Experimental study on high pressure combustion of decomposed ammonia: How can ammonia be best used in a gas turbine?

Hydrogen, a carbon-free fuel, is a challenging gas to transport and store, but that can be solved by producing ammonia, a worldwide commonly distributed chemical. Ideally, ammonia should be used directly on site as a fuel, but it has many combustion shortcomings, with a very low reactivity and a high propensity to generate NOx. Alternatively, ammonia could be decomposed back to a mixture of hydrogen and nitrogen which has better combustion properties, but at the expense of an endothermal reaction. Between these two options, a trade off could be a partial decomposition where the end use fuel is a mixture of ammonia, hydrogen, and nitrogen. We present an experimental study aiming at finding optimal NH3-H2-N2 fuel blends to be used in gas turbines and provide manufacturers with guidelines for their use in retrofit and new combustion applications. The industrial burner considered in this study is a small-scale Siemens burner used in the SGT-750 gas turbine, tested in the SINTEF high pressure combustion facility. The overall behaviour of the burner in terms of stability and emissions is characterized as a function of fuel mixtures corresponding to partial and full decomposition of ammonia. It is found that when ammonia is present in the fuel, the NOx emissions although high can be limited if the primary flame zone is operated fuel rich. Increasing pressure has shown to have a strong and favourable effect on NOx formation. When ammonia is fully decomposed to 75% H2 and 25% N2, the opposite behaviour is observed. In conclusion, either low rate or full decomposition are found to be the better options. Copyright © 2021 by ASME.publishedVersio

Investigations of microwave stimulation of a turbulent low-swirl flame

Irradiating a flame by microwave radiation is one of several plasma-assisted combustion (PAC) technologies that can be used to modify the combustion chemical kinetics in order to improve flame-stability and to delay lean blow-out. One practical implication is that engines may be able to operate with leaner fuel mixtures and have an improved fuel flexibility capability including biofuels. In addition, this technology may assist in reducing thermoacoustic instabilities that may severely damage the engine and increase emission production. To examine microwave-assisted combustion a combined experimental and computational study of microwave-assisted combustion is performed for a lean, turbulent, swirl-stabilized, stratified flame at atmospheric conditions. The objectives are to demonstrate that the technology increases both the laminar and turbulent flame speeds, and modifies the chemical kinetics, enhancing the flame-stability at lean mixtures. The study combines experimental investigations using hydroxyl (OH) and formaldehyde (CH2O) Planar Laser-Induced Fluorescence (PLIF) and numerical simulations using finite rate chemistry Large Eddy Simulations (LES). The reaction mechanism is based on a methane (CH4)-air skeletal mechanism expanded with sub-mechanisms for ozone, singlet oxygen, chemionization, electron impact dissociation, ionization and attachment. The experimental and computational results show similar trends, and are used to demonstrate and explain some significant aspects of microwave-enhanced combustion. Both simulation and experimental studies are performed close to lean blow off conditions. In the simulations, the flame is gradually subjected to increasing reduced electric field strengths, resulting in a wider flame that stabilizes nearer to the burner nozzle. Experiments are performed at two equivalence ratios, where the leaner case absorbs up to more than 5% of the total flame power. Data from experiments reveal trends similar to simulated results with increased microwave absorption

Drying and Pyrolysis of Logs of Wood

This work concerns log firing in small-scale boilers for house heating, which in Sweden meets a heat demand of 12 TWh a year. In boilers used for domestic central heating in the sizes of 10 to 30 kW the predominant combustion technique is grate firing. Prediction of the transient release of combustible gas from the log-fire in such boilers enables an optimised design of the combustion chamber to minimise emissions of unburnt and harmful emissions. The release of gas is governed by the heating of each log of wood. The heat flux to a log is given by the design of the furnace and by the surrounding logs. Exposure of a log to a heat flux will lead to evaporation of water (drying) and then to thermal degradation (pyrolysis) of the wood into volatiles (about 80%) and char (about 20%). Subsequently these constituents will burn. In order to predict the release of gas from a log of wood under combustion conditions, drying and pyrolysis of one log of wood, idealised to a cylinder, are studied. The complex heating situation in a log-fire in a small-scale boiler is simplified to a constant furnace wall temperature. The progress of drying and pyrolysis was monitored from measurements of interior temperature distribution and mass loss. The heat transfer properties of wood and charcoal were investigated. Based on comparison of measured data and simulations, using a numerical model, the applicability of measured thermal properties, the submodel of pyrolysis and the anisotropy of wood permeability are discussed. The results suggest that modelling of drying and pyrolysis of thermally thick wood particles should include: The structural changes during pyrolysis causing growth of pores, formation of cracks and global shrinkage of the sample. A pyrolysis reaction mechanism that accurately predicts initiation of pyrolysis at slow heating rates, such as in the particle\u27s interior. Drying governed by the liquid permeability in the longitudinal fibre direction at moisture contents above the fibre saturation point. Instant, non-thermal equilibrium release of gas from the particle at temperatures above 400 \ub0C (after pyrolysis). Due to the structural changes in conjunction with uncertainties in literature on thermal properties of wood and charcoal, it is recommended to use a simple approach, for instance, of constant thermal diffusivity. The application of the results of this study to other types of thermally thick wood particles needs data on thermal properties of wood and charcoal coupled to the structural variations in such particles

Drying and Pyrolysis of Logs of Wood

This work concerns log firing in small-scale boilers for house heating, which in Sweden meets a heat demand of 12 TWh a year. In boilers used for domestic central heating in the sizes of 10 to 30 kW the predominant combustion technique is grate firing. Prediction of the transient release of combustible gas from the log-fire in such boilers enables an optimised design of the combustion chamber to minimise emissions of unburnt and harmful emissions. The release of gas is governed by the heating of each log of wood. The heat flux to a log is given by the design of the furnace and by the surrounding logs. Exposure of a log to a heat flux will lead to evaporation of water (drying) and then to thermal degradation (pyrolysis) of the wood into volatiles (about 80%) and char (about 20%). Subsequently these constituents will burn. In order to predict the release of gas from a log of wood under combustion conditions, drying and pyrolysis of one log of wood, idealised to a cylinder, are studied. The complex heating situation in a log-fire in a small-scale boiler is simplified to a constant furnace wall temperature. The progress of drying and pyrolysis was monitored from measurements of interior temperature distribution and mass loss. The heat transfer properties of wood and charcoal were investigated. Based on comparison of measured data and simulations, using a numerical model, the applicability of measured thermal properties, the submodel of pyrolysis and the anisotropy of wood permeability are discussed. The results suggest that modelling of drying and pyrolysis of thermally thick wood particles should include: The structural changes during pyrolysis causing growth of pores, formation of cracks and global shrinkage of the sample. A pyrolysis reaction mechanism that accurately predicts initiation of pyrolysis at slow heating rates, such as in the particle\u27s interior. Drying governed by the liquid permeability in the longitudinal fibre direction at moisture contents above the fibre saturation point. Instant, non-thermal equilibrium release of gas from the particle at temperatures above 400 \ub0C (after pyrolysis). Due to the structural changes in conjunction with uncertainties in literature on thermal properties of wood and charcoal, it is recommended to use a simple approach, for instance, of constant thermal diffusivity. The application of the results of this study to other types of thermally thick wood particles needs data on thermal properties of wood and charcoal coupled to the structural variations in such particles

Experimental study on high pressure combustion of decomposed ammonia: How can ammonia be best used in a gas turbine?

Hydrogen, a carbon-free fuel, is a challenging gas to transport and store, but that can be solved by producing ammonia, a worldwide commonly distributed chemical. Ideally, ammonia should be used directly on site as a fuel, but it has many combustion shortcomings, with a very low reactivity and a high propensity to generate NOx. Alternatively, ammonia could be decomposed back to a mixture of hydrogen and nitrogen which has better combustion properties, but at the expense of an endothermal reaction. Between these two options, a trade off could be a partial decomposition where the end use fuel is a mixture of ammonia, hydrogen, and nitrogen. We present an experimental study aiming at finding optimal NH3-H2-N2 fuel blends to be used in gas turbines and provide manufacturers with guidelines for their use in retrofit and new combustion applications. The industrial burner considered in this study is a small-scale Siemens burner used in the SGT-750 gas turbine, tested in the SINTEF high pressure combustion facility. The overall behaviour of the burner in terms of stability and emissions is characterized as a function of fuel mixtures corresponding to partial and full decomposition of ammonia. It is found that when ammonia is present in the fuel, the NOx emissions although high can be limited if the primary flame zone is operated fuel rich. Increasing pressure has shown to have a strong and favourable effect on NOx formation. When ammonia is fully decomposed to 75% H2 and 25% N2, the opposite behaviour is observed. In conclusion, either low rate or full decomposition are found to be the better options. Copyright © 2021 by ASME