Lean, exhaust gas composition

These reactions occur especially with stoichiometric and with lean exhaust gas compositions. 

The nature of the hydrocarbon also effects the conversion of CO and NO (Fig. 56). With a lean exhaust gas composition, the level of 50% conversion of an alkanic hydrocarbon is reached at a higher temperature than the corresponding temperature for CO and NO. Alkenic hydrocarbons reach the 50% conversion level at the same temperature as CO. These phenomena strongly depend on the detailed kinetic mechanism of the eonversion reactions and are therefore influenced by the composition and the design of the catalyst. 

As of today, three types of lean burn engines are proposed (Fig. 94) [60, 61]. The first type always operates in the lean burn range, with a lambda value between 1.5 and 1.6. The exhaust gas composition will be net oxidizing at all vehicle driving... 

During stoichiometric testing, modulations in the exhaust gas composition, due to closed loop control on a vehicle, were simulated by pulsing additional O2 to simulate lean and CO to simulate rich perturbations into the reactor. For most tests the amplitude of these modulations was +/- 0.5 air/fuel ratio units and the frequency was 0.5 Hz. Space velocity was 46000h- ... 

The most popular use of CSZ is as a gas sensor (2 sensor) for automotive exhaust, the structure of which is shown in Fig. 3.2. The sensor with the standard electrode (air) is inserted in exhaust pipes. The relation between the ratio of air to fuel (RAF) and the EMF is depicted in Fig. 3.3. The EMF shows a sharp drop at RAF = 15 (equivalent composition), left of which is called the rich burn region and right of which is called the lean burn region. Because the bad gases, such as CO and NO , , in exhausted gas are minimized at the point of equivalent composition, the value of RAF is controlled by the feedback of EMF. 

The fourth emission control concept is the oxidation catalyst. Secondary air is added to the exhaust gas to assure a lean composition, independent on the engine operation condition. The catalyst is designed to promote reactions between oxygen and both carbon monoxide and hydrocarbons, which can be removed to a high extent. However, nitrogen oxides cannot be removed in this manner. 

The disadvantage of this diffusion barrier is that it slightly changes the gas composition at the electrode because of the different diffusion coefficients of the constituents of the raw exhaust gas. On average the rich gas components, especially the small H2, are faster than the bigger 02 or NOx molecules causing a lean shift that is typical for all sensors. The closer the exhaust gas is to equilibrium, the less lean shift is observed. Downstream of the catalytic converter the lean shift completely disappears. Furthermore, precatalytic layers in front of the electrode, or catalytic materials inside the protection layer, reduce the effect... 

A car drives on the road at full speed, so the exhaust gas has a rich composition. Suddenly the driver removes his foot from the accelerator the amount of oxygen in the combustion chamber immediately jumps up, and the amount of fuel injected decreases. The exhaust gas changes from rich to lean conditions, so... 

Several parameters that affect catalytic methane oxidation on a natural gas vehicle were investigated with laboratory aged noble metal catalysts and a simulated vehicle exhaust. These include the air/fliel control strategy, the noble metal loading, the role of base metals and the exhaust hydrocarbon composition. The catalytic performance of several formulations was compared both slightly fuel-rich of the stoichiometric point and under extreme lean conditions. 

Because automotive exhaust gas has a cyclic fluctuation of lean-rich composition, a component that can store oxygen and that readily undergoes redox cycles, can provide oxygen for oxidation of CO and hydrocarbons in the fuel-rich region. In the reduced state the component can remove oxygen from the gas phase when the exhaust gas cycles into the lean region. Thus, cerium oxide not only promotes the oxidation activity of the catalyst, but also widens the air-foel ratio composition range where all three major pollutants, CO, hydrocarbons, and NO can be removed. 

In terms of a cycle during fuel lean periods - (99% of the time), when the exhaust gas has significant amounts of O and relatively little CO and VOC, an NO molecule will oxidise (on oxidised nanoparticulate Pt) to form NOj(g). This will go on to undergo an acid - base interaction with the BaO (or BaCO ) component - recall NO is an acidic gas, and alkaline earth oxides are basic materials. This interaction eventually forms Ba(N03)2 displacing C02(g). The Ba component of the composite material serves no catalytic function. Its role is solely involved in the trapping of NOx. 

The signal of the lambda probe in the exhaust pipe is fed into an electronic control circuit which keeps the actual lambda value close to 1, i.e. so that nearly complete combustion is achieved. The fuel-air composition under these conditions is a rich rather than a lean mixture. The exhaust gas then contains CO, NO and residues of hydrocarbons. These components are converted by means of the three-way catalytic converter into the less harmful gases N2, H2O and CO2. If the lambda value is higher or lower than the optimum, then the poisonous gases canot be converted properly. The optimum APR is ca. 14.7, i.e. 14.7kg of air are needed to burn 1kg of fuel. 

OSC investigations on a time scale of a few seconds, such as those previously discussed, are usually interconnected with gas-phase diffusion (inter/intra particle) and desorption phenomena, the dynamics of which is slower than surface phenomena. In addition, the actual change in composition of the exhaust gas for a TWC between lean and rich occurs on a millisecond scale. In this respect, only a limited number of studies have presented OSC data on a millisecond scale. The surface science approach employs an ultrahigh vacuum with gas-pulsed valves, a catalyst on a planar substrate, and a turn over frequency (TOF) mass spectrometer. This set-up allows... 

Fig. 14.12 Stack concentrations of UHC as function of NO in the exhaust of a lean-burn natural gas fired engine [234], The following conditions can be assumed for the exhaust system inlet composition CH4 = 4410 ppm, C2H6 = 490 ppm, O2 = 8.5%, H2O =13%, CO2 = 6.5%, NO = 10-1813 ppm, NO2 = 3-544 ppm, balance nitrogen temperature = 680°C, reactor residence time = 210 ms, pressure = 1.7 bar.

---