# The Direct Effect of Enriching the Gaseous Combustible with 23% Hydrogen in Condensing Boilers’ Operation

^{*}

## Abstract

**:**

_{2}concentrations in flue gases. After testing a flattened pipes condensing boiler, a CO

_{2}emission reduction coefficient of 1.1 was determined when converting from methane gas to G222 as combustible. Thus, by inserting into the national grid a G222 mixture, an important reduction in greenhouse gases can be achieved. For a 28 kW condensing boiler, the annual reduction in CO

_{2}emissions averages 1.26 tons, value which was experimentally obtained and is consistent with the theoretical evaluation.

## 1. Introduction

_{2}emissions reduction using hydrogen as combustible. Zhang J. et al. [5] have done an exhaustive life cycle assessment for three methods of hydrogen production by using solar energy. Like the previous study, the environmental impact had been investigated.

_{2}and N

_{2}concentration in flue gases. Schiro F. et al. [13] theoretically evaluated the impact of hydrogen-enriched natural gas on domestic gas boilers. Some recommendations based on analytical conclusions have been done, such as the need to assure a special burner design for high hydrogen enrichment percentage, considering the increased tendency to unwanted ignition and flashing back. This deduction comes from the fact that the 23% hydrogen methane gas mixture (generically called G222) is used in boiler testing in view of the application of the CE marking which is like a flashback limit gas [14,15].

_{2}concentration in the mixture, to keep constant the boiler’s thermal efficiency. The boilers’ CO

_{2}emissions decreased by about 7%.

## 2. Materials and Methods

## 3. Results

_{2}emissions savings are obtained, for a condensing boiler one-year operation.

- -
- Ambient conditions—temperature, relative humidity
- -
- The pressure of the working agents (gaseous fuel and water from the round-trip circuit of the equipment)
- -
- The maximum limits for the various components in the combustion gases
- -
- The establishment of a stationary work regime during a period in which the operation is monitored

## 4. Discussion

_{2}). By reference to the detailed operating parameters from the numerical simulations for a range of constructive solutions of condensing boilers and the measuring data from their testing, an interesting conclusion was drawn: if a boiler can be considered as HPCB, then the nominal operating parameters from the overall performance point of view are similar, under acceptable engineering error conditions.

- -
- Burning process is characterized by low burning air excess coefficients;
- -
- Heat exchange surface is over-dimensioned by a figure of 2 to 3 (by comparison to the non-condensing equivalent classical boiler) and generates sensible heat transfer efficiencies of about 98.5–99%;
- -
- Heat exchange surfaces characterized by small characteristic lengths, mainly in the condensing section of the boiler (the one before the flue gas exhaust);
- -
- The flue gases, at least for the most part of the boiler and necessary for the final condensing surfaces, ensures a countercurrent for the flue gases against the heated agent
- -
- The condensation is efficiently drained, in order not to generate a film with significant thermal resistance that can rise the useful condensing dew point at the flue gases’ contact with the heat and mass transfer surface.

_{2}emissions, as the ones already modeled or tested.

^{3}. By evaluating the values displayed in Table 8, 1.26 tons of CO

_{2}emissions savings are obtained, for the flattened pipes condensing boiler one-year operation, value which is in line with the theoretical value of 1.3 tons of CO

_{2}. The difference of around 3% is related especially to the incomplete combustion products like carbon monoxide together with the gas analyzer measuring errors.

_{2}volume in the two studied situations was determined as a function of the real volume of dry combustion products and CO

_{2}percentage from Table 6 according to Equation (2):

## 5. Conclusions

_{2}emissions when G20 is replaced with G222. For a 28 kW condensing boiler, the annual reduction in CO

_{2}emissions averages 1.26 tons, and considering the potential market for this type of boiler, the global effect is significant.

_{2}reduction when using a mixture between H

_{2}and CH

_{4}and, therefore, a reduction coefficient was only determined for this type of gas. Considering the theory behind the HPCB concept and the extended existing database made by the authors for this type of boiler, an interesting future objective is to determine coefficients also for other types of pollutants and operating performances.

_{2}emission reduction, higher values for the H2 concentration in the mixture will be evaluated in future research.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## Nomenclature

B | Combustible volumetric flow $\left({\mathrm{Nm}}^{3}/\mathrm{s}\right)$ |

V_{g} | Specific flue gases’ volume $\left({\mathrm{Nm}}^{3}/{\mathrm{Nm}}^{3}\right)$ |

V_{air,min} | Stoichiometric air volume for combustion $\left({\mathrm{Nm}}^{3}/{\mathrm{Nm}}^{3}\right)$ |

V_{g,dry} | Specific dry flue gases’ volume $\left({\mathrm{Nm}}^{3}/{\mathrm{Nm}}^{3}\right)$ |

${\mathrm{V}}_{{\mathrm{CO}}_{2},\mathrm{flue}\mathrm{gases}}$ | CO_{2} gas volume as part of the dry flue gases $\left({\mathrm{Nm}}^{3}/{\mathrm{Nm}}^{3}\right)$ |

${\mathrm{V}}_{{\mathrm{H}}_{2}\mathrm{O},\mathrm{flue}\mathrm{gases}}$ | H_{2}O gas volume as part of the dry flue gases $\left({\mathrm{Nm}}^{3}/{\mathrm{Nm}}^{3}\right)$ |

## References

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**Figure 3.**The temperature variation during testing (

**a**) G20-outlet; (

**b**) G20-inlet; (

**c**) G222-outlet, and (

**d**) G222-inlet.

Sensor/Device | Measurement | Measurement Range | Accuracy |
---|---|---|---|

RTD (thermal resistance) | Temperature T (°C) | −20 to 100 °C | ±0.1% |

Propeller flow meters | Water velocity (m/s) | 0.2 to 10 m/s | ±0.5% |

Gas meter | Gas flow (m^{3}/h) | 0.6 to 6 m^{3}/h | ±0.5% |

Gas analyzer (Afriso) | Flue gases’ concentration (ppm) | 0 to 21% vol. O_{2}0 to CO _{2} max. for CO_{2}0 to 1000 ppm CO 0 to 500 ppm NO _{x} | ±0.2% CO_{2} and O_{2}±5 ppm CO ±0.1% NO _{x} |

Combustible Type | Volume Percentage Composition |
---|---|

G222 | ${\mathrm{H}}_{2}=23{\%\mathrm{CH}}_{4}=77\%$ |

G20 | ${\mathrm{CH}}_{4}=95.6\%$ ${\mathrm{C}}_{2}{\mathrm{H}}_{6}=3.24{\%\mathrm{C}}_{3}{\mathrm{H}}_{8}=0.54\%$ ${\mathrm{C}}_{4}{\mathrm{H}}_{10}=0.08\%$$;{\mathrm{CO}}_{2}=0.44\%$ |

Combustible Type | Net Calorific Value | Gross Calorific Value |
---|---|---|

G222 | $29,930\mathrm{kJ}/{\mathrm{m}}^{3}$ | $33,480\mathrm{kJ}/{\mathrm{m}}^{3}$ |

G20 | $36,879\mathrm{kJ}/{\mathrm{m}}^{3}$ | $40,970\mathrm{kJ}/{\mathrm{m}}^{3}$ |

**Table 4.**Calculated values for determining the maximum CO

_{2}percentage in flue gases (CO

_{2}max.).

Parameter | G222 | G20 |
---|---|---|

${\mathrm{V}}_{\mathrm{air},\mathrm{min}.}\left[\frac{{\mathrm{m}}_{\mathrm{N}}^{3}\mathrm{air}}{{\mathrm{m}}_{\mathrm{N}}^{3}\mathrm{comb}.}\right]$ | 7.88 | 9.798 |

${\mathrm{V}}_{{\mathrm{CO}}_{2},\mathrm{flue}\mathrm{gases}}\left[\frac{{\mathrm{m}}_{\mathrm{N}}^{3}{\mathrm{CO}}_{2}}{{\mathrm{m}}_{\mathrm{N}}^{3}\mathrm{comb}.}\right]$ | 0.77 | 1.045 |

${\mathrm{V}}_{{\mathrm{H}}_{2}\mathrm{O},\mathrm{flue}\mathrm{gases}}\left[\frac{{\mathrm{m}}_{\mathrm{N}}^{3}{\mathrm{H}}_{2}0}{{\mathrm{m}}_{\mathrm{N}}^{3}\mathrm{comb}.}\right]$ | 1.896 | 2.191 |

${\mathrm{V}}_{\mathrm{g}}\left[\frac{{\mathrm{m}}_{\mathrm{N}}^{3}\mathrm{flue}\mathrm{gases}}{{\mathrm{m}}_{\mathrm{N}}^{3}\mathrm{comb}.}\right]$ | 8.891 | 10.976 |

${\mathrm{V}}_{\mathrm{g}.\mathrm{dry}}\left[\frac{{\mathrm{m}}_{\mathrm{N}}^{3}\mathrm{flue}\mathrm{gases}}{{\mathrm{m}}_{\mathrm{N}}^{3}\mathrm{comb}.}\right]$ | 6.995 | 8.785 |

${\mathrm{CO}}_{2}\mathrm{max}.[\%]$ | 11 | 11.9 |

Combustible Type | ${\mathbf{V}}_{\mathbf{C}{\mathbf{O}}_{2}}\left({\mathbf{m}}_{\mathbf{N}}^{3}\right)$ | ${\mathbf{m}}_{\mathbf{C}\mathbf{O}}{}_{2}\left(\mathbf{t}\mathbf{o}\mathbf{n}\right)$ |
---|---|---|

G222 | 6862.5 | 13.6 |

G20 | 7549 | 14.9 |

Combustible Type | Water Flow (kg/h) | Gas Flow, B (m ^{3}/h) | Water Outlet (°C) | Water Inlet (°C) | CO_{2} in Flue Gas(%) | CO in Flue Gas (ppm) | NO_{X} in Flue Gas(ppm) | Flue Gas Temperature (°C) |
---|---|---|---|---|---|---|---|---|

G222 | 1194 | 3.383 | 49.78 | 30.3 | 8.8 | 54.5 | 8 | 58.9 |

G20 | 1246 | 2.754 | 50.04 | 29.7 | 9.45 | 74.8 | 9 | 58 |

Combustible Type | Air Excess Coefficient $\mathit{\lambda}(-)$ | Heat Load (kW) | Efficiency $(-)$ |
---|---|---|---|

G222 | 1.33 | 28.24 | 106.12 |

G20 | 1.32 | 28.28 | 105.69 |

Parameter | G222 | G20 |
---|---|---|

${\mathrm{V}}_{\mathrm{g}.\mathrm{dry},\mathrm{real}}\left[\frac{{\mathrm{m}}^{3}\mathrm{flue}\mathrm{gases}}{{\mathrm{m}}^{3}\mathrm{comb}.}\right]$ | 9.59 | 11.9 |

${\mathrm{V}}_{{\mathrm{CO}}_{2}}\left[\frac{{\mathrm{m}}^{3}{\mathrm{CO}}_{2}}{{\mathrm{m}}^{3}\mathrm{comb}.}\right]$ | 0.844 | 1.124 |

${\mathrm{m}}_{{\mathrm{CO}}_{2}}\left[{\mathrm{t}}_{{\mathrm{CO}}_{2}}\right]$ | 14.95 | 16.21 |

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**MDPI and ACS Style**

Calotă, R.; Antonescu, N.N.; Stănescu, D.-P.; Năstase, I.
The Direct Effect of Enriching the Gaseous Combustible with 23% Hydrogen in Condensing Boilers’ Operation. *Energies* **2022**, *15*, 9373.
https://doi.org/10.3390/en15249373

**AMA Style**

Calotă R, Antonescu NN, Stănescu D-P, Năstase I.
The Direct Effect of Enriching the Gaseous Combustible with 23% Hydrogen in Condensing Boilers’ Operation. *Energies*. 2022; 15(24):9373.
https://doi.org/10.3390/en15249373

**Chicago/Turabian Style**

Calotă, Răzvan, Nicolae N. Antonescu, Dan-Paul Stănescu, and Ilinca Năstase.
2022. "The Direct Effect of Enriching the Gaseous Combustible with 23% Hydrogen in Condensing Boilers’ Operation" *Energies* 15, no. 24: 9373.
https://doi.org/10.3390/en15249373