In this study, the transformation characteristics of sodium in Zhundong coal were investigated, following its combustion (oxidizing atmosphere) and gasification (reducing atmosphere) in a circulating fluidized bed (CFB) experimental system of 0.25 t/d high alkali coal thermochemical conversion. The equilibrium distribution of Na is predicted under the oxidizing and reducing atmospheres by the thermodynamic equilibrium calculation. Na in bottom ash and fly ash evidently decreases as the temperature increases in both the combustion and gasification processes. More Na is retained in the ash during the gasification process. Na exists as sodium aluminosilicate in both combustion bottom ash and gasification bottom ash, and mainly as sodium sulfate (Na2SO4) in combustion fly ash and sodium chloride (NaCl) in gasification fly ash. Na undergoes different transformation processes through combustion and gasification. Gaseous metallic sodium, Na2O and NaCl are released from the coal and condense directly on the cold tube surface during gasification. When SO2 is present, Na reacts with it, forming a large amount of gaseous sodium sulfates during combustion.

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Three types of Zhundong coal with different sodium contentsShenhuazhundong coal (SHZD), Shaerhu coal (SEH), and Tianchimulei coal (TCML)were investigated to understand their slagging behaviors during circulating fluidized bed gasification (CFBG). Generally, the operational stability of Zhundong coal decreases with the rise of sodium content. Zhundong coal with low sodium content is suitable as a gasification fuel of a circulating fluidized bed. For high-sodium-content coal, agglomeration, and even defluidization, should be the focus. Agglomeration or defluidization could be alleviated or avoided by controlling the bed temperature. There is a one-to-one correspondence between temperature fluctuation and sodium content in bottom slags. Slagging is affected by sodium occurrence modes and some ash components (e.g., SiO2 and Fe2O3). Due to different contents of chemical components, especially chlorine, silicon, and sodium, the three types of Zhundong coal cause different problems. Chlorine and sodium in SEH, mainly existing as NaCl, play an important role in corroding metal walls. Microanalysis of agglomeration particles in the tests with TCML shows that the SiO2-to-Na2O ratio gradually increases from surfaces to the inner part, which confirms the effect of the Na2O–SiO2 reaction on the agglomeration. The SiO2 content in high-alkali coal decisively impacts on the slagging morphology by the interaction between Na2O and SiO2 in coal itself for high-SiO2 coals or additional SiO2 from quartz sands for low-SiO2 coals.

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Coal is the basic resource underpinning energy generation in China, however, constant, large-scale mining of coal results in many problems such as ecological destruction of mining areas. As a result, backfilling of solid waste underground is proposed to control strata and surface subsidence and to protect the environment. At present, these materials, such as granular material, cemented material and high-water-content materials are mainly used for backfilling. This study summarised the types of backfill materials that are used in coal mines in China along with the backfilling process. Moreover, distribution and characteristics of mines backfilled with these backfill materials were obtained and analysed. Considering the socio-environmental aspects that affect backfilling, this research proposed a guideline for the selection of backfill materials and then analysed specific engineering cases of three backfill materials. In addition, the future development of backfill materials was discussed. With extensive extraction of shallow coal resources in China and, therefore, rapid depletion of coal resources in eastern regions of China, coal mining depth is increasing significantly. As a result, it is required to investigate new backfill materials suited for the deep high-stress environment.

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