AN OVERVIEW OF NOx MITIGATION TECHNIQUES BY SNCR/SCR/HYBRID TECHNOLOGIES IN POWER PLANTS

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1 AN OVERVIEW OF NOx MITIGATION TECHNIQUES BY SNCR/SCR/HYBRID TECHNOLOGIES IN POWER PLANTS Radesh Kumar, AGM(Chem.),NTPC-Vindhyachal Dr.Teena Bhagchandani, Asstt.Manager(Chem.),NTPC-Vindhyachal Department of Chemistry, NTPC Vindhyachal ABSTRACT According to BP s Energy Outlook (2016), fossil fuels will continue to remain the dominant source of energy powering the World s economy. Unfortunately, the combustion of fossil fuels inevitably leads to the production of various air pollutants, such as Sulfur dioxide, Nitrogen oxide (NOx), Particulate matter, heavy metals and volatile organic compounds etc. NOx is a generic term for mono-nitrogen oxides, namely NO and NO2, which are produced during combustion at high temperatures (above 1350 o C). NOx means the sum of NO & NO2 contents in flue gas recalculated on NO2 basis. Emissions of NOx from combustion are primarily in the form of NO while NO2 has a very less contribution.nox (x =1,2) emissions are largely responsible for the ozone decline in middle to high latitudes from spring to fall, and for the acid rain perturbing the ecosystems and the cause of biological death of lakes and rivers. In December 2015, the Ministry of Environment, Forest & Climate Change (MoEF&CC),GOI announced stringent emission standards for coal-based thermal power plants on parameters of particulate matter (PM), sulphur dioxide (SO2), oxides of nitrogen (NOx) and mercury. According to the new norms, the limit of NOx for units installed before 2003 will be 600 mg/nm 3, for units installed between , it will be 300 mg/nm 3 and for the units installed after 2017, the norms are even more stringent, i.e. 100 mg/nm 3. In light of the above facts, NTPC Vindhyachal has been chosen as one of the Project for Pilot/Demo studies of presently available DeNOx technologies(scr/sncr). NTPC and M/s GE Power India Limited has signed a test protocol for conducting Selective Catalytic Reduction (SCR), Selective Non-Catalytic Reduction (SNCR) and hybrid (SCR+SNCR) system studies at Unit # 13 of Vindhyachal STPP. The Demonstration study will be carried out with SNCR technology using Injection of Urea and the Pilot study will be accomplished with SCR technology using Anhydrous Ammonia & Catalytic reactor. Based on the results of these studies,further deliberations/decision regarding implementation of the DeNOx technologies will be taken at Corporate level in NTPC.This paper describes about the SCR/SNCR technologies, the equipments/methodologies etc. employed in these studies and also the comparison of SCR & SNCR technologies.

2 1.0 Introduction: Power generation remains the main driver of global coal demand and will therefore play a significant role in meeting the global energy needs. According to the International Energy Outlook 1, there are about 150 years of known world coal reserves economically exploitable with current technology at present consumption rates. The International Energy Agency (IEA) as well as the US Environmental Protection Agency (EPA) expected in the new policies scenario, that coal fired plants are expected to account for around 27 % of the total new additions to generating capacity worldwide between 2011 and 2020, and around 22 % between 2011 and In India, above 60 per cent of the power generation capacity is from coal-based plants, which estimate to around 167GW as of now (CEA, June, 2015). But it is worth mentioning here that the coal-based power sector is one of the most polluting sectors associated with the emission of particulate matters (PM), sulphur dioxides (SO2) and oxides of nitrogen (NOx), neurotoxins like mercury and carbon dioxide 3. The emissions have high impact on the environment and public health. Figure 1: Contribution of coal-based power sector to industrial emissions A study by American nonprofit Resources for the Future in 2012 says that every coal-based power plant in India is responsible for around 650 deaths every year: approximately 500 deaths are associated with SO2, 120 with NOx and 30 with PM 2.5. Nitrogen oxides (NOx) is one of the most troublesome pollutants and their damaging effects on our health and environment are substantial. A high concentration of NOx increases the tropospheric ozone levels via photochemical oxidation of NOx, while depleting the stratospheric ozone through thermal reaction of nitrous oxide with oxygen atoms in the upper atmosphere. This may lead to global warming. NOx also contributes substantially to acid rain (formed via reaction involving HO2 or OH radicals) leading to acid deposits which contribute to destruction of forests and coastal water life, deterioration of building materials, and causing the smog in urban areas (due to NO2 formed by the photochemical oxidation of NO). Peroxyacetylene nitrates (PAN) can also be formed from nitric oxide and contribute significantly to global photo-oxidation pollution 3-4. The direct health hazards relating to NOx range from bronchitis and pneumonia, to effects on the immune system resulting in increased susceptibility to viral infection and hay fever 5. The global emissions of NOx into the atmosphere have been increasing steadily since the middle of the last century. Although there are important natural sources for NOx, a significant amount of the increased emissions attributed to human activities, especially by the combustion of fossil fuels. Recognizing the central role thermal power plays in worsening air quality, the Ministry of Environment, Forest & Climate Change (MoEF & CC) announced in December 2015 tighter standards for coal-based thermal power plants. The new standards (Table-1) aim to drastically cut emissions of various pollutants. Table 1: Standards (in mg/nm 3 ) PM SO 2 NOx Mercury Current standards none none none New standards Units installed till <500 MW >=500 MW 0.03 >=500 MW 200 Units installed between 2004 and <500 MW >=500 MW 200 Units installed after Jan The term NOx refers to the sum of seven main compounds 6 as shown in Table 2. Table 2. NOx composition Valence Name Formula Properties 1 Nitrous oxide N2O Colorless gas, water soluble 2 Nitric oxide NO Colorless gas, slightly water soluble Dinitrogen dioxide N2O2 3 Dinitrogen trioxide N2O3 Black solid, water soluble, decomposes in water

3 4 Nitrogen dioxide NO2 Red brown gas, very water soluble, decomposes in Dinitrogen tetroxide N2O4 water 5 Dinitrogen pentoxide N2O5 White solid, very water soluble, decomposes in water 2.0 Mechanisms of NOx Formation during Coal Combustion: During coal combustion nitrogen oxides are produced by potentially tens of species and hundreds of reactions involved which are closely linked with the coal combustion mechanisms of devolatilization, char combustion, and volatile burning in a typically turbulent two phase flow condition with significant heat releases 7. The total amount of NOx emissions resulting from solid fuel combustion can be described by the summation of the three main identified reaction pathways. They consist of two dominant and one devote mechanism depending on their shares in the total amount and the source of nitrogen and chemical kinetics. Thermal NOx The fixation of atmospheric nitrogen by atomic oxygen, the latter formed by splitting of the molecular oxygen and nitrogen due to high available temperatures within the combustion zone. Fuel NOx The fuel bound organic nitrogen reacts with available hydrocarbons (CHx). Prompt NOx Then in the combustion air bound nitrogen reacts with available hydrocarbons or hydrocarbon fragments that originated in thermal decomposition in a reducing atmosphere. Figure 2. Simplified NOx formation pathways during coal combustion 3.0 NOx abatement and control: NOx abatement and control technology is a relatively complex issue. The effectiveness of pollution prevention measures in reducing NO and NO2 generation is expressed in terms of relative DRE (Destruction or Removal Efficiency); i.e., the amount NOx generation is reduced by using a prevention technology compared to NOx generation when not using that technology. Various technologies have been developed to control the emission of NOx 8 ; and these may be classified as: i) Primary technologies ( clean technologies ) are aimed at reducing the initial formation of nitrogen oxides, for example, by modifications of the fuel quality or the combustion processes. ii) Secondary measures are aimed at removing the produced nitrogen oxides from the flue or exhaust gases ( clean-up techniques or flue-gas treatments ). 4.0 Pilot project for NOx abatement at NTPC Vindhyachal NTPC Vindhyachal, the largest thermal power station of India is situated in a cluster of heavy industries around Rihand reservoir in Singrauli region, which has been identified as a critically polluted area (CPA) by the Union Ministry of Environment and Forests (MoEF). The emission and effluent discharges from these industries are responsible for the degradation of environmental quality affecting the bio-diversity of the region. In light of the above fact, the station has specifically undertaken various unique steps with respect to design, modifications, changes in operational methodology and introduction of new technologies for monitoring, controlling and mitigation of environmental impact in the region. The station has also been chosen for the installation of pilot/demo project for de NOx technologies. NTPC and GE Power India Limited has signed test protocol for Selective Catalytic Reduction (SCR), Selective Non-Catalytic Reduction (SNCR) and hybrid system and agreed to carry out pilot testing at Unit # 13 of Vindhyachal STPS. This paper will describe the technologies, the instruments/reactors used in this study and also the comparison of SCR and SNCR technologies.

4 4.1 SELECTIVE CATALYTIC REDUCTION (SCR): The SCR process has been commercially implemented in Japan since and the first installations of such units in the U.S., West Germany and Austria came about ten years later 10. SCR uses a catalyst to react injected ammonia to chemically reduce NOx. It can achieve up to a 94% DRE 11 and is one of the most effective NOx abatement techniques. This process has the following overall stoichiometry: 4NH3 + 4NO + O2 4N2 + 6H2O Fig 3 shows a schematic diagram of the SCR process. The main components of the SCR process consist of a reactor with the catalyst and an ammonia storage and injection system. The reducing agent can be either liquid, water-free ammonia under pressure or it can be a 25% aqueous ammonia solution at atmospheric pressure. Figure 3. Schematic diagram of SCR process In order to ensure efficient NOx removal and minimum NH3 slip ( leakage ) from the SCR reactor, it is very important that the flue gas-ammonia mixture has a uniform NH3/NOx ratio. In practice, the degree of conversion of the nitrogen oxides depends on the amount of ammonia added and it increases with increasing NH3/NOx ratio. However, NH3 should not be added above the stoichiometrically required amount in order to avoid the slip of unreacted ammonia. In this pilot project, the maximum allowable ammonia slip is 2 ppm. Figure 4. NOx reduction and NH 3 slip The main purpose/ objective of conducting SCR pilot test at NTPC Vindhyachal is to evaluate the selection & design criteria for the SCR system with respect to Indian Coal/ conditions. By conducting pilot test following design parameters/ criteria will be evaluated: formation. 2 to SO3 conversion. n of Anhydrous ammonia for desired DeNOx efficiency SCR Pilot Test Procedure: A) Trial Run: Trial run is to be carried out by the contractor in order to check the performance of its SCR pilot plant and rectify/ modify its system before start of Main Test. During the Trial run various parameters shall be established as given below so that same can be maintained during the main test: tlet. B) Main Test: The objective of the Main test is to collect information/ data wrt effect of Indian ash on various components of SCR system & pilot APH when inlet conditions are nearer to the actual boiler operating point.

5 C) Optimization/ Extended Test: The purpose of Extended test is to gather more information wrt effect of change in various inlet conditions to the test set up on various components of the SCR system & pilot APH. The optimization test will be carried out for at least 3 months. Rest of period extended test will be done where flue gas with ash will be allowed to flow through the test reactor to see the impact of Indian ash on the catalyst Tentative Requirements of Consumables: Consumables Requirement Anhydrous Ammonia 10 tons Compressed Air (at 5 bar) a) Service Air 40 kg/hr. b) Instrument Air 60 kg/hr. Service Water 10 tons/ hr. Power 50kW Anticipated SCR Pilot test schedule: Description Testing Period Commissioning of the Pilot Test set up/ Completion of Trial run Dec 2017 Main Test 06 months (4400 hrs.) Start of Pilot test Jun 2018 Completion of Main pilot test Nov 2018 Report Submission of Main Test results Dec 2018 Optimization/ Extended Test 06 months (4400 hrs.) Start of Optimization/ Extended Test Dec 2018 Completion of Optimization/ Extended Test Jun An illustration of Pilot Test Plant: Figure 5a. Schematic representation of pilot test reactor Figure 5b. Pilot SCR Reactor SCR Catalysts: SCR catalysts may be made in several ways 12. The catalyst material can either be extruded into honeycomb monoliths or be deposited onto perforated metal plates or nets. Another procedure involves wash coating a layer of catalyst onto a monolith matrix consisting of cordierite or corrugated ceramic paper. The catalyst may be manufactured with different channel diameters since this will influence the volume activity of the catalyst, the pressure drop and the dust deposition (plugging of channels). The most commonly used catalyst we have today for the reduction of NOx from flue-gas is titania-supported vanadia 8. This catalyst which is also being used in this pilot project has high SCR activity and is resistant to poisoning by SO2 13.

6 Figure 6a. Schematic sketch of an SCR reactor with Plate Type Catalyst Figure 6b. Honeycomb catalyst 4.2 SELECTIVE NON-CATALYTIC REDUCTION (SNCR): The SNCR process was initially designed, developed & demonstrated for smaller size units/boilers, over the period the same has been extended up to large size utility boilers with a NOx reduction efficiency 25% - 35 %. It is basically a chemical process which reduces nitrogen oxides (NOx) in flue gas by converting in to elemental nitrogen (N2) and water (H2O) by injecting a nitrogen-based chemical reagent (most commonly urea or anhydrous ammonia) without the presence of any catalyst. The reactions involved are: Using ammonia: 4NO + 4NH3 + 4O2 4N2 + 6H2O Using Urea 4NO + 2CO(NH2)2 + O2 4N2 + 4H2O + 2CO2 Highest NOx reduction is achieved at temperature of flue gas between o C. The reagent is injected mainly in the first pass of the furnace only Flue gas temperature Vs NOx removal efficiency: At high temperatures, ammonia is oxidized to NOx while at lower temperatures, the reaction rate is slowed down, causing an ammonia slip, which may result in the formation of ammonia salts in formation of ammonia salts in the further flue gas path that may lead to secondary problems. Figure 7. Graph depicting variation in NOx removal efficiency with flue gas temperature Effect of the size of droplets on NOx removal efficiency: Droplets, which are too small, would evaporate too fast and possibly lead to a reaction at a too high temperature so that more NOx would be formed. Droplets which are too large, would evaporate too slowly so that the reaction would take place at the lower side or outside the temperature window, which would lead to an increasing of the ammonia slip, and decreasing of the NOx reduction Nox reduction with Urea Vs Ammonia: Urea dissolved in water only can be decomposed into reactive NH2-species after the water enclosing the urea particles has been completely evaporated. The place in the flue gas where the reaction is to take place can be defined in advance by means of the water droplet size and the resulting penetration depth. If the water droplet is big enough, injection in a place that is too hot for a NOx reduction is possible because the reaction can take place downstream the injection point in a colder place within the flue gas. The mass of the dilution water, which is additionally used as a carrier medium for urea solution, ensures a high penetration depth at rather low energy consumption, and may cool down the flue gas to the desired temperature, if necessary.

7 In contrast, in case of ammonia water, the ammonia evaporates immediately when the ammonia water is being heated up after it has entered the furnace. To ensure an optimum penetration depth, more energy is required because of the lower mass of ammonia in gaseous form compared to a water droplet. Figure 8. NOx Reduction with Urea versus Ammonia Water SNCR System Process Schematic Mixing Module Urea Storage Tank

8 4.2.5 Tentative Requirements of Consumables: Consumables Solid Urea for entire test period in kg/hr. Service Air Service Water Power Requirement 2500 kg /hr. 45Nm3/hr liter/hr. 415 V, 3 Ph. 7.5kw feeder 4.3 HYBRID SYSTEM (SNCR+SCR): Hybrid system process is a multistage system, employing both urea-based Selective Non-Catalytic Reduction (SNCR) and a small Selective Catalytic Reduction (SCR) component. Figure 8. Schematic representation of Hybrid system (SCR+SNCR) 4.4 Comparison of SCR and SNCR: Design criteria SNCR SCR NOx reduction efficiency 30-70% 60-90% Temperature Window o C o C Reactant Ammonia or urea Ammonia or urea Reactor None Catalytic Waste disposal None Spent catalyst Energy consumption Low High Capital investment cost Low High Plot requirements Minor Major Maintenance Low 3-5 years (Typical Catalyst life) NH3/NOx (Molar ratio) Urea/NOx (Molar ratio) Not applicable Retrofit Easy Difficult Mechanical draft Not required Required 4.5 Limitations of SCR & SNCR technologies: SCR technology has a high initial cost. In addition, catalysts have a finite life in flue gas and some ammonia slips through without being reacted. SCR systems are sensitive to contamination and plugging during normal, and abnormal, operations. Certain pollutants in flue gas can render the system ineffective at NOx reduction, or cause oxidation of ammonia present (forming more NOx). SCR systems have operational difficulty with binding of the catalyst by fly ash. Because of these issues, SCR catalysts have a limited operational lifetime of 16 to 40 thousand hours in coal-fired power plants, depending on the flue gas composition. Though in theory SNCR systems can achieve roughly 90 percent removal rates, practical constraints like required minimum temperatures, time and mixing often lead to results ranging from 30 to 50 percent. Though SCR systems have been documented as more effective in NOx removal, SNCR systems are often favored due

9 to their lower cost since they do not use a catalyst. The SNCR reagent storage and handling systems are similar to those for SCR systems, but because of the higher stoichiometric ratios, both ammonia and urea SNCR processes require three or four times as much reagent as SCR systems to achieve competitive NOx reductions. 5.0 Advanced Emissions Control Technology: Table 3: Advanced emissions control technology With multi-pollutant emissions control technologies, a single system is able to remove multiple pollutants viz. NOx, SOx, mercury and other heavy metals, halogens and particulate matter from flue gas before they are released into the atmosphere. A considerable advantage of a multi-pollutant strategy is lower capital investment when compared with investing in several different technologies to address each pollutant. Likewise, the installation of a single unit is faster and requires less downtime. Also, multi-pollutant technologies generally require a smaller footprint since their processes are encapsulated in one system. A multi-pollutant emissions control system captures NOx, SOx and mercury while reducing particulates. With this system, a moving bed absorber provides contact between flue gas and activated coke pellets, where SO2, SO3, NOx and mercury are absorbed onto the carbon surfaces. Ammonia is then injected upstream in order to promote the SO2 and NOx reactions as the moving bed acts as a particulate collection step. The impingement of the flue gas on the activated coke pellets provides "polishing" control of particulate. These technologies represent the forefront of innovation in the fossil-power generation industry and provide a cost-effective option to continue operating aging plants that still have significant service time available. 6.0 Conclusion: Research and development will have to continue to search more effective measures of NOx control and try to balance them against cost and efficiency. The cost will decrease as technology advances, operating experience is gained, competition becomes sharper, design flaws are corrected, and better designs become available. Reliability can only be gained with time. Cost will be reduced with time and experience. This study will provide vital information towards making conclusions and anticipating the future growth of DeNOx technologies in the power industry. References: 1. International Energy Agency (2011): World Energy Outlook 2011, IEA, 696 p, ISBN: , November BP p.l.c. (2013): BP s Energy Outlook 2030, January Roy, S.; Hegde, M.S.; Madras, G. Appl. Energy 2009, 86, Granger, P.; Parvulescu, V.I. Chem. Rev. 2011, 111, White Paper: Selective Catalytic Reduction Controls to Abate NOx Emission, Industrial Gas Cleaning Institute, Inc., July 1989, Washington, D. C. 6. Eddinger, J., Grano, D., William, V., Ravi, S. EPA 456/F R, 1999, Wendt, J.O.L. Progress in Energy and Combustion Science 1980, 6(2), H. Bosch,; F. Janssen, Catal. Today 1988, 2, N. Nojiri,; Y. Sakai,; Y. Watanabe, Catal. Rev. -Sci. Eng. 1995, 31, F. Nakajima, Catal. Today, 1991, 10, Selective Catalytic Reduction Control of NOx Emissions, SCR Committee of Institute of Clean Air Companies, November P. Forzatti,; L. Lietti, Heterogeneous Chemistry Review, 1996, 3, J. Baker, Eur. Chem. News, 1991, 57, 16.