11.4.5: Electricity → Chemical Energy
- Page ID
- 84622
In this section, we want to focus on technologies in which energy “input” is converted into fuel that can be stored and someday converted back to energy in a zero-emission process – or used in industry to replace the CO2-spewing operations with carbon-neutral ones. The only such fuel is hydrogen, the most abundant element in the universe. The combustion reaction produces only water vapor, H2O, which in no way contaminates the air.
One may argue that water vapor is a greenhouse gas, just like CO2. It’s true, but man-made carbon dioxide accumulates in the atmosphere and oceans while Mother Nature always makes it sure that there is not too much water vapor in the air: if there is, the excess is simply removed in the form of precipitation.
However, there is one major problem: namely, hydrogen does not occur in nature in a “ready to use” form. It has yet to be produced at the expense of energy. Then it can be stored (which is not so simple, however) and finally the energy input can be recovered. That is why we are talking about the possibility of storing energy in the form of hydrogen. However, at all stages: energy conversion into hydrogen, hydrogen storage and finally energy recovery, there are serious challenges that have yet to be faced.
The idea that hydrogen will be the “fuel of the future” has been talked about for a long time. The author of this textbook remembers that he had read about it as long ago as in high school (from which he graduated in 1963). However, it turned out that these were mainly discussions between scholars. To some disappointment of the author, the industry was clearly not interested in these ideas. But in 2003, there seemed to be a clear shift in the situation: namely, President George W. Bush, announced in his “State of the Union Address” a program to open a policy which he called “Hydrogen Economy”.
He said, among other things: A simple chemical reaction between hydrogen and oxygen generates energy, which can be used to power a car producing only water, not exhaust fumes. With a new national commitment, our scientists and engineers will overcome obstacles to taking these cars from laboratory to showroom, so that the first car driven by a child born today could be powered by hydrogen, and pollution-free.
Unfortunately, after this declaration nothing has really changed. Indeed, the industry after Bush’s speech consumed significant amounts of hydrogen, but did the same before the speech. In 1975, hydrogen production in the US amounted to around 6 million tons, followed by a gradual increase to current production of around 10 million tons. But this hydrogen was not produced for energy storage or as a clean fuel for cars. It was entirely consumed by industry, mainly for the production of ammonia (NH3), which is an important fertilizer, and in oil refineries, where it is used to remove sulfur from gasoline and diesel fuel. Moreover, almost all this hydrogen is called “gray”, which means that it is produced from fossil fuel (natural gas) in a process called steam reforming of the gas. Natural gas, which is almost a pure methane CH44, is mixed with water vapor, heated to a high temperature, and then the chemical reactions occurs:
CH4 + H2O → CO + 3H2
The carbon monoxide molecule created then reacts with another water molecule, yielding one more hydrogen molecule:
CO + H2O → H2 + CO2
The two reaction equations can be combined into a single general equation:
CH4 + 2H2O → CO2 + 4H2
As follows from the last equation, four H2 molecules with a total molecular weight of 8 and one CO2 molecule with a molecular weight of 44 come out from the reaction. Translating to ordinary mass units, it means that as much as 44/8 = 5.5 tons of CO2 is produced per a single ton of the hydrogen acquired.
But this is not the end, because these reactions are endothermic. That is, to split the interatomic bonds in the reactants, more energy is needed than that which is released when new bonds are formed in the reaction products. In other words, an extra boost of energy – in the form of heat – is needed for the reaction to occur. The easiest way to provide this additional heat is by burning an extra portion of gas – which even further increases the CO2 emissions. In practice, as much as seven tonnes of CO2 are produced for acquiring a ton of hydrogen.
As can be readily calculated (see Chapter 3, Section 3.2.1), the methane input needed to produce 7 tons of CO2 is 2.54 tons. The energy contained in this amount of methane (i.e., the heat of combustion, see Section 3.2) is 2.54 tons 55.5 GJ/ton = 141.2 GJ. And the heat of combustion of one ton of hydrogen is 141.8 GJ – essentially, the same. In view of this, it is immediately apparent that producing hydrogen in such a way in order to store energy would make little sense: burning methane directly would release the same amount of energy and emit the same amount of CO2, without the need for going through the entire steam reforming operation.
The only solution acceptable from the viewpoint of future programs treating hydrogen as a “storable means” must be based on the production of the so-called “green hydrogen” in a completely zero-emission process. As will be discussed further on, water electrolysis, i.e. the decomposition of H2O into H2 and O2 by electric current, is a process that does not produce CO2 at all. This is a method known for over 200 years and it has been used in industry for well over 100 years. Now, it becomes a major player in the efforts of creating a “hydrogen economy” of global reach. Sometimes heat is easier available than electricity – therefore, a number of thermochemical H2O decomposition methods have been proposed and tested, but none has yet been implemented at industrial scale. A novel method worth consideration is the pyrolysis of methane. It uses fossil fuel, yes, but in contrast to the CO2-yielding steam reforming, in pyrolysis the CH4 molecule is broken down into hydrogen and carbon in solid form (a substance similar to sooth, a.k.a. “carbon black”) which can be disposed off using methods not harmful to the environment.