Determination of the temperature of an object irradiated by microwaves by the method of spectral pyrometry

We describe the results of currently topical investigations of the method used for the production of activated carbon from the biomass wastes with the help of microwave carbonization. As compared with ordinary carbonization in thermal furnaces, the procedure of microwave carbonization proves to be a more energy-efficient and environmentally friendly method. However, at present, the problem of guaranteeing the required reproducibility and thermal stability of this process remains unsolved. In the course of microwave irradiation, the temperature of the biomass samples continuously varies and, moreover, complex physicochemical processes run inside these samples. Therefore, in order to determine optimal modes of microwave treatment, it is necessary to know the temperature dynamics of biomass samples. It is proposed to determine the required temperature dynamics of biomass samples by the method of spectral pyrometry. The method is based on the separation of the sections of thermal radiation spectra of the samples that coincide with the Planck function with variable temperature. In these sections, the samples exhibit the properties of gray radiators. The method proves to be efficient for an unknown emission coefficient continuously varying as a function of microstructure, chemical composition, and phase state of the sample. The sample irradiated with microwaves is a weighted amount of cotton linter treated by orthophosphoric acid with a weight of 1 g. The source of microwaves is a magnetron-type generator with a maximum output power of 600 W and a working frequency of 2450 MHz. The thermal spectra of irradiated samples were recorded by using small-size spectrometers operating in visible (350–760 nm) and near infrared (650–1050 nm) bands. The time of irradiation of samples with microwaves varied within the range 60–180 sec. The obtained spectra are processed with the help of the Spectral Pyrometry program, which reads and processes the recorded spectrum, constructs the plots on the Planck and Wien coordinates, and computes temperatures. The analysis of the accumulated results reveals different types of spectra of thermal radiation of the irradiated sample, namely, the spectra similar to the spectra of a perfect radiator, spectra with different temperature zones of the sample, and spectra with atomic lines and molecular bands. The accumulated results prove to be useful for the investigation of the influence of microwaves on various objects, for the study of processes running in the course of carbonization of the biomass, and for the development of more efficient modes aimed at getting activated carbons by the microwave method.