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Effect of burner location on flow-field deflection and asymmetric combustion in a 600 MW e supercritical down-fired boiler
Abstract
To evaluate the burner location's role on the flow-field deflection and asymmetric combustion that occurring in a 600MWe supercritical down-fired boiler, cold-modeling gas/particle flow experiments and numerical simulations on coal combustion were performed with varying the furnace arch's burner location. Meanwhile, full-load industrial-size measurements at the boiler's design setup were performed to uncover the asymmetric combustion characteristics and verify the simulation validity. The boiler's design setup displayed a severely deflected gas/particle flow field with (i) a large downward gas/particle flow penetration difference appearing in the front- and rear-half sides and (ii) the upward flow fully deflecting towards the front-half side. Accordingly, a badly asymmetric combustion performance (much lower gas temperature levels appearing in the front-half side than in the rear-half side) occurred with poor burnout and high NO x emissions. The calculated coal/air penetration and flow-field deflection extent were found to be shallower than the cold-modeling experimental versions, despite the consistent flow-field deflection pattern respectively gained by the two methods. Positioning burners towards the front/rear wall greatly improved the flow-field deflection in the form of apparently decreasing the aforementioned penetration difference and redirecting the upward gas/particle flow in the furnace's central part. Consequently, asymmetric combustion sharply weakened to increase burnout and reduce NO x emissions. In contrast, moving burners towards the furnace centerline aggravated the flow-field deflection and asymmetric combustion to worsen the furnace performance in burnout and NO x production. These results suggest that in the absence of coal/air distribution with varying burner location, the local high gas temperatures incurred by asymmetric combustion facilitate the NO x production. Finally, burners are recommended to position towards the front/rear wall as much as possible for weakening the flow-field deflection and asymmetric combustion if a lowered manufacturing cost requires a short upper furnace where the asymmetric upper furnace configuration effect is aggravated.
... As shown in Fig. 10, when the GP flows from the arches reaches the furnace section Y/Y 0 of 0.369 below the staged air nozzles, the maximum V y of GP flows attenuates to 0.25 m/s, and the GP flows begin to turn back and flow upward. For the ESSACT, the maximum V y of GP flows is still as high as 2 m/s, and the downward depth and the residence time of PC particles in the lower furnace will increase, which benefits PC burnout [9,53,54]. Fig. 11 shows the distribution of the vertical root mean square (RMS) fluctuation velocity V y ′ of GP flows in the furnace under two combustion technologies. From the furnace section Y/Y 0 of 0.032 to 0.220, the V y ′ peaks of GP flows are formed along the burner jet, and gradually shift to the furnace centre with an increase in the downward depth of the GP flows from the arches under two combustion technologies. ...
... Compared with the BWCT, the BPA flow trajectory is closer to the water-cooled wall, and the position that the GP flows turn back and flow upward is delayed for the ESSACT. In Figs. 15 and 16, it can be concluded that the downward depth of GP flows from the arches and the space utilization ratio in the lower furnace both increase, and the PC burnout and the boiler thermal efficiency will be improved [9,53,54]. ...
To confirm the validity and progressiveness of the eccentric-swirl-secondary-air combustion technology (ESSACT)in improving the economic and environmental performance of the down-fired boiler (DFB)with swirl burners, the gas/particle (GP)cold-modelling experiments are conducted by using a particle dynamic analyzer (PDA)measurement system in a 1:10 scale model of 300-MWe DFB applying original Babcock & Wilcox combustion technology (BWCT)and novel ESSACT, respectively. The distributions of average velocity, fluctuation velocity and particle size are compared and analyzed under the two combustion technologies in this study, in addition to the decay of particle volume flux and the jet trajectory of downward GP flows. In comparison with the BWCT, the rotation intensity of the burner secondary air is greatly weakened, and the vertical velocities of downward GP flows decay more slowly under the ESSACT. Although the recirculation regions are both formed below the arches under the two combustion technologies, the horizontal velocities and the horizontal fluctuation velocities of GP flows in the recirculation regions are both larger for the ESSACT, which benefits pulverized coal ignition. The particle size distributions under the two combustion technologies reveal that the mixing between the primary air and burner secondary air is obviously delayed and the burner secondary air effectively ejecting the primary air downward is realized by the ESSACT, which is advantageous to improve pulverized coal burnout and inhibit NOx formation. In addition, the maximum particle volume flux in the lower part of the lower furnace under the ESSACT is obviously higher than that under the BWCT, and the downward depth of GP flows and the space utilization ratio in the lower furnace increase for the original BW DFB retrofitted by the ESSACT.
... It is difficult to establish a comprehensive theoretical model to calculate key combustion parameters and efficiency at this stage of development. Therefore, traditional methods for optimizing boiler combustion primarily rely on experiments and plant trails, e.g., direct measurement of parameters like excess air to assess the impact on heat loss and emissions [8], conducting cold airflow experiment [9], and retrofitting the boiler [10], or empirical knowledge facilitated with a type of control system mainly aimed at improving the load-following capability [11], stability of the steam pressure [12] and load adaptation of boilerturbines [13]. While these methods are useful to varying degrees for industrial process operations, they exhibit limitations across various types. ...
... However, it has several difficulties in terms of combustion optimization and emission reduction, because of its special structure and the use of anthracite [10][11][12]. Plenty of research related to arch-fired boilers mainly focuses on combustion optimization and NO X reduction through experiments and numerical simulations, including design of novel combustion system [13,14], optimization of staged air configuration [15,16], flow field distribution [17,18], and asymmetric combustion characteristic [19,20]. ...
The arch-fired boiler easily suffers from membrane wall deformation and rupture caused by high stress. Thus, it is necessary to predict thermal stress distribution on the membrane wall for the sake of early warning. In this study, a lab-scale arch-fired boiler was constructed to achieve thermal stress distribution on the membrane wall, and the reason for high-stress formation was found. Besides, machine learning technology was first applied to predict thermal stress based on experimental data. The results show that both high-temperature distribution and boiler configuration play an important role on high-stress distribution. As for the prediction, the regression neural network model presents an admissible result (R² = 0.724), but has a relatively poor performance on the high-stress prediction due to the skewed data distribution. Specifically, the relatively high prediction errors present on the bending and connection areas. Future work should consider the effect of boiler configuration and different heat loads. In contrast, the binary classification neural network model has a more accurate result with a high F1 score (0.893). It is the best choice for practical application considering the generality and accuracy. This work is meant to be valuable in optimizing the prediction model and applying it in practice.
... Numerous publications have investigated the characteristics of coal spontaneous combustion 12,13 and approach to enhance the combustion efficiency and reduce the emissions from pulverized coal. 14 The oxidative combustion properties of pulverized coal are the key factors affecting its combustion efficiency and emission amounts. Therefore, plenty of experimental techniques have been adopted to examine coal oxidation and combustion, including thermogravimetry, 5,15 thermal release, 16 and combustion reaction, 17 which have provided more detailed insights into the oxidative combustion properties of pulverized coal under a variety of oxygenenriched atmospheres and with different particle sizes. ...
Spontaneous combustion of pulverized coal has become a safety topic and has been extensively researched. This study using differential scanning calorimetry investigated the exothermic characteristics and spontaneous combustion risk of three metamorphic pulverized coal samples during oxidative combustion, for oxygen concentrations of 21, 19, 17, 15, 13, 11, 9, 7, and 5 vol %. Results indicated that decreased oxygen concentrations reduced exothermic intensity and substantially increased ignition temperatures. The oxidative thermal release observed during the combustion stage was conspicuously higher than during the low-temperature oxidation stage. Thermal release during low-temperature oxidation was low during low oxygen concentrations; however, when the oxygen concentration was less than 13.0 vol.%, it had a considerable influence on exothermic combustion. When the oxygen level was lowered from 21.0 to 5.0 vol %, spontaneous combustion risk indexes lessened from 2.07 (sample A), 1.85 (sample B), and 0.81 [J/(mg min °C²)] (sample C) to 1.08 (sample A), 1.13 (sample B), and 0.40 [J/(mg min °C²)] (sample C), respectively. Both apparent activation energy and spontaneous combustion risk indexes of the samples decreased saliently as oxygen concentration decreased. Thus, reducing oxygen concentration would be an effective method of inhibiting or possibly even preventing the spontaneous combustion of pulverized coal.
... The mass fraction of volatile-N and char-N is calculated by the CPD (Chemical Percolation Devolatilization) model and the conversion ratio of volatile-N to HCN and NH 3 (HCN/NH 3 = 9/1) as the temperature increased. More detailed descriptions of the models can be seen in [18,[23][24][25][26][27]. ...
Down-fired boilers are commonly used to burn low-rank coals for power generation. However, they often have operational problems such as low char burnout and high NOx emissions. This paper comprehensively investigates how to mitigate such problems in a 300 MWe down-fired utility boiler burning a low-volatile coal by optimizing the staged-air windbox structure. First, experiments and simulations are conducted on a 1:1-scaled model of the staged-air windbox to study the cold air flow in the original and two new windbox structures for understanding the effect of different staged-air windbox structures. Then, combustion simulations are performed for the utility boiler to further study the influence of the newly designed staged-air windboxes on the boiler performance, based on which the optimization scheme of the staged-air windbox in the boiler is more reliably concluded. Finally, the utility boiler is retrofitted by using a newly designed staged-air windbox. Performance tests are made for the boiler before and after the retrofit. The new staged-air windbox is confirmed to greatly improve the performance of the utility boiler, e.g., NOx reduction by 20.7% and thermal efficiency increase by nearly 2 percentage points (from 88.76% to 90.48%) at full load. The idea of the new staged-air windbox design can be readily implemented into down-fired boilers of this kind for more efficient and environmentally friendly firing of low-rank coal for heat and power generation.
... Another way to improve industrial furnaces without expensive modifications can be achieved by influencing the flow field inside the combustion chamber. This has be investigated by Kuang et al. [13,14] by varying the angle of the overfire air jets and the burner location in a down fired furnace. The resulting flow fields differ and reveal an influence on the fuel burnout and thus the flue gas emissions. ...
The call for an increase in efficiency and performance leads to constant developments of furnaces and burners. For this purpose, a propagated application of different burner types, often operating with pure oxygen or oxygen enrichment, can be determined. As a consequence numerical models have to be adapted incorporating industrial needs. This work has developed an approach based on the spatial separation of the whole domain into parts, capable of various oxidizer and fuels to be considered. Each of these parts operates with a single oxidizer and fuel each. The advantages are possible improvements on each small part separately, decreasing computational effort needed. Furthermore, different numerical models, for e.g. combustion and turbulence, can be applied. Moreover, the steady flamelet model (SFM), limited to the usage of a single oxidizer and fuel, can be applied. The results, using the SFM have been compared to the eddy dissipation (EDM) and eddy-dissipation concept model (EDC). Clear differences in the highly reactive areas become obvious using the different combustion models. Though the influence on the remaining combustion chamber is low. Consequently, the heating characteristic of the tubes revealed negligible differences. The calculated temperatures inside the combustion chamber and the transient tube temperature have been compared to measurements at the real size furnace disclosing a high consistency. Thus, the presented approach presents a good alternative for researchers and operators to investigate real-size furnaces.
... The PC ignition is delayed, and the initial stability of PC combustion and the PC burnout become worse. It is generally believed that the lower volatile matter of the burning coal, the smaller design value of the BPA velocity (the design value of the BPA velocity for the down-fired boilers with SBs or DFBs is lower, and basically varies from 11 to 17 m/s [35,36].). However, based on the characteristics of the W-shaped combustion mode of the down-fired boiler, with increasing the BPA velocity, the downward momentum of the coal/air flow from the arches increases. ...
To further reduce unburned combustible in fly ash of the improved down-fired boiler with the eccentric-swirl-secondary-air combustion technology, a breakthrough idea is proposed to significantly increase the primary air velocity by reducing the damper opening of vent air. In this study, the effects of the high primary air velocity on anthracite burnout and NOx formation are investigated via industrial experiments. With increasing primary air velocity, there is a significant change in the dominant role between the inhibition effect of pulverized coal concentration reduction and the promotion effect of the recirculation zone enlargement on the pulverized coal ignition. With increasing primary air velocity from 23.9 to 32.6 m/s, the flue gas temperature in the furnace hopper increases by approximately 75 °C. Besides, the increase of coal char burnout rate at the initial combustion stage is larger than that at the intense combustion stage, and the sum of NOx reduction rates at the initial and intense combustion stages varies from 57 to 61%. The boiler thermal efficiency basically rises linearly from 91 to 92.34% with increasing primary air velocity from 23.9 to 32.6 m/s, and the change of primary air velocity has little influence on the NOx emission at the furnace outlet. Compared with the original boiler, unburned combustible in fly ash decreases by approximately four percentage points, boiler thermal efficiency increases by 3.3%, and NOx emission decreases by 43.3% under the primary air velocity of 32.6 m/s for the improved boiler.
... Great progress has been made in higher control level of NO x emission in coal-firing power plant [6,9,12] because low-NO x burning can relieve stress of denitration system in the downstream of the flue. Therefore, burner development and the combustion organization by burner are researched and discussed frequently [13][14][15][16][17]. Many researchers have focused on the burner's structure and operating parameters. ...
... In work [14], Hodzic et al found that over 50% of NOx emission reduction can be achieved by reburning at 0.1 cofiring, and even 67% with additional effect of reducing process temperature. With regard to influence of furnace configuration to NOx emissions, combustion simetry factors such as dimensionless of upper furnace height, boiler nose depth, furnace arch's burner location and staged air angle were numerically studied in works [15,16] to investigate effects to unburnt and NOx emissions. Same authors in work [17] studied effect of primary air cone length on burner performance and NOx emissions. ...
In this work, various combinations of the NOx emission influencing factors and their combined effects in air staging combustion on level of furnace, using OFA, were investigated in an experimental lab-scale furnace. At this, process temperature were varied in the range from 950 °C to 1450 °C, excess air ratio in primary zone in the range λ = 0.9 - 1.2, while distance of OFA nozzles from the burner outlet varied until a given distance of 2/5 of total length of furnace. Basic fuel is brown coal from Middle Bosnia coal basin, mixed in two coal blends and one coal-woody biomass blend, to combine an effect of fuel characteristics variation on NOx emission. Results shows that an average reduction of NOx emission over tested temperature range, when using OFA against conventional air supply with OFA swiched off, is 26.5%. At this, much more NOx emission reduction for two coal blends were occured at higher temperatures - at 1350 °C and above, where an average NOx emission reduction is 32.5%. Furthermore, it was found that the NOx emission decreased with an increase in distance of OFA nozzles from the outlet level of burner until a distance of 1/3 of total furnace lengh; with further increase of the distance, NOx emission is stabilised and no further effect to NOx emission reduction was observed, while CO emission and unburnt increased.
... The NO x emission reduction is generally achieved using two approaches: low-NO x combustion technology and the treatment of postcombustion flue gas [8]. Low-NO x combustion technology is preferred as a low-cost and highly effective approach to reduce NO x emissions [21,22]. To date, the majority of reports on low-NO x combustion technologies have mainly focused on FW and MBEL down-fired boilers equipped with direct-flow burners [4,8,[23][24][25][26][27][28][29]. ...
A 300-MWe anthracite and down-fired boiler equipped with swirl burners, which was retrofitted with the previously proposed deep-air-staging combustion technology to reduce particularly high NOx emissions, still suffered from high carbon content in fly ash (despite this content being slightly lower than that before retrofit) on basis of significant NOx reduction. To comprehensively produce a high-burnout and low-NOx setting, a novel combustion technology was proposed in which relative to the axis of the primary air duct, the axes of the inner and outer secondary air ducts of the swirl burner shift away from the furnace center, and this technology is called an eccentric-swirl-secondary-air combustion technology. The coaxial symmetrical arrangements of primary and secondary air ducts for traditional swirl burners are broken. Full-scale industrial measurements under original, deep-air-staging and eccentric-swirl-secondary-air combustion technologies with respect to different loads (i.e., 180, 250, and 300 MWe) were carried out to compare and analyze the validity of and improvement offered by the eccentric-swirl-secondary-air combustion technology. Under different boiler loads, especially at low and middle loads, compared with original and deep-air-staging combustion technologies, eccentric-swirl-secondary-air combustion technology markedly extends the penetration depth of coal/air flow and recirculation regions below arches. The thermal resistance between primary coal/air flow and high-temperature flue gas at the furnace center is reduced further, and the ignition of pulverized coal is timelier. The heating gradient in the burner outlet zone is maintained throughout, and the flame fullness in the primary combustion zone increases. The above three aspects contribute to pulverized coal burnout. Similar to deep-air-staging combustion technology, a low-oxygen and strong reducing atmosphere was formed in the primary combustion zone to reduce NOx formation due to the introduction of overfire air. Compared with deep-air-staging combustion technology, for the eccentric-swirl-secondary-air combustion technology, the air staged combustion in the furnace is improved further, and pulverized coal combustion in the primary combustion zone is more well distributed, which is beneficial to further reducing NOx emissions. The results of low-NOx and high-efficiency performance show that compared with the original boiler, NOx emissions and carbon content in fly ash for the boiler with the eccentric-swirl-secondary-air combustion technology are significantly reduced by above 42% and 32.5 to 20.7% at loads of 180, 250 and 300 MWe, respectively.
The research object in this study is a W-shaped boiler with swirl burners (i.e. swirling W-shaped boiler), which is enhanced by multiple ejection and staged combustion technology (MESCT). Through the method of numerical simulations combined with industrial experiment verification, the influence of greatly increasing the primary air velocity (PAV) on the combustion and NOx emission characteristics is deeply explored. The results show that, as the PAV raises from 20 to 31 m/s, the high-temperature recirculation zone (HTRZ) under the arch expands, which effectively promotes the ignition of pulverized coal (PC) airflow and balances the inhibition effect of ignition on account of the reduction of PC concentration. At the same time, the flame center in the primary combustion zone moves down and the flame fullness increases. With the increase of PAV, the oxidizing atmosphere in the prophase of PC ignition is strengthened, and the formation of NOx is increased. The PAV increases from 20 to 31 m/s, and the combustible component of fly ash decreases significantly from 8.97% to 5.35%, with a decrease of about 40%. However, the NOx emission concentration increases only by 8.5% from 672 to 729 mg/m³ (O2 = 6%). Compared with the traditional swirling W-shaped boiler, when the improved W-shaped boiler with a PAV of 31 m/s, the combustible component of fly ash and NOx emission concentration are reduced by 51.8% and 43.4%, respectively.
Taking a 300-MWe arch-fired boiler as the research object, compared with original Babcock & Wilcox (B&W) combustion technology, the flow, combustion and NOx emission characteristics are comprehensively revealed under the novel biased-swirl-secondary-air combustion (BSSAC) technology with the help of cold-modeling experiments and numerical simulations. The results show that, for BSSA combustion technology, the dimensionless downward depth of the air flow from furnace arches is increased by 0.13, and the air flow fullness of whole the lower furnace is improved under the effect of burner secondary air near the water-cooled wall, vent air and staged air being injected step by step. The high-temperature reflux zone and gas reflux velocity under the furnace arch increase significantly, the ignition of primary pulverized coal air flow is advanced, and the combustion path of pulverized coal particles increases. The combustible content of fly ash decreases by 32.4%. At the same time, the ignition process of pulverized coal air flow near the center of the furnace is in a strong reducing and low oxygen atmosphere, and the multi-stage effective air supply is obtained in the downward process of pulverized coal combustion. The NOx emission concentration at the furnace outlet is reduced from 824 to 694 mg/m³ (O2 = 6%).
In the present work, computational fluid dynamic (CFD) models have been established to investigate a newly presented flue gas temperature deviation solution on the basis of ANSYS FLUENT16.0 considering a 700 °C tangentially fired pulverized-coal boiler, which confronts with severe flue gas temperature deviation. Orthogonal test method was used to complete the optimization parameters with deflected secondary air swing angle (A), separated over-fire air reverse tangent angle (B), burner upward swing angle (C); and the thermal deviation at the outlet of the furnace as the performance index. The orthogonal test table L9(3⁴) with three levels for each factor was designed. Using the range analysis method and the weight-based matrix analysis method respectively, the significance degree of the influence of different factors and levels on the deviation of the flue gas temperature at the furnace outlet was discussed, the optimal factor and level combination was determined, and the corresponding influence weights were given. It is found (1) Residual swirling flow in the upper furnace causes the flue gas velocity and temperature deviations in crossover pass. (2) The influence degree of various factors on the deviation of the flue gas at the furnace outlet is as follows: separated over-fire air reverse tangent angle > deflected secondary air swing angle > burner upward swing angle, the influence weights of different factors are A = 0.2184, B = 0.5962, C = 0.1560, and the best factor and level combination is A1B3C2. (3) In this test, when the deflection secondary air angle is 0°, separated over-fire air reverse tangent angle is 25°, and the burner upward swing angle is 11°, the flue gas temperature deviation of the furnace outlet section is the minimum, which is 29.297 K, compared with the maximum, the decrease is up to 86.22 K.
The thermal deformation of membrane walls is a severe safety issue in the arch-fired boilers, which is closely related to the combustion and flame inside the furnace. In this paper, various combustion schemes were conducted on a laboratory-scaled arch-fired boiler, exploring the correlations between the working status on membrane walls and the arrangement of burners. The influence mechanisms of the burners on the membrane walls were experimentally determined, with strong impact regions found on the front/rear wall in a relative wide of 0.6. The asymmetric combustion obtained in the furnace increased the temperature and strain/stress on the front wall with greater deformation risk, which should be avoided in the practical operation. The flameout of burners dramatically changed the local temperature and enhances the tearing risk of membrane walls with tension stress, which is consistent with the tearing track in the prototype boiler. Great correlations between the operation parameters (Tg, Tw, Tc, ε1 and σ1) and the and volumetric thermal load (Pv) were experimentally determined and considered applicable to the prototype boiler. While the impact range of burners helps estimating the uniformity of temperature and strain/stress on membrane walls, providing reference to the design and operation of similar boilers.
Numerical simulations are carried out to investigate the influence of burner nozzle–organization–mode (N–O–M) on flow field distribution and combustion characteristics of a 300‐MWe subcritical down‐fired boiler. Three typical N–O–Ms respectively designed with Mitsui Babcock Energy Limited (MBEL), fuel‐lean coal/air flow down‐setting (FD), and multi‐injection multistaging combustion (MIMSC) technology are studied, plus industrial‐size measurements on the original MBEL boiler. Vertical velocity attenuation index (η) and maximum dimensionless penetrating depth (γ) are introduced to estimate the transfer effect and penetrating capacity of pulverized coal/air flow. Results uncover that under the N–O–M based on MBEL technology, flow field and temperature field are deflective. η is 9.257. Under N–O–Ms based on FD and MIMSC technology, flow field and temperature field present obvious symmetry. η respectively are 4.365 and 2.921. γ are correspondingly about 0.8 and 1.36. Carbon content in fly ash and NOx emissions at furnace outlet are lowest (5.77% and 735.21 mg/m3 (O2 = 6%)) under the N–O–M with MIMSC technology. The bigger the η is, the worse penetrating characteristics of coal/air flow is, resulting in poor burnout of pulverized coal. Therefore, MIMSC technology is recommended for the boiler improvement, and η should be taken into account in the design of new down‐fired boiler N–O–Ms.
Using a phase Doppler‐anemometer measurement system, the cold gas/particle‐airflow behavior in a 1:40 scale‐model furnace was assessed to study the influences of adjusting the inner–secondary‐air ratio in a 600‐MWe multi‐injection and multistaging down‐fired boiler. Numerical simulations were also conducted to verify the results of the modeling trials and to provide heat‐state information. The results demonstrate that reducing the inner–secondary‐air ratio from 19.66% to 7.66% gradually enhances the downward velocity decay of the gas/particle airflow, while the inner secondary‐air downward‐entraining effect on the fuel‐rich flow is weakened. Lowering the inner–secondary‐air ratio greatly inhibits the decay of the near burner–particle volume flux. In addition, the fuel rich–flow ignition distance is reduced, from 1.02 to 0.87 m. A lower inner–secondary‐air ratio is harmful to restrain early NOx formation. Reducing the ratio also causes the fuel‐rich flow to turn upwards ahead, while the penetration depth of this flow gradually decreases and the maximum temperature in the hopper region falls from 1900 to 1800 K. On the basis of these data, an optimal inner–secondary‐air ratio of 13.66% is recommended. The influences of adjusting inner–secondary‐air ratio in a 600‐MWe multi‐injection and multistaging down‐fired boiler are studied in this work. The results reveal that reducing the inner–secondary‐air ratio gradually enhances the downward velocity decay of the gas/particle airflow, the penetration depth of fuel‐rich flow decreases, and the maximum temperature in the hopper region drops from 1900 to 1800 K. Reducing the inner–secondary‐air ratio promotes advanced coal ignition but is harmful to restrain the early NOx formation.
This paper reports the oxy-fuel combustion characteristics in a pilot-scale furnace with a bituminous coal. After discussing the design principles of oxy-fuel burners in detail, a swirling low-NOx burner system is specially designed to achieve the compatible combustion of air-fuel combustion and oxy-fuel combustion. The initial O2 concentrations vary between 22% and 30% by volume in oxy-fuel combustion. A reliable transition process is performed between different combustion modes. Measurements of flame images, burnout rate, heat transfer, and pollution emission are carried out. The results show that compatible and stable combustion with low NOx emissions is achieved in different combustion modes. A high time-averaged CO2 concentration of 81.5% by volume is achieved in the dry flue gas. In this test facility, compared to air-fuel combustion, the averaged burnout rate of pulverized coal slightly increases (approximately 97%), despite the decrease of flame temperature. There is an optimal initial O2 concentration between 26% and 30% by volume, which can achieve a similar heat transfer process with air-fuel combustion. By using a low-NOx oxy-fuel burner system and flue gas recycle, NOx emissions decrease to a level that is 30%–50% of that in air-fuel combustion. The oxy-fuel combustion with a low-NOx oxy-fuel burner system is confirmed for efficiently and cleanly burning pulverized coal.
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.
A set of comprehensive on-site experiments was conducted to assess the performance of the louver-type primary air (PA) concentrators on anthracite ignition and combustion in a 600 MW supercritical arch-fired boiler (AFB) equipped with slot burners. The temperature profiles along the PA stream from fuel-rich and fuel-lean streams were measured under various conditions. The separation efficiency of the PA concentrators of two selected burners was also measured and its effect on the ignition of the PA streams was studied. Experimental results revealed that separation performance of some louver concentrators was even opposite to design setting. The ignitability of PA stream, however, was more profoundly depended on the configuration of burner, the position of the nozzle and the SA distribution than the separation performance of the louver concentrators. On the other hand, confirmed by numerical simulation, the unsatisfied separation performance of the louver-type concentrators could result in overall poor ignition of PA, causing over-shooting of the flame penetration length, and high loss of ignition and difficulties for combustion organization. Some suggestions are given to the retrofit and new design of combustion system.
Soto de Ribera Thermal Power Station is located in Northern Spain near Oviedo. It is designed to burn coal with volatile matter ranging from 6 to 19%, moisture from 14 to 22%, and ash from 28. 5 to 45%. Better results are obtained if a reasonably homogeneous mixture combines the best characteristics of the various coals.
A new combustion system has been applied to a 300 MWe down-fired boiler with swirl burners to reduce NOx emissions. The unit provided the introduction of overfire air (OFA) and a decrease in the flow area of the inner and outer secondary air ducts of the swirl burners. Industrial experiments on the retrofitted boiler were performed at different loads. Full-scale measurements of the flue gas temperature distribution in the burner outlet region, the furnace temperature distributions measured by a pyrometer and the local mean gas species concentrations in the region near the sidewall were made at loads of 180, 250, and 300 MWe. The results show that the ignition distance increased with decreasing load, especially as the load decreased from 250 MWe to 180 MWe. At three different loads, the retrofitted coal/air flow could be all ignited in time at a distance in the range of 0.6–1.4 m from the burner outlet. Compared with the original combustion system, the ignition distance of the coal/air flow was significantly reduced at a load of 300 MWe. In addition, at a load of 300 MWe, the temperature of the boiler hopper was much higher than that at loads of 180 MWe and 250 MWe. Compared with the original combustion system, the upper furnace temperature decreased slowly with increasing measurement height at a load of 300 MWe after the retrofit. Measurements of the O2 and CO concentrations in the region near the sidewall indicate that the fullness degree of the coal flame in the furnace at different loads was different. After the retrofit, the reheat steam temperatures reached the design temperature of 541 °C at loads of 180 MWe and 250 MWe, and the average reheat steam temperatures increased by approximately 13 °C. Compared with the original combustion system, a significant NOx reduction (more than 40%) at different loads was achieved without increasing the levels of unburnt carbon in the fly ash.
Boilers manufactured by using the down-fired technology of British Mitsui Babcock Energy Limited Corp. (called as MBEL down-fired boiler) account for an about 20% Chinese down-fired boiler’s market share. The published work showed that MBEL down-fired boilers suffered similarly from problems of asymmetric combustion and high NOx emissions, while combustion status and burnout rate varied greatly among boilers manufactured at different times. To uncover the airflow, coal combustion, NOx emissions, and slagging characteristics with respect to various boiler’s parameters (including furnace configuration, burner layout pattern, and air-distribution model), cold-modeling airflow experiments and full-load industrial-size experiments under design parameters were performed for three typical MBEL down-fired boilers (with a boiler capacity of 350, 300, and 600 MWe, respectively). All boilers present flow-field deflection, with the downward airflow in one side penetrating much deeper than that in the other side. Consequently, asymmetric combustion appears in real operations, with large gas temperature gaps existing between the front- and rear-half furnace sides. The 350 MWe boiler, which has a leptosome furnace pattern and owns the highest secondary-air velocity and volumetric heat capacity, shows the weakest flow-field deflection and highest furnace gas temperature levels corresponding to the lowest burnout loss, but unfortunately, suffers from particularly high NOx emissions and serious furnace-arch slagging. On the contrary, the 300 MWe boiler generates the lowest gas temperature levels and NOx emissions and highest burnout loss, without such a slagging problem that appearing in the 350 MWe boiler. Although the 600 MWe boiler also presents a poor furnace performance, its combustion status, gas temperatures, and burnout rate are all a little better than those of the 300 MWe boiler, with the exception of higher NOx emissions and the deduced furnace-arch slagging problem. Finally, the three boilers’ performance comparison generates an appropriate parameter assemble recommended for new MBEL down-fired boiler designs.
To achieve significant reductions in NOx emissions without increasing the levels of unburnt carbon in the fly ash, a new combustion system was applied to a 300-MWe Babcock & Wilcox (B&W) down-fired boiler installed with swirl burners. The unit featured introduced overfire air (OFA) and decreased outlet area of the inner and outer secondary-air ducts of the swirl burners. Full-scale measurements (adjusting the inner secondary-air vane angle to 35°, 45°, and 55°) revealed that the influence of the high-temperature recirculating region under the arch upon the combustion and NOx emission characteristics of the boiler is greater than that of the high-temperature flue gas entrained by the swirl burner itself. The ignition distance of the coal/air flow is reduced by at least 1.8 m compared with that of the original combustion system. For the inner secondary-air vane angle of 35°, the coal/air flow ignites earlier than for the vane angles of 45° and 55°. The measurements of the gas species concentrations in the zone near the sidewall indicates that at inspection port 1, the coal flame of the burners does not spread across the entire furnace cross-section for all three vane angles; however, for the vane angle of 35° the flame is spread across the entire furnace cross-section at inspection port 2. For this optimal (35°) inner secondary-air vane angle, the NOx emissions and carbon content in the fly ash reached levels of 674 mg/m³ (6% O2), and 11.4%, respectively, achieving a significant NOx reduction of 51.9% without increasing the levels of unburnt carbon in the fly ash.
A novel combustion system has been applied to a 600 MWe down-fired boiler to reduce NOx emissions without producing an obvious increase in the carbon content of fly ash. The system mainly includes moving fuel-lean nozzles from the arches to the front/rear walls, and re-arranging the staged air, as well as introducing separated-over-fire air (SOFA). This paper evaluates the effects of the SOFA locations (on the arches, on the throat, and on the upper furnace) on the combustion and NOx emissions characteristics using simulations. The numerical results are in good agreement with the measured results. Compared to the original combustion system, significant NOx reduction (approximately 50%) is found for all three SOFA location settings. Taking economic efficiency and NOx emissions into account, the SOFA on the upper furnace is adopted in the actual modification, and no negative effects are observed.
The numerical simulations of the slagging characteristics under multi-operating modes in three types of W-flame boiler were carried out by use of the slagging models coupled with the gas-solid two phase flow and combustion models. Combined with the actual operational status, comparative analysis on the slagging position, extent and reasons was represented. The results show that when the coals with same slagging tendency are burned, the slagging characteristics in three types of W-flame boiler are obviously different due to the differences in furnace structural size, thermal parameters in the lower furnace, burner type and secondary air distribution mode. The side wall slagging can be effectively alleviated by means of cutting off the burners close to the side walls, reducing boiler load and burning coals with low slagging tendency. Based on the analysis above, some measures for preventing slagging in W-flame boilers were presented.
Deep air-staged combustion tests of Datong (DT) bituminous coal were carried out in a 20 kW down flame furnace (DFF) with the burner stoichiometric ratio (SR) ranging from 1.200 (unstaged) to as low as 0.696 (deep staged). The experimental results shown that the concentration of CO reach as high as 120,000 ppm (12 vol.%) and the NOx decrease to nearly zero in the reducing zone under deep staging conditions of SR = 0.696, which was never observed before. Thus, the extent of CO formation (i.e. char gasification) and the NOx reaction mechanism under deep staging condition were studied in order to understand the combustion process of coal. This paper presents a refined numerical simulation for reproducing the profiles of CO and NOx along the DFF under deep staging condition. The comparison between simulation and experimental results prove the reasonability of refined kinetic parameters of char gasification. The enhancement of char gasification by CO2 is proposed and validated. With the simulated CO profile in the DFF confirmed by experiment, the NOx profile could be further analyzed. The discrepancy of simulated NOx profile in the reducing zone (i.e. fuel-rich zone) indicates that there are some undetected nitrogenous species and undiscovered NOx transfer mechanism regardless of the consistence of final NOx emission between simulation and experiment. It is supposed by us that a majority of NOx immersing in high level of CO in the reducing zone is mainly transferred into undetected nitrogenous species (excluding HCN and NH3) which is then rapidly oxidized into NOx once the remaining oxygen is injected into the DFF.
The temperature across natural gas flames is measured wth Pt, Pt-Rd thermocouples of different diameters, and a theoretical expression is developed for calculating the true flame temperature by using the temperatures recorded by the three thermocouples. The flame temperature is calculated by using both two-element and three-element methods and the results are compared. It is observed that the three-element method gives a higher flame temperature.
A novel combustion system was applied into a 600 MWe Foster Wheeler (FW) down-fired pulverized-coal utility boiler to solve high NOx emissions, without causing an obvious increase in the carbon content of fly ash. The unit included moving fuel-lean nozzles from the arches to the front/rear walls, and re-arranging staged air, as well as introducing separated-over-fire air (SOFA). Numerical simulations were carried out under the original and novel combustion systems to evaluate the performance of combustion and NOx emissions in the furnace. The simulated results were found to be in good agreement with the in situ measurements. The novel combustion system enlarged the recirculation zones below the arches, thereby strengthening the combustion stability considerably. The coal/air downward penetration depth was markedly extended, and the pulverized-coal travel path in the lower furnace significantly increased, which contributed to the burnout degree. The introduction of SOFA resulted in a low-oxygen and strong-reducing atmosphere in the lower furnace region to reduce NOx emissions evidently. The industrial measurements showed that NOx emissions at full load decreased significantly by 50%, from 1501 mg/m3 (O2 at 6%) to 751 mg/m3 (O2 at 6%). The carbon content in the fly ash increased only slightly, from 4.13% to 4.30%.
Co-firing biomass is the principal means of mitigating the future energy crisis by expanding the use of renewable energy. Oxy-fuel combustion is the most capable technologies for carbon capture and storage (CCS) system. This paper presents a 3D numerical study considering co-firing concepts in a 550 MW tangentially fired furnace using a commercial CFD code AVL Fire ver.2009.2. Necessary subroutines were written and coupled with the code to account for chemical reactions, heat transfer, fluid and particle flow fields and turbulence. Due to irregularities of the biomass particle shape, a special drag effect was considered. Three different co-firing cases (20% biomass with 80% coal, 40% biomass with 60% coal and 60% biomass with 40% coal) were considered. All the co-firing cases were simulated under air-firing and three different oxy-firing cases (25% O2/75% CO2, 27% O2/73% CO2 and 29% O2/71% CO2). Level of confidence has been achieved by conducting a study on co-firing of biomass with coal in a 0.5 MW small scale furnace under air and oxy-fuel conditions. Similar findings have been observed in the present study which indicates the model can be used to aid in design and optimization of large-scale biomass co-firing under oxy-fuel conditions. This study enables the calculation of species transport and mixing phenomena and the simulation of ignition, combustion and emission formation in industrial furnace. Results were presented by the aerodynamics of burner flow, temperature distributions, gaseous emissions such as O2 and CO2 distributions. With the increase of biomass sharing, peak flame temperature reduced significantly. The dominant effect of the lower calorific value of biomass dampens the effect of volatile content contributing to lower temperature. Comparatively, improved burnout is observed for the improved oxy-fuel cases. But, the CFD model predicted a significant increase in unburned carbon in fly ash for the increase of biomass co-firing sharing. Overall, this study highlights the possible impact of changing the fuel ratio and combustion atmosphere on the boiler performance, underlining that minor redesign may be necessary when converting to biomass co-firing under air and oxy-fuel conditions.
A down-fired boiler burning pulverised coal operates in an intrinsically staged fashion, with the combustion characteristics having similarities to those deliberately engineered for NO//x emission control on other types of plant. A 500 MW(e) unit of this design has been tested with the aim of quantifying the effect of altering the degree of staging on NO//x emissions during commercial operation. It was found that for a given load, the primary parameter affecting the NO//x levels was the initial stoichiometry of the flame. Reductions of around 20% in emissions were possible on a long-term basis.
Low-volatile fuels such as anthracite and lean coal are widely used in power generators throughout the world. In comparison with tangential-fired and wall-arranged furnaces, down-fired boilers are thought to be more suitable for firing anthracite and lean coal. Currently, down-fired boilers are widely in service and have developed rapidly in China over the past 20 years. In this paper, a comprehensive review of investigations into the gas/particle flow, combustion and NOx emission characteristics within various types of down-fired boilers is presented. The published work disclosed that down-fired boilers suffered similarly from various problems such as late coal ignition, poor combustion stability, low burnout (carbon in fly ash typically in the range 7-15%), heavy slagging, high NOx emissions (typically in the range 1100-2100 mg/m3 at 6% O2), and asymmetric combustion. Again, the causes of these problems and various solutions in dealing with them were introduced in turn. Although causes of these problems are complicated, the reported deficiencies such as the premature mixing between high-speed secondary air and low-speed fuel-rich coal/air flow, short coal/air flow penetration depth, downward coal/air flow washing over walls, shallow air-staging conditions, and asymmetric flow-field formation contribute great efforts to develop these problems. To summarize experiences and the lessons in those reported solutions, a series of suggestions for organizing reasonable combustion in down-fired furnaces have been provided so as to achieve timely ignition, symmetric and stable combustion, weak slagging, good burnout, and low NOx emissions.
Deflected flow fields and large combustion differences between zones near front and rear walls have been found in down-fired pulverized-coal boilers under symmetric air distribution modes. To eliminate or mitigate the flow-field deflection and achieve relatively symmetric combustion in these boilers, the secondary-air distribution ejected through the front and rear arches was adjusted to construct an asymmetric secondary-air distribution mode. Cold-modeling airflow experiments over a wide range of asymmetric secondary-air distributions (i.e., differences in the ratio of secondary-air mass flow rate between the front and rear arches (Rd) of −16%, –8%, 0%, 5%, 8%, 16%, and 32%) were conducted within a small-scale model of a down-fired pulverized-coal 300 MWe utility boiler. Results revealed that a steady and symmetric flow field could not be achieved simply by adjusting the secondary-air distribution between the front and rear arches. To establish a flow field along with an appropriate airflow reach for more economical operation, an optimal setting of Rd = 5% was found for the secondary-air distribution between the front and rear arches. Industrial-size measurements revealed that a secondary-air distribution setting of Rd = 6.3% (i.e., approaching to the cold-modeling optimized result of Rd = 5%) was applicable if applied in the real furnace to deal with asymmetric combustion, low burnout, and high NOx emissions.
Synthetic fuels derived from non-oil feedstock are gaining importance due to their cleaner combustion characteristics. This work investigates spray characteristics of two Gas-to-Liquid (GTL) synthetic jet fuels from a pilot-scale pressure swirl nozzle and compares them with those of the conventional Jet A-1 fuel. The microscopic spray parameters are measured at 0.3 and 0.9 MPa injection pressures at several points in the spray using phase Doppler anemometry. The results show that the effect of fuel physical properties on the spray characteristics is predominantly evident in the regions close to the nozzle exit at the higher injection pressure. The lower viscosity and surface tension of GTL fuel seems to lead to faster disintegration and dispersion of the droplets when compared to those of Jet A-1 fuel under atmospheric conditions. Although the global characteristics of the fuels are similar, the effects of fuel properties are evident on the local spray characteristics at the higher injection pressure.
A 600 MWe down-fired pulverized-coal supercritical boiler, which was equipped with a deep-air-staging combustion system for reducing the particularly high NOx emissions, suffered from the well-accepted contradiction between low NOx emissions and high carbon in fly ash, in addition to excessively high gas temperatures in the hopper that jeopardized the boiler's safe operations. Previous results uncovered that under low-NOx conditions, strengthening the staged-air effect by decreasing the staged-air angle and simultaneously increasing the staged-air damper opening alleviated the aforementioned problems to some extent. To establish low-NOx and high-burnout circumstances and control the aforementioned hopper temperatures, a further staged-air retrofit with horizontally redirecting staged air through an enlarged staged-air slot area was performed to greatly strengthen the staged-air effect. Full-load industrial-size measurements were performed to confirm the availability of this retrofit. The present data were compared with those published results before the retrofit. High NOx emissions, low carbon in fly ah, and high hopper temperatures (i.e., levels of 1036 mg/m3 at 6% O2, 3.72%, and about 1300 °C, respectively) appeared under the original conditions with the staged-air angle of 45° and without overfire air (OFA) application. Applying OFA and reducing the angle to 20° achieved an apparent NOx reduction and a moderate hopper temperature decrease while a sharp increase in carbon in fly ash (i.e., levels of 878 mg/m3 at 6% O2, about 1200 °C, and 9.81%, respectively). Fortunately, the present staged-air retrofit was confirmed to be applicable in regulating low-NOx, high-burnout, and low hopper temperature circumstances (i.e., levels of 867 mg/m3 at 6% O2, 5.40%, and about 1100 °C, respectively).
A fundamental investigation has been conducted on the combustion behavior of single particles (75–150 μm) of four coals of different ranks: anthracite, semi-anthracite, medium-volatile bituminous and high-volatile bituminous. A laboratory-scale transparent laminar-flow drop-tube furnace, electrically-heated to 1400 K, was used to burn the coals. The experiments were performed in different combustion atmospheres: air (21%O2/79%N2) and four simulated dry oxy-fuel conditions: 21%O2/79%CO2, 30%O2/70%CO2, 35%O2/65%CO2 and 50%O2/50%CO2. The ignition and combustion of single particles was observed by means of three-color pyrometry and high-speed high-resolution cinematography to obtain temperature–time histories and record combustion behaviors. On the basis of the observations made with these techniques, a comprehensive examination of the ignition and combustion behaviors of these fuels was achieved. Higher rank coals (anthracite and semi-anthracite) ignited heterogeneously on the particle surface, whereas the bituminous coal particles ignited homogeneously in the gas phase. Moreover, deduced ignition temperatures increased with increasing coal rank and decreased with increasing oxygen concentrations. Strikingly disparate combustion behaviors were observed depending on the coal rank. The combustion of bituminous coal particles took place in two phases. First, volatiles evolved, ignited and burned in luminous enveloping flames. Upon extinction of these flames, the char residues ignited and burned. In contrast, the higher rank coal particles ignited and burned heterogeneously. The replacement of the background N2 gas of air with CO2 (i.e., changing from air to an oxy-fuel atmosphere) at the same oxygen mole fraction impaired the intensity of combustion. It reduced the combustion temperatures and lengthened the burnout times of the particles. Increasing the oxygen mole fraction in CO2 to 30–35% restored the intensity of combustion to that of air for all the coals studied. Volatile flame burnout times increased linearly with the volatile matter content in the coal in both air and all oxygen mole fractions in CO2. On the other hand, char burnout times increased linearly or quadratically versus carbon content in the coal, depending on the oxygen mole fraction in the background gas.
To determine the effect of the arch- and wall-air distribution on flow characteristics, experiments were conducted within a 1:15-scaled model for a down-fired 350 MWe furnace at various settings, i.e. ratios (denoted by Rd) of the mass flow rate of staged air to that of total secondary air of 0%, 7%, 16%, 23%, and 29%. Meanwhile, industrial scale measurements were performed at full load with staged-air damper openings of 15% and 35% (equaling to Rd at about 10% and 20%), respectively. At settings of Rd = 0% and 7%, an essentially symmetric flow field appeared. At the left three higher settings (i.e., Rd = 16%, 23%, and 29%), a deflected flow field developed, with the airflow near the front wall penetrating much further than that near the rear wall. At this time, increasing Rd deteriorates the flow-field deflection. By means of cold-modeling experiments to evaluate the flow-field symmetric extent and downward airflow penetration depths in the lower furnace, the appropriate Rd was found to be in the range of 7–16%. Real-furnace measurements revealed that although the 15% opening was inapplicable in boiler operations for a long time, relatively symmetric combustion could developed at the this opening setting. At the 35% opening setting, an asymmetric combustion pattern developed in the furnace, with temperatures near the front wall being clearly higher that those near the rear wall. However, particularly high NOx emissions and good burnout developed at both two openings. In considering that numerical simulations and industrial scale measurements in published work have confirmed the validity of a previously-proposed deep-air-staging combustion technology in achieving excellent furnace performance within down-fired furnaces, retrofitting the present furnace with the technology is thus recommended if symmetric combustion, good burnout, and low NOx emissions are to be achieved.
To study the aerodynamic characteristics, experiments were carried out on a laboratory-scale model for an arch-fired boiler. The airflow velocity was measured by using a two-dimensional PIV. The results show that: (1) as the momentum flux ratio increases, the arch air becomes stronger and reaches deeper into the furnace; however, the velocity distribution was non-uniform in the lower furnace, and the recirculation region expanded slightly. The ideal value of the momentum flux ratio is 2.54. (2) As the low layer secondary air is injected with an inclination angle, its mixing with the arch air is postponed. So, the gas after mixing can reach the lower furnace and the residence time of fuel is prolonged. As the inclination angle increases from 0° to 20°, 30° and 40°, respectively, higher penetration depth and fullness degree of airflow can be obtained as well as the larger recirculation region. The ideal inclination angle is 30°; and (3) Compared with increasing the momentum flux ratio, adjusting the secondary air inclination angle is more beneficial to improving the distribution of the airflow within the furnace.
Over-fire air is introduced to achieve deep staging conditions for combustion optimization in a 0.7 MW arch-fired furnace. The momentum flux ratio (M) of vertical to horizontal component is determined by variations of air distribution and the inclination angle of the F-layer secondary air. Two coals with relatively large characteristic differences are used in the pilot tests. The results show that M directly affects the arch air penetration length and the position of the flame center. As the inclination angle increases appropriately, the position of high temperature zone moves downward, and the variation amplitude becomes smaller. The distribution of the wall temperature becomes more uniform. An overlarge angle would cause severe dregs on the wall of hopper, however. Increasing the arch air ratio can also delay the mixing of air–fuel flows and stage air and lower downward the flame center. Too small M would cause much higher temperatures in the upper furnace. The unburned carbon in the fly ash and NOx emission both attain their minimum values with an inclination angle of 30° and a momentum flux ratio of 1.35, which is considered as the optimum operation condition in the experimental range.
The aim of this investigation is predicting the flow characteristics with real operating conditions of a boiler to better understand the phenomena occurring in the interior of the furnace and to validate the models chosen for the simulation. For this purpose, we have done a numerical study of the flow of a reactive gas mixture with pulverized coal combustion occurring in a tangentially fired furnace of a real power plant. These calculations were developed with the commercial software ANSYS Fluent. Furthermore, a home-made code was built to perform some necessary preprocessing and postprocessing calculus. In particular, this code solves zero-dimensional balances that have provided a good agreement with the calculated and measured flow at the exit of the furnace and it is also used to validate the convergence of the numerical algorithm for the three-dimensional simulation. The results obtained from this study show that models and numerical methods selected are appropriate to correctly predict the combustion processes within the furnace. In conclusion, the validation of this numerical model and our home-made code provides the user a complete tool to evaluate the performance of a boiler under different operating conditions, reducing the cost of experimental tests.
To acquire the gas/particle flow characteristics of an in-assembly down-fired 600-MWe supercritical utility boiler, experiments were conducted by using particle dynamics anemometer (PDA) measurement within a two-phase small-scale model at various staged-air ratio settings (i.e., 0%, 6%, and 12%). The mean velocity, particle volume flux, and particle number concentration along several cross sections, were discussed well in the present work, in addition to the decay and trajectories of the downward gas/particle flow. For all three settings, asymmetric gas/particle flow characteristics appeared in the lower furnace, with the gas/particle flow in the front-half furnace penetrating greatly further and occupying much more furnace volume than that in the rear-half furnace. The longitudinal-velocity components are clearly higher near the front wall than near the rear wall. Decreasing the staged-air ratio can only weaken the gas/particle flow asymmetries to some extent. An estimation on the furnace performance, i.e., severe asymmetric combustion, low burnout, and high NOx emissions, is given out for the boiler's commercial operation in the near future.
By using phase-Doppler anemometry (PDA) measurements within a cold small-scale model for a down-fired 600-MWe supercritical utility boiler, gas/particle flow characteristics were acquired at various staged-air declination angle settings (i.e., 0°, 15°, 30°, and 45°) so as to assess the availability of a staged-air declination method in improving the strong asymmetric gas/particle flow field. Detailed comparisons made in the mean velocity, particle volume flux, particle number concentration, gas/particle flow decays, and trajectories of the downward gas/particle flows with respect to different staged-air angle settings, revealed that enlarging the staged-air angle essentially did not change the strong asymmetric patterns in gas/particle flow characteristics. From measurements taken within a real furnace equipped with a designed horizontally-fed staged air (i.e., 0° for the staged-air angle), strong asymmetric combustion characterized by gas temperatures being much higher near the rear wall than near the front wall, low burnout, and particularly high NOx emissions, were found. By considering the appearance of a similar strong asymmetric gas/particle flow field at various staged-air angle settings in this study and at different staged-air ratios in a previous study, staged-air declination and staged-air ratio reduction are very likely to be inapplicable if applied in real furnaces to deal with these problems (i.e., strong asymmetric combustion, low burnout and high NOx emissions). Retrofitting a boiler with deep-air-staging combustion technology developed in our recent investigation is recommended if symmetric combustion, high burnout, and low NOx emissions are to be achieved.
Deposit samples for eight coals were collected on stainless steel probes in reducing and oxidizing regions of a 160 kWth, down-fired, pulverized coal reactor. Firing conditions in the reactor and probe temperatures were controlled to simulate an industrial furnace operating on an advanced ultra-supercritical steam cycle, 500 °C in the near burner region and 750 °C in the oxidizing region. The samples were analyzed using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) to determine particle sizes and elemental compositions. Deposited particles on the top or upstream side of the probes were larger and more irregular in shape. Particles that deposited on the bottom or downstream side of the probes were smaller and more spherical. The SEM–EDS analysis showed each particle consisted of a unique composition of elements. The sulfur concentrations measured in the deposits were averaged for the upstream and downstream side of each specimen and were not found to correlate with sulfur concentrations of their respective coals, but they were strongly correlated with the calcium and iron contents of their respective coals. The high-calcium, sub-bituminous and lignite coals produced high sulfur concentrations in the deposits, particularly in the downstream, oxidizing deposits. The high-iron, bituminous coals produced high sulfur concentrations primarily in upstream, reducing deposits. The relatively low-calcium and low-iron bituminous coals produced low sulfur concentrations in the deposits. Although high in iron, the Mahoning coal deposits contained low sulfur concentrations.
A deep-air-staging combustion technology was previously developed to achieve reduction in NOx emissions and to eliminate strongly asymmetric combustion found in down-fired boilers. Recently, one of two down-fired 600 MWe supercritical utility boilers using this technology (without applying overfire air) began commercial operations. To understand coal combustion and NOx emissions characteristics within the furnace, full-load industrial-size experiments were performed at different air-staging conditions with measurements taken of gas temperatures in the burner region, gas temperatures and species concentrations in the near wing-wall region, carbon content in fly ash, and NOx emissions. As expected, the furnace performance characterized by relatively timely coal ignition, symmetric combustion, and low levels of carbon in fly ash, developed in the furnace at all three settings. Deepening the air-staging conditions could reduce NOx emissions by one-fifth, but varied slightly carbon in fly ash. In view of the still high NOx production (i.e., 1036 mg/m3 at 6% O2), adding an overfire air system which was essentially a part of the technology, was recommended for the boiler to significantly reduce the present NOx emissions.
This paper describes the practical combustion issues encountered with biomass co-firing on a large scale trial in a 500 MW down fired utility boiler at Aberthaw power station. It also investigates and discusses the effect of biomass particle size and physical properties on devolatilisation; flame stability; and slagging by using the biomass energy crop miscanthus.During large scale biomass co-firing, the air flow around the injectors plays an important part in the fuel combustion and stable boiler operation. Secondary air flow at the point of biomass injection was adjusted during the experiments to provide higher airflows around the biomass injectors, hence the mixing, and air available for biomass combustion, was improved producing less unburnt material in the ash and reducing potential filter blockages. However, higher biomass air flows disrupt the temperature distribution in the boiler causing a wider variation of superheater temperatures compared to the baseline condition firing coal only. This reduces the combustion efficiency and can lead to localised hot spots, in the superheaters, with the potential to cause equipment damage. The trials indicated during the experiments that a secondary biomass air flow of 50% of its maximum, gave an optimum balance between air/biomass mixing and variations in the superheater temperatures.
In the present paper, a computational model is used to simulate the aero-dynamic, thermal, and chemical conditions inside an arch-fired coal boiler. The model is based on the Eulerian-Eulerian concept, in which Eulerian conservation equations are solved both for the gas and the particulate phases. A NOâ formation and destruction submodel is used to calculate the local concentration of NO. The model is used to simulate a range of operating conditions in an actual, 350 MW, arch-fired boiler, with the aim of reducing, using primary measures, the emissions of NOâ. The model results shed some light on the relevant NOâ-formation mechanisms under the several operating conditions. Furthermore, they correlate well quantitatively with the available field measurements at the plant, and reproduce satisfactorily the tendencies observed under the different operating modes.
Industrial experiments were performed for a retrofitted 660-MWe full-scale arch-fired boiler. The ignition position of the primary air/fuel mixture flow, furnace temperature and flue gas component after the air heater were measured with different anthracite ratios at loads of 660- and 380-MWe. As the ratio of anthracite increased, the gas temperature decreased in the fuel-burning zones of the C4 and C6 burners, indicating a farther ignition position of the primary fuel/air flow under the arches, the gas temperature of the lower furnace decreased while that of the upper furnace increased, NOx emissions decreased while both the exhaust gas temperature and carbon content in fly ash increased, and the boiler efficiency decreased. At the rated load, an anthracite ratio of 35% was optimal for economical efficiency. At low load, the negative gas pressure fluctuated remarkably, which led to unstable combustion in the furnace. To ensure safe and stable operation of the boiler, a mass ratio of anthracite of 25% is advisable. Crown Copyright
Measurements have been performed in a 300-MW e , front-wall-fired, pulverized-coal utility boiler. New data are reported for local mean gas species concentrations of O 2 , CO, CO 2 , and NO x , gas temperatures, and charburnout measured at several ports in the boiler including those in the burner region. They complement previously obtained data in the same utility boiler before being retrofitted with low-NO x burners and over-fire air ports. During the current experimental work, a considerable effort was made to assure minimum variations on boiler operating conditions and coal chemical and particle size characteristics so that the data presented are especially useful for three-dimensional mathematical model evaluation and development. The main conclusions are as follows: (1) As compared with our previousmeasurementsin this boiler, prior to the retrofitting, the results show lower local mean O 2 and higher CO concentrations and temperatures in the boiler as a consequence of the lower stoichiometry in the main combustion zone, after retrofitting. (2) At the lower levels of the boiler the measured NO x concentrations are comparable with those obtained prior to the retrofitting but they are inferior at the upper levels of the boiler, which is consistent with the lower NO x emissions observed at the boiler exit, after retrofitting; specifically, 620 compared with 997 mg/Nm 3 prior to the retrofitting. (3) At the lower levels of the boiler, the high local mean char burnout values measured are due to the combined effects of significant presence of O 2 and relatively large residence times of the particles, while at the upper levels of the boiler the particles' residence time is the critical quantity influencing char burnout. (4) Results indicate that carbon isreleased at about the same rate as total coal mass release while both hydrogen and nitrogen are released more rapidly.
Experimental investigations on the flexibility of a 300 MW Arch Firing (AF) coal-fired boiler when burning low quality coals is reported. Measurements of gas temperature and species concentration and char sampling using a water-cooled suction pyrometer were carried out along the furnace elevation. The carbon content and the size distributions of the char samples were obtained. The char morphology was examined using a field emission scanning electron microscope (FESEM). The char sampling was performed on this type of boiler for the first time. The results indicate that the flexibility of this boiler burning low quality coals under a moderate boiler load is better than its flexibility under a high boiler load. Because of the insufficient capacity of the coal pulverizers used, in case of low coal quality the pulverized coal fineness will drastically decrease under high boiler loads. This causes an increase in the loss due to incomplete mechanical and chemical combustion. This is the main cause of a low burnout degree of the pulverized coal and the decrease of the flexibility of this AF boiler under a high boiler load.
In this study, the combustion reactivity of different rank coals, such as peat, lignite, bituminous coal and anthracite samples, was investigated. Non-isothermal thermogravimetry has been used to determine the combustion reactivities of the samples. The differences observed in the burning profile of the samples were studied. The calculated activation energy values of the samples were investigated, regarding carbon content, volatile matter content and burning profile peak temperature. The relationship between ignition temperature and carbon content (dmf) of the coal samples was also studied. The weight loss percentages of the different rank coal samples were discussed, depending on temperature.
For simultaneous measurement of size and velocity distributions of continuous and dispersed phases in a two-phase flow a method is proposed and applied to increase the sensitivity of a phase-Doppler-anemometry (PDA) system. Design considerations are presented to increase the detectable size range of the PDA system to approximately 1:200 (in diameter) for simultaneous detection of tracers and particles. For this the optical properties for light scattering of the particles are properly adjusted to the measurement problem by using homogeneously coloured particles and a special signal processing procedure, which is developed to guarantee reliable signal processing of the tracer signals even with poor signal-to-noise ratios (SNR). The application of the experimental configuration is described by simultaneous measurements of gas and particle velocity and velocity fluctuation profiles in a two-phase jet arrangement with a particle diameter range from 1 (tracers) up to 160 μm (particles). In this two-phase flow at high Stokes numbers (St≫1) different turbulence structure modification effects are identified. The height of influence of these effects depends on the local position in the jet. Near the nozzle exit high gas velocity gradients exist and therefore high turbulence production in the shear layer of the jet is observed. Here the turbulence structure in the jet mainly depends on lateral turbulence transport. Due to a changed turbulence structure with reduced intensity of large eddies, this lateral transport in the two-phase jet is decreased in comparison to the single-phase jet in this area. In the area at greater nozzle distances where the jet is nearly developed, the velocity gradient in the shear layer is lower and due to this lateral turbulence transport effects become less important. Here axial turbulence transport along the jet dominates and turbulence intensity reduction is higher.
This paper describes the results of an experimental study undertaken in an 300 MWe, front-wall-fired, pulverized-coal, utility boiler. The data reported include local mean gas species concentrations of O-2, CO, CO2, NOx, and gas temperatures measured at several ports in the boiler including those in the burner region, and incident wall heat fluxes taken around the boiler periphery at 39 ports. The incident wall heat fluxes are reported for two boiler operating conditions. During the experimental work reported here, a considerable effort was made to assure minimum variations on boiler operating conditions and coal chemical and particle size characteristics so that the data presented are especially useful for 3-D mathematical model evaluation and development. The results reveal that: (i) the boundary air injected below the first row of burners leads to oxidizing conditions close to the back wall; (ii) local gas temperatures and CO concentrations in the boiler, near the burners, reached maximum values of about 1470 degrees C and 1.6%, respectively; (iii) above the boiler nose the measured NOx concentrations are reasonably uniform with averaged values of about 670 ppm; and (iv) wall radiant heat fluxes present maximum values at the side wall close to the intermediate row of burners.
Measurements have been performed in a 300 MWe, front-wall-fired, pulverized-coal, utility boiler. This boiler was retrofitted with boosted over fire air injectors that allowed the operation of the furnace under deeper staging conditions. New data are reported for local mean gas species concentration of O-2, CO, CO2, NOx, gas temperatures and char burnout measured at several ports in the boiler including those in the main combustion and staged air regions. Comparisons of the present data with our previous measurements in this boiler, prior to the retrofitting with the new over fire system, show lower O2 and higher CO concentrations for the new situation as a consequence of the lower stoichiometry in the main combustion zone associated with the present boiler operating condition. Consistently, the measured mean NOx concentrations in the main combustion zone are now lower than those obtained previously, yielding emissions below 500 mg/Nm(3) @ 6% O-2. Finally, the measured values of particle burnout at the furnace exit are acceptable being those measured in the main combustion zone comparable with those obtained with the conventional over fire system.
To overcome the problem of high carbon content in the fly ash of down-fired utility boilers using low-volatility coals, the combustion system of a 660 MWe full-scale down-fired boiler was retrofitted, with the direction of the secondary air under the arch being changed from horizontal to an angle of declination of 20°. Industrial experiments were performed using the boiler before and after the retrofit to determine the reconstruction effect. Data are reported for the gas temperature distribution along the primary air and coal mixture flow, furnace temperature, gas compositions, such as O2, CO, CO2, and NO x , and gas temperatures in the near-wall region. Comparisons between the two cases were made, and the results show that with the angled secondary air under the arches, ignition of the primary air and pulverized coal mixture was brought forward in the boiler. Gas temperatures rose in the fuel-burning zone, and the residence time of pulverized coal in the fuel-burning zone was extended. Thus, the quantity of unburned carbon in fly ash and the gas temperature at the furnace outlet decreased, and the boiler efficiency increased.
Ash deposition is one of the major problems encountered in coal-fired power plants. To understand its formation mechanism, six samples of the deposit were collected from different positions of boiler in the Zhuzhou coal-fired power plant in Hunan of China. The mineralogical, chemical compositions, and microstructure of ash samples were analyzed by X-ray diffraction spectrometry (XRD), X-ray fluorescence spectrometry (XRF), optical microscopy, and field scanning electron microscopy equipped with energy dispersive X-ray spectrometry (FSEM-EDX). The results show that the deposits were mainly composed of silica amorphous phases and their chemical compositions of different layers significantly varied. The identified minerals include mullite, cristobalite, hematite, quartz, hercynite, and anorthite. The silica-rich glass phases are derived from volatilization–recondensation of SiO and the interaction between aluminosilicates and other minerals. Several types of crystals were identified in the deposits, including iron-oxide crystals, Fe–Ca-bearing phase, Si-rich phase, and aluminosilicate phase. The deposition of crystals as well as the following melting is the main reason for the porosity structure of deposit. The interaction and eutectic of minerals in coal led to the serious deposition in the coal-fired power plant.
Cold airflow experiments were conducted within a small-scale furnace of a down-fired pulverized-coal 300 MWe utility boiler. With focus on the large combustion difference between the zones near the front and rear walls in down-fired pulverized-coal boilers, we investigate the aerodynamic field at different staged-air declination angles of 0°, 15°, 30°, 45°, and 55°. For declination angles of 0°, 15°, and 30°, a deflected flow field appeared in the lower furnace, with downward airflow velocities near the rear wall decaying more rapidly than velocities near the front wall. In addition to the downward airflow reach into the lower furnace, the turbulence intensity and longitudinal-velocity components at certain cross sections were lower near the rear wall than near the front wall. Through an increase of the declination angle from 0° to 30°, the flow-field deflection diminished, which was accompanied by a slower decay in the downward airflow near the rear wall and an increase in the reach (as measured by the dimensionless depth) of the downward airflow near the rear wall as well as longitudinal-velocity components within the associated cross section. Those near the front wall changed only slightly. For larger angles of 45° and 55°, the deflected flow field disappeared. Turbulence intensities in the staged-air zones near the front and rear walls increased steadily as the declination angle increased from 0° to 55°. The optimal setting for staged air would necessitate a declination angle of 45°.
In this paper, the influence of air distribution on aerodynamic field and combustion performance in a 0.9 MW arch-fired furnace has been investigated by analyzing the momentum ratio of air flows and the air stoichiometric ratio in the preceding stage combustion zone. It is found that the momentum ratio of air−fuel flows directly affects the arch air penetration length and the position of the flame center, which is critical to the temperature distribution in the furnace and on the furnace walls. The best combustion performance in the experimental range occurs when the momentum ratio of arch air to secondary air equals 1.34 and that of arch air to D&E-layer secondary air equals 4.42. In addition, it is found that the heat loss due to incomplete combustion, also called combustible loss, and NOx emission in the flue gas are related to the air stoichiometric ratio (SR). The minimum values of unburned carbon in fly ash, unburned carbon in the slag and NOx emission at the furnace outlet are attained when SR = 0.67, 0.63, and 0.59, respectively. Furthermore, both combustible loss and NOx emission obtain proper values simultaneously when SR = 0.634, which could be considered as the optimum operation condition.
Experiments with a small-scale furnace for a down-fired pulverized-coal 300-MWe utility boiler were carried out on a single-phase test facility to investigate the influence of different secondary air distributions on the aerodynamic field in the furnace. When the secondary air flux of tier E increased within a suitable range, it did not reverse the fuel-rich flow or shorten the residence time of coal particles in the furnace. Industrial experiments were also performed on a full-scale boiler. The gas temperature distribution along the primary air and coal mixture flow and in the furnace, and gas components such as O2, CO, CO2, and NOx in the near-wall region, were measured with damper openings of the E-tier secondary air box at 0% and 30%. At 0%, ignition of the primary air and pulverized coal mixture was delayed and the gas temperature peak was above the burner arch, with high NOx emission. Increasing the damper opening to 30% provided the oxygen necessary for the initial combustion. This was advantageous for stable combustion and also lowered NOx emissions and carbon content in the fly ash.
This paper presents a systematic study on improving the performance of a 300 MW down-fired pulverized-coal utility boiler by inclining downward the F-layer secondary air (SA). A numerical method was adopted to evaluate the effects of inclined angles on the characteristics of flow, combustion, and nitrogen oxide (NO x) emissions in the furnace. Retrofitting was conducted to incline the F-layer SA downward with an optimal inclined angle of 25°. Full-scale experimental measurements were carried out before and after retrofitting. The results indicate that inclining downward the F-layer SA can increase the flame penetration depth and lower downward the flame center. The residence time of pulverized-coal particles increases, and the SA staging level enhances in the furnace. The boiler performance is improved with absolute increases of 3.55, 3.31, and 2.20% in boiler efficiencies and relative reductions of 28.65, 19.07, and 12.53% in NO x emissions under 300, 240, and 190 MW loads, respectively.
Combustion of anthracites and other low-volatile coals in arch-fired boilers determines, due to its special characteristics, high levels of NO and emissions. Tests performed at Compostilla and La Robla arch-fired units show a very diverse situation with NOx emissions ranging. The coal properties and operation conditions are found to report NOx emission levels. Modifications of combustion features as boiler loads, oxygen excess, burners out of service, tertiary to secondary air ratio, and distribution of burners in service that results in the reductions in NOx emissions of around 20–30%. The trials reported shows that low-NOx operation does not necessarily imply higher fly ash carbon contents.
Numerical studies of the slagging characteristics under different operational conditions in a 300 MW down-fired boiler were carried out using slagging models coupled with gas–solid two phase flow and combustion models. Combined with the real operating conditions; comparative and detailed analysis on the slagging position, extent, and causes is presented. The results show that the serious slagging is mainly on the side walls of the lower furnace. Because of the more rapid expansion of the flue gas under the higher temperature, the flue gas in the furnace center makes the flue gas on both sides deflect and flow to the side walls; and the pulverized-coal flame impinges on the side walls. This results in the slagging on the side walls. Under off-design operating conditions, such as stopping some burners, the local flow field is asymmetric and impinges on the local arch burner, front and rear wall regions where the stopped burners are located. It leads to slight slagging on the arch burner regions and the front and rear wall regions of the lower furnace. Based on the investigation, it has been found that the serious slagging on the side walls can be effectively alleviated by cutting off the burners close to the side walls, reducing boiler load and burning low slagging-tendency coals.
Industrial experiments were performed for a retrofitted 660 MWe full-scale down-fired boiler. Measurements of ignition of the primary air/fuel mixture flow, the gas temperature distribution of the furnace and the gas components in the furnace were conducted at loads of 660, 550 and 330 MWe. With decreasing load, the gas temperature decreases and the ignition position of the primary coal/air flow becomes farther along the axis of the fuel-rich pipe in the burner region under the arches. The furnace temperature also decreases with decreasing load, as does the difference between the temperatures in the burning region and the lower position of the burnout region. With decreasing load, the exhaust gas temperature decreases from 129.8 °C to 114.3 °C, while NOx emissions decrease from 2448 to 1610 mg/m3. All three loads result in low carbon content in fly ash and great boiler thermal efficiency higher than 92%. Compared with the case of 660 MWe before retrofit, the exhaust gas temperature decreased from 136 to 129.8 °C, the carbon content in the fly ash decreased from 9.55% to 2.43% and the boiler efficiency increased from 84.54% to 93.66%.
In this paper, the flow field characteristics of over fired air (OFA) for novel low NOx pulverized coal combustion technology are studied. The research was conducted with a 300 MWe tangential firing boiler that was adapted for this technology, and a three-dimensional particle-dynamics anemometer (PDA) was employed on the model to measure the characteristics of gas flow in the burnout area and gas/particle flows under the front panel superheater. The impact of a positive offset at 15°, counter offset at 15° and design case without an offset the OFA relative to the direction of the secondary air jet in the main combustion were considered. With different OFA offsets, the deflection characteristics, the velocity and root mean square (RMS) fluctuation velocity of OFA jet are obtained, as well as the gas/particle flows characteristics under the front panel superheater. The results show that, with a positive offset, an over-large tangential circle is formed, which produces slagging and temp-bias under the panel superheater. However, with a counter offset, the OFA is sent into the center of the chamber, and the particle is forced to the water wall. Compared with the other two conditions and combined with the counterflow of primary air, OFA without an offset for the jet contains a proper tangential circle, strong inflexibility and turbulence, which prevents slagging and burn out.
The present paper presents a three-dimensional numerical investigation of a pulverized-fuel, tangentially-fired utility boiler located at Florina/Greece under air, partial and full oxy-fuel conditions. Heat and mass transfer and major species concentration, such as CO2, CO and O2 are calculated; whilst the results for the reference air case scenario studied are in good agreement with the corresponding operational data measured in the plant, both for combustion calculations and NOx emissions. Results for the partial and full oxy-fuel operation scenarios are in line with similar experimental and numerical investigations found in the recent literature. This numerical investigation of oxy-fuel conditions scenarios prior to their implementation under real scale conditions demonstrates the utmost of its importance, since significant results regarding the operation of a boiler in terms of lignite particle trajectories and burning rates are attained. Furthermore, NOx calculations have been performed for all the examined case studies.
The formation of nitrogen oxides (NOX) in combustion systems is a significant pollutant source in the environment, and the control of NOX emissions is a world-wide concern as the utilization of fossil fuels continues to increase. In addition, the use of alternative fuels, which are typically of lower quality, tends to worsen the problem. Advances in the science of NOX reactions, mathematical modeling, and increased performance of computer systems have made comprehensive modeling of NOX formation and destruction a valuable tool to provide insights and understanding of the NOX reaction processes in combustion systems. This technology has the potential to enhance the application of various combustion techniques used to reduce NOX emissions from practical combustion systems. This paper presents a review of modeling of NOX reactions in combustion systems, with an emphasis on coal-fired systems, including current NOX control technologies, NOX reaction processes, and techniques to calculate chemical kinetics in turbulent flames. Models of NOX formation in combustion systems are reviewed. Comparisons of measured and predicted values of NOX concentrations are shown for several full-scale and laboratory-scale systems. Applications of NOX models for developing technologies, in order to reduce NOX emissions from combustion systems are also reported, including the use of over-fire air, swirling combustion air streams, fuel type, burner tilt angle, use of reburning fuels, and other methods.
Within a Mitsui Babcock Energy Limited down-fired pulverized-coal 350 MW(e) utility boiler, in situ experiments were performed, with measurements taken of gas temperatures in the burner and near the right-wall regions, and of gas concentrations (O(2) and NO) from the near-wall region. Large combustion differences between zones near the front and rear walls and particularly high NO(x) emissions were found in the boiler. With focus on minimizing these problems, a new technology based on multiple-injection and multiple-staging has been developed. Combustion improvements and NO(x) reductions were validated by investigating three aspects. First, numerical simulations of the pulverized-coal combustion process and NO(x) emissions were compared in both the original and new technologies. Good agreement was found between simulations and in situ measurements with the original technology. Second, with the new technology, gas temperature and concentration distributions were found to be symmetric near the front and rear walls. A relatively low-temperature and high-oxygen-concentration zone formed in the near-wall region that helps mitigate slagging in the lower furnace. Third, NO(x) emissions were found to have decreased by as much as 50%, yielding a slight decrease in the levels of unburnt carbon in the fly ash.
A new technique combining high boiler efficiency and low-NO(x) emissions was employed in a 300MWe down-fired boiler as an economical means to reduce NO(x) emissions in down-fired boilers burning low-volatile coals. Experiments were conducted on this boiler after the retrofit with measurements taken of gas temperature distributions along the primary air and coal mixture flows and in the furnace, furnace temperatures along the main axis and gas concentrations such as O(2), CO and NO(x) in the near-wall region. Data were compared with those obtained before the retrofit and verified that by applying the combined technique, gas temperature distributions in the furnace become more reasonable. Peak temperatures were lowered from the upper furnace to the lower furnace and flame stability was improved. Despite burning low-volatile coals, NO(x) emissions can be lowered by as much as 50% without increasing the levels of unburnt carbon in fly ash and reducing boiler thermal efficiency.
Cold airflow experiments were conducted to investigate the aerodynamic field in a small-scale furnace of a down-fired pulverized-coal 300 MW(e) utility boiler arranged with direct flow split burners enriched by cyclones. By increasing the staged-air ratio, a deflected flow field appeared in the lower furnace; larger staged-air ratios produced larger deflections. Industrial-sized experiments on a full-scale boiler were also performed at different staged-air damper openings with measurements taken of gas temperatures in the burner region and near the right-side wall, wall heat fluxes, and gas components (O(2), CO, and NO(x)) in the near-wall region. Combustion was unstable at staged-air damper openings below 30%. For openings of 30% and 40%, late ignition of the pulverized coal developed and large differences arose in gas temperatures and heat fluxes between the regions near the front and rear walls. In conjunction, carbon content in the fly ash was high and boiler efficiency was low with high NO(x) emission above 1200 mg/m(3) (at 6% O(2) dry). For fully open dampers, differences in gas temperatures and heat fluxes, carbon in fly ash and NO(x) emission decreased yielding an increase in boiler efficiency. The optimal setting is fully open staged-air dampers.