Energy & Fuels 2005, 19, 453-458
453
Coal Blending with Petroleum Coke in a Pulverized-Fuel Power Plant Katia S. Milenkova, Angeles G. Borrego,* Diego Alvarez, and Rosa Mene´ndez Instituto Nacional del Carbo´ n, CSIC, Apartado 73, 33080 Oviedo, Spain
Henrik I. Petersen and Per Rosenberg Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen, Denmark Received July 28, 2004. Revised Manuscript Received November 5, 2004
The current work investigates the performance of petroleum coke (PC) as a blended fuel under pulverized-fuel combustion conditions. Three full-scale combustion experiments were carried out: a pure Carboniferous, high volatile bituminous coal and two blends of this coal with different proportions of PC. The samples studied included feed fuels and blends, fly ashes, chars taken at different positions of the combustion chamber, and chars prepared in a drop tube reactor to test the performance of the individual fuels. The addition of PC led to a substantial increase in the unburned carbon of the fly ashes. The petrographic analysis of the granulometric fractions of the fuels revealed that this increase cannot be attributed to an enrichment in coke of the coarser fractions, as reported in the literature. On the contrary, the finer fraction contained slightly more coke than the raw blend. The petrographic analysis of the chars collected with the suction probe and the fly ashes showed that the two blended fuels were strongly enriched in PC-derived material, indicating a poorer combustibility compared to the high volatile bituminous coal. It is concluded that the reactivity of the blends in the later stages of combustion is related with the contents of PC-derived chars and the burnout itself.
Introduction Fuel-grade petroleum coke (PC) is the major product resulting from petroleum refining and also the one having the lowest added value.1 This is because, although it is a cheap fuel that typically contains few volatiles and ashes, it also has a high sulfur content, which has prevented its utilization at large scale. On the other hand, the environmental concern on SOx emissions has led to the installation of flue-gas desulfurization units on an increasing number of power plants, where PC has become a firm candidate to steaming fuel. Indeed, a number of power plants are nowadays including PC as an additional component of their feed blends.2,3 The behavior of PCs in a given process is difficult to predict because their characteristics depend on the delayed coking conditions, which is a continuous process where large differences occur within the drum and, particularly, on the feedstock composition. Although most PCs seem to be fairly similar when looking at their proximate characterization data, they also have shown * To whom correspondence should be addressed. Phone: +34 985119090. Fax: +34 985297662. E-mail:
[email protected]. (1) Adams, H. A. In Introduction to Carbon Technologies; Marsh, H., Heintz, E. A., Rodriguez-Reinoso, F., Eds.; Universidad de Alicante: Alicante, Spain, 1997; pp 491-517. (2) Hall, M. L.; Livingston, W. R. J. Chem. Technol. Biotechnol. 2002, 77, 234-239. (3) Gao, Y. M.; Kulaots, G.; Chen, X.; Suuberg, E. M.; Hurt, R. H.; Veranth, J. M. Proc. Combust. Inst. 2003, 29, 475-483.
quite different behaviors when submitted to conditions similar to those occurring in pulverized-fuel boilers.4 Only a few studies have considered the behavior of PC under large-scale pulverized-fuel combustion, and these have shown that PCs tend to increase the carbonin-ash content5-7 when cofired with bituminous coals. This was attributed to their somewhat lower reactivity6,8 and to the fact that they tend to concentrate in the larger size fractions.6 In this study, the performance of PC as a blended fuel under pulverized-fuel combustion conditions is investigated through the study of a highly volatile bituminous coal and two blends containing different proportions of PC, which were fired in a power plant. Both the feed fuels and their collected combustion chars were extensively studied, mainly using optical microscopy techniques, to obtain insight into the fate of the PC in the boiler. All of the samples collected from the boiler were extensively burned (over 90%), and therefore some laboratory-scale combustion tests were also performed, (4) Milenkova, K. S.; Borrego, A. G.; Alvarez, D.; Xiberta, J.; Menendez, R. Fuel 2003, 82, 1883-1891. (5) Hower, J. C.; Robertson, J. D.; Roberts, J. M. Fuel Process. Technol. 2001, 74, 125-142. (6) Yu, J.; Ku¨laots, I.; Sabanegh, N.; Gao, Y.; Hurt, R. H.; Suuberg, E. S.; Mehta, A. Energy Fuels 2000, 14, 591-596. (7) Milenkova, K. S.; Petersen, H. I.; Rosenberg, P.; Borrego, A. G.; Alvarez, D.; Menendez, R. Proceedings of the 12th International Conference on Coal Science; The Australian Institute of Energy: Toukley, New South Wales, Australia, 2003; p CD-9. (8) Bryers, R. W. Fuel Process. Technol. 1995, 44, 121-141.
10.1021/ef049817u CCC: $30.25 © 2005 American Chemical Society Published on Web 12/30/2004
454 Energy & Fuels, Vol. 19, No. 2, 2005
Milenkova et al.
Table 1. Proximate and Ultimate Analyses of Single Fuels and Blends and Petrographic Analysis of the Coala code
Rr (%)
V
vol mmf % L
I
PF PC M1 M2
0.84
58.4
11.2
30.4
ash (db %)
CV (db kcal/kg)
VM
C
15.05 3.33 13.53 13.76
7210 6581 7014 6999
35.3 13.8 33.1 32.8
83.3 84.7 83.6 83.5
daf % H N 5.3 3.9 5.4 5.4
1.5 1.0 1.4 1.3
O
S
9.2 4.5 8.8 9.0
0.8 5.3 1.4 1.5
a Rr ) random reflectance, V ) vitrinite, L ) liptinite, I ) inertinite, CV ) calorific value, VM ) volatile matter, vol ) volume, mmf ) mineral-matter-free basis, db ) dry basis, and daf ) dry-ash-free basis.
aimed at getting some additional information about the behavior of the fuels in the earlier stages of combustion. Experimental Section Samples. The feed fuels considered in this study are a Polish high volatile bituminous coal (PF; 0.84% Ro) of Carboniferous age and a fuel-grade PC. The full-scale combustion tests were carried out at the Energi E2 power plant Stignaes Unit2 (Denmark) and comprised the combustion of the pure PF coal and of two blends, both premixed before grinding and composed of coal PF plus 8% (M1) and 11% (M2) PC, respectively. Samples from the individual fuels, the pulverized blends, the corresponding fly ashes, and chars collected at different locations in the flame zone in the boiler were obtained. The 285 MWe Stigsnaes Unit2 boiler has four burner levels, each supporting six burners. Full-scale experiments were carried out during normal commercial operation of the power plant and with the three upper burner levels (18 burners) in operation. The full-scale char samples were taken from three positions along the rear wall, which is opposite to the burners, and from one location in each of the two side walls at a distance of about 2 m from the burner wall. The char samples were collected using a 3-m-long water-cooled lance (suction pyrometer), which was inserted about 1.5 m into the furnace through small openings in the furnace walls. The probe was held 5-10 min inside the furnace, and the char particles were collected with a filter attached to the cold outlet of the pyrometer. A total of nine char samples were collected during the test runs. Preparation of Size Fractions for the Assessment of Fuel Grindability. As coal and PC were mixed before grinding, some size segregation due to the differences in grindability of the two fuels might be expected. To study this possibility, granulometric separations of the mill samples were carried out, and the various size fractions thus obtained (>150, 150-100, 100-75, 75-45, 45-20, and
