Early baghouse dust collectors only “blocked dust.” Later, it was discovered that high-temperature filter cartridges themselves are excellent chemical reaction platforms: when flue gas passes through the filter cake and filter media body on the filter cartridge surface, the gas-solid contact time is short, but the contact area is huge. If catalytic reduction and adsorption removal reactions can be completed within this small time window, one set of equipment can replace three to four sets of equipment. Thus, “catalytic filter cartridges,” “adsorption filter cartridges,” and their combinations emerged. These technologies are collectively referred to as multifunctional integrated high-temperature filter cartridges, with the core concept being: upgrading the filter medium from a “physical barrier” to a “physical + chemical reactor.”
I. Catalytic Filter Cartridge Technology
Principle:A denitrification catalyst is loaded onto the internal pores and surface of a conventional ceramic fiber filter cartridge (or intermetallic compound filter media). Dust in the flue gas is intercepted on the surface or in the filter cake layer, while gas molecules diffuse into the interior of the filter material, undergoing selective catalytic reduction (SCR) on the catalyst surface, achieving dust removal and denitrification in the same equipment and at the same temperature.
Catalytic active components: The most mature system is vanadium-based. Vanadium has the advantage of good sulfur resistance, but it may generate SO₃ at high temperatures, and spent catalyst is hazardous waste. In recent years, manganese-based low-temperature catalysts have attracted attention, suitable for placement after desulfurization. Cerium-based and copper-based catalysts have also been studied, but their sulfur and water resistance is generally not as good as vanadium-based systems.
Loading processes:
Immersion method:The ceramic filter cartridge is immersed in a solution containing the catalyst precursor, then removed, dried, and calcined. This process is simple, but the catalyst distribution is uneven, easily causing pore blockage.
Coating method:A catalyst slurry is pre-prepared and coated onto the surface or inner wall of the filter cartridge using vacuum suction or pressure spraying. The coating thickness is controllable, but it is prone to peeling when the bonding strength is insufficient.
In-situ synthesis method:The catalyst precursor is directly introduced during the filter cartridge preparation process, allowing the catalyst to grow in situ within the filter material framework. While exhibiting the highest bonding strength, this method is complex and costly.
Complexing Agent Regulation: Catalyst activity depends on particle dispersion. Direct impregnation easily leads to the aggregation of active components into micron-sized particles with low specific surface area. Adding citric acid or acetylacetone as complexing agents can form stable complexes with ions such as vanadium and manganese, which slowly decompose during calcination, inhibiting aggregation and ensuring a uniform nanoscale distribution of active components. Experiments show that citric acid-assisted impregnation can increase NO removal efficiency from 65% to over 85%.
II. Adsorption-Type Filter Cartridge Technology
Catalytic reactions require specific temperature windows and have limited effectiveness against certain pollutants (such as dioxins and heavy metals like mercury). Adsorption-type filter cartridges take a different approach: utilizing high specific surface area materials for physical or chemical adsorption of pollutants while simultaneously filtering dust.
Activated Carbon/Activated Coke Loading: Activated carbon powder or activated coke particles are added to the surface of the filter cartridge or to the blended fibers. Large organic molecules such as dioxins and furans are adsorbed into the micropores of activated carbon, where they partially decompose or are treated by subsequent catalytic layers at flue gas temperatures (200–300℃). For mercury, the oxygen-containing functional groups on the activated carbon surface can oxidize and adsorb elemental mercury. A drawback is that activated carbon slowly oxidizes and is lost above 250℃, requiring continuous carbon replenishment or the use of oxidation-resistant modified activated carbon for long-term operation.
MOF Functionalization: Metal-Organic Frameworks (MOFs) can have a specific surface area exceeding 3000 m²/g, and the metal centers can be directionally designed. For example, Cu-BTC has an extremely high adsorption capacity for SO₂; MIL-101(Cr) has a strong affinity for VOCs such as toluene and styrene. However, MOFs generally have poor thermal stability (most below 350℃), and their direct application to high-temperature flue gas is not yet mature. Current research mainly focuses on the fine treatment of low-temperature sections (<200℃) or cooled exhaust gases.
III. Integrated Dust Removal and Desulfurization
Desulfurization typically requires alkaline absorbents. The integrated dust removal and desulfurization approach involves injecting a finely powdered alkaline absorbent into the flue gas duct upstream of the filter cartridge. The absorbent powder reacts with SO₂/SO₃ in the flue gas to form sulfates. These solid products, along with unreacted absorbent, are intercepted by the downstream high-temperature filter cartridge and continue to react in the filter cake layer, extending the residence time several times. This process eliminates the need for a wet desulfurization tower and produces no wastewater, but it consumes a large amount of absorbent (the calcium-to-sulfur ratio or sodium-to-sulfur ratio is typically 1.5–2.5), resulting in higher operating costs than wet processes.
IV. Multifunctional Synergistic Effect of Dust Removal, Denitrification, and Desulfurization
It is impractical to cram catalysis, adsorption, and desulfurization into a single filter cartridge—catalysts are susceptible to alkali damage, and desulfurizing agents are strongly alkaline; direct contact would cause mutual poisoning. Therefore, industrial applications employ a synergistic process chain rather than a single-tube all-encompassing approach:
Scheme A (High-Temperature Route): Flue gas first enters a catalytic filter cartridge (300–420℃) for denitrification and dust removal, then NaHCO₃ is injected for dry desulfurization, followed by collection of desulfurization products using a bag filter. The advantage is that the catalyst is unaffected by alkali metals; the disadvantage is the need for two dust removal systems.
Scheme B (Medium-Low Temperature Route): Flue gas is first injected with Ca(OH)₂ for pre-desulfurization (removing most of the SO₂ to avoid catalyst poisoning), then cooled to 200–250℃ before entering the catalytic filter cartridge, simultaneously completing dust removal and denitrification. The remaining small amount of SO₂ is absorbed by the residual desulfurizing agent in the filter cake. The process is compact, but the catalyst still faces the risk of ammonium sulfate blockage due to prolonged exposure to a low-sulfur environment.
True “single-tube multi-functional” filter cartridges are currently more commonly found in the TrennTech filtration laboratory—for example, layered filter cartridge designs: an outer layer loaded with desulfurizing agent, a middle layer of adsorption (activated carbon), and an inner layer of denitrification catalyst. However, issues such as interlayer thermal expansion matching and uniform airflow distribution remain unresolved.
V. Technical Challenges and Breakthrough Directions
Challenge 1: Catalyst-Filter Media Bonding Strength
Ceramic filter cartridges are subjected to severe mechanical vibrations (acceleration up to 50–100g) during pulse backflushing. Coated catalysts are prone to peeling off, resulting in a sharp drop in denitrification efficiency. Breakthrough Direction: Embed the catalyst into the filter media fibers using in-situ synthesis or sol-gel methods to form an “anchored structure.” Some studies also use sintered metal fiber felt as a carrier, with oxide nanoarrays grown on the surface as the catalyst framework.
Challenge 2: Long-Term Operational Stability
Trace amounts of SO₂ in the flue gas react with NH₃ to form ammonium bisulfate (ABS), a viscous liquid in the 200–230℃ range, which clogs the catalyst micropores. Under long-term exposure, the catalyst may also undergo deactivation such as sintering (nanoparticle growth), alkali metal poisoning, and arsenic poisoning. Breakthrough Direction: Develop sulfur-resistant low-temperature catalysts and install online thermal desorption devices (300℃ hot nitrogen backflushing once a week). Challenge 3: Deactivation and Regeneration Mechanisms
Currently, there is a lack of online methods for diagnosing the deactivation of multifunctional filter cartridges. Downtime sampling and analysis takes several days, making it difficult to guide timely regeneration. Research Direction: Real-time monitoring of upstream and downstream pressure drop, denitrification efficiency, and SO₂ penetration concentration, combined with machine learning models, to infer the type of deactivation (poisoning, clogging, sintering). Regeneration technologies include water washing (to remove sulfates), acid washing (to remove alkali metals), and thermal regeneration (to burn off carbon deposits). For vanadium-based filter cartridges, the activity recovery rate after water washing regeneration can reach 80%, but it generates wastewater containing vanadium and sulfur.
In the next five years, with the maturation of high-temperature resistant MOFs, alkali-resistant catalysts, and intelligent regeneration technologies, multifunctional filter cartridges are expected to open up new prospects in fields such as steel sintering, cement kilns, and biomass power generation. However, for equipment operators, the biggest temptation remains unchanged: to perform the work of three sets of equipment with the space and maintenance of one set.