The SES Hydrogen boiler is a response to the needs of the changing thermal power sector in Poland and Europe, which like other areas of the economy is facing the challenge of decarbonization and increasing the share of RES in the production of heat. Polish heat plants are dominated by outdated equipment and systems that require immediate modernization. However, its direction remains unspecified.
Therefore, in the third part of the H2 Safety series, Aleksandra Tracz-Gburzyńska discusses the hydrogen boiler which, due to its technological features and the use of renewable sources, guarantees zero-emission heat generation.
Hydrogen boiler – the basics of the combustion process
Hydrogen burns in daylight in a clean atmosphere with a pale blue, almost invisible flame* and emits no visible light or smoke. Sunlight can further dim the visibility of hydrogen flames. The products of burning hydrogen in the air are water/vapor and heat energy. Compared to other fuels, hydrogen has a higher adiabatic flame temperature for a stoichiometric mixture* in the air (~2130°C).
*A stoichiometric mixture is a mixture in which both the fuel and the oxidant are fully consumed (complete combustion) with the formation of the product(s) of combustion. For example, two two-atom gases, hydrogen (H2) and oxygen (O2) can combine to form water as the only exothermic product of the reaction.
* Hydrogen flame – the zone of combustion of a gas from which light and heat are emitted.
For a combustion reaction to occur, hydrogen must be mixed with sufficient oxidant to form a combustible mixture. A mixture with an optimal or stoichiometric ratio of reactants ensures that all components of the fuel and oxidant can burn completely, producing reaction products and heat. For example, two molecules of hydrogen react with one molecule of oxygen to form two molecules of water. In volume terms, this is a mixture of two parts hydrogen (67%) with one-part pure oxygen. In the case of a mixture of hydrogen and air in which additional nitrogen is present, the stoichiometric ratio of the mixture is 29.5% hydrogen.
Non-stoichiometric mixtures that are fuel-rich (more fuel than required for optimum response) or fuel-poor (less fuel than required for optimum response) can also support combustion, but not all reactants will be consumed. For a given set of reactants and specific conditions (for example, temperature and pressures), combustion is limited to a specific range of mixture compositions. The limits of this mixture composition are the limits of combustibility.
Hydrogen ignition occurs if its content in the air is below the Upper Flammability Limit (UFL) and above the Lower Flammability Limit (LFL) and if an effective ignition source is present. The flammability range of hydrogen is much wider compared to hydrocarbons, i.e. 4 to 77 vol % [ISO/TR 15916:2015(E)] in a mixture with air at *NTP, and even wider is the flammability range of hydrogen burned in pure oxygen, i.e. 4.1 to 94 vol % in a mixture with O2 at NTP [ISO/TR 15916:2015(E)].
*NTP – Normal Temperature and Pressure (NTP): 293,15 K (20oC) and 101,325 kPa
The Hydrogen Boiler – principles of operation
The main features which distinguish the design and concept of operation of the SES Hydrogen Boiler are the type of fuel and the oxidant used, which are hydrogen and oxygen. The process of combustion and generation of heat energy takes place in a sealed system without access to atmospheric air. In this way, if the hydrogen produced from RES is supplied to the unit, it is characterized by zero emissions of exhaust gases and NOx, SOx, and COx particles into the atmosphere.
The main burner is subjected to indirect ignition of the hydrogen-oxygen mixture via a pilot burner, in which hydrogen is burned continuously in oxygen. The pilot burner is designed to form and appropriately direct the flame profile to the hydrogen and oxygen flow zones within the main burner structure.
Safeguarding the combustion process and the equipment itself
Combustion of hydrogen-oxygen mixtures in a boiler plant is a controlled process through dosing the amount of fuel and oxidant supplied and their controlled ignition – in such a way as to achieve conditions as close as possible to stoichiometric combustion. This is part of the normal operation of such a system. Gas boilers, irrespective of the type of fuel gas used, present fire and explosion hazards related to the fuel system, burner, and reactions inside the combustion chamber. Therefore, regardless of the type of unit, it is necessary to know and use tools to minimize the risks associated with their use.
Therefore, to ensure the stable operation of such a system, several process safeguards are used to mitigate the risks associated with controlled combustion technology. These include:
- Burner management system – designed to control the entire combustion process, which starts and stops the appliance and prevents it from malfunctioning. The system reacts to the presence or absence of flame in the combustion chamber using one or more flame detectors and ensures safe burner start-up, operation, and shutdown under normal and emergency operating conditions. Each burner shall be supervised, and flame monitored by flame detectors and combustion safety devices, which are interfaced with the burner management system.
- Flame detectors – are safety devices that directly respond to the presence or absence of a flame. They transmit a signal to the burner management system.
- Flashback arrestors are installed in the oxygen and fuel supply lines of the boiler to stop the spread of the flame, shut off the fuel and oxygen supply and relieve the pressure created by flashbacks.
- Flame arresters – devices prevent the transmission of flame through a flammable mixture of fuel and oxygen by extinguishing the flame.
- Shut-off valves – are a key safety control to protect the equipment from fire and explosion. They automatically shut off the supply of fuel and oxidizer to the burner system.
Hydrogen boiler – legislation
Currently, there are no technical and construction regulations or standards in the Polish legal system dedicated directly to facilities such as hydrogen boiler houses. From the legal point of view, the only general requirements are those which, given the specificity of our installation, are not sufficient. For the implementation of hydrogen solutions in district heating, it is, therefore, necessary to establish appropriate legal regulations. This is especially true since our hydrogen boiler system will ultimately be supplied with fuel (hydrogen) and oxidant (oxygen) directly from the electrolyzer. The entire combustion process will take place without access to air.
SES Hydrogen S.A. is the first entity in Poland and one of the first in the world to introduce its hydrogen boiler installation technology. As one of the few, we are already developing and implementing safety standards and requirements for hydrogen boiler house infrastructure. These standards will ultimately be applied to our installations, both those dedicated to commercial and industrial customers.
Reference material:
1. ISO/TR 15916:2015(E) – Basic considerations for the safety of hydrogen system
2. Own elaboration based on internal data of SES HYDROGEN S.A.
