Boiler efficiency Heat Engines & Boilers

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

Heat balance on boilers

Definition of boiler efficiency

Quseful = m  (hout – hin ) 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 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

Firing type losses

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 loss rad – radiation heat loss ashheat – 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 40-60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.

Calculation of fluegas loss factor

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:

Saturation temperature and pressure values 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

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 8,9 9,8 Liquefied petroleum gas 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

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

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

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

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 variation and assessment of seasonal efficiency

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

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