United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
:¥
Research and Development
EPA 600 S2-85 099 Dec. 1985
Project Summary
Summary Report on Corrosivity
Studies in Coincineration of
Sewage Sludge and Solid Waste
H. H. Krause, P. W. Cover, and W. E. Berry
Corrosion probe exposures were con-
ducted in the Harrisburg, Pennsylvania
Incinerator to determine the effects of
burning low-chloride sewage sludge
with municipal refuse. Probes having
controlled temperature gradients were
used to measure corrosion rates for
exposure times up to 816 hours. The
effects of exposure time, metal temper-
ature, and gas temperature were stud-
ied. The results demonstrated that the
addition of the sludge reduced the initial
corrosion rates of carbon and low-alloy
steels to about half of that from refuse
alone. Little effect was observed on the
rates for Types 310 and 347 stainless
steel. An aluminized coating on steel
resisted corrosion effectively and offers
promise as a cost-effective substitute
for expensive alloys. In the range 260-
482°C (500-900°F), corrosion rates
were significantly reduced and were
less dependent on metal temperature.
The addition of sludge to refuse is
recommended as a corrosion prevention
measure and a waste disposal tech-
nique.
This Project Summary was developed
by EPA's Hazardous Waste Engineering
Research Laboratory. Cincinnati, OH,
to announce key findings of the re-
search project that is fully documented
in a separate report of the same title (see
Project Report ordering information at
back).
Background
Construction of the Harrisburg Incin-
erator was started on December 22,1969,
and it began operation in October 1972.
This facility has two identical boilers,
each with a nominal capacity of 327
Mg/day (360 tons/day) of refuse. At this
rate of 13.6 Mg (tons) of refuse per hour,
the normal steam production is 42.0
Mg/hr (92,500 Ib/hr) with a peak of 54.5
Mg (120,000 Ib). Design values are also
generally maintained for superheater
steam quality of 18.6 kg/cm2 (250 psig)
outlet pressure at a temperature of 236°C
(456°F). These two multidrum boilers
with welded waterwalls are fed from a
single refuse pit and are exhausted
through individual electrostatic precip-
itators before discharge into the single
stack.
Figure 1 shows the location of the
superheaters where the first tube failure
occurred and where the corrosion probes
were located for this study. Although
Boiler Unit No. 1 started in operation
(October 1972) before Unit No. 2, the
shortest-lived superheater tube was in
Unit No. 2 where a failure occurred after
about 3000 hours. It failed with a ductile
split several inches long while the wall
thickness of adjacent tubes was reduced
to 0.051 cm (0.020 inch) from their
original 0.42 cm (0.165 inch) wall. Wall
thickness of tubes remote from the soot
blower was reduced to about 0.38 cm
(0.150 inch). By March 1974, the entire
bank of carbon steel superheater tubes
had been replaced with T22 alloy. Fortu-
nately, by this time the effect of soot
blowing on metal wastage had been
recognized and the blowers were used
sparingly. Then in June 1975 the burning
of sewage sludge with the refuse was
begun, and this mode of operation has
continued to the present.
Prior to delivery to the incinerator the
sludge is processed conventionally.
Wastes from the entire city and some of
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Superheater
Figure 1. Harrisburg, Pennsylvania, refuse burning facility.
the adjacent regions are treated first by
grinding, grit removal, and preliminary
settling. Subsequently anaerobic digest-
ers are used with a polymer treatment to
thicken the supernatant liquid. The coag-
ulant used in the sludge treatment proc-
ess is an organic-based filter aid, em-
ployed at the rate of 1 2.3 kg (27 Ib) per
4545 kg (10,000 Ib) of dry solids. As this
organic polymer is probably a combination
of amines and acrylates, it is much more
suitable for use in sludge to be incinerated
than is ferric chloride, which was used
previously.
About 13.6-18.2 Mg (15-20 tons) of
partially dried sewage sludge are deliv-
ered by truck to the incinerator every 24
hours, andthe sludge isdumped onto the
refuse in the pit. This material contains
20-22 percent solids, so the incinerator
handles the equivalent of about 4500-
5500 kg (5-6 tons) per day of dry solids.
During the course of these corrosion
studies the partially dried sludge was
delivered at an average rate of 15,000 kg
(1 6.5 tons) per working day.
Introduction
Corrosion of boiler tubes has been a
significant problem for waterwall incin-
erators in which bulk refuse is burned,
particularly if superheated steam is gen-
erated. During the initial operation of the
Harrisburg, Pennsylvania Incinerator,
when only bulk solid waste was burned,
tube failures occurred every three to five
months. However, for the past two and a
half years no tube failures have occurred,
but during this period municipal sewage
sludge has been burned along with the
solid waste. In addition, soot blowing was
reduced in frequency and some of the
carbon steel tubes were replaced with a
low chromium alloy. Consequently, the
reason for the reduced metal wastage at
the Harrisburg Incinerator was uncertain
and corrosivity studies were needed to
evaluate the effect of adding the sewage
sludge to the refuse combustion environ-
ment.
As currently practiced, the dewatered
sludge filter cake containing about 22
percent solids is mixed with the municipal
refuse in the receiving pit. Considerable
mixing occurs in the pit before the sludge
and refuse are fed into the furnace
charging chute.The weight ratioof sludge
to solid waste is about 1 to 10 on an as-
received basis, and it seemed likely that
the sulfur and silica in the sewage sludge
are present in sufficient quantity and are
burned effectively to overcome the cor-
rosive effects of chlorine in the refuse.
The objective of this research program
was to determine the effect of the addition
of low chloride sewage sludge to munic-
ipal solid waste on the wastage rates of
boiler tube metals. Both short- and long-
term wastage was measured during this
program
Conclusions
Corrosion probe exposures conducted
at the Harrisburg, Pennsylvania, Incin-
erator demonstrated that the addition of
low-chloride sewage sludge to municipal
refuse reduced the corrosion of suscept-
ible metals caused by chlorine in the
refuse. Specific conclusions from this
research program were:
1. The corrosiveness of the refuse
combustion environment to carbon
steel as indicated by 8-hour expo-
sures was only one half as great
with sewage sludge present. Less
reduction occurred with T22 steel,
although the corrosion rate with
sludge was the same as that for
carbon steel. Very little reduction
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occurred with Incoloy 825 and
essentially none with Types 310
and 347 stainless steel, which have
very low corrosion rates with refuse
alone.
2. The corrosion resistance of alloys to
the refuse-sludge combination in
decreasing order was Incoloy 825;
310 stainless steel; 347 stainless
steel; T22 low chromium alloy; and
A106 carbon steel. A diffusion-
bonded aluminum coating on steel
remained intact during the expo-
sures and could be a cost-effective
alternative.
3. The corrosion rates decreased with
exposure time in the presence of
sewage sludge and appeared to
level off at about 1000 hours.
4. For carbon and low alloy steels the
corrosion rates increased with met-
al temperature in the range 260-
482°C (500-900°F), while those for
stainless steel decreased.
5. The short-term corrosion rates at
gas temperatures of 593-649°C
(1100-1200°F) were only one-fifth
as great as at 815°C (1500°F) and
showed less increase with metal
temperature.
6. The increased superheater tube life
at the Harrisburg Incinerator is the
result of (1) addition of sewage
sludge, which contributes large
amounts of SiOz to the combustion
environment; (2) reduced frequency
of soot blowing, leaving protective
oxidation products on the metal;
and (3) use of T22 alloy tubing,
which showed greater resistance to
corrosion than carbon steel when
the refuse was burned without
sludge.
Recommendations
As a result of this research program, it
is recommended that cofiring of sewage
sludge with municipal refuse be encour-
aged as a means of reducing corrosion of
heat recovery surfaces and at the same
time providing sludge disposal. However,
it should be pointed out that the sewage
should not have been treated with ferric
chloride, because the chloride residue in
the sludge would promote corrosion.
It also is recommended that boilers
intended to burn refuse be designed so
that gas temperature in the superheater
zone is as low as possible. In this way
superheater metal temperatures can be
used with little increase in corrosion. The
design should also provide combustion
air m a manner that will minimize carbon
monoxide formation near tube metal
surfaces.
The corrosion resistance of aluminized
coatings on carbon steel to the refuse
combustion environment should be in-
vestigated more extensively. This type of
coating offers a less expensive alternative
to high chromium-nickel alloys to obtain
longer tube life in refuse burning.
The use of silica as an additive to
combat corrosion from refuse burning
should be investigated. The beneficial
effect of the sewage sludge observed in
this program can be attributed in part to
the silica content, because the total sulfur
in the system was not great enough to
account for the corrosion reduction.
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H. H. Krause, P. W. Cover, and W. E. Berry are with Battelle Columbus Laboratory,
Columbus, OH 43201.
Michael!. Black and Robert A. OlexseyaretheEPA Project Officers (see below).
The complete report, entitled "Summary Report on Corrosivity Studies in
Coincineration of Sewage Sludge and Solid Waste," {Order No. PB 85-243
731/AS; Cost: $11.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officers can be contacted at:
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-85/099
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