Journal of Scientific & Industrial Research Vol.60, January 200 I, pp 52-59

Pyrolysis of Solid Wastes

A N E Rahman, M A Khaleel Akmal and K B S Prasad

Indian Institute o f Chemical Technology, Hyderabad 500 007 , India

Rece ived: 19 June 2000; accepted : 18 September 2000

Pyro lysis of solid wastes like papers , rags, garden wastes. used tea leaves and mun icipal solid waste (MSWJ is carried out in a fix ed bed reactor for stud ying distri but ion or energy in pyrolytic products and their possibl e applicati ons as fuel. Pyrolysis is conducted between 450°C to 650°C and at different react ion times. The temperature of 600°C is found to be good forgett ing max imum yield or tar and is enhanced from 16 per cent to 27.6 per cent as the pyrolysi s time reduces from 35 min to 5 min. The average yield of cha r and gas is found between 30-34 per cent and 16-20 per cent respectively, irrespect ive of reaction time. The ca lori fic values or so lid , liquid and gaseous prod cts o r pyro lysis of· all the wastes show a good fuel potential. The ta r can be used as furnace oil , char can be utili zed as a feedstock for prod ucing ac ti ve carbon and gas as a fu el for industria l fu rnaces or internal combust ion engines.

Introduction

The rapid growth of industria li zati on and urban­ization culminated the problem of waste di sposal. The co llec ti on, transport and disposal of sol id wastes, th ough an important step and also a serv ice, to be rendered by any municipality, less attent ic.. n is being paid since it in­vo lves huge expenditure. Municipal so lid waste is the cheapest and abu ndantly avai lable renewable source of energy. Wastes in India are considerabl y diffe rent from those produced in advanced cou ntries due to food hab­its, cu lture and trad ition . The annual output of garbage in India is estimated at 20 million tonnes equivalent to I 0 million metric tonnes of coa l replacement 1

• The main energy component in the so lid waste with an average of 8000 Btu/lb ( 18 mJ/kg) is its cellulos ic content.

The usual method of wastes disposal as land fi !l ­ing causes land and air pollution. In the process of in­cinerat ion the heat re leased by complete combusti on can generate steam and power. Pyrolysis of waste materials yields so li d, liquid and gaseous fuels and also minimizes atmospheric pollution .

Various types of processes of pyrolys is for mu­nicipal refuse main ly employ fix ed beds viz., the Union Carbide Purox Process1

. The Andco Torrax fluidized bed process' and the Occidental Flash pyro lys is in en­trained bed are reported from Japan and theUS .The ob­ject ive of these processes is to produce fu el gas except for the Occidental Flash Pyrolysis process of the US wh ich produces fuel oi l.

In the present invest igation, laboratory sca le results are reported on both norma l and rapid pyro lyti c behaviour of some common wastes li ke paper, rags. gar­den wast e, used tea leaves and municipal sol id was tes, which mainly constitute the municipal wastes.

Materials and Methods

The laboratory set up for norma l pyrolys is studies is shown in Figure I. The reactor was made of pyrex glass (50 mm ID X 750 mm long). It was in serted into a horizontal electric furnace to maintain the required tem­perature. The volatile products (moisture, oil vapours and gas) were led to a U-tube kept i the ice bath for affecting condensat ion. The 1 ixture of non-condens­ab le gas and res idual oil vapours was scrubbed counter currentl y with acetone in a tower packed with glass rings . Acetone was found to be a better sol vent than benzene or tol uene as it left no traces of oil · nd the surface con­tainer. For the co llection of gas, a calibrated gas holder was prov ided. An open tube manometer, to ind icate the system pressure as the pyrolysis proceeded, was al so attached. The apparatus used fo r the rapid pyrol ys is was a standard Gray-King Assay assembl y.

Experimental Procedure

( i) Preparation of Feed Stock

(i) Waste paper. (i i ) Garden waste. (iii ) Munici pal so lid waste.

RAHMA et a/..·PYROLYS IS OF SOLID WASTES 53

Acetone

To Atmosphere --~

Stop Cock -.-\-~

Manometer

Furnace

Ice bath

Gas holder

Fi gure 1 -A lahoratory set- up for conducting pyrolysis or so li d wastes

(iv ) Rags. (v) Used tea leaves.

The first three materia ls were crushed in a mirac le mill to a s ize between I to 2 mm. The rags co ll ected from a tail or 's shop, compri sing most ly of sy ntheti c fi­bres , were cut w ith scissors to 2 mm. The muni c ipa l so lid waste was a sy nth e ti c bl e nd of 80 per ce nt compostab le matter (grass, garden waste and vegetables),

8 per cent waste paper, 8 per cent rags and 4 per cent plastics . The composi tion of blend was similar to that

of Hyderabad C ity refuse.

Waste mate ri a l (70 g) was taken in the g lass reac­tor and it was inserted into the e lectri c furnace. The

furnace was heated to 650°C at the rate of 20°C/mi n. The time, system pressure and vo lume of gas evo lved

were noted at inte rva ls of 25°C pyrolysis. As the py­ro lys is of pure cellul os ic materia l yie ld s carbon dioxide upto 3 10°C(ref. 4) the gas up to 250°C was let out into atmosphere for avoiding di luti on of pyro lytic gas . (The 250°C is assumed to be the safest temperature for the materia ls which are not cellul os ic). The assumpti on was va lid as indicated by the gas collected separatel y up to 250°C , contained up to 2 pe r cent carbon diox ide. Gas rate vs temperature and system pressure vs time (Figure 2 and 3) be long to a ll the mate rials under stud y. Ac­etone from the sc rubbin g tower was collec ted and dis­tilled. The tar thus recovered and co llected from the U­tubc trap was accounted for the total quantity produced

54 J SCIIND RES VOL.60 JAN UARY 2001

for each ex periment. The quantity of liquor from U­tube trap and char from the reactor were accounted fo r material balance.

In conducting rapid pyrolysis, about 5 g of mate­rial was filled in the si lica tube of standard Gray King appara tu s. The sliding electric furnace after gett ing heated at 450°C, 550°C or 650°C was moved /mounted over the sili ca tube and retained for 2.5, 5 an I 0 min durat ion, and then qu ickly withdrawn. The products of pyrolysis evolved du ring the reaction time were recov­ered and materi al balance drawn.

Results and Discussion

Proximate and ultimate analyses of the materials used in the present studies are presented in Tabl I. The data is in agreement with the values reported in the lit ­erature for waste material s, indicating a typical charac­teri st ics of cellulose material hav ing hi gh vo latile mat­ter and oxygen with low sulphur content. The materi als used in this stud y have a hi gh elemental carbon. The Gray King assay results are presented in Table 2. The feed materials yielded an average of 20 per cent tar, 28 per cent aqueous I iquor, 33 per cent char and 19 per cent gas by Gray King assay. In fact the yields are dependent upon the size of feed, rate of heating, pressure and tem­perature of pyrolysis5.

Table l -Ana lyses of wastes

Waste paper Rags

Proximate analysis (wt per cent)

Moisture 6.7 6.0 Ash 13.1 2.5 Volatile matter 71.5 80.8 Fixed carbon 8.7 10.7

2 Ulti mate analysis (wt per cent dat) Carbon 48 .0 56.8 Hydrogen 6.2 6.0 Nitrogen 0.4 0.4 Sulphur 0.3 0.3 Oxygen 45.1 36.4

3 C/H ratio 7.7 9.5 4 Ca lorifi c value( k cal/kg) 4444 4743

Pyrolysis

In the present study the pyrolysis us ing the reactor described, was carried out at 650°C and at atmo.-pheric pressure with 20°C/min rate of heati ng us ing feed size of 1-2 mm. It can be seen from the data on pyro lysis (Tab le 3) that the yield of product is in agreement with that generally reported values in literature for biomass and waste materi alr'. The low yie .i d of l< r, as compared to Gray King assay is due to the di fficul [y in its recovery from the reactor surface. The quantity of carbon diox­ide let out into the atmosphere up to 250°C caused a loss in gas balance. However the compos ition of gas is similar to that of Gray-King assay.

The rate of gas evo lution (Figure 2) for the ce llu­los ic materials ranged from 440 to 500 rnl/min but for rags of synthetic fibres, it dropped to 250 ml/min thereby shift ing the peak temperature of pyrolysis from 325°C to 425°C. The active pyro lys is ti me fo r cellulos ic mate­ri als was 5 to 8 min whereas for rags 12 min (Figure 3) . This indicates that pyrolysis of plastic or sy nthetic fibre requ ires higher duration and temperature than for cellu­los ic materials.

The characteristics of char and tar oi Is covering proximate and ultimate analyses, calorific va lue. etc., are presented in Table 4 and 5. It i~ evident that the chars have an average heating va lue equal to that of char­coal so it can be converted to shaped fue l by briquetting.

Garden Used tea leaves MSW* was te

15.3 8.3 14.3 11.4 4.0 7.4 6 1.6 65.5 64 .3 11.7 2 1.3 14.0

56.3 55.0 55 .7 6.5 6.7 6.7 2.5 3.7 2.4 0.5 0.7 0.6

34.2 34. 1 34.6 8.4 8.5 8.3

5118 4976 5264

RAHMAN et a/. : PYROLYSIS OF SOLID WASTES 55

/, 00 0 MSW(Hydl

500 6 Rags

X Tea Waste

• Gorden Was te 0 MSW ( Hydl

450 0 Waste Paper 6 Rags

X Tea Waste 3 00 • Gar den Waste

400 0 Waste Pape r

Cl

E E

c. ~ E 350 " ..... V1

u V1 200 u .. ~ .. a.

0 300 E ~

"' V1 0 ';!. ~ "'

250

100

6 0 0 5 10 20 30 1.0

Time (mrn }

300 350 400 450 500 550 600

Temperature ° C

Figure 2- Gas rate Ys temperature Figu re 3- System pressure Ys time

Tab le 2- Gray King carboni zation assay of was tes (600°C, I h )

Waste paper Rags Used tea Garden waste MSW leaves

Prod uc t yie lds (wt pe r cent )

Char 33. 15 35.3 1 28 .86 37.07 36. 13 Tar 20.63 19.80 2 1.43 2 1.97 20.65 Liquor 29.30 23 .99 29.56 20.69 25.2 1 Gas 16.98 20.41 19.87 19.87 18. 13 Gas (vo l per cent) 11.6 1 14.23 12.82 13.89 13.38 (NI/1 00 g)

2 Compositi on of gas (vo l per cent)

Carbon d iox ide 45 .0 49.0 54.8 49 .8 46.0 Unsaturates 1.2 2.0 1.2 2.2 2.6 Oxygen 0.8 0.4 1.2 2.2 2.6 Carbon monox ide 28 .0 2 1.0 22 .0 15.2 19.0 Hyd rogen 8.6 8.2 7.4 I 0.4 13.4 Methane 9.0 14.4 5.6 11 .4 14.4 Nitrogen 7.4 5.0 7.8 10.6 4.8

3 Ca lorilic va lue of gas (k ca l/ m·1)

2005 2386 1500 2047 2527

56 J SCI I D RES YOL.60 JANUARY 2001

Table :\ - Data on pyro ly> is of solid wastes at 650°C Waste Rags Used tea leaves Garden waste MSW

Product yields

(wt per cen t)

Char

Tar

Li quor

Gas

Losscs

2 Gas compos i li on

(vo l per cent)

Carbon d iox idc

Unsa turatt:s

Oxygen

Carbon monoxide

Hydrogen

Mcthane

Nitrogen

paper

:\4.4

10

28.6

14.0

8.7

46.5

1.4

0.6

27.4

8. 1

8.5

7.5

28 .6 :\4 .:\

20.7 17 .1

28. 6 21.4

14 .6 16.7

7.5 10.5

"i i.O 51.7

0.6 2. 1

O.X 1.0

25 .7 14.4

7.8 10.1

5.7 11.5

.4 9.2

:\ Ca lorilic va lue of gas

(k caVNnr1

)

1953 1537 2005

4 Time of pyrolysis 30 :\8 34

min

The chars from MSW and garden waste are a good feed­stock for active carbon as indicated by their methylene

blue absorption test. T he test has the same va lu ~ for the c har obtained by low te mperature ca rboni za ti o n of Rama g undam coal in Lurgi Squel gas p lant. Ramagundam char is being used in India as a raw mate­

ria l fo r the produc ti on of active carbon. The tar oil obtained by pyrol ys is of wastes con­

tains I 0 pe r cent oxygen (except rags and paper) whereas that got by flash pyrolysis of American MSW con tained 33 per cent which leads to corrosion of burners. The calorifi c value of tar is more than that obtained by flash pyrolysis. Hence, it can be used as a fue l o il o r alter­nate ly it can be thermally cracked for getting low mo­

lecul ar hydrocarbons.

Rapid Pyrolysis The effect s of time and temperature on pyro lysis

of MSW is presented in Figure 4 a and band S a and b and Tab les 6-8. It can be seen that 5 min and 550°C are the I imiting conditions for achieving 27 per cent tar, 32

:\0.0 :\0.0

15 .7 14.3

28.6 30 .0

16.0 14.6

9.7 I 1.1

53.7 48 .5

1.6 1. 8

0.8 0.7

15 .7 18.8

9.7 10.3

13.4 12.6

5.1 n

2 11 5 2191

34 34

pe r cent char and 22 per cent gas . Further inc rease in e ithe r time and temperature does not have any sig nifi­

cant effect on products yield, but with the inc rease in time (more than 5 min ) the calorifi c va lue of gas de­c reases and with the increase in tempe rature (above 550°C) the calorific value o f gas increa. es. This study forms a basis for the se lect ion and design o f an efficient pyro lysis r actor.

Overall the energy distribution in the products of solid wastes was found to be 5, 22 and 7 per cent in char, tar o il and gas respectively the re ma in ing 20 per cen t be ing the losses in the system . Thi s di st ribu tion pattern is in agreement with that of biomass and wastes gener­ally reported except losses of gas in our system which are at a hi gher s ide which presumabl y due to the un ac­countability of gas formed during py ro lysis up to 250 oc. Conclusions

• The munic ipal so lid waste, garden waste. and

2

3

500

"' E ...._ - 300 0

> 200 '-'

100

RAHMAN et a/.. PYROLYSIS OF SOLID WASTES

!Temp. soo•cr

GO

30 ""

"' >-

20

0 o~----------~2~. ~s------------~s~. o------------------------~,~o~.o__J 0

Time o f pyrolysis (min)

Figure 4-Time of pyrolysis Ys yield of product and caloriri c va lue of gas

T<~h i P. 4-- 0<11 ~1 on chms frnm ~n lid WllS IP.S ll nrl P. h;u· of I Tl.

Waste paper Rags Garden waste Used rca leaves MSW

Proximate analysis (wt per cent)

Moisture 3.7 3.0 4.2 4.3 -1.3

Ash 35.4 11 .3 24.9 19.1 23.3

Volat il e maller 10.4 16.4 14.8 12.3 14.5

Fixed carbon 50.5 69.3 56. 1 64.3 57 .9

Ultimate analysis (wt per cen t daf)

Carbon 90.1 90.6 82.2 86.4 10.8

Hydrogen 2.0 2.5 1.9 2.5 2. 1

Nitrogen 0.5 1.0 6.4 6.0 4.5

Sulphur 0.7 0.3 0.8 0.4 08

Oxygen 6.7 5.6 8.7 4.7 8.8

C/H ratio 45.0 36.2 43.3 34.6 39.9

Calorific value of char (k cal/kg) 7957 7992 6868 7476 6966

Methylene blue absorption (rng/g) 8 12 12 6 !4

Tab le 5- Analysi s of pyrolitic tars of so lid wastes and other materia ls

Waste paper Rags Used tea Garden MSW USA*

leaves waste MSW

Ultimate analysis (wt per cent dal)

Carbon 72 .9 76.3 75 .2 76.2 78.0 57.0

Hydrogen 10.3 9.5 10.0 10.5 10.5 7.7

Sulphur 0.4 0 .3 0.7 0.4 0.4 0.2

Nitrogen 0.8 0.3 4.6 3.7 3.3 1. 1

Oxygen 15 .6 13 .6 9.5 9.2 7 .8 33.5

. C/H ratio 7.0 8.0 7.6 7.2 7.4 7.4

Calorific val11e or tar (ca l/kg) 5165 5440 7865 7870 7430 5893

*Tar obtai ned by flash pyrolysis

57

LTC char

4.3

24.5

10.6

60.6

89.2

3.6

0.6

0.6

6.0

24.8

78 16

12

No.6 fuel oi l

85.7

10.5

0.5

2.0

8. 1

10119

sg J SCIIND RES YOL.60 JANUARY 200 1

(Time :Sminl

L.O 3000

10L-----------L-----------~----------J_ __________ _L___________ 0 1.00 450 500 550 600 650 .

Temperat ure ° C

Figun: :; - Temperawrc or pyro ly, is v, y ield or product caloriric va lue or gas

T ah le o - Ellec l llr dural ion on pyrol y, is or MSW al 600°C

Dural ion

2.5 :1.0 I 0 .ll

Yi elds o r prnducl s (W I per ccn l )

Char 3(L2 32.9 32 .::.

T ar 25.2 27.6 27.7

Liquor 17.2 17. 1 17 .7

Gas 2 1A 22.4 22.-+

--1 I 0.0 -- I 00.0 - I 00.0

V ol orgas (inl/ 100 g or ICed! 1-LRo 15.0 15 .2

Composi1 ion or gas (l'n l per ccnl )

Carbon diox ide 52.0 :10.0 51.2

Okrins .l .X -Ul 3. ()

Oxygens 0.6 0.2 0.-+

Carbon mnnox ide: 22 .0 21.0 20.2

Hydrogen 6 .0 5.-+ :1.6

i\'kthanc X.X 12.6 IUl

N i1 rogc n 6.X 7.2 7.2

Caloriric va lue or gas (k c.t i/Nm1

) 2 I I 0 2-l 2X 2:?X)

•

•

RAHM AN et a/.: PYROLYSlS OF SOLID WASTES 59

Tab le 7 - Effect or temperature on rapid pyro lys is of MSW, 5mi n

Details Temperatureoc

450 550 600 650 Product yields (wt per cent) Char 43.4 33.2 33.0 32.6 T ar 2 1.4 27.1 27.6 27.5 Liquor 17 .8 17.5 17.1 17.4 Gas 17.4 22.2 22 .3 22.5 Vol or gas ( in Ill 00 g of Iced) 9.8 14.2 1 15.0 1-U::

2 Composi ti on or gas (vo l per cent ) Carbon 73.0 54.0 50.0 46.6 Unsaturates 2.4 4.0 4.0 -Ul Oxygen 0.2 0.4 0.2 0. -1-Carbon monoxide 20.0 25.6 2 1.0 24.4 Hydrogen 0.4 4.0 5.4 5.0 M ethane 0.8 6.0 12.6 12.4

3 Calorili c va lue or gas (kca i/Nm') 982 1925 2420 251-1-

T ab le 8- Comparison or pyro lyti c yields and gas composi ti on Pyrolysis at 550°C and 5 min W aste paper Rags Garden waste MSW

Product y ields (wt per cent ) Char 34.8 Tar 22.7 Liquor 27.2 Gas 16.3

2 Compositi on or gas (vo l per cent ) Carbon dioxide 52.0 Unsaturates 2.6 Oxygen 1.6 Carbon monox id.:: 29.6 Hydrogen 6. -1-M eth ane 3.X Nitrogen -1- .0

3 Calori l:c ~a lu e or gas 17 17 (k cai/Nm')

similar materials can be pyrolysed to yield gas and tar of moderate heat ing va lue suitabl e for power generation or fu el for internal combusti on eng me.

It can be concluded th at tar obtained by normal pyrolysi s under reduced time is useful as fu el oil.

The presence of syn thetic fibres or plasti cs in wastes needs shift in the pyrolysis reaction time and temperatu re to a littl e hi gher side. This study forms a basis for the des ign of an effici ent reac­tor whi ch is under progress.

25.2 35. 1 26.9 24.6 28.7 18.0 19.2 2 1.9

59 0 5 1.0 3.0 4.0 0.6 1.2

23.2 23.8 4.8 3.8 5.2 9.4 4.2 6.8

1670 2 169

Acknowledgement

33.2 27.1 17.5 22.2

5-1-.0 4.0 0.-1-

25.6 -1- .0 6.0 6.0

1925

The authors thank Dr K V Raghavan, Direc tor. In­dian Institute of Chemical Technology, Hyderabad for giving permi ss ion to publish thi s paper. Thank s are also clue to Mr A P Rudradas for hi s assistance in anal yzi ng raw waste material and the char.

References Manject Kumar & Pande GN. U1ju. l ( I 985) 32

2 Mech Cug. 99 (December 1977 ) 22. 2-1-

3 M cG rowin R. Chc111 D1g Prog. (March I 9!:>5) 57

4 Graham R G . .I Au a/ Af'f' Prml. (September 6. 198-1- l 12 5 I S Method oJTestiug r!f'Coal aud Coke. IS 1353 ( I <J<J .~ ) (J

6 Pul z W & Chartie r K. l:·ucrgv.fiwu bit•111ass iul~·llmfJI ' . ( I') ' 7)

137