Modeling Pyrolysis of Large Coal Particles with Many Species

Coal currently supplies 40% of the world’s electricity needs, and is one of the most important energy sources. As the initial stage of coal combustion, pyrolysis is a thermal decomposition process which converts coal into light gases and tars, which are subsequently consumed in combustion reactions, as well as solid char. Recently there has been interest in using slow pyrolysis as a stand-alone process for the production of chemicals and fuels from large (mm-scale) coal particles. Simulations can be used to efficiently study the impact of pyrolysis conditions on gas, tar and char yields, as well as gas and tar species compositions, which are an important output for a coal-to-chemicals process. In order to simulate pyrolysis of large coal particles, the Chemical Percolation Devolatilization (CPD) model, which predicts the mass fractions of char, tar and light gas, has been modified and improved. A transient multicomponent vaporization sub-model has been developed to predict the partitioning of heavy species into gaseous tar and liquid metaplast. The Direct Quadrature Method of Moments (DQMoM) is introduced as a computationally efficient method to solve for the evolution of the distribution of tar species as a function of molar mass, and the full discrete tar species distribution can be reconstructed by a novel delumping procedure. Finally, a heat transfer model that can predict temperature gradients within the particles has been incorporated using the finite volume method to discretize the energy equation, with the improved CPD model implemented at every position within the particle. The results show the necessity of resolving large particles spatially, due to the impact of the local temperature evolution on tar and gas mass fractions and the production of certain species. Higher pyrolysis temperatures result in increased yields of gas and especially large tar species, while decreasing pressures also increase the production of heavier tar species. The agreement between the full discrete species model, which solves differential equations for every tar species, and DQMoM with delumping, which solves many fewer equations, is excellent, while yielding a large improvement in computational efficiency

Effect of Multiphase Radiation on Coal Combustion in a Pulverized Coal jet Flame

The accurate modeling of coal combustion requires detailed radiative heat transfer models for both gaseous combustion products and solid coal particles. A multiphase Monte Carlo ray tracing (MCRT) radiation solver is developed in this work to simulate a laboratory-scale pulverized coal flame. The MCRT solver considers radiative interactions between coal particles and three major combustion products (CO2, H2O, and CO). A line-by-line spectral database for the gas phase and a size-dependent nongray correlation for the solid phase are employed to account for the nongray effects. The flame structure is significantly altered by considering nongray radiation and the lift-off height of the flame increases by approximately 35%, compared to the simulation without radiation. Radiation is also found to affect the evolution of coal particles considerably as it takes over as the dominant mode of heat transfer for medium-to-large coal particles downstream of the flame. To investigate the respective effects of spectral models for the gas and solid phases, a Planck-mean-based gray gas model and a size-independent gray particle model are applied in a frozen-field analysis of a steady-state snapshot of the flame. The gray gas approximation considerably underestimates the radiative source terms for both the gas phase and the solid phase. The gray coal approximation also leads to under-prediction of the particle emission and absorption. However, the level of under-prediction is not as significant as that resulting from the employment of the gray gas model. Finally, the effect of the spectral property of ash on radiation is also investigated and found to be insignificant for the present target flame

Effects of Water Content and Particle Size on Yield and Reactivity of Lignite Chars Derived from Pyrolysis and Gasification

Water inside coal particles could potentially enhance the interior char–steam reactions during pyrolysis and gasification. This study aims to examine the effects of water contents on the char conversion during the pyrolysis and gasification of Shengli lignite. The ex-situ reactivities of chars were further analyzed by a thermo gravimetric analyzer (TGA). Under the pyrolysis condition, the increase in water contents has monotonically decreased the char yields only when the coal particles were small (\u3c75 μm). In contrast, the water in only large coal particles (0.9–2.0 mm) has clearly favored the increase in char conversion during the gasification condition where 50% steam in argon was used as external reaction atmosphere. The waved reactivity curves for the subsequent char–air reactions were resulted from the nature of heterogeneity of char structure. Compared to the large particles, the less interior char–steam reactions for the small particles have created more differential char structure which showed two different stages when reacting with air at the low temperature in TGA

Influence of FB Conditions on Processes Within a Large Fuel Particle During Initial Phases of Conversion

A theoretical study has been performed in order to investigate the interaction between fluidized bed (FB) temperature and original size of coal, and the pressure rise within a large coal particle during the initial phases of the combustion process. A mathematical model, describing the devolatilization process, has included the internal and external heat transfer, primary decomposition reactions and mass transfer. The model has shown how the FB temperature and original size of coal reflect on the pressure profile of the coal particle during the devolatilization. One of the major consequences of coal devolatilization in the fluidized bed is the primary fragmentation; hence, the special attention has been paid to this process. On the basis of the devolatilization model results, a physical model of the primary fragmentation in FB is proposed

ASSESSING THE IMPACT OF SPECIFIC WEIGHT OF DIFFERENT-SIZED PARTICLES ON OPERATIONAL PERFORMANCE OF COAL PREPARATION PLANT

In case of firing pulverized coal in power steam boiler certain fuel preparation process has to be taken in order to ensure stable and optimal combustion in boil-er furnace. Before entering furnace low caloric coal is introduced from coal bun-ker to the coal preparation system where milling/pulverization and drying pro-cesses of raw large coal particles are conducted. For the purpose of defining fineness of grinding of pulverized coal mill gaseous mixture at mill outlet is in-troduced to the separator where large coal particles, separated from outgoing mill gaseous mixture, are recirculated to mill for regrinding. In this paper char-acter of two-phase flow in inertial separator at milling plant in TPP Nikola Tesla Unit B under various operating conditions has been analyzed. The CFD ap-proach has been used for calculating two-phase flow in separators flow domain. Measurement data taken on site along with results of performed heat balance calculations of milling plant have been used for validating calculation model as well as for setting appropriate boundary conditions in CFD model. The CFD cal-culations has been performed for different positions of all regulating flaps, recir-culation rates of gaseous phase and values of specific weight of solid phase in two-phase mixture. For evaluating separators operating performance at different operating regimes changes of milling capacity and fineness of grinding of pulver-ized coal at separators outlet have been observed. Additionally, deviation rate of trajectories of different-sized particles to the streamlines of gaseous phase has been examined

Doctor of Philosophy

dissertationThe proliferation of nonconventional subsurface hydrocarbon production methods has turned some attention toward production from deep coal seams. There exists little research into coal pyrolysis under conditions relevant to subsurface processing (large coal domains, very slow heating rates, high hydrostatic pressure, volumetric confinement). Basic studies into the phenomena of mass transfer and devolatilization in a high-volatile Utah bituminous coal are described for very large particles (>1 cm) at very slow heating rates (< 10K/min) at atmospheric pressure. Studied systems included large coal blocks heated via immersion heaters and 2 cm-diameter coal cores heated in a tube furnace apparatus. Changes in char porosity during pyrolysis as a function of heating rate are described in large coal blocks. Coal core data show char porosity evolution as a function of temperature and heating rate and demonstrate a distinct threshold for plastic deformation. Volumetric confinement of core swelling was shown to dramatically affect char morphology. Devolatilization data from coal cores are presented, showing little impact of heating rate upon total volatile yield, but a substantial impact upon the yield of tars. A Knudsen flow analysis is also presented to argue that the driving force for mass transfer at very slow heating rates is pressure-driven flow. Several novel pyrolysis phenomena are described, including a pore plugging effect at very slow heating rates. The presented experimental work suggests that many common assumptions for conventional coal pyrolysis would not apply in a subsurface processing environment

Effects of intraparticle heat and mass transfer during devolatilization of a single coal particle

The objective of the present work is to elucidate the influence of intraparticle mass and heat transfer phenomena on the overall rate and product yields during devolatilization of a single coal particle in an inert atmosphere. To this end a mathematical model has been formulated which covers transient devolatilization kinetics and intraparticle mass and heat transport. Secondary deposition reactions of tarry volatiles also are included. These specific features of the model allow a quantitative assessment to be made of the impact of major process conditions such as the coal particle size, the ambient pressure and the heating rate on the tar, gas and total volatile yield during devolatilization. Model predictions are compared to a limited number of experimental results, both from the present work and from various literature sources