* Humans have always needed chemical fuels for heating, and eventually developed machinery that ran on chemical fuels as well, establishing petroleum as the basis for modern society. However, petroleum supplies are limited and will likely begin to fade out in a few decades, leading to considerations of alternatives.
* The oldest fuel used by humans is wood. It is widely available, easy to handle and store, and provides plenty of heat when burned. Indeed, it is enjoying something of a revival as a fuel source since it is renewable, at least over the long term, with plants turning out wood pellets optimized for combustion in power and heating plants. Dung of cattle and other animals has also been burned as fuel -- as discussed below, use of animal droppings for fuel, at least indirectly, is is also undergoing a bit of a renewal -- and of course, vegetable oils and animal fats have been used for lamps and candles -- another option that, also as discussed below, is undergoing a renewal as well.
Coal is the solid residue of beds of plant matter buried hundreds of millions of years ago, and in fact it's nothing unusual to find well-preserved plant fossils embedded in it. It was known as a fuel before the birth of Christ, but records of coal mining in Europe only go back to the 12th century CE. Early in the 17h century, England, which by then had relatively few forests, began to make use of coal in large quantities for heating. The British dependence on coal would serve the country well when mechanized industrialization began late in the next century, providing a source of energy for industrial steam engines, then for steam engines mounted on ships and locomotives.
Deposits of long-buried plant matter also could yield a liquid product, a black oil, which seeped to the surface in places and was occasionally used for lamplighting. It wasn't a particularly important source of energy until 1859, when an American railroad conductor named Edwin Laurentine Drake (1819:1880) decided to drill a well in Titusville, Pennsylvania, where the oil seemed to be in evidence. Drake's well managed to hit a deposit of oil on 28 August 1859. As with British coal deposits, for decades oil deposits were really only useful for household purposes such as lighting, but the invention of the automobile at the end the century -- followed by the invention of the airplane -- got the oil economy rolling in a big way.BACK_TO_TOP
* Although petroleum dominates vehicular applications, coal still remains in widespread use, almost exclusively for electric power generation: coal is cheap and widely available.
Coal is found in thick, widespread seams buried under layers of stone and dirt, and is generally obtained from "surface mines". In the western USA, the usual approach is the "strip mine", in which the terrain is dug up in a huge strip and the seam collected by huge mining machines. The strip is then filled in and landscaped while a neighboring strip is excavated. Coal mining in the eastern USA takes place in the Appalachian mountains, with surface mining taking the approach of simply leveling entire hills, a process that has met with some resistance on environmental concerns.
Underground mining is also used to extract eastern coal, and underground mines tend to predominate elsewhere. Underground mining of coal tends to be unusually hazardous, even by the standards of mining, since coal dust and emissions are explosively flammable. Incidentally, when coal seams catch on fire underground, they can go on burning slowly for decades, and it's all but impossible to put such fires out.
Processing coal for use involves crushing it down to a convenient size and then removing impurities, the usual method being centrifugal separation in a "cyclone" device, though liquid "washing" processes are also used. Once cleaned, the coal is carried by trains to a power station. There is it pulverized into a powder and blown by huge fans into a combustion chamber or "firebox", with the powderization and forced draft encouraging complete combustion. The heat from the firebox converts water into steam, which drives a steam turbine to provide electric power. The emissions from the firebox are run through pollution-control devices, such as electrostatic precipitators and sulfur dioxide scrubbers.
* Although pollution-control devices have greatly reduced the environmental impact of coal-fired power stations, such plants still pump out quantities of carbon dioxide -- in fact, it is estimated that a third of human CO2 emissions are produced by coal-fired power plants. This has led to the development of what is called "clean coal" technology.
Demonstration plants for clean coal technology are now in operation, the primary approach being known as "integrated gasification combined cycle (IGCC)". While conventional coal-burning power plants burn pulverized coal to ensure thorough combustion, IGCC does one better by burning coal converted to gas, which not only gives efficient combustion but better control over exhaust products.
The coal at an IGCC plant is powdered, mixed with water to form a slurry, and then pumped into a "gasification unit", which is pumped full of high-pressure oxygen at high pressure and temperature. Under such conditions, the coal doesn't burn, instead decomposing into "coal gas" or "synthesis gas (syngas)" composed mostly of hydrogen and carbon monoxide, along with various impurities. The syngas is filtered to remove sulfur, particulates, and other pollutants before being sent to a combustion chamber to be burned; it is easier to get rid of the pollutants before combustion than after.
An IGCC system does not use the fuel to turn water into steam to drive a steam turbine. Instead, it burns it directly in a combustion turbine -- like a fixed-site turboshaft engine -- with oxygen pumped in to ensure complete combustion, increasing efficiency, as well as minimizing emissions of CO and NOx. To further increase energy efficiency, the hot exhaust may be used to drive a secondary steam turbine.
Finally, the carbon dioxide emissions are sequestered underground in old coal mines, depleted oil and gas fields, or in various types of natural underground formations. Estimates show that there's enough capacity available underground to support carbon sequestration for centuries. However, it is more expensive to build a clean-coal power plant than a conventional power plant, and the general assumption is that clean coal won't take off unless governments provide incentives to power producers to adopt it.
Experiments are being conducted at present on an alternative to carbon sequestration: running the power-plant exhaust through a "bioreactor" containing algae that convert the carbon dioxide into biomass and oxygen. The algae could then be used as a feedstock for a diesel-type fuel, or dried and fed back into the firebox. The idea is conceptually elegant, but critics question its practicality. Large-scale tests are being planned, with advocates hoping to prove the critics wrong.
Coal, though still in widespread use, is in decline. It is the "dirtiest" hydrocarbon fuel in all regards, its "external diseconomies "undermining its low purchase cost. It has been particularly undermined in the US by the revolution in natural gas production, and is also slowly losing ground to renewable power.BACK_TO_TOP
* Oil is obtained, as it always has been, through wells drilled down into underground deposits -- though in modern days the drilling rigs may be sophisticated ocean platforms. The general public image of an underground oil deposit is an underground lake of oil, but the reality is that the oil is dispersed in a matrix of solid rock. The deposit has to be "fracked" or fractured using explosives, acids, or high-pressure water; then sand or a "proppant" made of synthetic granules is pumped in to keep the fractured stone from forming back up again.
Although sometimes the oil is under enough pressure to flow to the surface on its own, it is usually pumped to the surface by a "nodding donkey" or "hammerhead" pump. The oil has to undergo "field processing" to separate it into salt water, oil, and natural gas. The simplest way of doing this is with a "settling tank", a tank with one input pipe and three output pipes, with the output pipes at different levels -- the top for gas, the middle for oil, the bottom for salt water. The settling tank does a fairly good job of separating out the natural gas, but the oil and salt water mixture may also need to be run through a "heater treater", often run by the natural gas from the well, to segregate them. A settling tank also is slow in operation, so wells with higher throughput will use a pressure-fed separation system.
The oil or "crude", once cleaned up, is piped off to a refinery for processing. A refinery is a "continuous-flow" system, not a "batch" system: crude oil goes into one end and refined fuels of various sorts come up the other, on a more or less constant basis. The first stage of processing at the refinery is performed by a "crude unit", which consists of a "feed heater" that boils the crude, to then feed it to an atmospheric-pressure distillation column, and a vacuum distillation column or "flasher".
The atmospheric column contains a series of trays with holes in them capped by inverted cups. The vaporized crude drifts up through the trays and around inverted cups, with the appropriate fraction liquefying at each tray. "Light ends" -- methane, ethane, propane, and butane -- are drawn off from the top. Below that is "straight-run gasoline", which used to be the only gasoline there was, but there's more in the mix now, as discussed below. Other fractions drawn off further down the column include naptha, heavy naptha, kerosene, "gas oil" -- and finally what is known as "residual oil" or "resid", which won't boil in the atmospheric column.
Heating the resid up more would just break it down, so it is instead fed into the flasher, the vacuum tower, where it boils at a relatively low temperature. The flasher doesn't provide a hard vacuum, by the way -- it's at about a third of atmospheric pressure. The "tops" of the flasher are used to make motor oils and other lubricants. However, not all the resid will boil even in the flasher. The "flasher bottom" is partly used for asphalt for road paving, but it's mostly a nuisance.
* The light ends are sorted out in the "gas unit", which has a set of tall, narrow distillation towers. The height is needed because the light ends are hard to separate from each other. The towers are under pressure -- about 14 atmospheres. The light ends have various uses:
At the other end of the spectrum, the less desireable flow from the flasher is fed into a "catalytic cracking" unit, which breaks down tarry long-chain hydrocarbons into shorter, more useful molecules. The process is based on "zeolite" catalyst, with zeolites being a sort of crystalline clay, a molecular latticework of aluminum, silicon, and oxygen atoms that is full of small pores where hydrocarbon molecules can be cracked into smaller pieces. Think of the zeolites as providing a template or form where molecules can be broken apart more easily than if they were floating around freely. Dozens of different zeolites have been discovered, with an increasing variety of synthetic zeolites being developed for a wide range of applications.
To perform catalytic cracking, the zeolite is crushed into a fine powder that flows, with the powder and feedstock mixed at high temperature and fed into a reaction chamber. The catalysis takes place quickly, and then the products are separated from the catalyst using a cyclone system. The products end up in another distilling tower to be separated into fractions. The catalyst is reused -- catalysis by definition does not consume the catalyst -- but it has to be run through a "regenerator" first since the zeolite crystals are fouled by carbon. Hot air cleans up the crystals so they can be sent back through the cycle again.
The carbon fouling occurs because hydrocarbons, as their name implies, consist of carbon chains with hydrogen atoms around the "edges". The ratio of hydrogen to carbon atoms tends to get lower as the length of the chain increases, and so breaking a long chain into smaller molecules means that there's not enough hydrogen available to make use of all the carbon, leaving a residue. A scheme known as "hydrocracking" adds a hydrogen gas stream to eliminate the excess carbon. Since fouling is minimized, the catalyst is provided in a fixed bed, not cycled through a regenerator.
Catalysis is used in other parts of the refinery process as well. A "reformer" unit takes straight-chain molecules and turns them into branched chains or rings that burn better in piston engines. The reformer is a high pressure reaction vessel using precious-metal catalysts such as platinum and rhenium. The catalysts are expensive and have to be replaced every few years. The "alkylation" or "alkyl" unit performs something of the reverse function: it glues together small hydrocarbon molecules to make more useful molecules. Instead of a feed heater, it has a chiller unit that cools the flow down to the temperature of cold water, and instead of platinum and rhenium, it uses sulfuric acid as a catalyst.
The bottoms of the flasher unit are, as mentioned, a nuisance. They are usually fed into a "coking unit", where they are heated and turned into nearly pure carbon or coke, which is used in steel production. The coking unit looks like a row of huge drums with a drilling derrick, of all things, on top of each drum. The drums fill up with solid coke and the drill is then used to break up the coke so it can be removed. Coke obtained from refineries is often too fouled with pollutants to be used in countries with high air-quality standards, so it is usually shipped elsewhere.
* The liquid fuels resulting from this process include gasoline, kerosene, diesel oil, and so on. Relatively pure form of kerosenes, taken from narrow ranges of the fuel "stack", are used as "jet propellant (JP)" for jet aircraft. An even purer form of kerosene taken from a very narrow range of the stack is used as "rocket propellant (RP)", being burned with liquid oxygen (LOX). RP has to have consistent properties, since if it didn't the performance of the rocket engines would be unpredictable and the launch mass of a large booster might vary by tonnes, with disastrous results. There's nothing particularly magic about RP otherwise: few would be able to tell it from ordinary kerosene.
Automotive gasoline is a blend of gasoline and other fuels, butane being one major constituent, with the volatile butane making engine starting easier. It's the butane that is visibly vaporizing when a gas tank is filled up, not the gasoline. In cold climates, more butane is used in the blend since it it's otherwise harder to start cars, and it won't evaporate away as easily as it does in a cold climate. Ethanol is also often blended these days since it results in cleaner-burning fuel, reducing emissions.
It should be noted that even resid is used as a fuel, "bunker fuel", in big marine diesel engines used to power large vessels. These engines have very high compression ratios, and resid works fine in them -- if at the cost of high levels of pollution emission.
* A refinery will have a set of "flare stacks", tall pipes with a burner at the top, that burn off escaping methane and other gas. Flare stacks were once used continuously to get rid of gas that was too uneconomical to sell, but with rising energy prices, there's hardly any uneconomical fuels left. The flare stacks are now just an emergency venting system, since the buildup of gas would produce an explosive hazard. Interestingly, some of the burners on the flare stacks inject steam into the burning gas, which makes it burn more cleanly.
Sulfur in the crude oil generally ends up in the form of smelly hydrogen sulfide, H2S. Once upon a time, the H2S was generally vented to the atmosphere, but it was too much and too noticeable a pollutant. These days, a refinery will have a sulfuric acid production unit to turn the H2S into a useful and salable product. At some plants, it is turned into elemental sulfur instead.
The output of refineries is generally distributed in pipelines. However, these pipelines are more complicated than those that deliver crude, since one pipeline will carry different types of refined fuel: diesel, then kerosene, then gasoline, and so on. Interestingly, it's usually no problem to put a batch of one type of fuel in the pipeline right after a batch of another type of fuel, since there's little mixing. When mixing is a concern, an inflatable ball can be put into the pipeline.BACK_TO_TOP
* We tend to take natural gas somewhat more for granted than gasoline, for the simple reason that we don't normally have to pump natural gas out at a filling station. Instead, it arrives at our homes through the gas mains to be burned in a furnace or water heater, and we get a bill at the end of the month.
Municipal gas infrastructure has been around since the about the middle of the 19th century, when it provided lighting as well as heating. In those days, the system was based on syngas, produced by roasting coal in an oxygen-poor environment. (Roasting coal in a high-pressure pure oxygen environment to produce syngas, as described above, appears to be a modern invention.) Thanks to the carbon monoxide, syngas was extremely toxic and families were killed by it every now and then.
Syngas went away as household heating fuel in the 1950s, with natural gas taking its place. Natural gas is mostly methane, which burns clean and isn't anywhere near as toxic as coal gas. A gas leak still isn't welcome, and so traces of smelly mercaptans are mixed with natural gas to make sure a leak can be detected, methane being otherwise odorless.
The triumph of natural gas over coal gas was due to the construction of long-distance gas lines, since otherwise it's difficult, if hardly impossible, to transport natural gas in a cost-effective fashion. It is possible to ship "liquefied natural gas (LNG)" in tanker vessels, but it not only requires the tanker to be fitted with expensive pressure vessels, the pressure vessels have to be cooled to cryogenic temperatures.
Propane is another hydrocarbon fuel in common use, most recognizably as a heating fuel in portable applications, such as mobile homes. Although it boils at -42 degrees Celsius, it will remain liquid at high environmental temperatures under moderate pressure. It is actually in surprisingly wide use as a vehicle fuel, with forklifts in particular often being propane-powered, and in some countries, such as Australia, it is in fairly common use as an automotive fuel.
Natural gas in general is now catching on in the USA as a vehicular fuel, with natural gas-powered buses and fleet vehicles increasingly being powered by natural gas. Natural gas works very well for large vehicles, which can accommodate the big pressure tanks required to store the gas; it becomes increasingly troublesome as the vehicle size shrinks.
* Although there was an abrupt rise in global oil prices in the 1970s that led to a burst of enthusiasm for alternative energy sources, oil prices dropped just as abruptly again, and things more or less went back to business as usual. However, at the beginning of the 21st century, demand rose faster than supply and the energy crunch began to come back. Costs have been rising, though this has had the effect of increasing oil supplies, at least for the short term -- given higher prices, the oil companies find it profitable to squeeze more oil out of a well or search for oil in more difficult environments.
Natural gas has also been a beneficiary of higher fuel prices, since they make it cost-effective to tap what turns out to be huge reservoirs that were previously too hard to exploit. That's why natural gas is in increasing use as a vehicle fuel, at least for buses and other large fleet vehicles -- the fact that it burns very clean is another big advantage -- and it taking over an increasing bite of the market for electric-power generation from coal.
Still, the Earth's supply of fossil fuels is finite and it will have to run out sooner or later. The main question at present is whether this will be "sooner" instead of "later". While some pessimists think that oil production will peak in a few years, if it hasn't already, the mainstream opinion is that the peak won't occur for a few more decades. The result has been a scramble to find alternatives.BACK_TO_TOP
* One of the straightforward options for an oil replacement is to convert coal into liquid fuels. There's plenty of coal available and coal conversion has been around for a long time, having been invented by a German chemist named Friedrich Bergius (1884:1949) in 1912. The typical process is to convert coal into syngas, then use a catalytic process known as "Fisher-Tropsch synthesis" to produce methanol or, preferably, a diesel-like fuel. The Germans used synthetic fuel production in a big way in World War II, and the South African Sasol organization also built up a major coal conversion capability when the country was under embargo for its racial apartheid policies. Sasol still remains a leader in synthetic fuel technology.
The problem with coal-to-liquid conversion is that it is energy-intensive -- in effect, it uses up a good deal of fuel not useful for powering vehicles to produce a relatively small amount of fuel that is -- and, without sequestration, contributes to the atmospheric CO2 load, aggravating climate change. The more popular alternative has been "biofuels", synthesized from grains, sugar cane, grapes, and oilseeds. Grains, sugar cane, and grapes can be used to synthesize ethanol (grain alcohol), while oilseeds and other sources of plant oils can be used to synthesize a fuel very much like diesel known as "biodiesel". In principle, plants used as biofuel feedstocks absorb CO2 from the atmosphere, meaning that when the biofuels are burned they won't increase the CO2 load.
In the USA, ethanol from corn is the preferred biofuel. The production method is conceptually the same as it is for producing corn liquor or "moonshine", if generally conducted on a much bigger scale:
Pure ethanol is generally used as the main component of the "E85" blend, which is 85% ethanol and 15% gasoline. E85 has an "energy density" -- energy per volume -- only about 65% as great as pure gasoline, and so it takes a little more than half again as many liters of E85 as gasoline to drive the same distance. Ethanol does burn well and cleanly, but high-concentration blends are corrosive, demanding stainless steel and corrosion-resistant plastics in automobiles. Such vehicles are known as "flex fuel" machines.
Some advocates believe that butanol might be a better biofuel, since it has a better energy density and doesn't require special fittings; it can also be produced by fermentation, though using strains of bacteria instead of yeast. Alcohols up to octanol are seen as even better fuels -- alcohols heavier than octanol have a tendency to freeze too easily to be useful. Work is being done to genetically engineer microorganisms to synthesize octanol, or even true synthetic gasoline.
Critics have sniped at corn-based ethanol, charging that it takes more energy to produce than it provides, but the general opinion is that it does in fact provide a reasonable, though not generous, margin of net energy. Cane sugar is the preferred feedstock in Latin America, particularly Brazil; it is a much better feedstock than corn, since it consists mostly of sugars to begin with and doesn't require treatment with enzymes. Grapes are similarly being used in some southern European countries. Sorghum, a grain mostly used in the West for fodder and brewing beverages, is seen as a potentially valuable ethanol feedstock -- it's not unusually efficient, but it will grow on lands not well-suited to other crops.
Biodiesel is more popular than ethanol in Europe, partly because European nations have been more enthusiastic about diesel cars than the USA. The term "biodiesel" actually covers almost any form of fuel derived from biological sources used to run a diesel engine, including vegetable oil, cooking oil, and rendered chicken fat. Diesels are widely tolerant of the fuels they can burn; Rudolf Diesel actually ran his engine on peanut oil at the Paris Exposition in 1900. There are some people who are running diesel vehicles on waste cooking oil from fast-food restaurants as fuel.
Unlike ethanol, proper biodiesel does not require that a vehicle burning it have any special modifications. Almost any plant material used for vegetable oil will do as a feedstock for biodiesel, with soybeans, cottonseed, or rapeseed (rape is a mustardlike plant, with oil-rich variants known as "canola") being popular feedstocks. In Southeast Asia, palm oil is a common feedstock, and some South Pacific islands even use coconut oil. Some biofuel advocates are particularly enthusiastic about growing algae as a biodiesel source, though it's not yet being done commercially.
Whatever the source, the oil is modified through a process called "transesterification", which prevents it from clogging fuel lines -- incidentally, vehicles running waste cooking oil must have a switchable fuel system and run ordinary diesel fuel for a time before they're turned off, or their fuel lines will end up blocked.
* There are misgivings about using crop plants as biofuel feedstocks, primarily because production of such plants competes with production of food crops and tends to force up their price. The fact that supplies of crop plants are not adequate to provide a primary source of fuel is also an issue. Critics often claim, with particular support from the facts in the case of corn-based ethanol, that the biofuels boom amounts to much more of a farm-subsidy program than it does a search for "energy independence". The great hope at present is for "cellulosic" biofuels, sometimes nicknamed "grassoline", derived from non-food-crop plants such as plant waste, sawdust, prairie grasses, fast-growing trees such as poplars or willows, and the like. Feedstock plants can be grown on lands not suitable for farming and would not, in principle, compete with food production.
There are a number of schemes for producing cellulosic ethanol, but it's tricky. There is a straightforward way to do it by simply using the cellulosic material as a feedstock for syngas production -- heating it in a reactor along with steam or oxygen to produce carbon monoxide and hydrogen -- and then using Fisher-Tropsch synthesis to produce ethanol or a diesel fuel. However, this is generally seen as an inefficient process.
There is a somewhat comparable scheme in which biomass is heated to high temperature in an oxygen-free reactor to produce a "biocrude". It's highly acidic and cannot be used as fuel itself, but it could be refined by processes similar to those used to refine ordinary petroleum; there is some work towards including a catalytic system in the reactor itself, combining the biocrude production and refining into a single step, so its output will be a useful fuel.
Cellulosic materials can also be used as the basis for distillation processes to produce fuel, but that's troublesome as well. The problem is the cellulose, hemicellulose, and lignin in grasses and the like. The first issue is that the cellulose and hemicellulose have to be broken down for digestion -- the lignin is something of a nuisance and has to be disposed of. Current methods use acid treatments, but this approach is expensive as well as environmentally troublesome, and better processes are being developed. There is some work on use of concentrated ammonia, a strong base, under pressure to the same end; it is seen as a much less troublesome approach.
The second issue is that, while cellulose breaks down into six-carbon sugars along the lines of glucose that can be fermented by yeasts into ethanol, hemicellulose breaks down into five-carbon sugars, and no naturally-occurring microorganism can digest them into ethanol. Advocates claim that processes have improved enough to bring cellulosic biofuels within reach of competitive pricing, and believe that costs are not close to bottoming out. Work is underway on genetically modifying organisms to do the trick, as well as on genetic modification of feedstocks to make them easier to process.
Large-scale biofuel technology is still regarded as in its infancy and advocates are, with good reason, optimistic that progress is likely to be made. Indeed, Brazil has achieved a fair degree of energy independence even with sugar cane ethanol, with plenty for export left over, showing that biofuels can be a practical option -- at least if circumstances permit it. We also raise crops such as cotton that don't contribute to food production, and don't see that as any major problem. Effectively, we are tinkering with a range of options, some of which won't be workable over the long run, some of which may pan out better than expected. Even if we don't end up relying completely on biofuels, it seems likely they represent a viable business proposition and can make their own contribution to the energy mix.
* Another scheme that's worth mentioning here as a footnote is the use of manure digesters to produce methane for local power. Facilities to digest pig, cow, or turkey manure have been set up in areas in which such animals are raised in quantity. Nobody honestly thinks that manure is the answer to global energy problems, but the farmers have to get rid of the manure anyway, and a fair financial analysis shows the digester plants to be cost-effective -- all the more so because the methane produced by the manure in the digesters is burned into CO2, which is much less a nuisance as a greenhouse gas. The residue from the process is used as fertilizer. Use of biogas as a fuel is straightforward, since it's basically the same as natural gas; some large dairy farms fuel their trucks with biogas. There has also been investigation of natural gas for aircraft propulsion.
This discussion has ignored sources of renewable energy such as wind power, wave and tidal power, and solar power. All of these technologies are of increasing importance in electrical power generation, but they do not directly involve chemical processes and so are beyond the scope of this document.BACK_TO_TOP
* Some dedicated Greens believe that hydrogen will be the ultimate fuel. There is plenty of it available, locked up in water, and when burned it ends up being (in principle non-polluting) water again.
Liquid hydrogen is actually used as a fuel for certain types of advanced rockets, since when burned with liquid oxygen it produces highly efficient thrust. It is not, however, in serious use in any other vehicular application, and given its idiosyncrasies it's not hard to understand why. Gaseous hydrogen has very low energy density and requires a big tank designed to handle high-pressure gas to provide any range; liquid hydrogen must be kept under deep-cold conditions. There has been work on "metal hydride" materials that can store hydrogen in their solid lattice structure, as well as adsorbent materials such as carbon nanotubes, but such schemes are not economically practical at present.
H2 molecules are very small and they leak easily, and hydrogen tends to be highly flammable or explosive in the presence of atmospheric oxygen. There's also the problem of obtaining it in the first place. At present, most hydrogen is produced by the "steam reforming" of methane / natural gas, or in other words mixing the natural gas with high-temperature steam:
CH4 + H2O --> CO + 3H2
This is obviously not a satisfactory process for the long haul, since it uses up fossil fuels and produces carbon emissions -- worse, in the form of toxic carbon monoxide. The more conceptually elegant approach is to electrolyze water, but that means finding an energy source to do it with: there's no great virtue in obtaining hydrogen as fuel using electricity from a coal-fired power plant. The general Green dream for a hydrogen economy is to produce the hydrogen using renewable energy resources such as solar or wind power, but nobody sees renewables as up to the job any time soon. In fact, some critics even deny that hydrogen can be called a "fuel", instead referring to it, awkwardly, as an "energy-storage medium" -- meaning it's just a means of converting an electric power source into a form that can be used in a vehicle.
There's also the problem of distribution: a pipeline pumping gaseous hydrogen would be as big around as the fuselage of a jumbo jet, and trying to design a commercial filling station that could handle the material safely at reasonable cost would be challenging. There seems no prospect of converting to a hydrogen economy any time soon, but it there's no harm in giving the idea a careful looking over. It might be practical in the mid-term as an airliner fuel -- flight experiments have been conducted with jetliners burning liquid hydrogen -- and there's no saying that future developments may make hydrogen more appealing in practice, not just in concept.BACK_TO_TOP
* While the petroleum-processing system is the most visible component of industrial chemistry, other chemicals are produced in large quantity, with the "top ten" non-petroleum-related industrial chemicals given below in decreasing order of production quantity:
Other important industrial chemicals include hydrochloric acid (HCL); ammonium nitrate (NH4NO3), used as fertilizer or an explosive; various hydrocarbons such as ethylene (C2H4), the most heavily produced organic compound, or benzene (C6H6); and plastics.BACK_TO_TOP