May 22-27, 2016
Fluidized bed combustion is widely considered an advantageous technology managing moist and heterogeneous fuels due to the heat storage in the bed material and its favorable conditions regarding fuel and oxygen mixing throughout the combustion chamber. Even though mixing is often regarded as sufficient, the technology remains sensitive to variations in load and heterogeneity of fuels, leading to an uneven distribution of oxygen in time and space of the furnace. The remedy has nearby exclusively been related to the supply of air, particularly considering a surplus of air and positioning of injection ports.
An applicable combustion concept, oxygen carrier aided combustion (OCAC), has been developed and demonstrated in the Chalmers 12 MWth circulating fluidized bed boiler. The novelty of this concept is that it targets the distribution of oxygen inside the combustion chamber on contrary to prior accepted remedies. The rational is to replace the regularly used inert bed material by an oxygen carrying metal oxide which can take up oxygen where it is abundant and subsequently release it to combust unburned gases at oxygen depleted zones. Thus, the bed material functions as a buffer of oxygen allowing for lower surpluses of air and better ability to handle load variations and heterogeneous fuels.
This work contains the modeling of a system where inert bed material is compared to an oxygen carrier with the aim to show the effects of its oxygen-buffering ability. To a system with constant fuel feed, a pulse of instantaneous fuel increase is modeled. The outcome of the model was then verified by the results of experimental work conducted in Chalmers 12 MWth CFB boiler. The results consistently show that the introduced oxygen carrying bed material does have an oxygen-buffering ability and the distribution of oxygen is considerably improved throughout the combustion chamber. Due to the enhanced distribution of oxygen the system is less sensitive to fluctuations in load and fuel heterogeneity.
As the infrastructure of plants where the concept would be applicable are already in place, the use of active bed material for oxygen carrier aided combustion has great potential to reach full scale commercialization in a near future. Furthermore, naturally occurring ores that contain considerable amount of metals such as iron and manganese have proven to be promising candidates as oxygen carriers. Owing to the possibility of using natural ores, in place of manufactured materials, economic feasibility of the concept is promising.
Angelica Corcoran, Fredrik Lind, and Henrik Thunman, "Validation of the oxygen buffering ability of bed materials used for ocac in a large scale cfb boiler" in "Fluidization XV", Jamal Chaouki, Ecole Polytechnique de Montreal, Canada Franco Berruti, Wewstern University, Canada Xiaotao Bi, UBC, Canada Ray Cocco, PSRI Inc. USA Eds, ECI Symposium Series, (2016). http://dc.engconfintl.org/fluidization_xv/15