The study investigated the impact of fly ash particle size, furnace temperature, oxygen content, and trace element properties on the occurrence and volatilization behavior of these elements. Based on their behavior, 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 contains manganese (Mn). Selenium (Se) falls between the first and second groups, while chromium (Cr) exhibits characteristics of both the first and third groups. The research revealed that, except for Mn, the enrichment of other elements in fly ash increases as the particle size decreases. Fly ash demonstrates different adsorption mechanisms for various trace elements, and increasing furnace temperature promotes the volatilization of certain elements. Except for Mn and Cr, the relative enrichment coefficient of most elements in fly ash is higher than in bottom slag. Low oxygen levels do not necessarily enhance the volatilization of all trace elements, and the classification of elements should be based on specific conditions.
Coal is a complex mixture of aromatic fused ring compounds and mineral impurities, containing 84 elements, including many trace elements enriched in organic matter and minerals. Although these elements are present in low concentrations (typically less than 100 g/g), they pose significant environmental risks. Studies have shown that heavy metals around power plants can exceed EPA limits within a few kilometers or even hundreds of kilometers. Data from a 1983 joint investigation across Western Europe, the U.S., Canada, and the former Soviet Union highlighted the importance of controlling trace elements during coal combustion. In 1990, the U.S. Clean Air Act Amendments required monitoring toxic emissions from fossil fuel plants, leading to stricter regulations. China has also implemented standards for trace elements in agricultural fly ash and waste incineration. With advancements in acid gas control, managing trace elements has become a priority. However, comprehensive studies on trace element distribution in industrial pulverized coal boilers remain limited.
This paper analyzed the occurrence and volatility of eight trace elements in raw coal, bottom slag, and fly ash from a 220t/h pulverized coal boiler. By examining factors like particle size, combustion conditions, and element properties, the study calculated the distribution of trace elements using mass balance. Elements were classified based on their storage and volatilization 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 was analyzed using a laser particle size analyzer, and wind speeds were calibrated with a pitot tube. Furnace temperature and oxygen levels were monitored using optical and flue gas analyzers. Three different excess air coefficients and furnace temperatures were tested under stable load conditions. Sample collection lasted 23 hours per condition, with coal, bottom slag, and fly ash samples retained for one hour each. Fly ash was collected from three electric fields and mixed proportionally.
Trace elements were measured using techniques such as graphite furnace atomic absorption, cold vapor atomic absorption, and X-ray fluorescence. The study found that some elements, like Pb, Se, and Hg, are more enriched in fly ash than in the Earth's crust, indicating potential hazards and resource value. Magnetic separation and particle sorting could further enrich these elements for commercial use. Japan has focused on extracting Al, Pb, and Zn from waste incineration ash.
Relative enrichment coefficients were used to assess the distribution of elements. The improved Meij coefficient indicated whether elements were enriched or depleted in fly ash or bottom slag. Results showed that smaller fly ash particles had higher enrichment for most elements, except Mn. Physical adsorption dominated for most heavy metals, while chemical adsorption was important for Cr. Mercury’s behavior was influenced by unburned carbon and surface activity. Chromium’s enrichment remained relatively constant, suggesting chemical adsorption dominance.
Temperature and excess air coefficient significantly affected enrichment factors. Higher furnace temperatures increased the enrichment of Cr, Zn, and Hg in fly ash. Lower oxygen levels promoted the volatilization of Pb, Se, and Hg, but not all elements followed this trend. Hypoxic conditions did not uniformly increase element volatility, and further research is needed.
Elemental properties, such as boiling points and reactivity, also influenced enrichment. For example, Pb was more volatile than Cd due to its affinity for chlorine. Some Cr existed in organic forms, affecting its distribution. Classification of elements should consider their properties, combustion conditions, and coal type.
In conclusion, most trace elements except Mn were enriched in fine fly ash particles. Adsorption mechanisms varied, with physical adsorption dominant for Cd, Zn, Se, and Hg, and chemical adsorption for Cr. Furnace temperature and oxygen levels impacted enrichment coefficients differently. While some elements showed higher enrichment in bottom slag, others were mainly distributed in fly ash. Element classification remains context-dependent, requiring detailed analysis of each element’s behavior.
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