Effects of titanium dioxide as direct causticization agent on the pyrolysis and steam gasification characteristics of straw black liquor solid

Black liquor gasification process with direct causticization using titanium dioxide (TiO2) is considered a promising technology for black liquor recovery in the future. In this study, laboratory-scale pyrolysis and steam gasification experiments with direct causticization using TiO2 for straw black liquor solid were performed in a thermogravimetric analyzer and a tube furnace, respectively. Effects of TiO2 on the pyrolysis and steam gasification characteristics are discussed on the basis of product analysis. The results show that after adding TiO2 at temperature higher than 550°C, the direct causticization reactions happened, while the thermogravimetric behavior of the black liquor solid along with releasing pattern of carbon dioxide and carbon monoxide was changed. In the presence of TiO2, swelling and agglomeration were prevented during the pyrolysis process. Meanwhile, sodium emission was significantly decreased and the steam gasification rate of the black liquor char at 850°C was substantially improved. The melting process during gasification was suppressed with direct causticization. Moreover, the yield of hydrogen was kept constant while the yields of carbon dioxide and carbon monoxide were increased during the steam gasification process with direct causticization.