ABSTRACT
Fuels that are gaseous under normal ambient conditions are classified as gaseous fuels� Notable examples of these include natural gas, propane, and hydrogen� As indicated earlier, the gaseous nature of such fuels provides distinct advantages in their utilization in comparison to common liquid or solid fuels� The following may be considered to be some of the main positive features of gaseous fuels:
• High combustion efficiency because the gas phase does not require atomization, as with liquid fuels, and less time is needed for mixing the fuel with the necessary air for combustion�
• Relatively clean burning, with clean waste gases producing no solid products such as ash or particulates� This would save having expensive equipment such as those for ash handling and soot blowing as in coal firing�
• Elimination of expensive fuel-storing facilities and storage tanks due to ease of distribution of the fuel from production points to consumers via pipelines�
• Cleaner associated working surfaces of combustion equipment help to maintain maximum efficiency over long operational periods with better control and improved heat transfer�
• Lower tendency for corrosion with a negligible release of sulfur compounds�
• Easy control of the fuel supply with larger turndown ratios of burners and finer control of excess combustion air�
• A smaller combustion chamber than for liquid or solid fuels for the same thermal output, with smaller ancillary equipment�
• Efficient combustion over a wider firing range with improved flame stability limits�
• Simpler and cheaper burner equipment can be used with excellent low-temperature operation�
• In process furnaces, the atmosphere can be readily controlled to provide reducing or oxidizing conditions as required for superior emissions control�
• Clean and convenient supply of natural gas from the mains to burners with ease of automatic and remote regulation�
• No filtration equipment may be required� • Constant and reliable supply of piped gas from the mains in most
locations� • Contact with the combustion products does not affect adversely the
quality of product load such as in glass-heating or special metalmelting furnaces�
• The combustion rate and flame length can be controlled readily, mainly by suitable adjustment of the gas supply rate�
• Simpler and easier to light burners and equipment are common� • In domestic applications, the absence of a fuel tank and uninter-
rupted availability of the fuel gas is a distinct advantage�
The design and control equipment of gaseous fuel burners will depend of course on the properties of the fuel� Mixing between fuel and air after their introduction in burners normally takes place in relatively simple venturi-type mixers� Figure 15�1 shows a schematic representation of the main stages in the combustion of a gaseous fuel� After the fuel mixes with a sufficient amount of air, preignition reactions commence, leading under the right conditions eventually to ignition with a rapid rise in temperature and the release of energy through combustion� These events will be followed by some relatively slow postflame reactions before the emission of the final products from the combustion device�
One of the main disadvantages of gaseous fuel usage is the increased risk of fire, explosion, and toxic hazards associated with their use and after a leak� However, there is much experience with safe usage and there are strict safety codes that must be followed in their application�
Natural gas is commonly associated with the volatile portion of crude petroleum� It is normally present in the porous rocks in oil wells above the liquid fuel zone under high pressure (e�g�, above 350 bars)� The gas is also found in a dry gas form in nonassociated gas fields� Russia is considered to have the largest known reserves of conventional natural gas at present�
Natural gas has been known since ancient times, mainly through fires that occur when it escapes through fractures in the earth such as in Persia, Iraq, and China, producing the so-called eternal fires that were revered by some of the ancient civilizations� Its exploitation began industrially mainly in the 19th century� The gas was used initially for street lighting and occasionally for some domestic heating Around the 1920s, high-pressure gas pipelines, especially in North America, were laid over long distances, making the gas readily available to millions of domestic and industrial users� Mainly after the Second World War, long pipeline networks with large diameters, well in excess of 1 meter and internal pressures in excess of 100 bars, were laid in many parts of the world such as Russia, Canada, and the United States� Its increased availability and the need to meet increasingly lower emissions tend to enhance the usage of natural gas� Rapid progress has been made worldwide in recent years in the discovery of new natural gas deposits and its transportation around the globe both as a gas and in its liquid state (liquefied natural gas [LNG])�
Natural gas has become increasingly a premium fuel that is in much demand� It is a prime feedstock in the chemical and petrochemical industries for the production of a wide range of economically important products� Above all, it is primarily used indirectly via its production of hydrogen for the upgrading of common fossil fuels, especially those that are declining increasingly in quality, to make them more acceptable operationally and ecologically� Figure 15�2 shows an example of the rapid increase in the consumption of natural gas in North America, which is exceeding its rate of production� Figure 15�3 shows the distribution of the major natural gas reserves among the largest reserve holders� Russia, Iran, and Qatar have the bulk of the global reserves�
Natural gas initially was considered a nuisance when the prime objective was the production of oil, and the gas often was wasted by being flared off� Unfortunately, some significant amounts of gas are still being flared in some parts of the world when it cannot be utilized locally, pumped back into wells to enhance oil recovery, or transported to potential markets via pipelines over long distances� More recently, through the expenditure of huge capital and advances in technology, much effort is going on to exploit gas resources via liquefaction of the natural gas to permit its transport for export using suitably designed ships, as LNG� Such approaches are making natural gas resources widely accessible and increasingly a major supplement to
petroleum resources� Natural gas has the potential to become a major source of wealth for countries that can manage optimally to exploit their resources� There are excellent long-term prospects for increased importance and impact of future developments of gas resources, particularly in light of the increased need for continued availability of oil resources, while reducing the environmental negative impact of their exploitation�
The gradual growth over recent decades in the global consumption of natural gas by different user sectors (measured in one million tons of oil equivalent, Mtoe) is shown in Figure 15�4� The potential for significant growth of gas consumption by the transport sector is evident�
The price of natural gas was strictly controlled in North America for many years, which resulted in poor incentives for its production and finding new gas reserves� However, after deregulation and increasingly stricter environmental constraints on combustion devices, this situation has been reversed markedly with increases in its price� Today, natural gas is liquefied in large installations located in some producing countries and transferred long distances by special marine transport in the form of LNG, such as from the Middle East, North Africa, Indonesia, and Australia to Japan, Europe, and North America�
The development of natural gas reservoirs does not have in general a longterm environmental impact in comparison to that of oil or coal� However, there can be some environmental impacts associated with the construction, operation, maintenance, and abandonment of the associated facilities� Since oil and gas are often found together, many of the environmental impacts for one can apply to the other�
Natural gases are sometimes found in low-permeability tight formations� Exploiting these formations requires extensive hydraulic fracturing of the reservoir to improve the mobility of the gas, as shown schematically in Figure 15�5� Such approaches have a negative impact on the local environment of the reservoir� This has been the subject of intense activity to develop and apply acceptable remedial measures�
The composition of natural gas varies significantly depending on its source and whether it has been processed for pipeline distribution and consumption� Typically, natural gas as delivered to consumers is composed of mainly CH4 (about 90% by volume) with various concentrations of ethane, C2H6, propane, C3H8, butane, C4H10, and nonfuel diluent gases such as nitrogen and carbon dioxide� Methane, a gas that is much lighter than air, is odorless, nontoxic, and colorless� Odorants, such as mercaptans (e�g�, ethyl mercaptans), are added to pipeline natural gas at its distribution points to ensure easy detection by humans down to very low concentrations in case of a leak�
Processed pipeline natural gas, as can be seen in Table 15�1, tends to have a widely varying composition influenced by the origin of the gas and the processing it went through before being transported and rendered suitable for distribution and utilization� The processing of the gas must remove almost all the hydrogen sulfide present� It reduces to an acceptable level the water content as well as much of the inert gases and the economically valuable higher hydrocarbons� The processed gas is normally sold mainly on the basis of its heating value with comparatively less consideration given to its specific chemical composition� For thermal applications, such as in burners and furnaces, the main properties that are considered to be of much significance are its specific gravity and heating value through its Wobbe index, which is an indication of the rate of energy release by combustion of the fuel when discharged through an orifice such as that of a burner under the action of a specified constant pressure head� Accordingly, it is necessary to maintain the same range of values of the Wobbe index when a specific burner is required to burn gases with different compositions� However, the detailed nature of the constituents of natural gas has an important significance to its use as a feedstock for the chemical industry and its use as an engine fuel since it affects very significantly the resistance of the gas engine to knocking and the nature of its emissions� Engine operating conditions, design, and controls need to be optimized to cope with the consequences of the variation in composition, especially for highly power-rated engines�
The heating value of the gas can be quoted in terms of its lower or higher values� For engine and burner applications, the lower value is usually used that is associated with the more realistic condition of the water vapor remaining in the vapor state� Also, in general, the heating value per unit volume or mole, rather than by mass of the gas (MJ/m3 or MJ/kmol), is usually quoted for natural gas and almost all gaseous fuels at a specified temperature and pressure when they are dry�
Much attention is given also to controlling the water in the gas to reduce the potential for corrosion and the formation of solid hydrates, such as in pipes and storage vessels� To adjust for some of the variations in the heating value and other properties of the gas, such as in peak shaving applications, its composition may be varied seasonally by including very small amounts of propane or air, such as when the heating value needs to be increased or reduced, respectively� The presence of some propane or other higher hydrocarbon vapors even in small proportions in the predominantly methane gas can affect engine performance significantly and requires appropriate control and optimizing� Figure 15�6 shows schematically some of the processes used to treat the well gas to the state of a dry pipeline gas� Some of this treatment takes place in the vicinity of the well; others are employed some distance away in suitable gas-processing plants�
The most common sulfur compound in raw natural gas is hydrogen sulfide (H2S)� Some natural gases may contain relatively small concentrations of mercaptans, carbonyl sulfide (COS), and various other sulfur compounds�
Their removal from natural gas is made largely by chemical processes in gas-processing plants, such as by the Claus process, where the extraction takes place chemically via amines� The remaining sulfur content of the processed natural gas is reduced to extremely low values with virtually no sulfur remaining in the gas, for example, around 8 to 30 parts per million (ppm) on a mass basis� The remaining sulfur in the gas originates mainly from the mercaptans that are added to the gas to serve as odorants�
The expansion of the high-pressure gas, whether during fueling or before its mixing with the air for combustion, will result in very substantial cooling of the expanded gas� Not only will the associated temperature drop lead to cooling of device parts and lower effective combustion temperatures, but also the resulting change in density, unless accounted for, can produce variations in the quality of the mixture reaching the combustion device and the progress of the combustion process� Accordingly, for example, in engine applications remedial measures are usually incorporated, especially in mobile applications, such as providing some heating to pressure regulators or having a first-stage regulator located near the tank or fuel source while the second is close to the intake manifold so that heat can be picked up by the fuel flow along the exposed piping system�
Methane contributes a significant fraction of the global emissions of greenhouse gases� Much of this is due to escapes and releases, whether by
industry, especially the oil and gas industry, or naturally� To eliminate methane releases from equipment and instruments, compressed air needs to be used whenever possible instead� However, near gas wells pneumatic devices may still use, for convenience, safety, and ready availability, pressurized natural gas� Equipment leaks need to be monitored and also attended to, including using proper effective seals�
In general, the catalysts that may be used to treat the exhaust gas from natural-gas-fueled devices, including engines, are made up of specially prepared mixtures of platinum, palladium, and rhodium applied to an inert wash coat of aluminum oxide and supported on a ceramic substrate� The effectiveness of any catalyst depends on a number of operational factors, including temperature, residence time or velocity of the gases, equivalence ratio, composition, thermal cycling, and the extent of its deterioration and poisoning with usage� The key low-molecular gases found in natural gas, such as methane and ethane, are usually considered to be relatively difficult to oxidize catalytically in comparison with those associated with gasoline or diesel fuels� Also, the presence of some small amounts of sulfur compounds in the exhaust gas does constitute a serious limitation to the action of these catalysts� Since the exhaust gas from the combustion of the lean mixtures normally used in natural gas engines has a relatively low temperature, the effeteness of the catalyst, especially for methane applications, is reduced very substantially�
Shale gas is the gas trapped in low-permeability shale rock� It is made up primarily of methane� The gas, which is relatively difficult to extract from its reservoir, has been receiving a great deal of attention in recent years as a potential source of gas energy that may be economical to extract in certain locations� This is mainly driven by the notable advances made in recent years in the technology of fracking and in horizontal and directional drilling� Once the drilling of a well is completed, high-pressure fluids are injected along the well bore to fracture the adjoining rock matrix containing the gas� The fluids employed are usually water with sand added to it, together with small amounts of additives, mainly to increase the viscosity of the injected water� The sand helps to prevent the fractures from closing under the action of the high pressure applied� However, there are major environmental issues that need to be attended to effectively reduce the negative envrionmental consequences associated with shale gas production, especially in relation to contaminating local water resources�
The composition of natural gas varies widely from one well to another and depending on whether it has been processed or not� Raw gas, before being used, needs to be processed to remove impurities such as water and sand and to separate light hydrocarbons, vapors, and gases (Figure 15�7)� The processed gas is then transmitted, often for long distances, to where it is needed
by local customers via large-diameter pipelines under high pressure� Today, the proper operation of pipelines is ensured through the use of computerized control systems, continuous pipeline monitoring, and including the status of remote sections of the pipeline�
Natural gas in recent years is being increasingly transported over continents and seas through its rather costly liquefaction and transport via specially designed and constructed large marine LNG tankers� The transportation of LNG via pipeline remains limited locally to only relatively short distances� Natural gas reservoirs are often located at huge distances from where the gas is consumed� This requires the construction and maintenance of a massive infrastructure of pipeline networks over land and offshore, with liquefaction and subsequent marine transport� This is followed then by its storage at arrival terminals and subsequent vaporization to join the gas pipe networks�
Industrial large-scale consumers are usually supplied with the gas at high pressure, whereas residential users buy the gas from a local distribution company and it is supplied at a very low-pressure difference from that of the atmosphere� These distributors usually add odorants to the gas for safety and easy detection of any leaks at very low concentrations and to ensure that its quality and properties are consistent with specifications�
Continued inspection of the integrity of gas pipelines is a prime safety consideration� The periodic use of inspection slugs (known in the field as pigs) pushed through the inside of pipelines ensures that the pipeline interior walls remain clean and free of defects and corrosion while also removing any accumulated debris� Pipelines use extensively cathodic protection to reduce the tendency to corrode�
A pressure gradient develops along the pipeline as the gas is moved at the desired rates down the line� Maintaining the right average pressure distribution and supply flow rates while compensating for pressure losses requires the incorporation of gas compressor stations at intervals along the longdistance line� These compressors can be of the reciprocating or rotary types driven by gas-fueled large-capacity spark ignition engines, gas-fueled diesel engines, or gas turbines� These power units usually require and consume a relatively very small fraction of the fuel gas being transmitted� Consequently, the prime concern in their operation is ensuring utmost dependability with uninterrupted delivery of the gas supplies� The consideration of their specific fuel consumption, in comparison, is of insignificant priority and concern� The transmission of gases via pipelines still represents a very efficient, economic, and highly developed system that can be controlled remotely from long distances over the whole integrated network�
There have been various schemes to process suitably natural gas into synthesis gas (a gas mixture made mainly of H2 and CO) via partial oxidation and/ or reforming with steam either with or without catalysts, so as to produce liquid fuels, such as gasoline and diesel fuel, mainly for transport applications� This was pursued during the Second World War by Germany and Japan, after they were deprived of sufficient natural oil resources, using coal as the main raw material� However, today it is becoming increasingly evident that such attempts, in comparison to using the gaseous fuel directly or liquid petroleum sources, tend to be less attractive and much costlier�
Table 15�2 shows a listing of the main paraffinic components normally found in natural gases together with values of their corresponding boiling points� Table 15�3 shows the typical composition of some industrial and natural gases commonly employed as fuels� Table 15�4 shows a listing of
the maximum values of flame temperature achieved in the combustion of a range of pure fuels in air� The corresponding required concentrations of gaseous fuels for producing such temperatures are also listed�
Flaring is the disposal of waste fuel gases through their combustion in the open atmosphere� It has been the accepted solution for dealing with waste gases and disposing of gaseous fuel releases in emergencies� Safety and emissions are the main consideration of flaring� The large volumes of gases flared are a serious contributor to air pollution and greenhouse gas releases in some parts of the world� This is not only through the carbon dioxide emitted but also through methane and other hydrocarbon gases released that may not be fully burned, particularly in turbulent crosswind and cold ambient conditions� Compounds such as styrene, xylene, and other types of organic compounds that have negative health effects are produced, although in small concentrations, as well as a wide range of partial oxidation products�
A major contributor to flaring in the petroleum industry is well testing, when relatively large volumes of gas from a well are released and burned to provide information about the flow from the reservoir and its potential capacity� There are many controlling parameters that influence the proper flaring of fuel gases, including:
• Composition of the gas to be flared and its supply rate • Its temperature and pressure • Whether liquid droplets are present with the vapors or not • If pilot flames or other ignition sources are used • Whether steam injection is applied • What flame stabilization methods are employed • Location and prevailing wind speed and turbulence intensity
A knockout drum is usually fitted to the base of the flare pipe to remove some of the unvaporized liquids in the fuel gas to be flared� The performance of flare stacks usually is monitored; should it be extinguished accidentally, automatic reignition is applied� In addition, safety devices are fitted to flares such that in case of a back-occurring flame flash, the flare is quenched� Flares can generate a lot of heat radiation to the nearby surroundings, and there are codes governing how far buildings should be located away from flares� In offshore applications, it is very difficult to locate the flare far enough from the drilling platform� Water-injected flare tips are often employed in such applications�
To permit the long distance transportation of natural gas between continents and oceans, natural gas is liquefied and turned into a cryogenic fluid in large installations at the points of gas sources� The liquid fuel is then transported via specialized tankers to the points of usage and distribution after vaporization into the gas pipes network� At present, it is unlikely that the liquefaction of the gas can be made at the points of gas consumption for economic and technical reasons� Hence, a major problem associated with the use of LNG for combustion devices in general and transportation applications in particular is its limited widespread availability at present� It is in locations where LNG is stored or distributed, such as at marine terminals, where LNG is unloaded that some LNG may be diverted for use as a fuel locally� Table 15�5 shows a listing of the main properties of liquefied methane representing LNG� The heating value and composition of LNG can vary depending on the concentrations of some of its minor components� These, because of their higher hydrocarbon nature, tend to increase the heating value of the gas after vaporizing the liquid fuel�
LNG is usually transported by specially designed and constructed marine tankers suitable for transporting cryogenic liquid long distances safely� The tankers use for their propulsion are either turbine or dual-fuel compression ignition engines of the diesel type that can consume the boil-off fraction of the LNG during the voyage, supplemented when needed by conventional liquid fossil fuels� Ever larger-capacity tankers have been built to transport LNG�
LNG production is capital-intensive, yet its production from natural gas, which contains some components that are in the liquid phase of higher hydrocarbons, notably ethane and propane, is still economically attractive in many situations� Also, the lead time between the discovery of natural gas at a certain location and its export is usually quite long and may extend up to several years� In spite of these limitations, the demand for LNG, whether for domestic heating applications or for industrial use, including its use for
power generation in power stations or engines, is expected to continue to increase� Of course, the cryogenic nature of LNG, being a liquid at 111 K at atmospheric pressure, can be exploited whenever possible for cooling applications and occasionally for expansion of work production� In engine applications that employ LNG boil-off injection, improvement in power and efficiency can be expected�
A part of the relatively high cost of production, operation, and transport of LNG is the energy requirement to liquefy the gas, which can consume a sizeable fraction of the energy of the gas being liquefied� Moreover, viable LNG projects require usually a very large source of gas that needs processing to make it economically viable and return the high initial capital cost�
The transportation, fueling, and storage of LNG represent a potentially serious safety hazards due to the gas being liquid and at an extremely low temperature (111 K at atmospheric pressure) with the potential for fire and explosion after spillage in accidents� The hazards associated with an LNG tank rupture and fuel spillage in principle far exceed those associated with compressed natural gas (CNG) or gasoline� Tanks storing LNG are normally double-walled� The LNG is usually stored at pressures only slightly higher than atmospheric pressure� The tanks cost substantially more than those for CNG and many times more than those for gasoline� Heat transfer from the environment into the well-insulated LNG tank will boil off increasingly more fuel, raising the pressure within the tank� Modern tank designs will not release the remains of the gas through insulated relief valves until there is a significant buildup of pressure to a prescribed value over a protracted period, typically 3 weeks� LNG on arrival in marine terminals is stored in specialized tanks and underground caverns� It is then vaporized when needed to be transported by pipelines as a gas to augment conventional natural gas supplies�
The exploitation of natural gas resources and the reduction of flaring and consequent emissions have been made possible by advances in the technology of the liquefaction and transportation of the gas� Nevertheless, ensuring safe operation with LNG remains a paramount concern to the industry and public� Some of the procedures that can be used to mitigate the associated hazards are the following:
• Provision of all aspects of fire protection and foam in tanks and firefighting facilities, including the provision of explosion-proof equipment such as pumps, electrical equipment, and fittings with the sent-out stream of gas odorized�
• Provision of detectors and alarm systems at strategic locations to provides adequate protection against low-temperature embrittlement of steels that may be in contact with the cryogenic liquid or its cold vapor�
• Protection against accidental rupture of tanks and avoiding resonance in LNG tanks through sloshing of the liquid during its transportation, especially in tankers�
• Preventing hydrate and ice formation that may plug lines by ensuring the gas is dehydrated with only a minimum practical amount of water�
• Protection against accidental interruption of the working of the various components, such as due to a vapor lock being formed in a line�
Of course, there is a paramount need for the proper training of technical personnel and the public in all aspects of dealing with LNG� Accordingly, at present there is still some resistance from the public to locate LNG-receiving terminals locally�
The conversion of natural gas to the liquid state changes somewhat its composition and its properties, including its heating value� All the sour components, such as H2S and CO2 and any water vapor, are removed; some larger hydrocarbon molecules may remain� This is done usually near the gas source before transmission to liquefaction plants that are commonly located near the coast� The LNG is gasified shortly after arrival by marine tankers using seawater as the heating medium for transport as pipeline gas�
The LNG tank wall is normally constructed of aluminum alloy� Heat leakage into the tank is minimized by vacuum sidewall insulation� On land, a dike is constructed around the tank to protect the surroundings in case of a spillage, such as after an accident� Reliquification equipment is usually installed on large units to reliquify the boil-off gas and avoid releasing the gas into the open atmosphere�
It has been known for some time that some gases, such as methane or carbon dioxide, can have different combined forms of molecular structure with water at low temperatures and high pressures; these are called as hydrates� These dissociate endothermically, steadily releasing gas with an increase in temperature and/or a reduction in pressure� The molar methane content of its hydrates is typically less than 16%� It is reported that there are enormous reserves of methane in the form of hydrates located on some seafloors and under the permafrost of arctic regions� However, it is unlikely that such resources can be exploited effectively in the near future� Moreover, the uncontrolled release of methane from the hydrates, should it ever occur, does
represent a very serious source of release of the greenhouse gas methane into the atmosphere, contributing to the problem of increasing global warming� Many industrial operations try to prevent unwanted hydrate formation, such as in pipelines, through the use of suitable inhibiting agents, since hydrates can impede fluid flow and undermine safety�
There are a very large number of natural-gas-consuming engines that are used for power production in nonmobile applications, such as those used in natural gas pipeline compression or electric power generation that employ units of huge power capacity� In these applications, industrial gas turbines are often derived in their designs from their corresponding aviation countertypes and large-capacity multicylinder turbocharged spark ignition reciprocating engines� Each of these classes of prime movers has its positive features and limitations� The performance and reliability of nonaviation turbines are continually improving, drawing benefits from very significant recent advances made to the aviation types� Some of the main features of naturalgas-fueled gas turbines are
• Higher firing temperature and pressure are increasingly implemented, which contribute to improved efficiency and an increase in power�
• High reliability and availability over the years, incrementally increasing thermal efficiencies�
• They can use pipeline natural gases as fuels readily and tolerate well variations in the composition and quality of the fuel gas� Their emissions tend to be not too excessive�
• They are amenable to remote control and require relatively little maintenance with longer running time between services�
• Very high power output per equipment mass and volume� • They start and stop easily and rapidly, and are very quick to reach
full load, thus they are useful in providing power quickly in the case of emergencies�
• Steady and continuous operation is easily available and can drive natural gas compressors at high speeds�
• Their speed can be varied with load and have high turndown ratios� • Performance can be improved through reheating, intercooling, and
heat exchange� The use of cogeneration is quite possible, attractive, and often implemented�
• They are factory assembled and easy to install and remove, with less auxiliary equipment and water needed� Can be removed and replaced quite quickly in case of an emergency�
• High reliability with less downtime, requiring no constant supervision with lower operational costs�
• Rotaryless vibration with only simple foundations needed and fewer moving parts�
In comparison, some of the main important features of large-capacity stationary turbocharged spark ignition engines, when operating on natural gas, relative to the corresponding gas turbine equipment are
• They have very high efficiency and much longer life� • They require special fuel gas quality� • They are associated with very well-developed technology that is
being increasingly improved� • They require higher maintenance costs and constant supervision� • They have improved automation and monitoring, but they are in
comparison less amendable to remote control� • They are bulky and of high weight and volume, requiring a heavier
foundation� • Starting and stopping require more time than gas turbines� • Emissions of CO and hydrocarbon are higher� • Variable speed and reciprocating gas compressor direct drive are
possible�
Liquefied petroleum gas (LPG) is often identified loosely as simply propane� The gas is made up of essentially propane with some butane and other constituent vapors� In colder climates, LPG consists mainly of propane, but in warmer climates, a considerable amount of butane may be present also�
Propane and butane are usually present in natural gas, and they are removed from the gas before its high-pressure transmission through pipelines� Hence, the production of natural gas gives rise to the availability of large amounts of LPG� Propane and butane are also produced from the refining processes of crude petroleum� Such gases then will contain varying quantities of other related fuel components such as propylene, butylenes, and isomers of these fuels� The presence of these components in LPG in
relatively high concentrations and fluctuations in their concentrations can give rise to complications as far as the utilization of the fuel in general, and in particular in internal combustion engines by increasing the tendency to knock and changing the nature of exhaust emissions� On this basis, there can be normally a significant difference between the effective octane number of propane and commercial propane, with commercial propane having a higher tendency to knock and produce smoke than pure propane�
LPG can exist in a liquid state at ambient temperatures when under moderate pressure� This makes it more easily portable for a variety of applications and especially on board vehicles in comparison to CNG� The constituents of LPG tend to be more complex than those of CNG and produce more reactive objectionable compounds in the atmosphere, such as during the formation of smog�
LPG has been used as vehicular fuel for quite some time; much of the operation of vehicles on LPG is related to the commercial and public sectors, such as taxis, trucks, and vans� Supplies of propane, unlike those of natural gas, are limited to its separation in the processing of natural gases into pipeline gas and in the refining of petroleum for the production of liquid fuels�
The volumetric energy content of LPG is significantly less than that of gasoline (~0�73)� LPG is heavier than air and the vapor can constitute a greater fire hazard than methane� Leakage of the fuel, for example, tends to concentrate the fuel vapor near the accident site, whereas after a methane or a hydrogen leak, the gas will disperse very rapidly and mainly vertically due to its light and highly buoyant nature� However, in this respect, a gasoline spill tends to represent a greater fire and explosion hazard than either methane or propane� Since it is heavier than air, flammable vapors when released can linger for quite a long time�
A 9�0-kg (20-pound) LPG cylinder is usually used in many mobile installations in North America, whereas 45-kg (100-pound) cylinders are widely used in domestic stationary applications (Figure 15�8)� Larger tanks, filled usually from mobile tank trucks, are used in sizes of around 190 kg (420 pounds)� When larger loads are needed, capacities of several thousand liters may be installed� Figure 15�9 shows a typical commercial propane storage installation� Railroad tank cars containing some 30 m3 (8000 gallons) are employed for bulk transfer of the propane from a point of source to distributors’ storage tank locations�
A processed LPG from a refinery is made up of a mixture of n-propane and n-butane� Analysis showed that on complete combustion, 70% of the thermal energy comes from the propane, while the rest is provided by the butane�
The LPG gas mixture was burned with a deficiency of air such that only 80% of the air required for complete combustion was used� If the products were assumed to contain only the gases CO2, CO, H2, H2O, and N2 and were in equilibrium with respect to the water gas shift reaction (CO H O CO H2 2 2+ ↔ + ), with a Kp value of 0�30, calculate the volumetric dry products composition of H2 and CO produced� Under the operational conditions used, the lower heating value (LHV) for propane is 46�34 MJ/kg and for butane is 45�73 MJ/kg�
Answer: Consider one mole of fuel mixture containing yC3H8 and (1 – y)C4H10� The
heating value is given per unit mass� In order to solve for y, the heating values need to be evaluated on a molar basis� The molecular weight for propane is 44 kg/kmol and for butane is 58 kg/kmol�
The heating value for propane becomes 2�039 MJ/mol and for butane 2�653 MJ/mol
Q y Q y Q
Q y
2 38 and 2 653 1 and 2 38
2 653 = = −( ) = =� � �
� 0
7 3
0 1
72 and 1 28
−( ) = − =
y
y y0 0� �
The overall combustion equation for a stoichiometric mixture becomes
0 0 0� � �72 C H 28 C H O 3 76N CO H O3 8 4 1 2 2 2 2+ + +( ) → + +B a b f N2
Carbon balance: (0�72 × 3) + (0�28 × 4) = a
a = 3 28�
Hydrogen balance: (0�72 × 4) + (0�28 × 5) = b
b = 4 28�
Oxygen balance: 2B = 2a + b
B = 5 42�
However, only 0�90 of the air is supplied with the products containing only CO2, CO, H2, and H2O� Hence, the equation becomes
0 0 0� � � �72 C H 28 C H 4 88 O 3 76N
CO H 3 8 4 1 2 2+ + +( ) →
′ + ′a b 2 2 2 2H O CO N+ ′ + ′ + ′d e f
Carbon balance: (0�72 × 3) + (0�28 × 4) = a′ + e′ Hydrogen balance: (0�72 × 4) + (0�28 × 5) = b′ + d′ Oxygen balance: 4�88 × 2 = a′ + d′ + 2 e′ Nitrogen balance: 4�88 × 3�76 = f′
e a
d b
b a
d a
K
′ = ′
′ = ′
′ = ′
′ = + ′
=
3 28 4 28 1 8 3 2
� – � – � – � 0
=
×
× =
′ × ′
′ × ′
P P
P P b e a d
1 8 3 28
3 2 3
7 2
� – � – �
�
�
0 0
0 0
0 0
a a a a
a
′( ) × ′( ) + ′( ) × ′ =
′ – � �5 32 3 542a′ + = 0
a b d e f′ = ′ = ′ = ′ = ′ =0 0� , � , � , �732 342 3 938 2 542, and 18 349�
The dry products are 0�738 + 0�342 + 2�542 + 18�349 = 21�971
%
� �
�
% � �
�
CO 738
21 971 3 36
H 342
21 971 1 562
= =
= =
Since LPG is a gas heavier than air, any accidental release will tend to flow to low levels and persist much longer than with a similar release of buo yant natural gas� Propane cylinders are always equipped with pressure relief valves to release any excessive pressure build up, such as when cylinders are subjected to excessive heating� The cylinders should be maintained in an upright position at all times so that only gas is vented� When a cylinder is inverted or placed on its side so that the liquid fuel covers the outlet opening, then liquid propane could be discharged� This liquid will flash evaporate exceedingly quickly into gas on its discharge while the pressure drops through the control equipment� In extreme conditions, the liquid fuel could pool or reach the point of use to create a very serious fire and explosion hazard�
Propane cylinders should not be transported without proper caps covering the valve assembly to protect against breakage in case of an accident� Since LPG products are essentially odorless, an odorant, usually a mercaptan, is added in very small concentrations as a standard practice so that leakages can be detected readily� Propane is essentially nontoxic, but with butane and higher hydrocarbons, it has a tendency to act as an anesthetic when vapors are breathed over a protracted period�
Most of the gaseous fuels commonly available for consideration as exploitable energy resources are fuel mixtures of individual members that are gaseous under normal ambient conditions� These include natural and liquid petroleum gases and a very wide range of other fuel mixtures that can vary widely in composition and properties that are mostly products of industrial and natural processes� One broad classification of such fuels has been based on their heating values relative to that of natural gas (~1000 Btu/ft3, i�e�, 37�0 MJ/m3)� The classification is as follows:
• High heating value gases have heating values of >500 Btu/ft3 (18�6 MJ/m3)
• Medium heating value gases have heating values of 300-500 Btu/ ft3 (11�2-18�6 MJ/m3)
• Low heating value gases have heating values of 100-300 Btu/ft3 (3�7-11�2 MJ/m3)
Few problems are usually encountered in the exploitation of high heating value gases and, to a lesser extent, even medium heating value gases� However, those fuel mixtures of the low heating value variety represent an enormous potential energy resource; yet, much of it is wasted at present� Often, this is due to insurmountable technical, economic, and environmental problems� The severity of these, which vary widely depending on their origin and fuel mixture composition, arises mainly from the difficulties associated with their combustion and the environmental and toxic effects associated with their exploitation�
Some of the main factors that need to be considered when planning to exploit a low heating value fuel resource include
• The average composition and how it varies with time� • The need to ensure adequate and continued supply of the fuel� • What are its combustion and other properties? Is the gas toxic? Does
it need treatment before usage? Are its products environmentally unacceptable?
