Two-stage drying is also referred to as combination drying.
Considering the theoretical drying curve of paddy (Figure 4) and the requirements for quick drying immediately after harvest to a MC that is safe for temporary storage the two-stage drying system or combination drying system was developed.
A typical first stage dryer takes advantage of the different drying rates during the three drying periods and that surface moisture can be removed rapidly from very wet paddy without causing damage to the grains by using very high temperatures for a short period of time. Drying air temperatures in first stage dryers can reach over 100ºC in fluidized bed dryers where the grain is exposed to the drying air only for a few minutes. After this rapid pre-drying to a MC of 18%, the grain is considered safe for up to two weeks of storage. The grain is then transferred to a storage bin with aeration facilities where it is slowly dried to the desired moisture content of 14% or lower with only slightly pre-heated air or even ambient air if the climatic conditions are feasible.
Although two-stage drying has many advantages since it uses two different drying principles well suited to the different drying phases of paddy grains at different MC ranges the introduction of two stage drying in Southeast Asia has so far failed. This strategy is used only in Thailand by the commercial sector.
Decentralized drying to safe MC levels can be done close to the production in relatively small mobile pre-dryers extending the allowable time for handling. Final drying can be centralized in storage bins utilizing energy saving aeration.
Low specific energy requirement since the two different drying technologies are optimized with respect to maximizing the drying potential of the drying air in the respective drying phases.
The system can produce excellent quality since the last critical drying stage at low moisture contents is done with low temperature which prevents the kernel from cracking through heat stress or moisture adsorption.
Users who want to dry from harvest to safe storage MC need two machines to complete the job.
In-store drying only makes sense if the grain remains in the storage container for storage after drying. In most Southeast Asian countries, bulk handling and storage is not yet practiced. If, in addition, there is a need to sell the paddy as quickly as possible after drying, as it is the case in most cases in SE-Asia, a dryer with shorter drying time is more appropriate.
Economics of scale require a certain size of the in-store dryers. In farming environment with small scale farms where different varieties are grown often the necessary amount of the same variety for filling an in-store dryer cannot be collected.
In countries where electricity cuts are part of daily life extended drying translates to increased risk. In this case a dryer with short drying time and ideally with self propelled fan and conveyors using a combustion engine reduces the risk of spoilage.