NBSIR 81-2279

Alkali Vapor Transport in Coal

Conversion and Combustion

Systems

J. W. Hastie, E. R. Plante, and D. W. Bonnell

High Temperature Processes Group

Chemical Stability and Corrosion Division

Center for Materials Science

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

May 1981

Interim Report

QC

100 >• DEPARTMENT OF COMMERCE

, U 5 6 riONAL BUREAU OF STANDARDS

31-2279

1981

c.2

NBSIR 81-2279

»9* ALKALI VAPOR TRANSPORT IN COAL

CONVERSION AND COMBUSTION

SYSTEMS

jUL l 0 W81

nu o-cc

J. W. Hastie, E. R. Plante, and D. W. Bonnell

High Temperature Processes Group

Chemical Stability and Corrosion Division

Center for Materials Science

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

May 1981

Interim Report

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
• .M

n
Table of Contents

Page

1. Introduction 1

2. Combustion Systems of Interest 4

3. Coal Mineral Characteristics 4

4. Corrosion by Alkalies 6

5. Alkali Vapor Transport Modeling 8

6. Experimental Methods 9

6.1 The Transpiration Reactor 9

6.2 Partial Pressure Determination 10

7. Single Component Systems 11

7.1 NaCl(JEL) and Na2 S04 (£) Vaporization 11

7.2 K2 S04 (£) Vaporization 13

7.3 K0H(£) and KC1(£) Vaporization 14

8. Coal Mineral Systems 15

8.1 Alkali Benzoate/Carbonate Vaporization 15

8.2 Illite Vaporization 16

9. The Soda-Lime-Silica System 17

9.1 Background 17

9.2 KMS Measurements 18

9.3 TMS Measurements 19

9.4 Na2 C03 in Glass 20

10. Complex Oxide Systems and Slags 21

10.1 Background 21

10.2 K2 0-Al 2 0 3 -Si02 System 26

10.3 K2 0-Ca0-Al 2 0 3 -Si02 System

(Simplified "Western" Slag) 27

10.4 K2 0-Ca0-Mg0-Al 2 03 -Fe2 03 -Si02 System

(Synthetic MHD Channel Slags of "Eastern"

and "Western" Coal Types) 27

10.5 Real MHD Channel Slag (K x ) 29

10.6 Synthetic Low Melting Slag (K2 ) 32

10.7 Application of Vaporization Data to Seed-Slag

Interaction 33

10.8 Salt-Slag Alkali Exchange 35
11. Heterogeneous Reactive Gas Systems 36

11.1 Background 36

11.2 Na2 S0 4 -Na0H-H2 0 System 37

11.3 II 1 i te-H2 0-H2 System 37

11.4 Synthetic Slag (K2 )-H2 0-H2 System 39

11.5 Glass-Combustion Gas System 42

12. Summary 45

13. Acknowledgments 45

14. References 46

15. Figure Captions 51

16. Figures 54
Alkali Vapor Transport in Coal Conversion

and Combustion Systems

J. W. Hastie, E. R. Plante and D. W. Bonne!!

Center for Materials Science

National Bureau of Standards

Washington, D.C. 20234

ABSTRACT

Alkali metal -containing vapor species are ubiquitous in

coal conversion and combustion systems. These species originate from coal mineral and atmospheric impurities (organic and

inorganic) and from ceramic construction materials. Alternatively, they are present as additives, such as with potassium

seeding for MHD or with bulk glass as a particle absorbing

medium, or with dolomite in fluidized bed systems. Alkali

vapor transport over representative slag, glass, and simple

halide, hydroxide, and sulfate systems is discussed in relation

to materials and process limitations in coal -supported energy

systems. Problems associated with molecular-level vapor transport measurements are also considered.

Key words: Alkali species; coal conversion; combustion; glass; mass spectrometry; slag; thermodynamics; vaporization.

1. Introduction

Vapors containing alkali metal species have diverse implications to

high temperature processes (Stwalley and Koch, 1980). Potential new applications of alkalies in combustion systems include—their vapor

phase catalytic action in smoke reduction (see Haynes et al., 1978 and

the review of Hastie, 1975), their liquid phase catalysis of coal gasification (Gangwal and Truesdale, 1980), and their role as electron sources

for magnetohydrodynamic (MHD) combustion systems. In most combustion

systems, however, their presence is undesirable. This is particularly

true in fossil energy systems.
More efficient coal utilization can be realized with combined power

plant cycles. For instance, the post combustion gases of a conventional

combustor or an advanced MHD system can be further utilized to drive a gas or steam turbine. However, the sustained durability of downstream

turbine or heat exchanger components requires minimal transport of

corrosive fuel impurities. Control of mineral-derived impurities is

also required for environmental protection. For the special case of

open cycle-coal fired MHD systems, the thermodynamic activity of potassium is much higher in the seeded combustion gas (plasma) than in common

coal minerals and slags. This results in the loss of plasma seed by

slag absorption and is of critical concern to the economic feasibility

of MHD.

Empirical experience with conventional coal -fired power plants has

indicated minerals containing alkali metal (Na, K), sulfur- and chlorinebearing species to be the most aggressive fuel components leading

to fire-side or hot corrosion (e.g., see Rapp, 1981). Species con- taining these elements appear to act synergistically in degrading alloy

or ceramic materials. The mechanisms by which such species are released

from their mineral source, transported, and deposited are not known,

though the literature contains numerous speculative schemes (see Hastie,

1975, p. 216). Rational development of new control strategies, such

as gas clean-up or the use of fuel additives, requires a clear understanding of the role played by the active fuel impurities. For instance, new control systems based on scavenging (e.g., absorption of alkali by

glass or other oxide media) or chemical modification of the active

inorganic impurities will need as design criteria information, such as species identity, concentration profiles, dew points, thermodynamic

reactivity, nucleation and absorption rates and diffusivities. Such

data will also be pertinent to minimization of seed-slag interaction in

MHD systems.

Previous attempts to define the mode of release and transport of

fuel impurities have largely been unsuccessful, owing mainly to a lack

of knowledge concerning species identities. This has resulted from the

inability of molecular specific measurement techniques to function under

the combined aggressive conditions of high temperature, high pressure,

and high chemical reactivity. We have developed several new measurement

2
techniques ideally suited to such conditions. The first of these techniques is a High Pressure Sampling Mass Spectrometric method for the

spatial and temporal analysis of flames containing inorganic additives

(Hastie, 1973; Hastie and Bonnell, 1980). The second method, known as Transpiration Mass Spectrometry (TMS) (Bonnell and Hastie, 1979), allows

for the analysis of bulk heterogeneous systems over a wide range of

temperature, pressure and controlled gas composition. In addition, the now classical technique of Knudsen Effusion Mass Spectrometry (KMS) has

been modified to allow external control of ambient gases in the reaction

cell (Plante, 1979a). Supplementary to these methods are the application,

in our laboratory, of classical and novel optical spectroscopic methods

for i_n situ measurement of temperature, flow and certain simple species

concentration profiles (see Hastie and Bonnell, 1980). In combination,

these measurement tools allow for a detailed fundamental examination of

the vaporization and transport mechanisms of coal mineral components in a coal conversion or combustion environment.

As a long-term objective, we aim to define the mechanisms by which

inorganic fuel impurities (particularly K, Na, Cl, S, and heavy metals)

and additives (e.g., K in MHD) are released to or removed from the

environment, transported in a gas stream and deposited in cooler or chemically less reactive regions. To meet this objective, we are addressing the following basic tasks:

(a) Measurement of species vaporization rates and related thermodynamic

functions for wel 1 -characterized salt, oxide, mineral, slag, glass,

and ash samples under controlled gas conditions and as a function

of temperature, time, gas composition and total pressure.

(b) Development, from the basic data derived from task (a), together

with auxiliary literature thermochemical data, of computer-based

models for prediction of release or retention of alkali and other

inorganic components under actual coal combustion, gasification, or MHD conditions.

(c) Validation of models developed from task (b) through comparisons

with large-scale test data.

This paper summarizes the status of this activity. Emphasis is

given to systems showing unusual behavior, or where additional lines of

research are revealed. A more comprehensive presentation of data for

many of the systems considered here may be found in the cited references.

3
2. Combustion Systems of Interest

Alkali, in the form of Na and K-containing species, can lead to a dramatic reduction in the durability of metal (alloy) and ceramic reac- tor components through a complex process known as hot corrosion (Rapp,

ed. , 1981). Examples of energy systems where this process occurs include, coal -fired utility boilers, turbines, gasifiers, MHD generators,

and pressurized fluidized bed combustors (PFBC). In such cases, the

alkali enters the vapor phase by vaporization from coal minerals, dolomite

(Yannopoulos et al . , 1977) or limestone, as in sulfur-scrubbing processes,

or from air-ingested salt particles (e.g., in marine environments).

Hardesty and Pohl (1979) have recently reviewed the major problem areas

and data limitations relating to the properties of coal mineral matter

and ash.

Even a minor amount of alkali vapor transport can be significant,

as revealed by the turbine tolerance level of 0.02 ppm alkali needed

for corrosion control in pressurized fluidized bed combustors (Spacil

and Luthra, 1979). If we consider only the alkali halide content of

the dolomite component, this tolerance level would require an alkaliscrubbing efficiency of better than 99.9999 percent for PFBC. Even if

corrosion (alkali) resistant materials were available, uncontrolled

alkali vapor transport would still lead to unmanageable deposits on cool

downstream components. For instance, under typical coal gasifier conditions, a species partial pressure as low as 10 atm would lead to vapor

transport and deposition in metric ton quantities on an annual basis.

The need for a basic understanding of alkali vapor transport in

fossil energy systems can be appreciated when we consider the diversity

of conditions such as temperature, pressure, chemical composition, and

time scale, present in existing and developing fossil fuel technologies.

Table 1 summarizes some typical process conditions.

3. Coal Mineral Characteristics

Typically, coal contains about 10 wt.% mineral matter. There is a recognized need for improved understanding of coal mineral transformations and slag-forming processes (Kolodney et al .

,

1976). The chemical form of alkali and halogen in coal is of considerable importance to

4
Table 1

Typical Coal -Conversion and Combustion Systems

Process Temperature

K

Pressure

atm

e Comments

Conventional Steam Plants

1000-1500 1

Pressurized Fluidized Bed Combustion

1200 1-10 2 % excess 0

?

, 300 ppm S0

2

Coal Gasificationa,b,c

Cogas 570-2250 3-6 entrained/slag

Koppers-Totzekd 1750-2100 0 entrained/slag,

medium btu

Bi-gas 1200-1920 66-100 entrained/slag

Atgas 480-1750 0-0.3 molten iron

Kellogg 1100-1480 27-80 molten salt

C0

2 Acceptor 1100-1370 10-20 fluidized (dolomite)

Hygas 920-1300 66-100 fluidized, high btu,

high S

Synthane 370-1260 33-66 entrai ned/f1 ui di zed

,

high btu

Lurgi 870-1260 20-33 fixed bed, low btu

Magnetohydrodynami cs

Open cycle

hot walls

1500-3000 1-10 1 % K seed, fuel

rich or stoichiometric

aComparable conditions exist in ammonia plant secondary reformers,

e.g. , 1060K, 13 to 20 atm, 28 percent H2 , 50 percent H2 0, 6.4 percent CO,

.and 4 percent C02

. “After Crowley (1975).

For technological status, see Vorres (1980) and Lenzer and Laurendeau

d (1976).

“in commercial operation.

1 atm = 1.01325 x 102 kpascal.

5
the mode of alkali vapor transport. Analyses of Gluskoter and Ruch (1971)

suggest that halogen is present in two forms, NaCl and organic. Also,

most of the potassium is present in a halogen-free highly bound form,

such as for the mineral illite, or other potassium al umi no-si 1 i cates.

A number of coal combustion systems utilize limestone and dolomite additions for sulfur removal. These materials provide an additional source

of alkali. Alkali (Na + K) contents of 0.05 to 1 wt.% are usual and the

predominant mineral form is the chloride (Shearer et al., 1979).

Laboratory simulation of coal combustion indicates several modes of

mineral decomposition (Sarofim et al., 1977). Submicron-size particles

tend to be derived from vaporization with subsequent homogeneous and

heterogeneous condensation. These particles are rich in silica (SiO

vapor transport) but with large enhancements of trace metals, including

alkalies, Cd, As, and other heavy metals. Various mechanisms have been

suggested concerning the combustion history of the alkali components

(Boow, 1972). The principal alternatives are, that NaCl is vaporized

during combustion and is not incorporated into silicate minerals—at

least in the initial combustion phase--or, that NaCl reacts with the ash

thereby lowering the alkali activity and hence the extent of vapor phase

alkali transport. This latter statement remains qualitative pending the

determination of alkali activities for coal minerals and slags in combustion atmospheres.

4. Corrosion by Alkalies

Alkali vapor transport and deposition is a well-known, though

poorly understood, factor in the corrosion or fouling of alloys and

ceramics, both in established and developing technologies. Problem

areas include oil-fired glass melting operations (Pressley, 1970), blast

furnaces, boilers, turbines, coal gasification (Raymon and Sadler,

1976), MHD (Anthony, 1976; Bowen, 1979; Schneider et al., 1980) and

coal -fired pressurized fluidized beds (CFCC Development Program, 1978).

In general, the corrosive effects of alkali deposits result from

the high solubility of ceramic and oxide coatings (e.g. , for alloys) in

molten alkali sulfate, carbonate, chloride, or vanadate deposits. This

solubility results from the high stability of Na (or K)-A1 -silicates, or similar oxide phases. Formation of these silicates, for instance, leads

6
to a volume increase and loss of structural integrity in ceramic materials, e.g. , see Clews et al. (1940) and Rigby and Hutton (1962). In

alloy systems fluxing can also occur and this results in greatly increased

oxidation rates (Rapp, ed. , 1981). Even when hot corrosion is not a problem, alkali deposits can lead to fouling and thermal barrier effects.

For instance, in secondary naphtha reformers, oxide deposits containing

Na2 0 (21 wt.%), K2 0 (3 wt.%), plus Si02 , Al 2 03 and CaO, lead to fouling

of waste heat boiler tubes (Venable, 1969).

Alkali vapor transport and deposition places severe limitations on ceramic materials for MHD generator walls and electrodes. Here, the

corrosive action of K2 S04 and K2 C03 -containing liquids appears to be

the major problem. In the combined presence of potassium seed and coal

slag, the rate of electrochemical corrosion of MHD ceramic electrodes

increases by two to three orders of magnitude. Alloy corrosion at

intermediate temperatures (900 to 1200 K) also can be related to formation

of a liquid Na2 S04 phase, which prevents formation of a protective Cr2 03

scale and greatly enhances corrosion of Co- and Ni-Cr alloys (Luthra and

Shores, 1980).

The thermodynamics of corrosive alkali salt-oxide interaction is

not well established. In an assessment of research needs for materials

in coal conversion, the need for carbonate-silicate melt studies, including activity and phase equilibrium measurements, was stressed (Staehle,

ed. , 1974). The lack of thermodynamic data for fused salts, and their

reactions with oxides and alloys leading to models of hot corrosion, was also indicated.

Hot corrosion of Ni-base turbine alloys by Na2 S0 4 and K2 S04 is

strongly dependent on the Na2 0 activity in the salt (see, Huang et al .

,

1979; and cited work). Further activity measurements of this type are needed over a wider range of conditions. Raymon and Sadler (1976) have

reviewed evidence for reactions involving alkali vapors and refractory

lining materials for coal gasifiers. They indicate a pressing need for

studies of alkali attack in reducing atmospheres at pressures up to

30 atm and temperatures to 1500 K.

7
5. Alkali Vapor Transport Modeling

Despite the incomplete state of a thermodynamic data base and

limited mechanistic insight, several attempts to model alkali vapor

transport in reactive atmospheres have been made. The increased sophistication of modeling efforts in recent years is demonstrated by the

following examples:

(a) Coal gasification (Wilson and Redifer, 1974).

(b) Glass furnace corrosion (Kirkbride, 1979).

(c) Pressurized fluidized bed combustion (Spacil and Luthra, 1980).

In example (a), the gas composition was modeled assuming ideal solution

phases and neglecting known complex vapor species, such as K2 S04 , K2 C03

and alkali chlorides. These serious limitations resulted from the

non-availability of oxide solution-activity data, accurate vapor species

thermodynamic functions, and the inability of existing computer codes to

handle non-ideal solution multiphase, multicomponent equilibrium computations.

The more recent work of example (b) modeled Na vapor transport by

including NaOH, NaCl and Na2 S04 as vapor species, with the major uncer- tainties arising from neglect of solution non- ideality and inaccurate

thermodynamic functions for Na2 S04 (see Bonnell and Hastie, 1979).

In example (c), many of the limitations represented by (a) and (b)

were resolved; the greatest uncertainty resulted from the highly approximate nature of the al kal i-si 1 icate activity data (see also, comments in

Section 10.1). For this system, the data base requirements are more

critical since, in pressurized fluidized bed combustion, the alkali

tolerance levels for downstream turbine operation are of the order of

0.02 ppm. Spacil and Luthra (1980) compared their thermochemical predictions with observed combustion gas stream alkali concentrations. Here,

Na2 S04 , NaCl, NaOH, and the K-analogues were included as significant

molecular species in the thermochemical model data base. Albite and

sanidine were assumed to represent the coal ash alkali-getter substrates.

Fair, but encouraging, agreement between calculated and observed gas

phase alkali concentration was obtained.

8
6 . Experimental Methods

The primary experimental methods used in this study are the Knudsen

Effusion Mass Spectrometric (KMS) and Transpiration Mass Spectrometric

(TMS) methods, as described elsewhere (KMS, Plante, 1979a; TMS, Bonnell

and Hastie, 1979). Both are modulated molecular beam methods with phase

sensitive detection, and they allow for accurate measurement of both

gaseous and condensible species. The basic differences between the KMS

-4

and TMS methods are, the upper pressure limits of 10 and one atm,

respectively, and the upper limit gas residence times of about 0.04 and

20 sec, respectively. Thus, the TMS method will more closely approach

the equilibrium condition for systems exhibiting non-equilibrium behavior. As the TMS method is still relatively novel, a brief description is given here.

6.1 The Transpiration Reactor

The TMS facility consists of a transpiration reactor mounted in one

of two available multi chambered (two or four), differentially pumped, vacuum ystems with a quadrupole mass filter (cross-beam) located in the

high vacuum stage. Essential features of the reactor include: a sample

container or boat, a boat carrier, a thermocouple for temperature measurement, a carrier gas inlet system, and a gas extraction system or probe,

as shown in figure 1. The boat carrier allows for boat removal from the

reactor without need for a complete disassembling of the transpiration

system. Molecular beam sonic probes are typically conical nozzles with

design details determined by reasonably well established gas dynamic

criteria. However, for highly reactive systems, we also found it desirable to develop a more robust capillary probe, at the possible expense

of sampling fidelity. For the present study, all of these components

were fabricated from platinum metal. The reactor is usually operated at

total pressures of 0.2 to 1.0 atm, using N 2 or Ar as a carrier gas and

temperatures up to about 1700 K.

9
6.2 Partial Pressure Determination

For the KMS method, conversion of mass spectral ion intensities to

species partial pressures is made through the basic relationships,

P. = k.I. T

and

( 1 )

k

i

GA.(2nR/M.)V.

1 T 1 ac 2 !

i t(T)i5

( 2 )

where, for species i; P. is partial pressure, the corresponding ion

intensity, T the temperature, an instrument and system sensitivity constant, G a gravimetric factor, A. an isotope abundance factor,

R the gas constant, m. the weight loss, t the time, c the Clausing factor,

a the orifice area, and m. the molecular weight (or an average value for

Knudsen effusion), as described elsewhere (Plante, 1979a). In some cases,

k. for different species, but arising from the same experimental configuration, can be inter-related through known ionization cross sections, , and transmission/detector efficiencies, S^, i.e.,

k

i “ ( Vsi V

-1

(3)

as discussed elsewhere (Bonnell and Hastie, 1979). The degree of agreement

for the k

1 s obtained from weight loss [expression (2)] or cross section

data [expression (3)] provides a useful check on the internal consistency

of the partial pressure data.

Relation (1) can also be applied to TMS data. The sensitivity

factor k^ is obtained by several independent methods, thereby providing

a good test of internal consistency in the data. The basic relationships

are,

(4)

and

( 5 )

10
where, for species i and transport gas j (N2 or A r); n is the number of

moles of substrate transported, V the volume of gas transported, At the

transport time interval, and S an instrument gas scattering correction

(S = 0.6 to 1.0; Bonnell and Hastie, 1979). The reference or transport

gas sensitivity factor k. is readily obtained from equation (1), since

vJ the pressure of transport gas is known from an external manometric

determination.

7. Single Component Systems

In complex combustion systems, alkali vapor transport can occur as the metal or as molecular species, such as NaCl , (NaCl) 2 , NaOH, (NaOH) 2 , Na2 S04 , NaSO (x = 2,3), NaPO (x = 2,3) and, possibly, other yet-to-be

A /\ established postulated species, such as Na2 C0 3 and NaCl*2H2 0. Potassiumcontaining systems show an analogous behavior. Species identities, and

their basic thermodynamic functions, are usually best established by

vaporization studies over the single component systems. Considerable

uncertainty and literature disagreement has existed for most alkali

containing systems. We have obtained new thermodynamic vaporization

data for the most important systems in this category.

7.1 NaCl(£) and Na2 S04 (£) Vaporization

These systems are the best established and they serve, primarily,

as test cases for the TMS method. The principal TMS results for liquid

NaCl may be summarized as follows. Additional detail may be found elsewhere (Bonnell and Hastie, 1979). For the reaction,

NaCl (£) = NaCl (g)

log P NaCl (atm) = 4 ' 85 (± °‘ 3) " 8820 (± 20 °) /T »

AH

v (1300) = 40.4 ± 0.9 (42.7 ± 3.5) kcal/mol, and

AS

v (1300) = 22.2 ± 1.4 (24.2 ± 0.5) cal/deg mol ,

11
where T is in Kelvin. These second-law data compare favorably with the

JANAF (1971) evaluation of previous literature data, indicated in parentheses. Our second- law data, obtained by both TMS and KMS, for the

dimerization reaction,

2NaCl (g) = (NaCl)

2 (g) ,

give

- AH

d (1300) = 46.4 ± 0.7 (47.1 ± 3.5) kcal/mol , and

- AS

d (1300) = 30.2 ± 0.6 (29.7 ± 1.0) cal/deg mol ,

which compare favorably with the JANAF (1971) values, indicated in

parentheses. Thus, the vaporization data for this system are well

established.

The Na2 S04 (£) system has historically been difficult to characterize,

due largely to containment problems. However, from our TMS and KMS

second- law data (see Bonne! 1 and Hastie, 1979), and other recent literature results, a quantitative thermodynamic description of this system is now possible. For the major vaporization process,

Na

2SQ

4

(£) = 2Na + S0

2 + 0

2 ,

AH

y (1550) = 69.8 ± 3 (71.7 ± 0.6) kcal/mol, and

AS

v (1550) = 27.8 ± 3 (29.7) cal/deg mol ,

with the JANAF (1971) data given in parentheses. For the secondary

reaction,

Na

2

S0

4 (£) = Na2

S0

4

(g) ,

our data are consistent with those of Kohl et al. (1975), who give,

12
AH

y

(1267) = 65.9 ± 3 kcal/mol, and

AS

v (1267) = 22.3 ± 2.8 cal/deg mol

7.2 K2 S04 Vaporization

Potassium sulfate is a potentially important constituent in

coal burning systems, being formed by reaction of ash/slag potassium

with combustion S02. The solubility of K2 S04 (and Na2 S04 ) in coal ash

is small, but reaction can occur to form K2 0 (and Na2 0) dissolved in the

ash with release of S02 and 02

. However, the deposition of K2 S04 (and

Na2 S04 ) in hot corrosion processes indicates that K2 0 (and Na2 0) is only

partially removed by interaction with coal ash. This could result from

slow kinetic processes, competitive chemical reactions, transport of the

sulfate as gaseous molecules, or by a series of chemical reactions

involving alkali-containing gaseous molecules which lead to a net transport of K2 S04 (or Na2 S04 ) across a temperature/concentration gradient.

The thermodynamics of K2 S04 and Na2 S04 condensed phases are generally considered to be well known and are tabulated by JANAF (1971).

Thermodynamic data for the gaseous molecules are much less certain,

however.

Using separate gravimetric Knudsen effusion and KMS techniques, we have obtained data for the heat of vaporization to form K2 S04 (g),

and detailed results will appear elsewhere (Plante et al., 1981). From

thermal data, the dissociation pressure, as represented by the reaction,

K

2

S0

4

(s,il) = 2K(g) + S0

2 (g) + 0

2 (g) , [1]

can be readily calculated. The only other reaction of comparable importance is that of the sublimation or vaporization process;

K

2

S0

4 (s,£) = K2

S0

4

(g) . [2]

A number of alternative vaporization processes have previously been

suggested for K2 S04

. However, one or more of the postulated reaction

products [K2 0(£,g), S03 (g)] have a sufficiently positive free energy of

13
formation so that the proposed alternate reaction paths lead to an insignificant partial pressure of evaporation products compared to

reactions [1] and [2]. Most of the previous K2 S04 vaporization mea- surements are thought to be in error because of container material

reactions or, possibly, creep of liquid K2 S04 from the metal container

(usually a platinum metal).

In the present study, weight loss measurements of K2 S04 (£)

were made using a thermobalance equipped with a Pt Knudsen cell. Combining these data with the known dissociation pressures for reaction [1]

leads to partial pressure data for reaction [2], as shown in figure 2.

Complementary KMS data were also obtained. For comparison we have also

indicated, in figure 2, smoothed data extrapolated from the torsion effusion measurements of Lau, et al. (1979), and data from two mass effusion

measurements made by Efimova and Gorokhov (1979). The absolute pressure

data from these three independent recent investigations is in unusually

good agreement, but some significant differences still exist (see Plante

et al. 1981). These pressure data are about 400 percent greater than the

corresponding values of Ficalora et al. (1968).

7.3 KQH(£) and KC1(£) Vaporization

The thermodynamic stability of KOH in the vapor phase can be

obtained from the Second and Third Law analyses of K0H(£) vaporization

as the thermodynamic functions for the liquid phase are reasonably well

established. JANAF (1971) has evaluated the various disparate sets of

KOH vaporization data but with considerable uncertainty. Much of the

difficulty associated with obtaining reliable thermodynamic data for

this system arises from its reactivity with container materials, the

presence of carbonate impurity, and the coexistence of dimers and monomers. Previous studies have also been hampered by decomposition to K

and H2 0. In the present work, using the TMS technique, we have suppressed

this decomposition by addition of H2 0 to the carrier gas.

We have obtained extensive data for the K0H(£) and KC1(£)

systems, which will be presented in a formal publication elsewhere

(Hastie et al., 1981). Representative data for K0H(£) are presented here

in comparison with other recent results not considered by JANAF (1971).

Species partial pressure data are summarized in figure 3. Note that the

14
KOH species data are in good agreement with JANAF (1971), as might be

expected. However, there is no agreement between workers regarding the

(K0H) 2 species, except that the relative amounts of dimer to monomer

found in the present study agree quite well with the KMS results of

Gusarov and Gorokhov (1968). When the monomer and dimer partial pres- sures are summed, the total pressures are about a factor of two greater

than the JANAF (1971) data.

With regard to the KC1(£) system, the KC1 species data were found to be in good agreement with JANAF (1971). However, the dimer

species (KC1) 2 was found to have an appreciably greater enthalpy of

formation than the JANAF (1971) value.

8. Coal Mineral Systems

8.1 Alkali Benzoate/Carbonate Vaporization

Part of the alkali content in coal is organically bound and

the benzoate salts, NaCOO0 and KCOO0, have been selected to model alkali

release from such a state e.g. , see Stewart et al. (1980). For Na and K in coal, typical organic/inorganic distribution ratios lie in the range

2 to 9, and 0.02 to 0.1, respectively. Using the TMS approach, we have

obtained comprehensive data on the mechanism of Na and K release from

their benzoates and the related carbonate-char systems. The detailed

results will be reported elsewhere (Chakrabarti et al., 1981).

The principal findings are as follows. On heating to ~ 800 K,

sodium and potassium benzoates decompose to yield a carbonate plus char

residue. At temperatures of 'v 1000 K, this residue reacts to yield Na

in the vapor phase according to the reaction,

Na

2

C0

3

(c) + 2C(c) = 2Na + 3C0 ,

and similarly for potassium. Comparison of species partial pressures

with those of multicomponent equilibrium calculations indicate that this

reaction is at, or near, equilibrium, a surprising result for what is

formally considered a solid-solid interaction at these relatively low

temperatures. Thus, the organic alkali components of coal can be a

15
significant source of alkali in the gas phase. Future studies are planned in the presence of reactive combustion gases, e.g., S02 , H2 0,

C02 , H2 , and HC1

8.2 mite Vaporization

The clay mineral illite is frequently found as a mineral

constituent of coal and is considered a major source of potassium vapor

species in combustion systems. A "Beaver's Bend illite" sample was used

(provided by the Morgantown Energy Technology Center) with the following

composition for the major components, in wt.%: Al 2 03 (26.0), Fe2 03 (4.4),

Si02 (60. 2) , K2 0(7.4), Na2 0(0.2), Mg0(2.1), and S(0.1). A water analysis,

carried out by heating to 1300 K in air, indicated a water content of

7.3 percent. It should be noted that water is present in the illite

structure as OH groups and can be expelled only by heating to relatively

high temperatures.

The melting behavior of dehydrated illite can be expected to be

similar to that for an approximately 30-60-10 wt.% composition in the

Al 2 Q 3 -Si02 ”K2 0 ternary system where melting begins at about 1300 K. From this analogy we can assume that over the temperature range of the

present experiments 1540 to 1950 K), illite (dehydrated) will be

present as a reasonably homogeneous liquid.

During the initial heating period, significant release of Na and K to the vapor phase was noted over the temperature interval 1300 K to

-3

1500 K. The predominant vapor species were S02 (^ 10 atm),

Na ("v 10 ® atm), K (~ 10 ® atm), and possibly NaOH (^ 10 ^ atm). The mass spectral signals for the latter species were less certain, being

detected as signal/noise ratios of about two. These initial data also

tended to show time dependence in the form of pressure bursts and isothermal signal decay indicative of sample inhomogeneity. Also during

this initial period, the 0 2 pressures were in excess of the reaction [3]

level owing to reduction of Fe2 03 to Fe3 04

. Following this initial vaporization phase the principal reactions

were,

K

2

0 (sol n. ) = 2K(g) + 1/2 0

2 (g) , [3]

16
and

Si0

2

(soln.) = SiO(g) + 1/2 0

2 (g) . [4]

At the higher temperatures, Fe304 contributed to a small partial pressure

of Fe(g), of a magnitude similar to the SiO(g) pressure. Representative

vaporization data, obtained using the KMS and TMS methods, are given in

figure 4 (see also table 2 in Section 10). A more detailed account of

this study is given elsewhere (Hastie et al., 1979). Not all the 02 pressure data points appear in the figure for reasons of clarity. The

02 /K pressure ratios varied from about 2 to 1/2 during the experiment.

This behavior is not consistent with the vaporization stoichiometry

indicated by reactions [3] and [4]. Since the K pressure is about

25 times greater than for SiO, reaction [4] is a negligible source of

02 , and most of the 02 should result from K2 0 decomposition according to

reaction [3]. For this to be true, the 02 pressure should be only about

1/4 of the K pressure and have the same temperature dependence as K. The variation of 02 /K pressure with time may be due to excess 02 dissolved

in the illite or, more likely, to iron oxide decomposition. This timedependent oxygen activity could also account for the K pressure differ- ences between the KMS and TMS methods, as shown in figure 4.

9. The Soda-Lime-Silica System

9. 1 Background

A commercially common soda-lime-silica glass has been considered as an absorbing medium for removing fly ash particulates in combustion gas

streams (Gatti et al . , 1980). However, a possible limitation with this

application is the release of alkali from the glass into the gas stream.

Glass also has some common features with coal slag, and the basic thermodynamic data derived from glass-combustion gas studies will benefit our basic understanding of slag-gas interactions.

Description of alkali release, or retention, by glass requires

accurate Na2 0 activity data at various temperatures and glass compositions.

However, thermodynamic activity data for glass systems are surprisingly

sparse. No critical analysis, for instance of the type represented by

17
JANAF Thermochemical Tables (JANAF, 1971), has been made on the available data. Glass activity data for a common system can vary by several

orders of magnitude, or more, depending on the measurement method used.

In the present study, we have utilized the KMS and TMS methods to

obtain vaporization and activity data for a glass of initial composition (wt.%): Na2 0(17), Ca0(12), and Si

2 (71 ) (Hastie et al . , 1980a).

The sample, obtained from the Ceramic, Glass and Solid State Science

Division (NBS) , was prepared by melting together the appropriate mixture

of Na 2 C03 , CaC03 , and Si02 in a platinum container in air to about

1800 K until all visible C02 was expelled, except for a residual Na2 C03

impurity level as discussed below. This glass is similar to those

suggested as being useful for removing fly ash particulate matter from

fluidized bed coal combustion systems. In addition, the binary silicate

system (no CaO present) is one that has been extensively studied by a variety of methods and reliable Na2 0 activity data is available [e.g.

,

see Sanders and Haller, 1979]. There is also limited information on the

effect of CaO additions on the Na2 0 activity (Neudorf and Elliott,

1980). Thus, indirect comparisons can be made with activity data for

the ternary system.

9.2 KMS Measurements

Partial pressures for Na and 02 were obtained under vacuum vaporization conditions using the KMS technique. Following an initial heating

period, these pressures followed the stoichiometry of the reaction,

During the initial heating period, an excessive amount of Na, as well as C02 , was observed. Initially the C02 pressures approximated

those for 02 but decreased to a negligible level (C02 /02 < 0.1) in

subsequent experimental runs. Two processes appeared to be controlling

the release of C02 ,

Na^O (soln.) = 2Na(g) + 1/2 0

2 (g) [5]

Na

2

C0

3

(soln.) = 2Na + 1/2 0

2 + C0

2

and [ 6 ]

Na

2

C0

3

(soln.) = Na

2

0 (soln.) + C0

2

[7]

18
with the former predominating in the early and lower temperature phase

of the experimental runs, and the latter at higher temperatures and

later observation times.

Figure 5 summarizes Na partial pressure curves for two experimental

runs. Experimental details have been presented elsewhere (Hastie et

al., 1980a) and the data points closely followed the curves given in

figure 5. The vertical bar represents the maximum conceivable error

that could have arisen with the KMS (run 1) data. Several data sets

from the literature are indicated for comparison. Neudorf and Elliott

(1980) measured Na2 0 activities in the binary silicate solution, as well

as the effects of CaO on the Na20 activity, using an emf method. We

have extrapolated their data to our experimental conditions based on the

effects of Na2 0 and CaO content on the Na2 0 activity. The data point of

Cable and Chaudhry (1975) was obtained by a classical transpiration

method under conditions where surface segregation effects were negligible.

Similarly, the data point of Sanders et al. (1976) represents a stirredmelt transpiration experiment where surface depletion is also unlikely.

The curve of Argent et al. (1980) represents Knudsen effusion mass spectrometric data (without beam modulation).

9.3 TMS Measurements

Glass vaporization in a N2 atmosphere was monitored using the TMS

technique. Representative Na-partial pressure curves are given in

figure 5. Note that these partial pressures are more than an order of

magnitude greater than those obtained by the KMS technique. Ideally,

both sets of data should coincide. We believe that the explanation for

this apparent discrepancy is as follows.

Under the conditions of the KMS experiments, the rate of alkali

removal was about an order of magnitude greater than for TMS. This may

be seen by the calculated composition change in the respective glass

samples, i.e., KMS(3) final wt.% Na2 0 = 13.3 and TMS(2) = 16.9 as compared

with the initial composition of 17.0 wt.%. During the initial phase of

each type of experiment, excessive amounts of C02 and Na were released.

Only when about one percent of the glass Na2 0 was depleted did the excess C02 and Na become negligible in the KMS experiments. As this

level of alkali depletion was never reached during the TMS experiments,

19
we believe that these latter data correspond to the anomalously high Na

pressures found in the early phase of the KMS experiments. These high

alkali pressures can be attributed to the presence of unreacted Na2 C03

impurity in the original glass samples, even though care was taken to

avoid this in the glass preparation. Residual carbonate impurity is a common problem with glass experimentation e.g. , see Cable and Chaudhry

(1975).

We calculate from the time- integrated C02 and excess Na signals

that the initial concentration of impurity Na2 C0 3 was 0.45 wt.%. From

the relative amounts of Na, 02 , and C02 released prior to vaporization

from the silicate itself, two types of impurity- related vaporization

processes appear to be present, as represented by reactions [6] and [7].

Comparison of the TMS and KMS data indicates that the activity of Na2 0

(solution), produced by reaction [7], is substantially greater than that

for the silicate-bound Na2 0 characteristic of the pristine glass.

Apparently, the Na2 0 produced jm situ by carbonate decomposition is not

readily incorporated into the silicate matrix, at least on the time

scale of the vaporization measurements. Formally, we can consider

Na2 C03 as a solute, in a metastable glass solution, and with an activity

defined by reaction [6], as discussed in the following Section (9.4).

Alternatively, one could argue that under the higher vaporization rate

conditions typical of KMS, surface depletion of alkali led to the relatively low Na-pressures observed, e.g., see Cable and Chaudhry (1975).

However, no significant isothermal time dependent vaporization was noted

on the several minute time scale of individual KMS measurements. Also,

the KMS(l) pressures are greater than the stirred-melt data of Sanders

et al., (1976) which would not be the case in a surface-alkali depleted

system.

9.4 Na2 C03 in Glass

We have interpreted the anomalously high alkali vapor pressures of

the TMS experiments described in Section 9.3 in terms of impurity Na2 C03

decomposition in a glass solution. By monitoring the release of C02 , and integrating over time, we have determined the mole fractions of

Na2 C03 present at the various measurement temperatures and times.

Hence, from the observed partial pressures for reaction [6], and the

20
corresponding reference state values (JANAF, 1971), we can calculate

Na2 C03 activity coefficient data, as shown in figure 6. These data

appear to be thermodynamically reasonable and tend to support the alkali

carbonate impurity interpretation of alkali vaporization differences

between the KMS and TMS experiments (see Section 9.3). We can likewise

argue that the data of Cable and Chaudhry (1975), shown in figure 5,

also appear to suffer from this impurity problem, even though they also

took precautions to eliminate residual carbonate during the glass synthesis process.

Future studies should be pursued under controlled doping conditions

and in atmospheres containing C02 and 02

. The known synergistic effect

of C02 on 02 -solubility in silicate melts at very high gas pressures

has, in fact, been interpreted in terms of Na2 C03 formation in solution.

Effects of this type could significantly enhance alkali vapor transport

in practical combustion systems.

10. Complex Oxide Systems and Slags

10.1 Background

As part of a program of systematic studies on potassium-containing

slag systems, vaporization data have been obtained for a series of

synthetic-binary, ternary, quarternary and sexternary oxide mixtures, as well as an actual MHD coal-slag sample. Virtually no experimental

thermodynamic activity data exist for these systems. Even for the most

studied relatively simple K20-Si02 system, existing data is extremely

crude and incomplete. Comparison of our activity data for this system

with those reported by Charles (1967), as based on the moist- atmosphere

transpiration data of Preston and Turner (1933), indicate several orderof-magnitude differences, with the Charles's results being low. Likewise,

the estimates of Spacil and Luthra (1980), as based on the phase diagram,

yield activities one to two orders of magnitude lower than our experimental values (for details, see CFCC, 1978).

Table 2 contains a summary of selected potassium vaporization data

for these systems. With a few noted exceptions, the oxygen partial

pressures coupled stoichiometrical ly with potassium, in keeping with

reaction [3]. Hence, K2 0(£) activities can be derived, to a good to

21
excellent approximation, using the corresponding dissociation-pressure

data for pure liquid K2 0. That is,

a

k

2

0 = <V5/2/2K

p

• <«

where K is the dissociation constant (see reaction [3]) for pure liquid

P K2 0. We have derived a temperature dependent expression for K

p as follows. From JANAF (1971), K data for K2 0(s) are available. Combining

P these results with the fusion enthalpy and entropy data of Natola and

Touzain (1970), and an estimated C for liquid K2 G of 25 cal/deg mol,

r leads to the expression,

log Kp

[K

2QU)] =

- ^

Sl + n.7723 .

The potassium vapor pressure data were obtained under neutral

conditions using the TMS technique. For most coal-conversion and combustion systems of interest, iron will be present in the slag as Fe3 04

. Hence the data reported in table 2 were obtained, for the most part, at

temperatures (and run times) where Fe2 03 (slag) had essentially converted

to Fe3 04 (slag). Evidence of this reduction was conveniently established

by monitoring the 02 -pre$sure. These results can also be applied to

systems where 02 -sources other than reaction [3] are present. In this

case, the K-pressures are converted to activities using equation (6).

The activity data are then combined with the known equilibrium constants

for reaction [3], and the assigned 02 -pressures , to yield the new K-pressure data. For relatively low temperature oxidizing conditions,

where Fe2 Q3 (slag) may be present, we can reasonably assume that the

present alkali activity data will be valid. That is, the various forms

of iron oxide do not significantly affect the alkali activity.

Experimental details and an extended discussion of these potassium

vapor pressure results have either appeared, or will appear, elsewhere,

as indicated in table 2. In general, the potassium vaporization data

followed the expected Clausius-Clapeyron behavior, as indicated in

table 2. Exceptions to this behavior could be traced to:

(a) residual alkali carbonate impurities resulting either from the

sample synthesis method, or from condensation out of MHD plasmas

with actual slag samples;

22
Summary

of

Alkali

Vaporization

Data

for

K-Containing

Complex

Oxide

and

Coal

Slag

Systems

d>

</>

CDE

EOo

S-dJ /-n

Q)

S- 4- d> SOW)

W) 3

tO <0 U3

a> »-> o

t- *0 Q) a. tj XJ

O)

o

4-> c

(TJ G_

•r- (/) U) C C /-n O O O X)

q. u cn E •»-> o c j- a» u o> o «—

tj 4- w >* C

t- CD TJ CD

io aai+J

TJ <D +-> C

C T3 V) ro

lO O) (TJ CM

(TJ •<—+-> O £<- (TJ\

Q. V1 QX

T3 TJ

<D <D 4

-

> to

(TJ C

5- <D ^ -Q TJ (0

•<- C O r- O 00

•r- U CD

O +JN

c 4- (u ai O C r— r- (D JlC XI to d) m o a» -> X (TJ l/l C

O U (TJ (TJ i- <U X= r—

CL X) 0.0.

4-o

CDU

c

a> TJ

dJO)

TJ C

0> (Tj to x: 3 U

>> C

cn o

o <-

to -r- (0 c

<u

s- E

u

CD o Q 4J

CM

CD

in

cm

co

CD

II o

CD CO

CO Lf> O «fr in rr- CM

Om

CD

roCM

CDOCM

CO

CDoCM

CO o CD O CD 00 o cn

CM «—

CMOCM

CM o n*» *3- 00 00 CM

<3- m

co cm

CM CM CM LO O C"

CO

CDo

o ** in cd cd

CO CD *3

-

*3- oo CD *3" *3"

O

<DOOCD

CM O

CO

CM

CO OCM <CO

TJ

CD

+->

x:

CM O

23
*oc

eo

0>

cn

4-> C

QJE

EOo

P Q) O C

"O > ftl +-> «—

s- a> c *r- oo

ro O O i/l O'* C E +-> CL —• <TJ D -r- E

ro +-> ET3 o

d oo -a u

CM X <C - 03 I—

• E O —I <U cv U - «-» ^ * C +-> C i—

<V Q. O <D

c_> l__. l_j e e -o

a> ^ o

-o -

<u a

c -

•I- -Q

<V -

+-> ro

D Old) a 4J

<0 >> Q. O Or- w U <— O

O C -O ^

c *—

:*£ T3

co

<TJ O <U O C r—

r- Q_ <D O <— E s- oo

«— 03 D CM H U1 ^ l

CO

r>.

i cn oo

f"» oo cn

<u o

a>

cn e

CO +j

qj <o SCL.

I

00

<c

<3- cn

oo

r— ao

oo «3- cm

co r- oo

CO S r- rCNJ CM II II II

o00

CD

CO

CO

lO

O0

CM

f"- 00

CO

cno

cn

o

cno

o

oo

CDO

CO

O

r>. o

oao

oaoo

co

a. i E o •r- CM CO ^

cr>r— co

(V co

•— i—ii—

CO co O O <v r-1 <— CM O QJ r— I I O c < •«- - •> CO

CM

H3 ro

-C *> o ao - ao

. o

o cn • o

i—i o i—

r— O CM O fO CM QJ CD

QJ ^ Ll. S

>— ro

^ oCM *0- U ^ CM

CM O O *— CO •<“ c o co

ro co OCM O

QJ fO l_i O

U x: CM 00 CD

CM O O •— rv -r- C O co

•f- CM o

24

Liquid

1550-2085

6.286

20642

Hastie

et

al.

(1979).
Phases

3

* 0

’ 6

Temperature

K-Pressure

’

Comments

Q> t- «/) <v w a> iqo a» •— cm •»— ^ *-> 4-> W

U l—l <J>

•r- CO . o 0) cm

-C • 0) r—i +* N U. o C • i—i ai

>» 00 3E <0 * L__l

»—I t— CTO • • CO c CM e— CT\ • •r- #— • r". 4J U_l CO Z •• O ri M

_ CM O O i CM r— «T3 •*“ O < CJ CO

r- r- J

•r- a) *rTO <D •—

r- O

<D ClE

<D -r- Q. ftJ E

a» _q

- -r- 4-> *4—

u E

<DCL in

4J 4-> C

CO 0)

fO E

E

- O ^ a

in •*-

QJ — 1—

25

Compositions

constrained

to

mole

ratio

Al~0,/Si0.
(b) alkali carbonate and sulfate phases in real slag samples;

(c) non-equilibrium effects in the condensed phase;

(d) non-equilibrium between the condensed and vapor phase leading to an unsaturated vapor;

(e) changing phase boundaries due to incongruent vaporization;

(f) rapid loss of alkali pressure with time (T constant), possibly

resulting from surface depletion in highly viscous systems; and,

(g) Fe-control led redox reactions resulting in changing oxygen and,

hence, K-partial pressure data with temperature and time.

A systematic study of systems with a progressive increase in the number

of components was made in order to isolate and quantify such behavior.

Some of these exceptional cases are discussed in the following sections.

10.2 K2 0 -Al 2 03 -Si 02 System

The K2 0-Al 2 03 -Si02 system has the potential for forming several

stable or metastable phase assemblages in which the K2 0 activity is

fixed according to the phase rule (see table 2). However, in practice,

we observe the K pressure (and, hence, K2 0 activity) to be dependent on the bulk K2 0 concentration. This effect could result from several

non-equilibrium factors including, slow condensed phase kinetics attribu-

- table to the complex crystal chemistry, changing composition in the

p-alumina phase (which extends from K2 0/A1 2 0 3 ratios of 1/5 to 1/9), or

to dissolution of Si

2 in the 0-alumina phase. The vapor pressure

equation in table 2, for the three phase region, is based on the highest

alkali-pressures which were effectively independent of composition.

Other evidence for non-equilibrium behavior is shown in figure 7.

In the previous experimental run, alkali-pressure data were obtained as

a function of increasing temperature up to 1860 K. On decreasing the

temperature, lower pressures were found than for the increased- temperature

run, particularly at 1760 K and below. As shown in figure 7, continuation

of this experiment initially produced low pressures over the AB interval.

The temperature and K2 0 concentration at which this phenomenon was noted

is reasonably consistent with the phase diagram which shows eutectic

melting at 1829 K and 22 wt.% K2 0. (Levin et al., 1964). We attribute

this loss of al kal i-volati 1 i ty to formation at the sample surface of a

26
frozen eutectic melt which is probably glassy in nature. For an equilibrium system, this melt would recrystallize but for the present experimental conditions there was probably insufficient time. The phase being

depleted by vaporization is KAlSi04 (kalsilite) and the pressure-loss

results from the slow alkali transfer rate across the frozen eutectic

which results in partial isolation of the remaining KA1 Si

4

. Note, in

figure 7, the upward curvature of the AB interval with increasing temperature. This unusual behavior probably results from remelting of the

eutectic barrier-phase with an increased rate of alkali transport through

this barrier together with incorporation of additional KAlSi04 in the

eutectic melt. The BC interval of figure 7 represents vaporization from

this regenerated phase. This effect was noted only for experiments

where a small amount of the KAlSi04 phase remained and where a potassiumdeficient surface glaze acted as a diffusion barrier to vaporization.

Representative vapor pressure data for "normal" behavior in this system

are summarized in table 2.

10.3 K20-Ca0-Al 2 03-Si02 System (Simplified "Western" Slag)

These four component systems, designated as Simplified "Western"

slags in table 2, are relatively well behaved in terms of alkali vaporization and are useful model systems for sub-bituminous basic coal slags.

The data have been cast in analytical form, as summarized in table 2.

10.4 K20-Ca0-Mg0-Al 2 03-Fe2 03-Si02 System (Synthetic MHD Channel Slags of

"Eastern" and "Western" Coal Types)

Literature compositional analyses of several hundred coal ash and a few MHD channel slags, containing potassium seed, have been evaluated

for the purpose of selecting representative compositions for modeling.

Table 2 indicates the compositions selected as most representative of

MHD channel slags of "Eastern" and "Western" coal types. These slags

are non-glassy with KAlSi04 as the dominant crystalline phase (in both

slag types) and small amounts of KAlSi 2 06 and at least one unidentified

phase (for "Eastern" slag type only). Thus, the bulk composition is not

as meaningful a variable as for homogeneous glassy slags. Vapor pressure

27
measurements were made using a relatively small effusion orifice (0.34 mm

diameter) to prevent vapor unsaturation effects found earlier on similar

mixtures but with larger orifices (0.5 to 1.0 mm). Steady state pressures

were not obtained until about ten percent of the K2 0 content had been

depleted by vaporization. Excess oxygen was vaporized during this

Initial experimental phase with reduction of FeO^ (x = 1.5 to 1.33).

Following this initial reduction period, the oxygen partial pressures

were, within experimental error, what would be expected for K2 0 dissociation (reaction [3]).

Figure 8 compares vapor pressure data for "Western" and "Eastern"

slags. This comparison clearly demonstrates a basic difference between

these slag types in that "Eastern" slags require about twice as much

K2 0-content to achieve similar alkali pressures as for "Western" slags.

This effect results from the higher concentration of basic CaO and MgO

in "Western" slags leading to a less complex silicate structure in the

"Western" slag and, hence, a less bound form of K2 0.

The vaporization behavior of the "Eastern" slag, in the composition

range 24 to 21 wt.% K2 0, is very different to that for the "Western"

slag as shown in figure 9. Here, the vapor pressure curves show a significant break at location X, the position of which depends on the

amount of sample vaporized. Note the significant positive deviation of

. the observed K and 02 -pressures (XC interval) from those obtained by

extrapolating the higher temperature linear portion of the respective

curves. The curves labeled AB in figure 9 were obtained prior to the

curves labeled BC. At temperatures above 1620 K, these two curve sets are seen to merge into a single linear portion. This non-linear behavior

is attributed to a changing oxygen potential in the slag. At the higher

temperatures, the oxygen partial pressures were greater than predicted

by K20 dissociation alone (reaction [3]). However, at the lower temperatures (XC region), less oxygen was observed than expected for reaction [3].

Apparently, FeO

x undergoes reduction at temperatures above 1620 K with

release of additional 02 (above the K2 0 level). Below this temperature,

FeO^ is oxidized by the 0 2 resulting from K2 0 dissociation. It is

significant that thermodynamic equilibrium is maintained during this

redox process, as evidenced by a common K2 0 activity over the AX and XC

intervals (data not shown in fig. 9). For the lower Fe0

x content

"Western" slags this effect was not observed.

28
10.5 Real MHD Channel Slag (KJ

Detailed TMS and KMS studies were made of vapor transport over a high liquidus temperature 1700 K) potassium-enriched coal slag with

initial composition as indicated in table 2. This slag sample was obtained by combustion of Illinois No. 6 coal with additional potassium

added to the combustor (see Hastie et al., 1980b). Note that this slag

composition lies between those of the "Eastern" and "Western" coal-types.

For identification purposes, this slag is given the designation K x. X-ray diffraction data indicated that the bulk of the slag potassium was present as the compound KAlSi04

. TMS analysis indicated that about

two percent of the slag potassium was present in relatively volatile

form, mainly K2 S04 and K2 C03 .

10.5.1 Identity of Volatile Species

The as-received potassium-enriched coal slag was subjected to

a series of heating cycles (runs) in nitrogen carrier gas. During the

initial heating cycle, mass spectral scans, obtained using the TMS

technique, revealed many volatile species in addition to the expected K

and Na species. The following species were positively identified: H 2 0,

C02 , S02 , 02 , K, and Na. Some of the other low- intensity ion signals

can be very tentatively assigned to the species (some hypothetical): K0 or K0H, KS or KSH, SiS, SiSH, H2 $, H2 S04 , and KSiO. From JANAF (1971), we can expect to see K0H under these conditions. Some of these more minor species may result from slag occlusions and metastable phases and

most likely do not represent an equilibrium release from the slag.

Following this initial heating cycle, the only significant slag vapor

species were K and 02 , and these were present in the approximate stoichiometric ratio expected for K2 0 decomposition.

10.5.2 Initial Species Partial Pressure—Temperature Dependence

The initial volatiles showed a non-monotonic variation of

partial pressure with temperature, as shown in figure 10. These volatiles

constitute only a few percent of the total slag components and are not

29
representative of the bulk slag composition. However, they do provide a sufficiently high flux of alkali (Na, K) and S0 2 to be a potential

source of corrosion in downstream MHD components. The high initial

partial pressures of S02 , C02 , K, and Na are indicative of the presence

of alkali sulfate and carbonate in the slag. An additional contribution

to low temperature (T < 1300 K) alkali release could result from the

high H2 0 content leading to the formation of volatile hydroxide species

(KOH). However, no definitive hydroxide signals were observed. (The

discussion of Section 11 is pertinent in this regard.) Note that at

T > 1400 K, the potassium pressures fall below those expected from

KA102 , but that the S02 , C02 , and H2 Q pressures are still relatively

high. Apparently, at this stage, the K produced by sulfate and carbonate

decomposition is retained in the bulk slag. After further heating, the

sample was virtually depleted of Na, S02 , and C02 ; H2 Q also continued to

fall -off in pressure to a negligible level. Following this initial

clean-up period, the sample showed a more normal vaporization behavior

and representative data are summarized in table 2.

10.5.3 K2 Q Activity Coefficients

Most of the bulk-slag composition changes result from K2 0

dissociative vaporization. Therefore, a continuous monitoring of the

K-partial pressure (and 02 ) allows one to calculate the slag composition

at any stage of an experiment using relationships (1), (2), and (4) and

the known initial sample weight and composition. For both the TMS and

KMS methods, the basic experimental requirement for monitoring the bulk

composition is the measurement of significant species partial pressures

as a function of time during an experimental run. An independent check

on this approach can be provided by chemical analysis of the sample

remaining at the end of a run. In general, when all the significant

species are measured and the ionization cross sections are known (though

not necessary in the present case), this jjn situ approach to monitoring

composition changes provides a good mass balance at any stage of the

experiment, as was shown for the NaCl and Na2 S04 test systems reported

elsewhere (Bonnell and Hastie, 1979).

30
Since the mole fraction of K2 0 can be defined at any stage of an experiment, it is possible to convert K-partial pressures to K2 0

activity coefficients using equation [6]. By varying the amount of K2 0

present in the slag duri ng a vaporization run, we were able to follow

the dependence of the K2 0 "apparent" thermodynamic activity on temperature and composition. The term "apparent" is used to emphasize that the

slag system may not always be in a state of complete thermodynamic

equilibrium. Typical data, expressed in activity coefficient form

[ Y (K2 0)], are given in figure 11. Most of these data were obtained

below the liquidus temperature. Good agreement was obtained between the

TMS and KMS-based data at relatively high temperatures 1600 K). From

this observation, we can conclude that the high temperature data represent

thermodynamic equilibrium because of the greatly different residence

time scales involved, i.e., TMS ~ 10 sec and KMS ^ 0.04 sec. The largest

experimental uncertainty in comparing activity coefficient data from the

two different techniques (KMS and TMS) is the accuracy of the K2 0 mole

fraction, which is probably uncertain by ten percent in each case. Comparison of these data with those for the K2 0-Si02 system, for instance,

at 10 mole % K2 0 in each case and 1700 K, indicates a Kj slag activity

coefficient of about 30 times that for the binary system. This observation is consistent with the more basic character of the Kj. slag.

Note in figure 11, the non-monotonic nature of the activity

coefficient curves. For normal non- ideal solution behavior, we would

expect a linear monotonic relationship with a negative slope representing

a negative partial molar enthalpy of solution for K2 0 in the slag. This

type of behavior occurs for segments of each run (see fig. 11), e.g., for

run 1, up to about 1430 K, and for run 2 between 1430 and 1630 K. However, the run 1 data are anomalous as they were obtained during the

initial heating period when K2 S04 and K2 C03 decomposition was a significant

source of additional K. The rapid reduction in y(K2 0) as the temperature

is increased beyond 1430 K results from the virtually complete depletion

of these relatively volatile forms of potassium. For runs 2 and 3, the

initial reduction of y(K2 0) with increasing temperature is believed to

be due either to diffusion limited (in solid slag) K-transport to the

slag surface or to changes in the mode of 02 release from the slag, for

instance, through Fe3 04 dissociation. The onset of increasing y(K2 0)

31
with temperature is believed to arise either from an as-yet unspecified

physiochemical change in the slag, leading to a less viscous (but still

solid) form and increased diffusion, or to depletion of secondary 02 sources. At higher temperatures, the bulk composition changes rapidly

(see mole fractions in fig. 11) and leads to a peaking in y(K2 0). We

believe that the data for runs 2 and 3, at temperatures in excess of

1450 K, represent an equilibrium vaporization condition, particularly as the KMS and IMS data are in agreement for these conditions. Clearly,

these unusual trends in the y(K2 0) data indicate the difficulty involved

in making a priori predictions of real slag vaporization behavior.

10.6 Synthetic Low Melting Slag (K2 )

A lower liquidus temperature (^ 1480 K), less viscous (as compared

with the K x sample) synthetic slag was prepared for studies analogous to

those performed for the K* system. It was hoped that this slag would

not show the same anomalous activity behavior, at lower temperatures, as for the Ki-MHD coal slag sample.

Under free vaporization (KMS) or N2 -atmosphere (TMS) conditions,

the predominant vapor species in this system are K and 02 , as shown in

figure 12. The initial excess of 02 present in this slag is believed to

result from the following sources. First, the preparation procedure of

melting and pouring in room air may have led to oxygen absorption by the

sample. Second, pressure bursts of K and 02 (and C0 2 ) were noted in the

initial phase of the TMS experiments and particularly near the liquidus

temperature, e.g., at 1500 K in figure 12. This effect is attributed to

K2 C03 impurity. Third, reduction of Fe2 03 to Fe3 04 , with release of excess 02 , is favorable at these temperatures and, in fact, has a similar

temperature dependence to that of the initial 02 data shown in figure 12.

Using the JANAF (1971) thermochemical data for Fe304 and Fe 2 03 , it is

possible to predict 02 partial pressures for given condensed phase

activities. On this basis, the initial experimental data of figure 12

at 1600 K, for instance, are consistent with ~ 50 percent and 25 percent

Fe 2 03 reduction for the KMS and TMS experiments, respectively.

The KMS data were obtained using the integrated ion intensity-weight

loss method of pressure calibration [Eq. (2)], taking into account the

32
additional weight loss due to Fe2 03 reduction. Calibration of the TMS

data, on the other hand, was made using the relative ionization cross section approach [Eq. (3)]. The apparent difference between the KMS and

TMS data, indicated in figure 12, is related to the problem of additional sources of 02 already mentioned. That is, the TMS data were obtained at

an earlier stage of the sample history, where the high 02 pressure

depresses the K-pressure by the mass-action effect. In fact, if the

data are converted to K2 0 activities, the KMS and TMS data are in satisfactory agreement. Such agreement is good evidence of system thermodynamic equilibrium. At a later phase of the KMS experiments, the 0 2 /K

pressures were of the correct stoichiometry for K2 0(£) decomposition.

A second type of time dependent phenomenon was observed for this

slag using the TMS method, as shown in figure 13. Once an isothermal

condition was achieved, the K-pressure decreased with time. This result

could be taken as evidence of surface depletion of K (and 02 ) from the

sample, due to the bulk diffusion rate being too small relative to the

surface vaporization rate. Also, this effect was found to be much less

pronounced at higher temperatures where the diffusion rates are higher.

However, this initial interpretation (Hastie et al., 1980b) no longer

seems reasonable. The order-of-magnitude greater vapor transport rates

for KMS vs TMS experiments would indicate lower apparent activities in

the former case due to surface depletion effects. However, in practice

the KMS activities are somewhat higher. Hence, this time dependent

phenomenon is attributed to the combined effects of K2 C0 3 impurity

decomposition, as noted previously for the analogous glass system in

Section 9, and, to FeO

x reduction, as noted with the "Eastern" slag data

in figure 9. That this latter effect was more apparent in the TMS, versus the KMS, experiments can be attributed to the much higher transport

rates and shorter FeO reduction times for the latter case.

A

10.7 Application of Vaporization Data to Seed-Slag Interaction

A key factor for successful MHD operation is the degree of interaction between plasma potassium seed and the slag medium. Using slag

activity data from the present studies, it is possible to predict conditions under which plasma seed will be continuously depleted by slag

33
absorption of alkali. Plante et al. (1975) presented similar arguments

earlier, based on their data for the binary oxide systems. A more definitive analysis can now be made from the present data on complex

synthetic and actual slag systems.

The stability of K2 0 in slag solutions can be readily determined

by comparison of the K2 0 dissociation pressure-product (P

k

2 *Pq ' data

in the slag phase with that in the plasma phase, as represented in

figure 14. This dissociation pressure (DP) expression is a convenient

representation of slag activity data [activity = DP, soln./DP, K2 0(£)].

The DP curves for the plasma phase were calculated using a multicomponent

equilibrium computer program, assuming stoichiometric combustion of

CH

q j

with air (4/1 mole ratio N2 /02 ) and KOH, K, KO, K+

, and e as vapor

phase species. Pressures of ten and one atmosphere were chosen to be

representative of the MHB combustor and the channel -diffuser-downstream

seed recovery units, respectively. Corresponding curves for the actual

MHD slag (K x ) and the Synthetic "Western" and Simplified "Western"

slags, were calculated from the experimental vapor pressure data given

in table 2.

Comparison of the DP curves for plasma and slag indicate temperatures for K2 0 slag saturation in the range of 2100 to 2300 K at 10 atm,

and 1850 to 2050 K at one atm, depending on the slag type and composition.

The approximately 200 K equivalence temperature difference between

actual and model slags is attributed, primarily, to the low CaO concen- tration in the former case. Slag absorption of K2 Q at the channel and

diffuser surfaces depends strongly on the plasma-surface interface

temperature. Experimentally, these interface temperatures are difficult

to measure though the data reported by Self (1979) indicate that they

are in the region of 2100 to 2300 K. At these temperatures, and at

atmospheric pressure, slags with at least 18 wt.% K2 Q are predicted to

be stable with respect to the plasma phase, as indicated in figure 14.

Such a prediction is in accord with the observed 19.5 wt.% K2 0 content

of the actual MHD slag (Kj). Conceivably, calculations of this type,

together with actual slag analyses, could be used to infer the actual

interface temperatures.

34
Note that the DP equivalence temperatures in figure 14 are significantly lower for the atmospheric conditions characteristic of downstream

units. Also, as the temperature decreases, the slag dissociation pressureproduct decreases much faster than for the plasma, resulting in a supersaturated alkali vapor concentration at the plasma-slag interface.

Hence, additional K2 0 will be absorbed by the slag. Below the dewpoint,

this seed will tend to deposit on the slag surface in a relatively

non-bound form, such as the alkali sulfate or carbonate, which is desirable

from a seed-recovery point of view. As the temperature decreases, the

reduced slag species diffusion rates and the increased tendency for

phase separation to occur will allow K2 S04 and K2 C03 deposits to remain

at, or near, the slag surface, as is found in practice. Indirect evidence

of this type of deposition may be seen in figure 10.

10.8 Salt-Slag Alkali Exchange

The common disposition of alkali in coal minerals is Na as NaCl and

K as K2 0--bound in a low-activity silicate phase. Thus, during coal

conversion, Na is expected to be released to the vapor phase more readily

than K. However, the possibility of NaCl-K2 0 (slag) interaction to

produce KCl-Na2 0 (slag) could greatly enhance K-release to the vapor

phase. Also, in MHD slags, where about 20 wt.% K2 0 content is possible,

the problem of recovering this lost seed could likewise be resolved

through replacement by NaCl. The feasibility of such an exchange process

was tested by a TMS monitoring of the vapor phase over the system,

NaCl + Kx slag (19.4 wt.% K2 0). Details of this study will be given

elsewhere (Hastie et al., 1981b), but the main observations are as follows.

When a thin layer of powdered NaCl was present on the surface of

the K x slag, a rapid exchange reaction occurred near the melting point

of NaCl , i . e.

,

NaCl (£) + K

2

0 (slag) = KC1(£) + Na

2

0 (slag) .

This result is demonstrated in figure 15, where the observed partial

pressures of NaCl and KC1 are expressed in thermodynamic activity form.

35
Note the marked decrease in NaCl activity and concomitant increase in

KC1 activity just above the melting point of NaCl. After a heating

period of about 50 min., the NaCl sample was virtually depleted, as was

the KC1 product. Insufficient salt was present in the initial mixture

to convert all the available K2 0 to KC1. However, 90 percent of the

initial NaCl was converted to Na2 0 (slag) with stoichiometric release of

KC1. About six percent of the available K2 0 was converted to KC1 vapor,

and we expect that nearly complete removal of K2 0 from the slag would

have been possible if sufficient NaCl was present. The remaining ten percent NaCl was lost by vaporization before, and during, the exchange

process. During the isothermal, constant activity, phase of the exchange

process (20 to 40 min. region of fig. 15), a potassium vapor transport

enhancement factor of,

P K (no NaCl)

% 10

was observed. Also, during this period, the high KC1 activity suggests

formation of an essentially ideal solution of KCl-NaCl, as well as the

establishment of thermodynamic equilibrium. Note the near unit NaCl

activity in the initial phase of the experiment (fig. 15), which confirms

the calibration factors used to convert mass spectral ion intensities to

partial pressures and reflects establishment of thermodynamic equilibrium.

Additional study of this exchange process is in progress.

11. Heterogeneous Reactive Gas Systems

11.1 Background

In the previous sections, we have considered alkali vapor transport

from condensed phase systems in the absence of external influences, such

as reactive gases. However, some of the component gases of combustion

systems, such as H 2 0, HC1 , S02 , 02 , CO, and H 2 , can be expected to

significantly modify alkali vapor transport through mass action effects

or formation of new molecular species. Some representative cases are considered as follows.

36
11.2 Na2 S04 “Na0H-H2 0 System

From a thermodynamic viewpoint, H2 0 should react with Na2 S0 4 to

form NaOH vapor at high temperatures. However, surprisingly, on the

relatively long time scale of TMS measurements, no such reaction was observed. When a small amount (0.25 to 1 percent) of condensed NaOH was mixed with the Na2 S04 , reaction with H 2 0 vapor was observed. Also, the

alkali vapor pressure enhancement was thermodynamically consistent with

the process,

H

2

Q + Na

2

S0

4

(c) = 2NaOH(g) + S0

2 + 1/2 02

.

Details of this study will appear elsewhere (Bonnell et al . , 1981).

11.3 Illite -H2 0-H2 System

Vaporization of potassium from the highly acidic illite system, in

neutral atmospheres, is expected to provide a relatively insignificant

source of alkali in most coal combustion systems (see Section 8.2).

However, in the presence of reactive combustion gases, such as H2 0 and

H2 , thermodynamic considerations predict a significant KOH partial

pressure. In addition, an increase in the K-pressure should result from

a reduction in the 0 2 pressure, in the presence of H2

. However, KMS

experiments did not indicate formation of KOH or additional K in the

presence of H2 gas. Thus, thermodynamic equilibrium does not appear to

have been established in this heterogeneous system, even though the

temperatures were sufficiently high to have normally ensured a rapid

approach to equilibrium.

Further evidence of this lack of thermodynamic equilibrium was provided by monitoring formation of SiO by H2 reduction of Si02 , present

either as the silicate in illite or as pure silica. Figure 16 shows

that SiO production from both forms of Si02 is about one to two orders

of magnitude less than the equilibrium values represented by the

JANAF (1971) curve. In the absence of H2 or H 2 0, the SiO and 02 pressures were reasonably consistent with an equilibrium system.

37
The kinetic limitation does not appear to result from the heterogeneous reaction process but, rather, from a lack of equilibrium for the

homogeneous reaction,

H

2

0 = H

2 + 1/2 02

[ 8 ]

as shown in figure 17. When H2 was the added reactant, Kp for this

reaction was found to be about two orders of magnitude greater than the

accepted value. This discrepancy would result if a large fraction of H 2 effused through the Knudsen cell without reaction. However, when H2 0 was the added reactant, K was also greater than the known value and by

P

an order of magnitude. This case represents closer agreement over the

system where H2 was added. Some of this improvement may result from the

lower H2 0 pressures used and/or the possibility for H 2 0 to undergo

decomposition on the effusion cell surfaces. To form H 2 0, however, H2 must undergo one or more collisions with the oxide sample surface. The

points agreeing most closely with the theoretical H2 0 dissociation curve

in figure 17 were obtained by assuming that P u = 2Pn , rather than using

u2 the measured P u to calculate K . This assumption would be true for a

m2 p

static system in which H2 and 02 are formed only by decomposition of

water. For Knudsen effusion, the rate of escape is inversely proportional

to the molecular weight and, at steady state conditions, P u = 1/2 Pn . n2 u2 Under these dynamic conditions, K

p

would be 1/4 the static value. Vaues

of K calculated on the basis of this assumption are not shown in figure 17.

r The K values for the dissociation of H2 Q using either of these two

r assumptions scatter within a factor of two around the JANAF (1971) curve over most of the temperature range. At the highest temperatures, the

agreement is less satisfactory because of excess 02 production from

residual illite in the effusion cell. Values of Kp for water addition,

where the observed P u is used to calculate K , indicate that the mass

h2 p

spectrometric method is overestimating the H2 pressure by almost an order of magnitude. This error could result from H2 0 reduction on the

furnace element and/or the outer surface of the Knudsen cell. However,

we have observed this lack of equilibrium for the H2 0 decomposition

reaction in other systems and, more particularly, with the TMS method,

38
which has less opportunity for furnace decomposition reactions, as well

as a much longer characteristic gas residence time.

If we consider the rate limiting step in the homogeneous H2 0-02 -H 2 system as,

0

2 + H2 = 20H ,

then the literature kinetic data (Baulch et al., 1972) indicate reaction

times in the approximate range of 10 to 100 sec for our experimental

conditions (~ 1600 K). Thus, the TMS residence time range of 1 to

20 sec is marginal for equilibrium in the H2 0 system, and the KMS system

is even less favorable (as observed experimentally). However, at temperatures in excess of 2000 K, this limiting rate is several orders of

magnitude faster, and equilibrium can be attained, as noted earlier

(Hastie et al. 1980b).

Evidence of non-equilibrium in H2 0-H2 -condensed phase systems was also noted recently by Sasaki and Belton (1980). They observed 02 partial pressures one to two orders of magnitude less than the equilibrium

values at a temperature of 1423 K over liquid Cu. However, in the

presence of a Pt wire catalyst, they were able to closely approach

equilibrium at residence times not too different from those of the

present mass spectrometric studies.

11.4 Synthetic Slag (K2 ) - H2 0 - H2 System

In order to extend the vapor transport conditions in slag systems

to a reducing hydrous environment similar to that present in coal gasification, a series of TMS and KMS measurements were made using H 2 or H 2 0 as the initial reactant gas. With the TMS system, compositions of

H 2 -N2 -H 2 0 up to 10 vol % H 2 were attained prior to hydrogen- induced

corrosive loss of the transpiration reactor.

11.4.1 Effect of H 2 on K-Vaporization

As H 2 was introduced to the slag system, the 0 2 concentration

decreased and K and H 2 0 increased, as expected for the process,

39
K2 0(s1ag) + H 2 = 2K + H2 0. [9]

Typical IMS data are given in figure 18 where the H 2 partial pressure -4-2 . . was varied over the range 10 to 10 atm. Note the pronounced hysterisis

effect for increased versus decreased H2 and H 2 0-content. Though not

shown here, this effect is also present in the K2 0 activity data, as calculated from the observed K and 02 -pressures. Hence the system is

not at thermodynamic equilibrium. From the established equilibrium

constants for reaction [3] with K2 0 (slag) and K2 0 (pure liquid), together

with the measured K2 0 activity data, we calculate,

K [9] = 209 at 1650 K. P

The corresponding experimental value, obtained from the measured partial

pressures of K, H2 , and H2 0, is

K [9], obs. =4.2 at 1650 K. p

Thus the system is far from equilibrium.

11.4.2 Effect of H 2 Q on K-Vaporization

Similar TMS experiments were performed but with H2 0 as the

added reactant and a non-reducing atmosphere. An unexpected K-pressure

dependence on H2 0 was found, as shown in figure 19. No hysterisis

effects were observed in this case. A similar, though less pronounced

(factor of four less effect on K-pressure), H2 0- induced K vaporization

effect was noted in the more acidic and more viscous MHD (Kj) slag

sample (Hastie et al., 1980b).

For the H2 0-pressure and temperature conditions used, K0H

should have formed according to the process,

K2 0 (slag) + H 2 0 = 2K0H. [10]

However, no K0H was observed in the TMS mass spectra. We also established

the K+ precursor as atomic K, from the pure K0H data (Section 7.3) and

40
appearance potential measurements. A higher temperature study, using

the KMS method, did show the expected formation of KOH in the presence

of added H2 0. However, the KOH-pressures were about an order of magnitude

below predicted equilibrium values, even though the correct H 2 0 pressure

dependence was found, as shown in figure 20.

The apparently anomalous H2 0-induced increase in K-pressure

can be explained as follows. Literature water solubility data for

aluminate and silicate melts [e.g., see Schwerdtfeger and Schubert

(1978)] suggest solubilities of at least several hundred ppm for our experimental conditions. Various acid-base reaction mechanisms have

been suggested to explain water solubility in silicate melts, as summar- ized by Turkdogan (1980). For basic melts, H 2 0 acts as an acid and

enhances the silicate network structure, and vice-versa for acid (high

silica) melts. Though apparently not previously recognized, these

structural changes should be reflected in the alkali activity data.

Thus, we can reasonably expect an activity increase when water is incorporated into the silicate matrix of the relatively basic K2 slag.

Reaction sets of the type,

H

2

0 + 0

2 ‘ (slag) = 2(0H")

K

2

0 (slag) = 2K + 1/2 0

2

, and

0

2 + 2e = 20

2 “,

would be consistent with the observed one-half power dependence of

log P^ on log P^

Q

(see fig. 19). These structural changes should also

be reflected in viscosity data. The water-solubility viscosity enhancement

effects noted by Brower et al. (1980) for a similar slag are consistent

with the present activity trends. For more basic systems a decreased

alkali activity is possible. The recent observations of Gray (1980),

where water vapor decreased alkali vaporization rates in low silica

glasses, could be interpreted in this manner. We believe that a similar water vapor solubility enhancement

of alkali vapor transport is possible in soda-lime-silica glass systems,

and work is in progress to verify this. Some of the disparities between

41
various glass vaporization studies may well result from variations in

water content and, hence, alkali activities. The common explanation for

water vapor enhanced alkali vapor transport over silicates has revolved

around formation of volatile NaOH (Sanders and Haller, 1977) and KOH

(Charles, 1967) species. However, no direct test for the presence of

these species has been made, and the possibility of water vapor enhancement of atomic Na and K transport exists in these systems.

Mention should also be made of the possible effect of H2 0

dissociation (i.e. , reaction [8]) to yield vapor phase H2 , which was suggested by Horn et al . (1979) as a factor in ceramic degradation.

However, in the present system there is an additional source of 02 and,

at thermodynamic equilibrium, K should be H2 Q- independent except for the

noted H2 0-solubility effect.

11.5 Glass-Combustion Gas System

Certain combustion gas components can promote alkali vapor transport

in glass systems. Such transport is important in glass melting. Also,

glass had been suggested as a medium for trapping particulate material

in combustion gas clean-up processes, such as for pressurized fluidized

bed combustion (Gatti et al . , 1980).

Using our experimental activity data for Na2 Q in glass (see

Section 9.2), we have modeled the effect of a typical combustion gas

mixture on alkali vaporization (Hastie et al .

,

1980a). For this purpose we have acquired, and adapted to our computers, a code known as SOLGASMIX

(Eriksson, 1975) which is unique in its ability to deal with non- ideal

solution multicomponent heterogeneous equilibria. Previous attempts to

model this type of problem have been limited to ideal solution assumptions

(Kirkbride, 1979). As is demonstrated in table 3, if solution non- ideality

is neglected, drastic errors result in the prediction of alkali vapor

transport processes. Table 3 and figure 21 summarize the predicted

alkali species partial pressures. The thermodynamic data base was constructed mainly from the JANAF (1971) compilation. Additional details

of this study will be presented elsewhere.

42
Table 3

Effect of Atmosphere and Solution Non- Ideality

on Alkali Vapor Species Distribution

Glass : Na

2

0 (17 wt.%), CaO (12 wt.%), Si0

2 (71 wt.%)

Initial Gas Composition (mole percent): H„0 (4), 0

2 (5), C0

2 (12),

N

2 (76), S0

2 (2), HC1 (l)

d

Temperature: 1400 K

Total Pressure: one atm

Non-Ideal Soln. Ideal Soln. Non-Ideal Soln.

N

2

atm. Gas atm.

b

Gas atm.

b

Species Pressure (atm) Pressure (atm) Pressure (atm)

Na 3. l(-7) a2. 2(-4) 1.0(-7)

NaOH — 8.5(-2) 9.2(-5)

NaCl — 9.8(-3) 8. 8(-3)

Na

2

Cl

2 — 7.1(-4) 5.8(-4)

Na

2

S0

4

-- 4. 4(-6) 4.4(-6)

Na

2

0 Activity0 3.4(-8) 1 . 62(- 1

)

3.4(-8)

Total Na 3.K-7) 9. 6(-2) l.U-2)

No Halogen 3.K-7) 8.5(-2) 1.0(-2)

aComputer notation, e.g., 3.1 (-7) = 3.1 x

10" 7

.

^Refers to mixed gas composition given above.

Q Infinite supply of glass assumed and activity not affected by vaporization loss.

-j Halogen content represents an upper limit for combustion of a high-halogen

coal in a salty (marine location) atmosphere. For typical low-halogen

U. S. coals, the chloride concentration would be ~ 0.01 mol % and the

NaCl pressures correspondingly less.

43
The principal results of these calculations can be summarized as follows:

(a) The presence of one percent halogen enhances alkali transport

by more than two orders of magnitude at 1200 K, but only by a factor of four at 2000 K. (b) Only in a N 2 atmosphere is atomic Na a significant vapor

species.

(c) If glass is treated as an ideal solution, then halogen has

only slight enhancement effect on alkali transport, i.e., NaOH

is the predominant species.

(d) Glass non- ideality reduced alkali transport by an order of

magnitude in the presence of halogen but by three orders of

magnitude in a halogen-free reactive atmosphere.

(e) Sulfur has a much lower effect than halogen, or H2 0, on alkali

transport and for the lower temperature range (< 1500 K) most

of the sulfur is removed from the gas stream due to Na2 S04 (£)

formation. Note, however, (in fig. 21) the non-monotonic

production of Na2 S04 vapor species with temperature, leading

to significant alkali vapor transport over the temperature

interval 1300-2000 K. (f) At 1200 K, alkali vapor transport covers the range of 7 to

1200 ppm, depending on the absence or presence of halogen,

respectively.

From these results, it is clear that alkali release from glass

under combustion gas atmospheres will be a significant source of alkali

“6

(i.e., >10 atm) in the combustion gas stream at temperatures greater

than 900 K, even for low-halogen combustion conditions. We should

stress that these predictions are sensitive to the assumption that the

gas stream attains thermodynamic equilibrium with the glass substrate.

Our experimental data on sulfate and complex oxide systems indicates

that such an equilibrium is not always readily attained. Incorporation

of rate processes into heterogeneous reaction models must await further

experiments to develop the necessary data base and mechanistic understand!' ng.

44
12. Summary

During the past few years, substantial progress has been made in

developing a thermodynamic data base and in providing mechanistic insight

into the vaporization processes for alkali metal salt, oxide, silicate,

and complex slag systems. Experimental techniques have been improved or newly developed suitable for thermodynamic studies of complex, corrosive

alkali-containing systems, including coal slags. Computer codes are now available for thermodynamic calculations of heterogeneous non- ideal

solution multicomponent equilibrium systems, such as for combustion-coal

slag interactions. Vapor phase problems remaining include: (a) possible

formation of novel species at very high gas pressures (10 to 100 atm);

(b) lack of mechanistic understanding and rate data for non-equilibrium

heterogeneous systems; (c) non-availability of computer codes for heterogeneous rate-limited systems; and (d) development of diagnostic methods

and data for actual coal -conversion and combustion systems, including

MHD channels, coal gasifiers, pressurized fluidized beds, and gas turbines.

13. Acknowledgments

A significant portion of this work was supported by the Department

of Energy. Mr. Art Sessoms provided valuable technical assistance.

Valuable discussion with our colleagues, Drs. W. Horton and L. Cook, is

also acknowledged.

45
14. References

Anthony, A. M. (1976). Rev. Int. Hautes Temp. Refract. 230.

Argent, B. B. , Jones, K. , and Kirkbride, B. J. (1980). "Vapors in

Equilibrium with Glass Melts", in The Industrial Use of Thermochemical

Data , Barry, T. I. , ed. , p. 379, The Chemical Society, London, UK.

Baulch, D. L. , Drysdale, D. D. , Horne, 0. G. , and Lloyd, A. C. (1972).

"Evaluated Kinetic Data for High Temperature Reactions", Vol 1, Butterworths, London and Washington, DC.

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?

S04-H

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Boow, J., (1972). Fuel, 51., 170.

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be published.

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J. Am. Ceram. Soc. , 5^, 574.

46
Gangwal , S. K. , and Truesdale, R. S. (1980). Energy Res. 4, 113.

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449.

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Hastie, J. W. , Bonnell, D. W. , Plante, E. R. , and Horton, W. S. (1980b).

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NBSIR 80-2178.

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Hastie, J. W. , Plante, E. R. , and Bonnell, D. W. (1979). "Molecular

Basis for Release of Alkali and Other Inorganic Impurities from Coal

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published.

Hastie, J. W. , Plante, E. R. , and Bonnell, D. W. (1981b). "Slag-Alkali

Halide Exchange Reactions", to be published.

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48
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49
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50
15. Figure Captions

1. Transpiration reactor showing internal details.

2. Partial pressure mass effusion data for molecular K2 S0 4 over K2 S0 4 (£)

(Plante et al . , 1981).

Open cirlces—present study, open triangles--Lau et al . (1979),

closed triangles--Efimova and Gorokhov (1979).

3. Partial pressure data for KOH and (K0H) 2 over liquid KOH, obtained

by TMS. Curves labelled Gusarov refer to data of Gusarov and

Gorokhov (1968). JANAF curves are from JANAF (1971).

4. Partial pressure data for K, 02 , and SiO over illite. The triangular

points were obtained by the TMS method with a N2 atmosphere, all

other data were obtained by KMS under vacuum vaporization conditions.

The closed symbols refer to 02 with ticks indicating increasing (up) or decreasing (down) temperature chronology.

5. Comparison of glass melt Na partial pressure data obtained by

various workers (see text) for compositions (wt.%) similar to Na2 0(17),

Ca0(12) , and Si0 2 (71). KMS(l), 17.0 to 16.7 and KMS(3), 15.6 to

13.3 wt.%. TMS(l), 17.0 and TMS(2), 16.9 wt.%.

6. Activity coefficient data (TMS) for Na2 C03 (0.45 wt.%) in glass

(see caption 5 for glass composition). Open circles and squares

refer to run-chronology of increasing and decreasing temperature,

respectively.

7. Non-equilibrium vaporization effect in the K2 0-Al 2 03 -Si02 system.

Broken curve represents the pure KAlSi04 phase. Run chronology for

the partially decomposed KAlSi04 system follows the temperature

sequence ABC (KMS data).

8. Selected potassium partial pressure data (KMS data) for synthetic

"Western" (W) and "Eastern" (E) MHD-channel slags with K2 0 compositions (wt.%) of W2 (19 to 17.6), W3 (12.9 to 10.8) and E (23.3 to

22.1). "Eastern" slag data points omitted for clarity, but they are of similar precision to the "Western" slag data (see Plante and

Cook, 1978).

9. Selected potassium and oxygen partial pressure data (KMS) for

a synthetic "Eastern" MHD-channel slag with composition (K2 0 wt.%)

23.3 to 22.8 (AB interval) and 22.8 to 22.1 (BC interval). Run

chronology follows the temperature sequence ABC. Open and closed

circles (and solid curves) refer to potassium data, with increasing

and decreasing temperature run-chronology, respectively. Open and

closed triangles (and dashed curves) refer to oxygen data, with

increasing and decreasing temperature run-chronology, respectively.

51
10. Partial pressure variation of initial volatiles (K2 0, 19.5 to 19.1 wt.%)

as a function of temperature (and time) for the K x slag (liquidus

temperature ~ 1700 ± 30 K) using the TMS approach (run 1).

Conditions: 0.5 atm N 2 , capillary probe. Dashed comparison curves

represent K-pressures over the K2 C0 3 and KA10 2 phases.

11. KMS data for variation of K2 0 activity coefficient with temperature

and composition for the K x slag. The numbers, ranging from 0.154

to 0.08, refer to the mole fraction of K2 0 remaining in the sample

at each measurement point. Runs 1 to 3 were carried out consecutively

on the same sample. The open square data point at 1575 K, run 1, was obtained by TMS with additional 02 present.

12. Vaporization of K and 02 from the K2 slag. Open circles--02 (KMS),

closed circles--02 (TMS), open triangles--K (KMS), closed triangles--

K (TMS). Chronological order of data taken with increasing temperature except for ticked data points where the temperature was decreasing.

13. Typical K+ ion intensity signal-decay with time for K2 slag. TMS

conditions: temperature, 1570 K; N2 carrier gas pressure, 0.21 atm;

capillary nozzle.

14. Comparison of K2 0 dissociation pressure-product data for the MHD

plasma with 1 wt.% K2 C0 3 seed (broken curves) and slag (solid curves)

phases. Curve A is for the actual slag (Kj) with 14 wt.% K2 0

content, B is for the Synthetic "Western" slag with 18 wt.% K2 0,

and C is for the quarternary Simplified "Western" slag with 15 wt.%

K2 Q.

15. Thermodynamic activities (TMS data) for NaCl and KC1 in the K x slag-alkali exchange process. The indicated reference state

partial pressures were obtained from JANAF (1971).

16. Silica activity function (KMS data). K refers to the reaction,

Si02 (c) + H2 = Si 0 + H 2 0. Open circlespare for illite and closed

circles represent similar data obtained using quartz but with a much larger sample surface area. Dashed curve represents JANAF (1971)

data for unit activity silica.

17. Equilibrium constant for H2 Q dissociation (KMS data). Open circles,

data obtained during illite + H2 experiment; open squares, H 2 0

rather than H2 addition; closed circles, P u assumed equal to 2Pn n2 u2 for Kp calculation. Dashed curve, JANAF (1971) data.

18. Variation of K-pressure (open symbols) and 02 -pressure (closed

symbols) with H2 0-pressure for K2 slag in the presence of added

H2

. TMS conditions: temperature, 1655 K; N 2 carrier gas pressure,

0.18 atm; capillary nozzle. Arrows indicate run chronology.

19. Isothermal (1610 K) dependence of K-pressure on H2 0-pressure for

the K2 slag with added H2 0. TMS conditions: N2 carrier gas

pressure, 0.21 atm; capillary nozzle.

52
20. Isothermal dependence of KOH-pressure on H2 0 apparent pressure for

the K2 slag at 1794 K. KMS data with Pt-cell orifice diameter of

0.34 mm. The curve of slope 0.5 represents the theoretical pressure

dependence for reaction [10] (see text).

21. Computer-calculated distribution of al kal i -containing vapor species as

a function of temperature for the non- ideal solution glass-combustion

gas system specified in table 3.

53

Figure 1

BEAM
PARTIAL

PRESSURE

K

2

S0

4

(ATM)

TEMPERATURE (K)

1428 1250 1110

10 4 / T (K)

Figure 2
PARTIAL

PRESSURE

(ATM)

TEMPERATURE (K)

Figure 3
PARTIAL

PRESSURE

(ATM)

TEMPERATURE K

5.0 6.0 7.0

104 / T (K)

Figure 4
Na

PARTIAL

PRESSURE

(ATM)

TEMPERATURE (K)

1670 1430 1250

Figure 5
LOG

ACTIVITY

COEFFICIENT

10 4 / T (K)

Figure 6
POTASSIUM

PRESSURE

(ATM)

TEMPERATURE (K)

1923 1724 1515

Figure 7
PARTIAL

PRESSURE

K

(ATM)

TEMPERATURE (K)

10 4 / T (K)

Figure 8
PARTIAL

PRESSURE

OF

K

AND

Oo

(ATM)

TEMPERATURE K

1700 1500

Figure 9
PARTIAL

PRESSURE

x

10°

(ATM)

TEMPERATURE (K)

1620 1430 1250

Figure 10
KjD

ACTIVITY

COEFFICIENT

TEMPERATURE K

1900 1700 1500 1400 1300 1200

Figure 1
PARTIAL

PRESSURE

(ATM)

TEMPERATURE K

Figure 12
ION

INTENSITY

Figure 13
DISSOCIATION

PRODUCT

WALL

4.0 5.0

10 4 / T (K)

Figure 14
SALT

ACTIVITY

TEMPERATURE (K). NON-LINEAR

770 1074 1190

Figure 15
TEMPERATURE K

Figure 16
Figure 17
PRESSURE

(ATM)

Figure 18

0

2

PRESSURE

(ATM)
.PRESSURE

K

(ATM)

Figure 19
PRESSURE

(ATM)

APPARENT PRESSURE H20 (ATM)

Figure 20
TEMPERATURE (K)

Figure 21
NBS-114A (REV. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report NoJ 3. Publication Date

4. TITLE AND SUBTITLE

Alkali Vapor Transport in Coal Conversion and Combustion Systems

5. AUTHOR(S)

J. W. Hastie, E. R. Plante, and D. W. Bonnell

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. national bureau of standards __ DEPARTMENT OF COMMERCE 8. WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

Type of Report & Period Covered

Interim Report

10. SUPPLEMENTARY NOTES

PH Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

Alkali metal -containing vapor species are ubiquitous in coal conversion and

combustion systems. These species originate from coal mineral and atmospheric

impurities (organic and inorganic) and from ceramic construction materials.

Alternatively, they are present as additives, such as with potassium seeding for

MHD or with bulk glass as a particle absorbing medium, or with dolomite in fluidized

bed systems. Alkali vapor transport over representative slag, glass, and simple

halide, hydroxide, and sulfate systems is discussed in relation to materials and

process limitations in coal -supported energy systems. Problems associated with

molecular-level vapor transport measurements are also considered.

12. KEY WORDS (Six to twelve entries; alphabetical order, capitalize only proper names; and separate key words by semicolons)

13. AVAILABILITY

[~yi Unl imited

1 1 For Official Distribution. Do Not Release to NTIS ^ Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

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PRINTED PAGES

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