Effect of the COMBDry Dewatering Process on Combustion Reactivity and Oxygen-Containing Functional Groups of Dried Lignite

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 CO 2 ) 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.

Effects of Upgrading Treatment on the Physicochemical Structure, Moisture Re-Adsorption Ability, and NOx Emission Characteristic of Lignite Particles

“…By means of the application of a mini crusher, large lignite particles were crushed in advance. Subsequently, the lignite particles with a uniform particle size in the range of 3–6 mm were separated by a vibrating screen machine, and then upgraded on the ComDry upgrading system under different conditions . The picture of the ComDry upgrading system is shown in Figure , and the system consists of eight main parts: coal feeder, scuttle, cyclone separator, cooling system, duster, induced draft fan, ComDry reactor, and combustion reactor.…”

“…Meanwhile, the pore volume and pore diameter of each dried lignite sample was identified by the Barrett–Joyner–Halenda model (BJH). The distributions of the macropores, mesopores, and micropores were clarified by means of the utilization of the t -plot method to the N 2 adsorption results …”

“…Aiming to meet the demand of lignite upgrading, many researchers have investigated the lignite drying technology. , According to the difference in the way of moisture content removal, the lignite drying technology could be divided into two categories: evaporative and nonevaporative drying methods . In general, upgrading treatment of lignite would lead to apparent variations in specific surface area, pore structures, relative amount, and distribution of functional groups on the surface of lignite particles. ,, Because of the existence of massive pore structures (original and new pores) and hydrophilic groups on the particle surface, moisture re-adsorption of dried lignite from the surrounding environment is ineluctable .…”

“…When lignite particles with high moisture content were added into the combustion zone, vaporization of the moisture content occurred immediately and the steam would participate in the combustion process of the lignite. Because of the physicochemical properties of H 2 O are different from those of O 2 , the involvement of H 2 O molecules would lead to significant variations in the combustion mechanisms and the emission characteristics of nitrogen-containing gaseous pollutants (mainly NO). , Additionally, rapid devolatilization occurs when the lignite particles enter the combustion zone, generating volatile matters and chars. H 2 O molecules could decompose into hydroxyl groups and hydrogen free radicals under high temperature conditions, and then react with the volatiles and char promoting the emission of NO during the combustion process. , Previous results illustrated that the enhancement of the upgrading degree could promote the emission of nitrogen content in volatiles into HCN and NH 3 , which could accelerate the consumption of NO in the atmosphere under high-temperature conditions.…”

“…Previous research studies mainly focused on investigating the effects of upgrading condition/degree on the particle physicochemical behaviors and moisture re-adsorption characteristics in the process of different upgrading methods. ,, Because of the moisture re-adsorption of upgraded lignite is unavoidable, it is necessary to clarify not only the main factors affecting dried lignite re-adsorption behavior but also the effects of moisture re-adsorption on NO emission characteristics during the combustion process. Additionally, previous researchers conveyed that there were significant differences in the combustion rates between volatile matter/char and oxidizing agents, and the two reaction processes could be investigated separately .…”