**Evaluation of Waste Plastic Oil-Biodiesel Blends as Alternative Fuels for Diesel Engines**

#### **Chalita Kaewbuddee, Ekarong Sukjit \*, Jiraphon Srisertpol, Somkiat Maithomklang, Khatha Wathakit, Niti Klinkaew, Pansa Liplap and Weerachai Arjharn**

Suranaree University of Technology, 111 University Avenue, Suranaree Sub-District, Muang District, Nakhon Ratchasima 30000, Thailand; annen\_ch@hotmail.com (C.K.); jiraphon@sut.ac.th (J.S.); somkiat\_mai@outlook.co.th (S.M.); vkata@sut.ac.th (K.W.); niti\_nick@hotmail.com (N.K.); pansa@sut.ac.th (P.L.); arjharh@g.sut.ac.th (W.A.)

**\*** Correspondence: ekarong@sut.ac.th

Received: 27 April 2020; Accepted: 29 May 2020; Published: 2 June 2020

**Abstract:** This study examined the use of waste plastic oil (WPO) combined with biodiesel as an alternative fuel for diesel engines, also commonly known as compression ignition engines, and focused on comparison of the basic physical and chemical properties of fuels, engine performance, combustion characteristics, and exhaust emissions. A preliminary study was conducted to determine the suitable ratio for the fuel blends in consideration of fuel lubricity and viscosity, and these results indicated that 10% biodiesel—derived from either palm oil or castor oil—in waste plastic oil was optimal. In addition, characterization of the basic properties of these fuel blends revealed that they had higher density and specific gravity and a lower flash point than diesel fuel, while the fuel heating value, viscosity, and cetane index were similar. The fuel blends, comprised of waste plastic oil with either 10% palm oil biodiesel (WPOP10) or 10% castor oil biodiesel (WPOC10), were selected for further investigation in engine tests in which diesel fuel and waste plastic oil were also included as baseline fuels. The experimental results of the performance of the engine showed that the combustion of WPO was similar to diesel fuel for all the tested engine loads and the addition of castor oil as compared to palm oil biodiesel caused a delay in the start of the combustion. Both biodiesel blends slightly improved brake thermal efficiency and smoke emissions with respect to diesel fuel. The addition of biodiesel to WPO tended to reduce the levels of hydrocarbon- and oxide-containing nitrogen emissions. One drawback of adding biodiesel to WPO was increased carbon monoxide and smoke. Comparing the two biodiesels used in the study, the presence of castor oil in waste plastic oil showed lower carbon monoxide and smoke emissions without penalty in terms of increased levels of hydrocarbon- and oxide-containing nitrogen emissions when the engine was operated at high load.

**Keywords:** waste plastic oil; biodiesel; castor oil; emission; diesel engine

#### **1. Introduction**

The demand for and consumption of energy is expected to increase, especially for fossil fuels. In Thailand, fossil fuels, also known as conventional energy, are widely used in various forms of transportation and industrial plants because of their convenience and ability to provide a high heating value. Fossil fuels are a nonrenewable resource which continues to be used by humans, with demand steadily increasing. Thus, these fossil fuels will soon be entirely consumed. For this reason, many countries are beginning to rely more on alternative energy or renewable energy sources.

Thailand still lacks any significant alternative sources of energy. Moreover, it produces insufficient energy to meet the demand, resulting in the import of over 49% of its consumed energy in 2017 [1]. The value of crude oil and imported petroleum products in Thailand increased by 39.8% and 23.0%, in 2016 and 2017, respectively, due to the higher oil demand. Thailand has mainly imported crude oil

from Middle Eastern countries. In Thailand, the share of energy from renewable sources is expected to increase steadily. To increase this share and reduce primary energy consumption, waste plastic oil has been proposed as a new option for use in transportation. While there is less demand for transportation energy, this initiative represents a move toward the direction of diversification of fuels through energy conversion technologies. It also focuses on using oil from plastic waste in diesel engines. Plastic waste is a petroleum waste that comes from both household and industrial sectors, leading to a large amount of plastic waste. These wastes require hundreds of years for decomposition and are a burden to manage. Most plastic is recycled using mechanical recycling, while only 2% of chemicals are recycled [2]. Generally, the waste management process that is currently popular is the landfill method, which normally requires a lot of landfill space and has an impact on the environment, resulting in soil pollution.

Plastic waste is composed of hydrocarbons, which are the main component of conventional fuels. This raises the possibility of recycling these plastic wastes through their conversion into fuel. Products can also be obtained from the production process, in addition to being used as an energy source similar to conventional fuels. It is also able to provide environmental benefits in terms of waste management for maximum benefits and reduction in the amount of plastic waste, reduced plastic waste disposal, and also minimizing the problem of finding places for garbage landfills. The use of plastic waste as a renewable energy feedstock also helps in mitigating the energy crisis.

Several studies have investigated the use of waste plastic oil in diesel engines as an alternative fuel. Waste plastic pyrolysis oil has properties that are similar to diesel fuel, including the heating value, density, and cetane index, and can be used as a substitute for diesel fuel [3]. The literature also shows that diesel engines use waste plastic oil to provide stability in performance and a similar efficiency [4]. The different types of plastics are also basically impacted by their different compositions. Recent studies have shown that the oil product of HDPE (high-density polyethylene), mixed with LDPE (low-density polyethylene), has a higher heating value than LDPE, PP (polypropylene), and HDPE alone. It was revealed that LDPE produces the highest yields [5]. Waste plastic oil has also been studied with regard to engine power, and it was found that there was no significant difference from diesel fuel [6]. The thermal efficiency of waste plastic oil was higher when compared to diesel fuel [7,8]. However, one study examined the exhaust emissions of a four-cylinder, direct-injection diesel engine running on diesel blended with different ratios of waste plastic oil and found that the amount of nitrogen oxides increased because of the longer ignition delay [9] and that there was greater hydrocarbon emission in comparison to diesel fuel [10].

In addition, it is expected that biodiesel will be used as a renewable energy source in the energy transportation sector. A great deal of research supports the use of biodiesel as a suitable alternative in replacing diesel fuel. The presence of oxygen in fuel molecules is expected to result in cleaner biodiesel combustion, leading to improvements when considering emission. However, there are only a few reports on the use of biodiesel mixed with waste plastic oil. For example, Ramesha et al. [11] reported that B20 algae biodiesel blended with waste plastic oil can be a suitable fuel for diesel engines. The waste plastic oil-biodiesel blend showed an increase of 16% in brake thermal efficiency with respect to diesel engines. Additionally, the carbonaceous gas emissions, including hydrocarbons and carbon monoxide, were decreased, but nitrogen oxides slightly increased, as compared to diesel fuel. In the study by Senthilkumar et al. [12], waste plastic oil was mixed with Jatropha biodiesel for diesel engines. The brake thermal efficiency and brake specific fuel consumption of the waste plastic oil-biodiesel blend were higher than the waste plastic oil. The hydrocarbon and carbon monoxide emissions decreased when waste plastic oil was blended with Jatropha biodiesel.

In the present work, waste plastic oil-biodiesel blends were used as an alternative fuel in a diesel engine without any engine modifications. The selected biodiesels were produced from castor oil and palm oil through a transesterification process and were then blended with waste plastic oil. Palm is an important economic crop and main feedstock for biodiesel production in Thailand. To avoid the use of edible feedstock, castor oil was considered because of its benefits of high oxygen content in fuel molecules and excellent fuel lubricity. These properties are attributed to the presence of ricinoleic acid, which is the main component of castor oil [13]. The oxygen in the fuel molecules contributes to better combustion processes in terms of emissions. In this study, we evaluated the effect of biodiesel addition to waste plastic oil in terms of basic physical and chemical fuel properties of the resulting fuel mixture, mainly focusing on fuel lubricity and viscosity, engine performance, combustion characteristics, and exhaust gas emissions of a single-cylinder diesel engine. In the section of combustion characteristics, basic parameters comprised of in-cylinder pressure and crank angle were recorded during the engine test. After that, heat release rate of test fuels was calculated on the basic principles of the first law of thermodynamics, which the specific heat ratio was calculated based on the in-cylinder pressure and combustion chamber volume through the assumption of polytropic process.

#### **2. Materials and Methods**

As part of this research, some physical and chemical properties of test fuels were determined. Experimental tests were carried out on a single-cylinder diesel engine (model Kirloskar TV1 with product code 240PE). The engine was connected to an eddy current dynamometer to simulate the load and was tested at a constant speed of 1500 rpm under variable load conditions, i.e., low, medium, and high load conditions (25%, 50%, and 75% of the maximum torque). The gas detector was used to determine the levels of carbon monoxide (CO), nitrogen oxides (NOX), hydrocarbon (HC), and smoke. The fuel consumption of the engine was tested by adjusting the load of the diesel engine. Therefore, the scope of this research project was limited to the following:


#### *2.1. Materials*

In this experimental investigation, waste plastic oil (WPO) was used as the main fuel, whereas the other fuels included castor oil methyl ester (COME) and palm oil methyl ester (POME) as components for blending with WPO. The diesel fuel was commercial diesel fuel (B7) containing 7% biodiesel, according to the department of energy business in Thailand. The waste plastic oil used in this research project was produced from waste plastic by the pyrolysis method. The pyrolysis process is a chemical process of heating that decomposes plastics in the absence of oxygen. The master plant that processes waste plastic to oil is located at Suranaree University of Technology.

#### 2.1.1. Waste Plastic Oil

The raw materials used in this study were from plastic waste, such as plastic waste bags collected from waste in Suranaree Subdistrict, Nakhon Ratchasima, Thailand. The composition of these plastics includes polyethylene (PE) and polystyrene (PS) and about 70% was contaminated organic matter. The waste plastics obtained from mechanical biological treatment (MBT) were processed into raw materials using an agglomerator, which processed the plastic into small pieces that could be continuously fed into the oil processing plant. The waste plastic oil was recycled using pyrolysis and did not undergo distillation.

The pyrolysis process involves the breakdown of large molecules into smaller molecules by chemically decomposing organic matter through heating in an oxygen-free environment. Waste plastic is processed to maintain a temperature of 300–350 ◦C inside the reactor, where the waste plastic is then vaporized and the outlet gas condensed through the condenser unit at this high temperature. The obtained liquid was taken as fuel, and this process happened constantly in converting the waste plastic

back into usable oil. All gases from this process were treated before being released into the atmosphere. The exhaust gas was treated through scrubbers and chemical treatment for neutralization. From the pyrolysis process, the following output products were collected: Waste plastic oil (70%), gas (10%), and solid (20%), with values based on the weight of the input. The plastics yielded approximately 600 L per ton.

#### 2.1.2. Production of Castor Oil Biodiesel and Palm Oil Biodiesel

The experimental work was carried out in a laboratory at Suranaree University of Technology. Castor oil was used in the transesterification process to convert castor oil into castor oil methyl ester. Methanol and potassium hydroxide (KOH) catalyst were used for the reaction. The reaction was carried out using methanol and castor oil in a 9:1 molar ratio with 0.5% KOH (by weight of oil). The KOH was first dissolved in methanol and was then mixed with the castor oil. This mixture was heated and stirred using an electric heater and a magnetic stirrer. The reaction was carried out at a constant temperature of 50 ◦C for about 120 min. Then, the mixture was poured into a separating funnel to separate the methyl ester of castor oil and glycerol. The layers were separated and were allowed to settle for a minimum period of 8 h, with glycerol at the bottom layer and the ester at the top layer. The castor oil methyl ester was then washed with water to remove any traces of methanol or potassium hydroxide that was not reacted. The castor oil methyl ester was heated to 120 ◦C for moisture removal.

However, palm oil was also used with methanol and potassium hydroxide (KOH) in the reaction. The reaction was carried out by taking methanol and palm oil in a 12:1 molar ratio and 2% KOH (according to the weight of the oil). KOH was dissolved in methanol and this mixture was then mixed with palm oil. This mixture was heated and stirred using an electric heater and a magnetic stirrer. The reaction was carried out at a constant temperature of 60 ◦C for about 30 min. Then, the mixture was poured into a separating funnel to separate the methyl ester of the palm oil and glycerol. The layers were separated and allowed to settle for 24 h, with glycerol at the bottom layer and the ester at the top layer. The palm oil methyl ester was then washed with water to remove any traces of methanol or potassium hydroxide that was not reacted. The palm oil methyl ester was heated to 120 ◦C for moisture removal.

#### *2.2. Gas Chromatography Analysis*

The column for GC–MS analysis was a DB-wax capillary column (60 m length × 0.25 mm inner diameter, 0.25 μm film thickness). Helium was used as a carrier gas with a constant flow rate of 1.0 mL/min. The oven temperature was programmed to operate from 70 ◦C to 250 ◦C, with the initial temperature of 70 ◦C that was held for 3 min, followed by a rate of heating of 3 ◦C/min to a temperature of 180 ◦C and then a rate of 10 ◦C/min to a final temperature at 250 ◦C, which was held for 25 min. The inlet was held at 250 ◦C with a split ratio of 20:1. The injection volume was 1 μL per sample. The mass spectrometer was scanned from mass to charge ratio (m/z) of 35 to 550 with the source at 250 ◦C.

#### *2.3. Experimental Setup*

An experimental investigation was tested to evaluate and compare the results obtained for the use of different types of test fuels. This study aimed to investigate the effect of waste plastic oil blended with biodiesel on engine performance and the emission of a single-cylinder diesel engine. WPOC10 and WPOP10 were selected for experimental comparison based on initial experiment data regarding the lubrication and viscosity of the blended fuels, whereby 10% biodiesel and 90% waste plastic oil was determined to be the optimal ratio for further testing in the engine. The total number of samples was four test fuels (using either WPOC10, WPOP10, WPO, diesel), which were prepared for testing with the equipment and measuring tools used in the laboratory as follows: A four-stroke, single-cylinder diesel engine (Kirloskar TV1) with a water cooler system, direct injection, and a rated output power of 3.5 kW at 1500 rpm, unmodified and under different loading conditions. The engine was mounted on a fixed bed floor in the laboratory room and the load was applied on the engine. A picture of the experimental setup is shown in Figures 1 and 2, and the engine specifications are given in Table 1.

**Figure 1.** Experimental setup for engine testing.

**Figure 2.** A schematic diagram of the experimental installation.

**Table 1.** Test engine specifications.


The observation results during testing in the engine were used to evaluate the use of different fuel blends in comparison to the commercial diesel fuel and waste plastic oil as a reference. Three repetitions were carried out in each test to obtain the average values for analysis. The used confidence intervals corresponded to a 95% confidence level with respect to statistical significance of the result trends.

#### 2.3.1. Testing by Adjusting Engine Load

Engine tests were done by loading the engine to a level that simulated the workload condition of the engine. Three engine loading conditions (25%, 50%, and 75% of the maximum engine torque) were chosen for engine experiments, and the engine loads were adjusted by the eddy current dynamometer, which acted as a brake and an enabling measurement of the engine's torque. The knob on the dynamometer loading unit was rotated to gradually adjust the engine load to ensure that the load on the load indicator gradually changed for each load value before the test. The experiment was performed using various blends at different loads, from low to high loads, at a constant speed. Furthermore, before testing the engine output with different fuels, the engine was always cleaned prior to installing the fresh oil to ensure the new oils were tested according to the intended compositions.

The fuel consumption was measured using a burette and stopwatch to determine the change in the fuel volume divided by the testing time. The burette with a unit of volume in mL was used to determine the volume of spent fuel. The tests were performed by counting the time needed using a stopwatch to determine the length of time before the fuel ran out. This required a fixed test fuel volume in a burette of 10 mL, and this measurement was repeated three times. The determination of each output value, such as the brake-specific fuel consumption, thermal efficiency, combustion, and amount of gas emission, was based on recordings at each engine load change using the four tested fuels.

#### 2.3.2. Emission Testing

In each engine test, the engine load was gradually adjusted using the knob, and the actual engine speed was measured through encoder wheel monitoring, to measure the tested value of the exhaust gas emission of fuel blends.

The data corresponding to the parameters of engine combustion characteristics were collected through the signals from an in-cylinder pressure sensor and shaft encoder, both of which were analyzed using IC Engine Soft of Apex Innovations' software. The amount of nitrogen oxides (NOX), hydrocarbon (HC), carbon monoxide (CO), and smoke in emissions was measured by the engine exhaust emission analyzer using a Testo 350 Gas analyzer for CO, NOX, and HC, and a Testo 308 for smoke by installing the equipment for the experiment, as shown in Figure 2.

The Testo 350 and Testo 308 exhaust gas analyzers were used to measure the exhaust gas. The exhaust emission probe was placed in the tailpipe of the engine and the exhaust gas emissions were measured. The test engine was run to idle for approximately 10 min in order to ensure the stability of the engine before measuring the exhaust gas emissions. The Testo 350 was analyzed by using a nondispersive infrared and electronic chemical method. Furthermore, the Testo 308 was analyzed using the principles of absorption photometry.

#### **3. Results and Discussions**

#### *3.1. Test Fuels*

In order to improve the properties of waste plastic oil by combination with biodiesel, such as through increasing the oxygen content in the waste plastic oil for better combustion and to improve the viscosity and lubricity of the waste plastic oil, COME and POME were blended with waste plastic oil at different volumetric ratios, ranging from 0% to 15%, and the basic physical and chemical properties of the blended fuels were investigated, which mainly focused on fuel lubrication and viscosity. It can be concluded that the presence of 10% biodiesel in waste plastic oil is the optimal ratio because the smallest scar diameter was obtained after lubrication testing and the viscosity was within the acceptable criteria prescribed by the standard specification for diesel fuel, as shown in Figure 3. The lubricity testing was evaluated by a high-frequency reciprocating rig (HFRR) and was conducted according to EN ISO 12156 [14]. From the preliminary experiment, 10% biodiesel was enough to maintain the lubrication of the blended fuel and there was no significant improvement in the lubrication of the blend when exceeding this percentage of biodiesel in waste plastic oil. Therefore, a combination of either 10% castor oil biodiesel or 10% palm oil biodiesel with 90% waste plastic oil (WPOC10 and WPOP10, respectively) was selected for further investigation in the engine test to study the effect of biodiesel addition to waste plastic oil on engine performance, combustion characteristics, and exhaust emissions.

**Figure 3.** Lubricity and viscosity of biodiesel blending.

The fatty acid profile of castor oil and palm oil are summarized in Table 2, which confirms that the most abundant fatty acid in castor oil is ricinoleic acid, comprising about 85% by weight of the total fatty acid content, while the major constituents of palm oil are palmitic and oleic acid, at about 46% and 37% of the weight. Palmitic acid is a fatty acid that naturally occurs in vegetable and animals, and it is the main component of human milk fat. Furthermore, oleic acid is also the major component of many oils and fats.


**Table 2.** Fatty acid composition of palm oil and castor oil.

The physicochemical properties of fuels were carried out, based on the ASTM standards, and the properties of the test fuels are given in Table 3.


**Table 3.** Properties of the test fuels.

#### Characterization of Waste Plastic Oil

The waste plastic oil or pyrolysis oil used in this study was extracted from mixed plastic wastes. The chemical compounds contained in the waste plastic oil were analyzed by gas chromatography–mass spectrometry (GC–MS), using a gas chromatograph Agilent 7890A coupled to a mass spectrometer Agilent 7000B. The results of the GC–MS analysis of waste plastic oil and diesel are presented in Figure 4, and it is an important chemical compound contained in plastic oil and the percentage of the area is shown in Table 4.

**Figure 4.** Total ion current chromatogram for: (**a**) Waste plastic oil and (**b**) diesel.

**Table 4.** Components identified from waste plastic oil and diesel by GC–MS analysis.


The waste plastic oil consisted of different hydrocarbons contents, which separate according to the light and heavy fractions, from the lowest carbon atom (C4) to the highest carbon atoms (>C20) and can be divided into three groups. The C4–C11 group represented the light fraction or gasoline, and typical gasoline consists of hydrocarbons between five and nine carbon atoms. The C12–C20 group represented the middle fraction or diesel. Diesel has a high percentage of carbon atoms of C16–C20 [15]. Table 4 presents the results of the comparison between fuels, and a similar trend was observed between waste plastic oil and diesel. The waste plastic oil and diesel produced the highest C12–C20 fraction.

#### *3.2. Engine Performance*

Figure 5 illustrates the results of the brake specific fuel consumption (BSFC) of the engine for four kinds of test fuels according to three engine-operating loads. The results showed that the BSFC increased at the low loading (25% of the maximum torque) rather than at medium and high loading, respectively (50% and 75% of the maximum torque). The results also showed that the increment in the engine load seems to result in less specific fuel consumption for all the fuels [16,17]. The increase in engine loading resulted in an increase of fuel flow rate, brake thermal efficiency, and exhaust gas temperature while also decreasing the brake specific fuel consumption at the same time.

**Figure 5.** Variation of brake specific fuel consumption.

The brake specific fuel consumption was slightly increased for biodiesel-waste plastic oil. In general, brake specific fuel consumption was found to increase when the biodiesel quantity of the blends was increased, which was due to its lower heating value [16,18–20].

Figure 6 shows the variation of brake thermal efficiency (BTE) and engine load. Higher BTE was obtained with the use of waste plastic oil and its blends. The addition of biodiesel tended to improve the combustion of waste plastic oil. This may be attributed to the increase in oxygen content, due to oxygen in the fuel molecule of the fatty acid in biodiesel, resulting in more effective combustion [20–25]. In addition, the proper lubricating properties of biodiesel may play a role in reducing the friction to the level that the brake efficiency was enhanced from pure waste plastic oil [26,27].

**Figure 6.** Variation of the brake thermal efficiency.

#### *3.3. Combustion Characteristics*

The combustion characteristics were examined as in-cylinder pressure and the rate of heat release based on the basic principles of the first law of thermodynamics, as shown in Figure 7. It was found that the combustion of waste plastic oil was similar to diesel fuel for all tested engine loads. The addition of castor oil biodiesel to waste plastic oil caused a delay in the start of the combustion rather than the addition of palm oil biodiesel. This was explained by the lower cetane number in castor oil. Additionally, the higher viscosity of castor oil biodiesel can be used to justify the delay in the combustion process due to the difficulty of fuel injection and the quality of fuel spray [27,28]. When considering peak of heat release rate, it was found that a higher peak was obtained for fuel blends with castor oil as biodiesel compared to palm oil. The accumulation of fuel volume during the longer ignition delay, which impacted the higher peak of premixed combustion, was used to justify the higher peak of the heat release rate obtained by the combustion of WPOC10 with respect to WPOP10 [29,30].

**Figure 7.** *Cont*.

**Figure 7.** In-cylinder pressure and rate of heat release at different levels of maximum torque: (**a**) 25%; (**b**) 50%; (**c**) 75%.

#### *3.4. Emissions*

From Figure 8, the blends of the biodiesel-waste plastic oil showed a disadvantage in carbon monoxide emissions because of the higher viscosity and lower calorific value. Therefore, the combustion temperature was lowered due to ineffective atomization of the fuel blends, leading to an increase in carbon monoxide emissions. Comparing the two biodiesels used in this study, the presence of castor oil in fuel blends showed lower CO emissions. Although, castor oil possesses higher viscosity than palm oil, which can generate poor fuel atomization that results in more incomplete combustion. The higher oxygen content of castor oil may improve the quality of the combustion and can be compensated for by the effect of higher viscosity, leading to lower CO emissions by the addition of castor oil biodiesel to waste plastic oil as compared to the addition of palm oil biodiesel.

From Figure 9, it can be seen that the amount of nitrogen oxide emissions increased with increasing engine load, and the nitrogen oxide emission levels of plastic waste oil were higher compared to diesel fuel. Comparing waste plastic oil and its blends, lower NOx emissions were found when biodiesel was added. The reduction in the peak of the heat release in the premixed combustion phase was caused by the combustion of biodiesel blends, which tended to attenuate the increase in combustion temperature and did not favor NOx formation. The results of biodiesel addition were similar to diesel fuel blends and resulted in NOX reduction, which was also observed in another study by Pumpuang et al. [22] using blends of diesel with castor oil ethyl ester biodiesel. Considering the addition of castor oil and palm oil biodiesel, the castor oil biodiesel blends showed higher NOx emissions than those of palm oil biodiesel. The longer ignition delay due to the lower cetane value, caused by the addition of castor oil biodiesel, led to a higher combustion temperature, and this can explain the higher NOx emissions observed with the combustion of waste plastic oil blended with castor oil biodiesel.

**Figure 8.** Carbon monoxide emissions.

**Figure 9.** Nitrogen oxide emissions.

The variation of hydrocarbons with engine loads is shown in Figure 10. Higher levels of hydrocarbon emissions were found with the combustion of waste plastic oil with respect to diesel fuel. However, the addition of biodiesels may have contributed to reducing hydrocarbon emissions [31]. In the case of palm oil biodiesel, the shorter ignition delay due to the lower cetane index, in comparison to castor oil biodiesel, can improve (reduce) hydrocarbon emissions by allowing more time for the combustion process, resulting in lower levels of hydrocarbon emission. However, the blend with castor oil biodiesel, containing a higher oxygen content for the same volumetric percentage when comparing the two biodiesels, was blended with the waste plastic oil. This is expected to promote lower levels of hydrocarbon emission compared to the blend of palm oil biodiesel. The effect of the extremely high viscosity of castor oil, however, tended to increase the emission of hydrocarbons and could counteract the beneficial reductions in hydrocarbon emission due to the higher oxygen content [32]. These effects were more obviously seen at low engine operating loads where the temperature in the combustion chamber was not high enough to vaporize all the injected fuels. Consequently, WPOC10 produced higher hydrocarbons with respect to WPOP10.

**Figure 10.** Hydrocarbon emissions.

Figure 11 shows the smoke emissions obtained by the combustion of the tested fuels. The combustion of the waste plastic oil was evidently associated with less smoke emission than diesel fuel. The addition of biodiesels to waste plastic oil tended to result in increased smoke emissions of these fuel blends. This increase in smoke emissions was apparent when the engine was operated at higher load conditions. Comparing castor oil and palm oil biodiesel, it was found that lower smoke emissions were obtained with the use of castor oil biodiesel. The two main factors had opposing effects on particulate matter emission, which was directly related to the smoke emissions. First, the higher viscosity of castor oil biodiesel caused more difficulty in fuel injection. The ineffective fuel atomization resulted in more incomplete combustion, which was related to a higher amount of unburnt fuel, resulting in higher smoke emissions. Second, the higher oxygen content present in the castor oil biodiesel and hydroxyl group belonged to ricinoleic acid as the primary fatty acid of castor oil can contribute to the enhancement of combustion quality, leading to less smoke emission [33]. It was notable that the effect of higher oxygen content may be more likely to reduce smoke emissions in this study as a result of lower smoke emissions associated with the combustion of WPOC10 compared to WPOP10.

#### **4. Conclusions**

The effects of biodiesel addition on fuel properties, combustion characteristics, engine performance, and exhaust emissions of crude waste plastic oil were studied. Two types of biodiesel, palm oil biodiesel and castor oil biodiesel, were selected as components for blending with waste plastic oil. The findings can be summarized as follows:

• Considering fuel lubricity and viscosity, the presence of 10% (v/v) biodiesel was the optimal ratio for improving the waste plastic oil.


**Author Contributions:** Conceptualization, E.S.; investigation, C.K., S.M., and N.K.; writing—Original draft preparation, C.K.; writing—Review and editing, C.K. and E.S.; supervision, J.S. and W.A.; resources, K.W. and P.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflicts of interest.

#### **Nomenclature**


#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **Potentiality of Waste-to-Energy Sector Coupling in the MENA Region: Jordan as a Case Study**

**Qahtan Thabit 1,\*, Abdallah Nassour <sup>1</sup> and Michael Nelles 1,2**


Received: 3 May 2020; Accepted: 25 May 2020; Published: 1 June 2020

**Abstract:** Population growth, urbanization, and changes in lifestyle have led to an increase in waste generation quantities. The waste management system in the Middle East and North Africa (MENA) region is still considered an adolescent system, while developed countries have made great progress in this field, including regulation, financing, administration, separation at source, recycling, and converting waste to energy. At the same time, in the MENA region, the best performance of the recycling process is around 7–10% of total waste. Nowadays, many developed countries like Germany are shifting from waste management to material flow systems, which represent the core of a circular economy. Also, it should be stated here that all countries that have a robust and integrated waste management system include waste-to-energy (W-to-E) incineration plants in their solutions for dealing with residual waste, which is still generated after passing through the entire treatment cycle (hierarchy). Therefore, this paper illustrates the potentiality of embedding waste incineration plants in the MENA region, especially in large cities, and addressing the economic and financial issues for the municipalities. Cities in these countries would like to build and operate waste treatment plants; however, municipalities do not have the sustainable investment and operating costs. The solution is to maximize the income from the output, such as energy, recycling materials, etc. In addition, the MENA region is facing another dilemma, which is water scarcity due to climate change, increasing evaporation, and reduction of precipitation. This research illustrates a simulated model for a waste incineration plant in the MENA region. The EBSILON 13.2 software package was used to achieve this process. Furthermore, the simulated plant applies the concept of waste-to-energy-to-water, so that not only is waste converted to energy but, by efficient usage of multi-stage flash (MSF) technology, this system is able to generate 23 MWe of electric power and 8500 m3/day of potable water. A cost analysis was also implemented to calculate the cost of thermal treatment of each ton of municipal solid waste (MSW) during the life span of the plant. It was found that the average cost of treatment over 30 years would be around US\$39/ton.

**Keywords:** waste-to-energy; sector coupling; waste incineration; waste heat recovery in desalination; efficiency increasing in waste incineration

#### **1. Introduction**

To attain sustainable development, the need to decouple resource consumption from economic growth is critical. There are two levels that should be taken into consideration. First, the concept or the term "waste management" must be transformed to "waste and resource flow management" [1]. Second, the waste management system must not be treated as a system anymore but as a comprehensive industrial sector. With these concepts, a new methodology has appeared that shows how waste is a source of materials for the production of energy and goods [2,3]. The waste sector contributes to

sustainable production with high recycling and recovery rates (circular economy), which in turn helps to save raw materials and primary energy [4,5]. For example, in Germany, after about 30 years of connecting these sectors there are now 68 waste incineration plants in operation with a capacity of around 20 million tons and a calorific value of around 10 MJ/kg [6,7]. This means there is 200 <sup>×</sup> 109 MJ of energy to be harvested. Therefore, waste represents a source of energy, which should be used and recovered. The incinerators are divided according to the energy content of the waste, as shown in Figure 1.

**Figure 1.** Distribution of waste incineration plants in Germany according to heat content, adapted from the German Environment Agency report [2,7].

In the Middle East and North Africa (MENA) region, 95% of municipal waste goes to landfills without any pre-treatment process. It is recognized to have a high organic content of about 50% and this leads to an increased water content in the composition of mixed municipal waste [8,9]. Furthermore, the waste management system in the MENA region suffers from many problems, including lack of secured financial support (economic problems), formulated laws, instructions, and professional organized systems. For example, separation at source is a very important technique, and it must be implemented in waste management in developing countries; here, some barriers appear, as the infrastructure of the cities and municipalities are not able to embed such a concept.

Around 18 countries in the region (Egypt, Algeria, Bahrain, Iraq, Yemen, Jordan, Qatar, Kuwait, Lebanon, Libya, Morocco, Oman, Saudia Arabia, Syria, United Arab Emirates, Tunisia, Palestine) share the same composition of waste and also the same energy content, which is approximately 6.5–7 MJ/kg. The rate of waste generation per capita in the region is around 1–1.5 kg/day/capita [10]. This paper shows the potentiality of recovering energy content from waste via a waste incineration plant, and can be considered motivation to erect the first waste incineration plant in the MENA region. This research also introduces an efficient utilization of the waste heat (steam) coming out from the power plant as potable water generation by implementing once-through multi-stage flash (OT-MSF) technology. The simulated power plant was achieved using the EBSILON software package, which was developed by the STEAG Company in Germany. Many parameters were required to achieve the simulation process, such as precise element analysis and other requirements, as shown in the next parts.

Waste management systems in the MENA region have different problems and challenges [11]. Before trying to find sustainable solutions for the waste management sector in this region, it is first very important to state several parameters governing the final solutions and treatment. Characterization and composition, collection methods, and existing treatment approaches represent the main aspects that should be taken into consideration to verify the optimum treatment for the daily generated waste. The real situation in the MENA region is that all countries share the same characterization and same final treatment processes, with around 90–95% of waste going to landfill [8,12,13].

In developed countries, for example in Germany, the waste management system involves many streams of treatment [7] to reduce the amount of waste sent to landfill as much as possible. The main ideas behind these kinds of treatment are protecting our environment, harvesting the huge amount of energy existing in waste (by incineration), and reducing the amount of waste sent to landfill in order to increase the life span of the landfills and ensure that only unusable materials are sent there.

At the core of this research is a need to answer several important questions: Which treatment process can be used after materials have been recycled many times? What about unrecyclable materials, hazardous waste, or mixed waste (most common in the MENA region)?

It can be concluded that there is a persistent need for waste incineration plants in the MENA region. As mentioned before, although developed countries have different processes to handle daily generated waste, they also use a thermal treatment technique (waste incineration).

According to the above-mentioned information, it can be concluded that a waste incineration plant represents the optimum treatment process, especially in the case of the MENA region where a huge amount of the waste, more than 50%, is organic in composition [10,12], as shown in Figure 2. Landfill is currently the only main treatment process, due to an absence of legislation and a comprehensive management system. It should be restated here that waste incineration is capable of reducing waste volume by more than 75% [14]. In this paper, a software model has been built to simulate a waste incineration power plant in the MENA region. Jordan has been used as a case study for this model.

**Figure 2.** Waste composition in the Middle East and North Africa (MENA) region [9].

#### **2. Materials and Methods**

This section clarifies the methodology of the research and all required data, technical information, and assessments which have been included and used in the EBSILON model, where this research deals with three sectors: waste management, water scarcity, and power production.

#### *2.1. Waste Composition in Jordan as a Case Study in the MENA Region*

Waste fractions or composition is a very important factor for the experts and decision makers to understand when trying to find solutions for waste management and facing existing problems. Many studies have analyzed the divisions and fractions forming municipal solid waste (MSW) [15]. It should be noted here that the vast majority of analysis studies in this field have studied MSW, which is the most concerning source of waste all over the world. Jordan was selected as a representative

case study for MENA region countries for many reasons: first, availability of data, second, Jordan is a strategic partner to the German Federal Ministry for Economic Affairs and Energy, and third, Jordan was ranked second in the world for water scarcity [10]. In Jordan, 90–95% of the MSW generated is going to landfill. The main problem with landfills is that they are unsanitary, and they affect the water aquifers, which are considered the main sources of water in Jordan. Furthermore, Jordan is a non-producing country in terms of energy, with 96% of its energy supply in the country being imported [10,16,17].

Considering all of that, the waste management sector in Jordan should be developed using a sustainable solution to overcome all these challenges. A waste incineration plant represents one of the potential solutions to these problems and could convert MSW in Jordan into two sources: energy (power) and water. The waste divisions of MSW in Jordan are shown in Table 1. The high proportion of organic waste (approximately 50%), seen in the MSW of many developing countries, such as Jordan, has a high-water content. The huge amount of water is the main obstacle faced by a waste power plant, it lowers the recovery of materials and decreases the energy content of the matter (lower calorific value (LCV)). The LCV is the core of the combustion process inside the combustion chamber, thus conserving a high ignition value requires a higher calorific value.


**Table 1.** Waste fraction divisions in Jordan [18].

Du Long's Approximation was used to evaluate the heat and governing coefficients of the chemical elements of the volatile fraction (see Table 2). Du Long's Approximation is an empirical method used for essential elements of hydrocarbons (C, H, and O) that are connected with each other, as shown in Equation (1) [19]:

$$\text{Q} = 14,406\text{ C} + 67.276\text{ H}\_2 - 6187\text{ O}\_2 + 4142\text{ S} + 2433\text{ Cl}\_2 - 1082\text{ N}\_2 \tag{1}$$


**Table 2.** Elementary analyses of the composition of raw waste in Jordan.

Because of the requirements of this model, a sample of 4 kg was collected from a more than 52 ton heap of waste in the waste-converting station in Amman city, which receives waste from six regions, and was considered a representative sample for Jordan. The sample was shredded and prepared for

analysis in the laboratory. The results are shown in Table 2. As expected, the vast majority of the fraction was the water content (around 60%), with the volatile fraction making up 40%. This 40% includes two divisions: combustible materials and incombustible materials (ash). As shown in Table 2, of the 40% volatile compounds, 15% is ash, which is an incombustible fraction, so the combustible fraction which generates the heat (energy) in the flue gas is 25%. The lower calorific value was taken as an average for the whole year, i.e., 7 MJ/kg for the raw materials.

Studies show that the average per capita production of waste is between 1 kg and 1.5 kg/day [10,18]. In the case of Jordan, around 4000 ton/day is generated in the whole country, the vast majority of this is sent to Alghabawi landfill, which lies around 45 km east of Amman, and receives 60–70% of the waste generated in the kingdom (Jordan), around 2800–3000 ton/day [20], and the rest is taken to other landfills. There are presently 24 covered landfills in Jordan. The location of these landfills was not chosen according to the international standards but according to population density, so as to serve the largest possible number of municipalities. Apart from one landfill, the locations have not been based on feasibility studies for proper site selection. The only exception is Alghabawi landfill of the Greater Amman Municipality. The location of this landfill was selected after conducting an environmental impact assessment for best site selection [21].

Thermal treatment of waste, which is often used, is the incineration of unsorted waste on a so-called mass-burn grate. Sometimes, it is necessary to add fuel to such waste in order to increase its temperature, which will result in more efficient combustion. Often, natural gas, coal, and wooden biomass are used as additional fuels, particularly if the waste has not been previously dried [15]. The technology of waste combustion on a grate is a mature technology that has been used for hundreds of years.

The primary role of waste incineration is the reduction of mass (up to 75%) and volume (up to 90%) of waste, as well as the destruction of dangerous organic compounds and pathogens [22]. There is a long tradition of grate incineration in Europe, and extensive experience has been collected in more than 400 operational incinerators, processing 52 Mt/year of municipal solid waste (MSW) in 2003, which was around 20% of the total quantity of MSW [23,24].

#### *2.2. Water Scarcity in the MENA Region*

There are many reasons for the phenomenon of water scarcity in MENA countries, including population growth, developing economies, changing lifestyles, and climate change. This region has one of the greatest water scarcity situations in the world: it is accommodating almost 6% of the world's population, while getting only 2% of the planet's renewable freshwater supply [25,26]. The average water accessibility per capita in the MENA region is 1100 m3/year, which drops below the water security threshold of 1700 m3/year [25]. Therefore, for the purpose of this research, it is very important to illustrate the water situation for the vast majority of the countries in the region in terms of water consumption and water availability, as shown in Figure 3, which highlights the water scarcity in 19 countries. As can be seen in Figure 3, the countries are divided into 11 countries with great scarcity, like Bahrain and Kuwait where they do not have any water resources to compensate, and 8 countries with moderate scarcity, like KSA (Kingdom of Saudi Arabia), where they have around 5 billion m3 as surface and ground water, while the consumption is around 23.5 billion m3. Note the differences in the scale of the figure (*y*-axis) due to the huge difference in the available amounts of water in the great scarcity region and the moderate scarcity region.

**Figure 3.** Water scarcity in MENA countries, (**A**): 11 countries with great scarcity, (**B**): 8 countries with moderate scarcity (data adapted from References [27–29]).

#### *2.3. Design Path of Waste-to-Energy-to-Water System*

This section presents the pathway of the research and the configuration of the power plant as shown in Figure 4. The whole system has been simulated using the EBSILON 13.02 software package to achieve this work. EBSILON is the abbreviation for "Energy balance and simulation of the load response of power generating or process controlling network structures." It is used for engineering, attainment, preparation, checking, and plant optimization. It allows the arrangement of individual components, component groups, sub-systems, and complete systems within closed or open cycles.

**Figure 4.** Flowchart of the simulated power plant for waste incineration to produce power and potable water (W-to-W: waste to water by using heat in the multi-stage flash (MSF) process).

This system includes three main blocks. The first one is related to the waste incineration facility, which involves the combustion chamber (grate firing), a system for controlled and continuous input of waste to the grate, a duct for the flue gas, and bottom tanks to assemble residual unburnable materials (ash). There are different treatment streams for the residual ash. In developed countries, it is separated into two fractions, metallic and non-metallic residues, where the non-metallic fraction is used in the

buildings and streets industry, while the metallic fraction is considered as a source of metals and represents another revenue for the facility, otherwise it can be directly converted to landfills.

The second part is the core of this system, the steam Rankine cycle, which consists of high- and low-pressure steam turbines, a condenser, a deaerator to reduce the amount of oxygen that dissolves in the water and to increase the life span of the boiler and decrease maintenance costs, a pre-heater to increase the temperature of the water before it enters the boiler, harvesting thermal energy from the flue gas, and a super-heated steam generator (boiler).

The third block includes a once-through multi-stage flash plant, the design and technical aspects of which will be described later. This block consists of 16 stages.

An economic analysis of the cost of treatment for each ton of MSW has been calculated. The capital and operational costs of the plant were also assessed along with the cash flow during the lifetime of the plant (assumed to be 30 years) in terms of expenditure and income.

Finally, CO2 emissions were analyzed, to compare the emissions of the WI (waste incineration) and landfill.

As mentioned before, the second block consists of thermo-mechanical components. The technical properties of the main components are shown below in Table 3.


**Table 3.** Technical parameters for the main components in a power block.

The annual capacity of the waste incineration plant, LCV, and working hours throughout the year are shown in Table 4 below. The table also shows the live super-heated steam temperature and pressure, temperature of the flue gas, and steam temperatures of the extractions from the steam turbines.


**Table 4.** Properties of the power plant.

(QMSW: mass flow of MSW, LCV: Lower calorific value, Hp: High pressure, Lp: Low pressure).

MSF plants, where freshwater is separated from brine through evaporation, normally reach and exceed 20 stages. Before the first stage, a brine heater powered by hot steam from a steam generator is responsible for heating the liquid up to the first inlet temperature value, as shown in Figure 5. Basically, the higher this temperature is, the more the distillation rate rises, since a larger amount of vapor can then be extracted from the salt water [30,31].

**Figure 5.** The once-through multi-stage flash process.

In the simulated power plant, there were 16 stages of the MSF process, with one pre-heater before the stages. The capital cost of the multi-stage flash desalination process depends on different parameters, particularly the size of the facility and the salinity and temperature of the water. Table 5 below clarifies the variations in the capital and operational costs based on data from real projects in many different countries in the MENA region.


**Table 5.** Capital and operational costs for different MSF plants in different countries in the MENA region [32] (MLD: million liters per day, O & M: operation and maintenance, KSA: kingdom of Saudia Arabia).

According to the data in References [32,33], capital costs were taken to be US\$1100/m3 and operational costs US\$0.26/m3. As Jordan was chosen as a case study in this research, Aqaba city was selected to be a candidate city for the erection of such a project. Since it lies on the Red Sea, it would be a good source of seawater for the MSF plant.

Concentration salinity and temperature ranges used in this model are 42–46 ppt (parts per thousand) and 24–33 ◦C, respectively [32]. Table 6 illustrates all the parameters used in the EBSILON model to simulate the 16 stages of the MSF process. As can be seen, the salt concentration unit was converted to mg/L, and the feed seawater temperature was calibrated to 48 ◦C. This is higher than the original temperature of the seawater; as mentioned previously, a pre-heater was used to control and fix the influent temperature into the plant instead of working with a variant range. This is further discussed in the next sections.


**Table 6.** Technical parameters used in the model.

#### *2.4. Economical Evaluation of the Waste-to-Energy-to-Water System*

In general, cost and economic analysis include two main streams. The first one is related to expenditure representing the investment cost, which is returnable as annual payments during the first 12–15 years of the life span of the power plant [34], and operational costs, which are continued during the life span of the power plant as salaries of the working staff (engineers, technicians, managers, etc.), maintenance and replacement of the components, and many other things, as will be discussed later. The second stream is related to income, representing the income from power sales and water sales (for this case). Then, the financial budget (cash flow) can be calculated by taking the difference between the annual expenditure and annual income.

#### 2.4.1. Capital Costs

Capital costs vary with respect to several dominant factors: design of the power plant, its size (capacity), existence of the local infrastructure, and opportunities for selling energy (in terms of power prices). This system includes the power plant (waste incineration). There is also the cost of the multi-stage flash units to be added on to the investment. From that, the capital cost is divided into two parts:


Many studies [35,36] state the capital costs of waste incineration (WI) plants, but they vary greatly, as the cost of a waste-to-energy power plant changes depending on different dynamic parameters, such as plant capacity, waste composition, pre-treatment existence, and the flue gas cleaning system, which is related to the limitations and laws of air pollution emissions in each country. According to Reference [35], where the investment costs of all thermal treatment plants were embedded, including WI, the investment cost (capital cost) was in the range US\$400–700/ton of MSW/year. Of course, this range is related to the capacity (annual amount of waste which would be burned in the plant) and emission treatment technology. For this study, the capital cost of US\$400/ton has been selected to adjust and verify the economic analysis.

The last part of the capital cost analysis of the system is the investment in the water desalination (multi-stage flash) process. The capital cost of MSF is US\$1100/(m3/day). According to the specified capacities of the waste incineration plant and water production, the initial investment for approximately 650,000 ton/year and 8500 m3/day is US\$260 million and US\$10 million, respectively.

To calculate the distribution of capital cost investment, Equation (2) was used:

$$\text{CC}(t)\_{capital} = \text{CAPEX}.\frac{r\_i.(1+r\_i)^{t\_{dbtt}}}{(1+r\_i)^{t\_{dbtt-1}}}\tag{2}$$

where:

*C*(*t*)*capital* is the annual distribution of the capital cost, *ri*: Interest rate (%), *tdebt*: Year debt (year), *CAPEX*: Capital expenditure.

#### 2.4.2. Operational Costs

Operational costs are an important part of the economic analysis of any project, as these costs continue through the whole life span of the power plant, whereas capital costs vanish after a few years. For the simulated model, operational costs were very variant. In terms of waste incineration, these costs include: salaries, maintenance costs, a flue gas cleaning system, disposal of waste materials (since different technologies use various amounts of reagents and consequentially generate different quantities of waste material, such as ash and various residues from the flue gas cleaning process), and the operation of feed-water pumps and fans supplying primary and secondary combustion air [37].

Operational costs were assumed to be 10% of the total cost, including the operational costs of the MSF system. According to the literature [35,36,38], operational costs ranged between 4% and 11%, so for this work, they were taken to be 10%. The most variant component in the MSF operation system is the cost of preparing the water before it enters later stages, like de-aerating and adding chemicals for water purification [39].

The aim of this economic analysis is to show the potentiality of erecting a waste incineration plant in the MENA region and to connect it with producing desalinated water to utilize waste heat in an efficient pathway. Therefore, the cost for each ton of MSW to be treated in this power plant was calculated.

The levelized cost of electricity was analyzed according to Equation (3):

$$LEC = \frac{\sum\_{t=1}^{t} \frac{Lifc}{t-1} \quad \frac{C(t)\_{\text{capital}} + C(t)\_{\text{ipertim}}}{\left(1 + r\_d\right)^t}}{\sum\_{t=1}^{t} \frac{E\_{cl-y}}{\left(1 + r\_d\right)^t}} \tag{3}$$

where:

*LEC*: is the levelized cost of electricity,

*C*(*t*)*operational*: is the annual distribution of the operational cost,

*rd*: Discount rate (%),

*E\_*(*el*-*y*): Annual power production (MWe/year).

The inflation rate was taken into consideration as a fixed percentage in order to show its effect on the economic model. The equation below is to calculate the inflation through the whole lifetime of the plant:

$$P\_n = P(1+i)^n \tag{4}$$

where:

Pn: Total inflated estimated cost (US\$),

P: Base estimated cost (US\$),

i: Inflation rate (%).

To calculate income from power and water sales, the equation below was used:

$$I\_l = \text{365 D}\_t fAWMP \tag{5}$$

where:

*f*: Inflation rate (%),

*Il*: Income for first year (US\$),

*Dt*: Distillate water production (m3/day),

*AWMP*: Average water market price (US\$).

Many parameters have been applied in this model, many of them were assumed and others were found in the literature. Table 7 shows all parameters used in this model.


**Table 7.** Economic parameters used in the model.

#### *2.5. CO2 Emissions*

Waste incineration and landfilling processes come at the last two treatment steps in the hierarchy of waste management, with landfilling being the last option [40]. Both of these include different emissions of greenhouse gases with variant concentrations, like NOx, CO2, CH4, and others. For the purpose of this research, CO2 emissions were considered, as this gas is listed as a main greenhouse gas that needs to be mitigated or recycled according to the Kyoto Protocol and the Paris Agreement [41]. The variation in emitted concentration is related to the composition of the MSW in each country, which constitutes the element formation. Due to that, many studies gave different concentration values of CO2 emissions for each ton of MSW for each case (landfill and incineration) [42,43]. In this work, 840 kg CO2 for each ton of MSW to be landfilled was taken and 415 kg CO2 for each ton of MSW for waste incineration, according to References [43,44]. Unfortunately, as mentioned before, 90–95% of the MSW in the MENA region is going to landfill, so to show the effect of that in terms of greenhouse gas emissions, the capacity of the simulated WI power plant in this study of 650,000 ton/year has been used in landfilling and energy recovery (WI) cases. To calculate the amount of CO2 emission:

$$E\mathcal{L} = \sum \mu \mathcal{M} \mathcal{L}^{\xi} \tag{6}$$

where, *E*Ɖ is the emission concentration of the given gas, μƉ, *M* is the mass of the waste, and ξ is the exhaust gas volume.

#### **3. Results**

#### *3.1. Once Through-Multi Stage Flash Performance*

In this section, the results of the water plant are illustrated. The simulated facility was able to produce 23 MWe and 8500 m3/day of distillate water. With regard to the OT-MSF part of the process, the results show that the temperature of top brine (T0) reached its maximum value at the first stage (around 91 ◦C), then started to decrease in each stage within a rate of 2 ◦C, as the temperature dropped. It eventually dropped to 55 ◦C in stage 16, as shown in Figure 6. A pre-heater was also erected after stage 16 to increase and moderate the temperature of intake seawater, which ranged from 24 to 33 ◦C, as mentioned before. The temperature of intake seawater was increased from 48 ◦C in stage 16 to around 84 ◦C in stage 1 before entering the brine heater.

In terms of mass flow rate, distillate water was accumulated stage by stage, as presented in Figure 7. At stage 1, the amount of condensed water in the gathering tray was around 3 kg/s, at stage 2, it became 6.3 kg/s, and at stage 16, it was around 97 kg/s. At the same time, the mass flow rate of the brine water (seawater) was decreasing due to the evaporation process in each stage. After the evaporation in stage 1, the mass flow rate decreased to 1497 kg/s and continued to decrease until stage 16, where it was approximately 1400 kg/s. It can be concluded that the mass balance of the evaporated and condensed water was more or less 3 kg/s in each stage.

*Energies* **2020**, *13*, 2786

**Figure 6.** Temperature profile of the brine and intake seawater.

**Figure 7.** Flow rate distribution of brine water and distillate water in the MSF plant.

It should be noted here that the salt concentration of the seawater was increasing from 42,000 mg/L to 45,000 mg/L because during the evaporation of water through the stages, the water is transferring from a liquid phase to a saturated water vapor phase, leaving behind an increment in the concentration of salt in the brine water in the lower stages of the MSF process. It should be noted here that the produced water is ready to use in the industry sector, if the water produced was intended to be used as a source of potable water post-treatment and is required in order to comply with local health regulations, preventing the risk of biological growth. A number of drinking water regulations and guidelines define the concentration limits for several substances, which are potentially hazardous for human health. In terms of brine discharge, of course it will contain a high concentration of salinity and chemical compounds like calcium bicarbonate due to the pre-treatment process of the water, and rather, working with high temperatures in the evaporation stages of the plant, where the brine must be well-treated before recirculating it into the seawater source. The treatment process of the brine is not in the scope of this research. Figure 8 illustrates the whole simulated facility as waste

incineration plant and multi stage flash process, please note the pre-heater which was erected before the influent of seawater into the desalination plant to moderate its temperature around 48 °C.

#### *3.2. Economic Analysis*

The most important part of this study is the cash flow analysis in terms of expenditure, income, and configured treatment cost for each ton of MSW, in order to make the concept of this research affordable and applicable for the municipality. Note, this plant works on treating MSW by reducing its volume and recovering dispersed energy. This represents the core concept of the circular economy—to deal with waste as a source of fuel. Furthermore, this plant produces useful power and potable water, which is critical to solve the water scarcity in the MENA region, especially for countries like Jordan. Figure 9 shows the distribution of expenditure and income through the whole lifetime of the power plant. It should be noted here that the expenditures were illustrated with minus values and the incomes with plus values.

Expenditure represents the capital and operational costs, while the income includes sales of power and water. It can be seen that capital costs reduce over the first 15 years then completely vanish. At the first operational year of the plant, they are around US\$31 million and they reduce at a rate of around US\$2 million/year. The operational costs are the only fixed costs during the whole lifetime of the plant; as shown in Figure 9, this amount (operational costs) is approximately US\$26 million at the first year of operation. Please note that the effect of annual inflation rate has been embedded in this figure; therefore, there appears to be a yearly increase in operational costs and the same effect can also be noticed for the incomes. Also, the division of operational costs into fractional parts, i.e., maintenance, salaries, treatment of emission gases, and ash disposal, are detailed in the definition of Figure 9.

Formulated or configured gate fees, which should be paid by the municipalities to the WI investor (government sector, private sector, public private partnership), were analyzed and calculated by computing the annual expenditure and income, as illustrated in Figure 10.

As can be seen from Figure 10, one can calculate the cost of treatment for each ton of waste by taking the difference between expenditure and income then dividing the resulting value by the capacity of the WI plant in terms of MSW (in this paper, this was taken to be 650,000 ton/year). It can be seen from the figure that the cost of treatment decreases during the first 15 years, due to the paying back of capital costs, which represent the main component of expenditure.

Finally, it should be noted that in the first year, the treatment cost per ton is US\$64 and this decreases to reach US\$36 in year 15. At year 16, it is US\$25, which is a very important result in this year, when the *CAPEX* has been totally returned and the expenditure is only the operational costs. The increment starting from year 17 is related to the effect of the inflation rate, which was taken as a fixed value of 3%. According to the equations above, the levelized cost of electricity (LCOE) was also calculated and was found to be US\$241/MWh.

Finally, the CO2 emissions were also taken into consideration, as mentioned before. These were calculated according to Equation (6) and by taking the WI capacity (650,000 ton/year) as a scale factor to show the effect of landfilling according to that capacity. The available data about MSW treatment processes in the MENA region, where 7–10% of MSW is recycled and the rest is going to landfill, were also inserted in the calculations. The results show that the same amount of MSW going to landfill will produce approximately 490,000 Tons of CO2, while WI generates 269,000 Tons.

**Figure 9.** Expenditure and income distribution over 30 years. OPEX (20% salary, 40% emissions treatment, 23% maintenance, 15% ash disposal, 2% other). OPEX: Operation Expenses.

**Figure 10.** Annual distribution of treatment cost for each ton of MSW.

#### **4. Discussion**

The simulated plant in this research shows the ability to recover the content energy from waste and convert it to useful electric power through steam Rankine cycle power block. The system was able to produce 23 MWe and 8500 m3/day of desalinated water by utilizing the heat energy from one of the extractions of the steam turbine in Once-Through Multi Stage Flash, two configurations are available for MSF technology: OT-MSF and brine recirculation, where the amount of water production depends on the MSF configuration, number of stages, and temperature of the seawater; therefore, a pre-heater has been used to moderate the temperature of the seawater. By this, the efficiency of stages in terms of water production was increased and the effect of seasonal changes of the temperature of seawater was reduced.

Many studies have introduced the concept of Waste-to-Energy-to-Water. Udono et al. [45] developed a model in a simpler, understandable way to reduce efforts required for modeling complex multi-domain problems, which can be adapted to any local conditions by changing the local parameters. Jana et al. [46] utilized ASPEN Plus (AP for short, is the leading Chemical Process Simulator in the market) to model the polygeneration process of power and water from biomass. It was found

that biomass have strong potential for efficient delivery of several outputs with lesser/negative CO2 emission as a sustainable solution.

Dajnak et al. [47] stated that the concept of Waste-to-Energy-to-Water needs further study to optimize the conversion process and to assess the economy of the concept relative to competing desalination energy sources.

Pirotta et al. [48] investigated the potentiality of energy recovery from the MSW of Maltese for power generation and water desalination. The best scenario considered corresponds to a potential electric power of 10 MW or to a maximum 4.8 million m3/year of desalinated water, it was concluded that the incineration has the greatest potential to maximize revenues, due to the optimal combination of heat production and electricity generation.

Many studies suggested other sources of waste for power and water production. Mohammed et al. [49] used waste gases that emerged from oil refineries rather than burning them in the air, hybrid MSF-MED (Multi Effect Desalination) thermal desalination processes are utilized in this study to produce a total range of 100–40,000 m3/day. Ishaq et al.'s [50] trigeneration system for electricity, hydrogen, and freshwater production using waste heat from a glass melting furnace was illustrated in this work. It concluded that the world should be seeking for new sources of energy with lesser impacts on the environment to cope with all the challenges.

In terms of cost analysis and levelized cost of electricity, it was in the range between 235–87 US\$/MWh depending on the population density and on the analysis and the results which were shown in Reference [36]. While for a city with 600,000 inhabitants, the cost was found to vary between 113 and 183 US\$/MWh, Nordi et al. [51] studied various waste management scenarios considering incineration, recycling, and anaerobic digestion, and the generation cost was found to vary from 80 to 150 US\$/MWh.

However, the novelty of this research is that it introduced a comprehensive potential energy recovery from MSW for the MENA region generally and for Jordan specifically to produce power and desalinated water through a combined heat power cycle for waste incineration plants; furthermore, an integrated economic analysis for the treatment cost for each ton of waste has been illustrated and it was demonstrated how to calculate it by using cash flow, capital cost, and operational costs concepts.

#### **5. Conclusions**

In this study, many points have been highlighted while seeking to explore the concept of converting waste to power and generating water. There were two main targets of this work. The first was to illustrate an efficient usage for waste heat (steam) from the power plant to generate potable water. The MENA region suffers from great water scarcity, and Jordan, which was chosen as a case study in this work, is classified as the fourth worst-off country in the world in terms of water shortage. Note that the cost of this thermal energy is free of charge in the MENA region. Second, municipalities and responsible government institutions face many financial and technical problems in dealing with MSW; therefore, this research introduced an innovative concept to treat the MSW and also proposed a financial strategy through the economic analysis discussed earlier. It can be stated that the simulated system was able to generate 23 MWe and 8500 m3/day of potable water by recovering the energy content in MSW with 7 MJ/kg as LCV (elements fractions, as mentioned in Table 2). The mass flow rate of the distillate water was 97 kg/s as a total accumulation of 16 stages, average flow per stage was approximately 3 kg/s, and salt concentration increased from 42,000 mg/L to 45,000 mg/L, which is a normal effect due to evaporated water. In terms of economic analysis, it was found that the treatment cost for each ton of waste would be US\$64 during the first year of the plant's life, while in the second year, this would reduce to reach US\$61/ton. The cost would continue to decrease until, in year 15, it would be US\$36/ton. The lowest price would be in year 16, at US\$25/ton. Taking the average for 30 years, the annual treatment cost (gate fees) would be US\$39/ton.

**Author Contributions:** Conceptualization, Q.T. and A.N.; Methodology, Q.T. and A.N.; Software, Q.T.; Validation, Q.T. and A.N.; Formal Analysis, Q.T. and A.N.; Investigation, Q.T. and A.N.; Resources, Q.T.; Data Curation, Q.T. and A.N.; Writing—Original Draft Preparation, Q.T.; Writing—Review and Editing, Q.T. and A.N.; Visualization, Q.T. and A.N.; Supervision, A.N.; Project Administration, M.N.; Funding Acquisition, A.N. and M.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by DAAD German Academic Exchange service, as a part of the PhD scholarship grant for Qahtan Thabit.

**Acknowledgments:** The authors would like to thank Steag Company "Steag Energy Services GmbH" for their essential support through providing the Ebsilon software package to achieve this research.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **Research on the Kinetics of Pyrolysis of Wood-Based Panels in Terms of Waste Management**

#### **Tomasz Jaworski and Małgorzata Kajda-Szcze´sniak \***

Department of Technologies and Installations for Waste Management, Silesian University of Technology, 44-100 Gliwice, Poland; tomasz.jaworski@polsl.pl

**\*** Correspondence: malgorzata.kajda-szczesniak@polsl.pl; Tel.: +48-32-237-21-04

Received: 18 July 2019; Accepted: 26 September 2019; Published: 27 September 2019

**Abstract:** Currently, there is a lot of interest in implementing the idea of a circular economy along with searching for optimal methods of waste management in terms of raw materials and energy. Waste wood-based floor panels are part of this discussion with regard to its management. The interest in this waste results from statistics and the prediction of its future quantities on the waste market. The separation and testing of individual layers of the waste floor panel was undertaken to answer the following question: Is it reasonable to mechanically separate the contaminated upper panel layer from the remaining part (which is suitable for material recycling) and subject it to thermal transformation methods? Thermogravimetric studies did not confirm the rationale of mechanical separation of layers for further management. Therefore, the use of pyrolysis was proposed as an alternative by showing the advantages of this process in the thermal transformation of the tested waste. The analyzed kinetics of this process included: mass loss, the influence of heating rate on the decomposition process, the impact of volatile parts in the substrate on the rate of mass loss, and the share of coke residue. Empirical formulas of the tested substrates in the molecular formula C–H–O–N (carbon-hydrogen-oxygen-nitrogen) were also proposed to assess its energy usefulness by entering the analyzed waste into a Van Krevelen diagram.

**Keywords:** kinetics; pyrolysis; wood-based panels; Van KREVELEN systematics; TG-MS/FTIR analysis

#### **1. Introduction**

It is estimated that more than 10 million m3 of various types of wood materials are currently produced in Poland, over 3.5 million m3 of which are MDF (medium-density fiberboard) and HDF (high-density fiberboard) [1].

At a time of great emphasis on the recovery of materials according to the idea of a circular economy (CE) and following the hierarchy of waste management (obligatory for business entities) with regard to the post-consumer wood waste, such as floor panels, two options for their management were considered as below [2–5]:


The proposed methods fit into the idea of a circular economy, whose most important assumptions are presented below. The most popular definition of the circular economy was presented by the Ellen MacArthur Foundation: "A circular economy is one that is restorative and regenerative by design and aims to keep products, components, and materials at their highest utility and value at all times, distinguishing between technical and biological cycles" [6]. Another definition was provided in the publication [7], i.e., a circular economy is "a concept used to describe a zero-waste industrial economy that profits from two types of material inputs: (1) biological materials are those that can be reintroduced back into the biosphere in a restorative manner without harm or waste (i.e: they breakdown naturally); and, (2) technical materials, which can be continuously re-used without harm or waste" [8]. According to the authors of the publication [9], the circular economy shall be regarded as a "project and business modeling strategy that is, slowing, closing and narrowing the resource loop".

The circular economy in [10] is defined as "a regenerative system in which input resources and waste, emissions and energy leaks are minimized by slowing, closing and narrowing the material and energy loops. This can be achieved through long-term design, maintenance, repair, reuse, regeneration, renewal and recycling."

To a large extent, the circular economy in waste management is currently focused on resource recovery, the rational management of non-renewable resources, and environmental impact prevention. In the past, waste management was understood only in terms of storage and incineration [5,11].

The former of the aforesaid methods could include two processes. The first mechanical one involves the roughing (rupture, ...) of the upper layer, which is theoretically more contaminated with chemical impurities—mainly urea–formaldehyde resins—due to its exposure to direct use and requirements on abrasion, waterproofness, etc. This layer would require thermal transformation. The second process is related to the layers lying below the upper one. Since they seem to be less contaminated, they should be managed through material recycling. The latter method is to submit the entire waste to the process of thermal transformation. The choice of pyrolysis as a method of thermal transformation of waste is not accidental, since more and more attention is being paid to its nature and ability to gain products in the form of gas, combustible oils, and charcoal. Although the Polish legislation requires the combustion of pyrolysis products, there are other ways to use these raw materials, such as for example in the production of methanol: R3 Recycling or the recovery of organic substances that are not used as solvents (including composting and other biological transformation processes). This heading also includes gasification and pyrolysis using these components as chemical reagents [2]. This article does not consider the possibility (due to the lack of literature data) of the return of waste floor panels to the manufacturers of these products and their possible inclusion in the production cycle under the so-called internal recirculation.

A methodology for determining the kinetic parameters of waste pyrolysis for fuels with an unidentifiable chemical formula of the basic molecule (e.g. post-consumer floor panel) has been proposed. The methodology can be helpful for comparative studies between various solid fuels and also waste with the general formula CxHyOz. The location of the molecule, which was determined with this method, in the Van Krevelen diagram gives an excellent configuration of its energy status. This status can be changed subsequently through operations such as drying, mechanical drainage, heating, etc. in order to enhance energy properties (mainly calorific value by improving the ratio of fuel elements h/c (hydrogen/carbon) and o/c (oxygen/carbon)).

#### **2. Thermodynamic and Kinetic Aspects of Pyrolysis**

Pyrolysis is a process of thermochemical decomposition in an anaerobic atmosphere. It is a complex process of the decay of chemical compounds into smaller molecules under the influence of external thermal energy [12,13]. Most reactions that occur during pyrolysis are endothermic. In general, the pyrolysis process can be represented by reaction (1):

$$\text{CuHmOp}-\text{heat}-\text{CuHyOz} + \sum \text{CaHbOc} + \text{H}\_2\text{O} + \text{C (chracol) (liquid) (gas)}\tag{1}$$

As a result of the pyrolysis, three fractions are formed [12,13]:


• gas fraction, which is a mixture of CO2, CO, H2, and hydrocarbons, mainly methane.

The literature presents different conditions for the process and the impact of these parameters on the products obtained [14–16]. The basic division into types of pyrolysis is related to the rate of heating of the fuel molecule. Slow pyrolysis occurs when the time required to warm the molecule to the pyrolysis temperature is significantly longer than the characteristic pyrolysis reaction time (theating >> tr). When the situation is reversed, this process can be defined as fast pyrolysis. Other parameters that affect the quality and quantity of products are: a maximum temperature, the residence time of the primary decomposition products in the conversion zone, a degree of fuel fragmentation, the reactor heating method, and its construction. Table 1 presents types of pyrolysis and characteristic parameters influencing the decomposition, while Figure 1 shows the possibilities of the course of pyrolysis depending on the parameters in which the process is carried out.


**Table 1.** Types of pyrolysis and characteristic parameters influencing the decomposition [13].

Pyrolysis as a method of obtaining fuels is a promising technology for the conversion of indirect wood-based waste such as floor panels. This type of waste can be found in the waste catalogue under the codes: 03 01 05, 17 09 04, and 20 01 38. The code allocation depends on the source of the waste floor panels [17].

Pyrolysis is a very complex process and, depending on the parameters of its operation, it is possible to obtain solid, liquid, and gas products in various weight ratios. The schematic diagram of the possibilities of the process, depending on the pressure, temperature, and speed of the process, was presented in work [13,18].

#### *2.1. Heat Demand for the Pyrolysis Process*

Although basically pyrolysis is an endothermic process, during its duration, there are effects of both heat extraction and its emission. At the time of pyrolysis, pyrogenetic water is formed, which is the result of the reaction of hydrogen with oxygen, as well as the result of the disintegration of hydroxyl group side chains. However, decomposition reactions are not the only chemical processes that occur during pyrolysis. The gases formed from the solid penetrate the remaining part of the solid phase (most often the char) and react with them. Therefore, it is difficult to determine clearly what is the thermal effect of the pyrolysis itself, and what are the effects of secondary reactions occurring between pyrolytic gases and the surface of the char [13]. The energy balance of the pyrolysis process is prepared in accordance with the first law of thermodynamics, from which it follows that if any transformation in the system changes its state and the system exchanges energy with the environment only through heat transport, then the relationship follows the equation below (2) [19]:

$$Q = \Delta l v \tag{2}$$

where:

Δ*h* – increase of enthalpy of the system

*Q* – heat exchanged with the environment

The above indicates that the change of enthalpy of the system is equal to the heat supplied, the quantity of which corresponds simultaneously to the heat demand for pyrolysis. In this case, possible heat losses to the environment are not taken into account. Thus, it is possible to use the calculation methodology proposed by [19], which is based on the balance of the enthalpy of substance formation, and takes into account the physical enthalpy of products in appropriate physical states.

Pyrolysis heat is determined on the basis of temperature measurements in DSC (Differential scanning calorimetry) scanning calorimeters. The results obtained in this type of device are a net measure of pyrolysis and secondary reactions. Pyrolysis heat is the difference in the energy of the formation of reagents and products at the reference temperature (298.15 K). The heat values of pyrolysis reported in the bibliography differ significantly from 750 kJ/kg (endotherm) to 130 kJ/kg (exothermic reaction). For example, pyrolysis heat in the range from 274 kJ/kg to 353 kJ/kg was determined using DSC [13]. Bibliography [13] presents thermogravimetric measurements for pine wood up to 700 ◦C, with a heating rate of 20 ◦C/min. After cooling, the samples of charcoal being still in the thermogravimeter were heated once again at the same rate. Shown the power consumed by pine wood and the charcoal during pyrolysis in TGA (Thermogravimetric analysis) as a function of temperature. After integration in time with the power difference consumed by the material, the thermal effect of the process is obtained. The difference among the thermal effects for wood and charcoal relates to pyrolysis heat, and it amounts to 1473 kJ/kg. This effect is clearly higher than the effect resulting from the above data.

#### *2.2. Kinetics of the Pyrolysis Process*

Fuels formed on the basis of waste floor panels are a complex solid substance, and there are thermal transformations in the hard-to-define structure of C–H–O–N molecules (which changes texture during the process) because it is difficult to implement a model description including reaction range, autocatalytic effects, diffusion effects, and structural changes. In the bibliography, one can find various approaches to pyrolysis kinetics; however, the basis for any analysis is the distribution reaction model, which occurs according to the following entry (3). This is as a single irreversible reaction of the thermal decomposition of carbon, but it is also helpful in case of a carbon-rich substance:

$$\text{Carbon} \longrightarrow \text{k }\lambda\text{volatile\\_products} + \text{(1-\lambda)}\text{charcoal} \tag{3}$$

This entry is often regarded in the literature of the subject as too simplified, because it omits important transitional stages of the substance distribution, including, first of all, subsequent and possibly parallel reactions of the distribution of primary products, such as coal tar (metaplast). For this reason, many forms of kinetic equations have been formulated, also by dividing pyrolysis into individual stages. However, a comparison of various schemes of the decomposition of organic carbon leads to the conclusion that regardless of the method of receiving final products, the formula expressed by the dependence (3) reflects the final result of pyrolysis. As a consequence, many authors approximate the process of decomposition of a substance rich in organic carbon with the first-order reaction occurring uniformly in the entire volume of the particle [20]. The speed of extracting volatile parts can be described in this case by the Formula (4):

$$dV/dt = k\left(V\_0^{\text{daf}} - V\right) \tag{4}$$

The most common source of information on pyrolysis kinetics are data obtained from thermogravimetric and derivative thermogravimetric measurements (TG/DTG). The results of such measurements under dynamic conditions indicate the existence of several process stages characterized by different quantities of apparent activation energy. The stages of various kinetics are attributed to the pyrolysis of the individual components of which the floor panels are made, including: wood-based HDF (high-density fiberboard) boards, urea–formaldehyde resins, and laminates [21].

#### *2.3. Van KREVELEN Systematics and Wood-Based Waste*

The original intention and achievement of Van Krevelen [22] was the presentation of classical fuels in a plane coordinate system, where the atomic quotients of O/C were given on the abscissa x and on the ordinates y = H/C (excluding N, S, P, and micronutrients). Assuming that carbon is tetravalent, the maximum value of the ordinate is 4, and the abscissa x = O/C ≤2. The Van Krevelen diagram was later modified by Meunieur [23], who adopted the total chemical formula CHyOx (x, y coordinates from the diagram) for these fuels. Modification made by Meunieur showed the chemical structure more accurately than the percentage mass fraction of elements C, H, and O [24].

When analyzing wood-based waste (and knowing their chemical composition), it was possible to determine their picture in the Van Krevelen systematics. The aim was to define the place of this waste and its energy suitability in comparison to classical fuels. To determine the waste position in the Van Krevelen diagram, it is necessary to calculate the atomic quotients x and y for dry organic matter of waste without sulfur and phosphorus in accordance with the condition:

$$\rm C + N + O + H = 100\% \tag{5}$$

where:

C, N, O, and H – content of elements in the sample in percentage by weight.

At the stage of laboratory tests, it was possible to determine the content of all four elements required by the procedure. To find an illustration/point in the diagram for waste, it is necessary to separate the content of these elements [24,25]. Determination of the empirical formula of wood-based waste ground on the percentage content of elements was based on the conversion of this content into molars of elemental atoms. The obtained values were divided by the lowest number in the ratio, which was in this case 1.071. Ultimately, the empirical formula is as follows:

$$\text{C}\_{4}\text{H}\_{5}\text{NO}\_{2}\tag{6}$$

**Figure 1.** Illustration of the position of waste from floor panels in the Van Krevelen system (author's own study) [24,26].

Figure 1 presents the location of wood-based waste (floor panels) in the Van Krevelen diagram. The coordinates of the point characterizing wood-based waste were calculated according to the formulas [24,25]:

$$\text{\textbullet X} = \text{OM}\_{\text{o}} / \text{CM}\_{\text{o}} = 0.62 \tag{7}$$

$$\text{Y} = \text{HM}\_{\text{c}} / \text{CMH} = 1.27 \tag{8}$$

where:

MC, MH, and MO – molar masses of carbon, hydrogen, and oxygen;

C, H, O – content of elements in the dry organic mass of the residue in percentage by weight.

#### **3. Materials and Methods**

Tests TG-MS/FTIR were carried out on samples with the following assay [27]:


These tests were carried out by means of the STA 409 PG Luxx thermogravimetric analyzer from Netzsch on a TG (Thermogravimetry) carrier coupled with a quadrupole mass spectrometer QMS Aeolos and a medium infrared spectrometer FTIR (Fourier Transform Infrared Spectroscopy) Tensor 27 from Bruker. The tests were carried out in an argon atmosphere (purity class 5.0, flow 25 mL/min), from 40 to 1000 ◦C, with a heating rate of 10 K/min. The sample weight was 10 ± 0.1 mg. The test was performed in crucibles with Al2O3 with a lid. In order to eliminate traces of oxygen in the thermal mixer, the OTS (Oxygen Trap System) system was used. The measurement was corrected with corrective measurements (measurement without a sample). The corrective measurements were carried out three times.

This paper presents an analysis of the physicochemical properties of selected post-consumer wood waste and urea–formaldehyde resin. The goal of the analysis was to determine specific fuel properties, such as moisture, ash content, content of volatile components, calorific value, and elemental composition (C, H, O, N, S, Cl).

The characteristics of the physicochemical properties of the tested samples are presented in Table 2.


**Table 2.** Physicochemical properties of samples.

Ppo—below the limit of quantification.

The individual layers of the floor panel are characterized by similar humidity of 5–6%, while pine wood is characterized by humidity at the level of approximately 14%. All the samples had a low ash content of less than 4.12% and a high content of volatile matter in the range of 78–82%. The wastes are characterized by a calorific value at a similar level, i.e., above 17 MJ/kg. In all the layers of the panel, there was a high nitrogen content in the range from 5.19–8.08%, while a low nitrogen content was recorded in pine wood. The tested wastes had low sulfur and chlorine content.

#### **4. Results and Discussion**

Figure 2 shows the weight loss curves for individual samples. It reveals the unexpected similarity between the weight loss curves of the pure wood sample and the sample representing the upper layer of the panel, as well as three curves of samples: the bottom and the middle layers and the sample of the averaged complete panel composition. In the case of waste floor panel pyrolysis, usually three stages of thermal decomposition are identified: in the first stage (from environment temperature to about 140–180 ◦C), the samples lose moisture; in the second stage (from about 160 to 400 ◦C), the material pyrolysis takes place; the third stage, which is clearly slower than the preceding one, occurs at a temperature of about 400 to 700 ◦C. The divergence from the three-stage decomposition was observed at the sample of a urea–formaldehyde resin, which decomposed in five stages occurring in the following temperature ranges: 40–120 ◦C, 120–200 ◦C, 200–260 ◦C, 260–300 ◦C, and 300–450 ◦C.

**Figure 2.** TG (thermogravimetry) curves of all the samples tested (own research).

Figure 3 shows the weight loss rates of individual samples. All the samples of different layers of the floor panel have a similar velocity of mass loss ranging from 6.145 to 7.792%/min, and the largest distribution occurs at the temperatures of 35.4 to 357.1 ◦C. This creates good conditions for simultaneous decomposition at almost the same temperature. A slightly different rate of decay characterizes clean wood and it amounts to 4.097%/min. The chemical additives in the panel increase the weight loss rate almost twice.

Figures 4–6 present representative results regarding TG, DTG curves, and ionic current (MS). Studies show that water (ion m/e = 18, blue line) is emitted from ZMT/20-24/2019 samples in two stages: when the samples lose moisture in the temperature range from the environment temperature to about 180 ◦C, and in the range from about 250 to 450 ◦C. The other main products of pyrolysis, such as: carbon monoxide (m/e = 28, purple line), carbon dioxide (m/e = 44, yellow line), methane (m/e = 16, red line), and nitric oxide (m/e = 30, green line) are emitted from samples in the range from about 200 to 500 ◦C. In the case of the ZMT/25/2019 sample (urea–formaldehyde resin), the temperature ranges in which individual products are released are slightly different, and also, ions were recorded: m/e = 17 (most likely ammonia – light green line) and m/e = 57 (light blue line).

**Figure 3.** Derivative thermogravimetric measurements (DTG) curves of all samples tested (own research).

**Figure 4.** TG and DTG, curves and ion current intensity for the sample ZMT/20/2019 (own research).

**Figure 5.** TG and DTG curves, and ion current intensity for the sample ZMT/24/2019 (own research).

**Figure 6.** TG and DTG curves, and ion current intensity for the sample ZMT/25/2019 (own research).

The combination of thermoanalysis with the analysis of gaseous products has provided knowledge of the mechanisms that occur in the examined waste during thermal processes and will enable the qualitative assessment of gases emitted during heating with regard to the chemical composition.

Figure 7a,b show the course of the curves of the emission of selected gases as a function of temperature with the TG curves for the sample of the charcoal by using the FTIR spectrometer data during the pyrolysis process of the samples: ZMT/20/2019 and ZMT/24/2019. It is clearly evident that ammonia and nitrogen dioxide are emitted during the pyrolysis process of the waste floor panel. This phenomenon is not observed in the case of the pyrolysis of pure pine wood.

**Figure 7.** TG curve and curves of emissions of carbon monoxide, methane, nitrogen dioxide, and ammonia for the samples (**a**) ZMT/20/2019 and (**b**) ZMT/24/2019 (own research).

This phenomenon is caused by the addition of urea–formaldehyde resin to the floor panels, which contains about 37% nitrogen in the composition [28].

The products of pyrolysis depending on the conditions of the process (which include heating speed, temperature, residence time, granulation of the material, etc.) are as follows: gas (CO, CO2, CH4,...), pyrolysis oil, and carbonizate (carbonized solid). For example, increasing the heating rate directs pyrolysis products toward the production of bio-oil, and long-term heating is used to produce carbonized solids. All pyrolysis products have a positive chemical enthalpy (calorific value), so they can be used for energy, e.g., in the production of heat and electricity or cogeneration. However, pursuant to the Polish legislation, products of the waste pyrolysis process should ultimately be oxidized and incinerated. There is also a possible legal path for another use of pyrolysis gases for the production of chemicals, e.g., in methanol production.

#### **5. Conclusions**

Mechanical separation of the panel layers for the purpose of different management of the upper layer (thermal transformation) and lower layers (recycling) is not reasonable. This thesis is justified by the following:


The thermal transformation (combustion/gasification/pyrolysis) shall be indicated as the only reasonable method of management of floor panels.. All these methods must deal with the issue of emissions of the contaminants as a result of decomposition or the secondary reaction mechanisms generated by the addition of urea–formaldehyde resin.

The problem of the combustion of floor panels in the layer on the grate of the waste thermal treatment installation (ITPOK) was the subject of description in the publication [21,29–31].

The proposed methods of thermal decomposition of floor panels using pyrolysis indicate the advantages of this process as evidenced by:


The research on pyrolysis should be further developed with solutions reducing the NOx and NH3 emissions from this process.

Post-consumer wood in the form of floor panels, MDF boards, etc., is a hazardous waste due to the content of chemical substances; however, these substances improve their functional qualities (hardness, water resistance, etc.). In this study, an attempt was made to check how contaminated the waste is and whether at least part of it is suitable for such management that is different from thermal management. Analyses of subsequent sections of the tested waste showed almost equal contamination with urea–formaldehyde resin in the panel cross-section. Therefore, it is recommended to direct this waste to organized thermal disposal. Pyrolysis seems to be a good solution here, especially since its products, depending on the mode of its conduction and the heating rate applied), can be also used as raw material for the production of chemicals.

Kinetic studies should be related to the thermodynamic analysis of pyrolysis. It is suggested to continue research on the basis of compounds: the enthalpy of creating substrates—which is an energy of process activation—oxygen content, and volatile parts in substrates, as well as the heating rate in the process (type of reactor for conducting pyrolysis).

**Author Contributions:** Conceptualization, T.J.; formal analysis, T.J.; writing, T.J.; funding acquisition for research, M.K.-S.; investigaton M.K.-S.; project administration M.K.-S.; resources M.K.-S.

**Funding:** This research was funded by the InnoEnergy. Its publication was supported as part of statutory research carried out at the Silesian University of Technology 08/030/BK\_18/0045.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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