Retrofit of Waste to Energy Plants to reduce -70% of NOx Emissions
Author: Christian Helmreich, M.A.L. Umwelttechnik GmbH, Vienna 06.11.2015
Reduction of NOx emissions out of the waste combustion in an existing WTE boiler is still a topic.
Advanced SNCR Systems are able to reduce raw NOx emissions by -70% even as retrofit installation
into existing boilers. A short guideline to the 100/10 NOx emission-regulation (100mg/Nm³ NOx and
10 mg/Nm³ NH3 Slip)
NOx control at WTE boilers depends strongly on the boiler design and emission limits are getting more
and more restricted to protect the environment. Today many different boiler designs have been made
by several boiler suppliers over decades. Newer waste boiler designs are characterised by staged
combustion process as primary measures, a well-known design to avoid high flame peak temperature
which increase thermal NOx formation, reduced by local oxygen reduction.
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
2-staged combustion  typical raw NOx1 between 330 - 390 mg/Nm³
o Primary air through waste grid
o Secondary air above combustion area
3-stage combustion typical raw NOx1 between 250 - 280 mg/Nm³
o Primary air through waste grid
o Secondary air above combustion area
o Flue-gas recirculation gas from lower grid area into upper part of furnace at 1000°C
Note 1: NOx emissions referred to 11% O2
Primary air is usually ambient air temperature. The secondary air is either ambient or flue gas
recirculation air.
1)
Selection of NOx treatment system
The two available technologies to reduce NOx after NOx-formation out of combustion are SNCR or SCR
System. The SCR technology is usually situated as tail-end in the clean gas or as hot dust application
direct after the boiler.
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The pro`s and con`s of each system is below listed:
Design Criteria
NOx Reduction Efficiency
SNCR
SCR
non-catalytic reduction
catalytic reduction
40-75%
870°-1100°C
60-90%
180°- 450°C
Reactant
Reactor
Waste Disposal
Thermal Efficiency Debit
Energy Consumption
Capital Investment Costs
Plant Requirements (Installations)
Ammonia or Urea
None
None
0 – 0.3%
Low
Low
Minor
Maintenance
Low
Dust in FG
Ammonia/NOx (Molar Ratio)
Urea/NOx (Molar Ratio)
not sensitive
1.0 – 1.8
0.5 – 0.75
Ammonia or Urea
Catalytic
Spent catalyst
0%
Higher I.D. fan
High
Major
3 to 5 years
(typical catalyst life)
sensitive
0.8 to 1.2
Ammonia Slip
5 to 20 ppm
2 to 10 ppm
Plant Retrofit
Easy
Difficult
Mechanical Draft of FG + electric power
Project realisation time
Not Required
6-12 months
Required
12-24 month
FG-Temperature Window*
* depending on application
Figure1: comparison between SNCR vs SCR system
Finally, at retrofit installations on WTE boilers, the SNCR system has a clear advantage in lower
investment costs and much faster installation time. The total cost of ownership over 5 years is
favourable for SNCR, due to considerable costs for catalyser replacement or catalyser recovery need
to be considered. A past comparison on WTE boilers gave a 2-3 times higher operation cost of SCR
compared to SNCR Systems.
2)
Efficiency criteria of SNCR systems
With selection of an SNCR System there are major physical influences which are relevant for the
efficiency of NOx reduction:
a.
b.
c.
d.
Available flue gas temperature window (ideal between 850-1050°C)
Flue gas retention time (gas velocity) in the optimum temperature window
Flue gas profile in the boiler (variation with boiler load)
Reagent agent (24,5% Ammonia-water vs. 40% Urea-solution)
Common flue gas temperature variations by load- or fuel fluctuations and imbalances can cause an
insufficient NOx reduction. Unfavourable operating conditions can be found when incineration has
inhomogeneous constantly varying fuel-mix which results in fast and major changes in ignition
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behaviour and heat release, as a consequence to the furnace temperatures. Therefor it is important to
select a SNCR system which is able to react accordingly.
Below figure2 shows the relation of NOx reduction to gas retention time. It can be seen, that with
longer retention time in the ideal temperature window the NOx reduction an NH3 slip formation has
a strong influence in effectiveness of the SNCR system.
Figure2: relation between NOx reductions, NH3 slip and retention time in flue gas (NH4OH)
The ideal flue gas temperature window for ammonia is between 900-980°C, while the optimum
operation temperature for urea-solutions is between 950-1050°C.
Selection criteria for reactant between ammonia- and urea solution:
Design Criteria
hazardous liquid
operators protrection
storage tank material
special piping steel
safety standards
storage- and trace heating
legal permission
aggregat supply
reaction on flue gas
24,5% aqueous
ammonia (NH4 OH)
yes
yes
class 316
GRP
class 316
EN 12942-14
no
yes
liquid
immediatelly
40% urea solution 32,5% urea solution
no
no
class 304
GRP
class 304
n.a.
yes (min. +10°C)
yes
solid / liquid
delayed
no
no
class 304
GRP
class 304
n.a.
yes (min. +0°C)
yes
solid / liquid
delayed
Figure3: comparison table between reactant media
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3)
Adaptive SNCR system = ANCR®
In order to follow larger variations and imbalances in gas-temperature during operation, two or three
injection levels have proven to be successful, if the design considers adaptive controlled SNCR lances
which are able to follow the actual temperature profile of the WTE-boiler. M.A.L. is using their own
developed motorized SNCR lance types, which are able to follow by a tilting movement or by rotation.
A) Tilting Injection Lance (Type: TL) with a tilting angel of +/- 40°
B) Rotation Injection Lance (Type: RL) with a rotation angel of +/- 90°
Image 1: SNCR Lance - Type RL
Image 2: SNCR Lance - Type TL
These adaptive injection lances with replaceable nozzle head can be operated alternatively with
compressed air or steam as atomizing media. It’s mandatory that the reagent spray into the flue gas is
covering the majority of the flue gas flow. The lance openings will be placed on positions, calculated
and evaluated by a prior flue gas measurement.
Typical arrangement of an adaptive SNCR
system with adjustable spray pattern
(motorized SNCR injection lances) which is
controlled according to the gas temperature
profile of the WTE boiler.
Main components:

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Reagent unloading
Urea dissolving
Reagent storage-tank
Dosing pump stations
Metering and dosing cabinet
SNCR injection lances
FG-Temperature measuring system
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4)
Intelligent control of an adaptive SNCR system
An online flue gas temperature measurement is required. That can be achieved with a number of
pyrometers which are constantly measuring the actual flue gas temperature in different levels of the
boiler, in order to detect the thermo-profile and the location of the profile. These pyrometers are once
calibrated to indicate the real gas temperature. The software program controls the injected reagent
flow and the position of spraying angle of each lance in relation of thermos-profile detected by
pyrometers. That intelligent control optimizes the NOx reduction and NH3 slip of the boiler.
Conclusion
Past classic SNCR systems may come to a limit when being installed only in one or two levels of the
boiler. Due to operation variations and imbalances in combustion a flexible SNCR injection system is
favourable to react quickly according to the thermal profile. Flexible injection lances are able to offer
a wider operating range compared to fix installed injection levels.
Under proper design of an adaptive SNCR system, considering the specific boiler arrangement and
design, it is today possible to guarantee NOx clean gas values below 100 mg/Nm3 and slip of 10mg/Nm³
which is equal to an NOx reduction rate of up to -70%.
Modern adaptive SNCR systems are a cost-effective save solution for a retrofit project to guarantee
the emissions given by the authorities. The project lead time is about 6-12 month depending on
available time to install the required boiler wall openings for SNCR lances and pyrometers.
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