Elsevier

Thermochimica Acta

Volume 642, 20 October 2016, Pages 81-87
Thermochimica Acta

Combustion performance of Loy Yang lignite treated using microwave irradiation treatment

https://doi.org/10.1016/j.tca.2016.09.010Get rights and content

Highlights

  • How changes in physicochemical properties affect combustion performance.

  • Microwave irradiation (MI) decreases the reactivity of lignite.

  • Reactivity progressively decreases as MI time increases.

  • MI treatment modifies the combustion intensity.

Abstract

The combustion characteristics and activation energy of the microwave irradiation (MI) treated lignite were investigated by thermogravimetric analysis at different heating rates. Effects of the changes in physicochemical properties (amounts of volatile matter and fixed carbon, specific surface area, and pore volume) on the combustion performance were also discussed. The activation energy was calculated using the Kissinger–Akahira–Sunose isoconversional method. The results showed that ignition temperature of the MI treated samples increased significantly relative to that of raw lignite, and it gradually increased with increasing MI time, indicating that MI treatment decreases the reactivity of lignite, which progressively decreases as MI time increases. The maximum combustion rates of the treated samples were higher than that of raw lignite, suggesting that MI treatment modifies the combustion intensity of Loy Yang lignite. Moreover, the combustion processes of samples treated by MI for 5 and 7 min were single rate-limiting step kinetics, whereas raw lignite and samples treated by MI for 3 and 9 min followed multistep kinetics. Average activation energy for sample treated using MI for 5 min was the maximum value, and it decreased with further increasing MI time because of the increased effect of diffusion.

Introduction

The utilization of lignite is gradually becoming important because of the continuous increase in the requirements of energy, the decrease in high-rank coal reserves, and its wide availability and many advantages (e.g., low mining and market costs, easy access, and low pollution-forming impurities) [1], [2], [3], [4]. However, the high water content in lignite is a major problem for its efficient and environmentally friendly use. Therefore, dewatering for lignite is an important process to improve its widespread application and has attracted an increasing amount of attention.

Microwave irradiation (MI) dewatering for lignite is receiving considerable attention and is used as an alternative to conventional convective and conductive drying techniques because MI offers some unique advantages (e.g., rapid and selective heating, energy transfer instead of heat transfer, penetrating deep into samples, and enhancing water loss) [5], [6]. The drying mechanism of MI is different from that of conventional techniques. For conventional drying, water is gradually removed from the outside inward through convection, conduction, and radiation. This is a relatively slow process. To dewater the trapped water existed in the interior of lignite, it may overheat the surface. In terms of MI dewatering, MI energy can penetrate into coal particles and it is possible for water to be evaporated within the particles, leading to an increase in water vapor pressure inside coal particles. The increased pressure, which is absent in conventional drying techniques, is an additional mechanism for MI dewatering [7], [8]. Standish et al. [9] reported that the rate of water removal from a brown coal using MI treatment is one to two orders of magnitude faster than that using conventional convective dewatering.

Valuable studies on MI dewatering of low-rank coal have been done previously. Effects of coal particle size, microwave level, and samples size on drying rate of low-rank coal were investigated by Tahmasebi et al. [10]; Effects of MI dewatering on pore structure, coal composition, and combustion characteristics were reported by Ge et al. [11]; Physicochemical properties, including total organic matter, chemical oxygen demand, functional groups, pH value, electrical conductivity, and inorganic matter, of water obtained from MI dewatering process were investigated by Cheng et al. [12]; and that adding MI absorber carbon materials (activated carbon and graphite) and metal oxides (Fe3O4, MnO2, etc.) improve MI dewatering was reported by Cheng et al. [13]. Taken previous studies, effects of the changes in the physicochemical properties during MI process on the combustion performance, including combustion characteristics and activation energy (E), have not been elucidated.

Thermogravimetric (TG) analysis is an effective and simple technique to rapidly quantitatively investigate the combustion performance in the laboratory, which was employed to evaluate the overall combustion process; this is useful in engineering design and economic assessment [14], [15], [16], [17], [18], [19], [20], [21]. Note that different operating conditions (e.g., heating rate, sample size, reaction atmosphere, and water content of a sample) give a little different combustion performance [14], [22]. However, under the same conditions, TG analysis is a valuable tool to investigate changes in the combustion characteristics and kinetic parameters [14], [23].

In this study, TG analysis was used to study the combustion performance of solid products obtained from MI treatment considering the changes in the physiochemical properties, including volatile matter and fixed carbon contents, specific surface area (SSA), and pore volume. Non-isothermal thermogravimetry was carried out at four different heating rates and the Kissinger–Akahira–Sunose (KAS) isoconversional method was employed to calculate E.

Section snippets

Sample

An Australian lignite (Loy Yang, LY) was used as the coal sample. The lignite sample was ground to pass through an 840-μm sieve for the following experiments. The proximate and ultimate analyses are shown in Table 1.

MI treatment

MI experiments were performed in a microwave reactor (μReactor Ex, Shikoku, Japan) with a frequency of 2.45 GHz and the dimensions of its chamber are 280 × 280 × 250 mm. Raw lignite (12 g) was placed into a 1000 mL three-necked flask. The left and right necks were used as gas inlets and the

Effects of MI treatment on pore size distribution

Because pore structure significantly affects the combustion performance of lignite, it is necessary to discuss the effects of MI treatment on pore size distribution. Fig. 3 shows the pore size distributions of all of the samples and x-axis is expressed on a logarithmic scale. Based on the International Union of Pure and Applied Chemistry classification, the diameter ranges of micro-, meso-, and macropores are <2 nm, 2–50 nm, and >50 nm, respectively. To clearly analyze the pore size distribution

Conclusions

The Ti values for the MI treated samples were markedly higher than that for raw lignite, and it increased gradually as treatment time increased, suggesting that MI treatment decreases the reactivity of LY lignite, which progressively improves with increasing processing time. This is because high-boiling-point volatile matter and char contents in the treated samples increase compared with that in raw lignite, and they gradually increase with increasing time. The Tp2 and MCR of the treated

Acknowledgements

This work was supported by JSPS KAKENHI Grant Nos. 24246149 and 15H02333, and China Scholarship Council (No.201406420045). The authors are also appreciative the Research and Education Center of Carbon Resources, Kyushu University, which provides the lignite sample.

References (31)

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