Steam Reforming of Natural Gas

Large quantities of hydrogen gas are required in the petrochemical industry.  This hydrogen is used to change the chemical structure of crude oil.  Crude oil is a mixture of many different molecules.  Some are very light and will have low boiling points. Others are very large, and have much higher boiling points.  The mixture can be separated by distillation; however, if you do so you will get a lot of rather useless heavy tar, and not nearly as much of the light hydrocarbon fuel (gasoline) as you want.

Rather than just distilling the oil to separate it into its components, modern refining of oil relies extensively on:

• cracking: breaking down of the large hydrocarbon molecules in crude oil into smaller ones.
• reforming: adding the broken down molecules back together in order to make the compounds wanted.

As part of the cracking-reforming process, large quantities of hydrogen are used to modify the structure of the cracked molecules, and reform them into the molecules wanted.

The hydrogen is usually generated from steam reforming, which is basically the following reaction:

H₂O (g) + CH₄ (g) 3H₂ (g) + CO (g)

However, the reaction isn't nearly as simple to carry out as it would appear in the above reaction. 

• the reaction is highly endothermic (H = +206.2 kJ/mol CH₄) so you have to supply a lot of energy.  This is done by burning about 25% of the methane to provide the necessary heat:  CH₄ (g) + 2O₂ (g) CO₂ (g) + 2 H₂O (g), and a catalyst is also required to get a rapid reaction.

• you can't burn the methane in the above reaction without using oxygen, which will probably come from the air, and so be about 75% nitrogen.  The nitrogen will partially react to make NO_x (a mixture of nitrogen oxides which are air pollutants, and contribute to both photochemical smog and acid rain).

• methane usually contains some sulfur compounds.  Sulfur is a catalytic "poison", and as well burning it would create noxious sulfur oxides.  The sulfur compounds must be stripped from the methane before it is burned.

• the reaction is an equilibrium, so the reaction won't go to completion.   The water gas shift reaction: H₂O (g) + CO (g) H₂ (g) + CO₂ (g) is used in a second stage to increase the amount of hydrogen produced.

Nevertheless, complex as the process is, it can produce H₂ (g) that is more than 99.9% pure for use in further refining processes, or for the production of other products such as ammonia.

Hydrogen Fuel Cells

Shuttle Orbiter fuel cell (courtesy NASA)

Hydrogen fuel cells are potentially very pollution free and are proposed as a method of powering automobiles.  In them, H₂ and O₂ gases react to form water and release energy – not through combustion, but through electrochemical processes.

H2 (g) + O2 (g) H2O (g)

H = -241.8 kJ

(1)

This reaction produces more energy per unit mass than any other chemical reaction.  However, that is not the reason it would be used to power an automobile.  Since the only product of the reaction is water, the reaction is pollution free.  Fuel cell technology is well developed because of its use in the space program.  The potential for clean, noise free automobiles is enticing.

The problem is that H₂ cannot be considered a fuel.  There is virtually no free hydrogen on earth.  It is almost all combined into water, and hydrocarbons.  Thus in order to make use of the fuel cell we have to have a way of producing the hydrogen gas.  Steam reforming is an obvious potential candidate.

Suppose that the CH₄ were simply burned in an internal combustion engine.  Then, if we had 100% combustion, and no energy loss through friction, and other heat losses, we would get:

CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g)

H = -802.2 kJ

(2)

However, an internal combustion engine is actually only about 20% efficient, so we probably get closer to 160 kJ/mol CH₄ burned to move the car.  In addition we still get all the noxious NO_x (nitrogen oxides) which contribute to photochemical smog and acid rain air pollution, since these products are always formed in high temperature combustion reactions.

Now let's examine what happens if we reform the CH₄ (g) into hydrogen.

H2O (g) + CH4 (g) 3H2 (g) + CO (g)		H = +206.2 kJ		(3)
H2O (g) + CO (g) H2 (g) + CO2 (g)		H =     -41.2 kJ		(4)

which gives an overall reaction of

2H2O (g) + CH4 (g) 4H2 (g) + CO2 (g)

H = +165.0 kJ

(5)

Since the overall reaction produces 4 mol H₂ (g), if all this hydrogen were combined in the fuel cell, we would get a net energy production of:

2H2O (g) + CH4 (g) 4H2 (g) + CO2 (g)		H = +165.0 kJ		(5)
4H2 (g) + 4O2 (g) 4H2O (g)		H = -967.2 kJ		(6)

which, adding (5) and (6) gives an overall reaction of

CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g)

H = -802.2 kJ

(7)

Equations (2) and (7) are exactly the same.  You get the same amount of energy as if you just burned the CH₄ to begin with!  Actually this shouldn't be surprising – it is just the law of conservation of energy at work.  So why bother going the fuel cell route?

Hydrogen fuel cell powered bus (photo courtesy NASA)

There are at least five reasons:

• fuel cells are a lot more efficient than internal combustion engines.  Fuel cells can get up to 75% efficiency.  We have to burn about 25% of the CH₄ to provide the energy for the reforming reaction, but that still leaves us 75% of the 802.2 kJ or about 600 kJ.  If 75% of this can then be converted into useable work in a fuel cell, we will get about 450 kJ of useful work.  Compare that to the perhaps 160 kJ from burning the fuel directly.  Your car will be able to travel two to three times as far as on the same amount of hydrocarbon fuel.
• an efficient catalyst can lower the temperature required.  The lower the temperature the less NO_x will be produced when we burn the CH₄ to provide the energy required for reforming.  This causes less nitrogen oxide air pollution.
• we still produce the same amount of CO₂ per mole of CH₄ used.  But, since we can go much farther on the same amount of fuel, the net contribution to global warming causing pollution is much less.
• fuel cells raise the potential of using reuseable biomass which can be efficiently and cheaply converted into liquid fuels like alcohols.  Reusing biomass would not increase the global CO₂ balance.
• electric motors are quiet.

However, hydrogen is a very difficult substance to store.  Active research is ongoing to try and develop small catalytic reformers that can convert hydrocarbons, or alcohols directly into hydrogen.  These reformers would be in the car itself, allowing the car to use easily stored liquid fuels, much as they do today.  Even more exciting is the potential for direct conversion of liquid alcohol fuels without the need for a reformer; however, such fuel cells are still very experimental.