Compression increases the vapor pressure, but at the same time the vapor is

Also superheated. The vapor must be de-superheated by injection of condensate

Before feeding it to the steam side of the heating element in order to make the

Heat transfer effective. The temperature rise across the fan compressor and desuperheater

is typically around 6 °C. The resulting driving temperature difference

Is low, and hence vapor recompression plants require comparatively large heating

Surfaces.

Vapor recompression systems need steam from another source for start-up.

Depending on the electrical power input and thin liquor temperature, they may

Also need a small amount of steam make-up during continuous operation. The

Specific power consumption for evaporation in a vapor recompression plant

Depends mainly on the boiling point rise, the heat exchange surface, and the thin

Liquor temperature. Typical specific power consumption figures range from 15 to

25 kWh t–1 of water evaporated.

9.2.3

Kraft Chemical Recovery

Kraft Recovery Boiler

Processes and Equipment

A kraft recovery boiler converts the chemical energy of the black liquor solids into

High-pressure steam, recovers the inorganics from the black liquor, and reduces

The inorganic sulfur compounds to sulfides.

Chemical Recovery Processes

Air pre-heater

BLACK LIQUOR

AIR

FEED

WATER

Smelt spouts

Primary air ports

Secondary air ports

Tertiary air ports

SMELT

Furnace

Forced draft fan

Superheaters

Steam drum

HIGH PRESSURE STEAM

Boiler bank

Economisers

Liquor guns

ASH

Induced draft fan

Electrostatic

Precipitator

Stack

FLUE

GAS

Fig. 9.8 Schematic of a kraft recovery boiler with single-drum design.

The recovery boiler consists mainly of the furnace and several heat exchange

Units, as illustrated in Fig. 9.8. Pre-heated black liquor is sprayed into the furnace

Via a number of nozzles, the liquor guns. The droplets formed by the nozzles are

typically 2–3 mm in diameter. On their way to the bottom of the furnace, the droplets

First dry quickly, and then ignite and burn to form char. After the char particles

Reach the char bed situated on top of the smelt, carbon reduces the sulfate to

Sulfide, forming carbon monoxide and carbon dioxide gases. Most of the inorganic

Black liquor constituents remain in the char and finally form a smelt at the

Bottom of the furnace, consisting mainly of sodium carbonate and sodium sulfide.

Some of the inorganic material is also carried away as a fume by the flue gas. The

Liquid smelt leaves the furnace through several smelt spouts.

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