Micro Gas Chromats to meet Flare Emission Regulations
Steve Jackson MA CEng EurIng
Seasoned Influencer translating Technology into Customer Value
Abstract - Climate change is one of the most pressing worldwide challenges particularly following the 1997 Kyoto protocol. A major contribution to green house gases (GHG) originates from gas flaring: the World Bank estimates circa 150 bn cum gas was flared globally per year, producing 400m tons of CO2 which is equivalent to 25% of US gas consumption or 30% of EU consumption. In the EU the key regulation to limit flare GHG releases is the Emissions Trading Scheme (ETS) Carbon Credit Trading principle since 2005 for new plants (offshore flaring since 2009). Flaring is a controlled, open flame combustion process to burn unwanted or surplus hydrocarbon gases from different sources for over-pressure, maintenance, safety and other reasons.
The INEOS Grangemouth polymer plants have combined high pressure and low pressure flare stacks. Since 2013 the CO2 emissions had to be declared. Prior to this INEOS installed ultrasonic flowmeters on the flare lines to measure the mass flow.?These meters however require the volume of Nitrogen in the gas to be known, in order to calculate the mass flow.?The remaining gas was then assumed to be hydrocarbons to be combusted.?The Nitrogen Vol % at this point was calculated using the DCS via a complicated algorithm. This resulted in the potential to over-report emissions for EU ETS with significant financial impact.?The existing flare seal oxygen analysers had become obsolete, therefore it was decided to replace these and install micro gas chromatographs (GC) to analyse actual hydrocarbon and H2 composition to the stacks as well as measuring flare N2 and O2 levels. This new measured approach would have a short payback, and improve the volumetric to mass flow calculation of the flare flow meters by using a measured, not calculated, Nitrogen value.
This paper describes the implementation and operation of the new micro GCs at the INEOS Grangemouth chemical complex in Scotland during 2014.
Index Terms – Micro, Process Gas Chromatographs, Flare Stack, Emissions Regulations, Green House Gas (GHG), Emission Trading System (ETS).
INTRODUCTION
This paper will cover the carbon credit concept, an overview of the Grangemouth chemical plants, issues with the original set-up, the new analyser scope and lessons learnt.
The Carbon Credit Concept
During flaring combustion hydrocarbons react with atmospheric O2 to form CO2 which is the most active GHG. Venting without combustion often releases methane the second main GHG directly to the atmosphere.
Many countries in Europe and N America have established regulations to limit and reduce the amount of GHG emissions. Flares are specifically mentioned as a source of GHG and included in these regulations.
The ETS is the central part of the EU’s policy to reduce climate change and a key tool for reducing industrial GHG emissions. Introduced in 2005, it became the biggest international system for trading GHG emission allowances. It covers more than 11,000 facilities in 31 countries which mean around 45% of total GHG emissions from these countries. The EU ETS is now in its third phase, running from 2013 to 2020 with a major revision approved in 2009 to strengthen the system. This third phase also includes flaring from chemical production sites, and the allocation for routine and maintenance flaring.
The main legislative instrument to regulate the measurement of flare gas emission is the EU ETS Monitoring and Reporting Guideline. The EU allocates Carbon Credits (gas emission permits) as tradable commodities to the countries which limit the amount of CO2?that can be released to the atmosphere. To ensure compliance to each country’s allocated allowances; emissions must be monitored and reported. In each country, this allocated limit (“cap”) is cascaded down over the various industrial sites, which have to report their individual emissions at the end of the year. Depending whether more or less CO2 than allocated have been emitted, sites sell or must buy credits.
Introduction to the INEOS Grangemouth Chemical Plants
INEOS have a fully integrated production complex with a range of chemical plants at the Grangemouth site including polypropylene, ethylene, polyethylene and ethanol. The site’s ethane feedstock was taken predominately from the North Sea, but with declining volumes this is now being supplemented with imported US shale gas from a newly constructed ethane storage tank - fully commissioned in September 2016.
The polymer plants I4 (polyethylene) and PP3 (polypropylene) share a common flare system which includes a high pressure flare (HFL) and a low pressure flare (LFL). The flares are designed to safely achieve complete combustion of any gaseous waste hydrocarbon emitted from the polymer plants.
Inert N2 as well as being 80% of the composition of air is introduced into the polymer plants for blanketing and purging safety purposes but produces no GHGs so it is important to measure this gas to the flare line. Similarly H2 attracts no carbon credits but is injected to control the viscosity of the polymer. However, O2 has to be measured for safety reasons to ensure it has not seeped back down the flare stack and caused the O2 level in the flare line to be above 2% which would be potentially explosive if flare combustion is initiated.
Original Flare Analysis Set-Up
Material sent to each of the HFL and the LFL flares is metered by the recently installed, dedicated ultrasonic flow meters FI7021 and FI7022; these meters run an algorithm based on hydrocarbon flow to calculate the mass flow from a measured volumetric flow. The O2 content in the flare stacks was monitored by the on-line analysers AE7000 and AE7010.
As of the beginning of 2013 the CO2, emissions from both polymer plants had to be declared under regulations related to the EU ETS. The accuracy requirement of these regulations meant that the method previously used was no longer sufficient and a new, conservative method had to be introduced. The new method assumed that all flow to flare is hydrocarbon to be combusted, unless it is known to be N2.
Issues with Original Set-Up
As a major portion of the flared material is N2 which is not metered, a conservative estimate leads to over-reporting of carbon emissions. The difference between the old and new methods for the first two months of 2013 was substantial.
Under-estimating the mass of N2 in the flare stream not only costs money, it reduces the accuracy of the mass flow calculation performed by the flow computers FI7021 and FI7022. The computer calculates density based on the speed of sounds in the metered gas but because the algorithm used is designed for total hydrocarbon flow, so the presence of N2 in the line introduces an error into the calculation. The computer can compensate for the error if the volume fraction of N2 is known. As N2 to flare is under reported only a portion of this error is calculated. In addition, the O2 analysers, AE7000 and AE7010, were obsolete with limited spare parts available.?Due to this a suitable replacement analyser had to be identified, ideally this would be installed during the August 2014 turnaround (TAR).
New Instrumentation/Analyser Scope
The installation of a single analyser (on each flare) would provide analysis of both O2 and N2 content in the flare stream. The cost of installation would be minimised by allowing the use of existing sample lines and cabling for all of the required analysis.
The proposed new analysers were Micro Gas Chromatographs (GCs). These would also provide basic composition data for the hydrocarbon content of the flare stream as shown in the Table below.
Existing, obsolete, paramagnetic O2 analysers AT-7000 and AT-7010 would be removed along with redundant sample handling equipment.?There would also be a requirement to remove existing communications junction boxes that will no longer be required.?This would in turn provide the space for mounting the new Micro GCs which would provide N2, O2, and hydrocarbon analysis of each flare stream, along with the necessary power transformer and communication junction boxes.
The equipment is installed in a Zone 1 hazardous area and the GCs have integral Ex d flameproof enclosures suitable for zone 1 operation. The Power Supply Unit (PSU) and Ethernet switches were combined and mounted in Ex d boxes with separate Ex e (increased safety) junction boxes for cable marshalling.
领英推荐
The GCs would also require a new calibration system to be installed; this would make use of 24V dc relay outputs from the GCs to drive a solenoid valve arrangement.?This would allow remote calibration to be carried out from the control room, minimising the requirement for technicians to be in the controlled flare area.?The existing flowmeters would be reused but would be connected to the GCs and monitored as a digital input.?These alarms would be handled by the analysers and communicated via the new communications system.
Unfortunately the old cabling was insufficient to handle the signals from the new analysers so the existing Ethernet type analyser network on an adjacent plant was used to connect the GCs via new fibre optic runs. This gave the added benefit of providing a digital Modbus map of the new signals into the DCS for reporting, replacing the old 4-20mA analogue current loop signals.
Operational Results
The GCs were installed during the August 2014 TAR.
A remote calibration is run against a reference gas every 4 weeks. During 2015 the Scottish Environmental Protection Agency (SEPA) asked INEOS to have the GC system externally validated by an independent third party to receive Top Tier Reporting Accreditation. This verification was successfully carried out in September 2015 and 2016. Historical measured ranges were compared with the calibration gas mixture so that this could meet the requirements of ISO 17025. The whole set-up has been stable with very little drift against the 4 weekly calibrations. External validations will be carried out yearly to confirm continued performance.
The facility for remote calibration and diagnostics from the control room is a very important safety feature that reduces the time technicians spend in the vicinity of the flare.
The re-designed sampling systems had two parallel filters that guaranteed flow even if one filter was blinded, the set-up also utilised polymer drying tubes to eliminate all moisture before the micro GCs. The total installation has proved most robust with no flow blocks and no moisture reaching the GCs since start-up.
The new N2 measurement has been a great success. Originally because N2 was not measured the composition of the flare line had to be assumed to be all hydrocarbons. The measurements over two years have shown that the N2 level has always been above 60% (see Screenshots) and sometimes as much as 80% for long periods. This accurate measurement has made a massive saving on the ETS scheme and improved the volume flow to mass flow conversion calculation.
Benefits of Micro GCs
?Lessons Learnt
CONCLUSIONS
The Micro GCs were compact enough to easily fit in the space left by the obsolete, unsupported O2 analysers and redundant sampling systems. Yet the new GCs provided N2, hydrocarbon and H2 analysis in addition to the original O2 measurement. All these signals were now presented digitally to the DCS and provided a much more accurate composition of the stream to the flare and hence the emissions after combustion. The re-design of the sample system increased reliability, reduced maintenance and improved safety with a remote calibration facility from the control room. ETS over-reporting ceased and the whole project had a conservative payback of less than one year.
The equipment is still working well in 2022 but a decision was made by Siemens not to actively promote the Micro GC after 2018. INEOS now have a plan to replace the Micro GCs with Modular Maxum GCs which will be easier to service and maintain in the future, they also use a purge method of protection rather than explosion proof.
ACKNOWLEDGEMENTS?
The authors acknowledge the contributions made to the chemical plants over the years by such UK analyser luminaries as Dave McCrae and Dave Davenport.
REFERENCES
Business Development| Growing Client Asset & Building Relationships in Wealth Management| Helping people secure their future
1 年Steve, thanks for sharing!