| Lignite which accounts for nearly 13% of the coal reserves particularly plays an important role in supplying primary energy in China. The price of lignite is approximately one third of that of bituminous coal, thereby attracts lots of attention worldwide. However, lignite has high moisture content and low heating value, which resulted in low combustion efficiency and high transportation costs. In addition, lignite is rich in volatile matter and oxygen-containing groups, which increase the tendency to spontaneous combustion. This shortage poses challenges to the storage and transportation of lignite, thereby limits its utilization. Lignite upgrading is such a promising way for the purpose to use lignite more efficiently and safely. However, it is reported that the upgraded lignite may further absorb moisture from the surroundings, due to a large number of holes and crevices existing on the coal surface. It is called moisture re-adsorption property of upgraded lignite. The problem of moisture re-adsorption not only reduces the heating value of upgraded lignite, but increases the spontaneous combustion tendency as a result of the heat release and temperature rise during the moisture re-adsorption process. Consequently, the advantages of thermal upgrading would partially be limited by the moisture re-adsorption. Therefore, developing methods to prevent moisture re-adsorption during the storage of the upgraded lignite is of important for improving the quality of upgraded lignite.In this work, a Chinese Zhaotong lignite(ZT lignite, moistue content as-received is 52.3%) was studied in a lab-scale fixed-bed reactor. The lignite was thermally upgraded at the temperatures ranging from 200 to 500 oC. Before the upgrading, three kinds of pretreantment methods were introduced in this study. Asphalt is so repellent that was used as a coating material in lignite thermal upgrading to prevent moisture re-adsorption of the upgraded lignite. Hydrochloric acid and sodium borohydride pretreatment were used for changing the chemical structure, such as oxygen-containing groups, subsequently changing the moisture re-adsorption and combustion characteristics. The changes in chemical structures of the upgraded lignites were investigated using Fourier transform infrared spectroscopy(FTIR). The changes in physical structures were analyzed by N2 adsorption isotherm and scanning electron microscopy(SEM). The moisture re-adsorption and combustion characteristics of the upgraded lignites were studied by using a constant temperature/humidity chamber and a thermogravimetric analyer(TGA), respectively.The results indicate that the upgrading temperature and the pretreatment methods showed significant influence on the chemical-physical structure of the upgraded lignite, moisture re-adsorption and combustion characteristics. As the upgrading temperature increased, the abundance of oxygen-containing groups(i.e., hydroxyl and carboxyl) decreased, the pore volume and surface area of the upgraded lignites increased while pore diameter decreased. The moisture re-adsorption ratio and spontaneous combustion tendency to the upgraded lignites decreased with the increasing temperature. Hydrochloric acid pretreatment increased the heating value of upgraded lignite, reduced the abundance of hydroxyl in the lignite and subquently lowered the moisture re-adsorption ratio. Also it improved the combustion characteristics, for instance, it decreased the apontanous combustion tendency and reaction activity, and increased the combustion intensity and burn-out property. Sodium borohydride pretreatment showed no positive effect on reducing the moisture re-adsorption ratio and improving the combustion characteristics. SEM images revealed that some pores in upgraded lignites were covered by asphalt and the surface became much smoother with the increasing asphalt concentration which resulted in the decrease of the pore volume and surface area. of the upgraded lignite. However, the asphalt addition during the upgrading suggests two sides of effects on the moisture re-adsorption characteristics: the coating effect and the polar effect. When upgraded at lower temperatures(200-300 oC), the moisture re-adsorption ratio would not be reduced until the asphalt concentration reached 5 wt%. While at higher temperatures(400-500 oC), the moisture re-adsorption ratio decreased with the increasing asphalt concentration. While the influence of asphalt on the combustion characteristics were temperature dependent. |
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