consumes a lot of energy, typically 3.2e4.5 MJ kg1 of evaporated
water (30e50% more than the latent heat). There is little free water
and the mass transfer rates are low. The air leaving the dryer is not
saturated to maintain correct drying rates all through the dryer.
This means that the energy efficiency (energy consumed compared
to the latent heat of the evaporated water) is generally low (order of
50e70%) and a large amount of hot air is required.
Due to the high energy costs of the drying operation in biomass
plants, this part of the process has received a lot of attention.
Various solutions have been found. Process integration applying
thermo-economic optimisation is common practice [3,4]. In this
approach the complete heat balance of a process plant is examined
and the plant is fully integrated using pinch analysis and heat
integration tools. In practice, for industrial plants, the process
integration is far from complete and usually does not take into
account the requirements for start-up procedures. This approach
makes better use of the available energy, rather than reducing the
heat requirement for one particular process unit.
Most technology solutions to improve the energy balance of the
drying process are based on complex heat integration with heat
pumps [5,6] or more often with steam recompression [7e10]. These
solutions invariably lead to an additional electrical load, suffer from
fouling and are expensive. Improvements to the biomass drying are
ideally simple and robust. An interesting alternative are forced solar
dryers [11] interesting in localities with sufficient sunshine