United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-89/061 Mar. 1990
Project Summary
Municipal Waste Combustion
Assessment: Fluidized Bed
Combustion
L.P. Nelson
This report describes an assess-
ment of combustion control practices
to minimize air •emissions from
refuse-fired fluidized bed combustors
(FBCs). The three FBC configurations
are described along with the design
and operating characteristics of
each, and the manufacturers of
refuse-fired FBCs are identified. The
waste-fired FBC population is over-
viewed, including existing, planned
and/or projected, and recently can-
celled facilities. Operating and emis-
sions data are presented for the two
existing U.S. facilities and one
Swedish circulating bed plant
The good combustion practice
(GCP) design, operating/control, and
verification recommendations devel-
oped for FBCs as part of this
assessment are summarized. The
GCP recommendations comprise a
set of specifications and procedures
designed to minimize emissions of
organic compounds. Quantitative rec-
ommendations are provided for a
number of the components; where
lack of data or dependence on site-
specific factors precludes quantita-
tive guidance, generic recommenda-
tions are presented.
Two model plants developed to
represent the population of FBCs
projected to be placed in operation
after the end of 1989 are described.
The models are evaluated to
determine the extent to which the
GCP recommendations are incorpor-
ated within the projected population.
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
protect that is fully documented in a
separate report of the same title (see
Pro/ect Report ordering information at
back}.
Introduction
Based upon its analysis of Municipal
Waste Combustors (MWCs), EPA has
determined that MWC emissions may
reasonably be anticipated to contribute to
the endangerment of public health and
welfare and warrant further regulation. As
a result, EPA's Office of Air Quality
Planning and Standards is developing
emission standards for new MWCs under
Section 111(b) of the Clean Air Act (CAA)
and guidelines for existing MWCs under
Section 111(d) of the CAA. In support of
these regulatory development efforts, the
Air and Energy Engineering Research
Laboratory in EPA's Office of Research
and Development has conducted an in-
depth assessment of combustion control
practices to minimize air emissions from
MWCs. This report documents the results
of that assessment for one specific MWC
technology: fluidized bed combustors
(FBC).
Objectives of this report were to
identify the population of existing and
planned refuse-fired FBC facilities in the
U.S., examine the design characteristics
and operating practices employed, define
one or more model plants representative
of the projected population, and develop
recommendations for implementing good
combustion practices for the control of
organic emissions from FBCs.
Two Types of Combustors
Two types of FBCs are currently used
or projected for use for refuse firing in the
-------
U.S., bubbling bed FBCs and circulating
fluid beds (CFBs). The two are
distinguished primarily by the bed
configuration. The relatively low fluidizing
air velocity in the bubbling bed produces
a stationary bed (consisting of approx-
imately 99% inert material and 1% fuel)
that resembles a violently boiling liquid.
The higher fluidizing air velocity of the
CFB entrains a significant portion of the
bed material and carries it out of the
reactor vessel. Solids are disengaged
from the CFB gas stream in a hot
cyclone and reinjected into the reactor.
The existing population of waste-fired
FBCs in the U.S. consists of four units at
two facilities. All of the units in the
population are bubbling beds. The
Western Lake Superior Sanitary District
(WLSSD) co-fires 120 tons/day of RDF
and 350 tons/day of wood chips in two
Energy Products of Idaho FBC units at
their French Island Generating Facility in
La Crosse, Wl. Both facilities have
demonstrated total tetra- to octa-chlor-
inated dibenzo-p-dioxin and dibenzofuran
(CDD/CDF) emissions rates below 20
ng/Nm3 in compliance tests.
The planned and projected refuse-
fired FBC population includes two units at
one facility in the construction stage and
13 to 16 units at seven facilities in the
feasibility study or early planning stage.
Seven other facilities that were included
in EPA's list of planned and projected
facilities in May 1988 have either been
cancelled or altered the project in a way
that caused its removal from the FBC
population.
GCP Recommendations
QCP recommendations consist of
combustor design and operating
specifications and procedures designed
to minimize emissions of organic com-
pounds. GCP recommendations were
presented in the Report to Congress for
three MWC technologies; these recom-
mendations have subsequently been
updated to account for recent data and
expanded to cover the full range of MWC
technologies. The design and operating
parameters affected by the GCP
recommendations are: Fuel feeding,
temperature at fully mixed conditions,
combustion air capacity and distribution,
mixing, particulate matter carryover,
auxiliary fuel capacity, and downstream
gas temperature. These components
were selected because each is indi-
vidually necessary to the implementation
of proper organic emissions control and
the set, as a whole, is sufficient to ensure
adequate control. Quantitative GCP
recommendations were developed during
this assessment for the temperature at
fully mixed conditions, overfire air
capacity, downstream gas temperature,
and CO in flue gas; generic recom-
mendations were identified for the other
components.
GCP recommendations in EPA's 1987
Report to Congress for other MWC
technologies specified a mean fully
mixed temperature of 980°C; however,
these recommendations were based on
the need to maintain a conservative
minimum temperature of 900°C where
significant spatial and temporal
temperature variations have been
observed. FBC systems inherently have
extremely efficient mixing and uniform
temperature cross-sections, so the appli-
cation of a safety factor to account for
temperature variations is not necessary.
Compliance tests for the two existing U.S.
FBC facilities conducted at between 815
and 870°C resulted in low CDD/CDF
emissions. Also, precedence for a lower
temperature requirement has been estab-
lished in permit limits set by the
Pennsylvania Department of Environmen-
tal Regulation for the Erie County CFB
(790 °C) and the Wisconsin Department
of Natural Resources for the French
Island facility (815°C). These factors sup-
port the establishment of 815°C as the
GCP recommendation for fully mixed
temperature in FBCs.
The compliance tests at the NSP and
WLSSD FBC facilities were conducted
while the units were operating with
approximately 40 and 0% of total air as
overfire air, respectively. Both compli-
ance tests resulted in low CDD/CDF
emissions. However, the WLSSD system
is designed such that RDF is injected
near the bottom of the bed. In systems
where overbed feed is employed, overfire
air above the feeders may be essential to
ensuring good mixing of fuel and air.
Thus, FBC design recommendations
include incorporation of an overfire air
system capable of supplying 40% of the
total combustion air. The 40% figure is
based on engineering judgment of the
overfire air jet momentum requirements.
Pilot scale and field test data from a
wide variety of sources support the
existence of a mechanism whereby
CDD/CDF is formed downstream of the
combustor by the catalytic action of fly
ash constituents on the gas stream.
Formation has been observed at
temperatures between 200 and 400°C,
with maximum formation at about 300 °C.
Downstream formation is thought to be
minimized if the gas and solids residence
time at between 200 and 400 "C are
minimized, i.e., the quench rate through
these temperatures is maximized. Ir
terms of practical GCfi
recommendations, the system should b«
designed such that the gas temperature
at the inlet to the ESP or baghouse i:
maintained below 230 °C.
Flue gas CO concentration is a goo<
measure of combustion conditions. A!
such, it serves as a convenient, thougt
inexact, surrogate parameter fo
CDD/CDF emissions, which also hav<
been correlated to good combustion. Th<
Gotaverken CFB in Sundsvall, Sweden
maintained CO concentrations below 5(
ppm. The WLSSD FBC also producet
CO levels of less than 50 ppm. The NSF
CO emissions of nearly 300 ppm, havi
been attributed to the compactness of th<
freeboard. Less than 2.7 m of furnaci
height separates the top of the bed fron
the entrance to the first convectivi
section. While this space may bi
sufficient for oxidation of gas-phase or
ganics which occurs essential!
instantaneously (assuming the other GCI
recommendations are in place), it is nc
sufficient to complete oxidation of CO t
C02. Thus, 50 ppm (4-hour average) ha
been specified as the flue gas CO GCI
recommendation for FBCs.
Two FBC model plants wer
developed to provide the basis for EP,
health and economic impact analysis c
the MWC regulation. Separate model
were required because of the significar
variation between bubbling bed and CFl
CDD/CDF emissions. The bubbling be
model plant represents the population c
new bubbling bed facilities expected t
be placed in operation after regulatio
proposal at the end of 1989. Based o
existing performance and emissions dat;
the bubbling bed model plant was judge
to incorporate all of the GCP design an
operation/control recommendations. Th
CFB model plant was based on th
Gotaverken design, the only CFB desig
in the planned/projected population. Tr
CFB model plant was judged 1
incorporate all of the GCP recon
mendations except minimizing particula
matter (PM) carryover. If the primary P'
control device allows a significant fractic
of the PM to pass into the boiler, organ
and organic precursor material adsorbs
on the PM may escape destruction. Fie
test data from a mass burn waterwall ar
an RDF spreader stoker MWC ha\
shown a strong correlation between P
carryover and CDD/CDF emissions. Tl
relatively high CDD/CDF emissions fro
the CFB model plant are also attribuft
to this mechanism. Unless a mo
efficient primary PM collection devii
can be incorporated into the desig
-------
further reductions in organic emissions
may have to be effected through in-
furnace sorbent injection and/or add-on
pollution control devices.
-------
L.P. Nelson is 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:
Fluidized Bed Combustion," (Order No. PB 90-164 0541 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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
OmCIALMAfL"
U.S.POSTAGE
5 0 ,2 5 ;
Official Business
Penalty for Private Use $300
EPA/600/S8-89/061
000085833
»«iiCl
230
------- |