Methane Production by Anaerobic Digestion of ... - ACS Publications

12 Methane Production by Anaerobic Digestion of Bermuda Grass DONALD L. KLASS and SAMBHUNATH GHOSH

Downloaded by UNIV OF OKLAHOMA on October 27, 2014 | http://pubs.acs.org Publication Date: January 29, 1981 | doi: 10.1021/bk-1981-0144.ch012

Institute of Gas Technology, 3424 South State Street, Chicago, IL 60616

It is now clear that every technically and economically feasible source of additional methane must be tapped to meet the growing demand for natural gas. One potentially large-scale source of methane is land- and water-based biomass which can be converted to substitute natural gas (SNG) by a variety of techniques. Because biomass is a renewable nonfossil carbon source that derives its energy from photosynthetic fixation of ambient carbon dioxide, the concept could lead to the development of perpetually available SNG supplies (1). Perennial grasses have been suggested as one category of land-based biomass suitable for conversion to methane (2). Most perennial grasses can be grown vegetatively, and they reestablish themselves rapidly after harvesting. Also, more than one harvest can usually be obtained per year. The warm-season grasses are preferred over the cool-season grasses because their growth rate increases rather than declines as the temperature rises to its maximum in the summer months (3). In certain areas, rainfall is adequate to permit harvesting every 3 to 4 weeks from late February into November, and yields between 18 to 24 metric ton/ac-yr (8 to 10 short ton/ac-yr) of dry grass equivalent are believed to be attainable in managed grasslands (3).

0097-6151/81/0144-0229$0.5.25/0 © 1981 American Chemical Society

In Biomass as a Nonfossil Fuel Source; Klass, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


230

BIOMASS

AS

A

NONFOSSIL

FUEL

SOURCE

Our initial e x p e r i m e n t a l w o r k t o s t u d y t h e c o n v e r s i o n of grass t o m e t h a n e a n d t h e feasibility of d e v e l o p i n g small-scale installations for on-site use by t h e i n d i v i d u a l h o m e o w n e r w a s d o n e w i t h c o m m o n l a w n grass, w h i c h c o n s i s t e d p r e d o m i n a t e l y o f K e n t u c k y bluegrass g r o w n in N o r t h e r n Illinois (4). E x p e r i m e n t s carried o u t in laboratory digesters s h o w e d t h a t t h e grass can be c o n v e r t e d d i r e c t l y t o h i g h - m e t h a n e gas u n d e r c o n v e n t i o n a l anaerobic d i g e s t i o n c o n d i t i o n s . M e t h a n e yields of 2.5 a n d 3.1 SCF/lb volatile solids (VS) a d d e d w e r e o b s e r v e d at m e s o p h i l i c t e m p e r a t u r e s a n d s e m i c o n t i n u o u s d i g e s t i o n c o n d i t i o n s . This c o r r e s p o n d e d t o e n e r g y recovery efficiencies as m e t h a n e of a b o u t 2 8 % a n d 3 4 % . Alkali t r e a t m e n t of K e n t u c k y bluegrass before d i g e s t i o n gave a m e t h a n e yield a n d e n e r g y recovery e f f i c i e n c y of 4.6 SCF/lb V S a d d e d a n d 5 1 % . This paper s u m m a r i z e s t h e p r e l i m i n a r y e x p e r i m e n t a l w o r k carried o u t w i t h t h e w a r m - s e a s o n grass Coastal B e r m u d a grass (Cynodon dactylon) t o s t u d y its c o n v e r s i o n t o m e t h a n e by anaerobic d i g e s t i o n . B e r m u d a grass is w i d e l y d i s t r i b u t e d t h r o u g h o u t t h e t r o p i c a l a n d s u b t r o p i c a l c o u n t r i e s of t h e w o r l d , a n d in t h e U n i t e d States, is best a d a p t e d t o t h e states s o u t h of a line c o n n e c t i n g V i r g i n i a a n d Kansas (5). Coastal B e r m u d a grass, w h i c h tolerates m o r e frost, makes m o r e g r o w t h in t h e fall, a n d r e m a i n s g r e e n m u c h later t h a n c o m m o n B e r m u d a grass, g r o w s tall e n o u g h t o be c u t for hay on a l m o s t any soil (5). From an overall s t a n d p o i n t of d i s t r i b u t i o n a n d g r o w t h c h a r a c t e r i s t i c s . Coastal B e r m u d a grass is a g o o d c a n d i d a t e for biomass e n e r g y a p p l i c a t i o n s . MATERIALS AND METHODS Digesters T h e d i g e s t i o n runs w e r e carried o u t in t h e s e m i c o n t i n u o u s m o d e in w h i c h s e q u e n t i a l w a s t i n g of a p o r t i o n of t h e d i g e s t e r c o n t e n t s a n d f e e d i n g of grass slurry w e r e p e r f o r m e d o n a daily basis u n d e r a n a e r o b i c c o n d i t i o n s . C u s t o m m a d e . 7 - / , c y l i n d r i c a l , f l a t - b o t t o m e d . L u c i t e digesters (7.5 in. ID) h a v i n g w o r k i n g v o l u m e s of 5 / w e r e e m p l o y e d . M i x i n g w a s p r o v i d e d by t w o 3-in. d i a m e t e r , stainless steel, p r o p e l l e r - t y p e impellers m o u n t e d on a single, t o p d r i v e n , stainless steel shaft p o s i t i o n e d in t h e c e n t e r of t h e vessel. The impellers w e r e m o u n t e d 3 in. a n d 6 in. f r o m t h e b o t t o m of t h e d i g e s t e r a n d w e r e d r i v e n by an e x t e r n a l l y m o u n t e d 1 / 8 - h p A C m o t o r at 1 3 0 r p m . Four internal L u c i t e baffles 6 in. x 3 in. X 1 in. s p a c e d 9 0 ° apart w e r e a t t a c h e d t o t h e d i g e s t e r w a l l at t h e 6 in. x 1/4 in. surface. T h e l o n g d i m e n s i o n of each baffle w a s p e r p e n d i c u l a r t o t h e f l a t b o t t o m a n d t o t a l l y i m m e r s e d in t h e c u l t u r e . T h e t o p of t h e digester w a s e q u i p p e d w i t h a t h e r m o m e t e r w e l l , a shaft seal on t h e stirring shaft, a n d f e e d , gas r e m o v a l , a n d gas s a m p l i n g ports. T h e b o t t o m of t h e digester c o n t a i n e d an effluent w i t h d r a w a l port.


12.

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A N D GHOSH

231

Methane from Bermuda Grass

T e m p e r a t u r e w a s c o n t r o l l e d b y m o u n t i n g t h e digesters in a c o n s t a n t t e m p e r a t u r e c a b i n e t in w h i c h preheated air w a s c o n t i n u o u s l y c i r c u l a t e d . T h e gas c o l l e c t i o n a n d m e a s u r i n g s y s t e m f o r each digester w a s m o u n t e d o u t s i d e t h e c o n s t a n t t e m p e r a t u r e c a b i n e t a n d w a s similar in d e s i g n t o t h a t d e s c r i b e d previously (6).


Digester Feeds Coastal B e r m u d a grass w a s o b t a i n e d f r o m t h e N o r t h Louisiana Hill Farm E x p e r i m e n t Station in Homer, Louisiana. T h e s t a t i o n reported t h a t t h e soil f r o m w h i c h t h e grass w a s t a k e n is classified as a S h u b u t a fine s a n d y l o a m , a soil t y p e c o m m o n t o t h e Coastal Plains region o f N o r t h Central Louisiana. The area w a s fertilized w i t h 3 0 0 lb per acre o f Ν as N H N 0 o n A p r i l 2 2 , 1 9 7 7 , a n d w i t h 6 0 lb of K 0 a n d 9 0 lb of P 0 / a c r e o n A p r i l 2 8 , 1977. T h e grass w a s harvested o n M a y 2 3 - 2 4 , 1 9 7 7 w i t h a sickle bar m o w e r , left o n t h e g r o u n d t h e first day, raked into w i n d r o w s t h e next day, a n d baled o n t h e t h i r d day. The yield for t h i s c u t t i n g w a s a p p r o x i m a t e l y 1.5 t o n / a c r e . A b o u t 1 4 0 0 lb (20 bales) w e r e s h i p p e d t o IGT b y t r u c k f r e i g h t a n d arrived o n J u n e 3 0 . 1977. T h e bales w e r e stored under a m b i e n t c o n d i t i o n s in an enclosed trailer. A s received, t h e grass c o n t a i n e d s t e m s a n d blades 3 in. or longer in l e n g t h . 4

2

2

3

5

A 3 0 0 - l b s a m p l e of t h e grass w a s g r o u n d t w i c e in a n Urschel Laboratory Grinder (Comitrol 3 6 0 0 ) e q u i p p e d w i t h a 0.030-in. c u t t i n g head, d r y - m i x e d in a r i b b o n blender, a n d stored in a c o v e r e d 2 0 - g a l plastic d r u m at r o o m t e m p e r a t u r e . A t y p i c a l particle size analysis of t h e g r o u n d grass is s h o w n in Table I, a n d t h e p h y s i c a l a n d c h e m i c a l properties of t h e as-received a n d g r o u n d grass as w e l l as t h e g r o u n d grass after r o o m - t e m p e r a t u r e storage f o r 3 m o n t h s are s h o w n in Table II. Feed slurries w e r e prepared fresh daily b y b l e n d i n g t h e required a m o u n t s o f g r o u n d grass a n d d e m i n e r a l i z e d w a t e r . T h e properties o f feed slurries prepared a b o u t 4 m o n t h s apart are presented in Table III. T h e p H o f t h e d i g e s t e r c o n t e n t s w a s m a i n t a i n e d in t h e 6.8 t o 7.2 range as m u c h as possible by a d d i n g a p r e - d e t e r m i n e d a m o u n t of 1.0 Ν NaOH s o l u t i o n t o t h e feed slurry before d i l u t i o n t o t h e required v o l u m e w i t h w a t e r . W h e n a d d e d n u t r i e n t s o l u t i o n s w e r e used, t h e c o m p o s i t i o n s o f w h i c h are s h o w n in Table IV. pre selected a m o u n t s w e r e also b l e n d e d w i t h t h e feed slurries before d i l u t i o n t o t h e final feed v o l u m e . Analytical Techniques M o s t analyses w e r e p e r f o r m e d in d u p l i c a t e ; several w e r e p e r f o r m e d in t r i p l i c a t e or higher m u l t i p l e s . T h e p r o c e d u r e s w e r e either A S T M , S t a n d a r d


232

BIOMASS

AS A NONFOSSIL

FUEL

SOURCE


Table L PARTICLE SIZE ANALYSIS OF GROUND G R A S S U.S. Sieve Size, mm

Grass Retained on Sieve. wt%

1.18 0.60 0.297 0.250 0.212 0.180 0.149 0.105 0.063

0 0 36.2 55.4 74.6 83.3 87.9 92.6 98.5

Table II. PHYSICAL A N D C H E M I C A L CHARACTERISTICS OF G R A S S

A s Received Ultimate Analysis, w t % C H Ν S Ρ Ca Na Κ Mg Μη Fe Sr Zn Proximate Analysis. w t % Moisture Volatile Matter Ash Organic Components. w t % Crude Protein Cellulose Hemicellulose Lignin High Heating Value. Btu/dry lb Btu/lb (MAF) Btu/lb C Bulk Density, l b / f t 3

9.26 95.3 4.73

After Grinding

After Storage and Grinding

47.1 6.04 1.96 0.21 0.24 0.30 0.08 1.6 0.14 0.01 < 0.005 < 0.001 < 0.005

47.5 6.12

5.15 95.0 5.05

6.26 95.1 4.90

12.3 31.7 40.2 4.1

4.70

8.185 8.616 17.378 23.76

8.162 8.583 17.183


12.

KLASS

A N D GHOSH

233


Table ft C H A R A C T E R B T C S OF FEED SLURRY*


Jury 2 3 . 1977 Density, g / m l at 25°C Total Solids. w t % of slurry Volatile Matter. w t % of slurry Total Alkalinity, m g / / as C a C 0 Bicarbonate Alkalinity, m g / / as C a C 0

0.944 1.59 1.51 211

3

137

3

PH Conductivity, μ m h o / c m Volatile Acids, m g / / Acetic Propionic Butyric Isobutyric

6.02 1.030 104 0 0 _0

Total A s Acetic Chemical Oxygen Demand, m g / / A m m o n i a N. p p m as Ν *

104 35.000 3.5

Formulated for loading rate of 0.1 lb V S / f t volume.

3

November 14, 1.59 1.50 240 143 6.11 1.060 115 24 0 _0 136 32.630 5.0

-day. 12-day detention time. 5 /

culture

Table IV. COMPOSITION OF A D D E D NUTRIENT SOLUTIONS M i x e d Nutrient Component NH CI NaH P 0 KI FeCI 4

2

4

3

MgCI CoCI NaMo0 CuCI MnCI

3.0 20.0 2.0 2.0 2.0 0.25 0.10 0.10 0.10

2

2

2

2

2

Ν

Formulation, g//

concentration. mg/ml

7.85

Ammonium Chloride Solution, g// 120.0

- -

31.42


234 Methods,

BIOMASS

or

special

techniques

as

AS A

reported

NONFOSSIL

previously

FUEL

(6).

SOURCE

Cellulose,

h e m i c e l l u l o s e , a n d l i g n i n in t h e grass a n d d i g e s t e d solids w e r e d e t e r m i n e d by t h e m e t h o d s of Goering a n d v a n Soist (7). Data Reduction


Gas y i e l d , m e t h a n e y i e l d , volatile solids r e d u c t i o n , a n d e n e r g y recovery e f f i c i e n c y w e r e c a l c u l a t e d by t h e m e t h o d s d e s c r i b e d p r e v i o u s l y (6). A l l gas d a t a r e p o r t e d are c o n v e r t e d t o 6 0 ° F a n d 3 0 i n . of m e r c u r y o n a d r y basis. Inoculum, Start-Up and Operation D e v e l o p m e n t of t h e m e s o p h i l i c i n o c u l u m for t h e grass digester runs w a s s t a r t e d o n J u n e 2 2 a n d 2 9 . 1 9 7 7 by a c c u m u l a t i n g d a i l y e f f l u e n t s f r o m e x i s t i n g l a b o r a t o r y d i g e s t i o n runs o p e r a t i n g o n g i a n t b r o w n kelp a n d o n m i x e d p r i m a r y - a c t i v a t e d s e w a g e sludge. T h e kelp a n d s l u d g e digesters w e r e o p e r a t e d at 3 5 ° C a n d d e t e n t i o n t i m e s of 18 a n d 5.6 d a y s , a n d l o a d i n g rates of 0.1 a n d 0.8 lb V S / f t - d a y . respectively. T h e e f f l u e n t s f r o m these digesters w e r e c o l l e c t e d in o t h e r digesters u n t i l 8 . 7 5 / o f t h e b i o m a s s c u l t u r e a n d 7 . 5 0 / of t h e s l u d g e c u l t u r e h a d been a c c u m u l a t e d . Each d i g e s t e r w a s t h e n o p e r a t e d s e m i c o n t i n u o u s l y for 10 d a y s t o stabilize its p e r f o r m a n c e . On J u l y 15. 1 9 7 7 . 1.75 / o f b i o m a s s c u l t u r e a n d 0.75 / o f s l u d g e c u l t u r e w e r e anaerobically t r a n s f e r r e d t o each of t w o B e r m u d a grass digesters w h i c h w e r e t h u s started w i t h 2.5 / of m i x e d i n o c u l u m . These digesters w e r e t h e n o p e r a t e d in t h e s e m i c o n t i n u o u s m o d e w i t h a d a i l y f e e d i n g a n d w a s t i n g s c h e d u l e d e s i g n e d t o increase c u l t u r e v o l u m e s b y 1 0 % per d a y t o a v o l u m e of 5 . 0 / w h i l e m a i n t a i n i n g t h e d e t e n t i o n t i m e a n d l o a d i n g c o n s t a n t at a b o u t 15 days a n d 0.1 lb V S / f t - d a y . T h e digesters w e r e fed w i t h kelp, s l u d g e , a n d grass. The ratio of kelp VS t o s l u d g e VS w a s m a i n t a i n e d at 7 0 : 3 0 . The p e r c e n t a g e s of t h e kelp a n d s l u d g e w e r e g r a d u a l l y decreased w h i l e t h e p e r c e n t a g e of grass w a s g r a d u a l l y increased d u r i n g t h i s t r a n s i t i o n . The c u l t u r e v o l u m e of 5 . 0 / w a s a t t a i n e d o n J u l y 2 3 . 1 9 7 7 a n d t h e kelp a n d s l u d g e w e r e c o m p l e t e l y d i s p l a c e d b y B e r m u d a grass b y A u g u s t 19. 1 9 7 7 . D i g e s t i o n w a s t h e n c o n t i n u e d at t h e selected o p e r a t i n g c o n d i t i o n s w i t h grass feed only. 3

3

Steady-state

digestion

was

defined

in t h i s

work

as o p e r a t i o n

without

s i g n i f i c a n t c h a n g e s in t h e gas p r o d u c t i o n rate, gas c o m p o s i t i o n , a n d e f f l u e n t characteristics. Usually, o p e r a t i o n for t w o or t h r e e d e t e n t i o n t i m e s e s t a b lished steady-state p e r f o r m a n c e . W i t h t h e e x c e p t i o n of Run 1, w h i c h d i d not achieve steady state, selected steady-state results are s h o w n in Table V. Run 1 is t h e e x p e r i m e n t started as i n d i c a t e d above t o establish a baseline w i t h o u t a d d e d n u t r i e n t s . Runs 5 and


12.

KLASS AND GHOSH

235


Table V. SUMMARY Of SELECTED STEADY-STATE DATA Run 2

Run 3

Run 4

Run 5

Run 6

Run 7

Run 8

7 5

7 5

7 5

7 5

7 5

7 5

7 5

c

c

c

c

c

c

c

M D 0 35 68 0.10 12

M D MN 35 6.9 010 12

M D MN 35 6.9 010 12

7 5 C M D MN 35 69 0.10 12

M D Ν 35 6.8 0.10 12

M D Ν 35 6.8 010 12

M D Ν 35 6.8 0.10 12

M D Ν 55 6.4 0.10 12


Run1 Operating Conditions Digester Volume. / Working Volume. / Agitation Schedule Agitation Type Feeding Frequency Nutrients Added Temperature. C pH Loading Rate, lb VS/ft -day Detention Time, day Total Solids in Feed Slurry. wt% Volatile Solids in Feed Slurry. wt% C/N Ratio in Feed Slurry Caustic Requirements, m- q/f Feed Gas Production Gas Production Rate. vol/vol-day Gas Yield. SCF/lb VS added Methane Yield. SCF/lb VS added Methane Concentration. mol% Coefficient of Variation. Methane Yield Efficiencies Volatile Solids Reduction. % Energy Recovered as Methane. % 3

9

3

3

e

b c

3

d

1 59

1 59

1 59

1.59

1.59

1.59

1.59

1 59

1.51 240 72

1.51 16 3 66

1 51 12.3 66

1.51 83 103

1.51 12.3 81

1.51 83 81

1.51 6.3 36

1.51 12.3 42

0313

0.459

0407

0.398

0.350

0.464

0.587

0527

587 351 59.8

527 2.73 51.8

0

Effluent Volatile Acids, m g / / as HOAc c

a

b

0

d

3 13 1 92 61.4

459 268 583

407 245 603

398 2.45 61.7

3.50 2.20 637

464 290 62.4

10

7

6

10

9

13

9

10

200 22 6

293 31 5

260 288

25.4 289

22.1 258

29.7 34.1

37.5 41.2

33.7 32.1

1.989

2.056

1.300

2.123

2.540

1.159

354

2.273

"C" denotes continuous agitation " M " denotes continuous mechanical mixing "D" denotes daily feeding and wasting cycles "Ο" denotes no nutrients added to feed slurry. "MN" denotes mixed nutrient solution added to feed slurry " N " denotes ammonium chloride solution added to feed slurry pH maintained in indicated range by periodic NaOH additions. Mean values. Did not achieve steady state


236

BIOMASS AS A NONFOSSIL F U E L

SOURCE

6. 7 w e r e s e q u e n t i a l l y d e r i v e d f r o m Run 1. Runs 2, 3, a n d 4 w e r e s e q u e n t i a l l y d e r i v e d f r o m a replicate of Run 1. T y p i c a l p e r f o r m a n c e of one of t h e runs (Run 7) over an e x t e n d e d t i m e is s h o w n in Figure 1. T h e r m o p h i l i c Run 8 w a s i n i t i a t e d by a n a e r o b i c a l l y a c c u m u l a t i n g t h e e f f l u e n t f r o m Run 7 u n t i l 5.0 / h a d been c o l l e c t e d . T h e c u l t u r e w a s m a i n t a i n e d in t h e b a t c h m o d e at 5 5 ° C for a f e w d a y s , a n d t h e n s e m i c o n t i n u o u s o p e r a t i o n w i t h grass w a s started at a d e t e n t i o n t i m e of 5 0 days a n d a l o a d i n g rate of 0.02 lb V S / f t - d a y . T h e a m m o n i u m c h l o r i d e n u t r i e n t s o l u t i o n (Table IV) w a s a d d e d t o t h e feed slurry at a rate of 1.0 m l of s o l u t i o n per 0.02 l b / f t - d a y l o a d i n g . The l o a d i n g rate w a s g r a d u a l l y increased at a c o n s t a n t d e t e n t i o n t i m e ; 0.1 lb V S / f t - d a y l o a d i n g rate w a s a t t a i n e d in 10 days. T h e d e t e n t i o n t i m e w a s t h e n g r a d u a l l y r e d u c e d t o 12.0 days over a 10-day period. O p e r a t i o n of Run 8 w a s t h e n c o n t i n u e d at t h e t a r g e t c o n d i t i o n s . 3


3

3

Dewatering Testa Gravity s e d i m e n t a t i o n tests w e r e c o n d u c t e d b y a m o d i f i e d AEEP m e t h o d (8) in w h i c h a 4 0 0 - m I s a m p l e of e f f l u e n t w a s e x a m i n e d in a 1 - / g r a d u a t e d c y l i n d e r g i v i n g a f l u i d d e p t h of 140 m m . T h e h e i g h t of t h e interface b e t w e e n t h e t h i c k e n e d s l u d g e a n d clarified s u p e r n a t a n t is p l o t t e d versus t i m e . V a c u u m f i l t r a t i o n tests w e r e c o n d u c t e d by a m o d i f i e d AEEP m e t h o d j 9 ) in w h i c h a 0.05 f t c i r c u l a r L u c i t e leaf c o v e r e d w i t h an Eimco No. N Y - 4 1 5 m o n o f i l a m e n t filter c l o t h w a s used in a 1 - / beaker c o n t a i n i n g 4 1 7 m l of e f f l u e n t sample. 2

DISCUSSION Feed Properties A l l of t h e d i g e s t i o n runs w e r e carried o u t w i t h small particle-size grass t o f a c i l i t a t e m a x i m u m gas yields a n d p r o d u c t i o n rates in t h e laboratory w o r k . S o m e m o i s t u r e loss w a s o b s e r v e d on g r i n d i n g as e x p e c t e d , and no s i g n i f i c a n t c h a n g e s w e r e d e t e c t e d on r o o m - t e m p e r a t u r e storage of t h e g r o u n d grass (Table III). Several c h a r a c t e r i s t i c s of t h e p a r t i c u l a r lot of grass a n d r e s u l t i n g feed slurries e x a m i n e d in t h i s w o r k i n d i c a t e d t h a t a n a e r o b i c d i g e s t i o n u n d e r c o n v e n t i o n a l c o n d i t i o n s m i g h t not p r o v i d e g o o d m e t h a n e f e r m e n t a t i o n . The mass ratios of C / N (24), C/P (196), C/Ca (157). a n d C / M g (336) in t h e d r y grass solids a n d t h e C O D / N ratio (105-112) in t h e feed slurry appear t o o h i g h w h e n c o m p a r e d w i t h t h e c o r r e s p o n d i n g ratios s u p p l i e d by suitable substrates s u c h as s e w a g e s l u d g e a n d g i a n t b r o w n kelp. T h e q u a n t i t i e s of a m m o n i a Ν (3.5 m g / / ) . Ca (48 m g / / ) , Na (13 m g / / ) , a n d M g (23 m g / / ) in t h e feed slurries f o r m u l a t e d t o m e e t t h e desired loading rate a n d d e t e n t i o n t i m e


12.

KLASS AND GHOSH

237


c o n d i t i o n s are also less t h a n t h e s t i m u l a t o r y c o n c e n t r a t i o n s r e c o m m e n d e d for anaerobic t r e a t m e n t of w a s t e (10). The c o n c e n t r a t i o n s of N, P. Na, Ca, a n d Mg

might

thus

have

t o be a d j u s t e d

to promote

adequate

methane

p r o d u c t i o n . A l s o , t h e relatively l o w p H (6.0) a n d b i c a r b o n a t e alkalinity (137 mg//

as C a C O ^ of t h e feed slurry indicate poor b u f f e r i n g c a p a c i t y a n d

potential problems w i t h pH control during digestion.


Mesophilic Digestion Representative d a t a f r o m Run 1, w h i c h d i d n o t achieve steady state, as previously m e n t i o n e d , are s h o w n in Table V. This represents t h e baseline r u n w i t h o u t a d d e d n u t r i e n t s . It w a s f o u n d t h a t t o m a i n t a i n p H in t h e 6.8-7.2 range, 7 2 m e q N a O H / / o f feed slurry w a s r e q u i r e d ; this raised t h e s o d i u m ion c o n c e n t r a t i o n in t h e digester t o a b o u t 1 6 7 0 m g / / . Overall, t h e results o f Run I w e r e poor. The m e t h a n e y i e l d , volatile solids r e d u c t i o n , a n d energy recovery e f f i c i e n c y as m e t h a n e in t h e p r o d u c t gas w e r e l o w , a n d t h e volatile acids c o n c e n t r a t i o n in t h e digester e f f l u e n t w a s h i g h . T h e p e r f o r m a n c e of Run 1 a n d t h e c o m p o s i t i o n a l data i n d i c a t i n g possible n u t r i t i o n a l deficiencies led t o t h e e v a l u a t i o n of Runs 2, 3, a n d 4 at t h e s a m e o p e r a t i n g c o n d i t i o n s as Run 1 e x c e p t t h a t t h e m i x e d n u t r i e n t s o l u t i o n in Table IV w a s a d d e d t o t h e feed slurry t o raise t h e c o n c e n t r a t i o n s of t h e n u t r i e n t s . S u f f i c i e n t n u t r i e n t f o r m u l a t i o n w a s a d d e d t o reduce t h e C / N ratios of Runs 2. 3. a n d 4 t o 16.3. 12.3. a n d 8.3, r e s p e c t i v e l y Substantial i m p r o v e m e n t s w e r e o b s e r v e d in t h e p e r f o r m a n c e of these runs, b u t t h e volatile acids c o n c e n t r a t i o n s in t h e digester e f f l u e n t s w e r e still h i g h . Also, there d i d n o t seem t o be a correlation b e t w e e n gas p r o d u c t i o n a n d t h e concentration of added mixed nutrient solution. S i g n i f i c a n t i m p r o v e m e n t in digester p e r f o r m a n c e w a s also observed w h e n pure N H C I n u t r i e n t s o l u t i o n (Table IV) w a s a d d e d t o t h e feed slurry as s h o w n by t h e results in Table V f o r Runs 5. 6, a n d 7. In these e x p e r i m e n t s , t h e r e w a s a g o o d c o r r e l a t i o n of m e t h a n e y i e l d , volatile solids r e d u c t i o n , a n d energy recovery e f f i c i e n c y w i t h t h e c o n c e n t r a t i o n of a d d e d n i t r o g e n . The h i g h e r t h e a d d e d n i t r o g e n c o n c e n t r a t i o n u p t o t h e h i g h e s t c o n c e n t r a t i o n e v a l u a t e d (C/N ratio 6.3; c a l c u l a t e d a m m o n i a N, 1.190 m g / / ) . t h e h i g h e r t h e gas y i e l d . Figure 2. w h i c h i n c l u d e s t h e data f r o m Runs 1. 5, 6, a n d 7 as w e l l as d a t a f r o m t h r e e o t h e r runs n o t s h o w n in Table V, illustrates t h i s correlation. N o i n h i b i t i o n b y a d d e d n i t r o g e n w a s observed a l t h o u g h it m i g h t be e x p e c t e d at c o n c e n t r a t i o n s a b o v e 1.500 m g / / (JO). 4

Run 7 e x h i b i t e d t h e best m e t h a n e yield of 3.51 SCF/lb V S a d d e d a n d t h e h i g h e s t volatile solids r e d u c t i o n a n d e n e r g y recovery efficiencies o f Runs 1 t o 7. A l s o , t h e volatile acids c o n c e n t r a t i o n in t h e digester effluent is in t h e range


238


BIOMASS AS A NONFOSSIL F U E L SOURCE

Figure 2.

Effect of ammonium chloride added to feed slurry on methane yield: 35°C, 0.1 lb VS/ff-day, 12-day detention time


12.

KLASS AND GHOSH

239


e x p e c t e d f o r b a l a n c e d d i g e s t i o n . T h e plot in Figure 2 s u p p o r t s t h e c o n c l u s i o n t h a t t h e particular lot o f Coastal B e r m u d a grass evaluated in this w o r k w a s n i t r o g e n - l i m i t e d u n d e r anaerobic d i g e s t i o n c o n d i t i o n s , a n d t h a t c o n t i n u e d a d d i t i o n o f a m m o n i u m c h l o r i d e u p t o t h e h i g h e s t c o n c e n t r a t i o n s t u d i e d (C/N ratio o f 6.3) appeared t o have a s t i m u l a t o r y effect o n m e t h a n e p r o d u c t i o n . Since it is k n o w n t h a t fertilization m e t h o d s a n d dosage rates affect t h e


n i t r o g e n c o n t e n t of Coastal B e r m u d a grass (JJJ, a tradeoff analysis w o u l d have t o be p e r f o r m e d t o establish t h e i n c r e m e n t a l benefits of increased fertilization vs. n u t r i e n t a d d i t i o n if an i n t e g r a t e d p r o d u c t i o n - h a r v e s t i n g gasification system were designed t o manufacture methane. A reasonably g o o d linear c o r r e l a t i o n w a s f o u n d b e t w e e n energy recovery e f f i c i e n c y as m e t h a n e in t h e p r o d u c t gas a n d volatile solids r e d u c t i o n as s h o w n in Figure 3. This t y p e o f c o r r e l a t i o n has also been f o u n d t o exist f o r g i a n t b r o w n kelp after c o r r e c t i o n w a s m a d e f o r a n y h y d r o g e n in t h e p r o d u c t gas (6). Thermophilic Digestion One r u n . Run 8. w a s carried o u t t o s t u d y t h e effect of d i g e s t i o n at t h e r m o p h i l i c t e m p e r a t u r e s . T h e results s h o w n in Table V w e r e o b t a i n e d w i t h s u p p l e m e n t a l n i t r o g e n a d d i t i o n s at t h e same c o n c e n t r a t i o n as t h a t used in Run 5. Run 8 e x h i b i t e d a b o u t 5 0 % h i g h e r gas p r o d u c t i o n a n d volatile solids r e d u c t i o n t h a n Run 5. b u t d u e t o t h e l o w e r m e t h a n e c o n t e n t in t h e p r o d u c t gas, t h e e n e r g y recovery e f f i c i e n c y o f Run 8 w a s o n l y a b o u t 2 4 % higher t h a n t h a t o f Run 5. It is a p p a r e n t also t h a t t h e h i g h volatile acids c o n c e n t r a t i o n in t h e d i g e s t e r effluent o f Run 8 does n o t indicate b a l a n c e d d i g e s t i o n . Carbon a n d Energy Balancée It w a s d i f f i c u l t t o c a l c u l a t e c a r b o n a n d e n e r g y balances f o r t h e digester runs p e r f o r m e d w i t h Coastal B e r m u d a grass because o f t h e a d d i t i o n of relatively large q u a n t i t i e s of alkali f o r p H c o n t r o l a n d of n u t r i e n t s . These a d d i t i v e s c o n t r i b u t e d t o ash w e i g h t s . T w o t e c h n i q u e s w e r e used t o c i r c u m v e n t these p r o b l e m s . O n e a s s u m e d t h a t a d d e d NaOH w a s c o n v e r t e d t o N a H C 0 o n a s h i n g at 5 5 0 ° C a n d r e m a i n e d in t h e a s h , a n d t h a t a d d e d N H C I w a s c o m p l e t e l y volatilized o n a s h i n g . These a s s u m p t i o n s are p r o b a b l y n o t s t r i c t l y true. T h e o t h e r t e c h n i q u e relied u p o n e x p e r i m e n t a l m e a s u r e m e n t of ash a n d v o l a t i l e solids in t h e d r y d i g e s t e d solids. 3

4

T h e details of o n e set o f c a l c u l a t i o n s b y b o t h t e c h n i q u e s are illustrated in Chart 1 f o r Run 1 w h i c h w a s p e r f o r m e d w i t h o n l y a d d e d alkali. T h e best balance, 1 0 1 % c a r b o n a n d 1 0 0 % e n e r g y a c c o u n t e d for, w a s o b t a i n e d b y t h e



35°C, 0.10 lb VS/ft3-day, 12-day DT 40.0 meq NaOH/1. feed, 20.0% VS Reduction 0

Carbon 96.66 χ 0. 376 = 36.3 lb Energy 96.66 χ 6.237 = 602,900 Btu

Method 1* 72.09 lb VS 24.57 lb Ash 96.66 lb

Carbon 88.15 χ 0.376 = 33.1 lb Energy 88.15 χ 6.237 = 549,800 Btu

Method 2^ 72.09 lb VS 16.06 lb Ash 88.15 lb

Digested Solids

Carbon 0.744 χ 12.0 = 8.9 lb Energy 3.13 χ 0.614 χ 1,012 χ 90.11 = 175,300 Btu

282.0 + 379=0.744 mole gas

3.13 SCF gas/lb VS added x90.11 lb VS added =282.0 SCF gas

Gas

X

40.0 meq NaOH 0.417 1. feed 1. feed dajr 417 g feed 1.59 wt % TS in feed day 100 X

0.084 g NaHCOa formed meq NaOH 5.05 wt % ash in TS 100

/Ash and VS experimentally determined to be 18.22 wt % and 81.78% on dry digested solids.

4.74 lb original ash +

4.74 = 24.57 lb ash

•Ash calculated by assuming 40.0 meq NaOH added/1, feed slurry for pH control converted to NaHCCX; on ashing at 550°C and 0.417 1. feed equal 417 g feed. Thus:

Carbon 54.85 χ 0.471 = 44.7 lb Energy 94.85 χ 8,185 = 776,300 Btu

5.15 lb H2O 90.11 lb VS 4.74 lb Ash 100.00 lb

100 lb Grass

Accounted For: Feed Carbon 101%*, 94%/ Feed Energy 100%*, 93%^

Chart 1. Carbon and Energy Balance For Ron No. 1


3

>

>

>

δ

Ν) Ο

12.

KLASS AND GHOSH

241


t e c h n i q u e t h a t a s s u m e d t h e a d d e d alkali w a s c o n v e r t e d t o b i c a r b o n a t e in t h e ash. T h e t e c h n i q u e t h a t relied o n e x p e r i m e n t a l ash a n d volatile solids d e t e r m i n a t i o n s gave results t h a t a c c o u n t e d f o r less t h a n t h e feed c a r b o n a n d energy. But t h i s d i d n o t o c c u r in t h e o t h e r c a l c u l a t i o n s , t h e results o f w h i c h are s u m m a r i z e d in Table VI. Higher a n d l o w e r d e v i a t i o n s f r o m 1 0 0 % o c c u r r e d w i t h each t e c h n i q u e .


Properties of Effluent and Digeated Solids A s already p o i n t e d o u t , t h e volatile acids c o n c e n t r a t i o n s in t h e effluents f r o m m o s t of t h e runs w e r e h i g h . T h e detailed b r e a k d o w n o f t h e i n d i v i d u a l acids a n d o t h e r properties are s u m m a r i z e d f o r t h e feed slurry a n d effluents f r o m Runs 1, 7. a n d 8 in Table VII. The effects of t h e a d d e d NaOH a n d N H C I o n t h e alkalinity, a m m o n i a n i t r o g e n , a n d specific c o n d u c t i v i t y are in t h e e x p e c t e d d i r e c t i o n s . The c o n v e r s i o n of n o n - a m m o n i a n i t r o g e n t o a m m o n i a n i t r o g e n in Run 1 , w h i c h w a s c o n d u c t e d w i t h o u t a d d e d N H C I , d u r i n g t h e d i g e s t i o n process is e v i d e n t ; t h e a m m o n i a n i t r o g e n increased f r o m 3.5 m g / / in t h e fresh feed slurry t o 7 6 m g / / in t h e effluent. 4

4

Gravity s e d i m e n t a t i o n a n d v a c u u m f i l t r a t i o n tests w e r e c o n d u c t e d o n t h e u n c o n d i t i o n e d e f f l u e n t f r o m Run 7. T h e e f f l u e n t e x h i b i t e d rapid s e t t l i n g velocities (Figure 4) a n d t h e f i l t r a t i o n c h a r a c t e r i s t i c s w e r e excellent (Table X). T h e properties o f t h e d i g e s t e d solids f r o m Runs 1, 7, a n d 8 are c o m p a r e d w i t h t h o s e o f t h e d r y feed solids in Table IX. For t h e t o t a l d i g e s t e d solids, c a r b o n c o n t e n t a n d h e a t i n g values decreased a n d t h e ash c o n t e n t increased as e x p e c t e d d u r i n g d i g e s t i o n . The h e a t i n g values of t h e d i g e s t e d solids per mass u n i t o f c a r b o n , h o w e v e r , d i d n o t e x h i b i t a general decrease o n d i g e s t i o n . On t h i s basis, t h e h e a t i n g value o f t h e d i g e s t e d solids f r o m Run 1 w a s s l i g h t l y less t h a n t h e c o r r e s p o n d i n g value of t h e feed solids, w h i l e t h e h e a t i n g values of t h e solids f r o m Runs 7 a n d 8 w e r e higher. These t r e n d s are p r o b a b l y t h e result o f different rates o f b i o d e g r a d a b i l i t y of t h e o r g a n i c c o m p o n e n t s in t h e grass, each o f w h i c h w o u l d be e x p e c t e d t o have d i f f e r e n t h e a t i n g values. To a t t e m p t t o a c q u i r e i n f o r m a t i o n o n t h e d e g r a d a b i l i t i e s o f t h e major classes of o r g a n i c c o m p o n e n t s in t h e grass, t h e c o n v e r s i o n data s u m m a r i z e d in Table X w e r e derived f r o m t h e c o m p o s i t i o n a l d a t a in Table IX b y a s s u m i n g t h a t a decrease in t h e c o n c e n t r a t i o n o f a n y o r g a n i c c o m p o n e n t w a s caused b y g a s i f i c a t i o n . This a s s u m p t i o n is n o t s t r i c t l y t r u e , b u t it p e r m i t s firsta p p r o x i m a t i o n c a l c u l a t i o n s of relative biodegradabilities. Hemicellulose, w h i c h w a s present in t h e h i g h e s t c o n c e n t r a t i o n , w a s c o n v e r t e d t o gas at t h e h i g h e s t yield w i t h o n e e x c e p t i o n , w h i l e t h e c r u d e protein f r a c t i o n a n d t h e l i g n i n f r a c t i o n gave t h e l o w e s t gas yields. T h e e x c e p t i o n appears t o be


242

BIOMASS AS A NONFOSSIL FUEL SOURCE Table VI. S U M M A R Y OF C A R B O N A N D ENERGY B A L A N C E S Accounted For Feed Carbon, % 94.0 .101 a

b

93.4 .100

b

Run 7

85.7 ,115

b

96.7 . 116

b

Run 8

107 ,116

a

a

a

a

106 . 116

b

a

b

Calculated from experimental determinations for moisture, volatile solids, ash. carbon, and heating values of feed and digested solids, a n d yield and composition of product gas. Volatile solids in digested solids calculated f r o m percent volatile solids reduction.

a

b Downloaded by UNIV OF OKLAHOMA on October 27, 2014 | http://pubs.acs.org Publication Date: January 29, 1981 | doi: 10.1021/bk-1981-0144.ch012

Feed Energy, %

Run 1

Calculated from parameters in footnote " a " except that ash in digested solids estimated by assuming original ash in feed is in digested solids, that NaOH used for pH control is converted t o N a H C 0 on ashing at 500° C and remains in ash. that that N H CI. if added, is volatilized o n ashing. 3

4

Table VII. C O M P A R I S O N OF FEED A N D DIGESTER E F F L U E N T SLURRIES Parameter*

Feed

Run 1

Run 7

Run 8

pH

6.0 221

6.8 3.182

6.8 2.571

6.4 2.906

137 3.5 1.030

1.525 76 5.000

2.276 738 12.540

1.013 500 6.250

104 0 0 0 0 0 0 104

1.461 541 43 42 25 54 0 1.989

238 126 5 9 2 3 0 354

1.821 323 71

Total Alkalinity, m g / / as CaCU3 Bicarbonage CaC0

Alkalinity,

m g / / as

3

A m m o n i a Nitrogen, m g / / Specific Conductivity, μιτιηο/οπη Volatile Acids (Filtrate), m g / / Acetic Propionic Butyric Isobutyric Valeric Isovaleric Caproic Total A s Acetic *

125 72 18 14 2.273

Mean Values

Table VIII. VACUUM FILTRATION CHARACTERISTICS OF UNCONDITIONED DIGESTER EFFLUENT. RUN 7 TEST TEMPERATURE 26"C

TS.wt% 1

6

6

1

6

6

Effltftnt VS.wt%ofTS 788 78 8

Çjkj TS.wt% VS.wtttofTS 16 8 96 9 97 1 1

6

3

YirtdDry Cake, lb/ft -hr Filtrate, lb/lb dry cake 12.3 115 13.1 155 2

30 sec cycle time. 6 sec form time. 12 sec drying time. 12 sec removal time. 20 in. Hg.


KLASS A N D GHOSH


243


12.

Figure 4. Interface height vs. time for gravity settling of unconditioned effluent from Run 7


244

BIOMASS AS A NONFOSSIL

FUEL

SOURCE

Table IX. C O M P A R I S O N OF DRY FEED A N D DIGESTED SOLIDS

Dry Feed

Run 1

Digested Solids* Run 8 Run 7


Ultimate Analysis. w t % C H

47.1

37.6

30.2

43.9

6.04

5.43

Ν

1.96

4.78 2.34

6.65

5.50 2.34

81.8 18.2

78.8 21.2

80.2

14.6 17.7

15.9

37.9

13.6

19.5 13.6

Proximate Analysis, w t % 5.15 95.0

Moisture Volatile Matter Ash

5.05

19.8

Organic Components. w t % 12.3

Crude Protein (Kjeldahl Nx6.25) Hemicllulose

40.2 31.7

Cellulose Lignin

4.1

Heating Value Btu/dry lb

*

Btu/lb ( M A R

8.185 8.616

Btu/lb C

17.378

17.5 7.4

4.5

6.237

6.021

7.625 16.588

7.641 19.957

7.721 9.627 17.588

Prepared by evaporation of total effluent t o dryness o n steam bath, pulverization, and drying in an evacuated desiccator t o a constant weight.

Table X. C O M P A R I S O N OF ORGANIC C O M P O N E N T CONVERSION

Component

Hemicellulose Cellulose Crude Protein Lignin *

M a s s Ratio In Feed Solids

Run 1 Run 7 Gasified, Gasified, wt% Ratio* wt% Ratio* 1.0 0.63

0.30

55.1 43.8 0

0.10

0

0

1.0 0.79

Run 8 Gasified. Ratio* wt%

1.0 0.76

16.2

1.0

64.5

45.3

2.2

9.3

0.01

0

67.3

0 0

Expressed as mass of indicated component gasified per unit mass of hemicellulose gasified.



12.

KLASS A N D GHOSH

245


t h e r m o p h i l i c Run 8, in w h i c h less h e m i c e l l u l o s e w a s c o n v e r t e d t h a n cellulose. The h i g h e r level of u n c o n v e r t e d h e m i c e l l u l o s e t h a n cellulose in Run 8 m a y be an artifact c a u s e d b y c o n v e r s i o n of cellulose at t h e r m o p h i l i c c o n d i t i o n s t o d e r i v a t i v e s w h i c h are d e t e c t e d in t h e h e m i c e l l u l o s e f r a c t i o n . T h e e x p e r i m e n t a l d a t a also i n d i c a t e t h a t a small a m o u n t o f t h e lignin f r a c t i o n w a s c o n v e r t e d in Run 7. Overall, t h e p o l y s a c c h a r i d e f r a c t i o n is m o r e b i o d e g r a d a b l e t h a n t h e p r o t e i n a n d lignin f r a c t i o n s . T h e l o w b i o d e g r a d a b i l i t y of l i g n i n u n d e r a n a e r o b i c d i g e s t i o n c o n d i t i o n s is e x p e c t e d , w h i l e t h e h i g h e r b i o d e g r a d a b i l i t y of t h e h e m i c e l l u l o s e m i g h t be p r e d i c t e d because it has a h i g h e r r e a c t i v i t y t o a c i d a n d alkali t h a n cellulose. The l o w e r b i o d e g r a d a b i l i t y of p r o t e i n w i t h respect t o t h e m o n o s a c c h a r i d e s a n d p o l y s a c c h a r i d e s in g i a n t b r o w n kelp has been r e p o r t e d (12). Thermodynamic Estimates T h e e n t h a l p y o f d i g e s t i o n , p r o d u c t gas c o m p o s i t i o n , a n d m e t h a n e yield f o r t h e lot of grass e x a m i n e d in t h i s w o r k w e r e e s t i m a t e d as s h o w n in Chart 2. T h e process is p r o j e c t e d t o be s l i g h t l y e x o t h e r m i c , — 1 7 3 B t u / l b grass reacted, w h i c h agrees w e l l w i t h t h e slight e x o t h e r m i c i t y r e p o r t e d f o r kelp (6). A s s u m i n g t h a t 2 0 % o f t h e c a r b o h y d r a t e f r a c t i o n a n d 7% o f t h e p r o t e i n present in t h e grass w o u l d be c o n v e r t e d t o n e w bacterial cells b y anaerobic f e r m e n t a t i o n a n d h e n c e n o t be available f o r m e t h a n e p r o d u c t i o n b y a single pass t h r o u g h t h e f e r m e n t o r , t h e m a x i m u m t h e o r e t i c a l yield of m e t h a n e is g i v e n b y (12): (1 lb VS a d d e d -

0.176 lb VS t o cells) f \0.95 7

9

2

S

C

-

F CH

M lb V S

/

6.87

S

C

F

C

h

4

lb VS-pass

T h e h i g h e s t e x p e r i m e n t a l yield o b t a i n e d in t h i s w o r k is 3.5 SCF/lb V S a d d e d (Run 7) or 5 1 % o f t h e t h e o r e t i c a l m a x i m u m value. T h u s , c o n s i d e r a b l e yield i m p r o v e m e n t s are still possible. Comparison W i t h Other Substrates T h e gas yields a n d volatile solids r e d u c t i o n a n d e n e r g y recovery efficiencies f r o m Coastal B e r m u d a grass are c o m p a r e d in Table XI w i t h t h o s e o b t a i n e d f r o m o t h e r substrates u n d e r similar m e s o p h i l i c d i g e s t i o n c o n d i t i o n s . T h e n i t r o g e n - s u p p l e m e n t e d B e r m u d a grass w a s c o n v e r t e d t o m e t h a n e a t a b o u t t h e s a m e efficiencies as g i a n t b r o w n kelp a n d p r i m a r y s e w a g e sludge. T h e n o n - s u p p l e m e n t e d B e r m u d a grass, h o w e v e r , afforded c o n s i d e r a b l y l o w e r m e t h a n e yields a n d efficiencies t h a n these substrates. U n t r e a t e d K e n t u c k y bluegrass w a s s o m e w h a t b e t t e r in p e r f o r m a n c e t h a n Coastal B e r m u d a grass.



246

BIOMASS AS A NONFOSSIL F U E L

Chart 2.

Summary of Thennodynamk Calculations

I.

Crass Composition

C, 47.10 wt% H, 6.04 N, 1.96 S, 0.21 0, 39.64* Ash, 5.05

II.

E m p i r i c a l Formula

C

III.

SOURCE

3

q

2

(dry)

0 ^

^

N

S

Q u

Ashj , Mol. Wt. 100

Q Q Q J

Q

Heat of D i g e s t i o n , Gas Composition, Methane Y i e l d a.

Assuming Ν and S can be neglected and C1U and C 0 are the o n l y products 2

C

b.

3

H

92 5 99 °2 48

+

1

,

1

8

2 H

2

°

"

2

,

0

8

9

C

H

+

"

,

8

3

Gas composition:

53.3 moIX CH*,

Methane Y i e l d :

7.92 SCF CHw/lb grass r e a c t e d (max.)

For 1.00

46.7 molZ C 0

1

1C

°

2

2

l b dry grass r e a c t e d :

Input = 8,185

Btu

Output- 8,012

Btu

Heat of Reaction:

8,012

- 8,185

- -173

Btu

*By d i f f e r e n c e .


12.

KLASS AND GHOSH

247


Table XI. C O M P A R I S O N O F COMPOSITIONS A N D S E L E C T E D S T E A D Y - S T A T E ANAEROBIC DIGESTION RESULTS


Compositional Parameter

Coastal Bermuda Grass

C. w t % H. w t % N. w t % Moisture. w t % Volatile matter. w t % Ash. w t % High heating value. Btu/dry lb High heating value. Btu/lb (MAR High Heating value. Btu/lb C Digestion Conditions Mechanical A g i t a t i o n Feeding Frequency Nutrients A d d e d Temperature. °C 3

a

47.1 6.04

45.8 5.9

1.96 5.15 95.0 5.05

8.052

4.620

8.537

8.616

9.309

7.977

11.620

17.378

17.581

16.619

19.513

c D d

Efficiencies Volatile Solids Reduction. % 20.0 Energy Recovered as Methane. % 22.6

d

e

f

9

Sludge

8.185

0.313 3.13 1.92 61.4

c

f

4.8 7.8 86.5 13.5

Gas Production Gas Production Rate, vol/vol-day Gas Yield. SCF/lb VS added Methane Yield. SCF/lb VS added Methane Concentration. mol%

b

Brown Kelp

43.75 6.24

Loading Rate, lb V S / f r -day Detention Time, day C/N Ratio in Feed Slurry

a

Primary

Giant 0

27.8 3.73 1.63 88.8 57.92 42.08

O 35 6.8 0.10 12 24.0

b

Kentucky Bluegrass

C D N

C D 0

35 6.8 0.10 12 6.3

35 7.1

C D

3.16 94.12 73.47 26.53

C D 0

0.13 12 9.54

0 35 7.0 0.10 12 17.1

35 7.0 0.10 12 13.8

3.51 55.9-

0.55 4.20 2.54 60.4

0.662 6.62 3.87 58.4

0.78 7.8 5.3 68.5

37.5 41.2

25.1 27.6

43.7 49.1

41.5 46.2

e

0.587 5.87

9

" C " denotes continous agitation. " D " denotes daily feeding and wasting cycle. " O " denotes no nutrients added to feed slurry. " N " denotes a m m o n i u m chloride added t o feed slurry. RunBofRef.4. Run 1 of Table 5. Run 7 of Table 5. R u n 2 o f R e f . 12. Experimental data obtained w i t h primary thickened sewage sludge in laboratory digesters under standard high-rate conditions. Sludge obtained f r o m Metropolitan Sanitary District of Greater Chicago.

American Chemical Society Ubraiy Ï1M 169 St N. W.

In Biomass as a Nonfossil Fuel Source; Klass, D.; D. C. 2003· ACS Symposium Series;ttllBglm, American Chemical Society: Washington, DC, 1981.

248

B I O M A S S AS A N O N F O S S I L F U E L

SOURCE


S U M M A R Y AND CONCLUSIONS B e r m u d a grass (Cynodon dactylon) is o n e of t h e h i g h - y i e l d w a r m - s e a s o n grasses t h a t has been s u g g e s t e d as a p r o m i s i n g r a w material for c o n v e r s i o n t o m e t h a n e . E x p e r i m e n t a l w o r k p e r f o r m e d w i t h laboratory digesters t o s t u d y t h e a n a e r o b i c d i g e s t i o n of Coastal B e r m u d a grass h a r v e s t e d in Louisiana a n d h a v i n g a C / N ratio of 2 4 is d e s c r i b e d . M e t h a n e yields of a b o u t 1.9 SCF/lb of v o l a t i l e solids (VS) a d d e d w e r e o b s e r v e d u n d e r c o n v e n t i o n a l m e s o p h i l i c high-rate conditions. W h e n supplemental nitrogen additions were made, t h e m e t h a n e yields increased. This o b s e r v a t i o n a l o n g w i t h t h e c o m p o s i t i o n a l d a t a c o m p i l e d o n t h e grass used in t h i s w o r k i n d i c a t e d t h a t t h e n i t r o g e n c o n t e n t of t h e u n s u p p l e m e n t e d grass w a s i n s u f f i c i e n t t o sustain h i g h - r a t e d i g e s t i o n at t h e h i g h e r y i e l d level. H o w e v e r as t h e C / N ratio w a s r e d u c e d by a d d i t i o n of a m m o n i u m c h l o r i d e , t h e m e t h a n e yield c o n t i n u a l l y increased up t o 3.5 SCF/lb a d d e d at t h e l o w e s t C / N ratio e x a m i n e d (6.3) even after relatively h i g h c o n c e n t r a t i o n s of a m m o n i u m n i t r o g e n w e r e m e a s u r e d in t h e effluent. It appears t h a t t h e a d d e d n u t r i e n t had a s t i m u l a t o r y effect on methane production above the point where nitrogen w a s not limiting. Thermophilic digestion w i t h supplemental nitrogen additions afforded m e t h a n e yields of a b o u t 2.7 SCF/lb V S a d d e d . Carbon a n d e n e r g y balances w e r e c a l c u l a t e d a n d t h e relative b i o d e g r a d a b i l i t i e s of t h e o r g a n i c s w e r e estimated. It w a s c o n c l u d e d f r o m t h i s w o r k t h a t Coastal B e r m u d a grass c a n be c o n v e r t e d t o h i g h - m e t h a n e gas u n d e r c o n v e n t i o n a l a n a e r o b i c d i g e s t i o n c o n d i t i o n s T h e p e r f o r m a n c e of t h e p a r t i c u l a r lot of grass s t u d i e d w a s s u b s t a n t i a l l y i m p r o v e d by s u p p l e m e n t a l n i t r o g e n a d d i t i o n s . A C K N O W L E D G E M EN Τ T h e a u t h o r s w i s h t o express t h e i r a p p r e c i a t i o n for t h e f i n a n c i a l s u p p o r t of t h e w o r k d e s c r i b e d in t h i s paper b y U n i t e d Gas Pipe Line Co.. a n d especially for t h e m a n y v a l u a b l e d i s c u s s i o n s a n d s u g g e s t i o n s p r o v i d e d by Dr. V i c t o r E d w a r d s a n d Robert C h r i s t o p h e r of U n i t e d . T h e a u t h o r s also a p p r e c i a t e t h e assistance s u p p l i e d by M i k e Henry. A l Iverson. Frank Sedzielarz. a n d Janet Vorres. w h o p e r f o r m e d t h e e x p e r i m e n t a l d i g e s t i o n s t u d i e s , a n d by J a m e s I n g e m a n s o n a n d Robert Stotz a n d t h e i r staff w h o p e r f o r m e d m a n y of t h e c h e m i c a l analyses. Special t h a n k s is g i v e n t o Mr. D a w s o n J o h n s of N o r t h Louisiana Hill Farm E x p e r i m e n t S t a t i o n for s u p p l y i n g t h e B e r m u d a grass a n d i n f o r m a t i o n on its p r o d u c t i o n .


12. KLASS AND GHOSH Methane from Bermuda Grass

249


REFERENCES

1.

Klass, D. L. Chemtech 1974, 4, 161-68.

2.

Klass, D. L. 169th National Meeting, American Chemical Society, April 1975; Energy Sources 1977, 3 (2), 177-95.

3.

InterTechnology Corp. October 1975, American Gas Association Project IU-114-1, Final Report.

4. Klass, D. L.; Ghosh, S.; Conrad, J.R. "Symposium Papers", Clean Fuels From Biomass, Symposium sponsored by the Institute of Gas Technology, Orlando, Fla., January 1976; Institute of Gas Technology: Chicago, Ill., 1976, pp. 229-52. 5. Burton, G. W. In "Forages The Science of Grassland Agriculture"; Highes, H. D.; Heath, M. E.; Metcalfe, D. S., Eds.; The Iowa State College Press: Ames, Iowa; Chapter 24. 6.

Klass, D. L.; Ghosh, S. "Symposium Papers", Clean Fuels From Biomass and Wastes, Symposium sponsored by the Institute of Gas Technology, Orlando, Fla., January 1977; Institute of Gas Technology: Chicago, Ill. 1977; pp 323-51.

7.

Agricultural Research Service, Washington, D.C., December 1970, United States Department of Agriculture, Agriculture Handbook No. 379.

8.

Association of Environmental Engineering Professors, "Environmental Engineering Unit Operations and Unit Processes Laboratory Manual", O'Connor, J. T., Ed.,III-1-1,July 1972.

9.

Ibid., V-2.

10.

McCarty, P. L. Public Works 1964 (November), 91-4.

11.

Johns, D. M. "Fertilization of Coastal Bermuda Grass on a Coastal Plain Soil", North Louisiana Hill Farm Experiment Station, Homer, Louisiana.

12. Klass, D. L .; Ghosh, S.; Chynoweth, D. P. 175th National Meeting, American Chemical Society, Anaheim, Calif., March 1978. RECEIVED JUNE 18, 1980.


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