VICTORIAN BROWN-COAL; FIXED-BED REACTOR; ASH DEPOSITION; ALKALI REMOVAL; ATOMIC SODIUM; SORBENTS; GASIFICATION; TEMPERATURE; MECHANISMS; CORROSION
Zhundong coal represents a large portion of China's future energy supply, because of the large reserve capacity. Although Zhungdong coal has low ash and good ignition characteristics, it also contains large amounts of sodium, which can foul and corrode heat-transfer surfaces. For economically viable use of Zhundong coal or other high-alkali coals, the alkali release must be mitigated prior to or within the burner. This can be done either by washing the coal,, using sorbents to trap the sodium, or a combination of these methods. Additive influence on the release of sodium over the entire coal combustion process was measured using a calibrated laser-induced breakdown spectroscopy (LIBS) technique. The additives used were alumina, silica, and five mineral sorbents comprising blends of silica, alumina, and various other inorganic compounds; different coal/sorbent ratios were assessed (1%, 3%, and 5% additive, by weight). During the three stages of sample coal combustion, it was found that the first stage, devolatilization, releases only similar to 1.2% of the total sodium; the char combustion stage releases similar to 28% of the total sodium; and the ash stage releases the vast majority of sodium (similar to 70%). The peak instantaneous release rate occurs during the char combustion stage. The sodium retention efficiency for all of the sorbents tested was lowest during the devolatilization stage and there was no appreciable change to the duration of the stage. Sodium retention efficiency was greatest in the ash stage, primarily due to the sheer amount of sodium released during this stage. The ash stage was effectively shortened, in proportion to the retention efficiency of the sorbent. The sodium release during the char combustion stage showed an obvious difference in profile for the various types of sorbent tested. Alumina was the least effective in reducing sodium release and retained the characteristic shape of sodium release from raw coal during this stage. Silica and sepiolite demonstrated similar sodium release profiles, which were markedly different than the sodium release profiles of either alumina or aluminosilicate mineral samples. Time-resolved sodium retention plots show an enhancement in the sodium retention by alumina during the char burnout. The addition of sodium sorbent to the coal extended the duration of the char burnout stage, which may indicate that the reduction of available sodium reduces the alkali catalysis of the char decomposition.