Energy Recovery from Municipal Solid Waste and Sewage Sludge

9 Energy Recovery from Municipal Solid Waste and Sewage Sludge Using Multisolid Fluidized-Bed Combustion Technology WAYNE E. BALLANTYNE, WILLIAM J. HUFFMAN, LINDA M. CURRAN, and DANIEL H. STEWART

Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: August 29, 1980 | doi: 10.1021/bk-1980-0130.ch009

Battelle's Columbus Laboratories, 505 King Avenue, Columbus OH 43205 This study was initiated to investigate the potential for energy recovery from municipal solid waste and sewage sludge using Battelle's multi-solid fluidized bed combustion (MS-FBC) technology. The MS-FBC technology was chosen because it represents an advanced fluidized-bed combustion technology that is designed to operate with heterogeneous solids while achieving higher throughputs and larger turn-down ratios than conventional fluidization. The technology, originally developed for coal, was thought to be highly adaptable to the diverse solids that are inherently difficult to process when using municipal solid waste (MSW) and domestic sewage sludge (DSS). Steam generation from MSW and DSS has been studied by many investigators, but fouling of heat transfer surfaces can be a major problem. Thus, this research study also sought to investigate direct steam generation using hot sand to vaporize the water in DSS (4 percent solids) in a conventional fluidized bed that is an integral part of the MS-FBC technology. The research program incorporated several items and the two tasks discussed in this paper were: Task 1. Obtain experimental data using a MS-FBC pilot plant to validate calculated energy recovery. Task 2. Demonstrate the technical feasibility of direct steam generation using DSS as 4 percent solids. Background on MS-FBC Technology Multi-solid fluidized bed combustion (MS-FBC) was initially developed by Battelle(1) as an advanced fluidization technology for achieving high rates of combustion of high sulfur coal while eliminating the need for flue gas desulfurization or extensive coal cleaning. This coal technology was shown to be feasible during numerous runs in the 15 cm pilot plant system described in this paper and it was then further developed over a 1500 0-8412-0565-5/80/47-130-109$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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cumulative hour operating p e r i o d using a 4.4 metric ton/day p i l o t facility. Other feedstocks (e.g., wood) have a l s o been shown to be compatible with MS-FBC and the b a s i c processing c a p a b i l i t y has r e c e n t l y been adapted to the g a s i f i c a t i o n o f wood.(2). The b a s i c MS-FBC concept incorporates an entrained f l u i d i z e d bed superimposed on an i n e r t , dense-phase f l u i d i z e d bed as shown i n Figures 1 and 2 f o r the MSW/DSS a p p l i c a t i o n . As c u r r e n t l y operated i n the MSW/DSS experiments, the dense phase i s A f r i c a n i r o n ore. This dense bed remains i n the comb i t o r and i t s height e s s e n t i a l l y d e f i n e s the combustion zone (760-870 C); i t s high density permits dense-phase turbulent f l u i d i z a t i o n to be achieved at gas v e l o c i t i e s exceeding 9.0 m/sec. The entrained phase i s commonly s i l i c a sand and i t s a d d i t i o n to the dense-phase bed i s g e n e r a l l y c r e d i t e d with improving the q u a l i t y of the dense-phase f l u i d i z a t i o n . The entrained phase a l s o moderates the combustion by absorbing the heat o f r e a c t i o n . A f t e r passing through the dense-phase region, the entrained phase along with f l u e gas and ash pass through a free-board region and are separated. The heated sand i s then t r a n s f e r r e d a conventional f l u i d i z e d bed shown i n Figures 1 and 2. This bed i s operated at gas v e l o c i t i e s l e s s than 0.39 m/sec. Steam tubes can be i n c o r p o r ated i n t o t h i s f l u i d i z e d bed f o r the production o f high pressure steam (Figure 2). In one MSW/DSS a p p l i c a t i o n , no tubes are employed and d i r e c t - c o n t a c t heat t r a n s f e r between hot sand and a 3-4 percent DSS mixture i s accomplished (Figure 1). The cooled, ent r a i n e d m a t e r i a l i s then returned to the combustor where the c y c l e s t a r t s over again. The v a l v e shown i n the sand r e t u r n - l i n e i s used to c o n t r o l the r a t e of the entrained-phase r e c i r c u l a t i o n which, i n t u r n , c o n t r o l s the amount o f combustion that can be permitted i n the combustor. Some of the more s i g n i f i c a n t features of the MS-FBC process that have been experimentally proven a t the 4.4 metric ton/day s c a l e using c o a l as a feedstock i n c l u d e the f o l l o w i n g items: (a) High s p e c i f i c r a t e s o f combustion (grams feed/hr-cm^ grate area) due to the high amount o f heat removal achieved a t v e l o c i t i e s greater than 9 m/sec. (b) "Turn-down" of 3:1 can be achieved a t 20 percent excess a i r by a d j u s t i n g the r a t e o f r e c i r c u l a t i o n of the e n t r a i n e d phase; t h i s r e c i r c u l a t i o n c o n t r o l e l i m i n a t e s the need f o r bed-slumping that i s common to r e g u l a r f l u i d i z e d bed combustors. (c) D i f f e r e n t types and s i z e s of c o a l feed can be handled with minimum feed p r e p a r a t i o n ; lump-coal approximately 2.5 cm i n diameter i s p r e f e r r e d . D e s c r i p t i o n o f Equipment

and

A process flow schematic of the combustor, sand disengager, the EHE used i n t h i s i n v e s t i g a t i o n i s given i n Figure 3.

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

ET AL.

Energy Recovery Using MS-FBC

Technology

11


BALLANTYNE

Figure 1.

Battelle's MS-FBC process with contact evaporation of domestic sewage sludge


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Figure 2.

Β attelle's MS-FBC

process with steam generation in external heat exchanger


ET AL.


Technology

1


BALLANTYNE

Figure 3.

Thermocouple and pressure tap locations on MS-FBC combustor and EHE


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MSW Feed System. The MSW feed system c o n s i s t e d of a sealed weigh hopper supported by load c e l l s , a 10.2 cm diameter by 102 cm long screw feeder powered by a v a r i a b l e speed DC motor, and a 7.6 cm diameter feed chute f i t t e d with an expansion bellows and a k n i f e gate v a l v e . An a i r c o o l e r was i n s t a l l e d to keep the feed chute near the combustor from overheating. DSS Feed System. The DSS feed system c o n s i s t e d of an a g i tated, 55-gallon polyethylene supply tank f o r the sewage sludge and standard p i p i n g and pumps. The feed system s u p p l i e d two a i r sparged tubes l o c a t e d 25 cm and 46 cm above the d i s t r i b u t o r p l a t e i n the EHE. Each tube contained s i x , 2.36 mm diameter holes d r i l l e d h o r i z o n t a l l y (3 on each side) through which DSS and a i r were f e d . Because the c l a y and lime a d d i t i v e s used i n the study had a tendency to s e t t l e out i n the supply tank, a r e c i r c u l a t i o n l i n e was a l s o i n s t a l l e d . Combustor. The combustor assembly of the MS-FBC system cons i s t e d o f the combustion chamber and the g a s - f i r e d burner. The combustion chamber c o n s i s t e d of a v e r t i c a l , 15 cm I.D. by 6.7 m high pipe constructed o f Type 304 s t a i n l e s s s t e e l . The d i s t r i b u t o r p l a t e was a 0.63 cm t h i c k d i s c l o c a t e d on the bottom flange p e r f o r a t e d with 96 uniformly-spaced holes of 0.238 cm diameter. The r e c y c l e d s i l i c a sand entered the combustor through a 7.62 cm diameter pipe l o c a t e d approximately 10 cm above the d i s tributor plate. This pipe was i n s t a l l e d at an angle of approximately 20 degrees to the combustor and was equipped with a s l i d e gate valve to regulate the sand flow from the EHE i n t o the combustor. A l l m a t e r i a l s i n the combustor and gas burner were of Type 304 s t a i n l e s s s t e e l . Entry ports i n t o the combustor were a l s o constructed from s t a i n l e s s s t e e l . The combustor, sand disengager, EHE, and sand r e c y c l e l e g were wrapped with two l a y e r s of F i b e r f r a x ( r e g i s t e r e d trade name of Carborundum Corporation) i n s u l a t i o n to reduce heat l o s s e s . Sand Disengager. The sand disengager was a modified 61 cm cyclone equipped w i t h a s p o i l e r i n t o which a i r could be i n t r o duced to c l a s s i f y the f l y ash from the heavier entrained sand. Type 304 s t a i n l e s s s t e e l was used i n the c o n s t r u c t i o n of the disengager. EHE. The EHE c o n s i s t e d of a 35.6 cm diameter by 2.13 m high f l u i d i z e d bed. A sketch o f the EHE i s included i n Figure 1. The d i s t r i b u t o r p l a t e was 0.63 cm t h i c k and had 128 h o l e s , each 0.238 cm i n diameter uniformly spaced across the p l a t e . An a d j u s t a b l e overflow tube was l o c a t e d i n s i d e the EHE to regulate the height of the bed. The EHE was constructed from Schedule 10 carbon s t e e l pipe.


9.

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ET AL.


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C o n t r o l System. The operating c o n t r o l system contained s t a n dard rotameters, manometers, gauges, thermocouples, r e c o r d e r s , etc., f o r c o n t r o l l i n g / m o n i t o r i n g the MSW feed a i r , EHE nozzle sparge a i r , disengager s p o i l e r a i r , pressure tap purge a i r , n a t u r a l gas flow, combustor a i r flow, EHE f l u i d i z i n g a i r , bed pressures, DSS feed, and temperatures. The f l u e gas monitoring system c o n s i s t e d o f analyzers and recorders f o r continuous monitoring o f SO2, CO2, CO, NO, and 02 from e i t h e r the combustor o r EHE. A d i g i t a l readout i n d i c a t o r provided the weight o f MSW i n the hopper. SCR c o n t r o l s were pro vided f o r the MSW screw feeder and the DSS feed pump.


Materials The feedstock s e l e c t e d as the most a p p l i c a b l e f o r the p i l o t f a c i l i t y was p e l l e t i z e d MSW. The p e l l e t s , obtained from the Teledyne N a t i o n a l Company, had been preprocessed to remove metal and most o f the moisture before p e l l e t i z i n g . A representative a n a l y s i s based on an average o f s e v e r a l analyses i s presented i n Tables I and I I . The p e l l e t s were p r e f e r r e d over shredded MSW due to the r e l a t i v e ease o f handling i n the 10 cm diameter screw feeder used i n the p i l o t f a c i l i t y . A l a r g e r combustor system would not be l i m i t e d to the use of a p e l l e t i z e d feed.

TABLE I.

TYPICAL MSW ANALYSIS

Item C 0 (by d i f f . ) H 0 Ash H Ν S 2

Heating value (as received) Bulk density

Weight % 39.90 33.98 13.50 6.80 5.40 0.30 0.12 100.00 4252 c a l / g 0.48 g/cm3

The DSS i n j e c t e d i n t o the EHE was obtained from the concen t r a t o r underflow a t Columbus' Jackson Pike wastewater treatment p l a n t . Within a few hours p r i o r to each run, the sludge was loaded i n t o 55-gallon drums which were trucked to B a t t e l l e and emptied i n t o the DSS feed tank. The sludge was nominally 3 to 4 percent s o l i d s . A t y p i c a l a n a l y s i s of the metal content i s pro vided i n Table I I . The h e a t i n g value of the d r i e d DSS s o l i d was


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TABLE I I .

Compound or Element

Dried MSW Weight Percent

4


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TYPICAL CONCENTRATION OF METALS AND OTHER CONSTITUENTS IN MSW, SEWAGE SLUDGE, AND SCALE

S0 0.16 CI 0.08 Al 0.8 Fe 0.2 Si 2.0 Na 0.5 Ca 0.8 Κ 0.5 Mg 0.1 Mn, Pb 0.02-0.03 Cr Na20.3Ca0.6S1O2

.

The r e s u l t s o f adding limestone were encouraging as the system was operated with no o s t e n s i b l e plugging f o r 7 hours. The system was then dismantled f o r a thorough v i s u a l i n s p e c t i o n of the disengager and connecting l i n e s . Only s l i g h t t r a c e s of s c a l e formation were found. I t i s p a r t i c u l a r l y important to note that 2/3 of the sand' used i n t h i s run was from Run 215. The use o f t h i s " o l d " sand was thought to be a r a t h e r severe t e s t of the assumed chemistry and use of limestone. Thus, the absence o f s c a l e i n the disengager a f t e r Run 216 was thought to be q u i t e s i g n i f i c a n t . In both of these runs, a s t o i c h i o m e t r i c amount (no excess) o f limestone was added as i n d i c a t e d i n the above r e a c t i o n . The amount of sodium i n both the MSW and DSS (Table II) was used f o r c a l c u l a t i n g the s t o i c h i o m e t r y . The r a t i o o f limestone to a s - r e c e i v e d MSW and dry DSS was 0.047 g/g and 0.5 g/g, r e s p e c t i v e l y . Run 217 was intended to be an extension of Run 216 to demons t r a t e prolonged o p e r a t i o n of the system. The system was operated using sand from Run 216 f o r about 13 hours. Then, combustor pressures b u i l t - u p , the MSW became d i f f i c u l t to feed, and the system had to be shutdown. On d i s m a n t l i n g the system, a p l u g was found about half-way up the combustor. The plug contained high amounts of s i l i c a and s a l t and may have e x i s t e d p r i o r to the a d d i t i o n o f


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limestone, g r a d u a l l y growing i n s i z e with each run. The b e l i e f that the plug e x i s t e d p r i o r to Runs 216 and 217 was supported by the f a c t that there was minimal s c a l e formation elsewhere i n the combustor and disengager. A f t e r c l e a n i n g the combustor, Run 218 was made. The major goal of t h i s experimental run was to show that agglomeration/ s c a l i n g could be minimized over an extended p e r i o d by adding comm e r c i a l c l a y to the DSS feed s i n c e c l a y would be a more economical inhibitor. The c l a y i s b e l i e v e d to r e a c t with the a l k a l i s a l t s present i n the DSS to form a l b i t e or nephaline(5), double a l k a l i s a l t s (Mp-1108 and 1280 C, r e s p e c t i v e l y ) which melt above the comb u s t i o n temperature. The r e a c t i o n s f o r the c l a y were assumed to be as f o l l o w s : Na 0-3Si0 Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: August 29, 1980 | doi: 10.1021/bk-1980-0130.ch009

2

Na 0-3Si0 2

2

+ A1 0

2

+ A l ^ -> N a ^ - A ^ O y 2 S i 0

2

3

+ 3Si0

2

+ Na 0. A l ^ . 6 S i 0 2

2

+ SK>

2

2

Clay was added at the s t o i c h i o m e t r i c q u a n t i t y needed f o r the sodium r e a c t i o n s and the r a t i o s to MSW and DSS were 0.0235 g/g asr e c e i v e d MSW and 0.25 g/g dry DSS. During the run, considerable d i f f i c u l t y with the MSW feed system was encountered. Upon shutdown and i n s p e c t i o n of the MSW, i t was found that l a r g e m e t a l l i c objects were present i n the hopper and screw and were probably h i n d e r i n g the MSW feed to the combustor. During uninterrupted feed p e r i o d s , the MS-FBC system operated smoothly as evidenced by the temperature p r o f i l e i n Figure 4. Furthermore, as can be seen by the s t a b l e EHE bed temperature p r o f i l e , the s t a b i l i t y of MS-FBC permitted quick recovery a f t e r the i n t e r ruptions. T o t a l operating time with feed was 31 hours and the DSS feedrate was 38 1/hr or 0.4 1 / h r - l of bed volume. The o b j e c t i v e of minimizing s c a l e formation was met because no s i g n i f i c a n t build-up of sand or other m a t e r i a l s could be det e c t e d i n the combustor, e x t e r n a l b o i l e r , p i p i n g , and cyclones a f t e r shutdown and i n s p e c t i o n . The problem of agglomeration and build-up was apparently minimized by the a d d i t i o n of c l a y . Other Results R e l a t i v e l y high CO l e v e l s p r e v a i l e d throughout most of the runs. This was not regarded as a s e r i o u s problem and d i d not r e c e i v e much a t t e n t i o n . That i s , high CO l e v e l s are common to the 15 cm diameter p i l o t p l a n t because i t does not have s u f f i c i e n t combustor freeboard height to ensure combustion of CO. This high CO emission was a l s o a d e f i c i e n c y i n burning c o a l , but, i n runs made with c o a l i n a l a r g e r MS-FBC p i l o t p l a n t (4.4 metric ton/day) with a d d i t i o n a l freeboard, acceptable emissions of CO were achieved and the same r e s u l t i s f u l l y expected to be true when burning MSW. Emissions of N0 and S02 were w i t h i n F e d e r a l l i m i t s . The combustor e f f i c i e n c y of the MSW and DSS i n the e x p e r i mental system was high and approached 99+ percent i n some runs. X



Figure 4.

(a)

Six-inch MS-FBC operating conditions for run 218

Data i n t e r r u p t e d due to d i f f i c u l t i e s w i t h MSW caused by f o r e i g n o b j e c t s i n the MSW.

Time (hours)


feed

^ g*

CO

^

?3

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This high e f f i c i e n c y i s i n d i c a t e d by the low amount o f unburned carbon that was determined to e x i s t i n various streams during Run 212 (Table I I I ) . TABLE I I I .

ANALYSIS FOR ASH AND TOTAL CARBON IN SOLID STREAM FROM RUN 212


Stream Recycled sand Sand i n EHE Sand i n EHE cyclone Sand i n combustor cyclone Combustor Fresh sand Sand taken from disengager

Present Ash

Total Carbon

99.7 99.7 99.1 98.6 100.0 99.9 99.9

0.2 0.2 1.9 1.3

Energy Recovery from Municipal Solid Waste and Sewage Sludge

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