BLACK LIQUOR GASIFICATION
Black liquor gasification is a specialized field (see also Section 4.3.2). Particular difficulties are the high inorganic load, particularly of sodium, and the requirement for recovering this for recycling. In addition, the sodium poses problems for conventional refractory solutions. Add to this the concern about the potential for tar formation from the lignin content, and it is understandable why only a few companies have attempted to realize this technology.
In comparison with conventional combustion technology (Tomlinson Boilers), pressurized black liquor gasification can increase the energy recovery in the pulping process from 65% using the most modern combustion equipment to about 75%. Compared with much of the existing installed equipment, of which the majority is 30 years or older, electric energy generation can increase by a factor of two to three. The quality of green liquor from a gasifier can provide process advantages, since sodium and sulfur are recovered separately for recycling to the digester, offering the opportunity for increased pulp yield and quality. The causticizing load does increase, however, which is a disadvantage.
In addition, the risk of a smelt-water explosion involved in conventional boiler technology is absent when gasifying black liquor due to the small smelt inventory in the process.