Paper 80

Reprinted From

Proceedings of the

2009 ASME Power Conference

Albuquerque, New Mexico, July 21-23, 2009

POWER2009-81221

ERRORS IN BOILER EFFICIENCY STANDARDS

Fred D. Lang, P.E.Exergetic Systems, Inc.

San Rafael, California 94901 [email protected]

ABSTRACTThis paper presents both criticism and suggested

changes to boiler efficiency standards associated withfossil-fired steam generators. These standards include thewidely used ASME PTC 4.1 and DIN 1942. Although nowreplaced with ASME PTC 4:2008 and the European EN12952-15:2003, it is useful to review both old and new.The chief criticism lies with inconsistent application ofthermodynamic principles. Conceptual errors are madewith reference temperatures and with shaft powers.

When using computed fuel flow as a touchstone,it becomes obvious that arbitrary use of referencetemperatures and/or use of capricious energy credits cannotdictate a system’s computed fuel flow. Efficiency, calorificvalue and fuel flow must have fixed definitionsconcomitant with a system’s useful energy flow.Thermodynamics is not an arbitrary discipline, thecomputed fuel needs of a system must describe the actual. Boiler efficiency requires the same treatment, as anabsolute value, as actual fuel feed and emission flow.

Boiler efficiencies and associated calorific valueshave obvious standing when judging contractualobligations, for thermal performance monitoring, and forconfirming carbon emissions. Note that a 0.5 to 1% changein efficiency may well have significant financialconsequences when testing a new unit, or the on-goingcosts associated with fuel and carbon taxes. This paperdemonstrates that errors greater than 2% are entirelypossible if following the current standards. This paperappeals to the resolution of efficiency at the 0.1% level.

The power plant engineer is encouraged to read theIntroduction and Summary & Recommendations sectionswhile the thermodynamicist is requested to throughlyreview and critique the mid-sections. The author hopessuch reviews will advocate for improvement of theseimportant industrial standards. PAPER-80.WPD, Rev 29.

NOMENCLATURENote that much of the following nomenclature is takenfrom Exergetic Systems’ steam generator simulator, theEX-FOSS program (Lang, 2009a).

Molar Quantities Related to Stoichiometrics

j n = Molar quantity of product j, moles/base

j N = Molecular weight of substance j x = Moles of fuel/100 moles dry product (base)

2 4 2 y = Moles of H O as in CaSO @yH O

2 z = Moles of average H per mole gaseous fuel

k " = Moles of As-Fired fuel constituent k.

Quantities Related to System Terms CV = Calorific Value

Cal AF HBC = Firing Correction relative to T , )Btu/lbm HHV = Fuel gross internal energy (including moisture

AFand ash) at constant volume, Btu/lbm HHVP = As-Fired gross calorific value corrected for a

AFconstant pressure process, Btu/lbm HNSL = Non-Chemistry & Non-Stack Losses.

Act AFHPR = Enthalpy of actual products, Btu/lbm

Ideal AFHPR = Enthalpy of ideal products, Btu/lbm

Act AFHRX = Enthalpy of actual reactants, Btu/lbm

Cal Cal AFHRX = Enthalpy of reactants at T , Btu/lbm J = Energy conversion, 778.16926 ft-lbf/Btu

AF LHV = Fuel net internal energy, Btu/lbm LHVP = As-Fired net calorific value corrected for a

AFconstant pressure process, Btu/lbm

AF AF m = As-Fired fuel mass flow rate, lbm /hr

WF Q = Useful Energy Flow Delivered from thecombustion gases, Btu/hr

R = Gas constant, 1545.325 ft-lbf/lb-mole/R

Cal T = Calorimetric temperature, F

RA T = Ambient air temperature, the referencefor ASME PTC 4.1 and 4.4, F

1

Stack T = Exit (Stack) boundary temperature, F

ID W = Fan powers regards outlet streams, Btu/hr

A 0 = Boiler absorption efficiency, unitless

B 0 = Boiler efficiency, unitless

f-Cal Cal)H = Heat of Formation at T , )Btu/lbm.0

L/H )H = Enthalpy correction for net CV, )Btu/lbAF

V/P AF )H = Enthalpy correction for volume, )Btu/lb

L/H)PV = Specific flow energy correction from net

AFCV to gross, )Btu/lbm

L/H )U = Internal energy correction from net CV

AFto gross, )Btu/lbm

Subscripts Abs = Absolute temperature AF = As-Fired referring to fuel containing

moisture and mineral matter (ash), the boundary crossing total fuel.

Cal = Calorimetric, as in calorimetric temperature HHV = Gross calorific value, higher heating value LHV = Net calorific value, lower heating value.

INTRODUCTION TO BOILER EFFICIENCY STANDARDS

This section discusses boiler efficiency standards andsummarizes their problems. No present boiler efficiencystandard addresses calorimetric fundamentals. Althoughmore fully explained in subsequent sections, this statementmeans that the method of determining the energy contentof a fossil fuel dictates the method by which theperformance of that fuel is judged when burned to makesteam. The practical justification for such a statement lieswith fuel water plus fuel hydrogen (product water).

Boiler efficiency as taught by industrial standardsusing an Input-Output Method is fundamentally:

B 0 = [Useful Energy Flow Developed] / [Fuel Energy Flow Supplied] (1)

Although a simple expression, historically there are anynumber of interpretations of the terms comprising itsnumerator and denominator. For example, the newlyreleased ASME PTC 4 states (§3-1.2) that one maycompute over a dozen different values of boiler efficiencyusing various interpretations of the same data set! Thispaper argues there is only one method, a method leading toa consistent and unique computation of fuel flow.

Definitions must be explained concerning both asteam generator as a “system” and then its “state”. Appliedthermodynamics is defined by a boundary; most boilerefficiency standards err with their boundary assumptions.

The boundary of a fossil-fired steam generatorbeing studied for thermal efficiency does notencompass all physical equipment, but rather theconstraining volume of its interacting fluids:fuel conveyance, combustion gas confinement,the inside of air ducts and working fluid pipeID. This boundary derives from the principlethat the thermal efficiency of a steam generatoronly addresses how the as-fired fuel interactswith gas/air/working fluid.

Extraneous equipment has no impact on boiler efficiencyif not directly affecting the fuel’s interaction with thegas/air/working fluid. This definition is consistent with theconcept of calorific value. It immediately excludes suchequipment as pulverizers, steam driven pumps,recirculation pumps, and the like. ASME PTC 4 and DIN1942 would suggest that a higher pulverizer shaft powerwill increase boiler efficiency. However, pulverizer powerhas no impact on the interaction of fuel and gas/air/working fluid (the grinding of coal is emulated during labprep of samples). A recirculation pump has no impact onfired fuel heating working fluid. This metric, boilerefficiency, must solely guide the engineer towards reduced

2fuel flow and CO emissions while still making adequatesteam. Shaft powers are monitored through house load.

A system in equilibrium with its medium, from whichno power, even theoretical, can be extracted, is said to beat its dead state (Keenan, 1941). For an active steamgenerator, having the potential for work, the condition ofits dead state is often confused with the concept ofreference state. The dead state limits how much potentialpower is possible, an ultimate limit to actual output, thenumerator of Eq.(1). For steam generators, the dead stateshould be taken as the coldest medium associated with thelocal environment. For any steam generator its actualthermal power is of course an absolute, established byconsidering unambiguous differences between referencestates (main steam less feedwater, etc.). In engineering, allenergy levels are relative to a chosen reference state. Textbooks will argue that reference states are arbitrary, asimple enough concept when dealing with a single fluid.Water properties for example, normally referenced to thetriple point, could be referenced to the boiling point at 1atmosphere, resulting in the same useful energy flow viaEq.(1). However, for this discussion we are not concernedwith the numerator of Eq.(1), but rather treatment of thedenominator. Note that although Eq.(1) expresses an Input-Output approach, conversion to specific valuesimmediately invokes a Heat Loss Method (herein termedthe “Energy Balance Method”); but the two must produce

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identical efficiencies. The argument here is for absolutes inthe denominator, that the “arbitrariness” of reference statescan not be defined by the casual analyst but, indeed, isestablished by the person determining the energy contentof the fuel. The technician determining calorific value ofnatural gas may be required run his/her calculations usinga national standard, e.g., 0C for France, 15C for Irelandand the U.K., 25C for Germany, 60F (15.56C) for NorthAmerica, etc. (see ISO 12213, ISO 13343 and AGA ReportNo. 3), or another yet. For solid and liquid fossil fuels thereference temperature may be viewed by some as arbitrary,but in fact is also set by the technician running the bombcalorimeter, not the casual analyst. For the calorimeter, thereference temperature is the calorimetric temperature atwhich the bomb is brought into equilibrium.

It is well understood that calorific values vary withtemperature. Natural gas evaluated at 0C is different thanthat at 25C, a bomb run at 50F will yield a different valuefor the same coal as one run at 120F. Because as-firedconditions are relative to reference temperature, boilerefficiency will vary with reference temperature. That said,the objective of this paper - and the proposed objective ofall efficiency standards - is an absolute understanding of asteam generator’s thermal performance, the fuel itconsumes, consistent with a defined boundary. If Eq.(1)’snumerator is an absolute measure of thermal power, wecan not allow an arbitrary denominator. However,recognizing that boiler efficiency varies with temperatureis no justification for standards advocating relativeefficiencies, but rather, efficiency equations whichproperly correct for reference temperature. The test forabsolutes is computed fuel flow, a fuel mass flow whichjustifies the developed thermal output. A temperaturedependent calorific value leads to a temperature dependentboiler efficiency, corrected to actual firing conditions,producing a consistent and unique fuel flow.

The act of choosing a calorimetric temperature toeither compute (for gaseous fuels) or to measure (for solidand liquid fuels) a calorific value does not give the casualanalyst the liberty to then choose another for reference. Forany multi-fluid system, only a consistent thermodynamicreference state can be considered or the laws ofthermodynamics will not be satisfied. It is not acceptableto assume one energy level for the fuel, another for theworking fluid, another for the combustion gases, anotherfor the dry combustion air, and yet another for moisture inthe combustion air. As examples: the air’s nitrogen andfuel make combustion products; sorbents, tube leaks andsoot blowing add to products; the air’s moisture affectsproducts; and combustion products heat working fluid.

ERRORS, WHAT ERRORS ? Addressed are three types of errors and uncertainties

involving thermodynamic concepts, procedural problems,and measurements which impact efficiency:

1) Conceptual errors are made when: a) mis-using reference temperatures, and b) when treating certain system components, especially shaft power items;

2) Procedural errors are made which impactthermodynamics in a generic manner, e.g., thedefinition of dry air, molecular weights, enthalpicreferences for fluids mixed within the system,thermodynamic properties, etc.; and

3) Measurement uncertainties found in the labaffecting fuel chemistry and calorific values, certain

WFloss terms (HNSL), and in plant data affecting Qand thus a back-calculated fuel flow.

For all of these, the author argues for an accuracy criteria

Bat the ±0.1% )0 level. What this means is that conceptsand processes affecting a computational efficiency at $

B±0.1% )0 must be included. This is not to say allconcepts and processes are to be understood at this level,but are to be included given a computational sensitivitythus demonstrated. Although coal analysis betweenlaboratories should lie within ±100 )Btu/lbm (±233)kJ/kg), modern bomb calorimeters are quoted as having0.1% )HHV/HHV repeatability. It is this fact which addsimperative. The author’s EX-FOSS program allows for anerror calculation based on stoichiometric inconsistencies(as-tested emissions versus assumed fuel chemistry), which

Bif > ±0.1% )0 , and unexplained, results in repeating thetest. It is noteworthy that ASME PTC 4 (§5-7.3) expressesthis same opinion, stating that “a convergence limit of0.1% efficiency is sufficient”.

Set out below are errors made in traditional boilerefficiency standards. This list is by no means complete, butconcentrates on “credits” and reference temperatures.

ASME PTC 4.1 (United States):This code was superseded in 1998 by ASME PTC 4.

Although PTC 4.1 is no longer an ASME Code noreven an ANSI standard, it is included here given itsoccasional use in North America and Europe.

§1.04.5, shaft powers from pulverizers and circulatingpumps are included as energy credits.

§7.2.8.1, reference temperature is taken as the air’s

RAambient temperature (T ). §7.2.8.1 and Appendix, nitrogen content in dry air is

fixed at 76.85% (an assumed weight fraction).

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§7.3, “heat credits” appear in the numerator anddenominator.

The FD and ID fans are considered outside thethermodynamic envelope.

The standard’s Input-Output and Heat Loss Methods areinconsistent (see DIN 1942).

This standard applies to any fossil fuel.

ASME PTC 4 (United States): The original 1998 release was superseded with a 2008

release to which the following are referenced. §5-5, the defined fuel energy flow ignores the as-fired

state (if different from 77F), this would especiallydestroy any accuracy of gas-fired efficiencies giventypical firing from 45F to 60F.

§5-7, “fuel efficiency” is PTC 4's preferred method, fuelenergy is not corrected to the as-fired.

§5-7.1, “credits” appear only in the numerator. §5-8.1, the conversion from constant volume to constant

pressure ignores nitrogen and oxygen bound in thefuel, oxygen is an important term when consideringhigh oxygenated fuels (such as PRB); suchconversion is not temperature dependent.

§5-9, sorbent energy flows are normalized to the as-fired fuel, which ignores unique sensible energies. §5-9.5.1, §5-9.5.2, §5-11.1, §5-11.4.1, etc., oxygen

2content in dry air is fixed at 20.95% (Ar and COare not included, air/oxygen = 3.7733) .

§5-11.2, psychrometric properties should be extended to -40F, as applicable for steam generators found innorther climes; caution should be exercised whenusing reference psychrometric temperatures; thestandard employs a water to dry air ratio of 0.622(textbook), its own data would suggests 0.6398).

§5-13.1, the reference temperature is set constant at 25C.§5-15.5.1 and §5-15.5.2, shaft powers are included as

energy credits. §5-19.8, the enthalpy of natural gas should be computed,

and is easily done given known properties. §5-19.10, water vapor properties should employ ASME

standards. §5-19.11 and §5-19.12, “average” combustion gas

properties are erroneously employed, such use ishardly justified given the wide applicability thisstandard is assumed to have; the referenced 1971JANAF/NASA properties are out-of-date (othercitations, appearing political, are quite current).

The FD and ID fans are considered outside thethermodynamic envelope.

This new 2008 release (unlike the 1998 version) relatesto coal-, oil- and gas-fired steam generators.

DIN 1942 (German): This code was superseded by DIN EN 12952-15:2004.

The following nomenclature and comments arespecific to DIN 1942 (Feb. 1994).

b§6.2, the reference temperature (t ) is set at 25C. However, “other temperatures may be agreed upon”by correcting the net calorific value with fuel, airand combustion gas sensible heat terms.

Z§6.3.2.3, so-called “heat credits” (denoted as Q )includes shaft powers from pulverizers,recirculating gas fans, working fluid circulatingpumps and “power from any other motors”.

Z§6.4, DIN 1942 employs the Q term in both its Input-Output and Heat Loss Methods. In DIN 1942:

N WF ZBQ is the useful output (herein Q ); Q is the

AF Vtotfuel energy (m LHVP); and Q is the loss term.

B-LHV0 = (DIN-144)N Q

ZB ZQ + Q

B-LHV0 = 1.0 - (DIN-147)Vtot Q

ZB ZQ + Q

ZFor Eq.(DIN-147) an increase in Q will always

B-LHV Vtotincrease 0 provided Q > 0.0. However,

Zthe same increase in Q will always decrease theInput-Output efficiency of Eq.(DIN-144), thusguaranteeing inconsistent computed fuel flows.This same conundrum exists with PTC 4.1, withPTC 4 and its “gross efficiency” definition, andwith other standards.

§6.3.4.1, oxygen content in dry air is fixed at 20.938% (Ar is not included, air/oxygen = 3.7760) . This standard applies to any fossil fuel.

Draft European Standard: The following comments reference prEN 12952-15 of

Nov. 1999, which is now issued as EN 12952-15. The draft and the new closely follow DIN 1942,employing its nomenclature and general methods.

b§7.2, the reference temperature (t ) is set at 25C, but“other temperatures may be agreed upon” whichcorrects heat credits as done in DIN 1942.

§7.3.4.1, oxygen content in dry air is fixed at 20.938%

2(Ar is not included but apparently CO is included,air/oxygen = 3.7760).

§7.4.3.2, energy credits appear only in the denominator. The ID fan is considered outside the envelope; the FD

fan may be considered inside the envelope. This standard applies to any fossil fuel.

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BS 2885 (British): The following comments reference BS 2885:1974;

which has been superseded by the British StandardsInstitute (BSI) issue of BS EN 12952-15:2003(basically the revised DIN 1942).

§2 (bottom), all fuels shall use a “calorimetrictemperature” of 25C.

Section E and Section F, Item 434, the standard expects that flue gas nitrogen is to be measured, thus dry air

is not specified. Items 708, 804 and 907 (and Notes), the reference

temperature for sensible heats in the dry flue gas,moisture in the combustion air and fuel is thecombustion air temperature (per ASME PTC 4.1),not 25C as its stated “calorimetric temperature”.

Item 901 (and Notes) regards “Method A” (Input-Output) it does not consider energy credits; itinvokes a simple “fuel efficiency” in which the CVbears no Firing Correction. However Item 902,invoking the Heat Loss Method (“Method B”)considers the “heat equivalent of auxiliary power”as a loss, carrying the same sign as the radiation &convection loss. Such inconsistencies will result inimpossible differences in computed fuel flows.

Energy credits (shaft powers) appear only in thedenominator of its Heat Loss Method.

This standard applies to any fossil fuel.

Standard for Recovery Boilers Used in the Pulp & Paper Industry (United States):

This standard is based on a 1993 draft of ASME PTC 4using its energy balance method.

§0 (page 4), the reference temperature is set at 77F. §7.1.4.1 and §7.1.4.3, oxygen content in dry air is fixed

at 23.14% (a carry-over from ASME PTC 4.1, butused as a molar ratio and is not correct) .

For recovery boilers, burning black liquor fuel, is itcommon industrial practice to correct the measuredheating value for Heats of Formation for the

2 4reduction of Na SO . Such corrections address thedifference between ideal combustion productsassociated with a bomb calorimeter versus actualproducts associated with further reduction ofcertain black liquor compounds. Such correctionsare thermodynamically inconsistent, as the calorificvalue is corrected with a computed )HR term:(HHVP - )HR + HBC).

This standard applies only to recovery boilers burningsodium-laced black liquor.

CONCEPTUAL AND PROCEDURAL ERRORSCombustion produces water from the fuel’s entrained

water and from bound hydrogen. Conceptual errorsassociated with the calorimetric temperature for highlyenergetic fuels, with low product water, are slight and

Btypically do not meet the 0.1% )0 criteria. However,conceptual errors associated with fuels producing 10% ormore product water (such as high volatile B bituminous(hvBb), to PRB coal, to the lignites and peat) are

Bappreciable ranging from 0.2% to the 0.8% )0 levelassuming a 18 )F (10 )C) change in calorimetrictemperature. Such errors derive from assuming a referencetemperature for boiler efficiency calculations, while thecalorific value was determined at another. For example, a

B0.5% )0 error is produced for a Powder River Basin(PRB) coal when 25C is the assumed reference, while theCV was determined at 35C.

Conceptual errors may also exist when notrecognizing the sensitivity of the ratio of ambient oxygento dry air. A change of ambient oxygen from 21% to

B-HHV20.5% represents a decrease of 0.253% )0 for a

2typical PRB fired unit assuming a constant Stack O ; a

B-HHV1.128% )0 decrease for 19% ambient. Of the steamgenerators tested by the author, typically 1 in 10 werefound to have degraded ambient oxygen levels typicallycaused either by a weather inversion, still air or flue gasleakage into FD Fans. The NASA (1976) standard ambientoxygen is 20.9476% at sea level.

Conceptual errors are also made when following thecurrent standards (low water fuels aside) which involve thetreatment of shaft powers, discussed in the next section.

Procedural errors are made by not adhering to thelatest thermodynamic standards. The author finds it rarethat any two standards use the same molecular weights.Inconsistency is present in air psychrometric properties(see ASHRAE procedures, discussed below). One wouldexpect to use the same fundamental methods whenevaluating a combustion turbine versus a conventionalsteam generator (ASME PTC 22 assumes 60F as a base,conversion is allowed per its §4.12, versus ASME PTC 4).

Precise methods offer little comfort if laboratoriescannot record CVs with at least repeatability, if not also

Bwith accuracy. Although the 0.1% )0 criteria is meet inrepeatability when using the modern bomb calorimeter, wemust remember that variability found in CVs (say fromgrab sampling) may, indeed, be quite real. Standards mustpresent a practical statistical treatment of multiple labchemistries and CVs, as would be associated with testinga coal-fired unit. It is noteworthy that ASME PTC 4devotes its Section 7 to uncertainty analysis. AlthoughSection 7 is clearly amenable for academic pursuits, it is

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not something most power plant engineers are going toplace under their pillows. The coal-fired industry needsprocedures, fully integrated within the standards, in whicha test can be evaluated for: 1) Consistency of individual as-tested fuel chemistries

(rejecting any HHV given an outlying chemistry) [a possible solution is to apply the techniquesafforded in the Oxy-Hydrocarbon model (Lang &Canning, 2007)];

2) A defined steady state period by examining timeplots of feedwater and fuel flows (over at least 15

WFminutes), the averaged data resulting in Q [although such plots have been used by the authorand his colleagues for years, the precedence for thisis 125 years old! (Kent, 1884)];

3) Tolerance on a proper Energy Balance Methodefficiency, as based on items only affecting

WFcalorific value (i.e., sampling) and Q ; and4) A consistent boiler efficiency, allowing the

calculation of an absolute fuel flow.

CONSISTENT BOILER EFFICIENCYThe temperature used to operate a bomb calorimeter,

or to compute a gaseous CV, is the beginning point fordeveloping consistent thermodynamics. Calorific valuesfor solid or liquid fuels are obtained either by adiabatic orisoperibol bomb calorimetry following ASTM D5865 orISO 1928. An adiabatic bomb calorimeter detects the grossenergy liberated from ideal combustion by maintaining aconstant water bath temperature about the bomb, which

Caldefines the calorimetric temperature, T . An isoperibolbomb calorimeter detects the net energy liberated byaccurately monitoring the water bath temperature, its

Calresultant average value being T . Many modern bomb

Calcalorimeters are automated to run at a programmable T .The author has found various labs in North America andEurope using 27C (80.6F), 28.5C (83.3F), 30C (86F) and,commonly, 35C (95F). The author has yet to find any labin North America or in Europe determining coal calorificvalue at 25C, the reference for most standards!

Boiler efficiency should be a simple reflection ofwhat the technician, determining calorific value by eithercalculation or measurement, has produced. The processbegins with reactants, the combustion, and ends with ideal

2 2 2products of CO , SO and H O. This process is pathdependent, the traditional path is to maintain an essentiallyconstant temperature of the calorimeter’s heat sink. For thesteam generator, the thermodynamicist need only accountfor losses associated with the actual product streams, andsensible heat terms accounting for the fact that reactantsmay not be fired at the calorimetric temperature. This then

is the conversion efficiency of burning fuel, making a“Useful Energy Flow Developed” from combustion; i.e.,its interaction with gas/air/working fluid. If a consistent

WFfuel flow is to be computed from boiler efficiency, Qmust only reflect heating from combustion gases. Theexecution of these concepts is a bit more involved.

The definition of gross calorific value (higher heatingvalue) as based on a bomb calorimeter is the energyliberated from products formed relative to the calorimetrictemperature, this includes, of course, the water produced asreduced to the liquid state. We do not measure net values(lower heating values). The internal energy liberated froma constant volume bomb is relative to the equilibriumtemperature at which the bomb functioned, as described bythe following:

T-Cal � MQ = - HHV (2)

Note that Eq.(2) is path dependent, for a traditional bombcalorimeter, industrial practice sets this path as one havinga constant bath temperature. Conversion from a constantvolume internal energy (HHV) to a constant pressureenthalpy (HHVP), and its definition, are given by:

V/P HHVP = HHV + )H (3)

Ideal-HHV Cal-HHV HHVP = - HPR + HRX (4)

V/PA fuel’s calorific value, after conversion using )H , isthe difference between the enthalpy of ideal combustion

Ideal-HHVproducts (HPR ) and the enthalpy of the reactants

Cal-HHV(HRX ), both evaluated at that temperature at whichthese quantities interacted. If we assume the boilerefficiency of a calorimeter is unity (i.e., ideal combustion),it follows that Eq.(4) must serve as the basis for all boilerefficiency standards.

First, consider that the as-fired fuel’s energy contentmust be corrected for sensible heat. Note that firing fuel at10C or 100C cannot affect its interaction with the gas/air/

Cal working fluid, if properly referenced to T . As analystswe should be able to run calorimeters at any temperature,fire the fuel at any other temperature, without bias to theconcept of ideal combustion, without affectingthermodynamic principles, and still compute a unique fuelflow. Thus by simply adding a “Firing Correction” term(HBC) to each side of Eq.(4), we bring the reactants termto the as-fired condition, maintaining unity efficiency:

Ideal-HHV Cal-HHV HHVP + HBC = - HPR + HRX + HBC(5)

Note that the signs associated with Eqs.(4) & (5) yield to

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the convention of a positive calorific value (note that the

Ideal Calnumeric value of HPR is always < 0.0, and HRX istypically < 0.0). The efficiency of a system described byEq.(5) is unity. Eq.(5) is interesting in that no condition ofthe combustion air (bone dry or fogged), nor size of FDFan, nor steam-air heating, etc. will affect boiler efficiencyper se, as this equipment only affects inlet streams; we aresimply correcting (HBC) for the reactants’ sensible heat!

The next step is to degrade from the ideal byaccounting for losses. How thermodynamic losses aregrouped may be treated in any number of ways. For thiswork, they are based on specific energy terms (Btu/lbm),

AF and are described by 3Losses/m . If just subtracting

Ideallosses from HPR , Eq.(5) becomes unbalanced without

B-HHV B-LHVan efficiency term, the gross (0 ) and net (0 )becoming for the Energy Balance Method the following:

B-HHV Ideal-HHV 0 (HHVP + HBC) = [- HPR

AF Cal-HHV - 3Losses/m ] + [HRX + HBC] (6A)

B-LHV Ideal-LHV 0 (LHVP + HBC) = [- HPR

AF Cal-LHV - 3Losses/m ] + [HRX + HBC] (6B)

Computed boiler efficiencies follow directly from Eq.(6):

B-HHV 0 =

(7A)Ideal-HHV AF Act-HHV - HPR - 3Losses/m + HRX

HHVP + HBC

B-LHV 0 =

(7B)Ideal-LHV AF Act-LHV - HPR - 3Losses/m + HRX

LHVP + HBC

The individual terms in these expressions may now bedefined and expanded; see Lang (2009b, 2009c) for details.It is tacitly assumed that back-correcting an as-measuredcalorific value (HHV) of a solid or liquid fossil fuel to

Calsome chosen T is not possible given chemicalcomplexities of such fuels. It is for this reason that the

Calreactant term HRX of Eq.(6) is evaluated using Eq.(4),the ideal products term being well defined, HHV beingmeasured in a modern calorimeter.

Cal-HHV Ideal-HHV HRX = HHVP + HPR (8A)

Cal-LHV Ideal-LHV HRX = LHVP + HPR (8B)

In Eq.(8B) conversion to net (detailed below) is given by:

L/H LHVP = HHVP - )H (9)

The actual reactant term is then developed, noting that the

same Firing Correction applies to both the gross and net:

Act-HHV Cal-HHV HRX = HRX + HBC (10A)

Act-LHV Cal-LHV HRX = HRX + HBC (10B)

In summary, Firing Corrections are to be judged onlyif they correct to the as-fired condition. Since FiringCorrections can not affect departure from unity efficiency(only losses), any consideration of a Firing Correctionwhich does not maintain this concept will produce error.

P Any standard not specifying the use of a FiringCorrection term, based on calorimetrics, applied inboth numerator and denominator of a proper EnergyBalance equation, per Eqs.(7) & (10), is in error. Theconcept of “fuel efficiency” is meaningless forindustrial standards. It is remarkable that asreaffirmed in 1991, ASME PTC 4.1 employed “heatcredits” in both its numerator and denominator, andnot eight years later PTC 4 employed “credits” onlyin the numerator - undoubtedly reflecting theconciliatory nature of thermodynamics.

P A standard allowing for pulverizer electrical power tobe added as a “credit” is an error. Pulverizer shaftpower adds nothing to the process of fuel interactingwith gas/air/working fluid, other than grinding coal.However this action is replicated when preparing asample for bomb calorimetry. Grinding a brittlesubstance effects its surface energy; it may be viewedas the excess energy at the surface of a groundmaterial compared to its bulk condition (a potentialenergy). If HHV differences between sample grindingin the lab and pulverizing could be measured (i.e.,greatly differing mesh sizes), then a correction toHHV (not the Firing Corrections) is entirely justified.The author has attempted this, using lab studies,without notable differences (it should be repeated).

P A standard allowing the ambient air temperature to betaken as the reference is in error. For example, somestandards would allow a natural gas CV to becomputed at 60F, referenced at 95F and fired at 45Fwith a computed fuel flow known but to God. Theonly applicable reference is the calorimetric at 60F.

P A standard not appreciating that the reference statesof the various fluids (water, gas, dry air, air’smoisture) must be referenced to the same condition

Cal (the author argues for T ), is thus allowing for aFirst Law violation; steam Generators mix all fluids.

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For example, North America psychrometricproperties share a unique strangeness, where dry airis taken from 0.0F (-17.8C) and water from the triplepoint; which is absurd when addressing boiler in-leakage. For more strangeness, ISO 12213, reflectingnational standards, specifies that different referencetemperatures may be employed for natural gas CVversus volume metering (e.g., Germany computes theCV at 25C, but metered flows are reference to 0C).

P A boiler standard which allows, for example,circulating pump power to affect boiler efficiency isin error. A working fluid pump power associated with

WFthe steam generator must be subtracted from Q , as

WFQ must only reflect the useful energy flow addedfrom combustion gases. Such manipulations shouldbe kept apart from the efficiency equation.

As regarding loss terms, although differing techniquesmay be employed, the above fundamentals must befollowed; specifically anything affecting a product stream

Ideal(i.e., referenced to HPR ) is a loss and will decreaseefficiency and increase computed fuel flow. Differentprocedures divide the 3Losses term of Eq.(7) into differentcategories. Exergetic Systems separates 3Losses into stack

Actrelated quantities, reflected in its HPR , and non-stackquantities termed “Non-Chemistry & Non-Stack Losses”(HNSL).

B-HHV Act-HHV Act-HHV 0 = - HPR + HRX - HNSL HHVP + HBC

(11)

This allows definition of the actual product term withoutdifferentiating “stack losses” from routine combustionproducts, and eliminates formulation problems of DIN1942, etc. The actual quantities are evaluated usingEqs.(13) & (14) in which Heats of Formation are converted

Cal Calf -to the T reference of )H ; conventional “stack0

losses” may be back-calculated for user convenience. Notethat Eq.(13) includes water’s Heat of Formation, specifiedseparately for clarity. In Eq.(13) the term “Comb-H2O”relates to water formed from the hydrocarbon, “Fuel-H2O”is fuel water, “Lime-H2O” is captured water associatedwith the reaction of injected limestone, “CAir-H2O” ismoisture in combustion air, and “Leak-H2O” is in-leakage

j of water. HPR is the enthalpy of non-water product j at

Calthe boundary, all evaluated at T . Note that water leakage

Stack f-Calis relative to its outlet stream (h - h ) less its inlet

Steam f-Cal(h - h ). Eq.(12) is not used, listed only for clarity:

Act Ideal AFHPR = HPR + 3(Stack Losses)/m (12)

Act-HHV j Comb-H2O f-Cal/liq HPR = 3HPR + [ n ()H 0

Stack f-Cal+ h - h )

Fuel-H2O Stack f-Cal+ n (h - h )

Lime-H2O f-Stack f-Cal+ n (h - h )

CAir-H2O Stack g-Cal+ n (h - h )

Leak-H2O Stack Steam H2O AF + n (h - h )]N /(xN )(13A)

Act-LHV j Comb-H2O f-Cal/vap HPR = 3HPR + [ n ()H 0

Stack g-Cal+ h - h )

Fuel-H2O Stack g-Cal+ n (h - h )

Lime-H2O f-Stack g-Cal+ n (h - h )

CAir-H2O Stack g-Cal+ n (h - h )

Leak-H2O Stack Steam H2O AF + n (h - h )]N /(xN )(13B)

Stack T

j f-Cal/ j j j j AF HPR = [)H + I dh ] n N / (xN ) (14)0

CalT

AAn absorption efficiency, 0 , based on Eq.(11), isderived from HNSL, whose individual terms are evaluatedusing established procedures:

$ P d/Fly d/Prec d/Ca R ID AFHNSL = L + L + L + L + L + L - W /m (15)

HNSL bears the same numerical value for both gross or net

Acalculations, as does 0 (below). All terms of Eq.(15) are

AFspecific to the fuel (i.e., Btu/lbm ). Again, these termsaffect product streams, no term can be included which doesnot have such affect. Slight exceptions to standard usage

$include the following: L is referenced to the total gross(corrected) calorific input, (HHVP + HBC), not HHV or

WLHV; the L term (of PTC 4.1) is combined with the ash

P d/Fly d/Precpit term L ; L is sensible heat in fly ash; L is thesensible heat in stack dust at collection (the assumedelectrostatic precipitator), considered a separate stream

d/Fly d/Cafrom fly ash L ; L is the sensible heat of

4 2CaSO @yH O and CaO effluents given limestone injection;

Rand L is the fuel rejects term which is an outlet stream.

ID The ID or recirculating fan powers, W , negativelycorrects HNSL such that the fuel's calorimetrics areconserved. Such components affect the outlet gas streamby increasing boundary temperature and thus the negativecorrection. Most standards advise to leave the ID Fanoutside the boundary. This is possible providedtemperature and emissions measurements are collectedbefore the fan, which is unusual. It is suggested that all gasfans be included, correcting as indicated. One can arguethat correcting a stack temperature for the ID Fan )T doesthe same thing. Of course for many situations this might beadequate. However, an exception can be quickly be foundwith high water systems (e.g., a peat or lignite unit), in

8

which a 10 )F correction to stack temperature adds a

B-LHV0.02% )0 error given the affect of non-linear waterproperties.

From Eqs.(11) and (7) it becomes apparent that the

A0 term is defined as follows:

A 0 = 1.0 - (16) HNSL

Act-HHV Act-HHV - HPR + HRX

AThere is subtlety here. For consistency with Eq.(11), 0

Act Actmust be defined in terms of HPR and HRX and thus

A0 becomes a multiplicative affect on boiler efficiency.

BWhile this approach minimizes error in 0 , the individualterms comprising HNSL have the same weight as stack

Actlosses (via HPR ). In terms of boiler efficiency, Eq.(17)then becomes obvious:

B-HHV A 0 = 0 (17A)Act-HHV Act-HHV - HPR + HRX HHVP + HBC

B-LHV A 0 = 0 (17B)Act-LHV Act-LHV - HPR + HRX LHVP + HBC

Note that the “chemistry” portion of Eq.(17) is the

Ccommonly termed combustion efficiency (i.e., 0 , thus:

B-HHV C-HHV A 0 = 0 0 ). All gas/air/working fluid data to the system must be

corrected to a common energy level associated with the

Calfuel, T . In summary, Firing Corrections affectreactants, while loss terms correct ideal products - bothrelative to how these terms were established via a

Calspecified T . This is not to say that changes in a FiringCorrection might not affect losses; it says that starting fromEq.(4) lending to Eq.(7), Firing Corrections per se merelybalance the calorimetric equation. For the boiler efficiencyof Eq.(7), they appear in both numerator and denominator;losses only in the numerator. For ideal combustion, FiringCorrections can not impact efficiency. Without losses,boiler efficiency will always be unity. In the presence oflosses, Firing Corrections may impact efficiency but onlythrough the numerics of the efficiency equation. The termHBC is well defined in a previous work (Lang, 2009c).

Air leakage is a fine example of loss effects versusFiring Corrections. If, given an increase in air leakage,Stack temperature is falsely held constant, efficiency willdegrade but only because of an increased air flow needed

2 2to maintain Boiler O , but with higher Stack O and thushigher losses. However, when lowering Stack temperaturereflecting dilution, one then computes a slight reduction inefficiency, this as a result of differing gas capacitances andother terms addressed by Eqs.(13) and (14).

CALORIFIC CONVERSIONSAnother inconsistency in industrial standards for

boiler efficiency lies when converting from gross to netcalorific values, and from a constant volume to a constantpressure process. For example, a constant is many timesused to convert from constant volume HHV to the constantpressure HHVP (i.e., the as-fired). Fuel oxygen issometimes ignored. Underlying correct procedures, the

fg-Callatent heat at constant volume ()u ) and the latent heat

fg-Cal Calat constant pressure ()h ) at T must sever as basis.To convert solid and liquid fuel calorific heating valuesfrom a constant volume to a constant pressure process:

V/P HHVP = HHV + )H (18)

V/P L/H LHVP = LHV + )H - )PV (19)

V/P Cal-Abs H2 N2 O2 AF )H = R T (" /2 - " - " ) / (J N ) (20)

L/H fg-Cal/H2O C-H-Gas H2O )H = )h (" z/2 + "

H2 H2S H2O AF+ " + " ) N /N (21)

L/H fg-Cal/H2O C-H-Gas H2O )U = )u (" z/2 + "

H2 H2S H2O AF+ " + " ) N /N (22)

L/H L/H L/H )PV = )H - )U (23)

Cal-Abswhere T is the absolute temperature. Note that thenew ASTM D5828-07 standard employs these samerelationships under a work order initially prepared by theauthor, but employing weight fractions. To convert from agross (higher) to a net (lower) heating value, the followingare exact and consistent with the above:

L/H LHV = HHV - )U (24)

L/H LHVP = HHVP - )H (25)

The latent heat of water has obvious import whenconverting from a gross to a net heating value. Althoughlaboratories typically only report the gross value, when afossil fuel is burned its net energy is liberated. There canbe no distinction imposed at the time a fuel is burnedcommercially versus how its energy was determined in thelaboratory. If the calorimetric process were to end mid-path, with vaporized water, the derived energy liberatedmust be the same as would be determined from thecompleted combustion/vaporization/condensation process.Thus conversion to LHVP serves a useful example. LHVP

Calmust employ the T reference for conversions, to dootherwise implies capricious thermodynamics: one coulduse one latent heat for entrained water, another for waterformed from combustion, and yet another latent heat forwater in-leakage.

9

COMPUTED FUEL FLOW AS THE ABSOLUTE TEST

Why does adherence to such thermodynamicconsistency matter? Is not any definition of efficiencyvalid if taken as a relative measure (and more so whenusing high energy fuels)? If an efficiency test produces aboiler efficiency within 0.5 to 1.0% of guarantee, suchrefinement could matter greatly. If the performanceengineer computes an emission flow base on a relative orerroneous boiler efficiency, paying more carbon taxes as aresult, it could matter greatly. Such financial argumentsaside, industrial standards should be relied on for state-of-the-art methods.

As-fired fuel flow is not an arbitrary parameter, it isabsolute and solely dependent on the fuel’s interactionwith gas/air/working fluid (i.e., boiler efficiency). If,indeed, such conversion of fuel energy were dependent(say on an air temperature, an arbitrary 25C, etc.), thencomputed fuel flow based on that conversion would be

WFarbitrary. From the Input-Output Method, where Q is the“Useful Energy Flow Developed” of Eq.(1), fuel flowfollows by back-calculation:

AFm = (26A)WF Q

B-HHV 0 (HHVP + HBC)

AFm = (26B)WF Q

B-LHV 0 (LHVP + HBC)

WF AFFrom Eq.(26) it is obvious that the ratio [Q /m ] isconstant, gross and net. Also, from Eq.(11), consideringthat HNSL and HBC are defined the same for gross and netcalculations, the following are developed and consideredimportant identities; see Eq.(16).

B-HHV B-LHV0 (HHVP + HBC) / 0 (LHVP + HBC) (27)

Act-HHV Act-HHV- HPR + HRX

Act-LHV Act-LHV/ - HPR + HRX (28)

When following the guidelines advocated herein, thesame fuel flow will be computed given the same losses,using either gross or net efficiency, at any calorimetrictemperature and at any level of Firing Corrections. Fuelflow supplied to an in-situ system, interacting with itsgas/air/working fluid, is a fixed quantity; it is what it is - it cannot be a function of a user selected reference!!

SENSITIVITIESMany would argue that methods of any stripe - be

they consistent or tolerated for convenience - are viable ifproducing “acceptable errors”. However, the problem in

agreeing with the notion of an acceptable error is that ourresponsibility as engineers, when writing standards, shouldbe to produce state-of-the-art procedures based on soundthermodynamics as one can never anticipate how astandard might be used.

These things said, methods addressing consistentcalorimetrics are not needed if testing systems using fuelswith low entrained water or producing little water atcombustion (i.e., low product water fuels). It is of interestthat ASME PTC 4.1 whose pedigree runs back to 1964,with foundations in the Babcock & Wilcox’s Heat LossMethod (taking off another 50 years!), addressed an era inwhich high energy coal predominated in the U.S.

The source of conceptual calorimetric errors for highproduct water fuels is the sensitivity of the latent heat ofwater, and its Heat of Formation of the liquid, totemperature as seen in Tables 1A & 1B. The temperaturedependency for water’s latent heat is great indicating a 1%sensitivity for every 18 )F (10 )C). As seen in Table 2

2 2the SO sensitivity is slight, while CO is 10 times less

2sensitive than SO . For fuels with high water contents, thelatent heat has obvious import when condensing vapor ina bomb calorimeter.

It could be argued that a fossil fuel having lowhydrogen but high fuel water has little sensitivity tocalorimetric temperature if the bomb’s starting and endingtemperature is the same. Under the preceding scenario,could the CV change? Or, if burning pure graphite in abomb, if the starting and ending temperature is the same,could the CV change? The answer is that the CV may verywell change due to the path chosen for Eq.(2). By way ofexample, let’s assume that graphite’s CV is beingmeasured in a calorimeter capable of variable bathtemperature. If the bomb is started at 77F, then reduced to34F where the graphite is burned, then assume a return to

-CO2f77F - even taking zero change in )H - changes0

reflected in the heat capacity of graphite going to 34F, and

2the heat capacity of CO coming from 34F, are differentresulting in an observed CV which must be different from

-CO2f-)H at 77F. If water is added to the graphite, path0

dependency becomes even more obvious. Of coursetraditional bombs are run with constant temperature, andsince Eq.(2) and thus Eq.(4), etc. are path dependent ... wesimply must adhere to the process in our analytics! Asimple spread sheet has been prepared which demonstratesthe sensitivities of mixtures of graphite and water givenvarious path scenarios; see www.ExergeticSystems.com.

IdealThe ideal product of combustion, HPR , is definedby the Heats of Formation of the comprising ideal

-CO2 -H2O -SO2f f fproducts: )H , )H and )H . The sensitivity0 0 0

Ideal Idealof these terms is reflected in the ratio ()HPR /HPR )

10

per 1.0 )C, presented in Table 3 for all Ranks of coal.Note that the indicated arrow points to hvBb Rank fuel(and to those below) whose associated efficiencies can besufficiently affected by a 18 )F (10 )C) change in anassumed reference temperature. The sensitivity of the

Ideal Ideal ()HPR / HPR ) term explains why efficienciesassociated with the poor quality fuels, having very high

Calproduct water, have greater sensitivity to T , andconcomitantly have reduced uncertainty versus highenergy fuels. Of course the paradox is explained throughthe nature of thermochemical properties. The science ofthermodynamics associated with calorimetrics suggests aninteresting ambivalence associated with the Energy

2Balance Method. When reckoning the insensitivity of CO ,and that seen in Table 3, consider an example of burningpure graphite. Again let us turn to measuring graphite’sCV, but this time in a traditional, constant temperaturebomb. Using results from our graphite bomb, one wouldcompute a Energy Balance efficiency which is essentiallydevoid of temperature sensitivity. What does it mean thenhaving a system which produces no water and whose Heatof Combustion is insensitive to temperature? Eq.(7) willproduce different boiler efficiencies based on any chosencalorimetric bath temperature, using essentially the sameCV, but producing a variable fuel flow given arbitraryFiring Corrections. Does this imply that ourthermodynamic understanding of a graphite system isimpossible? No, however it does mean that the Energy

Balance Method of computing efficiency is crippled by thenature of thermochemical properties and our inability todiscern Heats of Combustion to the accuracy required.

Simply put, the Energy Balance (Heat Loss)Method has limitations. Table 3 inherentlysuggests that a lower limit of 10% fuel waterplus fuel hydrogen by weight be placed on theapplicability of the Energy Balance Method.

However, if forced to use an Input-Output approachfor a low product water fuel, we still have the problem ofviable Firing Corrections procedures. For the situation offuel having <10% fuel water plus hydrogen, it is proposedto place an error bound on the Input-Output determinedboiler efficiency which is dependent on the sensitivity anagreed temperature range has on Firing Corrections, andthus on boiler efficiency.

Tables 4 and 5 illustrate effects on methane andseveral coal fuels given a 18 )F (10 )C) change incalorimetric temperature. As seen, differences range from0.4 to 0.8% in efficiency per every 18 )F. The differencebetween running a bomb at 95F and a standard’s 77F is notconsidered uncommon. Note that these are only a portionof the conceptual errors. When mis-using loss or “credit”terms, errors have been observed to exceed an additional

B1% to 2% in )0 (depending on vendor practices andmanipulations of standards).

Table 1A: Sensitivities of Water’s Saturated Energies at Constant Pressure

ReferenceTemp.

Enthalpy,Sat. Liquid(Btu/lbm)

Change(%)

Enthalpy,Sat. Vapor(Btu/lbm)

Change(%)

Latent Heatfor Water

()Btu/lbm)Change

(%)

25C (77F) 45.093 0.000 1095.094 0.000 1050.001 0.000

30C (86F) 54.080 +19.93 1098.996 +0.356 1044.916 -0.487

35C (95F) 63.061 +39.99 1102.879 +0.711 1039.818 -0.979

Table 1B: Sensitivities of Water’s Heat of Formation

ReferenceTemp.

2H O Heat of

LIQFormation(Btu/lbm)

PerCentChange

(%)

2H O Heat of

VAPFormation(Btu/lbm)

Change (%)

25C (77F) -6821.142 0.0000 -5771.141 0.000

30C (86F) -6817.358 -0.0555 -5772.442 +0.023

35C (95F) -6813.584 -0.1109 -5773.766 +0.046

11

2Table 2: Sensitivities of SO Heat of Formation

ReferenceTemp.

2SO Heat of Formation(Btu/lbm)

Change (%)

25C (77F) -1992.0027 0.0000

30C (86F) -1992.1867 0.0092

35C (95F) -1992.3670 0.0183

Table 3: As-Received (wet) Properties of Fossil Fuels(data from Penn. State, Input/Loss installations & NIST)

IdealSubstance Fuel Fuel Fuel Avg. HHV HHV )HPR

Ideal or Coal Hydrogen Water Ash at 25C Temp.Coef. HPR Rank (% wt) (% wt) (% wt) (Btu/lbm) (x10 /1)C) (x10 /1)C) -6 -6

an 1.94 3.55 9.85 12799.75 19.56 376.6 sa 3.01 1.44 16.51 12466.17 30.10 285.0 lvb 3.97 1.69 13.22 13155.11 39.22 347.7 mvb 4.44 1.75 11.48 13371.75 41.88 380.5Benzoic Acid 4.95 0.00 0.00 11364.57 45.0 (NIST) --- hvAb 4.91 2.39 10.86 13031.61 47.77 444.2

hvBb 4.63 5.61 11.83 11852.63 56.53 446.7 ³ hvCb 4.26 9.89 12.32 10720.40 60.18 450.6 subA 3.94 12.93 7.06 10292.89 51.16 398.3 subB 3.76 17.87 9.57 9259.75 61.15 408.0 subC 3.50 23.79 10.67 8168.69 75.14 423.3 ligA 3.02 29.83 9.64 7294.66 83.56 439.4Methane 25.33 0.00 0 .00 23867.31 105.39 424.3Irish Peat 2.73 46.78 1.72 4856.07 112.00 (est.) 542.7ligB-PSU 2.16 28.84 22.95 4751.83 122.17 481.3ligB-Greek 1.39 54.04 16.93 2926.82 246.01 685.2

Table 4: Calorimetric Temperature Affects on a Methane System Given an 18 )F Mis-Use of Calorimetric Temperature

Computed CV for Methane

Gross EfficiencyCalculated at 60F

Gross EfficiencyCalculated at 77F

CalAll Terms f (T )

B-HHVaffecting )0

23891.00 at 60F 83.057% 83.560% 0.503%

23865.92 at 78F 83.039% 83.543% 0.504%

Table 5A: Effects on Gross Boiler Efficiency Given an 18 )F Mis-Use of Calorimetric Temp.

Cal Cal B-HHVUnit T =77F T =95F )0

110 MWe CFB, Coal

88.123% 88.574% 0.451%

800 MWe Coal Slurry

81.357% 81.892% 0.535%

12

Table 5B: Effects on Gross & Net Boiler Efficiencies Given an 18 )F Mis-Use of Calorimetric Temperature

Cal Cal B-HHV B-LHVUnit T = 77F T = 95F )0 )0

600 MWe PRB Coal, HHV = 8542.828 Btu/lbm

83.666% 84.179% 0.513%

600 MWe PRB Coal, LHV = 7958.786 Btu/lbm

90.197% 90.711% 0.514%

Table 5C: Effects on Gross & Net Boiler Efficiencies Given an 18 )F Mis-Use of Calorimetric Temperature

Cal Cal B-HHV B-LHVUnit T = 77F T = 95F )0 )0

300 MWe Lig-B, HHV = 2760.249 Btu/lbm

62.239% 62.909% 0.670%

300 MWe Lig-B, LHV = 2143.454 Btu/lbm

81.779% 82.589% 0.810%

ASME VERSUS THE WORLD In North America the gross (higher) heating value

based boiler efficiencies are the norm for conventionalsteam generators. In the rest of the world, net (lower)heating value based efficiencies are exclusively used. Netefficiencies are higher numerically. Although for the same

Bfuel, the )0 impacts are approximately linear with

Calchanging T (assuming the CV is held constant), theeffects of increasing product water is obviously non-linear.

BIt is interesting that for PRB coals the )0 impact isapproximately the same between gross and net. Note thatTable 5C indicates that for very high product water fuelsthis conclusion breaks down. Although fuel flows will

Calcompute identically for the same T between gross andnet, there is numerical subtlety. As the net is a higher

B-LHVvalue, the percentage change of a given )0 is, of

B-HHV course, less versus a percentage )0 . This conditionalso implies that the computed fuel flow of Eq.(11B) will

Calbe slightly less sensitive to mis-use of T , but does notmean Europeans can justify sloppy standards!

SUMMARY AND RECOMMENDATIONS A consistent boiler efficiency will result if only

considering the fuel’s interaction with the gas/air/workingfluid, an efficiency leading to a unique fuel flow. Withoutfollowing such principles, errors can exceed 2% whichmay have serious effects on power plant monitoring andeconomics.

The established standards for computing boilerefficiency require improvement. Conceptual errors occurwhen not adapting the fundamental process of how theenergy content of a fossil fuel is determined. In addition,conceptual errors are made in the treatment of major steamgenerator components.

It is the opinion of the author that standards need torecognize the repeatability accuracy of the modern bomb

Bcalorimeter and, in general, should strive for ±0.1% )0resolution. The following recommendations, not listed byimportance, are offered for consideration when preparingfuture revisions of the standards.

1) Boiler efficiency standards should publish, as integralto their documentation, data sets which can bereplicated by the analyst. Such data sets should beginwith a simple combustion system, with one heatexchanger, allowing base thermodynamics to beconfirmed and fuel flow computed, followed byincreasing complexity.

2) Boiler efficiency standards should be required toreport the reference temperature used in theircomputations. This reference should be thetemperature at which the fuel’s calorific value wasdetermined. If required to employ a specifiedreference temperature, then the coal-fired operatorshould specify to the laboratory the calorimetrictemperature to use.

13

3) Calorimetric standards such as ASTM D5865 andISO 1928 should require routine reporting ofcalorimetric temperatures. This author, sitting on theD5865 committee, was defeated in getting approvalof this requirement (his Work Item WK-5174 wasapproved by 66 members, the one negative voteresulted in failure of D5865 proposed modification).

4) Boiler efficiency standards must specify the correct(and consistent) formula for gross to net, and volume

Cal to pressure conversions, and all dependent on T .

5) Standards should specify that any common steamgenerator component which was included within thesystem boundary (as defined herein), should bereported and its use fully justified.

6) Standard committees should investigate the newNASA combustion gas properties which appear tohave a wider range than the older JANAF tables (seeMcBride, et al., 2002). The NASA work conveniently

j reports HPR of Eq.(14). Although combustion gasproperties appear remarkably consistent, it is stronglyadvised to employ Eq.(13) when describing water. Inaddition, all properties should normalize to theCODATA set of Heats of Formation (Cox, 1989).

7) Boiler efficiency standards must anticipate use ofcomputers and issue approved source coding(preferably in the simplest of FORTRAN, as wasdone in ASME PTC 22).

8) Consistent application should be made of Enthalpy ofProduct and Reactant terms as based on thecalorimetric process. At the minimum, the latent heatand the Heat of Formation for water must becomputed as a function of calorimetric temperature.All chemistry products should be considered whenenfolding Stack loss terms into a single formulation.Thus a form of Eq.(13) should be employed, reducinginconsistencies in methods; Eq.(12) is to be avoided.

9) For solid and liquid fossil fuels, boiler andcalorimetric standards must not allow correctionsmade to as-tested calorific values (other thanconventional conversions from gross to net, and fromconstant volume to a constant pressure process). Suchchanges to reference temperatures are allowed underDIN 1942 and are wrong. Laboratory results based onthe as-received sample (HHV) must stand as-is andbe reported, and employed, as uncorrected values.

10) Steam generator standards must strictly adhere to theprinciple that boiler efficiency can only be degradedby losses applied to outlet streams. Firing Correctionsare only applied to inlet streams.

11) Modern boiler efficiency standards should allow fora variable concentration of ambient oxygen. Suchvariability was common in the older Europeanstandards, and was found in test procedures before1970. Ambient oxygen is an extremely sensitiveparameter for all combustion equations, fixing thisvalue is ill-advised.

12) The Energy Balance (Heat Loss) Method: a) should be the only method allowed provided fuel

water plus fuel hydrogen is >10% by weight; b) evaluations must require the reference temperature

be equal to the calorimetric temperature; c) should invoke a tolerance statement on efficiency

based solely on fuel sampling (CV testing) and

WFthose quantities affecting the Q term; andd) efficiency should be used to back-calculate fuel

WFflow based on Q (from the Input-Outputdefinition) for comparison with the actual when-ever practical and as agreed between parties.

13) The Input-Output Method: a) should only be applied if fuel water plus fuel

hydrogen is less than 10% by weight;b) should use an agreed reference temperature; andc) should invoke a tolerance statement on efficiency

based on: fuel sampling (CV testing); quantities

WFaffecting Q ; and affects on Firing Correctiongiven an agreed spread in reference temperature.

14) It is recommended that boiler efficiency standardsprovide easily understood procedures for uncertainty

WFstatements affecting at least fuel sampling and Q .

15) As ASME PTC 4 is extremely well-detailed,addressing numerous and important testing practices,it is recommended that it serve, after correction ofconceptual and procedural errors, as the basis for DINand European re-writes of their standards.

16) It is recommended that ASME prepare a new PTCwhich specifies consistent thermodynamic propertiesto be used in all combustion efficiency standards,these would include: molecular weights, airpsychrometrics, water properties, gas properties,Heats of Formulation, air’s molar constituents, etc.

14

ACKNOWLEDGMENTS The author is grateful to his colleagues in Europe and

North America, especially ASME reviewers, for theirnumerous comments and suggestions made on this paperand related parent works. He is grateful for the manyquestions, and encouragement to publish, originating fromChina regarding parent works. He is grateful to Mr.Dimitrios N. Konidaris, Public Power Corporation ofGreece, for his detailed review of EX-FOSS and CVconversion techniques, and his suggestions on improvedproperties. Most especially, he would like to thank Mr.Tom Canning ([email protected]), formerly with theElectricity Supply Board, Republic of Ireland, for manylong hours of discussions and his many critical reviews.

REFERENCES“Acceptance Testing of Steam Generators”, DIN

1942: February 1994, DIN Deutsches Institut für Normunge.V., Berlin; a defunct German standard.

“Code for Acceptance Tests on Stationary SteamGenerators of the Power Station Type”, BS 2885:1974(also: ISBN: 0 580 08136 2); a defunct British Standard.

Cox JD, Wagman DD and Medvedev VA, CODATAKey Values for Thermodynamics, Hemisphere Pub., NewYork, 1989.

“Fired Steam Generators”, Performance Test Code 4:2008, ASME, New York, NY; the U.S. standard since firstreleased in 1998.

“Gas Turbine Power Plants”, Performance Test Code22: 1985, ASME, New York, NY.

“Gross Heating Value of Black Liquor”, TAPPI,Norcross, Georgia, USA, T684 om-02; published 2002.

Keenan JH, Thermodynamics, Wiley & Sons, NewYork: 1941; Chapter XVII.

Kent W, “Rules for Conducting Boiler Tests”, ASME1884 Transactions, pp.260-281, see FIG.66.

Lang FD, and Canning T, “An Oxy-HydrocarbonModel of Fossil Fuels”, ASME, 2007 Power Conference,Pwr2007-22007, San Antonio, TX, 2007.

Lang FD, EX-FOSS: A Program for Monitoring &Analysis of Fossil-Fired Boilers, Exergetic Systems, Inc.,San Rafael, CA. (March 2009a, Ver.2.8, Mod.17, firstpublished 1983).

Lang FD, “Discord in Boiler Efficiency Standardsand Calorimetric Temperature (a supplement to the ‘PartIII Input/Loss Paper’)”, a 2009b unpublished paper isavailable at: www.ExergeticSystems.com.

Lang FD, “Monitoring and Improving Coal-FiredPower Plants Using the Input/Loss Method - Part III”,ASME, 2000 International Joint Power Gen. Conference,IJPGC2000-15079, Miami Beach, FL; a 2009c revisionreflecting years of comments and corrections (see Notes &Errata, Item 1) is available at: www.ExergeticSystems.com.

McBride BJ, Michael J, Zehe MJ and Gordon S,“Thermodynamic Properties of Individual Species”,NASA/TP-2002-211556, 9/2002, Glenn Research Center,Cleveland, Ohio.

“Natural gas - Calculation of Compression Factor -Part 3”, ISO 12213-3:2006.

“Natural gas - Standard Reference Conditions”, ISO13443:1996 /Cor 1:1997.

“Orifice Metering of Natural Gas and Other RelatedHydrocarbon Fluids”, American Gas Association (AGA),Report No. 3, September 1985.

“Performance Test Procedure Sodium BasedRecovery Units”, CA Report No. 84041601, March 1996,TAPPI Press, Atlanta, GA. Also see “Recovery BoilerPerformance Calculation - Short Form”, TIP 0416-01,2001, TAPPI Press, Atlanta, GA.

“Solid Mineral Fuels - Determination of GrossCalorific Value by the Bomb Calorimetric Method, andCalculation of Net Calorific Value”, InternationalOrganization for Standardization, ISO 1928:1995(E).

“Standard Test Method for Gross Calorific Value ofCoal and Coke”, ASTM D5865-04, ASTM, 2005.

“Steam Generating Units”, Power Test Code 4.1:1964, 1973 (Reaffirmed 1991), ASME, New York, NY; adefunct U.S. standard.

“Water-Tube Boilers and Auxiliary Installations -Part 15: Acceptance Tests”, prEN 12952-15: November1999, European Committee for Standardization, Brussels(also: CEN/TC 269/WG 3N 337); the draft Europeanstandard (reviewed in 2002); now EN 12952-15:2003replacing the German and British standards.

U.S. Patent 6584429 was issued June 24, 2003. Thispatent is owned by the author's company and, uponrequest, will be made available for ASME’s use withoutroyalties; use outside the U.S. requires no request.

U.S. Standard Atmosphere, 1976, NASA-TM-X-74335, U.S. Gov. Printing Office, Washington, DC, 1976.

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