Carbonate formation during lignin pyrolysis under CO2 and its effect on char oxidation

This work aims to study the transition of the chemical form of the alkali minerals during pyrolysis under a CO2 atmosphere, and its effect on char oxidation. Lignin containing a high concentration of Na was heated under a CO2 or Ar atmosphere, and the chemical form of Na in both chars was characterized using water extraction and capillary electrophoresis. These chars were subsequently gasified in air at various temperatures. Capillary electrophoresis analysis demonstrated that the organic-Na component of raw lignin was transformed into carbonate during pyrolysis under CO2. Almost all the Na in char formed under CO2 was a water-soluble carbonate, while water-insoluble Na was found only in char prepared under Ar. Water-insoluble Na was assumed to be R-COONa or -CNa in which Na was strongly bound with the char-matrix, making it resistant to water extraction. It was clearly shown that the chemical form of Na in lignin char was strongly dependent on the presence of CO2 during pyrolysis. Considering these experimental findings, a mechanism for transition of the chemical form of Na during pyrolysis under CO2 was proposed. In addition, significant differences between chars produced under CO2 and Ar appeared during gasification. The lowest temperature at which the oxidation of char formed under CO2 and Ar was 1073 K and 923 K, respectively. The major differences between chars prepared under Ar and CO2 were chemical form of sodium. The increase in oxidation temperature, therefore, was caused by transition of the chemical form of Na under CO2. In fact, -CNa, the most important catalyst in char oxidation, was not formed; although carbonate was formed during lignin pyrolysis under CO2. These results have important implications for the combustion or gasification of not only lignin but also Na ion-exchanged coal. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.