Pradeep K. Agarwal

The fate of organically bound inorganic elements and sodium chloride during fluidized bed combustion of high sodium, high sulphur low rank coals

A series of experiments has been conducted to study the transformations of the organically bound inorganic elements and inherent sodium chloride under conditions relevant to fluidized bed combustion of low rank coals. Large coal particles (5.5–9.0 mm) were suspended in a convective gas flow in a single particle furnace operated at 700–830 °C. The experiments were performed under pyrolysis as well as combustion conditions. The product particles withdrawn from the furnace at various residence times were analysed using chemical methods, X-ray diffraction and electron microscopy. It was found that the transformations of the inorganic matter in high sodium, high sulphur low rank coals result in the formation of a molten ash layer on the char surface, providing evidence of the formation of low melting point eutectics. The molten ash consisted of a matrix of mixed alkali sulphates. The findings are expected to assist in explaining the role of the inorganic matter in causing operational problems during the fluidized bed combustion of low quality, low rank coals.

Mixing of homogeneous solids in bubbling fluidized beds : theoretical modelling and experimental investigation using digital image analysis

An automated non-intrusive image analysis method has been developed for following the course of solids mixing in two-dimensional bubbling fluidized beds. In this investigation, experimental data have been obtained on the axial mixing of uniform solids. Oscillations in the concentration response, resulting from the gross circulation of the solids, have been observed experimentally. These oscillations become increasingly more prominent as the bed particle size increases. These measurements have been used to evaluate the three-phase counter-current back-mixing model (Gwyn et al.). The bubble parameters required for the model were obtained from independent experiments conducted as a part of this investigation; the exchange coefficient however, was found by parameter estimation using the solids mixing data. With this choice of parameters, the counter-current flow model has been found to predict the experimental trends reasonably well. The estimated values for the exchange coefficient do not compare favourably with the predictions of the models available in the literature (Yoshida and Kunii, and Chiba and Kobayashi). These models predict that the wake exchange coefficient should increase with increase in the minimum fluidization of the bed particles. Our results, on the other hand, show that the wake exchange coefficient increases with UO/Umf for UO/Umf < 3 and the values, in this region are independent of the particle size. In line with these results, the experimental measurements of Chiba and Kobayashi, for injected bubbles in a two-dimensional fluidized bed of particles smaller than those used in this investigation, are found to be in excellent agreement with the lower bound of our estimations.

Agglomeration and defluidization under simulated circulating fluidized-bed combustion conditions

Abstract The role of inorganic matter in causing agglomeration and defluidization during the circulating fluidized-bed combustion of high-sodium, high-sulfur low-rank coals was reported in a previous paper. It was found that the ash formed during the combustion of these coals is transferred to the surface of the bed particles. The ash coating thus formed contains a molten phase at the furnace temperature, which makes the bed particles sticky. During the experiments, the effects of temperature and coal quality on the extent of agglomeration were also measured. The results, reported here, show that the tendencies for agglomeration and defluidization to occur increase markedly at temperatures above the initial ash sintering temperature when the ash coating on the bed particles exceeds a critical thickness. The proportions of the melt-forming inorganic elements and mineral inclusions in the ash also have a pronounced effect on agglomeration and defluidization tendencies. The results are considered to be of significance in the development of mechanistic models for agglomeration and defluidization.

The role of inorganic matter in coal in the formation of agglomerates in circulating fluid bed combustors

Circulating fluid bed combustion is increasingly being adopted for large scale power generation from low-rank coals. However, these coals often contain alkalis which are known to cause agglomeration and defluidization in CFBC systems through mechanisms that are not yet fully understood. A laboratory-scale fluid bed combustion system has been used to carry out experiments with coals containing various amounts of sodium chloride, organically bound alkali and alkali earth metals. The bed material and fine ash removed from the furnace have been characterized by various analytical techniques. It was found that, in the stirred environment of circulating fluid bed combustors, a portion of the ash is deposited on the surface of the bed particles. This ash coating contains a molten phase which causes the bed particles to stick together and/or defluidize. The ash formation studies reported in a previous paper and the interaction of ash with bed material reported in this paper describe a sequence of events that can explain the role of the inorganic matter in agglomeration and defluidization.

Measurement and modelling of bubble parameters in a two-dimensional gas-fluidized bed using image analysis

Abstract Bubble size is an important parameter in the design of fluidized bed reactors. A novel method based upon digital image analysis has been developed to automate the measurement of bubble properties in gas-fluidized beds. In order to identify the bubbles, the gray-level image is segmented by applying a global threshold. The distributions of various bubble properties as a function of bed position and fluidizing velocity are presented. Results are compared with theoretical predictions using a population balance model The study shows that the model in which coalescence is considered as the dominant growth mechanism may not be adequate.

Transport phenomena in multi-particle systems—II. Particle-fluid heat and mass transfer

Particle-fluid heat/mass transfer in multi-particle systems is considered. The approach developed extends the model for momentum transfer reported in an earlier paper. Based on a boundary layer formulation for individual particles in the sphere assemblage, the model enables a unified consideration of fixed, distended, fluidized beds as well as isolated spheres. A simple relationship between the heat/mass transfer characteristics and the hydrodynamics of multi-particle systems is obtained by linking Sh/She=1 to the voidage function. The predictions of the model are compared with reported experimental data.

Transport phenomena in multi-particle systems—IV. Heat transfer to a large freely moving particle in gas fluidized bed of smaller particles

Abstract In fluidized-bed combustion, relatively few larger and lighter coal particles are fluidized along with more dense and smaller sulphur-sorbent bed particles. Predictions for drying, ignition and devolatilization, as well as particle temperatures during char combustion, require knowledge of the external heat transfer coefficients. Several studies have dealt with mass transfer to freely moving ‘active’ particles. Heat transfer to fixed immersed objects in fluidized beds has also received wide research attention. Very few studies have addressed heat transfer to freely moving ‘active’ spheres. This paper proposes a mechanistic model for heat transfer with the effect of ‘active’ particle motion taken into account. The average heat transfer coefficients calculated from the model compare well with experimental data in the literature. The model predictions are also compared with available empirical correlations. The implications of the model predictions for modelling fluidized-bed combustion are also discussed.

A single particle model for the evolution and combustion of coal volatiles

Abstract A single particle model is proposed for the evolution and combustion of coal volatiles. The analysis is divided into preignition and postignition stages. The rate limiting steps for devolatilisation are assumed to be heat transfer (both to and through the coal particle) as well as chemical reaction (overall decomposition represented by the distributed activation energy kinetics). Approximate expressions are proposed for the estimation of volatile burnout times. The model results are compared with the experimental data reported for single coal particles in stagnant as well as convective oxidizing environments. The application of the model to fluidized beds is discussed. Model predictions are also compared with the volatile burnout times in fluidized beds.

Circulatory motion of a large and lighter sphere in a bubbling fluidized bed of smaller and heavier particles

Automated image analysis methods have been used to characterize the circulation pattern of the active particle and to measure its velocity in the bubbling bed

Transport phenomena in multi-particle systems—I. Pressure drop and friction factors: Unifying the hydraulic-radius and submerged-object approaches

Abstract A submerged-object model which accounts for the presence of stagnant zones is developed for predicting the pressure drop in random multi-particle systems. This model is then combined with the hydraulic-radius approach. With appropriate definition of the parameters involved, these approaches are found to yield complementary results. The tortuosity factor is considered as a velocity enhancement coefficient and is then related to the stagnant bed voidage. An expression for the tortuosity factor in random multi-particle systems is obtained. A definition for the multi-particle Reynolds number is developed which is applicable to the entire range of void fractions. A generalized friction factor is also developed from the definition of the friction factor for flow in a conduit and is related to the porosity-dependent drag coefficient for individual spheres. The friction factor reduces to the single-sphere drag coefficient in the limit e→1. The approach, in principle, permits the collapsing of the entire momentum transfer data for multi-particle systems to a single curve coincident with the single-sphere drag coefficient relation. Friction factors calculated from the experimental data reported in the literature on packed, fluidized and distended beds, as well as sedimenting systems, are in good agreement with the theory for laminar as well as turbulent regimes. An expression for the relative velocity of sedimenting suspensions, applicable to a wide range of Reynolds number and void fractions, is obtained. An approximation for the estimation of the minimum fluidizing velocity is also developed and compared with the data available in the literature.