Ottimizzazione del processo di ossicombustione tramite studio cinetico in un impianto pilota

Per lo sviluppo di tecnologie che consentano un migliore impatto ambientale nell'utilizzo di carbone per l'approvvigionamento energetico, occorre studiare nuovi approcci per la progettazione dei sistemi di combustione. L'obiettivo di questa tesi consiste nello studio delle cinetiche chimiche di devolatilizzazione del carbone in condizioni di ossicombustione. A questo scopo si utilizzano dati sperimentali sulla conversione e sul tempo di residenza, estratti da un impianto pilota di tipo Entrained flow reactor (EFR). Per l'ottimizzazione delle cinetiche chimiche sono necessari dettagli delle storie termiche e delle traiettorie delle particelle, difficilmente estraibili tramite sperimentazione. Per questo è necessario un approccio integrato con la fluidodinamica computazionale (CFD) per completare i dati a disposizione e sviluppare una metodologia di calcolo iterativa, che combini dati da EFR e da CFD. Tramite questo approccio, si confrontano modelli di devolatilizzazione di tipo SFOR, meno onerosi dal punto di vista computazionale, proponendone alcune versioni modificate. I modelli studiati con tale metodo hanno buoni valori predittivi, soprattutto per quelli che considerano il campo di volatili finale funzione della temperatura. L'accoppiamento tra dati sperimentali e CFD risulta quindi efficace per la predizione della devolatilizzazione, rendendo possibile applicare l'approccio metodologico utilizzato per tutti i combustibili solidi, quali carboni e biomasse. Questo può essere di supporto per la progettazione e l'ottimizzazione di sistemi di combustione quali fornaci industriali o sistemi di gassificazione

experimental investigation on the fixed bed of a small size biomass boiler

Abstract During the last decades, the increase of the world energy consumption promoted the renewable resource development and use. Together with wind, solar and hydro energy, biomasses play a key role in the reduction of the industrial environmental impact; moreover, the biomass combustion systems are very attractive for micro-generation purpose. In this paper, experimental tests on a 140kW small size fixed bed biomass boiler were carried out. The main goal was to study the thermal behavior and some chemical products, such as CO, CO2, Methane and Ethylene, from the combusting fixed bed of the system. In fact, despite the wide amount of literature for the laboratory scale systems, the commercial scale boilers have been seldom studied by the experimental point of view. The data were obtained by varying the operational parameters of the boiler, that are the air excess and the secondary to primary air feeding ratio. Furthermore, the collected data were analyzed and the relationship between the thermal-chemical data and the control variables was discussed

biomass early stage combustion in a small size boiler experimental and numerical analysis

Abstract The increment in the world energy consumption and the necessity for a sustainable industrial production, indicate that renewable resources may be key actors for future development. In this scenario, biomass appears fundamental for the smooth transition from fossil fuels to lower carbon footprint technologies, and as a moderator agent within the renewable market. The small size biomass combustion application appears as suitable for smart grid and distributed generation applications, but it is necessary to improve the design tools capabilities and the experimental knowledge of these systems. The present work aims at investigating the thermal behaviour of a 140 kW fixed-bed boiler sited at the Biomass to Energy Research Centre (CRIBE) of the University of Pisa and fed with woodchips. Experimental activities were conducted in order to acquire thermal and chemical data. Moreover, a computational fluid dynamic model was developed and validated. Attention was paid to the fixed bed analysis, and the results showed a good model prediction capability, with respect to the reduced computational demand required

Experimental analysis of overcharged Li-polymer batteries

Since safety hazards can occur during the life of a Li-ion battery, understanding its behavior under abusive conditions is important for the development of a safe cell. In this work, overcharge tests on commercial Li-polymer cells were conducted in a climatic chamber, resulting in gas evolution. A comprehensive post-mortem analysis of the abused cells was carried out: the exhaust gases were identified by gas phase chromatography coupled with a thermal conductivity detector (micro-GC/TCD), then flammable and toxic species were detected; the cathode and anode materials were analysed using scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS), while the electrolyte composition was studied by Fourier-transform infrared spectroscopy (FT-IR). Interestingly, the ambient temperature seemed to affect the degradation of the cell materials and hence the composition of the evolved gas

Small-scale fixed-bed biomass boilers: experimental and numerical characterization

Nel corso della ricerca sono state svolte attività legate alla caratterizzazione dei sistemi di combustione a biomassa di piccola taglia dal punto di vista sperimentale e modellistico. Utilizzando una caldaia a biomassa da 140kW a letto fisso, a disposizione presso il centro di ricerche per le biomasse da energie (CRIBE), ne è stato valutato il funzionamento dal punto di vista termico e delle emissioni inquinanti. Successivamente ne è stato analizzato il letto fisso dal punto di vista della composizione chimica dei volatili (raccolti nei pressi della superficie del letto) e termico in diverse condizioni operative. I dati sono stati successivamente utilizzati per la costruzione e la validazione di un modello per applicazioni di fluidodinamica computazionale applicato ai sistemi di combustioni di tipo fixed bed. In una fase successiva, è stato analizzato il comportamento della singola particella di combustibile dal punto di vista dimensionale in diverse condizioni di test. Infine, è stato studiato il comportamento fisco-meccanico complessivo del combustibile, sotto forma di cippato di biomassa, all’interno della zona di combustione tramite metodo ad elementi discreti, validato in via preliminare tramite test sperimentali in una riproduzione in scala 1:1 del sistema di alimentazione della caldaia presente presso il CRIBE

Experimental investigation on air displacement and air excess effect on CO, CO2and NOxemissions of a small size fixed bed biomass boiler

Biomass has a key role in the future of the energetic scenario, since it is a programmable renewable energy source which is suitable either to generate power and/or produce heat. This paper deals with biomass combustion processes in small size boilers, which is suited either for large residential buildings, or for small size industrial users. This kind of boilers is different from small pellet stoves and fireplaces and from large utility size boilers. In particular, those small size boilers have been seldom studied in the scientific literature about the combustion kinetics and pollutant formation mechanisms. Hence, the authors present the thermo-chemical data collected from a 140 kWth chipped wood boiler at the University of Pisa. The effect of the air excess and the ratio between and secondary feeding air and primary feeding air mass flow rate, on CO, CO2and NOxemissions was investigated. Results have shown that the air excess has the main influence on the pollutant produced. A comparison of the measured data with those presented in the literature and the discrepancies with other studies are also discussed

Acoustic Pyrometry Robustness to Time of Flight Estimation Errors

Acoustic pyrometry is a widely used technique for contactless temperature measurement. It may be used in several applications, especially when high temperatures and harsh environments are involved. For instance, it has been applied to measure the temperature distribution at gas turbine outlet. This technique is based on the measurement of the time of flight of an acoustic wave through a medium. If multiple emitter-receiver couples are used, by using a computational procedure a reconstruction of a temperature map is possible. On the other hand, a full assessment of the robustness of this technique to potential errors in time of flight estimation is still missing. In this study, the impact of an inaccuracy in time of flight estimation on the reconstruction of a correct temperature map is investigated by means of a statistical approach. As a general result, it was found that when the time of flight was measured without inaccuracies, temperature estimation errors may be lowered by simply increasing the number of cells in which the estimation is performed. However, when the estimation of the time of flight is affected by errors, an optimal configuration exists that minimize the temperature estimation errors

Measurement of GT exhaust gas temperature by acoustic pyrometry: Preliminary error investigation

Acoustic pyrometry is an interesting technique that may find several useful applications in turbomachinery. It is well known that the speed of sound in a medium is directly related to its temperature. Acoustic pyrometry estimates the temperature of a gas by considering the time of flight of an acoustic wave moving through it. If one acoustic emitter-receiver couple is used, only the average temperature along the acoustic path can be determined. If multiple emitter-receiver couples laying on the same plane are used, a reconstruction of the temperature map in the section is possible. In this last case, the analysis is based on the fact that the temperature of each sub portion of the section affects the time of flight of all the acoustic paths travelling across it. Many parameters affect the accuracy of the measurement. They are mainly related to the physic of the sound propagation in a medium, the accuracy of the instrumentation used, the interaction between the acoustic wave and the flow velocity and the hardware set-up. In this study, the impact of the measurement set up of an acoustic pyrometry for the measurement of the exhaust gas temperature in a gas turbine was investigated to determine the optimal solution in terms of accuracy and robustness to uncertainties

HETEROGENEOUS KINETICS FROM CFD DIAGNOSTICS IN AN ENTRAINED FLOW REACTOR

A Computational Fluid Dynamics model of a pilot-scale entrained flow reactor is developed with the aim to shed light into the cloud of solid fuel particles. An iterative procedure is suggested to derive devolatilization kinetics: particle average residence times and heating rates are estimated from the numerical model, and a linear dependence of the particle temperature with residence time is assumed. In this manner the volatile release equation can be integrated analytically and subsequently kinetic parameters can be obtained from experimental conversion data. The procedure is shown for a Sebuku type coal in oxy-fuel conditions