Air Preheat Temperature Research Articles

Combined Approach Supporting Integrated Furnace Design and Retrofit

Existing complicated and complex calculation procedures for the design of process tubular furnaces create the main reason why the connection of targeting and detail design stage of furnaces (i.e., the continuous design and/or retrofit approach) is a serious problem in design practice. A complex, newly developed approach of furnace design and/or retrofit is presented in this paper. This comprehensive approach involves the application of heat transfer (radiation and convection) in equipment design, thermodynamic analysis for process design from the point of view of furnace integration, and the latest approach consisting in involving process fluid flow and hydrodynamic analysis to avoid potential problems in operation. This new methodology bridges existing gaps between modern targeting methods based on pinch analysis (providing optimized parameters such as the temperature of preheated air, stack temperature, and excess air) and the detailed design of furnaces. It is partly interactive and based on three stages of design: targeting, synthesis, and detailed design. It can be applied for both grassroots and retrofits problems.

Measurements of droplets characteristics in a swirl-stabilized spray flame

Experiments have been performed in a kerosene airblast atomized spray flame where the fuel supply is sandwiched between two coswirling air streams. A phase-Doppler particle sizing system was used to measure fuel droplet size, velocity and turbulent kinetic energy of droplets and gas as well as the fuel volume flux within the combustor for two values of air preheat temperature, 200 °C and 400 °C. The results reveal that a small liquid fuel exists in the centre of the combustor and due to larger droplets with the swirl effect; the droplets characteristics (velocity and turbulent kinetic energy) did not follow the gas ones. Based upon these measurements, an optimum swirl number will exist with every atomization and burner arrangement of a liquid-fuelled flame associated and will be different from that associated with the corresponding gas-fuelled flame. The investigated atomizer shows a marked influence of the inlet air temperature on the measured droplet size which may be attributed to the design of the internal airflow, promoting prompt atomization at the dominant atomization mode.

The experimental study on regenerative heat transfer in high temperature air combustion

For the purpose of decomposing the processing gases CF4 from semiconductor manufacturers, ceramic honeycomb regenerative burner system is suggested by using the principle of HTAC. A simulated high temperature air combustion furnace has been used to determine the features of HTAC flames and the results of the decomposition of CF4. The preheat air temperature of it is above 900°C. The exhaust gas released into the atmosphere is lower than 150°C. Moreover, the efficiency of recovery of waste heat is higher than 80%, the NOx level in exhaust gas is less than 198 mg/m3 and the distribution of temperature in the furnace is nearly uniform. The factors influencing on heat transfer, temperature profile in chamber and NOx emission were discussed. Also some CF4 can be decomposed in this system.

Exergy analysis on combustion and energy conversion processes

When we consider exergy analysis on combustion and thermodynamic processes, we introduce another concept against energy analysis, which is supported by an evaluation of its temperature level. When a higher temperature energy than that an ambient level is taken into consideration, it can be put for some domestic or industrial purpose. A medium temperature energy of 30–60 °C is used for domestic heating, and a high temperature of 200 °C and above is suitable for power generation or process heating. Therefore, we study exergy concept supported by temperature level. When we discuss power generation, a high temperature energy of 1500 °C and above in combined cycle has a higher conversion efficiency than that of 500–600 °C in steam cycle. If we try to apply high temperature air combustion, a preheated air temperature of 1000 °C and above can be produced by exhaust heat recovery from stack gas, which has been developed as a new technology of energy conservation. In this study, the authors present an exergy analysis on combustion and energy conversion processes, which is based on the above-mentioned concept of exergy and energy supported by temperature level. When we discuss high temperature air combustion in furnace, this process shows a higher performance than that of the ambient air combustion. Furthermore, when we discuss the power generation and heat pump processes, the minimum ambient temperature would already be known for each season, and the conversion performance can be estimated by the maximum operating temperature in their cycles. So, the authors attempt to calculate the exergy and energy values for combustion, power generation and heat pump processes.

구형축열체를 이용한 축열기의 설계인자도출

Heat regenerator occupied by regenerative materials improves thermal efficiency of combustion system through the recovery of sensible heat of exhaust gases. By using one-dimensional two-phase fluid dynamics model, the unsteady thermal flow of regenerator with spherical particles, was numerically analyzed to evaluate the heat transfer and pressure losses and to derive the design parameter for heat regenerator. It is confirmed that the computational results, such as air preheat temperature, exhausted gases outlet temperature, and pressure losses, agreed well with the experimental data. The thermal flow in heat regenerator varies with porosity, configuration of regenerator and diameter of regenerative particle. As the gas velocity increases with decreasing the cross-sectional area of the regenerator, the heat transfer between gas and particle enhances and pressure losses decrease. As particle diameter decreases, the air is preheated higher and the exhaust gases are cooled lower with the increase of pressure losses. Assuming a given exhaust gases temperature at the<br/> regenerator outlet, the regenerator need to be linearly lengthened with inlet Reynolds number of exhaust gases, which is defined as a regenerator design parameter.

고온 예열 공기에 의한 난류 비예혼합 화염의 연소 특성

An experiment using high preheated air in a turbulent non-premixed flame was performed to investigate the effects of high preheated air on the combustion characteristics. Combustion using high preheated and diluted air with flue gas is a new technology which enables NO emission to be reduced. In this study, Na was used as diluent and propane as fuel. Combustion characteristics, especially the distributions of the flame temperature, NO concentration and OH radical intensity were examined under the condition of 300 K, 600 K, 1000 K in terms of the combustion air temperature, and also under the condition of the dilution level from 21% to 13% in terms of oxygen concentration. As the preheated air temperature increased, it appeared that the flame length became shorter, maximum flame temperature increased, the reaction region moved to upstream, and NO concentration increased, but the flame temperature`s fluctuation was reduced. In opposite, it was shown with decrement of oxygen concentration at the maximum temperature that both maximum value and the gradient of the flame temperature decreased, and NO emission also decreased considerably, but its fluctuation became larger, being inclined to be unstable.

Ignition Phenomena of PMMA Rod in Highly Preheated Air Flow.

Ignition process and burning characteristics of a polymethylmethacrylate (PMMA) rod in highly preheated convective environment are investigated experimentally. The main parameters are O2 concentration, flow velocity, preheated air temperature and oxygen dilution gas. The convective ignition can be classified into three steps : blue flame generation, attached flame formation and envelope flame development. Convective ignition of PMMA suddenly exposed to high temperature air is initiated in downstream and dim blue premixed flame is formed there. When the pyrolysis of solid fuel increases enough for flame propagation, the blue flame propagates to upstream to form an attached flame or luminous envelope flame. The ignition delay time is almost independent on oxygen concentration because the initial blue flame formation process is mainly controlled by pyrolysis rate. However, the following attached flame and envelope flame formation time increases with decrease in the oxygen concentration since these processes are chiefly dominated by burning velocity.

619 層流対向流火炎におけるNOx排出特性の数値シミュレーション : 予熱空気と伸張率の影響(GS-6 熱工学(シミュレーション))

Highly preheated air combustion by heat regeneration has been paid much attention in recent combustion industries. In this study, the effects of preheated air temperature and stretch rate on the formation of nitric oxide in flames were studied by using numerical calculation of laminar counter-flow flames. To exclude effects of other things, oxidizer was diluted by nitrogen. Results show that in the case of low stretch rate the effects of preheated air temperature, oxide mole fraction, and the highest temperature of flames on emission index of NOx were greater than the case of high stretch rate. It is also comforted that the highest temperature of flames and emission index were lower in the case of higher stretch rate.

Rapid Liquid Fuel Mixing for Lean-Burning Combustors: Low-Power Performance

Designers of advanced gas turbine combustors are considering lean direct injection strategies to achieve low NOx emission levels. In the present study, the performance of a multipoint radial airblast fuel injector Lean Burn injector (LBI) is explored for various conditions that target low-power gas turbine engine operation. Reacting tests were conducted in a model can combustor at 4 and 6.6 atm, and at a dome air preheat temperature of 533 K, using Jet-A as the liquid fuel. Emissions measurements were made at equivalence ratios between 0.37 and 0.65. The pressure drop across the airblast injector holes was maintained at 3 and 7–8 percent. The results indicate that the LBI performance for the conditions considered is not sufficiently predicted by existing emissions correlations. In addition, NOx performance is impacted by atomizing air flows, suggesting that droplet size is critical even at the expense of penetration to the wall opposite the injector. The results provide a baseline from which to optimize the performance of the LBI for low-power operation.

Modelling and experiments of straw combustion in a grate furnace

A two-dimensional mathematical model for the combustion of straw in a cross-current, moving bed was developed as part of a tool for optimizing operating conditions and design parameters. To verify the model and to increase the understanding of straw bed combustion, laboratory fixed-bed experiments were performed in a 15 cm diameter and 137 cm long vertical reactor. Air was introduced through the bottom plate and the straw was ignited from the top with an electrical radiation source. The temperature was measured at eight positions within the bed and gas analysis was performed for O 2, CO 2, CO, C x H y , NO x , NH 3 and SO 2. The inlet air flow rate and air preheat temperature were varied. The experimental data for ignition front temperatures and front propagation velocity compared reasonably well with the modelling results. A higher air inlet temperature resulted in lower maximum bed temperatures, which was successfully modelled. A limited parameter analysis study was performed. Measurement of NO from the bed showed that, within the air inlet flow rates investigated, the conversion of fuel nitrogen to NO decreased with increasing air flow rate during ignition front propagation, while the conversion increased during the char burnout phase.

Influence of BladeCooling on the Efficiency of Humid Air Turbine

The influence of an open system blade cooling on the efficiency of the humid air turbine has been analyzed. It has been assumed that three stages of the turbine are cooled by means of compressed air taken from the outlet of the compressor. The thermodynamic analysis showed, that the deleterious impact of blade cooling results mainly from the irreversibility of mixing of cooling air with the working fluid. The influence of the blade cooling on the optimum operational parameters of HAT turbine (compression ratio, excess air, air humidity and preheated air temperature) has been analyzed too. This paper was presented at the ECOS’99 Conference in Tokyo, June 8-10, 1999

Parametric study of no production via quantitative laser-induced fluorescence in high-pressure, swirl-stabilized spray flames

We report the influence of equivalence ratio and air preheat temperature on nitric oxide (NO) concentration in high-pressure heptane spray flames. The burner used was based on the lean direct-injection design and incorporated a pressure-atomized hollow-cone spray nozzle. Helical vanes in the air passage coupled with a divergent exit and preheated air produced a strongly swirling, clean blue flame. NO concentration profiles were measured at 4.27 atm using laser-induced fluorescence (LIF) at five axial heights above the burner. The uniformity of NO throughout the central region of the flame demonstrated the well-mixed nature of the recirculation zone. Measurements were taken at the 40 mm centerline height to determine the effects of primary equivalence ratio (φp=1.0 to 0.8) and air preheat temperature (Tair=375 K to 575 K). The results strongly suggest that NO formation occurs in near stoichiometric regions of the flame and is subsequently diluted with excess air. A residence time effect is evident and apparently scales as the mass flow rate of air relative to that for stoichiometric combustion, yielding a φP2 scaling of the NO (ppm). Moreover, moderate increases in preheat temperature produce significant increases in NO (ppm) levels, suggesting thermal NO production.

Radiative heat loss and NO x emission of turbulent jet flames in preheated air up to 1230 K

Characteristics of flame radiation and NO x emission of turbulent jet flame of ethylene and methane were investigated in highly preheated airflow of up to 1230 K. The radiative heat loss of the jet flame was obtained by summation of the local flame radiation measured by a narrow-view radiative heat flux meter. Radiant fraction, the ratio of the radiative heat loss to the heat of combustion, increases with preheated air temperature and exceeds 0.6 at 1200 K for a C 2 H 4 jet. This ratio shows that flame radiation gives a significant impact on the flame temperature and consequent NO x emission. As air temperature is increased, the NO x emission for CH 4 rapidly increases, but it does not in the case of C 2 H 4 up to 1100 K. The NO x emission level is as low as 3 g/kg-fuel at 1100 K, without detectable soot in the exhaust gas. The low NO x emission of the sooty flame in high-temperature airflow is due to the radiative heat loss and competing oxidation of carbon or CO and nitrogen. The important role of the radiation of flames in high-temperature air is also understood by diluting the fuels. The radiant fraction of the C 2 H 4 jet changed from 0.6 to 0.1 by N 2 dilution. The flame temperature of the undiluted C 2 H 4 jet was 250 K lower than that of 1005 diluted C 2 H 4 due to radiative heat loss. The NO x emission index also increases with dilution, as much as 10 times of that of undiluted C 2 H 4 . NO x production rate is scaled with representative flame temperature, which estimates the flame temperature considering the radiative heat loss and the fuel dilution. Good correlations between NO x production rate and representative flame temperature were obtained, indicating that the radiative characteristic is dominant for NO x emission in a highly preheated airflow.

Effect of Air Preheat Temperature and Oxygen Concentration on Flame Structure and Emission

The structure of turbulent diffusion flames with highly preheated combustion air (air preheat temperature in excess of 1150°C) has been obtained using a specially designed regenerative combustion furnace. Propane gas was used as the fuel. Data have been obtained on the global flame features, spectral emission characteristics, spatial distribution of OH, CH, and C2 species, and pollutant emission from the flames. The results have been obtained for various degrees of air preheat temperatures and O2 concentration in the air. The color of the flame was found to change from yellow to blue to bluish-green to green over the range of conditions examined. In some cases a hybrid color flame was also observed. The recorded images of the flame photographs were analyzed using color-analyzing software. The results show that thermal and chemical flame behavior strongly depends on the air preheat temperature and oxygen content in the air. The flame color was observed to be bluish-green or green at very high air preheat temperatures and low-oxygen concentration. However, at high-oxygen concentration, the flame color was yellow. The flame volume was found to increase with increase in air-preheat temperature and decrease in oxygen concentration. The flame length showed a similar behavior. The concentrations of OH, CH, and C2 increased with an increase in air preheat temperatures. These species exhibited a two-stage combustion behavior at low-oxygen concentration and single-stage combustion behavior at high-oxygen concentration in the air. Stable flames were obtained for remarkably low equivalence ratios, which would not be possible with normal combustion air. Pollutant emission, including CO2 and NOx, was much lower with highly preheated combustion air at low O2 concentration than with normal air. The results also suggest uniform flow and flame thermal characteristics with conditioned, highly preheated air. Highly preheated air combustion provides much higher heat flux than normal air, which suggests direct energy savings and a reduction of CO2 to the environment. Colorless oxidation of fuel has been observed under certain conditions.

Control of Heat Flux Profiles from Rotary Kiln Burners by Modification of Mixing

AbstractThe effect of flame shaping using a Precessing Jet (PJ) burner on heat flux profiles was measured at the semi‐industrial scale (2MW) in the cement kiln zone simulator at the International Flame Research Foundation. Natural gas was used as the fuel and measurements were taken at air preheat temperatures of 400°C, 640°C and 840°C. Results showed that the radiation from the shaped PJ flames was greater than that produced by momentum controlled flames from a multi‐channel burner. Increasing the proportion of shaping jet to Precessing Jet also “stretched” the shape of the heat flux profile such that the location of the peak heat flux was shifted down the kiln and reduced in magnitude slightly. The changes in the heat flux profiles are related to the changes in the mixing field of the non‐reacting jet flows recorded using a semi‐quantitative flow visualisation technique.

Effects of Air Dilution on Highly Preheated Air Combustion in a Regenerative Furnace

Air dilution in highly preheated air combustion has been found to be effective in controlling the NOx emission and improving the temperature uniformity. In this study, the effects of air dilution with different diluents on highly preheated air combustion have been numerically investigated. A regenerative furnace was simulated, where nitrogen, carbon dioxide, helium, argon, and flue gas were adopted as air diluents. It was found that the flame temperature, the temperature uniformity, and the NOx emission are different when different air diluents were adopted under the same preheated air temperature and oxygen concentration. On the basis of the analysis and comparison, it is deduced that the heat capacity of the diluted air, represented by the product of the density and specific heat, plays a main role in the effect of air dilution on the combustion process. The turbulent intensity, represented by the kinetic energy, also plays a role but less significant compared with the heat capacity. The difference in the momentum of diluted air flows with different diluents results in a change in the flame shape.

Modeling of combustion characteristics and NOx emission in highly preheated and diluted air combustion

The gas diffusion combustion in a regenerative furnace with highly preheated and diluted air has been numerically investigated in this paper. The highly preheated air combustion possesses high combustion intensity and high level temperature, but the NO x emission also has an unwanted high level. Decreasing the oxygen concentration in the highly preheated air could decrease the NO x emission and improve the uniformity of temperature distribution in the furnace. The combustion characteristics of highly preheated and diluted air combustion have been studied, including temperature distribution, soot formation, OH radical distribution, as well as NO x emission. The influence of the preheated air temperature, the oxygen concentration, and the air diluent has also been investigated. The optimal combinations of the preheated air temperature and the oxygen concentration have been predicted in the case of flue gas recirculation, which could provide the highest possible temperature in the furnace while keeping the NO x emission lower than the permitted value.

A process simulation model for a 2 ton h-1 incinerator (a combined bed combustion and furnace heat transfer model)

A process simulation model was constructed for a 2 ton h - incinerator. The simulation model was designed to provide system performance parameters according to various operating conditions. In accommodating the wide variation of quality and composition of input wastes, the plant operating parameters such as amount of excess air, preheated air temperature, waste feed rate and primary air distribution over the stoker, etc. must be carefully controlled. The proposed model calculates operating variables of each submodule, by employing steady-state thermal and material balance equations. Combustion of waste bed, and its radiative heat transfer in the combusion chamber are considered. The calculated results of the combustion chamber performance are evaluated, in terms of temperature, locations and width of the flame band, and mean residence time in the secondary combustion chamber. These results are compared with a limited set of field test measurements for verification of the model.

Numerical Study of NOx Emission in High Temperature Air Combustion.

The characteristics of NOx emission in the high-temperature air combustion technology were investigated numerically. Two kinds of methods were used. Detailed chemistry and transport properties were used for a two-dimensional laminar diffusion flame, and a simplified reaction scheme was used for calculating a turbulent flame in a furnace. Results show that the thermal mechanism dominates NOx emission in high-temperature air combustion. If only the preheated air temperature is increased, NOx emission will rise significantly. However, the combination of air preheating and flue gas recirculation not only improves the combustion efficiency, but also suppresses NOx emission in the combustion process.

Heat transfer characteristics of a rotary regenerative combustion system (RRX)

With a view to saving fuel, the use of regenerative burners as a heating source has been spreading in the field of industrial furnaces. By combining a burner with a regenerative air preheater, a second generation regenerative burner—the Rotary Regenerative Combustion System (RRX)—has been developed, which makes for lower emission of air pollutants and compactness, in addition to fuel savings. In this paper, heat transfer characteristics of RRX were deduced theoretically based on the heat transfer theory of a regenerative air preheater and investigated experimentally using two test rigs. A commercially operating fired heater was revamped in the summer of 1994 to install three sets of RRXs, and it has been successfully operated for one year. As a result, it was determined that the heat transfer rate in RRX can be predicted to within ±10%, by considering not only convective but also radiative heat transfer. Furthermore, it was confirmed both theoretically and experimentally that fuel efficiency exceeding 90% was stably attained in a commercial fired heater. Around 60 ppm of NOx emission (as dry, 6% O2) was also measured, although the preheated air temperature was calculated to be as high as 930 K. © 1999 Scripta Technica, Heat Trans Jpn Res, 27(8): 584–596, 1998