Oxygen carrier aided combustion: Implementation of oxygen carriers to existing industrial settings
Doctoral thesis, 2019
Utilization of biomass and waste to produce heat and power is necessary for a sustainable future energy mix. Thermal conversion of biomass is considered to yield CO2 neutral emissions and the utilization of waste reduces its volumes in a world struggling to dispose of ever-increasing amounts. However, these fuels can be difficult to combust as they are complex in their composition. One technology allowing for conversion of both biomass and waste is fluidized bed conversion. Commonly, fluidized bed boilers are operated with an excess of air which lowers the efficiency of the plant. Replacing the quartz sand used as bed material during biomass and waste conversion with an oxygen carrier is referred to as oxygen carrier aided combustion (OCAC). By this replacement, oxygen availability is increased throughout the combustion chamber with increased boiler efficiency as a consequence.
This thesis presents the implementation of oxygen carriers to existing industrial units. The development has been rapid due to the possibility of conducting research through integration of scales. Experiments on semi- and full industrial scale validated the improved distribution of oxygen. Bed materials extracted from the industrial units were analyzed and tested for their oxygen transferring capacity on laboratory scale. The evaluation of bed materials provided for an understanding of how oxygen carriers can be utilized for the concept. The oxygen carriers included in this work are two types of ilmenite: sand and rock, and a manganese ore.
This work provides a comprehensive understanding of how the bed material develops regarding oxygen transfer, as well as chemical and mechanical resistance. Sand and rock ilmenite show different characteristics when exposed to the process. When following their progression of iron and structural development after being subjected to OCAC, sand ilmenite develops cavities inside the particles to which iron migrates and which further causes mechanical instability and shattering of particles over time. Iron migrates to surfaces on rock ilmenite particles, which are decomposed by splitting. The materials interact similarly with main ash constituents of the fuel. A heterogenic outer layer is formed, consisting mainly of Ca but also traces of other elements from the fuel ash. Ca and K diffuse inward and are incorporated in the ilmenite structure. The ash interactions are not found to directly inhibit the oxygen carrying capacity, however, a decline in capacity is noticed as ash layers build up and become thicker.
This work shows that the oxygen carrier ilmenite can be implemented in existing industrial settings, without reconstruction of the current system. Optimization measures are proposed where magnetic separation allows for reuse of bed material that still contain oxygen transferring capacity and the regeneration of bed material can be decreased in comparison to quartz sand. Thus, the results of this thesis suggest that OCAC is a feasible concept for conversion of complex fuels.
Oxygen carrier aided combustion