7What is Combustion?A chemical reaction between fuel and oxygen producing heat.Air is usually the source of oxygen.The chemical reaction produces “flue gases”
8What Is Required For Combustion? Three Elements:FuelAirSource of Ignition
9Fuel ComponentsGas, Oil and Coal are all basically a mix of Hydrocarbons.During combustion these break down progressively as some parts burn more easily.The most important components are Carbon and Hydrogen compounds.
10Other ComponentsIn addition to the Carbon and Hydrogen many fuels contain Sulphur.Sulphur also burns but produces hazardous products.Liquid and solid fuels can contain other non-combustibles which form ash.Nitrogen may be present as a gas or in compound form in liquid/solid fuels.
11Chemical FormulasIn formulas we will use the following basic componentsCarbon = CHydrogen = H2Oxygen = O2Nitrogen = N2Water = H2OCarbon Dioxide = CO2Methane = CH4
12Note on CalculationsEach component in a formula is a Molecule (of gas)A Molecule of any gas occupies the same VolumeThe number of Molecules is therefore the same as the number of VolumesAll calculations are therefore Volumetric, including measured Gas Analysese.g. 2 CO = 2 volumes of CO
13Examples Of Combustion For Typical Fuel Components with Oxygen C + O2 ® CO22H2 + O2 ® 2H2OS + O2 ® SO2
14HeatWhere does the heat come from?Heat+C + O CO2
15But we don’t have Pure Oxygen available Oxygen in Air(by volume)Air ≈ 21% O2 + 79% N2Ratio1 O2 : 3.75 N2The other main component in air is Water Vapour. In humid conditions this can be 5% or more and affects efficiency
16Examples Of Combustion For Basic Fuel Components with Air C + O N2 ® CO N22H2 + O N2 ® 2H2O N2S + O N2 ® SO N2
17Example - Combustion Of Methane CH4 + 2O N CO2 + 2H2O + 7.5N2+ Heat
18StoichiometryThe technical term used to define the theoretical amount of air or oxygen required for complete combustion of a fuel is the Stoichiometric ratio.e.g. - for a typical Natural Gas the Stoichiometric Ratio is approximately 10 volumes of Air to one of Gas.
19Excess AirBecause of many factors, including imperfect mixing, extra air is always needed to ensure complete combustion.The extra air above the Stoichiometric amount required is known as the excess air.
20Stoichiometric Air Example CH4 + 2O N2® CO2 + 2H2O + 7.5N2 + HeatNote – no Excess Oxygen in Flue Gas
21Excess Air ExampleCH4 + ( )O2 + ( )N2 CO2 + 2H2O + 9N O2 + Heat0.4/2.0 = 0.2 or 20% excess air0.4/( )=0.032 or 3.2%O2 in flue gases (wet)0.4/( )=0.038 or 3.8%O2 (dry)
22Fuel Rich Examples (Sub-stoichiometric) 3C + O2 2 CO + C + heat4H2 + O2 2 H2O + 2H2 + heatProducts include Combustible Gases and free Carbon (soot)
23Some Dangers of operating below Stoichiometric Flue gases contain combustibles.When these gases find a supply of air they will burn.If this happens in the convection tubes it can damage the tubes.Pockets of gas can build up in ducting and cause explosions.Flames eventually back out of burners.
24Heater Control Problems with Sub-Stoichiometric Combustion Increasing fuel flow will reduce heat to the process as more combustibles are generated.This can lead to total loss of control and very high levels of unburned gases in the heater.
25How do you get out of this situation? Do not open up air suddenly, as this will cause unburned gas to burn rapidly and possibly explosively.Reduce the gas flow slowly until temperature starts to recover. This allows unburned gases to disperse safely.
26Flue Gas AnalysisWe control the excess air by measuring the excess Oxygen in the Flue GasThe amount of excess air we need to know is what goes through the burners.The ideal sample point is at the exit of the firebox, as there should be little or no air leaks in this box.
28On-Line Analysis The oxygen analyser is located in the stack. This analyser measures in the gas stream, so it indicates what we call a “WET” analysis since water vapour is present.Air leaks between the firebox and stack affect the readings.
29Portable (off-line) Analysis Portable analysers can be used to check gases wherever a test point is available.They draw a sample through a cold line so water condenses out. The analysis is therefore known as “DRY”. This gives higher O2 readings but standard compensations can be made.Analysers can also measure CO and NOx for combustion efficiency and emissions checks.
30Flue Gas LossesThe gases passing out of the stack are above the ambient temperature, so they carry unused heat into the atmosphere.Increasing Flue gas temperature increases these losses.Increasing Excess air increases the amount of flue gases, giving even more loss.
31Units Of Heat Flow British Thermal Unit BTU/hr Kilocalorie 1 KCal/hr=3.938 BTU/hrKiloJoule 1 KJ/hr = Btu/hrKilowatt 1 KW = 3,413 BTU/hr (1W = 1J/s)
32Gross and Net Heating Value Higher (Gross) Heating Value (HHV):The total heat theoretically available from combustion of a fuel.Lower (Net) Heating Value (LHV): the HHV less the latent heat used to convert the produced water to vapour.
34Wobbe IndexThis is a factor used in the design of Premix Burners only.It is based on Calorific Value and Density.If 2 gases have the same Wobbe index they should work equally well in the same premix burner.
35Products Of Combustion Water Vapour - H2OCarbon Dioxide - CO2Sulphur Dioxide - SO2, SO3Carbon Monoxide - COUnburned Hydrocarbons - UBCNitrogen Oxides - NO, NO2
36Flame Speed Another important factor in Combustion is the Flame Speed Each gas burns in air at a particular speed under reference conditionsA stable flame is produced when the Flame Speed and gas/air mixture velocity correspond
38Other Gas Characteristics All fuel gases will burn within a mixture range both below Stoichiometric and above Stoichiometric.The “flammability range” varies between gases, and is another indicator of how easily a gas will burn.Gas density affects burner design as heavier gases have higher pressure drops though gas jets.
40Basic Objects of a Burner The burner must mix the fuel and the air effectively to ensure complete combustion.The flame must be stabilised in a fixed position so that its heat can be absorbed effectively.The flame shape must be controlled to suit its working environment.
42Basic Burner Types Natural Draught PremixRaw Gas (Nozzle Mix)Combination Oil & Gas
43Natural DraughtAir is pulled through the burner by draft created by the heat in the furnace and stack (explained in a later section).Since air velocity is low we need to use the energy in the gas (typically at 1 barg) to improve the gas/air mixing.We have 2 basic ways we do this.
44Premix Burners Fuel pressure drop occurs in the gas jet. Gas velocity in venturi induces part of the air so air flow adjusts with gas flow.Fuel and primary air mix before the nozzle.Secondary air mixes in burner throat.All domestic gas burners are premix, including cooking appliances.
45Basic Burner Types Pre-Mix Heater Burner GAS NOZZLE
46Pre-Mix Burner Advantages Large fuel gas discharge orifice.Large ports in firing nozzle.Small flame volume.Automatic variation of air flow with varying fuel rates.
47Premix Burner Disadvantages Can only accept small variations in gas quality without adjustment (n.b. unless Wobbe Index is maintained)Limited turndown.Difficult to adapt for combination gas/oil firing (but not impossible)Maintenance more difficult.Hard to reduce NOx.
48Raw Gas Burners (Nozzle Mix) Gas and air are kept separate until discharged into the combustion zone.Fuel pressure drop occurs at the combustion zone.The energy in the gas helps mix fuel and air.
49Basic Burner Types Nozzle Mixing Gas Burner GAS NOZZLEBURNER THROATFLAME HOLDER
54Nozzle Mixing Gas Burner Advantages A high turndown ratioNo possibility of flashbackThe ability to burn a wide variety of fuels with differing heating valuesFlame shape can be controlled as required by gas tip and tile design.Can be adapted many ways to reduce NOx
55Nozzle Mixing Gas Burner Disadvantages Small fuel discharge ports"Large" flame volumeFuel/air ratio is dependent on operators
56Raw Gas CombinationDesigned to burn gas and fuel oil either separately or together.Inner tile stabilizes oil flame with controlled primary air.Gas burners stabilize in secondary tile throat.Oil guns remove easily for cleaning while gas burners are in service.Gas burners can also be maintained while oil burners are in service.
57Combination Natural Draught Gas and Oil Burner PRIMARY TILEGAS TIPS
58Combination Burner Limitations Oil guns need frequent maintenance.Oil firing problems can cause fouling of gas tips.Total capacity of burner is set by air flow available, so firing gas and oil at the same time requires both fuels to be limited to give correct total Heat Flow.
59Forced Draught Burners Basically similar to Natural Draught Raw Gas Burners (including Combination Oil/Gas Burners).Higher air velocities give better mixing and smaller flames.Air can be preheated, using various types of heat exchanger.Flames are hotter, giving higher rates of heat transfer.
60Gas pilotsMost process burners use a pilot to provide the basic source of ignition.Pilot is usually fully premixed.Pilot can be ignited manually or have a built-in spark ignition.Some pilots have flame rods to check flame is alight.
63FurnacesA furnace is basically an insulated box lined with tubes containing the process fluid.We fire burners inside the box to heat the tubes by a mixture of radiation and convection heat transfer.There are many different furnace designs depending on the process application and the companies involved.The next 2 slides show some basic types.
67Burner LocationsDepending on the heater layout burners may be installed up-fired, side-fired, end-fired and down-fired.Most heaters are up-fired, except for special types such as Ethylene Crackers and Reformers.
68Heat Transfer (a) - Radiation In the firebox we get heat transferred initially by direct radiation from the flames to the tubes.Additional heat is radiated to the back of the tubes from the hot furnace walls.Radiant efficiency depends on the emissivity of the flame and of the tube surfaces, plus the temperatures of both.
69Heat Transfer (b) - Convection Hot gases passing over tube surfaces heat the tubes mainly by Convection.Away from the Flames most heat is transferred by Convection.A Convection Bank is a section of the Heater where Radiation is insignificant, normally just below the Stack.
70Process FlowIn most heaters the coolest fluid is exposed to the coolest heat source.Fluid passes first through the Convection Tubes, where available.Fluid exits near the burners.
71Furnace DraughtNatural Draught burners depend on the air flow being created by the difference in air pressure between the inside of the heater and outside.The reason the pressure is different is that the air inside the heater is hotter than the air outside.Since hot air is lighter it rises and reduces the pressure inside the heater.
72Furnace Draught Typically the temperature in a firebox is 500 - 800°C. At this temperature the draft increases by about 2.5 mm water for every 3 metres of firebox height.If we have a convection section we need more draught above it to overcome the pressure drop through the tube bank.
73Where Draught comes from 10ft column of air at 1000degF =0.05”w.g.10ft column of cold air=0.15“w.g.DRAUGHT = 0.1” /2.5 m.m.
74Furnace DraughtThe temperature in the stack is lower, so we need more stack height to give us the required draught.The next chart shows what happens in our heater with a convection bank and a stack damper
76More on Draught We need just enough air to burn our fuel properly. We do not want any air to get in except through the burners.Any air which does not pass through the burners just absorbs some of the heat available and throws it away up the stack.
77Even more on DraughtWe need to keep draught negative all the way through the heater.If we get a positive draught then hot gases will find small holes and make them bigger.The critical point is usually at the top of the firebox – look at the chart again.Many heaters have alarms for positive pressure.
80Before TuningBefore tuning make a full check of the burner conditions.Ensure air doors are open equally and gas valves open completely.Check flame appearance / stability. Close all peep doors.Keep in Radio touch with panel operators.
81Heater Tuning Draught Calculation / Setting For a typical heater as in the sketch we should have about 2 mm draught at the arch.If the heater is 10 metres high we can expect an additional 8-9 mm at the floorThis gives us 12 mm total.Burners should have been designed for slightly less than this theoretical draught, so we close the air doors to control the excess air through the burners.After we close the air doors we may need to adjust the stack damper to maintain 2 mm at the arch.We check O2 and draught and repeat adjustments until we get both figures correct.
82HEATER ADJUSTMENT FLOW CHART TARGET DRAFT1 to 3 mm waterTARGETOXYGEN2 – 3 %STARTCHECK DRAFTHIGHLOWCHECK O2CHECK O2TARGETHIGHLOWHIGHLOWCLOSE STACK DAMPEROPEN AIR REGISTERSCLOSE AIR REGISTERSOPEN STACK DAMPERRETURN TO STARTRETURN TO STARTCHECK O2HIGHLOWON TARGETCLOSE AIR REGISTERSOPEN AIR REGISTERSRETURN TO STARTRETURN TO STARTGOOD OPERATION
83HEATER ADJUSTMENT FLOW CHART TARGET DRAFT1 to 3 mm waterTARGETOXYGEN2 – 3 %STARTCHECK DRAFTTARGETCHECK O2HIGHLOWON TARGETCLOSE AIR REGISTERSOPEN AIR REGISTERSRETURN TO STARTRETURN TO STARTGOOD OPERATION
84Heater Tuning Draught Control – General There are differences in approach depending on the type of burner, if the heater has a convection bank, and if there is a stack damper.If the burners are in a plenum and have their own air doors then we have an extra adjustment point. In such cases the individual burner air doors should be fixed open unless a burner is stopped, when they should be shut.Sinclair has almost every combination possible, so we have to look at all the possibilities.
85Heater Tuning Draught Control – Raw Gas Burners Basically the Flowchart given applies to this type of burner.If there is no stack damper we need to monitor the arch Oxygen – assuming that the furnace leaks have been fixed.We must still check that Draught is negative as putting too much air through burners can cause draft to go positive at the arch.
86Heater Start-upDuring start-up draught is low as temperatures are low.Pilots self-inspirate so should work normally.High excess air is used to control furnace temperature rise.Individual Burner light-off should be done with air doors nearly closed, so gas lights more smoothly.Increase air opening slowly so burner heats up quickly and flame can stabilize properly.
87Heater Tuning Fuel Gas Valves Valves fitted upstream of each burner are for isolation only.The only time a valve should not be opened fully is during light-off.If any valves are not completely open then the burners are not all firing at the same rate.Gas pressure trip settings are established on the basis that valves are fully open.If a trip setting interferes during normal operation it should be checked and may be changed, provided that the burner stability is checked at the revised setting.If an individual burner gives a problem with the valve open then the problem should be investigated. On many burners there are small gas jets which can plug easily and will affect flame stability.
88What can go Wrong?O2 falls too low – Temperature control is lost as fuel does not burn – flames search for air and blow back through registers – “Puffing” – CUT BACK ON FUEL FIRSTDraught goes positive – gas leaks out of any gaps and causes damage, but O2 still looks OK. Heaters should have an alarm for high pressure.
89Heater Tuning Flue Gas Analysis In general a good target for excess Oxygen is 3%We need this level in the firebox – that should mean we are getting the right amount of air through the burners.Gas samples taken above convection banks include any air which leaks in around the tubes.These leaks should always be minimised as they affect the convection bank efficiency.In serious cases the leaks can exceed our 3% target, so we could actually be firing below stoichiometric.
90Heater Tuning Flue Gas Analysis One way to check what is really happening is to also measure CO levels.Typically it is safe to run with a maximum of 50 ppm of CO in flue gases.Older burners will start producing CO at around 2% excess Oxygen, so we have a good indication of the actual excess air through the burners.On-line CO analysers allow burners to be run safely right down to their minimum achievable levels of excess air.
91Heater Tuning SummaryWe are aiming to have 3% excess oxygen in the firebox.We need all the burners in each heater to be operating with the same amount of fuel and air.This means air doors set equally, gas valves full open, and clean gas tips.If there is a stack damper, it should normally be set to give a draft of 0.1” maximum at the heater arch.Some heaters may still need more draft to get enough air through the burners.
93What is the Problem?All combustion processes produce some Nitrogen OxidesIn the atmosphere these oxides can form Nitric acid and fall as acid rainThey react with other gases and sunlight, producing ozone and smog
94NOx Formation in Combustion In ambient conditions Nitrogen is an inert gas
95NOx Formation in Combustion In hot flames we getThermal NOxFuel NOx
96Thermal NOx Created from atmospheric Nitrogen Formation controlled by the breaking of N2 moleculesto reactive nitrogen atoms by the supply of heat.The N atoms then react with available Oxygen to form NO.Thermal NOx formation rate is dependenton peak flame temperature and oxygen availability.
98NOx definitions The primary component formed in a flame is NO. In the atmosphere this NO converts to NO2, which is the harmful form.We define limits as NOx, where all measured levels are treated as having converted to NO2.Fired Heater limits are always expressed as the equivalent levels of NOx at 3% excess Oxygen.EPA bases limits on lbs/million Btu rather than on percentages.
100Fuel NOxSome fuels contain ‘fixed’ Nitrogen as compounds. Liquids and Solids contain more of these than most gases.These compounds break down in the combustion process and release the Nitrogen in a form which reacts easily to form NOx.Nitrogen as a gas component is not significant.NOx levels increase in direct proportion to the fixed Nitrogen in the fuel.NOx reduction techniques are also effective in reducing Fuel NOx.
101How can we reduce NOx? Reduce the Flame Temperature Reduce the Oxygen availableFlue Gas Treatment
102Reducing Flame Temperature Slow down fuel / air mixingInject cooler inert gases into the flame (steam or recycled flue gas)Increase the excess airReduce air below stoichiometricUnfortunately all of these things conflict with our requirement to get maximum heat from the flames to the process
103Reducing Available Oxygen Reduce the excess airInject Inert gases into the flame to reduce the oxygen concentration available (recycled flue gas again)
108Staged Air Burners Disadvantages Long FlamesComplicated Air AdjustmentFuel Composition affects PerformanceHigher Excess Air RequiredLimited NOx Reduction
109Staged Fuel Low NOx Burners Features / AdvantagesDisadvantages
110Staged Fuel Burner Features Two Stage Fuel InjectionGood Heat Transfer from Secondary FlameCombustion Product Injection"Compact Flame“Tolerates gas variations
1111. Two stage fuel injection Primary gas burns with high excess air, cooling the flameSecondary gas mixes into flame above the burner, where oxygen level is low, so burns at a lower temperature
1122. Heat Transfer from Secondary Flame Secondary Flame burns slowly above the burnerMaintains uniform radiant Heat transfer further up the furnace
1133. Combustion Product Injection Secondary gas pokers are above the burner tileThey induce furnace gases into the Secondary flameOxygen is reduced but temperature increases, maintaining flame dimensions well
1144. Compact FlameHigh excess air primary flame gives strong core to flameControlled secondary mixing and recirculation keeps flame relatively compact
1155. Tolerates Gas variations Balance of primary to secondary gas is fixed (typically 30-40% primary)Stoichiometry is not affected by fuel properties
119Staged Fuel Burner Disadvantages Turndown is limitedStability sometimes a problemSmall Gas Port SizeEffectiveness of NOx reduction depends on fuel properties
120Combination of Staged Fuel and Internal Flue Gas Recirculation Low Emission BurnersCombination of Staged Fuel and Internal Flue Gas Recirculation
121Low Emission Burner Based on Staged Fuel Burner Primary Gas induces furnace gases into Primary Flame“Zoning” of air in burner throat gives high stabilitySelf compensates for gas changes
122Internal Flue Gas Recirculation Recycle GasFlue GasBurnerRecycle GasFurnace
123Flue Gas Recirculation Hot flue gases rise fast up the centre of the furnaceCooler gases travel down wall around tubes to the floorGases have only Excess Oxygen and relatively low temperatureLighter fuel gases run at higher pressure / velocity, maintaining recirculation levels
124Flame RetentionPrimary gas induces ‘inert’ gas into the burner throat.Flame holder mixes limited air with fuel and recirculated gases to give a fuel-rich zone around the outside of the flame holder for high stabilityBalance of air passes through centre of flame holder to mix into the primary flame
125Staged Fuel Staged fuel induces more inert gases into flame Mixing is delayed by the fuel-rich zone on the outside of the primary flame