Study of a Biomass Sample by TGA-GC/MS and TGA-Micro GC/MS
Since about the year 2000, there has been a marked change to the use of more renewable and sustainable energies.
This change has been catalyzed by the prospect of limited resources of fossil fuels, the greatly increased awareness of environmental and atmospheric problems, and the unsolved problems associated with the operation and decommissioning of nuclear power plants. Renewable energy is energy derived from resources, which are naturally replenished on a short timescale. These include hydro- and wind-power, biomass, solar energy, geothermal power, and biofuels. Related technologies such as fuel cells, batteries or energy storage systems utilize special compounds that can be investigated by thermal analysis.
Biomass refers to renewable, non-fossil, organic substances from which energy can be obtained. Different kinds of biomass such as wood, straw, corn, sugar cane, eucalyptus, and rapeseed oil are readily available and fast growing. Before biomass can be used as an energy resource, it has to be suitably treated [1, 2]. For example, liquefied biofuels are used for transport, whereas solid biomass is needed for heat and electricity generation. Several complex processes are available to do this. Thermal analysis was used to characterize empty fruit branches of palm oil – a biomass material commonly used in biofuels; the sample was measured by thermogravimetric analysis (TGA) coupled to GC/MS and Micro GC/MS to determine its moisture content, dry mass, and the gases released during pyrolysis.
Empty fruit branches of palm oil
Biomass Sample by TGA
Figure 1. TGA analysis of the biomass sample derived from empty fruit branches of palm oil; the TGA curve is colored black, the derivative Thermogravimetric (DTG) curve in red.
Figure 1 shows the TGA mass loss curve in black and the DTG curve in red. The biomass sample exhibits two mass loss steps. In the first step, moisture is released (about 4.3%; Table 1) up to about 200 °C. This is followed by pyrolysis of the organic substances (approximately 67.9%; Table 1).
Table 1. TGA results
The TGA measurement was used to determine the temperatures at which the decomposition gases were to be analyzed in the TGA-IST-GC/MS experiment. The temperatures are shown in Table 2.
Table 2. The TGA temperatures at which gas samples were collected and stored in storage loops of the storage interface.
Total Ion Chromatography (TIC) at 420° C
Figure 2. TIC chromatogram corresponding to a TGA temperature of 420 °C; some of the peaks identified are labeled.
Figure 3. The first 12 minutes of the TIC chromatogram corresponding to a TGA temperature of 420 °C; the main peaks identified peaks are labeled.
Figure 2 shows the TIC chromatogram of the gas sample stored at 420 °C. Each peak in the TIC corresponds to a different compound. For better visualization, the first 12 min of the same TIC are shown on an expanded time scale in Figure 3. Over 50 different compounds were identified (see Table 3), mainly ketones, aldehydes, heterocyclic organic compounds, carboxylic acids, phenol, esters and furan. The most important detected compounds for biofuel production include acetic acid, phenol, acetone, BTEX, and furfural.
Table 3. Main evolved compounds detected by GC/MS in a TGA-GC/MS experiment.
TIC of Residues
Figure 4. GC/MS analysis of residue (evolved products that condensed in the protective tube).
After the TGA-IST-GC/MS experiment, the condensed products remaining in the protective tube between the TGA and IST were collected (see Section 4.3) and dissolved in isopropanol. A small volume of this solution (1 μL) was then injected manually into the GC using the same method as for the storage loops but with a 50:1 split ratio. Figure 4 shows the evolved compounds detected by GC/MS; these are mainly fatty acids and alkanes with high boiling points.
TCD at 300° C and TIC at 325° C
Figure 5. TCD chromatogram of Module A at about 300 °C.
Figure 6. TIC chromatogram of Module B at about 300 °C.
Figure 7. TIC chromatogram of Module C at about 325 °C.
Two additional TGA-Micro GC/MS experiments were performed. In the first experiment, the MS was connected to Module B and in the second to Module C. Figure 5 displays the chromatogram obtained at about 300 °C using Module A, Figure 6, at 300 °C using Module B, and Figure 7 at 325 °C with Module C. The results confirm that the first decomposition step between 100 and 200 °C corresponds only to the loss of moisture. During the second step, H20, CO, CO2, formaldehyde, acetaldehyde, and chloromethane are detected with Modules A and B. Compounds detected with Module C consisted mainly of light ketones such as acetone, light furans, acetic acid and methyl acetate (Table 4).
Table 4. Main compounds detected during a TGA-Micro GC/MS experiment.
DTG and Emission Profile Curves
Figure 8. TGA curve and emission profiles of selected evolved compounds.
The Micro GC/MS emission profiles in Figure 8 indicate the release of water during the first and second decomposition steps; most other compounds, excluding BTEX, are evolved in the second step (main peak on the DTG curve). BTEX compounds (benzene, toluene, ethylbenzene and xylenes) show a maximum emission peak at about 500 °C, corresponding to the shoulder in the DTG curve. Here it can also be seen that 2,3-butanedione is released in two stages: first in the second decomposition step (larger peak) and again at 500 °C (smaller peak).
Evolved compounds of biomass can be easily characterized by TGA coupled to a GC/MS (heavy and medium compounds) and Micro
GC/MS (permanent and light compounds). The results yielded information about the composition (moisture, ash, carbon) of the
biomass material as well as the type of gases that are produced during pyrolysis. These techniques can be used to characterize the
structure and composition of the various gases evolved from the thermal decomposition of biomass feedstocks, and hence help assess
their potential as a source of energy.
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