The development of instrumentation for application to carbon capture and storage measurements

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1 The development of instrumentation for application to carbon capture and storage measurements Nicholas A Martin National Physical Laboratory (NPL), Analytical Science Division, Environmental Measurements Group, Hampton Road, Teddington, Middlesex, TW11 0LW, United Kingdom Abstract New continuous emissions monitors (CEMs) must now be designed and operated so that end users can demonstrate that they meet the measurement requirements enshrined in the most recent environmental legislation. We report on the project C-Save, which was an industrial collaboration between Signal Group Ltd, NPL and BP plc, with aims of developing and improving instrumentation applicable to carbon capture and storage (CCS) measurements. The work involved the development of a prototype mid-infrared CO 2 gas sensor, based on the technique of gas filter correlation. This prototype underwent laboratory testing at NPL and then field testing at a pilot power plant in the UK employing CCS technology. In addition, we describe NPL s dedicated suite of facilities that are accredited under ISO to carry out tests to the MCERTS Performance Standards and Test Procedures for Continuous Emissions Monitoring Systems, together with the harmonised International Standard EN Introduction Carbon capture and storage (CCS) has been proposed as a means of reducing global greenhouse gas emissions of carbon dioxide (CO 2 ), thereby potentially enabling power stations which burn fossil fuels to continue to be employed for the generation of electricity. The CCS technologies currently being developed may be categorised into three main methods, pre- and post-combustion, and oxyfuel combustion [1]. The captured CO 2 must then be compressed and transported in pipelines to a suitable storage site where 99% of the injected sample must be retained over 100 years if the storage process is to be considered effective [2]. Directive 2009/31/EC: Geological Storage of Carbon Dioxide [3] provides the framework for the environmental regulation of CCS across the European Union (EU). This requires a mandatory Environmental Impact Assessment to be carried out, which should take into account of any potential leakages that may occur at the proposed storage location. A monitoring plan must also be set up to verify that the stored CO 2 remains in place, as intended. If CCS is adopted widely, then appropriate instrumentation is required to be developed to provide the measurements to implement and enforce regulations. C-Save, a collaborative project between Signal Group, NPL, BP, and the Environment Agency, with funding from the UK s Technology Strategy Board (TSB) and National Measurement System, was aimed at developing and improving instrumentation to meet the requirements of CCS. This includes a continuous emissions monitor (CEM), eventually to be used to measure CO 2 at a power station, before and after any capture process. 1

2 The prototype instrument was first tested at NPL s laboratory, and then in the field at Ferrybridge pilot power plant in the UK, which employed post combustion carbon capture technology. C-Save was in addition concerned with developing instrumentation, based on an infrared camera imaging system, capable of detecting localised leaks of CO 2. The project also used an infrared laser based system to make improved open path measurements to measure relatively small leaks of CO 2. This was in the presence of normal ambient background levels, and over practical distances of a few hundred metres that are relevant to end users. The focus of this article is the development of the CEM. 2. Design of Continuous Emissions Monitor for CCS Applications A review of potential technologies to employ indicated that a suitable and cost-effective option was an extension of the gas filter correlation (GCF) technique measuring CO 2 from its infrared absorption characteristics. GFC was advantageous because of the requirement for analysis of either wet or dry sample gas, which excluded a number of other conventional Non-dispersive infrared (NDIR) techniques. A programme of improvements identified components which could be optimised to increase the CEM s accuracy and include: the use of quartz glass gas filter cells (rather than metal), with a unique quartz to optical window material seal, minimising gas adsorption problems and leakages giving exceptional long term stability and reducing zero drift; spectral modelling of the optical components and spectral line broadening to enable the selection of suitable filters for either dry or wet sampling by minimising spectroscopic cross interference; the incorporation of a high speed IR detector with integrated electronics to deliver a higher accuracy timing of the signal processing; a gas sample cell operating at 110⁰C to enable direct measurement of hot sample gas either with or without moisture; an ultra linear automatic gain control to improve the mid- and long-term stability as the IR source ages and the sample cell potentially becomes contaminated. More recent upgrades that have been implemented include cell position sensing by laser to minimise small baseline shifts and improve general instrument instability, and a high resolution lineariser incorporated within the smart panel to provide better accuracy over the entire instrument measurement range. All such enhancements were targeted towards meeting the requirement of an allowable overall measurement uncertainty of ±2.5%, as defined [4] in EU document 2007/589/EC. A schematic diagram of the optical layout of the prototype CEM, which employed a single optical beam configuration, is shown below in Figure 1. The infrared source generated broad band radiation, which was then modulated by a gas filter wheel. This wheel incorporated two infrared transmitting gas cells, one filled with nitrogen and the other filled with carbon dioxide. As the wheel rotated at a constant speed it could either block the light from the source (zero signal), allow radiation to be transmitted through the cell filled with N 2 (the maximum intensity), or allow a reduced amount of light to be transmitted through the cell containing CO 2. A fixed optical band pass filter, centred at 4.3 µm, was also used in the optical path (after the rotating wheel) to reduce the optical energy transmitted, ensuring that wavelengths outside the spectral region where CO 2 has a strong absorption were minimised. 2

3 After the filter, the emerging radiation was passed through a lens, and then through either one of two infrared transmitting measurement cells (one for each range of CO 2 ) where test samples of gas could be introduced. Finally the emerging modulated beam energy was passed through a second lens and focussed onto a specialised pyro-electric solid state high sensitivity detector where it was converted into an electrical signal. Figure 1: Optical Layout of the CEM 3

4 In the absence of sample gas in either of the measurement cells (1% or 20% range) the ratio of the signals produced when the IR beam passed through the rotating nitrogen cell (and then beam blocked) and the rotating CO 2 cell (also then with the beam blocked) should remain constant. When the CO 2 sample gas was introduced into either of the measurement cells some of the radiation was absorbed and the ratio of the signals described above was altered. The difference between the ratios was measured and processed to produce the concentration value. A continuous small bleed of zero gas free of CO 2 and H 2 O was passed through the detector and source housing to prevent absorption of light due to CO 2 and certain potential cross interferents which are present naturally in the atmosphere. 3. Background to the Testing of CEMs In its role as the UK s national measurement institute, NPL has developed a dedicated suite of laboratories facilities which are accredited under ISO These are used for carrying out tests and calibrations for instrument manufacturers to the MCERTS Performance Standards and Test Procedures for Continuous Emissions Monitoring Systems, which also match the harmonised European Standard EN The MCERTS product certification scheme in the UK is currently operated by SIRA Environmental Ltd on behalf of the Environment Agency (EA)[5], with technical support from the Source Testing Association (STA). The principal legislation in the European Union that addresses emissions from industrial processes is Integrated Pollution Prevention and Control (IPPC), which is embodied in EC IPPC Directive 96/61/EC. Plant operators require that CEMs be reliable within the relevant mandatory CEN standard (EN 14181)-Quality Assurance of Automated Measuring Systems [6]. Compliance with EN is a legal requirement and it is applicable to processes covered by the EU directives on waste incineration (2000/76/EC) and large combustion plants (2001/80/EC). More recently Directive 2010/75/EC has replaced these and others, but the requirements remain the same. It standardises the quality of emissions monitoring instruments throughout Europe to ensure that the accuracy, relevance and quality of data presented by member states is consistent, and enables comparison. The CEN standard is split into 4 sections comprising of Quality Assurance Level (QAL) 1, QAL2, QAL3 and an AST (Annual Surveillance Test). QAL1 deals with the precautions, which must be taken prior to the purchase and installation of a CEM[7,8] but is not dealt with in detail in EN Under EN and the associated EN (which covers QAL1) a CEM cannot be installed until it has been proven to be suitable for the intended application. There are no specifications given for measurement principles or specific measurement techniques to be used by the CEM, the only requirement is that the system in question shall meet the stated performance criteria. This must be tested by comparing the uncertainty of the measurement obtained with the CEM with the uncertainty criteria defined in the relevant Directive. The overall measurement uncertainty is calculated using information about the application and detailed performance data relating to the product. This data must have been obtained in both laboratory and field trials, which requires the CEM to have completed performance testing in accordance with the relevant standards. 4

5 QAL2 addresses the installation, commissioning and validation of the CEM, and requires at least 15 parallel measurements to be acquired, spread over three days. These three days do not need to be consecutive but must be performed within a period of at most 4 weeks. The tests should be carried out using independent qualified personnel and equipment from ISO accredited laboratories. If there are validation failures, corrective action and full re-validation is compulsory. QAL2 must be repeated every three years in waste incineration plants, every five years for large combustion plants, and within six months following any rule contraventions, or following major changes to the CEM or to the plant. QAL3 is the next stage where plant operators are required to run a quality assurance programme to ensure accuracy and reliability of the CEM and the data it produces. It covers documentation, maintenance and regular zero and span calibration checks, and operation throughout the lifetime of the plant. It must demonstrate compliance with the criteria given by the CEN standard. Finally, the AST is a condensed version of the QAL2 tests, which also audits the QAL3 process. It ensures that operators are carrying out satisfactory operational and maintenance regimes, and passing the AST can spare the cost of a full repeat of the QAL2 tests. NPL emissions monitoring teams have all of the required MCERTS accreditation to carry out testing for both QAL2 and AST[9]. 4. NPL facilities for testing of CEMs The majority of the laboratory tests in EN have defined performance characteristics and performance pass-fail criteria [10]. For each pair of CEMs, the tests to be carried out include: determining the response time; repeatability standard deviation at zero point and span point; lackof-fit (linearity); zero and span drift; influence of ambient temperature at zero and span point; influence of sample gas pressure; influence of the sample gas flow (extractive CEMs); influence of voltage variations; and cross-sensitivity to interfering gas species. For the separate field tests a calibration function, response time, linearity, lack of fit, maintenance interval, changes in zero and span point with time, CEM availability, reproducibility and contamination check (for in situ CEMs) must be determined. The laboratory and field data are combined to deliver the overall expanded uncertainties, and these are compared with criteria defined by regulation (generally in EU Directive 2010/75/EC) for each determinand gas. There is an additional requirement that the uncertainties shall also be <75% of the limit value given in the performance standards. NPL has developed a suite of accredited facilities for such applications which include variable temperature test cells (probe chambers) that can operate up to 400⁰C, where accurate concentrations of target gases and water vapour can be introduced to simulate stack conditions and establish the performance of CEMs. The equipment allows for two instruments to be tested in parallel and can accommodate in-stack, cross stack or extractive configurations (See Figure 2). A patented gas dilution system which employs mass flow controllers with binary weighted flows is used to generate the required traceable gas concentrations, and an environmental chamber is available for testing those parts of the monitoring system exposed to temperature extremes relevant to indoor and outdoor conditions. 5

6 Figure 2: NPL s Probe Chamber Facility The tests described above may also be used as a pragmatic framework to evaluate sensor performance for certain new applications under development, this was the approach taken here for the CEM developed by Signal Group Ltd. During tests appropriate traceable gases were continuously introduced into NPL s probe chamber and maintained at a nominal temperature of 196 C in order to replicate a stack environment in the laboratory. The CEM was housed separately in NPL s environmental chamber (Figure 3), and sampled from the probe chamber through a separate heated line operated at nominally 100 C, and a pump. The CEM had two measurement ranges (20% and 1%), with the higher range covering the case where no removal of CO 2 was carried out, and the lower range applicable to where abatement measures may be in operation. 6

7 Figure 3: First Prototype CEM housed in Environmental Chamber In order to provide data for the manufacturer to optimise the design of the CEM, lack of fit tests were carried out on both concentration ranges, the influence of ambient temperature was measured (in the range 15⁰C-30⁰C),and the repeatability standard deviation at zero and span point were determined, together with the zero drift over 24 hours. With design changes implemented, Signal Group Ltd achieved an improvement by a factor of ten in analyser precision over their conventional sensors, with a maximum drift < 0.02% of full scale. Tests weree also carried out to establish if there was a cross sensitivity to carbon monoxide (CO), nitrous oxide (N 2 O) and water vapour (H 2 O) at key concentrations defined in EN , which might all be present in a stack. The dominant source of uncertainty of the CEM was found to be interference from H 2 O, most likely due to differences in spectral broadening when comparing a calibration cell filled with pure CO 2, and a sample cell containing this species in the presence of high concentrations of water vapour. To mitigate this effect, simultaneous measurement of H 2 O with a correction of the CO 2 concentration will be implemented, in preference to removal of water from the sample or dilution of the stack sample gas. After modifications the next prototype was configured into a standard 19 inch rack mount with full user interface as shown in Figure 4. The CEM was then deployed in the field over several months at a pilot power station fitted with carbon capture technology. The sampling port available was at the return flue of the plant, in order to measure any residual CO 2 left after the capture process (See Figure 5). 7

8 The plant was also equipped with a Fourier transform infrared (FTIR) analyser that sampled sequentially from 20 dedicated ports of interest. One of these ports was colocated (within 1 metre) with that employed by the CEM. Figure 6 shows results from a measurement intercomparison between the two instruments, where good agreement was observed when the sampling location and sampling periods coincided. The intercomparison also captured a measurement period (05:00 and 07:30) where the CO 2 concentration was variable due to the particular test conditions in the plant, related to absorber off-gas. Figure 4: Second Prototype CEM monitoring at Power Station 8

9 Figure 5: Sampling Point at Power Station Figure 6: Field Intercomparison Results 9

10 5. Summary The development of prototype technology to address certain aspects of the measurement and monitoring requirements for CCS applications has been carried out in a project called C-Save. With the combination of consortium expertise in the development of sensors, in-house test facilities together with field tests at a working carbon capture pilot plant, the current potential of these instruments has been demonstrated. References 1. "IPCC Special Report Carbon Dioxide Capture and Storage Summary for Policymakers". Intergovernmental Panel on Climate Change 2. The IPCC Special Report on Carbon Dioxide Capture and Storage 3. DIRECTIVE 2009/31/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 23 April 2009 on the geological storage of carbon dioxide and amending Council Directive 85/337/EEC, European Parliament and Council Directives 2000/60/EC, 2001/80/EC, 2004/35/EC, 2006/12/EC, 2008/1/EC and Regulation (EC) No 1013/ establishing guidelines for the monitoring and reporting of greenhouse gas emissions pursuant to Directive 2003/87/EC of the European Parliament and of the Council EN 14181:2004 Stationary source emissions Quality assurance of automated measuring systems. 8. Air Guidance Note on the Implementation of I.S. EN (AG3), Environmental Protection Agency, Office of Environmental Enforcement (OEE), Johnstown Castle Estate,Wexford, Ireland EN (E), Air Quality-Certification of automated measuring systems- Part 3: Performance criteria and test procedure for automated measuring systems for monitoring emissions from stationary sources. 10

11 Acknowledgements The C-Save Consortium gratefully acknowledges funding received from the UK Technology Strategy Board (TSB) and Department for Business, Innovation and Skills (BIS), and also the collaboration from Doosan Babcock Limited, SSE, and Vattenfall for making the power station and carbon capture facilities at Ferrybridge available for the key field validation measurements. 11