J. Anal. Appl. Pyrolysis 80 (2007) 283–288 www.elsevier.com/locate/jaap
Release and enrichment of 44 elements during coal pyrolysis of Yima coal, China Chen Yiwei a, Liu Guijian a,b,*, Gong Yanming b, Yang Jianli c, Qi Cuicui a, Gao Lianfei a a
CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China b The Energy Institute, The Pennsylvania State University, University Park, PA 16802, USA c State Key Laboratory of Coal Conversation, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan 030001, Shanxi, China Received 14 August 2006; accepted 15 March 2007 Available online 20 March 2007
Abstract The coal samples were collected from Yima coal district, China. The pyrolysis experiments were carried out in a simulated bed quartz reactor with a heating rate of 20 8C/min. The 44 elements in raw coal and chars were determined by inductively coupled-plasma mass spectrometry instrument (ICP-MS). The release and enrichment behavior of 44 trace elements during coal pyrolysis of Yima coal was studied. According to the transformation behaviors, chemical features and thermal features under different pyrolysis temperatures, the 44 elements can be categorized to 4 groups: light elements (Li and Be), nonmetal elements (Se, As, B, etc.), heavy metal elements (including 24 elements, Cu, V, Co, etc.) and rear earth elements (REE) (14 elements). The results showed that (1) the higher pyrolysis temperatures, the higher release ratio and release ratio of REE are very low; (2) the enrichment ratios of the elements in chars increase by the sequence of nonmetal elements < light elements < heavy metal elements < REE. The nonmetal elements, light elements and a few heavy metal elements will be emitted out from coal during coal pyrolysis and they will pollute environment. # 2007 Elsevier B.V. All rights reserved. Keywords: Coal; Transformation behavior; Elements
1. Introduction Trace elements (TE) are defined as an element concentration in coal lower than 0.1%. During the process of coal utilization, many trace elements, including B, Cr, Mn and Th, some hazardous elements, may release into atmosphere and result in severe pollution to environment. These elements may become easily accessible not only during coal mining, but also during the storage, transport, cleaning, combustion and other coal preparation processes, by which make the coal consistent in quality and suitable for selling [1,2]. More and more researches have been focused on the trace elements for the wide spread
* Corresponding author at: CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China. Tel.: +86 551 3603714; fax: +86 551 3621485. E-mail address: [email protected]
(L. Guijian). 0165-2370/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2007.03.004
presence and the great concern for their toxicological and environmental effects on the ecosystem and human health. In some area, severe endemic have happened for improper utilization of coal that had extremely high content of some hazardous trace elements [3,4]. It is important, therefore, to understand the toxicity and transformation mechanisms of the trace elements released during coal utilization processes, especially during coal combustion and pyrolysis. As a main process of coal utilization, which differs from coal combustion, pyrolysis is usually operated in close system at high temperatures under inert, reductive surrounding . Trace elements in coals can disperse into the environment through several pathways, such as, leaching into soil and adversely affecting plant nutrition and by creating runoff into waterways. Further more, some of them, particularly those easily to be volatilized during combustion such as As, F and Se, release into the atmosphere either as vapors or respirable fine particulates [3,6]. These toxic elements can easily enter into the
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human body either through the respiratory tract or by ingestion, and bioaccumulation in tissues. When the amount of trace elements released to air is higher than the allowed maximum limit from Environmental Protection Agency (EPA), they will adversely affect normal metabolism and destroy some physiological functions, which produce pathological phenomenon for human health. These health problems are severe (leading to death) and are widely spread around the world [3,4]. Emission of trace elements from coal combustion takes place both in gaseous phase and in solid phase . Their presence in dust particles is a result of their origin from minerals in coal as well as of condensation and absorption from gaseous phase during the waste gas transport from the source to the emitter. Only a few research studies have been reported
about the behavior of trace elements found in coal during coal pyrolysis so far. Guo et al. (our cooperation groups) [7–11] and others results [12,13] had studied the dynamics and mobility of trace elements released during coal pyrolysis in the past. However, only a few trace elements were studied. In this paper, we studied the 44 elements, and the goal of this study is focused on the investigation of transformation behavior of 44 trace elements during coal pyrolysis. 2. Sampling Yima Coalfield, an important industrial coal-production area, is located in the Southeast of Henan Province. The coal samples were collected from one underground mine of the
Fig. 1. Release behaviors of trace elements for Yima coal: (a) shows the release ratios of different trace elements, circled are the representative elements chosen for discussion (B, Se, Cu, Pb and Th); (b) shows the idiographic Lr of elements circled in (a).
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Yima Coalfield. The samples were taken by cutting channels downwards so that the samples can be representative characterization within a coal seam studied. The samples were stored immediately in sealed plastic bags to prevent contamination and weathering. 3. Experimental The pyrolysis processes were carried out in State Key Laboratory of Coal Conversion, Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences. The experiment were carried out in a pyrolysis reactor. It consists of mainly electrically heated tube furnace and reactor (which was made of a stainless steal reactor with a quartz liner in an electric furnace). The ends of reactor were closed with stoppers, having provisions for an inert gas sweep inlet tube, an exit gas tube and a thermocouple. The purpose of the inert gas was simply to sweep the vapor, through the bed of coal, and prevent the vapor from diffusing in the wrong direction away from the coal. The procedure also allowed the vapors produced from coal to be swept out of reactor and into two gas washing bottles. Prior to a reaction, about 2.5 g coal sample was placed in the reactor. The reactor was sealed and filled with a desired gas to a certain pressure. Then the furnace with the reactor was heated up. When the reactor reached to a desired temperature, the fast pyrolysis was started. After a set heating time (7 min), the reactor was pulled out from the furnace and quenched in cold water. This simulates the cooling process in a drop-tube reactor, where coal particles leaving the hot zone fall down into a cold environment. The tars condensed on the wall of the quartz tube liner were carefully remove by THF soaked cotton balls, and the chars were collected from the bottom of the quartz tube liner after drying off THF. The collected chars were weighted. In these experiments, three fixed temperatures (500, 700 and 900 8C) were used for pyrolysis under an initial N2 pressure of 0.1 MPa, and N2 inlet speed is 120 ml/min. The heating rate was set at 20 8C/s. When the temperature reached the desired temperature, it was kept for 7 min. 4. Analytical methods and results The bulk samples were air-dried, milled and split until a typical split of 0.5 kg was obtained. The samples were pulverized to less than 200-mesh and dried for 12 h in a desiccator for mineralogical, proximate, ultimate and chemical analyses. The proximate and ultimate analyses were performed following ASTM (1992) standard procedures . The proximate and ultimate analyses of the samples were performed at Laboratory for Coal Chemical Analysis at the Anhui University of Science and Technology, Huainan, Anhui Province. Trace elements were determined by the inductively coupled-plasma mass spectrometry instrument (ICP-MS, PE Elan 6000, America) and ICP-OES for As, Se. In this work, taking into account their toxic effect and the chemical features, 44 trace elements were selected. The data are listed in the Table 1.
Table 1 The contents of 44 elements in raw coal and chars under different temperatures (ppm)
Li Be B P Sc Ti V Cr Mn Co Ni Cu Ga Ge As Se Rb Sr Y Zr Nb Mo Pd Sb Cs Ba La Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ta Tl Pb Th U
20.2 1.41 90.7 181 28.2 711 144 91.5 171 23.8 7.9 43.5 57.1 0.91 68.9 1.75 107 249 42.2 193 19.6 1.48 3.66 2.89 10.9 429 1.41 9.6 49.1 8.3 2.03 8.5 1.26 8.13 1.65 4.94 0.69 4.43 0.68 1.27 5.00 50.4 15.5 7.74
21.7 1.69 96.2 186 25.5 725 164 104 183 25.7 8.72 54.6 63.2 1.14 52.5 1.49 118 295 47.4 228 23.5 1.82 3.91 3.12 12.3 507 1.69 11.1 50.4 10.4 2.49 10.3 1.53 9.98 2.04 6.15 0.86 5.89 0.90 1.72 6.37 66.8 21.7 10.6
23.0 1.60 99.5 206 31.0 777 176 115 191 29.2 9.66 64.0 68.9 1.25 88.7 1.01 131 336 56.9 238 26.0 1.94 3.90 3.54 13.8 562 1.60 13.4 60.1 12.2 2.88 12.1 1.83 12.1 2.51 7.37 1.04 7.0 1.09 2.05 7.06 76.8 27.7 11.9
25.0 1.67 101.0 202 32.8 809 180 118 203 30.8 9.96 60.9 74.0 1.32 99.8 1.54 142 358 61.0 249 27.2 2.07 4.08 3.14 14.7 585 1.67 10.8 64.3 12.9 3.13 13.1 2.01 13.1 2.60 8.02 1.08 7.76 1.20 2.13 7.10 44.9 29.3 12.7
5. Results and discussion In order to quantitatively evaluate the release and enrichment ability of the elements upon heat treatment, in this paper, Lr and Er are defined as concentration in char Lr ¼ 1 char yield 100% concentration in coal Er ¼
concentration in char concentration in coal
Apparently, Lr ¼ ð1 char yield ErÞ 100%:
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5.1. Release behavior Fig. 1a shows the release ratios of trace elements at various pyrolysis temperatures for Yima coal. The X-axis represents the elementary atomic sequence of the 44 elements and the atomic numbers increase from left to right. The Y-axis represents the release ratio. Different pyrolysis temperatures were represented by different signs. According to Fig. 1, the following can be concluded: (1) with pyrolysis temperatures increasing, the released ratio (Lr) increases, Lr(900 8C) > Lr(700 8C) > Lr(500 8C), showing a higher pyrolysis temperature leads to a more thorough coal decomposing, and trace elements existing in coal may release easier into the atmosphere. (2) The release ratios are different due to the different chemical features, thermal stabilities and occurrence of trace elements in coal. For examples, five (circled in Fig. 1a) elements were chosen for further discussion. (3) For REE, the release ratio (Lr) of trace elements is at a extraordinary low level (around 20%) because the chemical and thermal stabilities are very immovability and the elements are hard to release out during pyrolysis, and these properties finally lead to extraordinary low level of the release ratio (Lr). (4) The element Pb, the release ratio (Lr) greatly increased during the temperature range of 700–900 8C, showing that Pb
released rapidly during this period, further discussion of Pb will be given in the following related section. Fig. 1b shows the release ratios of B, Se, Cu, Pb and Th at various pyrolysis temperatures. At 900 8C, the release ratios of B, Se, Cu, Pb and Th are 43.76%, 55.56%, 29.3%, 55.01% and 4.54%, respectively. Most of the release behavior of B and Se evolved at relative low temperatures (<700 8C). And release ratio (Lr) of these elements was slightly affected by further increase of temperature. release ratio (Lr) of Pb was very low until about 700 8C. Once the heating temperature increases to over 700 8C, its release ratio increases instantly with the temperature increasing which shows that the main releasing process of Pb is more than the temperature of 700 8C. Cu and Th showed a relatively lower and more immovability release behavior during this temperature range. It means an immovability occurrence and chemical forms in coal. The volatile of trace elements in coal is impacted by many factors. Studies on the mobility of trace elements during coal combustion have shown that their volatility depends on their affinities and concentrations in coals, on the physical changes and chemical reactions of these elements with mineral, on the combustions (e.g., temperature, excessive air coefficient), boiler output and so on [6,2,15–20]. In this study, the factors are mainly temperatures and occurrences of trace elements
Fig. 2. Enriching behaviors of trace elements for Yima coal: (a) shows the enriching ratios of different trace elements, circled are the typical elements groups, from left to right: light elements, heavy metal elements and REE. Nonmetal elements are not circled in (a) for their spread distributions; (b) shows the Er of the chosen elements from each group circled above.
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during coal pyrolysis. However, the contents of trace elements in coal are no significant important. For example, the content of Se in coal is the lowest. However, its volatile ratio is highest. According to the Fig. 1a, we can see clearly that the emission ratios of the elements normally increase gradually with temperature increase. Liu et al. [21,22] and Finkelman [23,24] had reported about the occurrences of trace elements in coal samples. They concluded that the occurrences of trace elements in coal control the emission of trace elements during coal combustion [25,26]. The occurrences of trace elements in coal are important factors used in anticipating the behavior of the trace elements during coal cleaning, combustion, as well as during weathering and leaching of the coal and coal waste products . The elements in coal exist in a widely variety of forms. Some of the elements are primarily associated with organic matter, other elements commonly are associated with minerals in coal. Hence, different occurrences of trace elements in coal depend on their volatilization behavior. Many studies have explained that the occurrences of trace elements in coal are very important. It has been proposed that trace elements with an affinity to organic matter and sulfides are more easily volatilized than those with affinity to silicate minerals [15,16]. In our samples, The Se and B are mostly organic association. Some studies [21,23,24,27,28] had reported that B in coal is mostly organically bound, Se is mainly associated with organic matter and pyrite, some of Se is bounded with sulfide and selenides. When coal were heated, the bond of organic matter was destroyed, the structural molecules were decomposed, B and Se were emitted from coal very easy. The elements B exhibit high vapor pressure in their decomposition coal products and are probably released in the forms of hydrogen Boride during coal combustion. Pb is one of elements which are the most associated with minerals in coal and Pb occurs in the form of galena which is decomposed fast when temperature is about 900 8C. We can explain why volatile ratio of Pb is slightly increased at <700 8C. 5.2. Enrich ratio Fig. 2a shows the enriching ratio of elements at various pyrolysis temperatures for Yima coal. The X-axis represents the atomic number while Y-axis represents the enrichment ratios. The enrichment ratio (Er) of different elements is quite different. Some elements such as Cu, Mn and REE, showed notable enriching behaviors in chars, while others, like B, Se
Fig. 3. Average Er of four groups elements at different pyrolysis temperatures.
and As, waned in chars. This may directly related to the chemical forms and thermal immovabilities of the elements in coal. By considering the elementary features and comparing the differences of release behavior, these 44 elements can be divided into four groups: light elements, nonmetal elements, heavy metal elements and REE (Table 2). It can be clearly circled in Fig. 2a except the nonmetal elements for their spread distribution. The elements and the average enrichment ratio (Er) of the four groups are listed in the Table 2. Fig. 3 shows the average enrichment ratio (Er) per group of the 44 elements. It is shown clearly in Fig. 3 that the average enrichment ratio (Er) of the four groups follows: nonmetal elements < light elements < heavy metal elements < REE. Basically, enrichment ratio (Er) is determined by the chemical and thermal features and the form of the elements in coal. For nonmetal elements, the volatilization is very high since the chemical and thermal features are very active. And it is very easy for the elements to release from coal, which leads to a relative low enrichment ratio (Er) in chars. From nonmetal elements to light elements, heavy metal elements and then to REE, the volatilization change from high to low, and the chemical and thermal immovabilities vary form low to high. Therefore, the enrichment ratio (Er) are also become higher and higher from nonmetal elements to light elements, heavy metal elements and then to REE. Four elements from each group and Pb were chosen for further discussion. Fig. 2b shows the enrichment ratios of these chosen elements: Li, Se, Cu, Pb and Th. The enrichment ratio (Er) of Se is very low, basically no more than 1 due to high volatilization of Se during coal pyrolysis. The enrichment ratio (Er) of Pb and Li are quite immovable. Especially for Pb, its enrichment ratio (Er) varies 1 to 1.1 during the temperature
Table 2 The 44 elements and their average Er of the four groups
500 8C 700 8C 900 8C
Heavy metal elements
B, P, As and Se
Li and Be
0.925 1.025 1.140
1.136 1.137 1.211
V, Cr, Mn, Co, Ni, Cu, Ga, Ge, Rb, Sr, Zr, Nb, Mo, Pd, Sb, Cs, Ba, Ta, Tl, Pb, U, Th, Sc, Ti 1.173 1.301 1.351
Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er Tm, Yb, Lu 1.195 1.413 1.491
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(Lr) is at a extraordinary low level. (4) For element Pb, the release ratio (Lr) greatly increased at the temperature from 700 to 900 8C, showing that Pb released rapidly during this period. According to their enrichment ratios of 44 elements, the 44 elements can be divided into four groups: light elements, nonmetal elements, heavy metal elements and REE. And Er of the elements basically concerned to their immovabilities, following the sequence below: nonmetal elements < light elements < heavy metal elements < REE. Enriching ratios (Er) shows the concentrations of the elements in chars. Acknowledgements
Fig. 4. Responses of Er to temperature increasing, the two lines represent the increaing of Er to two temperature ascending range: 500–700 and 700–900 8C.
range of 500–900 8C. Cu and Th showed sizable enriching behavior, 1.40 for Cu and 1.89 for Th at 700 8C due to their immovable chemical and thermal features. Fig. 4 shows the relationship between enrichment ratio (Er) of different elements and a same range of temperature. The X-axis represents the atomic number while Y-axis represents the enrichment ratio (DEr = Er700 8C Er500 8C or Er900 8C Er700 8C) (from 500 to700 8C and from 700 to 900 8C). Apparently, if DEr > 0, it can be explained that the enrichment in char of the element became relatively stronger, while <0 means that the element in char is lower enriched. From 500 to 700 8C, for most elements, the enriching in char became much more because the most of the DEr (the filamentary line) was above zero except Be, Se, Sb and La. The high volatility of Be and Se made them volatile rapidly in a relative low pyrolysis temperature, thus the DEr < 0. However with the temperature increasing, the volatility of Be and Se became less, then the volatility got changed and the DEr got above zero. From 700 to 900 8C, fewer elements showed a slowing down capacity of enriching behaviors in char, only Pr behaviors need further discussion based on their chemical forms and other factors. Comparing these two lines (Fig. 4), DEr(500–700 8C) > DEr(700–900 8C), it was found that the increasing value of enrichment ratio (Er) is getting lower with the temperature increasing. It is to say the enriching ratios of elements are increasing when the enriching capacities in char are also getting stronger with the temperature ascending. Thus, it can be predicted that the DEr may get near zero, the line of DEr may get near to X-axis under a higher pyrolysis temperature. 6. Conclusion According to characterization and measurement of the distributions of 44 elements in raw coal and products of pyrolysis (chars), the results can be concluded: (1) with pyrolysis temperatures increasing, the release ratio (Lr) increases, Lr(900 8C) > Lr(700 8C) > Lr(500 8C). (2) The release ratios are different during different elements duo to the different chemical features, thermal immovabilities and occurrences of elements in coal. (3) For REE, the release ratio
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