Torrified PKS: An Attractive Biomass Commodity in West Africa

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.

palm-kernel-shells

 

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.

PKS-torrefaction

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.

nut-cracker-machine-palm-mill

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.

For more information about PKS trading opportunities and our technical consulting services, please email on salman@bioenergyconsult.com or eko.sb.setyawan@gmail.com

Torrefaction of Biomass: An Overview

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.

torrefaction-of-biomass

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.

steps-in-biomass-torrefaction

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.

torrified-biomass

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.

A Primer on Waste-to-Energy

Waste-to-Energy (also known as energy-from-waste) is the use of thermochemical and biochemical technologies to recover energy, usually in the form of electricity, steam and fuels, from urban wastes.These new technologies can reduce the volume of the original waste by 90%, depending upon composition and use of outputs.

Energy is the driving force for development in all countries of the world. The increasing clamor for energy and satisfying it with a combination of conventional and renewable resources is a big challenge. Accompanying energy problems in different parts of the world, another problem that is assuming critical proportions is that of urban waste accumulation.

The quantity of waste produced all over the world amounted to more than 12 billion tonnes in 2006, with estimates of up to 13 billion tonnes in 2011. The rapid increase in population coupled with changing lifestyle and consumption patterns is expected to result in an exponential increase in waste generation of upto 18 billion tonnes by year 2020.

Waste generation rates are affected by socio-economic development, degree of industrialization, and climate. Generally, the greater the economic prosperity and the higher percentage of urban population, the greater the amount of solid waste produced. Reduction in the volume and mass of solid waste is a crucial issue especially in the light of limited availability of final disposal sites in many parts of the world. Millions of tonnes of household wastes are generated each year with the vast majority disposed of in open fields or burnt wantonly.

The main categories of waste-to-energy technologies are physical technologies, which process waste to make it more useful as fuel; thermal technologies, which can yield heat, fuel oil, or syngas from both organic and inorganic wastes; and biological technologies, in which bacterial fermentation is used to digest organic wastes to yield fuel.

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 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.

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. In addition, wastes can also yield liquid fuels, such as cellulosic ethanol, which can be used to replace petroleum-based fuels. Cellulosic ethanol can be produced from grasses, wood chips and agricultural residues by biochemical route using heat, pressure, chemicals and enzymes to unlock the sugars in biomass wastes.

Waste-to-energy plants offer two important benefits of environmentally safe waste management and disposal, as well as the generation of clean electric power.  The growing use of waste-to-energy as a method to dispose of solid and liquid wastes and generate power has greatly reduced environmental impacts of municipal solid waste management, including emissions of greenhouse gases.

Pyrolysis of Municipal Wastes

Pyrolysis is rapidly developing biomass thermal conversion technology and has been garnering much attention worldwide due to its high efficiency and good eco-friendly performance characteristics. Pyrolysis technology provides an opportunity for the conversion of municipal solid wastes, agricultural residues, scrap tires, non-recyclable plastics etc into clean energy. It offers an attractive way of converting urban wastes into products which can be effectively used for the production of heat, electricity and chemicals.

Pyrolysis-MSW

Pyrolysis of Municipal Wastes

Pyrolysis process consists of both simultaneous and successive reactions when carbon-rich organic material is heated in a non-reactive atmosphere. Simply speaking, pyrolysis is the thermal degradation of organic materials in the absence of oxygen. Thermal decomposition of organic components in the waste stream starts at 350°C–550°C and goes up to 700°C–800°C in the absence of air/oxygen.

Pyrolysis of municipal wastes begins with mechanical preparation and separation of glass, metals and inert materials prior to processing the remaining waste in a pyrolysis reactor. The commonly used pyrolysis reactors are rotary kilns, rotary hearth furnaces, and fluidized bed furnaces. The process requires an external heat source to maintain the high temperature required.

Pyrolysis can be performed at relatively small-scale which may help in reducing transport and handling costs.  In pyrolysis of MSW, heat transfer is a critical area as the process is endothermic and sufficient heat transfer surface has to be provided to meet process heat requirements.

The main products obtained from pyrolysis of municipal wastes are a high calorific value gas (synthesis gas or syngas), a biofuel (bio oil or pyrolysis oil) and a solid residue (char). Depending on the final temperature, MSW pyrolysis will yield mainly solid residues at low temperatures, less than 4500C, when the heating rate is quite slow, and mainly gases at high temperatures, greater than 8000C, with rapid heating rates. At an intermediate temperature and under relatively high heating rates, the main product is a liquid fuel popularly known as bio oil.

Wide Range of Products

Bio oil is a dark brown liquid and can be upgraded to either engine fuel or through gasification processes to a syngas and then biodiesel. Pyrolysis oil may also be used as liquid fuel for diesel engines and gas turbines to generate electricity.

Bio oil is particularly attractive for co-firing because it can be relatively easy to handle and burn than solid fuel and is cheaper to transport and store. In addition, bio oil is also a vital source for a wide range of organic compounds and specialty chemicals.

Syngas is a mixture of energy-rich gases (combustible constituents include carbon monoxide, hydrogen, methane and a broad range of other VOCs). The net calorific value (NCV) of syngas is between 10 and 20MJ/Nm3. Syngas is cleaned to remove particulates, hydrocarbons, and soluble matter, and then combusted to generate electricity.

Diesel engines, gas turbines, steam turbines and boilers can be used directly to generate electricity and heat in CHP systems using syngas and pyrolysis oil. Syngas may also be used as a basic chemical in petrochemical and refining industries.

The solid residue from MSW pyrolysis, called char, is a combination of non-combustible materials and carbon. Char is almost pure carbon and can be used in the manufacture of activated carbon filtration media (for water treatment applications) or as an agricultural soil amendment.

Overview of Biomass Energy Technologies

A wide range of bioenergy technologies are available for realizing the energy potential of biomass wastes, ranging from very simple systems for disposing of dry waste to more complex technologies capable of dealing with large amounts of industrial waste. Conversion routes for biomass wastes are generally thermo-chemical or bio-chemical, but may also include chemical and physical.

Thermal Technologies

The three principal methods of thermo-chemical conversion corresponding to each of these energy carriers are combustion in excess air, gasification in reduced air, and pyrolysis in the absence of air. Direct combustion is the best established and most commonly used technology for converting wastes to heat.

During combustion, biomass is burnt in excess air to produce heat. The first stage of combustion involves the evolution of combustible vapours from wastes, which burn as flames. Steam is expanded through a conventional turbo-alternator to produce electricity. The residual material, in the form of charcoal, is burnt in a forced air supply to give more heat.

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.

Co-firing has the major advantage of avoiding the construction of new, dedicated, waste-to-energy power plant. An existing power station is modified to accept the waste resource and utilize it to produce a minor proportion of its electricity.

Gasification systems operate by heating biomass wastes in an environment where the solid waste breaks down to form a flammable gas. The gasification of biomass takes place in a restricted supply of air or oxygen at temperatures up to 1200–1300°C. The gas produced—synthesis gas, or syngas—can be cleaned, filtered, and then burned in a gas turbine in simple or combined-cycle mode, comparable to LFG or biogas produced from an anaerobic digester.

The final fuel gas consists principally of carbon monoxide, hydrogen and methane with small amounts of higher hydrocarbons. This fuel gas may be burnt to generate heat; alternatively it may be processed and then used as fuel for gas-fired engines or gas turbines to drive generators. In smaller systems, the syngas can be fired in reciprocating engines, micro-turbines, Stirling engines, or fuel cells.

Pyrolysis is thermal decomposition occurring in the absence of oxygen. During the pyrolysis process, biomass waste is heated either in the absence of air (i.e. indirectly), or by the partial combustion of some of the waste in a restricted air or oxygen supply. This results in the thermal decomposition of the waste to form a combination of a solid char, gas, and liquid bio-oil, which can be used as a liquid fuel or upgraded and further processed to value-added products.

Biochemical Technologies

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 a series of chemical reactions during which organic material is decomposed through the metabolic pathways of naturally occurring microorganisms in an oxygen depleted environment. In addition, wastes can also yield liquid fuels, such as cellulosic ethanol and biodiesel, which can be used to replace petroleum-based fuels.

Anaerobic digestion is the natural biological process which stabilizes organic waste in the absence of air and transforms it into biogas and biofertilizer. Almost any organic material can be processed with anaerobic digestion. This includes biodegradable waste materials such as municipal solid waste, animal manure, poultry litter, food wastes, sewage and industrial wastes.

An anaerobic digestion plant produces two outputs, biogas and digestate, both can be further processed or utilized to produce secondary outputs. Biogas can be used for producing electricity and heat, as a natural gas substitute and also a transportation fuel. Digestate can be further processed to produce liquor and a fibrous material. The fiber, which can be processed into compost, is a bulky material with low levels of nutrients and can be used as a soil conditioner or a low level fertilizer.

A variety of fuels can be produced from biomass wastes 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.

The largest potential feedstock for ethanol is lignocellulosic biomass wastes, which includes materials such as agricultural residues (corn stover, crop straws and bagasse), herbaceous crops (alfalfa, switchgrass), short rotation woody crops, forestry residues, waste paper and other wastes (municipal and industrial).

The three major steps involved in cellulosic ethanol production are pretreatment, enzymatic hydrolysis, and fermentation. Biomass is pretreated to improve the accessibility of enzymes. After pretreatment, biomass undergoes enzymatic hydrolysis for conversion of polysaccharides into monomer sugars, such as glucose and xylose. Subsequently, sugars are fermented to ethanol by the use of different microorganisms. Bioethanol production from these feedstocks could be an attractive alternative for disposal of these residues. Importantly, lignocellulosic feedstocks do not interfere with food security.

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.

Insights into MSW-to-Energy

You know the saying: One person’s trash is another’s treasure. When it comes to recovering energy from municipal solid waste — commonly called garbage or trash— that treasure can be especially useful. Instead of taking up space in a landfill, we can process our trash to produce energy to power our homes, businesses and public buildings.

In 2015, the United States got about 14 billion kilowatt-hours of electricity from burning municipal solid waste, or MSW. Seventy-one waste-to-energy plants and four additional power plants burned around 29 million tons of MSW in the U.S. that year. However, just 13 percent of the country’s waste becomes energy. Around 35 percent is recycled or composted, and the rest ends up in landfills.

MSW-to-Energy

Recovering Energy Through Incineration

The predominant technology for MSW-to-energy plants is incineration, which involves burning the trash at high temperatures. Similarly to how some facilities use coal or natural gas as fuel sources, power plants can also burn MSW as fuel to heat water, which creates steam, turns a turbine and produces electricity.

Several methods and technologies can play a role in burning trash to create electricity. The most common type of incineration plant is what’s called a mass-burn facility. These units burn the trash in one large chamber. The facility might sort the MSW before sending it to the combustion chamber to remove non-combustible materials and recyclables.

These mass-burn systems use excess air to facilitate mixing, and ensure air gets to all the waste. Many of these units also burn the fuel on a sloped, moving grate to mix the waste even further. These steps are vital because solid waste is inconsistent, and its content varies. Some facilities also shred the MSW before moving it to the combustion chamber.

Gasification Plants

Another method for converting trash into electricity is gasification. This type of waste-to-energy plant doesn’t burn MSW directly, but instead uses it as feedstock for reactions that produce a fuel gas known as synthesis gas, or syngas. This gas typically contains carbon monoxide, carbon dioxide, methane, hydrogen and water vapor.

Approaches to gasification vary, but typically include high temperatures, high-pressure environments, very little oxygen and shredding MSW before the process begins. Common gasification methods include:

  • Pyrolysis, which involves little to no oxygen, partial pressure and temperatures between approximately 600 and 800 degrees Celsius.
  • Air-fed systems, which use air instead of pure oxygen and temperatures between 800 and 1,800 degrees Celsius.
  • Plasma or plasma arc gasification, which uses plasma torches to increase temperatures to 2,000 to 2,800 degrees Celsius.

Syngas can be burned to create electricity, but it can also be a component in the production of transportation fuels, fertilizers and chemicals. Proponents of gasification report that it is a more efficient waste-to-energy method than incineration, and can produce around 1,000 kilowatt-hours of electricity from one ton of MSW. Incineration, on average, produces 550 kilowatt-hours.

Challenges of MSW-to-Energy

Turning trash into energy seems like an ideal solution. We have a lot of trash to deal with, and we need to produce energy. MSW-to-energy plants solve both of those problems. However, a relatively small amount of waste becomes energy, especially in the U.S.

Typical layout of MSW-to-Energy Plant

This lack may be due largely to the upfront costs of building a waste-to-energy plant. It is much cheaper in the short term to send trash straight to a landfill. Some people believe these energy production processes are just too complicated and expensive. Gasification, especially, has a reputation for being too complex.

Environmental concerns also play a role, since burning waste can release greenhouse gases. Although modern technologies can make burning waste a cleaner process, its proponents still complain it is too dirty.

Despite these challenges, as trash piles up and we continue to look for new sources of energy, waste-to-energy plants may begin to play a more integral role in our energy production and waste management processes. If we handle it responsibly and efficiently, it could become a very viable solution to several of the issues our society faces.

Energy Potential of Coconut Biomass

Coconuts are produced in 92 countries worldwide on about more than 10 million hectares. Indonesia, Philippines and India account for almost 75% of world coconut production with Indonesia being the world’s largest coconut producer. A coconut plantation is analogous to energy crop plantations, however coconut plantations are a source of wide variety of products, in addition to energy. The current world production of coconuts has the potential to produce electricity, heat, fiberboards, organic fertilizer, animal feeds, fuel additives for cleaner emissions, eco-friendly cutlery, health drinks, etc.

coconut-shell-biomass

The coconut fruit yields 40 % coconut husks containing 30 % fiber, with dust making up the rest. The chemical composition of coconut husks consists of cellulose, lignin, pyroligneous acid, gas, charcoal, tar, tannin, and potassium. Coconut dust has high lignin and cellulose content. The materials contained in the casing of coco dusts and coconut fibers are resistant to bacteria and fungi.

Coconut husk and shells are an attractive biomass fuel and are also a good source of charcoal. The major advantage of using coconut biomass as a fuel is that coconut is a permanent crop and available round the year so there is constant whole year supply. Activated carbon manufactured from coconut shell is considered extremely effective for the removal of impurities in wastewater treatment processes.

Coconut Shell

Coconut shell is an agricultural waste and is available in plentiful quantities throughout tropical countries worldwide. In many countries, coconut shell is subjected to open burning which contributes significantly to CO2 and methane emissions.

Coconut shell is widely used for making charcoal. The traditional pit method of production has a charcoal yield of 25–30% of the dry weight of shells used. The charcoal produced by this method is of variable quality, and often contaminated with extraneous matter and soil. The smoke evolved from pit method is not only a nuisance but also a health hazard.

The coconut shell has a high calorific value of 20.8MJ/kg and can be used to produce steam, energy-rich gases, bio-oil, biochar etc. It is to be noted that coconut shell and coconut husk are solid fuels and have the peculiarities and problems inherent in this kind of fuel.

Coconut shell is more suitable for pyrolysis process as it contain lower ash content, high volatile matter content and available at a cheap cost. The higher fixed carbon content leads to the production to a high-quality solid residue which can be used as activated carbon in wastewater treatment. Coconut shell can be easily collected in places where coconut meat is traditionally used in food processing.

Coconut Husk

Coconut husk has high amount of lignin and cellulose, and that is why it has a high calorific value of 18.62MJ/kg. The chemical composition of coconut husks consists of cellulose, lignin, pyroligneous acid, gas, charcoal, tar, tannin, and potassium.

The predominant use of coconut husks is in direct combustion in order to make charcoal, otherwise husks are simply thrown away. Coconut husk can be transformed into a value-added fuel source which can replace wood and other traditional fuel sources. In terms of the availability and costs of coconut husks, they have good potential for use in power plants.

Utilization of Date Palm Biomass

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.

date-wastes

Date palm biomass is found in large quantities across the Middle East

Date palm is considered a renewable natural resource because it can be replaced in a relatively short period of time. It takes 4 to 8 years for date palms to bear fruit after planting, and 7 to 10 years to produce viable yields for commercial harvest. Usually date palm wastes are burned in farms or disposed in landfills which cause environmental pollution in dates-producing nations.

The major constituents of date palm biomass are cellulose, hemicelluloses and lignin. In addition, date palm has high volatile solids content and low moisture content. These factors make date palm residues an excellent biomass resource in date-palm producing nations.

Date palm biomass is an excellent resource for charcoal production in Middle East

A wide range of physico-chemical, thermal and biochemical technologies exists for sustainable utilization of date palm biomass. Apart from charcoal production and energy conversion (using technologies like combustion and gasification), below are few ways for utilization of date palm wastes:

Conversion into fuel pellets or briquettes

Biomass pellets are a popular type of alternative fuel (analogous to coal), generally made from wood wastes and agricultural biomass. The biomass pelletization process consists of multiple steps including pre-treatment, pelletization and post-treatment of biomass wastes. Biomass pellets can be used as a coal replacement in power plant, industries and other application.

Conversion into energy-rich products

Biomass pyrolysis is the thermal decomposition of date palm biomass occurring in the absence of oxygen. The products of biomass pyrolysis include biochar, bio-oil and gases including methane, hydrogen, carbon monoxide, and carbon dioxide.

Depending on the thermal environment and the final temperature, pyrolysis will yield mainly biochar at low temperatures, less than 450 0C, when the heating rate is quite slow, and mainly gases at high temperatures, greater than 800 0C, with rapid heating rates. At an intermediate temperature and under relatively high heating rates, the main product is bio-oil.

Bio-oil can be upgraded to either a special engine fuel or through gasification processes to a syngas which can then be processed into biofuels. Bio-oil is particularly attractive for co-firing because it can be more readily handled and burned than solid fuel and is cheaper to transport and store.

Conversion into biofertilizer

Composting is the most popular method for biological decomposition of organic wastes. Date palm waste has around 80% organic content which makes it very well-suited for the composting process. Commercial-scale composting of date palm wastes can be carried out by using the traditional windrow method or a more advanced method like vermicomposting.

Comparison of MSW-to-Energy Processes

MSW-to-Energy is the use of thermochemical and biochemical technologies to recover energy, usually in the form of electricity, steam and other fuels, from urban wastes. The main categories of MSW-to-energy technologies are physical technologies, which process waste to make it more useful as fuel; thermal technologies, which can yield heat, fuel oil, or syngas from both organic and inorganic wastes; and biological technologies, in which bacterial fermentation is used to digest organic wastes to yield fuel. These new technologies can reduce the volume of the original waste by 90%, depending upon composition and use of outputs.

Components of MSW-to-Energy Systems

  1. Front-end MSW preprocessing
  2. Conversion unit (reactor or anaerobic digester)
  3. Gas cleanup and residue treatment plant
  4. Energy recovery plant (optional)
  5. Emissions clean up

Incineration

  • Combustion of raw MSW, moisture less than 50%
  • Sufficient amount of oxygen is required to fully oxidize the fuel
  • Combustion temperatures are in excess of 850oC
  • Waste is converted into CO2 and water concern about toxics (dioxin, furans)
  • Any non-combustible materials (inorganic such as metals, glass) remain as a solid, known as bottom ash (used as feedstock in cement and brick manufacturing)
  • Air pollution control system for fly ash, bottom ash, particulates etc.
  • Needs high calorific value waste to keep combustion process going, otherwise requires high energy for maintaining high temperatures

Anaerobic Digestion

  • Well-known biochemical technology for organic fraction of MSW and sewage sludge.
  • Biological conversion of biodegradable organic materials in the absence of oxygen at mesophilic or thermophilic temperatures.
  • Residue is stabilized organic matter that can be used as soil amendment
  • Digestion is used primarily to reduce quantity of sludge for disposal / reuse
  • Methane gas is generated which is used for heat and power generation.

Gasification

  • Can be seen as between pyrolysis and combustion (incineration) as it involves partial oxidation.
  • Exothermic process (some heat is required to initialize and sustain the gasification process).
  • Oxygen is added but at low amounts not sufficient for full oxidation and full combustion.
  • Temperatures are above 650oC
  • Main product is syngas, typically has net calorific value of 4 to 10 MJ/Nm3
  • Other product is solid residue of non-combustible materials (ash) which contains low level of carbon

Pyrolysis

  • Thermal degradation of organic materials through use of indirect, external source of heat
  • Temperatures between 300 to 850oC are maintained for several seconds in the absence of oxygen.
  • Product is char, oil and syngas composed primarily of O2, CO, CO2, CH4 and complex hydrocarbons.
  • Syngas can be utilized for energy production or proportions can be condensed to produce oils and waxes
  • Syngas typically has net calorific value (NCV) of 10 to 20 MJ/Nm

Plasma Gasification

  • Use of electricity passed through graphite or carbon electrodes, with steam and/or oxygen / air injection to produce electrically conducting gas (plasma)
  • Temperatures are above 3000oC
  • Organic materials are converted to syngas composed of H2, CO
  • Inorganic materials are converted to solid slag
  • Syngas can be utilized for energy production or proportions can be condensed to produce oils and waxes

MSW-to-energy technologies can address a host of environmental issues, such as land use and pollution from landfills, and increasing reliance on fossil fuels. In many countries, the availability of landfill capacity has been steadily decreasing due to regulatory, planning and environmental permitting constraints. As a result, new approaches to waste management are rapidly being written into public and institutional policies at local, regional and national levels.