experimental studies on low-temperature pyrolysis of municipal household garbage—temperature...

Technical note

Experimental studies on low-temperature pyrolysis

of municipal household garbage—temperature

influence on pyrolysis product distribution

Wang Yana,b, Zhang Shutinga,*, Zhang Yufenga, Xie Huia,Deng Naa, Chen Guanyia

aSchool of Environmental Science and Engineering, Tianjin University, Tianjin PB 300072, PR ChinabSchool of Automation and Energy Engineering, Tianjin University of Technology, Tianjin PB 300191, PR China

Received 16 May 2004; accepted 20 September 2004

Abstract

The low-temperature pyrolysis of the mixture of nine typical components from municipal

household garbage has been experimentally studied in an externally heated fixed-bed pyrolyser, at

temperatures ranging from 300 to 700 8C. The yields of final pyrolysis product varying with

temperatures are presented in this paper. The solid product yield decreases with the increase

of temperature in the test temperature range, and reduces quickly at 300–550 8C but very slowly

at 550–700 8C. However, the pyrolysis liquid yield increases with the increase of temperature,

but reaches the maximum at 550 8C, and afterwards begins to decline. Among liquid product,

cream-shaped tar is found, the yield of which also reaches the maximum at 550 8C. The pyrolysis

gas yield steadily increases with the increase of temperature at the whole test temperature range.

The above-mentioned experimental results could be helpful to the practical application of

low-temperature pyrolysis of municipal household garbage.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Low-temperature pyrolysis; Pyrolysis characteristics; Municipal household garbage; Production

distribution; Fixed bed

Renewable Energy 30 (2005) 1133–1142

www.elsevier.com/locate/renene

0960-1481/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.renene.2004.09.016

* Corresponding author. Tel./fax: C86 22 87402075.

E-mail address: [email protected] (Z. Shuting).

http://www.elsevier.com/locate/renene

W. Yan et al. / Renewable Energy 30 (2005) 1133–11421134

1. Introduction

Municipal household garbage has greatly increased in recent years, so that reducing

waste mass, making it innocuous, and converting it into fuel has become an important

issue which the Chinese government, scientific and technical personnel are eager to

address. The current waste treatment methods include direct landfilling, sanitary

landfilling, composting, incineration, pyrolysis, gasification, and so forth. Direct

landfilling, sanitary landfilling and composting, because of their disadvantages, are

gradually replaced by other methods. Incineration technology, which was widely used in

the past, is being disputed at present because it causes many problems such as producing

dioxins, discarding heavy metal, and wasting heat emitted from burning [1–6]. Applying

the pyrolysis technology to waste treatment can, however, avoid these problems [7]. The

pyrolysis is a complex process by which organic substance of thermal instability in waste

is decomposed by heating in the non-oxidation or anaerobic condition into intermediate

products, which is subsequently condensed into new gas, liquid, and solid. Solid organic

matter pyrolysis has a long-history technology [8] applied to waste disposal recently.

High-temperature pyrolysis and gasification have been already applied to engineering

projects [9,10], but low-temperature pyrolysis technology is still at an experimental stage

[11–16]. The pyrolysis temperature mentioned in literature is mostly higher than 800 8C,

and most test materials are single component. There are few documents referring to low-

temperature pyrolysis of mixed material. Low-temperature pyrolysis is a feasible

technology to make waste unharmful to the environment, to reduce their volume and to

convert them into fuel, so that it is a promising technology in waste treatment.

Present research on low-temperature pyrolysis has mostly focused on the kinetics of

waste by experimenting with a little bit of sample, so the temperature field in test sample is

uniform. In practice, however, great temperature gradients will occur in sample, that is to

say, material in diverse positions will have different temperatures, consequently different

pyrolysis characteristics, different products and different yields. Hence, it is necessary to

study the pyrolysis characteristics of sample at great temperature gradient. Experiment

facility used in this study was designed and manufactured in the lab to simulate the real

pyrolysis characteristics which occur in actual waste pyrolysis process. The experiment

sample is the mixture of typical organic components in municipal household garbage, and

the experiment temperature is in the temperature range of 300–700 8C. In this paper,

influences of temperatures on pyrolysis product yields are described.

2. Experimental facility and material

2.1. Experimental facility

The low-temperature pyrolyser is shown schematically in Fig. 1. The apparatus

includes heating furnace, pyrolysis reactor, condenser, temperature controller, U-tube

pressure gauge, flowmeter, and so on. The heating furnace is an externally heated, fixed-

kiln, and electric-heating furnace with 7.5 kW in power, 200 mm in diameter, and 350 mm

in height. The pyrolysis reactor is made from temperature-resistant stainless steel,

Fig. 1. Schematic of the experiment apparatus. 1Zfixed-bed pyrolysis furnace; 2Zpyrolysis reactor;

3Zthermocouple; 4Ztemperature controller; 5Znitrogen inflating pipe; 6Zpyrolysis liquid gatherer;

7Zthermometer; 8Zcondenser; 9ZU-tube pressure gauge; 10Zflowmeter; 11Zsampling vent.

W. Yan et al. / Renewable Energy 30 (2005) 1133–1142 1135

and there are one vent pipe, one nitrogen inflation pipe, and four thermocouples fixed on

the reactor cover to measure pressure changes in the reactor. A pressure gauge is

installed on the nitrogen inflation pipe. Three thermocouples used to measure material

temperature changes are fixed in the middle of reactor along the radial direction, while

one thermocouple applied to measure pyrolysis gas temperature is fixed on upside of the

test material. In order to completely condense condensable matter in the pyrolysis gas,

three U-shaped double-pipe condensers are installed, on the bottom of which there are

three peach-shaped liquid separating funnels as pyrolysis liquid gatherers to collect

pyrolysis liquid. Temperature measuring points are located on entrances to every

condenser. The pyrolysis gas flow is measured by a cumulative flowmeter. A U-tube

pressure gauge, used to measure pressure changes, is fixed between condensers and the

flowmeter.

2.2. Experimental material

Components of municipal household garbage greatly vary with regions and functions.

In this paper, the test material is waste synthesized artificially with reference to physics

composition of Beijing municipal household garbage [17], with moisture and inorganic

components such as metal and glass removed. Table 1 lists test material components and

their contents. Tables 2 and 3 show the proximate analysis and ultimate analysis of test

material, respectively.

Table 1

Components of test material

Component Content (wt%) Component Content (wt%)

Rubber 8.27 Vegetable 7.30

PVC 8.51 Cloth 5.35

PE 8.51 Paper 18.25

Rice 24.33 Wood chip 4.87

Fruit husk 14.60

Table 2

Proximate analysis of test material

Primary analysis wt%

Ad (ash) (%) 2.47

Vd (volatile matter) (%) 80.24

FCd (fix-carbon) (%) 17.29

Heating value (kJ/kg) 19,901


3. Experiment methods and conditions

Three steps to ensure the accomplishment of this test were taken prior to the

experiment. One, to break material into particles of 1 cm in diameter, desiccate them in the

oven for 11

2h at a temperature between 100 and 105 8C after natural air-drying, then put

600 g of pretreated material into the reactor. Two, to vacuumize the whole test system and

inflate nitrogen into this system. Three, to heat the pyrolysis furnace by electricity. The

material temperature is the most important parameter concerned, but it is very difficult to

measure accurately because large temperature gradients exist between particles, even in a

single particle, owing to the low thermal conductivity of the test material. Gauging the

hearth temperature is much more easy than gauging the actual material temperature, and

the change of hearth temperature can approximately reflect the material temperature

change, so that the hearth temperature can be thought as the ultimate pyrolysis

temperature. During the pyrolysis process, gas gathered from the sampling vent is

analyzed at regular intervals. Pyrolysis liquid is mainly collected by the liquid

gatherer, and the weight of tar adhering to pipe wall can be ascertained by weighing the

pipes before and after test. Pyrolysis gas combusts completely, and combustion products

are exhausted into the atmosphere. After the pyrolysis has finished, solid remains in the

reactor can be collected for further study. All tests were processed under the following

conditions: the range of ultimate pyrolysis temperatures between 300 and 700 8C, test

temperature points set in every 50 8C, slightly positive pressure in whole test system.

4. Experiment results and analysis

After a series of complex physical and chemical reactions, organic matter can be

transformed into solid, liquid and gas products. The factors influencing these products’

Table 3

Ultimate analysis of test material

Ultimate analysis (wt%)

Cd (Carbon) 46.54

Hd (Hydrogen) 6.24

Nd (Nitrogen) 0.67

Od (Oxygen) 43.12a

Cld (Chlorine) 3.44

Sd (Sulphur) 0.23

a The percent of O is obtained by difference.


yields include material component, furnace shape, heating mode, heating temperature,

material size, and gas-phase residence time. The pyrolysis temperature is the most pivotal

factor, so that the experiment goal is to know how the temperature affects the pyrolysis

product yields.

4.1. Temperature effects on pyrolysis solid yields

Solid product remaining in the reactor is carbon-contained residue, which is produced

by removing volatile matter from solid waste and carbonizing it. Owing to the volatile

matter being removed incompletely, solid product is commonly called semi-coke. Fig. 2

show variations of semi-coke yield with ultimate pyrolysis temperatures.

Fig. 2 indicates that semi-coke production decreases gradually with the temperature

rise. While temperature ranges between 300 and 550 8C, semi-coke yield reduces quickly

from 61.17% in weight in 300 8C to 34.83% in 550 8C, that is to say, the yield decreases by

10.54% when the average pyrolysis temperature increases by 100 8C. At the range of

temperature from 550 to 700 8C, the yield declines fairly slowly from 34.83% in 550 8C to

32.67% in 700 8C. Because of this tiny decrease, it is reasonable to think that ultimate

pyrolysis temperature at this range has little influence on the semi-coke yield at this stage.

The pyrolysis reaction of solid waste is a depolymerization and polymerization process.

Main reactions in the stage of temperature from 300 to 550 8C are depolymerization,

decomposition, and degasification. At this stage, a great amount of volatile matter

(condensable gas and non-condensable gas) is exhausted, and semi-coke is produced.

Simultaneously, the higher temperature is provided, the more volatile gas exhausts and

less solid matter remains. During the phase of temperature from 550 to 700 8C, most

volatile matter is removed from organic matter. The obvious characteristic of this stage is

that the secondary decomposition accompanies the polymerization. However, decompo-

sition occurs more frequently than polymerization does, so that semi-coke yield reduces

slowly in this phase. Related tests have indicated that the volatile matter content of semi-

coke decreases with temperature rise and that the volatile matter content rarely changes,

namely the semi-coke yield tends to be steady, when temperatures are more than 550 8C.

Fig. 2. Variations of semi-coke yield with ultimate pyrolysis temperatures.


Document [18], which has studied thermal weight loss of 11 municipal household garbage

components, indicates that thermal weight loss of every component is up to 80% at

the pyrolysis temperature of 500 8C, and does not change when temperatures are more than

500 8C. Experimental results of this test correspond with the above-mentioned findings.

4.2. Temperature effects on pyrolysis liquid yields

Volatile gas produced by the pyrolysis process is partly condensed into what is

commonly known as pyrolysis liquid. Variations of pyrolysis liquid yield with ultimate

pyrolysis temperatures are shown in Fig. 3. Pyrolysis liquid yield, in the whole reaction

stage (temperature from 300 to 700 8C), increases with the temperature rise in the

beginning, and attains the maximum at 550 8C, then declines in spite of the temperature

increase. The main components of pyrolysis liquid are tar and moisture. Moreover, tar is

composed of numerous compounds such as methanol, acetone, formic acid, acetic acid,

aldehyde, benzene, toluene, and other aromatic organic substances. Molecular weights of

these compounds vary from 32 to 10,000, and their boiling points range from 55 to 300 8C,

accordingly only half of these compounds can be distilled from pyrolysis liquid [19].

For dry test material, moisture in the tar is composed of a hydrogen element and an oxygen

element from raw material. It can also be observed from this test that to separate moisture

from the dissoluble tar is very difficult. Meanwhile, the mixture of dissoluble tar and

moisture not only has low heating value, but also gives off a strongly pungent odor, so that

they will pollute environment if not properly disposed of.

Only document [20] has mentioned that volatile pyrolysis products can be condensed

not only into the mixture of dissoluble tar and moisture but also into beige cream-shaped

tar. Variations of cream-shaped tar yields with temperatures are shown in Fig. 4. The yield

varies greatly with temperatures, that is to say, it increases when temperatures are less than

550 8C, and reaches the maximum at 550 8C, then gradually decreases. Moreover, the

cream-shaped tar yield is high at temperatures 450–600 8C. This kind of tar, in practical

pyrolysis process, is liable to condense in pipes and block up them, therefore some

measures, such as avoiding pyrolysis at this temperature range, can be taken to prevent

cream-shaped tar blocking up pipes. Cream-shaped tar, despite its low production, can be

Fig. 3. Variations of pyrolysis liquid yield with ultimate pyrolysis temperatures.

Fig. 4. Variations of cream-shaped tar yield with ultimate pyrolysis temperatures.


collected as fuel because of its high heating value more than 30 MJ/kg and low ash

content. The composition of cream-shaped tar is fairly complex. It is formed by four or five

oligomer groups of molecular weights from 100 to 1000, such as carbonyl group and

hydroxide group [20].

4.3. Temperature effects on pyrolysis gas yields

Gas product means non-condensable gas in pyrolysis products. Ultimate pyrolysis

temperature is the determinant factor of gas production. Fig. 5 shows variations of

pyrolysis gas yield with temperatures. Fig. 6 indicates that gas yield is in direct proportion

to temperatures during the entire pyrolysis process of temperature from 300 to 700 8C.

However, there exists a transitional point at 500 8C. With the temperature less than 500 8C

pyrolysis gas yield raises slowly and the increment is 52.37 L/kg. When temperatures are

more than 500 8C pyrolysis gas yield rises quickly and the increment reaches 135.00 L/kg

nearly 21

2times of the former. The reason for this phenomenon is that 500 8C is the

initiative temperature of secondary decomposition. With the temperature more than

500 8C, not only primary decomposition gas but also secondary decomposition tar will be

Fig. 5. Variations of pyrolysis gas yield with ultimate pyrolysis temperatures.

Fig. 6. Schematic of the organic substance pyrolysis process.


decomposed into gas. Typical reaction formulas of secondary decomposition refer to

formulas (1) and (2):

C2H6/C2H4 CH2 (1)

C2H4/CH4 CC (2)

From reaction formulas listed above, it can be concluded that secondary decomposition

results in the increase of pyrolysis gas yields.

5. Translations between gas mass yield and liquid mass yield

Figs. 3–5 clearly reflect the mass distribution trends of different pyrolysis products.

With temperatures less than 550 8C, semi-coke yield declines distinctly with the

temperature rise, while both gas yield and liquid yield increase at the same temperature

range. Change trends of these products differ from one another while temperature ranges

from 550 to 700 8C: semi-coke yield reduces continuously but more slowly with the

decrease of only 2.16%. Gas production keeps on increasing more rapidly during this

stage. However, pyrolysis liquid yield reaches the maximum at 550 8C, then begins to

decline. The decline of pyrolysis liquid yield is the main reason for the rise of pyrolysis gas

yield. Fig. 6 shows the before-mentioned pyrolysis process of organic substance.

6. Conclusions

The yields of different pyrolysis products from municipal household garbage follow the

following rules:

†
Semi-coke yields have two obvious variation trends at the range of temperature from
300 to 700 8C: while temperature ranges from 300 to 550 8C, semi-coke yields reduce

quickly. At temperature ranging from 550 to 700 8C, the yield reduces slowly.


†
Variation curves of pyrolysis liquid yield with temperatures at the same temperature
range accord with a parabola, and the yield reaches the max value at 550 8C. The yield

of cream-shaped tar in pyrolysis liquid varies greatly with temperature in the range of

450–600 8C, and similarly attains the maximum at 550 8C. However, the yield is less

than 5% while the temperature is under 450 8C and above 650 8C.

†
Volume yields of pyrolysis gas increase during the whole temperature range, and rise
rapidly when temperatures are above 500 8C.

†
When ultimate pyrolysis temperatures are less than 550 8C, solid pyrolysis is the main
reaction. When temperatures are more than 550 8C, the yields of tar, especially cream-

shaped tar, decrease obviously. At the same time, most tar in primary decomposition

products is decomposed into non-condensable gas so that the yield of it will increase.

Acknowledgements

The authors express sincere thanks to National Natural Science Foundation of China.

The research is funded by the NSFC(50378062)

References

[1] Kong HL, Jin BS, Wu HF, Zhong ZP. Dioxins generation hazard and control in MSWI. J Nanjing Architect

Civil Eng Inst 2002;2:31–7.

[2] Wang Z, Li AM, Ba DC, Yao W. HCl emission characteristic of PVC component from municipa1 solid

waste pyrolysis. J Northeast Univ 2002;7:711–4.

[3] Ma SB, Lu J, Gao JS. Treatment and recovery as resources of chlorine-containing organic waste. Pyrolysis

dechlorination of PVC in a fixed bed reactor. J East China Univ Sci Technol 2002;2:59–62.

[4] Tuppurainen K, Halonen I, Ruokojarvi P, Tarhanen J, Ruuskanen J. Formation of PCDDs and PCDFs in

municipal waste incineration and its inhibition mechanism. Chemosphere 1998;36(7):1493–511.

[5] Qiao SB, Liu WT. Hazard and control of secondary pollution from refuse incineration. Energy Conserv

Technol 2003;1:38–9.

[6] Li SY, Bie RS. Formation and control method of secondary pollutants from incineration of municipal solid

waste. Tech Equip Environ Pollut Contro1 2003;3:63–7.

[7] Zhang YF, Deng N, Ling JH. A new pyrolysis technology and equipment for treatment of municipal

household garbage and hospital waste. Renew Energy 2003;28(15):2383–93.

[8] Wang SR. Experimental studies on biomass pyrolysis for producing bio-oil and mathematical modeling.

Zhejiang University Doctoral Dissertation; 1999: 15.

[9] Zhang YF. The research for principle and apparatus of urban and medical garbage disposal. Sci Symp Eng

Thermophys Chin 2002;276–80.

[10] Wang H, He F, Ma WH, Bao GR. Dioxins zero emission technology for gas—producing with municipal

solid waste gasification and ash melting. Acta Energiae Solaris Siyica 2002;4:260–4.

[11] Su XY, Wang ZY, Ceng CM, Tang ST, Lu ZA, Li DK. Study on biomass pyrolysis and gasification in a

fluidized bed. J Fuel Chem Technol 2002;8:298–305.

[12] Li SQ, Li AM, Yan JH, Ren Y, Li XD, Li RD, Chi Y, Cen KF. Pyrolysis of the biomass wastes pyrolysis in a

rotary kiln. Influences of reaction conditions on pyrolysis product distribution. Acta Energiae Solaris Siyica

2000;10:333–9.

[13] Li J, Zhang Z, Yang XM, Yao JZ, Lin WG. TG—DSC study on pyrolysis characteristics of municipal solid

wastes. J Chem Ind Eng 2002;7:759–64.


[14] Li AM, Liu LF, Li RD. A test on yield and characteristic of biomass pyrolytic semi-cokes. J Agro-Environ

Sci 2003;22(2):214–6.

[15] Jin BS, Zhong ZP, Zhou SM. Study on pyrolysis characteristics and kinetics of municipal solid wastes. J Eng

Thermophys 1999;7:510–4.

[16] Chu H, Li AM. Research of municipal solid waste pyrolysis gas. J Saf Environ 2002;4:22–7.

[17] Zhang Y, Tao H. Waste treatment technology and engineering example: Chemical Industry Publishing

Company; 2002, p. 5.

[18] Zhang Y, Ran JY, Qu CS. Investigation on physical and pyrolysis characteristics of MSW. Acta Sci

Circumstantiae 2000;9:646–7.

[19] Zhao YC. The principle and technology of the renewable disposal for household waste treatment: Chemical

Industry Publishing Company; 2002, p. 1.

[20] Anderson LL. Flues from waste: Academic Press; 1984.

experimental studies on low-temperature pyrolysis of municipal household garbage—temperature...

Documents