Ash deposition behavior of a high-alkali coal in circulating fluidized bed combustion at different bed temperatures and the effect of kaolin

Abstract

High alkali and alkali earth metals (AAEMs) content in coal causes severe slagging and fouling during combustion in a boiler. In this study, the ash deposition behavior of a high-alkali coal at different bed temperatures and the effect of kaolin were investigated in a 30 kW circulating fluidized bed (CFB) test system using an ash slagging probe and deposition probe. The results show that the ash deposition tendency increases with the bed temperature. The condensation of Na 2 SO 4 is an important inducement for slag formation in the furnace. The melting or partial melting of slags is attributed to Na-Fe-Ca eutectics. At 920 C, Na 2 SO 4 will react with CaSO 4 to form the low-melting compound of Na 2 SO 4 -CaSO 4 . The deposited ash on the convection-heating surface consists of granular particles. On the windward side, the layered-structure ash deposits, i.e. the inner and outer layers, are formed at the bed temperature of 920 C but are absent at lower temperatures (820 C and 870 C). The formation of the inner layer consists of fine particles (<2 mm) and is closely related to Na 2 SO 4 . The size of the deposited ash in the outer layer is larger than 10 mm, while that on the leeward side is less than 10 mm. By adding kaolin in the coal, the slags are replaced by loose particles due to the absorption reactions between kaolin and alkali metals. The ash deposition tendency is improved and the optimal result is achieved when kaolin is added at an addition ratio of 3%.

Experimental Investigation into Ash Deposition and Na Migration Characteristics during Combustion of High Sodium Zhundong Lignite in a Circulating Fluidized Bed Operating at Low Temperatures

“…These Na species with low melting points would condense on heat transfer surfaces or get coated on the ash particles, , increasing the stickiness of the ash particles and the propensity of ash deposition. The release of Na during Zhundong lignite combustion is dependent on combustion temperature. , At combustion temperature 1200 °C, most Na would be released into the gas phase. However, as the combustion temperature decreased from 1050 to 850 °C, more Na would be retained in the residue ash. , This brings the benefits of circulating fluidized bed (CFB) combustion burning Zhundong lignite.…”

“…The release of Na during Zhundong lignite combustion is dependent on combustion temperature. , At combustion temperature 1200 °C, most Na would be released into the gas phase. However, as the combustion temperature decreased from 1050 to 850 °C, more Na would be retained in the residue ash. , This brings the benefits of circulating fluidized bed (CFB) combustion burning Zhundong lignite. As reported in the literature, − ash slagging in a traditional CFB furnace at typical temperatures 850–950 °C would be decreased due to the violent gas–solid flow within the furnace.…”

“…Meanwhile, ash fouling on heat-transfer surfaces at the flue path would be mitigated due to the retention of Na by the bed materials or other incorporated minerals within the furnace. Despite this, a noticeable ash deposition and bed agglomeration remain occurred during CFB combustion of Zhundong lignite. , The abundance of Fe and Na in bottom ash led to the agglomeration of bed material particles, and the Ca- and Na-bearing sulfates, aluminosilicates, or eutectics of low melting points were the culprits for its severe ash deposition. − Cofiring Zhundong lignite with other coals of low Na contents has been practiced in the current power plants, ,, yet transportation of such coals from Zhundong coalfield beyond would increase the operating cost significantly.…”

Effect of steel slag bed material on ash deposition and Na migration in circulating fluidised bed during Zhundong lignite combustion

“…16 In CFB in the presence of bed material, despite a portion of Na could be captured by bed particles in the furnace, 17,18 obvious ash deposition on heat transfer surfaces has been still found owing to the low melting-point fly ash. 7,19 Co-combustion of Zhundong lignite with coal gangue has been practiced in the current power plants for deposition mitigation, yet this is merely a matter of expediency once the operating cost is considered. 6,20 The use of additives including kaolin, SiO 2 , Al 2 O 3 , oil shale and its semi-coke and so forth 21,22 was also reported, whose effects on ash deposition however varied with the additives that being used.…”

“…The released Na would further react with SO x , silicon or other minerals, forming low melting‐points Na sulfates and silicates and aggravating ash sintering, melting and deposition 16 . In CFB in the presence of bed material, despite a portion of Na could be captured by bed particles in the furnace, 17,18 obvious ash deposition on heat transfer surfaces has been still found owing to the low melting‐point fly ash 7,19 . Co‐combustion of Zhundong lignite with coal gangue has been practiced in the current power plants for deposition mitigation, yet this is merely a matter of expediency once the operating cost is considered 6,20 .…”