Joakim Reimer Jensen

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.

Synthesis of nano-particles of ZnO/Al2O3 in a premixed flame

Abstract Materials with very high specific surface areas can be synthesized by combustion of volatilized precursors together with an additional fuel like methane, hydrogen etc. The structure and properties of the product powder can be varied depending on the temperature and particle residence time in the high temperature zone and on the precursor concentration. The vapour phase synthesis of zinc and copper oxide composites by combustion in a premixed flame of aluminum acetylacetonate and zinc acetylacetonate is investigated. The laboratory setup consists of a burner where premixed methane, air and precursor vapours are combusted above a flame arrestor and saturation units where the precursors are evaporated into a nitrogen stream. This burner setup provides a very homogenous flame environment and ensures a low precursor concentration in the flame zone. The synthesized particles are collected on polycarbonate filters and are characterized by their specific surface area, aggregate size distribution, their morphology as observed by TEM and by their atomic structure and crystalline dimensions by x-ray diffraction. This talk will focus on the relationships between the particle synthesis and processing and the morphology of the product particles. The composite particles consist of zinc oxide deposited on top of an γ-aluminum structure probably due to the different vapour pressures of the oxides. Thereby the alumima serve as a structural support for a high surface area zinc oxide powder. The specific surface areas are 25 m 2 /g and 200 m 2 /g for pure zinc oxide and pure alumina respectively and the surface area of the composite powders varies within these limits.