A climate-neutral future is only possible if the world grad­u­ally aban­dons fossil fuels and replaces them with clean energy sources. Heating is still a major contrib­utor to global CO₂ emis­sions. Although gas condensing boilers are already very effi­cient, a green fuel like hydrogen would make for a better carbon foot­print. It will be a while before all gas-based heating systems are fully converted to hydrogen, but it’s only a matter of time.

Two trends: Admix­ture or 100 percent H₂

Given the long service life of gas condensing boilers of over 15 years, it makes sense to develop future-proof solu­tions today. The United Kingdom is clearly the pioneer here: from 2025, all newly installed gas boilers in the UK must be designed to be convert­ible for 100 percent hydrogen oper­a­tion at no signif­i­cant expense. So a certain amount of time pres­sure exists. However, the industry is not unpre­pared. Heating tech­nology manu­fac­turers have been working on solu­tions for several years now, and there are various field tests throughout Europe that are trialing hydrogen use in local grids (see Box: H₂ boiler for tiny house). On an indus­trial scale, however, devel­op­ment is still in its infancy.

The market is currently working in two direc­tions: On the one hand, manu­fac­turers are making their equip­ment fit for blending hydrogen with conven­tional natural gas. Ten percent is now possible without any prob­lems, and 20 percent is currently estab­lishing itself as the new stan­dard. All in all, signif­i­cant CO₂ savings are possible. Although some­what higher admix­tures of around 30 percent are still manage­able, further increases beyond this limit are not tech­ni­cally feasible. There­fore, the second line of devel­op­ment is to build devices that can be oper­ated with both natural gas and one hundred percent hydrogen.

H₂ boiler for tiny house

Intergas devel­oped a boiler for a tiny house that uses 100% hydrogen (Photo | ebm-papst, Miquel Gonzales)

Dutch heating tech­nology manu­fac­turer Intergas has already tested what the future of heating could look like on a small scale. The company has devel­oped a boiler that heats a tiny house with one hundred percent hydrogen.

The special­ists from ebm-papst have contributed their know-how to the project from the very begin­ning and have also provided compo­nents. The gas valve and blower ensure that the boiler oper­ates safely. ebm-papst was also able to assist with certi­fi­ca­tion by the Dutch testing agency Kiwa.

Intergas is already imple­menting the next project: At the end of 2022, 33 houses near Groningen are to be connected to a hydrogen network.

What makes hydrogen special

In prin­ciple, manu­fac­turers can retain the oper­ating prin­ciple of their condensing units in both cases. However, due to the prop­er­ties of hydrogen, three aspects in partic­ular must be taken into account. Hydrogen is the lightest of all chem­ical elements with the lowest density. This means that it has a higher perme­ability than natural gas, and special atten­tion must be paid to the tight­ness of the compo­nents in the boiler. The second aspect concerns the choice of suit­able mate­rials. More crucial, however, is the combus­tion behavior. For one thing, the flame velocity when hydrogen burns is eight times higher than with methane, and for another, it ignites very quickly.

With an admix­ture of up to 20 percent hydrogen, combus­tion behavior remains of secondary impor­tance. However, it is still impor­tant to pay special atten­tion to combus­tion. Hydrogen has a lower calorific value than natural gas and the Wobbe index, which is impor­tant for fuel gas exchange, is also lower. This means that it is neces­sary to make adjust­ments to ensure that the system is as effi­cient as it is when oper­ated with natural gas. For the effi­ciency of the condensing units, it is there­fore all the more crucial that the compo­nents of the gas-air composite system are perfectly matched. Only with an optimum mixing ratio of natural gas, hydrogen and air is the energy yield perfect. The CleanEco (pneu­matic composite) and Clean­Vario (elec­tronic composite) system solu­tions from ebm-papst, consisting of blower, venturi, gas valve and control unit, are already prepared and certi­fied for the use of 20 percent hydrogen. Most of the gas blowers in the various perfor­mance classes, for example, have been tested by DVGW (Deutscher Verein des Gas- und Wasser­faches e. V.) and approved for H₂ admix­ture (Figure in the article header). The same applies to the gas valves from ebm-papst. The blowers ensure the optimum mixing ratio of air and gas in every oper­ating state (for more prod­ucts, see Box at the end of the article).

For the admix­ture, the Clean­Vario elec­tronic composite system in partic­ular can make the most of its advan­tages because it is gas adap­tive and can adapt to different fuels. The ideal mixture of fuel and air for low-emis­sion and also effi­cient oper­a­tion is achieved when the combus­tion air ratio is between 1.2 and 1.3 λ. To check whether combus­tion is proceeding at an optimum rate, an elec­trode directly on the flame measures the ioniza­tion current in the Clean­Vario. Depending on how high or low this is, the gas valve is elec­tron­i­cally controlled to increase or restrict the gas flow. This makes it possible to control combus­tion inde­pen­dently of the fuel. Here, we have succeeded in further devel­oping the reli­a­bility of the system beyond the state of the art by intel­li­gently linking all actu­a­tors and sensors. The ioniza­tion tech­nology works reli­ably up to an admix­ture of 30 percent hydrogen. At 100 percent, however, it does not. The ioniza­tion current is then barely measur­able.

Safe igni­tion, safe oper­a­tion

Fig. 2: The graph shows the flame velocity as a func­tion of the air concen­tra­tion. Hydrogen (green line) ignites much faster than methane (orange line), espe­cially at high concen­tra­tions. (Graphic | ebm-papst)

When 100 percent hydrogen is used, combus­tion behavior takes on a crucial impor­tance. Due to the element’s high flame velocity and reac­tivity, the heating tech­nology industry is primarily concerned with the issue of safety. However, if you have a precise knowl­edge of its combus­tion behavior, you can safely control the combus­tion of hydrogen.

The igni­tion process is partic­u­larly crit­ical. The danger is that a flash­back will occur. In this case, the flame travels from the burner back toward the gas-air composite system, against the direc­tion of flow. This happens when the flame spreads very quickly after igni­tion and the pres­sure is so high that new fuel mixture cannot flow into the burner fast enough. The flame there­fore finds itself a way to continue feeding and migrates towards the fuel mixture (Fig. 2). One way to make igni­tion safe is to increase the air content during the starting process. The advan­tage is that hydrogen ignites at very low concen­tra­tions compared to natural gas. Once the crit­ical igni­tion phase is over, the unit returns to effi­cient normal oper­a­tion after a frac­tion of a second.

However, a flash­back can also occur during oper­a­tion, namely when lower outputs are called up. The reason for this is the high flame velocity of the hydrogen. The flame comes to a stand­still where the flame velocity is as high as the efflux velocity. At rated output, this happens at a suffi­cient distance from the burner surface. If the output is reduced, the efflux velocity decreases and the flame migrates towards the burner surface (Fig. 3). Above a certain crit­ical level, there is a risk that the flame will enter the burner and migrate upstream. Steps must there­fore be taken to ensure that the efflux velocity does not fall below a certain level, even at low heating outputs.

Fig. 3: At full power (a), the efflux velocity and flame velocity are equal. The flame stabi­lizes at a suffi­cient distance from the burner surface. If the efflux velocity at low power (b) is too low, the flame may migrate into the burner and a flame flash­back may occur. (Graphic | ebm-papst)

Gas-air composite system for 100 percent hydrogen

ebm-papst is working on a new oper­ating concept to prevent flash­back. Since econom­ical and equally durable alter­na­tives to ioniza­tion tech­nology have yet to be found for the elec­tronic composite system, a pneu­matic solu­tion makes the most sense as of now. The pneu­matic composite system is usually set for a specific fuel-air ratio when it is installed. However, the system does not provide any infor­ma­tion on the mixture flow rate entering the burner. In order to reli­ably prevent flash­back during oper­a­tion, it is impor­tant to precisely control the efflux velocity. ebm-papst is currently working on an elec­tronic-pneu­matic composite system that can do just that and is also prepared for the require­ments, for example, by means of adapted startup behavior.

Ready for the future

It is diffi­cult to predict exactly what the future of heating will look like. What is certain, however, is that hydrogen will play a signif­i­cant role in enabling the global commu­nity to achieve its climate targets. For a tran­si­tional period, the admix­ture of about 20 percent to normal natural gas is likely. The compo­nents from ebm-papst are prepared for this appli­ca­tion. And they will be, too, when 100 percent hydrogen flows into the boilers.