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.

Rationale for Biomass Supply Chain

Biomass resources have been in use for a variety of purposes since ages. The multiple uses of biomass includes usage as a livestock or for meeting domestic and industrial thermal requirements or for the generation of power to fulfill any electrical or mechanical needs. One of the major issues, however, associated with the use of any biomass resources is its supply chain management.

The resource being bulky, voluminous and only seasonally available creates serious hurdles in the reliable supply of the feedstock, regardless of its application. The idea is thus to have something which plugs in this gap between the biomass resource availability and its demand.

The Problem

The supply chain management in any biomass-based project is nothing less than a big management conundrum. The complexity deepens owing to the large number of stages which encompass the entire biomass value chain. It starts right from the resource harvesting and goes on to include the resource collection, processing, storage and eventually its transportation to the point of ultimate utilization.

Owing to the voluminous nature of the resource, its handling becomes a major issue since it requires bigger modes of logistics, employment of a larger number of work-force and a better storage infrastructure, as compared to any other fuel or feedstock. Not only this their lower energy density characteristic, makes it inevitable for the resource to be first processed and then utilized for power generation to make for better economics.

All these hassles associated with such resources, magnify the issue of their utilization when it comes to their supply chain. The seasonal availability of most of the biomass resources, alternative application options, weather considerations, geographical conditions and numerous other parameters make it difficult for the resource to be made consistently available throughout the year. This results in poor feedstock inputs at the utilization point which ends up generating energy in a highly erratic and unreliable manner.

The Solution

Although most of the problems discussed above, are issues inherently associated with the usage of biomass resources, they can be curtailed to a larger extent by strengthening the most important loophole in such projects – The Biomass Resource Supply Chain.

World over, major emphasis has been laid in researching upon the means to improve the efficiencies of such technologies. However, no significant due diligence has been carried out in fortifying the entire resource chain to assure such plants for a continuous resource supply.

The usual solution to encounter such a problem is to have long term contracts with the resource providers to not only have an assured supply but also guard the project against unrealistic escalations in the fuel costs. Although, this solution has been found to be viable, it becomes difficult to sustain such contracts for longer duration since these resources are also susceptible to numerous externalities which could be in the form of any natural disaster, infection from pests or any other socio-political or geographical disturbances, which eventually lead to an increased burden on the producers.

Agricultural Wastes in the Middle East

Agriculture plays an important role in the economies of most of the countries in the Middle East.  The contribution of the agricultural sector to the overall economy varies significantly among countries in the region, ranging, for example, from about 3.2 percent in Saudi Arabia to 13.4 percent in Egypt.  Large scale irrigation is expanding, enabling intensive production of high value cash and export crops, including fruits, vegetables, cereals, and sugar.

The term ‘crop residues’ covers the whole range of biomass produced as by-products from growing and processing crops. Crop residues encompasses all agricultural wastes such as bagasse, straw, stem, stalk, leaves, husk, shell, peel, pulp, stubble, etc. Wheat and barley are the major staple crops grown in the Middle East region. In addition, significant quantities of rice, maize, lentils, chickpeas, vegetables and fruits are produced throughout the region, mainly in Egypt, Syria, Saudi Arabia and Jordan.

Agricultural Wastes in the Middle East

Large quantities of agricultural wastes are produced annually in the Middle East, and are vastly underutilised. Current farming practice in the Middle East is usually to plough these residues back into the soil, or they are burnt, left to decompose, or grazed by cattle. These residues could be processed into liquid fuels, solid fuels or thermochemically processed to produce electricity and domestic heat in rural areas.

date-palm-waste

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

Date palm is one of the principal agricultural products in the arid and semi-arid region of the world, especially Middle East and North Africa (MENA) region. The Arab world has more than 84 million date palm trees with the majority in Egypt, Iraq, Saudi Arabia, Iran, Algeria, Morocco, Tunisia and United Arab Emirates.

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. Some studies have reported that Saudi Arabia alone generates more than 200,000 tons of date palm biomass each year.

In Egypt, crop residues are considered to be the most important and traditional source of domestic fuel in rural areas. These crop residues are by-products of common crops such as cotton, wheat, maize and rice. The total amount of residues reaches about 16 million tons of dry matter per year.

Cotton residues represent about 9% of the total amount of residues. These are materials comprising mainly cotton stalks, which present a disposal problem. The area of cotton crop cultivation accounts for about 5% of the cultivated area in Egypt.

A cotton field in Egypt

Energy crops, such as Jatropha, can be successfully grown in arid regions for biodiesel production. Infact, Jatropha is already grown at limited scale in some Middle East countries and tremendous potential exists for its commercial exploitation.

Date Palm as Biomass Resource

Date palm is one of the principal agricultural products in the arid and semi-arid region of the world, especially Middle East and North Africa (MENA) region. There are more than 120 million date palm trees worldwide yielding several million tons of dates per year, apart from secondary products including palm midribs, leaves, stems, fronds and coir. The Arab world has more than 84 million date palm trees with the majority in Egypt, Iraq, Saudi Arabia, Iran, Algeria, Morocco, Tunisia and United Arab Emirates.

date-wastes

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

Egypt is the world’s largest date producer with annual production of 1.47 million tons of dates in 2012 which accounted for almost one-fifth of global production. Saudi Arabia has more than 23 millions date palm trees, which produce about 1 million tons of dates per year.

Biomass Potential of Date Palm

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. Some studies have reported that Saudi Arabia alone generates more than 200,000 tons of date palm biomass each year.

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. In countries like Iraq and Egypt, a small portion of palm biomass in used in making animal feed.

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 biomass an excellent waste-to-energy resource in the MENA region.

Technology Options for Date Palm Biomass Utilization

A wide range of thermal and biochemical technologies exists to tap the energy stored in date palm biomass to useful forms of energy. The low moisture content in date palm wastes makes it well-suited to thermochemical conversion technologies like combustion, gasification and pyrolysis which may yield steam, syngas, bio oil etc.

On the other hand, the high volatile solids content in date palm biomass indicates its potential towards biogas production in anaerobic digestion plants, possibly by codigestion with sewage sludge, animal wastes and/and food wastes. The cellulosic content in date palm wastes can be transformed into biofuel (bioethanol) by making use of the fermentation process.

The highly organic nature of date palm waste makes it highly suitable for compost production which can be used to replace chemical fertilizers in date palm plantations. Thus, abundance of date palm trees in the MENA and the Mediterranean region, can catalyze the development of biomass and biofuels sector in the region.

An Introduction to Biomass Harvesting

Biomass harvesting and collection is an important step involving gathering and removal of the biomass from field which is dependent on the state of biomass, i.e. grass, woody, or crop residue. The moisture content and the end use of biomass also affect the way biomass is collected. For crop residues, the operations should be organized in sync with the grain harvest as it occupies the centrestage in farming process.

biomass-harvesting

All of other operations such as residue management and collection take place after so-called grain is in the bin. On the other hand, the harvest and collection dedicated crops (grass and woody) can be staged for recovery of the biomass only. In agricultural processing, straw is the stems and leaves of small cereals while chaff is husks and glumes of seed removed during threshing.

Modern combine-harvesters generally deliver straw and chaff together; other threshing equipment separates them. Stover is the field residues of large cereals, such as maize and sorghum. Stubble is the stumps of the reaped crop, left in the field after harvest. Agro-industrial wastes are by-products of the primary processing of crops, including bran, milling offal, press-cakes and molasses. Bran from on-farm husking of cereals and pulses are fed to livestock or foraged directly by backyard fowls.

The proportion of straw, or stover, to grain varies from crop to crop and according to yield level (very low grain yields have a higher proportion of straw) but is usually slightly over half the harvestable biomass. The height of cutting will also affect how much stubble is left in the field: many combine-harvested crops are cut high; crops on small-scale farms where straw is scarce may be cut at ground level by sickle or uprooted by hand.

Modern combine-harvesters generally deliver straw and chaff together

Collection involves operations pertaining to gathering, packaging, and transporting biomass to a nearby site for temporary storage. The amount of a biomass resource that can be collected at a given time depends on a variety of factors. In case of agricultural residues, these considerations include the type and sequence of collection operations, the efficiency of collection equipment, tillage and crop management practices, and environmental restrictions, such as the need to control erosion, maintain soil productivity, and maintain soil carbon levels.

Biogas Sector in India: Perspectives

Biogas is an often overlooked and neglected aspect of renewable energy in India. While solar, wind and hydropower dominate the discussion in the cuntry, they are not the only options available. Biogas is a lesser known but highly important option to foster sustainable development in agriculture-based economies, such as India.

What is Biogas

Briefly speaking, biogas is the production of gaseous fuel, usually methane, by fermentation of organic material. It is an anaerobic process or one that takes place in the absence of oxygen. Technically, the yeast that causes your bread to rise or the alcohol in beer to ferment is a form of biogas. We don’t use it in the same way that we would use other renewable sources, but the idea is similar. Biogas can be used for cooking, lighting, heating, power generation and much more. Infact, biogas is an excellent and effective to promote development of rural and marginalized communities in all developing countries.

This presents a problem, however. The organic matter is putting off a gas, and to use it, we have to turn it into a liquid. This requires work, machinery and manpower. Research is still being done to figure out the most efficient methods to make it work, but there is a great deal of progress that has been made, and the technology is no longer new.

Fossil Fuel Importation

India has a rapidly expanding economy and the population to fit. This has created problems with electricity supplies to expanding areas. Like most countries, India mainly uses fossil fuels. However, as oil prices fluctuate and the country’s demand for oil grows, the supply doesn’t always keep up with the demand. In the past, India has primarily imported oil from the Middle East, specifically Saudi Arabia and Iraq.

Without a steady and sustainable fossil fuels supply, India has looking more seriously into renewable sources they can produce within the country. Biogas is an excellent candidate to meet those requirements and has been used for this goal before.

Biogas in India

There are significant differences between biogas and fossil fuels, but for India, one of the biggest is that you can create biogas at home. It’s pretty tricky to find, dig up and transform crude oil into gas, but biogas doesn’t have the same barriers. In fact, many farmers who those who have gardens or greenhouses could benefit with proper water management and temperature control so that plants can be grown year round, It still takes some learning and investment, but for many people, especially those who live in rural places, it’s doable.

This would be the most beneficial to people in India because it would help ease the strain of delivering reliable energy sources based on fossil fuels, and would allow the country to become more energy independent. Plus, the rural areas are places where the raw materials for biogas will be more available, such animal manure, crop residues and poultry litter. But this isn’t the first time most people there are hearing about it.

Biogas in India has been around for a long time. In the 1970’s the country began a program called the National Biogas and Manure Management Program (NBMMP) to deal with the same problem — a gas shortage. The country did a great deal of research and implemented a wide variety of ideas to help their people become more self-sufficient, regardless of the availability of traditional gasoline and other fossil fuel based products.

The original program was pioneering for its time, but the Chinese quickly followed suit and have been able to top the market in biogas production in relatively little time. Comparatively, India’s production of biogas is quite small. It only produces about 2.07 billion m3/year of biogas, while it’s estimated that it could produce as much as 48 billion m3/year. This means that there are various issues with the current method’s India is using in its biogas production.

Biogas_Animal

Biogas has the potential to rejuvenate India’s agricultural sector

The original planning in the NBMMP involved scientists who tried to create the most efficient biogas generators. This was good, but it slowed people’s abilities to adopt the techniques individually. China, on the other hand, explicitly worked to help their most rural areas create biogas. This allowed the country to spread the development of biogas to the most people with the lowest barriers to its proliferation.

If India can learn from the strategy that China has employed, they may be able to give their biogas production a significant boost which will also help in the rejuvenation of biomass sector in the country. Doing so will require the help and willingness of both the people and the government. Either way, this is an industry with a lot of room for growth.

Sugarcane Trash – A Wonderful Resource that Indian Sugar Industry is Wasting

In Indian sugar mills, the frequent cycles of ups and downs in the core business of selling sugar has led to the concentration towards the trend of ancillary businesses, like cogeneration power plant and ethanol production, becoming the profit centres. These units, which were introduced as a means to manage sugar mills’ own byproduct, like bagasse, are now keeping several sugar mills financially afloat. Thus, the concept of ‘Integrated Sugar Mill Complex’ has now become a new normal.

Limitations of Bagasse

Bagasse is a ubiquitous primary fuel in cogeneration plants in sugar mills, which adds more than 2,000 MW of renewable power to the Indian energy mix. The inclination of cogeneration plant managers towards bagasse is primarily because of its virtue of being easily available on-site, and no requirement to purchase it from the external market.

This remains true despite its several significant shortcomings as a boiler fuel, prime among which are very high moisture content and low calorific value. As a result, the fuel-to-energy ratio remains abysmally low and the consequent lesser power generation is depriving these sugar mills from achieving true revenue potential from their ancillary power business vertical, which is pegged at ~10,000 MW.

Sugarcane Trash – A Wonder Waste

Though, there is a much neglected high calorific value biomass which is available in proximity of every sugar mill and is also a residue of the sugarcane crop itself, which could enable the cogeneration units to achieve their maximum output potential. This wonder waste is sugarcane trash the dry leaves of sugarcane crop – which is left in the farms itself after sugarcane harvesting as it has no utility as fodder and generally burnt by farmers, which harms the surrounding air quality substantially.

Given its favourable properties of having very low moisture content with moderate-to-high calorific value, sugarcane trash could be used in most of the high pressure boiler designs in a considerable proportion along with bagasse.

cane-trash

Undeniably, sugar mills should not discontinue using bagasse as the primary fuel, but surely complement it with sugarcane trash as it would lead to an increase in their revenue generation and would also allow them to expand operations of their cogeneration plant to off-season, as using sugarcane trash with bagasse in season would leave more bagasse for off season usage.

Hurdles to Overcome

Despite these evident benefits, the major obstacle in development of sugarcane trash as an industrial boiler fuel has been its difficult collection from thousands of small and fragmented farms. Moreover, the trash becomes available and needs to be collected simultaneously during the operating season of the sugar mills, which makes deployment of resources, human or otherwise, for managing the procurement of trash very difficult for any sugar mill.

As a matter of fact, the sugar mills which initiated the pilots, or even scaled commercially, to utilise sugarcane trash along with bagasse, had to sooner or later discontinue its use, owing to the mammoth challenges discussed above.

The Way Forward

Thus, in order to utilise this wonder waste, there is a dire need to outsource its procurement to professional and organised players, like RY Energies and others, which establish the biomass supply chain infrastructure in the vicinity of the cogeneration units to make on-site availability of sugarcane trash as convenient as bagasse and enable them to procure the rich quality biomass at sustainable prices which leads to an increase in their profits.

sugarcane-trash-burning

Burning of cane trash creates pollution in sugar-producing countries

These biomass supply chain companies offer value to the farmers by processing their crop residues in timely manner, thus prevent open burning of the crop residue and contribute to a greener and cleaner environment.

Indeed, owing to its favourable fuel properties, positive environmental impact and now, with ease in its procurement, sugarcane trash biomass is the fuel of today and future for the Indian sugar mills.

The Top 7 Benefits of Composting

The impact of human activities on the environment is rapidly changing. One such activity gaining much attention is waste disposal. A lot of waste products go to landfills despite constituting a reasonable fraction of organic matter, such as paper materials, food wastes, and pet droppings.

The new preferred way to dispose of organic waste is composting. Composting refers to the process through which materials biodegrade. It is a means by which organic waste can be safely recycled. Composting can be effectively done with compost systems.

benefits-composting

Take note that this process of waste disposal is still in its early stages, especially when adopted in homes. Still, here are 7 benefits of composting:

1. Improved Soil Quality

Composted materials become humus, a known nutrient-rich constituent of soil. The newly formed humus replenishes soil nutrients and improves water retention in loose soil. Thus, soil quality considerably improves as a result of composting.

Composted materials are also rich in fungi and bacteria. These microbes prevent insect infestation and suppress weed growth. With these nutrient draining agents out of the way, your soil quality dramatically improves, too.

2. Saves Time and Money

It is a waste of time and money when a yard being cultivated does not experience normal growth, nor does it yield the expected harvest. Fortunately, you can save money and time in the long term with composting practices. This is possible because of the compost’s ability to fight insect infestation, weed growth, and to replenish the soil of lost nutrients.

The three nutrients that are sought in chemical fertilizers, Nitrogen, Phosphorus, and Potassium (NPK), are made available by humus. This directly saves you the cost of purchasing fertilizers. Without the presence of compost, farmers need to spend a lot of money to buy pesticides and weed killers.

3. Environment Friendliness

Composting is an environmentally friendly option compared to landfills. Landfills are currently the most common destination for organic waste. In landfills, organic waste cannot decay properly, so they generate a specific greenhouse gas called methane.

landfills-methane-gas

Methane is known to cause harmful effects on the environment – similar to that of carbon dioxide but even more dangerous. The more organic waste ends up in landfills, the more methane gas that is produced.

Composting solves this problem in a whiff by reducing the amount of methane produced while organic matter decays. Composting allows carbon to be retained in the soil, which lowers the carbon footprint caused by decaying matter.

The ability of compost to bypass the incineration of yard waste also makes it a preferred option for organic waste in yards.

4. Improved Human Health

There are several ways for composting to indirectly enhance human health. The reduction of greenhouse gas emissions, as mentioned above, by composting is not only good for the environment but also for people – a reduction of greenhouse gas means a healthier environment to live in.

Organic food production credited to composting also improves human health in significant ways. It reduces the number of chemicals from fertilizers and pesticides that end up in meals, translating to healthier humans.

5. Higher Agricultural Yield

A higher yield of crops is very important to farmers. Through its ability to increase soil quality, composting achieves a higher return in agricultural products. More plant yield accounts for more plants to be sold, which also means more money to be made.

Soil quality also translates to the quality of the food which is produced. Food produced from high-quality, organic soil is free from all toxins from chemical fertilizers and pesticides.

6. Reduced Erosion

Erosion is harmful to the soil because it makes soil matter and nutrients to be washed away. This is compounded by the fact that soils are loose.

Compost averts erosion by remedying the existing structure of the soil. It further prevents erosion by:

  • Aiding water infiltration in the soil structure.
  • Aiding water retention, thereby slowing runoff and loss of soil matter.
  • Allows for quicker vegetation growth.

7. Aids Biodiversity

Microorganisms present in the soil, such as bacteria, fungi, and protozoa, will cause the decay of organic material. Their presence is important because they aid soil aeration. Soil aeration on its own accelerates the composting process, making nutrients available in their usable state as quickly as possible.

Other organisms that are present in composted soil include worms and beneficial insects. All these aids the process of plant growth.

Conclusion:

Composting is a sustainable and environmentally friendly way to dispose of organic waste. It is particularly important even now as the world struggles with creating solutions to waste disposal.

Composting results in better soil quality. It is also a process that saves them time and money of farmers. Humans can benefit from composting through improved health. There is a higher yield of farm produce as a result of composting. Erosion is significantly reduced, and biodiversity is achieved in the soil through composting.

Overview of Biomass Handling Equipment

The physical handling of biomass fuels during collection or at a processing plant can be challenging task, particularly for solid biomass. Biomass fuels tend to vary with density, moisture content and particle size and can also be corrosive. Therefore biomass fuel handling equipment is often a difficult part of a plant to adequately design, maintain and operate.

Biomass_Conveyor

The design and equipment choice for the fuel handling system, including preparation and refinement systems is carried out in accordance with the plant configuration. This is of special importance when the biomass is not homogeneous and contains impurities, typically for forest and agricultural wastes. Some of the common problems encountered have been the unpopular design and undersized fuel handling, preparation and feeding systems.

The fuel handling core systems and equipment are dependent on both the raw fuel type and condition as well as on the conversion/combustion technology employed. The core equipment in a biomass power plant include the following:

  1. Fuel reception
  2. Fuel weighing systems
  3. Receiving bunkers
  4. Bunker discharge systems (stoker, screw, grab bucket)
  5. Fuel preparation
  6. Fuel drying systems
  7. Crushers
  8. Chippers
  9. Screening systems
  10. Shredding systems
  11. Grinding systems (for pulverised fuel burners)
  12. Safety systems (explosion relieve, emergency discharge, fire detections etc)
  13. Fuel transport and feeding
  14. Push floors
  15. Belt feeders
  16. Conveyers and Elevators
  17. Tube feeders
  18. Fuel hoppers and silos (refined fuel)
  19. Hopper, bunker and silo discharge
  20. Feeding stokers
  21. Feeding screws
  22. Rotary valves

To enable any available biomass resource to be matched with the end use energy carrier required (heat, electricity or transport fuels) the correct selection of conversion technologies is required. Since the forms in which biomass can be used for energy are diverse, optimal resources, technologies and entire systems will be shaped by local conditions, both physical and socio-economic in nature.

As the majority of people in developing countries will continue using biomass as their primary energy source well into the next century, it is of critical importance that biomass-based energy truly can be modernized to yield multiple socioeconomic and environmental benefits.

Biomass Pelletization Process

Biomass pellets are a popular type of biomass fuel, generally made from wood wastes, agricultural biomass, commercial grasses and forestry residues. In addition to savings in transportation and storage, pelletization of biomass facilitates easy and cost effective handling. Dense cubes pellets have the flowability characteristics similar to those of cereal grains. The regular geometry and small size of biomass pellets allow automatic feeding with very fine calibration. High density of pellets also permits compact storage and rational transport over long distance. Pellets are extremely dense and can be produced with a low moisture content that allows them to be burned with very high combustion efficiency.

biomass-pellets

Biomass pelletization is a standard method for the production of high density, solid energy carriers from biomass. Pellets are manufactured in several types and grades as fuels for electric power plants, homes, and other applications. Pellet-making equipment is available at a variety of sizes and scales, which allows manufacture at domestic as well industrial-scale production. Pellets have a cylindrical shape and are about 6-25 mm in diameter and 3-50 mm in length. There are European standards for biomass pellets and raw material classification (EN 14961-1, EN 14961-2 and EN 14961-6) and international ISO standards under development (ISO/DIS 17225-1, ISO/DIS 17225-2 and ISO/DIS 17225-6).

Process Description

The biomass pelletization process consists of multiple steps including raw material pre-treatment, pelletization and post-treatment. The first step in the pelletization process is the preparation of feedstock which includes selecting a feedstock suitable for this process, its filtration, storage and protection. Raw materials used are sawdust, wood shavings, wood wastes, agricultural residues like straw, switchgrass etc. Filtration is done to remove unwanted materials like stone, metal, etc. The feedstock should be stored in such a manner that it is away from impurities and moisture. In cases where there are different types of feedstock, a blending process is used to achieve consistency.

The moisture content in biomass can be considerably high and are usually up to 50% – 60% which should be reduced to 10 to 15%. Rotary drum dryer is the most common equipment used for this purpose. Superheated steam dryers, flash dryers, spouted bed dryers and belt dryers can also be used. Drying increases the efficiency of biomass and it produces almost no smoke on combustion. It should be noted that the feedstock should not be over dried, as a small amount of moisture helps in binding the biomass particles. The drying process is the most energy intensive process and accounts for about 70% of the total energy used in the pelletization process.

Schematic of Pelletization of Woody Biomass

Before feeding biomass to pellet mills, the biomass should be reduced to small particles of the order of not more than 3mm.  If the pellet size is too large or too small, it affects the quality of pellet and in turn increases the energy consumption. Therefore the particles should have proper size and should be consistent. Size reduction is done by grinding using a hammer mill equipped with a screen of size 3.2 to 6.4 mm. If the feedstock is quite large, it goes through a chipper before grinding.

The next and the most important step is pelletization where biomass is compressed against a heated metal plate (known as die) using a roller. The die consists of holes of fixed diameter through which the biomass passes under high pressure. Due to the high pressure, frictional forces increase, leading to a considerable rise in temperature. High temperature causes the lignin and resins present in biomass to soften which acts as a binding agent between the biomass fibers. This way the biomass particles fuse to form pellets.

The rate of production and electrical energy used in the pelletization of biomass  are strongly correlated to the raw material type and processing conditions such as moisture content and feed size. The The average energy required to pelletize biomass is roughly between 16 kWh/t and 49kWh/t. During pelletization, a large fraction of the process energy is used to make the biomass flow into the inlets of the press channels.

Binders or lubricants may be added in some cases to produce higher quality pellets. Binders increase the pellet density and durability. Wood contains natural resins which act as a binder. Similarly, sawdust contains lignin which holds the pellet together. However, agricultural residues do not contain much resins or lignin, and so a stabilizing agent needs to be added in this case. Distillers dry grains or potato starch is some commonly used binders. The use of natural additives depends on biomass composition and the mass proportion between cellulose, hemicelluloses, lignin and inorganics.

Due to the friction generated in the die, excess heat is developed. Thus, the pellets are very soft and hot (about 70 to 90oC). It needs to be cooled and dried before its storage or packaging. The pellets may then be passed through a vibrating screen to remove fine materials. This ensures that the fuel source is clean and dust free.

The pellets are packed into bags using an overhead hopper and a conveyor belt. Pellets are stored in elevated storage bins or ground level silos. The packaging should be such that the pellets are protected from moisture and pollutants. Commercial pellet mills and other pelletizing equipment are widely available across the globe.