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

The significance of beam steering on laser-induced incandescence measurements in laminar counterflow flames

Published online: 17 October 2018Beam steering is often encountered in laser diagnostic measurements, especially in flame environments, due to changes in refractive index caused by thermal and species gradients. It can negatively affect the accuracy of the results. In this work, the effects of beam steering on laser-induced incandescence (LII) measurements of pre-vaporized-liquid counterflow flames are assessed. The focus on counterflow flames is to facilitate future detailed experimental campaigns on one-dimensional nonpremixed sooty flames. It is found that the temperature and species gradients in the counterflow configuration have a much more significant impact on the beam profile than in laminar flat flames, especially for heavier fuels. As a result of the changes in the beam profile, for the same applied laser energy, the local fluence shifts markedly with fuel type, therefore, having a direct impact on the LII measurements. A procedure is developed for ensuring accurate measurements and it is shown that, for a specific fuel, it is possible to tailor the laser energy, such that the collected LII signal in the counterflow flames is nearly independent of beam-steering effects.S. Kruse, P. R. Medwell, J. Beeckmann, H. Pitsc

Effect of total solids content on anaerobic digestion of poultry litter with biochar

Methane production via anaerobic digestion of poultry litter provides a pathway for energy production from an abundant waste product. Recent studies have shown the use of biochar (pyrolysed biomass) can decrease methane production lag times and increase peak daily yields from ammonia-stressed low-solids anaerobic digesters. Due to the variety of feedstocks and digester configurations used, research to date has not yet determined the effect of biochar addition as a function of the digester total solids content. This study shows the addition of biochar reduces the lag time by a greater percentage in the digesters with a higher total solids content. There was a 17%, 27% and 41% reduction lag time due to biochar addition at total solids contents of 5%, 10% and 20%, respectively. The peak daily methane yield increased by 136% at 10% total solids. There was no significant increase in the peak yield at 5% total solids, while there was a 46% increase at 20% total solids. Real-time PCR analysis confirms the Methanosaetaceae family, which is a key methanogen due to its ability to facilitate direct interspecies electron transfer while attached to biochar, preferentially attaches to biochar. Furthermore, this research shows the attachment of the Methanosaetaceae family, does not decrease with increasing total solids content. A potential negative effect of biochar addition, a reduced volumetric efficiency, can be negated by using a shorter retention time. This new understanding will help to improve predictions of the impact of biochar addition for new digester designs operating in semi-solids and high-solids conditions.Mathu Indren, Cristian H.Birzer, Stephen P.Kidd, Paul R.Medwel

Generalisation of the eddy-dissipation concept for jet flames with low turbulence and low Damkohler number

Moderate or intense low oxygen dilution (MILD) combustion has been the focus of a range of fundamental experimental and numerical studies. Reasonable agreement between experimental and numerical investigations, however, requires finite-rate chemistry models and, often, ad hoc model adjustment. To remedy this, an adaptive eddy dissipation concept (EDC) combustion model has previously been developed to target conditions encountered in MILD combustion; however, this model relies on a simplified, pre-defined assumption about the combustion chemistry. The present paper reports a generalised version of the modified EDC model without the need for an assumed, single-step chemical reaction or ad hoc coefficient tuning. The results show good agreement with experimental measurements of two CH4/H2 flames in hot coflows, showing improvements over the standard EDC model as well as the previously published modified EDC model. The updated version of the EDC model also demonstrates the capacity to reproduce the downstream transition in flame structure of a MILD jet flame seen experimentally, but which has previously proven challenging to capture computationally. Analyses of the previously identified dominant heat-release reactions provide insight into the structural differences between a conventional autoignitive flame and a flame in the MILD combustion regime, whilst highlighting the requirement for a generalised EDC combustion model.M.J.Evans, C.Petre, P.R.Medwell, A.Parent

Characteristics of turbulent flames in a confined and pressurised jet-in-hot-coflow combustor

Available online 16 August 2022Combustion in hot and low oxygen environments—such as those encountered in practical devices including inter-turbine burners and sequential gas turbines—is not yet fully understood at a fundamental level, particularly in terms of the effects of pressure. To meet this gap in understanding, a confined-and-pressurised jet-in-hotcoflow (CP-JHC) combustor has been developed to facilitate optical diagnostics of turbulent flames in hot and vitiated coflows for the studies of flame stabilisation, structure and soot formation at elevated pressures. The CPJHC burner has been designed for steady operation at 10 bar with internal temperatures of up to 1975 K, with a water-cooled central jet issuing into a hot oxidant stream of combustion products from a non-premixed natural gas/H2 burner. This work describes the key features and operational capabilities of the CP-JHC burner and presents a selection of experimental results showing characteristics not previously available. Specifically, temperature measurements of the hot coflow are used to estimate the enthalpy deficit of the stream, revealing an increase in thermal efficiency with increasing heat input, and a decrease with increasing pressure. Chemiluminescence imaging of OH* and CH* is performed for turbulent jet flames to study the flame structure under various operating conditions, and true-colour imaging results are also included to highlight the change in soot formation under elevated pressures. The mean images indicate a change in stabilisation behaviour with changes in pressure and jet Reynolds number (Rejet), which is further investigated by a statistical analysis of the shortexposure CH* images. This analysis reveals that an increase in Rejet from 10,000 to 15,000 leads to an increase in the mean lift-off height (from the jet exit plane) from approximately 1.5 to 6 jet diameters at atmospheric pressure, while the flames at elevated pressures show significantly less variation and tend to stabilise at the jet exit for P > 3.5 bar(a). The experimental findings are complemented by numerical simulations of laminar opposed flow flames, providing additional insights into the fundamental chemical kinetics effects which influence these flames. In particular, a monotonic reduction in both the maximum and integrated OH* and CH* mass fractions is observed with increasing pressure. This reduction is particularly pronounced at lower pressures, with a reduction to 10% of the atmospheric-pressure value at 3 bar(a) for the integrated OH* mass fraction. Additionally, this behaviour is shown to be related to the combined effects of a shift in the formation pathways and the increased impact of collisional quenching.D.B. Proud, M.J. Evans, Q.N. Chan, P.R. Medwel

Small-scale autothermal thermochemical conversion of multiple solid biomass feedstock

The thermochemical conversion of four types of biomass in a batch-fed reverse downdraft process for heat generation in cookstoves is investigated. Fuel switching is widely considered inefficient because many combustion devices do not respond well to changes in fuel. Here, the use of agricultural by-products, represented by wheat straw, sheep manure, cow manure, and wood pellets is addressed. Two air supply rates within the oxygen-limited regime, where the fuel consumption is linearly dependent on the air supply, are investigated. At higher air supply rates, in the reaction-limited regime, low carbon yields lead to the exposure of the ash fraction to high temperatures, such that the resultant ash melting has detrimental effects on the process. Generally, no detrimental impact of the ash content on the conversion process within the oxygen-limited regime could be identified. The release of gaseous products, evaluated through cold gas efficiency, increases linearly from 24 to 54% with higher air flow, corresponding to increasing process temperatures from 690 to 980 °C, and is largely fuel type independent. The char produced from all feedstocks fall within the highest classification for biochars, based on its elemental composition and determined by international protocols. This emphasises the potential of the investigated process for a combined production of producer gas and biochar from a variety of low-value biomass feedstocks.Thomas Kirch, Paul R. Medwell, Cristian H. Birzer, Philip J. van Ey

Co-combustion characteristics and kinetics of microalgae chlorella vulgaris and coal through TGA

Published online: 26 Dec 2018The combustion characteristics and kinetics of microalgae (Chlorella vulgaris) and sub-bituminous coal blends (CCBs) are studied by a thermogravimetric analyzer (TGA), and those of pure Chlorella vulgaris (C. vulgaris) and coal were also taken respectively as control groups. The microalgae to coal blending ratio (MCR) is 3/7, 5/5 and 7/3. The results showed that three stages were observed during the combustion of CCBs. And the main combustion of CCBs was occurred at the second stage ranged from 254.6 ~ 389.4°C to 698.7 ~ 741.0°C. Both of the ignition temperature (Ti) and the final temperature detected when stabilization of samples mass occurred (Tf) were decreased as the C. vulgaris content increased in the CCB. The maximum combustion rate (Rmax) of C. vulgaris was maximum. The average reaction rate (Rv) was firstly decreased, and then increased as the content of C. vulgaris in CCBs increasing. With the increasing content of C. vulgaris, both of the ignition index (Di) and the comprehensive combustion characteristic index for the blends (SM) were increased. Some deviations from their expected characteristics indicate interaction. As the heating rate (β) increases, Ti, the peak temperature (Tp), the reaction rate at the peaks (Rp), Rv and Tf were all increased significantly, while the residual mass (Mr) was first increased, and then decreased. For CCBs, the activation energy (E) was the first decreased, and then increased, and the minimum E was obtained as MCR = 5/5. Among all the samples, E of pure coal was the minimum one. Finally, kinetic triplets were determined by the Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO), and master-plots method, they are respectively E = 62.90, 108.99, 85.28, 92.27, 104.98 kJ/mol, the reaction order (n) = 1.4, 4.1, 2.7, 3.2, 4 and the frequency factor (A) = 6.38 × 105, 1.05 × 106, 2.29 × 104, 8.73 × 104, 2.93 × 106 min−1 for the coal, blends with MCR of 3/7, 5/5, 7/3 and C. vulgaris combustion at β = 20°C/min.Chunxiang Chen, Qing Nian Chan, Paul R. Medwell and Guan Heng Yeo

Effects of biochar parent material and microbial pre-loading in biochar-amended high-solids anaerobic digestion

This study characterises the effect of biochar (pyrolysed biomass) produced from wood pellets, wheat straw and sheep manure on high-solids anaerobic digestion (HSAD) of poultry litter. Also, pre-loading biochar with microorganisms before addition to HSADs was investigated. The addition of wood pellet biochar provides a 32% increase to the methane yield compared with control digesters. The addition of biochar produced from either wheat straw or sheep manure has detrimental effects on digester performance compared with controls. The addition of wood pellet biochar pre-loaded by placing it in a high-solids digester for 90 days provides a 69% increase in the total methane yield, 44% increase in the peak daily methane yield and a 33% reduction in the lag time compared with controls. This study highlighted a need for careful selection of parent material for biochar production and, for the first time, the opportunities to re-use wood pellet biochar for further improvements.Mathu Indren, Cristian H. Birzer, Stephen P. Kidd, Tony Hall, Paul R. Medwel

Hydrogen addition to a commercial self-aspirating burner and assessment of a practical burner modification strategy to improve performance

Available online 5 July 2023. OnlinePublThe ability for existing burners to operate safely and efficiently on hydrogen-blended fuels is a primary concern for the many industries looking to adopt hydrogen as an alternative fuel. This study investigates the efficacy of increasing fuel injector diameter as a simple modification strategy to extend the hydrogen-blending limits before flashback. The collateral effects of this modification are quantified with respect to a set of key performance criteria. The results show that the unmodified burner can sustain up to 50 vol% hydrogen addition before flashback. Increasing the fuel injector diameter reduces primary aeration, allowing for stable operation on up to 100% hydrogen. The flame length, visibility and radiant heat transfer properties are all increased as a result of the reduced air entrainment with a trade-off reported for NOx emissions, where, in addition to the effects of hydrogen, reducing air entrainment further increases NOx emissions.Adam J. Gee, Douglas B. Proud, Neil Smith, Alfonso Chinnici, Paul R. Medwel

The influence of biochar position in a leach bed system anaerobically digesting chicken litter

As a consequence of the rapidly growing poultry industry, chicken litter is becoming an abundant and problematic waste. Anaerobic digestion of chicken litter can mitigate environmental issues while producing valuable by-products. Recent studies have shown that leach bed reactor (LBR) systems are suitable for processing chicken litter and that anaerobic digestion can be enhanced using biochar. This study investigates the influence of biochar position within an LBR system on anaerobic digestion of chicken litter. Compared to a system without biochar, application of biochar in both the LBR (mixed in with the feedstock or as a layer below the feedstock) and coupled leachate tank (LT) increased methane yield by 6 to 8% at 51 days and accelerated VFA degradation and methane production. More significant differences in methane yield were observed at shorter solid retention times. Biochar mixed in feedstock in addition to a filter in the LT performed best in terms of both methane and hydrogen sulfide production, with a 77% reduction in hydrogen sulfide yield and hydrogen sulfide contents maintained below 500 ppm. The enhanced rates of VFA degradation and methane production when applying biochar in both reactors corresponds with observed differences in the methanogen population. Biochar application in both reactors increased the abundance of Methanobacteriales in digestate and Methanosarcinaceae in leachate compared to the control. Microbial attachment and activity on biochar also increased when mixed in feedstock. Increased diversity of the methanogen population throughout the system, as well as increased activity on biochar, may have facilitated the syntrophic relationship between acetogenic bacteria and methanogens, thus accelerating VFA degradation and methane production. These results suggest mixing biochar in feedstock, in addition to a biochar filter in the LT, to enhance anaerobic digestion of chicken litter in this system.Ben A. Collins, Cristian H. Birzer, Stephen P. Kidd, Tony Hall, Paul R. Medwel