United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park.NC 27711
Research and Development
EPA/600/S8-89/059 May 1990
v>EPA Project Summary
Municipal Waste Combustion
Assessment: Fossil Fuel
Co-Firing
V.J. Land rum and R.G. Barton
Fossil fuel co-firing, defined as the
combustion of refuse derived fuel
(RDF) with another fuel (usually coal)
in a device designed primarily to
burn the other fuel, is generally
confined to commercial and utility
boilers. This report identifies RDF
processing operations and various
RDF types; describes such fossil fuel
co-firing technologies as coal fired
spreader stockers,pulverized coal
tangentially fired boilers, and cyclone
fired boilers; and describes the
population of coal fired boilers that
currently co-fire RDF, have previously
co-fired RDF but have ceased to do
so, and have been used in RDF co-
firing demonstration projects. Model
plants are developed and good
combustion practices are recom-
mended.
This Project Summary was
developed by EPA's Air and Energy
Engineering Research Laboratory,
Research Triangle Park, NC, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Fossil co-firing is defined as the
combustion of RDF with another fuel
(usually coal) in a device designed
primarily to burn the other fuel.This
report provides an overview of fossil fuel
co-firing technology and identifies the
extent to which fossil fuel co-firing is
practiced.
Currently Co-Firing
Four facilities currently co-fire RDF.
The Madison Gas and Electric's Blount
Street Power Plant includes two 50 MWe
front wall fired pulverized coal boilers
modified to co-fire RDF by installing RDF
injectors between the bottom two burner
levels. A drop grate was added to the
boilers to ensure complete burnout of the
ash generated by the RDF. RDF supplied
about 15% by weight of the fuel used by
the boilers (12% of total heat input). No
major problems have been reported.
Minor problems occurred with the
formation of clinkers on the grate.
Despite the lack of firing problems, the
additional operating costs associated with
co-firing RDF make it uneconomical
without special price concessions and
subsidies from the City.
The Ames, Iowa, City Power Plant
includes a 35 MWe tangentially fired
pulverized coal boiler modified to burn
RDF and a 65 MWe front wall fired
pulverized coal unit originally designed to
burn RDF Air transport RDF injectors on
both units are located just above the
primary combustion zone. In addition,
both units are equipped with drop grates.
The 35 MWe unit fires 18% by weight
RDF (12% of total heat input). The 65
MWe unit fires 20% by weight RDF (10%
of total heat input). Problems reported are
related to handling the RDF feed and the
resulting ash. The variable nature of RDF
makes it difficult to maintain continuous
feed with a relatively constant heat
content. Wires and non-ferrous metals
create significant ash handling problems.
The RDF feeding system frequently jams,
damaging the machinery. Originally, tube
wastage was observed around the drop
grate due to the reducing atmosphere
produced by ash burning on the grate.
This problem was remedied by installing
refractory around the grate. The plant
operators report that RDF co-firing m
-------
their boilers is not financially advanta-
geous because of high maintenance
costs.
Baltimore Gas and Electric Company
operates -two 200 MWe cyclone fired
boilers modified to co-fire RDF by
installation of RDF injection equipment in
the cyclone. Initially, erosion was
observed on the cyclone wall opposite
the RDF injection port. The installation of
a deflection plate eliminated the problem.
The RDF is an average of 5 to 10% by
weight (3 to 5% total heat input) of the
total fuel fed to the boiler. No major
problems have been reported operating
with RDF. The utility has not yet
identified the impact of firing RDF on
operating and maintenance costs;
however, it reports that, from a fuel cost
standpoint, RDF co-firing is financially
attractive. The RDF purchase contract
requires the utility to pay only 80% of the
price they would pay for an equivalent
amount of coal (on a heating value basis),
resulting in a fuel cost savings of 20%.
The City of Lakeland, Florida's
Mclntosh Power Plant includes a 364
MWe opposed burner, pulverized coal
fired boiler designed to co-fire RDF and
coal. The coal burners are located at four
elevations on the front and rear walls.
Two air transport RDF injectors are
located on each side wall at the same
elevation as the upper coal burners. The
boiler is also equipped with drop grates.
Only 1% of the mass fired in the boiler is
made up of RDF. Lakeland reports no
operating problems, no excess tube
wastage, slagging or fouling. They report
that firing RDF appears to be financially
viable when operating costs alone are
considered. However, when debt
retirement on the processing plant is also
considered, the unit does not quite break
even.
Dicontinued Co-Firing
Six facilities have previously co-fired
RDF but have discontinued this practice.
The Oscar Mayer Steam Generating
Plant in Madison, Wisconsin, consists of
a spreader stoker traveling grate system
modified to burn RDF. The facility had a
capacity to burn up to 60 tons (54
tonnes) of RDF per day. Oscar Mayer
reports that there was little economic
incentive to continue co-firing RDF. They
found overhead and maintenance costs
to be prohibitive. In addition, the quality
of the RDF varied excessively for
convenient use. However, the facility still
has the capacity to co-fire RDF if the
economic situation changes.
The Union Electric, St. Louis Meramec
Plant has two 125 MWe tangentially fired
pulverized coal boilers modified to co-fire
RDF. There are four vertically arranged,
tilting coal burners in each corner. RDF
injectors were installed, one in each
corner between the second and third coal
burners. Dump grates were not added.
The RDF supplied an average of 10% of
the total heat input. Problems
experienced included plugging and
jamming of the RDF feed system and
excessive abrasion from the RDF feed
particles due to the high glass and grit
content of the RDF. The excessive
abrasion caused frequent leaks in the
feed lines which had to be continually
repaired resulting in extremely high
maintenance costs. Boiler efficiency was
decreased due to the higher moisture
and ash content of RDF compared to coal
and increased unburned combustibles in
the bottom ash.
The Wisconsin Electric's Oak Creek
Station has two 310 MWe tangentially
fired pulverized coal boilers modified to
co-fire RDF. There are five tilting coal
burners in each corner. Four RDF
injectors were installed, one in each
corner, above the top coal burners. No
dump grates were added. The RDF
supplied an average of 15% of the total
heat input. The facility experienced
excessive abrasion of the RDF feed lines
because of the glass and grit in the RDF
as well as increased slagging. Solidified
slag collected at the bottom of the
furnace and choked off bottom ash flow.
Deslagging involved the use of shotguns
and dynamite to blast loose slag deposits
and required 4-5 hours. Deslagging was
required 12-15 times per month during
co-firing as opposed to 7-10 times per
month during operation with coal alone.
Four Rochester, New York, Gas and
Electric units are tangentially fired,
pulverized coal boilers with capacities of
42, 62, 62, and 75 MWe. Two RDF
injectors were installed in each unit in
opposite corners above the coal burners.
Two units received dump grates and two
did not. The RDF supplied an average of
15% of the total heat input. Problems
experienced included pluggage of RDF
feed lines and large clinker formation on
dump grates. Boiler efficiency decreased
because of the higher moisture and ash
content of RDF compared to coal,
increased excess air levels, and
increased amounts of unburned
combustibles in bottom ash.
The Commonwealth Edison, Chicago
Crawford Station includes one 200 MWe
and one 325 MWe tangentially fired,
pulverized coal boilers modified to co-fire
RDF. The modifications included installa-
tion of two RDF injectors in two opposite
corners of each furnace (four per unit).
Dump grates were not installed. The]
average heat input provided by the RDF
was 10%. Problems included plugging
and excessive abrasion of the RDF feed
lines, increased volume and particle size
of bottom ash, and increased slag
accumulation.
The United Illuminating Harbor Station
in Bridgeport, Connecticut, includes an
80 MWe cyclone boiler previously used
to co-fire RDF. The RDF was a dry
powder with a heat content of 7800 Btu/lb
(1.81 x 107 J/kg). The RDF provided ar
average heat input of 30%. No majoi
problems were associated with co-firing
RDF. The facility ceased co-firing
because the RDF processing plan"
closed.
Co-Firing Demonstrations
Two facilities were involved in RDF co
firing demonstration projects. The B.L
England Station includes one 129 MW<
cyclone fired unit which was used to co
fire RDF as part of an 18 day feasibility
demonstration study. The RDF wa:
injected through the secondary air duct
in the cyclone. The facility was able ti
burn 2550 tons (2313 tonnes) of RDI
during the 18-day test. Stable operatioi
was maintained only when RDF was les
than 15% of the total heat input to th
boiler. Severe fouling and slaggin
problems were experienced and the a
heater plugged rapidly, forcing
shutdown of the unit. In addition, a har
tenacious ash that could only be remove
with jackhammers fouled the reheat«
surfaces. Unit efficiency was significant!
affected by RDF co-firing. The un
gradually derated from 129 to 100 MWi
Emissions increased significantly whe
co-firing RDF. Particulate emissions ar
precipitator power levels increased by
factor of 3. Because of these problem
Atlantic Electric concluded that RDF c<
firing was unacceptable.
The Milwaukee County Institution
Power Plant includes one spreader stok
boiler rated at 110,000 Ib/hr (49,8J
kg/hr) of steam that was used to co-fi
RDF during a 9 day test with 3 days
performance testing. The RDF consist!
of about 21% by weight of the coal/RC
mixture and about 11 % by heating vali
No significant feeding problems we
encountered. The coal/RDF mixtu
clinkered more frequently than cc
alone. Also, some RDF fell into the a
pit unburned. The boiler was unable
achieve steam generation rates that h
been easily attained when firing c<
alone, reportedly due to the volumel
limitations of the spreader feed
-------
equipment when firing the less dense
Dal/RDF mixture. Erratic steam rates
jere also experienced, reportedly due to
non-uniform mixing of the coal and RDF.
Particulate carryover did not increase
significantly, reportedly due to using
densified RDF rather than fluff RDF.
Conclusions
RDF co-firing affects many aspects of
boiler operation and performance,
including boiler efficiency, flue gas flow
rates, stack emissions, bottom ash
production, slagging and fouling. Boiler
efficiency decreases with RDF co-firing
because of increased flue gas production,
increased unburned carbon, and fuel
moisture losses. Flue gas flow rates
increase as the amount of RDF co-fired
increases primarily due to the
requirement for increased excess air to
account for variability in RDF feeding.
Compared to coal, RDF typically has low
sulfur concentrations, high chlorine
concentrations, and a high ash content.
Thus, co-firing RDF typically increases
HCI emissions and decreases SOX
emissions. In general, particulate
emissions increase with RDF co-firing.
CO emissions from RDF co-firing with
coal are not significantly changed from
coal-fired boiler CO levels. NOX
missions from RDF co-fired systems are
.*ot expected to be significantly different
than those from coal-fired systems. The
only dioxin data available on co-fired
units were from a study co-firing about
20% RDF by weight. No dioxins or furans
were detected in the emissions. In,
addition, emissions tests have been*
performed at seven coal fired boilers. No
CDD or CDF emissions were detected
from any of these facilities. The available
emissions data from boilers burning
100% RDF indicate that highly variable
CDD/CDF emissions may occur under
normal operating conditions. However,
the conditions under which substantial
CDD/CDF emissions occur are not
expected to be encountered under
normal operation in an RDF co-fired
system.
Bottom ash production increases by af
factor of 2 or 3 when 10-20% RDF is co-'
fired on a heat input basis because RDF
has a higher specific ash content (Ib/Btu)
than coal. RDF co-firing may increase the
potential for slagging and fouling and
result in increased maintenance costs,
and decreased boiler efficiency and
availability.
-------
V. Landrum and R. Barton are with Energy and Environmental Research Corp.,
Durham NC 27707.
James D. Kllgroe is the EPA Project Officer (see below).
The complete report, entitled "Municipal Waste Combustion Assessment: Fossil
Fuel Co-Firing," (Order No. PB90 159 831/AS Cost: $17.00 (subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park. NC 27711
UNOFFICIAL MAIL;
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S8-89/059
MOTCTIO. IOEIICI
CHICAGO
------- |