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)/

12 = 1294 kg.

983

9 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

1.2 . 953 = 1144 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,

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.

9.2.3.1.3 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