Or manganates in combination with BLG. The reactions in the gasifier

Release carbon dioxide to the product gas. Titanates are more efficient at converting

Carbonates to carbon dioxide than are manganates. The metal oxides proceed

From the gasifier to a leaching step, where sodium hydroxide is formed in the

Presence of water. The insoluble metal oxides are then separated from the liquor

and returned to the gasifier [27].

Vision Bio-Refinery

Although a general breakthrough in novel recovery techniques is not expected

before 2010–2015, it is likely that over the next decades a number of technologies

Will emerge to match certain applications. Fully commercialized BLGCC applications

Will add substantial flexibility to pulp mill operations, and will represent a

Most important step towards the pulp mill as a bio-refinery. Developments in the

Future may then involve the production of liquid biofuels from product gas, export

of pure hydrogen or fabrication of hydrogen-based products [28].

9.2.4

Sulfite Chemical Recovery

The recovery of cooking chemicals from the sulfite pulping process can be split

Into primary and secondary recovery steps. This definition relates to the recovery

Of sulfur dioxide (see Fig. 9.17). In sulfite cooking, gas is continuously relieved

From the digester for pressure control during the time at temperature, and the

Cook is also terminated by a pressure relief. The resulting relief gas contains considerable

Amounts of sulfur dioxide together with a bulk of water vapor and some

NCG such as carbon oxides. In the primary recovery system, this gas is subjected

To countercurrent absorption by fresh cooking acid in a number of vessels operated

Under staged pressure levels.

Recovery

Cooking /

Washing COOKING

CHEMICAL

CYCLE

Evaporation

Recovery

Boiler

Secondary

Recovery -

SO2 absorption

From flue gas

Primary

Recovery -

SO2 absorption

From relief gas

RELIEF GAS

RECYCLE

Fig. 9.17 The sulphite cooking chemical cycle.

Following evaporation, thick spent sulfite liquor is usually fired in a recovery

Boiler under an oxidative environment. Sulfur leaves the boiler in the form of SO2

With the flue gas, and is subsequently absorbed from the flue gas in the secondary

Recovery system. The design of both recovery boilers and secondary recovery systems

Is largely different between sulfite cooking bases. While magnesium and

Sodium bases can be recovered from the spent cooking liquor and re-used for

Cooking acid preparation, the recovery of calcium and ammonium bases is not

Practicable.

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