Borate Autocausticizing During Char Bed Burning, 2004 International Chemical Recovery Conference

Borate has shown significant potential as an autocausticizing agent in the kraft recovery cycle. By reducing the causticizing load on the lime cycle, borate could increase the recovery cycle capacity at many mills. This project focused on obtaining additional data to understand the behavior of borate in the combustion environment of a recovery boiler char bed.

The IPST char bed reactor was designed to study and understand particulate formation during the combustion of black liquor char. In this study the reactor was used to generate data on the reaction of borate in a burning char bed environment. Testing was performed with various levels of borate (calculated as the amount required to reach 0%, 12%, 24%, 50% and 100% of autocausticizing), and different levels of oxygen (5%, 13%, 21%) in the feed gas.

Cooled smelt samples collected after the burning tests were analyzed to determine the chemical composition and calculate the extent of borate autocausticizing reactions. The addition of borate to black liquor in the char bed reactor resulted in higher levels of sodium hydroxide in the re-dissolved smelt, confirming that borate is acting as an autocausticizing agent. Sodium hydroxide levels increased from 5% to 35% (by weight) of the smelt at the maximum borate levels, while the sodium carbonate dropped from 65% to 10%.

The char burning rate and the bed temperature both increase with increasing amounts of oxygen in the combustion air. The rates and temperatures measured were very similar to earlier char burning work. The presence of borate does not affect the char burning rate or bed temperature at autocausticizing levels less than 50%; however, at autocausticizing levels at 50% and 100% the endothermic borate reactions result in a drop in the bed temperature.