The Production and Uses of Hydrogen
Today, hydrogen is used for the most part on a large scale as a feedstock in the chemical and petrochemical industry, to produce principally ammonia, refined petroleum products and a wide variety of chemicals. Its utilizations also include the metallurgic, electronic, and pharmaceutical industries (Fig. 9.4). Except as a propellant for rockets and space shuttles, hydrogen is still rarely used today as a fuel. Industrial facilities often build their own hydrogen production unit to ensure secure supply and safety, avoiding also transportation difficulties. Consequently, the hydrogen market is presently mainly a "captive" market. To cover the present needs, about 50 million tonnes of hydrogen are produced yearly worldwide, representing some 140 toe, or less than 2% of the world's primary energy demand. Using hydrogen as the main energy source would thus imply enormous investments to increase the production capacity and to establish the needed infrastructure for storage and distribution.
As mentioned earlier, hydrogen is not a primary energy source, but rather an energy carrier. It must first be manufactured before it can be used as a fuel. Currently, almost 96% of the world's hydrogen needs are produced from fossil fuels , with almost half being generated by the steam reforming of methane (Fig. 9.5). Although relatively inexpensive, this process relies on diminishing natural gas (or oil) resources and emits large amounts of CO2. At present, water electrolysis is much costlier, and represents only 4% of the production. It is preferentially used when high-purity hydrogen is needed. At the same time, as mentioned, it uses inexhaustible water resources and the energy required can come from any source, including in the future atomic and alternative energy sources, and not fossil fuel-based energy.
Total consumption ~ 50 million tons
Figure 9.4 The main hydrogen-consuming sectors in the world.
140 | Chapter 9 The Hydrogen Economy and its Limitations Hydrogen from Fossil Fuels
Hydrogen can be obtained from hydrocarbons by reforming or partial oxidation. Compared with other fossil fuels, natural gas is the most suitable feedstock for hydrogen production because of its wide availability, ease of handling and it has the highest hydrogen-carbon ratio, which minimizes the amount of CO2 produced as a byproduct. Methane can be converted to hydrogen by steam reforming or partial oxidation with oxygen, or by both in sequence (autothermal reforming). Steam reforming is presently the preferred method, accounting for 50% of the hydrogen produced worldwide, and for more than 90% in the United States. In this process, natural gas reacts with steam over a metal catalyst in a reactor at high temperatures and pressures to form a mixture of carbon monoxide (CO) and hydrogen. In a second step, the reaction of CO with steam (water gas shift reaction) produces additional hydrogen and CO2. After purification, hydrogen is recovered, while the CO2 byproduct is generally vented into the atmosphere. In the future, however, it could be captured and sequestered, if stricter measures to mitigate climate changes are imposed. Methane steam reforming can also be performed at a smaller scale using various converters. Hydrogen could thus be produced directly locally, such as in filling stations. These decentralized units would have higher hydrogen production costs and lower efficiency than larger industrial ones, but could avoid the cumbersome (and dangerous) transportation of hydrogen from distant centralized production centers. In this case however, the cost of CO2 capture would be prohibitively high. Partial oxidation and autothermal reforming are more efficient than simple reforming, but require oxygen, the separation of which from air at low cost is still technically difficult. The concept of producing hydrogen from petroleum, albeit established, is not attractive for the long run because it would not solve our energy dependence on diminishing oil reserves.
With the largest reserves compared to all other fossil fuels, coal could supply significant amounts of hydrogen well into the next century. The current technology to achieve this goal is the so-called integrated gasification combined cycle (IGCC). This "clean coal" technology would allow the co-generation of hydrogen and electricity and therefore significantly improve the overall energy efficiency compared to current commercial plants. In this process, much as in methane reforming, coal is gasified by partial oxidation with oxygen and steam at high temperature and pressure. The created synthesis gas, a mixture containing mostly CO and H2 (but also CO2), can be further treated with steam to use CO to increase the H2 yield by the water gas shift reaction. The gas can then be cleaned to recover hydrogen. However, because coal has a low hydrogen/carbon ratio, it also releases much more CO2 per unit of hydrogen or electricity produced than methane or even petroleum. In current projects, this issue is considered to be addressed by capturing and sequestering the CO2 emitted into geologic formations or depleted oil and gas reservoirs. Using this technology, Vision 21 and the billion-dollar FutureGen programs funded by the United States government through the Department of Energy (DOE) have the goal to co-produce hydrogen and electricity in zero-emission coal-fueled facilities . To validate the concept, a 275-MW prototype of such a power plant is now in the planning stage to be built and tested. This is justified by DOE citing the large amounts of coal reserves still available. Coal gasification is, however, a less mature technology than other hydrogen-generation processes, although the cost of hydrogen production using this technology is among the lowest available. In large centralized plants, the current cost of producing hydrogen is estimated to be just above $1 per kg , with substantial potential for improvement and further price reduction. However, the planned sequestration of CO2 produced in large amounts from coal and other fossil fuel-burning power plants will be technologically and economically very challenging. None of the existing CO2 separation and capture technologies has yet been adapted for a large-scale power plant, and costs are uncertain. CO2 sequestration has so far only been tested on a relatively small scale, and the environmental impacts of large-scale sequestration must be carefully assessed before we start pumping billions of tons of CO2 into subterranean cavities and under the seas. The energy needed for the capture and sequestration processes will also reduce the overall efficiency of the power plants by as much as 14% . Although coal, as long as it is readily available, is a viable alternative to generate hydrogen in large centralized plants, it is unsuited to decentralized hydrogen production.
The use of fossil fuels to produce hydrogen necessitates that the CO2 emitted as a byproduct is, according to present plans, captured and sequestered in order to reduce greenhouse gas emissions and to mitigate global climate change. Such storage, although possible, is not without dangers, as Earth movements or volcanic eruptions could lead to the catastrophic release of large amounts of CO2. Regardless, the use of fossil fuels would only be a temporary solution as oil, natural gas and eventually coal will all be depleted. Thus, more sustainable methods are needed, such as to generate hydrogen from biomass or from water by electrolysis using not only all renewable energy sources such as hydro, wind, solar and biomass, but also atomic energy.
Continue reading here: Hydrogen from Biomass
Was this article helpful?