Analysis of Gas Emissions During Thermal Testing Using Thermogravimetry and GC

This article discusses a new technique of coupling thermogravimetry to GC in order to automate the event-controlled or continuous characterization of gas emissions from biomass during thermal processes. Individual components in gas mixtures can hence be better qualified.

Thermal Treatment to Produce Solid Fuels from Biomass

Biomass has been subjected to comprehensive study mainly as a renewable and nearly CO₂-neutral energy source. Along with methods for direct gas generation (biogas plants), many thermal treatment steps will be studied to enable production of high-energy solid fuels from biomass, which can then be used as blending components in conventional fuels such as coal or as a single energy source in incineration plants. In order to achieve this, the volatiles must be thermally expelled so as to obtain fixed carbon, which is an efficient energy source. In order to model the thermal treatment that includes combustion or pyrolysis in the laboratory, thermal analysis provides a range of well-established test methods.

Thermogravimetry

Thermogravimetry (TG) enables accurate recording of temperature-dependent mass changes and differential scanning calorimetry (DSC), which are concurrently applied and enables determining energetic changes as a function of temperature. In conventional instrument combinations, such as directly coupled mass spectrometers (MS) and infrared spectrometers (FT-IR), the gases from the biomass are recorded and analyzed continuously, or they are all condensed into adsorption tubes, then thermally desorbed and subjected to a separate analysis. The decomposition processes of the main biomass components such as cellulose, hemi- cellulose and lignin overlap, so it is usually not possible to separate and detect individual components using directly coupled gas analysis systems. While previously studying products condensed in tiny adsorption tubes, on the other hand, the direct correlation with the gas separation temperature from the biomass is largely lost.

Gas Chromatography

Gas chromatography (GC) is a high-resolution method for separating volatile and semi-volatile compounds. The gas mixtures are separated based on differences in component distribution between a stationary and a mobile phase. Since gas separation in the GC separation column takes a specific amount of time, the duration is based on the sample properties, gas flow rate and length of the separation column and also the type of stationary and mobile phases. Hence, direct coupling of a GC-MS to the TG with a continuous gas flow is not possible.

Coupling of TG/STA to GC-MS

The new solution is the direct coupling of TG/STA to a GC-MS as shown in Figure 1, which is capable of implementing software-controlled gas sampling as well as gas injection in a quasi continuous operation mode, even at short intervals. Mass spectrometry is used as a detection system at the outlet of the GC separation column and records the time distribution of the separated gas components in the purge gas flow. This initial separation of the gases by means of GC along with the high sensitivity and resolution of the MS allows structural information to be obtained, in turn enabling reliable identification of most of the evolved gas components.

Figure 1. TG 209 F1 Libra® thermobalance with heated transfer line to the JAS valve box at the Agilent GC-MS

The advantages of this new coupling are:

• TG/STA-GC-MS coupling with automatic, event-controlled or temperature- dependent GC-MS triggering.
• Fully heated gas transfer (300°C) from the TG/STA furnace outlet to the valve box and the injector at the GC-MS.
• Fast MS with a broad mass range for analysis of the gas chromatogram.
• The GC-MS can be used as a standalone application for liquid/gas injection at any time.

Samples and Results

As a representative of energy grasses, reed (Phragmites australis) was analyzed in the continuous mode. Before the measurement, the dried grasses were chopped and ground for homogenization. The heartwood of a domestic oak (quercus robur) in the form of fine sawdust was subjected to event controlled TG-GC-MS analysis.

For thermogravimetry with GC-MS coupling, sample masses of approx.4 mg to 20 mg were placed in small alumina crucibles. Helium and nitrogen, respectively, were chosen as gas atmospheres in the sample chamber. The carrier gas for the GC was helium. Heating of the reed in the thermobalance resulted in the separation of volatiles and residual moisture in a multi-stage process as shown in Figure 2. The primary objective of the investigation was the analysis of organic components in the range from 200°C to 550°C; these are presented here by the total ion current curve at 1-min injection intervals and the mass spectrometer scan range of 45 u to 300 u, all highly analogous to the mass-change rate.

Figure 2. 19.79 mg of reed, pyrolysis in helium at 10 K/min, determination of residual moisture, volatiles and fixed carbon, with the profile of the total ion chromatogram (TIC) for volatiles.

The residual mass (here 33.74% evaluated at 899°C or, for more practical purposes, 37.42% at 600°C) is the value for the fixed carbon (energy carrier) and the mineral content (ash). Further analysis of the TIC curve, for e.g., in the DTG peaks at 277°C and 347°C – also enables good identification of the main components in the pyrolysis gas as shown in Figure 3, even with the short 1-min intervals in the GC separation column when kept at a uniform temperature of 250°C.

Figure 3. Reed, section of the total ion chromatogram at 287°C (25 min) with indication of the main components identified in the pyrolysis gas.

Changing the injection intervals, the GC capillary temperature and the carrier gas flow rate provides the user greater latitude, when necessary, in order to optimize the separation of gas mixtures for this quasi-continuous operation mode of the TG-GC-MS coupling. In the process, the exact correlation with the gas separation temperature is preserved as shown in Figure 4.

Figure 4. Temperature-dependence of selected pyrolysis products of reed with temperatures at maximum intensities (excerpt).

By triggering the GC-MS analysis via the Proteus software and importing the data into the TG evaluation, the relationship between the mass change of the reed sample and the temperature profile of the chosen molecule ions for furfural (m/z 96), substituted pyran (m/z 114), chloromethane (m/z 50) and guaiacol (m/z 124) can be shown very clearly. Since a direct temperature measurement at the sample was not done, the pyrolysis GC-MS, otherwise useful cannot offer the benefit of a reliable identification of overlapping gas separations including the accurate temperature (≤+0.5°C) and time correlation (≤+0.1 min) with the underlying mass loss in the temperature controlled TG measurement.

The new TG-GC-MS coupling additionally offers automatic recognition of mass losses with direct triggering of the GC-MS analysis (event-controlled mode). The oak wood sample was heated in a nitrogen atmosphere at 20 K/min until the preset threshold of a mass-loss rate of 7.8%/min was initially reached as shown in Figure 5.

Figure 5. Event-controlled TG-GC-MS experiment on oak wood (4.37 mg) with two automatically detected mass-loss steps, stop of the heating with simultaneous start of the GC-MS analysis.

At that moment, the heating was held (isotherm at 291°C) and a part of the content of the sample loop was injected into the GC separation capillary (split 10:1). With the GC furnace program (e.g., 0.5 min 60°C isothermal, heating 25 K/min to 310°C, duration approximately 10 min), an almost complete separation of the gas mixture which is released at 291°C was achieved as shown in Figure 6.

Figure 6. TIC chromatogram for oak wood at 291°C with indication of the retention times for the main peaks.

Analysis of the individual peaks in the registered TIC chromatogram and identification of the different components can be carried out automatically through the library search (integrated NIST 08 MS spectra library); here, for polycyclic aromatics, for example, the proof of substituted phenanthrene is shown in Figure 7.

Figure 7. Identification of 1-methyl-7(1-methylethlene)-phenanthrene during the pyrolysis of oak wood at 291°C at the TIC peak at 9.156 min; shown is the measured spectrum and the very good agreement (99%) with the NIST library spectrum for the substituted phenanthrene.

A click on the peak in the TIC chromatogram at 9.156 min will open the spectrum as recorded by the mass spectrometer at that point in time. Another click on the spectrum offers suggestions from the spectra library, sorted according to the quality of agreement with the measured spectrum. In the present case for the oak wood investigated, the substituted phenanthrene can clearly be detected. In order to quantify the identified gases, both GC-MS and thermoanalytical possibilities are available. Determination of the area using the total ion chromatogram offered by the GC-MS software is a simple technique and correlates with the corresponding mass changes (yielded by the TG software).

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