Enhancement of Sulfur Uptake by Hydration of Spent Limestone for Fluidized-Bed Combustion Application

Abstract

The reactivation of spent limestone generated by fluidized-bed desulfurization of simulated flue combustion gases was studied. The regeneration technique consisted of spent sorbent hydration by immersion in water under controlled conditions. The effectiveness of the reactivation process was assessed by re-injection of the hydrated material in a fluidized-bed reactor under simulated desulfurization conditions. At the same time, the influence of hydration on the propensity of limestone to undergo attrition was evaluated by following the changes of particle size distribution and by collection of elutriated fines during sorbent utilization in the fluidized-bed reactor. The total calcium sulfation degree of hydrated samples was nearly twice that of the original spent sorbent. Apparently, reactivation did not bring about any significant enhancement of limestone attrition upon re-injection into the fluidized bed. Mechanistic aspects of sorbent reactivation were investigated with the aid of computer-aided scanning electron microscope-energy-dispersive X-ray analysis mapping of polished cross sections of sulfated, hydrated, and resulfated limestone particles. This analysis highlighted a pronounced redistribution of sulfur in the particles upon hydration that apparently provides the key pathway to enhanced sulfur uptake.

Sulphation of limestones in a fluidized bed combustor: The relationship between particle attrition and microstructure

“…The two sulphated limestones display a bimodal distribution function, with a marked peak in the low-sulphur-content zone (denoted as the "core zone") and a second peak corresponding to the high-sulphur-content zone (denoted as the "shell zone"). This pattern is indicative of the formation of a core-shell structure upon sulphation of the particles (Scala et al, 2001;Montagnaro et al, 2004). The values of areas subtended by the curves (core-zone and shell-zone areas) clearly indicate a lower sulphation capacity of Limestone B (core-zone …”

“…Porosimetric data were worked out in terms of cumulative pore undersize distributions; (c) SEM-EDX analysis: particles cross sections of sulphated sorbent material (polished and embedded in epoxy resin) were analyzed by means of a scanning electron microscope (SEM)-Philips XL30-with a LaB 6 filament and equipped with an energy dispersive X-ray (EDX) probe-Edax DX-4-for elemental mapping of sulphur and calcium throughout the particle cross section; (d) computer controlled (CC) SEM-EDX analysis: an algorithm was set up for automated analysis of EDX elemental maps with the aim of obtaining semi-quantitative indicators of sulphur distribution throughout the particle cross section. This procedure generated probability density functions of pointwise sulphur contents that could be directly related to the sorbent particle sulphation pattern, and provided a reliable tool for semi-quantitative comparative analysis of samples of different nature (Scala et al, 2001;Montagnaro et al, 2004).…”

Flue gas desulfurization under simulated oxyfiring fluidized bed combustion conditions: The influence of limestone attrition and fragmentation

“…2 reports the degree of calcium conversion X Ca as a function of time during oxyfiring sulfation tests carried out in beds of either sand or corundum. The same figure reports, for comparison, data obtained in a test performed with the same limestone under non-oxyfiring conditions (1800 ppm SO 2 , 8.5% O 2 , balance N 2 ) at 0.75 m s −1 and with an inert (sand) particle size of 0.4-0.6 mm, all the other operating conditions being the same (taken from Scala et al, 2001). In the experiments the reactor could not be considered differential with respect to SO 2 .…”

Limestone Attrition under Simulated Oxyfiring Fluidized‐Bed Combustion Conditions

“…with all the other operating conditions being the same (taken from [16]). In the experiments, the reactor could not be considered differential with respect to SO 2 .…”

“…Data collected during simulated air-blown FB combustion in a bed of sand display a monotonically decreasing E vs. t pattern. This trend is determined by the concurring effects of particle rounding off (at the very beginning of the tests) and of the strengthening of the particle outer layer due to the progress of sulfation [1,3,16]. Attrition/elutriation of unconverted Ca present in the bed as lime in the early stage of tests carried out under simulated air-blown combustion conditions is far more extensive (by about two orders of magnitude) than attrition of uncalcined limestone in experiments carried out under simulated oxyfiring conditions.…”