3.1: biomass and bio-energy
Biomass: is any organic materials, i.e. decomposing, matter derived from plants or animals available on a renewable basis. Biomass includes all kinds of vegetative and organic waste. Good examples of biomass are (i) Agricultural waste left behind in the fields after harvesting (ii) plant material left in the forest after collection of timber (iii) bagasse in the sugar industry (iv) rice husk in rice mills (v) saw dust from timber milling machine wood (vi) groundnut shell (vii) municipal solid waste (MSW) (viii) energy crop that are separately cultivated for their fuel content Ghosh, K.P., 2002.
Bio-energy: is energy derived from the conversion of biomass where biomass may be used directly as solid fuel, or processed into liquids OECD, 2004.
(International Energy Association, IEA, 2009, biomass-based energy accounted for roughly 10%
of world total primary energy supply in 2009. Most of this is consumed in developing countries for cooking and heating using very inefficient open fires or simple cook stoves with considerable impact on health (smoke pollution) and environment (deforestation). Modern bio-energy supply on the other hand is comparably small, but has been growing steadily in the last decade. A total of 280 TWh of bio-energy electricity, i.e. 1.5% of world electricity generation, was generated globally in 2010, and 8 EJ of bio-energy for heat were used in the industry sector.
Analysis in the IEA Technology Roadmap: Bio-energy for heat and power suggests, that in order to achieve significant emission reductions in the energy sector, sustainably produced bio-energy will play an increasing role in the future with demand increasing three-fold to 2050.
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While several technologies for generating bio-energy heat and power already exist, there is a need to extend the use of the most efficient technologies available today, and to complete the development and deployment of a number of new technology options. Co-firing biomass with coal will be an important option to achieve short-term emission reductions, and make use of standing assets. In addition, new dedicated bio-energy plants will become increasingly important to meet growing demand for bio-energy electricity and heat.
While in favorable circumstances producing energy from biomass can be cost competitive today. In many cases, economic incentives are currently needed to off-set cost differences between bio-energy and fossil fuel-generated electricity and heat. Such support is justified by the environmental, energy security and socio-economic advantages associated with sustainable bio-energy, but should be introduced as transitional measure leading to cost competitiveness in the medium term. Support measures should be backed by a strong policy frame work which balances the need for energy with other important objectives such greenhouse-gas (GHG) reduction, food security and biodiversity, and socio-economic development.
3.2: Biomass Power Generation Technologies
There can be many advantages of using biomass instead of fossil fuels for power generation, including lower greenhouse gas (GHG) emission, energy cost saving, improved security of supply and demand, waste management/reduction opportunities and local economic development opportunities. However, whether these benefits are realized, and to what extent, depends critically of the sources and the nature of the biomass feedstock.
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Thermo- chemical conversion processes of biomass feedstock
Thermo-chemical process
Combustion process: the cycle used is the conventional rankine cycle with biomass being burned (oxidised) in a high pressure boiler to generate steam. The net power cycle efficiencies that can be achieved are about 23% to 25%. The exhaust of the steam turbine can either be fully condensed to produce power or used partly or fully for another useful heating activity. In addition to exclusive use of biomass combustion to power a steam turbine, biomass can be co-fired with coal in a coal power plant. (EPRI., 2012)
Direct co-firing is the process of adding a percentage of biomass to the fuel mix in a coal-fired power plant. It can be co-coal-fired up to 5-10% of biomass (in energy terms) and 50-80% with extensive pre-treatment of feedstock (i.e. torrefaction) with only minor changes in the handing equipment. For percentages about 10% or if biomass and coal are being separately in different boilers, known as parallel co-firing, the changes in mills, burners and dryers are needed, EPRI.,2012.
Gasification process: is achieved by the partial combustion of the biomass in a low oxygen environment, leading to the release of gaseous product (producer gas or syngas). So-called “allothermal” or indirect gasification is also possible. The gasifier can either be of a “fixed bed”, “fluidized bed” or “entrained flow” configuration. The resulting gas is a mixture of carbon monoxide, water, CO2, char, tar and hydrogen, and it can be used in combustion turbines, micro-turbines, fuel cells or gas turbines. When used in turbines and fuel cells, higher electrical efficiencies can be achieved than those achieved in a steam turbine. It is possible to co-fire a
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power plant either directly (i.e Biomass and coal are gasified together) or indirectly (i.e gasifying coal and biomass separately for use in gas turbines), EPRI. 2012.
Pyrolsis process: pyrolsis is a subset of gasification systems. In pyrolsis, the partial combustion is stopped at a lower temperature (450°c to 600°c) resulting in the creation of liquid bio-oil, as well as gaseous and solid products. The pyrolsis oil can then be used as fuel to generate electricity, EPRI, 2012.
3.3: How Briquetting Work.
There are two approaches to briquetting; both require the loose biomass to be ground to a coarse powder like sawdust.
High Pressure Briquetting: high pressure briquetting uses a power-driven press to raise the pressure of dry powdered biomass to about 1500 bar (150 Mpa). According to D. Fulford et al (2014)This compression heats the biomass to a temperature of about 120°C, which melts the lignin in the woody biomass material. The press forces the hot material through a die at a controlled rate, as the pressure decrease, the lignin cools down and re-solidifies the briquette and then binding the biomass powder into uniform, solid briquettes. This high pressure machines are produced in a wide range of sizes, for example a range capable of processing 30kg/hour to 2000kg/hour. Piston press, screw press and roller press are examples of high pressure briquetting machines.
Low Pressure Briquetting: can be used for materials with low amount of lignin, such as paper and charcoal dust. In this process, the powdered biomass is mixed with water to form a paste and then binder with starch or clay material. A briquetting press is used to push the paste into a mould or through an extruder, or it can simply by shaped by hand, the briquettes thus
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produced are left to dry, so that the binder sets and holds the biomass powder together. Low pressure briquetting machines are often hand operated, using a level that drives a piston to compress the paste. According to D. Fulford et al (2014)
3.4: Biomass Briquetting Technologies
The utilization of agricultural and forestry residues is usually cumber sum due to uneven and troublesome of their characteristics. But these have been overcome by means of densification, i.e. through compaction of the residues into products of high density and well define regular shape. Densification of biomass can be categorized into two main types: briquettes and pellets. Briquettes are of relatively large size (typically 5-6 cm in diameter and 30-40 cm in length) while pellets are small in size (about 1 cm in diameter and 2-4 cm in length), S.C.
Bhattacharya, 2008.
Biomass densification represents a set of technologies for the conversion of biomass residues into a convenient fuel. The technology is also known as briquetting or agglomeration. Depending on the types of equipment used, it could be categorized into five main types: Maninder et al;
5. Low pressure or manual presses.
Piston press densification: there are two types of piston press (1) the die and punch technology; and (2) hydraulic press. In the die and punch technology, which is also known as ram and die technology, biomass is punched into a die by a reciprocating ram with a very high pressure thereby compressing the mass to obtain a compacted product. The standard size of the
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briquette produced using this machine is 60 mm, diameter. The power required by a machine of capacity 700kg/hr is 25 kW. The hydraulic press process consist of first compacting the biomass in the vertical direction and then again in the horizontal direction. The standard briquette weight is 5 kg and its dimensions are: 450 mm x 160 mm x 80 mm. the power required is 37 kW for 1800 kg/h of briquetting. The technology can accept raw material with moisture content up to 22%. The process of oil hydraulics allows a speed of 7 cycles/minute (cpm) against 270 cpm for the die and punch process. The slowness of operation helps to reduce the wear rate of the parts.
The ram moves approximately 270 times per minute in this process.
Screw press: the compaction ratio of screw presses ranges from 2.5:1 to 6:1 or even more. In this process, the biomass is extruded continuously by one or more screws through a taper die which is heated externally to reduce the friction. Here also, due to the application of high pressures, the temperature rises fluidizing the lignin present in the biomass which acts as a binder.
The outer surface of the briquettes obtained through this process is carbonized and has a hole in the centre which promotes better combustion. Standard size of the briquette is 60 mm diameter.
Roller press: in a briquetting roller press, the feedstock falls in between two rollers, rotating in opposite directions and is compacted into pillow-shaped briquettes. Briquetting biomass usually requires a binder. This type of machine is used for briquetting carbonized biomass to produce charcoal briquettes.
Pelletizing: pelletizing is closely related to briquetting excepted that it uses smaller dies (approximately 30 mm) so that the smaller products are called pellets. The pelletizer has a number of die arranged as holes bored on a thick steel disk or ring and the material is forced into the dies by means of two or three rollers. The two main types of pellet presses are: flat/disk and
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ring types. Other types of pelletizing machines include the punch press and the cog-wheel pelletizer. Pelletizers produce cylindrical briquettes between 5 mm and 3 mm in diameter and of variable length. They have good mechanical strength and combustion characteristics. Pellets are suitable as a fuel for industrial applications where automatic feeding is required. Typically pelletizers can produce up to 1000 kg of pellets per hour but initially require high capital investment and have high energy input requirements.
Manual press and low pressure briquetting: there are different types of manual presses used for briquetting biomass feed stocks. They are specifically designed for the purpose or adapted from existing implements used for other purposes. Manual clay brick making presses are a good example. They are used both for raw biomass feedstock or charcoal. The main advantages of low-pressure briquetting are low capital costs, low operating costs and low levels of skill required to operate the technology. Low-pressure techniques are particularly suitable for briquetting green plant waste such as coir or bagasse (sugar-cane residue). The wet material is shaped under low pressure in simple block presses or extrusion presses. The resulting briquette has a higher density than the original material but still requires drying before it can be used. The dried briquette has little mechanical strength and crumbles easily. The use of a binder is imperative.
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Below is the portrait of different briquetting machines that are used for biomass densification.
Figure: 3.1: roller, screw press, piston press, pelletising machine
Sources: www.alibaba.com Ashen foundation http://www.ashen.org/briquettes
Figure: 3.2: manual press
Sources: Legacy foundation and engineers without borders
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The picture below shows the different briquette types produce from different technologies.
Figure3.3: shows the products of different machines.
Source: www. Google.com.
3.5: Economic analysis of briquetting technologies
Table 3.1, highlight high compaction technology or binderless technology consist of piston and screw press and are becoming more important commercially. In the screw extruder press, the biomass is extruded continuously by a screw through a heated taper die. In the piston press the wear of the contacts parts e.g, the ram and die is less compared to the wear of the screw and die in screw extruder press. The power consumption in the former is less than that of the latter. The briquette quality and production procedure screw press is definitely superior to the piston press. The piston presses are two type, the mechanical and hydraulic press. The mechanical are typically large scale installation 200kg/h to 1800kg/h and hydraulic presses are 50kg/h to 200kg/h and 400kg/h to 1500kg/h. the screw presses are usually available with capacity from 75kg/h to 250kg/h.
Screw press Piston press Pellets
Roller press Manual or low pressure
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Table 3.1: Economic comparison between piston press and screw press technology Piston press Screw press Optimum moisture content of raw
material
10-15% 8-9%
Wear of contact parts Low in case of ram and die High in case of screw
Output from the machine In strokes continuous
Power consumption 50 kWh/ton 60 kWh/ton
Density of briquette 1-1.2gm/cm 1-1.4gm/cm
Maintenance high Low
Combustion performance of briquettes
Not so good Very good
Carbonization of charcoal Not possible Make good charcoal
Suitability in gasifiers Not suitable Suitable
homogeneity Non-homogeneous homogeneous
Capacity 150-2000kg per/hr 75-250kg per/hr
Capital cost Us$ 20-30,000 Us$ 1,350
Sources: FAO, 1996 and EEP, 2013
3.6: Densification technologies in different countries.
Two common types of briquetting presses employed in developing countries are heated-die screw press and piston press. It appears that heated-heated-die screw press technology was invented in Japan in mid-1940s. the technology has spread to most of its neighbouring and nearby countries, particularly Korea, China, Taiwan, Vietnam, Thailand, Malaysia, Philippines, Bangladesh, etc. where heated-die screw-press briquetting machines are used almost exclusively.
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Also, the design of screw-press briquetting machines appears to have evolved and been adapted to suit local conditions in different countries.
The piston press technology is the dominant technology in India, Brazil, and Africa.
While these are locally made in India and Brazil, the African machine appears to be mostly imported. Compared to piston-press machines, heated-die screw press machines have smaller capacity but produce stronger and denser briquettes. Screw press technology is therefore more suitable if the briquettes are to be carbonized to obtain briquetted charcoal.
Besides, conventional binder less briquetting, low-pressure cold briquetting using binder has also been tried in some places. Most worthy among these is the carbonization-briquetting process; in which biomass is first carbonized and the resulting charcoal is briquetted using a suitable binder. The process has been tried for cotton stalks in Sudan and coffee husks in Kenya;
limited use of this technique has been reported in India and Nepal. Briquetting of bagasse using molasses as binder has been reported to have had limited success in Sudan.
Another low-pressure binderless briquetting process involves mixing pulverized chopped and decomposed biomass with water into a pulp. The pulp is pressed inside a perforated pipe to get 4-inch diameter cakes, which are sun-dried to get briquettes, Stanley, 2002. The basic press is made on site and the product is normally of lower density compared with conventional briquettes.
A non-profit organization, Legacy Foundation, is currently involved in dissemination of the technology.
Briquette made from a mixture of pulverized coal, biomass and slaked lime has been introduced by a Japanese company in two Asian countries, China and Indonesia. The briquettes, called coal-biomass briquettes are produced by using a roll-press. It is claimed that the use of the
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desulfurizing agent (slaked lime) and biomass results in cleaner combustion of the briquettes in stove and less of ash compared with coal or coal briquettes Kobayashi, 2002.
Densification technology employed in developed countries, capacity of these plants is much larger, being in the range 1-30 tons per hour.
3.7: Biomass briquetting processes.
There are two main types of briquettes carbonized and non-carbonized, produced by the application of two different processing techniques. Carbonized briquettes are made from biomass sources that have been processed through partial pyrolsis (which drives off volatile compound and moistures leaving a higher concentration of carbon per unit). Hereafter, they are mixed with binders, cast into appropriate shapes through pressing and finally dried. Un-carbonized briquettes are processed directly from biomass sources through various casting and pressing processes, which is known as solidification, EEP, 2013. Depending upon the type of biomass, three processes are generally required involving the following steps. FAO, 1996.
1. Sieving – Drying – Preheating – Densification – Cooling – Packing.
2. Sieving – Crushing – Preheating – Densification – Cooling – Packing.
3. Drying – Crushing – Preheating – Densification – Cooling – Packing.
Step 1, is used for saw dust, Step 2, is used for agro and mill residues which are normally dry.
Eg coffee husk, rice husk, groundnut shell etc. and Step 3, is used for materials like bagasse, mustard, coir path and other cereal stalks.
Converting residues into a densified form has the following advantages
1. The process increase the net calorific value per unit volume
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3. The process helps to solve the problem of residue disposal 4. The fuel produced is uniform in size, shape and quality.
3.8: Pretreatment Technologies to Improve Quality Attributes.
Pretreatment improved both the physical and chemical properties, thereby making the material easy to densify and transport.
1. Grinding: biomass is ground to a certain particle size. This grinding partially breaks down the lignin, increases the specific area of the materials, and contributes to better binding, Peleg, 1977.
2. Preheating: is widely used as it results in a higher quality product. Most briquette producers use preheating to form more stable and dense pellets or briquettes, Bhattacharya, 1993.
3. Stream conditioning and Explosion: an efficient method of pretreatment for both herbaceous and woody biomass, either for densification or ethanol production. The compressed hot water or steam is commonly used in this process. During steam explosion, which is a high-temperature, short-time process, the material is introduced into a rector and heated under pressure at elevated temperatures. This process produces significant physical, chemical, and structural changes in the biomass and makes more lignin sites available for binding during pelletization, Lin and Wyman, 2005.
4. Torrefaction: a method of changing the properties of biomass materials by slowly heating it in an inter-environment to a maximum temperature of 300 degree, Felfli et al.
1998. The process is also called mild pyrolysis as most of the smoke-producing
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compounds and other volatiles are removed resulting in a final product that has approximately 70% of the initial weight and 80-90% of the original energy content Arcate, 2000 and 2002. Thus, treatment yields a solid uniform product with lower moisture content and higher energy content compared to the initial biomass.
3.9: Common binders used in biomass densification.
Binders improve the binding features of the biomass and for long durability of the product.
Binder helps to reduce wear in production equipment and increase abrasion-resistance of the fuel.
Sometimes addition of binders can results in increase sulfur content of densified biomass.
Binders are allowed, but must be specified to final product. The following are the most common binders used for densification purposes, Tabil et al, 1997.
1. Lignosulfonate: are used in animal feedstuffs and have been considered the most effective and popular binder, Anonymous 1983; MacMahon 1984. The composition
1. Lignosulfonate: are used in animal feedstuffs and have been considered the most effective and popular binder, Anonymous 1983; MacMahon 1984. The composition