Energy In Transportation

Petroleum As The Energy Source

What is Petroleum?

Petroleum is derived from the words ‘petra’ and ‘oleum’ meaning rock oil.
Animal biomass buried under earth crust for millions of years under high pressure in the absence of oxygen led to the formation of sedimentary rock with complex hydrocarbons. This we call petroleum. Majorly it constitutes crude oil and natural gas.

Crude Oil

Crude oil is a complex mixture of hydrocarbons.
The different constituents of the crude oil mixture is simply different hydrocarbons with varying number of carbon atoms.
This gives them varying densities and thus have different boiling points.
Note: The basic factor that determines boiling temperatures of different components is the amount of carbon they have.
Higher the carbon content higher the density higher the boiling temperature.

Separating The Crude: Fractional Distillation

The different components of crude oil mixture with different densities boil at different temperatures.
Thus, in order to separate crude, we heat the mixture to about 6000 Celsius which boils the entire mixture.
This vapour mixture is then made to pass through a fractional distillation column which is maintained at different temperatures at different levels. (lower to higher from top to bottom)
Depending on their respective densities the constituents condense upon cooling down at different levels thereby separating itself from the mixture.
Thus, those hydrocarbons with high boiling point condenses low in the column and those with lower boiling point condense at the top.

Further higher the carbon atoms the fuel is found in solid form.
Hydrocarbons with 1 to 4 carbon atoms are gases at room temp. Eg: Methane, Ethane, Propane and Butane.
These components do not condense and thus are pressurised at 5 bar to make LPG.
Hydrocarbons with 5 carbon atoms to 14 carbon atoms are found in liquid form. Eg: Petrol, kerosene, diesel.
Hydrocarbons with more than 16 carbon atoms are normally found in solid form.
Note that since all the components are essentially hydrocarbons, they all are combustible. Only difference is they combust at varying temperature.
More the amount of carbon atoms more difficult it is to break the bonds and thus at higher temperatures they burn. For instance, petrol can burn very easily as compared to lubricant oil.

Components Of Crude Oil

Table and figures above and below provide a snapshot of components of crude oil, their properties and their application.

Typical Products

Products	Proportion	Boiling Point in degree Celsius	Application	Carbon Atoms
LPG	4%	Below 30	Cooking Fuel/ Transportation	Propane and Butane (C3-C4)
Naptha		70	Used in gasoline and Chemicals
Petrol	47%	100-150	Used in gasoline	C7 to C9
Kerosene (Paraffin Oil)	10%	170	Used as Jet Fuel and Heating and Lighting oil	C10 to C16
Diesel	23%	270-350	Transportation	C14 to C 20
Lubricating Oil		>350	Engine Oil, Polish, Wax	C20 to C50
Fuel Oil		600	Industrial Heating
Asphalt/Bitumen (liquid asphalt)	3%	Residue	Tar for roads, sound absorbers	>C70

Basics On Hydrocarbons

Straight chain and closed rings

The classification of hydrocarbons in general depend on how the carbon atoms have arranged themselves.
This decides their chemical and physical properties.

Aliphatic compounds

Carbon atoms in aliphatic are arranged in straight chain manner (open). Further aliphatic are classified based on how the carbon atoms are bonded with one another. (single or double bonds). This is important in all organic matter because single bonds are very hard to break and thus make stable compounds (saturated). Double bonds on the other hand are easy to break and thus make unstable compounds (unsaturated).

Aromatics and Naphthalene

These are hydrocarbons where carbon atoms are arranged in closed rings. Again, depending on single or double bonds they are classified into naphthalene and aromatics respectively. Eg: Benzene (details are not important for future civil servants).
Note that aromatics from any source react with sunlight and moisture in atmosphere to form ozone at the ground level which is a pollutant.

Composition of Crude oil

Crude oil has primarily hydrocarbons, but also some non-hydrocarbon component.

Hydrocarbons

Crude oil majorly contains saturated straight chain hydrocarbons called paraffins. Examples include C1 to C4 in the form of gases (methane, ethane, propane and butane) and C5 to C10 in the form of liquids, common examples including kerosene, petrol, diesel etc.
In minor amounts it contains closed ringed double bonded hydrocarbons which are volatiles like benzene. (which is unstable and therefore a pollutant)
The major difference between coal and oil & gas is the C-H ratio (no of hydrogen for every carbon). It is the C-H ratio that decides the amount of energy you can get out of a fuel (very important for you to appreciate the nature of fuels). So, methane (4 hydrogen for every carbon) will have higher energy compared to propane (C3H6: 2 hydrogen for every carbon).

Carbon	83-87%
Hydrogen	10-14% (up to 5.5% in coal)
Nitrogen	0.1-2%
Oxygen	0.1-1.5%
Sulfur	0.5-6%
Metals	<1000 ppm

Non-Hydrocarbons

Sulphur, Nitrogen, Oxygen, Metals. Table above gives you rough composition of crude oil components.

Characteristics of fuel for transportation

Amount of energy

The major difference between coal and oil & gas is the C-H ratio (no of hydrogen for every carbon). It is the C-H ratio that decides the amount of energy you can get out of a fuel (very important for you to appreciate the nature of fuels). So, methane (4 hydrogen for every carbon) will have higher energy compared to propane (C3H6: 2 hydrogen for every carbon).

Energy density

Most important characteristic that decides the suitability and favorability of a fuel in transportation is energy density. Energy density is simply how much energy is there in every gram of fuel.
Table below gives you an idea. (not important to remember but important to compare)

Coal	6 Cal/gm
Gasoline	10 Cal/gm
Natural gas	13
Hydrogen	26
U-235	20 million

Energy in volume

In addition, for automobiles, how much fuel can a box of 1cm x 1cm x 1cm hold become very important. (this is the factor that determines the size of the tank).
This is simply an indicator of how much fuel is there in 1 litre at atmospheric pressure. Table gives you an idea.

Petrol	740 g
Diesel	840 g
Hydrogen	71 g

This means you can only fill 71 grams of hydrogen in 1 litre bottle.
If you want to fill more, you need to compress it as a compressed gas or even liquify it which will require high pressure.

Knocking and Octane Number

Knocking is an important property of any internal combustion engine.
Petrol or diesel are not a homogenous mixture of fuel. It is a mixture of hydrocarbons (C7-C9 for petrol, C14 to C 20 for diesel) which have different boiling points.
As a result, some components of the fuel burn faster than others resulting in a lag in complete combustion of fuel.
This lag leads to shock waves in the engine cylinder causing a damage to the piston. This is called knocking.
Simple solution is we need to homogenize the fuel. This is done by using fuel with higher octane number (simply more C8 hydrocarbon).
Usually this is expressed as the percentage of Octane (91-94 etc.)

Combustion in IC engines

Combustion includes breaking of bonds between carbon and hydrogen and allowing them combine with oxygen in air resulting in liberation of energy in the form of heat in the combustion chamber of the engine. This heat pushes the piston which runs the crank shaft and thence to rotating of the wheels.
Air-fuel ratio
Hydrocarbons burn when air:fuel ratio is between 7:1 to 30:1
In IC engines air-fuel ratio is maintained at 15:1
If air-fuel mixture is more: Lean (In this case fuel is not completely burnt giving rise to more CO but it gives more mileage)
If air-fuel mixture is low: Rich (In this case fuel is completely burnt thereby give more power output)

Challenges in burning transportation fuels

Emissions from IC engines

Source of Emission

When combustion takes place inside the combustion chamber the fuel and air is mixing to form oxides. As they combine pollutants are emitted.
Air consists of 79% of nitrogen – Nitrogen does not participate in combustion and thus whatever comes in will go out of the exhaust.
CO₂ and H₂O will be produced because you are burning hydrocarbons.
Rest is pollutants. This concentration is about 1%.

Regulated and Unregulated Emissions

Emissions are classified into 2 types

Regulated Emissions
These are in large concentration.
Emissions are very perceptible, and you immediately feel the discomfort.
These include NOx, PM, CO, hydrocarbons.
Carbon Monoxide more in petrol emission.
Long-term exposure to CO prevents oxygen transfer in the blood because hemoglobin attracts CO more readily. When hemoglobin reacts with CO it produces carboxy hemoglobin which is very stable.
Hydrocarbon emissions are main problem in petrol engines.
NOX emissions are more prevalent in diesel engines. Can cause lung tissue damage, eye irritation because NOX reacts with water to cause nitric acid.
PM is more prevalent in diesel engine. They harm the respiratory tracts.
Unregulated Emissions
These are emission in low concentrations and are not perceptible. But long-term is very fatal.
These include formaldehyde, BTX (Benzene, Toluene, Xylene), Aldehydes, SO2, CO2, Methane, Poly-Cyclic Aromatic HC and Nitro PAHC.

How do you deal with Emissions?

Engine Design

Engine is designed to produce certain concentrations of emissions by calibrating the combustion at various speeds and load which varies with urban/rural road for example. This is called engine tuning.
Further optimize the combustion in order to get more heat output and thereby increasing efficiency.
This is done by the way fuel-air mixture is delivered to the engine.
Increase the number of valves: Earlier it used to be 1 intel 1 outlet not we have 2 inlet 2 outlet under 4-valve. This makes gas exchange more efficient.
Increase the pressure at which fuel is injected: Higher the injection pressure smaller will be the droplet size of the fuel. Smaller will be the droplet size better will be the mixing of fuel and air. This will lead to increased efficiency of combustion thereby reducing the pollutant emissions.

Emission Reduction Strategy for NOX and PM

EGR + Soot Trap

Exhaust Gas Recirculation is basically done in order to reduce NOx emissions.
Problem: Once EGR is done this produces lot of particulates. This is tackled by deploying soot traps.
This will meet Euro 4 or Euro 5

Exhaust After-treatment

NOx reduction is the goal using some strategy to treat the exhaust gases. 2 main technologies include
SCR (Selective Catalytic Reduction)
Here we use urea which is broken into ammonia and H2O. Ammonia reacts with NOx to produce nitrogen and water.
Lean NOx trap system
As a result of NOx treatment strategies, you can burn the fuel at high temperatures which reduces the PM formation.
Using combination of EGR + Trap + Exhaust after-Treatment we can meet BS 6 criteria.

Note: For after-treatment technologies to work we need to low-sulphur fuel (10-15ppm) because sulphur kills the catalysts like plutonium, rhodium etc. which are used in SCR technology.

Sulphur

In addition to killing the catalysts, sulphur is responsible for PM formation.
Crude oil has in it sulphur is 2 forms hydrogen sulphide and sulphur in free form.
This sulphur upon being heated reacts with oxygen to forms oxides of sulphur, SO2 and SO3.
The oxides of sulphur react with moisture in the engine to form H2SO4 vapours which leads to formation of sulphates which act as the nuclie for PM formation.
Thus, we need to reduce the sulphur in the fuel to arrest PM pollution.
But sulphur provides lubricity to the fuel. So, if we reduce sulphur, we need to add lubricants to the fuel.