Emission reduction techniques and economics. Part 1: NOx

Transcription

1 Emission reduction techniques and economics. Part 1: NOx N. Orfanoudakis 1, A. Vakalis *, K. Krallis, A. Hatziapostolou 2 3 & N. Vlachakis 1 Laboratory for Steam Boilers, Turbines & Thermal Plants, TEI-Chalkis, Psachna Evia 2 Energy Technology Dept., TEI Athens, Aigaleo Athens 3 Laboratory for Fluid Mechanics, TEI-Chalkis * The late A. Vakalis was director at lignite power plants Abstract It is clear that Greece shall rely on lignite for a very high percentage of its electricity production supplemented by Natural Gas. This approach requires that the relevant improvement measures for lignite combustion have to be clearly specified, decided upon and taken. The purpose of this work is to demonstrate the various methods of emission reduction, in connection with the cost of installation, operational cost and experience of operation as well as good environmental performance. The work has been performed mainly for the study of the BAT (Best Available Techniques) in the framework of the IPPC Directive, as this applies to Greece and its power plants (existing or planned in future). Clean Coal Technologies (CCTs) are those which facilitate the use of coal in an environmentally satisfactory and economically acceptable way. Among other aspects, clean coal technologies should meet various regulations covering emissions, effluents, and residues. In some situations, CCTs offer the possibility of satisfying even more stringent standards, at an acceptable cost. A basic approach to the cleaner use of coal is to reduce emissions by reducing the formation of pollutants such as NOx, and/or cleaning flue gases after combustion. Another approach is to develop more thermally efficient systems so that less coal is used to generate the same amount of power, together with improved techniques for flue gas cleaning, for effluent treatment and for residue use or disposal. Amongst all the emissions, particulate, NOx and SOx, in the current work only NOx emissions are considered. The reduction Measures considered are primary and secondary (end of pipe).

2 792 Air Pollution XII 1 NOx emissions reduction and control 1.1 NOx emissions abatement and control by primary measures Emissions of NO x can be either abated or controlled by primary measures or flue gas treatment technologies. Primary measures for NO x control may be divided into the following categories: Burner optimization (excess air control, burner fine tuning) Air staging (overfire air or two stage combustion) Flue gas recirculation Fuel staging (some burners out of service, reburning) Low NO X burners Burner optimisation techniques and low NO x burners are used to minimise the formation of NO x during combustion. In air staging and flue gas recirculation technologies, the aim is to reduce the oxygen availability to help reduce NO x formation and complete the combustion. In fuel staging, previously formed NO x is reduced to nitrogen and oxygen within the furnace. Primary measures for NO x control (burner optimisation, low NO x burners and overfire air) are now considered integral parts of a newly built power plant and existing units retrofit them whenever they are required to reduce their NO x emissions. SCR is accepted throughout the world as the proven commercial option to achieve high NO x removal. However, new developments in primary measures, such as application of the reburning process at large scale, advanced reburning as well as post-combustion flue gas treatment such as SNCR and combined SO 2 /NO x control systems should offer alternatives to SCR in the future Burner optimization for NOx control (excess air control, burner fine tuning) Burner optimisation is usually the first method used to control NO x formation. Optimisation is achieved by modifying boiler-operating conditions. Excess air control, boiler fine tuning and balancing the fuel and air flow to the various burners are in use and continue to be investigated to achieve minimum NO x formation in the burner. In the excess air control procedure, minimising air (oxygen) flow to the fuel during the initial stages of combustion leads to reduced NO x formation. As the oxygen level is reduced, combustion may become incomplete and the amount of unburned carbon in the ash may increase. In addition, the steam temperature may be decreased. Reducing the oxygen in the primary zones to very low amounts (< 1%) can also lead to high levels of carbon monoxide. The result of these changes can be a reduction in the boiler efficiency, slagging, corrosion and a counteractive overall impact on boiler performance. Potential safety problems, which may result from the use of this technique without a strict control system include, fires in air preheaters and ash hoppers as well as increases in opacity and in rates of waterwall wastage.

3 Air Pollution XII 793 Fine tuning the boiler settings include mill balancing, adjusting air registers, air and coal flow balancing, tuning firing configuration and improving the plant control system. It is known (for example in IEA Coal Research reports) that controlling the varying burner tilt angles to control steam temperature and changing oxygen flow, mill loading and air register settings during different burner loads can also contribute to reducing NO x formation. They reported that reducing excess air in combination with fine tuning the boiler could achieve as much as 39% less NO x formation. However, they also found that fine tuning the boiler could result in increased heat rate. They concluded that several maintenance and operational issues have to be addressed. These include studying the effects of sustained low oxygen operation on waterwall wastage and the possible damage the burner buckets may sustain as a result of close proximity of the flame fronts. Retention efficiency 20-30%. Installation availability: 100% Electricity consumption: none Installation cost: 1 EURO/kW th. Operational cost: 1 EURO/kW th Flue gas recirculation for NOx control Flue gas recirculation for NO x control includes gas recirculation into the furnace or into the burner. In this technology 20-30% of the flue gas (at C) is re-circulated and mixed with the combustion air. The resulting dilution in the flame decreases the temperature and availability of oxygen (vitiated air) therefore reducing thermal NO x formation. When flue gas recirculation into the burner is used in low NO x burners, the flue gas is usually re-circulated subject to the operational constraints of flame stability and impingement, as well as boiler vibration. Flue gas recirculation in combination with other primary measures for NO x control is installed at 49 pulverised coal-fired units on a total capacity of >15 GWe. Retrofitting an existing coal-fired unit with flue gas recirculation involves installation of a system to extract the flue gas from the boiler unit, additional ductwork, fan and a fly ash collecting device. The fly ash control device is necessary in order to clean the flue gas of particulate matter prior to recirculation. Heat distribution in the furnace may be affected due to the increase in throughput. Excessive flue gas recirculation can also result in flame instability problems and increased steam temperatures. Flue gas recirculation alone in coal-fired boilers achieves a low NO x reduction efficiency (<20%). This is because the ratio of thermal-no x to total NO x emissions is relatively low in coal-fired plants. The technique is being used on coal-fired units in combination with other primary measures for NO x control. Retention efficiency 15% (for solid fuel), 30% (for oil fuel), up to max. 75% (for gaseous fuel). Installation availability: 100%

4 794 Air Pollution XII Electricity consumption: none Installation cost: 2 EURO/kW th. Operational cost: Unknown Air staging for NOx control (overfire air (OFA) or two-stage combustion) Air staging or two-stage combustion, is generally described as the introduction of overfire air into the boiler or furnace. Staging the air in the burner (internal air staging) is generally one of the design features of low NOx burners. Furnace overfire air (OFA) technology requires the introduction of combustion air to be separated into primary and secondary flow sections to achieve complete burnout and to encourage the formation of N 2 rather than NO x. Primary air is mixed with the fuel producing a relatively low temperature; oxygen deficient, fuel-rich zone and therefore moderate amounts of fuel NO x are formed. The secondary of the combustion air is injected above the combustion zone through a special wind-box with air introducing ports and/or nozzles, mounted above the burners. Combustion is completed at this increased flame volume. Hence, the relatively low-temperature secondary-stage limits the production of thermal NO x. The location of the injection ports and mixing of overfire air are critical to maintain efficient combustion. Retrofitting overfire air on an existing boiler involves waterwall tube modifications to create the ports for the secondary air nozzles and the addition of ducts, dampers and the wind-box. This technique is currently used in 116 pulverised coal-fired units, on a total capacity of 50 GWe as a stand-alone measure. It is used in combination with other primary measures for NO x control, in 175 coal-fired units on a total capacity of 53 GWe. Retention efficiency 40% (for solid fuel), 45% (for oil fuel), up to max. 65% (for gaseous fuel). Installation availability: 100% Electricity consumption: none Installation cost: 5 EURO/kW th. Operational cost: Unknown Low NOx burners Low NO x burners are designed to control fuel and air mixing at each burner in order to create larger and more branched flames. Peak flame temperature is thereby reduced, and results in less NO x formation. The improved flame structure also reduces the amount of oxygen available in the hottest part of the flame thus improving burner efficiency. Combustion, reduction and burnout are achieved in three stages within a conventional low NO x burner. In the initial stage, combustion occurs in a fuel rich, oxygen deficient zone where the NO x are formed. A reducing atmosphere follows where hydrocarbons are formed which react with the already formed NO x. In the third stage internal air staging completes the combustion but may result in additional NO x formation. This however can be minimised by completing the combustion in an air lean environment.

5 Air Pollution XII 795 Low NO x burners can be combined with other primary measures such as overfire air, reburning or flue gas recirculation. A large number of low NO x burners have been developed and are currently used in over 370 coal-fired units (125 GWe). Retention efficiency 40%. Installation availability: 100% Electricity consumption: none Installation cost: 5 EURO/kW th. Operational cost: Unknown. 1.2 NOx emissions abatement and control by flue gas treatment Flue gas treatment for NOx control can be categorised into three areas: Selective catalytic reduction (SCR) Selective non-catalytic reduction (SNCR) Combined SO 2 /NO X control techniques The summarised descriptions of NOx control technologies is mainly drawn from the detailed IEA Coal Research review on NOx emissions abatement and control by Soud and Fukasawa (1996) Selective catalytic reduction (SCR) for NOx control In SCR systems, ammonia vapour is used as the reducing agent and is injected into the flue gas stream, passing over a catalyst. NOx emission reductions over 80-90% are achieved. The optimum temperature is usually between 300 C and 400 C. This is normally the flue gas temperature at the economiser outlet. There are three typical layout arrangements of SCR systems applied to coalfired power stations. High dust position is the most widely used SCR configuration, especially with dry bottom boilers, because it does not require particulate emissions control prior to the denitrification process. Low dust positioning has the advantage of less catalyst degradation caused by fly ash erosion, but requires a more costly hot-side ESP. Tail end position SCR has been used primarily with wet bottom boilers with ash recirculation to avoid catalyst degradation caused by arsenic poisoning. The configuration is also favoured with retrofit installations (due to SCR space requirements) between the economiser outlet and ESP. The efficiency of SCR process reactions (reagent stoichiometry and utilisation nearly 1.0) allows very close and effective reagent injection-control based on feedback, of measured NOx concentrations in the flue gas at the economiser outlet. The temperature of the flue gas in the SCR reactor is controlled by mixing the flue gas exiting the economiser with the flue gas from the economiser bypass. The ammonia injection grid is located in the ductwork leading to the SCR catalyst, far enough upstream to ensure optimum gas and reagent distribution across the catalyst cross-section. The catalysts can have different compositions: based on titanium oxide, zeolite, iron oxide or activated carbon. Most catalysts in use in coal-fired plants

6 796 Air Pollution XII consist of vanadium (active catalyst) and titanium (used to disperse and support the vanadium) mixture. However, the final catalyst composition can consist of many active metals and support materials to meet specific requirements in each SCR installation. Catalyst geometry may typically be a flat plate or honeycomb. A moving bed is used for granular activated carbon. German experience shows that plate types generally have a higher resistance to deposition and erosion than honeycombs. In this case, catalytic converters are used in an air preheater. SCR technology has been used commercially in Japan since 1980 and in Germany since 1986 on power stations burning mainly low-sulphur coal and in some cases medium-sulphur coal. There are now about 15 GWe of coal-fired SCR capacity in Japan and nearly 30 GWe in Germany, out of a total of about 53 GWe worldwide. During the 1990s SCR demonstration and full-scale systems have been installed in US coal-fired power plants burning high-sulphur coal. Their commercial use has followed the introduction of stringent limits to regulate NOx emissions in each country. Retention efficiency 95%. Installation availability: 95% Electricity consumption: increase 0.6% Installation cost: EURO/kW th. Operational cost: EURO/tn Selective non-catalytic reduction (SNCR) for NOx control In SNCR systems, a reagent is injected into the flue gas in the furnace within an appropriate temperature window. Emissions of NOx can be reduced by 30% to 50%. The NOx and reagent (ammonia or urea) react to form nitrogen and water. A typical SNCR system consists of reagent storage, multi-level reagent-injection equipment, and associated control instrumentation. The SNCR reagent storage and handling systems are similar to those for SCR systems. However, because of higher stoichiometric ratios, both ammonia and urea SNCR processes require three or four times as much reagent as SCR systems to achieve similar NOx reductions. The temperature window for efficient SNCR operation typically occurs between 900 C and 1,100 C depending on the reagent and condition of SNCR operation. When the reaction temperature increases over 1000 C, the NOx removal rate decreases due to thermal decomposition of ammonia. On the other hand, the NOx reduction rate decreases below 1000 C and ammonia slip may increase. The optimum temperature window generally occurs somewhere in the steam generator and convective heat transfer areas. The longer the reagent is in the optimum temperature window, the better the NOx reduction. Residence times in excess of 1 second yield optimum NOx reductions. However, a minimum residence time of 0.3 second is desirable to achieve moderate SNCR effectiveness. Ammonia slip from SNCR systems occurs either from injection at temperatures too low for effective reaction with NOx or from over-injection of reagent leading to uneven distribution. Controlling ammonia slip in SNCR

7 Air Pollution XII 797 systems is difficult since there is no opportunity for effective feedback to control reagent injection. The reagent injection system must be able to place the reagent where it is most effective within the boiler because NOx distribution varies within the cross section. An injection system that has too few injection control points or injects a uniform amount of ammonia across the entire section of the boiler will almost certainly lead to a poor distribution ratio and high ammonia slip. Distribution of the reagent can be especially difficult in larger coal-fired boilers because of the long injection distance required to cover the relatively large cross-section of the boiler. Multiple layers of reagent injection as well as individual injection zones in cross-section of each injection level are commonly used to follow the temperature changes caused by boiler load changes. However, it is difficult to make fine adjustments due to the complexity of these injection levels and zones. A potentially troublesome reaction is unreacted ammonia combining with SO 3 to form ammonium bisulphate. Ammonium bisulphate will precipitate at air heater operating temperatures and can ultimately lead to air heater fouling and plugging. Although no SO 2 is oxidised by the SNCR system, naturally occurring SO 3 concentrations are sometimes high enough (especially from higher sulphur coals) to be a concern with potentially high ammonia slip rates. An SNCR process can produce nitrous oxide (N 2 O), which contributes to the greenhouse effect. N 2 O formation resulting from SNCR depends upon the reagent used, the amount of reagent injected and the injection temperature. In Western Europe, Japan & the USA, SNCR systems have been used commercially on coal-fired power plants since 1980, 1970 & the early 1990s, respectively. Retention efficiency <50%. Installation availability: 95% Electricity consumption: increase 0.2% Installation cost: EURO/kW th. Operational cost: EURO/tn Combined SO2/NOx removal processes Combined SO 2 /NO x removal processes remain considered fairly complex and costly. However, emerging technologies have the potential to reduce SO 2 and NO x emissions for less than the combined cost of conventional FGD B for SO 2 control and selective catalytic reduction (SCR) B for NO x control. Most processes are in the development stage, although some processes are commercially used on low to medium-sulphur coal-fired plants. Retention efficiency up to 95% for SO 2 & 95% for NO X. Installation availability: Unknown Electricity consumption: increase 0.2% Installation cost: EURO/kW th. Operational cost: 600 EURO/tn.

8 798 Air Pollution XII 2 Conclusions The purpose of this work was to demonstrate the various methods of emission reduction, in connection with the cost of installation, operational cost and experience of operation as well as good environmental performance. The authors examined the various methods for pollutant reduction, for all kind of fuels but mainly focused on solid fuels, as set in the bibliography. Especially, the authors examined the various NOx pollution reduction methods on a common basis of: Method used Short Description Retention efficiency Electricity consumption Installation availability Installation cost per kw th Retention cost per tonne of pollutant Thus, this work was focused not only to the NOx emission reduction techniques themselves but, also to the cost of installation and operation of anti-pollution equipment. Finally, in the tables provided for each technique the experience from the operation of such equipment (capture efficiency etc.) as well as installed units, were reported. A basic approach to the cleaner use of coal is to reduce emissions by reducing the formation (primary measures) of pollutants such as NOx, and/or cleaning the flue gases after combustion (secondary measures). Another approach is to develop more thermally efficient systems so that less coal is used to generate the same amount of power, together with improved techniques for flue gas cleaning, for effluent treatment and for residues use or disposal. Clean Coal Technologies (CCTs) are those which facilitate the use of coal in an environmentally satisfactory and economically acceptable way. Among other aspects, clean coal technologies should meet various regulations covering emissions, effluents, and residues. In some situations, CCTs offer the possibility of satisfying even more stringent standards, at an acceptable cost. References [1] N. Papageorgiou, Steam Generators Ι & ΙΙ Simeon Editions, [2] Ε. Kakaras, Ν. Papageorgiou, Effect of the quality of Greek lignite on the ΝΟ Χ & SO 2 emissions, from power plants, Technical Chamber of Greece Conference (ΤΕΕ), [3] D. Chatzifotis, C. Papapavlou, E. Kakaras, Measurements of the Inlet Conditions in a large scale Lignite-fired Boiler, Lisbon [4] D. Chatzifotis, Α. Violos, Measurements Division PPC, The environmental impact of power plants., Environmental conference organized by the Hellenic Association of Mechanical and Electrical Engineers (ΠΣ ΜΗ), 1992.

9 Air Pollution XII 799 [5] Ν. Orfanoudakis Mathematical models for solid fuels furnaces, Diplom Arbeit, National Technical University of Athens (NTUA), Athens [6] N. Orfanoudakis Measurements of size and Velocity of burning Coal, PhD Thesis University of London, [7] G. Bergeles, F. Israel, N. Orfanoudakis, G. Papadakis, N.P. Sargianos, A.M.K.P. Taylor and J.H. Whitelaw (1994) Design and evaluation of coal burners. Final report of contract number JOUF C. [8] Orfanoudakis, N. and Taylor, A.M.K.P.: "Evaluation of an amplitude size anemometer and application to a swirl stabilised coal burner", Combustion Science Technology, Vol. 108, p , [9] Orfanoudakis N and Taylor, A.M.K.P.: "Measurements of size and velocity in a swirl stabilised coal burner", to appear in Combustion and Flame. [10] L. Georgoulis, Μ. Psofogiannakis, V. Tsadari, Development and Production Division. Methods for optimization of electrostatic precipitators for solid fuel fired power plants, Conference for lignite & solid fuels- TEE, Athens May [11] H. Reidick, W. Kessel, P. Fritz und K.R.G. Hein Feuerungstechnische Maßnahmen zur NO X -Minderung an ausgewählten braunkohlebefeuerten Dampferzeugern, VGB Kraftwerkstechnik, 71, Heft 5, pp , [12] H. Reidick, Erfahrungen mit primären und sekundären NO X - Minderung der Abgase von Dampferzeugern-Feuerungen, EVT Energie und Verfahrenstechnik GmbH, Register 45, pp , [13] H. Spliethoff, Large scale trials and development of fuel staging in a 160MW Coal fired Boiler, Joint Symposium on Stationary Combustion NO X Control Washington DC, March 25-28, [14] A. Kicherer, H. Spliethoff and K.R.G. Hein (IVD-Stuttgart), The effect of NO X reduction with different reburning fuels, Internal Report Universität Stuttgart - IVD, [15] H. Spliethoff, U. Greul, H. Maier and K.R.G. Hein (IVD- Stuttgart), Low NO X Combustion for pulverized coal - Comparison of air-staging and reburning, International Conference on Combustion and Emission Control 3-5 Dec. London, [16] Environmental Resources Management (ERM), Revision of the EC Emission Limit Values for new Large Combustion Installations (>50MW th ), Commission of the European Communities (DG XI), [17] Mechanical and Electrical Equipment of Power Plant SES Aghios Dimitrios V. Vol I of the Contract DMKT-156/ [18] Mechanical and Electrical Equipment, Vol I of the Contract DMKT- 156/ [19] Environmental Impact Study for power plant SES Kardia PPC, Production Division PPC, October 1994.

10 800 Air Pollution XII [20] Environmental Impact Study for power plant SES LKP-A PPC, Production & Mines Division PPC, October 1994 & [21] Environmental Impact Study for power plant SES Ptolemais PPC, Production Division PPC, October [22] Environmental Impact Study for power plant SES Aminteo-Filotas PPC, Production Division PPC, October [23] Environmental Impact Study for power plant SES Megalopolis PPC, Production Division PPC, February [24] Environmental Impact Study for power plant SES Aghios Dimitrios PPC, Production Division PPC, October 199