 |  | Design of a Suspension Burner System for Forestry and Agricultural Residues (NRI) | ||
 |  | Description of the unit | ||
|  | General | ||
| | Feeder unit | ||
| | Furnace | ||
| | Heat exchanger and cyclone | ||
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A diagram of the unit is given in Figure 1. The system consists of a feeder to meter particulate agricultural/forestry residues (feedstock) and a paddle fan which sucks the feedstock from the feeder and blows it tangentially into a cylindrical furnace. Inside the pre-heated furnace the feedstock travels in a cyclonic path and burns in suspension. The flue gases (and majority of ash residue) exit the furnace and can supply heat for processing, or be vented to atmosphere. For the experimental trials the flue gases were used to heat drying air via a shell-and-tube heat exchanger - flue gases on the tube-side and drying air on the shell side. A cyclone separated ash particles from the flue gases prior to exhausting to atmosphere.
Figure 1 Diagram of the experimental
system
The feeder tested in the furnace trials is a turntable system consisting of a 920 mm diameter revolving turntable, above which an open-ended drum is suspended centrally. The drum is supported by an adjustable arm so that the distance between the drum and the turntable can be varied. A 12 mm-square mesh is attached to the drum to screen the feedstock.
The turntable speed is adjustable between 0 r.p.m. and 10 r.p.m. and is driven by a 0.2 kW motor through a variable speed gearbox. The material is drawn off the turntable by a 180 mm diameter suction tube attached to a 0.6 kW mild-steel paddle fan capable of delivering up to 13 m3 of air per minute.
Tests were carried out on two other feeders, a screw auger and a vibratory system, and a description of these appears on pp.6-7.
The paddle fan is connected by ducting to a 0.3 m3-capacity cylindrical furnace. A flap valve is located in the duct to reduce the effect of any 'blowbacks' from the furnace during operation. The furnace casing, of overall dimensions 950 mm diameter by 1040 mm high, is fabricated from mild steel and has an insulated inner sleeve. The furnace chamber is lined with refractory cement to a depth of 100 mm for the wall and 60 mm for the floor. The removable furnace lid is constructed from heat-resistant ceramic boards, fixed onto an outer mild-steel sheet. The inlet and outlet ducting is manufactured from stainless steel for increased heat resistance.
The furnace outlet duct connects to a mild-steel heat exchanger outlet pipe. The outlet pipe, which has a heat-resistant ceramic lining, leads to a shell-and-tube heat exchanger unit housing 54 tubes, each 50 mm diameter by 2160 mm long. The flue gases pass on the tube side and exhaust to atmosphere through a cyclone to remove ash particulates. Ambient air, supplied by a 1.1 kW axial-flow fan rated for 70 m3/min at NTP, passes on the shell side. The resultant heated air exits the heat exchanger through a 380 mm diameter duct.
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