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Coal ash composition of Australian low rank coals

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  • Hyperfine Interactions 71 (1992) 1411-1414


    L . J . BROWN ~, J .D . CASHION ~ and R.C. LEDGER 2 [ . .

    tDepartment of Physics, Monash UmverstO,, Clayton 3168, Vie., Australia 2Research and Development Dept., S.E.C.V., Mulgrave 3170, Vie., Australia


    The iron--containing phases in nine precipitator ashes of widely differing composition have been analyzed by S?l,'e M6ssbauer spectroscopy. The importance of the relative proportion of calcium ferrite to magnesioferrite in determining the ash fusion characteristics was established and a procedure developed which reproduced the observed proportion, starting from the ash chemical analysis data.


    The fouling propensity of low rank coals, which is an important parameter in determining the usage of the coal, is difficult to predict from either coal or ash analyses using current knowledge. Bartlett recognized /1/ that standard ash fusion temperatvres measured on ashes from low rank coals were incompatible with the chemical analyses of the ashes, the phase diagrams involved or the behaviour of the ashes in the boilers, tluffman and lluggins /2/ maintain that most ashes are almost completely molten at initial deformation temperature.

    Identification of ash phases is made difficult by the very small particle size of many constituents, particularly the oxides, which makes them x-ray amorphous. M6ssbauer spectroscopy is particularly valuable in determining the iron-containing phases, which constitute from 10-70 wt% of the ashes studied here, since both fine particle and low intensity phases can usually be identified.

    2. SAMPLES

    Precipitator ash samples from nine different coals were used and their origins are shown in Table 1. The mixture of run of mine and new coals was chosen to provide cross references because there is no full scale experience of combustion of coals of the types of the three exploratory sites. Laboratory ash and precipitator ash samples were taken from all the experimental furnace runs but only the precipitator ash samples were studied here. The ashes studied covered a wide range of compositions as shown in Table 2, with the dominant element varying through calcium, magnesium, iron, aluminium, sodium to silicon.


    The ashes were all subjected to ti~ermomechanical analysis and simultaneous measurement of electrical conductance under various atmospheres for comparison of ash fusion characteristics with the standard measurement of ash fusion temperature. Although little correlation was found/3/ , one point of relevance to these'experiments was the observation that for the three samples measured under both reducing and static atmospheres, the llazelwood and Morwell precipitator ashes both unexpectedly gave larger reductions in length between approximately 7000C and 12000C under static atmospheres, while the Bowmans sample gave the expected result of an earlier contraction under a reducing atmosphere.


    Tile room temperature M6ssbauer spectra of four of the samples are shown in Fig. 1.

    9 J.C. Baltzer A.G., Scientific I:'ublishing Company

  • 1412 L.J. Brown et al. / Coal ash composition o f Australian low rank coals

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    995[ ~ d"

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    -12 -II -4 0 4 II 12 Velocity ( ram/see wrt a-Fe)






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    Fig. 1. Room temperature Fig. 2. 4.2 K

    M6ssbauer spectra of (a) Morwell, (b) Yallourn, (c) Loy Yang and (d) Leigh Creek samples

    The spectrum of tile Morwell ash sample (Fig. la) shows principally two sextets of approximately equal intensity, the outer of which is broadened and has been fitted with two sextets, with some poorly resolved doublet structure in the centre. The Ilazelwood power station and Hazelwood samples are similar to this. Fig. lb shows the spectrum of the Yallourn sample with a new sextet (B = 40.7 T) dominating the two principal sextets from the Morwell spectrum. The spectrum of the Maryvale sample is similar to this except that the mean field of the new sextet is 43.2T. The spectrum of the Loy Yang sample (Fig lc) shows a clean, but asymmetrical, doublet while the Leigh Creek spectrum (Fig. ld) showed one sextet with a hyperfine field of 51.2T, a poorly resolved sextet with a smaller field and an intense central doublet. The Bowmans spectrum was similar to this while the Vales Point spectrum had a better resolved inner sextet and a more complicated central doublet structure.

    Low temperature spectra were essential to identify and quantify the various components and Fig. 2 shows the spectra for the same four samples. The two sextets from Fig. la now have a clear, but weaker, additional sextet between them (Pig. 2a). This new sextet dominates the Yallourn spectrum (Fig. 2b) and also the Hazelwood spectrum, although at a lower intensity. The spectrum of the Loy Yang sample (Fig. 2c) has split into a sextet, with very broadened and asymmetric lines, and a doublet. The spectrum from the Leigh Creek sample is similar although the doublet clearly has a wider splitting. The Vales Point sample was the only one to show evidence of a divalent iron spectrum.

    Sample Coal Type Furnace

    Morwell Morwell 1981 o/cut lignite tlazelwood llazelwood 1586 bore lignite Yallourn Yallourn 1982 o/cut lignite Maryvale Maryvale 3103 bore lignite Loy Yang Loy Yang 1984 o/cut lignite Ilazelwood P/S Morwell 1988 o/cut lignite Leigh Creek Leigh Creek sub-bit. Bowmans Bowmans trial pit lignite Vales Point Vales Point sub-bit.

    1000 hr., S.E.C.,Vic. 1000 hr., S.E.C.,Vic. 1000 hr., S.E.C.,Vic. 1000 hr., S.E.C.,Vic. 1000 hr., S.E.C.,Vic. IIazelwood power stn., Vic Exp. furnace, E.T.S.A. Exp. furnace, E.T.S.A. Vales Point power stn., NSW

    Table 1. Details of the origins of samples used.

  • L.J. Brown et al. / Coal ash composition of ,'lttstraliatt low rank coals 1413

    Morwell If'wood P/S Hazelwood Yal lou r n Maryvale Loy Yang Leigh Creek Bowman Vales Point

    SiO2 AlO~.s FeO,.s CaO MgO NaOo.s K()o.s 'Fi02 SOa 0.19 1.21 11.57 29.95 24.12 1,1.29 0.60 {/.22 16.76 4.38 1.,19 6.50 36.74 29.95 7.35 0.51 0.12 11.,17 5.20 3.~7 10.14 15.97 33.,12 17.78 0.84 0.21 1{I.08 0.85 1.89 24.73 11.78 36.52 13.57 0.30 0.1l 1{}.23 3.18 1.09 41.68 11,56 22.8t 1074 0.58 0.12 5.47 7.52 26.02 9,67 4.67 20.11 19.83 0.25 /I.41 8.20

    32.80 24.66 3.49 16.50 6.56 12.35 1.13 1.42 1.09 10.57 5.01 2.58 5.09 8.23 43.43 1.02 0.10 16.77 59.44 27.,18 3.66 2.46 1.58 1.1 l 2.80 1.03 0.43

    1.,181 2.7~ I 0.01 i

    2.78 3.32 0.01 7.20 0.111

    Table 2. Chemical analysis of the ash samples in atomic weight percent.


    The dominant iron-containing species in all the samples is either the brownmillerite calcium ferrite, Ca2Fe2Os, or magnesioferrite, MgFe204, with some possible substitution by aluminium for iron in both cases. The minor constituents are iron substituted peric]ase, MgO:Fe 3., in the two Hazelwood samples, iron substituted spinel, MgAl2-xFexO4, in the Loy Yang sample, hematite, Fe2Oa, in the Leigh Creek and Bowmans samples and iron in aluminosilicate in all the last three samples including the Fe 2. part of the Vales Point spectrum

    The proportion of superparamagnetie oxides in the samples from the S.E.C.V. 1000 hour furnace was much lower than in previous experiments which we have performed on related ashes ]4-6/. The earlier ashes were produced in the experimental 35 kg/h furnace which has a faster rate of cooling of the flue gas and this does not allow as much time for solid state reaction or aggregation, hence resulting in a smaller particle size.

    Of particular interest in the present samples is the identification of the criteria which determine the ratio of calcium ferrite to magnesioferrite. We have previously observed /7/ that the presence of calcium ferrite is essential for good pelletizing of the ash as a possible means of waste disposal. In the present samples, we noted in Section 3 the unexpected fusion behaviour under reducing conditions of two of the ashes. Both of these are high in calcium ferrite and we believe that this phase enhances the stability under reducing conditions because there is no related structure for it to decompose into following the reduction of Fe 3' to Fe 2 llowever, magnesioferrite can convert into magnesiowustite without drastic rearrangement of the nearest neighbour configurations.

    After consideration of the relative chemical activities and origins of the species involved, the following steps are proposed for distributing the amounts of each element present (as determined by the chemical analysis) in a hierarchy of importance: / i I assume that all the CI-is associated with Na*, hence calculate the remanent Na

    assume that the remanent Na* reacts with SO3 =, hence calculate the remanent SOn =, the remanent SOn = reacts with Ca 2', hence calculate the remanent Ca 2', (denoted Caxs), this Caxs is available to compete with the Mg2* to form double oxides with Fen,, and the effect of aluminium is ignored at this stage. The data were then used to plot [Fe 3"] in calcium ferrite]/{[Fe 3'] in calcium ferrite +

    magnesioferrite]} against [Ca,s|/{[Caxd + 2[Mg]}. The results are shown in Fig. 3. If there were no preference by the iron for either calcium or magnesium, then we would expect the data to lie on the line y = x. llowever, three of the data points lie on the line y = 2.7x showing that, in the absence of other constraints, the formation of calcium ferrite is 2.7 times more probable than magnesioferrite.