Hydrogen turbulent nonpremixed flames blended with spray or prevapourised biofuels

Available online 6 April 2023The low radiant intensity of hydrogen flames may be enhanced by adding biofuels with a high sooting propensity. This paper reports the effect of biofuel concentration and phase on the combustion characteristics of turbulent nonpremixed hydrogen-based flames. The 0.2 and 1 mol% vapourised/spray biofuel surrogates blended flames exhibit limited soot loading, except for 1 mol% spray toluene and anisole blends where soot starts to form. Spray additives benefit the formation of soot by creating localised fuel-rich conditions. Blending 3.5 and 4 mol% vapourised toluene attains a sooting flame and significantly enhances the luminosity and radiant fraction. The global NOx emissions increase with prevapourised/spray biofuel surrogates due to the enhanced NO formation via thermal and prompt routes. Reducing the hydrogen concentration from 9:1 to 7:3 in H2/N2 (by mole) leads to large increases in luminosity and radiant fraction by 34 times and 135%, respectively, and a reduction in NOx emissions by 68%.Yilong Yin, Paul R. Medwell, Bassam B. Dall

Structural differences of ethanol and DME jet flames in a hot diluted coflow

This study compares the flame structure of ethanol and dimethyl ether (DME) in a hot and diluted ox- idiser experimentally and computationally. Experiments were conducted on a Jet in Hot Coflow (JHC) burner, with the fuel jet issuing into a 1250-K coflow at three oxygen levels. Planar measurements using OH-LIF, CH 2 O-LIF, and Rayleigh scattering images reveal that the overall spatial distribution and evolution of OH, CH 2 O, and temperature were quite similar for the two fuels. For both the ethanol and the DME flames, a transitional flame structure occurred as the coflow oxygen level increased from 3% to 9%. This indicates that the flames shift away from the MILD combustion regime. Reaction flux analyses of ethanol and DME were performed with the OPPDIF code, and ethane (C 2 H 6 ) was also included in the analyses for comparison. These analyses reveal that the H 2 /O 2 pathways are very important for both ethanol and DME in the 3% O 2 cases. In contrast, the importance of fuel-specific reactions overtakes that of H 2 /O 2 reactions when fuels are burnt in the cold air or in the vitiated oxidant stream with 9% O 2 . Unsteady laminar flamelet analyses were also performed to investigate the ignition processes and help interpret experimental results. Flamelet equations were solved in time and mixture fraction field, which was pro- vided by non-reactive Large-Eddy Simulation (LES).Jingjing Ye, Paul R. Medwell, Konstantin Kleinheinz, Michael J. Evans, Bassam B. Dally, Heinz G. Pitsc

Soot structure and flow characteristics in turbulent non-premixed methane flames stabilised on a bluff-body

The soot properties of methane in turbulent regimes are not well characterised but are highly desirable. Methane is the main constituent of natural gas that is broadly used in many industrial combustors. Investigation of turbulent methane flames under well-defined boundary conditions is therefore useful for interpreting soot formation in practical burners and can be used for further model development. This study presents a joint experimental and numerical study of a series of turbulent non-premixed bluff-body flames fuelled with pure methane for three values of the momentum flux ratio of fuel jet to co-flowing air. Soot volume fraction (SVF) and flowfield are measured simultaneously using planar laser-induced incandescence (P-LII) and 2D-polarised particle image velocimetry (P-PIV). Additionally, time-averaged temperature, mixture fraction, OH and C2H2 concentrations are estimated numerically using RANS models. The global flame structure for all three flames features a recirculation zone with a double-vortex structure, a jet-propagating zone, and a neck zone connecting the two regions. The soot distribution within the recirculation zone shows clear distinct features, which is attributed to the mean mixture fraction distribution in this zone. Increasing the momentum flux ratio shifts the location of the mean stoichiometric mixture fraction to the rich inner vortex core, leading to a distinct peak of the total integrated soot in the inner vortex of the recirculation zone that is not observed in other cases. Also, it is deduced that the soot inception starts earlier in the recirculation zone for the flame with the highest momentum flux ratio and in the jet zone for the other two flames. Much higher soot concentration and lower intermittency are found with ethylene-based flames stabilised on the same burner and with the same operating conditions. In addition, the study has generated a database of soot and flowfield results, which can be helpful for future model validations.Amir Rowhani, Zhiwei Sun, Alfonso Chinnici, Paul R. Medwell, Graham J. Nathan, Bassam B. Dall

Experimental investigation of the influence of solar-to-fuel ratio on performance and stability characteristics of hybrid solar-MILD hydrogen processes

This study presents an investigation of the influence of solar-to-fuel energy input ratio (S/F) on performance and stability characteristics of hybrid processes of solar and MILD combustion of H2. A laboratory-scale MILD Hybrid Solar Receiver Combustor was operated at 8-kWth capacity under MILD combustion and in the mixed-mode (MILD plus solar energy simultaneously). An 18-kWe three-lamp metal-halide solar simulator and the combustion of pure hydrogen were used as energy sources. The global combustion performance and stability limits for each mode of operation are reported for different levels of heat extraction and S/F values in the range 5–25%. It was found that similar thermal performance can be achieved for both modes across a wide range of conditions, together with steady operation in response to transients, indicating for the first time that MILD combustion can be used to efficiently compensate for variability in the solar resource, reduce thermal stresses and guarantees constant output. Steady solar-MILD operations retain similar features of conventional MILD processes (nearly-zero emissions, thermal field uniformity) even at relatively high S/F ratio. The global combustion characteristics, performance and stability limits are found to correlate with S/F in the mixed mode, while the operability region for which steady MILD processes can occur was found to increase significantly by adding high-flux concentrated solar radiation to the combustion process and by increasing S/F.A. Chinnici, G.J. Nathan , B.B. Dall

An experimental study of the stability and performance characteristics of a Hybrid Solar Receiver Combustor operated in the MILD combustion regime

This study describes the performance and stability characteristics of a Hybrid Solar Receiver Combustor operated in the Moderate or Intense Low oxygen Dilution (MILD) combustion regime, in which the functions of a solar receiver and a combustor are integrated into a single device. The device was built and tested at a nominal capacity of 20 kWth for both the combustion-only (MILD and conventional combustion) and mixed-mode (a combination of both solar and combustion). Here, a 5 kWel xenon-arc solar simulator and natural gas were used as the energy sources, while the combustion mode was operated in the MILD combustion regime. The thermal efficiency, wall cavity temperature, heat flux distribution within the cavity and pollutant emissions are reported for the two modes of operation for a range of energy input, equivalence ratio, heat extraction, air preheat and solar-to-fuel energy input ratio. The stability limits for stable operations are also identified for each mode of operation. It was found that MILD combustion can be successfully stabilised within the HSRC in a wide range of operating conditions with and without air preheating, and in the mixed-mode of operation, providing ultra-low NOx and CO emissions. Also, the stability limits were found to increase by adding concentrated solar radiation to the combustion process. The thermal performance was found to be similar in both combustion-only (conventional combustion and MILD) and mixed-mode (up to ≈ 88% assuming reasonable heat recovery from the exhaust gas), confirming that an overall benefit can be derived from the device.A.Chinnici, G.J.Nathan, B.B.Dall

Experimental and numerical study of the influence of syngas composition on the performance and stability of a laboratory-scale MILD combustor

This work presents the thermal performance, emissions and stability limits for steady-operations of a laboratory-scale combustor operating in the Moderate or Intense Low oxygen Dilution (MILD) combustion regime and fed with syngas fuels. The device was operated at 12-kWth using syngas with different H2/CO composition (H2/CO by v/v = 1–3). The global performance of the device, namely thermal efficiency, pollutant emissions, heat flux distribution and stability limits, were measured as a function of the heat extracted. It was found that the MILD combustion regime can be achieved over a broad range of fuel composition and operating parameters, with nearly-zero NOx and CO emissions. An increase in the H2/CO ratio of the syngas stream was found to increase both the stability limits for steady MILD processes and the NOx emissions. Irrespective of the fuel type, similar thermal efficiencies were measured for all cases investigated, providing evidence that the device can efficiently (i) operate with low-calorific fuels and (ii) accommodate for variability in the composition of the syngas fuel stream. In addition, the numerical analysis highlighted that the syngas composition strongly influences the rate of radiative heat transfer and reactions as well as the characteristics of the reaction zone.A. Chinnici, G.J. Nathan, B.B. Dall

Radiating biofuel-blended turbulent nonpremixed hydrogen flames on a coaxial spray burner

Available online 5 March 2024The low radiant intensity and luminosity of hydrogen flames can be enhanced by the addition of a small portion of sooting biofuels. To achieve higher effectiveness, the impact of blending turbulent nonpremixed hydrogen flames with liquid biofuels, by gas-assist atomisation, is investigated and compared with the introduction methods of prevapourisation and ultrasonic spray. The flame appearance, luminosity, radiant fraction, centreline temperature, and the near-field spray characteristics of four biofuel surrogates (eucalyptol, D-limonene, guaiacol, and anisole) blended into hydrogen flames are measured experimentally. Radiating biofuel/hydrogen flames are achieved on a coaxial needle spray burner by the addition of 0.1–0.3 mol% biofuel surrogates. Compared with the unblended hydrogen flame, the luminosity and radiant fraction are enhanced by 30%–500% and 2%–15%, respectively, with the addition of biofuel surrogates. The results show that adding the biofuel surrogates by gas-assist atomisation is more effective than prevapourisation and ultrasonic atomisation in luminosity and radiant fraction enhancement. It is found that the local fuel-rich conditions, which are beneficial for soot formation, are further facilitated by the larger droplets and spray objects generated by gas-assist atomisation. Of the additives tested, anisole is the most effective for luminosity and radiant fraction enhancement of a hydrogen flame while exhibiting the largest flame temperature drop due to the enthalpy of vapourisation and the radiative loss from the promoted soot formation. The viscosity and surface tension greatly influence the spray characteristics which in turn impacts the flame characteristics. Guaiacol, the representative of lignin, appears to have the lowest effectiveness in radiant fraction enhancement due to the presence of a hydroxy group, a higher bond dissociation enthalpy, and a coarser spray ascribed to higher viscosity and surface tension.Yilong Yin, Paul R. Medwell, Bassam B. Dall

First-of-a-kind demonstration of a direct hybrid between a solar receiver and the radiant burner technology

The use of hybrid solar thermal devices, which harness the energy from both concentrated solar radiation and combustion, is receiving growing attention due to their potential to provide a firm and dispatchable thermal energy supply while lowering the costs of energy systems and assisting the penetration of renewable energy. Direct hybrids, which integrate the functions of both a solar receiver and a combustor into a single device, feature the greatest potential in terms of high efficiency, cost-reduction and flexibility in operation, among all the hybrid design proposed to date. In this study, we propose a novel concept of direct hybrid, which integrate the functions of the radiant burner, RB, technology into a billboard-type solar receiver. RB technology was selected here given its potential to provide high radiative heat transfer and heat fluxes similar to those of concentrated solar radiation. First-of-a-kind, systematic measurements of the performance of a 20-kW laboratory-scale Hybrid Solar Radiant Burner Receiver, HSRBR, unit are reported here under simulated solar conditions and using natural gas as energy source for combustion and mixed operations. It was found that the device can achieve thermal efficiency of up to 80% and provide firm supply of hot air of up to 700 °C. Also, it was found that the RB can efficiently manage transients with a fast response time, so that the device can provide both a steady thermal output and a constant outlet temperature of the heat transfer fluid. The location of the RB within the solar receiver (either in the front or back of the billboard-like heat exchanger) was found to have only a small influence on the performance of the device, indicating a great design flexibility.Alfonso Chinnicia), Graham J. Nathan, and Bassam B. Dall

MILD combustions at different fuel-air patterns

This paper experimentally investigates the global performances of the non-premixed (NP), partially-premixed (PP) and fully-premixed (FP) MILD (moderate or intense low-oxygen dilution) combustions at a laboratory-scale furnace. Experiments are carried out at the firing rate varying from 7.5 kW to 15 kW and the equivalence ratio changing from φ = 0.5 to φ = 1. The furnace temperatures and NOx emissions of the three premixing patterns are compared and discussed. The PP MILD combustion produces the highest NOx emissions. The exhaust temperatures of all the cases are found to decrease as φ increases. For the present furnace and burner configuration at φ < 0.97, the variation of the premixing modes or firing rates does not influence the CO emissions, and the establishment of the MILD combustion can guarantee the fully oxidation of the fuel. The Damköhler number (Da) of the MILD combustion is found to be ∼O(1). Moreover, the reaction regime of the FP MILD combustion is found to be located on the flamelets-in-eddies regime of turbulent combustion.Pengfei Li, Jianchun Mi, Bassam B. Dall