chapter 2 combustion engineering for second semester
- 1. Chapter 3 A flame is a region of intense chemical reaction. This region of intense chemical reaction is characterised by the emission of heat and light • In the case of a premixed flame, we know that the fuel and oxidiser were mixed before entering into the combustion zone/combustor. • In another type of flame, fuel and oxidiser are fed to the combustor separately. Mixing of fuel and air occurs in the combustor. This type of flame is known as a non- premixed flame. As the mixing process is governed by diffusion of fuel and air, it is called a diffusion flame.
- 2. • Diffusion flames can be achieved easily. • If the fuel and oxidizer are initially separated and then transported towards each other(Diffusion) • As fuel and oxidiser come as separate streams, the safety of these flames is high. The industrial combustor flames are generally not a premixed one. • If the reactants ( fuel +oxidizer) are mixed at prior, then after ignition the reaction front propagates through the un burnt mixture with a definite speed. Such flames are known as Premixed flames eg: Bunsen burner and for SI engines , we use carburettor
- 3. A premixed flame propagates through the unburned mixture at a definite speed. Thus, it can be considered as a front separating burned and unburned mixture. At this level of resolution, the flame is a surface of infinitesimal thickness across which there is a jump of mixture properties from unburned gas temperature Tu to burned gas temperature Tb Fuel concentration in the unburned region yu to the fuel concentration in the burned region, yb = 0, This level of resolution is known as the hydrodynamic or flame sheet level. The next, more detailed level of resolution is the transport- dominated level.
- 4. • At this level, the flame is expanded to a finite thickness over which the temperature of the reactants increases from Tu to Tb and the fuel concentration drops from yu to 0. • This is due to transport of heat and fuel species from and to the reaction zone by convection and diffusion. • The flame is thus made of a finite preheat zone and an infinitesimal reaction sheet. • This level of resolution is the transport or reaction sheet level.
- 5. • The third and the most detailed level of resolution is called chemical level. • However, the chemical reaction is limited to a region close to the maximum temperature, and also the reaction is completed within a very thin region due to the high reaction rate. • At this level of resolution, the chemical reaction is confined to an infinitesimal region: the reaction sheet. • The rapid rate of property change within the reaction zone implies that diffusion has a much greater influence than the convection. • This level of resolution is referred to as the chemical level.
- 6. RANKINE–HUGONIOT RELATIONS A premixed flame may be considered as a wave propagating through a reacting mixture and separating the unburned region from the burned region. Let us consider a one-dimensional planar flame propagating as a wave through a quiescent ( temporary) reacting mixture with a velocity Vu. At the hydrodynamic level of resolution, the length scales are much larger than the preheat zone where the diffusion fluxes are important.
- 7. • The entire flame, including the preheat zone itself, appears as an infinitesimally thin surface. • Hence, on a flame-fitted coordinate system, the diffusive fluxes vanish at the boundaries of a typical domain encapsulating the flame. • Integrating the transport equations across the flame on a hydrodynamic length scale, the following conservation relations are obtained. Mass: = = f
- 8. Energy: = Detonation and Deflagration Waves Detonation is characterised by supersonic flame propagation velocities( around 2000 m/s) and substantial pressures up to 2mPa. The main mechanism of detonation propagation is of a powerful pressure wave that comprises the unburnt gases ahead of the wave to a temperature above the auto ignition temperature. The supersonic solution (Mu >1) is known as Detonation wave.
- 9. Deflagration is characterised by a subsonic flame propagation velocity ( below 1000m/s) and relative pressure below 50kPa. The main mechanism of combustion propagation is a flame front that moves forward through a gas mixture. The subsonic solution ( Mu<1) is known as deflagration wave.. FLAME PROPAGATION AND FLAME SPEED • A premixed flame is a self-sustaining localised combustion zone that propagates at subsonic speeds. • Thus, the speed at which the combustion zone propagates is crucial.
- 10. The velocity of the flame, Vflame is known as the flame propagation speed. However, the mixture through which the flame propagates can also have some velocity. • The normal component of the velocity of the fluid relative to the flame is important. • If Vflow is the local flow velocity at the flame location, the velocity of the flame relative to the flow is defined as the flame displacement speed, Su. • The flame displacement speed is defined as: Su = (Vflame − Vflow )nˆ where ˆn is the unit normal to the flame surface.
- 11. • This flame displacement speed is referred to as flame speed or burning velocity. • Although the two terms are often used synonymously, the former generally is used to denote flame motion , and the latter is used as a flame property. FACTORS AFFECTING FLAME SPEED • Dependence on temperature: The dependence of flame speed, and consequently flame thickness depends on reaction rate and transport properties like thermal diffusivity on temperature • Dependence on Pressure: Flame speed does not change with pressure.
- 12. • Dependence on Fuel Type: C2H4 have higher velocities but the flame speed of CH4 is lower. Hydrogen has a much higher flame speed due to high thermal and mass diffusivities of hydrogen and rapid reaction kinetics for hydrogen. FLAME QUENCHING • In case of flames inside ducts or tubes, heat loss to the duct wall is very often the deciding factor for the stability of the flame. • The basic condition for quenching of the flame is that the heat loss from the flame exceeds the heat generated inside the flame. • This would lead to a lowering of the temperature of the flame.
- 13. • Therefore, a critical condition exists when the heat loss and heat release balance each other. • This would be the limiting condition for survival of the flame. • Although a lower flame temperature implies reduction in both heat release due to chemical reaction and heat loss from the flame, the former is a much stronger function of temperature. • Hence the decrease in rate of heat release significantly exceeds the decrease in the rate of heat loss, causing a further reduction in temperature till the temperature becomes too low for the reaction to sustain.
- 14. • The minimum gap needed between the two walls for the flame to survive and is known as quenching distance. Minimum Energy for Ignition • A homogeneous fuel-air mixture is often ignited by a heat source. • Although electric spark is the most common external energy source for ignition, some experiments have also used focused laser beams to supply the energy. • For homogeneous reacting mixtures, the energy source generally ignites a small localised volume of the mixture. • Under favourable conditions, the flame kernel thus formed expands outward, consuming the reacting mixture.
- 15. • For flame generated by ignition to be sustained, the heat generated within the flame volume must exceed the heat lost from the flame surface to the surrounding unburned mixture. • Since heat loss is a surface phenomenon and the heat is generated from the entire flame volume, a high volume- to-surface ratio helps to sustain the flame. • On the other hand, energy needed for ignition increases with the volume ignited. • Hence, there exists a minimum volume that has to be ignited for sustenance of the flame and a corresponding minimum energy that has to be supplied.
- 16. FLAME PROPAGATION IN MICROSCALE COMBUSTORS • we saw that there exists a critical dimension of the duct below which a premixed flame gets extinguished. • However, there is a strong motivation for development of combustors of extremely small sizes, which can act as power sources for microscale devices. • The size limitations in such devices often demand construction of combustors, whose characteristic dimension is less than the critical dimension for quenching. • One of the ways in which flame is sustained in a combustor, whose characteristic dimension is smaller than the quenching distance, is by means of heat recirculation.
- 17. • Heat recirculation refers to transfer of some energy from the burned gas region to the unburned mixture. • This transfer of energy preheats the reacting mixture, leading to stronger reaction, which prevents extinction of the flame in spite of high heat loss due to high volume- to-surface ratio. • Under certain circumstances, the heat recirculation to the upstream mixture can lead to preheating of the reactant mixture to such an extent that the temperature of the burned gases exceeds the flame temperature of adiabatic premixed flames in spite of heat loss from the flames. These flames are called super-adiabatic flames.
- 18. • Premixed flames suffer from serious hazards like possibilities of different kinds of instabilities like flashback, lift-off and blowout. • Flashback refers to a condition when the flame propagates towards the unburned mixture. • In the case of burner flames, this condition implies entry of the flame into the burner tube carrying reacting mixtures and propagation upstream along the tube. • This condition not only disturbs the stability of the flame stabilised on the burner but is also a potential safety hazard as the flame can move upstream and ignite a large volume of reactants in the mixing chamber where the fuel and air are mixed. • This can lead to explosion.
- 19. • Lift-off, on the other hand, refers to a condition where the flame detaches itself from the burner and moves downstream away from the burner. • In lift-off, the flames are stabilised at some distance above the burner and are known as lifted flame. • Lifted flames can be undesirable in practical applications for several reasons . • First, with the flame stabilised away from the burner, some fuel may escape unburned. • Second, as it is difficult to stabilise the flame at a specific location, poor heat transfer may result from the flame.
- 20. • These flames can also be noisy and susceptible to external perturbations. • Flame lift-off occurs when the flame propagates in the direction of the fluid motion. • Beyond a certain distance, the flame can no longer be stabilised and it blows off completely. This phenomenon is known as blowout. • Blowout is an undesirable phenomenon as it can lead to shutdown or fatal accidents. • Since flame speed decreases as one moves away from stoichiometric mixture towards lean side, lean mixtures used for low emission characteristics are often susceptible to blowout. • Such blowout is called lean blowout.
- 21. • FLAME STRETCH • Flame speed is affected by several factors like flow non- uniformity, flame curvature and flame/flow unsteadiness • It is possible to combine these effects in terms of a single parameter called flame stretch.
- 22. Chapter 4 :Pollution Emissions The word pollution is derived from Latin word, which means to make dirty. Pollution is the introduction of harmful materials to the environment. These harmful materials are called as Pollutants. The major pollutants may be divided broadly into three categories • The first includes pollutant gases that contain emissions of CO, SO2, SO3,NOx, unburned and partially burned hydrocarbons
- 23. • Greenhouse gases may be considered the second category. This group contains those gases which absorb and re emit thermal radiation. • The main greenhouse gases are carbon dioxide, methane, nitrous oxide, hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) • The third group, particulate matter, consists mainly of soot, fly ash, metal fumes, and other aerosols • Pollutants may be classified as primary and secondary types also. • Primary pollutants are generated directly from the source.
- 24. • Secondary pollutants are formed via reactions of primary pollutants in atmosphere. • Both types affect our health as well as environment • Ammonia is the primary pollutant that comes from agricultural activities. • Ammonia enters the air as a gas and combine with other pollutants such as nitrogen oxide and sulphates created by automobiles and they form aerosols. • Aerosols are tiny particles that can penetrate deep into the lungs and causes heart diseases
- 25. EMISSION OF POLLUTANT GASES Oxides of Nitrogen: • Oxides of nitrogen that form during combustion can be of three types: nitrous oxide (N2O), nitric oxide (NO) and nitrogen dioxide (NO2). • Generally, NO and NO2 together and then are called NOx. • To form NO from nitrogen, a very high temperature is required. • At high temperature, the nitrogen molecules dissociate to reactive nitrogen atoms. • NO, formed during combustion, reacts with atmospheric oxygen, after it is exhausted from combustion system.
- 26. • This reaction forms NO2 as secondary pollutant. NO2 is a hazardous gas for plants and animals. • It generates photochemical smog. • Photochemical smog is a mixture of pollutants when nitrogen oxides and organic compounds react to sunlight, creating a brown haze in the atmosphere • NO2 further reacts with atmospheric water and oxygen to form nitric acid, as follows: 4NO2+2H2O+O2 → 4HNO3 • This nitric acid, when it precipitates through rain, reduces the pH value of rain to as low as 2 from its normal value, which is around 6. • This causes damage to soil and structures.
- 27. • NO released at the upper atmosphere, mainly from aviation exhausts, reacts with ozone to form NO2. • This converts ozone to oxygen, creating depletion in the earth’s ozone layer. • NO2 formed in atmosphere can break down into NO again and produces nascent oxygen(O) • Nascent oxygen reacts with atmospheric oxygen to form ozone. • Ozone in the lower atmosphere is a constituent of photochemical smog. • The lower level ozone causes health hazards for animals too.
- 28. Carbon Monoxide • Carbon monoxide (CO) is a toxic gas found in the exhaust of a combustion system. • Exposure to carbon monoxide for a small duration causes headache and nausea. • When the exposure is longer, it may lead to unconsciousness, even death. • The amount of exposure depends on a combination of CO concentration and time of inhalation. • Carbon monoxide is an indicator of incomplete combustion and is normally produced in the fuel-rich combustion zone.
- 29. • In the case of premixed combustion, CO can be produced due to inhomogeneity in mixing. • Inhomogeneous mixture leads to a dearth( scarcity) of oxygen at any part of the fuel air mixture and thus produces a rich premixed flame. • In such situations, incomplete combustion of carbon produces CO. • However, the tendency of CO production is low in the case of premixed combustion. • But in non-premixed combustion, it is difficult to avoid CO production. • Here, the fuel and oxygen mixes in the combustion area, and improper mixing is a probable situation. • This creates CO during combustion.
- 30. • Unburned Hydrocarbon Gases • Unburned hydrocarbons (UHCs) are considered gaseous pollutants for the atmosphere. • These are also an indicator for incomplete combustion, like CO. • If we study the elementary reaction steps of any hydrocarbon fuel, we find various hydrocarbons are generated as intermediate products. • If sufficient reactants are not present to complete the reactions or the amount is insufficient for reaction completion, UHCs may result. • UHCs are produced mainly from rich mixtures as well as from flame quenching.
- 31. • Oxides of Sulfur • Oxides of sulfur normally result from combustion of sulfur-containing fuels. • Sulfur is found in coal or fuel oil in different percentages. It can be present either in its elemental form or as a compound. • Sulfur combines with oxidising agents very fast compared to other reactions. Sulfur is first oxidised to sulfur dioxide (SO2). • SO2 subsequently combines with oxygen to form sulfur trioxide (SO3) either in the combustion device or in the atmosphere after coming out as exhaust.
- 32. • These two oxides together can be called SOx. • Both oxides are considered environmental pollutants and are hazardous for animal health due to coughing and choking. • EMISSION OF GREENHOUSE GASES • Among the greenhouse gases, CO2 and H2O are natural combustion products for hydrocarbon fuel, which is the major fuel of present use. • Around 70% of CO2 is generated by the energy and transport sector. • Nitrous oxide (N2O) is another greenhouse gas that is a combustion product.
- 33. • Amine radical (NH 2 )reacts with NO to produce nitrous oxide . • Amine radical may be generated from ammonia (NH3) or hydrogen cyanide (HCN) during combustion. • N2O is commonly known as laughing gas and is considered an atmospheric pollutant. • It is also responsible for stratospheric ozone layer depletion. • Fluidised bed combustion is more prone to N2O production. • Methane is treated as another greenhouse gas, and it also comes from the combustion route.
- 34. • Methane can be generated as an intermediate product for hydrocarbon combustion as the smallest single bond component. • Subsequent oxidation of methane results in consumption of methane during combustion. • Incompleteness in combustion may generate methane as an exhaust component. • Methane emission is very common in the case of bio- mass burning or low-temperature burning of coal.
- 35. EMISSION OF PARTICULATE MATTER • Soot is the most vulnerable contributor in particulate matter emission from flame. • Soot generation is present in non-premixed and partially premixed flames. • Soot particles are carcinogenic and cause bronchial trouble. They are considered an atmospheric pollutant. • As suspended particles, they contribute towards smog formation. • Coal-fired power plants are sources of another particulate matter known as fly ash. • The pulverised coal-fired systems are the major generators of fly ash.
- 36. • These particles are carried with the flue gas and discharged into the atmosphere through the chimney. • These particles are also harmful for the environment. • If released into the atmosphere, they normally precipitate down almost immediately to cover the whole area with ash. • Inhalation of these particles is harmful for animals and humans. ABATEMENT OF EMISSION • Energy generation from combustion produces certain undesirable components. • These components cause damage to our atmosphere, habitats and overall environment.
- 37. • With the growing need of energy, we are searching for new non-conventional sources. • But we have not yet ended our dependence on conventional sources, mainly fossil fuels. • The technology of fossil fuel combustion is more developed and better understood compared to others. • Existing devices cannot be scrapped overnight unless we find comparable new technology. • A number of statutory bodies in the world form regulations about the environment.
- 38. • These regulatory authorities are forming the limits for pollutant emissions • They keeping in view the present environmental position and threat on one hand and the available technologies for abatement on the other. • These authorities are tightening these norms to minimise harmful emissions. • Control of NOx Emission • Thermal power plants are a major source of NOx emission. • As the fuel used in thermal power plants, coal or fuel oil, contains some nitrogen, NOx can be emitted by these plants.
- 39. • Control of the emissions can be thought of in two ways. • The first is controlling the combustion; the second is capture after combustion. • During combustion, keeping the flame temperature lower may help in Nox reduction. • Supply of less oxygen in high-temperature zone can be one method of NOx reduction. • Another method is not to allow the flue gas to stay a long time in the oxidising region. • however, that these methods may lead to loss of combustion efficiency as well as incomplete combustion.
- 40. • Another common technique is the burner out of service method. • In this method, one or more burners at each burner level can be selected to supply air only. • Necessary fuel can be supplied through other burners. • This method generates fuel lean zones within the furnace to reduce NOx formation. Control of SOx Emission • Thermal power plants are the main sources of SOx emissions. • Coal and fuel oil contain organic and inorganic sulfur. • One way to reduce sulfur is by using low-sulfur fuel.
- 41. • Combustion control to reduce sulfur is also not a solution as sulfur dioxide formation is a very fast process. • Wet gas desulfurisation can be considered another post- combustion capture method. • Here, a slurry tower is made for wet scrubbing of the flue gas. • Slurry containing water and finely ground limestone is sprayed from the top of the tower. • Flue gas is passed in the opposite direction and interacts with the slurry. • SO2 is converted first to sulfurous acid (H2SO3). The acid subsequently reacts with limestone:
- 42. • CaSO3 generated in this process is further oxidised at the bottom of the chamber to CaSO4 and collected at the bottom as hydrated crystal (CaSO4, 2H2O) or gypsum. • Another method employs a spray of very fine limestone slurry before the electrostatic precipitator (ESP). • The sulphite and sulphates formed by the reaction of limestone and sulfur dioxide can be arrested in ESP. • This method is called dry gas desulfurisation.
- 43. Control of Carbon Dioxide Emission • To control CO2 emission, carbon capture and storage are employed to reduce carbon loading to our atmosphere. • CO2 is considered the major contributor to greenhouse gases. • Control of CO2 emissions thus reduces global warming due to greenhouse gas emissions. • However, the methods evolved are not really cost effective yet for the transport sector and small-scale power generation units. • Fossil fuels can be gasified to synthetic gas (syngas). In this process, the fuel can be gasified by substioichiometric oxygen or by steam.
- 44. • The syngas contains hydrogen, carbon monoxide, carbon dioxide, water, methane and other hydrocarbons along with tar. • Tar can be separated from the syngas by cooling. • The syngas then reacts with water to perform water gas shift reaction in a reactor. • The reaction oxidises carbon monoxide and produces CO2 and hydrogen: CO+H2O ↔ CO2+H • The product is first dried to remove water. CO2 can then be separated from hydrogen by pressure swing adsorption on a suitable adsorbing material, like zeolite.
- 45. • Subsequently it can be removed by a desorption process and stored. Control of Particulate Emission • Fly ash can be considered the major particulate coming out of a thermal power plant. • A small percentage of ash generated in pulverized fuel- fired boiler furnaces is deposited at the furnace bottom as bottom ash. • As the coal particles are very small in size, ash particles generated after combustion are also too small to precipitate at the bottom.
- 46. • The remaining large portion (as high as 80%–90%) of the ash generated is carried with the flue gas towards the stack. • It is deposited on various surfaces in the boiler, reducing the heat transfer efficiencies. • The soot generated during combustion also takes a similar path as fly ash. • A periodic cleaning of those surfaces is necessary. Normally soot blowers are employed for periodic cleaning of the surfaces.
- 47. • ESP is a common device to take the particulate matter from the flue gas. • In this device, two electrodes are present. • They are normally called the emitter and the collector. • A very high voltage is applied across these two electrodes to charge the particulate matters in the flue gas. • The charged particles are then attracted towards the collector and collected there. • The collected ash is sometimes deposited in the hopper, placed below the collector electrode, by shaking the collector electrode. • The collection efficiency of ESP can be as high as 99.9%.
- 48. EMISSION QUANTIFICATION • Pollution emissions are normally quantified in terms of concentration, which can be done either by volume or by mass. • On a mass basis, the concentration is expressed as a percentage. • The volume or molar basis concentration is usually expressed in parts per million (ppm) or parts per billion (ppb). • Normally the emission quantity is very small and thus the measure is expressed in such terms.
- 49. • The emission index (EI) can be specified for a species, and it is expressed as a ratio of mass of that species to the mass of fuel burned. • So it is defined as the mass of emission of ith species generated from unit mass of fuel burning: • The quantity is a dimensionless one. • The value will be very low, so it is often specified as g/kg like units. • It is a useful parameter to compare devices operating on the same fuel.
- 50. • The following measurements are taken during an exhaust test of an IC engine:CO2 = 12.5%, CO=0.15%, O2 = 2.4%, C6H14(equivalent) = 365 ppm, NO = 80 ppm. All measurements are taken on a dry basis. The fuel of the engine is iso-octane. Find the emission index of CO and UHC (which is expressed as hexane equivalent). • Molecular weight of CO = 28 • Molecular weight of iso-octane = 114 • Molecular weight of hexane = 86 • m = 8 Solution