Reducing alkali species from high-alkali coal by hydrothermal pretreatment

摘要

Despite its abundant resource, high-alkali coal (Na_2O in dry ash>2%, by weight) is limited in application due to its higher ash slagging and fouling tendency during coal thermal conversion, such as coal combustion or gasification. Thereby, it is of importance to reduce the undesirable species from the high-alkali coal. In this paper, a novel method named hydrothermal pretreatment (HTP) was introduced to remove the alkali species, which was conducted within a batch-type autoclave under various temperatures. The treated coal was analyzed, along with the oxygen-containing functional groups determined by Fourier transform infrared spectrometer (FT-IR). And the alkali species and other components in the coal ash were quantified by X-ray fluorescence (XRF), then the ash slagging and fouling tendency was evaluated. Apart from this, FactSage was adopted to simulate the behavior of alkali species during coal thermal conversion. The results show that the alkali species could be washed out, with the removal ratio up to 54.5% even under 50 °C, while the residual alkali content in treated coal exceeds 2% unless the HTP temperature reaching 300°C. HTP could remarkably reduce both the inorganic- and organic alkali from coal, and there is a positive correlation between the removal ratio and the temperature. With the temperature approaching 300°C, the alkali content could be lowered less than 2%, with a maximum removal ratio of 82.1%. The slagging and fouling tendency of the treated coal is depressed in view of the calculations based on the XRF results. Furthermore, the lowering of slagging and fouling tendency via HTP is also demonstrated by the FactSage simulations, where more high-melting-point species form rather than the low temperature eutectics for the raw coal. Besides, the coal treated via HTP is superior to the raw one in terms of certain remarkable changes, for example, the moisture, oxygen and sulfur of the treated coal decline obviously, while the calorific value rises sharply. In summary, HTP could effectively reduce the alkali species from the high-alkali coal. And the proposed mechanism may lie in that the alkali species in coal matrix became released due to the breakage of the coal functional groups and micropores during HTP.