Hydrogen as an Alternative Fuel
An ideal alternative fuel should be inexpensive, convenient to use, clean, and have lower carbon content. Among various alternatives, hydrogen fuel offers the highest potential benefits and possesses most of the key criteria for an ideal fuel. Similar to electricity, hydrogen is a high-efficiency energy carrier, which can lead to zero or near-zero emissions at the point of use. Therefore, for past 50 years, researchers and several industrial organizations have promoted hydrogen fuel as the solution to the global warming. However, hydrogen economy is not a new concept. In 1874, Jules Verne stated that “water will be the coal of the future”. After few decades, Rudolf Erren suggested the use of hydrogen as a transportation fuel. Hydrogen is a potentially emissions-free alternative fuel with a very high specific energy content of about 140.4 MJ/kg (gasoline has only 48.6 MJ/kg) . The huge interest for hydrogen production and utilization is based on the premise that the fuel cell is a proven technology and hydrogen is abundant on the Earth. However, hydrogen on the Earth is in its oxidized state (H2O), which has no fuel value, and there are no other natural resources for hydrogen. Fortunately, hydrogen can be created using both renewable and non-renewable resources.
Hydrogen as an Alternative Fuel , The available technologies for hydrogen production are the reforming of natural gas, gasification of coal and biomass, and the splitting of water by water electrolysis, photo-electrolysis, photobiological production, water-splitting thermochemical cycle, and high temperature decomposition. This image below illustrates the sources and methods of hydrogen production.
Now, steam methane reformation (SMR) is the least expensive method to create hydrogen. It is a two-step process. At first step, methane reacts with steam at temperature 700–1100 °C to form Syn gas (CO + H2), and then, carbon monoxide reacts with steam to create additional hydrogen.
Hydrogen as an Alternative Fuel , Hydrogen, when created from reforming of hydrocarbons, generates CO2 as a by-product. About 2.5 t of CO2 is vented into the atmosphere for each ton of hydrogen created from reforming of hydrocarbons. Similarly, coal gasification process is another such matured technology and has no exception to huge CO2 emissions. Here, coal (CHxOy) reacts with O2 or steam to create hydrogen. Such enormous emission of CO2 downgrades the use of the conventional techniques to create hydrogen. Thus, it is essential to develop methods that can create hydrogen without or with reduced CO2 emissions. A high CO2 emission during hydrogen production nullifies its inherent advantages. Thus, the focus is to review the modified or new hydrogen producing technologies that do not lead to carbon emissions. Another possibility is to capture the CO2 created in these processes to avoid its release to the atmosphere.
Interestingly, hydrogen can also be created using non-fossil resources, primarily water.Unlike hydrocarbons, water does not emit CO2[H2O to H2 + 1/2O2] directly during hydrogen production. However, the direct splitting of water is a very energy intensive process and generally requires very high temperature (>2,000 °C). Of course, generation of such high temperature involves the burning of fossil fuels. Various techniques have been suggested to directly split water and that includes electrolysis, photoelectrochemical, photocatalytic, photobiological, and thermal decomposition. Among these, electrolysis process has potential to be a viable process at large scale in the midterm future. Moreover, the efficiency of the electrolysis of water is favorable (app 75 %), but the cost of generation is several times higher than that from fossil fuels. The high endothermic water-splitting process, when assisted by burning fossil fuels, leads to huge CO2 emissions. Thus, renewable energy sources such as solar or wind need to be developed for the electrolysis of water in the foreseeable future.