Energy Recovery from Wood Residues by Fluidized-Bed Combustion

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Downloaded by EAST CAROLINA UNIV on January 3, 2018 | http://pubs.acs.org Publication Date: August 29, 1980 | doi: 10.1021/bk-1980-0130.ch007

Energy Recovery from Wood Residues by Fluidized-Bed Combustion J. W. STALLINGS1 Energy Incorporated, P. O. Box 736, Idaho Falls, ID 83401 MIKE OSWALD Energy Products of Idaho, P. O. Box 153, Coeur d'Alene, ID 83814 Dramatic increases in costs of energy during the decade of the seventies have provided the economic incentive for the forest products industry to recover energy from their wood residues rather than purchasing fossil fuels. The Fluid Flame® System was designed by Energy Incorporated and its subsidiary, Energy Products of Idaho, to accomplish this task utilizing fluidized-bed technology. Thirty-one of these units are now in commercial operation burning various types of wood residues and recovering energy in the form of steam, hot gases, or a combination of the two. The total installed output is equivalent to 1.62 x 10 1 2 J/hr (1,540 MM Btu/hr) of thermal energy or 559,000 kg/hr (1,232,000 lb/hr) of steam. Applications have varied according to the energy needs of the individual customer. In steam production, fire-tube boilers are generally employed for systems providing less than 13,600 kg/hr (30,000 lb/hr) of steam, while water-tube boilers are used for larger installations. Either steam or hot gases from the Fluid Flame® systems have been used in plywood dryers, tube dryers, and both direct and indirect hot-gas dry kilns. In one installation, a steam turbine generates up to 2 megawatts of electricity on demand. This amount is equivalent to fifty percent of the total energy available from the facility. Fluidized-Bed Technology The term fluidized bed refers to a layer of sand-like particles suspended by the upward flow of a gas stream. A point of equilibrium is reached at which the upward drag force of the gas is equal to downward force of the weight of the particles. The fluidized gas-solid mixture exhibits a high degree of turbulence similar to boiling water. Additional fluidizing air increases the circulation of the particles and further expands the height of the bed. Fluidized-bed technology is ideally suited to energy recovery from wood wastes for a number of reasons. The high heat 'Current address: SRI International, 333 Ravenswood Ave., Menlo Park, CA 94025 0-8412-0565-5/80/47-130-085$05.00/0 © 1980 American Chemical Society Jones and Radding; Thermal Conversion of Solid Wastes and Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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t r a n s f e r r a t e s and high s u r f a c e areas per u n i t volume ensure v i r t u a l l y complete combustion w i t h i n the burner. The l a r g e amount of energy stored i n the hot bed m a t e r i a l enables combust i o n of f u e l s with r e l a t i v e l y high moisture contents. The abras i v e a c t i o n of the turbulent bed m a t e r i a l continuously exposes unburned surface area of the f u e l to the intense heat and the oxygen supply. Thus, a wide s i z e d i s t r i b u t i o n of f u e l can be fed i n t o the burner without l o s s i n combustion e f f i c i e n c y . Wood residues c u r r e n t l y burned i n commercial u n i t s i n c l u d e plywood t r i m and sander dust, wood shavings, whole l o g c h i p s , bark, and l o g yard wastes. Coal or a g r i c u l t u r a l residues can a l s o be burned i n the system e i t h e r separately or i n combination with other f u e l s . System D e s c r i p t i o n A schematic diagram of a t y p i c a l system f o r steam product i o n , i n c l u d i n g approximate mass and energy balances as w e l l as c a p i t a l and operating c o s t s , appears i n F i g u r e 1. A f t e r screening to remove p a r t i c l e s l a r g e r than a threei n c h nominal s i z e , the wood residues are t r a n s f e r r e d i n t o a live-bottom storage hopper by means of a pneumatic conveyance system. Hopper s i z e s vary with the amount of storage d e s i r e d . Those manufactured by Energy Products of Idaho employ h y d r a u l i c d r i v e s to provide adequate torque and power to a sweep auger. From the bottom of the storage b i n hopper, a screw conveyor t r a n s f e r s the wood residues to a metering b i n . The r a t e of discharge from the storage b i n hopper to the metering b i n i s c o n t r o l l e d by l e v e l i n d i c a t o r s on the s i d e s of the metering b i n . The rate of f u e l fed to the burners from the metering b i n i s c o n t r o l l e d by burner demand. The feed p i p e enters the combustion c e l l from the top and protrudes a t an angle from four to eight f e e t i n t o the vapor space to i n s u r e even d i s t r i b u t i o n of the feed on the bed. The combustion c e l l i s f a b r i c a t e d out of carbon s t e e l and i s l i n e d with high-temperature block i n s u l a t i o n covered by a l a y e r of c a s t a b l e r e f r a c t o r y . The o r i g i n a l combustion systems employed sand as a bed m a t e r i a l . However, excessive a t t r i t i o n n e c e s s i t a t e d the search f o r m a t e r i a l s which were more thermally s t a b l e and chemically i n e r t and thus would not e l u t r i a t e from the bed. C u r r e n t l y , two p r o p r i e t a r y bed m a t e r i a l s are employed. One i s a n a t u r a l l y o c c u r r i n g s t a b l e m i n e r a l , and the second i s a raw m a t e r i a l used i n the production of f i r e b r i c k s . With these improved bed m a t e r i a l s , a t t r i t i o n i s minimal. In f a c t , p a r t i c u l a t e from the wood residues

Jones and Radding; Thermal Conversion of Solid Wastes and Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


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1 0 Kg/hr (11 SHORT T O N S / H R ) AS RECEIVED WOOD RESIDUES AT 5 0 % H_ 3 . 3 7 X 1 0 J/Kg ( 3 , 6 0 0 B t u / l b ) USEFUL ENERGY

Figure 1.

EFFICIENCY = 7 5 % CAPITAL COST * $ 1 . 5 MILLION INSTALLED ( 1 9 7 9 U.S. D O L L A R S ) OPERATING COST « $ 5 0 , 0 0 0 - 1 0 0 , 0 0 0 P E R Y E A R (INCLUDES BED MAKEUP, REFRACTORY REPAIR, POWER, CHEMICALS, AND EQUIPMENT MAINTENANCE) OPERATING T I M E « 8 , 0 0 0 H R S / Y R (91 % )

Flow diagram

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T R A M P MATERIAL REMOVAL SYSTEM

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FLUIDIZING AND COMBUSTION AIR B L O W E R

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COMBUSTION C E L L 8 . 4 4 Χ 1 0 J/hr ( 8 . 0 0 Χ 1 0 B t u / h r ) USEFUL ENERGY

MULTICLONE

EXHAUST

2.72 X 1 0 Kg/hr ( 6 0 , 0 0 0 lb/hr) 1.03 X 1 0 Pa ( 1 5 0 p s i a ) SATURATED STEAM



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o f t e n b u i l d s up i n the bed and n e c e s s i t a t e s removal of p o r t i o n s of bed m a t e r i a l . G r i d p l a t e s were used i n the i n i t i a l u n i t s t o d i s t r i b u t e f l u i d i z i n g a i r to the system. However, the growing need f o r a tramp removal system and the excessive thermal s t r e s s on the p l a t e s l e d to the design of a manifold f o r a i r d i s t r i b u t i o n i n s t e a d of p l a t e s . An i n v e r t e d v i b r a t i n g cone below the m a n i f o l d was i n c l u d e d to g r a d u a l l y remove bed m a t e r i a l from the system i n order to screen o f f l a r g e pieces of i n o r g a n i c s , such as rocks and stones. The screened bed m a t e r i a l i s then r e i n j e c t e d back i n t o the burner. In a f u r t h e r improvement of t h i s design, a double s t a t i c cone assembly u t i l i z i n g g r a v i t y flow has been i n s t a l l e d to cut costs and maintenance. This removal system was designed to allow tramp m a t e r i a l up to 10.2 cm (4 i n . ) i n cross s e c t i o n to be removed from the combustion c e l l without shutdown. Larger pieces cannot f i t i n the spaces between the m a n i f o l d sections. The system described here uses hot combustion gases from a f l u i d i z e d - b e d combustion c e l l i n conjunction w i t h a package water-tube b o i l e r f o r steam production. The combustion gases are then r e l e a s e d to the atmosphere. More recent designs have employed v e n t u r i scrubbers f o r gas cleanup when necessary. However, the f i n a l d e c i s i o n on a gas cleanup method depends on the s p e c i f i c r e g u l a t i o n s f o r each l o c a t i o n . Some advantages of the r a d i a n t e f f e c t on heat t r a n s f e r can be gained by p l a c i n g the b o i l e r d i r e c t l y on top of the combustion cell. Burning takes place both i n the combustion c e l l and i n the boiler. This approach i s e f f e c t i v e with both f i r e - t u b e and water-tube b o i l e r s . The F l u i d F l a m e ® energy systems are designed to run thems e l v e s without the need f o r f u l l - t i m e operators. The system i s f u l l y automated and i s equipped w i t h annunciators to a l e r t operators when problems a r i s e . A number of e l e c t r i c a l process c o n t r o l l e r s a r e used to maintain constant temperatures i n the bed and i n the vapor space of the combustion c e l l i n order to prov i d e steam a t constant pressure. S i g n a l s from these c o n t r o l l e r s d i c t a t e the feed r a t e from the metering b i n and the amount of excess a i r f e d i n t o the system. The temperature i s c o n t r o l l e d i n order to maintain constant parameters throughout the system and to keep temperatures below the s l a g g i n g temperatures of the ash. Normal temperature v a r i a t i o n i s l e s s than +5C° (9F°), and the response time to process v a r i a t i o n s i s r a p i d . The maximum turn-down r a t i o f o r an operating system i s threeto-one. Operation i n an on-off mode can a l s o be employed to e f f e c t i v e l y increase t h i s r a t i o . Another approach i s the use of modular u n i t s . However, t h i s l a t t e r o p t i o n w i l l increase c a p i t a l costs considerably. The advantages of a f l u i d i z e d - b e d system over conventional systems are many. Wood residues with moisture contents as l a r g e as 63 percent can be burned without the need of a supplemental


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f u e l , other than during s t a r t u p . The absence o f grates removes a major maintenance problem, as does the use of an a i r d i s t r i b u t i o n system w i t h i n the f l u i d i z e d bed. The l a r g e heat s i n k i n the bed allows f o r automatic s t a r t u p a f t e r shutdowns of up to s i x t e e n hours. The o l d e s t F l u i d F l a m e ® system has been i n commercial opera t i o n f o r approximately s i x years. The o r i g i n a l r e f r a c t o r y i s s t i l l i n place but a new c o a t i n g was added a f t e r f i v e years. With improved techniques i n r e f r a c t o r y i n s t a l l a t i o n , systems which have not been i n o p e r a t i o n f o r f i v e years have a p r o j e c t e d r e f r a c t o r y l i f e t i m e of approximately ten years, assuming proper care and maintenance. System

Efficiency

The c a l c u l a t e d e f f i c i e n c y of the steam system as described here i s approximately 75 percent. This value i s derived by using the t o t a l u s e f u l energy of the f u e l as the energy i n p u t . The heat content of the wood i s approximately 1.9 χ 1 0 J/kg (8300 Btu/lb) on a dry b a s i s , which i s equivalent to 9.6 χ 1 0 J/kg (4100 Btu/lb) on a wet b a s i s . I f the assumption i s made that 1.2 χ 1 0 J (500 Btu) i s needed to b o i l o f f the water i n each pound of f u e l , the the energy l e f t i n the f u e l which can be used f o r steam production i s 8.4 χ 1 0 J/kg (3600 B t u / l b ) . V a r i a t i o n s i n e f f i c i e n c y a r e caused by d i f f e r e n c e s i n feedwater temperature and moisture content of the wood r e s i d u e s . The e f f i c i e n c y could be increased by employing the waste heat from the combustion gases to preheat e i t h e r the f l u i d i z i n g a i r o r the b o i l e r feedwater. Waste heat could a l s o be employed to p a r t i a l l y dry the f u e l . 9

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Economics

Data f o r the payback p e r i o d i n years f o r the system described h e r e i n are presented i n F i g u r e 2. These r e s u l t s are f o r the 8.44 χ 1 0 J/hr (8.00 χ 1 0 Btu/hr) system and are based on r e placement of n a t u r a l gas with wood r e s i d u e s . The cost of the n a t u r a l gas i s assumed to be $2.09/10 J ($2.20/10 Btu). These c a l c u l a t i o n s were made to show the payout p e r i o d i n terms of r e placement of n a t u r a l gas. Labor and d e p r e c i a t i o n were not i n c l u d e d , and an operating cost of $100,000/year was assumed. F o r systems where the wood residues are owned by the operator, the payout p e r i o d has g e n e r a l l y been l e s s than two years f o r a twelve month operation. 1 0

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Thirty-one f a c i l i t i e s are now i n commercial o p e r a t i o n . Figure 3 i n c l u d e s a map showing the l o c a t i o n s of these burners. The c a p a c i t i e s vary from 1.27 χ Ι Ο J / h r (1.2 χ 1 0 Btu/hr to 1 0

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Figure 2. Payback from biomass replacement of natural gas at $2.09/10 J {$2.20/10* Btu) for a system with a capacity of 8.44 χ 10 J/hr (8.00 χ 10 Btu/hr) 10

Figure 3.

Fluidflameenergy systems


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1.27 χ 1 0 J/hr (1.2 χ 1 0 Btu/hr). As the perceived market value of organic residues i n c r e a s e s , changes w i l l be made to the b a s i c system to increase e f f i c i e n c y . Further developments are a l s o planned to improve the o v e r a l l economics of the system. I f operated i n a s t a r v e d - a i r mode w i t h a f i r e box downstream from the combustion c e l l , c a p i t a l costs f o r the burner and b o i l e r per u n i t throughput can be decreased, and more advantage can be taken of the r a d i a n t e f f e c t s on heat t r a n s f e r . As the costs of energy continue to i n c r e a s e r e l a t i v e to the b a s i c p r i c e i n d i c e s , the economics of f l u i d i z e d - b e d combustion of biomass to recover energy w i l l continue to improve. RECEIVED

November 16, 1979.


Energy Recovery from Wood Residues by Fluidized-Bed Combustion

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