Proceedings of the 10th International Conference on Environmental Science and Technology Kos island, Greece, 5 – 7 September 2007
SYSTEM DESIGN AND COST ESTIMATIONS FOR NOX SELECTIVE CATALYTIC REDUCTION (SCR) ON COAL- FIRED BOILERS M. TSITSIRIKI1, O. BEREKETIDOU1,2, H. LATSIOS1 and M.A. GOULA1 1
Pollution Control Technologies Department, Technological Educational Institute of Western Macedonia, Koila, 50100 Kozani, Greece 2 Department of Engineering and Management of Energy Resources, University of Western Macedonia, Bakola & Sialvera, Kozani, 50100,Greece e-mail:
[email protected],
[email protected]
EXTENDED ABSTRACT Nitrogen oxides (NOx) are gaseous pollutants that are primarily formed through combustion process. While flue gas is within the combustion unit, about 95% of the NOx exists in the form of nitric oxide (NO). The balance is nitrogen dioxide (NO2), which is unstable at high temperatures. Once the flue gas is emitted into the atmosphere, most of the NO is ultimately converted to NO2. NOx in the atmosphere reacts in the presence of sunlight to form ozone (O3), one of the criteria pollutants for which health-based National Ambient Air Quality Standards have been established. NOx is generated in one of three forms; fuel NOx, thermal, NOx and prompt NOx. Fuel NOx is produced by oxidation of nitrogen in the fuel source. Combustion of fuels with high nitrogen content such as coal and residual oils produces greater amounts of NOx than those with low nitrogen content such as distillate oil and natural gas. Thermal NOx is formed by the fixation of molecular nitrogen and oxygen at temperatures greater than 3600oF (2000oC). Prompt NOx forms from the oxidation of hydrocarbon radicals near the combustion flame and produces an insignificant amount of NOx. Selective Noncatalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR) are post-combustion control technologies based on the chemical reduction of nitrogen oxides (NOx) into molecular nitrogen (N2) and water vapour (H2O). The primary difference between the two technologies is that SCR utilizes a catalyst to increase the NOx removal efficiency, which allows the process to occur at lower temperatures. The technologies can be used separately or in combination with other NOx combustion control technologies such as low NOx burners (LNB) and natural gas reburn (NGR). SNCR and SCR can be designed to provide NOx reductions year-round or only during summer months, when ozone concerns are greatest. The paper presents design specifications and a costing methodology for SCR applications for large industrial boilers. The work describes the process chemistry, performance parameters, and system components of SCR. In addition, impacts to the boiler performance and facility operations resulting from the installation of SCR are presented. The paper also estimates important underlying design parameters including the normalized stoichiometric ratio, catalyst volume and reagent consumption. Lastly, it presents assumptions and equations for estimating capital costs, annual operation and maintenance costs, and annualized costs. Keywords: Selective Catalytic Reduction (SCR), coal-fired boilers, system design, cost estimation, large combustion plants
B-855
1.
INTRODUCTION
In general, for coal- and lignite-fired combustion plants, the reduction of nitrogen oxides (NOx) by using a combination of primary and/or secondary measures is considered to be BAT. The nitrogen compounds of interest are nitric oxide (NO) and nitrogen dioxide (NO2), collectively referred to as NOx, and nitrous oxide (N2O). A distinction of BAT has been made according to the boiler technology, i.e. pulverized or fluidized bed combustion, and whether coal or lignite is used as a fuel. The use of primary measures, either for coal or lignite, tends to cause incomplete combustion resulting in a higher level of unburned carbon in the fly ash and some carbon monoxide emissions. With good design and control of combustion, these negative impacts can mostly be avoided. The amount of unburned carbon-in-ash varies according to the fuel and is normally somewhat higher than without primary measures. For most of the utilization options for the fly ash, the associated BAT level of unburned carbon-in-ash is below 5%. Levels of unburned carbon below 5% can normally be achieved but with some coals only at the cost of somewhat higher NOx emissions. Primary NOx reduction measures also have an impact on the total energy efficiency of the process. If the combustion remains incomplete, the energy efficiency remains lower. A normal rise in the amount of unburned carbon due to low NOx combustion has a negative impact of approximately 0.1-0.3% on the unit efficiency. For pulverized coal combustion plants, the reduction of NOx emissions by the use of primary measures in combination with secondary measures such as SCR is BAT, where the separation efficiency of the SCR system ranges between 80 and 95%. There are different processes available today for the regeneration of used catalysts, which increases the catalyst lifetime considerably and which, therefore, reduces the operating costs. The economic feasibility of applying an SCR system to an existing boiler is primarily a question of the expected remaining lifetime of the plant, which cannot necessarily be determined by the age of the plant. The use of SCR has the disadvantage of a ‘slide’ ammonia emission (i.e. ammonia slip). With respect to the ammonia concentration when using an SCR, a level of less than 5 mg/Nm3 is the associated BAT level. This level also avoids problems in the future utilization of fly ash and the smell of the flue-gas in the surrounding area. Information on key aspects of the design of SNCR and SCR systems is considered proprietary by vendors, including methods for estimating certain design parameters and costs. This information is not available to develop cost methodologies for SNCR and SCR. In order to obtain estimates of the proprietary design parameters and costs, it is necessary to develop mathematical correlations from available data using regression and curve fitting techniques. These expressions are derived in EPA reports prepared by The Cadmus Group, Bechtel Power, Inc. and SAIC in SNCR and SCR techniques [1, 2, 3] from documented comprehensive SNCR and SCR performance data and costs based on quotations provided by suppliers and facilities. SNCR and SCR are secondary measures which have largely been applied to coal-fired combustion plants [5]. In Europe, SCR systems are particularly applied in Austria, Germany, Italy and the Netherlands. Outside of Europe they are mostly applied in Japan and the US. The SCR technology has proven to be successful for hard coal-fired power plants, but has not yet been applied to lignite-fired plants. In the few cases were an SCR system has been applied to lignite-fired power plants, it was shown that the catalyst’s lifetime was too short, because of the high quarts content in the ash which causes high abrasion of the catalyst.
B-856
2.
ENVIRONMENTAL ISSUES
In Greece, Public Power Corporation (PPC) operates 21 lignite-fired thermal units with installed capacity of nearly 4,900 MW and 4,500 MW net capacity. A total capacity of 4,050 MW is concentrated in Northern Greece and the remaining 850 MW in Southern Greece. All lignite-fired power plants use the pulverized coal combustion technology and the emissions normally vary from unit to unit as they also depend on the age of the unit, the used firing technology, the efficiency, the lignite pulverisation and the burner arrangement. The power plants of the Northern System have higher NOx emissions, as the local lignite permits higher firing temperatures in the furnace. The emissions registered by PPC are as follows: NOx 12,7505.5 t/a SO2 84,128.3 t/a CO2 39,900,000 t/a Dust 11,637.2 t/a CO2 412.7 t/a. The environmental legislation in Greece is harmonised with the European Union Directive L309/15 concerning air pollutants emission limitations. Especially, the NOx emission limit value to be applied by solid fuels existing plants (for outpout values >500 MWh) from 1st January 2016 is 200 mg/Nm3 [6]. The main quality features of the Greek lignites and the operational characteristics of the coal-fired power plants in Northern Greece are presented in Tables 1 and 2. From table 2 it is anticipated that the values of the uncontrolled NOx concentration for Ptolemaida, Amyntaio and Liptol plants are very close to the emission limits, therefore these plants will not be included in the SCR system design and cost estimations that follows. Table 1. Main quality features of the Greek lignites in Northern Greece Plant SES PTOLEMAIDAS SES AGIOU DIMITRIOU SES KARDIAS SES AMYNTAIOU SES LIPTOL
High Heating Value (Btu/lb) 2861 3093 2861 2321 2861
Sulfur Content (%) 0,40 0,40 0,40 0,35 0,40
Ash Content (%) 40 16 15 20 40
Moisture (%) 53 55 52 52 53
Table 2. Coal-fired power plants in Northern Greece and their operational characteristics
Plant
PTOLEMAIDA I & II III IV AGIOS DIMITRIOS I & II III & IV V KARDIA I II III IV AMYNTAIO I & II LIPTOL I & II
Maximum Fuel Consumption Rate (lbx10-5 /hr)
Average Annual Fuel Consumption (lb x 10-8)
Uncontrolled NOx Concentration (lb/MMBtu)
Plant Capacity Factor (%)
195 125 300
7,80 5,00 12,0
47,181 40,805 80,904
0,25 0,23 0,26
26,1 30,3 30,4
600
22,0
156,99
0,33
33,5
620 375 300 300 300 300 600 43
22,0 11,5 11,0 11,0 11,0 11,0 20,0 1,98
174,15 99,003 93,027 87,219 79,948 76,890 172,80 17,107
0,32 0,30 0,36 0,34 0,34 0,36 0,20 0,24
32,2 36,0 31,7 31,2 32,7 31,5 32,5 24,7
Outpout (MW)
B-857
3.
TECHNOLOGY APPLIED
3.1 Process Description Like SNCR, the SCR process is based on the chemical reduction of the NOx molecule. The primary difference between SNCR and SCR is that SCR employs a metal-based catalyst with activated sites to increase the rate of the reduction reaction. A nitrogen based reducing agent (reagent), such as ammonia or urea, is injected into the post combustion flue gas. The reagent reacts selectively with the flue gas NOx within a specific temperature range and in the presence of the catalyst and oxygen to reduce the NOx into molecular nitrogen (N2) and water vapour (H2O). The use of a catalyst results in two primary advantages of the SCR process over SNCR. The main advantage is the higher NOx reduction efficiency. In addition, SCR reactions occur within a lower and broader temperature range. However, the decrease in reaction temperature and increase in efficiency is accompanied by a significant increase in capital and operating costs. The cost increase is mainly due to the large volumes of catalyst required for the reduction reaction. Figure 2.1 shows a simplified process flow schematic for SCR. Reagent is injected into the flue gas downstream of the combustion unit and economizer through an injection grid mounted in the ductwork. The reagent is generally diluted with compressed air or steam to aid in injection. The reagent mixes with the flue gas and both components enter a reactor chamber containing the catalyst. As the hot flue gas and reagent diffuse through the catalyst and contact activated catalyst sites, NOx in the flue gas chemically reduces to nitrogen and water. The heat of the flue gas provides energy for the reaction. Nitrogen, water vapour, and any other flue gas constituents then flow out of the SCR reactor.
Figure 1. Selective Catalytic Reduction (SCR) Process Flow Diagram [7, 8]
B-858
3.2 SCR Performance Parameters The rate of the reduction reaction determines the amount of NOx removed from the flue gas. The major design and operational factors that affect the NOx removal performance of SCR are the following: • Reaction temperature range; • Residence time available in the optimum temperature range; • Degree of mixing between the injected reagent and the combustion gases; • Molar ratio of injected reagent to uncontrolled NOx; • Uncontrolled NOx concentration level; and • Ammonia slip. The majority of the discussion regarding SNCR design and operational factors is valid for the SCR process excepting small variations due to the use of a catalyst and the reaction chamber being separate from the combustion unit. Additional design and operational factors to consider, which are specific to the SCR process, include: • Catalyst activity • Catalyst selectivity • Pressure drop across the catalyst • Catalyst pitch • Catalyst deactivation • Catalyst management In Table 3 the general assumptions for performing the calculations and the estimated economic factors for the cost equations are presented. Table 3. General assumptions and estimated economic factors Number of SCR operating days Acceptable Ammonia Slip Fuel Volume Flow Rate ASR Stored Ammonia Concentration Number of Days of Storage for Ammonia Pressure Drop for SCR Ductwork Pressure Drop for each Catalyst Layer Temperature at SCR Inlet Equipment Life Annual Interest Rate Catalyst Cost, Initial Catalyst Cost, Replacement Electrical Power Cost 29% Ammonia Solution Cost Operating Life of Catalyst Catalyst Layers
155 days 2.0 ppm 547 ft3/min per MMBtu/hr 1.05 29 % 14 days 3 inches w.g. 1 inch w.g. 650 °F 20 years 7% 182,5 €/ft3 220,5 €/ft3 0,04 €/kWh 0,08 €/lb 24000 hours nfull=2, nempty=1
4. RESULTS 4.1 Design Parameters SCR system design is a proprietary technology. Extensive details of the theory and correlations that can be used to estimate design parameters such as the required catalyst volume are not published in the technical literature [4]. Furthermore, the design is highly site-specific. In light of these complexities, SCR system design is generally undertaken by providing all of the plant- and boiler-specific data to the SCR system supplier, who specifies the required catalyst volume and other design parameters based on prior experience and computational fluid dynamics and chemical kinetic modelling [1].
B-859
This section presents a step-by-step approach to estimate design parameters based on a procedure developed in the EPA report [1]. This procedure assumes SCR system size and cost are based on three main parameters: the boiler size or heat input, the required level of NOx reduction, and the catalyst volume (see Table 4). The approach to SCR sizing described in this section is based on the catalyst volumes for a base case and several sensitivity cases developed to support the cost estimating procedures in Reference [1]. Although this approach is based on SCR data for utility boilers, it provides sufficient accuracy and detail to develop the capital and annual cost estimates for SCR as applied to industrial boilers.
Table 4. Design of the SCR system parameters estimation
Plant AGIOS DIMITRIOS I & II III & IV V KARDIA I II III IV
Catalyst Volume (ft3)
Catalyst and SCR Crosssectional Areas (ft2)
Length and Width of the Reactor (ft)
Actual Catalyst Height (ft)
Number of Catalyst Layers
SCR Height (ft)
14219
3108
3574
59,8
3,3
3
40
14227 7996 7726 7325 7325 7726
3211 1942 1554 1554 1554 1554
3693 2234 1787 1787 1787 1787
60,8 47,3 42,3 42,3 42,3 42,3
3,2 3,1 3,5 3,4 3,4 3,5
3 3 3 3 3 3
40 39 40 40 40 40
4.2 Cost Analysis The cost-estimating methodology presented here provides a tool to estimate study level costs for high-dust SCR systems. Actual selection of the most cost-effective option should be based on a detailed engineering study and cost quotations from the system suppliers. The cost estimating equations presented in this section are based on equations developed by The Cadmus Group, Bechtel Power, Inc. and SAIC in the draft EPA report, Selective Catalytic Reduction for NOx Control on Coal-fired Boilers [1]. These equations follow the costing methodology of Electric Power Research Institute (EPRI) [9].In the EPRI method, both the purchased equipment cost (PEC) and direct installation cost are estimated together. This methodology is different from the EPA Air Pollution Control Cost Manual methodology, which estimates equipment costs and installation costs separately. The capital and annual cost equations were developed for coal-fired wall and tangential utility and industrial boilers with heat input rates ranging from 250 MMBtu/hr to 6000 MMBtu/hr (25 MW to 600 MW). The SCR system design is a high-dust configuration with one SCR reactor per combustion unit. It utilizes anhydrous ammonia as the reagent with an allowed ammonia slip in the range of 2 to 5 ppm. The catalyst is a ceramic honeycomb with an operating life of 3 years at full load operations. The cost equations are sufficient for NOx reduction efficiencies up to 90%. A correction factor for a new installation versus a retrofit installation is included to adjust the capital costs [1]. The cost information presented in this work is based on using ceramic honeycomb catalyst for the base case. In general, more catalyst volume is required for an SCR system using plate catalyst, although the unit cost of plate catalyst is lower than honeycomb. Thus, any difference in capital cost is expected to be within the accuracy of a study-level cost estimate.
B-860
Total Capital Investment (TCI) includes direct and indirect costs associated with purchasing and installing SCR equipment. Costs include the equipment cost (EC) for the SCR system itself, the cost of auxiliary equipment, direct and indirect installation costs, additional costs due to installation such as asbestos removal, costs for buildings and site preparation, offsite facilities, land, and working capital. In general, SCR does not require buildings, site preparation, offsite facilities, land, and working capital. Direct capital costs (DCC) include purchased equipment costs (PEC) such as SCR system equipment, instrumentation, sales tax and freight. This includes costs associated with field measurements, numerical modeling and system design. It also includes direct installation costs such as auxiliary equipment (e.g., ductwork, fans, compressor), foundations and supports, handling and erection, electrical, piping, insulation, painting, and asbestos removal. Indirect installation costs are those associated with installing and erecting the control system equipment but do not contribute directly to the physical capital of the installation. This generally includes general facilities and engineering costs such as construction and contractor fees, preproduction costs such as startup and testing, inventory capital and any process and project contingency costs. Total annual costs (TAC) consist of direct costs, indirect costs, and recovery credits. Direct annual costs are those proportional to the quantity of waste gas processed by the control system. Indirect (fixed) annual costs are independent of the operation of the control system and would be incurred even if it were shut down. Direct annual costs (DAC) include variable and semi variable costs. Variable direct annual costs account for purchase of reagent and electrical power. Semi variable direct annual costs include operating and supervisory labour cost, maintenance cost, and catalyst replacement cost. In general, indirect annual costs (fixed costs) include the capital recovery cost, property taxes, insurance, administrative charges, and overhead. Capital recovery cost is based on the anticipated equipment lifetime and the annual interest rate employed. An economic lifetime of 20 years is assumed for the SCR system. The remaining life of the boiler may also be a determining factor for the system lifetime. The total annual cost (TAC) for owning and operating an SCR system is the sum of direct and indirect annual costs. In Table 5 the direct capital and the total annual cost for the existing plants are presented.
Table 5. SCR systems cost estimation
Plant AGIOS DIMITRIOS I & II III & IV V KARDIA I II III IV
Direct Capital Cost (million €)
Total Indirect Cost (million €)
Total Plant Cost (million €)
Total Direct Annual Cost (million €/yr)
Indirect Annual Cost (million €/yr)
Total Annual Cost (million€/yr)
15,16
3,031
20,92
1,176
2,020
3,196
15,42 10,69
3,084 2,138
21,28 14,75
1,188 0,720
2,055 1,424
3,243 2,144
9,444
1,889
13,03
0,658
1,258
1,917
9,350 9,350 9,436
1,870 1,870 1,887
12,90 12,90 13,02
0,636 0,636 0,658
1,246 1,246 1,257
1,881 1,881 1,916
B-861
5. CONCLUSIONS To meet air quality requirements SCR can be applied with adapted NH3 consumption to reduce NH3 slip effects and to increase catalyst life. The investment costs of an SCR device are considerable and they depend on the volume of the catalyst which is fixed by the flue-gas volume, by the ammonia slip and by the NOx conversion rate which should be attained. The main factors for operating costs are the lifetime of the catalyst which is influenced by the fuel characteristic and the SCR configuration, demand on the reduction agent, energy consumption due to pressure loss and eventually energy for reheating of the flue gas. The main cost factors amongst investment and maintenance costs are costs for catalysts displacement, reduction agent (aqueous solution of ammonia) and electric energy. In the investment costs, costs for the whole flue gas channel are included (pipes, catalyst box, bypass, NH4OH unit consisting of tank, storage system, dosing device, evaporation and mixing system). The selective catalytic reduction (SCR) process is a widely applied process for the reduction of nitrogen oxides in exhaust gases from large combustion installations in Europe and in other countries throughout the world, such as Japan and the USA. As it seems very likely that the existing coal-fired power plants will still be operating in the coming years certain modifications, such as the installation of additional SCR units are needed in order to comply with the new European Commission regulations on power plant emissions.
REFERENCES 1.
2.
3.
4. 5.
6. 7. 8.
9.
‘Selective Catalytic Reduction for NOx Control on Coal-fired Boilers’, Report prepared for the U.S. Environmental Protection Agency by The Cadmus Group, Inc., Bechtel Power Corporation, and Science Applications International Corporation, May 1998. ‘Investigation of Performance and Cost of NOx Controls as Applied to Group 2 Boilers’, Revised Report, prepared for the U.S. Environmental Protection Agency by The Cadmus Group, Inc., Bechtel Power Corporation, and Science Applications International Corporation, August 1996. ‘Cost Estimates for Selected Applications of NOx Control Technologies on Stationary Combustion Boilers’, Report prepared for the U.S. Environmental Protection Agency by The Cadmus Group Inc., and Bechtel Power Corporation, June 1997. ‘Selective Catalytic Reduction (SCR) Control of NOx Emissions’, Institute of Clean Air Companies (ICAC), Prepared by the ICAC SCR Committee, November 1997. ‘Integrated Pollution Prevention and Control (IPPC)’, Reference Document on Best Available Techniques for Large Combustion Plants, November 2004, European Commission, Directorate General JRC, Institute for PROSPECTIVE Technologies Studies (Seville), Unit on Sustainability in Industry, Energy and Transport European IPPC Bureau European Union Directive, Official Journal of the European Communities, L 309/15, 27/11/2005 Rummenhpohl, V., H. Weiler and W. Ellison, ’Experience Sheds Light on SCR O&M Issues, Power Magazine, September 1992 Tonn, D.P. and T.A. Uysal, “2200 MW SCR Installation on New Coal-Fired Project’, Presented at the Institute of Clean Air Companies ICAC Forum’98, Durham, North Carolina, March 18-20, 1998 Electric Power Research Institute, Technical Assessment Guide. Vol. 1, Revision 7, 1993
Acknowledgments: The presented work is financially supported by the 3th PEP OF WESTERN MACEDONIA, REGION OF WESTERN MACEDONIA, PROJECTTITLE ‘Development and Application of an Operational Forecasting Model for Atmospheric Pollution and Tackling Actions in the Region of Kozani - Ptolemaida’, 0608.
B-862
