# Polyhydroxyalkanoates (PHAs): Biopolymers for Biofuel and Biorefineies
## 3 Biofuels Based on PHAs  
Due to the scarceness of traditional fossil fuels, increased emissions of combustion pollutants, and their costs, biomass-based fuels have become an attractive alternative option. Biofuels refer to the fuels produced from organic materials such as plants and animals. Table 5 lists the types of biofuels used for energy generation [119]. Studies have shown that the depletion of fossil fuels and environmental concerns increased the interest among scientists to explore renewable biofuels that are environmentally friendly and more acceptable. More importantly, in recent years the worldwide interest in biofuels has increased enormously [120]. To date, several biofuels derived from biomass have been explored including hydrogen (H\({}_{2}\)), methane (CH\({}_{4}\)), methanol (CH\({}_{3}\)OH), ethanol (C\({}_{2}\)H\({}_{5}\)OH), biodiesel, acetone (C\({}_{3}\)H\({}_{6}\)O), and several others. Globally, the transportation industry is a major source of carbon dioxide (CO\({}_{2}\)) emission and energy consumption. Liquid biofuels can replace fossil fuels in the transportation industry [121]. Liquid biofuels, such as biodiesel and bioethanol, have numerous advantages including ease of production from common biomass, eco-friendly, biodegradable, and sustainability. However, the extensive use of these biofuels created the food vs. fuel controversy, while their high production costs restricted their large-scale application in the transportation industry. In 2009, [PERSON] et al., [122] introduced a novel biofuel, hydroxyalkanoateates methyl ester (HAME) and hydroxybutyrate methyl ester (HBME), derived from bacterial PHAs (Figure 7). HA esters have chemical structures similar to those of biofuels, especially biodiesel, consisting of the methyl ester of long-chain fatty acids. HA esters are different from petroleum: petroleum is enriched with low oxygen (O\({}_{2}\)) and high nitrogen (N\({}_{2}\)) and sulfur (S) contents leading to the generation of environmental pollutants upon combustion. In contrast, HA esters have a high O\({}_{2}\) content without any N\({}_{2}\) or S. In general, oxygenated additives can reduce the exhaust smoke for diesel as well as decrease combustion times and ignition delay. However, oxygenated additives, like HA esters, have a low cetane number and a high heat of vaporization which makes it difficult to completely fuel diesel engines. Therefore, the introduction of HA esters as sustainable fuels or fuel additives contributed to the divergence of the biofuel market [122]. In contrast, the use of PHAs as biofuel sources becomes very promising, as highly purified PHAs are not needed. Thus, PHAs can be produced from the sewage or industrial wastewater, which is not in competition with human or animal food sources, leading to cost reduction. [PERSON] et al., [122] reported the production of HAME or HBME via the esterification of mcl PHAs and P3 HB by acid-catalyzed hydrolysis (Figure 8). Combustion heat is one of the most important benchmarks to assess the quality of a fuel. The combustion heat of HAME, HBME, and their blends with diesel, gasoline, and C\({}_{2}\)H\({}_{5}\)OH were compared. The results demonstrated that when HBME was blended with C\({}_{2}\)H\({}_{5}\)OH, the combustion heat of C\({}_{2}\)H\({}_{5}\)OH improved by up to 30%. However, the combustion heat was reduced when HBME was blended with gasoline and diesel. Furthermore, HBME and HAME were obtained via acid-catalyzed hydrolysis and the most commonly used acid catalyst is sulfuric acid (H\({}_{2}\)SO\({}_{4}\)). Moreover, until now, different concentrations of H\({}_{2}\)SO\({}_{4}\) with methanol were used to reach the maximum yield of HBME or HAME. [PERSON] et al., [123] documented the optimized conditions for HBME production from P3 HB by acid-catalyzed hydrolysis. They produce P3 HB from _A.