Modeling of Combustion Systems A Practical Approach 10 ppsx

A process heater is any device that makes use of a flame and hot combustion products to duce some product or prepare a feed stream for later reaction.. The diffusion burner comprises a fu

in a refinery or petrochemicals plant, or that provide heat forsteam We build upon this foundation by next considering arche-typical process units such as boilers, process heaters of varioustypes, and reactors such as hydrogen reformers and crackingunits.

In order to lay the groundwork for more detailed combustionmodeling, the chapter considers important combustion-relatedresponses such as NOx emissions, flame length, noise, etc., andthe factors that influence them Historically, practitioners havedefined a traditional test protocol for quantifying these effects,which we present We also consider some aspects of thermoacous-tic instability; this has become a more important topic with theadvent of ultralow NOx burners employing very fuel lean flames

In the latter third of the chapter, we develop stoichiometric andmass balance relations in considerable mathematical detail Wealso consider energy-related quantities such as heat and work,adiabatic flame temperature, and heat capacity As well, we relatethe practical consequences of a mechanical energy balance asapplied to combustion equipment Such things include draft pres-sure, incompressible airflow, compressible fuel flow, and practicalrepresentations thereof — capacity curves for air and fuel

2.1 General Overview

Combustion is the self-sustaining reaction between a fuel and oxidizer

charac-terized by a flame and the liberation of heat Usually, but not always, the flame

is visible A flame is the reaction zone between fuel and oxidizer; it typicallycomprises steep thermal and chemical gradients — the flame is often only amillimeter or so thick On one side of the flame, there is fuel and oxidizer atlow temperature; on the other side are combustion products at high tempera-ture Hydrogen and hydrocarbons in some combination are the typical fuels inthe petrochemical and refining industries Occasionally, due to some specialrefining operations, we find carbon monoxide in the fuel stream Oxygen (inair) is the usual oxidizer In practice, combustion reactions proceed to comple-tion with the fuel as the limiting reagent — that is, with air in excess

A burner is a device for safely controlling the combustion reaction It is typically part of a larger enclosure known as a furnace A process heater is

any device that makes use of a flame and hot combustion products to duce some product or prepare a feed stream for later reaction Examples of

pro-such processes are the heating of crude oil in a crude unit, the production of hydrogen in a steam–methane catalytic reformer, and the production of eth- ylene in an ethylene cracking unit A boiler is a device that makes use of a flame or hot combustion products to produce steam The furnace is the portion of the process unit or boiler encompassing the flame The radiant

section comprises the furnace and process tubes with a view of the flame In

contrast, the convection section is the portion of the furnace that extracts heat

to the process without a line of sight to the flame Every industrial tion process has some thermal source or sink

combus-2.1.1 The Burner

A burner is a device for safely controlling the combustion reaction A diffusion

burner is one where fuel and air do not mix before entering the furnace If

fuel and air do mix before entering the furnace, then the device is a premix

burner Premix burners may mix all or some of the combustion air with the

fuel If one desires to distinguish between them, a partial premix burner is

one that mixes only part of the combustion air, with the remainder providedlater Most pilots are of the partial premix type to ensure that they will lightunder high excess air conditions typical of furnace start-up

Diffusion burners supply most of the heating duty in refinery and boilerapplications; therefore, we discuss them first Figure 2.1 shows the mainfeatures for accomplishing this

The particular version of burner shown in Figure 2.1 is a natural draft

burner That is, a slight vacuum in the furnace (termed draft — 0.5 in water

column below atmospheric pressure is a typical figure) and a relatively largeopening (burner throat) allow enough air to enter the combustion zone to

support the full firing capacity The diffusion burner comprises a fuel

mani-fold, risers, tips, orifices, tile, plenum, throat restriction, and damper Each

diffu-sion burner type may differ in detailed construction, but all will possessthese main functional parts We discuss each in turn

2.1.1.1 The Fuel System

A fuel manifold is a device for distributing fuel In the figure, one fuel inlet admits fuel to the manifold while several risers allow the fuel to exit A riser

is a fuel conduit (In the boiler industry, risers are sometimes termed pokers,

but the function is the same.) Each riser terminates in a tip; for some boiler

burners the tip is called a poker shoe or just a shoe A tip is a device designed

to direct the fuel in a particular orientation and direction It has one or more

orifices, holes, or slots drilled at precise angles and size An orifice is a small

hole or slot that meters fuel — for a particular design fuel pressure, sition, and temperature, the orifice restricts the flow to the specified rate

compo-Together, these parts comprise the fuel system of a gas burner.

FIGURE 2.1

A typical industrial burner The typical industrial burner has many features, which can be classified in the following groups: air metering, fuel metering, flame stabilization, and emissions control Refer to the text for a discussion of each.

Burner Throat Primary Fuel Tip Flame

Secondary Fuel Tips Burner Front Plate Cone (Throat Restriction) Inlet Damper

2.1.1.2 About Fuels

There are two main gaseous fuels for combustion processes: natural gas andrefinery gas Commercially available natural gas has a stable compositionthe other hand, is capable of considerably more variation In a sense, refinerygas is the “garbage dump” for the gas products in the refinery Generally,whatever the refinery cannot use for some higher value-added process itconsumes as fuel It is quite typical for refineries to specify several differentrefinery fuels for combustion equipment — one representing normal condi-tions, another representing a normal auxiliary case, and perhaps two or threeupset scenarios The scenarios will vary in hydrogen concentration, typicallyfrom 10 to 60%, giving the fuels quite different combustion properties

It is very important that the refinery weigh the likelihood of scenarios thatrepresent widely varying heating values on a volumetric basis For example,suppose a particular process unit can receive fuel according to four differentscenarios:

• Fuel A: 620 Btu/scf, normal fuel representing 10% of run time

• Fuel B: 760 Btu/scf, stand-by fuel representing 84% of run time

• Fuel C: 840 Btu/scf, start-up fuel representing 5.98% of run time

• Fuel D: 309 Btu/scf, high hydrogen upset case representing 0.02%

of run time

Since fuel C is a start-up case, it does not matter how infrequently it occurs,the burners must operate on fuel C However, the difference in volumetricflow rate among the fuels A, B, and C is small On the other hand, fuel Drepresents a significant difference in hydrogen concentration This will mark-edly affect major fuel properties such as flame speed, specific gravity, andflow rate through an orifice Later, we shall develop the flow equations thatshow that fuel D represents the maximum flow (and maximum fuel pressure)condition

One can obtain a burner to meet all these fuel conditions However, forfuels A, B, and C, the pressure will necessarily be lower Some possibleconsequences are lower fuel momentum and “lazier” longer flames whenthe facility is not running fuel D In severe cases, the flue gas momentumwill control the flame path Thus, flames may waft into process tubes andwill be generally poorer in shape — all for the sake of preserving goodoperation for an operating case representing only 0.02% of the run time Afar better scenario would be to design the burner to handle fuels A, B, and

C Then fuel B is the maximum pressure case, but the facility will not haveenough pressure to make maximum capacity with fuel D Therefore, 99.98%

of the time the flames will be fine and the unit will operate properly; 0.02%

of the time the unit will not be able to fire the full firing rate In the opposingscenario, the operators will struggle with the unit 99.98% of the time, andthe burner will run optimally only 0.02% of the time

comprising mostly methane (see Appendix A, Table A.9) Refinery gas, on

2.1.1.3 Fuel Metering

A fuel control valve upstream of the burner generally does the fuel metering

A riser or poker delivers fuel to the burner tip The facility specifies the

pressure the burner will receive at maximum (design) firing rate The burnermanufacturer then sizes the fuel orifices in the tips to ensure that burner willmeet the maximum fuel capacity at the specified conditions The burnermanufacturer provides a series of capacity curves (one for each fuel) thatshow the firing rate vs the pressure Figure 2.2 gives an example

In natural draft burners, air is generally the limiting factor, and its trolling resistance is the burner throat Thus, the only way to increase theoverall firing capacity of the burner is to increase the throat (i.e., burner)size This may or may not be possible depending on the available space inthe heater and if the heater can handle the extra flue gas and heat that result

con-If not, the entire unit may require modifications, not just the burner

2.1.1.4 Turndown

Burners operate best at their maximum capacity One measure of the flame

stability of a burner design is the turndown ratio The turndown ratio is the

FIGURE 2.2

A typical capacity curve Fuel capacity curves give heat release as a function of fuel pressure They are quite accurate for a given fuel composition However, if there are a range of fuels, each needs its own capacity curve This example shows three The typical range is two to five fuel scenarios, all represented on the same graph As the flow transitions to sonic, the capacity curve becomes linear.

Fuel Pressure, bar(g)

ratio of the full firing capacity to the actual firing capacity The maximumturndown ratio is the max/min firing ratio The turndown ratio is higher ifone modulates the air in proportion to the fuel If the air dampers aremanually controlled, then one is interested in the maximum unmodulatedturndown ratio, because this is the more conservative case A typical maxi-mum turndown ratio for premix burners is 3:1 Diffusion burners often haveturndown ratios of 5:1 without damper modulation, i.e., leaving the damperfully open despite lower fuel flow One typically achieves 10:1 turndownratios with automatic damper and fuel modulation Multiple burner furnacesusually require a turndown ratio of somewhere between 3:1 and 5:1 withthe air dampers fully open To achieve greater turndown for the unit as awhole, one isolates some of the burners (fuel off, dampers closed) Turndown

is easiest for a single fuel composition Multiple fuel compositions alwaysreduce the available pressure for some fuels and reduce the maximum turn-down ratio

2.1.1.5 The Air System

For natural draft burners, air is the limiting reactant That is, sufficient fuelpressure is available to allow the burner to run at virtually any capacity, butonly so much air will flow through the burner throat at the maximum draft

The burner throat refers to the minimum airflow area; it represents the

con-trolling resistance to airflow Therefore, the airside capacity of the burnerdetermines the burner’s overall size

Air may enter from the side (as shown in Figure 2.1) or in line with theburner Analogous to the fuel orifice, there are two metering devices for the

airside The first is the damper, upstream of the plenum A damper assembly

is a variable-area device used to meter the air to the burner This is necessarybecause the maximum airside pressure drop is limited and the firing ratemodulates Therefore, one must modulate the air to maintain the air/fuelratio The damper may require manual adjustment, or one may automate it

by means of an actuator

The inlet damper unavoidably creates turbulence and pressure fluctuations

behind it The plenum is the chamber that redistributes the combustion

air-flow before allowing it to enter the burner throat This redistribution doesnot need to be perfect, and there is a trade-off between uniform flow (largerplenum) and burner cost In some cases, one may shorten the plenum by

means of a turning vane — a curved device designed to redirect the airflow

(not present in the figure)

Burners are available in discrete standard sizes To accommodate the nite variety of potential capacities, manufacturers adjust the limiting airflow

infi-by means of a restriction in the burner throat A choke ring is an annular blockage from the outside diameter inward A baffle plate is a flat restriction originating from the center outward A cone is an angled restriction originat-

ing from the center of the burner throat Figure 2.1 shows a cone, but baffleplates or choke rings are also very common The purpose of the throat

restriction is to provide a point of known minimum area and pressure dropcharacteristic.

Some codes actually specify what portion of the draft (airside pressuredrop across the burner) that the damper and the burner throat must take.For example, one guideline says that the burner must use 90% of the available

pressure and take 75% of the pressure drop across the throat.* The 90/75 rule,

as it has come to be known, aims to create turbulence in the burner throatrather than at the damper to aid fuel–air mixing In the author’s opinion,this is a misguided approach and the industry should abandon it for severalreasons First, if the burner performs well, the degree of turbulent mixing inthe throat is immaterial Second, if the burner performs well, it does notmatter where the pressure drop occurs so long as the total pressure drop iscorrect Third, this rule increases the time and cost of burner testing Moreimportantly, changes to the burner for the sake of meeting the 90/75 rulemay actually make the burner perform worse Therefore, one is consumingresources to meet an essentially useless rule

Very often, the end user will have to clean or inspect tips while the furnace

is running In multiple-burner furnaces comprising many burners, there isusually little danger in shutting off a single burner In petroleum refineries,the usual practice is to specify a burner having replaceable tips that do notrequire burner removal from the heater (of course, one must still shut offfuel flow to the individual burner before removing the tips) One must alsotake care to close the air damper during this procedure; otherwise, thefurnace will admit tramp air

Tramp air is air admitted out of place Air entering the furnace should

participate fully in the combustion process, and tramp air enters the bustion process too late to oxidize the fuel properly Tramp air may comenot only through unfired burners, but also through leaks in the furnace Onepossible sign of tramp air is a high CO reading even with supposedlysufficient excess oxygen CO is a product of incomplete combustion Depend-ing on the furnace temperature, 1 to 3% oxygen should represent enough excessair, and CO should be quite low under such conditions But with tramp air,significant CO (>200 ppm) may still occur — even with 3% oxygen (or more)

com-in the flue gas The oxygen has entered the furnace somewhere, but it is notparticipating fully in the combustion reaction As long as there is tramp air, thefurnace will require higher oxygen levels — enough to provide both effectualair through the burner and ineffectual tramp air In severe cases of tramp airleakage, even a wide-open damper cannot provide enough air to the burnerand CO persists, though stack oxygen levels are 5% or more

The exit of the furnace radiant section is the relevant place to measureemissions for combustion purposes The exit of the stack is the relevant place

to measure emissions for compliance purposes The stack exit is not generallyuseful for understanding what is happening in the combustion zone

* This requirement appears as a footnote to API Standard 560, Fired Heaters for General Refinery Service, 3rd ed., Washington, DC, American Petroleum Institute, May 2001, p 65.

2.1.1.6 The Flame Holder

Anchoring the flame to the burner is essential for the sake of performance

and safety A flame holder is a device designed to keep the leading edge (root)

of the flame stationary in space There are many different devices for

accom-plishing this The most common device is the burner tile A burner tile is a

refractory flame holder designed to withstand the temperature of direct

flame impingement The tile ledge is the portion of the tile that anchors the

flame (Figure 2.3)

One type of flame holder is the bluff body — a nonstreamlined shape in the

flow path — to present an obstruction to some of the flowing fuel–air mixture;the tile ledge qualifies This obstruction generates a low-pressure, low-velocityzone at its trailing edge The flame holder affects only a small portion of theflow, reducing its velocity to well below the flame speed The velocity upstream

of the flame holder is very low Thus, the hot combustion products recirculatethere, continually mixing fresh combustion products with an ignition source

— the hot product gases In this way, the flame holder anchors and stabilizesthe flame over a wide turndown ratio Figure 2.4 shows a 2-MW round-flameburner employing a tile-stabilized flame The shape of the burner and position

of the fuel ports mold the flame into the desired shape

2.1.1.7 Stabilizing and Shaping the Flame

A flame has a very fast but finite reaction rate One measure of the reaction

rate is the flame speed The laminar flame speed is the flame propagation rate

[L/θ] in a combustible mixture of quiescent fuel and air If the air and fuelmixture exceed the flame speed, then the flame will travel in the direction

of the stream — a phenomenon known as liftoff If the liftoff continues, the

FIGURE 2.3

The tile ledge as flame holder (From Baukal, C.E., Jr., Ed., The John Zink Combustion Handbook,

CRC Press, Boca Raton, FL, 2001.)

flame will be transported to a region of high flue gas concentration thatcannot support combustion and the flame will extinguish Air and fuelvelocities in a typical industrial burner far exceed laminar flame speeds But

in the vicinity of the bluff body, the fluid speed is low; therefore, the flameanchors over wide turndown range and the burner is quite stable throughoutits entire operation

2.1.1.8 Controlling Emissions

In the past, the only emission of concern was CO because it indicated plete combustion, combustion inefficiency, or a safety hazard Nowadays,life is more complex and other emissions such as nitric oxides are importantdue to their role in the formation of ground-level ozone and photochemicalsmog Technically, noise is also a regulated emission (e.g., <85 dBA) Emis-sions control is an active subject of interest Staging the combustion into

incom-distinct zones is one strategy (termed staging) to lower certain emissions

such as NOx If NOx emissions are not a concern, then the secondary fueltips of Figure 2.1 may be unnecessary and the burner carries out combustionusing only primary fuel tips

• Fuel–air mixing strategy:

– Fuel and air premixed (premix burner)

– Separately metered fuel and air (diffusion burner)

• Firing orientation

– Upfired (the burner fires from the floor upward)

– Downfired (the burner fires from the roof downward)

– Side-fired (the burner fires from the wall sideway)

• Emissions

– The burner design reduces combustion-related emissions

– The burner design has no special features for reducing tion-related emissions

combus-These five characteristics generally fix the burner design, and they willdefine an archetypical burner Neglecting solid-fired burners for now (e.g.,wood, municipal solid waste, pulverized coal, etc.), each of the above cate-gories has two possibilities, except for firing orientation, which has three.This leads to 24·3 = 48 different slots However, as is typical, not every slothas a filler, and some burner models fill more than one slot For our purposes,about a dozen burner types are of importance We should add that there aremany kinds of esoteric designs for special reactions, but as regards traditionalhow one burner manufacturer has chosen to fill them

It is, of course, possible to entertain other considerations For example,

• Type of draft: Is the motive force for air due to natural convection

in the heater (natural draft), from a fan outlet upstream of the burner(forced draft), from a fan inlet downstream of the burner (induceddraft), or both inlet and outlet fans (balanced draft)?

• More fuel-state variations Will the burner fire liquid and gaseousfuels at the same time or separately?

• Service: Will the heater serve in a boiler to generate steam, or will itserve in a process heater to refine petroleum or make petrochemicals?However, for the most part, design variants of the enumerated burnertypes will accommodate all of the above categories So, we will describe thefuel–air combustion, these categories will do Table 2.1 shows the slots and

burner types first and then make general remarks about other features whereappropriate.

2.1.2.1 Round-Flame Gas Diffusion Burners

Burners in these categories are gas fired and designed to produce roundflames These comprise the lion’s share of fired duty These also comprisethe largest single burner capacities — a typical refinery size fires about 2.5

MW (~8 MMBtuh) However, they can be as large as 8 MW (~35 MMBtuh),though this is uncommon — the traditional approach in the refinery hasbeen to use more but smaller burners rather than a few large burners In thepower generation industry, opposite sensibilities prevail Package boilers rep-resent the middle ground A 3-MW floor-fired burner is about as big around

as a man can circle his arms, and roughly his height Each weighs about 500

kg All kinds of process heaters and boilers use them Electrical utilities andcement kilns use the larger sizes (>10-MW heat release per burner)

2.1.2.2 Round-Flame Gas Premix Burners

Figure 2.5 shows an example of a round-flame floor-fired premixed burner.

Some furnaces use premix burners in larger upfired applications requiringround flames However, this has fallen into disfavor because the burners areusually loud (due to fuel jet noise) and sensitive to hydrogen concentrationvariations

In premix burners, air and fuel mixing occur prior to entering the furnace.Premixing has several advantages First, premixed burner flames tend to beshort, crisp, and well defined The fuel jet provides momentum for fuelmixing and air entrainment prior to the burner exit An advantage of this

Conventional Low NOx Conventional Low NOx

Flat SFFG XMR, PSFFR Oil Round LNC + EA DeepStar

arrangement is that increased fuel flow results in increased airflow However,premixed burners also have major disadvantages in the wrong application.First, the momentum of the fuel stream depends on the molecular weight ofthe gas and the fuel pressure As the H/C ratio of the fuel changes, so doesthe molecular weight, the fuel pressure for a given heat release, and theamount of educted air Higher pressures tend to educt less than the propor-tional air, while lower fuel pressures educt higher than ideal airflows Thus,the air/fuel ratio is not as constant as one might hope.

Another serious drawback of premixed combustion is the potential for

flashback Flashback is the upstream propagation of flame into the premix

chamber of the burner In a diffusion burner, it is impossible for a flame topropagate back into the fuel riser because there is no oxygen there to supportcombustion However, in a premix burner, there is a combustible mixtureinside the burner tip Premix burners also have a much larger tip and orificesbecause these have to accommodate a high-volume, low-pressure fuel–airmixture If the flame speed significantly exceeds the fuel–air exit velocity,then the flame may flash back into the burner tip

FIGURE 2.5

A gas premix floor burner (Courtesy of the American Petroleum Institute, Washington, DC.)

Secondary air

Pilot Gas

Primary air

Burner internals cannot withstand the high temperatures of combustionfor long Flashback is very sensitive to hydrogen concentration because thelaminar flame speed of hydrogen is about three times that of hydrocarbons.Various techniques moderate flashback One important consideration is the

quench distance — a characteristic length for a given orifice geometry through

which a flame cannot propagate For small-diameter orifices, the edges willabstract sufficient heat from a propagating flame to extinguish it The quenchdistance varies with orifice diameter — smaller orifices are more effectivemum slot widths for various fuels Generally, for a given area, a circularorifice is more efficient for quenching flames than a rectangular one How-tice, both geometries are commercially available

The removal of heat from the flame via thermal conduction through the

tip is a mechanism for quenching the flame and eliminating flashback Since

the tip temperature depends in some measure on the temperatures of thefurnace, fuel, and combustion air, the tip’s ability to conduct heat away from

a propagating flame also changes with temperature Moreover, at highertemperatures, the orifices in the tip expand and one must take account ofthis effect in burner design as well

2.1.2.3 Flat-Flame Gas Diffusion Burners

Figure 2.6 gives a typical example

These burners are usually floor fired against a flat wall that radiates heat

to the process tubes (Figure 2.7)

In other ways, these burners are similar to round gas diffusion burners.One uses these burners to maximize radiant heat transfer from the wall tothe process For example, high-temperature processes — such as production

of hydrogen or ethylene — use the hot wall to radiate to process tubescontaining feedstock Since the reaction occurs along the length of the tube,

the so-called heat flux profile can be important Figure 2.8 gives an example.

The burner manufacturer adjusts the heat flux profile according to furnacevendor specifications by changing the angle, size, and distribution of thefuel jets NOx reduction with these burners is a challenge because the processoperates at very high furnace temperatures (1000 to 1250°C)

Another type of flat-flame diffusion burner is side-fired The architecture

of the burner resembles that of the side-fired premix burner, but the burnermeters the air and fuel separately Figure 2.9 gives one example, and Figure2.10 shows some in operation

Ethylene reactors and wall-fired hydrogen reformers (described later) usethese burners The overall chemistry for hydrogen production is

CHψ + 1/2 O2 → CO + ψ/2 H2 (2.1)ever, smaller rectangular slots may have manufacturing advantages In prac-than larger ones Appendix A, Table A.4 gives critical diameters and mini-

Hydrogen plants often use pressure-swing adsorption (PSA) to purifyhydrogen and separate it from the CO2 by-product The off-gas from thisstream is mostly CO2, with some hydrogen, and to a lesser extent hydrocar-bons The high concentration of CO2 dramatically lowers the flame temper-ature Flat-flame side-fired diffusion burners have excellent stability, cannotflash back, and NOx emissions below 20 ppm are possible even with 400°Cair preheat and 1200°C furnace temperatures As Figure 2.9 and Figure 2.10show, the fuels travel down a central riser; the slotted tip projects the fuel

in a radial plane parallel to the wall Preheated air comes through the largeannular gap and enters the furnace in the same orientation At the highfurnace temperatures, the separate fuel and air streams react to generate aflame with very low NOx and uniform radiation

2.1.2.4 Flat-Flame Premix Burners

Flat-flame premix burners comprise side-fired ethylene or steam–methanereforming service (hydrogen production) almost exclusively Figure 2.11shows a common design

The burner tip is roughly 4 in in diameter and 8 in long Slots or holescover the end and admit premixed fuel and air to the furnace Premix burnersare prone to flashback, though proper design will ameliorate this

FIGURE 2.6

A flat-flame gas diffusion burner The burner creates a flat flame used to heat a wall that radiates

heat to the process.

2.1.2.5 Flashback

Flashback can only occur in premixed burners because they are the only typethat has a combustible mixture inside the tip Once the flame flashes backinto the burner tip it can destroy it in minutes Combustion inside the tipincreases the mixture temperature downstream of the eductor This back-pressures the eductor and further reduces the air/fuel ratio The richening

of the fuel mixture and higher temperatures increase the flame speed fore, once flashback occurs, there is no mechanism for moving the flameback to the furnace side The burner immediately experiences lower massflow due to the reduced density of the gas during flashback; one may hear

There-a gurgling sound from the combustion rumble in the tip

2.1.2.6 Use of Secondary Fuel and Air

The tip receives its premixed fuel and air from a venturi (Figure 2.12)

A fuel jet ahead of the venturi induces surrounding air via the fuel’sforward momentum Under some conditions the fuel educts only a portion

of the combustion air In that case, secondary air slots allow additional air

to bypass the venturi (shown in Figure 2.11) In some cases, the premixburner may have some nonpremixed fuel in addition to the fuel–air mixture,thus staging the fuel (Figure 2.11) Fuel not added to the immediate com-

bustion zone — the primary zone — is termed secondary fuel or staged fuel.

FIGURE 2.7

Floor-fired flat-flame burners The burners, John Zink Model PXMR, are shown firing against

a wall in an ethylene cracking furnace The wall radiates heat to the process tubes (not shown).

Secondary fuel injection is one technique for reducing NOx Side-fired mixed burners are typically much smaller than floor-fired burners and weigh

pre-a mere 50 kg or so, including the tile Firing rpre-ates for these burners pre-are pre-about

1 MMBtuh or 1/3 MW

2.1.2.7 Round Combination Burners

In some facilities, fuel oil can be a significant fuel stream Normally, a refinerywill want to burn as heavy a fuel as possible because other liquid fuels havegreater value (e.g., transportation fuels for automobiles, trucks, and aviation).When sold commercially, fuel oils are widely available and graded as eithernumber 2 or 6, with intermediates formed by blending Fuel oil 2 is similar

to automotive diesel Fuel oil 6 is much heavier (also called residual fuel oil

or, archaically, Bunker C oil) Marine and stationary boilers and some processheaters burn this fuel Sometimes, the liquid fuel comprises rejected oil fromother processes (waste oil) in whole or part One can also burn pitch — anondescript fuel from a variety of sources that is solid at room temperature.One must heat these fuels to reduce their viscosity in order for them to burnefficiently Heavy liquid fuels do not atomize well even under pressure

FIGURE 2.8

A typical heat flux profile Shown is a side view of one cell of a dual-cell floor-fired ethylene cracking unit (ECU) with its associated heat flux profile The proper heat flux profile is a trade- off between reduced fouling (flat heat flux profile) and maximized efficiency (heat release skewed toward furnace bottom) Burner vendors design floor-fired burners to provide the required flame shape for a given flux profile.

% of max heat flux process tubes

< floor burners >

50 40

70 80 90

(so-called mechanical atomization), so fuel guns make use of pressurized steam

to produce the requisite atomization Mechanical atomization is sufficientfor light oils such as fuel oil 2 Sometimes, light liquid fuels use compressedair for atomization This is the case if steam atomization could be detrimental

or there is insufficient fuel oil pressure for mechanical atomization For

FIGURE 2.9

A flat-flame diffusion burner Radial orifices admit fuel through the center pipe, while bustion air flows through the outer pipe Unlike premix designs, this radiant wall burner cannot flash back The design accommodates high forced draft air preheat applications.

com-FIGURE 2.10

Wall-fired diffusion burners in operation The photo shows an ethylene cracking furnace equipped with John Zink Model FPMR burners The process tubes (right) are receiving heat radiated from the burner firing along the wall (Photo courtesy of John Zink LLC, Tulsa, OK.)

example, light naphtha fractions can prevaporize in the fuel oil gun vaporization is unwanted because it leads to slug flow in the fuel gun, that

Pre-is, alternate slugs of liquid and gaseous fuel going to the burner This causeserratic flow and performance

FIGURE 2.11

A flat-flame premix burner The flame heats the refractory, which in turn radiates heat to the

process tubes inside the furnace Secondary air allows for a higher capacity, as the eductor need not inspirate all of the combustion air Also shown is a secondary fuel nozzle at the burner tip Some burners do not have all these features.

FIGURE 2.12

Venturi section of a premix burner The Venturi (more generically, an eductor) comprises an

inlet bell, throat, and expansion section The fuel jet induces a low-pressure zone along the jet surface The surrounding atmospheric pressure pushes air into the low-pressure zone The fuel and air mix and exit the venturi toward the tip outlet.

EXPANSION SECTION THROAT

INLET BELL

Practitioners use the term oil gun to refer to the liquid fuel delivery and

atomization assembly Figure 2.13 shows a typical design

Steam shears heated oil into fine droplets — the fuel oil vapor is the phasethat actually burns The burning vapor provides heat, vaporizing even moredroplets and recharging the combustion zone

Very often, one burns fuel oil and gas together (in so-called combination

burners) This is sometimes to add fuel flexibility — perhaps the gas and the

liquid fuel are available in different seasons However, the more typicalpractice is to use the less expensive heavy oil with the gas fuel serving tomake up the required process heat Thus, both fuels fire simultaneously Acombination burner is a gas-fired burner augmented with a fuel oil gun.Figure 2.14 shows one common arrangement

The typical combustion scenario is a single fuel oil gun in the center ofthe burner with gas firing at the periphery When both fuels fire at once,flame lengths tend to be longer than when either fires alone This is due tothe peripheral combusting gas reducing the available oxygen for the fuel oilstream To minimize (but not eliminate) this effect, separate air registersprovide individualized airflow to each zone

FIGURE 2.13

An oil gun Steam vaporizes oil droplets allowing for uniform combustion.

2.1.2.10 Downfired

Most burner models can be adapted to fire downward with some kind oftile case or support to hold the tile in place at the roof Figure 2.15 shows ageneric schematic

Hydrogen and ammonia reformers of this type are large furnaces oftenwith more than 200 burners, each firing at ~2 MW With so many burners

in a furnace, first cost is important

Downfired operation affects the flame shape because the firing direction

is opposite the buoyant force Hence, forced draft is the preferred option forthese burners Forced draft operation helps to minimize the flame bendingtoward the tubes The greater momentum of the forced air helps to overcomethe buoyant effects and furnace currents Furnace currents can be quitecomplicated Downfiring increases NOx emissions by 15% or so as the burnerreceives hotter convective air at the roof than at the floor Space is limitedand roof burners are more difficult to access than floor burners There arealso limitations on the total burner weight because the furnace roof cansupport only so much Therefore, a simple, reliable design is the order ofthe day

FIGURE 2.14

A combination burner John Zink Model PLNC combination gas–oil burner One may fire the

burner on gas only, oil only, or both (Rendering courtesy of John Zink LLC, Tulsa, OK.)

OIL GUN REGEN TILE

GAS PILOT

PRIMARY AIR CONTROL

GAS RISERS (FOR

2.1.2.11 Side-Fired

Some steam–methane reformers and ethylene cracking units (ECUs) useside-firing The general idea is to present a uniform heat flux to the reactortubes by using many small burners (<1 MMBtuh) in the furnace wall A largeECU may have hundreds of sidewall burners Figure 2.16 shows some inoperation

The flame travels parallel to the plane of the wall This path is necessarybecause the reactor tubes are less than 2 m away; flame impingement wouldoverheat them Reactor temperatures for these kinds of units are some of thehighest temperatures found in a petrochemical plant, 1200 to 1250°C

2.1.2.12 Balcony Fired

Figure 2.17 shows a typical balcony burner, also known by a lengthier and

more descriptive moniker — horizontally mounted, vertically fired (HMVF).

These burners penetrate the side of a furnace; however, the firing direction

is up A 90° bend in the air passage accomplishes this

2.1.2.13 Combination Side and Floor Firing

Some heater vendors fire ECU and related units with both floor and wallfiring The advantage of wall firing is a superior ability to tailor the heat flux

profile The heat flux profile is the radiant heat distribution along the vertical

reactor dimension A heat flux that decreases with elevation can improveunit efficiency However, a perfectly even heat flux distribution maximizes thetube life and the conversion of the feed can be easier to model Sidewall burners

FIGURE 2.15

A downfired burner for hydrogen reforming This burner is equipped with a center gun for waste gas from a pressure-swing adsorption (PSA) unit.

ensure an even heat flux profile However, they represent a higher first cost(due to the many burners), and operators must adjust the air doors row byrow, making process changes and start-up more labor-intensive.

One hundred percent floor firing does not have these drawbacks Theburner vendor can adjust the heat flux profile in the bottom two thirds of

FIGURE 2.16

Sidewall burners in operation The radial combustion pattern of the burners at left radiates along the wall, which in turn heats the process tubes (far right) The two rectangular spots on the end wall are sight ports.

FIGURE 2.17

Balcony burner This burner is horizontally mounted but vertically fired.

the furnace by changing tip drillings, in essence, changing the heat releasedistribution However, heat flux in the top third of the furnace is difficult toinfluence with tip geometry at the floor A heater comprising both floor andwall firing is one compromise to reduce first cost and operating labor whileretaining good control of the reactor heat distribution In practice, one seesall three firing scenarios.

Boilers, process heaters, and reactors comprise three main categories of cess units One may further differentiate among them as follows

pro-2.2.1 Boilers

A boiler is a device for generating steam There are two main configurations

for fired units: firetube and watertube

2.2.1.1 Firetube Boilers

In firetube boilers, the flue gas flows through the tubes and out the stack,transferring heat to surrounding water Nowadays, firetube boilers are gener-ally smaller units generating saturated steam They are usually fully automaticand unattended These provide facility steam and heat for schools, hospitals,and other commercial needs The household water heater is a firetube config-uration However, because it does not generate steam, it is not a boiler

2.2.1.2 Watertube Boilers

Watertube boilers are considerably larger and provide process steam forrefineries, pulp mills, electrical generation, etc As the name implies, watertubeboilers generate steam inside the tube In this respect, they are similar toprocess heaters but with water in the tube rather than process fluid Large-capacity, high-pressure, and superheated steam units are invariably of thewatertube design because small tubes can withstand higher internal pressurethan large shells for a given thickness Watertube boilers can grow to bequite large Coal-fired utility boilers are the largest watertube boilers Those

in the petroleum refinery and industrial plants are many times smaller

2.2.1.3 Fired Heaters and Reactors

A fired process heater is a combustion unit for heating any process fluid other

than water In a refinery, they comprise hot-oil heaters, crude heaters,

vac-uum heaters, and the like A fired reactor is a process combustion unit

designed to effect some thermochemical transformation One major

distinc-of the many process heater and fired reactor configurations

We further discuss some of them below

2.2.1.4 Vertical Cylindrical

Vertical-cylindrical (VC) units have tall right-circular shells (see Figure 2.18a

and b) The helical coil is the rarer of the two The heater designer may

arrange the tubes helically or vertically, and may accommodate several arate passes through the heater, depending on the process needs

sep-2.2.1.5 Cabin Style

A cabin (or box) unit has a rectangular profile and is usually shorter than a VC,

though it can be many times wider (Figure 2.18c to i) Tubes in a cabin heatertypically run horizontally at the walls (e.g., Figure 2.18d and e); they usuallyfire with one row of burners down the center of the heater floor (not shown).However, there may be two rows of burners, especially if there are center tubes(Figure 2.18d) Some process heaters are also end fired (Figure 2.18e)

FIGURE 2.18

Some process heater types Process and convection tubes are shaded Heaters may or may not have convection sections Round heaters are known as vertical cylindrical (VC), and rectangular heaters are known as cabin heaters (a) VC, (b) VC with helical coil Cabin type: (c) wicket tube

or arbor coil, (d) floor fired, (e) end wall fired.

Radiant Section

Radiant Section

Burners

Process

Tubes

Process Tubes

tion among fired units is their shape Figure 2.18a to 2.18i gives a sampling

FIGURE 2.18 (continued)

(f) Vertical tube, sidewall fired; (g) vertical tube, floor fired; (h) floor + wall fired — usually two

or three rows of sidewall burners + floor burners; (i) downfired (one of many cells), (j) terrace wall (horizontally mounted, vertically fired).

Side

Section

Radiant Section

Radiant Section

Radiant Section

Radiant Section

Radiant Section Burners

Burners

Burners Convection

Section

2.2.1.6 Fired Reactors

In the petroleum and petrochemicals industry, most chemical reactors usefired duty to effect their transformations We shall discuss ethylene crackingunits and hydrogen and ammonia reformers as illustrative

2.2.1.7 Hydrogen Reformers

Hydrogen reformers exist in side-fired (Figure 2.18f), downfired (Figure2.18i), or terrace wall-fired (Figure 2.18j) configurations Hydrogen is animportant commodity in fuels upgrading The general chemistry is

CH4 + H2O → 3H2 + CO (2.2)That is the reforming reaction The reaction temperature inside the processtubes is ~815°C for this step.1 Therefore, the bridgewall temperature of thereactor must be higher (~1050°C) The following water–gas shift reaction (or

simply, shift reaction) increases the H2 yield:

CO + H2O → CO2 + H2The shift reaction can occur at low temperatures (200 to 230°C) or hightemperatures (300 to 450°C), depending on the catalyst in the tube and thedesired conversion efficiency

Thermodynamically, the low-temperature shift reaction is more efficient.However, nowadays, pressure-swing adsorption (PSA) is the separation pro-cess of choice, and its natural companion is the high-temperature shift reac-tion because one can burn any unconverted CO as fuel As the name implies,PSA is a pressure cycle Solid adsorbents adsorb impurities at high pressureand release them at low pressure Using two adsorbent vessels, the processmay produce product on a continuous basis The depressurization cyclegives CO, CO2, N2, and some H2 as a fuel stream at low pressure For thispurpose, hydrogen reformer burners are equipped with a PSA tip For exam-ple, Figure 2.15 shows a burner equipped with a large center tip to use thePSA tail gas for process heat PSA tail gas tends to form very little NOx(owing to the inert content of the fuel) and good flames (owing to fast flamespeeds of H2 and CO)

2.2.1.9 Ethylene Cracking Units (ECUs)

Figure 2.18g gives an example of a floor-fired ethylene cracking unit (ECU);Figure 2.18h shows an ECU that is fired by both floor and wall burners Insuch a configuration, the floor burners have 70 to 80% of the heat release,with the wall burners comprising the balance Figure 2.18f gives an example

of a wall-fired steam–methane reformer, and Figure 2.18i shows one cell of a

downfired hydrogen reformer Such reformers comprise several cells andhundreds of burners

Modern ECUs contain tubes 10 to 12 m in length Each tube hangs fromthe roof with a single U-bend at the bottom Burners on both sides of theprocess tube heat it The heat converts the feed hydrocarbon into ethylene,

C2H4 Ethylene is the largest volume organic chemical, and almost all ene production is by the thermal process.2 High heat causes abstraction ofhydrogen and formation of a double bond For example,

ethyl-CH3—CH3→ CH2=CH2 + H2 (2.4)Ethane, propane, and naphtha are the most common hydrocarbon feed-stocks These reactors have the highest bridgewall temperatures of any com-mon petrochemical process, 1200 to 1250°C

The combustion-related responses that we wish to model are things like NOxand CO emissions, CO breakthrough point, flame length, heat flux profile,and the like These will be a function of one or more of the following factors:fuel composition, oxygen concentration, burner type, degree of staging, fur-nace temperature at some convenient point such as the bridgewall or floor,etc Tests for these kinds of things are now fairly standard

2.3.1 The Traditional Test Protocol

The American Petroleum Institute (API) formally defines various burner tests.3

We numerically index the important ones below To this, we add point 0, as itdoes not specifically appear in the API publication, but is normally tested

0 Cold light-off This refers to ignition of the burner in a cold furnace.

One should use the same method for ignition that is available in thefield This may comprise manual ignition by a torch, ignition by aburner pilot, or manual spark ignition The idea is to simulate the fieldstart-up condition, where the burners start up for the first time Onelooks for attached and stable flames, and a smooth transition as theburner ramps up The damper position at this condition is termed the

light-off position It is usually of interest and therefore recorded.

1 Normal This point refers to burner operation at the specified normal

firing rate and design excess air The damper position at the normal

firing rate is termed the normal position and is of interest.

2 Minimum This point refers to the maximum turndown of the burner

with its damper in the normal position

3 Maximum This point refers to the increased firing rate that causes

the excess oxygen to reach its minimum acceptable limit (often 1%)with the damper in the normal position

4 CO breakthrough This point refers to the firing rate greater than maximum, where CO just becomes greater than 200 ppm — the CO

breakthrough point or oxygen at CO breakthrough refers to the percent

oxygen at this point

5 Absolute minimum This point is the maximum achievable turndown

with adjustment of the air damper to give the design excess air.The API guidelines confirm suitability for purpose and are the usual andsufficient tests to predict acceptable burner operation in the field However,these points will not provide enough information for detailed analysis, test-ing, and characterization of burners Later chapters provide other experi-mental designs for these purposes

2.3.2 Instability, Thermoacoustic and Otherwise

API 535 characterizes unacceptable burner operation as burner instability at

any of the test points; it further characterizes instability as3

a Pulsation or vibration of burner flame, burner, or furnace

b Uncontrollable fluctuations in the flame shape

c Significant combustibles in the flue gas, in other words, over 2000ppmvd [parts per million on a volume dry basis] CO or over 0.5percent unburned combustibles

d Flashback into the venturi section of premix burners

e Loss of flame

While the API document has burner instabilities in mind, not all ities are due to burner design proper Specifically, thermoacoustic couplingbetween the burner and furnace can sometimes be the problem The lattercase may not indicate any problem with the burner per se, but may be due

instabil-to a coupling between the test furnace and the burner — a thermoacousticproblem

Thermoacoustic coupling is any resonant phenomenon propagated by a

ther-mal source The issue is a complex one and a specialty discipline in its ownright The predominant modes of burner–furnace interaction are quarter-wave and Helmholtz resonation Generally, if heat adds to an acoustic wave

in phase with a pressure rise, it will reinforce the resonance One can alsoreinforce resonance by removing heat in phase coincident with a fallingpressure wave.

usually small, generally,

where D S is the stack diameter.4

FIGURE 2.19

Resonant modes for furnaces (a) A furnace and stack in quarter-wave mode The pressure oscillates between minimum and maximum at the heater floor In half-wave mode (b) the pressure has maximum oscillations at the top and bottom end of the heater In Helmholtz resonant mode (c) the furnace acts as a spring and the stack as a piston In principle, the energy from the combustion reaction can excite any of these modes However, the heat release pattern

in a furnace tends to excite modes with (a) or (c), most commonly.

pressure oscillation

pressure oscillation

mass-spring analogy

D S

The bottom end of the heater (where the burners reside — not shown)corresponds to the closed end of a tube This would be the case when theburner throat represents a very small fraction of the total cross-sectional area

of the heater, as is typical in furnace design At the closed end of the tube,the velocity must be zero Since velocity and pressure are 90° out of phase,the absolute value of the pressure wave will be at a maximum (the pressureantinode) That is, the pressure wave will alternate between maximum andminimum at the heater floor If such an acoustic phenomenon were to occur,the length of the resulting wave would be one fourth of the heater + stackheight We can convert this to a frequency with the formula

(2.5)

where c is the speed of sound [L/θ], λ is the wavelength [L], and ν is the

frequency [1/θ] We may also calculate the speed of sound from

(2.6)

where γ is the heat capacity ratio, R is the gas constant [L2/θ2T], T is the

temperature [T], and W is the molecular weight (~28 for flue gas) For flue

gas, γ ≈ 7/5; for fuels, γ ≈ 4/3.* Substituting Equation 2.6 into Equation 2.5and solving for ν (Greek letter nu) gives

for a Quarter-Wave Resonator

Problem statement: Suppose an enclosed flare behaves as a

quarter-wave resonator Calculate the resonant frequency if the average

* The heat capacity ratio is approximately 5/3 for monatomic gases, 7/5 for diatomic gases like air, and 4/3 for simple polyatomic gases such as CH4, H2O, and CO2.

γλ

W c

RT W c L

flue gas temperature is 1450°F and the height of the unit is 60 fttall.

Solution: From Equation 2.6 we have

This is about double the speed of sound at room temperature inair From Equation 2.7 we have

Thus, the resonant frequency will be 9 Hz This is below thethreshold of human hearing (20 to 20,000 Hz) Therefore, observ-ers may feel such resonance but cannot hear it It may correspond

to the resonant frequency of large structures such as homes Insuch a case, residents have been known to mistake the phenomenafor an earthquake.*

Half-wave behavior (2.9)

2.3.5 Helmholtz Resonator Behavior

If the stack area is small compared with the cross-sectional area of the heater,then the heater may behave as a Helmholtz resonator In this mode, the stack

* Private conversation with Tim Hogue, supervising engineer at the Hyperion Wastewater ment Facility in Los Angeles, 1993.

2 2

1

s Hz[ ] .

RT W c L

acts as a piston and the furnace volume as a spring Equation 2.10 representssuch a case:

Helmholtz resonator (2.10)

where A S is the cross-sectional area of the stack, L S is the length of the stack,

and V S is the volume of the furnace For a cylindrical furnace and stack wemay write

Helmholtz resonator, VC heater (2.11)

The frequency of a heater in Helmholtz resonance will usually be lowerthan that for a half- or quarter-wave resonance

for a Helmholtz Resonator

Problem statement: Repeat Example 2.1 for a VC heater behaving

as a Helmholtz resonator with a furnace height of 40 ft and a stackheight of 20 ft Presume that the stack diameter is 20% of thefurnace diameter

Solution: The speed of sound will be identical to our first

exam-ple: 2165 ft/sec Then Equation 2.11 gives

2.3.6 Mechanism for Thermoacoustic Coupling

Earlier, we stated that the heat must add in phase with the pressure wave

in order for a burner to couple thermoacoustically with a furnace Uponreflection, this seems counterintuitive It seems that an increase in pressurenear the burner would attenuate the flow of air, and thus the heat release

2

L L

D D

RT W c

L L

D D

F s S F

F s

S F

νπ

ft ft

H

would be out of phase with an acoustic pressure wave (We can usuallyneglect any variation in fuel pressure because the fuel flow across an orifice

is usually in critical flow — also termed sonic or choked flow It is impossible

for a pressure wave to propagate upstream of a critical flow nozzle.) Tounderstand the mechanism for thermoacoustic coupling, we must examinethe transfer of heat

The velocity wave must be out of phase with the pressure wave becausewhen a high velocity encounters a wall, the velocity must drop to zero(velocity node) and the pressure must rise to maximum (pressure antinode).Therefore, the waves are 90° out of phase Now, imagine we are at theposition of the flame and pressure and velocity are fluctuating there Let usfocus on the velocity for a moment

The average velocity is the mean flow rate through the heater divided bythe cross-sectional area An acoustic wave superimposed on the mean flowwill alternately enhance or attenuate the flow as the velocity alternates Now

in the case of enhanced flow, the flue gas is colder than the mean temperaturebecause it comprises a greater portion of cool influent air In the case ofattenuated flow, the flue gas temperature is warmer than average becauseless cold air flows into the furnace during that period Clearly, we transfermore heat when the thermal gradient is larger, that is, when the flowingfluid is cooler than average In other words, more heat transfer results duringthe enhanced flow cycle than during the attenuated flow cycle

Now since pressure and velocity are not in phase, we can find a position

in the heater for the flame where the increase in heat transfer will coincidewith an increase in the pressure wave For a furnace resonating in quarter-

wave mode, that position is L/4 That is, if the position of our heat source (flame) is at L/4, then we will enhance thermoacoustic coupling Not only can we add heat at L/4, but also, if we remove heat at 3L/4, we will subtract

heat during falling pressure, which amounts to the same thing, as Rijkeshowed.4 So in fact, having a flame at L/4 and a convection section at 3L/4

reinforces thermoacoustic coupling The minor miracle seems not to be whysome heaters experience instability, but why not all heaters experience insta-bility The answer seems to lie in the following facts:

1 Flames are not concentrated at L/4.

2 Surfaces and volumes within the heater can cause destructive ference of the fundamental resonant mode

inter-3 Resistances in the heater, such as convection tubes, etc., reduce theefficiency of thermoacoustic coupling

2.3.7 Comments Regarding Thermoacoustic Resonance

Burner–furnace interactions are a cause for concern if the test furnace isacoustically similar to the field unit — even if the burner itself is not the

problem However, the test unit almost never bears acoustic similarity to thefield unit Therefore, thermoacoustic resonance in the test furnace does notusually signal a concern in the field, but burner instability does How can

we know the difference?

To be sure, one must construct an acoustically similar system, either ically or mathematically This is not trivial and requires expert assistance.However, if the following remedies are effective at eliminating or attenuatingthe instability, then thermoacoustic coupling is the likely culprit

phys-1 Reduce the stack damper opening and compensate with a steamsparger or induced draft (ID) fan, if available By closing the damper,one may possibly convert the furnace from quarter-wave to half-wave mode and decouple the burner and the furnace

2 Switch test furnaces If the same burner is stable in a furnace withdifferent dimensions, the culprit is likely thermoacoustic resonance

3 Measure the frequency of the instability Calculate the expected quencies of the resonant phenomenon for quarter-wave and Helm-holtz behavior If the frequencies match, this is a strong indicationthat thermoacoustic behavior is an issue

fre-4 Is there an induction period? Does the problem build? Resonance isthe constructive addition of multiple pressure waves Theoretically,the pressure waves add forever, becoming infinitely strong How-ever, nonlinear behavior and attenuating mechanisms put a ceiling

on the maximum amplitude Notwithstanding, during the inductionphase, pressure waves grow over time Usually some seconds arerequired to reach the maximum Although one cannot hear such fre-quencies by ear, the “huffing” of air in the 2- to 20-Hz range can beheard or felt Theoretically, the velocity lags the pressure wave by 90°,but the frequency is the same and thus an appropriate indicator

2.3.7.1 Resonance in the Field

Sometimes, heaters resonate Even if due to thermoacoustic resonance, it iseasier to redesign the burner rather than the furnace, although the addition

of quarter-wave tubes on the furnace roof can help These reflect the pressurewave 180° out of phase with the resonant frequency The destructive inter-ference eliminates the resonance The author has used this method success-fully for a process heater,* and the literature reports successful application

to boilers.5 Redesign of the burners could include changing the flame

dimen-sions Sometimes, shortening of the flame below the L/4 criterion can be

effective; however, shorter flames may elevate NOx

* Together with Mahmoud Fliefil, acoustic engineer, John Zink LLC, Tulsa, OK.

2.4 Mass Balance for Combustion in Air

With this brief discussion of combustion equipment concluded, we turn our

attention to the combustion reaction itself Reaction 2.12 shows the

combus-tion reaccombus-tion for methane:

CH4 + 2O2→ CO2 + 2H2O (2.12)

The single arrow (→) indicates a reaction proceeding unilaterally from

reactants to products at right Reaction 2.12 encodes many important aspects

of the combustion reaction First, the stoichiometry (law of proportions) is

apparent: one volume of methane (CH4) reacts with two volumes of oxygen(O2) to form one volume of carbon dioxide (CO2) and two volumes of water(H2O) Second, the combustion reaction alters molecular entities, but notatomic ones That is, reactants and products each have the same number of

C, H, and O atoms However, the identity of the molecular entities haschanged from CH4 and O2 to CO2 and H2O Therefore, combustion reactionsconserve mass: 1 kg of reactants will yield 1 kg of products without fail We

may augment Reaction 2.12 to account specifically for the nitrogen in the

air Nitrogen does not take part in combustion chemistry to any appreciableextent Combustion in air does form some nitrogen oxides in part per millionquantities, but they are a trivial part of the mass balance, although they form

an important class of regulated compounds, which we discuss later Reaction2.13 augments the combustion reaction to account for nitrogen The nitrogendilutes both reactants and products The ratio of nitrogen to oxygen in air

is approximately 79/21 by volume

CH4 + 2O2 + 2(79/21) N2→ CO2 + 2H2O + 2(79/21) N2 (2.13)

Reaction 2.13 gives the stoichiometric amount of air required for tion of one volume of CH4 However, the typical industrial practice is to usesome quantity of extra air to ensure complete combustion In principle, ifone were to mix the air and fuel thoroughly during the combustion process,then no excess air would be required However, adding a little excess air is

combus-the most cost-effective way to ensure complete combustion Reaction 2.14

accounts for this excess air (ε):

CH4 + 2(1 + ε)O2 + 2(79/21)(1 + ε) N2→

CO2 + 2H2O + 2(79/21)(1 + ε)N2 + 2ε O2 (2.14)

For the first time, oxygen appears in the products The reader may verifythat atomic entities are still equal for both products and reactants

One final step will transform Reaction 2.14 to a general equation for

hydro-carbon combustion: we rewrite CH4 as CHψ, where ψ [ ] is a generalizedsubscript used to account for any desired H/C ratio (ψ):

CHψ+ (1 + ψ/4)(1 + ε) O2 + (79/21)(1 + ψ/4)(1 + ε) N2→

CO2 + (ψ/2) H2O + (79/21)(1 + ψ/4)(1 + ε) N2 + ε(1 + ψ/4) O2 (2.15)Example 2.3 shows the equation’s utility

Dry Oxygen Calculations

Problem statement: Use Reaction 2.15 to account for the

combus-tion of a refinery gas with ψ = 2.5 and 15% excess air What arethe dry and wet oxygen concentrations that one would measure

in a furnace operating under these conditions? What is the ture fraction in the flue gas?

mois-Solution: With ψ = 2.5 and ε = 0.15, Reaction 2.15 becomes

In addition, the reader may verify the following atomic quantitiesfor both product and reactants: C = 1.0, H = 2.5, O = 3.74, N = 14.06.Atomic species (and therefore mass) are conserved However, molesare not conserved We begin with 9.9 kg/mol and end with 9.52 kg/mol However, mass is conserved at 271.32 kg/kg/mol_CHψ.

If not otherwise qualified, this text presumes excess oxygen refers to oxygen

on a dry volume basis One should not confuse the terms excess air and excess

oxygen; they differ by a factor of five, roughly; that is, 3% excess oxygen and

15% excess air specify approximately the same air/fuel ratio

2.4.1 Wet vs Dry Measurements

Why do the wet and dry readings differ? Since the wet products includewater, the wet volume is larger than the dry volume On the same volumebasis, water vapor reduces the other species’ concentrations Although therewill be no liquid water in the furnace, nonetheless, the oxygen concentration

is still referred to as wet So-called in situ oxygen analyzers measure oxygen

on a wet basis with a probe in contact with the actual flue gas Extractive

analyzers first extract a sample of flue gas through a heated sample line.Usually, the sample splits to several analyzers, e.g., O2, CO, and NOx (NO+ NO2) A heated sample line keeps water from condensing This is importantbecause NO2 is very water soluble; liquid water in the sample line willremove it before it reaches the analyzer, artificially lowering the NOx reading.For boilers and process units, NO2 is less than a few percent of the total NOx.However, for units that operate with high excess air, such as combustionturbines, about half the NOx may be NO2 Just before the analyzers, a sampleconditioning section quickly extracts the water from the heated sample Itdoes so in a way that minimizes both the contact surface and contact timebetween the water and extracted gas Therefore, an extractive oxygen ana-

lyzer will report higher oxygen concentration than an in situ analyzer, as it

is measuring species on a dry volume basis Extractive and in situ analyzer

readings will always differ so long as there is hydrogen in the fuel; thedifference will be greater for larger H/C ratios

2.4.2 Flue Gas Relations for Hydrocarbons

One advantage of having a single entity (ψ) represent the H/C ratio forthe fuel is that it allows the construction of flue gas species charts using ψ

= H/C as a single parameter Reaction 2.15 provides all the information

we need to derive flue gas species as a function of excess air From this, it

is easy to develop the equations for the concentrations of each species forwet and dry conditions The total wet products (TWP) and total dry prod-ucts (TDP) are

(2.16b)

TWP is the volumetric flue-gas-to-fuel ratio The volume or mole fractions

for each species become

Here, y refers to the mole fraction of the subscripted species Usually, the

excess air is unknown and we must calculate it from the oxygen tion and the fuel stoichiometry

concentra-We can also calculate flue gas concentrations on a mass basis by

multiply-ing by the molecular weight for each species We will use w to indicate mass fractions (as opposed to mole or volume fractions) and W to indicate the

molecular weight of various species This leads to the following:

y

TWP

CO wet2

1, =

y TWP

H O2

12

W

W W

N2 CH

W W

O2 CH

where TDP w and TWP w [ ] are the mass ratios of total dry and wet products

to fuel, respectively Then the mass fractions become

We may rearrange Equation 2.17 to solve for excess air Conveniently, forhydrocarbon combustion the excess air is a function of two separable quantities:fuel composition and excess oxygen We may collect the fuel composition

effects into a single constant, K.6 These provide convenient relations foroxygen and excess air

W

W W

W W

N2 CH

O2 CH

w

TDP

W W

w

TWP

W W

1, =

CH ψ

ψ

W g dry, =yCO2,dry WCO2+yN2,dry WN2+yO2,dry WO2

W g wet, =yCO2,wet WCO2+yH2O,wet WH2O+yN2,wet WN2++yO2,wet WO2

1 21 44

2

0 2, ,

=+εε y O wet K wet

2

0 21,

+

εε

=+(ψ ) ( +ε) y CO wet K wet

2

0 844,

=+(ψ ) ( +ε)

W W

W W

= +

+

1 21100