|Design of a Suspension Burner System for Forestry and Agricultural Residues (NRI)|
Sawdust at 12% moisture content was loaded into the turntable feeder drum. With the fan and turntable feeder switched off, a small fire was built on the furnace floor. To promote a strong fire, the fan was then switched on to supply a low volume of air. When the fire was well alight the furnace door was closed and the fan speed increased to an air-flow rate of approximately 8 m3/ min. This caused the flue gas temperature to rise quickly to about 600°C, then, as the fire subsided, the temperature started to drop. At this point the turntable was switched on at a preset feedrate of about 25 kg/in. Steady operating conditions were reached after about 10 minutes.
For all the experimental trials the unit was first run for 2-3 hours with sawdust at 12% moisture content and operating at a feed rate of 25 kg/in at an excess air value of approximately 250%.
The sample of coir residue received from a Sri Lankan coir factory consisted of pith and fibre at a moisture content of 86% (wet basis). During consideration of the potential use of this material as a fuel for a suspension burner a simple assessment was made of various methods of expelling the surface water. The results of this work and a general summary are given below.
Trials with a hydraulic press
A weighed amount of material was placed in a slotted cylinder and compacted by a hydraulic ram to extract the water. The load, sustained until the flow water ceased, was measured by a transducer and the overall moisture content of the pressed material was calculated. The pressed material was in the form of a solid cake. The results obtained are shown below:
Table - Trials with a hydraulic press
Note:* compaction ratio is defined as original depth of material divided by final depth of material. Cylinder area-333 cm2
Trials with a screw press
A known amount of wet material was placed in a perforated cylinder and a load was applied through a screw (50 mm diameter, 6 mm pitch) for approximately 5 minutes. The screw was turned manually, using in the first trial a short lever and in the second and third trials a longer lever.
An estimate of the applied peak load was calculated. The pressed material was weighed and the moisture content calculated. The results obtained are shown below:
Table - Trials with a screw press
Note: Cylinder area-485 cm2
It is possible to dewater coir residue to approximately 65% moisture content using a screw press. However this should be considered a reasonable practical limit when considering the repetitive manual batch-process required.
It should be noted that assuming a 40 kg/h feedrate, a volume of approximately 0.5 m3 of wet material (equivalent to 286 kg at 86% moisture content) at a bulk density of 600 kg/m3 will be required. If the pressing area can be increased by 25% (total loading about 19 tonnes), 8 pressings per hour would be necessary. At a compaction ratio of 4, the screw travel would be 750 mm, which can only be shortened by the use of 2 or more presses as the pressing area has been maximized.
Physical characteristics of the pressed material
The pressed material is in the form of a strongly bound cake. At 65% moisture content it has a calculated net calorific value of 5.0 MJ/kg, against its gross calorific value of 19.27 MJ/kg.
Approximate sieve analysis of non-pressed material and pressed material
A sieve analysis was carried out; the results are shown below:
Table - Sieve analysis
The material retained on the 4.0 mm mesh consisted mainly of long fibre, together with, particularly in the case of the pressed material, lumps of matted dust. The large fractions on 16 and 30 meshes consisted of dust or discrete particles together with very short fibre. Both types of material were sieved but the pressed material had to be teased apart first.
Trials with a centrifuge
A moisture content of 75% was achieved using a small spin dryer. It is possible that better results could be obtained on a faster and more powerful machine. However any such machine may prove expensive.
Trials with Protessor expeller and a Rosedown expeller
Dewatering proved unsuccessful as it was not possible to feed the wet coir residue into the expellers.
The need in the coir fibre industry is for low cost/capacity equipment; it is therefore clear that within this framework a purpose-built screw press would be acceptable. However, the final moisture content of the product would not be less than 65% and would be in the form of a compacted cake having the dimensions of the press body. This cake has considerable stability and would require significant mechanical action to break it down for suspension burner use and further drying if deemed necessary. Given the characteristics of a screw press operation, the pressed cake's relatively large dimensions, and the significant levels of large fibres present in the product, it is clear that, whilst this amount of dewatering may be acceptable on a cost/operation basis, it would present considerable difficulties for suspension burner operations.
Summary of results
Typical heat balance on suspension burner/timber drying unit
Typical heat balance
Note: * ignoring heat losses from the exposed surfaces of the heat exchanger system.
A heat balance was conducted for each trial so that an assessment of the major heat losses could be made. It served to highlight that the unit is very energy efficient in terms of the transfer of heat generated from the burner to the kiln, and actual losses from the burner are small in comparison to the rest of the unit.
Basic data for typical run
Basic data for typical run (cont.)
* Combustion gas temperature inside the burner were, in general,
* * Average daily ambient temperature 32°C Average nightly ambient temperature 22°C
Calculated results for a typical run
* gross calorific value x feedrate (dry basis). gross calorific
value taken as 20,500 kJ/kg
** net calorific value x feedrate (wet basis)
heat exchanger efficiency =
secondary heat exchanger efficiency =
where Tcg = temperature of suspension burner combustion
Tpfg - temperature of primary heat exhanger flue gas
Tsfg - temperature of secondary heat exchanger flue gas
Tamb - temperature of ambient air
using the relationship: W=
where W = mass flowrate of air (kg/h)
NCV = net calorific value of feed (kJ/kg)
FR = feedrate of woodwaste, dry basis (kg/h)
Cp = specific heat of air, calculated at the suspension combustion gas temperature (kJ/kg°C)
DT = temperature differences between suspension burner combustion gas and ambiant (°C)
then, excess air =
where Ws = stoichiometric air requirements, taken as 4.5 kg of air per kg of wet sawdust at 15% moisture content
Calculated heat losses for a typical run
* ambient temperature 27°C (300 K) average
** Using the relationship; Q=CE (Ts4-TA4)
where Q - radiated surface heat loss (kJ/h m2)
C - Stefan Boltzman constant - 2.0142 x 10 7 kJ/h m' K4
E - emissivity
Ts- surface temperature K
TA- ambient temperature K
Using the following relationships for natural convextion (IHVE Guide 1970); (a) horizontal surfaces facing up ho=2.5
(b) vertical surfaces ho= 1.9 (T) 0.25
(c) cylindrical surfaces ho= 1.32 (T/do) 0.25
where ho= heat transfer coefficient for natural convection (kJ/h m2 °C)
T - temperature difference between the surface and ambient, (°C)
do= diameter of cylinder
Record of kiln samples for a typical run
Record of kiln samples for a typical run (cont.)
Notes: moisture content =
estimated dry weight =
Seasonal quality record for a typical run
Notes:Category: 1 - no defect
2 - slight
3 - moderate
4 - severe
5 - very severe
Average timber grade: good
Additional remarks: timber load was weighted to reduce defects; quality assessment is purely subjective