Advanced Combustion Technologies

๐Ÿ”ฅAdvanced Combustion Technologies Unit 8 โ€“ Pollutant Formation & Emissions Reduction

Pollutant formation in combustion processes is a critical concern in environmental engineering. This unit explores key pollutants like CO, NOx, and particulate matter, examining their formation mechanisms and factors affecting emissions. Understanding these processes is crucial for developing effective control strategies. Emission standards and regulations play a vital role in limiting pollutant release. The unit covers primary and advanced emission control technologies, from fuel selection to oxy-fuel combustion. It also delves into measurement techniques and the environmental and health impacts of combustion-related pollutants.

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 (>>1,500ยฐC) 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


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ยฉ 2024 Fiveable Inc. All rights reserved.
APยฎ and SATยฎ are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.