Fabrizio Scala

Attrition of sorbents during fluidized bed calcination and sulphation

The attrition behavior of two different limestones during calcination and sulphation in fluidized beds has been investigated by a combination of experimental techniques. The aim of the study is to shed light on the interactions between sorbent attrition and the change of particle mechanical and morphological properties associated with the progress of chemical reactions. A number of different experimental techniques have been used to characterize breakage mechanisms relevant to particle attrition in different sections of industrial fluidized bed reactors operated at atmospheric pressure. Primary fragmentation and abrasive attrition were characterized in situ by means of experiments carried out in a bench-scale fluidized bed reactor operated batchwise. Fragmentation under high velocity impact conditions was studied ex situ by means of single particle impact tests on pre-conditioned samples at room temperature. Scanning electron and optical microscopy analyses of the particles and EDX mapping of polished particle cross-sections were used to relate topography and internal composition of sorbent particles to the attrition mechanism.

Modelling fluidized bed combustion of high-volatile solid fuels

Abstract A model of an atmospheric bubbling fluidized bed combustor operated with high-volatile solid fuel feedings is presented. It aims at the assessment of axial burning profiles along the reactor and of the associated temperature profiles, relevant to combustor performance and operability. The combustor is divided into three sections: the dense bed, the splashing region and the freeboard. Three combustible phases are considered: volatile matter, relatively large non-elutriable char particles and fine char particles of elutriable size. The model takes into account phenomena that assume particular importance with high-volatile solid fuels, namely fuel particle fragmentation and attrition in the bed and volatile matter segregation and postcombustion above the bed. An energy balance on the splashing zone is set up, taking into account volatile matter and elutriated fines postcombustion and radiative and convective heat fluxes to the bed and the freeboard. Results from calculations with a high-volatile biomass fuel indicate that combustion occurs to comparable extents in the bed and in the splashing region of the combustor. Due to volatile matter segregation with respect to the bed, a significant fraction of the heat is released into the splashing region of the combustor and this results in an increase of the temperature in this region. Extensive bed solids recirculation associated to solids ejection/falling back due to bubbles bursting at bed surface promotes thermal feedback from this region to the bed of as much as 80–90% of the heat released by afterburning of volatile matter and elutriated fines. Depending on the operating conditions a significant fraction of the volatile matter may burn in the freeboard or in the cyclone.

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.

Modeling flue gas desulfurization by spray-dry absorption

A detailed model for flue gas desulfurization by spray-dry absorption with a lime slurry is presented. The model combines a steady state one-dimensional spray-dryer model with a single-drop model for SO2 absorption with instantaneous irreversible reaction in a rigid droplet containing uniformly dispersed fine lime particles. The fate of the droplets is followed from atomization until formation of a porous coherent shell around the drying droplets. The model results were validated against available experimental spray-dry FGD results, showing excellent agreement at low to medium Ca/S feed ratios. The model was then used to study the relevance of the different resistances to SO2 absorption and to predict the influence of the main operating variables on the spray-dryer desulfurization performance. Analysis of variables profiles along the spray-dry column showed that the initial droplet velocity has no influence on model results and that the initial droplets decelerating phase always accounts for negligible SO2 capture. Results further showed that the controlling resistance to SO2 absorption shifts from a liquid-phase one near the atomizer to a gas-phase one at the column exit. The operating variables that exert the largest influence on the overall desulfurization efficiency are the Ca/S molar feed ratio, the mean initial droplet size and the mean lime particle size. In particular, careful control of the last two variables is critical in order to obtain a good spray-dryer performance.

Fluidized bed technologies for near-zero emission combustion and gasification

Part 1 Introduction to fluidization science and technology: Overview of fluidization science and fluidized bed technologies Particle characterization and behaviour relevant to fluidized bed combustion and gasification systems Properties of stationary (bubbling) fluidised beds relevant to combustion and gasification systems Properties of circulating fluidized beds relevant to combustion and gasification systems Heat and mass transfer in fluidized bed combustion and gasification systems Attrition phenomena relevant to fluidized bed combustion and gasification systems. Part 2 Fundamentals of fluidized bed combustion and gasification: Conversion of solid fuels and sorbents in fluidized bed combustion and gasification Conversion of liquid and gaseous fuels in fluidized bed combustion and gasification Pollutant emissions and their control in fluidised bed combustion and gasification Fluidized bed reactor design and scale up Modelling of fluidized bed combustion processes Modelling of fluidized bed gasification processes Economic evaluation of circulating fluidized bed combustion (CFBC) power generation plants. Part 3 Fluidized bed combustion and gasification technologies: Atmospheric (non-circulating) fluidized bed combustion Pressurized fluidized bed combustion (PFBC) Circulating fluidized bed combustion (CFBC) Fluidized bed gasification Measurement, monitoring and control of fluidized bed combustion and gasification. Part 4 Emerging CO2 capture technologies: Oxy-fired fluidized bed combustion: Technology, prospects and new developments Chemical looping combustion (CLC) Calcium looping for CO2 capture in combustion systems Sorption-enhanced gasification. Part 5 Other applications of fluidized bed technology: Applications of fluidized bed technology in processes other than combustion and gasification.

Attrition of Limestone During Fluidized Bed Calcium Looping Cycles for CO2 Capture

Attrition of a limestone during calcium looping cycles for CO2 capture was studied in a lab-scale fluidized bed apparatus. Batch experiments under alternating calcination–carbonation conditions were carried out to investigate the effect of chemical reactions and temperature changes on the attrition propensity of the sorbent particles. Attrition processes were characterized by following the modifications of bed sorbent particle size distribution and the elutriation rates of fines throughout conversion over repeated cycles. Different bed temperatures and CO2 inlet concentrations during the calcination stage were tested in the experiments. Results show that relatively large attrition rates were experienced by the sorbent particles only during the first cycle. From the second cycle on the attrition rate progressively declines, also during the calcination stage where the softer CaO is produced. It is inferred that the combined chemical-thermal treatment affects the particle structure making it increasingly hard. At the same time, the CO2 capture capacity decays toward an asymptotic level, possibly related to the very same structural modifications. The bed temperature and CO2 concentration both appear to influence the sorbent behavior in the tests.

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.

Dolomite attrition during fluidized-bed calcination and sulfation

The attrition behavior of a dolomite during fluidized-bed calcination and sulfation was investigated in a bench-scale apparatus. Operating conditions of the bed were those typical of atmospheric bubbling fluidized-bed combustion. Batchwise experiments were carried out in order to study the influence of the chemical reactions on the parallel phenomena of particle fragmentation and abrasion in the bed. To this end changes of particle size distribution and fines elutriation rates were followed throughout conversion during both sequential and simultaneous calcination and sulfation. The study was complemented by SEM/EDX analysis of polished sulfated particle cross sections in order to relate the sorbent attrition propensity to the changes of particle morphology upon sulfation.

Assessment of ettringite from hydrated FBC residues as a sorbent for fluidized bed desulphurization

Abstract The performance of synthetic ettringite as a sorbent in fluidized bed desulphurization has been assessed and compared with that of a commercial limestone. Experiments have been carried out in a bench scale fluidized bed reactor under simulated desulphurizing (steadily oxidizing) combustion conditions. Sorbent performance has been characterized in terms of desulphurization rate, maximum sulphur uptake and attrition propensity. Fluidized bed sulphation experiments have been complemented by microstructural characterization of solid samples, accomplished via X-ray diffraction analysis, scanning electron microscopy and sulphur mapping of cross-sections of particles embedded in epoxy resin. Experimental results show that both the rate and the maximum extent of sulphur uptake by ettringite significantly exceed those of the limestone. Maximum degree of free calcium utilization is 0.58 for ettringite compared with 0.27 for the limestone. Sulphation tests also indicate that attrition propensity of ettringite is larger than that correspondingly observed for the limestone. Microstructural characterization indicates that sulphation of ettringite takes place evenly throughout the particle cross-section, whereas sulphation of limestone mostly conforms to a core-shell pattern. Along a parallel pathway, the rate and yield of ettringite formation by hydration of fly ash from a utility fluidized bed boiler have been assessed. Formation of ettringite in these experiments appears to be quantitative upon curing of ash at 70 °C for times up to 4 days.

The influence of sorbent properties and reaction temperature on sorbent attrition, sulfur uptake, and particle sulfation pattern during fluidized-bed desulfurization

The influence of operating parameters such as sorbent properties and reaction temperature on sorbent attrition, sulfur uptake, and particle sulfation pattern during fluidized-bed desulfurization is assessed. Sulfur distribution throughout the particles is evaluated by means of a novel quantitative automated statistical procedure. With the aid of this technique, energy dispersive X-ray sulfur mappings of cross sections of sorbent particles are converted into sulfur distribution density functions, which can be directly related to the prevailing particles sulfation pattern. This procedure is applied to samples of three different sorbents (two limestones and one dolomite) sieved in three particle size ranges and batch-wise sulfated in a fluidized bed at three different temperatures. This analysis is complemented by parallel measurement of calcium conversion degree and elutriated calcium mass during the sulfation tests, as well as by visual inspection of scanning electron microscope micrographs of cross sections of spent sorbent particles discharged at the end of the tests. Experimental results show that the two limestones achieve a larger final sulfation degree than the dolomite. For one of the limestones it was found that the maximum sulfation degree was higher the smaller the particle size and the lower the bed temperature. Results of the statistical analysis on spent sorbent particles reveal that, for most samples, a core-shell sulfation pattern is established. Departure from the core-shell pattern is shown by the finest sorbent particles and by sorbent reacted at the lowest bed temperature investigated, in which a uniform sulfur distribution is achieved consistently with sulfation degree results. The influence of particle size and reaction temperature on sulfur uptake is interpreted in light of the significance of kinetic and intraparticle diffusional resistances assessed by the evaluation of particle Thiele moduli.