The Physics of Energy sources Pre-Nuclear energies Fossil fuels

Transcription

1 The Physics of Energy sources Pre-Nuclear energies Fossil fuels Pre-Nuc. En. Ref 1: Boyle, Everett and Ramage, Energy Systems and Sustainability (003) 1

2 Fossil fuels Ref 1! Various types! They result from fossilised remains of dead plants and animals! These processes occur over hundreds of millions of years Pre-Nuc. En.

3 Combustion Combustion is an exothermic chemical reaction between a fuel and an oxidant Fuel Oxydant Combustion product Energy (heat/light)! Fuels:! Solids (Coal, Wood) C O CO We assume perfect combustion: Stoichiometry respected Energy! Liquid, Gas ydrocarbons Alcohols Ex: methanol y y C x y x O xco O 4 O O CO 3 4 R O Energy C O 3O CO O Energy Energy O O Energy Fossil fuels: Carbon and ydrocarbon Pre-Nuc. En. 3

4 A bit of general chemistry! Chemistry is Physics! Due to the electronic structure of the atom! Rate of the reaction: Chemical kinetics depends mainly on! Physical conditions:! Activation energy (Arrhenius): Activation energy Δ = en. used in bond breaking reaction - en. released in bond making products Δ Δ <0 exothermic Δ >0 endothermic Pre-Nuc. En. Adapted from wikipedia 4

5 Carbon based combustion! Coal or wood is not made from Carbon only! Composition will depend on the type of fuel, its production etc C O CO Δ = 393.5kJ/mol Ref 1 If all the coal was made of Carbon and combustion was only giving CO then 1g of C in one mole 1tonne would produce ~ 33GJ Pre-Nuc. En. 5

6 Oil! The main constituents of oil are hydrocarbons. Example of hydrocarbons Alkanes (a.k.a. paraffins) Cyclo-Alkanes (a.k.a. naphthenes) C Methane C C C Propane C C C C Cyclo-Butane Cyclo-Pentane Aromatic hydrocarbons Benzene (simplest) Naphthalene Pre-Nuc. En. 6

7 From petroleum to oil! From Petroleum (crude oil) to refined fuels Ref 1 Propane, butane Largest fraction Petrol, jet fuel, kerosene Diesel, gas oil Petroleum Marine Diesel Lubrication oil, coke, bitumen Pre-Nuc. En. 7

8 ydrocarbon combustion General ydrocarbon: y y C x y x O xco O 4 Energy Example: Butane combustion C410 13O 8CO 10 O Energy( = ( 877.5kJ)) Name Formula Molar mass A few data examples Density Boiling point Enthalpy of combustion Methane C 4 16g/mol 0.717kg/m 3 (gas) C kJ/mol Butane C g/mol.48g/l (gas) -0.5C kJ/mol eptane C g/mol 0.684g/ml (liq) 98.4C -4817kJ/mol Benzene C g/mol 0.879g/cm3 (liq) 80.1C kJ/mol Toluene C g/mol 0.867g/ml (liq) 110.6C kJ/mol Iso-octane Trimethylpentane C g/mol 0.688g/ml (liq) 99.3C -5461kJ/mol Pre-Nuc. En. 8

9 Flue gases! So far we have considered an ideal combustion! A more realistic combustion will! appen in air 78% N, 1%O, 1% Ar! Not be complete fuel not burnt! Fuel like coal will contain impurities such as Sulphur! This will lead to other flue gases on top of CO! Some potentially represent a health risk CO toxic SO (coal), NOx will produce acid rain (Sufuric and Nitric)! Some will also contribute to greenhouse effect un-burnt gases such as Methane Pre-Nuc. En. 9

10 Combustion N O NO, NO,... Acid rain Several ways to form acid: here are a few examples in the atmosphere NO 3NO In the atmosphere (water vapour droplets) NO O NO oxidation O (Nitrous and Nitric acids) NO NO 3 NO NO 3 O (decompostion) S O SO ( g ) SO ( g ) ( aq) dissolution in water O SO. O ( aq) SO SO. 3 ydrolysis O SO SO 3 3 Pre-Nuc. En. 10

11 Effects Statue: stone like marble are easily attacked by acids So you can imagine the effects on your health! ow to avoid this? Pre-Nuc. En. 11

12 Remedies! SO is coming essentially from the Sulphur contained in coal What about other flue gases: Catalytic converters Some reactions that have a poor probability to happen are facilitated by the use of a catalyst. In this case catalyst: Platinum, Palladium or Rhodium Reduction of nitrogen oxides: NO x xo N Oxidation of carbon monoxide: CO O CO Oxidation of unburnt hydrocarbons: C x y (xy/)o xco y O Pre-Nuc. En. 1

13 What is catalysis? Without catalyst Let s say we have components, A and B we want them to react to give a product D: Ea for Eq 1 AB D (Eq1) Ea for same reaction with catalyst With catalyst (let s call it Ct) It will react with one or both of the reactants to form a intermediate component! ACt ACt B will react easily with the new component due to a lower resulting activation energy! BACt BACt This will then be followed by the reactions:! BACt DCt! DCt DCt Adapted from wikipedia The catalyst plays a major role in the reaction but the resulting equation is equivalent to AB D with a lower Ea Pre-Nuc. En. 13

14 Summary: What did we see?! Combustion of fossil fuels principle! Could you write the basic equations for hydrocarbons, C and other fuels?! Would you be able to calculate the energy generated by a combustion?! Products of combustion! Would you be able to tell what is released in the atmosphere?! Would you be able to tell how to decrease/avoid these?! Definition of activation energy of a reaction! Catalysis processes Pre-Nuc. En. 14