Secondary Air Ratio Research Articles

Optimization of Air Distribution to Reduce NOx Emission and Unburned Carbon for the Retrofit of a 500 MWe Tangential-Firing Coal Boiler

The use of separated overfire air (SOFA) has become a standard technique of air staging for NOx reduction in the coal-fired boiler and can also be applied to existing boilers by retrofit. This study was to optimize the air distribution for the proposed SOFA installation in a 500 MWe tangential-firing boiler that has 20 identical units in Korea. Using computational fluid dynamics (CFD) incorporating advanced coal combustion submodels, the reference case was established in good agreement with the design data, and different flow ratios of burner secondary air, close-coupled OFA (CCOFA), and SOFA were evaluated. Increasing the total OFA ratio effectively suppressed NO formation within the burner zone but had a negative impact on the boiler performance. With moderate air staging, NO reduction became active between the CCOFA and SOFA levels and, therefore, the OFA distribution could be optimized for the overall boiler performance. For total OFA ratios of 25% and 30% with respective burner zone stoichiometric ratios of 0.847 and 0.791, increasing the SOFA ratio to 15% and 20%, respectively, was ideal for decreasing the unburned carbon release and ash slagging as well as NO emission. Too high or low SOFA ratios rapidly increased the unburned carbon because of inefficient mixing between the strong air jets and char particles. Based on these ideal cases, the actual air distribution can be adjusted depending on the coal properties such as the ash slagging propensity.

A Study on the Secondary Air Injection Method of the Combustor Considering Characteristics of Solid Refuse Fuels

This study used a numerical simulation method to design a grate-type combustion boiler for solid refuse fuel (SRF). First, proximate analysis, ultimate analysis, calorific value measurement, and combustion characteristic tests such as thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG) were carried out to investigate the combustion characteristics of SRFs and municipal solid waste. To design an optimal SRF combustion steam boiler, the flow field, temperature distribution, chemical species concentration with varying secondary air injection angles and air ratios of the combustor need to be predicted. The secondary air injection angle affects fly ash generation on the grate, and the Cl compound in fly ash promotes high temperature corrosion in the superheater and heat exchanger of the boiler. The low heating values of two types of domestic waste are 13,062 kJ/kg and 14,402 kJ/kg, respectively, and that of fluff and densified SRFs are 19,552 kJ/kg and 29,014 kJ/kg, respectively. The concentration of Cl in densified SRF is 1.80 wt% and that of domestic waste and fluff SRF is 0.50 wt% or lower. The results of the TGA and DTG analyses reveal that the SRFs and domestic wastes complete their reactions at 500 °C or lower and at 540 °C, respectively. According to the numerical calculation results, it is optimal to inject the secondary air normally from the walls to suppress fly ash at the grate in the SRF combustor. Furthermore, an air ratio of 1.87 is recommended for stable combustion and prevention of high temperature corrosion on the walls of the heat exchanger tube. Contours of temperature in combustor for SRF-B (°K)

Numerical simulation on NOX emissions in a municipal solid waste incinerator

The simulations of combustion and NOX emissions process are conducted by computational fluid dynamics method with comprehensive physical and chemical models in a municipal solid waste incinerator. This study focused on the effects of primary NOX control methods (different loads, over fire air, the ratios of secondary air and over fire air) and secondary NOX control methods (selective non-catalytic reduction process) on NOX emission and combustion characteristic. Results showed that the moisture evaporation rate, volatile release rate, char burning rate, temperature and the strength of flow velocity decreased as decreasing the operating load. However, the NOX emission of furnace outlet increased from 273.18 mg/Nm3 to 602.10 mg/Nm3. 100% operating load was the best condition to obtain a lowest NOX emission and a best combustion characteristic among different loads. In addition, the combustion strategy combining secondary air and over fire air could improve the combustion performance and reduce NOX emission. Further, a higher over fire air ratio was conducive to the formation of strong swirl flow and the enhancement of flow velocity strength in the flue. The optimal over fire air ratio was 31%–35% with NOX emission of 267.05 mg/Nm3 to 273.18 mg/Nm3 and residence time of 2.75 s–3.03 s. Moreover, ammonia based selective non-catalytic reduction process, better than urea, achieved a NOX emission of 132.41 mg/Nm3, a NOX removal efficiency of 51.50% and NH3 slip of 11.08 mg/Nm3 or 6.71 ppm. The study indicated that the optimal NOX control method was a condition of 100% load, combining with over fire air and 31% and 35% over fire air ratio. Moreover, the best secondary NOX control method was the ammonia based selective non-catalytic reduction process.

3-D Numerical Study of CFB 110 MW: Fluidization in Furnace and Cyclone with Loads And Air Combustion Variation

Circulating Fluidized Bed (CFB) Boiler has advantages when uses low rank coal compared to Pulverized Boiler. It will be less operational cost but it has greater risk in degradation because of sands material inside the process. Air combustion between primary air and secondary air is one importance parameter that influence fluidization of CFB. Numerical simulation of commercial CFD was used with Eulerian multiphase model implemented to analyzed sand volume of fraction, air and sand velocity including distribution of pressure around furnace. It was used 55%-45% air combustion ratio of primary and secondary air as reference when boiler operated at 63% and 100% based on operation performance of CFB and then it was simulated with additional variation of air combustion ratio with 50%-50%. Simulation of 110% load was added using all of variation of air combustion. The simulation showed that fluidization with air combustion ratio 50%-50% and 55%-45% executed well when operated at 63%. Meanwhile, fluidization 100% and 110% with all those air combustion ratios would cause a great number of sands entered inlet cyclone and higher sands and air velocities.

Experimental investigation on heat transfer characteristics of partially premixed round methane-air impinging flame jet using Mach-Zehnder interferometry

In this study, premixed and partially premixed laminar co-flow impinging flames are studied experimentally. Methane and air were used as the fuel and oxidizer in a co-annular burner with axisymmetric configuration. For the premixed setup, the mixture of fuel-air flowed from the inner tube, while for partial premixing, the secondary air was added from the region between the inner and the outer tubes. The intention is to investigate the temperature field of impinging flame using Mach-Zehnder interferometry and finding the Nusselt number and heat transferred to the impingement surface using this temperature field. The Mach-Zehnder method was used to find temperature field of a transparent fluid by the refraction index. The effects of operational factors such as Reynolds number, equivalence ratio, the distance of the impingement surface to the burner, burner diameter, and secondary air ratio have been studied. For validation of the experimental data processing, the temperature at different locations was measured by thermocouples, which confirmed the interferometry method. The results indicate that the maximum heat flux was transferred to the impingement surface while the impingement surface was near to the inner reaction zone. Partially premixed impinging flame has better heat transfer characteristics and 4.5–6% higher maximum flame temperature than premixed flame. The average total heat transfer enhancement by partial premixing is 10.2–13.5%.

The effect of the secondary air injection on the gas–solid flow characteristics in the circulating fluidized bed

The design and operation of the secondary air (SA) have a great influence on the axial distribution of particles, the structure of the redox atmosphere, and the gas–solid mixture diffusion in the circulating fluidized bed (CFB) boiler. In this paper, the effects of the SA on the gas–solid flow in the CFB boiler, such as the axial particle distribution and the circulating rate were studied systematically through experimental analysis. All experiments were carried out under the ambient temperature and atmospheric pressure, and quartz sand was selected as the bed material in the 4.5m high CFB test rig made of Plexiglas. The results showed that the SA whose capability to carry particle is affected by the particle concentration at the SA injection port, has a weaker effect on the entrainment of particles than the primary air (PA). The particle concentration in the dense phase zone mainly depends on the PA rate. The increasing of the secondary air ratio (SAR) may lead to the accumulation of particles in the dense phase zone. In addition, one transitional state was found in the transition zone. The variation of particle concentration in the transition zone with the increasing of the SAR is determined by the magnitude of the particle concentration at the SA injection port. The circulating rate decreased with the increasing of SAR or the SA injection height. The effect of the SA injection on the circulating rate in a circulating fluidized bed can be predicted by a modified model with the dimensionless parameters.

Experimental investigation and prediction of heat transfer in a swirling fluidized-bed combustor

In this experimental study, the heat transfer coefficient in a twin-cyclonic swirling fluidized-bed combustor (T-FBC) with a conical shape bed was investigated to verify the effects of combustor design feature and operating parameters. The combustor was operated at a ratio of secondary and tertiary air to primary air ranging from 0 to 0.5 at primary air velocity (up) near minimum swirling fluidization condition (ums), to about 3ums by using silica sand with particle size diameters (dp) of 300–500, 600–710, and 710–1000 µm as bed material, for swirl number of an annular spiral air distributor of 2.76, and 2.98. The local heat transfer coefficients were measured in radial positions at the levels of 60, 125, 223, and 288 cm above the primary air distributor and the axial profiles of average heat transfer coefficient were characterized. The experimental results revealed that the radial profile of heat transfer coefficient was maximum at the center of the combustor for most of the test trials. Within the range of the tests, the heat transfer coefficient noticeably increased with the increase of the primary air flow rate and swirl number of the air distributor, and reached the almost constant value at 2.5–3ums. With design features of this combustor, the heat transfer coefficient in the bed region was comparable to the value in freeboard region though about 5–12% higher. Based on the test results, semi-empirical models for prediction of average Nusselt numbers were developed in the dense bed and freeboard zones for the different swirl numbers of 2.76 and 2.98, and the proposed models were in close agreement with the experimental data within ±20% error for both regions.

Effects of secondary air distribution in primary combustion zone on combustion and NOx emissions of a large-scale down-fired boiler with air staging

A new air-staged and low-NOx emission combustion technology has been applied to a 300 MWe anthracite- and down-fired boiler with swirl burners. To achieve the optimum air distribution in the primary combustion zone and further reduce NOx emissions, full-scale industrial experiments varying the ratios between burner secondary air ratio (Rbsa) and staged air ratio (Rsa) (i.e., Rbsa/Rsa ratios of 42.9/17.8, 47.1/13.5 and 51.5/9.3), on the premise that the overfire air (OFA) ratio was approximately 20%, were performed to evaluate the overall performance of retrofitted boiler. The improved ignition of coal/air flow was at a distance of 0.8–1.4 m to the burner outlet with increased Rbsa/Rsa ratio. Compared with the boiler before retrofit, under Rbsa/Rsa ratios of 47.1/13.5 and 51.5/9.3, the flue gas temperature in the primary combustion zone of the retrofitted boiler increased slightly. Oxygen and carbon monoxide concentrations in the near-sidewall region revealed the furnace flame fullness varied under different Rbsa/Rsa ratios. With increased the air staged level in the lower furnace, NOx emissions decreased continually and the carbon content in fly ash did not necessarily increased. Under the optimum Rbsa/Rsa ratio of 47.1/13.5, the NOx reduction efficiency was approximately 46% with increasing boiler thermal efficiency slightly.

Performance analysis of a novel thermoelectric assisted indirect evaporative cooling system

In this paper, a novel thermoelectric assisted indirect evaporative cooling system is proposed. Specifically, thermoelectric cooling (TEC) modules are sandwiched between channels of a flat plate cross flow indirect evaporative cooler. A mathematical model of the novel system is developed, and influences of main operating and geometrical parameters on the system's performance are analyzed in detail. Analytical results show that with the assistance of the thermoelectric cooling, the proposed system is able to cool primary air to a temperature much lower than inlet air wet bulb temperature, even dew-point temperature, meanwhile keep a relatively high coefficient of performance (COP), by selecting appropriate number and electric current of TEC modules. The dew point effectiveness and COP of the proposed system change monotonously with inlet mass flow rate, temperature and humidity ratio of primary air, respectively. The dew point effectiveness also varies monotonously with the mass flow rate ratio of secondary air to primary air. However, there exists an optimal mass flow rate ratio resulting in a maximum COP. Moreover, under variable number and electric current of TEC modules, there always exist optimal widths of primary air channel and secondary air channel, leading to a maximum COP.

Numerical modeling of co-emissions for gas turbine combustors operating at part-load conditions

A numerical model is presented for the precise prediction of carbon monoxide (CO) emissions in gas turbine combustors. All models are based on Computational Fluid Dynamics (CFD). This work starts with an introduction of fundamental mechanisms, which are responsible for CO emissions. As we will show, there is a need of CO-models as standard combustion models fail to predict CO-emissions precisely. For the purpose of validation, experiments are conducted. High ratios of secondary air is bypassing the burner in order to induce interaction of the flame front with secondary air, as the flame brush gets diluted and decreases in reactivity. Note, this is an important mechanism for elevated CO emissions in multi-burner systems with high staging ratio. Five operating points with each having a different adiabatic flame temperature were measured. They include equilibrium (complete burnout) and superequilibrium CO (incomplete burnout). In summary, it is shown that the prediction of CO with the presented models lead to a significant improvement as it captures the transition from equilibrium to superequilibrium CO. Furthermore, the strong underestimation of CO predicted by standard combustion models is shown.

Optimization of combustion characteristics of palm kernel-based biofuel for grate furnace

Grate firing is one of the main competing technologies in biomass combustion for steam and electricity generation. Ash generated in the furnace during combustion process would greatly reduce the boiler thermal performance and may lead to unscheduled shutdown. The focus of this study is to optimize the combustion characteristics of the mixture of palm kernel shell (PKS) and selected additives (Al2O3, MgO and CaO) to develop a fuel mixture of low ash yield and higher heating value (HHV). D-Optimal Design under Cross Methodology of Design Expert (6.08) was employed to mix the components alongside various particle sizes. The mixed samples were ashed in a muffle furnace (848 K) to a constant weight and their HHV were determined using Ballistic Bomb Calorimeter. Combustion test based on optimized PKS additive mixture was conducted with a 5 kW grate furnace from which the effects of varying the ratio of primary to secondary air flow rate on temperatures and flue gas compositions from the furnace were measured. The ash obtained after combustion process was characterized using X-ray diffraction (XRD) for the purpose of identifying the mineral phase compounds that are present in PKS and PKS-additive ash. The optimum composition obtained for the process was 2.5, 0.0, 5.0, 92.5% and 5.50 mm for additives (Al2O3, MgO, CaO), PKS and particle size, respectively. The composition resulted in lowest ash yield (0.56%) and HHV (20.64 kJ/g). The coefficient of determination (R2) (0.7951 and 0.7344) and least-square errors (0.19 and 0.024) of the prediction model indicated a close fitness to the experiment results obtained for ash yield and HHV. Primary to secondary air ratio of (40:60) recorded maximum temperature (1058 K), minimum level of CO (285 ppm) and 6% oxygen. XRD results showed excellent interaction between PKS and additives. The appearance of potassium-alumino silicate (KAlSiO4) in the PKS-additive ash prevented the release of potassium chloride which has the ability to increase ash deposition and corrosion.

Influence of increased primary air ratio on boiler performance in a 660 MW brown coal boiler

A computational fluid dynamics model was established based on a 660 MW brown coal boiler, to study the effects of primary air ratio (PAR) on boiler performance. To improve model prediction, moisture content in brown coal is specially considered. Confidence in the model was established by carrying out mesh independence test and validation against real life data and theoretical calculations. Then it was used to simulate 10 cases with different PAR. Results show that with PAR increasing from 0.325 to 0.55, overall boiler performance deteriorates and total radiative heat flux decreases by 35.2 MW. However, the temperature distribution and heat flux in main burners region and above separated over fire air region show a parabolic trend. The results indicate that boiler performance deteriorates with PAR increasing, and well explain why boiler thermal load is still reduced, even if PAR is increased to maintain the drying capacity in high moisture content cases. Momentum ratio of primary and secondary air is pointed out to be the main cause of this phenomenon. At last, a feasible solution is proposed to raise primary air temperature, not the ratio of it, to maintain the drying capacity. Therefore, the adverse effects caused by increased PAR can be avoided.

Experimental study on NOx emissions of pulverized bituminous coal combustion preheated by a circulating fluidized bed

This study investigated nitrogen oxide (NOx) emissions of pulverized coal combustion preheated by a circulating fluidized bed (CFB). During the test process, high-temperature fuel preheated in a CFB was burned in a down-fired combustor (DFC). The effect of air distribution on NOx emissions was studied in the DFC, including three types of secondary air nozzle structures, five secondary air ratios, and three tertiary air position arrangements. Under stable conditions, the conversion ratio of fuel-nitrogen to N2 in the CFB was 41.4%, which resulted in lower NOx emissions in the platform. In this study, secondary air could be injected into the combustor at the top (annular) or through the side wall (circular) of the DFC, both with high combustion efficiency. This means that the secondary air is completely separated from the burner, and burner structure is greatly simplified. NOx emissions from secondary air nozzle structures of center, annular, and circular ports were 565.66, 345.45, and 220.38 mg/Nm3 (@6% O2) respectively. NOx emissions initially decreased then increased with increases in secondary air ratio with the annular nozzle structure. NOx emissions could be further inhibited by rationally arranging tertiary air positions.

Experimental research on combustion characteristics of coal gasification fly ash in a combustion chamber with a self-preheating burner

Coal gasification fly ash (CGFA) is the by-product of coal gasification in a circulating fluidized bed (CFB) and is characterized by its low volatile-content, low carbon-content, and high ash-content, which can be used as a secondary fuel. In China, the annual output of CGFA is enormous; therefore, the realization of a highly efficient CGFA combustion method would be beneficial to achieve better overall coal utilization. Nevertheless, it is challenging to burn CGFA steadily with conventional technologies because of its low volatile content. However, the coal preheating combustion technique has been proven to be an effective method to burn semi-coke and anthracite, both of which are poorly flammable, and could also be applied to CGFA. We have used an advanced preheating combustion technique with a self-preheating burner and carried out experimental investigations into CGFA combustion in a 0.4-MW CGFA preheated combustion test rig. The results show that the preheated combustion system ran smoothly and the CGFA combustion efficiency reached 98.6% with NOX emissions of 155 mg/m3 (6% O2). Therefore, the highly efficient and clean combustion of CGFA was realized, and the combustion of poorly flammable fuels using a self-preheating burner was shown to be feasible. The effects of the primary air ratio, secondary air ratio, and the position of the tertiary air inlet on the combustion characteristics and NOX emissions of CGFA were also studied.

Effect of Operating Conditions on the Combustion Characteristics of Coal, Rice Husk, and Co-firing of Coal and Rice Husk in a Circulating Fluidized Bed Combustor

This study was performed to investigate the combustion characteristics of coal and rice husk and their co-combustion in a 100 kW circulating fluidized bed combustor (diam.: 150 mm, height: 6000 mm). Gaining insights into the effects that the primary and secondary excess air ratios (λPA and λSA), biomass fraction, and secondary air (SA) position have on CO and NOx emissions was also an aim of this study. In contrast to coal combustion, and co-combustion of coal and rice husk, the combustion of rice husk could not be sustained without the aid of an external heat source, and the released volatiles were found to be fired chiefly above the bottom bed. Air-staging substantially decreased NOx (40–75% reduction), but with a high CO penalty, compared to no air-staging. However, the effect of the SA position which was profoundly engaged with the char inventory in the bed was pronounced for coal combustion but not for rice husk combustion. Increase in λPA and λSA tended to reduce CO levels but enhanced NOx formation...

Co-Combustion of Tannery Sludge in a Bench-Scale Fluidized-Bed Combustor: Gaseous Emissions and Cr Distribution and Speciation

In this study, eleven coal mono-combustion tests and four co-combustion tests of coal and tannery sludge were conducted on a 35kW fluidized bed combustor. The combustion behavior and emission characteristics of the fuels were investigated on a bubbling fluidized bed (BFB) and a circulating fluidized bed (CFB). The effects of an excess air ratio, primary air rate, secondary air ratio and fuel type on flue gas emissions were studied. The results showed that the fluidization status and temperature distribution had direct influences on CO emission. Sufficient fluidization and high temperature effectively reduced CO emission. NOx emission was relatively sensitive to the excess air ratio and increased with increasing excess air ratio. By comparing BFB and CFB, we found that CFBs have an advantage in optimizing combustion and controlling emissions by enhancing mixing and increasing freeboard temperatures. The Cr speciation and distribution among different ash types were extensively investigated in four co-combus...

Industrial-scale investigations of anthracite combustion characteristics and NOx emissions in a retrofitted 300 MWe down-fired utility boiler with swirl burners

To burn anthracite and reduce NOx emissions, a new combustion system was applied to a 300 MWe Babcock & Wilcox (B&W) down-fired boiler that included overfire air (OFA) and a decreased flow area of the inner and outer secondary air ducts of the installed swirl burners. Industrial-scale measurements (adjusting OFA ratios from 15.4% to 22.6%) were conducted to evaluate the overall performance of the retrofitted boiler. The experimental results demonstrated that the new combustion system promoted coal/air flow ignition and combustion stability. When the OFA ratio was less than 19.7%, the influence of the burner secondary air ratio on coal/air flow ignition was greater than that of the recirculation region below the arches. The overall combustion level in the primary combustion zone depended on the oxygen content and the recirculation region in the lower furnace instead of the early or late ignition of the coal/air flow. Oxygen and carbon monoxide concentration measurements in the near-sidewall region revealed that the fullness degree of the coal flame varied under different OFA ratios. Considering both environmental and economic effects, 19.7% was chosen as the optimal OFA ratio, thereby achieving a significant NOx reduction of 47% without increasing carbon content in the fly ash.

Optimization of Combustion Organization of Regenerative Reheating Furnace

Regenerative reheating furnace is an important energy consuming equipment in petroleum industry. So it is very important to optimize the combustion organization of the heating furnace. In this paper, a gas consumption regenerative reheating furnace of oil field was taken as the research object. The impact of air excess ratio, preheating temperature of combustion-supporting air and the ratio of the primary air and secondary air on the generation of NOx in the radiant tube was analyzed by the combination of orthogonal test and numerical simulation. The conclusion was made that when the air excess ratio was 1.05, preheating temperature of combustionsupporting air was 900 °C, the ratio of primary air and secondary air was 20%: 80 %, the concentration of NOx in the radiant tube was only 1.1 ppm. The discharge quantity of NOx was reduced to the lowest limitation. And it is an ideal solution to reduce the NOx emissions.

Reducing CO emissions through a secondary air nozzle retrofit based on the jet penetration factor and the momentum flux ratio of a commercial wood waste incinerator

The combustion process in an incinerator inevitably generates pollutants such as carbon monoxide (CO). In this study, we looked at a grate-type commercial wood waste incinerator, whose CO emissions are above those of the regulation of 50ppm at a 12% O2 basis. In order to reduce the CO emissions, we modified the configuration of the secondary air nozzles of the incinerator plant, based on the jet penetration factor and momentum flux ratio of secondary air to primary air. A computational prediction and experimental measurements were performed to investigate the effect of this retrofit. A computational fluid dynamics (CFD) analysis showed that the retrofit made the secondary chamber flow more uniform, and decreased the standard deviation of the O2 and vertical velocity in the chamber. A comparison of the temperature and O2 concentration in the measurement port showed the CFD result can be used to predict the flow and combustion phenomena in the combustion chamber. The CO concentration at the furnace outlet of the retrofit decreased by 94.7% in the numerical study; a similar trend was observed in the experimental measurements, where the emissions decreased from 96.3ppm to 43.4ppm.

Factors affecting the downward flame depth in a 600 MW down-fired boiler incorporating multiple-injection and multiple-staging technology

Considering the excessively deep flame depth existing in a 600 MW down-fired boiler incorporating multiple-injection and multiple-staging technology, 1:20 scale aerodynamic tests were conducted to incrementally improve factors responsible for the deep flame depth. These trials demonstrated that, in all cases, the downward velocity near the wing walls decayed more rapidly than that near the furnace center. Increasing the mass ratio of pulverized coal in fuel-rich flow to that in fuel-lean flow and reducing the secondary air ratio while simultaneously increasing the tertiary air ratio was found to increase the penetration depth. Increasing the distance between adjacent burners, optimally lowering the fuel-rich and fuel-lean flow velocities, and reducing the secondary air velocity all decreased the penetration depth. The comprehensively improved downward airflow depth and air flux into the furnace hopper were reduced by 6.3% and 14.9%, respectively. Cold-state airflow tracing tests were performed in an actual boiler, the improved downward airflow depth reduced apparently, which was consistent with the results from modeling tests. Industrial-scale hot-state experiments determined improved hopper near-wall temperatures of 700–800 °C near the furnace center (values that were lower than those of 800–900 °C near the wing walls). These values were approximately 450 °C less than the prior temperatures at the same location, indicating the flame penetration depth was greatly reduced.