Influence of Periodically Changing CO₂ Partial Pressure on Sulfur Capture and Free Lime Content of Residues in PFBC

Abstract

PFBC technology is still under development. Further research into the capture of SO 2 by limestone during PFBC is needed. The present work investigates the influence of periodically changing partial pressure of CO 2 on the sulfation of limestone and also the free lime content present in residual products. Experiments were conducted in a fixed-bed reactor under conditions typical of PFBC, that is, limestone particles were alternately exposed to the conditions of calcination and recarbonation as the reaction of sulfation proceeded. It was found that the degree of sulfation achieved was considerably higher with periodically changing partial pressure of CO 2 than with a steady high partial pressure of CO 2 . This was true even when the average value of the pressure during its periodical change exceeded the calcination equilibrium pressure. These findings may partly explain the high conversion of limestone obtained in PFBC units. Furthermore, the free lime content proved remarkably different for various cycle times of changing CO 2 partial pressure and/or various periods of a cycle time elapsing in the calcining region. This indicates that the content of free lime in solid residues from PFBC boilers can vary considerably with respect to overall operating conditions.

Bench-Scale and Modeling Study of Sulfur Capture by Limestone in Typical CO2 Concentrations and Temperatures of Fluidized-Bed Air and Oxy-fuel Combustion

“…In a CO 2 atmosphere, the calcination–sulfation has resulted in higher conversion than the calcination–sulfation in air . Also, in a CO 2 atmosphere, higher conversion has been achieved in calcining conditions compared to non-calcining conditions . It appears that the optimal sulfur capture in fluidized-bed oxy-fuel conditions is not only dependent upon the sulfur capture mechanism (indirect/direct) but also the combination of several parameters, such as temperature, gas concentrations, and limestone type, determines in which conditions the highest capture capacity is obtained and in which conditions the “optimal” sulfation pattern is gained inside the particle.…”

“…Although the change of indirect sulfation to direct sulfation can result in a higher sulfation degree, 2 opposite results have been reported. 8 Recently, de Diego et al 9 and Garci ́a-Labiano et al 10 found that indirect sulfation gave better conversion degrees compared to direct sulfation and suggested that 1198−1223 K would be an optimal temperature for sulfur retention in oxy-fuel conditions (for gaining calcination).…”

Experimental and Modeling Study of Sulfur Capture by Limestone in Selected Conditions of Air-Fired and Oxy-fuel Circulating Fluidized-Bed Boilers

“…Also, direct sulfation has led to higher conversion compared to sulfation (in a N 2 atmosphere) . In a CO 2 atmosphere, higher conversion has been attained in calcining conditions compared to noncalcining conditions …”

“…In conventional coal combustion, the sulfation efficiency is usually low because of the pore blockage of the sorbent. Direct sulfation can lead to higher sulfation when compared to the calcination–sulfation process, but conflicting results have also been reported . It has been explained that the counter-diffusion of CO 2 generated during the direct sulfation reaction forms a porous product layer, offering a lower diffusion resistance than the essentially nonporous layer formed during the CaO–SO 2 sulfation reaction .…”

“…Direct sulfation can lead to higher sulfation when compared to the calcinationÀ sulfation process, 5 but conflicting results have also been reported. 9 It has been explained that the counter-diffusion of CO 2 generated during the direct sulfation reaction forms a porous product layer, offering a lower diffusion resistance than the essentially nonporous layer formed during the CaOÀSO 2 sulfation reaction. 10 This report has since been cited repeatedly by several authors to explain the porosity in the product layer and the high rate of the direct sulfation reaction compared to the indirect sulfation reaction at higher conversions.…”