High Temperature Air Combustion Research Articles

Novel conceptual design of a supercritical pulverized coal boiler utilizing high temperature air combustion (HTAC) technology

Technical and ecological aspects of implementation of High Temperature Air Combustion to power station boilers fired with pulverized coal have been considered. Several boiler concepts have been examined in the context of the following three key points: existence of an intensive in-furnace recirculation, homogeneity of both the temperature and the chemical species fields, and uniformity of heat fluxes. CFD-based numerical simulations have been performed in order to determine the shape of the boiler and its dimensions, to optimize both the distance between burners and location of the burner block. It was concluded that HTAC technology could be a realizable, efficient and clean technology for pulverized coal fired boilers.

A Constant-Volume Bomb and Co-Flow Reactor Investigation of Ignition Phenomenon of Hydrocarbon Fuels in High Temperature Oxidizer

The paper presents results of experimental studies on the methane and propane ignition process in various volumetric compositions of oxidizers heated to temperature higher than ignition point of analyzed fuels (HTAC conditions – High Temperature Air Combustion conditions). The increment of temperature D T and the ignition delay time t ’ are parameters characterizing the process of ignition under these conditions. These parameters are the function of air excess ratio l , temperature of the oxidizer t oxi and the volumetric composition of oxidizer z i . In order to achieve the minimum ignition delay time t ’ and the maximum increment of the temperature D T, the oxidizer temperature didn’t have to be maximized. There is an interval of t oxi in which analyzed parameters reaches its extreme values.

Characterisation of heat transfer and flame length in a semi-scale industrial furnace equipped with HiTAC burner

Abstract Abstract This paper investigates the effects of multiple burner nozzles on the combustion characteristics, such as flame volume, heat transfer and NOx emission in a high temperature air combustion (HiTAC) industrial furnace. Experiments were carried out in one semi-industrial furnace located in Kungliga Tekniska Hogskolan (Stockholm, Sweden). Three different types of burners were tested, including both regenerative and recuperative types. Variable flame temperature and oxygen concentration were applied in experiments. Heat transfer characteristics of HiTAC are studied in this paper, and the influences of a variety of inertial fuel/air jets are investigated for both flame length and NOx emission. One improved correlation between chemical flame length and flame Froude number is established for HiTAC with manifold nozzles. NOx emission is also correlated to the flame Froude number. The HiTAC recirculation system effects on flame shape, NOx emission and heat transfer were also examined.

263 高温空気燃焼の可燃限界の解明(OS4-2 燃焼現象の計測,予測,応用2,OS4 燃焼現象の計測,予測,応用)

263 高温空気燃焼の可燃限界の解明(OS4-2 燃焼現象の計測,予測,応用2,OS4 燃焼現象の計測,予測,応用)

In-Depth Experimental Study of Solid-Waste Destruction by High Temperature Air Combustion

The objective of this article is to investigate the application of high temperature air combustion in thermal destruction of solid waste. The secondary air was preheated at different temperatures and burned with low calorific value and high content of diluted inert gas released from solid waste. It was discovered that the combustion with the higher preheated secondary air temperature had affected an increase in average flame temperature, and reduction in supplementary fuel consumption, but caused higher NOX emission. The maximum percentage of energy saving and NOX emission increasing occurred in the case of preheated secondary air at 900°C with a value of 56% and 40%, respectively.

Experimental Research on Heat Transfer Performance of Honeycomb Heat Regenerator

Heat transfer characteristics of honeycomb ceramic regenerator has been examined experimentally. The influence of switching time, flow rate of air and fuel gas on temperature distribution under unsteady heat transfer process in regenerator has been investigated. The results obtained are of some reference value for application of high temperature air combustion (HTAC). Key words: honeycomb ceramic regenerator, heat transfer, HTAC technology, molten steel container

Numerical Simulation of Combustion Characteristics in High Temperature Air Combustion Furnace

The influences of air preheating temperature, oxygen concentration, and fuel inlet temperature on flame properties, and NO, formation and emission in the furnace were studied with numerical simulation. The turbulence behavior was modeled using the standard k-ɛ model with wall function, and radiation was handled using discrete ordi-nate radiation model. The PDF (probability density function)/mixture fraction combustion model was used to simulate the propane combustion. Additionally, computations of NO x formation rates and NO x concentration were carried out using a post-processor on the basis of previously calculated velocities, turbulence, temperature, and chemical composition fields. The results showed that high temperature air combustion (HiTAC) is spread over a much larger volume than traditional combustion, flame volume increases with a reduction of oxygen concentration, and this trend is clearer if oxygen concentration in the preheated air is below 10%. The temperature profile becomes more uniform when oxygen concentration in preheated air decreases, especially at low oxygen levels. Increase in fuel inlet temperature lessens the mixing of the fuel and air in primary combustion zone, creates more uniform distribution of reactants inside the flame, decreases the maximum temperature in furnace, and reduces NO x emission greatly.

Numerical simulation of high temperature air combustion in aluminum hydroxide gas suspension calcinations

The high temperature air combustion(HiTAC) process in gas suspension calcinations(GSC) was studied by using a CFD software FLUENT that can simulate the three-dimensional physical model of GSC with the k-epsilon turbulent viscous model, PDF non-premixed combustion species model, P1 radiation model, thermal and prompt NO pollution model. The simulation vividly describes the distributions of the temperature, velocity and consistency fields. Finally, the optimal operation conditions and igniter configuration of particular fuel combustion are obtained by analyzing and comparing the simulation results. And the emission quantity of NO x , CO and CO 2 deduced from computation can play a role as reference. These optimal and estimated values are beneficial to practical operation.

高温空気燃焼のNO_x排出特性に対するノズル間隔の影響(熱工学,内燃機関,動力など)

高温空気燃焼のNO_x排出特性に対するノズル間隔の影響(熱工学,内燃機関,動力など)

Characteristics of Pyrolysis and Combustion of Polymers in Stagnation-Point Flow for Preheated and Diluted Air with H2O and CO2

Combustion experiments on polypropylene (PP) and polyethylene terephethalate (PET) in stagnation-point flow were performed to investigate the fundamental characteristics of these polymers in high-temperature air combustion (HiTAC). Numerical study was also performed to estimate the regression rate and to determine the kinetic parameter of pyrolysis. In the experiments on PP combustion, extinction limits and sooting limits were found to be extended when highly preheated air was used. In the case of H2O and CO2 dilutions, the dilution enhanced regression rates at low stretch rates. In the case of PET combustion, results indicated that regression rates depended on the production rate of char. The kinetic parameters of PP pyrolysis under the combustion conditions were estimated using a new method in which experiments and numerical simulations were combined. The regression rates calculated using the kinetic parameters obtained in the present study were in good agreement with those of the experiments in contrast with the numerical results using the kinetic parameters obtained using the previously reported thermal gravimetric analysis (TGA).

Dilution effects of superheated water vapor on turbulent premixed flames at high pressure and high temperature

Methane/air turbulent premixed flames diluted with superheated water vapor at high-pressure and high-temperature were experimentally investigated to explore the effects of recycled water vapor on turbulent flame characteristics from the viewpoint of applying high-temperature air combustion (HiTAC) to high-load combustors as well as elucidating those effects of exhaust gas recirculation (EGR) in IC engine. A newly devised water evaporator was installed in a high-pressure chamber and superheated water vapor was successfully supplied to air up to 1.0 MPa and 573 K. The maximum dilution ratio defined as the ratio of the molar fraction of H 2O to those of air and H 2O was 0.1. Turbulent burning velocity, mean volume and the structure of the turbulent flame region were compared with those of flames diluted with CO 2, reported previously, which is another major species in recycled burnt gas. Results showed that the effects of superheated water vapor dilution on turbulent burning velocity, S T, normalized by laminar burning velocity, S L, was much weaker than that of CO 2 dilution. The mean volume of the turbulent flame region defined as the region between 〈 c〉 = 0.1 and 〈 c〉 = 0.9, was scarcely changed either. This means that the effects of recycled burnt gas on the structure of turbulent premixed flames at high pressure and high-temperature is predominated by CO 2 but not by superheated water vapor, indicating that suppression of combustion oscillation in premixed-type gas-turbine combustors by extension of the volume of heat release region is due to recycled CO 2. The emission indices of CO and NO x were also measured at high pressure, and it was proved that water vapor dilution is effective to restrain CO emission, which is a possible defect of HiTAC when it is applied to gas-turbine combustors.

Effects of fuel composition in high temperature air combustion

High temperature air combustion is of great interest since it simultaneously provides higher thermal efficiency together with low NOx emissions. These systems have been applied in numerous industri...

Sulfur Recovery from Acid Gas Using the Claus Process and High Temperature Air Combustion (HiTAC) Technology

Sulfur-bearing compounds are very detrimental to the environment and to industrial process equipment. They are often obtained or formed as a by-product of separation and thermal processing of fuels containing sulfur, such as coal, crude oil and natural gas. The two sulfur compounds, which need special attention, are: hydrogen sulfide (H2S) and sulfur dioxide (SO2). H2S is a highly corrosive gas with a foul smell. SO2 is a toxic gas responsible for acid rain formation and equipment corrosion. Various methods of reducing pollutants containing sulfur are described in this paper, with a focus on the modified Claus process, enhanced by the use of High Temperature Air Combustion (HiTAC) technology in the Claus furnace. The Claus process has been known and used in the industry for over 100 years. It involves thermal oxidation of hydrogen sulfide and its reaction with sulfur dioxide to form sulfur and water vapor. This process is equilibrium-limited and usually achieves efficiencies in the range of 94-97%, which have been regarded as acceptable in the past years. Nowadays strict air pollution regulations regarding hydrogen sulfide and sulfur dioxide emissions call for nearly 100% efficiency, which can only be achieved with process modifications. High temperature air combustion technology or otherwise called flameless (or colorless) combustion is proposed here for application in Claus furnaces, especially those employing lean acid gas streams, which cannot be burned without the use of auxiliary fuel or oxygen enrichment under standard conditions. With the use of HiTAC it has been shown, however, that fuel-lean, Low Calorific Value (LCV) fuels can be burned with very uniform thermal fields without the need for fuel enrichment or oxygen addition. The uniform temperature distribution favors clean and efficient burning with an additional advantage of significant reduction of NOx, CO and hydrocarbon emission.

High-Temperature Air Combustion Phenomena and Its Thermodynamics

Abstract The fundamentals and thermodynamic analysis of high-temperature air combustion (HiTAC) technology is presented. The HiTAC is characterized by high temperature of combustion air having low oxygen concentration. This study provides a theoretical analysis of HiTAC process from the thermodynamic point of view. The results demonstrate the possibilities of reducing thermodynamic irreversibility of combustion by considering an oxygen-deficient combustion process that utilizes both gas and heat recirculations. HiTAC conditions reduce irreversibility. Furthermore, combustion with the use of oxygen (in place of air) is also analyzed. The results showed that a system, which utilizes oxygen as an oxidizer, results in higher first and second law efficiencies as compared to the case with air as the oxidizer. The entropy generation for an adiabatic combustion process is reduced by more than 60% due to the effect of either preheating or oxygen enrichment. This study is aimed at providing technical guidance to further improve efficiency of a combustion process, which shows very small temperature increases due to mild chemical reactions.

Numerical Study of Radiation Effects on Polypropylene Combustion Using High-temperature Oxidizer Diluted with H2O and CO2

Combustion of polypropylene (PP) in a high-temperature, low-oxygen oxidizer enriched with H2O and CO2 in stagnation-point flow was studied numerically to explore fundamental characteristics of polymer incineration under the condition of high-temperature air combustion (HiTAC). The detailed chemistry of propene as a decomposition gas was used for the calculation. Two typical gas radiation models, i.e., the optically thin model (OTM) and the statistical narrow-band (SNB) model, were employed to clarify the effect of gas radiation on PP combustion as well as the validity of the use of these models under HiTAC conditions because the H2O and CO2 included in the burnt gas recirculated in HiTAC furnaces are highly radiative species. Results showed that, under HiTAC conditions, calculations using OTM overpredicts regression rates compared with those using SNB, indicating that OTM is not suitable for use with polymer combustion under HiTAC conditions, while the differences of these gas radiation models were slight when ordinary air at high temperature was used as an oxidizer. It was also shown that when SNB was used, CO2 enrichment in the oxidizer hardly enhanced the regression because CO2 near the PP surface behaves as a barrier against the radiative heat flux. The most effective condition to increase the regression rate is to maintain a higher concentration of H2O in the high-temperature oxidizer so as to enhance the gas radiation to the PP surface.

Electrostatic probe measurement in an industrial furnace for high-temperature air conditions

Electrostatic probe measurements are reported that identify flame location, displacement speeds of reaction region, and other flame properties within an industrial furnace that is operated with high-temperature preheated air. The electrostatic probe has advantages over other methods when a furnace is operated with high-temperature air. The probe consisted of a fine detection wire and a supporting tube that played a role of the reference electrode. The reaction regions were found to be widely dispersed and weakened as they moved downstream. However, the ion-current signals still included many sharp peaks, perhaps associated with the thin reaction thickness, contrary to the flame structure expected from the high-temperature air combustion. It was also possible to estimate the displacement speeds of reaction region by using the cross-correlation method between two ion current records detected by parallel detection components. The results demonstrate that the electrostatic probe is useful to detect the structure and state of the reaction mode in industrial furnaces even in the presence of high-temperature air combustion.

Structure of reaction zone of normal temperature air flameless combustion in a 2 ton/h coal-fired boiler furnace

By injecting normal temperature air into a coal-fired boiler furnace at an evaporation capacity of 2 ton/h, flameless combustion with little noise is achieved. Numerical simulations and experimental research shows that the combustion takes place in a wide and broad area, almost the whole furnace, resulting in a voluminal reaction. For this normal temperature air flameless combustion (NTAFC), when the excess air coefficient approaches 1, the fuel combusts fully. The concentrations of NO x and CO in flue gas are relatively low and are hardly affected by either an excess air coefficient or thermal load. In front of the jet is a low-temperature gas-mixing zone whose diameter is equivalent to that of the jet, and the temperature of this zone increases with the distance from the jet tip. A mild stable combustion reaction without a visible flame takes place in the part of the furnace where the temperature value is higher and the amplitude of variation is not large. In addition to the advantages shared with high-temperature air combustion (e.g. uniform temperature distribution, low NO x and CO emission, high combustion efficiency, and so on), NTAFC does not require a high-temperature air-preheating system and is easier to actualize.

Dynamic simulation on effect of flame arrangement on thermal process of regenerative reheating furnace

By analyzing the characteristics of combustion and billet heating process, a 3-D transient computer fluid dynamic simulation system based on commercial software CFX4.3 and some self-programmed codes were developed to simulate the thermal process in a continuous heating furnace using high temperature air combustion technology. The effects of different switching modes on injection entrancement of multi burners, combustion and billet heating process in furnace were analyzed numerically, and the computational results were compared with on-site measurement, which verified the practicability of this numerical simulation system. The results indicate that the flow pattern and distribution of temperature in regenerative reheating furnace with partial same-side-switching combustion mode are favorable to satisfy the high quality requirements of reheating, in which the terminal heating temperature of billets is more than 1 460 K and the temperature difference between two nodes is not more than 10 K. But since the surface average temperature of billets apart from heating zone is only about 1 350 K and continued heating is needed in soaking zone, the design and operation of current state are still needed to be optimized to improve the temperature schedule of billet heating. The distribution of velocity and temperature in regenerative reheating furnace with same-side-switching combustion mode cannot satisfy the even and fast heating process. The terminal heating temperature of billets is lower than that of the former case by 30 K. The distribution of flow and temperature can be improved by using cross-switching combustion mode, whose terminal temperature of billets is about 1 470 K with small temperature difference within 10 K.

LOCAL REACTION ZONE STRUCTURE OF NON-PREMIXED FLAMES OF PROPANE WITH HIGHLY PREHEATED LOW-OXYGEN AIR

To investigate the instantaneous structure of the local reaction zone of the HiCOT (High Temperature Air Combustion Technology) flames, the characteristics of ion current obtained from the non-premixed flames of propane with highly preheated low-oxygen air have been closely examined. Nonluminous (bluish-green) region is seen in the upstream part and luminous (bright yellow) region is seen in the downstream part of the non-premixed flames of propane with highly preheated low-oxygen air, while only the luminous region is seen in the non-premixed flames of propane with highly preheated regular air. The ion current fluctuations, obtained from the nonluminous region of the HiCOT flames, look quite similar with those obtained from turbulent premixed flames in the reaction sheet regime, implying that the unburnt mixture, burnt gas and local reaction zone exist separately. It has been clearly shown from the ion current that homogenous combustion does not occur but the local reaction zone exists separately from burnt and unburnt gases. The thickness of the local reaction zone in the nonluminous region of HiCOT flames is not broadened but is similar to that of the laminar premixed flames. Thickness of the local reaction zone in the non-premixed flames of propane with highly preheated regular air is estimated approximately twice as much as that in the nonluminous regions of the HiCOT flames.

Studies on a High-Temperature Air Combustion Burner for a Compact Fuel-Cell Reformer

A new burner configuration for a compact fuel-cell reformer with a high-temperature air combustion concept was studied. The burner was computationally designed for a 40 Nm3/hr hydrogen-generating reformer using natural gas-steam reforming method. In order to satisfy the primary requirements for designing a reformer burner (uniform distribution of temperature along the fuel processor walls and minimum heat losses from the reformer), the features of the present burner configuration included 1) a self-regenerative burner for an exhaust-gas-recirculation to apply for the high-temperature air combustion concept, and 2) an annular-type shield for protecting direct contact of flame with the processor walls. For the present design conditions, the predicted temperature distributions along the processor walls were found uniform within 100 K variation. Thus, the present burner configuration satisfied the requirement for reducing temperature gradients along the processor walls, and consequently demonstrated that the high-temperature air combustion concept could be applied to the practical fuel reformers for use of fuel cells. The predicted uniform temperature distributions along the processor walls were experimentally demonstrated for a test burner.