CHARACTERIZATION OF COAL DRYING IN PILOT SCALE PNEUMATIC DRYER

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 100^oC 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 200-350^oC 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 100^oC 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 200-350^oC 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:

1. Electric heater : 15 kW
2. Blower Fan: 330m3/h, 3kW
3. Induced Fan: 4 kW
4. Coal Feeder: 12-60 rpm
5. Pipe length: 5 m
6. 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 [%]