Heat Recirculation Research Articles

Reduction of CO2 emissions by recycling low-potential heat from the Benfield CO2 removal process at a natural gas hydrogen production plant

AbstractThe EU policies related to CO2 emission strictly define the stages of carbon neutrality achieving. According to these regulations, all production installations that emit carbon dioxide will be charged additional emission fees from 2026 to fully in 2035. Analysis of the increasing emission fees shows that in some industries incurring such additional costs will result in a lack of profitability of the products. Industries directly related to the food sector, such as nitrogen fertiliser production, are strategic in the economies of all countries. Nitrogen fertilisers are produced from ammonia, which is synthesised on a large scale from hydrogen and nitrogen. Hydrogen is produced by natural gas reforming with water vapour resulting in syngas (mixture of H2, CO, CO2, H2O), which CO in the next step reacts with water vapour (water gas shift reaction) producing H2 and CO2. CO2 is separated from hydrogen using the Benfield method. The analysis of the Benfield process (one process of hydrogen production) shows a possible way to reduce CO2 emission by optimising heat balance. It was shown that in the proposed technology the heat recovery reaches 89%, while the level below 30% was reported for other available technologies. The proposed solution is based on recirculation and reuse of heat, which is lost in other technologies. The analysis is for a process balance in a medium-sized hydrogen production installation. The analysis considers also the correlations with other installations thermally linked to hydrogen production. The economic balance showed the great financial benefits of this solution. In the scenario discussed, the CO2 emission factor was reduced by 20%. Graphical Abstract

Flame stabilization and emission reduction: a comprehensive study on the influence of swirl velocity in hydrogen fuel-based burner design

PurposeThis study aims to numerically analyse a full-scale burner across a wide range of operating pressure conditions and determine the effect of swirl velocity on flame stabilization, flame holding and combustion performance.Design/methodology/approachThis study uses a numerical analysis approach to investigate a three-dimensional full-scale burner. Modified governing equations are used to determine the effect of swirl velocity on flame stabilization and flame holding. The GR-Mech 3.0 chemical reaction mechanism is used to predict the combustion process. To validate the model, a grid independence study is performed.FindingsThe study reveals that swirl velocity enhances flame stability, resulting in better combustion rates. As the swirl velocity increases, higher flame temperatures are observed due to high convective heat recirculation. The heat transfer coefficient and high radiative extinction coefficient are found to vary based on fuel swirl velocity. The mass fraction of CH4 and CO emphasizes the role of swirl velocity on flame structure. Increasing velocity potentially improves combustion by delaying the process, leading to better combustion and lower emissions.Originality/valueThe findings of this study contribute to the understanding of swirl-stabilized combustion and can guide the development of advanced combustion technologies, making it a valuable addition to the existing combustion field.

Mean‐State and Seasonal Variability in Temperature Structure and Heat Transport in the East Australian Current System From a Multi‐Decadal Regional Ocean Model

AbstractWestern Boundary Currents (WBCs) such as the East Australian Current (EAC) are vital for moving heat from low to high latitudes, controlling regional weather and global climate. Previous EAC System studies have provided a general overview of temperature variability and heat transport, but they lacked spatial and seasonal detail. Using a high‐resolution, 26‐year‐long ocean model simulation, we systematically characterize the seasonal variability and structure in key temperature metrics and heat transport in the EAC System. Our findings reveal a clear seasonal cycle with a poleward expansion of key variables in summer (mean and eddy kinetic energy, upper ocean heat content, heat transport) and contraction equatorward with reductions in winter. We show that the dynamical regime transition, from jet to eddy‐dominated at the EAC separation zone (S), modifies the net meridional heat transport. Upstream of separation, transport is poleward, with some zonal input of heat. Downstream of EAC separation, there is an increase in heat recirculation by mesoscale eddies combined with a smaller zonal export of heat. This study highlights the significance of resolving spatio‐temporal variability in WBC systems driven by mesoscale dynamics, which will have implications for the representation of critical dynamics in future climate models.

Investigation on H2-fueled combustion characteristics and improvement of thermal performance in a micro-planar with arranged inlet-fins and bluff-bodies for micro-thermophotovoltaic

To improve the energy conversion and efficiency, a micro-planar with arranged inlet-fins and bluff-bodies is proposed for H2-fueled micro thermophotovoltaic system. Effects of inlet-fins and bluff-bodies setting and combustor dimensions on combustion properties, heat transfer and energy conversion are investigated and analyzed under various operating conditions. The results illustrate that the flame stability and heat transfer in combustion chamber with arranged inlet-fins and bluff-bodies are enhanced, where heat recirculation is formed and radiation temperature is boosted. According to the field synergy analysis, coupling effects of inlet-fins and bluff-bodies on burning and thermal process can be improved by the optimization of bluff-bodies setting, and the flow limit is also expanded. There is a balance of the burner dimensions effects on the augment of radiation surface area and the reduction of radiation temperature for micro power generation. Particularly, the micro-planar with La/L = 0.62 contributes to achieve a higher radiation density, and the highest electrical output Pe = 3.16 W is adopted in combustor C2-0.62 at mf = 6.751 × 10−5 kg/s. It is beneficial to promote energy output of micro-planar and the energy efficiency of micro-thermophotovoltaic system.

A numerical study comparing the performance of a self-aspirating domestic LPG porous burner and that of a conventional LPG burner

The study involves in the design of a domestic two-layer self-aspirating porous burner that operates at a thermal load of 1 kW using liquefied petroleum gas (LPG). The porous burner is intended to function within India’s standard domestic regulator fuel inlet pressure of 3000 Pa. For the same thermal load, this work also provides a numerical comparison between the domestic conventional burner and the designed porous burner. For both porous and conventional burners, a full-scale 3D model is developed to calculate the flow, combustion, heat transfer to the cooking vessel, thermal efficiency, and emissions. The combustion process of both burners is numerically computed using a detailed chemical kinetic mechanism of LPG combustion, the San Diego Mechanism (SDM) with 57 species and 268 reactions are used. The porous burner is simulated using a non-thermal equilibrium condition to better calculate the heat recirculation within the porous domain. The self-aspirated porous burner has an equivalence ratio ϕ of 0.75 at 1 kW and an efficiency of 84.2%; conventional burner at the same load had an efficiency of 68%. 10 and 6 parts per million (ppm), respectively, are the measured CO and NOx emissions from the domestic porous burner and 660 and 80 ppm for domestic conventional burner, respectively.

Flame Propagation of Premixed Gas with Enhanced Heat Recirculation: Dynamic Characteristics of Lean and Rich Combustion

Flame Propagation of Premixed Gas with Enhanced Heat Recirculation: Dynamic Characteristics of Lean and Rich Combustion

Roles of fuel composition on the ignition process of endothermic hydrocarbons

Due to the characteristics of heat absorption and decomposition, endothermic hydrocarbon fuels (EHFs) have been widely used in scramjets for thermal protection and heat recirculation. The understanding of ignition characteristics of EHFs is of great importance for their safe and efficient utilization. In this paper, the ignition processes of EHFs were numerically simulated at atmospheric pressure and with an initial temperature of 500 K. Three different ignition stages were identified based on the chemical heat release and flame kernel propagation. A 3-component kerosene surrogate model composed of n-dodecane, methyl cyclohexane and m-xylene was adopted, as well as the corresponding chemical kinetic model with 369 species and 2691 reactions. Results showed that the discrepant decomposition characteristics of n-alkanes and cycloalkanes affected the chemical heat release and propagation during the ignition process. Two-stage exothermic characteristic was observed in the time evolutions of chemical heat release rate and fuel decomposition. The mass production of molecules and accumulation of radicals dominated the first and second exothermic peaks, respectively. Furthermore, the minimum ignition energies (MIEs) of EHFs with various methyl cyclohexane were determined to quantify the effect of fuel composition on ignition performance. Characteristically, the MIE dramatically decreased from 10.2 to 2.15 mJ when 20% n-dodecane was replaced by methyl cyclohexane. However, it was slightly increased as methyl cyclohexane continued to increase. Analyses from both physical and chemical aspects were conducted to elaborate the dependence of MIE on fuel composition. The dominant effects of flame-dynamic and chemical effects on different ignition stages were analysed. The faster propagation speed and stronger endothermic ability of methyl cyclohexane led to the nonlinear variation of MIEs. The results in this study provide useful guidance for composition optimization and safety evaluation of EHFs.

Solar-driven gasification for syngas production at low temperatures using a rotary hybrid porous media reactor

Solar-based technologies play a crucial role in the energy transition towards a green circular economy. This work investigates the solar-driven steam gasification of carbon char particles at low temperatures using an indirectly irradiated rotary hybrid porous media reactor. The solar concentration system consists of a heliostat and parabolic dish with a focus on the emitter plate located at the bottom of the reactor, allowing allothermal conditions for hydrogen (H2) and syngas production. The hybrid bed is composed of carbon and alumina particles randomly mixed in a 50/50 proportion in mass. Three sets were tested varying rotation speed of the unit, i.e., Set A (0 rpm), Set B (15 rpm), and Set C (20 rpm). Numerical simulations were performed using a one-dimensional model assessing mass, energy and chemical conservation equations, considering a semi-global kinetic scheme, including three homogeneous and four heterogeneous chemical reactions. The optical design configuration of the solar system, in addition to the high heat recirculation enhanced by the rotation of the reactor, allowed maximum temperatures of 662 K (outer surface) and 573 K (reaction chamber). At this low-temperature range, a good agreement between numerical simulations and experiments was achieved, observing a H2/CO ratio of 0.088 (Set B) and 0.163 (Set C), representing a 5.7% and 3.1% deviation, respectively. Numerical results show an increase in the temperature would lead to a higher H2/CO ratio production. Further studies are necessary to assess hydrogen and syngas production under these operating conditions at high temperatures.

Thermoelectric Materials Based on Lead Telluride and Prospects for their Practical Application

Lead telluride (PbTe) is considered one of the most promising materials in thermoelectrics due to its unique thermoelectric properties. This semiconductor exhibits a high thermoelectric figure of merit (ZT) in certain temperature ranges, making it highly effective for converting heat energy into electricity. Additionally, PbTe is characterized by stability and low thermal conductivity, which further enhances the efficiency of thermoelectric devices. Another advantage of using PbTe is its relative affordability and high availability of raw materials. This makes it attractive for manufacturing mass thermoelectric devices such as thermoelectric modules for automobiles, industrial thermoelectric generators, heat recirculation, and others. The paper provides a review of works and an analysis of general approaches to semiconductor thermoelectric materials, including lead telluride.

Impact of H2 Blending of Methane on Micro-Diffusion Combustion in a Planar Micro-Combustor with Splitter

An investigation into the non-premixed combustion characteristics of methane in a planar micro-combustor with a splitter was performed. The impact of blending methane with hydrogen on these characteristics was also analyzed. Additionally, the effects of inlet velocity and global equivalence ratio on flame location, flame temperature, combustion efficiency and outer wall temperature were studied for three different fuel compositions: pure methane (MH0), 60% methane with 40% hydrogen (MH40), and 40% methane with 60% hydrogen (MH60)). A heat recirculation analysis of the combustor wall was conducted to determine the amount of heat recirculated into the unburnt gas at various inlet velocities for all three fuel compositions. The results demonstrated that the stability limit of methane in terms of inlet velocity (1–2 m/s) and global equivalence ratio (1.0–1.2) was significantly enhanced to 1–3 m/s and 0.8–1.2, respectively, with the addition of hydrogen. At an inlet velocity of 2 m/s, the flame location of 3.6 mm for MH0 was significantly improved to 2.2 mm for MH60. Additionally, outer wall temperature exhibited a rise of 100 K for MH60 compared to MH0. Furthermore, from heat recirculation analysis, when the ratio of heat recirculated to heat loss exceeded unity, the flame started exhibiting the lift-off phenomenon for all the fuel compositions.

Organosilicon elastomers of MWCNTs and nano-sized metals for heating purposes

Silicone elastomers are continuously gaining importance in a variety of industrial applications, including strain gauges, electrical heaters, electromagnetic interference, and electrostatic discharge protection coatings. But these materials have certain drawbacks such as high cost, poor resistance to solvents, low tear strength, limited high-temperature resistance, difficulty in bonding and limited abrasion resistance. Therefore, unique nanocomposites of organosilicon elastomers with metals have been prepared using MWCNTs. The prepared nanocomposites demonstrated nano-diameter structures (15–60 nm) with micron-sized fibrous structures (12–60 μm). Aluminum elongated inclusions (60 µm in size) and bronze spherical inclusions (12–30 µm) were observed in the nanocomposites. The composites containing MWCNTs, Co-Mo/Al2O3-MgO catalyst and bronze (3 wt. %) had the maximum thermal and electrical conductivity. The prepared nanocomposites were used to make the heaters. The heating of the heaters was carried out by applying a voltage of 12–24 V. The Highest temperature achieved was more than 50 °C in all the heaters. By supplying 12–24 V electricity to the heaters in the ventilation and heat recirculation system, the interior of a minibus or living space can be heated. The developed electric heater has another important economic application in temperature-controlled materials.

GJ 367b Is a Dark, Hot, Airless Sub-Earth

We present the mid-infrared (5–12 μm) phase curve of GJ 367b observed by the Mid-Infrared Instrument on the James Webb Space Telescope (JWST). GJ 367b is a hot (T eq = 1370 K), extremely dense (10.2 ± 1.3 g cm−3) sub-Earth orbiting an M dwarf on a 0.32 day orbit. We measure an eclipse depth of 79 ± 4 ppm, a nightside planet-to-star flux ratio of 4 ± 8 ppm, and a relative phase amplitude of 0.97 ± 0.10, all fully consistent with a zero-albedo planet with no heat recirculation. Such a scenario is also consistent with the phase offset of 11°E ± 5° to within 2.2σ. The emission spectrum is likewise consistent with a blackbody with no heat redistribution and a low albedo of A B ≈ 0.1, with the exception of one anomalous wavelength bin that we attribute to unexplained systematics. The emission spectrum puts few constraints on the surface composition but rules out a CO2 atmosphere ≳1 bar, an outgassed atmosphere ≳10 mbar (under heavily reducing conditions), or an outgassed atmosphere ≳0.01 mbar (under heavily oxidizing conditions). The lack of day–night heat recirculation implies that 1 bar atmospheres are ruled out for a wide range of compositions, while 0.1 bar atmospheres are consistent with the data. Taken together with the fact that most of the dayside should be molten, our JWST observations suggest that the planet must have lost the vast majority of its initial inventory of volatiles.

Direct pore-level simulation of hydrogen flame anchoring mechanisms in an inert porous media

This work addresses the stabilization of hydrogen flames inside porous media. Hydrogen is a peculiar fuel with a large molecular diffusivity, triggering preferential diffusion and other effects related to non-unity Lewis numbers. Consequently, the macro-scale behavior of hydrogen flames is highly influenced by the flame-wall interactions at the pore-scale. In this study, direct pore-level simulations are used for the investigation of anchoring mechanisms and burning-rate enhancement for lean hydrogen flames embedded in inert porous media. It is observed that preferential diffusion greatly enhances flame anchoring, in the wake of the struts, leading to a high increase in flame surface. Also, the local heat release rate is enhanced not only by the higher gas temperature resulting from the heat recirculation but also by the higher local equivalence ratio resulting from preferential diffusion. Furthermore, most of the heat produced by a hydrogen flame is released by the hydroperoxyl reactions, effective at low temperatures but necessitating radicals, mostly H. These radicals are produced by the shuffle reactions, effective at higher gas temperatures. The coupling between those two reaction groups is very dependent on the transport of radicals, which can be impacted by the pore scale velocity gradient, leading to hydrodynamic dispersion and flame wrinkling. Consequently, it is shown that the burning-rate enhancement observed within porous burners fueled with hydrogen is not only due to heat recirculation but also to increased transports of radicals and preferential diffusion effects, which should be accounted for in macroscopic models.

Computational insights into steady-state and dynamic Joule-heated reactors

Heat recirculation in Joule-heated reactors.

Experimental and computational (Chemical Kinetic + CFD) analyses of Self-Recuperative annular tubular porous burner for NH3/CH4 -air Non-Premixed combustion

Ammonia is now being explored as a potential gas turbine fuel because of its renewable and carbon-free nature. Ammonia combustion possesses various limitations, such as high NO emissions, low burning velocity, and high auto-ignition temperature. Moreover, dual-fuel combustion approaches can be used to mitigate these drawbacks. However, the limitations of available information about the resulting emissions are insufficient to encourage widespread adoption. In this work, an NH3/CH4-air fueled self-recuperative high aspect ratio annular tubular porous burner is developed for gas turbine power applications. In this burner, an annular zone surrounds the combustion chamber to preheat the incoming air, while two perforated discs are mounted between the combustor and annular zone for uniform distribution of air. Eight Zirconia foams (4 of 10 PPI and 4 of 20 PPI) are stacked at the upstream side of the combustor. Experimentally, air preheat temperature, combustor exit temperature, NO, and CO emissions are analyzed and measured for various ammonia addition (0–41 % by weight) percentages, equivalence ratio of 1.0, and heat inputs of 17 & 21 kW for 10 PPI and a combination of 10 and 20 PPI porous foams. A preheated air stream coming from the annular zone speeds up fuel dissociation and helps in stabilizing the flame on the upstream side of the combustor. Combustor exit temperatures are lower in sandwich cases of 10 + 20 PPI than the porous foams of 10 PPI. However, with porous foams of 10 PPI, the air preheat temperature is higher. The insertion of porous foams shows better recirculation of heat and stable combustion is observed for higher percentages of ammonia and higher thermal inputs. The OH radicals are distributed for larger areas inside the dumb combustor, while for porous combustion, it is distributed near the porous foams. The porous sandwich cases of 10 + 20 PPI shows higher combustion efficiency compared to 10 PPI and without porous cases. The chemical kinetic analysis shows that the HNO and HCO radicals act as the main intermediate channel for NO and CO production.

TADRP: Toward Thermal-Aware Data Replica Placement in Data-Intensive Data Centers

With the mushrooming growth of data volumes, data replica placement plays a key role in promoting the energy efficiency and Quality-of-Service (QoS) of data-intensive data centers. The existing data placement strategies mainly focus on storage performance improvement or QoS enhancement in data centers, but ignore the indispensable factor -heat recirculation. To bridge this gap, we propose a thermal-aware data replica placement strategy called TADRP, aiming to improve cooling efficiency and minimize the total power consumption of data-intensive data centers. TADRP leverages an ant colony optimization (ACO) algorithm coupled with Laplacian probability distribution to find a quasi-optimal disk sequence (or Disk Sequence for short), which consists of disks selected from different rack servers to place data replicas. TADRP categorizes disks of Disk Sequence into active and inactive ones, by placing hot and cold replicas on active and inactive disks, respectively. We quantitatively evaluate TADRP in terms of cooling costs, total power consumption, number of power-state transitions, and execution time. We compare TADRP with four alternative solutions, namely, Random, Hadoop, SRS, and CDP-NSGAII. Experimental results show that TADRP can reduce the cooling costs and the total power of the existing solutions by 14.7% -61.7% and 19.2 %-55.1%, respectively, without undesirable I/O performance drops.

On the design of a hydrogen micro-rectangular combustor for portable thermoelectric generators

Micro-combustion based thermoelectric generators have been proposed to power micro-electromechanical units. The present study aimed to optimize the thermal design of micro-combustor to best suit the requirement of thermoelectric generators. An equivalent heat transfer coefficient was used to consider the arranged thermoelectric generators and its effect was elucidated. Increasing the coefficient enhances combustor wall heat loss, reducing flame stability and uniformity index of temperature. It is thus impossible to constrain combustor outer wall temperature below the maximum allowable temperature of commercial thermoelectric generators (623K) simply through increasing equivalent heat transfer coefficient. To address this issue, three simple strategies of combustor channel modification, adding copper layer and using flue gas heat recirculation were developed. A notable improvement in uniformity index of temperature over 6.5% is achieved for the latter two strategies. Nevertheless, none of these three strategies can ensure outer wall temperature below 623K. Motivated by the different mechanisms to improve temperature uniformity and reduce wall temperature for the two simple strategies of adding copper layer and using flue gas heat recirculation, a hybrid strategy combining them was proposed. The hybrid strategy can maintain flame stability, improve uniformity index of temperature approach to 100% and control combustor outer wall temperature below 623K, making it a qualified strategy for designing high-performance micro-combustors.

Effects of channel height on the combustion efficiency and thermal performances of a micro Swiss-roll combustor

Micro combustors can be used as the heat sources for micro power generation systems. Recently, we developed a micro Swiss-roll combustor for non-premixed fuel and air. In the current work, the impacts of channel height (H2 = 7.5, 7.0, 6.5, and 6.0 mm) on the combustion efficiency of CH4, and the surface radiation efficiency of this combustor were numerically investigated under constant flow rates of the fuel and air. The results showed that both combustion efficiency and radiation efficiency varied non-monotonically with the decrease of channel height. The optimal channel height was found to be 6.5 mm under which a highest combustion efficiency of 99.05 % and a highest radiation efficiency of 27.03 % were obtained. These variation rules were theoretically analyzed in terms of the heat recirculation effect on fresh reactants, the buoyancy effect on flame structure and chemical reaction, and the non-dimensional residence time of the reactants (DaL). It was unraveled that the combustor with H2 = 6.5 mm achieved a best heat recirculation effect. In addition, the buoyancy effect was weakened as the channel height was decreased, resulting in a more symmetric flame structure in the vertical direction and a more intense chemical reaction. Furthermore, when the channel height descended from H2 = 7.5 mm to 6.5 mm, the region of DaL > 1 was broadened due to the intensified reaction, but narrowed as the channel height was further reduced to 6.0 mm due to increased flow velocity. The combined actions of these factors led to non-monotonic variations of combustion efficiency and radiation efficiency. The discoveries of the present study provided a guideline for optimal design of such kind of micro Swiss-roll combustors.

Ultra-lean premixed combustion of a holder-stabilized 80 %hydrogen–20 %methane–air mixture in a preheated mesoscale combustor

To achieve cleaner emissions, the ultra-lean premixed flames of 80 %H2–20 %CH4–air with the coupling effect of flow and heat recirculation were experimentally investigated. The flame behaviors for various Re values are observed and discussed in detail. The conventional stable flame, stable wedge-shaped residual flame, and wedge-shaped residual flame with upward and downward oscillations appeared in sequence as the equivalence ratio decreased. For a stable wedge-shaped residual flame with a decreasing Re value, the operating range broadens first and then narrows. By contrast, the operating range of the wedge-shaped residual flame with oscillations narrowed monotonically and slightly. With a decreasing Re value, the resident location of the conventional stable flame base remained nearly the same. When the equivalence ratio was reduced further, a stable wedge-shaped residual flame appeared. Subsequently, a wedge-shaped residual flame with oscillation occurred when the equivalence ratio was further reduced. Finally, the blow-off mechanisms were revealed.

Sustainable hydrogen production through coupling isopropanol-butanol-ethanol with enhanced heat recirculation

The utilization of isopropanol-butanol-ethanol (IBE) to clean energy effectively mitigated the problems of environmental pollution and sustainable energy regeneration. In order to improve the utilization efficiency of ethanol and isopropanol in IBE, a liquid double-layer porous media burner was designed. And the combustion characteristics with different porous pellets and isopropanol ratios were studied. The results indicated that the mole fraction of hydrogen decreased with the increasing pellet diameter, and the lower heating value of syngas reached the highest value of 82.0 MJ/Nm3 when the pellets possessed two holes. Meanwhile, the peak temperature reached the maximum of 1009 K when the water content was 20% due to the sufficient combustion. And the isopropanol was conducive to the improvement of the total energy conversion efficiency. In addition, the yields of hydrogen and carbon monoxide were improved by 23.7% and 30.5% when the velocity increased from 10 to 12 cm/s. These results showed that the combustion of ethanol and isopropanol in porous media burners achieved the sustainable production of hydrogen and efficient utilization of IBE biofuels.