Acoustic effects during the combustion of gaseous fuels in a bubbling fluidized bed (original) (raw)
Related papers
On the Mechanism of Bubbling Fluidized-Bed Combustion of Gas Oil
Industrial & Engineering Chemistry Research, 2003
A study has been carried out on atmospheric fluidized-bed combustion (FBC) of conventional liquid fuels at temperatures lower than the value classically adopted for FBC of solid fuels (i.e., 850°C). The study comprised an experimental program of steady-state tests for characterization of the combustion mechanism, mainly through proper acquisition of gas compositions and passive measurements of pressure signals. To this end, a bench-scale, bubbling-bed facility was used, and gas oil (diesel fuel) was chosen as a reference fuel and fed in a nonpremixed, submerged setup. Extending the bed operating temperature down to a value as low as 550°C allowed the relevant features of the combustion mechanism to become more evident and separately investigable. Operability of the bench-scale unit and feasibility of the combustion process were always ensured in the temperature range 550-800°C. However, tests carried out at T < 750°C always exhibited an irregular combustion behavior, such as the continuous random occurrence of uncontrollable micro-explosions. Despite an operation globally at steady state, the outlet gas composition and bed pressure were actual dynamic variables. The paper proposes an interpretation of the above fancy features in relation to the mechanism of liquid fuel combustion in a fluidized bed, the main steps of which are fuel atomization and vaporization, formation of a rising fuel vapor bubble, coalescence with air bubbles, and onset of ignition. Further, the paper introduces two characteristic variables, i.e., the average frequency of micro-explosion events (f e) and the average overpressure (P max), which are easily computable by the investigator and allow discrimination of the occurrence of a "regime with micro-explosions". Furthermore, another output of the study is a model for a first-approximation estimation of the frequency of microexplosion. The model development is simply based on the prediction of the coalescence between fuel and air bubbles, and the resulting ignition of the combustible mixture.
Overall Study of Air/Propane Combustion in Fluidized Beds
Commercial propane was used to study the combustion of volatiles released from coal particles burning in fluidized beds. A set of experiments was made with four different sizes of sand particles. In these experiments mixtures of air and propane were blown into the fluidized bed through the distributor. The fluidized bed was heated with an electrical resistance placed around the reactor. The following measurements were made: the dry molar fractions of CO 2 , CO and O 2 in the flue gases, the bed temperature and the intensity of the combustion noise. The objective was to determine the way fluidization disturbs the combustion mechanism of propane. For some experiments hematite, was introduced in the bed..A mathematical model concerning the tests performed without hematite, was developed to evaluate the concentrations of chemical species and temperature in bubbles, clouds and particulate phase, and also the mass and energy transfer between bubbles and particulate phase. The comparison between numerical and experimental results allowed a better understanding of the combustion of volatiles inside fluidized beds.
Propagation of Reaction Between Bubbles with a Gas Burning in a Fluidised Bed
Open Access, 2012
A bubbling fluidised bed reactor has been used for investigating how combustion propagates between bubbles of premixed fuel + air, rising from the distributor towards the surface. Earlier work has shown that when the temperature of the sand gradually rises, the gases first burn in flames above the bed, regime A, then combustion moves under its surface, to occur in bubbles travelling up the bed, under regime B. Above a certain temperature, characteristic of the gas mixture composition, combustion descends towards the bottom of the bed. Ignition then occurs in small bubbles near the distributor, under the stable regime C. The kinetic model, used to calculate the delay for thermal ignition inside gas bubbles rising through the bed, gives correct predictions for regimes A and C but not for B. Under regime B, bubbles of the mixture begin to ignite under the bed’s surface while their residence time in the bed is remains shorter than the delay times for thermal ignition derived from the kinetic model. As the temperature rises, the ignition delay rapidly decreases, and regime C is reached, in accordance with the model. In this work attention was focused on regime B. A laboratory reactor of quartz glass was used, with a bed of quartz sand. Fast video recording was employed to capture ignition phenomena as the bed’s temperature was raised or lowered. Records of freeboard concentrations of O2, CO and of total hydrocarbons, VOCs, were obtained, confirming the specific aerodynamic and chemical character of regime B. It has been shown that combustion spreads from the surface to bubbles near the bed’s surface and then to other bubbles close by. Such transfer of the reaction stabilizes combustion inside the bed, at temperatures appreciably lower than that for the thermal ignition of the mixture, given by the kinetic model. This is consistent with earlier findings, which have shown that the combustion of gaseous mixtures in bubbling fluidised beds is controlled by gas phase processes, as in flames.
Influence of Correlation between OEC and Acoustic Excitation in Natural Gas Flames
Procceedings of the 24th ABCM International Congress of Mechanical Engineering, 2017
The use of combustion in industrial and human activities is of paramount importance for economic and social development. Even with the promotion of energy alternatives to reduce the use of combustion, this will be a mechanism for the generation of energy still widely used, either by the use of natural gas as a less polluting fuel, but also by the use of synthesis gases of biomass gasification. However, combustion reactions are the main sources of emission of atmospheric pollutants, such as CO, NOx, volatile organic compounds and particulate matter. In view of this reality, it is necessary to study new techniques as well as the correlation between them in order to bring greater sustainability to combustion. The objective of the work is to correlate techniques such as oxygen-enhanced combustion (OEC) and the acoustic excitation of flames, forced by an external source, evaluating the effect of the use of these coupled techniques on the atmospheric emissions and thermal efficiency of the combustion in diffused and confined flames of natural gas. For this, an experimental prototype was developed with systems of acoustic performance and enrichment of the oxidant with oxygen. In the work is also presented experimental planning to identify the operational condition that optimizes together the emission levels of pollutants as well as the temperature of the exhaust gases in the flames studied. The results showed that the optimum operating point of the experimental device used is the simultaneous use of the two combustion techniques, which indicates the viability of the coupling between them. An increase in the stability of the flame acoustically excited with the use of OEC was also observed.
2004
Fluidized bed combustion is new, but industrially proven, clean energy technology enabling efficient and environmentally acceptable burning of low grade coals, biomass and industrial and municipal wastes. Book is devoted to the processes in bubbling fluidized bed combustion furnaces and boilers, suitable for small and medium size industrial units: • giving, from one side, deep fundamental explanation of the main physical and chemical processes in bubbling fluidized beds, book is useful for researchers and post diploma studies; • based on the fundamental knowledge of the processes main problems in the choice of the boiler concept and design, and main data for the calculation of regime parameters and boiler dimensions are explained giving engineering tools for design engineers; • in the part of the book discussing behavior of the furnaces and boilers in operation, advantages and deficiencies, book is useful for operational engineers explaining possible event in exploitation; and • finally, book is useful for all persons thinking or making decisions about the choice of the most suitable combustion technology, available on the world market, for energy production using wide range of solid fuels, low grade fuels, biomass, industrial and municipal wastes, in distributive energy production, and small and medium size utility units. Three chapters of the book are devoted to the recent results of the fundamental and applied research of the hydrodynamics of bubbling fluidized beds, heat and mass transfer and combustion of solid fuels. A special chapter is devoted to the concepts of the bubbling FBC furnaces and boilers, choice of concept, brief calculation of the main parameters and influence of fuel characteristics on boiler concepts and parameters. Separate chapter is devoted to the investigation of solid fuel behavior during fluidized bed combustion and determination of the engineering data for the choice of boiler concept and design. Last chapter defines emission characteristics of the bubbling FBC furnaces and boilers. In the first chapter, and throughout the whole book, place of the bubbling FBC technology among the conventional combustion technologies is discussed, pointing out the niche markets in which bubbling FBC evidently is technology of choice.
MODELING the TRANSIENT BUBBLING MECHANISM of a FLUIDIZED BED COMBUSTOR
The fluidized bed modeling and control is investigated, focusing in fluidized bed combustors. The two phase Davidson’s flow model is limited, since it considers constant bubble size and rising velocity. Recent research work in two and three dimensional (atmospheric,circulating and pressurized) fluidized beds concluded that the intrinsic transient motion of gas bubbles in freely bubbling fluidized beds, contains multiple mechanisms for both fluidization, combustion and erosion processes, such as, bubble size changes, interaction, bubbles velocity, normal and abnormal fluidization condition, wall effect on fluidization, etc. The background average function of bubbles signal corresponds to the emulsion phase density.
Improvements in the simulation of liquid fuel combustion in a low-temperature fluidized bed
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The paper presents a further work along the route towards a more thorough description of fundamentals and mechanisms governing the liquid fuel combustion in bubbling fluidized bed combustors (FBC), operating in a temperature range (i.e., 650-800°C) that is lower than the classical value for FBC of solid fuels. Three new sub-models have been added to an existing "system model": i) formation of a reacting gas-liquid flare inside the bed downstream from the liquid fuel nozzle; ii) motion and coalescence of the fuel vapor bubbles; iii) mixing and combustion in the splash zone. Among the other things, the simulation code predicts temperature and concentration of the unburned species and combustion products inside the bed, bubbles and splash zone.
Combustion of slugs of propane and air moving up through an incipiently fluidized bed
Combustion Theory and Modelling, 2007
A mathematical model is proposed to show the evolution of temperature, chemical composition and energy release or transfer in slugs, clouds and particulate phase, in a fluidized bed where there are slugs, of a mixture of air and propane, moving up through the particulate phase previously set in the state of incipient fluidization with air. The analysis begins as the slugs are formed at the orifices of the distributor, until they explode inside the bed or emerge at the free surface. The model also makes the analysis of what happens in the gaseous mixture that leaves the free surface of the fluidized bed until the propane is thoroughly burnt. It is essentially built upon a simple quasi-global mechanism for the combustion reaction and the mass and heat transfer equations from the two-phase model of fluidization. The aim was not to propose a new modelling approach, but to combine classical models, one concerning the reaction kinetics and the other the bed hydrodynamic aspects, to obtain a better insight on the events occurring inside a fluidized bed reactor, enhancing the understanding of this type of reactor. Experimental data to balance with the numerical model were obtained through tests on the combustion of commercial propane, in a laboratory scale fluidized bed, using four sand particle sizes: 400-500, 315-400, 250-315 and 200-250 μm. The mole fractions of CO 2 , CO and O 2 in the flue gases and the temperature of the fluidized bed were measured and compared with the numerical results.
Clean Air, 2005
A laboratory size fluidised bed reactor was used for the tests. The gas mixture was prepared by vaporization of liquids in hot air. The test fuels were benzene, toluene, m-xylene, methyl acetate, ethyl acetate and ethyl formate. The degree of fuel conversion was assessed in terms of CO and VOCs concentrations. The concentrations of NO and NO 2 were also measured. The results obtained indicate that emissions of all pollutants can be very low. The combustion of esters was cleanest, pollutants were highest when aromatic compounds were used. With methane fuel (reference) emission levels were moderate. The results obtained indicate that liquid organic substances can be effectively burned in fluidised beds and the flue gases can in most cases be cleaner than with the reference fuel. With sufficient air excess and bed temperature, low NO and CO emissions are possible with no unburned fuel. Depending on the needs, e.g. to utilize waste gases, it is possible to select appropriate process parameters. The best results are obtained if the combustion is flameless, i.e. takes place entirely inside the bed.
Experimental research of acoustically perturbed Bunsen flames
2006
This dissertation is related to experimental research of the response of Bunsen type laminar premixed methane-air flames to an acoustic excitation of the flow. This study is stimulated by the problem of acoustic instability of burners and combustors used in domestic and industrial heating devices where combustion is organized via laminar Bunsen type flames. The study and accordingly the dissertation can be divided into two parts. The first part is concentrated on a detailed parametric study of the heat release rate response of conical and tent shaped Bunsen flames to the acoustic perturbation of the gas mixture flow velocity. A Transfer Function (TF) concept which relates the response (heat release rate oscillation) with the stimulus (velocity perturbation) is used to characterize the flame’s susceptibility to the excitation. The measurement method is based on the simultaneous registration of the acoustic velocity oscillation via a hot-wire probe installed in the burner in the fresh...