Thermal and Emission Performance Evaluation of Hydrogen-Enriched Natural Gas-Fired Domestic Condensing Boilers

Abstract

The combustion of gaseous fuels in condensing boilers contributes to the greenhouse gas and toxic compound emissions in exhaust gases. Hydrogen, as a clean energy carrier, could play a key role in decarbonizing the residential heating sector. However, its significantly different combustion behavior compared to hydrocarbon fuels requires thorough investigation prior to implementation in heating systems. This study presents experimental and theoretical analyses of the co-combustion of natural gas with hydrogen in low-power-output condensing boilers (second and third generation), with hydrogen content of up to 50% by volume. The results show that mixtures of hydrogen and natural gas contribute to increasing heat transfer in boilers through convection and flue gas radiation. They also highlight the benefits of using the heat from the condensation of vapors in the flue gases. Other studies have observed an increase in efficiency of up to 1.6 percentage points compared to natural gas at 50% hydrogen content. Up to a 6% increase in the amount of energy recovered by water vapor condensation was also recorded, while exhaust gas losses did not change significantly. Notably, the addition of hydrogen resulted in a substantial decrease in the emission of nitrogen oxides (NOx) and carbon monoxide (CO). At 50% hydrogen content, NO_x emissions decreased several-fold to 2.7 mg/m³, while CO emissions were reduced by a factor of six, reaching 9.9 mg/m³. All measured NO_x values remained well below the current regulatory limit for condensing gas boilers, which is 33.5 mg/m³. These results highlight the potential of hydrogen blending as a transitional solution on the path toward cleaner residential heating systems.

1. Introduction

Natural gas is the most frequently used fuel for residential heating in the European Union (EU), accounting for about 30.9% of all energy sources. In several countries, this share is close to or exceeds 50% (The Netherlands—66.2%, Italy—49.8%, Hungary—49.2%, Luxembourg—46.2%). The EU is responsible for approximately 34% of energy-related emissions, with a significant share originating from the direct combustion of fossil fuels, including natural gas, in heating appliances. It is estimated that the annual use of gas fuel for residential purposes in the EU results in approximately 220 million tons of CO₂ emissions. Several initiatives are being implemented to reduce CO₂ emissions, including the replacement of solid fuel boilers with heat pumps and electric boilers, the increased use of biomass (wood chips, sawdust, pellets, and briquettes) [1], and the introduction of renewable gaseous fuels (biomethane, biogas, and hydrogen) [2,3,4,5].

Hydrogen is increasingly being considered as a viable alternative to natural gas in residential heating systems due to its potential to reduce carbon dioxide emissions. This is supported by key EU documents outlining the policies, strategies, and roadmaps for transitioning to a circular economy. It should be noted that the EU Directive 2024/1788 of 13 June 2024 permits the blending of hydrogen with natural gas; however, it does not specify a reference concentration for H₂ mixed with natural gas. Therefore, several European countries have now introduced their own rules regarding the maximum permissible concentration of hydrogen [6,7]. For example, in Germany, up to 10% hydrogen by volume is allowed in the gas network, provided there are no natural gas compressor stations (CNGs); in France, the limit is 6%; in Austria, 4%; and in Switzerland, 2% [8].

Hydrogen as a fuel differs significantly in its properties from hydrocarbon fuels, which affects the operation of energy devices, including condensing boilers, in several aspects, such as the combustion process, the emission of nitric oxides and carbon monoxide, thermal efficiency, and the safety of device operation. The thermodynamic parameters that differ significantly between H₂ and natural gas (NG) include laminar burning velocity (S_L), adiabatic flame temperature (T_A), flammability limits (FLs), and low heating value (LHV). Chen et al. [9] demonstrated that for combustion process parameters typical of low-power-output boilers (T_air = 293 K, P = 1 bar, ϕ = 0.8), the ratio of S_{L_H₂} to S_{L_NG} is 6.7. The higher velocity of the laminar flame makes hydrogen flame combustion more susceptible to the phenomenon of flashback in premix burners, which can raise serious safety concerns. Additionally, the higher hydrogen adiabatic flame temperature (T_{A_H₂/}T_{A_NG} = 1.1) can lead not only to increased nitrogen oxide concentrations in emissions but also to enhanced radiative heat transfer within the combustion chamber. In addition, hydrogen fuel has almost three times less calorific value per cubic meter compared to natural gas, which can limit the power output of heating devices or necessitate their modification [10]. However, the advantages of hydrogen include its wide range of flammability, which under normal atmospheric conditions in the air ranges from 4 to 75% vol. compared to NG (4.3–15% vol.), as well as its low ignition energy of 0.02 mJ, which is much higher for natural gas and reaches 0.28 mJ [11,12]. These properties of hydrogen ensure its stable combustion in much leaner mixtures; however, this feature is not used in small residential heating boilers [13]. Therefore, these distinctive thermodynamic parameters of hydrogen combustion necessitate further research to explore the possibility of its co-combustion with natural gas in heating devices. Cuoci et al. [14] conducted experimental and numerical studies on the operation and emissions of a 30 kW condensing boiler fueled with natural gas mixtures enriched with up to 35% hydrogen by volume. They observed stable operation of the device; however, as the hydrogen content (H₂) increased, NO_x emissions rose, while CO emissions decreased.

However, other studies have indicated a decrease in NO_x emissions at higher concentrations of hydrogen mixed with NG. In experimental studies, Coskun achieved a 37% and 40% reduction in NO_x and CO emissions, respectively, when burning fuel using a 24 kW domestic boiler with a porous burner operating on an NG/H₂ mixture of 20% vol. and 80% vol. [15]. In [16], the authors examined conventional storage water heaters operated on hydrogen-enriched natural gas with up to 30% vol. H₂. They achieved a slight reduction in nitrogen oxide and carbon monoxide emissions. However, the limit for stable operation of the device was only 10% hydrogen by volume in the fuel.

Studies on the co-firing of a mixture of H₂ (up to 45% vol.) and NG in a staged combustion system using a 15 kW domestic boiler showed a significant reduction in NO_x—by more than 10%—with a simultaneous increase in CO and C_xH_y emissions, as well as a slight decrease in thermal efficiency [17]. The possibility of using a mixture of H₂ with low calorific natural gas in selected domestic appliances was presented by Wojtowicz and Jaworski [18]. The combustion process of a mixture containing 13% hydrogen and 87% natural gas by volume in a gas stove, water heater, and gas boilers was analyzed. Fuel replacement resulted in a decrease in boiler efficiency by 0.5 to 1.8, depending on the temperature of the heat carrier, while a 10% increase in efficiency was observed for the water heater. Another observation about the change in the overall efficiency of a medium-sized condensing boiler (2.8 MW) was made by Yang. In his study [19], an increase in the hydrogen content of the fuel to 100% vol. led to an increase in the boiler efficiency compared to natural gas (NG), with an equivalence factor of ϕ = 0.91. The authors attribute this effect to a decrease in flue gas temperature, resulting in reduced heat loss from the boiler’s exhaust gas.

A review of the literature revealed a lack of a comprehensive and comparable experimental database on the co-combustion of hydrogen with natural gas in low-power condensing boilers (second and third generation). This prevents drawing clear and objective conclusions regarding the direct relationship between the increase in installation efficiency and the reduction in harmful emission concentrations as a function of hydrogen content in the NG/H₂ mixture. Moreover, there is a lack of knowledge regarding the influence of hydrogen on heat transfer phenomena within boiler combustion chambers. Therefore, the authors decided to conduct a thermodynamic and environmental assessment of the co-combustion process of hydrogen and natural gas using various hydrogen concentrations (up to 50% vol.) in two types of condensing boilers. The experimental studies were complemented by a theoretical analysis addressing the impact of fuel composition changes on heat transfer mechanisms in condensing boiler systems.

2. Materials and Methods

This study describes the sequence of tests and the methodology for assessing the effect of hydrogen on the performance of one type of condensing gas boiler from the second and third generations, in which a mixture of hydrogen and natural gas (NG) in different proportions was used as fuel. The combustion process in a second-generation boiler occurs in accordance with the manufacturer’s specifications from the production stage. The fuel flow is selected according to the heat demand, while the air intake is regulated by adjusting the fan speed according to a programmed curve. In comparison to the second-generation boiler, the third-generation boiler is equipped with an automatic combustion control system that includes an additional ionization electrode and a dedicated controller. The ionization rod signal regulates the air amount by selecting the appropriate fan speed, ensuring that the electrode measurement signal readings are within the specified range. The fuel flow is adjusted to the actual thermal load by a regulation valve. This allows us to ensure that the boiler operation is adjusted to the optimal range of equivalence coefficients, thereby maintaining flame stability. The technical specifications of the tested boilers are presented in

Table 1. Technical parameters of the tested condensing boilers.

Device	Nominal Power	Minimal Power	Efficiency 1,2	NOx limit	Flame Type
	kW	kW	%	mg/kWh	–
CB_2nd_GEN	25.7	5.4	106.91	37.9	Diffusion
CB_3rd_GEN	21.9	2.8	109.51	33.5	Diffusion

¹—at water temperature 50/30 °C; ²—according to EN 15502-1:2021-09 [20].

.

The experimental research was conducted on a test rig shown in Figure 1. The test setup consists of a fuel supply system, the tested boilers, a thermal energy extraction system, and measuring equipment (thermocouples, mass balance, and flow meters). The natural gas, which was supplied from the gas network, had an average composition of CH₄ = 94.2% vol., C₂H₆ = 4.2% vol., C₃H₈ = 0.4% vol., C₄H₁₀ = 0.1% vol., and N₂ = 1.1% vol. This was then mixed with hydrogen in a static mixer. The flow was controlled using two mass flowmeters. The list of measuring instruments is presented in

Table 2. List of measurement devices.

Sensor/Device	Measurement	Measurements Range	Accuracy
Gas analyzer NGA2000, Rosemont Inc., Shakopee, MN, USA	Flue gas	O2: 0–21% CO: 0–100 ppm NO: 0–100 ppm CO2: 0–25%	±1% FS
Mass flow controller Brooks SLA5853, Brooks Instrument BV, Veenendaal, The Netherlands	NG flow rate	0–300 Nl/min	±0.2% FS
Mass flow controller Bronkhorst EL-FLOW, Bronkhorst High-Tech B.V., Ruurlo, The Netherlands	H2 flow rate	0–200 Nl/min	±0.5% FS
Manometer MRU DM9200, MRU GmbH, Neckarsulm-Obereisesheim, Germany	Fuel pressure	350 mbar	±1% FS
Temperature sensor RTD PT100, Guenther GmbH, Schwaig, Germany	Temperature	−20 to 100 °C	±1% MV
Mass balance C315.30 Radwag, Radom, Poland	Water	0–30 kg	±0.01 kg

.

Experimental tests were performed to ensure stable operating conditions, where the monitored parameters (heating water) do not vary by more than ±1 °C over a 10-min time interval for two heat loads: nominal (P_nom) and minimal (P_min). The measured quantities (temperatures, fuel flow rates, concentrations of nitric oxides, carbon monoxide, oxygen, and carbon dioxide) were recorded at a frequency of 1 Hz and subsequently averaged over time. The hydrogen content in the analyzed mixtures was 10, 20, 30, 40, and 50% by volume. The key parameters of the tested fuels are presented in

Table 3. Parameters of the tested fuel mixtures.

Fuel Name	Fuel Composition		LHV	SAR	TA 1	SL 1	LFL	UFL
	CH4 % vol.	H2 % vol.	MJ/m3	m3_Air/m3_Fuel	K	cm/s	%	%
NG	100	0	35.8	9.6	1978	28.4	5.3	15.0
NG_H2_10	90	10	33.3	8.9	1983	30.1	5.1	16.3
NG_H2_20	80	20	30.8	8.2	1992	32.3	5.0	17.9
NG_H2_30	70	30	28.3	7.5	1998	34.8	4.8	19.7
NG_H2_40	60	40	25.8	6.7	2004	38.4	4.7	22.1
NG_H2_50	50	50	23.3	6.0	2011	43.3	4.6	25.0

¹ T_A and S_L values were determined for ϕ = 0.77 and T_in = 298 K and P = 1 bar.

, along with their calculated values. For the calculations, pure methane was assumed as the main component of natural gas.

Table 4. Selected test parameters.

Device	Power	Temperature				Gas Pressure	Test Duration
		Inlet Water	Outlet Water	NG + H2	Ambient
	kW	°C	°C	°C	°C	mbar	Sec.
CB_2nd_Gen	nominal	36	56	15	24	15–25	600
	low	23	30
CB_3rd_Gen	nominal	35	55
	low	24	37

presents the experimental values measured during the tests.

For both condensing boilers, it was possible to test at the rated power using the analyzed fuel mixtures while maintaining similar water temperatures at the boiler inlet and outlet. At the rated power, the minimum outlet water temperature for the CB_2nd_GEN boiler was 30 °C, while the CB_3rd_GEN boiler could operate at an outlet temperature of 37 °C. As a result, the temperature difference between the inlet and outlet water in the third-generation boiler was twice as high as in the second-generation boiler.

Based on measurements of thermodynamic parameters for the studied fuels, calculations were performed to determine the thermal efficiency of the boiler, the heat loss of flue gases at the boiler outlet, and the heat of the condensation of water vapor from flue gases. The effect of hydrogen content in fuel on the emission of NO_x and CO was also determined for the nominal operating parameters of the boiler.

The thermal efficiency of a gas boiler is defined as the ratio of its useful thermal power (Q_out) to the heat of the burned fuel (Q_in), as shown in Equation (1).

$${\eta }_b={\displaystyle \frac{Q_{out}}{Q_{in}}}·100[\mathrm{\%}]$$

(1)

The heat of combustion of the fuel mixture was determined from Equation (2), while the useful heat transferred to heated water was calculated from Equation (3).

$$Q_{in}=V_f· LHV \left[\mathrm{W}\right]$$

(2)

$$Q_{out}=m_w·c_w·\Delta T \left[\mathrm{W}\right]$$

(3)

where V_f denotes the volume flow rate of the fuel [m³/s]; LHV denotes the lower heating value of the fuel mixture [J/m³]; m_w denotes the mass flow rate of water; c_w denotes the average specific heat for water [J/kg·K]; and ΔT denotes the water temperature increase [K].

The flue gas heat loss to the environment was determined using the following Equation (4):

$$Q_{fg}={\displaystyle \frac{m_{fg}\sum _{i=1}^n{g}_i{h}_i\left(T_{fg}\right)}{Q_{in}}}·100\mathrm{\%} [\%]$$

(4)

where m_fg denotes the mass flow rate of flue gases [kg/s]; g_i denotes the mass fraction of flue gas components [–]; h_i denotes the enthalpy of flue gas components at the flue gas temperature [J/kg]; and T_fg denotes the flue gas temperature [K].

The emissions measured during the tests were normalized to a reference state corresponding to zero oxygen content in the exhaust gases and recalculated to mg/m³ of flue gas according to Equations (5) and (6) [21,22].

$${CO}_{O2=0\mathrm{\%}}={\displaystyle \frac{21-{\left[{\mathrm{O}}_2\right]}_{ref}}{21-{\left[{\mathrm{O}}_2\right]}_{mes}}}·1.25{\left[CO\right]}_{mes} [\mathrm{m}\mathrm{g}/{\mathrm{m}}_{\mathrm{f}\mathrm{g}}^3]$$

(5)

$${N{O}_x}_{O2=0\mathrm{\%}}={\displaystyle \frac{21-{\left[{\mathrm{O}}_2\right]}_{ref}}{21-{\left[{\mathrm{O}}_2\right]}_{mes}}}·2.05{\left[{NO}_x\right]}_{mes} [\mathrm{m}\mathrm{g}/{\mathrm{m}}_{\mathrm{f}\mathrm{g}}^3]$$

(6)

where [O₂]_ref denotes the reference oxygen concentration [%]; [O₂]_mes denotes the measured oxygen concentration [%]; [NO_x]_mes denotes the measured NO_x emission concentration [ppmv]; and [CO]_mes denotes the measured CO emission concentration [ppmv].

The heat flux transmitted in the combustion chamber, which heats water, includes the radiative heat flux from the flame and flue gases, the convective heat flux, and the condensation payload. The chemical composition of the fuel affects several thermodynamic parameters that describe the process of heat transfer to the working fluid (water) in the boiler, including flame temperature, gas and air mass flow rates, radiative coefficients, the degree of diffusion, and thermal conductivity. The calculation method for determining the heat energy transferred to the coolant in the condensing boiler is described by Equations (7)–(12). This enabled the theoretical analysis of the effect of changes in fuel composition on the heat exchange process indicators in the condensing boiler.

The convective heat flux was determined using the periodic ratio over the range 2300 < Re < 10,000 [23]

$$Q_c=0.00263\mathrm{F}{\displaystyle \frac{\mathrm{k}·{Pr}^{0.35}}{\mathrm{d}}}·{\mathrm{R}\mathrm{e}}^{0.8}·(T_{fg}-T_w) [\mathrm{W}]$$

(7)

where F denotes the heat exchange area [m²]; k denotes the thermal conductivity coefficient [W/m·K]; Re denotes the Reynolds number [–]; Pr denotes the Prandtl number [–]; d denotes the characteristic length [m]; and T_w denotes the boiler wall temperature [K].

The radiant heat flux transmitted by the flame was calculated according to Equation (8) [24]. The value of the flame emission coefficient (ε_fl) for the analyzed fuels was determined using the model described in [25,26] and is given in Equation (9).

$$Q_{R,fl}=5.67F_{fl}{\displaystyle \frac{1}{\frac{1}{{\epsilon }_{fl}}+\frac{F_{fl}}{F}\left(\frac{1}{{\epsilon }_w}-1\right)}} \left[{\left({\displaystyle \frac{T_{fl}}{100}}\right)}^4-{\left({\displaystyle \frac{T_w}{100}}\right)}^4\right] [\mathrm{W}]$$

(8)

$${\epsilon }_{fl}={\sum }_{i=1}^n{a}_i\left[1-\mathrm{e}\mathrm{x}\mathrm{p}(-k_{i}XPL\right]$$

(9)

where F_fl denotes the flame area [m²]; ε_fl denotes the flame emissivity coefficient [–]; ε_w denotes the wall emissivity coefficient [–]; a_i denotes the weighting factor [–]; k_i denotes the absorption coefficient [bar⁻¹ m⁻¹]; L denotes the path length [m]; p denotes the total pressure [bar]; X denotes the sum of the molar fraction of absorbing species [–]; and T_fl denotes the adiabatic flame temperature [K].

The amount of heat exchange that takes place between the gas layer with a temperature of (T_fg) and the surrounding wall of the furnace chamber with a temperature of T_w, taking into account the emissivity of the wall, can be described by Equation (10) [27],

$$Q_{R,fg}=5.67\mathrm{F}\left({\epsilon }_w+1\right) \left[{\epsilon }_{fg\left(T_{fg}\right)}{\left({\displaystyle \frac{T_{fg}}{100}}\right)}^4-{\epsilon }_{fg\left(T_w\right)}{\left({\displaystyle \frac{T_w}{100}}\right)}^4\right] [\mathrm{W}]$$

(10)

where F denotes the heat exchange area [m²]; ε_fg denotes the flue gas emissivity coefficient [–]; ε_w denotes the wall emissivity coefficient [–]; T_fg denotes the flue gas temperature [K]; and T_w denotes the wall temperature [K].

For condensing boilers, latent heat from the condensation of water vapor contained in the flue gases must also be included in the balance of energy supplied to the heat carrier. The amount of energy from the condensation process is determined based on the ratio (11) proposed by Osakabe in [28],

$$Q_{cond}=F·h_c·L_w\left(c_{fg}-c_w\right) \left[M\right]$$

(11)

where F denotes the heat exchange area [m²]; h_c denotes the mass transfer coefficient [m/s]; L_w denotes the latent heat [J/kg]; c_fg denotes the steam mass concentration per unit volume of flue gas [–]; and c_w denotes the mass concentration of saturated steam at the wall temperature [–].

The mass transfer coefficient for flue gases is determined based on Sherwood ratios, where the characteristic length (H) is assumed to be equal to the height of the combustion chamber of the mixture [29],

$$h_c=0.664{\displaystyle \frac{D}{H}} {Re}^{1/2}{Sc}^{1/3} [m/s]$$

(12)

where D denotes the mass diffusivity [m²/s]; H denotes the characteristic length [m]; Re denotes the Reynolds number [–]; and Sc denotes the Schmidt number [–].

When assessing the effect of changes in the chemical composition of the fuel on the heat transfer phenomena in the condensing boiler, a constant equivalence coefficient of ϕ = 0.77 was adopted, corresponding to a molar oxygen content of 5%. The calculations were carried out for a boiler power of Q_in = 25 kW and a constant flue gas temperature of 50 °C. The size of the combustion chamber for calculation was determined based on data from boiler manufacturers, with a diameter of Do = 20 mm and a height-to-diameter ratio (H/Do) of 3.5.

3. Results and Discussion

3.1. Analytical Analysis of the Effect of Fuel Composition on Heat Transfer in a Condensing Boiler

Studies of the effect of changes in the chemical composition of fuel on heat transfer phenomena in a condensing boiler were carried out for three types of fuel: natural gas (represented by methane) as a reference fuel and a mixture of NG/H₂ with a volume fraction of hydrogen of 10, 30, and 50% vol. The results of the analyses are normalized based on the values obtained for the methane gas-fired boiler and are presented in

Table 5. Heat transfer results normalized to NG.

Parameter/Fuel	Symbol	NG_H2_10	NG_H2_30		NG_H2_50
Convective heat flux	${\mathrm{Q}}_{\mathrm{c}}$	0.99	1.02	↑	1.04	↑
Radiative heat flux from flue gases	${\mathrm{Q}}_{\mathrm{R},\mathrm{f}\mathrm{g}}$	1.0	1.02	↑	1.05	↑
Flue gas emissivity coefficient in Tfg	${\epsilon }_{\mathrm{f}\mathrm{g}}$	0.99	1.01	↑	1.02	↑
Radiative heat flux from flame	${\mathrm{Q}}_{\mathrm{R},\mathrm{f}\mathrm{l}}$	0.85	0.87	↓	0.93	↓
Flame emissivity coefficient	${\epsilon }_{\mathrm{f}\mathrm{l}}$	0.81	0.83	↓	0.87	↓
Condensation heat flux	${\mathrm{Q}}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}}$	0.99	1.02	↑	1.08	↑

.

It can be observed that the addition of 10% vol. H₂ to natural gas slightly alters the amount of energy transferred by convection, flue gas radiation, and water vapor condensation. However, it leads to a reduction in the amount of energy transferred from the flame. A further increase in the H₂ content causes the amount of energy transferred by convection, flue gas radiation, and water vapor condensation to increase. Simultaneously, the amount of energy transmitted from the flame decreases, which is associated with a 13% and 17% decrease in emissivity for the hydrogen flame fuels NG_H2_50 and NG_H2_30, respectively. For higher H₂ shares, this effect is partially compensated for by the higher combustion temperature of the NG/H₂ mixture.

A higher combustion temperature also affects the increase in energy transferred by convection and flue gas radiation (T⁴). The increase in water vapor content in the flue gases, resulting from the combustion of NG/H2 fuel, also leads to a slight increase in emissivity, ranging from 1% to 2% for NG_H2_50 and NG_H2_30, respectively. The condensing heat load also increases. An additional factor that increases the energy transferred by convective heat exchange in the furnace chamber of the NG/H₂ fuel mixture boiler is the increase in the volume and flow rate of flue gases in the furnace, which increases the flow rate, and the amount of energy transferred.

An analytical analysis of the complex indicators of the heat transfer process revealed a positive effect of the hydrogen content in the mixture on the mechanisms of heat and mass transfer in these condensing boilers. The attenuation of energy transfer was observed only for radiant heat transfer from flame radiation, which accounts for approximately 40% of the total energy supplied to the boiler [30].

3.2. Analysis of Experimental Results

Experimental studies on the effect of the hydrogen content proportion in the fuel included determining the key operating parameters of the boiler necessary to calculate thermal efficiency and the emission of nitrogen oxides and carbon monoxide. The results of the thermal measurements are presented in

Table 6. Experimental measurement data for boilers’ rated power testing.

Parmeter/Fuel	Symbol	Unit	NG	NG_H2_10	NG_H2_20	NG_H2_30	NG_H2_50
			CB_2nd_Gen
Inlet water temperature	${\mathrm{T}}_{\mathrm{w},\mathrm{i}\mathrm{n}}$	°C	17.3	17.3	17.4	17.5	NA
Outlet water temperature	${\mathrm{T}}_{\mathrm{w},\mathrm{o}\mathrm{u}\mathrm{t}}$	°C	26.6	24.7	25.8	26.0	NA
Water mass flow rate	${\mathrm{m}}_{\mathrm{w}}$	kg/s	0.642	0.757	0.632	0.595	NA
NG volume flow rate	VNG	l/min	39.1	35.4	32.1	29.3	NA
H2 volume flow rate	VH2	l/min	0	4.1	8.2	12.4	NA
Mass of condensate	${\mathrm{m}}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}}$	kg	0.300	0.292	0.296	0.303	NA
Flue gas temperature	${\mathrm{T}}_{\mathrm{f}\mathrm{g}}$	°C	50.7	44.7	45.0	50.6	NA
O2 share in flue gases	XO2	%	5.5	6.2	7.3	8.1	NA
			CB_3rd_Gen
Inlet water temperature	${\mathrm{T}}_{\mathrm{w},\mathrm{i}\mathrm{n}}$	°C	17.6	17.7	17.6	17.5	17.0
Outlet water temperature	${\mathrm{T}}_{\mathrm{w},\mathrm{o}\mathrm{u}\mathrm{t}}$	°C	27.0	26.9	26.5	26.7	25.6
Water mass flow rate	${\mathrm{m}}_{\mathrm{w}}$	kg/s	0.658	0.646	0.669	0.662	0.676
NG volume flow rate	VNG	l/min	40.7	37.3	35.9	35.1	31.5
H2 volume flow rate	VH2	l/min	0	4.6	9.5	15.3	31.4
Mass of condensate	${\mathrm{m}}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}}$	kg	0.282	0.346	0.349	0.360	0.381
Flue gas temperature	${\mathrm{T}}_{\mathrm{f}\mathrm{g}}$	°C	51.7	51.1	50.9	51.2	49
O2 share in flue gases	XO2	%	4.8	5.1	5.5	5.8	6.3

NA—values were not determined due to the impossibility of achieving stable boiler operation for a mixture with a H₂ content of 50% vol.

and

Table 7. Experimental measurement data for low-power boiler tests.

Parmeter/Fuel	Symbol	Unit	NG	NG_H2_10	NG_H2_20	NG_H2_30	NG_H2_50
			CB_2nd_Gen
Inlet water temperature	${\mathrm{T}}_{\mathrm{w},\mathrm{i}\mathrm{n}}$	°C	17.3	17.5	17.4	17.5	17.4
Outlet water temperature	${\mathrm{T}}_{\mathrm{w},\mathrm{o}\mathrm{u}\mathrm{t}}$	°C	21.2	21.0	21.1	20.8	20.6
Water mass flow rate	${\mathrm{m}}_{\mathrm{w}}$	kg/s	0.402	0.455	0.432	0.451	0.428
NG volume flow rate	VNG	l/min	10.2	9.9	9.5	8.4	6.6
H2 volume flow rate	VH2	l/min	0	1.1	2.4	3.6	6.6
Mass of condensate	${\mathrm{m}}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}}$	kg	0.132	0.144	0.146	0.149	0.152
Flue gas temperature	${\mathrm{T}}_{\mathrm{f}\mathrm{g}}$	°C	40.5	39.1	38.0	37.6	36.5
O2 share in flue gases	XO2	%	5.5	5.7	6.2	6.9	8.0
			CB_3rd_Gen
Inlet water temperature	${\mathrm{T}}_{\mathrm{w},\mathrm{i}\mathrm{n}}$	°C	17.4	16.8	16.9	17.3	17.4
Outlet water temperature	${\mathrm{T}}_{\mathrm{w},\mathrm{o}\mathrm{u}\mathrm{t}}$	°C	21.2	20.3	20.5	20.9	21.0
Water mass flow rate	${\mathrm{m}}_{\mathrm{w}}$	kg/s	0.424	0.451	0.445	0.445	0.450
NG volume flow rate	VNG	l/min	11.0	9.6	9.3	8.3	7.7
H2 volume flow rate	VH2	l/min	0	1.0	2.3	3.9	7.7
Mass of condensate	${\mathrm{m}}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}}$	kg/s	0.142	0.138	0.145	0.150	0.164
Flue gas temperature	${\mathrm{T}}_{\mathrm{f}\mathrm{g}}$	°C	36.2	34.5	34.4	34.4	35.6
O2 share in flue gases	XO2	%

for the rated and minimum power conditions, respectively.

Based on the measured data (Table 6 and Table 7), the efficiency of the condensing boilers was determined using Equation (1). The results are shown in Figure 2 and Figure 3 for the rated and minimum power, respectively. For the rated power of both analyzed condensing boilers, an increase in the proportion of hydrogen in the fuel relative to the primary fuel (natural gas) increased the efficiency. For the boiler CB_3rd_Gen, the increase in efficiency was from 1.3 to 1.6 percentage points for NG_H2_10 and NG_H2_50, respectively. A similar trend of increasing efficiency was observed for boiler CB_2nd_Gen; however, the increments in efficiency were relatively lower, ranging from 0.4 to 0.7 for NG_H2_20 and NG_H2_30, respectively. During tests of the second-generation boiler, it was not possible to achieve stable operation at the rated load with a H₂ content of 50% vol. Five attempts were made to operate the boiler using NG_H2_50 fuel. In each case, the ionization probe signal, which is responsible for monitoring safe combustion, dropped below the minimum threshold (50% of the maximum range). This triggered an emergency shutdown of the boiler.

Similarly, for the minimum power, an increase in the thermal efficiency of the tested boilers was obtained with an increase in the hydrogen content in the fuel. Higher efficiency was achieved with the second-generation boiler. For example, at 50% by volume of H₂ in the fuel, the efficiency gain was 3.2 percentage points. The higher efficiencies observed for the boilers operating at low power were attributed to reduced flue gas outlet losses Q_fg (Figure 4). At low power, the proportion of latent heat in the total heat transferred to the working fluid was also higher (Figure 5). The estimated heat loss at the boiler outlet does not exceed 1% for the rated power and 0.5% for low power, while the share of condensing heat in the total energy transferred to water at the minimum power is twice the share of heat when the boilers are operating at the rated power.

As shown in Figure 4, the H₂ content in the fuel has a negligible effect on the amount of energy lost to the environment with the flue gases, while the amount of energy transferred to the coolant due to the condensation of the water vapor contained in the flue gas increases (Figure 5).

In addition to thermal studies, measurements were also made of the concentration of harmful emissions, including nitrogen oxides (NOx) and carbon monoxide (CO), resulting from the use of hydrogen in the fuel mixture with natural gas (NG). The results obtained at the rated power of the boilers are shown in Figure 6. The values of the emission concentrations of these harmful compounds measured during the experiments were recalculated to the initial oxygen content in the flue gases of 0% following Equations (5) and (6). It was observed that the addition of hydrogen to the fuel results in a decrease in both nitrogen oxide and carbon monoxide emissions for both boilers tested, and this reduction rises with the increase in the hydrogen content of the fuel. It can result from the following. Thermal NO formation, according to the Zeldovich mechanism, is dependent on flame temperature. When hydrogen is added to NG, the air demand and the specific heat of the water present in flue gases are increased, which reduces NO formation. Thanks to the higher burning rate of hydrogen, the residence time can be reduced, thereby limiting the formation of thermal NO.

In the case of CO emissions in the presence of hydrogen, the oxidation of CO to CO₂ is greatly accelerated by the presence of the OH radical. Hydrogen (H₂) in the combustion process forms this key OH radical. The primary reaction responsible for the oxidation of CO to CO₂ is the reaction CO + OH → CO₂ + H. Analysis of the combustion of syngases [31] reveals that the transformation between CO and CO₂ is largely dominated by this single reaction. The replacement of a substantial amount of the carbon present in the fuel can also contribute to reducing CO emissions.

Non-combustion kinetics-related mechanisms, such as the absorption of NO and CO by water, can contribute to reducing their presence in exhaust gases.

The NO_x concentrations measured for the tested devices were lower than the limits for second- and third-generation condensing boilers. For fuels with a higher hydrogen content, the NO_x concentration in the flue gases decreases. This observed trend is the opposite of that observed in high-temperature combustion processes, such as gas turbines [32] or gas engines [33]. This is due to the fact that condensing boilers belong to the category of low-temperature devices, where the average temperature of the walls of the combustion chamber does not exceed 60 °C, and the flame temperature in the boiler firebox is below 1300 °C due to the high density of heat flux transmitted from the flame to the walls of the chamber. In addition, due to its high reactivity, hydrogen burns very quickly, ensuring a more uniform temperature distribution in the combustion chamber and a reduction in the level and number of local temperature peaks, which are responsible for the intensive formation of nitrogen oxides in the flame.

4. Conclusions

The results of calculations and experimental measurements on the co-firing of natural gas with hydrogen (up to 50% by volume) in condensing boilers equipped with conventional and advanced combustion systems allow us to draw the following conclusions:

• The presence of hydrogen in the fuel mixture above 10% vol. enhances the heat transfer process in condensing boilers by increasing the energy transfer through convection and flue gas radiation.
• The amount of energy transferred by flame radiation to the furnace chamber walls decreases with increasing hydrogen content due to a reduction in flame emissivity. The emissivity factor decreases by nearly 20% compared to natural gas for a fuel containing 10% vol. H₂. For higher hydrogen shares in the fuel, the reduction in energy transferred from the flame is partially compensated for by an increase in flame temperature.
• Thermal calculations based on experimental data confirm an increase in the thermal efficiency of the tested boilers with an increase in the H2 share, while a more pronounced improvement is observed under nominal operating conditions. Thermal calculations based on experimental data confirm an increase in the thermal efficiency of the tested boilers with increasing H₂ content in the fuel, with a more pronounced improvement observed under nominal operating conditions. An efficiency increase of 1.3 to 1.6 percentage points was achieved for hydrogen shares of 10% and 50% volume, respectively (boiler CB_3rd_Gen).
• The efficiency of hydrogen-enriched fuel combustion in the studied boilers is positively influenced by the higher flue gas temperature and the greater amount of energy recovered through the condensation of water vapor from the flue gases.
• Analysis of flue gas composition shows that the addition of hydrogen to natural gas reduces the emissions of both carbon monoxide and nitrogen oxides. This reduction becomes more significant with a higher hydrogen content in the fuel mixture. For carbon monoxide, an increased H₂/CO ratio in the flame contributes to more complete carbon oxidation, with CO emissions decreasing by a factor of five for a fuel containing 50% H₂ compared to pure natural gas. For NO_x, the high reactivity of hydrogen contributes to a more uniform temperature distribution, which reduces the formation of local hot spots in the furnace—the primary site of NO_x generation by the thermal mechanism. For the tested boilers, NO_x emissions decreased several-fold—from 30 mg/m³ to as low as 3 mg/m³ (boiler CB_3rd_Gen).

Our study confirms that hydrogen can be efficiently co-fired with natural gas in condensing boilers to increase the thermal efficiency of energy conversion without compromising operational stability, especially in systems equipped with combustion process control. In addition, the addition of hydrogen helps to reduce the concentration of emissions of harmful carbon monoxide and nitrogen oxide compounds, thereby enhancing its potential as a low-carbon energy carrier in household heating systems and as an environmentally friendly renewable fuel.