Heating system

Fuels and heaters

Depending on the availability and cost different fuels can be used for heating the drying air such as kerosene, diesel, liquified petroleum gas (LPG), biomass like rice hull, or electricity (see table underneath).

In Southeast Asia kerosene burners are most common because of their simple design, availability and easy handling of the fuel but because of high fuel cost they are increasingly being replaced by rice husk furnaces.

Types of heaters

Kerosene burners

Kerosene burner attached at air heater of a re-circulating batch dryer

Rice husk furnace

Rice hull is a by-product in rice milling and is usually available for free or are cheaper compared to fossil fuels. It is also a regenerative form of fuel and therefore from the environmental and economic point of view rice hull would be an ideal fuel for drying. Unfortunately the physical properties of rice hull like low density, abrasiveness, and steep angle of repose make it a product that is difficult to store, handle, convey and to gravity-feed it into furnaces.

Available rice hull furnaces cover a wide range from simplest design where husk is piled on a grate to highly sophisticated types with conveyors and control devices. Because simple designs are generally very labor intensive and the more complex designs require large investments and are prone to breakdown, rice hull furnaces were not widely used as heat sources for drying, except in the Mekong delta in Vietnam, where they have gained much popularity. But since the turn of the century, which had seen an increase in fosil fuel prices, they are becoming more popular and several new designs have been developed since.

Direct and indirect heating

When using a system with direct heating the combustion products are mixed with the drying air meaning that they come in contact with the paddy. In western countries this is only allowed for products used to feed animals. In SE Asia direct fired heaters is not considered a problem because the flue gasses will only pollute the rice hull, which is not considered a problem since the hull is removed during the milling process. Indirect heating, on the other hand, involves a heat exchanger for heating up the drying air. It adds cost and decreases the total fuel efficiency of the dryer.

Direct heating of paddy, however, is acceptable since the husk, and with it the residues of the flue gas that might have accumultaed on the grains, is removed in the milling process.

Solar drying

The use of solar energy as a heat source (solar drying, solar assisted drying) has been evaluated intensively by many projects and institutions. While some solutions were proven to be technically feasible none was successfully commercialized for paddy drying because of the following reasons:

• In natural convection dryers that were considered promising because they don’t need a fan and thus no additional energy is needed the maximum layer depth of the grain is only 7 cm. Thicker layers provide too much resistance to the air that cannot be overcome by the small forces created by the thermal solar energy. The capacity of those dryers referred to the floor area they are using for the solar collector is far too small for any serious application;
• In solar tunnel dryers the drying air does not flow through the grain bulk but over the grain. The layer depth is therefore also limited to a few centimeters;
• Generally in solar dryers most heat is generated when it is not needed, at midday when the air RH is low anyway. Heating the air is needed at night when it rains or in early morning, when the RH of the ambient air is too high for drying. Some projects introduced additional devices to store the heat of the air during the day and release it when needed. This increases the capital cost beyond acceptable levels. The capacity is also limited;
• Solar collectors need to be made from durable materials (UV stabilized plastic), which makes them expensive. They consume a lot of floor area for a very limited heat accumulation. Feedback from the field indicated that cheap structures were often quickly destroyed by animals (e.g. village dogs), wind or solar radiation; and
• Generally capital cost per ton capacity is very high, which makes solar dryers suitable for high-value commodities that are dried in relatively small amounts (vanilla, spices, fruits, mushrooms, raisins etc.). Paddy is a high volume, low price commodity.

For above reasons solar drying is not recommended for paddy.