Riccardo Chirone

The influence of the frequency of acoustic waves on sound-assisted fluidization of beds of fine particles

Abstract Sound-assisted fluidization of nonfluent 0.5–45 μm catalyst particles has been studied with a 145 mm i.d. column. Different amounts of solids of weight W ranging from 1 to 3 kg have been charged in the column. A loudspeaker generated an acoustic field, above the bed, with a sound pressure level SPL (referred to 20 μPa) varying from 110 to 140 dB and a frequency f varying from 30 to 1000 Hz. The improvement of the quality of fluidization obtained with certain combinations of W , SPL and f has been attributed to the breakup of clusters originally forming the bed into subclusters. For given W and SPL, the ranges of frequency within which channel-free homogeneous fluidization could be obtained have been determined, and within these ranges the kinds of curves for sizes of subclusters d s as a function of the frequency have been outlined. The nonmonotonic form of these curves could not be explained by means of the original sound-assisted fluidization model, which assumes a rigid cluster-subcluster structure. The existence of elastic forces between clusters and subclusters, assumed by the cluster/subcluster oscillators model, yields theoretical d s versus f curves with the same trend as those from experiments.

The influence of fine char particles burnout on bed agglomeration during the fluidized bed combustion of a biomass fuel

Abstract The combustion of biomass char in a bubbling fluidized bed is hereby addressed, with specific reference to the influence that the combustion of fine char particles may exert on ash deposition and bed agglomeration phenomena. Experiments of steady fluidized bed combustion (FBC) of powdered biomass were carried out with the aim of mimicking the postcombustion of attrited char fines generated in the fluidized bed combustion of coarse char. Experimental results showed that the char elutriation rate is much smaller than expected on the basis of the average size of the biomass powder and of the carbon loading in the combustor. Samples of bed material collected after prolonged operation of the combustor were characterized by scanning electron microscopy (SEM)–EDX analysis and revealed the formation of relatively coarse sand–ash–carbon aggregates. The phenomenology is consistent with the establishment of a char phase attached to the bed material as a consequence of adhesion of char fines onto the sand particles. Combustion under sound-assisted fluidization conditions was also tested. As expected, enhancement of fines adhesion on bed material and further reduction of the elutriation rate were observed. Experimental results are interpreted in the light of a simple model which accounts for elutriation of free fines, adhesion of free fines onto bed material and detachment of attached fines by attrition of char–sand aggregates. Combustion of both free and attached char fines is considered. The parameters of the model are assessed on the basis of the measured carbon loadings and elutriation rates. Model computations are directed to estimate the effective size and the peak temperature of char–sand aggregates. The theoretical estimates of the effective aggregate size match fairly well those observed in the experiments.

Some issues in modelling bubbling and circulating fluidized-bed coal combustors

Abstract Fundamental aspects of coal combustion in the two types of atmospheric fluidized combustors, i.e. those based on bubbling and on circulating technologies, have been studied in order to determine those variables which play a dominant role in the reactor performance. The analysis is based on two relatively simple models, which take into account carbon surface reaction, intraparticle and external diffusion as well as phenomena of comminution of coal and its char. Performances of the two types of fluidized-bed combustors are discussed with reference to coal combustion only, without taking into consideration the emission issues. Sensitivities of main output variables to changes in fuel properties, design and operating variables are evaluated.

Combustion of Single Coal Char Particles under Fluidized Bed Oxyfiring Conditions

In this work combustion of single coal char particles was studied at 850°C in a lab-scale fluidized bed under simulated oxyfiring conditions. The burning rate of the particles was followed as a function of time by continuously measuring the outlet CO and O2 concentrations. Some preliminary evaluations on the significance of homogeneous CO oxidation in the reactor and of carbon gasification by CO2 in the char were also carried out. Results showed that the carbon burning rate increases with oxygen concentration and char particle size. The particle temperature is approximately equal to the bed one up to an oxygen concentration of 2%, but it is considerably higher for larger oxygen concentrations. Both CO2 gasification of char and homogeneous CO oxidation are not negligible. The gasification reaction rate is slow and it is likely to be controlled by intrinsic kinetics. During purely gasification conditions the extent of carbon loss due to particle attrition by abrasion (estimated from the carbon mass balance) appears to be more important than under combustion conditions.

The fate of fixed carbon during the fluidized-bed combustion of a coal and two waste-derived fuels

The fate of fixed carbon during fluidized-bed combustion of a bituminous coal and two alternative fuels. a refuse-derived fuel (RDF) and a tyre-derived fuel (TDF), was investigated. A simple model was developed based on the assumption that fixed carbon present in the bed could be lumped into a coarse particles phase and a fine carbon phase. The model is based on a network of paths representing the generation of coarse and fine char particles from the parent fuel by primary fragmentation, the fine particle production by comminution of coarse char, the combustion of the coarse and of the fine char particles, as well as the elutriation of fines. Results of computations of carbon conversion were in good agreement with those measured in batchwise experiments with each of the three fuels. Conversion of the coal takes place mainly via fuel devolatilization to coarse char which further reacts to gaseous products. Conversion of TDF occurs via the generation, upon devolatilization, of amultitude of fines which eventually undergo combustion and elutriation. The phenomenology associated with RDF fluidized-bed combustion is intermediate between those of coal and TDF. The resulting framework for the evaluation of fixed carbon balance helps in identifying key factors in the conversion of such widely different fuels.

Fluidized bed combustion of tyre derived fuel

Abstract Mechanistic aspects of the fluidized bed combustion of tyre derived fuel (TDF) have been analyzed both experimentally and theoretically. The time-resolved rates of carbon fines elutriation during the fluidized bed combustion of TDF have been measured during batch experiments in a bench scale reactor at 850 °C, under different operating conditions. Experimental results indicated that both gas superficial velocity and oxygen partial pressure exert influence upon the overall fixed carbon combustion efficiency. The efficiency increases slightly with the oxygen concentration and significantly if the gas superficial velocity decreases. Experimental data are further analyzed in the framework of a fluidized bed combustor model especially suited for high-volatile solid fuels feedings. The model takes into account phenomena that assume particular importance with this kind of fuels, namely fuel particle fragmentation in the bed and combustibles segregation and postcombustion above the bed. Experimental and model results indicate that the efficiency of the fluidized bed combustion of TDF is controlled by the competition between combustion and entrainment of char fines and volatile matter released in the early stage of fuel conversion.

Regeneration Strategies of Deactivated Catalysts for Thermo-Catalytic Decomposition Process in a Fluidized Bed Reactor

The present article addresses on the possibility of carrying out a two-stage operation for hydrogen production from methane Thermo-Catalytic Decomposition (TCD) in a fluidized bed, consisting into first operating the reactor for methane decomposition, until a defined catalyst deactivation degree is approached, and then operating the fluidized bed as a combustor or a gasifyer for catalyst regeneration by carbon removal. Three different strategies of catalyst regeneration, carbon combustion in air, CO2 gasification and steam gasification, have been analyzed. The regeneration strategies have been compared on the basis of the efficiency of carbon removal and the performances of regenerated catalyst with respect to the TCD process. The effect of multiple cycles of decomposition and regeneration steps has been also quantified. A reasonable cyclic process has been simulated switching between two different feeds, the first containing CH4 and the second containing air. The effect of different air regeneration times on the product distribution in the CH4 decomposition phase has been tested to individuate its optimal value.

Fluidized Bed Combustion of a Biomass Fuel: Comparison Between Pilot Scale Experiments and Model Simulations

In the present work the efficiency of the fluidized bed combustion (FBC) of high-volatile fuels and the extent of volatile matter post-combustion in the splashing zone and freeboard are investigated. A typical Mediterranean biomass (pine-seed shells) has been burned in a pilot-scale bubbling FB combustor (200 kWt) at different operating conditions. Both over-and under-bed fuel feeding options have been considered. A FBC model specifically developed for high-volatile fuels has been also applied to provide a comparison with bed carbon loading, in-bed heat release and splashing region temperature experimental data

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.

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.