Sulfur Uptake during Oxygen-Carrier-Aided Combustion with Ilmenite

Abstract

The feeding of sulfur during conventional biomass combustion can reduce ash-related corrosion attacks on heat-transferring surfaces. In this work, oxygen-carrier-aided combustion with ilmenite as the bed material and the feeding of elementary sulfur as an additive is addressed. More precisely, the sulfur uptake by the bottom bed material, i.e., the inventory of the oxygen carrier and its accumulation of the main biomass ash components calcium and potassium, is considered. The experiments, including time-resolved sampling of bed material, were carried out during continuous operation in conditions that can be considered to mimic those in a full-scale boiler. The aim with the investigation was to compare the sulfur uptake by the bed material based on two distinct conditions: (1) sulfur was introduced to the furnace in the absence of an ash layer on the particles (i.e., at the start of an experiment) and (2) sulfur was introduced with an already existing ash layer on the particle (i.e., after long time operation). Convectional ash characteristic methods where used to study the sulfur uptake, and the investigation showed that sulfur was mainly bound to the potassium and calcium phases on/in the particles. Differences in the elemental distribution of sulfur were observed depending upon the emergence of an ash layer coat on the bed material particles. Sulfur was located predominantly between the formed ash layer and the core when it was added from the start of the process and was located mainly in the particle core when an ash layer coated the particle before its addition.

Thermodynamic Analysis on the Fate of Ash Elements in Chemical Looping Combustion of Solid Fuels─Iron-Based Oxygen Carriers

“…There is a clear difference between the K 2 SO 4 concentration in the AR where the combination of iron oxide and BT causes the highest gas-phase release, whereas ilmenite does not due to the formation of potassium titanate K 2 Ti 6 O 13 or KTi 8 O 16.5 (see the discussion regarding potassium below). Calcium and potassium sulfates can accumulate in ilmenite during OCAC, which has been shown by Vigoureux et al A detailed sulfur phase distribution can be found in the SITable S7 for iron oxide and Table S14 for ilmenite.…”

“…Both the separation of Ti and Fe as well as the Ca layer have been observed previously under chemical looping conditions. , Other studies have found that, with increased time of exposure, K migrates into the ilmenite particle core and that, depending on the chemical form of K, it will react differently in the matrix . Research conducted on the ash layer build-up and sulfur interaction when using ilmenite in OCAC applications shows that calcium and potassium are bound to sulfur both in and on the ilmenite particles. , Limited research around the chemistry of heavy metals in chemical looping processes is available. In a series of experiments around OCAC with ilmenite and waste fuels, zinc ferrites were detected between the double calcium layer, and copper ferrites were identified on the particle surface .…”

“…29 Research conducted on the ash layer build-up and sulfur interaction when using ilmenite in OCAC applications shows that calcium and potassium are bound to sulfur both in and on the ilmenite particles. 27,30 Limited research around the chemistry of heavy metals in chemical looping processes is available. In a series of experiments around OCAC with ilmenite and waste fuels, zinc ferrites were detected between the double calcium layer, and copper ferrites were identified on the particle surface.…”

Investigating the Interaction between Ilmenite and Zinc for Chemical Looping

“…Furthermore, alkali sulfates at the particle surface, seen in Figure S9 in the SI, could also contribute to the release of SO 2 . Sulfur uptake during oxidizing conditions and the release during reduction has previously been established in ilmenite and attributed to the phases CaSO 4 , K 2 SO 4 , and K 2 Ca 2 (SO 4 ) 3 …”

“…One example is ilmenite concentrate, hence referred to as rock ilmenite, which is obtained after crushing and processing the iron and titanium rich mineral ore. Iilmenite fulfills the OC requirements considerably and has therefore been extensively studied in academia and also implemented in industrial applications. ,,− Considering the knowledge obtained around ilmenite as an OC and its wide implementation, it is considered a benchmark oxygen carrier. Ilmenite utilized in the combustion of biomass and waste has been studied concerning major ash-forming elements − and heavy metals. , With time, ilmenite particles will exhibit increased porosity accompanied by cracks in the particles, along with segregation of iron to the particle surface and enrichment of titanium in the particle core . The most recent study used thermodynamic equilibrium calculations to illustrate possible pathways for important heavy metals during CLC of biomass and waste.…”