Article pubs.acs.org/EF
Effect of the COMBDry Dewatering Process on Combustion Reactivity and Oxygen-Containing Functional Groups of Dried Lignite Yaying Zhao,†,§ Guangbo Zhao,*,† Rui Sun,*,† Hui Liu,† Zhuozhi Wang,† Lee Sihyun,‡ and Ming Kong† †
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Korea Institute of Energy Research (KIER), Yuseong-gu, Daejeon 34129, Korea § Northeast Electric Power University, Jilin132012, China ‡
ABSTRACT: Two typical types of Chinese lignite samples were employed to investigate the coal drying characteristics of an innovative COMBDry lignite drying system. The drying rate increased significantly with an increase in drying temperature and ratio of the flue gas to the lignite. Using a nitrogen adsorption instrument, it was found that the drying process promoted generation and enlargement of the surface pore structures of the particles, which can enhance coal combustion rate. A horizontally fixed bed furnace and a SIGNAL S4i pulsar NDIR (infrared (IR)) gas analyzers were used to investigate the combustion characteristics of coal samples after the drying treatment. In an attempt to analyze the variation in the sample surface chemical structure during the drying process, Fourier transform infrared (FT-IR) spectrometry and Raman spectroscopy were employed for the investigation. The results showed that the concentration of aliphatic hydrogen decreased with an increase in drying temperature and resulted from the decomposition of oxygen-containing complexes (released CO and CO2) and reaction with hydroxyl groups. The number of functional groups decreased when the drying temperature exceeded 210 °C. The amount of carbonyl and carboxylic esters initially increased and then (210 °C) decreased with an increase in flue gas temperature, and the content of aromatic carbon was unchanged with the treatment. The ordered crystalline carbon changed into the crystal defect structure and amorphous carbon and the degree of graphitization decreased and thus the combustion reactivity of the dried lignite was improved.
1. INTRODUCTION In recent years, with the rapid development of the economy and technology, the shortage of high-quality carbonaceous resources has gradually attracted attention.1 Because of the abundant reserves of lignite, upgrade technology for low-rank coal is gradually being developed. The moisture and volatiles contents in lignite are generally very high and the calorific value is relatively low. The thermal stability is so poor that lignite can easily undergo weathering and metamorphism, and easily succumb to spontaneous combustion.2 These factors present significant challenges in the course of utilization, particularly because of the high moisture content in lignite: (1) Long-distance transportation leads to a substantial cost. (2) Because of the high volatiles content, the thermal stability is not conducive to long-term preservation. Moreover, the efficiency of direct combustion is low and leads to a large amount of CO2 emissions.3 Thus, to expand the use of lignite, a drying treatment is necessary. Frequently used lignite drying technologies have been summarized by Allardice et al.4 and Osman et al.5 These include the rotary tube dryer,6 the fluidized bed dryer,7 mechanical thermal dehydration,8 high-temperature dewatering,9 and solvent dehydration.10 The rotary tube dryer is the most commonly used, but the energy consumption and construction cost of the device is high, and the equipment size is limited by the diameter of the rotary kiln. The high temperatures and high pressures required for water thermal drying technology makes this process expensive and limits its © 2017 American Chemical Society
use. Although the newly developed nonevaporation dehydration technology (HTD and MTE) has a high drying efficiency, the extraordinary conditions and complex construction process result in a very high cost.5 This research focuses on the application of COMBDry low-temperature flue gas counter flow drying and upgrading technology; this system has the following characteristics: (1) Coal particles move along a baffle under the action of gravity from the top to the bottom of a COMBDry dryer, which leads to a uniform drying effect and stable contact time between sample particles and a low-temperature flue gas. (2) The temperature of the inlet and outlet of the dryer are controllable, which is convenient for the removal of the intimal water of lignite particles. (3) The sample particles have less size limitations, and the COMBDry dryer is easy and flexible to operate and conductive to a large scale. Evans,11 as well as Androutsopoulos and Linaros,12 investigated the variation in pore structure of lignite particles during the drying process. Salmas et al.13 found that, despite the shrinkage of the lignite structure during the dewatering process, there was still a portion of the pore structure that was generated when the drying temperature was
