Combustion characteristics of lean premixed methane/higher hydrocarbon/hydrogen flames

Declining indigenous resources, increased dependence on imports and intermittent renewable energy, have resulted in an increasingly diverse energy-generation landscape. As a result, gas turbine operators face new challenges with respect to gas turbine flexibility in terms of combustion efficiency, safety and emission control. Increased reliance on liquefied natural gas, potentially containing high concentrations of heavier hydrocarbons, typically ethane and propane, coupled with the emerging prospect of hydrogen injection into national gas grids, presents associated combustion impacts not fully appreciated. This new reality underlines the necessity of developing understanding of fundamental combustion characteristics, ultimately guiding the design of future highly flexible gas turbines. This thesis aims to characterise fundamental combustion performance of methane/higher hydrocarbon/hydrogen fuels, through a combination of experimental and numerical techniques, with a focus on natural gas blends representative of fuel variations and at air fuel ratios expected in premixed low-carbon power generation facilities. The parameters identified to investigate fuel behaviour were the laminar burning velocity, Markstein Length and the Lewis Number, yielding essential physiochemical and thermo-diffusive flame information. These properties are of value when attempting to predict flame behaviour in turbulent environments, reflective of most practical gas turbine applications. The main components of natural gas, and relevant hydrogen enriched binary and tertiary mixtures were parametrically investigated, with respect to stretch-related and flame propagation behaviour at lean air fuel ratios, in addition to a comparison of numerically simulated results obtained from chemical kinetics. Effective Lewis Number models were appraised and compared to experimentally measured data, employing theoretical formulations relating Markstein Length to Lewis Number as proposed in literature. The influence of hydrogen and propane addition to the lean stability limits of premixed turbulent methane flames was examined, using a generic swirl burner, at various inlet temperature and thermal powers, with measured lean blow off limits in correlation with measured Markstein length behaviour

Combustion performances of premixed ammonia/hydrogen/air laminar and swirling flames for a wide range of equivalence ratios

Ammonia, a carbon-free source of hydrogen has recently gained considerable attention as energy solution towards a green future. Previous works have shown that adding 30VOL.% hydrogen with ammonia can eradicate the drawbacks of pure ammonia combustion but no study in the literature has investigated this blend across a wide range of equivalence ratios. The present work investigates 70/30VOL.% NH3/H2 blend from 0.55 ≤ Φ ≤ 1.4 for both premixed laminar spherically expanding flames and turbulent swirling flames at atmospheric conditions. A detailed chemistry analysis has been conducted in Ansys CHEMKIN-PRO platform using a chemical reactor network (CRN) model to simulate the swirling turbulent flames. NO and NO2 emissions have followed similar bell-shaped trends, peaking at around Φ = 0.8, while N2O emission rises at lean conditions (Φ ≤ 0.7). The results indicate that Φ = 1.2 is the optimum equivalence ratio with reduced NOX emissions and some ammonia slip

Nitrogen oxide emissions in ammonia combustion

Similar to hydrogen, ammonia is a zero-carbon fuel that can be synthesized from renewable energy sources such as solar and wind. Due to its better feasibility for production, preservation, and distribution, ammonia has been considered sustainable to meet the requirements of the future energy fields that are developing toward a low-carbon economy. However, the broad deployment of ammonia as fuel is limited by emissions. This chapter presents the pathways of ammonia mixture reactions and the production routes of emissions with different equivalence ratios. Some critical intermediate radicals are revealed for formation. It is found that many factors affect the chemical reaction pathways of ammonia-based fuels, such as equivalence ratio, fuel mixture, pressure and temperature, and so forth. Ammonia combustion and emissions have been investigated under different conditions on both laboratory and industrial scales. It was found that the productions peaked at Φ = 0.8–0.9 for various ammonia/hydrogen blends. The NO productions from ammonia-based flames were effectively decreased with rich blends because of more generated (i = 0, 1, 2) radicals. An overall equivalence ratio of 1.20 was suggested for two-stage combustion to improve combustion efficiency and emission performance. Furthermore, some practical controlling techniques, e.g., thermal , two-stage combustion, humidification, and plasma-assisted combustion, are introduced for mitigation

Humidified ammonia/hydrogen RQL combustion in a trigeneration gas turbine cycle

Ammonia is an example of a zero-carbon fuel of high interest for implementation in gas turbine technologies. Preliminary analyses showed that a basic humidified ammonia-hydrogen Brayton cycle can produce total plant efficiencies of ~34%. However, further improvements are required to make these units competitive to current fossil-based plants whose efficiencies are above 80%. Thus, this work seeks to numerically and analytically demonstrate the implementation of a complex cycle that will increase final efficiencies whilst using the full potential of ammonia as a cooling fluid, power fuel and heating gas (i.e. trigeneration cycle) with heat district distribution. Therefore, a basic gas turbine cycle was inserted into a two-shaft, reverse Brayton gas turbine plant facility. In order to improve combustion and reduce emissions, a Rich-Quench-Lean system was integrated into the analysis by resolving the combustion performance via CHEMKIN-PRO. Detailed sensitivity analyses were also conducted throughout the burner to identify the key reactions responsible for both flame stability and NO formation/reburn pathways, which are vital for future safe and efficient operation of these types of cycles. The study shows that the total efficiency has significantly increased when compared to the basic turbine facility, with a value ~59%. Moreover, low emissions were accomplished below current European NOx thresholds. The obtained values show a significant potential for the utilisation of ammoni-based blends with steam injection in gas turbine facilities through employment of novel cycles that consider lower dilution in the combustion sector in combination with novel ammonia combustion systems and trigeneration concepts