Zhundong coal with a large reserve is faced severe ash-related problems including slagging and fouling during pulverized-coal boiler combustion. The gasification in circulating fluidized bed is a good way to use Zhundong coal due to the low reaction temperature, while it is unknown for the slagging and ash deposition mechanism of Zhundong coal during circulating fluidized bed gasification. In this paper, the gasification of Zhundong coal was tested in a 0.4 t/d CFB test rig simulating the real industrial device. Ash deposition probes were installed along the gas flow direction to characterize its slagging and ash deposition at different wall temperatures. These probes were treated with three cooling methods (no cooling, air cooling or water cooling) to reach different wall temperatures (882–737, 665–429 and 81–45℃). The results reveal that deposition was affected greatly by wall temperature. Mineral elements including Na, Ca, Fe, Al and Si were likely to accumulate in deposits on high-temperature surfaces, and existed in the forms of silicates and sulfates at wall temperature above 882℃, resulting in slagging. Although the contents of most mineral elements in the deposits on cooled surfaces were always low, a large gas-wall temperature gradient triggered the condensation of gaseous species, such as Na, Cl and K. The CaSO4 and CaO in gases were condensed and restructured in an ordered crystalline array on the surfaces of ash particles, forming a coating layer that made the ash particles stickier. Chemical composition and crystallographic analyses show that the properties of deposits on the probes and gas-borne ashes were different. More mineral phases as well as sodium were present in deposits. Additionally, due to the Cl enrichment on air-cooled surfaces, the corrosion of the metal surfaces should be focused.
Conclusions
The effects of wall temperature on slagging and ash deposition during circulating fluidized bed gasification of Zhundong coal were investigated. The main conclusions are listed below.
(1) Deposition was greatly affected by wall temperature. Slagging occurred on the surfaces at above 882℃. As the gas-wall temperature gradient was enlarged, fine particles were captured and deposited onto cooled surfaces. Deposition affected wall temperature in return and thus destabilized the heat flux.
(2) The enrichment of Ca, Si, Na, Fe and S was the main cause of slagging on the high-temperature surfaces. Volatile species such as NaCl were condensed on cooled surfaces and thus contributed to deposition. Ca was condensed and restructured in an ordered array to form a coating layer on ash particle surfaces, making ash particles stickier.
(3) The contents of mineral elements decreased along the gas flow direction. The largest content changes occurred within the wall temperature of 882–804℃. The contents of these elements were always low at below 737℃. However, gaseous Na, Cl and K were condensed to deposits along the gas flow direction.
(4) The properties of deposits and ash were very different. More mineral phases were present in deposits. Mineral elements such as S, Ca and Fe tended to enrich in deposits on higher-wall-temperature surfaces. Attention should be paid to chlorinous corrosion under air-cooling condition.
The results have been published on Applied Thermal Engineering 106 (2016) 1127–1135.
