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A special issue of Energies (ISSN 1996-1073). This special issue belongs to the section "B: Energy and Environment".

Deadline for manuscript submissions: closed (31 March 2019) | Viewed by 47791

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Special Issue Editors

Prof. Dr. Derek Dunn-Rankin

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Guest Editor

Mechanical and Aerospace Engineering Department, University of California, Irvine, CA 92697, USA
Interests: combustion; sprays; particle transport; non-intrusive optical diagnostics

Dr. Yu-Chien Chien

E-Mail Website
Guest Editor

Mechanical and Aerospace Engineering Department, University of California, Irvine, CA 92697, USA
Interests: combustion; flames; energy systems; emissions

Special Issue Information

Dear Colleagues,

The Special Issue in Energies on “Cleaner Combustion” focuses on how the combination of fuel treatment, effective energy extraction, and emission mitigation in combustion can be optimized to reduce the environmental impact that threatens living systems because humans rely heavily on energy for basic necessities and economic development. We refer to “cleaner combustion” as an explicit acknowledgment that any improvements in this critical technology can have a significant global impact because of the ubiquitous nature of combustion. A growing population and growing standards of living have produced explosive growth in energy demand, and the environmental upset has started feeding back. This Special Issue includes a spectrum of research on cleaner combustion that helps balance our needs for energy with cognizant respect for the environment.

The goal of this collection is to identify the challenges and improvements of existing energy generation strategies in combustion, as well as reaching various pathways that resolve combustion emissions. The topics range from combustion and flame research that directly or indirectly impact the optimization of combustion processes using conventional carbon-based or hydrocarbon fuels, to options leading to a cleaner combustion outcome by using alternative fuels, biofuels, or low carbon fuels, or including new methods to process exhausted gases.

Prof. Dr. Derek Dunn-Rankin
Dr. Yu-Chien Chien
Guest Editors

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Keywords

cleaner combustion
low NOx
low carbon fuel
biofuel
alternative fuel
exhaust gas cleanup
emission control
MILD combustion
highly preheated combustion
oxy-fuel
chemical looping

Published Papers (13 papers)

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Research

18 pages, 9164 KiB

Open AccessArticle

An Experimental and Numerical Study on Supported Ultra-Lean Methane Combustion

by Ho-Chuan Lin, Guan-Bang Chen, Fang-Hsien Wu, Hong-Yeng Li and Yei-Chin Chao

Energies 2019, 12(11), 2168; https://doi.org/10.3390/en12112168 - 06 Jun 2019

Cited by 5 | Viewed by 2990

Abstract

With a much larger global warming potential (GWP) and much shorter lifespan, the reduction of methane emissions offers an additional opportunity and a relatively quick way of mitigating climate change in the near future. However, the emissions from coal mining in the form of ventilation air methane (VAM), usually in ultra-lean concentration, pose the most significant technical challenge to the mitigation of methane emission. Therefore, a better understanding of ultra-lean methane combustion is essential. With three 5 mm × 50 mm rectangle cross-section slot jets, a novel sandwich-type triple-jet burner is proposed to provide stable combustion of an ultra-lean methane–air mixture with equivalence ratios from 0.3 to 0.88, and 0.22 in extreme conditions. The ultra-lean methane flame in the center of the triple-jet burner is supported by the two lean outer flames at an equivalence ratio φ = 0.88. The flow field and combustion chemical reactions are predicted by detailed numerical simulation with GRI-Mech 3.0 reaction mechanisms. Two-dimensional numerical results are validated with those obtained by experimental particle image velocimetry (PIV), as well as visual flame height and temperature measurements. An ultra-lean methane–air mixture has to burn with external support. In addition, the ultra-lean flame is non-propagating with a relatively low temperature. The ultra-lean center flame is seen to start from the outer flame and incline perfectly to the post-flame temperature and OH concentration profiles of the outer lean flame. The adjacent stronger flame provides heat and active radicals, such as OH and HO₂, from the post-flame region and in the wall proximity of the gap between the adjacent flame and the central ultra-lean jet to initiate and maintain the combustion of the central ultra-lean flame. The outstanding wall-proximity radical of HO₂ is found to be the main contributor to the initiation and stabilization of the central ultra-lean flame by providing a low-temperature oxidation of fuel through the following reaction: HO₂ + CH₃ ⇔ OH + CH₃O. The major chemical reaction paths contributing to fuel decomposition and oxidation of the supported ultra-lean center flame are also identified and delineated. Full article

(This article belongs to the Special Issue Cleaner Combustion)

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11 pages, 3748 KiB

Open AccessArticle

Combustion Characteristics of Methane Hydrate Flames

by Yu-Chien Chien and Derek Dunn-Rankin

Energies 2019, 12(10), 1939; https://doi.org/10.3390/en12101939 - 21 May 2019

Cited by 38 | Viewed by 4691

Abstract

This research studies the structure of flames that use laboratory-produced methane hydrates as fuel, specifically for the purpose of identifying their key combustion characteristics. Combustion of a methane hydrate involves multiple phase changes, as large quantities of solid clathrate transform into fuel gas, water vapor, and liquid water during burning. With its unique and stable fuel energy storage capability, studies in combustion are focused on the potential usage of hydrates as an alternative fuel source or on their fire safety. Considering methane hydrate as a conventional combustion energy resource and studying hydrate combustion using canonical experimental configurations or methodology are challenges. This paper presents methane hydrate flame geometries from the time they can be ignited through their extinguishment. Ignition and burning behavior depend on the hydrate initial temperature and whether the clathrates are chunks or monolithic shapes. These behaviors are the subject of this research. Physical properties that affect methane hydrate in burning can include packing density, clathrate fraction, and surface area. Each of these modifies the time or the temperature needed to ignite the hydrate flames as well as their subsequent burning rate, thus every effort is made to keep consistent samples. Visualization methods used in combustion help identify flame characteristics, including pure flame images that give reaction zone size and shape and hydrate flame spectra to identify important species. The results help describe links between hydrate fuel characteristics and their resulting flames. Full article

(This article belongs to the Special Issue Cleaner Combustion)

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13 pages, 4100 KiB

Open AccessArticle

General Correlations of Iso-octane Turbulent Burning Velocities Relevant to Spark Ignition Engines

by Minh Tien Nguyen, Dewei Yu, Chunyen Chen and Shenqyang (Steven) Shy

Energies 2019, 12(10), 1848; https://doi.org/10.3390/en12101848 - 15 May 2019

Cited by 6 | Viewed by 2696

Abstract

A better understanding of turbulent premixed flame propagation is the key for improving the efficiency of fuel consumption and reducing the emissions of spark ignition gasoline engines. In this study, we measure turbulent burning velocities (S_T) of pre-vaporized iso-octane/air mixtures over wide ranges of the equivalence ratio (φ = 0.9–1.25, Le ≈ 2.94–0.93), the root-mean-square (r.m.s.) turbulent fluctuating velocity (u′ = 0–4.2 m/s), pressure p = 1–5 atm at T = 358 K and p = 0.5–3 atm at T = 373 K, where Le is the effective Lewis number. Results show that at any fixed p, T and u′, Le < 1 flames propagate faster than Le > 1 flames, of which the normalized iso-octane S_T/S_L data versus u′/S_L are very scattering, where S_L is the laminar burning velocity. But when the effect of Le is properly considered in some scaling parameters used in previous correlations, these large scattering iso-octane S_T/S_L data can be collapsed onto single curves by several modified general correlations, regardless of different φ, Le, T, p, and u′, showing self-similar propagation of turbulent spherical flames. The uncertainty analysis of these modified general correlations is also discussed. Full article

(This article belongs to the Special Issue Cleaner Combustion)

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16 pages, 5278 KiB

Open AccessArticle

Performance Analysis of Air and Oxy-Fuel Laminar Combustion in a Porous Plate Reactor

by Furqan Tahir, Haider Ali, Ahmer A.B. Baloch and Yasir Jamil

Energies 2019, 12(9), 1706; https://doi.org/10.3390/en12091706 - 06 May 2019

Cited by 10 | Viewed by 2731

Abstract

Greenhouse gas emissions from the combustion of fossil fuels pose a serious threat to global warming. Mitigation measures to counter the exponential growth and harmful impact of these gases on the environment require techniques for the reduction and capturing of carbon. Oxy-fuel combustion is one such effective method, which is used for the carbon capture. In the present work, a numerical study was carried out to analyze characteristics of oxy-fuel combustion inside a porous plate reactor. The advantage of incorporating porous plates is to control local oxy-fuel ratio and to avoid hot spots inside the reactor. A modified two-steps reaction kinetics model was incorporated in the simulation for modeling of methane air-combustion and oxy-fuel combustion. Simulations were performed for different oxidizer ratios, mass flow rates, and reactor heights. Results showed that that oxy-combustion with an oxidizer ratio (OR) of 0.243 could have the same adiabatic flame temperature as that of air-combustion. It was found that not only does OR need to be changed, but also flow field or reactor dimensions should be changed to achieve similar combustion characteristics as that of air-combustion. Fifty percent higher mass flow rates or 40% reduction in reactor height may achieve comparable outlet temperature to air-combustion. It was concluded that not only does the oxidizer ratio of oxy-combustion need to be changed, but the velocity field is also required to be matched with air-combustion to attain similar outlet temperature. Full article

(This article belongs to the Special Issue Cleaner Combustion)

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13 pages, 2355 KiB

Open AccessArticle

Numerical Study of the Structure and NO Emission Characteristics of N₂- and CO₂-Diluted Tubular Diffusion Flames

by Harshini Devathi, Carl A. Hall and Robert W. Pitz

Energies 2019, 12(8), 1490; https://doi.org/10.3390/en12081490 - 19 Apr 2019

Viewed by 2327

Abstract

The structure of methane/air tubular diffusion flames with

65 %

The structure of methane/air tubular diffusion flames with

65 %

fuel dilution by either CO₂ or N₂ is numerically investigated as a function of pressure. As pressure is increased, the reaction zone thickness reduces due to decrease in diffusivities with pressure. The flame with CO₂-diluted fuel exhibits much lower nitrogen radicals (N, NH, HCN, NCO) and lower temperature than its N₂-diluted counterpart. In addition to flame structure, NO emission characteristics are studied using analysis of reaction rates and quantitative reaction pathway diagrams (QRPDs). Four different routes, namely the thermal route, Fenimore prompt route, N₂O route, and NNH route, are examined and it is observed that the Fenimore prompt route is the most dominant for both CO₂- and N₂-diuted cases at all values of pressure followed by NNH route, thermal route, and N₂O route. This is due to low temperatures (below 1900 K) found in these highly diluted, stretched, and curved flames. Further, due to lower availability of N₂ and nitrogen bearing radicals for the CO₂-diluted cases, the reaction rates are orders of magnitude lower than their N₂-diluted counterparts. This results in lower NO production for the CO₂-diluted flame cases. Full article

(This article belongs to the Special Issue Cleaner Combustion)

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18 pages, 5138 KiB

Open AccessArticle

Study on Powder Coke Combustion and Pollution Emission Characteristics of Fluidized Bed Boilers

by Chen Yang, Haochuang Wu, Kangjie Deng, Hangxing He and Li Sun

Energies 2019, 12(8), 1424; https://doi.org/10.3390/en12081424 - 13 Apr 2019

Cited by 7 | Viewed by 3110

Abstract

The fluidized reactor is widely used in a number of chemical processes due to its high gas-particle contacting efficiency and excellent performance on solid mixing. An improved numerical framework based on the multiphase particle-in-cell (MP-PIC) method has been developed to simulate the processes of gas–solid flow and chemical reactions in a fluidized bed. Experiments have been carried out with a 3-MW circulating fluidized bed with a height of 24.5 m and a cross section of 1 m². In order to obtain the relationship between pollutant discharge and operating conditions and to better guide the operation of the power plant, a series of tests and simulations were carried out. The distributions of temperature and gas concentration along the furnace from simulations achieved good accuracy compared with experimental data, indicating that this numerical framework is suitable for solving complex gas–solid flow and reactions in fluidized bed reactors. Through a series of experiments, the factors affecting the concentration of NO_x and SO_x emissions during the steady-state combustion of the normal temperature of powder coke were obtained, which provided some future guidance for the operation of a power plant burning the same kind of fuel. Full article

(This article belongs to the Special Issue Cleaner Combustion)

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12 pages, 881 KiB

Open AccessArticle

The Impact of Fuel Type on the Output Parameters of a New Biofuel Burner

by Karol Tucki, Olga Orynycz, Andrzej Wasiak, Antoni Świć and Joanna Wichłacz

Energies 2019, 12(7), 1383; https://doi.org/10.3390/en12071383 - 10 Apr 2019

Cited by 15 | Viewed by 3528

Abstract

Intensified action aimed at reducing CO₂ emissions and striving for energy self-sufficiency of both business entities and individual consumers are forcing the sustainable development of environmentally friendly and renewable energy sources. The development of an appropriate class of equipment and production technology is not without significance in this process. On the basis of a proven design for a combustion burner for ecological fuels, a new biofuel burner, also dedicated to prosumers’ energetics, was built. The aim of the study was to determine the effect of the type of biofuel on a burner’s output parameters, especially gaseous emissions, during the combustion of four types of fuels, including three types of biomass. The combustion temperature was measured for lignite, wood pellets, straw pellets, and sunflower pellets. An analysis of exhaust gas composition was performed for lignite and wood pellets. The results of exhaust emissions and combustion temperatures were compared with the burners currently in use. The use of a new burner might contribute to cleaner combustion and reducing the emissions of some gaseous components. Full article

(This article belongs to the Special Issue Cleaner Combustion)

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15 pages, 14644 KiB

Open AccessArticle

Emissions from Solid Fuel Cook Stoves in the Himalayan Region

by Jin Dang, Chaoliu Li, Jihua Li, Andy Dang, Qianggong Zhang, Pengfei Chen, Shichang Kang and Derek Dunn-Rankin

Energies 2019, 12(6), 1089; https://doi.org/10.3390/en12061089 - 21 Mar 2019

Cited by 12 | Viewed by 3607

Abstract

Solid fuel cooking stoves have been used as primary energy sources for residential cooking and heating activities throughout human history. It has been estimated that domestic combustion of solid fuels makes a considerable contribution to global greenhouse gas (GHG) and pollutant emissions. The majority of data collected from simulated tests in laboratories does not accurately reflect the performance of stoves in actual use. This study characterizes in-field emissions of fine particulate matter (PM_2.5), carbon dioxide (CO₂), carbon monoxide (CO), methane (CH₄), and total non-methane hydrocarbons (TNMHC) from residential cooking events with various fuel and stove types from villages in two provinces in China (Tibet and Yunnan) in the Himalayan area. Emissions of PM_2.5 and gas-phase pollutant concentrations were measured directly and corresponding emission factors calculated using the carbon balance approach. Real-time monitoring of indoor PM_2.5, CO₂, and CO concentrations was conducted simultaneously. Major factors responsible for emission variance among and between cooking stoves are discussed. Full article

(This article belongs to the Special Issue Cleaner Combustion)

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13 pages, 2506 KiB

Open AccessArticle

Characterization of Performance of Short Stroke Engines with Valve Timing for Blended Bioethanol Internal Combustion

by Kun-Ho Chen and Yei-Chin Chao

Energies 2019, 12(4), 759; https://doi.org/10.3390/en12040759 - 25 Feb 2019

Cited by 6 | Viewed by 5734

Abstract

The present study provides a feasible strategy for minimizing automotive CO₂ emissions by coupling the principle of the Atkinson cycle with the use of bioethanol fuel. Motor cycles and scooters have a stroke to bore ratio of less than unity, which allows higher speeds. The expansion to compression ratio (ECR) of these engines can be altered by tuning the opening time of the intake and exhaust valves. The effect of ECR on fuel consumption and the feasibility of ethanol fuels are still not clear, especially for short stroke engines. Hence, in this study, the valve timing of a short stroke engine was tuned in order to explore potential bioethanol applications. The effect of valve timing on engine performance was theoretically and experimentally investigated. In addition, the application of ethanol/gasoline blended fuels, E3, E20, E50, and E85, were examined. The results show that consumption, as well as engine performance of short stroke motorcycle engines, can be improved by correctly setting the valve controls. In addition, ethanol/gasoline blended fuel can be used up to a composition of 20% without engine modification. The ignition time needs to be adjusted in fuel with higher compositions of blended ethanol. The fuel economy of a short stroke engine cannot be sharply improved using an Atkinson cycle, but CO₂ emissions can be reduced using ethanol/gasoline blended fuel. The present study demonstrates the effect of ECR on the performance of short stroke engines, and explores the feasibility of applying ethanol/gasoline blended fuel to it. Full article

(This article belongs to the Special Issue Cleaner Combustion)

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11 pages, 1043 KiB

Open AccessArticle

Efficiency of the Air-Pollution Control System of a Lead-Acid-Battery Recycling Industry

by Kyriaki Kelektsoglou, Dimitra Karali, Alexandros Stavridis and Glykeria Loupa

Energies 2018, 11(12), 3465; https://doi.org/10.3390/en11123465 - 11 Dec 2018

Cited by 8 | Viewed by 4551

Abstract

The air-pollution control system of a lead-acid-battery recycling industry was studied. The system comprised two streams with gravity settlers followed by filter bags for the factory indoor air and the metal-recycling furnace, respectively. Efficiency in particle removal according to mass was found to be 99.91%. Moreover, filter bags and dust from the gravity settlers were analyzed for heavy metals by Wavelength Dispersive X-Ray Fluorescence. The results showed high concentrations of Pb and Na in all cases. In the filter bag samples from the indoor atmosphere stream, Ca, Cu, Fe, and Al were found in concentrations higher than that in the filter bag samples from the furnace stream. The opposite was found for Na. Tl and K were only found in furnace stream bag filters. The elemental concentration of the dust from the furnace fumes stream contained mainly Fe, Na, Cd, Pb, Sb, and Cl, while the indoor main stream contained mainly P, Fe, Na, Pb, and Sb. In all cases, impurities of Nd, Ni, Rb, Sr, Th, Hg, and Bi were found. The high efficiency of the air-pollution control system in particle removal shows that a considerable reduction in emissions was achieved. Full article

(This article belongs to the Special Issue Cleaner Combustion)

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13 pages, 3394 KiB

Open AccessArticle

Density Functional Theory Study on Mechanism of Mercury Removal by CeO₂ Modified Activated Carbon

by Li Zhao, Yang-wen Wu, Jian Han, Han-xiao Wang, Ding-jia Liu, Qiang Lu and Yong-ping Yang

Energies 2018, 11(11), 2872; https://doi.org/10.3390/en11112872 - 23 Oct 2018

Cited by 14 | Viewed by 3365

Abstract

Doping of CeO₂ on activated carbon (AC) can promote its performance for mercury abatement in flue gas, while the Hg⁰ removal mechanism on the AC surface has been rarely reported. In this research, density functional theory (DFT) calculations were implemented to unveil the mechanism of mercury removal on plain AC and CeO₂ modified AC (CeO₂-AC) sorbents. Calculation results indicate that Hg⁰, HCl, HgCl and HgCl₂ are all chemisorbed on the adsorbent. Strong interaction and charge transfer are shown by partial density of states (PDOS) analysis of the Hg⁰ adsorption configuration. HCl, HgCl and HgCl₂ can be dissociatively adsorbed on the AC model and subsequently generate HgCl or HgCl₂ released to the gas phase. The adsorption energies of HgCl and HgCl₂ on the CeO₂-AC model are relatively high, indicating a great capacity for removing HgCl and HgCl₂ in flue gas. DFT calculations suggest that AC sorbents exhibit a certain catalytic effect on mercury oxidation, the doping of CeO₂ enhances the catalytic ability of Hg⁰ oxidation on the AC surface and the reactions follow the Langmuir–Hinshelwood mechanism. Full article

(This article belongs to the Special Issue Cleaner Combustion)

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14 pages, 2692 KiB

Open AccessArticle

Combustion Characteristics and NO_x Emission through a Swirling Burner with Adjustable Flaring Angle

by Yafei Zhang, Rui Luo, Yihua Dou and Qulan Zhou

Energies 2018, 11(8), 2173; https://doi.org/10.3390/en11082173 - 20 Aug 2018

Cited by 4 | Viewed by 4337

Abstract

A swirling burner with a variable inner secondary air (ISA) flaring angle β is proposed and a laboratory scale opposed-firing furnace is built. Temperature distribution and NO_x emission are designedly measured. The combustion characteristics affected by variable β are experimentally evaluated from ignition and burnout data. Meanwhile, NO_x reduction by the variable β is analyzed through emissions measurements. Different inner/outer primary coal-air concentration ratios γ, thermal loads and coal types are considered in this study. Results indicate that β variation provides a new approach to promote ignition and burnout, as well as NO_x emission reduction under conditions of fuel rich/lean combustion and load variation. The recommended β of a swirling burner under different conditions is not always constant. The optimal β_opt of the swirling burner under all conditions for different burning performance are summarized in the form of curves, which could provide reference for exquisite combustion adjustment. Full article

(This article belongs to the Special Issue Cleaner Combustion)

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16 pages, 4156 KiB

Open AccessArticle

Numerical Investigation of the Effects of Steam Mole Fraction and the Inlet Velocity of Reforming Reactants on an Industrial-Scale Steam Methane Reformer

by Chun-Lang Yeh

Energies 2018, 11(8), 2082; https://doi.org/10.3390/en11082082 - 10 Aug 2018

Cited by 4 | Viewed by 3057

Abstract

Steam methane reforming (SMR) is the most common commercial method of industrial hydrogen production. Control of the catalyst tube temperature is a fundamental demand of the reformer design because the tube temperature must be maintained within a range that the catalysts have high activity and the tube has minor damage. In this paper, the transport and chemical reaction in an industrial-scale steam methane reformer are simulated using computational fluid dynamics (CFD). Two factors influencing the reformer temperature, hydrogen yield and stress distribution are discussed: (1) the mole fraction of steam (Y_H2O) and (2) the inlet velocity of the reforming reactants. The purpose of this paper is to get a better understanding of the flow and thermal development in a reformer and thus, to make it possible to improve the performance and lifetime of a steam reformer. It is found that the lowest temperature at the reforming tube surface occurs when Y_H2O is 0.5. Hydrogen yield has the highest value when Y_H2O is 0.5. The wall shear stress at the reforming tube surface is higher at a higher Y_H2O. The surface temperature of a reforming tube increases with the inlet velocity of the reforming reactants. Finally, the wall shear stress at the reforming tube surface increases with the inlet velocity of the reforming reactants. Full article

(This article belongs to the Special Issue Cleaner Combustion)

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