3 RESULTS AND DISCUSSION
3.1 Product Distribution
Unlike conventional pyrolysis, which has gaseous, liquid and solid products, waste tyre particles are transformed only into gaseous and solid products in the process of thermal plasma pyrolysis. The key process parameters likely to affect the yields of product are power input, feed rate and steam injection, as illustrated in Figs.2 to 4.
It can be seen from Figs.2 and 3 that due to the heat transfer and system energy, the yield of gas increases as the input power is increased. From 30.8 to 48.4 kV.A, the yield of the gas increases from 39.98% to 45.00%, higher input power does not decrease solid yield in the process. It is suggested that the tyre decomposition at about 35.2 kV.A is completed. On the contrary, when increasing the feed rate, the yield of gas decreases obviously.
Water steam is injected into the plasma reaction chamber together with the tyre particles in order to improve product quality and get syngas so that the range of application can be extended and the process be more economical. The comparison of gas and char yields corresponding to the process with or without water steam injection is given in Fig.4. With water steam injection, the yield of the gas increases from 0.186 m3/min (42%, ω) to 0.226 m3/min (77%, ω) at the same input power and feed rate level (increased by 80%).
3.2 Gas Composition and Calorific Value
The gas composition from plasma pyrolysis of tyres is very different from that of conventional pyrolysis. The conventional pyrolysis of tyres at 700oC gives(%): CH4 20.6, C2H4 8.9, C2H6 8.1,C3H6 4.5, C3H8 3.2, C4H8 16, C4H10 3.8, CO 10.4, CO2 11.4. However, it can be seen from Table 3 that the main gaseous products of plasma pyrolysis identified by chromatography are H2, CO, CH4,C2H2, O2, CO2, C2H6, C2H4 and other hydrocarbons. Product selectivity is one of the advantages ofplasma pyrolysis; on a N2-free basis, the concentration of H2 can reach up to about 57%. H2 is a clean gas fuel which can be used in several applications. In this study, it is desirable to get more gas yield and higher gaseous calorific value. On the basis of the experimental data summarized in Table 3, the effects of plasma pyrolysis conditions on gas composition and gas calorific value are discussed as follows.
3.2.1 Influence of power input
As seen in Table 3, by increasing the input power from 30.8 to 39.6 kV?A, H2 concentration is increased from 12.07% to 15.23%, CO concentration is increased from 2.75% to 4.2%, and the solid conversion is increased from 39.68% to 44.30%; but from 39.6 to 48.4 kV?A, only minor influence of power input on the component of the gas was observed, and the efficiency did not increase further also. Concerning gas calorific value, one can see that the calorific value of the pyrolytic gas increases firstly to a maximum value of about 7.56 MJ/m3 with increasing input power from 30.8 to 35.2 kV?A, but it afterwards decreases, which suggests that the high calorific value gas such as C2H4, CH4 ulteriorly decomposed into H2 and elemental carbon with the elevation of the input power.Further increase of input power might increase the energy density in the system, but the maximum temperature achieved in the discharge zone increases only slightly because of increased loss of discharge power as radiation[5]. It can be suggested that within the range of this study, the power input is not the key parameter influencing the pyrolysis, so we can get the approximate result using lower power input with the otherwise same conditions.
3.2.2 Effect of feed rate
The second important parameter with influence on conversion was detected during the investigation of the change of particle feed rate into the plasma jet. Sample feed rates were varied over the range of 40～120 g/min. From Table 3 we can see the increase of H2 and CO concentration due to the increase of feed rate. The same trends are found about the calorific value of the gas, but the solid conversion is decreased from 60.60% to 30.98%. So we suppose that increasing the feed rate may decrease the amount of energy available for heating each particle and likely decrease the average temperature of particles; increasing the feed rate may also have influence on the efficiency of heat transfer. These two factors lead to decreased gaseous product volume and increased carbon residues.
3.2.3 Influence of steam injection
By water steam injection, oxygen content is increased in the system, and the carbon conversion can be increased. There are some chemical reactions occurring during plasma gasification when injecting water steam:
C + H2O → CO + H2.
Table 3 shows a trend of large increase of the amounts of H2 and CO and a slight decrease of C2H2 concentration. The summation of H2, CO concentrations reaches up to about 38.3% due to steam injection reaction; without water steam injection, the concentration of H2 is about 15% and there is scarcely CO. Water steam injection can elevate carbon conversion up to 77%. The gas product from plasma pyrolysis with steam can be utilized as syngas after separation of other gaseous products; also the calorific value of the pyrolytic gas has a large elevation to 8.96 MJ/m3. This measure counteracts the disadvantage of limited application of gaseous product of plasma pyrolysis and favors practical applications.
4 CONCLUSIONS
Experiments on plasma pyrolysis of tyre particles show that:
(1) Increasing the input power can increase the pyrolysis conversion and the yield of the
gaseous product, but the calorific value of the pyrolytic gas would firstly increase then decrease.
(2) Increasing the feed rate can increase the yield of the gaseous product and the gas calorific value, but a large feed rate would lead to a lower conversion.
(3) Water steam injection is the main parameter influencing the gaseous products distribution and the gas quality. By addition of steam, the concentration of syngas can reach up to 38.3% and the calorific value is 8.96 MJ/m3.
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