Pressure fluctuations as a diagnostic tool for fluidized beds

Detection of agglomeration and gradual particle size changes in circulating fluidized beds

High-density circulating fluidized beds (CFB) differ in several respects from low-density CFB systems. In high-density CFB risers, solids move upward throughout the entire riser cross-section, and the net downflow of particles at the wall, a commonly observed feature of fast fluidized beds, is absent. Hence there exists a transition regime from the low density to high density CFB where the net particle motion in the vicinity of the wall is changing from downwards to upwards. This was confirmed by experiments carried in a dual-loop high-density CFB facility with concentric-tube heat exchanger installed in the riser. Local suspension-to-wall heat transfer coefficient and suspension temperature distribution below and above the heat exchange section were measured. Experimental results elucidated that particles move both upwards and downwards in the vicinity of the wall for the operation conditions studied. This alternation of direction leads to higher heat transfer coefficients at both ends of the heat exchange.

The loop-seal (LS) of a circulating fluidized bed (CFB) reactor is the heart of the system and is still under consideration due to complexities in both two-phase flow and the geometry. The LS controls the solid circulation rate (SCR) of particles that is crucial for the stable operation of a CFB system. In this study, the effect of the aeration rate on the circulation rate of two-chamber LS for a 10 MWth CFB gasification system is examined. The influence of aeration rate supplied to the recycle and the supply chambers of the LS is investigated in detail with 44 different cases using computational particle fluid dynamics (CPFD) method. Barracuda VR software is used to model the whole CFB system with non-reacting isothermal flow assumption. The solid circulation rates are investigated for various aeration rates corresponding to different operating regimes. The stable and unstable operating conditions are identified according to monitored pressure fluctuations. Moreover, three regimes are revealed based on the analysis of differential pressure between two chambers of LS. It is found that the region for the stable operation, corresponding to higher SCR and lower pressure fluctuations, exists for a certain range of aeration rates and the aeration ratio between two chambers of LS.

Modeling of axial bed-to-wall heat transfer in a CFB combustor with abrupt riser exit geometry

The circulating fluidized bed (CFB) is one of the complex nonlinear systems which has gained acceptance in a wide variety of fields like catalytic cracking, power generation and mineral processing. Compared with conventional fluidized beds, CFB have many advantages including better interfacial contacting and reduced back mixing. CFB is a relatively new method of forcing chemical reactions to occur in the chemical and petroleum industries. In the absence of conventional means to derive a reliable model, we have devised a model of the circulating fluidized bed using neural networks, which have the ability to characterize such complex systems through their non-linear mapping. The main objective is to develop a real time NN model to simulate and control the CFB. It has been shown that the attempt has been a successful and the results are presented.

NO and N2O emissions from circulating fluidized bed (CFB) boilers are determined by their formation and destruction rates in the furnace. The effect of circulating ash from a CFB boiler on NO and N2O emissions were investigated in a laboratory-scale fluidized bed reactor. The results show that the residue char in circulating ash and the CO generated from the char play an important role in NO reduction and N2O formation; however, active components of circulating ash such as CaO, Fe2O3 accelerate the decomposition of N2O. Experiment was also conducted on a 75 t/h CFB boiler fueled with the mixture of anthracite and biomass. The lower residue carbon content of circulating ash in this experiment is lower; therefore, the reacting rate of NO deoxidize is limited. This result verified the conclusion of laboratory research.

This work presents an investigation of the application of chaotic invariants in different regions of an isothermal experimental fluidized bed column, capable of quantifying the fluidization regimes, emphasizing the identification of the fast regime. The work was divided in two stages, the bench design and study of regions characteristic of a fluidization column, from numerical simulation, and non-linear analysis from time series of static pressure of experimental CFB (circulating fluidized bed) column. The numerical simulation-assisted design phase brought together theories and fundamentals of computational fluid dynamics through the Two Fluid Model (TFM) model, to understand the multiphase flow in a circulating fluidized bed (CFB) and mapping study of its characteristic regions in several fluidization regimes. The experimental phase consisted of the assembly of bench and preliminary tests for noise attenuation for observation of time series of the signal of static pressure fluctuation in the characteristic regions. In the test stage, three types of particles were used (355 m glass, 1.0 mm sand and 1.2 mm sand), when subjected to specific operating conditions (incoming air flows), they developed the investigated fluidization regimes (expanded, bubbling, turbulent, and fast). The definition of the reference fluidization regimes is based on the literature description and corrected through the transparency of the experimental LFC column. The nonlinear analysis, which consisted in the evaluation of the evolution of chaos invariants such as the correlation dimension, Kolmogorov entropy and the Hurst exponent, of time series of static pressures, collected from three distinct regions of a fluidization column, subjected to different regimes and type of particles.

A new structural parameters model based on drag coefficient for simulation of circulating fluidized beds

Dynamic Modelling and Simulation of Fast Pyrolysis Process Integrated with Circulating Fluidized Bed Boiler

Comments on “propagation characteristics of pressure disturbances originated by gas jets in fluidized beds”

Experimental research on the unstable performances of parallel external loops in the circulating fluidized bed

In this research article, experimental study was carried out to obtain the heat transfer characteristics between a submerged horizontal tube bundle and a fluidized bed in a large-scale circulating fluidized bed (CFB) boiler with an external heat exchanger (EHE). The operational parameters in the tube EHE were measured during performance tests at variable load conditions. The average heat transfer coefficient (HTC) was calculated using a mechanistic heat transfer model based on packed renewal theory. The heat transfer characteristics are considered in terms of heat transfer mechanisms such as emulsion phase convection, gas convection and also thermal radiation. The obtained heat transfer data exhibit a maximum value with variation mean bed particle size irrespective of pressure. The results showed that the average HTC increases with a decrease of the Sauter mean particle diameter and with the increase of the fluidizing number as a result of good mixing dynamics in emulsion phase (i.e. emulsion wall contact time, bubble fraction in the bed). Based on the heat transfer data, empirical correlations are proposed for predicting a heat transfer coefficient from fluidized bed to horizontal tube bundle. The mechanistic heat transfer model predicted the average HTC in sufficiently good agreement with CFB boiler data accessible in the literature.

Chemical Looping for Energy Technology: A Special Issue

Hydrodynamic behaviour of aerogel powders in high-velocity fluidized beds

Enhanced combustion behavior and NOx reduction performance in a CFB combustor by combining flue gas recirculation with air-staging: Effect of injection position