Modification of gasification fly ash using calcium aluminate cements to produce sorbents for carbon capture

Abstract

As the link between climate change and carbon dioxide (CO2) emissions becomes increasingly harder to deny, carbon capture and sequestration has been promoted as a potential mitigation tool. One of the more well-established methods of carbon capture involves the use of calcium-based sorbents such as limestone.

Gasification fly ash (GFA) has the potential to be used as a calcium-based sorbent in carbon capture applications. Presented as a cheaper alternative to limestone, the implementation of GFA could drastically reduce operational costs. Raw, untreated GFA, however, is unsuitable for use in a fluidised bed reactor due to its corrosiveness and agglomeration under high temperatures. One of the simplest methods of corrosion removal is by washing GFA with water. Washed GFA, however, loses all its mechanical strength and is unable to be fluidised.

In this paper, we have investigated the suitability of modifying washed GFA with calcium aluminate cements (CACs) in hopes of improving its mechanical strength. A total of 25 experiments, with varying parameters such as CAC type and calcination gas, were tested. Sorbents were first tested for its fluidisability and mechanical strength in a cold fluidised bed as a preliminary test. Select sorbents were then placed in a hot fluidised bed to test its mechanical strengths under actual conditions, and its capture capacities were also recorded.

Results have shown that GFA-CAC sorbents calcined under a nitrogen atmosphere exhibit much greater mechanical strengths than unmodified, washed GFA. The addition of CACs to GFA have also demonstrated greater stability in its carbon capture capacity in the hot fluidised bed. It was also discovered through x-ray diffraction characterisation that the compound mayenite is not solely responsible for contributing to the mechanical strength. Overall, the binding of GFA with CAC for use in carbon capture applications is promising.