External Flue Gas Recirculation Research Articles

Numerical simulation on coking process of coke oven with external flue gas recirculation

The coking behaviour and NOx emission characteristics of the coke oven are of great significance for coke production. Based on the realisation k-epsilon turbulence model, the P-1 radiation model, the species transport model with eddy dissipation, and the thermal NOx model. A comprehensive model for traditional regenerative coke oven was established with reversing mode and external flue gas recirculation model considering temperature-dependent thermophysical properties. The convective term of model equations was the discretised using the second-order upwind scheme, while the others were applied by the central difference scheme. The coking behaviour of the coke oven with external flue gas recirculation was numerically solved through the SIMPLE algorithm, and the effects of reversing period and external flue gas recirculation ratio on the coking behaviour were explored. When increasing the reversing period, the mean temperature of the vertical flue increases, and the coking time decreases, but the temperature uniformity of the carbonisation chamber becomes worse. As the external flue gas recirculation ratio increases, the average temperature of the vertical flue decreases, and the NOx concentration at the outlet decreases, yet the coking time increases. These results would provide useful information for guiding the improvement of coke oven operation and reducing NOx emissions.

Thermodynamic and emission characteristics of a hydrogen-enriched natural gas-fired boiler integrated with external flue gas recirculation and waste heat recovery

Hydrogen-enriched natural gas (HENG) is a low-carbon fuel and its utilization in combustion devices has been under extensive discussion recently as it is a promising way to reduce the CO2 emission during combustion. The applicability of HENG in the existing natural gas-fired combustion equipment in terms of its efficiency and emissions attracts increasing attention, especially for industrial and domestic small-scale boilers. In this study, the thermal efficiency and pollutant emissions of a 4.2 MWth HENG-fired boiler integrated with external flue gas recirculation (FGR) were respectively evaluated based on the thermodynamic and heat transfer models and the Chemical Reaction Network model. Results suggested that NOx emission rose by ∼19% as the hydrogen volumetric fraction in the fuel increased from 0 to 0.4 at a constant excess air ratio and FGR rate. To suppress the NOx rise, the FGR rate was tuned higher while the system thermal efficiency decreased subsequently. To further improve the overall system thermal efficiency, a cascade flue gas waste heat recovery strategy including sensible heat recovery using an external economizer and latent heat recovery using dual-spray heat exchangers was proposed. The thermodynamic analysis demonstrated that the external economizer improved the overall system thermal efficiency by 0.4% - 0.7% and the dual-spray heat exchangers promoted the overall system thermal efficiency by over 5%. Thus, a comprehensive performance optimization strategy was developed for HENG-fired boilers in terms of their overall system thermal efficiency promotion, NOx emission control, and CO2 emission reduction.

A study on the combustion performance and NOx emissions characteristics of a novel U-type radiant tube based on MILD combustion theory

Based on the theory of moderate or intense low-oxygen dilution (MILD) combustion, a 3D steady-state CFD model coupling dual flue gas recirculation and dispersion combustion was developed to describe the flow field, combustion temperature field, wall temperature distribution, and NOx concentration field during the operation of a novel U-type radiant tube (N-URT). A user-defined function (UDF) method was implemented to simulate the dilution of O2 concentration in the combustion air due to external flue gas recirculation (E-FGR). The effects of the E-FGR ratio on combustion characteristics and NOx emission were evaluated. The simulated results were consistent with experimental results. The simulated NOx concentration at the outlet was reduced to 105.5 mg/m3 at 8% O2, which was about 35.2% lower than the conventional combustion method. The study found that when the E-FGR ratio was increased from 0 to 25%, the standard deviation of wall temperature decreased from 30.3 °C to 23.3 °C, and the non-uniformity coefficient decreased from 0.2598 to 0.2183, indicating an improvement in temperature uniformity. However, the thermal efficiency decreased from 82.1% to 76.2%. Moreover, at higher E-FGR ratios, the volume of the high-temperature region (above 1500 ℃) gradually decreased, and both the maximum flame temperature and the outlet NOx concentration decreased sharply. Regression equations for the E-FGR ratio (β) on the maximum combustion temperature (Tmax) and the outlet NOx concentration (CNOx) were obtained, and an E-FGR ratio greater than 13.4% was recommended to meet the emission requirements.

Effects of flue gas recirculation on the thermal performance of the gas remediation well

Gas thermal remediation (GTR) is a promising novel soil remediation technology. Many problems such as high energy consumption and unreasonable temperature distribution are still commonly found in the application of the gas remediation well. External flue gas recirculation (e-FGR) was applied to improve the thermal performance of the well. A 3D CFD model of the gas remediation well coupled with e-FGR was developed to analyze the heat transmission and combustion process. Four representative e-FGR ratios, 15 %, 30 %, 45 %, and 60 %, were selected to analyze the relationship between the e-FGR ratio and the well thermal performance. The results show that limited impact is obtained at a low e-FGR ratio and excellent results can only be achieved when the e-FGR ratio reaches 60 %. The maximum flame temperature was reduced by about 28 % at an e-FGR ratio of 60 % when compared with conventional combustion conditions. Localized high-temperature areas on the well surface are eliminated. The total heat transfer of the well increased from 4.69 kW to 6.16 kW when the e-FGR ratio increased from 0 to 60 %, which means a 23.9 % reduction in the remediation cycle and fuel consumption can be achieved at an e-FGR ratio of 60 %.

Combustion characteristic study with a flue gas internal and external double recirculation burner

A new low NOx burner for gas boiler is designed. A hollow tube is embedded in the burner axial center line, which is combined with the flared flame stabilizer to draw back the internal flue gas from inside the boiler, and a flue gas recirculation tube is installed outside the boiler to draw back the flue gas from the end of the boiler. The effectiveness of the burner is evaluated by industry experiments and extensive computational fluid dynamics (CFD) simulations. The research results show that the excess air ratio and external flue gas recirculation (e-FGR) rate significantly alter the temperature distribution and NOx emission of the boiler. The temperature and NOx emission do not vary monotonically with the excess air ratios, but decrease monotonically with the e-FGR rates. To achieve the low NO emission, the optimum excess air coefficient α should be 1.15 due to the better temperature uniformity, and the value of e-FGR rate should be in the range of 28%–40%. In addition, the maximum concentration of NO does not appear on the flame surface, but in the downstream region of the flame.

Optimization and control of NOx in coke oven vertical flue under flue gas recirculation

The orthogonal experimental design with CFD simulation was carried out to simulate combustion in the coke oven vertical flue at external flue gas recirculation. The control of nitrogen oxides (NOx) generation was analysed by different factors such as air excess coefficient, oxidant preheating temperature, flue gas recirculation and oxidizer inlet cross-sectional area ratio. The simulation results showed that the the orthogonal experimental design with CFD had a significant effect on the combustion process. The flue gas recirculation ratio effectively improved the temperature distribution uniformity in the vertical flue, and had a significant effect on NOx production in the vertical flue. Within the parameters of the orthogonal design, when the air excess coefficient was 1.1, the oxidant preheating temperature was 1200 K, the flue gas recirculation ratio was 30%, and the oxidant inlet cross-sectional area ratio was 0.5, the NOx production in the coke oven vertical flue was the smallest, lower than the industry emission standard.

Low NOX combustion mechanism of coke oven gas with excess air coefficient and flue gas recirculation

With the more stringent NOX emission limit from the coking chemical industry, the coking production with only one type of low NOX combustion technologies cannot meet the emission requirement. Thus a new low NOX experimental system was developed with the help of the external flue gas recirculation (FGR) and reasonable excess air coefficient, where the goal is to reduce NOX emission effectively. And an experiment was carried out on the 18 KW coke oven gas (COG) combustion device, and the effects of excess air coefficient, the ratio of flue gas recirculation and air preheating temperature on the combustion characteristics and NOX formation were extensively studied. The results show that there is an important effect of excess air coefficient, ratio of FGR and air preheating temperature on the temperature distribution inside the furnace and the formation of NOX. When the air excess coefficient of 1.2 and an external flue gas recirculation rate of 20% to 25%, the emission of thermal nitrogen oxides can be significantly reduced.

Experimental and numerical analysis of natural bio and syngas swirl flames in a model gas turbine combustor

Turbulent reacting flows in a generic swirl gas turbine combustor model are investigated both numerically and experimentally. In the investigation, an emphasis is placed upon the external flue gas recirculation, which is a promising technology for increasing the efficiency of the carbon capture and storage process, which, however, can change the combustion behaviour significantly. A further emphasis is placed upon the investigation of alternative fuels such as biogas and syngas in comparison to the conventional natural gas. Flames are also investigated numerically using the open source CFD software OpenFOAM. In the numerical simulations, a laminar flamelet model based on mixture fraction and reaction progress variable is adopted. As turbulence model, the SST model is used within a URANS concept. Computational results are compared with the experimental data, where a fair agreement is observed.

Influence of external flue gas recirculation on gas combustion in a coke oven heating system

In this study, external flue gas recirculation was considered to reduce nitrogen oxides (NOx) emissions in the heating flues of a coke oven battery. The thermal and prompt NO formation was numerically investigated employing an accurate 3-D representation of the heating flue geometry that the most popular Polish coke oven battery. Originally the developed model was experimentally validated as a transient coupled model for the representative heating flue and the two coke ovens. Then the coupled model was simplified to the heating flue model only with realistic boundary conditions on the heating flue and coke oven interface. As a result of the heating flue model simulations, the range of the reversed flue gas ratio was found for typical thermal loadings of the heating wall to effectively reduce NOx formation. The obtained results showed the significant effect of the considered flue gas recirculation on NOx formation reduction. Namely, the recirculation ratio of 0.2 resulted in 50% of the NOx reduction efficiency.

Numerical Analysis of Turbulent Combustion in a Model Swirl Gas Turbine Combustor

Turbulent reacting flows in a generic swirl gas turbine combustor are investigated numerically. Turbulence is modelled by a URANS formulation in combination with the SST turbulence model, as the basic modelling approach. For comparison, URANS is applied also in combination with the RSM turbulence model to one of the investigated cases. For this case, LES is also used for turbulence modelling. For modelling turbulence-chemistry interaction, a laminar flamelet model is used, which is based on the mixture fraction and the reaction progress variable. This model is implemented in the open source CFD code OpenFOAM, which has been used as the basis for the present investigation. For validation purposes, predictions are compared with the measurements for a natural gas flame with external flue gas recirculation. A good agreement with the experimental data is observed. Subsequently, the numerical study is extended to syngas, for comparing its combustion behavior with that of natural gas. Here, the analysis is carried out for cases without external flue gas recirculation. The computational model is observed to provide a fair prediction of the experimental data and predict the increased flashback propensity of syngas.

Flame temperatures and species concentrations in non-stoichiometric oxycoal flames

The oxyfuel technology offers the possibility for CO 2 sequestration from coal fired power plants. One drawback is the need for a high external flue gas recirculation to avoid inadmissible high flame temperatures. The concept of controlled staging with non-stoichiometric burners (CSNB) allows a significant reduction of the commonly proposed flue gas recirculation rate while fulfilling all requirements on temperature limitations. The concept aims at a more efficient oxyfuel process with a higher degree of freedom for heat-flux adjustments suitable for a new generation of oxyfuel boilers. The steam generator size could be reduced and in this way a more cost effective steam generator concept is possible. Additionally the energy demand for the flue gas recirculation is lowered. This paper presents the experimental investigations of non-stoichiometric oxycoal flames. Temperature and gas profiles were taken to analyze the combustion behavior of coal with high oxygen concentrations in the oxidizer under oxygen deficiency and accordingly oxygen excess. In addition an optical flame monitoring system allowed a comparison of ignition, flame shape and stability. In the test rig lignite was burned under different stoichiometries ranging from 0.5 to 2.5 and different oxygen concentrations in the oxidant ranging from 30 to 40 vol.%. The thermal input of the burner was 70 kW at a total thermal input of 140 kW and a dry flue gas recirculation was used. The results were compared to a conventional air-blown combustion and showed that similar temperature ranges can be reached even with oxygen concentrations in the oxidizer as high as 40 vol.%.