Fluidized bed superheated steam dryer for bagasse: Effects of particle size distribution

L. Stella Polanco, Ensley, Carlen  |  2/22/2014 1:56:47 AM

L. Stella Polanco

Feasibility studies of proposed biomass processing plants for production of biofuels or biochemicals require serious consideration of energy balances, especially steam generation. As in the cane sugar industry, biorefineries processing grassy biomass materials, such as energycane and sweet sorghum, will extract juice as the first stage. The residue left from this stage, bagasse, would be expected to contain about 50 percent moisture and can be used as a fuel to satisfy power requirements of the biorefinery. Excess bagasse can be used for further conversion to other products. Reduction of bagasse moisture to a range between 10 percent to 40 percent increases fiber preservation during storage, gives efficient and stable operation to those boilers using it as a fuel, lowers emission, and gives higher energy efficiency and higher product quality for thermochemical conversion technologies. This moisture content cannot be attained with conventional mechanical/milling methods. Alternative technologies for bagasse drying should be considered before biorefinery construction.

Physical properties of bagasse that are related to its origin make it a difficult material to process. Both the particle size distribution and the behavior of assemblies of bagasse particles are important considerations for the selection and design of equipment for feeding, collection, burning, depithing, pneumatic transportation, separation of dust, pelletizing and drying.
Bagasse drying has not been widely used because of additional costs and fire hazards; however, it is justified by plants with high energy demand. Flue gas dryers such as the rotary and the most recently pneumatic and cyclonic dryers have been the favored technologies for drying bagasse to a moisture content of 30 percent to 40 percent. The fluid bed pressurized superheated steam dryer, which is successfully used to dry beet pulp, offers a high evaporation capacity, reduction of emissions from the dryer, reduction of fire and explosion hazards and the possibility of heat recovery to increase the overall energy efficiency of factory operations. These features and the possibility of final moisture between 10 percent to 20 percent show the advantage of a superheated steam dryer over conventional bagasse dryers.

In a fluid bed dryer, the bagasse bed is expanded by the drying medium (pressurized superheated steam) which is injected upward. Steam and bagasse are transported together (like a single phase fluid) passing through several fluidization cells to a collection point where the dried bagasse particles are separated from the combined streams of superheated steam and evaporated moisture. The released vapor still superheated, entrains fine particles (dust) which are separated by internal cyclones. Temperature, pressure and velocity of the gas as well as particle size, density change (moisture content change), height and bed void are parameters that affect the stability of the fluid bed. Geometrical configuration and dimensions of the fluidization chambers, recirculation of coarse particles, dust separation system and design of grid plates and gas spargers are design details for the success of a dryer on a large scale.

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