Article pubs.acs.org/IECR
Lumped Kinetics for Biomass Tar Cracking Using 4‑Propylguaiacol as a Model Compound Elmer B. Ledesma,* Alyssa A. Mullery, Jacqueline V. Vu, and Jennifer N. Hoang Department of Chemistry and Physics, University of St. Thomas, Houston, Texas 77006, United States ABSTRACT: Lumped kinetics for the vapor-phase cracking of 4-propylguaiacol, a model compound representative of components found in primary tar derived from lignin, has been investigated. Analysis of the products from pyrolysis experiments in a laminar-flow reactor at temperatures between 300 and 900 °C and a residence time of 1 s revealed that the products can be lumped into three compound classes: oxygen-containing compounds, single- and multiring aromatic hydrocarbons, and permanent gases. Temperature was found to have a marked effect in governing the overall product composition. The oxygencontaining compounds peaked in yield between 500 and 700 °C. The aromatic hydrocarbons and permanent gases dominated the product composition above 600 °C, especially at 900 °C, the highest temperature investigated. A lumped kinetic model with three irreversible first-order reactions was developed to model the experimental data. This model was extended to one with eight first-order irreversible reactions. Optimized reaction-rate parameters for each reaction in both models were determined by fitting the experimental data using a plug-flow reactor model.
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INTRODUCTION Constituting about one-quarter the content of woody biomass and representing the largest renewable resource of aromatics found in nature, lignin is a valuable feedstock for the production of chemicals, liquid transportation fuels, and combustible gases.1−7 Pyrolysis and gasification are thermochemical conversion processes that can provide economically viable routes for the conversion of lignin into these globally important commodities.8 Pyrolysis of lignin results in the production of a liquid product that consists of a complex mixture of oxygenated compounds, collectively termed bio-oil or tar.9−11 This tar product can be used directly as a fuel or as a source of chemicals. It can also be upgraded to chemicals and synthetic fuels.1,12,13 The major product of gasification is syngas, which is utilized as a feedstock for the production of chemicals or synthetic fuels.14−16 To design commercial-scale units for the pyrolysis and gasification of lignin, reaction-rate data for the secondary vaporphase cracking of tar compounds is crucial. Tar cracking plays a significant role in both pyrolysis and gasification. In pyrolysis, secondary vapor-phase reactions of tar have the potential to control the distribution of products.17 In gasification, the presence of tar in product gases is of concern, as tar compounds can condense, thereby resulting in the fouling of heat exchangers, outlet pipes, and filters.14−16 To implement effective methods of tar mitigation in gasification, knowledge of tar-cracking kinetics is therefore important. Studies on lignin pyrolysis demonstrate that tar compounds undergo a progressive alteration and conversion with temperature.18−26 An initial volatile product, generally termed “primary tar”, is first released at temperatures between 400 and 600 °C. This primary tar is a complex mixture of highly substituted oxygen-containing compounds. It is considered primary because it has not undergone secondary thermal cracking.18 Between 600 and 800 °C, primary tar compounds undergo secondary thermal cracking, producing “secondary tar” and gases. Secondary tar mainly comprises lower-molecular© 2015 American Chemical Society
weight oxygen-containing compounds. At higher temperatures (>800 °C), secondary tar thermally cracks to produce more gases and “tertiary tar”, a mixture consisting primarily of aromatic hydrocarbons. Because of the structural complexity of lignin primary tar and its reactivity under thermal cracking conditions, kinetic studies on lignin tar cracking have primarily used global or lumped kinetics to obtain rate data.17,26−31 These studies show that rate data vary widely, thereby preventing the extraction of reliable rate parameters crucial for process design and optimization. In addition, the complex nature of the starting material in these studies precludes a mechanistic understanding of thermal cracking. To obtain reliable rate data and to gain insight into thermal cracking pathways, a useful approach is to examine compounds that are representative of components found in lignin primary tar. Studies show that major components of primary tar are aromatic oxygenates, especially guaiacols and syringols.1,18,21,23,32 However, few kinetic studies on the vaporphase cracking of guaiacols and syringols have been reported in the literature.33−37 Many kinetic studies are available on other aromatic tar compounds such as anisole, 38 phenol, 39 benzaldehyde,40 catechol,41 benzene,42 and toluene,43 but these are more representative of secondary and tertiary tars. Moreover, the detailed kinetic mechanisms developed for these compounds are only partially applicable to the modeling of industrial-scale pyrolysis and gasification of actual biomass fuel, where the nature of the fuel is substantially different and the reaction kinetics is coupled to transport processes. A global or lumped kinetic study using guaiacols or syringols as model primary tar compounds is therefore warranted. Received: Revised: Accepted: Published: 5613
March 17, 2015 May 11, 2015 May 14, 2015 May 14, 2015 DOI: 10.1021/acs.iecr.5b01022 Ind. Eng. Chem. Res. 2015, 54, 5613−5623
Article
Industrial & Engineering Chemistry Research Table 1. Percentage Yields of Oxygen-Containing Compounds as Functions of Temperature temperature compound
formula
phenol catechol 2-methylphenol 4-methylphenol 3-methylcatechol 4-methylcatechol 4-ethylphenol 4-ethylcatechol 5-vinylsalicylaldehyde 2-acetyl-4-methylphenol 4-vinylguaiacol 4-propylphenol 1-ethyl-4-methoxybenzene 4-propylcatechol 5-propylsalicylaldehyde 1-methoxy-4-propylbenzene 1-methoxy-4-(1-methylpropyl)benzene
C6H6O C6H6O2 C7H8O C7H8O C7H8O2 C7H8O2 C8H10O C8H10O2 C9H8O2 C9H10O2 C9H10O2 C9H12O C9H12O C9H12O2 C10H12O2 C10H14O C11H16O
400 °C
500 °C
0.11 0.33 0.16 0.11 1.44 1.73 0.04 12.13 15.29 0.93 1.20
0.13 0.13 0.07 0.27
550 °C
600 °C
700 °C
800 °C
0.06 0.20 0.09
0.19 0.25 0.17 0.27 1.23 4.31 0.29 5.51 0.50 0.36 0.22 3.68 0.19 6.98 12.66 0.43 0.43
0.78 0.21 0.26 1.13 1.16 6.25 0.37 0.49 0.07 0.08
2.46
0.98 1.81 0.19 4.20 0.56 0.38 0.62 3.48 0.21 16.87 17.45 0.81 0.89
0.18 1.14
1.38 0.09 0.04 0.68
acetone and dichloromethane as the solvent. An aliquot of the acetone/dichloromethane solution was then used for analysis. The permanent gas products were collected in a Tedlar gassampling bag. In this study, the amount of product obtained is expressed as percentage yield, defined as the percentage ratio of a product’s outlet mass flow rate to the inlet mass flow rate of 4-propylguaiacol. Analysis of Condensable Products. Condensables were analyzed by gas chromatography using an Agilent 6890 gas chromatograph equipped with an Agilent 5975 mass spectrometric detector. Separation of products was achieved using an HP-5MS capillary column. Quantification of tar compounds was performed by calibrating the gas chromatographic column with reference standards. Products were identified by matching each product’s mass spectrum with the mass spectrum of a compound in the National Institute of Standards and Technology 2005 Mass Spectral Library database. In some cases, product peaks were assigned by analysis of their mass fragmentation patterns using a commercially available software package.48 Analysis of Noncondensable Products. Permanent gases were analyzed by gas chromatography using an HP5890 Series II gas chromatograph equipped with a flame ionization detector for analysis of C1−C6 hydrocarbons and a thermal conductivity detector for carbon monoxide analysis. A gas-tight syringe was used to introduce a sample from the gas bag into the gas chromatograph. Separation of hydrocarbon gases was achieved using an alumina-BOND/Na2SO4 PLOT column, and carbon monoxide was separated using a molecular sieve PLOT column. Quantification of gas products was performed by calibrating the gas chromatographic columns with reference standards.
In this study, we investigate the lumped kinetics for the vapor-phase cracking of 4-propylguaiacol. Observed as a component in lignin primary tar, 4-propylguaiacol is a good model compound to represent primary tar in lignin.32,44 Only two studies in the literature have examined the pyrolysis of 4propylguaiacol.45,46 However, these studies were performed using 4-propylguaiacol in the liquid phase at low pyrolysis temperatures ( two-ring > three-ring > four-ring. Permanent Gases. Figures 1−4 present the percentage yields of permanent gases as functions of temperature. As
lower-molecular-weight compounds (from phenol to 4-ethylphenol in Table 1) with only one to three small functional groups began to form at temperatures above 500 °C and peaked in yield between 600 and 800 °C. Although not included in Table 1, at 900 °C, the highest temperature investigated in this study, phenol with a yield of 0.94% was the only oxygen-containing compound identified in the condensable products. The results in Table 1 show that the higher-molecular-weight compounds with larger substituent groups on the aromatic ring (below 4-ethylphenol in Table 1) began to form above 400 °C and peaked in yield between 500 and 600 °C. These compounds were no longer observed in the condensable products above 700 °C. Their region of peak yield temperature coincided with temperatures at which the lower-molecularweight compounds began to form. Although not presented in Table 1, two oxygen-containing compounds were observed at 350 °C: 5-propylsalicylaldehyde with a yield of 0.02% and 4-propylcatechol with a yield of 0.05%. As shown in Table 1, the percentage yields of 5propylsalicylaldehyde and 4-propylcatechol increased rapidly above 400 °C, peaking at 550 °C with 5-propylsalicylaldehyde having a maximum of 17.45% and 4-propylcatechol having a maximum of 16.87%. These peak yields make these two products the highest-yielding oxygen-containing compounds below 600 °C. Single- and Multiring Aromatic Hydrocarbons. Table 2 presents percentage yield data for single- and multiring
Figure 1. Percentage yields of methane and ethane as functions of temperature.
illustrated in Figure 1, methane and ethane were produced at temperatures above 400 °C. As temperature was increased, methane showed a continuous increase in yield, attaining a yield of 6.44% at 900 °C, whereas ethane reached a peak yield of 6.28% at 750 °C. Figure 2 shows that ethylene was produced
Table 2. Percentage Yields of Single- and Multiring Aromatic Hydrocarbons as Functions of Temperature temperature
a
compound
formula
600 °C
700 °C
800 °C
900 °C
benzenea phenylacetylene styrene ethylbenzene indene naphthalene 1-methylnaphthalene 2-methylnaphthalene acenaphthylene biphenyl fluorene phenanthrene anthracene fluoranthene pyrene
C6H6 C8H6 C8H8 C8H10 C9H8 C10H8 C11H10 C11H10 C12H8 C12H10 C13H10 C14H10 C14H10 C16H10 C16H10
0.25
1.70
0.13 0.06 0.03
0.37 0.19 0.15 0.05 0.14 0.06
6.75 0.19 1.63 0.24 0.98 0.74 0.14 0.10 0.19 0.04 0.06 0.05 0.02
11.26 0.64 2.11 1.79 2.52 0.11 0.10 0.51 0.07 0.29 0.42 0.15 0.06 0.06
Figure 2. Percentage yields of ethylene and acetylene as functions of temperature.
above 450 °C, and its yield continuously increased with temperature, reaching a peak value of 28.5% at 850 °C. As Figure 2 demonstrates, acetylene showed a rapid increase in yield above 700 °C, reaching a value of 10.9% at 900 °C. These high yields of ethylene and acetylene at 900 °C make these hydrocarbon gases two of the major gas products observed from the vapor-phase cracking of 4-propylguaiacol. The percentage yields of carbon monoxide and 1,3-butadiene are shown in Figure 3. The results in the figure illustrate that both carbon monoxide and 1,3-butadiene were produced at temperatures above 500 °C. 1,3-Butadiene reached a peak yield of 5.22% at 850 °C, whereas carbon monoxide attained a yield of 26.0% at 900 °C. This high yield of carbon monoxide makes this compound one of the major gas products observed at the highest temperature examined. Figure 4 shows the percentage yields of the C3 hydrocarbonspropane, propylene, and propadieneas functions of temperature. As illustrated in Figure 4, propane increased in yield above 450 °C and peaked at 700 °C with a yield of 3.89%.
Benzene data obtained from analysis of gas-phase products.
aromatic hydrocarbons as functions of temperature. The results in the table show that the aromatic hydrocarbons were produced only at 600 °C and above. What the data in Table 2 make clear is that there was a progressive temperature shift in the onset of production of these products: single-ring aromatic hydrocarbons and indene at 600 °C, followed by the two-ring aromatics at 700 °C, three-ring aromatics at 800 °C, and fourring aromatics at 900 °C. With the exception of ethylbenzene and the methylnaphthalenes, all of the aromatic hydrocarbons showed increases in yield with temperature. The data in Table 2 also show that, at the highest temperature investigated in this 5615
DOI: 10.1021/acs.iecr.5b01022 Ind. Eng. Chem. Res. 2015, 54, 5613−5623
Article
Industrial & Engineering Chemistry Research
the single- and multiring aromatic hydrocarbons, and the total yield of the permanent gases. As demonstrated in Figure 5, the discrepancy in the mass balance was marked at temperatures in the range of 550−800 °C, peaking at 650 °C. We observed a dark brown material along the entire length of the heated section of the reactor at the conclusion of experiments in the range 550−750 °C. Above 750 °C, the color of the deposit transformed from dark brown to black. In their study on the effect of vapor-phase reactions on lignin-derived oligomeric compounds, Zhou et al.49 found that cracking reactions and polycondensation reactions that lead to the formation of highmolecular-weight oxygen-containing products compete, with the latter becoming more pronounced at the highest temperature examined in their study (550 °C) and at low residence times (
