ORNL-TM-5682.txt

ORNL/TM-5682

WASTER

 

 

Survey of Technology for Storage
of Thermal Energy in Heat Transfer Salt

M. D. Silverman
J. R. Engel

 

 

 

OAK RIDGE NATIONAL LABORATORY

OPERATED BY UNION CARBIDE CORPORATION FOR THE ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION

 
 

 

Printed in the United States of America. Available from
National Technical Information Service
U.S. Department of Commerce
5285 Port Royal Road, Springfield, Virginia 22161
Price: Printed Copy $4.00; Microfiche $3.00

 

 

 

This report was prepared as an account of work sponsored by the United States
Government. Neither the United States nor the Energy Research and Deveiopment
Administration/United States Nuclear Reguiatory Commission, nor any of their
employees, nor any of their contractors, subcontractors, or their employees, makes
any warranty, express or implied, or assumes any legal liability or responsibility for the
accuracy, completeness or usefulness of any information, apparatus, product or
process dgisclosed, or represents that its use would not infringe privately owned rights.

 

 

 
ORNL/TM-5682

Contract No. W-7405-eng-26"

Engineering Technology Division

SURVEY OF TECHNOLOGY FOR STORAGE OF THERMAL
ENERGY IN HEAT TRANSFER SALT

M. D. Silverman J. R. Engel

Manuscript Completed — January 18, 1977
Date Published — January 1977

 

NOTICE

This report was prepared as an account of work
sponsored by the United States Government. Neither
the United States nor the United States Energy
Research and Development Administration, nor any of
their employees, nor any of their contractors,
subcontractors, or their employees, makes any
warranty, express or implied, or assumes any legal
liability or responsibility for the accuracy, completeness
or usefulness of any information, apparatus, product or
process disclosed, or represents that its use would not
infringe privately owned rights.

 

 

 

Prepared by the
OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee 37830
operated by
UNION CARBIDE CORPORATION
for the
ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION

(G OF THIS DOCUMENT 18 URLIMITES.

Cert R
prioditd %
iii

CONTENTS
Page

ABSTRACT ....cviveevenns teeenesunno . Ce e ee e ee et et 1
1 INTRODUCTION ...cveereennnnreeosnnsnesssnnsossseassnasss veseens 1
2. PROPERTIES OF HTS AND ALTERNATIVE SALT MIXTURES ....... Ceeaeens 5

2,1 HTS .....cveennnn cheiseeciasastsearare seeseae e 5

2.2 Alternative Salt8 ...veevcernnnoens et e e Ceee e 11

2.3 Summary ..... e st e e e st raeeseas e Gt s e s eensses e nen e 12
3. MATERIAL COMPATIBILITY .viveeerneensncnonsennsnsansesanssnsansnss 12

3.1 State of the Art ...cieeereeerones et et e et eneaerrranenens 12

3.2 Available Corrosion Data .......coueeu. Ch et tieiaae e 13
4. EQUIPMENT USED IN INDUSTRIAL HTS SYSTEMS ............. O
5. CONCEPTUAL SYSTEMS FOR THERMAL ENERGY UTILIZATION

INVOLVING NITRATE SALTS 1 iieetineenoonssnsnsontnssssssasonasnnn 17

5.1 Power-Generation Systems .......... ceesereaeans cesercsrans 17

5.2 Other Applications ..... coeass Gt erestaaccs st estaanaaas 19
6. CONCLUSIONS ...ccveereccancananannnss Ceteasrserssseannsoreanases 21

REFERENCES ...v.verenerernnsnnsns ceesetaanans cesssssbecas ceseeseeses 23
SURVEY OF TECHNOLOGY FOR STORAGE OF THERMAL
ENERGY IN HEAT TRANSFER SALT

M. D. Silverman J. R. Engel

ABSTRACT

The widespread use of nitrate-based fused salt mixtures
as heat transport media in the petroleum and chemical process
industries and in metallurgical heat-treatment operations has
led to the development of satisfactory equipment for handling
and containing these materials. A mixture known as heat trans-
fer salt (HTS), which is composed of 40% NaNO,, 7% NaNO;, and
53% KNO3: by weight, has been used commercially in large quanti-
ties as a heat transfer fluid. It has been suggested that this
salt be used for storing energy as sensible heat in the tem—
perature range 200 to 540°C (400 to 1000°F). The eutectic 547%
KNO3—46% NaNO3; by weight known as 'draw salt,'" which has under-
gone less testing but is more stable thermally and more attrac-
tive economically than HTS and has similar physical properties,
may be a desirable alternative. Several specific energy stor-
age applications, such as intermediate-load and peaking electric
power, solar energy, and energy from fluidized-bed coal burners,
are discussed. Long-term stability and corrosion data on these
salts are presently available only to "480°C. However, for the
design and construction of energy storage facilities to operate
over many years at temperatures up to "V540°C, long-term tests
of thermal stability and corrosion are needed. Means for ob-
taining such information are proposed.

1. INTRODUCTION

Thermal-energy storage systems are of substantial interest to the
Energy Research and Development Administration (ERDA) for both optimizing
use of available energy sources and providing additional flexibility for
utilizing heat from these sources. Systems based on storage of latent or
sensible heat in fused salts are being envisioned for such diverse appli-
cations as providing intermediate-load and peaking power by the utility
industry, a storage reservoir for coupling with a fluidized-bed coal-con-
version system, storage for solar energy, and a heat transport fluid for
process heat. These potential applications are discussed briefly below
and are covered in more detail in Section 5.

Nearly all electric utility systems have considerable electric gener-

ating capability that is operated intermittently to accommodate daily,
weekly, and seasonal variations in the system load demand. Frequently,
these intermediate~ and peak-load demands are met by thermal-electric
systems that consume premium~quality fossil fuel (oil and/or gas) or by
older units that are less efficient and more costly to operate than base-
load units. In such cases, the capacity of the thermal-energy generator
must be matched to the maximum electric generating capability of the unit.
Systems are being developed and installed by some utilities to store energy
(in various forms) during periods of low electrical demand to supply power
when demand is high.

One potentially attractive approach to the efficient use of energy
storage is to separate the thermal-energy-generation system from the ther-
mal- to electric—energy conversion system by means of a reservoir for
storing thermal energy. Thus the energy required by a large, intermittently
operated electric generator and its steam—turbine drive could be supplied by
a much smaller thermal-energy generator that is operated continuously.

This approach would incur any benefits associated with the high degree of
utilization (effectively base-load operation) of the thermal-energy pro-
ducer. The trade-off implied by this concept is the substitution of a
large thermal-energy storage reservoir and its associated hardware for one-
half to two-thirds of the instantaneous thermal-energy generation capa-
bility. This trade-off appears to be attractive if the cost of heat gen-
eration is dominated by capital charges or if it permits the substitution
of lower-cost fossil or nuclear fuels for the increasingly scarce and ex-
pensive premium-quality fuels (oil and gas) in systems that can be made
responsive to wide variations in load. Although this concept is still
under development, two thermal-electric systems have been examined for
which thermal-energy storage at high temperatures may be attractive.

Nuclear steam—-electric systems generally are characterized by high
capital costs for the nuclear heat source and relatively low fuel costs and
therefore must be operated at high load factors to obtain a satisfactory
return on investment. These systems would not be considered for interme-
diate-load electricity generation if the capacity of the nuclear heat source
were sized to the steam—-electric generating capacity. These systems become
more attractive if a small nuclear heat source can be coupled to a much

larger (in terms of power-generation capability) steam-electric generator
through a thermal-energy storage reservoir. In order to retain this at-
tractiveness, it is essential that the efficiency of thermal-energy utili-
zation not be excessively degraded by the interposition of the storage
step. Since gas-cooled reactors are capable of delivering heat at tempera-
tures well above those commonly used in steam power plants, these systems
appear to be particularly well suited to the use of thermal-energy storage
at high temperature for electricity generation.

A preliminary study has been made of a system in which a high-tempera-
ture gas-cooled reactor (HTGR) of the type developed by the General Atomic
Company (GAC) was coupled to a high-temperature thermal-energy storage
reservoir using sensible heat storage in HTS to generate intermediate-load
electric power. This system was found to be economically attractive when
compared with the more usual fossil-fired units used for the same electri-
cal load duty, and the efficiency of thermal-energy utilization was "90%
of that for direct use of the energy (without storage) in a modern steam-
electric power plant. Although this study considered a large HTGR and
therefore a very large bloc of electric power (which probably could not be
accommodated by most utility systems), the concept appears to retain much
of its attractiveness in smaller size systems that are being studied.

Fluidized-bed coal burners, with limestone added to the bed, provide
one means for reducing or eliminating the release of sulfur oxides from
the combustion of high-sulfur coal. Thus, such units may be more suitable
than other coal burners for siting close to electrical load centers. How-
ever, since fluidized-bed burners operate most effectively at constant
power output, they are not well suited for load following or intermediate-
load duty. The use of a high-temperature thermal storage reservoir with a
fluidized-bed coal burner could form the basis for supplying intermediate-
load and possibly peaking power. A preliminary investigation of this con-
cept showed that it may have significant potential for storing thermal
energy as sensible heat in HTS or a similar molten salt.

An important feature of current solar thermal power concepts is the
storage of some thermal energy to extend the duty cycle of the generating
system and to smooth out variations in solar-energy input. A number of
concepts are being considered in this program, including sensible heat

storage in nitrate~based molten salts.
Heat transfer salt has been used for many years in industrial circu-
lation systems to provide heating and cooling for certain chemical reac-
tions. Large-scale use as a heat transport fluid to supply process heat
over considerable distances (up to several miles) has been studied for
potential petroleum refinery use and for other applications. Such uses
may be important in situations where the energy source must be located some
distance from the load centers.

All the energy utilization concepts discussed above involve the use of
thermal energy at relatively high temperatures (to about 550°C). 1In all
cases the simplest approach would be to make use of the sensible heat ca-
pacity of a relatively inert high-temperature fluid. HTS, because of its
extensive use in industrial applications, and possibly other nitrate salt
mixtures appear to have substantial potential for relatively near-term
application to all the concepts. The realization of this potential depends
significantly on the technology that is available to implement the concepts.

This report covers the state-of-the-art technology that has been accu-
mulated for nitrate-based salts and especially for heat transfer salt
Hitec®. Hitec, a mixture of 407% NaNO;, 77 NaNO3;, and 53% KNO3 by weight,
was formerly marketed as HTS by DuPont Chemical Co. Areas where needed
information is inadequate for the contemplated applications will be dis-
cussed and recommendations for obtaining such data from a research and de-
velopment program will be proposed. _

Hitec (HTS) was developed by DuPont during the late thirties for chem-
ical process applications in the temperature range where water and organic
media (e.g., Dowtherm) are inadequate [i.e., above 375°C (V700°F)]. Phys-
ical and chemical properties, heat transfer data, and corrosion information
for HTS were first reported1 in detail in 1940. The salt was readily ac-
cepted by the petroleum and chemical process industries, and considerable
experience was gained in developing equipment for handling it. By the end
of the forties, millions of kilograms of a modified HTS mixture (45% NaNO,—
55% KNO3) were being used in Houdry fixed-bed cracking units to maintain
temperature conditions in the 425 to 485°C range. A discussion of the phys-
ical and chemical behavior of these nitrate-nitrite systems was reported,2
along with methods for plant control of salt composition. Rottenburg® dis-

cussed industrial use of HTS in a publication entitled "Heat Transfer Media
for Use at Elevated Temperatures,' but a later review article” by Vosnick
and Uhl, which concentrates almost entirely on HTS, supplied much more
detailed information on its properties, heat transfer, safety, corrosion,
and system design and operation. A licensed 'salt dilution process,"
which has been issued to American Hydrotherm Co. of New York,® involves
the use of steam both at startup and at shutdown to avoid solids handling
and freezing problems with HTS. A sizable number of plants (30 to 50)
employing this process are now in commercial operation.

HTS has been and is being used in sizable quantities (as much as V5 X
10° kg in one unit) and at numerous installations. However, quantitative
data on both long-term stability and corrosion are not available because
industry has replaced facilities often for process changes dictated by
economic factors and, in general, has not been motivated to obtain such
information.

In discussions with Park Chemical Company, a supplier of heat transfer
salt (under the trade name Partherm 290) for metallurgical heat treatment
operations, it was learned® that an alternative fused salt mixture, the
binary eutectic 547 KNO;—46% NaNOj3 by weight (known as "high-temperature
draw salt'"), possesses greater thermal stability and is less corrosive
than HTS, but quantitative data are not available. Although its melting
point of 220°C (426°F) is somewhat higher than that of HTS (142°C), other
desirable properties plus lower cost make this mixture attractive, and its

potential is also examined.

2. PROPERTIES OF HTS AND ALTERNATIVE SALT MIXTURES

2.1 HTS

Physical properties

 

The heat transfer salt HTS, or Hitec,’ marketed by Coastal Chemical
Co. (a DuPont subsidiary), contains 407% NaNOp, 7% NaNO3, and 537% KNOj by
weight. Although other compositions have been used industrially (e.g.,
the Houdry fixed-bed cracking process used a 45% NaNO,—55% KNOj3 mixture),
practically all the properties that have been determined’»® and listed in

Table 1 are for the 40-7-53 composition. This mixture melts at 142°C
Table 1. Physical properties of "selected" nitrate-based salts

for thermal-energy storage

 

 

 

Property Hitec Draw salt

Composition, wt % 40NaNO,, 7NaNO3, 53KNOj; 46NaNO3, 54KNO;
Melting point, °C 142 220
Density, kg/m?®

At 260°C 1890 1921

At 540°C 1680 1733
Specific heat, J kg=! (K)~! 1560 Unavailable -
Viscosity, Pa/sec

At 260°Ca 0.0043 0.0043

At 540°C 0.0012 0.0011
Thermal conductivity, W m~! (K)~! 0.61 0.57
Heat transfer coefficient, W m~2 (K)~! 4600—16, 700 4300—15,600b

aExtrapolated.

bEstimated from HTS values using known parameters for draw salt.
(288°F), but appreciable changes in composition do not affect the freezing
point markedly.9 HTS has essentially zero vapor pressure in the 142 to
450°C range, and its specific heat is appreciably lower than that of water
(v1/3). However, its thermal conductivity is approximately the same and
its density is approximately twice as large. The viscosity of HTS in its
useful temperature range is greater than that of water and the liquid
metals by an order of magnitude, but it compares favorably with other heat
transfer fluids on the basis of heat-transport capacity10 (i.e., the heat
transferred per unit time, over a given range of temperature, for varying
mass velocity). Fried!® has derived a "heat transfer efficiency factor"
for comparing various fluids as a function of temperature. Various heat
transfer media and the limitations of each are summarized in Table 2.

HTS possesses most of the desirable attributes required for a heat
transfer medium. Among its favorable '"handling" properties are reasonable
cost (33 to 45¢/kg) and ready availability, although the quantities (102
kg) required for the aforementioned applications would probably necessitate
expansion of capacity from only two domestic suppliers of NaNO, and one for
KNO;, HTS has a low melting point, although impurities formed by thermal
decomposition (largely from NaNO,) gradually elevate the melting point.
The salt mixture is stable in air and in the presence of moisture. It is
relatively nontoxic and is nonflammable; however, the molten salt must be
kept out of contact with easily oxidized organic materials.

Hitec does not explode spontaneously, and attempts to detonate it by
blasting gelatin have proved unsuccessful.® However, the molten salt must
be prevented from coming into contact with hot carbon since the mixture

explodes. Therefore, solid fuel furnaces should not be used.?

Chemical properties

 

It is known that the chemistry of the thermal stability of HTS is com-
plex but that the decomposition of HTS proceeds via several significant
reactions involving sodium nitrite (the least stable component of the three
compounds that form heat transfer salt). However, the generally accepted®

overall reaction for its decomposition is

5NaNO», - 3NaNO3 + Na,0 + N, . (1)
Table 2. Comparison of heat transfer media for use at high temperatures

 

 

Practical tem~ Cost Melting Heat capacit Film a
Medium perature range ($/kg) point (J k _1pK-1)y coefficient Limitations
(°c) & (°c) 8 h at 7.5 kW
Dowtherm Ab 180370 0.22 13 2760 600 Leaks readily at seals and glands; poor rate of
heat transfer; decomposition causes fouling
HTS 205540 0.44 142 1560 1400 Lines must be heated or steam traced because of
freezing point
Draw salt 260550 0.25 220 e 1320 Lines must be heated or steam traced because of
freezing point
Na 125760 0.57 98 1300 4000 Requires sealed system; reacts violently with H:0
- and other materials; needs special equipment
NaK 40—760 3.50 18 1050 3000 Requires sealed system; reacts violently with H;0
and other materials; needs special equipment
Lead 370930 0.44 327 159 2080 Forms solid oxides that foul heat transfer sur-
faces and cause corrosion; needs high power; is
toxic
Mercuryb 370540 8.8 -39 138 1100 Very toxic; installation requires a high inven-
tory cost

 

%At 315°C and at a velocity to require the indicated power per 300 m of 7.6-cm pipe.

bAll are used as liquid except Dowtherm A and mercury, with latent heats of evaporation at 1 atm of 3.25 X 10° and
2.92 x 10° J/kg, respectively.

®Value should be approximately that of HTS.
Nitrogen evolution measurements and the above stoichiometry have been

° to estimate the decomposition of HTS with time and

employed by Bohlmann
as a function of temperature. Although these data indicate that an HTS
system operating between 260 and 520°C might require replacement of about
half the nitrite in the mixture annually, industrial experience has been
much more favorable. One circulating HTS system12 has been operated under
a nitrogen purge at temperatures up to V500°C (930°F) for as long as five
years with "minimal" incident and "minor" salt replacement. Another in-
stallation'?® believes 10 years of operation at V480 to 510°C under such
conditions is achievable. It has been reportedlu that the decomposition
of alkali nitrate-nitrite mixtures 1s catalyzed by iron above 520°C but
not by stainless steel. Because the only long-term (18 to 30 months)
quantitative data presented9 were obtained in carbon steel circulation
loops, the long-term stability of HTS (of varying purity) should be inves-
tigated at elevated temperatures (450 to 550°C) in low-alloy steel and

stainless steel systems.

Effect of impurities

 

Technical-grade inorganic salts contain impurities. The purity that
may be required for an HTS circulating system can be obtained by examining
the specifications15 listed for metal heat treating operations. These list
0.10 and 0.30% for sulfates and chlorides, respectively, as maximum concen-
trations that should be present in HTS. |

The major impurities found in HTS systems that have been operating for
extended'pefiods at elevated temperatures are (1) sodium oxide (analyzed
as sodium hydroxide due to water ébsorption in the salt), which is a decom-
position product of the nitrite according to Eq. (1); and (2) sodium car-
bonate, which is formed by absorption of CO; (which may be present in the

cover gas phase as an impurity) by the free alkali present in the HTS:

Na,0 4+ CO, - Na2C0; . (2)

Formation of NaOH and/or Na;CO; in HTS depresses the freezing point at
first, but eventually increasing amounts of these species result in car-

bonate precipitation and elevated freezing points. A detailed discussion
10

of the phase relations in HTS systems and the effect of impurities on the
freezing point of the salt mixtures, along with suggested methods for con-
trolling these impurities and reconstituting HTS in commercial systems, has
been reported.2

To summarize briefly, these methods involve (1) treatment of the HTS
with nitric acid, which converts the hydroxide and carbonate back to ni-
trate (which in turn can be reduced to nitrite); (2) cooling the salt to
allow the carbonate to settle out and then withdrawing the precipitate;
(3) adding calcium nitrate to precipitate calcium carbonate and then fil-

tering out the insoluble carbonate.

Cover gases

Practically all the industrial systems that circulate HTS employ a
cover gas (e.g., steam, air, or nitrogen). Using steam as a cover gas or
inleakage of moisture leads to the formation of sodium hydroxide via the
sodium oxide formed from nitrite decomposition. In turn, sodium carbonate
is formed from the hydroxide by CO;, absorption. The presence of these
components is deleterious, leading to increased melting points, precipita-
tion, and/or increased corrosion.?

If purified air or oxygen is used as a cover gas, oxidation of nitrite
to nitrate will occur at measurable rates above 400°C (750°F), according to

the back reaction shown in Eq. (2):
2NaNO, + 0, -+ 2NaNOj . (3)

The major effect of this reaction would be to convert nitrite to nitrate in
HTS, %7 1% thus elevating the melting point of the mixture. However, no
other major change should occur since the nitrate salts are each thermally
more stable than nitrite. According to Eq. (1), a nitrogen overpressure
(via a mass action effect) should suppress the formation of both sodium
nitrate and sodium oxide. Some industrial installations have used nitrogen
as a cover gas in circulating systems, ostensibly to keep the decomposition
of nitrite as low as possible and the freezing point depressed by avoiding
the increased formation of nitrate. However, in actual practice, a purge
of nitrogen from which CO, and moisture have been removed is usually

employed.
11

2.2 Alternative Salts

The binary eutectic 54% KNO3—46% NaNO; (''draw salt') suggested above
as an alternate for HTS essentially possesses all the favorable "handling"
properties of HTS and, in addition, is lower in cost. Although the thermal
decomposition of the specific eutectic mixture has not been studied in
detail, considerable data have been reportedls’17 for the decomposition of
each of the individual salts. The major decomposition reaction for an

alkali nitrate is expressed by

2NaNO; <+ 2NaNO, + 0, . (3)

The equilibrium constant for this reaction is 7 x 1072 at 550°C (1022°F)

18 These values

and is V0.9 x 1072 for the corresponding potassium salt.
can be interpreted to indicate that at equilibrium and 1 atm, draw salt at
this temperature will contain approximately 53.4 parts of KNOj;, 0.5 part
KNO,, 43.5 parts NaNOj3, and 2.5 parts NaNO,.

It has been stated that the binary mixture is more stable than either
of its components.® However, if it is assumed that the stability of the
binary is only that based on the equilibrium constants given above for the
individual salts, then draw salt at equilibrium at 550°C would contain
about 3 parts of nitrite to 97 parts of nitrate. This is considerably less
than the 40% nitrite contained in HTS, so replacement costs for the binary
system, based on nitrite decomposition, should be appreciably lower than
those for HTS. Moreover, since oxygen is one of the products of nitrate
decomposition, air could probably be used as the cover gas.

Another alternative salt mixture, a ternary eutectic containing 44.5%
KNO3, 37.5% LiNO3, and 18% NaNO; and that melts at 120°C (248°F), was in-

20 that lithium nitrate decomposi-

vestigated.19 However, it has been shown
tion to yield the oxide, nitrogen, and oxygen is favored thermodynamically.
Furthermore, molten lithium salts apparently are more corrosive than the
corresponding sodium or potassium compounds, because of their greater ten-
dency to decompose.20 For these reasons and because the cost of lithium
salts is approximately 10 to 20 times that of the corresponding sodium and

potassium compounds, this mixture was not considered further.
12

2.3 Summary

HTS is assembled from inexpensive chemicals that are readily available
in large quantity: the list prices per kilogram are $0.15, "$0.30, and
"$0.44 for NaNOj, KNO3, and NaNO,, respectively.?! 1t is believed that HTS
in very large quantities could be obtained in the desired purity for
“$0.44/kg. Since sodium nitrite is the most expensive component of HTS,
draw salt could be obtained for approximately one-half to two-thirds the
cost of HTS. The known properties of this binary eutectic are also listed

in Table 1.

3. MATERIAL COMPATIBILITY

3.1 State of the Art

HTS has been used in numerous diverse applications, but mainly in the
chemical and petroleum process fields as a heat transport fluid and in the
metallurgical industry for metal treatment operations. It is almost axio-
matic, both in the chemical and metallurgical process industries, that un-
less a definitive argument can be made for the use of stainless steel and
chrome, nickel, or molybdenum alloys, mild carbon steel should be employed.

For operation at temperatures <450°C, practically all systems contain-
ing HTS have been built of carbon steel (for economics), and the corrosion
rates, although relatively high (>0.12 to 0.25 mm/year), have been toler-
ated because chemical equipment can be depreciated over short time periods.
Furthermore, in most of these applications, carbon steel has sufficient
strength for the purposes employed. However, at temperatures >450°C, the
corrosion rate for HTS in carbon steel rises appreciably, and carbon steel
does not possess the requisite strength often required for use over long
operation at the elevated temperatures. This may necessitate the use of
stainless steels and chrome-molybdenum alloys instead as containment mate-

rials, especially if a usable life of up to 30 years is desired.
13

3.2 Available Corrosion Data

 

Carbon steel

Corrosion information and data for HTS contained in steel systems are
quite sparse and come mainly from three sources: laboratory data, which
are usually for the short term and from which large extrapolations would
have to be made; industrial loops, some of which have operated for as long
as several years; and finally chemical plants, which usually supply only
qualitative data because they are often replaced for economic reasons.

® has summarized most of the available data; using the para-

Bohlmann
bolic rate law, he has extrapolated from short-term tests (up to 700 hr)
yearly corrosion rates for various steels. Rates of 0.1 to 0.4 mm/year
were obtained on carbon steel in the temperature range 450 to 540°C.
Earlier, Russian investigators®?? found corrosion rates of 0.1 to 0.2 mm/
year for steel exposed to HTS for 700 hr at 500°C. Recent Russian studies??
revealed corrosion rates of 0.02 and 0.04 mm/year for unstressed and
stressed specimens, regpectively, when exposed to HTS for 600 hr at 500°C.
The results of one-year plant tests in molten salt baths (assumed to be
exposed to air since no specific cover gas was mentioned) in which the
alkali content was not allowed to go over 0.3% revealed no corrosion damage

3 However, when the alkali content of the bath rose

to mild carbon steel.?
to Vv1.5%, the mild steel wvessgel walls burned through after 20 days at 550°C
(1022°F).

Most of the industrial applications (phthalic anhydride production,
acrylic acid manufacture, caustic soda concentrators, etc.) involve HTS at
temperatures below 450°C in carbon steel equipment. Additional assessments
of corrosion by heat transfer salt in these plant size systems were also
presented by Bohlmann.? The information is largely qualitative, that is,
"negligible corrosion was observed" and often equipment was replaced after
relatively short usage (one to two years) because of economic factors.
Recently, one item of quantitative data was obtained from an HTS system

24 Metallurgical examination of

used in a General Electric plastics plant.
a section of carbon steel piping from the discharge line of the salt pump,

exposed to temperatures between 450 and 500°C for 80% of the time and in
14

which flow was 2.5 m/sec, showed a corrosion rate of “0.025 mm/year (inter-
granular growth and oxide layer) after five years of exposure. HTS has
been used also as a heat transfer medium in concentrating caustic soda and
caustic potash. In this application, the 73% NaOH is circulated very
rapidly through single-pass heat exchangers with HTS (N, cover) on the
shell side. The temperature of the salt is not allowed to exceed 525°C.
Corrosion and/or erosion in the nickel tubes of the heat exchanger was ob-
served (attributed to very high velocities of the caustic), but corrosion

on the shell side by HTS was considered to be very low.'}

Alloz steels

Corrosion information on low-alloy (e.g., Cr-Mo) and stainless steels
has been obtained mostly from laboratory studies in short-term experiments
(up to 700 hr). These data, also presented in Bohlmann's report, indicate
that type 316 and possibly type 321 stainless steel would yield corrosion
rates of <0.025 mm/year at 540°C over long periods. Some Russian data??
obtained from 700-hr tests at 500°C with a stainless steel (0.1% C, 18% Cr,
9% Ni, and a small percentage of Ti) yielded a corrosion rate of 0.06 mm/
year. However, the latter study also revealed that under stress this type
of steel was subject to intergranular corrosion. More recent Russian
work??® showed corrosion rates of 0.007 and 0.013 mm/year for unstressed and
stressed specimens, respectively, of another stainless steel (18% Cr, 10%
Ni, and a small amount of Ti) exposed to HIS at 500°C for 600 hr. This
same steel was used in plant baths (assume open to air) for more than four
years at 450 to 500°C with essentially no corrosion damage. However, some
intergranular corrosion was found (V10 mils deep) at the intersection of
two seams where the metal had not fused. It was stated that '"this steel
was promising material but only for HTS applications at atmospheric pres-
éure and below 500°C where large load-bearing stresses would not be en-
countered and that the alkali content of the salt must be controlled
<0.3%Z." Heat-resistant titanium alloys VIS-1 and 480T3, which are char-
acterized by small creep, yielded corrosion rates?® of 0.001 to 0.002 mm/
year at 500°C (932°F). To reiterate, although HTS has been used in many
industrial applications, quantitative corrosion data on stainless steels

are extremely meager, especially with respect to long-term operation.
15

Water intrusion into nitrate-nitrite salt mixtures does not cause
serious corrosion effects, because nitrates have been used to passivate

25,26

steel surfaces; rather, the corrosion results from the presence of

impurities such as Naz0, which reacts with water to form NaOH. The latter
compound is known to aggravate corrosion in stainless steel systems,
especially with respect to intergranular effects. Hence long-term corro-
sion tests should be performed to investigate the effect of impurities,
such as NaOH, which may be present in large-scale HTS systems contained in
stainless steel.

For possible reactor applications, interaction of HTS with other mate-
rials should be considered. HTS is not compatible with graphite® or sodium,
both of which are strong reducing agents. Based on thermodynamic data, the
nitrate—sodium metal reaction should be as exothermic as the sodium~water
reaction. HTS is soluble in water; the salt is hygroscopic when granular.
This high solubility has been used in one type of process called the '"salt
dilution'" system to simplify handling problems with HTS at startup and

5

shutdown. Other systems have employed steam and electric tracing of proc-

ess lines.

4. EQUIPMENT USED IN INDUSTRIAL HTS SYSTEMS

Millions of kilograms of HTS have been used in industrial systems and
have led to the development of specific types of equipment for salt han-
dling. Since these have been discussed in detail by Vosnick and Uhl"* and
by Fried,'' they will only be described briefly here. Pumps used in salt
systems are of the submerged vertical-centrifugal type, specified to permit
no contact of liquid with the packing gland. Mechanical seals have seen
extended use, but newer bellows-type seals can be used well above 250°C
without the need of water-cooled jackets. Canned pumps are available at
higher cost than centrifugal pumps and offer the advantage of leak-free
operation at temperatures up to 540°C. Piping systems are constructed of
seamless carbon steel (e.g., ASTM Al06) for use below 450°C, and low-alloy
(Cr-Mo) or stainless steels (types 316, 321, and 347) are employed above

this temperature; usually these are welded systems containing ring-joint
16

flanges with soft metal or asbestos gaskets. Valves are normally used only
in bypass and bleed lines. Salt flow is seldom controlled by valves, but

instead by sizing the pump for maximum HTS flow.

Although numerous industrial installations have used excess heat from
HTS to generate steam (and others are preparing to do so in view of the in-
creasing cost of energy), the temperature of the steam produced has not
been above 315°C. Consequently, the steam generators (or heat exchangers)
used for this purpose have been constructed of carbon steel and have not
been subjected to very high temperatures and pressures. Applications that
are envisioned for HTS use (i.e., HTGR coupling for intermediate and peak-
ing power, storage sink for fluidized-bed coal conversion, and solar stor-
age) would require steam generators to withstand approximately 14 MPa (2000
psi), 500°C steam conditions over long periods of time (up to 30 years).
For these conditions, steam generators would be constructed of chromium-
molybdenum or stainless steel alloys.

Modern fossil-fired power plants use steam generators that are sub-
jected to these high temperatures and pressures, but they are not con-
structed of carbon steel or low-alloy steels. Hence, some development work
would be needed to ensure that the alloys selected for this purpose will be
compatible with HTS on one side and with high-pressure steam on the other.
Considerable development work now being pursued on the liquid-metal fast
breeder reactor (LMFBR) project toward the construction of a sodium-steam
generator can be utilized for designing an HTS steam generator because both
systems will have to contain materials at low pressure on one side and
high-pressure steam on the other. Because steam is compatible with HTS and
does not present reaction problems, development of an HTS steam generator
should not be difficult.

HTS has been and can be safely used in properly designed circulation
systems. The few accidents reported27 have been due to overheated pots or
reaction of hot salt with organic materials such as wood or graphite.
Recommendations for good practice with HTS have been made by the Factory

Insurance Association.?®
17

5. CONCEPTUAL SYSTEMS FOR THERMAL ENERGY
UTTLIZATION INVOLVING NITRATE SALTS

5.1 Power-Generation Systems

An HTIGR of the type developed by General Atomic can supply thermal
energy at temperatures considerably higher than that supplied by presently
operating nuclear reactors. A preliminary conceptual study indicates that
part of this energy could be coupled to a high-temperature storage reser-
voir using sensible heat in Hitec to generate intermediate load electric
power and/or peaking power during periods of heavy load demand. It is
recognized that the ratio of this intermediate load to base-load generation
would depend on the specific needs of the generating utility. A diagram-
matic sketch of a system conceived during this study is shown in Fig. 1.

In this type of system, two primary helium loops (i.e., two helium circu-
lators) and two intermediate loops would be employed. Each intermediate
helium loop provides heat to one steam generator and one salt system in
parallel. The temperatures and pressures shown are tentative. In this
particular arrangement, the efficiency of the peaking cycle is approxi-
mately 34.87%7 and that estimated for the base-load steam system is 35.77%,
compared to 39% for an all-base—~loaded HTGR system, thus yielding a thermal
utilization factor of 90%Z. This system appears economically attractive for
intermediate-load and peaking power generation when compared to fossil-
fired units for similar electrical load demand.

In this study a 2000-MW(t) HTGR is used as an example, where 1120
MW(t) would be base loaded and 2550 MW(t) would be stored as sensible heat
in the salt storage reservoir to be delivered on a daily 8-hr demand cycle.
However, additional studies now being made indicate that this type of sys-
tem retains its economic attractiveness even in smaller sizes. Early esti-
mates indicate that the capital costs of salt storage and inventory com-
prise about 207 of the total costs and that substitution of the binary
eutectic salt KNO3-NaNOj3; could lower this fraction appreciably.

Fluidized beds for converting coal into clean energy sources are being
tested by industry on a pilot-plant scale. This treatment has a unique

advantage over present methods of burning coal in power plants because
353°C

He CIRCULATOR

 

 

Y

316°C
5 MPa

He CIRCULATOR

ORNL-DWG 76-20782

 

 

 

 

 

REACTOR
CORE

 

He-He
HEAT

 

 

EXCHANGE

 

SALT
HEATER

 

 

 

 

 

760°C f

 

4.9 MPa

Fig. 1.

 

705°C

 

P |

o’

 

 

 

COLD
STORAGE

 

—

STEAM
GENERATOR

 

 

 

 

TO H.P. TURBINE

L 510°C, 16.8 MPa
§ T BASE LOAD

REHEATER

 

 

 

 

 

+ TO I.P. AND L.P.

L TURBINE
538°C, 3.8 MPa

 

288°C

 

 

 

543°C

STEAM
GENERATOR

 

 

TO H.P. TURBINE

L  482°C, 14 MPa
§ PEAK LOAD

REHEATER

 

 

 

 

 

 

HOT
STORAGE

TO I.P. AND L.P.
TURBINE
510°C, 3.8 MPa

 

1

—-——.’

 

 

Salt storage system with intermediate helium loop.

81
19

high-sulfur coal can be used in the feed to the bed, thereby negating the
need for large, expensive flue gas scrubbing systems to attain low-level
sulfur dioxide emissions that meet Environmental Protection Agency stan-
dards. If the vast supplies of high-sulfur coal cannot be used efficiently,
low~-sulfur coals will have to be used, thus resulting in higher fuel costs
to the consumer. Fluidized-bed burners operate more efficiently and more
smoothly when they are used at constant power output. Therefore, storing
sensible heat in a molten salt such as Hitec during low-demand periods
would provide flexibility and avoid variable loads to the coal combustion
system,

Two possible systems are sketched in Figs. 2 and 3. The need for an
intermediate heat exchanger would be determined by the temperature drop
across the fluidized bed and tube wall and the ability of the salt medium
to tolerate the resulting inner wall temperature. A salt leak into the
fluidized bed should not create any safety problems because of the large
volume of air flowing through the combustor at a linear velocity of V1.4

m/sec (residence time of 6 sec).

3.2 Other Applications

 

All solar energy systems will require an energy storage system to ex-
tend the duty cycle of the generating system when solar energy supply is
limited. Proponents of these systems propose either latent or sensible
heat storage. A report that discusses the application of current technol-
ogy to high-temperature thermal-energy storage, especially with respect to

28

solar energy application, presents several possible systems. A conceptual

design for a 10-MW(e) solar pilot plant involving a NaNO3;-NaOH latent heat

30 Another

thermal storage system has been proposed by the Honeywell Corp.
proposed solar energy design31 envisions storing energy as sensible heat
in either Hitec or draw salt. However, the overall economics of such sys-
tems remains to be determined.

Heat transfer salt has been used industrially in circulation systems
to provide heating and cooling for various chemical reactions. Large-~scale

potential use of HTS as a heat transport fluid for petroleum refinery ap-

plication has been investigated for transporting process heat over long
20

ORNL-DWG 76-20783
EXHAUST GASES

 

 

 

 

 

 

 

 

 

 

 

150°C
AlR
PREHMEATER AR
1._+
S 0%
315°C
SALT
PREHEATER HTS
260°C
870°C

 

("~ _FLUIDIZED BED —- )
_ COMBUSTOR .

—

PREHEATED AIR
PREHEATED HTS
336°C

 

 

 

 

 

 

 

- HTS
538°C
225°C BED MATERIAL
BED TEMP. 870°C
COAL LIMESTONE
(3.9% S) (Ca/S = 2)

Fig. 2. Fluidized-bed, coal-fired combustor for HTS energy storage
system (heat recovery from products).
21

ORNL -DWG 76--20784

FLUE GAS
870°C

 

" — — FLUIDIZED BED- — )
—~ -~ COMBUSTOR

HTS

— - -
(TS|

- . - - — . = HTS THETO
— . = —& THERMAL
— 1 TZ | 538°C  sTORAGE

 

 

AlR BED MATERIAL
20°C COAL , LIMESTONE BED TEMP. 870°C

(3.8% S) (Ca/S =

Fig. 3. Fluidized-bed, coal-fired combustor for HTS energy storage
system (no heat recovery from products).

distances (up to several miles). Such use may be desirable in situations
where the energy source must be located away from the load center. This
investigation32 showed that a transport system handling HTS (or draw salt)
is the most economical when compared to systems using other fluids such as
helium, air, N, CO,, steam, and the liquid-metal alloy NaK. The cost of

a heat transport system to supply and return HTS (or draw salt) through 2.1
km of piping was estimated to be approximately 407% of that estimated for
the next best choice, the alloy NaK.

6. CONCLUSIONS

Heat transfer salt has been used safely in many applications in the

process industries. The experience gained in such usage has resulted in
22

the development of equipment and components that can satisfactorily handle
large quantities of this mixture. The properties and handling of this
material have been well documented. Because of these factors and its low
price and ready availability, HTS (or draw salt) would serve as a satis-
factory sensible heat storage medium for diverse applications such as in-
termediate-load and peaking power, coupling with a fluidized-bed coal
burner, solar energy storage, and as a process heat fluid.

There are several areas of technology where further information is
needed so that HTS or draw salt can be used for these applications.

1. The long-term stability of HTS and draw salt at temperatures up to
550°C should be investigated, preferably in conjunction with long-term cor-
rosion testing.

2. Long~term corrosion tests are needed on materials that would con-
tain the nitrate salts; practically all the data now available for low-
alloy and stainless steels are based on short-term experiments that have
been extrapolated to yield so-called long-term corrosion rates. These
tests should investigate the effect of temperature, salt velocity, AT condi-
tions, and impurities on the corrosion rate. In addition, metallurgical
effects such as intergranular corrosion, stress—-corrosion cracking, creep,
etc., should be evaluated.

3. The steam generator that would be used for the above applications
will contain HTS or draw salt at low pressure on one side and high-pressure
steam on the other (about 14 MPa at "500°C). Since present information is
available only for carbon-steel steam generators containing HTS and steam
at lower temperatures and pressures (Vv325°C, 4 MPa), further development in
the area of alloy materials and stainless steels is needed for steam gen-
erator fabrication. It is believed that information obtained from the
large-scale development effort on the IMFBR project toward obtaining a
viable steam generator can be used to advantage in this regard. Informa-
tion gained from previous studies by the Foster-Wheeler Corporation toward
developing a steam generator for the molten-salt breeder reactor should be

utilized.
1.

lO.

i1.

12.

13.
14.

15.

16.

17.

23

REFERENCES

W. E. Kirst, M. W. Nagle, and J. B. Castner, "A New Heat Traunsfer
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H. P. Vosnick and V. W. Uhl, '"Molten Salt for Heat Transfer,'" Chem.
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H. W. Hoffman and S. I. Cohen, Fused Salt Heat Transfer — Part III:
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J. J. Carberry, '"Media for Heat Transport,'" Chem. Eng. 60, 225 (June
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J. R. Fried, '"Heat Transfer Agents for High-Temperature Systems," Chem.
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Private communication, Lee Bergmann, BASF Wyandotte, Detroit, Mich.

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E. S. Freeman, 'The Kinetics of the Thermal Decomposition of Sodium
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E. S. Freeman, 'The Kinetics of the Thermal Decomposition of Potassium
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24

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A. G. Bergman and K. Nogoev, "The CO(NH;),—LiNOj; K, Li, Na || NOs;
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=
O

1i-19.
20.
21.
22.
23.
24,
25,
26.
27.
28.
29,
30.
31.
32.
33.

70.
71.
72.
73.
74.
75.
76.

77.

Lo~ we =

25

 

ORNL/TM-5682
Internal Distribution
T. D. Anderson 34, W. J. McCarthy
S. E. Beall 35. H. E. McCoy
E. S. Bettis 36. J. P. Nichols
E. G. Bohlmann 37. H. Postma
S. Cantor 38. S. A. Reed
W. E. Cooper 39. R. C. Robertson
J. L. Crowley 40. M. W. Rosenthal
F. L. Culler 41. G. Samuels
J. H. DeVan 42, J. L. Scott
D. M. Eissenberg 43. M. R. Sheldon
J. R. Engel | 44-53. M. D. Silverman
G. G. Fee 54. 1I. Spiewak
E. C. Fox 55. D. Steiner
A. P. Fraas 56. J. J. Taylor
L. C. Fuller 57. D. B. Trauger
M. J. Goglia 58. J. S. Watson
R. H. Guymon 59. W. M. Wells
R. E. Helms 60. R. W. Werner
H. W. Hoffman 6l. W. J. Wilcox
R. S. Holcomb 62. ORNL Patent Office
W. R. Huntley 63-64. Central Research Library
J. E. Jones, Jr. 65. Document Reference Section
J. R. Keiser 66-68. Laboratory Records Department
E. J. Kelly 69. Laboratory Records (RC)
R. E. MacPherson

External Disgtribution

 

Director, Division of Nuclear Research and Applications, ERDA,
Washington, D.C. 20545

W. F, Savage, Division of Nuclear Research and Applications, ERDA,
Washington, D.C. 20545

K. 0. Laughon, Division of Nuclear Research and Applications, ERDA,
Washington, D.C. 20545

R. G. Oehl, Division of Nuclear Research and Applications, ERDA,
Washington, D.C. 20545

G. M. Kaplan, Division of Solar Energy, ERDA, Washington, D.C.
20545

M. Gutstein, Division of Solar Energy, ERDA, Washington, D.C.

20545

J. H. Swisher, Division of Energy Storage Systems, ERDA, Washington,
D.C. 20545

C. J. Swet, Division of Energy Storage Systems, ERDA, Washington,
D.C. 20545
78.

79.

80.

81.

82.

83.

84.

85.

86.

87.

88.

89.

90.

91.

92.

93.
94 .

95.
96.
97.
98.
99.
100.

101-127.

26

R. N. Quade, General Atomic Co., P.0. Box 81608, San Diego, Calif.
92138

D. L. Vrable, General Atomic Co., P.0. Box 81608, San Diego, Calif.
92138 |

0. H. Woike, General Electric Co., P.O. Box 15132, Cincinnati,
Ohio 45215

R. F. Altman, Engineering Experimental Station, SEMTD, Georgia
Inst. of Tech., Atlanta, Ga. 30332

J. D. Walton, Engineering Experimental Station, SEMID, Georgia
Inst. of Tech., Atlanta, Ga. 30332

T. T. Bramlette, Exploratory Materials Division, Sandia Labora-
tories, Livermore, Calif. 94550

J. J. Bartel, Exploratory Materials Division, Sandia Laboratories,
Livermore, Calif. 94550

T. D. Brumleve, Solar Energy Technology Division, Sandia Labora-
tories, Livermore, Calif. 94550

L. M. Murphy, Solar Energy Technology Division, Sandia Labora-
tories, Livermore, Calif. 94550

J. C. Powell, Systems and Research Center, Honeywell, Inc., 2600
Ridgeway Parkway, Minneapolis, Minn. 553413

W. J. Masicka, NASA Lewis Research Center, 21000 Brookpark Rd.,
Cleveland, Ohio 44135

J. P. Joyce, NASA Lewis Research Center, 21000 Brookpark Rd.,
Cleveland, Ohio 44135

A. A. Bruhn, American Hydrotherm Corp., 470 Park Ave., South,

New York, N.Y. 10016

F. D. DeNoyers, General Electric Co., Noryl Ave., Selkirk, N.Y.
12158

R. W. Foreman, Director of Research, Park Chemical Co., 8074
Military Ave., Detroit, Mich. 48204

L. H. Bergman, BASF-Wyandotte, Wyandotte, Mich. 48192

J. F. Cox, Nuclear Department, Foster-Wheeler Energy Corporation,
110 South Orange Ave., Livingston, N.J. 07039

C. D. Miserlis, Badger-America, Inc., 1 Broadway, Cambridge, Mass.
02142

J. W. Pepper, Electric Power Research Institute, 3412 Hillview
Ave., Palo Alto, Calif. 94304

V. W. Uhl, Chemical Engineering Department, University of
Virginia, Thornton Hall, Charlottesville, Va. 22901

H. Behrman, ERDA, ORO, Oak Ridge, Tenn. 37830

Director, Reactor Division, ERDA, ORO, Oak Ridge, Tenn. 37830
Research and Technical Support Division, ERDA, ORO, Oak Ridge,
Tenn. 37830 _

Technical Information Center, ERDA, Oak Ridge, Tenn. 37830