Energy & Fuels 2004, 18, 1883-1887
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Influence of Mineral Matters on the Calorific Value of an Anthracite under Oxygen Bomb Conditions Hong Zhang,* Ming Zhou, Chao Wang, Ming Sun, Mei Li, and Xianyong Wei School of Chemical Engineering, China University of Mining and Technology, Xuzhou 221008, Jiangsu, China Received April 22, 2004. Revised Manuscript Received July 27, 2004
Accurate calibration of mineral composition on calorific value (CV) is essential to gangue utilization, cement production, etc. In this paper the influence of typical minerals on the CV of an anthracite is quantitatively studied. The results indicate that the decomposing rate of calcite decreases with the increase of calcite content during the CV test process. The deviation of CV resulting from calcite decomposition can be calculated according to its decomposing rate, which does not change with the origin of calcite. Kaolinite is transformed mainly to an amorphous solid and some to mullite. Its deviation of CV can be deduced by assuming that kaolinite is totally transformed to an amorphous solid. The complete combustion of pyrite contributes to the increase of CV. The influence of quartz and gypsum can be neglected, as the former does not change during the test and the latter has little heat effect. The composite influence of different minerals on CV can be calibrated according to that of separate minerals when total mass fraction of minerals is high. If the mass fraction is relatively low, overall heat effect produced by each mineral is a little lower than the CV actually determined.
Introduction Calorific value (CV) of coal is one of the most important indexes in assessing the quality of coal, as coal is the most important and primary energy source for industrial process as well as for power generation in China. With persistent mining of coal resources, its grade is deteriorating sharply. Besides, in cement production with shaft-kiln technology, black raw meal is an artificial mixture of coal and minerals. In both cases, the influence of minerals on CV cannot be neglected.1,2 Some scholars have done extensive research about the statistical relation between CV of coal and ash content, and they attribute the influence of various kinds of minerals on CV of coal to ash content, completely ignoring the composition of minerals.3,4 For example, the empirical formula for the high ash-contained anthracite is
Qgr,ad ) 334.5FCad + 209.1Vad - 12.5Aad More than 40% of samples calculated with this formula have an error greater than 210-840 J/g.3 Actually, mineral composition has great influence on CV of coal, as they not only take up the content of coal but also * Corresponding author. E-mail:
[email protected]. Telephone: (86)-516-399-5018. Fax: (86)-516-399-1167. (1) Shirazi, A. R.; Bo¨rtin, O.; Eklund, L.; Lindqvist, O. Fuel 1995, 74, 247-251. (2) Bordoloi, D.; Baruah, A. Ch.; Barkakati, P.; Borthakur, P. Ch. Cem. Concr. Res. 1998, 28, 329-333. (3) Chen, W.-M. Calorific Value of Coal and Formulas for Its Calculation; Coal Industrial Press: Beijing, 1993. (4) Mason, D.; Gandhi, K. N. Fuel Process. Technol. 1983, 7, 1122.
undergo mineral conversion accompanying the heat effect during the CV test. For example, calcite decomposes at 0.1 MPa, 893 °C, absorbing up to 1645 J/g, while pyrite oxides produce 7491 J/g.5,6 Obviously, if the impact of mineral composition on CV of coal is taken into account, the accuracy of the formula will be improved greatly. The effect of typical inorganic minerals on the caloric value of anthracite coal under different conditions is studied in this article. Experimental Section Preparation of Ultraclean Coal. As the aim of this experiment is to study the influence of minerals on the CV of the organic part of coal, an ultraclean coal (UCC) is prepared. An anthracite from Taixi, Ningxia province in China, with low content of ash and sulfur, is chosen as the raw material, which is treated with the HCl-HF-HCl procedure, similar to that proposed by Steel.7 In this process, the coal sample is first powdered through 0.2 mm and then put into plastic beakers to react with (1 + 1) HCl at the boiling temperature of water for about 1 h. After it is leached and water-washed, it is treated with (1 + 1) HF using a similar procedure. In the end, UCC is obtained after it is dried. Table 1 shows the proximate and ultimate analysis results of the UCC obtained. Preparation of Mineral Matters. The mineral matters most frequently present in coal are carbonate minerals, clay minerals, sulfide minerals, silicate minerals, etc.8 Two crys(5) Knacke, O.; Kubaschewski, O; Hesselmann, K. Thermochemical Properties of Inorganic Substances; Springer-Verlag: Berlin, 1991. (6) Perry, J. H. Chemical Engineering Handbook, 4th ed.; McGrawHill Book Co.: New York, 1967. (7) Steel, K. M.; Patrick, J. W. Fuel 2003, 82, 1917-1920. (8) Smith, K. L.; Smoot, L. D.; Fletcher, T. H.; Pugmire, T. H. The Structure and Reaction Processes of Coal; Plenum Press: New York and London, 1994.
10.1021/ef049898u CCC: $27.50 © 2004 American Chemical Society Published on Web 10/08/2004
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Table 1. Proximate and Ultimate Analysis of Raw Coal (%) Ad (%)
Mad (%)
Vdaf (%)
Cdaf (%)
Hdaf (%)
St,d (%)
4.99
1.14
8.48
91.62
3.55
0.08
tallinely different calcites, with one analytic reagent and the other industrial limestone from Tongshan county, Jiangsu province, are chosen to compare the influence of their crystalline state on CV. A kaolinite from Jiahe coal mine, Xuzhou city, Jiangsu province, is chosen, containing little impurity determined with XRD analysis. The industrial pyrite chosen has a sulfur content of 52.53%, amounting to 98.5% in grade. The natural gypsum chosen is from Zaozhuang, Shandong Province, with SO3 content up to 45.7%, amounting to 99.5% in grade. The quartz chosen is an analytic reagent. Determination of Calorific Value. Powders of UCC and various kinds of mineral are weighed and carefully mixed and sent to determine their calorific value (CV) using a CT2100 calorimeter. Determination of the Decomposing Rate of Calcite during CV Test. As calcite may not completely decompose to lime, the determination of its decomposing rate is of great importance in calculating its heat effect. The residue of the CV test is powdered through 80 µm mesh. Its decomposing rate is calculated from CaO content in the residue, which is determined with the ethanol glycerin method.9 Identification of Mineral Conversion. The residues after determining CV are powdered through 320 mesh and analyzed using a Regaku D/MAX-3B XRD apparatus with Cu KR radiation, 35 kV and 30 mA.
Figure 1. Relationship between calcite decomposing rate and its content.
Figure 2. Relationship between mass fraction of CaCO3 and Qb.
Results and Discussion Effect of the Origin of Calcite on CV of Coal. There exist many kinds of calcites formed at differing ages and under differing conditions, resulting in different crystalline state and size, which may lead to different decomposing rates under the same conditions, thus affecting CV. An artificial analytic reagent CaCO3 and an industrial calcite are chosen because of their drastic crystalline difference. In a series of tests it is found that the CV differences between the two sources with sample weights 0.8, 1.0, and 1.2 g containing equal CaCO3 content are 146, 130, and 125 J/g respectively, which are within the instrumental tolerance limit of 150 J/g. It is thus concluded that the origin of calcite has no obvious effect on CV of coal in CV test. The reason possibly is that samples are powdered below 80 µm, reducing the differences of heat transfer from chemical reaction between different origins. Relation between Calcite Decomposing Rate and Its Content. Under oxygen bomb conditions, there are many factors affecting CaCO3 decomposing rate, among which the CaCO3 content in coal is most important. At low content, it breaks down completely. When the content goes up, it may partially break down. Obviously CaCO3 content must have an effect on the CV of coal. The CVs of 5%-85% CaCO3-contained samples are determined. The results are shown in Figure 1. The curve can be divided into four sections. When the calcite content is 5.31%, the decomposing rate is 100%. When the calcite content is 10.18%, the decomposing rate drops to 86.13%. It can be deduced that the highest (9) Cement Chemical Analysis; Building Materials Industry Press: Beijing, 1982.
Figure 3. XRD diagram of the ash of the sample containing 60% kaolinite and 40% UCC.
calcite content when calcite is completely decomposed is between 5.31% and 10.18%. When calcite content increases from 15% to 69.93%, its decomposing rate fluctuates between 79.75% and 82.44%, which is quite stable. When CaCO3 content reaches 80%-85%, its decomposing rate drops to 40%. Obviously, when CaCO3 content is low, CV of the sample is high and it can reach high combustion temperature. The calcite can decompose completely before the reaction reaches equilibrium. As CaCO3 content goes up, the reaction meets equilibrium and the decomposing rate is relatively stable. With CaCO3 content going up further, the marginal part of the sample cannot reach a certain temperature and the whole decomposing rate falls sharply. According to Liliedhal et al.,10 dolomite or calcite will be stable at a (10) Lieliedhal, T.; Sjostrom K.; Wiktorsson, L. P. Fuel 1992, 71, 797-803.
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Figure 4. Relationship between mass fraction of kaolinite and Qb.
given temperature, if the partial pressure of CO2 is high enough, and this explains the incomplete decomposition of calcite when the concentration is higher than 10% in another aspect. After the residue of a 60% CaCO3-contained sample is analyzed with XRD, it is found that the residue is made up of mainly decomposed CaO and some undecomposed calcite. There is no sign of other materials, which indicates that the CaO does not hydrate to Ca(OH)2 during the test. Quantitative Relation between CaCO3 Content and CV of Coal. If Qb is defined as the actual CV determined of certain CaCO3-contained sample in an oxygen bomb, Qpb as the theoretical CV calculated according to the proportion of UCC, Qpb ) Qb0 (the CV of UCC) × its content, and Qcb as the calibrated CV, Qcb ) Qb0 × its content - qCaCO3 (decomposition heat) × content of CaCO3 × decomposing rate of CaCO3, their relationship is shown in Figure 2. It should be noted that under oxygen bomb conditions there is no need to take into account the energy requirement for heating calcite from 20 °C to 900 °C, which is 792 J/g,11 besides that for calcite decomposition, as the reaction system returns to 20 °C at the end of tests. If CaCO3 in samples does not break down, proportionally calculated Qpb should be equal to actually determined Qb. The results indicate that when CaCO3 content is within 10%, the difference between Qpb and
Qb is within the tolerance of the instrument. However, when the content reaches 15%, their difference is 238 J/g, which goes beyond the tolerance. Further more, when CaCO3 content is 40%, Qb is 581 J/g lower than Qpb. It is too large. This partially explains why the formula for CV of coal gangue is inappropriate. If CaCO3 decomposing heat effect is taken into account, the result is satisfactory, as the difference between Qcb and Qb is within 150 J/g. Change of Kaolinite in CV-Determined Process. Kaolinite is one of the clay minerals that commonly exist in coal too. As clay minerals have similar physical and chemical characteristics, only kaolinite is chosen in this test. Theoretically when it is heated to 450-575 °C, [OH-] is taken off in the form of water and amorphous matter is formed, while the structure destroyed. When it is further heated to 950-1050 °C, mullite and glassy material or quartz are formed.12 Actual mineral compositions of the residue of a sample containing 60% kaolinite and 40% UCC after CV test are determined with XRD. Results are shown in Figure 3. From Figure 3, it can be found that highly scattered peaks appear when the XRD scanning angle is between 15 and 30°, indicating there is large content of amorphous solid. The strongest diffraction peaks are mullite (0.3396, 0.5400, 0.2206 nm), and no kaolinite peaks (0.7178, 0.3575 nm) appear. This result indicates that kaolinite decomposes fully during the CV test, most of which turns into amorphous solid and some to mullite. Effect of Kaolinite on CV. CV of mixtures containing 10-90% kaolinite and 90-10% UCC are determined. If Qb is defined as the actual CV of a certain kaolinite-contained sample in an oxygen bomb, Qpb as the theoretical CV calculated according to the proportion of the ultraclean coal, and Qcb as calibrated CV, Qcb ) Qb0 × its content - Qkaolinite(decomposition heat) × content of kaolinite, their relationship is shown in Figure 4. From Figure 4, it can be concluded that the difference between proportional Qpb and actual Qb exceeds the instrumental tolerance 150 J/g when kaolinite content is 10%. With the increase of kaolinite content, their difference becomes even larger. After the heat effect resulting from kaolinite transformation is taken into account, the difference between calibrated Qcb and actual Qb is within the limit of 150 J/g before kaolinite
(11) Hocking, M. B. Modern Chemical Technology and Emission Control; Springer-Verlag: Berlin, 1985.
(12) Crimshaw, R. W. The Chemistry and Physics of Clays and Allied Ceramic Materials; Ernest Benn, Ltd.: London, 1971.
Figure 5. Influence of mass fraction of FeS2 on Qgr,ad. Table 2. Influence of the Mass Fraction of Gypsum on Qb of Coal w(gypsum) (%) w(UCC) (%) Qpb - Qb Qcb - Qb
5.08 94.92 84 79
10.1 89.9 -117 -125
15.03 84.97 -100 -117
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Figure 6. XRD diagram of the ash of coal samples containing mixed minerals. Table 3. Influence of Mass Fraction of Minerals on Qgr,ad of Coal Q (J g-1)
w (%) no.
UCC
calcite
kaolinite
quartz
pyrite
gypsum
Qpb - Qb
Qcb - Qb
1 2 3
40.02 41.62 19.97
19.95 39.78 59.98
19.95 10.26 12.00
9.96 3.05 2.32
4.75 2.96 3.61
5.37 2.33 2.12
-96 +268 +631
-142 -163 -46
content exceeds 80%. However, with the growth of kaolinite content, the gap between Qcb and Qb changes from positive to negative. When kaolinite content reaches 90%, the difference between the two reaches as high as 493 J/g, which far exceeds the allowed error. As it is difficult to determine the decomposing rate of kaolinite and conversion rate of mullite, we assume kaolinite completely decomposed to amorphous solid, with no further change to mullite, which is not compatible with the experimental facts. On the other hand, the heat effect of mullite formation is so low that it usually has no obvious effect on CV. Effect of FeS2 on CV. Sulfur in coal usually exists in sulfides, mostly in pyrite, resulting from different geologic origins. FeS2 burns and turns into Fe2O3 and SO2 in excessive oxygen, giving out 7491 J/g of heat.5 As the content of sulfur is relatively low in the coal, 5%10% w(FeS2) is studied in the test, the results of which are shown in Figure 5. When w(FeS2) is 6.91%, its proportional Qpb is 623 J/g lower than its actual Qb. When FeS2 exothermic combustion of 518 J/g is taken into account, the gap between calibrated Qcb and Qb is narrowed to 105 J/g, which is quite satisfactory. Effect of Quartz on CV. Quartz exists almost in every coal. Although there are several crystal conversions in the course of heating, it restores to its original crystal structure when it is cooled to ambient temperature. Our experimental results confirm the theoretic prediction that there is no heat effect resulting from the conversion of quartz in samples. Effect of Gypsum on CV. Gypsum, or CaSO4‚2H2O, is one of the sulfates that usually appear in coal, although in low content. When it is heated to 70-90 °C, it partially dehydrates to CaSO4‚0.5H2O. When the temperature is raised to 100 °C, it dehydrates completely to anhydrate, or CaSO4. Its overall heat effect is 103 J/g. Samples containing 5%-15% gypsum in UCC are tested, and the results are shown in Table 2.
The results indicate that there is no obvious influence of gypsum on CV, apparently because of its low heat effect and low content. Composite Effect of Mixed Minerals on CV. Separate effects of five typical minerals on CV are illustrated above. Practically these minerals coexist in the coal, and they might react with each other, apart from their separate effects. Phase Composition of the Residue of Samples Containing Mixed Minerals. A sample is prepared with mineral composition shown in no. 3 of Table 3. After the CV of this sample is determined, its residue is analyzed with XRD, which is shown in Figure 6. Results show weak CaO peaks (0.3043, 0.2787, 0.2408 nm) and strong calcite peaks (0.3947, 0.3043, 0.2849 nm), indicating calcite partially decomposed to CaO. Also the highly scattered peaks show there is quite a bit of amorphous solid. Pay attention to that there are weak kaolinite peaks (0.7296, 0.3947, 0.3662 nm) and no mullite peaks, indicating kaolinite’s whole decomposition and no further transformation. It is worthy to note that there is no FeS2 peaks (0.1629, 0.1040, 0.2696 nm), indicating that FeS2 decomposes completely. As gypsum peaks (0.429, 0.306, 0.287 nm) disappear on the diffraction chart, it can be deduced that gypsum decomposes completely, although the peaks (0.349, 0.285, 0.164 nm) of anhydrate partially overlap those of calcite. In addition, the product of interactions among these minerals is not found strong, the reason for which is apparently the low combustion temperature and short lasting time under CV test conditions. Effects of Mixed Minerals on CV. The effects of mineral composition and total mineral content on CV are studied, the results of which are shown in Table 3. Samples 1 and 2 have similar UCC content and different mineral compositions. The gap between their actual Qb is 493 J/g, indicating the severe influence of
Influence of Mineral Matter on Calorific Value
mineral composition on CV. If the heat effects of mineral conversions are not taken in account, the gap between proportional Qpb and actual Qb can be as high as 631 J/g in this test. On the contrary, proper calibration of the influence of the conversions of minerals on Qb yields satisfactory results. Let us take sample 3 as an example. By taking into account the endothermic heat of the decomposition of calcite, which is 857 J/g, and that of kaolinite, which is 92 J/g, and the exothermal heat of pyrite, which is 272 J/g, neglecting the heat effect of quartz and gypsum, one finds the calibrated Qcb is 6262 J/g, which is only 46 J/g lower than that of the actual Qb. It is worthy of attention that Qcb - Qb in the tests are always negative, while statistically they should be randomly positive or negative. They are apparently resulted from the neglection of the exotherm of mullite formation or/and other mutual reactions. The higher the UCC content, the higher the burning temperature. At higher temperature, there would be more interactions, which will produce more heat. It seems that theory coincides well with all the results shown in Table 3. Conclusions The separate effects of five minerals on CV are studied. The content of calcite is the main factor
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affecting CV, while its source or sample weight has negligible effect. With growing calcite content in coal, its decomposing rates decrease. By taking into account its decomposing rate, we find the influence of calcite on CV can be well calibrated. Most kaolinite turns into amorphous solid and some to mullite during CV test. The influence of kaolinite can be well calibrated by assuming it is fully decomposed when its content is lower than 80%. Pyrite usually exists in low content in coals. Its influence can be well calibrated by assuming it burns completely. Quartz has no effect on calorific value, while the influence of gypsum can be ignored because of its minor content in coal and low heat effect during conversion. The composition of minerals has a remarkable influence on the CV of coal. The composite influence of minerals can be calibrated according to their separate effects, totally neglecting their mutual reactions. Although the calibrated Qcb is a little lower than the Qb actually determined when the total minerals content is low, it is a simple and effective method. As a matter of fact, a calorimeter specially designed according to this mechanism has been successfully and widely used in cement plants in China. EF049898U
