Boilers and burners in hydrogen plants
Hydrogen is now in increasing demand as a fuel for generating heat or electricity. The combustion systems that are needed for this production process differ from traditional systems that run on fossil fuels (such as natural gas). The simple reason is that the properties of the two mediums are different. Among other things, the boiler and burner require specific attention.
How hydrogen, when used as a fuel, differs from fossil fuels like natural gas is shown in the figure above. One striking property of hydrogen is its calorific value, which is about three times lower than natural gas and four times lower than gasoline. Note that this applies to the volume of gas under atmospheric pressure and at a temperature of 0 °C. Because the gas is also much lighter than natural gas, the energy content per kg of hydrogen is, in fact, around 2.5 times higher than natural gas and gasoline. In other words, to fit one kg of hydrogen into an acceptable volume (a normal car refuels about 5 kg of hydrogen), you need high pressures and low temperatures.
Another important difference lies in the ignition energy, which is almost fifteen times lower for hydrogen than for methane. This places the gas in the category “highly flammable” substances and basically means that combustion is quick and intense, with the associated high temperatures. Moreover, the flammability limits for hydrogen are significantly wider and therefore indicate that the gas can ignite with ambient air at a wider mixing range.
|Density gaseous H2 (0 °C, 1 atm)||0,090 kg/Nm3||14 times lighter than air|
|Boiling point (1 atm)||-252 °C (20 K)||Methane: -161 °C (112 K)|
|Density liquid H2||70,8 g/l||Gasoline: 720 g/l|
|Energy content gaseous H2 (lower calorific value)||120 MJ/kg||
Methane: 50 MJ/kg
|Energy content gaseous H2 (0 °C, 1 atm)||33.33 kWh/kg
|Methane: 36 MJ/Nm3|
|Energy content liquid H2 (lower calorific value)||8.5 MJ/l
|Gasoline: 33 MJ/l
Gasoline: 46 MJ/kg
|Flammability limits in air (25 °C, 1 atm)||4 - 75 vol %||Methane: 5,3 - 15,0 % vol|
|Detonation limits in air (25 °C, 1 atm)||15 - 59 % vol.||Methane: 6,3 - 13,5 % vol|
|Self-ignition temperature||585 °C||Methane: 540 °C|
|Ignition energy||0.02 mJ||Methane: 0.29 mJ|
Boiler modification, burner replacement
Because of the differences between hydrogen and fossil fuels we mentioned above, a traditional combustion system cannot simply run on hydrogen. When switching to hydrogen (or possibly a hybrid mixture) in existing systems, it is therefore essential to first identify where modifications are required.
This process begins with determining whether the plant still meets the requirements for which it is currently being used. It's a known fact that many combustion plants for heat or electricity production are ageing and, over time, are sometimes loaded more heavily or differently than originally intended. In other cases, the plants have been heavily over-dimensioned from the outset – possibly because of an accumulation of safety factors – which actually causes the plant to produce too much heat. An obvious solution to that last situation is to share the heat among other divisions, or possibly even with another company (“the neighbour”). Of course, there is also the option to downrate the plant.
The next step is to assess the hardware. Where the piping can remain, in most cases, replacing or modifying the boiler is inevitable. This mainly has to do with the higher combustion temperatures of hydrogen. A situation is created that changes the distribution of heat, whereby temperatures often rise above the limit for which the boiler is designed. Modifications can possibly be made by installing additional protection and, in any case, ensuring that combustion is controlled without extreme spikes in temperature. In specific cases, the higher flue gas temperatures also warrant modifications to the superheater.
Burners need to be replaced in virtually all circumstances. Again, the reason is the higher combustion temperature. Besides oxygen, normal outdoor air contains a lot of nitrogen (about 80% nitrogen and 20% oxygen), which reacts with the hydrogen more quickly at higher temperatures. If the design of the burner remains unchanged and the operating conditions also stay the same, significantly more NOx emissions will be produced with hydrogen combustion than with fossil fuel combustion.
The solution to these increased emissions isn't found in changing the operating principles of the burner. Nothing really changes there. But you can keep the formation of nitrogen compounds under control by “spreading out” the combustion of hydrogen further over the available space in the combustion chamber. This prevents the oxygen and hydrogen from finding the perfect mixture and therefore also stops any severe reaction from occurring where the flame temperatures can become very high in locations. Another option is to make use of flue gas circulation. Like with conventional natural gas, a proportion of cold flue gas is returned before it reaches the chimney and is mixed with the combustion air.
Finally, it is important to install extra safeguards that can detect possible faults in hydrogen gas-fired plants. Hydrogen molecules are smaller than natural gas molecules and can therefore escape more easily through valves and flanges. On the one hand, hydrogen gas is more dangerous because of its low ignition energy. On the other, this poses less of a problem because it is such a light gas. If it escapes into the normal atmosphere, it will dissipate immediately and there is very little chance of spontaneous ignition (explosion). But monitoring for safety is still crucial. In addition to monitoring, the currently installed and future instrumentation has to be suitable for hydrogen operations.
One party, all the know-how
Stork has years of experience in the construction and conversion of hydrogen facilities. A big advantage is that the company has all the necessary disciplines in-house. Not only for engineering and constructing the systems, but also designing and producing the components themselves.
The reverse engineering of the existing boiler often serves as the starting point for determining the modifications needed to switch to hydrogen. This allows Stork to tailor the boilers and burners to specific applications and thus always build turnkey solutions. We are also fully responsibility for the combustion system. Customers therefore have a single point of contact in the event of disruptions, but also when modifications to the installation are required. One contact person who, above all, knows all the ins and outs and can therefore guarantee the efficiency and safety of the hydrogen plant.