Re-engineering palm oil mills into centres of energy efficiency

Palm oil mills can unleash renewable energy at enormously high energy efficiency levels arising from the unique characteristics of their operating parameters.

Palm oil mills are in an enviable position to harness increased quantities of renewable energy at very high efficiencies, the potential of which remains largely unrealised.

At the moment, palm oil mills tend to focus on milling operations. They are predominantly concerned with the core business revolving around the processing of fresh fruit bunches (FFB) and oil extraction rates. However, a new premise envisions mills operating as centres of energy efficiency.

The bulk of the FFB mass from the field transported to the mill for processing is discharged as biomass residue. Of this residue, the energy content of the mesocarp fibre and palm kernel shells are being used in inefficient ways to provide the energy needs of the mill. In actual fact, palm oil mills can unleash renewable energy at enormously high energy efficiency levels arising from the unique characteristics of their operating parameters.

The extraction and utilisation of renewable energy inherent in the fibre and shells performed at high efficiencies can meet not only the energy needs of the mill but yield abundant surplus clean energy for other use. Optimising the harnessing of the renewable energy at the mill serves to reduce the carbon dioxide (CO2) emissions of palm oil production, and hence its carbon footprint.

Mills are still continuing to run processes that were designed at a time when the awareness of renewable energy and energy efficiency was not at the fore.

Today, however, innovative methods and systems are readily available to release this inherent renewable energy potential through vastly improved energy efficiencies at mills. A pioneering approach involves addressing four problem areas, namely, high steam consumption for process heating, rapid and sharp fluctuations in process steam demand, relatively high process steam temperature and low power-to-heat ratio of the Combined Heat and Power (CHP) plant employed in the mill energy system.

The current steam consumption of the palm oil extraction process is excessive arising from the sterilisation process. The initial process in the extraction of palm oil involves thermal treatment of the FFB in large steriliser vessels which consumes a major portion of process steam. The effective removal of residual air at the onset of the sterilisation process from within the stack of FFB placed in cages in the steriliser vessel is necessary for it to attain timely adequate and uniform temperature to effect thermal treatment. However, palm oil mills employ outdated air release methods that are not efficient and incur a vastly inefficient use of process steam leading to its high consumption. Innovation avails an opportunity for efficient air removal at the commencement of the sterilisation cycle at those mills using conventional horizontal sterilisers. This permits adequate thermal treatment with a shorter time for sterilisation, use of process steam at a lower temperature and a lower steam consumption, all of which translate into drastic improvements in energy efficiencies and water consumption.

Another major and common problem in palm oil mills today is the rapidly fluctuating process steam demand with sharp peaks and troughs, originating from the sterilisation of FFB as a batch process. This leads to an unsteady operation of the CHP plant and steam blow off to the atmosphere at several locations, wasting large amounts of energy and water. It could also lead to upsets in process steam pressure that may affect process temperatures. A simple method is available to tackle the fluctuations in process steam demand of the palm oil extraction process to allow the plant to operate in steady state and efficiently. The method involves isolating the fluctuations from affecting the steam boiler and the steam turbine of the CHP plant.

At present steam for process heating is set at a temperature of 143 ºC and 4 bar pressure. However, the palm oil extraction process requires temperatures of no more than 110 ºC. Hence the current temperature of steam use should be questioned. On the contrary, if the pressure is held steady at all times, the process steam temperature can be safely lowered close to 110 ºC with a lower pressure by way of improving heat transfer to various processes. The lowering of process steam pressure could vastly improve power output by the CHP plant and achieve a higher energy-efficiency usage of the biomass.

The most efficient way of exploiting biomass for energy is through the principle of Combined Heat and Power (CHP) or otherwise known as Cogeneration where the residual heat from a power generation process is recovered as usable heat for a downstream process. This allows making more efficient use of fuel inputs to achieve high energy conversion efficiencies and maximise carbon emissions reduction.

While palm oil mills have already been using the cogeneration principle for quite some time and are now mostly self-sufficient in energy supply, the full potential of the biomass energy has yet to be optimised.

At present CHP plants are configured to generate low power output to cater for the low power-to-heat demand ratio of the palm oil extraction process at the mill. However, the requirement to supply usable heat to the palm oil extraction process at a low temperature presents a potential to configure and operate the CHP plant at much higher power output, except that this capability is currently impeded by the fluctuating demand for process heat and high process steam pressure. Achieving a steady state operation of the CHP plant together with the lower process steam pressure as proposed above paves the way to generate large additional power at the mill for export commensurate with the process heat consumption levels and provides high energy-efficiency conversion for the biomass to optimise the energy usage at the mill. However this potential can only be realised in those locations where the additional power can be exported and distributed to the local networks and grid. A lack of power export facility may therefore be a limitation to achieving the optimal energy conversion potential of the CHP at the mill.

It is a misconception in the palm oil industry that mills cannot contribute to energy efficiency by way of additional power generation (via CHP using surplus biomass) unless there are adjacent heat consuming plants to utilise the additional heat output arising therefrom. On the contrary, the existing low power to heat demand ratio of the mill already provides the opportunity to generate more power, maximising energy conversion efficiency, regardless of whether or not there are nearby heat users. In other words, more electric power can be generated with the existing amount of steam flowing through a steam turbine.

CHP plant at the mills need only be optimally configured to fit the usable heat needs of the mill, while the surplus biomass could be conserved for CHP implementation at a location elsewhere, wherever there is a heat consumer. Such operating practice would not result in any loss in energy or carbon emissions reduction potential in the biomass utilisation. What matters for carbon emissions reduction is the high efficiency of energy usage at the mill rather than the quantum of renewable energy produced at the mill.

If the above innovative approach is implemented in a palm oil mill it could demonstrate that mills can be converted into centres of clean energy production unleashing its hidden potential. This can increase the availability of renewable energy at palm oil mill and thereby reduce the carbon footprint of palm oil production by 0.79 tCO2e per tonne of crude palm oil.

Additional revenue streams from the sale of electricity and biomass fuel can represent a significant hike in income for a mill at present energy prices. The twist here is that environmental improvements can surprisingly, also improve profits.

This article originally appeared in Sustainable palm oil conversation and debate

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