Nickel Catalyzed Air Gasification of Cellulosic CharsJump in Reactivity

Abstract

Cellulosic chars containing nickel species have been gasified in air in the temperature range 235-400 °C. The chars were prepared from Ni 2+ -exchanged carboxy methyl cellulose (NiCMC) by pyrolysis in N 2 at several heat treatment temperatures (HTTs) ranging from 400 to 900 °C. All the chars displayed a dramatic jump in reactivity when the gasification temperature (GT) was raised to a particular value (jump temperature, Tj). Above Tj, the rate did not vary significantly with a further increase in temperature. T j increased with HTT. The jump was accompanied by a transition from a rate-controlled, low reactivity, high activation energy region to a diffusion-controlled, high reactivity, low activation energy region. Chars of pure CMC did not show such a jump in gasification rate with increase in GT. XRD studies showed that elemental nickel is present in chars of HTT ≥ 500 °C, in agreement with thermodynamic calculations. A sudden enormous increase in the mobility of the catalyst species in the neighborhood of its Tammann temperature (T Tam ) appears to be the cause for the jump in char reactivity. This jump strongly influenced the dependence of char gasification rate on HTT, affecting the usual monotonic decrease in rate with an increase in HTT. A monotonic decrease was observed only when the GT lied below the Tj of the char. When GT ≥ T j , the rate did not decrease with an increase in HTT, but rather increased or remained almost invariant. On the other hand, a monotonic decrease in rate with an increase in HTT was obtained in the absence of nickel species in the char. The study emphasizes the important role of mobility of catalyst species in controlling the char reactivity.