WHAT TO DO BEFORE YOU "NUKE" YOUR MULTIPLE HEARTH FURNACE
PRACTICAL TIPS AND TECHNIQUES FOR IMPROVING OPERATION,
REDUCING EMISSIONS AND MEETING THE REGULATIONS
F. Michael Lewis
F. Michael Lewis, Inc.
319 West Grand Avenue
El Segundo, California 90245
Lee A. Lundberg
SE Technologies, Inc.
98 Vanadium Road
Bridgeville, Pennsylvania 15017
Harry E, Boslian
U.S. Environmental Protection Agency (MLK-443)
26 West Martin Luther King Drive
Cincinnati, Ohio 45268
Eugene P. Grumpier
Office of Air Quality Planning & Standards (MD-13)
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
William G. DeWees
DEECO, Inc.
203 North Harrison, Suite F
Gary, North Carolina 27513
For Presentation At
The Future Direction of Municipal Sludge (Biosolids) Management;
Where We Are and Where We're Going
WEF Specialty Conference
Portland, Oregon
July 26-30, 1992
INTRODUCTION AND BACKGROUND
s
Over the past several years, the U.S. EPA has conducted a series of studies to gather information relating to
emissions of metals, organics, and total hydrocarbons (THC) from sludge incinerators in preparation for final
issuance of regulations under 40 CFR Part 503. This extensive sampling and monitoring program
encompassed six (6) multiple hearth (MH) furnaces and two (2) fluidized bed (FB) furnaces and provided
continuous emission monitoring (CEM) information not previously available. 1.2,3 This scientific'data base,
coupled with practical, field experience in the design, startup and rehabilitation of sludge incinerators, serves
as the basis for this paper, which explores cause and effect process relationships between operating
conditions, physical facilities and system performance.
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THE PRIMARY PROBLEM
The primary problem with MH furnaces is that they are inherently unstable. Due to their basic nature, MH
furnaces tend to magnify even a momentary disturbance or perturbation, such as a swing in feed sludge
moisture or a change in feed rate, into a serious process upset. These upsets inevitably create severe
process oscillations, which adversely affect overall furnace operations and result in increased emissions.
MH furnaces have relatively large inventories of material, distributed among various process stages (drying,
combustion, fixed carbon burn-out and cooling). Each of these processes is very sensitive to changing
conditions and is interrelated and interdependent. Moisture evaporation rate in the drying zone, for example,
is primarily a function of radiation from the roof of the hearth and the hot gases, a process which varies in
proportion to absolute temperature raised to the fourth power. Therefore,, even small temperature
perturbations are quickly amplified. These factors must be addressed in developing an effective approach to
improving performance and reducing emissions. This paper explores ways to overcome these problems, by
modifying existing facilities, to enable "yesterday's" equipment to meet "tomorrow's" requirements.
A situation where feed characteristics do not change significantly over the short term and where feed
systems can be controlled to deliver material to the furnace at relatively constant rates would be ideal. When
such operating conditions can be established, a relatively stable hearth temperature profile will result and
attention can be focused on 'fine-tuning* furnace performance. The limitations of existing facilities often
make it difficult to achieve these goals, although it is possible to approach them through a carefully crafted
program. In the absence of a consistent and regular furnace feed, however, one must inevitably deal with
recurring upset conditions. When a process can only achieve targeted setpoint values when there are no
disturbances on the system, it does not meet the criteria of control stability. This statement is descriptive of
most MH furnaces. Some degree of stability, albeit modest, can be achieved for any MH furnace; however, it
can only be optimized within the context of the overall systems, operations and facilities present at each site.
Traditionally, when MH furnace operators speak of 'lining out the furnace", what they are referring to is an
attempt to bring about a long and sustained period of operation during which any changes in furnace feed
that do occur, either in rate or in thermodynamic properties, do not materially affect the operating parameters
of their particular furnace. This is an important concept, as there are a number of different ways in which a
MH furnace can be "lined out'. At best, some of these techniques are ineffectual; still others run contrary to
improving operations and reducing emissions.
Typically, the operator has little control over key thermodynamic properties of the feed material, such as
volatile solids (VS), total solids (TS) or heating value of the sludge and, hence, must learn to adapt to
changing conditions within the framework of the existing facilities. Often, for a variety of reasons, the
operator cannot even fully control furnace feed rate, which further confounds the problem. To attempt to
achieve a relatively stable hearth temperature profile through the furnace, operators often resort to using
very high levels of excess air (high 02), which acts as a heat sink for naturally occurring variations in the
feed sludge and tends to dampen out or mask the effects of these changing conditions. While this may help
to improve control over furnace temperatures, it does so at the expense of unnecessarily increasing auxiliary
fuel usage, particularly if one is attempting to maintain elevated exhaust temperatures, for emissions control
purposes. In addition, it will limit the sludge processing capacity of the furnace system, either by causing the
capacity of the combustion air fan or the I.D. fan to be exceeded or by increasing the hearth area required for
drying (due to reduced exhaust temperatures) to the point where there is insufficient hearth area available for
combustion, fixed carbon burn-out and ash cooling.
Each system has a "pinch point* which limits its ability to respond to upset conditions. This point is a function
of the basic configuration of the system; the capacities of the various system components; and the mode of
furnace operation. When upsets are the rule rather than the exception, capacity limitations of the furnace
and various system components (fans, burners, blowers, valves, pumps and scrubbers) become more
pronounced, thereby leading plant personnel to be more inclined to "play it safe" in their operating practices,
keeping various parameters at certain comfort levels. When this happens, not only will the process continue
to suffer the effects of repeated upsets, the effective capacity of the furnace will be reduced as well.
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HISTORICAL OPERATING PRACTICES CAUSING ORGANIC EMISSIONS
The MH furnace is a counter-current process, in which the solids flow downward and the flue gases flow
upward. The feed sludge, which typically enters the furnace at a TS content of 18% to 25%, will not ignite
until it has been dried to a TS content on the order of 35% to 45%, depending upon feed characteristics and
excess air levels.4 The hottest hearth (combustion hearth) is located somewhere in the center of the
furnace, below the drying hearths and above the fixed carbon burn-out and ash cooling hearths. In general,
to provide reasonably adequate combustion and safe conditions, the temperature of the combustion hearth is
maintained above 1,400°F. To avoid slagging and clinkering, and to minimize the enrichment of metals in
the paniculate matter.S it is desirable for combustion hearth temperature to be maintained below 1,600°F.
As a practical matter, the controlling parameter is the upper limit on combustion hearth temperature, which is
usually maintained by adjusting (increasing) excess air levels to reduce the temperature of the "hottest"
hearth. If no auxiliary fuel is added to the furnace above this hearth, the evaporation of moisture from the
incoming feed sludge will typically cod the flue gases down to temperatures of between 750°F to 850°F at
the furnace exhaust.
For many years, the designers and operators of MH furnaces believed that the "thermal jump" theory,
derived from the work of Rudolfs and Baumgartner.6 indicated that organics would not distill off of the
incoming sludge prior to the combustion zone. This concept prevailed as late as 1979.7 The high levels of
organic emissions measured during the previously mentioned EPA 503 Regulations background studies'! -2,3
has, however, essentially laid the "thermal jump" theory to rest.
Historically, auxiliary fuel has been added near the bottom of MH furnaces, just above the ash cooling
hearths. While this practice may minimize auxiliary fuel consumption, it will also, unfortunately, generate the
highest Carbon Monoxide (CO) and THC levels in the furnace exhaust gases. In general, MH furnaces
operated in this "classical1 manner will frequently have CO levels above 2,000 ppmv and THC levels often
above 100 ppmv. Furnaces operating under this mode are also quite easily identified by their characteristic
yellowish-brown plume and the blackish areas found around the furnace roof near the outlet, due to the
deposition of soot and condensed organics, smelling like creosote. In addition, these facilities will generally
be plagued with problems of I.D. fan imbalance, as a result of deposition of these same compounds on the
fan wheel.
Burn-Out
A term familiar to all MH furnace operators, but unfortunately not to all consulting engineers designing and
specifying these furnaces, is "burn-out". When a feed stoppage is sustained for any time period greater
than, perhaps, five (5) minutes, the rate of combustion within the furnace accelerates rapidly and the fire
begins to rise rapidly to the upper hearths. A more complete discussion of this phenomenon can be found
elsewhere.8
Burn-out is perhaps the best example of the "inherent instability" of a MH furnace and serves as a graphic,
albeit extreme, illustration of the manner by which externally applied stressors can induce prolonged and
severe process upsets. Generally speaking, the wet sludge feed to most MH furnaces acts as a 'coolant',
due to its high moisture content. When the furnace feed is stopped, this coolant is removed and
temperatures on the upper (drying) hearths increase. When this occurs, the rates of moisture evaporation
and combustion increase. As the combustion rate increases, hearth temperatures rise which, in turn, causes
more rapid drying of the sludge. This, again, causes the combustion rate to increase even further, until the
operator is finally unable to get sufficient air into the furnace to meet the instantaneous oxygen demand and,
hence, black smoke appears in the stack.
Even furnaces that appear to be operating at steady state continually experience 'mini' burn-outs. When a
MH furnace is operating at heavy load (and variations in composition are more important), with excess air
rates near 75% (9% 02, dry) and auxiliary fuel introduced below the combustion hearth, the point of
maximum combustion will move in a spiral path up and down the furnace. To a degree, this is simply an
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inherent manifestation of the characteristic behavior of a counter-current stagewise process such as that
which occurs in a MH furnace. On the other hand, these effects can be intensified by physical and
operational attributes of a particular facility, such as insufficient combustion air delivery capacity; the inability
to obtain representative hearth temperature or exhaust oxygen measurements; and/or the use of improperly
configured or ineffectual control logic. Although not published, part of the EPA studies of one MH furnace3
involved carefully tracking hearth temperature profiles for an entire day during this mode of operation. The
period of oscillation was approximately one (1) hour, during which the burning zone was observed to move
from the inner to the outer limits of one hearth. During this test, the feed rate was extremely constant and
consistent. A more complete discussion of phenomena that cause the fire to move up and down a MH
furnace can be found elsewhere.8
Black Rain
Another term which is familiar to operators, but not necessarily to engineers, is "black rain". When a MH
furnace is operated in the classical mode, with auxiliary fuel introduced below the combustion hearth and low
furnace exhaust temperatures, it will inevitably experience periods which exhibit these 'telltale" sinusoidal
cycles in instantaneous burning rate. At such times, the combustion air flow through the furnace (and,
hence, flue gas flow through the I.D. fan) will follow this same pattern, as operational adjustments are made
in an attempt to maintain flue gas oxygen at desired levels or to maintain combustion hearth temperatures
within acceptable limits. All of the physical and operational parameters discussed above can amplify these
oscillations, as a result of over-correction or under-correction by the operators or the automatic controls.
When, for instance, a furnace feed stoppage has caused a severe process upset, and the I.O. fan has
reached its maximum capacity, water flowing across the upper plate of the tray scrubber can become
entrained in the flue gas stream and pass through the I.D. fan and discharge duct, washing off some of the
previously accumulated soot and condensed organic compounds. When this contaminated water leaves the
stack, it appears as what the furnace operators affectionately call "black rain*. Those who must wash this
residue from their cars, parked in nearby lots, refer to this phenomenon using language and phrases less
suitable for publication. Once again, however, the symptom reflects the underlying fundamental problem,
namely: process instability.
CAUSES OF CO AND THC EMISSIONS
CO and THC are used by regulatory agencies as surrogates for the aggregate general category of organic
emissions. These emissions include both volatile organic compounds (VOC's) and organic compounds that
are products of incomplete combustion (PIC's). Volatilized organics primarily come from the uppermost
(drying) hearths, while PIC's mainly come from the combustion zone of the furnace. In one of the most
comprehensive studies of the theory of PIC formation.9 several theories and hypotheses were presented
regarding this issue. Statements of these theories and hypotheses, excerpted from this study, are as
follows:
Theory 1: PIC Emission Rates are Kinetically, Not Thermodynamically Controlled.
Theory 2: Deviation from Normal, Average Operating Conditions are Responsible for Most PIC
Emissions.
Hypothesis 1: Flame Reactions Control Bulk or Absolute Emissions of PIC's While Post-Flame
(Thermal) Reactions Control Relative Emission Rates.
Hypothesis 2: Most PIC Emissions are the Result of Pyrolysis Pathways.
Hypothesis 3: A Significant Fraction of the Observed PIC's are Formed Outside of the High
Temperature Zones.
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Hypothesis 4: A Significant Fraction of Observed PIC's are Due to Fuel/Waste Interactions.
Hypothesis 5: Carbon Monoxide and Total Unburned Hydrocarbons are Surrogates for PIC Emissions.
Hypothesis 6: Surrogates that are Indicative of Poor Waste/Air Mixing (viz. benzene emissions rate,
stack Oxygen concentration and waste/heat load) are indicators of PIC Emissions.
When viewed in the proper context, any or all of these theories and hypotheses could be directly applicable
to MH furnace operation. In order to understand why the surrogates of PIC's (CO and THC) are found in
such high concentrations in MH furnace flue gas, one need only review the above and then visualize the
operation of a typical MH furnace, with its counter-current flow of sludge and flue gas; low exhaust
temperature; extremely poor gas phase mixing within the hearths; and notorious instability. The first step of
any emissions reduction program should focus on means to prevent PIC formation by attempting to eliminate
the conditions which are favorable to their generation, a topic which will be discussed later herein.
A popular, but erroneous, perception of MH furnace operation is that it represents a "continuous" process. In
the broad, global sense this is true; however, when subjected to close scrutiny, it is not. A MH furnace has
rabble arms on each hearth, which serve to direct the solid material through the furnace. There are typically
four rabble arms on each of the drying and uppermost combustion hearths. On the lower combustion and
ash cooling hearths, there are usually only two rabble arms. Sludge is plowed forward, in its downward
spiraling path, each time a rabble arm passes over the sludge furrow. At a shaft speed of one revolution per
minute, sludge is moved forward (and turned/mixed) at best only once every fifteen (15) seconds. In
essence, the hearth drop holes represent the region of most intense gas/solid mixing and, hence, exhibit the
greatest combustion intensity. If one studies the extent of the overall process occurring even at a single drop
hole, it is readily apparent that a MH furnace is more accurately described as a continuous series of "batch"
processes, rather than a single, truly continuous process.
Continuing to focus on the intense activity at each drop hole, one can speculate on possible causes for CO,
THC and PIC emissions in general. In the time interval between rabble arm passes, the flue gases rising up
through a drop hole (on an OUT hearth) will have excess oxygen. On the other hand, during the period
when burning sludge is falling through the drop hole, this region quickly becomes locally devoid of oxygen
and pyrolysis conditions exist. This provides an environment favorable to the formation of PIC's. To a lesser
degree, a similar situation occurs at the inner edge of an IN hearth.
CORRELATION OF CO, THC AND TEMPERATURE
As part of the EPA studies, 1.2,3 and also sub-studies of this EPA-supported program,10 attempts were
made to correlate THC with CO and also to correlate these emissions with the temperature of the upper
hearth or afterburner. In some tests, correlations appeared good, while in others it seemed that there were
no relationships at all. It is important to bear in mind that correlations for a legal regulation must necessarily
meet a different set of criteria than those used by engineers trying to get a better understanding of the
process. The conclusion reached in the EPA study was that there was not a sufficiently "good" correlation
for CO measurements to be used as a regulatory substitute for THC.1 However, when one examines the
same data from a somewhat more liberal process perspective, the correlations can indeed be found which
are consistent with classical combustion theory and previous combustion research.
The inherent instability of MH furnaces, coupled with operating practices during some of the study periods
which placed a higher priority on minimizing auxiliary fuel consumption rather than emissions, may account
for a significant portion of the disparities in the data and the observed lack of correlation. It is currently
estimated that the final EPA 503 limits for THC will be about 30 ppm. When one considers that the THC
analyzers used in these studies had to have a maximum range of 2,000 ppm, nearly two orders of magnitude
higher, one can appreciate how significantly furnace instability affects performance.
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Some previous attempts at data correlation used linear correlations, whereas combustion theory tells us that
none should exist. Real world combustion is a combination of both rate and diffusion controlled
mechanisms. Full scale CO burn-out tests were conducted, under very controlled conditions, on the
municipal solid waste incinerator at Pittsfield, MA. 11 Thermokinetic analysis of similar situations suggests
that the measured data should fit an equation of the following general form:
y = a e Dx
where "y" represents THC (or CO) concentration and V represents the inverse of the absolute temperature.
Therefore, a graph of the natural logarithm of CO concentration versus the inverse of the absolute
temperature (1/°R) should plot as a straight line. The data from the Pittsfield, MA tests is presented in this
manner in in Figure 1, along with a regression line which exhibits the excellent correlation (r2 = 0.95). This
data is replotted using an arithmetic scale in Figure 2. From this, it is apparent that linear correlations will not
be found between CO (or THC) and temperature. Similar results were obtained using data taken from pilot
studies for the Hyperion plant in Los Angeles, CA.12
Using data from the afterburner runs at one facility tested during the EPA programs (Site THC-2). excluding
only those few points where emission excursions were extremely high due to feed stoppages which
occurred, excellent correlations (from a process perspective) can be found between the natural logarithms of
CO and THC concentrations, as shown in Figure 3. Some data may seem to exhibit linear relationships
between raw concentration figures, but linear correlations between THC and CO will generally be better
using logarithmic scales. Given the presumed relationship between THC and CO, one might expect the
natural logarithm of THC concentration to have the same type of correlation with the inverse of the absolute
temperature (1/°R) as was found for CO.
Because of poor instrumentation, the temperature readings at this site were not very accurate and the
normal temperature variations that one would logically expect were not present. As a result, the correlation
between THC emissions and temperature was not as good as would be expected. For the purposes of
illustration, a regression analysis was performed to develop a relationship between THC concentration
(corrected to 7% 02) and afterburner exhaust temperature, assuming the equation form described above.
The results of this analysis are presented in Figure 4, plotted on an arithmetic scale for comparison with
Figure 2. The THC values shown are in the correct 'ballpark' and the curve is comparable to that obtained
(for CO) under more carefully controlled, stable operating conditions.
AFTERBURNER DESIGN
As noted earlier, THC emissions from MH furnaces include VOC's from the uppermost hearths and PIC's
from the combustion zone. The combustion processes which reduce VOC's and PIC's take place in the gas
phase, either above the burning fuel bed or in an afterburner. The destruction efficiency of VOC's and PIC's
in an afterburner is not governed by temperature alone. Other factors, such as the degree of gas phase
turbulence (completeness of mixing), oxygen concentration and residence time play equally important roles.
The afterburner at EPA Site THC-2 was not designed to promote significant turbulence and had a residence
time of only one-half (1/2) second. The rule-of-lhumb of providing an afterburning zone at 1,400°F for one-
half second was derived to yield 90% carbon conversion in fume incinerators and can be found in the air
pollution control legislation introduced in Los Angeles in the 1960's.13 To obtain the destruction efficiency
necessary to meet proposed regulations, without excessive temperatures, requires longer residence times
and more attention paid to increasing turbulence and providing complete gas phase mixing.
The ideal way to minimize THC emissions from a MH furnace is by afterburning, preferably using an external
chamber designed specifically for this purpose. Figure 5 is a schematic of the external afterburner installed
on the MH furnace in Pittsburgh, PA, which has been in operation since 1985. This unit was designed to
provide a residence time of one second at 1,400°F. For operating flexibility, two burners were provided, one
large (12 MMBTU/Hr) and one small (3 MMBTU/Hr). Although sufficient burner capacity is available to
operate this unit as high as 1,600°F, it has never been necessary to exceed 1,400°F to achieve essentially
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complete combustion and destruction of odors.14 During tests of this afterburner unit, THC concentrations
were on the order of 200 to 300 ppm at the inlet and less than 10 ppm at the outlet. 13 Some of the key
design features which have made this possible include the long, narrow gas path and the reverse bend,
which induces a pressure drop at the midpoint to promote turbulence and mixing.
Numerous thermodynamic studies'! 5,16 have indicated that combustion temperatures above 1,400°F are
necessary for destruction of VOC's and PIC's, within residence times typically used for afterburners (one-half
second and greater). However, the results of the recent EPA tests of emissions from MH furnaces.3 and
other recent work 17,18 have indicated that acceptable destruction of VOC's and PIC's can be achieved,
sufficient to meet the expected final standards (30 ppm THC) with MH furnace flue gas exhaust temperatures
less than 1,200°F.
To understand what appears to be an anomaly in the data, it is again necessary to make a microscopic
examination of the thermodynamic process, this time focusing on the region above the combustion hearths.
This zone includes the upper hearths, where drying is taking place, as well as any afterburning facilities,
either external to the MH furnace or internal ("zero" hearth). From a purely technical viewpoint, an
afterburner is devoid of sludge feed; however, when auxiliary fuel is fired above the combustion hearth, some
of the benefits attendant with an 'afterburner' can be realized. Therefore, this discussion will apply, although
to a lesser degree, to "fired" drying hearths of any MH furnace.
Figure 6 provides a schematic representation of an afterburner which, in this case, is employed on a MH
furnace operating without any auxiliary fuel input. The Theoretical Temperature of the Products of
Combustion (TTPC) of the flue gas leaving this MH furnace, prior to the introduction of any auxiliary fuel, is
621 °F. For ease of explanation, this is shown as an external afterburner; however, it could also represent an
internal, zero hearth afterburner or simply any fired upper hearth, where drying is taking place. For
illustrative purposes, the MH furnace flue gas is shown as being proportionally blended into the burner
exhaust gases in ten (10) separate gas streams, to simulate the mixing that takes place between the burner
and furnace gases. In actual practice, there would generally be only one point of furnace flue gas
introduction, but mixing of burner exhaust with MH furnace flue gases, even under nearly ideal conditions, is
never instantaneous.
The TTPC of the typical auxiliary fuel burner exhaust would be approximately 3,340°F at 15% excess air. As
the 621 °F MH furnace flue gas is mixed in stagewise fashion into the burner exhaust, as illustrated
schematically, the temperature decreases. Figure 7 illustrates a series of temperature profiles which would
occur within the "afterburner' as the percentage of MH furnace flue gas mixed with the burner exhaust is
increased from 0% to 100%. Each curve represents a different burner firing rate for which the final mixed
exhaust temperature can be read at the 100% point on the graph. This curve illustrates the fact that, even
when the final exhaust temperature is only 1,200°F, over 65% of the MH furnace flue gas has been subjected
to a temperature of 1,400°F or more for at least some short period of time.
As commented earlier, this concept can be applied to the mixing of burner exhaust with MH furnace flue
gases even on fired upper hearths. Accordingly, one can understand how firing burners on the drying
hearths may serve to partially reduce THC emissions, even though there is no outright afterburner and final
mixed gas temperatures are below the 'magic' 1,400°F level. Moreover, the authors contend that auxiliary
fuel should always be added above the combustion (hottest) hearth, wherever possible.
Zero Hearth Afterburners
From a process perspective, there is generally insufficient natural turbulence and gas-phase mixing at any
point in a MH furnace. Further, unless an external combustion chamber is provided, the retention time
available for afterburning is usually minimal, and the design represents, at best, a geometrical compromise.
While the best approach would be to have a separate afterburner, this is not always possible.
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One of the most common afterburner styles used with MH furnaces is the so-called 'Zero Hearth"
afterburner. In this configuration, sludge is fed directly to the second hearth and the uppermost hearth is
equipped with burners sized to raise the exhaust gas temperature to the desired level. Unfortunately, the
design of many of these zero hearth afterburners does not achieve the desired goal, because the MH
furnace flue gas will always take the path of least resistance and much of the gas may short-circuit directly to
the exhaust breeching.
If the uppermost hearth is an "In" hearth, there is no easy solution to the problem of short-circuiting and each
application must be evaluated on a case-by-case basis. A much better, though not perfect, afterburner
arrangement can be configured when the uppermost hearth is an "Out" hearth. In this instance, drop holes
on the breeching side can be sealed with refractory, which will force the flowing flue gas to pass through
more of the hearth volume, as shown schematically in Figure 8. With this design, it is generally advisable to
maintain two or more rabble arms in operation, to keep the hearth clear of accumulating ash.
MH FURNACE OPTIMIZATION
To demonstrate the potential for reducing emissions and improving the performance of existing MH furnaces,
the most recent EPA study3 included a series of tests directed towards MH furnace optimization. Based on
the concepts presented herein, two (2) major goals were established for the MH Furnace Optimization
program, as follows:
1. To the maximum extent practical, with existing burner configurations, auxiliary fuel would be added
above the combustion hearth.
2. To the maximum extent practical, with existing or slightly modified MH furnace configurations, mixing
and turbulence within the hearths would be increased, to minimize the formation of localized oxygen-
starved regions (pyrolysis pathways) and reduce the kinetic limitations of VOC and PIC destruction.
The steps necessary to achieve the first goal are straightforward; however, the burner placements on some
of the existing furnaces were specifically designed to add auxiliary fuel below the combustion hearth and the
desired practice could not be implemented. In fact, problems related to existing burner placement may have
potentially caused some erroneous conclusions to be drawn from the results of earlier tests on MH furnace
emissions.1.2.5 During some of those tests, only a minimal amount of burner capacity was available on the
upper hearths; therefore, it was necessary to fire burners on the combustion hearths to achieve higher top
hearth temperatures. Data from these previous studies suggested that higher outlet temperatures could
increase heavy metal emissions. It must be emphasized that, in furnace retrofit situations, where
modifications are being made to existing MH furnaces to reduce THC emissions, the need to fire burners on
the combustion hearth can be completely eliminated, thereby addressing potential problems of increased
heavy metal emissions associated with achieving higher outlet temperatures.
Attempts at improving mixing within the hearths of a MH furnace have previously received only a limited
amount of attention. One proprietary method involves the addition of high velocity mixing jets. 19 While it
may be beneficial to consider installing such facilities as a part of a major retrofit project, it was beyond the
scope of the EPA testing effort and, therefore, another method was utilized. The air/fuel ratio controls used
with the auxiliary burners on most MH furnaces, including the one at EPA Site THC-2. employ what is termed
a 'cross-connected, pressure balanced" system. Special "On-the-Fly" air/fuel ratio control units were
purchased and installed on the combustion hearth burners as a part of the EPA testing program. These
units enabled the air/fuel ratio of the burners to be changed while the MH furnace was in operation. The On-
the-Fly air/fuel ratio control units were adjusted to lean out the flame so that the combustion hearth burners,
still operating in their usual hearth temperature control mode, would fire at or near maximum air flow rate,
even at low natural gas flow rates. When adjusted in this manner, a cyclonic motion was imparted to the flue
gases within the hearth, substantially improving mixing.
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Furnace optimization test runs using this technique are referred to as HI Turbulence runs in the referenced
EPA study, to differentiate them from the tests using the fired afterburner.3 Except for a single, momentary
emissions spike, caused by a brief stoppage of sludge feed, the 02 corrected THC emissions were within the
anticipated final regulations (30 ppm) for a five-hour run. During these tests, the top hearth and breeching
temperature was approximately 900T, significantly lower than was required to achieve compliance during
the afterburner tests previously described. The flap gate on the sludge feeder was inoperative and remained
open during the entire test period, making the downstream oxygen readings essentially meaningless with
respect to actual combustion conditions, due to air leakage into the furnace. It would have been possible to
make operational adjustments to reduce THC emissions even further, if the capacity of the burners on the
upper hearths had been greater and the oxygen measurements had been representative of actual
combustion conditions within the furnace.
Based on the significant reduction in CO and THC that was achieved during the HI Turbulence runs using the
On-the-FIy air/fuel ratio control units to enhance combustion hearth mixing, it can be reasonably postulated
that the improved mixing resulted in the reduction of localized, oxygen-starved regions which favor PIC
formation. It is important to note that this technique achieved a comparable level of THC emissions, in
compliance with the anticipated final limits, at a substantially lower temperature (900°F versus 1,100°F) and,
hence, lower auxiliary fuel consumption. The MH furnace did not have multiple thermocouples on each
hearth; however, visual observations confirmed that the circumferential uniformity of hearth temperatures
was greatly improved after these changes were made.
When using the On-the-FIy air/fuel ratio control units, burners on the hearths above the combustion hearth
should be adjusted to fire with a minimum (15% to 25%) of excess air. The combustion hearth location
varies with furnace feed rate, moving down under high loadings and up during periods of low feed rates. The
advantage of On-the-FIy air/fuel ratio controllers is that the burners can be varied between low and high
excess air combustion with a simple adjustment of a one-quarter turn valve.
IMPROVING WET SCRUBBING SYSTEM PERFORMANCE
The typical wet scrubbing system found on most MH sludge incineration systems consists of an adjustable
throat venturi scrubber, followed by a two (2) or three (3) plate impingement tray scrubber and an I.D. fan.
Typically, the venturi throat is adjusted to maintain a fixed scrubber pressure drop and the inlet damper to the
I.D. fan is modulated to control furnace draft. The impingement tray scrubber serves a number of process
purposes. One of its primary functions is to sub-cool the exhaust gases (to 100-11Q°F) for plume
suppression. In addition, the impingement tray scrubber can effect significant removals of water-soluble acid
gases, such as SO2 and NO2, and can accomplish some particulate removal.8
Practical means by which the performance of sludge incinerator wet scrubbing systems can be improved,
through physical modifications and operational changes, are discussed in detail in various other technical
papers.4,8,13,20 One of the most fundamental of these techniques involves the use of the venturi throat to
control furnace draft, leaving the I.D. fan inlet damper open. In this manner, additional pressure drop, which
would otherwise betaken across the I.D. fan inlet damper, is used to increase particulate removal efficiency
of the venturi scrubber. Since many of the heavy metals which are volatilized in the furnace will recondense
onto the fly ash particulates as the furnace exhaust gases are cooled, increased metals removal efficiencies
can also be achieved using this technique.
Sludge incineration systems at municipal wastewater treatment plants have a unique opportunity to exploit
condensation effects to reduce power requirements and to improve the efficiency of wet scrubbing systems.
First of all, there is typically an abundant supply of relatively cold secondary effluent that can be used both as
a scrubbing medium and as a heat sink. Further, 70% to 80% of the furnace feed is usually water; hence,
the furnace flue gases have naturally high adiabatic saturation temperatures (175 to 183°F). Consequently,
the due gases already have the high absolute humidity that is essential for effective condensation scrubbing.
The potential advantages of flux-force condensation (FF/C) scrubbing are described elsewhere.20
551
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Conventional practice throughout the wastewater treatment industry is to fix venturi scrubber water rates to
achieve Liquid to Gas (L/G) ratios of from four to six times that required for adiabatic saturation. This
represents a relatively low range of venturi scrubber L/G ratios; therefore, there is usually very little
condensation occurring in the venturi scrubber and, hence, most existing systems are not taking maximum
advantage of the enhanced paniculate (and heavy metal) removal efficiency that can be achieved with FF/C
scrubbing. The development and application of a Psychrometric Optimization Model that can be used to
derive approaches for improving wet scrubber performance is discussed in detail in a separate paper.20
SUMMARY AND CONCLUSIONS
Emerging regulations, particularly those related to THC emissions, will create formidable challenges to
owners and operators of most existing MH furnaces. This is due to the fact that MH furnaces are inherently
unstable. Even brief, minor operating disruptions may produce prolonged "upset" periods, inevitably leading
to increased emissions. Operating phenomena, such as burn-out and black rain, are symptomatic of the
process instabilities which plague MH furnaces. Despite these inherent problems, the authors contend that
the performance of virtually any existing MH'furnace can be improved, through a carefully crafted program of
operational changes and physical modifications. Whether such changes will enable a MH furnace system to
comply with emissions regulations will depend upon the particulars of each situation. Often, measures which
seem to impart some degree of stability to MH furnace operations may actually increase certain emissions
and, further, may unnecessarily increase auxiliary fuel consumption or limit furnace capacity.
CO and THC are used by regulatory agencies as surrogates for organic emissions. These include both
volatilized organics, primarily corning from the uppermost (drying) hearths, and products of incomplete
combustion, which mainly come from the combustion zone of the furnace. As noted earlier, it is expected
that the final EPA 503 limits for THC will be about 30 ppm. Achieving compliance with this limit may prove to
be problematic for MH furnaces operating in the classical manner, with auxiliary fuel introduced below the
combustion hearth and relatively low furnace exhaust temperatures.
This paper emphasizes several approaches developed to address these problems, based on fundamental
process theory, field testing and practical experience in the design, startup and rehabilitation of sludge
incineration systems. First of all, it is highly recommended that auxiliary fuel be added above the combustion
hearth, to the maximum practical extent. Secondly, means should be provided to increase gas-phase mixing
and turbulence within each hearth, to minimize the formation of localized oxygen-starved regions (pyrolysis
pathways) and reduce the kinetic limitations of VOC and PIC destruction. One possible mixing method
involves the use of special On-the-Fly air/fuel ratio control units on the furnace burners, to permit the burner
air/fuel ratio to be changed while the MH furnace is in operation. These units can be adjusted to lean out the
flame so that combustion hearth burners, still operating in their usual hearth temperature control mode, will
fire at or near maximum air flow^rate at all natural gas flow rates. When adjusted in this manner, a cyclonic
mixing motion will be imparted to* the flue gases within the hearth.
To truly minimize THC emissions from MH furnaces, to levels comparable with fluidized bed (FB) furnaces,
some afterburning facilities may be required; however, it is possible to achieve compliance with expected
final emissions limits at lower temperatures by other techniques, such as by using On-the-Fly air/fuel ratio
control units on the furnace burners, as described herein. These approaches will contribute substantially to
improving performance; however, some instances may require more extensive measures. Even when it is
possible to incorporate afterburning facilities at a low capital cost, auxiliary fuel consumption will increase
when higher exhaust temperatures are required. By first taking steps to optimize the existing system, the
impacts of rising operating (fuel) costs can be minimized. When looking at the overall system, one should
also consider means to improve wet scrubbing system performance, as it complements the combustion
process in controlling a variety of related emissions. The purpose of this paper has been to explore the
major problems associated with MH furnaces and to develop ways to overcome these problems, largely
within the framework of the existing facilities. By making the most of what is already available, one can often
make 'yesterday's' equipment meet "tomorrow's' requirements.
552
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REFERENCES
1, "Emissions of Metals and Organics From Municipal Wastewater Sludge Incinerators", Volume I -
Summary Report, EPA 600/2-91/0073, U.S. EPA, Cincinnati, OH (July, 1991).
2. "Emissions of Metals, Chromium and Nickel Species and Organics From Municipal Wastewater
Sludge Incinerators", Volume I - Summary Report, EPA 6QO/R-§2/003a, U.S. EPA, Cincinnati, OH
(March. 1992).
3. Chehaske, J.T., DeWees, W.G., & Lewis, F.M., Total Hydrocarbon Emission Testing of Sewage
Sludge Incinerators", Draft Report, EPA Contract No. 68-CO-0027, U.S. EPA, Cincinnati, OH
(December, 1991).
4. Lewis, P.M. & Lundberg, L.A.. "Modifying Existing Multiple Hearth Incinerators to Reduce
Emissions", Proc. Natl. Conf, on Municipal Treatment Plant Sludge Management, Palm Beach,
Florida, pp.81 -88, HMCRI, Silver Spring, MD (1988),
5. Bostian, H.E., Grumpier, E.P., PaJazzolo, M.A. Barnett, K.W., & Dykes, R.M., "Emissions of Metals
and Organics From Four Municipal Wastewater Sludge Incinerators - Preliminary Data", Proc. Natl.
Conf. on Municipal Treatment Plant Sludge Management, Palm Beach, Florida, pp. 71-76, HMCRI,
Silver Spring, MD (1988).
6. Rudolfs, W., & Baumgartner, W.H., "Loss of Volatile Matter By Drying Sewage Sludge Before
Ignition*, Water Works and Sewerage, 79, pp.199-201 (June, 1932).
7. Process Design Manual: Sludge Treatment and Disposal, EPA Technology Transfer Publication
625/1 -79-011, U.S. EPA, Cincinnati, OH (September, 1979).
8. Lewis, P.M., & Lundberg, L.A., "Design, Upgrading and Operation of Multiple Hearth and Fluidized
Bed Sludge Incinerators to Meet New Emission Regulations', Paper No. 90-30.1, Proc. AWMA 83rd
Annual Meeting and Exhibition, Pittsburgh, PA (June, 1990).
9. Dellinger, B., Taylor, P.H., & Tirey, D.A., "Minimization and Control of Hazardous Combustion
Byproducts*, EPA/6GGYS2-9Q/039, U.S. EPA, Cincinnati, OH (May, 1991)-.
10. Shamat, N., Grumpier, E.P., & Roddan, A., "Total Hydrocarbon Analyzer Evaluation Study", Water
Environment & Technology, 3 (10), pp.73-92 (October, 1991).
11. Lee, K.C., "Research Areas for Improved Incineration System Performance", JAPCA, 38 (12),
pp.1542-1550 (December, 1988).
12. Lewis, P.M., Haug, R.T., Choti, S.I., & Moghaddam, O., "Ultra-Low NOx Combustion of Solid, Liquid
and Gaseous Fuels in the Hyperion Fluid Bed Furnace System0, Paper No. 92-43.02 (Draft), For
presentation at the AWMA 85th Annual Meeting and Exhibition, Kansas City, MO (June, 1992).
t
13. Lewis, P.M., Haug, R.T., & Lundberg, L.A., "Control of Organic, Particuiate and Acid Gas Emissions
From Multiple Hearth and Fluidized Bed Sludge Incinerators*, Presented at the 61 st Annual WPCF
Conference and Exposition, Dallas, TX (1988).
14. Lundberg, L.A., & Marchese, N.J., "Design of an integrated sludge incineration and energy recovery
system," Proc. A&WMA Specialty Conference on Thermal Treatment of Municipal, Industrial and
Hospital Wastes II, Pittsburgh, PA, pp.154-170, (November, 1989).
15. Rdke, R.W., et al., "Afterburner Systems Study", EPA EHS-D-71-E, U.S. EPA (1972).
553
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16. "Fluidized Bed Combustion of Sludge Derived Fuel - Results of Demonstration Studies to Develop
Design Criteria and Air Emissions Factors", Report of the Bureau of Engineering, City of Los
Angeles, CA (January, 1982).
17. Waltz, E.W., "Technical Discussion Paper - Potential Changes in EPA 503 Regulations for Total
Hydrocarbons and Management/Operational Practices for Sludge Incinerators", Prepared for the
AMSA Sludge Management Committee Incineration Workgroup, Portland, OR (July, 1990).
18. Baturay, A., Total Hydrocarbon Emissions from Multiple Hearth Furnaces", Paper No. 91-36.12,
Proc. AWMA 84rd Annual Meeting and Exhibition, Vancouver, BC, (June, 1991).
19. "Municipal Wastewater Sludge Combustion Technology", U.S. EPA Seminar Publication, Technomic
Publishing Company, Inc., Lancaster, PA (1985).
20. Lundberg, LA., Lewis, F.M., Semrau, K.T., & Hoecke, D.A., "Improving the Performance of Existing
Wet Scrubbers on Sludge Incineration Systems', (Draft), For presentation at The Future Direction of
Municipal Sludge (Biosolids) Management: Where We Are and Where We're Going, WEF Specialty
Conference, Portland, OR (July, 1992).
Figure 1 Carbon Monoxide Bum-Out Data - Pittsfield, MA
1000-
:; CO Data Corrected to ?'/• Oxygen
Carbon 100::
Monoxide
Concentration
(ppm) 1i
FT2 = 0.95
0.00045 0.00050 0.00055 0.00060
Inverse of Absolute Temperature (1/Deg.R)
554
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Figure 2 Carbon Monoxide Bum-Out Data - PittsTieia, >vi«
300
2 50-.
Carbon 200
Monoxide
150
Concentration
(ppm) 100
50-
1200 1300
CO Data Corrected to 7% Oxygen
*•*
. -: •.,...*
= 0.95
1400 1500 1600
Temperature (Deg.F)
1700 1800
Figure 3 Correlation Between Total Hydrocarbons
and Carbon Monoxide - EPA Site THC-2
100
;; THC and CO Readings are Unconnected
Total
H y drocarbon s 10 £
100
= O.S14
1000
Carbon Monoxide (ppm)
55S
-------
Figure 4 Correlation Between Total Hydrocarbons
and Temperature - EPA Site THC-2
300
:: \
250--
200"
Total : I
Hydrocarbons 150 -
(ppm) ;;
100--
50-
\
\
- 0.766
700 800 900 1000 1100 1200 1300 1400 1500
Temperature (Deg.F)
Figure 5 - External Afterburner Arrangement
n.ue
-------
Figure 6 - Conceptual Afterburner Mixing Model
3.340 Deg.F
2.566 Deg.F
2.153 Deg.FJ
1.889Deg.FJ
1.703Deg.FJ
1.566Deg.pf
1.460Deg.FJ
1.375Deg.FJ
1.306Deg.FJ
*<
1.249Deg.F(
1.200 Deg.F
621 Deg.F
Figure 7 Mixed Gas Temperature Profiles as a Function of
Percent of Flue Gas Mixed in Afterburner
3500
3000
2500-
Mixed Gas
Temperature 2000
(Deg.F)
1500
1000?
500
l—l—l—l—l—»—l—l—I—I—l
_ 1.400
- 1,300
. 1.200
•--1,100
- 1.000
BOO
207. 40% 60% 80% 100%
Percent of Flue Gas Mixed in Afterburner
557
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Figure S - MH Furnace With Zero-Hearth Afterburner
(Top Hearth Is an Out Hearth)
ZERO HEARTH -f
AFTERBURNER ^L
HEARTH t (?)"-—
^£^Fj-p_l ^ ffiV j
"*»»
y
*« PUUQQED ON \
/^-*> BREECHING! SIDE \
"\ X
f. ^1
v •;
^ r
) \
( ^
^- FLUi OAS PATH ' -
"P . FLUE GAS
•" EXHAUST
SLUOQE FEED
TO HEARTH 1
.
558
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TECHNICAL REPORT DATA
(Please read Instructions on the reverie before eompler
REPORT NO,
EPA/600/A-93/066
2.
4. TITLE AND SUBTITLE
WHAT TO DO BEFORE YOU "NUKE1* YOUR MULTIPLE^ HEARTH
FURNACE - PRACTICAL TIPS AND TECHNIQUES FOR IMPROVING
OPESAf IOH, REDUCING EMISSIONS AMD MEETING REGULATIONS
5. REPORT DATE
6, PERFORMING ORGANIZATION CODE
. AUTHOR(S)
F. MICHAEL LEWIS, LEE A. LUNDBERG, HARRY E. BOSTIAH,
EUGENE P. CRDMPLER AND WILLIAM G. DEWEES
8, PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
RISK REDUCTION ENGINEERING LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OH 45268
10. PROGRAM ELEMENT NO.
11. CONTRACT/CHANT NO.
68-CO-0027
Work Assignment No. 0-4
12. SPONSORING AGENCY NAME AND ADDRESS
Risk Reduction Engineering Laboratory—Cincinnati, OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH 45268
13. TYPE OF REPORT AND PERIOD COVERED
Published Paper
14. SPONSORING AGENCY CODE
EPA/600/14
15.SUPPLEMENTARY NOTES project Officer = Dr. Harry E. Bostian (513)569-76195 Proceedings
of "The Future .Direction of Municipal Sludge (Biosolids) Management: Where We Are and
Where We're Going", Volume I, Portland. Oregon. 7/26-30/92. p:543-550
16. ABSTRACT
This paper emphasizes several approaches for improving the
operation of multiple hearth sludge incinerators. First of all,
it is highly recommended that auxiliary fuel be added above the
combustion hearth, to the maximum practical extent. Secondly,
means should be provided to increase gas-phase mixing and
turbulence within each hearth, to minimize the formation of
localized oxygen-starved regions (pyrolysis pathways) and reduce
the kinetic limitations on the destruction of volatile organics
and products of incomplete combustion. One should also consider
means to improve wet scrubbing system performance, as it
complements the combustion process in controlling a variety of
related emissions.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS C. COS AT I Field/Group
Water pollution, sludge disposal,
incinerator(s), organic compounds,
combustion products
Emissions,
multiple hearth,
total hydrocarbons,
continuous monitoring
IB. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (Tliis Report!
UNCLASSIFIED
21. NO. OF PAGES
18
20. SECURITY CLASS (Tliis page!
UNCLASSIFIED
22. PRICE
EPA Form 222C-1 {R«». 4-77) PREVIOUS EDI TION is OBSOLETE
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