Minimizing Ammonia Consumption in DeNOx Plants In situ Laser Gas Analyzer LDS 6 Monitors NH 3 Slip in Real-time

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1 Minimizing Ammonia Consumption in DeNOx Plants In situ Laser Gas Analyzer LDS 6 Monitors NH 3 Slip in Real-time Flue gas denitrification Combustion processes result in emissions that can be harmful to the environment, in particular carbon dioxide (CO 2 ), sulfur dioxide (SO 2 ), 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 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 NH 3, called NH 3 slip, must be minimized for several reasons. On the other hand, the amount of NH 3 must be large enough for full conversion of the nitogen oxides. Thus, the NH 3 slip is a very essential process parameter that must be monitored carefully and with high reliability. Case Study December 2007 The NH 3 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 NH 3 slip concentration data. This Case Study presents details of this application.

2 NH 3 slip indicates denitrification process conditions Application task Today, there are two major types of DeNOx processes known in industry: the Selective Catalytic Reduction (SCR) an 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 Nitrogen oxides (NOx) formed in combustion processes are efficiently reduced to water and nitrogen in the SCR process. Ammonia (NH 3 ) or urea (CO(NH 2 ) 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 C to 400 C. As its conversion efficiency and buffer capability is high, the NH 3 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. Fig. 1: LDS 6 in situ laser gas analyzer The SNCR process In the SNCR process, usually ammonia (NH 3 ) or urea (CO(NH 2 ) 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 temperature range of 800 C 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 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 NH 3 slip is a strong indicator of the current reaction conditions Ammonium Bisulphate formation Together with acidic flue gas components, the NH 3 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: 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. 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 1200 C Ex-version available (option) LDS 6 is designed for fast and nonintrusive measurements in many industrial processes. Measuring components include: O 2 /Temp., NH 3 /H 2 O, HF/H 2 O, HCl/H 2 O, CO/CO 2, low ppm H 2 O,... The amount of the NH 3 slip determines the total amount of ABS, as the SO 3 is usually in excess in combustion process. 2

3 LSD 6, the perfect solution for fast and reliable NH 3 slip measurement Application solution A single LDS 6 analyzer is able to monitor the NH 3 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. Since LDS 6 delivers NH 3 concentration data in real-time, very fast control of the NH 3 slip is achieved - runtimes with excess dosage are completely avoided. Another important measuring point is the emission monitoring directly in the stack. Here, the final emission of NH 3 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 NH 3 measurements 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 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. Fig. 2: LDS 6 installation points for DeNOx control Versatility LDS 6 offers the option to measure the water vapour concentration of the flue gas in situ and parallel with the NH 3 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 3

4 User benefits and measuring conditions Measuring conditions Typical measuring conditions for the NH 3 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. Notice User lists are available for different fields of application. Please contact the addresses below. User benefits Optimizing the SCR process by controlling the NH 3 -slip means: to minimize technological drawbacks, optimize maintenance intervals, decrease deterioration and replacement costs to reduce the total nitrogen (NH 3 and NOx) emission. An optimized process input is the base of minimized emission. Optimizing an SNCR process by controlling the NH 3 -slip means: to reduce the consumption of ammonia or urea while keeping the legislative threshold values for NOx (and NH 3 if required) to stabilize the process and avoid peak emissions to minimize technological drawbacks, increasing DeNOx efficiency at reasonable level of NH 3 slip to reduce the total nitrogen - NH 3 and NOx - emission. An optimized process input is the base of minimized emission. Conditions for NH 3 slip measurements in DeNOx plants using the LDS 6 SNCR principle SCR principle Gas to be measured NH 3, NH 3 /H 2 O NH 3, NH 3 /H 2 O Measuring range ppm ppm NH 3 resolution ± 1 ppm ± 0.5 ppm Option H 2 O measuring range H 2 O resolution % Vol. ± 0.1 % Vol. 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 % Vol. ± 0.1 % Vol. Dust load < 20 g/nm³ < 20 g/nm³ Temperature C C Typical optical path length m m Pressure Ambient Ambient Required response time < 30 s < 30 s Recommended purging mode Purging media Process side only, elevated flow Instrument air, bar MLFB gas code C, D C, D MLFB applicationcode E F Table 1: LDS 6 measuring conditions for NH 3 slip measurement Process side only, elevated flow Instrument air, bar 4

5 Case Study Siemens Process Analytics - Answers for industry If you have any questions, please contact your local sales representative or any of the contact addresses below: Siemens AG A&D SC PA, Process Analytics Östliche Rheinbrückenstr Karlsruhe Germany Siemens Energy & Automation Inc Hollister Road Houston, TX USA Siemens Pte. Limited A&D SC PS/PA CoC 60 MacPherson Road Singapore Phone: Fax: processanalytics.automation@siemens.com Siemens Ltd., China A&D SC, Process Analytics 7F, China Marine Tower No.1 Pu Dong Avenue Shanghai, P.R.China Phone: Fax: xiao.liu1@siemens.com Phone: Fax: saasales.sea@siemens.com Siemens LLC A&D 2B. PO Box 2154, Dubai, U.A.E. Phone: Fax: fairuz.yooseff@siemens.com Phone: Fax: splanalytics.sg@siemens.com Siemens AG Automation and Drives Sensors and Communication Process Analytics KARLSRUHE GERMANY Siemens AG 2007 Subject to change