Section 5 - Combustion Theory Flashcards
(Quiz 2 Content)
Common H-C fuel groups
- paraffins (CnH2n+2 - methane, ethane, propane, etc.)
- Olefins (CnH2n)
- Acetylenes (CnH2n-2)
Main alcohols used in engine fuels
Ethanol, methanol, butanol
Crude oil contains ______ H-C compounds
25000
Purpose of refining?
Separate crude oil via distillation, then chemically process into fuels and other products. Still is used to heat sample, so boiling off lighter components are condensed + recovered
Fractions
Group of compounds that boil off between two temps
Order of fractions as they leave the still
Naptha > distillate > gas oil > residual oil (subdivided using light, middle, heavy)
T/F: Light virgin naptha cannot be used as gasoline
F: opposite is true
Gasoline is a blend of ____ distillates with a range of bpts between ___ and ___ºC
H-C
25-225ºC
What molecule is often used to simulate the properties of gasoline?
Octane
There are ___ isomers of octane
18
Requirement of the Clean Air Act Amendment (1990) by the U.S.
U.S. required to use reformulated gas (RFG) year-round IOT reduce ozone by requiring a minimum O2 content of 2% by weight and max benzene content of 1%
As part of the reformulated gasoline program, sulfur is restricted to ___ ppm.
31
Sources for diesel fuel
Petroleum diesel
Synthetic diesel
Biodiesel
List reasons why alternative fuels have become very attractive
- Occasional high cost of oil
- Accessibility to oil reserves
- Environmental concerns such as global warming
Why are alcohols recieving a lot of attention?
Can be synthesized biologically
T/F: Since there is O2 in fuel, combustion of alcohols produces more CO than fossil fuel combustion, but less CO2.
F: produces more CO2, less CO
Drawbacks of alcohol vs. fossil fuels (6)
- Lower E density than gas
- Corrosive to fuel systems (meth > eth > but)
- Poor cold temp start up due to low vapour P
- Toxicity (meth)
- Use of food crops drives up world prices of food
- Production of eth is very E intensive and expensive
Other alternative fuels
Liquid petroleum gas (LPG) (mainly used on fork lifts and golf carts, mix of propane and butane, stored as liquid in steel tanks)
Dimethyl ether: good diesel fuel bc good auto-ignition quality
Gaseous alternative fuels
Hydrogen
Natural gas
Producer gas
Drawback of gaseous alternative fuels
Gaseous state at 25ºC and thus have low E density. Engines that use them have low volumetric efficiencies
Issues with H2 as a gaseous fuel?
- Safety (very reactive and volatile, only need 5% mixed with air and you can ignite)
- Lack of distribution structure (how would we transport H2 if it’s unsafe?)
- Production E (not found in nature but produced from natural gas or electrolysis)
Features of natural gas as a gaseous fuel alternative (5)
- Cleaner burning than gas
- Lower E content than LPG
- Used for power gen gas turbines and large SI engines
- Low soot, NOx if burnt lean
- Can’t be compression-ignited
For combustion, dry air is taken to be composed of ___ O2 and ___ N2 by volume
21%
79%
For every mol of O2, there are ___ moles N2
3.76
Amount of water in moist air at temp T is specified by the _______ and _______.
Specific humidity (omega = m_w/m_air)
Relative humidity (Phi = P_w/P_sat(T))
Chemical equation for complete combustion (sufficient O2 available)
[FUEL] + aO2 –> bCO2 + cH2O
T/F: If products are at “low” temperature, N2 is not significantly affected by the reaction and is considered inert.
T
Complete rxn equation for a general H-C fuel with air is:
[FUEL] + a(O2 + 3.76N2) –> bCO2 + cH2O + dN2
Fuel lean
Excess air for combustion
Combustion rxn equation with excess air:
[CaHb] + gamma(a+b/4)(O2 + 3.76N2) –> aCO2 + b/2 H2O + dN2 + eO2
Combustion rxn equation with fuel-rich mixture:
[CaHb] + gamma(a+b/4)(O2 + 3.76N2) –> aCO2 + b/2 H2O + dN2 + eCO + fH2
Equivalence ratio (phi)
Indicates if mix is stoichiometric, fuel-lean, or fuel-rich
phi = (A/F)s/(A/F)mix
OR
phi = (F/A)mix/(F/A)s
Conditions for equiv ratio
Stoichiometric: phi = 1
Fuel lean: phi < 1
Fuel rich: phi > 1
What is gamma in the rxn equation?
1 / phi
T/F: 110% stoichiometric air is equal to 110% excess air
F: 110% stoic = 110% theor = 10% excess
Heat of formation
Enthalpy/E required to form compounds from their elements at STP (T = 298K P = 1 atm)
Sensible enthalpy
Change in heat/E of a substance that we can feel
Heat of combustion
Max amnt of E released from fuel when reacted with a stoichiometric amount of air (and all H2 and C in fuel is converted to CO2 and H2O)
T/F: The heat of combustion stays the same regardless of water being vapour or liquid
F: 2 diff values for heat of combustion (H_R) based on if water is vapor or liquid
Higher heat of combustion/heating value (HHV)
Water in products is liquid (more heat is removed from combustion)
Lower heat of combustion/heating value (LHV)
Water in products is vapour
Difference between HHV and LHV
Heat of vaporization
What is one of the biggest problems with using H2 as a fuel wrt heating values?
A/F ratio must be immensely high. For every gram of H2, 34g of air must be added. Air doesn’t have a high energy content at all
Adiabatic Flame Temperature (AFT)
Final products temperature (for either cst V or P process)
T/F: Hand calculations can be used to solve balanced equations regardless of the conditions
F: At high temps, minor species will be present due to dissociation of major species, so computer equilibrium solvers can be used
Chain branching
Chain branching rxns lead to rapid production of radicals which causes the overall rxn to proceed extremely fast (explosively)
Chain termination rxns
Radicals come together to form final products
Explosion limits
Critical initial T above which an explosion occurs (no spark ignition tho)
Critical T is also referred to as autoignition T
Rapid compression machine
Piston-cylinder assembly to estimate explosion limits by raising P and T of F-A mix very quickly to predetermined P2 and T2
A flame
A thin region in space where chemical rxns convert F-A mix into combustion productions
At a given P, T and phi, flame has 2 basic proprties
- AFT or T_ad
- Laminar burning velocity (S_l)
What is burning velocity affected by? (3)
- Equivalence ratio
- Initial gas T and P
- AFT
T/F: Turbulent flame velocity is not a property of the gas but depends on the details of the flow
T
Turbulent burning velocity depends on…
Turbulent intensity
T/F: In a channel of width d, the flame can propagate in a channel as long as the tube diameter is larger than the flame thickness
T
Flammability limit
The energy at which a spark-ignited flammable mixture will ignite
What affects flammability limit? (2)
- Mixture
- Initial P and T
