disposal of waste tyres for energy recovery and safe environment—review

Pergamon Energy Convers. Mgmt Vol. 39, No. 5/6, pp, 511-528, 1998

© 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain

PII: S0196-8904(97)00044-7 0196-8904/98 $19.00 + 0.00

D I S P O S A L O F W A S T E T Y R E S F O R E N E R G Y

R E C O V E R Y A N D S A F E E N V I R O N M E N T - - R E V I E W

V. K. SHARMA, 1'* M. MINCARINI, I F. FORTUNA, 1 F. COGNINI 2 and G. CORNACCHIA ~

~ENEA C.R. Trisaia, Environment Department, AMB-TEIN-RIF Unit, 75025 Policoro (MT), Italy and 2ENEA C.R. Trisaia, Innovation Department, INN-NUMA-TECMA, 75025 Policoro (MT), Italy

(Received 3 March 1997)

Abstract--This article presents the essential features of different practical means applied for the disposal of waste tyres. Considerable attention has been devoted to new, more efficient and environmentally attractive methods, such as incinerators with energy recovery systems, pyrolysis using microwave energy, etc. © 1997 Elsevier Science Ltd.

Waste tyre Air pollution Energy and environment Incineration Pyrolysis

INTRODUCTION

The energy crisis and environmental degradation are the main problems mankind is facing today. These problems owe their origin to a growing population, rapid industrialisation and huge quantities of solid refuse which are generated daily. To alleviate part of our energy crisis and environmental degradation, it has become imperative to make use of appropriate technologies for the possible recovery of resources from non-conventional sources, like organic wastes, plastic wastes, used rubber tyres, etc.

The problems related to the disposal of organic wastes available from urban, agro-industries or agricultural farms, etc. using both aerobic and anaerobic biotechnology have already been addressed by several researchers and more recently by Sharma et al. [1, 2].

The problems related to the disposal of other solid wastes such as plastic and waste rubber tyres, have also been addressed. A variety of global and national policies are also being developed and proposed worldwide. It is, however, to be noted that, no doubt, considerable research and development efforts to recover energy and useful by-products, combat relevant disposal problems, minimise pollution effects, etc. have been made in many industrialised countries, but the fact remains that, even at present (at least in most of the developing countries), a considerable portion of waste tyres is still being disposed of by using the most common techniques, such as incineration (without the recovery of value) and landfill. Both these methods lead to a waste of potentially valuable raw materials together with many other problems. Stockpiling, the other current disposal option, is even less attractive--the waste tyres are unsightly and pose a considerable fire hazard.

In view of the problems caused by the combustibility of waste tyres, it is not surprising that moves are afoot to harness this energy. It has been observed that, by using novel processes for recycling of wastes, incinerators with energy recovery systems, the pyrolysis process using electric or microwave heating, etc., it is possible to conserve available resources, recover useful products (fuel, by products such as char and steel, etc.) and combat disposal problems and minimise pollution effects [3-8].

Available results indicate that the pyrolysis process has a promising future. A number of designs and their performances have already been described by various researchers [9-15], but there still seems to be many operational problems associated with the models available on the market.

*Visiting scientist from New Delhi, India; to whom all correspondence should be addressed.

511

512 SHARMA et al.: DISPOSAL OF WASTE TYRES--REVIEW

Given this context, the description of the most appropriate models, experiences had so far with recent developments, etc. are discussed in the present review article. Emphasis is, however, given to the disposal of waste tyres using thermo-chemical processes, such as incinerators and pyrolysis.

QUANTITY OF WASTE TYRES PRODUCED IN ITALY: AN ESTIMATION

Knowledge about the waste rubber tyres available in the country is essential to decide the size as well as type of disposal method. The quantity of waste rubber produced annually (in tons) can be estimated using the expression:

Waste rubber tyres produced annually (t /yr)= (No. of registered vehicles.14 + No. of old damaged (off the road) vehicles.28)/1000

The numerical values used in the above equation, i.e. 14 and 28, represent the average values for the waste tyres produced in a year (kg/yr), corresponding to each registered and old damaged vehicle, respectively. It is to be noted that the following parameters were duly considered while evaluating the contribution due from both registered and old damaged vehicles, i.e.

• knowledge about the registered and old damaged (off the road) vehicles • average miles covered by each tyre • average tyre life • unitary weight of tyre • weight reduction due to wearing • number of tyres used contemporaneously.

In addition, the facts mentioned below were also considered while evaluating the contribution from both registered and old damaged (off the road) vehicles.

For registered vehicles

Waste rubber produced from each car = 5 kg/yr Waste rubber produced from each heavy vehicle (e.g. truck) = 100 kg/yr Ratio between cars and heavy vehicles = 10:1

For old damaged (off the road) vehicles

Waste rubber produced from each car = 20 kg/yr Waste rubber produced from each heavy vehicle (e.g. truck) = 200 kg/yr Ratio between cars and heavy vehicles = 20:1

Using the expression mentioned above, it has been estimated that nearly 380,000--400,000 tons of waste tyres are produced each year in Italy [17]. It is, however, to be noted that, as the population and associated automotive industries grow, this number will certainly increase.

COMPOSITION AND CHARACTERISTICS OF USED TYRES

While considering the disposal of used tyres, it is essential to be aware of the different materials and substances used in the production of tyres. The production of tyres uses several materials and substances. Natural or synthetic rubber, mixed with several ingredients, originates the mixture that, upon vulcanisation and coupling with the wire gauze, forms the tyre.

A mixture is generally composed of:

• elastomer (natural or synthetic rubber); • reinforcing agents (carbon black); • plastificants (hydrocarbon oils); • vulcanising agents (sulphur and sulphur compounds); • accelerating agents (to facilitate the sulphur action); • protective agents (anti-oxidising, anti-oxonant, stabiliser).

It is, however, to be noted that, mainly because of the consumption of the tread, the composition

SHARMA et al.: DISPOSAL OF WASTE TYRES--REVIEW 513

of a used tyre is slightly different from the new one. This is especially true in the case of the rubber tyre for both car and truck as demonstrated in Table 1.

D I S P O S A L OF W A S T E T Y R E S

The scale of the disposal problem is considerable. It is, however, to be noted that, even today, the vast majority of waste tyres are simply dumped or buried in landfill sites. It is true that landfilling of waste tyres for disposal purposes has a reasonably low investment and is easy to design, develop and operate, but in reality, this is a waste of potentially valuable raw materials and leads to many other problems. The number of landfill sites willing to accept tyres is diminishing. Moreover, as the tyres do not degrade, if not shredded, they can cause instability in the landfill. Stockpiling, the other current disposal option, is even less attractive, mainly because the tyres are unsightly and pose a serious fire hazard causing atmospheric pollution, including zinc oxide, dioxins (toxic hydrocarbon) and poly-nuclear aromatic hydrocarbons- -potent carcinogens.

On the other hand, waste tyres present a significant recycling opportunity, as they are already collected and sorted by retreading companies. There has been a spate of interest in tyres as people realise their value as a resource and become aware of the problems arising from their inappropriate disposal. In view of the problems mentioned above, it is not surprising that moves are afoot to harness this energy for electricity generation. Over the past decade or so, several schemes to tackle the waste tyres problem have been proposed.

Keeping in view the facts mentioned above, an at tempt has been made to address the problem in question and to give readers (who, one way or the other, are involved in this technology) a broad and detailed overview of various aspects of the recovery of value from this waste. Description of the most appropriate models is discussed in the text following.

D I S P O S A L OF U S E D R U B B E R T Y R E S W I T H M A T E R I A L R E C O V E R Y

Because of the vulcanised nature of the rubber, used tyres are not directly reusable in the production cycle. In fact, the vulcanisation transforms the elastomer into a non-fusible and insoluble substance. For these reasons, it is necessary to treat the rubber with a devulcanisation process for regeneration and reuse as virgin materials. The concepts under investigation can be categorised as:

- - reuse of rubber material: using this methodology, the waste tyres, upon being pre-treated, are used to reproduce products similar to the initial ones;

- - recovery of materials and energy.

Using appropriate technologies, the used tyre is milled to obtain a powder or a granular material having a specific granulometry. Mechanical milling, cryogenic milling, devulcanisation processes, etc. are the different technologies used for this purpose. It is, however, to be noted that devulcanisation processes, because of their high operating cost, are rarely used.

Table 1. Composition of new and used rubber tyres [16]

Car Truck

Tyre New Used New Used composition (%) (%) (%) (%)

Elastomer 48 47 45 43 Carbon black 22 21.5 22 21 Steel 15 16.5 25 27 Textile fibres 5 5.5 - - - - Zinc oxide 1.2 1 2.2 2 Sulphur 1 1 1 1 Other oil 8 7.5 6 6


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1. Crushing chamber 2. Elevator 3. Primary granulator 4. Secondary granulator

5. Magnetic separator 6. Densimeter separator 7. Vertical depression column

Fig. 1. Description of a mechanical trituration plant.

TYRE REMOULDING

Tyre remoulding or retreading is nothing but the most direct form of tyre recycling. The concept in question is used to replace the worn rubber by a new tread section. In this process, the remaining tread is ground away from a tyre to be remoulded, and a new tread rubber strip is fused to the old carcass by vulcanisation. The literature survey very clearly demonstrates the economic potential of this process.

It is true that much of the early work on tyre remoulding or retreading has been done in Italy (with more than 30% of the waste tyres retreaded), but in later years, both due to the economic potential and significant R&D efforts, mainly in many industrialised countries, a considerable variation in terms of the technology employed has been obtained. It is, however, to be noted that further research and development efforts are still needed to meet the ever increasing demand of good product quality, reliability and many technical conditions set by the world market.

MECHANICAL MILLING

Using this technology, shown in Fig. 1, the milling is generally done by rotating blades. The principal aim of using a rotating blade is to separate the rubber from the metallic part. It has been observed that, by using the latest generation plants, it is possible to obtain very pure materials. No doubt, the low cost of the plant and the absence of air emissions are the major advantages, but high consumption of electric power and limited markets for the products obtained are the main drawbacks that need further research efforts.

CRYOGENIC MILLING

With this process (Fig. 2), the rubber is cooled using liquid nitrogen at a temperature ranging between - 6 0 and -100°C. As a result of this operation, the rubber becomes fragile and, thus,


is milled very easily into very fine powder either by a disk or hammer mill. The main advantage of this process consists in the possibility obtaining very fine powder (up to one hundred microns). High consumption of both energy and liquid nitrogen (approximately 0.9 kg to treat just 1 kg of rubber) makes the process very expensive.

DISPOSAL OF USED TYRES WITH ENERGY AND MATERIAL RECOVERY

In light of the overall environmental impact along with the drive toward energy and material conservation, new waste tyres disposal options are being developed and implemented. Resource recovery and thermal treatment (incineration, gasification, pyrolysis, etc.) seem to be the most advanced and significant disposal methods, capable of alleviating some of the more pressing problems observed in this field.

No doubt, a number of designs and their performance have already been described by various researchers, but there still seems to be many technical, economical and some social problems related to different models available in the market. It is in this context that the description of the most appropriate, cost effective and efficient designs will be discussed in the following paragraphs.

THERMAL TREATMENT

The thermal treatment process, in general, is subdivided into the following categories, i.e.

• Combustion (incineration).

1.1. Belt conveyer 1.2. Crusher 1.3. Vibrating conveyer 2.4 Elevator 2.5 Cryogenic rotary chillier 2.6 Hammer milling 2.7 Vibrating conveyer 2.8 Magnetic separator 2.9 Vibrating conveyer 2.10 Pne.msfic conveyer 2.11 Mechanical sieve 2.12 Gravimetric separator 2.13 Stocking bin 2.14 Granulator milling 2.15 Pneumatic conveyer 2.16 Stocking bin 2.17 Floating table 2.18 Pneumatic conveyer 2.19 Stocking bin 2.20 Floating table 2.21 Vibrating sieve 3.1 Stocking bin

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516 SHARMA et al.: DISPOSAL OF WASTE TYRES---REVIEW

• Gasification. • Pyrolysis.

The above mentioned thermal treatment processes have advantages, such as

1. The volume of waste tyres can be reduced by more than 90%. 2. The processes are net energy producers with possible material recovery. 3. Non-polluting and capable of destroying most of the organic substances harmful to human

health.

It is, however, to be noted that the following problems are associated with thermal treatment of waste tyres:

• Disposal of ash: lead and cadmium salts used as stabilisers in tyre production remain as ashes, thus causing disposal problems.

• Toxic gases: when tyres are burnt, some toxic gases, such as SO2, H2S, HC1, HCN, etc., are generated, thus requiring additional systems for their proper treatment.

• Soot: imperfect burning of the waste material will produce soot. In general, such materials have much higher heating values than municipal refuse, require more combustion area and higher flame temperature.

• Appropriate incinerators: to tackle the problems, such as higher temperature, insufficient oxygen supply, corrosive action of the gases, etc., it is necessary to design suitable incinerators using appropriate materials.

INCINERATION

In this process, the waste tyres are burnt under controlled high temperature combustion. In general, the incineration of waste tyres may be defined as the reduction of combustible wastes to inert residue by controlled high-temperature combustion. The combustion process is spontaneous above 400°C, highly exothermic and, once initiated, becomes self-supporting.

A typical incinerator is comprised of a set of scales, a storage pit and tipping area, incinerator cranes, charging mechanisms, a furnace and pollution control devices. The incinerator has to be designed for good burning and the prevention of soot delivery. The walls and furnace beds must be able to withstand the high temperatures (approx. 1150°C) generated by the combustion process. Here, the waste tyres, having a calorific value of 7500-8000 kcal/kg, are used as fuel in the incinerators.

The heat generated during incineration produces steam which may be used to heat and air condition buildings or for industrial processing or the production of electricity. Burning refuse in steam-generating incinerators and its use as a supplemental fuel are the most advanced and proven waste energy utilisation technologies.

In thermal technologies, it is the design of the furnace and its effective efficiency that play an important role concerning general combustion performance. At the design stage, it should, therefore, be kept in mind that solid fuels being rich in volatile matter develop long flames and thus require the firebox to be larger than in a traditional boiler.

Furthermore, it should be stressed that high combustion efficiency, defined as the ratio of thermal energy output to global energy input, usually depend upon interdependent factors, such as the fuel's physical characteristics, plant design, manufacture and operating conditions. Volatile matter content, moisture, mineral content, dimension and structural characteristics (density, area/volume ratio, structure, etc.), resin content, etc. are the main physical characteristics that affect boiler efficiency.

Temperature, heat exchange surface, excess air, COs content, etc. are the principal functional parameters of a combustion facility firebox. The use of refuse as a supplemental fuel in power producing plants offers many advantages, such as

(1) maximum heat recovery; (2) low air pollution emissions; (3) environmentally acceptable process;

SHARMA e t al.: DISPOSAL OF WASTE TYRES--REVIEW 517

Combustible gas to the burner

Tyres feeder

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Cooling air l Combust ion air

Cooling water 1 1 - Rubber feed box 2 - Mobile-grate kiln 3 - Post-combustion chamber

4 - Heat recovery boiler 5 - Ash removal system 6 - Flue-gas treatment system

7 - Chimney

Fig. 3. General layout of the plant process for tyres burning using a mobile-grate kiln.

(4) reduced power production cost, etc.

However, the disadvantages of incinerators are:

(1) large capital investment; (2) need of flue gas depuration; (3) relatively high operating cost; (4) skilled labour/operator is required to operate the system, etc.

The thermal technologies, in general, are discussed under the headings:

--Grate kiln. --Rotary kiln. --Fluidised bed kiln.

Combustion on grate kiln (Figs 3 and 4)

Combustion on a fire grate seems to be the most appropriate technology for waste tyres treatment. Here, the waste material is fed through the feed door and deposited on the fire grate. The gas ignited on the furnace floor is mixed with primary air. Continuous multistage combustion takes place in the plant, having a pre-combustion chamber large enough to reduce the problems relevant to thermal shock of the refractory. The gases produced during combustion contain particulate matter as well as incompletely oxidised compounds. Final oxidation of valorised products takes place in the secondary burning chamber, where an auxiliary burner may be employed if the temperature is low.

Concerning the size of the furnace, it is, however, to be noted that a large furnace generally causes high inertia, caused by the high volume of gases produced, and thus, reduces significantly the radiant effects of the flame temperature.

Use of the combustion on grate kiln technology is justified economically, especially for large-sized plants. It is also to be noted that the stresses on the refractory, when compared to other treatment technologies, are much lower. The need for a post-combustion chamber and problems relevant to grate packing (in the case of melting of metallic fraction), no doubt, are the disadvantages of such a plant, but experimental results, made available by different research groups working in this field, very clearly demonstrate the validity and acceptability of the technology.


Rotary kiln combustion (Figs 5 and 6)

The rotary kiln furnace consists of the rectangular drying and ignition zone and rotating combustion kiln. The kiln is a large metal cylinder lined with refractory material with its axis slightly inclined. It is mounted on rollers and slowly rotated by an electric motor.

The waste tyres, in different sizes, are passed through the drying and ignition zone, thus driving off the volatile constituents. The final combustion takes place in the kiln. The volatiles driven off in the ignition zone go through a passage above the kiln and join the hot gases effluent from the kiln in a secondary combustion chamber where the combustion is completed. The solid residue is discharged from the kiln into the refuse collecting device. Hot gases generated in the process pass through the heat exchangers, generating steam. So, to conclude, it can be stated that the rotary kiln combustion plant, generally, presents the following characteristics:

1. multistage combustion; 2. possibility maintaining the temperature around 800°C; 3. valorisation of volatile substances; 4. use of refractory having good thermal shock resistance.

Finally, low operating cost, possibility of using different wastes, continuous removal of ashes, proven technology, etc. are the main advantages, whereas the need of a post-combustion chamber and particulate filtration to control the gases, such as SO2 and NOx, the need of a minimal plant size to justify the capital investment, etc. are a few disadvantages that need further efforts to improve the system.

Fluidised bed combustion

The fluidised bed kiln (Fig. 7) is principally a combustion chamber. The material is continuously injected into the fluidised bed furnace where it burns while being kept in suspensionby air blowing from the underside. A fluidised bed plant foresees the following principal components, i.e.

6 7

! - Fan b l o w e r

4 - Flue gas h e a t e x c h a n g e r

7 = G e n e r a t o r

2 - A s h c o n v e y e r

5 - C l e a n i n g o f f lue gas

3 - Textile Pdter

6 - Vapour turbine

Fig. 4. Mobile grate kiln rubber burning plant.

S H A R M A et al.: D I S P O S A L O F W A S T E T Y R E S - - R E V I E W 519

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4 - Calmness chamber

2 - B u r n e r 3 - Rotary-kiln

5 - Post-combustion burner

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7 - Flue-gas scrubbing system and ash separator

9 - Sedimentation system for sludge treatment

A - Solution for scrubbing B - Waste tyres feed

8 - Scrubbing tower

C - Combustible oil to the burners

D - A i r to the burners E - W a t e r F - Recycling water for scrubbing

F i g . 5. G e n e r a l l a y o u t o f t he p l a n t p r o c e s s f o r w a s t e ty re s b u r n i n g u s i n g h o r i z o n t a l r o t a r y k i ln .

--Fuel system. --Fluidised bed heater. --System for sand and limestone supply for the abatement of SO2 and HC1 produced during the

combustion. --System for gas cleaning and its removal. --System for the removal of solid combustion residue (ash, gypsum, steel scraps, etc.). --Control and measurement unit.

The system can be operated using the elastomer together with other industrial wastes (sludge, wood, etc.) and coal. Hot gases and light particulates from the process are raised in the furnace and channelled to superheaters, the evaporator and economiser section of the boiler. The gas then passes through the mechanical dust collector, induced draft fan and baghouse, where fine particle loading is collected before discharge of the gas to the atmosphere.

Ash and sand material will be removed from the bottom of the unit and taken to the town landfill. The superheated steam generated in the boiler is directed to the steam turbine connected to the generator, producing electricity. By directing the steam exhaust into a condenser, the efficiency of the unit is improved because the cooling water produces a vacuum and, thus, a greater differential in pressure from the inlet to the outlet of the turbine.

The fluidised bed reactor has many advantages, such as

--feeding of different wastes; --low levels of harmful emission, i.e. uniform temperature inside the combustion chamber results

in reduced NOx emission (<400 ppm); --high combustion efficiency (> 98%); --high operating flexibility.

520 SHARMA e t al.: DISPOSAL OF WASTE TYRES--REVIEW

In addition, elimination of the post-combustion chamber makes the system more economical. No doubt, the technology seems to be the most appropriate for the treatment of waste tyres, but it still has a few disadvantages, such as

--high operating costs; --considerable feedstock preparation (necessity of trituration); --lack of experimental data at the commercial level; --possibility for the over packing of low melting ashes.

PYROLYSIS

Pyrolysis, an endothermic process (thermal degradation of materials caused by heating in an oxygen-free or oxygen deficient atmosphere), offers an environmentally attractive method of reducing the world's waste tyre backlog.

As mentioned above, the machines use high temperatures and an oxygen free environment to decompose tyres, chemically, producing lower emissions of nitrogen oxide and sulphur oxide than incinerators--the conventional technology. The products of pyrolysis represent about 50% of the initial volume of the organic matter, and they can be converted into energy to either sustain the process or produce excess power.

The process, as shown in Fig. 8, generally begins with preheating of the shredded materials fed to a retort. In the retort, the waste material is heated further, up to the required temperature, depending upon the process, for gas production. To remove oils or tars (which, in turn, are sent back to the retort for volatilisation), the gas is cleaned by liquid separators.

By recycling rather than burning, pyrolysis also allows valuable materials to be recovered. The oil and gas, for example, can be used as fuels within the pyrolysis system or for an adjacent plant such as a combined heat and power system. Char and steel are also recovered from the process.

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1. Chimney 2. Textile filter 3. Boiler 4. Vapour 5. Post combustion

6. Product to be recycled 7. Supply line 8. Primary reactor 9. Steel scrap

Fig. 6. General layout of Eneal's horizontal rotary kiln combustion plant installed at Marangoni (Rovereto, Italy).

S H A R M A et al.: DISPOSAL OF WASTE T Y R E S - - R E V I E W 521

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Secondary air

Effluent gaseous output

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Primary

I. Tyre grinder

4. Sand storage

7. Burner starter

2. Elevator

5. Limestone storage

8. Convective section

3. Tyre 's storage bin

6. Fluidized -bed furnace

9. Textile filter 10. Chimney

Fig. 7. Description of a fluidised bed tyres burning plant.

It is further to be noted that the oil fraction (mainly composed of olephinic and aromatic hydrocarbon) can condense at normal pressure and temperature, whereas the gaseous fraction is composed of non-condensable organics, H2, H2S, CH4, CO, NH3, etc.

The percentage of both oil and gaseous fractions depends upon the operating conditions, such as temperature and pressure in the pyrolysis reactor, residence time and type of catalyst employed. It has been observed from experiments conducted by researchers in different countries that an increase in reactor temperature results not only in a corresponding increase in the heating value of the product but also the percentage increase in H2, CH4, CO and other hydrocarbons. Generally, such products are available as:

- - 33% char fraction; - -35% oil residue; - -12% metallic fraction; - -20% gas.

The non-condensable gas, having a heating value of the order of 10,000 kcal/Nm 3, is mainly composed of light hydrocarbons and, hence, can be used to heat the pyrolysis reactor. The oil, on the other hand, can be used as a fuel in a conventional furnace. The other important by-product, char, is used as a fuel or for the production of activated carbons or as an inert additive in the rubber.

Pyrolysis is not a combustion process and is performed in closed vessels. The much larger volumes of air and vapours and grit and dust emissions associated with the incinerator are, therefore, not present. To date, the pyrolysis of waste tyres has had limited application world-wide but has been used at both pilot scale and full scale with a considerable degree of success.

IMPORTANT INSTALLATIONS

In order to exploit waste tyres as a potential resource for electricity generation as well as raw materials, several small to medium scale projects have been undertaken in different industrialised countries. Brief descriptions of a few such successful plants, still operating are given below.

522 SHARMA e t al.: DISPOSAL OF WASTE TYRES--REVIEW

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(A) 30 megawatt Elm's electricity generation plant

The 30 megawatt electricity generation plant, first of its kind in Europe as well as the largest individual generation of electricity from renewable energy sources, has been designed and constructed by Elm Energy and Recycling (U.K.) Limited.

The operation of the plant is based upon the technology that utilises waste rubber tyres, nearly 100,000 tons per annum, for electricity generation without polluting the environment while producing valuable reusable by-products, side by side. The concept of an incinerator with an energy recovery system has been used while constructing the plant based on five incinerators. Each of these consists of three pulse hearth furnaces designed to give a long combustion time at 800°C. These are followed by a radiant chamber which heats up to 30-40% of the steam.

A reburn tunnel at approximately 1000°C burns the remaining gases and then passes to a boiler plant. Use of advanced flue gas clean up systems ensures the emission level is well under control as well as within E.C. requirements. The emissions are considerably lower than those found in coal fired plants.

The operating design capacity of the plant is to generate nearly 30 megawatts of electricity. The plant is still under thorough experimentation. For more detailed technical and performance information, readers are requested to see Energy Digest, Vol. 21(2), 1992.

(B) AEA's lO00tpy tyre pyrolysis plant

A pyrolyser, developed at AEA Technology's Harwell Laboratory for Hebert Beven & Company, can pyrolyse up to 1000 tpy. A greater throughput of the batch process can obviously be achieved by running several units in parallel.

Here, a one tonne batch of whole or shredded tyres is charged into a crucible which is lowered into the reaction vessel. The airtight lid is lowered into the kiln, and the tyres are heated to 450-600°C for about 10 h. The vapour is passed through a condenser to give fuel oil, and the gas fractions can be fed back to the boiler to help heat the kiln. The residue left in the crucible consists of scrap steel, with a weight fraction of about 17%, and coke (roughly 40%).

(C) BRC Environmental's 5000tpy pyrolysis plant

A pyrolysis plant has been designed by BRC Environmental to handle 5000 tpy of waste tyres. The idea of using microwave energy has been proposed, for the very first time. It is felt that, with microwaves, there are no problems of heat transfer, and the process gives dry products.

In the process, whole or shredded tyres are fed continuously via two purge locks to the reaction zone where they meet hot recycled gas. The microwaves break down the tyres, and the vapour passes to a quench cooler and distillation column. This separates the vapour into fuel gas (hydrogen and methane) and light and heavy oils.

On a mass balance, BRC claims a carbon recovery of 36%. Any sulphur in the rubber from the vulcanisation process can be removed from the carbon by acid washing.

In contrast to Beven's thermal pyrolyser, the main product here in the system under investigation is the high value product called activated carbon. In addition, it is felt that, by using microwave techniques, it would also be possible to destroy other organic materials, notably clinical wastes, plastics and hazardous chemicals.

(D) Pyrolysis pilot-plant for the treatment of plastic/rubber wastes

A pilot scale pyrolysis plant has been installed at the ENEA research centre, Trisaia, in Italy. The pyrolysis unit, shown in Fig. 9, consists of a reactor of the rotary kiln type, 110 dm 3 in volume and 0.4 m in diameter. It is heated externally by using electric elements surrounding the reactor chamber. Waste material is introduced at one end of the kiln and rolls slowly down the length of the kiln after successive rotations.

Transport of the granular material can be assisted by tilting the axis of rotation of the kiln a few degrees so that the charge end is higher than the discharge end. The pyrolyser operates under a positive pressure of nitrogen (300 mm w.c.); air leakage through the system is negligible.

At the beginning of the experiment, the waste material is fed into a hopper with a maximum

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holding capacity of 400 dm 3, hermetically sealed and equipped with a screw feeding device. Feeding of the material across the reactor is continuous until the hopper becomes empty; the maximum feeding rate is 48 kg/h. The solid residue of pyrolysis (char) is continuously discharged in a water cooled tank.

The gaseous products formed during pyrolysis pass through a scrubber that removes tar and cools the gases to about 250'~C. To remove the fuel oils and cool the gaseous stream further, to about 30C, the output from the scrubber, mentioned above, is treated in a jacketed pipe condenser.

The non-condensable gases from the jacketed pipe condenser are filtered (using a demister filter) and are passed through a wet scrubbing system designed to remove acid components by NaOH (4%) injection. Subsequently, the cleaned fuel gases along with propane gas (as a supporting gas) are combusted in a flare to supply the thermal energy required to heat the process nitrogen used for oxygen removal.

Such gases are monitored on line, by process gas chromatography and with sampling equipment for laboratory analysis, both located before the wet scrubbing system.

(E) Gas([ication/pyrolysis plant .for treatment of hazardous waste

This system (Figs 10 and 11), using both the concepts of pyrolysis and gasification, has been designed, developed and experimented by researchers from VEBA OEL Technologies in Germany [18]. The principal objective of designing such a hybrid system was to overcome problems such as fineness of grinding and supply of homogeneous material, encountered in gasification. Because of the fact that, in pyrolysis, material of size up to 200 mm could be fed and the products obtained from the pyrolysis, i.e. oil and coke, could be mixed homogeneously, the problems mentioned above could be solved by combining both systems together.

(F) SahTt-Amable t,acuum pyrolysis pilot plant [19]

The objective of the pilot plant built and tested during the fall of 1987 in Saint-Amable near Montreal was to minimise secondary reactions such as thermal cracking, repolymerisation and recondensation reactions, gas phase collision, catalytic cracking and reduction and oxidation reactions, etc., using the concept of vacuum pyrolysis.

The system, shown in Fig. 12, was designed to decompose continuously 200 kg/h of steel belt tyres at a pressure of 1.3 kPa. The reactor was externally fired with gas and a small portion of the

Sewage sludge

Plastic waste

Shredder fluff

Tyres

Household waste

Fuel gas Gas [ I Oxygen Cleaning I

Gas cleaning

IM,++,s,p_oon c°" I st!° 1 Metals Slag Sulphur

Fig. 10. Block diagram of combined pyrolysis and gasification plant for waste treatment [18].

Waste water

526 S H A R M A e t al.: DISPOSAL OF WASTE TYRES- -REVIEW

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1. Feed conveyer 9. Light oil quencher 2. Vacuum reactor 10. Decanter I 3. Cooling screw 11. Vacuum pump I 4. Discharge screw 12. Flare stack 5. Crusher 13. Heavy oil storage 6. Vibratory screen 14. Light oil storage 7. Carbon black handling system 15. Magnetic separator 8. Heavy oil quencher 16. Steel recovery

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pyrolysis oil. The vapours were quenched by two scrubbers set in series. The carbon black was recovered at the bottom of another water head which connected the bottom of the reactor and the ground. It is, however, to be noted that, besides the main limitation of the system, i.e. low rate of the heat transferred to the rubber per unit area of reactor surface, till today, the project has been working continuously.

(G) Other installations

In California (U.S.A.), Ethos Rubber Inc. has decided to realise a new trituration plant for tyres available from trucks and cars. The plant has a capacity of 100,000 tpy of waste rubber tyres. The end products would be used in the preparation of asphalt mixtures to be used for road covering, for stadia and for new rubber articles.

In Great Britain, an incinerator for the treatment of waste tyres, with an overall capacity of 170,000 tons per year, is planned, to be installed at Wolverhampton. The purpose of this plant is to produce 20 megawatts of electric power. The flue gases are to be treated with a fabric filter and the powder, having a high content of zinc oxides, is to be used in the elastomeric mixture.

Two other incinerators, with overall treatment capacity of 24,000 and 9000 tons of waste tyres per year, are also in the construction phase, respectively, at Durham and Grantham.

In Germany, an incinerator for the treatment of waste tyres with an overall treatment capacity of nearly 50,000 tons per year is under thorough experimentation. The plant has been constructed jointly by Continental and Lurgi GmbH.

Similarly, in other European countries, research and development efforts are in progress concerning the use of waste tyres not only to solve our environmental problems but also to produce electricity.

528 SHARMA et al.: DISPOSAL OF WASTE TYRES---REVIEW

C O N C L U S I O N S

F r o m the discussion made so far concerning the use o f different t reatment technologies to tackle the waste tyre problem with energy and material recovery options, the following conclusions could be drawn, in general:

1. As far as pulverisation o f tyres is concerned, notwithstanding the problems encountered which show further development work to be undertaken, the cryogenic pulverisation process appears to be the mos t appropria te one.

2. In reference to the thermal destruction technologies, it is to be noted that, in short - to-medium term perspective, incinerators with energy recovery systems, no doubt , seem to be the most advantageous both technically and economically, but more air requirement, higher flame temperature, p roduct ion o f corrosive combust ion products, excessive heat generated during combust ion, etc. are a few problems that need futher attention. At present, it can, however, be stated that incineration with energy recovery is o f greater manufactur ing and operat ing complexity and is better suited for large power plants.

3. The pyrolysis process ( though in its initial R & D stages), having higher energy recovery values than incineration (70% vs 41%) and also being attractive environmentally, seems to be another potential solution for possible energy as well as useful by-products recovery. It is, however, to be noted that many problems, both technical and operational, need to be solved before the said process could satisfy the reliability and economic condit ions set by the world market.

Acknowledgements-43ne of the authors (VKS) would like to express his sincere thanks to Professor G. Furlan, from ICTP, Trieste for his constant encouragement throughout the course of this work. Published work cited in the present document is also gratefully acknowledged.

REFERENCES

1. Sharma, V. K., Canditelli, M., Fortuna, F. and Cornacchia, G., Energy Convers. Mgmt, 1997, 38(5), 453. 2. Sharma, V. K., Canditelli, M., Fortuna, F., Castelluccio, G. and Cornacchia, G., Renewable Energy, An Int. Journal,

1997. 3. Walter, R. C., Bass, K. C. and Roseveare, W. E., lnd. Eng. Chem., 1978, 48, 138. 4. Shang, J., Mei, J. and Notestein, J., Fluidized bed combustion of scrap tires, Technical notes from Annual Report No.

DOE/METC-8614068. Department of Energy, U.S.A., 1981. 5. Berti, L., Marani, M. and Scialdoni, R., Risparmio Energetico, 1990, 29, 23. 6. Allott, K., Process Engg Environmental Protection, 1991, 45. 7. Coronidi, M. and Ranaldi, E., Relazione Finale, Vol. 1, FISIA/A,B-TEIN-G-004, 1995, 248. 8. Rausa, R. and Pollesel, P., J. Analytical and Applied Pyrolysis, 40--41, 1997. 9. Kaminsky, W. and Sinn, H., Proc. ACS Symposium Series No. 130, 1980, p. 423.

10. Roy, C. and Unsworth, J., Pyrolysis and Gasification, ed. G. L. Ferrero, K. Maniatis, A. Buekens and A. V. Bridgwater. Elsevier Applied Science, London, 1989, 180.

11. William, P. T., Besler, S. and Taylor, D. T., Fuel, 1990, 69, 1474. 12. Williams, P. T. and Taylor, D. T., Fuel, 1993, 72, 1469. 13. Carugati, A., Pederzani, G. and Pollesel, P., Relazione tecnica, Eniricerche S.p.A., Milano, no. 94074, July, 1994. 14. Chang, Y., Resources, Conservation and Recycling, 1996, 17, 125. 15. Fortuna, F., Mincarini, M., Cornacchia, G. and Sharma, V. K., J. Analytical and Applied Pyrolysis, 1997, 40--41, 403. 16, Mincarini, M. and Scialdoni, R., Rapporto 92/03, Istituto per l'Ambiente, Working Group on Recycling, Milano, 1992,

107. 17. Corodini, M., RT/AMB/90, ISSN/1120-5555, 1991, 35. 18. Capetti, E., Crosio, M. and Senneca, O., Private communication, ENEA, CR Casaccia, Rome, 1994, 30. 19. Roy, C., Labrecque, B. and De Caumia, B., Resources Conservation and Recycling, 1990, 4, 203.

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