Evaluation of Vermiculite in Reducing Ash Deposition during the Combustion of High-Calcium and High-Sodium Zhundong Coal in a Drop-Tube Furnace

The additive injection technology has been effective in alleviating slagging and fouling during coal combustion. In this study, experiments were conducted in a drop-tube furnace to understand the influence of vermiculite injection on ash deposition during the combustion of Zhundong coal. The interaction between coal ash and vermiculite was investigated by inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray diffraction (XRD), scanning electron microscopy-energy-dispersive spectrometry (SEM-EDS). Results show that deposits formed at 1100 degrees C with vermiculite additive are more friable and polyporous than those without vermiculite additive, and these deposits can be easily removed. The mechanisms behind this phenomenon can be explained as follows: (1) The agglomeration of ash particles is inhibited by the partition of low viscosity slices formed through vermiculite swelling after heat treatment. (2) Intermetallic reactions occur on the surface of vermiculite particles, resulting in the formation of frizzle sheets, and these sheets are stacked in microscale to produce a macroscale polyporous structure. (3) The surface of sticky and spherical ash particles are covered by high-melting-point and less sticky minerals, including forsterite and enstatite. (4) The decomposition of anhydrite in ash to produce free CaO is inhibited by vermiculite injection, and therefore, the formation of a relevant low-temperature eutectic is reduced.