Rice Straw As Bioenergy Resource

The cultivation of rice results in two types of biomass residues – straw and husk – having attractive potential in terms of energy. Rice husk, the main by-product from rice milling, accounts for roughly 22% of paddy weight, while rice straw to paddy ratio ranges from 1.0 to 4.3. Although the technology for rice husk utilization is well-established worldwide, rice straw is sparingly used as a source of renewable energy. One of the main reasons for the preferred use of husk is its easy procurement. In case of rice straw, however, its collection is difficult and its availability is limited to harvest time.

Rice_straw

Rice straw can either be used alone or mixed with other biomass materials in direct combustion, whereby combustion boilers are used in combination with steam turbines to produce electricity and heat. The energy content of rice straw is around 14 MJ per kg at 10 percent moisture content.  The by-products are fly ash and bottom ash, which have an economic value and could be used in cement and/or brick manufacturing, construction of roads and embankments, etc.

Straw fuels have proved to be extremely difficult to burn in most combustion furnaces, especially those designed for power generation. The primary issue concerning the use of rice straw and other herbaceous biomass for power generation is fouling, slagging, and corrosion of the boiler due to alkaline and chlorine components in the ash. Europe, and in particular, Denmark, currently has the greatest experience with straw-fired power and CHP plants.

Because of the large amount of cereal grains (wheat and oats) grown in Denmark, the surplus straw plays a large role in the country’s renewable energy strategy. Technology developed includes combustion furnaces, boilers, and superheat concepts purportedly capable of operating with high alkali fuels and having handling systems which minimize fuel preparation.

A variety of methods are employed by the European plants to prepare straw for combustion. Most use automated truck unloading bridge cranes that clamp up to 12 bales at a time and stack them 4-5 bales high in covered storage. Some systems feed whole bales into the boiler. Probably the best known whole bale feeder is the “Vølund cigar feeding” concept, originally applied by Vølund (now Babcock and Wilcox-Vølund). Whole bales are pushed into the combustion chamber and the straw burned off the face of the bale.

However, the newer Danish plants have moved away from whole-bale systems to shredded straw feed for higher efficiency. For pulverized coal co-firing, the straw usually needs to be ground or cut to small sizes in order to burn completely within relatively short residence times (suspension fired systems) or to feed and mix upon injection with bed media in fluidized bed systems.

The chemical composition of feedstock has a major influence on the efficiency of biomass cogeneration. The low feedstock quality of rice straw is primarily determined by high ash content (10–17%) as compared with wheat straw (around 3%) and also high silica content in ash. On the other hand, rice straw as feedstock has the advantage of having a relatively low total alkali content, whereas wheat straw can typically have more than 25% alkali content in ash.

However, straw quality varies substantially within seasons as well as within regions. If straw is exposed to precipitation in the field, alkali and alkaline compounds are leached, improving the feedstock quality. In turn, moisture content should be less than 10% for combustion technology.

In straw combustion at high temperatures, potassium is transformed and combines with other alkali earth materials such as calcium. This in turn reacts with silicates, leading to the formation of tightly sintered structures on the grates and at the furnace wall. Alkali earths are also important in the formation of slag and deposits. This means that fuels with lower alkali content are less problematic when fired in a boiler.

Thermal Conversion of Biomass

A wide range of thermal technologies exists to harness the energy stored in biomass. These technologies can be classified according to the principal energy carrier produced in the conversion process. Carriers are in the form of heat, gas, liquid and/or solid products, depending on the extent to which oxygen is admitted to the conversion process (usually as air). The major methods for thermal conversion of biomass are combustion, gasification and pyrolysis.

biomass-gasification

Combustion

Conventional combustion technologies raise steam through the combustion of biomass. This steam may then be expanded through a conventional turbo-alternator to produce electricity. A number of combustion technology variants have been developed. Underfeed stokers are suitable for small scale boilers up to 6 MWth.

Grate type boilers are widely deployed. They have relatively low investment costs, low operating costs and good operation at partial loads. However, they can have higher NOx emissions and decreased efficiencies due to the requirement of excess air, and they have lower efficiencies.

Fluidized bed combustors (FBC), which use a bed of hot inert material such as sand, are a more recent development. Bubbling FBCs are generally used at 10-30 MWth capacity, while Circulating FBCs are more applicable at larger scales. Advantages of FBCs are that they can tolerate a wider range of poor quality fuel, while emitting lower NOx levels.

Co-Firing

Co-firing or co-combustion of biomass wastes with coal and other fossil fuels can provide a short-term, low-risk, low-cost option for producing renewable energy while simultaneously reducing the use of fossil fuels. Co-firing involves utilizing existing power generating plants that are fired with fossil fuel (generally coal), and displacing a small proportion of the fossil fuel with renewable biomass fuels.

types-of-biomass-cofiring

Co-firing has the major advantage of avoiding the construction of new, dedicated, waste-to-energy power plant. Co-firing may be implemented using different types and percentages of wastes in a range of combustion and gasification technologies. Most forms of biomass wastes are suitable for co-firing. These include dedicated municipal solid wastes, wood waste and agricultural residues such as straw and husk.

Gasification

Gasification of biomass takes place in a restricted supply of oxygen and occurs through initial devolatilization of the biomass, combustion of the volatile material and char, and further reduction to produce a fuel gas rich in carbon monoxide and hydrogen. This combustible gas has a lower calorific value than natural gas but can still be used as fuel for boilers, for engines, and potentially for combustion turbines after cleaning the gas stream of tars and particulates.

Biomass_Gasification_Process

Layout of a Typical Biomass Gasification Plant

If gasifiers are ‘air blown’, atmospheric nitrogen dilutes the fuel gas to a level of 10-14 percent that of the calorific value of natural gas. Oxygen and steam blown gasifiers produce a gas with a somewhat higher calorific value. Pressurized gasifiers are under development to reduce the physical size of major equipment items.

A variety of gasification reactors have been developed over several decades. These include the smaller scale fixed bed updraft, downdraft and cross flow gasifiers, as well as fluidized bed gasifiers for larger applications. At the small scale, downdraft gasifiers are noted for their relatively low tar production, but are not suitable for fuels with low ash melting point (such as straw). They also require fuel moisture levels to be controlled within narrow levels.

Pyrolysis

Pyrolysis is the term given to the thermal degradation of wood in the absence of oxygen. It enables biomass to be converted to a combination of solid char, gas and a liquid bio-oil. Pyrolysis technologies are generally categorized as “fast” or “slow” according to the time taken for processing the feed into pyrolysis products. These products are generated in roughly equal proportions with slow pyrolysis. Using fast pyrolysis, bio-oil yield can be as high as 80 percent of the product on a dry fuel basis.

Biomass-Pyrolysis

Bio-oil can act as a liquid fuel or as a feedstock for chemical production. A range of bio-oil production processes are under development, including fluid bed reactors, ablative pyrolysis, entrained flow reactors, rotating cone reactors, and vacuum pyrolysis.

Summary of Biomass Combustion Technologies

Direct combustion is the best established and most commonly used technology for converting biomass to heat. During combustion, biomass fuel is burnt in excess air to produce heat. The first stage of combustion involves the evolution of combustible vapours from the biomass, which burn as flames. The residual material, in the form of charcoal, is burnt in a forced air supply to give more heat. The hot combustion gases are sometimes used directly for product drying, but more usually they are passed through a heat exchanger to produce hot air, hot water or steam.

Combustion_Moving_Grate

The combustion efficiency depends primarily on good contact between the oxygen in the air and the biomass fuel. The main products of efficient biomass combustion are carbon dioxide and water vapor, however tars, smoke and alkaline ash particles are also emitted. Minimization of these emissions and accommodation of their possible effects are important concerns in the design of environmentally acceptable biomass combustion systems.

Biomass combustion systems, based on a range of furnace designs, can be very efficient at producing hot gases, hot air, hot water or steam, typically recovering 65-90% of the energy contained in the fuel. Lower efficiencies are generally associated with wetter fuels. To cope with a diversity of fuel characteristics and combustion requirements, a number of designs of combustion furnaces or combustors are routinely utilized around the world

Underfeed Stokers

Biomass is fed into the combustion zone from underneath a firing grate. These stoker designs are only suitable for small scale systems up to a nominal boiler capacity of 6 MWth and for biomass fuels with low ash content, such as wood chips and sawdust. High ash content fuels such as bark, straw and cereals need more efficient ash removal systems. Sintered or molten ash particles covering the upper surface of the fuel bed can cause problems in underfeed stokers due to unstable combustion conditions when the fuel and the air are breaking through the ash covered surface.

Grate Stokers

The most common type of biomass boiler is based on a grate to support a bed of fuel and to mix a controlled amount of combustion air, which often enters from beneath the grate. Biomass fuel is added at one end of the grate and is burned in a fuel bed which moves progressively down the grate, either via gravity or with mechanical assistance, to an ash removal system at the other end. In more sophisticated designs this allows the overall combustion process to be separated into its three main activities:

  • Initial fuel drying
  • Ignition and combustion of volatile constituents
  • Burning out of the char.

Grate stokers are well proven and reliable and can tolerate wide variations in fuel quality (i.e. variations in moisture content and particle size) as well as fuels with high ash content. They are also controllable and efficient.

Fluidized Bed Boilers

The basis for a fluidized bed combustion system is a bed of an inert mineral such as sand or limestone through which air is blown from below. The air is pumped through the bed in sufficient volume and at a high enough pressure to entrain the small particles of the bed material so that they behave much like a fluid.

The combustion chamber of a fluidized bed plant is shaped so that above a certain height the air velocity drops below that necessary to entrain the particles. This helps retain the bulk of the entrained bed material towards the bottom of the chamber. Once the bed becomes hot, combustible material introduced into it will burn, generating heat as in a more conventional furnace. The proportion of combustible material such as biomass within the bed is normally only around 5%. The primary driving force for development of fluidized bed combustion is reduced SO2 and NOx emissions from coal combustion.

Bubbling fluidized bed (BFB) combustors are of interest for plants with a nominal boiler capacity greater than 10 MWth. Circulating fluidized bed (CFB) combustors are more suitable for plants larger than 30 MWth. The minimum plant size below which CFB and BFB technologies are not economically competitive is considered to be around 5-10 MWe.

Waste to Energy Conversion Routes

Waste-to-energy is the use of modern combustion and biological technologies to recover energy from urban wastes. There are three major waste to energy conversion routes – thermochemical, biochemical and physico-chemical. Thermochemical conversion, characterized by higher temperature and conversion rates, is best suited for lower moisture feedstock and is generally less selective for products. On the other hand, biochemical technologies are more suitable for wet wastes which are rich in organic matter.

Teesside-WTE-plant

Thermochemical Conversion

The three principal methods of thermochemical conversion are combustion in excess air, gasification in reduced air, and pyrolysis in the absence of air. The most common technique for producing both heat and electrical energy from household wastes is direct combustion.

Combined heat and power (CHP) or cogeneration systems, ranging from small-scale technology to large grid-connected facilities, provide significantly higher efficiencies than systems that only generate electricity.

WTE_Pathways

Combustion technology is the controlled combustion of waste with the recovery of heat to produce steam which in turn produces power through steam turbines. Pyrolysis and gasification represent refined thermal treatment methods as alternatives to incineration and are characterized by the transformation of the waste into product gas as energy carrier for later combustion in, for example, a boiler or a gas engine. Plasma gasification, which takes place at extremely high temperature, is also hogging limelight nowadays.

Biochemical Conversion

Biochemical processes, like anaerobic digestion, can also produce clean energy in the form of biogas which can be converted to power and heat using a gas engine. Anaerobic digestion is the natural biological process which stabilizes organic waste in the absence of air and transforms it into biofertilizer and biogas.

Anaerobic digestion is a reliable technology for the treatment of wet, organic waste.  Organic waste from various sources is biochemically degraded in highly controlled, oxygen-free conditions circumstances resulting in the production of biogas which can be used to produce both electricity and heat.

In addition, a variety of fuels can be produced from waste resources including liquid fuels, such as ethanol, methanol, biodiesel, Fischer-Tropsch diesel, and gaseous fuels, such as hydrogen and methane. The resource base for biofuel production is composed of a wide variety of forestry and agricultural resources, industrial processing residues, and municipal solid and urban wood residues. Globally, biofuels are most commonly used to power vehicles, heat homes, and for cooking.

Physico-chemical Conversion

The physico-chemical technology involves various processes to improve physical and chemical properties of solid waste. The combustible fraction of the waste is converted into high-energy fuel pellets which may be used in steam generation. The waste is first dried to bring down the high moisture levels. Sand, grit, and other incombustible matter are then mechanically separated before the waste is compacted and converted into fuel pellets or RDF.

Fuel pellets have several distinct advantages over coal and wood because it is cleaner, free from incombustibles, has lower ash and moisture contents, is of uniform size, cost-effective, and eco-friendly.

Biomass Resources from Rice Industry

The cultivation of rice results in two major types of biomass wastes – Straw and Husk –having attractive potential in terms of biomass energy. Although the technology for rice husk utilization is well-proven in industrialized countries of Europe and North America, such technologies are yet to be introduced in the developing world on commercial scale.

Rice-Biomass

 The importance of Rice Husk and Rice Straw as an attractive source of energy can be gauged from the following statistics:

Rice Straw

  • 1 ton of Rice paddy produces 290 kg Rice Straw
  • 290 kg Rice Straw can produce 100 kWh of power
  • Calorific value = 2400 kcal/kg

Rice Husk

  • 1 ton of Rice paddy produces 220 kg Rice Husk
  • 1 ton Rice Husk is equivalent to 410- 570 kWh electricity
  • Calorific value = 3000 kcal/kg
  • Moisture content = 5 – 12%

Rice husk is the most prolific agricultural residue in rice producing countries around the world. It is one of the major by-products from the rice milling process and constitutes about 20% of paddy by weight. Rice husk, which consists mainly of lingo-cellulose and silica, is not utilized to any significant extent and has great potential as an energy source.

Rice husk can be used for power generation through either the steam or gasification route. For small scale power generation, the gasification route has attracted more attention as a small steam power plant is very inefficient and is very difficult to maintain due to the presence of a boiler. In addition for rice mills with diesel engines, the gas produced from rice husk can be used in the existing engine in a dual fuel operation.

The benefits of using rice husk technology are numerous. Primarily, it provides electricity and serves as a way to dispose of agricultural waste. In addition, steam, a byproduct of power generation, can be used for paddy drying applications, thereby increasing local incomes and reducing the need to import fossil fuels. Rice husk ash, the byproduct of rice husk power plants, can be used in the cement and steel industries further decreasing the need to import these materials.

Rice straw can either be used alone or mixed with other biomass materials in direct combustion. In this technology, combustion boilers are used in combination with steam turbines to produce electricity and heat. The energy content of rice straw is around 14 MJ per kg at 10 percent moisture content.  The by-products are fly ash and bottom ash, which have an economic value and could be used in cement and/or brick manufacturing, construction of roads and embankments, etc.

Straw fuels have proved to be extremely difficult to burn in most combustion furnaces, especially those designed for power generation. The primary issue concerning the use of rice straw and other herbaceous biomass for power generation is fouling, slagging, and corrosion of the boiler due to alkaline and chlorine components in the ash. Europe, and in particular, Denmark, currently has the greatest experience with straw fired power and CHP plants.

Incineration of Medical Waste: An Introduction

Incineration is a thermal process that transforms medical wastes into inorganic, incombustible matter thus leading to significant reduction in waste volume and weight. The main purpose of any medical waste incinerator is to eliminate pathogens from waste and reduce the waste to ashes. However, certain types of medical wastes, such as pharmaceutical or chemical wastes, require higher temperatures for complete destruction.

Medical waste incinerators typically operate at high temperatures between 900 and 1200°C. Developing countries of Asia and Africa usually use low-cost, high-temperature incinerators of simple design for stabilization of healthcare wastes. The most reliable and predominant medical waste incineration technology is pyrolytic incineration, also known as controlled air incineration or double-chamber incineration. The pyrolytic incinerator comprises a pyrolytic chamber and a post-combustion chamber.

Medical waste is thermally decomposed in the pyrolytic chamber through an oxygen-deficient, medium-temperature combustion process (800– 900°C), producing solid ashes and gases. The gases produced in the pyrolytic chamber are burned at high temperature (900– 1200°C) by a fuel burner in the post-combustion chamber, using an excess of air to minimize smoke and odours.

Small-scale decentralized incinerators used in hospitals, of capacity 200–1000kg/day, are operated on demand in developing countries, such as India. On the other hand, off-site regional facilities have large-scale incinerators of capacity 1–8 tonnes/day, operating continuously and equipped with automatic loading and de-ashing devices. In recent years, mobile incinerators are getting attraction in developing world as such units permit on-site waste treatment in hospitals and clinics, thus avoiding the need to transport infectious waste across the city.

However, the WHO policy paper of 2004 and the Stockholm Convention, has stressed the need to consider the risks associated with the incineration of healthcare waste in the form of particulate matter, heavy metals, acid gases, carbon monoxide, organic compounds, pathogens etc. In addition, leachable organic compounds, like dioxins and heavy metals, are usually present in bottom ash residues. Due to these factors, many industrialized countries are phasing out healthcare waste incinerators and exploring technologies that do not produce any dioxins. Countries like United States, Ireland, Portugal, Canada and Germany have completely shut down or put a moratorium on medical waste incinerators.

Cofiring of Biomass

Cofiring (or co-combustion) involves utilizing existing power generating plants that are fired with fossil fuel (generally coal), and displacing a small proportion of the fossil fuel with renewable biomass fuels. Cofiring of biomass with coal and other fossil fuels can provide a short-term, low-risk, low-cost option for producing renewable energy while simultaneously reducing the use of fossil fuels.

Biomass can typically provide between 3 and 15 percent of the input energy into the power plant. Cofiring of biomass has the major advantage of avoiding the construction of new, dedicated, biomass power plant. An existing power station is modified to accept the biomass resource and utilize it to produce a minor proportion of its electricity.

Cofiring of biomass may be implemented using different types and percentages of biomass in a range of combustion and gasification technologies. Most forms of biomass are suitable for co-firing. These include dedicated energy crops, urban wood waste and agricultural residues such as rice straw and rice husk.

The fuel preparation requirements, issues associated with combustion such as corrosion and fouling of boiler tubes, and characteristics of residual ash dictate the co-firing configuration appropriate for a particular plant and biomass resource. These configurations may be categorized into direct, indirect and parallel firing.

Direct Co-Firing

This is the most common form of biomass co-firing involving direct co-firing of the biomass fuel and the primary fuel (generally coal) in the combustion chamber of the boiler. The cheapest and simplest form of direct co-firing for a pulverized coal power plant is through mixing prepared biomass and coal in the coal yard or on the coal conveyor belt, before the combined fuel is fed into the power station boiler.

Indirect Co-firing

If the biomass fuel has different attributes to the normal fossil fuel, then it may be prudent to partially segregate the biomass fuel rather than risk damage to the complete station.

For indirect co-firing, the ash of the biomass resource and the main fuel are kept separate from one another as the thermal conversion is partially carried out in separate processing plants. As indirect co-firing requires a separate biomass energy conversion plant, it has a relatively high investment cost compared with direct co-firing.

Parallel Firing

For parallel firing, totally separate combustion plants and boilers are used for the biomass resource and the coal- fired power plants. The steam produced is fed into the main power plant where it is upgraded to higher temperatures and pressures, to give resulting higher energy conversion efficiencies. This allows the use of problematic fuels with high alkali and chlorine contents (such as wheat straw) and the separation of the ashes.

Pelletization of Municipal Solid Wastes

MSW is a poor-quality fuel and its pre-processing is necessary to prepare fuel pellets to improve its consistency, storage and handling characteristics, combustion behaviour and calorific value. Technological improvements are taking place in the realms of advanced source separation, resource recovery and production/utilisation of recovered fuel in both existing and new plants for this purpose. There has been an increase in global interest in the preparation of RDF containing a blend of pre-processed MSW with coal suitable for combustion in pulverised coal and fluidised bed boilers.

RDF

Pelletization of municipal solid waste involves the processes of segregating, crushing, mixing high and low heat value organic waste material and solidifying it to produce fuel pellets or briquettes, also referred to as Refuse Derived Fuel (RDF). The process is essentially a method that condenses the waste or changes its physical form and enriches its organic content through removal of inorganic materials and moisture. The calorific value of RDF pellets can be around 4000 kcal/ kg depending upon the percentage of organic matter in the waste, additives and binder materials used in the process.

The calorific value of raw MSW is around 1000 kcal/kg while that of fuel pellets is 4000 kcal/kg. On an average, about 15–20 tons of fuel pellets can be produced after treatment of 100 tons of raw garbage. Since pelletization enriches the organic content of the waste through removal of inorganic materials and moisture, it can be very effective method for preparing an enriched fuel feed for other thermochemical processes like pyrolysis/ gasification, apart from incineration.

Pellets can be used for heating plant boilers and for the generation of electricity. They can also act as a good substitute for coal and wood for domestic and industrial purposes. The important applications of RDF are found in the following spheres:

  • Cement kilns
  • RDF power plants
  • Coal-fired power plants
  • Industrial steam/heat boilers
  • Pellet stoves

The conversion of solid waste into briquettes provides an alternative means for environmentally safe disposal of garbage which is currently disposed off in non-sanitary landfills. In addition, the pelletization technology provides yet another source of renewable energy, similar to that of biomass, wind, solar and geothermal energy. The emission characteristics of RDF are superior compared to that of coal with fewer emissions of pollutants like NOx, SOx, CO and CO2.

RDF production line consists of several unit operations in series in order to separate unwanted components and condition the combustible matter to obtain the required characteristics. The main unit operations are screening, shredding, size reduction, classification, separation either metal, glass or wet organic materials, drying and densification. These unit operations can be arranged in different sequences depending on raw MSW composition and the required RDF quality.

Various qualities of fuel pellets can be produced, depending on the needs of the user or market. A high quality of RDF would possess a higher value for the heating value, and lower values for moisture and ash contents. The quality of RDF is sufficient to warrant its consideration as a preferred type of fuel when solid waste is being considered for co-firing with coal or for firing alone in a boiler designed originally for firing coal.

Trends in Utilization of Palm Kernel Shells

The palm kernel shells used to be initially dumped in the open thereby impacting the environment negatively without any economic benefit. However, over time, palm oil mills in Southeast Asia and elsewhere realized their brilliant properties as a fuel and that they can easily replace coal as an industrial fuel for generating heat and steam.

palm-kernel-shell-uses

Palm kernel shells is an abundant biomass resource in Southeast Asia

Major Applications

Nowadays, the primary use of palm kernel shells is as a boiler fuel supplementing the fibre which is used as primary fuel. In recent years kernel shells are extensively sold as alternative fuel around the world. Besides selling shells in bulk, there are companies that produce fuel briquettes from shells which may include partial carbonisation of the material to improve the combustion characteristics.

Palm kernel shells have a high dry matter content (>80% dry matter). Therefore the shells are generally considered a good fuel for the boilers as it generates low ash amounts and the low K and Cl content will lead to less ash agglomeration. These properties are also ideal for production of biomass for export.

As a raw material for fuel briquettes, palm shells are reported to have the same calorific characteristics as coconut shells. The relatively smaller size makes it easier to carbonise for mass production, and its resulting palm shell charcoal can be pressed into a heat efficient biomass briquette.

Although the literature on using oil palm shells (and fibres) is not as extensive as EFB, common research directions of using shells, besides energy, are to use it as raw material for light-weight concrete, fillers, activated carbon, and other materials. However, none of the applications are currently done on a large-scale. Since shells are dry and suitable for thermal conversion, technologies that further improve the combustion characteristics and increase the energy density, such as torrefaction, could be relevant for oil palm shells.

Torrefaction is a pretreatment process which serves to improve the properties of biomass in relation to the thermochemical conversion technologies for more efficient energy generation. High lignin content for shells affects torrefaction characteristics positively (as the material is not easily degraded compared to EFB and fibres).

Furthermore, palm oil shells are studied as feedstock for fast pyrolysis. To what extent shells are a source of fermentable sugars is still not known, however the high lignin content in palm kernel shells indicates that shells are less suitable as raw material for fermentation.

Future Outlook

The leading palm oil producers in the world should consider limiting the export of palm-kernel shells (PKS) to ensure supplies of the biomass material for renewable energy projects, in order to decrease dependency on fossil fuels. For example, many developers in Indonesia have expressed an interest in building palm kernel shell-fired power plants.

However, they have their concerns over supplies, as many producers prefer to sell their shells overseas currently. Many existing plants are facing problems on account of inconsistent fuel quality and increasing competition from overseas PKS buyers. PKS market is well-established in provinces like Sumatra and export volumes to Europe and North Asia as a primary fuel for biomass power plants is steadily increasing.

The creation of a biomass supply chain in palm oil producing countries may be instrumental in discouraging palm mills to sell their PKS stocks to brokers for export to foreign countries. Establishment of a biomass exchange in leading countries, like Indonesia, Malaysia and Nigeria, will also be a deciding factor in tapping the unharnessed potential of palm kernel shells as biomass resource.

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.

palm-biomass

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.