ASSESSMENT OF THE EFFECT OF BIOCHAR ON THE PROCESS OF THERMOLISIS OF BIOMASS OF PLANTS GROWN ON RECLAIMED LANDS

Abstract

Samples of soil contaminated with heavy metals for pot experiments with annual crops (maize, sweet and sudan sorghum) were taken near the mines "Pavlogradska" and "Blagodatna" in the coal-mining region of Western Donbass. The basis of the reclaimed areas was an 8-10 m thick mine rock (MR) dump, covered with different layers of black soil or red-brown clay by creating two types of soil artificial profiles with the addition of 30 cm of black soil (30 cm of BS +MR) and 50 cm red-brown clay (50 cm RBC + MR). Experimental soil samples were treated with 3.0% (w/w) biochar. The purpose of the study was to study the effect of the addition of biochar on the process of thermal destruction of the biomass of maize, sweet and sudan sorghum, grown on different substrates - components of artificial reclaimed profiles (upper layers of chernozem mass and red-brown clay). The process of thermolysis of biomass components of three agricultural crops was studied by the method of thermogravimetric analysis. Thermal destruction of the biomass of the three studied plant species occurred in two stages: evaporation of water and volatile compounds (stage 1) and decomposition of the main components: hemicellulose, cellulose and lignin (stage 2). The first stage took place in the temperature range of 50-180°С. The process was slow, the maximum speed did not exceed 5-8%/min, the extreme point was observed at a temperature of 100-110°C. Loss of body weight is insignificant, 4.5–7.5%. The second stage is divided into two phases: the decomposition of holocellulose with the beginning of lignin decomposition (phase 1) and the cessation of lignin decomposition, and the formation of non-combustible residue (phase 2). Destruction of holocellulose occurred in the temperature range of 190-390°С. Due to the large amount of hemicellulose in the biomass of the studied plants, its decomposition was shifted to the region of higher temperatures. Therefore, the degradation ranges of hemicellulose and cellulose overlapped, and only one extreme point was observed in the DTG curves. The process proceeded at high speeds with the peak of destruction in the temperature range of 280-310°C. Weight loss was also the most significant and was between 50 and 55%. Decomposition of lignin occurred rather slowly, with one insignificant peak in the temperature range of 420-440°C. A weight loss of 26-30% has been established. At the first stage, the process proceeded mainly with heat absorption; the reactions of the second stage were exothermic with noticeable thermal effects in the zones of cellulose and lignin decomposition. Differences in thermal characteristics of sugar sorghum biomass grown on different substrates and with the addition of biochar were determined in the process of thermolysis. Degradation of holocellulose occurred more slowly in biomass collected from a vessel with red-brown clay, in contrast to lignin, which decomposed faster than in the experiment where the substrate was a loose layer of chernozem. The share of non-combustible residues was almost 2 times smaller. A slight increase in the rate of the cellulose decomposition reaction (1,2 times) and a significant increase in the rate of the lignin destruction reaction (5 times) was observed. In the treatment with biochar, a more complete combustion of biomass was observed. The rate of cellulose decomposition in the experiment with red-brown clay became somewhat higher, although the decomposition of lignin occurred more slowly. The share of non-combustible residues increased by 1,8 times. Thermal destruction of Sudan grass biomass on both substrates occurred in a similar manner. The addition of biochar did not reveal significant deviations in the thermal behavior of biomass grown on chernozem, in contrast to the experiment with red-brown clay, where the addition of biochar contributed to a significant reduction in the duration of thermolysis. The first stage of thermolysis of corn biomass was somewhat shorter in the experiment with a bulk layer of chernozem compared to red-brown clay and was accompanied by a lower mass loss. The same trend was observed in the decomposition of holocellulose. At the same time, the destruction of lignin in the version with chernozem lasted longer, the extremum point was shifted to the region of higher temperatures, the speed of the process was almost twice as low as in the version with clay, and the proportion of non-combustible residue was 1,7. times higher. More complete combustion of corn biomass was also observed in the version with biochar.