Oil Palm Tree Wastes 3: Industrial and Agricultural uses of Oil Palm Trunk
Published Date: 19th October 2020
During the early growth stage of the oil palm tree, the trunk grows at a rate of about 35 to 75 cm per year and produces alternate rows of fronds made up of a central petiole, and leaflets. There are usually about 40 leaflets on a frond. The base of the old fronds surrounds the trunk and begins to fall off at the age of 12 to 15 years. By this time, growth, and production have slowed down. Under commercial plantation production, the average economic life-span of the oil palm trees is 25 to 30 years, after wish they are felled to allow for replanting. At this time of re-plantation, the heights of the oil palm trees range from 7 to 13 meters, with a width range of 45 to 65 centimeters, at 1.5 meters above the soil surface. Malaysian estimates have shown that the process of re-plantation generates about 8.36 million tons of dried biomass annually, consisting of 7.02 million tons of trunk, and 1.34 million tons of fronds. Oil palm fronds and trunks are therefore the two major biomass wastes generated at the palm plantation level, while the other wastes are byproducts of fresh fruit processing. Oil palm trunk accounts for about 5 percent of the total oil palm biomass waste and could yield as high as 75 tons of waste per hectare during the replantation period.
Oil palm industries generate millions of tons of biomass waste every year, which when appropriately recycled will not only be able to solve the waste disposal problems associated with the industry, but also can create value-added products for additional income. About 89 percent of these biomass wastes produced annually are however used as fuel, mulch, and fertilizer. The oil palm trunk is particularly high in moisture content (70 percent fresh weight), and because of this, the newly felled tree trunk cannot be used as fuel. When left in the field, the old trunks occupy the spaces being replanted and are known to harbors insects and the Ganoderma mushrooms that harm new palm trees. The oil palm trunk usually takes between five and six years to decompose under natural tropical conditions. It, therefore, constitutes a serious environmental problem when left unprocessed in the plantation. However, of all the solid wastes produced from oil palm processing, the empty fruit bunch, and the oil palm trunk has the highest potential for commercial exploitation.
Decades of Asian research have resulted in the development, and commercialization of a variety of oil palm trunk-based products, which could be adopted by other oil palm producing regions, especially West and Central Africa. The use of oil palm trunk for the production of different types of value-added products through chemical, physical, and biological innovations however continues to evolve and forms a major area of multi-disciplinary research engagement. The oil palm trunk as a lignocellulosic material has been used in the production of various types of wood materials such as saw-wood, ply-wood, lumber, and binderless particleboard. The felled oil palm trunk contains a large quantity of sap which is a good feedstock for the production of bioethanol. Studies have also shown that oil-palm trunks can be processed into compost and roughage comparable to rice straw for use in feeding ruminants.
Structural and chemical characteristics of the oil palm trunk
Transversely, the oil palm trunk is divided into the back, cortex, peripheral, inner, and central zones (Plate 2). The three zones make up the main components of the trunk which consists of the vascular bundles, and parenchyma. Spread over this main part in the entire trunk are brownish to blackish dots (vascular bundles) which are believed to be the main support structure of the plant. The peripheral part of the trunk contains thin layers of parenchyma, and the congested vascular bundles, that gives rise to a sclerotic zone, which provides functional support to the trunk. This latter zone is made up of large numbers of radially extended fibrous, and vascular bundles. The central zone is composed of slightly larger, and widely scattered vascular bundles surrounded by thin walls of parenchyma tissue. Thus, the three major constituents of the oil palm trunk are the vascular bundles, fiber, and parenchyma cells. The dry wood is very light in weight due to the loss of the high quantity of moisture present in the fresh trunk.
The oil palm trunk fiber is strong due to its high content of lignin in the form of lignified cellulose fibers, which contributes to its strength better than de-lignified fibers. This fiber is shorter and thicker than fibers found in the other parts of the palm tree. The chemical composition of the oil palm trunk (wt.%) has been reported to be 29 – 37 percent cellulose, 12 – 17 percent hemicellulose, 42 – 45 percent holocellulose, 18 – 23 percent lignin, 15 – 18 percent xylose, 30 – 32 percent glucose, and 2 – 3 percent ash. The oil palm trunk has a high moisture content, with the sap accounting for 67 – 82 percent of the overall trunk weight. Its structural carbohydrates in the form of cellulose, hemicellulose, and starch are also significant and can be hydrolyzed into simple sugars, which subsequently can be used in microbial fermentation. Efficient sugar recovery from the trunk is usually aided by pretreatment processes with dilute acids and enzymes.
Uses of the oil palm trunk
Several products can be derived from the oil palm trunk. The schematic diagram of palm trunk processing into different products shown in figure 1, highlights the production of veneer wood as the main product, while from the core of the trunk, sap used in bioethanol production, and sap residue used in pellet, syngas, and animal feed can also be generated.
Production of wood materials: The high moisture content of the trunk usually causes some defects in the form of excessive shrinkage, and the collapse of the wood product during processing. Recent studies have however shown that these defects can be overcome by the use of microwave drying techniques, which increases the drying time, and moisture removal. A lot of research has also focused on the manufacturing of binderless particleboard, and compressed panels from the oil palm trunk. Several preliminary trials have been carried out using the bark, mid-parts, and core-parts of the trunk to manufacture experimental binderless particleboard panels. Other studies have experimented on the production of hybrid plywood, mid-density fibers, polymer composites, particle boards, paper, pulp, furniture, and biofuels from oil palm trunk. Only the outer part of the oil palm trunk however can be used for plywood production, while the inner part is discarded because of its high moisture content, and degradability. The oil palm trunk has also been chipped and waxed with resin to produce pre-formed desktops, and chair seats for schools. The treatment makes the furniture to be resistant to knocks, scratches, ink, termites, and fungal attacks.
Palm trunk sap: The freshly felled oil palm trunk contains a large quantity of sap that accounts for approximately 75 percent of the whole trunk weight. The sugar content of the sap was found to increase remarkably from 83 to 153 mg ml after 30 days of storage, and thereafter followed by a gradual decrease. The total sugar concentration in the sap is comparable to that of sugar cane juice, with the major sugars being glucose, sucrose, fructose, and galactose, indicating that with proper aging, the sap can be a good feedstock for the production of bioethanol.
Researchers at the Forest Research Institute, Kepong, Ehsan, Malaysia, in collaboration with engineers at the Japan International Research Center for Agricultural Sciences Ibaraki, Japan have developed a prototype sap compressing system from oil palm trunk. The process involved first, the trimming off of about 50 cm of both ends of the trunk to eliminate any possible fungi and bacteria contamination from them. The remaining part was then sawn into small logs of about 1.2 meters, before peeling off the back, and outer layer with a rotary lathe. Thereafter, the peeled palm trunk core was ripped into small chips by a shredder, and the chips compressed by a mill in order to squeeze out the sap. The operation, therefore, involves the processing and compressing system for squeezing the sap out of the oil palm trunks in order to increase the utilization of the rich sugar content of the palm trunk. The equipment used was a rotary lathe, a shredder, and a compressing mill. The compressing yield was approximately 80 percent of the sap content of the trunk under slow rotation of the mill, while the sap yield was dependent on the rotation rate. Ethanol produced from the sap was of a good grade, indicating that the prototype system could contribute to improved utilization of the palm trunk, and sustainable biofuel production from a non-food resource. It is also possible to use the bagasse generated from this process in different ways including particle boards, compost, and animal feeds production.
Researchers at the Japan International Research Center for Agricultural Sciences, Ibaraki, Japan, have studied the chemical compositions of sap extracted from the felled oil palm trunk and reported that the sap contains a high percentage of glucose. The sap from the inner part of the trunk particularly accounted for more than 80 percent of the whole trunk weight in confirmation of earlier reports. The glucose concentration of the sap was found to be 85.2 g/L in the inner part and decreased towards the outer part. Other sugars found in relatively lower concentrations were sucrose, fructose, galactose, xylose, and rhamnose. In addition, the oil palm sap was found to be rich in various kinds of amino acids, organic acids, minerals, and vitamins as shown in table 1. The most abundant vitamins were inositol (640.0 µ/g sap), ascorbic acid (20.0 µ/g sap), niacin (2.5 µ/g sap), pantothenic acid (1.5 µ/g sap), and pyridoxine 1.1 µ/g sap). The sugars contained in the sap were readily converted to lactic acid with almost the same efficiency as the reference fermentation with glucose as a substrate.
Composting of oil palm trunk: The oil palm trunk can be shredded to an average size range of 10 to 50 mm, and used as both mulch, and manure. When used in the production of high-grade soil or growth medium, the 10 mm shredding is preferred, whilst in producing a mulch the 50 mm shredding is preferred. The shredded oil palm trunk can also be blended with nitrogen-rich materials such as animal dung, palm oil mill effluent, and palm fronds before composting to produce a higher quality organic manure. The shredded oil palm trunk could be blended with these materials in the range of about 10 to 20 percent by volume. The oil palm trunk has also been subjected to vermicomposting to produce organic manure. Vermicomposting is a process that involves the oxidation, and stabilization of organic wastes through the joint action of earthworms, and microorganisms in order to turn the waste into valuable soil amendment called vermicompost. Researchers at the Universiti Sains, Penang, Malaysia, evaluated the physical, and chemical properties of oil palm trunk subjected to vermicomposting with an exotic earthworm species (Eudrilus euginae), over an 84-day duration, and concluded that the earthworm growth was negatively affected by the presence of heavy metals in the substrate. It was however observed that the carbon-nitrogen (C: N) ratios decreased with time in the vermireactor, indicating the stabilization of the waste.
Production of animal feed from oil palm trunk: Waste oil palm trunk generated from replantation activities can be used as feedstock for animal feed pellet production because it has a nutritive value that is suitable for ruminant diet formulation. The roughage produced from the oil palm trunk is particularly high in metabolizable energy, probably because of its high sugar, and starch content. The waste trunk has been chopped, chipped, treated with NaOH, and ensiled. The high cost of processing the trunk is however a major constraint to it being widely used as animal feed.
Research at the Forestry and Forest Products Research Institute, Ibaraki, Japan on the chemical composition of oil palm trunks suggests that their conversion to cattle feed is one of the best ways of utilizing the trunks. An alkaline treatment to increase the digestibility was suggested as a means of improving the value of the material for cattle feeding. About 10 percent of sodium hydroxide has been found optimal for such a treatment. It has also been reported that the alkaline solution can extract the starch, xylan, and p-hydroxybenzoic acid from the trunk, while the extracted starch has been found suitable for use in the food industry or for fermentation into alcohol.
The oil palm trunk is a major biomass waste produced from oil palm production. It accounts for about 5 percent of the total oil palm biomass waste and could yield as high as 75 tons per hectare during the replantation period. Several products such as veneer wood, sap, and sap residue used in pellet, syngas, manure, and animal feed can be generated from the oil palm trunk. Technologies for converting the palm trunk into these useful products are being researched and developed particularly in Asian countries. Such technologies will need to be replicated in Africa in order to promote its conversion from waste to wealth.
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