Product Layer Development during Sulfation and Sulfidation of Uncalcined Limestone Particles at Elevated Pressures

Abstract

Fluidized bed combustion or gasification allows for in-bed sulfur capture with a calcium-based sorbent such as limestone or dolomite. Sorbent particle size, porosity, internal surface, and their variation during conversion have great influence on the conversion of the sorbent. The uptake of SO2 and H2S by five physically different limestones is discussed, for typical pressurized fluidized bed combustor or gasifier conditions: 850/950 °C, 15/20 bar. Tests were done in a pressurized thermogravimetric apparatus (P-TGA), the size of the limestone particles was 250−300 μm. It is stressed that the limestones remain uncalcined. A changing internal structure (CIS) model is presented in which reaction kinetics and product layer diffusion are related to the intraparticle surface of reaction, instead of the outer particle surface as in unreacted shrinking core (USC)-type models. The random pore model was used for describing the changing internal pore and reaction surfaces. Rate parameters were extracted for all five limestones using the CIS model and a USC model with variable effective diffusivity. Differences in the sulfur capture performance of the limestones were evaluated. Plots of the CaSO4 or CaS product layer thickness as a function of conversion are given, and the relative importance of limestone porosity and internal surface is discussed.

Kinetics of high‐pressure removal of hydrogen sulfide using calcium oxide powder

“…These reactions are well researched and a number of researchers have reported kinetic data pertaining to the reaction between CaO and H S at ambient and high pressures 2 using gravimetric techniques in the temperature range of Ž 650᎐1, 050ЊC Pell, 1971;Squires et al, 1971;Camp, 1979;Westmooreland et al, 1977;Borgwardt et al, 1984;Heesink and van Swaaij, 1995;Yrjas et al, 1996;Zevenhoven et al, . 1998 .…”

“…A similar effect of initial sorbent surface area on sulfidation extent has also been reported by other researchers for calcined and uncal-Ž . cined limestone sulfidation Zevenhoven et al, 1998 .…”

Morphological analysis of sulfated Ca-based sorbents under conditions corresponding to oxy-fuel fluidized bed combustion

“…This structure undergoes a slight change from 900 9 to 950 ºC but the most relevant change was observed in the sulfated sorbent at 975 ºC. Some researchers [25][26][27] found that high temperatures led to the sintering phenomenon of the sorbents decreasing their sulfation capacity. Therefore, this effect seems to be one of those responsible for decreasing the sorbent sulfation conversion at temperatures above 900 ºC obtained in TGA [7].…”

Dynamic Coupling of Pore Structure Evolution with Carbonation Kinetics of CaO-Based Sorbents: Experiments and Modeling

“…The unreacted shrinking core model hypothesizes the reaction to occur at a sharp interface that moves progressively toward the particle center. Zevenhoven et al 21 pointed out that the use of a USC-type model was somewhat self-contradicting when pore diffusion inside the particle is not a limiting mechanism, because a sharp interface at radial position is not present in this case. The USC model is therefore inappropriate for the carbonation behavior because the carbonation reaction and pore diffusion take place simultaneously over the sorbent particles.…”