The importance of rice husks

About 20% of the weight of the paddy is rice husk. World rice production in 2008 was 661 million tons, so 132 million tons of rice husks were also produced. While rice husk has some uses, it is still often considered waste from rice mills and is therefore often burned in the open or dumped on wasteland. Nutshells have a high calorific value and can therefore be used as a renewable fuel.

Production of rice husks

Rice husks are produced in the first step of the rice milling process, which is the removal of the husks from the grain during the husking stage of the rice mill.

Properties of Rice Husk and RHA

• Rice husks are difficult to ignite, and not easy to burn with an open flame unless air is blown through the rice husks. It is highly resistant to moisture penetration and fungal decomposition. Therefore, the husk is a good insulating material.
• Rice husks contain high silica (SiO₂) content, which means it slowly breaks down when brought back to the field. It also makes it a poor feed.
• Handling of rice husk is difficult because it is bulky and dusty. Its angle of repose is about 40-45°, which means its flow ability, for example, in a feed hopper is very poor.
• Rice husk has a low bulk density of only 70-110 kg/m³, 145 kg/m³ when vibrated, and 180 kg/m³ in block or granular form. Therefore, it requires a lot of storage and transportation, which makes long-distance transportation uneconomical.
• The ash content at the time of combustion is 17-26%, which is much higher than that of fuel (0.2-2% for wood and 12.2% for coal). This means that when used to generate electricity, a large amount of ash needs to be handled.
• The average calorific value of rice husk is as high as 3410 kcal/kg, so it is a good renewable energy source.
• Due to the high silica content, rice husks are very wear-resistant and wear out the conveying elements very quickly.
• Rice husks are not an easy fuel. One problem in rice husk firing is the behavior of the ash, ie the low melting point of rice husk ash leads to its tendency to slagging and scaling.

Application of rice husk in animal husbandry

Rice husks are sometimes used as animal feed and as litter material for pets. Untreated hulls are low in protein and digestible energy. The husks can be pretreated with 12% NaOH to reduce the silica content from 19% to 3-4% to improve digestibility and use as animal feed.

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