As minor streams are neglected, such as dust, sulfur dioxide, reduced sulfur compounds

(TRS), carbon monoxide and nitrogen oxides (NOx) in the flue gas, as well

As other inorganic matter and unburned carbon in the smelt.

Energy Balance

Once the material balance of the recovery boiler has been calculated, a rough energy

Balance is easily obtained (see Tab. 9.4). At first, the enthalpies of input and

Output streams to the boiler are listed. Output streams have negative enthalpies.

The reaction enthalpy is then calculated from the higher heating value (HHV) of

The black liquor solids. Since the major part of the sulfur leaves the boiler in a

Reduced state, the corresponding energies of reduction must be subtracted from

The HHV. The energy available for steam generation results from summing up all

The stream and reaction enthalpies. In our example, the heat to steam amounts to

9.9 GJ t–1 black liquor solids. We assume a feedwater of 120 °C and 95 bar, as well

as high-pressure steam of 480 °C and 80 bar. Then, the gross amount of steam

Generated is 3.5 tons per ton of black liquor solids. Note that some of the generated

steam is consumed by the boiler itself. Sootblowing steam, steam for air/

Liquor pre-heating and feedwater preparation need to be deducted from the gross

Steam generation to obtain the net steam quantity available for the mill.

The data in Tab. 9.4 show that the humidity of the flue gas accounts for a considerable

Energy loss from the boiler. The humidity comes mainly from the water

In the black liquor, from water formed out of hydrogen in organic material, and

From sootblowing steam. Increasing the dry solids concentration of the black

Liquor, and thereby reducing the water input to the boiler, leads to a higher steam

Generation per mass unit of black liquor solids (Fig. 9.9).

Chemical Recovery Processes

Tab. 9.4 Simplified recovery boiler heat balance.

System input/output Mass

[kg ton–1 dry solids]

Specific enthalpy

[kJ kg–1]

Enthalpy

[MJ ton–1 dry solids]

Enthalpy of input/output streams

Black liquor 1.333 2.8. 130 485

Pre-heated air (dry) 4.909 1.0. 120 589

Humidity of pre-heated air 70 2.725 190

Sootblowing steam 100 2.820 282

Flue gas (dry) 5.137 0.96. 180 –888

Humidity of flue gas 827 2.840 –2.349

Smelt 448 1.500 –672

Reaction enthalpy

HHVof black liquor solids 1.000 14.000 14.000

Reduction to Na2S 89 13.090 –1.170

Reduction to K2S 25 9.625 –244

Losses –300

Heat to steam 9.923

Feedwater/steam

Feedwater 3.494 510 1.782

Total steam generation 3.494 3.350 11.705

90%

95%

100%

105%

110%

60% 70% 80% 90%

Relative steam generation

Black liquor solids concentration, wt.-%

Fig. 9.9 Steam generation in a recovery boiler as a function of

The black liquor solids concentration; typical curve normalized

to 100% at 75% solids concentration.

Causticizing and Lime Reburning

Overview

The causticizing and lime reburning operations target at the efficient conversion

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