A significant amount (20%) of the paddy is rejected from the rice mills which is the rice husk. Thus 132 million tons of rice husk were wasted in the year 2008 mostly by burning them in the open field. Yet the rice husk has an attractive calorific value which can be used by reclaiming it from those waste materials.
2.1 Characteristics of Rice Husk An inherent quality of the rice husk is that it doesn’t ignite and burn easily and to do that one needs to blow air through the husk. It also has to more positive qualities that it resists the moisture to get into it and cannot be decomposed with fungal attach. Thus this is often used as a good insulation material. Silica (SiO2 ) content in rice husk is very high therefore it decomposes over a long period of time if left alone in the open field. The rice husk is bulky and dusty which makes it less affordable for handling. Having an angle of repose of about 40°-45° makes it fluent through the feed hoppers. Because of its low bulk density (70-110 kg/m³, 145 kg/ m³ when vibrated or 180kg/m³ in form of pellets) a large storage capacity by volume is required for transport which leads to an uneconomical situation. As mentioned earlier because of its good calorific value (3410 kcal/kg) it is an attractive option for renewable energy research.
2.2 Fabrication The gasifier was fabricated using the available materials in Bangladesh so that the system can be constructed at any corner of the country economically. The complete design of rice husk gasifier is shown in Figure 01.
2.4 Chemical Reaction Occur into Gasification System There are different reaction zones where different types of reactions take place. In the oxidation zone: C + O2 = CO2+ 401.9 kJ/mol (1) H + O2 = H2O + 241.1 kJ/mol (2)
3. FABRICATION The fabrication consists of preparing and assembling the major parts of the gasifier. It takes different types of metal working, welding, electrical wiring, programming etc.
3.2 Description About Individual Parts The following sections describe some technical details of the individual parts of the gasifier. However, some of these consists of some smaller components. 3.2.1 Fuel feeding system The feed hopper is a conical part that serves as the temporary storage for rice husk. It is located on the top of reactor. This device is provided with a door that can be opened for easy disposal of rice husk. The hopper has a capacity of 15kg which is shaped like a funnel, works as a supplier and a reservoir. 3.2.2 Air supply system Two blowers (7.08m3 /min) work as the air supply unit for the reactor. These blowers are accompanied by pressure valve, main valve, bypass valve, and supply pipe. The blower is equipped with 25µ filter to clean the inlet air from dust particles, water etc. The blower-1 is used to supply air to the after burner, while blower-2 is used carry the bed material (char) away.
3.2.3 Start-up system Like all other start-up systems the start-up burner starts up the initial burning of the rice husk. It works by a spark plug. After the air supply and the ignition systems are turned on the start-up unit comes into play. The rice husk needs to be combusted below 800°C as well as hot spots or localized high temperatures are not expected. Which, unfortunately, in practice (in directly fired furnaces/ gasifiers) is not possible. This requires a large grate area and quick dissipation of heat. 3.2.4 Reactor The operation of the gasifier leads to a very high temperature inside the reactor resulting in much lower amount of CO2, also known as the greenhouse gas. This ranges from 5 to 15% which is almost 50% of the amount produced by direct combustion. Other quantities are; heating value: 1200 to 1400 kcal/m3, black carbon content: 10 to 50 gm/m3 , and tar content: 56 to 100 mg/m3.
3.2.5 Char chamber The char is deposited in char chamber. The temperature in this zone is about 50°C to 87°C. It is produced in the gasifier and is completely black, which is good for soil conditioning. Rice husk contain about 75% organic volatile matter and the balance 25% of the weight of this husk is converted into ash during the burning process which is known as char or rice husk ash (RHA). This RHA in turn contains around 85% - 90% amorphous silica. 3.2.6 Scraper The scraper is one of the most important elements in this set up. Here it is used to remove the char from the combustion chamber and driven by a dc motor that rotates after every five minutes. It is made of a shaft and a net. The shaft is coupled with a motor by a chain sprocket arrangement. The net should have ability to resist high temperature and the weight of a mass of about 15 to 20 kg. The scrapping system arrangement of the fabricated gasifier, which includes a motor controlled by timer.