Release of Nitrogen Species during Rapid Pyrolysis of Model Coals

Article pubs.acs.org/EF

Release of Nitrogen Species during Rapid Pyrolysis of Model Coals Lei Deng, Xi Jin, Yu Zhang, and Defu Che* State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China S Supporting Information *

ABSTRACT: To gain an in-depth insight into the release of nitrogen species during rapid pyrolysis of coals, two model coals containing pyrrolic or pyridinic nitrogen are synthesized, and rapid pyrolysis of model coals is performed in an entrained flow system between 800 and 1300 °C. The effects of pyrolysis temperature, minerals, particle size, and the interaction between the two forms of nitrogen are discussed. The results show that a moderate amount of HCN and small amounts of NO and NO2 rather than NH3 are formed. As the temperature rises, the HCN release increases first and then decreases, and exhibits very similar trends for the two forms of nitrogen. Because of the better thermal stability of pyridinic nitrogen, the releases of both HCN and NOx are lower for pyridinic nitrogen than for pyrrolic nitrogen. When the two forms of nitrogen are pyrolyzed together, the interaction between the nitrogen species released makes the HCN release decrease and approach the HCN release from pyridinic nitrogen alone. For pyrrolic nitrogen, Fe addition suppresses the HCN release at all temperatures and Na addition promotes the HCN release obviously at 1000 °C or above, whereas Ca addition increases the HCN release with increasing temperature first and then decreases. For pyridinic nitrogen, all the metal additions suppress the HCN release and Fe has the strongest catalytic effect. As the catalyst content increases, the HCN release decreases drastically. The HCN release from pyridinic nitrogen increases slightly with increasing particle size at 800 or 1000 °C. When the temperature achieves 1200 or 1300 °C, the particle size dependence of HCN release is not observed.

1. INTRODUCTION In China, coal accounts for the main portion of energy consumption and combustion is still the main coal utilization approach.1 However, emission of nitrogen oxides (NOx and N2O) from coal combustion can cause serious environmental problems. NOx (NO and NO2) is a noxious gas that contributes to the formation of acid rain and photochemical smog, whereas N2O is a greenhouse gas that is implicated indirectly in the depletion of the ozone layer.2−4 As well-known to all, coal pyrolysis occurs at the primary stage of coal combustion, and the release of fuel nitrogen during pyrolysis is the prerequisite and foundation for nitrogen oxide formation during the subsequent conversion process. The nitrogen oxide emission limit becomes more and more stringent in China.5 Therefore, an increasing importance should be attached to the fate of fuel nitrogen during coal pyrolysis by the scientists. During coal pyrolysis, a part of the fuel nitrogen escapes as volatile-N in gas phase, such as tar-N, N2, HCN, and NH3, and the other part retains as char-N in the residual char.6−9 Numerous studies10−21 have identified HCN and NH3 as two major precursors of nitrogen oxides during coal combustion. However, formation mechanisms of HCN and NH3 during the pyrolysis of coal have been debated in the literature.9 Kambara et al.22,23 believed that HCN and NH3 were formed simultaneously, and they originated from different nitrogen functional groups. Many researchers24−28 have challenged the direct correlation between the release of fuel nitrogen and fuel nitrogen functionality during pyrolysis and combustion. It is acknowledged that the formation of NH3 requires the hydrogenation of fuel nitrogen.9 A number of hypotheses,6,29−31 which have a common assumption that HCN is formed first and NH3 is produced from the hydrogenation of © 2012 American Chemical Society

HCN, have been proposed. To verify or disprove these hypotheses, Tan and Li17 pyrolyzed a Victorian brown coal in a specially designed drop-tube/fixed-bed reactor. The results showed that HCN was not converted to NH3, but was converted into soot-N or N2 during its interactions with char. In subsequent investigations, Li and Tan18 proposed a new theoretical framework about the formation of HCN and NH3 during pyrolysis of coal at intermediate temperature levels. They pointed out that the NH3 formation from the reactions of N-containing heteroaromatic ring systems in the gas phase was difficult. Direct hydrogenation of the N-sites by the active hydrogen generated in situ in the pyrolyzing solid was the main source of NH3. Conceivably, a good understanding of the evolution mechanism of fuel nitrogen during coal pyrolysis would be of great help to minimize the emission of nitrogen oxides. Much work has been devoted to the investigation of the release of nitrogen species from fuel nitrogen during pyrolysis of various coals.3,10−21,32−34 However, there are some limitations on the relevant studies, because of the complexity of the coal structure. The influences of coal type, minerals, and pyrolysis products on nitrogen species emission are significant when coal is taken for research. To avoid these limitations, many researchers have taken model compounds as the research object to study the release of nitrogen species.12,35−54 The model compound retains the main structure and property of the aromatic heterocyclic ring, simplifies the coal macromolecule, has the definite chemical composition and nitrogen Received: August 27, 2012 Revised: December 14, 2012 Published: December 18, 2012 430

dx.doi.org/10.1021/ef3014053 | Energy Fuels 2013, 27, 430−439

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Figure 1. N 1s spectra of the two model coals.

extend the experimental database to develop and validate formation mechanisms of nitrogen species during pyrolysis. In this paper, two model coals containing pyrrolic or pyridinic nitrogen are synthesized in an autoclave by the compaction heat treatment of cellulose and phenolic nitrogen compounds (4-hydroxycarbazole or 8-hydroxyquinoline). An entrained flow pyrolysis system is built up to investigate the release of nitrogen species from fuel nitrogen of different forms during rapid pyrolysis of model coals in a wide temperature range. The effects of pyrolysis temperature, minerals, particle size, and the possible interaction between the two nitrogen functional groups, are examined in detail.

functional group, and makes the results of catalysis studies more precise. Thus, the model compound has obvious advantages over coal when the mechanism research is conducted.35−37 There are some potential limitations of using a model compound to mimic the complex heterogeneous structure and composition of coal. Because of the simplification of the coal macromolecule, the influence of some neighboring free radicals and functional groups on the nitrogen functional group in the coal sample may not be reflected during pyrolysis of the model compound. Research on the mechanisms of nitrogen release from coal employs three types of model compounds: (1) heterocyclic compounds, such as pyrrole, pyridine, quinoline, and carbazole; (2) polymers containing pyrrolic or pyridinic nitrogen, such as polyvinylcarbazole, polyvinylpyrrolidone, and polyvinylpyridine; and (3) model coals prepared from cellulose and phenolic nitrogen compounds by compaction heat treatment. Compared with heterocyclic compounds and polymers, model coals are closer to coals with regard to physical properties and chemical structures.53 Thus, model coals are the most appropriate model compounds that can simulate the conversion of fuel nitrogen during coal pyrolysis. Much attention has been paid to the evolution mechanism of fuel nitrogen during pyrolysis of heterocyclic compounds35−45 or polymers.12,46−52 However, there are few studies53,54 on nitrogen release during pyrolysis of model coals. In the studies of Wu et al.,53,54 the release of HCN, NH3, and N2 was discussed during temperatureprogrammed pyrolysis of model coals in a fixed bed reactor and the maximum temperature was 1000 °C. Whereas in pulverized coal boilers, coal pyrolysis occurs rapidly in the form of entrained flow at higher temperatures. Therefore, a comprehensive study on the release of nitrogen species during rapid pyrolysis of model coals under the conditions similar to those in pulverized coal boilers is essential to developing practical and efficient control technologies for nitrogen oxides. In addition, many investigators have examined the emission of nitrogen species during pyrolysis of pyrrolic or pyridinic nitrogen in prior research with regard to model compounds. The two nitrogen functional groups, which are the most abundant nitrogen forms in coals, may interact during pyrolysis. Unfortunately, the effect of interaction between the two nitrogen functional groups on nitrogen release has seldom been studied. Model compounds simplify the chemical environment of HCN and NH3 formation. The relationship between formation of these precursors and nitrogen functional groups was established. However, there still exist differences among the results of pyrolysis of model compounds, especially for NH3 formation.35−38,40−44,46,53 Hence, it is necessary to

2. EXPERIMENTAL SECTION 2.1. Model Coal Synthesization and Sample Preparation. Two model coals containing nitrogen are synthesized from cellulose and nitrogen compounds by compaction heat treatment. Cellulose is mixed with 4-hydroxycarbazole or 8-hydroxyquinoline (Tianjin Fuchen Chemical Regent Factory, analytical reagent grade, 98%) in a weight ratio of 4:1. The mixture is, first, heated from room temperature to 200 °C in an autoclave, in a helium atmosphere, at a heating rate of 0.5 °C min−1. The pressure in the autoclave is then raised to 20 MPa by a manual pump, and the sample is heated at 200 °C for 24 h. The compaction heat treatment is conducted to simulate the natural coalification.53 After being cooled down to room temperature, the solid materials are recovered and ground, and then the fine particles are submerged in tetrahydrofuran and stirred thoroughly to remove the unreacted nitrogen-containing reactant. Finally, two model coals, which have similar physical properties to coals, are recovered by filtration and arefaction. To meet the experimental requirements, the model coal samples are ground and sieved, then air-dried at 105 °C for 24 h and placed in a desiccator. In most cases, samples are sieved to 63−98 μm. In the other cases, the particle sizes of samples selected for the pyrolysis experiments include 50−63, 98−125, and 125−150 μm. Nitrogen forms in the model coals are determined through X-ray photoelectron spectroscopy (XPS) analysis. The N 1s spectra of model coals are obtained on an AXIS ULTRA X-ray photoelectron spectrometer (Kratos), as shown in Figure 1. According to the binding energies, the peaks in the N 1s spectra can be assigned to the corresponding nitrogen forms. It has been observed that most of the fuel nitrogen was present in pyrrolic and pyridinic forms, with XPS peaks at binding energies of 400.5 ± 0.3 and 398.7 ± 0.4 eV, respectively.48,49,55−58 It can be seen that only one peak appears at 400.7 eV in Figure 1a or appears at 398.4 eV in Figure 1b. Thus, nitrogen forms are totally pyrrolic and pyridinic nitrogen in the two model coals, respectively.55,59,60 The N 1s spectra of the two model coals also reveal that the compaction heat treatment does not change the nitrogen forms in 4-hydroxycarbazole and 8-hydroxyquinoline. The model coal sample containing pyrrolic nitrogen is denoted as M5, and the other one is denoted as M6 hereinafter. 431


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The proximate and ultimate analyses of M5 and M6 are listed in Table 1. Comparison of physical properties, chemical compositions,

total height of 720 mm. The reactor can operate at a temperature up to 1400 °C with a constant heating rate of 40 °C min−1. Before the pyrolysis experiments start, the electric heating furnace is heated to a preset temperature, a sample of 0.1 g is loaded in the microfeeder in advance, and the whole reaction and analysis system is in an argon atmosphere. The argon gas is split into two streams. One stream (800 mL min−1) enters the microfeeder to entrain the sample into the feeding tube with a feeding rate of 0.034 g min−1, and the other stream (200 mL min−1) enters the feeding tube directly. As shown in Figure 2, the exiting tube is fixed and the sample tube can be pushed forward by a stepping motor to keep a given distance between the surface of the sample bulk and the bottom of the exiting tube so that a constant feeding rate can be maintained. The preset temperatures of the electric heating furnace under different experimental conditions are from 800 to 1300 °C. The residence times of the model coal samples under different experimental conditions are approximately from 6.0 to 8.8 s (see section 2 in the Supporting Information). The concentrations of HCN, NH3, NO, and NO2 from the pyrolysis of model coal samples are detected downstream from the exiting tube by a GASMETTM DX4000 Fourier transform infrared (FTIR) gas analyzer (Temet Instruments Oy, Helsinki, Finland). The concentrations are expressed in volumetric fraction, and the gas profiles are recorded with a personal computer versus time. The estimated uncertainty limits of the FTIR concentrations are ±2%. All the pyrolysis experiments are performed twice for repeatability, and the discrepancies of HCN, NO, and NO2 measurements determined by repeated runs are within ±5%. 2.3. Data Analysis. To analyze and compare the conversion of fuel nitrogen to nitrogen species, the conversion ratios of fuel nitrogen to HCN, NO, NO2, and NOx during pyrolysis of model coal samples, XHCN−N, XNO−N, XNO2−N, and XNOx−N (%), are defined as

Table 1. Proximate and Ultimate Analyses of the Two Model Coals proximate analysis (wt %, dry)

ultimate analysis (wt %, dry)

sample

volatile matter

fixed carbon

C

H

N

Oa

M5 M6

31.53 31.10

68.47 68.90

68.12 65.39

3.09 2.86

2.83 3.45

25.96 28.30

a

By difference.

and pyrolysis reactivities between model and real coals are performed (see section 1 in the Supporting Information). According to Wu et al.,54 these model coals have a carbon structure similar to that of natural brown coal. It can be concluded that the synthetic model coals are remarkably similar to coals, and they can substitute for coals in the mechanism research. In general, pyrrolic and pyridinic nitrogen coexist in coals.55,59,60 To examine the interaction between the two nitrogen functional groups, some samples are prepared by mixing M5 and M6. The mass fraction of M5 in the mixed sample, P, is defined as mM5 P= mM5 + mM6 (1) where mM5 and mM6 are the masses of M5 and M6 (g), respectively. The values of P selected for the pyrolysis experiments include 0.00, 0.25, 0.50, 0.75, and 1.00. Six types of metal compounds are added to the model coal samples. The additives used are Na2CO3, CaCO3, Fe2O3, NaCl, CaCl2, and FeCl3, which are dried, ground, and sieved to 63−98 μm in advance. Each additive is mixed directly with M5 or M6 at room temperature, and the load rate is 3% by weight. To examine the effect of additive content, the load rates of Na2CO3, CaCO3, and Fe2O3 achieve 5% by weight for some samples. Each mixture is stirred well, dried at 105 °C for 24 h, and placed in a desiccator. 2.2. Pyrolysis and Gas Analysis. All the pyrolysis experiments are carried out in an entrained flow reactor. A schematic diagram of the entrained flow pyrolysis system is shown in Figure 2. The reactor is made of an alundum tube with an inside diameter of 32 mm. An electric heating furnace is installed along the alundum reactor for a

t0 +Δt

∫ F × C HCN × 10−6 × dt /22.4

XHCN−N =

t0

M × Nd /14

× 100

(2)

t0 +Δt

∫ F × C NO × 10−6 × dt /22.4

XNO−N =

t0

M × Nd /14

× 100

(3)

Figure 2. Schematic diagram of the entrained flow pyrolysis system. 432


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t0 +Δt

Conversion of tar-N to soot-N is an important fate of tarN.13,17,21,61−64 According to Xie et al.,21 it can be inferred that the formation of soot locks up some volatile-N in a form slightly more stable than tar-N, reducing the yield of HCN. Thermal cracking of char-N is also an important source of HCN.9 Several researchers65−67 pointed out that conversion of fuel nitrogen to N2 increased remarkably with increasing temperature during pyrolysis of coal samples and most of N2 arose from char-N. This may be another reason for the decreases in the yields of HCN when M5 and M6 are pyrolyzed at temperatures higher than 1000 °C. Besides the tar-N and char-N, it seems that part of HCN, which is produced at the preliminary stage of pyrolysis, may be converted to soot-N or N2 due to its reaction with char.17 Thus, the formation of sootN and N2 can lead to a decrease in HCN yield. In addition, the residence time of the model coal sample in the reactor decreases as the pyrolysis temperature rises (see section 2 in the Supporting Information), which can also lead to a decrease in HCN yield. Therefore, increasing the pyrolysis temperature would intensify the cracking reactions of free pyrrole radicals, free pyridine radicals, and other nitrogen-containing intermediates, but also reduce the residence time and favor the formation of soot-N and N2, combining to give a maximum in the HCN release at around 1000 °C. From Figure 3, it can also be seen that the conversion ratio XHCN−N for M5 is larger than that for M6 during pyrolysis of model coal samples at all temperatures. The largest discrepancy of XHCN−N between the two nitrogen forms even reaches 5.45% at 1000 °C. Wu et al.53 examined nitrogen release during temperature-programmed pyrolysis of two model coals that were pretty similar to those used in this study. However, some opposite results were obtained; that is, the release of HCN from pyridinic nitrogen was higher than that from pyrrolic nitrogen during pyrolysis. The reason for the differences of these results could be the different pyrolysis methods in the two studies. It is well-known that the thermal stabilities of pyridinic nitrogen (N-6) and quaternary nitrogen (N-Q) are better than that of pyrrolic nitrogen (N-5), and the conversions between different nitrogen functional groups may occur during pyrolysis.48,49,55−58 During rapid pyrolysis in this study, samples are heated at a relatively high temperature with a very short residence time (approximately from 6.0 to 8.8 s). It appears that changes in nitrogen functionality during rapid pyrolysis of M5 and M6 are relatively small.57,68,69 Thus, the unstable heterocyclic structures (N-5 and N-6) crack promptly, and then HCN is formed. Since pyridinic nitrogen shows better thermal stability, the HCN release will be higher for pyrrolic nitrogen than for pyridinic nitrogen. In the study of Wu et al.,53 model coal samples were pyrolyzed from room temperature to 800 or 1000 °C and soaked for 5 min. Because of the relative long residence time (approximately from 21 to 25 min), a large part of N-5 may be converted to N-6 or N-Q,48,49,55,56,58 which makes the HCN release much lower for pyrrolic nitrogen than for pyridinic nitrogen. During rapid pyrolysis of model coal samples at different temperatures, HCN is always found to be the primary precursor of nitrogen oxides, and no NH3 is detected. These results are consistent with earlier findings.36−38,40−43,53 Tan and Li16 reported that very significant proportions of NH3 were formed from the thermal cracking of char. As pointed out in the theoretical framework about the formation of NH3 during pyrolysis of coal, Li and Tan18 suggested that the main source of NH3 was the direct hydrogenation of the N-sites by the

∫ F × C NO2 × 10−6 × dt /22.4

X NO2−N =

t0

× 100

M × Nd /14

XNOx −N = XNO−N + XNO2−N

(4) (5)

−1

where F is the volumetric flow (L s ); t0 is the time at which measurement begins; Δt (see section 3 in the Supporting Information) is the measurement duration (s); M is the sample weight (g); Nd is the nitrogen content in the sample (%); and CHCN, CNO, and CNO2 are the concentrations of HCN, NO, and NO2 (μL L−1), respectively. The emission of HCN, NO, and NO2 is discussed, and the concentration profiles of a typical pyrolysis experiment are presented (see section 3 in the Supporting Information).

3. RESULTS AND DISCUSSION 3.1. Effect of Pyrolysis Temperature. 3.1.1. Release of HCN. Figure 3 shows the HCN release from rapid pyrolysis of

Figure 3. Conversion of fuel nitrogen to HCN from pyrolysis of the two model coals.

the two model coal samples. It is interesting to note that the conversion of fuel nitrogen to HCN exhibits extremely similar trends for M5 and M6. The release of HCN during pyrolysis tends to increase with increasing temperature first, reaching a maximum at 1000 °C, and then decrease. Qiao39 studied the formation mechanism of HCN during pyrolysis of pyridine in a tubular quartz reactor at the temperature of 600−1300 °C. Similar observations were found. The results in this study indicate that the pyrolysis temperature plays a crucial role in the HCN formation and the ring cracking processes may take place in a similar way for pyrrolic and pyridinic nitrogen. As shown in Figure 3, the conversion ratio XHCN−N increases as the pyrolysis temperature rises in the temperature range of 800−1000 °C. However, the tendency to increase for XHCN−N gradually decreases. A possible reason is that the decomposition reactions of free pyrrole radical, free pyridine radical, and other nitrogen-containing intermediates proceed more thoroughly and tend to complete with increasing pyrolysis temperature. It is noteworthy that the conversion ratio XHCN−N decreases drastically when the pyrolysis experiment is carried out at 1100 °C, and XHCN−N at 1300 °C is even lower than the value at 800 °C. When pyrolysis of M5 and M6 occurs at 1100 °C or above, large amounts of soot are formed. Ikeda and Mackie36 and Etemad-Rad and Metcalfe45 also found the soot formation during pyrolysis of some model compounds. As soon as M5 and M6 samples are heated up, the fuel nitrogen will be partitioned between volatiles and char. After the initial partition of fuel nitrogen, both volatile-N and char-N may be further thermally cracked into precursors of nitrogen oxides.9,16 433


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Figure 4. Conversion of fuel nitrogen to NO and NO2 from pyrolysis of the two model coals.

4 that, with increasing pyrolysis temperature, the release of NO decreases and the release of NO2 increases. Köpsel and Halang70 found that nearly 1.5% of the fuel nitrogen was released as NOx between 200 and 600 °C during temperature-programmed pyrolysis of the Lausitz lignite. Compared with the study of Köpsel and Halang,70 samples are pyrolyzed at higher temperatures, but with a shorter residence time, in this study. Consequently, the emission of NOx in the study of Köpsel and Halang70 is a little higher than that in this study. From Figure 4, it can also be noted that the conversion ratio XNOx−N for M5 is larger than that for M6 during pyrolysis of model coal samples at each temperature. As discussed in section 3.1.1, pyridinic nitrogen shows better thermal stability. Hence, the NOx release will be lower for pyridinic nitrogen than for pyrrolic nitrogen. In addition, the emission of NOx tends to decrease with increasing pyrolysis temperature first, reaching a minimum at 1000 °C, and then increase for both M5 and M6. One reason for this could be that more fuel nitrogen tends to be released as HCN as the temperature rises from 800 to 1000 °C. An additional reason could be the NO reduction on char. The effect of NO reduction may become weaker when pyrolysis of M5 and M6 occurs at 1100 °C or above. 3.2. Effect of Interaction between Two Nitrogen Functional Groups. Figure 5 shows the release of HCN from rapid pyrolysis of the mixed model coal samples. Whether the two model coal samples are pyrolyzed alone or mixed and pyrolyzed together, the release of HCN tends to increase as the

active hydrogen generated in situ in the pyrolyzing solid. Thus, the NH3 release is largely controlled by the slow rate of H radical generation in the char.9 In this study, there is insufficient residence time for chars to polymerize or crack. Therefore, no NH3 is formed during rapid pyrolysis of M5 and M6 due to the lack of the active hydrogen. In this study, the dry model coal samples are pyrolyzed in the absence of water vapor. Generally, water vapor is the major product during coal pyrolysis. Schmiers et al.49 and Schäfer and Bonn29 found that NH3 might be formed from the HCN hydrolysis in the presence of H2O. Additional pyrolysis experiments are carried out to examine the NH3 emission during pyrolysis of the model coal sample in an atmosphere of water vapor (see section 4 in the Supporting Information). From Figure SI-7 (Supporting Information), it can be seen that the NH3 release is only observed when the volume fraction of water vapor (14.69%) is relatively high. HCN is still the primary precursor of nitrogen oxides, and the conversion ratio XHCN−N increases as the volume fraction of water vapor increases. It appears that the active hydrogen generated from the interaction between water vapor and char favors the HCN formation.10 The minor conversion ratio XNH3−N (see eq SI-1 in the Supporting Information) is probably due to the short residence time of the model coal sample. It can also be seen that the conversion ratio XNH3−N for M6 is larger than that for M5 during pyrolysis of model coal samples with a volume fraction of water vapor of 14.69%. According to Li and Tan,18 the formation of NH3 will be favored for the thermally more stable N-containing structure (N-6). 3.1.2. Release of NOx. When pyrolysis experiments are performed at different temperatures, small amounts of NOx are detected. Figure 4 shows the release of NO and NO2 from rapid pyrolysis of the two model coal samples. It can be seen that the conversion ratio XNOx−N is 0.85−1.30% for M5 and 0.41−0.95% for M6. NO is the major component of NOx at lower temperatures, and NO2 makes the main contribution to the emission of NOx at higher temperatures. The dry model coal samples are entrained into the alundum reactor by the argon gas (section 2.2). After drying at 105 °C for 24 h, the samples have no residual water. Therefore, the oxygen in the entrained flow pyrolysis system is totally the fuel oxygen. In the model coal samples, part of the fuel oxygen may be linked with pyrrolic or pyridinic nitrogen. When the samples are heated, the N−O structures can be released as NO. As the pyrolysis temperature continues to increase, NO will be oxidized to NO2 with the oxygen in the sample. Thus, it can be seen from Figure

Figure 5. Relation between the HCN release and P at different temperatures. The dotted lines show the HCN release when M5 and M6 do not interact. 434


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Figure 6. Effect of mineral species on the HCN release during pyrolysis of M5.

Figure 7. Effect of mineral species on the HCN release during pyrolysis of M6.

of the interaction on the HCN release is more significant for the pyrolysis experiments performed at 900 or 1000 °C than for the other experiments. It is likely that, at the temperature of 900−1000 °C, more N-heterocyclic structures tend to open than at lower temperatures, and the produced polymers are more stable than at higher temperatures. 3.3. Effect of Minerals. 3.3.1. Effect of Mineral Species. Alkali, alkaline earth, and transition metals exist inherently in coals as mineral matter, and they catalyze the coal pyrolysis and affect the nitrogen release in different ways.32,33,65,75−77 In general, Na, Ca, and Fe are major components in coal ash. To examine the effect of mineral species on the nitrogen release, these mineral elements have been used intensively as catalysts to add to the coal and model compound samples. The method of catalyst addition is of great importance for the nitrogen species emission. Loading the catalysts onto the solid sample by the impregnation method (precipitation method) using an aqueous or organic solution has been widely adopted.3,32,65,75,78−82 This method makes it possible to exchange the metallic ions with oxygen-containing functional groups in low rank coals and incorporate finely dispersed particles of metal compounds into coal samples.83−85 The added minerals can well simulate the indigenous minerals in coal, and the catalytic reactions occur mainly in the solid phase.75 As suggested by Li et al.,78 the impregnation method might cause significant changes to the coal structure. Physically mixing the catalysts with the solid sample is another method of catalyst addition.81,86,87 This method makes a low degree dispersion of the catalysts in the solid sample.65,81 During rapid pyrolysis, the catalyst may prefer to react with the volatile-N rather than the char-N. Compared with the impregnation method, the physically mixing method is easier to be carried out and is

pyrolysis temperature rises, and then decrease. The maxima of the HCN release are achieved at 1000 °C. It can be seen that the conversion ratio XHCH−N does not change significantly when P rises in the range of 0.00−0.75, but increases abruptly when P is greater than 0.75 for all the temperatures. That is to say, the conversion ratio XHCH−N for the mixed sample is close to that for M6 at all pyrolysis temperatures. These results indicate that, when the mixed sample contains pyridinic nitrogen, even though its mass fraction is low, it can produce obvious inhibition for the conversion of fuel nitrogen to HCN from pyrolysis of the mixed samples. As shown in Figure 5, the dotted line indicates the conversion ratio XHCH−N from pyrolysis of the mixed sample at each temperature when M5 and M6 do not interact. It is obvious that all the real curves of XHCH−N are below the corresponding dotted lines. In other words, the actual HCN release is lower than the hypothetical HCN release for the mixed sample. These results confirm that the interaction between pyrrolic and pyridinic nitrogen occurs when M5 and M6 are mixed and pyrolyzed together. Pels et al.48 and Stańczyk et al.71 proposed that pyrrolic and pyridinic nitrogen were combined during the ring-opening process, and then some stable polymers were formed. In this study, the two model coals are physically combined. Because of the short residence time of the rapid pyrolysis, there is a slim chance for the solid−solid reaction during pyrolysis. During pyrolysis, a part of the fuel nitrogen may escape from the solid sample to form aromatic and heteroaromatic compounds and tarry materials.15,72−74 The possible reason for the reduction of the HCN release could be that some stable polymers are formed due to the reaction of one nitrogen species with another in the gas or tar phase following pyrolysis. It can also be seen that the negative effect 435


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Figure 8. Effect of mineral contents on the HCN release during pyrolysis of the two model coals at 1000 °C.

to the reduction of HCN formation may be larger. However, in this study, it is the former reactions that make the main contribution to the HCN reduction. As shown in Figure 6a,b, Na and Ca promote the HCN release at high temperatures. Besides the same effect as Fe, Na and Ca have some other effects during pyrolysis. Previous studies have revealed that Na can promote the conversion of char-N to volatile-N at a high temperature,80 whereas Ca can increase the reactivity of the devolatilized char.94,95 Although Na and Ca are physically mixed with the model coal samples in this study, it seems that increasing the pyrolysis temperature may favor the catalytic reactions in the solid phase, which makes catalysis of Na and Ca on HCN formation more significant than catalysis of them on HCN reduction during pyrolysis of pyrrolic nitrogen at high temperatures. Compared with pyrrolic nitrogen, pyridinic nitrogen shows better thermal stability (see section 3.1.1). Hence, the net HCN release decreases because catalysis of Na and Ca on HCN reduction may gain importance during pyrolysis of pyridinic nitrogen, as shown in Figure 7a. From Figures 6 and 7, it is noteworthy that metal oxide and metal chloride additions influence the HCN release with different degrees. According to Wei et al.,96 this may be attributed to the effect of chlorine during pyrolysis. It can also be noted that the change in conversion ratio XHCN−N at low temperatures is more significant than that at high temperatures. When M5 and M6 are pyrolyzed with catalysts at low temperatures, reactions proceed more slowly, which makes the catalytic effect more remarkable. 3.3.2. Effect of Mineral Contents. As discussed in section 3.1.1, the HCN release reaches a maximum at 1000 °C. To investigate the effect of additive content, two load rates of metal oxides are mixed with model coal samples, and then pyrolyzed at 1000 °C. The release of HCN from these pyrolysis experiments is shown in Figure 8. During pyrolysis at 1000 °C, the conversion ratio XHCN−N decreases dramatically as the additive content increases, irrespective of nitrogen form and metal type. A similar phenomenon was observed by Mori et al.3 They also found that the N2 conversion showed the reverse trend. Wu et al.54 studied the effects of Fe and Ca on the N2 formation during temperature-programmed pyrolysis of two model coals at a maximum temperature of 1000 °C. The mode coal samples in their experiments and in this study have fairly similar properties. When Fe and Ca additions were increased from 0.5 to 3.0 wt %, more N2 was released. Because of the different methods of catalyst addition in the two studies, the reason for the substantial decrease of XHCN−N in this study could be that

more likely to be applied in commercial boilers for the NOx reduction. In this study, six types of metal compounds are employed to investigate the effect of mineral species on the nitrogen release. Fe2O3 together with CaCO3 and Na2CO3 can be taken as representatives of metal oxides, because the two carbonates are heated to generate metal oxides and CO2 during pyrolysis. NaCl, CaCl2, FeCl3, can be taken as representatives of metal chlorides. Considering the feasibility of practical applications, each catalyst is mixed directly with M5 or M6 with the same particle size. Figures 6 and 7 show the effect of mineral species on the HCN release from rapid pyrolysis of the two model coals. It can be seen that, at the same temperature, the effect of different mineral species is different, and at different temperatures, the effect of the same metal compound is also different. During pyrolysis of M5 samples with catalysts, Na addition promotes the HCN release obviously at 1000 °C or above and Fe addition suppresses the HCN release at all pyrolysis temperatures, whereas Ca addition increases the HCN release with increasing temperature first and then decreases. As for M6 samples, all the metal additions suppress the HCN release, with a sequence of Ca < Na < Fe for metal oxides and with a sequence of Na < Ca < Fe for metal chlorides for all pyrolysis temperatures. These results indicate that catalyst addition considerably influences the HCN release. From Figures 6a and 7a, it can be noted that Ca addition decreases the conversion ratio of fuel nitrogen to HCN for pyrrolic nitrogen at 800 or 1000 °C and for pyridinic nitrogen at all temperatures. Compared with Ca addition, Fe addition suppresses the HCN release more remarkably for both pyrrolic and pyridinic nitrogen. Wu et al.53 have explored the effect of Fe and Ca on the HCN release during pyrolysis of two model coals containing pyrrolic and pyridinic nitrogen at 1000 °C. Guan et al.88 have also studied the effect of Fe and Ca on the release of nitrogen species during pyrolysis of model chars containing pyrrolic nitrogen at 950 °C. They found that Fe and Ca could suppress the HCN formation, and the catalytic effect of Ca was lower than that of Fe. The results in this study agree well with the findings in studies of Wu et al.53 and Guan et al.88 Several researchers have reported that the reduction of HCN formation was accompanied by a growth of N2 formation when coals were pyrolyzed with different additives.3,33,89,90 Fe and Ca can accelerate the decomposition of HCN to N2,3,91 and also decrease the fuel nitrogen conversion to HCN and increase the conversion to N2 at the same time.3,65,92,93 In the study of Wu et al.53 and Guan et al.,88 the impregnation method for catalyst addition was employed. The contribution of the latter reactions 436


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more HCN is catalyzed to form N2 as the additive content increases. 3.4. Effect of Particle Size. Figure 9 shows the HCN release from rapid pyrolysis of M6 with different particle sizes.

(1) HCN, but no NH3, is formed during rapid pyrolysis of the two model coals. The HCN release increases with increasing temperature first and then decreases, and exhibits extremely similar trends for the two nitrogen forms. Small amounts of NOx are also formed during pyrolysis. The releases of both HCN and NOx from pyrrolic nitrogen are larger than those from pyridinic nitrogen due to the worse thermal stability of pyrrolic nitrogen. (2) When the two model coals are mixed and pyrolyzed together, the total HCN release decreases due to the interaction between the nitrogen species released. The HCN release from the mixed nitrogen forms is closer to that from pyridinic nitrogen. (3) For pyrrolic nitrogen, Na addition promotes the HCN release obviously at 1000 °C or above and Fe addition suppresses the HCN release at each temperature, whereas Ca addition increases the HCN release with increasing temperature first and then decreases. As for pyridinic nitrogen, all the metal additions suppress the HCN release, and Fe has the strongest catalytic effect. When the additive content increases, the HCN release decreases dramatically. (4) The HCN release from pyridinic nitrogen increases slightly with increasing particle size at 800 or 1000 °C, but is changed little with different particle sizes at 1200 or 1300 °C.

Figure 9. Effect of particle size on the HCN release during pyrolysis of M6.

As discussed in section 3.1.1, the emission of HCN shows the similar trends for M5 and M6 at different pyrolysis temperatures. Hence, to examine the effect of particle size on the HCN release, only M6 is investigated and four particle size ranges are selected. As shown in Figure 9, the conversion ratio XHCN−N for M6 increases slightly with increasing particle size at 800 or 1000 °C. These results agree with the findings of Feng et al.97 It seems that the particle size dependence of fuel nitrogen conversion is a consequence of a competition between formation and destruction reactions of HCN. Liu et al.98 proposed that HCN can react with free radicals (such as H and OH) to produce large amounts of nitrogen atoms; then nitrogen atoms may combine with each other to form N2. Kidena et al.12 suggested another pathway for the N2 formation through a secondary reaction of HCN and char-N or tar-N. When M6 is pyrolyzed in a larger particle size, the diffusion resistance of the free radicals is greater in the particle, and the specific areas of samples are lower, both of which make the HCN destruction harder. Therefore, the larger the particle size, the higher the release of HCN. From Figure 9, it can also be noted that the effect of particle size on the HCN release is unconspicuous when the pyrolysis temperature achieves 1200 or 1300 °C. In such cases, the pyrolysis temperature may be the more important governing factor in formation and destruction reactions of HCN, compared with the diffusion of the free radicals and the specific areas of samples. Hence, the HCN release is changed little when M6 is pyrolyzed at relatively high temperatures with different particle sizes.

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ASSOCIATED CONTENT

S Supporting Information *

Details concerning data, methods, results, and discussion are presented in tables, figures, and text. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-29- 82665185. Fax: +86-29-82668703. Notes

The authors declare no competing financial interest.

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REFERENCES

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Release of Nitrogen Species during Rapid Pyrolysis of Model Coals

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