Calorimeter, whereas with kraft black liquor it always appears as sodium sulfide

(Na2S). The reduction process of Na2S to Na2SO4 is endothermic by 13 090 kJ kg–1

of Na2S. The NHV can be calculated according to the following expression:

NHV _ HHV _ 2440 _

_2 _ H__ 13090 _

S _ g_ RED_ _1_

Recovery

where NHV is the net heating value of black liquor solids (BLS; in kJ kg–1 BLS);

HHV is the higher heating value of BLS (in kJ kg–1 BLS); H and S are the weight

Fractions of hydrogen (H) and sulfur (S) in BLS; and gRED is the degree of reduction

Given as a weight fraction.

Typical values for the HHV of kraft black liquor range between 13 MJ kg–1 BLS

(predominantly derived from hardwoods) and 15.5 MJ kg–1 BLS (predominantly

Derived from softwoods), as indicated in Tab. 9.2.

It should be noted that the recycling of bleach (e.g., oxygen delignification) and

Purification (e.g., cold caustic extraction) filtrates has an impact on the composition

And heating value of the BLS due to a generally lower content of organic compounds.

Tab. 9.2 Chemical analysis and heating values of black liquor solids [5–8].

Components A B C D E F G

Wood species Unit hardwood hardwod hardwood softwood softwood softwood softwood

Elemental analysis

C wt% on DS 32.3 33.3 33.4 35.8 37.8 35.8 38.0

H wt% on DS 3.8 3.6 3.9 3.5 4.2 3.6 3.8

N wt% on DS 0.2 0.1 0.1 0.2 0.1

O wt% on DS 35.8 33.6

S wt% on DS 3.0 5.4 4.4 4.1 4.8 4.6 3.7

Na wt% on DS 18.2 19.9 20.7 19.9 17.9 19.6 19.2

K wt% on DS 3.0 1.5 1.7 1.1 1.2 1.8 0.6

Cl wt% on DS 0.7 0.6 0.3 0.2 2.9 0.5 1.0

HHV MJ kg–1 13.2 13.2 13.2 14.1 15.4 15.1

NHV (calculateda) MJkg–1 11.5 10.9 11.1 12.2 13.1 13.2

Reference [5] [6] [6] [6] [6] [7] [8]

a. Assuming a degree of reduction of 90%.

Characterization of Black Liquors 969

9.1.2

Physical Properties

The most important physical properties which affect evaporator and recovery boiler

Design and operation include liquor viscosity, boiling point rise, surface tension,

Density, thermal conductivity and heat capacity.

Viscosity

The viscosity of the black liquor is determined by its composition, the dry solids

Content, and temperature. At low shear rates, black liquor behaves as a Newtonian

Fluid, and the macromolecular components such as lignin and polysaccharide

molecules control the rheological properties. At a dry solids content of about 15%,

The viscosity of the black liquor is only three-fold that of water at a given temperature.

At about 50% solids content, however, black liquor behaves as a polymer

Blend with water as plasticizer, and the viscosity increases exponentially with solids

Content. The relationship between dry solids content and viscosity at a low

shear rate is expressed in Eq. (2) [8]:

Log

Lbl

Lw _ __

DS _ 373

T

DS _ 373

T

_2_

where lbl is the viscosity of black liquor (in Pa.s); lw is the viscosity of water (in Pa.s):

DS is the weight fraction of dry solids in black liquor; and T is temperature (in K).

According to Eq. (2), viscosity can change by five orders of magnitude over the

Range of dry solids contents typical for kraft black liquor recovery.

Дата добавления: 2015-10-21