Experience with the Gasification of Low‐ Grade Coal; A Case Study of Continuously Changing Temperatu

www.ijesci.org International Journal of Energy Science (IJES), Volume 6 Issue 1, 2016 doi: 10.14355/ijes.2016.0601.04

Experience with the Gasification of Low‐ Grade Coal; A Case Study of Continuously Changing Temperature inside Gasifier Han S. Uhma*, Young H. Naa, Yong C. Hongb, Dong H. Shinb, Chang H. Chob and Young K. Parkc Department of Electrical and Biological Physics, Kwangwoon University, 447‐1 Wolgye‐Dong, Nowon‐Gu, Seoul 139‐701, Republic of Korea a

Convergence Plasma Research Center, National Fusion Research Institute, 113 Gwahangno, Yuseong‐Gu, Daejeon, 305‐333, Republic of Korea b

Wintech, 5th floor, Daewoo Building, Bangi‐Dong, Songpa‐Gu, Seoul 138‐827 Republic of Korea

c

Corresponding Authors: Han‐Sup Uhm, Ph.D. Department of Electrical and Biological Physics, Plasma Bioscience Research Center, Kwangwoon University, Office) 82‐2‐940‐8374, Fax) 82‐2‐940‐5664 hsuhm@kw.ac.kr

Abstract Indonesian brown‐coal with high ash content is gasified by two microwave steam‐plasmas heating up a reaction chamber of 1145 liters in a swirl‐type gasifier for production of hydrogen‐rich synthetic gas. With additional heating of the gasifier by a partial oxidation of coal, the inner temperature of the gasifier can be increased from 1100C to 1700C. In this regard, the influence of the gasifier temperature on the gasification efficiency can be investigated in this experiment. The carbon conversion rate and cold gas efficiency are less than 90 percent and 65 percent, respectively, for the inner temperature of the gasifier below 1400C. On the other hand, the carbon conversion rate at the chamber temperature of 1600C is almost 100 percent, ensuring a complete gasification of carbons in a low‐grade coal. The cold gas efficiency of the hydrogen‐rich synthetic gas at the high inner temperature of the gasifier wall is 84%, very high in a relatively‐small gasifier like the experiment here. The total calorific power of the synthetic gas can be easily more than 500kW in this particular experiment.

Keywords Gasification, Low‐Grade Coal, Steam Plasma, Microwave Torch

Introduction Electricity is the most important and convenient energy source of mankind provided from the electrical power plant, which can operated by various energies including natural gas, nucleus, coals, biomass [1‐3], etc. However, most of the electrical power plants in the world operated by the high‐grade coals with an ash content of less than 12 percent are expensive. The conventional coal‐power plants emit an unacceptable level of pollutants and carbon dioxide into the atmosphere. In this regard, over the last few decades, the integrated gasification combined cycle (IGCC) [4–6] has emerged as the best available technology with which to utilize coal efficiently and environmentally‐friendly, as this method meets emission limits, which is not achievable by other means. A pure steam torch [7,8] generated by microwave power has gained attention lately, as this type of torch can serve as a heat source for environmental cleanup activities and due to its potential for use in the production of renewable types of energy. Therefore, coal gasification by microwave steam plasma was also conducted at a table‐top scale [9‐11], showing the possibility of efficient gasification at this scale. Additionally, a very‐high‐power microwave plasma torch which makes use of 915 MHz microwaves has been recently developed, producing an air torch flame with a diameter of 8 cm and a length of 120 cm [12]. The conventional coal gasification methods mentioned IGCC are typically carried out at a gas temperature of less than 1400C due to the limited partial oxidation of coal.

In order to investigate the influence of the gasifier‐wall temperature on the gasification of low‐grade coal, we used microwave steam plasma to heat the coal powder, with additional auxiliary heating from the partial oxidation of coal in a reaction chamber with a volume of 1145 liters. Because two heating methods (plasma and oxidation) are

36

International Journal of Energy Science (IJES), Volume 6 Issue 1, 2016 www.ijesci.org

used, the inner‐wall temperature in the gasifier can easily increase from 1200C to 1700C without a reduction in the gasification efficiency at a high temperature. The coal used for the gasification process in this study is Indonesian brown coal with a calorific value of 4640 kcal/kg and a total ash and moisture content of 33.17 percent.

The carbon conversion rate and cold gas efficiency are less than 90 percent and 65 percent, respectively, for the inner temperature of the gasifier below 1400C. On the other hand, the carbon conversion rate at the chamber temperature of 1600C is almost 100 percent, ensuring a complete gasification of carbons in a low‐grade coal. The cold gas efficiency of the hydrogen‐rich synthetic gas at the high wall‐temperature is 84%, very high in a relatively‐ small gasifier like the experiment here. The total calorific power of the synthetic gas can be easily more than 500kW in this particular experiment.

Apparatus, Methods and Materials The initial plan for the gasification device is a fabrication of a gasifier capable of producing a synthetic gas with thermal power of 500kW. Therefore, the estimated gasification parameters [13] of coal powder for 500 kW of thermal energy are as follows: (1) the gasified fuel in the composition is 40 percent hydrogen, 30 percent carbon monoxide, 10 percent nitrogen, and 20 percent carbon dioxide in this sample calculation; (2) the inner chamber size is 1 lube (m3) or more; (3) the required gasified fuel amount is L = 3500 lpm; (4) the required reforming energy is 90kW; (5) the oxygen requirement for partial oxidation is 342 lpm; (6) the water requirement for reforming is 1.1kg/min or 60kg/h; and (7) the coal powder required is 90 kg/hr.

Based on the estimation of the gasification parameters as outlined above, a 500 kW gasification system was fabricated. Two microwave systems with a maximum power of 75 kW operated at 915 MHz were procured and installed into a swirl‐type gasifier. The characteristics of the microwave plasma system were well described in the previous literature [13]. One microwave plasma system is installed at the upper part and the other is at the lower part of the gasifier. Figure 1 shows (a) the gasifier (at the upper part of the image) and (b) the installation of the microwave systems into the gasifier. The inner configuration of the swirl‐type gasifier is cylindrical with a diameter of 90 cm and a height of 180 cm. The inner volume of the gasifier is 1145 liters.The inner surface of the gasifier consists of a fire‐resistant ceramic material called HACT180, which can sustain temperatures up to 1800C. The next layer is an insulating cement called INCT120. Five thermometers are vertically installed along the inner surface of the gasifier to monitor the inner‐surface temperature. The plasma from the microwave system at the upper part of the gasifier enters the inner chamber of the gasifier sideways and slightly downward, creating a swirling plasma flame along the inner wall. Meanwhile, the plasma from the microwave system at the lower part enters the inner chamber of the gasifier sideways.

(a) (b) FIGURE 1 (COLOR ONLINE). IMAGES OF (a) THE GASIFIER AT THE UPPER PART OF THE PICTURE AND (b) THE INSTALLATION OF MICROWAVE SYSTEMS ONTO THE GASIFIER.

37

www.ijesci.org International Journal of Energy Science (IJES), Volume 6 Issue 1, 2016

Figure 2 shows an image of the overall gasification system, which is similar to the photo image in the previous literature [14]. One of the important issues pertaining to coal gasification by steam plasma is a steady supply of coal powder into the gasifier. The coal‐powder supply system in Fig. 2 provides coal powder into the gasifier by blown air. The synthesized gas from the gasifier is discharged through the synthetic gas exit shown in Fig. 2. The temperature of the synthetic gas is monitored at the bent point of the synthetic gas exit. The ash and molten rock are discharged into an ash tank located under the gasifier in Fig. 2. The coal‐powder storage tank, also shown in Fig. 2, contains coal powder, which is carried into the coal‐powder supply system by air blown from air compressors. Steam boilers supply hot steam into the microwave system as a swirl gas and additional steam into the gasifier. The substantial gasification of coal occurs at a gas temperature that can exceed 1200C. Therefore, it is necessary to preheat the inner temperature of the gasifier. Plasma flames alone may take a long time to preheat the gasifier; therefore, a kerosene burner for preheating is installed at the lower part of the gasifier. The kerosene burner and plasma flames preheat the gasifier for about one and a half hours to 1050C, after which the kerosene burner is turned off. The coal powder is then injected into the gasifier while the inner temperature is monitored.

FIGURE 2 (COLOR ONLINE). A PICTURE OF THE OVERALL GASIFICATION SYSTEM.

(a) (b)

FIGURE 3 (COLOR ONLINE). THE TEMPERATURE OF THE INNER WALL IS PLOTTED IN TERMS OF THE MEASUREMENT TIME. FIVE THERMOMETERS ARE VERTICALLY INSTALLED ALONG THE INNER SURFACE OF THE GASIFIER TO MONITOR THE INNER‐ SURFACE TEMPERATURE. USUALLY, THE TEMPERATURE AS RECORDED BY THE THERMOMETERS AT THE LOWER PART IS HIGHER THAN THAT AT THE UPPER PART.

The thermometers installed inside the gasifier monitor the inner‐wall temperature continuously after the preheating process. The inner‐wall temperature of the gasifier increases due to continuous input of coal powder

38

International Journal of Energy Science (IJES), Volume 6 Issue 1, 2016 www.ijesci.org

and oxygen, and the continuous operation of the microwave plasma. The thermometers are installed along the inner surface from the lower part to the upper part of the gasifier. The temperature of the inner wall is plotted in Fig. 3a in terms of the measurement time. Usually, the temperature as recorded by the thermometers labeled by 1 and 2 at the lower part is higher than that at the upper part labeled by 5. The temperature of the gas fluid element may decrease as it moves from the bottom of the gasifier to the top. Particularly, the temperature of the exiting synthetic gas from the gasifier is less than 1000C despite the very high wall temperature in the measurement time range of 500 to 650 min. A large fluctuation of the temperature at the beginning was caused by the unsteady input of the coal powder. The inner temperature labeled by 2 of the wall exceeded 1720C at the measurement time of t = 620 min, indicating possible damage to the inner wall. Therefore, the oxygen input is reduced to cool it down. We found later that there was no damage to the wall. Average temperature of five thermometers is also shown in Fig. 3b for clarity of inside temperature.

After the preheating of the gasifier, the upper and lower plasma flames were powered by 40 kW with 26 kg/hr of steam as a swirl gas and by 30 kW with 14 kg/hr of steam, respectively. An additional 20 kg/hr of steam enters the upper part of the gasifier. Indonesian brown coal at an injection rate of 90 kg/hr is injected into the gasifier through an injection port near the upper plasma flame. Fine coal powder with an average particle size of 70 m delivered in a waterproof bag is used in the experiment. An analysis of the Indonesian brown coal powder was presented in the previous literature [14], which shows that the ash and moisture content is 33.17 % and carbon content is 59.23 %.

The water content of 10.71 percent during the 90 kg/hr of coal injection means an additional water injection of 9.6kg/hr. Therefore, the total water injected into the gasifier is 69.6kg/hr, which is estimated to be 1444 lpm at room temperature. 400 lpm of air from the air compressors is used to blow the coal powder into the gasifier. The nitrogen amount is 320 lpm in this blown air and the oxygen amount is 80 lpm. An additional 330 lpm of oxygen is injected into the gasifier for the partial oxidation of coal. The total oxygen input from the water, air and the additional oxygen injection is calculated to be 1132 lpm at room temperature. The carbon content of 90 kg/hr of coal powders injected into the gasifier is 90 kg/hr  0.5923 = 53.3 kg/hr = 1.234 mole/s, which means that 1.234 mole of carbon enters the gasifier every second.

The synthetic gas was sampled at the synthetic gas exit and was analyzed through gas chromatography, which has error about of 1 percent for CO, CO2, O2 and CH4 measurements. However, this gas analyzer has error that can reach 3 percent for hydrogen measurements, and H2O and N2 are not measureable with this type of chromatography. Figure 4 shows plots of the relative concentrations of the synthesized gas species of H2, CO and CO2 in terms of the measurement time. Oxygen (O2) and methane (CH4) in synthetic gas are neglected because their concentrations are much less than 0.5 percent. The concentration of carbon monoxide is much higher than that of carbon dioxide at a high gas‐temperature, as expected. Based on the inner‐wall temperature, Figure 4 indicates that the concentration of the flammable gases (H2 and CO) is about 60 percent for all temperature range in this measurement time.

FIGURE 4 (COLOR ONLINE). THE RELATIVE CONCENTRATIONS OF GAS SPECIES IN THE SYNTHETIC GAS VERSUS THE MEASUREMENT TIME, SHOWING THE RELATIVE CONCENTRATIONS OF H2, CO, CO2, N2, AND H2O.

39

www.ijesci.org International Journal of Energy Science (IJES), Volume 6 Issue 1, 2016 Results and Discussions The 320 lpm of nitrogen gas entering the gasifier will exit without any loss or increase. Therefore, the total flow rate FT of the synthetic gas is related to the relative concentration n of the nitrogen gas as FT = 320/n lpm. The total oxygen input of 1132 lpm may exit the gasifier in the form of CO, CO2 and H2O, whose relative concentrations are denoted by cm, cd and w, respectively. The oxygen flow rate due to the exit of CO, CO2 and H2O is (1/2)cmFT + cdFT +(1/2)wFT, which will be balanced with the total oxygen input of 1132 lpm. Thus, the balanced equation for oxygen gas is expressed as 160(cm + 2cd + w) = 1132n. We also recognize that the sum of the all relative concentrations is unity as h + cm + cd + w + n = 1, where h is the relative concentration of hydrogen gas. We therefore eventually obtain the relative concentration of nitrogen in synthetic gas as n = 0.1238(1 + cd – h) in terms of hydrogen and carbon dioxide concentrations of cd and h, as presented in Fig. 4 for the entire operational time period. Once the nitrogen concentration n is known, the water concentration w can immediately be found from above equations which is also presented in Fig. 4.

The total flow rate of the synthetic gas is calculated from FT = 320/n = 2585/(1 + cd – h) in units of lpm and plotted in Fig. 5. The H2 and CO flow‐rates for total flow rate of FT lpm are hFT lpm and cmFT lpm, which can be translated into the calorific power of synthetic gas by making use of the emission energy for hydrogen and carbon monoxide given by 285.8kJ/mol and 283kJ/mol, respectively. The power of the synthetic gas is also plotted in Fig. 5 in units of kW. The time profiles of the power (kW) and flow rate (lpm) of the synthetic gas are very similar to each other. The fluctuation of the total gas flow FT and synthetic gas power in Fig. 5 may be caused by unsteady supply of the coal powders into the gasifier, although the coal powder supply was most cautiously carried out. Nevertheless, comparing Fig. 5 with Fig. 3, we can find that the power and flow rate of the synthetic gas increase as the inner‐ wall temperature of the gasifier increases.

The calorific power of the coal is calculated to be 487.2 kW for 90 kg/hr injection. The microwave power is 70kW. 60 kg/hr of steam entering into the gasifier as the swirl gas through the microwave plasma and additional steam. One gram of steam has the latent heat of 539 cal. Thus, the latent heat of 60 kg/hr of steam is calculated to be 38 kW. Therefore, the total input energy entering into the gasifier is 595 kW. The cold gas efficiency  at a high heating value is calculated, dividing the synthetic gas power in Fig. 5 by the total input power of 595kW. The carbon mole fraction in the synthetic gas per unit time could be calculated from Fig. 4 by (cm + cd)FT/22.4/60. Dividing the mole fraction of CO and CO2 in the synthetic gas by total carbon input of 1.234 mole/s, we can also calculate the carbon conversion rate R. The fluctuation of coal powder injection causes a severe difficulty in evaluation of the efficiency and conversion rate. In order to find a meaningful value, we averaged these values about 25 minutes around the measurement time. Shown in Fig. 6 are plots of the cold gas efficiency  and the carbon conversion rate R versus the measurement time t. We observed from the comparison of Fig. 6 with Fig. 3 that the cold gas efficiency of the gasifier increases from  = 0.65 to  = 0.82 after the inner‐wall temperature of the gasifier increases from about 1400C to 1600C during the time from t = 450 minute to t = 500 minute. The inner temperature of the gasifer plays a critical role in the gasification efficiency. Carbon conversion rate also increases drastically from R = 90% to almost R = 100% during this time interval. A major contribution to high gasification efficiency is the carbon dioxide reduction and carbon monoxide increase at high wall temperature of the gasifier in Fig. 4.

FIGURE 5 (COLOR ONLINE). PLOTS OF POWER (KW) AND FLOW RATE (LPM) OF SYNTHETIC GAS VERSUS THE MEASUREMENT‐ TIME T DURING THE OPERATIONAL PERIOD OF 10 HOURS.

40

International Journal of Energy Science (IJES), Volume 6 Issue 1, 2016 www.ijesci.org

FIGURE 6 (COLOR ONLINE). PLOTS OF AVERAGED COLD GAS EFFICIENCY  AND CARBON CONVERSION RATE R OF SYNTHETIC GAS VERSUS THE MEASUREMENT‐TIME T DURING THE OPERATIONAL PERIOD OF 10 HOURS. THEIR MEAN VALUES WERE AVERAGED OVER 25 MINUTES INTERVAL AROUND THE DATA MEASUREMENT‐TIME T.

Conclusions The influence of the gasifier‐wall temperature on the gasification of low‐grade coal using a swirl‐type gasifier equipped with two sets of microwave steam plasma for heating to the 1145‐liter reaction chamber was studied.

With the additional heating of the gasifier by the partial oxidation of coal, the inner‐wall temperature in the gasifier changes from 1200C to 1700C. Indonesian brown coal with a calorific value of 4640 kcal/kg, and a total ash and moisture content of 33.17 percent was used for the gasification process. The carbon conversion rate and cold gas efficiency were less than 90 percent and 65 percent, respectively, for the inner temperature of the gasifier below 1400C. On the other hand, the carbon conversion rate at the chamber temperature of 1600C was almost 100 percent, ensuring a complete gasification of carbons in a low‐grade coal. The cold gas efficiency of the hydrogen‐ rich synthetic gas at the high wall‐temperature was 84%, very high in a relatively‐small gasifier like the experiment here. Measuring the gas temperature inside the reaction chamber is almost impossible because of its very high temperature. However, it is believed that the gas temperature inside the reaction chamber may increase as the inner‐wall temperature of the gasifier increases. In this regard, we may guess that the carbon conversion rate and cold gas efficiency may enhance as the gas temperature inside the reactor increases.

The total calorific power of the synthetic gas can be easily more than 500kW in this particular experiment.

Therefore, this gasification system may serve as a moderately‐sized power plant due to its compactness and lightweight nature. A power plant utilizing low‐grade coal could be useful in rural or sparsely populated areas without access to a national power grid.

Burning or gasification of fossil fuels including the low‐grade coal may produce carbon dioxide, contributing the carbon dioxide concentration in the atmosphere and worsening the global warming problem. There are some suggestions including carbon‐dioxide capture and storage or utilizations. However, the best solution to carbon dioxide issue is the efficient use of a fossil fuel converting it into synthetic gas and activating fuel cells for electrical power generation. A combination of synthetic gas from coal gasification and solid‐oxide fuel cell may efficiently generate electrical power, resulting in a less emission of carbon dioxide. Therefore, the gasification of low‐grade coal in this article may be the first step to the solution of the global warming issue, providing the synthetic gas to fuel cells for an efficient generation of electrical power.

ACKNOWLEDGEMENTS

This work was supported by the SRC program (20100029421). This work was also supported by the Korea Micro Energy Grid project of the Ministry of Knowledge Economy in Korea and by the Agency for Defense Development through the Chemical and Biological Defense Research Center. This work was also supported by 2013 R&D Convergence Program funded by Korea Research Council of Fundamental Science & Technology (R&D Convergence‐13‐6‐NFRI)

41

www.ijesci.org International Journal of Energy Science (IJES), Volume 6 Issue 1, 2016 REFERENCES

[1]

V. K. Singh, R. Sairam, P. L. Raviteja, A. Naresh, R. Suresh, Performance Evaluation of Biomass Cooking Devices in Household Environment with Various Solid Biomass Fuel, IJES 4 (2014) 24

[2]

E. O. Köroğlu, B. Özkaya, A. Y. Çetinkaya, Microbial Fuel Cells for Energy Recovery from Waste, IJES 4 (2014) 28

[3]

S. L. Pal1, A. P. Vanerkar, S. Satyanarayan, Acceleration of Biogas Production in Cattle Dung Digestion through Jatropha Curcas Oil Cake as an Additive, IJES 5 (2015) 17

[4]

A. J. Minchener, Coal gasification for advanced power generation, Fuel 84 (2005) 2222‐2235.

[5]

Barnes, Next generation coal gasification technology, IEA Clean Coal Center CCC/187 (2011) 1‐49.

[6]

H. Watanabe, M. Otaka, Numerical simulation of coal gasification in entrained flow coal gasifier, Fuel 85 (2006) 1935‐1943.

[7]

H. S. Uhm, J. H. Kim, Y. C. Hong, Microwave steam torch, Appl. Phys. Lett. 90 (2007) 211502.

[8]

H. S. Uhm, J. H. Kim, Y. C. Hong, Disintegration of water molecules in a steam‐plasma torch powered by microwaves , Phys. Plasmas 14 (2007) 073502.

[9]

H. S. Uhm, Y. C. Hong, D. H. Shin, B. J. Lee, Plasma‐enhanced gasification of low‐grade coal for compact power plants , Phys. Plasmas 18 (2011) 104505.

[10] Y. C. Hong, Lee, S. J. D. H. Shin, Y. J. Kim, B. J. Lee, S. Y. Cho, H. S. Chang, Syngas production from gasification of brown coal in a microwave torch plasma, Energy 47 (2012) 36‐40.

[11] D. H. Shin, Y. C. Hong, S. J. Lee, Y. J. Kim, C. H. Cho. S. H. Ma, S. M. Chun, B. J. Lee, H. S. Uhm, A pure steam microwave plasma torch: Gasification of powdered coal in the plasma, Surface & Coating Tech. 228 (2013) S520‐S523.

[12] Y. C. Hong, D. H. Shin, S. J. Lee, Y. J. Kim, B. J. Lee, H. S. Uhm, Generation of high‐power torch plasma by a 915MHz Microwave system, IEEE Trans. on Plasma Science 39 (2011) 1958‐1962.

[13] H. S. Uhm, Y. H. Na, Y. C. Hong, D. S. Shin, C. H. Cho, and Y. K. Park, High Efficiency Gasification of Low‐Grade Coals by Microwave Steam‐Plasmas, Energy and Fuel, in press (2014).

[14] H. S. Uhm, Y. H. Na, Y. C. Hong, D. S. Shin, and C. H. Cho, Production of hydrogen‐rich synthetic gas from low‐grade coals by microwave steam‐plasmas, Int. Jour. Hydrogen Energy 39 (2014) 4351.

42