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Experiment study on entrained flow gasification technology with dry slag by second-stage water supply
Powder Technology 306 (2017) 10–16
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Experiment study on entrained flow gasification technology with dry slag by second-stage water supply Haiquan An a, Juan Yu a, Junjie Fan b, Yanchi Jiang a, Zhongxiao Zhang a,⁎ a b
School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Institute of Combustion and Gasification, College of Power Engineering, University of Shanghai for Science & Technology, Shanghai 200093, China
a r t i c l e
i n f o
Article history: Received 22 May 2016 Received in revised form 28 October 2016 Accepted 9 November 2016 Available online 11 November 2016 Keywords: Entrained flow gasification Second-stage water Syngas composition Temperature distribution
a b s t r a c t Experimental system of a new entrained flow gasifier with second-stage water supply (20 kg/h) was set up in this paper, and effect of molar ratio of O/C on temperature distribution, syngas composition, carbon conversion and effective syngas were analyzed with a typical high AFT coal (Guizhou LJ coal). The results of non-secondstage water and second-stage water with changing molar ratio of O/C indicate that, when the molar ratio of O/ C is larger than 0.96, gasification efficiency with dry slag by second-stage water would become better than non-second-stage water. Specifically, when the molar ratio of O/C was 1.1, the effective syngas fraction and carbon conversion of second-stage water gasification were 80.2% and 93.24% respectively. Correspondingly, the effective syngas fraction and carbon conversion of non-second-stage water gasification were 78.5% and 91.67% respectively. Furthermore, the gasification temperature was lower than the ash fusion temperature of test sample, the slag was non-melt during all the gasification experiments. The optimal gasification was conducted with the process temperature under 1450 °C, which was lower than the ash fusion temperature of LJ coal. These experimental results prove the applicability of entrained flow gasification technology with dry slag by second-stage water supply on using high AFT coals, which is helpful to generalize the large-scale gasification technology using abundant high AFT coals in China. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Entrained flow gasification technology is widely employed in chemical and energy industrial fields, such as manufacturing chemical products and IGCC power generation technology [1,2] in China. Due to the highly efficient and clean carbon conversion process, large-scale pressurized entrained-flow gasification has been considered as a crucial technology for clean coal utilization [3,4]. In recent years, research on entrained flow gasification with high ash fusion temperature (AFT) coals has been conducted gradually due to abundant high AFT coals deposits in China [5–7]. Generally, based on the coal feeding mode, the entrained flow coal gasification technology can be divided into coal water slurry (CWS) gasification and pulverized coal gasification. Both of the two type gasification technologies are widely employed in industry. In comparison with pulverized coal gasification [8], CWS gasification can obtain higher volume fraction of H2 composition in the syngas [1]. In addition, the preparation of CWS is more complex but the transport of CWS is more stable [9]. The process temperature of CWS gasification can be adjusted with the oxygen injection, but usually would not exceed 1400 °C at the ⁎ Corresponding author at: Room A215, School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dongchuan RD, Minhang District, Shanghai, China. E-mail address: [email protected] (Z. Zhang).
http://dx.doi.org/10.1016/j.powtec.2016.11.009 0032-5910/© 2016 Elsevier B.V. All rights reserved.
outlet of the gasification reactor [10]. Nowadays, mainly entrained flow gasification prefers coals with low ash content and low AFT (below 1400 °C) to achieve steady slag tapping, because they have appropriate slag viscosity during that temperature [11]. When the high AFT coals (especially for the AFT higher than 1500 °C) should be used as the fuel of entrained flow gasification technologies [1,3,4,8], it is impractical to simply introduce more O2 to raise the operation temperature because the tolerable temperature of refractory brick or membrane in gasifier is limited. Thus, blending with the low AFT coals or other additives to reduce the FT is generally acceptable. However, it is easy to generate nonmelting slag due to wrong blending ratio in industrial production. Besides, the cost of additives is usually expensive. Based on our research, a new idea is presented to solve the problem on entrained flow gasification with high AFT coals. Under the condition of oxygen needed to keep the high efficiency gasification, by adding two opposite nozzles as second stage O2/H2O injection, the oxygen injection by the primary nozzle is properly allocated. Reducing the oxygen injection from the primary nozzle, the regional violent combustion near the primary nozzle will be moderated and the temperature will be lowered, with the result that the coal ash would not melt. As a result of oxygen classification and water injection, the temperature during gasification process was lower than the AFT of using coal and the slag was solid when it was expelled from the gasifier. In detail, the gasifying agent refers to O2 and H2O, the specific composition and proportion of them
H. An et al. / Powder Technology 306 (2017) 10–16
depend on the gasification situation, coal type, syngas composition requirements, etc. The second-stage O2/H2O injection can greatly improve the flow field disturbance effect of the gasifier reactor. Tests and simulations of the flow field distributions in a cold model entrained flow gasifier have been conducted with different injecting velocity, position, number of nozzles. The results demonstrate that the flow field distributions in the gasifier would be greatly improved due to more reflux gas forming by two-stage O2/H2O injection [12]. Furthermore, the gasification reaction would be enhanced with the strong gas transfer in the boundary layer of the char particle caused by two-stage O2/H2O injection under high temperature. In this paper, based on other studies on the entrained flow gasification experiments [13–17], a 20 kg/h entrained flow gasifier with dry slag by second-stage water supplying (EGDSSW) was established, and experiments were conducted under 8 different operating conditions, 4 with second stage gasifying agent injection and 4 without. In this study, the second stage gasifying agent with oxygen atom refers to water only due to safety issues and equipment constraints. The experiments with second stage O2/H2O supply would be presented in next work. 2. Experimental apparatus and procedure 2.1. Coal water slurry preparation The entrained flow gasification technology with dry slag by secondstage water supplying was particularly for coals with high ash fusion temperature (AFT), therefore the Guizhou LJ coal (ash flow temperature above 1500 °C) was used in this experiment. The LJ coal was pulverized with a high speed ball mill and mixed with deionized water containing the additives [18,19], the loading of coal reached to 65% in the CWS. The mean diameter of the coal particles was approximately b50 μm. The proximate analysis, ultimate analysis, ash composition analysis and ash fusion characteristic data of LJ coal were presented in Table 1. 2.2. Experimental apparatus and procedure In this study, the entrained flow gasifier with dry slag by secondstage water supply (EGDSSW) was designed, constructed, and operated. The work of EGDSSW started in 2013, and the objective was to provide valuable information and optimal operating parameters for the entrained flow gasification technology with dry residue by secondstage water supply using typical high AFT coal. A schematic of the EGDSSW system was presented in Fig. 1. The EGDSSW consists of an alumina-based refractories lined reactor (0.46 m in inner diameter and 2 m in vertical reactor wall length) with a conical shaped outlet followed by a water sprayed quench chamber for syngas cooling and gas/solid-residue separation. The process temperatures in the reactor are recorded by five thermocouples placed vertically Table 1 Main properties of high AFT coal A. Coal type
LJ coal
Proximate analysis, %
Ultimate analysis, %
Mad
Aad
Vad
FCad
Cad
Had
Oad
Nad
Sad
2.49
13.00
31.99
52.52
73.28
4.52
4.32
1.36
1.03
11
inside the reactor, which are shielded by protective ceramic encapsulation. Three thermocouples were mounted in the combustion and gasification reaction region (upper part) of the reactor, two thermocouples were mounted in the gasification reaction region (lower part) of the reactor. The coal water slurry (CWS) was stirred in an agitation tank at least for 1 h before experiment, and kept stirring during the experiment proceeding. The CWS was transported by a screw pump and introduced in the top of the reactor. The mass of CWS was controlled by the frequency of the pump (from 1– 50 Hz) and calibrated for 20 min at certain frequency repeatedly. To ensure the CWS was transported as a liquid to the top nozzle, cooling water was flowed through a concentric vertical tube near to the primary nozzle. The first stage oxygen was evaporated in a liquid oxygen evaporator and introduced in the top of the reactor with the CWS. The accuracy of oxygen flow was controlled by a MFC and adjusted by a ball valve. The second stage water was introduced by two opposed nozzles located 0.5 m from the top of the reactor, the deionized water was controlled by a double plunger metering pump. To ensure the opposed nozzles closed in a safe condition, a stream of N2 was flowed constantly through a supply pipe that surrounded them into the reactor. The N2 came from a group of nitrogen cylinders (with 99.99% N2) and controlled by a MFC. Prior to beginning the reaction process, the reactor was heated by diesel fuel and air, which was introduced in the top-end of the reactor. The two nozzles were symmetrical and titled toward the center axis of the reactor. Meanwhile, the diesel and air was separately controlled by a turbine flowmeter. The temperature in the reactor increased rapidly to reach approximately 1000 °C (nearly 1 h), then the mass flows of diesel and air were decreased to warm the reactor uniformly (nearly 1 h). After that, the diesel was cut off, and a small amount of N2 replaced the air to cool the diesel nozzle. The gasification process started by feeding the CWS and first stage oxygen, and then the second stage water was added to cool the reaction temperature and enhance the blending the reactants. The raw syngas, cooling N2 and other particulates (mainly ash and char) were cooled by cooling water in the water quench vessel resulting in exit gas temperatures below 100 °C. Henceforward, the purged raw syngas was bubbled through the chimney to leave the gasifier. In order to measure the volume concentration of the exit gas (mainly CO, CO2, CH4 and H2) of raw syngas, a gas analysis meter (Gasboard 3100) was used after a two-step filter. After gas sampling, the produced syngas was collected to study for CO2 absorption and capture or burned out by a small burner. When the experiment was over, the particles (ash and char) were sampled from the black water flowing out at the bottom of the gasifier. The carbon conversion was derived using a Thermogravimetric Analyzer (TGA) with the particles samples. 2.3. Experimental condition The main experimental parameters for performing this gasification experiment were presented in Table 2. The CWS feeding rate was 20 kg/h for each experiment, approximately 13 kg/h LJ coal feeding. The experiments can be divided into two groups, Group A and Group B. Group A represented the experiments without second stage water supply, and Group B represented the experiments with water supply through the second stage nozzles. The molar ratio of O/C was defined as the ratio between O (including oxygen element of O2, steam and coal) and fixed carbon of coal, derived as [5,17,20,21]:
Composition of ash %
LJ coal
SiO2
Al2O3333
Fe2O33
CaO
MgO
Na2O
K2O
TiO2
SO3
53.96
30.23
5.77
2.87
0.89
0.37
0.64
2.92
1.62
Ash fusion temperature, K
LJ coal
Qnet,ar, (kJ/kg)
DT
ST
FT
1703
N1773
N1773
28,530
λ¼
no;c þ no;o þ no;s nc;c
ð1Þ
where no,c is the mole fraction of oxygen in coal sample, no,o is the mole fraction of oxygen in first stage and second stage O2, no,s is the mole fraction of oxygen in the second stage oxygen element, and nc,c is the mole fraction of fixed carbon in coal sample.
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H. An et al. / Powder Technology 306 (2017) 10–16
Fig. 1. The EGDSSW system.
The supplied oxygen was introduced by two steps. Approximately 75– 90% of total supplied oxygen would be injected from the first stage, and mixture of O2 and H2O would be injected from the second stage to adjust the gasification temperature and strongly blend the reactants. The oxygen element ratio between second stage and first stage was represented:
η¼
no;so þ no;s no;fo
ð2Þ
where, no,fo is the mole fraction of oxygen in first stage oxygen and no,so is the mole fraction of oxygen by the second stage nozzles.
In this study, effect of molar ratio of O/C (λ) and the second/first stage ratio (η) were analyzed. The selected λ values are 0.9, 0.95, 1.0 and 1.1, and the η values are 0 or 1/9. However, in order to keep the gasification system steady and safe, only water was supplied from the second stage O2/H2O nozzles in this study. When the second stage water was introduced into the reactor, the process temperature was over 1200 °C, so the apparent reaction rates of char-H2O and char-CO2 do not only depend on the chemical reaction rate, but also the pore and external diffusions should be considered, which have significant effects on the gasification efficiency. In this article, the external diffusion was linked to gas concentration by mass transfer in the boundary layer of a sphere char particle and was limited by the Fick's Law [22]. In addition, a gravimetric stoichiometric
Table 2 Operating condition parameters. Parameter Operating condition
CWS feeding rate kg/h
O/C molar ratio (λ) \
First-stage oxygen Nm3/h
Second-stage oxygen/steam (Nm3/h)/(kg/h)
Second/first stage ratio(η) \
System pressure Bar
1A 1B 2A 2B 3A 3B 4A 4B
20 20 20 20 20 20 20 20
0.9 0.9 0.95 0.95 1.0 1.0 1.1 1.1
7.6 6.8 8.1 7.3 8.6 7.7 9.4 8.5
0/0 0/1.22 0/0 0/1.30 0/0 0/1.38 0/0 0/1.51
0 1/9 0 1/9 0 1/9 0 1/9
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
H. An et al. / Powder Technology 306 (2017) 10–16
coefficient was used to relate the gas diffusion rate to the reaction rate of char, the apparent reaction rate under high temperature was presented by the reference [23]: r¼
6Ni DM;i Mi 2
d ρRT
pbulk;i −pC;i
2 þ 0:6Re0:5 Sc0:33
ð3Þ
where J is the mass flux of reactant gas at the particle outer surface, h is the mass transfer coefficient, Mi is the molar mass of the reactant gas, the cbulk,i and cC,i are molar concentrations in the bulk phase and at the outer particle surface, r is the apparent reaction rate under high temperature, Ni is the gravimetric stoichiometric coefficient of the reaction with char and reactant gas, d is the particle diameter, ρ is the apparent density of the char particle, DM,i is the molecular diffusion coefficient for the reactant gas, and Re and Sc are the Reynolds number and Schmidt number between reactant gas with the char particle in the reactor, respectively. The ash and char particles were collected when the gasification process was completed. Therefore, the particles sample was dried for 1 h at 105 °C and divided into three samples, each weight 20 mg. Each sample was heated to 800 °C for 1 h with O2 by a TGA. The carbon conversion rate (x) was derived with an average value from three tests: x¼
3 3 m0;i −mash;i wt;ash 1X 1X xi ¼ 1− 100% 3 i¼1 3 i¼1 mash;i wt;FC
ð4Þ
where x is the average carbon conversion rate of three tests, xi is the carbon conversion rate in test i, m0,i is total sample mass in test i, mash,i is terminal sample mass in test i, wt,ash is the ash mass fraction of LJ coal and wt,FC is the fixed carbon mass fraction of LJ coal. 3. Results and discussion Considering to the safety issues during the reaction process experiments were conducted under atmosphere pressure in this study. Furthermore, the experiments under pressurized condition and with H2O/O2 supplied as two-stage supply would be conducted later and presented elsewhere.
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oxygen depletion as shown in Fig. 2, called the Combustion and Gasification Region. Obviously, the rate of temperature decrease rate was rapid due to char gasification and then grew slow again until the reaction reaching the thermodynamic equilibrium, called the Gasification Region and Equilibrium Region respectively. Under the operating condition of Group A, the highest temperature was not measured and the process temperature was only slightly below the coal ash fusion temperature. The changing tendency of process temperature was in agreement with the literatures' [5,20,24]. Under the operating condition of Group B, the process temperature was lower than the corresponding level of Group A for oxygen deficiency, especially in the region around the second-stage water nozzle, called Second-stage Water Region. This is because H2O was only used as the second stage gasifying agent supply in this study. Certainly, H2O can avoid the high process temperature and cool the second stage nozzles by replacing the O2 as two-stage gasifying agent supply. Furthermore, the mean process temperature was higher with larger O/C molar ratio under each operating condition due to increased char-O2 combustion. In general, the process temperatures of all operating conditions were basically below the AFT of coal samples, as proven by the coal residue forms in the slag lock-hopper. As the Fig. 2 shows, when the molar ratio of O/C is 1.1, the main gasification process temperature is higher than 1300 °C and 1250 °C for Group 4A and 4B, respectively. Under these temperatures, the gasification reactions are relatively easy to react. Comparing with Group 4A, the temperature of Group 4B is 38 °C lower at the second-stage water nozzle position, and the region temperatures obviously reduce by the nozzle spraying. As a result of that, the end of gasification temperature with Group 4B is approximately 1228 °C, which is 42 °C lower than that of Group 4A. At other molar ratio of O/C situations, the temperatures vary similarly with Group 4A and 4B. The results demonstrate that the O2 supplying has a significant impact on gasification process temperature for the char combustion and gas combustion, and two-stage water plays an effective role on reducing the overall temperature in the reactor. 3.2. Syngas composition
Under all the eight operating situations, the highest temperature during the gasification process was higher than that taken by the first thermocouple, and located about 100 mm from the reactor primary nozzle due to volatile combustion with O2, called the Pyrolysis and Combustion Region. Then, the temperature was decreasing slowly until the
In this article, the raw syngas consisted primarily of CO, CO2, H2O, H2, N2 and CH4. In order to clearly indicate the syngas composition variation under different operating conditions, the original data was processed as presented in Fig. 3. The main studied gases were CO, CO2, H2 and CH4, which were also reported by other researchers [24,25]. As the molar ratio of O/C increased, the process temperature increased due to enhanced char combustion. Therefore, the endothermic reactions C + CO2, C + H2O were promoting to produce CO and H2, and consume CO2 at the same time. In the entrained flow gasifier, the proportion of
Fig. 2. Temperature distributions in the reactor.
Fig. 3. Syngas composition versus different O/C molar ratio.
3.1. Temperature distribution
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H. An et al. / Powder Technology 306 (2017) 10–16
CH4 in the syngas composition was very small and because less with increasing temperature [24]. Quantitatively, as the O/C molar ratio was increased from 0.9 to 1.1, the fraction of CO increased from 33.4% to 46.4% for Group A and from 32.8% to 44.0% for Group B, while the fraction of H2 increased from 26.9% to 32.2% for Group A and from 25.6% to 36.1% for Group B, and the fraction of CO2 decreased from 38.0% to 21.2% for Group A and from 39.4% to 19.5% for Group B. In other words, the increased O2 resulted in an increasing of the CO fraction by 38.9% and 34.1%, and increased the H2 fraction by 19.7% and 41.0% for the Group A and B, respectively. The results showed that increasing of the O/C molar ratio had a larger impact on the H2 generating with the second stage water injection. Comparison with Group A, Group B had an obvious promotion of H2 fraction in the syngas because the second stage water injection enhanced mass transport through the boundary layer of a char particle and promoted the primary water gas reaction (C + H2O → CO + H2) and water gas shift reaction (CO+ H2O → CO2 + H2). As a second way to quantify the effect of two-stage water injection on the syngas composition, the difference between each gas fraction between second stage water injection (Group B) and non-second stage injection (Group A) has been presented in Fig. 4 with different molar ratio of O/C. As the Fig. 4 shows, when molar of O/C is larger than 0.93, Group B has a higher fraction of H2 production in the syngas than Group A. It proved that the second stage water injection began to be advantage to produce more H2 under that O/C molar ratio situation. When molar ratio of O/C was N0.96, Group B has a higher fraction of CO + H2 production and a lower fraction of CO2 production than Group A, respectively. The results can be analyzed by the gasification reaction rate changing. When molar of O/C was larger than 0.93, the process temperature was higher than 1100 °C at the second stage point, the gasification reaction rate was governed by the chemical reaction rate mainly [26–28] and increased gradually with the disturbance caused by the second stage water injection. Under this temperature condition, the C + CO2 and C + H2O reactions began to be promoted by the diffusion effect, but the extent was different. Obviously, the impact on C + H2O reaction was larger than C + CO2 reaction. When molar ratio of O/C was larger than 0.96, the process temperature was higher than 1200 °C at the second-stage point and the external and internal diffusion influence gradually became an important factor on the gasification reaction rate, which can be expressed as Eq. (3). Meanwhile, the second stage water injection caused the airflow disturbance and the atmosphere around the char particle was not stagnant any longer. In another word, the Reynolds number between reactant gas with the char particle cannot be assumed as nearly zero as that in an entrained flow gasifier with non-second-stage injection [23]. In addition, as the molar ratio of O/C
increased, the more second-stage water was introduced. The higher H2O concentration helped more H2 to be produced at the final thermodynamic equilibrium, which was shown as a constant increase with the molar ratio of O/C increasing [29]. Finally, the higher temperature resulted in the lower CH4 fraction due to the significant influence by the process temperature on the methane-steam reforming reaction. 3.3. Carbon conversion and effective syngas The carbon contents in the ash were derived by a TGA and the carbon conversion was calculated by Eq. (4), presented in Table 3. Further, the changing of carbon conversion and fraction of CO + H2 has been plotted versus the molar ratio of O/C in Fig. 5. As the molar ratio of O/ C increases from 0.9 to 1.1, the carbon conversion of non-second stage water and second stage water is increased from 72.48% to 91.67%, and from 71.39% to 93.24%, respectively. Consistent with the above discussions and results, the char combustion and gasification were greatly promoted by increased H 2 O injection. Furthermore, comparing the carbon conversion plot versus the molar ratio of O/C between Group A and Group B, a crossover point was shown at a molar ratio of O/C equal to 0.95. It means that in conditions of lack of oxygen and low temperature, the promoting of char-gasification by second stage water injection was weaker than that of char-combustion by equal oxygen injection in the primary nozzle. However, when the molar ratio of O/C was larger than 0.95, the second stage water injection promoted the char-gasification rapidly and resulted in a 1.7% increase of carbon conversion with 1.1 O/C molar ratio compared to non-second stage water injection. The effective gas fraction in syngas presents a similar tendency with the carbon conversion plot in Fig. 5, and the optimum fraction of CO + H2 is 78.54% and 80.17% for non-second stage water and second-stage water, respectively. In other words, the second stage water injection made a positive effect on entrained flow gasification when the molar ratio was larger than 0.95 and obtained an optimum carbon conversion and effective syngas fraction at 1.1 O/C molar ratio. To certify applicability of the entrained flow gasification technology with dry residue by two-stage water supply, the optimum carbon conversion and effective syngas fraction is contrasted with the literatures' [1,3,31,32] in Table 4. The types of reactors, feedstocks, coals and carbon conversions are presented with raw data. In order to compare with each group results of oxygen consumption and syngas composition feasibly, the ratio of O2/coal (Nm3/kg) per hour for each experiment is used, and syngas composition is only considered to consist of CO, H2 and CO2 under dry basis. The operation temperature in entrained flow gasifier with pulverized-coal is always higher than for CWS due to lack of water. But, the gasification temperature needs to be increased by over 150 °C at least than the AFT of coals to ensure the liquid slag-off. The gasification of Baodian coals was operated up to 1600 °C, and the Nantun coal was blended with limestone to reduce the AFT [1]. The gasification Table 3 Carbon content in the ash and carbon conversion.
Fig. 4. Fraction of syngas change between second-stage water and non-second stage water.
Parameter
Slag mass
Carbon content mass in the slag
Operating condition
mg 1
2
3
1
2
3
–
1A 1B 2A 2B 3A 3B 4A 4B
20.15 20.08 19.88 19.95 20.22 19.89 20.23 19.85
20.23 20.45 19.95 20.24 19.96 20.06 20.17 19.98
19.97 20.11 20.16 20.14 20.07 20.13 20.28 20.23
10.58 10.78 9.64 9.70 8.45 8.04 5.15 4.24
10.71 10.97 9.88 9.96 8.42 8.11 5.12 4.28
10.48 10.76 9.97 9.88 8.45 8.16 5.01 4.36
72.48 71.39 76.06 76.25 82.06 83.18 91.67 93.24
mg
Average carbon conversion %
H. An et al. / Powder Technology 306 (2017) 10–16
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gasification. By raising the ratio of second stage water supply and well introducing proportional O2 into the second stage water, the carbon conversion and fraction of CO + H2 could be promoted further. That was because the second stage O2 can promote the region temperature and large airstream of second stage water can enhance the external diffusion rate around the char particles. Both of that can promote the char gasification to produce increased syngas of CO and H2. 4. Conclusions
Fig. 5. Effective syngas fraction and carbon conversion.
technologies with pulverized-coal obtained a higher fraction of CO + H2 obviously than that with CWS. However, even when steam is introduced into the reaction, the fraction of H2 production is still lower than gasification with CWS. The gasification with Shenfu CWS, a typical coal with low AFT, obtained a mean result with 98.4% carbon conversion and 81.0% fraction of CO + H2, under the ratio of O2/coal equal to 0.66 in a coking plant [3]. The high AFT coals are expected to be very unsuitable in entrained flow gasifier with CWS for difficult liquid slag-off. Thus, blending with low AFT coals to reduce the high AFT is a general solution. The EFG-R and OMB-EFG with blend coal obtained 83.5% and 82.5% of CO + H2 fraction in the syngas, respectively. They had a O2/coal equal to 0.66 and 0.64 [1,32]. In this study, LJ bitumite with high AFT was used in a CWS entrained flow gasifier with dry residue-off, not blending any additive to reduce the ash fusion temperature. Under the general operation methods with non-second stage water supply and O2 /coal of 0.72, the carbon conversion was 91.67% and the fraction of CO + H 2 was 78.8%. Though the results were affected by the heat loss from the reactor and insufficient residence time of char particles, they were unquestionably comparable to the gasification with low AFT coals. Then, when second-stage water was introduced into the gasification proceeding, the carbon conversion and the fraction of CO + H 2 rose to 93.24% and 80.4%, respectively. Meanwhile, the O2/coal reduced to 0.65 which was approaching the mean value in the CWS gasification with low AFT coals. Generally speaking, the entrained flow gasification technology with dry slag by two-stage water supply was found to be very suitable for high AFT coals
In this study, experimental system of a 20 kg/h entrained flow gasifier with dry slag by second-stage water supply was established. The operating conditions were chosen to verify the application of second-stage water gasification technology, by changing the overall molar ratio of O/C and the oxygen element ratio between second-stage and first-stage. The data of process temperature in the gasifier, syngas composition and carbon conversion were processed, analyzed and presented. The characteristics of entrained flow gasification technology with dry slag by secondstage water supply were discussed and following conclusions were drawn: (1) Increasing the molar ratio of O/C appropriately can increase the process temperature in the entrained flow gasifier, effective syngas fraction and carbon conversion with non-second stage and two-stage water supply. When the molar ratio of O/C is 1.1 for Guizhou JZ coals in this study, an optimal gasification result of entrained flow gasification technology with dry slag by secondstage water supply (effective syngas fraction is 80.2% and carbon conversion is 93.24%) is obtained, and the ash residue will not melt at the same time. (2) Increasing the molar ratio of O/C can make a larger positive impact on H2 generating and a smaller positive impact on CO generating with second stage water injection than non-second stage water supply. (3) As the molar ratio of O/C is larger than 0.93, the fraction of H2 product with second stage water injection begins to exceed that with non-second stage water injection. As the molar ratio of O/C is larger than 0.96, the fraction of effective syngas(CO + H2) product with second stage water injection begins to exceed that with non-second stage water injection and the difference value gradually tends to equilibrium. However, the fraction of CO product with second stage water injection is always lower than that with non-second stage water injection. (4) When the molar ratio of O/C is larger than 0.95, the carbon conversion with second stage water injection begins to exceed that with non-second stage water injection.
Table 4 Performance of different entrained flow gasification with literatures. Literature
[1] [1] [1] [3] [31] [31] [32] This article 4A This article 4B
Reactora & feeding typeb
EFG-R with CWS-fed EFG-M with dry-fed EFG-M with dry-fed EFG with CWS-fed EFG with dry-fed EFG with dry-fed OMB-EFG with CWS-fed EGDSSW with CWS-fed EGDSSW with CWS-fed
Coal typec
Blend coal with LAFT Nantun coal with HAFT Baodian coal with HAFT Shenfu coal no.6 with LAFT Tuncbilek lignite with LAFT Soma lignite with HAFT Blend coal with LAFT LJ bitumite with HAFT LJ bitumite with HAFT
O2/coal (Nm3/kg)
0.66 0.62 0.75 0.66 0.64 0.62 0.64 0.72 0.65
Carbon conversion (%)
98.3 99 100 98.4 – – – 91.67 93.24
Syngas composition dry basis (vol.%) CO
H2
CO2
46.5 66.3 65.7 45.1 60.1 62.4 44.9 46.5 44.2
37.0 27.3 24.9 35.9 20.5 24.2 35.6 32.3 36.2
16.5 6.3 9.6 17.7 19.4 13.4 19.5 21.2 19.6
a EFG is entrained flow gasifier, −R means gasifier with refractory brick, −M means gasifier with membrane; OMB-EFG is opposition multi-burner entrained flow gasifier; EGDSSW is entrained flow gasifier with dry slag by second-stage water supply. b CWS-fed is gasification with coal water slurry, dry-fed is gasification with dry pulverized-coal. c LAFT and HAFT is low ash fusion temperature and high ash fusion temperature, respectively.
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Experiment study on entrained flow gasification technology with dry slag by second-stage water supply Haiquan An a, Juan Yu a, Junjie Fan b, Yanchi Jiang a, Zhongxiao Zhang a,⁎ a b
School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Institute of Combustion and Gasification, College of Power Engineering, University of Shanghai for Science & Technology, Shanghai 200093, China
a r t i c l e
i n f o
Article history: Received 22 May 2016 Received in revised form 28 October 2016 Accepted 9 November 2016 Available online 11 November 2016 Keywords: Entrained flow gasification Second-stage water Syngas composition Temperature distribution
a b s t r a c t Experimental system of a new entrained flow gasifier with second-stage water supply (20 kg/h) was set up in this paper, and effect of molar ratio of O/C on temperature distribution, syngas composition, carbon conversion and effective syngas were analyzed with a typical high AFT coal (Guizhou LJ coal). The results of non-secondstage water and second-stage water with changing molar ratio of O/C indicate that, when the molar ratio of O/ C is larger than 0.96, gasification efficiency with dry slag by second-stage water would become better than non-second-stage water. Specifically, when the molar ratio of O/C was 1.1, the effective syngas fraction and carbon conversion of second-stage water gasification were 80.2% and 93.24% respectively. Correspondingly, the effective syngas fraction and carbon conversion of non-second-stage water gasification were 78.5% and 91.67% respectively. Furthermore, the gasification temperature was lower than the ash fusion temperature of test sample, the slag was non-melt during all the gasification experiments. The optimal gasification was conducted with the process temperature under 1450 °C, which was lower than the ash fusion temperature of LJ coal. These experimental results prove the applicability of entrained flow gasification technology with dry slag by second-stage water supply on using high AFT coals, which is helpful to generalize the large-scale gasification technology using abundant high AFT coals in China. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Entrained flow gasification technology is widely employed in chemical and energy industrial fields, such as manufacturing chemical products and IGCC power generation technology [1,2] in China. Due to the highly efficient and clean carbon conversion process, large-scale pressurized entrained-flow gasification has been considered as a crucial technology for clean coal utilization [3,4]. In recent years, research on entrained flow gasification with high ash fusion temperature (AFT) coals has been conducted gradually due to abundant high AFT coals deposits in China [5–7]. Generally, based on the coal feeding mode, the entrained flow coal gasification technology can be divided into coal water slurry (CWS) gasification and pulverized coal gasification. Both of the two type gasification technologies are widely employed in industry. In comparison with pulverized coal gasification [8], CWS gasification can obtain higher volume fraction of H2 composition in the syngas [1]. In addition, the preparation of CWS is more complex but the transport of CWS is more stable [9]. The process temperature of CWS gasification can be adjusted with the oxygen injection, but usually would not exceed 1400 °C at the ⁎ Corresponding author at: Room A215, School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dongchuan RD, Minhang District, Shanghai, China. E-mail address: [email protected] (Z. Zhang).
http://dx.doi.org/10.1016/j.powtec.2016.11.009 0032-5910/© 2016 Elsevier B.V. All rights reserved.
outlet of the gasification reactor [10]. Nowadays, mainly entrained flow gasification prefers coals with low ash content and low AFT (below 1400 °C) to achieve steady slag tapping, because they have appropriate slag viscosity during that temperature [11]. When the high AFT coals (especially for the AFT higher than 1500 °C) should be used as the fuel of entrained flow gasification technologies [1,3,4,8], it is impractical to simply introduce more O2 to raise the operation temperature because the tolerable temperature of refractory brick or membrane in gasifier is limited. Thus, blending with the low AFT coals or other additives to reduce the FT is generally acceptable. However, it is easy to generate nonmelting slag due to wrong blending ratio in industrial production. Besides, the cost of additives is usually expensive. Based on our research, a new idea is presented to solve the problem on entrained flow gasification with high AFT coals. Under the condition of oxygen needed to keep the high efficiency gasification, by adding two opposite nozzles as second stage O2/H2O injection, the oxygen injection by the primary nozzle is properly allocated. Reducing the oxygen injection from the primary nozzle, the regional violent combustion near the primary nozzle will be moderated and the temperature will be lowered, with the result that the coal ash would not melt. As a result of oxygen classification and water injection, the temperature during gasification process was lower than the AFT of using coal and the slag was solid when it was expelled from the gasifier. In detail, the gasifying agent refers to O2 and H2O, the specific composition and proportion of them
H. An et al. / Powder Technology 306 (2017) 10–16
depend on the gasification situation, coal type, syngas composition requirements, etc. The second-stage O2/H2O injection can greatly improve the flow field disturbance effect of the gasifier reactor. Tests and simulations of the flow field distributions in a cold model entrained flow gasifier have been conducted with different injecting velocity, position, number of nozzles. The results demonstrate that the flow field distributions in the gasifier would be greatly improved due to more reflux gas forming by two-stage O2/H2O injection [12]. Furthermore, the gasification reaction would be enhanced with the strong gas transfer in the boundary layer of the char particle caused by two-stage O2/H2O injection under high temperature. In this paper, based on other studies on the entrained flow gasification experiments [13–17], a 20 kg/h entrained flow gasifier with dry slag by second-stage water supplying (EGDSSW) was established, and experiments were conducted under 8 different operating conditions, 4 with second stage gasifying agent injection and 4 without. In this study, the second stage gasifying agent with oxygen atom refers to water only due to safety issues and equipment constraints. The experiments with second stage O2/H2O supply would be presented in next work. 2. Experimental apparatus and procedure 2.1. Coal water slurry preparation The entrained flow gasification technology with dry slag by secondstage water supplying was particularly for coals with high ash fusion temperature (AFT), therefore the Guizhou LJ coal (ash flow temperature above 1500 °C) was used in this experiment. The LJ coal was pulverized with a high speed ball mill and mixed with deionized water containing the additives [18,19], the loading of coal reached to 65% in the CWS. The mean diameter of the coal particles was approximately b50 μm. The proximate analysis, ultimate analysis, ash composition analysis and ash fusion characteristic data of LJ coal were presented in Table 1. 2.2. Experimental apparatus and procedure In this study, the entrained flow gasifier with dry slag by secondstage water supply (EGDSSW) was designed, constructed, and operated. The work of EGDSSW started in 2013, and the objective was to provide valuable information and optimal operating parameters for the entrained flow gasification technology with dry residue by secondstage water supply using typical high AFT coal. A schematic of the EGDSSW system was presented in Fig. 1. The EGDSSW consists of an alumina-based refractories lined reactor (0.46 m in inner diameter and 2 m in vertical reactor wall length) with a conical shaped outlet followed by a water sprayed quench chamber for syngas cooling and gas/solid-residue separation. The process temperatures in the reactor are recorded by five thermocouples placed vertically Table 1 Main properties of high AFT coal A. Coal type
LJ coal
Proximate analysis, %
Ultimate analysis, %
Mad
Aad
Vad
FCad
Cad
Had
Oad
Nad
Sad
2.49
13.00
31.99
52.52
73.28
4.52
4.32
1.36
1.03
11
inside the reactor, which are shielded by protective ceramic encapsulation. Three thermocouples were mounted in the combustion and gasification reaction region (upper part) of the reactor, two thermocouples were mounted in the gasification reaction region (lower part) of the reactor. The coal water slurry (CWS) was stirred in an agitation tank at least for 1 h before experiment, and kept stirring during the experiment proceeding. The CWS was transported by a screw pump and introduced in the top of the reactor. The mass of CWS was controlled by the frequency of the pump (from 1– 50 Hz) and calibrated for 20 min at certain frequency repeatedly. To ensure the CWS was transported as a liquid to the top nozzle, cooling water was flowed through a concentric vertical tube near to the primary nozzle. The first stage oxygen was evaporated in a liquid oxygen evaporator and introduced in the top of the reactor with the CWS. The accuracy of oxygen flow was controlled by a MFC and adjusted by a ball valve. The second stage water was introduced by two opposed nozzles located 0.5 m from the top of the reactor, the deionized water was controlled by a double plunger metering pump. To ensure the opposed nozzles closed in a safe condition, a stream of N2 was flowed constantly through a supply pipe that surrounded them into the reactor. The N2 came from a group of nitrogen cylinders (with 99.99% N2) and controlled by a MFC. Prior to beginning the reaction process, the reactor was heated by diesel fuel and air, which was introduced in the top-end of the reactor. The two nozzles were symmetrical and titled toward the center axis of the reactor. Meanwhile, the diesel and air was separately controlled by a turbine flowmeter. The temperature in the reactor increased rapidly to reach approximately 1000 °C (nearly 1 h), then the mass flows of diesel and air were decreased to warm the reactor uniformly (nearly 1 h). After that, the diesel was cut off, and a small amount of N2 replaced the air to cool the diesel nozzle. The gasification process started by feeding the CWS and first stage oxygen, and then the second stage water was added to cool the reaction temperature and enhance the blending the reactants. The raw syngas, cooling N2 and other particulates (mainly ash and char) were cooled by cooling water in the water quench vessel resulting in exit gas temperatures below 100 °C. Henceforward, the purged raw syngas was bubbled through the chimney to leave the gasifier. In order to measure the volume concentration of the exit gas (mainly CO, CO2, CH4 and H2) of raw syngas, a gas analysis meter (Gasboard 3100) was used after a two-step filter. After gas sampling, the produced syngas was collected to study for CO2 absorption and capture or burned out by a small burner. When the experiment was over, the particles (ash and char) were sampled from the black water flowing out at the bottom of the gasifier. The carbon conversion was derived using a Thermogravimetric Analyzer (TGA) with the particles samples. 2.3. Experimental condition The main experimental parameters for performing this gasification experiment were presented in Table 2. The CWS feeding rate was 20 kg/h for each experiment, approximately 13 kg/h LJ coal feeding. The experiments can be divided into two groups, Group A and Group B. Group A represented the experiments without second stage water supply, and Group B represented the experiments with water supply through the second stage nozzles. The molar ratio of O/C was defined as the ratio between O (including oxygen element of O2, steam and coal) and fixed carbon of coal, derived as [5,17,20,21]:
Composition of ash %
LJ coal
SiO2
Al2O3333
Fe2O33
CaO
MgO
Na2O
K2O
TiO2
SO3
53.96
30.23
5.77
2.87
0.89
0.37
0.64
2.92
1.62
Ash fusion temperature, K
LJ coal
Qnet,ar, (kJ/kg)
DT
ST
FT
1703
N1773
N1773
28,530
λ¼
no;c þ no;o þ no;s nc;c
ð1Þ
where no,c is the mole fraction of oxygen in coal sample, no,o is the mole fraction of oxygen in first stage and second stage O2, no,s is the mole fraction of oxygen in the second stage oxygen element, and nc,c is the mole fraction of fixed carbon in coal sample.
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H. An et al. / Powder Technology 306 (2017) 10–16
Fig. 1. The EGDSSW system.
The supplied oxygen was introduced by two steps. Approximately 75– 90% of total supplied oxygen would be injected from the first stage, and mixture of O2 and H2O would be injected from the second stage to adjust the gasification temperature and strongly blend the reactants. The oxygen element ratio between second stage and first stage was represented:
η¼
no;so þ no;s no;fo
ð2Þ
where, no,fo is the mole fraction of oxygen in first stage oxygen and no,so is the mole fraction of oxygen by the second stage nozzles.
In this study, effect of molar ratio of O/C (λ) and the second/first stage ratio (η) were analyzed. The selected λ values are 0.9, 0.95, 1.0 and 1.1, and the η values are 0 or 1/9. However, in order to keep the gasification system steady and safe, only water was supplied from the second stage O2/H2O nozzles in this study. When the second stage water was introduced into the reactor, the process temperature was over 1200 °C, so the apparent reaction rates of char-H2O and char-CO2 do not only depend on the chemical reaction rate, but also the pore and external diffusions should be considered, which have significant effects on the gasification efficiency. In this article, the external diffusion was linked to gas concentration by mass transfer in the boundary layer of a sphere char particle and was limited by the Fick's Law [22]. In addition, a gravimetric stoichiometric
Table 2 Operating condition parameters. Parameter Operating condition
CWS feeding rate kg/h
O/C molar ratio (λ) \
First-stage oxygen Nm3/h
Second-stage oxygen/steam (Nm3/h)/(kg/h)
Second/first stage ratio(η) \
System pressure Bar
1A 1B 2A 2B 3A 3B 4A 4B
20 20 20 20 20 20 20 20
0.9 0.9 0.95 0.95 1.0 1.0 1.1 1.1
7.6 6.8 8.1 7.3 8.6 7.7 9.4 8.5
0/0 0/1.22 0/0 0/1.30 0/0 0/1.38 0/0 0/1.51
0 1/9 0 1/9 0 1/9 0 1/9
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
H. An et al. / Powder Technology 306 (2017) 10–16
coefficient was used to relate the gas diffusion rate to the reaction rate of char, the apparent reaction rate under high temperature was presented by the reference [23]: r¼
6Ni DM;i Mi 2
d ρRT
pbulk;i −pC;i
2 þ 0:6Re0:5 Sc0:33
ð3Þ
where J is the mass flux of reactant gas at the particle outer surface, h is the mass transfer coefficient, Mi is the molar mass of the reactant gas, the cbulk,i and cC,i are molar concentrations in the bulk phase and at the outer particle surface, r is the apparent reaction rate under high temperature, Ni is the gravimetric stoichiometric coefficient of the reaction with char and reactant gas, d is the particle diameter, ρ is the apparent density of the char particle, DM,i is the molecular diffusion coefficient for the reactant gas, and Re and Sc are the Reynolds number and Schmidt number between reactant gas with the char particle in the reactor, respectively. The ash and char particles were collected when the gasification process was completed. Therefore, the particles sample was dried for 1 h at 105 °C and divided into three samples, each weight 20 mg. Each sample was heated to 800 °C for 1 h with O2 by a TGA. The carbon conversion rate (x) was derived with an average value from three tests: x¼
3 3 m0;i −mash;i wt;ash 1X 1X xi ¼ 1− 100% 3 i¼1 3 i¼1 mash;i wt;FC
ð4Þ
where x is the average carbon conversion rate of three tests, xi is the carbon conversion rate in test i, m0,i is total sample mass in test i, mash,i is terminal sample mass in test i, wt,ash is the ash mass fraction of LJ coal and wt,FC is the fixed carbon mass fraction of LJ coal. 3. Results and discussion Considering to the safety issues during the reaction process experiments were conducted under atmosphere pressure in this study. Furthermore, the experiments under pressurized condition and with H2O/O2 supplied as two-stage supply would be conducted later and presented elsewhere.
13
oxygen depletion as shown in Fig. 2, called the Combustion and Gasification Region. Obviously, the rate of temperature decrease rate was rapid due to char gasification and then grew slow again until the reaction reaching the thermodynamic equilibrium, called the Gasification Region and Equilibrium Region respectively. Under the operating condition of Group A, the highest temperature was not measured and the process temperature was only slightly below the coal ash fusion temperature. The changing tendency of process temperature was in agreement with the literatures' [5,20,24]. Under the operating condition of Group B, the process temperature was lower than the corresponding level of Group A for oxygen deficiency, especially in the region around the second-stage water nozzle, called Second-stage Water Region. This is because H2O was only used as the second stage gasifying agent supply in this study. Certainly, H2O can avoid the high process temperature and cool the second stage nozzles by replacing the O2 as two-stage gasifying agent supply. Furthermore, the mean process temperature was higher with larger O/C molar ratio under each operating condition due to increased char-O2 combustion. In general, the process temperatures of all operating conditions were basically below the AFT of coal samples, as proven by the coal residue forms in the slag lock-hopper. As the Fig. 2 shows, when the molar ratio of O/C is 1.1, the main gasification process temperature is higher than 1300 °C and 1250 °C for Group 4A and 4B, respectively. Under these temperatures, the gasification reactions are relatively easy to react. Comparing with Group 4A, the temperature of Group 4B is 38 °C lower at the second-stage water nozzle position, and the region temperatures obviously reduce by the nozzle spraying. As a result of that, the end of gasification temperature with Group 4B is approximately 1228 °C, which is 42 °C lower than that of Group 4A. At other molar ratio of O/C situations, the temperatures vary similarly with Group 4A and 4B. The results demonstrate that the O2 supplying has a significant impact on gasification process temperature for the char combustion and gas combustion, and two-stage water plays an effective role on reducing the overall temperature in the reactor. 3.2. Syngas composition
Under all the eight operating situations, the highest temperature during the gasification process was higher than that taken by the first thermocouple, and located about 100 mm from the reactor primary nozzle due to volatile combustion with O2, called the Pyrolysis and Combustion Region. Then, the temperature was decreasing slowly until the
In this article, the raw syngas consisted primarily of CO, CO2, H2O, H2, N2 and CH4. In order to clearly indicate the syngas composition variation under different operating conditions, the original data was processed as presented in Fig. 3. The main studied gases were CO, CO2, H2 and CH4, which were also reported by other researchers [24,25]. As the molar ratio of O/C increased, the process temperature increased due to enhanced char combustion. Therefore, the endothermic reactions C + CO2, C + H2O were promoting to produce CO and H2, and consume CO2 at the same time. In the entrained flow gasifier, the proportion of
Fig. 2. Temperature distributions in the reactor.
Fig. 3. Syngas composition versus different O/C molar ratio.
3.1. Temperature distribution
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H. An et al. / Powder Technology 306 (2017) 10–16
CH4 in the syngas composition was very small and because less with increasing temperature [24]. Quantitatively, as the O/C molar ratio was increased from 0.9 to 1.1, the fraction of CO increased from 33.4% to 46.4% for Group A and from 32.8% to 44.0% for Group B, while the fraction of H2 increased from 26.9% to 32.2% for Group A and from 25.6% to 36.1% for Group B, and the fraction of CO2 decreased from 38.0% to 21.2% for Group A and from 39.4% to 19.5% for Group B. In other words, the increased O2 resulted in an increasing of the CO fraction by 38.9% and 34.1%, and increased the H2 fraction by 19.7% and 41.0% for the Group A and B, respectively. The results showed that increasing of the O/C molar ratio had a larger impact on the H2 generating with the second stage water injection. Comparison with Group A, Group B had an obvious promotion of H2 fraction in the syngas because the second stage water injection enhanced mass transport through the boundary layer of a char particle and promoted the primary water gas reaction (C + H2O → CO + H2) and water gas shift reaction (CO+ H2O → CO2 + H2). As a second way to quantify the effect of two-stage water injection on the syngas composition, the difference between each gas fraction between second stage water injection (Group B) and non-second stage injection (Group A) has been presented in Fig. 4 with different molar ratio of O/C. As the Fig. 4 shows, when molar of O/C is larger than 0.93, Group B has a higher fraction of H2 production in the syngas than Group A. It proved that the second stage water injection began to be advantage to produce more H2 under that O/C molar ratio situation. When molar ratio of O/C was N0.96, Group B has a higher fraction of CO + H2 production and a lower fraction of CO2 production than Group A, respectively. The results can be analyzed by the gasification reaction rate changing. When molar of O/C was larger than 0.93, the process temperature was higher than 1100 °C at the second stage point, the gasification reaction rate was governed by the chemical reaction rate mainly [26–28] and increased gradually with the disturbance caused by the second stage water injection. Under this temperature condition, the C + CO2 and C + H2O reactions began to be promoted by the diffusion effect, but the extent was different. Obviously, the impact on C + H2O reaction was larger than C + CO2 reaction. When molar ratio of O/C was larger than 0.96, the process temperature was higher than 1200 °C at the second-stage point and the external and internal diffusion influence gradually became an important factor on the gasification reaction rate, which can be expressed as Eq. (3). Meanwhile, the second stage water injection caused the airflow disturbance and the atmosphere around the char particle was not stagnant any longer. In another word, the Reynolds number between reactant gas with the char particle cannot be assumed as nearly zero as that in an entrained flow gasifier with non-second-stage injection [23]. In addition, as the molar ratio of O/C
increased, the more second-stage water was introduced. The higher H2O concentration helped more H2 to be produced at the final thermodynamic equilibrium, which was shown as a constant increase with the molar ratio of O/C increasing [29]. Finally, the higher temperature resulted in the lower CH4 fraction due to the significant influence by the process temperature on the methane-steam reforming reaction. 3.3. Carbon conversion and effective syngas The carbon contents in the ash were derived by a TGA and the carbon conversion was calculated by Eq. (4), presented in Table 3. Further, the changing of carbon conversion and fraction of CO + H2 has been plotted versus the molar ratio of O/C in Fig. 5. As the molar ratio of O/ C increases from 0.9 to 1.1, the carbon conversion of non-second stage water and second stage water is increased from 72.48% to 91.67%, and from 71.39% to 93.24%, respectively. Consistent with the above discussions and results, the char combustion and gasification were greatly promoted by increased H 2 O injection. Furthermore, comparing the carbon conversion plot versus the molar ratio of O/C between Group A and Group B, a crossover point was shown at a molar ratio of O/C equal to 0.95. It means that in conditions of lack of oxygen and low temperature, the promoting of char-gasification by second stage water injection was weaker than that of char-combustion by equal oxygen injection in the primary nozzle. However, when the molar ratio of O/C was larger than 0.95, the second stage water injection promoted the char-gasification rapidly and resulted in a 1.7% increase of carbon conversion with 1.1 O/C molar ratio compared to non-second stage water injection. The effective gas fraction in syngas presents a similar tendency with the carbon conversion plot in Fig. 5, and the optimum fraction of CO + H2 is 78.54% and 80.17% for non-second stage water and second-stage water, respectively. In other words, the second stage water injection made a positive effect on entrained flow gasification when the molar ratio was larger than 0.95 and obtained an optimum carbon conversion and effective syngas fraction at 1.1 O/C molar ratio. To certify applicability of the entrained flow gasification technology with dry residue by two-stage water supply, the optimum carbon conversion and effective syngas fraction is contrasted with the literatures' [1,3,31,32] in Table 4. The types of reactors, feedstocks, coals and carbon conversions are presented with raw data. In order to compare with each group results of oxygen consumption and syngas composition feasibly, the ratio of O2/coal (Nm3/kg) per hour for each experiment is used, and syngas composition is only considered to consist of CO, H2 and CO2 under dry basis. The operation temperature in entrained flow gasifier with pulverized-coal is always higher than for CWS due to lack of water. But, the gasification temperature needs to be increased by over 150 °C at least than the AFT of coals to ensure the liquid slag-off. The gasification of Baodian coals was operated up to 1600 °C, and the Nantun coal was blended with limestone to reduce the AFT [1]. The gasification Table 3 Carbon content in the ash and carbon conversion.
Fig. 4. Fraction of syngas change between second-stage water and non-second stage water.
Parameter
Slag mass
Carbon content mass in the slag
Operating condition
mg 1
2
3
1
2
3
–
1A 1B 2A 2B 3A 3B 4A 4B
20.15 20.08 19.88 19.95 20.22 19.89 20.23 19.85
20.23 20.45 19.95 20.24 19.96 20.06 20.17 19.98
19.97 20.11 20.16 20.14 20.07 20.13 20.28 20.23
10.58 10.78 9.64 9.70 8.45 8.04 5.15 4.24
10.71 10.97 9.88 9.96 8.42 8.11 5.12 4.28
10.48 10.76 9.97 9.88 8.45 8.16 5.01 4.36
72.48 71.39 76.06 76.25 82.06 83.18 91.67 93.24
mg
Average carbon conversion %
H. An et al. / Powder Technology 306 (2017) 10–16
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gasification. By raising the ratio of second stage water supply and well introducing proportional O2 into the second stage water, the carbon conversion and fraction of CO + H2 could be promoted further. That was because the second stage O2 can promote the region temperature and large airstream of second stage water can enhance the external diffusion rate around the char particles. Both of that can promote the char gasification to produce increased syngas of CO and H2. 4. Conclusions
Fig. 5. Effective syngas fraction and carbon conversion.
technologies with pulverized-coal obtained a higher fraction of CO + H2 obviously than that with CWS. However, even when steam is introduced into the reaction, the fraction of H2 production is still lower than gasification with CWS. The gasification with Shenfu CWS, a typical coal with low AFT, obtained a mean result with 98.4% carbon conversion and 81.0% fraction of CO + H2, under the ratio of O2/coal equal to 0.66 in a coking plant [3]. The high AFT coals are expected to be very unsuitable in entrained flow gasifier with CWS for difficult liquid slag-off. Thus, blending with low AFT coals to reduce the high AFT is a general solution. The EFG-R and OMB-EFG with blend coal obtained 83.5% and 82.5% of CO + H2 fraction in the syngas, respectively. They had a O2/coal equal to 0.66 and 0.64 [1,32]. In this study, LJ bitumite with high AFT was used in a CWS entrained flow gasifier with dry residue-off, not blending any additive to reduce the ash fusion temperature. Under the general operation methods with non-second stage water supply and O2 /coal of 0.72, the carbon conversion was 91.67% and the fraction of CO + H 2 was 78.8%. Though the results were affected by the heat loss from the reactor and insufficient residence time of char particles, they were unquestionably comparable to the gasification with low AFT coals. Then, when second-stage water was introduced into the gasification proceeding, the carbon conversion and the fraction of CO + H 2 rose to 93.24% and 80.4%, respectively. Meanwhile, the O2/coal reduced to 0.65 which was approaching the mean value in the CWS gasification with low AFT coals. Generally speaking, the entrained flow gasification technology with dry slag by two-stage water supply was found to be very suitable for high AFT coals
In this study, experimental system of a 20 kg/h entrained flow gasifier with dry slag by second-stage water supply was established. The operating conditions were chosen to verify the application of second-stage water gasification technology, by changing the overall molar ratio of O/C and the oxygen element ratio between second-stage and first-stage. The data of process temperature in the gasifier, syngas composition and carbon conversion were processed, analyzed and presented. The characteristics of entrained flow gasification technology with dry slag by secondstage water supply were discussed and following conclusions were drawn: (1) Increasing the molar ratio of O/C appropriately can increase the process temperature in the entrained flow gasifier, effective syngas fraction and carbon conversion with non-second stage and two-stage water supply. When the molar ratio of O/C is 1.1 for Guizhou JZ coals in this study, an optimal gasification result of entrained flow gasification technology with dry slag by secondstage water supply (effective syngas fraction is 80.2% and carbon conversion is 93.24%) is obtained, and the ash residue will not melt at the same time. (2) Increasing the molar ratio of O/C can make a larger positive impact on H2 generating and a smaller positive impact on CO generating with second stage water injection than non-second stage water supply. (3) As the molar ratio of O/C is larger than 0.93, the fraction of H2 product with second stage water injection begins to exceed that with non-second stage water injection. As the molar ratio of O/C is larger than 0.96, the fraction of effective syngas(CO + H2) product with second stage water injection begins to exceed that with non-second stage water injection and the difference value gradually tends to equilibrium. However, the fraction of CO product with second stage water injection is always lower than that with non-second stage water injection. (4) When the molar ratio of O/C is larger than 0.95, the carbon conversion with second stage water injection begins to exceed that with non-second stage water injection.
Table 4 Performance of different entrained flow gasification with literatures. Literature
[1] [1] [1] [3] [31] [31] [32] This article 4A This article 4B
Reactora & feeding typeb
EFG-R with CWS-fed EFG-M with dry-fed EFG-M with dry-fed EFG with CWS-fed EFG with dry-fed EFG with dry-fed OMB-EFG with CWS-fed EGDSSW with CWS-fed EGDSSW with CWS-fed
Coal typec
Blend coal with LAFT Nantun coal with HAFT Baodian coal with HAFT Shenfu coal no.6 with LAFT Tuncbilek lignite with LAFT Soma lignite with HAFT Blend coal with LAFT LJ bitumite with HAFT LJ bitumite with HAFT
O2/coal (Nm3/kg)
0.66 0.62 0.75 0.66 0.64 0.62 0.64 0.72 0.65
Carbon conversion (%)
98.3 99 100 98.4 – – – 91.67 93.24
Syngas composition dry basis (vol.%) CO
H2
CO2
46.5 66.3 65.7 45.1 60.1 62.4 44.9 46.5 44.2
37.0 27.3 24.9 35.9 20.5 24.2 35.6 32.3 36.2
16.5 6.3 9.6 17.7 19.4 13.4 19.5 21.2 19.6
a EFG is entrained flow gasifier, −R means gasifier with refractory brick, −M means gasifier with membrane; OMB-EFG is opposition multi-burner entrained flow gasifier; EGDSSW is entrained flow gasifier with dry slag by second-stage water supply. b CWS-fed is gasification with coal water slurry, dry-fed is gasification with dry pulverized-coal. c LAFT and HAFT is low ash fusion temperature and high ash fusion temperature, respectively.
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