A. P. Iribarne

Hydration of combustion ashes — a chemical and physical study

Abstract This study deals with curing of mortars in which coal ashes from both PC and FBC boilers were the sole binder components. The work focuses on the hydration products and the hydration results are correlated with the physical properties of the cured beams from 10 formulations of mortar made of sand and ash blends. Curing was extended to two to three years and the beam specimens were cured in water for swelling measurements and in moist atmosphere for shrinking measurements. Mechanical strength and dimensional stability were tested periodically and porosity measurements were also made. Separation by density of pulverized mortar samples was performed to obtain the light fractions containing the hydrates formed. The cured mortars and these separated fractions were analysed and the hydrates identified included AFt, gypsum, portlandite, a variety of hydrated calcium aluminosilicates and amorphous C–S–H. The composition of C–S–H, as characterized by atomic ratios of various elements referred to Ca indicated considerable replacement of Si by Al and S. The degree of hydration, as indicated by the percentage of light fraction in the mortar or by other properties, and the chemical characteristics of the ash blends were related to the mechanical properties of the cured mortar using two novel indices. One of these relates the silicoaluminous to sulphocalcic components and the other the available free lime to sulphate content of the mortar.

Reuse of landfilled FBC residues

Two aspects of the possible reactivation of fluidized bed combustion (FBC) residues are considered. One is the behaviour of aluminosulfates such as ettringite, which may form when excess water is used in hydration methods designed to reactivate the lime in the FBC residues. The second is the behaviour of compounds such as calcium aluminates, silicates or ferrites (‘other calcium compounds’ or OCC) that are produced by reactions in the fluidized bed. In both cases, the possibility of recovering the CaO contained in these materials for retention of SO2 when they are reintroduced into a combustor is assessed. It was found that ettringite is an excellent sulfur sorbent under FBC conditions, with reactivity superior to normal limestones. The CaO moiety in the OCC is not easily reactivated by hydration. Unhydrated, they do react in a sulfating atmosphere like that of the combustor, but their performance as sorbents is much poorer than limestone and varies with the nature of the compound.

The phase analysis of coal combustion ashes

The phase composition of coal combustion ashes is important in regard to their potential uses. Here it is shown that a combination of phase separation techniques with X-ray diffraction, thermogravimetric analysis, and conventional chemical analysis, including elemental analysis and wet chemistry, can be used to obtain a fairly complete phase composition. The application of these techniques is illustrated with an FBC ash sample, indicating the procedures used to achieve the identification of calcium silicates, aluminates and ferrites, and their quantitative estimates. A new method for the quantitative determination of CaO in presence of large amounts of Ca(OH)[sub 2] has been developed which uses phase separation as a previous step to chemical titration.

A New Mechanism for FBC Agglomeration and Fouling in 100 Percent Firing of Petroleum Coke

In an effort to clarify the causes of agglomeration and fouling in fluidized bed combustion of petroleum coke, a detailed study was made of samples taken from different locations of an industrial-sized CFBC boiler, including deposits formed after 7 and 96 days of operation. It was found that vanadium, the suspected cause of the agglomeration, does not accumulate in fouled regions and that no low melting oxides were present. Neither could any low melting eutectics be expected from the vanadium compounds identified. Therefore, the high concentrations of vanadium in the petroleum coke fuel cannot explain the formation of agglomerates. Fouling is attributed to molecular and the absence of fuel-derived ash providing inert material, which could contribute discontinuities between the sintered anhydrite grains and prevent massive consolidation of deposits.

A scanning electron microscope study on agglomeration in petroleum coke-fired FBC boilers

Ten samples originating from different boiler FBC systems burning petroleum coke and one laboratory sample were chosen to perform a study on the development, structure, and composition of deposits formed by agglomeration in various locations. The work focused on examination by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis. The possibility of a contribution of liquid phases in the adherence to solid surfaces and in agglomeration was discussed and checks by SEM, EDX, and analysis by neutron activation were performed; no evidence could be found either for liquid phases or for any role of vanadium or alkaline element compounds. The agglomerations result from the continued sintering of CaSO4 particles until they build up a strong framework that is indefinitely extended, into which particles of different and complex compositions are bound, without contributing to the cohesion. Chemical sintering occurring by the sulphation of CaO into CaSO4 appears to be an important contribution while CaO is still available, but sintering also occurs by mass transfer mechanisms and continues after the depletion of CaO. Deposits formed in regions only reached by fly ash (convection section), and also in in-bed deposits, grow from particles <50 μm, mostly in the range of 10 μm or less. In regions collecting bed ash (e.g., J-valves), the deposit grows from the sintering together of particles on the order of 100–300 μm (originally bed ash particles), which themselves appear as conglomerates of extensively sintered smaller particles.

A Scanning Electron Microscope Study on Agglomeration in Petroleum Coke-Fired FBC Boiler

Ten samples, from different FBC boiler systems burning petroleum coke, were chosen to study the development, structure and composition of deposits formed by agglomeration at various locations in the boilers. The work focused on examination of the samples by scanning electron microscopy (SEM) and dispersive X-ray analysis (EDX). Chemical analysis and other techniques were also employed. The results obtained have not brought to light any evidence of the participation of the liquid phase or of vanadium or alkaline metal compounds. The CaSO4 of the deposits is high (80 to 100%) and the agglomeration results from the prolonged sintering of CaSO4 particles, until a strong 3-dimensional framework is formed, in which other, unrelated particles may be trapped, without contributing to cohesion. While CaO is still available, “chemical sintering” associated with its conversion to CaSO4 appears to be important, but sintering also occurs by a slower mass transfer mechanism and continues after the depletion of CaO. Deposits formed in regions only reached by fly ash (convection section), and also in-bed deposits, grow from particles <50 μm, mostly very small ones, < 10 μm. Where the bed ash can collect (e.g., J-valves), the deposits grow by the sintering together of larger particles, 100–300 μm, which themselves appear to be conglomerates of smaller particles sintered together.Copyright