Activated Carbon from Macadamia Nut Shell by Air Oxidation in Boiling Water

Abstract

A high-yield activated carbon is produced from macadamia nut shell charcoal by (i) carbonization of the charcoal at 1173 K, (ii) air oxidation of the carbonized charcoal in boiling water (AOBW) at 503−553 K, and (iii) activation (a second carbonization) of the oxygenated carbon. In step ii, air is bubbled through a sparger to maintain a relatively high concentration of dissolved oxygen in the water, and the boiling water serves to control the temperature of the carbon during its gasification by the dissolved oxygen. Carbon dioxide is observed to be the only gaseous product of the oxidation chemistry. The oxidation results and the properties of the activated carbons from AOBW are similar to those obtained by controlled atmospheric air oxidation. However, the rate of CO2 formation is observed to increase with time to a plateau for AOBW, whereas the gasification rate decreases with time for atmospheric air oxidation. Multiple cycles, involving AOBW followed by activation, efficiently increase the specific surface area of the carbon to values approaching 1000 m2/g. Increases in the specific surface area occur by the removal of carbon during the AOBW step(s) and the activation step(s). Our findings indicate that carbon removal by desorption of chemisorbed oxygen during the activation step creates a specific surface area more efficiently than a prolonged, low-temperature gasification of the carbon during the AOBW step. If we assume a simple kinetic model in which the gasification reaction is first order with respect to dissolved oxygen and zero order in carbon, the activation energy for AOBW is estimated to be 108 kJ/mol between 513 and 533 K, according to measured CO2 evolution rates and dissolved oxygen concentrations. This value is near the range of activation energies observed in gaseous air oxidation at low temperatures.

Performance of a First-Generation, Aqueous-Alkaline Biocarbon Fuel Cell

“…Conesa et al 14 reported almost identical results in a study of the oxidation of carbonized charcoal by oxygen dissolved in hot liquid water at 150°C. On the other hand, as mentioned previously, Tam et al 15 determined that carbonized charcoal particles burn steadily in boiling water that contains dissolved oxygen at temperatures at and above 240°C. The exact temperature at which ignition and steady combustion occursswith a rate of release of active sites that equals their rate of consumptionsis strongly influenced by the presence of catalytic mineral matter (e.g., potassium), 9,48 as well as the carbonization temperature of the charcoal 49,50,52,53 and its content of chemisorbed oxygen.…”

“…Carbonized charcoal particles burn steadily at 240°C and higher in boiling water that contains dissolved oxygen. 15 These findings suggest that biocarbons could be susceptible to electrochemical combustion at temperatures near 200°C. Aqueous-alkaline fuel cells function in this temperature range.…”

“…[7][8][9][10] Finally, carbonized charcoal is a good electrical conductor, 4 has a high surface area, and contains many dangling bonds that cause it to be extremely reactive at modest temperatures. 9,[12][13][14][15] Despite these attractive features, most researchers have emphasized carbon fuel cells that use fossil carbons (e.g., graphite, coke, and coal) as the consumable anode 1,2,16 with a variety of electrolytes as the charge carrier, including aqueous alkaline, [16][17][18][19] molten potassium hydroxide at >400°C, 16,[20][21][22][23][24][25][26] solid zirconia stabilized with magnesia or yttria at >1000°C, 27,28 molten lead at >500°C, 29,30 and molten carbonates at >560 °C. [31][32][33][34] In a recent paper, Cooper, Cherepy, and their coworkers 35 at the Lawrence Livermore National Laboratory described the performance of a molten carbonate carbon fuel cell that used nine different carbons (including two biocarbons) and a porous nickel electrode.…”

Mesoporous activated carbon prepared from macadamia nut shell waste by carbon dioxide activation: Comparative characterisation and study of methylene blue removal from aqueous solution

“…Air is the least expensive oxidant, and the highly reactive oxygen content enables activation at much lower temperatures than using other activators. However, it is difficult to control at industrial scale due to the strongly exothermic reactions . Steam activation is an established production technology but requires an industrial water supply and the steam generation that increase the capital and production costs.…”

Production and Characterization of Carbonaceous Adsorbents from Biomass Wastes by Aqueous Phase Carbonization

“…We focused on the carbonization of such moist residues in pressurized hot liquid water at moderate temperatures (<350 • C). On the other hand, a method to oxidize carbon in pressurized hot liquid water, which can provide an alternative to the conventional activation manner by steam or carbon dioxide treatment at higher temperatures, has been reported (Tam et al, 2001). Using some nut shell charcoals, activated carbons having specific surface areas of 500-1000 m 2 /g, were obtained in the aqueous method with less energy consumptions at remarkably high yields.…”