Introduction to Combustion

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101 101

22

Introduction to Combustion

Chapter Overview

This chapter overviews some practical and theoretical aspects of This chapter overviews some practical and theoretical aspects of combustion. We first discuss burners generically, including fuel combustion. We first discuss burners generically, including fuel and air metering, flame stabilization

and air metering, ﬂame stabilization and shaping, and some fun-and shaping, and some fun-damental techniques for control of emissions such as NOx. So damental techniques for control of emissions such as NOx. So prepared, we move on to consider archetypical burners — those prepared, we move on to consider archetypical burners — those repr

representative of the traditional classes of esentative of the traditional classes of burners one might findburners one might find in a refinery or petrochemicals plant, or that provide heat for in a refinery or petrochemicals plant, or that provide heat for steam. We build upon this foundation by next considering steam. We build upon this foundation by next considering arche-typical process units such as boilers, process heaters of various typical process units such as boilers, process heaters of various types, and reactors such as hydrogen reformers and cracking types, and reactors such as hydrogen reformers and cracking units.

units.

In order to lay the groundwork for more detailed combustion In order to lay the groundwork for more detailed combustion modeling, the chapter considers important combustion-related modeling, the chapter considers important combustion-related responses such as NOx emissions, flame length, noise, etc., and responses such as NOx emissions, flame length, noise, etc., and the factors that influence them. Historically, practitioners have the factors that influence them. Historically, practitioners have defined a traditional test protocol for quantifying these effects, defined a traditional test protocol for quantifying these effects, which we present. W

which we present. We also consider some aspece also consider some aspects of thermoacousts of thermoacous--tic instability; this has become a more important topic with the tic instability; this has become a more important topic with the advent of ultralow NOx burners employing very fuel

advent of ultralow NOx burners employing very fuel lean ﬂames.lean ﬂames. In the latter

In the latter third of the chapterthird of the chapter, we develop stoichiometric and, we develop stoichiometric and mass balance relations in considerable mathematical detail. We mass balance relations in considerable mathematical detail. We also consider energy-related quantities such as heat and work, also consider energy-related quantities such as heat and work, adiabatic ﬂame temperature, and heat capacity. As well, we relate adiabatic ﬂame temperature, and heat capacity. As well, we relate the practical consequences of a mechanical energy balance as the practical consequences of a mechanical energy balance as applied to combustion equipment. Such things

applied to combustion equipment. Such things include draft pres-include draft pres-sure, incompressibl

sure, incompressible airflowe airflow, compressible fu, compressible fuel flowel flow, and practi, and practicalcal representations thereof — capacity curves for air and fuel.

representations thereof — capacity curves for air and fuel.

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102

102 Modeling of Combustion Systems: A Modeling of Combustion Systems: A Practical ApprPractical Approachoach

2.1 2.1 General

General Overview

Overview

Combustion

Combustion is the self-sustaining reaction between a fuel and oxidizer charac-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 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 typically is visible. A flame is the reaction zone between fuel and oxidizer; it typically comprises steep thermal and chemical gradients — the flame is often only a comprises steep thermal and chemical gradients — the flame is often only a millimeter or so thick. On one side of the flame, there is fuel and oxidizer at millimeter or so thick. On one side of the flame, there is fuel and oxidizer at low temperature; on the other side are combustion products at high low temperature; on the other side are combustion products at high tempera-ture. Hydrogen and hydrocarbons in some combination are the typical fuels in ture. Hydrogen and hydrocarbons in some combination are the typical fuels in the petrochemical and refining industries. Occasionally, due to some special the petrochemical and refining industries. Occasionally, due to some special refining operations, we find carbon monoxide in the fuel stream. Oxygen (in refining operations, we find carbon monoxide in the fuel stream. Oxygen (in air) is the usual oxidizer. In practice, combustion reactions proceed to air) 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.

tion with the fuel as the limiting reagent — that is, with air in excess. A

A burnerburner is a device for safely controlling the combustion reaction. It isis a device for safely controlling the combustion reaction. It is typically part of a larger enclosure known as a furnace. A

typically part of a larger enclosure known as a furnace. A proces process s heaterheater isis any device that makes use of a ﬂame and hot combustion products to any device that makes use of a ﬂame and hot combustion products to pro-duce some product or prepare a feed stream for later reaction. Examples of duce some product or prepare a feed stream for later reaction. Examples of such processes are the heating of crude oil in a

such processes are the heating of crude oil in a crude unitcrude unit, the production of , the production of hydrogen in a steam–methane catalytic

hydrogen in a steam–methane catalytic reformerreformer, and the production of eth-, and the production of eth-ylene in an

ylene in an ethylene cracking unitethylene cracking unit. . AA boilerboiler is a device that makes use of ais a device that makes use of a ﬂame or hot combustion products to produce steam. The

ﬂame or hot combustion products to produce steam. The furnace furnace is theis the portion of the process unit or boiler encompassing the ﬂame. The

portion of the process unit or boiler encompassing the ﬂame. The radiantradiant section

section comprises the furnace and process tubes with a view of the ﬂame. Incomprises the furnace and process tubes with a view of the ﬂame. In contrast, the

contrast, the convection sectionconvection section is the portion of the furnace that extracts heatis the portion of the furnace that extracts heat to the process without a line of sight to the ﬂame. Every industrial to the process without a line of sight to the ﬂame. Every industrial combus-tion process has some thermal source or sink.

tion process has some thermal source or sink.

2.1.1

2.1.1 The The BurnerBurner

A

A burnerburner is a device is a device for safely controlling the combustion reaction. Afor safely controlling the combustion reaction. A diffusiondiffusion burner

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

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

burner. Premix burners may mix all or some of the combustion air with thePremix burners may mix all or some of the combustion air with the fuel. If one desires to distinguish between them, a

fuel. If one desires to distinguish between them, a partial partial premixpremix burner isburner is one that mixes only part of the combustion air, with the remainder provided one that mixes only part of the combustion air, with the remainder provided later

later. Most pilots are . Most pilots are of the partial of the partial premix type to ensure that they premix type to ensure that they will lightwill light under high excess air conditions typical of furnace start-up.

under high excess air conditions typical of furnace start-up.

Diffusion burners supply most of the heating duty in refinery and boiler Diffusion burners supply most of the heating duty in refinery and boiler applications; therefore, we discuss them first.

applications; therefore, we discuss them ﬁrst. Figure 2.1Figure 2.1 shows the mainshows the main features for accomplishing this.

features for accomplishing this.

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

burner. That is, . That is, a slight vacuum in a slight vacuum in the furnace (termedthe furnace (termed draftdraft — 0.5 in. water— 0.5 in. water column below atmospheric pressur

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

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Introduction to Combustion

Introduction to Combustion 103103

support the full ﬁring capacity. The diffusion burner comprises a

support the full ﬁring capacity. The diffusion burner comprises a fuel mani- fuel manifold

fold,, risersrisers,, tipstips,, oriﬁcesoriﬁces,, tile, plenumtile, plenum,, throat restrictionthroat restriction,, andand damperdamper. Each diffu-. Each diffu-sion burner type may differ in detailed construction, but all will possess sion burner type may differ in detailed construction, but all will possess these main functional parts. We discuss each in turn.

these main functional parts. We discuss each in turn.

2

2..11..11..11 TThhe Fe Fuueel Sl Syysstteemm

A fuel

A fuel manifoldmanifold is a device for distributing fuel. In the ﬁgure, one fuel inletis a device for distributing fuel. In the ﬁgure, one fuel inlet admits fuel to the

admits fuel to the manifold while several risers allow the fuel manifold while several risers allow the fuel to exit. Ato exit. A riserriser is a fuel conduit. (In the boiler industry, risers are sometimes termed

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

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

burners the tip is called is called aa poker shoe poker shoe or just aor just a shoeshoe. A. A tiptip is a device designedis a device designed to direct the fuel in

to direct the fuel in a particular orientation and direction. It has a particular orientation and direction. It has one or moreone or more oriﬁces, holes, or slots drilled at precise angles and size. An

orifices, holes, or slots drilled at precise angles and size. An orificeorifice is a smallis a small hole or slot that meters fuel — for a particular design fuel pressure, hole or slot that meters fuel — for a particular design fuel pressure, compo-sition, and temperature, the orifice restricts the flow to the specified rate. sition, and temperature, the orifice restricts the flow to the specified rate. Together, these parts comprise the

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

FIGURE 2.1 FIGURE 2.1

A typical industrial burner. The typical industrial burner has many features, which can be A typical industrial burner. The typical industrial burner has many features, which can be classiﬁed in the following groups: air metering, fuel

classified in the following groups: air metering, fuel metering, flame stabilization, and emissionsmetering, flame stabilization, and emissions control. Refer to the text for a discussion of each.

control. Refer to the text for a discussion of each.

B

Buurrnneer r TThhrrooaatt PPrriimmaarry y FFuueel l TTiipp _Flame_Flame

Secondary Fuel Tips Secondary Fuel Tips Burner Front Plate Burner Front Plate

Cone (Throat Restriction) Cone (Throat Restriction) Inlet Damper Inlet Damper Noise Muffler Noise Muffler Refractory Tile Refractory Tile Furnace Floor Furnace Floor Fuel Risers Fuel Risers Air Plenum Air Plenum

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104

104 Modeling of Combustion Systems: A Modeling of Combustion Systems: A Practical ApprPractical Approachoach

2

2..11..11..22 AAbboouut t FFuueellss

There are two main gaseous fuels for

There are two main gaseous fuels for combustion procecombustion processes: natural gas andsses: natural gas and reﬁnery gas. Commercially available natural gas has a stable composition reﬁnery gas. Commercially available natural gas has a stable composition the other hand, is capable of

the other hand, is capable of considerably more variation. In a sense, refineryconsiderably more variation. In a sense, refinery gas is the “garbage dump” for the gas products in the refinery. Generally, gas is the “garbage dump” for the gas products in the refinery. Generally, whatever the refinery cannot use for some higher value-added process it whatever the refinery cannot use for some higher value-added process it consumes as fuel. It is quite typical for refineries to specify several different consumes as fuel. It is quite typical for refineries to specify several different refinery fuels for combustion equipment — one representing normal refinery fuels for combustion equipment — one representing normal condi-tions, another represen

tions, another representing a normal ting a normal auxiliary case, and perhaps two or auxiliary case, and perhaps two or threethree upset scenarios. The scenarios will vary in

upset scenarios. The scenarios will vary in hydroghydrogen concentration, typicallyen concentration, typically from 10 to 60%, giving the fuels quite different combustion properties.

from 10 to 60%, giving the fuels quite different combustion properties. It is very important that

It is very important that the reﬁnery weigh the likelihood of scenarios thatthe reﬁnery weigh the likelihood of scenarios that repres

represent widely varying heating values ent widely varying heating values on a volumetric on a volumetric basis. For example,basis. For example, suppose a particular process unit

suppose a particular process unit can receive fuel according to four can receive fuel according to four differendifferentt scenarios:

scenarios: •

• FuFuel Ael A: 620 B: 620 Btu/tu/scfscf, no, normarmal fuel fuel rel reprpreseesentinting 10ng 10% of ru% of run timn timee •

• FuFuel B: 7el B: 760 Btu60 Btu/sc/scf, stf, standand-by f-by fuel ruel reprepreseesentinting 84ng 84% of ru% of run timn timee •

• FuFuel C: 84el C: 840 Btu/0 Btu/scfscf, star, start-ut-up fuel rp fuel reprepreseesentinting 5.ng 5.98% o98% of run tif run timeme •

• Fuel Fuel D: 3D: 309 B09 Btu/sctu/scf, hf, high igh hydrhydrogen ogen upseupset cast case re reprepresenesenting ting 0.02%0.02% of run time

of run time Since fuel C is a

Since fuel C is a start-up case, it does not matter how infrequently it occurs,start-up case, it does not matter how infrequently it occurs, the burners must operate on fuel C. However, the difference in volumetric the burners must operate on fuel C. However, the difference in volumetric ﬂow rate among the fuels A, B, and C is small. On the other hand, fuel D ﬂow rate among the fuels A, B, and C is small. On the other hand, fuel D repres

represents a ents a significant difference in hydrogen concentration. This will mark-significant difference in hydrogen concentration. This will mark-edly affect major fuel properties such as flame speed, specific gravity, and edly affect major fuel properties such as flame speed, specific gravity, and flow rate through an orifice. Later, we shall develop the flow equations that flow rate through an orifice. Later, we shall develop the flow equations that show that fuel D represents the maximum flow (and max

show that fuel D represents the maximum ﬂow (and maximum fuel pressure)imum fuel pressure) condition.

condition.

One can obtain a burner to meet all these fuel conditions. However, for One can obtain a burner to meet all these fuel conditions. However, for fuels A, B, and C, the pressure will necessarily be lower. Some possible fuels A, B, and C, the pressure will necessarily be lower. Some possible consequences are lower fuel momentum and “lazier” longer flames when consequences are lower fuel momentum and “lazier” longer flames when the facility is not running fuel D. In severe cases, the flue gas momentum the facility is not running fuel D. In severe cases, the flue gas momentum will control the flame path. Thus, flames may waft into process tubes and will control the flame path. Thus, flames may waft into process tubes and will be generally poorer in shape — all for the sake of preserving good will be generally poorer in shape — all for the sake of preserving good operation for an operating case representing only 0.02% of the run time. A operation for an operating case representing only 0.02% of the run time. A far better scenario would be to design the burner to handle fuels A, B, and far 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 have C. Then fuel B is the maximum pressure case, but the facility will not have enough pressur

enough pressure to e to make maximum capacity make maximum capacity with fuel D. with fuel D. ThereforTherefore, 99.98%e, 99.98% of the time the flames will be fine and the unit will operate properly; 0.02% 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

of the time the unit will not be able to fire the full firing rate. In the opposingto fire the full firing rate. In the opposing scenario, the operators will struggle with the unit 99.98% of the time, and scenario, the operators will struggle with the unit 99.98% of the time, and the burner will run optimally only 0.02% of the time.

the burner will run optimally only 0.02% of the time.

comprising mostly methane (see Appendix A,

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

In natural draft burners, air is generally the limiting factor, and its con-trolling resistance is the burner throat. Thus, the only way to increase the overall 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 in the heater and if the heater can handle the extra flue gas and heat that result. 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)

H e a t R e l e a s e , M W 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 subsonic flow sonic flow

F u e l A F u e l B F u e l C

transition to sonic flow

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106 Modeling of Combustion Systems: A Practical Approach ratio of the full firing capacity to the actual firing capacity. The maximum turndown ratio is the max/min firing ratio. The turndown ratio is higher if one modulates the air in proportion to the fuel. If the air dampers are manually controlled, then one is interested in the maximum unmodulated turndown ratio, because this is the more conservative case. A typical maxi-mum turndown ratio for premix burners is 3:1. Diffusion burners often have turndown ratios of 5:1 without damper modulation, i.e., leaving the damper fully open despite lower fuel flow. One typically achieves 10:1 turndown ratios with automatic damper and fuel modulation. Multiple burner furnaces usually require a turndown ratio of somewhere between 3:1 and 5:1 with the air dampers fully open. To achieve greater turndown for the unit as a whole, one isolates some of the burners (fuel off, dampers closed). Turndown is easiest for a single fuel composition. Multiple fuel compositions always reduce 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 fuel pressure is available to allow the burner to run at virtually any capacity, but only 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 burner determines the burner’s overall size.

Air may enter from the side (as shown in Figure 2.1) or in line with the burner. 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 necessary because the maximum airside pressure drop is limited and the firing rate modulates. Therefore, one must modulate the air to maintain the air/fuel ratio. 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 does not need to be perfect, and there is a trade-off between uniform flow (larger plenum) 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 infi-nite variety of potential capacities, manufacturers adjust the limiting airflow 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 baffle plates or choke rings are also very common. The purpose of the throat

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Introduction to Combustion 107 restriction is to provide a point of known minimum area and pressure drop characteristic.

Some codes actually specify what portion of the draft (airside pressure drop 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 throat rather 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 several reasons. First, if the burner performs well, the degree of turbulent mixing in the throat is immaterial. Second, if the burner performs well, it does not matter where the pressure drop occurs so long as the total pressure drop is correct. Third, this rule increases the time and cost of burner testing. More importantly, changes to the burner for the sake of meeting the 90/75 rule may actually make the burner perform worse. Therefore, one is consuming resources 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 is usually little danger in shutting off a single burner. In petroleum reﬁneries, the usual practice is to specify a burner having replaceable tips that do not require burner removal from the heater (of course, one must still shut off fuel ﬂow to the individual burner before removing the tips). One must also take care to close the air damper during this procedure; otherwise, the furnace 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 combustion process too late to oxidize the fuel properly. Tramp air may come

not only through unfired burners, but also through leaks in the furnace. One possible sign of tramp air is a high CO reading even with supposedly sufficient excess oxygen. CO is a product of incomplete combustion. Depend-ing on the furnace temperature, 1 to 3% oxygen should represent enough excess air, 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) in the flue gas. The oxygen has entered the furnace somewhere, but it is not participating fully in the combustion reaction. As long as there is tramp air, the furnace will require higher oxygen levels — enough to provide both effectual air through the burner and ineffectual tramp air. In severe cases of tramp air leakage, even a wide-open damper cannot provide enough air to the burner and CO persists, though stack oxygen levels are 5% or more.

The exit of the furnace radiant section is the relevant place to measure emissions for combustion purposes. The exit of the stack is the relevant place to measure emissions for compliance purposes. The stack exit is not generally useful 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.

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108 Modeling of Combustion Systems: A Practical Approach

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-velocity zone at its trailing edge. The flame holder affects only a small portion of the flow, reducing its velocity to well below the flame speed. The velocity upstream of the flame holder is very low. Thus, the hot combustion products recirculate there, continually mixing fresh combustion products with an ignition source — the hot product gases. In this way, the flame holder anchors and stabilizes the flame over a wide turndown ratio. Figure 2.4 shows a 2-MW round-flame burner 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 fuel} mixture 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.)

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flame will be transported to a region of high flue gas concentration that cannot support combustion and the flame will extinguish. Air and fuel velocities 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 flame anchors over wide turndown range and the burner is quite stable throughout its entire operation.

2.1.1.8 Controlling Emissions

In the past, the only emission of concern was CO because it indicated incom-plete combustion, combustion inefﬁciency, or a safety hazard. Nowadays, life is more complex and other emissions such as nitric oxides are important due to their role in the formation of ground-level ozone and photochemical smog. Technically, noise is also a regulated emission (e.g., <85 dBA). Emis-sions control is an active subject of interest. Staging the combustion into distinct zones is one strategy (termed staging) to lower certain emissions such as NOx. If NOx emissions are not a concern, then the secondary fuel tips of Figure 2.1 may be unnecessary and the burner carries out combustion using only primary fuel tips.

2.1.2 Archetypical Burners

When taxonomists classify things, they speak of slots and filler. Slots are the classes or categories, and filler is the stuff that populates the class. With respect to major considerations that affect burner design, we shall list five:

FIGURE 2.4

A gas burner in operation.

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110 Modeling of Combustion Systems: A Practical Approach • Fuel state: – Gas – Liquid – Solid • Flame shape: – Round – Flat

• 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 combus-tion-related emissions.

These five characteristics generally fix the burner design, and they will define 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 slot}

has 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 are many kinds of esoteric designs for special reactions, but as regards traditional how 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 (induced draft), or both inlet and outlet fans (balanced draft)?

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

• Service: Will the heater serve in a boiler to generate steam, or will it serve in a process heater to reﬁne petroleum or make petrochemicals? However, for the most part, design variants of the enumerated burner types will accommodate all of the above categories. So, we will describe the

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

2.1.2.1 Round-Flame Gas Diffusion Burners

Burners in these categories are gas fired and designed to produce round flames. These comprise the lion’s share of fired duty. These also comprise the 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 has been to use more but smaller burners rather than a few large burners. In the

power 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 and cement 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 requiring round flames. However, this has fallen into disfavor because the burners are usually loud (due to fuel jet noise) and sensitive to hydrogen concentration variations.

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

TABLE 2.1

Burner Sampling for One Manufacturera Mounting Orientation Fuel Type Flame Shape Diffusion Premix

Conventional Low NOx Conventional Low NOx

Up Gas Round SFG SMR HEVD LMX

Flat SFFG XMR, PSFFR Oil Round LNC + EA DeepStar

+ HERO Flat

Down Gas Round MDBP/PFLD DSMR HEVD DSMR MKIII Flat

Oil Round Flat

Horizontal Gas Round SFG SMR HEVD LMX Flat PSFFG HMVF FPMR PMS LPMW Oil Round LNC + EA DeepStar

+ HERO Flat

a _{All referenced burner models are trademarks of John Zink LLC, Tulsa, OK.}

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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 of the gas and the fuel pressure. As the H/C ratio of the fuel changes, so does the molecular weight, the fuel pressure for a given heat release, and the amount 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 to propagate back into the fuel riser because there is no oxygen there to support combustion. However, in a premix burner, there is a combustible mixture inside the burner tip. Premix burners also have a much larger tip and orifices because these have to accommodate a high-volume, low-pressure fuel–air mixture. 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

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Introduction to Combustion 113 Burner internals cannot withstand the high temperatures of combustion for long. Flashback is very sensitive to hydrogen concentration because the laminar 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 will abstract sufficient heat from a propagating flame to extinguish it. The quench distance varies with orifice diameter — smaller orifices are more effective mum slot widths for various fuels. Generally, for a given area, a circular orifice 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 the furnace, fuel, and combustion air, the tip’s ability to conduct heat away from a propagating flame also changes with temperature. Moreover, at higher temperatures, the orifices in the tip expand and one must take account of this 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 to the process. For example, high-temperature processes — such as production of hydrogen or ethylene — use the hot wall to radiate to process tubes containing 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 furnace vendor specifications by changing the angle, size, and distribution of the fuel jets. NOx reduction with these burners is a challenge because the process operates 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 burner meters the air and fuel separately. Figure 2.9 gives one example, and Figure 2.10 shows some in operation.

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

CH

ψ

+ 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

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mini-114 Modeling of Combustion Systems: A Practical Approach

Hydrogen plants often use pressure-swing adsorption (PSA) to purify hydrogen and separate it from the CO₂ by-product. The off-gas from this stream is mostly CO2, with some hydrogen, and to a lesser extent

hydrocarbons. The high concentration of CO₂ dramatically lowers the flame temper-ature. Flat-flame side-fired diffusion burners have excellent stability, cannot flash back, and NOx emissions below 20 ppm are possible even with 400°C air preheat and 1200°C furnace temperatures. As Figure 2.9 and Figure 2.10 show, 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 large annular gap and enters the furnace in the same orientation. At the high furnace temperatures, the separate fuel and air streams react to generate a flame 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–methane reforming service (hydrogen production) almost exclusively. Figure 2.11 shows a common design.

The burner tip is roughly 4 in. in diameter and 8 in. long. Slots or holes cover the end and admit premixed fuel and air to the furnace. Premix burners are 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.

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2.1.2.5 Flashback

Flashback can only occur in premixed burners because they are the only type that has a combustible mixture inside the tip. Once the flame flashes back into the burner tip it can destroy it in minutes. Combustion inside the tip increases 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. There-fore, once flashback occurs, there is no mechanism for moving the flame back to the furnace side. The burner immediately experiences lower mass

ﬂow due to the reduced density of the gas during ﬂashback; one may hear 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’s forward 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 premix burner 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 combustion 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).

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Secondary fuel injection is one technique for reducing NOx. Side-fired pre-mixed burners are typically much smaller than floor-fired burners and weigh a mere 50 kg or so, including the tile. Firing rates for these burners are 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 refinery will want to burn as heavy a fuel as possible because other liquid fuels have greater value (e.g., transportation fuels for automobiles, trucks, and aviation). When sold commercially, fuel oils are widely available and graded as either number 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 process heaters burn this fuel. Sometimes, the liquid fuel comprises rejected oil from other processes (waste oil) in whole or part. One can also burn pitch — a nondescript 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 burn efficiently. 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 p

r o c e s s t u b e s < floor burners > 50 40 60 30 20 10 70 80 90

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(so-called mechanical atomization), so fuel guns make use of pressurized steam to produce the requisite atomization. Mechanical atomization is sufﬁcient for light oils such as fuel oil 2. Sometimes, light liquid fuels use compressed air 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 combustion 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.

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.)

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example, light naphtha fractions can prevaporize in the fuel oil gun. Pre-vaporization is unwanted because it leads to slug ﬂow in the fuel gun, that is, alternate slugs of liquid and gaseous fuel going to the burner. This causes erratic 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

VENTURI

FUEL/AIR MIXTURE TO TIP FUEL

NOZZLE

EDUCTED AIR

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Introduction to Combustion 119 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 phase that actually burns. The burning vapor provides heat, vaporizing even more droplets 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 typical practice is to use the less expensive heavy oil with the gas fuel serving to make up the required process heat. Thus, both fuels fire simultaneously. A combination 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 of the 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 to the peripheral combusting gas reducing the available oxygen for the fuel oil stream. To minimize (but not eliminate) this effect, separate air registers provide individualized airflow to each zone.

2.1.2.8 Burner Orientations

One may fire burners in four basic orientations: up, down, sideways, and balcony fired (horizontally mounted to the wall but having a vertical flame

at right angles to the burner).

2.1.2.9 Upﬁred

This is the most typical firing arrangement (see Figures 2.1, 2.4, and 2.7). The burner mounts to the floor. Pillars support the heater and allow sufficient clearance for personnel to walk under the burners and inspect and maintain them. A 1.8-m gap from the lowest part of the burner to the ground is adequate for personnel to walk under the heater without stooping. However, some units do not have this kind of clearance. Clearance is important for initial installation and because one requires sufficient room to extract, clean, and reinsert fuel oil guns, risers, and tips.

FIGURE 2.13

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

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2.1.2.10 Downﬁred

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

Hydrogen and ammonia reformers of this type are large furnaces often with 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 for these burners. Forced draft operation helps to minimize the flame bending toward the tubes. The greater momentum of the forced air helps to overcome the buoyant effects and furnace currents. Furnace currents can be quite complicated. Downfiring increases NOx emissions by 15% or so as the burner receives hotter convective air at the roof than at the floor. Space is limited and roof burners are more difficult to access than floor burners. There are also limitations on the total burner weight because the furnace roof can support only so much. Therefore, a simple, reliable design is the order of the 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

COMBINATION FIRING)

SECONDARY AIR CONTROL TERTIARY AIR CONTROL

GAS RISER MANIFOLD (FOR COMBINATION FIRING)

AIR INLET PLENUM PRIMARY TILE

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2.1.2.11 Side-Fired

Some steam–methane reformers and ethylene cracking units (ECUs) use side-ﬁring. The general idea is to present a uniform heat ﬂux to the reactor tubes by using many small burners (<1 MMBtuh) in the furnace wall. A large ECU may have hundreds of sidewall burners. Figure 2.16 shows some in operation.

The ﬂame travels parallel to the plane of the wall. This path is necessary because the reactor tubes are less than 2 m away; ﬂame impingement would overheat them. Reactor temperatures for these kinds of units are some of the highest 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 wall firing. 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 improve unit efficiency. However, a perfectly even heat flux distribution maximizes the tube 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.

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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 by row, making process changes and start-up more labor-intensive.

One hundred percent floor firing does not have these drawbacks. The burner 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.

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Introduction to Combustion 123 the furnace by changing tip drillings, in essence, changing the heat release distribution. However, heat flux in the top third of the furnace is difficult to influence with tip geometry at the floor. A heater comprising both floor and wall firing is one compromise to reduce first cost and operating labor while retaining good control of the reactor heat distribution. In practice, one sees all three firing scenarios.

2.2 Archetypical Process Units

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

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 usugener-ally fully automatic and 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 for refineries, pulp mills, electrical generation, etc. As the name implies, watertube boilers generate steam inside the tube. In this respect, they are similar to process heaters but with water in the tube rather than process fluid. Large-capacity, high-pressure, and superheated steam units are invariably of the watertube design because small tubes can withstand higher internal pressure than large shells for a given thickness. Watertube boilers can grow to be quite 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

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124

124 Modeling of Combustion Systems: A Practical ApproachModeling of Combustion Systems: A Practical Approach designed to effect some thermochemical transformation. One major designed to effect some thermochemical transformation. One major distinc-of the many process heater and ﬁred reactor conﬁgurations.

of the many process heater and ﬁred reactor conﬁgurations. We further discuss some of them below.

We further discuss some of them below.

2.2.1.4

2.2.1.4 VVerticaertical l Cylindrical Cylindrical

Vertical-cylindrical (VC) units

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

and b).. The helical coil is the rarer of the two. The heater designer mayThe helical coil is the rarer of the two. The heater designer may arrange the tubes

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

arate passes through the heater, depending on the process needs.

2.2.1.5

2.2.1.5 Cabin Cabin StyleStyle

A

A cabincabin (or(or boboxx)) unitunit has a rectangular profile and is usually shorter than a VC,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 heater though it can be many times wider (Figure 2.18c to i). Tubes in a cabin heater typically run horizontally at the walls (e.g., Figure 2.18d and e); they usually typically run horizontally at the walls (e.g., Figure 2.18d and e); they usually fire with one row of burners down the center of the heater floor (not shown). fire 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 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 2.18d). Some process heaters are also end ﬁred (Figure 2.18e).

FIGURE 2.18 FIGURE 2.18

Some process heater types. Process and convection tubes are shaded. Heaters may or may not Some process heater types. Process and convection tubes are shaded. Heaters may or may not have convection sections. Round heaters

have convection sections. Round heaters are known as vertical cylindrical (VC), and are known as vertical cylindrical (VC), and rectangularrectangular heaters are known as cabin heaters. (a) VC,

heaters are known as cabin heaters. (a) VC, (b) VC with helical coil. Cabin type: (c) wicket tube(b) VC with helical coil. Cabin type: (c) wicket tube or arbor coil, (d) floor fired, (e) end wall fired.

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

Convection Convection Section Section Side Side View View Plan Plan View View Radiant Radiant Section Section Radiant Radiant Section Section Radiant Radiant Section Section Radiant Radiant Section Section B

Buurrnneerrss BBuurrnneerrss BBuurrnneerrss BBuurrnneerrss

Burners Burners Process Process Tubes Tubes Process Process Tubes Tubes

((aa)) ((bb)) ((cc)) ((dd)) ((ee))

tion among ﬁred units is their shape. Figure 2.18a to

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FIGURE 2.18 (continued) FIGURE 2.18 (continued)

(f) Vertical tube, sidewall fired; (g) vertical tube, floor fired; (h) floor + wall fired — usually two (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 or three rows of sidewall burners + floor burners; (i) downfired (one of many cells), (j) terrace wall (horizontally mounted, vertically fired).

wall (horizontally mounted, vertically ﬁred). Side

Side View

View RadiantRadiant

Section Section Radiant Radiant Section Section Radiant Radiant Section Section Radiant Radiant Section Section Plan Plan View View Burners Burners B

Buurrnneerrss BBuurrnneerrss

((ff)) ((gg)) Side Side View View Plan Plan View View (h) (h) (i)(i) (j)(j) Radiant Radiant Section Section Radiant Radiant Section Section Radiant Radiant Section Section Radiant Radiant Section Section Burners Burners B

Buurrnneerrss BBuurrnneerrss

Burners Burners Burners Burners Convection Convection Section Section

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126

126 Modeling of Combustion Systems: A Practical ApproachModeling of Combustion Systems: A Practical Approach

2.2.1.6

2.2.1.6 Fired Fired ReactorsReactors

In the petroleum and petrochemicals industry, most chemical reactors use In the petroleum and petrochemicals industry, most chemical reactors use ﬁred duty to effect their transformations. We shall discuss ethylene cracking ﬁred duty to effect their transformations. We shall discuss ethylene cracking units and hydrogen and ammonia reformers as illustrative.

units and hydrogen and ammonia reformers as illustrative.

2.2.1.7

2.2.1.7 Hydrogen Hydrogen ReformersReformers

Hydrogen reformers exist in side-ﬁred

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

important commodity in fuels upgrading. The general chemistry is CH

CH44 + + HH22OO

→

3H3H22 ++ CCOO ((22..22))

That is the reforming reaction. The reaction temperature inside the process That is the reforming reaction. The reaction temperature inside the process tubes is ~815°C for this step.

tubes is ~815°C for this step.11 _{Therefore, the bridgewall temperature of the}_{Therefore, the bridgewall temperature of the}

reactor must be higher (~1050°C). The

reactor must be higher (~1050°C). The following water–gas shift reaction (orfollowing water–gas shift reaction (or simply,

simply, shiftshift reaction) increases the Hreaction) increases the H22 yield:yield:

CO + H

CO + H22OO

→

COCO22 + + HH22

The shift reaction can occur at low temperatures (200 to 230°C) or high The shift reaction can occur at low temperatures (200 to 230°C) or high temperatures (300 to 450°C), depending on the catalyst in the tube and the temperatures (300 to 450°C), depending on the catalyst in the tube and the desired conversion efﬁciency.

desired conversion efﬁciency.

Thermodynamically, the low-temperature shift reaction is more efﬁcient. Thermodynamically, the low-temperature shift reaction is more efﬁcient. However, nowadays, pressure-swing adsorption (PSA) is the separation However, nowadays, pressure-swing adsorption (PSA) is the separation pro-cess of choice, and its natural companion is the high-temperature shift 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, 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 pressure PSA is a pressure cycle. Solid adsorbents adsorb impurities at high pressure and release them at low pressure. Using two adsorbent vessels, the process and release them at low pressure. Using two adsorbent vessels, the process may produce product on a continuous basis. The depressurization cycle may produce product on a continuous basis. The depressurization cycle gives CO, CO

gives CO, CO22, , NN22, and some H, and some H22 as a fuel stream at low pressure. For thisas a fuel stream at low pressure. For this

purpose, hydrogen reformer burners are equipped with a PSA tip. For purpose, hydrogen reformer burners are equipped with a PSA tip. For exam-ple,

ple, Figure 2.15Figure 2.15 shows a burner equipped with a large center tip to use theshows a burner equipped with a large center tip to use the PSA tail gas for process heat. PSA tail gas tends to form very little NOx PSA tail gas for process heat. PSA tail gas tends to form very little NOx (owing to the inert content of

(owing to the inert content of the fuel) and good flames (owing to the fuel) and good flames (owing to fast flamefast flame speeds of H

speeds of H22 and CO).and CO).

2.2.1.8

2.2.1.8 Ammonia Ammonia ReformersReformers

Most ammonia reformers resemble downﬁred hydrogen reformers (Figure Most ammonia reformers resemble downﬁred hydrogen reformers (Figure 2.18i) in shape and size. The chemical reaction is reversible:

2.18i) in shape and size. The chemical reaction is reversible: N

N22 + + 3H3H22

↔

2NH2NH33 (2.3)(2.3)

Since the reactants comprise four moles and the products two, pressure Since the reactants comprise four moles and the products two, pressure (which always favors the denser phase) increases the yield of NH

(which always favors the denser phase) increases the yield of NH33. Fertilizer. Fertilizer

comprises the largest market for ammonia. comprises the largest market for ammonia.22

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2.2.1.9

2.2.1.9 Ethylene Ethylene Cracking Cracking Units Units (ECUs)(ECUs)

Figure 2.18g

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

with the wall burners comprising the balance. Figure 2.18f gives an exampleexample of

of aa wall-fired steam–methane reformerwall-fired steam–methane reformer, and Figure 2.18i shows one cell of a, and Figure 2.18i shows one cell of a downfired hydrogen reformer. Such reformers comprise several cells and downfired hydrogen reformer. Such reformers comprise several cells and hundreds of burners.

hundreds of burners.

Modern ECUs contain tubes 10 to 12 m in length. Each tube hangs from Modern ECUs contain tubes 10 to 12 m in length. Each tube hangs from the roof with a single

the roof with a single UU-bend at the bottom. Burners on both sides of the-bend at the bottom. Burners on both sides of the process tube heat it. The heat converts the feed hydrocarbon into ethylene, process tube heat it. The heat converts the feed hydrocarbon into ethylene, C

C22HH44. Ethylene is the largest volume organic chemical, and almost all ethyl-. Ethylene is the largest volume organic chemical, and almost all

ethyl-ene production is by the thermal process.

ene production is by the thermal process.22 _{High heat causes abstraction of}_{High heat causes abstraction of}

hydrogen and formation of a double bond. For example, hydrogen and formation of a double bond. For example,

CH

CH33—CH—CH33

→

CHCH22=CH=CH22 + + HH22 (2.4)(2.4)

Ethane, propane, and naphtha are the most common hydrocarbon Ethane, propane, and naphtha are the most common hydrocarbon feed-stocks. These reactors have the highest bridgewall

stocks. These reactors have the highest bridgewall temperatures of any com-temperatures of any com-mon petrochemical process, 1200 to 1250°C.

mon petrochemical process, 1200 to 1250°C.

2.

2.33 Im

Impo

port

rtan

ant

t Fa

Fact

ctor

ors

s an

and

d Re

Resp

spon

onse

sess

The combustion-related respo

The combustion-related responses that we nses that we wish to model wish to model are things like NOxare things like NOx and CO emissions, CO breakthrough point, flame length, heat flux profile, and CO emissions, CO breakthrough point, flame length, heat flux profile, and the like. These will be a

and the like. These will be a function of one or more of the following factors:function of one or more of the following factors: fuel composition, oxygen concentration, burner type, degree of

fuel composition, oxygen concentration, burner type, degree of staging, fur-staging, fur-nace temperature at some convenient point such as the bridgewall or ﬂoor, nace temperature at some convenient point such as the bridgewall or ﬂoor, etc. Tests for these kinds of things are now fairly standard.

etc. Tests for these kinds of things are now fairly standard.

2.3

2.3.1.1 TThe he TTrradiadititionaonal l TTest Pest Prorotoctocolol

The American Petroleum Institute (API) formally deﬁnes various burner tests. The American Petroleum Institute (API) formally deﬁnes various burner tests.33

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

0.

0. Cold light-off Cold light-off . This refers to ignition of the burner in a cold furnace.. This refers to ignition of the burner in a cold furnace. One should use the same method for ignition that is

One should use the same method for ignition that is available in theavailable in the ﬁeld. This may comprise manual ignition by a torch, ignition by a ﬁeld. This may comprise manual ignition by a torch, ignition by a bur

burner ner pilopilot, t, or or manmanual ual spaspark rk ignignitioition. n. The The idea idea is is to to simsimulatulate te the he fielfieldd start-up condition, where the burners start up for the first time. One start-up condition, where the burners start up for the first time. One looks for attached and stable flames, and a smooth transition as the looks for attached and stable flames, and a smooth transition as the bur

burner rner rampamps ups up. The . The damdamper pper posiositiotion at thn at this cois condinditiotion is ten is termermed thed the light-off position.

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