Gaseous FuelsGaseous Fuels:

Benefits: No ash. SO2 easy to remove from fuel. Easy to achieve to achieve premixed combustion (low NOx, low soot)

Drawbacks: Difficult to store and transport.Natural gas:

High energy content per CO2. Sources: NG wells, LNG from overseas, Coalbed methane,

Shale gas. It is natural so varied composition. Vs Coal: less C, S, N, metals, ash per MJ. Vs Gasoline: Octane # of 120. Vs Diesel: Cetane # of 0. Transportation: Heavy/Light duty trucks, rail marine.

Syngas: Syngas: Gasification is a thermo-chemical process that

converts carbon containing feedstocks to:1) Feedstock + O2 → CO2 and H2O. 2) C + CO2 → 2CO 3) C + H2O → H2 + CO

Upgrading: Water Shift Reaction: 1) CO+H2O ↔ CO2 + H2 Particulate, tar, CO2 and SO2 Removal Catalytic Methanation.2) 3H2 + CO → CH4 + H2O Water Removal Result: High Energy Syn-NG

Syngas via Biomass Gasification:•Biomass is partially combusted (fuel rich, =3-5) φresulting in: •Low energy syngas (compared to NG) • N2, CO2, and H2O •Heat •Ash.Nexterra’s Gasification Technology at Kamloops BC: • Biomass is fed to base of fuel pile • Air is fed from the base • Air moves up, fuel moves down • Syngas exits at 260-371C

Syngas via coal gasification: Similar to biomass gasification: Used to reduce pollutants from coal (esp. ash, SO2, Hg, CO2, NOx) Integrated Gasification Combined Cycle • Gasification • Gas Cleaning • Combined Gas Turbine – Steam Cycles • Carbon Sequestration?

PlascoSyngas: Plasco (Ottawa) converts garbage (MSW) to electrical energy via syngas • The MSW waste stream is converted into a crude syngas using recycled heat. • Plasma torches refine the crude gas into a cleaner syngas. • Removal of sulphur, acid gases and heavy metals • Syngas is used to fuel internal combustion engines that efficiently generate electricity.

The plaso conversion system recovers 1.2MWH electricity, 300L potable quality water, 5-10kg commercial salt, 150 kg construction aggregate and 5kg agricultural fertilizer from every tonne of waste converted.

Solid Fuels Solid Fuels:

Benefits: High Density, relatively easy to store and transport, cheap (almost no processing).

Drawbacks: Many pollutants in fuel (sulfur, nitrogen, ash), difficult to remove pollutants from fuel, non-premixed combustion (high NOx, smoke).

Trend to more processing (Syngas, “Clean Coal”)Coal:

Benefits: High energy density, plentiful, relatively inexpensive. Greater fossil fuel reserve to production ratio than oil and natural gas.

Types of coal: 1. Low rank coals 47%:- lignite 17% Power Generation - Sub-bituminous 30% Power Generation, Cement

Manufacture, Industrial uses. (*)2. Hard coal 53% :- Bituminous 52%

Thermal-Steam Coal (*) Metallurgical-Coking Coal: Manufacture of iron and

steel.- Anthracite 1%: Domestic/Industrial including

smokeless fuel. Disadvantages: High CO2 emissions • “Coal-fired power

plants are responsible for 40 percent of all U.S. carbon dioxide emissions” Reuters, 2007. • Sulfur content results in SO2 • Nitrogen content results in NOx • Mercury • Radioactive compounds in fly ash • Ash • Difficult to remove pollutants from fuel.

Solid Biomass Fuels: Forestry Biomass e.g.: Mill Waste, Harvest Residues. Agricultural Biomass: Energy crops (switch grass),

agricultural waste (Corn Stover). Benefits: Renewable, Low net Greenhouse Gas Emissions,

Supports Rural Economy. Drawbacks: Dispersed supply far from markets, Existing

users, Impact on ecosystems, Low energy density, Expensive, High water content, Ash.

Challenges: Low Density: Solution: Pellets (Twice the density.)

Moisture Content: Solution: Drying (Via Solar, Via Biomass, Via Fossil Fuel). Drying helps reduce moisture to below 10% compared to 20 to 60%.

Torrefaction: Torrefaction removes moisture and volatiles from biomass, leaving bio-coal. Added value of torrefied biomass via • Higher energy density • More homogeneous composition • Hydrophobic behavior • Elimination of biological activity •Improved grindability.

Municipal Solid Waste: Benefits: Renewable, Uses waste, Source close to energy

users. Drawbacks: Variable fuel content, many potential pollutants

(Sulfur, Nitrogen, Mercury, Radioactive, Ash, Difficult to remove pollutants from fuel), High water content, Low energy density, Chlorine (dioxins), Metals, Odor.

Stoichiometry for Practical Fuels, Struct.Chemical Compounds and Structures: Alkanes (CnH2n+2):

Normal isomers (e.g. butane). (1o) 2+ (2o) 6 (less primary bonds, less knocking/catching fire under high pressure).

Branched isomers (isooctane, 2, 2, 4-trimethyl

pentane. More stable. (1o)5+(2o)1+(3o)1+(4o)1. More primary (C-C) bonds.

Alkenes (CnH2n): Ethylene (C2H4) Alkynes (CnH2n-2): Acetylene (C2H2) Aromatic Hydrocarbons: Ring

structure. Very stable. Soot precursor. E.g. Benzene.

Poly-Aromatic Hydrocarbons: e.g. Pyrene (C16H10)

Alcohols: -OH group. Esters: -O-C=O group. E.g. Methyl Oleate. Ethers: E.g. Dimethyl Ether: H3C-O-CH3, Methyl Tert-butyl

ether: (H3C)3-C-O-CH3.

Conservation of MassCombustion: exothermic process where fuels are rapidly oxidized.Flame: Premixed Flame: The thin zone propagates through a fuel-

oxidizer mixture. Non-premixed (diffusion) flame: Fuel and oxidizer, initially

separate, diffuse into a common region and sustain combustion, and combustion products diffuse out of that zone.

Partial Pressure, mole fraction, mass fraction equations:

Equivalence Ratio, Excess Air:

(50% or more EA used to ensure complete combustion and no CO/UHC emissions)Adiabatic Conditions: W=0, Q=0. Therefore, HR=HP.Heating Values of Hydrocarbon Fuels: The heating value of a fuel is the difference between the enthalpy of the reactants and the enthalpy of the products when the fuel burns completely with air, reactants and products being at the same temperature T and pressure P. This experiment is done in a calorimeter. The higher heating value (HHV) is obtained when all the

water formed by combustion is a liquid. ►The lower heating value (LHV) is obtained when all the water formed by combustion is a vapor. ►The higher heating value exceeds the lower heating value by the energy that would be released were all water in the products condensed to liquid ( HΔ vap WATER = 44.01 kJ/mol).

Heat of Combustion: • Consider a quantity of fuel and air at TO (298K) • React and cool products to TO • Measure heat release (q) •If work, kinetic energy and potential energy changes are negligible, then the equation.Adiabatic Flame Temperature Equation:

Equilibrium Gibbs free energy: At equilibrium, the Gibbs function, G, is at a minimum for an isothermal, isobaric fixed-mass system. Equilibrium Gibbs function change :

Factors affecting equilibrium: Pressure effect on equilibrium: Higher pressure will shift

equilibrium to side with lesser moles, hence reducing dissociation. This can be proved via examining the following

equation: For exothermic reactions, as temperature increases,

equilibrium favors left hand side and vice-versa. This can also be examined by considering changes in T in the equation above.

Dissociation rules of thumb: Dissociation of species must be taken into account to accurately calculate the correct Adiabatic Flame Temperature.

For T>2200K Add the species CO, H2 and OH 1) CO2 CO + ½ O2 ⇔2) 2 OH H2O + ½ O2 ⇔3) H2O H2 + ½ O2 ⇔

For T>2400K: Add the species H and O 1) H2 2 H ⇔2) O2 2 O⇔

For T>3000K 1) Add the specie NO 2) NO ½ N2 + ½ O2⇔3) NO important as a pollutant above 1700K

For T>3500K 1) Add the specie N 2) N2 2N CO and ⇔

H2 will be present at any temperature for fuel rich combustion.

Chemical KineticsGlobal Reaction Rate:

m+n denotes overall reaction order.

Global reactions are only accurate close to conditions used to determine their constants.

Global Reaction: A single reaction containing initial reactants and final products, representing an entire combustion process. e.g. 2H2 + O2 → 2H2O

Elementary Reaction: A chemical reaction in which species react directly to form products in a single reaction step and with a single transition state. e.g. H + O2 → OH + O

Kinetic theory equations: Assumes molecules react instantaneously when they collide if their kinetic energy >> the energy barrier.

Chemical Time Scales:

Explosive Characteristics of Fuels: A characteristic of combustion is that chemical reactions are fast thus explosive. An example is the

auto-ignition of fuel in an HCCI (homogeneous charge compression ignition) engine. Overall Reaction: H2+ ½ O2 → H2O Chain Initiation: This step produces relatively few radicals

yet gets the chain reaction going. Does not control overall explosive behavior.

Chain Branching: Chain branching provides exponential growth in the radical population which is essential to oxidize the fuel.

These steps provide the rapid growth in the radicals.

Radicals are highly reactive. Radicals consume the fuel. Chain branching controls explosive behavior and

produces the very fast reaction times Chain Propagation: H2 + OH → H2O + H

Produces product without consuming radicals. Chain Termination: These eliminate radicals and prevent

explosive behavior.

Third body reactions: Trimolecular reactions: Often use a third body M to remove

energy from the transition state and stabilize the molecule. E.g. A + B + M → C + M

Reverse Reaction Equations:

At equilibrium:

From before:

As xi P = [ ]i RT:

Combining equilibrium equation and above:

Types of reactions: Homogeneous Reaction: A reaction that occurs with gaseous

participants only. For example, auto ignition inside a rapid compression engine.

Heterogeneous Reaction: Involves the reaction of a gaseous species with a solid surface or large object such as a catalytic filter. For example, pollutant reduction occurring in a car’s catalytic converter.

Reaction Mechanisms: Reaction pathway for methane:

CH4 + OH → CH3 + H2O CH3 + O → CH2O + H CH2O + OH → HCO + H2O HCO + M → H + CO + M CO + OH → CO2 + H

Alkane oxidation: Three Main Steps:

1. Fuel → C2H4, C3H6, CH4, radicals 2. C2H4, C3H6, CH4, and others → CO, H2 3. CO → CO2 , H2 → H2O

Chain Initiation: RH → R + H where RH is an alkane This provides the first radicals (i.e. H)

H Abstraction: RH + OH → R + H2O RH + X → R + XH where X is any radical For alkane: CnH2n+2 + X → CnH2n+1 + XH

Scission: β The breaking up of a hydrocarbon during combustion based on where a radical site may be.

The molecule will sever bonds on the carbon atom one

away from the radical: C-C bonds weaker than C-H bonds. CnH2n+1 → C2H4 + Cn-2H2n-3. As Cn-2H2n-3 same as CmH2m+1, the cycle repeats. More C2H4 (ethylene) formed that any other species. Original alkane not that important as ethylene quickly

forms. Conversion to C2H4 occurs without significant

temperature increase.

CnH2n+1 → C3H6 + Cn-3H2n-5. As Cn-3H2n-5 same as CmH2m+1, the first cycle repeats. Thus less C3H6 formed than C2H4.

CO Oxidation: CO + OH → CO2 + H. Often rate limiting reaction for hydrocarbon oxidation. CO oxidation inhibited by the presence of hydrocarbons

due to the competition for OH. Peak temperature occurs as CO oxidized • Releases 2/3

of fuel energy. Fuel begins to break down to form UHCs. UHCs will start to become oxidized into CO CO will become oxidized into CO2 Radicals will favor UHC oxidation over CO oxidation.

Therefore, CO will not be destroyed until UHCs are burned out.

Thermal Oxidation: Convert Hazardous Volatile Organic Compounds (VOC) to

non-hazardous gases. E.g. C → CO2, H → H2O, S → SO2, Cl → HCl.

High temperatures used to convert VOCs to CO2. Reactions occur in the gas phase.

Typically the VOCs are very dilute in air thus little heat rise. Advantages: • Wide range of compounds can be removed •

Very high destruction efficiency Disadvantages: • High energy consumption • Other

pollutants can form (i.e.NOx) A high Destruction and Removal Efficiency (DRE) is

achieved by increasing the 3Ts: 1. Time (added length) 2. Temperature (added fuel) 3. Turbulent Mixing with Oxygen (added air)

T99: Temperature needed to destroy 99% of a compound in 1 second.

PFR: Steady state, steady flow. No mixing in the axial direction (i.e. diffusion << convection). Uniform velocity profile (1D flow). Frictionless flow. Ideal gas behavior.

NOX EmissionsGround Level Ozone:

Characteristics: A colourless gas with a strong smell. Major component of summer smog.

Effects: It causes lung irritation, difficulty in breathing and damage to crops, forests and vegetation.

Sources: Ozone is not directly emitted into the atmosphere. It is produced photochemically from:

Volatile Organic Compounds (VOC) which are emitted by evaporation and combustion.

Nitrogen Oxides (NOx) which are emitted by combustion.

Greatest level of ground level is at noon when NO2 forms NO and O in the presence of light. This O atom reacts with O2 to form O3 (Ozone).

Nitrogen oxides: Source (U.S. data): • Combustion engines for transportation

(40%) • Stationary combustion mostly from electric utilities (56%).

Composition: • Most important nitrogen oxide is NO but NO2 and N2O also can be present.

NOx Formation:1. Thermal NO (N2 from air) 2. Fuel NO (N from fuel) 3. Prompt NO (N2 reacting with HCs)

Thermal NO:

Forms at high temperatures during combustion (global reaction): 1/2 N2 + 1/2 O2 = NO

Zeldovich Mechanism (detailed reactions):1. N2+O ↔ NO + N 2. N + O2 ↔ NO + O 3. N + OH ↔ NO + H

Reaction (1) is the rate limiting step. It is very temperature sensitive due to its: 1. High activation energy Rfr = 1.8 ×1011 * exp (−38,370/T) [N2] [O] in kmol/m3/s 2. Strong temperature dependence of [O]

Thermal NO formation is kinetically controlled. Slow rate of formation. Slower than hydrocarbon combustion. Equilibrium values generally not reached.

Control: 1. Reduce Peak Temperature. Exhaust Gas

Recirculation (or Flue Gas Recirculation). Lowers temperature in cylinder and thus NOx but also reduces output power.

2. Reduce time. Quick quench.3. Reduce oxygen concentration at highest

temperature zone. Low excess air.4. Reduce nitrogen concentration. Oxy-fuel burners.

Prompt NOx: N≡N bond scission by flame radicals. Occurs only in flame fronts.

Fuel NOx: Forms from nitrogen in fuel during combustion: CxHyNz + mO2 → NO + …. Heavy oil contains 0.2-1.2% N and coal contains 1.2-1.6%

N. Mechanism:

1. Fuel N → HCN 2. Then, NO formation similar to Prompt NOx

These reactions are fast Typical conversion: 40-70% Fuel NO formation is controlled by stoichiometry NO formation is weakly dependent on temperature Under fuel rich conditions, conversion of fuel nitrogen to

N2 is favored: N + NO N2+O Control: Staged combustion. Operate under fuel rich

conditions to convert fuel N to N2 then burn off CO and hydrocarbons.

Staging Methods: 1. Overfire Air. Separate air injection. NOx Reduction

by 40 to 60 percent.2. Low NOx burners. Staging within burner. NOx

Reduction of 30 to 55 percent Catalytic Converter:

Application: Gasoline (SI) engines Engine Modifications: fuel air ratio controlled to near

stoichiometric (equivalence ratio of approx. 1), need oxygen sensor, computer control & air/fuel control Fuel Change

Lead and sulfur eliminated Problems Poor performance at low startup temperature

Selective Catalytic Reduction: NO converted to N2 by NH3 (ammonia) and a platinum

catalyst in the presence of oxygen: 4 NO + 4 NH3 +O2 → 4 N2 + 6 H2O T from 500-800K Over 90% removal can be achieved Catalysts can become fouled by aluminum sulfates from

fuel sulfur. Applications:

Gasoline (S.I.) engines: Conditions: Premixed Combustion, =1, high pressure.φNOx formation. Mostly thermal NOx with equilibrium O. Some prompt NO and thermal NOx with super equilibrium O. NOx Control: EGR (5-15%), Catalytic converter.

Stationary Gas Turbine Engines: Conditions: Premixed Combustion, <1, high pressure.φNOx formation: At =0.7, mostly thermal NOx with φsuper-equilibrium ONOx Control: Lean burn

Aviation Gas Turbine Engines:Conditions: Non-Premixed Combustion, <1, high φpressure.NOx formation: Mostly thermal NOx with equilibrium O.NOx Control: RQL: Rich Burn, Quick Quench, Lean Burn.

Furnaces and Boilers:Conditions: Non-premixed Combustion, overall <1, φatmospheric pressure.NOx Formation: Thermal NOx with equilibrium O, Fuel NOxNOx Control: EGR (Low excess air, Reburn),SCR Staged Combustion (low NOx burners)

Diesel Engines: Conditions: Non-premixed Combustion, overall <1, high φpressure.NOx Formation: Mostly thermal NOx with equilibrium O.NOx Control: EGR (up to 50%), SCR.