Which Is More Environmentally Friendly? A Comparative Analysis of the Environmental Benefits of Two Waste-to-Energy Technologies for Plastics Based on an LCA Model

Abstract

Waste plastics are low-value waste; thus, achieving high-value recycling is the most desirable goal. Scientific methods are required to evaluate the environmental benefits and support the technology optimization and market selection of waste-to-energy technology for plastics. This study selected catalytic cracking and incineration power generation of two typical examples of waste plastics energy technologies as the research objects, established a full life cycle assessment model, and used the mid-point method to analyze and compare the environmental benefits of the two technologies. The results showed that catalytic cracking technology is more environmentally friendly. The sensitivity analysis showed that the treatment units had a high impact on the results; process optimization predicted the efficiency of technology optimization and advocated future technological developments to explore the direction of using clean energy, upgrading equipment, and updating catalysts. The results of this study can provide ideas for the optimization of two kinds of waste plastics energy technology. In addition, the comparative data analysis intuitively demonstrated the advantages and disadvantages of waste-to-energy technologies and provided a practical path for the future development of high-value treatments for waste plastics.

1. Introduction

Plastics are a form of polymer material produced from petroleum-based raw materials. Plastics are widely used in production and everyday life at low cost, but the waste stock is huge and cannot easily be degraded naturally [1]. Therefore, finding suitable ways to dispose of waste plastics is a key initiative to achieve sustainable development and alleviate global environmental problems [2]. The energy utilization of waste plastics is one feasible direction for the high-value recycling of waste plastics [3,4]. Catalytic cracking technology (CCT) and incineration and power generation technology (IAPGT) are both energy utilization technologies; CCT has not yet achieved large-scale production, whereas IAPGT is more maturely developed [5,6].

The study of waste-to-energy technology is particularly important because it transforms waste into a green alternative equal to fossil energy and reduces the consumption of the Earth’s non-renewable resources [7,8]. International research on waste-to-energy technology for plastics is gradually being improved [9]. Jeong et al. [10] used a two-stage gasification process to gasify waste polyethylene pellets; their experimental results showed that the addition of an activated carbon catalyst could increase the yield of gas and reduce the yield of tar. Filip et al. [11] subjected waste polystyrene pellets to a cracking reaction at 400 °C with a catalyst of volcanic ash; their results showed that the conversion of polystyrene pellets was significantly higher and the product liquid hydrocarbons could be converted into qualified aviation fuel. Lei et al. [12] set different reaction temperatures for the high-temperature cracking conversion of waste plastics catalyzed by activated carbon; the optimal product result was a high-quality blend of 85% aviation fuel and 15% diesel fuel. Sharma et al. [13] performed the pyrolysis of high-density polyethylene plastics, during which silicates were added to catalyze the reaction; they obtained a high-utilization mixture of hydrocarbon products with similar boiling points to petroleum, with two-thirds of the liquid products and one-third of the gaseous solid products. This experiment proved that silicate and plagioclase catalysts were more effective in cutting off long-chain molecules such as polypropylene and polyvinyl chloride. Zhou et al. [14] added mixed waste plastics and coked wax oil together in a catalytic cracking experimental equipment, using coked wax oil as the heat conduction medium to overcome the disadvantages of poor heat transfer, viscous raw materials, and the poor recovery of liquid products: the fuel oil yield was 96.67%. This shows that the experimental development of waste-to-energy technology for plastics is more mature and can be used as a research object for large-scale applications [4].

In addition to advances in technology development, life cycle assessment (LCA) methods have been widely applied in the fields of waste plastics treatment and renewable energy. Using LCA models, Somoza-Tornos et al. [15] concluded that waste plastics cracking methods can reduce the negative impacts on resource scarcity, ecology, and human health by 164, 89 and 87%, respectively, compared with naphtha methods for the same quality of ethylene production. Ahmed et al. [16] used the LCA method to account for the environmental impacts of waste to electricity and compost for farming and showed that the conversion of waste to electricity could yield more significant environmental benefits. Floriana et al. [17] used the life cycle evaluation method to compare the carbon emissions of two different plastics production processes; their analysis showed that the fracking method to produce plastics was able to reduce carbon emissions by 3.6–5.6 t compared with the fossil method. The life cycle assessment method has also been used in other sustainable development areas: Florindo et al. [18] conducted a life cycle sustainability evaluation of four beef production processes in Brazil and demonstrated that the optimal benefits were achieved by the farm animal rotation approach, which could be attributed to a reduction in mechanized inputs in farming and a reduction in chemical fertilizer use in pastures. Tonini et al. [19] modeled the treatment and disposal of household food waste in Amsterdam, the Netherlands, weighting the model indicators based on stakeholder theory, and concluded that separate collection anaerobic digestion energy recovery was the most sustainable treatment method. Life cycle assessment methods have commonly been applied in the field of renewable energy, but there is still a gap in the research on the evaluation of the environmental benefits of waste-to-energy technology for plastics [20]. In addition, there have been few studies on the comparison of traditional and new energy technologies [21]. Thus, this study will fill this part of the research gap and provide data support for subsequent academic research.

Therefore, this study innovatively took two typical plastic waste energy technologies as the research object, evaluated and compared the environmental benefits of both in a comparative manner using the life cycle evaluation method, visually demonstrated the differences in environmental impacts between old and new technologies, proposed optimization suggestions for the weak links in providing ideas for the application of high-value recycling of waste plastics, and addressed the lack of research in this field.

2. Materials and Methods

2.1. Target Scope Determination

The life cycle of energy-based technology waste plastic processing can be broadly divided into three parts: the construction phase, operation phase, and decommissioning phase [22]. The construction phase refers to establishing a technical enterprise workshop, equipment, and constructing buildings [23]. The construction materials, energy input, and waste pollutants are the material flow boundaries of inputs and outputs. The operation phase refers to the operation cycle within the service life of the equipment, including energy input, material input, product output, and the waste pollutants generation phase [24]. The pre-processing stage has similar steps and similar input-output lists; therefore, this part was ignored in the comparative analysis [25,26]. The decommissioning stage refers to the stage of dismantling the equipment and plant when the waste plastics energization equipment can no longer operate normally for production [27]. The system boundary will define all the material inputs and outputs in the above three phases of the study system; thus, intermediate products and reused materials were not included in this study [28]. Based on the above conditions determined, the specific system boundary of this study is shown in Figure 1 and Figure 2, and the dashed line is the system boundary set in this study.

In determining the functional unit, the environmental impact of treating one ton of waste plastic was uniformly set as one functional unit. The service life of the reaction equipment was uniformly 20 years [29].

There is no waste plastic treatment process in the construction phase; the material flow in the construction phase was converted into each functional unit according to the equipment service life [30]. In this study, the decommissioning phase results were not considered in the analysis because of the relatively small material input and output and insignificant results [31].

2.2. Inventory Analysis

The CCT data in this study were obtained from enterprise operation data in a certain subject in Anhui Province, China, and the IAPGT data were obtained from the empirical literature on waste incineration power plants in the Yangtze River Delta region of China [32,33,34]. This CCT enterprise was an early adopter of catalytic cracking of waste plastics in China, with a large production scale, and had undergone five technological updates. Thus, this enterprise’s plant was representative of other plants in terms of the degree of technological development and production scale. The empirical literature selected for the IAPGT were all plants with more than ten years of operating experience, which had developed a mature operation mode and had the construction and production characteristics of the current mainstream waste-to-energy plants.

2.2.1. System Parameters

The production parameters of the two waste plastics energization technologies are presented subsequently.

The main parameters under 1 functional unit of the CCT process were: 2.5 tons of reinforced steel equipment; 3 kg of operation aids such as fixed metal racks; two concrete cooling pools 1.5 m in length and width and 2 m in height; 175 kg of water; 461 kg of cement; 512 kg of sand; and 1252 kg of stone used for one cubic meter of concrete components. During the operation of the CCT process, 400 kWh of electricity, 50 kg of fuel oil, 0.5 kg of activated carbon for the reaction catalyst, and 0.1 kg of sodium hydroxide for gas cleaning were used per functional unit. The output of the process was mixed oil, carbon black and mixed gas, with an output weight ratio of 46.1, 24.6, and 29.3%. Carbon dioxide and sulfur dioxide were emitted during the production process.

The main parameters of the IAPGT process were: 1.3 t of reinforced steel equipment under 1 functional unit; 3 kg of fixed metal frame and other operational aids; the same cooling pool; 40 degrees of electricity per functional unit during the operation of the IAPGT process; 0.45 kg of compound urea accelerant; 0.5 kg of activated carbon for the reaction catalyst; and 8 t of lime for gas cleaning. The output of the process was electrical energy. The production process emitted carbon dioxide, nitrogen oxide, sulfur dioxide, monoxide tower, hydrogen chloride slag, and fly ash.

2.2.2. Construction Phase Data List

Raw material inputs in the construction phase included materials such as metal and concrete for equipment and auxiliary facilities. The common C30 concrete standard for building construction is based on the following parameters: 1 m³ of concrete consists of 175 kg of water, 461 kg of cement, 512 kg of sand and 1252 kg of stone [35]. Electricity used for the construction was converted to electricity used for the construction of conventional industrial plants, and construction waste (solid waste and wastewater) was calculated according to the building construction waste emission rate. The list of material flows in the construction phase of the two waste plastic energy technologies is shown in

Table 1. Building materials.

Flow of Materials	CCT	IAPGT
(+) Reinforced steel for equipment(t)	2.5	1.3
(+) Operating auxiliary steel (kg)	3	3
(+) Concrete (m3)	2.475	2.475
(+) Electricity (kWh)	29.8	29.8
(−) Solid waste (kg)	1.065	1.065
(−) Wastewater (m3)	3.522	3.522

.

2.2.3. Operation Phase Data List

The material flow in the operation phase mainly involved the input of chemicals, energy consumption, product output, and waste discharge. The operational power consumption was derived from the enterprise’s operational data and empirical data from relevant literature, and the direct waste emission data were referenced from the enterprise’s environmental impact assessment report and empirical data from previous studies. The indirect GHG emissions from waste were mainly from the processes of recycling and transportation, power generation, and input material production; their environmental impacts were obtained from the built-in global database of Simapro software [36].

The input and output data for the operational phase of the two technologies under 1 functional unit (1 ton of waste plastic processed) are shown in

Table 2. Process operation data.

Flow of Materials	CCT	IAPGT
	① Pharmaceutical input
Activated carbon	0.5 kg	0.5 kg
Sodium hydroxide	0.1 kg	0.1 kg
Urea fueling agent	/	0.45 kg
Lime	/	8 t
	② Energy input
Electricity for production	400 kWh	40 kWh
	③ Product output
Mixed oil	0.461 t	/
Carbon black	0.246 t	/
Mixed Gas	0.293 t	/
Electricity	/	529.41 kWh
	④ Waste discharge
Carbon dioxide	153 kg	24 kg
Sulfur dioxide	0.0048 kg	5.62 kg
Nitrogen oxides	/	157.5 kg
Carbon monoxide	/	3.56 kg
Hydrogen chloride	/	1.9 kg
Slag	/	18 kg
Fly ash	/	2.7 kg

. The material flow was expressed in terms of weight in use or weight in production.

2.3. Environmental Impact Assessment Method

The midpoint method was chosen as the environmental impact evaluation method in this study, and the specific operational tool was the Recipe 2016 Midpoint (H) V1.04/World (2010) H model built in the Simapro software, which could cover three endpoint environmental impact categories and 18 midpoint environmental impact categories [37,38,39]. The 18 environmental impact categories covered are shown in

Table 3. Recipe 2016 midpoint model environment impact categories.

Environment Impact Categories	Unit
Global warming	kg CO2 eq
Stratospheric ozone depletion	kg CFC11 eq
Ionizing radiation	kBq Co-60 eq
Ozone formation, human health	kg NOX eq
Fine particulate matter formation	kg PM2.5 eq
Ozone formation, terrestrial ecosystems	kg NOx eq
Terrestrial acidification	kg SO2 eq
Freshwater eutrophication	kg P eq
Marine eutrophication	kg N eq
Terrestrial ecotoxicity	kg 1,4-DCB
Freshwater ecotoxicity	kg 1,4-DCB
Marine ecotoxicity	kg 1,4-DCB
Human carcinogenic toxicity	kg 1,4-DCB
Human non-carcinogenic toxicity	kg 1,4-DCB
Land use	m2a crop eq
Mineral resource scarcity	kg Cu eq
Fossil resource scarcity	kg oil eq
Water consumption	m3

.

2.4. Sensitivity Analysis

The sensitivity analysis was based on the range of variation in key parameters of the study subject, as shown in the following equation [40]:

$${\mathit{G}}_{\mathit{n}\mathit{i}}=\frac{\left(\Delta {\mathit{K}}_{\mathit{j}}/{\mathit{K}}_{\mathit{j}}\right)}{\Delta {\mathit{L}}_{\mathit{n}}/{\mathit{L}}_{\mathit{n}}}$$

where ${\mathit{G}}_{\mathit{n}\mathit{i}}$ indicates the sensitivity of ${\mathit{L}}_{\mathit{n}}$ to ${\mathit{K}}_{\mathit{j}}$, ${\mathit{K}}_{\mathit{j}}$ is the value of the life cycle evaluation index of the $\mathit{j}$th environmental impact type, and ${\mathit{L}}_{\mathit{n}}$ is the value of the $\mathit{n}$th inventory analysis data.

3. Results and Discussion

3.1. Results of LCA Model Evaluation

First, this study analyzed the overall environmental impacts of the technology options. Using the Simapro software, 18 environmental impact types were characterized and subjected to dimensionless analysis to obtain the characterization and standardization results of the environmental impacts, which were used to compare the two technologies on each environmental impact category. The characterization and standardization results are shown in

Table 4. Results of the characterization and standardization of environmental impacts.

Environment Impact Categories	Unit	CCT	Standardization	IAPGT	Standardization
Global warming	kg CO2 eq	6.071	0.758	11.851	1.481
Stratospheric ozone depletion	kg CFC11 eq	0.000	0.008	0.000	0.391
Ionizing radiation	kBq Co-60 eq	0.148	0.308	0.186	0.388
Ozone formation, human health	kg NOX eq	0.014	0.715	0.189	9.211
Fine particulate matter formation	kg PM2.5 eq	0.009	0.362	0.044	1.744
Ozone formation, terrestrial ecosystems	kg NOx eq	0.015	0.869	0.190	10.713
Terrestrial acidification	kg SO2 eq	0.014	0.345	0.127	3.121
Freshwater eutrophication	kg P eq	0.002	3.901	0.003	5.301
Marine eutrophication	kg N eq	0.000	0.040	0.001	0.411
Terrestrial ecotoxicity	kg 1,4-DCB	21.201	1.395	40.206	2.645
Freshwater ecotoxicity	kg 1,4-DCB	0.402	15.984	0.707	28.105
Marine ecotoxicity	kg 1,4-DCB	0.559	12.870	0.931	21.421
Human carcinogenic toxicity	kg 1,4-DCB	4.555	4.423	3.738	3.629
Human non-carcinogenic toxicity	kg 1,4-DCB	5.237	0.167	12.864	0.411
Land use	m2a crop eq	0.105	0.017	118.162	19.142
Mineral resource scarcity	kg Cu eq	0.199	0.001	0.173	0.001
Fossil resource scarcity	kg oil eq	0.061	0.062	2.342	2.389
Water consumption	m3	0.062	0.232	0.289	1.086

.

As shown in Table 4, the most significant environmental impacts of CCT are concentrated in terrestrial ecotoxicity, global warming, non-carcinogenic and carcinogenic human toxicity, and terrestrial ecotoxicity, whereas the other 14 types of environmental impacts do not show significant performance. It can be seen that the environmental toxicological impact of catalytic cracking technology is more obvious. The main environmental impacts of incineration and power generation technologies are in the areas of land use and terrestrial ecotoxicity, among which the impact on land use is obvious; this may be explained by the fact that waste incineration plants are mostly large-scale operations that occupy more land resources, and the landfill of fly ash after incineration will also have some impact on the use of land resources [41,42]. The total environmental impact of IAPGT is 111.59, which is substantially higher than the CCT at 42.46, which indicates that catalytic cracking technologies are more environmentally friendly at this stage.

Secondly, using the inventory data and the built-in global material flow database, the four treatment units of the construction phase, operation phase, power input and waste discharge were evaluated separately, and the types of environmental impacts that demonstrated significant performance were selected as evaluation indicators for the above results. The specific results of the analysis are as follows.

• Catalytic cracking technology.

The key environmental impact categories of CCT in relation to the treatment units are shown in

Table 5. Environmental effects of different treatment units in CCT.

Environment Impact Categories	Construction	Operation	Electricity	Waste
Global warming	6.228 (92.65%)	0.027 (0.41%)	0.456 (6.80%)	0.009 (0.14%)
Terrestrial ecotoxicity	22.285 (98.30%)	0.060 (0.27%)	0.233 (1.03%)	0.092 (0.41%)
Carcinogenic human toxicity	4.565 (99.65%)	0.001 (0.03%)	0.013 (0.30%)	0.001 (0.02%)
Non-carcinogenic human toxicity	5.273 (95.93%)	0.015 (0.28%)	0.201 (3.66%)	0.007 (0.13%)

.

Table 5 shows that the environmental impact of CCT throughout its life cycle mainly comes from the construction stage, accounting for more than 90%, which may be due to the environmental pollution impact of the metals and concrete used in the construction stage itself. During the construction phase, carcinogenic human toxicity is most evident, which can be attributed to the production of raw materials, transportation, and the assembly of equipment involving metal welding and other processes, which generate more intermediate products and pollutants containing toxicity [43,44].

The specific results of the environmental impact contribution analysis of catalytic cracking material flow are shown in Figure 3. In terms of negative environmental impact, reinforced steel for equipment contributed the most, accounting for 98.72%, followed by concrete, production input electricity, and fuel oil. In terms of positive environmental impacts, blended oil, blended gas, and carbon black products all contributed significant emission reduction impacts, with carbon black products showing the most obvious environmental damage offset. This may be attributed to the higher value of carbon black products for recycled product applications. Carbon black can be used to make asphalt and other industrial supplies and will be widely used in industrial production. Mixed oil and mixed gas are hydrocarbons and can be turned into fuel after refining.

2.

Incineration and power generation technology.

The key environmental impact categories of IAPGT in relation to the treatment units are shown in

Table 6. Environmental effects of different treatment units in IAPGT.

Environment Impact Categories	Construction	Operation	Electricity	Waste
Global warming	3.495 (27.45%)	9.228 (72.48%)	0.000 (0.01%)	0.006 (0.05%)
Terrestrial ecotoxicity	12.950 (30.90%)	28.945 (69.06%)	0.004 (0.01%)	0.012 (0.03%)
Carcinogenic human toxicity	2.385 (63.10%)	0.307 (8.15%)	0.000 (0.01%)	1.086 (28.75%)
Non-carcinogenic human toxicity	2.880 (20.91%)	10.867 (78.87%)	0.000 (0.01%)	0.029 (0.21%)
Land use	0.090 (0.08%)	118.152 (99.92%)	0.000 (0.00%)	0.000 (0.00%)
Fossil resource scarcity	0.683 (27.58%)	1.793 (72.38%)	0.000 (0.01%)	0.000 (0.04%)

.

Table 6 shows that the environmental impact of the whole life cycle of incineration and power technology mainly comes from the operation phase, accounting for 87.75%, which can be attributed to the fact that the incineration of waste plastics will use a large amount of slaked lime to neutralize the acidic gases produced by the incineration, resulting in the emission of gaseous pollutants [45]. The land use impact is evident in the operational phase, where the environmental impact is most significant, probably due to the fact that the scale effect of the incineration power generation process requires more production space [46].

The specific results of the analysis of the environmental impact contribution of the incineration and power material flow are shown in Figure 4. In terms of negative environmental impacts, the input of lime during operation contributes the most, where the environmental contribution of nitrogen oxides in the ozone layer, fine particulate matter, terrestrial ecosystems, and acidification evolution is greater than that of lime input, proving the damaging effect of nitrogen oxides on the ozone layer and biosphere, similarly to previous studies [47,48]. In terms of positive environmental impacts, electricity products contributed significant energy savings and emission reduction impacts, mainly because the electricity generated from waste incineration was used as a renewable energy source, which produced an environmental damage offset relative to electricity generation using fossil energy.

3.2. Sensitivity Analysis

In this study, assuming a 10% change in the above evaluation results, the change rates of the corresponding 18 environmental impact categories were obtained. Sensitivity analyses of the two techniques are shown in

Table 7. Sensitivity analysis of CCT.

Environment Impact Categories	Construction	Operation	Electricity	Waste
Global warming	3.420%	0.015%	0.752%	0.134%
Stratospheric ozone depletion	8.710%	0.277%	1.362%	0.059%
Ionizing radiation	3.934%	0.045%	0.041%	0.011%
Ozone formation, human health	3.523%	0.026%	0.874%	0.023%
Fine particulate matter formation	3.616%	0.027%	0.800%	0.011%
Ozone formation, terrestrial ecosystems	3.537%	0.026%	0.833%	0.022%
Terrestrial acidification	3.845%	0.049%	1.190%	0.017%
Freshwater eutrophication	3.713%	0.035%	0.334%	0.009%
Marine eutrophication	3.321%	0.008%	0.287%	0.081%
Terrestrial ecotoxicity	3.504%	0.010%	0.110%	0.022%
Freshwater ecotoxicity	3.400%	0.004%	0.098%	0.003%
Marine ecotoxicity	3.411%	0.004%	0.099%	0.003%
Carcinogenic human toxicity	3.341%	0.001%	0.030%	0.001%
Non-carcinogenic human toxicity	3.357%	0.010%	0.384%	0.007%
Land use	4.511%	0.078%	0.504%	0.027%
Mineral resource scarcity	3.355%	0.001%	0.005%	0.001%
Fossil resource scarcity	67.312%	3.007%	14.223%	0.396%
Water consumption	3.267%	0.265%	0.177%	0.270%

and

Table 8. Sensitivity analysis of IAPGT.

Environment Impact Categories	Construction	Operation	Electricity	Waste
Global warming	0.983%	2.589%	0.001%	0.003%
Stratospheric ozone depletion	0.102%	3.301%	0.003%	0.000%
Ionizing radiation	1.638%	4.233%	0.002%	0.001%
Ozone formation, human health	0.159%	0.426%	0.000%	1.039%
Fine particulate matter formation	0.413%	1.507%	0.000%	0.558%
Ozone formation, terrestrial ecosystems	0.166%	0.431%	0.000%	1.035%
Terrestrial acidification	0.241%	1.446%	0.000%	0.639%
Freshwater eutrophication	1.491%	3.703%	0.001%	0.003%
Marine eutrophication	0.176%	3.551%	0.000%	0.000%
Terrestrial ecotoxicity	1.074%	2.400%	0.001%	0.000%
Freshwater ecotoxicity	1.028%	2.365%	0.001%	0.027%
Marine ecotoxicity	1.090%	2.291%	0.001%	0.031%
Carcinogenic human toxicity	2.127%	0.275%	0.001%	0.363%
Non-carcinogenic human toxicity	0.746%	2.816%	0.001%	0.003%
Land use	0.003%	3.333%	0.000%	0.000%
Mineral resource scarcity	2.067%	1.399%	0.001%	0.000%
Fossil resource scarcity	0.972%	2.552%	0.001%	0.001%
Water consumption	0.376%	3.045%	0.054%	0.000%

.

As can be seen from the data in the table, the catalytic cracking construction phase has the most significant impact on the evaluation results. This may be because more reinforced steel and concrete are used in the construction phase, and the production process of both is accompanied by high consumption of resources. The waste discharge phase has a smaller impact on the results. Incineration and power generation technologies show the strongest and weakest effects in the operation phase and power input. This result can be attributed to the fact that these technologies use electricity as well as produces electricity, and there is a material flow back during the life cycle; therefore, it is less sensitive.

Thus, for CCT, the optimization can be focused on the construction phase, and for IAGPT, the optimization can be focused on the operation phase.

3.3. Technology Optimization

Based on the environmental impact results, both technologies exhibit high values of environmental impact in terms of electricity usage. In addition, CCT exhibits significant negative environmental impacts in terms of equipment construction, and IAPGT exhibits negative environmental impacts in terms of chemical inputs. Therefore, this study presents a discussion and prediction of the corresponding technology optimization.

3.3.1. Optimization of Electricity Input

The electricity used in both contemporary technology options is still from coal-fired power, which is in line with the current energy structure status quo and causes obvious environmental impacts. In terms of power input optimization, the energy structure was considered to be cleaned and optimized, and hybrid power was used as an energy input to predict the degree of environmental optimization, taking CCT as an example [49].

The power structure in the original LCA model is updated; the output data results and change rates are shown in Figure 5. The use of hybrid power has a significant effect on reducing the environmental impact, which can achieve a more than 80% reduction in environmental impact, and there is optimization potential.

3.3.2. Optimization of Catalytic Cracking Equipment

The current equipment feedstock used in CCT is reinforced steel with a complex production process aimed at achieving resistance to high temperature and pressure, as well as acid and alkali corrosion [50]. Therefore, in terms of equipment optimization, the equipment’s raw material was considered to be alloyed with alloys containing chromium and nickel.

The original strengthening steel for the equipment in the original LCA model was upgraded to alloy steel containing chromium and nickel; the output data results and change rates are shown in Figure 6. The use of chromium-nickel alloy steel has a significant effect on the reduction in environmental impact, which can achieve a more than 80% reduction in environmental impact, and there is potential for optimization. However, it will have a negative impact on freshwater eutrophication and freshwater ecotoxicity.

3.3.3. Optimization of Incineration and Power Generation Agents

The acid gas detergent currently used in incineration power generation technology is quicklime, which generates further environmental pollution; activated carbon also exhibits a similar acidic waste gas scrubbing capacity to lime [51,52]. Changing the acid gas detergent to a composite structure of activated carbon + limestone could be considered, as could increasing the proportion of activated carbon used.

The 100% lime agent structure in the original LCA model was replaced with an agent structure of 50% activated carbon and 50% lime; the output data results and change rates are shown in Figure 7. The use of compound acid detergent can achieve a reduction of approximately 50% of the environmental impact, and there is potential for optimization, but it will have some negative impact on human carcinogenic toxicity.

4. Conclusions

Based on the environmental impact results and process optimization discussions of the two waste-to-energy technologies for plastics, this study proposes the following conclusions:

(1)

The negative environmental impacts of CCT are lower compared with IAPGT, most of which are concentrated in the construction stage; the impact of reinforced steel on equipment is the most significant. The negative environmental impact of IAPGT is more clearly reflected in the operation stage; the impact of chemical input is the most significant. According to the standardized results of environmental impact, the total environmental impacts of CCT and IAPGT are 42.466 and 111.598 per functional unit, respectively.

(2)

Based on the sensitivity analysis, the environmental impact contribution of CCT is the most significant due to data fluctuations in the construction phase, and the environmental impact contribution of incineration and power generation is the most significant due to data fluctuations in the operation phase. Therefore, for CCT, optimization should be focused on the construction phase, and for IAPGT, optimization should be focused on the operation phase.

(3)

The process optimization discussion shows that a mixed power input, chromium-nickel composite steel, and composite agent structure can significantly reduce the negative environmental impact. However, there is also a negative environmental impact; thus, it is necessary to avoid environmental damage as much as possible and choose a technical solution with higher overall environmental benefits during the specific implementation. Relevant companies and R&D platforms could conduct research based on this idea and make sustainable improvements to the technology.

(4)

Contemporary research is still at the experimental stage, and the data sources are not extensive enough. Thus, subsequent studies could be based on more extensive enterprise research combined with actual production experience to drive the optimization of the technology from the perspective of production applications. In addition, future studies could apply more environmental analysis indicators.