Article pubs.acs.org/est
NOx and N2O Precursors from Biomass Pyrolysis: Nitrogen Transformation from Amino Acid Qiangqiang Ren†,‡ and Changsui Zhao*,† †
School of Energy and Environment, Southeast University, Nanjing 210096, Jiangsu Province, P.R. China Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, P.R. China
‡
ABSTRACT: Large quantities of NOx and N2O emissions can be produced from biomass burning. Understanding nitrogen behavior during biomass pyrolysis is crucial. Nitrogen in biomass is mainly in forms of proteins (amino acids). Phenylalanine, aspartic acid, and glutamic acid were used as the model compounds for the nitrogen in biomass. Release behavior tests of nitrogen species from the three amino acids during pyrolysis in argon and gasification with O2 and CO2 were performed using a thermogravimetric analyzer (TGA) coupled with a Fourier transform infrared (FTIR) spectrometer. The results indicate that although the influence of oxygen and CO2 in the atmosphere on nitrogen behavior is different for the amino acids, it is interesting to find some phenomenon in common. The presence of oxygen promotes NO and HNCO formation for all the three amino acids; HCN and HNCO formation are suppressed by introduced CO2 for all the three amino acids. This can reveal the Nconversion mechanism from biomass in depth under the same conditions.
1. INTRODUCTION Understanding the formation of nitrogen species during biomass pyrolysis is essential and continues to be a challenge for minimizing the emissions of NOx and N2O during biomass combustion. Nitrogen in biomass is mainly in the form of proteins (amino acids).1,2 It is accepted that the combined study of biomass and nitrogen-containing model compounds (amino acid, protein, etc.) is the best approach to grasp a more complete mechanism of N-chemistry.3−5 Various protein-rich model compounds were pyrolyzed with the aim of finding features that are protein-specific, making conclusions regarding the model compounds applicable for biomass fuels in general. Furthermore, the protein composition is very complex as the nature of the proteins (amino acids composition) varies greatly from one biomass species to another. Then the investigation of amino acid can bring us helpful information about the gaseous products released from protein and biomass fuels. In our previous studies, it was found that biomass intrinsic properties, such as the amino acid composition,6 mineral matter,7,8 and atmosphere9 influence the selectivity of Nconversion into NH3, HCN, and HNCO. The formation mechanisms of nitrogen-containing species from amino acids have been discussed and can be referred to in reference 6. Deamination, dehydration, and decarboxylation reactions were the main primary reactions during pyrolysis of protein or amino acid, and the secondary reaction during amino acid pyrolysis was very complicated. Strong interactions were observed during copyrolysis of amino acids with cellulose, hemicellulose, or lignin. It is an interesting finding that the presence of O2 in the atmosphere promotes the yields of HCN and HNCO evidently, and HNCO seems to be a favorable product from biomass-N. © 2012 American Chemical Society
The use of CO2 reduces the formation of HCN and the emission of HNCO is suppressed, however, the effect of conditions on the product formation was not clear. It is not clear how the nitrogen species are formed from amino acids under the conditions. The objective of this study is to understand the mechanism of N-conversion from amino acids and to interpret the nitrogen behavior from biomass found in our previous study by investigating nitrogen transfer for amino acids under the same condition.
2. EXPERIMENTAL SECTION Materials. Based on the amino acid composition in agricultural straw obtained in our previous work,7 phenylalanine (Phe), aspartic acid (Asp), and glutamic acid (Glu) are the major amino acids in agricultural straw. Besides, considering the structures of the amino acids, phenylalanine, which is an aromatic amino acid, aspartic acid and glutamic acid, which are aliphatic amino acids, were used in the study. So the three amino acids selected are representative. The amino acid samples were commercially available samples purchased from the Sigma Chemical Co. without further purification. All the compounds had a purity of 98.5% or more. Apparatus and Product Analysis. The pyrolysis and gasification of amino acid samples were carried out with a thermogravimetric analyzer (TGA). The experimental conReceived: Revised: Accepted: Published: 4236
November 20, 2011 March 16, 2012 March 17, 2012 March 23, 2012 dx.doi.org/10.1021/es204142e | Environ. Sci. Technol. 2012, 46, 4236−4240
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200−500 °C. Indole is a main product during pyrolysis of phenylalanine10 which produces HCN. Meanwhile, a large number of N-heterocycles, pyrrole and pyridine can be formed during phenylalanine pyrolysis at 250 °C, which are known to break down at high temperatures to give high levels of HCN.11 NO formation is enhanced at late stage (>500 °C). As a N2O precursor, HNCO has different release trend from HCN, NH3 and NO. The HNCO peak appearing at 265 °C is formed by the secondary decomposition of 2,5-piperazinedione (DKP) and a higher HNCO peak appearing at 536 °C is due to the catalytic thermal cracking of the N-containing char. It is because as an aromatic amino acid, the thermal stability of phenylalanine is relatively strong and the nitrogen in char is converted into HNCO at high temperature. For aliphatic amino acids, C−OH (C−C, etc.) bonds break readily, and HNCO is mainly formed via the cracking of piperazinedione. Compared with argon atmosphere, when oxygen is introduced into the system, all the four N-containing species start to release at lower temperature. For HCN, the emission is bigger than that in argon when the temperature is lower than 325 °C. The dehydration reaction is weak, and piperazinedione formation is minimal during phenylalanine pyrolysis.12 While in the presence of oxygen, random bond cleavage occurs to a greater extent for phenylalanine at early stage (∼300 °C) and 2,5-piperazinedione formation may be involved to a greater extent, which contributes to HCN formation. HCN curve shows trimodal distribution reaching peaks at 217, 325, and 542 °C, respectively. The former two peaks are due to the thermal decomposition of 2,5-piperazinedione, and the catalytic thermal cracking of the N-containing char results in the last peak. As discussed above, the greater DKP formation contributes to NH3 formation at early stage (∼300 °C) in the presence of oxygen. NH3 has a narrow release profile and has little emission when the temperature is higher than 400 °C in the presence of oxygen. NO is formed at low temperature due to the direct oxidation effect of oxygen as expected. In the presence of oxygen, concerted rupturing of C−C bonds occurs to a greater extent in phenylalanine pyrolysis. The catalytic thermal cracking of the N-containing char compounds may contribute to the higher HNCO emission. It is reported that these Ncontaining char compounds might react to form hydantoins, which can open to yield HNCO.13,14 HCN and HNCO are observed when oxygen passed over carbonized phenylalanine in TGA at late stage (>500 °C). The yields of N-containing species and the selectivity of N-conversion in different atmosphere are obtained and displayed in Table 1. HCN and NH3 formation are restrained in both 5%O2/95%Ar and 5% CO2/95%Ar. The presence of O2 promotes formation of NO and HNCO from phenylalanine. Compared with argon and 5%O2/95%Ar atmosphere, when CO2 is introduced into the system, the N-containing species are restrained at low temperature and start to release at higher temperature. It is likely that DKP formation and its cleavage reaction is restricted and weak. It is interesting to note that NO and HNCO are suppressed evidently in the presence of CO2. As can be seen from Table 1, almost no HNCO and NO are detected in 5%CO2/95%Ar atmosphere. It is because DKP formation is restrained in the presence of CO2, and the intermediacy of DKP provides an attractive explanation for HCN and HNCO formation,1,15 less HCN and HCNO from the secondary decomposition of DKP are formed. Nitrogen Behavior from Aspartic Acid (Asp). Figure 2 shows N-containing species release curves from aspartic acid
ditions were in accordance with those for biomass.9 Amino acid samples of around 10 mg were pyrolyzed at heating rate of 40 °C/min from room temperature to 800 °C. Argon with purity of 99.999% was used as the carrier gas to provide an inert atmosphere for pyrolysis which can eliminate the influence of atmosphere on the formation of N-containing species and to remove the gaseous and condensable products. Also, the fate of nitrogen during amino acid gasification in mixtures of 5%O2/ 95%Ar (V./V.) or 5%CO2/95%Ar (V./V.) was studied. The flow rate of carrier gas was 65 mL·min−1. Evolved gases from TGA were connected to a Bruker Vector 22 Fourier transform infrared (FTIR) spectrometer with a heated (about 180 °C) transfer-line to minimize condensation. The resolution of the collected spectra was set to 1 cm−1. Spectra were recorded with a temporal resolution of about 6s. In present work, a semiquantitative method was used to determine the components of the gaseous mixtures by IR absorbance spectrum. The N-containing species were identified by their characteristic IR bands: NH3 (N−H Bending vibration, 966 cm−1); HCN (C−H Bending vibration, 714 cm−1); NO (N−O Stretching vibration, 1900 cm−1) and HNCO (CN Stretching vibration, 2250 cm−1). The concentrations of the N-containing species were determined based on the integral values of their release curves. Detailed information about the TGA-FTIR apparatus and product analysis method can be referred to references 7 and 9.
3. RESULTS AND DISCUSSION Reproducibility of the System. The experiments showed good reproducibility of the peak absorbance between the consecutive runs. The mean run-to-run variability was estimated using the relative standard deviations (R.S.D.) by averaging the relative analytic peak area calculated from three runs with area normalization method. For the N-containing compounds, the RSD was satisfactory. The average RSD was 2.8%. Nitrogen Behavior from Phenylalanine (Phe). Figure 1 shows N-containing species release curves from phenylalanine pyrolysis and gasification with O2 and CO2. During pyrolysis process, HCN and NH3 are formed mainly in the range of
Figure 1. Nitrogen species release curves from phenylalanine under different atmosphere. 4237
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Table 1. Yields and Conversion of Nitrogen from Amino Acids under Different Atmosphere integral value of gas release curve/a.u.
N-selectivity
sample
atmosphere
HCN
NH3
NO
HNCO
HCN/NH3
HNCO/HCN
Phe
Ar 95%Ar/5%O2 95%Ar/5%CO2 Ar 95%Ar/5%O2 95%Ar/5%CO2 Ar 95%Ar/5%O2 95%Ar/5%CO2
15.645 9.014 8.627 17.324 30.374 8.912 13.598 43.555 13.255
10.640 3.888 4.697 18.630 1.613 4.588 6.427 4.611 9.040
4.652 5.110 0.354 0.634 4.024 2.050 1.091 1.886 3.731
10.060 12.985 0.712 2.993 31.460 0.363 4.185 26.600 0.555
1.470 2.318 1.837 0.930 18.831 1.942 2.116 9.446 1.466
0.643 1.441 0.083 0.173 1.036 0.041 0.308 0.611 0.042
Asp
Glu
increases. HNCO curve shows bimodal distribution, and the first peak appearing at 367 °C is formed via the secondary decomposition of DKP, which is identical with that in argon. The catalytic thermal cracking of the N-containing char contributes to the second peak in the presence of oxygen. The yields of N-containing species and the selectivity of Nconversion in different atmosphere are obtained and displayed in Table 1. Compared with Ar atmosphere, the presence of O2 promotes formation of HCN and NO, especially HNCO, but suppresses NH3 yield. When CO2 is introduced into the system, the N-containing species are restrained at low temperature and start to release at higher temperature. HCN formation is minor at low temperature and reaches maximum value at 642 °C. One possible explanation is that indole is negligible from aspartic acid in the presence of CO2,18 which leads to minor HCN at low temperature. The catalytic thermal cracking of the N-containing char compounds contributes to the NO formation at late stage. NH3 and NO do not have obvious emissions until the temperature is higher than 350 °C. As can be seen from Table 1, yields of HCN and NH3 decrease and NO yield increases in the presence of CO2. It must be noted that HNCO is suppressed evidently and has almost no yield in the presence of CO2. Nitrogen Behavior from Glutamic Acid (Glu). Figure 3 shows N-containing species release curves from glutamic acid pyrolysis and gasification with O2 and CO2. During pyrolysis process, like aspartic acid, glutamic acid underwent virtually no deamination at 300 °C. The NH3 formation in high temperature region is mainly produced as a result of the secondary reactions between imine and amine, which are the products from glutamic acid.19 Glutamic acid undergoes only monodecarboxylation and the other carboxyl is incorporated into a pyrrolidinone ring with subsequent formation of pyrrole.20 For glutamic acid, the products from the secondary pyrolysis were mainly N-heterocycles and one- to two-ring aromatic compounds, especially nitriles.16,20 Nitrogen-containing rings are known to break down at high temperatures to give high levels of HCN.21 The high level of HCN detected in the pyrolysis of glutamic acid could be due to the availability of more nitrogen in the molecule or to the breakdown of pyrrole and nitriles. Compared with argon atmosphere, when oxygen is introduced into the system, HCN curve shows evidently bimodal distribution, the first peak appearing at 325 °C is formed via the intense secondary decomposition of pyrrole and nitriles formed from glutamic acid. The catalytic thermal cracking of the N-containing char contributes to the second peak appearing at 561 °C in the presence of oxygen. NH3
Figure 2. Nitrogen species release curves from aspartic acid under different atmosphere.
pyrolysis and gasification with O2 and CO2. During pyrolysis process, NH3 formation is little at earlier stage (∼300 °C) due to weak deamination reaction for aspartic acid. It is reported that decarboxylation reaction of aspartic acid is relatively great.16,17 Amine, the major product of decarboxylation reaction produces NH3. The bigger NH3 emission at late stage is formed via the reaction of amines and imines, which are products of the secondary decomposition of DKP. It should be noticed that the DKP is volatile and does not remain in the heated crucible until 400 °C is reached. However, at that temperature some linear products, such as dipeptide and polypeptides, can remain in the crucible. Polypeptide is the common intermediate in the pyrolysis of aspartic acid;16 the decomposition of polypeptides at high temperature will also produce DKP. HCN, NH3, and HNCO reach their maximum emission at 380, 608, and 376 °C, respectively. Compared with argon atmosphere, when oxygen is introduced into the system, HCN formation has an obvious increase. The presence of oxygen enhances the decomposition of aspartic acid, and the strong secondary decomposition of polypeptides contributes to the formation of HCN. The presence of oxygen has a negative effect on the reaction of imines and amines to yield NH3, so NH3 formation is restrained. OH radicals formed by cleavage of C−OH bonds are promoted in the presence of oxygen, and NO formation 4238
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Therefore HCN and HNCO formation are suppressed by introduced CO2 for all the three amino acids. This is in good agreement with the results found in biomass pyrolysis under the same conditions.9 It is confirmed that the nitrogen behavior for biomass under different atmosphere is due to the amino acid behavior under different atmosphere. This helps to reveal the N-conversion mechanism from biomass during thermal utilization based on a comprehensive consideration of nitrogen behavior from amino acids.
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AUTHOR INFORMATION
Corresponding Author
*Phone: + 86-25-83793453; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by National Natural Science Foundation of China (51106157). REFERENCES
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Figure 3. Nitrogen species release curves from glutamic acid under different atmosphere.
formation has similar trend to that in argon, but the yield is lower. When the temperature is high than 500 °C, there are more NO and HNCO emissions reaching the maximum at 595 and 575 °C, respectively. The yields of N-containing species and the selectivity of N-conversion in different atmosphere are obtained and displayed in Table 1. Compared with Ar atmosphere, the presence of oxygen promotes formation of HCN, NO, and HNCO, but suppress NH3 yield. When CO2 is introduced into the system, HCN curve shows bimodal distribution, the first peak appearing at 302 °C is formed via the weakened decomposition of pyrrole and nitriles. The catalytic thermal cracking of the N-containing char contributes to the second peak appearing at 679 °C in the presence of CO2. NH3 and NO have obvious emissions at late stage. It must be noted that HNCO is suppressed evidently and has almost no yield in the presence of CO2. From Table 1, in the presence of CO2, HCN yield decreases although not sharply, and NH3 and NO yields increase, and almost no HNCO is detected. So according to the nitrogen behavior from the three amino acids, although all the three amino acids underwent dehydration, decarboxylation, and deamination reactions, the relative significance of these reactions was different in each case, and the products from glutamic acid, aspartic acid, and phenylalanine were different in each case due to the distinct structures. The influence of oxygen and CO2 in atmosphere on nitrogen behavior is different for the amino acids. Even so, some characteristics in common for N-conversion from the three amino acids can be summarized, and the deep mechanism for the effect of atmosphere on nitrogen conversion from biomass has been revealed for the first time. The presence of oxygen promotes NO and HNCO formation for all the three amino acids, the reason is random bond cleavage occurs readily, the secondary decomposition of DKP becomes intense, and the existence of oxygen enhanced the catalytic thermal cracking of the N-containing char. It is likely that DKP formation is restrained in the presence of CO2. 4239
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