Article pubs.acs.org/est
NOx and N2O Precursors from Biomass Pyrolysis: Role of Cellulose, Hemicellulose and Lignin Qiangqiang Ren†,‡,* and Changsui Zhao‡ †
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China School of Energy and Environment, Southeast University, Nanjing 210096, Jiangsu Province, China
‡
ABSTRACT: Cellulose, hemicellulose, and lignin play important roles in biomass. Nitrogen in biomass is mainly in forms of proteins (amino acids). Two amino acids, proline and glutamic acid, with different structures were selected as the nitrogen-containing model compound in biomass. Interaction between the two amino acids with cellulose, hemicelluloses, or lignin at different weight ratios was investigated to understand nitrogen chemistry. Considering the composition of wood and agricultural straw, proline and the mixture of cellulose, hemicellulose, and lignin were pyrolyzed under the same condition. Nitrogen transformation during copyrolysis of amino acid with the component at different ratios was identified to determine the role of cellulose, hemicellulose, and lignin. The emissions of HCN and NH3 were detected with a Fourier transform infrared (FTIR) spectrometer. The results indicate that although the structure of the amino acid has a significant effect on the nitrogen transformation during pyrolysis, it is interesting to find some characteristics in common for the aliphatic amino acid and heterocyclic amino acid. The effects of hemicellulose on NH3 formation from the two amino acids are similar, hemicellulose inhibits N−NH3 conversion and lignin promotes NH3 formation for the two amino acids.
1. INTRODUCTION Straw is a widely available herbaceous biomass fuel with relatively high nitrogen content compared with many woodderived fuels. Gasification- and combustion-based technology has become the most promising technology for large-scale utilization of biomass. Biomass nitrogen can be transformed into environmentally harmful gases under combustion.1−3 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. It is accepted that the combined study of biomass and nitrogencontaining model compounds (amino acid, etc.) is the best approach to grasp a more complete mechanism of Nchemistry.4−14 In our previous studies,15−18 it was found that biomass intrinsic properties, such as the amino acid composition, mineral matter and atmosphere influence the selectivity of Nconversion into NH3, HCN, and HNCO. It is well-known that cellulose, hemicelluloses, and lignin are the major components in biomass, and the effect of these components on the conversion of amino acid has been conducted.19 Amino acid sample was pyrolyzed with cellulose, hemicellulose, or lignin with 1:1 mixing ratio by weight at 800 °C in an argon atmosphere. The results indicate that the mixtures undergo solid-state decomposition reactions during copyrolysis. HCN and NH3 yields and nitrogen conversion pathway from amino acid pyrolysis are influenced by cellulose, hemicellulose, and lignin. What should be noticed is that the above results were based on the ratio of amino acid to cellulose, hemicellulose or lignin was 1:1. In fact, the weight fraction of amino acid in biomass is much smaller. Thus, nitrogen conversion at different © 2013 American Chemical Society
weight ratio of amino acid to cellulose, hemicellulose, or lignin should be further studied. On the other hand, cellulose, hemicellulose, and lignin are linked with each other in biomass, so the performance of nitrogen transformation during copyrolysis of amino acid and the mixture of cellulose, hemicellulose, and lignin is also of great interest. The objective of this study is to identify nitrogen transformation during copyrolysis of amino acids with the components (cellulose, hemicellulose, and lignin) at different ratios to determine the role of cellulose, hemicellulose, and lignin on biomass-N chemistry. The extent of interaction between the amino acid with cellulose, hemicellulose and lignin is determined by comparing the yields of HCN and NH3 from copyrolysis with those from individual pyrolysis of the amino acids under the same condition.
2. EXPERIMENTAL SECTION Samples. In order to avoid biased understanding of nitrogen chemistry from amino acid pyrolysis, the structures of the amino acids were taken into consideration. Two representative amino acids, Proline (Pro), which is a heterocyclic amino acid, and glutamic acid (Glu), which is an aliphatic amino acid, were used in the study. The amino acid samples were commercially available samples purchased from the Sigma Chemical Co. without further purification. The sample had a purity of 98.5% or more. Received: January 23, 2013 Accepted: June 28, 2013 Published: July 12, 2013 8955
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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 Here, xylan processed from Birchwood, in yellow powder form, was used as hemicellulose. Particle size of hemicellulose is averaged at ∼100 μm and those of cellulose and lignin are at ∼50 μm. Each amino acid sample of 10 ± 0.01 mg mixed with cellulose (1:1, 1:2, 1:3, w/w), hemicellulose (1:1, 1:2, 1:3, w/ w) or lignin (1:1, 1:2, 1:3, w/w) was prepared. Based on typical composition of agricultural straw and wood, cellulose, hemicelluloses and lignin were mixed with three mass ratios (50:30:20, 30:50:20, 20:20:60). The blend of cellulose, hemicellulose, and lignin was mixed with proline with ratio of 1:1 and was copyrolyzed to compare with those from copyrolysis of proline with cellulose, hemicelluloses, or lignin. Apparatus and Product Analysis. The pyrolysis experiments were performed in a horizontal fixed-bed tubular reactor (40 mm i.d., 60 mm o.d., and 400 mm length). The pyrolysis temperature was 800 °C for simulating the temperature of the pyrolysis zone of straw burning. 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 detailed information about the apparatus and gaseous product analysis method can be referred to ref 19.
Figure 1. NH3 and HCN releasing curves from copyrolysis of proline with cellulose.
3. RESULTS AND DISCUSSION Reproducibility of the System. Three repeated tests on pyrolysis of proline 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. Co-Pyrolysis of Proline Separately with Cellulose/ Hemicellulose/Lignin. NH3 and HCN release curves from copyrolysis of proline with cellulose at different mass ratios are displayed in Figure 1. The results are presented briefly here, and comparison is made with copyrolysis of proline and cellulose, hemicellulose, or lignin as well as the pyrolysis of proline in the absence of the component. HCN releases fast at early step during single proline pyrolysis and NH3 has no release at this stage due to the negligible deamination for proline itself, NH3 is mainly formed form the secondary thermal cracking of DKP.19,20 The dominant product from proline pyrolysis was pyrrole at 800 °C, and only a small amount of 2,5-diketopiperazine (DKP) remaining and small amounts of pyridine and pyrazine were found.20 It indicates that dehydration reaction is minor for pyrolysis of proline itself at 800 °C. So pyrrole, the degradation product from the 2,5diketopiperazine, contributes to the higher HCN for single proline. During copyrolysis of proline and cellulose, HCN emission decreases evidently, as cellulose mass share increases, HCN emission has a little increase, but is still far less than that for proline itself. It indicates that the presence of cellulose no doubt inhibits the thermal cracking of pyrrole. It is interesting to find that HCN releasing curve takes on bimodal shape with two peaks when cellulose is introduced. As can be seen from
the figure, the first peak appears earlier than that of individual proline, and the peak becomes weaker. As for the second peak, it occurs later than that of individual proline. The discussion should be combined with NH3 performance. As cellulose addition increases, NH3 releases faster and starts to release earlier during copyrolysis. The presence of cellulose inspires the conversion of nitrogen into NH3. NH3 releasing curve takes on bimodal shape with two peaks when the ratio of cellulose to proline is 1:1. The first peak during copyrolysis of proline and cellulose is small and may be formed through deamination reaction of individual proline. It is found that as the ratio of cellulose to proline increases to 2:1 or 3:1, the peak occurring at late period of pyrolysis process disappears, and the peak arising at earlier period goes stronger. Considering HCN formation discussed above, it is likely that the stronger deamination reaction of proline contributes to the earlier peak of NH3 and the secondary reactions between amine and imines prefer to form HCN rather than NH3 in the presence of cellulose. The conversion of N-HCN and N-NH3 are competitive reactions during copyrolysis of proline and cellulose. Table 1 shows the yield and conversion of HCN and NH3 during proline pyrolysis with cellulose. In the presence of cellulose, more NH3 and less HCN have been produced. As the cellulose share increases, this effect is more intense. When the ratio of cellulose to proline increases to 3:1, NH3 yield is the biggest and increases by almost 1.6 times compared with individual proline. As for HCN, the influence of cellulose is different, when the ratio of cellulose to proline is 1:1, HCN yield is the smallest and decreases by 89% compared with individual proline. 8956
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cracking of pyrrole. It is a bit odd to find that when the ratio of hemicellulose to proline is 2:1, HCN yield has a sharp increase, which is similar to that of cellulose and lignin at the same ratio. When the ratio of hemicellulose to proline is 1, NH3 releases faster and has the biggest yield. As the ratio continues to increase, NH3 emission decreases and has the least emission at the ratio of 3. It is different from the effect of cellulose. It seems that hemicellulose of certain amount promotes the reaction of amines to form NH3, but more hemicellulose inhibits the reaction. When the ratio of hemicellulose to proline is 1:1, NH3 yield has an increase, which is similar to that of cellulose at the same ratio. However, when the hemicellulose share further increases, it is a bit odd to find that NH3 yield has a prompt reduction. Repeated test results have clarified this phenomenon. NH3 formation during glutamic acid pyrolysis in the presence of hemicellulose will be focused to know the effect of hemicellulose. As can be seen from Table 1, the presence of hemicellulose has complicated effects on the formation of NH3 and less HCN. When the ratio of hemicellulose to proline is 3, both NH3 and HCN are suppressed and the yields of NH3 and HCN are the least, and the conversion of nitrogen into NH3 and HCN can reach 4.3% and 2.8%, respectively. NH3 and HCN releasing curves from copyrolysis of proline with lignin at different mass ratios are displayed in Figure 3.
Table 1. Yield and Conversion of HCN and NH3 during Proline Pyrolysis with the Component sample
NNH3/μmol
NHCN/μmol
XNH3/%
XHCN/%
pro pro+cellulose (1:1) pro+cellulose (1:2) pro+cellulose (1:3) pro+hemicellulose (1:1) pro+hemicellulose (1:2) pro+hemicellulose (1:3) pro+lignin (1:1) pro+lignin (1:2) pro+lignin (1:3)
3.954 8.184 8.187 10.269 10.191 3.968 3.714 12.861 8.918 10.394
22.644 2.469 7.389 3.848 2.986 10.603 2.400 4.370 9.094 4.732
4.547 9.412 9.415 11.809 11.719 4.563 4.271 14.790 10.256 11.953
26.040 2.840 8.497 4.425 3.433 12.194 2.760 5.025 10.458 5.442
For individual proline, the conversion of nitrogen into NH3 and HCN is about 4.5% and 26%, respectively. The cellulose favors to N-NH3 and suppresses N-HCN. In the presence of cellulose, the conversion of nitrogen into NH3 and HCN can reach 11.8% and 2.8%, respectively. The total nitrogen conversion to HCN and NH3 decreases compared with proline. The selectivity of N-conversion into HCN and NH3 for proline is affected by cellulose share. NH3 and HCN release curves from copyrolysis of proline with hemicellulose at different mass ratios are displayed in Figure 2. During copyrolysis of proline and hemicellulose, HCN emission decreases evidently. It is interesting to find that as the ratio of hemicellulose to proline increases to 2, HCN emission increases and is still less than that for proline itself. When the ratio increases to 3, HCN has the least emission. It indicates that the presence of hemicellulose has the same effect on HCN formation to cellulose and inhibits the thermal
Figure 3. NH3 and HCN releasing curves from copyrolysis of proline with lignin.
During copyrolysis of proline and lignin, HCN emission decreases evidently, as the ratio of lignin to proline is 2, HCN emission has a little increase, but still far less than that for proline itself. As lignin share increases, NH3 releases faster and starts to release earlier during copyrolysis. The presence of lignin has inspired the conversion of nitrogen into NH3. It indicates that the presence of lignin has the same effect on the
Figure 2. NH3 and HCN releasing curves from copyrolysis of proline with hemicellulose. 8957
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As can be seen from Figure 4, the peak value of NH3 release curve is bigger during copyrolysis of glutamic acid and cellulose. However, at late period of pyrolysis process, NH3 formation decreases with the increase of cellulose share. For HCN formation, it is clear the presence of cellulose promotes HCN formation. The formation of heterocyclic amines from decarboxylation reaction of glutamic acid is restrained which results in less NH3 formation, and the secondary decomposition of nitrogen-containing aromatic ring favors to HCN formation in the presence of cellulose. The effects of cellulose on glutamic acid and proline are totally different, and the difference is probably focused on the effect of cellulose on the selectivity of secondary cleavage of 2,5-diketopiperazine. Table 2 shows the yield and conversion of HCN and NH3 during glutamic acid pyrolysis with the component. For
conversion of N-HCN and N-NH3 compared with cellulose. As can be seen from Table 1, the presence of lignin promotes NH3 formation and reduces HCN formation. When the ratio of lignin to proline is 1:1, NH3 yield is the biggest and increases by almost 2.3 times compared with that of individual proline, meanwhile HCN yield is the smallest and decreases by 81%. The copyrolysis leads to bigger conversion into N-NH3 and smaller one into N-HCN. When the ratio is 3, the total conversion of nitrogen into NH3 and HCN is the least. The main difference is that the extent of the reaction between cellulose and lignin with proline is different, and it shows that lignin has stronger effect on promoting NH3 emission and cellulose has stronger effect on restraining HCN formation. The differences in the inherent structures and chemical nature of the three components possibly account for the different behaviors observed. Co-Pyrolysis of Glutamic Acid Separately with Cellulose/Hemicellulose/Lignin. Figure 4 shows HCN and
Table 2. Yields and Conversion of HCN and NH3 during Glutamic Acid Pyrolysis with the Component sample
NNH3/ μmol
NHCN/ μmol
XNH3/%
XHCN/%
glu glu+cellulose (1:1) glu+cellulose (1:2) glu+cellulose (1:3) glu+hemicellulose (1:1) glu+hemicellulose (1:2) glu+hemicellulose (1:3) glu+hemicellulose (1:10) glu+lignin (1:1) glu+lignin (1:2) glu+lignin (1:3)
3.619 2.475 1.599 1.733 8.008 5.455 4.144 0.287 13.527 12.506 11.917
6.828 9.459 11.480 14.044 8.876 7.609 8.631 13.496 10.849 6.987 7.244
5.319 3.638 2.350 2.547 11.770 8.017 6.090 0.422 19.882 18.381 17.515
10.038 13.906 16.876 20.646 13.049 11.187 12.688 19.841 15.949 10.272 10.649
individual glutamic acid, the conversion of nitrogen into NH3 and HCN is about 5.3% and 10%, respectively. The copyrolysis leads to less N-NH3 conversion and bigger N-HCN. When the ratio of cellulose to glutamic acid is 3:1, the conversion of nitrogen into NH3 and HCN can reach 2.5% and 20.6%, respectively. The total nitrogen conversion to HCN and NH3 increases with cellulose. NH3 and HCN release curves from copyrolysis of glutamic acid with hemicellulose are displayed in Figure 5. Because of the odd phenomenon occurring in proline pyrolysis with hemicellulose, the effect of hemicellulose on glutamic acid is also focused. As can be seen from Figure 5, when the ratio is 1:1, NH3 formation is higher than that of individual glutamic acid, and there is an increasing formation of NH3 formation at late pyrolysis period, which is quite different from the effect of cellulose. It seems that cellulose plays a role in the early stage of glutamic acid pyrolysis and hemicellulose has a big effect on the secondary reaction of glutamic acid. It is also observed that as the hemicellulose share increases, NH3 release is postponed and the formation decreases, especially when the ratio of hemicelluloses to glutamic acid increases by 10:1, little HCN is detected surprisingly. The general effect of hemicellulose on glutamic acid and proline is similar. As for HCN, it is clear that HCN formation increases with increasing hemicellulose share, which is different from proline. It seems that hemicellulose and cellulose have similar effects from the perspective of HCN formation from glutamic acid. From Table 2, it can be clear about the effect of hemicellulose share on the yields of HCN and NH3. When the ratio is up to 10:1, the conversion of nitrogen into NH3 and
Figure 4. NH3 and HCN releasing curves from copyrolysis of glutamic acid with cellulose.
NH3 release curves from glutamic acid pyrolysis with cellulose. During pyrolysis process, glutamic acid underwent virtually no deamination. Heterocyclic amines has been detected during glutamic acid pyrolysis with high performance liquid chromatography (HPLC).21,22 The NH3 formation is mainly produced from the secondary reactions between imine and amines. Glutamic acid underwent only monodecarboxylation reaction and the other carboxyl is incorporated into a pyrrolidinone ring with subsequent formation of pyrrole.23 Glutamic acid can form a relatively stable nitrogen-containing aromatic ring in the early steps of the pyrolysis, especially pyrrole and nitriles,11,13 which contribute to the formation of HCN. 8958
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Figure 6. NH3 and HCN releasing curves from copyrolysis of glutamic acid with lignin. Figure 5. NH3 and HCN releasing curves from copyrolysis of glutamic acid with hemicellulose.
cellulose, and lignin were performed to understand the role that the three components play part in. The mixed ratio of cellulose, hemicellulose, and lignin was basically according to different biomass composition. In the study, three mixed ratio is selected to represent hard straw, soft straw and woody biomass. 50%:30%:20% represents wheat straw, 30%:50%:20% represents cotton stalk and 20%:20%:60% represents wood. The results from copyrolysis of proline and the mixture at varying cellulose/hemicellulose/lignin weight ratios are presented in Figure 7. A comparison was made between the amounts of the identified nitrogen-containing species obtained from copyrolysis of proline with the mixture and the amounts of the same compounds from copyrolysis of the amino acid with one of cellulose, hemicelluloses and lignin. It is to judge which one plays the main part among the mixture during copyrolysis of proline and the mixture. The shape of copyrolysis of proline with the mixture, with respect to HCN and NH3 release curves, are very similar to those of copyrolysis of proline with one of the mixture, shown in from Figures 1−3. During copyrolysis of proline with the mixture, HCN emission decreases evidently, NH3 releases faster and starts to release earlier during copyrolysis. Table 3 shows the yield and conversion of HCN and NH3 during copyrolysis of proline with the mixture of cellulose, hemicelluloses, and lignin. C, H, and L represent cellulose, hemicellulose, and lignin, respectively. C50/H30/L20 means the mixture comprised of 50% cellulose, 30% hemicelluloses, and 20% lignin. Co-pyrolysis of proline with the mixture gives almost the same NH3 quantitatively as those obtained by copyrolysis of proline with cellulose or lignin. HCN yield is higher than that from copyrolysis of proline with anyone of the
HCN can reach 0.4% and 19.8%, respectively. Hemicellulose favors to the conversion of N-glutamic acid into HCN. NH3 and HCN release curves from pyrolysis of glutamic acid with lignin are displayed in Figure 6. It is clear that lignin has positive effects on the formation of HCN and NH3. From this view, lignin has different effects on glutamic acid compared with cellulose and hemicellulose. As can be seen from Table 2, lignin promotes the yields of NH3 and HCN, and it should be especially noticed that NH3 yield does not have great change when the lignin share increases. Generally, lignin favors to the conversion of N-glutamic acid into NH3. According to the above analysis, it is interesting to find some characteristics in common for the role of the components in nitrogen conversion from the two amino acids, one is aliphatic amino acid and the other is heterocyclic amino acid. The effects of hemicellulose on NH3 formation from the two amino acids are similar, and lignin promotes NH3 formation from the two amino acids. Of course, there are some distinct influences, for glutamic acid, the presence of cellulose, hemicellulose or lignin promotes the conversion of N-HCN. While for proline, the components suppress the conversion of N-HCN. This can help to understand why some kinds of biomass produce more NH3 than HCN, and in some cases more HCN than NH3 is produced during pyrolysis. Co-Pyrolysis of Proline with the Mixture of Cellulose, Hemicellulose, and Lignin. It is accepted that different compositions of biomass contribute to different N-conversion.5,15 In fact, cellulose, hemicellulose, and lignin are the main components and are linked with each other in biomass. So copyrolysis of proline and the mixture of cellulose, hemi8959
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Figure 7. NH3 and HCN releasing curves from copyrolysis of proline and the mixture.
Table 3. Yields and Conversion of HCN and NH3 during Proline Pyrolysis with the Mixture sample
NNH3/μmol
NHCN/μmol
XNH3/%
XHCN/%
pro pro +C50/H30/L20 pro +C30/H50/L20 pro +C20/H20/L60
3.954 9.362 10.202 6.831
22.644 8.396 10.863 14.078
4.547 10.766 11.732 7.855
26.040 9.655 12.492 16.189
cellulose, hemicelluloses, or lignin. It indicates that the reactions among cellulose, hemicellulose, and lignin have some effects on nitrogen conversion from biomass pyrolysis.
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AUTHOR INFORMATION
Corresponding Author
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[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 (No. 51106157). REFERENCES
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