Catalyst-embedded ceramic filters offer a way to remove NOx at lower temperatures, while simultaneously removing PM, SOx, and HCI. The technology also removes organic hazardous air pollutants, THC, dioxins, and mercury.
Applications include the Cement NESHAP; Boiler MACT; incinerator CISWI MACT; Hazardous Waste MACT; glass furnaces; ceramics manufacturing, including tracking proppants, kilns, and thermal oxidizer clean-up.
Typically, PM is removed to ultralow levels (90%.
Fitter Types: Standard and Catalyst
Standard UltraTemp fitters remove PM or PM plus acid gases and metals, including mercury; UltraCat catalyst filters remove those, plus O-HAPS, dioxins and NOx.
Catalyst filters feature the same fibrous construction as the standard version, but have nanobits of catalyst embedded throughout the filter walls. Distribution across the entire wall thickness, as opposed to just a catalyst layer, creates a very large catalytic surface area. The walls that contain the catalyst are about 3/4 inches thick. Ammonia is injected upstream of the filters and reacts with the NOx at the surface of the micronized catalyst to destroy the compound (Rgure 1). An analysis comparing the effectiveness of this nanocatalyst with that of conventional catalysts was summarized in a paper by Schoubye and Jensen of Haldor Topsoe A/S:
'The catalyst particles are micro-porous, and, due to their small size, they catalyze the gas-phase reactions without diffusion restriction (i.e., almost 100% utilization of the catalyst's intrinsic \activity), as opposed to pellet or monolithic catalysts. In industry, conventional catalyst types typically operate with 5-15% catalyst effectiveness in the SCR of NOx by NH3 and with even lower catalyst utilization in dioxin destruction.'
Another remarkable feature is low temperature activation. Substantial NOx removal is initiated at 350°F, with over 90% removal as the temperature exceeds 450°F.
System Design Criteria
Rlters are placed in a housing module configured like a reverse pulse jet baghouse. Polluted airstream enters the bottom of the housing. Process PM and reacted acid gas sorbent PM are captured on the filter surfaces, while NOX and injected aqua ammonia are transformed to nitrogen gas and water vapor. 0-HAPS (Cement NESHAP) and dioxins are broken down without ammonia additions. Cleaned air passes through the center of the filter tubes and out of the space above (Figures 1 -3).
The modular housing design allows filters to be configured for the largest gas flow volumes. The systems modular nature also provides redundancy so a single module can be taken offline while the other modules receive the flow.
Placing multiple plenums in parallel provides redundancy. If one plenum is taken offline for service, others treat the entire flow at a temporarily higher pressure with no change in performance.
Particulate is captured on the face of the filter and does not penetrate the filter. At start-up, the pressure drop is 6' w.g. Over the filter's life, the pressure undergoes a gradual increase, averaging 3% annually. Rlter life is generally over 10 years. Conventional reverse pulse jet methods are used for fitter cleaning.
Standard Filter: Typical Pollutant Control
Particulate: The typical level of particulate at the outlet of the ceramic filters is s 0.002 grains/dscf (5 mg/Nm3).
With the exception of mercury, heavy metals are captured at the same rates as other particulate (> 99%).
S02, SOfc HCI, other acid gases: Ceramic filters use dry injection of calcium or sodium-based sorbents for acid gas removal. Injected in the duct upstream of the filter modules, the additional sorbent particulate is captured with its pollutant gas. The reaction of the sorbent with the acid gas creates a solid particle that is captured on the filters alongside the unreacted sorbent and process particulate. The reaction occurs within the duct prior to the filter and on the cake on the filter surface.
The sorbent cake on the filters increases exposure of the SOj or HCI, and increases removal rate. For a given removal efficiency, filters require significantly less sorbent than ESPs, which minimizes sorbent costs.
With sorbent injection, SO2 removal is above 90%. S03 and HCI are preferentially removed at higher rates than S02. Sorbent injection of powdered activated carbon is an option for mercury control. The mercury chemistry and temperature of the application determine the formulation of PAC used and the resulting effectiveness.