plasma reactor with hybrid water/gas plasma torch. High enthalpy steam plasma was generated in dc arc torch with water stabilized (Gerdien) arc. Reactions of.
Steam Plasma Gasification of Pyrolytic Oil from Used Tires M. Hrabovsky, M. Konrad, V. Kopecky, M. Hlina, T. Kavka, O. Chumak and A. Maslani Institute of Plasma Physics ASCR, Praha, Czech Republic Abstract: Production of syngas by gasification of pyrolysis oil was studied in plasma reactor with hybrid water/gas plasma torch. High enthalpy steam plasma was generated in dc arc torch with water stabilized (Gerdien) arc. Reactions of steam, oxygen and carbon dioxide with oil produced by pyrolysis of used tires were studied for arc power 110 kW. Syngas with high content of hydrogen and carbon monoxide and very low content of carbon dioxide and other components was produced. Very low content of complex hydrocarbons and tar was detected. For steam gasification, water was added into the reactor together with treated material, in other experiments CO2 and O2 were supplied as oxidation medium. Keywords: thermal plasma, plasma gasification, waste treatment, syngas
1. Introduction Sustainable development in transport includes proper handling of used tires. The amount of waste tires in EU, USA and Japan is around 6 million tones per year and will increase in the future [1]. In EU countries landfill directive imposes a ban on the landfilling of tires from the year 2006. Retreading is environmentally positive solution but not widely used as the costs of new tires are the same as of retreaded tires. Recycling by mechanical shredding and subsequent grinding produces rubber granules. The process consumes lot of energy and has very limited market for application. Recovery of energy by incineration is another possibility (tire derived fuel), but the process brings problems with hazardous emissions and subsequent flue gas cleaning at high capital costs. Also some methods for rubber reclaiming were developed, but the products are of poor quality and the costs are similar to the costs of new rubber. The mentioned drawbacks of these technologies have led to research on other possibilities of recycling. One of the methods is pyrolysis of rubber from waste tires [2, 3]. The products of pyrolysis are solid char, liquid pyrolytic oil and synthetic gas. The pyrolytic products are of poor quality and therefore investigations on upgrading of properties of these products are taking place. In this study we have investigated the possibilities of pyrolytic oil upgrading by gasification in thermal plasma reactor. The pyrolytic oil as low grade fuel should be
converted to synthesis gas comprising mainly hydrogen and carbon monoxide as high grade fuel. Thermal plasma offers possibility of decomposition of organic materials by pure pyrolysis in the absence of oxygen, or with stoichiometric amount of oxygen added for complete gasification of carbon present in treated material. Oxygen needed for complete gasification of materials can be supplied either by addition of oxygen, air, steam or CO2. Utilization of air is the cheapest option but it results in dilution of produced syngas by nitrogen. In experiments presented in this paper we studied pyrolytic oil gasification in steam thermal plasma generated in dc arc torch with water stabilized (Gerdien) arc. The composition of plasma gas is suitable for plasma gasification as plasma gas does not contain gas components which would dilute produced syngas (mixture of H2 and CO), with the exception of small amount of argon. Water, carbon dioxide and oxygen were used as oxidizing media.
2. Experimental system The plasma gasification reactor (Fig.1) was designed for operation up to 1700°C wall temperature [4, 5]. The reactor has ceramic lining, the inner volume is 0.206 m3, and the outer steel walls of the reactor are water cooled with possibility of calorimetric measurements on cooling circuits. The pyrolytic oil was fed into the reactor through a nozzle 0.5 mm in diameter. The oil from a pump with the manual control of oil flowrate was fed through a hose into
feeding nozzle. Oxygen and carbon monoxide were supplied directly into the reactor by inputs at the reactor side, water was injected into the same nozzle as oil. The output of gas is positioned close to the plasma input and thus outgoing gas passes high temperature region with high level of uv radiation. Gaseous reaction products are fed into the quenching chamber where their temperature is reduced to 300 0 C in water spray with automatically controlled flow rate. The gas then flows into the combustion chamber at the output of the system.
Oil input
nozzle. The produced synthetic gas (syngas) was quenched in a chamber with water spray. Table 1. Operation parameters of plasma torch.
Arc current [A] Arc power [kW] Steam plasma flow rate [g/s] Argon plasma flow rate [slm] Torch efficiency [%] Mean plasma enthalpy [MJ/kg] Bulk plasma temperature [K]
400 110 0.3 7 59 129 14 200
The composition of the syngas was measured online by mass spectrometer Omnistar (Pfeiffer Vacuum) with mass range 0-100 amu. The selected gases for monitoring were hydrogen, carbon monoxide, carbon dioxide, methane, oxygen, and argon.
2. Results and Discussion
Figure 1. Scheme of plasma gasification reactor.
Plasma torch with hybrid gas/water stabilization of arc [6, 7] was attached at the top of the reactor. The torch generates an oxygen-hydrogen-argon plasma jet with extremely high plasma enthalpy and temperature. Plasma gas is a mixture of steam with small amount of argon. The values of basic parameters of plasma torch used in presented experiments are given in Tab. 1. Due to the principle of arc stabilization by a water vortex the flow rate of plasma gas is very low, plasma enthalpy and plasma temperature is very high. This is the main difference from gas plasma torches using steam as plasma gas with temperatures below 8 000 K. The utilization of high enthalpy, high temperature plasma is advantageous for the adjustment of a high reaction temperature and easy control of syngas composition. The flow rates of oxidizing gases were controlled by thermal mass flow controllers (Brooks Instrument, Aalborg). The stream of pyrolytic oil crosses the plasma jet about 30 cm downstream of the torch
The results of analysis of elemental composition of the studied pyrolytic oil, made on two samples in two laboratories, are presented in Tab. 2. The oil contained more than 21 wt.% of water, the density was 0.9 kg/l, and the heating value based on Dulong equation was 42.1 MJ/kg for oil without water and 39.5 MJ/kg for oil with water. From the elemental composition of the oil follows its molecular formula C5H8O. Table 2. Elemental composition of pyrolytic oil. C [wt.%]
H [wt.%]
S [wt.%]
N [wt.%]
Cl [wt.%]
85.8
10.2
0.72
0.62
8.2 x 10-4
88.18
9.39
1.18
0.85
≤ 5 x 10-3
For the gasification an oxidizing medium was fed into the reactor in stoichiometric ratio to oxidize the surplus of carbon to carbon monoxide. Water, oxygen or carbon dioxide were used, separately, or in a mixture, according to equations C5 H 8 O + 4H 2 O → 5CO + 8H 2
(1)
C5 H8O + 4CO2 → 9CO + 4H2
(2)
C 5 H 8 O + 2O 2 → 5CO + 4H 2
(3)
The arc current and power as well as feeding rates of material and flow rates of oxidizing gases are given in Tab. 3. The table also shows average temperatures Tr of inner reactor wall, which was measured at five positions, and temperature of syngas Tg, measured at the output tube from the reactor before quenching. Before each measurement of produced syngas composition, the reactor was run at least 5 min to reach steady state output of gas composition monitoring. Results of measurement of composition of produced syngas are shown in Tab. 4. The table does not include concemtrations of argon from the torch, that were in all cases below 1%. Several results for same conditions, taken in different times, are given in Tab. 4 to illustrate stability of the output. The table presents also low heating values of produced syngas and yields of carbon gasification, defined as ratio of carbon content in syngas to total carbon supplied in treated material and added gases. In Fig. 2 average values of syngas components concentrations are shown for cases of gasification with addition of water, oxygen, carbon dioxide and mixture of CO2 and O2. 100 90
19
27
80 70
48 58
Vol %
60
H2 CO
54
50
CO2 CH4
53
40 30
O2
47
32
20 10
4 5
0 water
25
16 0.5
3 CO2
0.5
1.5
3
O2
0.2
1.5
0.3
CO2+O2
Oxidation medium
Figure 2. Composition of syngas for various oxidizing media
It can be seen that efficiency of carbon gasification varied between 0.58 and 0.67 for gasification with water, the value 0.58 was obtained for gasification with CO2. The highest efficiency of gasification 0.85 was obtained if oxygen was used as gasification medium; values 0.68 and 0.77 were obtained for mixture of CO2 with O2. The heating values of syngas are high due to high content of hydrogen and carbon monoxide. Higher content of CO2 and consequently lower syngas heating values were obtained in case of gasification with CO2 as
oxidizing gas. The reactor temperatures as well as syngas temperatures in runs with feeding of O2 were higher compared to CO2 and water feeding, as in latter cases part of the input plasma energy is spent for dissociation of water and CO2. In all cases, no tar production was detected. This fact was confirmed by the analyses made in our previous experiments with steam plasma gasification of organic materials [4, 5].
3. Conclusions Management of waste tires represents important environmental issue. The both usual methods of tire treatment, grinding or combustion have many drawbacks. The pyrolysis is promising technology for treatment of waste tires, but techniques for the upgrade of products of pyrolysis have to be developed. The plasma gasification of pyrolytic oil is one of the possibilities as it produces high quality syngas with high heating value up to 12 MJ/Nm3. Plasma gasification of pyrolytic oil was studied in plasma reactor with hybrid water/gas dc arc torch. High enthalpy, high temperature steam plasma was generated in the torch with very low plasma flow rate. Plasma produced from steam does not dilute produced syngas by other plasma gases. Synthesis gas with high content of hydrogen and carbon monoxide, low content of carbon dioxide and methane, and high heating value was produced when water or oxygen were used as oxidizing medium. If carbon dioxide was used for oxidation of surplus of carbon in the oil, the content of CO2 in produced syngas was higher. The efficiency of carbon gasification was 0.6 to 0.7 for gasification with water and CO2 and 0.9 in case of gasification with oxygen. Acknowledgement The work was supported by the Grant Agency of the Czech Republic under the project P 205/11/2070.
References [1] M. Juma et al., Petroleum & Coal 48 (2006), pp. 15-26. [2] I.M. Rodriguez et al., Technology, 72(2001), pp. 9-2.
Fuel
Processing
[3] M.F. Laresgoiti et al., J. Anal. Appl. Pyrolysis 71(2004), pp. 917–934.
[6] M. Hrabovsky et al., IEEE Trans. Plasma Science, 34 (2006), pp. 1566-1575.
[4] M. Hrabovsky et al., High Temp. Mat. Process. 10 (2006), pp. 557-570.
[7] M. Hrabovsky, Pure and Appl. Chem. 74 (2002), pp. 429-433.
[5] M. Hlina et al., Czechoslovak J. of Physics, Vol. 56 (2006), Suppl. B, B1179-1184. Table 3. Torch parameters, reactor and syngas temperatures, and input flow rates. No. I [A] P[kW] Tr [oC] Tg [oC] oil [kg/h] H2 O [kg/h] CO2 [slm] O2 [slm] 1 400 110 1074 877 8.8 10.6 0 0 2 3
400 400
110 110
1072 1066
876 878
8.8 8.8
10.6 10.6
0 0
0 0
4
400
110
1199
1048
10.6
0
0
92
5 6
400 400
110 110
1194 1208
1042 1059
10.6 10.6
0 0
0 0
92 92
7 8
400 400
110 110
1202 1057
1054 958
10.6 10.6
0 0
0 182
92 92
9 10
400 400
110 110
1087 1056
968 991
10.6 10.6
0 0
182 182
92 0
Table 4. Syngas composition,low heating value and carbon gasification efficiency.
No. % H2 % CO 1 56.6 33.4
% CO2 %CH4 4.5 5.5
% O2 0.1
Cout /Cin LHV [MJ/Nm 3] 0.58 12.3
2 3
57.7 58.5
32.7 31.6
4.1 4.7
4.9 5.0
0.7 0.4
0.67 0.67
12.1 12.1
4 5 6
50.0 47.9 45.5
46.0 47.0 48.8
2.3 2.9 4.1
1.5 2.0 1.5
0.0 0.0 0.1
0.74 0.77
11.7 11.8 11.6
7 8
49.1 19.0
46.8 53.2
2.5 25.4
1.5 1.8
0.1 0.2
0.68 0.84
11.7 9.4
9 10
19.2 26.9
53.7 53.2
25.1 16.4
1.6 1.9
0.0 0.3
0.85 0.58
9.4 10.7
