D9.1 & D9.2 Literature research on low NOx hydrogen burners and developing design rules for low NOx burners

The energy-intensive industry is investigating the option to use hydrogen as a fuel to drastically reduce the carbon intensity of their manufacturing processes to meet the climate agreements. One of the challenges is to gain high efficient combustion while keeping the NO_x emissions low. Towards this end, this research is aimed to accelerate the development of LowNO_x hydrogen burners needed for the large-scale introduction of hydrogen in the high-temperature industry. The project is divided into two phases. In phase 1 a literature inventory was performed on the different burner types used in the industry and on the NO_x mitigating strategies that can be applied for these burner types. Furthermore, the differences between the NO_x emissions from natural gas and hydrogen combustion for various industrial high temperature burners were investigated. This information obtained in phase 1 was used for the development of new design rules for low NO_x hydrogen burners which was developed in phase 2 of the project.

The inventory on the different burner technologies reveal that many different burner types are used in industrial processes each designed for a specific process. For example, several burners are designed with the aim of generating mainly radiative heat transfer while other are designed to generate mainly convective heat transfer. Generally, the burners can be divided into premixed and non-premixed burners. In processes where the air and fuel are premixed prior entering the burner, switching to hydrogen can result in flashback and burner tip overheating. However, by increasing the combustion excess air, these issues can be overcome with the additional advantage that the NO_x emission will be reduced. Most burners used in the high temperature industry rely on the non-premixing concept referred to as diffusion or nozzle mix burners. The literature inventory show that a diversity of different designs of nozzle mix burners is large. Examples of nozzle mix concepts identified in the literature are swirl burners, pipe-in-pipe burners, FLOX burners, regenerative burners, recuperative burners and radiant heaters.

The literature inventory reveals the major challenges with blending hydrogen to natural gas (up to 100%) are higher flame temperature, wider flammability limits, faster diffusion and higher burning velocity. Altogether, upon hydrogen addition the flame moves closer to the burner tip causing lower internal flue gas recirculation and higher flame temperatures. As a result, it was found that for most of the burners tested, hydrogen addition to natural gas increases the NO_x emission. NO_x mitigating measures identified in the literature show that reducing the residence time, hot zones with high (local) temperature and oxygen concentration are effective measures to reduce the NO_x emission. For example, experiments performed on a forced draught burner present in an industrial boiler system show that switching from natural gas to hydrogen results in a three times higher NO_x emission than the legal NO_x limit allows. By applying external flue gas recirculation the NO_x emission, when using pure hydrogen, was successfully reduced below the Dutch  legal NO_x limits (70 mg/m³, 3% O₂). However, for burners present in high temperature processes reducing the NO_x emissions is still a challenge to be solved, since the flue gas temperatures are often too high for external flue gas recirculation. Prototype hydrogen burner designs found in the literature aim to lower the NO_x emission by creating more internal flue gas recirculation by, for example, increasing the distance between the fuel and air nozzles, changing the number of fuel and air nozzles and changing the diameter of the fuel and air nozzles. Furthermore, several dedicated burner designs are making use of, for example, the Coanda effect with the aim of creating an under-pressure to create more flue gas entrainment. Other techniques identified in the literature used are mild combustion (FLOX), staged fuel combustion, water injection and micro combustion.

The literature inventory reveals that the degree of mixing between hydrogen, air and flue gases is essential to reduce the NO_x emission. To find some general design rules for burners, it is important to have a better understanding of the mixing between hydrogen, air and flue gases. Towards this end a jet model was developed in this study to calculate the mixing of hydrogen, air and flue gases using different burner design parameters. The jet calculations are supplemented with numerical flame calculations. The simulation results reveal that due to the wider flammability range of hydrogen and faster burning velocity combustion takes place much closer to the burner head in comparison to methane and creating higher temperatures near the burner head. Consequently, the NO_x formation rate increases when switching from methane to hydrogen. Based on numerical flame simulations internal flue gas recirculation and staged combustion were found to be effective strategies in reducing NO_x. Both strategies are based on decreasing the burning velocity and flame temperatures. Comparison between the calculated amount of flue gas recirculation and measured values needed to reduce the NO_x emission back to that of methane combustion show excellent agreement. Simulations show that about 16% flue gas need to be present in the H₂/air mixture to get the same NO_x levels as that for methane  for a furnace temperature of 1000 °C. Furthermore, simulations show that combustion under fuel rich conditions results in low NO_x formations, when the equivalence ratio is higher that j=1.2 the NO_x formation is in the same range as for methane combustion.

Based on the knowledge gained in the study three different conceptual burner designs are proposed; 1) a pipe-in-pipe burner, 2) a swirl burner and 3) a staged combustion design. All three burner designs are equipped with a venturi to create internal flue gas recirculation. The burner design parameters, such as the fuel- and air nozzle diameter, diameter of the venturi and air pressure needed to create sufficient flue gas recirculation and/or staged combustion to supress the NO_x formation were calculated using the jet model.

It is recommended to construct the burner in such a way that the burner heads can be flexibly exchanged and test the burner in a (semi)-industrial furnace. During these tests, the effect of different burner heads and burner configurations on the NO_x emission will be studied. The information acquired during these tests will be used to create the optimum configuration for the three proposed low NO_x industrial high-temperature hydrogen burner designs. Together with industry partner(s) and burner manufacture(s), one burner design will be selected for a field test. The test will also give valuable insights for the design of hydrogen burners for various industrial applications.