Thermal analysis of switchgrass (PANICUM VIRGATUM L.) grown on reclaimed lands

Анотація

The steady decline reserves of fossil fuel and growing environmental problems lead to the need to produce pollution-free biofuel and chemicals from renewable resources such as wood and grassy energy plant. Processes such as pyrolysis and gasification are effective ways of converting biomass to fuel. Understanding the thermal reactions during the decomposition of biomass is important for designing the thermochemical installations and simulation of pyrolysis and gasification processes to predict the yields and properties of the product obtained. Thermogravimetric analysis (TGA) is one of the methods most commonly used for investigation the kinetics of weight loss that occurs through the biomass decomposition. In this research thermogravimetric analysis was used to study the processes occurring during the combustion of switchgrass biomass and to study the influence of various factors on the efficiency of its thermal decomposition. The work was carried out at Pokrov land reclamation station of Dnipro State Agrarian and Economic University. Two experimental variants were used: field and model. In the field variant, the seeds of switchgrass (Panicum virgatum L.) were sown on experimental sites, which were undergone a long phase of phytomelioration, with a mixture of loess-like loam and red-brown clay. To determine the effect on the
thermal characteristics from the different additional fertilizing agents the following five substances were used: (1) mineral fertilizer N60:P60:K60 kg ha-1; (2) ash of sunflower husk and (3) sewage sludge in the amount of 10 t ha-1 each; (4) mixture of ash with sewage sludge in the amount of 10 t ha-1 and (5) a double dose of sewage sludge (20 t ha-1). All amendments were put into the soil once in spring in a dry form.  In the model experiment, plants were grown in lysimeters with geochemically active dark gray schist clay (DGC). The clay was taken from a site, which has been at the stage of natural overgrowth for four decades, from the depths 0-20 cm, 20-40 cm and 40-60 cm. Lysimetric vessels were filled up by the DGC substrates with thickness of 60 cm. As an underlying substrate the sand was used. The thermal analysis of plant biomass was carried out using the derivatograph Q-1500D of the “F. Paulik-J. Paulik-L. Erdey” system. Differential mass loss and heating effects were recorded. The results of the measurements were processed with the software package supplied with the device. Samples of biomass were analyzed dynamically at a heating rate of 10°C/min in an air atmosphere. The reference substance was aluminum oxide.  It was found that the thermolysis of plants grown on a phytomeliorated mixture of loess loam and red-brown clay passes in three stages in the temperature range from 30oC to 580oC. In the first stage, which passes in the temperature range 30-190°C, mainly the endothermic processes of evaporation of water and volatile substances took place. Weight loss was insignificant (from 9.6 to 11.6%). The main pyrolysis process continues in the range from about 200°C to 370°C. At this stage, the processes of exothermic thermo-oxidative degradation of hemicellulose and cellulose components intensified. This stage is characterized by the highest rate and the largest percentage of weight loss (47.8-52.2%). Amendments application increases the reactivity of the biomass during the thermal decomposition of hemicellulose and cellulose. The best effect was obtained from the application of sewage sludge and mineral fertilizer. Thus, if the peak of degradation of these components in the control was related to as 24.0 %/min at the temperature 280°C, in the variant “double dose of sewage sludge” the peak of decomposition rate increased to 26.4 %/min at the lower
temperature, namely 260°C. In the variant with mineral fertilizers peak was 32.4 %/min at 260°C. The last stage of thermolysis passed within the temperature 350о-580оC and was characterized by the appearance of bright exothermic effect. The main component that decomposes at this stage was lignin. The weight loss at this stage was 29.6-33.8%. Addition of the fertilizing agents contributed to the more complete biomass combustion since the proportion of residual mass in experimental variants decreased by 3-6% in comparison with control samples. The thermal destruction of switchgrass biomass, grown on dark-gray schist clay, differs somewhat from similar processes in plants on the phytomeliorated mixture. Thus, on a layer of 0-20 cm it was longer and occurred in the temperature range from 40°C to 690°C while on the layer 20-40 cm it was shorter and in the temperature range from 30°C to 530°C.  The first stage of volatile components evaporation took place in all three variants almost identically, within the range of 30оС-180оС. Weight loss was from 7.6 to 10.9%. In the second stage, two peaks of hemicellulose and cellulose destruction  are clearly traced in the samples from the 40-60 cm layer, while in others the cellulose decomposition occurs earlier, therefore only one peak is observed in the DTG curves in this region. The weight loss on the second stage was within the range of 52.0-58.6 %. The last stage took place in the temperature range from 360°C to 690°C. Decomposition of the biomass samples taken from plants grown on the deeper layer was occurred faster and more complete than on the layer 020 cm. The weight loss was 24.0-29.2 %. In general, thermolysis is more effective in plants grown on deeper layers.