q&a on applied combustion-final

7/29/2019 Q&A on Applied Combustion-FINAL

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CAIRO UNIVERSITY ENGINEERING FACULTY MECHANICAL POWER ENGINEERING dpt.

Questions & Answers on

Material: Applied Combustion

Lecturer: prof. Mohamed Rashad

Written by Eng. Abdelhamid Ibrahim Abdelhamid

2nd Year of Internal Combustion Engines Diploma – 2008/2009

of 18 APPLIED COMBUSTION Abdelhamid Ibrahim A.



CHAPTER 4: COMBUSTION CHAMBERS

Q1. WHAT ARE THE MAIN REQUIREMENTS FOR COMBUSTION CHAMBER DESIGN?

A1. Minimum loss in pressure with the maximum heat release for the limited space available. The combustion chamber must also be capable of maintaining stable and efficient combustionover a wide range of engine operating conditions.

Q2. HOW CAN WE DESIGN THE COMBUSTION CHAMBER & HOW CAN THE AIRFLOW BE APPORTIONED?

A2. Air from the engine compressor enters the combustion chamber at a velocity up to 500 feet per second, but because at this velocity the air speed is far too high for combustion, the first thing that the chamber must do is to diffuse it, i.e. decelerate it and raise its static pressure.Since the speed of burning kerosene at normal mixture ratios is only a few feet per second, any fuel lit even in the diffused air stream, which now has a velocity of about 80 feet per second,would be blown away. A region of low axial velocity has therefore to be created in the chamber,

so that the flame will remain alight throughout the range of engine operating conditions.

It is arranged that the conical fuel spray from the nozzle intersects the recirculation vortex at itscentre. This action, together with the general turbulence in the primary zone, greatly assists inbreaking up the fuel and mixing it with the incoming air.

An electric spark from an igniter plug initiates combustion and the flame is then self sustained.

Q3. DESCRIBE THE METHODS OF INJECTING FUEL TO THE COMBUSTION CHAMBER?

A3. Fuel is supplied to the air stream by one of two distinct methods. The most common is theinjection of a fine atomized spray into the re-circulating air stream through spray nozzles. Thesecond method is based on the pre-vaporization of the fuel before it enters the combustionzone.

Q5. WHAT IS THE COMBUSTION CHAMBER PERFORMANCE?

A5. A combustion chamber must be capable of allowing fuel to burn efficiently over a widerange of operating conditions without incurring a large pressure loss. In addition, if flameextinction occurs, then it must be possible to relight. In performing these functions, the flametube and spray nozzle atomizer components must be mechanically reliable.




Q6. WHAT IS THE COMBUSTION INTENSITY?

A6. The heat released by a combustion chamber or any other heat generating unit isdependent on the volume of the combustion area. A combustion intensity is a goal in gasturbine combustion chamber.

Q7. DESCRIBE THE FLAME TUBE COOLING METHODS?

A7. A film of cooling air flows along the inside surface of the flame tube wall, insulating it fromthe hot combustion gases. A recent development allows cooling air to enter a network of passages within the flame tube wall before exiting to form an insulating film of air, this canreduce the required wall cooling airflow by up to 50 per cent. Combustion should be completed before the dilution air enters the flame tube, otherwise the incoming air will cool the flame and incomplete combustion will result.

FLAME TUBE COOLING METHODS

Q8. DEFINE COMBUSTION CHAMBER STABILITY?

A8. Combustion stability means smooth burning and the ability of the flame to remain alight over a wide operating range.




COMBUSTION STABILITY LIMITS

CHAPTER 10: FUEL SYSTEM




Q1. WHAT ARE THE MAIN REQUIREMENTS OF THE FUEL SYSTEM?

A1. The functions of the fuel system are to provide the engine with fuel in a form suitable for combustion and to control the flow to the required quantity necessary for easy starting,acceleration and stable running, at all engine operating conditions.

Q2. WHAT IS THE EFFECT OF POWER & ALTITUDE ON AIR & FUEL CONSUMPTION?

A2. A typical change of airflow with altitude is shown in the following figure

AIRFLOW CHANGING WITH ALTITUDE

To meet this change in airflow a similar change in fuel flow must occur, otherwise the ratio of airflow to fuel flow will change and will increase or decrease the engine speed from that originally selected by the throttle lever position.




FUEL FLOW CHANGING WITH ALTITUDE

Q3. DESCRIBE THE FIVE REPRESENTATIVE SYSTEMS OF AUTOMATIC FUELCONTROL?

A3. The five representative systems of automatic fuel control are the pressure control and flow control systems, which are hydro-mechanical, and the acceleration and speed control and

pressure ratio control systems, which are mechanical. With the exception of the pressure ratiocontrol system, which uses a gear-type pump, all the systems use a variable-stroke, multi- plunger type fuel pump to supply the fuel to the spray nozzles.

Q4. WHAT IS THE GENERAL AIM OF THE FUEL CONTROL SYSTEM?

A4. The usual method of varying the fuel flow to the spray nozzles is by adjusting the output of the H.P. fuel pump. This is effected through a servo system in response to some or all of thefollowing:

(1) Throttle movement.(2) Air temperature and pressure.(3) Rapid acceleration and deceleration.(4) Signals of engine speed, engine gas temperature and compressor delivery pressure.

Q5. DESCRIBE THE FLOW CONTROL UNIT (FCU) IN THE 4 CASES OF A TURBO-PROPELLER ENGINE?

A5. CASE 1




At steady running conditions, at a given air intake pressure and below governed speed, thespill valve in the F.C.U. is in a sensitive position, creating a balance of forces across the fuel pump servo piston and ensuring a steady pressure to the throttle valve.

CASE 2

When the throttle is slowly opened, the pressure to the throttle valve falls and allows theF.C.U. spill valve to close, so increasing the servo pressure and pump delivery. As the

pressure to the throttle is restored, the spill valve returns to its sensitive or controlling position,and the fuel pump stabilizes its output to give the engine speed for theselected throttle position. The reverse sequence occurs as the throttle is closed.

CASE 3

A reduction of air intake pressure, due to a reduction of aircraft forward speed or increase inaltitude, causes the F.C.U. capsule to expand, thus increasing the bleed from the F.C.U. spill valve. This reduces fuel pump delivery until the fuel flow matches the airflow and the reduced H.P. pump delivery (throttle inlet pressure), allows the spill valve to return to its sensitive position. Conversely, an increase in air intake pressure reduces the bleed from the spill valve

and increases the fuel flow. The compensation for changes in air intake pressure is such that fuel flow cannot be increased beyond the pre-determined maximum permissible for static International Standard Atmosphere (I.S.A.) sea-level conditions.

CASE 4

The engine speed governor prevents the engine from exceeding its maximum speed limitation.With increasing engine speed, the centrifugal pressure from the fuel pump rotor radial drillingsincreases and this is sensed by the engine speed governor diaphragm. When the enginereaches its speed limitation, the diaphragm is deflected to open the governor spill valve, thusoverriding the F.C.U. and preventing any further increase in fuel flow. Some pressure control

systems employ a hydromechanical governor.

Q7. DESCRIBE THE SERVO PRESSURE CONTROL BY KINETIC VALVE?

A7. The proportioning valve diaphragm is held open in a balanced condition allowing fuel to pass to the A.S.U (altitude sensing unit). This means that the restrictor outlet pressure is equal to the throttle outlet pressure and, as their inlet pressures are equal, it follows that the pressuredifference across the restrictors and the throttle are equal; therefore, a constant fuel flow isobtained.

When the throttle is slowly opened, the pressure difference across the throttle valve and the proportioning flow restrictors decreases and the proportioning valve diaphragm adjusts its position. This reduces the proportional flow, which closes the A.S.U. spill valve and increasesthe servo pressure. The fuel pump increases its delivery and this restores the pressuredifference across the throttle valve and equalizes the pressure difference across the restrictors.The proportional flow is restored to its original value and the balance of forces in the A.S.U.returns the spill valve to the controlling position.




Q8. DESCRIBE THE PLUNGER-TYPE & GEAR-TYPE OF FUEL PUMPS?

A8. Plunger-type fuel pump

This pump is of the single unit, variable-stroke, plunger-type; similar pumps may be used asdouble units depending upon the engine fuel flow requirements. The fuel pump is driven by theengine gear train and its output depends upon its rotational speed and the stroke of the

plungers. A single-unit fuel pump can deliver fuel at the rate of 100 to 2,000 gallons per hour at a maximum pressure of about 2,000 lb. per square inch. To drive this pump, as much as 60 horsepower may be required. The fuel pump consists of a rotor assembly fitted with several




plungers, the ends of which project from their bores and bear on to a non-rotating camplate.Due to the inclination of the camplate, movement of the rotor imparts a reciprocating motion tothe plungers, thus producing a pumping action. The stroke of the plungers is determined by theangle of inclination of the camplate. The degree of inclination is varied by the movement of aservo piston that is mechanically linked to the camplate and is biased by springs to give the full stroke position of the plungers. The piston is subjected to servo pressure on the spring sideand on the other side to pump delivery pressure; thus variations in the pressure differenceacross the servo piston cause it to move with corresponding variations of the camplate angle

and, therefore, pump stroke.

Gear-type fuel pump

The gear-type fuel pump is driven from the engine and its output is directly proportional to itsspeed. The fuel flow to the spray nozzles is controlled by recirculating excess fuel delivery back to inlet. A spill valve, sensitive to the pressure drop across the controlling units in the system,opens and closes as necessary to increase or decrease the spill.




CHAPTER 4: FUNDAMENTALS OF THE COMBUSTION PROCESS IN DIESEL

ENGINES

Q1. STATE THE PHASES OR PERIODS OF COMBUSTION PROCESS?

A1. The combustion process may be considered to take place in four phases or periods:

1. The delay period,2. The rapid pressure rise period,3. The mechanically controlled period,4. The after-burning period

Q2. WRITE SHORT ESSAY ABOUT DELAY PERIOD & FACTORS AFFECTING IT?

A2. The first combustion phase occupies the time between the initial discharge of fuel into thecylinder to the time where the spray droplets partially vaporize and then ignite i.e. physical and chemical delay.

The ignition is represented by the oxidation process initially establishing a nucleus of flame, theheat liberated then raises the temperature and pressure of the air mass until the burning air charge pressure exceeds the normal compressing pressure. This point defines the end of thefirst phase, known as the delay period and the start of the second rapid pressure rise period.

The time delay between the initial injection and ignition is influenced by the following:

1. The amount of penetration and atomization of the fuel spray,2. The temperature and pressure of the cylinder air mass at the moment of injection,3. The amount of pre-combustion turbulence,4. The ability of the fuel to ignite readily, this being related to the fuel's cetane number .

Q3. WHAT ARE THE FACTORS AFFECTING RAPID COMBUSTION?

A3. Since the rate of combustion increases with the speed of cylinder air swirl and this in turnincreases with engine speed, the second rapid pressure rise phase remains approximately constant over the entire engine speed range.

The second period's rapid pressure rise and peak pressure is greatly influenced by the durationof the delay period. Generally, a large delay between the beginning of fuel injection and when

actual ignition occurs a very high rate of pressure rise, whereas a small delay period results ina more gradual rate of pressure increase.

Q4. WRITE SHORT ESSAY ON THE MECHANICALLY CONTROLLED BURNING PERIOD &FACTORS AFFECTING IT?

A4. This period begins at the crank-angle movement point when the very steep pressureincrease has changed to a much slower pressure rise rate.

This period begins when the fuel injected in the rapid-pressure-rise second phase finds it more

difficult to seek out and react with fresh oxygen in the remaining and much reduced unburnt air mass.




The third phase is therefore mechanically controlled by the injection pump output characteristics which can shorten or extend the duration of fuel delivery. Thus, the prolongationof this variable period is determined directly by the amount of additional fuel supplied abovethat needed for no-load running.

Q5. WHAT IS THE EFFECT OF INJECTION ON COMPRESSION RATIO, HEATCOMBUSTION, INTENSITY OF TURBULENCE, INJECTION PRESSURE & DELAY PERIOD?

A5. To understand the relationship between the time occupied by the delay period and thecrank-angle movement over this same interval with increasing engine speed, the following factors should be appreciated:

1. The shorter time available for compression reduces the amount of gas escaping past the piston rings, enabling higher compression pressures to be attained,

2. The shorter time available for the heat of combustion to be transferred to the cooling system raises the temperature of the compressed charge,

3. Higher engine speed increases the intensity of the pulsating vortices which form theturbulence and in turn raise the rate of flame propagation throughout the combustion

chamber,4. Higher engine speed increases the injection pressure and the intensity and thinness of

atomization.

The conclusion drawn from these factors is that the delay period time interval will decrease withrising engine speed whereas the corresponding crank-angle movement for the same timeinterval increases.

Q6. DEFINE DIESEL KNOCK?

A6. Diesel knock is the sound produced by the very rapid rate of pressure rise during the early part of the uncontrolled second phase of combustion. The primary cause of an excessively high pressure rise rate is due to a prolonged delay period.

Q7. WHAT ARE THE FACTORS AFFECTING DIESEL KNOCK?

A7. An extensive delay period can be due to the following factors:

1. a low design compression ratio permitting only a marginal self-ignition temperature to bereached,

2. a low combustion pressure due to worn piston rings or badly seating valves,3. poor fuel ignition quality, that is a low cetane number fuel,4. a poorly atomized fuel spray preventing early ignition to be established, that is a worn

and badly seating injector needle or blocked nozzle hole or holes,5. an inadequate injector needle spring load producing coarse droplet formation,6. over-advanced injection causing the fuel spray to enter the cylinder and to accumulate

very early on in the compression stroke before adequate pressure and temperature hasbeen reached,

7. a very low air intake temperature in cold wintry weather and during cold starting.

Q8. WHAT DOES IGNITION QUALITY MEAN?




A8. Different kinds of fuel, even though similar in appearance may show difference in the timelag between the start of fuel injection and the commencement of ignition. The ease with whichdiesel fuels ignite in terms of the time interval between injection and ignition is called ignitionquality.

Q10. WRITE SHORT ESSAY ABOUT BOWL IN THE PISTON SHAPE?

A10. The bowl in the piston semi-open combustion chamber should ideally be positioned centrally but because the injector has to be located to one side of a two-valve cylinder-head,the chamber bowl is normally also offset so that it remains approximately concentric to theinjector nozzle. In general, an injector and chamber offset from the cylinder axis of up to 10%can be tolerated without it greatly hindering the organized air swirl or there being any loss of engine performance.

Q11. WHAT IS THE EFFECT OF ASPECT RATIO?

A11. A basic comparison between chambers which are recessed in the piston head is the

diameter (D) to depth (d) ratio, commonly known as the aspect ratio, that is AR=D/d of thechamber.

The extremes of chamber aspect ratios range from 5:1 for open shallow chambers to 2:1 for semi-open deep chambers.

Experience has shown that the best overall performance over a wide speed range occurs whenthe chamber aspect ratio lies between 2.5:1 to 3:1

Deep, low, aspect-ratio chambers have shorter spray paths and therefore require less spray penetration than shallow, high aspect-ratio chambers.

Fuel injection spray penetration pressure is critical and must match the chamber aspect ratio toobtain the optimum amount of chamber wall wetting. Thus, a deep, low, aspect-ratio chamber requires less spray penetration than would a shallow, high, aspect-ratio chamber.

Q12. WRITE SHORT ESSAY ON THE ADVANTAGE OF LIB ON ISUZU?

A12. A further refinement to the square-bowl chamber has been to add a lip to the vertical rimof the chamber which slopes at 30 o to the horizontal. This lip prevents the toroidal upward roll




movement of the air ejecting fuel particles over the edge of the chamber rim into the squishzone so that the majority of the mixing is completed and burnt inside of the piston bowl. At thesame time, the lip tends to create and impose micro-turbulence within the chamber bowl.

Q13. WRITE SHORT ESSAY ON THE ADVANTAGE OF OVERSWIRL & UNDERSWIRL?

A13. Overswirl results in less fuel reaching the walls where the mixing is more effective, it

tends to increase heat losses and can under certain conditions, spoil the mixing through theinterference of combustion products from the upstream spray.

In contrast, underswirl will restrict the rate at which the fuel spray mixes with the air stream.

However, overswirl reduces the mean effective pressure and increases the specific fuel consumption where the swirl levels are considerably above the optimum compared with thosein the underswirl conditions.




CHAPTER 11: DIESEL IN-LINE FUEL INJECTION PUMP SYSTEMS

Q1. WRITE SHORT ESSAY ON IN-LINE INJECTION PUMP FILLING AND PUMPING CYCLEOF OPERATION?

A1. Pressure chamber filling phase: Towards the end of the plunger downward stroke both thefeed and feed/spill ports are uncovered. Fuel surrounding the upper portion of the barrel at lift

pump pressure will immediately enter the pressure chamber through the ports to fill up thespace between the plunger and the underside of the delivery body.

Q2. WRITE SHORT ESSAY ON COMMENCEMENT OF FUEL DELIVERY (INJECTION)PHASE?

A2. After the plunger has reached its outer dead centre and commences its upward stroke, it begins to sweep over the port aperture. Initially it pushes back a small amount of fuel throughthe ports until the top edge of the plunger cuts off the space above the plunger from the fuel gallery and ports. The pressure build-up underneath the needle pressure shoulder eventually

lifts the needle off its seat. This causes fuel to be discharged from the nozzle sac into thecombustion chamber via the annular-formed spray hole or though several small holesdistributed around the nozzle nose.

Q3. WRITE SHORT ESSAY ON END OF FUEL DELIVERY (INJECTION CEASES) PHASE?

A3. The continuing plunger rise forces fuel through the delivery valve until the edge of the plunger helix uncovers the feed/spill port. Instantly, the fuel pressure in the barrel collapses asfuel escapes down the vertical slot and partly around the circumferential groove and out through the feed/spill port. Simultaneously, as the fuel pressure decreases, the delivery valve

spring load exceeds the delivery valve opening pressure causing the delivery valve to snapshut. The sudden drop in fuel pressure in the barrel causes a corresponding drop in pipeline pressure so that the injector needle valve also snaps down on its seat, it therefore prevents any more fuel discharging from the injector nozzle spray holes.

Q4. WRITE SHORT ESSAY ON FUEL DELIERY OUTPUT CONTROL?

A4. The plunger stroke is controlled by the cam lobe profile and is always constant but the part of it which actually pumps is variable. The point on the upward travel of the plunger at whichthe spill occurs can be altered by twisting the plunger relative to the barrel. This enables the position of the plunger helical spill groove to be varied relative to the fixed barrel spill port, it thereby increases or decreases the effective pumping stroke of the plunger.

The plunger can be positioned in the no-delivery shut-off position by rotating it until the helical groove uncovers the spill port in the barrel for the entire stroke, so that at no time can the fuel be compressed and displaced through the delivery valve.

Q5. WRITE SHORT ESSAY ON INJECTION ADVANCE WITH INCREASING ENGINE LOAD?

A5. For some indirect pre-combustion chambers, the temperature of the compressed air in thechamber may vary considerably from something like 400 o C to 750 o C from low to full engineload. It is therefore desirable to match the injection timing to the change in engine load which is




a measure of chamber air charge temperature. Thus, under light engine load, the peak chamber temperature is reached later in the compression stroke and therefore combustion isoptimized if injection is set back slightly. Conversely, with increased engine load, more heat energy is released so that the peak temperature will be higher and it will occur earlier in thecompression stroke. Accordingly, injection timing can be brought forward to improve theeffectiveness of the combustion process.

Q6. WRITE SHORT ESSAY ON THE TYPES OF CALIBRATION MECHANISMS FOR THE FUEL PUMP?

A6.

Split-toothed sector gear & sleeve

By slackening the split-toothed sector gear around each control sleeve in turn, the sleeve andits corresponding plunger can be rotated clockwise or anticlockwise independently of thepumping elements, which are all meshed to the control rack. Each pumping element cantherefore be separately adjusted relative to the control rack so that any movement of the rod

produces similar changes in fuel delivery from individual pumping elements. After adjustment,each split-toothed sector gear is retightened.

Plunger control arm clamp

Slackening, in turn, each control arm clamp relative to the control-rod permits the control sleeveand its corresponding plunger to be rotated in either direction, independently of the other pumping elements, which are interlocked ( via the control sleeve-peg and control-rod slottedclamp ) to the control rod. Accordingly, the individual pumping element fuel deliveries can beadjusted by moving the slackened control rod clamp in the left or right-hand directions, alongthe control rod relative to the other plunger helix settings.

Flanged barrel or sleeve with elongated holes

Slacken each pair of securing nuts ( which clamp the flanged barrel or sleeve to the pumpingelement housing ) so that the individual barrels can be rotated relative to their plungers withinthe limits of the elongated stud holes formed in the flanged end of the barrel.




A REVIEW: NOx CONTROL IN COMBUSTION PROCESSES

Q1. DISCUSS THE TWO MAJOR MECHANISMS OF NO x FORMATION?

A1. Thermal NOx is formed by the high temperature reaction of nitrogen with oxygen, via thewell-known Zeldovich mechanism as given by the simplified reaction:

N2 + O2 NO, NO2

Thermal NOx increases exponentially with temperature. Above 1100 o C, it is generally thepredominant mechanism in combustion processes making it important in most high-temperature heating applications. This means that this mechanism becomes more importantwhen air preheating or oxygen enrichment of the combustion air are used, which normallyincreases the flame temperature.

Prompt NOx is formed by the relatively fast reaction between nitrogen, oxygen and hydrocarbonradicals. Fenimore found that NO concentration profiles in the post-flame gases did not

extrapolate to zero at the burner surface. It is given by the overall reaction:

CH4 + O2 + N2 NO, NO2 , CO2 , H2O , trace species

Q2. DISCUSS THE FACTORS AFFECTING NO x FORMATION?

A2. There are many factors that have an impact on NOx formation. These include the oxidizer and fuel compositions and temperatures, the ratio of the fuel to the oxidizer, the burner andheater designs, the furnace and flue gas temperatures and the operational parameters of thecombustion system.

Fig. 5 shows the predicted NO as a function of the flame stoichiometry for air/fuel flames. NOincreases at fuel-lean conditions and decreases at fuel-rich conditions. NOx is dramaticallyreduced under fuel-rich conditions.

Fig. 6 shows the importance of the gas temperature on thermal NOx formation. The NOx risesrapidly at temperature above 1100 o C for all three fuels shown. This is a demonstration of theincrease in thermal NOx as a function of temperature.




Fig. 7 shows how NOx increases when the combustion air is preheated. Air preheating iscommonly performed to increase the overall thermal efficiency of the heating process.However, it can dramatically increase NOx emissions because of the strong temperaturedependence of NO formation.

Q3. WRITE SHORT ESSAY ABOUT COMBUSTION MODIFICATION TO REDUCE NO x ?

A3.

Combustion Modification

Combustion modification prevents NOx from forming by changing the combustion process.There are numerous methods that have been used to modify the combustion process for lowNOx.

1. Low Excess Air

Excess air increases NOx emissions. The excess air generally comes from two sources: thecombustion air supplied to the burner and air infiltration into the heater. Excess air produced byeither source is detrimental to NOx emissions.

2. Temperature Reduction




In many combustion devices, the combustion air is preheated by the hot exhaust gases toimprove thermal efficiency.

a. Air Preheat Reduction

One combustion modification technique is reducing the combustion air preheattemperature. Reducing the level of air preheat can significantly reduce NOx emissions.

b. Water or Steam Injection

Many of the combustion modification methods attempt to reduce the temperature of theflame to lower NOx emissions. In many cases, this may result in a reduction of thecombustion efficiency.

c. Gas recirculation

Furnace gas recirculation is a process that causes the products of combustion inside thecombustion chamber to be recirculated back into the flame. This is sometimes referredto as internal flue gas recirculation. External flue gas recirculation is similar.

3. Staging

Staged combustion is an effective technique for lowering NOx. Staging means that some of thefuel or oxidizer or both is added downstream of the main combustion zone. The fuel, oxidizer or both can be staged into the flame. The burner flame front should be a fuel-rich premixed flamedesigned to increase flame speed and, in turn, flame stability.RQL means Rich fuel, Quick mix & Lean burning

4. Burner Out-of-Service (BOOS)

This is a technique primarily used in boilers where the fuel is turned off to the upper burnerswhile maintaining the air flow to all. The fuel removed from the upper burners is then redirectedto the lower burners while maintaining the same air flow to the lower burners. Therefore, theoverall fuel & air flow to the boiler remains the same but is distributed. This makes the lower burners fuel-rich which is less conductive to the NOx formation.

5. Reburning

An example is methane reburn where some methane is injected in the exhaust gases usuallywell after the primary combustion zone in which the gases are at a lower temperature. Fuel-richconditions are not favorable to NOx. As the exhaust gases from the combustion process flow

through this fuel-rich reducing zone, NOx is reduced back to N2

6. Low NOx Burners

The use of various low NOx burners to achieve emissions reductions compared to standard gasburners. A study shows that ultra low- NOx burners are the most cost-effective means to reduceNOx.

Ib h A

q&a on applied combustion-final

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