This article is the fifth in a series examining alternative fuel production and source materials. Previously we looked at biodiesel, natural gas, propane, and ethanol. Next month’s article will be our last in this series, and we will study electricity as a vehicle fuel. This month, our focus is on hydrogen.
There are two ways hydrogen may be used as a fuel. Hydrogen can be burned in an internal combustion engine (ICE) or it may be used to produce electricity in a fuel cell. In both cases, the main byproducts are heat and water. This makes hydrogen-powered vehicles an environmentally conscious option for consumers.
The internal combustion engine of a hydrogen vehicle burns hydrogen gas as its fuel to create energy. This energy then powers the vehicle.
Designers of conventional ICE vehicles do not need to completely redesign conventional engines to create vehicles powered by hydrogen. However, some modifications are necessary to burn hydrogen efficiently.
The combustion chamber and cooling system of conventional ICE engines require modification to control problems with premature ignition. Hydrogen ICEs require the use of superchargers or turbochargers to supply sufficient quantities of air in the combustion chamber to allow the hydrogen to burn. Special fuel injectors and fuel delivery rails are needed to efficiently transfer hydrogen gas into the combustion chamber.
A fuel cell is a device that electrochemically produces energy by separating the protons and electrons in a hydrogen molecule and channeling their flow to create an electrical current. The electricity can be used to power the vehicle’s electric motor. Fuel cells are extremely efficient, even more so than hydrogen ICEs and can greatly reduce air pollution.
There are different types of fuel cells. In vehicles, the typical fuel cell is a proton exchange membrane (PEM). The type of fuel cell creates electricity by using a combination of hydrogen, oxygen, electrodes, a catalyst, and a special polymer membrane. A FCEV uses a fuel cell stack that is made up of multiple PEM fuel cells.
In the operation of a PEM fuel cell, hydrogen from the storage tank flows into the fuel cell where it contacts a catalyst on the anode side that splits the diatomic hydrogen (H2) into hydrogen protons (H+) and electrons. Oxygen molecules attract the hydrogen protons and electrons. The hydrogen protons pass through the PEM while the electrons travel around the membrane creating an electrical current. On the other side of the PEM, hydrogen protons and electrons combine with oxygen to produce water and heat.
The electrons produced in the first step travel from the anode to the cathode in the second step, producing electrical power.
Hydrogen is the most abundant element on earth, but the majority of hydrogen is bonded with other elements in compounds. In fact, “free” hydrogen does not exist on earth for very long, because hydrogen binds so easily with many other elements, such as oxygen and carbon.
To produce pure hydrogen, it is necessary to release it from these compounds and is the only way to use it as a fuel.
Hydrogen may be produced from any substance with hydrogen content. However, there are issues with cost versus yield when producing hydrogen. To produce hydrogen from renewable sources, it is best to use substances that have very high hydrogen content.
Biogas is typically about 60% methane, which makes it a fantastic source of hydrogen. Biogas can be made from any organic waste material. Biogas is produced by anaerobic methane digestion. This allows for methane gas to be made from organic waste materials. Biogas may be processed and made into hydrogen in almost the same fashion as natural gas. While natural gas is about 85 to 95% methane, biogas is only about 60% methane. This means that it will take more biogas than it does natural gas to make hydrogen. However, organic waste is just that- waste. The methane from biogas sources could be steam-reformed into hydrogen. Recent reports suggested that biomass may be able to produce 12 quadrillion BTU/year of energy by 2050. Current personal transportation energy consumption occurs at a rate of 16 quadrillion BTU/year.
The process of creating hydrogen from water is called electrolysis (water splitting). Hydrogen is produced via electrolysis by passing electricity through two electrodes in a water electrolyte such as potassium hydroxide, which is used because of its high conductivity as an alkaline electrolyzer. A membrane is placed between the cathode and anode, which separate the hydrogen and oxygen as gases are produced, but allows the transfer of ions.
Electrolysis cells are connected in a series. Hydrogen is produced on one side of the cell, oxygen on the other. The water molecule is split, releasing oxygen at the anode electrode and hydrogen at the cathode electrode. The hydrogen is then captured, compressed, and stored by a high-pressure system awaiting vehicle refueling. Oxygen is usually released into the atmosphere.
Hydrogen can be produced using a number of non-renewable fuel sources, including natural gas and coal gasification. Of those two courses, natural gas is arguably the best options since it contains more than 85% methane. However, coal gasification, followed by chemical conversion of the resulting fuel gas, is also a highly feasible option as it can yield large quantities of hydrogen at relatively low cost.
Hydrogen can be produced from catalytic reforming of naphthaa process used in oil refining that produces significant amounts of hydrogen along with high octane gasoline.
One of the most common and energy-efficient ways to produce hydrogen gas is known as steam-methane reforming. In this method, methane is pumped into a heat pressurized storage tank. A boiler is typically used to produce high-temperature steam. The boiler is fired by some of the methane gas. In the heated pressurized storage tank, steam and methane are mixed together in the presence of a catalyst, such as nickel. The steam combines with the methane to form carbon monoxide and hydrogen.
The carbon monoxide created in the above reaction can then be reacted with more steam to produce carbon dioxide and more hydrogen. The only waste products from the steam reforming process is stored in a pressurized tank to be purified, stored, and later used as a vehicle fuel.
This article is based on the Clean Cities Learning Program Petroleum Reduction Technologies curriculum. For more information, visit the Clean Cities Learning Program or contact the NAFTC.