Application of waste plastics to electric furnaces for steel making as thermal and carbon sources

Transcription

1 Application of waste plastics to electric furnaces for steel making as thermal and carbon sources I. Naruse 1, T. Kameshima 1 & H. Omori 2 1 Department of Ecological Engineering, Toyohashi University of Technology, Japan 2 R&D Division, Daido Steel Co., Ltd., Japan Abstract This study develops technology of effective utilization of the waste plastics instead of some parts of coke supplied in electric furnaces for steel making. In order to perform this technology for practical electric furnaces, it is necessary to solve such problems as difficulty of thermal supply to the molten iron and increase of thermal load in the downstream of the furnace due to high reactivity with the plastics. From these viewpoints, first, reaction rates of several types of plastics were measured under the pyrolysis and combustion conditions. Next, the methods to decrease the reaction rate of the plastics were experimentally studied, changing such parameters as size of the plastic pellet, mixing fraction of waste iron material in the pellet, the reaction atmosphere and the furnace temperature. Additionally, compositions of the pyrolized gases and total amounts of both soot and tar produced were analyzed. The results show that both the pyrolysis and combustion characteristics depend on the types of plastics, the pellet size and the mixing fraction of the waste iron in the pellet. The area index proposed in this study can estimate the reaction rate of the practical size of pellet used in the electric furnace. The gases produced from the waste plastics can be utilized as the thermal sources, while both the soot and char produced may contribute to the carbon source in the molten iron. Keywords: waste plastics, electric furnace for steel making, thermal recycle.

2 146 Waste Management in Japan 1 Introduction Recently, total amount of plastics production in Japan is about 13 million ton/year [1]. Waste plastics of 10 million ton/y have been treated by means of direct landfill and incineration. However, both the direct landfill and incineration have to be depressed due to lack of the landfill area as well as long-term environmental pollution. From these viewpoints, this study has developed technology of effective utilization of the waste plastics instead of some parts of coke supplied in electric furnaces for steel making. In order to perform this technology to the practical electric furnaces, however, it is necessary to solve such the problems as difficulty of thermal supply to the molten iron and increase of thermal load in the downstream of the furnace due to high reactivity with the plastics. In this study, therefore, reaction rates of several types of plastics were measured under the pyrolysis and combustion conditions, using a thermo-gravimetric analyser (TG) and an electrically heated batch furnace (BF). Next, the methods to decrease the reaction rate of the plastics were experimentally studied, changing such the parameters as size of the plastics pellet, mixing fraction of waste iron material in the pellet, the reaction atmosphere and the furnace temperature. Additionally, compositions of the pyrolized gases and total amounts of both soot and tar produced were analyzed. 2 Experimental Fundamental thermal properties of several kinds of plastics are measured, using a thermo-gravimetric analyser (TG). The experimental conditions are summarized in Table 1. Six kinds of plastics of polypropylene (PP), polyethylene (PE), polyurethane (PU), ABS resin (ABS), polyvinyl chloride (PVC) and polycarbonate (PC) as samples. In the TG tests, the power sample is pyrolyzed. Table 1: Experimental conditions in TG test. Conditions PP, PE, PU, ABS, PVC, PC Sample Temperature increasing rate [K/min] 5 Sample mass [mg] 5 Final holding temperature [K] 1073 Reaction atmosphere Ar gas Pyrolysis and combustion tests are conducted, using an electrically heated batch furnace (BF) shown in Fig. 1. The BF mainly consists of electric heaters, a process tube and instruments. The furnace temperature is controlled by temperature controllers. Ar gas and air is supplied from the bottom of the furnace for the pyrolysis and combustion test, respectively. In the process tube, alumina balls are packed to preheat the reaction gas. At the furnace top, an electric balance is installed to weigh the sample mass during reaction continuously.

3 Waste Management in Japan 147 Shape of the plastic sample is a cylindrical pellet. Before the tests, the single pellet is put on a nickel pan, which connects with the balance by a stainless steel chain. After the furnace temperature attains the given temperature, the furnace is moved up, and the reaction can be started. The experimental conditions in the BF test are shown in Table 2. Specification of the pellet employed in the experiments is summarized in Table 3. In the electric furnaces for steel making, generally, a lifting magnet is applied to supply a scrap iron metal to the furnace. As the plastics pellet itself does not have magnetism, a powder of waste iron is mixed with the plastics powder before making the pellet. In the experiments, both the mixture pellet and the plastics pellet are tested. When making the mixture pellet, the iron powder is completely mixed with the plastics sample. Com puter Mass Data Filter Air Dryer Electric Balance Exhaust PDF Reactor Tube Sam pling Gas Electric Heater Gas Analyzers Therm o C ontroller Thyristor Regulator G as C onc. Data Amplifier Computer Up Down Therm o C ontroller Thyristor Regulator Air,Ar Alum ina B all Packed Bed to Preheat A ir Figure 1: Schematics of electrically heated batch furnace. Table 2: Experimental conditions in BF test. Furnace temperature [K] 873, 1273 Reaction atmosphere Pyrolysis Ar gas Combustion Air Flow rate of reaction gas [l/min] 10

4 148 Waste Management in Japan When analyzing compositions of the pyrolyzed gas of the plastics, an electrically heated vertical furnace (VF) is used. The compositions are analysed by a gas chromatograph with TCD and FID detectors. In this test, only PP pellet is used as a representative sample. Table 3: Specification of the sample pellets employed. Shape Cylindrical pellet Size [mm] φ10, 20, 25, 30, 50 Aspect ratio [-] 1 Sample Plastics, Waste iron powder Mixing fraction of plastics with iron 100:0, 50:50 [wt%] Figure 2: Profiles of mass decrease of six kinds of plastics in TG. 3 Results and discussion 3.1 Fundamental thermal properties of plastics The thermal properties of the plastics are important information to select the plastic kind and/or elucidate the pyrolysis and combustion characteristics. Figure 2 shows profiles of mass decrease of six kinds of plastics employed in the TG tests. From the figure, the pyrolysis temperature for PVC is the lowest of all the samples. Although the pyrolysis of PE starts at the highest temperature, the total pyrolysis period is the shortest. There are two patterns on the tendency of mass decrease. For PP and PE, the pyrolysis reaction takes place at one step. For the

5 Waste Management in Japan 149 other plastics, on the other hand, tow or three steps reaction occurs. This is because carbonaceous materials may be produced after volatile matter evolution. 3.2 Fundamental pyrolysis and combustion characteristics of plastics pellet In the practical electric furnace for steel making, the increasing rate of temperature is much faster than that in the TG. Therefore, it is necessary to obtain the pyrolysis and combustion performance under the condition of high heating rate. Additionally, when the plastics pellets are initially charged into the electric furnace, the furnace temperature becomes relatively low. From these viewpoints, the dependence of temperature on the pyrolysis and combustion performance should also be tested. Figure 3: Fraction of mass decrease for five kinds of plastic pellet under the pyrolysis condition.

6 150 Waste Management in Japan Figure 4: Fraction of mass decrease for five kinds of plastic pellet under the combustion condition. Figures 3 and 4 show fraction of mass decrease for five kinds of plastic pellet under the pyrolysis and combustion conditions, using the BF, respectively. In both the figures, (a) shows the result at low temperature, and (b) does at high temperature. The pellet diameter in these experiments is 20 mm. Under the pyrolysis condition, the decreasing rate of mass at low temperature is slower than that at high temperature for all of the sample. Especially at low temperature, the decreasing pattern depends on the plastic kind as shown in Fig. 2 obtained by the TG tests. Under the combustion condition, while, effect of temperature on the mass decreasing profile is almost same as that under the pyrolysis condition. However, the mass decreasing profiles for all the samples are similar each other due to the combustion heat produced during reaction.

7 Waste Management in Japan 151 Based on the pyrolysis data obtained, Arrhenius plots of pyrolysis rate as shown in Figure 5 are calculated by means of the non-isothermal analysis [2]. From the figure, the activation energy for all of the samples is almost same order, but the frequency factor strongly depends on the plastic kind. PVC can pyrolyze easily at low temperature. On the contrary, PE hardly pyrolyzes. ln K PP PE ABS PU PC PVC -7 Figure 5: /T [K -1 ] Arrhenius plots of pyrolysis rate for six kinds of plastic Mass decrease fraction [-] Sample: PP(100) Atmosphere: Ar Temperature: 1273K φ10 φ20 φ25 φ30 φ Time [sec] Figure 6: Profiles of mass decrease at 1273 K under the pyrolysis condition, in varying the pellet size form 10 to 50 mm in diameter.

8 152 Waste Management in Japan 3.3 Methods for mild reaction Generally, the plastics pyrolyze fast at high temperature, and produce gaseous hydrocarbon compounds, tar and soot. In order to effectively utilize the plastic pellet as heat and carbon sources in the electric furnace, it is necessary to study the parameters to accomplish the mild reaction. In this study, size of the pellet and mixing of waste iron powder into the plastic pellet are selected as parameters for the mild reaction. Furthermore, shape and size of the plastic pellet in the practical electric furnace for steel making is cubic and 200 mm in length, respectively. As it is impossible to use it in a laboratory-scale furnace, an empirical index to estimate the pyrolysis rate is also proposed by the experiments of size effect on the reaction performance Effect of pellet size on mild reaction Figure 6 shows profiles of mass decrease at 1273 K under the pyrolysis condition, in varying the pellet size form 10 to 50 mm in diameter. PP is selected as a plastic sample. From the figure, the total reaction period increases with an increase of the pellet size even at high temperature. This is caused by difference of conductive heat transfer inside the pellet. In other words, a bigger pellet has smaller surface area as pellet mass basis. Mass decrease fraction of plastics [ ] M ass decrease fraction of plastics [-] Atmosphere: Ar Temperature: 1273K A tm osphere:a r Tem perature:1273k φ10 φ30 P P(50)-W Fe(50) P P(100) φ Time [sec] Tim e [sec] Figure 7: Profiles of mass decrease for the PP pellet and the mixture pellets of PP with waste iron powder Effect of mixing of waste iron powder on mild reaction In the electric furnaces for steel making, generally, a lifting magnet is applied to supply a scrap iron metal to the furnace. As the plastics pellet itself does not have magnetism, a powder of waste iron is mixed with the plastics powder before making the pellet. Figure 7 shows profiles of mass decrease for the PP

9 Waste Management in Japan 153 pellet and the mixture pellets of PP with waste iron powder. In this figure, WFe means the waste iron powder. Comparing the profiles for the PP pellet with those for the mixture pellet, the pyrolysis reaction rate for the mixture pellet is depressed in all of the pellet sizes. This result suggests that mixing the waste iron powder contributes to the mild reaction even at high temperature. This depression effect by mixing of the iron powder is caused by higher thermal diffusivity inside the mixture pellet than the plastic pellet. When the iron powder is mixed in the pellet, the thermal conductivity inside the pellet relatively increases. Therefore, the surface temperature does not rise up, compared with that for the plastic pellet. Reaction rate [-/sec] Atmosphere: Ar φ30 φ10 φ20 Area index [m 2 /g] Temperature: 1273K φ20 φ30 φ10 PP(100) PP(50)-WFe(50) Figure 8: Correlation between area index and the pyrolysis rate Prediction of pyrolysis rate of practical pellet in the electric furnace for steel making Shape and size of the plastic pellet in the practical electric furnace for steel making is cubic and 200 mm in length, respectively. As it is impossible to use it in a laboratory-scale furnace, an empirical index to estimate the pyrolysis rate is necessary. Based on the results obtained above, the following index of Area index is useful to estimate the pyrolysis rate of the practical pellet. Area index [m 2 / g] = a m a and b indicate outside surface area [m 2 ] except for contacting area with the nickel pan and the contacting area with the nickel pan [m 2 ]. m indicates the mass of plastics. This index means extent of heat transfer by both convection and conduction. b m (1)

10 154 Waste Management in Japan Figure 8 shows correlation between area index and the pyrolysis rate. In the figure, a green plot shows the real data in the practical electric furnace for steel making, other plots show the experimental data obtained in this study. Consequently, the area index can well estimate the pyrolysis rate in the practical electric furnace Carbon balance during pyrolysis When the plastics pyrolize, gaseous hydrocarbons, tar, soot and so forth will be produced. The gaseous hydrocarbons should be utilized to preheat the scrap as a fuel. The tar and soot may contribute to carburisation into the molten iron in the electric furnace. Figure 9 shows the carbon balance for PP during pyrolysis at 1273 K. From the figure, the main species produced during pyrolysis are char and soot. Fraction of hydrocarbons is only 9 %. When analyzing the gaseous species by the gas chromatograph, CH 4, C 2 H 6, C 2 H 4 and C 3 H 6 were detected as main gaseous hydrocarbons. This result suggests that char and soot produced will contribute to carburizing into the molten iron in the electric furnace. [%] 100 Non-recovery carbon: 35% Char and soot: 55.9% Recovery carbon: 65% Hydrocarbon: 9.1% Figure 9: Carbon balance for PP during pyrolysis at 1273 K. 4 Conclusions The results show that both of the pyrolysis and combustion characteristics depend on the types of plastics, the pellet size and the mixing fraction of the waste iron in the pellet. The area index proposed in this study can estimate the reaction rate of the practical size of pellet used in the electric furnace. The gases produced from the waste plastics can be utilized as the thermal sources. While, both the soot and char produced may contribute to the carbon source in the molten iron. References [1] The Society of Wasted Plastics Treatment, [2] Freeman, E. S. and Carroll, B., J. Physical Chemistry, 62, 394(1958).