Pyrolysis Study on Solid Fuels: From Conventional Analytical Methods

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Review

Pyrolysis study on solid fuels: from conventional analytical methods to synchrotron vacuum ultraviolet photoionization mass spectrometry Yu Wang, Yanan Zhu, Zhongyue Zhou, Jiuzhong Yang, Yang Pan, and Fei Qi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02234 • Publication Date (Web): 26 Jan 2016 Downloaded from http://pubs.acs.org on January 31, 2016

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Pyrolysis study on solid fuels: from conventional analytical methods to synchrotron vacuum ultraviolet photoionization mass spectrometry Yu Wang,† Yanan Zhu,† Zhongyue Zhou,§ Jiuzhong Yang,† Yang Pan,†* Fei Qi§* †

National Synchrotron Radiation Laboratory, University of Science and Technology of China,

Hefei, Anhui 230029, P. R. China. §

Key Laboratory for Power Machinery and Engineering of MOE, Shanghai Jiao Tong

University, Shanghai 200240, P. R. China

ABSTRACT: The demand of modern society for energy keeps increasing due to the rapid growth of population and urbanization. Pyrolysis of solid fuels including biomass, coal and polymer waste has recently received special attention as it can provide extra sources of fuels. In the past few years, a variety of analytical techniques have been used to detect the pyrolysis products of solid fuels, such as GC/MS, TG, FTIR, NMR, SEM, etc. However, on-line detection of gas release during the pyrolysis process, which is believed to be closely related to the thermal decomposition mechanisms of solid fuels, is rare. Recently, some progress has been made in real-time diagnostic techniques, the most important one of which is photoionization mass spectrometry (PIMS). This review focuses on recent developments in pyrolysis study of solid fuels, especially on those performed with synchrotron vacuum ultraviolet (SVUV) PIMS.

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1. INTRODUCTION Due to the energy crisis and the negative impact of fossil fuels combustion upon the environment, alternative energy sources are required. Recently, the potential value of solid fuels has become attractive to many countries, as its effective use can ease the shortage of fossil fuels to a large extent. Solid fuels mainly consist of biomass, coal and organic polymers from municipal solid waste (MSW).1 Among the current conversion processes of solid fuels, thermochemical means appears to be a promising alternative for multiple energy applications. The most commonly used thermochemical processes are combustion, pyrolysis and gasification. Unlike direct burning process which usually produces hazardous gases that are harmful to the environment, pyrolysis can convert solid fuels into liquid fuels and chemical feedstock which can be readily stored and transported.2 For instance, liquid pyrolysis products of biomass (biooil) are usually a complex mixture of carboxylic acids, esters, ethers, alcohols, ketones, aldehydes, diols, hydroxylated ketones and aldehydes, furans, sugars and anhydrosugars, phenolic compounds, and hydrocarbons.3 Useful components can be extracted after a series of separation and purification. Besides, light gaseous products and char are also two kinds of important products from biomass pyrolysis, which can be further converted to substitute natural gas and charcoal, respectively. As for the gasification process of solid fuels, pyrolysis is still a capital step as it is the first step of all the processes regardless of the gasification agent employed. Over the past two decades, research of solid fuels pyrolysis has generally focused on four aspects: understanding the reaction mechanisms, finding suitable pyrolysis conditions, developing kinetic models and designing reactors. Given the complexity of solid fuels pyrolysis, the major challenge in its application is to obtain products with high yield and purity. In order to


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achieve this goal, a better understanding of the thermochemical conversion process is needed above all. A number of analytical techniques have already been employed to investigate the pyrolysis processes of various solid fuels, which has contributed to the practical production of liquid fuels and chemicals from solid fuels nowadays, including methanol produced from coal and ethanol produced from biomass. In this review, the recent studies on solid fuels pyrolysis conducted with conventional analytical methods are reviewed in the first section. Then, typical works performed with photoionization mass spectrometry (PIMS) are summarized, especially those applications of synchrotron vacuum ultraviolet PIMS (SVUV-PIMS) to the pyrolysis of representative solid fuels. 2. CONVENTIONAL ANALYTICAL METHODS 2.1 Thermogravimetric analysis (TGA) TGA is the technique most commonly used for solid fuels pyrolysis studies.4-8 These experiments are usually performed with a small amount of samples (≤ 10 mg) at very low heating rates (≤ 20 °C/min) to minimize the heat transfer effects. TGA can provide critical information about the partial processes such as moisture evaporation, decomposition of major constituents and char formation. The thermal decompositions of cellulose, hemicellulose, and lignin (major biomass constituents) have been studied extensively with the help of TG. Two main stages of weight loss for hemicellulose and cellulose have been found during the slow pyrolysis process, while the decomposition of lignin lasts a wide temperature range and is overlapped with the decomposition of the other components.9 As for coal pyrolysis, although it is difficult to conclude the definitive mechanism simply from TGA, the pyrolysis characteristics of coal can be revealed by the experimental data. The particle size effect on the pyrolysis mechanism of coal was studied with TG by Zhang et al.10 Results show that the coal pyrolysis


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process is strongly affected by the heating rate and the particle size. Moreover, TG can also be used as a new method to study the co-pyrolysis behaviors among different kinds of solid fuels. Zhou et al. investigated the co-pyrolysis behaviors of polyolefin and coal with TGA. Results indicate that the synergistic effect during pyrolysis occurs mainly in the high temperature region.11 Co-pyrolysis characteristics of japonica and anthracite coal were studied with TGA by Lu et al.12 Analysis results suggest that the synergistic effects between the biomass and coal are slight because the pyrolysis characteristics of the mixtures of biomass and coal are very close to the combination of those of the individual materials. On the other side, due to the advantage of a fast and repeatable data acquisition of pyrolysis rate, a deep investigation of the kinetic parameters can be obtained with TGA. Knowledge of the thermal decomposition kinetics is useful for designing pyrolysis reactors and optimizing the conditions. Ranzi et al.13 analyzed the main kinetic features of biomass pyrolysis, devolatilization, and gas phase reactions of the released species based on the previous TGA experimental data. They concluded that biomass can be characterized as a mixture of cellulose, hemicellulose and lignin, and proposed a multistep lumped mechanism of cellulose pyrolysis. Yang et al.4 calculated the TG dynamics parameters of wheat straw enzymatic acidolysis lignin by using the methods of Kissinger14 and Ozawa15, respectively. Results show that the fitting degree of the Kissinger method is better than that of Ozawa. As a conclusion, variations can be observed in the kinetic parameters calculated, which may be caused by differences in methods, operating conditions, data analysis as well as the chemical composition of the raw materials. 2.2 GC/MS and LC/MS To obtain a comprehensive understanding of solid fuels pyrolysis, qualification and quantification of pyrolysis products are required. The analytical methods of biomass, coal and


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polymer waste pyrolysis are well-established. Due to the high sensitivity and good performance in separation, gas chromatography/mass spectrometry (GC/MS) is usually used to probe into pyrolysis product distribution.16-20 Liquid products of solid fuels pyrolysis conducted in fixed bed reactor, fluidized bed reactor and batch reactor are usually analyzed with GC/MS. Luo et al.21 analyzed the liquid products of cellulose pyrolysis with GC/MS. The experimental results show that the production rate of hydroxyacetaldehyde (HAA) as well as its proportions in bio-oil increase with the temperature. Wang et al.18 used GC and GC/MS to analyze the volatile fractions of tar obtained by the pyrolysis of coal under different atmospheres. Their results indicate that the integrated process of CO2 reforming methane and coal pyrolysis can obviously improve the tar yield compared with traditional pyrolysis processes. Both slow and fast pyrolysis processes of typical MSWs were studied by Velghe et al.16 Composition analyses of liquid products with GC/MS show that aliphatic hydrocarbons are the major compounds. However, analysis with GC/MS is relatively more time-consuming mainly due to the sample preparations, like products adsorption and extraction. Thus a simple, quick and reliable analytical technique named pyrolysis-GC/MS (Py-GC/MS) has been developed.22-24 Recent developments in PyGC/MS technology and instrumentation has been reviewed by Sobeih et al.,25 which exhibits the great advantage of Py-GC/MS in the investigation of solid fuels pyrolysis. Lu et al.22 used PyGC/MS to study the effects of pyrolysis temperature and time on the distribution of the pyrolysis products of cellulose. They found that levoglucosan is the dominant product and its formation is favored at elevated pyrolysis temperature and time. Fast pyrolysis of aspen lignin over microporous zeolite catalysts was investigated by Zhang et al.24 Thanks to Py-GC/MS, the effects of catalyst type, catalyst-to-lignin ratio, and positioning of catalysts on the fast pyrolysis products of lignin can be studied conveniently.


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Since the pyrolysis products with high boiling points over 300 °C are hard to be volatilized and detected by GC/MS, liquid chromatography/mass spectrometry (LC/MS) is usually used in such cases. LC/MS represents a novel and useful technique for both separation and identification of semi/nonvolatile and thermolabile compounds. It has already been proved to be an invaluable tool in the analysis of complex mixtures derived from various biopolymers, such as proteins and DNA. As for pyrolysis oil of solid fuels, LC/MS has been employed as a complementary analytical method of GC/MS in many studies.26-29 Patwardhan et al.27 analyzed the pyrolysis products of cellulose from the fluidized bed pyrolyzer reactor with various chromatographic methods, among which LC/MS was chosen to determine the presence of oligomeric sugars. Owen et al.28 applied a high performance liquid chromatography (HPLC) separation method for lignin degradation products, which is amenable to negative-ion-mode electrospray ionization. Combined with a high-resolution MS, a mixture of 12 lignin-related model compounds were separated and their structural information was obtained. In addition, gel permeation chromatography (GPC) is usually used to characterize the molecular weight distributions of liquid pyrolysis products of solid fuels,30,

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which could offer a better understanding of the

pyrolysis oil composition. 2.3 FTIR and MS As is well known, Fourier transform infrared spectroscopy (FTIR) is another important analytical tool for pyrolysis products of solid fuels.32-35 In contrast with GC/MS which is used for the volatile compounds identification, FTIR has already been utilized to study the pyrolysis products in different phases. Li et al.32 studied the formation of gaseous compounds during cellulose and levoglucosan pyrolysis with FTIR. They found that the products have shown great similarities and levoglucosan is the major precursor of formaldehyde. Pyrolysis oil of cellulose with addition


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of polystyrene was analyzed with FTIR by Rutkowski et al.34 The results showed that the oxygen functionalities and hydrocarbons contents highly depend on the composition of the cellulose/polystyrene mixture. Chars formed during biomass pyrolysis are also critical to the total product formation because reactions between the volatilized gases and the char are possible. FT-Raman spectroscopy was used to investigate chemical structural changes of char during the pyrolysis of Victorian brown coal by Li et al.33 Experimental results indicated that the presence of ion-exchangeable Na and Ca in brown coal affects the char-forming reactions greatly during pyrolysis process. However, FTIR analysis can only provide information of the functional groups of pyrolysis products. Besides, some gases including H2, N2, O2, and H2S have no or weak IR absorption, which makes them impossible to be detected with FTIR. Thus, a more powerful and comprehensive method is needed. MS is capable of providing the molecular weights and structural information of unknown components in complex mixtures. It can be easily connected to different kinds of pyrolysis reactors, such as tubular furnaces and micro-pyrolyzer reactors. Tracing back to nearly 30 years ago, Evans et al.36 used the technique of molecular-beam mass spectrometry (MBMS) to elucidate the molecular pathways in the fast pyrolysis of wood and its principal isolated constituents. With a low-energy (22.5 eV) electron ionization source, fragment ions on the mass spectra can be reduced to a certain extent, which makes the identification of pyrolysis products easier. Afterwards, Lattimer37 studied the pyrolysis of five polyolefins using pyrolysis MS (Py-MS) with a “soft” ionization source (field ionization). Free radical degradation mechanisms were proposed to explain the patterns of volatile pyrolyzates. During the past two decades, dozens of pyrolysis studies related to solid fuels have been conducted with Py-MS.38-42


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In order to reduce the difficulty in mass spectrum interpretation, “soft” ionization method is used in most of the studies. By virtue of the on-line sampling capability and high-speed data acquisition system, FTIR and MS are usually coupled to a thermogravimetric analyzer, known as TG-FTIR43-47 and TG-MS,4851

respectively. It has been proved that these two techniques have a huge potential to perform

accurate measurements of volatile species in real time. Yang et al.44 studied the behavior of gaseous products evolving from the pyrolysis of cellulose, hemicellulose and lignin using TGFTIR. It was found that CO2 release is mainly caused by the primary pyrolysis, while secondary pyrolysis is the main source for CO and CH4 release. The co-pyrolysis characteristics of giant reedgrass and lignite were investigated by Guan et al.51 with TG-MS. Gaseous products emission characteristics of the blends were close to the combination of those of the single fuels, which means that there is no obvious interaction between energy grass and lignite. Singh et al.46 studied the pyrolysis of four waste materials using TG-MS and TG-FTIR as complementary characterization techniques. The comparison between mass spectrum and IR spectrogram presents a valuable way to obtain more reliable results. 2.4 Other analytical methods Beside the most commonly used techniques mentioned above, many other analytical techniques like elemental analysis, nuclear magnetic resonance (NMR),52-55 X-ray diffraction (XRD)56, 57 and scanning electron microscope (SEM),58-60 etc. are also used to improve the understanding of solid fuels pyrolysis. For the analysis of some samples (e.g. wood and coal) containing various constituents, elemental analysis can be used to determine the ratio of elements in the samples and their purity, which plays a guidance role in the subsequent pyrolysis study. NMR analyses have usually been used to characterize the chemical compositions of pyrolysis oil and char.


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Specifically, carbon structural features of pyrolysis products can be determined with 13C NMR, while 1H NMR is a powerful tool for the measurement of hydrogen-containing functional groups in pyrolysis oil. XRD and SEM are other two commonly adopted techniques, which are applied to the characterization of pyrolysis char and sample itself. So far, the fundamentals of the pyrolysis of solid fuels have not been fully understood despite of all previously published studies. Therefore, there is growing interest in developing analytical tools that can be used to rapidly and accurately measure the chemical composition of solid fuels pyrolysis products. Recently, VUV-PIMS based on laser, rare gas discharge lamp and synchrotron light has been widely used to study the pyrolysis of solid fuels. Compared with conventional “hard” electron ionization (EI) method used in commercial mass spectrometers, photoionization enables the fragment-free or fragment-minimal mass spectra of the pyrolysis products to be obtained in real time. Therefore time/temperature evolution of products in pyrolysis reactors can be measured, which gives insight into the pyrolysis mechanisms of solid fuels. In this review, instruments based on VUV-PIMS and their applications will be discussed in detail. 3. PHOTOIONIZATION MASS SPECTROMETRY Generally, current PIMS techniques for the study of solid fuels pyrolysis can be classified into three groups according to their ionization sources. In this section, the instrumentations of three main PIMS techniques with VUV light generated by laser, discharge lamp and synchrotron radiation are presented. 3.1 Laser-based VUV or REMPI PIMS


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VUV radiation at 10.5 eV (118 nm) can be generated using xenon gas by the frequency tripling of the third harmonic (355 nm) of an Nd:YAG laser. It can ionize most of the aromatic compounds with ionization energies (IE) lower than 10.5 eV, thereby minimizing the possibility of producing fragment ions. Zoller et al.61 studied the volatile products evolved during the pyrolysis of 20 different rank coals using a laser-based PIMS coupled with TG (TG-PIMS). They found that the relative amounts of oxygen-containing compounds such as CnH2nO, phenols, and dihydroxybenzenes decrease with increasing coal rank. However, some gaseous products like SO2 and COS were not detected because their IEs are above 10.5 eV. Another study of coal pyrolysis was conducted by Tsuji et al.,62 in which FTIR was chosen as a complementary method of PIMS for the identification of pyrolysis products. In consideration of this major drawback, laser-based wavelength-tunable PIMS is developed through resonant four-wave sum-difference frequency mixing in krypton or xenon. However, the operation of wavelength-tunable process is not convenient. Additionally, it requires more sophisticated instrumentation compared with the conventional EI technique, and it is not easy to detect species with very high IEs. On the other hand, the resonance enhanced multiphoton ionization (REMPI) technique based on lasers has been successfully applied to solid fuels pyrolysis studies.63-67 The REMPI technique usually requires two or more photons for photoionization, which takes place through an optical resonance absorption step. Most aromatic pyrolysis products exhibit strong absorption bands in the region 220-300 nm, which is easily accessible by commercial laser systems. Mukarakate et al.65 designed a laser ablation/pulsed sample introduction/MS platform. As shown in Figure 1, this setup combines pyrolysis and/or laser ablation with REMPI reflectron time-of-flight mass spectrometry (TOFMS). Pyrolysis products of biomass samples can be formed by either a hot stage pyrolysis reactor or laser ablation using the third harmonic (355 nm) of an Nd:YAG laser.


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Then, the products are entrained by the carrier gas from a pulse valve. After passing through a conical skimmer, the products are finally introduced into an ionization chamber. The timing sequence of these two lasers (ablation and ionization) and the nozzle pulse used for sampling is well arranged. In contrast with conventional pyrolyzers, laser ablation is useful in volatilizing low vapor pressure biomass materials and minimizing the sample preparation time. Figure 2B) and 2C) exhibit the mass spectra of pyrolysis products of poplar obtained with this REMPITOFMS system in two different pyrolysis modes. In order to compare the characteristics of REMPI and EI, mass spectrum of pyrolysis products of poplar was recorded with 22.5-eV EI MBMS, as shown in Figure 2A). A number of fragment ions can be observed due to the impact of EI, while mass spectra obtained with REMPI show predominantly parent mass ions. In addition, almost all the determined products with REMPI-TOFMS are aromatic compounds. These results indicate that REMPI is a highly selective and soft ionization process compared with electron impact ionization.


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Figure 1. A schematic diagram depicting the pyrolysis/ablation, sample entrainment, ionization, and ion beam detection of a section of biomass material. (Reprinted from 65, with permission from the American Institute of Physics) Zimmermann et al. used on-line REMPI-TOFMS and analyzed the volatile organic compounds from different sources, such as industrial process gases,68 mainstream cigarette smoke69 and pyrolysis products of solid fuels.63,

64, 66, 67

This kind of experimental setup is sensitive in

detecting the volatile compounds but largely hinders the detection of the active compounds. Experimental results conducted with REMPI and single-photon ionization (SPI) technique were compared by Zimmermann et al. Similar to the conclusion made in previous studies, REMPI shows great selectivity to aromatic compounds, especially polycyclic aromatic hydrocarbons (PAHs). In light of this, PIMS with the ionization technique of REMPI can serves as a good complementary analysis method.


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Figure 2. Comparison of mass spectra obtained by MBMS and REMPI-TOFMS systems. Panel A was recorded during pyrolysis of poplar at 500 °C using EI-MBMS with electron energy of 22.5 eV. Panel B was recorded during hot stage pyrolysis of poplar at 500 °C using REMPITOFMS. Panel C was recorded during laser ablation of poplar using REMPI-TOFMS. (Reprinted from 65, with permission from the American Institute of Physics) 3.2 Discharge-lamp PIMS Discharge lamps have been widely used in recent years because of their simplicity and low cost. Beside laser, SPI process can also be performed using discharge lamps. For instance, deuterium lamps can produce the narrow intense Lyman α atomic line at 121.567 nm.70 The ionization selectivity of SPI is determined by the IEs of the compounds and the wavelength (or photon energy) used. SPI is a very “soft” ionization method and its combination with MS is suitable for on-line investigations. Compared with REMPI, SPI performed with discharge lamps is also sensitive to aliphatic compounds, oxygenated chemicals and some heterocyclic compounds,


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which is a good supplement to REMPI. Gao et al.71 successfully detected 54 kinds of PAHs in coal soot with PIMS using a RF-powered Kr discharge lamp (10.6 eV photon) as the ionization source. However, the pyrolysis products were introduced to the ionization chamber through capillary tube, which cannot ensure in-situ analysis. A series of in-situ pyrolysis studies of lignite were performed by Hu et al.72,

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using Py-PIMS (Kr discharge lamp). During experiments,

volatile species passed a repeller plate with an aperture, entered ionization region and finally got ionized by VUV light. Evolution profiles of determined pyrolysis products can be easily obtained, which are very useful for the understanding of the whole pyrolysis process. Wang et al.74 used SPI-TOFMS with a Kr discharge lamp as the ionization source to investigate the catalytic pyrolysis of polypropylene on-line in a tubular furnace. On the basis of the evolution profiles measured at a low temperature, a degradation mechanism was proposed for polypropylene pyrolysis with a low zeolite catalyst content. Recently, a new VUV photoionization source named electron-beam-pumped rare gas excimer lamp (EBEL) was applied for mass spectrometry.75,

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Different from those commonly used

discharge lamps based on rare-gas excimer emission, excimer molecules are formed via electron beam excitation of rare gases. VUV light emitted from EBEL has a good beam quality and high output power. Besides, it allows tuning the wavelength of VUV light in the region 100-200 nm by changing the filling gas. A more detailed description of EBEL can be found in a previous reference.75 With this newly developed light source, lots of works have been performed.64, 66, 77 As shown in Figure 3, Streibel et al.77 combined the SPI-MS using EBEL as ionization source with TG via a heated transfer line. Many kinds of solid fuels, like soft and hard wood,66 coal,77 as well as a copolymer (ABS)77 were investigated with this setup. As the temperature increases, volatile pyrolysis products will be formed and introduced into the ionization chamber. Both the


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mass spectra of pyrolysis products and the corresponding weight loss information are recorded simultaneously during the decomposition process. Furthermore, a two-dimensional contour plot of the measurement can be obtained, as shown in Figure 4. Due to the “soft” ionization characteristics of SPI-MS, most of the mass peaks can be attributed to the parent ions, which makes the identification of pyrolysis products easier. The productions of each major pyrolysis product as functions of the heating temperature (evolution profiles) can also be gained conveniently, which helps to understand the pyrolysis mechanisms. On the other hand, TG-SPIMS offers a way to conduct quantitative studies of solid fuels pyrolysis by combining the TG curve with the evolution profiles of products. The relationship between TG information and evolution profiles can also benefit the development of the decomposition kinetics. In conclusion, the discharge lamps have three major disadvantages: fixed wavelength, low photon flux and large emittance. The first one confines the capability of PIMS to identify some compounds with higher IEs, while the latter two drawbacks reduce the sensitivity of PIMS. A photoionization source with high photon flux and tunable wavelength is needed for the pyrolysis studies of solid fuels.


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Figure 3. Schematic representation of the TG-EBEL-SPI-oaTOFMS system. (Reprinted from 77, with permission from Springer Science and Business Media)

Figure 4. TG/SPI-oaTOFMS investigation of soft wood (spruce/fir mixture). The left column shows the TG- and DSC curve (above) and a two dimensional contour plot of the measurement (below). The right column depicts one selected single mass spectrum recorded at a temperature where heavy mass loss occurred. (Reprinted from 77, with permission from Springer Science and Business Media) 3.3 Synchrotron VUV-PIMS


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Synchrotron radiation is produced when high-energy particles such as electrons in rapid motion are forced to travel in a curved path by a magnetic field. It has a broad-spectrum energy distribution from microwaves to hard X-rays. Properties of synchrotron radiation mainly include high photon flux, high brilliance, polarization, and pulsed time structure. Benefitting from these inherent advantages, especially the broad photon-energy range and high photon flux, SVUV light has been proved to be a great choice of ionization source of PIMS. In the past few years, exciting works in different research fields have been successfully performed with SVUV-PIMS, such as combustion chemistry,78 plasma chemistry,79 and aerosol chemistry,80 etc. Recently, SVUV-PIMS has been applied to in-situ study of solid fuels pyrolysis at National Synchrotron Radiation Laboratory (NSRL).81-85 The available photon energy ranges from 7.8 to 24 eV with an average photon flux of ~1013 photons/s. Figure 5 shows the schematic diagram of the newly developed SVUV-PIMS system. A QSTAR Pulsar hybrid quadrupole-time-of-flight (QTOF) mass spectrometer (AB Sciex, Toronto, Canada) was modified for the study of biomass pyrolysis. Briefly, the experimental setup consists of a pyrolysis chamber with a tubular furnace, a photoionization chamber for the introduction of SVUV light, and the QTOF mass spectrometer. During the experiment, the furnace was heated to a specific temperature, and the sample was then introduced into the furnace with a quartz boat. Nitrogen serves as the carrier gas to bring the gas phase products to the Q0 chamber. After passing through the first skimmer, these products formed a molecular beam which cools down the species. The molecular beam interacted with SVUV light in photoionization chamber to generate ions, which were finally guided into QTOF MS for analysis through a repeller plate and the second skimmer. Nitrogen is also used for collisional cooling and for focusing the ions entering the instrument in the Q0 quadrupole. Moreover, after choosing one specific ion in Q1, collision-induced dissociation (CID) for this


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specific ion can be performed in Q2. Thanks to the CID experiment, valuable structural information of the selected ion can be obtained.

Figure 5. Schematic diagram of biomass pyrolysis apparatus. The pyrolysis and ionization chambers are enlarged as the inset. Parts I and II in the inset are the pyrolysis and photoionization chambers, respectively. Q0, Q1 and Q2 are the three quadrupoles for ion guide, ion selection and ion collision, respectively. The right part of the diagram is a RTOFMS. (Reprinted from 85, with permission from Springer Science and Business Media) Three specific experimental modes can be performed to study the pyrolysis process from different aspects. Firstly, structural assignments of some pyrolysis products can be deduced based on the measured photoionization mass spectra at different photon energies under a fixed temperature. Through comparison of the appearance energies and the known ionization energies (IEs) of pyrolysis products, the peaks in the mass spectra can be identified. Secondly, on the basis of the mass spectra measured under different heating temperature at a selected photon energy, the formation temperatures of various products as well as the effects of heating temperature on pyrolysis products can be obtained. Specifically, to find the appropriate photon energy for soft ionization, mass spectra of pyrolysis products at various photon energies need to


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be measured and compared. A moderate photon energy that both ionizes most organic compounds and minimizes fragmentation should be finally selected. Thirdly, the time evolution profiles of pyrolysis products can be recorded in real time, offering crucial information of pyrolysis mechanisms of solid fuels. 4. APPLICATIONS OF SVUV-PIMS ON SOLID FUELS PYROLYSIS So far, the research group in NSRL has already studied the pyrolysis processes of several representative solid fuels using SVUV-PIMS, such as cellulose, hemicellulose, lignin,82 wood,84, 85

coal83 and waste plastics.81

4.1 Biomass pyrolysis studies Pyrolysis products of biomass have extremely complex compositions (isomers, radicals, and reactive intermediates), which challenge the conventional analytical methods. Benefiting from the “freezing” effect of the molecular beam, SVUV-PIMS enables the diagnostics of many reactive systems. Figure 6 shows a sample mass spectrum obtained during the pyrolysis of microcrystalline cellulose. Compared with the low-energy (22.5 eV) EI mass spectra of cellulose pyrolysis in a previous study36, the spectrum shows higher sensitivity and less fragmentation. Almost all the detected mass peaks can be assigned to parent ions of pyrolysis products. Several typical cellulose pyrolysis products, like HAA (m/z 60), furfural (m/z 96), 5-hydroxymethylfurfural (m/z 126) and levoglucosan (m/z 162), can be determined based on their IEs. HAA was always detected with very minor yields, which indicates that it is probably a secondary pyrolysis product. In addition, thanks to the molecular-beam sampling and “soft” ionization, a previously non-reported compound (m/z 128) was observed on the mass spectrum. The mechanism of levoglucosan

dissociative

photoionization

excludes

its

formation

from

levoglucosan


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photoionization. It was finally deduced as a new intermediate (or primary product) from cellulose pyrolysis. This demonstrates that SVUV-PIMS is a powerful approach to detect reactive species in biomass pyrolysis process.

Figure 6. Mass spectrum of the volatile products from microcrystalline cellulose pyrolysis at the photon energy of 9.0 eV and pyrolysis reactor at 350 °C. (Reprinted from 82, with permission from the Royal Society of Chemistry)

Figure 7. CID spectrum of m/z 150 during pine wood pyrolysis at 300 °C and 10.5 eV, as well as the chemical structures of two possible isomers. (Reprinted from 84, with permission from Springer Science and Business Media) Wood is a typical biomass resource, which consists of three main components, i.e. cellulose, hemicellulose and lignin. The combining of the pyrolysis of the individual components and their synergistic effects increases the difficulty for studying wood pyrolysis. Isomeric identification is


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one of the problems due to their unknown IEs. In this case, structural information of isomers can be given according to previous studies36, 86 and the CID experiments. For example, one pyrolysis product with m/z 150 was detected during pine wood pyrolysis at 10.5 eV. The CID spectrum of this mass peak (m/z 150) is exhibited in Figure 7. The molecular formula determined with highresolution QTOF MS is C9H10O2. Coumaryl alcohol and 4-vinylguaiacol are two possible isomers of this peak based on previous works. Their chemical structures are also shown in Figure 7. It is much easier for 4-vinylguaiacol to lose a methyl radical from methoxy group than coumaryl alcohol. As a matter of fact, the peak at m/z 135, generated by the methyl loss from m/z 150, is clearly detected in the CID spectrum. Therefore, the C9H10O2 is assigned to 4vinylguaiacol during pine wood pyrolysis. In other words, CID experiments provide us with an optional choice in the identification of unknown species. On the basis of products identification, corresponding pyrolysis products of each component (cellulose, hemicellulose and lignin) can be selected to study their decomposition behaviors. And the time evolution profiles of these products can provide useful information of pyrolysis mechanisms of biomass. Figure 8 displays the time evolution profiles of the main markers of cellulose, xylan (major component of hemicellulose) and lignin in miscanthus. The time starts from the biomass injection in the tubular furnace. The time evolution profile of m/z 120 reveals that the earliest product is formed from lignin in miscanthus, followed by the xylan pyrolysis products. This finding is consistent with the previous TG analysis. Light gases are still produced from lignin under higher temperature after cellulose decomposition. Cellulose is the most stable constituent within miscanthus. Possible pyrolysis mechanisms include hydrogen transfer, the formation of double bonds, etc. This work indicates that SVUV-PIMS is a powerful technique for a deeper understanding of the mechanisms of biomass pyrolysis.


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Figure 8. Time evolution profiles of some typical markers for lignin, xylan and cellulose during miscanthus pyrolysis from biomass introduction in the preheated reactor at 550 °C, 10.5 eV photon energy. TIC, Total Ion Current. (Reprinted from 82, with permission from the Royal Society of Chemistry) 4.2 Coal pyrolysis studies More and more attention has been paid to more efficient coal pyrolysis technologies. Compared with biomass, coal is a more complex solid fuel, which consists of tens of thousands of organic and inorganic compounds. The understanding of the fundamental properties of coal requires advanced techniques. Recently, volatile compounds from pyrolysis of two kinds of bituminous coal, Huainan (HN) and Yima (YM), were investigated with tunable SVUV-PIMS at NSRL.83 Mass spectra of products at different photon energies and temperatures were measured during the pyrolysis processes. A series of isomeric products can be identified via SVUV-PIMS based on their different IEs. For instance, during the pyrolysis of HN coal, mass peaks m/z 106, 108, 110, 117 and 122 were identified as pure C2 alkyl benzene, C1 alkyl phenol, benzenethiol (or dihydroxybenzene), indole, and C2 alkyl phenol, respectively. When the temperature increased to 650 °C, the relative content of these ions exhibited a significant increment, which indicates that new compounds with the same m/z appeared at 650 °C. These mass peaks with m/z at 106, 108, 110, 117 and 122 probably also included benzaldehyde, benzyl alcohol, 4-hydroxy-


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benzaldehyde, benzyl nitrile, and benzoic acid, respectively. Additionally, some reactive species were detected during the pyrolysis process with SVUV-PIMS, such as benzyl radical. On the other hand, with the photoionization mass spectra recorded with SVUV-PIMS, the effects of the reaction temperature on the pyrolysis products of coal can be conveniently studied. Figure 9 shows the relative contents of three typical aromatic compounds (alkyl-phenol, alkylnaphthalene, and alkyl-phenanthrene) evolved from HN and YM coal pyrolysis at different temperatures. Different tendencies of these pyrolysis products can be observed as the reaction temperature increases, which are probably caused by the different structures of the macromolecular network between HN and YM coals. It seems that the macromolecular structure of HN coal is ruptured easier than that of YM coal. Although this is just a semi-quantification results, it can be found that it is much more efficient to study the temperature effects on pyrolysis products with SVUV-PIMS than with GC/MS. Furthermore, this technique could be useful for identifying coals from different resources.


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Figure 9. Relative contents of three aromatics evolved from HN and YM coal pyrolysis with a fixed photon energy of 10.5 eV and different temperatures of 450, 550, and 650 °C. (Reprinted from 83, with permission from the American Chemical Society) 4.3 Plastics pyrolysis studies Efficient recycling of waste materials and energy has attracted great interest in recent years. Waste plastics are the major component of MSW, which are also typical solid fuels. Different from biomass and coal, plastics are synthetic materials, making it easier to study their pyrolysis. Considerable efforts have been made to investigate their thermal decomposition products and their relative kinetic parameters. However, a real-time analysis technique of plastics pyrolysis is


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in urgent need for the study of decomposition mechanisms. The pyrolysis processes of two commonly used synthetic polymers, polyvinyl chloride (PVC) and polystyrene (PS), were studied with SVUV-PIMS81. About 30 products from m/z 36 to 290 were identified in the thermal decomposition of PVC. At low temperature, only HCl and a small amount of benzene were formed, which indicates the low-energy barriers for dehydrochlorination and benzene elimination from PVC. It is notable that HCl was not detected in previous mass spectrometric studies. The reason may be that most previous works used 118 nm laser SPI-MS which has a lower photon energy than the IE of HCl. With the help of the tunable SVUV photoionization, HCl was successfully detected in this work.

Figure 10. Photoionization mass spectra of the thermal decomposition products of PVC at a photon energy of 10.0 eV and various temperatures: (a) 300 °C, (b) 360 °C, and (c) 500 °C. The ion signals are amplified by a factor of 10 for the mass regions shown in the figure. (Reprinted from 81, with permission from John Wiley & Sons, Inc.)


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With the increase in temperature, a series of mass spectra can be measured (see Figure 10). Two major decomposition stages were deduced for the thermal decomposition of PVC. The first-stage thermal decomposition of PVC began at 215 °C. As the temperature increased to 300 °C, substituted aromatics such as toluene were produced, indicating the initiation of the second stage. As shown in Figure 10, the signal intensities of aromatic products were enhanced at higher temperatures. Some kinds of PAHs, like indene (m/z 116), anthracene and phenanthrene (m/z 178), were produced at the second decomposition stage. This work mainly demonstrates the good performance of SVUV-PIMS in pyrolysis product analysis, which could help understanding the thermal decomposition mechanisms of synthesized polymers. 5. SUMMARY AND PERSPECTIVES This review highlights a series of recent developments in solid fuels pyrolysis studies with conventional analytical methods and PIMS, especially the applications of SVUV-PIMS to the pyrolysis of biomass, coal and waste plastics. Different from lasers and discharge lamps, the tunable wavelength, high photon flux and good energy resolution of SVUV light enable the SVUV-PIMS to be a powerful and universal technique in pyrolysis study. Based on the molecular-beam sampling, reactive intermediates and key radicals produced during pyrolysis process can be detected by SVUV-PIMS, which provides valuable experimental results, especially the evolution profiles of pyrolysis products measured in real time. Moreover, the combination of SVUV ionization source and commercial high-resolution mass spectrometers could lead to more accurate and reliable experimental results, which is well suitable for the study of complex reactions.


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Although SVUV-PIMS has shown great advantages in studies of several complex pyrolysis processes, there is still a lot of space for further improvements. (1) A fundamental understanding of flash pyrolysis is still beyond our knowledge due to the difficulty in obtaining the time evolution profiles of products in micropyrolyzer reactors. Based on the rapid data acquisition rate of SVUV-PIMS, corresponding experiments could be designed to obtain the products evolution profiles. (2) Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) provides the highest mass resolution. FT-ICR-MS with SVUV ionization source will be a powerful tool for analyzing complex mixtures, like coal and crude oil. The difficulty in interpreting the spectra could be reduced with ultrahigh mass resolution. (3) A great deal of work needs to be done about the quantification of pyrolysis products of solid fuels. The combination of TG and SVUV-PIMS using molecular-beam sampling could quantify not only the reactants and products, but also the reactive intermediates and radicals, which offers valuable experimental data for reaction kinetics studies. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (YP) and [email protected] (FQ). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work has been supported by grants from the National Natural Science Foundation of China (51127002, 91545120 and U1432128), National Basic Research Program of China (973 Program) (2012CB719701) and Chinese Academy of Sciences. ABBREVIATIONS GC/MS, gas chromatography/mass spectrometry; LC/MS, liquid chromatography/mass spectrometry; TGA, thermogravimetric analysis; FTIR, Fourier transform infrared spectroscopy; NMR, nuclear magnetic resonance; SEM, scanning electron microscope; PIMS, photoionization mass spectrometry; SVUV, synchrotron vacuum ultraviolet; MSW, municipal solid waste; HAA, hydroxyacetaldehyde;

Py-GC/MS,

pyrolysis-GC/MS;

HPLC,

high

performance

liquid

chromatography; GPC, gel permeation chromatography; MBMS, molecular beam MS; Py-MS, pyrolysis-MS; XRD, X-ray diffraction; EI, electron ionization; REMPI, resonance enhanced multiphoton ionization; TOFMS, time-of-flight mass spectrometry; PAHs, polycyclic aromatic hydrocarbons; SPI, single-photon ionization; EBEL, electron-beam-pumped rare gas excimer lamp; NSRL, National Synchrotron Radiation Laboratory; QTOF, quadrupole-time-of-flight; CID, collision-induced dissociation; IE, ionization energy; HN, Huainan; YM, Yima; PVC, polyvinyl chloride; PS, polystyrene. REFERENCES (1) Cao, Y.; Casenas, B.; Pan, W.-P. Energy Fuels 2006, 20, (5), 1845-1854. (2) Zaman, A. U. Int. J. Environ. Sci. Technol. 2010, 7, (2), 225-234.


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Pyrolysis Study on Solid Fuels: From Conventional Analytical Methods

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