Kinetics of the Direct Causticizing Reaction Between Black Liquor and Titanates During Low Temperature Gasification, 2005 Engineering, Pulping & Environmental Conference

Gasification of kraft black liquor increases the amount of Na2CO3 which needs to be converted into NaOH in the lime cycle because one mole of Na2CO3 is created for each mole of gaseous sulfur produced during gasification, while the sulfur ends up as Na2S in conventional recovery. This disadvantage can be eliminated when kraft black liquor is gasified in the presence of titanates, leading to direct causticization of Na2CO3 in the gasification reactor. One of the black liquor gasification processes which is presently installed at the Georgia Pacific, Big Island is based on steam reforming of black liquor at about 600 °C. The purpose of the present research is to determine whether the sodium in black liquor reacts fast enough and at a sufficient yield with titanate compounds at relative low temperatures (≤ 750°C) so that the causticizing penalty of this steam gasification process may be eliminated. The kinetics of the direct causticizing reaction of mixtures of black liquor and TiO2 or Na2O.3TiO2 were determined in a novel reactor consisting of a stack of Al2O3 trays by on-line measurement of both the weight-loss of the mixtures and the composition of the off-gas after burning off most of the carbon by oxygen at 500°C. The variables studied are the CO2 concentration (0 to 10%), temperature (630°C to 740°C), and the type of liquor (reference material or soda black liquor). The final yield of NaOH from sodium in the black liquor was determined by titration. The results show high conversions of Na2CO3 at temperatures of 680°C are reached in the order of one hour, and confirm that direct causticization at low temperature gasification is possible. The kinetic data is modeled using different solid-solid reaction models.