Fly ash and adsorption of air-entraining agents in concrete: influence of combustion conditions
Multi-scale and/or multi-disciplinary approach to process-product innovation
Analysis of Environmental Issues (T3-3b)
Keywords: Fly ash, air entrainment, concrete, combustion
Fly ash, a by-product from combustion of pulverized coal or biomass, is utilized in the concrete manufacture, where it serves as partial replacement of Portland cement. The residual carbon in fly ash is known to adsorb the air-entraining agents (AEAs) added to the concrete to increase its workability and resistance toward freezing and thawing conditions. The recent year’s introduction of improved burner technologies, in order to reduce NOx emissions, has been reported to lower the fly ash quality due to the ash is produced with increased carbon content and higher AEA adsorptivity per gram of carbon. In future design and operation of new combustion technologies, it is important to understand the mechanisms in the combustion process causing the problem. The aim of this project is to obtain knowledge of how combustion conditions of pulverized fuel affect the fly ash quality.
In the present study, fly ash has been produced from combustion of pulverized coal in an entrained flow reactor in order to investigate the influence of combustion conditions on the fly ash quality. Various combustion conditions have been achieved by changing the total excess air and the ratio between feed air and total air. The collected fly ashes have been analyzed for carbon content and adsorption capacity of AEAs, the latter using the foam index test. The values have been compared with total excess air, feed/total air ratios and NOx emissions.
The ash analysis showed that the foam index of the produced fly ash increased with lower total excess air and lower feed/total air ratio. Part of the increment was due to elevated carbon content, but it was also found that the residual carbon was produced with higher AEA adsorptivity at lower excess air and feed/total air ratio. A linear relationship was found between the NOx emission and the feed/total air ratio, while the major changes in the foam index of the fly ash occurred at low ratios. Thus, small variations in the oxidizing conditions appear to affect the fly ash quality at lower NOx emissions, while enhanced oxidizing conditions result in higher NOx emissions, but have no affect on the fly ash quality. Determination of the optimal combustion conditions, where high fly ash quality and low NOx emission are achieved, will be beneficial in operation and future design of combustion technologies.
Presented Tuesday 18, 09:05 to 09:25, in session Analysis of Environmental Issues (T3-3b).
