Classification of Gaseous Fuels
Gas fuels are the most convenient requiring the least amount of handling and simplest and most maintenance free burner systems. Gas is delivered "on tap" via a distribution network and so is suited to a high population or industrial density. However large consumers do have gas holders and some produce their own gas.
The following is a list of the types of gaseous fuel:
(A) Fuels naturally found in nature:
Methane from coal mines
(B) Fuel gases made from solid fuel
Gases derived from Coal
Gases derived from waste and Biomass
From other industrial processes (Blast furnace gas)
(C) Gases made from petroleum
Liquefied Petroleum gas (LPG)
Gases from oil gasification
(D) Gases from some fermentation process
When deciding whether an alternative gas can be used in an appliance, three factors must be considered:
- For the same pressure drop is the heat release roughly the same
- For the same air and fuel flows is the flame shape the same
- For the same heat release conditions are pollutants within a specified tolerance
The first criteria is best summarised by consideration of the Wobbe Index.
Consider the flow of gas through the control valve. It may be considered as an orifice of area, A
The heat release Q is calculated knowing the CV of the gas
The pressure drop is calculated from Bernoulli's equation.
Continuity gives the volume flow rate:
The combination of these gives the heat release in terms of CV (J/m3), pressure drop, density and area
Now if the fuel gas is changed on the same burner (i.e. Area and pressure drop remain the same) the new heat release rate may be calculated as the ratio of the CV multiplied by the square root of density.
Thus, a number may be derived which gives an indication of the interchangability of the gases, the Wobbe number. In practice, the specific gravity with relation to air is used instead of density and Wo = CV/Ö sp gr.
A second factor is used to define the propensity of the gas to react. This is called the Weaver flame speed factor. It is defined as the ratio between the laminar flame speed of the gas of interest with relation to hydrogen. Thus Hydrogen has a value of 100. The lower the number the lower the flame speed. Weaver speed factor is greatly influenced by the amount of hydrogen in the mixture.
If the Wobbe number and flame speed factor are identical for two gases they are completely interchangeable. Unfortunately, this still doesn't guarantee the emissions will be the same.
Gases are classified according to Wobbe and Weaver numbers:
The international gas union assign the following gas families:
Family 1 - Wo = 17.8 - 35.8MJ/Nm3 - Coke Oven gas, Low CV gas
Family 2 35.8 - 71.5 - Natural gases, Town gas
Family 3 71.5 - 87.2 - Liquefied Petroleum Gas (LPG)
High flame speed gases We = 32 to 45
Intermediate speed We = 25 to 32
Low flame speed We = 13 to 25
Types of Gaseous Fuel
Naturally occurring gas found in oil fields and coal fields (Fire damp). The quantities of the constituents vary but the principle component is methane. Other components include higher hydrocarbons which can be separated out as a condensate. Some gases also contain hydrogen sulphide.
Terms used to describe gases:
dry or lean - high methane content (less condensate)
wet - high concentration of higher hydrocarbons (C5 - C10)
sour - High concentration of H2S
sweet - low conc. of H2S
residue gas - gas remaining after the condensing process
casing head gas - gas extracted from an oil well by extraction at the surface.
Table * gives the composition of Nat gas from various sources.
Total world production of nat gas in 1986 was 100 trillion m3. It is used as feed stock as well as fuel. It is preferred due to its high CV. Gas from coal mines is of equal quality to oil fields however it is much more difficult to extract. In 1961 220 mill m3 of coal nat gas were extracted in the UK. The North Sea gas has smashed the industry.
Natural gases can be liquefied for distribution by tanker. Liquefied natural gas (LNG) contains mostly methane, LPG (Liquefied petroleum gas) mostly butane and propane.
These are gases which are chemically made by some process.
Increased interest presently in power generation due to the gasification properties of waste and biomass.
Main methods of synthesis:
Producer gas: The gas is produced by blowing air and sometimes steam through an incandescent fuel bed (the process is self heating). The reaction with air is exothermic but insufficient air is added hence CO is produced. Steam addition results in the formation of hydrogen by the water gas reaction. This is endothermic and hence balances out the exothermic air reaction.
Producer gas is low CV and is hence is only usually used on site
Blue Gas or Water Gas - This is produced in a similar manner to above but allows the production of a higher CV fuel by intermittently blasting the incandescent bed with air and steam such that the overall heat balance is maintained. The products of the air blast contain the nitrogen which reduces CV. These are discharged to atmosphere. The products of the steam blast are kept since they have a higher CV. CV is virtually doubled in this way. Often used as a synthesis gas in the chemical industry.
Oil Gas This is the gas formed by the thermal cracking of crude oil. If oil is sprayed onto heated checker work (refractory) it cracks to form lower gaseous hydrocarbons. These depend entirely on the feed stock but calorific values can increase to as much as 25MJ/m3 but can be as low as half of this.
Carburetted Water Gas - Water gas has still to low a CV for most purposes and this makes it unattractive to distribute. Carburetted water gas is the result of combining the water gas and oil gas methods. Oil is sprayed into the hot water gas chamber to result in a good quality gas. The ratio of the two determines the quality. This was the method used to produce the "Town gas" of old and has largely been superseded by natural gas in countries with an abundant supply. AS supplies of natural gas diminish, however, it will become more important again.
Coal and Coke Oven Gas - As mentioned previously, gases are liberated in the high temperature carbonisation (coking) of coal. These are cleaned, de tarred and scrubbed and used as fuel. If coke is not required (coal gas), steam injection at the end of the cycle reacts with the coke to form blue water gas. This reduces the CV of the gas produced but the thermal efficiency of conversion rises.
The main advantage of liquid fuels over their gaseous alternatives is the extremely high thermal energy / volume ratio. Typically they would have a heating value of 40MJ/kg which in volumetric terms is equivalent to 33000MJ/m3 compared with natural gas at about 34! Hence they have become the conventional gas for transport applications. Liquid fuels (light and heavy fuel oils) are also used in furnaces and boilers.
Not surprisingly the most heavily specified of all fuels are the transport fuels. Some common specification parameters follow. Most are self explanatory:
Cloud point - The temperature at which a sample has visible cloudiness
Pour Point - The temperature at which the fuel will no longer pour
Flash point - The temperature above which the vapour above a fuel in air becomes flammable
Gum content The quantity of remaining solid residue after a sample has been heated and evaporated for a prescribed period of time in air
Trace metal content - Va, Cu
Aromatic content - The percentage of aromatic hydrocarbon content
Thermal stability- The resistance of the fuel to thermal degradation measured by heating a sample to a specified temperature, filtering and comparing residue with standard coloration filters.
Relevant Properties of Liquid Fuels
There are two varieties of reciprocating engines, Spark Ignition (gasoline) and Compression Ignition (Diesel).
Spark Ignition -
OCTANE NUMBER - Vital that the fuel does not auto-ignite before the flame front reaches it. This causes knock. Knock is the single most important parameter which limits gasoline engine efficiency and power. 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. Additives such as tetra - ethyl - lead (TEL) improve octane numbers of fuel by suppressing 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.
The max power of a spark ignition engine is determined by the RON. As compression ratio increases so does power up to the point where autoignition takes place. This is also dependent on mixture ratio.
VOLATILITY / BOILING RANGE - Fuel must be burned in vapour phase but is supplied as liquid
If volatility too low, problems with lubrication. Too high, problems with vapour lock
Typical boiling range 30 - 150C
VAPOUR PRESSURE - Again if vapour pressure is too low there may be a problem with vapour lock
CALORIFIC VALUE - Maximum power per unit weight and volume of fuel is best.
Compression Ignition -
CETANE NUMBER - High speed Diesel engines also suffer from a variant of knock. There is a delay between the fuel injection at high p and T and the ignition. This is again related to the fuel spontaneous ignition behaviour. If this delay is too great a large percentage of the fuel charge has already been injected resulting in a sudden increase in pressure and a rough, bumpy cycle. 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 mixture / methyl - napthalene mixture which has the same ignition delay time as the sample. As the speed of the engine reduces, the requirement of a high cetane number reduces. For high speed diesels the value should be greater than 50.
VOLATILITY - Much lower than for gasolines. Effects ignition and vapourisation during combustion
VISCOSITY / FREEZING + POUR POINT - Diesels and kerosines contain heavier fractions and hence are more viscous and can freeze in winter.
The most highly specified of all fuels.
Table * shows typical aviation fuel specification. The additional parameters over and above transport fuels are the smoke point and flash point. The smoke point gives an indication of the tendency of the fuel to produce soot. This is important to the life of the combustion chambers since the majority of heat transferred to them is via radiation which itself is greatly affected by soot. The smoke point is very sensitive to the level of aromatics in the fuel and these are typically limited to 20-25%.
Another major concern with aero fuels is safety. Flash point defines the temperature above which the fuel vapours become flammable and is clearly an important parameter. Other fuel additives are sometimes used to reduce flammability such as Gelling and anti misting agents which are destroyed on pumping.
These are the heavier fractions from distillation and are classed into three categories light, medium and heavy. They are not attractive as general transport fuels due to their physical characteristics:
Viscosity / Pour point - They have high viscosity (depending on the grade) and often require heating. The minimum heating temperature is defined by the pour point.
Sludge Formation - Some oils, particularly cracked oils, do not store well and oxidizable components result in sediment or gum formation which blocks filters.
High C/H ratio - Resulting in sooty, highly luminous flames. This is an advantage for heat transfer applications
Sulphur - Residual oils have sulphur contents between 0.2 and 4%. This increases the dew point and corrosion. Accelerates gum formation.
Ash - Maximum amount usually present is 0.2%. The content is important. High vanadium ashes have reduced melting temperature and can build up as deposits and attack refractories.
Uniformity - An agreed specification is laid out to guarantee uniformity of quality.
Vanadium / Sodium - These compounds in ash cause eutectic compounds with very low melting points ( as low as 600C). This results in a liquid, sticky ash which congeals onto the heat exchanger surfaces resulting in faster build up, corrosion and problems.
The main advantage of these fuels is price. They are cheap. However, the capital cost of a plant which successfully utilises fuel oil is considerably higher. The oil must be cleaned, heated, filtered and stored. The interest in these oils and similar derived oils will increase as supplies reduce.
A COMPARISON BETWEEN HEAVY FUEL OIL AND COAL FURNACES
HEAVY FUEL OIL
- Higher calorific value - smaller furnace - Require 50% less mass of oil than coal for the same rating.
- 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
- 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.
- More expensive - Especially true for the lighter oils (not true for orimulsion)
- 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.
Gaseous Fuels ReVISITED
These fuels can be broken down into the following groups:
1 Naturally Occurring Gases
(i) The most volatile components of fossil fuel reserves
(a) From oil reserves
- gas direct from the oil fields (mostly methane)
- most volatile fraction from the distillation of oil, petroleum gas (mainly butane, propane)
(b) From coal reserves (Firedamp)
(ii) From the digestion of biomass by bacteria - Landfill gas, digester gas
2 Manufactured Gases
Gases derived from other feed stocks (solid or liquid)
(i) Pyrolysis, Carbonisation - The product of the heating of a solid or liquid to devolatilise the substance to form a char, coke or charcoal
- Coke oven gas
- Charcoal ovens
(ii) Gasification - The product of complete gasification of a solid or liquid feedstock - i.e. including the char. Only residue is ash.
- Producer gas - Low CV gas from partial combustion of coal in air
- Blue or Water Gas - Med. CV gas from gasification of coal with steam.
- Carburetted Water gas - Med. to High CV gas. Town gas.
- Oil gas - Formed by the cracking of heavy fuel oil.
The Importance of Gaseous Fuel
- Generally VERY clean burning. Little soot. Operate with low XSA.
- Easy to burn - No grinding or atomisation. Excellent mixing
- No problems with erosion or corrosion
- No ash
- The gas is easy to clean. E.g. if sulphur is present, it may be easily removed prior to combustion.
- Simplest combustion plant of all - Burners
- Control system
- No ash problems
- Heat exchangers
- Can be started up and shut down very easily and quickly.
- Problems with distribution and storage
- Explosion risk - very volatile.
- Relatively costly. Offset by cheaper and more efficient plant.
Combined Cycle Power Plant.
These are setting the goals for efficient and clean power plant globally. Any competitor must compete with the combined cycle:
- Already these systems are achieving thermal efficiencies of conversion of gas calorific value to shaft power of 57% in the UK. (Killingholme)
- By the year 2000, this is estimated to be as high as 60%.
- Comparable steam plant (c.f. Eggborough) 39%.
- PLUS the Combined Cycle (CC) plant is simpler.
- Low NOx combustors are available and no FGD is required
- Plant can only run with a clean fuel. e.g. gas or gas oil. No chance with coal or heavier fuel oils.
- Economics are better with gas in the UK.
Characterisation of a Gas
There are relatively few parameters which must be considered. The most important as always is calorific value. Specified for gases usually in MJ/Nm3. This is not most appropriate parameter.
Wobbe index is normally used by preference.
Consider a burner:
Heat release rate = Vol flow x CV
Q = V x CV
Pressure drop from Bernoulli's eqn
From Mass continuity, Volume Flowrate = Area x velocity = Av
So, if we change a fuel gas supply to a burner but keep the same burner control and supply system ( area and D p constant). Then the heat release rate, Q will be proportional to:
Specific gravity = density of gas / density of air
This is termed the Wobbe Index and determines the interchangeability of gases.
FLAME SHAPE AND STABILITY
Once it is determined that two gases are similar, the next most important consideration is whether they will behave in the same way in combustion. In particular, flame shape and stability.
This is determined by the aero-thermodynamics of the system. Very complex. First order estimation may be made based on laminar flame speed.
The fastest flame speed of any gas is Hydrogen. A flame speed factor, the Weaver factor, is defined as the ratio (percentage) of the laminar flame speed of the fuel gas with Hydrogen. Always less than 100.
If "We" is the same for two fuels, they will burn in a similar manner.
Flame speed may be adjusted by varying the proportion of the fuel mixture.
Increase H2 = increase We
Increase CO2, CO, N2= decrease We
For manufactured gases from coal or oil, We was very important due to the variable Hydrogen content. We for natural gas is not so variable. All saturated hydrocarbons have a very similar flame speed.
FINAL PROOF CAN ONLY BE MADE WITH A TRIAL
INTERNATIONAL GAS UNION CLASSIFICATION OF GASES
Family 1 - Wo = 17.8 - 35.8MJ/Nm3 -Coke Oven gas, Low CV gas
Family 2 35.8 - 71.5 -Natural gases, Town gas
Family 3 71.5 - 87.2 -Liquefied petroleum gas (LPG)
High flame speed gases We = 32 to 45
Intermediate speed We = 25 to 32
Low flame speed We = 13 to 25
NATURALLY OCCURRING GASES
- Two types - Fossil fuel gases and gases occurring naturally from some biological process.
- Natural gas is the term for gases found in oil and coal fields.
- A very important fuel gas in modern society. (1986 - 100 trillion cu.m world-wide). Its use is growing rapidly.
1974 - 16% of the world energy demand from Nat Gas
1994 - 22%
- Consisting mainly of methane. It has a low C/H ratio (75%) and burns easily and cleanly.
- It is a very valuable chemical feedstock and in the UK its use as a primary fuel was limited until the 1990's.
- It's use is set to increase with CCGT systems.
TYPES OF NATURAL GAS FROM OIL FIELDS
- Dry or lean - high in methane
- Sour / Sweet - high / low in hydrogen sulphide
- Wet - high in higher hydrocarbons (C5-C10)
- Natural gas is also found in coal fields (firedamp).
- UK production in 1961 of coal gas was 220million cu.m. Industry killed by the discovery of North Sea gas.
Biological, Natural Gases
- Action of certain bacteria on biomass in the absence of oxygen breaks down the H/Carbons in organic compounds.
- Temperatures of around 37C.
- Examples: - Cow's stomach
- Landfill gas
- Bio reactors (Sewage sludge)
- Typical composition:
Typical % vol
Sulphur / CO / Alcohols
- Med. to high CV gas - Wobbe Index of order 27MJ/Nm3
- Very attractive regenerable energy source.
- Digestion is not complete. Residue must be settled and disposed of.
- Gases may be manufactured from either a liquid or solid feedstock. The main commercial methods were developed for coal but the same technology applies to waste and biomass.
- CARBONISATION / PYROLYSIS
- If a coal is heated to high temperatures in the absence of oxygen it devolatilises.
- Gases and tars are released leaving a solid char residue or coke.
- The composition of the gases depends on the temperature, rate of heating and coal rank and composition.
- This is done in coke ovens. The gas is used partially to heat the coal.
Carbonisation is a viable process but is of no use if the primary requirement is a fuel gas since tars and a solid char residue are also produced. Yield of gas is therefore low.
Complete gasification of the coal is required. A variety of techniques are available which allow this leaving only the ash residue.
Basic concept: Partial combustion of the feed-stock.
Simplest method of gasification. Available for well over 100 years.
Air is blown through an incandescent bed of coal.
Enough air to maintain temperature.
Not enough air to complete the combustion reaction.
RESULT: A mixture of order 50% Nitrogen (from air)
29% CO (incomplete combustion)
4% CO2 (equilibrium)
Very low CV gas is produced. Not suitable for distribution. Can be used on site. Wo of order 5-6MJ/m3.
Steam is added to modify the products. Endothermic reactions (water gas) result in the formation of hydrogen.
Efficiency in terms of potential energy in the cold gas is of order 75%.
REACTIONS IN A PRODUCER
Initially we get the oxidation of coal in the presence of available oxygen:
2C + O2 ---> 2CO +240GJ (exothermic)
Then competition for oxygen between the carbon and the water. The water-gas reactions.
C + H2O -----> CO + H2 -121GJ (endothermic)
C+2H2O -----> CO2 + 2H2 -82.4GJ
CO +H2O ----> CO2 + H2 +41.9GJ
The ratio of steam and air may be used to modify the composition of the gas produced.
NB: Problem with this technique is the use of air. This results for all conditions in a large Nitrogen (diluent) content in the fuel.
BLUE WATER GAS
Same idea as producer gas except the problem of Nitrogen dilution is overcome.
The bed of coal is simultaneously blasted with air followed by steam.
Air reaction is exothermic so the bed heats up
Steam reaction is endothermic so the bed cools down again.
The products from the air cycle contain the Nitrogen which is exhausted.
The products of the steam cycle are kept as the gas.
Efficiency reduces to order 62%.
Mixture of gas produced is higher in quality.
Wobbe Index = 15.64MJ/m3
If Oxygen is used instead of air, the process can be continuous. c.f. Lurgi Gasifier.
Coke is preferred to coal because coal can continue to devolatilise in the blow period reducing efficiency. Also problems with caking.
CARBURRETTED WATER GAS
Blue water gas is still not of sufficient quality for distribution.
It may be enriched by adding a carburettor.
Fuel oil is sprayed into a brick lined chamber during the blow period of the BWG plant with air. This heats the bricks to around 1000C
During the make period, the air is turned off and the oil is cracked into smaller hydrocarbons in the now heated chamber.
This produces a cocktail of hydrocarbons (mainly methane) which enrich the gas. Percentage H/C = 17%
Wobbe index of the resultant gas of order 24.9MJ/m3. This is now suitable for distribution and is known as town gas.
Efficiency of cracking is of order 75%.
Cost is relatively high because oil must be used as well as coal.
The highest technology commercially used.
As well as the reactions already described. There is a further endothermic reaction possible.
CO2 + 4H2 -----> CH4 + 2H2O
C + 2H2 ------> CH4
Consideration of equilibrium shows that these reactions favour high pressure.
Operating pressures of 30-35bar are used and the stock is gasified with an oxygen steam mix (1:8) at 900C.
Result is a small plant because of the pressure and a high CV gas without the need of a carburettor.
Wobbe Index = 23MJ/m3
Efficiency = 72%
Advantages of a gas of reasonable quality with no oil
Problems with plant complexity
-Gaseous fuels play a vital part in modern energy demand
-Gas fuelled plant sets the standard of efficiency
-Of all fuels, gases are the easiest to burn and to clean
-Natural gas dominates all markets at present because of its abundance and quality.
-Gasification will begin to play an important role again in the future as gas reserves dwindle.
-Coal gasification technology is being developed on biomass plants with an aim to make a renewable gas source.