Abstract
A circulating fluidized bed (CFB) boiler generates energy by burning petroleum coke. Because burnt petroleum coke has a high sulfur content, limestone is added to the boiler to reduce the emittance of sulfur dioxide through desulfuration. The residue collected from the boiler is called CFB ash. CFB boilers in Taiwan can produce 328,000 tonnes of CFB fly ash per year. In this study, the pozzolanic characteristics of CFB fly ash were investigated by blending CFB fly ash and ordinary Portland cement (OPC). The CFB fly ash was mainly composed of CaO, SO3, and SiO2 in concentrations of 37.8%, 9.2%, and 2.2%, respectively. The crystals of CFB fly ash contained 3CaO.SiO2, 2CaO.SiO2, Ca(OH)2, C-S-H (Tobermolite), and Ettringite. The results revealed that applying the toxicity characteristic leaching procedure to CFB fly ash renders it suitable for use in blended cement. At later curing ages (90 days), the pore volumes of both the OPC and the CFB-fly-ash-blended cement pastes (CFBFABCP) decreased as the curing time increased. A possible explanation is that C3S and C2S were consumed to form C-S-H gel, resulting in an increase in the Q1 and Q2 groups identified by 29Si Nuclear Magnetic Resonance (NMR) spectroscopy. Furthermore, the peak of the Q0 group decreased, but those of the Q1 and Q2 peaks increased with an increasing curing time. The pozzolanic activity of the CFBFABCP containing 10% CFB fly ash indicates that it is a suitable substitute for OPC in blended cement.
Highlights
Circulating fluidized bed (CFB) combustion is a clean technology for coal burning, which has the advantages of adaptability to numerous fuels, high combustion efficiency, low NOx emissions, and stable operation over a wide range of load regulations [1]
The results revealed that when the CFBFABCP contained 10%-40% CFB fly ash, the corresponding compressive strength decreased by 5.6%-22.2%
Compressive strength development increased at later ages in the CFBFABCP samples than in the ordinary Portland cement pastes, at higher concentrations of CFB fly ash
Summary
Circulating fluidized bed (CFB) combustion is a clean technology for coal burning, which has the advantages of adaptability to numerous fuels, high combustion efficiency, low NOx emissions, and stable operation over a wide range of load regulations [1]. CFB fly ash usually contains a high content of calcium, as both an oxide and a sulfate [2]. CFB fly ash differs from typical coal combustion by-products in that (1) it lacks the high content of f-CaO present in atmospheric fluidized bed combustion ash, preventing it from reacting vigorously with water, and lacks the high content of SO3 (usually anhydrite) present in both atmospheric fluidized bed combustion ash and pressurized fluidized bed combustion ash; (2) it consists of few spherical particles because the temperature in CFB combustion boilers (often 800-900 °C) is lower than that in pulverized coal-fired boilers (often 1300-1500 °C); and (3) it self-cements [4, 5]. Technological advancements would improve recovery and reduce the environmental impact of CFB fly ash
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