Environmental Engineering Quiz: Emission Control

Pollutant-Specific Treatment Techniques

• Pollutant-specific treatment techniques are used to target and remove specific pollutants from exhaust gases.
• Different techniques are used, including desulfurization, neutralization, physical sorption, adsorption catalysis, denitrification, reduction processes, and oxidation processes.
• Other emissions, such as mercury, volatile organic compounds (VOCs), dioxins, furans, and odors, can also be reduced using specific methods.

General Considerations

• Evaluating the efficiency of a treatment process is crucial to selecting the best approach.
• Choosing the right process for a specific pollutant is essential for effective treatment.
• Criteria for selecting the most suitable treatment process include performance level, economy, operational safety, suitability given operational limits, and recycling opportunities and cost of disposal.

Desulfurization Overview

• Desulfurization aims to minimize the SO2 and SO3 concentration in exhaust gases.
• Chemical sorption processes, including neutralization (wet, quasi-dry, and dry methods), regenerative processes, and limestone method, are used for this purpose.
• Physical sorption and adsorption catalysis are also employed for desulfurization.
• Examples of these methods include the BF method and Sulfacid method.

Chemical Sorption (Wet Method)

• The wet method uses an alkaline chemical wash to remove SO2.
• SO2 reacts with water to form sulfurous and sulfuric acids.
• Hydrogen sulfite is then subsequently oxidized to sulfate using air.
• The addition of OH ions shifts the equilibrium toward the deprotonated form.
• Alkaline agents like CaO, Ca(OH)2, CaCO3, NaOH, Na2CO3, MgO, and Mg(OH)2 are utilized in this process.

The Limestone Method

• Limestone (CaCO3) is a cost-effective neutralizing agent for SO2.
• A large volume of scrubbing fluid and finely ground limestone are required.
• Limestone is mixed with water to form a thick suspension.
• This mixture is sprayed into a scrubber where SO2 reacts to form gypsum (CaSO4).

Limestone Method Conditions

• The scrubber solution ratio (including circulation) is approximately 10 (scrubber solution/m³ flue gas).
• Gypsum is separated from the solution in a hydro-cyclone and can be used for construction material.
• Optimal pH for absorption of SO2 is above 6 and the ideal pH for the process is 5.5-6.
• Maintaining high enough separation coefficient ( ~ 90%) is crucial to ensure the effectiveness of the process

Physical Adsorption (B-F Method)

• The B-F method uses a moving bed of active coke to adsorb SO2.
• Adsorption occurs at temperatures ranging from 120-150 °C.
• Thermal desorption at 500-650 °C produces a SO2-rich gas.
• This method is particularly suitable for treating furnace and combustion chamber exhaust gases.
• Further expansion of this process (BF-Uhde) allows for simultaneous NOx removal.

Adsorption Catalysis (Sulfacid Process)

• Activated charcoal adsorbers are employed at 60-75 °C to remove SO2, O2, and H2O.
• The components react within the pores of the adsorber to form H2SO4.
• Regeneration of the saturated adsorber can be done either via a chemical reduction process or washing.

De-SOx for Marine Ships

• Large marine diesel engines commonly run on heavy fuel oil (average sulfur content of 2.7% w/w).
• Emission control areas (ECAs) enforce sulfur oxide limits for fuel oil to limit air pollution from ships.
• Emissions should be reduced to 6 g SOx/(kWh) or less.
• Different ECAs are defined, with various regulations in place to reduce SOx emissions.

Denitrification Overview

• Dry processes are preferred for denitrification due to NO's low solubility in water.
• Denitrification processes can be categorized as reductive or oxidative.

Reductive Processes

• NOx reduction involves the reaction of NOx with NH2 radicals to form N2.
• Aqueous ammonia solutions are used, but gaseous ammonia poses safety concerns.
• Urea solutions are viable alternatives for NOx reduction in these processes.
• Simplified equations demonstrate the reactions involved.
• Unwanted reactions like excessive NH3 consumption might occur

SNCR Process (Thermal Reduction)

• Urea or ammonia solution is injected into the combustion chamber.
• Mixing with exhaust gas and maintaining optimal temperatures (900-1000 °C) is crucial.
• NOx reduction efficiency ranges from 60-80%.
• Sensitive point is ammonia slip (unconverted ammonia being emitted into the atmosphere).
• Suitable for cement factories and waste incinerators

Temperature Dependence

• Insufficient temperatures hinder the reaction, and extremely high temperatures promote the conversion of ammonia to NO.
• The optimal reaction temperature is crucial for efficient NOx reduction.

SCR Process (Catalytic Reduction)

• Catalytic reduction of NOx uses materials like V2O5, WO3, zeolites, and activated coke.
• The operating temperature is in the range of 200-300 °C, and space velocity ranges from 5000-10000 h⁻¹.
• This process is used in automobile three-way catalytic converters.
• This can also be used with SO2 removal.

SCR Characteristics

• Unwanted parasitic reactions at catalysts, such as NH3 reaction with O2 or SO3, can form ammonium salts, which can cause blockages.
• Effective NOx reduction efficiency in this process generally lies between 80-90%.
• Suitable for waste incinerator plants and stationary diesel engines.

Catalyst Arrangement (Warm/Cold)

• Warm SCR layout uses electrostatic filters upstream of the denox catalyst, while cold SCR systems have the scrubber upstream.
• Catalyst materials for both warm and cold SCR systems can vary, including specific metal oxides.
• Ammonia handling is easier in the cold layout, but the gas must be reheated resulting in potential issues. Additional considerations include the need for heat exchangers in the warm SCR systems which can negatively impact operational life if the gas stream is highly concentrated in SO2.

Oxidative Processes (EBDS and Purasiv-N Methods)

• Electron Beam Dry Scrubber (EBDS) uses an electron beam to generate radicals in the exhaust gas stream to oxidize NOx and SO2 to form acids.
• Purasiv-N involves adsorbing NOx onto zeolite, catalytically oxidizing it to NO2, and then using thermal desorption to recover the NO2-rich gas.

Reducing Mercury Emissions

• Mercury is a harmful pollutant that needs to be addressed.
• Adsorption methods are used for mercury recovery.
• Treating mercury with substances like activated charcoal or selenium filters is used to remove/recover mercury emissions. Various methods of regeneration, including thermal ones, can be employed.

Reducing VOC emissions

• Volatile Organic Compounds (VOCs) can be separated using absorption, gas permeation methods.
• Oxidation of VOCs is effective in ways including thermal post-combustion, catalytic post-combustion, and biological scrubbing.
• Adsox process is one suitable approach to VOC removal

Persistent Organic Pollutants (POPs) and Dioxins/Furans

• POPs, such as PCDD/PCDFs, are persistent in the environment and pose significant health concerns.
• Dioxins and furans are produced from precursors in combustion conditions above 200°C.
• Metals, like copper(II) chloride, can catalyze this process.
• High O2 concentration above 12 vol % also contributes to dioxins creation.

Primary Measures for Dioxin/Furan Reduction

• Optimization of the combustion process is essential, including reducing excess air and increasing temperature.
• Exhaust gas recirculation reduces oxygen concentration, thus inhibiting dioxins/furans formation.
• Selective non-catalytic reduction (SNCR) uses ammonia or urea to inhibit metal catalyst functions relevant to dioxin/furan production.
• Optimal temperature conditions in the exhaust gas (cooling to 250-400°C) are also important.

Secondary Measures for Dioxin/Furan Reduction

• Dust removal is important as dioxins and furans bind to dust particles.
• Adsorption onto activated charcoal or coke, often combined with other processes for effective removal.
• Catalytic oxidation using the SCR method converts these pollutants to less harmful substances (CO2, H2O, HCI).

Comparison of SNCR and SCR for Denitrification and Dioxins/Furans Reduction

• Both processes affect the production of NOx and the reduction of dioxins/furans, but via different methods.
• SNCR inhibits catalytic effects, which reduces dioxins and furans formation within the combustion chamber.
• SCR process, on the other hand, is an external catalyst reaction process to oxidatively decompose pollutants to less harmful substances

POPs from Plastics (HBCD Example)

• Hexabromocyclododecane (HBCD) is a flame retardant commonly found in polystyrene applications.
• HBCD releases during thermal cutting, mainly loading onto ultrafine particles.
• The emission rate is substantial.

Impact of HBCDs

• HBCDs persist in the environment and can travel over great distances.
• They bioaccumulate in various organisms, including humans.
• Significant health risks associated with HBCD exposure include developmental, reproductive, or endocrine disruption.

Reducing Olfactory Emissions

• Olfactory emissions, often VOCs, can be addressed through methods like bioscrubbers, chemical scrubbers, adsorption onto active charcoal, and thermal/catalytic post-combustion.
• OUE (odor unit) calculations are used to quantify odor intensity levels and emission limits.

Olfactometer for Odor Testing

• Olfactometers are instruments used for odor testing.