Conventional and in-situ transesterification of Annona squamosa seed oil for biodiesel production: Performance and emission analysis

Abstract

This research paper deals with the production of biodiesel from renewable Annona squamosa (A. squamosa) seed oil. Reactions were carried out through conventional and in-situ processes in the presence of H${}_2$SO₄ as a catalyst. The methyl ester content was quantified by H¹NMR (Proton Nuclear Magnetic Resonance spectroscopy). The highest biodiesel conversion of 88% was obtained from conventional reaction and the optimized process parameters were catalyst concentration of 3.5 wt%, 9:1 of methanol to oil molar ratio and reaction temperature of 60 °C. Similarly, 84% conversion was obtained for in-situ process, where the optimum values were catalyst concentration of 5 wt%, 15:1 of methanol to oil molar ratio, 60 °C and 30 wt% of co-solvent. The fuel properties of A. squamosa biodiesel were assessed as per American Society for Testing and Materials (ASTM) standards. It was found that the conversion in conventional process was higher and the same was used for experimental investigation on the compression ignition engine to analyze the performance and the emission characteristics of A. squamosa biodiesel and its blends with diesel. Comprehensive analysis revealed that B20 improved the thermal efficiency of the engine and had lower exhaust emissions.

Introduction

Developing an alternative and sustainable energy supply is the framework of every country because fuel plays an important role in its economy. The need for alternative fuels is increasing steadily because of the depletion of conventional petroleum reserves and the subsequent impact of exhaust gases from petroleum engines on the environment.

A promising renewable fuel comprising mono-alkyl esters of long chain fatty acids derived from vegetable oils is known as biodiesel (Zahan and Kano, 2018). Biodiesel has received more attention because it is biodegradable, non-toxic, renewable and environmental friendly. Blending of diesel–biodiesel​ results in improved combustion efficiency because the oxygen content increases by the presence of ester group. Modification of engines is not required and it can be mixed with petroleum diesel in any proportion. Transesterification of vegetable oils with low molecular weight alcohols like methanol with the help of acid or base catalysts results in the production of biodiesel. Generally, methanol and basic catalysts are used across the globe to produce biodiesel. When compared to petroleum fuel, biodiesel has a lower emission of carbon monoxide, particulate matter, oxides of sulfur and unburned hydrocarbons. The higher flash point of biodiesel (150 °C), makes it less volatile and safer to transport or handle. Carbon dioxide emission of B100 is 78.45% and B20 is 15.66% which are lower than that of petroleum diesel (Srivastava and Prasad, 2000).

Relatively high yield per hectare and widespread production of renewable energy sources, such as sunflower oil, palm oil, corn oil, and soybean oil have gained much attention and are being studied intensively by numerous investigators (Carraretto et al., 2004, Dias et al., 2021, Hamza et al., 2021, Melikoglu and Cinel, 2020, Vicente et al., 2004). However, these sources cannot meet the present day demand for fuels because of the vast use of farm land and inflated food prices (Chhetri et al., 2008, Chisti, 2007, Kim et al., 2004, Patil et al., 2008, Thompson, 2012). Various organizations have objected to biodiesel production from edible oils because biodiesel contends with food industry. The overall cost for biodiesel production from refined vegetable oils is about 70% (Haas et al., 2006). Biodiesel production from waste cooking oil may affect biodiesel quality. The cost and the availability of the feedstock are the major challenges that biodiesel faces in spite of these significant advantages (Barua et al., 2020). However, the critical issues faced are the cost of the vegetable oils and the production capacity. The use of cheaper raw materials such as substrates for biodiesel production is preferred because the monoesters of vegetable oils are more expensive than the corresponding petroleum products.

To overcome this problem, non-edible oils play an important role in providing a new alternative source of feedstock such as karanja, jatropha, castor, neem, rapeseed, etc. (Canoira et al., 2010, Mogilicharla and Reddy, 2020, Sarin et al., 2007, Tapanes et al., 2008, Thapa et al., 2018). Vast land and technology are required for the development of these alternative raw materials. Free fatty acids (FFA) concentration and water content increase because of improper handling like exposure to air. Low yield, soap formation and products separation will be very difficult when the FFA content is more than >1% w/w. Two alternative approaches are normally used for feedstocks with a higher FFA content. The first step is to reduce the FFA content of the oil by acid-catalyzed esterification, followed by a base-catalyzed transesterification of the triglycerides (Ghadge and Raheman, 2005, Veljković et al., 2006). The second is the acid catalyst process that simultaneously catalyzes both esterification and transesterification reactions (Thangaraj et al., 2019). An alternative process for higher FFA content is the acid-catalyzed esterification of the oil but it is much slower than base-catalyzed reactions (Berchmans and Hirata, 2008, El-Mashad et al., 2008). Reactor corrosion, substantial generation of wastes, by-products and salt formation are the major drawbacks in the acid-catalyzed process (Corma and García, 2003). In large scale production of biodiesel, transesterification is the most commonly used in which the oil combines with an alcohol to form biodiesel and glycerol as a by-product. To catalyze this reaction, both alkali and acid catalysts are used.

Transesterification reaction is conventionally carried out in the presence of alkaline or acid catalyst by subjecting the pre-extracted oil to treatment with an appropriate alcohol (Georgogianni et al., 2008). Depending on the oil, the catalyst amount required differs. 3:1 molar ratio of alcohol to triglyceride is necessary to complete the transesterification reaction (Musa, 2016). To move the reaction towards ester formation, excess alcohol is used because transesterification is an equilibrium reaction. The major advantage of the transesterification reaction is that it can be carried out not only at any laboratory scale using a few liters of oil but also in a large industrial scale capable of producing millions of liters of biodiesel per year. In-situ transesterification differs from conventional reaction in which the oil bearing material mixes with the acidified or alkalized alcohol directly wherein the alcohol acts as both the extraction solvent and the esterification reagent. In in-situ transesterification, oil extraction and transesterification occur simultaneously by eliminating the extraction, separation and transesterification steps. In-situ esterification method is preferred to conventional methods because the reaction time, solvent and energy requirement are reduced (Georgogianni et al., 2008, Khan et al., 2018, Samuel and Dairo, 2012, Yousuf et al., 2017).

A. squamosa belongs to the species of the Annonaceae family and is commonly known as sugar apple. Sugar apple was introduced to India by the Portuguese in the 16th century. It is grown in lowland tropical climates worldwide. The height of the sugar apple tree ranges from 3 to 6 m and the leaves are aromatic when crushed. The compound fruit is nearly round or ovoid and it has thick rinds composed of knobby segments. The fruit is creamy-white, juicy and sweet. An average fruit contains about 20 to 38 seeds which are 1.25 cm long and black or dark-brown in color (Yathish et al., 2013).

The present work aims to study the production of biodiesel from A. squamosa oil using an acid catalyst. A. squamosa oil is an effective way of reducing the raw material cost in biodiesel production. The fatty acid methyl ester (FAME) conversion of A. squamosa oil by acid-catalyzed transesterification and in-situ transesterification is compared. Optimal conditions for the transesterification and in-situ transesterification reactions have been investigated and identified. The resultant biodiesel properties were analyzed according to ASTM standard. Furthermore, the biodiesel was mixed with different diesel blends in engine test rig equipped with data acquisition system after which the comparative analysis of the performance and the emission characteristics were carried out.