––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– Address correspondence to Ryo Moriyama, The Institute of Applied Energy, Shinbashi SY BLDG. 14-2, Nishishinbashi 1-chome, Minato-ku, Tokyo, 105-0003, Japan. E-mail: moriyama@iae.or.jp

Upgrading of Low Rank Coal as Coal Water Slurry and Its Utilization ––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– Ryo Moriyama, Shohei Takeda, and Yukuo Katayama In this paper, a new brown coal utilization process, combining the techniques of coal/water slurry (CWS) production and CWS in-line vaporization, was proposed. In the process, brown coal was converted to CWS with high-pressure kneader under mechanical shear stress and moderate heating. Water in obtained CWS was vaporized with special designed pre-heater, termed in-line vaporizer, processing about 2.0 t/day (dry coal base). Indonesian brown coal containing 22% water and Australian brown coal containing 59% water were successfully converted to the CWS. CWS of Australian brown coal was pre-heated with the in-line vaporizer. The CWS was converted to coal/steam mixture and directly burned in a combustion furnace. Keywords Brown coal; Low rank coal; Coal water slurry; CWS; CWM INTRODUCTION

It is known that Australian brown coals, such as Loy Yang and Yallourn, have very high

moisture content from 60 to 70%. When these brown coals are utilized for coal combustion or a coal gasification process, the thermal efficiency of the process significantly decreases as a result of the water evaporation. More CO2 is emitted and lager volume of a combustion chamber is required based on the heating value of the brown coals. The brown coals must be used at a power plant adjacent to an open cut mine since the high moisture raises calorie-based cost of transport. Furthermore, the spontaneous combustibility of the brown coals also restricts their transport. Inexpensive and safe drying technologies are required for utilization of the brown coals.

Several coal-dewatering technologies have been developed for effective utilization of the brown coals [1-8]. Hydrothermal Dewatering (HTD) process is one of the coal-dewatering technologies [1-6]. In the process, brown coal containing 62% moisture is mixed with water and is converted to coal/water slurry (CWS) of 75% moisture. The CWS is heated to around 570 K under a sufficient pressure to prevent evaporation of water. After the cooling and

Upgrading of Low Rank Coal

depressurizing processes, excess liquid water is separated from the CWS. The product is pumpable CWS containing 50% moisture and can be fired in a conventional boiler. During the dewatering process, the latent heat of water evaporation is not expended since the water is removed as a liquid. Water-soluble inorganic compounds are also removed with the water.

Mechanical Thermal Expression (MTE) process is being developed [1,7,8]. In the process, water is expressed from the brown coal by means of mechanical pressure (4 to 6 MPa) and moderate heating (450 to 475 K) with comparatively low energy consumption. The mechanical energy required to remove the water from the brown coal is less than 1% of the thermal energy required for an equivalent drying by evaporation. However, wastewater treatment raises processing costs for HTD and MTE processes. Furthermore, products obtained with HTD and MTE processes still contain 50% and below 25% moisture, respectively.

KEM Corporation developed a new process, which converts the brown coals to CWS. In this process, the brown coal is subjected to moderate heating (< 525 K) and high shear stress. By removing a part of water during the process, the heating value of the CWS will be equivalent to that of conventional CWS from bituminous coal.

From an energy point of view, coal gasification or combustion process using CWS as feed has a disadvantage in the heat required for the processing. Considerable amount of coal burns in order to compensate the latent heat of water evaporation, and this results in the generation of more CO2. For many years, increasing the thermal efficiency of the coal gasification or combustion with the pre-heating of CWS has been challenged.

Miyatani [9] proposed heat recovery system of a coal gasifier. Heat recovered from the system can be applied to the heat for evaporation of water in CWS. Roffe et al. [10] tried to evaporate water in CWS with pre-heating and found line plugging in a pre-heater using CWS of 40-60 wt.%-coal as feed. Usui at al. [11] also found that CWS of 65 wt.% coal caused line plugging in a pre-heater at a temperature range of 423 to 433 K due to increase in the viscosity of the CWS. They concluded that the combined effect of the swelling of coal and the change of surface condition of coal was mainly responsible for the increase in the viscosity. Novack et al. [12] reported CWS vaporizer development with useful experimental observations and conclusions. However, there is no commercialized process of the CWS vaporizer, and direct injection of CWS without vaporization is commonly used.

The authors designed the CWS pre-heater, termed CWS in-line vaporizer, which accommodates CWS pressure and flow change as well as phase change in the same transfer line leading the feed from a CWS tank to a gasifier [13]. In the present paper, the authors propose a brown coal utilization process combining the CWS production process licensed by KEM Corporation and the CWS in-line vaporizer. EXPERIMENTAL

Upgrading of Low Rank Coal

Sample

Indonesian (ID) and Australian (AU) brown coals were used as samples, whose properties are shown in Table 1. Commonly, commercialized CWS contains moisture over 30%. This content is greater than the moisture of ID coal while less than that of AU coal. Considering CWS produced from the brown coals, a known amount of water should be added into ID coal and be removed from AU coal. Diameter of AU coal was greater than that of ID coal. Mass mean diameter of AU coal was 0.46 mm. CWS production process

Three kneaders of different performance and one autoclave were used for CWS production experiment. Their names and performances were listed in Table 2.

The experimental kneader processing 2.5 kg was used for CWS production. The kneader was equipped with a jacket filled with hot oil around the pressure vessel, of which the design pressure was 1.0 MPa-G. The kneader had two sigma-shaped blades that gave mechanical shear stress of < 0.2 MPa to the sample.

2.5 kg of the sample was heated in the closed vessel with kneading. During the heating, the pressure in the vessel went up due to the increase in the vapor pressure of water. When the temperature of the sample reached 457 K, the pressure reached 0.98 MPa-G. After maintaining the pressure for 1 or 5 hours, the vessel was cooled down to an ambient temperature, and product slurry was collected. The obtained slurries from ID coal and AU coal were termed ID slurry and AU slurry, respectively.

In order to investigate the effect of shear stress on the CWS production, AU coal was processed with the autoclave and the high-shear kneader. The autoclave equipped with a stirrer, which gave little shear stress to the sample. AU coal introduced into the autoclave was heated to 457 K, and the temperature was maintained for 1 h. Sample collected after the cooling process was termed AU-Test. The high-shear kneader had a specific shaft [14] with a special designed stir wing screws and blades to give shear stress of < 1.0 MPa to the sample. Photograph of high-shear kneader is shown in Figure 1.

The large-scale kneader with the capacity of about 200 kg was used to produce a large amount of AU slurry for experiment using CWS in-line vaporizer. The kneader also had a specific shaft and gave shear stress of < 1.0 MPa to the sample without an external heater and a pressurization system. 180 kg of AU coal was introduced into the kneader and was kneaded for 4.5 h. Characterization of original coals and obtained slurries

Original coals and obtained slurries were subjected to several instrumental analyses. The

Upgrading of Low Rank Coal

viscosities of the obtained slurries were measured with a rotary viscometer (shear rate: 6.7–1008 s-1) at room temperature. Distributions of types of water for AU coal, AU slurry and AU-Test were measured with a 1H-NMR spectrometer (JEOL: Mu-25) on the basis of freezing properties of water [15]. About 1 g of sample was inserted to 8 mm I.D. NMR sample tube. One pulse method was used to obtain the entire free induction decays (FID). The FID was measured at temperatures of 296, 268, and 223 K. The signal at each temperature was recorded under temperature equilibration.

Pore size distribution of dried AU coal and dried AU slurry were measured by argon gas adsorption and desorption at 78 K. AU coal and AU slurry were dried at 333 K under the vacuum condition for 1 day and subjected to the measurement. CWS in-line vaporizer

The CWS in-line vaporizer processing about 2.0 t/day (dry coal base) [13] with a combustion furnace is schematically shown in Figure 2. This apparatus was mainly composed of a CWS tank, a high-pressure pump and three heaters. Temperature of heating oil and flow rate of slurry were 570–585 K and 100–140 L/h, respectively. AU slurry was heated through a feed tube soaked in heating oil in the three heaters, termed 1st, 2nd or 3rd heater. Diameters and lengths of the feed tube were determined in such a manner to steadily heat up the feed slurry and to completely vaporize the water in the slurry. The coal/steam mixture was directly introduced into a combustion furnace and burned. Prior to the CWS feed, the temperature in the furnace was maintained with a fuel burner. An exhaust line was prepared for start-up with vaporization of water, monitoring exhaust of coal/steam mixture, and sampling of dried coal particles.

The inner diameters of the feed tubes were 6 mm at the 1st heater, 2 to 8 mm at the 2nd heater, and 10 mm at the 3rd heater. At the 1st heater, CWS was preheated to the temperature about 10 K below that of the heating oil under high pressure in order to maintain slurry phase without vaporization. The length of the feed tube in the 1st heater was about 70 m, that is, the residence time of the CWS in the heater was calculated to be 80 sec. At the 2nd heater, a part of water in CWM evaporated. The inner diameters of tubes discreetly increased so as to maintain the velocity in a range to cause atomizing and preventing the tubes from erosion. Finally, the rest of water was evaporated and was super heated in the 3rd heater. Total length of tubes in the 2nd and the 3rd heaters is 47.9 m, that is, the residence time of the CWS in the heaters was calculated to be less than 10 sec. The temperatures and the pressures at the outlet of each heater were measured in order to estimate the state of the fluid and analyze them from fluid dynamics point of view. RESULTS

Upgrading of Low Rank Coal

Production of the slurries from brown coals

ID slurry was produced with the experimental kneader. 1.1 kg of water was added to 2.0 kg of ID coal and then total moisture of the mixture became about 50%. Figure 3a shows photographs of the CWS production for ID coal at the start and the end of the operation. The pumpable ID slurry with low viscosity was obtained by this operation.

AU coal was also kneaded with the experimental kneader. In this case, no water was added since AU coal had high moisture content. The slurry was obtained as shown in Figure 3b.

In both cases, any surfactant was not added. Therefore, stability of the slurry was observed by a precipitation test. The slurry was left at rest in a vessel. Precipitation was not found 2 or more days after the experiment for both slurries. This characteristic might be attributed to the hydrophilic property of the brown coals and/or the effect of organic acid dissolved in the liquid phase.

The AU slurry was also produced with the large-scale kneader. About 180 kg of sample was introduced into the kneader and was kneaded. Despite low temperature and atmospheric pressure, viscous AU slurry was obtained by this operation. Properties of the slurries obtained from brown coals

The viscosities of obtained slurries were measured with a rotary viscometer at room temperature. AU-Test obtained with the autoclave was too hard to measure the viscosity. It seemed to be a kind of gel. Plots of flow curves for slurries are shown in Figure 4. In the figure, shear stress linearly increases with increasing shear rate in a range of small shear rate while the slope lowers with increasing shear rate. This behavior is typical characteristic of the pseudo-plastic fluid, which is one of the non-Newtonian fluids. Apparent viscosity of AU slurry was relatively higher than that of ID slurry. This difference might be caused by the difference in the particle size of coal particles in the slurry.

Apparent viscosity of AU slurry processed with experimental kneader for 1 h is an order of magnitude higher than that for 5 h. This result implies that processing time is one of the key factors for producing slurry with low viscosity. Flow curve of AU slurry processed with high-shear kneader for 1 h is similar to that processed with experimental kneader for 5 h. This result suggests that increase of shear stress shorten experimental period. Furthermore, effect of particle size was investigated. Coarse particles with size larger than 0.84 mm were removed from AU slurry. Apparent viscosity of the slurry was considerably decreased by means of removal of coarse particles.

Distributions of types of water for AU coal and AU slurry were measured using 1H-NMR spectroscope on the basis of freezing properties of water. 1H-NMR relaxation measurement was performed at 296, 268, and 223 K. In Figure 5, FID intensities for AU coal at 296, 268, and 223 K are plotted versus decay times. FID intensity at decay time zero corresponds to the

Upgrading of Low Rank Coal

total amount of 1H in the sample while slope of the intensity corresponds to molecular mobility of 1H. Slope of FID intensity seems to be composed of two or more components. The slowly decaying component appeared after 30 µs for each temperature is described by an exponential function and it corresponds to 1H in water. If a part of water freezes, the intensity of this component decreases. Therefore, the authors defined FID intensity at decay time 40 µs to be relative amount of water in the sample. From above analysis, water was classified on the basis of freezing properties of water [15].

Figure 6 shows the distributions for AU coal, AU slurry and AU-Test. AU slurry and AU-Test were obtained with experimental kneader operated for 1 h and with autoclave, respectively. Free water is identical to bulk water that freezes at around 273 K, bound water is defined as pore condensed water that freezes at a temperature range of 273 to 223 K, and non-freezable water does not freeze even at 223 K. The comparison demonstrates that the CWS production process decreases amount of non-freezable water and increases amount of free water. Fluidity of AU coal increases with increasing free water content. From the above results, both heat and shear stress are found to be important factors for producing pumpable slurry with low viscosity.

Figure 7 shows pore size distributions for dried AU coal and dried AU slurry. The pore volume with 5-20 nm of pore radii was decreased and that with smaller than 5 nm was increased by the CWS production process. Destruction of coal pores might cause restriction of removed water from recovery.

Preheating and combustion of CWS obtained from AU coal

The CWS in-line vaporizer shown in Figure 2 was used for pre-heating of AU slurry. The slurry produced by the large-scale kneader contained about 10% of particles with diameter of >0.5 mm. Water was added and large coal particles were removed from the slurry by precipitation before the experiment since the minimum inner diameter of the feed tube was 2 mm. The water content of the slurry was about 75% and apparent viscosity of the slurry was less than 1000 mPa･s.

The AU slurry was pre-heated and exhausted into atmosphere for several minutes in order to monitor the condition of the exhaust and to measure the water content of exhausted coal particles. The temperature of heating oil and the flow rate of sample were 573 K and 120 L/h, respectively. Figure 8 shows the exhaust of steam and steam/coal mixture. The moisture in obtained coal particles was 5-6 wt.%. Water in the slurry was completely evaporated with this vaporizer.

The AU slurry was pre-heated and directly introduced into the combustion furnace. At the experiment, the temperature of heating oil and the flow rate of sample were 583 K and 120 L/h, respectively. The log data at the experiment is shown in Figure 9. The AU slurry was steadily fed into the combustion furnace for 90 min without plugging of the feed tube. The

Upgrading of Low Rank Coal

experiment was terminated because 200 L of sample in the CWS tank was completely fed. From read out the log data, followings were found:

1. Fluid was in a solid/liquid (slurry) phase at the outlet of the 1st heater since the pressure and the temperature at the outlet of the heater were 8.5 MPa and 558 K, respectively. (vapor pressure of water is 8.5 MPa, at 573 K)

2. Fluid was in a solid/liquid/gas phase at the outlet of the 2nd heater since the temperature at the outlet of the heater was 473 K and pressure of the outlet was close to the vapor pressure of water.

3. Fluid was in a solid/gas phase at the outlet of the 3rd heater since the pressure and the temperature at the outlet of the heater were 0.8 MPa and > 473 K, respectively. (vapor pressure of water is 0.8 MPa, at 449 K)

The temperature in the combustion furnace increased from 803 to 873 K and the consumption of the fuel oil decreased with feeding the AU slurry. This means that the AU slurry steadily burned in the furnace. SUMMARY

From the above results, AU coal was converted to slurry using their own moisture and without any surfactant. Obtained slurry was pumpable. Water in the slurry was completely evaporated with the in-line vaporizer, and coal in the slurry burned in the combustion furnace during 1.5 h of experimental period. ACKNOWLEDGMENTS

The research and development of CWS vaporizer are a part of technological project, termed Production of Methanol from Coal, Natural Gas and Carbon Dioxide using Solar Energy, funded by Ministry of Economy, Trade and Industry (METI) in Japan. Production and test of preheating of CWS were carried out in collaboration with Hokkaido Center of International Institute of Advanced Industrial Science & Technology in Japan. The experiment of large-scale kneader was performed with Japan Systematization Laboratory and KEM Corporation. Preheating and combustion of CWS were carried out with the cooperation of Tokai-Pulp & Paper Co., Ltd. REFERENCES 1. D. J. Allardice and B. C. Young, Utilisation of low rank coals, 18th Annual International

Pittsburgh Coal Conference (2001). 2. G. Favas and W. R. Jackson, Hydrothermal dewatering of lower rank coals. 1. Effects of

process, Fuel, vol. 82, pp. 53–57 (2003).

Upgrading of Low Rank Coal

3. G. Favas and W. R. Jackson, Hydrothermal dewatering of lower rank coals. 2. Effects of coal characteristics for a range of Australian and international coals, Fuel, vol. 82, pp. 59–69 (2003).

4. G. Favas, W. R. Jackson and M. Marshall, Hydrothermal dewatering of lower rank coals. 3. High-concentration slurries from hydrothermally treated lower rank coals, Fuel, vol. 82, pp. 71–79 (2003).

5. L. Racovails L, M. D. Hobday and S. Hodges, Effect of processing conditions on organics in wastewater from hydrothermal dewatering of low-rank coal, Fuel, vol. 81, pp. 1369–1378 (2002).

6. D. J. Allardice, L. M. Clemow, G. Favas, W. R. Jackson, M. Marshall and R. Sakurovs, The characterization of different forms of water in low rank coals and some hydrothermally dried products, Fuel, vol. 82, pp. 661–667 (2003).

7. C. Bergins, C. Kinetics and mechanism during mechanical thermal dewatering of lignite, Fuel, vol. 82, pp. 355–364 (2003).

8. C. Bergins, Mechanical/thermal dewatering of lignite. Part 2: A rheological model for consolidation and creep process, Fuel, vol. 83, pp. 267–276 (2004).

9. K. Miyatani, Heat recovery technique on entrained gasifier, J. Fuel Soc. Japan, vol. 62, pp. 874-881 (1983).

10. G. Roffe and G. Miller, Thermal preconditioning of coal/water mixtures for gas turbine applications, ASME Paper, 85-GT (1985).

11. H. Usui, Y. Yamasaki and Y. Sano, Heat transfer of coal-water mixtures in a round tube flow, J. Chem. Eng. Japan, vol. 20, pp. 65–70 (1987).

12. M. Novack, G. Roffe and G. Miller, Combustion of coal/water mixtures with thermal preconditioning, ASME Paper, 87-GT (1987).

13. R. Moriyama, K. Aiuchi, S. Takeda, S. Kitada, M. Onozaki and Y. Katayama, Preheating feed of coal-water mixture in green fuel production, 12th International Conference on Coal Science (2003).

14. T. Inoue, Japanese patent No. P2000-169274A, 2000. 15. K. Norinaga, Ji. Hayashi, N. Kudo and T. Chiba, Evaluation of effect of predrying on the

porous structure of water-swollen coal based on the freezing property of pore condensed water, Energy Fuels, vol. 13, pp. 1058–1066 (1999).

TABLE 1 Properties of raw materials

wt.% (dry basis)

Moist.

(wt.%)*

Particle size

(mm) Ash C H O N S

Indonesian

(ID) Coal 22.2 < 0.25 6.34 66.03 5.19 20.59 1.37 0.39

Australian

(AU) Coal 59.0 < 2.0 0.90 67.28 4.97 25.34 0.60 0.28

* as received

TABLE 2 Performance of facilities

Sample amount

(kg/batch)

Pressure

(MPa-G)

Temperature

(K)

Shear Stress

(MPa)

Autoclave 0.33 10.0 570 ‒

Experimental kneader 2.5 1.0 460 < 0.2

Large-scale kneader 200 0.0 ‒ < 1.0

High-shear kneader 20 3.0 500 < 1.0

FIGURE 1. Photograph of high-shear kneader.

1. CWS tank

2. Pump

3. Water pump

4. High-pressure pump

5. 1st heater

6. 2nd heater

7. 3rd heater

8. Let down valve

9. Combustion furnace

10. Exhaust line

11. Oil heater

FIGURE 2. Schematic diagram of CWS in-line vaporizer.

CWS+Steam

CWS

Dry Coal+ Steam

1

23

4

5 6 7

89

10

11

(a) Indonesian (ID) coal

(b) Australian (AU) coal

Start End

Start End

FIGURE 3. Photographs of CWS production process.

(a) ID coal; (b) AU coal .

AU/1h/exp. AU/5h/exp. ID/5h/exp. AU/1h/high AU/1h/high/mod.

Coal/Period/Apparatus

0 200 400 600 800 10000

200

400

600

800

1000

1200

FIGURE 4. Flow curves for obtained slurries.

Shea

r stre

ss [P

a]

Shear rate [s-1]

0 50 100 150 2000.1

1

FIGURE 5. Free induction decay (FID) curves for AU coal obtained at 296, 268 and 223 K.

Inte

nsity

[A.U

.]

Decay time [µs]

296 K 268 K 223 K

AU coal

AU-Test

AU slurry

0.0 0.5 1.0 1.5

FIGURE 6. Distribution of types of water in AU coal and its derivatives.

Water content [g-water/g-dry coal]

Free water Bound water Non-freezable water

0.1 1 10 1000.0

0.5

1.0

1.5

2.0

2.5

3.0

FIGURE 7. Pore size distributions for dried AU coal and AU slurry

Po

re v

olum

e [1

0-3 c

m3 /g

]

Pore radius [nm]

AU coal AU slurry

FIGURE 8. Exhaust of steam/coal mixture.

Steam exhaust Steam/coal exhaust

0 20 40 60 80 100 1200

2

4

6

8

10

12

Pres

sure

[MPa

-G]

350

400

450

500

550

600

Picked up

Operation period

Tem

pera

ture

[K]

Outlet of 1st Outlet of 2nd Outlet of 3rd

25 30 35 40

FIGURE 9. Log data of in-line vaporizer during feeding of AU slurry for 100 min.

Time [min]

Outlet of 1st Outlet of 2nd Outlet of 3rd Inlet of burner