Palm kernel shells are widely used in fluidized bed combustion-based power plants in Japan and South Korea. The key advantages of fluidized bed combustion (FBC) technology are higher fuel flexibility, high efficiency and relatively low combustion temperature. FBC technology, which can either be bubbling fluidized bed (BFB) or circulating fluidized bed (CFB), is suitable for plant capacities above 20 MW. Palm kernel shells (PKS) is more suitable for CFB-based power plant because its size is less than 4 cm.
Palm kernel shells is an abundant biomass resource in Southeast Asia
With relatively low operating temperature of around 650 – 900 oC, the ash problem can be minimized. Certain biomass fuels have high ash levels and ash-forming materials that can potentially damage these generating units.
In addition, the fuel cleanliness factor is also important as certain impurities, such as metals, can block the air pores on the perforated plate of FBC unit. It is to be noted that air, especially oxygen, is essential for the biomass combustion process and for keeping the fuel bed in fluidized condition.
The requirements for clean fuel must be met by the provider or seller of the biomass fuel. Usually the purchasers require an acceptable amount of impurities (contaminants) of less than 1%. Cleaning of PKS is done by sifting (screening) which may either be manual or mechanical.
In addition to PKS, biomass pellets from agricultural wastes or agro-industrial wastes, such as EFB pellets which have a high ash content and low melting point, can also be used in CFB-based power plants. More specifically, CFBs are more efficient and emit less flue gas than BFBs.
The disadvantages of CFB power plant is the high concentration of the flue gas which demands high degree of efficiency of the dust precipitator and the boiler cleaning system. In addition, the bed material is lost alongwith ash and has to be replenished regularly.
A large-scale biomass power plant in Japan
The commonly used bed materials are silica sand and dolomite. To reduce operating costs, bed material is usually reused after separation of ash. The technique is that the ash mixture is separated from a large size material with fine particles and silica sand in a water classifier. Next the fine material is returned to the bed.
Currently power plants in Japan that have an efficiency of more than 41% are only based on ultra supercritical pulverized coal. Modification of power plants can also be done to improve the efficiency, which require more investments. The existing CFB power plants are driving up the need to use more and more PKS in Japan for biomass power generation without significant plant modifications.
Europe is targeting an ambitious renewable energy program aimed at 20% renewable energy in the energy mix by 2020 with biomass energy being key renewable energy resource across the continent. However, the lack of locally-available biomass resources has hampered the progress of biomass energy industry in Europe as compared with solar and wind energy industries. The European biomass industry is largely dependent on wood pellets and crop residues.
Europe is the largest producer of wood pellets, which is currently estimated at 13.5 million tons per year while its consumption is 18.8 million tons per year. The biggest wood pellet producing countries in Europe are Germany and Sweden. Europe relies on America and Canada to meet its wood pellet requirements and there is an urgent need to explore alternative biomass resources. In recent years, palm kernel shells (popularly known as PKS) from Southeast Asia and Africa has emerged as an attractive biomass resources which can replace wood pellets in biomass power plants across Europe.
What are Palm Kernel Shells
Palm kernel shells 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.
Advantages of Palm Kernel Shells
PKS has almost the same combustion characteristics as wood pellets, abundantly available are and are cheap. Indonesia and Malaysia are the two main producers of PKS. Indonesian oil palm plantations cover 12 million hectares in Indonesia and 5 million hectares in Malaysia, the number of PKS produced from both countries has exceeded 15 million tons per year. Infact, the quantity of PKS generated in both countries exceeds the production of wood pellets from the United States and Canada, or the two largest producers of wood pellets today.
Interestingly, United States and Canada cannot produce PKS, because they do not have oil palm plantations, but Indonesia and Malaysia can also produce wood pellets because they have large forests. The production of wood pellets in Indonesia and Malaysia is still small today, which is less than 1 million tons per year, but the production of PKS is much higher which can power biomass power plants across Europe and protect forests which are being cut down to produce wood pellets in North America and other parts of the world.
PKS as a Boiler Fuel
Although most power plants currently use pulverized coal boiler technology which reaches around 50% of the world’s electricity generation, the use of grate combustion boiler technology and fluidized bed boilers is also increasing. Pulverized coal boiler is mainly used for very large capacity plants (> 100 MW), while for ordinary medium capacity uses fluidized bed technology (between 20-100 MW) and for smaller capacity with combustor grate (<20 MW). The advantage of boiler combustion and fluidized bed technology is fuel flexibility including tolerance to particle size.
When the pulverized coal boiler requires a small particle size (1-2 cm) like sawdust so that it can be atomized on the pulverizer nozzle, the combustor grate and fluidized bed the particle size of gravel (max. 8 cm) can be accepted. Based on these conditions, palm kernel shells has a great opportunity to be used as a boiler fuel in large-scale power plants.
Use of PKS in pulverized coal boiler
There are several things that need to be considered for the use of PKS in pulverized coal boilers. The first thing that can be done is to reduce PKS particle size to a maximum of 2 cm so that it can be atomized in a pulverized system. The second thing to note is the percentage of PKS in coal, or the term cofiring. Unlike a grate and a fluidized bed combustion that can be flexible with various types of fuel, pulverized coal boilers use coal only. There are specific things that distinguish biomass and coal fuels, namely ash content and ash chemistry, both of which greatly influence the combustion characteristics in the pulverized system.
PKS has emerged as an attractive biomass commodity in Japan
Coal ash content is generally greater than biomass, and coal ash chemistry is very different from biomass ash chemistry. Biomass ash has lower inorganic content than coal, but the alkali content in biomass can change the properties of coal ash, especially aluminosilicate ash.
Biomass cofiring with coal in small portions for example 3-5% does not require modification of the pulverized coal power plant. For example, Shinci in Japan with a capacity of 2 x 1,000 MW of supercritical pulverized fuel with 3% cofiring requires 16,000 tons per year of biomass and no modification. Similarly, Korea Southeast Power (KOSEP) 5,000 MW with 5% cofiring requires 600,000 tons per year of biomass without modification.
PKS cofiring in coal-based power plants
Pulverized coal-based power plants are the predominant method of large-scale electricity production worldwide including Europe. If pulverised fuel power plants make a switch to co-firing of biomass fuels, it will make a huge impact on reducing coal usage, reducing carbon emissions and making a transition to renewable energy. Additionally, the cheapest and most effective way for big coal-based power plants to enter renewable energy sector is biomass cofiring. Palm kernel shells can be pyrolyzed to produce charcoal while coal will produce coke if it is pyrolyzed. Charcoal can be used for fuel, briquette production and activated charcoal.
Even though palm kernel shell has many similarities with wood pellets, it is not easy to reduce its size which makes it difficult for its optimum cofiring with coal in power plants and industries. Few years ago, Indonesia had exported PKS to Poland for cofiring purposes but because PKS was difficult to make powder (low grindability) it made cofiring performance poor, so the use of PKS for cofiring is currently discontinued.
To improve the quality of PKS, especially for the use of cofiring, PKS must be processed with torrefaction (mild pyrolysis). With the torrefaction process, it becomes easier to make powder from PKS, so that the desired particle size for cofiring is easier to obtain. Another advantage of the torrefaction process is that the caloric value of PKS will also increase by about 20%, Torrified biomass is hygroscopic which means ease in indoor as well as outdoor storage.
During the torrefaction process, PKS is heated at a temperature of around 230 to 300 °C in the absence of oxygen. With continuous pyrolysis technology, torrified PKS production can be carried out at large capacities. The need for biomass fuel for electricity generation is also large, usually requiring 10 thousand tons for each shipment. PKS torrified producers must be able to reach this capacity. The production of 10 thousand tons of PKS that are burned can be done per month or several months, for example, to reach 10 thousand tons it takes 2 months because the factory capacity is 5000 tons per month.
In general, the advantages of the PKS torrefaction process are as follows:
It increases the O/C ratio of the biomass, which improves its thermal process
It reduces power requirements for size reduction, and improves handling.
It offers cleaner-burning fuel with little acid in the smoke.
Torrefied PKS absorbs less moisture when stored.
One can produce superior-quality PKS pellets with higher volumetric energy density.
Pelletizing of torrefied PKS can be an option to increase the energy density in volume basis. The pelletizing process resolves some typical problems of biomass fuels: transport and storing costs are minimized, handling is improved, and the volumetric calorific value is increased. Pelletization may not increase the energy density on a mass basis, but it can increase the energy content of the fuel on a volume basis.
Africa, especially West Africa, which has many palm oil plantations and also the location where the palm oil trees originate, can supply torrified PKS to Europe to meet its rapidly-increasing biomass fuel demand.
In Africa, palm kernel shell is generally produced from PKO mills. CPO production is generally carried out on a small scale and only processes the fiber portion of the palm oil fruit. This palm oil mesocarp fibre is processed to produce CPO, while the nut that consist kernels and shells are processed elsewhere to produce the main product of PKO (palm kernel oil). PKO mills are usually quite large by collecting nuts from these small scale CPO producers. PKS is produced from this PKO mills.
The nut cracker machine separates kernel and shell
The distance between Africa and Europe is also closer than Europe to Malaysia and Indonesia. Currently, even though Europe has produced wood pellets for their renewable energy program to mitigate climate change and the environment, the numbers are still insufficient and they are importing wood pellets from the United States and Canada in large quantities. European wood pellet imports are estimated to reach more than 1.5 million tons per year. Torrified PKS from West Africa can help in meeting the biomass fuel demands for power plants across Europe.
To improve the quality of biomass, especially for cofiring purposes, biomass waste can be processed with torrefaction (also known as mild pyrolysis). With the torrefaction process, it becomes easier to make powder (high grindability) so that the desired particle size for cofiring of biomass is easier to obtain. Another advantage of the torrefaction process is that the caloric value of biomass increases by about 20%. Torrified biomass is essentially hydropobic which means ease in storage including outdoor storage. This condition also makes it easier to handle and use, in addition to reduction in transportation costs.
What is Torrefaction
Torrefaction, which is currently being considered for effective biomass utilization, is also a form of pyrolysis. In this process (named for the French word for roasting), the biomass is heated to 230 to 300 °C without contact with oxygen. For comparison, pyrolysis of biomass is typically carried out in a relatively low temperature range of 300 to 650 °C compared to 800 to 1000 °C for gasification. Torrefaction is a relatively new process that heats the biomass in the absence of air to improve its usefulness as a fuel.
Torrefaction, a process different from carbonization, is a mild pyrolysis process carried out in a temperature range of 230 to 300 °C in the absence of oxygen. During this process the biomass dries and partially devolatilizes, decreasing its mass while largely preserving its energy content. The torrefaction process removes H2O and CO2 from the biomass. As a result, both the O/C and the H/C ratios of the biomass decrease.
Advantages of Biomass Torrefaction
Torrefaction of biomass improves its energy density, reduces its oxygen-to-carbon (O/C) ratio, and reduces its hygroscopic nature. Torrefaction also increases the relative carbon content of the biomass. The properties of a torrefied biomass depends on torrefaction temperature, time, and on the type of biomass feed.
Torrefaction also modifies the structure of the biomass, making it more friable or brittle. This is caused by the depolymerization of hemicellulose. As a result, the process of size reduction becomes easier, lowering its energy consumption and the cost of handling. This makes it easier to cofire biomass in a pulverized coal-fired boiler or gasify it in an entrained-flow reactor.
Another special feature of torrefaction is that it reduces the hygroscopic property of biomass; therefore, when torrefied biomass is stored, it absorbs less moisture than that absorbed by fresh biomass. For example, while raw bagasse absorbed 186% moisture when immersed in water for two hours, it absorbed only 7.6% moisture under this condition after torrefying the bagasse for 60 minutes at 250 °C (Pimchua et al., 2009). The reduced hygroscopic (or enhanced hydrophobic) nature of torrefied biomass mitigates one of the major shortcomings for energy use of biomass.
In biomass, hemicellulose is like the cement in reinforced concrete, and cellulose is like the steel rods. The strands of microfibrils (cellulose) are supported by the hemicellulose. Decomposition of hemicellulose during torrefaction is like the melting away of the cement from the reinforced concrete. Thus, the size reduction of biomass consumes less energy after torrefaction. During torrefaction the weight loss of biomass comes primarily from the decomposition of its hemicellulose constituents. Hemicellulose decomposes mostly within the temperature range 150 to 280 °C, which is the temperature window of torrefaction.
As we can see from figure above, the hemicellulose component undergoes the greatest amount of degradation within the 200 to 300 °C temperature window. Thus, hemicellulose decomposition is the primary mechanism of torrefaction. At lower temperatures (< 160 °C), as biomass dries it releases H2O and CO2. Water and carbon dioxide, which make no contribution to the energy in the product gas, constitute a dominant portion of the weight loss during torrefaction.
Above 180 °C, the reaction becomes exothermic, releasing gas with small heating values. The initial stage (< 250 °C) involves hemicellulose depolymerization, leading to an altered and rearranged polysugar structures. At higher temperatures (250–300 °C) these form chars, CO, CO2, and H2O. The hygroscopic property of biomass is partly lost in torrefaction because of the destruction of OH groups through dehydration, which prevents the formation of hydrogen bonds.
There is a huge demand for charcoal briquettes in the Middle East, especially in Saudi Arabia, Egypt and UAE. However the production of charcoal in the Middle East is in nascent stages despite the availability of biomass resources, especially date palm biomass. The key reason for increasing demand of charcoal briquettes is the large consumption of meat in the region which uses charcoal briquettes as fuel for barbecue, outdoor grills and related activities.
The raw materials for charcoal briquette production are widely available across the Middle East in the form of date palm biomass, crop wastes and woody biomass. With a population of date palm trees of 84 million or 70% of the world’s population, the potential biomass waste from date palm trees is estimated at 730,000 tons / year (approximately 200,000 tons from Saudi Arabia and 300,000 tons from Egypt). Date palm trees produce huge amount of agricultural wastes in the form of dry leaves, stems, pits, seeds etc. A typical date tree can generate as much as 20 kilograms of dry leaves per annum while date pits account for almost 10 percent of date fruits.
The fronds and trunks of date palm trees are potential raw materials for charcoal because of the potential to produce high calorific value and low ash content charcoal. Leaf waste will produce a low calorific value due to high ash content. In addition, woody biomass waste such as cotton stalks that are widely available in Egypt can also be a raw material for making charcoal. The contribution of the agricultural sector in Egypt is quite high at 13.4%.
Charcoal is compacted into briquettes for ease in handling, packaging, transportation and use. Briquettes can be made in different shapes such as oval, hexagonal, cube, cylinder or octagonal. An adhesive (called binder) is needed for the manufacture of the briquette. Two common binders are saw dust and corn starch.
Date palm biomass is an excellent resource for charcoal production in Middle East
Continuous pyrolysis is the best technology for charcoal production. Continuous pyrolysis has the ability to handle large biomass volumes, the process is fast and smoke production is negligible. When using conventional pyrolysis technology (or batch carbonization), the process is lengthy, processing capacity is small and there are concerns related to harmful smoke emissions.
Apart from charcoal, continuous pyrolysis also gives bio oil, wood vinegar and syngas. Syngas can be converted into electricity by using a gas engine or converted into a wide variety of biofuels through different processes. Bio oil can be used as boiler fuel and marine fuel. Wood vinegar can be used as biopesticide and liquid organic fertilizer. Low water content in date palm waste fronds and trunks make it very suitable for thermochemical conversion technologies, especially pyrolysis and gasification.
Charcoal can also be used for the production of activated charcoal/carbon. Activated carbon is used by a lot of industries for purification processes. In addition, a number of industries that are using petcoke as fuel can switch to charcoal due to its better combustion properties and eco-friendly nature.
Japan’s biomass fuel requirement is estimated to be tens of millions of tons each year on account of its projected biomass energy capacity of 6,000MW by the year 2030. To achieve this capacity, more than 20 million tons of biomass fuel will be needed every year which will be mainly met by wood pellets and palm kernel shell (PKS). The similarity of the properties of wood pellets with PKS makes PKS the main competitor of wood pellets in the international biomass fuel market.
PKS has emerged as an attractive biomass commodity in Japan
Canada and USA are the biggest suppliers of wood pellets to the Japanese biomass market while PKS mainly comes from Indonesia and Malaysia. With the size of the material almost the same as wood pellets, but at a cheaper price (almost half the wood pellets) and also available in abundance, PKS is the preferred biomass fuel for the Japanese market. PKS can be used 100% in power plants that use fluidized bed combustion technology, while wood pellets are used in pulverized combustion.
Although there is abundant PKS in CPO (crude palm oil) producing countries, but fluctuations in CPO production and increase in domestic demand has led to reduction in PKS exports in Southeast Asia. In palm oil plantations, it is known as the low crop season and peak crop season. When the low crop season usually occurs in the summer or dry season, the supply of fruit to the palm oil mills decreases so that the CPO production decreases and also the supply of PKS automatically reduces, and vice versa in the peak crop season. When demand is high or even stable but supply decreases, the price of PKS tends to rise.
In addition, a wide range of industries in Indonesia and Malaysia have also began to use PKS as an alternative fuel triggering increased domestic demand. In recent years, PKS is also being processed into solid biomass commodities such as torrified PKS, PKS charcoal and PKS activated carbon. Thus, there is very limited scope of increasing PKS supply from Southeast Asia to large-scale biomass consumers like Japan and South Korea.
Palm oil mills process palm oil fruit from palm oil plantations, so the more fruit is processed the greater the PKS produced and also more processors or mills are needed. At present it is estimated that there are more than 1500 palm oil mills in Indonesia and Malaysia. Palm kernel shells from Indonesia and Malaysia is either being exported or used domestically by various industries. On the other hand, in other parts of the world PKS is still considered a waste which tends to pollute the environment and has no economic value.
Top palm oil producers around the world
West African countries, such Nigeria, Ghana and Togo, are still struggling to find a sustainable business model for utilization of PKS. Keeping in view the tremendous PKS requirements in the Asia-Pacific region, major PKS producers in Africa have an attractive business opportunity to export this much-sought after biomass commodity to the Japan, South Korea and even Europe.
Simply speaking, PKS collected from palm oil mills is dried, cleaned and shipped to the destination country. PKS users have special specifications related to the quality of the biomass fuel used, so PKS needs to be processed before exporting.
PKS exports from Indonesia and Malaysia to Japan are usually with volume 10 thousand tons / shipment by bulk ship. The greater the volume of the ship or the more cargo the PKS are exported, the transportation costs will generally be cheaper. African countries are located quite far from the Asia-Pacific region may use larger vessels such as the Panamax vessel to export their PKS.
Biomass is being increasingly used in power plants in Japan as a source of fuel, particularly after the tragic accident at Fukushima nuclear power plant in 2011. Palm kernel shell (PKS) has emerged as a favorite choice of biomass-based power plants in the country. Most of these biomass power plants use PKS as their energy source, and only a few operate with wood pellets. Interestingly, most of the biomass power plants in Japan have been built after 2015.
Palm Kernel Shells
Palm Kernel Shell is generating very good traction as a renewable energy resource and biomass commodity in Japan. This is because PKS is the cheapest biomass fuel and is available in large quantities across Southeast Asia. PKS, a biomass waste generated by palm oil mills, can be found in plentiful quantities in Indonesia, Malaysia and Thailand.
PKS must meet the specifications before being exported to Japan. Some key specifications for PKS exports are: moisture content, calorific value and impurities or contaminants (foreign materials). All three variables must meet a certain level to achieve export quality. Japanese markets or their consumers generally require contaminants from 0.5 to 2%, while European consumers of PKS need 2% – 3%.
Japan usually buys with a volume of 10,000 tonnes per shipment, so PKS suppliers must prepare a sufficient stockpile of the PKS. The location of PKS stockpile that is closest to the seaport is the ideal condition to facilitate transportation of shipment.
PKS has emerged as an attractive biomass commodity in Japan
Wood pellets are mostly produced in from wood waste such as sawdust, wood shaving, plywood waste, forestry residues, and related materials while using tools like track saws, table saws, circular saws, miter saws, etc. The development potential for quantity enlargement is also possible with energy plantations. Technically the properties of wood pellets are not much different from the PKS.
Wood pellet price is more expensive than PKS. Wood pellet production process is more complex than PKS, so wood pellet is categorized as finished product. The quality of wood pellet is generally viewed from its density, calorific value and ash content. Indonesia wood pellet export is not as big as PKS, it is also because of the limited producers of wood pellet itself.
Japan buys wood pellets from Indonesia mostly for testing on their biomass power plants. Shipping or export by container is still common in wood pellet sector because the volume is still small. Currently, the world’s leading producer of wood pellets come from North America and Scandinavia. Even for Indonesia itself wood pellet is a new thing, so its production capacity is also not big.
For a short-term solution, exporting PKS is a profitable business. Wood pellets with raw materials from energy plantations by planting the legume types such as calliandra are medium-term solutions to meet biomass fuel needs in Japan. Torrefaction followed by densification can be a long-term orientation. Torrified pellet is superior to wood pellet because it can save transportation and facilitate handling, are hydrophobic and has higher calorific value.
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