Devolatilization Of Large Coal Particles Research Articles

Fast pyrolysis of large coal particles in a novel hot plate reactor: Effects of the reaction atmosphere on the porous structure and char reactivity

Fast pyrolysis of large particles of coal semi-anthracite (SA) in a novel Hot Plate Reactor (HPR) was carried out. Advantages of gas flow control of the HPR were used to compare the effect of the reaction atmosphere on the fast pyrolysis process, mainly in aspects such as chemical and porous structure, also in char reactivity. The effects of the heating rate and the reaction atmosphere were evaluated through characterization of the surface of the char by using Fourier Transform Infrared (FTIR). When CO2 reaction atmosphere was used, evidence was found of thermoplastic effects, and such effects might have caused retention of aliphatic structures typical of volatile material. This shows that different processes apply to devolatilization of large coal particles compared to devolatilization under conventional combustion conditions. In addition, the possible presence of thermal annealing and graphitization of chars may explain their poor performance under combustion, due to carbonaceous structure chemical deactivation, which was stronger in those obtained under CO2 atmosphere than those obtained under N2 atmosphere. This allowed demonstrating that when it comes to large coal particles combustion, it is not advisable to assume as true the knowledge acquired during devolatilization under conventional combustion conditions.

98/03216 Prediction and measurement of devolatilization of large coal particles under combustion conditions

98/03216 Prediction and measurement of devolatilization of large coal particles under combustion conditions

97/01616 Devolatilization of large coal particles at high pressure

97/01616 Devolatilization of large coal particles at high pressure

97/00748 Devolatilization of large coal particles at high pressure

97/00748 Devolatilization of large coal particles at high pressure

96/06381 Modeling the devolatilization of large coal particles

96/06381 Modeling the devolatilization of large coal particles

Devolatilization of large coal particles at high pressure

Devolatilization times of large (0.1 and 0.2 g) Utah hvBb and North Dakota lignite coal particles, in the range 15–30 s, were measured in air at 101 and 507 kPa, at air temperatures of 900 and 1200 K in a convective flow reactor. Visual observations indicated infrequent heterogeneous ignition of the lignite prior to devolatilization and occasional explosion of bituminous coal particles during devolatilization. Devolatilization times were correlated with temperature, pressure and particle size. Power-law exponents for tests at 101 kPa and 900 K were determined to be 2.5 for Utah hvBv coal and 2.2 for North Dakota lignite. At 507 kPa and 900 K, exponents decreased to 1.6 for both Utah and North Dakota coals. At 101 kPa and 1200 K, exponents were 1.9 for Utah coal and 1.6 for North Dakota lignite.

96/04193 Sulfur release during the devolatilization of large coal particles

We show that the explosion of the first supernovae can trigger low-mass star formation via gravitational fragmentation of the supernova-driven gas shell. If the shell mass does not exceed the host galaxy gas mass, all explosions with energies ESN⩾1051 erg can lead to shell fragmentation. However, the minimum ambient density required to induce such fragmentation is much larger, n0>300 cm−3, for Type II supernovae than for pair-instability ones, which can induce star formation even at lower ambient densities. The typical mass of the unstable fragments is ∼104−7M⊙; their density is in the range 110–6×107 cm−3. Fragments have a metallicity strictly lower than 10−2.6Z⊙ and large values of the gravitational-to-pressure force ratio f≃8. Based on these findings, we conclude that the second generation of stars produced by such self-propagating star formation is predominantly constituted by low-mass, long-living, extremely metal-poor (or even metal-free, if mixing is suppressed) stars. We discuss the implications of such results for Pop III star formation scenarios and for the most iron-poor halo star HE0107-5240.

Sulfur release during the devolatilization of large coal particles

Experimental data are reported on sulfur release rates during the flash pyrolysis of coal particles sized up to 2 mm, at heating rates and temperatures found typically in fluidized bed reactors. Predicted sulfur yields are also reported for pyrolysis conditions studied using an existing model for devolatilization of large coal particles, coupled with kinetics from an earlier study. The predictions of the experimental data were found to be sensitive to the value of the thermal diffusion coefficient used in the model, with each coal and pyrolysis temperature requiring a specific value in order to accurately predict the experimental results. The rate of heat transfer is also shown to be an important rate-controlling mechanism and the transition from reaction-controlled to heat-transfer-controlled pyrolysis is determined as a function of thermal diffusivity, particle size and sulfur release kinetics.

A general model for devolatilization of large coal particles

There have been several studies on the devolatilization of large coal particles under laboratory conditions simulating large-scale fluidized or fixed-bed combustion conditions, all of which demonstrate that the process of devolatization of large coal particles occurs over a much longer timescale than that of pulverized coals under the same conditions. This paper reports the development of a model for large coal particle devolatilization under fluidized and fixed-bed combustion conditions. The model combines a heat transfer model with a general and comprehensive coal devolatilization model, FG-DVC. It inherits the generality of FG-DVC and can be applied to coals of various types without extensive prior knowledge of the coals. This model includes a detailed treatment of coal pyrolysis reactions including the yields of individual gas species, tar yields, and tar molecular weight distributions. The changes in coal physical properties are modeled with the application of Merrick's coal physical property submodels for specific heat and thermal conductivity. Both the convective and radiative heat transfer between the gas phase and coal particles are considered and Gunn's correlation for the surface heat transfer coefficient is used. Model predictions are compared with the data of large coal particle devolatilization measured in fluidized beds and tubular reactors, for particle thermal response, total weight loss, and individual gas evolution rates. In general, the agreement with the data is very good for the particle center temperature and weight loss curves and is fair for the individual gas evolution curves.

95/03931 Mathematical modelling of devolatilization of large coal particles in a convective environment

95/03931 Mathematical modelling of devolatilization of large coal particles in a convective environment

Mathematical modelling of devolatilization of large coal particles in a convective environment

A generalized model for devolatilization of large coal particles typically encountered in a gasifier has been developed. The model predicts a time-temperature history of the particle in a convective heat transfer environment. Thermal decomposition of volatile matter is treated as a series of independent chemical reactions in which the rate constants differ only in activation energy. The activation energy is expressed as a Gaussian distribution. Experimental data obtained in a flow reactor are presented and used to validate kinetic and transport parameters of the model. Devolatilization time predicted by the model with updated parameters is also compared with that obtained from a pilot fixed-bed gasifier.

Devolatilization of large coal particles under fluidized bed conditions

The rate of devolatilization of various sizes of coal particles is an important parameter in the design and interpretation of the performance of fluidized bed combustors. Combined with data on char combustion kinetics, bed mixing and residence times of the gaseous and particulate phases, devolatilization data are a primary input to combustion models. It is proposed here that the phenomenon of volatiles release from a coal can be represented by a single reaction rate equation of the nth order Arrhenius type: dV dt = k (V ∞ − V) n , where n is the order of reaction, not zero, k is the devolatilization rate constant, V is the fractional amount of volatiles evolved to time t, and V ∞ is the fractional amount of volatiles evolved as t → ∞. A group of nine coals ranging in type from semianthracite to subbituminous, having particle sizes from 1.4 to 29 mm were devolatilized under simulated fluidized bed combustion conditions. Order of reaction and devolatilization rate constants were calculated for each coal type and particle size combination and a relationship between rate and size determined. The order of the devolatilization reaction was constant for each coal over the size range examined, ranging between 0.4 and 0.6 for the various coals. A relationship to estimate total devolatilization time of various sizes of coal particle was derived from the nth order rate expression and shown to be of similar form to a previously published semi-empirical relationship, τ = K d N . The rate relationships are used to estimate the coal particle size below which substantial freeboard volatiles will occur.

Drying and devolatilization of large coal particles under combustion conditions

An experimental and modelling investigation was carried out on the drying and devolatilization (under combustion conditions) characteristics of low-rank coals for particle sizes of interest in fluidized bed combustion. Experiments were conducted on single particles, suspended by a flow of air in a quartz reactor, of low grade coal (Bowmans deposit) for an extensive range of operating conditions. The residence time of the coal particle in the reactor was varied to obtain time resolved histories on the drying and devolatilization behaviour under combustion conditions. To model drying and devolatilization under combustion conditions, the analysis was divided into pre-ignition and post-ignition stages. The pre-ignition stages corresponded to drying and devolatilization under pyrolysis conditions, while post-ignition stages were characterized by the formation of a diffusion flame around the coal particle. The energy feed back from the flame, at a higher temperature than the air, enhances the rate of subsequent drying and devolatilization. The model result, although in qualitative agreement with experimental data, indicate the necessity for more detailed modelling, including: a more rigorous estimate for the wet-dry interface temperature; and the effect of heterogeneous ignition and combustion of volatiles trapped in the solid matrix for larger particles (initial radius ≈5 mm) prior to the formation of the volatiles flame.

The effects of fragmentation on devolatilization of large coal particles

Spherical coal particles (5 to 12 mm diameter) from three Australian bituminous coals were devolatilized in an inert atmosphere of nitrogen at 850 °C, using a specially designed thermogravimetric rig based on an electrically heated stainless steel tubular reactor. Weight loss histories of the coal particles were recorded and the simultaneous concentrations of gases evolved were determined chromatographically. It was concluded that the particle size had no effect on the ultimate yields of gas, tar plus water and char. Tar was the major volatile component and evolved before the gaseous volatiles. Therefore, it is recommended that the devolatilization time should be measured as the time for 95% of the ultimate weight loss, rather than the time for 95% of the ultimate gaseous volatiles yield. Fragmentation of the coal particles, which varied with different coals depending on the coal plasticity, occurred early in the devolatilization process and increased the rate of devolatilization and decreased the devolatilization time. The increase in devolatilization time correlated with particle diameter by an empirical equation t v = Ad p n for each coal. Comparison of the data with previous work using a fluidized bed reactor showed that the devolatilization time was shorter in the fluidized bed because of a greater degree of fragmentation.

A study on devolatilization of large coal particles

A generalized devolatilization model for large coal particles is proposed in this paper. The difference between the present model and previous ones is that the equivalent activation energy, E, and the equivalent frequency factor, K, of coal particle devolatilization are independent of coal type, and depend only on the final temperature, T ∞, of the coal particles. The relationship between E K and T ∞ has been obtained and is found to be universal for all coal types. If the final volatile yield, V ∞, is determined, the devolatilization processes of a large coal particle for any kind of coal can be predicted by using the present model. This model has been tested with five Chinese coals with quite different properties, diameters ranging from 3 mm to 9 mm, and temperatures ranging from 1173K to 1733K. The calculated results agree with experimental data.

A model for the devolatilization of large coal particles

A model for the devolatilization of large coal particles

Devolatilization of large coal particles in fluidized beds

Analysis of devolatilization of predried large coal particles in fluidized beds requires consideration of both the chemical kinetics of coal decomposition and transport processes. Models available either assume the devolatilization particle to be isothermal (whereas it may be shown that, in general, large temperature gradients may exist within the particle) or require extensive numerical integration procedures. This Paper describes a model which permits formulation of analytical and easy-to-use equations for the estimation of the devolatilization history of a large predried coal particle in a fluidized bed. The model predictions are compared with experimental data collected for Mississippi lignite. A correlation is proposed for the estimation of the total devolatilization time. The analytical solutions presented may be used with ease in coupling the devolatilization process to the other phenomena, such as drying and/or combustion of volatiles and residual char, occuring during fluidized bed combustion of coal.