Minimizing Ammonia Consumption in DeNOx Plants · Minimizing Ammonia Consumption in DeNOx Plants ... Ammonia or urea are used as reducing agents to convert nitrogen oxides to nitrogen

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Minimizing Ammonia Consumption in DeNOx PlantsIn situ laser gas analyzer LDS 6 monitors NH3 slip in real-time

The NH3 slip is an essential process parameter in denitrifi-cation 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

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.

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 tempera-tures. 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.

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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 instal-lations 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

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.

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.

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 reduc-ing agent, the SNCR process is usually operated in the tem-

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 oxida-tion 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 consid-erable changes in the temperature distribution and the composition of the flues gas, the efficiency of the SNCR-DeNOx process is strongly dependent on the temperature and NOx distribution in the reaction zone.

At a constant NOx level behind the reaction zone, the NH3 slip is a strong indicator of the current reaction conditions.

Ammonium Bisulphate formation

Together with acidic flue gas components, the NH3 injected to the flue gas (or formed from an injected ammonia deri-vate like urea) can lead to salt formation. Mainly ammo-nium bisulphate (ABS) causes difficulties in the process:

• 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 effi ciency.

• 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.

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 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.

Full network connectivity via ethernet allows remote maintenance.

Key features include:

• In-situ principle, no gas sampling• Three measuring points simultaneously• Temperature up to 1 200 °C• Ex-version available (option)

LDS 6 is designed for fast and nonintrusive measurements in many industrial processes. Measuring components include:

O2, NH3/H2O, HF/H2O, HCl/H2O, CO/CO2 et al.

Fig. 1: LDS 6 in situ laser gas analyzer


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LDS 6, the perfect solution for fast and reliable NH3 slip measurement

Application solution

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 concen-tration in situ directly after the catalyst or the high tem-perature reaction zone, see fig. 2.

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.

Another important measuring point is the emission moni-toring directly in the stack. Here, the final emission of NH3 and therefore the total nitrous emission is observed.

LDS 6 advantages for DeNOx control at a glance

Performance

Faster regulation than with other control instruments (e.g. FT-IR) and therefore most efficient optimization. The in situ approach allows representative NH3 measure-ments without side effects or cross interference.

Easiness

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.

Robustness

The sensor pair at the measuring point contains a mini-mum of electrical and optical components to ensure highest reliability and availability. The residual mainte-nance is reduced to the cleaning of the sensor windows after several months of continuous operation. No optical realignment is necessary after cleaning.

Versatility

LDS 6 offers the option to measure the water vapour con-centration 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 compen-sation 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.

Fig 2: LDS 6 installation points for DeNOx control


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Änderungen vorbehalten© 09/2013, Siemens AG

The information provided in this Case Study contains descriptions or characteristics of perfor-mance 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 obliga-tion to provide the respective features shall only ex-ist if expressly agreed in the terms of contract. Availability and technical specifications are subject to change without prior notice.All product designations may be trademarks or product names of Siemens AG or supplier companies whose use by third parties for their own purposes could violate the rights of the owners.

User benefits and measuring conditions

Measuring conditions

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.

User benefits

Optimizing the SCR process

by controlling the NH3 slip means:

• to minimize technological drawbacks, optimize mainte-nance 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

effi ciency 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.

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.

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

Temperature 250 ... 350 °C 300 ... 400 °C

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

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.


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Minimizing Ammonia Consumption in DeNOx Plants · Minimizing Ammonia Consumption in DeNOx Plants ... Ammonia or urea are used as reducing agents to convert nitrogen oxides to nitrogen