Energy & Fuels 2007, 21, 1173-1180
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NOx and N2O Precursors (NH3 and HCN) in Pyrolysis of Biomass Residues Michae¨l Becidan,* Øyvind Skreiberg, and Johan E. Hustad Department of Energy and Process Engineering, NTNU, Kolbjørn Hejes Vei 1A, NO-7491 Trondheim, Norway ReceiVed August 23, 2006. ReVised Manuscript ReceiVed December 14, 2006
Nitrogen release is a little known aspect of pyrolysis of biomass. In this study on thermally thick samples of three biomass residues with high N-content, the NOx precursors NH3 and HCN were measured with a Fourier transform infrared (FTIR) analyzer at different heating rates (low and high) and temperatures (400900 °C). The feedstocks investigated have been given scarce or no attention. At a high heating rate, (1) NH3 is the main N-compound with increasing yield with increasing temperature until reaching a plateau at 825900 °C at a conversion level of 31-38%; (2) HCN release is increasing sharply with temperature to reach a conversion of 9-18%; (3) the (HCN + NH3) conversion levels of all samples are close; (4) N-selectivity is affected by temperature and particle size; (5) release patterns and thermal behaviors of N and C are different and influence of fuel properties (intrinsic and physical) may be inferred; (6) the intricate structure of biomass indicates that decomposition paths may include (N-compounds + non-N-compounds) reactions. At a low heating rate, (1) NH3 is the main N-compound; (2) HCN and NH3 release are significantly different for the various fuels (7.9-19.2%) and fuel properties (intrinsic and physical) might be of importance; (3) the release pattern of N is affected by fuel properties.
1. Introduction On the one hand, nitrogen (N) can be considered a minor component of biomass; on the other hand, N is a major challenge both directly through NOx and N2O emission and its environmental consequences (increase of the greenhouse effect, acid rain, etc.) and indirectly through involvement in production of ground-level ozone and stratospheric ozone destruction. The main volatile NOx precursors are NH3 (ammonia) and HCN (hydrogen cyanide). The study of NH3 and HCN release, and their relative importance and yields, is a key to optimize NOx reduction as this will affect N-chemistry after the devolatilisation/pyrolysis step. The main motivation of this work is to study the influence of the operating parameters on N-release as NH3 and HCN during pyrolysis (primary step of combustion and gasification) for thermally thick samples as they depict practical conditions encountered in industrial equipment (combustor and gasifier) better. An extensive study of the thermal history of the samples can be found elsewhere.1 The biomass residues selected have never been studied before. NH3 and HCN release from biomass pyrolysis has been studied in two ways: the study of biomass itself and the study of model compounds, i.e., chemical compounds depicting a single chemical functionality of N. Pyrolysis of N-containing model compounds has been used to characterize the main N-decomposition products and decomposition paths. The main classes of model compounds studied are amino acids/proteins and pyrrole/pyridine (aromatic N-heterocycles) as they are two representative biomass N-functionalities. During the pyrolysis of amino acids/proteins at 700-1000 °C, HCN was identified * To whom correspondence should be addressed. Telephone: (+47) 73 59 29 11. Fax: (+47) 73 59 83 90. E-mail:
[email protected]. (1) Becidan, M.; Skreiberg, Ø.; Hustad, J. E. Experimental study on pyrolysis of thermally thick biomass residues samples: intra-sample temperature distribution and effect of sample size (“scaling effect”). Manuscript submitted for publication.
as the main product in most cases.2-4 The main decomposition pathway for amino acids and proteins is the formation of cyclic amides by dehydration, the most common one being diketopiperazine (or DKP).2,5-8 The pyrolysis of pyrrole and pyridine at 800-1100 °C also produces HCN as the main N-product.3,9-17 The N-aromatic compounds’ decomposition mechanism may be described as such: breaking of bonds in the ring and/or rupture of the C-N bond followed by random cleavage/ recombination of the resulting diradical.17 Yet, the decomposition of pyridinic- and pyrrole-type compounds10 reports significant amounts of NH3. This shows that the vicinity of functional (2) Haidar, N. F.; Patterson, J. M.; Moors, M.; Smith, W. T. J. Agric. Food Chem. 1981, 29, 163-165. (3) Johnson, W. R.; Kang, J. C. J. Org. Chem. 1971, 36, 189-192. (4) Hansson, K.-M. Principles of biomass pyrolysis with emphasis on the formation of the nitrogen-containing gases HNCO, HCN and NH3. Ph.D. Thesis, Chalmers University of Technology, Go¨teborg, Sweden, 2003. (5) Chiavari, G.; Galletti, G. C. J. Anal. Appl. Pyrolysis 1992, 24, 123137. (6) Simmonds, P. G.; Medley, E. E.; Ratcliff, M. A.; Shulman, G. P. Anal. Chem. 1972, 44, 2060-2066. (7) Ratcliff, M. A.; Medley, E. E.; Simmonds, P. G. J. Org. Chem. 1974, 39, 1481-1490. (8) Basiuk, V. A. J. Anal. Appl. Pyrolysis 1998, 47, 127-143. (9) Patterson, J. M.; Tsamasfyros, A.; Smith, W. T. J. Heterocycle Chem. 1968, 5, 727-729. (10) Ha¨ma¨la¨inen, J. P.; Aho, M. J.; Tummavuori, J. L. Fuel 1994, 73, 1894-1898. (11) Axworthy, A. E.; Dayan, V. H.; Martin, G. B. Fuel 1978, 57, 2935. (12) Ikeda, E.; Mackie, J. C. J. Anal. Appl. Pyrolysis 1995, 34, 47-63. (13) Mackie, J. C.; Colket, M. B., III; Nelson, P. F. J. Phys. Chem. 1990, 94, 4099-4106. (14) Houser, T. J.; McCarville, M. E.; Biftu, T. Int. J. Chem. Kinet. 1980, XII, 555-568. (15) Houser, T. J.; Hull, M.; Always, R. M.; Biftu, T. Int. J. Chem. Kinet. 1980, XII, 569-574. (16) Mackie, J. C.; Colket, M. B., III; Nelson, P. F.; Elser, M. Int. J. Chem. Kinet. 1991, 23, 733-760. (17) Lifshitz, A.; Tamburu, C.; Suslensky, A. J. Phys. Chem. 1989, 93, 5802-5808.
10.1021/ef060426k CCC: $37.00 © 2007 American Chemical Society Published on Web 02/15/2007
1174 Energy & Fuels, Vol. 21, No. 2, 2007
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Table 1. Model Compounds versus Biomass: Literature Results from Pyrolysis.2-5,9-17,25-33
groups such as sOH or sCdO may influence N-chemistry but no clear mechanism has been established yet. N-release study of model compounds provides interesting information but should be looked upon carefully because even if biomass-N is mainly found in amino acids/proteins and/or aromatic N-heterocycles, it is also true that N-compounds are only a minor part of the macro structure of biomass. It has been stated that proteins are reacting independently from the main biomass components (hemicellulose, cellulose, and lignin)18 because of their compact structure, but it must be argued that reality is more complicated as proteins can be linked with other biomass components in a variety of ways (H bonds, covalent bonds, ionic bonds)19-24 and therefore intimately integrated into the biomass. Their integration and interaction are difficult to comprehend but show that N-compounds are most probably not devolatilisating independently from the rest of the biomass. This fact brings limitations to the validity of N-model compounds results (decomposition mechanisms) as they do not represent fully the intricate structure of biomass. This calls for the study of biomass itself, in spite of its complex constitution which hinders clear mechanistic insights. N-release during pyrolysis of thermally thick samples has not been as extensively studied as coal N-release and remains little known. The results from the literature2-5,9-17,25-33 are exhibiting obvious discrepancies. However, some conclusions may be extracted and compared with results from model compounds according to some key features listed in Table 1. The main scatter is the substantial amount of NH3 produced by most biomasses. Several sources of NH3 are foreseeable,34 but no clear answer has been provided yet. However, a strong correlation between nascent char/char (18) Samuelsson, J. I. Conversion of nitrogen in a fixed burning biofuel bed. Licentiate thesis, Chalmers University of technology, Go¨teborg, Sweden, 2006. (19) Grøndal, J. Utilization of brewers spent grains fractions as ingredients in the food and feed industry; project No. 244, an industrial research education programme under the Danish academy of technical sciences, 1990. (20) Egi, A.; Speers, R. A.; Schwarz, P. B. MBAA TQ 2004, 41, 248267. (21) Rimsten, L. Extractable cell-wall polysaccharides in cereals, with emphasis on β-glucan in steeped and germinated barley. Doctoral dissertation, Swedish University of Agricultural Sciences, Uppsala, Sweden, 2003. (22) Monro, J. A.; Bailey, R. W.; Penny, D. Phytochemistry 1976, 15, 175-181. (23) Monro, J. A.; Bailey, R. W.; Penny, D. Phytochemistry 1974, 13, 375-382. (24) Redgwell, R. J.; Trovato, V.; Curti, D.; Fischer, M. Carbohydr. Res. 2002, 337, 421-431.
formation and NH3 production has been observed,18,25,34-36 but more work is necessary to assess the origin(s) of this NH3. The object of the present investigation was twofold: (1) to study the temperature and heating rate dependence of the formation of NH3 and HCN during pyrolysis of thermally thick biomass residues samples in an in-house designed and fabricated batch reactor; (2) to study little known biomass residues with high N-contents. 2. Experimental Section 2.1. Reactor and Procedures. The reactor and setup were described previously;37 the reactor is basically a vertical tube with an Al2O3 ceramic coating. Two procedures were carried out and comprehensively described in a previous work:37 “fast/high” heating rate pyrolysis experiments (sudden introduction in a hot reactor) and “slow/low” heating rate pyrolysis experiments (application of a 10 K/min heating rate at the reactor walls). The Fourier transform infrared (FTIR) analysis of the gases was performed with a Bomem 9100 analyzer. The instrument is equipped with a deuterated triglycine sulfate (DTGS) detector. The FTIR was used to quantify HCN, NH3, CO2, CO, CH4, C2H2, and C2H4. The gas samples were also quantified online using a Varian CP-4900 micro gas chromatograph (CO2, CH4, C2H2 + C2H4, C2H6, H2, O2, CH4, CO, and N2). Results for non-N-compounds are presented elsewhere.37 2.2. Gas Identification. 2.2.1. FTIR. Table 2 shows the main characteristics of the FTIR analyzer. The uncertainties were calculated from the experimental data according to the procedure proposed in ref 38. 2.2.2. FTIR: HCN and NH3. The HCN IR-spectrum was (25) Li, C.-Z.; Tan, L. L. Fuel 2000, 79, 1899-1906. (26) Tian, F.-J.; Yu, J.-l.; Mckenzie, L. J.; Hayashi, J.-i.; Chiba, T.; Li, C.-Z. Fuel 2005, 84, 371-376. (27) Predel, M.; Kaminsky, W. Bioresour. Technol. 1998, 66, 113-117. (28) Hansson, K.-M.; Samuelsson, J.; Tullin, C.; A° mand, L.-E. Combust. Flame 2004, 137, 265-277. (29) Tan, L. L.; Li, C.-Z. Fuel 2000, 79, 1883-1889. (30) Leppa¨lahti, J. Fuel 1995, 74, 1363-1368. (31) Aho, M. J.; Ha¨ma¨la¨inen, J. P.; Tummavuori, J. L. Combust. Flame 1993, 95, 22-30. (32) Vriesman, P.; Heginuz, E.; Sjo¨stro¨m, K. Fuel 2000, 79, 1371-1378. (33) Ha¨ma¨la¨inen, J. P.; Aho, M. J. Fuel 1996, 75, 1377-1386. (34) Tian, F.-J.; Wu, H.; Yu, J.-l.; McKenzie, L. J.; Konstantinidis, S.; Hayashi, J.-i.; et al. Fuel 2005, 84, 2102-2108. (35) Hansson, K.-M.; Samuelsson, J.; A° mand, L.-E.; Tullin, C. Fuel 2003, 82, 2163-2172. (36) Glarborg, P.; Jensen, A. D.; Johnsson, J. E. Prog. Energy Comb. Sci. 2003, 29, 89-113. (37) Becidan, M.; Skreiberg, Ø.; Hustad, J. E. J. Anal. Appl. Pyrolysis 2007, 78, 207-213.
Pyrolysis of Biomass Residues
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Table 2. FTIR Characteristics species
NH3
HCN
calibration range (ppm)
0-3006
0-1990
wavenumbers (cm-1) used for quantification
1142.26-1138.88 1124.30-1120.00
4006.00-4000.00 3374.70-3372.29
quantification method
height and area
height and area
uncertainty high range: relative standard deviation (%)
2.8 (over 300 ppm)
8.4 (over 200 ppm)
uncertainty low range: standard deviation (ppm)
8 (0-300 ppm)
45 (0-200 ppm)
Table 3. Proximate Analysis and Ultimate Analysis of the Samples brewer spent grains volatile matter fixed carbon ash
coffee waste
fiberboard
proximate analysis (wt %, dry basis) 78.75 76.67 16.22 16.75 5.03 6.58
81.95 17.61 0.44
ultimate analysis (wt %, dry ash free basis) carbon 51.59 51.33 hydrogen 7.07 6.79 nitrogen 4.15 3.02 sulfur 0.23 0.21 oxygen (by difference) 36.96 38.65
48.80 6.33 3.62
