© Siemens AG 2013
Process Analytics
Minimizing Ammonia Consumption
in DeNOx Plants
In situ laser gas analyzer LDS 6 monitors NH3 slip in real-time
The NH3 slip is an essential process parameter in denitrification plants and should be continuously monitored for
process optimization.
The in situ measuring principle is best suited for this kind
of monitoring task because it provides measuring data in
real-time for fast reaction.
The LDS 6 in situ laser gas analyzer offers best possible
capabilities for this application. It is installed directly in
the process gas flow and delivers fast and accurate NH3
slip concentration data.
This Case Study presents details of this application.
Flue gas denitrification
For flue gas denitrification, besides front-end primary
measures such as special furnaces with optimized air
supply, back-end measures are used, based on reduction
processes.
Ammonia or urea are used as reducing agents to convert
nitrogen oxides to nitrogen and water at high temperatures. The agent is fed via spray nozzles to the gas flow,
whereby the dosed volume must be continuously adjusted
to the current NO content. The unused amount of NH3,
called NH3 slip, must be minimized for several reasons.
On the other hand, the amount of NH3 must be large
enough for full conversion of the nitogen oxides. Thus,
the NH3 slip is a very essential process parameter that
must be monitored carefully and with high reliability.
Combustion processes result in emissions that can be
harmful to the environment, in particular carbon dioxide
(CO2), sulfur dioxide (SO2), nitric oxides (NOx), and dust.
Therefore, measures are implemented in power stations
and other plants comprising combustion processes to
reduce emissions.
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© Siemens AG 2013
NH3 slip indicates denitrification
process conditions
Application task
Today, there are two major types of DeNOx processes known
in industry: the Selective Catalytic Reduction (SCR) and the
Selective Non-Catalytic Reduction (SNCR). SCR DeNOx installations are common for large scale combustion plants like
coal fired power utilities, whereas SNCR technology can
often be found in small to mid-size incineration plants like
municipal waste incinerators (MWI).
LDS 6 can be used for optimization of either technology.
The SCR process
perature range of 800 to 950 °C. At temperatures below
the optimum temperature, the reaction rate is too slow,
resulting in an inefficient NOx reduction and too high
ammonia slip. Above the optimum temperature, the oxidation of ammonia to NOx is getting significantly high and
the process tends to produce NOx instead of decreasing it.
As combustion processes normally show fast and considerable changes in the temperature distribution and the
composition of the flues gas, the efficiency of the SNCRDeNOx process is strongly dependent on the temperature
and NOx distribution in the reaction zone.
Nitrogen oxides (NOx) formed in combustion processes
are efficiently reduced to water and nitrogen in the SCR
process. Ammonia (NH3) or urea (CO(NH2)2) is introduced
to the flue gases upstream of a heterogeneous catalyst
where the reduction takes place. Depending on the
amount of dust, type and concentration of acidic gas
components in the flue gas, the SCR process is normally
operated in the temperature range of 300 to 400 °C.
At a constant NOx level behind the reaction zone, the
NH3 slip is a strong indicator of the current reaction
conditions.
As its conversion efficiency and buffer capability is high,
the NH3 slip behind a SCR catalyst is normally very low,
e.g., in the range of 1 ppm or below.
• ABS has a melting point of 147 °C and will consequently
be present as a liquid or solid accumulated on surfaces.
It might plug parts of the catalyst, increasing the pressure
drop and causing catalyst deactivation. It might also plug
the air preheater (AP), decreasing its efficiency.
• ABS is hygroscopic at lower temperatures and will cause
corrosion when absorbing moisture from the gas.
• ABS formed on ash particles can cause sticky ash on
the electrostatic precipitator’s (ESP) hoppers which are
problematic to handle.
An increase of the slip at constant process conditions is a
precise indicator of a decrease in the catalyst’s activity.
The SNCR process
In the SNCR process, usually ammonia (NH3) or urea
(CO(NH2)2) is introduced to the flue gases in the hot
combustion zone where the reduction of NOx takes place
spontaneously. Depending on the type of the used reducing agent, the SNCR process is usually operated in the tem-
Ammonium Bisulphate formation
Together with acidic flue gas components, the NH3 injected
to the flue gas (or formed from an injected ammonia derivate like urea) can lead to salt formation. Mainly ammonium bisulphate (ABS) causes difficulties in the process:
The amount of the NH3 slip determines the total amount of
ABS, as the SO3 is usually in excess in combustion process.
The analyzer LDS 6
LDS 6 (fig. 1) is a diode laser-based in situ gas analyzer for
measuring specific gas components directly in a process
gas stream.
LDS 6 is designed for fast and nonintrusive measurements
in many industrial processes. Measuring components
include:
LDS 6 consists of a central unit and up to three pairs of
cross duct sensors in a transmitter/receiver configuration.
The central unit is separated from the sensors by using
fiber optics. Regardless how hostile the environment is, the
analyzer can always be placed outside any hazardous areas.
Measurements are carried out free of spectral interferences
and in real-time enabling pro-activ control of dynamic
processes.
O2, NH3/H2O, HF/H2O, HCl/H2O, CO/CO2 et al.
Full network connectivity via ethernet allows remote
maintenance.
Key features include:
•
•
•
•
2
In-situ principle, no gas sampling
Three measuring points simultaneously
Temperature up to 1 200 °C
Ex-version available (option)
Fig. 1: LDS 6 in situ laser gas analyzer
© Siemens AG 2013
LDS 6, the perfect solution for fast and
reliable NH3 slip measurement
Application solution
Easiness
A single LDS 6 analyzer is able to monitor the NH3 slip
in up to three measurement points simultaneously.
One sensor pair is used to control the ammonia concentration in situ directly after the catalyst or the high temperature reaction zone, see fig. 2.
The central unit can be placed in the control room several
hundred meters away from the measurement points by
using fiber optic cables. Three measuring points can be
handled simultaneously with one single central unit.
No calibration is necessary in the field.
Since LDS 6 delivers NH3 concentration data in real-time,
very fast control of the NH3 slip is achieved – runtimes
with excess dosage are completely avoided.
Robustness
Another important measuring point is the emission monitoring directly in the stack. Here, the final emission of NH3
and therefore the total nitrous emission is observed.
The sensor pair at the measuring point contains a minimum of electrical and optical components to ensure
highest reliability and availability. The residual maintenance is reduced to the cleaning of the sensor windows
after several months of continuous operation. No optical
realignment is necessary after cleaning.
LDS 6 advantages for DeNOx control at a glance
Versatility
Performance
LDS 6 offers the option to measure the water vapour concentration of the flue gas in situ and parallel with the
NH3 slip. This additional information is useful to detect
leaks in the boiler’s steam pipes faster and in an earlier
stage than by, e.g., the pressure drop. Also the compensation for the volume error in the result of extractive
analyzers (e.g. as part of the CEM system) in the stack
measuring at dry gas conditions becomes possible.
Faster regulation than with other control instruments
(e.g. FT-IR) and therefore most efficient optimization.
The in situ approach allows representative NH3 measurements without side effects or cross interference.
Fig 2: LDS 6 installation points for DeNOx control
3
© Siemens AG 2013
User benefits and measuring conditions
Measuring conditions
SNCR principle
SCR principle
Gas to be measured
NH3, NH3/H2O
NH3, NH3/H2O
Measuring range
0 ... 50 ppm
0 ... 50 ppm
Option
• H2O measuring range
• H2O resolution
0 ... 30 % Vol.
± 0.1 % Vol.
0 ... 30 % Vol.
± 0.1 % Vol.
Dust load
< 10 ... 3.5 g/Nm³
at 2 ... 6 m
< 5 ... 2.5 g/Nm³
at 4 ... 8 m
User benefits
Temperature
250 ... 350 °C
300 ... 400 °C
Optimizing the SCR process
Typical optical path
length
2 ... 6 m
4 ... 8 m
Pressure
1013 ± 50 mbara
1013 ± 50 mbara
Required response time
down to 3 s
adjustable
down to 3 s
adjustable
Recommended
purging mode
Process side only,
moderate to
elevated flow
Process side only,
moderate to
elevated flow
Purging media
Instrument air,
2 ... 8 bar
Instrument air,
2 ... 8 bar
MLFB gas code
C, D
C, D
MLFB application code
E
F
Typical measuring conditions for the NH3 slip measurement
in SCR or SNCR DeNOx installations are given in table 1.
If the ranges of typical values are kept unchanged, the
MLFB codes given in the last line of table 1 can be used for
ordering the analyzer. In other cases, please use the given
contact addresses for technical clarification.
Note:
User lists are available for different fields of application.
by controlling the NH3 slip means:
• to minimize technological drawbacks, optimize maintenance intervals, decrease deterioration and replacement
costs
• to reduce the total nitrogen (NH3 and NOx) emission.
An optimized process input is the base of minimized
emission.
Optimizing an SNCR process
by controlling the NH3 slip means:
• to reduce the consumption of ammonia or urea while
keeping the legislative threshold values for NOx (and NH3
if required)
• to stabilize the process and avoid peak emissions
• to minimize technological drawbacks, increasing DeNOx
efficiency at reasonable level of NH3 slip
• to reduce the total nitrogen - NH3 and NOx - emission.
An optimized process input is the base of minimized
emission.
Table 1: Conditions for NH3 slip measurements in DeNOx plants
using the LDS 6 . Other preconditions (temperature, pressure etc.)
please inquire with your local Siemens sales support.
Note:
LDS 6 is also widely used as tool to monitor SCR catalysts in engine labs.
For this application a separate Case Study is available.
Siemens AG
Industry Sector
Sensors and Communication
Process Analytics
76181 KARLSRUHE
DEUTSCHLAND
Änderungen vorbehalten
© 09/2013, Siemens AG
The information provided in this Case Study
contains descriptions or characteristics of performance which in case of actual use do not always
apply as described or which may change as a result
of further development of the products. An obligation to provide the respective features shall only exist if expressly agreed in the terms of contract.
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