Article pubs.acs.org/EF
Nitrogen in Biomass Char and Its Fate during Combustion: A Model Compound Approach Leilani I. Darvell,*,† Celeste Brindley,‡ Xiaomian C. Baxter,† Jenny M. Jones,† and Alan Williams† †
Energy Research Institute, School of Process, Environmental and Materials Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom ‡ Department of Chemical Engineering, University of Almería, 04120 Almería, Spain ABSTRACT: The emission of nitrogen oxide (NOx) from biomass combustion is still a concern, even though the nitrogen content of biomass in general is relatively small. To elucidate the pathways of fuel N conversion during combustion processes, many studies have focused on the pyrolysis of amino acids and/or model compounds. In contrast, the model compound char nitrogen and its fate during combustion have not been investigated. The partitioning of nitrogen between the volatiles and char and the type of nitrogen species released during char combustion are very important, because volatile N is better controlled by low NOx burners than the nitrogen retained in the char. In this study, low heating rate biomass model chars were prepared from a combination of either cellulose or glucose and five of the amino acids most commonly found in terrestrial biomass: L-proline, Lglutamine, L-histidine, L-asparagine, and L-tryptophan. The chars were characterized by X-ray photoelectron spectroscopy (XPS), and the partitioning of nitrogen was also calculated. Furthermore, the impact of protein nitrogen on emissions of nitrogen species from combustion processes was investigated by thermogravimetric analysis−mass spectrometry (TGA−MS) combustion tests, which were used to calculate the char combustion kinetics and conversion of char nitrogen to different nitrogen-containing species. Results indicate that there may be a correlation between the proportion of pyridinic N and pyrrolic N (and/or possibly some quaternary N) in the char and the N precursor. However, further correlations to the N species formed during the char combustion to the N functionalities in the chars were not obtained and merit further investigation. Finally, the char yields obtained and N partitioning of the model chars are comparable to those reported in the literature for biomass chars.
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INTRODUCTION The combustion of sustainable biomass can lead to an overall reduction in CO2, nitrogen oxide (NOx), and sulfur oxide (SOx) levels. The magnitude of CO2 reduction depends upon the source of the biomass, while NOx and SOx reductions arise from biomass (in general) having relatively lower concentrations of nitrogen and sulfur compared to coal, usually 99.5%) from Sigma. The five amino acids used to prepare the mixtures were L-(−)-proline (Pro) and L-(−)-tryptophan (Trp), both >99% from Acros Organics, L-histidine (His) and L-asparagine monohydrate (Asn), both >98% from Alfa Aesar, and L-glutamine (Gln), >99.5% from Sigma. The mixtures of cellulose and glucose with the different amino acids were prepared in a 1:1 mol ratio. The C, H, and N contents of the compounds and mixtures were determined in duplicates using a CE Intruments Flash EA 1112 Series elemental analyzer. The percent relative error in the duplicates was better than ±3% for nitrogen and carbon and better than ±6% for hydrogen. Char Preparation and Analysis. Low-heating-rate, low-temperature chars were prepared from the cellulose, glucose, and their equimolar mixtures with amino acids in a tube furnace, by heating under flowing argon at 10 °C min−1 to 600 °C, with a holding time of 15 min. The chars were characterized for their C, H, and N contents as detailed in the previous section. Combustion of Chars by Thermogravimetric Analysis−Mass Spectrometry (TGA−MS). Temperature-programmed combustion (TPC) experiments of the chars produced were conducted in a Netzsch STA 449C Jupiter coupled to a Netzsch QMS 403C Aëolos quadrupole mass spectrometer for the analysis of the evolved gases, to determine the conversion of char nitrogen to nitrogen-containing species during combustion. For this purpose, approximately 5 mg of char was heated in flowing 12.5% O2/He, at a heating rate of 10 °C min−1 to a final temperature of 900 °C, and up to 26 different species were monitored simultaneously and scanned approximately every 15 s. Pure graphite (99.999%, Aldrich-Sigma Co.) was used as a control test material to enable calibration of the mass spectrometer for semiquantitative analysis of N compounds from the chars. More details of this technique can be found in the studies by Jones et al. and Darvell et al.18,19 The moisture and ash contents of the chars were estimated from their TGA combustion runs. X-ray Photoelectron Spectroscopy (XPS) of Chars. XPS measurements of the chars were performed using a VG Escalab 250 XPS with a monochromated Al Kα source. The chars investigated were pressed onto adhesive carbon tape. The spot size was
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RESULTS Char Characterization and N Partitioning. The amino acids used as N precursors have a mixture of amino and cyclic N functionalities, as shown in Figure 1. The char data are listed Table 1, which shows the ash contents (from the TGA combustion analyses), elemental analyses, and relative intensities (%) of XPS N (1s) peaks representing pyridinic N (N-6), pyrrolic N (N-5), and/or quaternary N (N-Q). The nitrogen partitioning and char yields from the pyrolysis experiments in the tube furnace (unless otherwise stated) are also included in Table 1. It can be observed that the main differences regarding the char elemental contents lie in their N concentrations. The cellulose and glucose starting material contain trace nitrogen (zero in theory), and the chars were prepared under flowing argon. Hence, the N content of the model chars prepared from their mixtures is due to the amino acid added. With regard to the model compound chars, both chars from histidine resulted in the highest N content (15.2− 17.5 wt % dry), while the chars prepared from the proline mixtures resulted in the lowest N contents (4.8−7.6 wt % dry). While this is no doubt because of histidine having the highest N content at 27.8 wt % N, with proline having a N content of 12.5 wt %, the N partitioning behavior of histidine is quite different from the other amino acids when mixed with cellulose. This may be related to the different functionalities in histidine. The nitrogen partitioning data show that approximately 9−46% of the model compound N was retained in the char, under the pyrolysis conditions used. Overall, a higher retention of N in the char was observed for the model compounds prepared from 6483
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Figure 2. X-ray photoelectron spectra of chars prepared from mixtures of amino acids with cellulose and glucose, where (a) cellulose + Pro char, (b) cellulose + His char, (c) cellulose + Gln char, (d) cellulose + Asn char, (e) cellulose + Trp char, (f) glucose + Pro char, (g) glucose + His char, (h) glucose + Gln char, (i) glucose + Asn char, and (j) glucose + Trp char.
corresponding spectra for chars produced with glucose. Two N (1s) XPS peaks were identified in the spectra of the chars studied: one at the binding energies of 400.4 ± 0.1 eV and one at the binding energies of 398.7 ± 0.5 eV, which have been attributed in the coal literature to pyrrolic nitrogen and pyridinic nitrogen in association with an oxygen functionality
glucose than from cellulose, except in the case of histidine. It is to be noted that all of the mixtures, except those with proline, resulted in higher amounts of char than those of the cellulose and glucose alone. Panels a−e of Figure 2 show the XPS spectra for the cellulose model chars, and panels f−j of Figure 2 present the 6484
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(N-5) and to pyridinic nitrogen (N-6), respectively.4,6,20,21 It is to be noted that there are also two or three more types of nitrogen that have been distinguished in carbonaceous material by XPS: (1) a quaternary nitrogen (N-Q) at 401.4 ± 0.5 eV and (2) nitrogen oxide and nitrate structures (NX-1 and NX-2) at 402−405 eV.4,6,18,19 The concentrations of the latter usually fall within the accuracy limits of the deconvolution and, therefore, are generally not reported.7 With regard to the N-Q nitrogen peak, its deconvolution from the N-5 spectra is not straightforward, because their binding energies are very close; consequently, the spectra from N-Q tend to fall under the peak for N-5. Moreover, the exact nature of the quaternary nitrogen (N-Q) is not completely understood, but it has been suggested that it may correspond to several functionalities with slightly different binding energies of nitrogen atoms incorporated within the graphene sheets.22,23 As a result, it is not possible to accurately measure this peak by the use of standards. Consequently, in this instance, the N-5 peak fitted will be referred to as N-5 and N-Q to acknowledge the possibility that it may also include contributions from the N-Q peak. The relative intensities of the N-6 and N-5 and N-Q peaks are listed in Table 1 for all of the chars. From Figure 2 and Table 1, it can be observed that the relative intensities of the two peaks fitted appear to be specific for the amino acid used as a N precursor and that the precursor for the carbohydrate (i.e., cellulose and glucose) does not seem to have a noticeable effect on the nitrogen functionality of the resultant char. The only exception was proline, where a change in nitrogen functionality on the resultant chars was observed when it is mixed with glucose than when it is mixed with cellulose, resulting in the cellulose + proline char containing a higher proportion of N-5 and N-Q (59%), while the glucose + proline char contains 49%. It can also be noticed that the char N is almost evenly split between these two functionalities for the glutamine and asparagine chars, both precursors which have linear structures. However, this split is different for the rest of the chars, with His chars containing ∼56−57% N-6, while the tryptophan chars contain ∼60−62% N-5 and N-Q. Combustion Profiles and Char Reactivities. The plots of the time derivative of the mass loss (DTG) with temperature from the TGA combustion experiments are shown in Figures 3 and 4 for the cellulose chars and glucose chars, respectively. From the DTG curves shown in Figure 3, it can be observed
Figure 4. Plot of the time derivative of mass loss (DTG) for the combustion of chars prepared from the mixtures of amino acids with glucose.
His and cellulose + Trp chars. Both of these chars start to burn at higher temperatures, and it is clear that the combustion rate is the slowest for the latter. Similarly, the DTG curves for the glucose chars (in Figure 4) show that also the chars prepared from glucose with both His and Trp present a slower rate of combustion. It is to be noted that the combustion of both pure cellulose and glucose chars is completed at lower temperatures than the chars from the amino acid mixtures. For the cellulose chars (in Figure 3), only one peak can be observed for their oxidation, except in the case of the cellulose + Trp char, where at least two peaks can be observed. The number of steps involved in the combustion of the cellulose + Trp char cannot be deduced with certainty from its DTG curve, because the peaks are not well-resolved. Similarly, the glucose chars also present only one peak for their combustion reaction (in Figure 4), with the exception of the glucose + Trp char, where several peaks with various degrees of overlapping can be observed in its DTG curve. The apparent first-order kinetic parameters, which all fall within the range of 415−590 °C, were calculated from their respective TGA burning profiles. A first-order reaction is assumed, and the reaction rate constant method has been applied, as detailed in the study by Saddawi et al.24 The resultant activation energies (Ea), pre-exponential factors (A), and corresponding coefficients of determination (R2) are presented in Table 2. Table 2 lists the global kinetics for all of the chars that clearly showed one main mass loss region. Note that, in the case of the chars from the tryptophan model compounds, more than one step can be observed in their burning profiles, as mentioned above. Hence, for these two chars, only the kinetics for the initial step of char oxidation are reported in Table 2, and further char combustion steps are not discussed here. The rates of combustion calculated at 500 °C (k500) are also listed in Table 2 for comparison purposes. When the activation energies are compared, it can be observed that all of the chars prepared from the model compounds present lower activation energy values than those from the pure cellulose and glucose chars, with the exception of the glucose + histidine char and both tryptophan chars, with the latter resulting in the highest activation energy values. These higher Ea values result in lower char reactivities under the conditions studied, because both of these chars present the slowest combustion rates, as estimated at 500 °C. The lower reactivities of these chars are also evidenced by the temperature of maximum combustion rate, Tmax, as the Trp chars present the highest peak temperatures, which are listed in Table 2. The
Figure 3. Plot of the time derivative of mass loss (DTG) for the combustion of chars prepared from the mixtures of amino acids with cellulose.
that the cellulose chars prepared from the mixtures with amino acids start combustion at around the same temperature as the char from pure cellulose, with the exception of the cellulose + 6485
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Table 2. Char Combustion Kinetic Parameters for All of the Weight Loss Region (Global) or for the First Weight Loss Region (First Step) in the TGA Combustion Experiments char cellulose cellulose + Pro cellulose + His cellulose + Gln cellulose + Asn cellulose + Trp glucose glucose + Pro glucose + His glucose + Gln glucose + Asn glucose + Trp
kinetics global global global global global first step global global global global global first step
T range (°C) 455−525 415−520 430−535 420−515 415−490 480−520 390−590 460−545 450−525 420−540 430−535 495−535
Ea (kJ mol−1)
A (s−1)
162.3 150.6 141.2 141.5 136.9 204.3 142.1 115.4 143.7 129.1 119.1 168.2
× × × × × × × × × × × ×
6.96 8.69 1.14 2.80 1.32 1.06 1.86 3.16 1.52 3.47 7.31 4.50
Tmax repeatability is approximately ±3 °C, as shown by previous experiments.25 Char Nitrogen Conversions. The relative conversions of char nitrogen to some of the different nitrogen-containing species have been determined by calculating the corresponding ratios of NO/N, NO2/N, etc. from measurements of the areas under the spectral peaks for the species monitored and from the char elemental analysis. The method used and species monitored by mass spectrometry have been detailed in the studies by Jones at al. and Darvell et al.18,19,26 and do not represent absolute values but rather can be used comparatively between the different chars. Typical gas evolution profiles from the combustion of the chars are shown in panels a−j of Figure 5 for the cellulose and glucose model chars. The gas evolution profiles obtained from the oxidation of the chars show a few differences. Single peaks are detected for the formation of CO, CO2, NO, N22+ (which gives an indication of N2 formation), and C2N2 from the combustion of all of the cellulose and glucose chars, except for the Trp char. Moreover, for the cellulose + His, cellulose + Gln, and cellulose + Asn chars, the release of the N-containing species occurs at T > 500 °C, with a broad peak. In contrast, for the cellulose + Pro and cellulose + Trp chars, the NO and C2N2 are released in a sharper peak and at higher temperatures (∼600 °C for the cellulose + Pro char and ∼550 °C for the cellulose + Trp char). For both tryptophan chars, the gas evolution profiles are more complex and at least two unresolved peaks can be observed. This was expected, because at least a two-stage char combustion was observed in their DTG plots (see Figure 3 and 4). In the case of the glucose chars, the release of NO, N22+, and C2N2 appears to occur at T > 500 °C with a broad peak for the Pro, His, Gln, and Asn chars. For the glucose + Asn char, the release of these N species happens at a slightly higher temperature (T > 550 °C). However, in all cases, NO, N22+, and C2N2 appear to be released simultaneously (i.e., no obvious sequential release) but later in combustion at T > 500 °C. This is true for all but the tryptophan mixture char, where there is an initial release of C2N2, followed by more N22+ associated with the lower T peak of the N22+ profile, and more NO associated with the higher T peak. The N22+ profile follows the CO2 profile, but the C2N2 profile and the NO profile do not. This behavior indicates that there is more than one type of char formed during the pyrolysis of the tryptophan model compounds. Note that NH3 and N2O cannot be detected using this technique because of the lack of resolution between the NH3 species and water contribution to m/z 17, while in the case of N2O, its signal cannot be resolved
R2 7
10 106 106 106 106 1010 106 104 106 105 104 107
0.9992 0.9999 0.9997 0.9997 0.9994 0.9990 0.9993 0.9995 0.9993 0.9990 0.9997 0.9985
k500 (s−1) 7.54 5.75 3.28 7.63 7.38 1.65 4.63 5.02 2.94 6.54 6.58 1.93
× × × × × × × × × × × ×
10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4
Tmax (°C) 560 562 591 548 554 644 582 575 593 561 556 660
from CO2. The calculated char N conversions for the fuels are listed in Table 3. The most abundant N-containing compounds detected from char combustion were NO and N22+. The histidine chars showed the lowest amount of char N conversion to N22+, while the proline and asparagine chars showed the highest. Smaller amounts of C2N2 were detected from the combustion of almost all of the amino-acid-containing chars; in general, it appears that the histidine chars show the largest conversions and the tryptophan chars result in the lowest conversions, as compared to the other chars. In contrast, HCN was detected from the combustion of the glucose + His char only, while very small amounts of HNCO were detected from the glucose + Pro and glucose + His chars.
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DISCUSSION Formation of Char Nitrogen. A considerable number of studies have been made of pyrolysis of biomass and model chars and the way that it influences the amount of nitrogen in the char.11−17,19,27−31 While a number of studies have been undertaken on fuel N partitioning, very few studies have been concerned with the oxidative reaction of the N-containing chars.19,30 However, there is considerable information on the oxidation of N-containing coal chars, which has set out the importance of nitrogen compounds. The chemical structure of biomass and coal and their corresponding N functionality are different, because nitrogen in coal is found in heterocyclic compounds, such as pyridine, pyrrole, and pyridone,20−22,32 while nitrogen in biomass is mostly in the form of proteins. The formation of the N-containing volatiles has been extensively discussed, and as a consequence, two main reaction routes for the decomposition of protein N have been proposed in the literature.28 One route leads to the formation of NH3, and the other route leads to the formation of HCN. In the first route, the formation of NH3 simultaneously leads to char formation, while the second route leads to the formation of volatile cyclic amides, which in turn may result in HCN or HNCO upon cracking. These reactions influence the nature of the remaining char nitrogen, and the details of these remain to be resolved. From the experimental data here, there is a correlation (R2 = 0.68) between the initial N content of the model compounds and the percentages of nitrogen in the resultant char (a high initial N content gives a high char N content), as shown in Figure 6. Moreover, in the case of the cellulose-based model compounds, the nitrogen partitioning also appears to strongly follow this trend; i.e., the model compounds with the highest N content result in the highest char N/fuel N ratios. However, it 6486
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Figure 5. continued
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Figure 5. Gas evolution profiles during the temperature-programmed combustion of chars in 12.5% O2/He at 10 °C min−1, where (a) cellulose + Pro char, (b) cellulose + His char, (c) cellulose + Gln char, (d) cellulose + Asn char, (e) cellulose + Trp char, (f) glucose + Pro char, (g) glucose + His char, (h) glucose + Gln char, (i) glucose + Asn char, and (j) glucose + Trp char.
is not the case for the glucose-based compounds, with the exception of proline, which had the lowest N content and also resulted in the lowest char N/fuel N ratio. As far as the char yields are concerned, it appears that the addition of amino acid to the carbohydrate models promotes the formation of char, except in the case of proline, where it appears to actually promote devolatilization reactions, with the proline-containing model compounds resulting in lower char yields than the pure carbohydrates, both cellulose and glucose. This effect is discussed further below. In the case of chars studied here, it is not clear whether there is a relationship between the amino acid and the nitrogen functionality determined by XPS. The XPS results show that there may be a correlation between the proportion of pyridinic N and pyrrolic N (and possibly some quaternary N) in the char and the N precursor, i.e., amino acid. Both histidine chars show a higher proportion of pyridinic N than any other char (∼57% N-6), while both tryptophan chars contain more pyrrolic and/ or quaternary N (∼61% N-5 and N-Q) than pyridinic N. All of the chars from asparagine and glutamine, both of which have linear structures, result in ∼50% pyridinic N and pyrrolic N (and possibly some quaternary N). Interestingly, in the case of proline, there appears to be complex interactions with either cellulose or glucose (or maybe both) that significantly change the proportion of these N functionalities in the resultant chars, as mentioned in the Results. Results from pyrolysis studies of proline with both cellulose and glucose have shown that glucose, in particular, catalyzes the pyrolysis reactions.12−15,17 This has been evidenced by a nonadditive behavior with regards to its pyrolysis products12−15 and, more recently, by the pyrolysis kinetics derived by our group, which have shown a markedly faster decomposition reaction when mixed with glucose than cellulose.17 With regard to the pyrolysis reactions, numerous studies have shown that Maillard or browning chemistry also plays an important role in the decompostion of mixtures of amino acids and carbohydrates, such as cellulose and glucose.12−15 The Maillard chemistry concerns the reactions between the carbonyl groups of carbohydrates and the amino groups in proteins or free amino acids. The chemistry of the Maillard reactions has been
Table 3. Char Nitrogen Conversions from the TGA−MS Combustion char cellulose + Pro cellulose + His cellulose + Gln cellulose + Asn cellulose + Trp glucose + Pro glucose + His glucose + Gln glucose + Asn glucose + Trp a
NO/N NO2/N N22+/N HCN/N C2N2/N HCNO/N 0.60
nd
0.12
nd
0.002
nd
0.33
0.001
0.10
nd
0.002
nd
0.31
nd
0.12
nd
0.001
nd
0.53
nd
0.16
nd
0.001
nd
0.29
nd
0.11
nd
trace
nd
0.50
nd
0.16
nd
0.002
0.003
0.38
nd
0.13
0.001
0.005
0.002
0.22
nd
0.14
nd
0.001
nd
0.44
nd
0.16
nd
0.002
nd
0.23
nd
0.13
nd
0.001
nd
nd = not detected.
Figure 6. Relationship between the nitrogen content of the model compounds and the resultant chars. 6488
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surface area, porosity, and reactivity for NOx destruction, have more bearing on the conversion of char N to nitrogen oxides than the N functionalities in the char.8,32,35 A relative comparison of the ratios estimated for the release of the different N species to char N appear to show that the release of these species is dependent upon the amino acid or their N functionalities, because the release of NO/char N follows the trend of Pro > Asn > His, for both cellulose and glucose model chars, with Gln and Trp chars producing the least amounts. Similarly, in the case of N22+, both cellulose and glucose model chars follow exactly the same trend: Asn > Pro > Gln > Trp > His. The correlation of char N emissions to the N functionality in the chars is not straightforward, and the fact that other N species, such as NH3 and N2O, cannot be measured by this method further aggravates this; however, the following can be observed: both histidine chars show a higher proportion of pyridinic N than any other char, and they also show the lowest conversion to N22+ and the highest conversion to C2N2. Furthermore, both tryptophan chars contain more pyrrolic N and possibly quaternary N (∼61% N-5 and N-Q) than pyridinic N (and had the slowest reactivities), and both showed the lowest conversion to both NO and C2N2. However, similar conversions to those obtained with the tryptophan chars (for NO and C2N2) can also be observed for the glutamine chars, which contain almost equivalent amounts of both N functionalities. Also, the distributions of N functionalities in both asparagine chars are similar to the glutamine chars, but the asparagine chars show the highest conversions to N2. Proline is an interesting case because the distribution of N functionalities in both of its chars are different, yet the conversion to N species from both chars follow the same trends (i.e., higher NO conversions, etc.). Plots of the relative intensities of the N-5 and N-Q and N-6 peaks against their corresponding char nitrogen conversion to the different nitrogen species detected (not shown) show a poor correlation between the nitrogen functionality and the conversion of char N to the nitrogen species formed during combustion. However, a weak correlation can be found for the peak N-6 and the formation of C2N2, as shown in Figure 7, where a determination coefficient (R2) of 0.40 was obtained for a linear fit and 0.46
extensively studied in food science because of its importance regarding changes in color and flavors in food during cooking, which tend to involve temperatures
