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microlithotype partitioning with particle size of pulverized coal: Examples from power plants burning Central Appalachian and Illinois basin coals
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International Journal of Coal Geology 73 (2008) 213 – 218 www.elsevier.com/locate/ijcoalgeo
Maceral/ microlithotype partitioning with particle size of pulverized coal: Examples from power plants burning Central Appalachian and Illinois basin coals James C. Hower ⁎ University of Kentucky Center for Applied Energy Research, 2540 Research Park Drive, Lexington, KY 40511, United States Received 27 March 2007; received in revised form 5 June 2007; accepted 18 June 2007 Available online 12 July 2007
Abstract Pulverized coals from eleven power plants burning Central Appalachian coal blends and eight power plants burning Illinois Basin coal blends were studied in order to assess the petrographic nature of industrial-scale coal grinding. All coals were high volatile bituminous. Coals were wet screened at 100 (150 μ), 200 (75 μ), 325 (about 40 μ), and 500 (about 25 μ) mesh. Petrographic analysis of the whole coals and size fractions consisted of a combined maceral and microlithotype analysis. Microlithotype analysis, in particular, provides a reasonable approximation of the whole-particle composition at the scale of utility coal pulverization. In the size fractions, duroclarite, the most abundant trimaceral microlithotype, is most abundant in the coarsest fraction and least abundant in the finest fraction. Vitrite, the most abundant monomaceral microlithotype, exhibits the opposite trend. Duroclarite becomes more enriched in vitrinite towards the finer sizes. The partitioning of microlithotypes and the partitioning of macerals within the microlithotypes is indicative of the relative brittle nature of vitrite compared to the hard-to-grind trimaceral microlithotypes. Increased vitrinite in duroclarite is an indication that the microlithotype within the particular size fraction is more brittle than relatively vitrinite-depleted duroclarite in coarser fractions. The relative grindability of microlithotypes will, in turn, impact combustion efficiency. © 2007 Elsevier B.V. All rights reserved. Keywords: Coal; Combustion; Pulverization; Petrology; Maceral
1. Introduction Pulverization of coal in coal-fired power plants is at the interface of coal science and engineering. Aside from the actual selection of the coals to be burned, pulverization is the first step in determining the efficiency of combustion, of the capture of trace elements by coal-combustion products, and of the release of SO2 and trace elements ⁎ Tel.: +1 859 257 0261; fax: +1 859 257 0360. E-mail address: [email protected]. 0166-5162/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2007.06.005
with the stack gas. The goal of utility-scale coal pulverization is to produce a coal feed with a particle size N70% passing 200 mesh (75 μ). Scott (1995) and Kitto and Stutz (2005) describe the different kinds of coal pulverizers and the principles of their operations. In all of the units sampled for this study, coal is pulverized in a ball and race mill with the pulverized coal entrained in the air and swept out of the top of the pulverizer and into the boiler and the excessively hard material. The pulverizer rejects or “pyrites”, are removed from the bottom of the
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pulverizer. The rejects, typically less than 1% of the total feed, can remove over 10% of elements such as Hg and As from the feed coal (Mardon and Hower, 2004). The relationship between coal hardness, grindability, petrology, and efficiency of combustion was reviewed by Hower (1998) and will not be repeated in detail here. In general, the finer the coal, the more efficient the combustion (Conroy et al., 1989; Maier et al., 1994; Abbas et al., 1994; Wu, 2005). Hardgrove grindability index (HGI), although an inherently flawed test because of the petrographic biases in the sample preparation (discussion in Hower, 1998), can be used to predict pulverization behavior. Fundamentally, pulverizers are typically rated on the basis of a 50 HGI coal. The capacity chart for a Combustion Engineering Bowl Mill relating HGI, percent grinding mill capacity (from 40 to 150%), and the desired fineness (in percent passing 200 mesh) is a frequently used nomogram (example published by Savage, 1974). For example, a 53 HGI coal requires 2.5 times the numbers of revolutions in a ball mill than a 110 HGI coal to achieve 80% passing 200 mesh (Fitton et al., 1957). As an approximation of mill behavior, Lowe (1987) estimated a 1% loss of mill throughput per 1 HGI unit decrease. Rogers et al. (1987) found that the behavior of dry (air-swept) closed-circuit ball mills could be predicted from the HGI, the relationship being nearly linear in the 40 to 85 HGI range. In order to achieve a given fineness as defined by the percent passing 200 mesh, Scott (1995) isolated grinding pressure (GP) in a vertical spindle mill as the controlling factor influencing fineness and power consumption. Grinding power was related to HGI by: GP ¼ ð0:091HGI 0:256Þ1 ;
ð1Þ
with power consumption, in kWh/t, as: P ¼ 1:09GP þ 5:15;
ð2Þ
with r2 of 0.88 and 0.85, respectively. This translates into a decrease in power consumption from 5.59 kWh/t to 5.26 kWh/t over an HGI range from 30 to 110, the range found in banded bituminous coals. Fineness increases with an increase in grinding pressure but is also a function of the rank of the coal, lower rank (subbituminous in study cited) coals producing a coarser size consist than relatively higher rank coals (Conroy and Trenaman, 1990). The “optimum grinding pressure,” the optimization of fineness with mill power consumption (Conroy et al., 1989), for specific coals is a function of a variety of rank and compositional factors, not all properly expressed in a single variable such as HGI (Conroy and Trenaman, 1990).
Bengtsson (1986) noted several general parameters that influenced the number of combustion phases, two of which, vitrinite reflectance and petrographic composition, are directly related to the coal petrology, and a third, particle size, investigated in this study, that is a function of both of the latter parameters. Man et al. (1998), in a flash-pyrolysis study of a limited number of 38- to 300-μm size fractions of a vitrain, suggested that maceral segregation among particle sizes was more important than particle size itself in determining devolatization yields. The amount of carbon burnout increases (Yu et al., 2005; Barranco et al., 2006) and the efficiency of NOx reduction decreases (Maier et al., 1994) with poorer pulverization. Hower et al. (1996, 1997a) found that fly ash carbon was reduced following conversion to low-NOx combustion, contrary to other low-NOx conversions followed (Hower et al., 1996, 1997b, 1999a), due to a rigorous overhaul of the pulverizers during the outage. In this study, a series of coals from Kentucky and Tennessee power plants is investigated with the objective of determining the distribution of macerals and microlithotypes in the size fractions of the pulverized coal. 2. Procedure Samples of pulverized feed coal were obtained from several power plants burning Central Appalachian (eastern Kentucky and central West Virginia) or Illinois Basin (generally western Kentucky) high volatile bituminous coal blends. The samples came from previous studies of power plants (Hower et al., 1997a, 1997b, 1999a, 1999b; Trimble and Hower, 2000; Sakulpitakphon et al., 2003; Hower et al., 2006, and unpublished petrology results from the study by Hower and Thomas, 2006). The pulverized coal from each plant was wet screened at 100 (150 μ), 200 (75 μ), 325 (about 40 μ), and 500 (about 25 μ) mesh. The screened splits were weighed and divided for petrographic and proximate analysis. Petrographic analysis consists of a combined maceral/microlithotype analysis (nomenclature on Table 1) and vitrinite maximum reflectance. In this analytical procedure, previously used at the CAER in studies of coals tested for HGI or pulverized at power plants, both the maceral and the maceral association (microlithotype) are counted together, providing more information than the standard maceral count. The maceral counted is the maceral at the cross hair of the field reticle and the microlithotype comprises the maceral assemblage within a 25-micron radius of the
J.C. Hower / International Journal of Coal Geology 73 (2008) 213–218 Table 1 Maceral group composition of microlithotypes
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3. Discussion
microlithotype, is shown on Figs. 1 and 2. The Appalachian coals (Fig. 1) have more duroclarite and less vitrite than the high-vitrinite Illinois Basin coals (Fig. 2), but the overall trends are similar: the coarse + 100-mesh fractions have abundant duroclarite. Finer size fractions have progressively less duroclarite, and trimaceral microlithotypes, in general, and less durite, the liptinite- and inertinite-rich bimaceral microlithotype. The b 500-mesh fractions are vitrite-rich, with much of the remainder being the brittle inertite. As a further example of the change of composition of the size fractions, Table 2 illustrates the change of maceral and group maceral composition of duroclarite by size fraction for three Appalachian-source coal blends. The-500-mesh fractions are not present in sufficiently significant quantities to allow comparisons, but, in general, the coarser fractions can be compared. In general, liptinite decreases content and vitrinite and inertinite increase from the +100-mesh fraction to the 325 × 500-mesh fraction. This parallels the overall decrease in duroclarite amount, both the decrease in abundance of duroclarite and the shift in maceral composition of the microlithotype being indicative of the more brittle nature of the finer fractions. Trimble and Hower (2000) noted a similar trend in progressivegrinding experiments. In addition to the petrographic trends noted above, note that pulverization generally exceeded or was close to the 70% passing 200 mesh target (Appendix A Notable exceptions are two of the three pulverized coals, each representing one ball mill, collected at plant I for the Hower and Thomas (2006) study. Both mills might have been in need of adjustments at the time of
Microlithotypes are important in grinding and pulverization properties of coal, with the more liptinite-rich varieties contributing to tougher-to-grind coals. Grindability studies (Hower, 1998; Hower and Calder, 1997; Padgett and Hower, 1997; Trimble and Hower, 2000, 2003) have demonstrated that, as grinding progresses, the most brittle microlithotypes, such as vitrite and inertite, will preferentially partition to the finer fractions. The remaining coal is enriched in the harder microlithotypes, the liptinite-and inertinite-rich bimaceral and trimaceral varieties (Table 1). Some of these patterns are evident in the utility combustion grinds examined in this study (Appendix A, available electronically). The − 500-mesh fraction contains a high percentage of the monomaceralic microlithotypes vitrite and inertite. The distribution of vitrite, representing the an abundant brittle monomacerite, and duroclarite, representing an abundant harder-to-grind trimaceral
Fig. 1. Distribution of vitrite and duroclarite for 11 sized pulverized feed coals burning high volatile bituminous Appalachian Basinsourced coal blends.
Monomaceral microlithotypes Vitrite (Vt) Liptite (Lp) Inertite (In) Bimaceral microlithotypes Clarite (Cl) Vitrinertite (Vi) Durite (Du) Trimaceral microlithotypes Duroclarite (Dc) Clarodurite (Cd) Vitrinertoliptite (Vl) Carbominerite (Cm) 20% b silicates, carbonates b 60% (vol.) 5% b sulfides b 20% (vol.)
Vitrinite (V) N 95% Liptinite (L) N 95% Inertinite (I) N 95% V + L N 95% V + I N 95% I + L N 95% V N L, I; each N 5% I N V, L; each N 5% L NV, I; each N 5%
cross hair. Group maceral compositions of microlithotypes are given on Table 1. The CAER procedure differs from the procedure discussed in Taylor et al. (1998) in counting particles less than 50 μ in diameter. In this manner, there is no bias against the very fine particles prevalent in pulverized coal. Overall, the microlithotype, as a cross-section of the particle, is a proxy for the particle volume composition and the accompanying maceral analysis allows tracking of the changes in microlithotype composition between size fractions. Further, microlithotype size is at the same scale as the particle sizes produced in coal pulverization, therefore, microlithotype analysis, particularly in combination with maceral analysis, is a suitable measure of the nature of the particles.
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Fig. 2. Distribution of vitrite and duroclarite for eight sized pulverized feed coals burning high volatile bituminous Illinois Basin-sourced coal blends.
sampling, accounting for the large 100 × 200-mesh fraction in the pulverized feed coal. Trimble and Hower (2000) investigated an aspect of coal grinding not accessible in the current study. In their investigation of progressive grinding, using a modified HGI scheme, they found that successive iterations of the HGI test, particularly in the case where the fines were removed after each iteration, resulted in trimaceral microlithotypes that were progressively depleted in vitrinite. With progressive grinding, the most brittle parts of particles such as vitrite and vitrinertite would be chipped off, leaving a remnant of a harder-to-grind particle. Their fines-removed model would resemble utility-scale pulverization where fines are continually removed from the system. For example, a duroclarite particle would preferentially break along a plane of
weakness, such as a vitrinite-rich band, fragmenting into vitrite and vitrinite-depleted trimaceral microlithotypes, possibly no longer in the duroclarite range, depending on the amount of vitrinite lost chipped from the larger remnants. Based on the HGI studies of Trimble and Hower (2000), it should be noted that vitrinite-rich duroclarite could be one of the products of this breakage mechanism. In the case of pulverizers, the coarser (albeit reduced in size from the original feed), harder-to-grind particles will report to the + 200-mesh fractions, if they are not partitioned to the pulverizer rejects. The dependence of the partitioning of particle composition on pulverized particle size reinforces the assumption, as demonstrated by the improvement in carbon burnout following conversion to low-NOx combustion coincident with pulverizer maintenance (Hower et al., 1997a, 1997b), that attention to pulverizer performance has the potential to increase combustion efficiency. In various ways, this has been expressed in the literature, sometimes with attention to petrographic detail (Bengtsson, 1986; Oka et al., 1986; Thomas et al., 1993; Cai et al., 1998; Alvarez et al., 1998; Yu et al., 2003), at other times with more attention strictly to the engineering characteristics (Zhang et al., 2007). Petrographic studies, however, are not always part of the evaluation of coal combustion, so the detail provided by investigations of the current study is generally not available. 4. Conclusions The combined maceral and microlithotype investigation of whole pulverized feed coals and the wet-
Table 2 Maceral and maceral group composition of duroclarite in sized fractions of pulverized feed coal from three Kentucky power plants burning high volatile bituminous Appalachian Basin-sourced coal blends
93,152
93,168
93,181
Mesh size
% of total
vit
fus
sfus
mic
mac
Total inertinite
ex
res
total liptinite
min
+100 100 × 200 200 × 325 325 × 500 − 500 +100 100 × 200 200 × 325 325 × 500 − 500 +100 100 × 200 200 × 325 325 × 500 − 500
43.3 36.6 25.6 18.6 4.6 46.4 36.3 25.6 15.2 5.8 41.0 31.8 29.2 17.4 2.6
85.2 83.1 85.9 90.3 73.9 73.7 84.6 93.0 89.5 75.9 76.1 80.5 77.4 81.6 84.6
2.3 2.7 3.9 2.2 8.7 3.9 2.2 0.8 5.3 3.4 2.0 4.4 5.5 4.6 7.7
3.5 4.4 0.8 1.1 4.3 3.4 1.7 0.8 2.6 3.4 6.8 1.9 3.4 2.3 0.0
1.2 0.5 3.1 1.1 4.3 3.9 5.5 0.8 1.3 0.0 1.5 1.3 2.1 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
6.9 7.7 7.8 4.3 17.4 11.2 9.4 2.3 9.2 6.9 10.2 7.5 11.0 6.9 7.7
7.4 8.7 6.3 4.3 8.7 14.7 6.1 4.7 1.3 17.2 13.7 11.3 11.6 11.5 7.7
0.5 0.0 0.0 1.1 0.0 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
7.9 8.7 6.3 5.4 8.7 15.1 6.1 4.7 1.3 17.2 13.7 11.3 11.6 11.5 7.7
0.0 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.6 0.0 0.0 0.0
Key: vit — vitrinite; fus — fusinite; sfus — semifusinite; mic — micrinite; mac — macrinite; ex — exinite; res — resinite; min — minerals.
J.C. Hower / International Journal of Coal Geology 73 (2008) 213–218
screened size fractions from splits of the whole coals was conducted on a series of samples from Kentucky and Tennessee power plants burning either Central Appalachian or Illinois Basin high volatile bituminous coal blends. The original studies were part of, in general, previously published investigations of the power plants. Most pulverizers in the study produced pulverized coal meeting the desired ca. 70% passing 200 mesh objective. Two mills from the same plant, collected on the same date, had too much coal in the 100 × 200-mesh fraction. Brittle microlithotypes, such as vitrite, as illustrated on Figs. 1 and 2, partition to the finer fractions while harder-to-grind microlithotypes, such as duroclarite (also illustrated on Figs. 1 and 2), partition to the coarser sizes or, in some cases, get eliminated from the boiler feed and are sent to the pulverizer rejects (Hower et al., 2005, 2006). Duroclarite, the most abundant of the harder microlithotypes, also illustrates the change in maceral composition within the harder microlithotypes. Vitrinite in duroclarite generally is more abundant in the finer sizes compared to the +100-mesh fraction. Comparison of these results to Trimble and Hower's (2000) progressive HGI grinding study would suggest that the breakage of particles in pulverization results in populations of brittle particles such as vitrite, inertite, and vitrinite-rich bimaceral and trimaceral microlithotypes and in populations of hard-to-grind vitrinite-depleted biand trimaceral microlithotypes. Portions of the latter will, as noted above, be rejected from the pulverizer feed. Other parts of the hard remnants will be the particles measured in the +100-and 100 × 200-mesh size fractions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j. coal.2007.06.005. References Abbas, T., Costa, M., Costen, P., Godoy, S., Lockwood, F.C., Ou, J.J., Romo-Millares, C., Zhou, J., 1994. NOx formation and reduction mechanisms in pulverized coal flames. Fuel 73, 1423–1436. Alvarez, D., Borrego, A.G., Meneńdez, R., Bailey, J.G., 1998. An unexpected trend in the combustion behavior of hvBb coals as shown by the study of their chars. Energy and Fuels 12, 849–855. Barranco, R., Colechin, M., Cloke, M., Gibb, W., Lester, E., 2006. The effects of pf grind quality on coal burnout in a 1 MW combustion test facility. Fuel 85, 1111–1116. Bengtsson, M., 1986. Combustion behavior for a range of coals of various origins and petrographic composition. PhD dissertation, Stockholm, Sweden, The Royal Institute of Technology, variously paginated.
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International Journal of Coal Geology 73 (2008) 213 – 218 www.elsevier.com/locate/ijcoalgeo
Maceral/ microlithotype partitioning with particle size of pulverized coal: Examples from power plants burning Central Appalachian and Illinois basin coals James C. Hower ⁎ University of Kentucky Center for Applied Energy Research, 2540 Research Park Drive, Lexington, KY 40511, United States Received 27 March 2007; received in revised form 5 June 2007; accepted 18 June 2007 Available online 12 July 2007
Abstract Pulverized coals from eleven power plants burning Central Appalachian coal blends and eight power plants burning Illinois Basin coal blends were studied in order to assess the petrographic nature of industrial-scale coal grinding. All coals were high volatile bituminous. Coals were wet screened at 100 (150 μ), 200 (75 μ), 325 (about 40 μ), and 500 (about 25 μ) mesh. Petrographic analysis of the whole coals and size fractions consisted of a combined maceral and microlithotype analysis. Microlithotype analysis, in particular, provides a reasonable approximation of the whole-particle composition at the scale of utility coal pulverization. In the size fractions, duroclarite, the most abundant trimaceral microlithotype, is most abundant in the coarsest fraction and least abundant in the finest fraction. Vitrite, the most abundant monomaceral microlithotype, exhibits the opposite trend. Duroclarite becomes more enriched in vitrinite towards the finer sizes. The partitioning of microlithotypes and the partitioning of macerals within the microlithotypes is indicative of the relative brittle nature of vitrite compared to the hard-to-grind trimaceral microlithotypes. Increased vitrinite in duroclarite is an indication that the microlithotype within the particular size fraction is more brittle than relatively vitrinite-depleted duroclarite in coarser fractions. The relative grindability of microlithotypes will, in turn, impact combustion efficiency. © 2007 Elsevier B.V. All rights reserved. Keywords: Coal; Combustion; Pulverization; Petrology; Maceral
1. Introduction Pulverization of coal in coal-fired power plants is at the interface of coal science and engineering. Aside from the actual selection of the coals to be burned, pulverization is the first step in determining the efficiency of combustion, of the capture of trace elements by coal-combustion products, and of the release of SO2 and trace elements ⁎ Tel.: +1 859 257 0261; fax: +1 859 257 0360. E-mail address: [email protected]. 0166-5162/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2007.06.005
with the stack gas. The goal of utility-scale coal pulverization is to produce a coal feed with a particle size N70% passing 200 mesh (75 μ). Scott (1995) and Kitto and Stutz (2005) describe the different kinds of coal pulverizers and the principles of their operations. In all of the units sampled for this study, coal is pulverized in a ball and race mill with the pulverized coal entrained in the air and swept out of the top of the pulverizer and into the boiler and the excessively hard material. The pulverizer rejects or “pyrites”, are removed from the bottom of the
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pulverizer. The rejects, typically less than 1% of the total feed, can remove over 10% of elements such as Hg and As from the feed coal (Mardon and Hower, 2004). The relationship between coal hardness, grindability, petrology, and efficiency of combustion was reviewed by Hower (1998) and will not be repeated in detail here. In general, the finer the coal, the more efficient the combustion (Conroy et al., 1989; Maier et al., 1994; Abbas et al., 1994; Wu, 2005). Hardgrove grindability index (HGI), although an inherently flawed test because of the petrographic biases in the sample preparation (discussion in Hower, 1998), can be used to predict pulverization behavior. Fundamentally, pulverizers are typically rated on the basis of a 50 HGI coal. The capacity chart for a Combustion Engineering Bowl Mill relating HGI, percent grinding mill capacity (from 40 to 150%), and the desired fineness (in percent passing 200 mesh) is a frequently used nomogram (example published by Savage, 1974). For example, a 53 HGI coal requires 2.5 times the numbers of revolutions in a ball mill than a 110 HGI coal to achieve 80% passing 200 mesh (Fitton et al., 1957). As an approximation of mill behavior, Lowe (1987) estimated a 1% loss of mill throughput per 1 HGI unit decrease. Rogers et al. (1987) found that the behavior of dry (air-swept) closed-circuit ball mills could be predicted from the HGI, the relationship being nearly linear in the 40 to 85 HGI range. In order to achieve a given fineness as defined by the percent passing 200 mesh, Scott (1995) isolated grinding pressure (GP) in a vertical spindle mill as the controlling factor influencing fineness and power consumption. Grinding power was related to HGI by: GP ¼ ð0:091HGI 0:256Þ1 ;
ð1Þ
with power consumption, in kWh/t, as: P ¼ 1:09GP þ 5:15;
ð2Þ
with r2 of 0.88 and 0.85, respectively. This translates into a decrease in power consumption from 5.59 kWh/t to 5.26 kWh/t over an HGI range from 30 to 110, the range found in banded bituminous coals. Fineness increases with an increase in grinding pressure but is also a function of the rank of the coal, lower rank (subbituminous in study cited) coals producing a coarser size consist than relatively higher rank coals (Conroy and Trenaman, 1990). The “optimum grinding pressure,” the optimization of fineness with mill power consumption (Conroy et al., 1989), for specific coals is a function of a variety of rank and compositional factors, not all properly expressed in a single variable such as HGI (Conroy and Trenaman, 1990).
Bengtsson (1986) noted several general parameters that influenced the number of combustion phases, two of which, vitrinite reflectance and petrographic composition, are directly related to the coal petrology, and a third, particle size, investigated in this study, that is a function of both of the latter parameters. Man et al. (1998), in a flash-pyrolysis study of a limited number of 38- to 300-μm size fractions of a vitrain, suggested that maceral segregation among particle sizes was more important than particle size itself in determining devolatization yields. The amount of carbon burnout increases (Yu et al., 2005; Barranco et al., 2006) and the efficiency of NOx reduction decreases (Maier et al., 1994) with poorer pulverization. Hower et al. (1996, 1997a) found that fly ash carbon was reduced following conversion to low-NOx combustion, contrary to other low-NOx conversions followed (Hower et al., 1996, 1997b, 1999a), due to a rigorous overhaul of the pulverizers during the outage. In this study, a series of coals from Kentucky and Tennessee power plants is investigated with the objective of determining the distribution of macerals and microlithotypes in the size fractions of the pulverized coal. 2. Procedure Samples of pulverized feed coal were obtained from several power plants burning Central Appalachian (eastern Kentucky and central West Virginia) or Illinois Basin (generally western Kentucky) high volatile bituminous coal blends. The samples came from previous studies of power plants (Hower et al., 1997a, 1997b, 1999a, 1999b; Trimble and Hower, 2000; Sakulpitakphon et al., 2003; Hower et al., 2006, and unpublished petrology results from the study by Hower and Thomas, 2006). The pulverized coal from each plant was wet screened at 100 (150 μ), 200 (75 μ), 325 (about 40 μ), and 500 (about 25 μ) mesh. The screened splits were weighed and divided for petrographic and proximate analysis. Petrographic analysis consists of a combined maceral/microlithotype analysis (nomenclature on Table 1) and vitrinite maximum reflectance. In this analytical procedure, previously used at the CAER in studies of coals tested for HGI or pulverized at power plants, both the maceral and the maceral association (microlithotype) are counted together, providing more information than the standard maceral count. The maceral counted is the maceral at the cross hair of the field reticle and the microlithotype comprises the maceral assemblage within a 25-micron radius of the
J.C. Hower / International Journal of Coal Geology 73 (2008) 213–218 Table 1 Maceral group composition of microlithotypes
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3. Discussion
microlithotype, is shown on Figs. 1 and 2. The Appalachian coals (Fig. 1) have more duroclarite and less vitrite than the high-vitrinite Illinois Basin coals (Fig. 2), but the overall trends are similar: the coarse + 100-mesh fractions have abundant duroclarite. Finer size fractions have progressively less duroclarite, and trimaceral microlithotypes, in general, and less durite, the liptinite- and inertinite-rich bimaceral microlithotype. The b 500-mesh fractions are vitrite-rich, with much of the remainder being the brittle inertite. As a further example of the change of composition of the size fractions, Table 2 illustrates the change of maceral and group maceral composition of duroclarite by size fraction for three Appalachian-source coal blends. The-500-mesh fractions are not present in sufficiently significant quantities to allow comparisons, but, in general, the coarser fractions can be compared. In general, liptinite decreases content and vitrinite and inertinite increase from the +100-mesh fraction to the 325 × 500-mesh fraction. This parallels the overall decrease in duroclarite amount, both the decrease in abundance of duroclarite and the shift in maceral composition of the microlithotype being indicative of the more brittle nature of the finer fractions. Trimble and Hower (2000) noted a similar trend in progressivegrinding experiments. In addition to the petrographic trends noted above, note that pulverization generally exceeded or was close to the 70% passing 200 mesh target (Appendix A Notable exceptions are two of the three pulverized coals, each representing one ball mill, collected at plant I for the Hower and Thomas (2006) study. Both mills might have been in need of adjustments at the time of
Microlithotypes are important in grinding and pulverization properties of coal, with the more liptinite-rich varieties contributing to tougher-to-grind coals. Grindability studies (Hower, 1998; Hower and Calder, 1997; Padgett and Hower, 1997; Trimble and Hower, 2000, 2003) have demonstrated that, as grinding progresses, the most brittle microlithotypes, such as vitrite and inertite, will preferentially partition to the finer fractions. The remaining coal is enriched in the harder microlithotypes, the liptinite-and inertinite-rich bimaceral and trimaceral varieties (Table 1). Some of these patterns are evident in the utility combustion grinds examined in this study (Appendix A, available electronically). The − 500-mesh fraction contains a high percentage of the monomaceralic microlithotypes vitrite and inertite. The distribution of vitrite, representing the an abundant brittle monomacerite, and duroclarite, representing an abundant harder-to-grind trimaceral
Fig. 1. Distribution of vitrite and duroclarite for 11 sized pulverized feed coals burning high volatile bituminous Appalachian Basinsourced coal blends.
Monomaceral microlithotypes Vitrite (Vt) Liptite (Lp) Inertite (In) Bimaceral microlithotypes Clarite (Cl) Vitrinertite (Vi) Durite (Du) Trimaceral microlithotypes Duroclarite (Dc) Clarodurite (Cd) Vitrinertoliptite (Vl) Carbominerite (Cm) 20% b silicates, carbonates b 60% (vol.) 5% b sulfides b 20% (vol.)
Vitrinite (V) N 95% Liptinite (L) N 95% Inertinite (I) N 95% V + L N 95% V + I N 95% I + L N 95% V N L, I; each N 5% I N V, L; each N 5% L NV, I; each N 5%
cross hair. Group maceral compositions of microlithotypes are given on Table 1. The CAER procedure differs from the procedure discussed in Taylor et al. (1998) in counting particles less than 50 μ in diameter. In this manner, there is no bias against the very fine particles prevalent in pulverized coal. Overall, the microlithotype, as a cross-section of the particle, is a proxy for the particle volume composition and the accompanying maceral analysis allows tracking of the changes in microlithotype composition between size fractions. Further, microlithotype size is at the same scale as the particle sizes produced in coal pulverization, therefore, microlithotype analysis, particularly in combination with maceral analysis, is a suitable measure of the nature of the particles.
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Fig. 2. Distribution of vitrite and duroclarite for eight sized pulverized feed coals burning high volatile bituminous Illinois Basin-sourced coal blends.
sampling, accounting for the large 100 × 200-mesh fraction in the pulverized feed coal. Trimble and Hower (2000) investigated an aspect of coal grinding not accessible in the current study. In their investigation of progressive grinding, using a modified HGI scheme, they found that successive iterations of the HGI test, particularly in the case where the fines were removed after each iteration, resulted in trimaceral microlithotypes that were progressively depleted in vitrinite. With progressive grinding, the most brittle parts of particles such as vitrite and vitrinertite would be chipped off, leaving a remnant of a harder-to-grind particle. Their fines-removed model would resemble utility-scale pulverization where fines are continually removed from the system. For example, a duroclarite particle would preferentially break along a plane of
weakness, such as a vitrinite-rich band, fragmenting into vitrite and vitrinite-depleted trimaceral microlithotypes, possibly no longer in the duroclarite range, depending on the amount of vitrinite lost chipped from the larger remnants. Based on the HGI studies of Trimble and Hower (2000), it should be noted that vitrinite-rich duroclarite could be one of the products of this breakage mechanism. In the case of pulverizers, the coarser (albeit reduced in size from the original feed), harder-to-grind particles will report to the + 200-mesh fractions, if they are not partitioned to the pulverizer rejects. The dependence of the partitioning of particle composition on pulverized particle size reinforces the assumption, as demonstrated by the improvement in carbon burnout following conversion to low-NOx combustion coincident with pulverizer maintenance (Hower et al., 1997a, 1997b), that attention to pulverizer performance has the potential to increase combustion efficiency. In various ways, this has been expressed in the literature, sometimes with attention to petrographic detail (Bengtsson, 1986; Oka et al., 1986; Thomas et al., 1993; Cai et al., 1998; Alvarez et al., 1998; Yu et al., 2003), at other times with more attention strictly to the engineering characteristics (Zhang et al., 2007). Petrographic studies, however, are not always part of the evaluation of coal combustion, so the detail provided by investigations of the current study is generally not available. 4. Conclusions The combined maceral and microlithotype investigation of whole pulverized feed coals and the wet-
Table 2 Maceral and maceral group composition of duroclarite in sized fractions of pulverized feed coal from three Kentucky power plants burning high volatile bituminous Appalachian Basin-sourced coal blends
93,152
93,168
93,181
Mesh size
% of total
vit
fus
sfus
mic
mac
Total inertinite
ex
res
total liptinite
min
+100 100 × 200 200 × 325 325 × 500 − 500 +100 100 × 200 200 × 325 325 × 500 − 500 +100 100 × 200 200 × 325 325 × 500 − 500
43.3 36.6 25.6 18.6 4.6 46.4 36.3 25.6 15.2 5.8 41.0 31.8 29.2 17.4 2.6
85.2 83.1 85.9 90.3 73.9 73.7 84.6 93.0 89.5 75.9 76.1 80.5 77.4 81.6 84.6
2.3 2.7 3.9 2.2 8.7 3.9 2.2 0.8 5.3 3.4 2.0 4.4 5.5 4.6 7.7
3.5 4.4 0.8 1.1 4.3 3.4 1.7 0.8 2.6 3.4 6.8 1.9 3.4 2.3 0.0
1.2 0.5 3.1 1.1 4.3 3.9 5.5 0.8 1.3 0.0 1.5 1.3 2.1 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
6.9 7.7 7.8 4.3 17.4 11.2 9.4 2.3 9.2 6.9 10.2 7.5 11.0 6.9 7.7
7.4 8.7 6.3 4.3 8.7 14.7 6.1 4.7 1.3 17.2 13.7 11.3 11.6 11.5 7.7
0.5 0.0 0.0 1.1 0.0 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
7.9 8.7 6.3 5.4 8.7 15.1 6.1 4.7 1.3 17.2 13.7 11.3 11.6 11.5 7.7
0.0 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.6 0.0 0.0 0.0
Key: vit — vitrinite; fus — fusinite; sfus — semifusinite; mic — micrinite; mac — macrinite; ex — exinite; res — resinite; min — minerals.
J.C. Hower / International Journal of Coal Geology 73 (2008) 213–218
screened size fractions from splits of the whole coals was conducted on a series of samples from Kentucky and Tennessee power plants burning either Central Appalachian or Illinois Basin high volatile bituminous coal blends. The original studies were part of, in general, previously published investigations of the power plants. Most pulverizers in the study produced pulverized coal meeting the desired ca. 70% passing 200 mesh objective. Two mills from the same plant, collected on the same date, had too much coal in the 100 × 200-mesh fraction. Brittle microlithotypes, such as vitrite, as illustrated on Figs. 1 and 2, partition to the finer fractions while harder-to-grind microlithotypes, such as duroclarite (also illustrated on Figs. 1 and 2), partition to the coarser sizes or, in some cases, get eliminated from the boiler feed and are sent to the pulverizer rejects (Hower et al., 2005, 2006). Duroclarite, the most abundant of the harder microlithotypes, also illustrates the change in maceral composition within the harder microlithotypes. Vitrinite in duroclarite generally is more abundant in the finer sizes compared to the +100-mesh fraction. Comparison of these results to Trimble and Hower's (2000) progressive HGI grinding study would suggest that the breakage of particles in pulverization results in populations of brittle particles such as vitrite, inertite, and vitrinite-rich bimaceral and trimaceral microlithotypes and in populations of hard-to-grind vitrinite-depleted biand trimaceral microlithotypes. Portions of the latter will, as noted above, be rejected from the pulverizer feed. Other parts of the hard remnants will be the particles measured in the +100-and 100 × 200-mesh size fractions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j. coal.2007.06.005. References Abbas, T., Costa, M., Costen, P., Godoy, S., Lockwood, F.C., Ou, J.J., Romo-Millares, C., Zhou, J., 1994. NOx formation and reduction mechanisms in pulverized coal flames. Fuel 73, 1423–1436. Alvarez, D., Borrego, A.G., Meneńdez, R., Bailey, J.G., 1998. An unexpected trend in the combustion behavior of hvBb coals as shown by the study of their chars. Energy and Fuels 12, 849–855. Barranco, R., Colechin, M., Cloke, M., Gibb, W., Lester, E., 2006. The effects of pf grind quality on coal burnout in a 1 MW combustion test facility. Fuel 85, 1111–1116. Bengtsson, M., 1986. Combustion behavior for a range of coals of various origins and petrographic composition. PhD dissertation, Stockholm, Sweden, The Royal Institute of Technology, variously paginated.
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