Development of Innovative Gasification Process of Used Plastic by Using Fluidized Bed

Abstract

Used plastic waste flowing into oceans has become a worldwide problem. Since international trade in used plastics has been regulated in recent years, a large amount of used plastic now requires domestic disposal. On the other hand, used plastics with a high calorific value could be used as an energy source. Therefore, the authors developed a new used plastic gasification process utilizing a fluidized bed. In this process, used plastics are decomposed in a fluidized bed reactor at around 600°C, which is a lower temperature than that used in current commercial processes. A higher calorific value gas could be obtained by gasification reaction control at the lower temperature. Hydrogen enriched gas generated by the water-gas shift reaction (WGSR) of basic oxygen furnace gas was used as the fluidizing or gasifying agent, since hydrogen was considered to have an effect of promoting the decomposition reaction of the hydrocarbons in used plastics. As the fluid medium in the reactor, catalysts were used to improve gasification efficiency. In this study, the effect of the gasification temperature and type of catalyst on the calorific value of the produced gas and gasification efficiency were investigated. High calorific value gas (LHV: 5000 kcal/Nm³) could be successfully produced by pyrolysis of used plastics by using an appropriate gasification temperature and catalyst.

1. Introduction

In recent years, marine pollution caused by used plastics has become a serious issue, with an estimated 8 million tons of plastic waste entering the world’s oceans annually.¹⁾

The potential significant impact on ecosystems due to the ingestion of these plastics by marine organisms is a global concern.²⁾ In 2018, the “Ocean Plastics Charter” was adopted at the G7 Summit held in Canada, calling for all signatory countries to make all plastics reusable, recyclable, and recoverable by 2030. Furthermore, in 2019, the Basel Convention was amended to prohibit the transboundary movement of contaminated used plastics. As a result, there is an urgent need for domestic processing of the large quantities of used plastics that were previously exported overseas for recycling purposes.

On the other hand, used plastics have a high calorific value equal to or greater than that of coal, making them a valuable potential energy source for Japan, which is a resource-limited country. One method of utilizing used plastics as a solid fuel is known as Refuse Derived Paper and Plastics Densified Fuel (RPF). RPF is produced by mixing and heating used plastics with wastepaper and is primarily used as a fuel for boilers in paper mills.³⁾ However, there are also challenges to using RPF, such as its slow combustion rate and limited versatility as an energy source, which restrict its applications.

To address these challenges, this study aims to develop a process for the gasification of used plastics through thermal decomposition. Gasification is a technology that converts solid or liquid materials (such as coal, waste, biomass, heavy oils, etc.) into gaseous components (carbon monoxide, hydrogen, carbon dioxide, methane, hydrocarbons). In this study, gasification involves the production of gaseous components (primarily light hydrocarbons ranging from C₁ to C₄) from waste materials (used plastics). A higher calorific value would make the generated gas a more suitable fuel for various processes in steelmaking plants and could also allow effective utilization as a reducing agent in blast furnaces. Therefore, the target for the lower heating value (LHV) of the generated gas was set to be equal to or greater than the calorific value of the coke oven gas commonly used in steelmaking plants (5000 kcal/Nm³), with the aim of producing a high-calorific gas.

One representative example of a gasification process using used plastics as a feedstock is the EUP (Ebara-Ube Process) developed by Ebara Corporation and Ube Corporation in Japan. This process is characterized by gasification in a fluidized bed reactor and a gas reformer using oxygen and air at temperatures ranging from 1300 to 1500°C to produce H₂ and CO.⁴⁾ However, the gas obtained through the EUP process has a low calorific value due to its predominant contents of H₂ and CO. In 2005, JFE Engineering Corporation began operation of a Thermoselect gasification reforming process facility, which generates fuel gas by combusting waste materials such as combustible waste, incineration ash, and sewage sludge at a processing rate of 2.0 to 9.0 t/h at a temperature of 1200°C using oxygen.⁵⁾ However, the representative composition of the generated gas was reported to be N₂/CO₂/H₂/CO = 3.3/31.9/33.2/31.4 vol%, and the estimated LHV was approximately 1800 kcal/Nm³, indicating a low calorific value. Therefore, in this study, we focused on the decomposition temperature to increase the calorific value of the generated gas. Previous research reports on the gasification of used plastics have often set the decomposition temperature at 700°C or higher, resulting in a generated gas LHV ranging from 1000 to 3200 kcal/Nm³.^6,7,8,9) Thus, to produce high-calorific gas with an LHV of 5000 kcal/Nm³ or higher, the aim in this study was to suppress the decomposition of light hydrocarbons with higher calorific values compared to H₂ and CO by conducting gasification at a lower temperature.

However, low-temperature conditions also suppress the decomposition of hydrocarbons, resulting in a lower gasification rate (proportion of carbon in the plastic that transitions to the generated gas). Therefore, in this study, we focused on catalysts and gasifying agents to improve the gasification rate. The catalysts commonly used to decompose used plastics include γ-alumina, dolomite, and olivine.¹⁰⁾ Slapak et al. reported that use of γ-alumina as a catalyst instead of silica sand at approximately 1000°C increased the gasification rate from 25 wt% to 69 wt%.¹¹⁾ Furthermore, according to reports by Li et al. and Murakami et al., iron oxide acts as a catalyst for tar decomposition.^12,13) According to a report by Martínez-Lera et al., successful gasification of PP and PE at a gasification rate of approximately 50% was achieved using olivine as a catalyst at 850°C.¹⁴⁾ However, the calorific value of the generated gas was low, as the higher heating value was 1000 kcal/Nm³. According to a report by Akkache et al., gas with a calorific value of approximately 4800 kcal/Nm³ was generated from used plastics at 850°C without a catalyst. However, their evaluation was based only on the moment when the largest amount of methane gas was generated during the test, and they did not achieve consistent and stable production of a high-calorific gas.¹⁵⁾ Therefore, in this study, OG (Oxygen converter Gas recovery system) dust, which is recovered from the oxygen converter gas recovery system in a steelmaking plant, was used as the catalyst, and was compared with γ-alumina and silica sand, which does not have catalytic activity. Since OG dust is an iron oxide powder which is recovered from the converter gas recovery system, it is readily available and cost-effective in steelmaking plants. It can also be recycled as a sintering raw material even after deactivation.

A gasifying agent is a gas such as air, oxygen, water vapor, carbon dioxide, etc. which is used to facilitate thermal decomposition reactions and other processes.¹⁰⁾ While some studies have been conducted under an inert gas atmosphere such as nitrogen (N₂) and did not use a gasifying agent, as in Hall et al., the gasification rate was relatively low at around 3 to 5 wt%.¹⁶⁾ Martínez-Lera et al. used water vapor as a gasifying agent, while Friengfung et al., on the other hand, used a gasifying agent consisting of a mixture of water vapor and oxygen with a 1:1 ratio.¹⁴⁾ Ogino and Tabe have reported that H₂ contributes to the decomposition reaction of chain hydrocarbons.^17,18) The reason why H₂ is believed to enhance gasification is as follows: Plastics are composed of chain hydrocarbons, and during decomposition, the straight chains are broken. When these chains are broken, hydrogen atoms are added to the broken ends, but in an environment with a low hydrogen concentration, double or triple bonds are formed. During this process, high-boiling point aromatic hydrocarbons are produced through a reaction called intramolecular cyclization.¹⁹⁾ Aromatic hydrocarbons tend to form tar at room temperature, resulting in a decrease in the gasification rate. Since this suggests that the use of a hydrogen-rich gasifying agent could suppress tar formation and promote gasification, in this study, hydrogen was chosen as the gasifying agent to facilitate the thermal decomposition reaction.

In this study, converter gas, a byproduct gas of steelmaking, was used to incorporate H₂ into the gasifying agent. Converter gas is a gas which is emitted from the converter during the refining process. While it contains very little H₂, it has a high CO concentration of 65 vol%. Therefore, to increase the H₂ concentration, a shift reaction, as shown in Eq. (1), was performed by adding water vapor before introducing the gas into the reactor.

  

$$\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2$$(1)

In this paper, we investigated the effects of catalysts and the decomposition temperature on the gasification of used plastics using a gasifying agent obtained by subjecting converter gas to a shift reaction with the aim of producing a high-calorific gas with a lower heating value (LHV) of 5000 kcal/Nm³ or higher.

2. Experimental Method

2.1. Fluidized Bed Gasification Equipment

To elucidate the fundamental behavior of plastic gasification, we created a test apparatus for gasification of used plastics using a fluidized bed reactor and conducted experiments. The fluidized bed reactor is characterized by a uniform temperature distribution compared to other reactors.²⁰⁾ Figure 1 shows the configuration of the experimental apparatus.

Fig. 1. Experimental equipment. (Online version in color.)

The apparatus consisted of a fluidized bed gasifier, a gasifying agent supply system, a plastic feed system, and a purification device for the gas product. The gasifier can be heated to 650°C by an external heating system. A gas dispersion plate made of sintered metal with 5-mesh openings was installed at the bottom of the gasifier, and the gasifying agent was supplied from below the dispersion plate to fluidize the bed of solid particles loaded on it. Water, N₂, CO₂, and H₂ were sent to a preheater through flow control devices and preheated to 150°C. The water was vaporized in the preheater before being supplied. The plastic feed system was designed to supply plastic quantitatively from a feeding hopper installed on top of the gasifier by means of a vibrating feeder. Figure 2 provides detailed information on the size of the fluidized bed gasifier, the placement of the temperature gauges, and the placement of the pressure gauges.

Fig. 2. Fluidized bed gasifier. (Online version in color.)

The fluidized bed gasifier consisted of a freeboard, a fluidized bed, and a gasifying agent preheating section. The diameter of the freeboard was increased compared to the fluidized bed and gasifying agent preheating section to reduce the gas velocity in the freeboard and decrease the amount of char flowing into the gas outlet line.²¹⁾ The height (L) to diameter (D) ratio of the fluidized bed, defined as L/D, was used as an indicator determining the flow stability of the fluidized bed. The amount of catalyst was adjusted to maintain a constant value of L/D in all experiments. The gasifying agent preheating section was provided to prevent a decrease in the temperature of the fluidized bed due to the supply of the gasifying agent.

To understand the flow state of the fluid medium, the temperature at two points (T1, T2) and the pressure difference (ΔP) were measured vertically inside the gasifier. T1 and T2 were measured using K-type thermocouples inserted from the top. Thermocouple T1 was placed at the center of the fluidized bed to measure the temperature of the fluid medium, while thermocouple T2 was placed at the center of the freeboard. The pressure difference ΔP was measured to assess the pressure drop within the gasifier and confirm the flow state. When the flow state was stable, the pressure difference remained relatively constant. However, when the flow state became unstable, the flow of the gasifying agent was disrupted, resulting in significant fluctuations in the pressure difference.

2.2. Experimental Conditions

2.2.1. Fluid Medium

The fluid medium used in the experiments and its properties are shown in Table 1.

Table 1. Properties of fluid mediums.

Fluid Medium	Size (μm)	Average size (μm)	Real density (kg/m3)	Bulk density (kg/m3)	Minimum fluidization velocity (m/s)
Quartz sand	80–300	190	2600	1240	0.018
OG dust	45–200	122.5	5000	2690	0.014
γ-alumina	75–150	112.5	3300	750	0.010

In this experiment, silica sand, which is widely used as a fluid medium in fluidized beds, was employed together with OG dust and γ-alumina, which are expected to promote gasification due to their catalytic effects. Silica sand of grade 7 with particle sizes ranging from 80 to 300 μm was used. OG dust is an iron-rich dust which is discharged from a converter, but in this study, it was pre-classified to remove particles with sizes under 45 μm to prevent ejection from the gasifier. γ-alumina, which is commonly used as an adsorbent and industrial catalyst, was used in the form of spherical γ-alumina. The true density, which affects the flow state, showed variations depending on the type of fluid medium. OG dust, which contains iron, had the highest true density, followed by γ-alumina and silica sand. The bulk density of γ-alumina was smaller than that of silica sand. The minimum fluidization velocity is the velocity at which the fluid medium starts to flow and is one of the indicators determining the flow state. The minimum fluidization velocity is determined by the true density, particle size, and particle shape. The results of fluidization tests using a cold test apparatus showed that silica sand had the highest minimum fluidization velocity, while γ-alumina had the lowest minimum fluidization velocity.

2.2.2. Pyrolysis Temperature

The pyrolysis temperature of plastics in the fluidized bed was determined considering the thermal decomposition mechanism of plastics. The thermal decomposition mechanism of plastics, estimated based on reports by Moon et al. and Narobe et al., is shown in Fig. 3.^22,23)

Fig. 3. Mechanism of used plastic gasification process by thermal decomposition. (Online version in color.)

Plastics are composed of polymer chains of hydrocarbons, and when heat is applied, these chains are broken and decomposed. Initially, solid plastics, as shown in Fig. 3(a), are heated to around 400°C, resulting in decomposition into liquid oils and tars with carbon numbers ranging from 5 to 50, as shown in Fig. 3(b). Further heating to 500 to 600°C leads to decomposition into light hydrocarbon gases with carbon numbers ranging from 1 to 4, as shown in Fig. 3(c), while heating above 700°C results in CO₂, H₂, and CO, as shown in Fig. 3(d). The lower heating value (LHV) of the major gases produced during the thermal decomposition of plastics is given in Table 2.

Table 2. LHV of main gas product.

Gas	LHV (kcal/Nm3)
CO2	0
H2	2583
CO	3022
CH4	8573
C2H6	15241
C3H8	21788
n-C4H10	28348

To produce gas with a calorific value of more than 5000 kcal/Nm³, thermal decomposition should not progress beyond the state in Fig. 3(c), considering the low calorific values of the CO₂, CO, and H₂ which are generated when the decomposition temperature exceeds 700°C. Therefore, assuming that one characteristic feature of the process developed in this study is suppression of the decomposition temperature to below 700°C to limit the progress of thermal decomposition to the state in Fig. 3(c), the gasification behavior of plastics at decomposition temperatures of 550°C, 600°C, and 650°C was investigated.

2.2.3. Gasifying Agent

The quantity and composition of the gasifying agent were determined as the supply conditions for the agent. First, the gasifying agent quantity was examined. Ikeda gives the gas flow rate for stable operation of a fluidized bed empirically as follows.²⁴⁾

  

$$3U_{mf}<U_0<10U_{mf}$$(2)

U₀ (m/s) represents the gas flow rate, while U_mf (m/s) represents the minimum fluidization velocity. The gasifying agent quantity is calculated from U₀, considering the cross-sectional area of the apparatus. The range of gasifying agent quantities that can be obtained under the conditions of 650°C and atmospheric pressure was calculated when U_mf varied within the range of Eq. (2), as shown in Fig. 4.

Fig. 4. Appropriate range of gasifying agent quantity. (Online version in color.)

As a condition for stable operation with all the fluid media used in this experiment, the gasifying agent quantity was fixed at 4.0 NL/min based on Fig. 4. Next, as the gasifying agent composition, hydrogen was used because it was considered effective in promoting the decomposition of plastics, as mentioned earlier. For this purpose, converter gas (LDG) was reformed by shift conversion to amplify the hydrogen concentration, and the resulting gas was used. The shift conversion of the converter gas was calculated by using a general process simulator to determine the gas composition at equilibrium. The calculation conditions are shown in Fig. 5.

Fig. 5. Conditions of shift reaction. (Online version in color.)

LDG at room temperature (25°C) and steam heated to 250°C were supplied to the shift converter at a mixing ratio determined so as to achieve a temperature of 430°C after the shift reaction, which is the maximum usable temperature of the shift converter. The gas composition before and after the shift reaction are shown in Table 3.

Table 3. Gas composition.

Gas	Flow rate (NL/min)	N2 (vol%)	CO2 (vol%)	CO (vol%)	H2 (vol%)	H2O (vol%)
LDG	1.63	18	15	65	1	1
Steam	2.37	0	0	0	0	100
Gasifying agent	4.00	7	30	2	25	36

Based on the calculated gas composition after the shift reaction, the gasifying agent composition for this experiment was determined. Since CO is not used as a gasifying agent and is not expected to have a significant impact on the experimental results, the gasifying agent composition was set as N₂/CO₂/H₂/H₂O = 9/30/36/35 vol%.

2.2.4. Bed Height-to-diameter Ratio (L/D)

The ratio of the bed height to the bed diameter (L/D) was set as one of the parameters that determine the fluidization stability of the fluidized bed. In determining the L/D ratio, the following equation proposed by Keairn et al. was used.²⁵⁾

  

$$Z_s=D\left(\frac{1.9}{{\left(d_p\cdot {\rho }_p\right)}^{0.5}}+\frac{0.0025}{{\left(U_0-U_{mf}\right)}^2}\right)$$(3)

The height of the slugging initiation fluidized bed, Z_s (m), the diameter of the fluidized bed, D (m), the particle diameter, d_p (m), the true density of the particles, ρ_p (kg/m³), the gas velocity, U₀ (m/s), and the minimum fluidization velocity, U_mf (m/s), were defined. Slugging refers to the growth of bubbles induced by the introduction of a gasifying agent until the bubble diameter approaches the diameter of the fluidized bed. Because slugging leads to an unstable flow regime and a significant decrease in reaction efficiency, it was necessary to keep the bed height, Z_s, below the height at which slugging occurs. For D = 0.066 m, Z_s is 0.222 m, 0.207 m, and 0.227 m for silica sand, OG dust, and γ-alumina, respectively, and it is necessary to hold the L/D ratio to less than 3.14. To maximize the residence time of the gasifying agent in the catalyst bed, L/D was set to 3. The weight of each fluidized medium was determined based on their bulk densities, which were 948 g for silica sand, 1827 g for OG dust, and 576 g for γ-alumina.

2.2.5. Plastic Materials

Table 4 shows the plastics used in this study and their properties.

Table 4. Properties of plastics.

Plastic	Size (mm)	LHV (calculated) (kcal/kg)	C (wt%)	H (wt%)	N (wt%)	O (wt%)	Cl (wt%)	Fe (wt%)
Crashed Plastics	0.6–2.0	9754	77.5	12.7	0.1	4.5	0.1	0.1
Ash (wt%)	Moisture (wt%)	PE (wt%)	PP (wt%)	PS (wt%)	PVC (wt%)	PET (wt%)	Insoluble (wt%)
5.1	0.3	77	16	<1	1	<1	5

The crushed plastics used in this study were obtained by grinding and classifying used plastics into particles with sizes ranging from 0.6 to 2.0 mm. LHV ranged from 8470 to 9530 kcal/kg, and the carbon content ranged from 74.4 to 78.6 wt%.

To determine the amount of plastic to be loaded, equilibrium calculations were performed using the general process simulator. The impact of plastic loading on the generated gas was calculated under the conditions shown in Fig. 6.

Fig. 6. Conditions of equilibrium calculation. (Online version in color.)

Since plastics are polymeric materials, they were simulated by using a relatively high molecular weight straight-chain hydrocarbon, C₃₀H₆₂. As the gasifying agent, a gas with the composition shown in Table 3 was introduced at 150°C, and the heat generation and gasification efficiency were determined by varying the loading amount of C₃₀H₆₂. Because it was necessary to optimize the amount of plastic loading according to the quantity of gasifying agent, a plastic loading amount of 300 g/h was chosen as the condition that achieves both high heat generation and high gasification efficiency. A summary of the experimental conditions is presented in Table 5.

Table 5. Experimental conditions.

Plastics		Catalyst
Type	Feed rate (g/h)	Type	Temp (°C)	L/D
Crashed plastics	300	Quartz sand	600	3
		OG dust	550
			600
			650
		γ-alumina	600
Gasifying agent
Flow rate (NL/min)	N2 (vol%)	CO2 (vol%)	H2 (vol%)	H2O (vol%)
4	9	30	26	35

2.3. Experimental Method

After heating the fluidized bed to the specified temperature with an external heater, a gasifying agent consisting of preheated N₂, CO₂, H₂, and H₂O at 150°C was introduced from the bottom of the fluidized bed. Plastic was then supplied to the fluidized bed at a rate of 300 g/h using a vibrating feeder. The gas generated by the decomposition of the plastic was collected periodically through a gas backflow system, and the contents of N₂, CO₂, H₂, CO, and light hydrocarbons (with carbon numbers ranging from 1 to 4) were measured using a gas chromatograph with a thermal conductivity detector (GC-TCD). The evaluation parameters for the generated gas were the LHV in kcal/Nm³ and the carbon-to-gas conversion ratio in wt%. LHV is an indicator of the heat generation capacity of the generated gas and was defined by the following equation based on the calculation method specified in JIS K2301 8.2.

  

$$H_0=\frac{\sum \left(H_i\times R_i\right)}{100}$$(4)

H₀ represents the lower heating value (LHV) of the generated gas (kcal/Nm³), H_i represents the LHV of each gas component in the generated gas (kcal/Nm³), and R_i represents the volumetric content of each gas component in the generated gas (vol%). The carbon-to-gas conversion ratio indicates the proportion of carbon in the plastic that is transferred to the generated gas and serves as an indicator of gas generation efficiency. The carbon-to-gas conversion ratio was defined by the following equation based on the calculations by Koyama et al.²⁶⁾

  

$${\eta }_c=\frac{\sum \left(V_i\times \frac{12}{22.4}\times C{n}_i\right)-\sum \left(V_j\times \frac{12}{22.4}\times C{n}_j\right)}{W_p\times \frac{C_p}{100}}\times 100$$(5)

η_c represents the gasification rate of the feedstock (wt%), V_i represents the production rate of each gas component in the generated gas (Nm³/h), C_ni represents the number of carbon atoms in each gas component in the generated gas (–), V_j represents the quantity of each gas component in the gasifying agent (NL/h), C_nj represents the number of carbon atoms in each gas component in the gasifying agent (–), W_p represents the amount of plastic used in the experiment (kg/h), and C_p represents the carbon content of the plastic (wt%). The number of carbon atoms in each gas component refers to the number of carbon atoms contained per molecule of the gas, which is 1 for CH₄ and 3 for C₃H₈.

3. Experimental Results

3.1. Trends in Plastic Gasification Experiments

Figures 7 and 8 shows the temperature and pressure profiles of the experimental setup, respectively, when crushed plastic was used as the feedstock, OG dust was used as the fluidizing medium, and the temperature was set at 600°C.

Fig. 7. Trend of fluidized bed temperature.

Fig. 8. Trend of fluidized bed pressure.

As soon as the plastic was introduced, the temperature T1 in the central part of the fluidized bed decreased. This was presumed to be due to the progress of plastic decomposition (endothermic reaction). On the other hand, the temperature T2 in the freeboard increased. It is estimated that gas was generated by the decomposition of the plastic and pushed the high-temperature gas near T1 into the upper freeboard. ΔP remained constant and stable after the introduction of the plastic, indicating that slugging did not occur and the fluidized bed remained stable and flowing.

Next, Fig. 9 shows the composition of the generated gas (feedstock supply time: 1.5 hours, sampling interval: 30 minutes, average data from 3 samplings).

Fig. 9. Composition of gas product.

In this experiment, formation of light hydrocarbons with high calorific values in the gas product was confirmed, resulting in a significant increase in heat generation. Since water vapor was removed by a trap, the generated gas only contained saturated water vapor, which was present in trace amounts and therefore was not included in this graph. When compared to the gas produced by the Thermoselect method, it can be observed that the low gasification temperature suppresses the production of CO and H₂.

When the gasifying agent was changed to 100% N₂, LHV was 2713 kcal/Nm³, and when it was changed to 26 vol% H₂ and 74 vol% N₂, LHV was 4361 kcal/Nm³. The content of light hydrocarbons (C₁–C₄) was 17.3 vol% when N₂ was used as the gasifying agent, and 25.1 vol% when the gasifying agent included H₂, indicating that H₂ has an amplifying effect on heat generation (amplification of light hydrocarbons).

3.2. Influence of Decomposition Temperature and Catalyst

To investigate the influence of the decomposition temperature on the heat generation of the generated gas and the gasification rate (carbon-to-gas conversion ratio), an experiment was carried out using OG dust under the conditions shown in Table 5. The results are presented in Figs. 10 and 11, respectively.

Fig. 10. Effect of temperature on LHV.

Fig. 11. Effect of temperature on carbon to gas conversion.

At a gasification temperature of 550°C, both LHV and the gasification rate were low. The gasification rate also remained low across the entire temperature range. This was presumed to be due to the low catalytic activity of the OG dust.

Next, to investigate the influence of the catalyst on heat generation and the gasification rate, an experiment was conducted with different catalysts at a decomposition temperature of 600°C. The results are shown in Fig. 12 (feedstock supply time: 1.5 hours in all cases).

Fig. 12. Effect of catalyst on gasification rate.

The effect of improving LHV was highest in the order of OG dust, γ-alumina, and silica sand because, when OG dust and γ-alumina were used, there was an increase in the content of light hydrocarbons compared to when silica sand was used. The improvement of the gasification rate was higher in the order of γ-alumina, OG dust, and silica sand. An examination of the composition of the generated gas revealed that, when OG dust was used, there was a slight increase in H₂ and a decrease in CO₂ compared to when silica sand was used. On the other hand, when γ-alumina was used, both H_2, and CO₂ increased, compared to when silica sand was used.

4. Discussion

4.1. Effect of Catalyst

By using OG dust as a catalyst during the thermal decomposition and gasification of used plastics, it was possible to generate a gas with a high calorific value. Therefore, the catalytic mechanism of OG dust was investigated.

The main component of OG dust is iron oxide. According to Funai et al., iron oxide acts as a catalyst for the decomposition of plastics and heavy oils in the presence of high-temperature steam.²⁷⁾ A conceptual diagram of the catalytic behavior of iron oxide is shown in Fig. 13.

Fig. 13. Catalytic behavior of iron oxide.

It is estimated that the decomposition of plastics occurs through an alternating process of consumption of the lattice oxygen in the iron oxide and replenishment of oxygen generated by the decomposition of water molecules. The hydrogen generated in this process is believed to be utilized in the hydrogenation decomposition reaction of plastics, thereby promoting the production of light hydrocarbon gases. Upon examining the composition of the generated gas in the experiments using OG dust, it was found that the amount of light hydrocarbon gas increased compared to when silica sand was used. Therefore, use of OG dust was considered to be effective for generating a high-calorific gas.

Since the main component of OG dust is iron oxide, it was hypothesized that if the iron oxide in the OG dust could be effectively utilized as a catalyst, further improvement in the gasification rate could be achieved. The following experiment was conducted to verify this assumption.

A small-scale experimental apparatus with a fluidized bed diameter of 19 mm was used. Polypropylene (PP) was supplied at a rate of 3 g/h for one hour, and the decomposition temperature was set at 600°C. The gasifying agent was a mixture of Ar/CO₂/H₂ = 40/30/30 vol%, which was supplied at a flow rate of 60 ml/min. Silica sand, FeO, Fe₃O₄, and Fe₂O₃ iron oxide reagents were used as catalysts, with 1 g of each mixed with 10 g of fluidized silica sand. The experimental results are shown in Fig. 14.

Fig. 14. Effect of catalyst on carbon to gas conversion.

The gasification rate was found to be higher in the order of silica sand, FeO, Fe₃O₄, and Fe₂O₃. However, considering commercialization, the gasification rate was still low, indicating the need for further development of gasification rate improvement methods.

The gasification rate in this experiment was lower than when using γ-alumina. To elucidate the mechanism, cross-sectional SEM observations of OG dust were conducted. The results, visualized as 2D images, are shown in Fig. 15.

Fig. 15. EDX mapping of OG dust.

OG dust was found to have a metal-Fe core, which is surrounded by iron oxide, with most of the surface covered by calcium. The structure of OG dust, as inferred from the XRD (X-Ray Diffraction) analysis results, is shown in Fig. 16.

Fig. 16. Structure of OG dust.

OG dust was estimated to have a three-layer structure consisting of metallic Fe (M–Fe), FeO or Fe₃O₄, and Ca (OH)₂ from the innermost layer outward. The Ca (OH)₂ was believed to be derived from steel slag. The outermost layer of Ca (OH)₂ undergoes thermal decomposition to produce CaO. Additionally, since CO₂ was present in the generated gas, CaCO₃ was formed through the reactions. It was therefore believed that the catalytic activity of the OG dust decreased due to the formation of a CaCO₃ shell.

According to Knozinger, γ-alumina had been reported to exhibit catalytic activity due to its ability to exchange oxygen in the presence of high-temperature steam.²⁸⁾ The advantage of γ-alumina over OG dust in terms of the gasification rate may be attributable to the numerous micropores on the alumina surface, resulting in a large specific surface area of 200 m²/g and high contact efficiency with tar, leading to enhanced catalytic effects.²⁹⁾ Based on these reports, it was believed that the presence of oxygen-containing gases such as CO₂ and CO promotes an increase in H₂ and light hydrocarbons through the decomposition of aromatic hydrocarbons. Indeed, when examining the experimental results of γ-alumina, increased levels of CO₂, H₂, CO, and CH₄ can be observed compared to the results obtained with silica sand and OG dust, confirming the expected mechanism.

Therefore, like γ-alumina, OG dust also showed the potential to improve catalytic performance by enhancing contact efficiency.

4.2. Future Plans

This study revealed that high-calorific gas can be generated by gasification of used plastics at low temperatures using a fluidized bed reactor with a catalyst as the fluid medium. Since γ-alumina is expensive, costing over 20000 yen/kg, we would like to explore the improvement of gasification efficiency using OG dust. In the future, we aim to enhance the catalytic performance of OG dust to improve gasification efficiency.

Furthermore, for commercialization of this process, it is necessary to scale up the fluidized bed reactor to increase the used plastic processing capacity. Because heat transfer with external heaters becomes insufficient when the diameter of a fluidized bed reactor is increased, it will be necessary to develop a method for circulating and heating the fluid medium externally. Therefore, in the future, we will investigate efficient circulation and heating methods for the fluid medium.

5. Conclusion

An experimental investigation was conducted with the aim of developing a technology for gasification of used plastics, and the following findings were obtained:

(1) This study demonstrated that hydrocarbons can be produced and high-calorific gas (LHV > 5000 kcal/Nm³) can be generated by controlling the decomposition temperature to 600 to 650°C using a fluidized bed and introducing a gasifying agent obtained by shift conversion of converter gas during the gasification of the used plastics.

(2) When OG dust was used as a catalyst for gasification of used plastics in the fluidized bed, it was found that gasification efficiency improved in comparison with use of silica sand as the catalyst. However, gasification efficiency was still low. Investigation of the cause revealed that the OG dust was coated with a calcium shell, leading to a decrease in catalyst activity.

Nomenclature

D: Fluidized bed diameter (m)

L: Fluidized bed height (m)

ΔP: Pressure drop (Pa)

U₀: Fluidizing gas linear velocity (m/s, at STP)

U_mf: Minimum fluidization velocity (m/s, at STP)

Zs: Slugging bed height (m)

d_p: Fluidizing medium particle diameter (m)

ρ_p: Fluidizing medium density (kg/m³)

H₀: Lower heating value of produced gas (kcal/Nm³, at STP)

H_i: Lower heating value of each gas component in produced gas (kcal/Nm³, at STP)

R_i: Concentration of each gas component in produced gas (–)

η_c: Gasification ratio (wt%)

V_i: Flow rate of each gas component in produced gas (L/h, at STP)

V_j: Flow rate of each gas component in gasifier (L/h, at STP)

C_ni: Carbon number of each gas molecule in produced gas (–)

C_nj: Carbon number of each gas molecule in gasifier (–)

W_p: Feeding rate of plastic (g/h)

C_p: Carbon concentration in plastic (wt%)

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