High-Efficiency Gasification of Low-Grade Coal by Microwave Steam

Jul 1, 2014 - High-power steam plasma for heating the coal powders were ... up the gas temperature in a reaction chamber of 1145 L in a swirl-type gas...
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High-Efficiency Gasification of Low-Grade Coal by Microwave Steam Plasma Han Sup Uhm,*,† Young Ho Na,† Yong Cheol Hong,‡ Dong Hun Shin,‡ Chang Hyun Cho,‡ and Young Ki Park§ †

Department of Electrical and Biological Physics, Kwangwoon University, 447-1 Wolgye-Dong, Nowon-Gu, Seoul 139-701, Republic of Korea ‡ Convergence Plasma Research Center, National Fusion Research Institute, 113 Gwahangno, Yuseong-Gu, Daejeon 305-333, Republic of Korea § Wintech, Fifth Floor, Daewoo Building, Bangi-Dong, Songpa-Gu, Seoul 138-827, Republic of Korea ABSTRACT: High-power steam plasma for heating the coal powders were developed, where the magnetron power at 915 MHz was available up to 75 kW. The steam plasma itself is an impedance load, which depends on physical conditions, including the microwave power. By monitoring the minimum reflected power, the optimum injection rate of the steam and the corresponding reflected power ratio in terms of the microwave power was found, showing that most of the microwave power is absorbed by the torch plasma with a minimal reflected wave-power of less than a few percent, once the plasma torch was ignited. Indonesian brown coal with high ash content is gasified by two microwave steam plasmas heating up the gas temperature in a reaction chamber of 1145 L in a swirl-type gasifier. With additional heating of synthetic gas from a partial oxidation, the inner temperature of the gasifier can reach to 1700 °C. The carbon conversion rate at the average chamber temperature of 1640 °C is almost 100%, ensuring a complete gasification of carbon in a low-grade coal. The cold gas efficiency is 84%, very high in a relatively small gasifier like the experiment here. The total calorific power of the synthetic gas is 500 kW. 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 would be useful in rural or sparsely populated areas without access to a national power grid.

1. INTRODUCTION The gasification of coal may have started in the early 19th century, heating coal without air and producing coal gas, known then as town gas, which is rich in CH4 and which has a high calorific value of 20 kJ/L. Later, steam was applied to the heated coal, producing a gas called water gas, which is mostly composed of carbon monoxide (CO) and hydrogen (H2), releasing roughly 11 kJ/L by combustion. On the basis of high potential of CO2 capture and sequestration, power plants of Integrated Gasification Combined Cycle (IGCC)1−3 are currently operating commercially, and more are under construction. Stimulated by the new research initiative of IGCC, coal gasification research has been recently revitalized for electrical power plants,4−7 methanol and hydrogen production,8,9 high-temperature fuel cells,10 etc. The conventional coal gasification operated by partial oxidation was mostly carried out with the gas temperature less than 1400 °C. Further input of oxygen may cause efficiency reduction due to its excessive coal consumption although the gas temperature might increase, leading to operational restrictions and ending up with a moderate gasification efficiency.3 The steam plasma11,12 powered by microwaves provides abundant oxidizing radicals, including oxygen atoms and hydroxyls, which may very effectively gasify coal. In this regard, coal gasification by microwave steam plasma was conducted in tabletop scales,13−15 showing a possibility of efficient gasification. In this context, the gasification of low-grade coal by microwave steam plasma has been recently carried out and reported in a short communication.16 © 2014 American Chemical Society

This article presents an in-depth study of the gasification of low-grade coal by microwave steam plasma, discusses characteristic properties of the microwave system, and provides a systematic evaluation method of gasification performance. To increase gas temperature for efficient gasification of a low-grade coal, one has to develop microwave steam plasma for heating the coal powders. Therefore, high-power steam plasma operated by microwaves at 915 MHz was developed, where the magnetron power at 915 MHz is available up to 75 kW. The steam plasma developed by a microwave discharge in a microwave system acts like an impedance in an electrical circuit, which depends on physical conditions, including the microwave power. In this regard, the optimum value of steam injection was experimentally found in terms of the microwave power by monitoring the minimum reflected power. It was also shown later that most of the microwave power is absorbed by the torch plasma with a minimal reflected wave power of less than a few percent, once the plasma torch was ignited at the optimum value of steam injection. Indonesian brown coal with high ash content is gasified by two microwave steam plasmas heating up the gas temperature in a reaction chamber of 1145 L in a swirl-type gasifier. Making use of microwave steam plasma with additional heating of synthetic gas by a partial oxidation, the inner temperature of the gasifier can reach 1700 °C without reduction of gasification Received: March 18, 2014 Revised: June 16, 2014 Published: July 1, 2014 4402

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no ohmic heating in the microwave system except intense radiation from plasma. Table 1 presents the optimum steam rate and

efficiency at high temperature. This unique heating system may lead to high-efficiency gasification with almost complete carbon-conversion of low-grade coal.

Table 1. Optimum Injection Rate of the Steam and the Corresponding Reflected Power Ratio in Terms of the Microwave Power

2. EXPERIMENTAL APPARATUS AND METHODS 2.1. Microwave Plasma System. The atmospheric microwave plasma system17 consists of a magnetron, waveguide components (WR-975 for 915 MHz), and a microwave plasma torch. The typical power of the magnetron is a few tens of kilowatts at 915 MHz. In general, the dimensions of a system used for generating microwave plasma are determined by the operating microwave frequency. Compared with the waveguides used at 2450 and 915 MHz, as a simple example, the dimensions of the waveguide at 915 MHz are 2.7 times larger than those of 2450 MHz. Therefore, the size of the discharge tube to confine the plasma can be greatly increased by making use of a 915 MHz microwave system, eventually generating a plasma column with a large volume. Also, 915 MHz microwave generators with power capabilities up to 100 kW are available commercially. These upgrades of the plasma volume and microwave power can make the microwave plasma torch at 915 MHz a useful tool for practical applications in the areas of environmental cleanup and energy regeneration, including the gasification of hydrocarbon fuels, coal, and biomass. For this reason, we use a microwave power system operating at a frequency of 915 MHz, which generates microwave plasma in a configuration without electrodes. Particularly, a steam plasma torch can be a heat source for environmental cleanup and renewable energy production due to its containment of highly active species, such as electrons, ions, and radicals, which serve to enhance the chemical reaction rate, eliminating the need for catalysts in the material processing step. In this context, a pure steam-plasma torch was developed and reported,11,12 presenting an in-depth study of the pure steam-plasma torch and discussing its characteristic properties. The waveguide used in the microwave plasma torch was tapered to deliver the microwave power into the discharge tube effectively. The propagation of microwaves through a waveguide may depend on the mode structure, categorized as transverse electromagnetic (TEM), transverse electric (TE), and transverse magnetic (TM) waves. According to a simple simulation study,18 the TE10 mode is the dominant mode in a rectangular waveguide; it also has the lowest attenuation among all other modes in a tapered rectangular waveguide. The WR-975 waveguide (248 mm × 124 mm) for 915 MHz used in the microwave plasma torch is tapered to a shorted cross-section of half of its original height to obtain a stronger electric field in the discharge tube. Here, the word shorted means short-circuit reaction of the electric field. The discharge tube is inserted vertically, perpendicular to the wide wall of the waveguide. The center of the discharge tube was located 110 mm (a quarter wavelength) away from the short-circuited end. The plasma torch generated inside the discharge tube consisting of fused quartz was stabilized by injecting a swirl gas, which enters the discharge tube sideways, creating a vortex flow in the tube and keeping the torch flame of 5000 °C off the discharge tube wall. A steam generator provides steam at a temperature higher than 150 °C that enters the discharge tube as a swirl gas.12 A directional coupler and a three-stub tuner are useful for the initiation of the plasma torch, which controls the reflected power. A 915 MHz microwave system with a maximum power of 75 kW was used to generate the steam plasma. Steam (14 kg/h) was injected for initial stabilization at a microwave power of 30 kW. The steam plasma itself is an impedance load, which depends on physical conditions, including the microwave power. A microwave plasma system that consisted of a magnetron with 915 MHz frequency17 was built and operated by steam.12 The optimum steam rate must be found in terms of the microwave power by monitoring the minimum reflected power in this microwave system. According to the microwave plasma system,17,18 the incoming and reflected microwave powers are monitored simultaneously for experimental conditions. The minimum reflected power ensures the efficient power delivery to plasma. We remind the reader that there is

no.

microwave power (kW)

optimum injection rate of steam (kg/h)

reflected power ratio (percent)

1 2 3 4 5 6 7 8 9

20 25 30 35 40 45 50 55 60

12 13 14 15.5 18 20 24 29 33

3 3 3 2.5 1 2 1 1.5 1

corresponding reflected-power rate in terms of the microwave power. Clearly, once the plasma torch is ignited, most of the microwave power is absorbed by the torch plasma with a minimal reflected wave power of less than a few percent. Figure 1 shows steam-

Figure 1. Steam-plasma torches inside a quartz tube with a diameter of 8.6 cm and a length of 120 cm, surrounded by a steel mesh with a diameter of 16 cm, where photo images of (a) a torch of 40 kW with 18 kg/h steam, (b) a torch of 50 kW with 24 kg/h steam, and (c) a torch of 60 kW with 33 kg/h steam are presented. plasma torches inside a quartz tube with a diameter of 8.6 cm and a length of 120 cm; the tube is surrounded by steel mesh with a diameter of 16 cm, where photo images of (a) a torch of 40 kW with 18 kg/h steam, (b) a torch of 50 kW with 24 kg/h steam, and (c) a torch of 60 kW with 33 kg/h steam are presented. 2.2. Estimation of Operation Parameters for 500 kW Thermal Energy. We like to study coal gasification in a reaction chamber at T = 1900 K (= 1627 °C) as a high-temperature gasification. Residence time for coal gasification: the carbon in coal may actually participate in the breaking down of water molecules at 4403

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high temperatures according to C + H2O → CH + OH with a reaction constant19 of 3.86 × 10−17 cm3/mol/s at the temperature of T = 1900 K (= 1627 °C). Assuming that water molecules are a substantial fraction of neutrals in the reaction chamber, the rate of this reaction can be theoretically calculated to be on the order of 10/s at this gas temperature, requiring a fractional second of residence time in the chamber. In reality, carbons exist in particulate forms, thereby requiring much longer residence time in chamber than a fraction of a second. The reaction constant20 of CH + O2 → CO + OH is 8.3 × 10−11 cm3/mol/s at 298 K. The reaction constant21 of CH + O → products is 9.96 × 10−11 cm3/mol/s at 298 K. The reaction constants of these reactions may increase at high temperature. The ratio of the oxygen density to the neutral density in steam at temperatures around 1900 K is 8 × 10−5.12 Therefore, the rate of the reaction CH + O2 → CO + OH at 1900 K can be theoretically calculated to be 2 × 105/s or higher, requiring a very short residence time of much less than 1 s. In this context, coal gasification in steam at T = 1900 K can be completed within a few seconds. Determination of the reaction chamber size: we assume that the synthetic gas is composed of 40% hydrogen, 30% carbon monoxide, 10% nitrogen, and 20% carbon dioxide, which are hypothetical numbers based on operational parameters for gas engines. The enthalpy changes due to the combustion of each of the fuel components are as follows: ΔH = −285.8 kJ for H2 + (1/2)O2 → H2O and ΔH = −283 kJ for CO + (1/2)O2 → CO2. Therefore, the enthalpy change due to the combustion of 1 mol of gasified fuel is calculated as follows: ΔH = −285.8 kJ × 0.40 − 283 kJ × 0.30 = −199.2 kJ. The required gasified fuel in units of liters per minute (lpm) for a 500 kW system is L = (60s/min)(22.4liter/mol) × (500 kJ/s)/(199.2 kJ/mol) = 3373.5 lpm, which is equivalent to L = 56.2 L/ s. Considering that the temperature inside the reaction chamber for gasification is T = 1900 K for high-temperature gasification, the gas flow inside the chamber is L = 56.2 × 1900/300 = 356 L/s. We assumed a residence time of 3 s for gasification, which requires a chamber volume of 1068 L. Reforming energy of coal powder in steam: to simplify the analytical calculation, we assume that the coal powder provides carbon atoms in the form of graphite. Therefore, there are two possible coal gasification scenarios in hot steam. The first one is C + H2O → CO + H2 with enthalpy and entropy changes of ΔH = 131 kJ/mol and ΔS = 139.5 J/ mol, respectively, making the reaction temperature T = 939 K. The second is C + 2H2O → CO2 + 2H2 with these respective changes being ΔH = 90.1 kJ/mol and ΔS = 91.7 J/mol at the reaction temperature of T = 983 K. These reactions require energy for gas formation from coal powder. We also note from the above reactions that the enthalpy requirement for hydrogen formation is also about 85 kJ/mol. Approximately 35 kJ of energy is needed for the gasification of 1 mol of our synthetic fuel. 56 L/s of gasified fuel corresponds to 2.5 mol/s. Therefore, the reforming energy is estimated to be 88 kW. In estimation of residence time, reaction chamber size, and reforming energy, the primary reactions of carbon (C), H2O, and O2 were considered, neglecting all the secondary reactions including CO2 + C ⇄ 2CO, 2H2 + O2 ⇄ 2H2O, etc. The secondary reactions of byproducts from the primary reactions may also be important for precise determination of the operational parameters for the gasification system. The carbon dioxide may disintegrate to CO and oxygen atoms at the temperature of 3600 K based on Gibbs free energy. The resultant oxygen atom may produce a CO molecule by C + O → CO. These reactions may require a very high temperature. A substantial fraction of water molecules may also disintegrate into various species at the temperature of 5400 K,12 requiring a very high temperature. However, a rough estimation of the system parameters may be enough for the purpose of the present experiment. Partial oxidation of coal: microwave energy of 70 kW is clearly not enough for coal gasification; therefore, the partial oxidation of coal powder is essential. This partial oxidation can provide 18 kW of power and 60 kW more for the additionally required energy. The partial oxidation occurs in two forms. The first is C + O2 → CO2 with enthalpy energy of ΔH = −393.5 kJ/mol, and the second is 2C + O2 → 2CO with enthalpy energy of ΔH = −221 kJ/mol. It is not known

which reaction prevails. Perhaps the second reaction prevails at a high temperature. Nevertheless, the enthalpy energy from these exothermic reactions is about −307 kJ/mol on average. The requirement of 78 kW corresponds to (78 kJ/s)/(307 kJ/mol) = 0.25 mol/s = (5.7 L/s of oxygen), which is equivalent to 342 lpm. Water requirement for reforming: according to the two possible scenarios of coal gasification in hot steam as mentioned above, each mole of hydrogen synthesis requires 1 mol of water. Therefore, the water requirement is 0.4 mol for 1 mol of gasified fuel, corresponding to 1350 lpm (= 3374 lpm × 0.4) of steam, which is 1.085 kg/min or 65 kg/h. The coal powder requirement can also be calculated if the coal ingredients are known. The carbon requirement can be calculated. Again, according to the possible scenarios of coal gasification in hot steam mentioned above, we note that each mole of carbon monoxide or carbon dioxide requires 1 mol of carbon. Therefore, the carbon mole number required in this gasification step is 0.5 mol or more for 1 mol of gasified fuel. Because of the partial oxidation, the carbon mole number may be more than 0.5 mol for 1 mol of gasified fuel. 2.3. A Swirl-Type Gasifier for Low-Grade Coal. A swirl-type gasifier for 500 kW of thermal energy was fabricated with two microwave systems with a maximum power of 75 kW to operate at 915 MHz. One microwave plasma system is installed at the upper part, and the other one is installed at the lower part of the gasifier.16 The inner configuration of the swirl-type gasifier is a cylindrical shape with a diameter of 90 cm and a height of 180 cm. The inner volume of the gasifier is 1145 L. The inner surface of the gasifier is made of a fireresistant ceramic material called HACT180, which can sustain 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 from the bottom of the gasifier to the top. 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. Therefore, the plasma flame from the upper part swirls downward and meets the plasma flame from the lower part, thereby repeatedly gasifying the coal powders. Figure 2 shows the movement of the gas

Figure 2. Computer-generated movement plot of the gas fluid element obtained from a simple fluid simulation in the swirl-type gasifier. fluid element obtained from a simple fluid simulation in the swirl-type gasifier. This simulation indicates that the plasma flame from the upper part swirls downward and meets the plasma flame from the lower part, thereby repeatedly gasifying the coal powder. Here, the upper plasma flame is designed such that it angles 15° downward. To confirm this, we test fired two plasma flames into the gasifier and observed the process through its two view ports. Figure 3 shows the plasma flames inside the gasifier. The upper plasma flame reaches the lower part of the gasifier, as shown in Figure 3a, and the two plasma flames meet, 4404

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the gasifier. The synthetic gas was sampled at the synthetic gas exit and analyzed through gas chromatography, which has about 1% of error for CO, CO2, O2, and CH4 measurement. But, this gas analyzer has up to 3% error for hydrogen measurement, and H2O and N2 are not measurable through this chromatography. The relative concentrations of synthesized gas species of H2, CO, and CO2 during the operation time of 2.5 h were measured in terms of the data measurement time t16 and were presented in Figure 4. Oxygen (O2) and methane (CH4) in

Figure 3. The plasma flames inside the gasifier as seen through view ports in the gasifier. The upper plasma flame reaches the lower part of the gasifier, as shown in (a), and two plasma flames meet, swirl, and move together toward the exit of the gasifier in (b). swirl, and move together toward the exit of the gasifier, as shown in Figure 3b. One of the important issues pertaining to coal gasification by steam plasma is a steady supply of coal powder into the gasifier. The coalpowder supply system installed at the top of the gasfier provides coal powder into the gasifier by blown air. The synthesized gas from the gasifier is discharged through the synthetic gas exit. The ash and molten rock are discharged into an ash tank located under the gasifier. The coal-powder storage tank 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 be required to preheat the gasifier for a long time; therefore, a kerosene burner for preheating is installed at the lower part of the gasifier to heat the gasifier within a short time. Thus, the kerosene burner and plasma flames preheat the gasifier for about 1.5 h to 1100 °C, after which the kerosene burner is turned off. The coal powder is then injected into the gasifier, while the inner temperature is monitored. After preheat of the gasifier, the upper and lower plasma flames were powered by 40 kW with 26 kg/h of steam as swirl gas and by 30 kW with 14 kg/h of steam, respectively. Additional 20 kg/h of steam enters the upper part of the gasifier. Indonesian brown coal with the injection rate of 90 kg/h is injected into the gasifier through an injection port (60 kg/h) near the upper plasma flame and the other injection port (30 kg/h) near the lower plasma flame. Fine coal powders with an average particle size of 70 μm were delivered from a coal company through waterproofed bags and used in the experiment. The analysis of the Indonesian brown coal powders shows that the ash and moisture content is 33.17%. The 10.71% of water content in 90 kg/h of coal injection means an additional water injection of 9.6 kg/h. Therefore, the total water injection into the gasifier is 69.6 kg/h, which is estimated to be 1444 L/min (lpm) at the room temperature condition. 400 lpm of air from air compressors is used to blow the coal powders into the gasifier. The nitrogen is 320 lpm in this air blow, and oxygen is 80 lpm. Additional 330 lpm of oxygen is injected into the gasifier for the partial oxidation of coal. The total oxygen input from water, air, and additional oxygen injection is calculated to be 1132 lpm at the room temperature conditions. The inner temperature of the gasifier increases due to continuous input of coal powders and oxygen, and then it reaches 1700 °C after a few hours continuous operation of microwave plasma. The temperature of the inner wall of the gasifier was measured during the operation period of the last 2.5 h16 in terms of the data measurement time t in units of minute. Usually, the temperature at the lower part is higher than that at the upper part. The temperature of the gas fluid element may decrease as it moves from the bottom of the gasifier to the top. In particular, the temperature of exiting synthetic gas from gasifier is less than 1000 °C even for the very high wall temperature in

Figure 4. Plots of the relative concentrations of synthesized gas species vs the data measurement time t during the operational period of last 2.5 h. synthetic gas are neglected because their concentrations are much less than 0.5%. The concentration of carbon monoxide is much higher than that of carbon dioxide at a high gas temperature as expected. Figure 4 indicates that the concentration of the flammable gases (H2 and CO) is more than 70% during most of the operation time. The other parameter to be measured for the gasification performance evaluations is the total flow rate of the synthetic gas, which is extremely difficult in this experiment due to large gas volume, high gas temperature, ash powders, etc. Nevertheless, we tried to measure the total flow rate of synthetic gas by making use of a flow meter equipped with a Pitot tube of 1.5 cm diameter and 2 m long. The total flow rate measured was in the range of 3000−3500 lpm with large fluctuations. Therefore, the only reliable data to be used in the gasification performance evaluations are the concentrations of H2, CO, and CO2 in Figure 4.

3. RESULTS AND DISCUSSION The gasification performance of the present gasifier system can be evaluated by estimating the cold gas efficiency and the carbon conversion rate of gasification of the low-grade coal used in this case. The nitrogen gas entered the gasifier at 320 lpm through the air blow for coal powder injection and some extra amount in coal, of which it is not known well what happens at a high temperature. Nitrogen at high temperature may produce nitric oxides, which disappear immediately in an environment of high concentration of hydrogen22 without any loss of nitrogen gas. In this context, we assume that 320 lpm of the nitrogen gas that entered the gasifier will exit without any loss or increase. Any extra overestimation of nitrogen in synthetic gas may lead to an overestimation of the cold gas efficiency. The total flow rate FT of synthetic gas is related to the relative concentration n of nitrogen gas as 320 FT = (1) n and is in units of lpm. The next consideration is the oxygen gas, which enters the gasifier as steam, as water content in coal, and as oxides, which may still be in oxide materials in ash after gasification. Influence of any oxygen emission from oxide materials on the gasification will be discussed later. Thus, the 4405

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total oxygen input of 1132 lpm may exit the gasifier as a form of CO, CO2, and H2O, whose relative concentrations are denoted by cm, cd, and w, respectively. There is almost no oxygen and methane exit as mentioned above. 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. The balance equation of oxygen gas is then expressed as 160(cm + 2cd + w) = 1132n

(2)

where use is made of eq 1. We finally remind the reader that the sum of the all the relative concentrations is unity, as h + cm + cd + w + n = 1

(3) Figure 5. Plots of power (kW) and flow rate (lpm) of synthetic gas vs the data measurement time t during the operational period of last 2.5 h.

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)

(4)

The calorific value and the ash and moisture content of the Indonesian brown coal are 4640 kcal/kg and 33.17%,16 respectively. The calorific power of the coal is calculated to be 487.2 kW for 90 kg/h injection. The microwave power used in this gasification is 70 kW. In addition, 60 kg/h of steam enters 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/h of steam is calculated to be 38 kW. Therefore, the total input energy entering the gasifier is 595 kW. According to element analysis of the Indonesian brown coal,16 the carbon content is 59.23%. Therefore, the carbon content of 90 kg/h of coal powders is 53.3 kg/h =1.234 mol/s, which means that 1.234 mol of carbon enters the gasifier every second. A systematic evaluation method of gasification performance on time is discussed here. Substituting the relative concentrations of hydrogen (h) and carbon dioxide (cd) into eq 4, the relative concentration of nitrogen (n) can be calculated and presented in Figure 4 during the operation period of the last 2.5 h. The water concentration, which is considerably less than 0.5% throughout the operation period, is also calculated from eq 3 and plotted in Figure 4. The total flow rate (FT) of the synthetic gas is calculated, substituting the nitrogen concentration n into eq 1. 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 285.8 and 283 kJ/mol, respectively. Figure 5 shows plots of the power (kW) and flow rate (lpm) of the synthetic gas versus the data measurement time t during the operation period. The time profiles of the power (kW) and flow rate (lpm) of the synthetic gas are very similar to each other, indicating that synthetic gas power increases as the synthetic gas flow rate increases, being obvious for the predetermined input conditions in this experiment. The cold gas efficiency η is calculated by dividing the synthetic gas power in Figure 5 by the total input power of 595 kW. The carbon mole fraction in the synthetic gas per unit time can be calculated from Figure 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 mol/s, we can also calculate the carbon conversion rate R. Here, the total carbon input of 1.234 mol/s was obtained from the coal powder injection of 90 kg/h. Plots of the cold gas efficiency η and the carbon conversion rate R during the operation period in Figure 6 show fluctuations,

Figure 6. Plots of cold gas efficiency η and carbon conversion rate R of synthetic gas and their mean values averaged over the time range from t = 0 to t of the data measurement time.

which may be caused by unsteady injection of the coal powders into the gasifier although the coal powder supply was most cautiously carried out. The carbon conversion rate cannot be more than unity, from a physical point of view. The coal powder injection has about 2% fluctuation around the mean value of 90 kg/h injection rate. In this regard, the average values of the cold gas efficiency and carbon conversion rate up to the data measurement time shown in Figure 6 are meaningful and useful for industrial applications. These average values were obtained over the time range from t = 0 to t of the data measurement time. The average value of the carbon conversion rate increases from 0.955 to 0.998, as the data measurement time t approaches t = 150 min. Similarly, the average value of the cold gas efficiency increases from 0.7 at t = 0 to 0.835 at t = 150 min. The slow increase of cold gas efficiency to the end of the operation time may be caused by a slow increase of the inner temperature of the gasifier. The cold gas efficiency averaged over the entire operation time is about 84% for the average temperature of 1640 °C. The unity of the carbon conversion rate ensures a complete gasification of low-grade coal by microwave steam plasma. Some of the oxygen emission from oxide materials due to high temperature in the gasifier may increase total volume of the synthetic gas, leading to an increase of hydrogen, carbon monoxide, and water volumes. Therefore, the cold gas efficiency based on calorific values of the hydrogen and carbon monoxide in the synthetic gas may be overestimated for the measured concentrations of hydrogen and carbon monoxide by considering the oxygen emission from oxide materials. It is also 4406

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Figure 7. Element analysis (a) of the coal powder before gasification and (b) of the ash after gasification according to energy dispersive spectrometry (EDS).

efficiency improve drastically as the gas temperature increases due to microwave steam plasma. The steam content in the synthetic gas is minimal (10 lpm or less). It is important to note that the operation parameters in the gasification studied above may not be the best choices for electrical power generation due to the large consumption of electrical power needed to generate microwave plasma. A typical combined system of a gas engine and a steam turbine to generate electrical power is required, though this would convert 43% of the synthetic gas energy to electrical energy. This indicates that 70 kW of electrical energy for the microwaves translates into 163 kW in terms of synthetic gas energy. The total input energy therefore may be 487 kW + 38 kW + 163 kW = 688 kW instead of 595 kW. As a result, the estimated cold gas efficiency may be 501 kW/688 kW = 0.73, which is substantially less than 0.84, as mentioned above. Owing to the low-grade coal in this gasification system, consumption of the primary ingredients of the coal, water, and air used in this gasification may not be the major issue, because they are inexpensive. The only major concern is the consumption of electrical power. Therefore, new operation parameters must be devised to reduce the electrical power consumption if using this gasification system for electrical power generation. The coal gasification system investigated here, which utilizes a microwave steam torch, can serve as a moderately sized power plant due to its compactness and lightweight nature. This can be useful in rural or sparsely populated areas where access to a national power grid is not available.

very difficult to correctly estimate the amount of the oxygen emission from oxide materials if there is any emission. In this regard, we neglected the oxygen emission from oxide materials in this gasification analysis. Shown in Figure 7 is an element analysis of (a) the coal powder before gasification and (b) of the ash after gasification from an energy dispersive spectrometer (EDS). The carbon content dominates before gasification, whereas carbon is virtually nonexistent in the ash, where oxide materials dominate. The most oxygen content in the ash may exist as the oxide materials. According to Figure 7, 69% corresponding to carbon disappears leaving 31% of oxide materials during gasification, which may be mostly in the ash after gasification. About 21% of oxygen content in coal powder before gasification may still be in oxide materials in ash. In this context, the oxygen percentage in the ash after gasification may be calculated to be 21 × 100/31 = 67%, which is slightly higher than 62% observed from EDS in Figure 7b. The discrepancy between the calculation and observation may be caused by irregular evaporations of oxide materials during high-temperature gasification. Very high temperature of T = 1640 °C in the gasifier due to the high temperature of microwave steam plasma causes almost 100% carbon conversion (R = 0.998) of the low-grade coal with a high content (33.17%) of ash and moisture, as predicted in the literature.13 The calorific power of the synthetic gas averaged over the entire operation time is about 502 kW in this experiment. It is also remarkable to observe that the cold gas efficiency in this experiment is 84% at T = 1640 °C, which is extremely high in a relatively small gasifier like the one described here. The difference between the calorific powers of input value and synthetic gas in this experiment is 595−502 = 93 kW, which may have disappeared due to mostly convective loss of synthetic gas exit and other losses including radiation, conduction, ash heat, etc. The temperature of the synthetic gas is measured at the synthetic gas exit and is about 1000 °C. The flow rate of the synthetic gas averaged over the entire time is FT = 3299 lpm. Therefore, the convective energy loss E is calculated to be E = 64 kW. However, the insulation of the gasifier was very good. We can touch the outer wall of gasifier even if the inner wall is 1640 °C. According to these analyses, we conclude that the carbon conversion rate and cold gas

4. CONCLUSIONS In summary, we developed microwave steam plasma for heating the coal powders. High-power steam plasma operated by microwaves at 915 MHz was developed, where the magnetron power at 915 MHz was available up to 75 kW. The steam plasma itself is an impedance load, which depends on physical conditions, including the microwave power. By monitoring the minimum reflected power, the optimum injection rate of the steam and the corresponding reflected power ratio in terms of the microwave power was presented in Table 1. It was found that most of the microwave power is absorbed by the torch plasma with a minimal reflected wave power of less than a few 4407

dx.doi.org/10.1021/ef500598u | Energy Fuels 2014, 28, 4402−4408

Energy & Fuels

Article

(14) Hong, Y. C.; Lee, S. J.; Shin, D. H.; Kim, Y. J.; Lee, B. J.; Cho, S. Y.; Chang, H. S. Energy 2012, 47, 36−40. (15) Shin, D. H.; Hong, Y. C.; Lee, S. J.; Kim, Y. J.; Cho, C. H.; Ma, S. H.; Chun, S. M.; Lee, B. J.; Uhm, H. S. Surf. Coat. Technol. 2013, 228, S520−S523. (16) Uhm, H. S.; Na, Y. H.; Hong, Y. C.; Shin, D. H.; Cho, C. H. Int. J. Hydrogen Energy 2014, 39, 4351−4355. (17) Hong, Y. C.; Shin, D. H.; Lee, S. J.; Kim, Y. J.; Lee, B. J.; Uhm, H. S. IEEE Trans. Plasma Sci. 2011, 39, 1958. (18) Kim, J. H.; Hong, Y. C.; Kim, H. S.; Uhm, H. S. J. Korean Phys. Soc. 2003, 42, S876. (19) Mayer, S. W.; Schieler, L.; Johnston, H. S. Symp. (Int.) Combust., [Proc.] 1967, 11, 837−844. (20) Lichtin, D. A.; Berman, M. R.; Lin, M. C. Chem. Phys. Lett. 1984, 108, 18−24. (21) Herron, J. T. J. Phys. Chem. Ref. Data 1988, 17, 967−1027. (22) Uhm, H. S.; Cho, S. C.; Park, I. G.; Hong, M. S. J. Korean Phys. Soc. 2008, 52, 1800.

percent, once the plasma torch was ignited. By making use of two microwave steam plasmas, we fabricated a swirl-type gasifier for heating up the gas temperature in a reaction chamber of 1145 L. With additional heating of synthetic gas by a partial oxidation of coal, the inner-wall temperature in the gasifier can easily reach 1700 °C. The coal for this gasification was Indonesian brown coal with calorific value of 4640 kcal/kg and ash and moisture content of 33.17%. The carbon conversion rate at the chamber temperature of 1640 °C was almost 100%, ensuring a complete gasification of carbon in a low-grade coal with a high ash content. The cold gas efficiency was 84% at 1640 °C, which is extremely high in a relatively small gasifier like the one used in the experiment here, as shown in the previous literature.3 The total calorific value of the synthetic gas was 500 kW. This gasification system can serve as a moderately sized power plant due to its compactness and lightweight nature. This power plant for low-grade coal would be useful in rural or sparsely populated areas, which do not have access to a national power grid.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82-2-940-8374. Fax: +822-940-5664. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the SRC program (20100029421) and by the Basic Science Research Program (2012R1A1A2007903) through the National Research Foundation funded by the Korea government (MSIP). This work was also supported by the Korea Micro Energy Grid project of the Ministry of Knowledge Economy in Korea and by 2013 R&D Convergence Program funded by Korea Research Council of Fundamental Science & Technology (R&D Convergence-13-6-NFRI). The present research was partially funded by the Research Grant of Kwangwoon University of 2014.

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REFERENCES

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dx.doi.org/10.1021/ef500598u | Energy Fuels 2014, 28, 4402−4408