23 Biodiesel from Plant Vegetable Oils and its application as Engine fuel

Prof. A.K Jain

Learning Objectives

This modules helps to understand

• Vegetable plant oils and production
• Extraction of plant oils.
• Scope and limitation in use of vegetable oils as engine fuel.
• Chemistry of biodiesel production.
• Characteristics of plant oils and biodiesel,
• Commercial production of biodiesel.
• Engine operation using biodiesel as fuel

1.  INTRODUCTION

Biodiesel is a product of transesterification of vegetable plant oils. The characteristics (discussed later) of biodiesel resemble to that of petroleum diesel. Thus it can be used as a replacement to petroleum diesel for operating agricultural machinery such as engine pump-sets, tractors, harvest combines etc. and also the automobiles. Engine emissions contain less sulfur oxides, carbon monoxide hydrocarbons, thus the engine operation using biodiesel is more environment friendly. Moreover it is a renewable energy source and a farm produce. Its application for using IC engines reduces the dependence on imported petroleum fuels. Use of biodiesel will also tend to make Indian Agriculture sustainable in term of energy. The following aspects are discussed in brief in this chapter.

2.VEGETABLE PLANT OILS AND PRODUCTION

Plant oils are the esters of glycerol and fatty acids (mono, di and tri glycerides). Fatty acids consists a hydrocarbon chain bounded by single (saturated) or double triple bonds (unsaturated) bounded with a carboxyl group (COOH). The increased length of carbon chain in the fatty acids increases the molecular weight, viscosity and melting point, whereas it decreases the density of the oil. The unsaturation tends to increase the melting point of the oil. The number of carbon atoms and double bonds are represented by the general formula n:m where n indicates the number of carbon atoms and m represents the number of double bonds in the acid chain. The fatty acids found in different oils are; stearic acid (18:0), Oleic acid (18:1), Linoleic acid (18:2), Linolenic acid (18:3), Palmitic acid (16:0), lauric acid (12:0) etc. The free fatty acids content of oils is small, normally 2-3%. Apart from FFA, mono-glycerides, di-glycerides and tri-glycerides, the plant oil contains phosphatidies, waxes, terpenes, Gums etc. The presence of terpenes causes bad smell on combustion of oil and can occlude manifolds and filters of the engine.

Some important plant vegetable oils and their major constituent fatty acids are as follows:

Sunflower, Soyabean, Maize and Cotton Oils	Linoleic Acid	(18:2)
Olive, Peanut and Rape	Oleic Acid	(18:1)
Palm	Palmitic Acid	(16:0)
Coconut	Lauric Acid	(12:0)

Structural Formula of Vegetable Oil

Glycerol is trihydric alcohol, slightly viscous at room temperature, soluble in water, insoluble in hydrocarbons, sweet smelling. The melting and boiling temperature at normal pressure are 18.7 oC and 290 oC respectively and has a density of 1.47 kg/dm3. Structural formula of glycerol and fatty acids are shown below

Jatropha Circus oil is a non-edible oil having potential for energy application. The oil content of the Jatropha seeds is around 50%. It can be grown over even less fertile/ wasteland. Annual rainfall of around 540 mm is sufficient for its growth. Jatropha plant give fruits twice a year. The Jatropha Circus plant, its fruits, seeds kernel are shown in Photograph/Figure1.

4.Plant oils Production Statistics of the Country

Although India is one of the major oil producing countries in the world accounting for about one-fifteenth of the total world oil production. Her per capita oil consumption of around 12.7 kg/year is much below that of developed countries whose oil consumption level is around 23.5 kg/year. The current annual consumption of vegetable oils of the country stands around 22 MT. Indigenous production with an annual growth rate of 2.5 % is about 8.9 MT.

The breakup of the use pattern of vegetable oils in the country is as follows:

Edible purpose

Refined liquid oil                                                                     55%

Vanaspati (Hydrogenated oil)Industrial raw material       20%

Soap, glycerin, detergents                                                     10%

Cosmetics                                                                                 9%

Paints, varnish, energy                                                           6%

The above statics indicate that the country is deficient in production of edible oils (for human consumption) and is importing edible oils to meet its requirement. Under the present scenario the production and application of Non-edible oils such as Jatropha, Jajoba, Karanji, Mahua etc. have potential to be used as energy source.

5.EXTRACTION OF PLANT OIL

Oil from the seeds is extracted either by mechanical pressing and expelling (hot or cold) or by solvent extraction. The selection of the process depends on the oil content of the seed: for seed with low oil content, chemical solvent extraction is used. Where as for seeds with high oil content mechanical extraction is used which may follow solvent extraction. The residue left after mechanical pressing is called cake. It contains 7-15% oil and is used as animal feed. The residue left after the oil extraction is called meal that contains 0.5 to 3% oil.

6.SCOPE AND LIMITATIONS IN USE OF PLANT OILS AS ENGINE FUELS

Scope and limitations in use of plant vegetable oils as an energy source especially for engine application are as follows:

6.1 Scope:

1. Renewable energy source with a crop rotation period of one year or less..
2. Good energy value, almost 90% to that of liquid diesel fuel.
3. Does not disturb the ecological balance of the atmosphere as the Carbon-dioxide used for the production of oil is released back into the atmosphere when the oil is used for energy purpose.
4. Since the sulfur content of plant oils is very low or negligible, the emissions have practically no Sox.
5. Compatible with the existing CI diesel engines and can be used without any alteration or modification of the engine.
6. Engine exhaust gases contain less CO, hydrocarbons and smoke due to bounded oxygen in the oil molecule.
7. Safe handling and storage mainly due t%o high flash point of the vegetable oils.

6.2 Limitations:

The major problems observed with the use of unaltered vegetable plant oil in a diesel engine are as follows:

1. Injector fouling
2. Increased carbon deposits on combustion chamber
3. Varnish formation on combustion chamber
4. Ring sticking
5. Cylinder wall scoring
6. Increased fouling of lubrication oil

Properties such as high viscosity and high content of unsaturated fatty acids in the plant oils are responsible for these above said problems.

7.CHEMISTRY OF BIODIESEL PRODUCTION PROCESS

Trans-esterification (Biodiesel production) is the process in which mono-glyceride, diglyceride and triglyceride structure of the vegetable plant oil is converted to simple esters of alcohol (Methanol or Ethanol) and fatty acids. This reduces the viscosity and melting point and make the plant oil as suitable fuel for compression ignition engines. The other important variables are catalyst and process temperature. The chemical reaction representing the trans-esterification reaction is as under.

The process parameters that affect the trans-esterification reaction are:

1. Stoichiometric ratios of the reactants.
2. Alcohol (Methanol or Ethanol)
3. Catalysts
4. Process Temperature

7.1 Stoichiometric ratios of the reactants

In the process theoretically one molecule of plant oil reacts with three molecules of alcohol to form three molecules of ester and one molecule of glycerol as shown in the chemical reaction above.

To shift the equilibrium of the reaction to the right, it is necessary to provide excess alcohol to obtain above 97 % yield of ester. Therefore, 1:6 molecular ratio of oil to alcohol is recommended in the actual process reaction.

7.2 Alcohol

Trans-esterification of vegetable plant oils can occur with either methyl alcohol (methanol) or ethyl alcohol (ethanol). Although the rate of reaction is faster in the initial stage of the process with methanol but after one hour the yield of ester is the same both in case of ethanol and methanol.

Methanol is commercially produced from the thermal cracking of Neptha which is a product obtained during the fractional distillation of crude petroleum. Therefore, methanol is petroleum based product. Its use for biodiesel production will lead to continuous dependence on petroleum products. On the other hand Ethanol is produced from Agricultural products. Therefore the use of ethanol for biodiesel production will totally reduce the dependence of petroleum products. Ethanol is therefore a preferred raw material for the production of biodiesel compared to methanol.

7.3 Catalyst

The rate of reaction is increased with the presence of a catalyst. It is possible to use either an alkaline or an acidic catalyst. Alkali catalysts are considered to be a better choice compared to acidic catalysts in the reaction. A part of alkali is used for neutralization of free fatty acid in the plant oil. Therefore, the quantity of alkali catalyst depends on FFA content of the oil. For most refined oils 1% (based on the weight of the oil) KOH or NaOH are used commercially for trans-esterification reaction. For production of ethyl esters the recommended catalyst is KOH. Sodium methoxide is another catalyst that increases the reaction rate but is not used commercially due to its high cost. The alkali catalyst that have been used in the process are:

Sodium hydroxide

Potassium hydroxide

Sodium methoxide

7.4 Temperature

The temperature has a marked effect on the rate of esterification reaction. Higher temperature increases the reaction rate. The effect of temperature on the yield of methyl ester when the oil and methanol ratio in the reaction was 1:6 is shown in Figure 5. After one hour, the yields obtained at 45 and 60 oC are the same, while the yield at 32 oC is only slightly lower. In all cases, after 4 hours of residence time the yield is 98-99%. In batch processing unit the temperature is linked with the boiling point of the alcohol used. In case of methanol the maximum recommended temperature is 60 oC and in case of ethanol it is possible to go to a temperature of 70 oC. This is to avoid the excessive loss of alcohol due to evaporation. In continuous operating plants even a temperature of 250 oC is reported. At such high temperature the residence time is very short.

8.CHARACTERISTICS OF PLANT OILSAND BIODIESEL

• Specific Gravity: Specific gravity of plant oil is slightly higher than diesel and is related to the origin of the oil. It decreases with the increasing temperature.
• Viscosity: Viscosity of plant oils and other liquid fuels is measured in term of Kinematic Viscosity. The kinematic viscosity is the ratio of absolute viscosity to the density. It’s unit is cSt
• (cm2/s). Kinematic viscosity of the plant oil increases with increasing saturation and length of carbon chain. It also depends on temperature and decreases with increase in temperature. The viscosity of plant oils is about 10 times higher compared to that of the diesel and is the main cause for creating problems during the engine operation.
• Cloud and Pour Point: Cloud point is the temperature at which solids starts appearing in the oil. Pour point is the temperature at which oil flow tend to stop because of occlusion of filters and high viscosity. The pour point and cloud point of vegetable plant oils is higher than diesel but still meet the BIS limits of 6 oC
• Cetane Number: This is an index related with the pressure at which the air-fuel mixture burns in the absence of a spark or a flame. The cetane number for diesel fuel is 48-51. For vegetable plant oils it is about 40 and for oil esters it is 49. This is also related to the degree of unsaturation and the length of carbon chain. Long chain with two or more double bonds has higher bond energy thus needing higher temperature to vaporize.
• Heating Value: Heating value of vegetable plant oils is about 10% lower compared to diesel mainly due to bonded oxygen. The heating value affects the performance curve of the engine as specific fuel consumption increases and power output decreases.
• Distillation Curve: This refers to the fuel volatility and provides information concerning the air-fuel mixing system to be used and the regularity of distribution in the combustion chamber. For
• vegetable plant oils it starts at 310-360 oC (160-200 oC for diesel) and ends at 810 oC (less than 400 oC for diesel). The distillation range for esters is 300-360 oC.
• Flash Point: It is defined as the minimum temperature at which the vapors formed catch fire on ignition. The flash point of vegetable oils is much higher compared to diesel. High flash point ensures the safety during storage and handling of fuel.
• Conradson Index: This gives information regarding the carbon residue during combustion of the fuel. It is higher for plant oils compared to diesel.
• Ash: Ashes are derived from the solid particles of fuel or from soluble metallic compounds. These are harmful for engine operation.
• Iodine Number: It is mg of Iodine that can be bonded to 100g of oil. This indicates the number of double bonds (degree of unsaturation). Unsaturation is good as far as the filterability, flow is concerned but negative in terms of oxidation stability. High iodine number fuel have high conradson index, increasing the solid residue. Iodine number of certain plant oils is given in Table 1.

Table 1. Iodine number of certain plant oils.

• Total Acidity: This is mg of KOH required for saponification of free fatty acids in the oil sample. Crude plant oil has an acidity of about 3 mg KOH/g whereas for refined oil it is about 1 mg KOH/g and for esters it is less than 0.5 mg KOH/g.

Characteristics of refined sunflower oil, its methyl ester, ethyl ester (both biodiesels) and petroleum diesel fuel are given in the Table 2.

9.COMMERCIAL PRODUCTION OF BIODIESEL

The trans-esterification process can be performed continuously or in batch mode. Continuous process are suitable for capacities greater than 20,000-25,000 t/year of esters. The temperature and time required are 70 oC and 1 hour respectively. After the reaction is completed, the two phases are separated either by centrifuging or by settling. The hydrophilic phase is more dense contains glycerin, excess of methanol and catalyst. Whereas hydrophobic phase contain ester and floats over the hydrophilic phase. The ester thus produced contain impurities (traces of hydrophilic phase) which can be removed by water washing of the ester. The excess methanol can be recovered by evaporation and condensation from both the phases.

9.1 Batch Process for Plant Oil Ester Preparation

Batch process is simple and suitable for small capacity plants (less than 3000 t/year). The batch esterification process flow sheet diagram is shown in Figure 3. Various steps in the process are:

Alcohol catalyst mixing: Alcohol (methyl or ethyl) and catalyst are mixed in a stainless steel mixing tank.

First batch: Oil and alcohol (stoichiometric 1:6) are mixed for 1 hour at 70 oC in a tank with conical base. Then it is left undisturbed. After 7 hours the two phases are separated and hydrophilic phase is separated by decantation.

Second batch reactor: the hydrophobic phase containing ester from the first reactor is transferred to the second reactor. The process is repeated with the same quantity of methanol and catalyst and with the same duration as for the first batch. This second esterification is aimed at achieving quality levels.

Figure 6. Batch process for methyl ester formation

Catalyst neutralization: alkali catalyst can be neutralized with phosphoric acid. Since potassium phosphate can be used as a fertilizer, it is preferred to use potassium hydroxide as catalyst.

Methanol recovery: the excess methanol is recovered from both ester and the hydrophilic phase by evaporation and condensation. The hydrophilic phase from the two reactors can be processed to recover glycerol.

10.Farm Level process developed by PAU for production of bio-diesel

This is a simple technology developed at PAU for the use of farmers for generation of bio-diesel at farm level. The vegetable plant oil is heated to a temperature of 60 oC and simultaneously methanol is mixed with catalyst (1% based on the weight of oil) in a separate tank. The two are mixed in a stainless steel tank and stirred for about 5 minutes. The reactants are thereafter, transferred to a plastic container with conical bottom and allowed to remain undisturbed for 2 hours. Hydrophilic phase settled at the bottom is removed by decantation and can be used for glycerin recovery. The hydrophobic phase from the top is removed and transferred to another tank for water washing. The ester phase is mixed with water, stirred and allowed to settle for 5 minutes. The top layer is removed by decantation and is water washed again. After three washings, the ester layer is shifted to another tank and heated to remove traces of water and unused methanol. After removal of traces of water clear ester is obtained which can be used for running diesel engines. A line diagram showing various steps during the process is given in Figure 7.

Free Glycerin	0.02%
Total glycerin	0.24%
Flash point	100 oC
Water and Sediment	0.05%
Kinematic Viscosity at 40 oC	1.9 to 6 cSt
Ash	0.02%
Sulfur	0.05%
Cetane number	> 40
Carbon residue	0.05%
Total acidity	0.80 mg KOH/g

12. ENGINE OPERATION USING BIODIESEL AS FUEL

The existing diesel engines with direct injection system can be operated on biodiesels. Short-term tests on engine operation with biodiesel have been carried. The performance parameters include the engine efficiency, power recovery, engine exhaust gases and specific fuel consumption. The engine efficiency is comparable to that of diesel. Specific fuel consumption is higher compared to diesel mainly due to lower calorific value of biodiesel oils compared to diesel. Power recovery also drops to about 5% in case of biodiesel operation compared to diesel. The engine exhausts contains less carbon monoxide, hydro-carbons and very small or no sulfur oxides compared to diesel operation.

In addition to good short-term performance, an acceptable alternative fuel must be able to provide sustained engine operation without excessive wear or fouling.

Long term testing of methyl or ethyl esters of plant oils indicates that it is good replacement for diesel fuel. A new tractor (Mahindra & Mahindra) was operated using biodiesel as fuel in field conditions for Agricultural operations for 600 hours. The tractor was tested thereafter in the workshop of Mahindra & Mahindra and the engine was found in better condition than the diesel operation. The performance of the engine with biodiesel is very similar to that of diesel operation and there appear to be no deleterious effects on the engine or the lubricating oil.

CONCLUSION:

Biodiesel can be used as a replacement of petroleum diesel for running agricultural machinery such as Engine pump sets and tractors in fact all agricultural machinery and also auto mobiles that are being run on petroleum diesel. Engine performance in term of engine exhaust is environment friendly due to lesser carbon monoxide and hydro carbon emissions in the engine exhaust. Commercial use of plant vegetable oils may not be attractive today due to constrained oil production coupled with easy availability of other petroleum fuels, but the technology should be kept alive so that it could be make use of as and when the situation demands.