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 • Chem. Composition • Resistance T,P,Q, Chem. Composition 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 Mi Me Li Le calculated on mass h Mi Li 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: h h where hj: collection efficiency for the jth size mj: mass percent of particles in the jth size j mj Example 3.1 from the book PM Control Devices 1. 2. 3. 4. 5. Gravity Settler Cyclones ESP Filters and Baghouses Wet Scrubbers Settling Chamber • Efficient for particles with diameter of 1050 mm (depending on its density) • Velocity through chamber < 0.3-3 m/s (to prevent reentrainment) V H L Settling Chamber v H L • Settling time < transit time through chamber vH • t = H/vt = L/v v (d ) t p vt ( d p ) L Q Lw Settling chambers are cheap to build and operate but not preferred due to their large space requirement Settling Chamber • Assuming unit density sphere at STP, vt and chamber Lw are tabulated below: Assumed flow rate Q = 150 m3/min vt ( d p ) Q Lw Dp (um) 0.1 Vt (m/s) 8.6 (10)-7 Required Area 3 km2 0.5 1.0 5 1.0 (10)-5 3.5 (10)-5 7.8 (10)-4 0.25 km2 71000 m2 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 Cyclone Theory dragforce 3m d pV t mV i 2 D /2 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 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 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: hj 1 1 ( d pc / d pj ) 2 Example 4.5 Example 4.5 Example 4.5 ESP ESP ESP Geometry ESP Theory Corona Power vrs Efficiency ESP Theory ESP THEORY ESP Theory ESP Theory ESP Theory ESP Theory ESP Theory ESP Theory ESP Theory ESP Theory ESP Theory Efficiency Efficiency 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 Sparking 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 Drift velocity Channel (Duct) Width, D Range of Values 1-10 m/min 15-40 cm Specific Collection Area Plate area/Gas Flow Gas velocity u 0.25-2.1 m2/(m3/min) Aspect Ratio (R) Duct Length/Plate Height Corona Power Ratio Pc/Q 0.5-1.5 Corona Current Ratio (Ic/A) 50-750 mA/m2 Plate area per electrical set As Number of electrical sections • Table 5.1 460-7400 m2 2-6 1.2-2.5 m/s 1.75-17.5 W/(m3/min) 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 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. Collection Efficiency vrs Particle Diameter An Example Example Example POWER REQUIREMENT 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 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 SOLUTION 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