TG, TG-MS, and FTIR characterization of high-yield biomass charcoals

The pyrolysis and combustion characteristics of various high-yield charcoals manufactured in a process development unit were studied by thermogravimetry (TG), thermogravimetry-mass spectrometry (TG-MS), and Fourier transform infrared spectrometry (FTIR), Charcoals resulting from two runs with a macadamia nutshell feedstock and one run with a eucalyptus wood feedstock are compared. The peculiarities of devolatilization, oxygen gasification (temperature-programmed combustion), and the formation of nitrogen oxides are discussed. Small sample sizes (0.2-4 mg) were employed to minimize the effects of heat and mass transfer limitations; nevertheless the results offered a reliable characterization of kilograms of charcoal. Samples produced in different runs or taken from different parts of the reactor behaved similarly with only minor differences that we attribute to reactor inhomogeneities and variations in feedstock composition. In the presence of oxygen, two partial processes were detected. The lower temperature weight loss may be due to devolatilization of the char and oxidation of the volatile matter that results in a carbonized residue. The residue burns off around 450-500 degrees C. The rate of both processes evidenced strong dependence on oxygen concentration, indicating an unexpected influence of oxidation on the low-temperature devolatilization processes. The intensity of the mass-spectrometric ion signal for NO+, which represents the formation of nitrogen oxides, did not follow the overall mass loss rate curves (DTG). The peak temperatures of the NO+ and DTG curves differed by 3-9 degrees C. These differences were influenced by the feedstock composition but did not depend on such operating conditions as the heating rate and oxygen concentration. Elevated pressure thermogravimetry revealed that a very high partial pressure of CO2 in the carrier gas (577 kPa) has only negligible influence on the rate of the temperature-programmed combustion.