efficiency improvement of small boilers for domestic use: energetic

Località Piacenza

Doc. n. R 1.2/1

CONSORZIO LEAP

Laboratorio Energia Ambiente Piacenza Progetto ECATE

Rev 0.

0 Prima emissione

Prof. Marchesi Ing. Rinaldi

Aprile 07

REV DESCRIZIONE ELABOR VERIFICATO APPROVATO DATA

Progetto E.C.A.T.E. Efficienza e Compatibilità Ambientale delle Tecnologie Energetiche

> EFFICIENCY IMPROVEMENT OF SMALL

BOILERS FOR DOMESTIC USE: ENERGETIC AND EXERGETIC ANALYSIS

NOTA : IL PRESENTE DOCUMENTO E’ EMESSO IN REVISIONE 0, IN PRIMA EMISSIONE. ESSO PUO’ ESSERE SOGGETTO A FUTURE REVISIONI

LEAP Laboratorio Energia e Ambiente Piacenza _ via Nino Bixio 27, 29100 Piacenza _ tel. +39.0523.579774 _ www.leap.polimi.it

1

LEAP / Relazione del mese 9 Sottoprogetto 1 GENERAZIONE DI ENERGIA TERMICA AD ALTA EFFICIENZA Obiettivo Realizzativo 1.2 MISURA DELL’ENERGIA TERMICA Risultato R1.2/1 ARTICOLO RELATIVO ALLE MISURE DI ENERGIA TERMICA


2

INDICE

Pag

INTRODUCTION 3

2. ENERGETIC AND EXERGETIC ANALYSIS OF A TRADITIONAL BOILER 3

2.1. Energetic analisys 4

2.2. Exergetic analisys 6 3. ENERGETIC AND EXERGETIC ANALYSIS OF A TRADITIONAL BOILER WITH A HEAT RECOVERING SYSTEM 7

3.1 Energetic analisys 7

3.2. Exergetic analisys 9 4. CONCLUSIONS 9 REFERENCES 10


3

EFFICIENCY IMPROVEMENT OF SMALL BOILERS FOR DOMESTIC USE: ENERGETIC AND EXERGETIC ANALYSIS

The 98 % of domestic natural gas boilers sold every year in Italy are traditional ones, only 2 % of them are condensing boilers. This paper presents results coming from the improvement of traditional boilers whit a heat recovering system. This system has been studied and optimized by the authors with CFD analysis and experimentally tested on a traditional boiler. Efficiencies of boilers fired with natural gas have been calculated by means of an energetic and exergetic analysis. The datas necessary for theoretical calculations have been measured in a special facility that join the actual technical normes in terms of boiler’s certification. INTRODUCTION The analisys of italian market in the field of domestic boilers, shows that the quite totality of them are traditional ones. The 98 % of domestic natural gas boilers sold every year in Italy are traditional ones, only 2 % of them are condensing boilers. Energy consumptions for heating systems in civil applications, grow of about 1,1% every year; during the year 2004 have been expended about 136 Million of TEP, 40 millions in the civil sector and 26 millions for the heating sector. The foresight for year 2010 anticipates an energetic consuption, in the field of domestic heating, of 28 millions of TEP, with about 70 millions of tons of CO2 wasted in atmosphere . In Italy there are about 21,8 millions family unit. Feeding of heating plants is as follow: natural gas 13,4 millions 61%, liquid fuels 4,2 millions 19%, gas bottles, coal , wood 3,6 millions 17%, other fuels 0,6 millions 3%. Despite to the fact that the great part of boilers manufacturers in Italy can build and offer condensing boilers, whether in case of new buildings or in case of restructure, these are not installed. This work compares performances of a traditional boiler with the performances of a traditional boiler with a heat recovery system. The comparison has been made by means of an energetic and an exergetic analisys. The heat recovery system has been studied with CFD method and tested by the authors in a special facility that can assure a good accuracy and reproducubility of the tests. In the next future this facility will provide certifications according to technical normes [1] [2]. 2. ENERGETIC AND EXERGETIC ANALYSIS OF A TRADITIONAL BOILER Hypothesis assumed for calculations: Fuel is methane (CH4) in standard conditions (P0 = 101325 Pa and at T0 = 298,15 K). The High Heat Value is HHV = 890,4 kJ / mol, the Low Heat Value is LHV = 802,3 kJ/mol. The molar mass is MmCH4 = 16,043 kg / kmol, the volumetric mass is ρCH4 = 0,72 kg / m3. Burning is air in standard conditions, it could be schematized as composed only by oxygen and nitrogen. The volumetric composition is XO2 = 0,21 and XN2 = 0,79, where XX is the volumetric fraction of the component x. Combustion takes place with an excess of air respect to the stoichiometric conditions. If λ is defined as the ratio between tha mass of air during the combustion and the mass of air in


4

stoichiometric condition, (λ-1) is the excess of air. As indicated by many manufacturers for new type of domestic boilers, the execss of air is about 0,25. For this reason, in our calculations, we consider λ = 1,25. The boiler works in steady state condition, its furnace power is Pfoc = 30 kW. Water goes out from the boiler at a pressure PWout = 202650 Pa and at a temperature TWout = 80°C, it comes back to the boiler at a temperature TWin = 60 °C and about at the same pressure. Gaseous emissions go out from the exhaust-pipe at the temperature Tf = 180 °C.

For the combustion of a mole of Cα Hβ , with an excess of air of (λ-1), we can write:

( ) ( ) 222222 O4

1N76,34

OH2

CON76,3O4

HC ⎟⎠⎞

⎜⎝⎛ β

+α−λ+⎟⎠⎞

⎜⎝⎛ β

+αλ+β

+α→+⎟⎠⎞

⎜⎝⎛ β

+αλ+βα

For methane that burns with air at λ = 1,25, we can write:

( ) ( )

2222224

2222224

O5,0N4,9OH2CON4,9O5,2CH

O441125,1N76,3

44125,1OH

24CON76,3O

44125,1CH

+++→++

⎟⎠⎞

⎜⎝⎛ +−+⎟

⎠⎞

⎜⎝⎛ +++→+⎟

⎠⎞

⎜⎝⎛ ++

2.1. Energetic analisys Since that gaseous emissions leave the boiler at Tf > T0, the heat that can be transferred to the water (Qb→w) is smaller than the LHV.

Qb→w = HR(T0, P0) –HP(Tf, P0) = LHV – [HP(Tf, P0) - HP(T0, P0)]

HR(Tx Py) and HP(Tx Py), are respectively the enthalpy of reactants and products at temperature Tx and at pressure Py.

Tha term [HP(Tf, P0) - HP(T0, P0)] has been calculated by means of literature datas [3]

Qb→w = 802,300 – 64,165 = 738,135 kJ / molCH4

100LHVQ%Q wb

wb ⋅= →→ % 00,92100

300,802135,738%Q wb =⋅=→

Qf = LHV – Qb→w Qf = 802,300 – 738,135 = 64,165 kJ / molCH4

Qf is the loss of heat during the combustion process. The increase of massic enthalpy of water is:

Δhw = cw (TWout – TWin) Δhw = 4,2 (80 – 60) = 84 kJ / kg


5

If we assume that Qwall is the loss of heat through the walls of the boiler, the energy balance of the boiler can be written as follow:

LHV = Δhw + Qf + Qwall

Figure 2.1 – Schematic design of a traditional boiler.

If we indicate with mw the mass of water to the heating system, with NCH4 the mole of methane burned during combustion and if we assume that Qwall is about the 2 % of LHV, the energy balance become:

NCH4 LHV – NCH4 Qf – NCH4 Qwall= mw Δhw NCH4 LHV – NCH4 Qf – NCH4 0,02 LHV= mw Δhw

NCH4 802,3 – NCH4 64,165 – NCH4 0,02 802,3 = mw 84 NCH4 / mw = 0,1163 molCH4 / kgH2O

The energetic efficiency of the boiler can now be calculated as:

8101,04,890

089,722 HHV

QQ HHV

h.en

wallwbw.en ≅=η

−=

Δ=η →

The furnace power Pfoc = 30 kW can be obtained, in steady-state conditions, with a volumetric consumption of methane of:

Methane

Air

TWin = 60 °C TWout = 80 °C

Tf = 180 °C QWall


6

3

34CH

4CHm

wb

foc4CH h

m 3,26 hs3600

72,0110043,16

135,73830

hs36001M

QP

≅⋅=Γρ

=Γ −

→ 2.2. Exergetic analisys The methane adiabatic flame temperature for λ = 1,25 is Taf = 2031 K, the methane exergy is exCH4 = 830,2 kJ / mol. The combustion products exergy can be calculated by means of literature datas [3][4][5], it is:

exprod = NCO2 exCO2 + NH2O exH2O + NN2 exN2 + NO2 exO2 exprod = 1 ⋅ 62 + 2 ⋅ 50 + 9,4 ⋅ 38 + 0,5 ⋅ 41 = 539,7 kJ / mol

The exergetic efficiency of combustion is therefore:

65,02,8307,539

exex

COMBCOMB ex4CH

prodex ==η=η

If methane were burned in stoichiometric conditions, the exergetic efficiency should be of 0,717. It can be affirmed that the efficiency reduction for a non stoichiometric (λ = 1,25) combustion is not so great. For every mole of methane, the exergy transferred to the water is:

Δexw = Δhw – T0 Δsw Δexw = 12,4786 kJ / kgH2O Δexw = 107,3 kJ / molCH4

The exergy of emission at Tf = 180 °C can be calculated by using the following equation:

∑ ∑ =−−−=i i

4CHf0ii00iif mol/kJ 56,12ex )ss(NT)hh(Nex

The exergetic efficiency of emissionprocess is:

9767,07,539

56,121 exex1

ff exprod

fex =−=η−=η

The exergetic efficiency of the heat exchange process is :

2035,056,127,539

3,107 exex

exEXEX

exchange heatexchange heat exfprod

wex =

−=η

−Δ

==η +

−


7

The global exergetic efficiency is:

1292,02035,09767,065,0 exexexexex exchange heatfCOMB=⋅⋅=ηη⋅η⋅η=η .

3. ENERGETIC AND EXERGETIC ANALYSIS OF A TRADITIONAL BOILER WITH A HEAT RECOVERING SYSTEM The heat recovering system is placed in the exhaust-pipe. This system provides to a air combustion pre-heating from 25 °C to about 70 °C, to a water pre-heating from 60 °C to about 62 °C. At the same time the emissions leave the boiler exhaust-pipe at about 80 °C. 3.1 Energetic analisys The heat that can be transferred to the water (Qb→w) is smaller than the LHV.

Qb→w = HR(T0, P0) –HP(Tf, P0) = LHV – [HP(Tf, P0) - HP(T0, P0)]

HR(Tx Py) and HP(Tx Py), are respectively the enthalpy of reactants and products at temperature Tx and at pressure Py. The term [HP(Tf, P0) - HP(T0, P0)] has been calculated by means of literature datas [3][4][5]

Qb→w = 802,300 – 12,03 = 790,27 kJ / molCH4

100LHVQ%Q wb

wb ⋅= →→ % 50,98100

300,80227,790%Q wb =⋅=→

Qf = LHV – Qb→w Qf = 802,300 – 790,27 = 12,03 kJ / molCH4 Qf is the loss of heat through the exhaust –pipe during the combustion process. It doesn’t comprise the entalpy of evaporation of water. The increase of massic enthalpy of water is:

Δhw = cw (TWout - TWin) Δhw = 4,2 (80 – 60) = 84 kJ / kg

If we assume that Qwall is the loss of heat through the walls of the boiler, the energy balance of the boiler can be written as follow:

LHV = Δhw + Qf + Qwall

If we indicate with mw the mass of water to the heating system, with NCH4 the mole of methane burned during combustion and if we assume that Qwall is about the 2 % of LHV, the energy balance become:

NCH4 LHV - NCH4 Qf - NCH4 Qwall = mw Δhw NCH4 LHV - NCH4 Qf - NCH4 0,02 LHV = mw Δhw


8

NCH4 802,3 - NCH4 12,03 - NCH4 0,02 802,3 = mw 84 NCH4 / mw = 0,1084 molCH4 / kgH2O

The energetic efficiency of the boiler can now be calculated as:

8695,04,890

224,774 HHV

QQHHV

h.en

wallwbw.en ≅=η

−=

Δ=η →

The furnace power Pfoc = 30 kW can be obtained, in steady-state conditions, with a volumetric consumption of methane ΓCH4 of:

33

4CH4CH

mwb

foc4CH h

m 3,045 hs3600

72,0110043,16

27,79030

hs36001M

QP

≅⋅=Γρ

=Γ −

→

Figure 3.1 – schematic design of a traditional boiler with a heat recovery system.

Methane

Air pre-heated at 70°C

T’Win = 62 °C TWout = 80 °C

Tf = 80 °C QWall

TWin = 60 °C


9

3.2. EXERGETIC ANALISYS As calculated in the former case, the exergetic efficiency of combustion is:

65,02,8307,539

exex

COMBCOMB ex4CH

prodex ==η=η

For every mole of methane, the exergy transferred to the water is:

Δexw = Δhw – T0 Δsw Δexw = 12,4786 kJ / kgH2O Δexw = 115,116 kJ / molCH4

The exergy of emission at Tf = 80 °C can be calculated by using the following equation:

∑ ∑ =−−−=i i

4CHfii0iif mol/kJ 7463,2ex )ss(NT)hh(Nex

The exergetic efficiency of emission process is therefore:

9949,07,539

7463,21 exex1

ff exprod

fex =−=η−=η

The exergetic efficiency of the heat exchange process is :

2144,07463,27,539

116,115 exex

exEXEX

exchange heatexchange heat exfprod

wex =

−=η

−Δ

==η +

−

The global exergetic efficiency is:

1386, 0 2144 , 0 9949 , 0 65 , 0 exexchangeheat ex f ex COMB ex ex = ⋅ ⋅ = η η ⋅ η ⋅ η = η .

4. CONCLUSIONS It is important to point out that, in this work, all the energetic efficiencies have been calculated on the High Heating Value (HHV). From the thermodynamic point of view this is the only and proper way to calculate the energy efficiency of a system in wich a combustion process takes place. However, in the common technical language, the energetic efficiency is often calculated as the ratio between the usable energy and the Low Heating Value (LHV). From a scientific point of view this definition is incorrect. If this definition is applied to the condensing boiler its energetic efficiency is more than 100 %, but thermodynamic efficiency is a parameter which value is between 0 and 1.


10

TRADITIONAL BOILER

TRADITIONAL BOILER WITH

HEAT RECOVERY SYSTEM

ηen [%] 81,01 86,95 ηex [%] 12,92 13,86

γCH4 [m3 / kgH2O] 0,00259 0,002234 ΓCH4 [m3 / h] 3,26 3,04

γCO2 [gCO2 / kgH2O] 5,12 4,77 Table 4.1 – Energetic, exergetic and pollution comparison between a traditional boiler and a traditional boiler with the heat recovery system.

Results obtained from this work shows that the energetic efficiency for a traditional boiler is about 81 %. The 19 % of the wasted energy is as follow divided: 10 % is due to the water condensation hentalpy, less than 8 % is due to the emissions at 180 °C and about 2 % is due to the loss of heat through the walls. The installation of the heat recoverring system in a traditional boiler, can improve its energetic efficiency, even if the emissions temperature is higher than dew point. This system increase the heat exchange surface, in this way gaseous emissions go out through the exaust-pipe at the temperature of 80 °C. At the same time the air temperature rises to about 70 °C. The energetic efficiency reached from a boiler with a heat recovery system is about 87 %. The wasted energy is as follow divided: 9 % is due to the water condensation hentalpy, 2 % is due to gaseous emissions at 80 °C and 2 % is due to the loss of heat through the walls. The exergetic efficiency rises from the value of about 13 %, obtained with a traditional boiler, to the value of about 14 % in a boiler with the heat recovery system.

REFERENCES [1] Italian norm UNI 10389, Generatori di calore. Misurazione in opera del rendimento di

combustione.

[2] European norm UNI EN 483, Gas-fired central heating boilers Type C boilers of nominal heat input not exceeding 70 kW.

[3] Annaratone, Generatori di vapore, Ed. CLUP Libreria. [4] T.J. Kotas, The exergy method of thermal plant analysis, Butterworths. [5] Galliani Pedrocchi, Analisi energetica, CUSL.

efficiency improvement of small boilers for domestic use: energetic

Documents