Ignition and combustion characteristics of single particles of Zhundong lignite: Effect of water and acid washing

The ignition and combustion characteristics of Zhundong lignite (ZDL), with and without washing to remove different forms of inherent sodium, were investigated. Water washed (ZDL-WW) and acid washed (ZDL-AW) samples were prepared by soaking the raw lignite (ZDL-Raw) in ultrapure water and 0.5 M hydrochloric acid (HCl), respectively, at 60 °C for 24 h. A single particle of a ZDL sample, ca 2.5 mm in diameter, was suspended on a silicon carbide fibre (142 µm) tip and burned in air at 1123 K in a furnace. The time-resolved ignition and combustion behaviours of the single particles were observed with the aid of combined use of a shortwave infrared camera, a CCD camera, which enabled the determination of the ignition mechanism, ignition time, burnout time and burning rate. A flame emission spectrometer was used to identify the presence of sodium in the flame. The ignition of all ZDL samples followed the joint hetero-homogeneous mechanism in the present work. Upon the homogeneous ignition, ZDL-Raw exhibited a soot free yellowish translucent flame while ZDL-WW and ZDL-AW showed sooty flames. The ignition time followed the order of ZDL-Raw>ZDL-WW>ZDL-AW while the burning rate followed the opposite order. These observations were attributed to the catalytic effect of sodium in the lignite whose amount was varied due to the water and acid washing. Sodium ions were detected in the flame of ZDL with and without washing and the intensity of sodium signal also followed the order of ZDL-Raw>ZDL-WW>ZDL-AW. It is believed that sodium ions released in the flame promoted catalytic cracking of large tar fragments and oxidation of soot precursors.