Thermal decomposition of sodium bicarbonate and its effect on the reaction of sodium bicarbonate and sulfur dioxide in a simulated flue gas

The objective of this study was to ascertain the effect of thermally decomposing sodium bicarbonate while simultaneously reacting with sulfur dioxide. The study was performed by quantitatively determining the rate of thermal decomposition as a function of particle size in a sulfur dioxide free gas stream. The rate of reaction of sodium carbonate (product of the thermal decomposition) with sulfur dioxide was then studied and the data applied to a pore plugging model which accounts for the loss in reactivity with increased reaction time. The reaction of sodium bicarbonate with sulfur dioxide was then studied and the results compared to that for sodium carbonate.

From the analysis of the data, the activation energy for the thermal decomposition reaction, the sulfur dioxide-sodium carbonate and sulfur dioxide-sodium bicarbonate reaction were derived. The thermal decomposition reaction of sodium bicarbonate was found to be similar to that of calcium carbonate below the point where heat transfer is rate limiting.

The degree of conversion of sodium bicarbonate was found to be 12-17 times greater (depending on particle size) than that of sodium carbonate in the temperature range 250°-350°F (120°-177°C). This greater conversion was qualitatively explained by hypothesizing the formation of an activated species during thermal decomposition which would be more chemically reactive. By developing a relationship between the fluxes of the three diffusing species (CO₂, H₂O and SO₂), it was shown that the rate of generation of CO₂ and H₂O formed during thermal decomposition had little or no effect on the counter diffusion of sulfur dioxide at temperatures below 250°F (120°C). However, at higher temperatures (>350°F, 177°C) the generation of CO₂ and H₂O would tend to inhibit sulfur dioxide diffusion into the particle.

For the particle sizes tested for sodium bicarbonate (51µm-140µm), the optimum sulfur dioxide reaction temperature was found to be 300°F (149°C). For the particle sizes tested for sodium carbonate (20µm-200µm), the optimum sulfur dioxide reaction temperature was found to be 650°F (343°C).