Wet Scrubbers for meeting the new HMIWI MACT Standard
In my previous blog post I outlined new rules that were promulgated on September 15th, 2009 for the hospital, medical, and infectious waste incinerator (HMIWI) maximum achievable control technology (MACT) standard. Wet scrubbers are used on many of the existing medical and hazardous waste incinerators to meet this MACT standard. The unfortunate news is that new control strategies are required to meet the more stringent standards.
The new emission limits present challenges for both existing and new systems. These challenges relate primarily to the following pollutants.
- Lead, Pd
- Cadmium, Cd
The particulate limits for new systems are reduced from 0.015 to 0.008 gr/dscf. This is a 50% reduction. The lead (Pd) emission limits are reduced from 1.2 to 0.036 mg/dscm for existing systems and from 0.07 to 0.00069 mg/dscm for new systems. The cadmium (Cd) limits are reduced from 0.16 to 0.0092 mg/dscm for existing systems and from 0.04 to 0.00013 mg/dscm for new systems. The new Pd and Cd limits for both existing and new systems are nearly a 100% reduction.
There are 4 keys to meeting these more stringent standards with wet scrubber systems.
- Add-on particulate polishing package
- Venturi Scrubber
- Mist elimination
Add-on particulate polishing package
The new emission limits will exceed the design capability of most of the existing wet scrubber systems today. This will require an add-on polishing control to meet the more stringent standards. Envitech has had success achieving higher removal efficiencies by integrating an add-on particulate polishing package (PPP) into incinerator wet scrubber systems. The PPP is comprised of a skid mounted package that provides slight reheat of the gas temperature to slightly above saturation in combination with a filter system. The reheat eliminates the potential for condensation build-up in the filters.
This strategy has been used for both commercial and industrial waste incinerator scrubbers (CISWI) and low level radioactive waste incinerator scrubbers. Removal efficiencies of > 99.8% was achieved for both Pd and Cd at the outlet of the Venturi scrubber which already has a very low particulate load < 0.015 gr/dscf. This has proven to be a cost effective strategy to meet the new standards.
Sub-Cooling - Sub-cooling the gas in a medical or hazardous waste incinerator scrubber provides several advantages. It makes use of condensation effects to enhance particulate control in a downstream Venturi scrubber. The water vapor in the gas condenses onto the particulate and grows them in size. A particulate that is 0.3 microns will grow to about 0.7 microns after condensing the water vapor. This makes it easier to collect in the downstream Venturi. A second advantage of sub-cooling is condensing metals (i.e. Pd and Cd) as much as possible from a gas phase to a particulate. This allows them to be collected downstream in the scrubber. The final advantage is steam plume suppression. Removing the water vapor eliminates a steam plume under most meteorological conditions. This reduces the visibility of the system in the surrounding community.
Venturi Scrubber - The particulate capture efficiency of a wet scrubber system is determined by the pressure drop across the Venturi scrubber. Higher removal requires higher pressure drop. Sub-cooling discussed above enhances the Venturi performance by growing the size of particulate. Often times this reduces the power consumption by half for most medical and hazardous waste incinerator wet scrubbers. The new HMIWI standards, however, exceed the practical capability of a Venturi scrubber. This can be overcome with an add-on particulate polishing package discussed previously. It is recommended to optimize the Venturi scrubber performance to minimize the load on the PPP. This reduces the annual operating expense by increasing the life of the filter elements.
Entrainment Separator - An entrainment separator or mist eliminator is used after the Venturi scrubber to knock out water droplets in the gas stream. Any water droplets that escape the mist eliminator will contain pollutants which can cause a stack test failure. A horizontal, chevron style mist eliminator is commonly used in incinerator wet scrubber systems. Effective mist elimination is important for the add-on particulate polishing package discussed previously. Water droplets can lead to fouling of the add-on control.
As facilities get their arms around the new rules for the HMIWI MACT, they will need to consider all of the above items for complying with the new standard using a wet scrubber system.