**5. Conclusion**

186 Renewable Energy – Trends and Applications

Amount / kmol

0.0

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

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

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

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,

temperatures and at higher content of unsaturated FAMEs in BDF.

0.5

C

1.0

1.5

2.0

100 200 300 400 500 600 700 800 900 1000

CO2(g) CH4(g)

CO(g) H2O(g) N2(g)

Fuel: air-mixed biogas (CH4/CO2 = 1.5, Air/Biogas = 1.0)

H2(g)

Temperature / oC

0 50 100 150 200 250 300 350 400 450 500 0.0

Temp: 800 oC

Current: 2.0 A

Uf : 13 %

kmol CO2 + 0.2 kmol O2 + 0.8 kmol N2.

higher than 1.0 is required practically.

**4.3.3.2 Biodiesel fuels (BDFs)** 

Time / h

Current density: 125 mA cm-2

(200-500 h)

Cell type: 5 x 5 cm2 square-shaped cell Fuel: air-mixed simulated biogas (CH4/CO2 = 1.5, Air/Biogas = 1.0)

Degradation rate 2.6 % / 1000 h

(a) (b)

0.2

0.4

0.6

Cell voltage / V

0.8

1.0

The fuel flexibility of SOFCs has been demonstrated in this study with biofuels, biogas and biodiesel fuels (BDFs). In the course of this study, roadblocks for the realization of an internal reforming SOFC (IRSOFC) operating on biofuels, *Bio*-SOFC, which is promising candidate for a distributed generator in a coming carbon-neutral society, were uncovered. A biogas-fueled SOFC can be operated internal reforming mode if an adequate amount of air is added to the biogas. To suppress carbon deposition and thermal stress caused by the internal reforming reaction, Air/Biogas > 1.0 is required. Although the feasibility of an IRSOFC running on biogas has been demonstrated in the previous research, there is an urgent need to collect more practical data using a real stack. On the other hand, performance of an IRSOFC operating on BDFs has also been evaluated. The deactivation of the anode was accompanied by significant carbon deposition on the Ni-based anode material, which occurs more for lower operating temperature and higher concentration of unsaturated fatty acid methyl esters (FAMEs) in BDF. Only the IRSOFC operating on palm-BDF with the lowest degree of unsaturation operated at 800 oC exhibited stable performance without severe coking. The concentration of unsaturated FAMEs in BDF is quite an important factor to determine SOFC performance, and therefore it should be controlled carefully. In terms of fuel injection performance, a higher concentration of unsaturated components resulting in lower viscosity of BDFs is preferable, however significant carbon deposition will occur as mentioned above.

Our final goal is to operate an IRSOFC using low grade biofuels. BDF derived from waste cooking oil which has a similar chemical composition to that of palm-BDF is quite an attractive candidate for SOFC operation, although the fuel composition fluctuates depending on the origin of the oil and kind of food cooked by the oil. On the other hand, south-east Asian nations near the Mekong basin are very interested in the efficient use of their abundant fishery resources as alternative fuels. BDF derived from discarded catfish oil is one of the most attractive alternative fuels, however it contains not only FAMEs with high number of carbon atom between 20 and 24 in a molecule, but also several trace impurities which promote carbon deposition. In future work, the dependence of IRSOFC performance on composition of BDFs will be investigated.
