C.M.H. Brereton

Microstructural aspects of the behaviour of circulating fluidized beds

Abstract Local instantaneous and time-averaging suspension densities were determined in a 152 mm diameter by 9.3 m tall circulating fluidized-bed riser using a needle capacitance probe inserted from the side. Radial profiles were obtained at three different heights and for three different solids circulation fluxes for sand particles of mean diameter 148 μm. The results confirm that local densities are greater near the wall than in the interior of the column. The fast-fluidization regime is shown to consist of a developing flow with transfer of particles to and from the wall region. An intermittency index, which would be zero for perfect core—annulus flow and one for perfect cluster flow, is used to characterize the flow; it shows that the behaviour is always between these two limits but tending towards the former with increasing height up the column.

Particle velocity profiles in a circulating fluidized bed riser of square cross-section

Abstract A five-fibre optical particle velocity measuring system was used to determine vertical velocities and the fractions of rising and falling particles in a circulating fluidized bed riser of 146 mm × 146 mm square cross-section. The superficial gas velocity and solids circulation rate are shown to affect lateral and axial profiles of particle velocity and the fraction of particles moving upwards. The thickness of the outer annulus downflow region first decreased with height until a minimum was reached and then became thicker towards the top of the riser. Because of the exit effect, lateral profiles of particle velocity were asymmetric at the top of the riser and the wall downflow layer was thicker on the side opposite to the exit than on the exit side. The corners were also found to be regions of greater downflow.

Modelling of circulating fluidised-bed solids flow and distribution

Abstract A comprehensive, predictive model is presented that describes the flow and distribution of solids within a circulating fluidised bed (CFB). Dense particle sheets or streamers are assumed to fall at the walls, surrounding a rapid dilute suspension upflow. Mechanisms and relationships for interchange of solid particles between the streamers and dilute suspension are given. The fraction of solids reflected internally at the reactor exit and the suspension density at the solids return location constitute the model boundary conditions. Variations with reactor height in axial suspension density, fractional wall coverage by streamers, and renewal rate of solids at the wall are predicted. Although developed primarily for prediction of circulating fluidised-bed combustor operation, the model may be used for other CFB applications. Model results may be incorporated in detailed modelling of bed-to-wall heat transfer. Model relationships utilise data obtained from the University of British Columbia 115 kWth pilot-scale unit. The models predictive capability was tested against data provided by Studsvik Energy of Sweden obtained from their 2.5 MWth combustor. Good prediction of the Studsvik density profiles was obtained.

Voidage profiles in a circulating fluidized bed of square cross-section

Abstract Using an optical fiber particle concentration measuring system, an extensive study was carried out of voidage profiles in a circulating fluidized bed riser of square cross-section. Both lateral and axial voidage profiles were obtained. Particle concentration is not necessarily lowest at the axis of the riser, but may go through a minimum between the wall and the axis. The study also reveals the influence of the corners on the voidage profile. Because of the exit effect, profiles of voidage at the top of the riser are asymmetric. Bimodal and even trimodal probability distributions of particle concentration are found. The intermittency index is used to characterize the heterogeneity/homogeneity of the flow.

Instantaneous local heat transfer and hydrodynamics in a circulating fluidized bed

Local heat transfer mechanisms and hydrodynamics are studied in a 9.3 m tall, 152 mm i.d. transparent cold model circulating fluidized bed for 171 μm Ottawa sand. Instantaneous measurements at the wall are made with platinum-coated heat transfer probes. For some conditions, simultaneous local voidages are determined using a capacitance probe. Results show that the sudden and dramatic peaks in the measured instantaneous heat transfer coefficients are directly caused by the arrival of strands of particles at the heat transfer surface. Analyses of the capacitance probe signals indicate that these strands possess wide distributions of voidages which vary with the local time-averaged area-averaged suspension density. Simultaneous heat transfer probe measurements further suggest the existence of characteristic residence lengths for these strands. The average falling velocity of the strands is 1.26 m s−1 using high-speed cinematography.

Gas concentration profiles and NOx formation in circulating fluidized bed combustion

Abstract Experiments were conducted to understand competing NO x formation and destruction reactions better in circulating fluidized bed combustion. Both longitudinal and lateral gradients were measured for NO x , O 2 , CO and CH 4 during combustion of solid fuels of differing reactivity in a CFBC pilot unit 152 mm square by 7.3 m tall. Fuels tested were anthracitic, bituminous, subbituminous and lignitic coals and petroleum coke. In all cases there were significant lateral and longitudinal gradients of gaseous species. Profiles of NO x were correlated to a large degree with profiles of oxygen and local char concentration, which in turn were coupled to the CFB hydrodynamics. High partial pressures of oxygen at the base of the unit and in the char-lean core favour NO x formation reactions, while low O 2 partial pressures in the solids-rich annulus and towards the top of the unit favour NO x reduction. When NO x formation and destruction are limited to the char, profiles are readily interpreted. The addition of volatile nitrogen makes the profiles considerably more complex. Limestone used for sulfur capture can augment or reduce overall NO x formation, depending on the percentage volatile matter of the fuel.

The effect of halides on emissions from circulating fluidized bed combustion of fossil fuels

Combustion tests involved addition of chlorides and bromides while burning Highvale coal were carried out in a pilot-scale (152 mm square × 7.3 m tall) circulating fluidized bed combustor (CFBC). The halogens were added in the form of hydrochloric acid (HCl) and calcium bromide (CaBr2.12H2O) solutions (10 wt%). The thermodynamic equilibrium composition of CFBC products was calculated using the ASPEN database package. The calculations predict 12% conversion of HCl to CaCl2 for an HCl feed rate of 9.78 kg h−1 and complete conversion of CaBr2 to HBr in the vapour phase throughout the range of CaBr2 solution feed rate investigated. The experimental results indicate that chloride and bromide additions increase the CO and SO2 concentrations in the flue gases, with corresponding decreases in the NOx level. The halides have no significant effect on N2O emission. The CO level increased from 27 to 230 ppmv when the chloride concentration in the reactor was ∼4200 ppmv (Cl/fuel = 4.58 wt%). The effect of bromide on CO emission was more dramatic, the CO emission jumping from 56 to 480 ppmv for lower concentrations of bromide (20–400 ppmv). The experiments confirm previous work showing that halide-containing species inhibit CO oxidation through interaction with the hydrogen-oxygen radical pool. The decrease in NO concentration with chloride addition can be explained by surface modification of CaO particles due to formation of a liquid calcium chloride phase favoured by high HCl concentrations near the feed point. The formation of the liquid calcium chloride phase has the potential to make the CaO surface unavailable, thereby reducing catalytic oxidation of volatile nitrogen to NO.

Influence of operating parameters on NOitx emissions from a circulating fluidized bed combustor

A parametric study of NOitx emissions has been carried out on a 150 mm2 × 7.3 m tall pilot scale circulating fluidized bed combustor with coals of various ranks, including anthracite, bituminous, subbituminous and lignite, as well as one petroleum coke. Increasing either temperature or excess air was found to increase NOitx emissions significantly in the ranges studied. The effect of staged combustion on NOitx emission depended on the level at which the secondary air was introduced. A reduction of NOitx emission was usually, but not always, observed when secondary air was introduced 3.4m above the base. Limestone addition promoted NOitx formation for high volatile bituminous coal, due to the catalytic effect of limestone on oxidation of volatile nitrogen. However, adding limestone during petroleum coke combustion reduced NOitx emissions. A relationship exists between fuel nitrogen to NOitx conversion and the volatile content of the fuel, with fuels of higher volatile content emitting more NOitx.

Suspension-to-membrane-wall heat transfer in a circulating fluidized bed combustor

Suspension-to-membrane-wall heat transfer was studied in a pilot scale circulating fluidized bed combustor at 700 and 800°C for suspension densities of 10–70 kg/m3 with 286 μm silica sand particles. The total suspension-to-membrane-wall heat transfer coefficient is determined using the exposed-pipe-radial, insulated-pipe-tangential, fin-one-dimensional heat transfer model of Bowen et al. [B.D. Bowen, M. Fournier, J.R. Grace, Heat transfer in membrane waterwalls, Int. J. Heat Mass Transfer 34 (1991) 1043–1057]. Suspension-to-pipe and suspension-to-fin transfer is also determined from temperatures measured by embedded thermocouples. The fin accounted for about one third of the total heat transfer, and most of this portion passed through the insulated portion of the pipe.

Local heat transfer, solids concentration and erosion around membrane tubes in a cold model circulating fluidized bed

Abstract Measurements of local solids concentration, heat transfer and erosion were obtained in a 152 mm diameter x 9.3 m tall circulating fluidized bed operated at room temperature with 200 μm sand particles at a superficial gas velocity of 7.0 m s−1 with different solids circulation rates. Local solids concentrations were found to be higher in the fin region than on the crests of membrane tubes. For the small heat transfer surfaces investigated, this led to higher local heat transfer coefficients in the fin region, a trend which is the reverse of that for long heat transfer surfaces. Erosion rates were higher on the tube crests than on the fins.