The long term behaviour of CFBC ash-water systems

Area of Science:

• Materials Science
• Environmental Chemistry
• Geochemistry

Background:

• Circulating fluidized bed combustion (CFBC) ashes exhibit significant chemical activity when exposed to water, persisting for extended periods.
• The setting and behavior of CFBC ash-water mixtures are critically influenced by the formation of gypsum and ettringite.

Purpose of the Study:

• To investigate the chemical reactions and transformations occurring in CFBC ash-water systems.
• To understand the role of ettringite formation, carbonation, and their impact on ash behavior and environmental implications.

Main Methods:

• Experimental analysis of CFBC ash-water mixtures.
• Use of model systems (CaO-Al2O3-CaSO4-H2O) to simulate ash behavior.
• Observation of ettringite crystallization, carbonation, and decomposition processes.

Main Results:

• Ettringite formation causes expansion and water absorption, while the overall system shrinks.
• Carbonation of free lime reduces ettringite formation and is enhanced by air contact, leading to decomposition of ettringite into calcite and gypsum.
• Model systems replicate key behaviors, though real ashes are complicated by silicates and alkaline interactions.

Conclusions:

• Carbonation of lime in CFBC ashes is environmentally beneficial, reducing landfill chemical activity and CO2 emissions.
• The long-term chemical activity of CFBC ashes is influenced by ettringite stability and carbonation processes.
• Understanding these reactions is crucial for managing CFBC ash in environmental applications.