Black Liquor
Black liquor is an integral part of the papermaking process and represents about half the mass of the wood entering the pulping plant. It is the principal source of energy for the pulping process. In addition to the lignin and hemicellulose from the pulp wood, it also contains the cooking chemicals, which contain significant quantities of sodium and sulfur, the recovery of which for recycling to the pulp digesters is essential for the economy of the process.
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Table 4-15 Properties of Animal Biomass |
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HHV |
Moisture |
Volatiles |
Ash |
|
|
MJ/kg |
wt% |
wt% |
wt% |
|
|
Meat and Bone Meal (SRM) |
20.1 |
2.54 |
65.27 |
26.13 |
|
Meat and Bone Meal (HRM) |
18.9 |
5.03 |
63.7 |
25.54 |
|
Meat and Bone Meal (Blood) |
23.3 |
14.96 |
93.82 |
1.45 |
|
Meat and Bone Meal (MBM) |
20.0-28.0 |
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|
Poultry Litter |
13.0-14.0 |
63 |
25 |
20 |
|
Source: Schellberg and Pena 2002; Arbon 2002; and Bajohr 2002 |
Total black liquor production worldwide is an energy resource of about 600TWh/y or 51 million toe/y. This represents about 0.5% of the total world energy consumption. In a country such as Sweden, with a strong pulping sector, black liquor can be as much as 6% of the national gross energy consumption. An average-sized mill produces 1000 air-dried tons of pulp per day, generating some 1800tDS/d (tons dry solids per day) of black liquor. On an HHV basis this is about 300 MW.
In a typical pulping process (see Figure 4-6) wood chips are cooked with sodium hydroxide and NaHS in a pulp digester where lignin and other organic material, which contributes almost half the mass of the wood, is dissolved into the white liquor
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Figure 4-6. Gasification of Black Liquor |
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Table 4-16 |
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Composition of Black Liquor (Dry) |
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|
Element |
wt% |
|
C |
36.40 |
|
H |
3.50 |
|
о |
34.30 |
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N |
0.14 |
|
s |
4.80 |
|
Cl |
0.24 |
|
Na |
18.60 |
|
К |
2.02 |
|
Total |
100.00 |
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Source: Marklund 2001 |
containing the caustic. The solids content of the weak black liquor leaving the digester is about 15 wt%. This is increased to about 70 wt% in evaporators to leave a combustible pumpable material that in traditional processes is completely combusted in a special (black liquor) recovery or Tomlinson boiler. The inorganic material, mostly NaS and NaC03, leaves the boiler as a smelt that is recovered by causticizing for reuse in the digester.
The use of black liquor gasification is a relatively new development. It is located in the same position in the liquor circuit as the boiler (see Figure 4-6). In some applications black liquor gasifiers have been built in parallel with existing boilers as part of a de-bottlenecking project.
Typically, the strong (evaporated) black liquor has a viscosity of 70-120 cP and a density of 1400 kg/m3. The heating value is about 13.5-14 MJ/kg (HHV).
A typical elemental analysis of the dry material in black liquor is shown in Table 4-16.