Paper 11 icaer_2013modified


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Paper 11 icaer_2013modified

  1. 1. Effect of Ambient Air Temperature on the Performance of Regenerative Air Preheater of Pulverised Coal Fired Boilers Chittatosh Bhattacharya Deputy Director (Tech.) National Power Training Institute, E. Region, India Bidesh Sengupta Design Engineer, Engineering - Rotary Air Preheaters, Alstom India Ltd., India Proceedings of ICAER 2013: IIT Bombay Day – 2, Session – D (FN) , Paper Ref:011 December 10-12, 2013; Mumbai; INDIA
  2. 2. World Energy Outlook 2010 Starting with Electric Power Energy Realities The shift from coal power is not as fast as the growth of RE over the years !!
  3. 3. To Meet Sustainable Power Demand Coal Power has A Proven Past A Progressive Present & A Promising Future With the support of low cost, low to high quality of coal resources to run at least for another Century (if not more) to provide affordable quality power for all with negotiable emission.
  4. 4. Energy & Environment Challenges & Needs For Sustainable Coal Power Generation at Affordable PRICE with Environment protection must have two straight forward approaches 1 Objective - Reduce coal & auxiliary power consumption :lesser amount fuel to produce same amount of energy with lesser emission. 2 Benefit – Acceptable environmental impact from the pollutants produced by coal power generation process Input Air Water Coal OBJECTIVE - Electricity Output OBJECTIVE Reduce Auxiliary Power & Losses BENEFIT Reduction in emission of pollutants Reduce Coal, Water, Air Consumption per unit power produced
  5. 5. Energy & Environment Modeling for Sustainable Coal Power R E D U C T I O N O F C OA L & A U X I L I A RY P OW E R C O N S U M P T I O N Few Identified Primary Actions 1 Increase “As Fired” Coal Quality by adequate Drying 2 Complete burning of Coal & maximizing heat trapping in Boiler 3 Reduce Induced & Forced Draught Fans Power Consumption 4 Reduce Heat lost through stack & improve Furnace heat transfer All the above actions are linked to the Performance of one Boiler Component better known as Air Preheater
  6. 6. Regenerative Air Preheater Basics....... HOT Secondary Air OUT HOT Primary Air OUT APH Hot END Hot Flue Gas IN Boiler 130℃ ESP APH Mill ID Fan 340℃ STACK FD Fan PA Fan APH Cold END Cold Primary Air IN COLD Flue Gas OUT COLD Secondary Air IN Arrangement of Regenerative APH The APH accounts for 10-12 % of a unit’s thermal efficiency and is a critical component of plant system. Ambient Air Temperature has a major impact on overall APH performance and therefore the intent is to focus on the factors affecting APH performance and how to overcome or eliminate the issues.
  7. 7. Performance Impact of Air Preheater on Overall Equipment Efficiency Boiler Input Devices FD & PA Fans Pulverizers Burners Economizer Boiler Output Devices Environmental Emission Control Devices Boiler Operation Electrostatic Precipitators Combustion Control Bag houses SCR FGD ID Fans WB/ Mill 1 Evaluation of APH Leakage 4 1 Hot Flue Gas 2 Hot Air APH Bypassing APH Gas Side 3 3 ID Fan P FD FAN + P PA FAN APH Gas Side 4 2 Ambient Cold Air Ambient Cold Air APH Bypassing APH helps to dry up pulverized coal with hot primary air, rapid attaining of ignition temperature and creating turbulence with more volumetric flow of Secondary air.
  8. 8. Understanding APH Leakage……. The leakage of Flue Gas/Air across the circumference bypassing APH is not creating any fan power loss but causing a heat rate penalty of ƞboiler loss of 1% for every ~ 220 C rise in Texit gas . With increase in ΔTCold Hot side (average 185-2000C ), the leakage increases more in large dia. Regenerative APH. An air leakage of 5-7% is already acknowledged in the design and supply condition of the regenerative APH and a loss of 10 -13% reduction in ƞoverall APH is already observed for every 10% increase in leakage. 2 3 increases The leakage of air to flue gas side & loading of ID/FD fan power without contributing to improve ƞboiler rather causes LOI. Though these two leakages are measurable with O2 concentration in flue gas before & after APH, the Flue Gas /Air bypassing APH are not accurately measurable as per ASME PTC 4.3 neither it is a part of PG test, and only approximated.
  9. 9. APH Heat Exchange Performance Evaluation Regenerative APH Heat Exchange Mechanism Hot side Heating Surface On Rotation Heating Surface Flue Gas H/E Storage H/E Flue Gas & Air Side Heat Balance Heat Transferred by Flue Gas qg: 1. qg = mge x Cpg x (TgeTgl) Cold side Heating Surface Transfer H/E in Air Air Leaving Tal mal qal Gas Entry qge +Qa Tge mge Heat Transferred to Air qa: 2. qa = mal x Cpa x (Tal- Tae) +Qa - Qg qae - Qg qgl m : Mass of Fluid [ Flue Gas / Air ] (kg/h) Cp: Mean Specific Heat between Tae and Tgl (kcal/kg /℃) T : Temperature of Hot/Cold Fluid [ Flue Gas / Air ] (℃) Heat Balance: qg = qa mgex Cpg x (Tge- Tgl) = mal x Cpa x (Tal- Tae Air Entry Tgl Gas Leaving
  10. 10. APH Thermal efficiency Evaluation Matrix ηThermal: Ratio of ΔT between Gas inlet and Air inlet and between Gas/Air inlet and Gas/Air outlet . 1. ηgas - Gas side Efficiency ⊿Tmax = Tge- Tgl 350 Tge- Tae 300 2. ηair -Air side Efficiency ηa = ⊿Ta ⊿Tmax = Tal - Tae Tge - Tae Temperature (℃) ηg = ⊿Tg Tge 400 250 Tal ΔTg 200 150 ΔTa Tgl 100 50 Tae 0 1000 Cold Length of Heating Elements 2000 Hot ΔTmax
  11. 11. Xratio: APH Thermal efficiency Indicator Xratio ( XR) : heat capacity ratio of air Vs flue gas XR = Cmin Cmax = mal x Cpa mge x Cpg Alternately may be expressed as below from the equation of qg=qa XR = mal x Cpa mge x Cpg = Tge – Tgl (Tal – Tae ) Relation for thermal efficiency - = ⊿Tg ⊿Ta ηa x X R = ηg Thus we find ambient air temperature (AAT) is a determining factor of regenerative air pre heater efficiency and its performance – the effect of which is evaluated in next few slides……………..
  12. 12. Effect of AAT in Performance Ambient air temperature (AAT) affects - mass flow rate (ma), coefficient of heat capacity (Cpa); density, humidity ratio (moisture content capacity of air on RH) ・ Quantity of Air flow(ma, )、 when (Tae) AAT ・ (Tae) AAT ・ Humidity Ratio ・ (Tae) AAT ・ As (Tae) AAT Humidity Ratio Boiler (Air/Flue gas) Hot Air ma Flue Gas DP h mg Tge Heat loss to dry air ηAPH ΔPh(Δpa :O/L – ΔPg I/L) thus Air leakage increases The result of Plant Findings as ΔPa Tae ΔPg m L FD/PA Fan Tgl ESP
  13. 13. FIELD STUDY ON THE EFFECT OF AAT IN ηAPH A set of operating parameters governing APH performance was collected having a considerable difference in AAT . All data are taken at nearly equal power generating condition (having a variation in PLF within + 0.5%) and collected within 24 hours span to avoid variation in RH factor on the same day, (~ 40%).
  14. 14. Effect of AAT (Tae ) on mass flow (ma) variation Va = Constant: Space available for the fluid flow through the device is constant. Pa = Constant: atmospheric pressure. Mass Flow rate through the device is; ma = Pa Va / R Tae Tae(AAT) ma Almost 15% drop in mass flow rate of air for 8 0C rise in AAT ma ma.Cpa As decreases ; decreases too, reducing the capability of air to extract the heat from APH element matrix absorbed from flue gas..!!!
  15. 15. Effect of AAT (Tae ) on Relative Humidity (RH) Tae(AAT) RH constant : humidity ratio Humidity ratio ha The moisture in air does not help in any heat transfer from the element matrix through flue gas and therefore is considered as loss HL= heat loss due to moisture/kg of air Cm = specific heat content of moisture For 8 0C rise in AAT there is ̰12 KJ/kg loss of energy……
  16. 16. Effect of AAT (Tae ) on APH efficiency (η ) APH The APH efficiency : The APH efficiency is calculated without considering the effect of moisture in air. For 8 0C rise AAT , ηAPH decreases by almost 14% of the initial η.
  17. 17. Effect of AAT (Tae ) on APH modified efficiency (η ) APHm The modified efficiency is given by: = The modified efficiency is calculated considering the moisture of air . Decrease of 22% of ηAPHm for 8 0C rise in AAT or for 1 0C rise in AAT the efficiency decreases by 2.74%. Means more input is required to obtain rated output: ultimately causing Commercial LOSS!!
  18. 18. Effect of APH Performance Deterioration..!!!  Inadequate drying of coal in coal mills.  Increase of AAT increases leakage; increases ID / FD fan power consumption.  Due to leakage of secondary air in flue gas reduces O2 in the furnace causing incomplete combustion .  Due to incomplete combustion; more fuel is required to obtain the required output, also incomplete combustion badly affects the environment ultimately leading to commercial losses.  Auxiliary power is supplied from plant. Increase in consumption of auxiliary power leads to decrease the net
  19. 19. CONCLUSION The use of regenerative APH in tropical countries with higher AAT is not an energy efficient option where RH of air is substantially high and considerable fuel energy is wasted to dry up air-moisture beside the high moisture low-rank coal used for pulverised coal fired power generation system. As such, high moisture coal drying can be more economically achieved through atmospheric fluidized bed drier using waste heat of flue gas upstream of ID fan and before exhaust through stack. The partial flue gas recirculation through pulverizer (PFGR) system can reduce the APH heat transfer loading, since coal drying capacity sometimes get restricted due to insufficient hot primary air temperature and thereby causes power generation capacity restriction. Besides, a reduction in coal drying need in turn increases more secondary air temperature at APH ensuring better combustion with lower NOx generation potential with less excess air.
  20. 20. THANK YOU ???????