Carbon Capture and Storage (CCS), the sequestration of CO2, is being proposed as a method of reducing global greenhouse gas emissions and meeting future emission targets and proposed atmospheric limits, thereby potentially enabling power stations which burn fossil fuels to continue to be employed for the generation of electricity.

Background

There are a number of different technologies which are proposed for capturing CO2 from the combustion of fossil fuels. These fall into three broad categories: pre or post combustion capture, and oxy fuel combustion.

After capture, CO2 is most commonly transported under high pressure in pipelines to the storage location. For pipeline transportation, the CO2 will be compressed to ensure that single-phase flow is achieved. The CO2 is then injected underground via one or more wells under pressure in a supercritical fluid phase to a suitable location, on or off-shore, where geological features will contain the CO2. It is considered that for CO2 storage to be effective, 99% of the stored CO2 must be retained over a period of 100 years.

The EU Directive 2009/31/EC: Geological Storage of Carbon Dioxide, forms the framework for environmental regulation of CCS across EU Member States. An Environmental Impact Assessment is mandatory and leakage scenarios must be included for any proposed location. A monitoring plan must also be set up to verify that the injected CO2 is behaving as expected.

Emissions Trading Allowances must be surrendered for any leaked CO2, to compensate for the fact that the stored emissions were credited under the Emissions Trading Scheme (ETS) as not emitted when they left the source. Instrumentation to monitor capture, injection, transport and storage of CO2 is not yet available commercially.

In order to implement and enforce these regulations, instrumentation will have to be developed, evaluated, calibrated and certified. In its role as the UK’s national measurement institute, the National Physical Laboratory (NPL) is working to develop measurement and calibration tools that will be needed to meet the challenges of CCS implementation.

NPL is in a unique position where, as well as the capability to investigate new measurement techniques, it also has MCERTS accreditation to EN 15267-34 to meet the requirements of performance standards and test procedures for continuous emission monitoring systems (gaseous, particulate and flow-rate) in the field. It also has close links with the industry, trade associations and regulators, and represents UK interests at the European Committee for Standardization (CEN), influencing existing and developing new methods along with validating European and National standards.

Introduction

At each stage of the CCS process emissions must be monitored to comply with the CCS directive.

Detection, monitoring and measuring of CO2 will be required throughout the process for a number of purposes:

• Measuring the amount of CO2 captured • Measuring the amount of CO2 stored • Accounting for CO2 lost during the process • Detecting fugitive emissions • Determining the effectiveness of the storage process • Detecting leaks for safety reasons

Measurement of contaminants in the CO2 stream must also be identified and measured.

Types of emission

1. Residual gas emissions The residual CO2 emissions are defined as those after the capture process which are not captured and emitted to the atmosphere. It is expected that the capture process will be 85 to 95% efficient.

2. Vented emissions Vented emissions can be deliberate and pre-planned or for emergency reasons.

3. Fugitive emissions Fugitive emissions are those which are unintended or irregular, such as leaks from valves, pipe connections, and mechanical seals. They will be located where CO2 handling equipment is situated such as compression stages and pipelines, and will generally be of a point source nature, although accumulation and pooling may occur around leaks.

Fugitive emissions may also occur from the geological storage area itself, such as leaks through geological faults or unknown/unsealed wells and migration through gaps in cap rock. The CO2 emission may take place outside the vicinity of the injection site as it may be transported through porous layers below the surface. The type of emission can therefore be both from point sources, associated with the injection site or old wells or diffuse covering a wide area.

Monitoring technologies

A range of potential measurement technologies has been assessed by NPL and a leading manufacturer of gas analysers, systems and accessories for monitoring emissions to air, through funding from the Technology Strategy Board and National Measurement System, to judge their applicability to CCS monitoring requirements. A crucial part of this work has been the development of appropriate validation facilities.

Three technologies have been developed and evaluated: • A modified thermal imager for the detection and visualisation of CO2 leaks • A tuneable diode laser (TDL) spectrometer for measurement of atmospheric CO2 • A continuous emission monitor (CEM) to precisely measure the CO2 content in the residual gas emissions, determining the capture efficiency

Facilities at NPL have been used for the evaluation of these techniques, including our dedicated suite of test and calibration facilities that is accredited under ISO 17025. These are used for carrying out instrument tests to the MCERTS Performance Standards and Test Procedures for Continuous Emission Monitoring Systems and EN 15267-3, together with Performance Standards for Continuous Ambient Air Quality Monitoring Systems, including EN 14662-36.

Modified thermal imager

The modified thermal imager, has been developed to address the need to identify the location of any emissions from the CCS process primarily associated with plant, valves, compressors, pipelines and well heads. This could be used as part of a periodic survey, or where leaks have been identified from flow or other measurements but the location is unknown.

Principle

The technique of modifying the spectral response of a thermal imager to detect gases is not new and cameras are commercially available to detect volatile organic compound (VOC) emissions, refrigerant gases, SF6 and CO but not CO2. By filtering the camera to be sensitive to wavelengths that are absorbed by the gas of interest the presence of the gas is seen as a dark ‘smoke’ in the image – assuming hot is white.

Initial work determined the sensitivity required to detect the concentrations calculated to be produced from a number of fugitive emission scenarios from the CCS process. A model was developed to investigate the CO2 absorption produced from an emission in front of a thermal background and the effects of filtering this on the thermal imager’s response was calculated.

The model was based on spectroscopic information from the HITRAN 2008 atmospheric database7. The camera chosen for this application was a mid-infrared imager with an InSb 640 x 512 array fitted with an integral filter wheel, allowing filters to be rotated in front of the camera’s focal plane array.

The modelling showed that to achieve the highest sensitivities the filter is required to be cooled and stabilised to cryogenic temperatures. This is because the filter will emit light at wavelengths not being transmitted and lead to an increase in the background levels seen by the camera, and hence reducing the contrast of any absorbing gas. An external evacuable housing was built to house a filter to enable it to be cooled without encountering condensation problems.

Tests

A filter was chosen with a suitable transmission coinciding with the absorption of CO2 at wavelengths around 4 µm. This was used both in the cooled housing and the integral filter wheel. Tests in the laboratory showed that with a suitable thermal background CO2 could be visualised with the use of the filter in the integral filter wheel. This was done by simply releasing pure CO2 from a 6mm diameter pipe at flow rates as low as a few litres per minute.

The camera was then field tested at the Ferrybridge carbon capture plant (CCPilot100+) in West Yorkshire. The project, which is a collaboration between Doosan Babcock Limited, SSE and Vattenfall, supported by the Technology Strategy Board, DECC (Department of Energy and Climate Change) and Northern Way, is the first of its size to be integrated into a live power plant in the UK.

Imagery was recorded around the site in the way of a survey where no emissions of CO2 were detected. Leaks were then simulated at the site using a sample port from the captured CO2 stream. CO2 was released at flow rates of 20 to more than 300 litres per minute depending on the diameter of pipe used.

Leaks were simulated by directing the pipe into valve flanges or joints in front of plant at various temperatures. The camera was positioned at a distance of approximately 6m on an adjacent platform.

The CO2 gas could be seen in all cases, it being more obvious with a hotter background producing greater contrast. Identifying leaks is far easier when viewing video images where the gas plume moves and changes in front of a static background, as can be seen by logging on to this YouTube Link: https://www.youtube.com/watch?v=pq81fzvIYr0&feature=youtu.be

Image processing techniques can be employed to enhance this effect.

This work has demonstrated the potential for the spectrally filtered camera to visualise CO2 leaks of tens of litres per minute in an industrial CCS environment from practical distances.

Tuneable diode laser (TDL) spectrometer

An open path TDL spectrometer, has been developed to measure the concentration of CO2 in the atmosphere. This would enable the detection of increases in the levels of CO2 resulting from fugitive emissions from all aspects of the CCS process.

In particular the instrument should be able to detect diffuse leaks such as those associated with an underground storage area. It is estimated that the instrument will need to have a sensitivity of around 1 ppm in a background level of 385 ppm. Over open atmospheric paths variations in absorption of this level will be similar to those caused by variations in temperature and pressure.

In addition there will be diurnal and seasonal and variations in the normal background level of CO2 in the atmosphere. To measure the differences in the measured absorption caused by temperature and pressure variations along the laser path length, the TDL simultaneously measures the concentrations of oxygen, which is considered to be constant. To account for natural variations, upwind and downwind measurements will have to be made and ideally recordings of the CO2 concentration variations before CCS operations start.

The open path TDL built uses a transeiver unit containing the transmission and receiving optics, detectors and amplifiers. The two distributed feedback near infrared laser diodes and optical isolators are contained in an electronics unit also housing the fibre isolators, laser diode drivers and data acquisition hardware. The lasers are connected to the transceiver unit via optical fibres. The unit also has temperature and pressure sensors.

The laser beams are directed from the transceiver unit to a corner cube, or array of corner cubes, which reflects the beam back to the transceiver. The laser beam is expanded at the exit of the system which reduces any laser eye hazard. The system is mono-static where the beam expanding optics are also the receiving telescope. The corner cube can be positioned from 30 to 200m or more from the transceiver unit. The concentration of CO2 can be calculated by measuring the absorption along the laser’s path using methods based on the Beer–Lambert law. The laser output is scanned in wavelength across an absorption feature and as the absorption is relatively strong at the wavelengths used a direct absorption technique can be applied.

TDL testing

Testing of the TDL has taken place over ranges up to a laser round trip of 320m and releases of CO2 have been simulated using a 10m long open path test cell.

The 10m cell is 1m diameter with extraction in the centre and gas outlets at each end where a controlled flow can be applied. Samples ports are available along the length of the cell where gas concentrations can be sampled. Tests have shown the detection of the increased levels of CO2 in the cell when running.

Residual CO2 CEM

The document 2007/589/EC Commission Decision of July 18, 2007 establishing guidelines for the monitoring and reporting of greenhouse gas emissions pursuant to Directive 2003/87/EC and states an allowable overall uncertainty of ± 2.5%5 including the flow metering errors. This is approximately a tenfold improvement on the current MCERTs specification of 20% for SO2 and NOx from large combustion plants.

The requirement for either wet or dry analysis excluded a number of non-dispersive infrared (NDIR) analysis techniques. The remaining techniques were compared and gas filter correlation (GFC)9 was chosen as the most suitable and cost effective solution to meet the very high accuracy requirement.

Instrument description

The components and electronics of an existing GFC analyser were evaluated and a programme of improvements was investigated to increase the instrument’s accuracy. These included:

• Quartz gas filter cells – eliminates zero drift • Spectral modelling of optical components by NPL • High speed IR detector with integrated electronics • Gas sample cell operates at 110º C – direct measurement of scrubber outlet • Ultra linear automatic gain control • Laser gas cell position sensing • High resolution linearisation incorporated within smart panel

Testing

The CEM prototype system was tested at NPL’s MCERTs facility where the following tests were carried out.

Carbon dioxide (CO2) to cover the two required measurement ranges (20% and 1%).

• Lack of fit (linearity) tests • Effect of cross sensitivity with CO, N2O and H2O • Influence of ambient temperature in the range 15° C – 30° C • Repeatability standard deviation at zero and span point were determined • Zero drift test over 24 hours

The results of these tests were used to assess initial performance and identify the main sources of measurement uncertainty. With the design changes implemented an improvement by a factor of ten in analyser precision over their conventional sensors was achieved, with a maximum drift < 0.02% of full scale. Tests were also carried out to establish if there was a cross sensitivity to carbon monoxide (CO), nitrous oxide (N2O) and water vapour (H2O) at key concentrations defined in EN 15267-3, which might all be present in a stack.

Field testing

A prototype unit was fitted to the return flue after the capture of CO2 to measure the residual concentration of CO2 at the CCS Pilot Plant Ferrybridge.

It was operated on site for an initial six months period. It was then removed for review of components, service and further H2O testing by NPL. The analyser was reinstalled on site for the remainder of the project. The analyser’s sample components that were exposed to the sample showed no signs of deterioration thus were deemed suitable for long term operation.

The data collected was compared to the results for a multipoint sampling FTIR analyser that had 20 sample location points and produced three minute averages per scan. The data was produced once every 30 minutes, thus direct comparison was not possible but the data we have, showed good agreement for the periods when the sampling periods and location were coincident.

Conclusions

The evaluation and field testing of three potential technologies to address measurement and monitoring requirements for CCS applications has been successfully carried out. With the combination of expertise in the development of instruments, in-house test facilities and field tests at working carbon capture facilities, the potential of these instruments has been demonstrated. The CO2 measurement of the CEM was improved during the project, but further work is required to establish the uncertainty of flow measurements in CO2 streams to support accurate CO2 mass emission reporting.

Published: 27th May 2014 in AWE International