Developing Fuels

1. Developing Fuels Chemical Storylines notes

2. DF1 – The Vehicle of the Future? • This is one of the most modern electric cars. • It has a maximum speed of 130mph • It can recharge in as little as 3 hours • In terms of energy transfer, that’s about 150 J/s • However… • A petrol pump delivers petrol at about 1 litre per s • That’s about 34 000 000 J/s • Petrol is a very concentrated source of energy!

3. The problems with petrol... • It is finite; it probably won’t last more than 100 years • It is needed as a feedstock to make fibres, detergents, medicines, paints, plastics, and many other things • It may become too precious to simply burn • It pollutes – when burnt it releases large amounts of CO2 (which is a major cause of global warming) plus numerous other pollutants

4. Our jobs as chemists… • Look for alternative fuels • Get the fuels to burn as efficiently as possible • Improve the performance of the car engine • Deal with the pollutants

5. DF2 - Getting energy from fuels • To understand this, we need to understand burning (combustion) and energy changes – Thermochemistry • DF2.1 “Measuring the enthalpy change of combustion” • CI1.3 “Reacting Masses” • CI4.1 “Energy Changes” • DF2.2 “Determining an enthalpy change of reaction” • Combustion always has a negative enthalpy change as it is exothermic… • …it also must have oxygen present • As a result, we tend to talk about the fuel-oxygen system as the energy source rather than just the fuel

6. DF2.3 “Calculating an enthalpy change” • CI4.2 “Bond Enthalpies” • Assignment 1 • The enthalpy change of combustion (ΔHc) depends on.. • The number of bonds broken and made • The type of bonds broken and made • The energy released during combustion comes from bonds forming with oxygen, • Comparing methane (CH4) to methanol (CH3OH)… • …methanol already has one of these. It is an oxygenated fuel or partially oxidised. • Hence methanol releases less energy than methane.

7. In general, oxygenated fuels (those with oxygen atoms as part of their structure) are less energy-rich than hydrocarbons… • …however there are other ways in which they are better than hydrocarbons (see later) • However, because we are looking at carrying this fuel around, the energy released per kg may be more important • We call this the Energy Density • This can be calculated using the ΔHc and the Mr (see table 1 p24) • Assignment 2

8. DF3 – Focus on petrol and diesel • Petrol and diesel are complex mixtures of many different compounds almost all of which have been obtained from crude oil. • Crude oil is a mixture of hundreds of chemicals made of just carbon and hydrogen – hydrocarbons • These are separated and are then carefully blended to give fuels with exactly the right properties • This section looks at what these properties are and how we can produce the chemicals which meet the requirements

9. Separating Crude Oil • Crude oil is separated by fractional distillation (see next slide) • This produces fractions • Each fraction is still a mixture • However, it will have a specific boiling range and contain molecules made of specific numbers of carbon atoms (see table 2 p 25) • The main source for petrol is the gasoline fraction (5-7 C atoms) – this is called straight run gasoline • The main source for diesel is the gas oil fraction (14-20 C atoms) • CI12.1 “Alkanes”

11. The problems… • Straight run gasoline is a very poor petrol • Supply and demand don’t match up.. • Demand is greater than supply for petrol and diesel • The task of the oil refinery is to convert crude oil into useful components • This means altering the alkanes structures to suit our needs • Before going any further, we need to consider what our needs are..

12. DF3.1 “Winter and summer petrol” One important property is its volatility In the car’s cylinders a mixture of air and petrol vapour are burnt. This mixture is made in the carburettor The petrol needs to vaporise whatever the temperature outside. If it doesn’t, the car won’t start To overcome this, petrol companies make different blends for different countries and different times of the year In winter, blends need to be more volatile so they have more short, branched hydrocarbons In summer they need to be less volatile to reduce evaporation from the petrol tank which is wasteful and polluting Winter and summer petrol

13. Knocking • DF3.2 “Auto ignition” • A second important property is the tendency of the petrol to “auto-ignite” or “knock” • To understand this we need to see how a petrol engine works… • http://auto.howstuffworks.com/diesel1.htm • As the fuel-air mixture is compressed, it can ignite before the spark from the spark plug occurs (this is called auto-ignition). • If this is the case there will be two “explosions” in the cylinder • This lowers engine performance and damages the cylinder • A molecule’s tendency to auto-ignite is measured by its octane number; • Low number = very likely to auto-ignite • Assignment 5 • To get a high octane number we need; • Short chains • Branched chains

14. What about diesel? • Diesel is made of longer hydrocarbons (C10 to C15) and aromatic hydrocarbons(see later) • Diesel engines work differently; • http://library.thinkquest.org/C006011/english/sites/diesel.php3?v=2 • they compress just air, • the fuel gets injected into this compressed air • the mixture ignites spontaneously • Therefore, in a diesel engine we want auto-ignition!

15. DF4 Making petrol– refining and blending How can we get our high octane petrol? • Lead compounds? • Used until 1992 • Toxic exhaust fumes • Poison catalytic convertors • Short + branched molecules? • Having some improves the volatility which is good, however… • …can’t have too many or the petrol will be too volatile • Modifying the structures of the hydrocarbons? • Isomerisation • Reforming • Cracking • Adding other things? • Oxygenates

16. Pentane O.N. 62 MethylbutaneO.N. 93 Equilibrium - Reaction does not go to completion Isomerisation • CI3.3 – “Structural isomerism” • DF4.1 – “Drawing and naming alkanes” • This turns straight chain alkanes into branched isomers; • The process is done using an Al2O3 catalyst with platinum finely dispersed on it • The products then pass over a zeolite • This acts as a molecular sieve which separates the branched alkanes from the straight chain ones • Assignment 6

17. Hexane (C6H14) O.N. 25 Cyclohexane (C6H12) O.N. 83 An aromatic hydrocarbon or arene can be thought of as having this structure Also drawn as… Reforming + H2 • Turns straight chain alkanes into cycloalkanes and then into aromatic compounds: + 3H2 Benzene (C6H6) O.N. 106 Cyclohexane (C6H12) O.N. 83 • Reforming is done at 500ºC using the same catalyst as for isomerisation; an Al2O3 catalyst with finely dispersed platinum – it is therefore known as platforming • A single platformer may contain £5 million of platinum

18. C=C double bond Catalytic cracking • DF4.4 – “Cracking Alkanes” • Uses alkanes that are too big to use in petrol (commonly C30-C40) • Uses high temperatures and a zeolite catalyst • Basic reaction is… Alkene Very, very useful in making plastics, medicines and dyes Long chain alkane Shorter alkane • Cracking reactions are very varied. • Some examples are; • long alkane branched alkane + branched alkene • long alkane shorter alkane + cycloalkane • cycloalkane alkene + branched alkene • Assignment 8

19. Adding oxygenates • These are molecules similar to hydrocarbons but they contain oxygen atoms within them • Common ones used are alcohols and ethers • CI13.2 – “Alcohols and ethers” • DF4.5 – “A closer look at alcohols” • Both have high octane numbers and reduce the amount of carbon monoxide released • One commonly used is MTBE • However it is banned in some states • in USA as it can pollute water supplies

20. DF5 Making diesel fuel • Remember; • Diesel fuels use much larger hydrocarbons • We want them to auto-ignite • Instead of using octane numbers, diesel fuels use cetane number. • However, they are not equivalent • Cetane (C16H34) ignites very easily under compression so is given a cetane number of 100 • As with octane numbers, a higher cetane number makes a more suitable fuel • However, it is for completely opposite reasons

21. Other important features of diesel are; • Lubricity – nowadays additives provide lubrication • Flash point – The lowest temperature at which the vapours of a liquid can catch fire. Must be low for petrol but high for diesel • Cloud point – components of diesel can solidify in cold weather making fuel look cloudy. Winter diesel has a lower cloud point Why do hydrocarbons mix? • CI4.3 “Entropy” • DF5.1 “Why do hydrocarbons mix? • Mixing increases the disorder in a system • This is called the entropy of the system • An increase in entropy is a favourable change • Therefore substances will tend to mix with one another unless there is something preventing this from happening • e.g. forces of attraction hold water molecules together and so they don’t mix with oil molecules • DF5.2 “Check your notes part 2”

22. DF6 Trouble with emissions • As chemists, once when we have decided on our blend of fuel we have a second major area of concern… • The products of the chemical reactions that occur; the emissions • Know as the primary pollutants… particulates N2 H2O CxHy SOx NOx CxHy CO2 CO O2 N2 1000ºC

23. Where do these emissions come from? • CO2 and H2O • Products of combustion of hydrocarbon • CO • Incomplete combustion of hydrocarbons • NOx • N2 and O2 in air react together at high temperatures within the engine to form NO (which may then turn into NO2) • SOx • fuels will contain some sulphur compounds. These react with O2 to form SO2 and SO3 • CxHy • some is unburnt as it passes though system • some (short chain hydrocarbons) evaporate from the petrol tank and carburettor (evaporative emissions) • Particulates • Tiny carbon particles produced by the incomplete combustion of hydrocarbons

24. Why should we be concerned? • CO2 is a major contributor to global warming • CO is toxic to humans • NOxreacts with water to form acid rain and causes the formation of photochemical smog • SOxreacts with water to form acid rain • CxHycauses the formation of photochemical smog • Particulatesirritate our lungs

25. Photochemical smog Primary pollutants CO, NOx, CxHy Secondary pollutants NOx, CxHy, peroxyacetyl nitrate (PAN), O3, H2O2, • These are not the only pollutants caused by car engines… • Others are produced by reactions between primary pollutants these are known as secondary pollutants • e.g. ozone (O3) is produced when sunlight shines on a mixture of NOx , CxHy , O3 , H2O2 • Reactions that require light energy to occur are known as photochemical reactions • These reactions take place in photochemical smogs • The composition of photochemical smog varies greatly • It is affected by things such as weather, geography and time of day as well as the exact primary pollutants • Winds will cause ozone to reach DF6.1 Some reactions of NO2 and SO2

27. The effects of photochemical smog • Ozone is very good in the upper atmosphere where it filters out uv radiation • In the lower atmosphere it also helps to break down harmful substances and prevent their build up • However, it is also an irritant and toxic to humans. • It weakens the immune system and attacks lung tissue • It also acts as a greenhouse gas • In general, photochemical smogs cause eye and nose irritation and breathing difficulties especially for asthmatics and the young and old • Ozone also affects plants and animals as well as any compounds with C=C double bonds; plastics, textiles and plants

28. What can chemists do? • Monitoring • Find out what is present, in what concentrations and how this varies • Studying reactions • Atmospheric chemistry is very complicated • Many reactions involve ‘fragments’ of molecules called radicals • Computer simulations help us to predict what will happen in smog • Chemical simulations in huge smog chambers • DF6.2

29. DF7 Tackling the emissions problems • There are two key ways chemists can tackle the emissions problems • Develop car technology • Change the fuel used Engine technology • CI1.4 Calculations involving gases • DF7 “What is the volume of one mole of hydrogen?” • Assignment 10 • We can look at the air : fuel ratio • The ratio needed for petrol to burn completely is called the stoichiometric ratio • This is normally about 15:1 (by mass) • Less air is called a rich mixture • More air is called a lean mixture

30. Lean-burn engines use an 18:1 mixture • Make less NO and CO and give better fuel economy • However make more CxHy and can cause the engine to misfire • Lean-burn engines have to be specially designed to avoid this misfiring

31. Using catalysts • A catalyst speeds up a reaction but is not used up in that reaction… • …it does this by providing an alternative route with a lower activation energy • We can use a catalytic convertor in the exhaust system to speed up reactions involving some of the pollutants 2CO + O2 2CO2 C7H17 + 11O2 7CO2 + 8H2O 2NO + 2CO N2 + 2CO2 • CI10.5 Catalysis

32. Assignment 11 2CO + O2 2CO2 C7H17 + 11O2 7CO2 + 8H2O 2NO + 2CO N2 + 2CO2 • CO and CxHy require an oxidation catalyst but this won’t remove NO as this needs to be reduced • This is less of an issue in a lean burn engine as they produce little NO • This can be addressed using a three way catalyst system

33. Getting the best from the catalyst • The catalyst works best when hot… • Pt catalyst starts working at 240ºC • However using a Pt/Rh alloy brings this down to 150ºC • It also is best if there is a large surface area • It is finely powdered and adsorbed onto a ceramic support • This ceramic support then has a honeycomb structure • The surface area of catalyst available will be as much as three football fields • The surface of the catalyst must be ‘clean’ • Anything else binding to the catalyst will prevent the pollutant gases from binding • These things are called catalyst poisons • Lead will poison the catalyst and so only lead-free fuels can be used • Assignment 12

34. DF8 Using Diesel Fuel • No clear cut advantage to diesel or petrol • Diesel engines use a diesel oxidation catalyst • This uses O2 to convert CO into CO2 and CxHy intoCO2 and H2O • However, this prevents NOx from being reduced to N2 • Catalytic converters are also unable to remove carbon particulates • Assignment 13

35. DF9 Other Fuels • Alternatively we could change the fuel used by cars • Aromatic hydrocarbons make up about 40% of petrol. • These increase CO ,CxHy and NOx emissions and some are carcinogens • Short hydrocarbons such as butane increase evaporative emissions and so photochemical smogs • Both are becoming increasingly strictly controlled but both are essential for high octane fuels • This is why petrol companies have looked at oxygenates

36. Other hydrocarbon fuels • LPG (liquefied petroleum gas) is a mixture of propane and butane • It is kept under pressure to keep the fuel liquid • LNG (liquefied natural gas) is mainly methane • It must be cooled (below -160ºC) and is therefore most suitable for larger vehicles • Both have a high octane number • LPG produces 20% less CO2 and also less CO and NOx

37. Biofuels • Ethanol is often added to petrol • High octane number • Less polluting • It can be made from ethene (from cracking) • It can also be made by fermenting cane sugar • ⅓rd of Brazil’s cars run on pure ethanol or gasohol (a mixture of petrol and ethanol) • Advantages: • It is an oxygenate so produces less CO, NOx andSOx • The CO2 produced is balanced by that absorbed as the sugar cane grows

38. Biodiesel • Diesel engines can also run on biodiesel • This is made from vegetable oil and animal fat (even oil chip fat!) • Biodiesel is made by converting the oils and fats into “esterified oil”…

39. Advantages: • It is biodegradable so less harmful if spilled • It produces lower emissions than diesel • It is made from plants and animals (so is renewable) • Sometimes described as “Carbon neutral” (if we ignore the energy used in its production) • Virtually no SOx released • Less CxHy , CO and particulates released • NOx is higher but is easier to deal with using catalytic convertors • Disadvantages • The crop needs to be grown somewhere.. • Instead of a normal crop (food shortages) • Deforestation • Doesn’t address the need to be more energy efficient • There are still emissions

40. DF10 Hydrogen – a fuel for the future? • Projected rates of oil and coal consumption • Today we consume an average of 85 million barrels of oil each day. • The world has enough "proven" reserves to provide 40 years of consumption at current rates.(BP Statistical Review of World Energy) • Estimates put the amount of coal remaining at around 300 years • Alternative fuels will be vital… • …some favour hydrogen

41. If we could produce hydrogen without using fossil fuels it would be very beneficial… • The most likely way is by electrolysis of water (with the electricity generated by renewable sources) • The advantages of this are • Hydrogen can be stored • It can be used in modified car engines • One major issue for cars is how we store the hydrogen in the car • http://www.hydrogencarsnow.com/audi-a2h2-hydrogen-car.htm