Article pubs.acs.org/EF
Influences of Hydrothermal Modification on Nitrogen Thermal Conversion of Low-Rank Coals Qian Li,† Zhihua Wang,*,† Zhenmin Lin,‡ Yong He,† Kang Zhang,† Ronald Whiddon,† and Kefa Cen† †
State Key Laboratory of Clean Energy Utilization, Zhejiang University, 310027, Hangzhou, China Jiangsu Power Design Institute Co., Ltd. of China Energy Engineering Group, 211102, Nanjing, China
‡
S Supporting Information *
ABSTRACT: The influence of hydrothermal modification on thermal conversion of coal-N in low-rank coals was investigated in this paper. Hydrothermal modification has been performed using three typical Chinese low-rank coals at final modification temperatures of 200, 250, and 300 °C. An analytical pyro-probe coupled to a pyrolysis−gas chromatography mass spectrometry (Py-GC/MS) system was used to study the release characteristics of nitrogen-containing compounds from raw and modified coal. In addition, a tube furnace apparatus was used to study the release characteristics of nitrogen pollutants during pyrolysis and combustion of Zhaotong raw and modified coal. Results showed that hydrothermal modification affected the content and type of nitrogen-containing compounds. Some triple bonds present in the coal structure are broken to form amides. The presence of azoles decreases sharply after modification, but it increases as the hydrothermal modification temperature increases. During tube furnace pyrolysis, the HCN yield decreases after modification; similarly, with the increase of modification temperature, NH3 is released in greater quantity after modification, but the release decreases as the modification temperature increases. NH3 may be converted from HCN through secondary reactions. Hydrothermal modification is conducive to NO emission reduction in coal combustion process; however, increasing the modification temperature restrains the NO emission to some extent.
1. INTRODUCTION The demand for energy increases year after year with the fast development of the global economy. High-quality energy is limited and precious; there is no doubt that low-rank coals will have increased importance for prospective energy utilization, because of the high capacity of reserves. However, because of inherent deficiencies of low-rank coals, upgrading and modification are pivotal technologies for low-rank coal utilization. There are various methods for altering low-rank coal combustion characteristics: drying with solar, steam, and microwave energy, as well as hydrothermal modification (also called hydrothermal upgrading, hydrothermal treatment), which is a nonevaporative dehydration technique. The process of hydrothermal modification influences the coal molecule structure and alters the form of functional groups in the coal, inhibiting the reabsorption of water. Hydrothermal modification research was initiated by Graff and Brandes.1 The process of hydrothermal modification proceeds in a mixture of coal and deionized water at high temperature and high pressure. The moisture in the low-rank coal sample is partitioned into the liquid phase during the upgrading process. It has been found that the moisture and oxygen content of coal is reduced while the carbon content and heating value increase after hydrothermal modification.2,3 Favas4 found that the moisture content in coal decreased 72% after modification. It was also discovered that the postmodification process water had a high sodium content, indicating that alkali metal was also removed from the coal during modification. Umar5 researched the effects of hydrothermal modification process temperature on combustion characteristics for Indonesian low-rank coal and found that the ignition temperature increases as the hydrothermal © XXXX American Chemical Society
modification temperature increases; however, the maximum combustion rate, which reflects coal reactivity, is not significantly changed after modification. Sakaguchi6 found that additional water was unnecessary: using the moisture of the coal itself was sufficient to support the modification process if the modification temperature is elevated to 350 °C. Large-scale utilization of coal is commonly associated with high levels of pollution, especially NOX emission during the thermal transformation of coal. Control of nitrogen release is thus a prominent research topic. N2, NH3, and HCN are the primary gaseous nitrogen species produced by thermal transformation, with tar-bound nitrogen, which is a significant nitrogen-containing liquid compound. Nelson7 concluded that the formation of HCN resulted from the secondary decomposition reactions of nitrogen-containing volatiles. The transformation from coal-N to nitrogen-containing species relates to the coal structure,8 researchers have used an X-ray photoelectron spectroscopy (XPS) technique to detect the structure of coal-N.9,10 Obras-Loscertales11 found that decreasing the temperature and increasing the coal moisture content and the oxygen concentration fed to the combustor will reduce the NO formation in oxy-fuel combustion of coal in a bubbling fluidized-bed combustor. However, there are few reports about the thermal conversion of nitrogen species after hydrothermal modification. Therefore, the present work investigates the influence of hydrothermal modification on the evolution of nitrogen during coal pyrolysis Received: May 25, 2016 Revised: August 3, 2016
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DOI: 10.1021/acs.energyfuels.6b01255 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Diagram of the hydrothermal modification system. library. The content of nitrogen compounds in released volatiles was calculated using the area normalization method. All the experiments and measurements are repeated three times, and the average of the results is presented in this study. The GC/MS system is an accurate test instrument and has good measuring repeatability; the standard deviations for the measurement results in this study are ZT200 > ZT250 > ZT300
This trend is also shown in Table 3. From Figure 5b and Table 3, it can be seen that ZT200 shows a highest NH3 release after hydrothermal modification, which is slightly higher than that of ZT raw coal, and ZT300 shows the lowest. Previous work2 showed that the amount of N-5 and N-6 decreased after hydrothermal modification, and it decreased continually as the final hydrothermal modification temperature was increased. The content of N-Q was highest in ZT200, and it also decreased as the final hydrothermal modification temperature increased. The change of HCN and NH 3 correspond to the content of N-5, N-6, and N-Q after
Figure 5. Evolution of N-containing species during 1000 °C pyrolysis conditions after hydrothermal modification: (a) HCN and (b) NH3. F
DOI: 10.1021/acs.energyfuels.6b01255 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 6. Evolution of CO and NO during 1000 °C combustion conditions after hydrothermal modification.
heterogeneous reactions between HCN and char which form the NH3. 3.3. Effect of Hydrothermal Modification on Nitrogen Release during Combustion Process. Hydrothermal modification also influences the release of nitrogen during coal combustion. In a high-temperature furnace with an oxidizing atmosphere, both volatile-N and char-N is oxidized to NO and N2O. HCN is the main source of N2O; NO is formed primarily from NH3 and HCN.26,27 In addition, there are also some secondary reactions whereby a small fraction of HCN and NH3 in volatiles react with NO in the gas phase, producing N2. The release of CO and NO during the combustion of ZT coal is shown in Figure 6. NO, NO2, and N2O are the most significant pollutants arising from coal combustion. However, in this study, NO2 and N2O show much lower emissions than NO; the peak NO2 and N2O concentrations are both
