Condensing Gas Boiler Research Articles

Investigation of a high-temperature combination heat pump for lower-cost electrification in multifamily buildings

The development of space and water heating combination heat pumps capable of generating water temperatures high enough for convective heat emitters will enable more cost-effective and equitable decarbonization solutions for electrifying multifamily buildings. In this paper, multifamily building models and a charge-sensitive mechanistic cycle model of a combination heat pump are developed, and the system performance is predicted based on the models. Unlike other state-of-the-art residential heat pumping equipment, the modeled combination heat pump using an economized, fluid-injected variable-speed compressor can achieve higher temperature lifts of 40° – 85°C, with lower installation costs and complexity. The model predicted heating coefficient of performance (COPh) is 2.1 at an ambient temperature of −15°C with a high-temperature lift of nearly 85°C, and a seasonal coefficient of performance in heating mode (SCOPh) ranges from 2 – 4 for different locations. The system shows 30% – 90% lower CO2eq emissions over a condensing gas boiler and 9% – 13% lower projected installation costs than two separate space and water heat pumping appliances.

Assessment of the impact of setting the heating curve on reducing gas consumption in a residential building while ensuring thermal comfort

Due to the large share of the residential building sector in CO2 emissions, it is necessary to look for quick-to-implement and cost-free solutions for heating systems aimed at increasing their efficiency and reducing fuel consumption. The aim of the study is a multi-aspect assessment of the proper setting of the heating curve in the weather controller of a low-temperature floor heating system with a condensing gas boiler, and an analysis of the impact of the heating curve slope on the natural gas consumption, the emission of gaseous and dust pollutants and on maintaining appropriate indoor air parameters: air temperature, operative temperature and PMV and PPD. A wide range of simulation tests were carried out using the TRNSYS software for an existing energy-efficient single-family residential building, with mechanical ventilation with heat recovery, located in north-eastern Poland in Bialystok. Changing the heating curve slope by 0.1 can reduce gas consumption and pollutant emissions by 1.6–10.3 %. Low heating curve slopes may result in failure to reach the temperature set by users (difference between the set and achieved indoor temperature up to 1.9 K), while maintaining standard thermal comfort parameters (−0.5<PMV<+0.5; PPD<10 %). Reducing the indoor air temperature by 2 K can contribute to reducing gas consumption by 11 % while maintaining standard thermal comfort parameters. Changing the heating curve slope is one of the effective methods of reducing fuel consumption in the building. Results are useful for users of buildings equipped with low-temperature heating systems with floor heating and installers of these systems.

Analysis of the Hybrid Power-Heating System in a Single-Family Building, along with Ecological Aspects of the Operation

This study evaluates a hybrid heating system in a single-family building in northeastern Poland, which has a temperate continental climate. The analysis covers two heating seasons in 2021/2022 and 2022/2023. The hybrid heating system includes an air heat pump HPA–08 CS Plus with a heating power of 8.2 kW (AHP), a condensing gas boiler VC146/5–5 with a power of 14 kW (GB–Condens.), and a solid fuel boiler with a power of 11 kW for central heating. Additionally, hot water is heated by a Basic 270 (DHW’s AHP) air–water heat pump with a power of 2 kW, utilizing a tank with a capacity of 270 dm3 equipped with two heating coils. The building’s average electricity consumption is around 5400 kWh/year. A 4.96 kWp photovoltaic installation is installed on the building’s roof at a 40° angle towards the south to supplement the hybrid system. The study aims to assess whether the PV installation can adequately cover the energy needs of the hybrid heat source for heating and hot water. Furthermore, the study calculates the emission of pollutants (CO2, SOx, NOx, CO, and PM10) into the atmosphere. The total annual electricity production from PV installations was 5444.9 kWh in 2021/2022 and 5684.8 kWh in 2022/2023. The excess electricity was stored in the PGE power grid as per the Prosumer settlement rules. The installed PV installation is sufficient to power the following devices annually: AHP, DHW’s AHP, and GB–Condens. However, the daily electricity production from the PV installation is not enough to cover the energy needs of the heat pump for heating during the cold months in Poland (I–III, XI–XII). It can meet the power needs of a PC all year round and can also be stored during the summer months, for example, in energy warehouses or by directly storing it in the PGE power grid. The use of the PV installation resulted in an average reduction in pollutant emissions into the atmosphere: CO2—94.1%, SOx—91.8%, NOx—95.6%, CO—9.7%, and PM10—32.1%.

Theoretical and experimental study on total heat recovery of condensing gas boiler flue gas by gas engine-driven compression heat pump

How to improve the utilization efficiency of natural gas and achieve the goal of carbon reduction has gradually become the focus of people’s attention. The exhaust temperature of the condensing gas boiler is between 50℃ and 80℃, which still contains some heat that is not used, so it is necessary to recover this part of the heat. This paper mainly studies the total heat recovery and utilization of waste heat from low-temperature flue gas of condensing gas boiler. In the research field of gas boiler flue gas waste heat recovery, the use of gas compression heat pump to recover gas boiler flue gas waste heat is still very few. Based on the principle of reducing the cold source temperature, a low-temperature flue gas recovery scheme of a gas engine-driven compression heat pump coupled condensing gas boiler is proposed. The low-temperature chilled water produced by gas engine-driven compression heat pump is used to recover flue gas waste heat from condensing gas boiler and gas engine, which can improve the utilization efficiency of natural gas and eliminate the “white smoke phenomenon” simultaneously. In addition, the waste heat of flue gas is used as the low-temperature heat source of gas engine-driven compression heat pump to ensure the stable operation of the heat pump in the low-temperature environment. The flue gas waste heat recovery technology has been improved compared with other flue gas waste heat recovery technologies in terms of primary energy utilization efficiency, system stability, corrosion resistance of flue gas heat exchangers, and investment recovery cycle. The experimental results show that when the water inlet temperature of the flue gas heat exchanger is 15℃, the overall power of the unit can reach 645 kW, and the final exhaust temperature is below 25℃. The overall primary energy utilization rate of the GEHP&CB can reach 110 %, which is 10 %∼20 % higher than that of the condensing gas boiler. The payback period of the increased initial investment is 1.4 heating seasons. Due to the improvement of natural gas utilization efficiency significantly reduces natural gas consumption and pollutant discharge.

Experimental study of emission characteristics for fully premixed combustion of dimethyl ether and hydrogen based on flame ions

The detection of flame ion current is a promising approach in fully premixed combustion. However, how to control the excess air coefficient (α) by ion current still needs an effective method. For this reason, the application of natural gas (12T), hydrogen-doped 12T (12T-H) and dimethyl ether (DME) in fully premixed combustion is experimentally studied in this paper. The emission characteristics of NOx, CO and CO2 from fully premixed condensing gas boilers (FPCGB) were analyzed. The detection voltage generated by the flame ion current is used to monitor the NOx emissions of different gas types. A novel combustion control method based on flame ion current is proposed. The results show that hydrogen addition can reduce NOx, CO and CO2 emissions of 12T. The highest NOx emission is observed at a hydrogen volume ratio (HVR) of 15 % in the experiment. The detection voltage gradually decreases with increasing HVR. The CO2 of DME is always lower than 12T. When α exceeds 1.21, 1.31 and 1.12, respectively, DME exhibits higher values in NOx, CO and detection voltage compared to that of 12T. In addition, two combustion characteristic values are identified and proposed for combustion feedback control. The peak voltage (Up) can be used as a parameter to identify the type of gas. The emission voltage (Ue) can be used to determine whether the NOx emission concentration exceeds the 40 ppm threshold.

Multi-Indicator Assessment of a Thermal Insulation Investment, Taking into Account the Pre-Set Temperature

The article proposes indicators to evaluate a thermal insulation investment in a building, such as net present value (NPV), profitability index, discounted payback period, and ecological cost efficiency. Economic and ecological aspects were taken into account. Life Cycle Assessment (LCA) was used in the ecological analysis. The following heat sources in the building were considered: condensing gas boiler and heat pump. The developed indicators also depend on the pre-set temperature in residential premises. A methodology to determine the optimum thermal insulation thickness for both economic and ecological reasons was also proposed. A case study was analyzed, and a reference building, typical for Polish construction conditions, was used for research. Various solutions were suggested regarding the type of thermal insulation material and heat sources. The values of the indicators were determined for the proposed variants and for the economically and ecologically optimum thermal insulation thicknesses. Based on the conducted research, it was found that air temperatures maintained in the rooms of the building undergoing thermal modernization should be taken into account in the energy audit. The energy demand of the building for a room temperature of 26 °C is higher by 61% compared to the demand for the same building at the design temperature (20 °C). The innovation in the proposed approach to the economic and ecological assessment of a building is the combination of a wide range of temperatures potentially maintained in living spaces with ecological cost-effectiveness.

Key Performance Indicators for Decision Support in Building Retrofit Planning: An Italian Case Study

To achieve climate and energy goals in the building sector, the current pace of renovating existing structures must double, overcoming prevailing barriers. Key Performance Indicators play a pivotal role in science-based decision making, emphasizing both direct and indirect benefits of building retrofits. The authors aim to contribute to proper metric identification for multi-perspective building performance assessment and formulate a methodology supporting energy planning decisions. They introduce the Global Cost per Emission Savings (GCES), an aggregated indicator encompassing both public (CO2 emissions) and private (global cost) perspectives of diverse retrofit technologies for building HVAC systems. Applied to the Italian residential building stock via the Reference Building approach, the methodology is tested using condensing gas boilers, biomass boilers, and electric heat pumps, revealing diverse environmental and economic performances. Addressing the establishment of effective decision-support tools for policymakers, the paper explores the potential impact of various policies on the favorability of technologies. Different policy scenarios are delineated to analyze how distinct approaches may influence the attractiveness of technologies. Notably, in the baseline scenario, biomass boilers hold an advantage over heat pumps according to the GCES index. However, scenarios involving technology-specific incentives or a greenhouse gases emission tax failed to alter the technological ranking, leaving heat pumps financially uncompetitive. In contrast, the TXPM scenario positions heat pumps as the most financially appealing option, penalizing biomass boilers for high particulate matter emissions.

Potential of Energy Saving of Propane Heat Pump as replacement of gas boilers with low and high temperature emitters

This work deals with the analysis of the energy performance and the environmental impact of a Heating, Ventilation and Air Conditioning (HVAC) system based on an innovative Air-to-Water electrical Heat Pump (AWHP) using propane (R290) as the refrigerant. A building of the University of Bologna located in Forlì (North of Italy) is considered for replacing a condensing gas boiler and a conventional chiller with an AWHP using R290. To evaluate the efficiency of the existing heating system and the potential savings linked to the adoption of the propane AWHP, the building energy model was created and calibrated by collecting monthly thermal and electrical consumptions as a function of the actual climate data. In this paper, the main features of the R290-based AWHP are described in detail by emphasising the device performance as a function of the operating conditions (i.e., air and water temperature and speed of the scroll compressor). A series of scenarios have been studied to evaluate the energy performance of the propane AWHP with respect to the reference scenario under various operating conditions. The results show that while the total primary energy demand increases adopting the propane AWHP with respect to the case of a gas boiler, the non-renewable primary energy fraction decreases significantly, with a dramatic increase in the renewable quote. From an economic point of view, lower annual costs are obtained by adopting a propane AWHP coupled to fan coils, mainly when the electrical heat pump is used in a thermally insulated building in which a photovoltaic system is installed.

Influence of different heating systems on thermal comfort perception: a dynamic and CFD analysis

In this paper, we investigate the influence of different heating systems on the thermal comfort indexes, Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD), for a residential apartment located in Bologna (Italy). The apartment has an area of 40 m2 and is located on the ground floor of 4 floors building. The envelop consists in horizontal perforated bricks with internal thermal insulation material and two windows. The analyses are performed employing Trnsys, a commercial dynamic simulation software and Simcenter STAR-CCM+, a multiphysics computational fluid dynamics (CFD) software. The CFD analysis regards a steady condition of a typical winter day in Bologna. Thermal comfort indexes and thermal energy demand are studied comparing two different heating generation systems existing in the considered apartment: a condensing gas boiler coupled with radiators as terminal emitters and an air-to-air heat pump. By crossing the results obtained by the dynamical approach and by the CFD simulations, a two-objective methodology where energy consumption is minimised while thermal comfort is obtained, is presented.

Performance characterisation and design considerations for a domestic ammonia/salt resorption heat pump

Heat pumps will play a key role in the future provision of low carbon domestic heating and the re-use of industrial waste heat. Adsorption cycle heat pumps are advantageous in that they can use the existing natural gas network to avoid electricity supply limitations across the UK. A 2 kW domestic-scale ammonia/salt heat pump demonstrator is currently being tested at the University of Warwick as a replacement for a conventional condensing gas boiler. This paper describes analysis work in support of this testing which will lead to design refinements in follow-on developments.A Matlab-based 2D transient simulation package was developed to study heat transfer and reaction rate within a pair of linked reactors. Heat conduction and sorption rate are modelled together with inter-reactor gas flows and parasitic heat loss. Novel features include the use of Matlab's linked ODE solvers for convergence (ODE15S was found to be fastest) and the script file input configuration which combines clear visibility of parameters with the ability to run multiple simulations to show the effect of parametric variations. The code facilitates rapid design optimisation.Eleven cycle parameters have been investigated, including filling pressure, heat transfer coefficients, salt ratio, source temperatures, void space and heat capacity. The choice of cycle period involves a compromise between coefficient of performance and power output. A water/glycol heat transfer fluid gives better COP and output power than thermal oil. Insulation within the reactor shell has the potential to limit shell transient heat exchange but void space effects are likely to be more significant. The heat capacity of fluid in pipes and manifolds should be minimised.COP = 1.31 is achieved at 45 °C delivery; 60 °C for hot water is possible but with lower COP. The best results for space heating are obtained with source temperatures above -5 °C.

ПОКАЗНИКИ ЕНЕРГОЕФЕКТИВНОСТІ ГРОМАДСЬКОЇ БУДІВЛІ

The relevance of paying attention to the energy efficiency of buildings, which will reduce the consumption of fossil fuels and electricity and reduce harmful emissions into the environment, is shown. The energy efficiency indicators of the public building were analyzed, namely the thermal resistance of the enclosing structures, the specific energy demand for heating, cooling and hot water supply, the specific energy consumption of the building for heating and cooling, the specific consumption of primary energy and the specific emissions of greenhouse gases.The effect of increasing the normative thermal resistance of the outer wall of the building on the required thickness of the insulation was evaluated. It is indicated that under the conditions of a large facade glazing ratio and compliance with the regulatory thermal resistance of the fences, the building does not meet the requirements for specific energy consumption for heating, cooling and hot water supply.The influence of the heat source for heating the building on its specific energy consumption, energy efficiency class, specific consumption of primary energy and specific emissions of greenhouse gases was studied. Considered options for using a gas boiler, a condensing gas boiler, electric heaters, a biomass pellet boiler and connection to a centralized heat supply system. The effectiveness of the implementation of reversible heat pump equipment for heating and cooling of buildings is substantiated

The energy performance of dwellings with heat pumps of Dutch non-profit housing associations

ABSTRACT Achieving energy efficiency in the built environment requires extensive efforts in the renovation and adaptation of housing stock. A promising design solution is the heat pump. While gas boiler systems are commonly used in Dutch non-profit housing stock, the share of dwellings with a heat pump grew from 1.6% in 2017 to 3.2% in 2021. However, building characteristics and the energy consumption of dwellings with a heat pump are unclear. Therefore, a dataset of 69,422 dwellings with different types of heat pumps has been examined and compared to dwellings with a traditional HR107 condensing gas boiler. This research reports average characteristics and the average actual energy consumption of dwellings with all-electric, hybrid and gas absorption heat pump systems. Dwellings with a heat pump system are on average of higher building quality, their gas consumption is lower and their electricity consumption is higher than dwellings with an HR107 condensing gas boiler. Detailed insight is provided for dwellings with different heat pump systems and for dwellings with different building characteristics. Further research to determine the energy performance of dwellings with specific heat pump configurations is recommended in light of the energy transition in the built environment.

Life Cycle Assessment of an Air-Source Heat Pump and a Condensing Gas Boiler Using an Attributional and a Consequential Approach

Emissions from heating systems in the building sector significantly contribute to anthropogenic environmental pollution in Germany. Heat pumps are often considered to reduce these pollutions. Therefore, the aim of this study is to compare the environmental impacts of an air-source heat pump with the most commonly used technology, a condensing gas boiler. For this purpose, a comparative life cycle assessment is used. To determine the life cycle inventory of the use phase, an energetic simulation is carried out for both heating systems, with a new single-family house serving as the reference building. The gas boiler is beneficial in 8 out of 11 impact categories. However, the carbon intensity of 76.9 g CO2-eq/MJ heat is 15% higher than for the heat pump. Most environmental impacts for both heating systems occur during operation. The application of the consequential life cycle assessment approach leads to an average reduction of 46% in the environmental impact of the heat pump compared to the attributional approach. In contrast, the gas boiler is only slightly affected by changing the modeling approach. Nevertheless, the heat pump still has higher environmental impacts in 7 impact categories compared to the gas boiler. However, since the reduction of greenhouse gases of the natural gas-fired system is limited, the carbon intensity of the air-source heat pump is 70% lower compared to the condensing gas boiler.

Research on Gas Type Discrimination and Emission Regulation of Fully Premixed Condensing Gas Boiler Based on Flame Ion Signal

Research on Gas Type Discrimination and Emission Regulation of Fully Premixed Condensing Gas Boiler Based on Flame Ion Signal

An Environmental Assessment of Heat Pumps in Poland

Greenhouse gas emissions are regarded as the cause of the rise in mean Earth temperature, the further increase of which may cause dramatic irreversible global changes. Their levels in Poland are some of the highest in the world. In this study, to evaluate the “Energy policy of Poland until 2040”, greenhouse gas and particulate matter emissions were compared in all Polish climatic zones using a typical meteorological year. A comparison was made between air-to-water, brine-to-water, and water-to-water devices. The electrical energy consumption and coefficient of performance were determined hourly in the heating season. The determining algorithm took a buffer tank or separating heat exchanger into consideration, when they were required, and electricity transmission efficiency. Subsequently the emissions were estimated for the present Polish energy mix and a mix proposed in the “Energy policy of Poland until 2040”. After implementing this policy all heat pumps will be energetically viable devices. However, only in one location will water-to-water heat pumps cause less emissions than a condensing gas boiler. In other cases, condensing gas boilers will be significantly less emissive devices. Therefore, this policy is insufficient for reducing emissions. Hence, it is necessary to replace almost all coal-fired power plants with natural gas-fired gas turbines in a transitional period.

Alternative Heating and Cooling Systems for the Retrofit of Medium‐Rise Residential Buildings in Greece

The European Union recently set the target of doubling building retrofit rates through the European Green Deal. Currently, more than half of the households’ energy consumption is accounted for space conditioning, with southern European countries experiencing increasing demand for cooling systems over the past decade. Herein, the performance of market‐available heating and cooling systems that can replace the existing low‐efficiency systems in multiresidential buildings in Greece is compared. The study's objectives are to minimize the operating greenhouse gas emissions and the life‐cycle cost. Results demonstrate that air‐to‐air heat pumps have the lowest life‐cycle cost. In areas where natural gas is not available, the replacement of the diesel‐oil boiler with a biomass boiler leads to a 48% to 73% decrease of the operating greenhouse gas emissions. In areas where natural gas is available, the gas absorption heat pump has the lowest operating greenhouse gas emissions, demonstrating a reduction between 40% and 54% when compared to a conventional gas boiler; however, it dramatically increases the life‐cycle cost, making it less attractive than heat pumps and condensing gas boilers. The findings are in line with the current residential space conditioning market, while indicating the potential of biomass boilers and gas absorption heat pumps.

3D modeling framework and investigation of pollutant formation in a condensing gas boiler

Condensing gas boilers exhibit complex global flow-field patterns, which lead to noteworthy inhomogeneities of the burnt gas temperature distribution and levels of local pollutant formation in the combustion chamber. To enable numerical simulations of realistic heating systems, a modeling framework was developed. Challenges like high computational times and the combined multi-physics aspects that occur in quite complex geometries were addressed. The flamelet progress variable (FPV) model was applied, which tabulates detailed reaction kinetics as a function of progress variable and enthalpy. As the larger timescales of CO and NO formation inhibit a direct lookup of local CO and NO mass fractions from the chemistry table, the so called NOMANI model was utilized for NO and a rescaling model was applied for CO. All models were validated against resolved simulation results and experimental measurements. The multi-hole burner was modeled as a porous medium, because it was computationally unfeasible to resolve each burner hole. While local gas acceleration and preheating was considered, the exact flame structure as well as the recirculation zone behind the flame front could not be represented. To still capture the burnt gas temperature, the missing heat losses from the recirculation zone were modeled and coupled to the conjugate heat transfer (CHT) model. The modeling framework was applied to a commercial condensing gas boiler device at nominal load utilizing a lean methane-air mixture. The simulation shows reasonable agreement with experimentally measured emission levels and critical regions with CO and NOx emissions twice as high as in other regions were identified. Temperature variations of 200 K in the flue gas throughout the combustion chamber and 450 K in the solid region of the multi-hole burner were found. Based on these results, design aspects are correlated to CO and NO formation and suggestions on possible design modifications are discussed to further reduce pollutant emissions of such condensing gas boilers.

Environmental life cycle assessment of heating systems in the UK: Comparative assessment of hybrid heat pumps vs. condensing gas boilers

Residential space heating is one of the major contributors to greenhouse gas (GHG) emissions and hence a priority sector to decarbonise in the transition to Net Zero target by 2050 in the UK. To assess environmental impacts of a current heating system and potential alternatives in the UK, this study conducted a comparative LCA of a condensing gas boiler and a hybrid heating pump for a common type of UK’s existing houses (a semi-detached house). The functional unit of this study is defined as delivering space heating for the whole lifetime (20 years) of heating system. The results suggest that the hybrid heat pump potentially saves 30% of GHG emissions as compared to the condensing gas boiler in the core scenarios (4.5E + 04 kg CO2-eq/FU vs 6.4 E + 04 kg CO2-eq/FU respectively). The hybrid heat pump also shows 13% to 48% emission reduction as compared to the condensing gas boiler in terrestrial acidification, photochemical oxidant formation, particulate matter formation and fossil depletion. However, the hybrid heat pump emits 3 to 6 times more emissions in terms of human toxicity, water depletion and metal depletion than the condensing gas boiler. The production phase contributes around 50% of the impact for metal depletion and human toxicity in both core scenarios, while the use phase dominates in other selected impact categories. The combustion of natural gas and the electricity production are the major causes for the dominance of the use phase for all selected impact categories excepting metal depletion and human toxicity. The sensitivity scenarios support the robustness of the results. Further work is needed to understand the role hybrid heat pumps can play in the residential sector decarbonisation under different scenarios of residential uptake, household behaviour and wider UK energy system decarbonisation.

Study on the in situ performance of a 60 kW condensing gas boilers thermal installation and economic assessment

Condensing gas boilers currently represent the most advanced solution for heating based on combustion of fossil fuels from both efficiency and pollution points of view. Besides, compared with the district heating, condensing gas boilers make possible the entire management of the heat supply function by the real heating needs. Subsequently, minimization of the heating cost is possible. In this view, one of the buildings of the “Gheorghe Asachi” Technical University of Iasi was disconnected from the district heating and two condensing gas boilers – one of 25 kW and the other of 35 kW nominal output – were connected to the heating network to ensure the heat supply. A performance and economic analysis of the thermal installation with the two boilers, based on in situ experiments, was performed. The analysis was performed for nine operating regimes described by boilers flow water temperatures from 80 °C to 40 °C. Efficiency (in terms of higher heating value of the fuel) had varied from 86.7 % to 97.5 % and the price of thermal energy produced with the two boilers proved to be 21…30 % lower than the price of thermal energy coming from the district heating system. The annual heating savings are estimated to be 642…907 Euro/year, which induce a payback period of 1.6…2.3 years with strict reference to the investment represented by the two boilers.

Investigation of nitric oxide formation in methane, methane/propane, and methane/hydrogen flames under condensing gas boiler conditions

In this study, NOx formation in lean premixed burner-stabilized flames was investigated under condensing gas boiler operating conditions for pure methane, methane/propane, and methane/hydrogen mixtures. Temperature and NOx species profiles were experimentally obtained from thermocouple and wet chemiluminescence detector measurements using in-situ extracted gas samples. Resolved simulations utilizing different NO sub-mechanisms from the GRI 2.11, the GRI 3.0, the CRECK mechanism, and a recent mechanism from Glarborg and co-workers were performed, where the mechanism from Glarborg showed the best agreement with the measured NOx concentrations. While the other mechanisms also agreed fairly well with the experiments, a pathway analysis revealed that this results from error compensation, where the prompt pathway was underestimated, which was balanced by an overestimated contribution of the NNH pathway. Interestingly, the prompt pathway was found to be the major NOx formation route for condensing gas boiler conditions despite the lean mixture. While the temperature level in the post-flame region was too low for the thermal pathway to become dominant, the NNH and N2O pathways reached their chemical equilibrium just downstream of the flame front and therefore did not further contribute to the overall NOx emissions.The partial substitution of methane by hydrogen slightly reduced NOx emissions, which was similarly found in experiment and simulation. The post-flame temperature for methane/hydrogen remained nearly unchanged, because the higher adiabatic flame temperature compared to pure methane was compensated by increased heat losses due to a higher laminar burning velocity. The addition of hydrogen decreased the amount of CH radicals in the flame front, which led to a lower contribution of the most important prompt pathway.Substituting methane by propane increased the temperature level due to a higher adiabatic flame temperature and a slightly smaller laminar burning velocity. While increasing NOx emissions were found in the experiment, the simulation predicted a minimally smaller NOx level compared to pure methane. The higher temperature caused increasing contributions of the thermal, NNH, and N2O pathways. For the prompt pathway, which is again the most important NOx formation route, the temperature trend was superimposed by a decreasing CH concentration in the flame front.