STS - Siloxane Monitor for Biogas Feeds to CHP Engines
From Siloxane Monitoring
The STS Siloxane monitor is designed to output a total siloxane concentration in mg/m3 in gases derived from waste water treatment, food waste digestion and landfills. Understanding the levels of Siloxane in the gas allows operators to manage gas clean up systems such as activated carbon filters and regenerative filter systems. The monitor provides assurance to those exporting gas to grid that their supply is within the tolerance limit set by the grid network for Siloxane concentration.
The STS Siloxane Monitor has been designed in house by STS to provide the most cost effective , reliable and simple monitor available. STS were able to approach the issues of detecting and measuring siloxanes from a completely conceptual starting point - the monitor is not based on an existing device modified to also 'do siloxanes', it was conceived , designed , built and tested with only that purpose in mind.
This has enabled the design to be both results led and cost driven- resulting in the use of robust but simple design with an emphasis on practicality.
The monitor works by taking a sample of the biogas, usually approx 1litre which is passed through a GC concentration column and then to vent. This GC column media selectively separates the siloxane molecules from the other contaminants in the gas which pass straight through the media.
The column is then heated which releases the siloxanes from the media and they are carried by a N2 carrier gas into the measurement cell. The cell comprises a Infrared source, filters and an pyroelectric detector mounted behind specialised windows. As the carrier gas passes through the cell the siloxanes cause absorbance of the IR signal so the received signal by the detector falls and produces and equivalent voltage drop. This is recorded and processed to produce a mass of material which when calculated with the sample volume produces a reading in mg/m3.
Siloxanes are particularly difficult to measure due to their 'sticky' and persistent nature. Part of the challenge of making accurate measurements is ensuring that the measurement cell and concentrator are not contaminated after taking a reading. To alleviate this the monitor is purged with nitrogen to remove any residues and the concentrator is heated to drive off any residue which may not have been released leaving it clean for the next cycle.
The monitor will record one reading per hour under normal conditions - of that approx 15mins are the sampling phase, 10 minutes the measurement phase and the rest is cleaning and purging ready for the next sample.
The monitor produces a total siloxane reading which is the sum of the various species from L2 through to D6. The unit is usually calibrated using either D4 or D5 dependent on the site as these tend to form 80%+ of the siloxanes seen and are also the most damaging being the larger molecules.
The STS Siloxane Monitor is CE certified and complies with IGE SR25 requirements.
The Siloxane monitor has the following requirements:
- 110V 6A – (IEC rear connector) wired to flex outlet from Distribution box.
- N2 carrier gas line: ¼” Nylon/PTFE/SS piping (1/4” Swagelok connector supplied) Set at 1Bar at cylinder regulator Technical Grade 99.999%.
- Biogas feed line - ¼” PTFE/SS piping (1/4” Swagelok connector supplied) Maximum 1Bar input pressure.
- Vent line - ¼” PTFE/SS piping (1/4” Swagelok connector supplied) Vented ~3m high , end of Line flame arrester to be fitted.
The water Traps require the following:
- 24V supplied from instrument via Ancillary Box for auto drain (WT)
- Water Drain line - ¼” PTFE piping (Push fit connector)
- Water Vented external to kiosk at ground level.
- Biogas Gas connections – The water trap (WT) uses a 1/4” Swagelok connection. The Coalescing filter is a ¼” Swagelok & the Flame arrester is a ¼” Swagelok.
The heated Line requires the following:
- 110V 10A – Directly wired from distribution box.
- Temperature set to 60degC.
- Cut to length - Requires sealing at cut end.
110V , 250W , 6A or 400W 10A– via thermostat set at 20C
Provides frost protection and maintains gas above condensation point to prevent siloxanes being deposited in pipework
The instrument samples approx. 2L of biogas during a 20minute window every 1hour, at a flow of approx. 100ml/min. This is passed through the instrument then vented to atmosphere.
N2 Flow rate:
The instrument consumes approx. 4L of N2 every hour. Resulting in 0.1m3 of N2 per day. This is passed through the instrument then vented to atmosphere.
- STS recommend using the largest N2 cylinder to avoid regular replacement.
- Air Products: X47S: 80kg 200bar 8.9m3. Technical Grade.
- Regulator : BOC 8500 or similar, 300Bar inlet , 0-4 bar outlet.
- This should last approx. 90days, 3 months.
A GRP kiosk is required as per the manual specification.
The kiosk will require 6 external connections:
- Biogas IN
- N2 IN
- Vent OUT
- Drain OUT
- Heated Line out
- Heated Line Sensor out.
Regulated in the kiosk to 35mbar, normal inlet pressure to the regulators should not exceed 350mbar. If a higher pressure is present, the regulator will require re-specification.
The electrical supply:
A 110V earthed connection is required to the externally mounted switch. This is routed to a Distribution box in the kiosk where there are a 6A RCD to the instrument, and 2 10A RCDs directly wired to the heated line and cabinet heater. All earthed. Alternatively a 240V supply may be supplied to the external isolator switch and a suitably rated transformer sited in the kiosk to convert the supply to 110V to the distribution board.
- The instrument uses an air purge system to ensure safe operation, this is achieved with 2 high capacity fans mounted in the instrument which force a constant stream of air through the case. The instrument fans are connected to vent plates in the kiosk by flexible hoses. The flow of air through the instrument ensures that even in a catastrophic joint failure that the LEL of CH4 would not be exceeded. The purge fans are monitored so that if they are inactive the instrument will be placed into “Safe Mode”
- The instrument has an on-board methane sensor which will place the instrument into “Safe Mode” if it exceeds 10% of the LEL. An on-board temperature sensor will place the instrument in “Safe Mode” if the temperature in the cabinet exceeds 50C.
- Incoming bio-gas lines are controlled by Solenoid valves which are in the Normally Closed position unless activated, the instrument also has a solenoid valve on the incoming biogas line which is Normally Closed, any power failure will therefore close these valves stopping gas flow. “Safe mode” shuts the incoming gas valves, turns off all heaters and runs the case fans to vent.
- The kiosk itself is cross ventilated from a further two ventilation grilles.
The Siloxane Monitor & Kiosk is not EX or Atex rated and as such should be sited outside of any zoned area.