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The KOH-capture reaction by coal fly ash at suspension-fired conditions was studied through entrained flow reactor (EFR) experiments and chemical equilibrium calculations. The influence of KOH-concentration (50-1000 ppmv), reaction temperature (800-1450 °C), and coal fly ash particle size (D 50 = 6.03-33.70 μm) on the reaction was investigated. The results revealed that, at 50 ppmv KOH (molar ratio of K/(Al+Si) = 0.048 of feed), the measured K-capture level (C K ) of coal fly ash was comparable to the equilibrium prediction, while at 250 ppmv KOH and above, the measured data were lower than chemical equilibrium. Similar to the KOH-kaolin reaction reported in our previous study, leucite (KAlSi 2 O 6 ) and kaliophilite (KAlSiO 4 ) were formed from the KOH-coal fly ash reaction. However, coal fly ash captured KOH less effectively compared to kaolin at 250 ppmv KOH and above. Studies at different temperatures showed that, at 800 °C, the KOH-coal fly ash reaction was probably kinetically controlled. At 900-1300 °C it was diffusion limited, while at 1450 °C, it was equilibrium limited to some extent. At 500 ppmv KOH (molar ratio of K/(Al+Si) = 0.481), and a gas residence time of 1.2 s, 0.063 g K/(g additive) and 0.087 g K/(g additive) was captured by coal fly ash (D 50 = 10.20 μm) at 900 and 1450 °C, respectively. Experiments with coal fly ash of different particle sizes showed that a higher K-capture level were obtained using finer particle sizes, indicating some internal diffusion control of the process.

 Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

 Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

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.