Wayne E. Marshall, U.S. Department of Agriculture—Agricultural Research Service, Southern Regional Research Center, New Orleans, Louisiana
RICE: Chemistry and Technology, Third Edition
In the 1985 edition of Rice Chemistry and Technology, Juliano (Juliano, 1985) wrote a comprehensive chapter on the structure, composition, properties, and uses of rice hull and rice straw entitled, appropriately enough, “Rice Hull and Rice Straw.” My objective in this “update” of that chapter is to present a narrower focus, zeroing in on the utilization of rice hull and straw as value-added products—specifically, value-added products in the form of adsorbents. Information on the structure, composition, and properties of hull and straw was thoroughly covered by Juliano up to 1985. Little new information in these areas has emerged since then compared to the number of literature reports in the burgeoning field of utilization. This chapter takes the utilization route because this is an area of hull and straw chemistry and technology that underwent perhaps the greatest change during the 1990s. This area involves technology and resultant adsorbent products that may be useful in a variety of environmental and industrial applications.
The total rice production in the United States for crop year 2002 was 21.1 billion pounds (9.6 billion kilograms) of rough rice (NASS, 2003). Therefore, approximately 4 billion pounds (1.8 billion kilograms) of rice hulls and 26 billion pounds (11.8 billion kilograms) of rice straw were potentially produced. Thus, a major issue facing the rice processing industry is what to do with this prodigious quantity of co-products. A large quantity of hull is burned by the rice miller to produce steam for processing, particularly parboiling (e.g., Producers Rice Mill, Inc./Riceland Foods, Stuttgart, AR) or for production of electricity (e.g., Agrilectric Power Partners Ltd., Lake Charles, LA). Hull burning results in rice hull ash, a by-product that consists of varying amounts of silica and carbon and minor amounts of alkali oxides (K2O, Na2O) and alkali earth oxides (MgO, CaO), depending on the amount of air used for hull burning. However, the net result is both unburned rice hull and rice hull ash for either disposal or utilization.