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Coal/biomass characterization by thermogravimetric analysis (TGA)

The fine optimization of gasification technologies requires, among other things, extensive knowledge of the thermochemical processes occurring in the gasifier (i.e. devolatilization, pyrolysis, gasification and combustion reactions). In particular, it is well established that a knowledge of thermal decomposition of coal and biomass is essential to assess the performance of carbonization, combustion and gasification processes. Non-isothermal thermogravimetric analysis (TGA) is the simplest, least expensive and most effective technique to observe both the pyrolysis and combustion profiles of a fuel.

Several fuels have been characterized: coal from Sardinia, South Africa, Alasha, Hungary and lignocellulosic biomass (several kinds of wood chips, including pine and eucalyptus). After a preliminary characterization (to detect proximate, ultimate and calorimetric analyses according to ASTM standards), samples (about 10 µg) are characterized by using a Mettler Toledo TGA/DSC 3+ thermogravimetric analyzer, installed at the Sotacarbo labs.

The pyrolysis process is simulated by conducting the analysis in an inert atmosphere (pure nitrogen); combustion tests are performed using air (provided in bottles, with a controlled concentration); gasification tests can be performed fluxing the TGA with a mixture of nitrogen, CO2 and, in several cases, oxygen. The temperature is increased according to a pre-defined heating rate up to 800-1000 °C (depending on the specific aim of each test) and weight loss is constantly measured by the instrument, together with the heat fluxes.

The direct processing of the experimental data allows to determine the key process parameters (such as ignition, peak and burnout temperatures, the burning rate and time and the main combustibility indexes).

Moreover, kinetic characterization is carried out by comparing different isoconversional methods (such as the Flynn-Wall-Ozawa and the Kissinger-Akahira-Sunose methods).