COMBUSTION FUNDAMENTALS: OIL, ITS ORIGIN AND REFINING

 

The Origin of Oil

Petroleum is was formed as a result of the laying down of organisms such as plankton and bacteria millions of years ago on the sea floor. The main differences between coal and oil are that

 

Formation - Over the years a layer of partially decomposed material was formed (source rock horizon) which was subsequently covered with layers of mud / sediment (or strata). Subsequent compaction of this matter forms "kerogen", a complex mixture of hydrocarbons which under special conditions of high pressure and temperature produces oil and gas. Temperature, pressure and earth movement cause the oil to migrate to favourable locations. That is porous sedimentary rock called "Reservoir Rocks".

Because these substances are less dense than their surrounding rocks they tend to rise until they reach impermeable rock above where they are trapped preventing escape. This is called the cap rock.

 

Refinery Operations

Every crude oil has very different characteristics. It may be heavy (with more heavier hydrocarbons) or light (more smaller H/Cís), it may be aliphatic (many long chain molecules) or asphaltene (more ring structures).

It certainly contains a complex mixture of many different chemicals which must be separated and treated. Refinery operations fall into three categories:

  1. Physical Separation - Distillation, Solvent extraction, desulphurisation.
  2. Breaking Down - Cracking, Visbreaking, Coking
  3. Rebuilding Processes - Reforming

The first process is distillation. The different components of the mixture are seperated in a distillation column by their boiling range. The crude oil is vapourised and passed into a vertical column. The vapours rise until they reach a temperature where they condense onto trays and are taken off. See fig 1. Table 1 gives the approximate boiling range of the smaller hydrocarbons

 

 

 

Approx. boiling pt range (C)

Number of C atoms

Description

20

1-4

Gas (LPG)

20-60

5-6

Gasoline

40-180

6-10

Naphtha

180-260

10-14

Kerosine

260-340

14-20

Gas Oil / Diesel

340 plus

20 plus

Fuel oils, waxes, bitumen, coke.

 

The amount of each fraction depends on the crude. Table 2 gives the composition of various crudes (expressed on a gravimetric basis)

Source

Naphtha

Kerosine / Gas oil

Fuel Oil

UK North Sea

30

40

30

Nigeria

25

37

38

Middle East

20

32

48

North Africa

30

40

30

Venezuela

1

19

80

 

Amongst other factors this depends on the degree of permeability of the cap rock and the proximity of the oil to ground level. For example in Venezuela, the oil is relatively close to the surface and hence all the lighter fractions have evaporated off over the years leaving the heavier residue.

Of course the levels above are not necessarily those demanded by the market place. The most significant user of petroleum are the transport and industrial sectors which demand lighter fractions. Very heavy fuel oils are only valuable in power generation where they compete with the cheaper alternative coal.

The problem is solved by cracking the heavier components. Cracking is the process of reducing the size of the molecules to form lighter ones.

 

CRACKING

Thermal Cracking - is the oldest, simplest but least effective method where the molecules are broken down by the action of heat. When heavy petroleum is heated above its decomposition temperature, the molecules are broken down and rearranged. This results in an increased yield of gasoline. Gas and petroleum coke are also formed.

 

Catalytic Cracking - is the next most sophisticated method. Thermal cracking of heavy distillates is not very selective and produces substantial quantities of gas and fuel oil as well as gasoline. The gasoline is also of not very good quality. The heavy oil is heated to a lower temperature under pressure with a catalyst. Catalysts are usually natural or artificial clays (e.g. bentonite, montmorillonite activated by sulphuric acid or aluminium silicates). More recently zeolites are used due to higher activity. The catalysts are in the form of pellets, beads or powder depending on the process.

The result is a higher liquid yield (lower coke and gas) but more importantly a better quality of gasoline due to higher levels of iso-paraffins and aromatic H/Cís. All new cracking units for gasoline production from heavy oil are of this type.

The main effect of the catalyst is to direct the cracking of the alkanes towards the molecule and to convert alkenes into the corresponding alkanes. Naphthalenes are converted to the corresponding alkanes and alkenes. Aromatics are largely inert although a small proportion do get converted to coke. This collects on the catalyst surface and deactivates it. Catalyst regeneration is vital to the economics of the process.

Crackers are either of the fixed bed (batch) , moving bed (continuous) or fluidised bed type. Fluidised beds are preferred nowadays due to uniformity of temperature and higher heat and mass transfer rates. A typical yield for a modern plant (10,000 Tonnes per day of feedstock) is 50-60% gasoline of octane number 90.

 

Visbreaking - is the process used if to crack the remaining very heavy residues left from the refining process. These substances are very viscous and are not easily transported. They may be diluted with high value gas oil to reduce viscosity but this is expensive. However, it is possible to subject this substance to a mild cracking which breaks down enough of the heavier compounds to lower boiling point, less viscous ones to greatly reduce the need for gas oil. Care must be taken to minimise the formation of excessive coke.

Coking - is undertaken in certain refineries (patents CONOCO) to break down the final residuals to coke. Under severe temperature conditions of thermal cracking, the liquid feed is converted to gas, naphtha, fuel oil, gas oil and coke. Conditions can be adjusted to either produce gas oil or coke as the main product. Petroleum coke produced from petroleum of low sulphur is extremely valuable for the production of electrodes for aluminium and steel manufacture. Otherwise it is used to raise steam in the refinery.

 

REFORMING

Once the basic products are distilled and cracked, they still may not have the ideal formulation. Reforming may be used to adjust their composition.

 

Thermal / Catalytic reforming - of gasoline is similar to thermal cracking in principle and is used to improve of octane number. Temperatures and pressures are generally higher than for thermal; cracking. Products of the process are gasoline, residual oil and gases. The amount of gasoline is dependent on temperature but also on the catalyst. Catalysts not only accelerate the process but increase the yield of reformate. The most effective is platinum on purified alumina. Companies have developed their own special catalysts. Can be fixed or moving bed type.

 

Hydroforming (Hydrocracking) - is the process whereby the reduction in H to C ratio of a high boiling point product. As the number of C atoms in the molecule increases so it hydrogen content falls. Heavy, high boiling point fractions may be cracked in the presence of high pressure hydrogen with catalyst to result in saturated, lower boiling point products. Temperatures of 480-540C are used at pressures of 15 - 20 atm. High duty equipment is necessary for these conditions but also a hydrogen plant is required.

Platinum may be used if the feed has been pretreated to remove sulphur (Platforming). Platinic Chloride is the best but must be replaced periodically for regeneration. Catforming allows occsional regeneration in situ

 

 

 

COMBUSTION FUNDAMENTALS: LIQUID FUELS

 

Consider the energy density of some typical fuels:

Fuel

Calorific Value (MJ / Nm3)

Bituminous Coal

41,600

Lignite Coal

19,000

Kerosine / Diesel

36,700

Gasoline / Petrol

31, 320

Heavy / Residual Fuel Oil

40,000

Natural Gas

33.8

Producer Gas

6.0

 

Liquid fuels combine the flexibility of transporting a fluid fuel with the massive energy densities of coal. It is not surprising that the dominant market for liquid fuels is transportation. As with other fuels, CV, sulphur content, ultimate analysis are relevant properties. However, there are a number of properties of liquid fuels which are unique and depend on the fuel utilisation.

 

MOTOR FUELS

The fuel type depends on the engine: Spark Ignition (Gasoline), Compression Ignition (Diesel). Both require very different properties.

Spark Ignition Engines

These run with a premixture of fuel and air at nominally stoichiometric ratio. The principle requirements are a very high volatility to allow the fuel to vaporise and a high resistance to autoignition.

Octane Number - It is vital that the fuel does not auto-ignite before the flame front reaches it. This is dependent on the spontaneous ignition temperature. SIT is lowest for large alkanes and higher for aromatics, i.e. aromatics are the best. An "Octane" number is defined for each fuel as the percentage of iso-octane in an iso-octane heptane mixture which gives the same knocking tendency as the fuel. There are two ways of increasing octane number: increase aromatic content (e.g. benzene) or use additives such as tetra - ethyl - lead (TEL) which suppress auto-ignition. It is cheaper to produce a high octane gasoline by the use of TEL instead of increasing the aromatic content. 4* petrol has an octane number of 98, 2* = 90.

Other factors play a role in the fuelís performance make up. Sulphur results in emissions of sulphurous oxides and also plays a role in corrosion, additives such as detergents help to keep the engine clean and therefore running better for longer.

 

Compression Ignition Engines

These engines work with a non premixed, variable air fuel ratio. Importantly, they do not rely on the fuel being prevapourised as with spark ignition devices and do not require an external ignition source (a spark) but rely on the autoignition of the fuel and air mixture as it is brought up to above it SIT. The main technical requirement is therefore that the fuel should have a low SIT.

Cetane Number - If a high SIT fuel is used then a version of knock is experienced. As the gases are compressed in the compression cycle there is a delay before autoignition temperature is reached. If this delay is too great a large percentage of the fuel charge has already been injected resulting in a sudden detonation and a rough, bumpy cycle. The situation is far worse for high speed diesels. The best fuels thus have the lowest SIT temperature - Completely the opposite of spark ignition engines!

The ideal Diesel fuels are long chain aliphatic (alkane) compounds such as cetane (C16H34). A cetane number is thus defined based on a cetane / methyl-napthalene mixture which has the same ignition delay time as the fuel. As the speed of the engine reduces, the requirement of a high cetane number reduces. For high speed diesels (rpm>1500) the value should be greater than 45. For low speed Diesels 25 to 30.

Other factors to be considered with diesel fuels are Viscosity (for good fuel atomisation) and also pour point and cloud point.

Pour Point and Cloud Point - are relevant for fuel oils in cold surroundings. The pour point is defined as the temperature 2.8C (5F) above which the oil ceases to flow when cooled under prescribed conditions. Obviously, this is vital to winter driving conditions. Cloud point is the temperature at which waxy crystals are formed in the fuel making it cloudy. This is relevant since these crystals can block filters long before the pour point is reached.

 

 

 

AVIATION FUELS

If the aircraft uses a piston engine then the relevant properties are similar to those above. Modern aviation turbines use kerosine which has other relevant properties. These focus on two areas: safety and integrity of the engine.

Flash Point - This is a very important parameter for safety. For any liquid fuel to burn it must first vaporise and mix with air (oxygen). Before combustion, this vapour must be produced by natural evaporation. If evaporation is too low, then a flammable mixture will not be generated. This evaporation is governed by temperature. Flash point is the temperature at which the oil evolves just sufficient vapour to allow a flame and is tested by the formation of a momentary flame (or flash) when sparked. By the same token, the fire point is the temperature when at which the oil vapours will continue to burn once ignited. This is higher than the flash point. Fuels with flash points below 23C (e.g. gasolines) are considered dangerous and highly flammable.

Smoke Point and Char Value - are used to indicate the burning quality of the fuel. If the flame contains high soot and smoke levels it is very luminous. This can result in the combustion chambers being overheated due to excessive radiation and is potentially dangerous. The smoke point is the maximum flame height in millimeters to which a kerosine will burn in a standard candle flame apparatus without emitting smoke. Char value is similar, it is the amount in mg / kg of char produced when a kerosene is burned under prescribed conditions in a standard lamp.

 

FUEL OILS

Fuel oils fall into a number of categories ranging from light fuel oils (gas oil) which is very similar to Diesel oil through to heavy fuel oil and residual fuel oil. They are discriminated on the basis of their boiling range in the distillation column and are referred to by the ASTM standard number. e.g. No.1 fuel oil, No.2 etc.

Lighter oils are used for heating and power in remote areas where gas and electricity are not available. Heavier oils are used in steam raising and large marine engines. In general they are not the preferred fuel in any situation due to price. It is far more economic to crack them to lighter oils for transport if at all possible.

The heavier oils are essentially the residue of the distillation process and are very variable and subject to few specifications. They are characterised by progressively higher viscosities, sediment and sulphur and ash.

Ash levels - tend to be far lower than for example coal (normally 0.2% up to 1% for the residual fuel oils). This creates few problems in terms of ash handling but can cause major problems if certain compounds are present. Petroleum ash tend to have high levels of Vanadium and sodium. This results in the formation of ash which is of very low melting point (some as low as 250C up to 680C). This causes problems since this is in the range of operation of modern superheaters. A semi-molten ash particle becomes sticky and adheres to the tube. These soon build up resulting in an insulating layer and therefore increased temperature. This damages the tubes and reduces their life very significantly. Another problem is that the sodium / vanadium ashes are corrosive and enhance oxidation thus reducing life further. Ash deposition can reduce the life of a superheater from 18 months to 1 or 2 months in severe cases. in some cases can attack refractories. Additives may be employed to reduce the problems of corrosion and ash build up. These basically change the nature of the ash produced allowing more effective cleaning and sootblowing. Examples are dolomite, alumina and magnesia.

Viscosity - Fuel oils, especially the heavier ones have very high viscosities. This creates problems of pumping but also of atomisation. Often preheating of the oil is required to reduce viscosity. This adds significantly to the cost of the plant. There is an optimum temperature since increasing it has the effect of improving fuel delivery and atomisation but also of reducing density therefore resulting in lesser delivery of oil.

Fuel Composition - has a great effect on fuel performance. These fuels have a very high C/H ratio resulting in sooty, highly luminous flames. This can be an advantage for heat transfer applications. A major problem with some oils, particularly cracked oils, is gum and sludge formation. They do not store well and oxidizable components produce sediment or gum which blocks filters and cause problems. Perhaps the most significant component of oils to affect their behaviour is sulphur. Fuel oils may have sulphur contents up to 4.5% depending on the crude oil. This is far higher than for example coal and is further aggravated since with coal much sulphur is trapped with the ash. This causes further problems of corrosion and pollution.

 

ORIMULSION

A relative newcomer to the fuel market. Orimulsion originates in the Orinoco basin in Venezuela. This oil field is extremely close to ground level and as a result of years of evaporation only the very heaviest of compounds remain which are solid at normal temperatures (asphalt and bitumen). This is very difficult to extract using normal procedures but methods have been developed using detergents. Water with a low grade surfactant (detergent) is pumped into the deposit at very high pressure. The bitumen emulsifies to form a liquid emulsion which is then pumped to the surface for use. The simplicity of the technique and the accessibility of the reserves make the product extremely cheap. Its price has been therefore linked to coal prices (not oil) and it is marketed as "liquid coal".

Orimulsion has similar properties to heavy fuel oil except that it has a lower CV (due to the water). It is being burned at Pembroke power station in the UK as a demonstration unit.

 

 

 

A COMPARISON BETWEEN HEAVY FUEL OIL AND COAL FURNACES

HEAVY FUEL OIL

COAL

PROS

- Higher calorific value - smaller furnace - Require 50% less mass of oil than coal for the same rating.

PROS

- Lower sulphur content and some sulphur contained in ash - Less FGD equipment

- Can be burned with less excess air - smaller fan and furnace. Lower fan costs.

- Lower Vanadium and sodium content - Fewer problems with ash sticking and corrosion on boiler tube

- Less ash content - smaller ash disposal

equipment

- Cheaper

- Lighter fuel oil burners may be designed for much lower loads. Coal plant is necessarily always big

- Because the furnace is operating leaner due to the excess air requirement there is less chance of local rich pockets and soot formation.

- Oil is easier to ignite than coal and since start up is easier, furnaces may be banked at a faster rate to meet demand.

- Storage relatively simple.

- Easier storage

 

- Oil is easier to atomise and contains less soild carbon. The char particles left after devolatilisation are much smaller and easier to burn out. NB: This char burn-out is still the longest step in the reaction.

- Since residence time is reduced, the furnace can be made smaller.

 

 

 

 

HEAVY FUEL OIL

CONS

- More expensive - Especially true for the lighter oils (not true for orimulsion)

COAL

CONS

- Grinding of coal expensive and complex

- Handling complex due to the requirement to heat the oil before combustion.

- Larger furnace

- Higher Vanadium causes problems with deposition and corrosion.

- Less easy to bank and load change

- Higher SOx emitter. Higher sulphur content than coal plus coal can retain 10% of sulphur in ash.

- Greater ash handling costs

- Problems due to sooting and acid smuts around the heat exchanger.

- Transportation- complex, bulky machinery required.