Rice husk

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What is rice husk?

The outermost layer of the paddy grain is the rice husk, also called rice hull. It is separated from the brown rice in rice milling. Burning rice husk produced rice husk ash (RHA), if the burning process is incomplete carbonized rice husk (CRH) is produced.

	Rice husk	Burning rice husk	Rice hus ash (RHF)	Carbonized rice husk (CRH)

Importance of rice husk

Around 20% of the paddy weight is husk. In 2008 the world paddy production was 661 million tons and consequently 132 million tons of rice husk were also produced. While there are some uses for rice husk it is still often considered a waste product in the rice mill and therfore often either burned in the open or dumped on wasteland. Husk  has a high calorific value and therefore can be used as a renewable fuel.

Production of rice husk

Rice husk is produced in the first step in the milling process when the husk is removed form the grain in the husking stage of the rice mill.

Characteristics of rice husk and RHA

• Rice husk is difficult to igninte and it does not burn easily with open flame unless air is blown through the husk. It is highly resistant to moisture penetration and fungal decomposition. Husk therefore makes a good insulation material.
• Rice husk has a high silica (SiO₂) contents which means that it decomposes slowly when brought back to the field. It also makes it a poor foder
• Handling of rice husk is difficult because it is bulky and dusty. It has angle of repose is about 40-45° which means that it's flow ability, e.g. in feed hoppers is very poor.
• Rice husk has low bulk density of only 70-110 kg/m³,  145 kg/m³ when vibrated or 180kg/m³  in form of brickets or pellets. It thus requires large volumes for storage and transport, which makes transport over long distances un-economical.
• When burned the ash content is 17-26%, a lot higher than fuels (wood 0.2-2%, coal 12.2%). This means when used for energy generation large amounts of ash need to be handled.
• Rice husk has a high average calorific value of an 3410 kcal/kg and therefore is a good, renewable energy source.
• Because of the high silica contents rice husk is very abrasive and wears conveying elements very quickly.
• Rice husk is not an easy fuel. One concern in rice husk firing is the behavior of the ash, i.e., its slagging and fouling tendency caused by a low melting point of the rice husk ash.

Click here for detailled properties of rice husk.

Utilization of rice husk and rice husk ash

Rice husk is produced centrally at rice mills and has low moisture content since the paddy is dried to 14% or less before milling. The disadvantage is that rice husk has very low density and therefore transport over longer distance is expensive. The most obvious use of rice husk is therefore the use as fuel at or close to the rice mill. But rice husk is also used for some non-energy purposes.

Typical uses fo rice husk

Depending on the energy conversion efficiency the final product can be either white rice husk ash or black carbonized rice husk.

Typical uses of rice husk ash and carbonize rice husk

Examples for non-energy applications

Use of rice husk in animal husbandry

Rice husk is sometimes used as animal feed stuff  and as litter material for pet animals . Untreated hull is low in protein and digestible energy. Husk can be pretreated with 12% NaOH to reduce the silica content from 19 to 3-4% to improve digestibility and used as animal feed.

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Examples for energy applications

Cook stoves

		Simple rice husk cook stoves are used for a long time in the Greater Mekong Subregion. The origin of the desing is unkown, some sources mention Cambodia, others Vietnam.  After a Vietnamese scholar had brought a unit from Vietnam for testing to IRRI in the Philippines many national systems and projects have adapted the technology to local conditions and similar stoves can now be found in most Asian countries. The cook stoves come in different sizes and usually can boil a pot of water in around 10-15 minutes. The fuel is cheap and the stove itself very inexpensive but the disadvantage i is that the user is exposed to small, silica rich ash particles in the flue gas. To generate a cleaner flame the cook stove principle was modified into a gasifyer burner that has a clean flame. Commerical applications exist in the Philippines and in India.
Rice husk cook stove in Myanmar

Process heat generation for rice drying and parboiling

Rice husk furnaces for heating the drying air in rice dryers is the most obvious application for using rice husk since the husk is available as waste at the rice mill and drying of paddy is often done by millers. But only recently with increasing fosil fuel prices there has been a major shift away from kerosene towards rice husk furnaces. For some examples from the RKB Drying module click here.

In India and Bangladesh rice husk is often used as fuel for parboiling paddy before milling.

Mechanical power generation

In Myanmar rice husk is still used as fuel for boilers which generate heat for steam engines. These steam engines, most of them being produced in the beginning of the last century, still power many rice mills in Myanmar though belt transmissions. While this is old technology we still believe that this is worth mentioning here as an example for sustainably using rice husk as energy source over decades in an environment where many millers are not connected to the electicity grid. Click here for an external link with an impressive documentation of Myamar rice mills that still use steam engines.

Gasification plant with 30kW powering a rice mill in Battambang province, Cambodia (installed with assistance from SME Cambodia)

High fuel prices have led more recently to a revival of rice husk gasifiers, partly for electricity generation (see below) but also ain may cases coupled with a modified internal combustion engine that drives power a rice mill through a belt transmission. Several commercial units were developed in India and expecially in Cambodia rice millers have installed several units with 20-70 kW capacity. Click here for schematic diagram of a rice husk gasifyer powered rice mill.

Electricitiy generation

Small scale applictions between 10-200 kW usually use a rice husk gasifyer coupled with a modified internal combustion engine that drives a generator. These are common in Myanmar, were common in India and have recently been installed at several rice mills in Cambodia.

Larger commercial power plants using rice husk typically have a capacity of 1-4 MW and consist of an advanced burner, a boiler and a steam turbine coupled with a generator. Several units have been installed worldwide.

Utilization of carbonized rice husk

Studies conducted by (Haefele et al., 2008) indicate that the bio-char produced by small scale energy applications through incomplete combustion, can improve poor soils but may have little effect on fertile soils, but also lower CO2 emissions.

Further reading

Belonio, A. 2005. Rice husk gas stove handbook. Iloilo City, Philippines: APPROPRIATE TECHNOLOGY CENTER, Department of Agricultural Engineering and Environmental Management, College of Agriculture, Central Philippine University. 155.

Olivier, P. 2004. The rice hull house [Online] http://www.thelaststraw.org/backissues/articles/Rice%20Hull%20House.pdf.