Kinetic model for natural gas reburning

Kinetics of natural gas reburning is modeled on the basis of GRI3.0 and the method of Gear. The calculation results indicate that the model agrees with experiments. HCN, NH_i and HCCO play an important role in the reburning system. Injecting NH₃ and increasing the fraction of HCCO in the reburning system will promote NO reduction. Injecting natural gas at the places wherein the fraction of NO is highest is a benefit to de-NOx. There exists an optimal value of ratio of natural gas: NO for reburning system. The influences of temperature, excess oxygen ratio and pressure are also discussed in the reburning system.     

Reburning and burnout simulations of natural gas for heavy oil combustion

Reburning and burnout simulations were carried out through PLUG code of CHEMKIN-III using a reduced mechanism, in order to determine preliminary experimental parameters for achieving maximum NO_x reduction to implement the reburning technology for heavy oil combustion in pilot scale equipments in Brazil. Gas compositions at the entrance of the reburning zone were estimated by the AComb program. Simulations were performed for eight conditions in the usual range of operational parameters for natural gas reburning. The maximum NO reduction (ca. 50%) was reached with 10 and 17.5% of power via natural gas and 1.5 and 3.0% O₂ excess, respectively, at 1273 K. The model predicts 250 ppm of NO, 50 ppm of CO and air mass flows in the range of about 50-130 kg/h for burnout.     

An experimental parametric study of gas reburning under conditions of interest for oxy-fuel combustion

An experimental parametric study on the NO reduction efficiency by reburning under oxy-fuel conditions has been performed through the gas-phase interactions between different gas mixtures and NO in a CO₂ atmosphere, under flow reactor conditions in the 800-1800 K temperature range. The study provides a wide amount of experimental data on reburning under oxy-fuel conditions to be further used in modeling studies. A higher NO reduction is attained in a N₂ atmosphere compared to CO₂ under fuel-rich and stoichiometric conditions, although the efficiency is similar in both atmospheres under fuel-lean conditions. The formation of HCN in a fuel-rich environment is higher in N₂ than in CO₂ but comparable for other stoichiometries. The influence of the main parameters of the process under oxy-fuel conditions has been found to present similar trends to those observed in literature for reburning in air. A significant NO reduction can be obtained at moderately high temperatures, fuel-rich conditions, high values of the reburn fuel/NO ratio, sufficiently high residence times and low water vapor contents. C₂H₆ has been detected to act as a better reburn fuel as CH₄, whereas CO itself is unable to reduce NO.     

Detailed measurements in a laboratory furnace with reburning

This article presents a detailed experimental characterization of the reburning process in a large-scale laboratory furnace. Natural gas, pine sawdust and pulverized coal were used as reburn fuels. Initially, the study involved the collection of in-flame combustion data, without reburning, in order to define appropriate locations for the injection of the reburn fuels. Next, flue-gas data were obtained for a wide range of experimental conditions using the three reburn fuels and, subsequently, detailed measurements of local mean O₂, CO, CO₂, HC and NO_x concentrations, and gas temperatures have been obtained in the reburn zone for three representative furnace operating conditions, one for each reburn fuel studied. The flue-gas data revealed that the sawdust reburning leads to NO_x reductions comparable or even higher than those attained with natural gas reburning, while coal reburning yields much lower NO_x reductions. The detailed data obtained in the reburn zone indicates that the reburning process remains active throughout all the reburn zone in the cases of natural gas and sawdust reburning, while in the case of coal reburning its relatively low volatile matter content is insufficient to establish an effective reburn zone. In the cases of the sawdust and coal reburning the burnout levels remain approximately constant, regardless of the NO_x emissions reduction, with the sawdust reburning leading to higher particle burnout performance than the coal reburning.     

Optimisation of NOx reduction in advanced coal reburning systems and the effect of coal type

The advanced reburning process for NO_x emission control was studied in a down-fired 20 kW combustor by evaluating the performance of 15 pulverised coals as reburning fuels. The proximate volatile matter contents of the coals selected ranged from around 4 to 40 wt% (as received) with elemental nitrogen contents from around 0.6 to 2.0 wt%. The effects of reburn fuel fraction, reburning zone residence time, ammonia agent injection delay time (relative to the reburn fuel and burnout air injection points) and the nitrogen stoichiometric ratio are reported in detail and the optimum configurations for advanced reburning, established as a function of operating condition and coal type. The experimental results show that advanced reburning can reduce NO_x emissions up to 85%. The maximum benefits of advanced reburning over conventional reburning were observed at the lower reburn fuel fractions (around 10%). The results demonstrate that under advanced reburning conditions equivalent or higher levels of NO_x reduction can be achieved while operating the reburn zone closer to stoichiometric conditions compared with conventional reburning operating at high reburn fuel fractions (20-25%). Thus the practical problems associated with fuel-rich staged operation can be reduced. The effect of coal properties on the advanced reburning performance was also investigated. As with conventional reburning, the fuel nitrogen content of the coal used was found to have little influence on the NO_x reduction efficiency except at the highest reburn fuel fractions. There was, however, a strong correlation between the effectiveness of advanced reburning and the volatile content of the reburning fuels, which not only depended on the reburn fuel fraction, but also the mode (rich or lean) of advanced reburning operation. These parameters are mapped out experimentally to enable the best operating mode to be selected for advanced reburning as a function of the reburning fuel fraction and volatile content.     

废轮胎的燃料特性及用于燃煤锅炉再燃脱硝

再燃脱硝技术是燃烧过程中控制燃煤锅炉氮氧化物排放的有效方法。首先分析讨论了常用再燃脱硝的燃料,即天然气和煤粉的局限性,然后详细地讨论了废轮胎作为再燃燃料的可行性和优点,并介绍了作者有关废轮胎再燃脱硝的研究结果。当废轮胎与电厂褐煤灰组成混合燃料用于再燃脱硝时,其脱硝效率可达到86%,当废轮胎与Fe₂O₃组成混合燃料用于再燃脱硝时,其脱硝效率可达到88%。废轮胎用于再燃脱硝,一方面有利于它的能源资源化处理,另一方面能够非常有效地减少燃煤NO_x的排放,具有重要的工程应用价值。     

Formation and reduction of nitric oxide in fixed-bed combustion of straw

Mechanisms of nitric oxide (NO) formation and reduction in fixed-bed combustion of straw have been modeled mathematically and verified experimentally. The model for the straw combustion and nitrogen chemistry consists of sub-models for evaporation, pyrolysis, tar and char combustion, nitrogen conversion, and energy and mass conservation. Twenty chemical reactions are included, of which 12 belong to the fuel nitrogen reaction network. Volatile nitrogen is assumed to be NO, NH₃, HCN and HNCO, and char nitrogen is converted to NO during char oxidation. The model predictions are in qualitative agreement with the measurements during the ignition phase, i.e. when the combustion front passes through the un-burnt fuel. The yield of NO can be reduced considerably by using a low primary air flow due to the longer gas residence in the fixed-bed, while the NO exhaust concentration is insensitive to the bed temperature. The NO exhaust concentration initially reaches a maximum and then decreases towards a stable value after the straw bed is ignited. Variations of NO, NH₃, HCN, and HNCO concentrations in the ignition flame front indicate that a large quantity of NO can be reduced in the thin flame front zone. The developed model is further validated by separate experiments in which NO or NH₃ was added at the middle through tubes or at the bottom of the bed with the primary air flow. Both the simulations and measurements showed that the variation of the NO exhaust concentration is small as compared with the injected NO or NH₃ concentration. According to the simulations and experiments, it is proposed that flue gas recirculation may be a very effective method of reducing NO emissions from flue gas in the fixed-bed combustion of straw. Calculations indicated that about 20% of the flue gas may be recirculated without significantly affecting the combustion behavior.     

Effect of Mixing on Natural Gas Reburning

The purpose of this work is to understand the influence of the mixing process on reburning performance. Modeling is done in a one-dimensional approximation by utilizing a detailed chemical mechanism, staged addition of reactants, and mixture stratification. Natural gas is used as both the main fuel and the reburning fuel. The most important parameters that affect the efficiency of the reburning process are: the amount of the reburning fuel, the initial NO level, the initial temperatures of the injected reburning fuel and overfire air (OFA), temperature of flue gas at the points where reburning fuel and OFA are injected, and intensity of mixing of the reburning fuel and OFA with the stream of flue gases. It is shown that fuel stratification in the mixing zone improves reburning efficiency for small heat inputs of the reburning fuel and degrades reburning efficiency for large heat inputs. Initial temperatures of the reburning fuel and OFA affect NO reduction and can be optimized for deeper NO control. Reactions of N-containing species in the burnout zone play an important role in NO reduction for large heat inputs of the reburning fuel.     

Experimental and kinetic modeling study of the reduction of NO by hydrocarbons and interactions with SO2 in a JSR at 1 atm

The reduction of nitric oxide (NO) by a mixture of methane, ethylene and acetylene with and without addition of SO₂ has been studied in a fused silica jet-stirred reactor operating at 1 atm in simulated conditions of the reburning zone. The temperatures were ranging from 800 to 1400 K. In these experiments, the initial mole fractions of NO and SO₂ were 0 or 1000 ppm, that of methane, ethylene and acetylene were, respectively, 2400, 1200 and 600 ppm. The equivalence ratio has been varied from 0.5 to 2.5. It was demonstrated that the reduction of NO varies as the temperature and that for a given temperature, a maximum NO reduction occurs slightly above stoichiometric conditions. The addition of SO₂ inhibited the process of reduction of NO under the present conditions. The present results generally follow those obtained in previous studies involving simple hydrocarbons or natural gas as reburn fuel. A detailed chemical kinetic modeling of the present experiments was performed using an updated and improved kinetic scheme (1006 reversible reactions and 145 species). An overall reasonable agreement between the present data and the modeling was obtained. Also, the proposed kinetic mechanism can be successfully used to model the reduction of NO by ethane, ethylene, a natural gas blend (methane-ethane 10:1). The kinetic modeling indicates that the reduction of NO proceeds via the following sequence of reactions: HCCO+NO=HCNO+CO; HCCO+NO=HCN+CO₂; HCN+O=NCO+H; HCN+O=NH+CO; HCN+H=CN+H₂; HCNO+H=HCN+OH; CN+O₂=NCO+O; NCO+H=NH+CO; NCO+NO=N₂O+CO; NCO+NO=CO₂+N₂; NH+NO=N₂O+H; NH+NO=N₂+OH. The inhibition of this process by SO₂ is explained by the sequence of reactions H+SO₂+M=HOSO+M and HOSO+H=SO₂+H₂ that acts as a termination process: H+H+M=H₂+M.     

Releases of NO and its precursors from coal combustion in a fixed bed

Experiments have been carried out to investigate the emissions of nitrogen species including NO and its precursors during temperature-programmed coal combustion by TG/EGA method. Experimental results show that the conversion ratio of fuel nitrogen to NO is the highest, followed by that of fuel nitrogen to HCN and the conversion ratio to NH₃ is negligibly small. Nitrogen is retained in the char and released mainly as NO at the later stages of coal combustion. HCN and NO are both primary products from coal char oxidation. Coal rank, heating rate, indigenous minerals and external additives are the major influential factors of the nitrogen species release. Higher rank coals with higher fuel ratio have higher NO releases. HCN release decreases as fuel ratio increases for most coals. The fuel nitrogen conversion to NO increases and the fuel nitrogen conversion to HCN decreases with the increase of heating rate, which may imply that the char nitrogen prefers to react with oxygen to form NO instead of HCN while coal char is combusted at higher temperatures. Different metallic additives show different effects on nitrogen species emission and the effects of indigenous minerals on nitrogen release can be qualitatively estimated by ash analyses.     

Reduction of NO in the exhaust gas by reaction with N radicals

N atoms can serve as a reducing agent for NO in the exhaust gases from fossil fuel power plants. In order to investigate the reduction of NO, synthetic exhaust gas was plasma treated in a dielectric barrier discharge (DBD) (direct treatment) as well as treated by adding nitrogen atoms generated in a pure nitrogen DBD (remote treatment). A DBD with a coaxial electrode geometry was used. Oxygen could be added to the synthetic exhaust. A complete NO reduction was achieved in dry, oxygen free exhaust gas with direct treatment, but oxygen and humidity in the exhaust gas promotes formation of N_xO_y and reduces the efficiency of NO reduction. A 90% NO reduction was achieved with remote treatment. Remote treatment is insensitive to oxygen in the exhaust, but requires large amounts of nitrogen and energy. Fourier transform infrared spectroscopy and ultraviolet absorption spectroscopy were employed in order to detect NO, NO₂, and N₂O in the treated exhaust gas stream.     

Application of gaseous fuel reburning for controlling nitric oxide emissions in boilers

An experimental study was performed on a 36 kW_th down-fired furnace in order to obtain a wider and more detailed knowledge of the influence of the key parameters on the gaseous fuel reburning process when typical Chinese coals served as the primary fuel. The experimental results show that both above 50% nitric oxide reduction and low carbon loss can be obtained when reburn zone residence time is about 0.6 s–0.9 s, gaseous reburn fuel percent is about 10%–15% and average excess air coefficient of reburn zone is about 0.8–0.9. According to the optimizational reburning operating parameters, a 350 MW_e coal-fired boiler was retrofitted for reburning application. The retrofitted boiler had about 0.7 s reburn zone residence time and about 2 s residence time after the burnout air was injected. The results of the demonstration on the full-scale boiler showed that the nitric oxide emission was about 235 mg/Nm³ (O₂ = 3%, dry) from the retrofitted boiler and above 60% nitric oxide reduction efficiency could be achieved with no adverse effects on the boiler operability when the coke-oven gas used as the reburn fuel. This successful demonstration project filled the gaps in the gaseous fuel reburning application on 300 MW_e grade unit boiler in China and showed that the values of the optimizational reburning parameters obtained from experiments could be successfully applied to the designing, retrofitting and optimizing for the gaseous fuel reburning process.     

The evaluation of waste tyre pulverised fuel for NOx reduction by reburning

The combustion of coal for power generation will continue to play a major role in the future, however, this must be achieved using cleaner technologies than we use at present. Scrap tyre arisings in the UK are 400,000 tonnes per year amounting to 30 million tyres and in the EU as a whole, more than 2.5 million tonnes of tyres per year are scrapped. The recent EC Waste Landfill Directive (1999) sets a deadline for the banning of whole and shredded tyres from landfill sites by 2006. Consequently, there is an urgent need to find a mass disposal route for tyres. We describe, in this paper, a novel use for tyre rubber pulverised fuel in a NO_x reburning process which may have an application in power station boilers. This method of disposal could represent a way of combining waste disposal, energy recovery and pollution control within one process. A preliminary study of micronised tyre combustion was undertaken to identify the suitable size ranges for application in NO_x reduction by reburning. Tests were performed in a down-fired, pulverised fuel combustor (PFC) operating at about 80 kW. Superior combustion characteristics, i.e. burnout were achieved with particle sizes less than 250 μm. A South African coal was used as the primary fuel in the reburn tests and the tyre was fed pneumatically via a separate feed system. Parameters studied, were, reburn zone stoichiometry and reburn fuel fraction. Additionally, the carbon content of the ash was carefully monitored for any effect on burnout at the fuel rich reburn stoichiometries. The NO_x reductions achieved with tyres are compared with reburning with coal. NO_x reductions up to 80% were achieved with tyres at half of the reburn fuel feed rate required to achieve the same reductions by coal. The results have been evaluated within the context of other studies available in the literature on NO_x reburning by bituminous coal, brown coal, gas and biomass.     

Catalytic effect of biomass ash on CO, CH4 and HCN oxidation under fluidised bed combustor conditions

In fluidised bed combustion heterogeneous reactions catalysed by the bed material, CaO, and char are significant for the emission levels for instance of NO, N₂O, and CO. The catalysts present in the bed affect significantly the selectivity of HCN and NH₃ oxidation, which are known as precursors of NO_x (i.e. NO and NO₂) and N₂O emissions from solid fuel combustion. Thus the catalytic activity of biomass ashes may also be responsible for the negligible N₂O emissions from biomass combustion due to the presence of a large amount of solids in fluidised bed combustion, homogeneous oxidation may be suppressed within the bed by the quenching of the radicals. For this reason the catalytic oxidation of hydrocarbons and CO on the bed material may be of significance for the total burnout within the fluidised bed combustor.Within this study the effect of different ashes from spruce wood, peat, and for comparison bituminous coal on the oxidation of CH₄, CO, and HCN was studied. The different ashes were shown to have a strong catalytic activity for the oxidation of CH₄, CO, and HCN. In HCN oxidation the selectivity towards NO is high, whereas very little N₂O is formed. The activity of the ashes is strongly dependent on the fuel, which may be explained by their composition.The kinetics of the oxidation of CO and HCN in the temperature range relevant for fluidised bed combustion, i.e. 800-900 °C, has been evaluated for spruce wood ash.     

铁基催化剂脱除NO气体的研究现状

对金属铁及其化合物在烟气脱硝过程中的催化作用的研究进展进行了综述。金属铁能够直接催化还原NO为N2,同时铁被氧化为铁的氧化物。当在烟气中补充一定量的还原气体CO,则CO通过还原铁的氧化物以保证金属铁和NO的连续反应,从而提高NO的脱除效率。在典型的模拟烟气条件下铁丝网在900℃以上可达到90%以上的脱硝效率。Fe2O3能有效地还原再燃脱硝过程的中间产物HCN/NH3,从而大幅度提高再燃脱硝的效率。此外,Fe-ZSM-5分子筛也具有催化还原NO的性能。铁催化还原NO的微观反应机理尚需深入研究。     

Evaluation of the use of different hydrocarbon fuels for gas reburning

Gas reburning is a NO_x reduction technique that has been demonstrated to be efficient in different combustion systems. An experimental study of gas reburning performance in the low temperature range (at and under 1100°C) has been carried out. An evaluation of the use of different hydrocarbon fuels, such as natural gas, methane, ethane, ethylene and acetylene was performed and the influence of the temperature and stoichiometry is considered. The results show that the reburning process is effective under appropriate conditions at the low temperatures used in this work. However, as the temperature diminishes, the influence of the reburn fuel becomes more marked and the use of acetylene or ethane and ethylene leads to better performance than natural gas or methane, the classical reburn fuels for high temperature applications.     

Calcium-based sorbents for simultaneous NOx/SOx reduction in a down-fired furnace

The performance of a suite of different carboxylic salts of calcium, have been assessed as dual NO_x/SO_x reducing agents. The salts studied include, calcium magnesium acetate (CMA), calcium acetate (CA), calcium formate (CF), calcium benzoate (CB), calcium propionate (CP) and magnesium acetate (MA). The primary fuel was propane operating with a primary zone stoichiometry fixed at λ₁=1.05 and the reburn zone stoichiometry, λ₂, was varied between 1.03 and 0.86. Overall stoichiometry, λ₃, was 1.15. CMA was also tested using a US Blend coal as the primary fuel. Experiments were performed in a down-fired pulverised coal furnace operating at an output of 80 kWth. Results showed that CMA and CP were the best dual NO_x/SO₂ performers followed by CB, CA, MA and CF. Also, the co-injection of urea with the carboxylic salts as an advanced reburning agent was studied. The results showed that real improvements in NO reduction over basic reburning of greater than 70% could be obtained depending to a large extent on the initial effectiveness of the reburn fuel as well as nitrogen stoichiometric ratio within the reburn zone. Decomposition of the carboxylic salts was studied by thermo-gravimetric analysis (TGA) yielding information on the release of organic fractions important as precursors for CH_i radical formation. Examination of structural and thermo-chemical properties of the carboxylic salts identified a correlation of NO reduction under reburning conditions with volatile organic content. Calcium magnesium acetate and calcium propionate showed superior SO₂ capture ability with reductions greater than 70% at Ca/S above 2, around 20% higher than calcium acetate and calcium formate. Magnesium acetate achieved reductions of less than 10% at Mg/S ratios up to 2.5. There is a clear difference in the potential effectiveness of the sorbents as dual NO_x/SO₂ reductants, since the organic input for a given Ca input varies according to the composition of the sorbent. Some compromise may have to be made when choosing the correct operating conditions since good reductions in SO₂ may not give acceptable NO_x reductions. However, the application of advanced reburning under these conditions has been shown here to compensate for low initial NO_x reductions by basic reburning.     

Heavy fuel oil combustion in a cylindrical laboratory furnace: measurements and modeling

The finite-volume based commercial CFD-code Fluent was used to simulate the reacting flow in a heavy fuel oil fired laboratory furnace. Both the standard k−ε turbulence model and the Reynolds stress model (RSM) were tested. The combustion model was based on the conserved scalar (mixture fraction) and prescribed probability density function approach. The heavy fuel oil droplet trajectories were predicted by solving the momentum equations for the droplets using the Lagrangian treatment. The soot distribution in the furnace was calculated by solving a transport equation for the soot mass fraction. Simple expressions for the soot formation and oxidation rates were employed. The radiation heat transfer equation was solved using the finite volume method. The formation of thermal NO from molecular nitrogen was modeled according to the extended Zeldovich mechanism. Fuel-based NO was modeled assuming that all the nitrogen in the fuel is released as hydrogen cyanide (HCN), which then further reacts forming nitric oxide NO or molecular nitrogen N₂, depending on the local combustion conditions. The formation of prompt NO was also included in the calculations. The CFD-code was validated against experimental data for a combustor fired by an industry-type swirl burner for which the initial conditions of the spray have been characterized. It was found that the standard k−ε model does not satisfactorily predict the highly swirling flow field in the furnace. The RSM was able to improve the prediction of the flow field. The predicted gas species concentrations were found to be in a reasonable agreement with the measurements, except near the burner and in the vicinity of the furnace axis where discrepancies were found.     

NO prediction in natural gas flames using GDF-Kin®3.0 mechanism NCN and HCN contribution to prompt-NO formation

The first objective of this work was to complete the previous version of our natural gas combustion mechanism (GDF-Kin^®2.0) with nitrogen chemistry in view of NO prediction in natural gas combustion. Two initiating main routes of Prompt NO have been evaluated separately: CH+N₂NCN+H(GDF-Kin^®3.0_NCN) and CH+N₂HCN+N (GDF-Kin^®3.0_NCN). In fact, recent literature tends to consider that the initiation of prompt no proceeds via NCN while current chemical mechanisms still attribute HCN and N as products of the reaction of CH with N₂. The two versions of mechanism have been validated by modeling our experimental results (CH and NO profiles obtained in various natural gas flames [1]) and the CH/NO database available in the literature. Both versions of the mechanism exhibit similar predictions for CH and NO and an overall good agreement with experiments was observed even under lean conditions where CH and NO were formed at low levels. The predictions of GRI3.0 mechanism are also reported.The reaction path analyses show that production of NO is controlled by Prompt-NO mechanism. The set of Thermal-NO, NNH and N₂O mechanisms contributes to less than 3% of the total NO measured in these flames. In GDF-Kin^®3.0_HCN model, the formation of NO is controlled by (i) the two reactions of the extended Zeldovich mechanism (N+OHNO+OH and N+O₂NO+O) converting the N atoms produced by the reaction CH+N₂HCN+N, and by (ii) the oxidation of HCN. In GDF-Kin^®3.0_NCN model, NO is mainly formed by the oxidation of HCN itself issued from NCN and by NCN oxidation yielding directly NO. N atoms still play a significant role in the formation of NO due to the activation of the reaction NCN+H→HCN+N which generates N atoms in this mechanism.     

Catalytic Cracking of n-Dodecane and Diesel Fuel to Improve the Selective Catalytic Reduction of NOx in Automotive Exhaust Containing Excess Oxygen

The low exhaust gas temperatures of diesel engines and the chemical composition of diesel fuel is responsible that only a small amount of nitrogen oxides is converted into nitrogen over SCR catalysts based on Pt zeolites. In order to improve the NO_x reduction over these types of catalysts, it is investigated if a catalytic cracking process is suitable to convert the diesel fuel into more reactive molecules which should act as reducing agent in the HC-SCR process.