**1. Introduction**

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Waste plastics, such as polystyrene (PS), polypropylene (PP), and polyethylene (PE), are usually disposed together and so require time for sorting. Polystyrene articles (e.g., Styrofoam, food trays, and packing materials) occupy a large volume at a very low weight. This results in inefficient transportation of waste plastic. In addition, subjecting waste plastics to pyrolysis, which involves conversion of plastic to oil requires energy and is expensive. It would be useful to reduce waste plastic volume and to recover the energy of waste polystyrene recycling by a simple process. In this chapter, a simple process is proposed to obtain liquid fuel dissolved polystyrene in biodiesel at lower energy costs.

As shown in Figure 1, waste polystyrene such as expanded polystyrene is selectively dissolved in biodiesel derived from waste cooking oil, and biodiesel-dissolved polystyrene is utilized as diesel fuel for a cogeneration system. This chapter describes the potential for selective dissolution of polystyrene in biodiesel, which consists of fatty acid methyl esters (FAME), by using Hansen solubility parameters and for properties and combustion characteristics as diesel fuel by using fuel ignition analyzer.

The results obtained by Hansen solubility parameters indicated that methyl oleate, the main component of biodiesel, provides selective solubility for polystyrene. Results of experiments that examined solubility properties revealed that the kinematic viscosity increased with polystyrene concentration in FAME derived from soybean cooking oil. The cause of the increase in kinematic viscosity was due to the presence of a polymer with a molecular weight of up to approximately 107 .

On examining the use of dissolved polystyrene as fuel for diesel engines, experiments showed that the cetane number decreased with an increase in polystyrene concentration; however, the cycle-to-cycle fluctuations in combustion pressure and ignition timing when polystyrene dissolved in FAME was used were less than those found for neat FAME. Polystyrene dissolved

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in FAME crystallized and precipitated in the gel upon addition of hydrocarbons such as engine lubricant oil. Therefore, the use of FAME-dissolved polystyrene as diesel fuel requires attention to tribology.

**Figure 1.** Energy recovery using biodiesel fuel (FAME) derived from waste cooking oil selective-dissolved waste poly‐ styrene (PS).

Biodiesel consists of fatty acid methyl esters (FAME) and can be produced from a great variety of feedstocks including vegetable oil (e.g., soybean, palm, rapeseed oil) and animal fats, as well as waste cooking oils (e.g., used frying oils). FAME has been used as alternative fuel of diesel fuel. Many research projects have been carried out and have been published in books and journals [1-3]. Most of all are concerning to fuel quality, combustion and exhaust emission characteristics, which regards the utilization with the diesel engines.

Effective use of waste plastics is important in establishing a recycling-oriented society. FAME can dissolve plastics and rubbers, which are used as fuel system parts of diesel engines, resulting in the need to replace these parts. If a method to utilize FAME as a solvent to dissolve plastics can be developed, waste plastics could be recovered and could be utilized as liquid fuel.

Several studies on solubility report that *n-*alkenes and di-*n-*alkyl are better solvents for the low and medium molecular weight samples of polystyrene than the corresponding *n*-alkanes [4]. Also, it is shown that certain food items have been shown to be incompatible with the expanded polystyrene (EPS) used for the manufacture of food containers. Citronella, limonene and terpinene, which are constituents of many flavor oils, are excellent solvents for polystyrene [5].

Studies of solubility for volume reduction and waste management of polystyrene recycling have been conducted earlier [6,7]. Solubility values of extrude poly-styrene(XPS) in several solvents such as benzene, toluene, xylene, tetrahydrofuran, chloroform, 1,3-butanediol, 2 butanol, linalool, geranoil, *d*-limonene, *p*-cymene, terpinene, phellandrene, terpineol, metha‐ nol, eucalyptus, cinnamaldehyde, nitrobenzene, N,N-dimethylformamide, and water have been determined. The solubility of the polymer in the mentioned solvents at different tem‐ peratures has been investigated. The solvent can be easily recycled by distillation.

Polystyrene dissolved in FAME has a greater heating value as fuel than that of FAME alone, so that a reduction of the fuel consumption per output and the increase in the use of diesel and boiler cogeneration can be expected. The diesel combustion characteristics of soybean oil FAME-dissolved packing peanuts has been investigated by Kuzhiyil et al. [8]. The results showed that engine power increased initially with the polystyrene concentration and then decreased at concentrations greater than 5%. The decline in engine power at high polystyrene concentrations could be caused by the poor spray atomization and deteriorated combustion efficiency due to the high viscosity of polystyrene mixtures.

Studies on the use of soy methyl ester-polystyrene (SME-PS) blends to increase durability of concrete have been conducted [9,10]. Experimental results show that SME-PS reduces water absorption of concrete (by up to 75%), protects from damage caused by freezing and thawing (reduces damage by 66%, reduces chloride ingress depth by up to 50%, and slows the rate of the alkali-silica reaction by 50%.

The method for solubilizing polystyrene in a fatty acid ester has been granted as a US patent [11]. The purpose of the invention is to produce commodity material useful for the production of polymers from polystyrene dissolved in fatty acid esters. This patent also provides the method for disposing of polystyrene solubilized in a fatty acid ester. A fatty acid ester composed of a variety of alkyl and alkene esters is used and heated to temperatures between 100° C and 180° C.

Expanded polystyrene (EPS) is used for packaging material to cushion appliances and containers of seafood and agricultural products. Extruded polystyrene (XPS) board is used as insulating material in building. The production of EPS in the world came up to 10 Mt in 2010. Presently, various EPS organizations from more than 25 countries around the world have subscribed to the international agreement on recycling of EPS. In Japan, EPS of 1.1 Mt was produced and 88% of waste EPS was recovered by means of material recycling and energy recovery in 2010; however, the EPS trays with food residue were not recovered. Almost all were sent to landfill sites or were incinerated. EPS and XPS are low density, high volume materials, which consume a significant amount of space in a landfill. The transport efficiency of waste polystyrene is poor because its volume-to-mass ratio is large. Reducing the volume of waste EPS and XPS will reduce CO2 emissions and decrease the cost of energy per volume due to the transportation of waste plastic.
