What is an Air Clean-Up System?
In an extractive continuous emissions monitoring system (CEMS), instrument-grade air is brought to the CEMS cabinet for several purposes:
- To use as purge air to clean out the CEMS probe
- To purge the calibration gas lines before, in between, and after calibration cycles
- To purge Analyzer cases (most commonly Carbon Monoxide (CO) Analyzers) to eliminate any interfering gases from the ambient atmosphere that might get into them
The air brought to a CEMS should be instrument grade, oil-free, with a dew point less than -40˚F. This can be sourced from the plant or produced by a compressor specifically for the CEMS.
The air clean-up system is a series of components to ensure that the instrument air supplied is clean and moisture-free. It can also have additional components to remove gases being measured by the CEMS. Different components can be utilized depending on the quality of the instrument air being supplied. In cases where compressed air is supplied instead of instrument-grade air, additional oil and moisture removal systems are required to prepare it for use by CEMS.
The Air Clean-Up System can be located in the CEMS cabinet or is often wall-mounted near the CEMS.
Air Clean-Up System Components
The instrument air is brought to the air cleanup system at 80 – 90 psig where a pressure regulator brings it down to 40 – 60 psig at the beginning of the Air Clean-up. The most basic extractive CEMS Air Clean-up System travels through a series of three components to ensure all moisture (water, oil, etc.) is removed before it is sent to the umbilical. One more component is added if a Carbon Monoxide (CO) analyzer is used.
The instrument air is brought to the air cleanup system at 80 – 90 psig where a pressure regulator brings it down to 40 – 60 psig at the beginning of the air clean-up. In the most basic Extractive CEMS Air Clean-up System, it travels through a series of three components to ensure all moisture (water, oil, etc.) is removed before it is sent to the umbilical. If a Carbon Monoxide (CO) analyzer is used, one more component is added.
Coalescing Filter/Auto-Drain – The first component the instrument air is introduced into is typically a coalescing filter. This is normally an auto-drain but also could be a manual-drain. The instrument air travels through the Type “C” filter, where moisture is removed. All moisture is collected at the bottom of the canister.
As an auto-drain filter fills, a floater moves up. The floater is attached to the bottom plug, so as the floater moves up, so does the plug. This allows the moisture to be released. The moisture should be minuscule, so there is no need to drain it to a vent (i.e., it can be left to drip on the floor).
- Canister with Drierite: After the coalescing filter, the air moves into a canister filled with drierite. The air travels down the stainless-steel tube in the center of the canister and spills out through a screen at the bottom. The air then travels up through the drierite and onto the next component. The drierite absorbs the moisture still in the instrument air. It has a distinctive property that when it absorbs moisture, it turns from blue to pink. This is used as an indicator to warn if the instrument air is not moisture/oil free. It is not a cost-effective method to eliminate moisture.
- Coalescing Filter/Manual- Draw Filter: This filter works just like the initial coalescing filter, except the bottom plug is manually opened instead of the water automatically draining. This manual-drain filter uses a Type “A” particulate filter to remove any particles down to .01 microns.
- Canister with Purafil (NOx and SO2): After the coalescing filter, the air moves into a canister filled with Purafil. The air travels down the stainless-steel tube in the center of the canister and spills out through a screen at the bottom. The air then travels up through the Purafil and onto the next component.
Purafil absorbs the NOx and SO2 in the instrument air. It has a distinctive property: it turns from pink to brown when it absorbs the two gases. This is only used if NOx and/or SO2 are being measured.
- Canister with Charcoal (THC): The air brought in must be free of hydrocarbons if a dryer is used and/or if hydrocarbons are being measured. If the instrument air is not already hydrocarbon free, then a canister filled with charcoal is used. Just like the canister with Purafil, the air travels down a stainless-steel tube and disperses out through a screen at the bottom. It then travels up through the charcoal, removing any hydrocarbons in the instrument air.
- Dryer Towers (CO2 & Moisture): These two tall towers are used to absorb CO2 and any moisture still in the instrument air. It operates on 115V, plugged directly into an outlet. It uses solid-state controls to operate the two solenoids that open and close the inlets/outlets to each tower. Much like the canisters, the air travels up a central tube surrounded by a media that removes CO2 and moisture. It then travels back down the outside of the tube, inside the tower, and on to the next component. When it gets about 75% depleted, the solenoids switch, allowing the air to begin filling the 2nd tower. The other 25% is pushed through the exhaust out the bottom, removing the moisture and CO2 collected. For the dryer towers to work properly, the air introduced into them must be oil and hydrocarbon free.
Quarterly maintenance is required on air clean-up systems.
- Each of the coalescing filters will have their respective filters and O-rings replaced.
- The old drierite is removed and new drierite is put in. The amount of drierite that is pink is noted to make sure that the air does not contain an excessive amount of moisture.