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NOx and N2O precursors (NH3 and HCN) from biomass pyrolysis: Co-pyrolysis of amino acids and cellulose, hemicellulose and lignin
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Proceedings of the
Proceedings of the Combustion Institute 33 (2011) 1715–1722
Combustion Institute www.elsevier.com/locate/proci
NOx and N2O precursors (NH3 and HCN) from biomass pyrolysis: Co-pyrolysis of amino acids and cellulose, hemicellulose and lignin Qiangqiang Ren a, Changsui Zhao a,*, Xiaoping Chen a, Lunbo Duan a, Yingjie Li a, Chunyuan Ma b b
a School of Energy and Environment, Southeast University, Nanjing 210096, PR China National Engineering Laboratory for Coal-Burning Pollutants Emission Reduction, Shandong University, Jinan 250061, PR China
Available online 6 August 2010
Abstract Nitrogen in biomass is mainly in forms of proteins (amino acids). Glycine, glutamic acid, aspartic acid, leucine, phenylalanine and proline are the major amino acids in agricultural straw. The six amino acids were pyrolyzed individually at 800 °C in a tubular reactor in an argon atmosphere. Each amino acid sample was then pyrolyzed individually with cellulose, hemicellulose or lignin with 1:1 mixing ratio by weight under the same condition. The emissions of HCN and NH3 were detected with a Fourier transform infrared (FTIR) spectrometer. The extent of interaction between the amino acids with cellulose, hemicellulose or lignin was determined by comparing the yields of HCN and NH3 from co-pyrolysis with those from single amino acid pyrolysis under the same condition. The results indicate that the structure of the amino acid has a significant effect on the nitrogen transformation during pyrolysis. The mixtures undergo solid-state decomposition reactions during co-pyrolysis. The extent of interaction between the amino acids with cellulose, hemicellulose or lignin depends on the amino acid types and the components in biomass. Although single proline and leucine form no char, they give a significant amount of nitrogen-containing char when co-pyrolyzed with cellulose, hemicellulose and lignin. HCN and NH3 yields and nitrogen conversion pathway from amino acid pyrolysis are influenced by cellulose, hemicellulose and lignin. Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Amino acid; Biomass; Co-pyrolysis; Nitrogen-containing species; FTIR
1. Introduction To provide renewable and CO2-neutral resources for energy production, biomass has
* Corresponding author. Address: 2# Sipailou, School of Energy and Environment, Southeast University, Nanjing 210096, PR China. Fax: +86 25 83793453. E-mail address: [email protected] (C. Zhao).
attracted great interest. Most of biomass fuels, such as agricultural straw and wood, contain nitrogen. Although the nitrogen content in biomass fuels is low, it is still important since the biomass nitrogen can be transformed into environmentally harmful gases under combustion [1,2]. In order to minimize the emissions of NOx and N2O, a better understanding of the primary volatile nitrogen species during biomass pyrolysis is essential and continues to be a challenge.
1540-7489/$ - see front matter Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2010.06.033
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Nitrogen functionalities in biomass are different from those in coal. Nitrogen in biomass is mainly in forms of proteins (amino acids) [3,4]. Recent research on the formation of NOx precursors (mainly HCN and NH3) for biomass was through biomass and nitrogen-containing model (amino acid, protein, etc.) [3–11]. In biomass pyrolysis studies, both HCN and NH3 were usually found. In some studies, more ammonia than HCN was found and in some cases the opposite was found. Due to the complicated intrinsic properties of biomass, it is difficult to understand fully N-conversion mechanism from biomass only. The mechanism for the formation of NOx precursors during biomass pyrolysis is still unclear. Since most of the nitrogen in biomass is bound in proteins, 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 the pyrolysis of amino acid can bring us helpful information about the gaseous products release from protein and biomass fuels. The pyrolysis of amino acids has also been extensively studied in last two decades [12–19], which mainly concerned thermal decomposition mechanism of amino acids. Few literatures were focused on nitrogen species release during pyrolysis of amino acid. It is still not clear how or under what conditions the HCN and NH3 are formed from amino acids. It is accepted that the combined study of biomass and model compounds is the best approach to grasp a more complete mechanism of N-chemistry [3,6]. It is true that N-structures (amino acids, so-called model compounds) are only a part of the biomass matrix, and they can be linked with other biomass components in a various ways, which may significantly influence N-chemistry of model compounds. This brings limitation to the validity of the results obtained through model compounds studies and their direct transfer to biomass. The results obtained from model compounds alone should be looked upon carefully. However, there is very little information on the product distribution from the co-pyrolysis of amino acids with cellulose, hemicellulose and lignin.
The objective of this study is to investigate the selectivity between HCN and NH3 for several amino acids and to identify nitrogen transformation during co-pyrolysis of amino acids with the components (cellulose, hemicellulose and lignin) to help understanding the nitrogen release pathways. The extent of interaction between the amino acids with cellulose, hemicellulose and lignin is determined by comparing the yields of HCN and NH3 from co-pyrolysis with those from individual pyrolysis of the amino acids under the same condition. 2. Experimental 2.1. Materials Based on amino acid composition in agricultural straw in our previous work [7], six major amino acids including glycine (Gly), glutamic acid (Glu), aspartic acid (Asp), leucine (Leu), phenylalanine (Phe) and proline (Pro), as shown in Fig. 1, were used in the study. 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. Cellulose, hemicellulose, and lignin were purchased from commercial chemical shop (Sigma– Aldrich). Cellulose is in powder fibrous form, and lignin is alkali lignin in brown powders. A commercial hemicellulose can hardly be purchased whereas xylan, although it might have different physical and chemical properties, has been widely used as a representative component of hemicellulose in pyrolysis processes [20–22]. Here, xylan processed from Birchwood, in yellow powder form, was used as hemicellulose. Particle size of hemicellulose is averaged at 100 lm and those of cellulose and lignin are at 50 lm. Aspartic acid, proline and leucine were selected to study nitrogen transformation during co-pyrolysis of amino acid with the components (cellulose, hemicellulose and lignin). Each amino acid sample of 10 ± 0.01 mg mixed with cellulose (1:1, w/w), hemicellulose (1:1, w/w) or lignin (1:1, w/w) was prepared. Although amino acid contents are far less than those of cellulose, hemicellulose and lignin in straw, it is just to judge whether the interaction exists and the extent of interaction between amino acids with the components from co-pyrolysis.
Fig. 1. Structures of the amino acid samples used in the study (left to right): aspartic acid, glutamic acid, glycine, leucine, phenylalanine and proline.
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2.2. Pyrolysis apparatus The pyrolysis experiments were performed in a horizontal fixed-bed tubular reactor (40 mm i.d., 60 mm o.d., and 400 mm long). The pyrolysis temperature was 800 °C for simulating the temperature of the pyrolysis zone of straw burning. The mass of each amino acid was 10 ± 0.01 mg. Samples were introduced into the center of the preheated reactor with a porcelain boat. 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. The argon flow rate was 1500 mL min1 (20 °C, 1 atm). 2.3. Product analysis Gaseous samples emitted from amino acid pyrolysis were continuously measured by Gasmet DX-4000 FT-IR Gas Analyzer which was composed of two units: the sampling unit for gases sampling and cleaning, and the measuring unit for continuous analysis with FT-IR analyzer. The analyzer detects concentrations exceeding approximately 1 ppm for each of these species under the experimental conditions. Infra-red spectra obtained were processed by Calcmet Software 2005. A water-cooled tar trap of quartz glass was placed between the reactor and the FT-IR instrument. By lowering the gas temperature in the tar trap to well below the temperature in the gas cell, tars (if any in the pyrolysate) were prevented from condensing on the walls and mirrors of the gas cell and thereby blocking the IR beam. The interior of the tar trap was free from water. A rubber tube connected the tar trap with the FT-IR. The rubber tube was heated in order to avoid ammonia absorbing on the walls of the rubber tube. A filter prevented soot and unreacted sample that may follow the gases, from entering the FT-IR. Nitrogen containing species HCN and NH3 were detected on-line. The gas yield was determined based on the integral values of the gas releasing curve according to Eq. (1). Xi is used to represent the ratio of nitrogen converted to HCN and NH3 during pyrolysis process to the nitrogen contained in the sample according to Eq. (2). Unfortunately, no quantification of HNCO was available due to lack of calibration spectra for this species. Moreover, HNCO is only a minor compound among NOx precursors [3]. Z t0 þDt C i 106 ðV =1000=60Þ dt=22:4 Ni ¼
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where i is the ith gas species, HCN and NH3; Ni is the gas yield, mol; Ci is the gas concentration in flue gas, lmol/mol; V is volumetric flow, mL/ min; t0 is the time at which measurement begins, s; Dt is the measurement duration, s; m is the amino acid sample weight, g; Nd is the nitrogen content in the sample, %. Char was affected by the pyrolysis reaction. For char analysis, amino acids of 200 mg and the mixtures of 400 mg (1:1, w/w) were pyrolyzed under the same conditions. 3. Results and discussion 3.1. Pyrolysis of single amino acid Three repeated tests on pyrolysis of leucine were carried out to find out the uncertainty quantification of the experimental results, and the relative standard deviation was 2.27%. So the results reported are quite reproducible. Figure 2 shows HCN and NH3 releasing curves from pyrolysis of the six amino acids. The results indicate that the structure of the amino acid has a significant effect on the nitrogen transformation to nitrogen species during amino acid pyrolysis. The light gases found in this study can be formed through direct pyrolysis of the amino acid
t0
ð1Þ Xi ¼
Ni 100% m N d =14
ð2Þ
Fig. 2. HCN and NH3 releasing curves from pyrolysis of the six amino acids: (a) NH3, (b) HCN.
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chain, and are also probably formed through cracking of primary tar products. Tar components from amino acids are 2,5-diketopiperazine (DKP) [3], which is cyclic amide. The decomposition of dipeptide and polypeptides at high temperature produce DKP. NH3 can be formed from individual molecular reaction of amino acid and may also be produced as a result of the secondary reactions [3,5,12, 15,19]. Both the decomposition reaction of primary amines and the bimolecular reactions between imine and amine can produce NH3 [3,13]. The intermediacy of 2,5-piperazinedione (DKP) provides an attractive explanation for the formation of HNCO, HCN and CO [3,13]. Imine is mainly formed through the decomposition of DKP. The formation of primary amines is governed by the decomposition of DKP into cyanide and amide, where the amide produces amine by loss of carbon monoxide [3]. The yields and conversion of nitrogen into HCN and NH3 during pyrolysis of the six amino acids are listed in Table 1. Significant yield of ammonia is observed from leucine pyrolysis and little ammonia is from aspartic acid. Meanwhile, proline has the most HCN yield and leucine has the least one during pyrolysis. From Table 1, phenylalanine and leucine undergo significant decomposition to HCN and NH3, more than 50% nitrogen in the amino acids is released as NH3 and HCN. There are only about 3.5% nitrogen contents in aspartic acid released as HCN and NH3 from pyrolysis. The total conversion of nitrogen into HCN and NH3 decreases in the order of leucine, phenylalanine, proline, glycine, glutamic acid and aspartic acid during pyrolysis. Aspartic acid has the biggest HCN/NH3 ratio about 12.231, while leucine has the smallest HCN/NH3 ratio about 0.004. The results indicate that XHCN and XNH3 have an obvious dependence on amino acid composition and different amino acids have distinctive N-conversion selectivity during pyrolysis. Experiments by our group on leucine, glycine and proline, all of which have non-reactive side chains, revealed that glycine produces char, while the other two amino acids do not produce any char. Amino acids with reactive side chains, glutamic acid, aspartic acid, and phenylalanine give
char yields. Based on the nitrogen emission characteristics of single amino acid, three representative amino acids, aspartic acid, proline and leucine were selected to study the interaction between amino acid with the component (cellulose, hemicellulose and lignin) and to investigate whether the HCN and NH3 yields from amino acid depend on the main constituents in biomass. 3.2. Co-pyrolysis of amino acid with cellulose, hemicellulose and lignin 3.2.1. Co-pyrolysis of aspartic acid with cellulose/ hemicellulose/lignin Figure 3 shows HCN and NH3 releasing curves from co-pyrolysis of aspartic acid and cellulose/ hemicellulose/lignin. NH3 releases ahead obviously in the presence of cellulose or lignin. NH3 releasing curve has one peak for single aspartic acid, while two peaks for co-pyrolysis of aspartic acid/cellulose or aspartic acid/lignin. For aspartic acid alone, the deamination reaction is minor, but the higher NH3 peak value (occurring at 14 s) indicates that there is a substantial deamination reaction during co-pyrolysis of aspartic acid/cellulose. The second NH3 release peak appearing at 56 s is from the secondary
Table 1 Yields and conversion of nitrogen into HCN and NH3 during amino acid pyrolysis. Sample
NNH3/lmol
NHCN/lmol
XNH3/%
XHCN/%
Asp Glu Gly Leu Phe Pro
0.199 3.619 19.217 61.707 20.080 3.954
2.434 6.828 9.508 0.260 11.967 22.644
0.264 5.319 14.412 80.836 33.131 4.547
3.238 10.038 7.131 0.341 19.745 26.040
Fig. 3. Nitrogen species releasing curves from co-pyrolysis of aspartic acid and components in biomass: (a) NH3, (b) HCN.
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decomposition reaction of tar (DKP), the reaction of imines and amines to yield NH3, which is promoted during co-pyrolysis of aspartic acid and cellulose [5]. Compared with the effect of cellulose, the first NH3 release peak in the earlier period (occurring at 14 s) during co-pyrolysis of aspartic acid/lignin is also formed directly through deamination reaction of aspartic acid. The higher NH3 peak value indicates that aspartic acid undergoes intense deamination reaction in the presence of lignin. The phenomenon that the second NH3 release peak appearing at 36 s is earlier and its value is higher than that in the co-pyrolysis of aspartic acid and cellulose is due to the more intense secondary reactions of tar (DKP) in the presence of lignin and more hydrogen availability from higher H2 and CH4 yields generated from lignin pyrolysis [20]. As discussed above, the intermediacy of 2,5piperazinedione (DKP) provides an attractive explanation for HCN formation. As can be seen from Fig. 3, HCN releasing curves have higher peak values during co-pyrolysis compared with that from aspartic acid alone. One possible explanation is that indole, one nitrogen functionality of tars, is negligible from single aspartic acid pyrolysis [22], but has a high yield from co-pyrolysis of aspartic acid with cellulose, hemicellulose or lignin. The thermal cracking of indole contributes to increasing HCN formation. The yields and conversion of nitrogen into HCN and NH3 during co-pyrolysis of aspartic acid with cellulose, hemicellulose and lignin are illustrated in Table 2. C, H and L in the Table represent cellulose, hemicellulose and lignin, respectively. It indicates clearly that the conversion of nitrogen into NH3 and HCN increases remarkably during co-pyrolysis of aspartic acid and the components. But the extent is not the same. HCN yield from co-pyrolysis of aspartic acid/cellulose is the highest, and NH3 yield from co-pyrolysis of aspartic acid/lignin is the highest. HCN/ NH3 ratio decreases sharply from 12.231 for Table 2 Yields and conversion of N into HCN and NH3 from single amino acid and the mixture during pyrolysis. Sample
NNH3/lmol
NHCN/lmol
XNH3/%
XHCN/%
Asp Asp/C Asp/H Asp/L Pro Pro/C Pro/H Pro/L Leu Leu/C Leu/H Leu/L
0.199 7.012 9.244 15.826 3.954 8.185 10.190 12.860 61.707 48.922 32.316 42.802
2.434 16.988 11.161 10.775 22.644 2.469 2.986 4.371 0.260 2.460 7.390 4.396
0.264 9.326 12.295 21.048 4.547 9.412 11.719 14.789 80.836 64.088 42.334 56.070
3.238 22.594 14.844 14.331 26.040 2.840 3.434 5.026 0.341 3.223 9.681 5.759
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aspartic acid alone to 2.423, 1.207 and 0.681 for aspartic acid/cellulose, aspartic acid/hemicellulose and aspartic acid/lignin, respectively. It is interesting to note that HCN/NH3 ratio is less than one from co-pyrolysis of aspartic acid/lignin. It indicates that lignin has a different effect on the pathway of DKP cleavage from that of cellulose and xylan during aspartic acid pyrolysis. Lignin facilitates the secondary decomposition of DKP into amines rather than indole formation. The results make clear that the selectivity of N-conversion has an evident dependence on the cellulose, hemicellulose and lignin. Interactions between aspartic acid with cellulose, hemicellulose and lignin were further studied by analyzing the chars. Individual pyrolysis of aspartic acid, cellulose, hemicellulose and lignin at 800 °C yields chars of 9.15%, 1.85%, 14.63% and 47.71%, respectively. While the char yields from co-pyrolysis of aspartic acid/cellulose, aspartic acid/hemicellulose and aspartic acid/lignin are 8.87%, 19.07% and 33.32%, respectively. The results reveal that the weight losses of mixtures of aspartic acid/cellulose, aspartic acid/hemicellulose and aspartic acid/lignin are different from that of the individual sample, indicating the mixture undergoes solid-state reactions to produce volatile products and carbonaceous residue (i.e. char). Larger char yields could result in larger N-containing species yields since N-containing species can be released from pyrolysis of char [23]. The thermal history of the char formed from pyrolysis of mixtures of carbohydrates and amino acids can have an impact on the release of volatile products [24]. The catalytic thermal cracking of the N-containing char compounds from co-pyrolysis of aspartic acid and cellulose/hemicellulose/lignin may also contribute to the higher HCN emission. It is reported that these N-containing char compounds might react to form hydantoins (or imidazoledinediones, 5-membered cyclic amides), which can open to yield HCN [25,26]. Thus, to gain mechanistic insight into the formation of HCN and NH3 from the pyrolysis of amino acids and cellulose/hemicellulose/lignin, solid-state chemistry of the char and the gas phase chemistry need to be investigated in more detail. 3.2.2. Co-pyrolysis of proline with cellulose/hemicellulose/lignin NH3 and HCN releasing curves from co-pyrolysis of proline/cellulose, proline/hemicellulose and proline/lignin are displayed in Fig. 4. It can be seen that HCN releases fast at early step during single proline pyrolysis and NH3 has no release at this stage. It indicates that extent of deamination seems to be negligible for proline, and NH3 is mainly formed from the secondary thermal cracking of DKP.
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Fig. 4. Nitrogen species releasing curves from co-pyrolysis of proline and components in biomass: (a) NH3, (b) HCN.
The research indicated that at 600 °C, the only major product formed from proline pyrolysis was 2,5-diketopiperazine, which was formed by condensation of two proline molecules. The major products from proline pyrolysis at 700 °C were the 2,5diketopiperazine and pyrrole. Compared with the 600oC run, more low molecular weight products were formed, and traces of pyridine were found. At 800 °C, the dominant product from proline pyrolysis was pyrrole. There was only a small amount of 2,5-diketopiperazine remaining and small amounts of pyridine and pyrazine were found [27]. It indicates that dehydration reaction is minor for pyrolysis of proline at 800 °C. So pyrrole, the degradation product from the 2,5-diketopiperazine, contributes to the higher XHCN for single proline. During co-pyrolysis of proline and the components, HCN emission decreases evidently, indicating that the thermal cracking of pyrrole is suppressed in the presence of cellulose, hemicellulose or lignin, while NH3 releases faster and increases during co-pyrolysis. It appears that the cleavage of DKP is inclined to produce amines rather than pyrrole, the dominant amines decompose to NH3 during co-pyrolysis. In the presence of cellulose, NH3 releasing curve takes on bimodal shape with two peaks
occurring at 24 and 41 s, respectively. The first peak during co-pyrolysis of proline and cellulose may be formed through deamination reaction of single aspartic acid. The second peak is from thermal cracking of amines. 2,5-Diketopiperazine prefers to form imines and amines rather than pyrrole during the interaction of proline and cellulose. The co-pyrolysis of proline/cellulose, proline/ hemicellulose and proline/lignin mixtures are particularly interesting since single proline does not form any char at 800 °C. The char yields from co-pyrolysis of proline/cellulose, proline/hemicellulose and proline/lignin share 3.77%, 8.87% and 22.78%, respectively. The interesting finding is that char from the mixtures contains N. It should be noticed that cellulose, hemicellulose and lignin do not contain any nitrogen content. So the nitrogen in the char originates from the proline. The ratio of nitrogen converted to char-N is 0, 3.15%, 4.92% and 7.00% for single proline, proline/cellulose, proline/hemicellulose and proline/ lignin, respectively. The N/C ratios (w/w) in the char from proline/cellulose, proline/hemicellulose and proline/lignin are 0.070, 0.064 and 0.031, respectively. It seems that the extent of deamination is not negligible during co-pyrolysis. Even a small deamination can have a significant effect on ammonia yield and makes amino acids relatively less stable [28]. The results strongly confirm the occurrence of the solid-state reactions between proline and cellulose, hemicellulose or lignin during pyrolysis. It can be seen from Table 2 that the proline/ cellulose, proline/hemicellulose and proline/lignin blends produce significantly more NH3 and less HCN than proline alone. It seems that NH3 and HCN are formed via competitive reactions during pyrolysis of proline and the blends. Lignin enhances NH3 yield, and cellulose reduces HCN yield to the biggest extent. The co-pyrolysis leads to smaller nitrogen conversion to HCN and NH3 compared with the single proline. It should be noted that HCN/NH3 ratio decreases from 5.727 for single proline to less than one for co-pyrolysis of the blends. The selectivity of N-conversion into HCN and NH3 for proline is affected by cellulose, hemicellulose and lignin. 3.2.3. Co-pyrolysis of leucine with cellulose/hemicellulose/lignin Figure 5 shows the HCN and NH3 releasing curves from co-pyrolysis of leucine/cellulose, leucine/hemicellulose and leucine/lignin. It is clear that HCN release is far less than NH3 release, NH3 is the dominant nitrogen species for leucine. Decarboxylation is the main primary reaction for leucine pyrolysis [18], and amine is the main product from the decarboxylation reaction. Meanwhile, it was verified that amines, nitriles and alkenes were the major products and amine was detected in large quantities from leucine pyrolysis
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Fig. 5. Nitrogen species releasing curves from co-pyrolysis of leucine and components in biomass: (a) NH3, (b) HCN.
verified by GC–MS [18,29]. Each amine, therefore, decomposes by the loss of two molecules of hydrogen to yield the corresponding nitrile [30]. So, besides the minor individual molecular reaction, the decomposition reaction of primary amines contributes to the dominant NH3 for leucine pyrolysis. The peak value of NH3 releasing curve decreases in the order of cellulose, lignin and hemicellulose, while that of HCN increases in the order of cellulose, lignin and hemicellulose during co-pyrolysis. As can be seen from Table 2, co-pyrolysis reduces NH3 yield and promotes HCN yield. It seems that NH3 and HCN formation is competitive each other during co-pyrolysis. Hemicellulose enhances NH3 yield, and cellulose reduces HCN yield to the biggest extent. The co-pyrolysis leads to smaller nitrogen conversion to HCN and NH3 compared with the single leucine. It should be noted that HCN/NH3 ratio increases from 0.004 for single leucine to 0.050, 0.229 and 0.103 for co-pyrolysis of leucine/cellulose, leucine/hemicellulose and leucine/lignin, respectively. The selectivity of N-conversion into HCN and NH3 for leuline is affected by cellulose, hemicellulose and lignin.
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Leucine does not form any char at 800 °C, which is similar to proline. The char yields from co-pyrolysis of leucine/cellulose, leucine/hemicellulose and leucine/lignin share 2.10%, 7.77% and 22.97%, respectively. Nitrogen element is also found in the char from the mixtures. The ratio of nitrogen converted to char-N is 0%, 0.88%, 3.28% and 4.39% for single leucine, leucine/cellulose, leucine/hemicellulose and leucine/lignin, respectively. The N/C ratios (w/w) in the char from leucine/cellulose, leucine/hemicellulose and leucine/lignin are 0.027, 0.043 and 0.017, respectively. This suggests that the interactions between leucine and cellulose/hemicellulose/lignin during co-pyrolysis are relatively mild compared with those for proline. It is likely that the secondary decomposition reactions of primary-formed amines prefer to nitrile and HCN formation through an imine intermediate from co-pyrolysis of leucine and cellulose, hemicellulose or lignin [5,30]. In other words, compared with single leucine, the secondary decomposition path of amines is inclined to HCN formation in the presence of cellulose, hemicellulose or lignin. Hemicellulose seems to have the strongest effect on decomposition of DKP, and cellulose has the weakest effect. Although proline and leucine are similar in some aspects, the effect of cellulose, hemicellulose or lignin on the pyrolysis of each of the two amino acids aforementioned is different. The extent of interaction depends on variety of the amino acids and the components in biomass. Hemicellulose has the strongest effect on the secondary reaction of DKP for leucine, while lignin has the strongest effect on the secondary reaction of DKP for proline. 4. Conclusion The study on N-behavior during pyrolysis of amino acid and the mixtures of amino acid individually with the main components in biomass provided several interesting findings. Amino acids with different structures have different nitrogen transformation during pyrolysis. The yields of NH3 and HCN increase during co-pyrolysis of aspartic acid individually with cellulose, hemicellulose or lignin. During co-pyrolysis of proline individually with cellulose, hemicellulose or lignin, HCN emission decreases remarkably, NH3 emission rises. During co-pyrolysis of leucine individually with cellulose, hemicellulose or lignin, HCN emission increases evidently, NH3 emission decreases Strong interactions are observed in the copyrolysis of amino acids individually with cellulose, hemicellulose or lignin. A significant yield of nitrogenous char is obtained from co-pyrolysis of cellulose, hemicellulose or lignin with proline as well as leucine, although proline or leucine volatilizes completely at 800 °C. Co-pyrolysis of amino
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acids with cellulose, hemicellulose or lignin mainly alters the secondary decomposition pathways of DKP. The magnitude of interaction is dependent on the variety of amino acid and the components in biomass. To obtain the mechanism of nitrogen transformation during biomass pyrolysis, further research is needed on the solid-state and gas phase reactions of amino acids and the main components in biomass. Acknowledgements This work was financed by the Special Fund of Transformation of Scientific and Technical Achievements in Jiangsu province, China (Project No.: BA2007023) and the Scientific Research Foundation of Graduate School of Southeast University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10. 1016/j.proci.2010.06.033. References [1] F. Winter, C. Wartha, H. Hofbauer, Bioresour. Technol. 70 (1) (1999) 39–49. [2] H. Zhou, A.D. Jensen, P. Glarborg, A. Kavaliauskas, Fuel 85 (5–6) (2006) 705–716. [3] K.M. Hansson, J. Samuelsson, C. Tullin, L.E. ˚ mand, Combust. Flame 137 (3) (2004) 265–277. A [4] F.J. Tian, J.L. Yu, L.J. Mckenzie, J. Hayashi, C.Z. Li, Energ. Fuel 21 (2) (2007) 517–521. ˚ mand, A. Habermann, F. [5] K.M. Hansson, L.E. A Winter, Fuel 82 (6) (2003) 653–660. [6] M. Becidan, Ø. Skreiberg, J.E. Hustad, Energy Fuel 21 (2) (2007) 1173–1180. [7] Q.Q. Ren, C.S. Zhao, X. Wu, C. Liang, X.P. Chen, J.Z. Shen, G.Y. Tang, Z. Wang, J. Anal. Appl. Pyrol. 85 (1–2) (2009) 447–453. [8] Q.Q. Ren, C.S. Zhao, X. Wu, C. Liang, X.P. Chen, J.Z. Shen, G.Y. Tang, Z. Wang, Bioresour. Technol. 100 (17) (2009) 4054–4057.
[9] Q.Q. Ren, C.S. Zhao, X. Wu, C. Liang, X.P. Chen, J.Z. Shen, Z. Wang, J. Therm. Anal. Calorim. 99 (1) (2010) 301–306. [10] Q.Q. Ren, C.S. Zhao, X. Wu, C. Liang, X.P. Chen, J.Z. Shen, Z. Wang, Fuel 89 (5) (2010) 1064–1069. [11] N. Paterson, Y. Zhuo, D. Dugwell, R. Kandiyoti, Energy Fuel 19 (3) (2005) 1016–1022. [12] J. Li, Y.W. Liu, J.Y. Shi, Z.Y. Wang, L. Hu, X. Yang, C.X. Wang, Thermochim. Acta 467 (1–2) (2008) 20–29. [13] J. Li, Z.Y. Wang, X. Yang, L. Hu, Y.W. Liu, C.X. Wang, J. Anal. Appl. Pyrol. 80 (1) (2007) 247–253. [14] J.N. Leichtnam, D. Schwartz, R. Gadiou, J. Anal. Appl. Pyrol. 55 (2) (2000) 255–268. [15] S.F. Wang, B.Z. Liu, Q.D. Su, J. Anal. Appl. Pyrol. 71 (1) (2004) 393–403. [16] A. Keyhani, V.A. Yaylayan, J. Agric. Food Chem. 44 (1) (1996) 223–229. [17] S.F. Wang, B.Z. Liu, K.J. Sun, Q.D. Su, J. Chromatogr. 1025 (2) (2004) 255–261. [18] J. Li, Z.Y. Wang, X. Yang, L. Hu, Y.W. Liu, C.X. Wang, Thermochim. Acta 447 (2) (2006) 147–153. [19] N.F. Haidar, J.M. Patterson, M. Moors, W.T. Smith, J. Agric. Food Chem. 29 (1) (1981) 163–165. [20] H.P. Yang, R. Yan, H.P. Chen, D.H. Lee, C.G. Zheng, Fuel 86 (12–13) (2007) 1781–1788. [21] Y. Zheng, A.D. Jensen, P. Glarborg, K. Sendt, B.S. Haynes, Proc. Combust. Inst. 32 (2) (2009) 1973–1980. [22] N.W. Tame, B.Z. Dlugogorski, E.M. Kennedy, Proc. Combust. Inst. 32 (1) (2009) 665–671. [23] P. Glarborg, A.D. Jensen, J.E. Johnson, Prog. Energy Combust. Sci. 29 (2) (2003) 89–113. [24] R.K. Sharma, W.G. Chan, M.R. Hajaligol, J. Anal. Appl. Pyrolysis 86 (1) (2009) 122–134. [25] V.A. Basiuk, J. Douda, J. Anal. Appl. Pyrolysis 55 (2) (2000) 235–246. [26] J.I. Samuelsson, Conversion of Nitrogen in a Fixed Burning Biofuel Bed. Licentiate Thesis, Chalmers University of Technology, Go¨teborg, Sweden, 2006. [27] P.F. Britt, A.C. Buchanan, C.V. Owens Jr., J.T. Skeen, Fuel 83 (11-12) (2004) 1417–1432. [28] J. Bjarnason, K.J. Carpenter, Brit. J. Nutr. 24 (1) (1970) 313–329. [29] M.A. Ratcliff, E.E. Medley, P.G. Simmonds, J. Org. Chem. 39 (11) (1974) 481–1490. [30] P.G. Simmonds, E.E. Medley, M.A. Ratcliff Jr., G.P. Shulman, Anal. Chem. 44 (12) (1972) 2060– 2066.
Proceedings of the
Proceedings of the Combustion Institute 33 (2011) 1715–1722
Combustion Institute www.elsevier.com/locate/proci
NOx and N2O precursors (NH3 and HCN) from biomass pyrolysis: Co-pyrolysis of amino acids and cellulose, hemicellulose and lignin Qiangqiang Ren a, Changsui Zhao a,*, Xiaoping Chen a, Lunbo Duan a, Yingjie Li a, Chunyuan Ma b b
a School of Energy and Environment, Southeast University, Nanjing 210096, PR China National Engineering Laboratory for Coal-Burning Pollutants Emission Reduction, Shandong University, Jinan 250061, PR China
Available online 6 August 2010
Abstract Nitrogen in biomass is mainly in forms of proteins (amino acids). Glycine, glutamic acid, aspartic acid, leucine, phenylalanine and proline are the major amino acids in agricultural straw. The six amino acids were pyrolyzed individually at 800 °C in a tubular reactor in an argon atmosphere. Each amino acid sample was then pyrolyzed individually with cellulose, hemicellulose or lignin with 1:1 mixing ratio by weight under the same condition. The emissions of HCN and NH3 were detected with a Fourier transform infrared (FTIR) spectrometer. The extent of interaction between the amino acids with cellulose, hemicellulose or lignin was determined by comparing the yields of HCN and NH3 from co-pyrolysis with those from single amino acid pyrolysis under the same condition. The results indicate that the structure of the amino acid has a significant effect on the nitrogen transformation during pyrolysis. The mixtures undergo solid-state decomposition reactions during co-pyrolysis. The extent of interaction between the amino acids with cellulose, hemicellulose or lignin depends on the amino acid types and the components in biomass. Although single proline and leucine form no char, they give a significant amount of nitrogen-containing char when co-pyrolyzed with cellulose, hemicellulose and lignin. HCN and NH3 yields and nitrogen conversion pathway from amino acid pyrolysis are influenced by cellulose, hemicellulose and lignin. Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Amino acid; Biomass; Co-pyrolysis; Nitrogen-containing species; FTIR
1. Introduction To provide renewable and CO2-neutral resources for energy production, biomass has
* Corresponding author. Address: 2# Sipailou, School of Energy and Environment, Southeast University, Nanjing 210096, PR China. Fax: +86 25 83793453. E-mail address: [email protected] (C. Zhao).
attracted great interest. Most of biomass fuels, such as agricultural straw and wood, contain nitrogen. Although the nitrogen content in biomass fuels is low, it is still important since the biomass nitrogen can be transformed into environmentally harmful gases under combustion [1,2]. In order to minimize the emissions of NOx and N2O, a better understanding of the primary volatile nitrogen species during biomass pyrolysis is essential and continues to be a challenge.
1540-7489/$ - see front matter Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2010.06.033
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Nitrogen functionalities in biomass are different from those in coal. Nitrogen in biomass is mainly in forms of proteins (amino acids) [3,4]. Recent research on the formation of NOx precursors (mainly HCN and NH3) for biomass was through biomass and nitrogen-containing model (amino acid, protein, etc.) [3–11]. In biomass pyrolysis studies, both HCN and NH3 were usually found. In some studies, more ammonia than HCN was found and in some cases the opposite was found. Due to the complicated intrinsic properties of biomass, it is difficult to understand fully N-conversion mechanism from biomass only. The mechanism for the formation of NOx precursors during biomass pyrolysis is still unclear. Since most of the nitrogen in biomass is bound in proteins, 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 the pyrolysis of amino acid can bring us helpful information about the gaseous products release from protein and biomass fuels. The pyrolysis of amino acids has also been extensively studied in last two decades [12–19], which mainly concerned thermal decomposition mechanism of amino acids. Few literatures were focused on nitrogen species release during pyrolysis of amino acid. It is still not clear how or under what conditions the HCN and NH3 are formed from amino acids. It is accepted that the combined study of biomass and model compounds is the best approach to grasp a more complete mechanism of N-chemistry [3,6]. It is true that N-structures (amino acids, so-called model compounds) are only a part of the biomass matrix, and they can be linked with other biomass components in a various ways, which may significantly influence N-chemistry of model compounds. This brings limitation to the validity of the results obtained through model compounds studies and their direct transfer to biomass. The results obtained from model compounds alone should be looked upon carefully. However, there is very little information on the product distribution from the co-pyrolysis of amino acids with cellulose, hemicellulose and lignin.
The objective of this study is to investigate the selectivity between HCN and NH3 for several amino acids and to identify nitrogen transformation during co-pyrolysis of amino acids with the components (cellulose, hemicellulose and lignin) to help understanding the nitrogen release pathways. The extent of interaction between the amino acids with cellulose, hemicellulose and lignin is determined by comparing the yields of HCN and NH3 from co-pyrolysis with those from individual pyrolysis of the amino acids under the same condition. 2. Experimental 2.1. Materials Based on amino acid composition in agricultural straw in our previous work [7], six major amino acids including glycine (Gly), glutamic acid (Glu), aspartic acid (Asp), leucine (Leu), phenylalanine (Phe) and proline (Pro), as shown in Fig. 1, were used in the study. 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. Cellulose, hemicellulose, and lignin were purchased from commercial chemical shop (Sigma– Aldrich). Cellulose is in powder fibrous form, and lignin is alkali lignin in brown powders. A commercial hemicellulose can hardly be purchased whereas xylan, although it might have different physical and chemical properties, has been widely used as a representative component of hemicellulose in pyrolysis processes [20–22]. Here, xylan processed from Birchwood, in yellow powder form, was used as hemicellulose. Particle size of hemicellulose is averaged at 100 lm and those of cellulose and lignin are at 50 lm. Aspartic acid, proline and leucine were selected to study nitrogen transformation during co-pyrolysis of amino acid with the components (cellulose, hemicellulose and lignin). Each amino acid sample of 10 ± 0.01 mg mixed with cellulose (1:1, w/w), hemicellulose (1:1, w/w) or lignin (1:1, w/w) was prepared. Although amino acid contents are far less than those of cellulose, hemicellulose and lignin in straw, it is just to judge whether the interaction exists and the extent of interaction between amino acids with the components from co-pyrolysis.
Fig. 1. Structures of the amino acid samples used in the study (left to right): aspartic acid, glutamic acid, glycine, leucine, phenylalanine and proline.
Q. Ren et al. / Proceedings of the Combustion Institute 33 (2011) 1715–1722
2.2. Pyrolysis apparatus The pyrolysis experiments were performed in a horizontal fixed-bed tubular reactor (40 mm i.d., 60 mm o.d., and 400 mm long). The pyrolysis temperature was 800 °C for simulating the temperature of the pyrolysis zone of straw burning. The mass of each amino acid was 10 ± 0.01 mg. Samples were introduced into the center of the preheated reactor with a porcelain boat. 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. The argon flow rate was 1500 mL min1 (20 °C, 1 atm). 2.3. Product analysis Gaseous samples emitted from amino acid pyrolysis were continuously measured by Gasmet DX-4000 FT-IR Gas Analyzer which was composed of two units: the sampling unit for gases sampling and cleaning, and the measuring unit for continuous analysis with FT-IR analyzer. The analyzer detects concentrations exceeding approximately 1 ppm for each of these species under the experimental conditions. Infra-red spectra obtained were processed by Calcmet Software 2005. A water-cooled tar trap of quartz glass was placed between the reactor and the FT-IR instrument. By lowering the gas temperature in the tar trap to well below the temperature in the gas cell, tars (if any in the pyrolysate) were prevented from condensing on the walls and mirrors of the gas cell and thereby blocking the IR beam. The interior of the tar trap was free from water. A rubber tube connected the tar trap with the FT-IR. The rubber tube was heated in order to avoid ammonia absorbing on the walls of the rubber tube. A filter prevented soot and unreacted sample that may follow the gases, from entering the FT-IR. Nitrogen containing species HCN and NH3 were detected on-line. The gas yield was determined based on the integral values of the gas releasing curve according to Eq. (1). Xi is used to represent the ratio of nitrogen converted to HCN and NH3 during pyrolysis process to the nitrogen contained in the sample according to Eq. (2). Unfortunately, no quantification of HNCO was available due to lack of calibration spectra for this species. Moreover, HNCO is only a minor compound among NOx precursors [3]. Z t0 þDt C i 106 ðV =1000=60Þ dt=22:4 Ni ¼
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where i is the ith gas species, HCN and NH3; Ni is the gas yield, mol; Ci is the gas concentration in flue gas, lmol/mol; V is volumetric flow, mL/ min; t0 is the time at which measurement begins, s; Dt is the measurement duration, s; m is the amino acid sample weight, g; Nd is the nitrogen content in the sample, %. Char was affected by the pyrolysis reaction. For char analysis, amino acids of 200 mg and the mixtures of 400 mg (1:1, w/w) were pyrolyzed under the same conditions. 3. Results and discussion 3.1. Pyrolysis of single amino acid Three repeated tests on pyrolysis of leucine were carried out to find out the uncertainty quantification of the experimental results, and the relative standard deviation was 2.27%. So the results reported are quite reproducible. Figure 2 shows HCN and NH3 releasing curves from pyrolysis of the six amino acids. The results indicate that the structure of the amino acid has a significant effect on the nitrogen transformation to nitrogen species during amino acid pyrolysis. The light gases found in this study can be formed through direct pyrolysis of the amino acid
t0
ð1Þ Xi ¼
Ni 100% m N d =14
ð2Þ
Fig. 2. HCN and NH3 releasing curves from pyrolysis of the six amino acids: (a) NH3, (b) HCN.
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chain, and are also probably formed through cracking of primary tar products. Tar components from amino acids are 2,5-diketopiperazine (DKP) [3], which is cyclic amide. The decomposition of dipeptide and polypeptides at high temperature produce DKP. NH3 can be formed from individual molecular reaction of amino acid and may also be produced as a result of the secondary reactions [3,5,12, 15,19]. Both the decomposition reaction of primary amines and the bimolecular reactions between imine and amine can produce NH3 [3,13]. The intermediacy of 2,5-piperazinedione (DKP) provides an attractive explanation for the formation of HNCO, HCN and CO [3,13]. Imine is mainly formed through the decomposition of DKP. The formation of primary amines is governed by the decomposition of DKP into cyanide and amide, where the amide produces amine by loss of carbon monoxide [3]. The yields and conversion of nitrogen into HCN and NH3 during pyrolysis of the six amino acids are listed in Table 1. Significant yield of ammonia is observed from leucine pyrolysis and little ammonia is from aspartic acid. Meanwhile, proline has the most HCN yield and leucine has the least one during pyrolysis. From Table 1, phenylalanine and leucine undergo significant decomposition to HCN and NH3, more than 50% nitrogen in the amino acids is released as NH3 and HCN. There are only about 3.5% nitrogen contents in aspartic acid released as HCN and NH3 from pyrolysis. The total conversion of nitrogen into HCN and NH3 decreases in the order of leucine, phenylalanine, proline, glycine, glutamic acid and aspartic acid during pyrolysis. Aspartic acid has the biggest HCN/NH3 ratio about 12.231, while leucine has the smallest HCN/NH3 ratio about 0.004. The results indicate that XHCN and XNH3 have an obvious dependence on amino acid composition and different amino acids have distinctive N-conversion selectivity during pyrolysis. Experiments by our group on leucine, glycine and proline, all of which have non-reactive side chains, revealed that glycine produces char, while the other two amino acids do not produce any char. Amino acids with reactive side chains, glutamic acid, aspartic acid, and phenylalanine give
char yields. Based on the nitrogen emission characteristics of single amino acid, three representative amino acids, aspartic acid, proline and leucine were selected to study the interaction between amino acid with the component (cellulose, hemicellulose and lignin) and to investigate whether the HCN and NH3 yields from amino acid depend on the main constituents in biomass. 3.2. Co-pyrolysis of amino acid with cellulose, hemicellulose and lignin 3.2.1. Co-pyrolysis of aspartic acid with cellulose/ hemicellulose/lignin Figure 3 shows HCN and NH3 releasing curves from co-pyrolysis of aspartic acid and cellulose/ hemicellulose/lignin. NH3 releases ahead obviously in the presence of cellulose or lignin. NH3 releasing curve has one peak for single aspartic acid, while two peaks for co-pyrolysis of aspartic acid/cellulose or aspartic acid/lignin. For aspartic acid alone, the deamination reaction is minor, but the higher NH3 peak value (occurring at 14 s) indicates that there is a substantial deamination reaction during co-pyrolysis of aspartic acid/cellulose. The second NH3 release peak appearing at 56 s is from the secondary
Table 1 Yields and conversion of nitrogen into HCN and NH3 during amino acid pyrolysis. Sample
NNH3/lmol
NHCN/lmol
XNH3/%
XHCN/%
Asp Glu Gly Leu Phe Pro
0.199 3.619 19.217 61.707 20.080 3.954
2.434 6.828 9.508 0.260 11.967 22.644
0.264 5.319 14.412 80.836 33.131 4.547
3.238 10.038 7.131 0.341 19.745 26.040
Fig. 3. Nitrogen species releasing curves from co-pyrolysis of aspartic acid and components in biomass: (a) NH3, (b) HCN.
Q. Ren et al. / Proceedings of the Combustion Institute 33 (2011) 1715–1722
decomposition reaction of tar (DKP), the reaction of imines and amines to yield NH3, which is promoted during co-pyrolysis of aspartic acid and cellulose [5]. Compared with the effect of cellulose, the first NH3 release peak in the earlier period (occurring at 14 s) during co-pyrolysis of aspartic acid/lignin is also formed directly through deamination reaction of aspartic acid. The higher NH3 peak value indicates that aspartic acid undergoes intense deamination reaction in the presence of lignin. The phenomenon that the second NH3 release peak appearing at 36 s is earlier and its value is higher than that in the co-pyrolysis of aspartic acid and cellulose is due to the more intense secondary reactions of tar (DKP) in the presence of lignin and more hydrogen availability from higher H2 and CH4 yields generated from lignin pyrolysis [20]. As discussed above, the intermediacy of 2,5piperazinedione (DKP) provides an attractive explanation for HCN formation. As can be seen from Fig. 3, HCN releasing curves have higher peak values during co-pyrolysis compared with that from aspartic acid alone. One possible explanation is that indole, one nitrogen functionality of tars, is negligible from single aspartic acid pyrolysis [22], but has a high yield from co-pyrolysis of aspartic acid with cellulose, hemicellulose or lignin. The thermal cracking of indole contributes to increasing HCN formation. The yields and conversion of nitrogen into HCN and NH3 during co-pyrolysis of aspartic acid with cellulose, hemicellulose and lignin are illustrated in Table 2. C, H and L in the Table represent cellulose, hemicellulose and lignin, respectively. It indicates clearly that the conversion of nitrogen into NH3 and HCN increases remarkably during co-pyrolysis of aspartic acid and the components. But the extent is not the same. HCN yield from co-pyrolysis of aspartic acid/cellulose is the highest, and NH3 yield from co-pyrolysis of aspartic acid/lignin is the highest. HCN/ NH3 ratio decreases sharply from 12.231 for Table 2 Yields and conversion of N into HCN and NH3 from single amino acid and the mixture during pyrolysis. Sample
NNH3/lmol
NHCN/lmol
XNH3/%
XHCN/%
Asp Asp/C Asp/H Asp/L Pro Pro/C Pro/H Pro/L Leu Leu/C Leu/H Leu/L
0.199 7.012 9.244 15.826 3.954 8.185 10.190 12.860 61.707 48.922 32.316 42.802
2.434 16.988 11.161 10.775 22.644 2.469 2.986 4.371 0.260 2.460 7.390 4.396
0.264 9.326 12.295 21.048 4.547 9.412 11.719 14.789 80.836 64.088 42.334 56.070
3.238 22.594 14.844 14.331 26.040 2.840 3.434 5.026 0.341 3.223 9.681 5.759
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aspartic acid alone to 2.423, 1.207 and 0.681 for aspartic acid/cellulose, aspartic acid/hemicellulose and aspartic acid/lignin, respectively. It is interesting to note that HCN/NH3 ratio is less than one from co-pyrolysis of aspartic acid/lignin. It indicates that lignin has a different effect on the pathway of DKP cleavage from that of cellulose and xylan during aspartic acid pyrolysis. Lignin facilitates the secondary decomposition of DKP into amines rather than indole formation. The results make clear that the selectivity of N-conversion has an evident dependence on the cellulose, hemicellulose and lignin. Interactions between aspartic acid with cellulose, hemicellulose and lignin were further studied by analyzing the chars. Individual pyrolysis of aspartic acid, cellulose, hemicellulose and lignin at 800 °C yields chars of 9.15%, 1.85%, 14.63% and 47.71%, respectively. While the char yields from co-pyrolysis of aspartic acid/cellulose, aspartic acid/hemicellulose and aspartic acid/lignin are 8.87%, 19.07% and 33.32%, respectively. The results reveal that the weight losses of mixtures of aspartic acid/cellulose, aspartic acid/hemicellulose and aspartic acid/lignin are different from that of the individual sample, indicating the mixture undergoes solid-state reactions to produce volatile products and carbonaceous residue (i.e. char). Larger char yields could result in larger N-containing species yields since N-containing species can be released from pyrolysis of char [23]. The thermal history of the char formed from pyrolysis of mixtures of carbohydrates and amino acids can have an impact on the release of volatile products [24]. The catalytic thermal cracking of the N-containing char compounds from co-pyrolysis of aspartic acid and cellulose/hemicellulose/lignin may also contribute to the higher HCN emission. It is reported that these N-containing char compounds might react to form hydantoins (or imidazoledinediones, 5-membered cyclic amides), which can open to yield HCN [25,26]. Thus, to gain mechanistic insight into the formation of HCN and NH3 from the pyrolysis of amino acids and cellulose/hemicellulose/lignin, solid-state chemistry of the char and the gas phase chemistry need to be investigated in more detail. 3.2.2. Co-pyrolysis of proline with cellulose/hemicellulose/lignin NH3 and HCN releasing curves from co-pyrolysis of proline/cellulose, proline/hemicellulose and proline/lignin are displayed in Fig. 4. It can be seen that HCN releases fast at early step during single proline pyrolysis and NH3 has no release at this stage. It indicates that extent of deamination seems to be negligible for proline, and NH3 is mainly formed from the secondary thermal cracking of DKP.
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Fig. 4. Nitrogen species releasing curves from co-pyrolysis of proline and components in biomass: (a) NH3, (b) HCN.
The research indicated that at 600 °C, the only major product formed from proline pyrolysis was 2,5-diketopiperazine, which was formed by condensation of two proline molecules. The major products from proline pyrolysis at 700 °C were the 2,5diketopiperazine and pyrrole. Compared with the 600oC run, more low molecular weight products were formed, and traces of pyridine were found. At 800 °C, the dominant product from proline pyrolysis was pyrrole. There was only a small amount of 2,5-diketopiperazine remaining and small amounts of pyridine and pyrazine were found [27]. It indicates that dehydration reaction is minor for pyrolysis of proline at 800 °C. So pyrrole, the degradation product from the 2,5-diketopiperazine, contributes to the higher XHCN for single proline. During co-pyrolysis of proline and the components, HCN emission decreases evidently, indicating that the thermal cracking of pyrrole is suppressed in the presence of cellulose, hemicellulose or lignin, while NH3 releases faster and increases during co-pyrolysis. It appears that the cleavage of DKP is inclined to produce amines rather than pyrrole, the dominant amines decompose to NH3 during co-pyrolysis. In the presence of cellulose, NH3 releasing curve takes on bimodal shape with two peaks
occurring at 24 and 41 s, respectively. The first peak during co-pyrolysis of proline and cellulose may be formed through deamination reaction of single aspartic acid. The second peak is from thermal cracking of amines. 2,5-Diketopiperazine prefers to form imines and amines rather than pyrrole during the interaction of proline and cellulose. The co-pyrolysis of proline/cellulose, proline/ hemicellulose and proline/lignin mixtures are particularly interesting since single proline does not form any char at 800 °C. The char yields from co-pyrolysis of proline/cellulose, proline/hemicellulose and proline/lignin share 3.77%, 8.87% and 22.78%, respectively. The interesting finding is that char from the mixtures contains N. It should be noticed that cellulose, hemicellulose and lignin do not contain any nitrogen content. So the nitrogen in the char originates from the proline. The ratio of nitrogen converted to char-N is 0, 3.15%, 4.92% and 7.00% for single proline, proline/cellulose, proline/hemicellulose and proline/ lignin, respectively. The N/C ratios (w/w) in the char from proline/cellulose, proline/hemicellulose and proline/lignin are 0.070, 0.064 and 0.031, respectively. It seems that the extent of deamination is not negligible during co-pyrolysis. Even a small deamination can have a significant effect on ammonia yield and makes amino acids relatively less stable [28]. The results strongly confirm the occurrence of the solid-state reactions between proline and cellulose, hemicellulose or lignin during pyrolysis. It can be seen from Table 2 that the proline/ cellulose, proline/hemicellulose and proline/lignin blends produce significantly more NH3 and less HCN than proline alone. It seems that NH3 and HCN are formed via competitive reactions during pyrolysis of proline and the blends. Lignin enhances NH3 yield, and cellulose reduces HCN yield to the biggest extent. The co-pyrolysis leads to smaller nitrogen conversion to HCN and NH3 compared with the single proline. It should be noted that HCN/NH3 ratio decreases from 5.727 for single proline to less than one for co-pyrolysis of the blends. The selectivity of N-conversion into HCN and NH3 for proline is affected by cellulose, hemicellulose and lignin. 3.2.3. Co-pyrolysis of leucine with cellulose/hemicellulose/lignin Figure 5 shows the HCN and NH3 releasing curves from co-pyrolysis of leucine/cellulose, leucine/hemicellulose and leucine/lignin. It is clear that HCN release is far less than NH3 release, NH3 is the dominant nitrogen species for leucine. Decarboxylation is the main primary reaction for leucine pyrolysis [18], and amine is the main product from the decarboxylation reaction. Meanwhile, it was verified that amines, nitriles and alkenes were the major products and amine was detected in large quantities from leucine pyrolysis
Q. Ren et al. / Proceedings of the Combustion Institute 33 (2011) 1715–1722
Fig. 5. Nitrogen species releasing curves from co-pyrolysis of leucine and components in biomass: (a) NH3, (b) HCN.
verified by GC–MS [18,29]. Each amine, therefore, decomposes by the loss of two molecules of hydrogen to yield the corresponding nitrile [30]. So, besides the minor individual molecular reaction, the decomposition reaction of primary amines contributes to the dominant NH3 for leucine pyrolysis. The peak value of NH3 releasing curve decreases in the order of cellulose, lignin and hemicellulose, while that of HCN increases in the order of cellulose, lignin and hemicellulose during co-pyrolysis. As can be seen from Table 2, co-pyrolysis reduces NH3 yield and promotes HCN yield. It seems that NH3 and HCN formation is competitive each other during co-pyrolysis. Hemicellulose enhances NH3 yield, and cellulose reduces HCN yield to the biggest extent. The co-pyrolysis leads to smaller nitrogen conversion to HCN and NH3 compared with the single leucine. It should be noted that HCN/NH3 ratio increases from 0.004 for single leucine to 0.050, 0.229 and 0.103 for co-pyrolysis of leucine/cellulose, leucine/hemicellulose and leucine/lignin, respectively. The selectivity of N-conversion into HCN and NH3 for leuline is affected by cellulose, hemicellulose and lignin.
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Leucine does not form any char at 800 °C, which is similar to proline. The char yields from co-pyrolysis of leucine/cellulose, leucine/hemicellulose and leucine/lignin share 2.10%, 7.77% and 22.97%, respectively. Nitrogen element is also found in the char from the mixtures. The ratio of nitrogen converted to char-N is 0%, 0.88%, 3.28% and 4.39% for single leucine, leucine/cellulose, leucine/hemicellulose and leucine/lignin, respectively. The N/C ratios (w/w) in the char from leucine/cellulose, leucine/hemicellulose and leucine/lignin are 0.027, 0.043 and 0.017, respectively. This suggests that the interactions between leucine and cellulose/hemicellulose/lignin during co-pyrolysis are relatively mild compared with those for proline. It is likely that the secondary decomposition reactions of primary-formed amines prefer to nitrile and HCN formation through an imine intermediate from co-pyrolysis of leucine and cellulose, hemicellulose or lignin [5,30]. In other words, compared with single leucine, the secondary decomposition path of amines is inclined to HCN formation in the presence of cellulose, hemicellulose or lignin. Hemicellulose seems to have the strongest effect on decomposition of DKP, and cellulose has the weakest effect. Although proline and leucine are similar in some aspects, the effect of cellulose, hemicellulose or lignin on the pyrolysis of each of the two amino acids aforementioned is different. The extent of interaction depends on variety of the amino acids and the components in biomass. Hemicellulose has the strongest effect on the secondary reaction of DKP for leucine, while lignin has the strongest effect on the secondary reaction of DKP for proline. 4. Conclusion The study on N-behavior during pyrolysis of amino acid and the mixtures of amino acid individually with the main components in biomass provided several interesting findings. Amino acids with different structures have different nitrogen transformation during pyrolysis. The yields of NH3 and HCN increase during co-pyrolysis of aspartic acid individually with cellulose, hemicellulose or lignin. During co-pyrolysis of proline individually with cellulose, hemicellulose or lignin, HCN emission decreases remarkably, NH3 emission rises. During co-pyrolysis of leucine individually with cellulose, hemicellulose or lignin, HCN emission increases evidently, NH3 emission decreases Strong interactions are observed in the copyrolysis of amino acids individually with cellulose, hemicellulose or lignin. A significant yield of nitrogenous char is obtained from co-pyrolysis of cellulose, hemicellulose or lignin with proline as well as leucine, although proline or leucine volatilizes completely at 800 °C. Co-pyrolysis of amino
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acids with cellulose, hemicellulose or lignin mainly alters the secondary decomposition pathways of DKP. The magnitude of interaction is dependent on the variety of amino acid and the components in biomass. To obtain the mechanism of nitrogen transformation during biomass pyrolysis, further research is needed on the solid-state and gas phase reactions of amino acids and the main components in biomass. Acknowledgements This work was financed by the Special Fund of Transformation of Scientific and Technical Achievements in Jiangsu province, China (Project No.: BA2007023) and the Scientific Research Foundation of Graduate School of Southeast University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10. 1016/j.proci.2010.06.033. References [1] F. Winter, C. Wartha, H. Hofbauer, Bioresour. Technol. 70 (1) (1999) 39–49. [2] H. Zhou, A.D. Jensen, P. Glarborg, A. Kavaliauskas, Fuel 85 (5–6) (2006) 705–716. [3] K.M. Hansson, J. Samuelsson, C. Tullin, L.E. ˚ mand, Combust. Flame 137 (3) (2004) 265–277. A [4] F.J. Tian, J.L. Yu, L.J. Mckenzie, J. Hayashi, C.Z. Li, Energ. Fuel 21 (2) (2007) 517–521. ˚ mand, A. Habermann, F. [5] K.M. Hansson, L.E. A Winter, Fuel 82 (6) (2003) 653–660. [6] M. Becidan, Ø. Skreiberg, J.E. Hustad, Energy Fuel 21 (2) (2007) 1173–1180. [7] Q.Q. Ren, C.S. Zhao, X. Wu, C. Liang, X.P. Chen, J.Z. Shen, G.Y. Tang, Z. Wang, J. Anal. Appl. Pyrol. 85 (1–2) (2009) 447–453. [8] Q.Q. Ren, C.S. Zhao, X. Wu, C. Liang, X.P. Chen, J.Z. Shen, G.Y. Tang, Z. Wang, Bioresour. Technol. 100 (17) (2009) 4054–4057.
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