Rice hulls (or rice husks) are the hard protecting coverings of grains of rice. In addition to protecting rice during the growing season, rice hulls can be put to use as building material, fertilizer, insulation material, or fuel.
Rice hulls are the coatings of seeds, or grains, of rice. To protect the seed during the growing season, the hull is formed from hard materials, including opaline silica and lignin. The hull is mostly indigestible to humans.
Winnowing, used to separate the rice from hulls, is to put the whole rice into a pan and throw it into the air while the wind blows. The light hulls are blown away while the heavy rice fall back into the pan. Later pestles and a simple machine called a rice pounder were developed to remove hulls. In 1885 the modern rice hulling machine was invented in Brazil. During the milling processes, the hulls are removed from the raw grain to reveal whole brown rice, which may then sometimes be milled further to remove the bran layer, resulting in white rice.
Rice hull ash
Combustion of rice hulls affords rice husk ash (acronym RHA),. This ash is a potential source of amorphous reactive silica, which has a variety of applications in materials science. Most of the ash is used in the production of Portland cement When burnt completely, the ash can have a blaine number of as much as 3,600 compared to the blaine number of cement between 2,800 to 3,000, meaning it is finer than cement. Silica is the basic component of sand, which is used with cement for plastering and concreting. This fine silica will provide a very compact concrete. The ash also is a very good thermal insulation material. The fineness of the ash also makes it a very good candidate for sealing fine cracks in civil structures, where it can penetrate deeper than the conventional cement sand mixture.
A number of possible uses for RHA include absorbents for oils and chemicals, soil ameliorants, a source of silicon, insulation powder in steel mills, as repellents in the form of "vinegar-tar" release agent in the ceramics industry, as an insulation material.
Fertilizer and substrate
Rice hulls can be composted, but their high lignin content can make this a slow process. Sometimes earthworms are used to accelerate the process. Using vermicomposting techniques, hulls can be converted to fertilizer in about four months.
Rice hulls that are parboiled (PBH) are used as a substrate or medium for gardening, including certain hydrocultures. The hulls decay over time. Rice hulls allow drainage, and retain less water than growstones. It has been shown that rice hulls do not affect plant growth regulation.
Rice hulls are coated with fine-grained gunpowder and used as the main bursting charge in aerial fireworks shells.
With proper techniques, rice hulls can be burned and used to power steam engines. Some rice mills originally disposed of hulls in this way. Unfortunately the direct combustion of rice hulls produces large quantities of smoke. An alternative is gasification. Rice hulls are easily gasified in top-lit updraft gasifiers. The combustion of this rice hull gas produces a blue flame, and rice hull biochar makes a good soil amendment.
Pet food fiber
Rice hulls are the outermost covering of the rice and come as organic rice hulls and natural rice hulls. Rice hulls are an inexpensive byproduct of human food processing, serving as a source of fiber that is considered a filler ingredient in cheap pet foods.
Rice hulls are used as pillow stuffing. The pillows are loosely stuffed and considered therapeutic as they retain the shape of the head.
Goodyear announced plans to use rice hulls ash as a source for tire additive. 
- Otto W. Flörke, et al. "Silica" in Ullmann's Encyclopedia of Industrial Chemistry, 2008, Weinheim: Wiley-VCH, . doi:10.1002/14356007.a23_583.pub3.
- J. Chumee et al. "Characterization of platinum–iron catalysts supported on MCM-41 synthesized with rice husk silica and their performance for phenol hydroxylation" Sci. Technol. Adv. Mater. 9 (2008) 015006 free download
- Wallheimer, Brian (October 25, 2010). "Rice hulls a sustainable drainage option for greenhouse growers". Purdue University. Retrieved August 30, 2012.
- "Growstones ideal alternative to perlite, parboiled rice hulls". http://esciencenews.com. 2011-12-04. Retrieved 2013-06-06.
- Olivier, Paul; Hyman, Todd (2012-03-27). "Biomass Gasification and the Benefits of Biochar". Engineering, Separation and Recycling LLC. Retrieved 2013-06-06.
- Press aids
- "Ingredients to avoid". The Dog Food Project. Retrieved 2013-06-04.
- "SiC Whisker-Reinforced Ceramic Composites". Materials Science and Technology Division - Physical Sciences Directorate. Oak Ridge, TN, USA: Oak Ridge National Laboratory. Archived from the original on 2009-08-16. Retrieved 2013-06-06.
- Ma, Jian Feng; Kazunori Tamai; Naoki Yamaji; Namiki Mitani; Saeko Konishi; Maki Katsuhara; Masaji Ishiguro; Yoshiko Murata; Masahiro Yano (2006). "A silicon transporter in rice". Nature 440 (7084): 688–691. doi:10.1038/nature04590. PMID 16572174.
- Mitani, Namiki; Jian Feng Ma; Takashi Iwashita (2005). "Identification of the silicon form in xylem sap of rice (Oryza sativa L.)". Plant Cell Physiol. 46 (2): 279–283. doi:10.1093/pcp/pci018. PMID 15695469. Retrieved 2008-05-05.
- Mitani, Namiki; Jian Feng Ma (2005). "Uptake system of silicon in different plant species". J. Exp. Bot. 56 (414): 1255–1261. doi:10.1093/jxb/eri121. PMID 15753109. Retrieved 2008-05-05.
- The Rice Hull House where rice hulls are used for insulation
- Uses for rice husk ash, or RHA
- Rice hulls used in cutting tool industry