Fundamentals of Catalytic Tar Removal over in Situ and ex Situ Chars in Two-Stage Gasification of Coal

Abstract

Intrinsic fundamentals of catalytic tar removal over coal char were studied in a laboratory two-stage gasification facility. The in situ char from coal pyrolysis (without cooling) exhibited higher activity for tar removal than the char experiencing its slow cooling in N2 to room temperature. The tar content in dry producer gas can be as low as 100 mg/Nm3 over the in situ char in 1.2 s of gas residence time inside the char bed at 1100 °C. Brunauer–Emmett–Teller surface area results showed that the in situ char has a larger specific surface area and more micropores than the ex situ char, while X-ray diffraction analysis clarified that the ordering of the carbon crystallite structure in the char increased with raising the temperature of tar removal. The latter is considered to be due to the facilitated secondary pyrolysis occurring to the ex situ char at increased temperatures. The secondary pyrolysis of the ex situ char also decreased its specific surface area and the amount of micropores. Severe coke deposition on the surface of spent char was observed, especially at temperatures above 1000 °C. The tar coking ratio defined as the mass ratio of coked tar over the totally converted tar reached as much as 0.7 when the temperature was 1100 °C and the gaseous tar residence time was 1.2 s. The coke formed on the char surface reduced the difference of structural properties between the in situ and ex situ chars and, in turn, decreased the difference in their performances for cracking tar.