CH4 oxidation

Thus indeed CH4 oxidation in a SOFC with a Ni/YSZ anode results into partial oxidation and the production of synthesis gas, instead of generation of C02 and H20 as originally believed. The latter happens only at near-complete CH4 conversion. However the partial oxidation overall reaction (3.12) is not the result of a partial oxidation electrocatalyst but rather the result of the catalytic reactions (3.9) to (3.11) coupled with the electrocatalytic reaction (3.8). From a thermodynamic viewpoint the partial oxidation reaction (3.12) is at least as attractive as complete oxidation to C02 and H20. 

Figure 4.28. Electrophobic behaviour Effect of catalyst work function on the activation energy E and catalytic rate enhancement ratio r/r0 for C2H4 oxidation on Pt p02 4.8 kPa, Pc2H4 0.4 kPa (a) and CH4 oxidation on Pt p02 =2.0 kPa, Pch4 =2.0 kPa (b)."4 Reprinted with permission from Elsevier Science.

It has been known since the early days of electrochemical promotion that upon varying Uwr and thus , not only the catalytic rates, r, are changing in a frequently dramatic manner, but also the activation energy of the catalytic reaction is also significantly affected. An example was already presented in Fig. 4.28 which shows that both C2H4 and CH4 oxidation on Pt/YSZ conform to equation (4.50) with an values of -1 and -3, respectively. 

Figure 8.22. Effect of step changes in applied positive and negative currents on Uwr and r during CH4 oxidation on Pt/YSZ29 at two different volumetric flowrate Fv showing that x is influenced by I but not by Fv.29 Reprinted with permission from Academic Press.

The CH4 oxidation on Pd31 exhibits a very pronounced NEMCA behavior at much lower temperatures (380-440°C) compared with those on Pt catalysts (650-750°C). In this temperature range the reaction exhibits inverted volcano behavior.31 For positive overpotentials the p values are as high as 89, with A values up to 105.31 Negative overpotentials also enhance the rate31 with p values up to 8. 

Figure 8.23. Effect of catalyst potential and work function on the rate of CH4 oxidation to C02 on Pt for a low (1 1) CH4 to 02 feed ratio. Maximum methane conversion is 4%. pCH4= Po2as2 kPa, T, °C, r0, mol O/s.29 Reprinted with permission from Academic Press.

Figure 8.24. Effect of catalyst potential Uwr and work function change (vs 1=0) on the activation energy E and preexponential factor K° of the kinetic constant K of CH4 oxidation to C02 an average T value of 948 K is used in the rhs ordinate p°H4 =p°2 =2kPa, 29Reprinted with permission from Academic Press.

Figure 8.26. Reaction rate dependence on pCH4 at constant po2 -2 kPa (a) and p02 at constant Pch4=2 kPa (b) for open circuit conditions (circles), UWr=+500 mV (squares) and Uwr=+1000 mV (triangles) during CH4 oxidation on Pd/YSZ, T=400°C Reprinted with permission from Elsevier Science.

With respect to CO oxidation an activity order similar to that described above for CH4 combustion has been obtained. A specific activity enhancement is observed for Lai Co 1-973 that has provided a 10% conversion of CO already at 393 K, 60 K below the temperature required by LalMnl-973. This behavior is in line with literature reports on CO oxidation over lanthanum metallates with perovskite structures [17] indicating LaCoOs as the most active system. As in the case of CH4 combustion, calcination at 1373 K of LalMnl has resulted in a significant decrease of the catalytic activity. Indeed the activity of LalMnl-1373 is similar to those of Mn-substituted hexaaluminates calcined at 1573 K. Dififerently from the results of CH4 combustion tests no stability problems have been evidenced under reaction conditions for LalMnl-1373 possibly due to the low temperature range of CO oxidation experiments. Similar apparent activation energies have been calculated for all the investigated systems, ranging from 13 to 15 Kcal/mole, i.e almost 10 Kcal/mole lower than those calculated for CH4 oxidation. 

From the results discussed above as well as from the literature data [5-10,12-14] it follows that an important role of O2 in the SCR process is to convert NO into NOj. The latter then initiates methane oxidation into CO, and is itself reduced into NO and N2. Both NO, and O2 may participate in CH4 oxidation (Fig. 1B) and the ratio between the rates of these competitive oxidation reactions will be critical for the selectivity of the SCR process. Hence, the absolute rates of CH4 oxidation by Oj were compared with those occurring in the SCR process. The rates of these reactions were determined under different reaction conditions (using the... 

Figure 2. Arrhenius plots of differential rates of NO reduction ( ) and of CH4 oxidation (o) during the SCR reaction, and for CH4oxidation by Oj alone (A) over CoZSM-5 (A) and HZSM-5 (B) catalysts. Feed contained 0.28% CH4, 0.21 % NO (when used) and 2.6% O2 in He.

Fig. IB shows that at all temperatures the rate of CH4 oxidation by O2 alone is lower than the rate of CH4 oxidation during the SCR reaction, e.g., at 400 C with CoZSM-5 catalyst the difference between these rates is about 10 times. With increasing temperature this difference diminishes due to the different activation energies of these reactions (Fig. 2). At high temperatures these rates become comparable (in considering Figs. IB and 2 recall that the rate of CH4 oxidation during the SCR process includes a contribution from the rate of CH4 oxidation by O2 alone). These data suggest that below 500 C O2 does not compete effectively with NO, for CH4, but that at high temperatures such a competition must exist. The data of Table I support this view. At ADO C an increase in 62 concentration results in an increase in conversions of both NO into N2 and CH4 into CO2. At the same time, variation of Oj concentration by a factor of 13 has practically no effect on the...

Cultivar differences, however, extend beyond the impact of biomass production on emissions. Ma et al. [131] observed a 67% increase in CH4 oxidation from a hybrid cultivar accompanied... 

Kruger M, Eller G, Conrad R, Frenzel P. Seasonal variation in pathways of CH4 production and in CH4 oxidation in rice fields determined by stable carbon isotopes and specific inhibitors. Global Change Biol. 2002 8 265-280. 

Bender M, Conrad R. Kinetics of CH4 oxidation in oxic soils exposed to ambient air or high CH4 mixing ratios. FEMS Microbiol. Ecol. 1992 101 261-270. 

The free-radical chemistry was studied using a zerodimensional box-model based upon the Master Chemical Mechanism (MCM). Two versions of the model were used, with different levels of chemical complexity, to explore the role of hydrocarbons upon free-radical budgets under very clean conditions. The detailed model was constrained to measurements of CO, CH4 and 17 NMHCs, while the simple model contained only the CO and CH4 oxidation mechanisms, together with inorganic chemistry. The OH and HO2 (HOx) concentrations predicted by the two models agreed to within 5-10%. 

The simple model contained the same inorganic and CO-CH4 oxidation schemes as the detailed model, taken from the MCMv3. The model was completed with heterogeneous loss and dry deposition terms, as described in the following section. The chemical mechanism employed in the simple model contains 75 gas-phase reactions, 9 heterogeneous and 8 deposition processes and is shown in Table 7. 

The mechanisms of CH4 oxidation covered in this section appear to be most appropriate, but are not necessarily definitive. Rate constants for various individual reactions could vary as the individual steps in the mechanism are studied further. 

Figure 6-15 Average Cell Voltage of a 0.37m 2 kW Internally Reformed MCFC Stack at 650°C and 1 atm. Fuel, 100% CH4, Oxidant, 12% C02/9% 02/77% N2 (12)...

This reaction is thermally neutral. The heat absorbed in the CH4 reforming reaction is released by the subsequent reaction of the H2 product at the anode of the fuel cell. If, therefore, the reforming process can be carried out in close proximity to and in thermal contact with the anode process, the thermal neutrality of the overall CH4 oxidation process can be approximated. And the heat removal and recovery process for the fuel cell system can deal merely with the heat produced by its operational irreversibilities. 

Reaction Potentials. The reaction potentials Vm. Vq and Vr are the rates at which methanogenesis, CH4 oxidation and oxic respiration would proceed in situ were all enzymes saturated with the necessary substrates. They depend on in situ enzyme concentrations and hence on in situ microbial populations. They change over time. 

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