Trace Element Deportment in Combustion Processes

Coal combustion is an important anthropogenic source of semi-volatile trace elements such as As, Hg, Cd, Se, Pb and Zn to the environment. These emissions are of significance because of potential environmental impacts on human and ecosystem health.

These metals are known to be volatile to some extent in the hot zone of combustion furnaces and to heterogeneously condense on to available surfaces such as ash particles in cooler zones downstream in the combustion process. Due to their relatively large surface area, fine ash particles can be preferentially enriched in trace metals. Depending on the trace metals deposition mechanism, different enrichment patterns can be observed. An understanding of the mechanism and factors that influence the partitioning of the trace metals between the different product streams of the combustion process is an important step in predicting and mitigating the release of these elements into the environment.

This project is a component of a larger CCSD program to develop a model to predict the vaporisation and subsequent partitioning of trace metals during the combustion of Australian coals, based on theoretical thermodynamic equilibrium, theoretical trace element vaporisation, experimental data and data correlation.

The main components of the ACARP project are:

1. Bench scale combustion and pyrolysis experiments on specific coals at different oxygen concentrations.
2. Summarise the experimental program by defining the coal and mineral character determining trace element partitioning to gas and particulate phases, and establish any impact of the combustion conditions.

Small-scale experiments were conducted in a laminar flow drop tube furnace on several Australian coals under various combustion and pyrolysis conditions. The coals used in this investigation were sourced from the CRC coal bank and represent a broad range of trace element and chemical compositions. All ash and aerosol products were analysed for the target trace metals by ICP-MS and the degree of trace metal enrichment was determined. Calcium-metal and metal-metal correlations were examined to determine the extent of these interactions and their impact on trace metal volatility.

The following observations can be made from the experimental results:

1. Mercury was found to be highly volatile in all coals, with large concentrations appearing in the sub-micron and aerosol size fractions. A strong correlation was also observed between levels of "unburnt carbon" in the "bottom" ash or char and mercury retention in the coarse material (> 30 micron).
2. The proportion of selenium found in the gas phase and in the submicron ash fell in the range of 60 to 80 % of the coal's starting selenium content.
3. The other trace elements showed a broad range of partitioning to the submicron material, eg lead (10 to 90%), cadmium (0 to 70%) and nickel (0 to 50%). The trace element partitioning trends observed in this study are consistent with partitioning information obtained on US coals.
4. Arsenic - calcium interaction appears to play an important role in the heterogenous transformation of arsenic from the vapour phase to the submicron particles, which is consistent with US studies. However, contrary to some work reported overseas (Seames, 2000), no other correlation between calcium and the trace elements studied were observed.
5. The application of an air column for the preparation of "ash rich" and "ash depleted" coal fractions proved successful. Analysis of the fractionated coals revealed selective enrichment and depletion for some of the trace elements, notably mercury and selenium. This result may indicate a strong mineral association for these elements.
6. Determination of "modes of occurrence" of the trace metals in the feed coals will be undertaken as part of the ongoing CCSD program. Once established the "modes of occurrence" of the trace elements will be used to determine the impact this has on partitioning behaviour of the elements.