New Requirements for the Measurement of Ammonia Emissions

Transcription

1 New Requirements for the Measurement of Ammonia Emissions Dr. Berthold Andres Consulting für Emissions- und Prozessmesstechnik Biebergemünd, Hufeisenstrasse 9 Germany 1. Summary The first draft for the revised BREF (Best Available Techniques Reference Document) for LCPs (large combustion plants) was published in June As the IED 2010/75/EU puts now much more weight on BREFs and its BATCs (Best Available Technology Conclusions), BREFs are getting more attention in the public. The draft BREF for LCP now requires the measurement of ammonia (NH 3 ) as slip control for LCPs using the DeNox technologies SNCR and SCR. Low limit values in the range of a few mg/m³, salt formation and high solubility of ammonia in water are bearing high challenges for the measurement solutions. The paper gives a short introduction to the new BREF draft requirements and summarizes the current CEN and ISO activities for emission monitoring of ammonia. Main focus of the paper is a review of the state of the art technology and measuring principles, resp. their strength & weakness for different measuring tasks. The review includes Fourier Transform Infra-Red spectroscopy, NDIR Photometer with gas filter correlation, Chemiluminescence detectors with ammonia converters and Tunable Diode Laser Spectroscopy. The paper gives also an overview on AMS certified according to EN by MCERTS and / or UBA/TÜV. 2. Ammonia Emission Monitoring required for LCP by BREF draft The European IPPC Bureau published in June 2013 the draft 1 for the Best Available Techniques (BAT) Reference Document [1] for large combustion plants. The proposed BAT-AEL (best available technology - associated emission limit value) is both for existing and new plants in most LCP applications 5 mg/m³ or less as yearly average. The monitoring frequency is a continuous measurement. Table 1 gives some typical emission limit values for different plant sizes and types. Combustion plant rated thermal input [MW] BAT-AEL for Ammonia (NH3) as yearly average [mg/m³] Combustion Combustion Combustion Combustion of of Coal and of solid of HFO and HFO in Lignite biomass and LFO in reciprocating / or peat boilers engines Waste co-incineration in coal and lignite fired combustion plants < 100 < < 1-5 < 5 < < 1-3,5 1-5 < 1-5 < 5 < 6 > 300 < 1-3,5 1-5 < 1-5 < 5 < 6 Table 1: Typical emission limit values for different plant sizes and types

2 3. Measurement of Ammonia in a DeNOx plant Measurement of ammonia (NH 3 ) is used as slip control for large combustion plants and waste incinerators using the DeNOx technologies SNCR and SCR. The emission of ammonia does not result from the combustion of fossil fuels, but rather as a consequence of the incomplete reaction of ammonia in the denitrification process. Ammonia is used as additive as pure ammonia or in solution with water in SCR and SNCR units. Ammonia chemically reacts also to form ammonia bisulfate (NH 4 HSO 4 ), which is mainly removed together with the fly ash from the system. With no dust removal downstream, the ammonia slip is emitted along with the flue gas to the atmosphere. The ammonia slip at SCR and SNCR installations increases with an increasing NH 3 /NOx ratio, but also with decreasing catalyst activity. In order to avoid problems in the utilization of the fly ash and possibly the smell of the flue gas in surrounding areas, the ammonium concentration associated with the use of SCR or SNCR is usually kept below 5 mg/m³. The formation of ammonia bisulfate (NH 4 HSO 4 ) in case of too high concentration of ammonia may also result in blockage of downstream flue gas treatment systems. Picture 1 gives an overview of typical process and emission measuring points at DeNOx plants. Picture 1: Typical measuring points at a DeNOx plant (Source Siemens AG, Industry Automation Division)

3 4. Measurement systems fort the emission and process monitoring of ammonia Salt formation and high solubility of ammonia in water make the measurement of ammonia quite demanding, especially in the low range of 5 mg/m³. Following issues must be considered: Loss of ammonia in the sample gas must be avoided through high temperature or insitu measurement Salt formation must be avoided Hot measurement & sample conditioning > 180 C required Hot measurement requires the determination of water vapor content for the calculation to standard conditions Process control and optimization require fast ammonia measurement Emission monitoring at regulated sources requires the application of AMS with certification according to EN Following measuring principles are usually used today for the measurement of ammonia FTIR (Fourier Transform Infra-Red spectroscopy) FTIR Spectroscopy is based on an interferometer. The measuring signal is a function of the travelling distance of the light between a fixed and a movable mirror. The absorption spectrum is generated with a Fourier Transformation. AMS based on FTIR technology are usually hot and wet measuring systems. For the measurement of NH 3 as a very water soluble component, it is very important, that the whole sampling system including all gas wetted parts is heated to a minimum of at least 180 C. Picture 2 shows the typical system design of a FTIR AMS. Major advantage of FTIR AMS is the simultaneous measurement of different components like NO, SO 2, HCl, NH 3, CO, CO 2 or CH 4 with one analyzer. These components are usually required for the emission monitoring at waste incinerators. Several manufacturers offer QAL3 test units based on reference material according to EN to check the analyzer during continuous operation. These test units may be used instead of regular test gases and can reduce the amount of test substantially. FTIR AMS are usually high priced systems. Out of this reason, its use is currently limited to applications with many measuring components, e.g. for waste incinerators. With the additional requirement for the measurement of ammonia, its application may be extended to LCPs. Picture 2: System design of a hot/wet FTIR (Source ABB Automation GmbH)

4 4.2. NDIR Photometer with GFC (Gas filter correlation) This measuring principle is based on a single beam photometer, which allows the use of optical and gas filter correlation. The light emitted from a light source goes through the chopper, measuring cell, filter wheels and finally to the detector. Picture 3 shows a typical system design of a NDIR filter wheel photometer. The measurement is hot and wet, comparable to FTIR spectrometer. Some manufacturers offer for this measuring principle also automated internal test units. These devices can be used instead of test gases and allow the ongoing test of the AMS according to QAL3 of EN These test devices can substantially reduce the amount of test gases used to check the reference point. Too low sensitivity for some components, e.g. HF is a major disadvantage of these AMS. This may require an additional AMS for these components, increasing substantially the complexity of a complete AMS. Picture 3: Measuring principle of a NDIR filter wheel analyzer (Source Sick AG)

5 4.3. CLD (Chemiluminescence Detector) with ammonia converter This system uses a high temperature converter (approx. 800 C) to convert NH 3 to NO direct at the sample probe. With NOx also directly in the sample gas, this measurement must be done alternated. One sequence measures the total NO (NO generated from NH 3 conversion plus NO from NO x originally in the sample gas). A second sequence measures the background NO x, bypassing the converter. The NH 3 concentration is calculated from the difference of both measurements. This design is currently described in an ISO standard. AMS based on this principle are mainly used in Japan but less in the Europe resp. USA. Picture 4 shows the schematic design of a measuring systems based on CLD and converter. Picture 4: Schematic design of an extractive CLD measurement with converter Conversion of NH 3 to NO at the tip of the probe is the major advantage of these systems. False measurements caused by the high solubility may be reduced. Measurement of water vapor is not required, as a cooler can be used in front of the analyzer. The alternating measurements results in longer response times for this system design and makes it less suitable for direct process measurements. Additional disadvantages are quenching effects caused by higher and varying concentration of water vapor and CO 2. Another disadvantage is the complexity of this design. It requires sample switching and two converters for NH 3 and NO 2 conversion. Currently there is no EN certified AMS on the market based on this measuring principle TDLS (Tunable Diode Laser Spectroscopy) In these analyzers, a laser beam travels from a laser diode through the measuring cell to a sensitive detector (photo diode). The wave length of the laser diode is adjusted to an absorbance line of the measured component. The absorbance line is scanned by wave length modulation, as shown in picture 5. Signal processing based on the wave length specific absorbance of the measuring components gives the gas concentration. The TDLS-principle (Tunable Diode Laser Spectroscopy) allows the very selective measurement of one component in a complex gas mixture.

6 Picture 5: Typical scan range for the measurement of ammonia with TDLS analyzer (Source Sick AG) TDLS analyzer can measure up to two components, if both absorbance lines are close enough together. This allows the measurement of water vapor together with ammonia in one instrument. This is important for the standardization to water vapor of a hot and wet measurement. TDLS can be used as an insitu analyzer directly at the stack and is therefore very suitable for fast process measurements. Another advantage of the installation directly at the stack is, that no sampling components like sample lines, pumps, etc. are required, which results in a rather simple system design. This makes it very suitable for difficult to measure components like ammonia. A modular design with several probes as shown in picture 1, allows the measurement of several sample points with one instrument. Based on the principle, only one or two components can be measured with a TDLS analyzer. This disadvantage makes TDLS less suitable for emission monitoring systems, where additional components like HCl, SO 2 or NO must be measured. Additionally it is difficult to apply test gases for the QAL3 procedure according to EN TDLS analyzers usually require for QAL3 dismounting from the stack and additional test benches. 5. Certified AMS for Ammonia IED 2010/75/EU and national regulations require in combination with the EN the use of certified AMS at regulated sources. With MCERTS in the UK and UBA / TÜV in Germany there are currently two independent certification systems in Europe. Both are based on EN and differ on very little. Unfortunately both countries require for their own area the local EN certification. Germany allows currently still the use of AMS tested according to Bundeseinheitlichen Praxis bei der Überwachung der Emissionen (shortened with BEP in Table 2) [3] and the VDI 4203 part 2 [4]. Table 2 gives an overview of MCRTS resp. TÜV / UBA certified AMS as of February 2014 with the appropriate certification basis.

7 Manufacturer AMS Type Measuring principle Basis for certification Certified ranges ABB ACF-NT FTIR MCERTS PS V mg/m 3 Automation Products BEP & VDI VDI 4203 Dr. Födisch AG MCA 04 NDIR with GFC MCERTS PS V mg/m mg/m 3 BEP & VDI 4203 VDI Environnement MIR-FT FTIR MCERTS PS V mg/m 3 SA dated Gasmet Gasmet FTIR MCERTS PS V mg/m 3 Technologies OY CX4000 CEMS II BEP & DIN EN to mg/m mg/m 3 General Gigas 10 M FTIR BEP & 0-15 mg/m 3 Impianti srl DIN EN to 3 MKS Instruments Inc. MGS300 FTIR MCERTS PS V mg/m mg/m3 BEP & DIN EN to 3 NEO Monitors AS LaserGas II Monitor TDLS MCERTS PS V3.1 dated mg/m mg/m 3 BEP & VDI VDI 4203 Opsis AR 602Z UV MCERTS PS V mg/m 3 dated Protea Ltd ProtIR FTIR MCERTS PS V2.1 MCERTS 0-15 mg/m 3 204M FTIR dated Servomex Group Ltd. Servotough Laser TDLS BEP & DIN EN to mg/m mg/m 3 Model 2930 MCERTS PS V3.4 SICK MAIHAK MCS 100 E NDIR with MCERTS PS V mg/m 3 GmbH HW GFC dated BEP & VDI VDI 4203 SICK MAIHAK MCS 100 FTIR MCERTS PS V mg/m 3 GmbH FT dated BEP & DIN EN to mg/m mg/m 3 Siemens Process Analytics LDS 6 TDLS MCERTS PS V3.4 BEP & DIN EN to mg/m mg/m mg/m 3 Table 2: Certified AMS for Ammonia (MCERTS [5], TÜV / UBA [6, 7])

8 6. Comparison of different measuring principles for Ammonia Table 3 shows a comparison of several performance criteria for AMS based on different measuring principles. Performance criteria FTIR TDLS NDIR with GFC CLD with converter No NH 3 loss in sampling system (e.g. insitu, > 180 C or conversion) + *) Suitable for multi component systems Suitable for single component systems Suitable for process measurement (fast, high dust and water vapor load) Suitable for emission monitoring (with certification acc. EN or national) Simple extension with additional measuring components Suitable for additional HF measurements for e.g. waste incinerators Simple use of test gas for QAL Use of other reference materials for QAL instead of test gases *) + means suitable, 0 means suitable with restrictions, - means less or not suitable Table 3: Comparison of relevant measuring principles for AMS 7. Literature [1] Reference Document on Best Available Techniques for the Large Combustion Plants, Joint Research Centre, Institute for Prospective Technological Studies, Sustainable Production and Consumption Unit, European IPPC Bureau, draft 1 (June 2013) [2] Directive 2010/75/EU of the European Parliament and of the council of 24 November 2010 on industrial emissions (integrated pollution prevention and control) [3] Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit, Bundeseinheitliche Praxis bei der Überwachung der Emissionen - RdSchr. d. BMU v Az.: IG I /5 und RdSchr. d. BMU v Az.: IG I /0 [4] VDI 4203 Blatt 2, : Prüfpläne für automatsiche Messeinrichtungen, Prüfprozeduren für Messeinrichtungen zur Messung gas- und partikelförmiger Emissionen (zurückgezogen) [5] [6] [7]