Combustion particle structure effect

Changes in the particle structure have a strong effect on combustion behavior, influencing the particle temperature, mass transfer and pore diffusion rates, and consequently the rate-control regime of the process ( 5 ) Tlie changes in size and density of particles that have a homogeneous pore structure and small pore sizes (relative to particle size) are related to fractional burn-off, u, by... 

The major mechanism of generation of both the natural and anthropogenic StA is a pyrolysis in the gas- or condensed-phase of the carbon-containing material. The microphysical characteristics of the resulting StA depend on the specific nature of the source so, for instance, the particles produced by oil combustion have a coral-like structure and an effective spherical shape the pyrolysis of coal gives particles in the form of spheres, with a great amount of smaller, randomly oriented spheroids inside them [11]. 

As discussed earlier, analysis of temperature profiles obtained by microthermocouple measurements have elucidated the unique conditions associated with the combustion synthesis process. However, this approach does not directly identify the composition or microstructure of the phases formed. It is important to recognize that most published investigations in the field of combustion synthesis only address the final product structure. Considerably less has been reported about the structure formation processes leading to the final product. Most results that describe the evolution from the initial reactants to the final product are inferred by the effects of processing variables (e.g., density, dilution, particle size) on the final microstructure (see Section V). To date, only a few investigations have directly identified initial product structure. As discussed earlier, identification of this structure is important since the initial structure represents the starting point for all subsequent material structure formation processes. Thus, the focus of this section is on the initial stages of the structure formation mechanisms in combustion synthesis and novel methods developed especially for this purpose. 

Flame turbulence should not affect soot formation processes under premixed combustion conditions, and the near correspondence of the results from Bunsen flames [52] and stirred reactors [55] tends to support this contention. However, the effect of turbulence on sooting diffusion flames can be very complex and unclear in most experiments unless the effect of the intensity (and scale) of turbulence on the flame structure, the temperature-time history of the pyrolyzing fuel, the rate of incipient particle formation and particle growth, and, in the case of some fuels, the transport of oxygen to the fuel stream are known. 

Electrostatic precipitation is also very efficient for retention of very fine particles, as long as these are not highly electrically conductive. The structural features of these units dictate that they are only cost effective for particulate and aerosol emission control for low pressure drop clean-up of very large volumes of gas hence their extensive use for the treatment of combustion gases of fossil-fueled power stations. Next generation developments have been reviewed [27]. 

Experimental and computational aeroacoustics and emissions of modern swirl combustor flows are underway. Preliminary measurements of turbulent non-premixed flame sound highlight the influence of combustion as a sound source. Particle Image Velocimetry measurements in swirl combustors reveal the influence of heat release and its effect on the complex spatial structures that are present. Acoustic measurements in confined turbulent jets are used to better understand sound sources in such flows. Computational aeroacoustics studies of unconfined and confined flows and flames have allowed acoustic source identification. Preliminary LES of diffuser and swirl combustor flowfields serve as benchmarks for future combustion simulations. 

From all of the results reviewed in this section, it is clear that particle size may have an important effect on activity, but measurement of particle size alone cannot be used to predict the combustion activity. It is apparent that the structure and composition of the surface plays a decisive role in activity, and it greatly depends on the thermal history of the sample. As a result, catalysts with similar particle size may exhibit very different activities. Therefore, plots like Figures 1 and 2 are only valid when the samples have been pretreated under similar conditions. 

The Pd-O bond also varies with the extent of oxidation of Pd. During the methane combustion reaction, the catalyst surface is a non-equilibrium, kineti-cally controlled structure. The oxygen concentration profile in the particle results from a combination of particle reconstruction, oxygen adsorption, bulk diffusion, and oxygen removal. This concentration profile varies as a function of time, and as the oxygen content increases, the Pd-O bond strength decreases. This increase is accompanied by an increase in the specific activity. The most widely accepted reaction pathway is the Mars and van Krevelen redox mechanism, which involves lattice oxygen and uneoordinated Pd centers as active species. Inhibition by products (H2O and CO2) and impurities (SO2) is a major drawback for low temperature combustion. The effect of sulfur is particularly important for catalytic converters for NGV applications because it drastically reduces the methane combustion activity. 

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