Hydrogen has always played a crucial role in industrial processes ranging from hydrocarbon refining to pharmaceuticals manufacture, float glass production, food processing and much more in between. Now with moves towards developing hydrogen as an alternative to fossil fuels, it's set to become more important than ever. As new uses for hydrogen come into play, gas companies are working to develop new methods to produce and supply this essential gas.
Hydrogen is an abundant element in all types of hydrocarbons, water (H2O) and industrial by-products such as ammonia (NH3). It can be produced in a variety of ways; the aim is to match product quality with a customer's requirements at the right price. It is typically manufactured from hydrocarbons, ammonia or water. The method used depends on the volumes and purities required and what raw materials are available. Whether by-products, such as steam, carbon monoxide (CO) or carbon dioxide (CO2), can be used elsewhere on the site also figures into the process economics. So does energy efficiency, typically expressed in terms of natural gas equivalent (NGE); a lower NGE rating indicates a more efficient process.
The major manufacturing process used to produce hydrogen for large scale users - such as refineries that require supplies of more than 10,000 Nm3/h high-purity hydrogen - include steam reforming, partial oxidation (POX) and coal gasification.
Hydrogen for large-scale users is often produced in on-site plants. In choosing the best method for a particular customer, the gas companies balance the amounts required against the feedstocks available and the value of the by-products produced.
Steam Reforming - a common method used in large hydrogen production units. It involves a catalytic reaction in which a hydrocarbon feedstock is first heated, then purified to remove sulphur, mixed with steam and passed across a nickel oxide catalyst. Suitable feedstocks are natural gas, butane, propane, liquid petroleum gas (LPG), naphtha or other hydrocarbon products available on site. As the feedstock passes over the catalyst, an endothermic reaction takes place (see text box: The right reaction) producing a syngas which contains roughly 70% H2, along with 10-15% CO, 5-10% CO2, 5% CH4, and smaller amounts of oxygen and water. This crude hydrogen stream is then purified.
If only H2 is required purification is generally carried out using pressure swing adsorption (PSA). However, the syngas can also be processed using a solvent recovery system in order to recover CO, CO2 and methane, and the remaining hydrogen product purified using a PSA unit, to produce purities in the range of 99.95%. If higher purities are required the product is liquefied by cooling it to cryogenic temperatures and distilled further within a cold box, in a similar way to an air separation unit.
Partial Oxidation (POX) - In POX plants, hydrocarbons are mixed with oxygen and burned at very high temperatures to produce a synthesis gas (syngas), which is then purified. In coal gasification, coal is gasified to produce a syngas, which is then again purified to the required level.
Smaller is getting more beautiful
Not all hydrogen customers require such large amounts of hydrogen gas. Small and medium sized users include float glass manufacturers, who use hydrogen to mop up oxygen in molten glass, and pharmaceutical producers and some food processors whose processes include a hydrogenation step. In addition, there are an increasing number of projects - for example, the EU Clean Urban Transport for Europe aimed at establishing a hydrogen infrastructure for fueling cars and transportation systems - where small scale on-site hydrogen generation is essential to the success of the scheme.
Although some of those requiring only small amounts of hydrogen will continue to receive their supplies by trailer or in cylinders, there is a cut off point - around 300-400 Nm3/h or around 2-3 trailers per day - where it becomes more cost-effective to install a dedicated hydrogen plant on-site. As a result, there is a growing trend to develop new types of hydrogen plant aimed specifically at smaller users.
Methods suitable for smaller scale users include processes such as electrolysis of water, ammonia dissociation, and small-scale steam reformers. Ammonia dissociation is only a good option where the raw material - ammonia (NH4) - is readily available in the waste stream on an industrial site. In contrast, water is freely available almost everywhere, and the use of on-site electrolysis to produce transport fuel is being piloted in projects such as CUTE.
In water electrolysis, hydrogen is produced by electrochemically splitting water molecules (H2O) in an electrolysis cell. This includes an anode and a cathode placed in an ion-containing electrolyte, typically an aqueous alkaline solution with 30 percent potassium hydroxide (KOH). The anode and cathode are separated from each other by a gas-tight, ion-conducting diaphragm membrane. When a current is passed through the cell, gaseous hydrogen is produced at the cathode, and gaseous oxygen is produced at the anode. To achieve the necessary production capacity, a number of cells are connected in series to form a module. Larger systems can be made up by connecting several modules together. The electricity needed to drive the reaction can be drawn from the electricity grid, or produced using renewable sources.
Scaling down steam reformers
Smaller-scale steam reformers, with production rates in the order of 20-200 Nm3/h are available from some manufacturers (see for example: www.h2gen.com). These rely on a similar catalytic reaction as the big industrial steam reformers, but are more compact, and have a modular design which means that several units can be linked together to increase the capacity incrementally, if required, at a low cost. H2Gen, for example, claims that thanks to their turn-down capability and potential for installation in series, its HGM series generators are an economic way to produce 27 - 1350 Nm3/h hydrogen for on site use.
Other steam reformer designs for smaller scale generation - less than 400 Nm3/h - rely on systems such as autothermal reforming (ATR). This employs a different, bi-functional catalyst, that drives both an exothermic partial oxidation reaction, as well as an endothermic steam reforming reaction, to produce hydrogen gas. (see text box: The right reactions). The hydrogen gas produced is then purified using a small PSA system. Like other steam reformers aimed at smaller scale users, ATR plants can be very compact and multiple units can be installed if higher hydrogen capacity is needed.
Thanks to new designs, modern hydrogen plants are more efficient than ever, and customers are now more likely to be able to source a plant better suited to their particular needs. But in environmental terms, challenges remain. There are not yet any commercial-scale systems available that are completely non-polluting and that don't rely in some way on fossil fuels. The current steam reforming process, for example, produces carbon dioxide, a greenhouse gas that contributes to climate change. Even if the hydrogen is made by electrolysis, the power/electricity used is typically generated from fossil fuels, with a similar CO2 effect. The $quot;holy grail$quot; would be to develop a renewable technology, for example, photochemical hydrogen production using solar power. Small scale projects, like CUTE, are a good start - but there is still a long way to go to reach the goal of carbon-neutral hydrogen production on a large scale.
Users large and small
Volumes are normally expressed as normal cubic metres per hour (Nm3/h). Purities are expressed as percentages of pure hydrogen, so that 99.995% purity indicates that 0.005% of the volume consists of molecules other than hydrogen. Typically gas companies analyse for impurities such as CO, CO2, methane, oxygen and nitrogen in amounts greater than 50 parts per million (ppm). The total energy consumption for the various generation methods is measured in terms of natural gas equivalents (NGE), calculated by comparing how many Nm3 of natural gas is required to produce a Nm3 of H2 gas. The most efficient processes have total energy consumption values of less than 0.5 NGE.
Large-scale users, such as refineries, typically require hydrogen generation in the order of 10,000 to 200,000 Nm3/h. Small scale users, including pharmaceutical manufacturers and float glass producers, require anything up to 10,000 Nm3/h.