CALCULATING FIRED HEATER EFFICIENCY

James T. Harriz and Edward L. Ritter

Waterland, Viar & Associates, Inc.

Newark, Delaware

ABSTRACT

Tracking utility and process heater efficiency leads to increased operator awareness and decreased fuel bills. A review of the Heat Loss and Input/Output methods of efficiency analyses is presented, along with a discussion of their relative accuracy. Examples cited include refinery mixed fuel gas, coal, and chemical recovery fueled applications, using HHV and LHV calculations. The techniques presented can be applied to improve operator awareness, moni tor equipment performance, and evaluate capital improvement opportunities.

INTRODUCTION

What is Fired Heater Efficiency?

Efficiency is the ratio of delivered divided by the work bo i ler / furnace language, th is delivered to process divided by heat

useful applied. becomes applied.

work In

heat

Why are efficiency calculations important?

In practically any process, high level heat is provided by the combustion of fuel. Dollars not spent on fuel can be returned to the shareholders as profits, and maximizing efficiency at this point has great leverage in overall process economics. Routine monitoring of combustion efficiency for various fired equipment can be used to improve daily operation, and locate opportunities for maintenance or capital improvements.

A theoretical minimum threshold of high level heat is required to initiate an endoth'ermic process . The practical minimum is dictated by

655

design economics, heat transfer technology and safety criteria which define m1n1mum approach temperatures, engineering materials and equipment design.

The actual minimum depends on final design, production rate, and insulation condition. Since fuel pricing is not usually controllable by the individual user, only fuel savings from heater efficiency improvements will reduce operating costs.

How Is Efficiency Calculated?

Two methods are available for evaluating fired heater efficiency. These are commonly referred to as the Input/Output and Heat Loss methods.

The Input/Output method directly determines the net heat delivered. This is a very general method, evaluating only the useful heat delivered to process against fuel supplied. Delivered heat is the sum of process stream energy increases.

INPUT/OUTPUT HEAT LOSS METHOD METHOiD

HEATTOj PROCESS x100 1- . x100EFF= ~ tLOSSES.....]FUEL TOFUEL TO HEATERHEATER

The Heat Loss method evaluates flue gas thermal losses, determining efficiency by difference. Independent calculations address radiant and convective setting losses and residual combustibles losses to determine overall heater efficiency.

While neither of these two methods is flawless, the Heat Loss method is inherently less subject to measurement errors, and the auxiliary corrections for setting and combustibles losses are not difficult.

ESTABLISHING THE RULES

Combustion engineers recognize two distinct interpretations of fuel heating value: Higher Heating Value (HHV) and Lower Heating Value (LHV~~, The difference between them addresses the availability of the latent heat of vaporization of water in the flue gasses.

ESL-IE-86-06-106

Proceedings from the Eighth Annual Industrial Energy Technology Conference, Houston, TX, June 17-19, 1986

ESTABLISHING THE RULES

GROSS NET

HIGHER LOWER HEATlNG HEATING VALUE VAWE (HHV) (LHV)

LATENT HE"T LATENT HEAT OF VAPORIZAnON OF VAPORIZATION

(Waler 01 Combustion) (Waler in Fuel)

Higher Heating Value - The primary components of fuel are carbon and hydrogen. In the chemistry of combustion, hydrogen reacts with oxygen to form water vapor. The Higher Heating Value is the heat delivered by cooling all the products of combustion to their initial temperature, condensing any water vapor in the flue gas. This definition includes both sensible heat and latent heat of vaporization as available (recoverable) heat.

Lower Heating Value Lower Heating value represents the sensible heat recoverable by returning the products of combustion to their initial temperature. It neglects the latent heat of vaporization as inherently unavailable, recognizing the difficulty and inadvisability of condensing water from the product gasses. The degree of unavailability depends on the phase of any moisture included in the composition of the fuel supply. The latent heat of vaporization for included moisture is not considered if the fuel is in the gaseous 0 phase, since the moisture is already vaporized, and only sensible heat will be added in combust ion. However, if the fue 1 is a solid or0 liquid, then the initial moisture will also be liquid. The heat of vaporization must be added and becomes unavailable to process streams.

The classical definition of LHV implies that on ly the water formed as a product of combustion be forgiven as inherent ly unavai lab Ie. The more prac t ica1 interpretation recognizes all heat which vaporizes water as unavailable. The distinction is clarified in the figure above.

TWO CALCULATION METHODS

Input/Output Method This method is straigh t -forward; add up the produc ts of each heated stream mass flow rate times its enthalpy increase across the heater, and divide by heat release. For boilers, the heated streams include steam and blowdolffi flows; for process heaters, this may include several product passes. The applicable variables are identified below in bold relief.

INPUTIOUTPUT METHOD

~ PREHEATED FEED WATER

@ ATIEMPERA rlON WATER

/

Flue gas composition is determined from fuel analysis, and measured free oxygen in the flue gas. These allow precise calculation of stoichiometric flue gas composition and excess air on the per-pound-of-fue I bas is. From compos it ion and ideal gas properties, the enthalpy increase from air inlet to final stack is dependent only on the end temperature difference for the N2 and COt formed, and excess 02 measured. The enthalpy 0 water vapor is determined from the temperature and calculated partial pressure in the flue gas.

Residual oxygen measurements for fuel/air mixture controls are correctly taken at the heater flue gas outlet, ahead of any air heaters. However, it is crucial for efficiency calculations that the exit temperature and residual be measured in well-mixed flue gas, downstream ot the last heat trap in the stack. This will include the effects of air heater leakage on final temperature and 02 concentration.

Setting losses include radiant and convective losses from the heater exterior, and are relatively constant (fixed Btu' s/hr) regard less of firing rate. They are a function of surface area, interior and exterior temperatures, insulation thickness, type and condition. These variables are not subject to rapid change, and typically cons t i tute one to five percent of FULL LOAD heat release. This means that a two percent loss at full load becomes a four percent loss when running at half load.

Combustibles losses are highly dependent on fuel type. Natural gas and No. 2 fuel oil are easily fired and flue gas residual combustibles are usually negligible. Coal, biomass and trash burners will usually have measurable unburned carbon in the fly ash. The losses associated with carbon loss are deducted from the heating value of the fuel at the heating value of elemental carbon. If high (-1,000 ppm) carbon-monoxide or methane levels are detected by flue gas analysis (ahead of the air heater), then attention should be paid to improving combustion, rather than calculating efficiency.

PRACTICALITY & SENSITIVITY

The Input/Output and Heat Loss methods provide, in principle, the same answer. In fact, each method has limitations on overall accuracy. Efficiency is an intangible value; you can't pick up the heater and carry it to the Bureau of Standards for precise calibration, and process environments are not always kind to sensitive measuring devices. The fa llowing tab Ie summarizes the measurements required for each method, and the typical error associated with each.

Where a range of error is ind icated, consider the medium being measured. Natural gas is more accurately measured than coal for fuel rate, and enthalpy. for a single-phase process stream is easier to determine than for a multi-phase stream.

RELATIVE ACCURACY

EQUATION VARIABLE 1/0 HEAT LOSS TYPICAL ERROR TYPICAL ERROR

A"BIENT TEftP. DEGREES F' 1X 12 F'J

FUEL FLOW 2 lOX - HEATING VALUE o 1X a 1X - CO"POSITION o a

PROCESS - FLOW 2X - INLET ENTHALPY 2 - ~x - OUTLET ENTHALPY 2 - 5X

FLUE GAS TEltPERATURE - \02

2)( 2X

(10/500 (0-1.5:

Refinery Mixed Gas Reviewing the data vaporization for the formed water and corrected below, the refinery gas sample is mostly dry setting loss [(HHV/LHV)*(2.5% Btu loss/Btu HHV)]. methane, propane, and ethane with a carbon/hydrogen mass ratio of 3.3:1. This is mostly hydrogen, on a Coal - A coal sample has 4.3 percent moisture molal basis, and we eKpect two pounds of water to in ""t'h'e rough plus a small amount formed by be formed for each pound of fue 1 fi red. combustion of the 14.16:1 carbon/hydrogen ratlo Combustibles losses for clean gases are negligible, fuel. The coal has 5.3 percentage points higher and setting losses are given at full load. combustion efficiency than plant gas (HHV S&C

losses). This is because the stoichiometric air Using either me thod, the Higher and Lower for the coal sample is 10.40 Ib./lb. fuel versus

Heating Value basis efficiencies differ by 7.6 16.23 Ib./lb. fuel for the gas sample. The lower percent, attributable to the latent heat of air flow reduces stack heat loss at constant

RIXED REFINERY GAS

COAL COIIPONNT FDRftUU XlT VT. 18Y YOLo REACTAJITS I BY n. NOLES

p-r .,1. of dr, {Yel FUEL ANA1.YSIS IFlIEL COJIIPOSITIOI

-_---- --- .--- - ---_. - e. __ _.

HYDROGEN 1.76 16.80 CARIOI 74.28 1.000 FORMULA VT. REACTANTS I IY "'T.R2 COJlPOIIDT IIY IIY VOL. "OLES CARBO.. DIOX CO2 3.20 1.40 KYOROGElI 22.49 3. EIJ1 p-r -.ol_ ETHYLENE C2 Ht 1. 31 0.90 OXYGEI 0.00 0.000 af dry fuel THAiE C2 H6 2O.~9 13.20 SULF'UR 0.03 0.000 IIETHAII. CR4 47.86 ~7.50 RITROGJI 0.00 0.000 C 72.20 CARlON 77. J8 1. ODDC"_I-HEX"IIE C6 H14 0.89 0.20 CARBOI DIDI 3.20 0.012 R2 ~.10 HYDROGDl :5.47 0.842 rRCP,..E C3 H8 14.41 6.30 SULFUR 0101 0.00 0.000 02 9.00 OUGEI

RYOROOEll 9.~ 0.094OXYGS3I 0.006PROPyLJtE C386 3.93 1.80 JIOISnUtE 0.00 0.000 .2 1. 60 SUU'U1t 1.18RIYROllEll lSOBUTAME C4 HID 1.~1 0.50 5 1.10 RITllOGE1I 1.71 0.009SULFUR

10IlmIIlEI-BUTAI C4 HID 1.~1 0.50 Il2 0 4.30 CARBO. 0101 0.00 0.000 lSOPEIiTAII C~ "12 1.50 0.40 6.70 SULFUR PIOX 0.00 0.000ASH 4.61 0.040I-PEIITANE 1.50 0.40 ROISTURE

" H12 SUU'UR 5 0.03 0.02

COIIUSTIOIII COIilSTAlT X- 97.29 HMY- 12.990 LKY-12.436 BTU/U CO"BUSTIOI COIlS1'AlT Xa 90.23 HNY' 21,6a2 LHV-19.6..3 ITUtU STAeI TOP- 300.0 De9 F DATUI TEIIP 80.0 o.a r C/N",SS RATIO - 14.16:1 STACk TEltP. 300.0 De9 r DATUII ttIIp 80.0 o.a FeIN IIASS .ATIo 3.3Otl H2O 1111 CoiUnaaUon PRODUCTS 0. ~ U/U lu.l H2O 1ft CoabuaUan PRODUCTS 2.01 LIlLI :t....l

IIOu:CUUR WEIGHT OF F1IEL' 19.27 .... -_ -- _.- -~~- __e. - _.; --~. -- --- -. ~~ - - - .----~~~-~;;-;--_._------~--------~~--------;~--------;--------~~---------- -~~-~~~-~-----------~---------------------_._------------------------------------------------------------------------------------------------..--

12.48 13.~2 14.56 22. 73 UI lEG"D, LIlLI FUELl0.40 11.44 All REg'D. LI/U F"UL16.23 17.86 19,48 21.10 14.:i2 15.~ COU PROD, LilLI FUEL17.23 18.86 0.037 0.034

12.44 13.4820.48 22.10 23. 73 COJlI PROD, LIIU F'UEL11."0 0.043 0.0400.107 O,09a 0.091 O.oalI IOI9TUlE, LIlLI PROD 0.047ftOISTURE, LilLI PROD 0.117 0.070.09 0.04 0.040.00 VOUIIl 1 SO:2 O. 100.00 0.00 0.00'CLUKE I S02 0.00 13.2~ 1:2.34

YOWftE I CO2 10.14 9.30 8,:5. 7.97 6.10 ~...

1~.:l4 14.307.44 VOLU.S: I CO2 17. 02 7.16 6.~9YOLUILI 1 H:20 1. a414.21 13.26YOLURI I H2O 18.04 U.~ 1~.3O 4.65 ~.77

1.1:5 3.23 4.~ ~.:59 YOLUIlI: I 02 O. 00 1.82 3.34 YOlUIlE I 02 0.00 1~.93 76.1473.70 YOWKE I R2 75. O~ 75.39 75."12.89 73.32YOlUftl I 12 71.77 72.31 3.:18 4.9:5 6.12

VOlUNE I 0:2 (DIY BASE) 0.00 2.10 3.81 0.2:16 0.2~~:!t.24 6.4~ OLUlIE I 02(01' lASE} 0.00 1.96

0.2:18 CP CBTU/LBr heu.t) 0.298 0.293 0.289 89.43 89.000.28~ 0.282 CP (ITU/LIF heu.t) 0.261 0.2" 83.71 I COlli IFF' (HM') 90.70 90.27 89.8~ X CO"I ur (HMY) 8~.37 84.9:t 84.~4 84.12 92.50I COJlI EFF (LHY) 94. :24 93.80 93.37 92.93

ASSURED JIiIERT RESIDUAL I 02 IIlI ,LUE OA5 cORY BASE)---. __ .. _------.

ASSUJlED INERT 3.~ 5.2:!t 1.00 (DRY lASE) STACK 0.00 ._ -.-_.. -._..RESIDUAL I 02 IN FLUE QAS

-_._- - -- _. -- _. - -- -- --TEIP. ~

MMY 'l EF'FICIENCYSTACK 0.00 3.:.0 :5.2~ 7.00 89.17 8'.32 a8.'"90.70 90.32 TfIP.

300 92.60 LKV I EFFICIENCY

300 a~. :17 8~.03 84.ft2 64.12 83.50 300 94.14 94.~ 300 94.24 93.88 93. 4~ 92.93 92.28 LlY I EFFICIENCY

aHV I EFFICINCY 93.88 93.31

REFINERY KIXED GAS COAL

HEAT LOSS1/0YARIABLE 110 HEAT L.OSS VARIABLE

A"BIENT, 09 F 80 AK8IENT, O.g F' 80

FUEL FLOW, 1000 SCF/HR 128.3 FUEL FLO'll, 1000 LB/HR 12,842 HHY. BTulscr 1.100 1,100 HHV, BTU/LB 12,1390 1:l.990 12,436LHY, BTU/SCF 997 997 LHY. BTU/L.B 12,436 14. 16J 1C/H "ASS RATIO 3.30.1 CIH "ASS RATIO 1.1SULFUR CONTENT. WTX 0.03 SULFUR CONTENT, WTX

PROCESS STREA ttS PROCESS STREAKS 8FW FLOW, 1000 L8/HR 103 BFW FLOW, 1000 LS/KR 103

ENTHALPY, BTUILB 286.0 ENTHALPY, STU/LB 286. a

ATTE"P FLow 1 ATTE"P FLO'll I ENTHALPY 217.8 ENTHALPY 217.8

100STEA" FLOW 100 5TA" FLOW 1438.2ENTHALPY 1438.2 ENTHALPY

BLOW DOWN FLOW SL.OWDOWN FLOW ENTHALPY 580.2 ENTHALPY S80.2

300FLUE OAS TEttP. Ovg F 300 TEKP. 0.0 F'FLUE GAS X02 DRY BASE 1. 7:5 XQ2 DRY BASE 1.7~

2.5SETTl NG LOSSES, X HHY 2. " SETTING LOSSES, X HHY . 0COft8USTIBLES LOSSES. X HHY

------- ---------

temperature. However, the inherent combustibles loss for coal-fired boilers reduces their overall efficiency. The difference between the HHV and LHV is only 3.8 percent since less water is evolved by burning the low-hydrogen coal than plant gas.

Black LiQour - This "fuel" is fired both for heat and the liberation of residual chemicals. Black liquor is a by-product of the Paper Pulp industry. Most of the chemicals used in cooking wood chips to pulp remain in solution, and are recoverable. The concentrated solution has a low, but useful, heating value. The lliV efficiency is similar to coal, but black liquor contains 40 percent water. Little water is formed in burning the low hydrogen fuel, but the high initial water content brings the HHV efficiency down to 69 percent, even assuming complete combustion.

BLACK LHlOUR

FUEL AIIALYSISI

CORPONEN! FDRJlUU 18Y WT. IIY VOL. REACTANTS I &Y VT. 1l0LES peor .01.

at dry lu.l

CARBON 2&.44 1.000 HYDROGEJI H2 2.20 HYDROGEl 2.~ 1.140 SULFUR 5 2.60 oXTGtM 60 0.131 ASH 13.00 SULFUR 2." 0.0'2 DUGEI 02 '.00 NITltOGU 17.47 0.213 UTllOGtM Il2 1~.20 CARBOM D[OlI o.DO 0.000 "DISTURt .2 0 40.00 SULFUR DIDI 0.00 0.000

CARBOJI C 23.00

MOISTURE .~. !II 1.1"

COftBUSTION CONSTANT K- 101.12 HHY- 3.649 LHY 2,", BTU/LI STACK TDtP. 300.0 ~ r DATU" TEI'Ip. 80.0 n.v' elM ltASS RATIO 10.4',1 H2O la Co_bueUon PRODUCTS. 0.69 LIlLI fve-l

EXCESS All I o 10 20 30 40

AIR RUI'O, LIlLI FUEL 3.83 4.22 4.60 '.91 ~.31 COll8 PROD, LI/LI FUEL 4.83 '.22 ~.'O ~. 91 '.31 "DISTURE, LB/LI PROD O. 142 0.131 0.122 0.115 0.101

VOLU"E 'l 502 O.~. O.~ 0.47 0." 0.41 VOlURE I CO2 12.81 11.90 11.10 10.'1 9.7'J YOLU"E I H2O 22.1' 20. ~1 19.20 11. DO 1'.9' YOLUIlE I 02 0.00 1.~ 2.10 3.94 4.9. YOLU": I 12 '4.49 ~.~3 &&.43 &1.22 &1.91

YOLU"E 1 02fDRT BASE) 0.00 1.19 3.47 ~.9~'.10 CP CBTU/Lar h....t I 0.309 0.30' 0.300 o.m 0.292 x co"a Err (HHVl 12.00 71.43 10.1~ 10. 2~ &9." . ASSU"ED I)(ERT

RESIDUAL X 02 IN FLUE OAS fOI' BASE) STACIC 0.00 3. ~ 5. 2~ 1.00 1[1'1'.

300 12.00 71.48 10.84 10.03 69.04 HHY I EFnCIENCY 300 88.81 88.2' 81.49 8&.'5 e~.40 LHY I EFFICIENCY

VARIABl.E 110 HEAT LOSS HEAT l.OSS (AS fIRED)

----------

---

MAINTENANCE PLANNING

>\) z w \) u. u. w

. . . . . . .

AT NOMINAL LOAD

TIME

Efficiequipment preheater

ency can be with

compvery a

arison~ against useful. Replacing

new design, or

upgraan old adding

ded air an

economizer to existing equipment will reduce flue gas losses predictably, and should be evaluated against the existing equipment. How about a completely new unit? The old furnace might be right on spec, just as good as new, but somebody has an idea for something new; better; more efficient; uses cheaper fuel; etc.

When considering a replacement, be careful of the basis of comparison. More efficient? Cheaper fuel? Higher Btu's per pound? Know the basis for claims: Dry fuel or as-delivered; HHV or LHV, etc. Remember that high-hydrogen fuels have potentially good LHV but poor HHV efficiency (which is what counts); high-moisture fuels soak up a lot of

EVALUATING CAPITAL IMPROVEMENTS

NEW TECHNOLOGY

>\) z EXISTING EQUIPMENTw \) u. u. w

LOAD

la ten t heat, too; some fue Is are cheap per pound, but are harder to burn completely like coal, wood and garbage and have measurable carbon losses.

SUMMARY

The Heat Loss method is the most reliable and sensitive indicator of fired heater efficiency. Given the same operating conditions, di fferent fuels are not capable of equal operating efficiency (HHV or LHV). Recognizing this, rout ine monitoring of fired heaters can be used to keep equipment running as efficiently as possible. This will save money by substantially improving daily operations, maintenance planning, and capital improvement decisions. Money not spent on fuel adds to profits, and that's what it's all about.

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ESL-IE-86-06-106

Proceedings from the Eighth Annual Industrial Energy Technology Conference, Houston, TX, June 17-19, 1986

1986 Proceedings Volume II.pdf