Boiler efficiency

Heat Engines & Boilers

Contents

• Heat balance on boilers

• Efficiency determination

• Loss categories

• Fluegas condensation principals

• Seasonal efficiency

Heat balance on boilers

Input power sum is equal with output power sum: Qin = Qout Input heat components:

Input heat in chemical bound of fuel. Qfuel = B Hi

Input physical heat of fuel: Qfuelphysical = B cpfuel (tin - tamb )

Input heat of hot air: Qair = B Lo’ cpair (tin – tamb ) Other

Input heat: Qin = Qfuel + Qfuelphysical + Qin + Qother

otherambfuelfuelambairpair'Loiin

Q))tt(c)tt(cH(BQ

Definition of boiler efficiency

Output power can be divided into two categories:

Qin = Quseful + Qloss

Quseful = Qin – Qloss , Two forms of boiler efficiency determination can be gained.

QQ

Q

Q

in

loss

in

useful

boiler1

direct indirect

Direct efficiency

• Useful heat power can be determined from mass flow rate of heat transfer medium and from inlet and outlet enthalpy:

• Quseful = m (hout – hin )

• 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

Indirect efficiency

Different types of loss can be separated into two groups:

• Firing type lossesare originated from not total or not complete combustion of the fuel, which means that unburnt combustible parts remaining after combustion end

• Heat exchanger type lossesmeans that some part of generated heat by combustion goes to waste, not to useful purpose, not to heat transfer medium

Firing type losses

Different forms of firing losses: gas - unburnt gas (CO,CxHy)

soot - soot

coke - coke

flyash – combustible part of flying ash

ash - combustible part of bottom ash Considering above mentioned losses can be calculated the firing efficiency:

F = 1 - ( gas+ soot+ coke+ flyash+ ash)

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.

Bfg = F B

• Loss quantity can be determined from operational measurement results.

Qloss = massflow burnable content heating value of burnable part

• Loss factor is given by the ratio of loss heat power and input power.

= Qloss / Qin

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:

• fg – fluegas heat lossrad – radiation heat lossashheat – ash physical heat loss

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 fg = 5 - 10 % • At earlier constructions it is in between fg = 10 - 15 % • When fluegas is cooled below water vapor dew-point

temperature (which is generally in between 40-60C) 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.

Calculation of fluegas loss factor

fg = Qfg / Qin Qfg = mfg (hfgout - hfgamb) = B (Vo’+(-1) Lo’) cpfg (tfgout – tamb)

))tt(c)tt(cH(B

)tt(c))1((B

ambfuelfuelambairinpair'Loi

ambfgoutpfg'Lo'Vo

fg

)tt(c)tt(cH

)tt(c))1((

ambfuelfuelambairinpair'Loi

ambfgoutpfg'Lo'Vo

Fluegas heat loss variation in case of

fuel oil S firing

Gőzfejlesztők anyag és

energia áramai

Tüzelőberendezés anyag és energia áramai

Hőhasznosító rész anyag és energia áramai

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:

'L)1('V

waterH12.11p

'V

Vpp

00abs

O2HabsO2H

Saturation temperature and pressure values

Saturation temperature Saturation pressure

100 C 1 bar

60 C 0.2 bar

55 C 0.157 bar

50 C 0.12 bar

45 C 0.094 bar

40 C 0.074 bar

30 C 0.042 bar

20 C 0.023 bar

10 C 0.012 bar

0 C 0 bar

Saturation temperature and pressure values

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.220

5

10

15

20

25

30

35

40

45

50

55

60

65

Saturation pressure [bar]

Sat

urat

ion

tem

pera

ture

[°C

]

65

0

Tsat Psat( )

0.220 Psat

Water vapor dew point variation

Heating value ratio variation

Unit Lower Heating Value (LHV)

Higher Heating Value (HHV)

Conversion factor

Natural gas kWh/m3 10,4 11,5 1,11

Liquefied natural gas

kWh/m3 8,9 9,8 1,11

Liquefied petroleum gas

kWh/m3 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

Calculation of fluegas loss factor considering condensation

fg = Qfg / Qin

Qfg = (mfg – mcond) cpfg (tfgout - tfgamb) - mcond (h’’w – h’w) [kW] mfg = B (Vo’+(-1) Lo’), [kg/s] mcond = B H2O (H2Ostart - (H2Oend)/ H2Ostart [kg/s]

))tt(c)tt(cH(B

)hh(m)tt(c)mm(

ambfuelfuelambairinpair'Loi

'w

''wcondambfgoutpfgcondfg

fg

)tt(c)tt(cH

)hh()tt(c))1((

ambfuelfuelambairinpair'Loi

'w

''w

Ostart2H

Oend2HOstart2HO2Hambfgoutpfg

Ostart2H

Oend2HOstart2HO2H'Lo'Vo

Exhaust gas loss based on LHV in case of natural gas firing[%]

Exhaust gas loss based on LHV in case of LPG firing

20 40 60 80 100 120 140 160 180 200

10

5

5

10

fg tfg 1.0 fg tfg 1.167 fg tfg 1.313

tfg

O2fg

5 %3 %0 %

[oC]

[%]

Exhaust gas loss based on LHV in case of light fuel oil firing

20 40 60 80 100 120 140 160 180 200

5

5

10

fg tfg 1.0 fg tfg 1.2 fg tfg 1.4

tfg

O2fg

6 %

3,5 %

0 %

[oC]

[%]

Exhaust gas loss based on LHV in case of wood firing

[%]

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 rad = 0.5 - 1.0 % referring to maximal load. But the heat loss power is independent from load level, it is constant. (Qrad = 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)

Ash physical heat loss

• It is only in case of solid fuel firing, where bottom ash removed from fire chamber in hot condition.

• For loss factor determination bottom ash quantity and temperature needs to be measured

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.

Boiler efficiency variation at part load

Heating and cooling demand variation over a year in Europe

Load-duration curve of the heating season

Burning cycle and energy losses of boiler

Efficiency variationand assessment of seasonal efficiency

Load independent losses

Load dependent losses

Effective energy

1/ K

0 1 Workload

qB/ K

Sta

nda

rdiz

ed fu

el in

put q

F

qF = Q´F/(Q`K*tB)

= Q´H/(Q`K*tB) with QK: Nominal boiler capacit tB: Running time of the boiler QF: Firing power QH: Useful power From these values the average efficiency a() can be calculated

a() = *K/(-*qB + qB)

Summary

You are already familiar with

• Heat balance on boilers

• Efficiency determination

• Loss categories

• Fluegas condensation principals

• Seasonal efficiency

Thank You for Your Attention !