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Cashew Nut Shell Liquid as a Fuel for Compression Ignition Engines: A Comprehensive Review Shiva Kumar, Dinesha P., and Marc A. Rosen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00579 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018
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Energy & Fuels
Cashew Nut Shell Liquid as a Fuel for Compression Ignition Engines: A Comprehensive Review
Shiva Kumara, Dinesha P.a* and Marc A. Rosenb a
Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal - 576104, India. b
Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario, L1H 7K4, Canada. *Corresponding author email:
[email protected] Abstract In view of escalating concerns about energy (fuel) supply and environmental degradation, biodiesel is viewed as a potential replacement for petro-diesel, thereby reducing the dependences of many nations on importing crude petroleum. Cashew nut shell liquid (CNSL) is a biofuel that is renewable and biodegradable and has similar properties of petro-diesel fuel. In the present paper, the status of CNSL is reviewed in order to provide an updated source of information on CNSL and its use for researchers, engineers and policy makers. The paper summarizes information on cashew crop cultivation, oil extraction methods, and fuel modification techniques. Further the combustion behavior and emission characteristics of CNSL in a compression ignition engine are discussed. It is concluded that CNSL has the potential to be used as an alternative to diesel fuel in a compression ignition engines, and thereby to mitigate challenges associated with high dependences on imported petroleum and environmental impact due to emissions of greenhouse gases and other pollutants.
Key words: biofuels; cashew nut shell liquid; cardanol; diesel engines; emission; performance.
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Nomenclature and Abbreviations BSFC
Brake specific
fuel
consumption
(kg/kWh) BTE
Brake thermal efficiency (%)
CA
Crank angle
CI
Compression ignition
CNSL
Cashew nut shell liquid
CO
Carbon monoxide
deg.
Degree
IC
Internal combustion
HC
Hydrocarbon
NOx
Oxides of nitrogen
Introduction The world is facing growing demands for petroleum products with increased industrialization and use of vehicles. There are limited options available to obtain petroleum- based fuels, and these options are concentrated to some areas in the world. Hence, countries which lack these resources have to import them, often leading to foreign exchange problems. The use of conventional fuel affects economic growth and the demand for the same is increasing enormously due to modernization [1, 2]. The continuous use of petro-fuels results in environmental degradation due to the emission of greenhouse gases (GHGs) and other pollutants. Government policies in most countries have become more stringent regarding the emission of pollutants from transport vehicles, and vehicle manufacturers are challenged to meet environmental regulations. In recent years, much research has been carried out to achieve more environmental friendly transport, relying on technology development and cost optimization [38]. Since diesel engines are economical in terms of fuel usage and resilient in nature, they are widely used in transport and agriculture. Their higher compression ratios and lean combustion
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Energy & Fuels
characteristics are responsible for higher thermal efficiencies than other internal combustion (IC) engines [9]. The increased consumption of diesel fuel in the transportation and agricultural sectors may lead to a scarcity in the near future. The emissions from the diesel engines impact the environment and are also responsible for health hazards in living beings. Concerns over energy supplies and environmental degradation have fostered research on alternative fuels for IC engines. There are various forms of alternative fuels, viz., gaseous and liquid fuels. Biofuels typically are liquid fuels derived from biomass sources such as plant and animal material [1014]. From the literature it clear that vegetable oils are most suitable alternative liquid fuel sources that can replace fossil diesel, at least partially [1]. Vegetable oils include edible and non-edible oils. Although there are various methods of utilizing vegetable oil in compression ignition (CI) engines, the most commonly used are transesterification [15], blending [16], micro emulsions [17], and thermal cracking [18]. Among these, transesterification is widely used to convert oils to biodiesel [19-22]. One biodiesel, known as fatty acid methyl ester (FAME), is a mixture of methyl esters with long-chain fatty acids and is typically made from non-toxic, biological resources such as vegetable oils, animal fats, or even used cooking oils. Biodiesel shares some similar physiochemical properties with petroleum derived diesel fuel, namely, cetane number, energy content, viscosity, and phase changes [23]. Biodiesel has less knocking tendency since it has a reasonable cetane number. Biodiesel can be mixed in any proportion with fossil-based diesel to form a stable blend; even though it contains no petroleum products, it is compatible with standard diesel fuel. Several studies have been reported concerning the use of biofuels derived from sunflower oil [24], pongamia oil [25], palm oil [26], jatropha oil [27], neem oil [28], mahua oil [29], and others. However, studies related to cashew nut shell liquid (CNSL) are sparse. Hence, in this paper we review the work on and prospects of CNSL as a biofuel source in IC engines. The paper summarizes research on site selection for cashew crops, cashew yield, extraction techniques for CNSL and the effects of CNSL blends on engine performance and emissions.
Cashew crops Botanical description of cashew tree The cashew tree (Anacardium Accidentale) is a tropical widespread evergreen tree which grows to a height of 10-12 m and has a spread of 25 m. Cashew trees are commonly found in coastal
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areas. Cashew nut is the main product derived from the cashew tree, and is used for snack and confectionery products. Cashew apple is used to prepare fruit jams and beverages. The cashew tree, cashew fruit, parts of the cashew fruit, and the cashew shell are shown in Figure 1(a - d). Compared to the other nut bearing fruits, cashew fruit is unusual as the nut grows outside the fruit. Cashew nut is covered with a hard shell which is leathery in its raw state. Cashew shell contains a thick viscous dark brown liquid, known as cashew nut shell liquid (CNSL), within its sponge-like interior, which is very rich in non-isoprenoid phenolic lipids [30, 31].
Cashew Apple
Kernel
Nut shell
Testa skin
Fig.1: (a) Cashew tree; (b) Cashew fruit; (c) Parts of cashew fruit; (d) Cashew shells [30] Site requirements Soil requirements for cashew cultivation are moderate and the cashew tree can adopt to varying types of soil conditions without much effect on cashew yield. Deep and well drained sandy loams are ideal for this type of cultivation. Water stagnation and flooding are not beneficial for cultivation. Heavy clay soil with a pH more than 8 is not suitable for cashew tree growth. Thus, red sandy loam, lateritic soil coastal, and slightly acidic soil are ideal for cashew cultivation.
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Cashew is a tropical plant and can survive even at high temperature. It can be grown at altitudes as high as 700 m above sea level and where the temperature does not fall below 20⁰C. Areas where the temperature ranges from 20-30⁰C with an annual precipitation of 1000–2000 mm are ideal for cashew cultivation. A climate with a well-defined dry season of four months produces best yield from cashew cultivation. Excessive rainfall with high humidity may result in flower/fruit drop [32].
CNSL extraction and composition There are several practical methods for the extraction of CNSL from the cashew shell, namely mechanical, thermal, chemical extraction by solvent and pyrolysis methods. The following section describes the methodology involved in each type of oil extraction process. Mechanical extraction method The mechanical extraction method results in CNSL known as natural CNSL, which contains extra anacardic acid [33]. In this extraction method, CNSL is extracted from cashew nut shells using a mechanical press [34, 35]. High pressure is exerted in order to release CNSL. The pressure screw press is the most commonly used mechanical extraction method. Here the raw cashew nut shells are loaded in a hydraulic press, and high pressure is exerted by operating the screw. It is simple, time saving and effective. The percentage of CNSL from cashew nut shells is up to 21% and its purity is around 85% by weight. The composition of CNSL obtained from this method follows: cardol (15-31%), anacardic acid (46-65%), cardanol (10-22%) and traces of methyl cardol [35]. The main drawback of this method is that the residue from the extraction still contains 10-15% CNSL. Thermal extraction method The thermal extraction method produces CNSL known as technical CNSL, which contains more cardanol due to the conversion of anacardic acid to cardanol [33]. In this method, CNSL is extracted by applying heat to cashew shells. There are several thermal extraction techniques such as roasting, hot oil bath and solar cooker method [36, 37]. The most common being the roasting method and the hot oil bath method [36]. Commercially available technical CNSL is obtained by roasting shells, and contains mainly cardanol and cardol [38]. Roasting is the traditional method in which CNSL is removed by roasting shells in a drum. During roasting the shells, CNSL oozes
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out, making the shells brittle. To recover CNSL completely the shells are roasted again in baths at temperatures of 180-185⁰C. Due to the high temperature bath, this method recovers up to 90% of CNSL. High temperature condition prevents the decarboxylation reaction, thus converting anacardic acid present in natural CNSL into cardanol [39]. The hot oil bath is the main commercial method of extraction of CNSL, and is widely used in industry [36]. In this method, the cashew nut shells are placed in a cylinder and heat is supplied by steam at a temperature of 200-250⁰C for a duration of 2-3 minutes. Due to this heating, CNSL is released yielding up to 12% of CNSL. To extract all of the CNSL, the residue shells are subjected to heating through a bath of hot CNSL. Chemical extraction method There are two available techniques in this category: solvent extraction and the supercritical carbon dioxide method. In the solvent extraction method [40], fresh broken cashew shells are placed in an Erlenmeyer flask and pentane is added to cover the shells. The extracted CNSL is filtered and fresh pentane solution is added every 12 hours. The collected CNSL is evaporated in a rotary evaporator and reduced to a temperature below 30⁰C. Saito [41] suggest the use of supercritical fluids as alternative solvents to organic fluids. Smith et al. [42] proposed a method of supercritical carbon dioxide for the extraction of CNSL. In the supercritical method, cashew nut shells are placed in the extractor and it is sealed. The extractor is heated to the extraction temperature. Compressed carbon dioxide is passed through the extractor at a flow rate of 0.5-5 litre/minute at standard temperature and pressure. The CNSL is collected periodically, usually at intervals of about one hour. This method yields a maximum of 94% CNSL. Pyrolysis method Das et al. [43] investigated the pyrolysis method of extraction of CNSL from cashew nut shell. Pyrolysis is a thermochemical conversion process in which the reaction takes place in the absence of or with limited oxygen. The authors studied the yield of CNSL for both low and high temperature pyrolysis. In low temperature pyrolysis, the oven is kept at a temperature of not more than 200⁰C. In high temperature pyrolysis, the oven temperature is maintained at 500⁰C. The physical properties of CNSL obtained by both the methods are compared in the Table 1. No
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significant differences are observed in properties like ash content, moisture and density. But, it can be observed that viscosity changes drastically as the temperature rises from room temperature to a higher temperature (60oC to 80oC). It is observed that a slightly higher calorific value and lower flash point result from high temperature pyrolysis of CNSL compared to low temperature pyrolysis. A 100% miscibility is observed with methanol and diesel fuel, showing good potential for blending with these fuels.
Table 1. Physical properties of CNSL from high and low temperature pyrolysis [43]. Property
CNSL by low temperature CNSL by high temperature pyrolysis
pyrolysis
Ash (% wt)
0.01
0.01
Moisture (% wt)
3.5
3.0
0.993
0.987
30⁰C
159
166
60⁰C
33
39
80⁰C
17
16
Flash point (⁰C)
180
164
Pour point (⁰C)
-5
-5
C
76.4
79.9
H
10.5
11.8
N
