Energy Conservation in Textile and Polymer Processing - American

10 Energy Conservation in Cotton Ginning ROY V. BAKER

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USDA, SEA South Plains Ginning Research Laboratory, Lubbock, TX 79401 OLIVER L. McCASKILL USDA, SEA U. S. Cotton Ginning Research Laboratory, Stoneville, MS 38776

Cotton ginning is the first process in a long chain of processing and manufacturing operations required to convert raw seed cotton into usable consumer products. Although the primary function of cotton ginning is separation of cotton fibers from cottonseed, the modern ginning process also includes seed cotton drying and cleaning, lint cleaning, and bale packaging. The various ginning operations are connected by an intricate materials-handling system that forms a continuous flow process from seed cotton input to bale output. In the United States about 2,900 cotton gins process our annual 10- to 12-million bale cotton crop (1). A gin's materials-handling equipment and its cleaning and ginning machinery are normally powered by induction-type, squirrel-cage electric motors. Heat for the seed cotton drying process is obtained by burning natural or liquefied petroleum gas in direct-fired burners in pneumatic conveying lines. The amount of energy consumed by the ginning process varies over a wide range due to differences in size and design, to fluctuations in the moisture and foreign matter content of cotton, and to variations in operating procedures. The energy consumption values given i n Table I are r e p r e s e n t a t i v e o f moderate-size p l a n t s that g i n machine-picked cotton under optimum c o n d i t i o n s . No allowances were made i n Table I f o r energy consumed during i d l i n g or other nonproductive p e r i o d s . Excessive i d l i n g or u n d e r u t i l i z a t i o n o f g i n n i n g c a p a c i t y w i l l produce a p p r e c i a b l y higher energy consumption values {2_ 3.) than those obtained under optimum c o n d i t i o n s . Of the 177 kwh o f f o s s i l f u e l equivalent energy consumed i n ginning a bale o f c o t t o n , 89 kwh, or 50% o f the t o t a l , i s r e q u i r e d f o r seed cotton d r y i n g (Table I ) . This consumption value i n c l u d e s e l e c t r i c energy f o r the operation o f air-moving fans and equivalent o f 66 kwh ( 2 2 5 , 0 0 0 Btu) o f energy f o r heating the d r y i n g a i r . The m a t e r i a l s - h a n d l i n g o p e r a t i o n s , which are mostly pneumatic, consume the second l a r g e s t amount o f energy i n a cotton g i n . The gin's pneumatic conveying systems consume 9

This chapter not subject to U.S. copyright. Published 1979 American Chemical Society

Vigo and Nowacki; Energy Conservation in Textile and Polymer Processing ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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ENERGY CONSERVATION IN TEXTILE AND POLYMER PROCESSING

Table I.

Representative Energy Requirements Per Bale, f o r Ginning C o t t o n . » a

Ginning f u n c t i o n

Energy requirements Electric Drier energy, fuel, kwh 1,000 Btu d

Cleaning Drying Ginning Packaging M a t e r i a l s handling Total

5 8 7 3 L5 38

225

225

b

F o s s i l f u e l equivalents Consumption, D i s t r i b u t i o n , kwh % 15 89 20 9 44 177

9 50 11 5 25 100

'Energy requirements were based on a f u l l y u t i l i z e d model g i n that processes machine-picked c o t t o n a t r a t e s of 12 t o 14 b a l e s / hr and do not i n c l u d e energy consumed during i d l i n g o r other periods o f downtime. energy requirements d e r i v e d from Tables A-17 and A-21 o f reference (4) and Table 3 o f reference (5). The c a l c u l a t i o n o f f o s s i l f u e l e q u i v a l e n t s f o r e l e c t r i c energy was based on a generation e f f i c i e n c y of 34% (10,000 Btu o f f o s s i l f u e l energy r e q u i r e d t o generate 1 kwh o f e l e c t r i c i t y . ) l

F u e l requirements f o r d r i e r s are v a r i a b l e . They range from zero f o r dry cotton t o as much as 450,000 Btu/bale f o r wet cotton.

The Cotton Gin and Oil Mill Press


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about 44 kwh of energy/bale, which represents 25% of the gin's t o t a l energy consumption. Lesser amounts of energy are consumed by the other ginning operations. The l i n t - s e e d s e p a r a t i o n , seed cotton and l i n t c l e a n i n g , and bale packaging operations account for 11, 9, and 5 percent, r e s p e c t i v e l y , of the gin's energy requirements. The seed cotton d r y i n g o p e r a t i o n , because of i t s high energy consumption i n r e l a t i o n t o that o f other ginning operations appears to be the most l i k e l y candidate f o r energy conservation i n a cotton g i n . The r e c o g n i t i o n of t h i s p o t e n t i a l has r e s u l t e d i n the development and i d e n t i f i c a t i o n o f s e v e r a l a p p l i c a b l e energy conserving techniques. The purpose of t h i s paper i s to d e s c r i b e some of the most promising techniques f o r conserving energy at cotton g i n s . Energy Conservation

Techniques

Drying Requirements. The amount o f energy consumed during seed cotton d r y i n g v a r i e s d i r e c t l y with d r y i n g temperature. I t i s important to c o n t r o l d r y i n g temperature a c c u r a t e l y , not only to insure s a t i s f a c t o r y d r y i n g , but a l s o to avoid excessive d r y i n g , which wastes energy and lowers f i b e r q u a l i t y . The amount o f h e a t r e q u i r e d to achieve s a t i s f a c t o r y d r y i n g depends upon the f i b e r ' s i n i t i a l moisture content and the optimum f i n a l moisture content to which the f i b e r should be d r i e d . Since a ginner has l i t t l e c o n t r o l over i n i t i a l f i b e r moisture content, i t i s important that we i d e n t i f y the "optimum moisture content" t o e s t a b l i s h a r a t i o n a l basis for drying decisions. Past research on seed c o t t o n d r y i n g has e s t a b l i s h e d an optimum range o f f i b e r moisture content w i t h i n which s a t i s f a c t o r y c l e a n i n g and ginning can be achieved while the inherent q u a l i t i e s of the f i b e r s a r e maintained (6, _7, 8 ) . The c u r r e n t l y accepted optimum f i b e r moisture content l i e s w i t h i n the 6% t o 8% (wet b a s i s ) range. T h i s optimum range i s a compromise between e f f e c t i v e c l e a n i n g and q u a l i t y p r e s e r v a t i o n on the one hand, and q u a l i t y p r e s e r v a t i o n and smooth ginning on the other. Cotton that contains moisture i n excess of 8% w i l l not c l e a n and g i n p r o p e r l y and w i l l r e c e i v e low grades due to excessive t r a s h and rough p r e p a r a t i o n (7). Cotton having moisture contents below the optimum range i s subject t o excessive f i b e r breakage during ginning (8), and the low moisture contents c o n t r i b u t e to the generation of s t a t i c e l e c t r i c i t y , causing chokages and decreased operating e f f i c i e n c y (9). Information i n Table I I d e s c r i b e s the performance c h a r a c t e r i s t i c s o f t y p i c a l seed cotton d r y i n g systems and i l l u s t r a t e s some o f the e f f e c t s o f d r y i n g f i b e r s to a l e v e l below the optimum. G e n e r a l l y , these data show that f i b e r s ginned at moisture contents below 6 percent were s h o r t e r and produced lower yarn break f a c t o r s than f i b e r s ginned at moisture contents above 6%. Obviously, the amount of energy r e q u i r e d f o r d r y i n g


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Table I I .

Some E f f e c t s o f Seed Cotton Drying on F i b e r and Yarn P r o p e r t i e s . 3

Fiber properties Moisture Nonlint Mean contents, content length, % % in.

Amount o f seed cotton d r y i n g

b

7% to 8% i n i t i a l c

2 stages, amb. 2 stages, 150°F 2 stages, 200°F

6.3 4.8 4.2

Yarn p r o p e r t i e s Break f a c t o r Appearance units index

f i b e r moisture content 2.3 2.1 2.0

0.93 0.91 0.88

1903 1800 1737

96 94 90

8% t o 9% i n i t i a l f i b e r moisture content c

2 stages, amb. 2 stages, 180°F 2 stages, 275°F

7.0 5.1 4.9

3.1 2.5 2.3

1.03 0.99 0.98

2004 1828 1781

93 103 100

9% t o 10% i n i t i a l f i b e r moisture content c

2 stages, amb. 1 stage, 250°F 2 stages, 250°F a

9.0 6.7 4.9

8.6 7.3 6.4

0.98 0.95 0.92

1686 1658 1585

D a t a from references ( 6 ) , (10), and (11).

^At the g i n stand. °Ambient a i r a t about 70°F.


79 79 76

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to low moisture l e v e l s i s c o n s i d e r a b l y greater than that r e q u i r e d to achieve the 6% to 8% moisture range. Drying to l e v e l s below 6% tended to enhance t r a s h removal, but t h i s advantage was s m a l l and research has shown that i t i s o f t e n negated as a r e s u l t of decreases i n s t a p l e l e n g t h and bale weight (6). Thus, the f i r s t tenet i n an energy conservation program f o r seed c o t t o n d r y i n g may be simply stated as "use only the amount of d r y i n g required to lower f i b e r moisture content to the 6% to 8% range." Although t h i s concept may sound elementary and obvious, i t i s s u r p r i s i n g how o f t e n i t i s ignored. Management of Drying Systems. The management of seed cotton d r y i n g systems to minimize f u e l usage has not been an o v e r r i d i n g c o n s i d e r a t i o n i n the past because of r e l a t i v e l y low f u e l p r i c e s . However, with i n c r e a s i n g f u e l costs and the c o n t i n u a l p o s s i b i l i t y of f u e l c u r t a i l m e n t , a c r i t i c a l e v a l u a t i o n of a l t e r n a t i v e s a v a i l a b l e to managers to reduce f u e l consumption i s of increased importance. A t y p i c a l seed c o t t o n d r y i n g system i s composed of two c e n t r i f u g a l fans, a burner, a tower d r i e r , and connecting a i r l i n e s (Figure 1). The f i r s t f a n , c a l l e d a push f a n , takes ambient a i r from the g i n room and discharges i t through the burner and conveying l i n e s to the tower d r i e r . Seed cotton i s dropped i n t o the heated a i r stream between the burner and tower drier. The hot conveying a i r t r a n s p o r t s the seed cotton through serpentine passageways i n the tower d r i e r and d e l i v e r s i t to an a i r - f e d cleaner or separator, while the hot a i r i s routed to the second c e n t r i f u g a l fan ( p u l l f a n ) . The p u s h - p u l l fan arrangement i s necessary to overcome the a i r f l o w r e s i s t a n c e of the tower d r i e r and a i r l i n e s . A t y p i c a l g i n i s u s u a l l y equipped with two d r y i n g systems of t h i s type, which we commonly r e f e r to as a twostage d r y i n g system. Because of the f i x e d nature of current d r y i n g systems, a gin manager's c o n t r o l of the system i s l i m i t e d to temperature r e g u l a t i o n . The manager has l i t t l e c o n t r o l over i n i t i a l moisture content, ambient r e l a t i v e humidity, or the amount of equipment used. There i s l i t t l e l i k e l i h o o d that a manager w i l l ever have an a p p r e c i a b l e amount of c o n t r o l over i n i t i a l moisture content, but w i t h minor m o d i f i c a t i o n s to the d r y i n g system, he can e x e r c i s e greater c o n t r o l of equipment usage and can o f t e n take advantage of low ambient humidity c o n d i t i o n s to reduce f u e l consumption. During p e r i o d s of low r e l a t i v e humidity, cotton o f t e n a r r i v e s at a g i n with a f i b e r moisture content w i t h i n or s l i g h t l y below the optimum moisture range. Under these c o n d i t i o n s seed cotton d r y i n g i s not r e q u i r e d and a c o n s i d e r a b l e amount of f u e l can be saved by t u r n i n g o f f the burners. However, i n most gins i t i s s t i l l necessary to route seed c o t t o n through the tower driers. In these i n s t a n c e s a tower bypass (Figure 1) can be used to f u r t h e r reduce energy consumption. The a i r f l o w r e s i s t a n c e



114

SEED COTTON INPUT \ PUSH FAN

BURNER r

TOWER I BYPASS^,

SEPARATOR

Figure 1.

TOWER DRIER

PULL FAN

Flow diagram of a typical single-stage, seed cotton drying system equipped with a tower bypass


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of the drying system i s g r e a t l y diminished by bypassing the tower d r i e r — s o much so that one of the c e n t r i f u g a l fans can be e l i m i n a t e d . Therefore, i n a g i n having a two-stage d r y i n g system two fans can be e l i m i n a t e d , thereby saving about 12 kwh/bale (Table I I I ) . Table I I I .

Energy Consumption of Air-Moving Fans In a T y p i c a l Two-Stage Seed Cotton Drying System, With and Without Bypasses. a

Air-moving fan

Two push fans Two p u l l fans Total kilowatt-hours

Energy consumed (kwh/bales) f o r t y p i c a l two-state d r y i n g system with: A i r f l o w through Drying towers bypassed d r y i n g towers 12 12 24

0 12 12

of f o s s i l f u e l e q u i v a l e n t s .

Close a t t e n t i o n to equipment u t i l i z a t i o n and d r i e r temperature can g r e a t l y reduce energy consumption during seed cotton drying. Although the amount of f l e x i b i l i t y a v a i l a b l e to a manager w i l l depend upon operating c o n d i t i o n s and equipment design, a few of the many p o s s i b i l i t i e s a v a i l a b l e are given i n Table IV. The wide range i n energy consumption r a t e s f o r the v a r i o u s a l t e r n a t i v e s shown i n t h i s t a b l e i n d i c a t e s that c a r e f u l management of a d r y i n g system can be a v a l u a b l e conservation t o o l . Insulated Drying Systems: Seed cotton d r y i n g systems are t y p i c a l l y constructed of 16- to 22-gage uninsulated s t e e l sheet. These systems, because of l a r g e surface areas and high thermal c o n d u c t i v i t i e s , l o s e l a r g e q u a n t i t i e s o f heat through the w a l l s of the conveying conduits and tower d r i e r s . I t i s not uncommon f o r the d r y i n g a i r temperatures t o drop 100° to 200°F while passing through the d r y i n g system (12). U n t i l about 25 or 30 years ago, many drying systems at cotton gins were i n s u l a t e d , p a r t i c u l a r l y those at which tower d r i e r s were outside the g i n b u i l d i n g . Over the years outside d r y i n g i n s t a l l a t i o n s were g r a d u a l l y r e l o c a t e d t o the i n s i d e of g i n b u i l d i n g s , and i n s u l a t e d systems l o s t t h e i r appeal. A l s o , with low f u e l p r i c e s there was l i t t l e f i n a n c i a l i n c e n t i v e f o r i n s u l a t i n g d r y i n g systems. However, recent increases i n f u e l p r i c e s and the increased p o s s i b i l i t y of f u e l curtailment during the ginning season have prompted a r e e v a l u a t i o n of these systems. Studies have r e c e n t l y been completed i n which an i n s u l a t e d seed cotton d r y i n g system was compared with an i d e n t i c a l


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Table IV.

Drying Needs

Ambient humidity range

3

High Moderate

Low

None

A l t e r n a t i v e Equipment Arrangements f o r Various Seed Cotton Drying Requirements and Ambient R e l a t i v e Humidities.

All Medium to h i g h Low High Medium Low High Medium to low

Equipment arrangement No. 2 No. 2 No. 1 No. 1 burner d r i e r burner d r i e r output usage output usage

Energy consumption range, kwh/bale b

110-150

High

Use

High

Use

Medium High Low Low Off Off

Use Use Use Use Use Bypass

Medium Off Low Off Off Low

Use Bypass Use Bypass Use Use

65-110 60-80 45-65 30-40 20-30 30-40

Off

Bypass

Off

Bypass

10-15

c

Q u a l i t a t i v e d e s c r i p t i o n of the amount o f d r y i n g needed t o l o v e r f i b e r moisture content t o the optimum range o r t o maintain moisture content w i t h i n the optimum range. Estimated per-bale energy consumed i s expressed i n kwh o f f o s s i l f u e l equivalents. F i b e r s w i t h i n or s l i g h t l y below the optimum moisture range may r e q u i r e some heat to maintain these moisture l e v e l s when ginning under h i g h r e l a t i v e humidity c o n d i t i o n s .


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i n i n s u l a t e d system (13). Each d r y i n g system c o n s i s t e d of a d i r e c t - f i r e d burner, a p u s h - p u l l fan system, a 24-shelf tower d r i e r , and about 90 l i n f t o f conveying l i n e . For the i n s u l a t e d system, a l l conveying l i n e s were covered with 1 - i n - t h i c k f i b e r g l a s s b a t t , and the tower was covered w i t h 1 - i n - t h i c k , r i g i d f i b e r g l a s s board. The thermal conductance of the f i b e r g l a s s i n s u l a t i o n equaled 0.25 Btu/hr/ft /°F and the i n s u l a t i o n was rated f o r a maximum o p e r a t i n g temperature of 450°F. Drying a i r temperatures were monitored at the burner o u t l e t , c o t t o n mixpoint, and tower o u t l e t while p r o c e s s i n g seed cotton a t a r a t e of 8 b a l e s / h r . Drying response was determined from moisture samples taken before and a f t e r d r y i n g . Appropriate data from these s t u d i e s are summarized i n Tables V and VI. 2

Table V.

Drying Temperatures and Performance C h a r a c t e r i s t i c s of I n s u l a t e d and Uninsulated Single-Stage Drying Systems.

A i r temperature (°F) a t : Temperature Burner Cotton Tower drop, °F h o u t l e t mixpoint outlet

Type of system d

Insulated Uninsulated

350 350

300 281

160 122

140 159

Drying response «r/lb c

120 92

The d r y i n g systems were operated at a seed cotton drying r a t e of 8 b a l e s / h r . D i f f e r e n c e i n temperature between c o t t o n mixpoint and tower outlet. Amount o f moisture removed from seed cotton w i t h an i n i t i a l moisture content o f 8.6%, i n g r a i n s / l b o f seed cotton. One-inch-thick f i b e r g l a s s i n s u l a t i o n , k»0.25 B t u / h r / f t /°F.

At equal burner o u t l e t temperatures, the i n s u l a t e d system operated at higher d r y i n g a i r temperatures than the u n i n s u l a t e d system, both at the c o t t o n mixpoint and tower o u t l e t (Table V ) . At a 350°F burner temperature, the temperature drop from mixpoint to tower o u t l e t averaged 140°F f o r the i n s u l a t e d system and 159 F f o r the uninsulated system. These temperature drops were due not only t o l o s s e s o f heat from the d r y i n g systems, but a l s o , i n p a r t , to h e a t i n g the cotton. The i n s u l a t e d system removed 120 g r a i n s of moisture per pound of c o t t o n ( g r / l b ) , whereas the uninsulated system removed o n l y 92 g r / l b — i n d i c a t i n g an i n c r e a s e i n d r y i n g e f f i c i e n c y due to i n s u l a t i o n . Drying response data were obtained f o r each system over a range of o p e r a t i n g temperatures to estimate energy savings. At equal l e v e l s of d r y i n g , the i n s u l a t e d system operated at lower


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temperatures and consumed l e s s energy than d i d the u n i n s u l a t e d system (Table V I ) . The saving i n energy r e q u i r e d to heat d r y i n g a i r ranged from 20.6% to 26.9%, depending on operating temperature and d r y i n g l e v e l . Since the energy saving tended to decrease with i n c r e a s e s i n o p e r a t i n g temperature, i t appeared that t h i c k i n s u l a t i o n would be advantageous f o r the h i g h temperatures.

Table VI.

E f f e c t of I n s u l a t i o n on Operating Temperatures and Energy Consumption of a Single-Stage Drying System. 3

L e v e l of d r y i n g and type of system^ 70 g r / l b : Insulated Uninsulated 88

105

Burner outlet temp.,°F

Cotton mixpoint temp.,°F

Energy consumption kwh/bale

0

Energy savings**

%

211 274

192 238

29.3 40.1

26.9

gr/lb: Insulated Uninsulated

260 337

230 271

37.7 50.9

26.0

gr/lb: Insulated Uninsulated

310 381

268 304

46.3 58.5

20.9

Seed cotton d r y i n g r a t e of 8 b a l e s / h r . L e v e l of d r y i n g i n g r a i n s of moisture removed/lb of cotton.

seed

T h e o r e t i c a l energy r e q u i r e d to heat ambient a i r to burner o u t l e t temperature. The percentage of r e d u c t i o n i n energy consumption f o r heating gained through use of i n s u l a t i o n .

An energy saving of the magnitude experienced i n t h i s study would represent a cost saving of 10 to 15 cents/bale f o r ginners who i n s u l a t e two d r y i n g systems. At an estimated i n s t a l l e d cost of $1,500, gins could recover t h e i r investment a f t e r ginning 10,000 to 15,000 bales of cotton. Most gins could achieve these break-even volumes i n about 1 to 3 years. Heat Recovery Devices: The heated a i r used f o r seed cotton d r y i n g i s normally exhausted to the atmosphere a f t e r one pass through a tower d r i e r . Since the exhaust a i r i s s t i l l r e l a t i v e l y


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Ginning

warm (100°-200°F), t h i s p r a c t i c e wastes about 10% t o 20% of the thermal energy used f o r d r y i n g (14). U n f o r t u n a t e l y , d i r e c t reuse of the exhaust a i r i s complicated by the need t o c l e a n the a i r before i t i s reheated i n d i r e c t - f i r e d burners. During drying and the subsequent seed cotton s e p a r a t i o n process, drying a i r becomes contaminated w i t h l e a f t r a s h , f l y l i n t , and dust. A d d i t i o n a l a i r c l e a n i n g devices and more elaborate conduit arrangements increase the complexity and cost of r e c i r c u l a t i n g systems. Nevertheless, experimental r e c i r c u l a t i n g systems have been s u c c e s s f u l l y demonstrated (15), but these systems have not gained commercial acceptance. Another p o s s i b i l i t y f o r r e c o v e r i n g heat from exhaust a i r i n v o l v e s the use of a heat exchanger i n the exhaust l i n e . A heat exchanger can be used t o t r a n s f e r some of the heat from the d i r t y exhaust a i r t o f r e s h a i r e n t e r i n g the burner. T h i s t r a n s f e r process r a i s e s the temperature of the f r e s h a i r and reduces the heating load on the burner. P o t e n t i a l f u e l savings r e s u l t i n g from use of heat exchangers depend upon the e f f i c i e n c y of the heat t r a n s f e r process and the amount of r e c o v e r a b l e energy a v a i l a b l e i n the exhaust a i r (Table V I I ) . C h i l d e r s (14) reported that a f u e l saving of 3% to 6% was gained when a heat exchanger was used on the exhausts of u n i n s u l a t e d and i n s u l a t e d d r y i n g systems. In that study a c t u a l heat exchanger e f f i c i e n c i e s ranged from 18% to 27%. M c C a s k i l l (16) reported an e f f i c i e n c y of about 31% f o r a s p e c i a l cyclone-type heat exchanger used i n h i e h temperature a p p l i c a t i o n s . A l s o , commercial heat-pipe exchangers are f r e q u e n t l y rated as high as 50% e f f i c i e n t . Thus, depending on heat exchanger e f f i c i e n c y and temperature of exhaust a i r , a f u e l saving of 3% to 10% appears p o s s i b l e .

Table V I I .

P o t e n t i a l F u e l Savings f o r Insulated and Uninsulated Drying Systems at Various Operating E f f i c i e n c i e s of Heat Exchangers.

Type of system

Energy a v a i l a b l e from exhaust a i r , %

Insulated Uninsulated

20 15

P o t e n t i a l f u e l savings (%) at heat exchanger efficiencies of: 20%

30%

40%

50%

4.0 3.0

6.0 4.5

8.0 6.0

10.0 7.5

P e r c e n t a g e of t o t a l h e a t i n g requirements f o r d r y i n g that i s a v a i l a b l e i n exhaust a i r from r e p r e s e n t a t i v e d r y i n g systems.


120


U t i l i z a t i o n of Ginning Waste I f waste byproducts from the cotton ginning operation are not u t i l i z e d i n some manner, they c r e a t e a d i s p o s a l problem. Conversely, ginning waste i s an e x c e l l e n t source of raw m a t e r i a l that can be converted i n t o energy, o n - s i t e , and u t i l i z e d c o n c u r r e n t l y during the processing operation. Research has shown that the heat value of cotton g i n waste averaged 7,928 Btu/lb (dry weight) or about 7,000 Btu/lb at 11% moisture content (17). S e v e r a l heat-recovering i n c i n e r a t i o n systems are being evaluated at cotton gins across the n a t i o n (18). One of these systems, i n s t a l l e d i n 1973 at the U.S. Cotton Ginning Research Laboratory, S t o n e v i l l e , M i s s i s s i p p i , i s shown s c h e m a t i c a l l y i n F i g . 2. T h i s system i s composed of a continuous t r a s h feeder, two burning chambers, a heat exchanger i n the stack, a modulating h o t - a i r mixing v a l v e , and a conventional g i n - d r y i n g system (16). The t r a s h feed system c o n s i s t s of a h i g h - e f f i c i e n c y cyclone that i s equipped with a vacuum feeder that discharges i n t o a screw conveyor. As the waste moved p r o g r e s s i v e l y through the lower chamber due to the a d d i t i o n a l charges of t r a s h , i t i s reduced to ash and discharged from the i n c i n e r a t o r by the automatic ash removal c y l i n d e r . The i n c i n e r a t o r has operated on the c o n t r o l l e d - a i r p r i n c i p l e with minimum excess a i r . The lower chamber of the i n c i n e r a t o r i s equipped with two n a t u r a l gas burners to i g n i t e the t r a s h and help preheat the chamber during i n i t i a l s t a r t u p s . Those burners can be completely turned o f f when preheating has been accomplished The upper chamber i s equipped with one burner to insure i g n i t i o n of the smoke i n the upper chamber. T h i s burner i s a u t o m a t i c a l l y turned o f f by a c o n t r o l l e r at about 1,400°F. I g n i t i o n i s s e l f s u s t a i n i n g above t h i s temperature. A v a n e - a x i a l fan was i n s t a l l e d at the i n l e t to the heat exchanger to insure p o s i t i v e pressure of the ambient a i r i n the heat exchanger, to overcome the s t a t i c - p r e s s u r e l o s s caused by the heat exchanger and to insure continuous flow of ambient a i r through the heat exchanger to prevent damage by excess heating. The ambient a i r moved i n a cyclone path through the heat exchanger as i n d i c a t e d i n F i g . 3. S p i r a l f i n s i n the stack gas chamber and inner ambient a i r chamber improve the heat t r a n s f e r process by i n c r e a s i n g the exposed surface area and by extending the dwell time of stack gas and ambient a i r . The c o n t r o l system i s composed of a modulating mixing v a l v e , a conventional gin-type gas burner, and a conventional gin-type temperature c o n t r o l l e r . The heated a i r d e l i v e r e d from the heat exchanger enters a s p e c i a l l y designed, modulating h o t - a i r mixing v a l v e ( F i g . 2). T h i s v a l v e i s c o n t r o l l e d by the gin's d r y i n g system c o n t r o l l e r and i s capable of d i s c h a r g i n g heated a i r to the atmosphere, of d i r e c t i n g i t to the gin's d r y i n g system, or of mixing i t with ambient a i r i n the d e s i r e d p r o p o r t i o n and d i r e c t i n g


10.


Figure 2.

Cotton

Ginning

Schematic of heat-recovery incineration system


121



10.

BAKER AND M C C A S K i L L

Cotton

Ginning

123

it

to the gin's d r y i n g system. The c o n v e n t i o n a l gas burner was connected i n s e r i e s with the heat exchanger ( F i g . 2). The gas burner then could a s s i s t with i n i t i a l s t a r t u p and could supplement the heat exchanger i f i t became necessary. Both the modulating v a l v e and the gas burner were regulated by the same temperature c o n t r o l l e r . The i n c i n e r a t o r heat exchanger was the primary heat source, and the gas burner was the secondary source. The c o n t r o l system a u t o m a t i c a l l y turns o f f the gas burner when the i n c i n e r a t o r heat i s s u f f i c i e n t to s a t i s f y the demand. The average temperatures i n the heat-recovery system are shown i n Table V I I I . The average a i r volume d e l i v e r e d to the c o t t o n mixpoint was 6,630 s t d ft^/min. The heat recovered by the heat exchanger and d e l i v e r e d to the feed c o n t r o l was almost one m i l l i o n Btu/hr from the burning of only 450 l b of waste/hr. The o v e r a l l system recovery e f f i c i e n c y was almost 31%. Data r e l a t i v e to the a v a i l a b l e heat and the heat recovered are as follows: A v a i l a b l e heat from combustion (450 l b / h r ) 3,150,000 Btu/hr Recovered heat at c o t t o n mixpoint (309°F) 1,759,000 Btu/hr A v a i l a b l e heat from ambient a i r (91°F) 794,000 Btu/hr System's recovered heat 965,000 Btu/hr System e f f i c i e n c y 30.63%

Table V I I I .

Temperatures

i n Heat-Recovery

Description

system a

High

Low

Average

Stack gas at Heat exchanger i n l e t , °F Heat exchanger o u t l e t , °F

2,357 795

2,116 620

2,251 701

Heated a i r at Heat exchanger i n l e t , °F Heat exchanger o u t l e t , °F Cotton Mixpoint, °F

94 376 318

88 277 261

91 337 302

a

Average f o r 3 hr of burning at a t r a s h feed r a t e of 450 l b / h r . Temperatures were recorded at 1-min i n t e r v a l s .

The s u c c e s s f u l o p e r a t i o n of t h i s system demonstrated the p o t e n t i a l f o r heat recovery from i n c i n e r a t i o n of cotton g i n t r a s h . At a 30% recovery r a t i o , enough heat can be recovered from the i n c i n e r a t i o n process to supply most of the energy r e q u i r e d f o r seed cotton d r y i n g , even i n low-capacity gins (Table IX). Only the s i z e and volume of the ginning o p e r a t i o n w i l l d i c t a t e whether such recovery w i l l be economically f e a s i b l e .


124

ENERGY CONSERVATION IN TEXTILE AND POLYMER PROCESSING Table IX.

Processing rate, bales/hr

P o t e n t i a l f o r Heat Recovery from I n c i n e r a t i o n of Gin Trash. Heat from combustion, m i l l i o n Btu/hr

6 8 10 12 15 20 25 30

8.4 11.2 14.0 16.8 21.0 28.0 35.0 42.0

30% heat recovery for drying, m i l l i o n Btu/hr 2.5 3.4 4.2 5.0 6.3 8.4 10.5 12.6

Based on 200 l b of t r a s h / b a l e , w i t h a heat value of 7,000 B t u / l b .

Summary and Conclusions Many o p p o r t u n i t i e s e x i s t f o r energy conservation i n the ginning o f cotton, p a r t i c u l a r l y f o r seed cotton d r y i n g — a process that accounts f o r a large percentage of the energy consumed at modern cotton gins. S e v e r a l energy conservating techniques a p p l i c a b l e to the d r y i n g process have been i d e n t i f i e d and are a v a i l a b l e to the ginning i n d u s t r y . Improved c o n t r o l of the d r y i n g process through c l o s e management would*eliminate the problem of overdrying, which wastes energy and lowers f i b e r q u a l i t y . Ginners can a l s o reduce energy consumption by taking advantage of low r e l a t i v e - h u m i d i t y c o n d i t i o n s that e x i s t p e r i o d i c a l l y i n many areas of the cotton belt. Under such c o n d i t i o n s , unheated ambient a i r has considerable drying p o t e n t i a l . Some cotton a r r i v e s at the g i n with a moisture content w i t h i n the optimum range f o r ginning, and such cotton r e q u i r e s no a d d i t i o n a l d r y i n g . The tower d r i e r s can be bypassed i n these s i t u a t i o n s . Such bypassing would r e s u l t i n s u b s t a n t i a l savings i n e l e c t r i c energy. Drying e f f i c i e n c y can be improved by reducing heat l o s s e s from the d r y i n g system. R e l a t i v e l y inexpensive i n s u l a t i o n on conveying l i n e s and tower d r i e r s can reduce f u e l consumption as much as 27%. F u e l savings could a l s o be r e a l i z e d by r e c o v e r i n g waste heat from the exhausts of d r y i n g systems. Heat exchangers i n d r y i n g system exhaust l i n e s can reduce f u e l consumption by 3% t o 10%, depending on heat exchanger e f f i c i e n c y . Ginning waste i s a v a l u a b l e source of energy f o r cotton g i n s . Each pound of g i n waste has a heat value of about 7,000 Btu. A 30% recovery of heat from the i n c i n e r a t i o n of g i n waste i s


10.

BAKER AND MC CASKiLL

Cotton

Ginning

125

s u f f i c i e n t t o provide most o f the heat r e q u i r e d f o r seed cotton drying. The energy conserving techniques d e s c r i b e d i n t h i s paper are t e c h n i c a l l y f e a s i b l e , but economic f e a s i b i l i t y o f t e n depends on g i n s i z e and annual volume. Although a l l o f the techniques d e s c r i b e d may not be a p p l i c a b l e t o a l l g i n s , most gins could e f f e c t i v e l y use one o r more o f the techniques t o reduce f u e l consumption and p r o c e s s i n g c o s t s .

Literature Cited 1.

USDA, Cotton Gin Equipment, USDA, Agr. Market Serv. Cotton Div. Rpt. (1977), 3. 2. Fuller, S. and Washburn, Μ., Factors Affecting Consumption and Cost of Energy Utilized in the Cotton Ginning Process, presented at Symposium on Managing Energy Wisely in Cotton Gins at Lubbock, Texas, October 3, 1977, 12. 3. Wilmont, C. A. and Watson, Η., Power Requirements and Costs for High-Capacity Cotton Gins, USDA Market Res. Rpt. No. MRR 763 (1966), 23. 4. Shaw, D. L . , Cleveland, Ο. Α., and Ghetti, J. L . , Economic Models for Cotton Ginning, USDA, Econ. Res. Serv., Texas Tech Univ. Pub. No. T-1-158 (1977), 60. 5. Willcutt, Η., Effects of Feeding Systems on Gin Output and Energy Consumption, The Cotton Gin and Oil Mill Press (1976) 77 (18), 14-16. 6. Childers, R. E . , and Baker, R. V., Effect of Moisture Conditioning on Ginning Performance and Fiber Quality of High Plains Cotton, Trans. ASAE (1978), 21 (2), 379-384. 7. Leonard, C. G., Ross, J. E . , and Mullikin, R. Α., Moisture Conditioning of Seed Cotton in Ginning as Related to Fiber Quality and Spinning Performance, USDA Market Res. Rpt. (1970) No. MRR 859, 16. 8. Moore, V. P. and Griffin, A. C., The Relationship of Moisture to Cotton Quality Preservation at Gins, USDA-ARS (1964), ARS 42 105, 11. 9. Leonard, C. G., Controlling Static Electricity on Cotton During Ginning With an Antistatic Agent, USDA Agr. Res. Serv. Rpt. (1960), ARS 42-39, 16. 10. Cocke, J. B., Kirk, I. W., and Wesley, R. Α., Spinning Performance and Yarn Quality as Influenced by Harvesting, Ginning, and Mill-Processing Methods, USDA Market Res. Rpt. (1977), No. MRR 1066, 25. 11. Mangialardi, G. J. and Griffin, A. C., Moisture Restoration to Cotton at the Gin: Effects on Fiber and Spinning Proper ties, USDA Market Res. Rpt. (1965), No. MRR 708, 10. 12. Cocke, J. B., Effects of Input Temperature and Air Volume on Moisture Removal from Seed Cotton, USDA Agr. Res. Serv. Rpt. (1975), ARS-S-67, 8.



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13. 14. 15. 16. 17. 18.

Childers, R. Ε., Insulated Drying System: Key to Conserving Fuel, Cotton Ginners' Journal and Yearbook (1978), 46 (1), 6-9. Childers, R. Ε., Heat Recovery Systems for Cotton Gin Driers, presented at Southwest Regional Meeting ASAE, February 5-8, 1978. Leonard, C. G. and Gillum, Μ. Ν., The Monoflow Air System for Cotton Ginning, The Cotton Gin and Oil Mill Press (1968), 69 (11), 10,11 and 23,24. McCaskill, O. L. and Wesley, R. Α., Energy From Cotton Gin Waste, Cotton Ginners' Journal and Yearbook (1976), 44 (1), 5-14. Griffin, A. C., Fuel Value and Ash Content of Ginning Waste, Trans. ASAE (1976), 19 (1), 156-158, 167. Lalor, W. F., Jones, J. Κ., and Slater, G. Α., Test Results from Waste-Fired Gin Dryers, Cotton Inc., Agro-Industrial Rpt. (1977), 3 (7), 1-16.

RECEIVED

March 6, 1979.


Energy Conservation in Textile and Polymer Processing - American

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