Petroleum Plants

There are some species of certain families which accumulate the photo-synthetic products (hydrocarbons) of high molecular weight (10,000). They are commonly known as petroplants or petroleum plants. In 1979, Dr. M. Calvin of the University of California reported for the first time the collection and use of photosynthetically produced hydrocarbons from the plants (Calvin, 1979). Furthermore, he suggested it as a substitute for conventional petroleum sources. Calvin and coworkers screened most of the plants of Euphorbiaceae, especially Euphorbia (containing 2,000 species) which reduce CO₂ beyond the carbohydrates. The petroplants have lactiferous canals in their stem and secrete a milky latex. The latex can be either continuously tapped like Hevea latex and stored or extracted from the biomass by using the organic solvents. The product rich in hydrocrackable hydrocarbon is called as 'biocrude'. Biocrude yields about 70.6% energy ; out of which 22% as kerosene and 44.6% as gasoline.

About 400 plant species, belonging to different families are known which grow in different part of the country. It is hoped that petroplants can yield petroleum more than 40-45 barrel/acre.

Hydrocarbon from Higher Plants
Euphorbiaceae has been extensively screened, which has shown the fruitful results. In addition, the most useful plant families to be investigated are Asclepiadaceae, Apocyanaceae, Leguminosae, Sapotaceae, Moraceae, Dipterocarpaceae, Compositae, etc.

However, the members of Euphorbiaceae possess high amount of hydrocarbons. Plants producing rubber and other hydrocarbons are given in Table 20.1.

Hevea Rubber
Rubber plant, (Hevea brasiliensis) commonly known as Hevea rubber is the principal source of rubber which is restricted in distribution in South-East Asia. This plant meets one third of total world demand of rubber. The synthetic rubber elastomers from petroleum have not replaced the demand of natural rubber, due to its low cost build up, resilience, elasticity and good performance in automobiles and aeroplanes. Rubber is tapped from stem of trees by making incision and collecting the latex from it. The latex is further processed to get rubber.

Euphorbia
In Italy, Euphorbia Gasoline Rifinery was set up to tap vegetative gasoline. Euphorbia lathyris is an annual herb and E. tirucalli is a perennial one. E. lathyris can produce 20 t dry matter/ha/yr. Chemical analysis of this plant in organic solvents revealed that heptan extract and ether soluble fraction constituted about 8% terpenoid extract.

By using zeolite catalyst, it could be converted into high grade transportation fuel. Of the 85% converted materials, about 10% is in the form of natural gas and 75% in gasoline-like fractions (Nemathy et al, 1980). Calvin and co-workers estimated that 10 tonnes of biomass could yield 5.3 barrels of crude extract convertable to gasoline.

Table 20.1. Plants producing hydrocarbons. Till now, much emphasis has been given on crop plants. At present, there is need to encourage cultivation of petroplants, in waste lands. There is need for joint efforts of botanists, phytochemists, engineers, economists, and geneticists to make successful research on these lines.

Guayule and Russian Dandelion
Guayule (Partheniuum argentatum) and Taraxacum koksaghyz of family compositae are sources of rubber. Guayule a shrub is indigenous to North Central Mexico and South-West U.S.A. Guayule generally grows in arid, semi-arid and desert areas. The U.S. Government encouraged the cultivation of this plant after World War II to reform the economy of the country. It can tolerate temperature ranging from 32 - 38°C, and can grow in Indian conditions.

Like Hevea, guayule contains cis-polyisoprene and identical physical properties. There is need to develop technologies for the production of hydrocarbon to be used as alternative fossil fuel.

Aak (Calotropisprocera)
Aak (Family Asclepiadaceae), a shrub of 1-2.5 meters in height, occurs in hot and dry parts of India on waste dry places, river beds, roadsides and forest clearings. It secretes latex which causes irritation to skin. Latex contains high amount of extractable hydrocarbons. The ratio of C, H, O in the hexane extract has been found as 78.03%, 11.22% and 10.71% respectively. The ratio of C and H is similar to crude oil, fuel oil and gasoline. Hydrocarbon yield and energy value of C. procera are comparable to those of E. lathyris. Therefore, this plant can be used as a substitute of petroleum. Researches on it are being done at the Central Arid Zone Research Institute, Jodhpur.

In India, cultivation of petroleum plants needs to be encouraged and suitable technologies should be developed for extraction of crude oil to be used as fuel. NBRI (Lucknow), and Indian Institute of Petroleum (Dehra Dun) have started preliminary screening programme of such plants. Over 400 plant species are to be tested for growth conditions, habitat performance, biomass yield and hydrocarbon content.

Algal Hydrocarbons
Dead algal scum of Botryococcus braunii, an unicellular alga of Chlorococcales of green algae, contains about 70% hydrocarbons. Percentage of hydrocarbon may vary. The algal hydrocarbons closely resemble the crude oil, and therefore, can be used as a good source of direct production of hydrocarbons.

B. braunii grows in fresh or brackish water as well as in tropical and temperate zones. When in full growth, it becomes apparent in water as the small dots. The alga appears in two forms, as far as pigmentation and structure of synthesized hydrocarbons are concerned. The first form is of green color and contains linear hydrocarbons with an odd number of carbon atom (25-31) low in double bonds. The second form of alga is red in color which contains hydrocarbons with 34-38 carbon atoms and several double bonds, the 'botryococcenes'. Significance of these two forms are not known (Sasson, 1984).

This alga is composed of proteins, carbohydrates, and lipids, the percentage of which varies. However, it has proved to be a source of hydrocarbons. As a result of metabolic activity, the hydrocarbons are synthesized during growth phase of the alga.

Hydrocarbon is accumulated as globules on outer walls and cytoplasm of the cells. On cell wall, a major portion of hydrocarbon (95%) is located, whereas a: small amount (0.7%) of it is present within the cells. Hydrocarbons are recovered from the cells by centrifugation. The cells are again added in the fresh culture medium as inoculant. For the production of hydrocarbons in high amount, it is necessary to increase the algal biomass. However, it could be achieved by characterizing the culture medium and light and shade conditions for its growth and biomass production.

In addition, Chlorella pyrenoidosa, a fresh water alga, is known to be converted into hydrocarbons. Hydrogenation is done in a steel reactor at high temperature (> 400°C) and pressure (12,000 p.s.i., pound per square inch) in the presence of a catalyst (cobalt molybdate). The alga is suspended in a mineral oil in the reactor. Hydrogenation is carried out for about one hour. Consequently, 50% of algal biomass is converted into oil with a little amount (12-14%) of a byproduct, ammonium carbonate. Oil is a clear golden liquid which is separated from the reactor, blended with light gas oil in refineries and processed before its use.