Nueva Ecija, 19 Aug 2009. One of the milling by-products of rice is the rice hull. It constitutes about 20-22% by weight of the grain. It coats the seeds of the rice plant to protect the seed during the growing season. As such the rice hull is made of hard materials.
The hull is indigestible to humans and considered a waste dumped usually in open fields and waterways or burned in dumping grounds. However, rice hull should not be treated as useless. There are several uses of rice hull.
Rice hull is a good source of energy. A ton of rice hull is equivalent to 318 liters of fuel. Yes, you read it right. With a heating value of 13,900 kilojoules per kilogram, rice hull can sum up to this volume given ideal conditions.
Rice hull is used as burner fuel for drying activities and domestic stoves in raw form or briquettes. In Nueva Ecija, where rice hull is abundant, residents find it a good source of cooking fuel. In Davao del Sur and other parts of Mindanao, some farmers use rice hulls as bedding or litter for poultry operations. Others utilize it as feeding material for ruminant animals and poultry.
Some studies show that rice hull is a good energy source for hot water heaters and boilers to generate steam. Some rice mills originally dispose off the hulls in this way. In San Jose City, Nueva Ecija, Wideon Zacal has developed a rice hull-fueled steam engine for this purpose.
The rice hull can be used to produce mesoporous molecular sieves, which are applied as catalysts for various chemical reactions, as a support for drug delivery system and as absorbent in waster water treatment. As an absorbent, rice hulls can suck up liquid and provide an anti-skid surface, and an abrasive to keep surfaces from wear and tear due to friction.
In some parts of the world, rice hulls are also used as pillow stuffing. The pillows are loosely stuffed and considered therapeutic as they retain the shape of the head. In China, where these pillows have become popular, one pillow costs approximately US$6 and is considered a luxury item. The high price is testament to its being trendy, as one ton of rice hulls, even in the US, costs approximately US$5.
Rice hulls are organic material and as such they can be composted. However, their high lignin content can make this a slow process. In some instances, earthworms are used to accelerate the process. Using vermicomposting techniques, the hulls can be converted to fertilizer in about four months.
In some industries, rice hulls are also useful. In brewing beer, for instance, they are used to increase the lautering ability of a mash. In the United States, rice hulls are used as a “press aid” to improve extraction efficiency of apple pressing.
Rice hull is also used in the steel industry because of its remarkably high refractory capability and good insulation qualities. Likewise, rice hulls are a low-cost material from which silicon carbide “whiskers” can be manufactured. These whiskers are then used to reinforce ceramic cutting tools, increasing their strength tenfold.
As building material, rice hulls top. They are a class A insulating material because they are difficult to burn and less likely to allow moisture to propagate mold or fungi. It was found out that rice hull when burned produced amounts of silica. For these reasons it provides excellent thermal insulation.
Meanwhile, the ash produced after the husks have been burned is high in silica. A number of possible uses are being investigated for this, among them: aggregates and fillers for concrete and board production; absorbents for oils and chemicals; soil ameliorants; as a source of silicon; and as an insulation material for homes and refrigerants.
Some Filipino researchers have discovered rice hull ash (RHA) as a suitable cement binder in the manufacture of hollow blocks. Adding 10-20% ground gray or white ash to ordinary Portland cement does not affect the strength of the construction materials compared to 100% cement. This is because, researchers say, rice hull contains more than 87% silica. Adding 30% RHA by weight or 50% RHA by volume to ordinary cement in the manufacture of roof tiles makes the materials light, porous, and hard but brittle.
Another product that is becoming increasingly popular is the carbonized rice hull (CRH). To produce CRH, raw rice hull is burned without air so that it will not turn into ash. CRH is sterile and is free from disease organisms.
At the Philippine Rice Research Institute in Nueva Ecija, there’s a showcase of a pigpen where CRH, about one foot deep, serves as flooring instead of the usual cement floor. The pigpen does not have to be washed with water everyday. In fact, it does not get washed for the entire growing period of four months. The manure and urine of the pigs get buried in the carbonized rice hull. The usual foul smell is practically eliminated. And when the pigs attain market size and are sold, the litter is collected and used as organic fertilizer for vegetables, rice and other crops.
CRH is also very useful in rice farming. Twenty bags of CRH combined with organic fertilizer or compost may be applied in one hectare. It could be plowed in during land preparation. This will make the land not only more porous for better plant growth; it will also enable the soil to retain the moisture much longer. Thus, when there is a prolonged dry spell, the rice plants will be able to survive the rainless period longer.
“Intensifying rice hull use will have a great impact on the economy,” commented one researcher. “Tapping more of this ‘waste’ and less of fossil-based fuels will significantly reduce energy costs. In the rice processing industry, this usually represents 50-60% of the total drying cost.”