Lecture 6 Design of ESP

Transcription

1 Leture 6 Design of ES

2 DESIGN OF ELECTROSTATIC RECIITATOR Introdution An eletrostati preipitator (ES) is a partile ontrol devie that uses eletrial fores to move the partiles out of the flowing gas stream and onto olletor plates. The partiles are given an eletrial harge by foring them to pass through a orona, a region in whih gaseous ions flow [1]. INFORMATION REQUIRED FOR DESIGNING OF ES The effiieny of an ES depends upon two fators The size of the unit i.e. total square ft. of the olleting plate area Amount of independent eletrial energisation In addition following details are required for designing an ES 1. Soure of the emission : roperties of the proess by whih the pollutants are produed. artile size distribution 3. Chemial analysis of dust in relation to partile size 4. Speifi eleti resistivity of dust 5. Dust onentration of lean gas 6. Required dust onentration of lean gas(effiieny) 7. roperties of gas: omposition, temperature, pressure. 8. Corrosive properties of gas 9. Gas flow rate Apart from these variables the design of ES also inlude the determination of anillary fators suh as rappers to shake the dust loose from the plates, automati ontrol system, measures for ensuring high-quality gas flow, dust removal system, provisions for strutural and heat insulation and performane monitoring system []. Firstly size distribution of dust is determined; from the information of size distribution of dust the migration veloity is alulated. After that number of harge on a partile is alulated by using appropriate equation. On the basis of preipitation rate the olleting surfae area for a given effiieny at a partiular flow rate is alulated by using Deutsh-Andersen relationship.

3 ARTICLE CHARGING Aording to kinetis, the eletron energy Q d of an originally neutral dust partile is given by [3]: Q d d p KT d ne ln 1 E CNe KT (.6.1) Where, N=number of eletroni harges; d p =dust partile dia.; K=1.38*10-3 (J/K); T=Absolute Temp.; E= [C] eletrons; N=free ion density; C=(kT/m i ) 1/ Charging veloity is very fast at the beginning but slow down with time. Qd ned NeCt (.6.) 4 Field Charging: Q f ne r 9 E (.6.3) Where, n=number of eletroni harge; r p =Radius of partile; E=Average field intensity, E=1.60*10-19 MIGRATION ELOCITY The veloity of harged partile suspended in a gas under the influene of an eletri field is known as migration veloity. The partile migration veloity is the most important parameter and is funtion of a large number of operation quantities suh as- Eletri field strength, partile size, gas visosity, properties of the dust [3]. rinipal fores ating on partile are gravitational fore, eletri fore, visous fore and inertial fore. pm qec 3d (.6.4) Where, pm =artile migration veloity towards the olletor eletrode; q=ne, value of n depends upon types on harging (diffusion or field); C=Cunningham orretion fator; E=Colletor eletri field; =gas visosity; and d=partile size (µm).

4 Table.6.1 Effetive migration veloity (m/s) for various type of dusts. Dust Migration eloity (m/s) Zin Oxide Sulfuri Aid Metal Oxides Calium Carbonate Smoke Fume pit oal furnae Fly ash from lignite furnae Blast furnae dust 0.05 Smelter dust Blast furnae dust 0.05 COLLECTION EFFICIENCY OF ES The olletion effiieny of an ES as a funtion of gas flow rate and preipitator size is given by the Deutsh-Andersen Equation Assumptions: a. Repulsion effet is negleted. b. Uniform gas veloity throughout the ross setion. artiles are fully harged by field harging. d. No hindered settling effet. Colletion effiieny mathematially expressed as follows [3]: pm A 1 exp Q (.6.5) =Frational Colletion Effiieny; A =Area of the olletion eletrode; pm =artile migration veloity; Q=Av=olumetri flow rate of gas; v=gas veloity. 1 exp A L v pm Where, =olume of preipitator. For ylindrial type olletor: (.6.6)

5 A 4 D For parallel plate: A S (.6.7) (.6.8) Where, S=distane between the two parallel plates; =Gas olume; W=reipitation Rate arameter; A=late Area; Q=olumetri Flow Rate of Gas. When the harged partile passing a harge q p is in a region where an eletri field strength of E is present, a fore F will at on partile. Fq E (.6.9) The migration of partile towards the olletor is resisted by a drag fore and the net fore on the partile is zero when it moves with a onstant drift veloity (v pm ) q E C D d pg 8C pm For small partile stokes law is appliable. Hene C D 4g d g pm (.6.10) (.6.11) Substitute C D in above equation it gives v pm pm qec 3 d g qca Em 1exp 3Q (.6.1) Where, q p =artile harge; A = olletor surfae area; E m =eletri field strength; d=partile size; µ=gas visosity 0.55d C Cunningham orretion fator e d For standard air, λ=0.066µm For standard air, above equation beomes (.6.13)

6 5766q CA Em 1 exp Qd (.6.14) Colletion effiieny of different types of preipitators Ψ=(Mp1-Mp)/Mp1. (.6.15) Where, Mp1 and Mp being the mass of dust per unit volume of the gas stream at the entrane and exit of the preipitator. The volumetri flow rate g in a preipitator by the equation g d v 4 For a plate preipitators g (.6.16) n a h v (.6.17) Where; h=height of the plate whih is equal to the height of the hannels The equation is related to gas veloity v and number n of the element for pipe preipitators. If partiles are of a solid, the olleted partiles are removed from eletrode by shaking it in a proess known as rapping. If partiles are of a liquid, after olleting on the eletrode the liquid then flows down the eletrode by ation of gravity and ollets at the bottom. The partile harging proess is done by means of orona surrounding a highly harged eletrode, suh as wire [4]. DUST RESISTIITY Dust resistivity is the most important dust property. With high dust resistant a large voltage in the dust layer is observed aumulated by a dereasing urrent. Speifi Dust Resistivity: is the resistivity of a layer of dust will a layer thikness of 1 m over a olletion area of 1m The Speifi dust resistivity is designed the symbol ρ rs and measured in Ωm rs A R m (.6.18) Where, A =olletion area; R m =mean eletri resistivity; δ=dust layer thikness.

7 reipitation rate Channel width, D arameter late area Speifi olletion area Gas flow rate Gas veloity u Aspet ratio R= Corona power ratio Dutlength Height Table.6.1. Ranges for ES design parameters Range of values m/min m m /(m 3 /min) m/s (Not less than 1 for >99%) W/(m 3 /min) Corona urrent ratio µa/m late area per eletrial set m Number of eletrial setion Ns a. In the diretion of gas flow -8 b. Total number of setions 1-10 bus setions/(1000 m 3 /min) Spaing between setions 0.5-m L en, L ex -3m late height; length 8-15m; 1-3m roblem.6.1: A plate type ES use in a ement plant for removing dust partiles onsist of 10 equal hannels. The spaing between plates is 15 m and the plates are 3 m high and 3 m long. Unit handles 0,000 m 3 /h of gas. a) What is the effiieny of olletion plates? b) What is the olletion rate of partiles having density 9. gm/m 3? ) What should be the length of the plate for ahieving effiieny of 99% keeping other parameter same? Solution: 1 exp A Qg n h l pm A

8 q Qg Given: artile migration veloity ( pm )=0.1 m/se; Number of plates (n)=10; Height of plate (h)=3 m; Length of plate (l)=3 m; Gas flow rate (Q g )=0000 m 3 /h=.7778 m 3 /se; Density of partile (ρ)=9. g/m 3. Total area of olletion plates (A )= 10 3 m 3 m=180 m exp % The olletion rate q g / se Let us assume the length of plate is l m. Therefore, the total olletion area of plate (A ) beomes 40 l m. The length of plate an be obtained from following equation: 0.140l exp l m a) The effiieny of olletion plate is 96.08%. b) The olletion rate of partiles having density is g/se. ) Length of the plate for ahieving effiieny of 99% keeping other parameter same is m. roblem.6.: An ES handles 10 7 ft 3 /min of gas. It uses 3.6 Amp urrent and has 8000 ft olletion plate areas. At the present operating temperature, the dust resistivity is Ω-m. It has been suggested that the gas ooled to redue the dust resistivity to Ω-m assuming that average dust thikness is 0.45 inh and that voltage differene between the harging walls and outer surfae of the dust layer is 30 k must be maintained in both ases. Estimate the redution in power requirement that ooling the gas to get neglet the effet of gas temperature on harging and drift veloity. Solution: I A I

9 Given: Gas flow rate (Q g )=10 7 ft 3 /min; Current (I)=3.6 A; Total area of olletion plates (A )=8000 ft = m ; Dust resistivity ( 1 )= Ω-m; Dust resistivity ( )= Ω-m; Dust thikness (=0.45 inh=1.143 m. Assuming that 1 is the voltage when dust resistivity is 1 (k) and voltage is when dust resistivity is (k) and is the power input (kw) For a dust resistivity of Ω-m, I k 6 A Total applied voltage is 30 k k= k Therefore, the power input when dust resistivity is 1 11I kw For a dust resistivity of Ω-m, I A k Total applied voltage is 30 k k= k The power input I kw Therefore by ooling of gas, power input gets redued to Δ= 1 -. Δ= = kw. REFERENCES [1] [] Liu, D. H. F., Liptak, B. G., Bous,. A. Environmental Engineering Handbook, nd edition, CRC ress, LLC, Florida, [3] Theodore, L. Eletrostati reipitators in Air ollution Control Equipment Calulations, Wiley, 008. [4] eavy, H. S., Rowe, D. R., Thobanoglous, G. Environmental Engineering, MGraw- Hill, 1985.