Clean Combustion Research Center

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  • Article

    Characterizing turbulent non-premixed flame structure and pollutant formation of cracked ammonia jet flames using simultaneous NH and NO PLIF

    (Elsevier BV, 2024-03-14) Wang, Guoqing; Roberts, William L.; Guiberti, Thibault; Clean Combustion Research Center; Physical Science and Engineering (PSE) Division; Mechanical Engineering Program; School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China

    The combustion of cracked ammonia is crucial for enhancing flame stability and reducing pollutant emissions in ammonia-based combustion systems. This study investigated the flame structure and pollutant formation in cracked ammonia jet flames (CAJFs) to improve our understanding of the turbulence-chemistry interactions in ammonia combustion. A simultaneous NH and NO planar laser-induced fluorescence (PLIF) technique was employed to analyze the flame structure of turbulent non-premixed CAJFs, emulated using ammonia-hydrogen–nitrogen mixtures. Experimental measurements were conducted across a range of pressures (1–5 bar) and cracking ratios (7 %-28 %). The results revealed the significant influence of the cracking ratio on the chemical reactivity and NH-layer characteristics of ammonia-hydrogen flames. NH effectively marked the heat release layer of CAJFs. Reducing the cracking ratio from 28 % to 7 % resulted in increased local extinction-induced NH-layer fragmentation and reduced reaction area. The reaction layer thickness exhibited low sensitivity to the cracking ratio and pressure, while the broadening of the NH layer displayed a slowdown pattern with increasing height, differing from premixed flames. The pollutant NO rapidly formed within the reaction zone, persisting in the outer hot products until reduced by turbulent mixing. The fragmented flame structure promoted decreased NO formation and potentially lower NO concentration. A novel approach of multiplying NH and NO was proposed to reflect the formation characteristics of another important pollutant N2O. The effect of turbulent disturbances on N2O formation needs to be considered in turbulent CAJFs because N2O formation is strongly suppressed when turbulent transport reduces local NO concentration. This research enhances understanding of turbulence-chemistry interactions in cracked ammonia combustion, benefiting ammonia flame stability and pollutant emission control.

  • Article

    Direct learning of improved control policies from historical plant data

    (Elsevier BV, 2024-03) Alhazmi, Khalid; Sarathy, Mani; Chemical Engineering Program; Physical Science and Engineering (PSE) Division; Clean Combustion Research Center

    The continuous optimization of the operational performance of chemical plants is of fundamental importance. This research proposes a method that utilizes policy-constrained offline reinforcement learning to learn improved control policies from abundant historical plant data available in industrial settings. As a case study, historical data is generated from a nonlinear chemical system controlled by an economic model predictive controller (EMPC). However, the method’s principles are broadly applicable. Theoretically, it is demonstrated that the learning-based controller inherits stability guarantees from the baseline EMPC. Experimentally, we validate that our method enhances the optimality of the baseline controller while preserving stability, improving the baseline policy by 1% to 20%. The results of this study offer a promising direction for the general improvement of advanced control systems, both data-informed and stability-guaranteed.

  • Article

    Cutting off the upstream and downstream costs for CO2 electroreduction by upcycling fermentation emissions into ethanol

    (Royal Society of Chemistry (RSC), 2024) Sun, Ruofan; Zhao, Jiwu; Lu, Xu; Material Science and Engineering Program; Physical Science and Engineering (PSE) Division; Mechanical Engineering Program; Clean Combustion Research Center; KAUST Solar Center (KSC)

    Electrochemical reduction of CO2 (CO2RR), when powered by renewables, opens up a new avenue to mitigate the greenhouse gas while producing value sustainably. Nevertheless, this technology has been largely limited by the high costs of the upstream CO2 feed and downstream product separation. Here we report a hybrid bio-electrochemical system, integrating yeast fermentation with CO2RR in one single cell, that upcycles the fermentation-emitted CO2 into ethanol. We engineer a CuO-Ag tandem electrocatalyst with rationally designed CuO-Ag interfaces that pose minimal impact on the yeast, while efficiently converting CO2 into ethanol against side reactions, such as hydrogen evolution and glucose reduction. We showcase the win-win model enabled by this hybrid system—the CO2RR cost can be cut by 17.8% because the fermentation process provides a free, high-purity CO2 source and free ethanol distillation and in return, the CO2RR reduces the CO2 emissions of fermentation and increases the final ethanol product concentration. This proof-of-concept procedure sheds light on a tempting possibility for a cost-effective CO2 value chain.

  • Article

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

    (Elsevier BV, 2024-03-05) Yin, Yilong; Medwell, Paul R.; Dally, Bassam; Mechanical Engineering Program; Clean Combustion Research Center; Physical Science and Engineering (PSE) Division; School of Electrical and Mechanical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia

    The 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.

  • Conference Paper

    Combustion analysis of ammonia and methanol fuels and their blends in an optical spark-ignition engine

    (CRC Press, 2023-11-07) Uddeen, Kalim; Tang, Qinglong; Shi, Hao; Almatrafi, Fahad; Turner, James W.G.; Mechanical Engineering Program; Physical Science and Engineering (PSE) Division; Clean Combustion Research Center; School of Mechanical Engineering, Tianjin University, Tianjin, China; College of Physical Sciences and Engineering, Cardiff University, Cardiff, UK

    In order to meet global decarbonization targets, modern internal combustion engine operations will demand the use of either carbon-neutral fuels or fuels with lower fossil hydrocarbon content. There are several potential fuels such as ammonia (NH3), hydrogen (H2), and methanol (CH3OH), which are considered potentially revolutionary fuels for advanced IC engines in terms of their being able to be made in a netzero- carbon or carbon-neutral manner. This study investigates the combustion characteristics of pure ammonia, methanol, and their blends in a spark-ignition research engine. The results showed that pure ammonia combustion produced highly unstable combustion with lower engine efficiency even at advanced spark timing (ST) due to its lower burning velocity. However, methanol combustion exhibited higher engine performance for the same operating conditions due to its higher burning speed. In addition, this study also investigated the effect of methanol usage with ammonia fuel. Adding methanol to ammonia improved the combustion characteristics significantly because it increased the reactivity of the mixture, which further raised the higher heat release rate (HRR) and in-cylinder pressure. Furthermore, a highspeed natural flame luminosity (NFL) imaging technique was used to capture the flame propagation for different ammonia/methanol blending cases. Moreover, it was also observed that increasing the methanol fraction mixture leads to higher NOx emissions because of both fuel-bound NOx from arising from the nitrogen in ammonia and thermal the NOx due to the higher in-cylinder temperature generated by the higher heat release.