FACTORS AFFECTING SEASONING OF TIMBER USING

SAWDUST OPERATED KILN

P L A M C Wijewarnasuriya

H S Amarasekera

University of Sri Jayewardenepura, Sri Lanka

Introduction

Seasoning is a value addition to timber.

Cost of seasoning is not affordable to small scale saw millers.

Senadheera (2009) developed a sawdust burner for seasoning of timber and it has been coupled to a kiln chamber (50ft3) situated in University of Sri Jayewardenepura.

In this study , performance and feasibility of that sawdust operated kiln was investigated with a view to improve its efficiency.

Objectives

To improve the efficiency and to investigate the performance of sawdust operated kiln

To compare the university kiln with some selected industrially operating kilns in the country

For the first objective

(01) Effect of particle size of sawdust on the efficiency of the burner

Sawdust was divided into two main groups based on particle size using a mesh having 1mm wide pores.

Each sawdust group was burnt separately under a same feeding rate and water was heated. Temperature of water after a certain time period was measured.

Variation of temperature in water Vs time for different sawdust types

Water – 1000 ml Time – 30 min Feeding rate – 20 g min-1

(02) Effect of sawdust feeding rate on the efficiency of the burner

Water was heated from the heat generated by the most efficient sawdust type (particle diameter > 1mm) burner under different sawdust feeding rates.

Variation of temperature in water Vs time for different sawdust feeding rates

Water - 2000 ml Ti me - 30 min Sawdust particle size - > 1 mm

(03) Arrangement of heat transferring pipe in the kiln chamber

heat transferring

pipe Timber stack

(04) Drying of rubber wood using sawdust operated kiln

Dimensions of timber boards – 100 cm * 15 cm * 2.5 cmSticker thickness – 25 mm

Sampling – 8 boards were randomly selected

Drying rate

Mass of water removed from timber per hour during the kiln run

Drying rate = MW / T

MW - mass of water removed from timber in each stage

T - time taken

Drying rate = %MC / T

%MC - percentage moisture content reduced in a certain period of time

T - time taken

Drying efficiency

Percentage of water removed in a certain time period in relation to total water content in timber.

Drying efficiency = ms / mt * 100%

ms - weight of water removed from each stage

mt - total water content in the timber

Drying characteristics of rubber wood

Variation of moisture content in rubber wood with the time

Time Final MC in wood Drying rate Drying efficiency

hrs

0-8

%

54.66

g of water hr-1

1181.25

% MC hr-1

0.90

%

14.34

8-16 48.38 1031.25 0.79 12.52

16-24 43.93 900.00 0.56 10.93

24-32 40.16 774.00 0.47 9.40

32-40 36.50 666.00 0.46 8.09

40-48 32.62 600.00 0.49 7.29

48-56 29.42 525.00 0.40 6.37

56-64 26.79 431.25 0.33 5.24

64-72 24.28 412.50 0.31 5.01

72-80 21.88 393.75 0.30 4.78

80-88 20.05 300.00 0.23 3.64

88-96 18.45 262.50 0.20 3.19

96-104 17.20 206.25 0.16 2.50

104-112 16.17 168.75 0.13 2.05

112-120 15.14 150.00 0.13 1.82

120-128 14.23 114.00 0.11 1.38

128-136 13.89 84.00 0.04 1.02

136-144 12.97 54.00 0.11 0.66

0-144 13.00 458.58 0.34 81.3

According to Ratnayake (1998); a study has been done for same volume (25 ft3) of rubber wood using the same kiln chamber; it has taken only 62 hours for the kiln run under a dehumidifying system.

Average drying rate was found to be 459 g of water evaporated per hour for present study while dehumidifying system has exhibited an average drying rate of 1290 g of water per hour

When drying rate is expressed as reduction of %MC per hour, in respect of present study it ranged 0.04 – 0.90 %MC hr-1 and in respect of previous study (Senadheera, 2009) it ranged 0.16 – 0.60 %MC hr1.

(05) Investigation of drying behaviour of different timber species

Selected species

Rubber - Hevea brasiliensis

Albizia - Paraserianthes falcataria

Lunumidella - Melia dubia

Mahogany - Swietenia macrophylla

Dimensions of timber boards – 100cm * 15cm * 2.5cm

Sampling – 3 boards for each species was randomly selected

Variation of moisture content in different timbers with the time

Comparison of kilns

Performance of sawdust operated university kiln (UNI) was compared with

State Timber Coporation – Kaldemulla (STC)

Rowood Lanka Ltd – Nelundeniya (RWD)

For second objective

STC kiln: chamber capacity 4096 ft3, operated with two boilers (furnace oil and wood waste)

RWD kiln: chamber capacity 920 ft3, operated with a boiler (sawdust)

Comparison of kilns was done based on costs and benefits generated by kilns when seasoning 25mm thick rubber wood boards under 25mm thick stickers.

In order to dry rubber wood from green conditions upto 13% of moisture content, UNI, STC and RWD kilns consume 144, 144 and 216 hours respectively.

Costs and benefits were estimated for each kiln for annual wise assuming that number of operating days per year is 320.

Therefore, number of possible kiln runs per year for UNI, STC and RWD kilns are 53, 53 and 35.

  UNI kiln STC kiln RWD kilnEstablishment cost (Rs)

Building 42,123.00 326,305.00 102,400.00

Technology 32,500.00 5,506,000.00 356,000.00

Costs (Rs yr-1)

Annualised cost 8,573.00 723,895.00 46,805.00

Timber 212,000.00 17,367,040.00 2,576,000.00

Labour 180,000.00 1,968,000.00 540,000.00

Maintenance 15,000.00 75,000.00 40,000.00

Electricity 133,290.00 196,705.00 790,326.00

Fuel wood -

2,880,000.00 -

Sawdust 20,670.00 - 106,312.00

Furnace oil -

4,032,000.00 -

CO2 emission 6,155.00

1,078,271.00

36,495.00

Estimated costs and benefits for selected kilns

Benefit (Rs yr-1) UNI kiln STC kiln RWD kiln

Revenue from timber

477,000.00

39,061,440.00

5,796,000.00

Savings due to use of wood waste

383,125.00

2,746,046.00

1,970,490.00

CO2 benefit (neutral emission)

107,748.00

1,657,655.00

554,181.00

(01) Cost per unit volume (C1)

C1 = C / V

C- total cost of kiln run V- volume of timber

Cost efficiency increases : UNI < STC < RWD

However, cost efficiencies of STC and RWD kilns are almost same.

Kiln

UNI

Cost (Rs ft-3)

Without CO2 emission

cost

429.84

With CO2 emission

cost

434.50

STC 260.98 260.90

RWD 254.31 256.60

Costs (Rs yr-1) % Contribution 

Annualised cost

UNI

1.5

STC

2.6

RWD

1.0

Timber 36.8 61.3 62.4

Labour 31.3 7.0 13.1

Maintenance 2.6 0.3 1.0

Electricity 23.2 0.7 19.1

Fuelwood 0.0 10.2 0.0

Sawdust 3.6 0.0 2.6

Furnace oil 0.0 14.2 0.0

CO2 emission 1.1 3.8 0.9

Total 100.0 100.0 100.0

Percentage contribution of each cost on total cost of seasoning

(02) Cost per unit volume per unit time (C2)

C2 = C / (V*T)

C- total cost of kiln run V- volume of timberT- time taken for the kiln run

Kiln

UNI

Cost (Rs ft-3 hr-1)

3.02

STC 1.81

RWD 1.19

Cost efficiency increases : UNI < STC < RWD

Kiln Comparison

 

UNI

Cost for unit

volume

1.7

Cost for unit volume and

time

2.5

1.5STC 1.0

RWD 1.0 1.0

Comparison of two cost comparisons

Here, two kilns were compared with the kiln which possesses the least cost of seasoning.

(03) Cost-benefit analysis

Net Present Values were calculated for 15 years based on following assumptions.

Annual discount rate is 10%.

Annual depreciation rate for buildings is 6%.

Technology life time for UNI, STC and RWD kilns are 5, 15 and 15 years respectively.

Timber supply and sale during the concerned period of time is consistent.

Kiln

UNI

Net Present Value

(Rs millions)

2.68

STC 104.43

RWD 28.84

Conclusions

The efficient sawdust feeding rate for the burner is 60 g per minute and sawdust piraticle size should be more than 1 mm.

Under the existing performance the kiln is suitable only for fast drying timber species such as rubber wood and albizia.

The kiln is not suitable to run as an enterprise as it is not cost efficient.

Existing heat transfer technology (hot exhaust gas) should be changed to steam or hot water. (The maximum temperature obtained by the kiln chamber was 420C under the existing heat transfer technology.)

Identified design error regarding the burner should be corrected. (Total area of combustion chamber should be utilised for sawdust burning.)

Electricity consumption during kiln operation should be minimised. (One circulatory fan should be used out of two fans.)

Recommendations

References

Gjerdrum, P. (2000). Cost efficient timber drying, Proceedings of 2nd Workshop on Quality Drying of Hardwoods, Sopron, Hungary.

Perry, R.H. and Chilton, C.H. (1973). Chemical Engineer’s Hand Book, 4th edition. pp 245.

Ratnayake R.S.S. (1998). Development of drying schedules for rubber and pine timbers for the dehumidification kiln drying, M.Sc. thesis, Department of forestry and environmental science, University of Sri Jayewardenepura, Sri Lanka.

Senadheera, D.K.L.K. (2009). Development of sawdust burner for kiln seasoning of timber, B.Sc. dissertation, Department of forestry and environmental science, University of Sri Jayewardenepura, Sri Lanka.

Moisture content in moisture content test pieces (MCTP)

MC = (m1 – m0) / m0 * 100%

m1 –average initial weight of MCTP

m0 – average oven dry weight of MCTP

Estimated oven dry weight of sample boards

MO = M1 / ( MC / 100 + 1)

M1 – initial weight of the sample board

Current moisture content of sample boards

MP = ( M2/M0 ) – 1) * 100%

M2 - current weight of the sample board

Annualised cost = PV x r [1 – (1 + r)-t]

PV - present value of investment for technology (Rs)r - annual discount rate (yr-1)t - life time of machinery (yrs)

PV - present value of investment for technology (Rs)r - annual discount rate (yr-1)t - life time of machinery (yrs)

Cost of Timber = PG x N x VT

VT - timber volume per kiln run (capacity for timber in kiln chamber) (ft3)N - number of kiln runs per yearPG - unit price of green timber (Rs. ft-3)

NL - number of labourersS - monthly salary (Rs. month-1)

Cost of labour = NL x S

AE - amount of energy source per kiln run (kg or ℓ or kWh)N - number of kiln runs PE - unit price of energy (Rs kg-1

or Rs ℓ-1 or Rs kWh-1)

Cost of energy = AE x N x PE

Af - amount of furnace oil per kiln run (ℓ)N - number of kiln runs per year Df - density of furnace oil (kg ℓ-1)F - factor for carbon dioxide emission per unit mass of furnace oil (kg kg-1)VC - annual value of carbon dioxide emission (Rs Mt-1)

Cost for CO2 emission = Af x N x Df x F x VC 1000

CO - unit cost of furnace oil (Rs ℓ-1)CW - annual cost of wood waste (Rs)HO - calorific value of furnace oil (MJ ℓ-1)HW - calorific value of wood waste (MJ kg-1)MW - annual requirement of wood waste (kg)

Cost saving due to use of wood waste = [ MW x HW x CO ] – CW

HO

RT - annual revenue from timber (Rs)VT - timber volume per kiln run (capacity for timber in kiln chamber) (ft3)N - number of kiln runs per year PS - unit price of seasoned timber (Rs. ft-3)

Revenue from seasoned timber = 0.9 x VT x N x PS

BV - present value of the building (Rs)DR - depreciation rate for buildings (yr-1)LT - time period (yrs)

Building resale value = BV – (BV x DR x LT)

Df - density of furnace oil (kg ℓ-1)F - factor for carbon dioxide emission per unit mass of furnace oil (kg kg-1)HO - calorific value of furnace oil (MJ ℓ-1)HW - calorific value of wood waste (MJ kg-1)MW - annual requirement of wood waste (kg)VC - annual value of carbon dioxide emission (Rs Mt-1)

Benefit due to neutral CO2 emission = MW x HW x Df x F x VC 1000 x HO

The density of furnace oil is 1.005 kg ℓ-1 and burning of 1 kg of furnace oil emits 3.15 kg of CO2

(http://numero57.net/2008/03/20/carbon-dioxide-emissions-per-barrel-of-crude/ as at 10/08/2010). The annual cost of CO2 emission is US $ 20 Mt-1 (Source: New south Wales

Environmental Protection Authority, 1998) calculated in 1998. 1 US $ is Rs 111.58 (www.cbsl.gov.lk as at 10/08/2010). CO2

emission cost was converted for year 2010 according to following formula.

GDP deflators for year 1998 and year 2010 are 85.51 and 109.77 respectively. Therefore, estimated CO2 emission value for year 2010 is US $ 26.44.

Value in 1998 = GDP deflactor for 1998

Value in 2010 GDP deflactor for 2010

However, 60% of electricity generation in Sri Lanka is based on fossil fuels mainly oil (http://www.energy.gov.lk/pdf/Sri%20Lanka%20Energy%20Balance%202007.pdf as at 12/08/2010). Therefore, electricity consumption (60%) is responsible for emission of CO 2. Therefore, there is a cost of CO 2 emission due

to use of electricity for all three kilns. In order to generate 1 kWh; fossil oil releases 0.24 kg of CO2

(http://www.engineeringtoolbox.com/co2-emission-fuels-d_1085.html as at 12/08/2010).