Both agglomeration of bed material and corrosion of heat transfer equipment are issues related to combustion of biomass in a fluidized bed boiler. The biomass-ash component potassium is considered a major contributor for both phenomena. In this study, the conventionally used bed material, silica sand, was replaced with up to 40 wt % by the natural ore ilmenite in Chalmers 12 MW th circulating fluidized bed (CFB) boiler. In this study the purpose was to evaluate the physical and chemical changes ilmenite undergoes during this process. Close observations revealed that ilmenite underwent segregation of iron to the surfaces and an enrichment of titanium in the particle core. The ash formed a calcium-rich double layer on the particle, surrounding the iron layer. A diffusion of potassium into the particle core was also seen which led to the formation of KTi 8 O 16 . In addition to evaluating how ash components interact with the material, the ilmenite was leached and investigated as a possible potassium capturer. Leaching experiments on the used ilmenite showed that calcium and potassium were leachable to a very limited degree, namely, to less than 0.2 and 1 wt %, respectively, of the total content. The diffusion of potassium into the core of the particle could reduce both agglomeration and corrosion issues and could thereby be of great value for the improvement of the resistance of the bed material agglomeration in the fluidized bed boiler.
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.
Abstract:Oxygen Carrier Aided Combustion (OCAC) is realized by using an active oxygen-carrying bed material in fluidized bed boilers. The active material is reduced in fuel rich parts of the boiler and oxidized in air rich parts. Advantages could be achieved such as new mechanisms for oxygen transport in space and time. Here calcined manganese ore has been used as active bed material in a 12 MW th circulating fluidized bed boiler. The fuel was wood chips and the campaign lasted more than two weeks. From an operational point of view, manganese ore worked excellently. From the temperature profile of the boiler it can be concluded that fuel conversion was facilitated, especially in the dense bottom bed. The effect did not always translate to reduced emissions, which suggests that final combustion in the cyclone outlet was also influenced. Substituting 10% of the sand bed with manganese ore made it possible to reduce the air to fuel ratio without generating large amounts of CO. The use of 100% manganese ore resulted in higher emissions of CO than the sand reference, but, when combined sulphur feeding, dramatic reductions in CO emissions, up to 90% compared to sand reference, was achieved.
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