Biomass Rotary Dryer System

System Description

The dryer system would a conventional rotary dryer. The part of hot gas will first mix with wet material in rotary dryer .Drying would be achieved by intimate mixing of hot gas with the wet material as they travel together in the dryer drum. 

The dryer would consist of the following sub-systems :

  1. Hot Gas Generator
  2. The hot gas generator consists of a furnace (solid fuel fired ), which generates hot flue gas at 800 - 1000 oC. The temperature of the flue gas is reduced to 400 oC by mixing it with fresh air after it exits the furnace system. The furnace is of refractory construction with one layer of refractory brick, followed by a layer of insulation bricks. The temperature of furnace surface would be restricted to around 100 oC only, by proper insulation. The furnace roof shall be of arch type construction for adequate strength.

    The choice of the fuel to be fired would depend on the users requirement and economics of utilisation of the fuel.

    The solid fuel furnace would have a horizontal grate for combustion of the fuel fired. The hot gas generator would be divided into two compartments, the one in which fuel would be fired and the second for completing the combustion and reducing the possibility of ash and fire particles carryover into the dryer column.

  3. Material Conveyor
  4. The material conveyor would transfer the wet biomass from the feed hopper into the dryer system. The material conveyor would be a belt conveyor of sufficient capacity. It shall be located between the hot gas generator system and the rotary dryer system.

    The belt conveyor would be driven by a 7.5 hp AC motor through reduction gear. The motor is provided with a variable speed drive so that the speed of the conveyor can be varied and the quantity of material fed into the system is controlled. Thus, if the moisture content of the wet material is much more than 40%, then the material feed may be reduced and thus the moisture content at the exit can be maintained at the desired levels, and vice versa.

  5. Rotary Dryer & Flash Dryer Columns
  6. The rotary dryer will be employed to reduce or minimize the liquid moisture content of the material it is handling by bringing it into direct contact with a heated gas. The dryer is made up of a large, rotating cylindrical tube, usually supported by steel beams. The dryer slopes slightly so that the discharge end is lower than the material feed end in order to convey the material through the dryer under gravity.

    Material to be dried enters the dryer, and as the dryer rotates, the material is lifted up by a series of internal fins lining the inner wall of the dryer. When the material gets high enough to roll back off the fins, it falls back down to the bottom of the dryer, passing through the hot gas stream as it falls. This hot gas stream can either be moving toward the discharge end from the feed end (known as co-current flow), or toward the feed end from the discharge end (known as counter-current flow). The hot gas stream is made up of a mixture of air and combustion gases.

    The wet material would be fed into the system just ahead of the dryer. Adequate spark quenching arrangements would be provided at the entry to the dryer to avoid chances of fire.

    Immediately on entering the system, due to the high temperature gradient available there would be instantaneous drying of surface moisture which would bring down the temperature of the hot gas to 200 - 250 oC. The removal of the remaining moisture would be completed in the dryer by direct heat transfer between the material and the hot gas which carries it.

    All parts of the dryer would be well insulated by mineral / glass wool type insulation for reducing the heat loss and restrict the surface temperatures to 70 oC.

    The entire system would be powered by a centrifugal blower . The blower would be belt driven by an electric motor.

  7. Electricals
  8. All electrical motors are controlled from the main panel. The feeder motor speed control shall also be from the panel. The panel would also include temperature indicators of the various drying zones. The electric motors would be provided with Star Delta starters and the remaining smaller motors shall be connected through DOL starters.

    The each RAV's shall be driven by motors through a speed reducing gear.

  9. Instrumentation
  10. The electrical panel would contain voltmeters and ammeters for the blower motor.

    The dryer system would be fitted with four thermocouples with the indicator mounted on the electrical panel. These would greatly reduce the risk of fires .

    The system shall also be provided with two pressure gages, placed before and after the blower.

  11. Supports
  12. All supports for the furnace, feeder, dryer drum etc would be from the ground. The foundation for all supports shall be provided by the client.

    The blowers with their motors would be mounted on their own concrete foundations which shall be provided by the client.

  13. Descriptive Specification Quantity
  14. The dryer is rated to produce 1500 kg/h of dried biomass with a moisture content of < 15% from an initial moisture content of 50 %.

    The rotary dryer can handle materials with a higher initial moisture content, but with a reduced output. The heat required for the dryer can be obtained from either a hot air generator, or from
    the exhaust of the gas engine or from other sources (not in scope of supply).

  15. Input raw materials specs
  16. Maximum size : < 40 mm
    Maximum humidity : < 50 %

  17. Output
  18. Biomass ouput : 1500 kg/h
    Maximum humidity : < 15%

Performance Parameters

dryer Model RD 1500
Dry Material Output kg/h 1500
Material to be dried Wood Chips/ Biomass
Max Input Heat kWth 1064
Hot Air Requirement at 140 Deg C m3/h 42,000
Size powdery / granular / fibrous
Moisture content of material at inlet of the dryer 50
Moisture content of material at exit of the dryer Mg/mm3 15
Dust content in exhaust gas mg/nm3 < 600
Blower/Centrifugal Fan motor Rating hp 25
Rotary air lock motor rating hp 5
Feeder motor rating hp 3
Discharge Valve/ motor rating hp 3
Total Power consumption kw 23
Fuel consumption* @ 30 % inlet moisture kg/h 130
Fuel consumption* @ 45 % inlet moisture Kg/h 270
* Based on Fuel Calorific value of 4000 kcal/kg