ABSTRACT
In 1998 ninety million tonnes of oil palm fruit production was recorded, 43-45% of this will be mill residues in the form of empty fruit bunches (EFB), shell and fibre. Additional wastes generated include palm fronds and stems. These wastes are currently underutilised. Maximising energy recovery from the wastes is desirable for both economic and environmental reasons. Most crude palm oil mills harness the energy from the shell and fibre in their own low pressure boilers and the EFB's are usually burnt causing air pollution or returned to the plantation. A 60T/FFB/hour mill based within a 10,000 hectare plantation, can generate enough energy to be self sustaining and supply surplus electricity to the grid if it utilises all of its wastes. Boiler technology, gasification, pyrolysis and anaerobic digestion are alternate conversion technologies available to maximise energy recovery. The Clean Development Mechanism will promote the reduction of emissions and sustainable development by encouraging energy projects for the long term, utilising local skills and creating employment.
INTRODUCTION
This paper discusses how energy recovery can be maximised from palm oil wastes. We will look at the current palm oil production climate, in order to establish the amount of palm oil residues generated and where. The residues will be identified and their current usage and disposal assessed along with alternative uses. This then leads on to how energy recovery from the residues can play an essential role in the economics of the mills and how surplus electricity can be fed to the grid. We will discuss the appropriate technologies available. Finally, the implications of the Kyoto Protocol and carbon trading will be addressed.
BIOENERGY IN THE WORLD CONTEXT
Biomass has a significant role to play in solving the worlds energy needs. Biomass combustion is carbon neutral. The carbon dioxide released in combustion is recycled by trees and crops which may provide fuel for the future. By utilising biomass as a fuel instead of a non renewable fossil fuel, the net carbon dioxide released into the atmosphere is deemed to be reduced. Biomass is capable of replacing fossil fuels in order to provide electrical power and generate heat in those areas where it is abundantly available. Commercial, community and environmental benefits also ensue. However, before the biomass projects can be implemented, the commercial viability of projects has to be proven. The cost effectiveness and availability of the resource must be weighted as evenly as the green issues. A project needs to be viable in the long term in order to maintain an economic return and for the project to achieve its environmental, social and technological objectives.
In order for biomass to be widely accepted, overall cost factors have to be fully analysed. Comparing the use of biomass to other established fossil fuels has to be taken into account, together with the investment and infrastructure already in place. Incorporating bioenergy into a holistic framework which assesses the total cost including the environment shows that biomass can be a competitive energy source (Shell, 1999).
THE ENVIRONMENT AND THE FUTURE
It is difficult to estimate the potential contribution of biomass to world energy supplies because it depends on assumptions concerning price, socio-political influences and technical developments. The Riges model (Renewable Intensive Global Energy Scenario) suggested that by 2050 around half the worlds primary energy consumption could be met by biomass. According to the report, promoting energy efficiency and renewables aggressively could reduce global carbon dioxide emission levels to 75% of 1985 levels by 2050. Other assumptions suggest that biomass would expand to produce 18% of electricity requirements during the period 2025-2050. Biomass plantations are envisaged as the main sources of material for modern biomass conversion technologies with degraded and surplus land providing the areas needed. However, existing plantations such as those of oil palm will contribute significantly to this figure, especially in the early years, since the resource is already available.
As populations expand and develop, industrialising economies will require additional energy sources, much of which will be provided by renewable sources. Within twenty five years renewable energy will provide between 5-10% of the worlds energy sources and maybe 50% by the middle of the century (Shell, 1999).
By 2020 electricity use is expected to increase by 97% on the 1995 figure, with demand more than tripling in developing countries. In developed countries an increase of 42% is predicted as new uses are found for electric power. Global energy in that year will be about three times that consumed in 1970. By the year 2020 fossil fuels will have reached a plateau and renewable energy will become significant. Renewable energy will grow to compete with traditional fuels. The energy supply will become more diversified.
PALM OIL PRODUCTION AND RESIDUES
The growth of the oil palm industry is rapid with over 8.8 million hectares under production in 1998. Oil palm fruit production has risen at a huge rate from 11 million tonnes in 1990 to 94 million tonnes in 1998, FAO 1999. By next year yields will be over 100 million tonnes, FAS online 1990, 43-45% of this will become mill residue in the form of Empty fruit bunches (EFB), shell and fibre.
The following chart illustrates the actual and forecast production of residues from the palm oil milling process.
Oil palm residues will continue to accumulate. Residues will not only accumulate from the mills but from the extensive replanting programmes ongoing in plantations. Substantial additional biomass in the form of oil palm fronds and stems will also be generated. Recovering additional energy from palm wastes offers extra financial and environmental benefits without interfering with primary production of oils for the market.
Normally a palm oil mill can achieve a recovery of approximately 93% of the available palm oil from the fresh fruit bunches (FFB's). In general the FFB contains approximately:
Crude palm oil | 21 - 23% |
Palm kernel | 6 - 7% |
Fibre | 14 - 15% |
Shell | 6 - 7% |
EFB | 23% |
The energy which can be generated from these wastes depends on the moisture content. Empty fruit bunches are wet and have a lower calorific value.
Residue | Moisture content | Calorific value |
EFB | 65% | 4.4 MJ/kg |
Fibre | 40% | 9.6 MJ/kg |
Shells | 25% | 13.4 MJ/kg |
Fronds | 3.0 MJ/kg | |
Palm wood | 7.4 MJ/kg |
PRINCIPLES OF WASTE MANAGEMENT
The Fundamental principles of waste management are to 'Minimise, Recycle, Recover (energy) and finally Disposal'. These principals apply as much to agro-industrial wastes such as palm oil residues as they do to municipal waste. We can simply no longer afford to dispose of, or dump the residues when there is an economically useful alternative.
Whilst our aim here is to address the potential for recovery of energy in the palm oil industry we must first consider the current uses and disposal of mill residues.
CURRENT USES AND DISPOSAL OF MILL RESIDUES
- Fibre and shell
Typically crude palm oil mills use fibre and shell as a boiler fuel to produce process steam for sterilisation, etc and also possibly for electricity generation as an alternative to diesel generation. The power house may also have to supply electricity for other parts of the mill complex. These residues make oil palm mills self sustainable in energy. The fibre and shell alone can supply more than enough energy to meet the mill's requirements using low pressure relatively inefficient boilers.
- Empty Fruit Bunches
The empty fruit bunches have traditionally been burnt in simple incinerators, as a means of disposal and the ash recycled onto the plantation as fertiliser. This process causes air pollution and has now been banned in some countries, for example Malaysia. Under this route of disposal no energy is recovered. Alternatively EFB can be composted and returned to the plantation, or returned directly as mulch.
Using the EFB's as a mulch or compost can reduce the applied fertiliser cost and is a step towards environmental conservation by reducing dependence on fossil fuel required for the manufacture of inorganic fertiliser. However, inorganic fertilisers are still necessary for growth. Oil palm makes heavy demands on the nutrient supplies of the soils, especially K and N. Therefore it is essential that nutrients are returned to the soil.
Empty fruit bunches are a good sources of organic matter and plant nutrients. It has been calculated that EFB mulching at 27 tonnes per hectare is equivalent to the current fertiliser practice involving inorganic fertilisers. It is claimed that using the EFB as mulch has several advantages for the nutritional sustainability of the plantation. Mulch benefits crop production because it releases nutrients slowly to the soil via microorganisms therefore effectively recycling the plant nutrients. It improves the soil structure due to better aeration, increases the water holding capacity and increases the soil pH. It is claimed that this also increases the FFB yield over and above the increase due solely to the fertiliser value.
However, since FFB return only just over 20% EFB from an average production of 25tonnes/FFB/Ha only about 5 tonnes of EFB can be returned to the field as nutrients. So only 1/5 to 1/6 of the plantation can be maintained without inorganic fertilisers (Loong et al, 1987). Some plantation owners claim that the benefits of EFB as fertiliser and as a soil conditioning agent are significant, whilst other mill owners welcome alternate methods of disposal. This is due to the inconvenience of handling and transporting, as well as the costs and problems concerning disposal of the wastes onto the plantation.
On the optimistic side, the value of returning the EFB to the plantation can be quantified as follows:
- Typically a hectare yields about 25T FFB, which results in 5.5T EFB.
- At 35-50T/ha/y spreading rate, only 11-16% plantation can be treated
- 1T/ FFB/ha/yr is gained due to soil conditioning effect above the pure fertiliser benefit
- Total benefit of spreading EFB is $ 60/ha (value of fertiliser saved/ha)
plus $120/ha/yr (Value of increase yield of FFB)
$180/ha/yr - Estimated benefit in tonnes EFB $ 3.6 to $5.1/T EFB
- Allowing for transport and spreading costs
the net benefit value is: $ 2.1 to $3.3/ T EFB
This can also be considered to be the cost of EFB if it is alternatively used as a fuel.
The following conclusions can be drawn. The net value of EFB as a fertiliser and soil improver, allowing for transporting the EFB back to the plantation varies depending on the soil type/conditions. For a mature plantation in Indonesia, for example, the net value of the EFB can be expected to range form zero to as high as $6.9/T/EFB with perhaps a typical value of $2-3/T/EFB. Many owners find the transporting and handling of the wastes a nuisance and often the wastes are not properly spread around the plantations. For many the process of transporting the wastes into plantations over 10,000 hectares is not feasible time wise or economically. The other conclusion is that EFB's can only cover an average one fifth of the plantation and there is still a need to have inorganic fertilisers bought in to supplement the rest of the plantation, in any case.
EFB are a resource which has huge potential to be used for power generation, currently not being utilised.
- Palm Oil Mill Effulent (POME)
POME is generated mainly from the sterilisation and clarification processes. It is made up of @ 95-96% water, 0.6%-0.7% oil and 4-5% total solid including 2-4% suspended solids which are mainly debris from palm mesocarp.
Most palm oil mills and refineries have their own treatment systems for POME, which due to its high organic content is easily amenable to biodegradation. The treatment system usually consists of anaerobic/aerobic ponds. However, because of silting and short circuiting many do not reach discharge standards to water courses. This situation can be significantly improved by introducing enclosed anaerobic digestion systems which reduce the BOD/COD of the effluent and capture methane, one of the more potent greenhouse gases. The energy in the methane can then be recovered, either as a supplementary boiler fuel, or in a biogas engine generator. When the BOD level is reduced to below 5000 ppm, the digested material can be applied to land as a fertiliser. This can also create savings on fertiliser costs within a plantation (Ngan, 1995).
Other uses of POME include use as an animal feed equivalent to rice bran.
- Stems and fronds
One hectare supports approximately 130-143 trees. A hectare can also produce 10 tonnes of palm fronds. Estimates from 1993-1994 suggested that 44,000 hectares had to be replanted in Malaysia which involved felling approximately 6 million palms. This leaves a lot of waste wood and fronds. These wastes are currently often left on the ground as mulch, composted or burnt. There is an obvious benefit for soil fertility and structure, if properly managed. The wastes can also be harnessed for significant energy recovery.
ALTERNATIVE USES OF WASTES
- Pulp and paper from oil palm fibres
The fibre from EFB resembles the short fibred hardwoods like Eucalyptus. The high number of fibres/unit weight indicates the paper from EFB would have good printing properties and a good formation within paper making. Studies show that EFB could produce thin, high quality printing paper, speciality papers for example for cigarette and photographic papers and security papers.
- Preformed furniture from trunks and fronds
Oil palm fronds and trunks have been chipped and waxed with resin to produce pre formed desk tops and chair seats for schools. The furniture is characterised for resistance against knocks, scratches, ink, termites and fungus (Hassan et al, 1997)!
- Planting medium/substitute for rock wool
Oil palm wastes can be reduced in size and processed into blocks of planting medium. The medium can be used to grow a wide range of vegetables. The product is biodegradable, has good water retention and fertiliser qualities. It is readily available and produced from renewable resources (Kamarudin, 1995).
- Charcoal briquettes
Proprietary proposals have been made to chip and dry EFB and then pyrolyse it to produce charcoal, which in turn can be briquetted for use as a fuel.
MAXIMISING ENERGY RECOVERY
Palm oil mill residues are currently underutilised. Maximising energy recovery from the wastes is desirable for both economic and environmental reasons.
As mentioned before, most crude palm oil mills harness the energy from the shell and fibre in their own low pressure steam boilers. The introduction of advanced cogeneration (combined heat and power) can play a major role in combatting climate change, as well as introducing significant economic benefits. Cogeneration cuts energy/fuel costs, uses fuels at high conversion efficiencies which results in an overall reduction of emissions of carbon dioxide and other pollutants. However, it is only worth doing if one can sell the additional surplus energy (electricity) to a customer at an economic rate. Nowadays the ability to sell electricity into the local grid provides an opportunity to turn waste into a valuable commodity.
- Potential electricity generated from waste – world wide!
Forty countries are involved in palm oil production with a world cultivation in 1998 of nearly 9 million hectares. The worldwide production capacity is 94 million tonnes of oil palm fruit.
A approximate estimate of the potential electricity which could be generated is:
94,000,000 T x 132 kWh = 12.4 TWh
Self consumption in palm oil mills is:
94,000,000 T x 12 kWh = 1.1 TWh
Potential surplus electricity that could be used outside the mills is 11.3 TWh annually. At current per capita consumption this could provide electricity for 12 million people in countries with utilisation similar to Thailand.
CASE STUDY – A WORKING MODEL – SUPPLYING ELECTRICITY TO THE GRID (INDONESIA)
It has been estimated in a particular case study that a 60T/FFB/hour mill, located within a 10,000 hectare plantation, can generate enough energy to be self sustaining and to supply surplus electricity to the grid if it utilises all of its wastes. The mill has a gross electrical output of 7843 kW and when the mill is not operating a maximum net output of 7216kW. There is a high process steam demand which causes the electrical output to fall to 4128kW. After deducting the energy utilised by the mill the net quantities available to sell to the grid are 7096kW when the mill is not operating and 3428 kW whilst in operation.
Factory profile (10,000 hectare plantation) 7.8 MW plant | ||
Installed capacity | 60 T FFB/hour | |
Actual production target | 250,636T/FFB/Year | |
Daily Quantity | 964 T/FFB | |
Effective working days | 260 | |
Effective working hours | 16 | |
Products: | 54,137T/year CPO (21.60T/100TFFB) | |
Residues generated | ||
EFB | 60,162T | 24.0 |
Fibre | 30,578 | 12.2 |
Shell | 16,792 | 6.7 |
Liquid waste | 72,528 | 28.9 |
Unrecovered oil/kernel etc | 16,439 | 6.6 |
Type | Medium pressure boiler system | |
Process steam requirement | 425-500kg/T of FFB | |
Electricity produced: | ||
Mill shut down | ||
Elec.output (gross) (kW) | 7,843 | |
Elec.output (net) (kW) | 7,216 | |
Elec. to grid (kW) | 7,096 | |
Mill operating | ||
Elec. Output (gross) (kW) | 4,755 | |
Elec. Output (net) (kW) | 4,128 (after steam demand) | |
Elec. to grid (kW) | 3,428 | |
Elec. To host | 3,502,008 (kWh/year) | |
Elec. To grid | 41,841,505 (kWh/year) | |
Total Elec. | 45,343,513 (kWh/year) | |
- Additional sources of biomass and effect on case study
The palm fronds have the potential energy quantity of approximately equal to that of fibre or about 2.5MW in electrical base load equivalent. However, they are not always available since they are usually left in the plantation as mulch.
The usage of dead palm trunks could increase the electrical output by about 3.3MW. The additional cost of collecting and preparing the palm trunks has to be allowed for.
Energy can also be recovered from the anaerobic digestion of the POME although this is less significant at 260 kW. However, the biogas may help combustion stability and reduce the need for any oil firing when the fuel moisture content is high.
The above information suggests that a typical 10,000 hectare plantation could result in a power plant with a output of between 5MW and 11 MW (at a better boiler heat rate) produced entirely from its own biomass waste. These figures also take into account the steam requirements. If wastes are also utilised from other mills the power output is much higher.
ALTERNATIVE CONVERSION TECHNOLOGIES
In order for the plant to run more efficiently various options within the plant can be considered.
- Boiler technology
A traditionally designed palm oil mill burns all its fibre and about 50% of its shell in order to produce the electrical and process steam needs of the plant while the mill operates. The boilers are normally low pressure, typically 20 bar and feed back pressure steam turbines. A 60 T/hr FFB mill requires approximately 700kWe and 21,000kWth (30 te/hr process steam at 3.5kg/cms and 270°C*). This represents a very low cycle efficiency for the production of electricity approximately 3.5% but a very high overall cogeneration efficiency which exceeds 90%.
By raising the boiler pressure and introducing condensing steam turbines with extraction to provide process steam, the use of palm oil residues can be optimised in more energy efficient systems. Using a boiler pressure of 45 bar, medium pressure, with a condensing steam turbine and the same process steam as before, results in a cycle efficiency for electrical production of about 13%. In addition, the boiler can be designed to burn all of the wastes not just the fibre and shell. At 13% efficiency, the fibre from one tonne of FFB will produce about 41kWhe, shell will produce about 47kWhe and EFB will produce about 46kWhe. The total waste from 1T of FFB will, therefore, produce about 132kWe. This compares to only about 12kWe per 1T of FFB for the waste from the traditionally designed plant. For example a 60 T/h FFB mill could produce 8040 kWe. With mill use remaining at 700kWe, the surplus available for export would be 7340 kWe.
A high pressure boiler system, 65 bar, 450°C, costs about 35% more than a 45 bar system for both capital and O&M costs. The increase in efficiency going from 47bar/370°C to 65 bar/ 485°C will be about 25%. Alternatively, the MW output should increase from 7.34 MWe (net) to about 10.78 MWe (net). Table1 summarises the above.
Table 1 Boiler summary
Low pressure | Medium pressure | High pressure | |
Pressure/ boiler outlet conditions | 20bar/270°C* | 45 bar/370°C | 65 bar/485°C |
Capital cost | Base cost | 35% more | |
Cycle efficiency:process | 17% | 25% | |
Mwe (net) | 7.3MWe | 10.78Mwe |
- Boiler combustion and EFB
Utilising EFB's in the boiler can create combustion problems due to the moisture content of the fuel (60-65%), its fibrous nature and the amount of soluble alkalis contained within it. A solution to the problem of machine wear is the use of a shredder to reduce materials and moisture content. The moisture content needs to be reduced to 40% or less before it can be used in a boiler. A special screw press can be used for this.
Solutions concerning the combustion of palm oil wastes include:
The fluidised bed approach. This approach however, is not useful when the soluble alkalis are as high as they are in palm oil since the temperatures in the bed exceed the eutectic temperature, indicating that slagging in the bed will result. It is also more expensive than a stocker approach.
Combustion should be staged in order to minimise the production of nitrogen oxides and to promote better burnout of the carbon in a fuel. This approach has been refined to handle high moisture wastes, such as EFB, where temperatures in the lower furnace remain low.
- Large scale gasification
This process involves heating solid or semi solid carbonaceous material with steam and limited amounts of air. Some of the material burns to maintain the temperature and the rest reacts chemically with the residual oxygen present and with the steam to produce fuel rich gases. The gas produced from biomass has a calorific value of about 14% of natural gas. The resulting gases can be burned in conventional boilers after suitable clean up but are most efficiently used in gas turbines as part of an integrated gasification combined cycle (IGCC) process. The use of gas turbines with steam turbines in a combined cycle generation for electricity generation, coupled with biomass gasification offers theoretical efficiency gains over the use of conventional steam boilers and turbines.
However, gasification has not yet been commercialised successfully and has never been used for palm oil wastes. The technology requires a consistent fuel with a low (10-15%) moisture content and is unlikely to be economically competitive with combustion technologies below a capacity of about 30MW.
- Pyrolysis
Pyrolysis involves heating biomass or other organic material in the absence of oxygen so that volatile materials are driven off leaving a solid residue or char behind. These volatiles are then condensed as hydro-carbon rich liquids or bio-oils which can be used as a fuel for diesel engines or gas turbines. The residual char can be burnt as it is or sold as charcoal. Pyroloysis has not been used for the disposal of palm oil wastes. However, there are potentially major advantages in locations where it is not possible to sell electricity economically or at all. Bio-oil and charcoal can be transported elsewhere where they can be economically used. Conversion of biomass can be de-coupled from energy recovery.
- Anaerobic digestion
Anaerobic digestion, whilst treating effulent, produces a valuable product, in captured methane rich biogas.
In a 60 T/FFB per hour palm oil mill operating for 20 hours per day, approximately 20,000 cubic metres of biogas can be obtained per day. The gas has a calorific value of 53,000kcal per cubic metre. The gas contains about 65% methane, 35% carbon dioxide and less than 2000mg/L of hydrogen sulphide, (Ma 1999).
CLEAN DEVELOPMENT MECHANISM AND CARBON EMISSIONS TRADING
The Kyoto Protocol was adopted in December 1997 in response to the Earth summit held at Rio de Janeiro in 1992. It reflects the objectives of the UNFCC (United Nations Framework Convention on Climate Change) to stabilise the concentration of green house gases in the atmosphere. Developed countries (Annex 1) are responsible for 80% of the worlds emissions of green house gases. The Kyoto Protocol caused the Annex 1 countries to reduce their emissions by 2008-2012 to levels 5% lower than those emitted in 1990. In order for parties to achieve this, a number of flexible mechanisms were employed including 'emissions trading and the Clean Development Mechanism (CDM). Each Annex 1 country has been assigned a given amount which will be equal to the greenhouse gas emission limit for that party. The tradeable units are one tonne of carbon dioxide equivalent. At the moment a price of $20/metric tonne is being considered for forward sales, with the possibility that this will rise to over $100 when the mechanism is fully operational. The Annex 1 countries will be able to buy or sell permits. The mechanisms reduce the overall price of meeting the Kyoto targets by allowing countries with high reduction costs to acquire "emission credits" from countries where reductions are cheaper or free.
The purpose of CDM is to assist developing countries to achieve sustainable development by encouraging energy projects which are long term, utilise local information and technologies and create employment. It will also assist the Annex 1 countries in achieving compliance with their quantified emission limitation and reduction commitments by paying money to finance climate friendly projects in non Annex 1 countries. The CDM will benefit developing countries which are rapidly industrialising by reducing emissions and creating cleaner industrial environments. It will keep a focus on the environmental consequences of industrial activity. At the same time external investment in developing countries will also be encouraged.
Developing renewable energy projects, using palm oil residues for example, in countries that are rapidly becoming industrialised affords a challenge. The countries are seeking to grow economically whilst being confronted with the environmental lessons learned in the process of economic growth by the industrialised countries. Within developing countries much of the infrastructure that would support an industrialised economy is not yet in place. This allows greater scope for alternative energy sources and patterns of end use consumption as this new capital stock is put into place. Of course rapidly industrialising economies are not going to adopt patterns of energy production and consumption if they are perceived as policies which are more costly and likely to undermine impending growth objectives. However, in certain locations, palm oil residues are already competitive with existing fossil fuels, so the combined objectives of economic growth and reduced environmental impact can be achieved.
