Energy studies (Marsh)

Card Set Information

Author:
chloe_h
ID:
255583
Filename:
Energy studies (Marsh)
Updated:
2014-01-09 10:15:54
Tags:
energy
Folders:

Description:
Combustion
Show Answers:

Home > Flashcards > Print Preview

The flashcards below were created by user chloe_h on FreezingBlue Flashcards. What would you like to do?


  1. What is stoichiometric?
    • A flame is stoichiometric if the reactants contain the right
    • amount of oxidant to burn the fuel completely.

    Excess of fuel = fuel rich system.

    Excess of oxygen = fuel lean system
  2. Equivalence ratio
    • The ratio of the stoichiometric AFR to
    • the actual AFR in the system under consideration.

    Φ = 
  3. Equiv ratio for stoichiometric combustion?
    Φ = 1, stoichiometric combustion.

    Φ <1, lean mixture, lean combustion.

    Φ > 1, rich mixture, rich combustion
  4. Neutral flame
    Φ is close to 1.

    No soot.

    Practically speaking not always complete combustion.
  5. Rich flame.
    • Φ > 1
    • Soot usually forms. (incomplete combustion)
    • Flame tip rising due to convecting soot.
    • Glowing soot usually makes for orange flame
  6. Lean flame
    • Φ < 1.
    • Flame temperature much higher.
    • Excess oxidant makes for complete combustion.
    • Too much oxidant will chill the flame.
  7. The physical effects of varying Φ
    • Varying Φ allows us to balance a variety of effects occurring within the flame. As such
    • we are trying to minimise losses from these conflicting effects:

    –Losses due to excess amounts of air.

    –Losses due to unburnt fuel.

    • Based on a simple model, there must be a
    • point where our losses are minimised.
  8. Excess air required
  9. Bluff body (diagram)
  10. The 3 T’s of combustion
    • •Sufficient Time
    • for complete chemical reaction. (reaction
    • time is often known as residence
    • time)

    • •Sufficient Temperature
    • to heat the fuel through decomposition to ignition.

    • •Sufficient Turbulence
    • to mix the oxygen and fuel completely. But turbulence additionally causes
    • pressure drop = losses.
  11. Why excess air?
    • To ensure complete combustion (we rarely
    • get a perfectly mixed system in reality).

    • To reduce the product temperatures (for
    • example gas turbine combustion – damage to turbine blades).

    • Excess air can be staged in order to give
    • a stable flame initially, with a cooler turbine inlet temperature.
  12. What is combustion?
    Combustion (or burning) is a complex sequence of chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames.
  13. Emissions Index
    Emissions Index (EI) is a measure of the quantity of pollutant produced per unit mass flow of fuel. For example, NOx can be expressed as g (NOx) per g of fuel combusted.

    EI is useful because it allows engineers to make comparisons between fuels and systems based on an equal comparison, i.e. per unit mass of fuel. Therefore the environmental performance different plants or engines can be compared directly.
  14. Particulate Matter
    Particulate Matter (PM) will occur over a range of sizes, so it is important to express the average size using accepted particle size statistics.

    • Engineers must always be cautious when inspecting size data, because there can
    • potentially be huge differences in the statistics between ‘per mass’ and ‘per
    • number’.

    • For example if a large number of comparatively small particles have an
    • average distribution, distributed closely around one mean size, this could be
    • altered if a small number of very large particles are added into the set. An
    • analogy exists in the teaching notes depicting a single apple being added to
    • 5000 grains of rice: based on number the set has hardly changed, but based on
    • mass it is now significantly different. Therefore it is sensible to present PM
    • data as distributions where possible.
  15. Adiabatic flame temperature
    Adiabatic flame temperature is the maximum achievable temperature with no heat loss, work transfer or dissociation with stoichiometric, complete combustion of the reactants.
  16. Why is adiabatic flame temperature never reached?
    Adiabatic flame temperature is never reached in practice because of 4 major factors, which are :

    •  
    • (1)   Incomplete mixing of fuel and air to give a perfect mixture (for diffusion flames)

    • (2)   Heat loss through the combustor walls and flame stabiliser via radiation, similarly there can be convective heat losses. 
    •  
    • (3)   Incomplete combustion, even if very small. This results in CO and unburnt hydrocarbons.

    (4)   Dissociation, especially at high  temperatures (normally >1500 ºC).

    • i.e. CO2 → CO + 1/2O2, and 2H2O → 2H2 + 1/2O2
    • This leads to reduced heat release.
  17. Diffusion flames
    • Usually the simplest form of flame.
    • Combustion takes place at the same time as mixing between the fuel and oxidant.
    • The rate of mixing of the fuel and oxidant therefore controls the rate of combustion.
    • This can be additionally dependent on the temperature of the combustion itself.
    • Clearly, the rate of turbulence has a significant effect on the rate of combustion.
  18. Flame heigh v nozzle velocity
  19. Mole fraction
    Relative proportion of the molecules or atoms belong to a component in a mixture
  20. Velocity of gas jet in diffusion flames
    • Below a critical value of the jet velocity the flow is laminar. So, mixing is by
    • molecular diffusion only. A thin, steady flame results.

    As the velocity increases, the flame elongates and the flame tip becomes unsteady, resulting in flutter.

    • Further nozzle velocity increases results in a noisy turbulent flame, which can be seen
    • at the point where transition from laminar to turbulent flow occurs. (The breakdown point).
  21. The breakdown point
    Where transition from laminar to turbulent flow occurs.
  22. Breakpoint length
    Distance to breakdown point
  23. What causes transition in flames?
    Laminar-turbulent transition dependent on Re.
  24. Laminar flame speeds
    Increase in pressure and temperature leads to an increase in flame speed
  25. Pros and cons of premixed flames
    Pros:

    –Less turbulence needed to mix the fuel and air.

    –Flames usually shorter and more intense than diffusion flames.

    –Temperatures are higher.

    –Improved burnout (i.e. less unburned amounts) of fuel.

    Cons:

    • –Can become unstable and prone to flashback, auto-ignition and non-steady
    • combustion, or worse acoustically-coupled combustion (known as “buzz”)
  26. Basic explanation of flame types
    Premixed flames tend to be cleaner, achieving better fuel burnout.

    Premixed flames are governed by fluid mechanics and can be prone to auto-ignition, flashback and instability.

    Diffusion flames are governed by the diffusion of the products. They tend to becontrolled by chemistry.Diffusion flames tend to be simpler to control, hence their combustion systems are also simpler.
  27. Diffusion flames
    –Flame height can be calculated.

    –Flame height is dependent on the fuel nozzle velocity.

    –Transition to turbulent flow will cause the flame to flicker and eventually blow out.
  28. Premixed flames
    • –Flame front location depends on the velocity of the premixed gas stream.
    • –Reactants are consumed as they approach the reaction zone.
    • –Unless the flame speed and gas speed are balanced, the flame front will move.
    • –Flame speed is also influenced by pressure, temperature and the molecular structure of the fuel being used.
  29. Turbulence uses
    • –Improves oxidant/fuel mixing.
    • –Can be used to stabilise flames via recirculation
  30. Turbulent combustion
    • •Turbulent burning velocities are often
    • many times greater than laminar.

    •This is due to the intensity of the mixing processes generated by turbulent flow.

    •Laminar – flame front moves due to diffusion only.

    •Turbulent – flame front is ‘helped’ by the mixing of the flow field.
  31. Diffusivity
    • “Tendency to become diffused; tendency,
    • as of heat, to become equalized by spreading through a conducting medium.”

    • i.e. rate at which something diffuses
    • (moves via diffusion) through a fluid medium.
  32. Turbulent burning velocity dependency?
    Turbulent burning velocity is independent of the scale of the turbulence.

    Seen when the graph of ST vs Re levels off.

    At high turbulent intensities or high inlet velocities, ST is roughly proportional to turbulent intensity and inlet velocity. i.e. ST is the result of the flow field plus turbulence.

    At high turbulent intensities or high inlet velocities, ST is independent of AFR.
  33. Turbulence theory
    • Experimental results support large scale turbulence theory at low flow velocities.–ST is calculated by laminar flame speed and turbulent intensity.
    • ST will therefore be dependent on AFR (since SL is dependent on AFR)
  34. Two elements of stabilisation
    •Flame propagation in flow regions with high velocity.

    •Aerodynamics of the wake.
  35. Stabilise
    To keep the flame alight for high mass flows of reactants and to avoid blow out from crossflows such as wind or random events.
  36. Bluff body
    • A bluff body creates a reverse flow zone in the path of the fluid that is to be
    • combusted.

    Flow velocity is reduced near a solid wall. This is an area where the flame speed has more chance of matching the local (low) flow velocity 

    This promotes stabilisation of the flame (keeping the flame alight)

    • The combustion system must not allow this reverse flow zone to be too large as it
    • will dilute the combustion process too much.
  37. Reverse flow zone
    • Recirculates heat and active chemical species to the root of the flame, promoting stabilisation.
    • This zone is referred to as a "well stirred" or "partially stirred" reactor because concentration and temperature levels are fairly uniform. 

    Reverse flow zones carry back the hot burnt products - this heats up and mixes the incoming species

    Too much recirculation will weaken the incoming mixture.
  38. Gas oil bluff body
    • With gas oil a hollow cone spray is used to inject fuel into the most intense region
    • of recirculation

  39. What are the three main sources of NOX from
    combustion systems?
    • •Thermal or Zeldovich
    • NOX.

    •Fuel NOX.

    •Prompt NOX
  40. Thermal NOX
    • In high temperature processes, such as metals, glass or cement manufacture.
    • Produced when molecular nitrogen (N2) and oxygen (O2) in the combustion air disassociate into their atomic states and participate in a series of reactions.
  41. Adiabatic flame temperature
    •The constant volume adiabatic flame temperature is the temperature that results from a complete combustion process that occurs without any work, heat transfer or changes in kinetic or potential energy.

    • •This is the maximum temperature that can be achieved for given reactants because any heat transfer from the reacting substances and/or any incomplete combustion
    • would tend to lower the temperature of the products. 
  42. Adiabatic flame (constant pressure)
    •The constant pressure adiabatic flame temperature is the temperature that results from a complete combustion process that occurs without any heat transfer or changes in kinetic or potential energy.

    •Its temperature is lower than the constant volume process because some of the energy is utilised to change the volume of the system (i.e. generate work).
  43. Non-adiabatic factors
    – Heat transfer through combustor walls, flame stabiliser, radiative and convection losses.

    – Incomplete combustion

    – Incomplete mixing of air and fuel (in diffusion flames).

    –Dissociation, especially at temps exceeding 1500°C. This reduces flame temps and smears the flame. The dissociated compounds then re-react in lower temperature zones.
  44. Dissociation
    • Dissociation in chemistry is a general process in which molecules separate or split into smaller molecules, ions, or radicals, usually in a reversible manner.

    • This process occurs in combustion systems when flame temperatures are high.
  45. Fuel NOX
    When there is a high nitrogen content bound into the fuel, such as in coal and many fuel oils. During combustion, the nitrogen bound in the fuel is released as a free radical and ultimately forms free N2, or NO. 
  46. Prompt NOX
    Occurs when there is some fuel rich combustion giving rise to HCN and similar compounds. These compounds then oxidise to produce NO. This occurs often in natural gas burners.
  47. Methods of NOX Reduction?
  48. Ultimate analysis
    •The composition of a fuel can be expressed as a chemical formula or as an ultimate analysis.

    •An ultimate analysis tells us the percentage of the common species in the fuel.
  49. Composition of photochemical smog
    In the 1950s a new type of smog, known as photochemical smog, was first described. This is a noxious mixture of air pollutants including the following:

    •        nitrogen oxides, such as nitrogen dioxide

    •        tropospheric ozone

    •        volatile organic compounds (VOCs)

    •        peroxyacyl nitrates (PAN)

    •        aldehydes (R'O)
  50. Effects seen from pollution that arises from combustion systems:
    –Acid rain

    –Photochemical smog

    –Ozone depletion

    –Climate change

    •These are mostly caused by particulates, SOx, NOx, VOCs, Halogen compounds, unburnt fuels, and CO2.
  51. Photochemical smog is reaction of..
    • Sunlight, nitrogen oxides (NOx) and volatile
    • organic compounds (VOC's) in the atmosphere, which leaves airborne particles
    • and ground-level ozone. 
  52. Smog health problems
    Smog is a problem in a number of cities and continues to harm human health.

    • Ground-level ozone is especially harmful for senior citizens, children, and people with
    • heart and lung conditions such as emphysema, bronchitis, and asthma.

    This problem can be made worse in areas surrounded by higher ground or conditions that encourage temperature inversion.
  53. What can reduce smog?
    •        low NOx technologies.

    •        particulate clean-up

    •        ozone reduction by oxidation or removal of NOx (reducing ozone precursors).
  54. Behaviour of a hydrocarbon jet flame operating in diffusion mode.
    When fuel gas discharges at velocities below a critical value from the nozzle into stagnant air surroundings flow is laminar. Therefore mixing of air/gas is by molecular diffusion only. 

    • The result is a thin long flame surface fixed
    • in space.  As nozzle velocity is increased the diffusion flame increases in length until a critical velocity is reached and tip of flame becomes unsteady and starts to flutter.

    Further increase in jet velocity produces unsteadiness, which develops into noisy turbulent brush of flame starting at a definite point along the flame where breakdown of laminar flow occurs. The distance to breakdown point is called breakpoint length.

    • Eventually the gas jet velocity will be too high to sustain the
    • combustion reaction, since the fuel is travelling faster than the reactants can
    • mix. This is known as ‘blow-out’.

    With some fuels flame length decreases slightly in fully developed turbulent region.  Beyond the transition region. There is virtually no effect on flame length of increase in velocity, except that flame noise increases and luminosity decreases.
  55. Effects of Reynolds number on the shape and characteristic of a diffusion flame?
    The higher the reynolds number the more likely it is to be in turbulent flow, and as diffusion flames are affected by turbulence
  56. NOX Reduction (EXPANDED)
    •Exhaust gas recirculation

    To reduce maximum temperature of the system, whilst keeping the oxygen concentration low. This can be applied to both stationary combustors and reciprocating engines.

    •Direct heat removal from the flame front to give a low flame temperature (<1200ºC). This is only really applicable to gas-fired systems.

    • •Air staging. This is when a fuel rich first stage is produced running with normally
    • only 70-80% of the required stoichiometric air. The resulting reducing atmosphere reduces any NOX to N2.
    • Some heat removal is normally made at this point. The rest of the combustion air is then added either in 1 or 2 stages to give complete fuel burnout. This is widely applied using a stratified charge in reciprocating engines, as well as stationary combustion systems.

    •Fuel staging. This is when the fuel is added sequentially into the system with some cooling between the stages so as to lower overall maximum temperatures.

    •Reburn. This is when the fuel is added at the end of a combustion process to form a fuel rich zone where NOX can be reduced to N2. This is being used as a retrofit application in many power stations. Extra air must be added at some time to burn the extra fuel.

    •Minimisation of residence time in high temperature regions. The formation rate of NOX is very dependent on time and its formation can be reduced by lowering the overall system residence time, i.e. not giving sufficient time for the NOX forming reactions to take place.
  57. NOx reduction (bullet points)
    • •Exhaust gas recirculation
    • •Direct heat removal from the flame front to give a low flame temperature (<1200ºC)
    • •Air staging
    • •Fuel staging
    • •Reburn
    • •Minimisation of residence time in high temperature regions.
  58. Exhaust gas recirculation
    To reduce maximum temperature of the system, whilst keeping the oxygen concentration low. This can be applied to both stationary combustors and reciprocating engines.
  59. Direct heat removal from the flame front 
    Direct heat removal from the flame front to give a low flame temperature (<1200ºC). This is only really applicable to gas-fired systems
  60. Air staging. 
    This is when a fuel rich first stage is produced running with normally only 70-80% of the required stoichiometric air. The resulting reducing atmosphere reduces any NOX to N2. Some heat removal is normally made at this point. The rest of the combustion air is then added either in 1 or 2 stages to give complete fuel burnout. This is widely applied using a stratified charge in reciprocating engines, as well as stationary combustion systems.
  61. Fuel staging
    This is when the fuel is added sequentially into the system with some cooling between the stages so as to lower overall maximum temperature
  62. Reburn
    This is when the fuel is added at the end of a combustion process to form a fuel rich zone where NOX can be reduced to N2. This is being used as a retrofit application in many power stations. Extra air must be added at some time to burn the extra fuel.
  63. Minimisation of residence time in high temperature regions
    The formation rate of NOX is very dependent on time and its formation can be reduced by lowering the overall system residence time, i.e. not giving sufficient time for the NOX forming reactions to take place.
  64. Main methodologies for pollution reduction 
    –Particulates reduction through post-combustion cleaning systems (e.g. filters)

    –SOx reduction through flue gas desulphurisation.

    • –NOx reduction through catalytic reduction,
    • premixed and staged combustion systems.

    –CO2 reduction through biomass fired combustion and sequestration.
  65. Combustion pollutant clean up
    •Combustion pollutants can be addressed either by changing the combustion system, the combustion conditions or cleaning the flue gas.

    •Always remember that pollution abatement technologies come at a (normally very high) financial cost and they will usually reduce overall energy generation efficiencies.

    •In some cases this leaves a paradox, because some cleanup systems will lower efficiency and therefore increase the amount of CO2 generated per unit of electricity.

What would you like to do?

Home > Flashcards > Print Preview