Chapter 5 Energy from Combustion What are fuels? How is electricity generated? What are the positive and negative consequences of burning carbon-based fuels? How much heat is released when fuels are burned? What are biofuels, and what are the benefits of using them as a transportation fuel?

Combustion There are three requirements to generate a fire: a source of heat, a fuel, and an oxidizer: Fuel + Oxidizer ℎ𝑒𝑎𝑡 Products The majority of fuels are hydrocarbons (compounds made up only of hydrogen and carbon atoms)

Hydrocarbons Carbon forms four bonds in hydrocarbon molecules. A hydrocarbon can be represented many different ways. Shown here are structural formulas and condensed structural formulas that are used to convey the bonding arrangement of carbons and hydrogens in a butane (C4H10) molecule.

Hydrocarbon Combustion A combustion reaction features a fuel and oxygen as reactants (left side of the arrow) and carbon dioxide and water as products (right side of the arrow). For instance, the reaction for the combustion of butane is: 2 C4H10(g) + 13 O2(g) → 8 CO2(g) + 10 H2O(g) Note the proper balancing of this equation, which has 8 carbons, 20 hydrogens, and 26 oxygens on either side of the equation. The actual combustion of a fuel is never this simple. Fuels contain a variety of different hydrocarbons, as well as other compounds of sulfur. In addition, at high temperatures, nitrogen from the atmosphere also reacts with oxygen to produce a variety of nitrogen oxide pollutants.

What Is ‘Energy’? Some important definitions: Energy is the capacity to do work. There are two primary types of energy: Potential energy is energy due to position or composition. Kinetic energy is energy due to movement. Work is movement against a force: work = force  distance

Heat vs. Temperature Heat is energy that flows from a hotter to a colder object. Temperature is a measure of the average kinetic energy of the atoms and/or molecules in a substance. Note: Heat is a consequence of motion at the molecular level, whereas temperature is a measure of the average speed of that motion.

Units of Heat 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 calorie is the amount of heat required to raise the temperature of 1 g of water by 1 C. 1 calorie = 4.184 J 1 kilocalories (kcal) = 1000 calories (cal) = 1 Cal (1 dietary calorie)

A Contextual Comparison of Energies

Measuring Energy Changes: Calorimetry A calorimeter is used to measure the quantity of heat energy released in a combustion reaction. The heat of combustion is the quantity of heat that is given off when a specified amount of a substance burns in oxygen. Heats of combustion are reported in units of energy (kilojoules or kilocalories) per mole or gram.

Exothermic Reactions Burning a fuel can be compared to water flowing from the top of a waterfall. Both undergo a conversion from potential to kinetic energy. When energy is released during the course of a chemical reaction, it is said to be an EXOTHERMIC reaction. The combustion of methane (CH4) gas releases 50.1 kJ/g of energy. This is the equivalent of 802.3 kJ/mol: 802.3 kJ 1 mol CH4 × 1 mol CH4 16.0 g CH4 =50.1 kJ/g CH4

Not All Fuels Are Equal! Due to differences in their chemical composition, the combustion of different fuels will release different amounts of heat energy. The fuels with the highest heats of combustion are hydrocarbons. As the ratio of hydrogen-to-carbon decreases, the heat of combustion decreases. As the amount of oxygen in the fuel increases, the heat of combustion decreases.

Endothermic vs. Exothermic Not all reactions are exothermic; some reactions absorb energy, such as photosynthesis. These are known as endothermic processes. Endothermic: the heat added to reactants is greater than the heat evolved by formation of products Exothermic: the heat added to reactants is less than the heat evolved with products

Energy Changes at the Molecular Level The energy changes are due to the rearrangement of the atoms of the reactants and products. Breaking bonds requires energy, whereas forming bonds releases energy. The breaking and forming of bonds dictates if a reaction will be endothermic or exothermic. If it takes more energy to break bonds than form new ones, then the reaction is endothermic overall. If more energy is released by forming new bonds relative to breaking bonds, then the reaction is exothermic.

Some common bond energies (all in kJ/mol): H-H: 436 C-H: 416 C-C single bond: 356 C-C double bond: 598 C-C triple bond: 813 N-H: 391 C-N: 285 N-N: 160 N-N triple bond: 946 O-H: 467 C-O single bond: 336 C-O double bond: 803 N-O: 201 Bond Energies Bond energy is the amount of energy that must be absorbed to break a chemical bond. Breaking bonds ALWAYS requires energy!

Energy Changes in a Reaction 2 H2 + O2 →2 H2O Bonds breaking 2 H-H single bonds (+872 kJ) 1 O=O double bond (+498 kJ) Bonds forming 4 O-H single bonds (1868 kJ) Total Energy: 872 + 498 −1868 = −498 kJ

Fossil Fuels and Electricity The First Law of Thermodynamics Energy is neither created nor destroyed, but may be transformed from one form to another. Second Law of Thermodynamics The entropy (randomness) of the universe is increasing.

Power Plant Efficiency No electric power plant can completely convert one type of energy into another: Net efficiency (%) = electrical energy produced heat from fuel  100 Some of the energy is transferred into useless heat. The higher the temperature of the steam, the more efficient the power plant.

Coal: An Ancient Fuel Source Coal is a complex mixture of substances. Although not a single compound, coal can be approximated by the chemical formula C135H96O9NS. Higher grades of coal contain more energy. For instance, anthracite contains 30–35 kJ/g, bituminous 26–35 kJ/g, and lignite contains 9–19 kJ/g of energy.

Coal Use In the U.S., wood was used as the major source of energy until the 1960s, when coal became the largest source. Today, 92% of all U.S. coal consumption is due to electrical power generation. Asia Pacific countries are the largest users of coal worldwide, followed by Africa, Europe, and the U.S. Drawbacks include: mine safety, environmental harm by mining and its combustion. Fly ash is released that pollutes the air and carries other toxic components such as mercury, lead, and cadmium.

The Shift to Petroleum The shift from coal to oil occurred in the mid-1950s, brought on by the Texas oil boom. 1950 marked the first year that petroleum surpassed coal as the major energy source in the U.S. Petroleum generates about 40-60% more energy per gram than coal. There are 1.7 trillion barrels of proven oil reserves worldwide. The rate of extraction is the key threat; we currently use over 91 billion barrels per day, but only produce 87 billion barrels! The world oil production-to-consumption ratio varies dramatically each year.

Squeezing Oil from Rock Oil is not found in underground pools, but within the pores of geologic rock formations such as sandstone. Once oil is extracted from oil-rich rock formations, known as reservoirs, more expensive and time-consuming methods must be used. Examples of such secondary recovery methods includes using pressurized water or carbon dioxide, sequestered from a power plant, to push oil to the surface.

Fracking! Fracking is used to obtain natural gas or petroleum from hard rock formations such as shales. This consists of drilling down into these rock formations 1-3 miles beneath Earth’s surface. A fracking fluid containing a variety of substances is then injected under pressure in order to create cracks into which natural gas and oil can flow. This is highly controversial; many believe that this method destabilizes rock formations, which may lead to increased seismic activity.

Crude Oil Crude oil is a mixture of several thousand compounds, with the majority being hydrocarbons composed of 5-12 carbon atoms per molecule. Alkanes are hydrocarbons with single bonds between carbon atoms. Alkenes feature at least one double bond, and alkynes contain at least one triple bond. The names of alkanes feature an –ane suffix, with prefixes of meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, and dec-, which indicate the numbers of carbons, from 1-10, respectively.

Boiling Points The volatility of a liquid refers to how easily it is transformed into its gaseous phase (vaporization). Intermolecular forces known as London dispersion are broken between liquid molecules during vaporization of hydrocarbons. The stronger these forces, the higher the boiling point of the liquid. The boiling point of a liquid is the temperature at which the vapor pressure of a liquid equals the ambient pressure. At sea level, atmospheric pressure is 1 atm.

Distillation Hydrocarbons will have different volatilities and boiling points due to differing strengths of their intermolecular forces. A process known as distillation is used to separate crude oil into its various components. As the temperature of the boiler increases, hydrocarbons with higher boiling points begin to vaporize. Eventually, the vaporized components travel up the distillation tower and are collected. Over 87% of each barrel (42 gallons) is used for transportation and heating.

An Oil Refinery: Not Your Ordinary Distillation Columns!

Cracking! In order to meet the large demand for gasoline, the higher-boiling fractions are cracked into smaller molecules by heating them to a high temperature. Catalytic cracking uses a catalyst to lower the temperature required for splitting.

Reforming Catalytic reforming involves the rearrangement of atoms within a molecule, usually starting with linear molecules and producing ones with more branches. Molecules with the same molecular formula but different chemical structures and properties are known as isomers. Molecules with linear structures have stronger London dispersion forces. This corresponds to higher boiling points than their linear isomers. A branched hydrocarbon known as iso-octane is used a gasoline additive to prevent engine “knock”.

Catalysts The energy needed to initiate a chemical reaction between reactants is known as the activation energy. A catalyst lowers the activation energy for a reaction by providing an alternate pathway. This causes the reaction to proceed faster than an uncatalyzed process. An important catalytic process is Fischer-Tropsch, which is used to produce gasoline from coal.

Ethanol Biofuels are renewable fuels derived from a biological source such as trees, grasses, or agricultural crops. Ethanol is an alcohol, with an –OH functional group. To obtain ethanol from corn, a “soup” of corn kernels and water is made. Then, enzymes are used to catalyze the breakdown of starch molecules to make glucose, a sugar. The last step involves fermentation to convert glucose to ethanol, followed by purification by distillation. Ethanol can also come from plants containing cellulose such as cornstalks, switchgrass, wood chips, and other materials that are inedible by humans.

Oxygenated Fuels Oxygenated fuels such as ethanol contain a lower amount of energy per amount burned than the hydrocarbons found in gasoline. Ethanol (C2H5OH) releases 1,240 kJ/mol of energy, whereas octane (C8H18) releases 5,060 kJ/mol of energy. Automobile engines cannot run on ethanol by itself, so gasoline blends are used, which often contain up to 10% ethanol. This increases the octane rating of the fuel and reduces vehicle emissions that produce ground-level ozone.

Biodiesel Another important biofuel is biodiesel, which is generated from fats and oils, such as waste cooking oil. Biodiesel molecules contain a hydrocarbon chain, typically with 1620 carbon atoms. The hydrocarbon chains usually contain at least one C=C double bond. In addition to the hydrocarbon chains, biodiesel molecules also contain oxygen as an ester functional group. Glycerol is a byproduct of biodiesel production, which has applications, or can be converted to other alcohols.

Renewable Energies (2015) Note: Traditional biomass includes wood, agricultural waste, and animal dung.

Ethical Principles for Biofuels Biofuels development should not be at the expense of people's essential rights (including access to sufficient food and water, health rights, work rights and land entitlements). Biofuels should be environmentally sustainable. Biofuels should contribute to a net reduction of total greenhouse gas emissions and not exacerbate global climate change. Biofuels should develop in accordance with trade principles that are fair and recognize the rights of people to just reward (including labor rights and intellectual property rights). Costs and benefits of biofuels should be distributed in an equitable way. If the first five Principles are respected and if biofuels can play a crucial role in mitigating dangerous climate change, then depending on additional key considerations, there is a duty to develop such biofuels. These additional key considerations are: absolute cost; alternative energy sources; opportunity costs; the existing degree of uncertainty; irreversibility; degree of participation; and the notion of proportionate governance. Source: Nuffield Council on Bioethics, Biofuels: Ethical Issues, 2011, 84.

Are Biofuels Really Sustainable? Biofuels are potentially more carbon-neutral since they were derived from modern-day crops, grasses, and trees. The carbon released on combustion of biofuels is partially offset by the carbon these plants once absorbed via photosynthesis. Hard to gauge the net reduction in carbon dioxide emissions for biofuels: Direct and indirect changes in the use of land Waste products from biofuel production How much energy was required to produce the biofuel, including that used to plant and harvest the crop, produce fertilizers, and water the crops.