Publications

The 15 short chapters that form this 2023 ammonia-for-energy roadmap provide a comprehensive assessment of the current worldwide ammonia landscape and the future opportunities and associated challenges facing the use of ammonia, not only in the part that it can play in terms of the future displacement of fossil-fuel reserves towards massive, long-term, carbon-free energy storage and heat and power provision, but also in its broader holistic impacts that touch all three components of the future global food-water-energy nexus.

Ammonia-hydrogen blends have gained significance as they are carbon-free energy-dense fuels. However, NOx emissions have been a significant concern. In this study, the emissions from the premixed combustion of 70/30VOL.% NH3/H2 blend is studied using the computational fluid dynamics (CFD)- chemical reactor network (CRN) approach. The velocity, temperature, and species field are first obtained using CFD, based on which, a network consisting of perfectly stirred reactors (PSR) and plug flow reactor (PFR) is constructed. Three mechanisms have been implemented in the CRN to predict the NO and N2O emissions. It is shown that the trend of NOx is correctly predicted by the CRN over a wide range of equivalence ratios (φ) of 0.65–1.2 as compared to the authors’ recently published experimental data. It is demonstrated that a single CRN (based on the CFD for a specific φ) can be run to cover the range of φ  = 0.65 to 1.2 b y scaling the temperature input to each reactor of the CRN. To contrast the NO pathways at different φ, quantitative reaction pathway diagrams (QRPD) are constructed, and dominant production and consumption pathways of NO for lean and rich combustion are established. The shifts in reaction pathways with φ are noted and found to be governed by OH, O, and H radicals. Next, the effect of stoichiometry on these radicals is established. Finally, the experimental trend of high NO close to stoichiometric combustion and high N2O in very lean combustion along with their respective pathways are explained.

The use of ammonia in fuelling applications has been recognized as a method to support energy transition towards a zero-carbon economy. Ammonia, a chemical that contains large quantities of hydrogen, can be employed in applications that go from furnaces to aerospace engines. However, handling, power outputs, emissions and maintenance procedures still need to be properly defined before ammonia can substitute fossils in the production of energy. Therefore, this chapter presents four attempts in which ammonia has been employed to replace fossil fuels for powering purposes. From theoretical studies to technological developments, the chapter focuses on the challenges that each case faced whilst highlighting the novelties that the research groups developed to overcome those constraints. The experience gained over studies and developments combined with unique selection methods have enabled the progression of these ideas and rigs, establishing them as first demonstrators worldwide, thus providing the first insights into the complexity and potential of using NH3 as a fuel.

Laminar burning velocity (LBV) is a crucial parameter in describing the engine combustion characteristics, such as flame stability and thermal efficiency. In this study, n-dodecane was used as a surrogate fuel for diesel. Unique LBV data for n-dodecane/NH3/air mixtures at various initial temperatures (T u  = 488–650 K), ammonia blending ratios (X NH3  = 0∼0.9), and equivalence ratios (ϕ = 0.6–1.4) were measured by the Externally heated diverging channel (EHDC) method. Additionally, an improved mechanism incorporating a subset of C-N interaction reactions was developed, which predicts LBV of n-dodecane/NH3/air flames with high accuracy. The results show that LBV increases with initial temperature, while it decreases with increasing X NH3 . When X NH3 increased from 0.1 to 0.9 (ϕ = 1), the LBV decreased by 24.1 cm/s (from 81.3 cm/s to 57.2 cm/s) at an initial temperature of 488 K, and by 35.0 cm/s (from 139.4 cm/s to 104.4 cm/s) at an initial temperature of 650 K. This study also investigated the sharp decline in LBV at high X NH3 from the perspectives of chemical, thermal, and diffusion effects. The results indicate that at high ammonia concentrations, the influence of the adiabatic temperature increase becomes more significant, with the thermal effect proportion increasing by nearly 20 % as X NH3 rises from 70 % to 90 %. During the combustion of NH3 with large n-alkanes, key C/N compound intermediates play a critical role in explaining both LBV changes and nitrogen-containing product formation. C-N interaction reactions, such as R8403(CH3 + H2NO = CH3O + NH2), R8392(CH3 + NH2 = CH3NH2), R8414(CH + NO = HCN + O), and R8433(C2H + NO = HCN + CO), play a crucial role in optimizing nitrogen product formation.

Swirl combustors have proven to be effective flame stabilisers over a wide range of operation conditions thanks to the formation of well-known swirl coherent structures. However, their employment for lean premixed combustion modes while introducing alternative fuels such as high hydrogenated blends results in many combustion instabilities. Under these conditions, flame flashback is considered one of the major instability problems that have the potential of causing considerable damage to combustion systems hardware in addition to the significant increase in pollutant levels. Combustion Induced Vortex Breakdown is considered a very particular mode of flashback instability in swirling flows as this type of flashback occurs even when the fresh mixture velocity is higher than the flame speed, a consequence of the interaction between swirl structures and swirl burner geometries. Improvements in burner geometries and manipulation of swirling flows can increase resistance against this type of flashback. However, increasing resistance against Combustion Induced Vortex Breakdown can lead to augmentation in the propensity of another flashback mechanism, Boundary Layer Flashback. Thus, this paper presents an experimental approach of a combination of techniques that increase Combustion Induced Vortex Breakdown resistance, i.e. by repositioning a central injector and using central air injection, while simultaneously avoiding Boundary Layer Flashback, i.e. by changing the wall boundary layer characteristics using microsurfaces on the nozzle wall. Results show that using these techniques together has promising potentials regarding wider stable operation for swirl combustors, enabling them to burn a broader variety of fuel blends safely, while informing developers of the improvements obtained with the combined techniques.

With developing interest in NH3 as a prospective energy carrier, combustor designs and fuelling concepts require optimisation to reduce NOx emissions. Through the introduction of staged combustor concepts, pathways have previously been identified that limit NOx production whilst improving combustor efficiency and reducing unburned NH3. However, the efficacy of secondary air staging is sensitive to the primary flame behaviour, and whilst low NOx emissions can be achieved at rich conditions, high unburned NH3 leads to greater global NOx concentrations from downstream production. Here, time-resolved OH*, NH2* and NH* chemiluminescence were employed together for the first time for NH3-air and NH3 H2-air flames to investigate a primary flame configuration that produced the lowest combined emissions concentration. A generic, fuel-flexible burner was developed to enable partial and full premixing, together with operation of a swirl-stabilised non-premixed flame. Initially, NH3 H2-air flames were employed in a range of configurations and produced markedly different chemiluminescence and emissions results as functions of global equivalence ratio. The performance of a pure NH3-air flame was subsequently investigated and compared to the blended fuel results. Optical trends complemented changes in sampled exhaust emissions, enabling analysis of intermediate chemistry. Burner inlet temperature and pressure were then increased proportionally to maintain equivalent bulk nozzle exit velocities. Contrasting trends were identified as functions of fuel composition and equivalence ratio, with a comprehensive database of optical and analytical results generated. Results obtained for NH3 H2-air suggest the most favourable configuration resulted from a partially premixed flame employing H2 as a pilot, operating under rich conditions (Φ=1.2). However, at higher temperatures and pressures, the trends observed for non-premixed NH3-air flames will lead to superior performance, particularly with a small increase in equivalence ratio.

Experimental and numerical investigations were performed to study the combustion characteristics of synthesis gas (syngas) under premixed swirling flame mode. Four different type of syngases, ranging from low to high H2 content were tested and simulated. The global flame structures and post emission results were obtained from experimental work, providing the basis of validation for simulations using flamelet generated manifold (FGM) modelling approach via a commercial computational fluid dynamic software. The FGM method was shown to provide reasonable agreement with experimental result, in particular the post-exhaust emissions and global flame shapes. Subsequently, the FGM method was adopted to model the flame structure and predict the radical species in the reaction zones. Simulation result shows that H2-enriched syngas has lower peak flame temperature with lesser NO species formed in the reaction zone.

The increasing demand for clean and sustainable energy sources provides the impetus for the development of alternative fuels. Recent development of fuel-flexible gas turbine technologies enables the use of alternative non-fossil fuels that could play key roles in contributing to the global efforts in meeting emissions targets. This review highlights the current state-of-the-art production and properties of alternative fuels such as straight vegetable oil (SVO), biodiesel, bioethanol, bio-oil, hydrogenated vegetable oil (HVO) and Fischer-Tropsch (FT) fuel. This is followed by the evaluation of combustion performances in gas turbines. All of the alternative liquid biofuels have shown their potentials in reducing regulated emissions such as NOx, CO and soot under favourable operating conditions. Both HVO and FT fuels show comparable performance as that of jet fuel and can be used in aviation gas turbines, although the present day high production cost restricts the large-scale adoption, limiting its utility. They also have considerably higher cetane number than the rest, making it easier for the fuel to ignite. As for stationary power generation gas turbines that need not carry payloads, the other four alternative biofuels of biodiesel, bioethanol, bio-oil and SVO are possible candidates despite the physics-chemical properties variations when compared to fossil fuels. Amongst them, the use of SVO and bio-oil in gas turbines would require the parallel development of fuel supply systems and atomisation technologies to improve the combustion of the fuels. In all, the alternative liquid fuels reviewed provides realistic opportunities for cleaner and more sustainable operation of aviation and power generation gas turbines. Profound understanding on the fundamental combustion characteristics of the fuels are essential to expedite their mass adoption in gas turbine applications.