BiographyName :Pichai Chaibamrung BASIC DESIGN OF CIRCULATING FLUIDIZED BED BOILER 8 FEBRUARY 2012 Education 2009-2011, Ms.c, Thai-German Graduate School of Engineering 2002-2006, B.E, Kasetsart Univesity Pichai Chaibamrung Asset Optimization Engineer Reliability Maintenance Asset Optimization Section Energy Division Thai Kraft Paper Industry Co.,Ltd. Work Experience Jul 11- present : Asset Optimization Engineer, TKIC May 11- Jun 11 : Sr. Mechanical Design Engineer, Poyry Energy Sep 06-May 09 : Engineer, Energy Department, TKIC Email:
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[email protected] By Chakraphong Phurngyai :: Engineer, TKIC Content 1. Introduction to CFB 2. Hydrodynamic of CFB 3. Combustion in CFB 4. Heat Transfer in CFB 5. Basic design of CFB 6. Cyclone Separator Objective • • • • • • To understand the typical arrangement in CFB To understand the basic hydrodynamic of CFB To understand the basic combustion in CFB To understand the basic heat transfer in CFB To understand basic design of CFB To understand theory of cyclone separator By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 1. Introduction to CFB 1.1 Development of CFB 1.2 Typical equipment of CFB 1.3 Advantage of CFB 1.1 Development of CFB • 1921, Fritz Winkler, Germany, Coal Gasification • 1938, Waren Lewis and Edwin Gilliland, USA, Fluid Catalytic Cracking, Fast Fluidized Bed • 1960, Douglas Elliott, England, Coal Combustion, BFB • 1960s, Ahlstrom Group, Finland, First commercial CFB boiler, 15 MWth, Peat By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 1.2 Typical Arrangement of CFB Boiler • CFB Loop - Furnace or Riser - Gas – Solid Separation (Cyclone) - Solid Recycle System (Loop Seal) • Convective or Back-Pass - Superheater - Reheater - Economizer - Air Heater 1.2 Typical Arrangement of CFB Boiler By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 1.2 Typical Arrangement of CFB Boiler 1.2 Typical Arrangement of CFB Boiler • Air System - Primary air fan (PA. Fan) - Secondary air fan (SA. Fan) - Loop seal air fan or Blower By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 1.2 Typical Arrangement of CFB Boiler • Flue Gas Stream - Induced draft fan (ID. Fan) 1.2 Typical Arrangement of CFB Boiler • Solid Stream - Fuel Bunker - Bed Bunker - Sorbent Bunker - Bottom ash Bunker - Fly ash Bunker Feed Drain By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 1.2 Typical Arrangement of CFB Boiler • Water- Steam Circuit - Economizer - Steam drum - Evaporator - Superheater 1.3 Advantage of CFB Boiler • Fuel Flexibility By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 1.3 Advantage of CFB Boiler • High Combustion Efficiency - Good solid mixing - Low unburned loss by cyclone, fly ash recirculation - Long combustion zone • In situ sulfur removal • Low nitrogen oxide emission 1.3 Advantage of CFB Boiler • In Situ Sulfur Removal Calcination Sulfation By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 1.3 Advantage of CFB Boiler • Low Nitrogen Oxide Emissions 2. Hydrodynamic in CFB 2.1 Regimes of Fluidization 2.2 Fast Fluidized Bed 2.3 Hydrodynamic Regimes in CFB 2.4 Hydrodynamic Structure of Fast Beds By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 2.1 Regimes of Fluidization • Fluidization is defined as the operation through which fine solid are transformed into a fluid like state through contact with a gas or liquid. 2.1 Regimes of Fluidization • Particle Classification Distribution Foster 100% 75% 50% 25% 100% <600 <250 <180 <130 Size (micron) HGB <1000 <550 <450 <250 >100 PB#15 <1680 <1190 <840 <590 >420 By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 2.1 Regimes of Fluidization • Particle Classification 2.1 Regimes of Fluidization • Comparison of Principal Gas-Solid Contacting Processes By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 2.1 Regimes of Fluidization • Packed Bed The pressure drop per unit height of a packed beds of a uniformly size particles is correlated as (Ergun,1952) 2.1 Regimes of Fluidization • Bubbling Fluidization Beds Minimum fluidization velocity is velocity where the fluid drag is equal to a particle’s weight less its buoyancy. Where U is gas flow rate per unit cross section of the bed called Superficial Gas Velocity By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 2.1 Regimes of Fluidization • Bubbling Fluidization Beds For B and D particle, the bubble is started when superficial gas is higher than minimum fluidization velocity But for group A particle the bubble is started when superficial velocity is higher than minimum bubbling velocity 2.1 Regimes of Fluidization • Turbulent Beds when the superficial is continually increased through a bubbling fluidization bed, the bed start expanding, then the new regime called turbulent bed is started. By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 2.1 Regimes of Fluidization 2.1 Regimes of Fluidization • Terminal Velocity Terminal velocity is the particle velocity when the forces acting on particle is equilibrium By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 2.1 Regimes of Fluidization • Freeboard and Furnace Height - considered for design heating-surface area - considered for design furnace height - to minimize unburned carbon in bubbling bed the freeboard heights should be exceed or closed to the transport disengaging heights 2.2 Fast Fluidization • Definition By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 2.2 Fast Fluidization • Characteristics of Fast Beds - non-uniform suspension of slender particle agglomerates or clusters moving up and down in a dilute - excellent mixing are major characteristic - low feed rate, particles are uniformly dispersed in gas stream - high feed rate, particles enter the wake of the other, fluid drag on the leading particle decrease, fall under the gravity until it drops on to trailing particle 2.3 Hydrodynamic regimes in a CFB Cyclone Separator : Swirl Flow Back Pass: Pneumatic Transport Furnace Upper SA: Fast Fluidized Bed Lower Furnace below SA: Turbulent or bubbling fluidized bed Return leg and lift leg : Pack bed and Bubbling Bed By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 2.4 Hydrodynamic Structure of Fast Beds • Axial Voidage Profile Secondary air is fed 2.4 Hydrodynamic Structure of Fast Beds • Velocity Profile in Fast Fluidized Bed Bed Density Profile of 135 MWe CFB Boiler (Zhang et al., 2005) By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 2.4 Hydrodynamic Structure of Fast Beds • Velocity Profile in Fast Fluidized Bed 2.4 Hydrodynamic Structure of Fast Beds • Particle Distribution Profile in Fast Fluidized Bed By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 2.4 Hydrodynamic Structure of Fast Beds • Particle Distribution Profile in Fast Fluidized Bed 2.4 Hydrodynamic Structure of Fast Beds • Particle Distribution Profile in Fast Fluidized Bed Effect of SA injection on particle distribution by M.Koksal and F.Hamdullahpur (2004). The experimental CFB is pilot scale CFB. There are three orientations of SA injection; radial, tangential, and mixed By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 2.4 Hydrodynamic Structure of Fast Beds • Particle Distribution Profile in Fast Fluidized Bed Increasing SA to 40% does not significant on suspension density above SA injection point but the low zone is denser than low SA ratio Increasing solid circulation rate effect to both lower and upper zone of SA injection point which both zone is denser than low solid circulation rate 2.4 Hydrodynamic Structure of Fast Beds • Effects of Circulation Rate on Voidage Profile No SA, the suspension density i s prop ortional l to solid circulation rate With SA 20% o f PA, the solid particle is hold up when compare to no SA higher solid recirculation rate By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 2.4 Hydrodynamic Structure of Fast Beds • Effects of Circulation Rate on Voidage Profile 2.4 Hydrodynamic Structure of Fast Beds • Effect of Particle Size on Suspension Density Profile - Fine particle - - > higher suspension density - Higher suspension density - - > higher heat transfer - Higher suspension density - - > lower bed temperature Pressure drop across the L-valve is proportional to solid recirculation rate higher solid recirculation rate By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 2.4 Hydrodynamic Structure of Fast Beds • Effect of Bed Inventory on Suspension Density Profile 2.4 Hydrodynamic Structure of Fast Beds • Core-Annulus Model - the furnace may be spilt into two zones : core and annulus Core - Velocity is above superficial velocity - Solid move upward Annulus - Velocity is low to negative - Solids move downward core annulus By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 2.4 Hydrodynamic Structure of Fast Beds • Core-Annulus Model 2.4 Hydrodynamic Structure of Fast Beds • Core Annulus Model - the up-and-down movement solids in the core and annulus sets up an internal circulation - the uniform bed temperature is a direct result of internal circulation core annulus By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 3. Combustion in CFB 3.1 Stage of Combustion 3.2 Factor Affecting Combustion Efficiency 3.3 Combustion in CFB 3.4 Biomass Combustion 3.1 Stage of Combustion • A particle of solid fuel injected into an FB undergoes the following sequence of events: - Heating and drying - Devolatilization and volatile combustion - Swelling and primary fragmentation (for some types of coal) - Combustion of char with secondary fragmentation and attrition By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 3.1 Stages of Combustion • Heating and Drying - Combustible materials constitutes around 0.5-5.0% by weight of total solids in combustor - Rate of heating 100 °C/sec – 1000 °C/sec - Heat transfer to a fuel particle (Halder 1989) 3.1 Stages of Combustion • Devolatilization and volatile combustion - first steady release 500-600 C - second release 800-1000C - slowest species is CO (Keairns et al., 1984) - 3 mm coal take 14 sec to devolatilze at 850 C (Basu and Fraser, 1991) By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 3.1 Stages of Combustion • Char Combustion 2 step of char combustion 1. transportation of oxygen to carbon surface 2. Reaction of carbon with oxygen on the carbon surface 3 regimes of char combustion - Regime I: mass transfer is higher than kinetic rate - Regime II: mass transfer is comparable to kinetic rate - Regime III: mass transfer is very slow compared to kinetic rate 3.1 Stage of Combustion • Communition Phenomena During Combustion Attrition, Fine particles from coarse particles through mechanical c ontract like abrasion with other particles Volatile release in non-porous particle cause the high internal pressure result in break a coal particle into fragmentat ion Volatile release cause the particle swell Char burn under regime I which is m ass transfer is higher than kinetic trasfer. The sudden collapse or other type of sec ond fr agmentation call percolative fragmentation occurs Char burn under regime I, II, the pores increases in size à weak bridge connection of carbon until it can’t withstand the hydrodynamic force. It will fragment again call “ secondary fragmentation” By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 3.2 Factor Affecting Combustion Efficiency • Fuel Characteristics the lower ratio of FC/VM result in higher combustion efficiency (Makansi, 1990), (Yoshioka and Ikeda,1990), (Oka, 2004) but the improper mixing could result in lower combustion efficiency due to prompting escape of volatile gas from furnace. 3.2 Factor Affecting Combustion Efficiency • Operating condition (Bed Temperature) - higher combustion temperature --- > high combustion efficiency Limit of Bed temp -Sulfur capture -Bed melting -Water tube failure High combustion temperature result in high oxidation reaction, then burn out time decrease. So the combustion efficiency increase. By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 3.2 Factor Affecting Combustion Efficiency • Fuel Characteristic (Particle size) 3.2 Factor Affecting Combustion Efficiency • Operating condition (superficial velocity) - high fluidizing velocity decrease combustion efficiency because Increasing probability of small char particle be elutriated from circulation loop -The effect of this particle size is not clear -Fine particle, low burn out time but the probability to be dispersed from cyclone the high -Coarse size, need long time to burn out. -Both increases and decreases are possible when particle size decrease - low fluidizing velocity cause defluidization, hot spot and sintering By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 3.2 Factor Affecting Combustion Efficiency • Operating condition (excess air) - combustion efficiency improve which excess air < 20% Combustion loss decrease significantly when excess air < 20%. 3.2 Factor Affecting Combustion Efficiency • Operating Condition The highest loss of combustion result from elutriation of char particle from circulation loop. Especially, low reactive coal size smaller than 1 mm it can not achieve complete combustion efficiency with out fly ash recirculation system. However, the significant efficiency improve is in range 0.0-2.0 fly ash recirculation ratio. Excess air >20% less significant improve combustion efficiency. By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 3.3 Combustion in CFB Boiler • Lower Zone Properties - This zone is fluidized by primary air constituting about 40-80% of total air. - This zone receives fresh coal from coal feeder and unburned coal from cyclone though return valve - Oxygen deficient zone, lined with refractory to protect corrosion - Denser than upper zone 3.3 Combustion in CFB Boiler • Upper Zone Properties - Secondary is added at interface between lower and upper zone - Oxygen-rich zone - Most of char combustion occurs - Char particle could make many trips around the furnace before they are finally entrained out through the top of furnace By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 3.3 Combustion in CFB Boiler • Cyclone Zone Properties - Normally, the combustion is small when compare to in furnace - Some boiler may experience the strong combustion in this zone which can be observe by rising temperature in the cyclone exit and loop seal 3.4 Biomass Combustion • Fuel Characteristics - high volatile content (60-80%) - high alkali content à sintering, slagging, and fouling - high chlorine content à corrosion By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 3.4 Biomass Combustion • Agglomeration SiO2 melts at 1450 C Eutectic Mixture melts at 874 C 3.4 Biomass Combustion • Options for Avoiding the Agglomeration Problem - Use of additives - china clay, dolomite, kaolin soil - Preprocessing of fuels - water leaching - Use of alternative bed materials - dolomite, magnesite, and alumina - Reduction in bed temperature Sintering tendency of fuel is indicated by the following (Hulkkonen et al., 2003) By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 3.4 Biomass Combustion • Agglomeration 3.4 Biomass Combustion • Fouling - is sticky deposition of ash due to evaporation of alkali salt - result in low heat transfer to tube By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC PB#11 : Fouling Problem (7 Aug 2010) 1.Front water wall upper opening inlet - Overlay tube (26Tubes) - Replace refractory August 2010 4. Screen tube & SH#3 Slag PB11 Fouling May2010 6 months Aug2010 2 months Oct2010 2 months 2.Right water wall - Change new tubes (4 Tubes) May 2010 Aug 2010 3.Front water wall - Add refractory 2 m. (Height) above kick-out 5.Roof water wall -Change new tubes (4 Tubes) - Overlay tube - More erosion rate 1.5 mm/2.5 months Severe problem in Superheat tube fouling •Waste reject fuel (Hi Chloride content) •Only PB11 has this problems •this problems also found on PB15 (SD for Cleaning every 3 months) By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 3.4 Biomass Combustion • Corrosion Potential in Biomass Firing - hot corrosion - chlorine reacts with alkali metal à from low temperature melting alkali chlorides - reduce heat transfer and causing high temperature corrosion Foster Wheeler experience Wood/Forest Residual Straw,Rice husk Waste Reject By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 3.5 Performance Modeling • Performance of Combustion - Unburned carbon loss - Distribution and mixing of volatiles, char and oxygen along the height and cross section of furnace - Flue gas composition at the exit of the cyclone separator (NOx,SOx) - Heat release and absoption pattern in the furnace - Solid waste generation 4. Heat Transfer in CFB 4.1 Gas to Particle Heat Transfer 4.2 Heat Transfer in CFB By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 4.1 Heat Transfer in CFB Boiler • Mechanism of Heat Transfer In a CFB boiler, fine solid particles agglomerate and form clusters or stand in a continuum of generally up-flowing gas containing sparsely dispersed solids. The continuum is called the dispersed phase, while the agglomerates are called the cluster phase. The heat transfer to furnace wall occurs through conduction from particle clusters, convection from dispersed phase, and radiation from both phase. 4.1 Heat Transfer in CFB Boiler • Effect of Suspension Density and particle size Heat transfer coefficient is proportional to the square root of suspension density By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 4.1 Heat Transfer in CFB Boiler • Effect of Fluidization Velocity 4.1 Heat Transfer in CFB Boiler • Effect of Fluidization Velocity No effect from fluidization velocity when leave the suspension density constant By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 4.1 Heat Transfer in CFB Boiler • Effect of Fluidization Velocity 4.1 Heat Transfer in CFB Boiler • Effect of Vertical Length of Heat Transfer Surface By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 4.1 Heat Transfer in CFB Boiler • Effect of Bed Temperature 4.1 Heat Transfer in CFB Boiler • Heat Flux on 300 MW CFB Boiler (Z. Man, et. al) By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 4.1 Heat Transfer in CFB Boiler • Heat transfer to the walls of commercial-size 4.1 Heat Transfer in CFB Boiler • Circumferential Distribution of Heat Transfer Coefficient Low suspension density low heat transfer to the wall. By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 5 Design of CFB Boiler • • • • • 5.1 Design and Required Data 5.2 Combustion Calculation 5.3 Heat and Mass Balance 5.4 Furnace Design 5.5 Heat Absorption 5.1 Design and Required Data • The design and required data normally will be specify by owner or client. The basic design data and required data are; Design Data : - Fuel ultimate analysis - Weather condition - Feed water quality - Feed water properties Required Data : - Main steam properties - Flue gas emission - Flue gas temperature - Boiler efficiency By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 5.2 Combustion Calculation • Base on the design and required data the following data can be calculated in this stage : - Fuel flow rate - Combustion air flow rate - Fan capacity - Fuel and ash handling capacity - Sorbent flow rate 5.3 Heat and Mass Balance • Heat Balance Heat input Main steam Heat output Radiation Feed water Blow down Flue gas Moisture in fuel and sorbent Fuel and sorbent Unburned in fly ash Combustion air Unburned in bottom ash Moisture in combustion air By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 5.3 Heat and Mass Balance • Mass Balance Mass input Mass output 5.4 Furnace Design • 1. 2. 3. The furnace design include: Furnace cross section Furnace height Furnace opening 1. Furnace cross section Criteria - moisture in fuel - ash in fuel - fluidization velocity - SA penetration - maintain fluidization in lower zone at part load Make up bed material Solid Flue in Flue gas Solid gas Fuel and sorbent Moisture in fuel and sorbent Fuel and sorbent Make up bed material bottom ash bottom ash By Chakraphong Phurngyai :: Engineer, TKIC fly ash fly ash By Chakraphong Phurngyai :: Engineer, TKIC 5.4 Furnace Design 2. Furnace height Criteria - Heating surface - Residual time for sulfur capture 3. Furnace opening Criteria - Fuel feed ports - Sorbent feed ports - Bed drain ports - Furnace exit section 6. Cyclone Separator • 6.1 Theory • 6.2 Critical size of particle By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 6.1 Theory • The centrifugal force on the particle entering the cyclone is 6.1 Theory • The drag force on the particle can be written as • Vr can be considered as index of cyclone efficiency, from above equation the cyclone efficiency will increase for : - Higher entry velocity - Large size of solid - Higher density of particle - Small radius of cyclone - Higher value of viscosity of gas • Under steady state drag force = centrifugal force By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC 6.2 Critical size of particle • The particle with a diameter larger than theoretical cut-size of cyclone will be collected or trapped by cyclone while the small size will be entrained or leave a cyclone 6.2 Critical size of particle Effective number • Actual operation, the cut-off size diameter will be defined as d50 that mean 50% of the particle which have a diameter more than d50 will be collected or captured. Ideal and operation efficiency By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC References • • • • • • Prabir Basu , Combustion and gasification in fluidized bed, 2006 Fluidized bed combustion, Simeon N. Oka, 2004 Nan Zh., et al, 3D CFD simulation of hydrodynamics of a 150 MWe circulating fluidized bed boiler, Chemical Engineering Journal, 162, 2010, 821-828 Zhang M., et al, Heat Flux profile of the furnace wall of 300 MWe CFB Boiler, powder technology, 203, 2010, 548-554 Foster Wheeler, TKIC refresh training, 2008 M. Koksal and F. Humdullahper , Gas Mixing in circulating fluidized beds with secondary air injection, Chemical engineering research and design, 82 (8A), 2004, 979-992 THANK YOU FOR YOUR ATTENTION By Chakraphong Phurngyai :: Engineer, TKIC By Chakraphong Phurngyai :: Engineer, TKIC