Ind. Eng. Chem. Res. 2007, 46, 3891-3903
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Advances in Potassium Catalyzed NOx Reduction by Carbon Materials: An Overview A. Bueno-Lo´ pez, A. Garcı´a-Garcı´a, M. J. Illa´ n-Go´ mez, A. Linares-Solano,* and C. Salinas-Martı´nez de Lecea Department of Inorganic Chemistry, UniVersity of Alicante, Spain, Apdo. 99, E-03080 Alicante, Spain
The research work conducted in our group concerning the study of the potassium-catalyzed NOx reduction by carbon materials is presented. The importance of the different variables affecting the NOx-carbon reactions is discussed, e.g. carbon porosity, coal rank, potassium loading, influence of the binder used, and effect of the gas composition. The catalyst loading is the main feature affecting the selectivity for NOx reduction against O2 combustion. The NOx reduction without important combustion in O2 occurs between 350 and 475 °C in the presence of the catalyst. The presence of H2O in the gas mixture enhances NOx reduction at low carbon conversions, but as the reaction proceeds, it decreases as the selectivity does. The presence of CO2 diminishes the activity and selectivity of the catalyst. SO2 completely inhibits the catalytic activity of potassium due to sulfate formation. 1. Introduction Energy is fundamental to our society, but, unfortunately, its dependence on fossil fuels contributes significantly to the current environmental problems, such as the greenhouse effect, air pollution, and water and soil contamination.1,2 We can reduce our dependence by reducing the consumption, but this would need deep changes in the current social models which are not expected to occur in the coming years. Additional options are improving energy efficiency, which means using less energy to accomplish the same task, and producing energy with cleaner (i.e., renewable) technologies. Unfortunately, both methods have important limitations; i.e., the available amount of renewable energy is shown to be insufficient to meet the increasing demand, and in the near future, we have to continue depending on fossil fuels. Hence, it is necessary to develop new technologies to use them cleanly and efficiently. The major sources of NO and NO2, generally termed NOx, are fossil fuel combustion in industry (stationary sources) and the transport sector (mobile sources). Thus, fossil fuel combustion processes contribute significantly to NOx emission. These oxides are major atmospheric pollutants, because they promote ground level ozone formation, photochemical “smog”, and acid rain and contribute to serious public health problems (e.g., respiratory and cardiovascular disease, asthma, chronic bronchitis, and decrease of lung functions). The strategies to avoid NOx emissions are different depending on the type of source. Thus, there are efficient postcombustion techniques available for stationary sources such as the selective catalytic reduction (SCR) by NH3.3-5 However, SCR is expensive and presents other disadvantages such as the reductant slip, the formation of ammonia salts that accelerate duct deterioration, and the potentially dangerous transport and handling of NH3. Carbon materials have also been proposed as potential reducing agents for NOx reduction under suitable operating conditions.6-8 The use of carbon for this purpose presents advantages over gaseous reactants, such as the simplicity of the process, lower cost, and elimination of the environmental problems related to reductant slip. * To whom correspondenceshouldbeaddressed.E-mail:
[email protected]. Tel.: +34 965 90 35 45. Fax: + 34 965 90 34 54.
It is well-known that alkali metals are suitable catalysts for carbon gasification by a number of reactive gases. Meijer et al. reported a series of papers dealing with potassium-catalyzed carbon gasification by H2O and CO2,9-13 and Moulijn et al.14 proposed a unified theory of reactions of carbon with oxygencontaining molecules including H2O, CO2, O2, N2O, and NO. In this work, we review the elimination of NOx in stationary sources (e.g., in carbon-combustion power plants and/or smaller stationary engines). It analyzes the advances made by our research group since the early nineties in the study of the potassium-catalyzed NOx reduction by carbon materials. We report an overview that covers the early results obtained with catalyst-free activated carbons through the present results obtained with potassium-containing coal char briquettes and pellets. Conformed materials are necessary for the application of this technology in real facilities, and a manufacturing process has been developed using a binder, a commercial humic acid solution, which inherently contains potassium. The effect of the different variables affecting the NOx-carbon reactions is discussed: carbon porosity, coal rank, potassium loading, influence of the binder used in the conformed material preparation, pyrolysis temperature, and effect of the gas composition (partial pressure of NOx and effect of O2, H2O, CO2, and SO2). 2. Experimental Section 2.1. Preparation. 2.1.1. Powdered Carbon Materials. Table 1 summarizes the activated carbon materials used. Activated carbons KUA10, NaUA1, KUA1, and CCV were prepared from coals by chemical activation with alkaline hydroxides, V74 and V9 from olive stones by steam activation, A and A19 by CO2 activation from phenol-formaldehyde polymer resin and almond shells, respectively. Details about the chemical and physical activation processes were reported elsewhere.15 B is an almondshell char, and F is a pitch-based carbon fiber. All these samples were granular-shaped with a particle size ranging from 0.7 to 2 mm, except for sample F, a pitch-based carbon fiber presenting a characteristic size of 30-50 µm, and samples KUA1, KUA10, and NaUA1, which were powders of less than 0.7 mm. Several samples were oxidized with HNO3 following the experimental procedure previously described16 and “(ox)” has been added to the name of the carbon. The carbon materials described were
10.1021/ie061005t CCC: $37.00 © 2007 American Chemical Society Published on Web 11/09/2006
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Table 1. Powdered Activated Carbon Samples carbon nomenclaturea NaUA1 KUA1 KUA1(ox) KUA1-2.8 KUA1-4.6 KUA1-7.4 KUA10(ox) KUA10 CCV V74 V9 A A(ox) A19 B B(ox) F
raw material anthracite anthracite anthracite anthracite anthracite anthracite high-volatile bituminous B coal high-volatile bituminous B coal lignite olive stones olive stones phenol-formaldehyde polymer phenol-formaldehyde polymer almond shell almond shell char almond shell char pitch-based carbon fiber
activation agent
SN2 (m2/gC)
SCO2 (m2/gC)
NaOH KOH KOH KOH KOH KOH KOH
1977 1790 1078 2102 1765 1749 1550
1740 1785 853 998 691 644 956
KOH
2078
1570
KOH H2O H2O CO2
1807 1744 734 723
1125 898 698 696
CO2
878
712
CO2
1298 457 547 ndb
864 567 527 507
a The term (ox) is used in HNO -oxidated samples. The - symbol means 3 No activated. b Nondetected.
Table 2. Powdered Coal Char Samples carbon nomenclature
raw material
SN2 (m2/gC)a
SCO2 (m2/gC)
1442-500 1442-700 1442-900 1548-500 1548-700 1547-500 1547-700 1546-500 1546-700 A3-700 A3w4.2-700
Lignite Lignite Lignite Lignite Lignite subbituminous subbituminous subbituminous subbituminous high-volatile bituminous A coal high-volatile bituminous A coalb
nd 175 267 nd 174 nd nd nd nd nd 1382
294 385 628 105 256 166 244 338 363 354 982
a
Here, nd stands for nondetected. b Potassium-loaded char.
used to analyze the effect of the carbon porosity on the NOcarbon reaction. Three potassium-loaded activated carbons have been used to analyze the effect of the catalyst in the NO-carbon reaction. KUA1-2.8, KUA1-4.6, and KUA1-7.4 were prepared by impregnation of the activated carbon KUA1 with increasing amounts of potassium acetate (2.8, 4.6, and 7.4 indicate the potassium loading in mass percentage, respectively). Details about preparation of these samples were reported elsewhere.17,18 Table 2 includes the char derived from raw coals used in this study. Two lignites (denoted by 1442 and 1548), two subbituminous coals (denoted by 1547 and 1546), and a hvA bituminous coal (denoted by A3) were used as precursors. Chars were prepared by pyrolysis of these raw coals under N2 (5 °C/ min, 80 mL/min) until 500, 700, or 900 °C. The nomenclature of the obtained chars contains the raw coal name along with the pyrolysis temperature, as included in Table 2. A3 was also loaded with potassium by soaking 10 mg of the powdered raw coal in 10 mL of KOH solution for 15 min. The KOH/A3 weight ratio used was 1/1 (KOH solution 18 M). The KOH-A3 slurry was dried at 110 °C in static air for 2 h, pyrolyzed at 700 °C, and washed with water at 40 °C for 30 min. This potassiumloaded A3 char is denoted as A3w4.2-700, 4.2 being the potassium weight percentage. 2.1.2. Briquettes and Pellets. Most briquettes and pellets were prepared with the hvA bituminous coal denoted by A3
(coal particle size 0.1-0.2 mm), otherwise indicated. The powdered coal was impregnated with a solution of KOH (ranging from 0 to 0.31 gKOH/gcoal) and a binder. Most briquettes and pellets were prepared with a commercial solution of humic acid as binder (1.2 mL/gcoal).19-22 This commercial solution is a liquid with a density of 1.12 g/cm3, which has a total humic extract of 16 wt % and a potassium content of 0.049 g/cm3. Coal/humic acid/potassium hydroxide slurries were stirred for 30 min, dried at 110 °C, and conformed in pellets (2 mm in diameter with an average length of 8 mm) or briquettes (32 mm in diameter and 15 mm height). Finally, the briquettes and pellets were pyrolyzed in N2 for 2 h at 500 or 700 °C. Briquettes containing a coal tar-formaldehyde resin (resole) binder were also prepared.23 The general procedure to prepare the resoles was reported elsewhere.24 To prepare the briquettes, approximately 10 wt % of the resole solution was used to wet the coal with sufficient KOH being used as the catalyst to give the required range of potassium contents in the final briquettes. These briquettes were pyrolyzed in N2 for 2 h at 500 or 700 °C. The mechanical strength values of all these briquettes are about 2500 KPa, similar to those of briquettes prepared with molasses, which is widely used as carbon binder. The target value for practical application of briquettes in real facilities has been proposed to be 350 KPa.24 For NOx reduction tests, briquettes were ground to a particle size between 0.2 and 1.2 mm in order to insert them into the reactor. It is important to note that similar results were obtained with ground briquettes and whole pellets of similar compositions, and therefore, most experiments presented in this study have been carried out with pellets, as they are more convenient for laboratory scale experiments. Briquettes and pellets are denoted by A3-Binder-K(%)Temperature, being • A3 the carbon precursor used; • Binder HA for humic acid and RES for resole; • K(%) the potassium loading percentage in brackets; • Temperature the pyrolysis temperature of briquette and pellet preparation. 2.2. Characterization. The apparent surface areas (calculated from Dubinin-Radushkevich (DR) and BET equations, respectively) were determined by physical adsorption of CO2 at 0 °C and N2 at -196 °C in an automatic volumetric system (Autosorb-6, Quantachrome). The potassium content of samples was determined by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). For this purpose, potassium was extracted from samples by refluxing them in 1 M HCl for 8 h in a Soxhlet apparatus. A Fourier transform infrared (FTIR) spectrophotometer, model Infinit MI60 from Mattson, with a diffuse reflectance accessory model COLLECTOR from Spectra Tech, was used for in situ monitoring of isothermal reactions. 2.3. NOx Reduction Tests. Two different experimental setups denoted by R-GC/CA and R-GA were used for NOx reduction tests, carried out at atmospheric pressure. R-GC/CA. This setup is formed by a cylindrical fixed bed reactor (13 mm of inner diameter) coupled to a gas chromatograph (GC) and a chemiluminiscence NOx analyzer (CA). The former is a Hewlett-Packard 5890 Series II Chromatograph, equipped with a switched dual columns system Porapak Q 80/ 100, for separation of CO2 and N2O, and molecular sieve 13X, for O2, N2, CO, and NO joined by a six-way valve with a restriction that avoids a pressure drop when the second column is bypassed. The CA is a Thermo Environmental Inc. model
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42 H analyzer. Typical experiments were carried out in this setup with a gas flow of 60 mL/min (0.02-0.5% NOx + 0 or 5% O2/He balance) and 0.3 g of sample. Under these experimental conditions, the residence time of the gas on the solid bed was 0.27 s. R-GA. This setup is formed by a cylindrical fixed bed reactor (10 or 23 mm of inner diameter, depending on the sample shape) coupled to three Fisher-Rosemount Non-Dispersive InfraRedUltraViolet (NDIR-UV) specific gas analyzers (GA) for NO and NO2 (BINOS 1004), CO and CO2 (BINOS 100), and O2 and SO2 (BINOS 1001). Typical experiments were carried out in this setup with a gas flow of 620 mL/min (0.2% of NOx + 0 or 4% CO2 + 0 or 20% H2O + 0 or 400 ppm SO2 + 5% O2 + N2 balance) and 0.1-1 g of sample. The residence time of the gas on the solid bed ranged from 0.075 to 0.17 s. Two types of NOx reduction tests were carried out: temperature programmed reactions (TPR) and isothermal reactions. TPR experiments consisted of heating the sample at 10 °C/min under the selected reactive mixture. The isothermal reactions consisted of heating the sample under inert gas flow (He in R-GC/CA and N2 in R-GA) and, once the reaction temperature is stabilized, the selected reactive mixture replaces the inert gas. Isothermal reactions were typically extended for 2 h or until total consumption of the carbon sample (lifetime test). Experimental details (setup used, gas composition, and flow, etc.) of each experiment are included in the text and caption of figures.
Figure 1. Relationship between the amounts of NO reduced (µmolNO/gC) over 2-h isothermal reactions at 600 °C and the total surface area as determined by N2 adsorption at -196 °C (m2/g). The experimental setup was as follows: R-CG/CA; (gas flow) 60 mL/min (0.5% NO in He).
3. Results and Discussion 3.1. NO Reduction. N2 and CO2 surface areas of the different powdered activated carbons have been included in Table 1, and those corresponding to nonactivated ones (coal chars), in Table 2. A wide range of values is observed. Activated carbons exhibit large N2 surface areas while coal chars usually present diffusion problems for this adsorbate at 77 K. Samples with SN2 equal or lower than SCO2 are mainly microporous as coal chars, B, B(ox), A, V9, and KUA1, while the others exhibit an important contribution of supermicroporosity (7-20 Å) and mesoporosity, as indicated by the large differences in their N2 and CO2 surface areas.25 Two-hour isothermal reactions at 600 °C were performed with the samples included in Table 1 under 0.5% NO in He using the R-GC/CA setup. The activity curves (not shown) for most of the activated carbons are qualitatively similar, exhibiting an activity decrease for approximately 40 min or less, after which a constant level, different for each sample, is achieved. The total amounts of NO reduced within the 2-h experiment have been determined and plotted in Figure 1 versus the total apparent surface areas of the samples, as obtained from N2 adsorption. The higher the total surface area, the higher the carbon reactivity toward NO reduction. Even the carbon fiber sample F, without N2 uptake but significant uptake of CO2, follows the trend and exhibits near zero reactivity. The HNO3-oxidized carbons also lie into the same straight tendency, indicating that the effect of the oxidation process is not as important as the effect of the surface area. The unique carbons located above the line are KUA1 and CCV. This high activity was attributed to the catalytic effect of the potassium species remaining in these carbons after the activation process and the subsequent washing step. The activities of these samples fall on the straight line after complete removal of the metal by acid wash. The catalytic effect of potassium is not surprising as alkali metals are wellknown catalysts of carbon gasification reactions.9
Figure 2. Temperature-programmed reaction profiles of NO reduction for a potassium-free (KUA1) and potassium-loaded (KUA1-K%) activated carbons. The experimental setup was as follows: R-CG/CA; (gas flow) 60 mL/min (0.5% NO in He).
These results point out that the reaction of carbon materials with NO, excluding any catalytic effects, seems to occur over the entire surface area of the carbon and all the available surface area of the carbon is effective for the reaction.25 Furthermore, these results imply that the considerable differences in pore size distribution of the carbons compiled in Figure 1 do not produce any significant effect on their activity toward NO reduction at 600 °C. In fact, all of our attempts to correlate their activity with their narrow micropore volume, or with the supermicropore volumes of the samples, failed. 3.1.1. Effect of Potassium. The potassium content effect is shown in the TPR runs of Figure 2, where the NO conversion curves of samples with different potassium loadings are plotted (samples KUA1, KUA1-2.8, KUA1-4.6, and KUA1-7.4). As mentioned above, these samples were prepared by impregnation of the activated carbon KUA1 with increasing amounts of potassium acetate and, once dried, samples were pyrolyzed under He at 900 °C in situ before the TPR experiments.17,18 The profile of the potassium-containing sample is quite different from that of the parent carbon (KUA1). The fundamental differences with respect to the parent carbon are the appearance of the conversion at low temperature and the significant decrease in the temperature at which 100% reduction is achieved. Figure 3 features the product evolution during TPR for KUA1-4.6, as an example of the general behavior. The analysis of reaction products showed that N2 and N2O are the only products observed during low-temperature NO conversion. CO and CO2 are not observed, and oxygen from NO is retained
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Figure 3. Evolution of products during temperature-programmed reaction with KUA1-4.6. The experimental setup was as follows: R-CG/CA; (gas flow) 60 mL/min (0.5% NO in He).
Figure 4. Correlations between NO reduction activity at 400 °C and catalyst loading. The experimental setup was as follows: R-CG/CA; (gas flow) 60 mL/min (0.5% NO in He).
by the potassium-carbon system. This stage involves irreversible chemisorption of NO, oxygen retention, and evolution of nitrogen-containing compounds. In the second stage, N2 evolves (N2O was not detected) together with CO2, which evolves from 250 °C, and CO evolves above 550 °C, indicating the carbon gasification by NO. The potassium content is seen to have an important effect in the first stage (chemisorption). In contrast, the reaction (T > 325 °C) is practically independent of the potassium content under the experimental conditions used in TPR. However, differences are appreciated in 2-h isothermal reactions. Figure 4 summarizes the specific activity, that is, the amount of NO reduced per second and gram of potassium at 400 °C. Note that a sample with 1.9% of potassium, prepared by ionic exchange procedure, has been included. Higher potassium dispersion is expected in this sample than in those prepared by impregnation of the parent carbon with different amounts of catalyst precursor. A clear trend is observed in Figure 4; the decrease in specific activity with increasing catalyst loading suggests a concomitant decrease in catalyst dispersion and highlights the importance of catalyst/substrate interfacial area in determining the concentration of catalytically active sites. 3.1.2. Coal Briquettes. As shown above, activated carbons show activity for NO reduction in gas flows and potassium is an active catalyst for such a reaction. To make the process more reliable from an industrial point of view, we next analyze the use of briquettes and pellets to improve carbon handling. A novel manufacturing process was developed21 to prepare
Figure 5. NO reduction activity at 600 °C versus potassium content for activated carbons and coal briquettes. The experimental setup was as follows: R-CG/CA; (gas flow) 60 mL/min (0.5% NO in He).
briquettes and pellets with a suitable shape for practical application. The process has the advantage of using a binder agent (commercial solution of humic acid) which inherently contains potassium. Briquettes and pellets were prepared from coals of different rank, since coals are cheaper raw carbon materials than activated carbon, therefore being more suitable candidates to be consumed in large facilities. Figure 5 compiles the amounts of NO reduced, after 2-h isothermal reactions at 600 °C, by briquettes of a high volatile C bituminous coal and those corresponding to different KOHactivated carbons. Briquettes were ground to resole binder > humic acid binder This sequence indicates that binders decrease the reactivity of samples. In accordance with the results obtained with powdered carbon materials, the activity is larger for the 500 °C-pyrolyzed sample than for 700 °C-pyrolyzed one, if samples with similar binders are compared. However, the average selectivity during these tests, included in Table 7, provides values of about 45% for most of the samples, suggesting that the main variable affecting the selectivity is the potassium loading. The only sample that shows a higher selectivity (58%) is A3-HA-K(16.8)-700; this sample exhibits the best NOx
Figure 14. Effect of H2O and CO2 in NOx reduction during lifetime tests at (a) 350 and (b) 450 °C: (symbols) experimental data ((solid) NOx reduction; (open) sample weight); (lines) continuous for model fitting and dashed to guide the eyes. The experimental setup was as follows: R-GA; (gas flow) 620 mL/min; (sample) A3-HA-K(16.8)-700.
reduction capacity among the samples tested. These data indicated that although the activity of the samples is affected by the binder and also by the pyrolysis temperature, the potassium loading mainly controls the selectivity toward NOx reduction. 3.2.6. Effect of H2O. The presence of H2O in the gas mixture is analyzed in Figure 14 which plots the NOx reduction and the sample weight profiles during lifetime tests performed. These runs have been obtained under gas mixtures of different composition with pellets A3-HA-K(16.8)-700. Table 8 compiles the average selectivity and the amounts of NOx reduced during these experiments along with results of similar experiments performed with the coal char A3-700. As a general trend, NOx reduction and selectivity decrease in the presence of H2O. At 450 °C, the selectivity of pellets for NOx reduction with regard to O2 combustion was 31 and 18% under NOx/O2/N2 and NOx/ H2O/O2/N2, respectively. However, results obtained with pellets are better than those obtained with the potassium-free char, whose selectivity was only 3% at 450 °C. In addition, the pellets
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Table 8. Effect of H2O and CO2 on NOx Reductiona sample A3-700
A3-HA-K(16.8)-700
a
reactive mixture (temp °C)
Saverage (%)
gNOx/gsample
gNOx/gash-free_sample
NOx/O2/N2 (450) NOx/H2O/O2/N2 (450) NOx/CO2/O2/N2 (450) NOx/CO2/H2O/O2/N2 (450) NOx/O2/N2 (450) NOx/H2O/O2/N2 (450) NOx/CO2/O2/N2 (450) NOx/CO2/H2O/O2/N2 (450) NOx/O2/N2 (350) NOx/H2O/O2/N2 (350)
3 3 3
