Coal Swelling Model for High Heating Rate Pyrolysis Applications

et al. 2014

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The evolution of pressure in a lignite particle during pyrolysis was simulated on the basis of the gas motion equation in porous media and considering the Klinkenberg effect. The chemical percolation devolatilization (CPD) model was used to describe pyrolysis. The pore diameter in the particle is close to the average free path of volatile gas molecules; therefore, the solid phase restrains the movement of gas. Because flow out of the particle is restricted, the internal pressure rises. The internal pressure first increases and then decreases during pyrolysis, and the evolution is influenced by heating conditions. The pressure rise is larger at a higher heating rate, but the duration of the pressure peak is shorter. The total pressure in the particle is larger at a higher ambient pressure, but the pressure difference between the inside and the surroundings decreases with the increasing ambient pressure. A growing internal pressure can restrain tar gasification, which can lead to the increase of the Metaplast content and the decline of the tar yield.

Section: Introductionmentioning

confidence: 99%

Simulation of the Evolution of Pressure in a Lignite Particle during Pyrolysis

et al. 2014

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“…4 Under the surface tension, the bubble shrinks, and then with the generation of volatiles, the pressure in the bubble increases and the bubble swells again. The criteria for bubble rupture are defined using wall strength 4 13 Eastern Bituminous, 13 Australian Bituminous, 19 Adaville No. 1, 15 and Kentucky No.…”

Section: Change In Bubble Numbermentioning

confidence: 99%

Simulation of the Swelling of High-Volatile Bituminous Coal during Pyrolysis

et al. 2014

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In this paper, a model is established to predict the swelling ratio of high-volatile bituminous coal during pyrolysis, based on the assumption that the structure of bubble distribution in the particle at the beginning of the plastic stage is a central bubble surrounded by many surrounding bubbles. The initial number and size of the bubbles when the particles become plastic are calculated by the pressure in the particle. The chemical percolation devolatilization (CPD) model is used to describe pyrolysis. The pyrolysis of eight types of high-volatile bituminous coals is simulated, and the results are compared with experimental results to verify the model. The particle size during pyrolysis increases then decreases during pyrolysis. The model predicts experimentally observed trends in swelling ratio with heating rate; particle swelling during pyrolysis increases with heating rate, up to ∼10 4 K/s, and then decreases with further increases in heating rate. Predictions of increasing then decreasing swelling with increases in ambient pressure also agree with trends that have been observed experimentally.

“…4 However, studies with drop-tube furnaces and flat-flame burners show that the swelling ratios decrease at rapid heating rates. 5,6 Studies in elevated pressure reactors indicate that the coal swelling ratio also has the same increasing and then decreasing trend with increasing ambient pressure. 2,7,8 However, less work has examined the influence of T max during pyrolysis.…”

Section: Introductionmentioning

confidence: 99%

Simulation of the Swelling of High-Volatile Bituminous Coal during Pyrolysis. Part 2: Influence of the Maximum Particle Temperature

et al. 2015

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A model was established previously to predict the swelling ratio of high-volatile bituminous coal during pyrolysis based on the assumption that the structure of bubble distribution in the particle at the beginning of the plastic stage is a central bubble surrounded by many surrounding bubbles. The initial number and size of the bubbles when the particles become plastic are calculated by the pressure in the particle, and the chemical percolation devolatilization (CPD) model is used to describe pyrolysis. In this paper, to obtain accurate results at low pyrolysis temperatures, the previous model is improved and the following parts in the model are adjusted: (1) the method for estimating the volume of macropores in the particle at the beginning of swelling and (2) a correlation between the initial bubble number and the particle diameter. The swelling behavior of eight bituminous coals from the literature spanning a wide range of gas temperatures and gas pressures was simulated to test the suitability of the model. The influence of the maximum particle temperature during pyrolysis (T max ) on swelling is analyzed. The predicted particle swelling for a Pittsburgh #8 bituminous coal particle during pyrolysis increases with T max up to about 950 K, decreases as T max increases from 950 to 1100 K, and then changes little with further increases in T max . The influence of T max coupled with the heating rate makes the swelling ratio in experiments from the literature increase from 1.07 to 1.51 as T max increases from 840 to 952 K and the heating rate increases from 1.38 × 10 4 to 2.18 × 10 4 K/s. Particle swelling decreases from 1.51 to 0.95 as T max decreases from 952 to 1662 K and the heating rate increases from 2.18 × 10 4 to 2.24 × 10 5 K/s. At a constant heating rate of 1 × 10 4 K/s, when T max is larger than 950 K, the predicted particle swelling during pyrolysis increases and then decreases with increasing ambient pressure. However, when T max is smaller than 950 K, the predicted particle swelling during pyrolysis decreases with increasing ambient pressure.