Nickel catalyzed air gasification of cellulosic chars-jump in reactivity

Cellulosic chars containing nickel species have been gasified in air in the temperature range 235-400 degreesC. The chars were prepared from Ni2+-exch anged carboxy methyl cellulose (NiCMC) by pyrolysis in N-2 at several heat treatment temperatures (HTTs) ranging from 400 to 900 degreesC. All the cha rs displayed a dramatic jump in reactivity when the gasification temperatur e (GT) was raised to a particular value (jump temperature, T-j). Above T-j, 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 r ate-controlled, low reactivity, high activation energy region to a diffusio n-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 greater tha n or equal to 500 degreesC, in agreement with thermodynamic calculations. A sudden enormous increase in the mobility of the catalyst species in the ne ighborhood of it;; Tammann temperature (T-Tam) appears to be the cause for the jump in char reactivity. This jump strongly influenced the dependence o f char gasification rate on HTT, affecting the usual monotonic decrease in rate with an increase in HTT. A monotonic decrease was observed only when t he GT lied below the T-j of the char. When GT greater than or equal to 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 t he char. The study emphasizes the important role of mobility of catalyst sp ecies in controlling the char reactivity.