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This study reports measurements of stability limits and exhaust NO mole fractions of technically-premixed swirl ammonia-air flames enriched with either methane or hydrogen. Experiments were conducted at different pressures from atmospheric to 5 bar, representative of commercial micro gas turbines. The full range of ammonia fractions in the fuel blend, xNH3, was considered, from 0 (pure methane or hydrogen) to 1 (pure ammonia), covering very lean (φ = 0.25) to rich (φ = 1.60) equivalence ratios. Results show that increasing pressure widens the range of stable equivalence ratios for pure ammonia-air flames. Regardless of pressure, there is a critical ammonia fraction above which the range of stable equivalence ratios suddenly widens. This is because flashback does not occur anymore when the equivalence ratio is progressively increased towards stoichiometric and rich blowout occurs instead. This critical ammonia fraction increases with pressure and is larger for ammonia-hydrogen than for ammonia-methane. Provided that enough hydrogen is blended with ammonia (xNH3

Hydrogen is a promising carbon-free fuel for power generation in gas turbines. However, this raises some challenges associated with the storage and distribution of pure hydrogen. Storing hydrogen chemically in the form of ammonia is a safe and efficient alternative. However, ammonia as a fuel features a low chemical reactivity compared to hydrogen and natural gas and, as a consequence, stabilizing turbulent ammonia-air flames is challenging. Offsetting this low reactivity by enriching ammonia with some amount of hydrogen, which is much more reactive, is a promising strategy. In this study, the stability limits of technically-premixed ammonia-hydrogen-air flames are measured in a laboratory-scale swirl combustor for a wide range of ammonia fractions in the ammonia-hydrogen fuel blend. Results are compared to that obtained in the same combustor for reference methane-hydrogen-air mixtures. Data show that increasing the ammonia fraction in the fuel blend promotes lean blowout but reduces the flames’ propensity to flashback. The latter effect is even more pronounced if the volume fraction of ammonia in the fuel blend exceeds 0.7. In that case, increasing the equivalence ratio at a fixed bulk velocity does not yield flashback and rich blowout occurs instead, yielding a much wider range of stable equivalence ratios. This study also reports exhaust NO mole fractions, measured for large ranges of equivalence ratio and ammonia fraction in the fuel blend. Regardless of the ammonia fraction, data show that competitively low NO emissions occur for slightly rich equivalence ratios of φ ≥ 1.05, which is consistent with earlier studies. Stable flames and good NO performance are also found for very lean ammonia-hydrogen-air mixtures with φ ≤ 0.50, demonstrating the strong potential of fueling gas turbines with ammonia-hydrogen blends.

To investigate the oxidation of ammonia (NH3)/hydrogen (H2) mixtures at intermediate temperatures, this work has implemented jet-stirred reactor (JSR) oxidation experiments of NH3/H2 mixtures at atmospheric pressure and over 800-1280 K. The H2 content in the NH3/H2 mixtures is varied from zero to 70 vol% at equivalence ratios of 0.25 and 1.0. Species identification and quantification are achieved by using Fourier-transform infrared (FTIR) spectroscopy. A kinetic model for pure NH3 and NH3/H2 mixtures is also developed for this research, and validated against the present experimental data for pure NH3 and NH3/H2 mixtures, as well as those for pure NH3, H2/NO, H2/N2O, NH3/NO, NH3/NO2 and NH3/H2 mixtures in literature. The model basically captures the experimental data obtained here, as well as in literature. Both measured and predicted results from this work show that H2 blending enhances the oxidation reactivity of NH3. Based on the model analysis, under the present experimental conditions, NH3 + H = NH2 + H2 proceeds in its reverse direction with increasing H2 content. The H atom produced is able to combine with O2 to produce either O and OH via a chain-branching reaction, or to yield HO2 through a chain-propagation reaction. HO2 is an important radical under the present intermediate-temperature conditions, which can convert NH2 to OH via NH2 + HO2 = H2NO + OH; H2NO is then able to convert H to NH2 and OH. In this reaction sequence, NH2 and H2NO are chain carriers, converting HO2 and H to two OH radicals. Since the OH radical is the dominant radical to consume NH3 under the present conditions, the enhanced OH yield via H + O2 = O + OH, NH2 + HO2 = H2NO + OH and H2NO + H = NH2 +OH, with increasing H2 content, promotes the consumption of NH3. For NOx formation, non-monotonous trends are observed by increasing the content of H2 at the 99% conversion of NH3. These trends are determined by the competition between the dilution effects and the chemical effects of H2 addition. Nitrogen related radicals, such as NH2, NH and N, decrease as H2 increases, and this dilution effect reduces NOx formation. For chemical effects, the yields of oxygenated radicals, such as O, OH and HO2, are enhanced with increasing H2 content, which results in enhancing effects on NO formation. For N2O formation, the enhanced oxygenated radicals (O, OH and HO2) suppress its formation, while the enhanced NO promotes its formation.

An ilmenite oxygen carrier was tested in a laboratory scale chemical-looping reactor with a nominal thermal capacity of 300 Wth. Ilmenite is a mineral iron-titanium oxide, which has been used extensively as an oxygen carrier in chemical-looping combustion. Two different kinds of fuels were used, a sulfur-free kerosene and one kerosene that contained 0.57 mass% sulfur. Both fuels were continuously evaporated and directly fed into the chemical-looping reactor. Experiments were conducted for 50 h with the sulfur-free kerosene and for 30 h with the sulfurous kerosene. CO2 yields above 99% were achieved with both types of fuel. A significant and lasting improvement in the oxygen carrier's reactivity was observed, presumably an effect of using sulfurous kerosene. No evidence of sulfur was found on the particles' surface.

A project devoted to establishing chemical-looping combustion with liquid fuels currently being conducted by Chalmers University of Technology with support from Saudi Aramco is presented. The ultimate goal of the project is to develop technology capable of utilizing and processing heavy residual oils with inherent CO2 capture. Up to this point, a continuously operating reactor with the nominal effect 300W has been designed, constructed, and successfully operated with nickel-, manganese-, copper- and iron-based oxygen-carrier particles using non-sulfurous and sulfurous kerosene as fuel. The results so far are very promising, and 99% conversion of fuel carbon to CO2 has been demonstrated with all four kinds of oxygen carrier materials. Chemical-looping reforming for synthesis gas production was also demonstrated using kerosene and the Ni-based oxygen carrier. Here, complete hydrocarbon conversion to CO and H2 was achieved. Future work includes design and construction of a larger 10kW reactor system for direct combustion of heavier fuels such as fuel oil and heavy vacuum residues, as well as a techno- economic study. This paper provides an overview of the project and presents the main results and conclusions so far.