Fuel Production from Wastes Using Molten Salts - American Chemical

17 Fuel Production from Wastes Using Molten Salts R. L. GAY, Κ. M. BARCLAY, L. F. GRANTHAM, and S. J. YOSIM

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: August 29, 1980 | doi: 10.1021/bk-1980-0130.ch017

Rockwell International Corporation, Energy Systems Group, 8900 De Soto Avenue, Canoga Park, CA 91304

The disposal of municipal and industrial wastes has become an important problem because the traditional means of disposal, landfill, has become environmentally much less acceptable than previously. In addition, special incinerator systems are required to meet environmental standards for disposal by incineration. Disposal of wastes by landfill or incineration also includes a potential loss of energy sources and, in some cases, valuable mineral resources. New, much stricter regulation of these dis posal methods will make the economics of waste processing for resource recovery much more favorable. One method of processing waste streams is to convert the energy value of the combustible waste into a fuel. One type of fuel attainable from wastes is a low heating value gas, usually 3.7-5.6 MJ m - 3 (100-150 Btu/scf), which can be used to generate process steam or to generate electricity. Described here are some results which show the feasibility of processing wastes in molten salts into usable fuels. The molten salt acts as a reaction medium for the conversion of the waste into a low heating value gas (by reaction with air) and the simultaneous retention of potential acidic pollutants in the molten salt. The waste is converted to a fuel gas by reacting it with deficient air, that is, insufficient air for complete con version to CO2 and H2O. Results are presented for a high-sulfur o i l refinery waste, rubber, wood, leather scraps, and waste X-ray film. These waste streams represent a small segment of the large variety of wastes which may be processed using molten salts. Process Description A flow diagram of the Rockwell International process for fuel production from wastes is shown in Figure 1. In this pro cess, combustible waste and air are continuously introduced beneath the surface of a sodium carbonate-containing melt at a 0-8412-0565-5/80/47-130-227$05.00/0 © 1980 American Chemical Society In Thermal Conversion of Solid Wastes and Biomass; Jones, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

THERMAL CONVERSION OF SOLID WASTES AND BIOMASS

228

Off-Gas Stack

Baghouse o r V e n t u r i Scrubber

Fuel-Gas

Waste Molten Salt Gasif1er


Air

Figure 1.

Melt Processing for Carbonate Recovery

Molten salt process for fuel production from wastes

Figure 2.

Bench-scale molten salt gasifier

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


17.

GAY ET AL.

Fuel from Wastes Using Molten Salts

229

temperature of 900-1000°C. The waste r e a c t s c h e m i c a l l y w i t h the molten s a l t and a i r t o produce a p o l l u t a n t - f r e e combustible gas c o n t a i n i n g mainly CO, H 2 , N 2 , and a small amount of CH4 and C 2 H 6 . The product gas i s cleaned of p a r t i c u l a t e s u s i n g a baghouse or v e n t u r i scrubber. I t i s then burned i n a b o i l e r to produce steam or as an a l t e r n a t i v e , the product gas may be burned i n a gas t u r b i n e as p a r t of a combined-cycle process. Sodium carbonate i s a s t a b l e , n o n v o l a t i l e , inexpensive, and nontoxic m a t e r i a l . I t i s the i n t i m a t e contact of the waste m a t e r i a l w i t h the molten s a l t and a i r that provides f o r complete and r a p i d r e a c t i o n o f the waste. Any gas that i s formed during g a s i f i c a t i o n i s forced t o pass through the b a s i c carbonate melt. Halogens i n the waste ( f o r example, c h l o r i n e from c h l o r i n a t e d o r g a n i c compounds) form the corresponding sodium h a l i d e s a l t s . Any s u l f u r i n the waste i s r e t a i n e d as sodium s u l f i d e , and any ash i n the waste w i l l a l s o be r e t a i n e d i n the melt. The temperatures o f o p e r a t i o n are too low to permit a s i g n i f i c a n t amount of n i t r o g e n oxides to be formed by f i x a t i o n of the n i t r o g e n i n the a i r . A l s o , a t these operating temperatures, odors and i n f e c t i o u s m a t e r i a l are completely destroyed. A p o r t i o n o f the sodium carbonate melt i s withdrawn from the molten s a l t r e a c t o r , quenched, and processed i n an aqueous recovery system. The recovery system removes the ash and i n o r g a n i c combustion products (mainly sodium s a l t s such as NaCl and N a 2 S ) r e t a i n e d i n the melt. Unreacted sodium carbonate i s returned t o the molten s a l t furnace. The ash must be removed when the ash c o n c e n t r a t i o n i n the melt approaches 20 to 25 wt % i n order to preserve the melt f l u i d i t y . The i n o r g a n i c combustion products must be removed b e f o r e a l l o f the sodium carbonate i s completely converted to noncarbonate s a l t s . For the case o f a waste c o n t a i n ing a v a l u a b l e m i n e r a l resource, the v a l u a b l e m i n e r a l resource i s r e t a i n e d i n the melt during the g a s i f i c a t i o n process and may be recovered as a by-product of the r e g e n e r a t i o n process. T h i s molten s a l t g a s i f i c a t i o n process i s the b a s i s f o r the Rockwell I n t e r n a t i o n a l molten s a l t c o a l g a s i f i c a t i o n process. A 900-kg-h~l ( i ton/h) process development u n i t p i l o t p l a n t has been b u i l t and i s being t e s t e d under contract from the Department of Energy. T h i s p l a n t i n c l u d e s the g a s i f i e r and a complete sodium carbonate recovery and r e g e n e r a t i o n system. Experimental Bench-Scale Test Apparatus. A schematic diagram of the bench-scale t e s t apparatus i s shown i n F i g u r e 2. T h i s apparatus i s used t o t e s t the f e a s i b i l i t y o f g a s i f y i n g wastes b e f o r e p i l o t s c a l e t e s t s are performed. The waste throughput of the benchs c a l e apparatus i s 1 t o 3 kg h-1 (2 t o 6 l b / h ) . Approximately 5.5 kg (12 l b ) of molten s a l t are contained i n a 15-cm ID by 76-cm h i g h alumina tube placed i n s i d e a type 321 s t a i n l e s s s t e e l



230

THERMAL

CONVERSION

OF SOLID WASTES AND

BIOMASS

r e t a i n e r v e s s e l . T h i s s t a i n l e s s s t e e l v e s s e l , i n t u r n , i s cont a i n e d i n a 20-cm ID, four heating zone Ohio Thermal furnace. Each h e a t i n g zone i s 20 cm i n height, and the power to each zone i s c o n t r o l l e d by a s i l i c o n - c o n t r o l l e d r e c t i f i e r c i r c u i t . An a d d i t i o n a l f l a t p l a t e heater i s l o c a t e d a t the bottom of the v e s s e l . Furnace and r e a c t o r temperatures are recorded on a 12-point Barber-Colman chart r e c o r d e r . S o l i d s are p u l v e r i z e d i n a Wiley l a b o r a t o r y k n i f e - b l a d e m i l l to l e s s than 1 mm i n s i z e . These s o l i d s are then metered i n t o a 1.3-cm OD s t a i n l e s s s t e e l i n j e c t i o n tube by a screw feeder. The speed of the screw feeder i s v a r i a b l e between 0-400 rpm. The s o l i d s are mixed w i t h process a i r i n the i n j e c t i o n tube. The s o l i d s - a i r mixture comes out of the end of the 1.3-cm OD tube and i n t o the annulus of a 3.7-cm ID alumina tube. T h i s alumina tube extends to w i t h i n 1.3 cm of the bottom of the 15-cm ID alumina containment v e s s e l . During normal operation of the r e a c t o r , the depth of the molten s a l t expands from a quiescent l e v e l of 15 cm to an expanded depth of about 30 cm. In the case of l i q u i d s , a l a b o r a t o r y p e r i s t a l t i c pump i s used to pump the l i q u i d i n t o the 1.3-cm OD s t a i n l e s s tube. The feed r a t e of the l i q u i d i s c o n t r o l l e d by the pump speed. Heat balance of the bench-scale r e a c t o r i s maintained i n one of two ways. I f the feed m a t e r i a l i s of r e l a t i v e l y h i g h heating v a l u e (greater than 14.0 MJ kg-1, 6000 B t u / l b ) , a small a i r c o o l e r at the bottom of the furnace i s used to maintain the v e s s e l a t the operating temperature. I f the waste m a t e r i a l i s low i n h e a t i n g v a l u e , the melt temperature i s maintained by an e l e c t r i c a l l y heated furnace, or by adding a u x i l i a r y f u e l to the waste. Off-Gas A n a l y s i s . Gas samples are i n i t i a l l y cleaned of p a r t i c u l a t e s and d r i e d to 2% moisture b e f o r e a n a l y s i s . Carbon monoxide and carbon d i o x i d e are measured continuously using a Horiba Mexa-300 CO analyzer and a Horiba Mexa-200 CO2 a n a l y z e r . Syringe samples are taken downstream of the CO2 analyzer f o r gas chromatographic a n a l y s i s . A room temperature molecular s i e v e 13X column i s used to analyze f o r carbon monoxide, oxygen, and n i t r o gen. A Poropak Q column at 130°C i s used to analyze f o r carbon d i o x i d e , methane, ethane, ethylene, s u l f u r d i o x i d e , and hydrogen sulfide. 1

Molten S a l t Test F a c i l i t y . A 90-kg-IT (200 lb/h) p i l o t p l a n t at the Rockwell f i e l d l a b o r a t o r y at Santa Susana i s used f o r l a r g e r s c a l e t e s t i n g . The molten s a l t g a s i f i c a t i o n v e s s e l i s made of Type 304 s t a i n l e s s s t e e l and i s 4.6 m h i g h w i t h a 0.9 m i n s i d e diameter. The i n s i d e of the v e s s e l i s l i n e d w i t h 15-cmt h i c k r e f r a c t o r y b l o c k s . The g a s i f i e r contains 900 kg of sodium carbonate, which has an unexpanded bed height of 0.9 m. During o p e r a t i o n , the bed height approximately doubles. A n a t u r a l gas preheater i s used to heat the v e s s e l on s t a r t - u p .



17.

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231

A flow schematic f o r the Santa Susana molten s a l t t e s t f a c i l i t y i s shown i n F i g u r e 3. S a l t i s loaded i n t o the v e s s e l from a carbonate hopper and a weigh b e l t feeder. Waste m a t e r i a l s a r e shredded and crushed u s i n g a r o t a r y k n i f e shredder and a hammerm i l l and a r e then metered i n t o a pneumatic feed l i n e w i t h a v a r i a b l e - s p e e d auger. Product gases produced i n the molten s a l t v e s s e l u n i t flow through r e f r a c t o r y - l i n e d d u c t i n g to a spray c o o l e r and i n t o a secondary combustor. The gases are burned i n the secondary combustor and the o f f - g a s i s cleaned of any p a r t i c u l a t e matter w i t h a baghouse o r a v e n t u r i scrubber. The o f f - g a s i s then r e l e a s e d t o the atmosphere. A photograph o f the molten s a l t t e s t f a c i l i t y i s shown i n F i g u r e 4. Results G a s i f i c a t i o n o f A c i d P i t Sludge. Crude or p a r t i a l l y r e f i n e d crude i s t r e a t e d w i t h s u l f u r i c a c i d during r e f i n i n g . T h i s produces a waste product which contains s u b s t a n t i a l amounts of water (12-40 wt % ) , s u l f u r (2-14 wt % ) , ash (1-55 wt %) , and s i g n i f i c a n t combustible m a t e r i a l (11.6 to 20.9 MJ k g ~ l ) (5000-9000 B t u / l b ) . Normal i n c i n e r a t i o n of t h i s waste i s not economic due t o the high water, ash, and s u l f u r content. Therefore, the waste i s g e n e r a l l y ponded and not processed. The n i c k e l and vanadium content and energy v a l u e of t h i s waste product, as w e l l as the v a l u e of the l a r g e area o f land dedicated t o current d i s p o s a l techniques, o f f e r s i g n i f i c a n t economic p o t e n t i a l f o r improved d i s p o s a l processes. Data on the g a s i f i c a t i o n o f a c i d p i t sludge a r e given i n Table I . I n t h i s t e s t , the water-sludge mixture was heated to the c o n s i s t e n c y o f l i g h t o i l (about 90 to 95°C) and f e d to the r e a c t o r without removing any water from the a s - r e c e i v e d m a t e r i a l . A gas c o n t a i n i n g a higher heating value of 8.57 MJ m~3 (230 Btu/ s c f ) was produced. During t h i s t e s t , some a u x i l i a r y heat was f u r n i s h e d by the outer furnace to maintain the operating temperat u r e . I t i s estimated that a steady-state t e s t without a u x i l i a r y heat would produce a gas w i t h a higher heating v a l u e o f 6.33 MJ m~3 (170 B t u / s c f ) . Emissions of H 2 S were below the l i m i t of d e t e c t a b i l i t y (40 ppm). G a s i f i c a t i o n o f Rubber T i r e s . Conventional i n c i n e r a t i o n of rubber t i r e s produces a l a r g e amount of p a r t i c u l a t e s which c o n t a i n unburned hydrocarbons. As an a l t e r n a t i v e to i n c i n e r a t i o n , molten s a l t g a s i f i c a t i o n was t e s t e d . Two g a s i f i c a t i o n t e s t s were made u s i n g b u f f i n g s from a rubber t i r e . The r e s u l t s of these t e s t s were averaged and a r e presented i n Table I . Since the rubber contained organic s u l f u r that would form N a 2 S i n the melt, the N a 2 C 0 melt o r i g i n a l l y contained 6 wt % Na S to approximate steadys t a t e c o n d i t i o n s . Using a s t o i c h i o m e t r y of 33% t h e o r e t i c a l a i r 3

2




232

I 1 δ

.ο

v. S


GAY ET AL.



17.

Figure 4. Santa Susana molten salt test facility


233


2>

^Auxiliary heat provided by the e l e c t r i c furnace during testing. produced without auxiliary heat.

Not determined.

Remainder i s N

Estimated 6.33 MJ m

Gas volumes are defined at conditions of 101.3 kPa (1 atm) pressure and 15 .6°C (60'°F).

3.99 6.67

4.03

6.74

5.77

8.57

d

Higher Heating Value (MJ m-3)

a

gas would be

0.2 1.2 2.6 5 .2 11.7 14.1

1015 958

12.0 18.3

16.5 16.0

7.2 4.0

2.4 3.0

935

Leather Offal

2.2

12.9

C

0.9 ND

3.0

21.1

13.7

16.5

3.6

20.3

14.5

3.3

1.0 2.0

951

Wood

1.1

2.4

16.0

18.4

4.0

2.6

0.8

C

2

2.5

920

a

4

8.4

Rubber Tires

Film

2

b

15.8

7.0

16.0

4.0

3.4

935

2

Composition of Off-Gas (Vol % ) H CH C C0 CO

Acid P i t Sludge

3

Temperature (°C) 1

Air Feed Rate (m3 h " )

Waste Material

Waste Feed Rate (kg h-1)

GASIFICATION OF WASTES

TABLE I


17.

GAY ET AL.

235


(100% t h e o r e t i c a l a i r i s r e q u i r e d to o x i d i z e the m a t e r i a l comp l e t e l y to CO? and H 0 ) , a product gas w i t h a higher heating v a l u e of 5.77 MJ m"3 (155 Btu/scf) was made. No H S o r s u l f u r - c o n t a i n ing gases ( l e s s than 30 ppm) were detected i n the o f f - g a s . 2

2

G a s i f i c a t i o n of Wood. Pine wood was chosen as r e p r e s e n t a t i v e of t y p i c a l biomass. A t y p i c a l composition of pine wood on a dry b a s i s i s 51.8 wt % carbon, 6.3 wt % hydrogen, 0.5 wt % ash, and 41.3 wt % oxygen. The pine wood t e s t e d here was sawdust with a moisture content o f 2.8 wt %. The heating v a l u e i s t y p i c a l l y 21.2 MJ kg-1 (9100 B t u / l b ) . A pure sodium carbonate melt was used w i t h 30% t h e o r e t i c a l a i r . G a s i f i c a t i o n of wood was r a p i d and complete w i t h p r o d u c t i o n of a gas w i t h a higher heating value of 6.74 MJ m~ (181 B t u / s c f ) , as given i n Table I .


3

G a s i f i c a t i o n of Leather O f f a l . Chrome-leather tanning scraps o r o f f a l a r e a waste product of the tanning and l e a t h e r i n d u s t r i e s . These wastes a r e u s u a l l y the r e s u l t of trimming o p e r a t i o n s and c o n t a i n 3.5 wt % chromium and about 50 wt % water. The wastes cannot be i n c i n e r a t e d because malodorous substances and c a r c i n o g e n i c chromium-containing p a r t i c u l a t e s a r e formed. L a n d f i l l d i s p o s a l i s p r e s e n t l y used. The tanning scraps used f o r these t e s t s were a i r - d r i e d and contained 37.2 wt % carbon, 5.7 wt % hydrogen, 12.8 wt % n i t r o gen, 2.5 wt % chromium, and approximately 5 wt % nonchrome ash. The heating v a l u e of the chrome-waste was approximately 18.8 MJ kg" (7800 B t u / l b ) . The a n a l y s i s of the o f f - g a s from l e a t h e r g a s i f i c a t i o n i s given i n Table I . Approximately 50% of t h e o r e t i c a l a i r was used. The product gas had a heating v a l u e of 4.03 MJ m"" (108 B t u / s c f ) . The N 0 emissions were only 40 ppm even through the l e a t h e r contained 12.8 wt % organic n i t r o g e n . The chromium emission i n the form of p a r t i c u l a t e s was 0.3 mg m~ (0.00013 g r a i n / f t ) , which corresponds to a chromium r e t e n t i o n of 99.998%. T h i s chromium could be recovered and r e c y c l e d to the tanning process by processing the s a l t bed. 1

3

X

3

3

G a s i f i c a t i o n of Waste F i l m . Two s e r i e s of t e s t s were conducted w i t h waste X-ray f i l m . The f i r s t s e r i e s was performed i n the bench-scale g a s i f i e r ; the second s e r i e s was run i n the molten salt test f a c i l i t y . Bench-scale g a s i f i c a t i o n of f i l m a t 51% and 22% t h e o r e t i c a l a i r produced product gases of 3.99 MJ m" (107 Btu/scf) and 6.67 MJ m" (179 B t u / s c f ) , r e s p e c t i v e l y . Small p e l l e t s of pure elemental s i l v e r (greater than 99.9% pure Ag) were recovered from these t e s t s . In the p i l o t t e s t , 6,800 kg (15,000 l b ) of waste X-ray f i l m were burned a t the r a t e of 90-105 kg t r (200-230 l b / h ) . The average a i r feed r a t e was 290 m h"* (180 scfm). The a i r / f i l m r a t i o corresponded to 50% t h e o r e t i c a l a i r . Since the main purpose o f t h i s t e s t was to recover s i l v e r , no attempt was made to 3

3

1

3


236


analyze the o f f - g a s . A f t e r the f i l m had been g a s i f i e d , the s i l v e r and s a l t were drained from the v e s s e l and 160.5 kg (354 l b ) of 99.97% pure m e t a l l i c s i l v e r were recovered.


Conclusion The Rockwell I n t e r n a t i o n a l molten s a l t process f o r g a s i f i c a t i o n of wastes w i t h resource recovery has been shown here to be w e l l - s u i t e d f o r the p r o c e s s i n g of a v a r i e t y of wastes. A v a r i e t y of waste forms may be processed, t h a t i s , s o l i d s , l i q u i d s , and s o l i d - l i q u i d mixtures. The process i s s u i t a b l e f o r a p p l i c t i o n s which i n v o l v e e i t h e r s m a l l or l a r g e throughputs. The g a s i f i c a t i o n medium, sodium carbonate, i s s t a b l e , n o n - v o l a t i l e , inexpensive, and nontoxic. S u l f u r - c o n t a i n i n g p o l l u t a n t s are r e t a i n e d i n the melt when s u l f u r - c o n t a i n i n g wastes are g a s i f i e d . In the same manner, halogen-containing p o l l u t a n t s are r e t a i n e d during g a s i f i c a t i o n of halogen-containing wastes. The g a s i f i c a t i o n of a h i g h n i t r o g e n - c o n t e n t waste ( l e a t h e r scraps) produces very l i t t l e N0 i n the o f f - g a s . V a l u a b l e minerals may be recovered by p r o c e s s i n g of the s a l t a f t e r g a s i f i c a t i o n of m i n e r a l - l a d e n wastes. In g e n e r a l , the molten s a l t process i s best a p p l i e d to waste materi a l s i n v o l v i n g p o t e n t i a l p o l l u t a n t s (such as s u l f u r or chromium) or to wastes where g a s i f i c a t i o n and resource recovery are important (such as the recovery of s i l v e r w i t h simultaneous g a s i f i c a t i o n of X-ray f i l m ) . X

RECEIVED November 16,

1979.


Fuel Production from Wastes Using Molten Salts - American Chemical

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