Mechanism for Migration and Layer Growth of Biomass Ash on Ilmenite Used for Oxygen Carrier Aided Combustion

Abstract

Biomass is recognized as a CO2-neutral energy resource. However, biomass is a challenging fuel to combust because of its heterogeneity with regard to the content of inorganic constituents, volatiles, and moisture. Oxygen carrier aided combustion (OCAC) is a process advancement that provides enhanced combustion in existing circulating fluidized bed (CFB) units. The oxygen carrier has a central role in the OCAC concept through the oxygen transport it provides. The natural mineral ilmenite (FeTiO3) has been identified as a promising potential oxygen carrier. In order to ensure the feasibility even for long-term operation in industrial-scale processes, it is imperative to understand the evolution of the material during an OCAC process. In the present study, ilmenite was used as the bed material in the Chalmers 12 MWth CFB boiler during OCAC with woody biomass as fuel. Bed material samples were extracted from the bed inventory at different time intervals ranging from 5 to over 300 h. This paper proposes a mechanism for migration and layer growth of biomass ash on the ilmenite used as the oxygen carrier in a CFB combustor. It was found that with increased time of exposure, potassium migrated into the particle core. Longer process times led to the formation of a calcium layer around the particle, and simultaneously, migration of calcium inward on the particle was observed. Thermodynamic calculations were used along with analysis techniques in order to build a hypothesis for the possible mechanism of ash–bed material interaction.

Understanding the Interaction of Potassium Salts with an Ilmenite Oxygen Carrier Under Dry and Wet Conditions

“…In comparison to real ash interaction, phosphorus from bio ash has also been observed to accumulate on the surface of the ilmenite particles used in large scale experiments. 44 …”

Investigation of the Oxygen Supply and Distribution in a Bubbling Fluidized Bed by Using Natural Ilmenite for Oxygen Carrier Aided Combustion

“…Together with TiO 2 (rutile) it builts up KTi 8 O 16 . On the contrary, introduced calcium formed a two-layer structure around the Fe-rich outer layer with increasing operation time building up CaTiO 3 . During further experiments, Corcoran et al examined the mechanical development of sand and rock ilmenite over 15 days of OCAC operation.…”

“…Nevertheless, the reactivity compared to other, e.g., Ni-based oxygen carrier is quite low, , while the reactivity with syngas is higher as for methane . The increasing porosity of the ilmenite particles during the activation process results in reduced mechanical stability and increased attrition risk in the FBC . However, ilmenite shows good fluidizability and stability in long-term tests …”

“…31 The increasing porosity of the ilmenite particles during the activation process 26 results in reduced mechanical stability and increased attrition risk in the FBC. 14 However, ilmenite shows good fluidizability and stability in long-term tests. 18 The reactions ilmenite undergoes in the OCAC reactor especially depend on the temperature range applied.…”

Developing Oxygen Carriers for Chemical Looping Biomass Processing: Challenges and Opportunities

“…Experimetal observations suggest that c) is the most encountered mode of ash-OC interaction. [184,203] For example, Corcoran et al [183] reported the migration of potassium and calcium from the ash layer into the ilmenite particle, forming KTi 8 O 16.5 and CaTiO 3 , respectively. At the same time, a calcium layer mixed with other ash components (e.g., Si and P) gradually developed over the surface of the ilmenite particles.…”