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Energy & Fuels 2007, 21, 3130–3133
Impact of Petrographic Properties on the Burning Behavior of Pulverized Coal Using a Drop Tube Furnace S. Biswas,*,†,‡ N. Choudhury,†,‡ S. Ghosal,§,| T. Mitra,§,⊥ A. Mukherjee,†,‡ S. G. Sahu,†,‡ and M. Kumar†,‡ JadaVpur UniVersity, Kolkata 700032, India, and Central Fuel Research Institute, P.O.- FRI, Dist. Dhanbad 828108, Jharkhand, India ReceiVed NoVember 17, 2006. ReVised Manuscript ReceiVed July 13, 2007
The combustion behavior of three Indian coals of different rank with wide variation in ash content and maceral compositions were studied using a drop tube furnace (DTF). Each coal was pulverized into a specific size (80% below 200 mesh) and fed into the DTF separately. The DTF runs were carried out under identical conditions for all of the coals. The carbon burnout was found out from the chemical analyses of the feed coals and the char samples collected from different ports of the DTF. Char morphology analyses was carried on the burnout residues of the top port. The top port results show better burnout of the lower rank coals which however was not observed in the last port. An attempt has been made to account for this variation in terms of rank and petrographic parameters of the respective coals.
1. Introduction The combustion behavior of coal is a complex function of different chemical, petrographic, and inorganic constituent of coals. Indian coals being used in coal based thermal power plants are usually highly heterogeneous in their chemical makeup and petrographic mix. Traditional chemical parameters often fail to make a realistic assessment of the relative behavior of such highly heterogenous coals. Laboratory scale investigations in a drop tube furnace coupled with the chemical and petrological data may provide better insight into the burning behavior. Experimental furnaces are generally used to simulate combustion behavior in pulverized fuel boilers. The drop tube furnace (DTF) is a valuable tool when attempting to simulate coal combustion on a small scale.1 With the help of a DTF, conditions similar to those found during pulverized fuel combustion could be simulated. The maceral and microlithotype composition of coal, along with the mean reflectance percent, are acknowledged characteristics that account for the burning behavior of coal, and numerous research efforts have been made in this direction.2–8 An understanding of the * Corresponding author. Phone: 91-326-2381001-10, Extn. 269, +919431541959 (M). Fax: 91-326-2381113. E-mail:
[email protected]. † Central Fuel Research Institute. ‡ Scientists. § Jadavpur University. | Professor. ⊥ Reader. (1) Anderson, A. P.; Hedley, A. B. The development of a small scale pulverized coal combustion facility for research and commercial application. J. Inst. Energy 1983, 145–148. (2) Jones, R. B.; McCourt, C. B.; Morley, C.; King, K. Maceral and rank influences on the morphology of coal char. Fuel 1985, 64. (3) Oka, N.; Murayama, T.; Matsuoka, H.; Yamada, S.; Shinozaki, S.; Shibaoka, M.; Thomas, C. G. The influence of Rank and Maceral composition on Ignition and char burnout of pulverized coal. Fuel Process. Technol. 1987, 15, 213–224. (4) Crelling, J. C.; Skorupska, N. M.; Marsh, H. Reactivity of coal macerals and lithotypes. Fuel 1988, 67. (5) Furimsky, S.; Palmer, A. D.; Kalkreuth, W. D.; Cameron, A. R.; Kovacik, G. Prediction of coal reactivity during combustion and gasification by using petrographic data. Fuel Process. Technol. 1990, 25, 135–151.
maceral composition of coals would therefore seem to be a useful prerequisite before utilization of coals in the combustion process. In general, liptinite and vitrinite are more reactive and burn more effectively than inertinite. This, however, is not always true because macerals like inertinite are not always poor combustibles and the feature that gives an indication of its reactivity is its reflectance value.6,7,9–11 The presence of different macerals and microlithotypes has been related to the type and morphology of pyrolyzed chars, chars present in burnout residues, and fly ash.11–15 Image analysis techniques have been used to identify the unreactive/ less reactive content in the coal, which has been subsequently (6) Cloke, M.; Lester, E. Characterisation of coals for combustion using petrographic analysis. Fuel 1994, 73. (7) Chaudhuri, S. G.; Choudhury, N.; Chatterjee, C. N. Role of Petrography in combustion of pulverized coal, A review, MGMI, 1999– 2000, 96. (8) Choudhury, N.; Mukherjee, A.; Choudhury, A.; Biswas, S.; Sen, K. Influence of Rank and Macerals on the Burnout Behaviour of Pulverized Indian Coal in a Drop Tube Furnace. Presented at the 20th Annual International Pittsburgh Coal Conference, Sept 15–19, 2003. (9) Thomas, C. G.; Holcombe, D.; Shibaoka, M.; Young, B. C.; Brunckhorst, L. F.; Gawronski, E. Determination of Effect of coal Rank and Maceral compositions on pf combustion reactivity. Proceedings of the 1989 International Conference on Coal Science; NEDO: Tokyo, 1989; p 257. (10) Phon-anant, D.; Salehi, M.; Thomas, C.; Baker, J.; Conroy, A. Burnout and reactivity of coal Macerals. Proceedings of the 1989 International Conference on Coal Science; NEDO: Tokyo, 1989; p 253. (11) Chatterjee, C. N.; Chaudhuri, S. G. A preliminary study on the characterisaion of unburnt carbon in fly ash by microscopic method. Fuel Sci. Technol. 1987, 6, 65–71. (12) Bailey, J. G.; Tate, A.; Diessel, C. F. K.; Wall, T. F. A char morphology system with applications to coal combustion. Fuel 1990, 69. (13) Stephen, L.; Blend, I.; Edwards, A. S.; Marsh, H. The influence of rank upon char morphology and combustion. Fuel 1992, 71. (14) Canada-Thomas, G.; Chambers, A. A microscopic study of the combustion residues of subbitumnous and bituminous coals from Alberta. Int. J. Coal Geol. 1993, 24, 245–257. (15) Alonso, M. J. G.; Borrego, A. G.; Alvarwz, D.; Menendez, R. A reactivity study of chars obtained at different temperatures in relation to their petrographic characteristics. Fuel Process. Technol. 2001, 69, 257– 272.
10.1021/ef060582r CCC: $37.00 2007 American Chemical Society Published on Web 09/07/2007
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Energy & Fuels, Vol. 21, No. 6, 2007 3131
linked with the formation of low reactive unfused chars.16,17 However, such a study has rarely been carried out on Indian coals, which are distinctly different from coals of the Northern hemisphere but are somewhat akin to high inertinite coals of Australian origin. 2. Experimental Section Three coals of different ranks and petrographic compositions have been used in the study. One of them is an inertinite rich subbituminous coal, and the other two coals are bituminous coals with different petrographic makeups and ash contents. All of the coals were first crushed in a double roll crusher and then ground in a ball mill for preparation of the desired (80% below 200 mesh) size for charging in the DTF. Proximate and ultimate analyses were done following the standard procedure. A BIS polarized light microscope (Leica make – erstwhile Leitz W. Germany) was used for measuring reflectance percent (mean Ro %) and maceral composition, and microlithotype analyses were done using both white light and fluorescent light following the BIS procedure. The methods of analysis of maceral composition by BIS and ISO are more or less the same, except in BIS “semivitrinite”, a separate maceral, transitional between vitrinite and semifusinite (sub-maceral of the inertinite group), is taken into consideration. This semivitrinite is available mostly in Gondwana coals.18,19 The reflectance measurements of inertinites were carried out by QWIN image analysis software in a DMRXP HC microscope. The char samples collected from the DTF were mixed with epoxy resin and hardener with suitable proportion and then poured into a 1 cm3 size mould. These were left for 24 h at room temperature for hardening, and then, subsequent grinding and polishing were done with different polishing materials. Using the above mentioned microscope with an oil objective of ×50 magnification, observation and categorization of char particles were made, which conforms to the generalized classification scheme of common char types used by various workers. However, all of the char types used by several workers were not included in the classification.7 The char types were clubbed into three broad groups: (I) tenui balloon, tenui sphere, tenui network, and tenui fragment; (II) crassi balloon, crassi sphere, and crassi network; and (III) inertoid and fusinoid. Normally, the group I char will be more reactive than the group II char. The group III char will be less reactive. Drop Tube Furnace Study. The drop tube furnace (DTF) used in the study consists of a ceramic tube, with a length of 2500 mm and an i.d. of 100 mm, having five zones (Figure 1). All five zones were heated electrically by externally heated canthal wire, and the temperature in all five zones could be raised up to 1100 °C. After each zone, there was provision of sample (solid and gases) collection through a water-cooled probe. The specially designed water-cooled probe having a vacuum pump and cyclone was used for collection of solid samples. The details of the DTF have been reported elsewhere.8,20 Pulverized coal was first dried at 110 °C separately in an air oven for 1 h and fed into the drop tube furnace through the vibratory feeder at a rate of 1.5 kg/h. The primary air (30%) and preheated (180 °C) secondary air (70%) considering 20% (16) Cloke, M.; Lester, E.; Gibb, W. Characterisation of coal with respect to carbon burnout in p.f.-fired boilers. Fuel 1997, 76, 1257–1267. (17) Cloke, M.; Lester, E.; Belghazi, A. Characterisation of the properties of size fractions from ten world coals and their chars produced in a drop tube furnace. Fuel 2002, 81, 699–708. (18) Chaudhuri, S. G.; Ghosh, S. A preliminary study of reactive semifusinite of Indian coking coals. J. Mines, Met. Fuels 1978, 26, 137– 141. (19) Duguid, K. Semivitrinite in South African Coals. World Coal 1980, 19. (20) Biswas, S.; et al. The development of Drop Tube Furnace, a laboratory scale pulverized coal combustion facility for research and commercial applications. Presented at International Coal Seminar, CFRI, April, 2004.
Figure 1. Drop tube furnace.
excess air were fed into the combustor, and all of the relevant parameters including air flow were kept more or less the same in the DTF for the set of experiments. Furnace temperature and wall temperature were measured by the thermocouples at the center of the furnace and embedded inside the refractory of the furnace wall, respectively. It was used to have a constant check on the furnace temperature while performing the experiment. Gas temperature was measured along the length of the reactor using thermocouples inserted into the DTF. After reaching the steady state condition as observed from the results of oxygen and carbon monoxide in an online gas analyzer, the sample collection was started. The sample collection probe was inserted externally at the end of each zone along the length of the reactor to collect char samples. The probe was water jacketed in order to quench the hot particles during collection. The chemical data of the coal and chars were used to calculate the carbon burnout of the char sample collected from the port. carbon burnout ) [1 - (A0Cc ⁄ AcC0)] × 100 where C0 and A0 are the carbon % and ash % of the parent coal and Cc and Ac are the carbon % and ash % of the char on a dry basis, respectively. This is based on the assumption of constancy of ash content in the parent coal and char.
3. Results and Discussion The data of proximate, ultimate, and calorific value are shown in Table 1, and the maceral compositions, microlithotype, and char morphology results are provided in Tables 2, 3, and 4, respectively. The chemical analyses of the char samples and
3132 Energy & Fuels, Vol. 21, No. 6, 2007
Biswas et al. Table 1. Chemical Analysis of Coal Samplesa
Sl. no.
moisture
ash
FC
VM
C
H
N
S
GCV (kCal/kg)
H/C ratio
coal 1 coal 2 coal 3
8.6 4.2 3.0
17.4 27.8 42.7
45.7 42.4 35.4
28.4 25.6 19.0
80.7 82.8 84.4
5.5 6.1 6.2
2.3 2.7 1.7
0.30 0.44 0.41
5380 5180 3875
0.81 0.88 0.89
a
Proximate analysis (wt %, air-dried basis), ultimate analysis (wt %, dmf basis), GCV (air-dried basis).
Table 2. Petrographic Composition of Coalsa maceral (Vol %)
a
Sl. no.
vitrinite
semivitrinite
liptinite
inertinite
mineral matter
mean Ro %
coal 1 coal 2 coal 3
29.4 (33.0) 58.2 (68.4) 45.9 (63.2)
0.3 (0.4) 0.5 (0.6) 1.6 (2.2)
9.9 (11.1) 7.9 (9.3) 7.7 (10.6)
49.4 (55.5) 18.5 (21.7) 17.5 (24.0)
11.0 14.9 27.3
0.45 0.56 0.63
Results in parentheses are on a visible mineral matter free basis.
Table 3. Microlithotype Analyses (Vol %)a sample
vitrite
liptite
inertite
clarite
vitrinertite V>I
vitrinertite V I), liptite derived from vitrinite, and liptinite macerals was highest in coal 2 (59.6%). Coal 3 having nearly the same inertinite content (17.5% including mineral matter) and higher reactive macerals (55.2% including mineral matter) than coal 1 (39.6% including mineral matter), however, showed poor burnout. This could be due to the fact that because of the comparatively higher reflectance value the inertinite and vitrinite fractions were less reactive than their counterparts in the other two coals. The volatile matter content was also less compared to other coals.
Burning BehaVior of PulVerized Coal
The relatively better burnout of coal 1, despite its low vitrinite content, suggests the contribution of the inertinites to combustion. Several authors4,6,7,17 have identified the reactivity of macerals and the significant role of low rank inertinites in combustion. It was reported by Cloke and Lester6 that for subbituminous coals inertinites having a reflectance of
