AS A SOLID FUEL
What is biomass? Biomass is the term used to describe all the organic matter, produced by photosynthesis that exists on the earth’s surface. The source of all energy in biomass is the sun, the biomass acting as a kind of energy store. To make use of biomass for our own energy needs we can simply burn it in an open fire to provide heat for cooking, warming water or warming the air in our home. More sophisticated technologies have been developed for extracting this energy and converting it into useful power and heat in more efficient and convenient ways. Until relatively recently it was the only form of energy which was used by humans and is still the main source of energy for more than half the world’s population for their domestic energy needs. The extraction of energy from biomass is split into 3 distinct categories: Figure 1: Domestic biomass use in Solid biomass - the use of trees, crop Sri Lanka. Photo: Jean Long / Practical residues, animal and human waste (although Action. not strictly a solid biomass source, it is often included in this category), household or industrial residues for direct combustion to provide heat. Often the solid biomass will undergo physical processing such as cutting, chipping, briquetting, etc. but retains its solid form. Biogas - biogas is obtained by anaerobically (in an air free environment) digesting organic material to produce a combustible gas known as methane. Animal waste and municipal waste are two common feedstocks for anaerobic digestion. See the Biogas Technical Brief for more details. Liquid Biofuels – these are obtained by subjecting organic materials to one of various chemical or physical processes to produce a usable, combustible, liquid fuel. Biofuels such as vegetable oils or ethanol are often processed from industrial or commercial residues such as bagasse (sugarcane residue remaining after the sugar is extracted) or from energy crops grown specifically for this purpose. Biofuels are often used in place of petroleum derived liquid fuels. See the Liquid Biofuels and Sustainable Development Technical Brief.
This technical brief looks at the use of solid biomass fuels, and their associated technologies.
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Solid biomass is widely used in developing countries, mainly for cooking, heating water and domestic space heating. Biomass is available in varying quantities throughout the developing world - from densely forested areas in the temperate and tropical regions of the world, to sparsely vegetated arid regions where collecting wood fuel for household needs is a time consuming and arduous task. In past decades the threat of global deforestation, provided a focus for the efficient use of biomass (as well as introducing alternative fuels) in areas where woodfuel was in particular shortage. Although domestic fuelwood users can suffer greatly from the effects of deforestation, it often arises because of land clearing for agricultural use or for commercial timber. There have been many programmes aimed at developing and disseminating improved stove technologies to reduce the burden, primarily borne by women, of fuelwood collection as well as reducing health risks associated with smoke from burning fuelwood. Technologies have also been introduced to help with the processing of biomass to improve efficiency, allow for easy transportation or to make it more useable. Crop and industrial biomass residues are now widely used in many countries to provide centralised, medium and large-scale production of process heat for electricity production or other commercial end uses. There are several examples in Indonesia of timber processing plants using wood waste-fired boilers to provide heat and electricity for their own needs, and occasionally for sale to other consumers. There are also small scale options to utilising crop residues.
For solid biomass to be converted into useful heat energy it has to undergo combustion. Although there are many different combustion technologies available, the principle of biomass combustion is essentially the same for each. There are three main stages to the combustion process:
Drying - all biomass contains moisture, and this moisture has to be driven off before combustion
proper can take place. The heat for drying is supplied by radiation from flames and from the stored heat in the body of the stove or furnace. Pyrolysis - the dry biomass is heated and when the temperature reaches between 200ºC and 350ºC the volatile gases are released. These gases mix with oxygen and burn producing a yellow flame. This process is self-sustaining as the heat from the burning gases is used to dry the fresh fuel and release further volatile gases. Oxygen has to be provided to sustain this part of the combustion process. When all the volatiles have been burnt off, charcoal remains. Oxidation - at about 800ºC the charcoal oxidises or burns. Again oxygen is required, both at the fire bed for the oxidation of the carbon and, secondly, above the fire bed where it mixes with carbon monoxide to form carbon dioxide which is given off to the atmosphere. It is worth bearing in mind that all the above stages can occur within a fire at the same time, although at low temperatures the first stage only will be underway and later, when all the volatiles have been burned off and no fresh fuel added, only the final stage will be taking place. Combustion efficiency varies depending on many factors; fuel, moisture content and calorific value of fuel, etc. The design of the stove or combustion system also affects overall thermal efficiency and table 1 below gives an indication of the efficiencies of some typical systems (including non-biomass systems for comparison).
Type of combustion technology Three-stone fire Improved wood-burning stove Charcoal stove with ceramic liner Sophisticated charcoal-burning stove Kerosene pressure stove LPG gas stove Steam engine
Source: Adapted from Kristoferson, 1991
Percentage efficiency 10 - 15 20 - 25 30 - 35 up to 40 53 57 10 - 20
Table 1: efficiencies of some biomass energy conversion systems
Much of the research and development work carried out on biomass technologies for rural areas of developing countries has been based on the improvement of traditional stoves. This was initially in response to the threat of deforestation but has also been focused on the needs of women to reduce fuel collection times and improve the kitchen environment by smoke removal. There have been many approaches to stove improvement, some carried out locally and others as part of a wider programmes run by international organisations. Figure 2 below shows a variety of successful improved stove types, some small, portable stoves and others designed for permanent fixture in a household. Some of the features of these improved stoves include: a chimney to remove smoke from the kitchen an enclosed fire to retain the heat careful design of pot holder to maximise the heat transfer from fire to pot baffles to create turbulence and hence improve heat transfer dampers to control and optimise the air flow a ceramic insert to minimise the rate of heat loss a grate to allow for a variety of fuel to be used and ash to be removed metal casing to give strength and durability multi pot systems to maximise heat use and allow several Figure 2: Examples of improved stoves. pots to be heated simultaneously Improving a stove design is a complex procedure which needs a broad understanding of many issues. Involvement of users in the design process is essential to gain a thorough understanding of the user’s needs and requirements for the stove. The stove is not merely an appliance f or F society), variety of Improved social focus, heating food (as it has become in Westernigure 2: Abut is often acts as aCookstoves a means of lighting and space heating. Tar from the fire can help to protect a thatched roof, and the smoke can keep out insects and other pests. Cooking habits need to be considered, as well as the lifestyle of the users. Light charcoal stoves used for cooking meat and vegetables are of little use to people who have staple diets such as Ugali (Cornmeal commonly made from maize flour), 3
which require large pots and vigorous stirring. Fuel type can differ greatly; in some countries cow dung is used as a common fuel source, particularly where wood is scarce. Cost is also a major factor among low-income groups. Failing to identify these key socio-economic issues will ensure that a stove programme will fail. The function of an improved stove is not merely to save fuel.
Local manufacture of stoves
Since 1982, the Kenya Ceramic Jiko (KCJ), an improved charcoal-burning stove aimed at the urban market has been developed and manufactured by large numbers of small producers. The KCJ has two main components; metal and fired clay. Both these parts are made by entrepreneurs; the metal part (cladding) being made by small-scale enterprises or individual artisans, while the clay part (liner) is manufactured by slightly larger and more organised enterprises or women’s groups. The KCJ is sold by the artisans directly to their customers or through commercial outlets such as retail shops and supermarkets. The stove was initially promoted heavily to develop the market, by the NGO KENGO and by the Kenyan Ministry of Energy, through the mass media, market demonstrations and trade fairs. As a result of this substantial promotion, there are now more than 200 artisans and microenterprises manufacturing some 13,600 improved stoves every month. To date, it is estimated that there are some 700,000 such stoves in use in Kenyan households. This represents a penetration of 16.8% of all households in Kenya, and 56% of all urban households in the country.
Source: Dominic Walubengo, Stove Images, 1995
Charcoal production is the most common methods for processing wood to make them cleaner and easier to use as well as easier to transport but charcoal does not increase the total energy content of the fuel - in fact the energy content is decreased. Charcoal is often produced in rural areas and transported for use in urban areas. The process can be described by considering the combustion process discussed above. The wood is heated in the absence of sufficient oxygen which means that full combustion does not occur. This allows pyrolysis to take place, driving off the volatile gases and leaving charcoal (carbon). The removal of the moisture means that the charcoal has a much higher specific energy content than wood.
Figure 3: Charcoal Kilns, Malawi. Photo: Practical
Action / Paul Harris.
Other biomass residues such as millet stems or corncobs can also be converted to charcoal. Charcoal is produced in a kiln or pit. A typical traditional earth kiln will comprise of the fuel to be carbonised, which is stacked in a pile and covered with a layer of leaves and earth. Once the combustion process is underway the kiln is sealed, and then only once process is complete and cooling has taken place can the charcoal be removed. A simple improvement to the traditional kiln is also shown in Figure 5. A chimney and air ducts have been introduced which allow for a sophisticated gas and heat circulation system and with very little capital investment a significant increase in yield is achieved. 4
Figure 4: Improved Charcoal Kiln found in Brazil, Sudan and Malawi. Pro-Natura has developed a process based on the continuous carbonisation of renewable biomass, savannah weeds, reeds, straw of wheat or rice, cotton and corn stems, rice or cof fee husk and bamboo to produce green charcoal.
Briquetting is carried out on many materials to make them more suitable to be used as an energy source. Nearly all biomass has the potential to be briquetted into a hard stable fuel that has a high energy density and provide more consistent combustion and improved storage and transportation. The important factures in making briquette are the ash content or non combustible components and the moisture content. The raw materials that are commonly made into briquettes and pellets include: Wood & Sawdust Biomass waste such as rice husk, cotton stalks etc. Bagasse fro sugar cane Although briquetting is often a large scale commercial activity most waste biomass can be used as a fuel source either by directly briquetting or through the production of charcoal that is then briquetted on a small scale. Binders used for direct briquetting include starch paste, cellulose from woody material, cowdung and clay, which can be extruded of formed by hand into balls. One example of briquetting sawdust with a binding agent in Malaysia first carbonised the sawdust then uses starch as a binder. The starch paste is made in a separate cooking tank. Charcoal = 73% Starch = 5% Calcium carbonate = 2% Water = 20%. These charcoal briquettes can be made with a low-pressure mould. Research by Chardust Ltd Into making charcoal briquettes from various crop wastes including sisal waste. One report concluded that carbonising sisal was technically quite difficult in respect to regulating the temperature resulting in non-homogenous carbonisation but once the sisal waste had been carbonised it was relatively easy to produce briquettes. These were made by producing a paste of carbon dust and water which is then combined with 15% clay.
Published by Practical Action on 02/02/08
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