Gasification and Gas Reforming in Underground Caverns

Creating our liquid fuels from natural gas makes sense in many different ways. Such liquid fuels are free of sulfur contamination, which is very difficult to achieve for any diesel fuel made from petroleum or from coal. Sulfur-free liquid fuels would enable a great reduction in the pollution load, especially in smoggy cities.

It is also true that the initial liquid products created in a gas-to-liquid plant (GTL) are waxes. Wax could be transported by conventional oil tankers to refineries in the US and elsewhere. Any modern refinery could take in waxes rather than crude oil to produce liquid fuels and lubricants. Feeding our refineries with sulfur-free waxes would enable the production of high-quality sulfur-free liquid fuels from these waxes.

If an oil tanker should sink carrying molten wax, no seabirds or fish will die and the wax will be easily recoverable. This cannot be said of crude oil.

The reason that sulfur-free fuels are important is in part that sulfur poisons catalysts. Because of this, it is not possible to use a catalytic converter on a diesel engine burning conventional diesel fuel because of the sulfur content of the diesel fuel.

It would improve the overall efficiency of our transportation system if more cars had diesel engines because those engines are more efficient. The nitrogen oxide pollution from diesel engines undergoes chemical reactions with other contaminants in the atmosphere leading to smog. In addition, the nitrous oxide produced by this process is a very potent and long lasting greenhouse gas. It is possible to destroy these nitrogen oxides using a catalytic converter but this catalyst is easily poisoned by sulfur dioxide. Because of that catalytic converters can’t be used if the fuel has more than a few parts per million of sulfur. The current definition of ultra-low sulfur diesel fuel allows no more than 5 parts per million, but a lower sulfur level than that would be desirable for the long term effectiveness of catalytic converters on diesel engines.

There is no viable way to convert natural gas directly to liquid fuels such as gasoline, jet fuel, or diesel fuel. Instead, the first step in the synthesis is to make a synthesis gas (syngas) containing carbon monoxide and hydrogen.

This syngas is used in a catalytic process known as Fischer-Tropsch synthesis to produce liquid fuels.

The chemical plants that produce liquid fuels from natural gas have several different production units. The production unit that produces the syngas is the most expensive unit. This unit alone typically requires half of the total capital for the plant. At the same time, the syngas unit consumes electrical power.

Gas-phase reactions between natural gas, oxygen, and steam lead to the creation of the syngas intermediate which is needed to convert natural gas to liquid fuel. Normally these reactions occur over a catalytic bed at modest pressures around 10 to 20 atmospheres, at temperatures below 900 Celsius, and at a pressure less than 25 atmospheres. In our process, because the gasification chamber is deep underground the pressure can be much higher. You can see a process flow diagram in the figure below.

This shows the rocket motor syngas process. It was initially conceived as a literal rocket motor but it became apparent upon reflection that the hot synthesis gas need not come from something shaped like a rocket combustion chamber. The source of the supercritical syngas could just as easily be an underground cavern, and the turbine could resemble a supercritical steam turbine as is used in power plants around the world.

As long as the pressure in the cavern does not exceed the lithostatic pressure, there will never be a blowout at the top of the cavern. One does need to make sure the rocks around the cavern do not melt and are not permeable. There are methods for that which I don’t intend to discuss here.

Such a high-pressure underground reaction chamber can be a tiny fraction of the cost of a conventional gasification chamber in terms of dollars per unit volume. By increasing the volume greatly, it is possible to have a much longer residence time for equilibration of the gases in the chamber, and also to make the turbine used for pressure reduction dispatchable in response to an electrical load. Making this electrical power-producing turbine dispatchable greatly increases the value of the electricity produced and exported to the grid.

Catalyst helps the gas mixture move towards equilibrium, but higher temperature and pressure also improve equilibration speeds. At sufficiently high temperature and pressure, no catalyst is needed.

Coal ash is a known catalyst for the gas phase reactions that must occur after partial oxidation of natural gas in order to produce synthesis gas with the correct ratio of hydrogen to carbon monoxide, particularly the reaction between carbon monoxide and water vapor to produce carbon dioxide and hydrogen (the water-gas shift reaction).

By moving the gasification and/or gas reforming unit deep underground, it becomes possible to use much higher pressures in the gasification unit than have ever been practiced before. We have shown that it is thermodynamically feasible to create synthesis gas at pressures high enough to drive a turbine and generate electricity. By harvesting useful energy out of the synthesis gas production process, the electricity helps to pay for the capital cost of the GTL plant.

We filed a patent recently describing a new option in terms of conversion of natural gas to liquid fuels. We call this process cavern gas reforming and partial oxidation, and this is a part of a larger invention that we call Cavern Gasification.

Cavern Gasification is based on a large chamber deep underground which may be first used for the gasification of solid fuels such as biomass, municipal solid waste, or coal. After the reactor is used for gasification, it can be used for gas reforming by feeding with natural gas, oxygen, and steam. There is however no need for the gas reforming chamber to first be used for solid fuel gasification.

When coal is gasified, a coal ash filled cavern may be produced in which the coal ash itself is a catalyst for gas reforming reactions to make synthesis gas from natural gas.

Cavern gas reforming greatly reduces the operating cost of the plant, because of the electricity that is co-generated as the syngas is being produced. If one writes off part of the capital as being an investment in the power generation facilities, the capital cost of the GTL plant will be reduced.

Cavern gas reforming can use excess steam to drive most of the carbon content of the gas to be CO2 which is easier to sequester from the syngas compared to pulling it out of combustion byproducts.

This method can be used to create synthesis gas with the correct ratio of carbon monoxide to hydrogen to feed directly into a Fischer-Tropsch synthesis of liquid fuels.

It is also possible to operate the gas reforming underground reactor so as to maximize the hydrogen content of the gas, by means of making most of the carbon present in the synthesis gas carbon dioxide rather than carbon monoxide. After removal of the carbon dioxide for sequestration, the resultant high hydrogen fuel can be used in a combustion turbine which releases less carbon dioxide than if the turbine burned natural gas.