Potential to change the face of kraft chemical recovery are black liquor gasification

(BLG) and in-situ causticization. Major achievements have been made in these

fields since the 1990s, and commercialization is currently in progress [19–22].

Black Liquor Gasification

Black liquor gasification is founded chemically on the pyrolysis of organic material

Under reducing conditions, or on steam reforming with the objective to form

A combustible product gas of low to medium heating value. The main gasification

Products are hydrogen, hydrogen sulfide, carbon monoxide, and carbon dioxide.

Gasification processes are divided into low-temperature techniques, where the

Inorganics leave the reactor as solids, and into high-temperature techniques,

Which produce a slag. Both gasification types allow the separation of sodium and

sulfur, and both bear insignificant risk of smelt/water incidents.

High-temperature gasification occurs at about 1000 °C in an entrained flow reactor.

Air is used as oxidant in low-pressure gasifiers, whereas high-pressure systems

Operate with oxygen. The black liquor decomposes in the reactor to form product

Gas and smelt droplets, both of which are quenched after exiting the reaction

Chamber. The smelt droplets dissolve in weak wash to form green liquor. The

product gas serves as fuel after particulate removal and cooling [23].

Low-temperature gasification is carried out in an indirectly heated fluidized bed

reactor, with sodium carbonate bed material and at a temperature around 600 °C.

Superheated steam provides for bed fluidization, and the required energy is supplied

By burning a portion of the product gas in pulsed tubular heaters immersed

In the bed. Green liquor is produced from surplus bed solids. The product gas

proceeds to cleaning and further on to utilization as a fuel [24].

Due to the separation of sulfur to the product gas, the salts recovered from gasification

Have a high carbonate content. Despite the flexibility in producing cooking

Recovery

Liquors of different compositions, the overall mill balance for sodium and sulfur

Must be observed. Selective scrubbing of hydrogen sulfide from the product gas

And absorption in alkaline liquor is a must for kraft mills which operate at typical

Sulfidity levels. Depending on the set-up of hydrogen sulfide absorption, mills

may run into increased loads on causticizing and lime kiln processes [25].

BLG is particularly energy efficient when applied together with combined-cycle

technology – that is, when the product gas is burned in a gas turbine with subsequent

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