Hans Livbjerg

Computational fluid-particle dynamics for the flame synthesis of alumina particles

Abstract A mathematical model for the dynamics of particle growth during synthesis of ultra fine particles in diffusion flames is presented. The model includes the kinetics of particle coalescence and coagulation, and when combined with a calculation of the temperature, velocity and gas composition distribution in the flame, the effluent aerosol characteristics are calculated. The model is validated by comparison with an experimental study of the synthesis of alumina particles by combustion of Al-tri-sec-butoxide. Two parameters of the coalescence kinetics are estimated by regression of the model predictions to the measured specific surface area of the product particles. The estimated kinetics can be used to predict the surface area and shape of the particles for a wide range of synthesis conditions.

Computational analysis of coagulation and coalescence in the flame synthesis of titania particles

Abstract A method of combining computational fluid dynamics with a mathematical model for the particle dynamics has been applied to simulate experimental data from the synthesis of TiO 2 -particles in diffusion flames. Parameters of the coalescence kinetics are estimated by fitting the model predictions to the measured specific surface area of the product particles. The estimated kinetics can be used to predict the surface area and aggregate structure of the particles for a wide range of synthesis conditions. The regular equation for the rate of coagulation is modified to take into account the effect of dilution. The accuracy of the results, especially the degree of aggregation, i.e. the aggregate size, is highly dependent on the inclusion of this effect. When the dilution is accounted for, the predicted aggregate sizes (numbers of primary particles per aggregate) compare well with reported data based on small-angle X-ray scattering measurements.

A field study of submicron particles from the combustion of straw

ABSTRACT The evolution of small aerosol particles accompanying the combustion of straw for energy production is investigated. A sampling equipment specially designed for field measurements is described and characterized. The aerosol is studied by low-pressure cascade impactor and scanning mobility particle sizer, the particle morphology by transmission electron microscopy, and the chemical composition by energy dispersive x-ray analysis. The combustion gas contains 3–500 mg/Nm3 of submicron particles with a mean diameter of approximately 0.3 μm. The particles consist of almost pure potassium chloride and sulphate. The formation mechanism is analyzed by a theoretical simulation of the chemical reactions and the aerosol change during cooling of the flue gas. It is concluded that some sulphation of KC1 occurs in the gas phase although the sulphate concentration is much lower than predicted by an equilibrium assumption. The theoretical simulation proves that the fine mode particles can be formed by homogeneou...

Formation and emission of fine particles from two coal-fired power plants

The generation and emission of combustion particles from two full-scale coal-fired power plants was studied by field measurements during which particles are sampled for size classification and chemical analysis simultaneously at three positions in the plants: before the electrostatic precipitator, before the desulfurisation plant, and in the stack. The following sampling techniques are used: scanning mobility particle sizer, low pressure cascade impactor, dichotomous PM 2.5 sampler, and total particle filter. The so-called multi-platform method used in this work proves useful for gaining insight into the many particle-affecting processes in a power plant. In the boiler the size of particles extends over four decades, from approximately 20 nm to 200 µm, with the largest mass contained in particles in the size range 10-100 µm. Approximately 99.9% of the particles are removed in the electrostatic precipitator and the desulfurization scrubber. The mass fraction of submicron particles, i.e., PM 1 , increases from ∼0.3% at the exit of the boiler, just before the electrostatic precipitator, to ∼30% in the aerosol emitted via the stack. In the stack aerosol 50-80% of the particles are in the PM 2.5 range. The emitted particles primarily stem from the coal ash with a minor contribution of particles of entrained, dried-out droplets of scrubber slurry. The large emitted particles are compact, almost-spherical single particles originating from the ash mineral inclusions in the coal. The small ones, corresponding in mass approximately to the PM 0.45 fraction--in terms of number concentration by far the dominant fraction--are dendritic clusters, each consisting of several partly fused, even smaller particles. The cluster particles are generated in the burner by volatilization of Si, Al, Ca, and Fe under reducing conditions. Several other ash-contained elements, e.g., P, Ba, Co, Cu, Mn, Ni, Pb, V, and Zn, are partly vaporized in the boiler and enrich the small particles when they condense during cooling of the flue gas. Due to the higher penetration of the small particles through the filter and scrubber, these elements tend to have an enhanced concentration in the emitted particles.

A plug flow model for chemical reactions and aerosol nucleation and growth in an alkali-containing flue gas

The paper presents a numerical model for the simulation of gas to particle conversion and the chemical changes during cooling of a flue gas from the combustion of fuels rich in volatile alkali species. For the homogeneous nucleation of alkali species the model uses the classical theory modified by the Tolman coefficient for the size dependent surface tension. A special adaptation of the theory to the nucleation of seeds of chloride salt from an equilibrium mixture of monomer and dimer chloride vapor molecules is also made. The growth of particles occurs by condensation and agglomeration. Different multicomponent growth models are treated. The local gas phase composition is determined from a gas phase chemical equilibrium calculation combined with finite reaction rate kinetics for slower reactions. The model is useful in the analysis of boiler operation with respect to the formation of particles, HCl, SO2, and deposits.

The nucleation of aerosols in flue gases with a high content of alkali: A laboratory study

The formation of particles during cooling of a synthetic flue gas with vapors of sodium and potassium species is studied in a laboratory tubular reactor with laminar flow. It is shown to agree well with a theoretical model for the process. The kinetics of homogeneous nucleation of the pure chloride vapors is described by the classical nucleation theory, adapted to include the participation of stable dimer as well as monomer vapor molecules. The Tolman equation is used to describe the curvature-dependence of the surface tension of small nuclei. The values of the Tolman parameter for NaCl and KCl are determined from the measurements. The homogeneous nucleation of the pure chlorides is suppressed by even relatively small concentrations of foreign seed particles and is therefore unlikely to contribute to the creation of new particles in real flue gases. The addition of SO2 to the chloride vapor feed, in the presence of oxygen and water vapor, increases the number concentration of effluent particles significantly and affects their composition to include sulphate in addition to chloride. The sulphate content is independent of the peak temperatures of the flue gas but increases with increasing content of oxygen and SO2. The study proves that the alkali sulphates are formed by the sulphation of vapor phase rather than solid, alkali chloride. The sulphate vapors are formed in high supersaturation and show a pronounced tendency towards homogeneous nucleation, which is identified as the likely source of the submicron particles formed in alkali rich flue gases.

Preparation of ZnO–Al2O3 Particles in a Premixed Flame

Zinc oxide (ZnO) and alumina (Al2O3) particles are synthesized by the combustion of their volatilized acetylacetonate precursors in a premixed air–methane flame reactor. The particles are characterized by XRD, transmission electron microscopy, scanning mobility particle sizing and by measurement of the BET specific surface area. Pure (γ-)alumina particles appear as dendritic aggregates with average mobile diameter 43–93 nm consisting of partly sintered, crystalline primary particles with diameter 7.1–8.8 nm and specific surface area 184–229 m2/g. Pure zinc oxide yields compact, crystalline particles with diameter 25–40 nm and specific surface area 27–43 m2/g. The crystallite size for both oxides, estimated from the XRD line broadening, is comparable to or slightly smaller than the primary particle diameter. The specific surface area increases and the primary particle size decreases with a decreasing flame temperature and a decreasing precursor vapour pressure. The combustion of precursor mixtures leads to composite particles consisting of zinc aluminate ZnAl2O4 intermixed with either ZnO or Al2O3 phases. The zinc aluminate particles are dendritic aggregates, resembling the alumina particles, and are evidently synthesized to the full extent allowed by the overall precursor composition. The addition of even small amounts of alumina to ZnO increases the specific surface area of the composites significantly, for example, zinc aluminate particles increases to approximately 150 m2/g. The gas-to-particle conversion is initiated by the fast nucleation of Al2O3 or ZnAl2O3, succeeded by a more gradual condensation of the excess ZnO with a rate probably controlled by the cooling rate for the flame.

Kinetics and effectiveness factor for SO2 oxidation on an industrial vanadium catalyst

Abstract An experimental study of the reaction rate for SO 2 oxidation on a commercial V 2 O 5 catalyst is presented. Results in the purely kinetic region are correlated by means of 12 published rate expressions. It is concluded that most rate expressions are adequate only in a narrow temperature and composition range and that probably no single rate expression can be applied in the whole range of industrial operating conditions. Experiments with large size pellets are used to determine catalyst effectiveness as a function of temperature and conversion. It is found that a serious transport resistance occurs for T ⩾ 450°C and that the effectiveness decreases somewhat at high conversion. The effective diffusivity of SO 2 is determined and finally the tortuosity of the catalyst is calculated. The results show that the effective diffusivity can be determined on the basis of a gas-phase diffusion model and that the tortuosity factor falls within the range of previously published values with a slight increase for high conversion and possibly also for low temperature.

COMBUSTION AEROSOLS FROM MUNICIPAL WASTE INCINERATION—EFFECT OF FUEL FEEDSTOCK AND PLANT OPERATION

Abstract Combustion aerosols were measured in a 22 MW (thermal energy) municipal waste incinerator. Different types of waste fractions were added to a base-load waste and the effect on aerosol formation was measured. The waste fractions applied were: PVC plastic, pressure-impregnated wood, shoes, salt (NaCl), batteries, and automotive shredder waste. Also, runs with different changes in the operational conditions of the incinerator were made. Mass-based particle size distributions were measured using a cascade impactor and the number-based size distributions were measured using a Scanning Mobility Particle Sizer. The plant is equipped with flue gas cleaning and the penetration through this was determined. The particle morphology was investigated by Transmission Electron Microscopy (TEM) and chemical analysis of the aerosol particles was made by Energy Dispersive X-ray Spectroscopy (EDS). The mass-based particle size distribution was bimodal with a fine mode peak around 0.4 µm and a coarse mode peak around 100 µm. The addition of NaCl, shredder waste, and impregnated wood increased the mass concentration of fine particles (aerodynamic diameter below 2.5 µm). In general the mass concentration was stable and close to the reference PM2.5-value of 252 ± 21 mg/m3 (std.T,P). The total number concentration deviated during runs and between runs spanning from 43 · 106 to 87 · 106 #/cm3(std.T,P). The aerosols formed were mixtures of dense and aggregated particles in all tests. The fine particles are mainly composed by alkali salts, zinc, and lead. The heavy metals Cu, Cd, Hg, and Pb are significantly enriched in the fine particles.

Models of pore diffusion in porous catalysts

Abstract Four different pore diffusion models are fitted to isobaric counter-diffusion fluxes measured in technical catalysts by a Wicke-Kallenbach cell over a 30-fold pressure range with different gas pairs. The models differ significantly with respect to predictive capabilities and computational requirements. Of the models studied, only the cross-linked pore model ( Johnson and Stewart, 1965 ; Feng and Stewart, 1973 ), can predict diffusion fluxes reliably solely from pore size distribution. The study emphasizes the versatility and accuracy of the cross-linked pore model.