Card Set Information
Energy Conversion Systems
Energy Conversion Systems - Mechanical Engineering
What effects the finite rate of combustion?
1) Fluid mechanics (mixing)
2) Heat transfer
3) Chemical reaction rates
What are the three types of combustible fuels?
1) Gas combustion (Easy)
2) Liquid combustion (Fuel must vaporize first)
3) Solid combustion (Determined by carbon combustion)
What are the requirements for combustion (3Ts)?
2) Temperature (T > Tign)
3) Turbulence (mixing)
What are the components of solid fuel?
What are the steps of solid fuel combustion?
1) Drying and heating (3s)
2) Volatile release (12s)
3) Ignition (25s)
4) Secondary fragmentation (100s)
What are the two types of volatilzation?
1) Volatile burns close to the surface (O2 is readily available, and VM release is moderate).
2) Volatile burns away from the surface (O2 is limited, and VM release is fast).
What is the char burning process and equation?
1) O2 diffuses to the surface of the particle.
2) The O2 reacts with C to form CO or CO2
q = Cg/(1/Hm +1/Rc)
Hm is the mass transfer coefficient
Rc is the Arrhenius reaction rate.
What is the mechanism factor
Phi is a term in the mass transfer coefficient which is different depending on if CO or CO2 is the main product of the combustion reaction.
Phi = 1 when we have primarily CO2 (Low Temp)
Phi = 2 when we have primarily CO (High Temp)
What are the two types of ignition?
1) Forced ignition (uses an external source of energy).
2) Self ignition (reactants are raised to there Tign).
Give three types of industrial combustors.
1) Pulverized coal (Fine suspension) ~1300C
2) Stoaker fired (Fixed grate) ~1100C
3) Fluidized bed (Semi-fluid) ~850C
What are some benefits to fluidized bed boilers?
1) Highly fuel flexible (Large thermal mass).
2) Low acid gas emissions (mixing allows fir low temperature combustion).
What are the two types of flame groupings?
1) Named by mixing
Premixed flame - Mixed before ignition(Blue, intense)
Diffusion flame - Fuel is ignited and air mixes with it as it burns (Orange, Less intense)
2) Name by fluid flow
Laminar - Longer flame
Turbulent - Shorter flame
What are the major characteristics and issues with flames?
1) Burning velocity
2) Flammability limit
3) Flame quenching
4) Flame stabilization
5) Adiabatic flame temperature
What does the flame velocity depend on?
Air fuel ratio (Highest close to stochiometric ratio)
Inert gas (N2) amount
Fluid dynamics of flow
What effects the flammability limit?
1) Type of fuel
LFL = 1/sum(yi/LFLi) - yi is mol fraction
As T Up, LFL down, UFL up
What are the two aspects of flame quenching
1) Hot gases and flames lose heat if they come in contact with a cooler surface.
2) Free radicals tend to diffuse towards the surface of solid walls.
Name two ways to get flame stabilization.
1) Opposed jet systems
2) Flame stabilization rings
What is the adiabatic flame temperature equation?
HrTr = HpTp
Tp ~ Tr + fHc/Cp
Hc - heat of combustion of fuel
Cp - specific heat of the products
f - mass ratio of products and reactants
Why is the actual flame temperature lower than adiabatic?
1) Combustion is not instantaneous and heat is lost to surroundings.
2) Above 1650C CO2 and H2O dissociate absorbing heat.
What are the types of air pollutants?
1) GHGs (CO2, N2O CH4, CFC, HFC)
2) Acid Rain (SO2, NO, NO2)
3) Others (Hg, Pb, Cd)
Briefly describe smog, VOC, smoke, haze
Smog - Ozone that forms through reaction of NOx and VOC with other chemicals, especially in sunlight.
VOC - Unburnt hydrocarbons.
Smoke - Solid + Liquid + Gas (Near source).
Haze - Far away from source.
Give three ways which NOx forms
1) Thermal NOx - Oxidation of atmospheric N2 at high temperatures (T > 1000C)
N2 + O2 -> 2NO
2) Prompt NOx - Reaction with hydrocarbon radicals (T > 1000C)
3) Fuel Nitrogen - N + O -> NO2 at all temperatures
Give 5 (3 and 2) ways to control SO2 emissions.
Low surfer fuel
Desulferize the fuel (coal cleaning)
Capture SO2 in combustion using sorbents
Ca/S = 1.5, produces high quality CaSO4 (gypsom), disposal of wet slurry is an issue.
Contains untreated CaO, Ca/S =2.5
Give 5 (3 and 2) ways to control NOx emissions.
(1) Fluidized combustion --best
(2) Low NOx burners with staged air supply (40 -60%)
(3) Flue gas recirculator
Selective catalytic reactor (SCR)
>90%, uses amonium and catalysts (Ti - oxides)
Selective non-catalytic reactor (SNCR)
Cheaper alternative, not as effective
Give 4 ways to control particulates post combustion.
(1) Fabric filter (bag house) --best
(2) Electrostatic precipitator
(4) Inertia separator