structure and behaviour of inorganics in …users.abo.fi/rbackman/515_516.pdf · structure and...

LIEKKI 2 Technical Review 1993 - 1998 1073

STRUCTURE AND BEHAVIOUR OF INORGANICS INRECOVERY BOILERS - A MODELING APPROACH

Rainer Backman, Sonja Enestam, Ron Zevenhoven

Åbo Akademi UniversityCombustion Chemistry Research GroupLemminkäisenkatu 14-18 BFIN-20520 Turku, Finland

Phone: +358 (0)2 2154762Fax: +358 (0)2 2154780Internet: http://www.abo.fi/ktf/ccrg

The dust in the flue gases of a recovery boiler consists of two major fractions: a)

coarse (10 - 1000 µm) carryover particles, generated from black liquor droplets

carried over with the flue gases to the upper furnace, and b) submicron-sized

fume, generated either from the smelt bed or at the pyrolysis stage of the black

liquor combustion. The amount of the two dust fractions varies with location in

the boiler. The highest amount of carry over particles are found in the upper

furnace and superheaters. Fume is the dominating fraction in the colder parts of

the flue gas channel.

The melting behavior of the two dust fractions plays an important role in the

formation and growth of superheater and boiler bank deposits. Two characteristic

temperatures have been identified to describe the tendency of deposit formation

[1]. These are the sticky temperature and the flow temperature, also called the

initial deformation and radical deformation temperatures. The sticky temperature

is the temperature where the amount of liquid in the material is high enough for

the mixture to stick on a metal surface. The flow temperature is that where the

amount of liquid is high enough for the mixture to flow down a vertical surface. It

has been shown [2, 3] that the sticky temperature is reach at 15% melt content and

the flow temperature at 70% melt content. These two characteristic temperatures

are also referred to as T15 and T70.

1074 LIEKKI 2 Technical Review 1993 - 1998

The first melting point, T0, is of particular interest for the study of corrosion of

superheater tubes or floor tubes. Surface temperatures near the first melting point

can be exposed to reactive molten phase due to local temperature fluctuations. In

the smelt bed, the melting properties play an important role as well for the

physical processes as for the heat transfer. Knowledge of the temperature range

where both liquid and crystalline material can be present is valuable.

In this paper the theoretical background for a thermodynamic melting model for

alkali salts is presented. The model has been developed based on literature data,

laboratory scale measurements, and large scale measurements in boilers. It can

predict the melting range as well as other thermodynamic properties of mixtures

of sodium and potassium carbonates, sulfates, sulfides and chlorides. The model

has been used in a variety of applications in recovery boiler design, operation

optimization and trouble shooting. In this paper we particularly present an

application to the dynamic thermal modeling of the smelt bed.

Projects

515 The Structure and Behaviour of Salts in Kraft Recovery Boilers

516 New Black Liquor Combustion Characteristics III

523 The Fate of Liquor Droplet Residues Carried Over from the Lower Furnace

in Kraft Recovery Boilers


PHASE CHEMISTRY OF SALT MIXTURES

The pure salt components in the system (Na, K)(CO3, SO4, Cl, S), including

carbonates, sulfates, chlorides and sulfides of sodium and potassium, have their

melting points in the temperature range 771 - 1180°C (Table 1). Potassium

chloride is the lowest melting component and sodium sulfide the highest melting.

At pure component melting points the melting is sharp and complete, i.e. all

crystals are transformed to liquid when the melting point is reached. Vice versa

the crystallisation is sharp on cooling a pure salt liquid. Pure salts of sodium and

potassium exhibit no tendency of supercooling.

Table 1. Melting points for pure salt components [4, 5].

Component

KCl 771NaCl 801Na2CO3 858Na2SO4 884K2CO3 901K2S 948K2SO4 1069Na2S 1180

Mixtures of several alkali salts behave very differently compared to the pure

components. Extensive solid solubility is observed between many of the

components [6]. Table 2 summarizes the solution phases identified. In liquid

phase, all components are miscible in each other, i.e there is only one liquid

present in multicomponent systems. At temperatures above some 500°C, Na2SO4,

K2SO4, Na2CO3 and K2CO3 are miscible with each other in the whole composition

range. They form a solid solution with hexagonal crystalline structure. NaCl and

KCl are completely soluble into a halite phase above some 500°C. The crystalline

stucture is cubic. Recent work [7] indicates that also Na2S and K2S are completely

soluble into each other. These three solid solutions, the hexagonal, halite and

sulfide are the once in equilibrium with liquid at the first melting point of

multicomponent salt mixtures. At lower temperatures, some other solid solutions


can form. The orthorombic sulfate/carbonate is stable up to 626°C in potassium

sulfate rich mixtures and the α-form of K2CO3 up to 422°C in potassium carbonate

rich mixtures. Glaserite is a solid solution of sodium and potassium sulfate near

the composition 2K2SO4·Na2SO4. It is stable up to 470°C. The α-forms of Na2SO4

viz. Na2CO3, both can dissolve a few percent of the corresponding potassium

salts. The low temperature solid solutions are not important for the melting

behavior, but they can play a significant role in sintering processes.

In order to study the phase relationsships and the melting behavior in

multicomponent salt systems, a consistent method of dividing the bulk system

into subsystems must be used. One important feature of the salt systems is the

reciprocity. This means that in a recation between two components, the anions (or

cations) switch places:

Na2SO4 + K2CO3 = K2SO4 + Na2CO3

This reaction can take place both in solid and liquid state. A reciprocal system is

ternary to its nature, in spite of the four components reacting. Taking into account

the extensive solid solubility of alkali salt components, it is convenient to divide

salt mixtures into a) common cation binaries, b) common anion binaries, and c)

reciprocal systems (Table 3). Higher order systems are regarded as combinations

Table 2. Chemical phases and their constituents in the system (Na,K)(CO 3,SO4,S,Cl) [6, 7, 8].

Na2SO4 Na2CO3 Na2S NaCl K2SO4 K2CO3 K2S KCl

Liquid x x x x x x x x

Hexagonal ss x x x x

Halite x x

Sulfide ss x x

Orthorombic ss x x x x

K2CO3-a x x x x

Glaserite x x

Na2SO4-a x x

Na2CO3-a x x


of these subsystems. Thus, the system Na2SO4-KCl, is not a true binary system,

but a ternary, represented by the reciprocal (Na,K)(SO4,Cl). (Both K2SO4 and

NaCl can form if Na2SO4 and KCl are mixed and heated.)

The composition of the salt system considered with 8 "end component salt", can

be described unambigously by four characteristic number, choosen in this work to

be:

Sulfidity SU = portion of alkali as sulfur compounds = (S+SO4)/(Na2+K2)

Reduction RE = portion of total sulfur as sulfid = S/(S+SO4)

Potassium KA = portion of alkali as potassium = K/(Na+K)

Chloride CL = portion of alkali as chloride = Cl/(Na+K).

The symbols are (molar scale in mixture):

Na2 = content of Na2 S = molar content of sulfide ions

K2 = content of K2 SO4 = molar content of sulfate ions

Na = content of Na (2Na2) Cl = content of chloride ions

K = content of K (2K2).

Table 3. All true binary and some ternary subsystems in the system(Na,K)(CO3,SO4,S,Cl).

Common cation binariesSodium Potassium

Na2CO3-Na2SO4 K2CO3-K2SO4

Na2CO3-Na2S K2SO3-K2SNa2CO3-NaCl K2CO3-KClNa2SO4-Na2S K2SO4-K2SNa2SO4-NaCl K2SO4-KCl

Na2S-NaCl K2S-KCl

Common anion binaries Some reciprocals (ternaries)

Na2CO3-K2CO3 Na2CO3-Na2SO4-K2CO3-K2SO4

Na2SO4-K2SO4 Na2CO3-NaCl-K2CO3-KClNa2S-K2S Na2SO4-NaCl-K2SO4-KClNaCl-KCl Na2CO3-Na2S-K2CO3-K2S


Based on these numbers the bulk composition in terms of salt components can be

calculated as demonstrated in Table 4. The carbonate content is calculated based

on the charge balance. Note that the same bulk composition can be obtained in

several ways by mixing different salt components. The bulk composition given

like this by the minimum number of componets required, is not connected to the

equilbrium composition, which is a function of temperature. Table 4 also shows

the equilibrium composition at 400°C.

Table 4. Equivalent bulk compositions (wt %) for a salt mixture with thecharacteristic numbers: SU = 0.43, RE = 0.11, KA = 0.10 and CL = 0.05.Composition IV corresponds to chemical equilbrium at 400 °C.

I II III IV

Na2CO3 45.3 45.3 36.7 40.4

Na2SO4 32.6 38.4 44.2 39.4

Na2S 3.0 3.0 3.0 3.0

NaCl 4.8 4.8 4.8

K2CO3 7.2 11.3 6.5

K2SO4 14.3 5.9

K2S

KCl 6.1

100.0 100.0 100.0 100.0

Modelling the melting behavior

The melting behavior of complex salt mixtures can be studied theoretically by

thermodynamic phase modelling. This, again, is based on experimental work

involving determination of thermodynamic properties for pure components as

well as for mixtures. The occurrence of solid solutions and first melting points far

below the pure component melting points, make it necessary to use non-ideal

phase descriptions in order to get a good estimate for the melting behavior of

multicomponent mixtures. Reasonably good experimental data are available for

binary systems, but on the higher order systems very few experimental studies

have been done.


In the present work, first melting and final melting temperatures for 30 well

defined samples in the system (Na,K)(CO3,SO4,Cl) were determined by thermal

analysis. A detailed description is given elsewhere [9]. These results, complemented

by studies of multicomponent mixtures containing sulfide are the basis for the

model work reported here.

The solution model used for the liquid phase and solid solution phases in the salt

mixtures is the two-sublattice equivalent fraction formalism [10]. Using this

model the binaries in (Na,K)(CO3,SO4,Cl) was descibed by Sangster and Pelton

[6]. They used altogether 28 non-ideal parameters to describe the equilibrium

between liquid, hexagonal solid solution, and halite solid solution. They also

added some ternary interaction terms. Backman [11] evaluated the system Na2CO3-

Na2SO4-Na2S by the subregular model. These literature data have been the basis

for the present evaluation, which has added 10 binary interaction coefficients to

the model (Table 5). Some additional ternary interaction coefficients are also

added. The final numerical values of these are still to be optimized based on

experiments involving sulfides. The optimization and all calculations in this work

Table 5. Non-ideal interaction parameters for the two-sublattice equivalentfraction formalism in the system (Na,K)(CO 3,SO4,S,Cl).

Interaction coefficients (J/mol)

BinaryLiquidNa2CO3-Na2S 1 1 4256Na2CO3-Na2S 1 2 -7614Na2S-Na2SO4 1 1 3293Na2S-Na2SO4 2 1 -7836Na2S-NaCl 1 1 -3350 +35*T/KK2CO3-K2S 1 1 -2400K2S-K2SO4 1 1 -9000Sulfide ssK2S-Na2S 1 1 12500K2S-Na2S 1 2 500TernaryLiquid(Na,K)(SO4,CO3) 1 1 1 1 -18000


Figure 1. Comparison between measured and calculated first melting (solidus) and finalmelting (liquidus) temperatures. a) Salt mixtures prepared in laboratory, b) differentsamples from boilers.


have been done with ChemSage [12]. Figure 1 shows a comparison between

measured and calculated first melting and final melting temperatures. Figure 1 a

contains the results from this study where the samples were mixed carefully in

laboratory conditions. In Figure 1 b results reported in literature from different

boilers [13, 14] are summarized. Thus, analytical accuracy can vary considerably

between these samples. The binary phase diagrams for the sodium salts are given

in Figure 2. The common anion systems with both sodium and potassium are

Figure 2. Calculated binary phase diagrams for the system (Na)(CO 3,SO4,S,Cl).


collected in Figure 3. Pure component data in the model have been compiled from

different sources [4, 6, 11].

APPLICATIONS OF THE MODEL

The thermodynamic model, called MELTEST, is a useful tool for estimating the

amount of melt as a function of temperature and composition of different dust

fractions and deposits in kraft recovery boilers. A number of applications have

recently been reported [15-23] . Not only the first melting and final melting points

can be estimated, but the whole melting curve as a function of temperature. Figure

4 shows such curves for some different carry over compositions. The compositions

were estimated based on differences in black liquor composition. The big impact

Figure 3. Calculated solidus curves for Na-K common anion systems.


of composition on the flow temperature can clearly be seen. In Figure 5 a

calculated diagram is shown for the effect of chlorine and potassium content on

the sticky temperature and the flow temperature.

DYNAMIC THERMAL MODELLING OF A RECOVERY BOILER SMELTBED

Objectives

Very little is known about the response of the smelt in a kraft recovery boiler to a

thermal shock or to altered local temperature profiles and heat fluxes in general.

From the viewpoints of floor and floor tube corrosion, and boiler safety in

Figure 4. Calculated melting curves for 10 carry over compositions estimated based onblack liquor composition.


general, quantitative knowledge is needed. A handful of studies have been published

so far [24, 25], fortunately the issue is currently being addressed also elsewhere

[26, 27]. The purpose of the work described here is to analyse the thermal

response of a typical kraft recovery smelt to sudden changes which lead to a

change in the temperature profile in the smelt bed. These changes could be a

sudden increased (or decreased) heat flux to the top of the smelt, a sudden

increase (or decrease) in the top temperature of the smelt, a sudden outflow of the

liquid and partial melt or sudden change in the chemical composition of the smelt.

In all cases, the parameter of major interest is the temperature at the boiler floor,

Figure 5. Calculated diagram showing the effect of chlorine and potassium on the stickytemperature (T 15) and flow temperature (T 70) of salt mixtures containing sulfate andcarbonate.


as a result of such a change. This then allows for a further analysis considering

corrosion behaviour and thermal shock resistance of the boiler floor and floor

tubes.

Approach

In our approach, a 1-dimensional temperature profile is considered along the

vertical centre line of the boiler, ranging from the boiler floor to the top of the

smelt bed. Carbon or reacting carbon are not considered. The smelt is described

by the following characteristic numbers:

SU = 0.3, RE = 0.95, KA = 0.05, CL = 0.02.

In order to see the effect of potassium and chloride, an ideal Na2CO3/Na2S 70/30

mixture will be refered as an ideal case: i.e., SU = 0.3, RE = 1, KA = CL = 0.

The evaluation of the thermal response requires a defined steady-state situation as

a starting point. Thus, a smelt bed has to be defined by the above-mentioned 4

characteristic numbers, the steady-state boiler floor temperature plus two of the

three following parameters: the total bed height, the heat flux through the smelt

bed and the temperature at the top of the smelt bed. (From the two that are given,

the third is calculated.) Given this system definition, the position (= height from

the boiler floor) of the temperatures which corrrespond to the solidus T0, and

liquidus temperature, T100, of the smelt mixture are determined, allowing for

separating the smelt in three zones: 1) solid with temperature Tfloor < T < T0, 2)

partial melt with temperature T0 < T < T100 and 3) liquid with T100 < T < Ttop.

After having defined the bed, various other steady states of this bed and the

response of the bed to a chemical or thermal shock can be evaluated as a function

of time.

Possibilities for dynamic thermal modelling are 1) a sudden new bed surface

temperature, 2) a sudden new bed floor temperature or 3) a sudden removal of the

liquid, the liquid + partial melt, or another mass fraction of the bed.


Numerical procedure and input data

The differential equation for temperature, T, that is solved is the following [28]:

∂T∂t

=λρcp

∂2T∂m2 + ( λ

cp

∂ρ∂T

+ρcp

∂λ∂T

) ( ∂T∂m

)2

= A(T) ∂2T∂m2 + B(T) ( ∂T

∂m)

2

In order to account for changes in the volume of the bed, the height coordinate, x,

is replaced by a mass coordinate, m, i.e., the bed is divided in sections with a mass

∂m (which is constant) instead of sections with height ∂x (which varies due to

density changes). The values for the specific heat, cp, the density, ρ, and the

thermal conductivity, λ, were all taken as a function of temperature, giving the

'lumped' variables A(T) and B(T).

The equation was discretised in 200 mass (m = i∆m) sections and 100 time

sections (t = j∆t), and the resulting temperature profiles T(i,j) were calculated.

Given a number of mass sections of the bed, ∆m, the maximum allowable time

step, ∆t, for numerical stability was calculated using amax∆t /∆x2 < 1/2, is used

where amax is the maximum value for the heat diffusivity a = λ/ρcp in this system.

The initial conditions are a steady state solution for a bed defined by its chemistry,

floor and surface temperature and heat flux, boundary conditions are the bed floor

and surface temperature.

Software RBSMELT was produced using FORTRAN 77. The calculation of the

solidus and liquidus temperatures for the smelt beds was carried out by linking the

ChemApp routine (the non-interactive version of ChemSage) version 1.1 [12],

based on Gibbs' energy minimisation, to the software. This routine is also used to

calculate how much of each chemical compound in the partial melt is, at a certain

temperature, in a liquid phase or in a solid phase. Literature data for the density

and specific heat for all 8 compounds (Na,K) (CO3,S,SO4,Cl) was used, for their

liquid and solid state, respectively. (When data for a potassium salt KX was

missing, the value for the corresponding sodium salt NaX was used). For the heat

conductivity, temperature dependencies were not available so constant values for

a solid and liquid bed were used [24, 25]. The thermodynamic data used were the

enhanced MELTEST data described above.


Results and discussion

The important difference between an 'ideal' smelt bed (Na2CO3 + Na2S) and a

more realistic smelt bed, containing all 8 salts (Na,K) (CO3,S,SO4,Cl) is given in

Figure 6. The range from solidus temperature to liquidus temperature is not only

moved to lower temperatures but becomes also much wider when certain amounts

of potassium and chlorine salts are present as well.

For the same two chemical compositions as in Figure 6, the build-up of a bed with

floor temperature 300°C, and a surface temperature 1000°C at a heat flux of 300

W/m2 is given in Figure 7 and Figure 8, respectively, together with 19 other

values, giving 0, 30, 60,...,600 W/m2. At this heighest heat flux the surface

temperature is above 1200°C. It is rapidly seen that the presence of potassium and

chloride compounds gives a must wider height range for the partial melt, which

also extends much deeper into the bed.

Considering the dynamic modelling, Figure 9 gives the response at the surface of

a bed (RE = 1, SU = 0.3, KA = CL = 0, height 2 m, floor temperature 300°C) to a

sudden increase of surface temperature from 1000°C to 1200°C. After approx. 1

hour, the temperature has changed to approx. 1150°C immediately below the

surface, and only in the top 10 cm of the bed the temperature has changed. A

Figure 6. Smelt trajectories for an idealised (2 components) and a more realistic (8components) smelt bed.


Figure 7. The build-up of an idealised smelt bed (2 components) at various heat fluxes.Bed height 2 m at 300 W/m 2 and 1000°C surface temperature. Floor temperature 300 °C.

similar calculation was conducted for a change at the floor (RE = 1, SU = 0.3, KA

= CL = 0, height 2 m, surface temperature 1000°C) when the floor temperature is

suddenly changed from 300°C to 100°C, see Figure 10. Again, after 1 hour only in

the lowest 10 cm of the bed a temperature change is found. Finally, the effect of a

sudden, instantaneous removal of the liquid smelt from the top of the bed is shown

in Figure 11 (RE = 1, SU = 0.3, KA = CL = 0, height 2 m, floor temperature

300°C, surface temperature 1000°C). The bed height is reduced to below 1.7 m

Figure 8. The build-up of a more realistic smelt bed (8 components) at various heat fluxes.Bed height 2 m at 300 W/m 2 and 1000°C surface temperature. Floor temperature 300 °C.


and after 1 hour the temperature of the new surface has increased to approx.

950°C, and the top 15 cm of the bed has warmed up.

Repeating this for a bed that contains 8 compounds (SU= 0.3, RE = 0.95, KA=

0.05, CL= 0.02) gives practically the same results for a surface heat-up and a floor

cool-down. This is due to the small effect of the presence of a few % sulphates,

Figure 9. Thermal response of a smelt bed surface to a sudden temperature increase:1000°C → 1200°C (2 components). Initial state 2 m bed height, floor temperature 300 °C.

Figure 10. Thermal response of a smelt bed at the floor to a sudden temperaturedecrease: 300 °C → 100°C (2 components). Initial state 2 m bed height, surfacetemperature 1000 °C.


chlorides and potassium salts on the density and specific heat of the bed

(conductivities were taken constant). Since, however, these compounds have a

significant effect on the melting behaviour of the bed, the amount of material

removed at an instantaneous liquid smelt removal is much larger than for the ideal

case given in Figure 11: see Figure 12. The thermal response of the resulting

material, however, is again very much the same.

Figure 11. Thermal response of a smelt bed to a sudden removal of liquid smelt (2components). Initial state 2 m bed height, surface temperature 1000 °C, floor temperature300°C.

Figure 12. Thermal response of a smelt bed to a sudden removal of liquid smelt (8components). Initial state 2 m bed height, surface temperature 1000 °C, floor temperature300°C.


It is concluded that the smelt bed in a typical recovery boiler has very good

insulating properties so that floor corrosion problems are very unlikely to be

correlated to smelt bed surface temperature excursions. However, the analysis

suffers from some uncertainties considering thermal conductivities. Linking up

with the ChemSage software for rigourous calculations of the bed thermodynamics

appears very powerful.

ACKNOWLEDGEMENT

Financial support from Ahlstrom Machinery Oy, Kvaerner Pulping Oy, and the

Nordic Energy Research Program is acknowledged.

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