🔥Advanced Combustion Technologies Unit 8 – Pollutant Formation & Emissions Reduction

Key Pollutants in Combustion

• Carbon monoxide (CO) forms when there is incomplete combustion due to insufficient oxygen or low temperatures
• Nitrogen oxides (NOx) produced through thermal NOx mechanism at high temperatures and fuel NOx from nitrogen-containing fuels
• Sulfur oxides (SOx) generated when sulfur-containing fuels are burned, primarily forming sulfur dioxide (SO2)
• Particulate matter (PM) consists of small solid or liquid particles suspended in the exhaust gas
  • Includes soot, ash, and condensed organic compounds
• Unburned hydrocarbons (UHC) result from incomplete combustion and can contribute to smog formation
• Volatile organic compounds (VOCs) are organic chemicals that easily vaporize and can react with NOx to form ground-level ozone
• Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds that can have carcinogenic and mutagenic properties
• Dioxins and furans are highly toxic persistent organic pollutants (POPs) that can form during combustion processes

Pollutant Formation Mechanisms

• Thermal NOx forms at high temperatures ($>$1,500°C) through the oxidation of atmospheric nitrogen (Zeldovich mechanism)
• Fuel NOx is produced when nitrogen-containing compounds in the fuel are oxidized during combustion
• Prompt NOx forms quickly in fuel-rich regions of the flame through reactions involving hydrocarbon radicals and atmospheric nitrogen
• Soot forms through the incomplete combustion of hydrocarbons, involving nucleation, surface growth, and agglomeration processes
  • Nucleation creates initial soot particles from gas-phase precursors like acetylene and polycyclic aromatic hydrocarbons (PAHs)
  • Surface growth occurs as gas-phase species adsorb and react on the surface of soot particles, increasing their size
• CO forms in fuel-rich regions where there is insufficient oxygen for complete combustion to CO2
• UHC emissions result from flame quenching near cold surfaces, crevice volumes, and incomplete mixing
• SOx formation is directly related to the sulfur content of the fuel, with almost all sulfur converting to SO2 during combustion

Factors Affecting Emissions

• Fuel composition significantly impacts emissions, with higher nitrogen and sulfur content leading to increased NOx and SOx
• Air-fuel ratio affects the formation of pollutants like CO, UHC, and soot, with fuel-rich conditions promoting their formation
• Combustion temperature influences thermal NOx formation, with higher temperatures leading to increased NOx emissions
• Residence time in high-temperature regions affects the formation and destruction of pollutants
  • Longer residence times can increase thermal NOx but also allow for more complete combustion, reducing CO and UHC
• Mixing quality between fuel and air is crucial for minimizing local fuel-rich regions that promote pollutant formation
• Combustor geometry can impact flow patterns, mixing, and temperature distributions, all of which affect pollutant formation
• Load conditions and operating modes (startup, shutdown, transient) can lead to different emission characteristics compared to steady-state operation

Emission Standards and Regulations

• Emission standards set limits on the allowable levels of specific pollutants from combustion sources
  • Standards vary by country, region, and application (power generation, transportation, industrial processes)
• United States Environmental Protection Agency (EPA) sets National Ambient Air Quality Standards (NAAQS) for criteria pollutants
• European Union (EU) has emission standards for various sectors, such as the Industrial Emissions Directive (IED) and Euro emission standards for vehicles
• International Maritime Organization (IMO) regulates emissions from ships through the MARPOL Convention, including limits on sulfur content in marine fuels
• Kyoto Protocol and Paris Agreement aim to reduce greenhouse gas emissions, which can be affected by combustion processes
• Emission trading systems (cap and trade) and carbon taxes are market-based approaches to incentivize emission reductions
• Continuous Emission Monitoring Systems (CEMS) are often required to demonstrate compliance with emission limits

Primary Emission Control Strategies

• Fuel selection and treatment can reduce pollutant formation by choosing fuels with lower nitrogen, sulfur, and ash content
  • Examples include using low-sulfur coal, natural gas, or biomass fuels
• Combustion process optimization aims to minimize pollutant formation by controlling factors like air-fuel ratio, temperature, and mixing
  • Staged combustion (rich-lean or lean-rich) can reduce NOx formation by avoiding high-temperature regions
• Flue gas recirculation (FGR) lowers flame temperature and oxygen concentration, reducing thermal NOx formation
• Water or steam injection can lower peak flame temperatures, reducing thermal NOx but potentially increasing CO and UHC emissions
• Low-NOx burners employ staged combustion, fuel staging, and air staging to minimize NOx formation
• Selective Catalytic Reduction (SCR) uses a catalyst and ammonia injection to reduce NOx to nitrogen and water in the exhaust gas
• Selective Non-Catalytic Reduction (SNCR) involves injecting ammonia or urea directly into the high-temperature flue gas to reduce NOx

Advanced Emission Reduction Technologies

• Oxy-fuel combustion uses pure oxygen instead of air, reducing NOx formation and enabling easier CO2 capture
• Chemical looping combustion (CLC) uses metal oxides as oxygen carriers, avoiding direct contact between fuel and air
  • Enables inherent CO2 capture and reduces NOx formation
• Pressurized fluidized bed combustion (PFBC) operates at elevated pressures, improving efficiency and enabling combined cycle operation
• Integrated gasification combined cycle (IGCC) gasifies fuel to produce syngas, which is then cleaned and used in a combined cycle power plant
  • Allows for pre-combustion removal of pollutants like sulfur and particulates
• Plasma-assisted combustion uses non-thermal plasma to enhance combustion stability and reduce pollutant formation
• Catalytic combustion employs catalysts to enable complete combustion at lower temperatures, reducing NOx formation
• Advanced particulate filters, such as ceramic filters and electrostatic precipitators (ESPs), can remove fine particulate matter from the exhaust gas

Measurement and Monitoring Techniques

• Continuous Emission Monitoring Systems (CEMS) provide real-time measurements of pollutant concentrations in the exhaust gas
  • Includes analyzers for NOx, SO2, CO, CO2, and oxygen
• Portable emissions measurement systems (PEMS) are mobile devices used for on-site testing and compliance monitoring
• Fourier-transform infrared (FTIR) spectroscopy can measure multiple pollutants simultaneously by analyzing the absorption of infrared light
• Gas chromatography (GC) separates and quantifies individual compounds in the exhaust gas
• Particulate matter (PM) can be measured using gravimetric methods, which involve collecting PM on filters and weighing the collected mass
• Opacity meters measure the reduction in light transmission through the exhaust gas, providing an indication of particulate emissions
• Source testing involves collecting samples from the exhaust stack and analyzing them in a laboratory to determine pollutant concentrations
• Predictive emission monitoring systems (PEMS) use process parameters and models to estimate emissions in real-time, serving as an alternative to CEMS

Environmental and Health Impacts

• NOx contributes to the formation of ground-level ozone and acid rain, which can harm respiratory health and damage ecosystems
• SO2 can cause respiratory issues, particularly in children and the elderly, and contributes to acid rain formation
• Particulate matter (PM) can penetrate deep into the lungs, causing respiratory and cardiovascular problems
  • Fine particulates (PM2.5) are particularly harmful due to their ability to enter the bloodstream
• CO reduces the blood's ability to carry oxygen, leading to headaches, dizziness, and in severe cases, death
• Ozone, formed from NOx and VOCs in the presence of sunlight, can cause respiratory irritation and aggravate asthma
• Polycyclic aromatic hydrocarbons (PAHs) and dioxins are known carcinogens and can accumulate in the food chain
• Mercury emissions from coal combustion can bioaccumulate in aquatic ecosystems, posing risks to human health through fish consumption
• Greenhouse gases, such as CO2 and methane, contribute to climate change, leading to rising temperatures, sea levels, and extreme weather events