Environ Sci Pollut Res DOI 10.1007/s11356-014-3890-8
RESEARCH ARTICLE
Fuel nitrogen conversion and release of nitrogen oxides during coal gangue calcination Yingyi Zhang & Xinlei Ge & Lili Liu & Xidong Wang & Zuotai Zhang
Received: 10 July 2014 / Accepted: 19 November 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract The pollution emission during the widespread utilization of coal gangue in construction industry has long been neglected. In present study, the NOx release behaviors in a simulation experiment of coal gangue calcination in construction industry were systematically investigated. The corresponding evolution of nitrogen functionalities in coal gangue was also discussed. Results showed that pyrrolic (N-5) and pyridine N-oxide (N-6-O) forms nitrogen were relatively abundant in the raw gangue. During calcination, the N-5 and N-6-O form nitrogen greatly decreased and converted to quaternary nitrogen (N-Q). It was found that NO2 was formed under slowly heating-up condition and at 600 °C under isothermal condition, while only NO was detected with further increase of temperature. From 600 to 1000 °C, the conversion ratio of fuel nitrogen to NOx increased from 8 to 12 %. The char nitrogen was found greatly contribute to NO formation, which may bring difficulty to the abatement of NOx emission during coal gangue calcination. Keywords Coal gangue . Calcination . Nitrogen oxides . Nitrogen functionality . XPS . Emission Responsible editor: Angeles Blanco Electronic supplementary material The online version of this article (doi:10.1007/s11356-014-3890-8) contains supplementary material, which is available to authorized users. Y. Zhang : L. Liu : X. Wang : Z. Zhang (*) Beijing Key Laboratory for Solid Waste Utilization and Management and Department of Energy and Resource Engineering, College of Engineering, Peking University, Beijing 100871, People’s Republic of China e-mail: [email protected] X. Ge Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, People’s Republic of China
Introduction Utilization of waste material has emerged as a worldwide trend, for its beneficial effects on not only reducing environment hazards caused by waste disposal, but also offering an alternative resource of progressively dwindling fossil fuels and mineral. However, it is necessary to be cautious as the use of waste materials may introduce new challenges in environmental protection. Coal gangue is a problematic by-product of coal mining and beneficiation, which is a mixture of rock debris from the roof, dirt bands, and floor of coal seam (Frias et al. 2012; Querol et al. 2008; Zhou et al. 2012, 2014a, b, c). The global increase of coal production over the past decades results in considerable amounts of coal gangue dumped in every coalproducing country, causing environmental problems (Ardejani et al. 2010; Nichol and Tovey 1998; Zhao et al. 2008; Zhou et al. 2014a, b, c). On the other hand, since coal gangue contains a high content of mineral species, especially clay minerals (Yao and Sun 2012), it can be used as raw material for the synthesis of the construction materials such as brick, cement, and concrete (Frias et al. 2012; Zhou et al. 2014c). In China, the stockpiles of coal gangue reached 4.5× 1010 Mg (Zhou et al. 2014b). Under the encouragement of subsidies from the government, the utilization of coal gangue in construction material manufacture was up to 5× 107 Mg in 2011 (The China Resources Utilization Annual Report 2012). For other countries, the coal gangue dumps in Poland reached 5× 108 Mg till 2007 (Misz-Kennan and Fabianska 2010). In India, almost 6×107 Mg of coal gangue is produced per year, while the amount is around 9×107 Mg in Russia and 3.5×106 Mg in the Santa Catarina State of Brazil. The development degrees of coal gangue utilization in different countries are varied. For example, the utilization of coal gangue in India is majorly in construction industry, whereas in
Environ Sci Pollut Res
Russia, only 1 % of coal gangue is used in construction industry (Lemeshev et al. 2004; Pappu et al. 2007). Despite the difference of development degrees at current stage, the application of coal gangue in construction industry is expected to receive fast development (Lemeshev et al. 2004). Alongside the rapid development of coal gangue utilization, secondary environmental pollutions might be another concern. During utilization in building and construction industry, coal gangue is often subjected to calcination to exclude carbon and structural water in clay minerals (Zhou et al. 2012, 2014c). Emission of NOx during calcination of coal gangue is one of the severe challenges to be dealt with. Recently published studies show increasing awareness of the secondary environmental pollutions during high temperature processing of coal gangue, majorly focusing on trace elements release (Querol et al. 2008; Zhang and Ouyang 2014; Zhou et al. 2012, 2014a, b, c). However, little attention has been given to the NOx emission during coal gangue calcination. To comply with increasingly stringent limits of NOx emissions and requirement of environmental protection, an in-depth understanding of the NOx emission from coal gangue calcination is strongly desirable to prevent secondary pollutions to the local environment as well as protect the development of coal gangue utilization. Coal gangue is generally calcined at 600–1000 °C for construction material synthesis (Saxena and Jotshi 1994; Zhou et al. 2014c). At such temperatures, the thermal-NOx (i.e., formation by fixation of molecular nitrogen in combustion air) is negligible (Saxena and Jotshi 1994) and the fuel NOx (i.e., oxidation of chemically bound nitrogen in coal gangue) is a major contributor to NOx emission. A vast amount of studies have been conducted on fuel nitrogen conversion of coal and biomass, municipal waste as well as their mixtures during combustion (Bai et al. 2013; Courtemanche and Levendis 1998; Kazanc et al. 2011; Shao et al. 2013; Wang et al. 2012). However, to the best knowledge of the present authors, no studies focused on coal gangue. Previous investigations find that the conversion of fuel nitrogen is associated with both the nitrogen functionalities in fuel and the process conditions (Glarborg et al. 2003). As coal gangue contains more ash and less carbon, the chemical bondings of nitrogen and their distribution may be different from those in coal, which may significantly affect the fuel nitrogen conversion behavior and corresponding NOx release characteristics during calcination. In this regard, the present study aims to elucidate NOx release behaviors of coal gangue during calcination as well as the corresponding nitrogen functionalities evolution. The simulated calcination experiments were conducted at both isothermal and non-isothermal conditions. The effect of calcination temperature on the release behaviors of NO and NO2 was investigated. The nitrogen functionalities in coal gangue
before and after calcination were studied by using the X-ray photoelectron spectroscopy (XPS).
Materials and methods Materials Two coal gangue samples, denoted as CG1 and CG2, were collected separately from centralized coal waste dumps near Ningwu and Xishan coal mines in Shanxi Province, China. The samples were milled and sieved to a grain size of 150 μm before further analysis and experiments. Proximate and ultimate analyses were performed by the LECO TGA-701 and Elementar vario Macro CHNS analyzers, respectively. The major chemical compositions were determined by the Bruker S4 Explorer. The properties of coal gangue samples were presented in Table 1. The coal gangue samples were characterized by low carbon content and high ash content. The nitrogen content in CG1 and CG2 is 0.60 and 0.55 wt%, respectively. Calcination experiments The calcination experiments of coal gangue were carried out in a vertical fixed-bed reactor system (Fig. 1). The reactor is 100 cm long with an internal diameter of 7 cm. The NO and NO2 concentration in outlet gas was measured continuously by a flue gas analyzer (Testo pro350, Testo) at an interval of 2 s. The calcination experiments were performed under both temperature-programmed conditions and isothermal conditions. In the temperature-programmed calcination experiments, the samples were placed into the furnace at ambient temperature and heated up to 1200 °C at 5 °C/min. In the isothermal calcination tests, the furnace was first heated up and maintained at a specific temperature (600, 700, 800, 900, and 1000 °C). Then the samples were quickly fed into the furnace. For each run under both conditions, 500 mg sample was used and air was employed as the carrier gas with a constant flow rate of 1 L/min. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) was used to identify the nitrogen functionalities in coal gangue samples. The XPS spectra were obtained by an Imaging Photoelectron Spectrometer (AXIS-Ultra, Kratos Analytical), with monochromatic Al Kα radiation (225 W, 15 mA, 15 kV) and lowenergy electron flooding for charge compensation. Binding energies were calibrated using the C 1s hydrocarbon peak at 284.8 eV. The accuracies of binding energy values determined by the XPS were within ±0.1 eV.
Environ Sci Pollut Res Table 1 Chemical compositions of coal gangue
CG1 CG2
Proximate analysis (wt%) Moisture, ad Ash, d 1.27 64.96 0.87 74.01 Major composition (wt%) SiO2 Al2O3
CG1 CG2
31.8 41.2
28.1 26.3
Volatile matter, d 16.36 11.12
Fixed carbon, d 18.68 14.88
Ultimate analysis (wt%) C, ad H, ad 20.37 2.14 15.78 1.41
N, ad 0.60 0.55
Fe2O3
CaO
MgO
K2O
Na2O
TiO2
1.35 1.28
0.13 2.40
RESEARCH ARTICLE
Fuel nitrogen conversion and release of nitrogen oxides during coal gangue calcination Yingyi Zhang & Xinlei Ge & Lili Liu & Xidong Wang & Zuotai Zhang
Received: 10 July 2014 / Accepted: 19 November 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract The pollution emission during the widespread utilization of coal gangue in construction industry has long been neglected. In present study, the NOx release behaviors in a simulation experiment of coal gangue calcination in construction industry were systematically investigated. The corresponding evolution of nitrogen functionalities in coal gangue was also discussed. Results showed that pyrrolic (N-5) and pyridine N-oxide (N-6-O) forms nitrogen were relatively abundant in the raw gangue. During calcination, the N-5 and N-6-O form nitrogen greatly decreased and converted to quaternary nitrogen (N-Q). It was found that NO2 was formed under slowly heating-up condition and at 600 °C under isothermal condition, while only NO was detected with further increase of temperature. From 600 to 1000 °C, the conversion ratio of fuel nitrogen to NOx increased from 8 to 12 %. The char nitrogen was found greatly contribute to NO formation, which may bring difficulty to the abatement of NOx emission during coal gangue calcination. Keywords Coal gangue . Calcination . Nitrogen oxides . Nitrogen functionality . XPS . Emission Responsible editor: Angeles Blanco Electronic supplementary material The online version of this article (doi:10.1007/s11356-014-3890-8) contains supplementary material, which is available to authorized users. Y. Zhang : L. Liu : X. Wang : Z. Zhang (*) Beijing Key Laboratory for Solid Waste Utilization and Management and Department of Energy and Resource Engineering, College of Engineering, Peking University, Beijing 100871, People’s Republic of China e-mail: [email protected] X. Ge Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, People’s Republic of China
Introduction Utilization of waste material has emerged as a worldwide trend, for its beneficial effects on not only reducing environment hazards caused by waste disposal, but also offering an alternative resource of progressively dwindling fossil fuels and mineral. However, it is necessary to be cautious as the use of waste materials may introduce new challenges in environmental protection. Coal gangue is a problematic by-product of coal mining and beneficiation, which is a mixture of rock debris from the roof, dirt bands, and floor of coal seam (Frias et al. 2012; Querol et al. 2008; Zhou et al. 2012, 2014a, b, c). The global increase of coal production over the past decades results in considerable amounts of coal gangue dumped in every coalproducing country, causing environmental problems (Ardejani et al. 2010; Nichol and Tovey 1998; Zhao et al. 2008; Zhou et al. 2014a, b, c). On the other hand, since coal gangue contains a high content of mineral species, especially clay minerals (Yao and Sun 2012), it can be used as raw material for the synthesis of the construction materials such as brick, cement, and concrete (Frias et al. 2012; Zhou et al. 2014c). In China, the stockpiles of coal gangue reached 4.5× 1010 Mg (Zhou et al. 2014b). Under the encouragement of subsidies from the government, the utilization of coal gangue in construction material manufacture was up to 5× 107 Mg in 2011 (The China Resources Utilization Annual Report 2012). For other countries, the coal gangue dumps in Poland reached 5× 108 Mg till 2007 (Misz-Kennan and Fabianska 2010). In India, almost 6×107 Mg of coal gangue is produced per year, while the amount is around 9×107 Mg in Russia and 3.5×106 Mg in the Santa Catarina State of Brazil. The development degrees of coal gangue utilization in different countries are varied. For example, the utilization of coal gangue in India is majorly in construction industry, whereas in
Environ Sci Pollut Res
Russia, only 1 % of coal gangue is used in construction industry (Lemeshev et al. 2004; Pappu et al. 2007). Despite the difference of development degrees at current stage, the application of coal gangue in construction industry is expected to receive fast development (Lemeshev et al. 2004). Alongside the rapid development of coal gangue utilization, secondary environmental pollutions might be another concern. During utilization in building and construction industry, coal gangue is often subjected to calcination to exclude carbon and structural water in clay minerals (Zhou et al. 2012, 2014c). Emission of NOx during calcination of coal gangue is one of the severe challenges to be dealt with. Recently published studies show increasing awareness of the secondary environmental pollutions during high temperature processing of coal gangue, majorly focusing on trace elements release (Querol et al. 2008; Zhang and Ouyang 2014; Zhou et al. 2012, 2014a, b, c). However, little attention has been given to the NOx emission during coal gangue calcination. To comply with increasingly stringent limits of NOx emissions and requirement of environmental protection, an in-depth understanding of the NOx emission from coal gangue calcination is strongly desirable to prevent secondary pollutions to the local environment as well as protect the development of coal gangue utilization. Coal gangue is generally calcined at 600–1000 °C for construction material synthesis (Saxena and Jotshi 1994; Zhou et al. 2014c). At such temperatures, the thermal-NOx (i.e., formation by fixation of molecular nitrogen in combustion air) is negligible (Saxena and Jotshi 1994) and the fuel NOx (i.e., oxidation of chemically bound nitrogen in coal gangue) is a major contributor to NOx emission. A vast amount of studies have been conducted on fuel nitrogen conversion of coal and biomass, municipal waste as well as their mixtures during combustion (Bai et al. 2013; Courtemanche and Levendis 1998; Kazanc et al. 2011; Shao et al. 2013; Wang et al. 2012). However, to the best knowledge of the present authors, no studies focused on coal gangue. Previous investigations find that the conversion of fuel nitrogen is associated with both the nitrogen functionalities in fuel and the process conditions (Glarborg et al. 2003). As coal gangue contains more ash and less carbon, the chemical bondings of nitrogen and their distribution may be different from those in coal, which may significantly affect the fuel nitrogen conversion behavior and corresponding NOx release characteristics during calcination. In this regard, the present study aims to elucidate NOx release behaviors of coal gangue during calcination as well as the corresponding nitrogen functionalities evolution. The simulated calcination experiments were conducted at both isothermal and non-isothermal conditions. The effect of calcination temperature on the release behaviors of NO and NO2 was investigated. The nitrogen functionalities in coal gangue
before and after calcination were studied by using the X-ray photoelectron spectroscopy (XPS).
Materials and methods Materials Two coal gangue samples, denoted as CG1 and CG2, were collected separately from centralized coal waste dumps near Ningwu and Xishan coal mines in Shanxi Province, China. The samples were milled and sieved to a grain size of 150 μm before further analysis and experiments. Proximate and ultimate analyses were performed by the LECO TGA-701 and Elementar vario Macro CHNS analyzers, respectively. The major chemical compositions were determined by the Bruker S4 Explorer. The properties of coal gangue samples were presented in Table 1. The coal gangue samples were characterized by low carbon content and high ash content. The nitrogen content in CG1 and CG2 is 0.60 and 0.55 wt%, respectively. Calcination experiments The calcination experiments of coal gangue were carried out in a vertical fixed-bed reactor system (Fig. 1). The reactor is 100 cm long with an internal diameter of 7 cm. The NO and NO2 concentration in outlet gas was measured continuously by a flue gas analyzer (Testo pro350, Testo) at an interval of 2 s. The calcination experiments were performed under both temperature-programmed conditions and isothermal conditions. In the temperature-programmed calcination experiments, the samples were placed into the furnace at ambient temperature and heated up to 1200 °C at 5 °C/min. In the isothermal calcination tests, the furnace was first heated up and maintained at a specific temperature (600, 700, 800, 900, and 1000 °C). Then the samples were quickly fed into the furnace. For each run under both conditions, 500 mg sample was used and air was employed as the carrier gas with a constant flow rate of 1 L/min. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) was used to identify the nitrogen functionalities in coal gangue samples. The XPS spectra were obtained by an Imaging Photoelectron Spectrometer (AXIS-Ultra, Kratos Analytical), with monochromatic Al Kα radiation (225 W, 15 mA, 15 kV) and lowenergy electron flooding for charge compensation. Binding energies were calibrated using the C 1s hydrocarbon peak at 284.8 eV. The accuracies of binding energy values determined by the XPS were within ±0.1 eV.
Environ Sci Pollut Res Table 1 Chemical compositions of coal gangue
CG1 CG2
Proximate analysis (wt%) Moisture, ad Ash, d 1.27 64.96 0.87 74.01 Major composition (wt%) SiO2 Al2O3
CG1 CG2
31.8 41.2
28.1 26.3
Volatile matter, d 16.36 11.12
Fixed carbon, d 18.68 14.88
Ultimate analysis (wt%) C, ad H, ad 20.37 2.14 15.78 1.41
N, ad 0.60 0.55
Fe2O3
CaO
MgO
K2O
Na2O
TiO2
1.35 1.28
0.13 2.40
