Solids flow

Drying Heating Calcination

GAS FLOW

FLUE GAS

LIME MUD

FUEL, AIR

BURNED LIME

Flame

Rotary kiln AIR

Satellite

Cooler

Temperature,.C

Solids

Gas

Fig. 9.15 Schematic of a lime kiln with temperature profiles of solids and gas.

The energy supply for the very endothermic calcination reaction usually comes

From firing of fuel oil or natural gas. Approximately 150 kg of fuel oil or 200 Nm3

Natural gas are needed per ton of lime product. The oxygen for fuel combustion is

Supplied by air. The flame extends into the calcination zone, where the major part

Of the energy is transferred by radiation. As the flue gas passes through the kiln,

Its temperature falls gradually. Only about one-half of the chemical energy in the

Fuel is consumed by the calcination reaction, while about one-quarter is needed

For evaporation of water from the lime mud. The remainder of the energy is lost

With the flue gas and via the kiln shell. The flue gas which exits the kiln carries

Dust and, depending on the type of fuel, also sulfur dioxide. It is cleaned in an

Electrostatic precipitator for the elimination of particulates and, if needed, in a wet

Scrubber for SO2 removal.

Chemical Recovery Processes 991

The Future of Kraft Chemical Recovery

9.2.3.3.1 Meeting the Industry’s Needs

The core technology of the Tomlinson-type chemical recovery boiler was developed

In the 1930s. Various improvements have been made since then, and especially

The energy efficiency has improved dramatically. However, certain inherent disadvantages

of today’s recovery systems are inflexibility regarding the independent

Control of sodium and sulfur levels in white liquor, as well as the safety risk connected

to explosions caused by smelt/water contact.

Future recovery technologies are challenged by the technical requirements of

Modern kraft cooking processes with regard to liquor compositions, by increasingly

stringent environmental demands, and last – but not least – by the industry’s

Everlasting strive for improving the economic efficiency of pulping. In fact,

The ongoing developments address all of these issues, and novel recovery techniques

Provide for the appealing long-term perspective that product diversification

Will once make pulp mill economics less dependent on pulp prices alone.

With regard to the near future, the two technologies which have conceivable

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