Characterization of Tar from a Fluidized Bed Steam Reformer of Black Liquor

Black liquor is a big energy source and a crucial component for the economy of the pulp and paper industry. Black liquor is conventionally burned in the Tomlinson recovery boiler. However, black liquor gasification combined cycle (BLGCC) has some advantages over the conventional technology, like superior energy efficiency, that makes this technology suitable to replace the traditional boiler. Low-temperature steam gasification converts the black liquor to a quality synthesis gas. This gas contains H2 and CO and, in addition, some gaseous impurities and inert gases such as CO2, H2O and N2. These gaseous impurities, like tar, might condensate and cause downstream problems such as plugging and corrosion, among others. Especially when gasifying at low temperatures, these impurities are a big drawback for the commercialization of this technology. The steam reformer of black liquor at the University of Utah was operated at different operating conditions to evaluate tar production and some tar characteristics. Two sample collecting systems were tested: an impinger sampling train and a so-called Petersen column. Total tar concentration for different gasification conditions was satisfactorily obtained using the impinger train. Increasing the reformer temperature, decreasing the liquor feed rate, increasing the fluidizing steam temperature and co-injecting air in to the system all decreased tar production. On the other hand, changing the superficial gas velocity did not seem to affect tar concentration for the range of velocities considered. Tar concentrations varied from 36.2 g/m to 140.2 g/mof dry gas. Tar characterization was performed using different techniques that would provide a better understanding of its physical and chemical properties. A series of analyses were performed at the University of Utah; gas chromatography-flame ionization detection (GC-FID) analysis, gas chromatography-mass spectrometry (GC-MS) analysis and, thermogravimetric analysis (TGA). In addition, selected samples were sent to Huffman Laboratories, Inc. for elemental analysis. These analyses suggested that the chemical composition of the tar samples were very similar for all the experimental conditions tested. Tar samples mainly showed the presence of phenolics and both substituted and unsubstituted multi-ring structures with high concentrations of single-ring aromatics and phenolic compounds. It was concluded from this study that tar produced in a fluidized bed black liquor steam reformer is a very complex and broad mixture of hydrocarbons, and that the product gases would require strict temperature control to avoid tar condensation downstream. Gasification temperature seemed to be the parameter that most affected tar production, followed by black liquor flow rate.