Polycyclic aromatic hydrocarbon (PAH) emissions from a coal-fired pilot FBC system

Area of Science:

• Environmental Science
• Chemical Engineering
• Combustion Science

Background:

• Polycyclic aromatic hydrocarbons (PAHs) are hazardous compounds, with 16 listed on the Environmental Protection Agency (EPA) Priority Pollutant List.
• Fluidized bed combustion (FBC) is a widely used coal combustion technology, necessitating an understanding of its pollutant emissions.
• Controlling PAH emissions is crucial for environmental protection and regulatory compliance.

Purpose of the Study:

• To quantify the emissions of 16 priority polycyclic aromatic hydrocarbons (PAHs) from coal combustion in a bench-scale fluidized bed combustor (FBC).
• To investigate the influence of operating parameters, including combustion temperature, excess air ratio, limestone addition, coal chlorine content, and Ca/S molar ratio, on PAH emissions.
• To evaluate the effectiveness of secondary air injection in mitigating PAH formation.

Main Methods:

• Conducted four 1000-hour combustion runs using four different coals in a 0.1 MWth FBC.
• Developed and utilized an on-line sampling system comprising a glass wool filter, condenser, glass fiber filter, Teflon filter, and Tenax trap.
• Analyzed collected samples using Gas Chromatography/Mass Spectrometry (GC/MS) in selective ion monitoring (SIM) mode after extraction with methylene chloride and hexane.

Main Results:

• PAH emissions were primarily dependent on combustion temperature and excess air ratio.
• Injection of high-velocity secondary air into the freeboard significantly reduced PAH emissions.
• Increased limestone addition promoted PAH formation, while coal chlorine content potentially led to larger benzene ring PAHs.
• Higher sulfur content in coal correlated with increased total PAH emissions.
• Incomplete combustion favored PAHs with four or more benzene rings, whereas high-efficiency combustion produced PAHs with two or three rings.

Conclusions:

• Combustion temperature, excess air, and secondary air injection are key factors controlling PAH emissions in FBC.
• Limestone addition and coal chlorine content require careful consideration to manage PAH formation.
• Optimizing combustion conditions and operational strategies can effectively minimize hazardous PAH emissions from FBC systems.