Fragmentation Of Char Particles Research Articles

Evolution of PM2.5 from biomass high-temperature pyrolysis in an entrained flow reactor

In this study, wheat straw pyrolysis was conducted in an entrained flow reactor at 900–1300 °C, and PM2.5 were sampled from the flue gas through a heated sampling system. Multi-phase PM2.5 including carbonaceous matter, potassium-containing particles, and ash particles, was separated and quantified using a thermogravimetric analyzer (TGA). The micro-morphologies and chemical compositions of these three phases were characterized by scanning electron microscopy (SEM), scanning transmission electron microscope (STEM), energy dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD). Results show that PM2.5 yields during biomass pyrolysis are in the range of 7–34 g/kg (dry-basis biomass) and increase with the increase of pyrolysis temperature. At low pyrolysis temperatures (900–1000 °C), the carbonaceous matter is dominated by char-carbon. When the pyrolysis temperature increase from 1000 °C to 1100 °C, the production of soot is greatly enhanced and soot becomes dominant in PM2.5, and the amorphous morphologies of soot are replaced by the concentric graphitic layers. With the further increasing in pyrolysis temperature, soot particles become more spherical and onion-like. Above 1100 °C, the KCl content in PM2.5 declines, which is because of the capture of KCl and the formation of low-melting potassium aluminosilicates in large char particles. At 1300 °C, the fragmentation of char particles is significantly strengthened, resulting in more ash in PM2.5.

Characterizing char particle fragmentation during pulverized coal combustion

A particle population balance model was developed to predict the oxidation characteristics of an ensemble of char particles exposed to an environment in which their overall burning rates are controlled by the combined effects of oxygen diffusion through particle pores and chemical reactions (the zone II burning regime). The model allows for changes in particle size due to burning at the external surface, changes in particle apparent density due to internal burning at pore walls, and changes in the sizes and apparent densities of particles due to percolation type fragmentation. In percolation type fragmentation, fragments of all sizes less than that of the fragmenting particle are produced. The model follows the conversion of particles burning in a gaseous environment of specified temperature and oxygen content. The extent of conversion and particle size, apparent density, and temperature distributions are predicted in time.Experiments were performed in an entrained flow reactor to obtain the size and apparent density data needed to adjust model parameters. Pulverized Wyodak coal particles were injected into the reactor and char samples were extracted at selected residence times. The particle size distributions and apparent densities were measured for each sample extracted. The intrinsic chemical reactivity of the char to oxygen was also measured in experiments performed in a thermogravimetric analyzer. Data were used to adjust rate coefficients in a six-step reaction mechanism used to describe the oxidation process.Calculations made allowing for fragmentation with variations in the apparent densities of fragments yield the type of size, apparent density, and temperature distributions observed experimentally. These distributions broaden with increased char conversion in a manner that can only be predicted when fragmentation is accounted for with variations in fragment apparent density as well as size. The model also yields the type of ash size distributions observed experimentally.

石炭ガス化過程におけるチャーの粒度分布および比重の変化における挙動解明とモデル化

The entrained flow coal gasifiers, which are adapted for almost all IGCC technologies, are operated under the high-temperature and pressurized condition. It is necessary to clarify the coal behavior in such a condition for the design of the gasifier and the estimation of the gasifier's performance. In this work, the changes of char morphology, such as particle size distributions and bulk density, in the coal pyrolysis or the char gasification reaction were examined using a pressurized drop tube furnace facility (PDTF). Though a lot of pulverized coal particles have swelled by the rapid pyrolysis, the tendency was shown that the initial pyrolysis reaction of a coal with a high fuel ratio was controlled by pressure and the swelling was also suppressed. In the char gasification process, the change of the char particle diameter was few in the early stage of the gasification, because the surface was eroded by the reaction and the pore grew in the surface of char particles. The structure of char particles became fragile and the particles fragmented into smaller parts in the middle and latter stages of the gasification. The numerical model of particle size distributions and particle density of gasified char was proposed to explain the shrinkage and the fragmentation of char particles during gasification, and the validity of the model was shown.

Percolative Fragmentation of Char Particles during Gasification

Percolative fragmentation was confirmed to occur during gasification of three microporous coal chars. Indirect evidence obtained by the variation of electrical resistivity (ER) with conversion was supported by direct observation of numerous fragments during gasification. The resistivity increases slowly at low conversions and then sharply after a certain conversion value, which is a typical percolation phenomenon suggesting the occurrence of internal fragmentation at high conversion. Two percolation models are applied to interpret the experimental data and determine the percolation threshold. A percolation threshold of 0.02-0.07 was found, corresponding to a critical conversion of 92-96% for fragmentation. The electrical resistivity variation at high conversions is found to be very sensitive to diffusional effects during gasification. Partially burnt samples with a narrow initial particle size range were also observed microscopically, and found to yield a large number of small fragments even when the particles showed no disintegration and chemical control prevailed. It is proposed that this is due to the separation of isolated clusters from the particle surface. The particle size distribution of the fragments was essentially independent of the reaction conditions and the char type, and supported the prediction by percolation theory that the number fraction distribution varies linearly with mass in a log-log plot. The results imply that perimeter fragmentation would occur in practical combustion systems in which the reactions are strongly diffusion affected.

Combustion behaviour of coal and carbon in spouted and spout-fluid beds

This paper presents the experimental study of batch combustion of Thai lignite and pyrolysed electrode carbon in spouted bed and spout-fluid bed combustors. The combustion behaviours of evolved coal volatiles and char particle fragmentation are also investigated and compared with pyrolysed electrode carbon

Secondary fragmentation of char particles during combustion in a fluidized bed

Batchwise fluidized bed experiments have studied secondary fragmentation of char particles during combustion. Two chars, one from a nonswelling and the other from a swelling coal, have been tested for different particle sizes and oxygen concentrations in inlet gas. Statistical functions expressing the probability of particle fragmentation and size distribution of fragments have been determined. Model calculations show that secondary fragmentation of chars significantly influences the performance of fluidized bed combustors only if it occurs in early stage of combustion and if the fraction of particles subject to breakage is above 75%.