Heat recovery by steam generation (BLGCC; see Fig. 9.16). In such a case,

The yield of electrical power from pulp mill operations can be increased by a factor

Of two compared with the conventional power generation by steam turbines alone.

The related benefits range from the income generated from selling excess electrical

Power to the environmental edge of replacing fossil fuel elsewhere.

BLACK LIQUOR

GASIFIER

GAS COOLING

AND CLEANING

SULPHUR

RECOVERY

Product gas

Condensed phase

(salts, smelt)

Thick

Black

Liquor

Clean syngas

GAS TURBINE

Sulphur

COOKING LIQUOR

PREPARATION

EVAPORATION HEAT RECOVERY STEAM TURBINE

Exhaust gas

Electricity

Steam

MILL USERS

AND EXPORT

Electricity

Flue gas to stack Steam to process

Fig. 9.16 The principle of black liquor gasification with combined cycle (BLGCC).

At present, some BLG installations are operating on a mill scale, mostly providing

Incremental capacity for handling black liquor solids. The encountered difficulties

Are mainly the adequate carbon conversion for the low-temperature process

And the choice of materials for the high-temperature process. With regard to

Black liquor solids, the capacities of the currently installed systems are less than

10% of today’s largest recovery boilers. When the process and material issues are

Settled, appropriate scale-up will be the next challenge. Nevertheless, it is expected

That gasifier-based recovery systems will operate a number of reactors in parallel

Because physical limitations restrict the maximum size of a unit.

With regard to the combined-cycle systems, the BLG processes must be followed

By efficient gas-cleaning steps. Cleaning of the synthesis gas is especially needed

Chemical Recovery Processes 993

Because some volatile tar is formed during gasification, and this must be kept

From entering the gas turbine.

In-Situ Causticization

The second group of technologies on the brink of commercialization includes

Autocausticization. This is based on the formation of sodium hydroxide in the

Recovery furnace by means of soluble borates circulating in the cooking chemical

Cycle. Under certain conditions of causticizing or lime kiln limitations, partial

Autocausticizing can remove a bottleneck. Mill trials have demonstrated the technical

Feasibility, improved causticizing efficiency, and energy savings in lime

reburning, and the process has been applied in one mill [26].

Another technique of in-situ causticization is that of direct causticizing. The process

Is still in the conceptual phase, and builds on the formation of sodium titanates

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