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EG3539 2013. Dr. Jones’ contribution.

Combustion.

3 lectures. 1 tutorial.

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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.

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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.

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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.

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● 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.

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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

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Element C Hmol/kg fuel 857/12 = 71.4 143/2 = 71.4

(expressed as H2)

O2 requirement/kg fuel and oxidation

product

71.4for C + O2 → CO2

71.4 mol CO2

35.7for H2 + 0.5O2

↓ H2O

35.7 mol H2Ovapour

Accompanying nitrogen

3.76 × 71.4 mol = 268.5 mol

3.76 × 35.7 mol = 134.2 mol

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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.

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● 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.

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● 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

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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)] × 106 p.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.

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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.

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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.

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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.

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● 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.

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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.

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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.

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● 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

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● 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.

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● 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.

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

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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

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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%.

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● 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.

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● 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?

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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 heatIf 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.

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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.

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● 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.

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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.

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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.

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Carbon footprints of petrol engine cars .

Make and model Carbon footprint/g mile-1

Volkswagen Golf 1.6 litre 165

Vauxhall Corsa 1.2 litre 200

Toyota Avensis 1.8 litre 248

Mazda MX5 2 litre 303

Aston Martin DB9 593

Average for all new cars sold in the UK in 2011 223

1955 Cadillac Series 62 (next slide) 722

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1955 Cadillac Series 62

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● 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.

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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 gallonsAnnual fuel use of biodiesel in the UK: 185 million gallons

* Would be given to you in an exam if needed.

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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

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● 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.

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● 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.

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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.

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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.

Country Details

UK ● Usage 2010-2011 about 130 million gallons. ● A lower value than might be expected from the population and

number of vehicles. ● Produced from feedstocks including wheat straw (cellulose).

France ● The most abundant producer of ethanol in the EU, with an annual output of 1250 million litres.

● Sugar beet and cereal are common feedstocks for ethanol production in France.

● Ethanol-gasoline blends, most notably E10, are widely available in France.

The Netherlands

● High level of manufacture, including an ethanol plant at Rotterdam producing 127 million gallons of ethanol annually for

blending with gasoline.● E85 the most widely used.

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Continued from previous page.

Germany ● 752 million litres in 2010, from feedstocks including sugar beet and rye.

● E5 a common blend.

Belgium ● Ethanol plant at Wanze, Belgium (illustration next slide) commenced production in 2009 and produces up to 300000 cubic

metres of ethanol per year, thermally equivalent to 1.9 million barrels of oil.

● It receives feedstock from diverse sources, making for flexibility. These include syrup from Wanze Sugar Refinery, who are in fact the

owner of the bioethanol plant, so one might see this as an example of ‘integration’.

Czech Republic

● Produces, imports and exports ethanol. Automotive fuels as high in ethanol as E95 are available there.

● The ethanol plant at Vrdy in central Bohemia receives 150000 tonnes of cereal annually as feedstock for ethanol manufacture.

Romania ● Ethanol from ‘Black Sea grain’

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Ethanol plant at Wanze.

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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.

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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.

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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.

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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.

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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

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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.

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● 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 kg 1. 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.

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● 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

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● 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.

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Coal or coke when gasified with steam will react according to:

C + H2O → CO + H2

forming a gas equimolar in CO and H2

Heat of combustion of CO = 283 kJ mol-1

Heat of combustion of H2 = 286 kJ mol-1

A gas equimolar in the two will have heat of combustion:

0.5(283 + 286) kJ mol-1 = 284.5 kJ mol-1

● Having regard to the fact that any gas or gas mixture at 1 bar pressure and room temperature contains approximately 40 moles, the calorific value of this gas will

be:

284.5 kJ mol-1 × 40 mol m-3 = 11.4 MJ m-3

which is just under a third the calorific value of typical natural gas. ● It is quite suitable for ‘heavy duty’, e.g. in a steelworks, but would have to be used

on a suitably designed and adjusted burner.

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Another form of coal gasification: with air.

C + ½O2 (+ 1.88N2) → CO (+ 1.88N2)

Resulting gas of composition (molar basis):

1/2.88 CO = 35% COCalorific value 0.35 × 40 mol m-3 × 283 kJ mol-1

= 4 MJ m-3

● Notwithstanding its very low calorific value, such a gas (‘producer gas’) can melt steel. Again a suitable burner required.

● Considerable revival in producer gas recently, in particular with wood

instead of coal as feedstock making the carbon monoxide constituent carbon-neutral.

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Miscellaneous calculations.

Teaching value:Some practice with routine calculations.

A few new ideas introduced.Some perspective on international trends in hydrocarbons.

‘Warm-up’ question.The Bonaparte gas pipeline in Australia’s Northern Territory is 287 km

length. It is wholly within the Territory, not crossing any boundaries. The pipe supplier gives the delivery as 30 PJ per annum – petajoules per year, where peta denotes 1015 – of gas. This is recalculated in volume

terms below.

[30 × 1015/365] J day-1/37 × 106 J m-3 = 2 million cubic metres per day

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Further calculation.

Oil and gas often together in a reservoir, and if they ascend a well together the gas once separated is called associated gas.

The Prirazlomnoye Field offshore Russia is a shallow field, water depth 20 m, and produces 14000 barrels per day of oil and 106 m3 per day of gas. How much heat can a day’s oil and a day’s gas from the field release?

Would be given to you in an exam: see slide 49.Solution: ↓

A barrel of oil on burning releases 6 GJ of heat, therefore a day’s oil from Prirazlomnoye is capable of releasing 8 × 1013 J (80 TJ) of heat.

↓The calorific value of natural gas is about 37 MJ m-3 (the m3 referred to 1 bar, 288K). A day’s gas from Prirazlomnoye can therefore release 4 × 1013 J (40 TJ) of heat, half

that released by the oil.

Note the point on slide 49 that ‘natural gas always attracts a much lower price on a heat [rather than quantity] basis than crude oil’. In the above example, as the gas releases about half the heat that the oil does it will be worth about a tenth the

sale price of the oil.

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Natural gas is often supplied in liquefied form as LNG (liquefied natural gas) and is transported as such by tanker. At the Dabhol LNG reception

terminal in India 1.2 million tonnes per annum of LNG is converted back to gas and used to generate electricity. At what rate will it so generate if

the efficiency is 35%?

Rate of electricity production = 1.2 × 109 kg × 55 × 106 J kg-1* × 0.35/(3600 × 365 × 24) W

= 730 MW

Notes.● India receives a great deal of LNG from countries including Oman.

● The world’s largest exporter of LNG is Malaysia. The world’s largest importer is Japan.

● The US exports no LNG at all: all that produced goes to the domestic market and the deficit is imported.

* This calorific value would be given to you in an exam.

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At the Deborah Field in the southern part of the UK sector of the North Sea approval has been given for creation of a natural gas storage facility at the

field which, it is hoped, will enter service in 2015. The storage facility, operated by Eni, will have a capacity of 4.6 billion cubic metres (measured at 1 bar,

288K: this is an indication of amount and does not express the storage conditions) and its introduction will double the gas storage capability of the

whole UK. To how many barrels of oil is the gas so stored thermally equivalent?

Solution.

(4.6 × 109 m3 × 37 × 106 J m-3/44 × 106 J kg-1) × (1/900 kg m-3) × (1/0.159 m3 bbl-1)

= 27 million barrels

Note.● Clearly the magnitude is such that the facility would, if kept full, act as a

strategic reserve, which by government authority could be drawn on in a contingency if there were interruption to supply from UK or Norwegian fields.

57

In 2010 the Kazakhstan-China oil pipeline, the total length of which is 2205 km, exported a quantity of the order of 70 million barrels of oil from Kazakhstan to China. At one

of its junctions the pipeline changes from 24 to 32 inches diameter, but the mass flow rate must by the principle of continuity be the same in pipe of either diameter.

The speed in the 24 inch (0.61 m) segment can therefore be estimated in the following way.

70 × 106 barrels year-1 = 70 × 106 bbl × 0.159 m3 bbl-1/(365 × 24) hours year-1 m3 hour-1

= 1270 m3 hour-1

Flow speed = volumetric flow rate/pipe cross section =

[1270 m3 hour-1/(π × 0.3052) m2] × 10-3 km hour-1 = 4.3 km hour-1 (2.7 m.p.h.)

Note.The value obtained above is at the low end of the range of speeds of oil in pipelines – 3

to 8 m.p.h. is the ‘rule of thumb’ range often quoted – and it will be lower still in 32 inch pipline.

58

The Patos-Marinza Oil Field in Albania has been producing since the 1930s. The cumulative production by the middle of the first decade of the 21st Century (that is, about 70 years after production began)

was 120 million barrels.

That the field started to become moribund over time was, given its location, inevitable and there was well workover to increase the

productivity . It was reported in April 2011 that in the first quarter of that year Patos-Marinza had been producing at an average rate of 11894 barrels per day. Compare this with the average rate over the

seventy year period previously referred to.

[120 × 106/(365 × 70)] barrels per day = 4700 barrels per day

Notes. ● Recent production is over twice that.

● The simplest form of ‘well workover’ is just pouring acid down it!

59

Rio de Janeiro-Belo Horizonte Gas Pipeline II a.k.a. Gasbel II, entered service in 2010 and is entirely entirely within Brazil. It is 267 km long, the pipeline diameter 18 inches in diameter and its delivery is 5 million cubic metres per day. Determine the Reynolds

number.

Speed of flow of the gas = {[5 × 106/(24 × 3600)] m3s-1/[π × (9 × 0.0254)2 m2} m s-1

= 350 m s-1

↑Very high – see notes below

Using a value of 1.4 × 10-5 m2s-1 for the kinematic viscosity of methane gives a Reynolds number of:

(350 × 18 × 0.0254/1.4 × 10-5) = 107

Notes added 19.2.● Gases exiting compressor stations are frequently ‘supersonically ejected’.

● There are three compressor stations along Gasbel II to sustain flow. One or two compressor stations would be more common for that length.

60

LNG usage began in the USA in the 1940s, and the first transport of LNG be sea was from Louisiana to the Thames estuary in 1959 in a converted naval vessel called ‘Methane Pioneer’. Negishi LNG terminal, in Japan is

operated by Tokyo Gas and began receiving LNG in November 1969. With a shipment in November 2008 it attained a cumulative amount of

100 million tons.

Averaging over the period and approximating tons to tonnes for simplicity, this terminal performance expressed in thermal terms is:

100 × 106 × 103 kg × 55 × 106J kg-1/(39 × 365 × 24 × 3600) s = 4.5 GW

● Note the point made earlier that Japan is the world’s largest importer of natural gas. It was also one of the earliest importers of LNG.

61

Statpipe is a natural gas pipeline, linking gas from fields in the Norwegian sector of the North Sea to sales destinations in mainland Europe. It was opened in October 1985 and has conveyed 120 billion cubic metres of

gas since, a heat supply averaging to:

120 × 109 m3 × 37 × 106 J m-3/(25 × 365 × 24 × 3600)] W = 6 GW approx.

● The pipe is of total length 880 km, 25% shorter than the much more recently laid Langaled pipeline, which conveys natural gas from Norway

to England.

62

An oil field with associated gas is by no means rare. One seldom encounters the term in reverse – ‘gas field with associated oil’ – but it would not be incorrect thus to describe the Troll Field, which is in the Norwegian Sector of the North Sea. The 2008 production figures are 138000 barrels per day of oil and 120

million cubic metres of gas. A related calculation follows.

heat releasable from one day’s gas/heat releasable from one day’s oil

= 120 × 106 m3 × 37 × 106 J m-3/(138000 bbl × 0.159 m3 bbl-1 × 900 kg m-3 × 44 × 106 J kg-1)

= 5 to the nearest whole number.

Note.

● When pricing is factored in the half-an-order-of-magnitude higher figure for oil production will be eroded in that gas and oil do not sell equivalently on a heat basis, the latter being the more expensive as

noted. This makes the ‘associated oil’ able to raise more than a proportionate revenue.