U.S. patent application number 12/751506 was filed with the patent office on 2010-07-29 for dynamic control of selective non-catalytic reduction system for semi-batch-fed stoker-based municipal solid waste combustion.
This patent application is currently assigned to COVANTA ENERGY CORPORATION. Invention is credited to Stephen G. Deduck, Mark L. White.
Application Number | 20100189618 12/751506 |
Document ID | / |
Family ID | 39588932 |
Filed Date | 2010-07-29 |
United States Patent
Application |
20100189618 |
Kind Code |
A1 |
White; Mark L. ; et
al. |
July 29, 2010 |
DYNAMIC CONTROL OF SELECTIVE NON-CATALYTIC REDUCTION SYSTEM FOR
SEMI-BATCH-FED STOKER-BASED MUNICIPAL SOLID WASTE COMBUSTION
Abstract
The present invention controls reagent flow levels in a
selective non-catalytic reduction (SNCR) system by more accurately
predicting Nitrogen Oxides (NOx) production with a municipal waste
combustor. In one embodiment, the reagent levels correspond with
measured furnace temperatures. The reagent levels may have a
baseline level from prior measured NOx that is then modified
according to temperatures measurements. A slow controller may use
NOx measurements over an extended period to define a base regent
level, and a fast controller may use additional information such as
the furnace temperature to modify the base regent level. The fast
controller may further receive two additional signals that are
added individually or together to maximize NOx control while
minimizing ammonia slip from the reagent. The two signals are a
feed-forward signal from the combustion controller and a feedback
signal from an ammonia analyzer downstream of the combustion
zone.
Inventors: |
White; Mark L.;
(Tannersville, PA) ; Deduck; Stephen G.; (Scotch
Plains, NJ) |
Correspondence
Address: |
HOGAN LOVELLS US LLP;IP GROUP, COLUMBIA SQUARE
555 THIRTEENTH STREET, N.W.
WASHINGTON
DC
20004
US
|
Assignee: |
COVANTA ENERGY CORPORATION
Fairfield
NJ
|
Family ID: |
39588932 |
Appl. No.: |
12/751506 |
Filed: |
March 31, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11960148 |
Dec 19, 2007 |
7712306 |
|
|
12751506 |
|
|
|
|
60876559 |
Dec 22, 2006 |
|
|
|
Current U.S.
Class: |
423/235 ;
422/110 |
Current CPC
Class: |
B01D 53/56 20130101;
B01D 2251/2062 20130101 |
Class at
Publication: |
423/235 ;
422/110 |
International
Class: |
B01D 53/56 20060101
B01D053/56; G05D 7/00 20060101 G05D007/00 |
Claims
1. A system for controlling NO.sub.x levels comprising: a
temperature sensor; an amount of reagent for reducing NO.sub.x
levels; a reagent supplier; a reagent controller connected to the
reagent supplier adapted to receive temperature data from the
temperature sensor; wherein the reagent controller is configured to
adjust the amount of reagent based solely upon the temperature data
obtained from the temperature sensor.
2. The system of claim 1, wherein the system does not contain any
components to measure NO.sub.x levels directly.
3. The system of claim 1, wherein the system is a SNCR system.
4. The system of claim 1, wherein the system is a SCR system.
5. The system of claim 4, wherein the temperature sensor is not
configured to measure the catalyst temperature.
6. The system of claim 1, wherein the temperature sensor is
configured to measure the temperature of a furnace.
7. The system of claim 6, wherein the furnace is configured to
receive an amount of fuel and contains a flue and a grate for
moving the fuel through the furnace.
8. The system of claim 1, wherein the temperature sensor is a
pyrometer.
9. The system of claim 1, wherein the temperature sensor is
configured to measure the temperature of gases in the furnace.
10. The system of claim 1 further comprising: a combustion
controller connected to the reagent controller, wherein the
combustion controller is configured to provide combustion data
relating to combustion conditions upstream of the reagent
controller.
11. A system for controlling emission levels of NO.sub.x
comprising: a. a reagent controller; b. a furnace for receiving an
amount of fuel, said furnace containing a flue and a grate for
moving the fuel through the furnace; c. a continuous emissions
monitoring (CEM) system comprising: i. a probe for determining one
or more of the composition of pollutants, NO.sub.x levels, or
levels of un-reacted reagents emitted by the furnace; ii. a link to
the reagent controller for providing information on one or more of
the pollutants, NO.sub.x levels, or un-reacted reagents contained
in the emissions from the furnace; d. a temperature sensor for
providing temperature information; e. a combustion controller to
monitor the amount of fuel entering the furnace and transfer this
information to the reagent controller; f. wherein the reagent
controller has programmable logic stored on computer readable media
for causing the reagent controller to perform the step of
controlling reagent supply to the furnace based upon one or more of
the CEM information, the combustion information or the temperature
information.
12. The system of claim 11, wherein the system is a SNCR
system.
13. The system of claim 11, wherein the system is a SCR system.
14. The system of claim 11, wherein the CEM probe is positioned
inside the furnace downstream of the flue.
15. The system of claim 11, wherein the reagent controller has
programmable logic stored on computer readable media for causing
the reagent controller to perform the step of distributing the
reagent into an area of high turbulence to homogenize distribution
of the reagent in the furnace based upon information received from
the CEM system.
16. The system of claim 11, wherein the reagent controller has
programmable logic stored on computer readable media for causing
the reagent controller to perform the step of distributing the
reagent when the temperature is high enough to allow NO.sub.x to
react with the reagent.
17. The system of claim 11, wherein the CEM system comprises a
first probe for determining NO.sub.x levels and a second probe for
determining ammonia slip levels.
18. The system of claim 17, wherein the link is capable of
transmitting a signal containing information on both the NO.sub.x
levels and the ammonia slip levels.
19. The system of claim 11, wherein the combustion controller
comprises an output for informing the reagent controller when
additional fuel or additional air enters the furnace.
20. A method for controlling emission levels of NO.sub.x comprising
the steps of: a. providing a reagent controller; b. receiving an
amount of fuel in a furnace containing a flue and a grate; c. using
the grate to move the fuel into the furnace; d. providing a
continuous emissions monitoring (CEM) system; said CEM system: i.
analyzing data received from a probe to determine one or more of
the composition of pollutants, NO.sub.x levels, or levels of
un-reacted reagents emitted by the furnace; ii. providing
information on one or more of the pollutants, NO.sub.x levels, or
levels of un-reacted reagents contained in the emissions from the
furnace to the reagent controller; g. providing a temperature
sensor that provides the reagent controller with temperature
information; e. providing a combustion controller to monitor the
amount of fuel entering the furnace and transfer this information
to the reagent controller; f. wherein the reagent controller has a
programmable logic stored on computer readable media for causing
the reagent controller to perform the step of controlling the
amount of reagent supplied to the furnace based upon one or more of
the temperature information, the information received from the CEM
system and the information received from the combustion controller.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/960,148, which claims priority under 35
U.S.C. .sctn.119(e) from U.S. Provisional Patent Application No.
60/876,559 filed on Dec. 22, 2006, the subject matter of which is
herein incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to an improved control system
for a selective non-catalytic reduction (SNCR) system that uses a
reagent such as ammonia or urea to reduce nitrogen oxides (NOx)
emissions from a waste-to-energy boiler. Specifically, the improved
control system allows the SNCR system to achieve desirable NOx
reductions while also better minimizing the undesired excess
application of the reagent, thus reducing ammonia emissions from
the stack.
BACKGROUND OF THE INVENTION
[0003] The combustion of solid waste in a Municipal Waste Combustor
(MWC) generates some amount of nitrogen oxides (NOx). NOx is the
generic name for a group of colorless and odorless but highly
reactive gases that contain varying amounts of NO and NO.sub.2. The
amount of NOx generated by the MWCs varies somewhat according to
the grate and furnace design but typically ranges between 250 and
350 ppm (dry value at 7% O.sub.2 in the flue gas).
[0004] The chemistry of NOx formation is directly tied to reactions
between nitrogen and oxygen. To understand NOx formation in a MWC,
a basic understanding of combustor design and operation is useful.
Combustion air systems in MWCs typically include both primary (also
called undergrate) air and secondary (also called overgrate or
overfire) air. Primary air is supplied through plenums located
under the firing grate and is forced through the grate to
sequentially dry (evolve water), devolatilize (evolve volatile
hydrocarbons), and burn out (oxidize nonvolatile hydrocarbons) the
waste bed. The quantity of primary air is typically adjusted to
minimize excess air during initial combustion of the waste while
maximizing burnout of carbonaceous materials in the waste bed.
Secondary air is injected through air ports located above the grate
and is used to provide turbulent mixing and destruction of
hydrocarbons evolved from the waste bed. Overall excess air levels
for a typical MWC are approximately 60 to 100% (160-200% of
stoichiometric (i.e., theoretical) air requirements), with primary
air typically accounting for 50-70% of the total air.
[0005] In addition to destruction of organics, one of the
objectives of this combustion approach is to minimize NOx
formation. NOx is formed during combustion through two primary
mechanisms: Fuel NOx from oxidation of organically bound elemental
nitrogen (N) present in the municipal solid waste (MSW) stream and
Thermal NOx from high temperature oxidation of atmospheric
N.sub.2.
[0006] More specifically, fuel NOx is formed within the flame zone
through reaction of organically bound N in MSW materials and
O.sub.2. Key variables determining the rate of fuel NOx formation
are the availability of O.sub.2 within the flame zone, the amount
of fuel-bound N, and the chemical structure of the N-containing
material. Fuel NOx reactions can occur at relatively low
temperatures (<1,100.degree. C. (<2,000.degree. F.)).
Depending on the availability of O.sub.2 in the flame, the
N-containing compounds will react to form either N.sub.2 or NOx.
When the availability of O.sub.2 is low, N.sub.2 is the predominant
reaction product. If substantial O.sub.2 is available, an increased
fraction of the fuel-bound N is converted to NOx.
[0007] In contrast, thermal NOx is formed in high-temperature flame
zones through reactions between N.sub.2 and O.sub.2 radicals. The
key variables determining the rate of thermal NOx formation are
temperature, the availability of O.sub.2 and N.sub.2, and residence
time. Because of the high activation energy required, thermal NOx
formation does not become significant until flame temperatures
reach 1,100.degree. C. (2,000.degree. F.).
[0008] However, NOx emissions are generally undesirable and are of
environmental significance because of their role as a criteria
pollutant, acid gas, and ozone precursor. Direct health concerns of
NOx center on the gases' effects on the respiratory system because
NOx reacts with moisture, ammonia and other compounds to form
nitric acid and related particles that may damage lung tissue.
These and other particles produced from NOx penetrate deeply into
sensitive parts of the lungs and can cause or worsen potentially
fatal respiratory diseases such as emphysema and bronchitis.
[0009] In addition, the emissions of NOx pose other environmental
concerns. For example, ground-level ozone is formed when NOx and
volatile organic compounds (VOCs) react with heat and sunlight.
Children, asthmatics, and people who work or exercise outside are
susceptible to adverse effects from the ozone, and these effects
include lung tissue damage and decreased lung function. Ozone also
damages vegetation and reduces crop yields.
[0010] Furthermore, the reaction of NOx and sulfur dioxide with
other substances in the air to form acids, which fall to earth with
rain, fog, snow or dry particles as acid rain. Acid rain damages or
deteriorates cars, buildings and monuments, as well as causes lakes
and streams to become unsuitable for fish.
[0011] In addition, NOx are indirect greenhouse gases that affect
the atmospheric amounts of hydroxyl (OH) radicals. Specifically,
the breakdown of NOx gases gives rise to increased OH
abundance.
[0012] Consequently, various laws and regulations have been passed
to limit the emissions of NOx from MWCs and other sources. For
example, the Unites States Environmental Agency is authorized in 40
C.F.R. Part 60 to monitor and limit NOx from MWCs. Similar rules
and regulations to limit NOx emissions likewise exist
internationally, such as in Europe, Canada, and Japan. It should be
appreciated that a complete understanding and knowledge of various
rules and laws on NOx emissions are outside the scope of the
current discussion.
[0013] NOx control technologies can be divided into two subgroups:
combustion controls and post-combustion controls. Combustion
controls limit the formation of NOx during the combustion process
by reducing the availability of O.sub.2 within the flame and
lowering combustion zone temperatures. These technologies include
staged combustion, low excess air, and flue gas recirculation
(FGR). Staged combustion and low excess air reduce the flow of
undergrate air in order to reduce O.sub.2 availability in the
combustion zone, which promotes chemical reduction of some of the
NOx formed during primary combustion. In FGR, a portion of the
combustor exhaust is returned to the combustion air supply to both
lower combustion zone O.sub.2 and suppress flame temperatures by
reducing the ratio of O.sub.2 to inerts (N.sub.2 and carbon dioxide
(CO.sub.2)) in the combustion air system.
[0014] Post-combustion controls relate to removing NOx emissions
produced during the combustion process at solid waste fired
boilers, and the most commonly used post-combustion NOx controls
include selective non-catalytic reduction (SNCR) systems, which
typically reduce the NOx significantly, or selective catalytic
reduction (SCR) systems, which typically reduce the NOx even more
effectively than SNCR systems. As described in greater detail
below, SCR systems are many times more expensive to build, operate,
and maintain than SNCR systems and are consequently not
economically feasible for use on waste-to-energy (WTE) plants in
many parts of the world.
[0015] SCR is an add-on control technology that catalytically
promotes the reaction between NH.sub.3 and NOx. SCR systems can use
aqueous or anhydrous NH.sub.3 reagent, with the primary differences
being the size of the NH.sub.3 vaporization system and the safety
requirements. In the SCR system, a precise amount of a reagent is
metered into the exhaust stream. The reagent decomposes into
ammonia and reacts with NOx across a catalyst located downstream of
the injection point. This reaction reduces NOx to elemental
nitrogen and water vapor. SCR systems typically operate at
temperature of approximately 500-700.degree. F. In terms of waste
disposal fee impact and cost effectiveness, SCR generally has
higher costs resulting from high capital costs, as well as the cost
of catalyst replacement and disposal.
[0016] In contrast, SNCR reduces NOx to N.sub.2 without the use of
catalysts. Similar to the SCR system, the SNCR system injects one
or more reducing agents (or "reagents") into the upper furnace of
the MWC to react with NOx and form N.sub.2. Without the assistance
of a catalyst, these reactions occur at temperatures of
approximately 1600-1800.degree. F. When the reagent is introduced
in low amounts, virtually all of the reagent is consumed, and
increasing the reagent amount in the SNCR systems may result in
further NOx reductions. When operating the SNCR systems near the
upper end of their performance range, however, excess reagent may
be added to the reactor chamber, and the excess reagent passes
through the MWC and ultimately escapes into the atmosphere, an
undesirable phenomena known as ammonia slip.
[0017] SNCR systems are well known and disclosed, for example, by
Lyon in U.S. Pat. No. 3,900,554 and by Arand et al in U.S. Pat.
Nos. 4,208,386 and 4,325,924. Briefly, these patents disclose that
ammonia (Lyon) and urea (Arand et al) can be injected into hot
combustion gases within specific temperature windows to selectively
react with NOx and reduce it to diatomic nitrogen and water. While
described herein in connection with MWC systems, SNCR are also used
to reduce NOx emissions from other combustion facilities, such as
coal and oil furnaces and diesel engines.
[0018] The current SNCR controls typically use a slow-acting
controller to adjust ammonia flow based on stack NOx emissions. In
other words, the amount of ammonia introduced in a current time
period generally depends on the average amount of NOx measured in
the MWC emissions during one or more time periods. This approach
works well with processes that have little variation in NOx
emissions, such as coal or oil-fired boilers. Even when NOx
emissions vary significantly on a minute-to-minute basis, this
known approach works well to meet current regulatory limits because
the regulatory limits are based on a long-term average NOx levels,
such as a daily average, and are set at levels that are readily
achievable with current control approaches. If tighter NOx limits
or shorter averaging periods are required, however, this known
approach using measured NOx emissions levels to control reagent
levels results in potentially diminished NOx reduction and higher
ammonia slip.
[0019] In particular, simply speeding up the response of the
ammonia flow to the stack NOx signal is ineffective because of the
time delay between NOx generation in the furnace and NOx
measurement in the Continuous Emissions Monitoring (CEM) system
that monitors stack emissions from the MWC. A control system that
simply uses a faster response criteria will direct the SNCR system
to respond to a temporary increase in NOx emission by increasing
ammonia flow, even though the measured high NOx levels have already
left the furnace area with the SNCR system. When the additional
reagent is applied during subsequent periods of lower NOx levels,
the increased ammonia flow may be excessive, causing increased
ammonia slip. Likewise, the SNCR system responds to a temporary
decrease in NOx stack emissions by decreasing reagent flow, and the
decreased levels of reagent flow may be inadequate to optimally
address relatively higher NOx furnace levels. In short, past NOx
levels are a good indicator of current NOx levels for processes
with little variation, or when controlling to readily achievable
limits over relatively long time periods. When controlling to
stricter limits in processes with highly variable NOx emissions,
past NOx levels are no longer a good indication of current NOx
levels.
[0020] Similarly, current reagent levels may depend upon other
measurements. For example, in another known SNCR system control,
the CEM system measures ammonia slip to determine the amount of
un-reacted reagent contained in the stack emissions. The detected
levels of current ammonia slip are then used to modify the amount
of reagents applied in the SNCR system. However, ammonia slip
levels, in themselves, may have little relevance to NOx levels, so
adjusting the reagent level to minimize ammonia slip may provide
relatively poor NOx reduction performance. In addition, the ammonia
slip criteria of controlling SNCR system suffers from a similar
deficiency to the NOx-based control systems in that the measured
levels of current ammonia slip in the emissions, in itself,
provides limited guidance about the reagent flow needed to address
current future furnace conditions and resulting NOx levels in the
furnace.
SUMMARY OF THE INVENTION
[0021] In response to these and other needs, embodiments of the
present invention provide a system and method for controlling
reagent flow levels in a SNCR system in MWCs by basing reagent
levels on measured aspects that more accurately predict current
furnace NOx levels over the short term. In one embodiment, the
reagent levels correspond with measured furnace temperatures. The
new approach uses a rapidly responding ammonia flow to increase
ammonia during high NOx periods and to reduce it during low NOx
periods, but relies on a real-time temperature measurement in the
furnace as a surrogate for NOx. This eliminates the delay inherent
in the NOx measurement device. As a result, ammonia flow is
increased during the high temperature portion of the combustion
cycle when NOx generation is higher and then reduced during the low
temperature portions corresponding to lower NOx generation, thus
improving NOx reduction and reducing ammonia slip by minimizing the
excess application of the reagent.
[0022] Similarly, the reagent levels may have a baseline level that
is then modified according to furnace temperature measurements. For
example, a slow controller may use NOx measurements over an
extended period (such as several hours) to define a base reagent
level using the average NOx levels. A second, fast controller,
using additional information about the current condition of the
furnace such as the furnace temperature, predicts changes to the
furnace NOx levels and then makes modifications to the base reagent
level as needed to address the predicted changes to the NOx
levels.
[0023] Linking a combustion control system to the SNCR system to
provide a feed-forward signal to the SNCR control can further
enhance the SNCR control process. This linkage would allow reagent
flow to be increased in anticipation of higher NOx levels and
decreased in anticipation of lower NOx levels. In this embodiment,
the fast controller may use other collected data to more accurately
predict changes in the NOx levels and to make appropriate
corrections to the reagent levels. For example, another embodiment
of the present invention includes a fast controller that include
two additional signals that are added individually or together to
maximize NOx control while minimizing slip. The two signals are a
feed forward signal from a combustion controller and a feedback
signal from an ammonia analyzer downstream of the combustion
zone.
[0024] Thus, in one embodiment of the invention, a method for
controlling an amount of a NOx reducing reagent in an MWC is
provided. The method includes the steps of measuring temperature
changes; using the measured temperature changes to predict changes
in NOx levels in real or near-real time; and using the predicted
changes in NOx levels to define the amount of the NOx reducing
reagent.
[0025] In another embodiment of the invention a system for reducing
NOx emissions from an MWC is provided. The system includes a
temperature sensor producing temperature data; means for applying
an amount of a reagent for reducing NOx emissions, the reagent
applying means being positioned downstream from the temperature
sensor; and a reagent amount controller connected to the reagent
applying means, the reagent amount controller adapted to receive
the temperature data from the temperature sensor, the reagent
amount controller adjusting the amount the reagent in response to
said received temperature data.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] A more complete understanding of the present invention and
advantages thereof may be acquired by referring to the following
description taken in conjunction with the accompanying drawings in
which like reference numbers indicate like features, and
wherein:
[0027] FIG. 1 (PRIOR ART) is a flow chart depicting a known method
for controlling reagent levels in a selective non-catalytic
reduction (SNCR) system;
[0028] FIGS. 2A-2C are charts depicting problems caused by the
known method presented in FIG. 1 for controlling SNCR system
reagent levels;
[0029] FIGS. 3-6 are flow charts depicting an improved method for
controlling reagent levels in a SNCR system in accordance with
embodiments of the present invention;
[0030] FIG. 7 is a high-level schematic diagram of a municipal
waste combustor implementing an improved SNCR control system of
FIG. 8 in accordance with embodiments of the present invention;
[0031] FIG. 8 is a high-level schematic diagram of an improved SNCR
control system in accordance with embodiments of the present
invention;
[0032] FIG. 9 is a graph illustrating the relations among furnace
temperature, NOx emissions, and ammonia slip with conventional NOx
control techniques; and
[0033] FIG. 10 is a graph illustrating the relations among furnace
temperature, NOx emissions, reagent flow, and ammonia slip with
improved control methods according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] As depicted in the figures and as described herein, the
present invention provides an improved method and system for
controlling selective non-catalytic reduction (SNCR) systems in
municipal waste combustors (MWCs) to reduce both Nitrogen Oxides
(NOx) emissions and ammonia slip.
[0035] Turning now to FIG. 1, a known method 100 for controlling
SNCR systems is described. In the known SNCR control method 100, a
MWC facility is operated in step 110. The stack NOx emissions from
the MWC over one or more periods is then measured in step 120. In
step 130, a proportional-integral-derivative (PID) controller is
used to identify the error between the measured NOx emissions level
and a desired setpoint. As known in the art, the PID controller
calculation involves three separate parameters: the Proportional,
the Integral and Derivative values. The weighted sum of these three
parameters is used to adjust the process via a control element.
Then, in step 140, a corrective reagent level (i.e., ammonia) is
calculated and outputted to adjust the process accordingly. The
process can then repeat, starting at step 110, with the MWC being
operated with the SNCR system applying the reagent levels at the
level associated with the measured NOx levels.
[0036] The limitations of the known SNCR control method 100 are
summarized in FIG. 2A, which contains a chart 200 depicting stack
NOx levels 210, 220 for two time periods, T1 and T2. The two
measured stack NOx levels 210, 220 may be used to determine an
average NOx level 230, and the average NOx 230 may be used to
determine a corresponding SNCR reagent level. It can be seen that
the average NOx level 230 is lower than the T1 NOx level 210 and
greater than the T2 NOx level 220. Consequently, the reagent level
designed to address the average measured NOx 230 is insufficient
for the NOx level 210 for period T1 and is excessive for the NOx
level 220 for period T2. The area 240 between the T1 NOx level 210
and the average NOx level 230 represents excess NOx emissions that
could otherwise be reduced by the SNCR system through higher levels
of reagents. Similarly, the area 250 between the T2 NOx level 220
and the average NOx level 230 indicates that excess reagent is
applied by the SNCR system, some of which may be emitted as ammonia
slip.
[0037] Speeding up the response of the reagent flow to the stack
NOx signal is ineffective because of the time delay between NOx
generation in the furnace and stack NOx measurement in the
Continuous Emissions Monitoring (CEM) system that monitors stack
emissions from the MWC. A control system that simply uses a faster
response criteria will direct the SNCR system to respond to a
temporary increase in NOx emission by increasing reagent flow, even
though the measured high NOx levels have already left the furnace
of the MWC. When the additional reagent is applied during
subsequent periods of relatively lower NOx levels, the increased
flow will cause increased ammonia slip due to the un-reacted
reagent. Likewise, the SNCR system responds to a temporary decrease
in NOx emission by decreasing reagent flow during subsequent
periods, and the decreased levels of reagent flow would be
inadequate to optimally address relatively higher NOx levels during
subsequent periods.
[0038] Turning now to chart 200' of FIG. 2B, the implications of
basing reagent levels on accelerated measured stack NOx levels are
described. For reasons described above, there is a significant time
lag between furnace production and stack measurement of NOx. FIG.
2B, depicts a situation in which the T1 NOx level 210 is used to
define the reagent levels for T2. In this example, the reagent
level associated with T1 NOx level 210 would be even more excessive
for time period T2, as indicated by the relatively larger area
250'. The area 250' represents even more excess reagent applied by
the SNCR system that will likely be emitted as increased ammonia
slip. Thus, basing reagents levels on a peak NOx measurement would
likely produce increased ammonia slip. In the same way, basing
applied reagent levels on a low level of measured NOx (such as T2
NOx level 220) would result in excess NOx emissions (area 240) that
could otherwise be reduced through the SNCR system.
[0039] In addition to the above-stated limitations, the NOx levels
may also vary greatly within any particular time period.
Specifically, NOx emissions from a MSW combustion system are very
dynamic and are directly linked to a combustion cycle with a
non-continuous waste feeding system. Consequently, the NOx level
varies significantly from minute-to-minute as the MWC is fed,
ignited, and burned. The known SCNR control method 100 disclosed in
FIG. 1 keeps the ammonia flow rate relatively constant, and does
not attempt to chase these NOx spikes up and down. The reason for
this approach is the delay between the time of peak NOx generation
in furnace, and the time it shows up on the stack analyzer, which
is commonly about 1 to 3 minutes. Because a typical combustion
cycle may be two to three minutes, this means that the peak NOx
generation may be occurring at about the time of minimum indicated
NOx, and vice-versa. Thus, chasing NOx spikes with ammonia may
simply result in higher ammonia rates when NOx levels are low and
lower ammonia rates when NOx levels are high, the opposite of the
desired result from a SNCR control system. Causes for the
temperature variations in the MWC are described in greater detail
below.
[0040] Turning now to chart 200'' of FIG. 2C, the implications of
the rapidly changing NOx levels are described. In particular, it
can be seen that the actual NOx level 260 varies continuously over
periods T1 and T2. The T1 NOx level 210 and the T2 NOx level 220
then represent average values over periods T1 and T2. Thus, even if
the reagent can be applied accurately at the T1 NOx level 210 and
the T2 NOx level 220, the reagent level may be insufficient or
excessive at any particular time. Furthermore, as described above,
the measured changes in the stack NOx levels 260 occur well after
the production of the NOx in the furnace. Thus, even with rapid
measurements of current NOx levels 260, the application of the
reagents will not occur until well after the creation of the
NOx.
[0041] To address these and other limitation, the present invention
provides a new approach that uses a rapidly responding reagent flow
to increase reagent during high-NOx periods and reduce it during
low-NOx periods by relying on a real-time or near real-time
temperature measurement in the furnace as a surrogate for levels of
NOx emissions. This configuration helps to eliminate the delay
inherent in the NOx measurement device. As a result, reagent flow
is increased during the high temperature portion of the feeding
cycle when NOx generation is higher, and reduced during the lower
NOx generation intervals, thus reducing ammonia slip.
[0042] Referring now to FIG. 3, a SNCR control method 300 comprises
the steps of measuring the furnace temperature at prespecified
location in step 310, and associating the measured furnace
temperature with predicted furnace NOx level in step 320. A reagent
level corresponding to the predicted furnace NOx is then determined
and applied in step 330. The measurement of the furnace
temperatures in step 310 may be performed using a known temperature
probe as described below.
[0043] It is known that temperature changes correspond to changes
in NOx production. Specifically, a change in temperature indicates
a change in the waste burn cycle. For example, following
introduction of new waste into a furnace, the temperature will
initially decrease as the new waste is heated up and its water
vaporized. The NOx levels in the furnace are low at this point
because not as much nitrogen-bearing fuel is being burned. As the
volatile portion of the newly-fed waste starts to combust and
release heat energy, both the furnace temperature and NOx levels
increase. As the volatile fraction of the waste completes
combustion, NOx generation in the furnace will decrease and the
furnace will start to cool.
[0044] FIG. 9 illustrates the relations among furnace temperature,
NOx emissions, and ammonia slip with conventional NOx control
techniques. Beginning at approximately 13:50 on the time axis,
there is a rapid reduction in furnace temperature, accompanied by a
sharp reduction in NOx emissions and an increase in ammonia slip at
the stack. This chart also shows a general agreement between
furnace temperature and NOx, with the NOx level increasing when the
furnace temperature increases and vice-versa. It is also apparent
that the NOx emissions signal lags behind the temperature signal by
several minutes. This is due to the time delay between the time NOx
is generated in the furnace, and measured in the CEM system.
[0045] MWC have varying designs, thereby operating at different
temperatures and producing different levels of NOx depending, for
example, on the waste capacity, combustion process, and the design
of the MWC. The MWC can be evaluated to determine the NOx emissions
levels following furnace temperature changes. With this data, any
changes in temperature measured in step 310 may be accurately
associated with changes in NOx levels in step 320. While the
present discussion may focus on absolute temperature to predict NOx
levels, the SNCR control method 300 may likewise use relative
changes in temperature, with the temperature changes used to
calculate changes in NOx emissions.
[0046] Once the NOx levels are predicted in step 320, the amount of
reagent levels needed to best address the predicted furnace NOx
levels is calculated in step 330. Again, this amount of reagent
will depend greatly on the design of the MWC and may be determined
empirically through trial and error from previous reagent
applications. Likewise, the timing of application of the reagent
may be determined empirically through an analysis of prior waste
combustion to determine an expected delay between temperature
changes near the grate, and the subsequent arrival of changed NOx
levels downstream at the SCNR system.
[0047] Other embodiments of the present invention disclose SNCR
control methods that incorporate temperature measurements with
other collected data to better control the SNCR system. For
example, referring now to FIG. 4, a second SNCR control method 400
uses both temperature and NOx measurements to control the
application of the reagent by the SNCR system. The SNCR control
method 400 generally includes the steps of measuring temperature in
step 410 and measuring NOx levels in step 420, corresponding to
above-described steps 120 and 310. Next in step 430, the
temperature and NOx measurements are used to predict furnace NOx
levels near the SNCR system where the reagent is applied. For
example, the measured NOx levels may be used to determine prior NOx
levels at the SNCR system (since there may a significant time delay
between the flue gases passing by the SNCR system and the flue
gases reaching a downstream CEM system that measures the NOx
values). The prior NOx levels at the SNCR system may be used to
form a baseline prediction of current NOx levels at the SNCR system
in step 430, with the temperature changes being used to modify the
prior NOx levels. For example, the NOx levels likely increase if
temperatures increase, the NOx levels likely decrease if
temperatures decrease, and the NOx levels likely remain stable if
the furnace temperatures are stable. The particular relationship of
temperature and NOx levels to current NOx levels at the SNCR system
may be determined empirically through trial and error. Then, in
step 440, an appropriate amount of the reagent may be applied by
the SNCR system to address the predicted NOx levels determined in
step 430. Again, the levels of reagent will depend on the design
and operation of the MWC and the SNCR, and the specific amount of
reagent, and timing of the changes in reagent rate, can likely be
determined from historical collected data from past operations of
the MWC.
[0048] Referring now to FIG. 5, in another embodiment of the
present invention, a third SNCR control method 500, is provided. In
this embodiment, combustion controller data is collected in step
510. The combustion controller data generally relates to the amount
and time that waste and combustion air are introduced into the MWC
furnace. The combustion controller data may further provide
information, for example, on the nature of the waste, such as its
moisture content, general composition, and particle size; or
further information about combustion air, such as its distribution
among various injection points, its temperature, or its oxygen
content in a system employing recirculated flue gas or oxygen
enrichment This combustion controller data from step 510 may be
used in step 530 to predict furnace NOx levels following combustion
of the waste. Additionally, other information about the current
condition of the furnace, such as its current operating
efficiencies, outside weather conditions, etc. may be used as well.
As before, the NOx prediction in step 530 generally depends on
historically collected data from one or more MWCs, where current
emissions conditions are correlated with similar prior conditions,
and then measured NOx outputs during these periods of similar
conditions may be used to estimate NOx levels in the furnace. Once
the NOx levels are predicted, then an appropriate reagent level may
be defined in step 540, typically based upon historical data. The
historical data may be in the form of recent data, continuously
collected and analyzed, from the MWC unit being controlled, thus
providing near-real-time adjustment to the correlation between
furnace conditions and NOx levels.
[0049] It should be appreciated that similar to the SNCR control
method 400, NOx conclusions from the combustion control data in
method 500 may be adapted according to other measured data,
including the measured NOx emissions data collected in known SNCR
control method 100 and the temperature data collected in the first
SNCR control method 300. Thus, the controller may also receive
additional furnace data in optional step 520, and the prediction of
NOx levels at the SNCR system in step 530 may incorporate this
additional data. The combustion controller data from step 510 may
be combined with temperature data in step 520 to modify NOx levels
measured downstream to predict current NOx in the furnace. For
example, the combustion controller data from step 510 may provide
information on when the municipal waste was added to the MWC, and
corresponding temperature readings from step 520 may provide useful
information on the effect of the additional waste on the NOx
levels.
[0050] The combustion controller data from step 510 would direct
the reagent flow to increase when or shortly after new fuel is
introduced to the combustion zone but before an increase in
temperature occurs. This would eliminate any delay in the reaction
and ensure that increased reagent is available as soon as needed.
The same combustion controller data would allow reagent flow to be
reduced when or shortly after the feeding of new fuel pauses, thus
ensuring that excessive reagent is not present when not needed.
[0051] Referring now to FIG. 6, in another embodiment, a fourth
SNCR control method 600, following a prior operation of furnace and
SNCR systems in step 610 (for example, operating according to the
SNCR control method 300 using temperature data), may further
include collecting data on the levels of ammonia slip from the MWC
in step 620. The ammonia slip is typically measured in a flue
downstream from the furnace. The ammonia slip levels from step 620,
while not directly relevant to NOx levels in the furnace or in the
MWC emissions, can be used to determine whether excess reagent
levels is being provided by the SNCR system. For example, excess
reagent levels may be applied because of furnace conditions
preventing proper operation of the SNCR reagent such as a blockage
preventing proper mixing and distribution of the reagent.
Decreasing the reagent levels will momentarily reduce the undesired
ammonia slip. Conversely, optimal furnace conditions may allow for
higher reagent levels without excess ammonia slip. In this way, the
ammonia slip data may be used in step 630 to modify the reagent
levels, established otherwise as described above in SNCR control
methods 100, 300, 400, and 500. In this way, the real time ammonia
slip concentration in the flue gas downstream of the combustion
zone can be used to immediately reduce reagent flow when excessive
ammonia slip is occurring, and provide a permissive to increase
reagent flow when acceptable values of ammonia slip are
occurring.
[0052] Referring now to FIG. 7, a MWC furnace 700 in accordance
with embodiments of the present invention includes a SNCR system
710. As described above, the SNCR system 710 is well known in the
field of emission controls to reduce NOx emissions. The SNCR system
710 generally relies on the addition of a reagent such as ammonia
or urea to reduce NOx emissions. Specifically, the SNCR system 710
applies the reagent at one or more locations of the furnace having
a specific temperature range needed for the reaction of the NOx
with the reagent. While the SNCR is depicted as having a single
input valve into the interior of the furnace 700, it should be
appreciated that the SNCR system inputs are typically positioned
around the periphery of the furnace 700, along three exterior
surfaces, with the fourth surface being a wall shared in common
with the flue system. Multiple elevations may be used to
accommodate variations in gas temperature within the furnace. The
SNCR inputs are configured to distribute the reagent evenly in the
flue gases to better homogenize the NOx and reagent contents. The
SNCR input locations may be placed in a region of high turbulence
to further mix the reagent with the flue gases, encouraging the NOx
reducing reactions.
[0053] The SNCR system generally includes a SNCR controller 715 to
direct the timing, amount, and location of reagent applied to the
furnace 700. The SNCR controller 715 generally includes
programmable logic designed to adjust the flow of reagent in
response to various data inputs, as described above in the SNCR
control methods 100, 300, 400, 500, and 600. The SNCR controller
715 is connected to various components, as desired, to receive the
data signals. The SNCR controller 715 is described in greater
detail below in FIG. 8.
[0054] Continuing with FIG. 7, the MWC typically includes a CEM
system 720. While the CEM system 720 is depicted as being
positioned in the furnace 700 near the SNCR system, it should be
appreciated that the CEM system 720 is generally positioned
downstream in the flue, following various emissions treatments.
Because of the distance between the grate 750 and the CEM, as well
as the response time of typical gas analyzers, there may be a
significant time delay between increased NOx emissions from the
combustion of the waste 701, and detection of this increase by the
CEM 720.
[0055] Government agencies, such as the Environmental Protection
Agency (EPA), may require MWCs, along with other power generating
plants and industrial facilities to report pollutant emissions.
Conventionally, the CEM system 720 is used to analyze and correct
data received from a probe located in or adjacent to a stack or
ducts to determine the contents of gas that is emitted from the
MWC. The CEM system 720 commonly uses a probe that is inserted into
the stack or ducts to obtain sample emissions of the flue gas. The
sampled gas containing pollutant and/or other combustion
by-products is typically referred to as flue gas, sample stack gas
or emission gas and can also be considered emitted material. The
probe can be located anywhere in the ductwork, air pollution
equipment or stack where a representative volume of flue gas can be
obtained. The sample gas is delivered to an analyzer via the sample
gas line, and the analyzer determines the concentration of emitted
pollutants in the sample gas.
[0056] In operation, operators may use the CEM system 720 to
monitor the status of the furnace 700. The CEM may provide
information on measured amounts of pollutants, for example, levels
of NOx and un-reacted reagents contained in the emissions from the
MWC (i.e., ammonia slip). This and other information from the CEM
can be provided to the SNCR controller 715, which uses this data to
modify the reagent flow as needed.
[0057] The furnace 700 further comprises a temperature probe 730
positioned at a desired location within the furnace 700. The
particular location of the temperature probe 730 in the furnace may
depend on the performance characteristics and needs of the
temperature probe. The positioning of the temperature probe 730 may
affect the timing of the application of the reagent from the SNCR
system 710. Specifically, gases in the furnace require a certain
amount of time to travel between the grate 750 and the temperature
probe 730, and the flue gas may take a certain additional time to
reach the SNCR system. Therefore, it may be advantageous to
position the temperature probe 730 before the SNCR system 710.
[0058] FIG. 10 illustrates the relations among furnace temperature,
NOx emissions, reagent flow, and ammonia slip at the stack while
operating with the improved control method as described by this
invention. Beginning at approximately 20:50 on the time axis, there
is an increase in furnace temperature. In accordance with this
invention, the reagent flow is increased, reaching a value almost
50% greater than its initial value, which keeps NOx emissions low
and does not increase slip at the stack. Beginning at approximately
21:00, there is a reduction in furnace temperature. The control
system automatically reduces the reagent feed rate. Shortly after
21:10 the temperature reaches a minimum, then increases rapidly.
Reagent flow also increases rapidly to control NOx. At the minimum
temperature point, the reagent flow is approximately 50% of its
initial flow and only a trivial increase in ammonia slip is
measured.
[0059] In this way, the reagent flow from the SNCR system 710 may
be dynamically adjusted based on the combustion process. Presumably
the best signal available is from a fast-responding temperature
sensor 730, such as an IR or optical pyrometer. This signal is
directly related to the combustion intensity, and hence the NOx
generation rate, and can be used by the SNCR controller 715 to
dynamically adjust the reagent flow to better follow the combustion
process.
[0060] Continuing with FIG. 7, a combustion controller 740 controls
and/or monitors the amount of waste 701 introduced into the furnace
700. For example, the combustion controller 740 may be used to
direct a semi-batch-fed stoker-based furnace. Linking the
combustion control system into the SNCR system, thereby providing a
feed-forward signal to the SNCR controller 715, can further enhance
the NOx reduction process. This input from the combustion
controller 740 may allow the SNCR controller 715 to adjust reagent
flow in anticipation of changed NOx levels. In other words, the
SNCR controller may adapt the levels of the reagent according to
the combustion controller 740. For example, the combustion
controller 740 may provide information to the SNCR controller 715
about the amount and timing of waste 701 introduced to the furnace
700 at the grate 750, or changes in combustion air flows. Using
this information, the SNCR controller 715 may predict any changes
to the NOx levels. The travel time of the NOx between the high
temperature area of NOx product near the grate 750, and the cooler
area near the SNCR system 710 is also known, and this information
may be used by the SNCR controller 715 to apply an appropriate
amount of the reagent at an appropriate time.
[0061] In a preferred embodiment of the present invention depicted
in FIG. 8, the control configuration includes two controllers 810
and 820. The first controller 810 is slow acting, essentially
similarly to the current controller used in known SNCR systems. The
first controller 810 relies on measured NOx levels in the MWC
emission and a desired NOx setpoint 811. The first controller 810
is typically a slow-acting PI controller adjusting an ammonia flow
setpoint or valve position in response to NOx level data acquired
from a NOx analyzer 812, such as the CEM system 720.
[0062] The second controller 820 is typically a fast-acting PD
(proportional-derivative) controller reacting to the difference
between the current temperature 821 and some reference temperature
822. The PD controller may be, for example, a conventional PID
controller configured to respond primarily or exclusively to the
proportional and derivative measurements. Optionally, the input to
the second controller 820 may be a reference temperature in the
form of a rolling average temperature 822 over a time period of
sufficient duration (i.e. 10 to 60 minutes) to smooth out
combustion fluctuations. The second dynamic controller 820 may
generate an output signal representing a change to the reagent flow
or valve position with a range dependent on the current output of
the main controller 810. For example, it might range from -50% of
the current output to +50%. The signals from the two controllers
810 and 820 would then be added together by an adder 830 to
generate the actual reagent flow setpoint or valve position
840.
[0063] Continuing with FIG. 8, another embodiment of the present
invention includes two additional optional signals that may added
individually or together to maximize NOx control while minimizing
slip. The two signals are a feed forward signal 823 from the
combustion controller and a feedback signal 824 from an ammonia
analyzer downstream of the combustion zone. The combustion
controller signal 823 would cause reagent flow to increase when, or
shortly after, new fuel or additional air is introduced to the
combustion zone but before an increase in temperature. This control
configuration thereby eliminates any delay in the reaction and
ensures that increased reagent levels are available as soon as
needed. Similarly, the combustion controller signal 823 allows
reagent flow to be reduced when, or shortly after, the feeding of
new fuel (i.e., waste) pauses or combustion air is reduced, thus
ensuring that excessive ammonia is not present when not needed.
[0064] The real-time ammonia concentration 824 in the flue gas
downstream of the combustion zone can be used to immediately reduce
reagent flow when excessive ammonia slip is occurring, and provides
a permissive signal to increase reagent flow in response to a
measurement of acceptable values of ammonia slip.
[0065] Overall, it can be seen the embodiments of the present
invention provide a SNCR control system and method that
significantly reduces NOx emissions and ammonia slip with minimal
cost, enabling lower permit limits and a possible sale of NOx
credits.
CONCLUSION
[0066] While the invention has been described with reference to
exemplary embodiments various additions, deletions, substitutions,
or other modifications may be made without departing from the
spirit or scope of the invention. Accordingly, the invention is not
to be considered as limited by the foregoing description, but is
only limited by the scope of the appended claims. For example, it
should be appreciated that the principles of the present invention,
although adapted for SNCR systems, may likewise be adapted for
other NOx control technologies that rely upon the addition of a
reagent to reduce produced NOx, such as Selective Catalytic
Reduction (SCR). Likewise, it should be appreciated that the
principles of the present invention, although present in the
context of MWC systems, may be applied to other sources of the NOx,
such as hydrocarbon fuel burning energy facilities and other large
industrial facilities.
* * * * *