and Environmental Protection
http://ago.helion.pl ISSN 1733-4381, Vol. 8 (2008), p-51-62
Application of FTIR absorption spectroscopy to characterize
waste and biofuels for pyrolysis and gasification
Kalisz S. 1,2, Svoboda K.1,3, Robak Z.4, BaxterD.1, Andersen L.K.1
1
Joint Research Centre of the European Commission, Institute for Energy - Petten, the Netherlands,
2
Silesian University of Technology, Institute for Power Engineering and Turbomachinery -Gliwice, Poland
3
Institute of Chem. Process Fundamentals, Academy of Sciences of Czech Republic - Praha, Czech Republic
4
Institute for Chemical Processing of Coal - Zabrze, Poland Streszczenie
Zastosowanie spektroskopii absorpcyjnej FTIR do określania własności biopaliw i odpadów w procesach pirolizy i zgazowania
W artykule omówiono możliwości zastosowania spektroskopii absorpcyjnej FTIR do ok-reślania własności odpadów i biopaliw w procesach pirolizy i zgazowania. W badaniach posłużono się spektrometrem FTIR pozwalającym na analizowanie zarówno stałych jak i płynnych próbek odpadów lub biopaliw. Ponadto urządzenie wyposażone jest w moduł z celą pomiarową do analizy próbek gazowych, które otrzymano z uniwersalnego reaktora pozwalającego na przeprowadzenie pirolizy lub zgazowania próbek biopaliw. Reaktor ten wyposażony jest w ruszt stały, a jego podstawową cechą jest możliwość generowania dużych ilości produktów pirolizy/zgazowania do dalszej analizy. W pracy przedstawiono pierwsze wyniki badań spektroskopowych FTIR próbek gazowych uzyskanych z pirolizy trzech biopaliw: bawełny (celulozy), peletów drewnianych i mieszanki biomasy z tworzy-wami sztucznymi (ROFIRE®). Uzyskane spektra posłużyły początkowo do identyfikacji charakterystycznych dla danego paliwa grup składników, a następnie do próby powiązania zidentyfikowanych związków gazowych ze składnikami zawartymi w paliwach poddanych pirolizie.
Abstract
The paper discusses the various applications of FTIR absorption spectroscopy as a tool for characterizing waste biofuels for pyrolysis and gasification. The FTIR spectrometer used in the study allows for analysis of solid and liquid waste and biofuel samples. Further, an attached dedicated gas cell is used in the characterization of gases evolving during pyrolysis in a versatile pyrolyser/gasifier attached to the FTIR. The pyrolyser operates in a batch
mode and generates large quantities of product samples suitable for further chemical and physical analysis. The paper presents the preliminary results from investigation of the pyro-lysis gases from three different biofuels: pure cotton, wood and fuel made from a mixture of biomass and plastics (ROFIRE®). First, certain characteristic classes of components are identified in the gas, and second, an attempt is made to explain the origin of the gas compo-nents based on the known chemical constituents of the waste/biofuel.
1. Introduction
Fourier Transform InfraRed (FTIR) absorption spectroscopy is a powerful tool for characte-rization of complex mixtures of solid, liquid and gas samples in the infrared part of the optical spectrum (100 µm to 800 nm, or 100 to 12500 cm-1). All organic species will show specific absorption bands in this spectral region originating from vibrational transitions in the molecules. These absorption bands are characteristic for the chemical compound (e.g. phenol, water, benzene etc.), or for the class of compound (e.g. aliphates, aromates, olefi-nes, alcohols, acids, esters, amides etc.) and can be used for both qualitative and quantita-tive analysis of complex samples containing a mix of organic compounds.
Some advantages of the FTIR technique include: (1) simultaneous detection of different species by recording of the full spectral range (100 to 12500 cm-1), (2) quasi continuous measurement especially suited for dynamic processes, and (3) the destructive and non-intrusive character of the measurement (i.e. the sample is not destroyed in the measuring process and the observed process is not disturbed). In part these advantages derive from the Fourier-transform technique with its very high optical through-put, high wavelength resolu-tion and precision combined with the large spectral range recorded simultaneously. This further enables the recording of full spectra separated just milliseconds and even nanose-conds in time by using a special control of the spectrometer interferometer [1, 2]. Detailed information on FTIR spectroscopy may be found in [3].
Despite the general versatility of FTIR spectroscopy in analysis, recognition and subsequ-ent quantification of overlapping spectroscopic bands related to the many molecular species encountered in thermal processing of biomass, rather complex and time-consuming calibra-tion of the FTIR system is required. This is a major limitacalibra-tion of the FTIR technique. Ho-wever, sometimes there is no need for a rigorous analysis of individual molecular species. Instead a group of spectral bands, e.g. the C-H stretch bands around 3000 cm-1, can be used
to estimate the total amount of e.g. hydrocarbons. An example is given in [4] where the total amount of hydrocarbons is estimated by calibrating the system using a hexane equiva-lent concentration method. Another practical problem is that the linearity of the FTIR met-hod is not always given. Careful calibration over the full detection range is thus needed [5]. Application of FTIR absorption spectroscopy in biomass/waste research includes various examples ranging from raw biomass analysis through to analysis of biomass decomposition products and finally to studies on reaction kinetics. Versatility of FTIR in terms of the phy-sical state of the sample being analyzed allows for analysis of different pyrolysis products including oil [6], char [7] and gases [8].
With respect to solid samples, a common technique is to ground the sample and mix it with KBr powder in order to form pellets which are transparent to IR beam. Other technique
based on reflected light e.g. diffuse reflectance infrared FT (DRIFT) spectroscopy, has been applied for characterization of olive wastes [9], and in [10] near infrared (NIR) reflectance spectroscopy has been reported to serve in an industrial application for quick control of recycled paper.
Liquid bio-oils derived from various crops play an important role as biofuels along with pyrolysis oil to a lesser extent. The liquid samples are readily analysed with FTIR applying a droplet between KBr plates. As described in [6], a separation of the pentane, toluene and methanol soluble oil fractions may precede the analysis.
FTIR gas analysis on the evolving gases during pyrolysis is of great importance. Various dedicated gas analysis equipment based of FTIR spectroscopy is commercially available. However, most research applications are based on general purpose FTIR spectrometers equipped with a cell for gas analysis. Especially, coupling TGA (thermogravimetric analy-sis) with FTIR analysis of the evolving gases (TGA-FTIR) has become a very popular technique [8, 11-15]. This technique allows for detailed study of pyrolysis kinetics. Very often however, the TGA instruments operate with masses of only a few hundred milligrams and these small amounts can potentially lead to various errors especially with complex samples like waste and mixed biomass. A macro-TGA-FTIR with a sample mass of up to 150 g has been developed and this should overcome some of these problems [16]. Further, this enables the use of representative large compacted and palletized samples of bio-mass/waste. In these samples mass and heat transport phenomena play an important role in the release of gaseous species from the sample, and secondary reactions within the sample pellets are likely to play a much more pronounced role in the over all chemistry compared to minute samples of a few hundred milligram size.
FTIR absorption spectroscopy has also played – and continues to play - an important role in the elucidation of the chemical structure of coal and coal chars (coke). This research ties closely with the application of FTIR in biofuel/waste pyrolysis research since the chemical groups are similar and char is among the final products in the pyrolysis of biomass/waste. The chemical characterization of the evolved chars in the pyrolysis process by e.g. FTIR spectroscopy is hence also an important tool in understanding the properties of the bio-mass/waste itself.
FTIR spectroscopy of solid fuels has also been used in the attempts to predict individual fuel properties such as total pyrolysis weight loss of an unknown coal by correlating FTIR spectra and pyrolysis data from 26 coal standards. This method was successful when the pyrolysis was performed in a wire mesh-reactor. Similar agreement was however not achieved for CO2-gasification experiments in a fixed-bed reactor leading to the conclusion
that the statistical model developed in the study is sensitive to reactor geometry [17, 18]. In the same fashion, it would be of interest to predict pyrolysis properties like weight loss, tar yield, gas yield etc. using a fast and reliable spectroscopic technique based on FTIR. We thus recently set up a broad research programme to address these issues. The research will likely be similar to research presented for coal [17] as well as biomass [11], but will be extended to cover other biofuels/biomass and in particular various types of waste including municipal solid waste.
In this paper we present some preliminary results within this programme where we have used FTIR spectroscopy to analyse the pyrolysis gases from three different biofuels: pure cotton, wood and fuel made from mixture of biomass and plastics (ROFIRE®). First, certa-in characteristic classes of components are identified certa-in the gas, and second, an attempt is made to explain the origin of the gas components based on the known chemical constituents of the waste/biofuel. In these studies a special pyrolysis reactor was build to facilitate inve-stigations on real large biomass/waste samples.
2. Experimental setup and materials
The experimental setup used in the present study consists of a batch pyrolysis unit specially designed for handling of large biomass samples allowing for generation and collection of large quantities of pyrolysis products in a representative and well controlled manner [19]. The pyrolysis chamber has a volume of 330 cm3 and typically samples of 25 g or more are
pyrolysed. The unit is coupled with a Bruker Equinox 55 FTIR spectrometer equipped with a liquid-nitrogen cooled MCT (mercury-cadmium-telluride) detector and silicon coated KBr beam splitter. A dedicated TGA-IR module comprising a 123 mm path length gas cell with KBr windows is used for analysis of the evolving gases (Fig. 1).
TGA-IR module fiber filter collecting bottles Ar reactor furnace cold water bath
FTIR spectrometer gas purge T T p T MFC 2 N
Fig. 1 Experimental set-up
In contrast to standard TGA-FTIR applications the evolving gases are cooled prior to enter-ing a gas cell and condensed tars are analyzed separately by non-spectroscopic methods (not discussed here). All absorbance spectra are calculated using a background obtained just before each experimental run while the gas cell is purged with Ar. The spectra are recorded with 1 cm-1 resolution to reveal rotational band structures of the gases and 16 scans are
averaged for a given spectrum. The spectra are recorded from 6000 to 370 cm-1. Prior to the
experiments, calibration curves are obtained from gas samples with known composition. The gas cell is held at 150°C to prevent condensation of residual tars not condensed in the cold traps.
Each pyrolysis experiment starts with an initial 20 min purge of the pyrolysis reactor conta-ining the biofuel sample with Ar at 360 ml/min to remove any residual gas from the reactor. Next, the purge flow is reduced to 36 ml/min and the heating sequence is initiated with first a heating rate of 18.7°C/min, then a heating rate of 6.7°C/min, next 10°C/min and again 6.7°C/min. The time intervals corresponding to each heating rate are 15, 30, 30 and 15 minutes accordingly. The final furnace temperature of 900°C is kept for 30 min after which the reactor is left to cool by itself. In practice, the final temperature recorded just above the sample exceeded the furnace set point by 40-50°C.
Table 1. Feedstock properties
Cotton (cellulose)* Wood ROFIRE® [21] Proximate analysis
Moisture content at 105°C, wt.% N/A 8 2.9
Ash content at 550°C, wt.% (dry) N/A 0.5-0.6 6.0
LHV, MJ/kg (as received) 17520 17.76 26.704
Volatile matter, wt.% (dry) N/A 84 84.4
Density, kg/m3 N/A 630-650 472
Ultimate analysis, wt.% (dry composition)
Sulphur S N/A 0.01-0.02 0.09
Carbon C 44.4 50 63.3
Hydrogen H 6.2 6.0-6.2 8.9
Nitrogen N N/A <0.1 0.3
Oxygen O 49.4 43-44 20.95
Chlorine Cl N/A N/A 0.52
* Phyllis database: www.ecn.nl/phyllis/
Approximately 25 g samples of biofuels are used for each run. The feedstocks chosen for the study are standard 6 mm diameter wood pellets of 15 mm average length, and a com-mercially available solid fuel being marketed as ROFIRE® [20]. ROFIRE® is a solid fuel made from paper fibre mixed with other substances such as fabric fibre, wood chips and about 50 % plastics pressed into pellets of 8 mm diameter and 20 mm average length. The exact content of this fuel varies from one batch to the next (personal communication with the producer of ROFIRE®). For comparison purposes both feedstocks are compared with cellulose in the form of cotton wool (cosmetic pads). Some data on the three samples are presented in Table 1. Note that ROFIRE® has a non-negligible content of chlorine due to the presence of PVC. The ROFIRE® has already been intensively investigated during gasi-fication trials elsewhere [21].
3. Results and Discussion
The FTIR software allows for a diversity of result manipulations which ideally should reve-al the nature of the process. First, an identification of characteristic species was done using search function against the EPA FTIR spectra library supplied by Bruker. Since the search function does not necessarily identify the species correctly the intuitive intervention of an experienced operator is essential. Fig. 2 shows the spectra and the identification results for
the three samples being investigated. In the spectra there are certain bands which are clearly distinguishable in all spectra and there are other bands only present in the wood and ROFI-RE® spectra. Further, the spectra exhibit growing complexity going from pure cotton to wood to ROFIRE® showing the most complex spectrum.
Cotton 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 0 .0 1 .0 2 .0 3 .0
Wood C=O stretch
(formaldehyde) aromatic rings (benzene and derivatives) 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 0 .0 1 .0 2 .0 3 .0
ROFIRE O-H stretch(methanol water) C-H stretch (CH4) CO2 CO ethylene propene
CH4 (ethylene, propene)C=C stretch propene ethylene 1-butene 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 0 .0 1 .0 2 .0 3 .0 Wavenumber cm-1 A b s o rb a n c e U n it s
Fig. 2 Comparison of gas spectra from pyrolysis of cotton, wood, and ROFIRE® at 550-600°C
The common bands are:
at 500-1000 cm-1 aromatic rings rocking bands which expose the presence of ben-zene and its derivatives (e.g. BTX - benben-zene, toluene, xylene and phenol, cresol etc.); these are tar species of very low dew point and are thus not condensing in the cold traps
at 1200-1400 cm-1 C-H twisting and rocking bands (PQR branches of CH4)
at approx. 1750 cm-1 carbonyl stretching band (e.g. formaldehyde and hydroxyace-taldehyde)
at 2000-2200 cm-1 CO band
at 2300-2400 cm-1 CO 2 band
around 3000 cm-1 characteristic C-H stretching bands (PQR branches of CH4 seen)
at 3550-3750 cm-1 O-H stretch (e.g. methanol, water) The bands which are exclusively found in ROFIRE® are:
at 900-1000 cm-1 combined bands of ethylene and propene, C=C stretch
at 1400-1500 cm-1 combined bands of ethylene, propene and 1-butene
around 1800 cm-1 ethylene band
at 2800-3150 cm-1 wide band associated with various species with C-H bonds.
Fig. 3 FTIR absorbance spectra stack plot from the pyrolysis of wood
Plotting FTIR absorbance spectra in series in a 3D stack allows for quick observation of gas evolution patterns (Fig. 3). In these plots each spectrum given by its number Z [Points] in the sequence of spectra corresponds to a certain time and temperature in the reactor. The time (or hence temperature) evolution of a specific gas species can be followed at their characteristic absorption at a given wavenumbers [cm-1] corresponding to specific species as shown in Figs. 4-7. It should be noted that absorbances above 2.0 (1 % light transmis-sion) are likely to be uncertain.
Fig. 4. CO evolution profiles for a) cotton, b) wood, c) ROFIRE® taken at 2110 cm-1
Fig. 5 CO2 evolution profiles for a) cotton, b) wood, c) ROFIRE® taken at 2330 cm-1
Fig. 7 BTX evolution profiles for a) cotton, b) wood, c) ROFIRE® taken at 675 cm-1 When comparing evolution profiles for CO, CO2, CH4 and BTX, as shown in Figs. 4-7 we
note that profiles obtained for ROFIRE® are generally more flat and wider than those obta-ined for cotton. This is likely because ROFIRE® has many more constituents and hence there are many more processes running in parallel and consecutively during its pyrolysis. Moreover, the large compressed pellets of ROFIRE® facilitate secondary reactions within the fuel pellets. Secondary reactions automatically prolong the gas evolution profile and are hard to avoid if special measures are not undertaken [12]. The question whether these reac-tions affect the process or not depends greatly on the heating rate, type of fuel, and the reactor design. This is why it is sometimes hard to compare results obtained from different experiments. In our case it could be assumed that at least those secondary reactions taking place outside the fuel pellets are minimized to some extent since the evolving gases are continuously swept away from the reactor chamber by the carrier gas.
With respect to cotton (cellulose) pyrolysis the evolution of CO and CO2 starts at about
350°C and reaches a peak between 370 and 430°C. This is consistent with TGA results as reported in [13] where a continuous region of cellulose mass loss is observed between 350 and 540°C with the exception that the CO2 peak is reported at 290°C. Similarly, BTX is
released rather early at around 300°C. CH4 evolution starts somewhat later and the profile is
less steep at the initial stage. A characteristic second CO peak is observed at 800°C which could be attributed to the high temperature reaction of char with CO2: CO2 + C → 2 CO
[13], but could also result from secondary reactions.
Wood decomposition, similarly to cotton (cellulose), starts around 350°C since a main constituent of wood is cellulose. Also, similarly to cotton decomposition, two distinguisha-ble peaks are visidistinguisha-ble in all profiles. In other slow heating rate (10°C/min) biomass pyrolysis experiments [22] it is reported that a double-maxima release profile is common and can be attributed to the constituents of wood biomass. The main components of wood biomass are hemicellulose, cellulose and lignin which decompose in slightly shifted temperature win-dows. First, decomposition of hemicellulose is observed (200-375°C), next cellulose is decomposing (275-380°C), and finally lignin decomposition products are observed over a wide temperature range (180-550°C). This may certainly also explain the double maxima in CO and CO2 observed here. Nevertheless, we speculate that char reactions may also play an
important role in the formation of the double maxima.
Plastic containing biofuels have recently been investigated and it was demonstrated that certain features of pure cellulose decomposition are not affected by the presence of plastics such as PE, PP and PS while at the same time there might occur significant differences when PVC is added [23, 24]. As reported in [23], cellulose changes behavior in mixture with PVC (even in low concentrations) and degrades at lower temperatures with increased formation of easily cracked volatile species and char due to acid catalysis by HCl formed by pyrolysis of PVC. In our experiments with ROFIRE® containing traces of PVC, similar phenomena should be observeable in earlier formation of pyrolysis products characteristic for cellulose. Pyrolysis of cellulose causes formation of formaldehyde, hydroxyacetaldehy-de, CO, and CO2 among other species [12]. The evolution of CO and CO2 started, similarly
ROFIRE® started at a temperature about 100°C higher when compared to pure cellulose pyrolysis. This observation, which is also in contrast to suggestions given in [24] may indi-cate that the small amount of PVC present in ROFIRE® (as chlorine: 0.5 wt.% see, Table 1) is not sufficient to catalyse the low temperature pyrolysis of the cellulose fraction in this mixed fuel.
For ROFIRE® we further note the presence of ethylene and propene in the pyrolysis gas which is associated with the large polyethylene (PE) and polypropylene (PP) content in the fuel, since PE and PP are the most commonly used plastics.
It should be noted that the observed unique features of ROFIRE®, e.g. extended tempo-ral/temperature gas evolution patterns, their shape, enhanced ethylene and propene bands may be utilized for waste feedstock characterization in a combined pyrolysis – gas analysis FTIR unit.
Nitrogen containing compounds, e.g. HCN and NH3, even though they should be easily
detectable by the FTIR technique [25], were not recognized in the spectra likely due to adsorption of NH3 in the unheated gas line as well as in the cold traps unit.
4. Conclusions
We have reported on our preliminary FTIR spectroscopic analysis of the pyrolysis gases evolving from three biofuels: pure cotton, wood and ROFIRE® (a mixture of waste plastics and biomass). The obtained FTIR spectra and temporal/temperature gas evolution patterns show some common features for all feedstocks as well as some unique features only seen in the waste feedstock. These unique features, e.g. extended temporal/temperature evolution patterns, their shape, enhanced ethylene and propene bands may be utilized for waste feed-stock characterization in a combined pyrolysis – gas analysis FTIR unit.
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