Ruth Ann Yongue Roxann Laird Senior Engineer Assistant Project Director rayongue@southernco.com rfleonar@southernco.com (205) 670-5088 (205) 670-5863 Southern Company Services Power Systems Development Facility P.O. Box 1069 Wilsonville, AL 35186 GASIFICATION OF HIGH MOISTURE MISSISSIPPI LIGNITE AT THE POWER SYSTEMS DEVELOPMENT FACILITY December 2010 INTRODUCTION Lignite coal, a resource which remains largely untapped in many areas such as the U.S. Gulf Coast region, is a fuel that can substantially impact future energy generating strategies given its abundance, low cost, and accessibility. However, the high moisture and ash contents and low heating value of lignite limit its use in many power generation designs. One viable option for power generation from lignite is integrated gasification combined cycle (IGCC) technology using the Transport Integrated Gasification (TRIG TM ) process, which is particularly well suited to utilize low rank coals. The TRIG process was developed at the Power Systems Development Facility (PSDF), a flexible test center sponsored by the U.S. Department of Energy and operated by Southern Company Services. The gasification process features the Transport Gasifier, a dry-feed, non-slagging fluidized bed gasifier, which operates at lower temperatures than other commercially available coal gasifiers. Because of the high volatility of low rank coals, high carbon conversions are achieved with the TRIG process without the operational and maintenance issues arising from higher temperature operation. The use of an efficient fluid bed coal dryer and innovative, low-maintenance dry ash removal equipment further optimize the process for low rank fuels. While the PSDF gasification process has operated with a variety of coals, testing of lignite coals has been a major focus, and nine test campaigns utilizing lignite coals have been conducted since 2003. Four of these test campaigns featured high moisture lignite from the Mississippi Red Hills mine. This lignite proved easily gasified, yielding carbon conversions averaging over 99 percent and syngas heating values adequate for operation with a commercial turbine. The successful testing at the PSDF provided the design basis for a 582 MW TRIG plant to be constructed at the Mississippi Liberty Fuels lignite mine. PROJECT DESCRIPTION The PSDF is a key national asset for ensuring continued, cost-effective, environmentally acceptable energy production from coal. The facility is an engineering scale test center located in Wilsonville, Alabama. Since the facility began operation in 1995, the PSDF staff has effectively met the major project objectives related to advanced coal-fired power systems. Building on its previous success, PSDF now houses the National Carbon 1
Capture Center to address the nation s need for cost-effective, commercially viable CO 2 capture options from conventional pulverized coal power plants and from the new generation of coal gasification power plants. While current research at the PSDF focuses largely on developing new CO 2 capture technologies, related work continues in areas specific to IGCC processes, such as gasifier fuel flexibility and plant capital cost reduction. The PSDF routinely operates its gasification process, represented by Figure 1, which contains key components of an IGCC plant. These components include coal preparation and dry feed systems, a Transport Gasifier, syngas coolers, a hot gas particulate filter, continuous ash depressurization systems, and a recycle syngas compressor. The facility also accommodates slipstream units to test various syngas conditioning technologies. TRANSPORT GASIFIER SLIPSTREAM TESTING SYNGAS RECYCLE COAL SYNGAS COOLING PARTICULATE FILTRATION SYNGAS COOLING COAL PREPARATION AND FEED AIR / OXYGEN STEAM FINE ASH REMOVAL ATMOSPHERIC SYNGAS COMBUSTOR COARSE ASH REMOVAL ASH DISPOSAL STACK DISCHARGE Figure 1. PSDF Process Flow Diagram. Coal Dryer. The coal preparation equipment includes a fluid bed coal dryer system, which is essential for stable process operation with high moisture coals. The system was added after the initial testing of Mississippi lignite, as the existing equipment proved incapable of drying the high moisture lignite (about 40 wt% moisture) sufficiently for reliable coal feeding. Figure 2 provides a flow diagram of the fluid bed dryer system, which was manufactured by Schwing Bioset, Inc. This system offers economic advantages in commercial applications, as it uses waste heat to increase the process gas temperature. Also, the dryer does not require internal moving parts, and it operates with high thermal efficiency, using less energy per pound of water removed than conventional drying methods. It is capable of drying even higher moisture content fuels (higher than Mississippi lignite) by reducing coal throughput, increasing hot water supply temperature, and/or increasing hot water supply flow rate. 2
WET COAL FEED COOLING WATER CONDENSER FEED BIN BAG HOUSE CIRCULATING WATER HOT WATER RETURN HOT WATER SUPPLY DRYER NITROGEN DRIED PRODUCT OVERSIZE TO MILLING Figure 2. Fluid Bed Coal Dryer for the PSDF Gasification Process. HOT WATER RETURN Transport Gasifier. The Transport Gasifier is a pressurized, circulating fluidized bed reactor presented in Figure 3. Key features of the Transport Gasifier include: Simple design based on technology in use for 70 years which does not require expansion joints Equally effective gasification in either air- or oxygen-blown modes of operation, making it suitable for power generation or production of liquid fuels and chemicals High reliability non-slagging design, which allows a 10- to 20-year refractory life Operation without burners for high reliability and minimal maintenance Use of coarse, dry coal feed, which requires fewer, lower power START UP BURNER RISER UPPER MIXING ZONE FIRST SOLIDS SEPARATION DEVICE COARSE ASH REMOVAL SEAL LEG J LEG LOWER MIXING ZONE SYNGAS TO COOLING AND FILTRATION SECOND SOLIDS SEPARATION DEVICE Figure 3. Schematic of Transport Gasifier. STANDPIPE 3
pulverizers, and less drying than other dry-feed gasifiers Cost-effective operation and high carbon conversion with high moisture, high ash, and low rank fuels, including subbituminous and lignite coals Excellent heat and mass transfer due to a high solids mass flux, with a solids circulation rate up to 100 times greater than the coal feed rate Ash Removal Systems. Researchers on-site successfully developed proprietary continuous ash removal systems that feature: Reliable and continuous operation High capacity No moving parts and no pressurizing gas Low vent flows of clean gas Solids transport using inherent gas These systems include one for removal of coarse ash from the gasifier, the Continuous Coarse Ash Depressurization (CCAD) system, and one to process ash from the particulate filter, the Continuous Fine Ash Depressurization (CFAD) system. The two systems operate using pressure differentials to move the solids, controlled in pressure let-down devices. Heat transfer is achieved by heat exchange tube bundles. Operating conditions for the systems are given in Table 1. One of these systems is shown in Figure 4. Figure 4. CFAD System. PRESSURE LET DOWN DEVICE ASH TRANSPORT Table 1. Operating Conditions for Continuous Ash Removal Systems. CCAD CFAD Total Operating Hours 7,400 11,000 Ash Flow Rate, lb/hr 100 to 1,500 150 to 6,000 Inlet Temperature, F 600 to 1,600 400 to 800 Inlet Pressure, psig 160 to 260 150 to 250 Particle Size MMD, microns 70 to 850 5 to 20 MISSISSIPPI LIGNITE The coals found in Mississippi are lignite coals, which are brownish black coals, as seen in Figure 5, with heating values between those found in peat and subbituminous coals. Mississippi lignite is included in the Gulf Coast Region, a band of lignite that extends from south Texas through Louisiana, Arkansas, Tennessee, Mississippi, and into central Alabama (see Figure 6). Mississippi lignite resources equal about Figure 5. Mississippi Lignite. 13 percent of the total U.S. lignite resources of 40 billion tons. Typically, lignite beds or seams which can be mined economically range from 2 to 9 feet in thickness. Estimates of the total lignite resources for Mississippi in lignite beds 2 feet thick or greater and less than 200 feet deep are 5 billion tons. Mississippi lignite is 4
expected to be used primarily for electricity generation in the near future (www.osmre.gov). Figure 6. U.S. Lignite Reserves in the Gulf Coast Region. The Mississippi lignite tested at the PSDF originated from the Red Hills mine. Selected coal properties of the lignite are listed in Table 2. Table 2. Typical Red Hills Mississippi Lignite Properties. As-Received 5,700 11 42 33 0.4 11 2 Higher Heating Value, Btu/lb Ash, wt% Moisture, wt% Carbon, wt% Sulfur, wt% Oxygen, wt% Hydrogen, wt% As-Fed 7,900 15 19 45 0.5 15 3 TEST RESULTS The PSDF gasification process has operated with Mississippi lignite during four major test runs for a total duration of 2,300 hours. Table 3 lists these test runs. 5
Table 3. Mississippi Lignite Test Runs at the PSDF. Test Run Test Date Duration, hours TC22 March 2007 543 TC25 July 2008 742 R01 January 2009 510 R04 April 2010 509 Fluid Bed Coal Dryer. During the first Mississippi lignite test (which was conducted prior to the addition of the fluid bed dryer system), the high moisture content of the as-fed lignite caused numerous coal feeder trips. High moisture coal is especially problematic for dry, pressurized feed systems, as the coal moisture binds particles together, inhibiting flow from the feeder lock hopper to the pressurized dispensing vessel. Other coal characteristics, such as particle size and particle morphology (generally specific to coal type), can effect coal feeder operation, but comparisons of high moisture lignite feed system operation both before and after the fluid bed dryer installation suggest that moisture content was the leading factor affecting coal feeder performance. After installing the fluid bed dryer, coal feeder reliability significantly improved. Operation of the fluid bed coal dryer with Mississippi lignite achieved the design target for product moisture content, averaging about 19 wt% with no operational issues. Figure 7 demonstrates the consistency of the dryer operation. The operating envelope (pressure, temperature, and flow rates) was fully established with an understanding of the relationship between coal moisture and bed outlet temperature and an understanding of bed fluidization characteristics on product size and moisture content. The system proved easily operable in automatic mode. The dryer feed rate was varied from 5,500 to 14,000 lb/hr. Figure 7. Mississippi Lignite As-Fed Coal Moisture Content with and without Fluid Bed Dryer Operation. 6
Transport Gasifier. Operation of the Transport Gasifier with Mississippi lignite showed that the high reactivity of the coal is a good match for the relatively low temperature and minimal residence time of the gasifier. Carbon conversion for the test runs are given in Figure 8. The syngas heating value, adjusted for a commercial process (subtracting excess nitrogen content, heat losses, and water content) was expected to range from 125 to 150 BTU/scf, which is suitable for commercially available IGCC gas turbines. Figure 8. Steady State Carbon Conversions during Mississippi Lignite Testing. To establish boundary conditions and optimize Transport Gasifier operation with high moisture lignite, a number of parametric tests were performed. Except where noted, the parametric test results presented below were obtained from TC22, TC25, and R01 steady state operating periods selected when the coal feed rate and the air-to-coal mass ratio fell in a small range (4,100 ± 300 lb/hr and 2.9 ± 0.4 lb/lb, respectively). The duration of the steady state operating periods was typically five hours, and during these periods, the coal, steam, and air/oxygen feed rates, system pressure and temperature, carbon conversion, and syngas lower heating values were generally constant. Figure 9 plots the carbon conversion versus gasifier temperature for R01 and the preceding Mississippi lignite test (TC25). The figure shows that carbon conversion was not a strong function of temperature, and that high carbon conversions can be achieved at a range of temperatures due to the extremely high reactivity of the fuel. 7
Figure 9. Carbon Conversion as a Function of Gasifier Temperature during Lignite Operation. Figure 10 plots the gasifier circulation rate as a function of standpipe level during lignite operation. This figure shows some spread of data (particularly at around 190 inh 2 O standpipe level) due to other operating factors such as fluidization flow in the standpipe and J-leg. In general, though, the data showed a positive correlation and demonstrated good controllability of the gasifier. Figure 10. Gasifier Circulation Rate as a Function of Standpipe Level. In Figure 11, the gasifier differential temperature (the lower mixing zone temperature minus the outlet temperature) is plotted as a function of the circulation rate. As demonstrated by the data, more uniform and overall higher gasifier temperatures are achieved at higher circulation rates. Optimizing the circulation rate results in consistent 8
syngas quality and hydrocarbon cracking by providing high temperatures and sufficient residence times. Figure 11. Gasifier Temperature Differential as a Function of Circulation Rate. Ash Removal Systems. The gasification ash resulting from Mississippi lignite operation is low density about 20 to 25 lb/ft 3 and has no tendency to agglomerate, making it suited for processing with the continuous ash removal systems. These systems operated with 100 percent reliability during the Mississippi lignite testing. Particle size distribution curves for Mississippi lignite gasification ash are plotted in Figure 12. The fine ash produced had an average median diameter of 13 microns, and the coarse ash removed from the gasifier had an average median diameter of 88 microns. Percent Passing 100 90 80 70 60 50 40 30 20 10 0 Fine Ash Coarse Ash 0.1 1 10 100 1000 10000 Size, micron Figure 12. Typical Particle Size Distributions of Mississippi Lignite Gasification Ash. 9
Other Equipment. Gasification of Mississippi lignite offered advantages to auxiliary systems as well. While gasification of some higher rank coals in the past has produced syngas containing long chain hydrocarbons or tars that are problematic to downstream equipment, the syngas produced from Mississippi lignite contained essentially no tars. The fine gasification ash produced had very low drag (about 40 for Mississippi lignite as compared to about 110 for Powder River Basin subbituminous coal, in units of inwc/(lb/ft 2 )/(ft/min), i.e., pressure drop divided by areal loading and face velocity). The lower drag ash would require less filter surface area compared to higher drag coal ashes. Biomass Co-Gasification. During the latest Mississippi lignite test run (R04), biomass co-gasification was achieved for the last 100 hours of the run. The biomass fuel consisted of wood pellets purchased from Green Circle BioEnergy. The pellets were milled on-site (to about 850 microns, mass median diameter) and fed at a rate of 12 to 20 weight percent of the combined coal and biomass feed rate. Figure 13 provides a photograph of the milled pellets. During the co-gasification operation, carbon conversion remained high, averaging 98.4 percent during steady state periods. The syngas heating value did not change significantly after co-gasification began. Inspection of the gasifier solids showed no agglomeration, as demonstrated by Figure 14. There were also no operational problems with any of the auxiliary equipment. Figure 13. Milled Biomass. Figure 14. Co-Gasification Ash. FUTURE PLANS FOR COMMERCIAL OPERATION Based on the successful operation of the PSDF Transport Gasification process with Mississippi Lignite, Mississippi Power, a subsidiary of Southern Company, is constructing a TRIG plant to generate 582 MW of electricity. The plant will be located in Kemper County, Mississippi, and will utilize coal from North American Coal Company s new Liberty Fuels mine. The Liberty Fuels mine is approximately 60 miles southeast of the Red Hills mine (see Figure 6) and the lignite coals from the two mines have very similar properties. The planned Kemper County plant, a conceptual rendering of which is provided in Figure 15, will utilize two Transport Gasifier trains, two Siemens SGT6-5000F combustion turbines, and one Toshiba tandem compound double flow steam turbine. The plant is designed for 65 percent capture of the CO 2 produced using a Selexol process. This capture rate will result in emissions equivalent to 800 lb CO 2 /MWh, making it roughly equivalent in CO 2 emissions to natural gas combined cycle units, which emit between 800 and 850 lb CO 2 /MWh. The captured CO 2 will be used for enhanced oil recovery in nearby Jasper County, Mississippi. The plant will also feature greater than 90 percent removal of mercury, 99 percent removal of sulfur dioxide, and 99 percent removal of particulate. 10
Figure 15. Concept Rendering of the Planned Kemper County IGCC Project. Federal funding set aside for clean coal power development will be used to help the project participants overcome the economic hurdles associated with demonstration of first-of-a-kind technology. Mississippi Power has received a $270 million grant from the Department of Energy, $133 million in investment tax credits approved by the Internal Revenue Service (IRS) provided under the National Energy Policy Act of 2005, and loan guarantees from the federal government. Mississippi Power also received an additional $279 million in IRS tax credits. The Kemper County plant is currently under site development. Construction will begin in 2012, and commercial operation is scheduled to commence in May 2014. ACKNOWLEDGEMENTS This material is based upon work supported by the Department of Energy (DOE) under award #DE-FC21-90MC25140 and DE-NT0000749. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE. 11