Chapter 4: Energy from Combustion

ENERGY?

Like the energy of a crowd, you can’t see it,can’t measure it,but you know it is there.

What do you think of when you hear the word

Complete Combustion – produces more energy:

fuel + oxygen → CO2 + H

2O + energy

Incomplete Combustion – not as efficient:

fuel + oxygen → CO + H2O + energy

Some energy is still stored in CO, so:

CO + oxygen → CO2 + H

2O + energy

Energy is the capacity to do work.

Work is movement against a force (w = f x d).

Terms that need to be defined:

Heat is energy that flows from a hotter to a colder object.

Temperature determines the direction of heat flow.

4.1

Heat is a consequence of motion at the molecular level; temperature is a measure of the average speed of that motion.

There are many different forms of Energy:

Potential – stored energyGravitational, electrical, coiled springs

Chemical – energy stored in chemical bonds and vibration/rotation of atomsNuclear – energy stored as matter or in binding subatomic particles togetherElectrical – energy in motion and electrical potential of electrons

Kinetic – energy of motion: Anything with mass that is moving, vibrating, or spinning

Mechanical energy – moving machineryThermal energy – motion of atoms

Temperature measures average kinetic energy of matter: at absolute zero there is no motion

4.2

What are units of heat?

The joule (J): 1 J is the amount of energy required to raise a 1-kg object 10 cm against the force of gravity.

The calorie (cal): 1 cal is the amount of heat required to raise the temperature of 1 g of water by 1 oC.

1 calorie = 4.184 J1 kcal = 1000 cal = 1 Cal (1 dietary cal)

So that 450 Cal doughnut is really 450,000 calories!

Conservation of energy

(First Law of Thermodynamics)

Energy may be converted from one form to another, but the total amount of energy remains constant.Energy is often measured in:

Joules (J)calories (cal): 1 cal = 4.18 JCalories (Cal) = food calories 1Cal = 1000 cal

Coal is used to create electricity in a power plant:

4.1

1. Combustion2. Boil water in a closed, high pressure system3. Turn turbine to create electricity

Coal Power PlantsConvert energy to various forms:

Potential (chemical energy in coal)Thermal (burning coal, heating water)Kinetic (moving steam through pipes)Mechanical (steam turns turbines)Electrical (turbines produce electricity)??? : homes and businesses turn electrical energy back into many other forms (light, heat, mechanical...)

4.1

Potential energy(chemical bonds)

Kinetic energy

Mechanical energy

Electrical energy

burner

gas turbine

generator

The First Law of Thermodynamics Energy is neither created nor destroyed, but may be transformed from one form

to another.

Taking random, thermal energy and transforming it into ordered work goes against the Second Law of Thermodynamics.

The Second Law of Thermodynamics The entropy of the universe is

increasing.

Power plants are inevitably inefficient.

There is no free lunch!

The stored chemical energy in fossil fuels (coal, oil, gas,...) and wood comes from photosynthesis.

Photosynthesis is basically the reverse of a combustion reaction:

6CO2 + 6H

2O + energy → C

6H

12O

6 + 6O

2

versus combustion:

C6H

12O

6 + 6O

2 → 6CO

2 + 6H

2O + energy

CO2 and H

2O have very little potential

chemical energy.

The suns energy is stored in the products: saccharides (sugars) and oxygen.

Sugar molecules link together to form other sugars, starches and cellulose.

With time, pressure, and heat, dead plant material is converted into fossil fuels.

Coal

• Coal is a complex mixture of substances.

• Although not a single compound, coal can

be approximated by the chemical formula

C135H96O9NS.

4.3

4.3

History of U.S. Energy Consumption by Source 1800–2008

1 EJ = 1018 J

4.3

Coal use around the world:

Green = 1999

Gold = 2009

The unit is million metric tons oil equivalent, the approximate energy released in burning a million metric tons of oil.

Efficiency of Energy Conversion

We are not able to completely convert energy from one form to anotherDuring the conversion, energy is often also changed into other formsMost if this energy is 'lost', since it is not in a useful form.Some examples of this are friction, heat, light, and kinetic energy lost to the environment

For coal power plants:

Each energy conversion step may be 5-90% efficient, depending on the technology used.

Overall, most coal power plants are 35-50% efficient, with most energy lost as heat.

Higher temperature steam more efficient

Efficiency=electrical energy producedheat from fuel

∗100%

For coal power plants:

Each energy conversion step may be 5-90% efficient, depending on the technology used.

Overall, most coal power plants are 35-50% efficient, with most energy lost as heat.

Higher temperature steam more efficient

Efficiency=electrical energy producedheat from fuel

∗100%

4.6

Energy Changes at the Molecular Level

The energy changes are due to the rearrangement of the atoms of the reactants and products; it is the breaking and forming of bonds that dictates if a reaction will be endothermic or exothermic.

Bond energy is the amount of energy that must be absorbed to break a chemical bond.

energy

Breaking bonds ALWAYS requires energy!

4.6

4.6

Consider: 2 H2 + O2 2 H2O

Bonds breaking2 H–H + O=O

Bonds forming4 O–H

(2 H–O–H)

Usually, reactions occur to lower the stored chemical (potential) energy.

For these reactions, the potential energy lost is usually released as heat and light.

Some reactions actually absorb thermal energy from the surroundings.

For this to happen, the overall Entropy must increase.

Entropy – measure of disorder or chaos:more chaos → more entropy.

Second Law of Thermodynamics – the entropy of the universe is increasing.

The entropy and energy change both determine whether or not a reaction occurs.

Examples of increasing entropy(increasing disorder):

solid → liquidliquid → gascold object → warm objectone compound → 2 compounds

Coal – a major driving force behind the industrial revolution.Much more energy per gram than woodMore abundant – not enough wood to meet energy needsThere are different grades of coal

Some produce more energy per gramSome are more pure carbon, while most contain many impurities, such as sulfur.

4.3

Not all coal is created equal:

Some problems with coal fuel:Mine safety

mine collapsedangerous gasesblack lung disease from breathing coal dust

Environmental damagestrip mining

Pollution from combustion:particulatesSO

x – lung irritant, leads to acid rain

CO2 production – in recent years coal has

accounted for up to 40% of anthropogenic CO

2 production

Some ways to reduce pollution:Coal washing – remove sulfur from coal before burningGasification – process used to turn coal into CO and H

2 gases, which is a much cleaner

fuel, though some potential energy is lostWet scrubbing – scrubs the exhaust gases to remove SO

x.

This waste can be used for commercial products, such as gypsum drywall.

Hydrocarbons (HCs) – any compound containing only hydrogen and carbon

Crude oil – a mixture of HCs, such as methane, ethane, propane, octane, kerosene, waxes...

This mixture is separated by distillation:

Smaller molecules turn into gases at lower temperatures, so each compound is boiled off at a different temperature.

4.4

A Port Arthur, TX

Oil Refinery

An Oil Refinery

4.4

One of the drawbacks to petroleum is that it must be refined before use.

Distillation Tower:

4.4

How do we use each barrel (42 gal) of petroleum?

This 7.3 gal includes nonrenewable feedstocks for all plastics, pharmaceuticals, fabrics and other carbon-based products.

Over 87% of each barrel is used for transportation and heating.

Heat of Combustion – energy released per mole or per gram of fuel.

Different HCs have different Heats of combustion, so some produce more energy per gram than others.

This energy comes from making/breaking chemical bonds.

The products have lower potential energy, and the lost energy is released as heat/light

4.5

Bomb calorimeters can be used to determine the heat of combustion.

If you test a reaction that releases heat, the temperature of the water will increase.

4.5

CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) + ENERGY

Hydrocarbon fuels like methane (CH4) burn in the presence of oxygen to produce carbon dioxide and water.

Energy is released in this process called combustion.

When energy is released during the course of a chemical reaction, it is said to be an EXOTHERMIC reaction.

The combustion of methane gas releases 50.1 kJ/g of CH4.This is the equivalent of 802.3 kJ/mol CH4.

4.5

CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l)

Exothermic reaction

The products are lower in energy than the reactants.Exothermic reaction: E is a negative value.

4.5

Are all fuels created equal?

Gasoline Compared to combustion of octane, C

8H

18

(Octane rating)Not a pure HC, but a mixture of HCs, isomers, and additives

Isomers – same formula, different structure

Crude oil does not have enough octane to be used for fuel, and with mixtures, the combustive properties can be adjusted

Gasoline Additives

Additives improve propertiescombustion rate (knocking)combustion efficiencyresidue left in enginepollution

Gasoline AdditivesTetra ethyl lead (TEL)

Used to be a common additivePotential problem for lead poisoning

can cause sterility, insanity, loss of teeth...children more susceptible than adults

Absorbed by inhalation of exhaust fumesData shows a significant drop in lead concentrations in children after TEL was banned

Gasoline AdditivesMTBE used to replace TEL

methyl tertiarybutyl ether (H, C, and O)improves octane rating provides oxygen for combustionpossible carcinogen

Ethanol often used to replace MTBEAlso contains only H, C and ORenewable energy sourceBut attracts water – can cause rusting if left for prolonged periods

4.7

Gasoline Additives

Elimination of octane enhancing tetraethyl lead (TEL) created a need to find substitutes.

H3CO

CCH3

CH3

CH3

CH

H

CO

H H

HH

MTBE, methyl tertiary-butyl ether

Ethanol (ethyl alcohol)

Human health effects of exposure to MTBE are not known.

Fuel Alternatives

Biodiesel fuel use is on the rise. Made from natural, renewable sources (vegetable oils, animal fats), it can be used as pure fuel or blended with petroleum.

Ethanol is renewable, but more expensive than gasoline.

• Some believe it takes more energy to produce a gallon of ethanol than you will obtain from burning it.

4.9

Additives may be used to raise/lower the activation energy of the combustion.

Activation energy – an energy barrier that needs to be overcome for the reaction to proceed.

The activation energy controls how easily the fuel ignites, and how quickly it burns.

Catalysts – something that lowers the activation energy (speeds up the reaction), but is not consumed.

The catalytic converter in automobiles is a hot metal (Pt) surface that:

catalyzes reactions in the exhaust gasesCO + O

2 → CO

2

NOx → N

2 + O

2

Energy released in this converter is 'lost' as heat

4.11

Historic and projected energy consumption worldwide:

Note: All three projections slope upward.