impact of 1 ppm H2S contamination of biogas on the performance of IRSOFC running on biogas deduced by the 22 h poisoning test. Fuel: simulated biogas mixture (CH4/CO2 = 1.5), Cell: Ni-ScSZ/ScSZ/LSM-ScSZ (Shiratori et al., 2009a).

#### **4.3.2 Strong temperature gradient (a case study of biogas operation)**

Ni-ScSZ/ScSZ/LSM-ScSZ square-shaped cells with an area of 25 cm2 simulating a real SOFC were used for the evaluation of thermomechanical reliability of a *Bio*-SOFC. When the simulated biogas (CH4/CO2 = 1.5) was directly fed to the square-shaped cell, long term operation could not be performed at 800 oC (Shiratori et al., 2010b). This is due to a locallydecreased cell temperature caused by the strong endothermicity of internal reforming (Fig. 15).

Fig. 18. (a) The temperature distribution caused by internal dry reforming of methane and (b) resulting cell fracture detected by FESEM, showing brittleness of electrolyte thin film sintered on the anode support versus direct supply of hydrocarbon fuel.

Fig. 19. The temperature distribution in the Ni-ScSZ anode support when simulated biogases with CH4/CO2 = 1.5 were fed directly with a furnace temperature of 800 oC under open circuit condition; (a) Air/Biogas =0, (b) 1.0 and (c) 1.5.

To estimate the endothermicity of reaction 1 in Table 7, an equimolar mixture of CH4 and CO2 was supplied to the square-shaped anode-supported half cell, and the temperature distribution generated in the half cell was measured from the electrolyte side by thermography. As shown in Fig. 18a, a large temperature gradient was generated at the fuel inlet side, and the temperature around the cooled area considerably increased within 20 min, indicating the formation of an electrolyte crack (Fig. 18b) from which biogas leaked and burned. When simulated biogas with CH4/CO2 = 1.5 was used as a fuel, at a furnace temperature of 800 oC, as shown in Fig. 19a, a strong temperature gradient formed in the half cell and led to an electrolyte crack within 15 min after starting the supply of simulated biogas. The temperature gradient became more moderate with air addition to the biogas, as can be seen in Fig. 19b and c, for air/biogas of 1.0 and 1.5, respectively, at the same furnace temperature.


Table 11. Main reactions involved in the reformig of air-mixed biogas and their reaction enthalpies at 1100 K (D.R. Lide (Ed.), 2009).

The above results indicate that cell performance can be stabilized by addition of air to biogas as a result of the exothermicity of partial oxidation of CH4. Here, heat absorption accompanied by the internal reforming of air-mixed biogas was calculated thermodynamically for the feed of 1 kmol C considering the reactions listed in Table 11. Reactions 1-3 are the predominant reactions determining the amount of heat absorption. Reaction 1 is dry reforming of CH4, producing H2 and CO, which is a strong endothermic reaction. Reactions 2 and 3 are oxidation reactions of CH4 which can contribute to the suppression of the strong endothermicity of reaction 1. The results of the thermodynamic calculation are plotted on Fig. 20.

Fig. 20. Endothermicity of reforming of air-mixed biogases with Air/Biogas = 0, 1.0 and 1.5.

Fig. 20 suggests that heat absorption or endothermicity can be suppressed by increasing the air mixing ratio or by decreasing the operational temperature. A homogeneous temperature distribution obtained for Air/Biogas = 1.5 (Fig. 19c) as compared to Air/Biogas = 0 (Fig. 19a) is due to a 48 % lower heat absorption. Here, it is noted that the addition of excess air will result in the reduction of reforming efficiency, and lower operating temperature tends to cause carbon formation. The optimum Air/Biogas and operating temperature must be determined carefully, considering these points.

#### **4.3.3 Carbon deposition**

#### **4.3.3.1 Biogas**

184 Renewable Energy – Trends and Applications

In Out In Out In Out

In Out In Out In Out

In Out In Out In Out

To estimate the endothermicity of reaction 1 in Table 7, an equimolar mixture of CH4 and CO2 was supplied to the square-shaped anode-supported half cell, and the temperature distribution generated in the half cell was measured from the electrolyte side by thermography. As shown in Fig. 18a, a large temperature gradient was generated at the fuel inlet side, and the temperature around the cooled area considerably increased within 20 min, indicating the formation of an electrolyte crack (Fig. 18b) from which biogas leaked and burned. When simulated biogas with CH4/CO2 = 1.5 was used as a fuel, at a furnace temperature of 800 oC, as shown in Fig. 19a, a strong temperature gradient formed in the half cell and led to an electrolyte crack within 15 min after starting the supply of simulated biogas. The temperature gradient became more moderate with air addition to the biogas, as can be seen in Fig. 19b and c, for air/biogas of 1.0 and 1.5, respectively, at the same furnace

Fig. 19. The temperature distribution in the Ni-ScSZ anode support when simulated biogases with CH4/CO2 = 1.5 were fed directly with a furnace temperature of 800 oC under

1 CH4 + CO2 → 2H2 + 2CO Dry reforming of methane 259 2 CH4 + 1/2O2 → 2H2 + CO Partial oxidation of methane -23 3 CH4 + 2O2 → 2H2O + CO2 Complete oxidation of methane -802 4 CH4 + H2O → 3H2 + CO Steam reforming of methane 226 5 CH4 → C + 2H2 Methane cracking 90 6 2CO → C + CO2 Boudouard reaction -170 7 CO + H2O → H2 + CO2 Water-gas-shift reaction -34 8 CO + 1/2O2→ CO2 Oxidation of carbon monoxide -282 9 H2 + 1/2O2→ H2O Oxidation of hydrogen -248

Table 11. Main reactions involved in the reformig of air-mixed biogas and their reaction

enthalpies at 1100 K (D.R. Lide (Ed.), 2009).

Electrolyte crack

Crack15 min 30 min

15 min 30 min

15 min 30 min

**Reaction Reaction enthalpy** 

825oC

800oC

775oC

825oC

800oC

775oC 825oC

800oC

775oC

**at 1100 K / kJ mol-1**

CH4/CO2 = 1.5 Air/Biogas = 0

(a)

(b)

(c)

CH4/CO2 = 1.5 Air/Biogas = 1.0

CH4/CO2 = 1.5 Air/Biogas = 1.5

temperature.

5 min

5 min

5 min

open circuit condition; (a) Air/Biogas =0, (b) 1.0 and (c) 1.5.

The result of a long term test with a square-shaped cell is shown in Fig. 21a (Shiratori et al., 2010b). A degradation rate of 2.6 %/1000 h (200-500 h) was achieved by feeding air-mixed simulated biogas. The reasons for the degradation were a decrease in OCV (contribution ratio = 38 %) caused by incomplete gas sealing around the cell and increases in IR loss (34 %) and overvoltage (29 %) caused by insufficient electrical contact between the anode support and current collector. These voltage losses are related to technical difficulties of single cell testing with a square-shaped cell.

Fig. 21. Long-term test of an IRSOFC operating on biogas using a 5 x 5 cm2 square-shaped cell: (a) galvanostatic measurement of cell voltage in air-mixed simulated biogas (CH4/CO2 = 1.5, Air/Biogas = 1) under *U*f = 13 % at 800 oC and (b) the equilibrium gas composition calculated by HSC 5.1 software (Outokumpu Research Oy, Finland) for 0.6 kmol CH4 + 0.4 kmol CO2 + 0.2 kmol O2 + 0.8 kmol N2.

As shown in Fig. 21b, at 800 oC (operational temperature in the present study) direct feeding of air-mixed biogas (CH4/CO2 = 1.5, Air/Biogas = 1) thermodynamically does not cause carbon deposition. The safe temperature region for this fuel is above 750 oC, and a decrease in cell temperature over 50 K from the operational temperature of 800 oC may cause carbon deposition. After the 500 h durability test, carbon deposition was observed only at the fuel inlet side where the anode-support may be cooled down by more than 50 K as a result of the endothermic reforming reaction (Shiratori et al., 2010b). According to the thermodynamic calculation shown in Fig. 20, only 26% reduction of heat absorption is expected by the addition of an equimolar amount of air to biogas (Air/Biogas = 1.0). These results suggest that Air/Biogas = 1.0 is a thermodynamically safe composition if the temperature of 800 oC is kept anywhere in the anode-support, however taking the temperature gradient caused by the internal reforming into account, Air/Biogas = 1.0 leads to a decrease in the cell temperature at the fuel inlet side by more than 50 K, but is insufficient, Air/Biogas ratio higher than 1.0 is required practically.

#### **4.3.3.2 Biodiesel fuels (BDFs)**

Figure 22 shows the results of galvanostatic measurements at three different operating temperatures for palm-, jatropha- and soybean-BDFs under the condition of 200 mA cm-2 and S/C = 3.5. Stable voltage with little oscillation was obtained only for palm-BDF at 800 oC. In contrast, jatropha- and soybean-BDFs resulted in unstable cell voltage. Especially, at 700 oC, cell voltage for the SOFCs operating on jatropha- and soybean-BDFs dropped abruptly within 40 h and 47 h, respectively. No severe coking was observed at 800 oC in the case of palm-BDF, whereas jatropha- and soybean-BDFs led to significant amount of carbon on the anode surface. Carbon deposition tended to be more significant at lower operating temperatures and at higher content of unsaturated FAMEs in BDF.

Figure 23 shows the anode surface after the operation with real biodiesels at 800 oC. Nearly no carbon was observed at this temperature in the case of palm-BDF, whereas jatropha- and soybean-BDFs led to significant amount of carbon on the anode surface. Carbon deposition tended to be more significant for the fuels with a higher degree of unsaturation (Nahar, 2010). The rapid degradation is due to the carbon deposition promoted with a higher degree of unsaturation which can cover a portion of active reaction sites or block open pores.

Fig. 22. Cell voltage of IRSOFCs operating on (a) palm-, (b) jatropha- and (c) soybean-BDFs at different temperatures under the condition of S/C = 3.5 and 200 mA cm-2.

Fig. 23. Pictures of the anode after the 50 h tests of IRSOFCs operating on BDFs at 800 oC shown in Fig. 22; (a), (b) and (c) are for palm-, jatropha- and soybean-BDFs, respectively.
