Biomass Wastes from Palm Oil Mills

The Palm Oil industry generates large quantity of wastes whose disposal is a challenging task. In the Palm Oil mill, fresh fruit bunches are sterilized after which the oil fruits can be removed from the branches. The empty fruit bunches (are left as residues, and the fruits are pressed in oil mills. The Palm Oil fruits are then pressed, and the kernel is separated from the press cake (mesocarp fibers). The palm kernels are then crushed and the kernels then transported and pressed in separate mills.

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In a typical palm oil mill, almost 70% of the fresh fruit bunches are turned into wastes in the form of empty fruit bunches, fibers and shells, as well as liquid effluent. These by-products can be converted to value-added products or energy to generate additional profit for the Palm Oil Industry.

Palm Kernel Shells (PKS)

Palm kernel shells (or PKS) are the shell fractions left after the nut has been removed after crushing in the Palm Oil mill. Kernel shells are a fibrous material and can be easily handled in bulk directly from the product line to the end use. Large and small shell fractions are mixed with dust-like fractions and small fibres.

Moisture content in kernel shells is low compared to other biomass residues with different sources suggesting values between 11% and 13%. Palm kernel shells contain residues of Palm Oil, which accounts for its slightly higher heating value than average lignocellulosic biomass. Compared to other residues from the industry, it is a good quality biomass fuel with uniform size distribution, easy handling, easy crushing, and limited biological activity due to low moisture content.

Press fibre and shell generated by the Palm Oil mills are traditionally used as solid fuels for steam boilers. The steam generated is used to run turbines for electricity production. These two solid fuels alone are able to generate more than enough energy to meet the energy demands of a Palm Oil mill.

Empty Fruit Bunches (EFBs)

In a typical Palm Oil mill, empty fruit bunches are abundantly available as fibrous material of purely biological origin. EFB contains neither chemical nor mineral additives, and depending on proper handling operations at the mill, it is free from foreign elements such as gravel, nails, wood residues, waste etc. However, it is saturated with water due to the biological growth combined with the steam sterilization at the mill. Since the moisture content in EFB is around 67%, pre-processing is necessary before EFB can be considered as a good fuel.

In contrast to shells and fibers, empty fruit bunches are usually burnt causing air pollution or returned to the plantations as mulch. Empty fruit bunches can be conveniently collected and are available for exploitation in all Palm Oil mills. Since shells and fibres are easy-to-handle, high quality fuels compared to EFB, it will be advantageous to utilize EFB for on-site energy demand while making shells and fibres available for off-site utilization which may bring more revenues as compared to burning on-site.

Palm Oil Mill Effluent (POME)

Palm Oil processing also gives rise to highly polluting waste-water, known as Palm Oil Mill Effluent, which is often discarded in disposal ponds, resulting in the leaching of contaminants that pollute the groundwater and soil, and in the release of methane gas into the atmosphere. POME could be used for biogas production through anaerobic digestion. At many Palm-oil mills this process is already in place to meet water quality standards for industrial effluent. The gas, however, is flared off.

In a conventional Palm Oil mill, 600-700 kg of POME is generated for every ton of processed FFB. Anaerobic digestion is widely adopted in the industry as a primary treatment for POME. Liquid effluents from palm oil mills can be anaerobically converted into biogas which in turn can be used to generate power through gas turbines or gas-fired engines.

Conclusions

Most of the Biomass residues from Palm Oil Mills are either burnt in the open or disposed off in waste ponds. The Palm Oil industry, therefore, contributes significantly to global climate change by emitting carbon dioxide and methane. Like sugar mills, Palm Oil mills have traditionally been designed to cover their own energy needs (process heat and electricity) by utilizing low pressure boilers and back pressure turbo-generators. Efficient energy conversion technologies, especially thermal systems for crop residues, that can utilize all Palm Oil residues, including EFBs, are currently available.

In the Palm Oil value chain there is an overall surplus of by-products and their utilization rate is negligible, especially in the case of POME and EFBs. For other mill by-products the efficiency of the application can be increased. Presently, shells and fibers are used for in-house energy generation in mills but empty fruit bunches is either used for mulching or dumped recklessly. Palm Oil industry has the potential of generating large amounts of electricity for captive consumption as well as export of surplus power to the public grid.

Energy Potential of Empty Fruit Bunches

A palm oil plantation yields huge amount of biomass wastes in the form of empty fruit bunches (EFB), palm oil mill effluent (POME) and palm kernel shell (PKS). In a typical palm oil mill, empty fruit bunches are available in abundance as fibrous material of purely biological origin. Energy potential of empty fruit bunches is attractive as it contains neither chemical nor mineral additives, and depending on proper handling operations at the mill, it is free from foreign elements such as gravel, nails, wood residues, waste etc.

EFB

However, EFB is saturated with water due to the biological growth combined with the steam sterilization at the mill. Since the moisture content in EFB is around 67%, pre-processing is necessary before EFB can be considered as a good fuel.

Unprocessed EFB is available as very wet whole empty fruit bunches each weighing several kilograms while processed EFB is a fibrous material with fiber length of 10-20 cm and reduced moisture content of 30-50%. Additional processing steps can reduce fiber length to around 5 cm and the material can also be processed into bales, pellets or pulverized form after drying.

There is a large potential of transforming EFB into renewable energy resource that could meet the existing energy demand of palm oil mills or other industries as well as to promote sustainability in the palm oil industry. Pre-treatment steps such as shredding/chipping and dewatering (screw pressing or drying) are necessary in order to improve the fuel property of EFB.

Pre-processing of EFB will greatly improve its handling properties and reduce the transportation cost to the end user i.e. power plant. Under such scenario, kernel shells and mesocarp fibres which are currently utilized for providing heat for mills can be relieved for other uses off-site with higher economic returns for palm oil millers.

The fuel could either be prepared by the mills before sell to the power plants, or handled by the end users based on their own requirements.  Besides, centralized EFB collection and pre-processing system could be considered as a component in EFB supply chain. It is evident that the mapping of available EFB resources would be useful for EFB resource supply chain improvement. This is particular important as there are many different competitive usages. With proper mapping, assessment of better logistics and EFB resource planning can lead to better cost effectiveness for both supplier and user of the EFB.

A covered yard is necessary to store and supply a constant amount of this biomass resource to the energy sector. Storage time should however be short, e.g. 5 days, as the product; even with 45% moisture is vulnerable to natural decay through fungi or bacterial processes. This gives handling and health problems due to fungi spores, but it also contributes through a loss of dry matter trough biological degradation. Transportation of EFB is recommended in open trucks with high sides which can be capable of carrying an acceptable tonnage of this low-density biomass waste.

For EFB utilization in power stations, the supply chain is characterized by size reduction, drying and pressing into bales. This may result in significantly higher processing costs but transport costs are reduced. For use in co-firing in power plants this would be the best solution, as equipment for fuel handling in the power plant could operate with very high reliability having eliminated all problems associated with the handling of a moist, fibrous fuel in bulk.