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Energy & Fuels 2003, 17, 940-945
Nitrogen Release from Low Rank Coals during Rapid Pyrolysis with a Drop Tube Reactor Naoto Tsubouchi,* Miwa Abe, Chunbao Xu, and Yasuo Ohtsuka Research Center for Sustainable Materials Engineering, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan Received September 4, 2002. Revised Manuscript Received April 9, 2003
Nitrogen release from two low rank coals during rapid pyrolysis at 1300 °C has been studied with a graphite-made drop tube reactor, in which a graphite filter is installed for controlling a residence time of coal particles. Nitrogen distribution depends strongly on the time. At 0 s, 6065% of coal-N is retained in the chars, and the rest is released as tar-N, HCN, NH3, and N2. When the time is prolonged to 120 s, N2 yield increases dramatically and reaches 40-55%, whereas char-N decreases mainly. There is the reverse time dependence between N2 and char-N. Demineralization with HCl washing decreases N2 but increases char-N, and the addition of 1 wt % Ca to the demineralized coal shows almost the reverse effect on N2 and char-N. The deconvolution results of C(002) XRD lines reveal that the proportion of crystallized carbon in every char increases with increasing time. The linear relationship between the proportion and N2 yield exists among all of the samples used, which strongly suggests that N2 formation from char-N occurs in the process of carbon crystallization. Such a relationship is discussed on the basis of interactions between CaO and char matrix.
Introduction The present authors’ research group has recently shown that Fe and Ca cations, which are naturally present in low rank coals and externally added to them, change nitrogen distribution upon pyrolysis, that is, at the primary stage of combustion and promote N2 formation from the devolatilized chars.1-5 Although all pyrolysis runs have been carried out under slow heating conditions of 10-700 °C/min, coal particles are actually heated at a very fast rate of 104-105 °C/s in practical combustion processes that always include the pyrolysis stages. In fact, the fate of the nitrogen in coal (coal-N) during pyrolysis under such rapid heating conditions has been studied extensively using various methods,6-12 such as a wire mesh heater, fluidized bed and entrained flow reactors, and a Curie point pyrolyzer. However, no * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Ohtsuka, Y.; Mori, H.; Watanabe, T.; Asami, K. Fuel 1994, 73, 1093-1097. (2) Mori, H.; Asami, K.; Ohtsuka, Y. Energy Fuels 1996, 10, 10221027. (3) Wu, Z.; Ohtsuka, Y. Energy Fuels 1997, 11, 902-908. (4) Tsubouchi, N.; Ohshima, Y.; Xu, C.; Ohtsuka, Y. Energy Fuels 2001, 15, 158-162. (5) Tsubouchi, N.; Ohtsuka, Y. Fuel 2002, 81, 1423-1431. (6) Solomon, P. R.; Colket, M. B. Fuel 1978, 57, 749-755. (7) Nelson, P. F.; Buckley, A. N.; Kelly, M. D. Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1992; p 1259. (8) Kambara, S.; Takarada, T.; Yamamoto, Y.; Kato, K. Energy Fuels 1993, 7, 1013-1020. (9) Cai, H.-Y.; Guell, A. J.; Dugwell, D. R.; Kandiyoti, R. Fuel 1993, 72, 321-327. (10) Niksa, S. Energy Fuels 1995, 9, 467-478. (11) Genetti, D.; Fletcher, T. H. Energy Fuels 1999, 13, 1082-1091. (12) Kidena, K.; Hirose, Y.; Aibara, T.; Murata, S.; Nomura, M. Energy Fuels 2000, 14, 184-189.
information about catalytic effects of metal ions on nitrogen release in the rapid pyrolysis has been provided so far. If such effects are observed at a short solid residence time of a fraction of a second, the observations may be related with the mechanism of fuel-NOX emissions in pulverized coal-fired plants, where coal particles are rapidly pyrolyzed and then burned. As well-known, most of the NOX emitted from such plants originates from fuel-NOX.13,14 The present work first makes clear the feature of nitrogen distribution in the fast heating pyrolysis of low rank coals with a drop tube reactor, then examines the effects of solid residence time, demineralization, and subsequent addition of Ca ions on nitrogen release, and approaches possible mechanisms for remarkable formation of N2 observed at a longer residence time. Experimental Section Coal Samples. Zalainuoer coal from China and Adaro coal from Indonesia, denoted as ZN and AD, respectively, were used in this work. These samples were air-dried at ambient temperature, ground, and sieved to coal particles with size fraction of 250-350 µm. The ultimate and proximate analyses are given in Table 1, where the contents of Ca ions inherently present in these coals are also provided as inherent Ca. Demineralization and Calcium Addition. AD coal was demineralized with 18% HCl solution at 60 °C overnight to decrease the ash content to 0.1 wt %(db). The suffix “-dem” is (13) Unsworth, J. F.; Barratt, D. J.; Roberts, P. T. In Coal Quality and Combustion Performance; Coal Science and Technology Vol. 19; Elsevier: Amsterdam, 1991; pp 579-590. (14) Boardman, R.; Smoot, L. D. In Fundamentals of Coal Combustion for Clean and Efficient Use; Smoot, L. D., Ed.; Coal Science and Technology Vol. 20; Elsevier: Amsterdam, 1993; pp 433-509.
10.1021/ef020196j CCC: $25.00 © 2003 American Chemical Society Published on Web 05/23/2003
Nitrogen Release from Low Rank Coals
Energy & Fuels, Vol. 17, No. 4, 2003 941
Table 1. Ultimate and Proximate Analyses of Coals ultimate analysis wt %(daf) coal
code
Zalainuoer ZN Adaro AD
C
H
N
S
proximate analysis wt %(db) Ob
ash VM FC
68.7 4.7 1.7 0.3 24.6 4.0 44.1 51.9 68.8 4.7 1.3 0.1 25.1 1.4 46.7 51.9
inherent Ca wt%(db)
a
0.84 0.21
a Determined by the inductively coupled plasma method after acid leaching of high-temperature ash. b By difference.
Figure 2. A detailed sketch of a graphite reactor.
Figure 1. Schematic diagram of a drop tube reactor. added to the code AD to denote the demineralized coal. With Ca addition, AD-dem was first mixed with a saturated solution of Ca(OH)2 in a rotary evaporator to incorporate the Ca2+ ions as exchanged forms.15 Then, the resulting sample was dried under vacuum at 60 °C. Ca loading in the dried sample was 1.2 wt % as the metal. Pyrolysis. Pyrolysis runs were carried out with a graphitemade drop tube reactor, as shown in Figure 1. The reactor contained a graphite tube (495 mm × 25 φ) inside a quartz tube as the main body. The detailed sketch of the graphite tube is given Figure 2. A graphite filter with pore size of 100 µm was installed inside the tube. A thermocouple (Pt/Pt87%: Rh13%) was inserted at the bottom of the filter to determine pyrolysis temperature, and another (Pt/Pt87%:Rh13%) was contacted on the outside of the quartz tube to control the temperature. After evacuation of the whole system including the reactor and tar traps, high-purity He (> 99.9999%) flowed at 200 cm3(STP)/min, and the exit gas was analyzed to ensure that N2 concentration in the system was 15%. This might suggest that nitrogen functionality is not the important factor for the formation. It should be noted that a strong reverse correlation between N2 and char-N exists, irrespective of coal type, which points out that N2 originates mostly from char-N (and/or precursors). These observations agree with previous results in slow heating rate pyrolysis, in which N2 formation proceeds after almost all of the volatile matter is released.3,5 Figure 5 shows the changes in yields of tar-N, HCN, and NH3 between 0 and 120 s. When the time was increased from 0 to 120 s, NH3 yield increased but tar-N yield decreased for both coals, the trend for HCN being unclear as a result of the slight changes of
