Investigation of Ilmenite from Oxygen Carrier Aided Combustion: Sulfur Interactions and Recovery of Elements

Licentiate thesis, 2022

Oxygen- Carrier- Aided Combustion (OCAC) is a promising technology for the combustion of solid heterogeneous fuels such as biomass and waste in fluidized-bed (FB) units. In OCAC the conventional bed material silica sand is replaced by an oxygen carrier (OC). Replacing the bed material with an OC has many advantages. It allows the use of heterogeneous fuels, provides a better oxygen distribution inside the combustion chamber and thereby a more even temperature distribution. In this thesis, ilmenite is used as OC bed material.
Even though ilmenite has shown to be a promising OC, the higher price per kg of material compared to the traditionally silica sand is a drawback. In fluidized bed units, the attrition of the bed particles and therewith the loss of bed particles as fines in fly ashes leads to the need of a continuous addition of new bed material. At the same time, because of the ash interactions, parts of the bed material are also removed as a waste stream. Combined these two effects result in a large consumption of bed material. For the case of ilmenite, the loss of bed material by both mechanisms can result in significantly higher cost, if compared with the scenario where sand is used as bed material. Thus, ways to prolong the residence time of the bed material in the boiler and its value should be investigated.
Biomass which can be used as a renewable source of energy is a fuel which is hard to handle. Notably, its alkali content can cause corrosion of the equipment and increase of maintenance costs. One way to counteract corrosion is through addition of sulfur. The effect of sulfur on ilmenite performance has not been investigated earlier which is one of the main questions of the present work.
Furthermore, to increase the value of the ilmenite after its use in boilers, a modified process for the recovery of elements from the ilmenite waste flows has been examined and adapted to already existing process.
The work presented in this thesis is based on a two-week campaign which was carried out in the 12 MWth Circulating Fluidized Bed (CFB) semi-industrial unit. During the campaign, sulfur addition was tested. A range of methods were utilized to study the sulfur effect on the bed particles, including use of SEM-EDX and ICP to characterize the obtained samples. Further use of a laboratory scale quartz reactor to mimic oxidizing and reducing conditions was also performed.
For waste ilmenite generated during the campaign, digestion of ilmenite samples with sulfuric acid has been carried with the aim of recovering Ti. This work shows that sulfur is captured by the ilmenite particles and is mainly bind to the ash elements and that sulfur can also be released, depending on the redox conditions. This allows for a better management of the composition of used bed material. Furthermore, it was found that the used ilmenite from OCAC can represent an advantage for Ti extraction and recovery due to development of pore and cracks, and due to the natural separation of the Ti and Fe phase occurring under FB combustion conditions.