Gas/air ratio for hydrogen: a flex­ible system for eco-friendly heating

Effi­cient, eco-friendly heating appli­ances are essen­tial if the energy tran­si­tion in the heating sector is to succeed. The CleanEco Sense gas/air ratio control system from ebm-papst makes it possible to build effi­cient boilers that can run on natural gas, biogas, and entirely on hy-drogen – making for a flex­ible and future-proof solu­tion.

If the inter­na­tional commu­nity is to succeed in stop­ping climate change or at least slowing it down, the heating sector will need to massively reduce its CO2 emis­sions. Effi­cient boilers and eco-friendly fuels such as hydrogen can play an impor­tant role in this. The regu­la­tions vary from country to country, but in Europe it appears that in future, it will only be permitted to install appli­ances that can in prin­ciple also run entirely on hydrogen.

In Great Britain, the plan is to already imple­ment this by 2025, and there is also a require­ment prohibiting high addi­tional costs during the changeover. Today, new condensing units can already be oper­ated with 20 to 30 percent hydrogen, but a mass-market gas/air ratio control system for the combus­tion process has not been avail­able so far for oper­a­tion with 100 percent hydrogen.

Optimum air/fuel ratio

As a general rule, the combus­tion of hydrogen is no different to the combus­tion of other types of gas – the condensing boiler will only work effi­ciently if there is an optimum air/fuel ratio. This is always the case when the ratio is λ = 1.3. To achieve this reli­ably, it is crucial that the compo­nents of the gas/air ratio control system are perfectly matched. Two prin­ci­ples have proven effec­tive in this respect: The mechan­ical-pneu­matic ratio control system and the elec­tronic ratio control system.

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Effi­cient compo­nents & systems: blowers, venturi, valves and auto­matic burner controls from ebm-papst.

Our solu­tions for different appli­ca­tions.

Both ratio control systems comprise a blower, venturi, gas valve, and burner controls. The most impor­tant differ­ence is that, unlike pneu­matic ratio control systems, the gas valve in elec­tronic ratio control systems can be actively controlled. The advan­tage of this is that it is gas-adap­tive and can auto­mat­i­cally adapt to the combus­tion mixture. However, these two prin­ci­ples avail­able on the market are not suit­able for ensuring safe and effi­cient combus­tion if the unit runs entirely on hydrogen.

Elec­tronic ratio control systems would not work because the hydrogen flame is not conduc­tive and cannot, there­fore, be used as a control vari­able for the ioniza­tion process. In contrast, the estab­lished mechan­ical-pneu­matic ratio control system would work in prin­ciple, but the only way that the unit could run on hydrogen is with an inef­fi­cient gas/air ratio. A new system solu­tion is there­fore required.

A new gas/air ratio control system: CleanEco Sense

With CleanEco Sense, ebm-papst has devel­oped a new oper­ating concept that is suit­able for both conven­tional and all-hydrogen oper­a­tion (Fig. 1). Due to the laws of physics, the burner always needs to be replaced when switching over to hydrogen. However, the compo­nents supplied by ebm-papst (including the blower, venturi, gas valve, and burner controls) remain unchanged, meaning the changeover is afford­able for end users.

Fig. 1: CleanEco Sense oper­ating prin­ciple: The CleanEco Sense elec­tronic-pneu­matic gas/air ratio control system comprises an elec­tronic gas valve, the gas blower, venturi, and control unit. The stepper motor in the valve enables the gas quan­tity to be flex­ibly adjusted; the blower speed deter­mines the volu­metric flow. Intel­li­gent sensors in the valve contin­u­ously measure the posi­tion of the stepper motor and the pres­sure. The control unit constantly eval­u­ates this measure­ment data and can make read­just­ments at any time. For all-hydrogen oper­a­tion, only the burner needs to be replaced.

To create the new concept, the combus­tion special­ists at ebm-papst have combined the advan­tages of the mechan­ical-pneu­matic and elec­tronic ratio control systems and devel­oped a so-called elec­tronic-pneu­matic ratio control system in which the gas valve can be actively controlled. The tried-and-tested F01 range of elec­tronic gas valves (Fig. 2) is used for this purpose, expanded to include intel­li­gent sensors. The combus­tion behavior of hydrogen was inves­ti­gated in detail to enable safe oper­a­tion.

Hydrogen burns differ­ently to natural gas

The element is very reac­tive and has a high flame velocity. If it is only mixed in, this is not a problem provided the content is no higher than 30 percent. However, the situ­a­tion changes if the unit runs on 100 percent hydrogen. The greatest risk in this case is “flash­back”, which is when the flame migrates back from the burner toward the gas/air ratio control system in the oppo­site direc­tion to the flow, thereby damaging the boiler (Fig. 3).

This is partic­u­larly crit­ical during the igni­tion process as the flame spreads out very quickly after igni­tion. High pres­sure is produced as a result, meaning new fuel mixture cannot flow into the burner quickly enough.

    Fig. 2: The G02 range of elec­tronic gas valves enables the combus­tion air ratio to be controlled, including in pneu­matic ratio control systems. When combined with the built-in sensors, the unit can, there­fore, run on 100 percent hydrogen.
Fig. 3: 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.

In turn, when the unit is running, a crit­ical state can occur in partic­ular when lower outputs are requested. 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 the rated output, this happens at a suffi­cient distance from the burner surface. If the output is reduced, the efflux velocity decreases but the flame velocity remains roughly the same. The distance between the flame and burner reduces and there is a risk of flash­back (Fig. 4).

Fig. 4: At full output (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 output (b) is too low, the flame may migrate into the burner and a flash­back may occur.

Safe igni­tion, safe oper­a­tion

As a result, the key is to master precisely these two aspects. The CleanEco Sense and the built-in intel­li­gent sensors in the elec­tronic gas valve make it possible to prevent flash­back during igni­tion and oper­a­tion.

To make the igni­tion safe, the amount of air is adjusted for the brief period of the start-up process in such a way that the reac­tion is less intense. In a conven­tional mechan­ical-pneu­matic ratio control system, a fixed setting for the ratio is set once during instal­la­tion using the main flow throttle and it can then no longer be changed during oper­a­tion. However, with the CleanEco Sense it is possible to vary the λ value for the crit­ical igni­tion phase. This solves the problem of flash­back during start-up.

That then leaves the flash­back during oper­a­tion. To prevent this, it is impor­tant to know the volu­metric flow for the mixture as it enters the burner. In a mechan­ical-pneu­matic ratio control system, this can only be roughly esti­mated based on the blower speed. However, in order to reli­ably prevent flash­back during oper­a­tion, it is impor­tant to precisely control the efflux velocity.

The gas valve of the CleanEco Sense was equipped with intel­li­gent sensors that perma­nently monitor the pres­sure in the system and the posi­tion of the stepper motor.

Both measured vari­ables provide infor­ma­tion about the quan­tity of gas conveyed. An algo­rithm devel­oped by ebm-papst eval­u­ates this data in the control unit and regu­lates the blower speed accord­ingly, ensuring that the efflux velocity does not fall below a crit­ical level.

As a blockage in the exhaust and intake air system can also cause poor air perfor­mance, the system checks whether the vacuum gener­ated by the blower corre­sponds to the setpoint before igni­tion takes place. If it does not, the safety valve does not open in the first place. As an addi­tional benefit, the smart gas valve also makes it easier to control the combus­tion of natural gas or biogas.

High modu­la­tion level, simple to install

Regard­less of the combus­tion mixture, the CleanEco Sense has managed to almost double the modu­la­tion depth of the blower to 1:15. As a result, the boiler output can be adjusted much more precisely to suit the actual demand. This reduces the amount of fuel required, which means lower CO2 emis­sions when the unit runs on natural gas. The advan­tage is that this high modu­la­tion depth can be achieved without the manu­fac­turer or installer needing to configure complex settings on site.

The advan­tages of the CleanEco Sense at a glance

  • Unit can run on natural gas, biogas, and 100 percent hydrogen
  • To switch over to all-hydrogen oper­a­tion, only the burner needs to be replaced
  • Simple to install as the system adjusts itself
  • Short devel­op­ment time as stan­dard compo­nents are used
  • Turn-sown ratio of 1:15 over the entire output range from 2 to 385kW
  • Fami­lies of gases are auto­mat­i­cally detected
  • All compo­nents have been certi­fied for hydrogen oper­a­tion

Although the system is not gas-adap­tive, the intel­li­gent sensors make it possible to detect when the combus­tion mixture changes. In such cases, the boiler does not start at all, consti­tuting an addi­tional safety feature. The system has a modular design comprising stan­dard compo­nents.

ebm-papst’s product port­folio offers the appro­priate sizes of blower, venturi, and gas valve to cover the entire output range from 2 to 385 kW, meaning CleanEco Sense is suit­able for both private house­holds and larger commer­cial appli­ca­tions. In a nutshell, CleanEco Sense is a future-proof and flex­ible system that can play a major role in the energy tran­si­tion in the heating sector.

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