Characterization of Biomass Using TGA/DSC Coupled to a Mass Spectrometer

During the last ten to twenty years, 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 problems, and unsolved problems associated with the operation of nuclear power plants.

Alternative or renewable energies encompass a very large field. Besides wind energy and photovoltaic power, biomass is an important possibility for generating energy.

Two examples from the field of biomass illustrate the use of thermal analysis for the characterization of biomass. This work was performed using a TGA/DSC coupled to a mass spectrometer.

Introduction

Biomass refers to renewable organic substances from which energy can be obtained. This includes products such as wood, straw, corn (maize), eucalyptus, rape, sugar cane and many other non-fossil raw materials.

Before biomass can be used as an energy resource, it must be suitably processed - it can be gasified, liquefied or solidified. There are many complex procedures for doing this [1].

This article describes how corn (maize) and eucalyptus biomass was characterized using a TGA/DSC and a TGA/DSC coupled to a mass spectrometer (MS).

The main purpose was to determine the moisture content, the dry mass, and the nature of the gases produced in pyrolysis.

Experimental Details

Two different biomass materials were available, namely corn (maize) and eucalyptus. Both samples were present in coarse powder form and were analyzed by TGA/DSC and TGA/DSC-MS. This was done using a TGA /DSC  1 (large furnace and DTA sensor) coupled to a Pfeiffer Thermostar™ mass spectrometer (MS). The MS interface was installed at the outlet of the TGA. 

Samples of 4 to 20  mg powdered material contained in 70 μl aluminum oxide crucibles were heated from 35 to 1000 °C at a heating rate of 10 K/min. The measurements were carried out in an iner t at mosphere up to 6 0 0  °C using 70 mL/min nitrogen as purge gas.

After this, the purge gas was switched to synthetic air (oxidative atmosphere, 70  mL/min air). The TGA/DSC-MS measurements were performed in an argon atmosphere (100 mL/min, 35 to 700 °C, 10 K/min).

Measurements and Evaluations

Figures 1 and 2 display the results of the TGA measurements. They provide information about the composition of the two samples. The TGA curves are colored black and the DSC curves red. Both biomass samples exhibit three mass loss steps. Up to about 200 °C, moisture is released (about 5%). This is followed by the pyrolysis of the organic substances. At 600  °C, the purge gas was switched to an oxidative atmosphere. The pyrolysis carbon (char) burns and inorganic substances such as calcium oxide, magnesium oxide or silicon oxide remain behind as a residue.

Table 1 summarizes the TGA results. The DSC curve provides information about the energy content of the pyrolysis carbon black (char) produced during pyrolysis.

This information is important in order to classify and compare different types of biomass with regard to their possible energy potential and to select them later on for use in biomass thermal power stations.

Besides the pyrolysis carbon black, pyrolysis gases are also expected (carbon monoxide, hydrogen and methane). To obtain more information about this, additional TGA-MS measurements were performed.

Samples of the materials were heated in an inert atmosphere (argon) from 35 to 700 °C. The results are displayed in Figures 3 and 4. The black dashed curve is the TGA curve. The other curves are the mass spectrometer multiple ion detection (MID) curves for carbon monoxide (m/z 28), water (m/z 18), methane (m/z 16) and hydrogen (m/z 2).

The curves show the temperature at which particular gases are evolved. In the first mass loss step of both samples up to about 150 °C, only water was detected. The main difference between the two samples is that corn (maize) produces more methane (between 400 and 600 °C) than eucalyptus.

Methane is the most important gas in the gasification of biomass. A high content of methane is desirable because it is later burned and produces energy. The gaseous products produced were not quantified in this example.

Conclusions

Renewable energies have become more and more important in recent years due to different reasons such as the limited resources of fossil fuels, global warming or accidents in nuclear power stations.

Biomass is recognized as an important renewable energy source. Biomass can be easily characterized by TGA/DSC coupled to a mass spectrometer. The results yield information about the composition (moisture, ash, carbon) of the biomass and the type of gases that are produced during the pyrolysis. This information allows the potential of a particular biomass material as an energy source to be estimated.

_{Characterization of Biomass using TGA/DSC Coupled to a Mass Spectrometer | Thermal Analysis Application No. UC 405 | Application published in METTLER TOLEDO Thermal Analysis UserCom 40}