Unburned Fuels Research Articles

Achieving high flame stability with low NO And Zero N2O and NH3 emissions during liquid ammonia spray combustion with gas turbine combustors

Liquid ammonia spray direct injection is more desirable than gaseous ammonia injection in large scale gas turbine combustors because it enables a larger volumetric energy density and allows a reduction in cost and size of the fuel supply system. However, previous studies have shown that the large evaporative cooling effects of liquid ammonia droplets vaporization impair flame stabilization, and inhibit efficient control of emissions such as NO, N2O and unburned fuels. This article reports the development and testing of a novel two-stage gas turbine combustor for pure liquid ammonia spray, which achieves approximately zero N2O and unburned fuel emissions with NO emissions as low as 282 ppmv (@16% O2) in a 50 kW micro gas turbine combustor test rig. The combustor design focused on optimization of the secondary injection holes to prevent the dilution of the primary zone (PZ) by air injected into the secondary zone; improving mixture formation using a multi-nozzle twin-fluid atomizer; and elongation of the combustor length to allow sufficient fluid residence time for N2O reduction as well as droplets vaporization and combustion. It was found that the mitigation of PZ dilution resulted in a significant increase in the combustion temperature thereby encouraging efficient combustion of the liquid droplets. On the other hand, the multi-nozzle twin fluid atomizer resulted in a more uniform combustor wall temperature, a more efficient combustion and lower NOx emissions than a single-nozzle pressure swirl atomizer. It is considered that the multi-nozzle injector allowed for an improved droplets dispersion, which is necessary to mitigate large local heat extraction from the flame. These features of the combustor enabled a substantial improvement in liquid ammonia spray flame stability and a better simultaneous control of NO, N2O and unburned fuel emissions never reported before.

Methodologies for quantitatively comparing the effectiveness of chemical retardants for direct and indirect wildfire suppression using a combustion wind tunnel

Retardant chemicals are often used during wildfire fighting operations where they are primarily dropped from an aircraft onto unburned fuels in a fire's path. While there are established laboratory methods used to determine retarding effectiveness capacity relative to a standard, there are no specific methods for comparing the effectiveness of different retardant formulations in different operational applications. This paper develops four methodologies to assess and compare retardant effectiveness in a variety of tactical uses including retarding durability and effectiveness at different application depths. The resultant assessment methodologies use the consistent controlled environment of a combustion wind tunnel in which conditions reasonably represent field conditions and have low variability enabling fair comparisons between treatments. Evaluations of the assessment methodologies for two commercially available retardants and water are presented and demonstrate low variability in evaluation conditions and consistent, reliable results. These methodologies provide valuable and cost-effective insights into chemical performance and will help wildfire suppression agencies to make informed procurement decisions.

Effect of ammonia and coal co-firing on the formation mechanism and composition characteristics of particulate matter

Ammonia-coal co-firing is considered to be an effective method to reduce carbon dioxide emissions and the existing researches mainly focus on the combustion characteristics and NOx emission characteristics. In this study, co-firing of coal and ammonia was carried out in the high temperature drop tube furnace system with a large co-firing ratio of 0–50%. Firstly, the unburned fuels and the main composition of flue gas were measured as a baseline result. The results showed that ammonia co-firing will lead to the increase of NO and NOx concentration. Further, the particulate matters generated under different conditions were collected and tested. It was found that ammonia co-firing will affect the formation characteristics of particulate matter, especially sub-micron particulate matter. With the increase of co-firing ratio, the concentration of PM1 showed an overall increasing trend, but decreased at 20%. And ammonia co-firing will have an effect on the content of Si, Al, S and trace elements (As, Pb) in submicron particles. The content of water vapor, NOx in flue gas and combustion temperature have changed and affected the formation of SO3 and the gasification of silica and trace elements.

Radiation effects of CO2 and H2O dilution on laminar flame speeds of syngas flames under elevated pressures

The abundant CO2 and H2O in circulating flue gases both chemically affect the combustion characteristics significantly. Meanwhile, the intense radiation effects induced by CO2 and H2O also strongly coupled with chemical reactions by preheat unburned fuels. Thus, the comparison and analysis of dilution effect and interaction mechanism with CO2 and H2O blending is of great significance, especially when considering radiation reabsorption. In this study, laminar flame speeds of premixed syngas flames considering radiation reabsorption were simulated under elevated pressures. It found that radiation reabsorption effect induced by CO2 and H2O dilution both greatly increased the flame speed of syngas flames. Because of the discrepancy in dominant influence mechanisms of radiation reabsorption, different relative enhancements of flame speed as a function of pressure were observed. Moreover, the co-dilution of CO2 and H2O further improved the radiation reabsorption effect by means of the coupling inhibition of OH radical's consumption pathway.

Sensor for operational control of oxygen and combustible gases concentration in waste gases of thermal units

A new sensor has been developed for continuous monitoring of oxygen and combustible gases content in the waste gases of thermal units. The target application of the sensor is its installation in shunt pipes of thermal units, directly into the waste gas flow. The sensor is characterized by one reference gas electrode and three measuring electrodes applied on the surface of a solid electrolyte tube made of a zirconia electrolyte (e.g., 8YSZ). The reference gas electrode and one of the measuring electrodes were made of silver, the second measuring electrode was made of platinum, the third measuring electrode was made of a mixture of zinc oxide (95 wt %) and lanthanum-strontium manganite (5 wt %). The oxygen content in the gas mixture was determined by the well-known potentiometric method in accordance with the Nernst equation, i.e. an Ag|8YSZ|Ag electrochemical cell was used. To determine the products of incomplete combustion of fuel, the method of mixed potential between Pt- and Zn-based electrodes was used; the obtained potential value was determined by the difference in the oxidation rate of carbon monoxide as the main component of unburned fuels, on different materials of measuring electrodes. The experimental results of the sensor for the determination of carbon monoxide, hydrogen, and methane in a gas mixture are presented.

Numerical simulation of detonation re-initiation in a 90° bifurcated channel filled with n-heptane/air mixture

This paper aims to study the detonation re-initiation through multiple reflections when the detonation enters the horizontal channel holding liquid fuel from the vertical branch. Based on the Eulerian-Lagrangian method, the detonation diffraction, reflection and re-initiation of gas phase (C7H16/Air) and two-phase (n-heptane/Air) are compared, and the effects of droplet diameters on re-initiation and the interaction between shock wave and droplets are analyzed. The results show that compared with the detonation propagating in pure gaseous mixture, detonation is easier to be decoupled in two-phase mixtures during the diffraction stage due to the droplet fragmentation and evaporation process, and lots of unburned fuels are left behind the detonation wave. As the droplet diameter increases, the detonation wave propagates slower and the cell size increases. The number of transverse waves formed after the first reflection gradually decreases, and the transverse detonation in mixture with larger droplets is decoupled during the downstream propagation. The shock wave interacts with the droplet, leading to the occurrence of droplet breakup. At this stage, the total mass of the droplets remains basically unchanged. After that, the droplet evaporation process occurs, and the droplet mass decreases rapidly.

Effects of lubricant-fuel mixing on particle emissions in a single cylinder direct injection spark ignition engine

Gasoline direct injection (GDI) engines emit less carbon dioxide (CO2) than port fuel injection (PFI) engines when fossil fuel conditions are the same. However, GDI engines emit more ultrafine particulate matter, which can have negative health effects, leading to particulate emission regulations. To satisfy these regulations, various studies have been done to reduce particulate matter, and several studies focused on lubricants. This study focuses on the influence of lubricant on the formation of particulate matter and its effect on particulate emissions in GDI engines. An instrumented, combustion and optical singe-cylinder GDI engine fueled by four different lubricant-gasoline blends was used with various injection conditions. Combustion experiments were used to determine combustion characteristics, and gaseous emissions indicated that the lubricant did not influence mixture homogeneity but had an impact on unburned fuels. Optical experiments showed that the lubricant did not influence spray but did influence wall film formation during the injection period, which is a major factor affecting particulate matter generation. Particulate emissions indicated that lubricant included in the wall film significantly affected PN emissions depending on injection conditions. Additionally, the wall film influenced by the lubricant affected the overall particle size and its distribution.

Effect of diesel-palm biodiesel fuel with plastic pyrolysis oil and waste cooking biodiesel on tribological characteristics of lubricating oil

Engine oil's lubricity and tribological properties are changed when it is diluted with unburned fuels. SAE40 lubricating oil (LO) samples were contaminated with known percentage (5%) of fuels (commercial diesel (B10), diesel-palm biodiesel (PB) blend (B30a), and diesel–PB–plastic pyrolysis oil (PPO)–waste cooking biodiesel (WCB) blends (B20, B30b, and B40)). Utilizingfour-ball tester, the influence of each of these fuels on wear and frictional properties of LO was measured, and wear type on worn surfaces was examined. LO diluted by B10 had a high coefficient of friction (COF) (11.52%) with severe abrasive and adhesive wear than mineral lubricant followed by B30a (3.00%). LO mixed with quaternary fuels had a lower COF, wear scar diameter (WSD) and polishing wear. When compared to SAE40–B30a, SAE40–B30b had fewer frictional properties with adhesive wear. When compared to SAE–40 mineral lubricant, SAE40–B20 had the smallest increase in COF value (1.90%), followed by SAE40–B30b (2.17%), and SAE40–B40 (2.63%). While the WSD values for all tested samples are reduced by 3.12, 5.68, 6.33, 11.11, and 17.33%, respectively, when compared to pure lubricant for SAE40–B10, SAE40–B20, SAE40–B30a, SAE40–B30b, and SAE4–B40. PPO and WCB in B10are found to reduce lubricant degradation when compared to diesel-PB blend and commercial diesel.

Numerical Investigation on Combustion-Enhancement Strategy in Shock–Fuel Jet Interaction

Multidimensional numerical simulations are performed to investigate the evolution and formation of unburned fuels for a shock–fuel jet interaction scenario. A full set of Navier–Stokes equations with detailed chemical mechanisms are solved, and the results are analyzed through the Lagrangian method with the goal of improving combustion efficiency in supersonic flows. The flame morphology of a two-dimensional (2-D) non-premixed reactive shock–bubble interaction is first simulated and studied, in which unburned hydrogen is found to prevent efficient combustion. By applying the Lagrangian particle tracking method, most of the unburned hydrogen wrapped into the primary vortex turns out to be initially located upon the symmetry line of the bubble. Motivated by the idea of breaking the primary vortex, this study designs a novel geometry of a concentric bubble, which improves combustion efficiency to 94.4% in contrast to a solid fuel bubble (74%) due to multivortex interaction and a thick bridge structure. With the consistency between qualitative and quantitative 2-D and three-dimensional (3-D) flow dynamics, the idea of a 2-D concentric-bubble configuration is effectively extended to a 3-D coaxial jet interacting with oblique shock despite the existence of Kelvin–Helmholtz instability.

Geometrical inlet effects on the behavior of a non-premixed fully turbulent syngas combustion; a numerical study

This study numerically evaluates a three-dimensional, turbulent, non-premixed syngas (a mixture of H2 and CO) flame issued from a round nozzle. The flow Reynolds number on the basis of the inlet velocity and the nozzle diameter is 16,700 and the flame is shrouded by a coflow under atmospheric conditions. A computational tool with a modified k-omega turbulence model and eddy dissipation model is used for simulating the reacting flow and its immediate surroundings. The outcomes are first compared against the existing experimental data. This reveals that when β*, as a constant coefficient in the k-omega turbulence model, is 0.073, the numerical results match the experimental data with high accuracy. Subsequently, the effects of variations in the diameter of the inlet nozzle and its geometry are investigated. This shows that an increase in the inlet nozzle diameter leads to the downstream movement of the high-temperature region diminishing combustion efficiency. Importantly, it is shown that using the elliptical inlet nozzle with the large diameter of 1.2 based nozzle diameter (4.48 mm) assists the combustion performance and increases the maximum combustion temperature by up to 9.6%. It also results in a considerable reduction of the unburned fuels and enhances the flow residence time significantly.

Assessing Flare Combustion Efficiency using Imaging Fourier Transform Spectroscopy

Flaring plays a critical role in reducing the environmental impact of upstream oil and gas processing by converting methane and other gaseous hydrocarbons into CO2, which has a lower global warming potential. This process is highly efficient under ideal conditions but efficiency may be significantly lower under certain scenarios such as fuel stripping under crosswind and emission of volatile organic compounds and unburned fuels due to over-aeration or over-steaming in assisted flares. This study assesses the potential of using imaging Fourier transform spectrometers (IFTSs) to directly measure combustion efficiency by combining species column densities estimated from a spectroscopic model with intensity-weighted velocities found using an optical flow model. Simulated measurements using a computational fluid dynamics (CFD)-large eddy simulation of a flare in a crosswind are used to establish the technique's viability, followed by experimental measurements on a heated gas vent to validate the optical flow model. Finally, preliminary measurements are carried out on a laboratory-scale steam- and air-assisted flare. While the simulated measurements and heated vent experiments support the feasibility of this approach, experimentally-derived spectra from the lab-scale flare were contaminated with artifacts attributed to turbulent fluctuations, which complicates the quantitative interpretation of the IFTS data.

Design and analysis of the scramjet nozzle with contact discontinuity

A scramjet nozzle design method that utilizes Rao's maximum thrust theory and the slip-line unit process is proposed to reduce the negative effect of the internal contact discontinuity on the aerodynamic performance. The proposed method divides the nozzle flowfield into a lower region and an upper region. The compatibility between the two regions with different aerothermal properties is achieved by the constraints of the flow deflection angles and the static pressure along an inviscid slip line. Due to the high computational expense allied with finite rate reactions, particularly for heavy hydrocarbon fuels, the basic studies of scramjet nozzles in this paper are conducted with the chemically frozen flow assumption. It underestimates the influence of the dissociated species of combustion product or unburned fuels to some extent, but still reveals nozzle performance trends efficiently, and serves as a helpful reference. The initial expansion arc radius influences the evolution of the inviscid slip line, thereby changing the expansion processes of the upper and lower regions. However, the position deviation between the nozzle entrance and the exit cannot change the shape of the inviscid slip line except retreating of the terminal point. Notably, a critical position deviation maximizes the axial thrust coefficient and minimizes the lift. Compared with the nozzle designed by the truncated method, the nozzle designed by the proposed method shows increases in the axial thrust coefficient, lift, and pitching moment of 6.22%, 166.3%, and 164.8%, respectively, at the design point. Numerical results also show that only a minor decrease in the nozzle performance appears when 3D effects are considered, further revealing the reliability of the proposed method.

Effect of palm-sesame biodiesel fuels with alcoholic and nanoparticle additives on tribological characteristics of lubricating oil by four ball tribo-tester

Dilution of engine oil with unburned fuels alters its lubricity and tribological properties. In this research paper, SAE-40 lubricating oil samples were contaminated with known percentages (5%) of fuels (diesel, palm-sesame biodiesel blend (B30), B30 + ethanol, B30 + dimethyl carbonate, B30 + carbon nanotubes and, B30 + titanium oxide). The effect of all these fuels on wear and frictional characteristics of lubricating oil was determined by using a 4-ball tribo tester and wear types on worn surfaces were analyzed by using SEM. Lubricating oil diluted with B10 (commercial diesel) showed highest COF (42.95%) with severe abrasive and adhesive wear than mineral lubricant among other fuels. Lubricating oil diluted with palm-sesame biodiesel (B30 blend) with alcoholic additives showed comparatively less COF, less wear scar diameter and polishing wear due to presence of ester molecules. Lub + B30 + Eth exhibited increment in COF value (35.81%) compared to SAE-40 mineral lubricant. While lubricating oil contaminated with B30 with nanoparticles showed least frictional characteristics with abrasive wear. Lub + B30 + TiO2 showed least increment in COF value (13.78%) among all other contaminated fuels compared to SAE-40 mineral lubricant. It is concluded that nanoparticles in biodiesel blends (B30) helps in reducing degradation of lubricants than alcoholic fuel additives and commercial diesel.

Jet A and Propane gas combustion in a turboshaft engine: performance and emissions reductions

The Paris Agreement has highlighted the need in reducing carbon emissions. Attempts in using lower carbon fuels such as Propane gas have seen limited success, mainly due to liquid petroleum gas tanks structural/size limitations. A compromised solution is presented, by combusting Jet A fuel with a small fraction of Propane gas. Propane gas with its relatively faster overall igniting time, expedites the combustion process. Computational fluid dynamics software was used to demonstrate this solution, with results validated against physical engine data. Jet A fuel was combusted with different Propane gas dosing fractions. Results demonstrated that depending on specific propane gas dosing fractions emission reductions in ppm are; NOx from 84 to 41, CO2 from less than 18,372 to less than 15,865, escaping unburned fuels dropped from 11.4 (just Jet A) to 6.26e-2 (with a 0.2 fraction of Propane gas). Soot and CO increased, this is due to current combustion chamber air mixing design.

Role of accumulation for ignition of fuel beds by firebrands

Large outdoor fires are one of the prominent fire problems in the world. Spot fires, caused by firebrands, are known as a key mechanism of rapid fire spread. Firebrands ignite unburned fuels far ahead of the fire front. In large outdoor fires, firebrands are thought to accumulate and ignite unburned fuel which cannot be ignited by a single firebrand. Experiments are performed to investigate the ignition behavior of fuel beds with changing fuel moisture content (FMC) and wind speed. It was found that accumulated firebrands could cause ignitions on fuel beds with high FMC and the upper limit of FMC for ignition increased from 6 m/s to 8 m/s.

Exergy losses in premixed flames of dimethyl ether and hydrogen blends

A second-law thermodynamic analysis was conducted for stoichiometric premixed dimethyl ether (DME)/hydrogen (H2)/air flames at atmospheric pressure. The exergy losses from the irreversibility sources, i.e., chemical reaction, heat conduction and species diffusion, and those from partial combustion products were analyzed in the flames with changed fuel blends. It is observed that, regardless of the fuel blends, chemical reaction contributes most to the exergy losses, followed by incomplete combustion, and heat conduction, while mass diffusion has the least contribution to exergy loss. The results also indicate that increased H2 substitution decreases the exergy losses from reactions, conduction, and diffusion, primarily because of the flame thickness reduction at elevated H2 substitution. The decreases in exergy losses by chemical reactions and heat conduction are higher, but the exergy loss reduction by diffusion is slight. However, the exergy losses from incomplete combustion increase with H2 substitution, because the fractions of the unburned fuels and combustion intermediates, e.g., H2 and OH radical, increase. The overall exergy losses in the DME/H2 flames decrease by about 5% with increased H2 substitution from 0% to 100%.

Effect of low load combustion and emissions on fuel dilution in lubricating oil and deposit formation of DI diesel engines fueled by straight rapeseed oil

The objective of this study is to apply neat biomass fuel to a DI diesel engine and investigate the effect of in-cylinder gas flow and combustion on the deposit formation and the fuel dilution in lubricating oil. The study focuses on the low load combustion and emissions considering that low load exhaust contain much unburned fuels and the unburned fuels are the source of the deposit formation and the fuel dilution. Piston configuration and swirl velocity were altered in the engine test. The engine was fueled by neat rapeseed oil. The test was carried out through the four hours continuous engine operation with keeping low load. After the operation, state of deposit formation and fuel dilution in lubricating oil were investigated. Results indicate that Re-entrant piston which creates strong reverse squish and high swirl forms the deposit annular on the piston top. Toroidal piston easily produces deposit on the undersurface of cylinder head. The deposit in the cavity accumulates where initial rapeseed oil spray impinges regardless of piston types. The carbonization of the deposit is promoted on the wall surface where the burned gas with high temperature and high velocity comes into contact. It is important to avoid extremely strong reverse squish to the cylinder liner in order to control the fuel dilution. The deep-bowl chamber changes the direction of reverse squish from the cylinder liner direction to the cylinder head direction. The low velocity outflow from the piston cavity reduces the adhesion of unburned fuel on the cylinder liner, resulting in the smaller amount of unburned fuel scraped off by a piston ring.

Synergistic effects between iron-graphene and melamine salt of pentaerythritol phosphate on flame retardant thermoplastic polyurethane

A comparison of melamine salt of pentaerythritol phosphate (MPP), and a synergistic agents, iron–graphene (IG) was performed in thermoplastic polyurethane (TPU) by masterbatch-melt blending on thermal and flame retardant properties. The flame retardant properties of TPU composites were characterized by limiting oxygen index (LOI), UL 94 and cone calorimeter test (CCT). The CCT results revealed that IG can significantly enhance flame retardant properties of MPP in TPU. The peak heat release rate of neat TPU and flame retardant TPU/MPP composites decreased from 2192.6 and 226.7 to 187.2 kW/m2 compared with that of TPU containing 0.25 wt% IG. The thermal stability and thermal decomposition of TPU composites were characterized by thermogravimetric analysis (TGA) and thermogravimetric/Fourier infrared spectrum analysis (TG-IR). The results indicated IG and MPP can improve the thermal stability of TPU. The formation of thermal conductive network by IG can promote the decomposition of MPP into nonflammable melt, which can play the role of heat barrier and restrict the diffusion of fuels into combustion zone and access of oxygen to the unburned fuels. Copyright © 2016 John Wiley & Sons, Ltd.

Observations of energy transport and rate of spreads from low-intensity fires in longleaf pine habitat – RxCADRE 2012

Wildland fire rate of spread (ROS) and intensity are determined by the mode and magnitude of energy transport from the flames to the unburned fuels. Measurements of radiant and convective heating and cooling from experimental fires are reported here. Sensors were located nominally 0.5 m above ground level. Flame heights varied from 0.3 to 1.8 m and flaming zone depth varied from 0.3 to 3.0 m. Fire ROS derived from observations of fire transit time between sensors was 0.10 to 0.48 m s–1. ROS derived from ocular estimates reached 0.51 m s–1 for heading fire and 0.25 m s–1 for backing fire. Measurements of peak radiant and total energy incident on the sensors during flame presence reached 18.8 and 36.7 kW m–2 respectively. Peak air temperatures reached 1159°C. Calculated fire radiative energy varied from 7 to 162 kJ m–2 and fire total energy varied from 3 to 261 kJ m–2. Measurements of flame emissive power peaked at 95 kW m–2. Average horizontal air flow in the direction of flame spread immediately before, during, and shortly after the flame arrival reached 8.8 m s–1, with reverse drafts of 1.5 m s–1; vertical velocities varied from 9.9 m s–1 upward flow to 4.5 m s–1 downward flow. The observations from these fires contribute to the overall understanding of energy transport in wildland fires.

Influence of Fe2O3 on smoke suppression and thermal degradation properties in intumescent flame-retardant silicone rubber

This article examines the feasibility of intumescent flame retardant (based on phosphorus acid, melamine, and pentaerythritol) as a flame retardant and Fe2O3 as a smoke suppression agent in silicone rubber (SR) composites. Smoke suppression of Fe2O3 on intumescent flame-retardant SR composites was investigated using smoke density test and cone calorimeter test (CCT). And, the flammability of SR composites was characterized using CCT at an incident heat flux of 50 kW m−2. The test results further revealed that Fe2O3 could increase smoke suppression efficiency and thermal degradation temperature. Fe2O3 can promote early cross-linking of polymer during decomposition to increase char formation. The silica ash layer integrity governs the efficiency of diffusion barrier that restricts the diffusion of fuels into combustion zone and access of oxygen to the unburned fuels.