|Economic Uses for
Rice Straw - 84
The Rice Research Board funded several research projects from 1979 until
1983 to find economic uses for rice straw. The principal objective of these
projects was to reduce or eliminate rice straw burning as the means for
disposal. Uses for straw that were studied included livestock feed, making
fiberboard, energy generation, conversion to sugar syrup and yeast protein,
and making pulp for paper and for various industrial products.
All of these projects proved to be technically feasible, but unfortunately, none are economically promising. With present costs of better quality livestock feeds and of other raw products for pulp and energy generation, rice straw is not competitive. The possibility still reains that increasing costs of energy and wood chips for pulp at some future time could improve the prospects of using rice straw for these purposes. Until such time, the research results have been reported or published and are available if and when needed.
Feeding trials to determine the value of rice straw for livestock feed were conducted by the Department of Animal Science at UC Davis in conjunction with the USDA Western Regional Research Center in Albany, California.
Studies focused on the value of rice straw in mixed rations for beef calves and lambs and whether feeding value could be improved by treating the straw with ammonia (NH3) and sodium hydroxide (NaOH).
Results show that rice straw must be supplemented with other feeds, even if used as a maintenance ration for livestock. It is too low in digestible energy, crude protein, calcium, and phosphorus to be used alone. It also has low levels of cobalt, copper, manganese, and sulfur, suggesting possible borderline dietary deficiencies of these minerals.
Rice straw differs from most other roughages. It has a relatively low lignin content and a high silica content. Like lignin, silica has no nutritive value and may physically interfere with digestion.
Feeding value of rice straw is significantly improved when it is treated with sodium hydroxide or ammonia, both of which improve the digestibility of cellulose, which is 35 to 40 percent of the straw. Feeding trials demonstrated that treated rice straw is consumed more readily than untreated straw by cattle and sheep. It is also a better source of carbohydrate and produces more weight gain. Lambs fed diets containing 65 percent rice straw that had been treated with NaOH gained substantially more weight than did those receiving untreated straw. Treatment of the straw with NH3 produced comparable gains. Clearly, chemical treatment improved the feeding value of rice straw.
The practicality of feeding rice straw is questionable. The untreated straw has limited value as an energy source for livestock. While treatment substantially improves feeding value, treated straw cannot compete with other available livestock feeds at current prices.
Transportation costs critical
The cost of baling and transporting this low-value material from the field to a feeding location is certainly a significant consideration. If there is an economical potential for rice straw as a livestock feed, it will be in rice growing districts with short hauling requirements.
A Cooperative Extension project that involved treating rice straw with NH3 during the field baling operation was discontinued after two years. Livestock feeders have shown little interest in feeding rice straw.
About 3,000 pounds of rice straw was used in experiments incorporating rice straw with wood chips in making fiberboard. The research was done by the Washington Iron Works. The fiber production was done at the C.E. Bauer Process and Equipment Laboratory, Springfield, Ohio.
Rice straw proved to be a difficult material to work with, but a medium-density fiberboard was successfully produced from a 50/50 mixture of rice straw and California hardwood chips. The fiberboard was somewhat dark, taking on the color of the wood chips. The straw was chopped into short lengths and screened to remove dust and fine particles. It was then mixed with wood chips and processed with a steam pressurized refiner designed for refining wood chips. The resulting fibers were trucked to the Washington Iron Works in Seattle for drying and conversion into medium-density fiberboard panels.
Several different resin binders were tried in making the fiberboard panels. Isocyanate resin produced boards with the best overall properties. The research concludes that a durable exterior grade panel suitable for house siding or highway signs can be manufactured from rice straw/ wood fiber combinations and isocyanate resin.
Fiberboard manufacturers, however, have shown no interest in using rice straw with wood chips for fiberboard and probably will not as long as wood chips, which make a superior board, are readily available at present costs.
PAPER AND DISSOLVING-GRADE PULPS
Pulp is used in making paper and cellulose products that have many industrial uses. Although rice straw has never been used as a source of pulp in the United States, it has been used for many years in Egypt for making commercial grade paper and in China for high grade artistic paper. Neither of these countries have the wood resources of the United States. Paper made in Egypt is from a mixture of rice straw pulp and a high grade pulp from wood imported from Scandinavia.
The pulp and paper making project in California was a three-year effort conducted cooperatively by the UC Forest Products Laboratory in Richmond and the Division of Textiles and Clothing at UC Davis. When the project was terminated by the Rice Research Board in 1983, the investigators had produced a dissolving-grade pulp and paper with unusually high tensile strength but only moderate resistance to tear and bursting.
The dissolving-grade pulp had a color component that resisted all of the bleaching systems that were studied. The color caused serious yellowing of the bleached pulps and was expected to affect utilization. Although this problem and others encountered were probably solvable, the Rice Research Board terminated the project because it concluded that paper mills were not interested in using rice straw while they had access to a good supply of wood for paper making. Dissolving pulps
Rice straw pulps have been prepared which have alpha-cellulose contents and degrees of polymerization comparable to those found in dissolving pulps manufactured from wood. Dissolving pulps are normally made from wood or cotton sinters and have a variety of industrial uses, including man-made fiber production and manufacture of cellulose derivatives. Cellulose derivatives are used by many industries, such as the food, detergent and textile industries.
Significant differences were found between the properties of rice straw cellulose and those of wood and cotton cellulose. It is possible that these differences could be exploited to either manufacture superior products from rice straw cellulose or to manufacture the products more easily.
GASIFICATION FOR ENERGY
Preliminary analyses were made in 1973 of various systems for harvesting, transporting, stockpiling, and processing rice straw for energy generation. In subsequent years, studies suggested that baling was the most practical method for collecting the straw and that the moisture content must be near or below 17 percent, wet basis, for safe storage. All standard balers required some modification for handling rice straw. All encountered difficulties in wet field conditions.
Weather data were analyzed for several locations to determine the potential for harvesting rice straw at an acceptable moisture content. In the Chico area, for example, the probability was that 80 to 90 percent of the rice straw would be dry enough to bale between September 7 and October 17. Within this period, two wet periods of 2 to 3 days could be expected which could reduce the probability of harvesting at an acceptable moisture content. Energy generating companies find the lack of certainty of an annual supply of rice straw a troublesome basis for using rice straw as a fuel source.
The gasification process
Gasification is the thermo-chemical process required to convert rice straw to a gas type of fuel that could replace natural gas and dieselFluidized bed gasification has been investigated since 1981 as a method to produce low Btu gas from rice straw. The system uses a bed of sand inside a refractory-lined cylinder reactor. The fuel (rice straw) is injected into the sand bed which is fluidized by air injected from below. The injected air provides only one-fifth to two-fifths of the amount needed for total combustion.
The straw is processed through a hammer mill before entering the fuel-feed system. The system can convert 500 to 1,000 pounds of rice straw per hour to hot raw producer gas that captures 60 to 65 percent of the energy in the raw fuel.
Producer gas is a mixture of combustible gases carbon dioxide, hydrogen, methane, and a small amount of higher carbon gases. It also contains water vapor and nitrogen gas. The combustible gases range from 25 to 40 percent by volume of total gases. The amount is influenced by the type of solid fuel being gasified.
The unit has been operated for about 400 hours using eight different crop residues. Approximately 60 percent of the fuel used was rice straw. A hot-gas clean up system was developed for engine operation, and extensive test data were obtained for operation of an LP gas engine on producer gas. The engine is rated at 75 kilowatts at 1800 rpm on LP gas. Sustained operation of the engine on producer gas with naturally aspirated carburetors of three types produces 35 to 45 kilowatts.
The gas-cleaning system removes solid particulates from the producer gas to less than 10 milligrams per normal cubic meter (35 cubic feet and equivalent to mountain air quality). However, the amount of condensible gases is too high for low maintenance operation.
The entire gas-producer, engine system design and data on its performance is now available. This information and the experience gained in the development and operation of the equipment can provide the basis for the design and cost analysis of commercial systems to determine the economic viability of a new installation.
The Rice Research Board concluded that the research has proven the technical feasibility of converting rice straw to usable producer gas and that the next step is up to private industry and depends on the cost and availability of natural gas and diesel fuel. With present costs, this source of energy from rice straw does not appear economically feasible.
CONVERSION TO SUGAR SYRUP AND YEAST PROTEIN
Sugar syrup and yeast protein were produced from rice straw in the laboratory by the Department of Environmental Toxicology at UC Davis. The most successful procedure produced 25 grams of sugar from 100 grams of rice straw. The sugar was used to produce a food grade yeast. The yeast single-cell protein was comparable to other protein sources when the yeast was fed to mice at 50 percent of the total protein source. A lower nutritional value was obtained when the yeast single=cell protein was fed as the sole protein source.
It was not possible to obtain reliable figures for the cost per unit weight of sugars produced, as this would require experiments run at the pilot plant scale.
Although the technical feasibility for converting rice straw to sugar and single-cell protein has been proven, no commercial firms have developed an. interest in using these procedures with rice straw.