United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/S8-89/060 May 1990
x>EPA Project Summary
Municipal Waste Combustion
Assessment: Waste Co-Firing
V.J. Landrum and P.J. Schindler
This report provides an overview of
waste co-firing and auxiliary fuel
fired technology and identifies the
extent to which co-firing and auxiliary
fuel firing are practiced. Waste co-
firing is defined as the combustion of
wastes (e.g., sewage sludge, medical
waste, wood waste, and agricultural
waste) In a unit designed to burn
municipal solid waste (MSW) or
refuse derived fuel (RDF) as a major
fraction of total fuel Input Auxiliary
fuel firing is defined as the practice
whereby coal, fuel oil, or natural gas
is fired in a municipal waste
combustor under condititions when
waste feed quantities are interrupted.
This is a fairly common practice for
dedicated RDF boilers, and there may
be additional mass burn MWCs that
meet the definition of auxiliary fuel
firing. This report includes
descriptions of technologies used by
facilities that meet these definitions,
characterizes the population, and
discusses design and operating
practices and available emissions
data from each facility. THe report
concludes with a discussion of
recommended good combustion
practices for waste co-firing
combustors and auxiliary fuel fired
MWCs.
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
Waste co-firing is defined as the
combustion of wastes (e.g..sewage
sludge, medical waste, wood waste, and
agricultural waste) in a unit designed to
burn MSW or RDF as a major fraction of
total fuel input. All MWC designs have
the potential to be waste co-firing units:
however, some facility operating permits
specify the extent to which waste can be
co-fired. Auxiliary fuel firing is defined as
the practice whereby coal, fuel oil or
natural gas is fired in a municipal waste
combustor under conditions when waste
feed quantities are interrupted. This is a
fairly common practice for dedicated
RDF boilers, and there may be additional
mass burn MWCs that meet the definition
of auxiliary fuel firing.
Refuse Derived Fuel Firing
RDF combustion technology includes
suspension firing, semi-suspension firing,
and fluidized bed combustion.
Suspension firing is restricted largely to
fossil fuel co-fired units. Nearly all
dedicated RDF combustors are spreader
stokers, which burn RDF in a semi-
suspension mode. The RDF is normally
injected into the combustor through
airswept distributors on the front wall of
the furnace. A portion of the fuel burns in
suspension and the remainder falls to a
traveling grate where burnout is
completed. All RDF spreader stokers use
waterwall boilers to produce steam and,
in some cases, generate electricity, and
most of these units are equiped with oil
or natural gas burners that can provide
up to 100% of design heat input.
A Fluidized Bed Combustor (FBC) is a
reaction vessel containing a bed of inert
solid particles (typically sand, limestone,
or alumina), which is fluidized by the
upward flow of gas (air). Combustion in
the FBC occurs primarily within the bed,
which is interspersed with the fluidizing
air and particles of fuel and ash. FBCs
are characterized by extremely efficient
mass and heat transfer, i.e. very uniform
temperatures and mass compositions are
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observed in both the bed and the
freeboard. Efficient mass and heat
transfer allows both conventional and
waste fueled FBCs to operate at lower
excess air and temperature levels than
conventional combustion systems. Waste
fired FBCs typically operate at excess air
levels between 30 and 100% (5-10% 02
in the dry flue gas) and at bed
temperatures around 815°C.
There are many types of mass burn
MWCs in the existing population of
facilities, including large field-erected
units with individual capacities of 100-
1000 tpd (91-907 tonnes/day), and
modular shop-fabricated units of 5-120
tpd (4.5-109 tonnes/day). The large mass
burn systems include conventional mass
burn waterwall combustors, rotary
waterwall combustors, and refractory wall
incinerators. All of the large mass burn
units operate in an excess air mode, with
typical excess air levels of 30-150%.
Most modular mass burn units include
multiple combustion chambers, with the
waste burning in the primary chamber
and burnout of combustible products
being completed in a secondary
chamber.
Several MWC facilities meet the
definition of waste co-firing combustors
by burning sewage sludge, medical
waste, or wood waste, in a unit designed
to burn MSW or RDF as a major fraction
of total fuel input. Six plants that routinely
burn sewage sludge with municipal solid
waste have been identified in the existing
MWC population. The plants include units
using conventional mass burn waterwall
combustors, mass burn refractory wall
combustors, modular excess air com-
bustors, and FBCs. The average sewage
sludge mass input ranges from 1-10% for
five facilities and is 74% for the FBC
facility which was primarily designed as a
sewage sludge incinerator. An additional
facility has sewage sludge handling
equipment, although it has never been
used, and plant personnel report that
there are no future plans to co-fire
sludge. At least nine facilities in planning,
permitting, and construction phases may
burn a mixture of MSW and sewage
sludge.
At least 15 MWC facilities have
previously accepted or are currently
accepting medical waste, including three
mass burnwaterwall, three mass burn
refractory, three modular starved air, five
modular excess air, and one FBC.
Medical waste ranges from less than 1-
50% by weight of the total feed. The
design and operating characteristics of
these facilities are described in the
"Municipal Waste Combustion
Assessment, Medical Waste Combustion
Practices at MWC Facilities, July 1989,
EPA-600/8-89-062." Many new MWC
projects are planned for construction in
the next few years. All of these facilities
have the potential to accept medical
waste along with MSW. Existing facilities
also have the potential to begin accepting
medical waste with their MSW. There are
strong economic incentives for other
MWCs to accept medical waste; much
higher tipping fees can be charged for
medical waste than for MSW. As a result
of these incentives, there is a high
potential for additional MWCs to accept
medical waste routinely.
Wood waste is routinely fired in three
MWC facilities. The annual average mass
input of wood waste is 10-40%. At least
13 facilities in planning, permitting, and
construction phases may burn a mixture
of wood and MSW.
Auxiliary Fuel Firing
Auxiliary fuel firing is limited to MWCs
that produce steam, and is generally
practiced only as necessary to maintain
steam load when waste supplies are
curtailed or interrupted. Although some
mass burn facilities may fire auxiliary fuel
when waste is not available, auxiliary fuel
firing is more commonly practiced by
RDF spreader stokers than by other
designs. Because of the manner in which
feeding takes place (air swept
distributors), plugging of feed
mechanisms is a more persistent
problem in RDF spreader stokers.
Therefore, it is necessary to have the
ability to fire an auxiliary fuel in order to
maintain steam load when these
problems occur. These facilities are
described briefly below. Most RDF
spreader stoker facilities are equipped
with natural gas as the auxiliary fuel with
rated capacities of 14-100%. Coal can
provide 100% of the rated capacity for
these facilities.At least one additional
RDF-fired facility was designed to burn
RDF and/or coal. However, the units have
not burned coal for several years, and
there are no plans to resume this
practice. Two additional facilities have
been identified that plan to co-fire RDF
and coal in FBC boilers. Currently, no
chlorinated dibenzo-p-dioxin/chlorinated
dibenzofuran (CDD/CDF data are
available from RDF spreader stokers co-
firing coal. Testing at Mid-Connecticut in
1988-89 should provide these data.
Emissions were tested at the Albany,
NY facility in 1984 while firing 100% RDF
and also while firing RDF with 15%
natural gas. CDD/CDF emissions were
measured in the stack during both tests.
The tetra- through hepta-CDD/CDF
homolog groups were quantified (no octa-
CDD/CDF) for both test conditions. Total
CDD/CDF emissions were 440 ng/dscm
while firing 100% RDF, and 840 ng/dscm
while firing 85% RDF with 15% natural
gas. The natural gas burners are on the
rear wall of the boiler about half way
between the traveling grate and the over
fire air ports. It is suspected that, when
the gas burners are operating, mixing
patterns may be disrupted in the boiler,
increasing vertical gas velocities in the
lower portion of the combustion chamber.
This may allow pockets of unburned
material (gaseous and solid) to escape
from the boiler without being properly
mixed, resulting in higher CDD/CDF
emissions in the stack.
The existing population of waste co-
fired MWCs includes all combustor
designs, and each of these combustors
can potentially burn a wide variety ol
waste and fossil fuel mixtures. The
currently available emissions data base
does not support any conclusion that, foi
a given combustor design, CDD/CDF
emissions from waste co-firing facilities
will differ substantially from 100% MSV\
or RDF combustion. CDD/CDF emission;
have been measured from man}
combustion sources, including units tha
burn sewage sludge, wood, coal, ant
medical waste. However, for these fuels
CDD/CDF emission levels appear t(
depend more on combustion condition:
than on waste feed characteristics.
Good Combustion Practices
The good combustion practice
developed for MSW and RDI
combustors are based on two mai
concepts:
1) Sufficient mixing must be achieve
at a temperature adequate t
maximize the destruction of trac
organics.
2)Conditions that promote lo
temperature formation of trac
organics must be minimized.
These basic concepts apply to all MW
systems, and hence, to all waste co-firir
MWCs. They are necessary conditior
for minimization of CDD/CDF emissions.
The requirements that will minimu
CDD/CDF emissions do not change wi
waste feed. However, when firing
mixture of wastes and/or fossil fuel
conditions may arise that will lirr
operation. For example, the high moistu
content of sewage sludge may impose
maximum sludge firing rate to mainte
the required temperature at the fu
mixed location. A typical sewage sludi
with 20% solids and a dry heating val
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of 7000 Btu/lb (1.63 x 10? J/kg) will have
net as-fired heat input near zero.
.'herefore, the heat required to maintain
temperatures in the combustor must be
contributed from an additional source. It
may be necessary for mass burn
systems to have air preheat or to fire an
auxiliary fuel when burning MSW/sludge
mixtures. These considerations are
important when designing new systems
or retrofitting existing MWCs to fire
sludge along with MSW. Sometimes,
excess air levels and combustion air
distributions will also change slightly
when co-firing waste.
Combustion of wood waste, particularly
in RDF spreader stokers, may contribute
to high carryover of entrained particulate
matter, which may contribute to low-
temperature downstream formation
mechanisms that lead to higher
CDD/CDF emissions. As a result, it is
important to design all systems to
minimize the potential for these
conditions.
Auxiliary fuel firing in MWCs should be
designed so that adequate mixing at
necessary temperatures occurs for all
firing conditions. This can
beaccomplished by conducting flow
modeling studies on new and existing
boilers to determine the optimum location
and firing rate for auxiliary fuel burners
that will be used in combination with
MSW or RDF combustion. By applying
appropriate design and operating
practices to ensure good mixing, auxiliary
fuel firing MWCs can maintain low
emissions of air pollutants.
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V. Landrum and P. Schindler are with Energy and Environmental Research Corp.,
Durham, NC 27707.
James D. Kilgroe is the EPA Project Officer (see below).
The complete report, entitled "Municipal Waste Combustion Assessment: Waste
Co-Firing," (Order No. PB90-161 001/AS; Cost: $15.00, 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
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