Forward and reverse combustion gasification of coal with production of high-quality syngas in a simulated pilot system for in situ gasification

Applied Energy 131 (2014) 9–19

Contents lists available at ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Forward and reverse combustion gasification of coal with production of high-quality syngas in a simulated pilot system for in situ gasification Cui Yong a, Liang Jie a,⇑, Wang Zhangqing a, Zhang Xiaochun a, Fan Chenzi a, Liang Dongyu b, Wang Xuan a a b

School of Chemical and Environmental Engineering, China University of Mining & Technology (Beijing), Beijing 100083, China School of Resources and Environmental Engineering, East China University of Science and Technology, Shanghai, China

h i g h l i g h t s  A new forward and reverse combustion process for in-situ gasification was proposed.  Lignite and bituminous coal, were gasified, producing high-quality syngas.  Controlling conditions for reverse combustion gasification were identified.  Inject gas flow and velocity of gasification flame were linearly related.

a r t i c l e

i n f o

Article history: Received 10 January 2014 Received in revised form 16 May 2014 Accepted 1 June 2014

Keywords: Underground coal gasification Reverse combustion gasification Forward combustion gasification Syngas oxygen–steam gasification

a b s t r a c t This research focused on the feasibility and stability of applying the forward and reverse combustion approach to the in situ gasification of lignite and bituminous coal with oxygen or oxygen–steam mixtures as gasification agents, especially reverse combustion gasification. A high-quality syngas (H2 and CO) could be obtained using the reverse combustion gasification technique combined with forward combustion gasification in a pilot system for in situ gasification. The gasification time was extended more than 25% using the reverse combustion approach. The controlling conditions for reverse combustion gasification were obtained by comparing and analyzing experimental data. The results show the relationship between the inject gas flow within certain limits and velocity of the gasification flame was linear during reverse combustion. The underground conditions of the coal seam and strata were simulated in a pilotscale underground gasifier during experiments. The combustion gasification of coal was carried out experimentally for over 5 days. The average effective content (H2 and CO) of syngas was in the range of 60–70%, meeting the requirement of synthesis gas. The optimal ranges of gasifying lignite and bituminous coal were found to be 1.5–2.0 and 1.3–1.75, respectively. The product gas flow was proportional to oxygen blast. These are expected to provide useful guidance on practical underground coal gasification operations and to give experimental evidence in support of theory. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction At recent underground coal gasification (UCG) technique has attracted increasing attentions [1]. This technique exhibits many potential advantages of technology and economy [2]. It mentions here to increase utilization efficiency of coal and improve economic environment-friendly performance as compared to the currently applied technologies of conventional coal exploiting and using [1–4]. Especially, Prabu and Jayanti have used the UCG technology in the carbon-neutral power generation and solid oxide fuel cell system [5]. These represent a huge potential and bright prospect ⇑ Corresponding author. Tel.: +86 10 62339209; fax: +86 10 62331601. E-mail address: [email protected] (J. Liang). http://dx.doi.org/10.1016/j.apenergy.2014.06.001 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.

for application and development of UCG technology. Moreover, the hydrogen manufacturing from underground coal gasification is one of effective and feasible methods for hydrogen production [6–8]. UCG technology differs from conventional coal gasification in surface reactors, in that it is an invisible process, so experimental simulation of underground coal gasification is essential to research the process, phenomena, theory and technology of UCG. The researchers of different countries have obtained important achievements using their respective experimental simulated UCG units [9– 11]. The Laboratory and pilot scale simulated experiments were performed by shaftles-type underground coal gasification technique which is one of the main directions of the UCG research. Generally, forward and reverse combustion approaches are used in shaftless-type underground coal gasification (UCG)

10

Y. Cui et al. / Applied Energy 131 (2014) 9–19

processes [12–14]. In practical operations, reverse combustion has been widely applied to link the injection and production wells in UCG, and forward combustion has usually been applied to gasify the coal seam, using appropriate gasification agents [15–17]. Different gasification techniques should be selected to gasify coal according to coal seam thickness [18]. In essence, the single-phase forward gasification technique is performed to gasify a thin coal seam in-situ. Multi-phase forward and backward gasification techniques are carried out on thick coal seams to improve the efficiency of underground coal gasification. The operation of reverse combustion gasification is similar to backward gasification, viz., the product borehole is converted to an injection borehole [8]. Gasification agents are fed into the product borehole and production gas exits from the injection borehole under modern UCG conditions. Reverse combustion gasification differs from backward gasification in the direction between the flame (gasification working face) propagation and main gas flow, as shown in Fig. 1 [19]. The flame propagates in the opposite direction to the gas flow within the channel in the reverse combustion gasification process, whereas in backward gasification, the direction of the gasification flame follows the gas flow towards the outlet of the production gas. Thus, backward gasification is consistent with forward combustion gasification. During forward combustion gasification, when the flame gradually moves to the product borehole, a large cavity will be formed in the coal seam through coal combustion gasification and overburden roof spalling. This reduces coal gasification efficiency and decreases the quality of production gas, for the following reasons [8]: (1) the dry distillation zone becomes increasingly shorter in the late stages of forward gasification. (2) The reactivity of coal significantly decreases after coal seams undergo dry distillation during forward gasification. (3) The reaction rates of coal combustion and gasification fall because of a decreasing concentration of gasification agents absorbed on the coal surfaces of the cavity wall. To increase coal seam gasification and enhance syngas quality, the injection borehole and product borehole should be exchanged during practical operation. In this way, gasification of coal seams will continue by shifting the direction of the injection gas and new gasification conditions are again formed. If the gasification flame moves in the direction of the injection gas flow, residual coal seams around the former product borehole would not be gasified, which wastes coal resource. To maintain gasification in the residual dry distillation zone, reverse combustion gasification could be applied in UCG processes and the production of highquality syngas could be sustained. Reverse combustion is an unstable process, in which the flame front is regarded as a displacement front [20]. The broad combustion flame will propagate to form a tube-like cavity of partially combusted fuels through coal seams [12]. Therefore, the reverse combustion approach is more suitable to the linkage stage in the UCG process. Moreover, all theories, models, field tests and laboratory experiments on coal reverse combustion with air in the literature are based on well linkage techniques [21,22]. Researchers in

the former USSR invented combustion linking techniques in 1941 [23]. Subsequently, reverse combustion linking was successfully used by many countries in UCG field trials, including the former USSR, the United States, Canada, Uzbekistan, Belgium, Australia, and South Africa. During operation of these trials, high pressures were used to enhance linkage efficiency [24]. Skafa and Kreinin et al. described the characteristics of reverse combustion and summarized the results of their tests [12,25]. Laboratory experiments on reverse combustion are very difficult to carry out, particularly with coal and some of the reasons for this are summarized by Britten, as follows [20]: (1) after coal undergoes pre-oxidation, its structural integrity will be altered, greatly affecting the combustion characteristics of the coal. (2) RC in a combustion tube is affected by factors such as the permeability or thermal conductivity of the coal and the diameter of the tube. It is required that the permeability or thermal conductivity of coal are relatively low, and the dimensions of the tube is adequately large. (3) If the blasting rate of the gas is quite high, convective losses to the gas phase will inhibit thermal conduction in cold coal. In this case, the RC will revert to forward combustion. However, a number of laboratory experiments on reverse combustion in small-diameter tubes have been performed using various combustible media by workers in different countries, and the results of these studies can help to qualitatively understand the development of RC [26,27]. None of the industrial-scale UCG plants worked, except for the YuzhnoAbinsk UCG plant, because of the oil and natural gas used widely throughout the world in the late 1980s. Accordingly, field trial research and laboratory experiments did not continue to develop [18]. However, theoretical developments and reverse combustion models have been constantly studied. Most recently, Blinderman et al. presented detailed theories for forward and reverse combustion and discussed gasification flame propagation in the channel of an underground coal gasifier [16,19,22]. They used mathematical models to simulate forward and reverse combustion in a gasification channel. The models include transport phenomena, and chemical reactions governed by conservation of mass, energy and species. The theories explain the relevant combustion phenomena in UCG processes and consider the effects of primary factors of flame propagation, the supply gas rate and other parameters. Nevertheless, there is a lack of field tests and laboratory experiments to support the theories. At present, there are no literature reports in which reverse combustion has been applied to gasify coal seams with gasification agents other than air in UCG operations. In this paper, we focus on the feasibility and stability of applying the forward and reverse combustion approach to the in situ gasification of coal with the production of high-quality syngas, especially reverse combustion gasification. A large-scale pilot system was designed and established to simulate UCG conditions. To validate the reliability and repeatability of the forward and reverse combustion gasification experiments, laboratory experiments on lignite and bituminous coal gasification with a mixed oxygen–steam agent were carried out in the simulated pilot system for in situ gasification, according to the characteristics of forward and reverse

Fig. 1. Diagrams of reverse (a) and forward combustion gasification (b) [19].

Y. Cui et al. / Applied Energy 131 (2014) 9–19

combustion. The results of these experiments are expected to give useful guidance in practical UCG operations and to provide experimental evidence to support theory. 2. Materials and methods 2.1. Description of a simulated pilot system for in situ gasification A diagram of the simulated pilot system for UCG is shown in Fig. 2. This consisted of a simulated underground gasifier for UCG, supply gas equipment and a production syngas purification system. A pilot scale underground gasifier was constructed with a rectangular shape and internal dimensions 4.45 m (length)  1.17 m (width)  1.57 m (height). External walls of the gasifier were composed of fire retardant layers, thermal insulation layers, stainless steel sealing layers and reinforced concrete, anti-pressure layers, in order from inside to outside. Several holes were included for measurement and observation, structured on the gasifier body: 19 holes were used to measure experimental temperatures, six holes were used to monitor the gasifier’s pressures, and four boreholes were used to observe the internal conditions of the gasifier using installed cameras. Furthermore, four holes were used as injection gas inlets or production gas outlets. Gasification agents such as oxygen or an oxygen–steam mixture gas were fed into the pilot system by the gas supply equipment. The oxygen agent was supplied from ten steel cylinders connected in parallel, under 0.2 MPa pressure. Water was converted into saturated aqueous vapor at 0.7 MPa in an electric steam generator with a rated evaporation capacity of 80 kg/h. Considering pilot system safety, a valve and steel nitrogen cylinder were installed in the supply system to extinguish a fire in the reactor in the case of an emergency. The product gas was transported into the pilot-scale purification system by a 0.10 m diameter pipe. The purification system was composed mainly of a spray tower, gas washer and desulfurizer. The product gas exited from the purification process and was combusted in the flare stack. A portion of the syngas flow was fed to an online gas chromatograph for component analysis. Before the sample gas entered the chromatographic columns, it was further purified by a dehumidifier and quartz gas filter. 2.2. Control and monitoring system To collect complete experimental data and identify the characteristics of forward and reverse combustion gasification in the

11

simulated pilot system for in situ gasification, the control and monitoring system mainly consisted of gas flow meters and regulators, gas temperature, pressure sensors, stress sensors, displacement sensors and 85 Ni–Cr/Ni–Si (K type) armored thermocouples. Injection and production gas were measured by vortex flow meters. The supply flow rates of gasification agents were adjusted by gas regulators. The pressure in the gasification channel was monitored and displayed using pressure instrumentation installed into six holes in the gasifier. The syngas quantity was continuously analyzed by feeding to the two-TCD online gas chromatograph (Shimadzu GC2014) every 15 min. The sampling interval was 15 min. Intouch 10.0 industrial HMI software of Invensys Systems, Inc. (Houston, TX, USA) was used to collect and treat all experimental data information, then the original data were deposited into Historian 10.0 runtime database (Fig. 3). 2.3. Simulation and structure of the coal seam and strata The key usefulness of the model pilot is to simulate an underground coal seam and the surrounding strata. To simulate the actual conditions of the coal seam geology, the preparation and selection of coal and strata samples are of prime significance. Considering the internal dimensions of the gasifier, the prefabricated coal seam were designed with maximal dimensions of 4.45  1.1  0.5 m. The consecutive coal seams were constructed from 22 raw coal blocks closely integrated together, in which the dimensions of each single coal block sample were 0.4  0.55  0.5 m. The gaps between the individual blocks were filled by small block coals mixed with adhesive. The raw coal blocks joined together created a continuous coal seam. The gasification channel was excavated in the bottom of the coal seam, with a length of 4 m and a diameter of 0.1 m. The strata, including roof and floor, were established during the construction of the reactors, whose design was as follows: grit stone and shale were laid on the strata floor, and two grit stones and shales were laid in the strata roof, one-by-one. Before the coal seams were created, the floor comprised rock and clay. The roof comprised a layer with a thickness 0.5 m that was filled with rock blocks and clay above the coal seam. Expanded perlites of thickness 0.1 m were used as an insulating layer above the roof. The coal block sample and its corresponding rock sample were manually extracted from the same underground mine to maintain sample integrity and consistency. The simulated lignite seams were completed using about 2987 kg of coal, and the total weights of bituminous coal seams were about 3115 kg. The amount of bituminous coal was slightly higher than that of lignite because of their different densities. A schematic view of the simulated coal seam and strata is shown in Fig. 2.

Fig. 2. Diagram of the units used for the simulated pilot system for in situ gasification.

12

Y. Cui et al. / Applied Energy 131 (2014) 9–19

Fig. 3. The monitoring and control system.

2.4. Two-dimensional temperature fields nephograms of horizontal coal seam According to the physical and chemical properties of coal and the simulated mining geology conditions of coal seam, the percolation gasification channel can be formed in the gasification panel [1]. To follow the gasification flame propagation and the temperature changes in the coal seam during gasification experiments, there were 85 Ni–Cr/Ni–Si (K type) armored thermocouples uniformly located in a plane within the simulated coal seam, as seen in Fig.4. 2D nephograms of temperature fields were used to visualize the temperature changes during the UCG process. This approach used Tecplot 360 CFD Visualization software to solve the conservation equations according to real temperature data from the 85 thermocouples in the coal seam, and horizontal temperature fields nephograms in the coal seam as a function of time are presented. The physical dimensions of 2D temperature fields nephograms were 4.45  1.1 m equaled to sizes of horizontal coal seam. The computational domain on the level of the gasification channel was meshed using approximately 720 quadrilateral control volumes [28]. The thermal conditions of coal seams at different

horizontal locations were colored differently according to temperature gradient in the nephograms of temperature fields. A color map from black to white represents temperature from low to high, respectively. The nephograms showed that yellow areas in which the combustion/gasification reaction mainly took place were high temperature zones in the coal seam in the range 1000–1200 °C. The orange areas of the gasification flame front were the preheating zone in the coal seam, where coal was distilled and pyrolyzed to form char or semi-coke. The black areas, before the preheating zone, represented an unreacted coal seam. The pilot temperature fields provide important information for monitoring of gasification development. The in-situ coal gasification experiment using the forward and reverse combustion gasification technique is a continuous process. The representative 2D nephograms of temperature fields in different phases change over time can reflect gasification flame propagation and thermal status of coal seam during the whole experiment. The operational parameters such as volume ratio of steam to oxygen and oxygen blast should be adjusted to obtain high-quality syngas according to the changes of temperature fields. But practical operation of the UCG process is difficult with respect to the measurement of temperature in an

Fig. 4. Diagram of the 85 thermocouples horizontally located in coal seam. (The black dots represent the thermocouples.)

13

Y. Cui et al. / Applied Energy 131 (2014) 9–19

underground gasifier. Therefore, the 2D nephograms of temperature field could help to develop a qualitative understanding of UCG field trials. 2.5. Materials The coal seam comprised blocks of lignite and bituminous coal samples from the Dayan and Xinhe coal mines, respectively. The proximate and ultimate analyses of the lignite and bituminous coal samples acquired according to appropriate standards are presented in Table 1. 3. Experimental procedures

the oxygen flow rate required at steady-state to obtain high-quality syngas. In phase III, 93% oxygen and steam were supplied to the gasifier in reverse to maintain syngas quality in the last stage of the UCG process. The characteristics and feasibility of reverse combustion gasification were investigated in phase III. Using this approach, stable and feasible control parameters could be obtained in reverse combustion gasification experiments. Experimental combustion gasification of lignite was carried out for almost 130 h, and the bituminous coal gasification experiment was performed for about 120 h. In order to keep steam the form of gas, the temperature of gasification gas agents were heated up 120 °C before entering into the gasifier. 4. Results and discussion

3.1. Cold-state test Cold tests were performed to check that all systems were leakfree. All valves (both inlet and outlet) were open. The air was blasted in from the injection inlet by a blower, and the product gas was released from the outlet. The air flows at both inlet and outlet were recorded to calculate reactor leakage. Additionally, the pressures at the inlet and outlet and in the reactor were monitored. Reactor leaks could be detected using soap-water bubbles. When the blast volume was in the range of 2.5–10 m3/h, and the reactor leakage was less than 3%, the test data could be recorded. In the cold-test, the leak rate was 2.8%. When the static pressure of the reactor reached 25 kPa, pressure release took 35 min, meaning that the rate of pressure release was 0.714 kPa/min, which satisfied ignition requirements. 3.2. Ignition Initially, an electric igniter was placed inside the gasification channel through the injection borehole. Pure oxygen was blown into the coal seam, and the electricity turned on. It could be seen that the coal seam were ignited because the temperature in the gasification channel exceeded 600 °C [29]. Subsequently, the ignition process was terminated. The electric igniter was then removed and pure oxygen was passed to the reactor. The gasification process began after a 2 h ignition period. 3.3. Performing the gasification experiments The experiments were divided into three phases. In phase I, 98% oxygen was fed to the gasifier, combusting with coal to preheat the coal seam and to accumulate sufficient heat energy before oxygen– steam gasification. In phase II, 93% oxygen and steam were continuously supplied to the reaction zone, and the forward combustion gasification experiment was performed. The primary purpose of phase II was to explore the optimal ratio of steam to oxygen and

Table 1 Proximate and ultimate analyses of Dayan lignite and Xinhe bituminous coals. Parameter

Lignite

Bituminous coal

Proximate analysis w (%) as received 1. Total moisture (M) 2. Ash (A) 3. Volatiles (V)

32.5 16.32 46.24

4.9 7.3 32.8

Ultimate analysis w (%) 4. Carbon (C) 5. Hydrogen (H) 6. Oxygen (O) 7. Nitrogen (N) 8. Sulfur (S) Calorific capacity (MJ/kg)

74.46 4.8 18.34 1.41 0.98 15.58

77.25 5.25 7.59 1.43 0.45 29.87

Model experiments in a pilot scale underground gasifier were performed to investigate coal gasification with oxygen or oxygen–steam in the forward or reverse combustion process. 98% oxygen was supplied into the gasifier to preheat the coal seam after the coals were ignited successfully. When the temperature field in the gasifier increased to the levels required for coal gasification, an appropriate ratio of oxygen and steam was simultaneously fed to the simulated pilot system. The composition and production rate of syngas, temperature field in the coal seam and the operation parameters were recorded in real time under different experimental conditions. To obtain high-quality syngas, the gasification techniques were adjusted according to changes in temperature field and syngas composition. 4.1. Oxygen gasification using coal seam moisture (phase I) 98% oxygen was fed into the gasifier at a flow rate of 3.6–5 m3/h and phase I was initiated. After supplying oxygen for 2 h, highquality syngas was obtained. The changes in composition of the product gas over time are shown in Fig. 5. The quality of the product syngas was relatively high, although steam was not fed during this phase. In this phase, the volume fractions of effective syngas composition (CO and H2) markedly increased. The H2 composition accounted for about 23–38% of the product gas, and CO composition stood at roughly 20–48% of the syngas volume. The sum of effective syngas composition (CO and H2) reached about 58–76%, as seen in Fig.4. The average heating values of the syngas from gasifying lignite and bituminous coal were 9.10 and 11.67 MJ/m3, respectively. The temperature field are presented in Fig.6. The temperature of the coal seam close to the injection borehole significantly increased, to approximately 1000 °C. The higher content of hydrogen is attributable to the steam gasification reaction, favored at temperatures above 800 °C [30], although the steam was not supplied into the gasifier because there was plenty of moisture contained within the coal seam. At high temperatures, the moisture changed into steam and reacted with the coal to generate hydrogen and CO. The temperature in the combustion zone was relatively high, attributable to the release of a great deal of heat and volatiles by reaction of coal with 98% oxygen. The combustion of volatiles significantly heated the coal. The process is referred to as the main overall reaction [8,31]:

Pyrolysis : Coal ! C þ CO þ CO2 þ H2 þ CH4

þ 44:7 MJ=kmol; ð1Þ

Carbon Oxidation : C þ O2 ! CO2

 394 MJ=kmol:

ð2Þ

Volatiles combustion:

2CO þ O2 ! 2CO2

 571 MJ=kmol;

ð3Þ

14

Y. Cui et al. / Applied Energy 131 (2014) 9–19

Fig. 6. The 2D nephogram of horizontal temperature fields after 15 h of oxygen gasification.

chemical reactions. Initially, the gasification reactions require abundant thermal energy arising from combustible material (volatiles and char) reacting with oxygen. Exothermic combustible reactions release heat to sustain a higher gasification temperature in the underground gasifier. Meanwhile, the steam gasification reactions must be carried out at high temperature, and steam decomposition is a highly endothermic reaction. In addition, the water-gas-shift reaction has a significant impact on the composition of the syngas because of presence of residual water vapor in the product gas. The composition of the syngas is determined by three main reactions [7,32]:

Steam Gasification reaction : C þ H2 OðgÞ ! CO þ H2 þ 131:5 MJ=kmol; C þ H2 OðgÞ ! CO2 þ H2 þ 90 MJ=kmol: Fig. 5. Percentage composition of product gas during phase I (a) bituminous coal and (b) lignite.

H2 þ 2O2 ! H2 O

 242 MJ=kmol;

CH4 þ 2O2 ! CO2 þ 2H2 O

 890 MJ=kmol:

ð4Þ ð5Þ

The coal seam temperature and composition of syngas were crucial factors in deciding the time at which to switch to pure oxygen. The gasified lignite experiments were continued for 23 h using 98% oxygen as gasification agent, while the bituminous coal gasification experiments only lasted for 16 h, because lignite contains a relatively high moisture content compared with bituminous coal. Although the major purpose of this step was to preheat the coal seam, gasifying high-moisture content coal could produce hydrogen-rich syngas during phase I. However, the composition of the product syngas was not always consistent during this phase. 4.2. Effect of ratio of steam to oxygen on syngas composition The practical operation of the UCG process is difficult with respect to the measurement of temperature in an underground gasifier and the control of the optimal operational temperature. Therefore, the volume ratio of steam to oxygen became a crucial factor in the control of gasification temperature. Coal gasification processes using oxygen and steam as agents involve complex

ð6Þ ð7Þ

Water-gas-shift reaction : CO þ H2 OðgÞ ! CO2 þ H2

 41 MJ=kmol:

ð8Þ

Hence, the optimal volume ratios of steam to oxygen should be investigated, to obtain high-quality product syngas. The gasification experiments were performed under conditions of different volume ratios of steam to oxygen. The composition of the syngas as a function of the ratios of steam to oxygen are presented in Fig. 7. The H2 content of the syngas gradually increased, but the CO content of the syngas decreased with an increasing ratio of H2O(g)/O2. The (CO and H2) content of the syngas clearly decreased rapidly after increasing. This was explained by the fact that water decomposition and excess steam would consume massive amounts of heat if an excess water vapor flow was supplied, causing the gasification temperature in the underground gasifier to drop. The steam gasification reaction should proceed according to chemical Eq. (15) at a lower gasification temperature. On the other hand, the water-gas-shift reaction, chemical Eq. (16), should become more intense at lower temperature. This could also result in higher H2 and CO2 contents and lower CO content in the product gas. Thus, the volume fraction of CO2 obviously increased and the sum of effective syngas composition (CO and H2) noticeably decreased. The CH4 composition remained steady because it mainly arose from coal pyrolysis. The combustion and gasification reactions have only a slight impact on CH4 composition in the syngas [8]. The appropriate gasification temperature differs for various

Y. Cui et al. / Applied Energy 131 (2014) 9–19

15

Fig. 8. Percentage composition of product gas during phase II (a) bituminous coal and (b) lignite.

Fig. 7. Changes in syngas composition at different volume ratios of H2O/O2 (a) bituminous coal and (b) lignite.

types of coal, because of their differing reactivities, and the feasible ratio of H2O(g)/O2 should be adjusted accordingly. The optimal volume ratio of steam to oxygen depended on actual conditions in the underground gasifier during the UCG process. Hence, the optimal ranges for lignite and bituminous coal were found in the ranges 1.5–2.0 and 1.3–1.75, respectively.

4.3. Stability experiments of oxygen–steam forward combustion gasification (phase II) To produce a stable flow of consistent high-quality syngas, the oxygen–steam mixture were continuously supplied as gasification agents into the simulated underground gasifier. The stability gasification experiments for two different types of coal were carried out over different ranges of oxygen flow rate and ratios of steam to oxygen, according to their characteristics. Thus, oxygen flow for gasifying lignite was in the range 4.6–8.0 m3/h, with an average H2O/O2 ratio of 1.69, and the oxygen–steam mixture was fed to the gasifier with the oxygen flow at 6.5–18 m3/h, with an average H2O/ O2 ratio of 1.44 in bituminous coal gasification experiments. The

changes in composition of the product gas over time are shown in Fig. 8. A moderate heating value syngas can be obtained using oxygen–steam as a gasification agent under an appropriate ratio of H2O/O2. The (H2 + CO) content in the syngas was in range 60– 70%, in which H2 accounted for 35–45% and CO occupied 20–30% of the total product gas. The average heating value of lignite gas was 9.23 MJ/m3, slightly below that of bituminous coal, which was 10.35 MJ/m3. In oxygen–steam gasification experiments, the levels of oxygen in the gasification agents promoted the oxygen concentration on the gasified coal surface, in contrast to air gasification. The combustion of coal was accelerated [12], so that gasification was further strengthened, thereby maintaining the higher temperature fields of the gasification zone in the coal seam, as shown in Fig. 9. This had the benefit of enhancing the reaction rates for steam reduction and decomposition. Simultaneously, the H2 and CO contents in the product gas increased. However, the compositions of product syngas from gasifying lignite and bituminous coal differed. The average H2 composition in lignite product gas was higher than that from bituminous coal because the gasification reactivity of lignite is superior to that of bituminous coal [33]. The CO2/CO ratio was greater than 1 in lignite syngas. However, the average CO2 composition of bituminous coal syngas was nearly equal to that of CO. The value of the CO2 /CO ratio was determined by the combustion temperature. Gasification theory [34] indicates

16

Y. Cui et al. / Applied Energy 131 (2014) 9–19

Fig. 9. The 2D nephogram of horizontal temperature fields after 50 h of forward combustion gasification.

face) moving towards the product borehole. The direction of the gasification flame in the experiment propagated the same as before. Moreover, high-quality syngas was continued to be produced, and the residual coal seam was gasified completely, as far as possible. To verify the direction of gasification flame propagation, two 2D nephograms of temperature fields at different gasification times were used to observe the position of the main high temperature zone in the coal seam in each experiment. The movement of the high temperature zone should reflect the direction of flame development directly, as seen from Figs. 10 and 11 which are the 2D Nephograms of horizontal temperature fields after 15 and 25 h of phase III, respectively. Comparing the main high temperature zone in phase III with phases II and I, the results showed the gasification flame propagated in the same direction. This suggests that experiments on reverse combustion gasification were carried out successfully. During the reverse combustion gasification experiment, the relationship between gasification flame velocity, s, and supply gas flow, U, in the gasification channel was studied to identify the controlling parameters. s, which is the consumption of gasified

that the value of the CO2 /CO ratio is close to 1 when the combustion temperature equals 1200 °C. If the gasification temperature is lower than 1200 °C, the oxidation reaction should dominate, making the CO2 composition higher than that of CO. The temperature fields of the lignite seams combustion zone were between 1000 and 1100 °C, and the average combustion temperature of the bituminous coal seam slightly exceeded 1200 °C, as shown in Fig. 9. These results agree with those described in references [33,34]. The two stability experiments on oxygen–steam forward combustion gasification lasted for around 70 h. In the continuous gasification process, the composition of product syngas remained roughly constant in the range of 5%. The quality of experimental product coal gas met the requirements for synthesis gas. The effects of oxygen flow rate on the gasification process are discussed in detail in Section 4.5. 4.4. Control experiments for oxygen–steam reverse combustion gasification (phase III) The quality of syngas declined in the latter stage of phase II because a large cavity was formed along the gasification channel. The oxygen concentration on the surface of the incandescent coal was so low that the coal combustion reaction rate declined. In addition, the reactivity of the coal declined because the residual coals were partially pyrolyzed and were converted into char, which possessed a lower reactivity because of the larger pore diameter [8,35]. At the instant the injection and product boreholes were switched, oxidants were injected into the product borehole and product gas was removed from the injection borehole. At the beginning of phase III, steam was blown into the two boreholes to remove the product gas. Oxygen-enriched air was injected from the product borehole to improve the temperature in the coal seam. After 0.5 h, the temperature of the reactor center returned to above 1000 °C, and the oxygen–steam mixture was fed continuously until the total process was terminated. The low injection gas flow in the lignite gasification experiment was in the range 13–20.7 m3/h, with an H2O/O2 ratio of 1.68, and the high injection gas flow in the bituminous coal gasification experiment was approximately 40 m3/h, with an H2O/O2 ratio of 1.48. The main aim of the experiments in phase III was to investigate the feasibility of reverse combustion gasification and obtain the parameters that control stability. Thus, in the phase III experiments, it was essential to keep the flame (gasification working

Fig. 10. The 2D nephogram of horizontal temperature fields after 15 h of reverse combustion gasification.

Fig. 11. The 2D nephogram of horizontal temperature fields after 25 h of reverse combustion gasification.

Y. Cui et al. / Applied Energy 131 (2014) 9–19

coal per unit cross-sectional area, A0, of the coal seam, is given by [36]:

s¼

Vc ; A0

ð9Þ

where Vc denotes the coal consumption per hour, m3/h. The value of Vc can be calculated through a carbon mass balance for the gasification process [37]:

Vc ¼

0:5357GðuCO þ uCO2 þ uCH4 þ 2uC 2 H2 Þ þ GT T C þ GA AC þ GD DC ; qCoal MC ð10Þ

where G, GT, GA and GD are the production rates of syngas, tar, ash and dust of product gas, respectively. uCO ; uCO2 ; uCH4 and uC 2 H2 are volume fractions of the subscripted gases in the total syngas. TC, AC and DC are the percentage carbon contents in tar, ash and dust in the product gas. These data can be obtained from analyzing the syngas and byproducts of the gasification process. The velocity of the gas phase, vg, and the solid phase, vs, in the moving gasification working face are presented as follows [20,38,39] (noting that if vg is always assumed to have a positive value, s is positive in forward combustion gasification, and negative in reverse combustion gasification):

vg ¼ 

K

lmix

Pn

rP ¼ Dtr H2 OO2 ; v s ¼ s; X i  li  M 1=2 i

i¼1 lmix ¼ P n

i¼1 X i

 M 1=2 i

;

K ¼ 0:228  0:01041T þ 0:000178T 2 ;

ð11Þ

ð12Þ

ð13Þ

where the oxygen–steam mixed gasification agents had ideal gas behavior and were ordered by Darcy’s law during the combustion gasification process; K is the effective permeability of the coal seam, which is related to temperature by Eq. (12) [36]; rP originates from a difference in pressure between the injection gas inlet and the production gas outlet; lmix is the viscosity of oxygen–steam mixture at 120 °C; li, Mi and Xi are regarded as the viscosity, molar mass and mole fraction of species i in the mixture gas, respectively. It is assumed that the oxygen–steam mixture reacted with coal according to single-step Arrhenius kinetics. The gasification reaction rate was a first-order kinetic process in both reactants [40]. Dt is a stoichiometric coefficient quantifying the mass of gasified solid per mass of gasification agents reacted. The reaction rate expression per unit cross-sectional area A0 of the coal seam is given by:

17

oxygen–steam mixture. The values of pre-exponential factor and activation energy of 8.6  102 h1 and 135 kJ/mol, respectively, were derived from the related literature [42–44]. The relationship between vg and s was used to analyze the characteristics of reverse combustion in Fig. 12. vg could be calculated from Eq. (11) for different conditions of injection gas flux. The injection gas flow was adjusted, corresponding to the pressure of the injection gas inlet and the production gas outlet, measured by pressure sensor changes, while the range of pressure gradient rP become larger with increasing injection gas flux (as seen in Fig. 13). Fig. 12 highlights the salient features of reverse combustion gasification, in which vg was almost equal to the flame velocity. Thus, it was one of the critical control parameters for RC to maintain the seepage speed of the gas phase consistent with the consumption rate of the gasified coal per unit cross-sectional area of the coal seam, in agreement with the theory reported in the literature [19,20]. In the experiments, s could be obtained by tracking the position x of the main high temperature zone, where the isotherm was about 900 °C, at different times. The flame speed rate was determined by [45]:

s¼

Dx Dt

ð16Þ

From the results, it can be seen that the actual speed of the flame front was slightly faster than the analytical gasification flame velocity, because the temperature of the flame front in coal seam was above the analytical temperature of 1000 °C in reality. The agreement between experimental and analytical gasification flame velocities as functions of the injection gas flux is quite good over a wide flow range. The composition of syngas was continuously monitored during phase III. A high-quality syngas was then obtained using the reverse combustion gasification technique in addition to forward combustion gasification in the pilot system for in situ gasification, as seen in Fig. 14. The average (H2 + CO) content in the syngas was about 61% in the two RCG experiments. It could be explained that the residual coals were partially pyrolyzed and converted into char in the last stage of whole gasification pilots, but the reactivities of the chars arising from the two different types of coals were remarkably similar. The average heating values of lignite and bituminous coal gas in phase III were 8.37 and 9.73 MJ/m3, respectively. These were lower than that of the product gas in phase II because the CH4 content in the syngas in phase III apparently decreased. During the last several hours of reverse combustion gasification, the CO2 content increased rapidly, indicating the impending end of the experiments.

E

r H2 O—O2 ¼

AUe RT ; A0

ð14Þ

where A and E are the pre-exponential factor and the activation energy for the combustion gasification reaction, respectively. Generally, Darcy’s law is only applied to laminar flow in porous media. The particle Reynolds number is used to judge the flow pattern of the supply gas in the seepage channel, written by [37]:

Re ¼

Kvg dp m

ð15Þ

If Re > 2.5, gas flow in the porous medium does not conform to Darcy’s law, as pointed out by references [37,41]. Therefore, the injection gas flow must fulfill the specific condition that Re < 2.5. The value of Re calculated from the high flow rate of injection gas was less than 2.5.Fig. 12 shows the velocity of the gasification flame and gas phase in the moving flame as functions of the injection gas flow, calculated by the above equations. Dt was a constant value of 0.84 for the coal gasification reaction with an

Fig. 12. The velocity of the gasification flame and gas phase in the moving flame as functions of the injection gas flow for steady development.

18

Y. Cui et al. / Applied Energy 131 (2014) 9–19

Fig. 15. Change in product gas rate with growth of oxygen blast. Fig. 13. Changes in pressure gradient rP with growth of injection gas flow.

syngas under specific controlled conditions of the inject gas flow parameter. Furthermore, the use rate of coal seams can be improved. 4.5. Effect of oxygen blast on syngas flow rate Fig. 15 demonstrates the effect of oxygen blast on the product gas rate. The change in oxygen blast resulted in fluctuation of syngas flow, the product gas flow was essentially proportional to the oxygen blast, and the syngas production rate increased with increasing oxygen blast [7]. The ratios of syngas flow rate to oxygen blast of gasified lignite and bituminous coal were 3.7 and 3.1, respectively, and are often determined by the intrinsic characteristics of coal quality. Generally, oxygen consumption is relatively low for the high activity and low fixed carbon content of coal [34]. Thus, oxygen blast during bituminous coal gasification was higher than for lignite at the same production gas rate. Whatever the technique applied, such as oxygen gasification, forward or reverse combustion gasification in the UCG process, a suitable increase in oxygen blast should be beneficial in promoting the syngas production rate in practical operations under conditions of reasonable controlling parameters. 5. Conclusions

Fig. 14. Percentage composition of product gas during phase III (a) bituminous coal and (b) lignite.

The two controlling experiments on oxygen–steam reverse combustion gasification were performed for over 30 h. The gasification time was extended by more than 25% using the RC approach. The relationship between the inject gas flow within certain limits and the velocity of the gasification flame was linear. The reverse combustion gasification process can produce high-quality

(1) The pilot scale experiments proved the feasibility and stability of producing high-quality and moderate heating-value syngas by applying forward and reverse combustion gasification techniques in UCG process. In particular, the RC approach was used to successfully gasify two types of coal. A syngas composition was obtained using reverse combustion gasification in addition to forward combustion gasification. The gasification time was extended by more than 25% using the RC approach and the use rate of coal seams can be improved. (2) The essential controlling conditions for reverse combustion gasification indicated that velocity of the gas phase in the moving gasification flame was consistent with the consumption rate of the gasified coal per unit cross-sectional area in the gasification channel, governed by Darcy’s law. The agreement between experimental and analytical gasification flame velocities as functions of the injection gas flux is quite good over a wide flow rate range. (3) An oxygen and steam continuous gasification approach, in which a mixed oxygen-steam gas is fed simultaneously, can steadily maintain the composition of the syngas

Y. Cui et al. / Applied Energy 131 (2014) 9–19

produced during the complete UCG process. The average H2 + CO content in syngas was more than 60%, regardless of whether lignite or bituminous coal was gasified. The production syngas could thus be used as synthesis gas. (4) The volume ratio of steam to oxygen is a critical parameter for maintaining the stability and quality of syngas production during practical operations. The optimal ranges for gasifying lignite and bituminous coal were found to be 1.5–2.0 and 1.3–1.75, respectively. (5) Syngas can be produced using coal seam moisture during the oxygen gasification phase, but the composition of production syngas is not always consistent. (6) Oxygen blast affects the product gas rate. The product gas flow is proportional to oxygen blast. The ratios of product gas rate to oxygen blast for gasified lignite and bituminous coal are 3.7 and 3.1, respectively.

[16] [17]

[18] [19] [20]

[21]

[22] [23]

Acknowledgements This work was supported by the National High Technology Research and Development Program 863 of China (ratification No. 2011AA050106) and the Fundamental Research Funds for the Central Universities (ratification No. 2012YH01). References [1] Bhutto A, Bazmi A, Zahedi G. Underground coal gasification: From fundamentals to applications. Prog Energy Combust Sci 2013;39(1):189–214. [2] Olateju B, Kumar A. Techno-economic assessment of hydrogen production from underground coal gasification (UCG) in Western Canada with carbon capture and sequestration (CCS) for upgrading bitumen from oil sands. Appl Energy 2013;111:428–40. [3] Kapusta K, Stan´czyk K. Pollution of water during underground coal gasification of hard coal and lignite. Fuel 2011;90(5):1927–34. [4] Prabu V, Jayanti S. Laboratory scale studies on simulated underground coal gasification of high ash coals for carbon-neutral power generation. Energy 2012;46:351–8. [5] Prabu V, Jayanti S. Underground coal-air gasification based solid oxide fuel cell system. Appl Energy 2012;94:406–14. [6] Krzysztof S, Krzysztof K, Marian W, Jerzy S, Adam S, Jan R, et al. Experimental simulation of hard coal underground gasification for hydrogen production. Fuel 2012;91(1):40–50. [7] Stanczyk K, Smolinski A, Kapusta K, Wiatowski M, Swiadrowski J, Kotyrba A, et al. Dynamic experimental simulation of hydrogen oriented underground gasification of lignite. Fuel 2010;89:3307–14. [8] Yang L, Zhang X, Liu S, Yu L, Zhang W. Field test of large-scale hydrogen manufacturing from underground coal gasification (UCG). Int J Hydrogen Energy 2008;33:1275–85. [9] Stanczyk K, Howaniec N, Smolinski A, Swiadrowski J, Kapusta K, Wiatowski M, et al. Gasification of lignite and hard coal with air and oxygen enriched air in a pilot scale ex situ reactor for underground gasification. Fuel 2011;90:1953–62. [10] Prabu V, Jayanti S. Simulation of cavity formation in underground coal gasification using bore-hole combustion experiments. Energy 2011;36:5854–64. [11] Prabu V, Jayanti S. Heat-affected zone analysis of high ash coals during ex-situ experimental simulation of underground coal gasification. Fuel 2014;123:167–74. [12] Skafa P, editor. Underground coal gasification. Gosgortechizdat: Moscow; 1960. [13] Gregg DW, Edgar TF. Underground coal gasification. AIChE J 1978;24:753–81. [14] Yang LH, Liu SQ, Yu L, Liang J. Experimental study of shaftless underground gasification in thin high angle coal seam. Energy Fuels 2007;21:2390–7. [15] Hommert PJ, Beard SG. Description of reverse combustion linking and forward gasification during Underground Coal Gasification. In: Proc 3rd Underground

[24]

[25] [26] [27] [28]

[29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]

[40] [41] [42]

[43] [44] [45]

19

Coal Conversion Symposium. Lawrence Livermore Laboratories Report. CONF770652; 1977. p. 267–79. Blinderman M, Saulov D, Klimenko A. Forward and reverse combustion linking in underground coal gasification. Energy 2008;33:446–54. Britten JA, Kzantz WB, Gunn RD. Modeling studies of reverse combustion linking at high pressures. In: Proc 8th underground coal conversion symposium, Sandia National Laboratories Report. Sand 82–2355; 1982. p. 515–23. Evgeny S, Arvind V. Underground coal gasification: a brief review of current status. Ind Eng Chem Res 2009;48(17):7865–75. Dmitry N, Ovid A, Klimenko A. Flame propagation in a gasification channel. Energy 2010;35:1264–73. Britten JA. Activation energy-asymptotic modeling of reverse combustion instabilities in porous media: application to in-situ coal gasification. Ph.D. thesis, University of Colorado Denver, CO; 1984. Glaser RR, Gunn RD, Kzantz WB, Breidung KP, Gudenau HW. Unexpected aspects of reverse combustion: effects of pressure, volatility and chemical reactivity. In: Proc 9th underground coal conversion symp. U.S. DOE Report. DOE/METC/84-7; 1983. p. 219–26. Blinderman M, Klimenko A. Theory of reverse combustion linking. Combust Flame 2007;150:232–45. Gregg DW, Hill RW, Olness DU. An overview of the Soviet effort in underground gasification of coal. Lawrence Livermore Laboratories. Report No. UCR-50024; 1976. Su RY, Engleman VS. Reverse combustion linking phenomena in bituminous coal at high pressure. In: Proc 5th underground coal conversion symp. U.S DOE Report CONF-790630; 1979. p. 323–30. Kreinin EV, Fedorov NA, Zvyagintsev KN, Pyankova TM. Underground gasification of coal seams. Moscow: Nedra; 1982. Cupps CQ, Land CS, Marchant LC. Field experiment of in-situ oil recovery from a Utah tar sand by reverse combustion. AICHE Symp Ser 1976;155(72):61–7. Gumm RD, Krantz WB. Reverse combustion instabilities in tar sands and coal. Soc Pet Eng J 1980;20:267–77. Perkins G, Sahajwalla V. Modelling of heat and mass transport phenomena and chemical reaction in underground coal gasification. Chem Eng Res Des 2007;85:329–43. Hongtao L, Feng C, Xia P, Kai Y, Shuqin L. Method of oxygen-enriched twostage underground coal gasification. Min Sci Techno (China) 2011;21:191–6. Yang L, Liu S. Numerical simulation on heat and mass transfer in the process of underground coal gasification. Numer Heat Tr A-Appl 2003;44:537–57. Shu-gin L, Yuan-yuaon W, Ke Z, Ning Y. Enhanced-hydrogen gas production through underground gasification of lignite. Min Sci Tech 2009;19:384–94. Gao F. Manufacture of fuel gas. Beijing: Press of Chinese Construction Industry; 1995. p. 123–30. Takarada T, Tamai Y, Tomita A. Reactivities of 34 coals under steam gasification. Fuel 1985;64(10):1438–42. Xie KC. Coal structure and its reactivity. Beijing: Science Press; 2002. Alejandro M, Fanor M. Reactivity of coal gasification with steam and CO2. Fuel 1998;77(15):1831–9. Yang L. Study on the model experiment and numerical simulation for underground coal gasification. Fuel 2004;83:573–84. Liang J. Stability and controlling technology of underground coal gasification process. Beijing: China University of Mining & Technology Press; 2002. Winslow AM. Numerical model of coal gasification in a packed bed. LLC Preprint UCRL-77626; 1976. Thorsness CB, Wang SW.Further Development of A General-Purpose, PackedBed Model for Analysis of Underground Coal Gasification Processes, Lawrence Livermore National Laboratory, Report UCRL-92489; 1985. p. 1–19. Cogoli JG, Gray D, Essenhigh RH. Flame stabilization of low volatile fuels. Combust Sci Technol 1977;16(3–6):165–76. Kong XY. Advanced fluid mechanics in porous medium. Hiefei: University of Science and Technology China Press; 1997. Nettleton MA, Stirling R. The combustion of clouds of coal particles in shockheated mixtures of oxygen and nitrogen. In: Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences, 1971; 322(1549): 207–21. Essenhigh RH, Froberg R, Howard JB. Combustion behavior of small particles. Ind Eng Chem Res 1965;57(9):32–43. Smoot LD, Smith PJ. Coal combustion and gasification. U.S. Plenum Press; 1985. Hans V, Jeroen V, Rob B, Philip D. Reverse combustion: kinetically controlled and mass transfer controlled conversion front structures. Combust Flame 2008;153:417–33.