3.1. Thermochemical Characteristics of the ASR
The elemental analysis of the ASR revealed a carbon, hydrogen, oxygen, and nitrogen contents of 52.75, 7.02, 1.78, and 15.81 wt.%, respectively. Sulfur, which can be converted into H
2S, was detected at concentrations below 1 wt.%. Furthermore, chlorine, a source of erosion and dioxins, was present at only 1.37 wt.%. The higher heating value (HHV) of the ASR was 21,680 kJ/kg (
Table 7).
The results of the elemental analysis revealed that the ASR composition was similar to those of other combustible wastes such as biomass or municipal solid waste, thus making it a suitable solid refused fuel (SRF) or alternative fuel [
38,
39,
40]. To confirm this, the characteristics of the ASR used in the present study were compared with those of other combustible wastes reported in the literature. In addition, the heating value and elemental analysis results of the ASR was compared with those of plastic industrial waste from a company in Korea [
41,
42]. It was found that sawdust has lower contents of carbon and hydrogen (45.93 and 6.65 wt.%, respectively) than the ASR, whereas the proportion of oxygen in sawdust is significantly higher at 45.96 wt.%. Moreover, the proportion of sulfur in sawdust is low. In addition, the HHV of sawdust is 17,623 kJ/kg, which is approximately 4200 kJ/kg higher than that of the ASR. Based on these results, ASR exhibits strong potential to be used as a fuel. In contrast, plastic, which has a high molecular weight, has significantly higher carbon and hydrogen contents (80.16 and 12.34 wt.%, respectively) than the ASR. However, the nitrogen and oxygen contents in plastic are exceptionally low (0.73 and 0.16 wt.%, respectively). The plastic waste also had the highest chlorine content of the three types of waste (2.76 wt.%).Besides, the SRF demonstrated a similar elemental composition to that of sawdust. More specifically, sulfur was not detected in the SRF, but further analysis is needed to verify the samples, since it is an industrial waste.
Table 8 presents the proximate analysis results for each waste material. The ASR consisted of 1.17 wt.% moisture, 63.90 wt.% volatile compounds, 18.80 wt.% fixed carbon, and 16.13 wt.% ash. Volatile compounds and fixed carbon are particularly important for the gasification process because a higher proportion of combustible compounds results in a higher conversion rate of hydrocarbons into syngas [
43,
44]. In addition, the lower moisture content in the ASR than sawdust is also beneficial for its use as a fuel in a thermochemical process; additionally, a low moisture content offers higher economic efficiency because a pre-drying process is not required.
The TG analysis results determine the characteristics of the feedstock or fuel, such as the temperature of the final reaction; therefore, the results are important for determining experimental conditions [
27]. In the present study, a TG plot was generated under reduction conditions in the presence of nitrogen as the temperature increased at a rate of 10 °C/min. A TG analysis was conducted under both reduction and oxidation conditions (with nitrogen and air, respectively), because it was necessary to measure the weight reduction under similar conditions to those in the gasification process.
Figure 4 shows the results of the TG analysis. The ASR, wood, and plastic waste exhibited similar reducing trends, regardless of the presence of oxygen. Unlike the other types of waste, the ASR consisted of various materials, such as synthesis resins, rubber, and plastic. As a result, its thermal reaction was lower than that of wood and plastic waste [
28]. The TG plot for the ASR was dominated by the graph of light fluff since light fluff was the major component of the ASR (~80 wt.%). In contrast, heavy fluff had a similar TG graph to that of plastic waste [
45]. Based on the ASR TG plot, it can be predicted that the optimal operating temperature for ASR gasification is above 900 °C. The residues were more frequently converted to gasification products such as hydrogen, carbon monoxide, and methane above this temperature. The main purpose of gasification is to produce syngas, such as hydrogen and carbon monoxide, which are produced from hydrocarbons [
46]. Thus, the remaining residues should be thermally cracked at temperatures higher than 900 °C.
Unlike wood and plastic, the ASR did not exhibit rapid weight reduction at lower reaction rates because the light fluff in the ASR contains plastic, synthesis resin, and rubber, all of which have polyurethane as a raw material. Compared to other ASR materials, polyurethane requires the highest temperature for thermal reduction. Finally, it can be concluded that the ASR TG results were influenced by the polyurethane content [
28]. In addition, based on the combustible residue that remained at a temperature of 950 °C, it was clear that higher temperature, higher pressure and longer residence times were required for the ASR-gasification process.
3.2. Results for ASR Gasification in a Fixed-Bed Reactor
Figure 5 presents the results for syngas production at gasification temperatures of 800, 1000 and 1200 °C. The hydrogen and carbon monoxide levels increased with the increasing temperature and ER, whereas the carbon dioxide levels decreased. The amount of syngas increased with the increasing temperature because the devolatilization reaction occurred first in the thermochemical process for the combustible waste. Thereafter, two main reactions occurred, combustion and the water–gas-shift reactions. In general, gasification reactions cannot be summarized by specific equations because of their complexity; however, the results obtained from the ASR gasification can be explained using the main equations for general gasification. Equations (1), (2) and (5) indicate that the endothermic reactions are facilitated by an increase in the temperature. It was also found that the proportion of carbon monoxide was higher than that of hydrogen with an increase in the ER at the same gasification temperature. Moreover, it was observed that carbon monoxide and hydrogen are predominant, and reactants are more prevalent with the increasing temperature in an exothermic reaction, whereas products are more prevalent with the increasing temperature in an endothermic reaction [
47]. Overall, syngas production was not associated with the ER for hydrogen, whereas carbon monoxide exhibited complex reaction pathways that are affected by the temperature and steam ratio.
Unlike carbon monoxide, carbon dioxide production is affected by a change in the ER. This is because higher oxygen levels promote oxidation and facilitate the conversion reaction to carbon dioxide [
43]. In contrast, methane production decreased with the increasing temperature, indicating that homogeneous or secondary reactions (CH
4 + 2O
2 → 2H
2O + CO
2) were preferred [
42].
For each temperature range, the syngas proportion increased with a decrease in the ER, which is advantageous for the ASR gasification. However, the total amount of syngas produced increased with the increasing ER, while the solid residue content decreased. The main goal of gasification is to obtain the highest yield of hydrogen and carbon monoxide, which can be estimated from the dry-gas yield. Therefore, a temperature of 1200 °C and an ER of 0.5 would be the optimum conditions for optimum gas production in ASR gasification. For a gasification power plant, the amount of syngas and solid residue produced is an important factor because increasing the syngas yield and reducing the solid-residue content can improve the power-generation efficiency and ensure continuous operation. The HHV of the produced syngas was estimated, and the results were found to vary with temperature. As a result, the highest HHV of syngas was shown at a temperature of 800 °C (approx. 3500 kcal/kg), since the thermal cracking would be activated at increasing temperatures. The trend observed in the HHV was similar to that of methane, ethane, and propane. The HHV was also affected by changes in the volume of carbon dioxide. It is not economical to transport syngas from its production facility over long distances because it is a low-calorific fuel. However, syngas does have the advantage of being directly usable in the production area as a gas fuel.
Figure 5 shows variations in the syngas composition and HHV with increasing ER during the ASR gasification.
During the ASR gasification process, the tar content decreased by less than 10 wt.% with increasing temperature and ER. The conversion rate from high-molecular-weight compounds to low-molecular-weight compounds increased because the compounds cracked to a greater extent at higher temperatures. Additionally, solid residues, including ash, were reduced by 26%. Although combustible ASR compounds did not react completely, a high ASR gasification efficiency can be achieved by controlling oxygen, unlike in incineration (
Figure 6). It was also apparent that the dry-gas yield was affected by changes in the temperature and ER; in particular, the dry gas yield increased with the increasing ER and temperature, indicating that the ASR reaction improved under these conditions. Thus, it can be concluded that an increased ER and high temperatures increase the yield of syngas [
48,
49].
The cold-gas efficiency, calculated based on the syngas composition and cold-gas efficiency equation, decreased with an increase in the temperature and ER. The heating value of the syngas decreased because the production of carbon dioxide increased with an increase in the temperature and ER. Carbon conversion and cold gas efficiency, which are used to evaluate the gasification process, can help to measure the carbon converted into in the ASR, because the hydrocarbon gas and the dry-gas yield are used as calculation factors. This carbon conversion was calculated using the equation provided in
Table 9. The results in
Figure 6 indicate that the carbon conversion increased with the increasing temperature and ER; however, the cold-gas efficiency exhibited an opposite trend. As the temperature and ER increased, the production of carbon dioxide and other hydrocarbon gases increased and decreased, respectively.
3.3. Quality Assessment of Clay Brick Manufactured from Melting Slag
The clay brick manufactured using the melting slag was subjected to quality-control testing for compressive strength and absorption, based on the Korean Industrial Standard (
Figure 7). The manufactured clay brick exhibited a compressive strength of over 22.54 N/mm
2 and an absorption ratio of less than 10%, thus satisfying the Korean Industrial Standard. The clay brick also had a higher compressive strength than the standard clay brick, although this decreased as the melting temperature increased. In addition, as illustrated by the SEM analysis, the porosity of the melting slag increased with higher melting temperatures (
Figure 8), indicating that porosity affects the compressive strength of the clay bricks. However, the compressive strength increased at a higher melting slag content, reaching 153.35 N/mm
2 with a melting slag content of 10 wt.% and a melting temperature of 1300 °C. Additionally, foaming of the slag was only apparent in the clay brick when more than 10 wt.% of the melting slag was used. Overall, based on the results of the quality control testing, it was concluded that the optimum melting slag content was 10 wt.%. However, it should be noted that all clay bricks, regardless of the melting slag content, met the absorption ratio standard. Moreover, the contents of heavy metals in melting slag have to be measured before applying a commercial plant. According to advanced research, the content of copper was revealed to 34,000 mg/kg in ASR ash [
50,
51]. In general, the content of heavy metal is regulated by the leaching test in Korea, but slag has been often used to aggregate, for application in embankments, and so on. Therefore, further study is required in future, since the contents of heavy metal could be concentrated in melting slag.