A great number of people in Bangladesh rely on rice husk as an important source of fuel. However, its use is often inefficient, with detrimental effects on the health of the users and workers involved. This wasteful use continue because people do not have access to relatively simple and cheap technology that can make more productive use of rice husk as a fuel. To help alleviate this problem, the International Rice Research Institute have helped develop an improved stove design that meets the users’ needs while improving efficiency and safety.
Rice and Biomass in Bangladesh
Bangladesh is an environmentally threatened country suffering from a scarcity of fuels, especially biomass fuels. Biomass fuels include all fuels that are derived from organic matter, produced in natural conditions of photosynthesis. Biomass is constantly undergoing a complex series of physical and chemical transformations (such as decomposition) while giving off heat energy into the atmosphere. To benefit from biomass for energy needs, it is possible to tap into this energy source. Most basically, this can be in the form of an open fire used to provide heat for cooking, warming water or warming the air in homes. More sophisticated technologies also exist for extracting this energy and converting it into useful heat or power in an efficient way.
In Bangladesh, rice is a staple food and accounts for about 93 per cent of the total food produced in the country. Rice is the main source of biological energy for the population, with 75 per cent of necessary calories and 66 per cent of protein met by rice. It is also a source of heat energy: 64 per cent of the total energy supply is derived from biomass. The total biomass used per year is over 39 million tonnes, about half of which is derived from agricultural residues such as rice by-products. Rice husk is the largest source of biomass, contributing 22 per cent of the total.
Bangladesh produces on average 28 million tonnes of paddy per year, giving approximately 6 million tonnes of risk husk. Paddy processing in Bangladesh takes two forms: 10 per cent of the rice produced is hulled immediately, while the remaining 90 per cent is parboiled and then hulled, predominantly in small rice mills. The by-product from rice hulling is then used as a fuel to generate steam to parboil the paddy. All regions in Bangladesh, apart from the eastern districts, parboil the paddy.
The use of agricultural residues is often difficult due to their uneven and troublesome characteristics (in loose form they can be difficult to handle and to safely manage). The process of compacting these residues into a product of higher density is known as densification or briquetting. In Sylhet and Chittagong in the east of Bangladesh, rice is dry-processed and husk is made into briquettes. Poor households and small food retailing units in these districts have shifted their use from firewood to briquettes. Converting residues into a densified briquette form has the following advantages.
- The process increases the net calorific value per unit volume – i.e. it is more efficient.
- The densified product is easy to transport and store.
- The process helps to solve the problem of residue disposal.
- The fuel produced is uniform in size and quality.
Screw extrusion press with a heated die
The major parts of the machine are a driving motor, screw die, die-heaters and a power transmission system. When the motor is started and raw material (rice husk) is fed to the screw (housed in a circular tube), the rice becomes compressed and is extruded through the die (heated by an external electrical coil heater). Lengthways on the inner surface, the cylindrical die has five grooves, which prevent the densified material from rotating with the screw. The design of the screw results in the formation of a central circular hole in the briquette, which acts as an escape route for steam produced during briquetting.
There are several methods available for creating the briquettes, but screw extrusion press briquetting is a popular choice suitable for small-scale applications in developing countries. The process can produce denser and stronger briquettes than those produced by piston presses. There are two types of screw-press: a conical screw extrusion press; and, a screw extrusion press with a heated die. A screw extrusion press has a capacity of about 90 kg/hr and is driven by a 20 horsepower electrical motor (1 hp = 746 watts). Major maintenance problems are mainly concerned with the wear of the screw.
The lower end of the screw extrusion press market provides a small press that can be easily transported and can be utilised in rural communities. Where the capacity does not exist to provide such, communities often hand-make their briquettes. This involves manually pressing the rice husk on metal trays (e.g. baking trays). This is often more cost-efficient, but far less efficient in terms of volume, and therefore time. With women usually occupying the role of such household briquetting, it is vital that the time spent on the procedure is reduced in order to minimise their burden.
Rice Husk Combustion Stoves
There are three basic combustion configurations found in the rice husk sector of biomass fuel use: semi-cylindrical with a flat bottom; cylindrical; and, rectangular boxes. A common characteristic is fabrication in small, local workshops from low-grade scrap sheet metal. The materials used are of variable quality. A collaborative research initiative between UK’s DFID (Department for International Development), Bangladesh Rice Research Institute, The Energy Resources Institute (TERI), and the University of Greenwich Natural Resources Institute established the following drawbacks of traditional stoves.
- Much of the heat loss from the furnace (or firebox) is from the large fuel inlet port; several openings on the sidewalls, and a large port for removing ash.
- High levels of flue gas exit from all ports and openings that carry away a large amount of heat. This also results in high temperatures at working level.
- The design of the furnace is such that the required amount of air for complete combustion of husk is not available. This insufficient supply of air and consequent build-up of carbon monoxide pressure in the furnace cause it to backfire from the fuel inlet port. This makes for hazardous working conditions for the person feeding the furnace manually.
- There is no instrumentation in the system to monitor the pressure/temperature/water levels etc. Compounding this, there is no safety valve to avoid any untoward accidents.
- While the furnace is in operation, ash is removed at an interval of every two hours. The ash contains a substantial amount of heat and un-burnt rice husk.
- The heat-absorption area provided is not adequate. A major portion of the vessel is exposed to the atmosphere.
Much research and development work has been carried out on biomass technologies for rural areas of developing countries, with the intention of improving traditional stoves. This has mainly focused on the need to increase efficiency while bearing in mind the threat of deforestation (for wood-based biofuel), the needs of women to reduce the time spent on fuel collection, and the reduction of smoke emissions and accidents. According to Bangladesh Centre for Advanced Studies (BCAS), the increased use of biomass fuels in Bangladesh is depriving the soil of organic matter and essential nutrients such as zinc. If this trend is not reversed, it is likely that fertility will decrease, resulting in barren land within the next 50-100 years. Since biomass fuels are often the only option in rural areas, with a scarcity of wood fuel, people are increasingly turning to agricultural residues.
To alleviate environmental pressures and to make cooking and heating a faster, safer and more cost-effective process in rural communities, it is essential that the efficiency of biomass fuel use be increased. A study by the Institute of Fuel Research and Development (IFRD) of the Bangladesh Council of Scientific Industrial Research (BCSIR) has demonstrated that the efficiency of biomass fuels in cooking stoves varies from 5 per cent to 15 per cent. This means that 85 to 95 per cent – almost all – of the heat generated is lost through the processes outlined above.
At the International Rice Research Institute (IRRI), they have tested several models of improved cooking stoves, which are reported to save 50 to 70 per cent of the fuel that is wasted by traditional stoves. The maximum overall efficiency is estimated at 30 per cent. Some of the features of improved stoves include:
- A chimney to remove smoke from the kitchen;
- An enclosed fire to retain the heat;
- Careful design of the pot holder to maximise the heat transfer from the fire to the 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 fuels 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 pots to be heated simultaneously.
Improving a stove design is a complex procedure that requires a broad understanding of many issues. Involvement of the users in the design process is essential to gain a thorough understanding of their needs and requirements for the stove. The stove is not merely an appliance for heating food (as it has become in Western society), but often acts as a social focus, 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 habitats need to be considered as well as the lifestyle of the users. Failing to identify the key socio-economic issues will cause a stove programme to fail. The function of an improved stove is not merely to save fuel.
The improved stoves in Bangladesh have reduced the fuel consumption rates, resulting in rice husk savings of 44 to 54 per cent. The improved insulation of the stoves not only increases efficiency, but also makes working conditions more tolerable and less harmful, as Dr Mohammad Ahiduzzaman explains: “I can easily stand here [next to the boiler] because there is no heat loss by radiation because there is proper insulation”. The stoves are also safer and emissions have been reduced to acceptable levels. A pressure gauge and safety valve have been added to prevent the boiler from exploding and backfiring, thereby reducing the risk of injury.
Improved Stoves, Improved Lives
IRRI has developed an improved stove design by including its users in the design and development process. This has resulted in a stove design that is not simply more efficient, safer and cost saving; it also meets the needs of its user. By saving on rice husks, households not only save energy and money, but also valuable time in collecting rice husk and creating briquettes. By improving the insulation of the stoves, energy and heat is saved, as well as improving the working conditions.
ITDG would like to thank the International Rice Research Institutefor providing information and helping in this case study.
The case study draws on articles written by the International Rice Research Institute and DFID.
International Rice Research Institute (IRRI) www.irri.org
IRRI Library ricelib.irri.cgiar.org
PhilRice: Philippine Rice Research Institute www.philrice.gov.ph
Rice On-Line www.riceonline.com
Donor and Supporting Organisations
Department for International Development (DFID) www.dfid.gov.uk
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