Effects of metal salts on char oxidation in pectins/uronic acids and other acid derivative carbohydrates

Extensive experimental investigations on oxidative reactivity of pectins and related carbohydrates have revealed a high temperature oxidation reaction above 600 °C. Most biomass chars typically oxidize in the temperature range from 400 to 500 °C. This work reports characterization of this high temperature oxidation process, which is not limited to pectins, but seems to be specific to acid carbohydrate salts. Oxidation studies were conducted with a thermogravimetry/differential scanning calorimetry/mass spectrometry instrument. Significant temperature variations in pectin oxidation behavior were dependent on free sugar content, inorganic content, degree of esterification, and pyrolysis conditions. Results indicate alkali metals, such as sodium or potassium, bound in pectins initially as the salt of the uronic acid groups, decompose during pyrolysis between 200 and 400 °C to produce a metal complex in the char which apparently inhibits/shields the carbon char from oxidation at the ‘typical’ oxidation temperatures. Temperatures in excess of 600 °C are needed, in either oxygen or inert gas, to decompose the metal–char complex and allow oxidation of surrounding carbon to proceed.