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)?
- 1) Time
- 2) Temperature (T > Tign)
- 3) Turbulence (mixing)
What are the components of solid fuel?
- 1) Ash
- 2) Moisture
- 3) Volatiles
- 4) Char
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?
- Fuel type
- Air fuel ratio (Highest close to stochiometric ratio)
- Inert gas (N2) amount
- Ambient pressure
- Fluid dynamics of flow
What effects the flammability limit?
- 1) Type of fuel
- LFL = 1/sum(yi/LFLi) - yi is mol fraction
- 2) Temperature
- 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)
- 4) Particulates
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.
- In Situ
- Low surfer fuel
- Desulferize the fuel (coal cleaning)
- Capture SO2 in combustion using sorbents
- Post Combustion
- Wet scrubbing
- Ca/S = 1.5, produces high quality CaSO4 (gypsom), disposal of wet slurry is an issue.
- Dry scrubbing
- Contains untreated CaO, Ca/S =2.5
Give 5 (3 and 2) ways to control NOx emissions.
- In Situ
- (1) Fluidized combustion --best
- (2) Low NOx burners with staged air supply (40 -60%)
- (3) Flue gas recirculator
- Post Combustion
- 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
- (3) Cyclone
- (4) Inertia separator