Three Way Catalysts for Methane

Hannu Jääskeläinen

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Abstract: Methane emissions from stoichiometric natural gas engines can be converted using three-way catalysts (TWC). The engine air-to-fuel ratio must be accurately controlled to achieve high conversion rates. Both Pd/Rh and Pt/Rh catalyst systems have been used in natural gas applications. Methane slip and ammonia emissions may occur in some TWC systems.

Methane Conversion over TWC

In the absence of excess oxygen in the exhaust gas, methane can be controlled using a three-way catalyst (TWC). When applied to stationary engines, the technology is also known as Non-Selective Catalytic Reduction (NSCR). In the TWC, methane, other HCs, CO and NOx can be efficiently converted to CO2, H2O and N2. However, the engine must be operated within a narrow air-to-fuel ratio window—near stoichiometric conditions—to ensure high conversions.

While three way catalysts (TWC) have been a common feature on gasoline fueled vehicles for many decades, their application to natural gas engine exhaust presents some unique challenges. Figure 1 presents the post catalyst concentrations in simulated exhaust gas under static composition conditions. Propene (C3H6) and methane are used to simulate the unburned hydrocarbons from gasoline and natural gas engine exhaust respectively. Under warmed up conditions, CO and NOx conversion for both cases follow similar trends: CO conversion is close to 100% lean of stoichiometry and NOx conversion is close to 100% on the rich side. One notable difference is the point at which the conversion of both is maximum; this occurs at stoichiometry for C3H6 but rich of stoichiometry for CH4 [3686].

Figure 1. Post catalyst concentrations of CO, NO, C3H6, CH4, O2, H2, NH3 and N2O at 425°C

Continuous feed of (a) 7000 ppm CO, 1600 ppm NO, 500 ppm C3H6 (equivalent to1500 ppm C1), 5 vol% H2O and (b) 7000 ppm CO, 1600 ppm NO, 1500 ppm CH4, 5 vol% H2O while decreasing the O2 concentration from 7000 to 0 ppm. Full-size honeycomb Pd-only three-way catalyst (56.6 g/ft3 Pd; 600 cpsi) from Umicore containing Al, Ce, Zr and promoters in unknown ratios. Degreened in static air at 600°C for 10 h. The vertical dash lines mark the position of λ = 1.

The hydrocarbon conversion is quite different for the two cases. For C3H6, the hydrocarbon conversion is quite high on both sides of stoichiometry. Steam reforming, Equation (1), is responsible for the high conversion under reducing conditions while oxidation, Equation (2), likely dominates under oxidizing conditions.

(1)CH4 + H2O = CO + 3H2

(2)CH4 + 2O2 = CO2 + 2H2O

The maximum CH4 conversion (~60%) is lower than for C3H6 and is maximum rich of stoichiometry at a very narrow band of [O2] and decreases as one moves leaner or richer from here. The drop on the lean side is very rapid. Inhibition by NO was considered to be the primary reason for the loss of activity lean of the point of maximum CH4 conversion [3686].

Figure 1 shows a more gradual drop in CH4 conversion as one moves richer from stoichiometry that has been suggested as being due to inhibition by CO. The maximum CH4 conversion point corresponds to the point where CO is completely oxidized (i.e., RO2/nM = 2[O2]/[CO] =1]. At a lower temperature of 350°C (not shown), the CO/NO crossover point and the maximum conversion point for methane shift to lower [O2] than RO2/nM = 1. This suggests that the active sites for CH4 conversion need to be freed from competing reaction involving CO and NO [3686].

The very narrow range of air fuel ratio over which significant CH4 conversion is possible implied by Figure 1 would be a very challenging control problem. However, in commercial engines operating near stoichiometry, air fuel ratio oscillates about an average value. The effect of this oscillation is shown in Figure 2 which compares the static conversion of methane in Figure 1 to conversions where [O2] is varied at differing amplitudes about an average value. Clearly, oscillating the air-fuel ratio (AFR) is beneficial for methane conversion. Increasing the amplitude of the oscillations increases the maximum conversion, widens the AFR window to achieve a given conversion and moves the maximum conversion closer to the stoichiometric AFR. An increased amplitude of [O2] lowers the CO levels at low [O2] and shifts the CO/NO crossover point to higher [O2]; thus the trends in CH4 conversion are consistent with inhibition by CO and NO.

Figure 2. Effect of oscillations in O2 concentration on CH4 and NO conversions at 425°C

Other feed gas components: 7000 ppm CO, 1600 ppm NO, 1500 ppm CH4, 5 vol% H2O. (□) Continuous feed of O2 (○) ±1000 ppm O2, (△) ±2000 ppm O2 and (◇) ±3000 ppm O2. Symmetric O2 pulses with 5 s period. Same catalyst as in Figure 1. The vertical dash lines mark the positions of RO2/nM = 1 and λ = 1.