TAILORED CALCIUM OXIDE SORBENT REACTIVATION PROCESS AND PILOT PLANT SORBENT PROPERTIES

Transcription

1 TAILORED CALCIUM OXIDE SORBENT REACTIVATION PROCESS AND PILOT PLANT SORBENT PROPERTIES T. Thomas The Ohio State University Columbus, Ohio Abstract The work described in this paper provides an overview of a sorbent tailored to produce ideal SO 2 sorbing properties for coal-fired flue gas. The sorbent is produced by the Ohio State Carbonation Ash Reactivation (OSCAR) process. The status of a pilot scale process under development at The Ohio State University to evaluate the use of the tailored calcium sorbent to remove SO 2 and trace elements in a utility boiler is described briefly. Recent findings with respect to Hg++, and properties of the sorbent initial produced at the pilot plant are shown. Introduction Background of calcium based scrubbers In the 1970's, regulation of power plants moved rapidly forward. Prior to the establishment of the federal EPA, regulation of power plant emissions had been accomplished at the state/local level, frequently by local health departments, and sulfur dioxide regulations generally limited the sulfur content of fuels. In the early 70's, the Clean Air Act move primary regulation of sulfur dioxides from power plants to statelevel agencies, and in the late 70's, the federal EPA imposed standards of performance on new sources that essentially required the use of scrubbers for new plants.

2 Several technologies were available at the time, and these can be roughly grouped into the categories below. While the following list focuses on calcium-based sorbents, other (more costly) materials may be used as sulfur dioxide sorbents and are not included in the discussion. Wet Limestone These scrubbers use a gas-liquid contactor to scrub sulfur dioxide from flue gas. The solution is reacted with limestone to create a calcium sulfite precipitant. Post-reaction oxidation may be used to convert the precipitant to calcium sulfate. While other technologies were possibly superior, in the 70's wet scrubbing offered reasonably assured 90% control using inexpensive limestone at Ca/S ratios close to 1 Semi-wet, Lime Lime spray driers were an alternative to wet limestone scrubbers, and were proven in other industries. With an LSD, lime is slaked, and a solution of water and dissolved lime is sprayed into the tail gas. The contacting tower provides about 2 seconds residence time for the reaction. Reaction products and fly ash are removed after the LSD with particulate control technology. In order to achieve 90% control, Ca/S ratios have to be significantly larger than 1. During the 70's, the cost of lime was 3-4 times higher than limestone, and with the energy crunches of the 70's, the cost of lime (which is produced by heating limestone) was expected to increase significantly in the following decades. Thus, the utility community

3 preferred the projected cost structure of Wet Limestone systems over the LSD systems, even though they were more complex and expensive. The intervening decades did not prove out lime cost projections. The lime industry moved to much more energy-efficient calcining operations and even to coal use rather than natural gas. At the same time, demand for lime in paper-making diminished. The net result is that lime, today, has nearly the same price as it did in the 70's. Dry limestone There are two typical ways to use limestone in a fully dry process. One uses limestone in a fluidized bed combustor, where sulfur dioxide is sorbed as it is generated. The other, which has attracted scores of researchers, has been the introduction of limestone postcombustion, allowing the (existing) ductwork to serve as the reaction vessel. The first use is well known and is typically found in combustors in Europe Fluidized bed temperatures are typically lower than pulverized coal combustors, and as a result NOX emissions are also much lower. The second use has never demonstrated its promise. For a number of reasons, calcium used in this matter does not fully react with sulfur, with the consequence that the Ca/S ratio of such systems is typically over 4 just to achieve a 50% reduction of sulfur dioxide. Achievement of 90% control was more easily achieved in wet systems. Findings about limestone Even though the use of wet limestone scrubbers has become the dominant sulfur dioxide removal technology in the United States, the potential for the lower cost and complexity

4 of a dry limestone system has lead to significant further research and some further development. Calcination and Sintering Naturally occurring limestone has some porosity, and tests generally show the pores to be micro-scale pores, less than say 10 A in effective diameter. When limestone introduced into the combustion or post-combustion zones of a furnace (boiler) the heat quickly calcines limestone into lime by driving off CO 2. The release of carbon dioxide opens up additional porosity in the residual lime for naturally occurring lime, most of the pores are also micro-scale. Sulfation may occur on the surface of the lime or within the pore structure (which contains most of the surface area) If the temperature is too high, the lime sinters, and the porous structure is lost, limiting the ability of the lime to react with sulfur dioxide in the brief time available in a boiler. Sulfation layer The sulfation reaction forms a product layer on the surface of the lime particles. The product layer may bridge the pores, or fill the pores, rendering the pore surface area unavailable to further sulfation. And as the layer thickens, the reaction between calcium metal in the lime and the sulfur dioxide in the flue gas slows. Key finding - free ca++ migration thru calcium With research supported by the Ohio Coal Development Office of the Ohio Department of Development, the Chemical Engineering Department of the Ohio State University began an extensive investigation into the effectiveness of limestone for sulfur dioxide

5 control beginning around Up until this work, researchers had assumed that the product layer formed at the interface of the lime and the product layer, and therefore the product layer inhibited the reaction by serving as a barrier to the migration of sulfur dioxide to the surface of the particle. In two separately constructed radio-tracer experiments, OSU was able to turn that paradigm around. It is now generally accepted that sulfation occurs on the surface of the product layer, which in turn inhibits the migration of calcium and oxygen ions from the surface to the particle through the product layer to the flue gas interface. Other work, using an ultra-fast reactor designed and built by OSU, carefully began unraveling other findings about lime sulfur dioxide reactions. One interesting finding is that the size of the porosity is a significant determinant of the lime reactions. Very small, micro-scale pores, as discussed before, either bridge or plug, rendering the unreacted calcium underneath to be unavailable to the reaction. More ideal porosity is found in larger, meso-scale sizes. Conceptually, the ideal pore size and distribution of porosity would be found in a condition such that the free calcium available between pores could be fully consumed from the pore side walls just as the covering product layer became thick enough to inhibit the reaction. While such a condition is theoretically amenable to mathematical analyses, the real world does not distribute pores uniformly, nor does it cause pores to be of uniform size. Experimentally, OSU has found that pores need to be above 20 A in effective diameter, but less than 200 A. And for a given effective pore size, OSU has found that the higher the specific surface area, the better the reaction proceeds.

6 Patented Processes The foregoing discussion applies to the porosity of calcined lime, which is turn is driven by the underlying structure of the limestone. Most limestone has derived from the formation of calcium carbonate crystals in open oceans, followed by sedimentation and several hundred million years of heat and compression. It is not surprising, then, that lime produced from such limestone does not have the morphology desired for sulfur dioxide capture. OSU found that a product called precipitated calcium carbonate (PCC) had been introduced into the market. PCC is made by the (slow) solution of slaked lime into water, coupled with the (fast) reaction of the dissolved calcium with bubbled carbon dioxide. The produced calcium carbonate then precipitates out of solution, where it can be dried and post-processed. The produced PCC can have various forms of crystalline grains, with differing porosities and specific surface areas, simply by changing reaction temperature and by using small quantities of surfactants to reduce the surface charge on grains. Without a surface charge, grains tend to agglomerate and precipitate forming large pores, while with a surface charge, grains tend to grow as crystals prior to precipitation. Sorbent from lime Laboratory testing confirmed the efficacy of PCC in the removal of sulfur dioxide from flue gas (for example, 70 percent conversion of PCC in 500 ms for PCC v 20 percent conversion for natural limestone). Laboratory work also demonstrated that the PCC

7 would scavenge selenium (in the form of calcium selenite) and arsenic (in the form of calcium arsenate). OSU filed for and received a patent for the production and use of PCC for the removal of sulfur dioxide and heavy metals from flue gases, and developed preliminary designs for the a PCC scrubber that promised significant potential savings compared to wet scrubbers. Sorbent from ash One of the potential stumbling blocks to the use of a pure PCC was uncertainty with respect to the cost of sorbent that begins with lime. Limestone has a delivered price of ~ $15/ton, while lime has a delivered price of ~$70 per ton. Because PCC is produced with lime, PCC would have to be priced above the cost of lime, and OSU advisors suggested that other sources of lime be found 1. One such source is the lime available in lime spray drier ash. Lime spray driers use a surplus of lime in order to achieve acceptable sulfur dioxide reduction rates. The lime spray drier used at Ohio State University produces an ash consisting of about 20-25% CaO equivalent. The remainder of the ash is about 50% coal ash, and 25% calcium sulfate. 1 Actually, the cost has to be expressed on a common basis. Limestone at $15/ton can be represented as $37.50 per ton mole of calcium. Lime at $60 can be represented as $84 per ton mole calcium. Calcium carbonate requires 168 kj/gram mole to calcine, or roughly 4 mmbtu/ton mole calcium. (Clean) heat being worth around $4/mmBTU, the raw material/energy cost to manufacture lime appears to be around $53.50/ton mole. The heat of calcination is lost in the OSCAR precipitation step. If the PCC is injected into a boiler, it again calcines, absorbing heat. However, the heat release from sulfation is about three times greater than the heat of calcination, and so the net energy balance provides a release of heat If the heat is captured, (in the economizer, say) a heat credit should properly be accounted for.

8 OSU tested the use of CaO-rich ash in a manner similar to lime. That is, the ash stream was solubilized (at least the soluble portion) and bubbled with carbon dioxide to produce a PCC precipitant. The resultant material was found to also effectively capture sulfur dioxide. In fact, the capture is in excess of what would be predicted by the underlying PCC. Sulfur is also captured by the chemical processes within the coal ash. The OSCAR process and status Design and Schedule of Pilot Scale Process A pilot-scale demonstration project is being developed at OSU to address several questions key to the potential commercial use of the tailored sorbent. The specific objectives of the project are to: 1) Demonstrate a novel ash/spent sorbent reactivation technique based on a patented carbonation precipitation process, for increased SO 2 and arsenic removal from combustion of high-sulfur coals; 2) Integrate the ash reactivation process with SCR [selective catalytic reduction for nitrogen oxides] technology; 3) Analyze solid by-products under different desulfurization process configurations; 4) Demonstrate effective utilization of solid by-products in various construction project 5) Quantify process economics; 6) Gather operational and design data that will allow integration of this technology at commercial scale.

9 THE PILOT SCALE PROCESS In this project, a sidestream from the McCracken Power Plant (OSU's power plant) coalfired boiler (nominal capacity, 5 tons coal per hour) will be used. Fly ash mixed with spent FGD materials from the existing (and typical) lime spray dryer will be hydrated, carbonated, dried, and used to simulate (separately) a retrofit coal combustor with convective pass sorbent injection and a retrofit circulating fluidized bed reactor with sorbents.. A simplified schematic of the flow chart of the process is shown in Figure 1. Figure 1. Schematic of Pilot Scale Process Ball mill crusher Lime silo 2 S2 24 C S3 1 B D 8 LSD Stack A S K 6 25 E S4 7 F S5 Baghouse S6 0 To ash disposal 11 Legend: A Riser Sox Reactor B Cyclones C Air Heater D SCR Reactor (with Soot -blower) E Air Heater F Filter S# Sampling location S1 From the Furnace Existing Bottom-ash Disposal line G Slurry Reactor H Water holding tank I Filtration Unit J Solids Dryer K Solid Hoppers Gas Tap Solid Tap Gas/Solid Tap (Recommended venting to main Breaching) J I H Filtration unit Detailed schematic of OSCAR process as applied to McCracken Plant G

10 This diagram shows (unshaded) the existing lime sprayer dryer, bag house, and stack. The modifications (shaded) show (on the left-hand side) a "riser reactor" and cyclone. On the top of the shaded area is equipment for the selective catalytic reduction (SCR) testing. The right hand lower side shows the carbonator and dryer for the tailored sorbent. The "riser" reactor, for scale, is about 30 feet high and 2 feet in diameter. Heat removal for SCR testing will simulate steam tubes in boilers. All process residuals are returned back to the existing system. The process simulation will be tested in detail for process chemistry throughout the treatment system, and for a wide range of varying conditions. PROCESS TESTING The process shown in Figure 1 has substantial flexibility for testing the ability of the tailored sorbent. In one basic configuration, flue gas and solids pass through the riser reactor (A) with the tailored sorbent injection at the bottom of the reactor. This configuration simulates a typical pulverized coal combustor with sorbent injection. In the other basic configuration, flue gas solids are captured in the cyclone (B) and re-injected into the bottom of the riser reactor (A) along with tailored sorbent. This configuration simulates a circulating fluidized bed combustor. The testing program will evaluate both basic configurations over a range of inlet (point "0") conditions and recirculation rates. Two separate coal types will be tested. The results of this testing will show the effectiveness of sorbent treatment of flue gas over a wide range of operating conditions (flow rate, temperature, high and medium sulfur coals), and for two separate boiler configurations.

11 The process provides for the testing of an SCR in conjunction with SO 2 removal. Testing of the SCR will include variation of SCR inlet temperature (3), ammonia injection rate (5), and SCR catalyst integrated with the process testing. The results of this testing is expected to show the effectiveness of an SCR operating in conjunction with the OSCAR (Ohio State Coal Ash Regeneration) process when used with medium to high sulfur coal, and the potential effects of poisoning or blinding of catalyst material by the flue gas and sorbent. Spent flue gas sorbent material (from the existing dry scrubbing system) will be used as the source material for the tailored sorbent generation. The spent sorbent will enter a slurry reactor (G) for hydration and carbonation (using flue gas as a CO 2 source). Tailored sorbent and non-soluble ash will precipitate out and will be dried (I+J) for injection into the riser reactor (A). Process controls will allow tuning of the regeneration process for optimal process parameters. In addition, the process will provide for use of fresh lime as an additive to the spent flue gas. This testing will provide a comprehensive basis for design and evaluation of full-scale sorbent production systems. Planned testing of the process includes measurement of Ca, S, As, and Se species at up to 6 sampling locations (S1-S6); ammonia at S4; and particulate/so 2 /NO X at S4. The selection of these points and analytes was made for the purpose of process optimization and evaluation.

12 CCP TESTING The coal combustion products (CCPs) generated from the process (S5 and S6) will be evaluated to document how process changes affect the chemical, physical, and engineering properties of the CCPs in relation to various beneficial re-use applications. The possibility of whether the process can be tuned to provide CCPs with preferential properties will be tested. The engineering properties to be tested include as-collected moisture content and grain size distribution, optimal moisture content and dry density, unconfined compressive strength, permeability, swell and consolidation, and the effect of freeze-thaw on engineering properties. After optimal process conditions are identified, larger samples will be collected for two field demonstrations, one to evaluate flowable fill and the other structural fill. Mineralogical testing will be conducted on these samples. The results of these tests will be a comprehensive database of CCP engineering properties as produced over a wide range of operating conditions. Chemical analyses will be performed to determine the elemental composition and leaching behavior of the spent OSCAR FGD. Organic characterization of CCPs will be performed for products of incomplete combustion such as dioxins, furans and PAHs to determine how the sorbent alters their formation. Both organics and inorganics evaluations will consider the effectiveness of the sorbent in the capture of selected chemical species and the ability of the sorbent to chemically or physically bind the species when the spent sorbent is re-used and placed in the environment.

13 PROJECT SCHEDULE The original schedule for the project is provided for 3 years, start to finish. The project began in March 2000, and remained on schedule until the summer of As the process testing began, a lightning strike "fried" the electronics of the boiler's continuous emission monitoring system. On shutdown, other problems with the boiler were found. The boiler was shut down and is scheduled for restart in With this schedule, chemical and physical characterizations will essentially begin in January A parametric testing program will evaluate the pilot scale equipment over a range of test conditions (as previously identified). Following the parametric testing program, the pilot scale equipment will be configured to the conditions representing process optimals, and the equipment will be operated over an extended long-term test program during Findings to date The OSCAR system generated around 50 pounds of regenerated. Testing of the sorbent to date has shown the specific surface area of the sorbent (based on calcium) to be higher than the initial laboratory work suggested, and that the increased surface area is in the desirable pore size range. Other OSCAR reactivity Other publications have shown that the sorbent effectively scavenges selenium and arsenic, two potential SCR poisons. Recent laboratory work on pure sorbent and its capacity to capture mercuric chloride has shown that the sorbent may have 10 times the

14 capacity of activated carbon, suggesting that the economic use of the sorbent for mercury control is promising. Possible applications Various configurations of the OSCAR process have been considered, and include: Retrofit systems for sulfur dioxide and mercury control on the existing fleet of unscrubbed power plants. Pending Clear Skies proposals suggest that about 75% of the existing fleet will require sulfur controls, where only 25% are now scrubbed. A retrofit system (with offsite production) offers a low cost, low footprint means of meeting Clear Skies requirements Retrofit trimmers for sulfur dioxide and mercury control on the existing fleet of scrubbed power plants. Pending Clear Skies proposals suggest that most power plants will require mercury control. It is desirable to remove the mercury before it reaches the (existing) scrubber. A retrofit system (with offsite production) offers a low cost, low footprint means of achieving this goal while at the same time increasing the total sulfur dioxide capture.. Retrofit/new trimmers on fluidized bed combustors. FBC combustors are used because of low nitrogen oxide emissions. A desire exists to increase operating temperatures of FBC s, and doing so would increase nitrogen oxide emissions as well as sulfur dioxide emissions. The sulfur dioxide emissions can be controlled within the FBC by adding addition calcium, which in turn catalyzes the formation of addition nitrogen oxides. A post control trimmer would avoid these problems, while using FBC ash as the source of free calcium, and further while providing mercury controls

15 Controls for repowering projects. In repowering, a partial combustor is provided at the front end of an existing boiler. Product gas is sent to a turbine. The existing boiler receives partially combusted coal and turbine waste heat. Because it is possible to run a turbine without removing sulfur compounds, the boiler can oxide the various sulfur species and a post combustion OSCAR process can be used to remove sulfur and mercury. Conclusions As-mined calcium sorbents have morphologies that are not ideal for removal of sulfur oxides and other flue gas constituents. The morphologies can be tailored to produce sorbents with the desirable properties of high specific surface area, meso-scale pores, and small grains. The use of tailored calcium sorbents has been shown to be beneficial in laboratory testing to more effectively remove sulfur oxides as well as arsenic and selenium, and its process economics look promising compared to other forms of sulfur control. A demonstration project has been constructed to evaluate the sorbent performance in a coal-fire boiler on a prototype scale, and to comprehensively evaluate the resulting CCP's. Testing work will begin in January, In the interim, various configurations of the OSCAR process are being defined, and potential relationships are being explored for further commercialization. Acknowledgements This paper presents work which is being is co-sponsored by the Ohio Department of Development's Ohio Coal Development Office and The Ohio State University under

16 Grant Agreement CDO/D The research is being conducted at OSU's Department of Chemical Engineering, Department of Civil and Environmental Engineering and Geodetic Science, and the McCracken Power Plant. The assistance provided by the OSU Engineering Experiment Station and OSU Research Foundation is appreciated. References Used R. Agnihotri, S. Chauk, S. Mahuli, L.-S. Fan, Sorbent/Ash Reactivation for Enhanced SO 2 Capture Using Novel Carbonation Technique", Ind. Eng. Chem. Res., 38, Borgwardt, R.H., Bruce, K.R., Blake, J. "An Investigation of Product-Layer Diffusivity for CaO Sulfation". Ind. Eng. Chem Res., 26, Electric Power Research Institute, "Economic Evaluation of Flue Gas Desulfurization Systems", EPRI GS-7193, Feb A. Ghosh-Dastidar, S. Mahuli, R. Agnihotri, and L.-S. Fan, "Investigation of High- Reactivity Calcium Carbonate Sorbent for Enhanced SO 2 Capture, Ind. Eng. Chem. Res., 35(2), 1996b. Gullet, B. K., Bruce, K. R., "Pore Distribution Changes of Calcium Based Sorbents Reacting with Sulfur Dioxide". AIChE J., 33, 1987 OCDO (Ohio Coal Development Office of the Ohio Department of Development) Grant Agreement CDO/D-98-18, S. Mahuli, R. Agnihotri, S. Wei, S. Chauk, and L.-S. Fan, Pore Structure Optimization of Calcium Carbonate for Enhanced Sulfation", AIChE J., 43(9), S. Mahuli, R. Agnihotri, S. Chauk, A. Ghosh-Dastidar, and L.-S. Fan, "Mechanism of Arsenic Sorption by Hydrated Lime", Env. Sci & Tech, 31(11), Wei, S. -H., Mahuli, S. K., Agnihotri, R., Fan, L. -S., "High Surface Area Calcium Carbonate: Pore Structural Properties and Sulfation Characteristics". Ind. Eng. Chem., 36, U.S. Patent and Trademark Office, Patent Number 5,779,464, "Calcium Carbonate Sorbent and Methods of Making and Using Same", 1998.