Multi-Pollutant Control Demonstration In a Pilot Plant with Dry Sorbent Injection

Transcription

1 Technical Paper BR-1866 Multi-Pollutant Control Demonstration In a Pilot Plant with Dry Sorbent Injection Authors: B.B. Sakadjian A.A. Silva E.J. Campobenedetto Babcock & Wilcox Power Generation Group, Inc. Barberton, Ohio, U.S.A. Presented to: Air Quality VIII Date: October 24-27, 2011 Location: Arlington, Virginia, U.S.A.

2 Multi-Pollutant Control Demonstration In a Pilot Plant with Dry Sorbent Injection B.B. Sakadjian A.A. Silva E.J. Campobenedetto Babcock & Wilcox Power Generation Group, Inc. Barberton, Ohio, U.S.A. Presented at: Air Quality VIII October 24-27, 2011 Arlington, Virginia, U.S.A. BR-1866 Abstract New regulations on emissions from fossil fuel powered combustion facilities require tighter controls on sulfur dioxide (SO 2 ), hydrochloric acid (HCl), particulate and mercury (Hg). Dry sorbent injection (DSI) technology offers a low capital-cost solution to control these emissions or in the case of Hg, to enhance its capture. DSI is therefore being considered as part of a utilities overall multi-pollution mitigation strategy. This paper describes DSI tests for acid gas control conducted at the Babcock & Wilcox Research Center (BWRC). Separate test programs focused on multi-pollutant control targeting SO 2, sulfuric acid (H 2 SO 4 ) and HCl emissions. Sorbents used for mitigation include trona, hydrated lime and sodium bicarbonate. A description of the test program and results, including competing reactions of these pollutants, is discussed. The test programs used several novel continuous emissions monitoring techniques such as a laser-based system for H 2 SO 4 and a Differential Mobility Analyzer (DMA) for sub-micron particulate and H 2 SO 4 measurement, and Fourier Transform Infrared (FTIR) instrumentation for monitoring HCl. Computational fluid dynamics models make use of resulting test data from bench, pilot and field test campaigns. Introduction Dry sorbent injection (DSI) technology for acid gas control has been in existence since the 1960s. The initial focus was on sulfur dioxide (SO 2 ) control. The beginning of the millennium led to a resurgence in interest in DSI technology. This time, the focus was not control of SO 2, but controlling sulfur trioxide (SO 3 ) from coal-firing combustion systems. Sulfur from coal is primarily released in the form of SO 2 during the combustion process. Some of the SO 2 is oxidized to SO 3 as the flue gas travels through the convection pass, air heaters and ducting. Implementation of selective catalytic reduction (SCR) technology to mitigate nitrogen oxides (NO x ) resulted in higher SO 2 to SO 3 conversion due to active catalyst constituents such as vanadium oxide. SO 3 then reacts with moisture in the flue gas to form sulfuric acid (H 2 SO 4 ). As little as 5 to 7 ppm of H 2 SO 4 at the exit of wet flue gas desulfurization (FGD) systems can result in the formation of a blue plume and impact the ability of activated carbon to control mercury emissions. New regulations such as the Industrial and Utility Maximum Achievable Control Technologies (MACT) and the Cross State Air Pollution Rule (CSAPR) require additional controls on pollutants such as SO 2, hydrogen chloride (HCl) and total particulates. DSI technology offers a low capital-cost solution for controlling some of these emissions. Table 1 summarizes emission limitations for coal-fired plants as set by the Utility MACT. DSI may be relevant to all three emission limits listed. Particulate matter (PM) incorporates both filterable and condensable particulate. Sorbent injection is effective in mitigating H 2 SO 4 which accounts for a large portion of the condensable portion of particulate matter. Mitigating H 2 SO 4 also allows Hg control technologies such as activated carbon systems to be more effective. Babcock & Wilcox Power Generation Group 1

3 Table 1 Utility MACT Emission Limitations on Coal-Fired Energy Generating Units Subcategory Total Particulate Matter Hydrogen Chloride Mercury Existing coal-fired unit designed for coal >= 8,300 Btu/lb Existing coal-fired unit designed for coal < 8,300 Btu/lb lb/mbtu (0.30 lb/mwh) lb/mbtu (0.30 lb/mwh) lb/mbtu (0.020 lb/mwh) lb/mbtu (0.020 lb/mwh) 1.0 lb/tbtu ( lb/gwh) 11.0 lb/tbtu (0.20 lb/gwh) 4.0 lb/tbtu * (0.040 lb/gwh*) In addition to the Utility MACT, CSAPR requires additional control of SO 2 and NOx. The current CSAPR regulation requires SO 2 and NOx reductions as early as January DSI can be implemented quickly to control both HCl and SO 2 emissions on Powder River Basin (PRB) or heavy PRB blended fuels for compliance with the initial phase of CSAPR or as a temporary solution while longer lead-time SO 2 control technologies are implemented. The performance of sorbents is a function of a number of variables such as particle size, morphology, temperature, sorbent dispersion, reactivity and residence time. All of these factors must be considered when making predictions on sorbent injection rates to meet emission limits. Field and pilot tests have been performed to determine performance of sorbents and to generate the information necessary for the design of commercial systems. Tests have been performed on coal and biomass combustion flue gas and using simulated flue gas. Tests performed on bench and pilot units aid in the development of reaction mechanisms and improve the understanding of competing reactions with acid gases and other flue gas constituents. The equations below show the series of reactions that occur during the conversion of SO 2 to sulfuric acid mist/ blue plume. SO 2 (g) + 1/2O 2 (g) SO 3 (g) Some of the SO 2 in flue gas is oxidized to SO 3. Metal surfaces and active sites on catalysts can promote this oxidation SO 3 (g) + H 2 O (g) H 2 SO 4 (g) SO 3 reacts with water vapor in flue gas to form H 2 SO 4 Babcock & Wilcox Research Center test facilities Babcock & Wilcox Power Generation Group, Inc. (B&W PGG) is equipped with theoretical computational tools and modeling capabilities as well as bench, pilot and field test systems to develop effective DSI solutions for its customers. Babcock & Wilcox has been performing sorbent injection tests for several decades with early interest on SO 2 control. During the early 2000s, sorbent testing focused on the control of acid mist, H 2 SO 4. Testing continues at the Babcock &Wilcox Research Center (BWRC) in Barberton, Ohio, to improve understanding of reaction mechanisms involving flue gas constituents such as the acid gases of interest, gas-solid chemical reactions and condensation reactions. To accomplish this, researchers make use of tools varying from theoretical themodynamic calculations to bench scale experimentation and pilot-scale testing. Bench-Scale System The BWRC is equipped with multiple bench-scale systems to study and evaluate energy generation technologies, environmental pollution control, and materials evaluation and research. Figure 1 shows a bench-scale system designed to study the dynamics of SO 3 transformations. Tests are geared to improve understanding regarding acid gas condensation, aerosol formation phenomena, as well as control mechanisms for sulfuric acid. The system uses simulated flue gas with the ability to introduce controlled concentrations of H 2 O, SO 2, HCl, SO 3 and NH 3 in heated and humidified H 2 SO 4 (g) H 2 SO 4 (l) or (s) Quench of H 2 SO 4 leads to formation of aerosol liquid or solid particles responsible for blue plume Gaseous sulfuric acid reacts with alkali sorbents to form sulfate or bisulfate compounds as shown in the equations below: Na 2 CO 3 (s) + H 2 SO 4 (g) Na 2 SO 4 (s) + H 2 O (g) + CO 2 (g) Na 2 CO 3 (s) + 2H 2 SO 4 (g) 2NaHSO 4 (s) + H 2 O (g) + CO 2 (g) HCl and SO 2 undergo similar chemical reactions with alkali sorbents such as trona, sodium bicarbonate and hydrated lime. Fig. 1 Aerosol dynamics/acid gas control bench-scale test system. 2 Babcock & Wilcox Power Generation Group

4 Fig. 2 Small Boiler Simulator II combustion and emissions control facility. air. The generation of SO 3 is accomplished using a catalyst packed bed reactor which converts SO 2 and O 2 to SO 3. Rapid quench of flue gas which occurs in wet FGD systems is simulated with water spray systems. The gas is sampled and analyzed from multiple locations along the heated test loop to provide information on the various gaseous species at varied residence times. The system is equipped with the necessary flow control, temperature monitoring and gas analysis capability. A differential mobility analyzer (DMA), which uses electrostatic classification and an optical particle counter system, is used to count and provide size distribution information of submicron aerosol particulate (10 to1000 nm). The system is also equipped with a sorbent injection system to perform screening tests on sorbents. Small Boiler Simulator II Pilot-Scale Facility BWRC s 6 MBtu/hr Small Boiler Simulator- II (SBS-II) is a totally integrated, state-of-the-art, pilot-scale combustion and emission control facility enabling researchers to conduct performance tests and scale results for utility and commercial use. (See Figure 2.) The SBS-II is designed to accurately represent the time and temperature profile of a utility boiler. The furnace can be operated in either a wallfired or cyclone-fired mode and is equipped to fire natural gas, fuel oil, coals ranging from bituminous to lignite, and alternative fuels such as biomass and fuel slurries. The SBS- II fuel handling system includes a dryer, crusher, pulverizer, and feed systems for specialized fuel handling requirements and independent studies. The furnace is designed for two levels of overfire air study. Multiple sample ports are located throughout the facility for visual observation, in-furnace and in-situ measurements, and extractive sampling. Gas analyzers are located in a number of locations throughout the furnace to collect emissions data. Environmental equipment, including both wet and dry scrubbers, selective catalytic reactors, fabric filter baghouses, and a condensing heat exchanger, are integrated in the SBS-II for post-combustion control and study. Flue work is arranged so the flue gas bypass any piece of equipment. All equipment at the SBS-II can be used for both full facility operation or as an individual piece of test equipment. A sorbent injection system is in place at the SBS-II that houses various solid/liquid sorbent reservoirs, wellcontrolled pneumatic injection controls, and flexible hoses that deliver sorbents to numerous locations throughout the facility for studies of emissions reductions for NOx, SO 2, SO 3, HCl, HF and Hg. A number of different additives have been utilized at the facility including ammonia, urea, limestone, hydrated lime and trona. Emissions control testing on the SBS-II facility can be performed using flue gas generated from the combustion of coal, biomass, natural gas or fuel oils. Since different fuel sources provide different SO 2, SO 3 and HCl concentrations, additional injection of acid gases may be necessary to simulate the range of concentrations in commercial units. Alternately, tests can be carried out with simulated flue gas using heated air humidified and spiked with the desired concentrations of SO 2, SO 3 or HCl. The Acid Gas Injection (AGI) system consists of compressed cylinders, flow control and delivery equipment and an SO 3 generator to deliver desired quantities of acid gases to the main unit operations and ducting of the SBS-II facility. Figure 3 shows a flow diagram of the SBS-II system in a combustion mode configuration with the DSI and AGI systems connected to various locations along the flue gas path as the gas travels through the convection pass, air heater/flue gas cooler outlet into the baghouse. The location of the injection point determines the gas-solid reaction residence time. RESULTS AND DISCUSSIONS Continuous Emissions Monitoring Continuous emissions monitoring systems (CEMS) are readily available for some of the major flue gas constituents (SO 2, NOx, CO, CO 2, etc.). While there is constant development and improvement in emission measurement tools, Fig. 3 Flow diagram: SBS-II pilot-scale test facility including acid gas injection (AGI) and dry sorbent injection (DSI) systems. Babcock & Wilcox Power Generation Group 3

5 Fig. 4 DMA results with air. systems are often not readily available to monitor trace emissions during the early stages of research and development. SO 3 /H 2 SO 4 is one such case where continuous measurement has been a challenge. The sections below cover several types of measurement tools that have been tested at the BWRC to provide data on SO 3 and HCl. Sulfuric Acid/SO 3 Sampling and Analysis The BWRC Bench-Scale Aerosol Dynamics test system was used to generate information and provide better understanding regarding the dynamics of aerosol formation. One of the challenges in setting up a bench-scale system to study SO 3 was that continuous sampling systems were not readily available to measure SO 3 /H 2 SO 4. Instead of measuring SO 3 or H 2 SO 4 directly, the system was designed to provide zones for quenching the gas. When H 2 SO 4 undergoes rapid quenching in a gas-liquid contactor, it forms condensed sulfuric acid sub-micron aerosols. A differential mobility analyzer (DMA) based system from TSI Incorporated was used to sample and quantify the gas for sub-micron particles. Figures 4 and 5 show the number concentration versus diameter of sampled gases from the system. Figure 4 is a sample analysis from the differential mobility analyzer when the system is essentially free of sub-micron particles. When SO 3 is injected into the system and subjected to a rapid quench in a gas liquid contactor, sub-micron aerosol particles are formed. The mobility analyzer detects the formation of these condensed particulates as shown in Figure 5, where the number concentration is several orders of magnitude higher than that detected in air. These plots can be translated to concentrations of SO 3 /H 2 SO 4 taking into consideration factors such as dilution ratios. The analyzer provides an efficient way to get continuous readings of aerosol counts, thereby producing sorbent performance data without resorting to time-consuming batch sampling techniques such as the EPA test method relying on controlled condensation. The DMA provides dynamic information averaged over time scales spanning several minutes compared to controlled condensation that provides single averaged data points with sampling intervals of 1 hour. Test campaigns at BWRC s SBS-II pilot scale facility recently made use of a prototype SO 3 CEMS unit from ThermoFisher Scientific. This system utilizes a Quantum Cascade Laser (QCL), coupled with a heated White cell and detector to measure SO 3. The system uses a sampling and conditioning system to initially convert H 2 SO 4 to SO 3. Figure 6 shows a schematic of the extractive probe and laser based SO 3 analysis system. The extractive probe also includes a built-in SO 3 generator that is used to calibrate the system. Figure 7 is a photo of the extractive probe installed at the SBS-II pilot facility. The test campaigns used heated and humidified air, spiked with desired concentrations of SO 3 from an SO 3 generator. Figures 8 and 9 are representative SO 3 readings obtained during the test campaign from the CEMS unit. Figure 8 provides information regarding the types of dynamic response times experienced with SO 3. Between the start of SO 3 injection into the flues of the SBS-II facility and when steady readings are obtained by the SO 3 CEMS can be as long as two hours. In contrast, the system responds fairly quickly to SO 3 injected from the built-in SO 3 generator which is integral Fig. 5 DMA results with air/so 3 /H 2 O. Fig. 6 SO 3 CEMS system. 4 Babcock & Wilcox Power Generation Group

6 Fig. 7 Sampling probe of prototype unit. to the sample probe (auto system span). The long response time experienced with the sampling of SO 3 from the pilot system can be attributed to system and component dynamics. Transients include start-up and heat-up of the pilot system s SO 3 generator, SO 3 absorption/soaking on flue wall surfaces, absorption by residual sorbent in the flue and the presence of cold spots in the system. Bench-scale tests using the differential mobility analyzer displayed similar responses and lag in steady-state readings. Figure 9 shows a comparison between SO 3 CEMS readings and SO 3 controlled condensation sampling information generated using NCASI Method 8 sampling methodology. The figure shows that the readings are fairly close for the two methods. The prototype laser-based CEMS unit has since undergone further improvement. Among the changes, includes the ability to do dynamic spiking of SO 3 in order to determine any measurement bias. HCl Sampling and Control A number of SBS-II combustion tests were conducted which included sampling HCl from coal or biomass combustion flue gas. Tests were also performed at BWRC pilot-scale Fig. 9 SO 3 readings comparison. facilities that used simulated flue gas. The AGI system was used to deliver metered quantities of HCl in heated and humidified air. In addition, the system has been used to supplement and vary concentrations of HCl in combustion flue gas. Both Fourier Transform Infrared (FTIR) and EPA Method 26A sampling techniques were used to measure concentrations of HCl at various points along the gas duct. Figure 10 shows the comparison between FTIR and Method 26A sampling results. The method 26A sampling results shown in the figure were taken at the baghouse inlet location. The FTIR curve shows sampling information from different locations along the duct with a portion of the test period sampling from the baghouse inlet as is shown in the figure. While the FTIR detects fluctuations in the concentration of HCl, Method 26A sampling can only provide average information for a longer time duration. Both sampling methods are used to generate HCl removal information from the SBS-II pilot scale facility. Sorbent Performance Testing SO 2 and HCl Control A number of sorbents have been tested to determine their performance towards SO 2 and HCl capture. Among the sorbents tested include trona, sodium bicarbonate, and Fig. 8 SO 3 CEMS system. Fig. 10 Measurements of HCl with an FTIR and EPA Sampling Method 26A. Babcock & Wilcox Power Generation Group 5

7 Fig. 13 Competing reactions sodium-based sorbent. Fig. 11 SO 2 control with injection of sodium-based sorbents across a baghouse. hydrated lime. Figure 11 shows representative SO 2 removal versus stoichiometry curves for some of the sorbents tested. The tests were performed in the SBS-II on coal combustion flue gas. The fuel source was a bituminous coal with an uncontrolled SO 2 concentration of 600 to 700 ppmv. Sodium bicarbonate and trona (raw and milled) with two different particle size distributions were tested and were able to achieve greater than 90% SO 2 capture. Similarly, hydrated lime was tested to determine its ability to capture HCl. Data was generated to determine sorbent performance, both in terms of in-flight capture of HCl as well as capture across a baghouse. Figure 12 shows HCl removal as a function of stoichiometry with hydrated lime as the sorbent. Hydrated lime can also achieve high removals. Competing Reactions H 2 SO 4, HCl and SO 2 are all acid gases that react competitively with sodium and calcium based sorbents. Among the three gases, H 2 SO 4 has the highest reactivity. Parametric tests were performed to characterize sorbent performance when subjected to simulated flue gas with varying concentrations/ ratios of both HCl and SO 2. Figure 13 shows the performance of sodium-based sorbents towards SO 2 removal with varying ratios of HCl to SO 2. When HCl accounts for a significant portion (greater than 25%) of the acid gas concentration (HCl+SO 2 ), it is preferentially absorbed and affects the extent of SO 2 capture. Further studies are underway to quantify the competitive nature of sorbents towards HCl removal in the presence of SO 2, when HCl accounts for less than 25% of the acid gas content. Computational Fluid Dynamics Modeling Design of DSI systems make use of B&W PGG s CFD modeling capabilities. Figures 14 and 15 show models created for sorbent injection systems at two commercial combustion units. Figure 14 shows the distribution of gas velocity and Figure 15 shows the distribution of particles in the flue duct. Information generated in pilot and field test campaigns provide data to validate and enhance B&W PGG s CFD models. Fig. 12 In-duct removal of HCl with hydrated lime. Fig. 14 CFD model gas speed distribution. 6 Babcock & Wilcox Power Generation Group

8 Fig. 15 CFD model particle distribution. Summary DSI offers a low capital-cost solution to multi-pollutant control. DSI can mitigate acid gas pollutants such as H 2 SO 4, HCl and SO 2 with the ability to achieve high extent of removals. These gases undergo competing reactions; therefore, all acid gases must be considered and taken into account when making performance predictions. DSI test campaigns carried out at the Babcock & Wilcox Research Center provide performance data including in-flight capture and removal across a baghouse. SO 2 removals of 90% and higher were achieved with sodium bicarbonate or trona injection upstream of the baghouse. Under the test conditions, the acid gases displayed different degrees of reactivity with the sorbents with H 2 SO 4 displaying the highest reactivity followed by HCl and SO 2, respectively. Bench and pilot test campaigns made use of various online sampling and analysis tools. A Differential Mobility Analyzer proved to be effective in providing useful information on H 2 SO 4 aerosol particle concentrations and size distribution. A quantum cascade laser CEMS from ThermoFisher Scientific was able to measure SO 3 during BWRC pilot test campaigns and an FTIR was used to monitor HCl concentrations. DSI solutions make use of pilot and field test results which are incorporated in B&W PGG s CFD modeling tools. References 1. EPA Promulgated Test Methods- Method 8 and Method 26A: 2. TSI Incorporated: 3. Draft Utility MACT (Ref), ENVIRONMENTAL PRO- TECTION AGENCY, 40 CFR Parts 60 and 63, [EPA- HQ-OAR ; EPA-HQ-OAR , FRL ], RIN 2060-AP52 - National Emission Standards for Hazardous Air Pollutants from Coaland Oil-fired Electric Utility Steam Generating Units and Standards of Performance for Fossil-Fuel-Fired Electric Utility, Industrial-Commercial-Institutional, and Small Industrial-Commercial-Institutional Steam Generating Units 4. Cross State Air Pollution Rule (CSAPR): epa.gov/airtransport/ Acknowledgement The authors would like to acknowledge the Babcock & Wilcox Research Center engineers, scientists and technicians who have actively participated in test programs, and the B&W PGG CFD modeling experts who utilize data generated to enhance modeling capabilities. BWRC test participants include R. Bailey, J. Warchol, X. Guo, S. Hu, A. Mackrory, L. Mohr, J. Dudley, V. Burch, B. Clark, J. Peterson, J. Fennell, J. Feess, D. Panaia, R. Porter, M. Shea, R. Schreckengost, S. Ulbricht, T. Wilson and T. Bert. B&W modeling experts include R. Wessel and M. Hopkins. The authors would also like to acknowledge Thermo- Fisher Scientific for providing the prototype QCL-based SO 3 CEMS and J. Socha from ThermoFisher Scientific for providing his expertise with the QCL. Babcock & Wilcox Power Generation Group 7

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