CFB Ash

Other types of coal ashes (or Coal Combustion Products (CCPs)) are generated by fluidized bed boilers, wet flue gas desulfurization (FGD) scrubbers, spray dryers, and limestone or lime injection boilers. The CCPs generated from these units primarily consist of calcium oxide (CaO), calcium sulfate (CaSO4), calcium sulfite (CaSO3) and calcium hydroxide (Ca(OH)2). Fly ash may also be mixed as well with these materials. This article is a compilation of several articles that have focused on Circulating Fluidized Bed (CFB) ashes and their options for utilization in commercial and environmentally acceptable applications. For the sake of simplification the term 'CFB' will be used to denote the various types of Fluidized Bed Combustion processes.

CFB material can be generated from Atmospheric Fluidized Bed Combustion (AFBC) and Pressurized Fluidized Bed Combustion (PFBC). The combustion can also be in the original 'circulating' or in the 'bubbling' bed process. With the exception of relatively a high mineral carbon, the chemistry of the PFBC ashes is typical of CFB material.

The lack of lime (CaO) in the PFBC ashes also is distinctly different from AFBC ashes, which normally contain large amounts of lime. In the PFBC systems, the partial pressure of carbon dioxide (CO2) favors the equilibrium conditions of both calcination and recarbonization. This results in low lime and high carbonates (calcite or dolomite) in pressurized FBC ash as compared to high lime and low carbonates in the atmospheric FBC ash.

The CFB combustion process introduces ground limestone (or lime) into the combustion chamber that burns the finely pulverized coal. Due to the wide range of fuel that can be used in this process, as well as the interaction of the variables affecting sulfur capture in the fluidized bed combustor, CFB ash characteristics can vary widely. However we will focus on the ash material that would be generated with a coal fuel source. The calcium in the limestone attaches itself to the sulfur oxides released from the burning of the coal thereby reducing the sulfur dioxide released into the atmosphere. Thus this by-product (ash) will contain more calcium compounds and sulfur products than ash from a conventional combustion system. Consequently the levels of silica (SiO2), alumina (Al2O3) and ferric oxide (Fe2O3) will be somewhat lower than ash from a pulverized coal or conventional system.

The ash is formed at lower temperatures which leaves the grains unfused, i.e., in a more leafy or flake type shape, rather than spheroidal or round. When the ash is tested for Loss On Ignition (LOI) by ASTM methods, the loss will typically range from 2.5% to 8.5%. This loss on ignition is the result of the release of any free moisture, chemically combined 'lattice' or 'hydroxy' water, carbon dioxide (CO2), sulfur dioxide (SO2), and any volatile pyrolytic products of any organic material present. Due to the presence of calcined lime in the ash, the pH will be higher than for typical coal ashes. Since this material contains silica, alumina and lime and because of its small particle size and shape (thus high surface area), it tends to be self-cementing when hydrated

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