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Accurate Analysis of Boiler Efficiency

100 VGB PowerTech 12/2008

Accuracy Improvement Analysisof the Standard Indirect Method for Determininga Steam Boilers Efficiency

Andrej Senegac nik, Igor Kutrin and Mihael Sekavc nik

Authors

Dr. Andrej Senega cnik, Assistant Professor

Dr. Igor Kutrin,Teaching Assistant

Dr. Mihael Sekav cnik,

Assistant Professor University of LjubljanaFaculty of Mechanical EngineeringLjubljana/Slovenia.

Kurzfassung

Verbesserte Methoden zurBestimmung des

Keselwirkungsgrades

Der Kesselwirkungsgrad von Kohlekraftwerkenwird gewhnlich nach standardisierten Verfah-

ren bestimmt. Bei Einsatz eines Festbrenn- stoff s, wird der Massenstrom indirekt nachdem in DIN 12952-15 beschriebenen Verfah-

ren berechnet . In dem vorliegenden Beitragwerden bestimmte Vernderungen an der

standardisierten Methode DIN 12952-15 hin- sichtl ich der Konsistenz von Berechnungenund die Genauigkeit der Ergebnisse mit derSensitivittsanalyse vorgeschlagen, prsentiertund bewertet,. Die wesentlichen Berechnungs-verfahren bleiben identisch mit DIN 12952-15.Es werden sechs Vernderungen bercksich-tigt. Vier davon beeinflussen die Bestimmungder entsprechenden Rauchgasmasse, nm-

lich: nicht Vernachlssigung der partiellen Oxi-dation des Kohlenstoffes zu CO, nicht Ver-

nachlssigung von Unverbrann tem in der Asche und Schlacke bei der Bestimmung des

Luftberschusses mit zwei Modellen und nichtVernachlssigung von Restwasser in derRauchgasmessprobe. Die fnfte Vernderung

behandelt den Einfluss der kalten Falschluf tund die sechste Vernderung bercksichtigtdie Verwendung der Berechnungsmethodeder Enthalpie fr Rauchgas und Verbrennungs-

luft nach VDI 4670. Der Einfluss der vorge- schlagenen Vernderungen auf den Kesselwir- kungsgrad wurde mit zwei realen Datenreihenfr Stein- und Braunkohle bewertet.

Introduction

The efficiency of coal-fired power plants isusually determined according to standardised

procedures. Currently, the Guidelines VDI3986 and standard DIN EN 12952-15 [1] arevalid. If solid fuel is used, its mass flow is cal-culated indirectly according to the proceduresstated in DIN 12952-15. The indirect methodrequires the determination of boiler losses.Losses can be measured directly except forradiation and convection losses which are es-timated according to the standard procedurestated in DIN 12952-15. The article at hand

proposes, presents and evaluates some modi-fications of the standardised indirect methodDIN 12952-15 regarding the consistency ofcomputations and accuracy of the results.Most of the modif ications were introduced re-cently in reference [2] where they were treated

generally, i.e. from a stoichiometric calcula-tion point of view. Additional research pre-sented in this paper is focused on incorporat-ing additional calculations into the standard

procedure [3] and to evaluate their impactsusing sensitivity analysis. The main flow ofcomputations remains identical to DIN 12952-15. Six modifications are considered. Four ofthem affect the referred flue gas mass deter-mination, i.e.: partial oxidation of carbon toCO, including unburned matter in ash andslag while determining the excess air ratiowith two models and including residual watervapour content in the flue gas sample. Thefifth modification studies the impact of colduncontrolled leakage air. The sixth modifica-tion is applying the VDI 4670 procedure [4]for flue gas and combustion air enthalpies cal-culation. The impacts of modifications werestudied for two real data sets for hard coaland lignite.

Air Ratio Determination

Pa r t i a l Ox i da t i on o f Ca rbonto CO ( I )

If some of the carbon from fuel burns to COinstead to CO 2 the actual oxygen consump-tion is lower than the calculated consumptionaccording to DIN 12952-15. To evaluate the

CO impact, the carbon from fuel should bedivided into quantities of carbon combustedto CO 2 and to CO, respectively.

Let the factor CO2 be a quantity of carbonforming CO 2. The factor CO2 can be calculat-ed exactly with the next derivative formulae:

CO2d 1,8650 C VAod CO2Ad

COd CO2 = (1) CO2d C 1,8534 + 1,865 COd

Factor CO2 is dependent on , COd and CO2d (if the O2d is measured in the flue gas, theCO 2d value needs to be calculated with theformulae (6) and (7)).

Lambda , is calculated from the basic pa-rameters which are given in DIN 12952-15and are complemented with the carbon mon-oxide (CO) coefficients which are calculatedaccording to [5]. The complemented equa-

tions are:Aod = 11,5122 CO2 C + 5,7561 (1 CO2)

C + 34,2974 H + 4,3129 (2)S 4,3129 O

God = 12,5122 CO2 C + 6,7561 (1 CO2)C + 26,3604 H + 5,3129 (3)S 3,3129 O + N

VGod = 8,8930 CO2 C + 5,3847 (1 CO2)C + 20,9724 H + 3,3190 (4)S 2,6424 O + 0,7997 N

CO2 = 3,6699 CO2 C + 0,0029 (1 CO2)C + 0,0173 H + 0,0022 ( S O) (5)

If CO2d is measured is calculated accordingto: VGod CO 2d CO2d = +1 (6) VAod CO2d CO2Ad if y O2d is measured is calculated accordingto: VGod O2d = +1 (7) VAod O2Ad O2d The upper formulae for are given in [5].Iteration is needed since factors CO 2 and are correlated. The CO impact on referredflue-gas mass G is always negative. Thismeans that in presence of CO in flue gas,actual flue gas mass flow is lower than

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VGB PowerTech 12/2008 101

flue gas mass flow calculated according toDIN 12952-15. F i g u r e 1 shows the impactof CO content in dry flue gas on referred fluegas mass G. Actual operating points are alsomarked. The CO impact on G is negative. Ifthe CO content would rise to 1000 ppm, thedifference of referred flue gas mass G would

be in the case of hard coal 0.28 % and in thecase of lignite 0.22 %. The gradient of COimpact is slightly higher in the case of hardcoal due to the larger mass fraction of carbonin coal.

Unb u rned Combus t i b l e sin Ash a nd S l ag

The air ratio is calculated on the basis of themeasured flue gas composition at the boilersoutlet and the fuel composition. Standard pro-cedure for the flue gas mass flow calculationassumes that all the carbon from the burnedfuel is burned completely and that the compo-

sition of the burned fuel matches the composition of the supplied fuel i.e. fuel sample.In practice, some of the larger coal dust parti-cles do not burn completely; they leave the

boiler with ash as coke. Therefore, the unburned carbon does not part icipate in CO 2 formation. Actual oxygen consumption istherefore lower. In order to determine the ex-cess air ratio more accurately, unburned car-

bon should be subtracted from the carbonmass fraction in the fuel and added to the ashmass fraction. The air ratio calculation shouldthen be repeated using the new apparent fuel

composition. Two explanatory models are in-troduced in the following sections.

In the following paragraphs annotationscoal X means composition of the fuel sam-

ple while X means apparent fuel composi-tion (i.e. composition that actually burned).

Model 1 Unburned Combustible Matter in Ash and Slag is Pure Carbon (II)

Model 1: The assumption is that the combus-tible matter in ash and slag is pure carbon.

New apparent fuel composition, which ac-tually burns, is therefore:

C = coal C lu (1 coal Ash coal H2O) (8)H = coal H (9)S = coal S (10)O = coal O (11) N = coal N (12)Ash = coal Ash + lu

(1 coal Ash coal H2O) (13)

H2O = coal H2O (14) Model 2 Composition of UnburnedCombustible Matter in Ash and Slag Matches

the Combustible Component of Fuel Linear Model (III)

Model 2: The assumption is that the combus-tible matter in ash and slag has the same com-

position as the combustible component of thefuel. New apparent fuel composition istherefore:

C = (1 l u) coal C (15)

H = (1 l u) coal H (16)

S = (1 l u) coal S (17)

O = (1 l u) coal O (18)

N = (1 l u) coal N (19)Ash = coal Ash + lu (coal C + coal H

+ coal S + coal O + coal N) (20)

Sensitivity Analysis

F i g u r e 2 shows the impact of unburnedcombustibles in ash and slag to referred fluegas mass G. The x-axis of Figure 2 representsthe amount of unburned combustible in ash

because there were no unburned combustiblesin slag. To get a more objective impressionabout the impact of unburned combustibles inash and slag, the total amount of unburnedcombustible can be also expressed as a massfraction of carbon in the supplied fuel [6]. In-dex unburned carbon is calculated by expres-sion (21).unburned carbon

mSL SL + mFA FA= 100 % (21) mFo CThe impact of unburned combustibles in ashand slag on referred flue gas mass G is al-ways negative. This means that in a presenceof unburned combustibles in ash and slag, ac-tual flue gas mass flow is always lower than

flue gas mass flow calculated according toDIN 12952-15.

The results of Model 1 and Model 2 are pre-sented in Figure 2. The proposed models are

extreme cases. The actual impact lies some-where in between. From the results we canconclude that the average impact gradient ofunburned combustibles to referred flue gasmass is

hard coal: 0.87 % of referred flue-gasmass per 1 % of unburned carbon

lignite: 0.71 % of referred flue-gasmass per 1 % of unburned carbon

The impact is larger for hard coal due to ahigher mass fraction of carbon in the fuel. Inthe case of lignite, when the ratio C/H islower, the gap between pure carbon and thelinear model becomes wider.

Res idual Vapour Content in GasSample ( IV)

The standard procedure for the flue gas massflow calculation requires a dry flue gas sam-

ple. The gas sample is usually dried by cool-ing to about 4 C. Therefore, volume contentof water vapour in the flue gas leaving the

cooler is about 0.8 %. This means that themeasured volume contents of the other fluegas components are lower than they would beif the flue gas sample was completely dry. Toestablish volume contents in completely dryflue gas sample the following correctionshould be applied:

saturation pressureof water at 4 C Sat = (22) flue gas pressure in the cooler

*CO2dCO2d = (23) 1 Sat

*O2dO2d = (24) 1 Sat *COdCOd = (25) 1 Sat

DIN 12952-15

Actual operatingpoint

Hard coal

Lignite

R e l a t

i v e c h a n g e o f r e

f e r r e d

f l u e g a s m a s s

w h e n

C O i s c o n s

i d e r e

d , %

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012

CO content in dry flue gas

Figure 1. Impact of CO content on referred flue gas mass G.

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where asterisk annotates measured volumecontents of flue gas components in flue gascontaining residual vapour. The annotationswithout an asterisk are used for volume con-tents of flue gas components in completelydry flue gas.

Sensitivity Analysis

The Bunte triangle in this point is used for ex-

planatory purposes only because the triangleis valid only for completely dry gases. Fromthe formulae (22) to (25) it is evident that dur-ing the correction procedure, values of CO2d or O2d are increased by an equal percentage.Therefore, the absolute variation CO 2d orO2d , formulae (26)O2d = O2d *O2d or CO2d = CO2d *CO2d (26)is thus proportional to the value of CO 2d orO2d . Usually in flue gas resulting from coalcombustion the value CO 2d is 4 to 5 timeslarger than O2d . Therefore, the absolute vari-ation CO 2d is also four to five times largerthan O2d .From the Bunte triangle, F i g u r e 3 , whichgraphically shows the relations between CO 2,O2 content in flue gas and , it is obvious thatwe should distinguish between two cases withopposite effects:

a) oxygen content y O2d is used to calculate thereferred flue gas mass G,

b) carbon dioxide content y CO2d is used to cal-culate the referred flue gas mass G.

Figure 3, Detail B, explains this graphically.Origin point C moves to D in the case whenCO2 is considered/measured (case b). If O 2 isconsidered/measured the origin point C movesto point E (case a) and the impact is positive.

This means that if the residual water vapour inthe flue gas sample is not neglected the actualflue gas mass flow is greater than flue gasmass flow calculated according to DIN 12952-15. In case b, when CO 2 is considered/meas-ured the impact changes sign and magnitude.For example the magnitude in hard coal case bis 2.6 times larger than in hard coal case a,F i g u r e 4 .

F i g u r e 5 shows the sensitivity of the excessair ratio which is evaluated with partial deriv-atives (27). It shows how quickly the excessair ratio is changing if the content of CO 2 orO2 are changing.

( 1) VGod (CO2Ad CO2d ) = (CO 2d ) VAod (CO2d CO2Ad )2

or (27) ( 1) VGod O2Ad = (O2d ) VAod (O2Ad O2d )2

Sensitivity of the excess air is weakly depend-ent on the fuel composition and therefore onFigure 5 only the results for lignite are pre-sented. Essentially the absolute impact of bothgases O 2 and CO 2 is quite similar and that thesensitivity is increasing when air ratio is in-creasing. From Figure 3 it can also be con-cluded that in case a the impact of residualwater vapour in the flue gas sample decreaseswhen the y O2d approaches zero. In case b, theimpact of residual water vapour in the flue gassample maximises when CO2d approaches thestoichiometric referred CO 2 volume. In thislimiting case the ratio between the CO 2 andO2 impact limits to ( ).

Combustion Air Heat Input (V)

Standard procedure assumes that combustion

air enters the boiler in a controlled mannerthrough air heaters. Of course some of thecombustion air enters the boiler elsewhere(leakage air) usually having a lower tempera-ture than the controlled air. Usually controlledair temperature used in air-heat-input calcula-tions is measured at the boilers envelope afterthe forced draft fan.

The standard [1] includes a correction formu-la but it should be stated more clearly that atleast a rough energy balance of the air heatershould be made to estimate the quantity ofleakage air.

From the provided data, for a hard coal-fired power plant, a simple energy and mass bal-ance for the air heater was established. Theresult shows that there is about 13.5 % ofleakage air. To evaluate the impact of airleakage, the following assumptions wereused:

the mass content of leakage air is xLA = 0.135

the temperature of leakage air is 15 to 30 C lower than controlled-air temperature.

DIN 12952-15

Actual operatingpoint, unburnedcarbon 0.52 %

Linear model

Pure carbon model

R e l a t

i v e c h a n g e o

f r e f e r r e

d f l u e g a s m a s s ,

%

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.00 0.02 0.04 0.06 0.08 0.10 0.12

Unburned matter in ash

Unburned carbon1.73 %

Hard coal

Figure 2. Impact of unburned combustibles in ash and slag, hard coal.

Detail B Detail B

21

^yCO 2

CO 2

CO 2 + O 2210

yCO 2dyO2d

A

1 / l

y CO 2d

yCO2d

yO2d

y O2d

E

D

C

Figure 3. Bunte triangle.

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Sen s i t i v i t y a na ly s i s

Leakage combustion air has an impact just onthe heat input proportional to the fuel burnedQ(N)ZF. Therefore, there is no impact on thereferred flue gas mass G.

Different amounts of delivered heat to the boiler at the same usefu l heat output Q N ,means also different boiler efficiency andconsequently also different mass flow of

burned fuel. F i g u r e 6 presents the impactof leakage air on the flue gas mass flow m G,leaving the air heater. In Figure 6 several tem-

perature differences between controlled andleakage air are introduced. The impact of notneglecting the cold air leakage is positive, i.e.the flue gas mass flow leaving the air heater isincreased.

Air and Flue Gas EnthalpyCalculation (VI)

DIN 12952-15 calculates the specific heat offlue gas and combustion air using polynomialequations. This calculation procedure takesinto account just air, water vapour and CO 2 content. More accurate calculation procedurestake into account all flue gas components, i.e.

N2, O2, Ar, Ne, H 2O, CO 2, CO and SO 2 asgiven in VDI-Richtlinien, VDI 4670 Ther-modynamische Stoffwerte von feuchter Luftund Verbrennungsgasen [4].

For the VDI 4670 calculation of mass frac-tions of flue gas and the combustion air ofthe components (N 2, O 2, Ar, Ne, H 2O, CO 2,CO, SO 2) should be known. All the neces-sary formulae for the determination of re-ferred masses i (kg/(kg of fuel)) were takenfrom Brandt [5].

AD = Aod (28)

CO2 = CO2o + ( 1) Aod CO2Ad (29)

O2 = ( 1) Aod O2Ad (30)

N2 = N + Aod N2Ad (31)

Ar = Aod ArAd (32)

CO = 2,3321 (1 CO2) C (33)

SO2 = 1,9981

S (34)

H2O = H2OF + Aod H2OAd (33)

Se ns i t i v i t y Ana ly s i s

The application of the more accurate proce-dure VDI 4670 only affects the amount of en-

ergy of the flue gas leaving the boiler theflue gas loss l G. Figure 7 presents the im-

pact of VDI 4670 as used on the boilers fluegas loss. It is evident that the VDI 4670 en-thalpy calculation lowers flue gas loss. In thelignite case, the influence is almost twice aslarge as for hard coal. The reason for this is

the water vapour content in the flue gas. Thewater content amounts 4.5 % for hard coaland 14.0 % for lignite. The overall impact ofVDI 4670 as used in DIN 12952-15 is there-fore dependent on the flue gas composition orfuel composition.

Data

Some major input data used in the computa-tions are summarised in Ta b l e 1 .

Results

F i g u r e 8 graphically represents the mainresults of this research the absolute changeof boiler efficiency for particular modifica-tion and all for modifications simultaneously.The data from Table 1 were used in the calcu-lation.

Modification I p artial oxidation of car-bon to CO: the change is small due to avery low CO content in the flue gas.

Modifications II and III composition of

unburned combustibles in ash and slag: asexpected, these two modifications have im-

portant effects. The effect is propor tionalto the content of the unburned combusti-

bles in ash and slag. Unburned combustibles affect the computed referred flue gasmass due to the application of the apparent

DIN 12952-15

Hard coal

R e l a t

i v e c h a n g e o f r e

f e r r e d

f l u e g a s m a s s ,

%

Temperature of flue gas sample, C

O2 consideration

CO2 consideration

0.6

0.4

0.2

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

-1.21 2 3 4 5 6 7 8 9 100

Figure 4. Impact of residual vapour content in gas sample, hard coal.

S e n s

i t i v i t y o

f e x c e s s a

i r r a

t i o

( l - 1

)

Air ratio, l

10

20

5

15

25

0

-10

-20

-5

-15

-251 1.2 1.4 1.6 1.8 2

CO 2

O2

Lignite

Figure 5. Sensitivity of excess air ratio, lignite.

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Table 1. Major input data.

Input Data Units Hard coal LigniteO2 content in flue gas(by volume, dry), AH outlet

- 0.058 0.030

CO content in flue gas(by volume, dry), AH outlet

- 3.274E-05 9.500E-05

Flue gas temperature, AH outlet C 137.1 168.7

Air temperature, AH inlet C 55.5 51.91

Calorific value of coal kJ/kg 23,150 10,597

Carbon content, fuel (by mass) - 0.5978 0.2906

Hydrogen content, fuel (by mass) - 0.0361 0.0259

Sulphur content, fuel (by mass) - 0.0074 0.0169

Oxygen content, fuel (by mass) - 0.0511 0.0621Nitrogen content, fuel (by mass) - 0.0124 0.0027

Ash content, fuel (by mass) - 0.1603 0.0780

Water content, fuel (by mass) - 0.1349 0.5238

Ash collection efficiency - 0.390 0.053

Unburned combustibles content(by mass), slag

- 0.0000 0.0027

Unburned combustibles content(by mass), ash

- 0.0323 0.0010

Calorific value of unburnedcombustibles, slag

kJ/kg 33,000 27,200

Calorific value of unburnedcombustibles, ash

kJ/kg 33,000 27,200

Slag temperature C 1500 330

Volatile matter content, ash - 0.05 0.05

fuel composition instead of the actual fuelcomposition. Losses directly connected tothe referred flue gas mass are therefore af-fected. Modifications II and III cannot beapplied simultaneously. The difference inresults of modifications II and III is notsignificant.

Modification IV residual water vapourin the flue gas sample: magnitude and di-rection of change depends on which gas isconsidered in the calculation and the flue-gas sample temperature (4 C is used inFigure 8). Due to opposite impacts, bothresults for CO 2 and O 2 can be introduced

simultaneously. It is evident that the impactof CO 2 is the largest and most importantfactor.

Modification V combustion-air heat input: in cases when the amount of leakageair is less than 20 %, the flue gas loss in-creases up to 0.01 %.

Modification VI VDI 4670 guidelinesapplication: flue gas loss is lower up to0.02 %. The water vapour content in theflue gas dictates the deviation magnitude.

All modifications simultaneously: FromFigure 8 it is clear that the particular modi-fications cause deviations in both directions

positive and negative. Therefore, theycompensate for each other to some degree.The final overall deviation is somehow pro-

portional to the total sum of the particulardeviations. The final overall deviation ofthe boiler efficiency is positive.

It can be seen from Figure 8 that just the in-clusion of leakage air always lowers the boilerefficiency. Residual water vapour in the fluegas sample can lower or raise efficiency, de-

pending on whether CO 2 or O 2 is used in the boiler eff iciency calculation. If O 2 is use,we get the overall results shown in columnALL 1, Figure 8. If CO 2 is used we get dra-matically different results as shown in columnALL 2.

For these two studied examples with hard coaland lignite, with particular input data fromTable 1, the overall absolute change in boilerefficiency due to the proposed modificationsis relatively small. In this particular case whenthe oxygen content is used for boiler efficien-cy determination, the overall change of effi-ciency is +0.009 % for hard coal, and+0.002 % for lignite (Figure 8). If the carbondioxide content is used, the overall change ofefficiency is +0.07 % for hard coal and+0.06 % for lignite.

It can be concluded from Figure 8 that the proposed modif ications, which provide moreaccurate calculations, increase the calculated

value of boiler efficiency in almost all cases.This means that the actual (more accurate) boiler efficiency is higher than the value ofefficiency given according to DIN 12952-15.In some extreme cases with very cold air leak-age, high O 2 value with high residual watervapour with low unburned carbon etc. canlead to the opposite situation.

Conclusion

The article at hand presents and evaluatessome modifications of the standardised indi-rect method for steam boiler efficiency deter-mination, DIN 12952-15. The proposed modi-fications improve consistency of computa-tions and accuracy of results. Six modifica-

DIN 12952-15 R e l a t

i v e c h a n g e o f

f l u e - g a s m a s s

f l o w

l e a v

i n g

a i r

h e a t e r , %

Ratio of preheated air in air heater

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00

15 C

20 C

25 C

30 C

Temperature differencebetween controlled air

and leekage air

Hard coal

Actual amount of

controlled air = 0.865

Figure 6. Impact of air leakage on computed flue gas mass flow, hard coal.

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tions are studied. Four of them relate to moreaccurate air ratios, flue gas mass determina-tion including partial oxidation of carbon toCO; two models including unburned combus-tibles in ash and slag; the fourth including theresidual water vapour content in the flue gassample. The fifth modification is to includethe heat input of leakage air and the sixth isapplying the VDI 4670 procedure in the en-thalpy calculation of flue gas and combustionair.

Sensitivity analysis was made for all six mod-ifications. The impact of the proposed modi-fications to the boiler efficiency was evaluat-ed on two real data sets for hard coal and lig-nite. It is demonstrated that the impacts ofunburned combustibles in ash and slag andthe impact of residual water vapour in the fluegas sample are the most influential factors in

accurate boiler efficiency determinations re-garding the above stated modifications. Theimpact sign and magnitude in the case of re-sidual water vapour in the flue gas sample de-

pends on which gas is considered in calcula-tions, CO 2 or O2 and the flue-gas sample tem-

perature in the cooler. It is evident that theimpact of CO 2 has the greatest effect.

In two studied cases for hard coal and lignite,the proposed modifications increase the calcu-lated boiler efficiency. If O 2 is usedin the calculation, the average deviation is h B < +0.01 %; if CO 2 is used, the deviations in-crease to h B +0.07 %. Deviations of boilerefficiency are small due to low CO contentand low content of unburned matter in ash andslag. If the amount of unburned carbon fromfuel would be 1 %, the deviation of boilerefficiency would be higher than +0.1 %.

The analysis shows that boiler efficiency isdecreased only by including leakage air andresidual water vapour in the flue-gas sample ifO2 is used in the calculation. All other mod-ifications increase the boiler efficiency com-

pared to the standard procedure. The overallimpact of all modifications is expected to be

positive. This means that the actual boiler ef-ficiency is higher than the efficiency calcu-lated according to DIN 12952-15. The actualflue gas mass flow leaving the air heater isalso lower up to 2 %.

Acknowledgment

The authors of the article at hand would liketo thank VGB-FORSCHUNGSSTIFTUNGfor supporting this research.

References

[1] DIN 12952-15, Wasserrohrkessel und Anlagen-komponenten Tiel 15: Abnahmeversuche;Deutsche Fassung EN 12952-15:2003, Januar2004.

[2] Sekavcnik, A., Kutrin, I., and Oman, J.: FiveEnhancements to the Standard Indirect Methodfor Determining a Steam Boilers Efficiency,VGB PowerTech, vol. 86, no. 3, 2006, pp. 82-86.

[3] Senegacnik, A., Kutrin, I. , Sekavcnik, M., andOman J.: Analysis and Validation of Theoreti-cal Optimisation Potentials of Steam Boiler Ef-

ficiency Determination by Use of Real Meas-uring Data VGB FORSCHUNGSSTIFTUNGresearch project, Faculty of Mechanical Engi-neering, Ljubljana, 2008.

[4] VDI 4670 Thermodynamische Stoffwerte vonfeuchter Luft und Verbrennungsgasen, VDIRichtlinien, Oktober 2000.

[5] Brandt, F.: Brennstoffe und Verbrennungsrech-nung, 3. Auflage, Vulkan-Verlag GmbH, Essen,1999.

[6] Kutrin I. , Oman J., and Senegacnik, A.: Thecollection of technical reports on tests of steam

boiler s of Slovenia, Faculty of MechanicalEngineering, Ljubljana, 1993-2008.

List of Symbols

Nomenclature and meanings of symbol smatches the nomenclature in DIN 12952-15

Symbol Unit Description kg/kg Share of carbon forming CO 2coal kg/kg Fuel content fuel sample

(by mass)lu Ratio of unburned combusti-

bles to supplied fuel massflow

m kg/s Mass flowQ kW Heat flowV m 3/kg Combustion air and flue gas

volume (per unit mass of fuel)

R e l a t

i v e c h a n g e o f

t h e

f l u e g a s

l o s s , %

Lignite

Hard coal

DIN 12952-150.00

-0.02

-0.04

-0.06

-0.08

-0.10

-0.12

-1.14

-1.16

-1.18

-0.20

Figure 7. Impact of VDI 4670 enthalpy calculation in flue gas loss computation.

DIN 12952-15 A b s o l u t e c h a n g e o f

b o i l e r e f

f i c i e n c y ,

%Hard coalLignite

-0.020

-0.010

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0.080

CO 2

O2

Ipartial oxida.

of C to CO

II unburned combustibies in fuel pure carbon linear model

III IVresidual water

in fl. gas sample

Vuncontrolledcold air leak.

VI VDI 4670

applied

ALL 1 ALL 2all cases

simultaneously

O2consi-

deration

O2consi-

deration

Figure 8. Absolute change of boiler efficiency.

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m3/m3 Specific volume kg/kg Flue gas/combustion air com-

ponents by massAd kg/kg Combustion air content by

mass m3/m3 Content by volume Difference kg/kg Combustion air/flue gas mass

to fuel mass ratio Air ratio kg/kg Fuel content actual burned

(by mass)

Subscripts and superscripts * Measured value^ MaximumA Air Ar ArgonAsh AshC CarbonCO Carbon monoxideCO2 Carbon dioxided Dry (basis)F Fuel, burned fuelFA Flue dust (dry ash)Fo Fuel supplied

G Flue gas (combustion gas)H HydrogenH2O Water LA Leakage (infiltrated) air / tramp air

N, N 2 Nitrogen N Useful, effective Ne Neon

o StoichiometricO, O2 OxygenS Sulfur Sat SaturationSL SlagSO2 Sulfur dioxideZ Heat input h

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