The study investigated the impact of fly ash particle size, furnace temperature, oxygen content, and trace element properties on their occurrence and volatility. Based on these characteristics, trace elements were categorized into three groups: the first group includes mercury (Hg); the second group consists of lead (Pb), zinc (Zn), and cadmium (Cd); the third group involves manganese (Mn). Selenium (Se) falls between the first and second groups, while chromium (Cr) exhibits properties of both the first and third groups. It was found that, except for Mn, the enrichment of other elements in fly ash increases as particle size decreases. Fly ash shows different adsorption mechanisms for various trace elements, and higher furnace temperatures promote the volatilization of some elements. Except for Mn and Cr, the relative enrichment coefficient of other elements in fly ash is greater than in bottom slag. Low oxygen content does not enhance the volatilization of all trace elements, and classification should be based on specific conditions.
Coal is a complex mixture of aromatic clusters and mineral impurities, containing 84 elements, including many trace elements enriched in organic matter and minerals. Although these elements are typically present in low concentrations, they can cause significant environmental pollution. For example, heavy metals in soil near power plants have been found to exceed EPA limits within a few kilometers. Data from a 1983 joint investigation by Western Europe, the US, Canada, and the former Soviet Union revealed that arsenic emissions from coal combustion have become a critical issue. In 1990, the US Clean Air Amendment required estimating toxic emissions from fossil fuel plants and developing control strategies. National standards for agricultural fly ash and waste incineration also include trace element emission guidelines. With advances in acid gas control, trace element management has become a priority. While many researchers in China have studied this area, comprehensive research on trace element storage, volatility, and classification in industrial pulverized coal boilers remains limited.
This paper quantitatively analyzed eight trace elements in raw coal, bottom slag, and fly ash from a 220t/h pulverized coal boiler. The study examined the volatility of trace elements, fly ash particle size, and combustion conditions, using mass balance calculations to determine their distribution in combustion products. Based on these findings, elements were classified according to their storage and volatility characteristics.
The test involved the No. 6 boiler at Yangzi Petrochemical Thermal Power Plant, a high-pressure solid-state slag boiler with a square furnace and DC oscillating burners. Coal samples were analyzed using a HYDAO2000 laser particle size analyzer, with an average particle size of 35.1 µm. Before testing, wind speeds were calibrated, and furnace temperature and oxygen levels were adjusted to ensure consistent conditions. Fly ash samples were collected from the electrostatic precipitator, with ash from different electric fields mixed in a 80:15:5 ratio. Sample analysis included trace element determination via atomic absorption spectroscopy and X-ray fluorescence.
Fly ash contains trace elements such as Pb, Se, and Hg in higher concentrations than the Earth’s crust, making it a potential resource for extraction. Magnetic separation and particle sorting can further enrich these elements for commercial use. In Japan, extracting Al, Pb, and Zn from incineration ash has gained attention.
To evaluate the occurrence and volatility of trace elements, several relative enrichment coefficients were used, including the geochemical enrichment factor (K) and the improved Meij coefficient. These metrics help assess whether elements are enriched or depleted in fly ash versus bottom slag. Results showed that most elements, except Mn, tend to concentrate in smaller fly ash particles. Smaller particles have a larger surface area, which enhances physical adsorption of heavy metals. Mercury, however, is mainly gaseous and less likely to be adsorbed effectively. Chromium showed minimal variation with particle size, indicating chemical adsorption dominance.
Temperature and excess air coefficient significantly influence the enrichment factors. Higher furnace temperatures increased the enrichment of Cr, Zn, and Hg, while lower oxygen levels promoted the volatilization of Pb, Se, and Hg. However, this effect varied among elements, suggesting the need for further research. Trace element behavior depends on their nature and coal composition, with some elements more volatile under reducing conditions.
Based on the study, trace elements were classified into three groups: highly volatile (e.g., Hg), moderately volatile (e.g., Pb, Zn, Cd), and less volatile (e.g., Mn). Selenium fell between the first and second groups, while chromium exhibited dual characteristics. As was excluded due to low concentration. The classification should consider element properties, combustion conditions, and coal type.
In conclusion, the study highlights the importance of understanding trace element behavior in coal combustion. Most elements, except Mn, are enriched in fine fly ash, and their distribution depends on combustion parameters. Physical and chemical adsorption mechanisms play key roles, and the classification of elements must be context-specific. This knowledge is essential for managing environmental risks and exploring resource recovery opportunities from fly ash.
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