EG3539 2013. Dr. Jones’ contribution . Combustion. 3 lectures. 1 tutorial.

1. EG3539 2013.Dr. Jones’ contribution. Combustion. 3 lectures. 1 tutorial.

2. To be covered : Re-visiting of Rankine, Otto and Diesel cycles. Refining with Grangemouth as an example. Review of fossil fuels including mass balance and carbon footprint. Concept of carbon neutrality. Carbon mitigation in electricity production. Carbon-neutral transport fuels. Fuels from oil shale. Coal and coal products. Miscellaneous calculations, ten in all, in which new ideas will be introduced. (It is possible that these will be completed in the tutorial slot.) The above outline is an expansion of the course outline put on MyAberdeen by the course co-ordinator.

3. Recall from Year 2 the Diesel and Otto cycles. In those, fuel was introduced at a particular stage of the thermodynamic cycle. The chemical energy in the fuel becomes heat some of which is converted to work. In a Rankine cycle fuel is used to raise steam condensation of which leads to work. All the above done in previously.

4. Fuels from crude oil: The world uses 80 million barrels of oil per day. 1 barrel = 0.159 m3 ↑ No need to memorise. Crude oil ↓ Refining Liquid fuels including gasoline, kerosene, diesel and fuel oil. All of these have calorific values in the range 40-45 MJ kg-1 All approximate to the empirical formula CH2 ↑ Widely used in such things as mass balances. Densities range, according to the parent crude, from about 800 kg m-3 for gasoline to about 950 kg m-3 for heavy fuel oil.

5. ● Grangemouth refinery: has been in operation since the 1920s and is still the only refinery in Scotland. ● It now receives oil from the North Sea. ● Capacity of Grangemouth 0.7 million barrels per day.

6. Mass balance on the combustion of a kilogram of a petroleum liquid fuel when burnt under stoichiometric conditions. Recall that the empirical formula is CH2 formula weight 14g. Per kg of the fuel (12/14) kg carbon = 857 g carbon (2/14) kg hydrogen = 143 g hydrogen

8. Total gas from combustion of 1 kg: 71.4 mol CO2 (14%) 35.7 mol H2O vapour (7%) (268.5 + 134.2) mol N2 = 402.7 mol (79%) ----------------------------------------------------- Total 510 mol Notes on the above: ● It is for stoichiometric conditions, no excess air. Extension to excess air a possible tutorial exercise.

9. ● On cooling to room temperature the water will condense and the only water in the gas will be that in phase equilibrium with the liquid. This gives ≈ 470 mol post-combustion gas. ● 1 m3 of gas at ordinary temperatures and 1 bar pressure contains about 40 moles, so the quantity above converts to just under 12 m3.

10. ● Carbon footprint in the above mass balance 71.4 mol of carbon dioxide (3.1 kg) ● If only the carbon footprint is of interest, it can be calculated more directly as follows. From the stoichiometry: 1 molar unit CH2→ 1 molar unit CO2 14 g fuel → 44 g carbon dioxide Carbon footprint from burning 1 kg = (44/14) kg = 3.1 kg

11. Greenhouse gas emissions from fuels derived from crude oil scaled internationally. Its burning can be represented as: CH2 →CO2 1 kg oil = (1000/14) molar units ↓ (1000/14) = 71 moles of CO2 80 million barrels (bbl) oil = ≈ 80 × 106 bbl × 0.159 m3 bbl-1 × 900 kg m-3 = 1.15 × 1010 kg of oil  ↓   = 8 × 1011 moles CO2 Mass of the atmosphere  = 5.1 × 1018 kg of air or (5.1 × 1018/0.0288) moles of air = 1.8 × 1020 mol ↑ Would be given to you in an exam  Rise in CO2 from a year’s burning of oil =   [8 × 1011/(1.8 × 1020)] × 106p.p.m. × 365 day year-1 = 1.6 = p.p.m. molar basis This is for petroleum derived fuels only and takes no account of emissions from coal and natural gas.

12. Imagine a steam turbine working at 35% efficiency producing electricity at 500 MW with petroleum oil as the fuel. How much carbon dioxide will be produced in 30 days’ operation of the turbine? Use a value of 44 MJ kg-1 for the calorific value of the oil.

13. Rate of production of electricity = 500 × 106 J s-1 Rate of production of heat = 500 × 106/0.35 J s-1 = 1425 × 106 J s-1 Rate of utilisation of fuel = [1425 × 106 /(44 × 106)] kg s-1 =32.5 kg s-1 Rate of production of carbon dioxide =32.5 × (44/14) kg s-1 = 100 kg s-1 Amount over 30 days = 100 × 30 × 24 × 3600 × 10-3 tonnes = 0.26 million tonnes.

14. Now imagine that fuel for the turbine had been not petroleum oil but natural gas. For this, the calorific value is ≈ 55 MJ kg-1 and the stoichiometry such that: 1 molar unit CH4 → 1 molar unit CO2 16 g fuel → 44 g carbon dioxide Redoing the calculation and putting the changed numbers in a different colour: Rate of utilisation of fuel = [1425 × 106 /(55 × 106)] kg s-1 =25.9 kg s-1 Rate of production of carbon dioxide =25.9 × (44/16) kg s-1 = 71 kg s-1 Amount over 30 days = 71 × 30 × 24 × 3600 × 10-3 tonnes = 0.18 million tonnes.

15. ● There is therefore about 30% less carbon dioxide when natural gas is substituted for petroleum oil. That natural gas releases less CO2 other things being equal is a general result which follows from the calorific values and the stoichiometry. ● Comments on the importance of this later. Now imagine that fuel for the turbine had been neither petroleum oil nor natural gas but coal* of carbon content 80% and calorific value 20 MJ kg-1 For this the stoichiometry is: 1 kg coal containing 0.8 kg carbon → (44/12) × 0.8 kg carbon dioxide = 2.9 kg carbon dioxide * More on coal per se later in the lectures.

16. Re-doing the calculation: Rate of utilisation of fuel = [1425 × 106 /(20 × 106)] kg s-1 = 71.3 kg s-1 Rate of production of carbon dioxide = 71.3 × 2.9 kg s-1 = 207 kg s-1 Amount over 30 days = 207 × 30 × 24 × 3600 × 10-3 tonnes = 0.54 million tonnes. So the coal is the least favourable of the three for CO2 emissions.

17. The distinction between fossil fuel and non-fossil fuel carbon dioxide. ● Carbon dioxide in the atmosphere is taken up by plants and converted to glucose and from there to cellulose. ● When the cellulose is burnt the carbon dioxide is simply being put back where it came from and there is no net increase in the carbon dioxide level of the atmosphere and the fuels are said to be carbon neutral. ● By contrast when fossil fuels are burnt carbon dioxide not having, on any time scale of interest, previously existed in the atmosphere is put there. Combustion of such fuels therefore raises the carbon dioxide content of the atmosphere and these fuels are not carbon neutral. A related calculation follows.

18. ● Suppose it is required to produce heat at 10 MW. The amount of carbon dioxide per hour will this produce if the fuel is bituminous coal of carbon content 85% and calorific value 30 MJ kg-1 is calculated below. Rate of requirement of fuel = (107 J s-1/30  106 J kg-1)  3600 s hour-1 =1200 kg hour-1 1020 kg hour-1 of carbon burnt 3740 kg CO2 ● If the fuel is instead wood waste of carbon content 55% and calorific value 17 MJ kg-1 the hourly carbon dioxide production is: (107 J s-1/17  106 J kg-1)  3600 s hour-1 = 2117 kg hour-1 1164 kg hour-1 of carbon burnt 4270 kg CO2

19. ● There is the anomaly that the carbon neutral fuel produces significantly more carbon dioxide than the fossil fuel per unit heat produced. The above calculation uses arbitrary though typical values for the quantities involved, and that wood fuel produces more carbon dioxide than coal other things being equal is in fact a general result. ● The wood fuel is preferred on carbon dioxide emission terms because, unlike the carbon in the coal, that in the wood fuel was in the recent past carbon dioxide in the atmosphere. This means that when the wood is burnt carbon dioxide is simply being put back where it came from, as discussed in the previous slide.

20. ● The 1990 release of carbon dioxide in the UK was 590 million tonnes: ● The 2010 emission was 496 million tonnes. The UK Kyoto target for 2010 was 12.5% below the 1990 level, whereas in fact it was almost 16% below it. ● The most important reduction was in the electricity sector. This was partly by increased use of natural gas (see previous notes). As a result the UK has become a net importer of natural gas. ● Another successful measure has been co-firing of biomass with coal. An example follows.

21. Drax power station, West Yorks.: the scene of coal-biomass co-firing.

22. At one of its turbines Drax is producing 500 MW of electricity by co-firing 12.5% biomass, balance coal. This is examined below. 500 MW of electricity from say (500/0.35) = 1425 MW of heat. Let the coal supply rate be α kg per second and the calorific values of the coal and the biomass as fired be respectively 25 and 17 MJ kg-1  ↓   (α kg s-1 × 25MJ kg-1) + [(12.5/87.5) α kg s-1 × 17MJ kg-1] = 1425 MW ↓ α = 52 kg s-1 52 kg s-1 of coal and 7.4 kg s-1 of biomass. Coal of that calorific value would be expected to have about 80% carbon, so the rate of production of carbon dioxide is: 52 kg s-1 × 0.8 × (44/12) = 153 kg s-1

23. The carbon dioxide from the biomass combustion is non fossil fuel carbon dioxide, and is simply being put back where it came from when the biomass is burnt. It need not therefore be added to the above figure. For carbon accounting purposes, CO2 production from a day’s operation of the turbine is therefore: 153 kg s-1 × 3600 s hour-1 × 24 hour day-1 × 10-3 tonne kg-1 = 13200 tonne per day. If that amount of electricity had been produced by the coal only without biomass the rate of burning of the coal would have been: (1425 MW/25 MJ kg-1) = 57 kg s-1 giving over a day’s operation: 57 kg s-1 × 0.8 × (44/12) × 3600 s hour-1 × 24 hour day-1 × 10-3 tonne kg-1 = 14446 tonne of CO2. The reduction due to the co-firing with biomass is therefore 9%.

24. ● The reduction of carbon dioxide emissions due to co-firing with biomass is clearly demonstrated by the above figures relating to Drax. ● As noted, carbon dioxide in the biomass was in the fairly recent past in the atmosphere having been converted to cellulose by photosynthesis, so when the biomass is burnt it is being returned to where it came from and does not add to the carbon dioxide level of the atmosphere.

25. ● Contrast with France: much lower emissions of CO2 there from power generation than in the UK because of the wide use of nuclear plants which, of course, do not release carbon dioxide. ● A related calculation follows. In 2011 nuclear reactors in France provided 421 billion kWh. If an equivalent amount had been raised from natural gas, what would have been the carbon footprint?

26. Solution: 421 billion kWh = 421 × 1012 J s-1 × 3600 s = 1.5 × 1018 J of electricity. Efficiencies of generation depend on turbine performance not on whether nuclear and or chemical fuels were used, so the electricity above must have been obtained from about: (1.5 × 1018/0.35) J of heat = 4.3 × 1018 J of heat If this had been raised from natural gas, the carbon dioxide release would have been: Molar heat of combustion of methane. Would be given to you in an exam. ↓ [4.3 × 1018 J/(889 × 103 J mol-1)] × 0.044 kg mol-1 × 10-3 tonne kg-1 = 213 million tonnes So this amount of CO2 is eliminated by the use of nuclear fuels. ● The original motive for development of nuclear fuels for electricity in France was lack of oil. ● In a typical month the UK will itself import of the order of one terawatt hour of electricity from France: the transmission cable goes along the Eurotunnel.

27. Aside on nuclear fuels. Contrast between thermochemical (combustion) and thermonuclear processes. Example of a thermochemical process: natural gas combustion. CH4 + air  CO2/water Heat released per molecule of methane = 1.5  10-18 J Example of a thermonuclear process: fission of uranium 235. U235 + n  Ba141 + Kr92 + 3n Heat released per atom of uranium = 3.2  10-11 J Ratio = (3.2  10-11)/(1.5  10-18) ≈ 20 million In addition to thermochemical and thermonuclear processes there are isothermal devices for producing energy. Probably the most important such device at the present time is the wind turbine.

28. ● The factor of the order of millions or tens of millions between heat release in chemical and nuclear processes can be offset by quantities. ● The bomb dropped on Hiroshima in 1945 (image of a model above) weighed about 4.5 tonnes. ● If a supertanker containing one million barrels of oil were to break open and release all at once its contents which then ignited, the blast would exceed that at Hiroshima. Detailed calculation on request from the lecturer but not within the course. ● A refinery can have an inventory comparable to that of a supertanker.

29. Returning to non-nuclear and considering Belgium: ● In Belgium some former coal-fired power stations have been adapted to biomass alone as the fuel. An example is the Rodenhuize Power Station at Ghent which produces 180 MW of electricity from solid biomass. ● The fuel is in the form of compressed wood pellets with calorific value 18 MJ kg-1 The national generator Electrabel has a total of 341 MW installed capacity for electricity from solid biomass, necessitating a quantity of biomass annually of: [(341 × 106 J s-1/0.35)/(18 × 106 J kg-1)] × (365 × 24 × 3600) s year-1 × 10-3 tonne kg-1 ↓ 1.7 million tonne per year ● Like all responsible biomass fuel users worldwide, Electrabel ensures that the wood it receives is from suppliers with Forest Stewardship Council (FSC) recognition for sustainability. Such suppliers are monitored by the FSC for replacement of trees felled with new plantings.

30. Transport fuels. Imagine a motor vehicle operating at 50 miles per (Imperial) gallon of gasoline fuel. Per mile travelled the quantity of fuel used is: 1/50th of a gallon  9  10-5 m3 The density of such a fuel will be  800 kg m-3, so the weight of gasoline consumed per mile is:  9  10-5 m3 800 kg m-3 = 0.07 kg We saw in previous calculations that a liquid fuel from crude oil will to a fair approximation have empirical formula CH2. In a quantity of 1 kg of such a fuel there are 857 g of carbon and 143 g of hydrogen. When therefore 0.07 kg of the fuel is burnt (that is, the car travels 1 mile) there are: 0.07  857  44/12 g carbon dioxide produced = 219 g CO2 So the carbon dioxide release is 219 g per mile or 137 g per km.

31. Carbon footprints of petrol engine cars .

32. 1955 Cadillac Series 62

33. ● There are said to be of the order of 70000 powered vessels on the oceans of the world using 200 million tonnes of hydrocarbon fuel per year. The carbon footprint can be calculated as: From a previous slide, rounded up ↓ 200 million tonnes fuel → (0.86 × 200 × 44/12) million tonnes CO2 = 630 million tonnes. ● An approximately equivalent amount is released annually from commercial aircraft. Where there is R&D into fuels other than conventional jet fuel (e.g., biodiesel, see following slide) for aircraft the motive is usually saving oil rather than mitigating CO2.

34. Carbon-neutral engines for transport: For SI engines: ethanol, or blends of ethanol with gasoline. Calorific value of ethanol* 29.7 MJ kg-1 For CI engines: biodiesels, or blends of biodiesels with mineral diesel. Calorific value of biodiesels* ≈ 37 MJ kg-1 Sources of ethanol: sugars, starches, polysaccharides. Sources of biodiesel: Plant oils. Animal fat. Annual fuel use of ethanol in the UK: 130 million gallons Annual fuel use of biodiesel in the UK: 185 million gallons * Would be given to you in an exam if needed.

35. Ethanol/gasoline blends available in the range: E5 to E95 ↑ ↑ 5% EtOH 95% EtOH Octane number of ethanol alone 105 Calorific value of (for example) E85 = [(0.85 × 29.7) + (0.15 × 45)] MJ kg-1 = 32 MJ kg-1

36. ● Some variation from the nominal percentage of ethanol might be necessitated to meet octane number and vapour pressure requirements. ● Whether such variations occur and if so to what degree depends on the gasoline: gasoline fractions differ widely from each other in such properties according to the nature of the crudes from which they were obtained.

37. ● The automotive fuel E10 provides a suitable context for illustration of carbon balance for a fuel partly carbon-neutral. ● Carbon dioxide from the carbon-neutral part is being put back where it came from before being used in photosynthesis and does not contribute to rises in the atmospheric level when such a fuel is burnt. ● The working on the next slide is concerned with this.

38. Heat released on the burning of 1 kg of gasoline = 45 MJ, releasing:  (44/14) kg CO2 = 3.14 kg Taking E10 to be 10% by weight of ethanol (calorific value 29.7 MJ kg-1), balance gasoline, its calorific value is [(0.9 × 45) + (0.1 × 29.7)] MJ kg-1 = 43.5 MJ kg-1 So 45 MJ are released by (45/43.5) kg = 1.03 kg Fossil fuel derived CO2 = (1.03 × 0.9 × 3.14) kg = 2.91 kg Using the stoichiometry: C2H5OH + 3O2 + (11.3 N2) → 2CO2 + 3H2O + (11.3 N2) Non fossil fuel derived CO2 = (1.03 × 0.1 × 44/23) kg = 0.20 kg ↑ Half the molar mass of ethanol A drop in the fossil fuel derived CO2 of (3.14 – 2.91) kg = 0.23 kg or 7%. Students are encouraged to attempt for themselves the equivalent calculation for E50, E65 and so on.

39. Ethanol production and usage in selected EU countries.Some of the key facts underlined. Retain these as the gist of the information. No need to remember precise numerical information.

40. Continued from previous page.

41. Ethanol plant at Wanze.

42. Biodiesels. ● These are for compression ignition engines, not for spark ignition engines, and are carbon-neutral. Obtained from plant oil or from animal fat. ● Biodiesels for EU use have to conform to the standard EN 14214 2008 which specifies inter alia a minimum cetane number of 51. ● We note as a point of interest that the US standard ASTM D 6751-07b, also for biodiesels, sets the less stringent value of 47. ● If a biodiesel is of too low a cetane number, it can be modified by reaction with methanol (not addition of methanol) to raise the cetane number. This is in fact quite widely done and the process is called trans-esterification.

43. A biodiesel manufacturing plant at Le Havre in France takes animal fats as feedstock and also spent cooking oil for refining. The output of the plant is 75000 tonnes per year of biodiesel. Assigning this a calorific value of 37 MJ kg-1, the amount of fossil fuel derived carbon dioxide eliminated by its use in preference to mineral diesel can be calculated. Amount of petroleum material thermally equivalent to the biodiesel =   (75000 × 103 × 37/43) kg releasing on burning: [(75000 × 103 × 37/43) × 44/14] × 10-3 tonnes of CO2 = 0.2 million tonnes of carbon dioxide. This then is the carbon mitigation attributable to the plant at Le Havre. An alternative way of putting it is that the carbon footprint of a quantity of oil thermally equivalent to the biodiesel is eliminated.

44. Returning to fuels which are not carbon-neutral: Oil from shale: ● Shale consists of a band of organic material called kerogen within a rock structure. ● The shale can be crushed and retorted whereupon crude shale oil is yielded by decomposition of the kerogen. ● The term ‘shale oil’ means crude oil derived from shale in this way and this is usually hydrogenated before refining. There might also have been sulphur removal. It is then known as syncrude. ● It will attract a price equivalent to that of crude oil only after such processing. ● Refining of this material gives fractions corresponding to those from crude oil and interchangeable with them in fuel utilisation.

45. Availability of shale: It is very plentiful! If one approximates the lower 48 states of the US to a rectangle and draws a diagonal from NE to SW that diagonal represents a continuous band of shale. The shale reserves of Colorado, Wyoming and Utah are such that if all of the oil from it were converted to syncrude the quantity would exceed the known oil reserves of the entire Middle East.

46. The two factors most important in the viability of a fuel in the early 21st Century are EROEI* and CO2 emissions. Comparable emissions to fuels from crude oil. Difficult to conceive a ‘shale boom’ in the 21st Century in spite of the huge reserves. * Energy-return-on-energy-invested

47. Coal and coal products Preamble. ● In some ways coal belongs much earlier in the discussion, since its use long predates that of oil. 1801 coal production in Britain was about 12 million tons per year. That is more than half the current production rate . ● A training engineer in 2013 will have a much greater consciousness of oil and of ‘renewables’ than of coal. ● Even so, coal production continues on a large scale across the world, and its use has benefited from advances in combustion hygiene and, more recently, carbon sequestration. ● A thermodynamicist/fuel technologist needs some knowledge of coal.

48. ● There are a number of ranks of coal, ranging from lignite through sub-bituminous and bituminous to anthracite. All find application, and the most important is electricity generation. ● Bituminous coal is the most important and this discussion will be restricted to it. ● Bituminous coal will have a carbon content of around 80% and a calorific value, depending on the ash content, of up to 30 MJ kg­1. Uses include direct combustion, carbonisation to make coke and gasification to make retort coal gas or blue water gas. ● Bituminous coal has not only served industry for generations but played an important part in industrialisation itself by providing a material for the manufacture of coke subsequently used to make iron and steel for the construction of machinery.

49. ● Natural gas always attracts a much lower price on a heat basis than crude oil, a point which will be touched on later in the lectures. Such a comparison of bituminous coal with oil will be attempted here . All data necessary for the following approximate calculation would be given to you in an exam if necessary. Working in non-SI units is often unavoidable and indeed preferable. The prediction has recently been made in the professional literature that the barrel will never be replaced by an SI unit. ● A good example of a reference price for bituminous coal is the CAPP –Central Appalachian – price which on 12th February 2013 was: $US3.23 per million BTU Now 1 million BTU ≈ 1 GJ (109 J) A barrel of crude oil releases when burnt ≈ 6 GJ ● OPEC price of a barrel of oil on 12th February 2013 was $US115 per barrel so a quantity capable of releasing 1 GJ would have cost $US19.17. Price of unit heat from oil/ Price of unit heat from bituminous coal = (19.17/3.23) = 5.9

50. ● The most obvious application of coal – its direct burning – remains important and provides heat for some of the cycles discussed earlier in the course, e.g. steam raising for a Rankine cycle. ● Its conversion to gas is also widely practised. Calculations on this are within the scope of a thermodynamics course and will be included. ● Not only coal but also coke – made by heating coal – can be so gasified. It is usually possible for calculation purposes to treat this as being 100% carbon.