Boiler efficiency measurement. Department of Energy Engineering

Transcription

1 Boiler efficiency measurement Department of Energy Engineering

2 Contents Heat balance on boilers Efficiency determination Loss categories Fluegas condensation principals Seasonal efficiency Emission evaluation

3 Stoichiometry Combustion is an oxidation process, which is an exotherm chemical reaction: Exotherm - heat is produced by reaction Endotherm - heat is needed for keeping up reaction Authoterm - neutral from heat viewpoint, (reaction do not need or produce any heat) Base equations of combustion: C + O 2 = CO 2 + heat 2H 2 + O 2 = 2H 2 O + heat S + O 2 = SO 2 + heat

4 Air as Oxygen source for combustion For combustion two component is needed: Combustible element or material Oxygen But for feeding oxygen generally ambient air is used except for some missile, military and welding techniques Composition of air: Oxygen O 2 21% [V/V, mol/mol] Nitrogen N 2 78% [V/V, mol/mol] Others (CO 2, rare gas, etc.)1% [V/V, mol/mol] Simplification for combustion calculations: Composition of air: Oxygen O 2 21% [V/V, mol/mol] Nitrogen N 2 79% [V/V, mol/mol] By mass: Oxygen O % [m/m] Nitrogen N % [m/m]

5 Excess air factor λ A stoichiometric mixture of air and fuel is one that contains just sufficient oxygen for the complete combustion of the fuel - a mixture which has an excess of air is termed a weak mixture, - and one which has a deficiency of air is termed a rich mixture. Normally it is going to be burnt fuel totally in order to utilize all the possibility for energy generation, which needs at least theoretical air quantity or generally a bit more. The excess air factor: λ = Air actually use Air stoichiometrically necessary = A A 0 [-]

6 Determination of optimal excess air factor

7 Excess air calculation from measured data From equations: V d = V o d + (λ-1) L o ', and (λ-1) Lo' = O 2fluegas. (V o d + (λ-1). L o ' ) taking into account that V o d y L o ' can get: λ = O 2 fluegas

8 Excess air factor variation

9 Determination of optimal excess air factor Depends on several conditions: Fuel type, combustion system, burner construction, pollutant emission limits, etc. Some usual values of the excess air factor : fuel gas oil coarse solid fuel pulverized solid fuel λ

10 Heat balance on boilers Input power sum is equal with output power sum: ΣQ in = ΣQ out Input heat components: Input heat in chemical bound of fuel. Q fuel = Σ B H i Input physical heat of fuel: Q fuelphysical = Σ B c pfuel (t in - t amb ) Input heat of hot air: Q air = Σ B λ µ Lo c pair (t in t amb ) Other Input heat: Q in = Q fuel + Q fuelphysical + Q in + Q other & = B ( H + λ µ c ( t t ) + c ( t t )) + Q in i Lo' pair air amb fuel fuel amb Q& other

11 Definition of boiler efficiency Output power can be divided into two categories: Q in = Q useful + Q loss Q useful = Q in Q loss, Two forms of boiler efficiency determination can be gained. η boiler = Q& Q& direct useful in = 1 Q& Q& loss in indirect

12 Direct efficiency Useful heat power can be determined from mass flow rate of heat transfer medium and from inlet and outlet enthalpy: Q useful = m (h out h in ) For determination of direct boiler efficiency fuel and heat transfer medium flow rate needs to be measured in addition to inlet and outlet medium pressure measurement. Direct efficiency does not give information about reasons of boiler efficiency variation. It does not give any idea how to reduce loss and increase efficiency

13 Indirect efficiency Different types of loss can be separated into two groups: Firing type losses are originated from not total or not complete combustion of the fuel, which means that unburnt combustible parts remaining after combustion end Heat exchanger type losses means that some part of generated heat by combustion goes to waste, not to useful purpose, not to heat transfer medium

14 Firing type losses Different forms of firing losses: ξ gas - unburnt gas (CO,C x H y ) ξ soot - soot ξ coke - coke ξ flyash combustible part of flying ash ξ ash - combustible part of bottom ash Considering above mentioned losses can be calculated the firing efficiency: η F = 1 - ( ξ gas + ξ soot + ξ coke + ξ flyash + ξ ash )

15 Loss calculation In case of oil and gas firing, when it fulfils environmental protection requirements, firing loss is neglectable. In case of solid fuel firing generally it is worth to take into account. In this case it is necessary to distinguish inlet fuel flow from actually burning, fluegas-developing fuel flow. B fg = η F B Loss quantity can be determined from operational measurement results. Q loss = massflow burnable content heating value of burnable part Loss factor is given by the ratio of loss heat power and input power. ξ = Q loss / Q in

16 Heat exchanger type losses Heat exchanger type loss is the common name of heat produced by combustion, but going another direction than heat transfer medium, which is actually loss. Different forms of heat exchanger type losses: ξ fg fluegas heat loss ξ rad radiation heat loss ash physical heat loss ξ ashheat

17 Fluegas heat loss Heat delivered to the ambient air because flue gas has higher temperature than initial or ambient one. In all of the cases this is the largest loss, which determines mainly the boiler efficiency. At an up to date boiler it is generally in between ξ fg = 5-10 % At earlier constructions it is in between ξ fg = % When fluegas is cooled below water vapor dew-point temperature (which is generally in between C) extra heat can be gained. It can cause that overall boiler efficiency can be above 100 % in case when input heat is calculated from LHV.

18 Calculation of flue gas loss factor ξ fg = Q fg / Q in Q fg = m fg (h fgout - h fgamb ) = B (µ Vo +(λ-1) µ Lo ) c pfg (t fgout t amb ) ξ fg = µ + λ µ [ µ + λ 1) µ K ] fg 0 L0 c fg t fgout K * ( r λ µ H i L0 c air t amb

19 Fluegas heat loss variation in case of fuel oil S firing

20 Utilization of condensation heat

21 Condensation of fluegas water content Fluegas can be considered as ideal mixture of different gas components Accoding to Dalton s law he pressure of a mixture of gases can be defined as the summation of partial pressure of each components: When fluegas temperature drop down below saturation temperature belonging to partial pressure of water in the fluegas Partial pressure of water in the fluegas: p H 2O = p abs V V' H 2O = p abs H + water V ' + ( λ 1) L ' 0 0

22 Saturation temperature and pressure values Saturation temperature 100 C 60 C 55 C 50 C 45 C 40 C 30 C 20 C 10 C 0 C Saturation pressure 1 bar 0.2 bar bar 0.12 bar bar bar bar bar bar 0 bar

23 Variation of flue gas dew point with excess air factor 60 t h ( λ h ) λ h.

24 Heating value ratio variation Unit Lower Heating Value (LHV) Higher Heating Value (HHV) Conversion factor Natural gas kwh/m 3 10,4 11,5 1,11 Liquefied natural gas kwh/m 3 8,9 9,8 1,11 Liquefied petroleum gas kwh/m 3 30,4 32,8 1,08 Light fuel oil kwh/l 10,0 10,6 1,06 Pellets/wood bricks kwh/kg 4,9 5,5 1,12

25 Radiation type loss Radiation type loss is called the heat transferred to the ambient air by outer surface of the boiler. The name originates from ancient boiler construction, where brick works actually radiated heat to the ambient. Nowadays this heat is transferred mainly by convection, but the name remains the same. Actual value can be calculated according to heat transfer rules considering actual insulation solution. This loss factor varies in between ξ rad = % referring to maximal load. But the heat loss power is independent from load level, it is constant. (Q rad = const.). This cause that loss factor is in inverse proportionality with load. ( 1% loss at nominal load increases up to 5% at 20% part load)

26 Comparison of direct and indirect boiler efficiency Both methods shall give the same value. But in real some difference can be experienced because of measurement inaccuracies. Generally determination by indirect method is simpler, because fuel and heat transfer medium measurement is not needed. Furthermore indirect method gives information on waste heat distribution and can be information base of efficiency increment. Direct method cannot be used for this purpose, but it can be good control of indirect method.

27 Boiler efficiency variation at part load

28 Efficiency variation and assessment of seasonal efficiency q F = Q F/(Q`K*t B ) β = Q H/(Q`K*t B ) with Q K : Nominal boiler capacit t B : Running time of the boiler Q F : Firing power Q H : Useful power From these values the average efficiency η a (β) can be calculated Standardized fuel input qf Effective energy 1/ K η a (β) = β*η K /(β-β*q B + q B ) Load dependent losses q B / K Load independent losses 0 Workload 1

29 Atmospheric premix type burner

30 Atmospheric premix type burner with open combustion system wall hung boiler

31 Atmospheric premix type burner with open combustion system floor standing boiler

32 Atmospheric premix type burner with open combustion system floor standing boiler

33 Examples for different combustion air supply Open Closed Condensing type Combustion system

34 Combustion system of condensation type boilers

35 Condensation type wall-hung boiler

36 Measurement arrangement

37 Direct efficiency calculation Q useful ( ) V cw ρ cw c w t cwout t cwin where: Q useful useful power; ρ w density of water at t cwin temperature c w t cwin specific heat of water at t cwout t cwin + t cwout temperature 2 in- and outlet temperature of cooling water Q firing V gas H i where V gas Volume flow of natural gas; Direct efficiency of the boiler: H i := η D Q useful Q firing kj m 3 lower heating value

38 Parameter dependence on temperature Density of water ρ w (20ºC) = kg/m 3 Specific heat variation of water: c vm ( t) t.

39 Flue gas heat loss calculation ξ fg = [ µ + λ 1) µ K ] ( r λ µ fg 0 L0 c fg t fgout K * H i L0 c air t amb where µ air = [kg/kg] µ fg0 = [kg/kg] c fg = [kj/kg] c air = 1.04 [kj/kg] K: specific quantity of condensate water for 1 m 3 or 1 kg natural gas (from measured data) r = 2510 kj/kg evaporization heat of water H i Lower Heating Value by mass [kj/kg]

40 Density variation of natural gas ρ g0 := 0.72 kg m 3 ρ g t, p g ( ) 273 := ρ g0 t p g ρ g ( t, 2000) 0.7 ρ g ( t, 2500) ρ g ( t, 3000) t.

41 Indirect efficiency calculation η ind = 100%-ξ fg -ξ rad ξ rad = α.a outer.(t wall -t amb )/Q firing [-] Where: α = 20 W/m 2 K - heat transfer coefficient A outer outer surface of the boiler [m 2 ] t wall outer surface temperature [ºC] t amb ambient temperature [ºC]

42 Evaluation of emission measurement Measured oxygen content in fluegas: O 2 := 4 % Operational excess air factor: λ := O 2 λ = Refeence oxygen content of flue gas for evaluation of emission limits: Measured emission values of combustion process dependent pollutants: O 2r := 3 % Nitrogen oxydes: Carbon monoxyde: Unburnt hydrocarbons: NO x := 150 CO := 70 CxHy:= 18 ppm ppm ppm

43 Evaluation of emission measurement M NO2 := 46 Reference value: kg/kmol NOx m := M NO2 NO x O 2r NOx rm := NOx 21 O m 2 NOx m = NOx rm = mg/m 3 mg/m 3 M CO := 28 Reference value: M C4H10 := 58 kg/kmol kg/kmol Reference value: M CO CO m := CO O 2r CO rm := CO 21 O m 2 M C4H10 CxHy m := CxHy O 2r CxHy rm := CxHy 21 O m 2 CO m = CO rm = CxHy m = CxHy rm = mg/m 3 mg/m 3 mg/m 3 mg/m 3

44 Summary and comparison of emission values Emission values are refernced to dry (water free) fluegas at 273K, 101,3 kpa, 3% oxygen content. NOx rm = mg/m 3 < 350 mg/m 3 CO rm = mg/m 3 < 100 mg/m 3 CxHy rm = mg/m 3 < 50 mg/m 3

45 Summary You are already familiar with Heat balance on boilers Efficiency determination Loss categories Fluegas condensation principals Seasonal efficiency Emission evaluation

46 Thank You for Your Attention!