J. V. Iribarne

Reactivity of calcium sulfate from FBC systems

Abstract A relative estimate of the reactivity of calcium sulfate in a number of coal combustion ash samples was obtained, using the rate of solution in water as a parameter. Measurements were also performed on standard samples of calcium sulfate prepared in different ways, for comparison. The temperature of previous treatment appeared as the most important factor determining the reactivity of CaSO 4 ; the grain size distribution was less important, and the duration of heating (even to 105 days) had very little influence. No correlation between specific surface of ash samples and their reactivity was apparent. Calcium sulfate in FBC ash samples was much more reactive than that contained in high-temperature ashes, and than calcium sulfate heated, alone or with various additions, at 850°C for 2 days. Of the six FBC samples tested, five showed similar behaviour, including a sample from a pressurized system; only a deposit from 96 days operation of an industrial CFBC boiler burning petroleum coke showed considerably less reactivity. Surprisingly, CaSO 4 from two FBC samples placed in an oven for 60 days under sulfating conditions showed a very similar rate of solution to that of the other FBC samples, while a third sample kept in the oven for 105 days also showed no decrease in reactivity. Only when one of these samples had agglomerated (which occurred between 60 and 105 days) did it show decreased reactivity, suggesting that the agglomeration process rather than duration is significant in promoting sintering and reducing the sulfate reactivity.

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.

Agglomeration and Fouling in Petroleum Coke-Fired FBC Boilers

Experiments have been done subjecting ashes from industrial-scale FBC boilers to sulphating conditions in an oven for up to 105 days. These show that sulphation by itself causes agglomeration in the virtual absence of V, K, and Na, the elements normally associated with ash softening and classical fouling. In addition, it has been demonstrated that sulphation goes to completion over long periods of time and, at a specific level which differs from one ash to another, results in agglomeration. These experiments have also shown that there is a size range (75-300 μm) in which the agglomeration is worst, and particles that are smaller or larger either do not agglomerate or agglomerate more weakly. Added inert coal-derived ash decreases or prevent s the agglomeration. However, this ash does not appear to chemically combine with the sulphate, but acts by mechanically separating the sulphating particles. Finally, if alkali metals are present they can cause agglomeration at levels lower than those at which either the alkalis or sulphation separately cause agglomeration, i.e., they operate synergistically to cause fouling. Current work is being directed at examining these phenomena at higher temperatures (900°C and above).

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