煤粉再燃过程对煤焦异相还原NO的影响

利用高温携带流反应装置,研究了煤种(包括褐煤、烟煤和贫煤)、再燃区内反应温度、煤粉粒径、一次燃烧区空气过量系数SR1和再燃区空气过量系数SR2对煤焦异相还原NO作用的影响,探讨了煤焦异相还原NO的机理.结果表明:随着SR2和煤粉粒径的减小以及再燃区反应温度的提高,煤粉NO还原效率增加;在相同的SR2下,随着煤中挥发分含量的提高,煤粉粒径的增加和再燃区反应温度的降低,煤焦异相还原NO贡献上升;对于相同再燃燃料份额:SR1=1.0和SR1=1.2时煤焦异相还原NO的贡献均大于SR1=1.1时的异相还原NO的贡献.     

温度对超细煤焦再燃还原NO效率的影响

以超细煤粉制作的煤焦作为再燃燃料,用N2、O2、CO2、NO配制模拟烟气,在立式管式携带炉中,研究了温度对再燃降低NO效率的影响。结果表明,在实验温度范围内,随着再燃区温度的增加,再燃还原NO效率增大,化学动力学是控制超细煤粉再燃还原NO化学反应速率的重要因素;提高再燃区温度可以适当缩短停留时间,但不能低于0.6 s,否则NO还原效率会大幅度下降,同时燃尽率也会下降;在煤粉再燃过程中,煤焦再燃还原NO占有重要地位。     

煤再燃过程中燃料特性对NO还原的影响

在小型流动反应器中研究了不同煤粉及其煤焦对富燃料再燃区NO异相还原的影响。实验中选用的再燃燃料为小龙潭褐煤、富拉尔基褐煤和大同烟煤及其煤焦,以及添加不同催化剂的大同烟煤焦。实验在初始NO浓度为1000×10     

Modelling NOx emissions during staged combustion

Staged combustion has been accepted as an effective way to reduce NO_x emission. Based on the comparison of calculated results using Miller and Bowman's (1989, Progr. Energy Combust. Sci. 15 , 287) detailed elementary reaction model with experimental data, it is found effective to apply this model in the simulation of NO formation and destruction during staged combustion. Sensitivity analysis shows that C, CH, CH₂ and HCCO play an important role in NO destruction and reduction under fuel staging. NO generated in the primary zone can be reduced greatly by staged combustion. Besides the air–fuel ratio in the primary combustion zone, the combustion temperature in the reburning zone and the mass factor of the reburning fuel in the overall fuel, the main factors which affect NO destruction and reduction are the position where reburning is introduced and the types of reburning fuel. It is found that reburning cannot be introduced too close to the primary combustion zone. The reburning fuels that can effectively stimulate NO to HCN are H₂ and C₂H₄. Copyright © 1999 John Wiley & Sons, Ltd.     

不同煤种再燃过程中脱硝煤耗的试验研究

定义还原1 gNO 消耗的煤量为脱硝煤耗.在煤粉携带炉上进行了再燃试验,对不同煤种、不同工况下的脱硝煤耗进行了研究,分析了挥发分含量、再燃区温度、氧浓度、再燃燃料比等因素对脱硝煤耗的影响.结果表明:脱硝煤耗不仅能直观反映出不同煤种在还原 NO 方面的特性差异,而且还能有效反映再燃过程投入与收益之比;脱硝煤耗随着挥发分含量增加呈线性降低;再燃区氧浓度越低,脱硝煤耗也就越低;在49/6和6%氧浓度条件下,提高再燃燃料比,脱硝煤耗显著下降;在2%氧浓度条件下,提高再燃燃料比,脱硝煤耗增加;再燃区温度升高时,脱硝煤耗下降,并且挥发分越高的煤,脱硝煤耗随温度的变化越显著.     

煤粉再燃过程中NO异相还原机理的重要性

用两种褐煤的一种烟煤及其煤焦作为再燃燃料，实验研究了再燃环境下NO的还原。通过比较煤粉和煤焦对NO还原过程的不同，分析了异相机理对NO还原的重要性。结果表明，褐煤及其煤焦是有效的再燃燃料。褐煤作为再燃燃料时，煤焦的异相还原机理对NO还原起重要作用。煤中的金属氧化物对NO还原具有催化作用。     

Mechanism study of nitric oxide reduction by light gases from typical Chinese coals

Coal devolatilization plays an important role in NO formation and reduction. In this study, the coal pyrolysis experiment was performed in an entrained flow reactor to obtain the light gas release characteristics. Six typical Chinese coals with volatile content ranged from 8.8% to 38.3% were studied. The pyrolysis temperature was in the range from 600 to 1200 °C. A significant rank dependence of HCN, CO and C₂H₂/C₂H₄/C₂H₆ was observed and their release for high volatile coals was higher than that for low volatile coals. The HCN–N/NH₃–N ratio ranged from 0.00 to 0.66 for anthracite coals and ranged from 1.63 to 3.90 for high volatile coals. Based on the experimental results, the effect of coal pyrolysis gas on NO reduction in a plug flow reactor at reducing atmosphere was kinetically calculated. The optimal excess air ratio(α_opt) corresponding to the maximum NO removal efficiency decreased with an increase in reduction temperature. For the light gas from the HL coal pyrolyzed at 800 °C, the α_opt decreased from 0.73 to 0.17 when the reduction temperature increased from 927 to 1327 °C. The rate of production analysis indicated that NO removal efficiency was determined by 3 competing reaction paths: NO reduction, NO formation and oxygen consumption by combustible species.     

再燃过程中HCN对NOx还原的重要性

在降低ＮＯｘ排放的一系列方法中,燃料再燃是重要措施之一。通过对再燃区不同的空气过量系数和再燃温度条件下的数值计算,研究了天然气（ＣＨ４）作为再燃燃料时ＨＣＮ对ＮＯ再燃过程和再燃率的影响。再燃区模拟烟气成分为：ＣＯ２＝１６．８％,Ｏ２＝２％,ＮＯ＝０．１％和平衡气体Ｎ２。研究发现,再燃燃料中含氮组分的存在,以及再燃区的工况条件都对ＮＯｘ的再燃率有很大的影响。因此,在实施降低ＮＯｘ排放的再燃技术过程中     

NO and SO2 removal and pore structure evolution during reburning with calcium magnesium acetate blended peanut shell

Calcium magnesium acetate (CMA) was an effective material for the NO reduction and SO₂ removal simultaneously by using reburning technology. To further improve its NO reduction rate, the peanut shell blended with CMA was used as the reburning fuels. Effects of the Ca/S molar ratio and temperature on the NO reduction and SO₂ removal were investigated. Pore characteristics of solid samples at different temperatures were also studied. Results show that peanut shell addition improves the NO reduction and SO₂ removal during the CMA reburning process. A Ca/S molar ratio ranging from 2 to 3 is a reasonable range for this technology. At lower temperatures, NO reduction and SO₂ removal ratios are much smaller because of the poorly developed pore structure and tar blocking. With increases in temperature, both ratios increase significantly due to the well-developed pore structure caused by volatile matter release and CMA calcination. At a temperature of 900 °C, the SO₂ removal ratio increases slowly because of the partial surface sintering of CaO and because some pores are blocked by the pollutants.     

Nitric oxide destruction during coal and char oxidation under pulverized-coal combustion conditions

A modified drop-tube reactor that allows particle distribution over the reactor cross-sectional area, and oxidation of chars produced in situ, was used to study the conversion efficiency of char nitrogen to nitric oxide (α_NO). The results confirm previous findings by other investigators that α_NO decreases as the weight of char burned increases. α_NO for coal was the same as (at 4% O₂) or lower than (at 20% O₂) that for an equal mass of char during oxidation. Since coal will yield approximately half its mass as fixed carbon, these results suggest that the local stoichiometry surrounding the particle is responsible for the observed reduction in α_NO as sample size increases. The analysis of the exhaust gases showed increases in HCN concentration and a decrease in CO₂/CO ratio as sample size increased, suggesting that local stoichiometry influences α_NO. Additional experiments showed that α_NO decreased as the background NO concentration was increased, at rates that diminished as the oxygen concentration increased, independent of particle size. The steep reduction in NO production as the background NO concentration increased was explained by the destruction of NO in the gas phase.     

Experimental study on nitric oxide reduction through calcium propionate reburning

Performances of calcium propionate (CP) on nitric oxide (NO) reduction are experimentally investigated on a drop tube furnace system from basic reburning (BR), Thermal De-NO_x and advanced reburning (AR) and it is demonstrated to be feasible of using CP as reburning fuel. BR could supply about 80% efficiency with reburning fuel fraction (Rff) and residence time (τ) kept 20-25% and 0.7 s, respectively. Also, oxygen concentration is required to be less than 4%. However, initial NO concentration is not important to reduction. Characteristics of Thermal De-NO_x are also studied. The maximum efficiency of 85.34% could be achieved at 1273 K with mole ratio of ammonia to nitric oxide (β) equaling to 1.75. The corresponding “temperature window” is 1215-1341 K. From 2% to 6% of oxygen concentration, the efficiency of Thermal De-NO_x is constantly depressed by 16.17%. The performances of advanced reburning are greatly optimized and higher efficiency could be achieved using less calcium propionate and ammonia. At 1273 K, efficiency of 93.37% is supplied by AR with Rff = 19.83% and β = 0.8. Also, the corresponding “temperature window” is broadened to 1195-1355 K which is 1.27 times of the one in Thermal De-NO_x at β = 1.75. Meanwhile, the impact of oxygen concentration on NO reduction is weakened in AR.     

再燃与烟气再循环技术协同作用下NOX降解机理

常规的解决NOX 降解的办法 ,有其实现的局限性。发挥再燃与烟气循环的协同作用 ,可在克服这些局限性的同时 ,增强其可实现性 ;烟气再循环的应用 ,在降解NOX 的同时 ,由于其改变了炉膛的局部环境 ,强化了再燃技术降解NOX 的作用。再燃技术和烟气再循环技术的协同 ,形成炉内NOX 的二级降解 ;再燃与烟气循环协同系统的关键 ,在于再燃燃料的种类、喷入量与烟气再循环率大小的确立。     

Investigation of fuel lean reburning process in a 1.5 MW boiler

This paper examines a detailed study of fuel lean reburning process applied to a 1.5 MW gas-fired boiler. Experimental and numerical studies were carried out to investigate the effect of the fuel lean reburning process on the NO_X reduction and CO emission. Natural gas (CH₄) was used as the reburn as well as the main fuel. The amount of the reburn fuel, injection location and thermal load of boiler were considered as experimental parameters. The flue gas data revealed that the fuel lean reburning process led to NO_X reduction up to 43%, while CO emission was limited to less than 30 ppm for the 100% thermal load condition. The commercial computational fluid dynamics code FLUENT 6.3, which included turbulence, chemical reaction, radiation and NO modeling, was used to predict the fluid flow and heat transfer characteristics under various operational conditions in the boiler. Subsequently, predicted results were validated with available measured data such as gas temperature distributions and local mean NO_X concentrations. The detailed numerical results showed that the recirculation flow developed inside the boiler was found to play an important role in improving the effectiveness of fuel lean reburning process.     

Detailed modeling of hybrid reburn/SNCR processes for NOX reduction in coal-fired furnaces

Mechanism reduction has made the detailed kinetic modeling of combustion problems much easier; it also offers potential improvement of modeling accuracy and flexibility in comparison to global mechanisms. The present work applies mechanism reduction in conjunction with the CHEMKIN library and develops an automatic reduction program code. Regarding the hybrid re-burn/selective non-catalytic reduction (SNCR) (“advanced re-burning”) conditions in coal-fired furnaces and based on a full mechanism “GADM98,” a skeletal mechanism with 39 species, 105 reactions, and further a 10-step/14-species reduced mechanism were established. The reduced mechanism was implemented into a 3D-combustion computational fluid dynamics (CFD) code. The eddy-dissipation-concept model was used to describe the influence of turbulence on the combustion chemistry. A large number of simulations for reburning and hybrid reburn/SNCR processes in a coal-fired reactor were executed; the predicted results were compared with experimental measurements. The reduced mechanism and the comprehensive modeling give quite satisfactory results over a wide range of mole ratios for β = [NH₃]/[NO] and air/fuel equivalence ratios λ₂ in the reburn zone. From the modeling results, it was found that adding ammonia premixed with reburn fuel (CH₄) effects no further reduction of NO_x or even impairs the reduction efficiency compared to pure reburning, and in contrast, staged addition of ammonia downstream of the CH₄ injection in the reburn zone provokes a significant further reduction of NO_x over a wide range of parameters. According to the predictions, NO_x-reduction rates of 50-60% and of 70-80% can be achieved through pure reburning and hybrid reburn/SNCR approaches, respectively, at λ₂ = 0.95 and β = 1.5. Concerning the computational procedure, essential measures were taken to optimize convergence and computing time. The computing time with the present reduced mechanism is ∼2.5 times that with the traditional global mechanism for the same iteration number. Tabulation of the rate constants reduced the computing time of the reaction kinetics by ∼50%.     

Emission characteristics of biomass combustion under oxyfuel conditions

The purpose of this paper was to study the emission of three important pollutants (NO, SO₂, and H₂S) during the combustion process. The process was carried out in a counterflow fixed bed system, which consisted of a feeding system, a filtering system, a thermocouple, a fixed counter flow bed and an air compressor. Results were showed that the increased oxygen ratio leads to NO and H₂S decrease and an increase in reaction heat and CO₂ declines. As a result, the particle size is not much effective in CE because particle size change does not change gas composition. Particle size is effective in combustor hydrodynamics because the change in particle size is responsible for the change in combustor size and length.     

NO emission from non-premixed MILD combustion of biogas-syngas mixtures in opposed jet configuration

In this paper NO emission from MILD combustion of the mixture biogas-syngas is deeply elucidated, five NO routes were considered, specifically: thermal, prompt, NNH, N₂O and reburning. Several operating conditions are studied namely: fuel mixture composition, oxygen concentration in the oxidizer and injection velocity or strain rate. Biogas is modeled by a mixture of methane and carbon dioxide; while, syngas is considered to be composed by hydrogen and carbon monoxide, this gives a fuel mixture of CH₄/CO₂/H₂/CO. Volume of methane and hydrogen are varied alternatively from 0 to 50% in fuel mixture. Oxidizer is composed by O₂/N₂ mixture where oxygen volume is increased from 4 to 21%. Finally, injection strain rate is varied from apparition to vanishment of combustion. Atmospheric pressure is considered with constant fuel and oxidizer injection temperatures of 300 K and 1200 K respectively. Chemical kinetics of such complicated system is handled by a composed mechanism from the USC C₁–C₄ and the Gri 2.11 N-sub mechanism. It is found that under MILD regime, temperature intervals and levels are enhanced by hydrogen compared to methane. Furthermore, temperature levels keep relatively low which guarantees MILD regime. Contrariwise, when oxygen increases in oxidizer, temperature grows up rapidly and the MILD regime disappears. However, if strain rate augments, temperature shows a steep increase then reduces monotonically. It is observed that for low methane volume in the fuel mixture, NNH route dominates NO production. Whereas, when CH₄ increases, the prompt route is enhanced and exceeds NNH one at a methane volume of 12%. When hydrogen increases, prompt and NNH routes are enhanced with a domination of the prompt route until 44% of hydrogen volume. Oxygen increasing in the oxidizer improves thermal mechanism which surpasses prompt one at 17% of oxygen volume and governs NO production. Globally, the third most important route in NO production is the reburning one which is enhanced by all parameters except strain rate.     

Staged combustion properties for pulverized coals at high temperature

Staged combustion properties for pulverized coals have been investigated by using a new-concept drop-tube furnace. Two high-temperature electric furnaces were connected in series. Coal was burnt under fuel-rich conditions in the first furnace, then, staged air was supplied at the connection between the two furnaces. Reaction temperature (1800–2100 K) and time (1–2 s) were similar to those used in actual boilers. When coal was burnt at the same stoichiometric ratio as in actual boilers, similar combustion performance values as for actual boilers were obtained regarding NO_x emission and carbon in ash. The most important factor for low NO_x combustion was to raise the combustion temperature above the present range (1800–2100 K) in the fuel-rich zone. The NO_x emission was significantly increased with decrease of burning temperature in the fuel-rich zone when the temperature was lower than 1800 K. But, NO_x emission was cut to around 100–150 ppm, for sub-bituminous coal and hv-bituminous coal, in the latest commercial plants by forming this high-temperature fuel-rich region in the boilers. If the temperature and stoichiometric ratio could be set to the most suitable conditions, and, burning gas and air were mixed well, it would be possible to lower NO_x emission to 30–60 ppm (6% O₂). The most important NO_x reduction reaction in the fuel-rich zone was the NO_x reduction by hydrocarbons. The hydrocarbon formation rate in the flame was varied with coal properties and combustion conditions. The NO_x was easily reduced when coals which easily formed hydrocarbons were used, or, when burning conditions which easily formed hydrocarbons were chosen. Effects of burning temperature and stoichiometric ratio on NO_x emission were reproduced by the previously proposed reaction model. When solid fuel was used, plant performance values varied with fuel properties. The proposed drop-tube furnace system was also found to be a useful analysis technique to evaluate the difference in combustion performance due to the fuel properties.     

The economics of reburning with cattle manure-based biomass in existing coal-fired power plants for NOx and CO2 emissions control

Coal plants that reburn with catttle biomass (CB) can reduce CO₂ emissions and save on coal purchasing costs while reducing NO_x emissions by 60–90% beyond levels achieved by primary NO_x controllers. Reductions from reburning coal with CB are comparable to those obtained by other secondary NO_x technologies such as selective catalytic reduction (SCR). The objective of this study is to model potential emission and economic savings from reburning coal with CB and compare those savings against competing technologies. A spreadsheet computer program was developed to model capital, operation, and maintenance costs for CB reburning, SCR, and selective non-catalytic reduction (SNCR). A base case run of the economics model, showed that a CB reburn system retrofitted on an existing 500 MW_e coal plant would have a net present worth of −$80.8 million. Comparatively, an SCR system under the same base case input parameters would have a net present worth of +$3.87 million. The greatest increase in overall cost for CB reburning was found to come from biomass drying and processing operations. The profitability of a CB reburning system retrofit on an existing coal-fired plant improved with higher coal prices and higher valued NO_x emission credits. Future CO₂ taxes of $25 tonne⁻¹ could make CB reburning as economically feasible as SCR. Biomass transport distances and the unavailability of suitable, low-ash CB may require future research to concentrate on smaller capacity coal-fired units between 50 and 300 MW_e.     

大型褐煤锅炉煤粉再燃技术的数值模拟

考虑煤焦还原NO的反应动力学模型,利用Fluent软件对元宝山电厂3号锅炉超细煤粉再燃的不同配风方式进行了炉膛整体的燃烧数值模拟。模拟结果表明,再燃燃料比例、再燃风中的风煤比、再燃区的大小等因素对燃烧效率和NOx排放具有重要影响。通过优化计算得到,当主燃区空气过量系数控制在1．1时,再燃燃料占总燃料的15％,再燃风中的风煤比为2,烟气在再燃区的停留时间为0．5S左右的方案是一种较好的再燃组织方式。     

超细煤粉再燃技术的探讨

煤粉再燃是一种投资不高的脱硝技术 ,其脱硝率可达 5 0 %～ 70 %。着重阐述了采用超细煤粉作为二次燃料的再燃技术的原理 ,以及超细煤粉再燃技术的发展状况 ,并分析了在我国采用超细煤粉再燃技术的可行性和发展前景。