Antonio Cammarota

L-valve behaviour with solids of different size and density

Abstract A small diameter L-valve, 27 mm i.d., was inserted in a circulating fluidized bed loop with no solids reservoir and operated under different experimental conditions. Five materials (three sands, a spent FCC (fluid cracking catalyst] and a magnetite), belonging to groups A and B of the Geldart classification, were selected in order to investigate separately the effect of particle size and density. For each solid, the workability range of the apparatus was determined by varying the flow rate of the aeration gas and measuring the corresponding solids flow rate through the valve. The static pressure balance around the loop was determined for each experimental condition. Results were compared with those of previous investigations in order to correlate the pressure drop throughout the L-valve with the solids discharge flux.

THE HYDRODYNAMIC BEHAVIOR OF TWO CIRCULATING FLUIDIZED BED UNITS OF DIFFERENT SIZES

ABSTRACT The hydrodynamic behaviors of two Circulating Fluidized Beds (CFB) with fast columns of 0.041 and 0.120 m ID have been compared. Fluid Cracking Catalyst and Glass Beads 70 and 90 μm average size respectively have been used as bed solids. Bed Blow Out experiments have also been performed. They indicate the close relations between CFB and fluidized beds subject to vigorous elutriation. S-shaped voidage axial profiles were observed in the fast columns. Derivatives at the inflection point of profiles obtained in these and previous works have been correlated to flow variables.

The phenomenology of comminution in the fluidized bed combustion of packaging-derived fuels

Abstract Two commercially available packaging-derived fuels (PDFs), obtained as a result of recycling programs for municipal solid wastes, were fed batchwise into two laboratory scale bubbling fluidized bed combustors. One fuel tested came from a mono-material collection o PET bottles while the other was obtained by means of a mechanical selection of plastic containers. Two experimental procedures were set up to separately investigate primary fragmentation of fuels and carbon fines generation from devolatilized particles. Results were compared with those obtained with three other waste-derived fuels, namely a refuse-derived fuel, a tyre-derived fuel and an ebonite. After devolatilization, PDF particles generated char-send aggregates that greatly increased the fixed carbon residence time in the fluidized bed reactors. Carbon fines generation rates, under both inert and oxidizing conditions, were much lower than those measured in the tests with other high-volatile fuels. A new descriptive model was proposed to describe the overall combustion-comminution pattern of fixed carbon during the fluidized bed combustion of the packaging-derived fuels tested.

Secondary fragmentation of a char in a circulating fluidized bed combustor

Secondary fragmentation of Kentucky No. 9 chars, obtained with two different procedures of devolatilization, has been studied in a laboratory scale circulating fluidized bed combustor. The apparatus essentially consists of a 41 mm ID, 1.92 m high riser, a solids collecting system, a 41 mm ID recirculation column. Gas velocity and size of inert material have been varied in ranges of practical interest. An appropriate experimental technique has been developed to isolate secondary fragmentation effects from those related to other comminution phenomena taking place during fluidized bed combustion of chars. Particle multiplication factor, i.e. the number of particles generated per one char particle by secondary fragmentation, has been determined for each set of experimental conditions. The probability that a shrinking particle of a given size breaks into fragments and the fragments size distribution under similarity hypothesis have also been determined. These fragmentation functions are embodied into carbon particle population balances which are the basis of circulating fluidized bed combustion modelling.

Break-up of cylindrical clusters of solid particles under gravity flow in a two-dimensional column

Abstract A 0.012 m × 0.12 m two-dimensional column has been used to study gravitational motion and break-up of 0.032 m diameter cylindrical clusters of solid particles. Clusters were analysed by means of a KODAK SPIN PHYSICS SP2000 Motion Analysis System. Shape deformations, volume expansions and surface perturbations have been estimated for clusters of various solids at different distances from the point of release. Aspect and expansion ratios, times for onset of surface perturbations, and also cluster fall-velocities have been measured. Depending on the nature of the solids, three types of cluster have been considered: undeformable clusters, gas-impermeable clusters and gas-permeable clusters. Particle-to-particle forces are considered responsible for compacting solids of undeformable clusters. Two dimensionless numbers have been developed for the case of gas-impermeable clusters: the first is related to the aspect ratios of the cluster, whereas the second is related to the time needed for the appearance of surface perturbations on the cluster front. It has been suggested that the ratio of particle to cluster size controls expansion and break-up of gas-permeable clusters.

Primary and secondary fragmentation of coals in a circulating fluidized bed combustor

Primary and secondary fragmentation of two Kentucky No. 9 coals, having similar proximate and ultimate analyses but different swelling indexes (2.5 and 9, respectively), were studied in a laboratory scale circulating fluidized bed combustor (CFBC). The apparatus, having a 41-mm i.d. and 1.92-m-high riser, was operated keeping fixed the gas velocity and the size of inert bed material at values of practical interest. Two experimental procedures were used to separately investigate primary and secondary fragmentation effects taking place during fluidized bed combustion of coals. Particle multiplication factor, i.e., the number of particles generated per one mother particle, was used to quantify these effects. Statistical functions of fragmentation (the probability of breakage by primary fragmentation, the probability density that a shrinking particle of a given size breaks into fragments, and the size distribution of subparticles produced by secondary fragmentation) were also determined and embodied into an available model for circulating fluidized bed combustion of coals. On the basis of this mathematical model, the relevance of primary fragmentation on some output variables chosen to characterize the performance of a circulating fluidized bed combustor was quantified.

Dual-Fuel Fluidized Bed Combustor Prototype for Residential Heating: Steady-State and Dynamic Behavior

Fluidized bed combustion of biogenic fuels can be recognized as an attractive option for an ecologically sustainable use of biofuels in residential applications. Nevertheless, biomass combustion in fluidized bed reactors presents some drawbacks that are mainly related to mixing/segregation of fuel particles/volatile matter during devolatilization inside the bed and in the freeboard or to bed agglomeration. A prototype of a 30–50 kWth fluidized bed boiler for residential heating has been designed to burn either a gaseous combustible or a solid biomass fuel or both fuels at the same time. The prototype has been equipped with a gas burner located in the wind-box to optimize the start-up stage of the boiler and with a fluidized bed characterized by a conical geometry (“Gulf Stream” circulation) to improve the mixing of the fuel particles during both devolatilization and char burn-out. The operation of the combustor adopting wood pellets as fuel has been investigated to evaluate their use in residential combustion applications. Steady-state thermally stable regimes of operation have been recognized analyzing both boiler temperatures and gaseous emissions. The optimization of the steady-state operation of the boiler in terms of gaseous emissions has been achieved by varying the nominal thermal power and air excess. An ad-hoc experimental campaign has been carried out to analyze the dynamic performance of the prototype as a response to changes of the demanded thermal power. On the basis of the experimental data, an interpretation of the dynamic behavior of the fluidized bed boiler has been proposed.

Bed Agglomeration During the Fluidized Bed Combustion of Olive Husk

The fluidized bed combustion of a biomass residue (olive husk) common in the Mediterranean area has been investigated in a bench scale reactor. The focus of the study was the high propensity of this fuel to give rise to bed agglomeration problems during combustion, as a consequence of the high alkali content of the ash. Bed agglomeration characteristic times as well as temperature and pressure gradients were measured at different operating conditions. In addition, a diagnostic tool based on the measurement of the dynamic pressure signal inside the bed was tested for its capability to predict the bed agglomeration onset.Copyright