DISTRIBUTION OF CHEMICAL ELEMENTS WITHIN LIGNEOUS PARTS OF VARIOUS TREES

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1 DISTRIBUTION OF CHEMICAL ELEMENTS WITHIN LIGNEOUS PARTS OF VARIOUS TREES (1) H. KOVÁCS, (2) O. BÁNHIDI, (1) K. SZEMMELVEISZ (1) University of Miskolc, Institute of Energy and Quality Affairs (2) University of Miskolc, Institute of Chemistry 3515 Miskolc Egyetemváros The potential of energetic utilization of woody biomass fuels is influenced by their composition. The mineral salts dissolved in groundwater can migrate from the root to leaves in the transport tissue during the growth of the plants. Newly lumbered wood contains high amounts of water, which is vaporized during drying but the migrated salts are left behind. Before the energetic utilization the examination of heavy-metal content and chemical composition of plants may provide information on the behaviour of the raw materials during combustion, and may yield information about the toxicity of combustion products. Potentially toxic chemical elements are not distributed evenly in the plant parts therefore technologies firing different plant parts have different emission trends as well. According to our researches certain metals are partitioned in the soft parts (sprout, twig), while others in the bark or heartwood. Samples (poplar, oak) contaminated with heavy metals and collected from an abandoned mining area were analyzed and the results are presented in this study. After sampling the chemical composition analysis of the sprout, twig, bough, heartwood and bark parts has been carried out. The results of the examinations show that the heartwood can be burned in traditional furnaces without the risk of forming hazardous residues. The ash of foliage (sprout, twig) generated during combustion may contain dangerous chemical elements (primarily heavy-metals) in higher quantities, therefore it is necessary to handle the combustion remains separately. Keywords: ligneous plants, chemical composition, ICP spectrometry Introduction The average concentrations of main elemental components in ligneous biomass are: carbon (C) 50%, oxygen (O) 43% and hydrogen (H) 6%. These quantities do not differ significantly between different species. Other nonmetallic and metallic chemical elements form the remaining 1%. The most important is nitrogen, which occurs in trees in approximately 0,2% and is an important nutrient. Other chemical elements, for example F, S, Cl, Si etc. are also important in low amounts for the processes of metabolism. In the natural state of wood the metallic elements (trace elements) such as K, Na, Ca, Mg, Fe, 41

2 Cu, Mn etc. occur only in traces in the plants. The metabolic processes deviate from the ideal behaviour both in the case of too low or too high concentrations of these elements. The distribution of certain elements heavy metals and halogens being the most important - inside the plant is also critical from the aspect of energetic utilization either from an environmental or operational point of view. Based on operating experience, the most unwanted components are besides the moisture content of the fuel inorganic elements that can cause slagging or fouling problems. Biomass fuels contain high amounts of these components, for example K, Na, Si, Al, Ca, etc. Higher concentrations of these elements usually lead to strongly adhering, dense deposits formed on heat exchanger surfaces. The amount of low-volatile, or non-volatile trace elements (Ba, Co, Cr, Cu, Mo, Mn, V) is an important factor in ash utilization [1]. The aim of our examinations is to analyze the distributions of certain chemical elements, alkaline metals and alkaline earth metals in the plants. Based on the results partitioning of these elements between plant parts can be further studied. The analyzed elements were Pb, Cd, Cr, Zn, Tl, Ba, Ca, K, Mg, Na, Al, Co, Cu, Fe, Ni, As, Sn, Mn and Cl. 1. Chemical composition of ligneous plants The wet chemical composition of biomass strongly depends on the moisture and ash content and the type of biomass. On the other hand, the dry compositions of different plant types are not significantly different [2]. The main components which can be found in biomass, from the most concentrated to the least concentrated are C, O, H, N, Ca, K, Si, Mg, Al, S, Fe, P, Cl, Na, Mn and Ti. According to past studies, the zinc content of the polluted soil that cannot be absorbed in the root accumulates in the bark and leaves [3,4]. The results of numerous studies show that the pollutants picked up during accumulation partition in the actively growing plant parts like sprouts and young leafs. Studies regarding poplar species show that Zn and Cd concentrations are the highest in the foliage of these trees [5]. In case of willow species grown on soil polluted with treated sludge, the largest Cu, Pb and Cr concentrations were observed in the heartwood, while Zn, Cd and Ni concentrations were the highest in the foliage [6]. The distribution of metals is not uniform even in specific parts like heartwood [7, 8]. The bulk composition of the heartwood is often not representative for the whole plant as toxic elements tend to be concentrated in the soft parts [9]. 42

3 2. The composition of and and poplar samples have been collected from an abandoned mining area polluted by heavy metals near Gyöngyösoroszi, Hungary. A contaminated area has been chosen as the subject of this study in order to have more distinct concentration values to describe partitioning. The compositions of the plant parts have been determined by chemical analysis. Figure 1 shows the sampled plant parts. 1 Sprout 2 Twig 3 Bough 4 Trunk 5 - Bark Figure 1 Plant samples 2.1 Methodology for determination of measured elements Except for chlorine, all the elements were determined by ICP-AES, using a 720 ES instrument manufactured by Varian Inc., which is an axially viewed simultaneous multielement ICP spectrometer. Calibration was done by matrix matched calibration solutions, using CertiPUR IV multielement ICP CRM solution of MERCK Ltd. In order to dissolve the samples, they were heated in a closed PTFE bomb in concentrated nitric acid at 130 o C for 120 minutes. After cooling the bombs to room temperature, the dissolved samples were transferred into a volumetric flask and were filled up to the final volume of 50 cm 3. Chlorine was determined by direct potentiometry using a chlorine selective electrode. The calibration was performed by the multiple standard addition method. In the course of the measurements of chlorine, the same sample solutions were used as for ICP analysis. 2.2 Results of examination The measurement results indicated in Table 1 show the distribution of chemical elements in plant parts. Separating sprouts from twigs is difficult, therefore they are also discussed as one group further in this study. 43

4 Table 1 The distribution of elements in plants (percentage) Al Mg Cu Pb Cd Co Cr Mn Ni Zn % Sprout Twig Bough Tree-trunk Bark

5 TL Fe Ba Ca K Na Cl % Sprout Twig Bough Tree-trunk Bark Figures 2-7 show the distributions in mg/kg units. To improve visibility, secondary axes are used. The secondary axes and their corresponding datapoints are plotted y are plotted in red. Grouping the chemical elements into diagrams has been done based on the orders of magnitude of their concentrations. 45

6 Figure 2 The comparison of Mn, Zn, K, Mg, Cl and Ca contents of poplar plant parts Figure 3 The comparison of Mn, Zn, K, Mg, Cl and Ca contents of oak plant parts 46

7 Figure 4 The comparison of Cr, Al, Cu, Ba, Na, Fe contents of poplar plant parts Figure 5 The comparison of Cr, Al, Cu, Ba, Na, Fe contents of oak plant parts 47

8 Figure 6 The comparison of Cd, Co, TL, Pb, Ni contents of poplar plant parts Figure 7 The comparison of Cd, Co, TL, Pb, Ni contents of oak plant parts 48

9 3. Conclusions Based on the results it can be stated that the elements critical from the point of combustor operation pile up in the sprout, twig and bark. For the examined samples, 71.6 to 84 % of their Ba, Ca, K, Na and Mg content was found in young plant parts (sprout, twig, bark). The chlorine concentration of examined plants is also the highest in the twig and shoot, which altogether represent 64.8 % of the plants whole chlorine contents. The same can be stated in the case of Pb, Cr, Mn and Zn. Young plant parts contain % of the Cr and Pb and 79,6-85 % of the Zn and Mn content. It can also be stated that the heartwood parts were the least enriched in elements Cr, Mn, Ba and Fe in the case of poplar and elements Mn, Zn, Ca and Mg elements in the case of oak (<6%). Aluminum has been found to be distributed evenly in the soft parts of oak, however it was heavily partitioned poplar sprouts and twigs. The average concentration of Cu, Cd, Co, Ni, Tl in sprouts and twigs have been found to be 58.4 %, 43.6 %, 41.4 %, 50.5 % and 42.4 % of the total amounts in the plants respectively. Concerning Fe content the examined tree species show significant differences. In the case of poplar 77.5% of the total Fe content is present in sprouts and twigs, while in the case of oak this value is only 20.9%. Further examinations are necessary to explain this phenomenon. The results of examinations also prove that logs of these trees can be burned in traditional boilers without the risk of formation of hazardous ash. However, the ash of foliage (sprout, twig) generated during combustion may contain dangerous chemical elements (primarily heavy-metals) in a higher quantity, therefore it is necessary to handle the combustion remains separated and to take note of the deposition regulations. The ash content of ligneous plants is usually below 1%, however the examined tree species may contain % of ash mostly due to absorbed pollutants. The amount of ash formed during combustion is not significant, but the harmful elements found in the soft parts may become significantly concentrated in the ash. In conclusion it can be stated that from the point of energetic utilization the firing of tree-trunks carries the lowest risk of equipment damage. Firing logs would also lead to the generation of less harmful combustion products. Acknowledgements The described work was carried out as part of the TÁMOP B-10/2/KONV project in the framework of the New Hungarian Development Plan. The realization of this project is supported by the European Union, co-financed by the European Social Fund. 49

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