Haihui Xin

An In Situ Testing Method for Analyzing the Changes of Active Groups in Coal Oxidation at Low Temperatures

ABSTRACT This study details an in situ Fourier transform infrared spectroscopy analytical system that was employed to follow chemical variations in the functional groups on coal surface during the oxidation process at low temperatures. In the reported in situ Fourier transform infrared spectroscopy system, a special chamber was used to contain the coal powders, and a gas inlet tube and a programmable heater were used to simulate different reaction atmospheres and temperatures. The comparisons between in situ and ex situ Fourier transform infrared spectroscopy spectra indicate that the in situ Fourier transform infrared spectroscopy data offer a more accurate reflection of changes in the functional groups. The real-time changes of aliphatic hydrocarbon groups and oxygen-containing groups in a lignite coal sample were analyzed from 30°C to 220°C using in situ Fourier transform infrared spectroscopy. The experimental results indicate that the chemical variations in the functional groups are affected by their relative chemical activities. The results show that the presence of aliphatic groups on the coal surface varies with temperature. Over the range of 30–70°C the presence of these groups decreases, but then their abundance increases over the range of 70–180°C and finally decreases again when the temperature is increased to between 180°C and 220°C. With respect to oxygen-containing functional groups, three various trends were observed as the test temperature was varied. Our conclusion was that these variations are a function of the reaction activities of the various oxygen-containing functional groups.

The Infrared Characterization and Mechanism of Oxygen Adsorption in Coal

ABSTRACT The remarkable differences between the infrared spectra of oxygen adsorption and nitrogen desorption in coal have been experimentally and theoretically investigated. Density functional theory calculations were performed to better explain the mechanism of oxygen adsorption using six different molecular models of coal. In addition, the remarkable differences of infrared spectra between oxygen adsorption and nitrogen desorption was defined as the index V, which was used to classify the spontaneous combustion tendency of coal. The experimental data indicated that the spectra in the 4000–2000 cm−1 and 1250–1050 cm−1 regions exhibited significant changes. These results suggest that the mechanism of oxygen adsorption is the alteration and transfer of charge density around the activated sites, which leads to the observed changes of the infrared spectra. The V, which is related to the alteration of spectral intensity, is found to decrease with the increase of adiabatic oxidation time and the relative spontaneous combustion period of coal. This observation solidifies the connection between sample spectral intensity and oxygen chemisorption. These data suggest that the hydroxy in hydroxy and carboxyl groups on the surface of coal particles is the site of oxygen chemisorption, and the V can be used to rapidly and accurately categorize the spontaneous combustion tendency of coal.

Oxidation and Self-Reaction of Carboxyl Groups During Coal Spontaneous Combustion

Carboxyl groups play an important role during the development of coal self-heating. The real-time changing characteristics of carboxyl groups were obtained using an in situ Fourier Transform Infrared Spectroscopy method. They are significantly influenced by reaction atmospheres. Under dry-air atmosphere, their quantity obviously decreases before reaching 120°C and then increases with temperature rise. Under oxygen-free atmosphere, it decreases before reaching 80°C and then increases with temperature rise. In contrast with direct oxidation process, the quantity of carboxyl groups is smaller and the decrease phenomenon at the beginning almost disappears during the oxidation process following oxygen-free reaction. The results indicate that the carboxyl groups have different reaction pathways during coal self-heating. These pathways can be divided into two kinds, that is, oxidation pathways and self-reaction pathways. The study proposed the reaction sequences of carboxyl groups during coal self-heating, which are helpful for revealing the mechanism of coal spontaneous combustion.

The effects of poly(ethylene glycol) on the low-temperature oxidation reaction of coal as monitored using in situ series diffuse reflectance FTIR

In situ series diffuse reflectance FTIR was used to study the effects of poly(ethylene glycol) as a potential chemical additive inhibiting coal oxidation process at low temperatures. Two coals with different volatile content and, different ash percentages were examined following treatment with 5 wt% poly(ethylene glycol) 200. The surfaces of samples both with and without the additive were analyzed at temperature up to 200 °C in air using in situ diffuse reflectance FTIR. The results showed that poly(ethylene glycol) 200 is capable of inhibiting the oxidation of aliphatic moieties such as methyl and methylene groups, and also reducing the quantity of surface hydroxyl groups through reactions that form more stable ether linkages, thus improving the thermal stability of the coal. A mechanism by which the additive interacts with the coal surfaces is proposed.

In-situ Series Diffuse Reflection FTIR Used in Studying the Oxidation Process of Coal

A comparison of in-situ series diffuse reflection Fourier transform infrared and non-series Fourier transform infrared in studying the conversion of functional groups in the process of coal oxidation is studied. It is found that non-series Fourier transform infrared could not monitor the changes of functional groups precisely while in-situ Fourier transform infrared can. In-situ Fourier transform infrared was also used to monitor the oxidation processes of low ranks of coal at low temperatures (30–220°C) and a series data was obtained. The variation regularity of six main functional groups was discussed and found that in-situ Fourier transform infrared would make it possible to reveal the mechanism of coal oxidation.

Oxygen consumption and chemisorption in low-temperature oxidation of sub-bituminous pulverized coal

ABSTRACT Studying the effect of oxygen in coal oxidation is very important for understanding and controlling coal spontaneous combustion. However, the oxygen effect is not very easy to determine clearly due to the large effect of heat source on coal oxidation in temperature rising experiments. Here, focused on sub-bituminous coal, the oxygen effect was separated from coal oxidation by continuously measuring FTIR spectra of coal with respect to varying temperatures and under oxygen and nitrogen. The active groups’ real-time changes of coal oxidation, thermal treatment and oxygen effect were measured. The carboxylic ester and carboxyl units are the main functional groups that increase with temperatures increasing under oxygen and nitrogen, while the other functional groups decrease in quantity. The oxygen effect promoted the consumption of aliphatic hydrocarbons and hydroxyl groups and also promoted the formation of oxygen-containing groups (except hydroxyl). Four characteristic temperature stages involved in the oxygen effect and their key functional groups were identified. Simultaneously, the relationship of oxygen consumption and chemisorption in oxygen effect was analyzed. The starting temperature of oxygen chemisorption is between 50 and 60°C. The maximum contribution of oxygen effect was observed in methyl and methylene groups. These results are important for chemical control of coal spontaneous combustion. The oxidation of aliphatic hydrocarbon should be controlled before oxygen chemisorption. The value of oxygen consumption between 70 and 80°C can be measured accurately due to the constant chemisorption rate, which help to identify the tendency for spontaneous combustion. These results will help in better understanding of the reaction mechanism of coal oxidation, especially the oxygen effect.

PLS Regression on Wavelet Transformed Infrared Spectra for Prediction of Coal Contents

Study on multivariate calibration for infrared spectrum of coal was presented. The discrete wavelet transformation as pre-processing tool was carried out to decompose the infrared spectrum and compress the data set. The compressed data regression model was applied to simultaneous multi-component determination for coal contents. Compression performance with several wavelet functions at different resolution scales was studied, and prediction ability of the compressed regression model was investigated. Numerical experiment results show that the wavelet transform performs an effective compression preprocessing technique in multivariate calibration and enhances the ability in characteristic extraction of coal infrared spectrum. Using the compressed data regression model, the reconstructing results are almost identical compared to the original spectrum, and the original size of the data set has been reduced to about 5% while the computational time needed decreases significantly.