6.3 Particulate Matter Control Systems

6.3 Particulate Matter Control Systems

Particulate matter (PM) control is a fundamental aspect of air pollution engineering. PM consists of solid particles and liquid droplets suspended in the air. The EPA regulates PM based on its aerodynamic equivalent diameter, primarily focusing on $PM_{10}$ (particles with an aerodynamic diameter $\leq 10 \mu m$) and $PM_{2.5}$ (particles with an aerodynamic diameter $\leq 2.5 \mu m$). The aerodynamic diameter ($d_p$) is the diameter of a spherical particle with a density of $1000 \text{ kg/m}^3$ (the density of water) that has the same settling velocity as the irregular particle in question. Selecting the appropriate control technology depends heavily on the particle size distribution of the exhaust gas stream, the required collection efficiency, and gas stream characteristics (temperature, moisture, corrosiveness). The primary PM control devices are settling chambers, cyclones, baghouses, electrostatic precipitators, and wet scrubbers.

Settling Chambers

Gravity settling chambers are the simplest and oldest form of PM control. They consist of a large, expanded chamber where the gas velocity is significantly reduced, allowing particles to settle out of the gas stream by gravity. The design of a settling chamber relies on calculating the terminal settling velocity ($v_t$) of the particles, often determined using Stokes' Law for particles in the laminar flow regime (Reynolds number $< 1$): $v_t = \frac{g d_p^2 (\rho_p - \rho_g)}{18 \mu}$ Where:

• $v_t$ = terminal settling velocity (m/s)
• $g$ = acceleration due to gravity ($9.81 \text{ m/s}^2$)
• $d_p$ = aerodynamic particle diameter (m)
• $\rho_p$ = density of the particle ($\text{kg/m}^3$)
• $\rho_g$ = density of the gas ($\text{kg/m}^3$)
• $\mu$ = dynamic viscosity of the gas ($\text{kg/(m}\cdot\text{s)}$)

Settling chambers are only effective for very large particles (typically $> 50 \mu m$) and are mostly used as pre-cleaners to remove large, abrasive particles before they enter more sophisticated control devices.

Cyclones

Cyclones use centrifugal force rather than gravity to separate particles from the gas stream. The dirty gas enters tangentially near the top of the cyclone, creating a downward spiral (vortex). The centrifugal force pushes particles toward the cyclone walls, where they slide down into a hopper. The cleaned gas reverses direction at the bottom and travels up through a central inner tube.

The performance of a cyclone is often characterized by its cut diameter ($d_{p50}$), which is the particle size that is collected with 50% efficiency. According to the Lapple equation, the cut diameter is: $d_{p50} = \sqrt{\frac{9 \mu b}{2 \pi N_e v_i (\rho_p - \rho_g)}}$ Where:

• $b$ = width of the cyclone gas inlet (m)
• $N_e$ = effective number of turns the gas makes inside the cyclone (typically 5 to 10)
• $v_i$ = inlet gas velocity (m/s)

Pressure drop ($\Delta P$) is a critical operating parameter for cyclones. Higher inlet velocities increase collection efficiency but also significantly increase the pressure drop, leading to higher fan energy costs. Cyclones are highly effective for particles $> 10 \mu m$ and are commonly used in woodworking, agricultural processing, and as pre-cleaners for baghouses.

Baghouses (Fabric Filters)

Baghouses, or fabric filters, are highly efficient control devices capable of removing more than 99% of particulate matter, including submicron particles ($PM_{2.5}$). The dirty gas flows through porous fabric bags, which trap the particles on the surface, forming a "dust cake." The dust cake itself does most of the filtering, achieving extremely high efficiencies.

A critical design parameter is the air-to-cloth ratio ($A/C$), which is the volumetric flow rate of gas ($Q$) divided by the total fabric area ($A$): $\text{A/C Ratio} = \frac{Q}{A}$ Also known as the filtration velocity, typical A/C ratios range from 1 to 10 ft/min depending on the cleaning mechanism and the type of dust.

As the dust cake builds up, the pressure drop across the filter increases according to Darcy's filter law: $\Delta P = \mu (K_1 v + K_2 W v)$ Where $K_1$ is the resistance of the clean fabric, $K_2$ is the specific resistance of the dust cake, $v$ is the filtration velocity, and $W$ is the areal dust loading.

To maintain continuous operation, baghouses utilize various cleaning mechanisms:

• Reverse-Air: Gentle cleaning where clean air is blown backward through the bags. Used for delicate fiberglass bags (e.g., in high-temperature applications).
• Pulse-Jet: A short, high-pressure burst of compressed air is injected into the bag, creating a shockwave that knocks the dust cake off. This allows for continuous cleaning without taking the compartment offline and permits higher A/C ratios.
• Shaker: Mechanical shaking of the bags to dislodge the dust. Requires the compartment to be isolated from the gas flow during cleaning.

Electrostatic Precipitators (ESPs)

Electrostatic Precipitators (ESPs) use electrical forces to remove particles. The gas stream passes between high-voltage discharge electrodes (wires) and grounded collection electrodes (plates). The high voltage creates a corona discharge that ionizes the gas, which in turn charges the particles. The electric field then drives the charged particles to the collection plates, where they are periodically removed by mechanical rapping.

The theoretical collection efficiency ($\eta$) of an ESP is given by the Deutsch-Anderson equation: $\eta = 1 - e^{-\frac{w A}{Q}}$ Where:

• $w$ = particle migration velocity (m/s)
• $A$ = total collection plate area ($\text{m}^2$)
• $Q$ = volumetric gas flow rate ($\text{m}^3\text{/s}$)

The migration velocity ($w$) represents how fast the charged particle moves toward the collection plate. ESPs are very efficient, can handle huge gas volumes with very low pressure drop, and can operate at high temperatures. However, they are sensitive to particle resistivity. If resistivity is too low, particles lose their charge and re-entrain into the gas stream. If resistivity is too high, particles insulate the collection plate, causing "back corona" which severely degrades performance.

Wet Scrubbers

Wet scrubbers for PM control use a liquid (usually water) to capture particles. The Venturi scrubber is the most common type for high-efficiency PM removal. In a Venturi scrubber, the gas accelerates through a converging section to a high-velocity "throat," where water is injected. The intense turbulence atomizes the water into tiny droplets, which collide with and capture the particles. The droplets are then removed from the gas stream by a cyclonic separator.

Venturi scrubbers are highly effective for fine particles and have the added advantage of handling sticky, flammable, or explosive dusts, as well as absorbing some gaseous pollutants simultaneously. The collection efficiency is directly proportional to the pressure drop across the Venturi throat. High-energy Venturi scrubbers can achieve $>99%$ efficiency for submicron particles but require pressure drops of 30 to 80 inches of water, resulting in substantial fan energy costs.