Biomass fuel

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Introduction to the Characteristics of Rice Husk-Fired Circulating Fluidized Bed Boilers Institute of Engineering Thermophysics, Chinese Academy of Sciences December 27, 2005 I. Introduction As society’s demand for energy continues to increase, fossil fuels, which serve as the main source of energy, are declining rapidly. Therefore, finding a renewable alternative energy source has become a matter of widespread concern in society. Biomass energy is an ideal renewable energy source; it is widely available, as large amounts of industrial, agricultural, and forestry waste are generated every year. In the world’s current energy consumption, biomass energy accounts for 14% of the total global energy use, ranking fourth after oil, coal, and natural gas. In developing countries**, biomass energy (mainly corn, wheat and rice straws, rice husks, straw, wood, etc.) accounts for a large proportion, exceeding 50%. According to statistics, biomass energy accounts for 35% of renewable energy resources worldwide, ranking first among renewable resources. In 1996, China’s biomass production (mainly crop straws) was 705 million tons, while the utilization rate that year was less than 30%, indicating that there is still great potential for the use of biomass energy in China. Generating electricity using biomass energy is one of the important ways of utilizing biomass. The large cities in Sweden and Denmark use biomass to provide centralized district heating through combined heat and power generation. Compared to fossil fuels, biomass has the following advantages: 1. Renewability ； 2. Low pollution: Low emissions of SOx and NOx ； 3. When used as fuel, biomass results in a near-zero net carbon dioxide emission into the atmosphere during its growth cycle, thereby helping to reduce the greenhouse effect effectively. Circulating fluidized bed boilers have a wide range of fuel adaptability and can burn multiple types of fuels simultaneously ； It boasts excellent environmental properties, with emissions that meet **standards ； Even heat exchange within the furnace, high heat recovery efficiency, and stable operation ； The ash has high utility value. Given the aforementioned advantages of circulating fluidized bed boilers, it is natural for people to turn to using circulating fluidized bed technology to utilize biomass energy. Rice husk is a lightweight, ash-rich fuel with a high ash fusion point, moderate calorific value, and uniform natural particle size, making it suitable for comprehensive utilization of thermal energy through circulating fluidized bed combustion. II. Basic combustion properties of rice husks: In terms of their appearance, rice husks have rough surfaces that are not smooth at all; it is easy for them to stick together, resulting in poor fluidity. Additionally, rice husks are quite hard. The ignition and combustion temperature of rice husk is around 300–400°C. It has a high volatile matter content, and the volatiles evaporate and cause rapid ignition; combustion mainly occurs in the gaseous phase due to these volatiles. The fixed carbon has poor combustion properties, as can be seen from the thermogravimetric curve of rice husk, as shown in Figure 1. The ash component of rice husks is mainly silicon dioxide, with the silica content in the ash reaching over 90%. The resulting ash particles are quite hard; therefore, during the transportation of rice husks and also when transporting the ash after combustion, there is significant wear on the pipe walls. Anti-wear measures must be given special consideration for the heated surfaces inside the furnace and in the exhaust flues. Figure 1 shows the thermogravimetric curve of rice husk combustion, while Figure 2 depicts the state of the ash formed after the combustion of rice husk at different temperatures in a muffle furnace. At low temperatures, rice husk ash is white; when the temperature exceeds 800°C, it turns black. This is because the composition of rice husk ash changes at different temperatures. Table 1 shows the basic physical properties and elemental analysis of rice husks from a certain location. Based on the components and physical characteristics listed in the table, it can be seen that rice husks are a lightweight, high-in-ash fuel with moderate calorific value and uniform natural particle size. It takes a relatively long time for them to burn completely, and the combustion temperature should be maintained at around 850°C. Table 1: Basic physical property parameters of rice husk
Parameter Name | Unit | Value
Particle size range | mm | 0–10
Equivalent average particle size | mm | 1.6
Natural bulk density | kg/m³ | 123
True density | kg/m³ | 500

Table 2: Elemental analysis of rice husk (on a received basis)
Parameter Name | Unit | Value
C% | % | 36.6
H% | % | 4.59
O% | % | 31.92
N% | % | 1.88
S% | % | 0.09
Mg% | % | 8
Al% | % | 16.92
Vdaf% | % | 51.98
Qnet,arkcal/kg | cal/kg | 3200

III. Combustion characteristics of rice husk in a circulating fluidized bed
For 100% combustion of rice husk, its ash-forming properties require the regular addition of a small amount of fluidizing material; sand is commonly used as such material in circulating fluidized beds. Once the rice husks enter the furnace, under the action of the circulating material and flue gas, they exhibit excellent fluidization properties in the circulating fluidized bed. First, the rice husks move upward and tumble rapidly; during this process, their volatile components are released and burned quickly, while the fixed carbon begins to burn as well. Therefore, the combustion of rice husks in the furnace mainly occurs above the feeding point, with most of the heat being released in the upper-middle region of the furnace. Through the circulation of materials, the temperature distribution within the furnace can be made more uniform, thus creating a uniformly distributed temperature environment inside the furnace. Meanwhile, the high-temperature cyclone separator separates the unburned rice husks from the recycled ash, which is then sent back to the furnace via a return conveyor for further combustion, ensuring that the rice husks are completely burned out. The specific combustion characteristics are as follows: 1. The main component of rice husk ash is silica, which has a relatively high ash fusion temperature, exceeding 1200°C. At the combustion temperature in a circulating fluidized bed (where the temperature in the furnace remains at 850°C), burning rice husks does not cause coking issues. 2. The circulating fluidized bed enables a stable combustion temperature within the furnace, which is highly beneficial for the rapid combustion of the volatiles in rice husks in the fluidized bed as well as for the complete combustion of the fixed carbon. Actual combustion experiments of rice husks on a circulating fluidized bed test rig show that the carbon content in the fly ash resulting from the combustion of rice husks is less than 7%. The combustion efficiency can exceed 98%. 3. The ash component of rice husks is mainly silica, and the ash particles are relatively hard; therefore, the wear issue in rice husk circulating fluidized beds needs to be given special consideration. In addition to the conventional anti-wear measures for circulating fluidized beds, special care must be taken in selecting the gas flow velocity at the rear of the circulating fluidized bed. An appropriate gas flow velocity is key to addressing the wear problem in rice husk circulating fluidized beds. 4. Thanks to the unique advantages of the circulating fluidized bed combustion method, which provides a wide range of fuel compatibility, in a circulating fluidized bed system, rice husks can be burned alone, coal can be burned alone as well; the two can also be burned in different proportions. Additionally, gaseous fuels can be mixed in according to the user’s requirements. 5. During the co-firing of rice husks and coal, it is not necessary to add a circulating material; rice husks can be burned on their own. However, when burning only rice husks, a circulating material must be added. 6. In addition, the flue gas emissions resulting from the combustion of rice husks meet ** environmental standards. The ash residue left after burning rice husks can be utilized in various ways. The Institute of Engineering Thermophysics, Chinese Academy of Sciences, conducted experiments on burning rice husks purely using a circulating fluidized bed combustion system. This system was originally designed for burning coal. The experimental conditions were as follows: combustion temperature of 840°C, fluidization velocity of 2–3 m/s; the rice husks were fed into the system using a spiral feeder, and the carbon content in the fly ash was around 6–7%. The test results show that rice husks burn stably in a circulating fluidized bed and exhibit good burnout performance. The temperature distribution in the furnace is generally uniform, but the temperature at the bed surface is low; the temperature in the upper and middle parts of the furnace is relatively high (around 910°C), while the temperature at the furnace outlet is moderate (around 840°C). It also shows that rice husks can be burned alone in a circulating fluidized bed or mixed with coal and gaseous fuels, with little impact on the boiler’s load parameters. IV. Design features and co-firing characteristics of rice husk circulating fluidized bed boilers: In response to users’ requirements, the combustion and flow properties of rice husks are taken fully into account. Drawing on the successful experience of coal-fired circulating fluidized bed boilers, designs for circulating fluidized bed boilers that use rice husks as fuel are developed. The specific features are as follows: 1. A rational combustion system is established, including air distribution plates, furnace structure, ratios of primary/secondary air, selection of an appropriate fluidization speed, and determination of the appropriate separator performance. This ensures proper heat release and heat exchange, resulting in a uniform furnace temperature around 850°C, thereby enabling the boiler to meet the needs of burning rice husks. 2. The use of high-efficiency cyclone separators helps maintain an adequate ash concentration in the furnace. Minimize the amount of rice husk used as a fuel bed, and effectively keep the carbon content in the fly ash below 8%, thereby improving the combustion efficiency of the boiler. 3. The ash component of rice husks is mainly silica, and the ash particles are relatively hard. In addition to the conventional anti-wear measures used in circulating fluidized bed boilers, additional anti-wear measures should be taken in areas where there is severe erosion of the heated surfaces, thereby improving the reliability of the boiler. 4. The design and calculation of the furnace chamber and the rear heating surfaces should be carried out in a rational manner, taking fully into account the operating conditions of burning only rice husks, burning only coal, and burning a mixture of the two, so as to maximize fuel adaptability and meet the requirements of users. Since the amount of flue gas generated when burning rice husks differs from that when burning coal, it is essential to use an effective soot blowing device. 5. During the co-combustion of rice husks and coal, it is not necessary to add any circulating material; the rice husks can be burned on their own. If burning them separately, a bed material such as river sand needs to be added. 6. The flue gas emissions resulting from the combustion of rice husks meet ** environmental standards. The ash residue left after burning rice husks can be utilized comprehensively.