Black liquor, which accounts for the majority of the fuel consumed in Kraft mills, is usually combusted in recovery boilers to recovery chemicals and to produce process steam and on-site electricity (via a steam turbine). The efficiency of such boilers is, however, low (around 65-70%). Black liquor gasification is a process in which a clean synthesis gas (syngas) is produced from black liquor by converting its biomass content in to gaseous energy carrier. The syngas subsequently can be used in boilers or in combined cycle processes (utilizing gas turbines) to generate on-site electricity and/or process steam. The gasification produces a fuel gas that needs to be cleaned to remove undesired impurities for the power system and to recover pulping chemicals.

The potential advantages of black liquor gasification are the greater end use flexibility offered by a gaseous fuel, reduced air pollutant content, and higher electricity-to-heat ratios in combined cycle systems than standard recovery boiler steam turbine systems. Potential disadvantages of gasification combined cycle systems include the energy investments required for achieving sufficient black liquor solids concentration and higher lime kiln and causticizer loads (and associated fuel inputs) compared to Tomlinson recovery boiler systems. Additionally, since combined cycle systems generate electrical power more efficiently than steam turbine based systems, meeting the facility's steam demand may require the use of more fule. However, this additional fuel use also results in more available electricity for facility use or export to the grid.

The two main types of gasification – low temperature/solid phase and high temperature/smelt phase – are the two main categories of proposed options. In low temperature gasification, the gasifier operates below the melting point of the inorganic salts (700-750 °C). Fluidised beds are suitable for a low temperature gasification process, and are used in all of the low temperature processes under development. The other category includes those processes which operate above the melting point (~ 950°C) and use a water quench to cool and dissolve the molten sodium salts (BREF, 2010. p.308). Higher temperatures lead to higher carbon conversion rates but also may lead to more corrosion in the reactor vessel. The synthesis gas is water quenched (producing low-pressure steam) and cleaned before being fired in the turbine (Kramer et al., 2009. p.106-107).

 A study analyzing the potential of black liquor gasification for the US indicated that that on a thermodynamic basis, high-efficiency Tomlinson recover boiler systems would be more efficient at generating steam and power than low-temperature, mill-scale gasification systems. However, the study results also suggested that high-temperature, mill-scale gasification systems would be more efficient than high-efficiency Tomlinson boiler systems (Kramer et al., 2009. p.107).

Black liquor gasification technologies and applications are in continuous states of research and development. The potential benefits and costs of black liquor gasification – both environmental and economic – are likely to depend highly on the characteristics of individual installations and will be better understood as the technologies and applications are demonstrated and evaluated over time (Kramer et al., 2009. p.107).