Effects of wall temperature on slagging and ash deposition of Zhundong coal during circulating fluidized bed gasification

Abstract

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 0.4t/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°C). 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°C, 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.