Kurzfassung Optimierung des SNCR-DENOX- Verfahrens mittels Diodenlaser-Spektroskopie Beim SNCR-DENOX-Verfahren werden die Stickoxide in Rauchgasen von Verbrennungs- anlagen durch eine dosierte Zugabe von NH 3 - haltigen Reduktionsmitteln bei Gastemperatu- ren um 950 °C deutlich vermindert. Eine über- stöchiometrische Zugabe von NH 3 führt hier zu unnötigen Ammoniakverbrauch und zu NH 3 -Schlupf durch unverbrauchtes Reduk- tionsmittel. Die Folge sind unerwünschte Effekte wie Geruchsbelästigung oder Salzab- lagerungen an nachgeschalteten Anlagentei- len. Der vorliegende Artikel zeigt, wie SNCR- DENOX-Anlagen in der verbrennungstechni- schen Praxis mittels einer schnellen, Dioden- laser gestützten NH 3 -Schlupfmessung und einem Fuzzy-Regler hinsichtlich Effektivität, Verbrauchsmittelkosten und Schlupfminimie- rung entscheidend verbessert werden können. Bei der in der beschriebenen Weise optimier- ten SNCR-DENOX-Anlage der Müllverwer- tungsanlage Rugenberger Damm (MVR) Ham- burg konnte im Regelbetrieb an zwei Verbren- nungslinien der Reduktionsmittelverbrauch im Mittel um etwa 25 % und der NH 3 -Schlupf um etwa 50 % reduziert werden, bei gleichzeitig maximalem Reaktionsumsatz der Stickoxide von >80 %. Das moderne Regelkonzept mit der schnellen In-Situ-NH 3 -Schlupfmessung mit dem Sie- mens Diodenlaser-Spektrometer LDS 3000 und einem speziellen Fuzzy-Regler wurde bei der MVA Hamburg im Test- wie im Regel- betrieb validiert. Die positiven Langzeiterfah- rungen hinsichtlich Regelverhalten und Be- triebsaufwand lassen die hier beschriebene Schlupfregelung als Standardverfahren für die Regelung von Hochleistungs-SNCR-Anlagen geeignet erscheinen. Introduction The SNCR-DENOX method is a widely used, efficient method with relatively low in- vestment and operating costs for the removal of nitrogen oxides from flue gases of large- scale furnace systems. Liquid reducing agents containing ammonia are sprayed di- rectly into the hot flue gases of a combustion plant. The spontaneous chemical reactions of the reducing agent with the nitrogen oxides NO and NO 2 , which take place at tempera- tures of about 950 °C, reduce these to nitro- gen and water. Overstoichiometric reducing agent or agent introduced in the wrong tem- perature window remain unused as NH 3 slip in the flue gas and causes problems – in addi- tion to unnecessarily high cost for such con- sumables and ammonia emissions – due to the formation of ammonia salt deposits on the plant parts downstream of the DENOX. Municipal waste incineration plants with their constantly changing combustion condi- tions and flue gas compositions place high demands on the control of an SNCR plant if high NO x turnover rates and simultaneously low NH 3 consumption and slip values are to be achieved. This article describes a modern control concept based on laser technology and fuzzy logic for SNCR-DENOX plants and their application in industrial processes using the Rugenberger Damm waste inciner- ation plant (WIP) in Hamburg as an example. The Plant Concept of the WIP The Hamburg WIP operates two identical combustion lines for thermal decomposition of primary domestic waste with a nominal annual capacity of 320 000 t/a at the Rugen- berger Damm site (F i g u r e 1 ). About 21.5 tons of waste per combustion line are inciner- ated every hour on a two-level grate firing. VGB PowerTech 5/2003 113 DENOX Optimisation Optimisation of the SNCR-DENOX Method Using Diode Laser Spectroscopy Dr.-Ing. M.W. Markus Siemens AG Automation & Drives, Hamburg/Germany. Autoor Author Industrial steam Boiler Slag preparation Adsorbent feed Boiler dust Filter dust HCI rectification Reci- fan Water Preparation of lime milk Hearth type furnace coke silo Gypsum treatment Gypsum silo Lime HCI scrubber SO 2 scrubber Stack Fabric filter 1 Fabric filter 2 Ammonia water Waste SNCR Figure 1. The plant concept of the Hamburg WIP.
Transcript
Kurzfassung
Optimierung des SNCR-DENOX-Verfahrens mittels
Diodenlaser-Spektroskopie
Beim SNCR-DENOX-Verfahren werden dieStickoxide in Rauchgasen von Verbrennungs-anlagen durch eine dosierte Zugabe von NH3-haltigen Reduktionsmitteln bei Gastemperatu-ren um 950 °C deutlich vermindert. Eine über-stöchiometrische Zugabe von NH3 führt hierzu unnötigen Ammoniakverbrauch und zuNH3-Schlupf durch unverbrauchtes Reduk-tionsmittel. Die Folge sind unerwünschteEffekte wie Geruchsbelästigung oder Salzab-lagerungen an nachgeschalteten Anlagentei-len. Der vorliegende Artikel zeigt, wie SNCR-DENOX-Anlagen in der verbrennungstechni-schen Praxis mittels einer schnellen, Dioden-laser gestützten NH3-Schlupfmessung undeinem Fuzzy-Regler hinsichtlich Effektivität,Verbrauchsmittelkosten und Schlupfminimie-rung entscheidend verbessert werden können.Bei der in der beschriebenen Weise optimier-ten SNCR-DENOX-Anlage der Müllverwer-
tungsanlage Rugenberger Damm (MVR) Ham-burg konnte im Regelbetrieb an zwei Verbren-nungslinien der Reduktionsmittelverbrauch imMittel um etwa 25 % und der NH3-Schlupf umetwa 50 % reduziert werden, bei gleichzeitigmaximalem Reaktionsumsatz der Stickoxidevon >80 %.
Das moderne Regelkonzept mit der schnellenIn-Situ-NH3-Schlupfmessung mit dem Sie-mens Diodenlaser-Spektrometer LDS 3000und einem speziellen Fuzzy-Regler wurde beider MVA Hamburg im Test- wie im Regel-betrieb validiert. Die positiven Langzeiterfah-rungen hinsichtlich Regelverhalten und Be-triebsaufwand lassen die hier beschriebeneSchlupfregelung als Standardverfahren für dieRegelung von Hochleistungs-SNCR-Anlagengeeignet erscheinen.
Introduction
The SNCR-DENOX method is a widelyused, efficient method with relatively low in-vestment and operating costs for the removalof nitrogen oxides from flue gases of large-scale furnace systems. Liquid reducingagents containing ammonia are sprayed di-rectly into the hot flue gases of a combustionplant. The spontaneous chemical reactions ofthe reducing agent with the nitrogen oxidesNO and NO2, which take place at tempera-tures of about 950 °C, reduce these to nitro-
gen and water. Overstoichiometric reducingagent or agent introduced in the wrong tem-perature window remain unused as NH3 slipin the flue gas and causes problems – in addi-tion to unnecessarily high cost for such con-sumables and ammonia emissions – due tothe formation of ammonia salt deposits onthe plant parts downstream of the DENOX.Municipal waste incineration plants withtheir constantly changing combustion condi-tions and flue gas compositions place highdemands on the control of an SNCR plant ifhigh NOx turnover rates and simultaneouslylow NH3 consumption and slip values are tobe achieved. This article describes a moderncontrol concept based on laser technologyand fuzzy logic for SNCR-DENOX plantsand their application in industrial processesusing the Rugenberger Damm waste inciner-ation plant (WIP) in Hamburg as an example.
The Plant Concept of the WIP
The Hamburg WIP operates two identicalcombustion lines for thermal decompositionof primary domestic waste with a nominalannual capacity of 320 000 t/a at the Rugen-berger Damm site (F i g u r e 1 ). About 21.5tons of waste per combustion line are inciner-ated every hour on a two-level grate firing.
VGB PowerTech 5/2003 113
DENOX Optimisation
Optimisation of the SNCR-DENOX Method UsingDiode Laser Spectroscopy
Dr.-Ing. M.W. Markus
Siemens AG Automation & Drives,Hamburg/Germany.
AutoorAuthor
Industrial steam
Boiler
Slagpreparation
Adsorbentfeed
Boilerdust
Filterdust
HCIrectification
Reci-fan
Water
Preparation oflime milk
Hearth type furnacecoke silo
Gypsumtreatment
Gypsumsilo
Lime
HCIscrubber
SO2scrubber
Stack
Fabric filter
1
Fabric filter
2
Ammoniawater
WasteSNCR
Figure 1. The plant concept of the Hamburg WIP.
The hot flue gases pass the four vertical fluesof the steam generator before being trans-ferred to the respective flue gas purificationplants. Up to 68 tons of fresh steam are gen-erated per combustion line and hour whichare used in the form of process steam andelectricity.
Waste gas purification begins already in theboiler with the SNCR-DENOX and a high-temperature cyclone between the 2nd and 3rdor the 3rd and 4th boiler flue. After leavingthe boiler, weakly contaminated open hearthcoke (OHC) from the particle filter 2 is addedto the waste gas which is removed togetherwith the rest of the fly ash of the flue gas inparticle filter 1.
To separate the halogen content, the flue gasruns through a double-stage acid scrubberand to separate the sulphuric oxides througha 1-stage alkaline scrubber after which itreaches the post-purification stage. Here,OHC is added as an adsorbent and separatedagain in fabric filter 2 before the flue gasesare fed into the stack by suction fans. As aspecial process engineering feature, the WIPoperates an HCl-rectification plant in whichthe 10 to 12 % raw hydrochloric acid pro-duced in the acid washer is concentrated andpurified into marketable hydrochloric acid ofat least 30 %. As can be seen from the plantconcept, the WIP has no problems at all withgaseous ammonia emissions due to too highslip values of the SNCR-plant because theseremain in the scrubber. However, the ammo-nia separated here causes problems due to theformation of ammonia salts mainly on certainplant components of the HCl rectificationwhich demands the best possible slip moni-toring and control equipment.
The SNCR-DENOX Plant( F i g u r e 2 )
The WIP has three spray nozzle openingseach in three levels of the first boiler flue fora 25 % watery solution of ammonia whichcan be controlled individually. The optimumreaction window for the reduction of nitrogenoxides in the flue gas by NH3 is at about 950 °C. Too low temperatures lead to an in-complete reaction turnover whereas at toohigh temperatures the reducing agent ammo-nia reacts with oxygen to produce NOx andtherefore increases instead of decreasing thenitrogen oxides. The WIP uses an acousticmeasuring method to determine the flue gastemperature distribution over the cross-sec-tion of the 1st flue in order to identify therespective optimum injection level. Theamount of ammonia added is controlled con-ventionally by the reaction turnover of the ni-trogen oxide, i.e. by the NOx concentrationbehind the DENOX. The WIP has an extrac-tive gas tap at the transition between the 3rdand the 4th boiler flue. However, the measur-ing times of commercially available extrac-
tive NO analysers are too long incomparison with the high dynamicof the combustion process to allowgood control of the current flue gasstoichiometries. Additionally, usingNO as the control variable is un-favourable since NO has a very flatgradient at the operating point of theDENOX. Unlike this, NH3 slip canonly be measured at an overstoi-chiometric addition of ammonia andis therefore a clear indication of ex-cessive ammonia injection. There-fore, a set point is chosen with a slipcontrol at different NH3 slip valueswhich are as small as possible butnot zero to obtain a maximum reac-tion turnover of the nitrogen oxide.
The In i t i a l S i t ua t i on
The operating permit of the WIP provides fora limit value of 100 mg/Nm3 NOx (s.t.p.) as adaily average. This value is half of the generallimit value for waste incineration plants pre-scribed by the 17th BImSchV (Federal Immis-sion Control Act). If one wishes to stay belowthis low limit in the long term, the SNCR-DE-NOX needs to be operated with maximumpossible efficiency. The goal here is to addammonia at sub-stoichiometric conditions.However, as a result of the slow conventionalcontrol system, an NH3 lead value is obtainedwhich results in a usually considerably over-stoichiometric addition, i.e. too high ammoniaconsumption and relatively high NH3 slipamounting to an average of 25 to 30 mg/Nm3.
Although this ensures that the maximum NOx
daily averages are safely maintained, theprice is high ammonia costs and significantmaintenance expenses on parts of the HCl-rectification contaminated by ammonia salts.
The Op t imi sa t i on Concep t
The initial idea behind the optimisation con-cept was to overcome the sluggishness of the
existing control system and to be able toachieve an ammonia addition which con-stantly meets the requirements by means of afast, close-to-the-process control of ammoniaaddition in the SNCR plant on the basis of alaser-diode fiber-optic in-situ measurementof the NH3 slip and a special fuzzy controller.
NH 3 S l ip Con t ro l Us ing D iodeLase r Spec t rome t ry
By using optical spectroscopic gas analyserslike the Siemens LDS 3000 diode laser spec-trometer, it is possible to determine the ammo-nia concentration in situ, i.e. without extrac-tive gas tapping directly in the process andthus to achieve response times of a few sec-onds at verification limits below 1 ppm NH3.
The measuring principle which gets its infor-mation from the spectral analysis of a singleNH3 molecular absorption line is virtuallyfree of influences from the variable composi-tion or fluctuating dust-laden condition of theprocess gas (F i g u r e 3 ). The laser is locat-ed in the central unit CU 3000 together withits control, the evaluation computer and the
114 VGB PowerTech 5/2003
DENOX Optimisation
Laser-based NH3 measurement
Temperaturemeasurementbased onspeed-of-sound
SNCR
NH3
Figure 2. The SNCR plant of the WIP.
Measurement volumeAbsorption from dust etc.: σAbsorption from gases: αi(ν)
Figure 3. Structure of the LDS 3000 diode laser spectrometer: basic unit with laser module;CU 3000, fiber-optic cable FC 3001, in-situ sensors CD 3002.
user interface. The laser light and the usefulsignal are transmitted to the in-situ sensorheads CD 3002 at the measuring point by aspecial fibre-optic cable FC 3001. The powersupply to the detector electronics in the re-ceiver is also carried by this connecting cableby means of two shielded copper wires. Thesensors merely form the optical-mechanicalinterface to the process as a transmitter/re-ceiver configuration. This is the best possibleseparation of the measuring technology fromthe frequently very harsh conditions at themeasuring point. This concept increases thedegree of robustness and flexibility of thelaser measurement for applications in indus-trial process plants. With a fibre-optic beamsplitter in the laser module inside the centralunit it is possible to control and simultane-ously evaluate three measuring points withjust one analyser. At the measuring point, only the process flange of dimensionDN65/PN6 and a connection facility for apurging medium such as instrument air needto be set up. Maintenance is restricted to oc-casional cleaning of the process side surfacesof the sensor windows made of chemically-and mechanically-resistant quartz glass. Byusing a maintenance-free reference gas cellin the basic device there is no need for cali-bration of the measuring instrument at the ap-plication site. The LDS 3000 measures with arepetition rate of 50 spectra per second andtherefore achieves a real time measurementof the current process conditions regardingthe NH3 slip. Four NH3 measuring points areset up per combustion line. One measuringpoint is in the transition between the 3rd and4th flue of the boiler and is used for measur-ing the total NH3 slip. Three measuringpoints in the 1st flue mounted above the lastammonia injection opening allow location-independent measurement of the local NH3
distribution at the end of the actual reactionzone. This provides enough information to-gether with the acoustic measurement of thetemperature distribution in the 1st boiler flue
to be able to control the 9 NH3 injections in-dividually in position and amount.
Fuzzy Con t ro l l e r
To efficiently transfer this high informationdensity to a fast, close-to-the-process control,a fuzzy controller specially designed for thispurpose by Babcock Borsig Power Environ-ment GmbH, Gummersbach, is used. Basedon the available process variables such as tem-perature distribution, ammonia addition, am-monia slip at the four measuring points at theend of the reaction zone and at the end of theboiler and the NOx concentration after the DE-NOX, the control gives both the best choice ofactive injection openings, the individual NH3
quantity control and the limit value monitor-ing for the nitrogen oxides in real time.
Results of a Test Run/Long-term Experience
The amount of improvement of the SNCR-DENOX in comparison with the convention-al control with an extractive NO concentra-tion measurement in raw gas was impressive-ly proven by a test run, F i g u r e 4 . First theplant was operated in optimised mode withNH3 laser measurement and fuzzy controller.It exhibited a very stable operation with lowNH3 slip values and a NOx concentrationalways well below the limit value of100 mg/Nm3. The ammonia addition wasthen switched off to determine the NOx rawgas content. After a short time a value ofabout 500 mg/Nm3 NOx was established.
The SNCR-DENOX plant was then restartedinitially in conventional mode and left in thisoperating mode for a certain time. In compar-ison with the optimised operation at the be-ginning of the test run, the ammonia con-sumption was now higher by almost a factorof two which was also reflected in muchhigher NH3 slip values. The NOx concentra-tion values remain safely below the limit for
the daily mean values on average but a few“strays” are apparent which briefly drop wellbelow or considerably exceed the mean val-ue. Both events, the result of ammonia under-injection or over-injection, can be accuratelypredicted on the basis of the NH3 slip signalwhich behaves oppositely because this infor-mation is available a few minutes earlier.
Resuming the optimised mode clearly showsthe effect of the fast slip control which has animmediate positive effect on both the ammo-nia consumption, the NOx values and onthe NH3 slip. A 20 to 30 % reduction in theamount of ammonia consumed was achievedin this way. The set point for the slip was cho-sen at approximately 12 mg/Nm3 NH3 whichis equivalent to a reduction of 50 to 70% in re-lation to the slip values in conventional opera-tion. Daily averages of 80 mg/Nm3 for thenitrogen oxides were still safely maintainedwith a simultaneously marked reduction andsmoothing of the half hourly averages . Theoptimised SNCR plant has been in regular op-eration in both combustion lines since May2001 and has been operating reliably and withminimum maintenance since then. The reduc-tions in consumption and slip illustrated in thetest run could also be verified under the long-term operation. The aim of the optimisationprocess, to considerably reduce the NH3 slipand the ammonia consumption without nega-tively affecting the NOx reduction was suc-cessfully achieved.
Summary
A modern control concept for SNCR-DENOX plants, based on a fast in-situ NH3
slip measurement with the Siemens LDS3000 diode laser spectrometer and a specialfuzzy controller was successfully introducedat the WIP Rugenberger Damm in Hamburgand validated in tests and in regular opera-tion. The envisaged goal of a significant re-duction in slip and ammonia consumption atfull performance of the nitrogen oxide reduc-tion was achieved by individual dosing of thereducing agent constantly adapted to prevail-ing local requirements. It seems that the posi-tive long-term findings in terms of controlbehaviour and operating expenses make theslip control described here a suitable standardmethod of controlling high-performanceSNCR plants.
