Modeling Potassium Capture by Aluminosilicate, Part 1: Kaolin

Abstract

Additives rich in Si and Al such as kaolin may be applied in PF biomass boilers to fix alkali metals in species that are more benign than the alkali salts released in biomass combustion. In this study, models for the reaction of gas-phase potassium salts with kaolin particles are developed for the conditions appearing in PF boilers such as short residence times, high temperatures, and small size of the additive particles. The reaction between gaseous potassium components and kaolin particles has been modeled with a shrinking core model (SCM) and a uniform conversion model (UCM). The SCM takes into account the effects of chemical kinetics, diffusion in a gas film surrounding the particle, diffusion in a product layer, and thermochemical equilibrium on the reaction progress. The UCM covers diffusion in a gas film surrounding the particles, chemical kinetics, and thermochemical equilibrium. Both models are able to accommodate the effects of change in temperature, particle size, reaction time, and potassium component concentration. Literature data from experiments in an entrained flow-reactor (EFR) at 800−900 °C were used to derive the chemical kinetic rate coefficients of the reaction between kaolin and KOH. The models were then evaluated against experimental data for alkali salts of KCl, KOH, K 2 CO 3 , and K 2 SO 4 covering temperatures of 800−1450 °C, kaolin particle sizes of 4−14 μm, residence times of 0.8−1.9 s, and salt/additive molar ratios of 0.05−0.96. The evaluation indicated that both SCM and UCM were suitable for a wide range of conditions, but the UCM captured the effect of particle size better. The modeling outcomes suggested that if the reaction time was long enough, the thermochemical equilibrium would be the major limitation in capturing potassium by kaolin at high temperatures and high potassium concentrations. At lower temperatures, however, the conversion was mainly limited by chemical kinetics. The mass-transfer limitation was less critical under the investigated conditions. The developed models can account for the reaction of gas-phase potassium salts with kaolin at local conditions relevant to PF boilers using biomass.

Modeling Potassium Capture by Aluminosilicate, Part 2: Coal Fly Ash

“…Figure shows the binary diffusion coefficient of KOH ( D KOH ) and KCl ( D KCl ) in a nitrogen environment versus temperature, taken from ref . The D eff of both KOH and KCl in the product layer formed from reacted kaolin and reacted CFA are also shown here.…”

“…After establishing transport and kinetics equations (see ref for details), the final model can be represented by a set of ordinary differential equations (ODE) given in eqs and here, t is time, N CFA is the mole of unreacted CFA in an individual particle, [K] ∞ is the free-stream concentration of potassium (as K atoms), β is the number of CFA particles per gas volume, k overall,UCM is the overall rate constant, k g is the mass transfer coefficient of the potassium salt in the gas phase, and r i is the initial radius of the CFA particle.…”

“…The surface area of the unreacted core is represented by A . The final model can be represented by an ODE set given in eqs and k overall,SCM is the overall rate constant, MW CFA is the molar mass of CFA, ρ CFA is the density of CFA, r s is the radius of the unreacted core, and D eff represents the effective diffusion coefficient of potassium salt in the product layer.…”

“…The variations of a i ’s and b i ’s alter the stoichiometric coefficient (Ψ) in reaction . To facilitate modeling of such complicated behavior, a two-step scheme developed in the earlier work is implemented. In the first step , which is controlled by kinetic and diffusion resistance, the reaction proceeds irreversibly to give a pseudo component called products*, which itself dissociates immediately in the second step to yield the final products here, a constant nominal Ψ is assumed for all conditions.…”

“…In our earlier work, the shrinking core model (SCM) and the uniform conversion model (UCM) have been developed to simulate potassium scavenging by kaolin under conditions of PF boilers. Here, we aim to adapt the developed models in ref to predict the reaction of gaseous potassium salts with CFA particles to form thermally stable products. The condition commonly found in PF boilers, that is, temperatures of 800–1700 °C and the residence times of a few seconds, is the main focus of the present study.…”