Black Liquor Gasification

Modern chemical pulp mills produce black liquor that is roughly 80 percent solids. This solid is burned in recovery boilers to provide the steam needed to run the mills or to produce electricity through turbines. Another critical task of burning the solids in specialty Tomlinson boilers is to begin the process of recovering pulping chemicals to reduce the economic costs of operation. Black liquor gasification is an emerging commercial technology founded on decades of research and development. Its goal is to produce a combustible mixture of raw gases as well as separate out the inorganic pulping chemicals for recycling for the pulping process. The processes can take place at low, 600 degrees Celsius, or high temperatures around 1000 degrees Celsius (Larson et al. 2003).

ThermoChem Recovery International has commercialized the low-temperature process, where black liquor is indirectly heated to produce a hydrogen-rich gas. This process also produces a dry solid inorganic smelt that reduces the potential for smelt-water explosion hazards. ThermoChem has two commercial plants installed; Trenton, Ontario and one in partnership with Georgia Pacific in Big Island, Virginia. A Swedish company, Chemrec AB, uses a high temperature gasification system. This system can work in parallel with existing Tomlinson recovery boilers or serve as a replacement for old boilers. This replacement system has the capacity to double the biobased electricity produced and increase pulping capacity by 5 percent. Chemrec systems have been used across Sweden as well as in partnership with the Weyerhaeuser mill in New Bern, North Carolina.

The raw gases produced by the system can be converted in biobased syngas which can be used on-site or sold to the market (Brown 2003). The soilds that are produced in the black liquor can be problematic, particularly if the pulping chemicals were high in sulfur and sodium content. Gasification of black liquor has shown significant improvements over Tomlinson boilers with regards to SOx and NOx emissions and total reduced sulfur levels, thus improving future pulp yields. Detailed studies have also demonstrated that, when combined with a gas turbine, black liquor gasification can produce enough energy to make the pulping industry a net exporter of energy (Larson et al. 2003).

Chemrec AB's Black Liquor Gasifier