Fuel-oxygen Mixtures Research Articles

Gasification of Liquid Hydrocarbon Waste by the Ultra-Superheated Mixture of Steam and Carbon Dioxide: A Thermodynamic Study

The thermodynamic modeling of waste oil (WO) gasification by a high-temperature gasification agent (GA) composed of an ultra-superheated H2O/CO2 mixture is carried out. The GA is assumed to be obtained by the gaseous detonation of fuel–oxidizer–diluent mixture in a pulsed detonation gun (PDG). N-hexadecane is used as a WO surrogate. Methane or the produced syngas (generally a mixture of H2, CO, CH4, CO2, etc.) is used as fuel for the PDG. Oxygen, air, or oxygen-enriched air are used as oxidizers for the PDG. Low-temperature steam is used as a diluent gas. The gasification process is assumed to proceed in a flow-through gasifier at atmospheric pressure. It is shown that the use of the detonation products of the stoichiometric methane–oxygen and methane–air mixtures theoretically leads to the complete conversion of WO into a syngas consisting exclusively of H2 and CO, or into energy gas with high contents of CH4 and C2-C3 hydrocarbons and an LHV of 36.7 (fuel–oxygen mixture) and 13.6 MJ/kg (fuel–air mixture). The use of the detonation products of the stoichiometric mixture of the produced syngas with oxygen or with oxygen-enriched air also allows theoretically achieving the complete conversion of WO into syngas consisting exclusively of H2 and CO. About 33% of the produced syngas mixed with oxygen can be theoretically used for PDG self-feeding, thus making the gasification technology very attractive and cost-effective. To self-feed the PDG with the mixture of the produced syngas with air, it is necessary to increase the backpressure in the gasifier and/or enrich the air with oxygen. The addition of low-temperature steam to the fuel–oxygen mixture in the PDG allows controlling the H2/CO ratio in the produced syngas from 1.3 to 3.4.

Transition from Fast-Flame to Detonation

Flame acceleration within a channel is facilitated by repeating obstacles that generate turbulence. As the flame accelerates to supersonic velocities, it generates compression waves that coalesce to form a strong leading shock. This shock-flame complex continues to accelerate until it reaches a maximum velocity equal to the speed of sound in the combustion products; the complex is referred to as a fast-flame. Shock reflection driven onset of detonation may occur, characterized by an instantaneous jump in velocity to the theoretical ‘Chapman-Jouguet’ velocity. A more fundamental problem is the transition from a fast-flame to detonation without the presence of obstacles. 
 In the current study, a 7.6 cm square ‘optical section’ housing a perforated plate was used to study the transition from fast-flame to detonation. A spark- and glow- plug served as ignition sources. The optical section had windows installed on its side and end walls so that Schlieren- and direct- photography could be captured. Soot foils were also utilized to record detonation cell structure. The fast-flame was generated by the passage of a detonation wave through the perforated plate, after which the transition to detonation could be studied. 
 Gaseous fuel-oxygen mixtures of varying degrees of detonation stability over a wide range of pressures were tested. In low-pressure tests, delayed onset of detonation was observed, and detonation initiation sites were located at the channel walls indicating that flame-generated turbulence alone is not sufficient for the transition process. As pressure was increased, onset occurred immediately after the perforated plate at various locations across the channel cross-section, not strictly at the walls. For this abrupt ignition mode, transverse shock collisions played an important role in the transition to detonation. 

Ion Sensors for Pulsed and Continuous Detonation Combustors

Presented in the article are the design and operation principles of ion sensors intended for detecting the propagating reaction fronts, the deflagration/detonation mode, apparent subsonic/supersonic propagation velocity of the reaction front, and duration of heat release by measuring the ion current in the reactive medium. The electrical circuits for ion sensors without and with intermediate amplifiers, with short response time and high sensitivity, as well as with the very wide dynamic range of operation in the reactive media with highly variable temperature and pressure, are provided and discussed. The main advantages of ion sensors are their very short response time of about 1 ms, versatility of design, and capability of detecting and monitoring reaction fronts of different intensities directly in combustion chambers. Several examples of ion sensor applications in sensing deflagration-to-detonation transition in pulsed detonation engines and developed detonations in rotating detonation engines operating on different fuel–air and fuel–oxygen mixtures are presented and discussed.

Symbiosis of deflagration and detonation in one jet system – A hybrid detonation engine

For the first time, a model of a hybrid detonation engine has been developed and experimentally tested. It consists of two autonomous (independent of each other) jet plants: a rotating detonation module (RDM) and a well-known turbojet engine. The features of the RDM operation and the conditions for the existence of a detonation mode when using mixtures: jet fuel-oxygen and jet fuel-oxygen-air are studied. It has been established that the detonation mode in the jet fuel-oxygen mixture occurs soon after the start of the RDM (approximately after 3–5 ms), but the detonation wave velocities are relatively low. Further, the detonation mode is enhanced, and the detonation waves are accelerated to sufficiently high velocities – 2000 m/s or more. The air supply to the RDM contributes to the intensification of the detonation mode. In this case, the wave makes up to 12 rounds around the annular chamber at velocities up to 2700 m/s against 9 rounds (maximum) at velocities up to 2200 m/s when burning a jet fuel–oxygen mixture. The thrust created by the detonation module reaches 25% of the thrust of the turbojet engine associated with it.

Detonation and its limit in small tubes with ozone sensitization

Ozone as an ignition promoter was used to study the detonation limit behavior in small, round tubes. Experimentally, detonation cellular structures and velocity deficits of fuel-oxygen mixtures with and without 3000 PPM of ozone were measured in tubes of 32 and 6 mm in inner diameter. Both hydrogen-oxygen and methane-oxygen mixtures were tested. The addition of ozone reduces the detonation cell size and extends the transition to spinning detonation to lower initial pressures. Ozone addition, however, produces a negligible effect on velocity deficit far from the detonation limit while providing a measurable extension of the limit. The change in deficit caused by ozone does not correlate with the reduction in cell size, suggesting that the velocity deficit behavior is not solely a function of the detonation structure. By analyzing the velocity deficit data in the d/λ (the ratio of tube diameter to cell width) domain, two regimes of detonation limits are identified: a loss-governed regime where velocity deficiency results from various losses (mass, momentum, and heat) and is insensitive to detonation structure, and a geometry-limited regime determined by the geometric accommodation of the detonation structures. Two velocity deficit models were evaluated against the experimental results: a modified ZND model with heat and momentum losses (HML) and the flow divergence (FD) model. Velocity deficits in the loss-governed regime are well predicted by the HML model but not by the FD model. Velocity deficits in the geometry-limited regime can be correlated with the critical condition of d/λ=1/π and the transition point between the two regimes. Ozone addition extends the detonation limit by moving the reactive mixtures away from its geometric detonation limit, and promotes detonation propagation in miniaturized geometries.

Electron Transport Coefficients in Nonequilibrium Plasmas of Water Vapor and Fuel–Oxygen Mixtures

The electron transport coefficients in weakly ionized nonuniform nonequilibrium plasmas of water vapor and fuel–oxygen mixtures are calculated as functions of the degree of fuel oxidation. The calculations are performed for hydrogen and hydrocarbon fuels. It is shown that the production of water vapor during fuel oxidation strongly affects the electron transport coefficients. Using the calculated coefficients, the parameter ranges are determined in which thermocurrent instability can develop in the gas mixtures under study. The domain of reduced electric fields at which this instability develops extends with increasing the degree of fuel oxidation and becomes the largest under complete fuel oxidation.

Parametric Computational Studies of NOx Emission Reduction in Staged Combustion of Ideal Fuel-Air Mixtures1

This paper is the second of series of articles devoted to computational studies for optimization of the combustion of fuel-oxygen mixtures with combustion chamber exhaust temperatures of 1600°C or more while meeting pollution emission standards. Calculations of the combustion of ideally mixed fuel-air mixtures in a combustor consisting of two sequentially positioned (staged) combustion volumes, each with its own burner, are presented. The first burner is a traditional low-emission combustion chamber with combustion of a pre-mixed fuel-air mixture with a swirler and a pre-mixing chamber, and pilot and main burners. The second burner lies downstream in the flow and a fuel-air mixture with a different composition is fed into it through a special aperture; this mixture is burnt in a medium with reduced oxygen content and a high temperature. The distribution and composition of the fuel-air mixture feeds between the two burners are optimized to attain minimum NOx emissions for outlet temperatures of 1700°C. The optimum length of the second combustion volume is determined for complete burnup of the fuel with minimum concentrations of the oxides of nitrogen and carbon at its outlet. The results can be used to evaluate the minimum possible level of nitrogen oxide emissions attainable in the sequential combustion scheme for ideally mixed fuel-air mixtures.

An experimental study on the detonability of gaseous hydrocarbon fuel–oxygen mixtures in narrow channels

This study was performed to address the detonability related to gaseous hydrocarbon fuel–oxygen mixtures (i.e., CH4–2O2, C2H6–3.5O2 and C3H8–5O2) in different ducts (36-mm circular tube and annular channel heights with 2-mm, 4.5-mm and 7-mm gaps). The results show that detonation can achieve self-sustained propagation with a steady velocity because the conditions are within the detonability in C1–C3 alkanes/oxygen mixtures; the velocity decreases as the detonation transmits into the annular channel due to increased heat and momentum losses from the wall. A modified theoretical model is proposed to predict the detonation velocity deficit for different size channels. Through a comparison with the experimental results, this model was determined to be suitable for predicting the velocity deficit in C1–C3 alkane fuel with oxygen mixtures within reasonable accuracy. Both experimental and theoretical predicted data indicate that velocity deficit is more significant in the smaller channel gap, which verifies that losses are more prominent in the narrower channel. Furthermore, the detonation cellular structure from the smoked foil was measured to analyze the propagation mechanism. The results show detonation has a single-headed spinning structure when detonability is approached. The onset of single-headed spinning detonation of CH4–2O2, C2H6–3.5O2 and C3H8–5O2 in a 36-mm circular tube occurs from p0=3–7 kPa, 4.2–5 kPa and 3.5–3.6 kPa, respectively. This suggests the pressure range of the single-headed spinning phenomenon is narrower for hydrocarbon fuels with more carbon atoms.

An experimental study on vapor cloud explosion of propane-oxygen stoichiometric mixture

Vapor cloud explosions (VCEs) are serious hazards in refining and petrochemical industries. Statistics indicate that 75% of the total losses were caused by explosion so that considerable research effort has been focused on this subject. Consequences of an explosion are aerial overpressure and impulse, responsible for injury to humans and structures. Many studies were performed with fuel-air explosions but few works focused on fuel-oxygen mixtures explosion. In this study, experiments were performed on a large experimental field. In order to represent realistic conditions, the shape of the gas envelope was elongated like the dispersion shape from leakage with small wind. The flammable cloud was ignited with SEMTEX explosive charges put at the center of the gas volume envelopes. Overpressure values were collected at different positions on the field. Flame propagation was recorded by high speed camera and gave a constant velocity of 2384 m.s-1 , indicating a detonation regime.

The Basic Results on Reinitiation Processes in Diffracting Multifront Detonations. Part I.

The basic results of experimental investigations on reinitiation processes in diffracting multifront detonation wave (DW) are discussed in connection with problem of practical application of new ecological technology for worn-out tire destruction, where the cooled worn-out tire destroys with the help of gaseous detonation. The experimental results for various fuel-oxygen and fuel-air mixtures are presented at wide range of mixture compositions, initial pressure and temperature, geometrical sizes of experimental equipment, symmetry types, dilution of inert gases, etc. Classical and nontraditional schema of DW-diffraction are investigated, such as multipointed initiation, DW-excitation by circular charge, initiation space-oriented longitudinal charges, initiation by circular charge, diffraction on concave boundary, diffraction on contact surface of different mixtures, flame diffraction, etc. The main characteristic parameters are identified for each diffraction schema. The physical processes taking place directly in the DW-front plays the governing role in reinitiation. The most important among these processes are collisions of transverse waves, which stick out as microscopic initiators. The optimization problem of DW-initiation from spatial and temporary distribution of energy is discussed carefully. This data can be used at hazard estimation also.

Ignition of ethanol-containing mixtures excited by nanosecond discharge above self-ignition threshold

The kinetics of ignition in lean and stoichiometric C2H5OH:O2:Ar mixtures by a high-voltage nanosecond discharge are studied experimentally and numerically for gas temperatures above self-ignition threshold. Autoignition delay time and ignition delay after a high-voltage nanosecond impulse discharge are measured using reflected shock-wave technique. It is shown that the additional excitation by the discharge plasma leads to an order of magnitude decrease in ignition delay. Discharge processes are simulated on the basis of the measured time–resolved discharge characteristics. The densities of atoms, radicals and excited and charged particles produced in the discharge phase are calculated and used as initial conditions for chemical modeling. Calculated ignition delay times agree well with measurements with and without the discharge plasma. Calculations show that the effect of the discharge plasma on ignition of the ethanol-containing mixtures is primarily associated with active species production in the discharge phase; the role of fast gas heating during the discharge and in its afterglow is relatively small. A method is suggested to compare the effect of nonequilibrium pulse discharge plasma on ignition in different fuel–air and fuel-oxygen mixtures above self-ignition temperatures. It is shown that this effect is more profound at low gas temperatures when the thermal production of initial radicals is not efficient. Due to O2 dissociation in the discharge phase, the influence of nonequilibrium discharge plasma on ignition is more pronounced for fuels with high activation energy of thermal dissociation (like H2 and CH4). The role of N2 in favoring plasma-assisted ignition owing to additional O2 and fuel dissociation is demonstrated.

Possibilities of the Computer-Controlled Detonation Spraying method: A chemistry viewpoint

This article is aimed to discuss the chemical aspects of detonation spraying of powder materials. In this method of coating deposition, ceramic, metallic or composite powders are injected into the barrel of a detonation gun filled with an explosive gaseous mixture. When the latter is ignited, the powders are heated and accelerated toward the substrate. Subjected to high temperatures, the powders are prone to chemical reactions, the reaction products possibly becoming the major phase constituents of the coatings. What types of reactions are possible? Can these reactions be carried out in a controlled manner? We answer these questions considering the interactions of the sprayed powders with the gaseous environment of the barrel as well as those between the phases of a composite feedstock powder. In Computer-Controlled Detonation Spraying (CCDS), the explosive charge and stoichiometry of the fuel-oxygen mixtures are precisely measured and can be flexibly changed. Our studies demonstrate that with the introduction of a highly flexible process of CCDS, detonation spraying has entered a new development stage, at which it can be considered as a powerful method of composition and microstructure tailoring of thermally sprayed coatings. During CCDS of TiO2-containing powders, chemical reduction of titanium dioxide can be carried out to different levels to form either oxygen-deficient TiO2−x or Ti3O5 suboxide. CCDS of Ti3Al can produce titanium oxide coatings when oxidation by the detonation products dominates or titanium nitride-titanium aluminide coatings when oxidation is hindered but the interaction of the powders with nitrogen—a carrier gas component—is favored. During detonation spraying of Ti3SiC2–Cu composites, the Ti3SiC2 phase is preserved only in cold conditions; otherwise, Si de-intercalates from the Ti3SiC2 phase and dissolves in Cu resulting in the formation of the TiCx–Cu(Si) composite coatings.

Velocity of deflagration combustion at high pressures and temperatures

The article presents additional calculated data on combustion velocity for typical gas mixtures at pressures up to 6000 atm and temperatures up to 4000 K. It was determined that combustion velocity is mainly influenced by initial temperature, and the dependence on pressure is rather low. For fuel-oxygen and air-fuel mixtures in three-dimensional space (pressure-temperature-velocity of deflagration combustion), we have found a single surface of combustion velocities and continuous dependence of flame velocity on initial conditions.

Experimental study of spark-discharge initiation of detonation

The mechanisms of action on a high-current spark discharge that make it possible to reduce the total expenditure of electric energy on initiating detonation have been proposed. The action is based on the control of a discharge current with the aid of an external circuit and on the addition of easily ionizable admixtures to a fuel-oxygen mixture. Results of experimental investigations into the spark-discharge initiation of detonation with the action on the discharge process have been presented; the results were obtained in mixtures of hydrogen with oxygen and of commercial propane-butane with oxygen at atmospheric pressure.

Methods of re-initiation and critical conditions for a planar detonation transforming to a cylindrical detonation within a confined volume

The conditions necessary for the successful transformation of a planar detonation into a diverging, cylindrical detonation after diffraction into a confined volume have been investigated for four different fuel–oxygen mixtures over a wide range of gap sizes. The test mixtures were H2+0.5O2, C2H2+2.5O2, C2H2+4O2 and C2H4+3O2, and the range of gap size (w) varied from 1 to 45.5mm. This range of mixtures and gap sizes plus the higher fidelity owing to pressure measurements along the endwall allowed greater precision when interpreting the results than what was seen in earlier studies, leading to the identification of a wider range of conditions for the successful transformation of the detonation than what was heretofore assumed. The spontaneous re-initiation and continuous reflected re-initiation mechanisms previously observed by Murray and Lee [8] were confirmed, but the present study improves upon the prior one through the conclusive observation that successful propagation of detonations after multiple reflections from the front and back walls of the confined volume is also possible. When compared with literature data, the current results are in fair agreement when only spontaneous and single-reflected re-initiation modes are considered, but when multiple reflections are included, the go/no-go boundary for successful transmission is much wider than found from the limited literature data available. The demarcation boundary was found to fit the relationship w/λ=0.14w+0.24, with λ being the cell size in the axial tube. In addition to significantly expanding the database on the axial-to-cylindrical detonation transition in a confined volume and obtaining a new correlation for the demarcation condition, the present paper also shows that there is an optimum distance from the exit of the detonation tube to the endwall, approximately equal to one tube diameter, which promotes successful transmission.

Critical energy of direct detonation initiation in gaseous fuel–oxygen mixtures

The critical energy of some gaseous fuels and oxygen mixtures at different initial pressure and equivalence ratios are measured based on an experiment system we developed. The determination of direct initiation of detonation is based on the overpressure signal from a pressure transducer. The effective energy responsible for the direct initiation is considered as the first quarter cycle of the current discharge. The effect of initial pressure and equivalence ratio on the critical energy of direct initiation is firstly investigated in some typical gaseous fuel (i.e., C2H2, C2H4, C3H8, H2) and oxygen mixtures. It shows that the relationship between critical energy and initial pressure is inversely exponential and it is ‘U’ shaped between critical energy and equivalence ratio. The study of the effect of different oxygen concentration and argon dilution on the critical energy of stoichiometric acetylene and oxygen is carried out, the results indicate that oxygen concentration significantly affects the critical energy, and the critical energy increases with increasing amount of argon dilution. This study finally quantitatively research on the detonation hazard parameter (DH) of each stoichiometric mixture. The value of DH in increasing order for the mixtures are as following: C2H2–O2, C2H2–O2–50%Ar, C2H4–O2, C2H2–O2–65%Ar, C2H2–O2–70%Ar, C3H8–O2 and H2–O2, the results also indicate the critical energy is different by orders of magnitude if fuels do not belong to the same group.

Experimental research on the rotating detonation in gaseous fuels–oxygen mixtures

An experimental study on rotating detonation is presented in this paper. The study was focused on the possibility of using rotating detonation in a rocket engine. The research was divided into two parts: the first part was devoted to obtaining the initiation of rotating detonation in fuel–oxygen mixture; the second was aimed at determination of the range of propagation stability as a function of chamber pressure, composition, and geometry. Additionally, thrust and specific impulse were determined in the latter stage. In the paper, only rich mixture is described, because using such a composition in rocket combustion chambers maximizes the specific impulse and thrust. In the experiments, two kinds of geometry were examined: cylindrical and cylindrical-conic, the latter can be simulated by a simple aerospike nozzle. Methane, ethane, and propane were used as fuel. The pressure–time courses in the manifolds and in the chamber are presented. The thrust–time profile and detonation velocity calculated from measured pressure peaks are shown. To confirm the performance of a rocket engine with rotating detonation as a high energy gas generator, a model of a simple engine was designed, built, and tested. In the tests, the model of the engine was connected to the dump tank. This solution enables different environmental conditions from a range of flight from 16 km altitude to sea level to be simulated. The obtained specific impulse for pressure in the chamber of max. 1.2 bar and a small nozzle expansion ratio of about 3.5 was close to 1,500 m/s.

Application of propane-butane in detonation deposition facilities

Extensive data on the length of the deflagration-to-detonation transition (DDT) are obtained for a wide range of explosive gas mixtures of acetylene, propane-butane, and methane with oxygen in tubes with constant and constricting cross sections. For the fuel-oxygen mixtures examined, it is found that the DDT length can be reduced to one tube diameter by using a volume system of obstacles with the size and distance between the obstacles being commensurable with the cell size of multifront detonation in these mixtures. The optimal mixture for deposition is found to be an explosive propane-butane mixture with oxygen in which the fraction of propane-butane is ≈25%.

Catalytic Partial Oxidation Pilot Plant Study

A new high-pressure catalytic partial oxidation reactor was developed for high-pressure production of synthesis gas from natural gas and pure oxygen, and the integration of the new reactor with the gas to liquids process was investigated. The experimental work was performed at Praxair’s laboratory, process design and economic estimates were performed by Foster Wheeler, and theoretical simulations were preformed by University of Delaware. The reactor system was based on a millisecond contact time mixer capable of delivering a fuel-oxygen mixture to the catalyst without onset of homogeneous reactions. Significant differences were observed between high- and low-pressure operation. With proper mixer and catalyst bed design the system was able to operate stably at pressures up to 18 bar with performance comparable to low-pressure operation. Process economic analysis predicted a small economic advantage for a catalytic partial oxidation reactor compared to conventional technology.

Reliability of kinetic data used for estimating multifront detonation parameters

A faster increase in the cell size and other important parameters of multifront detonation than that predicted by kinetic calculations is obtained for fuel-oxygen mixtures of hydrogen and typical hydrocarbons diluted by nitrogen. In a stoichiometric hydrogen-oxygen mixture diluted by an additional amount of oxygen or hydrogen, experimental and calculated data are also found to diverge with increasing concentration of the added species. This effect, however, is not observed if these mixtures are diluted by helium or argon. An assumption about the reason for this difference in data is put forward. A conclusion is made that kinetic data should be corrected as applied to detonation conditions.