Control of nitrogen oxide emissions from coal combustion

The paper gives a discussion of the regulatory framework currently being used to limit nitrogen oxide (NO_x) emissions, with emphasis being placed on EC and UN measures. It then follows with a discussion of the procedures being adopted at present to control or limit these emissions. The topics discussed include combustion modification measures, new combustion technologies, flue gas treatment, selective catalytic reduction, selective noncatalytic reduction and combined SO_x/NO_x processes.     

Modeling fuel NOx formation from combustion of biomass-derived producer gas in a large-scale burner

This study investigates the characteristics of fuel NO_x formation resulting from the combustion of producer gas derived from biomass gasification using different feedstocks. Common industrial burners are optimized for using natural gas or coal-derived syngas. With the increasing demand in using biomass for power generation, it is important to develop burners that can mitigate fuel NO_x emissions due to the combustion of ammonia, which is the major nitrogen-containing species in biomass-derived gas. In this study, the combustion process inside the burner is modeled using computational fluid dynamics (CFD) with detailed chemistry. A reduced mechanism (36 species and 198 reactions) is developed from GRI 3.0 in order to reduce the computation time. Combustion simulations are performed for producer gas arising from different feedstocks such as wood gas, wood + 13% DDGS (dried distiller grain soluble) gas and wood + 40% DDGS gas and also at different air equivalence ratios ranging from 1.2 to 2.5. The predicted NO_x emissions are compared with the experimental data and good levels of agreement are obtained. It is found out that NO_x is very sensitive to the ammonia content in the producer gas. Results show that although NO–NO₂ interchanges are the most prominent reactions involving NO, the major NO producing reactions are the oxidation of NH and N at slightly fuel rich conditions and high temperature. Further analysis of results is conducted to determine the conditions favorable for NO_x reduction. The results indicate that NO_x can be reduced by designing combustion conditions which have fuel rich zones in most of the regions. The results of this study can be used to design low NO_x burners for combustion of gas mixtures derived from gasification of biomass. One suggestion to reduce NO_x is to produce a diverging flame using a bluff body in the flame region such that NO generated upstream will pass through the fuel rich flame and be reduced.     

Application of the exergy method to the environmental impact estimation: The nitric acid production as a case study

In this work the exergy method is used to compare various methods for removal of NO_x from waste (tail) gas released into the atmosphere from nitric acid production plants with respect to their overall environmental impact. Three basic methods for NO_x abatement are analysed: selective catalytic reduction (SCR), non-selective catalytic reduction (NSCR) and extended absorption. The positive and negative effects and the net effect from the NO_x abatement are calculated. The following exergy-based indicators are used for comparing the energy efficiency and the environmental impact of different treatment processes as a result from pollutants removal: reduction of the exergy of the emissions from the whole process route (ammonia and nitric acid production units); exergy of the additional emissions, arising as a result of the treatment process; total net reduction of the exergy consumption, Cumulative Energy Consumption (CEnC) and Cumulative Exergy Consumption (CExC) of natural resources as a result of the waste flows treatment.     

NOx reduction and NO2 emission characteristics in rich-lean combustion of hydrogen

Hydrogen is a clean alternative to conventional hydrocarbon fuels, but it is very important to reduce the nitrogen oxides (NO_x) emissions generated by hydrogen combustion. The rich-lean combustion or staged combustion is known to reduce NO_x emissions from continuous combustion burners such as gas turbines and boilers, and NO_x reduction effects have been demonstrated for hydrocarbon fuels. The authors applied rich-lean combustion to a hydrogen gas turbine and showed its NO_x reduction effect in previous research. The present study focused on experimental measurements of NO and NO₂ emissions from a coaxial rich-lean burner fueled with hydrogen. The results were compared with diffusion combustion and methane rich-lean combustion. Significant reductions in NO and NO₂ were achieved with rich-lean combustion. The NO and NO₂ reduction effects by rich-lean combustion relative to conventional diffusion combustion were higher with hydrogen than with methane.     

Catalysis for NOx abatement

Research in the field of NO_x abatement has grown significantly in the past two decades. The general trend has been to develop new catalysts with complex materials in order to meet the stringent environmental regulations. This review discusses briefly about the different sources of NO_x and its adverse effect on the ecosystem. The main portion of the review discusses the progress and development of various catalysts for NO_x removal from exhaust by NO decomposition, NO reduction by CO or H₂ or NH₃ or hydrocarbons. The importance of understanding the mechanism of NO decomposition and reduction in presence of metal ion substituted catalysts is emphasized. Some conclusions are made on the various catalytic approaches to NO_x abatement.     

Promoting hydrocarbon-SCR of NOx in diesel engine exhaust by hydrogen and fuel reforming

A hydrocarbon-selective catalytic reduction (HC-SCR) silver–alumina monolith catalyst has been prepared and tested for NO_x emissions control in a diesel engine. The work is based on ongoing laboratory experiments, catalyst research, and process development. Hydrogen and actual reformate (i.e. H₂ and hydrocarbon species produced in a partial and exhaust gas fuel reformer) significantly improved the passive control (i.e. no externally added hydrocarbons) NO_x reduction activity over the SCR catalyst using the whole engine exhaust gas from a single-cylinder diesel engine. Optimisation of the reforming process is required for various engine conditions in order to maximise H₂ production and minimise fuel penalty. When diesel fuel partial oxidation and exhaust gas reforming for SCR were implemented, the calculated fuel penalty was in the range of 5–10%, which is relatively high, as both reformers were not optimised yet. During HC-SCR of NO_x over silver–alumina, the known promoting effect of H₂ has been found to be sensitive to various factors, especially the engine exhaust gas temperature, H₂ concentration, HC concentration, HC:NO_x ratio, and space velocity. Under active control (i.e. hydrocarbon injection) SCR operation, powdered Ag–Al₂O₃ catalysts gave significantly higher initial NO_x reduction, but the catalyst activity deteriorated rapidly with time due to poisoning species adsorption (e.g. HCs, nitrates, particulate matter (PM), etc.), whilst for the Ag–Al₂O₃-coated monolithic catalysts, NO_x reduction activity was lower but remained constant for the duration of the tests. The improved physical (mass transfer, filtering of C-containing species) and chemical (reaction kinetics) processes during HC-SCR over powders compared to monoliths led to better initial catalyst activity, but it also accelerated catalyst deactivation which led to increased diffusion limitations.     

Ammonia chemistry in a flameless jet

In this paper, the nitrogen chemistry in an ammonia (NH₃) doped flameless jet is investigated using a kinetic reactor network model. The reactor network model is used to explain the main differences in ammonia chemistry for methane (CH₄) containing fuels and methane-free fuels. The chemical pathways of nitrogen oxides (NO_x) formation and destruction are identified using rate-of-production analysis. The results show that in the case of natural gas, ammonia reacts relatively late at fuel lean condition leading to high NO_x emissions. In the pre-ignition zone, the ammonia chemistry is blocked due to the absence of free radicals which are consumed by methane-methyl radical (CH₃) conversion. In the case of methane-free gas, the ammonia reacted very rapidly and complete decomposition was reached in the fuel rich region of the jet. In this case the necessary radicals for the ammonia conversion are generated from hydrogen (H₂) oxidation.     

Effects of ammonia substitution on hydrogen/air flame propagation and emissions

In order to evaluate the potential of partial ammonia substitution to improve the safety of hydrogen use and the effects on the performance of internal combustion engines, the propagation, development of surface cellular instability and nitrogen oxide (NO_x) and nitrous oxide (N₂O) emissions of spark-ignited spherical laminar premixed ammonia/hydrogen/air flames were studied experimentally and computationally. With ammonia being the substituent, the fundamental unstretched laminar burning velocities and Markstein numbers, the propensity of cell formation and the associated flame structure were determined. Results show substantial reduction of laminar burning velocities with ammonia substitution in hydrogen/air flames, similar to hydrocarbon (e.g., methane with a similar molecular weight to ammonia) substitution. In all cases, ammonia substitution enhances the NO_x and N₂O formation. At fuel-rich conditions, however, the amount of NO_x emissions increases and then decreases with ammonia substitution and the increased amount of NO_x and N₂O emissions with ammonia substitution is much lower than that under fuel-lean conditions. These observations support the potential of ammonia as a carbon-free, clean additive for improving the safety of hydrogen use with low NO_x and N₂O emissions in fuel-rich hydrogen/air flames. The potential of ammonia as a suppressant of both preferential-diffusional and hydrodynamic cellular instabilities in hydrogen/air flames was also found particularly for fuel-lean conditions, different from methane substitution. However, it should be noted that the use of ammonia also imposes considerable technological challenges and public concerns, particularly those associated with toxicity and the specific properties such as high reactivity with container materials and water, which should be completely resolved.     

A technical method to improve NOx/NH3 mixing ratio in SCR system and its engineering applications

A technical method for diagnosing the distribution of NO_x flux within the cross-section area in front of ammonia injection grid (AIG) was proposed for guiding the valve-tuning of AIG branch-pipes, in order to optimize the NO_x/NH₃ mixing ratio in the selective catalytic reduction (SCR) system of power plant. The weight coefficient of each branch-pipe in AIG system can be quantitatively determined with regard to the distribution of NO_x flux in the corresponding sub-zone of the cross-section area. The control strategy of the valves for different AIG branch-pipes can be achieved for guiding the NH₃ injection and improving the NO_x/NH₃ mixing ratio within the whole cross-section area in front of AIG. The technology has been applied on one side of the SCR system flue-gas tunnels (normally two tunnels for the SCR system called as A-side and B-side) of a 660 MW plant for more than one year. The ammonia consumption rate of the SCR system was reduced about 12.62% and the uniformity of outlet NO_x distribution was estimated to be greatly improved by about 79.01% with regard to the standard deviation. The rising rate of the flue gas resistance of the air preheater was slown down by 39.18% compared to that of the other flue-gas tunnel of SCR system. This implied that the formation of the sticky ammonium bisulfate (ABS) on air preheater was significantly inhibited through the application of this technology.     

Modeling of NOx formation in diesel engines using finite-rate chemical kinetics

A fast, physics-based model to predict the temporal evolution of NO_x in diesel engines is investigated using finite-rate chemical kinetics. The temporal variation of temperature required for the computation of the reaction rate constants is obtained from the solution of the energy equation. NO_x formation is modeled by using a six step mechanism with eight species instead of the traditional equilibrium calculations based on the Zeldovich mechanism. Fuel combustion chemistry is modeled by a single-step global reaction. Effects of various stages of combustion on NO_x formation is included using a phenomenological burning rate model. The effects of composition and temperature on the thermophysical properties of the working fluid are included in the computations. Comparison with measured single-cylinder engine-out NO shows good agreement with experimental data. The validated model is then used to demonstrate the impact of various operating parameters such as injection timing and EGR on engine-out NO_x. This fast, robust model has potential applications in model-based real-time control strategies seeking to reduce feed gas NO_x emissions from diesel engines.     

Effects of ammonia substitution on extinction limits and structure of counterflow nonpremixed hydrogen/air flames

The potential of partial ammonia substitution to improve the safety of hydrogen use was evaluated computationally, using counterflow nonpremixed ammonia/hydrogen/air flames at normal temperature and pressure. The ammonia-substituted hydrogen/air flames were considered using a recent kinetic mechanism and a statistical narrow-band radiation model for a wide range of flame strain rates and the extent of ammonia substitution. The effects of ammonia substitution on the extinction limits and structure, including nitrogen oxide (NO_x) and nitrous oxide (N₂O) emissions, of nonpremixed hydrogen/air flames were investigated. Results show reduction of the high-stretch extinction (i.e., blow-off) limits, the maximum flame temperature and the concentration of light radicals (e.g., H and OH) with ammonia substitution in hydrogen/air flames, supporting the potential of ammonia as a carbon-free, clean additive for improving the safety of hydrogen use in nonpremixed hydrogen/air flames. For high-stretched flames, however, NO_x and N₂O emissions substantially increase with ammonia substitution even though ammonia substitution reduces flame temperature, implying that chemical effects (rather than thermal effects) of ammonia substitution on flame structure are dominant. Radiation effects on the extinction limits and flame structure are not remarkable particularly for high-stretched flames.     

Effects of NH3 on N2O formation and destruction in fluidized bed coal combustion

The NH₃ oxidation and reduction process are experimentally and kinetically studied in this paper. It is found that NH₃ has contributions not only to N₂O formation, but also to N₂O destruction in certain conditions. The main product of homogeneous NH₃ oxidation is found to be NO rather than N₂O, but some bed materials and sulphur sorbents have catalytic contributions to N₂O formation from NH₃ oxidation. In reduction atmosphere, NH₃ can promote the KC destruction. It is deduced that the ammonia injection into fluidized bed coal combustion flue gas can decrease both NO_x and N₂O emissions. The ammonia injection process is kinetically simulated in this study, and the reduction rates of NO_x and N₂O are found to depend on temperature, O₂ concentration, initial NO_x and N₂O concentrations, and amount of injected ammonia.     

Numerical model to analyze Nox reduction by ammonia injection in diesel-hydrogen engines

Nowadays, the even increasing stringent environmental legislations have promoted interest in alternative fuels for internal combustion engines. Particularly, hydrogen is becoming a promising fuel due to its high specific energy and low emissions production. Environmentally, the main disadvantage of hydrogen is the high level of nitrogen oxides (NO_x) which produces. In this regard, this work proposes a NO_x reduction method which consists on direct injection of ammonia (NH₃) into the combustion chamber. A numerical model validated with experimental measurements was carried out to analyze emissions and brake specific consumption in a commercial engine operating with diesel-hydrogen blends. Comparing to diesel operation, a 10% hydrogen content increases a 5.3% the peak pressure and 5.7% the maximum temperature. The CO₂, CO and HC emissions are reduced but NO_x emissions increase up to 18.3%. Several injection instants and ammonia flow rates were analyzed, obtaining more than 70% NO_x reductions with a negligible effect on other emissions and brake specific consumption. It was found that the start of ammonia injection is too critical since the maximum NO_x reduction takes place when the temperature is around 1200 K. The NO_x reduction increases with the ammonia flow rate but an excessive quantity of ammonia can lead to un-reacted ammonia slip to the exhaust.     

Soot oxidation and NOx reduction over BaAl2O4 catalyst

This study addresses soot oxidation and NO_x reduction over a BaAl₂O₄ catalyst. By XRD analysis, the catalyst was shown to be of spinel structure. Temperature Programmed Oxidation (TPO) and Constant Temperature Oxidation (CTO) at 673 K show that the presence of O₂ decreases the ignition temperature of soot, and it enhances the conversion of NO_x to N₂ and N₂O. The kinetic features of soot oxidation in the TPO test are similar to that in the TG-DTA analysis. Analysis by Diffuse Reflectance Fourier Infrared Transform Spectroscopy (DRIFTS) indicates that the nitrates formed from NO_x adsorption and the C(O) intermediates from soot oxidation are the key precursors of the redox process between soot and NO_x over surfaces of the BaAl₂O₄ catalyst. Moreover, DRIFTS tests suggest that nitrates act as the principal oxidants for C(O) oxidation, through which nitrates are reduced to N₂ and N₂O. The O₂ in the gas mixture presents a positive effect on the conversion of NO_x to N₂ and N₂O by promoting the oxidation of nitrites into nitrates species.     

Combustion and NOx emissions characteristics of a down-fired 660-MWe utility boiler retro-fitted with air-surrounding-fuel concept

Air-surrounding-fuel is a well-known concept used within tangential and wall-fired boilers. Here, we report for the first time on industrial experiments performed to study the effects of this concept on a 660 MW_e full-scale down-fired boiler. Data are reported for the gas temperature distributions along the primary air and coal-mixed flows, furnace temperatures, gas compositions, for example O₂, CO and NO_x, and gas temperatures in the near-wall region. The influence of concentration control valve (CCV) opening on combustion and NO_x emission in the furnace were determined. The results show that the flame stability, temperature distribution, unburnt carbon are influenced by both concentration ratios and fuel-rich flow velocities. As CCV opening increases, NO_x emissions decrease from 2594 mg/m³ to 1895 mg/m³. Considering altogether economic benefits and environmental protection issues, 30% is the optimal value for the CCV opening.     

NOx formation and selective non-catalytic reduction (SNCR) in a fluidized bed combustor of biomass

Caledonian Paper (CaPa) is a major paper mill, located in Ayr, Scotland. For its steam supply, it previously relied on the use of a Circulating Fluidized Bed Combustor (CFBC) of 58 MW_th, burning coal, wood bark and wastewater treatment sludge.It currently uses a bubbling fluidized bed combustor (BFBC) of 102 MW_th to generate steam at 99 bar, superheated to 465 °C. The boiler is followed by steam turbines and a 15 kg/s steam circuit into the mill. Whereas previously coal, wood bark and wastewater treatment sludge were used as fuel, currently only plantation wood (mainly spruce), demolition wood, wood bark and sludge are used.Since these biosolids contain nitrogen, fuel NO_x is formed at the combustion temperature of 850–900 °C. NO_x emissions (NO + NO₂) vary on average between 300 and 600 mg/Nm³ (dry gas). The current emission standard is 350 mg/Nm³ but will be reduced in the future to a maximum of 233 mg/Nm³ for stand-alone biomass combustors of capacity between 50 and 300 MW_th according to the EU LCP standards. NO_x abatement is therefore necessary.In the present paper we firstly review the NO_x formation mechanisms, proving that for applications of fluidized bed combustion, fuel NO_x is the main consideration, and the contribution of thermal NO_x to the emissions insignificant.We then assess the deNO_x techniques presented in the literature, with an updated review and special focus upon the techniques that are applicable at CaPa. From these techniques, Selective Non-catalytic Reduction (SNCR) using ammonia or urea emerges as the most appropriate NO_x abatement solution.Although SNCR deNO_x is a selective reduction, the reactions of NO_x reduction by NH₃ in the presence of oxygen, and the oxidation of NH₃ proceed competitively.Both reactions were therefore studied in a lab-scale reactor and the results were transformed into design equations starting from the respective reaction kinetics. An overall deNO_x yield can then be predicted for any operating temperature and NH₃/NO_x ratio.We then present data from large-scale SNCR-experiments at the CFBC of CaPa and compare results with the lab-scale model predictions, leading to recommendations for design and operation. Finally the economic impact is assessed of implementing SNCR-technology when applying an NH₃ SNCR or urea SNCR to the CFBC at CaPa.     

一种控制燃煤机组SCR烟气脱硝系统氨逃逸的优化调整方法

以某烟气脱硝工程3号机组为例,针对选择性催化还原(SCR)脱硝反应器出口NO_x质量浓度偏差大、氨逃逸率高的问题,提出了一种喷氨均匀性调整优化方案。采用该方案对喷氨格栅每路供氨支管上的手动调节阀进行调整,最终在反应器出口建立均匀、稳定的NO_x分布。优化调整前后系统A、B两侧的NO_x不均匀度分别从55.1%和19.2%,降至27.6%和14.7%;SCR脱硝系统的平均氨逃逸率从3.289 6 m L·m~(-3)降至1.265 7 mL·m~(-3),表明采用该优化调整方法可使反应器出口NO_x分布均匀,降低氨逃逸率,有利于系统的安全、稳定运行。     

Experimental determination and analysis of CO2, SO2 and NOx emission factors in Iran’s thermal power plants

Emission factors of CO₂, SO₂ and NO_x emitted from Iran’s thermal power plants are fully covered in this paper. To start with, emission factors of flue gases were calculated for fifty thermal power plants with the total installed capacity of 34,863 MW over the period 2007–2008 with regard to the power plants’ operation characteristics including generation capacity, fuel type and amount and the corresponding alterations, stack specifications, analysis of flue gases and physical details of combustion gases in terms of g kWh⁻¹. This factor was calculated as 620, 2.57 and 2.31 g kWh⁻¹ for CO₂, SO₂ and NO_x respectively. Regarding these results, total emissions of CO₂, SO₂ and NO_x were found to be 125.34, 0.552 and 0.465 Tg in turn. To achieve an accurate comparison, these values were compared with their alternatives in North American countries. According to this comparison, emission factor of flue gases emitted from Iran’s thermal power plants will experience an intensive decline if renewable, hydroelectric and nuclear types of energy are more used, power plants’ efficiency is increased and continuous emission monitoring systems and power plant pollution reduction systems are utilized.     

Studies on properties of laminar premixed hydrogen-added ammonia/air flames for hydrogen production

In order to evaluate the potential of burning and reforming ammonia as a carbon-free fuel in production of hydrogen, fundamental unstretched laminar burning velocities, and flame response to stretch (represented by the Markstein number) for laminar premixed hydrogen-added ammonia/air flames were studied both experimentally and computationally. Freely (outwardly)-propagating spherical laminar premixed flames at normal temperature and pressure were considered for a wide range of global fuel-equivalence ratios, flame stretch rates (represented by the Karlovitz number) and the extent of hydrogen substitution. Results show the substantial increase of laminar burning velocities with hydrogen substitution, particularly under fuel-rich conditions. Also, predicted flame structures show that the hydrogen substitution enhances nitrogen oxide (NO_x) and nitrous oxide (N₂O) formation. At fuel-rich conditions, however, the amount of NO_x and N₂O emissions and the extent of the increase with the hydrogen substitution are much lower than those under fuel-lean conditions. These observations support the potential of hydrogen as an additive for improving the burning performance with low NO_x and N₂O emissions in fuel-rich ammonia/air flames and hence the potential of using ammonia as a clean fuel. Increasing the amount of added hydrogen tends to enhance flame sensitivity to stretch.     

Reducing inefficiency and emissions of large steam generators in the United States

Combustion process modifications to reduce inefficiency and emissions in contemporary large boilers are constrained by a number of important design and operating considerations such as steam temperature, ash fusion and material limitations. Plant efficiency gains, obtainable by further increasing combustion heat release efficiency, are in many cases negligible. However, depending upon standard operation, a large potential exists for increasing overall efficiency by lowering excess air levels, while at the same time decreasing pollutant emissions. Dramatic increases in fuel prices have now made investment in such improvements more attractive in the United States than it was in the past. Reduction of excess air level can in addition effect reductions in emissions of NO_x and SO₃, and even of particulates, through equipment and operating improvements. High fuel prices have also made investment in air preheaters and economizers attractive for a larger number of boilers than ever before.Air pollutants can originate from components of the fuel, from incomplete combustion, or even through complete combustion processes. These various forms of pollution can be controlled in varying degrees during the combustion process by wet ash handling, SO₂ to SO₃ conversion minimization, or mixing and temperature control in the case of combustibles and nitric oxide. NO_x reduction techniques, applied to a large gas-fired utility boiler, resulted in NO_x emissions of 33 ppmV or less at power outputs up to 355 MW. For coal- and oil-fired boilers, NO_x scavenging by ammonia injection, recently developed, can give dramatic reductions in NO_x emissions without affecting the combustion process.