Excess Enthalpy Combustion Research Articles

Numerical study on combustion characteristics of ammonia mixture under different combustion modes

Ammonia, an emerging carbon-free fuel, exhibits promising potential within the energy field. Nevertheless, it faces challenges such as unstable combustion and high NOx emissions that necessitate resolution. In this paper, numerical simulation methods are used to compare the combustion characteristics under different modes. Furthermore, the numerical research is carried out by adding ammonia gas in different combustion modes. The results indicated that regardless of the combustion mode, induction of ammonia elicits a significant surge in NOx emissions, but the combustion characteristics of different combustion modes are different. When ammonia is introduced into conventional combustion mode, ignition is significantly delayed. It is proved that the excess enthalpy combustion technology is effective in facilitating the combustion of ammonia mixtures. However, excess enthalpy combustion temperature will lead to elevate NO emissions. In contrast, MILD combustion and oxy-fuel MILD combustion technologies effectively suppress NO formation within ammonia mixtures. This is because in MILD combustion mode, the reaction rate becomes slow, which inhibits the formation of NO.

Hydrogen-air premixed combustion in a novel micro disc-burner with an annular step

A novel micro disc-burner with an annular step using the hydrogen/air premixed mixture is developed in this work, which can act as a heater or heat source with a high stability for the miniature power generators. The combustion and thermal performance of this combustor is studied by the means of numerical simulation. The results show that the excess enthalpy combustion appears at a small equivalence ratio. The thermal output power of the combustor increases with the increased Reynolds number. It is interesting to find that the upper and lower limits of thermal output power and its ratio in the total heat release amount change approximately linearly with the Reynolds number. It is found that the thermal output power and its ratio in the total heat release amount under any conditions are smaller than 102.8 W and 31.41%, respectively. The effects of the thermal conductivity and surface emissivity of solid wall on the combustion characteristics are studied. Both thermal output power and its ratio in the total heat release amount decrease with the increased thermal conductivity. Additionally, the thermal output power remarkably decreases with the decreased surface emissivity. The current work not only provides the guideline for applying this combustor but also helps us to gain insight into the heat transfer characteristic in the radial micro-channel.

Catalytically stabilized combustion characteristics of methane-air mixtures in micro-scale heat-recirculating systems

In applying catalysis to real combustion systems, the modeling of chemistry and transport interactions may be the key to understanding combustion characteristics. The catalytically stabilized combustion characteristics of a methane-air mixture in a micro-scale heat-recirculating system were investigated to gain a greater understanding of the mechanisms of flame stabilization and to gain new insights into how to design the system with improved robustness and stability. The system comprised catalytically-active and catalytically-inactive channels in a counterflow heat exchange relationship. The essential factors for design considerations were determined with improved flame stability and combustion characteristics. Two primary mechanisms responsible for the loss of flame stability were discussed. The results indicated that both chemical and thermal environments are improved with the catalytically stabilized combustion method and the heat-recirculating structure. The design incorporates the best features of both catalytic combustion and thermal flame methods. The system is essentially free of mass transfer limitations. Excess enthalpy combustion can occur in an efficient and rapid manner, resulting from the injection of free radicals and heat produced by the catalytic reaction. The flow velocity, wall thermal conductivity, equivalence ratio, exterior heat losses are important factors in determining the performance of the system. Stable operation of the system is limited to a relatively wide flow regime, and the flow velocity is critical to achieving flame stability. There is an optimum wall thermal conductivity in terms of flame stability. The system with a moderate wall thermal conductivity will be most robust against the surrounding conditions. Blowout shifts homogeneous combustion downstream significantly without substantially reducing the reaction rate. Finally, engineering maps denoting flame stability were constructed, and design recommendations were made.

Excess enthalpy combustion of methane-air in a novel micro non-premixed combustor with a flame holder and preheating channels

A novel micro non-premixed combustor with a flame holder and preheating channels is developed to improve the non-premixed flame stabilization for the first time. The numerical results indeed show that the methane-air non-premixed flames can maintain stable with no flashback or pulsating flame even at the stoichiometric ratio compared to the premixed flame, which indicates that the flame stabilization in the present combustor is indeed excellent. The maximum flame temperature at each operating condition is higher than the corresponding adiabatic flame temperature. The excess enthalpy ratio (Reer) is larger at a smaller nominal equivalence ratio (ϕ) and a bigger Reynolds number (Re), and the maximum Reer reaches 36.7%. With the decrease of ϕ, the area of excess enthalpy zone increases at smaller Re (<160), but expands firstly and then shrinks at a larger Re (≥160). In addition, some new non-premixed flame propagation characteristics are found. At the larger ϕ (>0.6), the flame top presents the “Λ” shape, which is significantly different from the flame topology structure of methane/air premixed flame. The standoff distance (Sroot) between the flame root and the flame holder at the larger Re decreases firstly and then increases with the decrease of ϕ, and the anomalous and non-monotonic Sroot are both found. These interesting flame propagation features are reported for the first time for the micro non-premixed flame. Researchers can gain insight into micro non-premixed flame propagation characteristics under the competitive effect between the combustion reaction and mixed performance through the present work.

Effect of Dimethyl Ether as an Additive to Liquefied Petroleum Gas Flame in SiC–Al2O3-Based Porous Inert Burner

To establish the applicability of dimethyl ether (DME-CH3OCH3) as an alternative fuel additive to liquefied petroleum gas (LPG: 48% n-C4H10, 25% i-C4H10, 23% C3H10, 4% C2H6) for enhancing combustion and reducing hazardous emissions, in this article, the flame behavior of LPG–air mixture blended with DME is studied within a highly conducting and radiating porous inert burner (PIB) under excess enthalpy combustion conditions. The experiments are performed in a two-section PIB comprising a silicon carbide (SiC) matrix and aluminum oxide (Al2O3) balls. The numerical model for solving the species continuity equation, energy balance equations for gas and solid phases, and the mass conservation equation is compared to the experimental data from the present work. New filtration velocity data are found for the stable combustion of LPG–DME–air mixtures inside the PIB. The effect of various DME volume fractions in the LPG–DME blends on temperature distribution, radical pool concentration, CO emission, reaction zone thickness, and syngas production within the PIB are investigated. The results show that with the addition of DME the peak radical pool concentration increases, which enhances the filtration velocity and thus the operating limit of the PIB. It is further demonstrated that at the same input conditions, the solid-phase temperature and preheating temperature of the PIB are greater for the higher value of DME volume fraction. Contrary to previous findings, increasing the DME level increases the peak mole fraction of CO, CH4, and CH3CHO; however, it decreases the equilibrium CO concentration. Furthermore, in the case of a pure DME flame, an order of magnitude decrease in C2H2, C3H3, and C6H6 is observed; however, the flame with higher DME fraction generates more CH2O and CH3.

A numerical study on heat-recirculation assisted combustion for small scale jet diffusion flames at near-extinction condition

In order to prove the existence of heat-recirculation (through burner wall) assisted flame stabilization mechanism for miniaturized jet diffusion flames, a series of two-dimensional axisymmetric numerical computations are performed for methane–air flames over a sub-millimeter (constant inner diameter of 0.8mm) jet. A skeletal mechanism including 17 species and 58 steps is employed for the chemical kinetics and detailed transport properties are considered. Burner is considered as thermally conductive and chemically inert. Heat fluxes responsible for heat recirculation (feedback) (Qre) and heat loss (Qloss) through the burner surfaces are calculated respectively from the numerical results. An effective excess-enthalpy (H) is defined and used to measure the global thermal effect of the burner on the flame. A wide range of fuel jet velocities (V, from the minimum value to sustain a steady state flame to 3.2m/s) for different burners with various thermal conductivities (kb, from 1.0W/m-K to 100W/m-K, which covers most of the realistic burner materials) and wall thicknesses (cb, from 0.2mm to 0.8mm) are considered in this work. The results show that, as V increases, the ratio of Qre to Qloss monotonically increases, resulting in a critical fuel jet velocity Vc, above which Qre exceeds Qloss and thus the heat recirculation effect could cancel out the heat loss effect by the presence of the burner. Further, it is suggested that the Vc can be reduced by reducing kb and cb, hence, heat-recirculation assisted combustion is found to be promoted at near extinction condition under such burner system. Flame temperatures are significantly elevated as kb or cb decreases at near extinction conditions, revealing that the extinction limit in fuel jet velocity can be extended under specific condition. Computations are then conducted for flames of hydrogen and dimethyl ether to examine the fuel dependence. It is indicated that the flame–burner thermal interaction is influenced by both physical properties of the fuel (Sc number) and chemical kinetics. For the fuels with Sc > 1, such as dimethyl ether (Sc = 1.2), flame is readily to be lifted, resulting in negligible flame–burner interaction (H ≈ 0). For the fuels with Sc < 1, such as methane (Sc = 0.7) and hydrogen (Sc = 0.2), chemical kinetics that dominating the flame base structure becomes the key factor. Due to the flame–burner attachment, an extremely high H can be expected for a hydrogen jet diffusion flame to achieve excess-enthalpy combustion at certain condition. Overall, this study implies that, given the narrow operation flow rate range restrained by the quenching and the blow off limits in a premixed combustion system, heat recirculation assisted stable combustion could be possible in a relatively wider flow rate (power output) range by choosing a diffusion combustion mode.

Flame Stability of Methane/Air Mixture in a Heat-Recirculating-Type Mesoscale Channel with a Bluff-Body

To extend the stable combustion range of micro-combustor, a heat-recirculating-type planar micro-combustor fitted with a bluff-body was proposed in the present work. Numerical simulation on CH4/air premixed combustion in this combustor was performed and the stable combustion range was determined, which showed that the blow-off limit increases with the equivalence ratio and the lower flammability limit was extended. Effect of the equivalence ratio and inlet velocity on combustion efficiency and maximum temperature were investigated. The numerical results showed that combustion efficiencies were higher than 99%, and the maximum temperatures were larger than the corresponding adiabatic flame temperature due to the excess enthalpy combustion effect. However, flashback emerged when the inlet velocity was too small and the equivalence ratio is relatively high.

Experimental study on premixed CH 4/air mixture combustion in micro Swiss-roll combustors

Excess enthalpy combustion is a promising approach to stabilize flame in micro-combustors. Using a Swiss-roll combustor configuration, excess enthalpy combustion can be conveniently achieved. In this work, three types of Swiss-roll combustors with double spiral-shaped channels were designed and fabricated. The combustors were tested using methane/air mixtures of various equivalence ratios. Both temperature distributions and extinction limits were determined for each combustor configuration at different methane mass flow rates. Results indicate that the Swiss-roll combustors developed in the current study greatly enhance combustion stability in center regions of the combustors. At the same time, excess enthalpy combustors of the Swiss-roll configuration significantly extend the extinction limits of methane/air mixtures. In addition, the effects of combustor configurations and thermal insulation arrangements on temperature distributions and extinction limits were evaluated. With heat losses to the environment being significant, the use of thermal insulations further enhances the flame stability in center regions of the Swiss-roll combustors and extends flammable ranges.

Numerical simulation of excess-enthalpy combustion flame propagation of coal mine methane in ceramic foam

Based on the assumption of a local non-equilibrium of heat transfer between a solid matrix and gas, a mathematic model of coal mine methane combustion in a porous medium was established, as well the solid-gas boundary conditions. We simulated numerically the flame propagation characteristics. The results show that the flame velocity in ceramic foam is higher than that of free laminar flows; the maximum flame velocity depends on the combined effects of a radiation extinction coefficient and convection heat transfer in ceramic foam and rises with an increase in the chemical equivalent ratio. The radiation extinction coefficient cannot be used alone to determine the heat regeneration effects in the design of ceramic foam burners.

New technology for coalbed methane power generation based on Stirling engine driven by porous burner

The development of coalbed methane (CBM) power generation is restricted by the gas source instability. The existing technologies such as steam turbine, gas turbine, and gas engine are not suitable for the instability. So a new technology of CBM power generation is put forward which is a Stirling engine system driven by porous burner. The Stirling engine is an external-combustion engine and its working fluid is not involved in combustion. Stirling engine works as long as heat supply is guaranteed, and therefore it is not conditioned by the gas source instability in itself. Porous burner is a premixed combustion technology in porous medium based on the excess enthalpy combustion. The porous medium is characteristic of the strong heat storage capability and the small combustion pore diameter, and accordingly it adapts well to the gas source instability. So the new CBM power generation technology, which combines the external-combustion distinction of Stirling engine and well adaptability to the gas source instability of porous burner, will agree with the coalbed gas source instability.

Physical properties of a LPG flame with high-temperature air on a regenerative burner

Oxygen-diluted Combustion (OdC) technology has evolved from the concept of Excess Enthalpy Combustion and is characterized by reactants of low oxygen concentration and high temperature. Recent advances in this technology have demonstrated significant energy savings, high and uniform thermal field, low pollution, and the possibility for downsizing the equipment for a range of furnace applications. Moreover, the technology has shown promise for wider applications in various processes and power industries. The objectives of this thesis are to analyze the thermodynamic aspects of this novel combustion technology and to quantify the enhancement in efficiency and heat transfer inside a furnace in order to explore the potentials for reduced thermodynamic irreversibility of a combustion process and reduced energy consumption in an industrial furnace. Therefore, theoretical and experimental investigations were carried out. The 2nd law of thermodynamics analyses of OdC systems have been carried out for cases in which the oxidizer is either oxygen (Flameless-oxy-fuel) or air (High Temperature Air Combustion, HiTAC). The analyses demonstrate the possibilities of reducing thermodynamic irreversibility of combustion by considering an oxygen-diluted combustion process that utilizes both gas- and/or heat-recirculation. Furthermore, the results showed that an oxygen-diluted combustion system that utilizes oxygen as an oxidizer, in place of air, results in higher 1st and 2nd law efficiencies. Mathematical models for heat regenerators were developed to be designing tools for maximized heat recovery. These models were verified by heat performance experiments carried out on various heat regenerators. Furthermore, experiments were performed in a semi-industrial test furnace. It was equipped with various regenerative burning systems to establish combustion and heat transfer conditions prevailing in an industrial furnace operating based on HiTAC. The tests were carried out at seven firing configurations, two conventional and five HiTAC configurations, for direct and indirect heating systems. Measurements of energy balance were performed on the test furnace at various configurations in order to obtain the 1st law efficiency. Moreover, local measurements of temperature, gas composition, and heat fluxes in the semi-industrial test furnace were performed to find out the main characteristics of HiTAC flame and the effects of these characteristics on the heating potential, i.e., useful heating in the furnace. In the case of HiTAC, these measurements showed uniformities of chemistry, temperature, temperature fluctuation, and heat fluxes profiles. The values of fluctuations in temperature were small. The high speed jets of the fuel and air penetrated deep into the furnace. The fuel gradually disappeared while intermediate species gradually appeared in relatively high concentrations and at broader regions inside the furnace. These findings indicate: a large reaction zone, low specific combustion intensity in the flame, low specific fuel energy release, and high heat release from this large flame. In addition to the thermodynamic limitations to the maximum temperature of the Oxygen-diluted Combustion, the low specific energy release of the fuel and the high heat release from the flame to its surroundings cause this uniform and relatively moderate temperature profile in a HiTAC flame, consequently suppressing thermal-NO formation. Heat flux and energy balance measurements showed that heating potential is significantly increased in the case of HiTAC compared to that in the conventional case, implying much more energy savings than the apparent heat recovery from the heat regenerators, and consequently much less pollutants emissions. Therefore, it is certain that this large HiTAC flame emits more thermal radiation to its surroundings than the conventional flame does, in spite of the moderate-uniform temperature profile of the flame. This intense heat flux was more uniform in all HiTAC configurations, including the indirect heating configuration, than that of the conventional-air combustion configuration.

Thermal Characteristics of Gaseous Fuel Flames Using High Temperature Air

Recent advances on high temperature air combustion (HiTAC) have demonstrated significant energy savings, higher and uniform thermal field, lower pollution, and smaller size of the equipment for a range of furnace applications. The HiTAC technology has evolved from the conception of excess enthalpy combustion (EEC) to high and ultra-high preheated air combustion. In the HiTAC method, combined heat regeneration and low oxygen methods are utilized to enlarge and control the flame thermal behavior. This technology has shown promise for much wider applications in various process and power industries, energy conversion, and waste to clean fuel conversion. For each application the flow, thermal, and chemical behavior of HiTAC flames must be carefully tailored to satisfy the specific needs. Qualitative and quantitative results are presented on several gas-air diffusion flames using high-temperature combustion air. A specially designed regenerative combustion test furnace facility, built by Nippon Furnace Kogyo, Japan, was used to preheat the combustion air to elevated temperatures. The flames with highly preheated combustion air were significantly more stable and homogeneous (both temporally and spatially) as compared to the flames with room-temperature combustion air. The global flame features showed the flame color to change from yellow to blue to bluish-green to green over the range of conditions examined. In some cases hybrid and purple color flame was also observed. Under certain conditions flameless or colorless oxidation of the fuel has also been demonstrated. Information on global flame features, flame spectral emission characteristics, spatial distribution of OH, CH, and C2 species and emission of pollutants has been obtained. Low levels of NOx along with negligible levels of CO and HC have been obtained using high-temperature combustion air. The thermal and chemical behavior of high-temperature air combustion flames depends on fuel property, preheat temperature, and oxygen concentration of air. Waste heat from a furnace in high-temperature air combustion technology is retrieved and introduced back into the furnace using regenerator. These features help save energy, which subsequently also reduce the emission of CO2 (greenhouse gas) to the environment. Flames with high temperature air provide significantly higher and uniform heat flux than normal air, which reduces the equipment size or increases the process material throughput for same size of the equipment. The high-temperature air combustion technology can provide significant energy savings (up to about 60%), downsizing of the equipment (about 30%), and pollution reduction (about 25%). Fuel energy savings directly translates to a reduction of CO2 and other greenhouse gases to the environment.

A study on combustion characteristics of superadiabatic combustor in porous media

Because of the energy resource exhaustion, the aggravating environmental air pollution, the smoke phenomena and so on, the recent trends and targets in designing combustor are reduction of pollutant emissions and improvement of combustor efficiency. Therefore many combustion methods and emission control technologies have been proposed by many researchers through numerical and experimental analyses, One of the most available and effective combustion methods is the excess enthalpy combustion, so called, the superadiabatic combustion. In this study, the superadiabatic combustion with the reciprocating flow in a porous media has been investigated with the variation of equivalence ratio, flow velocity and reciprocating cycle time. In this system, the flow direction is reversed regularly by the solenoid valves. The results of this study show that the maximum gas temperature is remarkably higher than the theoretical adiabatic flame temperature and the emission characteristic is very excellent. The analyses reveal several attractive characteristics of the flame and the proposed idea is promising to burn mixtures of low heat content in a reciprocating type combustor. This combustor can be applied to the elimination of unburned compound, with more intensive and continuous study.