The Nucleation of Aerosols in Flue Gases with a High Content of Alkali - A Laboratory Study

Abstract

ABSTRACT. The formation of particles during cooling of a synthetic ue gas with vapors of sodium and potassium species is studied in a laboratory tubular reactor with laminar ow. 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 ue gases.The addition of SO 2 to the chloride vapor feed, in the presence of oxygen and water vapor, increases the number concentration of ef uent particles signi cantly and affects their composition to include sulphate in addition to chloride. The sulphate content is independent of the peak temperatures of the ue gas but increases with increasing content of oxygen and SO 2 . 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 identi ed as the likely source of the submicron particles formed in alkali rich ue gases.

The effect of oxygen and volatile combustibles on the sulphation of gaseous KCl

“…The heterogeneous sulphation is not a part of the present investigation and is discussed in more detail in [13]. The homogeneous mechanism involves the formation of an alkali sulphate aerosol in the gas phase [5,17]. The overall reactions of the homogeneous mechanism may be described as follows: KCl is primarily sulphated by SO 3 through the gas-phase reaction,…”

“…Gas concentrations were measured in locations after the convection path (22) and before the stack (23) during all test cases. Additional gas measurements were also carried out during the transient tests to characterise the composition of the flue gas without injection of AS at measurement locations (17), (18), (19), (20), (22) and (23) in Fig. 1.…”

Thermal Dissociation of SO3 at 1000−1400 K

“…[5][6][7] In the presence of nitrogen oxides, there is a kinetic coupling of NO x and SO x chemistry via the radical pool. 5,7 While the overall mechanisms for SO 3 formation are fairly well-known, kinetic modeling of the process still suffers from a lack of accurate kinetic data and refinements are required in order to establish a reliable mechanism. 4,5 Flow reactor studies of SO 2 oxidation under post-flame conditions 5,7 indicate fractional conversions of SO 2 to SO 3 of the order of a few percent.…”

“…During the combustion of sulfur-containing fuels, most of the sulfur is converted to sulfur dioxide (SO 2 ) but a small fraction is oxidized further to form sulfur trioxide (SO 3 ). The presence of SO 3 enhances the corrosion potential of the flue gas, 1 and it has implications for the formation of aerosols in biomass-fired systems.…”

“…The presence of SO 3 enhances the corrosion potential of the flue gas, 1 and it has implications for the formation of aerosols in biomass-fired systems. [2][3][4] The conversion of SO 2 to SO 3 is kinetically limited 1 . The reactions that may contribute to the formation of SO 3 involve the interaction of SO 2 with the radical pool.…”