Overview of trace element partitioning in flames and furnaces of utility coal-fired boilers

Abstract

Some coal constituents are potentially toxic, trace metals/metal compounds bound with the coal's mineral and organic matter components. These trace species are released during the combustion process and may pose an environmental and human health risk depending on their abundances, physicochemical forms, toxicity, partitioning behavior in the combustion and environmental control systems, and their ultimate disposal/deposition in the local and regional ecosystems associated with the coal combustion system. This paper provides an overview of recent research efforts to characterize the combustion process in large, utility-scale boilers as it affects the release, transformation and partitioning of coal's trace inorganic components in both the combustion zone and the heat transfer sections of the boiler. Trace element partitioning is initiated at the burner front and continues in the high-temperature radiant section of the furnace as the coal particles are pyrolized and burned. The various trace elements are interdispersed among the fly ash, bottom ash, and combustion flue gas; this depends upon the degree of volatilization of their particular geochemical modes of occurrence within the coal and the extent to which they may be physically or chemically bound to the carbon matrix or the primary aluminosilicate minerals. Those elements (major and trace) that are not volatized during combustion will comprise the matrix of both fly ash and the bottom ash in the form of a homogeneous “melt” as well as crystalline phases; the split between bottom ash and fly ash is determined primarily by the type of furnace design and to a lesser extent by operating conditions and coal rank. Once combustion gases are carried downstream of the combustion zone of a coal-fired boiler, the key factors which influence the final trace constituent transformation/partitioning behavior are the conversion of vaporized components into various solid forms and their collection along with the fly ash. The former is determined, basically, by three complex and interrelated processes — adsorption, condensation, and chemical transformation. While these simultaneous processes compete along the entire boiler gas pathway, conversion will be complete for all but the most volatile species before the combustion gases reach the particulate and desulfurization control systems. Collection of the particulate-based trace species in flue gas cleanup devices, such as ESPs and fabric filters, will depend to a large degree on the size fractionation of the particulate and, therefore, on the original transformation of the major mineral components of the coal into fly ash. Collection of vapor-phase components will ultimately depend upon the flue gas temperature and contact with various scrubber types and sorbents. Recent combustion tests with different coals indicates that both the composition and distribution of mineral matter in the coal matrix, not coal rank, are the most important criteria for predicting volatilization behavior and fume formation. It is theorized that the extent of volatilization, and subsequent condensation, of particular elemental components of the fume depend on their mode of occurrence and their extent of dispersion, as associated with the organic matter or a nonorganic extraneous component. Modes of occurrence help establish high-temperature thermodynamic properties, including volatility. Trace element distribution and the extent to which they are bound (or encapsulated) in the coal matrix influence the probability of volatilization.