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Energy & Fuels 1997, 11, 292-298
NO Reduction by Potassium-Containing Coal Briquettes. Effect of Mineral Matter Content and Coal Rank Avelina Garcı´a-Garcı´a,† Servando Chincho´n-Yepes,‡ Angel Linares-Solano,*,† and Concepcio´n Salinas-Martı´nez de Lecea† Departamento de Quı´mica Inorga´ nica, Facultad de Ciencias, and Departamento de Construcciones Arquitecto´ nicas, Escuela Polite´ cnica de Arquitectura, Universidad de Alicante, Alicante, Spain Received August 31, 1996. Revised Manuscript Received October 15, 1996X
This paper reports the NO reduction activity of several potassium-containing coal briquettes obtained from different coal precursors (ranging from anthracite to lignite) with the purpose of understanding the effect of coal rank. Also, two fractions of a bituminous coal, with two very distinct ash contents, were selected to determine the influence of the mineral matter content of coals in NO reduction. The catalytic effect of potassium in this reaction was evaluated in a fixedbed flow reactor at atmospheric pressure using two types of experiments: (i) temperature programmed reaction in a NO/He mixture; and (ii) isothermal reaction at 300-600 °C. The reaction products were monitored in both cases, thus allowing detailed oxygen and nitrogen balances to be determined. The effect of coal rank is manifested in two ways: larger amounts of potassium incorporated in coal briquettes and higher NO reduction activity as the coal rank precursor decreases, for all of the reaction temperatures tested. Ash contents of the coals (quartz, silicates) seem to have a negative effect in the NO-carbon reaction, acting as a sink that sinters and deactivates the potassium, forming potassium carbonate and silicates, both of them inactive for the process.
Introduction Because of stringent legislation, the removal of NOx from exhaust streams of various combustion sources has become increasingly important in recent years. Thus, as an example, EC countries must reduce significantly their NOx emissions from large combustion plants.1 The reduction of NOx can be carried out by means of suitable reducing gases such as NH3 (see, for example, the wellknown selective catalytic reduction, the technology for which is commercially available2,3), CO, H2, hydrocarbons, etc.4,5 However, other solid reductants could be used, as is the case of carbonaceous materials (coals, chars, activated carbons, etc.) under suitable operating conditions. Thus, carbons and activated carbons have been proposed as potential inexpensive reducing agents for NO reduction.6-11 The use of carbon for this purpose †
Departamento de Quı´mica Inorga´nica. Departamento de Construcciones Arquitecto´nicas. Abstract published in Advance ACS Abstracts, January 1, 1997. (1) Hjalmarsson, A. K. NOx Control Technologies for Coal Combustion; IEACR/24; International Energy Agency Coal Research: London, 1990. (2) Cho, S. M. Chem. Eng. Prog. 1994, January, 39. (3) Ju¨ntgen, H.; Ku¨hl, H. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Dekker: New York, 1989; Vol. 22, p 145. (4) Sato, S.; Yu-u, Y.; Yahiro, H.; Mizuno, N.; Iwamoto, M. Appl. Catal. 1991, 70, L1. (5) Hamada, H.; Kintaichi, Y.; Sasaki, M.; Ito, T.; Tabata, M. Appl. Catal. 1990, 64, L1. (6) Mochida, I.; Ogaki, M.; Fujitso, H.; Komatsubara, Y.; Isa, S. Fuel 1985, 64, 1054. (7) Teng, H.; Suuberg, E.; Calo, J. M. Energy Fuels 1992, 6, 398. (8) Suzuki, T.; Kyotani, T.; Tomita, A. Ind. Eng. Chem. Res. 1994, 78, L1. (9) Illa´n-Go´mez, M. J.; Salinas-Martı´nez de Lecea, C.; LinaresSolano, A.; Phillips, J.; Radovic, L. R. Abstracts of Papers, 211th National Meeting of the American Chemical Society; American Chemical Society: Washington, DC, 1996; Catl 002. ‡
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could present advantages over gaseous reactants used in conventional technologies, such as low cost, elimination of the environmental problematic “slip” of the gaseous reducing agent (e.g. ammonia),12,13 and high availability of coals. However, to reduce the reaction temperature of the NO-carbon reaction, catalysts have to be used, potassium being one of the most promising catalysts among all of those studied in the literature.14-26 (10) Illa´n-Go´mez, M. J.; Salinas-Martı´nez de Lecea, C.; LinaresSolano, A.; Phillips, J.; Radovic, L. R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41, 174. (11) Illa´n-Go´mez, M. J.; Radovic, L. R.; Linares-Solano, A.; SalinasMartı´nez de Lecea, C. Extended Abstract, International Carbon Conference, Carbon ’96; The British Carbon Group: Newcastle, U.K., 1996; p 703. (12) Bosch, H.; Janssen, F. Catal. Today 1988, 2, 369. (13) Guo, F.; Hecker, W. C. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41, 179. (14) Kapteijn, F.; Mierop, A. J. C.; Moulijn, J. A. Proceedings of the 17th Carbon Conference, Kentucky; 1985; p 181c. (15) Kapteijn, F.; Mierop, A. J. C.; Abbel, G.; Moulijn, J. A. J. Chem. Soc., Chem. Commun. 1984, 1084. (16) Okuhara, T.; Tanaka, K. J. J. Chem. Soc., Faraday Trans. 1986, 82, 3657. (17) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C.; Calo, J. M. Energy Fuels 1993, 7, 146. (18) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartı´nez de Lecea, C. Energy Fuels 1995, 9, 97. (19) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartı´nez de Lecea, C. Energy Fuels 1995, 9, 104. (20) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartı´nez de Lecea, C. Energy Fuels 1995, 9, 112. (21) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartı´nez de Lecea, C. Energy Fuels 1995, 9, 540. (22) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Energy Fuels 1995, 9, 976. (23) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartı´nez de Lecea, C. Energy Fuels 1996, 10, 158. (24) Garcı´a-Garcı´a, A.; Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Spain Pat. P9400104, 1994. (25) Garcı´a-Garcı´a, A.; Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Proceedings of the 8th International Conference on Coal Science; Pajares, Tascon, Eds.; Oviedo, Spain, 1995; p 1787.
© 1997 American Chemical Society
NO Reduction by K-Containing Coal
Since 1990, we have been dealing with the reduction of NO and more recently with the much more complex NOx reduction reaction by coals, chars, and activated carbons, paying attention, among other experimental variables, to the role of several metal catalysts on the NO reaction.9-11,17-26 The results of studies carried out with activated carbons17-23 show that a high activity can be reached with potassium-loaded activated carbons; however, with the purpose of avoiding the timeconsuming method involved in the preparation of activated carbons and making the process cheaper, other approaches using coal-based materials have also been studied.10,11,24-27 One of these materials consists of a composite prepared by mixing of coal and a binder agent (humic acid), which inherently contains potassium. The resulting artifacts, named briquettes, present the advantage of having both good mechanical properties and activity for NO reduction.24-27 In a previous paper,26 the NO reduction activity of potassium-containing coal briquettes was analyzed in terms of different potassium loadings using a bituminous coal as precursor, to obtain basic knowledge to investigate in a subsequent step the more complex NOx reduction in the presence of oxygen. Two aspects, not analyzed previously with these briquettes, are the object of the present paper: (i) the effect of the coal rank, which has to be important since the NO-carbon reaction is a gasification reaction; and (ii) the influence of the mineral matter content in the NO reduction activity of potassium, because its influence during this reaction has been hardly studied. Furthermore, possible chemical interactions between specific components of the mineral matter present in the raw coal with the potassium species added as catalyst, during the pyrolysis treatment or the NO reaction, should be explored because catalyst deactivation may occur. Experimental Section Four coals of different ranks [three Spanish and one Australian (LY)] have been selected as raw materials for the briquette preparation: an anthracite (UA1), a high-volatile A bituminous (A3), a high volatile C bituminous (P), and a lignite (LY). To study the effect of the mineral matter content, two fractions of coal A3 with very different ash contents (8% and 25%), denoted A3 and A3′, respectively, have been used. These fractions were obtained using two portions of the raw coal with different particle sizes (0.71 < L < 1.40 and L < 0.71 mm, respectively). To prepare the briquettes, raw coals were ground and sieved to a particle size of 0.1 < L < 0.2 mm. Commercial humic acid, from Jiloca Industrial S.A. (liquid with a density of 1.12 g/cm3 and a potassium content of 0.049 g/cm3), has been used as binder agent for briquette preparation. The method was previously described.24,26 In summary, 10 g of coal is impregnated with a variable binder volume, depending on the humic acid/coal ratio (HA/C) desired, mixed for 30 min with stirring, dried at 110 °C, pressed (1-2 kg/ cm2), and pyrolyzed in N2 for 2 h at 700 °C. Potassium content was determined after the pyrolysis step by ICP-AES. For this purpose, potassium was extracted from the samples by refluxing them in 1 M HCl for 8 h. The briquettes are designated by the parent coal used, followed by their potassium content (26) Garcı´a-Garcı´a, A.; Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Fuel, submitted for publication. (27) Garcı´a-Garcı´a, A.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41, 293.
Energy & Fuels, Vol. 11, No. 2, 1997 293 Table 1. Characteristics of Raw Coals and Briquettes Used coal UA1 A3 A3′ P LY
ash (%)
rank
HA/C pyrolysis ash briquette (mL/g) yield (%) (%)
7.0 anthracite UA1-1.3 7.7 bituminous hVA A3-1.8 A3-4.7 25.1 bituminous hVA A3′-3.6 bituminous hVC P-1.5 16.7 P-3.2 P-3.9 lignite LY-1.9 0.5 LY-6.0 LY-7.2
1.20 0.40 1.20 1.20 0.40 0.80 1.20 0.25 0.78 1.20
88.1 73.2 72.0
11.3 14.3 19.6 40.1 19.9
65.2 52.1 53.4 53.4
30.6 6.8 16.5 19.8
(in weight percent). Ash contents, both in coal precursors and briquettes, were determined by burning samples in a muffle furnace at 650 °C over a period of 12 h. Some of the briquette characteristics, along with those of the parent coals, are summarized in Table 1. After pyrolysis, a test was conducted to determine mechanical strength of the briquettes. All of the samples studied in this paper have satisfactory values in the impact strength test,26 independent of their HA/C ratios. The kinetics of the NO-carbon reaction was studied at atmospheric pressure in a fixed-bed flow reactor (15 mm i.d.; ca. 300 mg of sample) connected to a gas chromatograph (Hewlett-Packard, Model 5890 Series II). The reactant mixture used was 0.5% NO in He using a 60 mL/min flow rate, which resulted in a bed residence time of 0.56 s. NO, N2, N2O, CO2, and CO were analyzed using a Porapak Q 80/100 column and a thermal conductivity detector. Briquettes were ground to
