SCR and SNCR optimization.pdf - Siemens

Optimizing
SCR/SNCR PROCESSES
Ammonia Slip – Power Plants

The Challange – Optimizing the SCR and SNCR DeNOx Processes
The SCR process
The nitrogen oxides (NO x ) formed
in combustion processes is effi -
ciently reduced to water (H 2 O) and
nitrogen (N 2 ) in the SCR process
(Selective Catalytic Reduction). Ammonia
(NH 3 ) or urea (CO(NH 2 ) 2 ) is
introduced to, and mixed with, the
fl ue gases upstream of a heterogeneous
catalyst over which the reduction
takes place. Depending on the
amount of dust, type and concentration
of acidic gas components in the
fl ue gas, the SCR process is normally
operated in the temperature range of
250 °C – 400 °C.
Ammonia
tank
Do you use SCR – or SNCR – or both?
Combustion air
The SNCR process
In the SNCR process (Selective Non
Catalytic Reduction), usually ammonia
(NH 3 ) or urea (CO(NH 2 ) 2 ) is
introduced to, and mixed with, the
fl ue gases in the hot combustion
zone where the reduction of NO x
takes place. Depending on type of
the used reducing agent, different
additives etc., the SNCR process is
usually operated somewhere in the
temperature range of 800 °C – 950
°C. At this temperature range many
competing reactions, consuming
ammonia, takes place e.g. oxidation
to nitrogen (N 2 ) and oxidation to
nitrogen oxides (NO x ). At temperatures
below the temperature window
the reaction rate is too slow resulting
in too high ammonia slip, and above
the temperature window the oxidation
of ammonia to NOx is too high
thus the process tends to produce
NOx instead of decreasing it.
There are several reasons, not only
to measure, but also for control
of the ammonia slip after a DeNOx
process.
As the ammonia slip contributes to
the total nitrogen emission from the
plant, the emission of NH 3 is usually
regulated to a maximum value
by legislation. And a too high NH 3
emission is of course negative also
from an NH 3 consumption cost point
Air preheater
of view. But in order to optimize
the NOx reduction reaction in the
DeNOx process, the effectiveness is
increased by an increased NH3 supply
to the process which gives an
increased NH3 slip. Optimizing the
NOx reduction is of high importance
in cases where there is a NOx fee or
tax on NOx emission by legislation.
Outside this discussion of legislation
and direct costs for NOx /NH 3
emissions and the NH3 consumption,
there are also some process specifi c
items that have to be taken into account.
Ammonium BiSulphate (ABS)
The NH 3 injected to the fl ue gas, or
being thermally produced from
Ammonia slip and water
vapour measurements for
catalyst optimization and
boiler tube surveillance
Temperature catalyst: 250 – 400°C
Temperature stack: 110 – 140°C
Ammonia level: 0 – 10 ppm
Water vapour level: 5 – 15 %
n Resolution NH3 : ±0,3 ppm
n Resolution H2O: ±0,1 %
Dust load catalyst: ~20 g/Nm 3
Dust load stack: ~5 mg/Nm 3
n Response time: 1 – 30 sec.
(depending on dust load)
ESP FGD BH
an injected ammonia derivate, can
give ammonium salt formation from
acidic gas components in the fl ue
gas at lower temperatures. Theoretically
several salts can be formed,
e.g. NH 4 Cl, NH 4 NO 3 , but the main
problems related to ammonium salt
formation is caused by ammonium
bisulphate (ABS), NH 4 HSO 4 in the
power producing industry from NH 3 ,
H 2 O and SO 3 in the fl ue gas. Some
formation of ammonium sulphate,
(NH 4 ) 2 SO 4 will also occur, especially
when the ammonia is in excess of
the SO 3 concentration. Ammonium
bisulphate has a melting point of 147
C, and will consequently be present
as a liquid accumulated on surfaces
or as a liquid aerosol in the fl ue gas
at normal operating temperatures.
Such an aerosol will contribute to
the visibility of the fl ue gas plume –
referred to as a “blue haze” phenomena
– if the particles are able to emit
through the stack. Further more, the
ABS is hygroscopic at lower temperatures
and will cause a corrosive solution
when absorbing moisture from
the gas.
If the ABS is formed in the SCR catalyst
in low temperature catalyst, it
might plug parts of the catalyst, increasing
the pressure drop over the
catalyst and cause catalyst deactivation.

ABS on Air Preheater surface
When the fl ue gas passes the air
preheater, AP, the temperature is
decreased and the ABS condensation
temperature is reached somewhere
inside the AP. Most of the
ABS is formed and accumulated on,
relative the fl ue gas, the cold heat exchange
surfaces, but a part of it will
be formed in the fl ue gas thus forming
a liquid aerosol.
The amount of the NH 3 slip will give
the total amount of ABS as the SO 3
usually is in excess in combustion
processes. As the accumulation will
partially plug the AP the pressure
drop will increase and the effi ciency
decrease.
The AP has to be designed by using
material (e.g. enamel-plates)
which can withstand the corrosive
ABS. Knowing the NH 3 slip and SO 3
content in fl ue gas the condensation
Ammonia slip control is important
The process has to be controlled in
order to keep the ammonia slip low
and keep the total nitrogen emission
low from the plant. This will avoid
ammonium salt formation with potential
dust plugging, corrosion and
catalytic deactivation. A slip control
will also minimise the ammonia consumption.
In order to control the SCR
process and the ammonia slip, a fast
and accurate in-situ measurement of
NH 3 has to be applied. LDS 3000 supplied
by Siemens Laser Analytics will
meet these requirements.
temperature, and hence the height
in the AP, can be predicted so that
the condensation occurs in the part
covered with enamel. This part will
be equipped with (high pressure)
water washing lances in order to
wash away the ABS salt at outages.
The washing will create an effl uent
water which may require neutralization
and possible other waste water
treatment.
An accurate measurement of the
ammonia slip gives the possibility
to predict or plan outage and
AP design – corrosive protection and
washing equipment operation.
ABS on the fl y ash
ABS formed on the fl y ash particles
collected in an electrostatic
precipitator, ESP, may cause two
different problems. Ammonia accumulated
to the fl y ash may be a
problem if placed on a refuse tip. If
LDS 3000
Siemens Laser Analytics provides
gas analyzers – LDS 3000 – for
process control applications based
on tunable diode laser. Due to the
early Swedish deNOx legislation we
focused on ammonia analyzers in
the late eighties and grew later into
combustion control applications. Filter
optimization and emission monitoring
are also substantial parts of
our core applications.
Our fi rst commercial order came
1992 and today we have more
than 60 installations in Europe, USA
and Japan.
accumulated on fl y ash, collected in
an ESP which operates at a temperature
above the ABS melting point, or
if the heaters in the bottom hoppers
locally create a temperature above
this melting point the ABS can give
a sticky ash from the ESP hoppers
which can be problematic to handle.
If the ash collector is of a fabric fi lter
type the salt can cause other operational
problems, e.g. increased pressure
drop.
Flue Gas Desulphurization
plant, (FGD)
If the plant is equipped with a fl ue
gas desulphurization plant, FGD,
parts of the ammonia slip will also
be absorbed on dust and particles
and powder products from that process
and parts of it will be solved into
the process water in wet scrubber
processes for removal of SO 2 , HCl
etc. is used.
The Solution – Ammonia Slip Measurement using LDS 3000 from
Siemens Laser Analytics AB – Laser Based Gas Analysis.
Central unit – CU 3000
LDS 3000
Sensor pair
CD 3002
The central unit CU 3000
All critical components are housed
in the central unit and it can be
placed several hundred meters away
from the measurement point. The
communication with the sensor is
through optical fi bers.
Sensor CD 3002
The sensor consists of a pair mounted
opposite of each other. An optical
component (lens or window) is facing
the process and protected with
purging air or other gas. The sensor
pair contains a minimum of electrical
or optical components to improve
reliability and availability.
Related documents
Additional documents can be downloaded
from our web site at:
http://www.siemens.se/sla
Navigate to Service & Support and
Documentation
1 Product Information
2 LDS 3000 Overview
3 LDS 3000 Basic Operation
4 LDS 3000 Data Sheet
5 Measurement Principle
6 Dust Compensation
Contact
Siemens Laser Analytics (headquarters
in Göteborg, Sweden)
Marketing Director
Jan Grimbrandt
jan.grimbrandt@siemens.com
Marketing Assistant
Michaela Hermansson
michaela.hermansson@siemens.com
Phone: +46 (0)31 50 68 50
web: http://www.siemens.se/sla

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402 73 Göteborg
Phone: +46 31 50 68 50
fax: +46 31 22 21 19
mail: info@lds3000.com
web: http://www.siemens.se/sla