Cahyadi, Budianto. D
Energy Technology
Center, BPPT,
Kawasan Puspiptek
Serpong Tangerang 15314
Abstract
Many low rank coal deposits with high moisture content
are available in Indonesia.
The challenge for low rank coal utilization is how to reduce moisture content
with coal drying process. There are many type of coal dryer in the market. This
paper discussed about coal drying with pneumatic drying method. Pneumatic
drying is a simple drying method only with hot air as heat transfer and water
removal media. The application of coal drying with pneumatic dryer in Indonesia
is not available yet, the pilot scale pneumatic dryer is required for
characterization coal drying and evaluating financial cost. This paper still focuses on low temp drying condition
around 100oC in pneumatic dryer. The advantage of pneumatic drying
is the residence time is very short so that the hot air temperature has
potential to be increased until 200-350oC for shortening the drying
time without sacrificing the coal quality and then capacity of dryer will
increase. This work presents the results
of research in the pilot scale test which can be useful in designing and
construction of such dryers in coal utilization.
Keywords: heat transfer, coal drying, pneumatic drying
1. Introduction
Indonesian low rank coal
contain relatively large amounts of moisture content typically ranging from 45
to 55%, mostly in Kalimantan and Sumatra. The
challenge for low rank coal utilization is how to reduce moisture content with
coal drying process.
High fuel moisture has several
adverse impacts on the operation of a pulverized coal generating unit, for it
can result in fuel handling problems and it affects heat rate, stack emissions
and maintenance costs.
Many type of coal dryer are
offered in the market. This paper discussed about coal drying with pneumatic
drying method. Pneumatic drying is a simple drying method only with hot air as
heat transfer and water removal media. The application of coal drying with
pneumatic dryer in Indonesia
is
not available yet, the pilot
scale pneumatic dryer is required for characterization coal drying and
evaluating financial cost.
Many papers of pneumatic
dryer presented for food industry and other porous material for mining
industry. D.Tolmac (2008,2009) presented application for starch, Skuratovsky
(2004) for PVC, and Narimatsu (2004) for alumina. Some mathematical model for
pneumatic drying was also presented by (Suratovsky and Levy, 2004). Rocha, S.C.S.
(1988), Pelegrina, A.H.; Crapiste, G.H. (2001). This pneumatic drying research for coal drying in Indonesia is
still in the beginning and the work will contribute in development of pneumatic
dryer for high moisture low rank coal.
This paper still focuses on
low temp drying condition around 100oC in pneumatic dryer. The
advantage of pneumatic drying is the residence time is very short so that the
hot air temperature has potential to be increased until 200-350oC
for shortening the drying time without sacrificing the coal quality and then
capacity of dryer will increase. This
work presents the results of research in the pilot scale test which can be
useful in designing and construction of such dryers in coal utilization
In these dryers, a
continuous drying of loose materials is being made, the concentration being
(0.05 – 2.00) kg of material per 1 kg of air. Average particle size of the
drying material can be 76 um. The gas velocity (air or gas) in the dryer is (5-6)
m/s. The initial moisture of the material dried can be about w1 = 25 %, and the
remaining moisture after drying is usually about w2 = 14 %. The specific
consumption of energy is usually about to 4500 kJ/kg of evaporable water.
Efficiency of such dryers is evaluated according to the thermal degree of
utilization which is about of 75 %, depending of the drying system (indirect or
direct drying). The drying time in these dryers is very short, only several
seconds, therefore they can be used for drying of the materials susceptible to
high temperatures in a short drying period of time.
2.EXPERIMENTAL SET UP
This research is performed
in the convection pneumatic dryer, Fig. 1. Fresh air is heated with electric
heater. Drying is performed in the direct contact of warm gases with the moist
material. The principle of direct drying is represented here. The drying
material is coal.
Dosing of moist material to
the dryer is performed through a coal feeder with the capacity of m1 = 78 kg/h,
through the screw conveying system, as given in the scheme of experimental
equipment in Figure 1. In such a way, a homogenous moist material is obtained
at the dryer inlet. The characteristics of convection pneumatic dryer are given
as follows:
- Electric heater : 15 kW
- Blower Fan: 330m3/h, 3kW
- Induced Fan: 4 kW
- Coal Feeder: 12-60 rpm
- Pipe length: 5 m
- Diameter of Pipe: 0.14m
Figure 1. Scheme of Pneumatic dryer facility
Moist coal material is transported via hot air – the drying agent through the dryer pneumatic pipe, it passes through the dryer head and goes to the cyclone and bag filter separators for separation of dried material and the hot gases are exhausted by a induced draft fan, into the atmosphere. The dried material is transported from the cyclone via vane valve, and through a separate line of pneumatic transport, up to the material warehouse department.
Table 1. Average value of measuring hot air
temperature and coal moisture at each points.
Measuring point
|
0
|
1
|
2
|
3
|
4
|
5
|
Temp hot air
|
100
|
75
|
65
|
60
|
56
|
52
|
Moisture of dried coal
|
25
|
22
|
19
|
17
|
15
|
14
|
Table 1 contains average values of the results of measuring the air temperature – the drying agent and moisture of dried material. Experimental measuring is being made in the approximate stationary conditions of the dryer operation. The stationary conditions mean the stationary conditions during a longer period of the dryer operation and a greater number of measuring (where non-stationary conditions of the process are excluded during the realistic conditions of the dryer operation).
3.
Result and Discussion
The coefficient of heat
transfer is under the dynamic conditions of the dryer operation (feeding rate
of wet material to be dried, variation in the initial moisture content,
temperature of drying, volume rate of hot air, etc.). For heat air through
pneumatic pipe of dryer V = 330 m3/h, and diameter of pipe dryer d =
140 mm, the quick rate transport is v = 6 m/s. Taking the length (height) into
consideration, the pneumatic pipe drying is hz = 5 m, and time for drying is t
= 0.5 s. Based on the results of energy balance and those of the drying
parameters measuring, a total coefficient of the heat transfer is determined to
be during convection drying ht = 83.3 W/m2K. Based on the results of
research, the coal and air flow ratio is
0.23, the drying capacity is 78 kg/h, and the air temperature at the dryer
inlet is 100 °C.
The Figure 2 presents
drying curves which demonstrate dependency of humidity changes on the time of
drying. In the beginning of drying period, dependency of moisture changes and
drying time is almost linear in character, where adequate time period of drying
is t = 0 – 0.15 s. It is the first
drying period and its drying rate is constant, z = 3 m. In the second drying
period, at time interval t = 0.15 – 0.24 s, dependency of moisture changes and
drying time is not linear in character, which is demonstrated by the polynomial
of
the second order. In the
end of drying, the content of balanced moisture is w2 = 13 %, z = 5 m.
Figure 2. Drying curve of distribution temperature
and
coal mositure content at each point measurement.
Figure 3. Kinetic drying curve at each point
measurement.
The Figure 2 also presented
drying curves rates. The first stage of drying rate is constant but in the
second period the drying rate is falling. When material moisture is reduced to
the balanced moisture w = 14 %, moisture evaporation rate is dw/dt = 0.2 (kg –
w / kg – dried solid).
The Figure 3 presents
thermal drying curves. Rate of moisture content change time of drying is
demonstrated by polynomial of the second order. In his case, the ratio of
drying material and the air is 0.23 (kg material / kg air). Thermal drying
curves, Fig. 3, may have thermal approximate linear character. In the beginning
of the period drying temperature of drying agents is 100 °C and in the end of
drying it is 52 °C.
• dependency equation of
moisture material from the time of drying, Fig. 2:
w = 25.214 – 3.864 z + 0.3214
z2 (12)
• equation of thermal
drying curves, Fig. 2:
T = 97.25 – 20.146 z +
2.304 z2 (13)
• dependency equation of
drying rate from time, Fig. 3:
dw/dt = 0.92 – 0.167z + 0.0036z2 (14)
Figure 4. Drying rate and material humidity
dependency
Empiric equations based on
experimental research define the character of drying process course.
Dependency change of drying
rate and moisture material is shown in Figure 4.
dw/dt = 0.3186w-0.007x2-2.90
In the beginning of the
period of drying, the surface moisture material covered by a thin layer of
water has the same feature as when moisture free. Due to the contacts of
colloid moisture material with heat drying agents, the process of liquid
evaporation starts. In that case, in the first period of drying, liquid
evaporation is accelerated, taking physical bound moisture into consideration,
Fig. 4, but in the second period, the rate of drying is visible lower, taking
physical-chemistry bond moisture into consideration.
4. Conclusion
This work presented about the performance of coal drying in pilot scale pneumatic dryer. Based on the results of research, the coal and air flow ratio is 0.23, the drying capacity is 78 kg/h, and the air temperature at the dryer inlet is 100 °C. With the length (height) of the pneumatic pipe drying is hz = 5 m, and time for drying is t = 0.24 s, the total coefficient of the heat transfer is 83.3 W/m2K. Raw coal moisture content of 25% with the size of 75um can be removed to 14% moisture content.
The future research works
are increasing the hot air temperature, varying in coal size, varying in coal
moisture content and increasing drying time with recycle the dried coal from
cyclone into drying chamber.
5. REFERENCES
[1] I. Skuratovsky, A Levy
and I. Borde: Pneumatic drying solid particles, Proceedings of the 14th
International Drying Symposium (IDS 2004)
São Paulo, Brazil, 22-25 August 2004, vol. A, pp. 366-373.
[2] Prvulovic, S., Tolmac,
D. and Lambic, M.: Determination of energetic characteristics of convection
drying place on pneumatic transportation material, Journal of Process Technique,
Vol. 1. 70-74, 2001.
[3] Tolmac, D., Prvulovic,
S. and Lambic, M.: Mathematical Model of the Heat Transfer for Contact Dryer,
FME Transactions, Vol. 35, No. 1, pp. 15-22, 2007.
[4] Holman, J.P.: Heat
Transfer, McGraw-Hill, New York, 1981.
[5] Prvulović, Tolmac,D:
Experimental Research on Energetics Characteristics of Starch Dryer, FME
Transactions (2009) 37, 47-52
[6] Liu, Q. and Bakker-Arkema, F.W.: Capacity Estimation
of High-Temperature Grain Dryers – A
Simplified Calculation
Method, Agricultural Engineering International: the CIGR Ejournal, Vol.
I, pp. 1-17, 1999.
NOMENCLATURE
d the dryer
pipe diameter [mm]
k thermal air
conductivity [W/mK]
G mass speed
stream air [kg/sm2]
μ dynamic
viscosity warm air [kg/sm]
hu
coefficient of heat transfer [W/m2K]
hc
coefficient of convection heat transfer [W/m2K]
H enthalpy
[kJ/kg]
t1 air
temperature at the inlet of dryer [°C]
t2 air
temperature at the outlet of dryer [°C]
Cp specific
air heat [kJ/mn3K]
W quantity of
evaporated water [kg/h]
m1 quantity
of moist material [kg/h]
w1 content
of the wet material at the inlet of dryer [%]
Lucky 88 Casino & Hotel: Tunica, MS - JTG Hub
BalasHapusLucky 88 Casino 문경 출장마사지 & Hotel Tunica. With 5500 포천 출장안마 slot machines and 5 restaurants, this หารายได้เสริม Tunica casino resort is a 남양주 출장마사지 favorite among 목포 출장샵 visitors to Tunica's locals.