Clean Energy from Waste and Coal - American Chemical Society

Chapter 7

Biomass-Fueled Gas Turbines Joseph T. Hamrick

Downloaded by CORNELL UNIV on July 25, 2016 | http://pubs.acs.org Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch007

Aerospace Research Corporation, 5454 Aerospace Road, Roanoke, VA 24014

Biomass fueled gas turbines offer a low cost means of generating power in areas where fossil fuels are scarce and too costly to import. This chapter presents results of a ten year research and development effort to advance biomass fueled gas turbines to commercial status. Aside from trees the most promising sources of biomass are sugar cane and sweet sorghum. Fuel alcohol can be produced from the sweet juices and grain for food is produced by the sweet sorghum. As the gas turbine start-up and shut-down effort is much less involved than the start-up/shut-down of a steam turbine power plant, the biomass fueled gas turbine is the preferred system for meeting the peak electrical power demands of any system. As CO2 is extracted from the atmosphere during biomass growth, and ultimate replacement through the gas turbine combustion process is in exact balance, the entire process results in no net increase in CO2 in the atmosphere.

Use of biomass to produce power in the five to thirty megawatt range is attractive because of higher hauling costs associated with power plants with higher outputs. The higher hauling costs result primarily from the low density of biomass. The ability to fuel gas turbine power generating systems is economically advantageous due to the quick start up and shut down capability. This capability allows restriction of operation to periods of peak usage for which higher prices are paid for power. Steam power generation systems by comparison cost substantially more per installed kw output, and they cannot be economically started up and shut down on a daily basis. Research and development of biomass fueled gas turbine systems over a ten year period has culminated in successful operation of an aircraft derivative gas turbine manufactured by the Allison Division of the General Motors corporation. With the cooperation of the Tennessee Valley Authority the power generating system was successfully integrated into their distribution grid for f u l l demonstration of the system as a small power producer using sawdust as the fuel. The Allison gas tur-

0097-6156/93/0515-0078$06.00/0 © 1993 American Chemical Society Khan; Clean Energy from Waste and Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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bine served well i n the research and development phase, but i t i s economically marginal as a power producer i n biomass fueled applications. As a r e s u l t , a larger gas turbine produced by the General E l e c t r i c Company i s the current selection for commercial use. A system using that engine i s being prepared for i n s t a l l a t i o n at Huddleston, V i r g i n i a , where a contract for sale of the output power to the V i r ginia Power Company has been negotiated. The R&D system which i s located at Red B o i l i n g Springs, Tennessee w i l l be upgraded f o r operation with the General E l e c t r i c engine. Sugar cane bagasse, an a l t e r native fuel to sawdust which has been tested i n the R&D f a c i l i t y , promises to be a major source of f u e l i n the future. Sweet sorghum bagasse can serve equally w e l l . Sweet sorghum and sugar cane juices are readily converted to a l cohol by yeast fermentation. Sweet sorghum can be grown throughout the United States as well as the tropic and temperate zones of the earth. These plants have the highest conversions of solar energy into biomass of any of the species i n the plant kingdom, substantially greater than trees. With the use of bagasse as a f u e l for gas turbines in the generation of power, i t i s possible for the income from power sales to reduce the cost of ethyl alcohol well below that for gasol i n e . The sorghum grain can be used for fermentation or food. The high volume, high temperature exhaust gases from the turbine can be used to concentrate the j u i c e , make alcohol, dry the bagasse or generate steam for i n j e c t i o n into the turbine. There i s adequate heat to concentrate the j u i c e and dry the bagasse for year-round use during the harvest period. Growth of sugar cane and sorghum on the 66.4 m i l l i o n acres of land taken out of production i n the U.S. between 1981 and 1988 can supply enough energy to generate 34 percent of the nonnuclear power that was generated i n 1986, enough to supply increased power demands into the next century. At the current rates paid by V i r g i n i a E l e c t r i c Power Company for power generated with renewable fuels, 25.4 b i l l i o n gallons of alcohol can be produced from the p r o f i t s earned on power sales, enough to supply gasohol to the entire nation. The system, which can be located at any point where there i s a power d i s t r i b u t i o n l i n e and a sorghum or sugar cane source, can provide jobs i n the area and an alternative crop for farmers while saving b i l l i o n s of dollars on set-aside payments. At $20/barrel, approximately $8 b i l l i o n could be saved on the trade imbalance by the reduction of o i l imports by use of the set aside acres. Background Information Research on wood burning gas turbines was started by Aerospace Research Corporation i n 1978. It culminated i n the operation of an A l l i s o n T-56 gas turbine power generating system at a f a c i l i t y l o cated i n Red B o i l i n g Springs, Tennessee. Over two m i l l i o n dollars i n U.S. Department of Energy funds and a matching amount i n private funds were spent i n carrying out the program. In addition, gas turbine engines were furnished by the A i r Force and Naval A i r Systems Command. Results of the research and development e f f o r t are provided i n Reference 1. A schematic diagram of the system i s provided i n Figure 1 and a view of the R&D f a c i l i t y i n Figure 2. The gas turbine, elect r i c generator, and e l e c t r i c a l switch gear configuration are the same as that for the thousands of a i r c r a f t derivative gas turbine

Khan; Clean Energy from Waste and Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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COMBUSTOREXHAUST CYCLONE FILTER

Figure 1. Schematic Diagram of Biomass Fueled Gas Turbine Power Generating System

Figure 2.

View of F a c i l i t y at Red B o i l i n g Springs,

Tennessee



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generator sets now i n use throughout the world. The difference i s i n the fuel system. The normal arrangement for the combustion chambers of commercial gas turbines derived from those used on a i r c r a f t i s the same as that used on the a i r c r a f t , namely a configuration sandwiched i n between the compressor and turbine. Because of the slower burning rate for biomass s o l i d s , the greater d i f f i c u l t y i n feeding the f u e l into the combustion chamber, and the need for cleaning the ash from the combustion products, a d i f f e r e n t arrangement i s necessary. To meet that requirement, the space between the compressor and turbine must be adequate to provide room for ducting to take a i r o f f the compressor for movement to an offset combustion system and room for return ducting to the turbine. Both the A l l i s o n T-56 gas turbine used in the R&D system and the General E l e c t r i c LM1500 gas turbine meet that requirement. Meeting that requirement allowed rotating components i n both gas turbines to remain unchanged, thus providing the same mechanical r e l i a b i l i t y for the biomass fuels as for l i q u i d fuels without p r o h i b i t i v e l y expensive modifications. At the outset of the R&D program reports on work involved with s o l i d s fueled gas turbine systems (2-5) were thoroughly reviewed. It was determined from research work with coal performed by the B r i t i s h at Leatherhead, England (2) that single large cyclones operating alone or i n series would be best for ash cleaning. The combustion chamber design was based upon information provided by the Australians i n Reference 3. An i n d i c a t i o n of problems that might be encountered with biomass were provided by the report from Combustion Power Company, Inc. of C a l i f o r n i a (4). From experience gained by Y e l l o t et a l (5) i n coal f i r e d gas turbine research i n the United States on solids feeding i t was learned that attempts to use lock hoppers to feed solids into a combustor resulted i n e r r a t i c operation of the gas turbine. As a r e s u l t , rotary a i r lock feeders, which were used i n a l l of the systems reported i n References 3, 4, and 5> were selected as the feeders i n the R&D program. Operational d i f f i c u l t i e s which resulted i n learning curves peculiar to the system such as wood processing, conveying, drying, combustion ash removal, engine s t a r t i n g , synchronization with the TVA power d i s t r i b u t i o n grid, and development of emergency procedures are covered i n Reference 1. Feeding a pulverized s o l i d into a high pressure chamber and dealing with turbine blade fouling presented the greatest challenge. An anticipated problem that was most feared at the outset, eroding of the turbine blades, never materialized. In over 1500 hours of operation, no erosion has been detected. The measures taken to resolve the two problems and the approach taken with the General E l e c t r i c LM1500 gas turbine i n meeting the problems are presented. Modern a i r c r a f t engines which require very high power to weight r a t i o s are designed for high turbine i n l e t temperatures and high compressor discharge pressures. As turbine blade cooling techniques, advanced materials and more sophisticated design methods have become available the pressure ratios and allowable turbine i n l e t temperatures have increased to high l e v e l s . As a r e s u l t , the modern a i r c r a f t derivative gas turbines are less suitable for operation with biomass than the e a r l i e r models. The current need for low turbine i n l e t temperature and low combustor pressure with biomass makes e a r l i e r models more compatible. The LM1500 gas turbine f i t s well into the biomass picture.


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The Rotary A i r Lock Feeder The rotary a i r lock feeder i s also referred to as a rotary valve. A schematic view of a rotary feeder i s shown i n Figure 1. Referring to Figure 1, the tips and sides of the vanes are f i t t e d with seals that compartmentalize p a r t i c l e s fed into a low pressure sector for movement around to a zone of high pressure and thence into the combustor. A major e f f o r t was directed toward development of long l a s t i n g s e a l ing methods and materials. Sawdust i s an extremely abrasive material that requires special techniques that were developed i n the program. To meet the 130 psig requirement of the R&D i n s t a l l a t i o n two a i r lock feeders operating i n series proved adequate. To provide conservative design margins, i t i s planned to use two feeders i n series for the 90 psig pressure requirements of the Huddleston i n s t a l l a t i o n as well as i n succeeding i n s t a l l a t i o n s up to 6000 kw. Turbine Blade Fouling The primary problem with coal f i r e d turbines was erosion of the turbine blades. A secondary problem was fouling of the blades. In work performed by the Coal U t i l i s a t i o n Research Laboratory at Leatherhead, England (2) i t was determined that single cyclones i n series adequately cleaned the ash from the combustion gases to prevent erosion. Therefore, i t was decided to use only single cyclones i n the wood burning program. As a r e s u l t , there has been no erosion of the turbine blades i n the more than 1500 hours of operation with the gas turbines used i n the R&D program. In the R&D program performed by the Australians (3) on brown coal i t was found necessary to l i m i t the turbine i n l e t temperature to 1200°F to avoid deposition of ash on the turbine blades. In the R&D performed at Leatherhead, England with stationery blades there was no s i g n i f i c a n t deposition at 1450°F after 1000 hours of operation with black coal. In tests with pine sawdust in early operation at Roanoke with a small Garrett turbine no s i g n i ficant deposition occurred at 1450°F i n 200 hours of operation. In tests with the A l l i s o n T-56 at Red B o i l i n g Springs i t was found necessary to p e r i o d i c a l l y clean the turbine blades with milled walnut h u l l s when f i r i n g with a mixture of oak and poplar sawdust at 1450°F turbine i n l e t temperature. Above 1450°F the p a r t i c l e s adhered to the blades and could be removed only by scraping. The 1248°F turbine i n l e t temperature needed to produce 4000 kw with the LM1500 gas turbine i n the Huddleston i n s t a l l a t i o n i s well below any problem zone for disposition with sawdust. Discussion of LM1500 Gas Turbine Performance And Factors Favoring Its Selection When the advancement was made from the Garrett 375 kw gas turbine to a larger engine, the A l l i s o n T-56-9 gas turbine selection was made on the basis of i t s perceived easy adaptability to the system and the a v a i l a b i l i t y of used engines from the U.S. A i r Force. As the R&D program advanced, i t became clear that the turbine i n l e t temperature would have to be r e s t r i c t e d to 1450°F to avoid excessive turbine blade f o u l i n g . The turbine i n l e t temperature of the T-56-9 i s 1700° F at i t s normal rated o v e r a l l e l e c t r i c a l output of 2332 kw. With a



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1450°F turbine i n l e t temperature the output drops to 1500 kw, a value too low for economical operation. A search for a more suitable gas turbine from standpoints of a v a i l a b i l i t y , adaptability to wood fueling, and e l e c t r i c a l output led to selection of the General E l e c t r i c J-79 gas generator and companion power turbine. The combined gas generator and power turbine was given the designation LM1500 by General E l e c t r i c . For a i r c r a f t propulsion the hot gases leave the engine at high v e l o c i t y , ^propelling the a i r c r a f t forward. For use i n power production the hot gases are ducted to a power turbine. A favorable feature of a two shaft arrangement, such as this one, i s that the gas generator can operate e f f i c i e n t l y at part load by adjusting i t s speed downward while the power turbine operates at the required constant speed f o r power generation. The compressor e f f i c i e n c y i s high over a broad range. This i s made possible by adjustment of variable stators i n the f i r s t s i x stages of the compressor. By adjustment of the stators to match the compressor speed and a i r flow, rotating s t a l l i s avoided and good compressor e f f i c i e n c y i s maintained. Rotating s t a l l i s a phenomenon associated with flow separation on the compressor blades as the angle of attack on the blades increases with changes i n rotative speed and a i r flow. Compressor e f f i c i e n c y over the speed range results i n economical operation over a wide range of power production. Detailed information on turbine i n l e t temperature, compressor discharge pressure, and wood feed rate as a function of power output was derived from General E l e c t r i c s p e c i f i c a t i o n MID-S-1500-2. Turbine Inlet Temperature. Figure 3 shows a straight l i n e r e l a t i o n ship between turbine i n l e t temperature and generator output. This c h a r a c t e r i s t i c provides a s i g n i f i c a n t amount of l a t i t u d e i n operation with untried species of plants or sources of fuels such as clean waste. For example, i t can be safely predicted that i n the worst case the turbine i n l e t temperature of 1200°F required for a 3400 kw output w i l l not result i n excessive or d i f f i c u l t to clean accumulations on the turbine blade. Minimum performance guarantees would be warranted i n such cases. With most wood species a 7000 kw output probably can be tolerated. Compressor Discharge Pressure. Figure 4 shows a straight l i n e r e l a tionship between compressor discharge pressure and generator output. The primary concern with pressure i s the feeding of s o l i d f u e l into the combustion chamber. The demonstrated maximum sustained pressure in the R&D system i s 130 pounds per square inch.. Thus, the a b i l i t y to sustain feeding of sawdust from 3000 kw to approximately 7500 kw i s assured. The pressure required to produce the 4000 kw projected for the production f a c i l i t y now being prepared for i n s t a l l a t i o n at Huddleston, V i r g i n i a i s only 90 psig. Wood Feed Rate. The wood feed rate in Figure 5 i s based upon a heat value of 8,200 Btu/lb. for sawdust. The heat value ranges from 8100 Btu per pound for oak to 8,600 Btu per pound for yellow pine. Green sawdust as delivered from the m i l l averages approximately 45 percent water content. T r a i l e r s 40 f t . long normally deliver on the order of 25 tons of green sawdust per load. For the 4000 kw output projected for the Huddleston f a c i l i t y f i v e t r a i l e r loads per day w i l l be required. Shelter for approximately f i f t y truck loads w i l l be needed to assure continued operation i n the winter months.


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Clean Energy from Waste and Coal - American Chemical Society

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