Low-Cost Potassium-Containing Char Briquettes for NOx Reduction

Energy & Fuels 2002, 16, 997-1003

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Low-Cost Potassium-Containing Char Briquettes for NOx Reduction A. Bueno-Lo´pez,† A. Garcı´a-Garcı´a,† C. Salinas-Martinez de Lecea,*,† C. McRae,‡ and C. E. Snape§ Department of Inorganic Chemistry, University of Alicante, Spain, Department of Pure & Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, U.K., and School of Chemical, Environmental and Mining Engineering, University of Nottingham, University Park, NG7 2RD U.K. Received February 13, 2002

The activity of potassium-containing char briquettes, for NOx reduction, has been investigated at 350 °C, using a mixture of 0.2%NO/5%O2/N2. The samples have been prepared from a Spanish bituminous coal and a coal tar-formaldehyde resin (resole) as binder. The influence of a number of variables relating to the preparation process, such potassium loading and pyrolysis temperature, on their reduction capacity has been analyzed. For comparative purposes, a set of binder-free samples with different potassium contents has been prepared. Potassium contents highly affect the selectivity of the briquettes. Samples prepared with high potassium loading exhibit satisfactory selectivity values with very low burnoff. Lifetime tests, until complete consumption of the sample, have been performed with the samples exhibiting high selectivity values. The most important finding is that high selectivity values are maintained during the whole life of the briquette. The binder affects the reaction kinetic and the lifetime of the sample. A binder-free sample presents higher activity with regard to binder samples.

Introduction NOx removal from high-temperature combustion sources has attracted increasing attention due to mounting environmental concerns,1 and for many years NOx control measures have been developed and implemented. However, soon we will have to further reduce the NOx emissions both from coal combustion and from automobile exhausts.2 NOx reduction can be achieved catalytically with ammonia for which technology is commercially available3 and also with hydrocarbons and hydrogen,4 among other reducing agents. However, carbonaceous materials (coals, chars, and active carbons) are also effective under suitable operating conditions.5,6 The use of carbon for this purpose presents advantages over gaseous reactants including the simplicity of the process, potentially low cost, and elimination of the environmental problematic “slip” of the gaseous reducing agent.7 * Author to whom correspondence should be addressed. † University of Alicante. ‡ University of Strathclyde. § University of Nottingham. (1) Yamashita, H.; Tomita, A.; Yamada, H.; Kyotani, T.; Radovic, L. R. Energy Fuels 1993, 7, 85. (2) Tomita, A. Fuel Process. Technol. 2001, 71, 53. (3) Juntgen, H.; Kuhl, H. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Dekker: New York, 1989; Vol 22, p 145. (4) Paˆrvulescu, V. I.; Grange P.; Delmon, B. Catal. Today 1998, 46, 233. (5) Garcı´a-Garcı´a, A.; Chincho´n-Yepes, S.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Energy Fuels 1997, 11, 292. (6) Illa´n Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartı´nez de Lecea, C. Energy Fuels 1996, 10, 158. (7) Bosch, H.; Janssen, F. Catal. Today 1987, 2, 369.

To reduce the temperature of the NOx-carbon reaction and to minimize the undesired carbon combustion by the abundant oxygen present in the exhaust gases,1,8 a number of catalysts have been explored. Among them, potassium has been demonstrated to be very effective.1,8-12 In previous work, NOx reduction was achieved to a considerable extent by using potassium-containing coal briquettes prepared with humic acid as binder agent,13-15 which inherently contains the catalyst (potassium). The alkali was also added in a combined manner (adding humic acid and KOH). Extremely promising results with respect to activity and selectivity were obtained for NOx reduction using this type of sample, mainly at high potassium contents. The potassium-containing coal briquettes improved the performance of the original char (without potassium) in the following aspects: (i) (8) Yamashita, H.; Yamada, H.; Tomita, A. Appl. Catal. 1989, 78, L1. (9) Inui, T.; Otowa, T.; Takegami, Y. Ind. Eng. Chem. Prod. Res. Rew. 1982, 21, 56. (10) Illa´n Go´mez, M. J.; Raymundo-Pin˜ero, E.; Garcı´a-Garcı´a, A.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Appl. Catal. B 1999, 20, 267. (11) Illa´n Go´mez, M. J.; Salinas Martı´nez de Lecea, C.; Linares Solano, A.; Radovic, L. R. Energy Fuels 1998, 12, 1256. (12) Kapteijn, F.; Mierop, A. J. C.; Abbel, G.; Moulijn, J. A. J. Chem. Soc. Commun. 1984, 1085. (13) Garcı´a-Garcı´a, A.; Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Energy Fuels 1999, 13, 499. (14) Garcı´a-Garcı´a, A.; Illa´n Go´mez, M. J.; Linares Solano, A.; Salinas Martı´nez de Lecea, C. SECAT 97; p 141, 1997, Jaca (Espan˜a). (15) Garcı´a-Garcı´a, A.; Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Energy Fuels 2002, 16, 569.

10.1021/ef0200121 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/04/2002

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increasing the NOx conversion, (ii) decreasing the CO/ CO2 ratio in the reaction products, and (iii) enhancing the selectivity toward NOx reduction against oxygen combustion by oxygen.14,15 However, the search of inexpensive binders is desirable to lower the price of the briquettes for the successful economical exploitation of the process. In this sense, the utilization of the structural attributes of the lowtemperature coal tar produced in processes such as that operated by Coalite could be a possible solution and on the other hand, it could help establish new markets for coal tar-based products as binders.16 It has already been demonstrated that the whole tar acid fraction from low-temperature tar produced by the Coalite process is very effective as feedstock to produce phenolic resoles that cure with acid at ambient temperature.16 The resole was prepared by the condensation of the coal tar acid and formaldehyde with alkali catalyst. Phenol-formaldehyde resins are known to be effective binders to produce briquettes with good thermal and mechanical properties that cold cure (other convenient polymers for low-temperature curing include poly(vinyl alcohol) and poly(vinyl acetate)),17,18 but all of these present the drawback of their relatively high cost. Using the whole tar acid fraction as opposed to pure phenol for producing phenolic resoles, potentially either reduces the cost of the resulting product to a considerable extent or enables more binder to be used improving product specification. The present work describes the development of lowcost potassium-containing char briquettes from a bituminous coal and a coal tar acid fraction for the NOxcarbon reduction reaction. The influence of a number of variables related to the preparation process, such catalyst loading and carbonization temperature, on their reduction capacity has been analyzed. These results have been compared with those for mechanically weak samples prepared without any binder. Experimental Section Sample Preparation and Characterization. The briquettes have been prepared using a Spanish high volatile A bituminous coal designated A3 (7.7 wt % ash content) and coal tar-formaldehyde resin (resole) as binder. The general procedure to prepare the resoles has been described previously.16 The formaldehyde to tar acid mole ratio was kept constant (1.5:1.0) in all the resoles prepared. To prepare the briquettes, approximately 10% w/w of the resole solution was used to wet the coal with sufficient KOH being used as catalyst to give the required range of K contents in the final briquettes. The mixture was compressed in a laboratory scale hydraulic press for around 10 s and a pressure of around 14 MPa. All briquettes were carbonized green except one which was prepared under base curing conditions with a proprietary curing agent present (designated A3-16.1-BTA-500). The individual briquettes were carbonized to 500 and 700 °C in a quartz sintered tube with a heating rate of approximately 5 °C min-1 and under a stream of nitrogen. The nomenclature of the samples includes the name of the original coal, A3, the potassium content (in weight %), the letters TA, if the sample has been prepared with a tar acid resole under thermal curing, and the carbonization temperature. (16) Thoms, L. J.; Snape, C. E.; Taylor, D. Fuel 1999, 78, 1691. (17) Coal Industry (Patents) Ltd., Eur. Pat. 1988, 0284252 A1. (18) Yorkshire Dyeware (Patents) Ltd., GB Pat. 1962, 1031723.

Bueno-Lo´ pez et al. Table 1. Nomenclature, Preparation Conditions, and Characterization of the Samples nomenclature A3 (original coal) A3-500 A3-2.8-TA-500 A3-6.1-TA-500 A3-16.5-TA-500 A3-16.1-BTA-500 A3-3.4-TA-700 A3-5.0-TA-700 A3-20.4-TA-700 A3-2.7-500 A3-5.7-500 A3-18.3-500 a

% K % N % C % H % S ash (% dry basis) a 1.7 a 2.4 2.8 2.0 6.1 16.5 1.3 16.1 1.3 3.4 1.5 5.0 1.6 20.4 1.3 2.7 2.0 5.7 1.8 18.3 1.2

82.1 1.2 0.7 75.5 2.5 0.1 71.8 2.6 1.2 50.1 2.3 0.0 49.7 1.9 0.1 74.7 1.1 0.8 73.1 1.1 1.3 51.6 1.2 0.7 71.5 2.6 0.7 66.6 2.2 0.6 52.4 1.7 0.5

7.7 10.5 15.9 16.6 36.7 34.5 17.4 18.6 40.2 16.2 21.4 36.3

Negligible.

A set of binder-free samples have been also prepared by impregnating the coal with a KOH solution with the desired amount of catalyst dissolved in the minimum volume of water, (the amounts employed have been 0.04, 0.10, and 0.29 g KOH/g coal). The resulting briquettes were carbonized at 500 °C, presenting very poor mechanical resistance when compared with the binder samples. Table 1 lists the nomenclature of all the samples. The amount of potassium in the briquettes studied was determined by extraction with 1 M solution of HCl in a Soxhlet reflux system and analyzing the resulting solution by ICPAES (induced coupled plasma atomic emission spectroscopy). The ash contents of all the briquettes were determined by burning the samples at 650 °C in a muffle furnace for 12 h. A Carlo Erba Instruments model EA 1108-elemental analyzer was employed to determine C, N, S, and H contents of the samples. The textural characteristics of selected samples were determined by physical adsorption of CO2 (at 273 K) and N2 (at 77 K) in an automatic volumetric system Autosorb-6, Quantachrome. Mercury porosimetry was conducted in some selected samples. The identification of the different potassium species in selected samples was carried out by X-ray diffraction (2002 Seifert powder diffractometer, using a Cu-R radiation with graphite monochromator and Na (Tl) scintillation detector, 35 mA and 42 kV). The scanning rate was 2°/min for 2θ from 6° to 90°. The surface potassium/carbon ratio (expressed in wt %) in selected samples was determined by XPS. The spectra were obtained with a VG-Microtech Multilabel electron spectrometer, by using the Mg KR (1253.6 eV) radiation of twin anode in the constant analyzer energy mode with pass energy of 50 eV. Pressure of the analysis chamber was maintained at 5 × 10-10 mB. The binding energy and the Auger kinetic energy scale were regulated by setting the C1s transition at 284.6 eV. The accuracy of BE and KE values was (0.2 and (0.3 eV, respectively. The BE and KE values were obtained by using the Peak-fit Program implemented in the control software of the spectrometer. Finally, surface oxygen groups of selected samples were evaluated by TPD (temperature-programmed desorption) experiments in inert atmosphere (N2-620 mL/min) up to 870 °C, monitoring the evolution of CO and CO2, directly related to the surface chemistry of the carbon. These analyses were performed using the BINOS analyzers described in the next section. NOx-Carbon Reaction Study. The NOx reduction tests have been carried out at 350 °C and atmospheric pressure in a tubular quartz reactor. 0.5 g of ground briquette, (particle size between 0.2 and 1.2 mm), and a gas mixture (620 mL/ min) which contains 0.2% NO + 5% O2/N2 were used. The reactor is coupled to NDIR-UV specific gas analyzers for NO, NO2, CO, CO2, and O2 (models BINOS 1004, 100, and 1001,

K-Containing Char Briquettes for NOx Reduction

Energy & Fuels, Vol. 16, No. 4, 2002 999

respectively). The samples were previously heated in nitrogen until the reaction temperature was reached and then the reaction mixture replaced the inert gas. Most of the catalytic tests were conducted for 2 h. However, the lifetime tests were performed on selected samples by extending the reaction until the sample was completely consumed (only leaving the ash residue). The isothermal reactivity results have been expressed as: the amount of NOx reduced after 2 h of experiment (NOx red), the burnoff percentage, integrated value based on carbon loss, as CO and CO2, over a 2 h period (% BO), and the selectivity factor, F, which has been used to determine the extent of NOx reduction against oxygen combustion.15 This value is calculated according to the expression

F ) (µmol NOred)/(2 µmol CO2 + µmol CO)

(1)

From the data obtained from lifetime tests, the average selectivity value has been estimated according to the mathematical expression

Faverage ) ((gNOred/PmNO)/2*(1 - (%ash/100))/PatC) (2) Both factors show values in the range of 0-1, a selectivity factor of 1 means that the carbon consumption is due to the NOx reduction, while a factor of 0 means that the carbon is consumed only by combustion with oxygen. Finally, oxygen balances for selected samples, for the 2 h reactions period, have been performed with an experimental device consisting of a fixed bed reactor coupled to a gas chromatograph Hewlett-Packard 6890 Series II, equipped with a switched dual columns system (Porapak Q 80/100, for separation of CO2 and N2O, and Molecular Sieve 13X, for O2, N2, and CO) joined by a six-way valve with a restriction that avoids a pressure drop when the second column is by-passed. A chemiluminiscence NOx analyzer (Signal model 4000 VM) has also used for NO and NO2 determination. A flow rate of 100 mL/min and 0.3 g of sample was employed in this experimental setup. (Using this experimental system it has been confirmed that the evolution of N2O as a reaction product is negligible, with nitrogen being the main reduction product).

Results and Discussion Sample Characterization. Table 1 lists the set of samples along with their elemental compositions (N, C, H, and S), potassium, and ash contents. The potassium contents increase in proportion to the amount of KOH used in the preparation of the binder-free samples. The potassium amounts of the briquettes prepared with the tar acid resole were as expected, considering the values obtained by the series of briquettes prepared without binder. The effect of carbonization temperature is again as expected, higher potassium contents being obtained at higher pyrolysis temperatures, the ash content also increasing gradually with the potassium content. NOx-Carbon Reaction. Activity. Figure 1 presents the NOx and O2 reduction capabilities (at 350 °C) of the briquettes prepared with the tar acid resole carbonized at 500 °C. The sample with the lowest potassium content (A32.8-TA-500; Figure 1a) gives rise to an uncontrolled increase of the temperature at the beginning of the reaction, of nearly 100 °C. The desired reaction temperature is stabilized after 45 min of reaction. The briquettes with higher potassium contents (6.1 and 16.5), (Figure 1b,c), do not exhibit this behavior, only the temperature for sample A3-6.1-TA-500 was found to slightly increase (10 °C) when the reactive gases were

Figure 1. NOx and O2 reduction profiles for samples with different potassium contents (a) A3-2.8-TA-500, (b) A3-6.1-TA500 and (c) A3-16.5-TA-500.

introduced into the reactor, and then remaining at this temperature during the rest of the experiment. The uncontrolled increase in temperature is clearly associated with high O2 and NOx conversions, as a consequence of the exothermicity of the O2 combustion. As the potassium percentage is increased in samples, the NOx conversion is clearly favored against oxygen combustion.15 Constant values of NOx reduction (throughout the 2 h of experiment) are observed for the briquettes with the high potassium contents. Table 2 summarizes the activity data. The amount of NOx reduced, the percentage of carbon lost during the tests (% BO), the selectivity factor (F), which determines the extent of NOx reduction against oxygen combustion, and the CO/CO2 ratio (estimated as the integrated values emitted over the 2 h reaction) are included. Data concerning the original coal (A3) carbonized at 500 °C have also been collected for comparative purposes. Concerning the effect of potassium, the data reveal that the samples with the lowest potassium contents, independent of the method of preparation, give uncon-

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Table 2. Kinetic Data for NOx Reduction at 350 °C (2 h reaction period) sample A3-500 A3-2.8-TA-500 A3-6.1-TA-500 A3-16.5-TA-500 A3-16.1-BTA-500 A3-3.4-TA-700 A3-5.0-TA-700 A3-20.4-TA-700 A3-2.7-500 A3-5.7-500 A3-18.3-500 b

NOx reduced (µmol/gsample) %BO 1496a 2572a 2001b 1529 1344 2565a 1197 1052 1978a 2677a 2464

36.8 84.2 24.0 2.4 2.5 100 8.1 1.6 54.4 83.3 4.8

F

CO/CO2

0.03 0.02 0.05 0.39 0.38 0.01 0.09 0.44 0.02 0.02 0.31

0.71 0.21 0.11