Is required before the boiler ash is mixed. In addition, any chemical make-up

Must be considered and the resulting composition is taken as the starting point

For the calculation.

The computation is performed line by line. First, it is assumed that potassium

And chlorine react completely to potassium sulfide and sodium chloride, respectively.

In this simplified model, all the potassium from the black liquor (18 kg t–1

Of black liquor solids) turns into K2S in the smelt. Using the molecular weights of

potassium (39 kg kmol–1) and sulfur (32 kg kmol–1), the sulfur bound in K2S is

then 18. 32/(2. 39) = 7 kg t–1 of black liquor solids. The remaining sulfur,

Kg, is distributed between sodium sulfide and sodium sulfate according

to the degree of reduction, DR, also termed the “reduction efficiency”:

DR _

Na2S

Na2S _ Na2SO4 _11_

Values for the chemicals in Eq. (11) can be inserted on a molar basis, equivalent

basis or sulfur weight basis, all of which give the same result. Assuming 95%

Chemical Recovery Processes

Tab. 9.3 Simplified calculation of smelt and flue gas constituents from black liquor solids.

System

input/output

Composition

[wt.%]

Smelt constituents

[kg ton–1 dry solids]

Flue gas constituents

[kg ton–1 d.s.]

Na2CO3 Na2S K2S Na2

SO4 NaCl N2 H2O CO2

O2

Black liquor solids 100%

Potassium, K 1.8% 18

Chlorine. Cl 0.5% 5

Sulphur. S 4.6% 37 7 2

Sodium. Na 19.6% 137 53 3 3

Carbon. C 35.8% 36 322

Hydrogen. H 3.6% 36

Oxygen. O 34.1% 143 4 288 859 –953

Smelt total 316 89 25 9 8

Air 100.0%

Nitrogen. N 75.6% 3.765

Oxygen. O 23.0% 1.144

Humidity 1.4% 70

Water and steam

Water in black liquor 333

Soot blowing steam 100

Flue gas total 3.765 827 1.181 191

reduction efficiency, 0.95. 39 = 37 kg sulfur are with Na2S, and the remaining

2 kg are with Na2SO4. Next comes sodium, with 37. (2. 23)/32 = 53 kg bound to

Na2S, 2. (2. 23)/32 = 3 kg in Na2SO4 and 5. 23/35.5 = 3 kg in NaCl. The

remaining sodium is converted to sodium carbonate: 196 – 53 – 3 – 3 = 137 kg.

Na2CO3 binds 137. 12/(2. 23) = 36 kg carbon. The rest of the carbon is oxidized

To CO2. Hydrogen from the black liquor is converted to water vapor. Finally, the

Oxygen demand can be calculated by summing up oxygen bound in carbonate,

sulfate, water vapor and carbon dioxide:

137. (3. 16)/(2. 23) + 2. (4. 16)/32 + 36. 16/(2. 1) + 322. (2. 16)/

Kg.

Recovery

As the black liquor solids contain just 341 kg of oxygen per ton, 953 kg must be

provided with combustion air. Assuming 20% excess air, the oxygen in air is

Kg. Nitrogen and humidity follow from the air composition. For

Calculating the total flue gas flow, we need to consider the water content of the

black liquor and the steam used for sootblowing. Supposing 75% black liquor solids,

the water coming with 1 ton of solids is 1000/0.75 – 1000 = 333 kg. The final

Total is about 450 kg of smelt and 6000 kg of wet flue gas per ton of dry liquor

Solids. The flue gas mass is equivalent to a volume of around 4800 standard cubic

Meters. Note that the above is a quite rough approach to the boiler mass balance,

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