Particle Control Technologies Lecture notes adapted from Prof. Dr. Benoit Cushman-Roisin Thayer School of Engineering at Dartmouth

Design Criteria Design of a system which remove solid or liquid particulate matter from a gaseous medium Chacacteristics of gaseous medium and particulate matter to consider in the design: Size T,P,Q, Chem. Composition Chem. Composition Resistance Pressure Drop PM Control Device

Collection Efficiency Considering the wide range of size of particulates, efficiency will be different for each size. The overall efficiency (h) can be calculated on a basis of total number (or mass) of particles Generally regulations are written based on mass, and efficiencies are calculated on mass basis.

Collection Efficiency Efficiencies calculated on mass basis: h: overall collection efficiency (fraction) Mi: total mass input rate (g/s or equivalent) Me: total mass emission rate (g/s or equivalent) Li: particulate loading in the inlet gas to the device (g/m3) Le:particulate loading in the exit gas stream, (g/m3)

Collection Efficiency When the particulate size distribution is known, and the efficiency of the device is known as a function of particle size, the overall collection efficiency can be calculate: where hj: collection efficiency for the jth size mj: mass percent of particles in the jth size

Example 3.1 from the book

PM Control Devices Gravity Settler Cyclones ESP Filters and Baghouses Wet Scrubbers

Settling Chamber Efficient for particles with diameter of 10-50 mm (depending on its density) Velocity through chamber < 0.3-3 m/s (to prevent reentrainment) V H L

Settling Chamber Settling time < transit time through chamber v H L Settling time < transit time through chamber t = H/vt = L/v  Settling chambers are cheap to build and operate but not preferred due to their large space requirement

Settling Chamber Dp (um) Vt (m/s) Required Area 0.1 8.6 (10)-7 3 km2 Assuming unit density sphere at STP, vt and chamber Lw are tabulated below: Assumed flow rate Q = 150 m3/min Dp (um) Vt (m/s) Required Area 0.1 8.6 (10)-7 3 km2 0.5 1.0 (10)-5 0.25 km2 1.0 3.5 (10)-5 71000 m2 5 7.8 (10)-4 3200 m2 10 3.1 (10)-3 810 m2

Settling Chamber Baffled Settling Chamber Large particles can not make sudden direction change and settle into dead space of chamber Baffle chambers are used as precleaners

Cyclones :

Cyclones :

Cyclone Geometry

Cyclone Geometry

Cyclone Theory

Cyclone Theory 2 3

Cyclone Theory

Cyclone Theory

Collection Efficiency

Collection Efficiency (i) increase Vt (expensive, since DP a Vt2, as we will see in the next slides

Collection Efficiency

Collection Efficiency

Pressure Drop =Hv(rgasu2)/2Q K: a constant depends on cyclone configuration and operating conditions. Theoretically K can vary considerably but for air pollution work with standard tangential-entry cyclones values of K are in the range of 12 to 18 Power needed = Energy per unit volume of gas x volume of gas per unit time Power=DPxQ =Hv(rgasu2)/2Q Cyclone pressure drops range from about 0.5 to 10 velocity heads (250 to 4000 Pa)

Cyclone Analysis

Example

Example Conventional Type (No:3) N=(1/H) (Lb+Lc/2) = (1/0.5)(2+2/2)=6 Vi=Q/WH =150 /(0.25*0.5) =1200 m/min 20 Vi=20 m/s

Example Calculate efficiency for each size range witch dpc = 5.79 um:

Example 4.5

Example 4.5

Example 4.5

ESP

ESP

ESP Geometry

ESP Theory

ESP Theory

+ - - + Time = 0 Time = t1, x=x1 e M P e M M+ P e-+MM+ + 2e- e e e e Negative electrode Positive Plate e e e M e e e Electrons are accelerated to the plate, on the way collide with gas molecules (much less frequently to particles) If electron carries sufficient energy ionize the molecule, produce 2nd free electron.By this way, each seed e- can create thousands of e- and M+ e

ESP Theory

- + + - Inter Electrode Area Particle Charging e M M+ P M- P- e M M- P Gas molecules away from the negative electrode, they start slowing down. Instead colliding with them they bump up to them and are captured. e P e e M- P Negative electrode e e e e P e e e e P M- e e Next , negative gas ions stick to the particles. Small particles (Dp<1 um) can absorb tens of ions, larger ones with tens of thousands of ions acquire. Now molecules are negatively charged and they too want to move to the positive plate.

ESP Theory

Charging of Particles Ions are transported by the electiric field and/or thermal diffusion. Field Charging: Ions are transported to particles along with the field lines as shown in left figure. Important mechanism for larger particles (dp>2 um) The ions will continue to bombard a particle until the charge on that particle is sufficient to divert the electric lines away from it. A particle is calle «saturated « when it no longer receives an ion charge.

Charging 2. Diffusion charging: random Brownian motion of the negative gas ions charges the particles. The random motion is related to the velocity of the gas ions due to thermal effects, the higher the temperature, the more movement. collision with gas ions due to thermal Brownial motion. Important for smaller particles (dp<0.2 um)

Particle Collection Particles reached to the plate can be removed either by water sprays or rapping of the plates In rapping deposited dry particles are dislodged from the plates by sending mechanical impulses or vibrations to the plates. The plates are rapped while the ESP is on-line. Rapping is applied when the dust layer is relatively thick (0.08 cm-1.27 cm) so that they will fall off as large aggregate sheets (no dust reentrainment) Dust is collected in the hoppers. Dust should be removed from here as soon as possible to avoid dust packing which makes removal very difficult. Charges are slowly leaked to the grounded collection plate. Particles are held onto plates by intermolecular cohesive and adhesive forces

Effect of Resistivity

Resistivity Resistivity (P) is resistance to electrical conduction and can vary widely P of a material is determined experimentally by establishing a current flow through a slab (of known geometry) of the material P = (RA/L)=(V/i)(A/L) [ohm-cm] R:resistance, ohm A: area normal to the current flow, cm2 L:path length in the direction of current flow, cm, V: voltage, i: current, A

Resistivity

Resistivity

Resistivity

Sparking

ESP THEORY

ESP Theory

ESP Theory Amount of charge of the particle controls its velocity. What is the maximum charge a particle can carry? As the more charge a particle collects, it produces its own electiric field and charging process slows down due to growing repulsion. The maximum surface charge, qsat, is reached when the electric field at particle surface is zero. Saturated charge for a sphere with a diameter of dp : qsat increases with E and dp2

ESP Theory

ESP Theory

ESP Theory

Efficiency

Efficiency

Collection Efficiency vrs Particle Diameter

Internal Configuration Internal configuration design is more art than science The even distribution of gas flow through the ducts is very important to the proper operation of an ESP The number of ducts (Nd) is equal to one less than the number of plates (n-1) Nd = Q/uDH (eq 5.15) u: linear gas velocity (m/min) D: Channel width (plate separation), m H: plate height, m

Internal Configuration At the start of the design, use 5.15 to estimate Nd by assuming a value for H and choosing representative values of u and D

Typical Values for the Fly-Ash ESP Parameter Range of Values Drift velocity 1-10 m/min Channel (Duct) Width, D 15-40 cm Specific Collection Area Plate area/Gas Flow 0.25-2.1 m2/(m3/min) Gas velocity u 1.2-2.5 m/s Aspect Ratio (R) Duct Length/Plate Height 0.5-1.5 Corona Power Ratio Pc/Q 1.75-17.5 W/(m3/min) Corona Current Ratio (Ic/A) 50-750 mA/m2 Plate area per electrical set As 460-7400 m2 Number of electrical sections 2-6 Table 5.1

Internal Configuration The overall length of the precipitator (Lo) Lo=NsLp + (Ns-1)Ls + Len +Lex Lp:: length of plate Ls: spacing between electrical sections (0.5-1.0 m) Len: entrance section in length (several meters) Lex:exit section in length (several meters) Ns: number of mechanical fields Ns ranges between 2 and 6. Ns=RH/Lp R is the aspect ratio (Duct Length/Plate Height)

Internal Configuration When the numbers of ducts and sections have been specified, the actual collection area (Aa) can be calculated as: Aa=2HLpNsNd During the design process several plate sizes and numbers of ducts are tried until one combination is found such that Aa is equal to the required collection area.

ESP Types Collection Plate Tubular Precipitator

ESP Types The method of charging Single-stage Two-stage (Charging and collection is in different places. Prefererd when dust loadings less than 7.35 mg/m3 and to collect finely divided liquid particles

ESP Types The temperature of operation Cold-side (T<204 C) Good for high sulfur coal. Hot-side Good for low sulfur coal boilers since high resistivity of these particles can be lowered by higher temperatures

ESP Types The method of particle removal from collection surfaces Wet Dry

An Example

Example

Example

POWER REQUIREMENT Power requirement: DP across ESP is very small (< 1 in. H2O). Therefore Corona is the main power requirement: Corona Power (W) = IcxV Ic:Corona current in Coul/s (amper) V:Average voltage (a function of resistivity) Corona power input may range from 50 to 500 W per 1000 cfm

POWER REQUIREMENT k: an adjustable constant in the range of 0.5-0.7 for we in ft/sec and Pc/A in W/ft2

Corona Power vrs Efficiency, Figure 5.9

Problem 5.10 Provide a reasonable design for a 99.4% efficient ESP treating 30,000 m3/min of gas. The dust has a resistivity of 7.1 (10)10 ohm-cm. Specify the total plate area, channel width, number and size of plates, number of electrical sections (total and in the direction of flow), and total corona power to be supplied, and estimate the overall dimensions.

SOLUTION

SOLUTION

SOLUTION H can be 1.5 to 3 times of plate height including hoppers, controls, and so forth.

SOLUTION

http://www.youtube.com/watch?v=y5w0IGuLR3A ** Video Demonstration on Electrostatic Precipitation http://www.youtube.com/watch?v=y5w0IGuLR3A ** http://www.youtube.com/watch?v=9kauc7OmmLQ http://www.youtube.com/watch?v=x5YFK8mmeRQ http://www.youtube.com/watch?v=BdRk3op2zpE ** http://www.youtube.com/watch?v=iUXHzYLgrB0