MANAGING THE ENVIRONMENTAL HAZARDS OF WASTE TIRES

This paper presents an experimental approach on understanding and managing the environmental hazards of co-products resulted during energy recovery processes applied on scrap tires. As tire combustion faces serious problems related to harmful emissions, pyrolysis appear as a process that allow the management of toxic compounds. Thus, for the reactions that occur during pyrolysis and combustion of tires organic matters the main intensive degradation thermal ranges have been established. The work was carried out by coupling thermogravimetric analysis (TGA) of tire samples with bench scale reactor in order to identify the relationships between thermochemical behaviour and products distribution.


INTRODUCTION
The energy crisis and environmental degradation are important problems that we are is facing today.Their origin lies on growing population, rapid industrialization 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 use appropriate technologies for energy recovery from non-conventional sources, like municipal and/or industrial wastes, refused plastics, used tires etc.The disposal of these organic solid wastes from human activity is a growing environmental problem for modern society.Thinking on environmental hazards, images of chemicals in waters or air pollution coming out of industrial furnaces are most often seen.There are some hazards that are overlooked and one of them is scrap tires, one of the very common and most important hazardous solid wastes all over the world.Without a good management, scrap tires treatment can threaten not only our environment, but the public health as well.For instance, run-off from scrap tire fires can contaminate groundwater and surface water and scrap tire sites are an ideal habitat for the breeding of insects carrying diseases [1 -3].
The disposal of waste tires is a major environmental problem, because waste tires in open areas demand valuable landfill space and may result in accidental fires with pollution emissions.Such fires are difficult to manage because of the tires' high flammability.For instant, one fire in Huntington (United States), burned for nine months [3].Waste tires cause also major environment problems because they are not affected by the biological degradation With the continued word-wide increase in production of automotives and trucks, the generation rate of waste tires is increasing dramatically.For example, the word generation of used tires in 2005 was over 2.5 million tons in North America, 2.5 million tons in Europe and 0.5 -1.0 million tons in Japan.More than 1 million tons of waste tires were generated within USA, Europe and Japan in 2007 [4 -6].According to these estimations, an increase of over 17 million tons was expected by 2012 (Figure 1).In China as well, it was 3.2 million tons in 2004 [7].It is estimated that globally one billion used tires arise each year [8 -11].Although waste tires account for only 2 wt % of total waste [7], their management is attracting growing interest in developed countries because of the environmental problems they may generate through inappropriate management, on the one hand, and the increasing demand for recycling raw materials, on the other.
European Union has introduced directives for the management of waste tires including the European commission's landfill Directive (1999) which has banned the disposal of waste tires to landfill since 2006 and the European End Life Vehicle Directive 2000 which requires that the 80 % in weight of an end of life vehicle is recycled [1].In another meaning, it proposes the following routine: reduce -reuse -recycle and finally to perform an energy valorisation process.
Although it would be desirable to implement only the three first strategies, waste tire generation is so high that it is unavoidable to make use of energy valorisation process.In Europe, the main methods for waste tires management are materials recovery (38.7 %), energy recovery (32.3 %) or retreating (11.3 %).But globally, wide spread in the word, approximately 64 % of used tires went to landfill or were illegally dumped or stockpiled, while only 13 % were recycled [12].
Tires are built to be tough and durable, the properties that ensure a safe ride and long service life, make scrap tire disposal a difficult task.Waste tires are recalcitrant to natural degradation.The vulcanized rubber consists of long chain polymers (poly-isoprene, poly-butadiene, and styrene-butadiene copolymers) that are cross-linked with sulphur bonds and are further protected by antioxidants and antiozonants that resist degradation.Combustion of tires produces toxic gases which contain carcinogenic and mutagenic chemicals.So tires incineration requires expensive air emission control system [13].
Natural rubber (NR), styrene-butadiene rubber (SBR) and butadiene rubber (BR) are the most common rubbers used for tires.Sulphur is used to vulcanizing, steel and carbon black are used as reinforcing agents and aromatic extender oil is used to soften and enhance the workability of rubber.All of these components are 100 % recyclable.Scrape tires' high volatile, high fixed carbon, moderate sulphur and low-ash contents with a calorific value greater than that of coal or biomass make it an ideal material for thermo chemical treatments (combustion, pyrolysis and gasification) [8, 9, 14 -18].With the increasing emphasis on recovery and recycling of waste, there is an increasing interest in developing new technologies for used tires treatment [4,8,10,11,13,19,20,21].

Material recycling / reusing
The properties of used tires (such as elasticity, stability in air atmosphere and high level of humidity) have led to a number of alternative applications like [17]:  In agriculture: as weights for silage cover sheets;  In landscaping: as erosion protection for dam walls and slopes;  In shore protection: as breakwaters;  In harbours and docks: as dock bumpers and ship fenders;  In the fishing industry: as artificial reefs for fish breeding;  In household and communities: as bumpers in garages, playground equipment;  Shoemaking: soles, heels and straps of sandals can be made from tire materials;  Road construction: tires are mixed with asphalt cement forming asphalt rubber, asphalt rubber filler costs 40% more than conventional material.

Retreating
Retreating of used tires is the most preferable way of making use of old tires.On average only 15 % [17] of crude oil is needed to produce a retread instead of a new tire.Thus the price of a tire is reduced up to 45 % [17] without any loss in quality.But, tire for passenger cars normally are retread once, tires for Lorries are twice retread, while aeroplane tires even up to seven times.In Germany, 12 % of all passenger cars, 20 % of all Light Duty Vehicles and 48 % of all Lorries use retreads and 90 % of the world's airlines use retread tires.
However, two aspects might be of negative impact on this process, which are: (a) The acceptance of retreated materials by the customers: people tend to think that "retreads" don't have the same quality compared to new tires.All investigations have shown that the retreads show similar qualities as the new tires.Moreover, some German manufacturers of retreads give 2 years guaranty on their products and allow their tires to be driven up to 190 Km/h [17]; (b) The availability of old tire with good quality: in countries where tires are valuable products compared to the average income, people want to drive a tire as long as possible, then tires are often driven until the threads or other internal layers become visible.These tires are no longer use in re-manufacturing process [17].
Nevertheless, these measures -alternative applications and retreads -are not capable of dealing with the massive numbers of waste tires being produced.Thermal valorisation is emerging as a possible solution for reprocessing huge amounts of this material.The three main applied technologies for thermochemical valorisation are: pyrolysis (degradation in absence of oxygen), gasification (degradation in presence of water vapour) and combustion (degradation in presence of oxygen).
Actually, in order to manage increasingly amount of waste tire, many studies have been carried out on these materials by converting them into useful source of raw materials (recycle, retread ...) or useful sources of energy (incineration, distillation, gasification, and pyrolysis) [2,10,15].
While waste tire management by recycling or retreating does not consume more than 11.3 % of its big disposal [13], waste to energy conversion method can reach a much higher proportion.
On the other hand, energy conversion has different difficulties.As presented above, direct combustion or incineration put serious emission problems.
A much manageable energy recovery technology is pyrolysis.Extensive studies on pyrolysis as a way to convert waste organic materials (oily sludge: residue inside the bottom of a fuel tank, fuel residue accumulates on the fuel filter, residue results from the purification of lubricating oil, oily waste; organic materials in municipal waste: oil, soap, waste tires, plastic, textile…) [8] into useful products have been carried out for decades.
Pyrolysis of tires involves the thermal degradation of the tire rubber at temperature (300 -900 °C) in an inert atmosphere.The pyrolysis of waste tires has received increasing attention since the process conditions may be optimized to produce high energy oil, gas and residual char in addition to the steel casing of tire.Steel is recycled back into iron or steel industry.The other three products are used as energy sources (fuel) or/and chemical resources [10,14,18,19].
Pyrolytic products of waste tires have high calorific values: the solid has around 30 MJ/kg, the liquid exhibits the same values as petroleum-like fuels, 41 -44 MJ/kg and while the gas has about 37 kJ/Nm 3 .It means the three products could be used as good fuel [10,14].
In this study the pyrolysis of light duty passenger vehicles waste tires are used and tests have been carried out in the batch reactor system under inert atmosphere (nitrogen).The effects of operating temperature, flow rate of nitrogen (N 2 ) and heating rate on the yields of pyrolysis products were investigated.Since we target the optimization of tire conversion and the gas production, a detailed characterization of the pyrolysis gas obtained at different temperature has been carried out including physical properties.

Raw materials
Raw materials are supplied a by French company dealing with waste tires.The raw material is a mixture of light duty vehicles non-shredded waste tires with piece sizes varying from 1 to 10 cm.For raw material characterisation two different tire pieces have been selected.The rubber higher heating value varies from 35 to 36 MJ/kg, while the ash content has a larger variation, 5.2 -9.4 % (wt).
The elemental analysis is presented in Table 1.Results confirm the above observation related to the large variability in rubber composition; and the main interest for energy recovery techniques is related to compounds with high potential for toxic emission during thermochemical pathways.On the other hand, tires have high contents of carbon and hydrogen that make them a considerable source of energy.

Laboratory pilot system for pyrolysis tests
Pyrolysis tests have been performed in a fixed bed, batch reactor with a total volume of 2L, heated by electric heating coil (Figure 2, a).The heating rate and temperature stabilization are ensured by a PID controller.A stream of nitrogen is introduced by metal tube to the bottom of the reactor in order to carry out the generated gases through a duct in the top of the reactor to a condenser cooled with water at 15 °C.
The condenser is connected to a double necked flask that receives condensable compounds (Figure 2, b).Nitrogen and non-condensed gases stream flows through the second neck of the flask that is connected to an exit tube where a small valve is used to take samples of gas every 20 min during the reaction.
The pyrolysis experiments were performed at 500 °C by varying the heating rate within the range of 5 -15 °C/min.Pyrolysis vapours were passed through two sets of condenser.The liquid was collected into the glass bottles and the uncondensed gases were recuperated into Tedlar gas sampling bags and analysed.The produced char was collected at the end of process.Three heating programs (Table 2), from ambient temperature to 500 °C, were used in this study for the pyrolysis of waste tires: 5, 10 and 15 °C/min.The process is stopped when no vapour are observed.In order to ensure its repeatability, each test was repeated three times.The liquid and solid fractions obtained at each reaction were weighted and the gaseous product weight was deduced from mass difference between raw material and liquid and solid products.All values presented in this work are the mean values of three repeated reactions.

Analysis methods and apparatus
For the characterization of the thermal decomposition of rubber a TG/DTG instrument (SETARAM 2005) was used.Samples of 50 -150 mg were used for each analysis, and the nitrogen flow rate varied from 5 to 95 mL/min.
An elemental analysis was performed on raw materials before reaction and on solid and liquid products by using a CHNS and O analyser (THERMO FINNIGAN, Flash EA 1112 series).The produced gas composition was analysed by gas-chromatography using an AGILENT micro-GC.The estimated measuring error on gas composition is around 0.5 %.

Pyrolysis products yields
As said above, three types of products are usually obtained from pyrolysis of tire rubber: solid char, liquid and gas.The product distributions obtained in this work from selected waste tires pyrolysis are presented in Table 3.
It can be seen that the heating rate has not a constant influence on liquid and gas fractions.
On the other hand, when heating rate increases from 5 to 15 °C/min, the yield of solid product continuously decreases.In other words, at 10 °C/min de decomposition process favours the formation of larger molecular weight, while at 15 °C/min smaller molecules are more likely produced.

Gas analysis
Using a gas-chromatograph with thermal conductivity detector (TCD/GC), the gas has been analysed at each 20 min during the thermal range of tyres degradation.It was found that the gases are mainly formed starting from 400 °C and the major compounds are H 2 and CH 4 (as seen in Figure 3 -Figure 5).In concordance with the tendency on gas evolution identified in Table 3, the gas composition exhibits inconstancy when heating rate is changed.Furthermore, at all heating rate, there are significant increasing of temperature inside the reactor even if the reference temperature was set at 500 °C.
This phenomena is more prominent at 10°C/min.Without doubt this is the effect of exothermic reactions development, resulting in a prompt rise of CH 4 and H 2 concentration: from 18 to 26 % for CH 4 and from 12 to 24 % for H 2 , when the heating rate was 5 °C/min.
However, this tendency seems to change at higher heating rates, which suggest that under these specific conditions the synthesis of CH 4 and H 2 is limited.
It is interesting to note that after the exothermic moment, the temperature inside the reactor returns at around 500 °C, but the concentration of CH 4 and H 2 remain at the same high level.It appears that the exothermic reactions offer the energy needed to unlock certain reaction mechanisms, which once started will continue even if the temperature inside the reactor begin to decline.

TG/DTA Analysis
TG/DTA analysis was performed in order to characterise these materials and to understand the influence of some parameters on thermochemical decomposition behaviours.TG results show that the range of temperature where the pyrolysis takes place is between 200 °C and 500 °C.
As Figure 6 illustrates, TG/DTG results show that waste tires pyrolysis performed with a heating rate of 10° C/min and under 20 mL/min N 2 is completed at 500 °C.
In addition, there are two identified pyrolysis zones: the first one takes places at T max1 = 370 -380 °C while the second one happens at T max2 = 470 -480 °C.
Taking into account the variation registered during tests performed in laboratory pilot installation, two parameters were varied: heating rate and N 2 flow rate (FR).The heating rate (HR) was established at 1, 5 and 9 °C/min and the N 2 flow rate (FR) was: 5, 50 and 95 mL/min.In Table 4 are extracted the main information obtained during TG/DTA analysis.Fig. 6.Typical TG/DTA curves registered for tested tire rubber.
In Figure 7 -Figure 9, the influence on thermochemical behaviour of tire rubber of varied parameters is observed.Analysing the TG/DTA data it was established that both varied parameters has influence on the rubber behaviours during the pyrolysis and, once again, the discontinuity in variation in detected.For the same heating rate, when the N 2 flow rate increases, the temperature of the main degradation peak remain practically constant, but does bring variation in total conversion.
Thus, 50 mL/min N 2 appear to be the optimum flow rate when operating with 5 °C/min.Furthermore, in these conditions the overall process starts earlier, which can be an important point in managing the condensing vapours.
Nevertheless, the total conversion appears to have a very discontinue variation and the chemical mechanisms governing the process need to be investigate further.Working at high N 2 flow rate ensure a very well extraction of pyrolysis vapour, so there is no risk related to accumulation of gases during pyrolysis.On the other hand, the appearance of large zone with mass loss perturbations on corresponding TG/DTG curves (especially when the heating rate increases) can be a warning sign in process control and management.
The heating rate influence is easily identified.Whatever N 2 flow rate was used, low heating rate give one step process, with no intensity in degradation peak.These are those situations when gas and liquid production and recovery during the pyrolysis occur with manageable heat and mass transfer phenomena.
In the same time, most polymer materials such as synthetic or/and natural rubber usually give solid structure more resistant to heat and so, in order to keep the same conversion level higher temperature and residence time are necessary.This is the reason why, at low heating rate the total conversion of polymers into gas and liquids rarely exceeds 45 %.Then, more the heating rate increases, more spontaneous reactions occur.This tendency is proven by the curves shape, which became deeper and narrower when the heating rate grows.

CONCLUSIONS
In this work pyrolysis has been considered an appropriate way to manage the reduction amount of scrap tires simultaneous with the energy recovery.TGA results afford the study of the kinetic parameters while the laboratory facilities allow the comprehension of tires behaviour in real conditions.The processing temperature was limited at 700 °C and the measurements focused on the mass balance determination and gaseous products analysis.
Variation in process parameters at small and laboratory pilot scale ensure that the total conversion of rubber tire does not register significant changes.It was found that the three obtained products (gaz, liquid and solid fractions) have a good energetic potential: the solid (20 -32 MJ/kg), the liquid (41 -43 MJ/kg) and the gas (32 -36 MJ/m 3 ).Nevertheless, the liquid needs to be refined in order to be used as Diesel-like fuel and gases should be treated to remove sulphur compounds.
Regarding the results provided by the larger pyrolysis installation we learned that at small heating rate the gas composition exhibits an increased heating value due to a high concentration in H 2 and CH 4 .Meanwhile, at high heating rate the amount of gas increases and the total conversion, too.The decrease in liquid yields and increase in gas yields at 15 °C/min are probably due to the decomposition of some oil vapours into permanent gases, and secondary carbonization reactions of oil hydrocarbons into char.Thus, at higher temperatures the gas yields gradually become dominating.
Therefore results from different authors are many times difficult to compare, mainly due to the variability in rubber composition and so different behaviours during the thermal decomposition.As a common point it was identified that the heating rate is the most influencing parameter, and its influence is highly recommended to be known before scale-up any pilot installation.
It is also known that pyrolysis product yields and their distributions over the whole range of temperature depend not only on the feedstock composition and operating temperature used for the experiments, but also on the specific characteristics of the system used, such as geometry and type of reactor which are closely related to the efficiency of heat transfer from the hot reactor surface to and within the tire mass.Also, feed particle size, vapour residence time and applied catalytic system make the difference from a work to another.
As the obtained gas and oil have a high potential for energy use, the authors consider that the evaluation of waste tire rubber pyrolysis in catalytic systems is a more appropriate approach either from economic and environmental point of view.An efficient catalytic system need to improve the solid conversion and also to reduce the formation of harmful molecules such as sulphur compounds.

Fig. 2 .
Fig. 2. Used pyrolysis installation for waste tires: (a) fixed-bed reactor and (b) condensing system.Before experiments, the reactor was purged with N 2 , the used flow of gas being 2 L/min for 10 min in order to remove the inside air.

Fig. 3 .
Fig. 3. Variation of CH 4 , H 2 , CO at 5 °C/min.Thus, as seen in Figure3, when the process is running with 5°C/min the amount of CH 4 and H 2 increases continuously, with two significant slopes at 440 -470 °C and 530 -540 °C.At 15°C/min the amounts of CH 4 and H 2 is considerably reduced compared with values registered for tests performed at 5 and 10°C/min.Also, a loop in CH 4 and H 2 evolution is identified, but this appears in different thermal ranges: 425 -530 °C for 5°C/min, 450 -570 °C for 10°C/min, and 450 -500 °C, for 15°C/min.

Table 2 .
Three heating programs for the pyrolysis of waste tires.

Table 3 .
The yield of pyrolysis products.

Table 4 .
Results obtained from TG/DTA Analysis.