Synthesis of a high-yield activated carbon by oxygen gasification of macadamia nut shell charcoal in hot, liquid water

Abstract Macadamia nut shells (‘macshells’) were pyrolyzed at elevated pressure to produce a high-yield charcoal. This macshell charcoal was carbonized at 950°C, reacted with oxygen dissolved in flowing hot liquid water at 100 bar and temperatures between 100 and 200°C, and carbonized again at 950°C to produce an activated carbon. Using this procedure, activated carbons with iodine numbers as large as 615 were manufactured with burnoffs as low as 13% (by weight of the carbonized charcoal). Doubling of the water flow rate and halving of the particle size did not affect the measured rates of oxygen chemisorption and gasification. These observations, together with calculated small values of the Thiele modulus, are consistent with the conclusion that the chemisorption and gasification rates were not mass transfer limited. Variations in the dissolved oxygen concentration had little effect on the oxygen chemisorption and gasification rates, suggesting that the order of reaction in oxygen was near zero. The overall yield of the activated carbon from the dry, raw macadamia shell feedstock was about 25 wt%.