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Technical Market Support

Effect of Iron Dosing on Boiler Performance

Technical Market Support » Thermal Coal

Published: October 04Project Number: C13070

Get ReportAuthor: Allen Lowe | A&SJ Lowe Consulting Services Pty Ltd

The combustion chamber of a pulverised coal fired boiler is designed to accept a range of heat transfer rates. However, should the heat transfer with a particular coal deviate from a range set by the control mechanisms on the boiler, then operational problems can be experienced with consequent increases in O&M costs and reductions in boiler efficiency.

Certain coal types appear to have a propensity to produce low furnace heat absorption. These can include bituminous coals with non slagging refractory ashes high in silica and low in iron content. High calcium sub-bituminous coals have also been associated with reduced furnace heat transfer.

Heat transfer in the combustion chamber (or furnace) is dominated by radiation heat transfer. It therefore depends, in the first instance, on the emissivity of the furnace walls. These, in normal operation, and in the absence of any slag formation, will be covered by a thin layer of powdered ash that will effectively determine the wall emissivity. It has therefore been suggested that the reduced furnace heat transfer may be associated with low furnace wall emissivity.

Emissivity of coal ash samples has been shown to increase with increasing iron in ash. This raises the possibility of using an iron based additive to increase ash emissivity with coals where low furnace heat absorption is experienced. This project sought to test the proposition that dosing of coals with iron sulphate would increase ash emissivity and thereby furnace heat absorption. Further, to examine other factors that may differentiate furnace heat transfer between different coals, such as the physical properties of the dust layer forming on the walls.

Two Australian coals with low (2 to 2.5 %) iron in ash were procured. One of these had previously been associated with instances of reduced furnace heat transfer while the other had not. The chemistry of the ash in both cases was similar with both coals having high ash fusion temperatures. The coals were test fired in the ACIRL Combustion Test Facility (CTF) and the properties of the dust layer forming on simulated furnace wall panels (termed slag panels) were examined. One coal was also dosed with iron sulphate to increase the iron oxide content in ash to approximately 5 % and the testing repeated. A specially designed probe was used to collect samples of the dust deposit such that ex situ measurement of emissivity could be performed on the undisturbed dust layer.

The testing showed a small but consistent difference between the heat transfer performance of the two coals. The coal previously associated with reduced heat transfer also produced the lowest heat transfer in the pilot scale facility. Dosing with iron was successful in increasing heat transfer performance however the effect was relatively small and insufficient to eliminate the difference in heat transfer performance between the two coals. Subsequent measurement of emissivity showed that the coal with the lowest heat transfer also had the lowest ash emissivity, dosing with iron increased the ash emissivity but not to the level of the alternative coal.

The dust deposit formed on the slag panels in the CTF was of similar thickness (1 to 2 mm) with both coals, both coals also produced deposits with porosity of the order of 90 %. A significant difference between the coals was observed in the size distribution of the dust collecting on the slag panels, the coal producing the lowest heat absorption also producing the finest ash particles. Both emissivity and thermal conductivity of dust deposits are expected to reduce as particle size of the dust in the deposit reduces.

Dosing with iron did not produce any significant changes in the thickness, particle size or porosity of the dust layer forming on the slag panels. There was no evidence that the iron dosing promoted slag formation in this test.

Computer modelling was carried out to quantify the impact on boiler performance of the differences in emissivity measured for these coals. A notional 660 MWe tangentially fired boiler burning NSW Central Coast coal was assumed. Ash deposit thickness and emissivity were varied to span the range of values measured on the CTF.

The model applied a 3x3x15 zoning arrangement to the furnace chamber for the calculation of heat transfer. The model appears to be unique for pulverised coal in that radiative transfer is calculated at a number of discrete wavelengths followed by a spectral integration to obtain total heat transfer. The detailed spectral radiative properties of the combustion gases, soot, char, fly ash and ash deposits were all modelled. This properly represents the radiative properties of the radiating species and avoids the need to make the common, but invalid, assumption that all radiating species are grey (emissivity independent of wavelength).

The model predicts that both of the coals tested here would perform satisfactorily in the boiler, depending on the thickness of the ash layer on the furnace walls. The coal with the lower emissivity produces a furnace heat transfer close to the minimum acceptable with a clean furnace while a dust layer of about 1 mm in thickness is sufficient for the heat absorption to fall below the minimum acceptable value. The coal with higher heat transfer produces heat absorption above the design limit for a clean furnace wall but falls back within the control range with a 0.5 millimetre thick deposit on the wall. Here a clean wall is taken to be one where the dust layer is enough to determine the emissivity but insufficient to cause a measurable thermal barrier to heat conduction to the boiler wall.

Dosing with iron sulphate is shown to produce a small increase in furnace heat absorption but insufficient to allow a significantly thicker dust deposit to form before the minimum heat absorption level is reached, as compared to that for the undosed coal.

The model assumes an effective thermal conductivity for the dust deposit of 0.2 W/m °K in line with published data. Thermal conductivity was not measured for these samples. The model predicts absorbed heat fluxes in line with that measured both in the CTF and in full scale plant but also requires an unrealistically large temperature gradient across the dust layer at those heat fluxes. This indicates that thermal conductivity as measured in laboratory apparatus does not well represent the heat transfer processes taking place in a pf furnace.

Subject to substantial revision of current understanding of ash thermal conductivity under pf furnace condition, the modelling indicates that a layer of dust a mere two millimetres in thickness will exert a controlling effect on heat transfer in the furnace. Observation of operating boilers suggests that dust thicknesses of two millimetres or more are commonly present, it is therefore postulated that furnace heat transfer with these coals is dominated by the thermal resistance of the dust layer rather than the emissivity of the dust.

Under normal circumstances a layer of dust will establish on the furnace walls. This, in the absence of any slagging or sintering, will stabilise at a thickness controlled by the balance between dust adhesive and cohesive forces and vibration and gravitational forces tending to cause the dust layer to fall from the wall. Further study to understand these processes may assist in designing better strategies to manage the reduced furnace heat transfer. For example, as the forces bonding the dust layer to the wall are typically weak, non contact mechanisms such acoustic excitation or, alternatively, some form of mild mechanical vibration of the boiler superstructure, may be sufficient to cause the dust layer to stabilise at a thickness that is compatible with furnace heat transfer requirements.

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