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THE PRESENT INVENTION RELATES TO A TREATMENT FOR REACTIVATING LIGHT ASHES, OBTAINED FROM THE COMBUSTION OF FOSSIL FUELS OR FROM THE CO-COMBUSTION THEREWITH CDR OR BIOMASSES INSIDE DUST PHASE BOILERS.

Il 9 gennaio 2001 un incendio ed una esplosione provocarono tre morti, sei feriti e danni ingenti ad una primaria industria laniera operante nel nord-ovest d'Italia. La sequenza degli eventi è stata analizzata sulla base degli atti processuali e di sopralluoghi sul teatro dell'incidente. Inoltre sono stati condotti test sperimentali ad hoc per contribuire a chiarire la dinamica dell'incidente e la sua eziologia. La ricostruzione dell'incidente ha mostrato che esso fu verosimilmente innescato da una iniziale stadio di combustione a brace (smoldering) di polveri deposte all'interno del reparto di stoccaggio lappole, un prodotto di scarto della manifattura laniera generata durante la cardatura usualmente considerato non infiammabile. Le fasi successive dell'incidente si sono sviluppate con modalità tipiche dell'"effetto domino" attraverso deflagrazioni multiple di polveri combustibili derivanti dal processo di lavorazione della lana, seguite da un incendio.

The study addresses the competition between the course of purely pyrolytic processes and heterogeneous oxidation during oxidative pyrolysis of non-conventional fuels. This feature is one key, together with volatile matter flammability and effectiveness of oxygen transport to the particle, to establish whether flaming or glowing ignition of the fuel takes place. A selection of six non conventional high volatile solid fuels is considered in the study, namely two plastics (polyethylene and polyethylene terephthalate), two lignocellulosic materials (Robinia Pseudoacacia and waste wood) and two rubber-derived materials (scrap tyres and ebonite). The analysis is based on the comparison of the behaviour of fuels when heated in a thermogravimetric apparatus under inert and oxidizing atmospheres. Further analysis is directed to assess the heterogeneous combustion kinetics of chars obtained by pre-pyrolyzing in inert conditions samples of the fuels. The study is complementary to a companion paper (Penergy Fuels 2002, 16, 653) where pyrolysis in nitrogen of the same fuels was addressed. Results indicate that heterogeneous oxidation and pyrolytic processes play different roles depending on the nature of the fuel. A general feature of all fuels tested is that burnoff cannot be simply described as sequential reaction paths corresponding to purely thermal degradation and heterogeneous oxidation. Sinergistic effects between these processes are significant and need to be taken into account.

The High Temperature Annular Shear Cell was used was used to characterise the flow properties of five ceramic powder samples with different particle size distributions at ambient temperature and at 500°C. A significant increase of powder cohesion was observed with increasing temperature. A model combining a continuum approach and a particle–particle interaction description was used to correlate the powder tensile strength with the interparticle forces. The dependence of the tensile strength on powder consolidation and temperature is correctly described by the model.

Two ceramic powder samples having different compositions of surface impurities and particle size distributions were considered. These two samples resulted from a high temperature fluidized bed reactor which in its operation showed changes of working condition that might be attributed to the onset of strong interparticle forces. The flow behaviour of these powders was characterized by the High Temperature Annular Shear Cell (HT-ASC), between ambient temperature and 500 °C. Furthermore, a model is developed to relate the change of the powder flowability to the formation of a liquid phase due to the melting of particle impurities present on the particle surface. In particular, the model is used to predict, on the base of the salt composition, the intensity of the interparticle forces at different temperatures. The interparticle forces predicted by the model can be compared with those that can be inferred from the powder flow properties measured with the HT-ASC. Therefore, it is demonstrated that it is possible to derive a theoretical model to predict interparticle forces in a particulate material relevant to fluidized bed reactor, on the basis of the impurities composition. Furthermore, it is demonstrated the possibility to correctly estimate the intensity of average interparticle forces in the same kind of material by the interpretations of bulk flow properties measured with a shear tester, even in the case in which capillary forces take the place of the much weaker van der Walls forces. More in general, the paper suggests a method by which powder rheology can be used to indirectly evaluate the effects of the interparticle forces on fluidization processes even in case in which strong capillary interaction occur.

Concentrating solar thermal (CST) technologies for power production can play a major role in the future portfolio of renewable energies. Limestone calcination/carbonation (Calcium Looping (CaL)), is an appealing reaction whose integration with CST is widely investigated for thermochemical energy storage (TCES) and carbon capture and storage/utilization (CCSU). Experimental data under realistic CST conditions/reactors currently lacks, since most of the experimental activities have been performed in thermogravimetric analyzers. In this study, CaL-CST integration was investigated in a lab-scale directly irradiated fluidized bed reactor, able to mimic the operating conditions required for industrial implementation of the technology. Three different techniques to improve the performance of CaL-CST for TCES and CCSU were investigated: i) lowering of calcination temperature; ii) precalcination; iii) use of dolomite instead of limestone. Experimental results revealed that all the strategies moderately improve system performance. After 20 cycles, depending on the technique applied, the mean carbonation degree ranges within 28.1–37.1% (TCES) and 15.3–18.7% (CCSU) with limestone, and values 61.5% (TCES) and 36.7% (CCSU) with dolomite. Figures of energy storage density are less sensitive to the different techniques, as pay for the lower calcination temperature (limestone), or for the presence of an inert MgO fraction (dolomite). Corresponding values range within 941–1065 MJ m−3 (TCES) and 777–872 MJ m−3 (CCSU), for loose-packed conditions. N2-physisorption analyses revealed that the increased reactivity arises from better microstructural properties in terms of specific surface. Optimal choice among the different strategies should consider the intrinsic peculiarities of each investigated technique.

Thermochemical energy storage is gaining widespread consideration to increase energy dispatchability in concentrating solar thermal power plants. Accordingly, excess solar energy input drives an endothermic reaction, accomplishing high energy densities and virtually unlimited storage times. As gas–solid reactions are usually involved, multiphase reactor design is essential for the success of this technology. A novel concept of directly-irradiated fluidized bed autothermal reactor is investigated for applications in concentrated solar thermal technologies. The device can be operated as a rechargeable battery, alternating a charge phase, during which solar energy is collected and stored by an endothermal gas–solid reaction, and a discharge phase, during which the stored chemical energy is released by the reverse exothermic reaction. The autothermal operation, during the charge process, consists in the recovery of the sensible heat of the reaction products to preheat the reactants by means of an internal double-pipe countercurrent heat exchanger. This operation allows to increase the overall efficiency, reducing the required solar energy input. A compartmental model to simulate the operation of the thermochemical battery is developed and closed with constitutive equations and parameters obtained by previous experimental studies on lab-scale test facilities. Limestone calcination/carbonation has been considered as model reversible reaction. Both the charge and the discharge steps were assessed investigating the effect of the design and operational variables. For the charge operation, an optimal temperature was found around 900 °C with thermal efficiencies close to 90%. For the discharge operation, thermal efficiency was found to depend almost solely on the reactor temperature, reaching values as high as 80%, whereas the gas flowrate can be set independently. The upper limit for reaction temperature is set to the reaction equilibrium condition corresponding to the inlet concentration of carbon dioxide. The obtained results represent the basis for the realization of a new prototype, that will serve for the complete proof-of-concept of the directly-irradiated fluidized bed autothermal reactor.

The flow properties of five samples of a ceramic powder, characterized by different particle size distributions were measured at ambient temperature and at 500 °C with the High Temperature Annular Shear Cell. A significant increase of powder cohesion was observed at high temperature. A model combining a continuum approach and a particle–particle interaction description was used to correlate the powder tensile strength with the interparticle forces. The dependence of the tensile strength on powder consolidation and temperature is correctly described by the model.

Fluidization technology displays a long record of success stories, mostly related to applications to thermal and thermochemical processes, which are fostering extension to novel and relatively unexplored fields. Application of fluidized beds to collection and thermal storage of solar radiation in Concentrated Solar Power (CSP) is one of the most promising, a field which poses challenging issues and great opportunities to fluidization scientists and technologists. The potential of this growing field calls for reconsideration of some of the typical design and operation guidelines and criteria, with the goal of exploiting the inherently good thermal performances of gas-fluidized beds at their best. “Creative” and non-conventional design and operation of fluidized beds, like those based on internal and external solids circulation, may be beneficial to the enhancement of thermal diffusivity and surface-to-bed heat transfer, improving the potential for application in the very demanding context of CSP with thermal energy storage. This paper investigated: i) a fluidized bed configuration with an uneven distribution of the fluidizing gas to promote vortices in the scale of bed height (internal solids circulation); ii) a dual fluidized bed configuration characterized by an external solids circulation achieved by the operation of a riser and a bubbling fluidized bed. CFD simulations showed the hydrodynamics conditions under which the internal solids circulation was established. The hydrodynamic characterization of the external solids circulation was achieved by an experimental study carried out with different cold models. The dual fluidized bed system was optimized in terms of operating conditions and geometrical features of the connections between two fluidized beds.Fluidization technology displays a long record of success stories, mostly related to applications to thermal and thermochemical processes, which are fostering extension to novel and relatively unexplored fields. Application of fluidized beds to collection and thermal storage of solar radiation in Concentrated Solar Power (CSP) is one of the most promising, a field which poses challenging issues and great opportunities to fluidization scientists and technologists. The potential of this growing field calls for reconsideration of some of the typical design and operation guidelines and criteria, with the goal of exploiting the inherently good thermal performances of gas-fluidized beds at their best. “Creative” and non-conventional design and operation of fluidized beds, like those based on internal and external solids circulation, may be beneficial to the enhancement of thermal diffusivity and surface-to-bed heat transfer, improving the potential for application in the very demanding context of CSP with therm...

The focus of the present study is the reduction of the residual unburnt carbon contained in fly ash from PC-fired boilers by reburning and/or beneficiation. More specifically, the study addresses the potential of enhancing oxyreactivity of the residual carbon contained in fly ash by mechanochemical activation in order to improve the effectiveness of ash reburning. The concept is tested with reference to a high-carbon (nearly 50% by mass) fly ash sampled at the last field of the electrostatic precipitator of a utility boiler. The raw fly ash displayed moderate-to-low combustion reactivity and extensive development of anisotropic order possibly associated with extended molecular-ordered carbon domains. Mechanochemical activation was performed by ball milling in a lab-scale apparatus at 500 rpm and durations variable between 5 and 30 min. Both raw and mechanochemically activated samples have been characterized from the standpoint of particle granulometry and morphology, proximate and elemental analysis, ash constitution. Moreover, the influence of mechanochemical activation on the ability of ash samples to build up surface oxides has been characterized by temperature-programmed-desorption experiments. The assessment of the effectiveness of mechanochemical activation was based on the characterization of the intrinsic combustion kinetics of residual carbon by non-isothermal and isothermal thermogravimetric analysis. Experimental results indicate that a noticeable modification of DTG profiles, indicative of enhanced reactivity, is observed as a result of mechanochemical activation. © 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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. © 1992 Combustion Institute.

The feasibility of adopting an oxy-combustion stage to regenerate a spent catalyst proposed for methane thermo-catalytic decomposition has been investigated in a laboratory scale bubbling fluidized bed reactor operated at 800 C and different inlet oxygen concentrations. The efficiency of carbon oxy-combustion regeneration strategy has been evaluated on the basis of the efficiency of carbon removed from the catalyst and the performance of regenerated catalyst. The effect of multiple cycles of decomposition and regeneration steps has been also quantified. Experimental activity confirmed the possibility of producing a CO2 stream that can be finalized to a sequestration unit but also indicated the requirement of a good temperature control of catalytic particles. 2009 Elsevier Inc. All rights reserved.

The multiplication of carbon particles inside the bed of a fluidised coal combustor can be related to fragmentation and attrition. Attrition generates finer carbon which is rapidly elutriated from the bed. Previous work on attrition was carried out on a non-swelling coal. The present paper reports an extension of this work to a metallurgical coke during fluidised bed combustion. In comparison with the coal, metallurgical coal is not subject to fragmentation and is less reactive. This enabled attrition to be examined separately and at higher bed carbon content for a given fluidising velocity and excess air.

The management of bed solids inventory and particle size distribution is critical to the efficient and reliable operation of fluidized bed combustors (FBC), as it affects bed hydrodynamics, heat extraction, cyclone performance, bottom vs fly ash balance, fouling, etc. The population of bed ash material establishing at steady state in a FBC is the result of concurring processes among which particle size reduction due to combustion and different modes of attrition plays a key role. A simplified theoretical framework is presented for the assessment of the inventory and particle size distribution of ash material establishing in the bed during steady operation of a fluidized bed combustor. Key properties are represented by the amount and the particle size distribution of the Primary Ash Particles (PAPs) liberated by the fuel along burn-off. The propensity of the Primary Ash Particles to undergo further secondary attrition represents another important variable in the assessment procedure. The experimental procedure for the characterization of the amount and size distribution of PAPs liberated from a solid fuel is presented. Results of the application of the procedure to a variety of fossil and biogenous fuels are reported. The impact of the PAPSD of the different fuels on the build-up of bed material during steady fluidized bed combustion is discussed. The study further addresses the assessment of the fractional ash reporting to fly ash during FB combustion of a bituminous coal and a granulated sludge. These fuels have been selected on account of the different conversion patterns they feature during burn-off: the shrinking particle/constant density conversion pattern is exhibited by bituminous coal particles; the shrinking core/constant particle size is instead the typical conversion pattern for the granulated sludge. The relative importance of the primary generation of ash material (PAPs) and of secondary attrition of PAPs is discussed in the light of the relevant conversion pattern of the parent fuel.