Soot combustion reactions

The presence of sulphur in diesel exhaust gases or particles has to be considered as a poisoning agent for the catalysts used in soot combustion reactions. Copper oxide has been reported to be sensitive towards sulphur dioxide (7) which implies a deactivation of the solid and then eventual modifications of its sinface properties. In this way, lCulCel073 sample was treated in a microflow reactor under SO2 flow (2L.h ) at room temperature for 30 minutes. 

In this work, it is shown that the copper-cerium oxide catalysts are active in the diesel soot combustion reaction. The isolated Cu ions seem to be the most active sites in such catalysts. The activity depends on the copper concentration and the pre-treatment of solids. Among all the tested catalysts, the lCulCe673 oxide, calcined at a relatively low temperature and containing the highest copper concentration, is the most active. 

In addition to these particular features of the soot combustion reactions, the reaction rate also depends on general variables, such as temperature for isothermal reactions and heating rate for ramp experiments, nature and partial pressure of gases in the stream, space velocity/residence time of gases in the solid bed, soot-to-catalyst ratio, and so on. 

Organic compounds are a major constituent of the FPM at all sites. The major sources of OC are combustion and atmospheric reactions involving gaseous VOCs. As is the case with VOCs, there are hundreds of different OC compounds in the atmosphere. A minor but ubiquitous aerosol constituent is elemental carbon. EC is the nonorganic, black constituent of soot. Combustion and pyrolysis are the only processes that produce EC, and diesel engines and wood burning are the most significant sources. 

As illustrated on Figure 7.8, the DPF regeneration step is just a soot particles combustion reaction (soot is mainly composed by a carbon matrix), which requires a temperature in the range of 600°C and oxygen presence in the exhaust gases. 

Similarly, the emission of soot from many practical devices, as well as from flames, is determined by the rate of oxidation of these carbonaceous particles as they pass through a flame zone and into the post-combustion gases. As mentioned in the previous chapter, the soot that penetrates the reaction zone of a co-annular diffusion flame normally bums if the temperatures remain above 1300K. This soot combustion process takes place by surface oxidation. 

Incomplete combustion will occur if there is not enough oxygen for the reaction to continue. This is much more common than complete combustion. Unlike complete combustion reactions, incomplete combustion reactions result in other products besides carbon dioxide and water. The byproducts of incomplete combustion reactions can include soot, which is elemental carbon (C). Other byproducts include nitrous oxides, sulfur oxides, and deadly carbon monoxide. 

Combustion reactions are needed to heat homes and run cars. Since most of these reactions involve incomplete combustion, they should always take place in well-ventilated areas. Carbon monoxide (CO) can be deadly. And soot (C), nitrogen oxides (NxOx), and sulfur oxides (SxOx) are all pollutants that can harm health and the environment. 

Incomplete combustion A combustion reaction in which not enough oxygen is present, resulting in unwanted byproducts such as soot, nitrous oxides, sulfur oxides, and carbon monoxide. 

Many working groups have modeled the performance of diesel particulate traps during the past few decades. Concentrated parameter models (CSTR assumption) have been applied for the evaluation of formal kinetic models and model parameters. The formal kinetic parameters lump the heat and mass transfer effects with the reaction kinetics of the complicated reaction network of diesel soot combustion. Those models and model parameters were used for the characterization of the performance of different filter geometries and filter materials, as well as of the performance of a variety of catalytically active coatings and fuel additives [58],... 

The regeneration curves are often derived from CO2 generation data in closed-loop laboratory systems. It is assumed that the formal kinetics of soot combustion may be described by the oxidation of carbon with oxygen. A typical formal kinetic model comprises two parallel reactions of n-th order ... 

Moisture can sometimes be observed on a metal spoon held over a flame, but water will not be the only material observed on the spoon. There will also be soot. Combustion is usually not the clean, simple reaction outlined above. Combustion is usually a complicated conglomerate of many reactions in which fragments of hydrocarbons, or soot, are produced. Some of the most interesting behaviors of combustion reactions are a result of incomplete combustion and more complicated reactions. 

When enough oxygen is not available, the combustion reaction is incomplete. Carbon monoxide and unbumed carbon (soot), as well as carbon dioxide and water vapor are made. 

The gas-solid reaction involved in non-catalytic soot combustion is a relatively slow process.76,84 Catalytic assistance (gas-solid-solid process) provides an increase in the rate of soot oxidation, although the process efficiency is mainly determined by the type of contact (tight or loose) established between the soot and the catalyst.76,78,84,85 As mentioned... 

The most convenient way to remove particulates from the exhaust gas is combustion. Diesel particulates are, however, relatively unreactive and oxidation occurs moderately at normal exhaust temperatures for both passenger cars and heavy-duty trucks. In the non-catal3rtic as well as the catal3rtic oxidation of soot the reaction rate under all circumstances is rather slow in comparison to the residence time in the exhaust system. To achieve complete oxidation, the reaction... 

In order to obtain more fundamental catalytic activity data of the catalytic materials of interest a number of model catalysts consisting of alkali metal and precious metal were prepared and tested for their ability to promote the reactions of water and carbon dioxide with solid carbon. These tests provide basic information about the ability of the catalysts to catalyse soot combustion with CO2, H2O and O2. Results are summarized in Table 2. Both alkali metal and precious metal (PM) doped supports were used. Two supports were used which can be categorised as an inert and a reducible oxide support. Clearly the presence of the alkali metal has a significant effect on catalysing the soot combustion as anticipated. The effect of the reducible oxide support is not significant. In addition to the experiments summarised in Table 2 two further samples of alkali metal supported on an alumina foam and cordierite wall flow filter were prepared and coated with soot in a similar manner to that described above. Measurement of the soot combustion characteristics of these samples in O2, CO2 and H2O were very similar to the powder samples. 

It was previously proposed that surface-carbonate compounds are reaction intermediates during catal34ic soot combustion (1,2). In this vein. High Frequency CO2 Pulses technique provides not only information about the stability of the surface, but also about the dynamics of the intermediate compound formation and decomposition. While XPS characterization shows that K/La surface ratio increases when increasing calcination temperature, results shown in Fig. 1 indicate that the CO2- surface interaction decreases. 

The most widely used model is a gas-solid reaction, assuming a carbon spherical particle. The most common cases are the regeneration and diesel soot combustion. In this model, carbon combustion is admitted and a cinder layer remains. Therefore, there is gas diffusion through the cinder layer and reaction on the carbon particle surface, which moves inward until total consumption. CO2 is formed during combustion and must diffuse through the cinder in the opposite direction, as shown in Figure 19.12. 

Long and Sackman 105) reported that the combustion of trimethyl-antimony in a static bomb calorimeter gives Sb204 as the main solid product, admixed with Sb203, unburned Sb metal, and some soot. After correction for the products of incomplete combustion. Long and Sackman obtained AH ° = —698.0 3.1 kcal/mole for the pure combustion reaction... 

Comparison of the soot catalytic combustion results obtained at different laboratories is not easy because of the particular features of the soot oxidation reactions, and these aspects must be taken into account while reading this chapter and also the specific literature about this topic ... 

Soot properties change with time during combustion (amount of oxygen, size, and shape of particles, graphitic structure, surface area, etc.). For this reason, the kinetic parameters of the catalyzed soot oxidation reactions (reaction rates, activation energies, and pre-exponential factors) also change with time, and they are vahd only for very specific soot combustion conditions. This drawback has had the result that, in most articles devoted to the study of soot combustion catalysts, kinetic parameters are not reported. 

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