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

Abstract

The influence of different conditions on the sulfur-capture efficiency during fluidized-bed desulfurization was studied using both experimental and modeling methods. The effects of the temperature (∼1120 or ∼1200 K) and gas atmosphere (90% N2 or 90% CO2) were studied using one limestone type. The CO2 atmosphere increased the degree of conversion compared to traditional air combustion conditions using both calcination–sulfation and direct sulfation methods. The scanning electron microscopy–energy-dispersive spectrometry analysis of spent sorbent particles revealed different sulfation patterns in different conditions. The N2 atmosphere produced a network sulfation or core–shell sulfation structure depending upon the temperature. Direct sulfation produced a core–shell structure with a thicker sulfate layer. A uniform pattern was observed for many particles in the CO2 atmosphere using indirect sulfation. The experimental results were analyzed using a time-dependent one-dimensional particle model that can accommodate simultaneous reactions. The model was used to interpret the test results and to determine the magnitude of reactions and diffusion rates as a function of the radius and time. The development of a Thiele number, conversion curve, and conversion profile during the reactions was used to explain the observed results.

Development of 2nd Generation Oxyfuel CFB Technology – Small Scale Combustion Experiments and Model Development Under High Oxygen Concentrations

“…The dense CaSO 4 is the main reason for relatively low calcium utilisation for sulphur capture in fluidised bed combustion applications. It has been observed also in earlier experimental work [5][6] that the sulphur capture is worse in direct sulphation region but capture is enhanced by higher CO 2 partial pressure in indirect sulphation region. Only conditions in test 5 were clearly on CaCO 3 side of CaCO 3 -CaO equilibrium curve, when sulphur is directly captured by CaCO 3 without calcination.…”

A Review of Thermal Co-Conversion of Coal and Biomass/Waste

“…Organically associated Ca can recapture the H 2 S to form sulfidic sulfur at temperatures above 873 K. Pyrolysis at temperatures of 873-1073 K, of ion-exchanged acid-washed coals with calcium acetate solution has revealed a significant increase of the sulfidic sulfur in the char [253]. In fluidized bed gasification/combustion, the role of Ca-based sorbent such as limestone or dolomite in capturing H 2 S/SO 2 has been well documented [254][255][256][257][258]. Given the high content of AAEM in some biomass fuels, sulfur capture by these metals could be an added advantage of co-conversion of coal and biomass based fuels.…”

Simultaneous calcination and sulfation of limestone in CFBB

“…A calcination and sulfation model established by Keener et al [32] showed that calcination had an influence on sulfation of limestone in typical air-fired CFBB conditions, but how calcination was influenced by sulfation was not described. Several experiments were conducted by Rahiala and Takkinen [33,34] with a bench-scale bubbling fluidized-bed reactor to study the influence of temperature and CO 2 concentration on sulfur capture of limestone, but bubbling fluidized-bed reactor cannot give precise chemical kinetics curves to analyze every stage of the reaction, and more importantly, their tests did not focus on the condition in real air fired CFBBs. An investigation by Olas et al [35] on mechanical activation of limestone under high concentrations of CO 2 in CFBB found that both the diffusion of SO 2 and CO 2 could be limited by the formation of CaSO 4 layer on the surface of the limestone particles, and mechanical treatment may increase both the sorbent calcination and sulfation rates.…”