The horizon of converting coal to gasoline on the large scale is looking nearer by the day, particularly in those energy-competing nations, China and the U.S. Since one tonne of coal can be made to produce 1.5 barrels of gasoline or of aviation fuel (spirit), the potential hydrocarbon resource that might be so made available is vast, and well in excess of the known world oil reserve. The latest vocal proponent of coal-oil is Brian Schweitzer the state governor of Montana who believes that its vast southeastern coal reserves could be used to produce gasoline and other petroleum products. Montana owns some 600 million tonnes of coal, which is located along with 600 million tonnes owned by Great Northern Properties, and 1.2 billion tonnes owned by the Northern Cheyenne tribe. Schweitzer had originally said that the synthetic fuel technology would become economically viable once the price of crude oil reached $35 a barrel, and the market is running way above (almost twice) that now, so the economic argument (if all the sums have been done on extraction and processing and they add up?) appears compelling.
The first stage is "coal gasification", which is as it sounds: the conversion of solid coal into a gaseous feedstock that can be processed into a hydrocarbon mixture. "Coal gas" was made more than two centuries ago, and is a gas rich in methane, CH4, hence generating up to 20.5 kJ per litre when burned. The product was also known as "town gas" and gained popularity to the extent that most major cities and many small towns installed a local gas house to generate it. Gas lanterns (which employed the device of playing a gas-air flame onto a gauze of thorium oxide, which emits a bright white light when it is heated - the only use for thorium before the days when the "atom" was understood and that it had a nucleus) were eventually replaced by electric lights. However, coal gas remained in use for cooking and heating until the more efficient natural gas (which yielded 38.3 kJ/litre) became readily available, in the U.K. in the early 1970's.
A slightly less efficient fuel known as "water gas" can be made by reacting the carbon contained in coal with steam. Depending on the quality of "coal" it can contain up to 95% of carbon ("anthracite grade"), but anywhere down to around 65% for some grades of "brown coal", with its plant-origin clear enough in the leaf structures etc. that are instantly apparent to the eyes, at least after cracking a lump of it open with a hammer:
|C + H2O CO + H2||Ho = 131.3 kJ/mol|
Water gas burns to give CO2 and H2O, releasing roughly 11.2 kJ per litre of gas consumed. Note that the enthalpy of reaction for the formation of water gas is positive, which means that this reaction is endothermic (absorbs heat). As a result, the preparation of water gas typically involves alternating blasts of steam and either air or pure oxygen through a bed of white-hot coal. The exothermic (heat releasing) reactions between coal and oxygen to produce CO and CO2 provide enough energy to drive the reaction between steam and coal, which cools it down again, until the next blast of air or pure oxygen gas.
When water gas is formed by the reaction of coal with oxygen and steam, it is a mixture of CO, CO2, and H2. The ratio of H2 to CO can be increased by mixing in more steam, to exploit the "water-gas shift reaction", which reduces water to form more hydrogen:
|CO + H2O CO2 + H2||Ho = -41.2 kJ/mol|
The concentration of CO2 can be decreased by reacting the CO2 with coal at high temperatures (since heat is absorbed to make it go) to form CO:
|C + CO2 2 CO||Ho = 172.5 kJ/mol|
The resulting CO2 depleted gas is called "synthesis gas" because it can be used in the manufacture of a variety of organic and inorganic compounds; for example as the source of H2 in the synthesis of ammonia:
N2 + 3 H2 2 NH3
It can also be used to make methanol:
CO + 2 H2 CH3OH
Methanol can then be used as a starting material for the synthesis of alkenes, aromatic compounds, acetic acid, formaldehyde, and ethyl alcohol (ethanol). Synthesis gas can also be used to produce methane, or synthetic natural gas (SNG):
CO + 3 H2 CH4 + H2O
2 CO + 2 H2 CH4 + CO2
Synthetic fuels are made from synthesis gas using the Fischer Tropsch process, in which essentially, the mixture of CO and H2 is passed over a catalyst of iron or cobalt (other metals e.g. nickel may also be employed). The process is an old one, dating back to 1923, when Franz Fischer and Hans Tropsch, working at the Kaiser Wilhelm Institute for Coal Research, developed a catalyst that converted CO and H2 at 1 atmosphere pressure and at a temperature of just 250 to 300 degrees C. into a mixture of liquid hydrocarbons. By 1941, Fischer-Tropsch plants produced 740,000 tons of petroleum products per year in Germany. It is interesting that the "Allies" initially laughed at Hitler's plans for world invasion believing that Germany would run out of fuel long before their army got very far, especially under the naval blockade they imposed which precluded imports of fuel from the Middle East into Germany. That the German war machine was kept running to the extent that it did is a testament to German determination and ingenuity.
However, it is the greatest tragedy of human history that so much effort and sacrifice was wasted by both the Allies and the Axis countries in this senseless conflict, as it slid increasingly out of hand; rising on the wave of the Great Depression in the aftermath of WW1, which had already cost so much.
Fischer-Tropsch type technology (it is no longer a single process, and many variants have been developed in the past 80 years) is based on a complex series of reactions that use H2 to reduce CO to CH2 groups attached to the metal catalyst surface, which link up with each other to form hydrocarbon chains:
|CO + 2 H2 (CH2)n + H2O||Ho = -165 kJ/mol|
The water produced in this reaction reacts with CO via the "water-gas shift reaction" to form more H2 and CO2:
|CO + H2O CO2 + H2||Ho = -41.2 kJ/mol|
The overall Fischer-Tropsch reaction is therefore described by the following equation:
|2 CO + H2 (CH2)n + CO2||Ho = -206 kJ/mol|
Since both reactions have a negative enthalpy (i.e. both give out heat), the process is overall a highly favourable one.
By the end of WWII, most of the industrial nations were undertaking research into synthetic fuel production using Fischer-Tropsch technology; however, the ready availability of cheap crude oil led to a decline in such efforts to convert coal into liquid fuel. The only commercial plants which currently employ the technology are based in the Sasol complex in South Africa, and consume 30.3 million tons of coal per year. They were built as a political expedient to keep the country replete with fuel during the trade sanctions imposed as part of a world protest against the apartheid regime.
As I have mentioned "zeolites" in previous postings, it is worth pointing out an alternative approach to the manufacture of liquid fuels, which is based on the reaction between CO and H2 to form methanol, CH3OH:
CO + 2 H2 CH3OH
Methanol can be used directly as a fuel, as employed in internal combustion engines or in fuel cells (mindful that being partially oxidised, it only delivers around 50% of the energy "punch" that say methane does). It can also be converted into gasoline using catalysts, notably the zeolite ZSM-5 zeolite developed by Mobil Oil Company in the 1970's, which underpinned the "Methanol to Gasoline (MTG) Process."
As we now witness a dwindling supply of cheap crude oil to process into transportation fuel and as a chemical feedstock for almost everything that modern life has come to depend upon, governments may well shift toward the manufacture of hydrocarbons from coal. This would provide a "home supply" of petroleum rather than running political, geological and military guantlets to bring the "black gold" back from the Middle East, and furthemore increase the world supply in total of a resource that is probably at or past its "Peak". One final point is that while "Peak Coal" is a very long way off, unlike "Peak Oil", can we physically extract and process sufficient of it on the scale required to substitute for a substantial fraction of the 90 million barrels that the world uses daily? That amounts to about 32 billion barrels every year, which could in principle be got from around 20 billion tonnes of coal. Current combined extraction of coal annually in the U.S. and China amounts to around 3 billion tonnes, as a benchmark; clearly, the infrastructure needed to be emplaced to seriously provide that amount of coal-oil is stupendous. Energy efficiency and a break in our heavy oil-dependency by implementing sustainable alternatives remains the best way forward. Coal-oil is at best a sticking plaster on the wound. And what about the CO2 emissions...?