Zumao Chen

Steam-drying of coal. Part 1. Modeling the behavior of a single particle

Abstract Steam-drying of coal is an attractive technology that offers, besides reduction of fire and explosion hazards, improved thermal efficiency. In addition, steam-dried coals appear to be less prone to spontaneous combustion. In this paper, a single-particle model has been developed to simulate the drying of granular media in superheated steam. The calculations from the model are compared with experimental data reported in the literature on the steam-drying of ceramic spheres and water droplets. The validated model is then used to investigate, parametrically, the effects of system variables on the drying behavior of coal in steam. The model can be extended with ease to compare results for steam-drying with those for drying of coal in humidified air (or any other inert gas medium). Calculations are presented to illustrate the effects of thermo-physical properties and operating variables on the inversion point temperature; above this temperature, the vaporization rate in steam is greater than that in air. This single-particle model is the first step in the development of a mathematical model, described in an accompanying paper, for the simulation of the operation of a large-scale fluidized bed dryer.

Steam drying of coal. Part 2. Modeling the operation of a fluidized bed drying unit

Abstract Drying of coal in steam offers, besides reduction of fire and explosion hazards, improved thermal efficiency. In addition, steam-dried coals appear to be less prone to spontaneous combustion. In a previous paper, a mathematical model was developed for the drying of a single porous particle in steam. In this paper, the single particle model has been integrated with a two-phase hydrodynamic model to simulate the continuous drying of coal in a bed fluidized with superheated steam. The model incorporates: (i) variation of particle temperature and moisture content with residence time within the bed; (ii) the effect of the initial condensation during the heating period of coal fed to the fluidized bed; and (iii) variation in the superficial gas flow—arising from the vaporization of moisture from the wet coal or the condensation of inlet steam—for various operating conditions. Model studies show that, for a specified reactor size, system performance is most sensitive to the steam-tube duty and the initial moisture content of the wet coal. The conceptual problems in defining an inversion temperature, above which vaporization rate in steam is greater than that in air, for the continuous drying of porous particles are discussed. It is shown that the inversion temperature is a consequence of departure from thermal equilibrium between the gas and solid phases. Model studies show that the inversion temperature depends on design and other operating parameters.

Influence of horizontal tube banks on the behavior of bubbling fluidized beds: 1. Bubble hydrodynamics

Abstract Experimental data on bubble characteristics—averaged size and rise velocity—were obtained using digital image analysis methods from two-dimensional (thin) bubbling fluidized beds with and without simulated horizontal tube bundles. These data were used to develop semiempirical correlations for bubble size and rise velocity. These mechanistic correlations can be extrapolated readily to three-dimensional fluidized beds, and calculations for bubble size, velocity and bubble fraction compare favorably with relevant literature data. Application of these correlations for bubble behavior in the interpretation of experimental data on the mixing of solids—in two- and three-dimensional fluidized beds containing horizontal tubes—is illustrated in an accompanying paper.

Mathematical modeling of fluidized bed combustion. 4: N2O and NOX emissions from the combustion of char

Abstract Batch experiments were conducted to investigate the emissions of carbon oxides and nitrogen oxides from the combustion of char prepared from a commercial coal in a bubbling fluidized bed. Combustion gases escaping from the surface of the bed were sampled and analyzed for N 2 O, NO, NO 2 , CO and CO 2 as a function of time by means of a Fourier Transform Infrared Spectrometer. The experimental variables include char size and loading, inlet oxygen concentration, inlet CO concentration and bed temperature. A single particle reaction–diffusion model was developed first. The detailed chemistry of NO and N 2 O formation destruction is complex. Homogeneous and heterogeneous reactions considered most pertinent were included in an extension of the single particle char combustion model reported earlier. This single porous char particle model was integrated into a three-phase hydrodynamic description of the fluidized bed reactor. This system model for fluidized bed combustion was developed in response to the observation that most previous system models, including those that incorporate details of NO X emissions, are based on the two-phase theory of fluidization. However, two-phase models are unable to predict the gas back-mixing and the recycle peak in solids-mixing. The non-isothermality of the bed resulting from the gas-phase reactions was taken into account through inclusion of an energy balance for the bubble phase. The effect of the variation in superficial gas velocity on bubble properties and cross-flow was included through an overall mass balance. Calculations from the system model, including details of homogeneous NO X reactions far from the char particles, compared well with data on the emissions of CO, CO 2 , NO and N 2 O for various experimental conditions. The validated model was then used to investigate the influence of operating conditions on the conversions of char-nitrogen to NO and N 2 O from the simultaneous combustion of char and propane in a fluidized bed with an air/propane mixture as the fluidizing medium. The most significant factors were found to be bed particle size and char diameter.

Influence of horizontal tube banks on the behavior of bubbling fluidized beds. 2. Mixing of solids

Abstract Video image analysis techniques were used to investigate the solids mixing behavior in a two-dimensional (thin) fluidized bed containing “simulated” horizontal tube banks. Colored glass beads were used as the tracer, and the image intensity was used to deduce the variation of axial concentration profiles with time. A three-phase counter-current back-mixing model was developed for data interpretation. The model accounted for the axial variations in bubble properties and in flow (of gas and solids) in the presence of tubes. These variations induce cross-flow of material between phases (for volume conservation). Mathematical form for the cross-flow term for inclusion in the conservation equations was deduced. The model equations were solved using finite difference techniques; bubble parameters for use in the model have been described in a previous paper, part 1 in this series. Calculations compare favorably with experimental data. The results also show that both tube position and tracer feed location have significant effects on the mixing time. Tubes located in the top and the bottom of the bed lead to smaller mixing times; tracer fed in the middle of the bed provides the fastest mixing rate.

The role of the diffusion of oxygen and radiation on the spontaneous combustibility of a coal pile in confined storage

Abstract This paper deals with the spontaneous combustibility and ignition behavior of coal stored in confined spaces. The geometry, which views the stockpile to be sealed from all sides except the top (which is exposed to the ambient conditions), is of interest in the transport of coals in barges or in cars by rail. Semi-analytical solutions are developed with the assumption that the coal pile is isothermal. Finite-difference numerical calculations show that the steady-state temperature at the exposed surface from a nonisothermal model is approximately the same as that from the isothermal formulation. Based on this approximation, the maximum temperature in the nonisothermal stockpile is linked analytically to the temperature calculated from the isothermal formulation. This approximation also permits rapid evaluation of the critical behavior of the nonisothermal stockpile in terms of the isothermal formulation. Detailed numerical solutions are necessary, however, to evaluate the transient behavior of the nonisothermal stockpile. Results in previous literature suggest that diffusion in a one-dimensional stockpile can lead to a maximum temperature rise of ≈ 80 K. This conclusion is shown to be a consequence of the unrealistic boundary conditions imposed on the oxygen and energy balance equations—diffusion alone can lead to dangerous self-heating. It is also shown that inclusion of the radiation at the boundary is extremely important in determining the stability of the stockpile. Finally, the maximum temperature within the pile at steady state, as well as the safe transit time, are calculated for the transport of coal in barges. These calculations confirm the importance of bed compaction in enhancing the safety of the coal pile.

A device for measuring solids flowrate in a circulating fluidized bed

Abstract The on-line measurement of solids flowrate is important to numerous industrial processes. This paper considers a variation of impact-type solids flow meters suitable for use in numerous applications, including circulating fluidized beds (CFBs). The solids flowrate meter introduced herein is on-line, capable of operation in high temperature environments, and useful for a broad range of flowrates with good linearity, accuracy and fast response time. The flow meter works by measuring the torque that results on a hinged plate when falling solids impact the plate. A theoretical model of the device is developed and its results are compared to experimental data for the operation with various solids.

Continuous drying and dehydration of sodium carbonate monohydrate in a fluidized bed

A two-phase model has been developed to simulate the dynamic behavior of the drying and dehydration of sodium carbonate monohydrate in a fluidized bed. The model includes details on the variation of the particle temperature and moisture content with residence time within the bed. In addition, the variation of superficial gas flow and the minimum fluidization velocity, with time, has also been accounted for. Calculations show that the accumulation of liquid moisture and the cohesive forces between bed particles in the presence of free water on their surface can, under certain circumstances, lead to defluidization. The system performance is most sensitive to the steam-tube duty, feed rate of the wet cake, and the variation of the minimum fluidization velocity with liquid moisture content of the bed particles.