Feedstock Recycling of Plastics Waste for Electricity or Fuel: An Exergy Approach

Waste plastics contribute to many environmental and social problems due to the loss of natural resources, environmental pollution, depletion of landfill space, and the various demands of an environmentally oriented society. The consumption of plastics waste increases annually, particularly in developing countries. Feedstock recycling of scrap polymers by thermal and chemical methods is well known and environmentally acceptable. However, new technologies for waste utilization as well as the methods that would enable an objective and broad assessment of these processes are strongly needed. Selecting the best method for thermal processing of waste polymers can be done based on a thermodynamic analysis of the process. In the paper, the process of thermal degradation of waste plastics (that is carried out in the new type of a tubular reactor with molten metal) is described and evaluated from the therodynamic poin of view. Depending on the final product (a fuel-like mixture or electricity), the calculated exergy efficiency of the proposed method ranged from 79% to 82%. These results mean that feedstock recycling of this type of waste by thermal degradation is a beneficial process from an energetic and ecological perspective as compared to other processes, particularly incineration.


Introduction
Waste management systems and energy production systems will have to undergo changes due to energy crises and the threat of global climate change. Reduction of the volume of plastic waste at the source, which is the favored method among sustainable development strategies, is very difficult to implement. However, it is believed that reuse, recycling (by different technologies), and incineration (with heat recovery) can lead to good results and thus should be regarded as primary methods in waste management. It is essential, therefore, that environmental strategies in which scrap polymers are considered an energy source be connected with the larger energy systems worldwide. Prior to choosing plastic utilization technology and making investment decisions, it is important to have knowledge of the consequences of these different choices. Thus far, several methods focusing on different technological, economic, and environmental aspects have been developed to estimate the benefits and losses of such decisions. Comparing the energy efficiency of waste utilization processes based on exergy calculations is one of the best methods for the assessment.
Feedstock recycling by thermal and chemical methods can be used to convert scrap polymers into useful products. These methods, which include gasification, liquefaction, liquefaction with hydrogenation, hydrolysis, pyrolysis, and thermocatalytic degradation, are well known and environmentally acceptable. Practically, each of the mentioned methods enables to reduce the impact of waste plastics on the environment as well each of them may be a cheap source of energy and useful raw materials. The processes are generally based on the thermal or catalytic cracking or on the typical pyrolysis. Several commercial technologies, available for thermal degradation of postconsumer plastics allow to obtain a fuel-like liquid mixture of hydrocarbons as a basic product [1,2]. However, industrial plants using this process are rare or were in operation for only a very short time. The proposed technologies and reactors are still far from perfect, and their profitability is weak. With these shortcomings in mind, we consider that new technologies (and reactors) should have the following features: (a) The process should be carried out without catalysts due to difficulties and the cost of the their recovery. (b) Low operating costs and investment costs are needed because the plastics, waste plastics, and products of degradation are inexpensive, and the conversion process must be profitable. (c) Required frequency of cleaning of a reactor should be low. (d) The industrial-scale plant should have modular construction to allow for greater flexibility of waste processing. (e) The most profitable, seem to be such a technology, which will allow to use all obtained products for electricity production. In this case, the yield of liquid product, is not the essential process parameter. If the required product should to be a fuel-like mixture of hydrocarbons the yield of liquid product has to be high.

Exergy Analysis -Methodological Remarks
A number of environmental indicators can be defined that are related to the concept of sustainability. However, it seems that indicators based on thermodynamic considerations can better illustrate whether a development is sustainable. Environmental systems and industrial metabolisms obey the fundamental laws of thermodynamics. Whereas the first law of thermodynamics states that for every process, no energy can be destroyed or created, the second law notes that the quality of energy decreases. This quality, expressed as "useable energy," is called exergy. Exergy is the maximum amount of work that can be obtained from a material, taking into account the state of the surroundings [3,4]. Therefore, exergy analysis is useful for assessing the genuine energetic efficiency of different industrial processes, including the conversion of waste to fuel or energy. The concept of exergy (as a general measure of work) is given by Z. Rant and H. D. Baehr, but the sources of the concept can be found in S. Carnot and J. H. Gibbs's earlier works.
A basic diagram of the exergy efficiency of a generic process is presented in Figure 1. The exergy efficiency η B of the process is expressed as follows: where B i denotes the exergy of every input stream of materials, fuels, energy, capital, and labor (i.e., index Ij; j =1,2..5) as well as the output streams for the products, waste products, produced energy, and waste energy (i.e., index Oj). It is obvious that the higher the recycling ratio is (denoted by α, β, and γ), the higher the exergy efficiency of the process will be. Sciubba [5] proposed calculating the specific exergy Bi of the multicomponent (I =1.. N) mixture j at a certain thermodynamic state (1) and a selected reference state (0) as follows: where h is the specific enthalpy (J/kg), s denotes specific entropy (J/kg/K), T 0 is the environmental temperature (for the reference state), μ is the specific chemical potential (J/kg), and c is the mass concentration. Thus, the stream exergy can be expressed as follows: where F j is the mass stream of the multicomponent mixture j. Exergy analysis is used for energetic and ecological assessments of different processes occurring in the environment [6][7][8][9][10] and at the industry level [11]. The basic principles of the methodology used to assess the impact of industrial processes on the environment and their energy efficiencies have been discussed in numerous scientific articles. Many scientists have analyzed the energy efficiency of thermal processes [12][13][14][15], energy production, fuel gasification and/or combustion [16][17][18][19][20][21][22] and biomass gasification [23][24][25][26][27][28]. Some studies have examined waste utilization methods and waste management systems based on comparative exergetic assessments [29][30][31][32][33][34] and the energetic/exergetic efficiency of selected processes [35,36]. This paper presents a simplified assessment of the feedstock recycling process of waste polyolefins, in which the products of their thermal degradation are used for electricity generation.

Feedstock Recycling of Polymers in Molten Metal
There are also technologies that use molten metals or molten inorganic salts in the process of waste degradation. In the case of using in the process of molten metals, sometimes this method is called the Clementi Process [37]. The process is carried out usually at 600ºC. Such molten metals as tin, lead, and bismuth (or their alloys),` in the liquid state, allow to create the reaction environment, what enables to stabilize the process.
Some years ago, a new type of vertical tubular reactor with a molten metal bed has been proposed (by the author of this paper) and useed in the laboratory-scale to convert waste polymers (plastics, rubber and etc.) into a valuable product, namely, a fuel-like mixture of hydrocarbons. The temperature of the process was decreased to 380°C -420°C, and the surface of the molten metal is very small by comparison of the typical Clementi process (in ratio about 1:1000). The basic product is a mixture of hydrocarbons (i.e., paraffins and olefins C4-C24). The description of the laboratory setup as well as the results of the laboratory thermal degradation of waste polyethylene (PE) and polypropylene (PP) can be found in the previous papers [38,39]. A very short summary of those experimental laboratory results (only for PE) is presented in Table 1. The experimental reaction rate of the degradation process was between 951 kg/(m3h) and 9183 kg/(m3h) depending on the process conditions. These experimental results were used to design pilot-scale plant and to create a mass balance of the process and next to calculate the exergy efficiency.

Exergy Efficiency Calculations for the Described Technology with Electric Power Generation
In the previous article [40], an exergy analysis was conducted in the case the process of thermal degradation of the blend of plastics that consisted of PE and PP. It was assumed That the final and the most required product was a (fuel-like) mixture of liquid hydrocarbons C4-C24, with the same composition which was obtained in laboratory experiments.
The scheme of the proposed pilot plant based on the vertical tubular reactor with molten metal was presented in the mentioned paper. It was assumed that the reaction rate was to be equal to 2877 kg/ (m 3 h) -for the purposes of the exergy analysis. This assumption was made based on the results obtained in the case of a similar demonstration plant and is consistent with the results of laboratory experiments in relation to the assumed process conditions. It was assumed that biomass would be the main source of the energy required to heat the reactor. Based on the experience from the demonstration plant -it was assumed that 28 kg/h of sawdust (11.7 kJ/kg) ought to be gasified in the gasifier and combusted to heat the reactor. Combustion of the gaseous product (obtained by the thermal degradation of HDPE) was a supplementary source of energy. The temperature of flue heating gases was equal to 650°C. Water was used to cool the HC product and for purification of flue gases in the scrubber. Electric power was needed for the feeders, fans, water and product pumps, control and acquisition data systems, and the lighting system. The exergy of required water streams and electric power were included in the analysis. The pilot reactor was 2 m high with an inner diameter of 0.2 m. The mass of the molten metal (30% tin and 70% lead by mass) was equal to about 320 kg. The labor of six persons was needed to operate the plant. The results of the mass balance of the thermal degradation process of the HDPE (50 kg/h) as well as the exergy efficiency of this process (equal to 79,5%) were presented in the mentioned paper.
The aim of this study is to compare the previous analyzed installation (the main product -a mixture of liquid hydrocarbons) with another one in which the resulting product of the thermal degradation) is used only for electricity generation. In this last case, the only source of energy (for thermal degradation) is gasification of biomass and only HDPE is processed in the reactor. The scheme of the alternative plant -that is evaluated in this article -is presented in Figure 2. No cooling system is needed, and therefore smaller amounts of water and energy are required to perform the process. Assessment and calculation of the exergy efficiency of the process were based on the following assumptions: (1) No mechanical disintegration of the waste was performed, as no additional energy was needed. (2) Generally, the total exergy of the stream is the sum of kinetic exergy, potential exergy, nuclear exergy, physical (or thermomechanical) exergy, and chemical exergy. However, only chemical and physical exergy are taken into consideration efficiency calculations in this study. Equation (4).
(10) The exergy of SO 2 , NO x , construction materials, labor, and capital was not considered. (11) A continuous process is analyzed in the study.. (12) The values of the chemical exergy of the system components are presented in Table 2. (13) As electrical energy (active power) can be fully converted into work, the exergy content of electricity is equivalent to the energy content of the electrical flow. It can be considered a particular case of the exergy of a work stream [41]. In other words, the exergy associated with electricity is equal to the energy [42]. The generated electric power (P) has been calculated using Equation (5) where, F mol,CO2 is the molar stream of carbon dioxide (kmol/h); R is the gas constant (kJ/kmol/K); T 0 is the temperature of the environment (298 K); and p CO2,out and p CO2,atm are the partial pressures of carbon dioxide in flue gases and in the environment, respectively. It was assumed that the partial concentration of CO 2 in the environment was equal to 385 ppm (Vol) . Ideal conditions for combustion were assumed. All gases were considered ideal.
(1) There was complete combustion and no exergy loss, with the exception of the cooling water. (2) The total waste degradation products flowing from the reactor without condensation are used to produce electricity in the electric power generator. No cooling system is needed. (3) The amount of sawdust has to be increased from 28 kg/h (that was required in a similar plant converting wastes to liquid product) to 34 kg/h. The stream of flue gases also increased to 1129.8 kg/h. (4) The reaction rate was assumed to be 2877 kg/ (m 3 h).
The pilot reactor was 2 m high with an inner diameter of 0.2 m. The mass of the molten metal (30% tin and 70% lead by mass) was about 320 kg. The labor of six persons (as the plant consisted of four reactors) was needed to operate the process in the plant -similarly as in the previous study. The idea of the mass balance, performed for the plant is presented in Figure 3. The detailed mass balances of the process of gasification of sawdust and the process of combustion of hydrocarbons (obtained by degradation) are presented in Table 3 and in Table 4. Water is used by the scrubber and for cooling of the electric power generator.
Electric power is also needed for feeders (of wastes and sawdust), fans, control and data acquisition systems, and lighting systems. The required water streams and electric power are presented in Table 5 and the overall mass balance of the plant in Table 6.   The generated electric power P was equal to 906.5 kW (with a power generation efficiency = 70%) and the stream of exergy (B O3 ) was equal to = 2284 MJ/h. The exergetic efficiency of the plant described in this case to be equal to 81.9% -in comparison to 79,5% (calculated for the previous installation). The exergy efficiency of the process is high, which means that thermal cracking is a more valuable and ecological technology than the incineration of waste plastics. However, the setup with an electric power generator may be the best solution for utilization of this type of waste because of cutting down the costs and energy required for transport and further processing of the liquid mixture of hydrocarbons in the refinery. The exergetic efficiency of the presented process, carried out in the described plant, is over 81% in the case of electricity generation and about 79% in the case of fuel production.
The exergetic efficiencies calculated seem to be too high too, but similar values to them have been calculated by other authors for various other processes, as, for example, for recycled exhaust gases and fuel cell technology (~78%; [43]), the solid-oxide fuel cell (SOFC) systems (~75%; [44]), combustion of methanol with utilization of solar energy or waste heat (~75-78%; [17] and combustion systems using flameless mode (70-90%; [45]. If exergy of construction materials, labor, and capital is included in the calculations, the true efficiency of the total process will not be as high-even if the waste heat generated in the process is recovered. However, the efficiency ought to be essentially higher than the efficiency of the incineration process of waste plastics, which usually amounts to 25-35%. It seems that a setup with electric power generator has more advantages due to higher efficiency, elimination of the environmental costs of transport of the products to refineries, and the higher usefulness of electricity than obtained gaseous and liquid products (hydrocarbons). The profiles of all streams and their exergy values for the considered case are shown in Table 7. Construction of a device (a motor or a turbine) for the combustion of the gaseous mixture of hydrocarbons (CH4 to C24H50) is not the subject of this paper. In the opinion of the author, construction of a proper engine for this purpose is possible. Some publications (with an engineering profile) and information from technical literature also show that it is possible. Among them, Jones et al. [46] and Rahm et al. [47] have pointed to such possibilities in their publications. They demonstrated that different technical solutions are possible, although no specific engine or another thermal machine is indicated (in these publications). Examples of technical solutions of engines that use fuels (of a wide range of hydrocarbons) are diesel engines with a dual power-supply system for compression ignition: Many companies have carried out studies on the possibility of introducing or mixing LPG (Liquefied Petroleum Gas) and/or LNG/CNG (Liquefied Natural Gas/Compressed Natural Gas) with the diesel fuel. The system Dual Fuel Diesel (DFD)simultaneous supply of the diesel motor by two different fuels, diesel and LPG-had already been used by Rudolf Diesel in order to the optimization of his engine. Such a solution could even gain some advantages, such as an increase in driving dynamics (increase in power and torque), reducing the pressure in the fuel system components, and decreasing the emission of pollutants into the atmosphere. Currently, the modern system of DFD can be applied to all diesel engines, both with turbo and without turbo, as well as with the electronically controlled throttle and is really used in the latest designs of diesel engines, including turbocharged. For example, some installations of Electronic Fuel Control (EFC) can dose fuel with the ratio 85:15 (85% LNG, 15% diesel). Some modern diesel engines are capable of running using crude oil and LNG. They are used in the oil and gas exploration and production industries. MAN Diesel & Turbo develops and manufactures dual-fuel engines that run on both gas and diesel fuel, which are based on diesel engines. So, these cited examples point to the feasibility of construction of a combustion engine that allows the use of a mixture of hydrocarbons (C1-C24) in practice.

Conclusions
The known technologies for utilizing waste plastics, particularly incineration, introduce high entropy streams into the environment and require a great deal of attention. At present, there are a great variety of options to use and recycle waste. Depending on local conditions, specific situations require the selection of the best technology that takes into account several implications: ecological, social, economic, legal, and technical consequences. The key to the decision should be the energy or exergy efficiency of the considered methods or processes. Therefore, it seems that the best way to assess and compare different technologies (or even waste management systems) for waste utilization is performing a thermodynamic analysis, as most of these implications may be taken into account in the evaluation process of the processes considered. The application of exergy analysis to account for the amount of raw materials as well as the wastes and waste energy used for energy production is effective in screening alternative technologies for realizing sustainable development. This also allows for indicating the best ways for direct or indirect waste recycling that may be defined for a single industrial process as well as for a waste management system at the national or international levels to be identified.
The method of the thermal decomposition of waste PE or PP in a molten metal bed is a promising process in comparison with catalytic cracking in the vessel or tubular flow reactors. In the proposed method, catalysts and stirring are not needed. Over 90% of scrap polyolefins may be converted into a liquid product. This liquid product may be used for producing fuel or other chemical production.
Assuming that the real efficiency may be not so high (as was calculated) because some input exergy streams (of labor, capital, and construction materials) were neglected in the calculations performed, it should yet be essentially higher than the efficiency of the incineration processes of wastes (which usually does not exceed 30-40%). This means that the thermal degradation process of waste plastics, particularly using electric power generation, could be considered environmentally friendly.
It is obvious that the obtained liquid mixture has to be processed in further processes -to fuels or to electricity. However, the exergetic efficiencies calculated for the proposed installations (with electricity generation or without it) are very similar. The choice of one of the possible solutions will depend primarily on the demand for the type of energy (fuel or electricity), the possibility of connecting to the power grid and the distance of the processing plant to the refinery, where the mixture of the hydrocarbons could be reprocessed to gasoline or diesel. This means that the thermal degradation process of waste plastics using the tubular reactor with the molten metal bed and with the electric power generation should be considered profitable and environmentally friendly if on the one side the long distance to the refinery causes the increase of the costs and on the other side there is a possibility of connecting the electric power generator to the grid and the price of energy is profitable for the owner of the processing plant