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Pilot scale entrained flow gasification of Turkish lignites
Journal of the Energy Institute xxx (2015) 1e7
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Pilot scale entrained flow gasification of Turkish lignites € nül a, M. Ziypak b, A. Akça b A. Ünlü a, *, U. Kayahan a, A. Argo a b
TUBITAK Marmara Research Center Energy Institute, p.o.b. 41470, Gebze, Kocaeli, Turkey Turkish Coal Enterprises, Hipodrom Cad. No:12, p.o.b. 06330, Yenimahalle, Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 June 2015 Received in revised form 10 August 2015 Accepted 6 September 2015 Available online xxx
As a part of a coal to methanol project, experimental results obtained from gasification of five different lignites, namely Saray, Çan, Orhaneli, Tunçbilek and Soma lignites, in a 6 ton/day pilot scale entrained flow gasifier are presented. The gasification experiments were conducted using pure oxygen as the gasification agent. The proximate and ultimate analyses for the lignites were performed and presented at the beginning of the study. The gasifier had been operated at an equivalence ratio (ER) of around 0.5 for all the runs. The syngas compositions (H2, CO and CO2 concentrations) and steady state reactor temperatures were recorded; cold gas efficiencies and lower heating values of produced gases were calculated for each run. The ash flow temperatures and lower heating values of lignites played an important role in the effective gasification of the lignites. Among the five lignites studied, two, namely Tunçbilek and Soma, were found to be appropriate for gasification with the entrained flow gasifier. The Tunçbilek lignite produced a syngas with 60.1% CO and 20.5% H2, which had a lower heating value (LHVmax) of 9.8 MJ/Nm3. The Soma lignite produced a syngas with 62.4% CO and 24.2% H2, which had a lower heating value (LHVmax) of 10.5 MJ/Nm3. For Saray, Çan and Orhaneli lignites, CO compositions were between 41 and 50.1%, H2 compositions were between 12.2 and 19.3% and the LHVmax values ranged between 6.6 and 8.3 MJ/Nm3. The cold gas efficiencies (hcold gas) for Saray, Çan, Orhaneli, Tunçbilek and Soma lignites were found to be 41, 42, 41, 64 and 72%, respectively. A short discussion about operational problems encountered during the gasification of Çan, Saray and Orhaneli lignites are also given in the discussion part. © 2015 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Coal Gasification Entrained flow gasification Pilot scale experiments
1. Introduction For their abilities to achieve high carbon conversion efficiencies, to produce tar free syngas, to reach high production capacities and to produce syngas with higher concentrations of H2 and CO [1,2] entrained flow gasifiers are attractive for chemicals production purposes. Entrained flow gasifiers mostly operate at high temperatures (1200e1500 C). The ash melts at these temperatures and flows out of the gasifier chamber as slag [3]. Since finely crushed coal particles (below 100 mm) are used in entrained flow gasifiers [4], short residence time gives satisfactory carbon conversion value in the gasifier. According to fuel feeding technology, entrained flow gasifiers are divided into two categories as dry-fed and slurry-fed [5]. In slurry-fed gasifiers, the fuel is mixed with some amount of water (around 40%) and fed to the gasifier as slurry (fuel þ water) by a slurry pump. Since feeding dry (solid) powder into high pressure chambers brings various difficulties, slurry-fed gasifiers are preffered for higher operating pressures. On the other hand, for slurry feeding, the energy penalty arising from water usage has to be considered, since significant amount of energy is required for the evaporation of the water [6,7]. Due to the energy penalty of slurry-fed gasifiers, it can be claimed that dry-fed gasifiers would be much more efficient for low rank coals such as lignite [8]. However, dry feeding also has its own disadvantage. It uses an inert gas, generally nitrogen, to blanket and transport pulverized fuel to the gasifier. This may cause a dilution of the produced syngas and a decrease in the lower heating value of the syngas produced. But the negative impact brought by the dilution effect, is expected to decrease as the capacity of the gasifier is increased. Even though entrained flow gasifiers are quite commonly used in commercial scale, parametric studies based on important parameters such as oxygen/fuel ratio [9], heating value, volatile matter and moisture content [10] which strongly affect the gas composition and carbon
* Corresponding author. Tel.: þ90 262 677 27 37; fax: þ90 262 641 23 09. E-mail address: [email protected] (A. Ünlü). http://dx.doi.org/10.1016/j.joei.2015.09.001 1743-9671/© 2015 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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conversion efficiency must be studied for each new fuel. Experimental data obtained from laboratory and pilot scaled gasifiers are still required for each type of fuel for modeling [11] design, and feasibility studies. Especially pilot scale testing (1e5 MWth), of coal plays an important role in improving the process understanding, validation, application of fundamental experimental and gasification technology development E.g. Roberts et al. compared the results of the laboratory and pilot scale entrained flow gasification tests and led a way to the refinement of the laboratory test procedures, measuring the properties such as slag viscosity and char reactivity [12]. Yun et al. performed pilot scale gasification testing of nine coals and obtained a “coal properties guideline” of ash melting temperature range and coal reactivity for IGCC plants [13]. Guo et al. investigated the effects of oxygen/carbon ratio and steam/carbon ratio to the gasification performance of the coal along with the operational performances of the instruments in a 30e45 t/day pilot scale pressurized entrained flow gasifier [14] and Shinada et al. investigated reliability and operating experience of a 200 t/d pilot entrained flow IGCC plant for scaling purposes [15]. Determination of optimum operation conditions and learning more about the transient behavior of the system for evaluation of low rank coals are crucial en route to ongoing R&D studies [16]. The data in the literature mostly generated from laboratory scaled gasifiers. Usually the effects of equivalence ratio (ER), gasification temperature, residence time, particle dimensions and different types of fuel on the measured gas composition and carbon conversion can be found in these studies. Generally, high residence time and gasification temperature increase the performance of the system but altering these parameters is not an easy task in pilot scale gasifiers due to the system dimensions and autothermal nature of the pilot gasifiers [17]. Another important parameter is the ash composition of the particular fuel studied. It has strong impact on the performance of entrained flow gasifier in two ways. It affects the fuel suitability, which is dependent on the ash flow temperature [18] and, it affects the H2/CO ratio of the produced syngas due to the catalytic effect of some minerals (Fe, Zn, Ni) for the water-gas shift reaction [19]. For example, for methanol production the so called M module (or stoichiometric ratio), which is defined as the (H2eCO2)/(CO þ CO2) mole fraction ratio, must be adjusted to a value just above 2 [20] by the water-gas shift reactor down-stream. The size of this reactor is thus affected by the original H2/CO ratio. Among all of these parameters equivalence ratio (ER) has major impact on gasification performance. When oxygen flow decreases, the concentrations of CO and H2 increase and carbon conversion decreases [21]. Reduction in ER also leads to a decrease in the reactor temperature. It is known that effective slag flow is very important and working temperature of the reactor must be adjusted at least 100 C above the ash flow temperature for continuous operation [22,23]. The entrained flow gasifier used in this study is located at the Turkish Coal Enterprise's (TKI) R&D facilities in Tunçbilek/Kütahya region of Turkey. The experiments were performed as a part of the “Optimization of Tunçbilek Gasification Systems” project, which is a joint project of TKI and TUBITAK Marmara Research Center Energy Institute. Currently, a continuation project between the same parties, namely the “Tunçbilek Coal to Methanol” project, is under progress where the gasifier is going to be integrated with a more through gas-cleaning process (COS hydrolysis and H2S removal), subsequently connected to gas conditioning (water-gas shift reactor and carbon dioxide removal system), syngas to methanol and distillation units. Presently, the gas-cleaning process is under construction, and the subsequent processes are under detailed design phase. The whole coal-to-methanol pilot-facility is expected to be operational at the second quarter of 2017. Although, gasification of coal is not a new concept, literature data about pilot scale gasification of Turkish lignites is very scarce. To the best of our knowledge, the pilot scale entrained flow gasifier used in this study is the only operational one in Turkey. Therefore, the main motivation of this study was to produce literature data for the entrained flow gasification of various Turkish lignites and to screen out the best candidates for further processing, i.e. for methanol production in the continuation project. 2. Pilot experiments A photograph of the pilot scale entrained flow gasification system can be seen in Fig. 1. The Tunçbilek gasifier is a dry-fed, autothermal pilot scale (6 ton/day) entrained flow type reactor operated at around atmospheric pressures. It is a cylindrical reactor with an inside diameter of 1 m and a height of 2.5 m. The vertically oriented gasifier has a top-down flow design with an integrated quencher (another 2.5 m of height) at the bottom. The pilot scale experimental procedure is as follows (see Fig. 2): at the start-up phase the solid fuel (lignite) was crushed (down to <100 mm) and dried (aimed to reach a moisture content around 10%) by the coal preparation facility. The gasifier was initially heated up to
Fig. 1. A photograph of the pilot scale entrained flow gasification system.
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Fig. 2. Schematic drawing of the pilot scale system.
around 600 C by an oil burner. After that point, oxygen and coal were started to be fed into the gasifier at proportions appropriate for combustion. At around 1100 C the oil burner was turned off and taken out of the gasifier. Finally, the coal to oxygen ratio was adjusted gradually to the desired value and the reactor was gradually switched from combustion to gasification mode. The fuel feeding rate was adjusted by using a screw feeder. Since pulverized coal and pure oxygen become very reactive when mixed, nitrogen was used as the carrier gas inside the fuel spray feeder. Oxygen and coal were only allowed to come into contact just before entering the reactor. The gasification reactions took place in the upper chamber of the reactor, and the produced gas was then led to the lower chamber where quenching occurs. In the quench chamber, water was sprayed into the syngas, which leads to a temperature drop to around 400 C. Later on, the syngas was further cooled down to 70 C by using a shell & tube heat exchanger. A spray tower located after the heat exchanger was used to remove remaining particulates from the gas stream. A mist eliminator located right after the spray tower was used to separate the mist in the gas stream. Finally, the syngas was directed to the flare through an ID fan, where it was combusted before releasing to the atmosphere. The maximum operating temperature for this particular reactor is around 1600 C, which is actually limited by the maximum operating temperature of the refractory lining. It should also be noted that the minimum operating temperature for any lignite should be 100 C above its ash flow temperature, since otherwise operating problems arise due to unmelted and partially softened ash. Thus, the operating temperatures were set accordingly. The syngas composition measurements were done by on-line analyzers. CO and CO2 were measured by SIEMENS ULTRAMAT 23 infrared analyzer system, H2 was measured by using SIEMENS CALOMAT 67 analyzer system and O2 was measured by using SIEMENS OXYMAT 61 analyzer system. _ from mines in various provinces of Turkey; Tekirdag /Saray mine, Çanakkale/ The lignites were provided by Turkish Coal Enterprises (TKI) € Çan mine, Orhaneli/Gümüs¸pınar mine, Tunçbilek/Omerler mine and Soma/Imbat coal washery. Table 1 lists the proximate analysis, ultimate analysis, heating values and ash flow temperatures of the lignites used. For the proximate analysis ASTM D-5142-09, for the ultimate analysis ASTM-D-5373-08 and for the determination of heating value ASTM-D- 5865-10 standard methods were used.
3. Results and discussion With respect to their Lower Heating Values (LHV), the lignites used in this study could be sorted as follows: Saray < Çan < Orhaneli < Tunçbilek < Soma. Coals with higher LHV values tend to have lower ash contents. When sorted according to ash content, the lignites turned out to be in the following order: Saray > Çan > Tunçbilek > Orhaneli > Soma. Another important parameter for entrained flow gasification is the ash flow temperature. The ash flow temperatures determined under reducing environment were in the following order: Saray < Soma < Orhaneli < Tunçbilek < Çan. In Fig. 3, the relation between equivalence ratio (ER), the reactor temperature value and operation time were given for all lignites. For each lignite, the ash flow temperature was also indicated on the corresponding plot. The last points on the plots were taken to represent the steady state values, after which the reactor temperatures are expected to stay constant with respect to time. Please cite this article in press as: A. Ünlü, et al., Pilot scale entrained flow gasification of Turkish lignites, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.09.001
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Table 1 LHV, ash flow, ultimate and proximate analysis of fuels tested. Fuel specifications
Çan lignite
Orhaneli lignite
Tunçbilek lignite
Soma lignite
Original based Dry based Moisture (wt. %) 14.35 e Ash (wt. %) 47.41 55.35 Volatile matter (wt. %) 26.27 30.67 Fixed carbon (wt. %) 11.97 13.97
Original based Dry based 9.90 e 34.63 34.62 31.81 35.31 27.09 30.10
Original based 21.85 10.41 38.57 29.17
Dry based e 13.32 49.35 37.32
Original based Dry based 8.20 e 19.91 21.69 31.79 34.63 40.01 43.68
Original based Dry based 7.05 e 11.38 12.24 36.54 39.31 45.03 48.44
C (wt. %) H (wt. %) N (wt. %) O (wt. %) S (wt. %) LHV (kcal/kg) Ash flow temperaturea
26.85 2.26 0.51 5.41 3.21 2196 1241 C
38.40 2.95 0.81 8.34 4.93 3444 >1500 C
53.05 4.14 0.86 8.01 1.75 4326 1302 C
67.88 5.3 0.99 10.26 2.25 5535
61.53 4.1 3.07 1.8 1.36 5391 1329 C
65.93 3.95 1.3 9.3 1.09 5658 1288 C
Highlightsb
Low LHV High amount of ash Low ash flow temperature
a b
Saray lignite
31.35 2.64 0.60 6.32 3.75 2593
42.63 3.28 0.91 9.26 5.48 3822
67.03 4.47 3.35 1.97 1.49 5872
Medium LHV High LHV High LHV Medium amount of ash Low amount of ash Low amount of ash Very high ash flow High amount of moisture High ash flow temperature Medium ash flow temperature temperature
70.93 4.25 1.4 10.01 1.17 6087
High LHV Low amount of ash Medium ash flow temperature
Measurements are made under reducing atmosphere. Low, medium and high terms used in this table shows relative values of a specification to each other and must not be understood globally.
For Saray lignite, due to its low lower heating value, an operating temperature 100 C above ash flow temperature could not be achieved. The high ash flow temperature of Çan lignite could not be surpassed, although a high value for the operating temperature (around 1440 C) was achieved. Due to operation below ash flow temperatures, these two lignites caused operational problems such as partial blockage of the reactor throat, problems for ash disposal system due to partial agglomeration of softened but not melted large ash pieces, fly ash build-up in heat exchanger tubes (Fig. 4). For the reasons listed above, these two lignites, namely Saray and Çan lignites, were found to be inappropriate for the pilot scale entrained flow gasifier. On the other hand, for the Tunçbilek and Soma lignites, the operation temperatures have well reached values that were at least 100 C higher than the corresponding ash flow temperatures. As a result, from operational point of view, these two lignites turned out to be suitable for the pilot scale entrained flow gasifier used. It should be noted that the coal-drying equipment was designed to reduce the moisture content of the pulverized coal on a single-pass basis. Therefore, Orhaneli lignite, which had high original moisture content, could not be dried down to the desired values before gasification with only single-pass drying. This cast a shadow on the performance of the Orhaneli lignite by decreasing the gasifier efficiency. With an appropriate drying process (that can decrease the moisture content to around 10%), most probably a much better gasification performance would have been achieved for the Orhaneli lignite. The compositions of the syngas for each lignite can be seen in Fig. 5. For an efficient gasification process, it is expected to have relatively higher CO and H2, and lower CO2 values in the syngas produced. With 60.1% CO and 20.5% H2 the Tunçbilek lignite and with 62.4% CO and 24.2% H2 the Soma lignite produced preferable syngases when compared with the other three lignites (see Table 2). Furthermore, the CO2 concentrations for Tunçbilek (19.4%) and Soma (13.4%) were much lower than the rest of the lignites (31.9%, 37.7% and 45.8% for Orhaneli, Çan and Saray, respectively). Therefore, for Tunçbilek and Soma lignites lower capacity CO2 separation units can be designed and total installation costs can be decreased. The actual gas-composition values, LHV of the syngases produced and cold gas efficiencies are listed in Table 2. Lower heating value of the syngas (LHVgas) was calculated by the following equation [24]
LHVgas ¼
283 CO þ 241:83 H2 100 22:4
And the cold gas efficiency of the syngas (hcold
hcold gas ¼
gas)
was calculated using the following formula [25]
Qgas LHVgas 1000 100 mcoal LHVcoal 4:18
The were calculated based on the syngas compositions including nitrogen. The cold gas efficiencies calculated for Tunçbilek and Soma lignites, as 64% and 72% respectively, were promising. The LHVgas values produced from Tunçbilek and Soma lignites were 9.8 MJ/Nm3, 10.5 MJ/Nm3 respectively and these values were suitable for subsequent processing technologies. With a 41% cold gas efficiency and a syngas lower heating value of 8.3 MJ/Nm3, the true potential of the Orhaneli lignite could not be realized. The main reason of this poor performance was wasting of the energy for the evaporation of moisture in the fuel. Although Saray lignite had a reasonable cold gas efficiency value of 41% and a syngas lower heating value of 6.6 MJ/Nm3, it did not seem to be cut out for entrained flow gasification technology. A similar performance issue could also be noticed for the gasification of Çan lignite, which has reached to 42% cold gas efficiency and 7.6 MJ/Nm3 syngas lower heating value. Saray and Çan lignites also caused ash melting related issues (as mentioned above). The gasification temperatures of Saray and Çan lignites could have been increased above the ash flow temperature to overcome the ash melting related blockage problems, if these coals were gasified at higher ER values i.e. close to the combustion zone. However, such an increase in the ER will lead to significant loss of lower heating value of syngas and therefore instead of gasification, direct combustion turns out to be the appealing alternative technology for Çan and Saray lignites.
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Fig. 3. Reactor temperature and ER values with respect to time for each lignite.
4. Conclusions Lignite type coals are located at the lowest boundary of the suitable fuel scope of entrained flow gasifiers, and, in order to advance and spread this technology, to utilize lignites throughout the world, further R&D work is crucial. An experimental investigation of five different Turkish lignites has been conducted using a pilot entrained flow gasifier. Results obtained from the lignite samples that have lower heating values, ranging from 2196 kcal/kg to 5658 kcal/kg, have reassured that an operation temperature above 100 C of ash flow temperature is crucial to achieve a smooth operation. Gasification results of the Tunçbilek and Soma lignites showed that lignites that have ash flow Please cite this article in press as: A. Ünlü, et al., Pilot scale entrained flow gasification of Turkish lignites, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.09.001
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Fig. 4. Partially blocked reactor throat (left) and fly ash build-up in shell & tube heat exchanger (right).
Fig. 5. Obtained syngas (nitrogen free) composition for each lignite.
temperature below 1300 C, LHV above 5000 kcal/kg, moisture below 15% and ash below 10% can be gasified without any significant problems. The moisture content of Orhaneli lignite could not be reduced to around 10% and thus the cold gas efficiency was far from the desired value. For this reason, in order to prevent feeding problems and to avoid energy penalties arising from evaporation of moisture during gasification, the lignites with moisture content higher than 15% needs to be dried further when dry-fed type fuel feeding system is used. This also indicates that slurry-fed type entrained flow gasification process is not a suitable choice for lignite type coals, which have lower heating values. Lastly, gasification of the Saray and Çan lignites in the entrained flow gasifier verified that lignites, having ash above
Table 2 Summary of experimental results for lignites tested. Condition
Saray lignite
Çan lignite
Orhaneli lignite
Tunçbilek lignite
Soma lignite
Temperature ( C) ER () a Syngas LHVmax (MJ/Nm3) b Syngas LHVgas (MJ/Nm3) mcoal (kg/h) Qgas (Nm3/h) hcold gas (%) As produced gas compositions CO (vol. %) H2 (vol. %) CO2 (vol. %) N2 (vol. %) Nitrogen free gas compositions CO (vol. %) H2 (vol. %) CO2 (vol. %)
1217 C 0.50 6.6 5.7 478 314 41
1437 C 0.53 7.6 6.6 381 351 42
1469 C 0.53 8.3 6.1 318 390 41
1518 C 0.51 9.8 9.3 280 430 64
1465 C 0.47 10.5 10.0 242 410 72
35.6 11.4 39.9 13.1
43.2 10.4 32.5 13.9
36.1 14.3 23.6 26.0
57.2 19.5 18.5 4.8
59.7 23.1 12.8 4.4
41.0 13.2 45.8
50.1 12.2 37.7
48.8 19.3 31.9
60.1 20.5 19.4
62.4 24.2 13.4
a b
Values calculated from nitrogen free gas composition values. Values calculated from as produced gas composition values (including fuel purge nitrogen).
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30% require higher ER operation in order to achieve required temperatures and this significantly drops the LHVgas and the efficiency, therefore Saray and Çan lignites are not suitable for entrained flow gasification and must be utilized by other means such as fluidized bed gasification or direct combustion methods. Acknowledgment _ for the financial support in the “Optimization of Tunçbilek Authors would like to acknowledge The Turkish Coal Enterprises (TKI) Gasification Systems” project in which this study was carried out. Nomenclature
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Pilot scale entrained flow gasification of Turkish lignites € nül a, M. Ziypak b, A. Akça b A. Ünlü a, *, U. Kayahan a, A. Argo a b
TUBITAK Marmara Research Center Energy Institute, p.o.b. 41470, Gebze, Kocaeli, Turkey Turkish Coal Enterprises, Hipodrom Cad. No:12, p.o.b. 06330, Yenimahalle, Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 June 2015 Received in revised form 10 August 2015 Accepted 6 September 2015 Available online xxx
As a part of a coal to methanol project, experimental results obtained from gasification of five different lignites, namely Saray, Çan, Orhaneli, Tunçbilek and Soma lignites, in a 6 ton/day pilot scale entrained flow gasifier are presented. The gasification experiments were conducted using pure oxygen as the gasification agent. The proximate and ultimate analyses for the lignites were performed and presented at the beginning of the study. The gasifier had been operated at an equivalence ratio (ER) of around 0.5 for all the runs. The syngas compositions (H2, CO and CO2 concentrations) and steady state reactor temperatures were recorded; cold gas efficiencies and lower heating values of produced gases were calculated for each run. The ash flow temperatures and lower heating values of lignites played an important role in the effective gasification of the lignites. Among the five lignites studied, two, namely Tunçbilek and Soma, were found to be appropriate for gasification with the entrained flow gasifier. The Tunçbilek lignite produced a syngas with 60.1% CO and 20.5% H2, which had a lower heating value (LHVmax) of 9.8 MJ/Nm3. The Soma lignite produced a syngas with 62.4% CO and 24.2% H2, which had a lower heating value (LHVmax) of 10.5 MJ/Nm3. For Saray, Çan and Orhaneli lignites, CO compositions were between 41 and 50.1%, H2 compositions were between 12.2 and 19.3% and the LHVmax values ranged between 6.6 and 8.3 MJ/Nm3. The cold gas efficiencies (hcold gas) for Saray, Çan, Orhaneli, Tunçbilek and Soma lignites were found to be 41, 42, 41, 64 and 72%, respectively. A short discussion about operational problems encountered during the gasification of Çan, Saray and Orhaneli lignites are also given in the discussion part. © 2015 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Coal Gasification Entrained flow gasification Pilot scale experiments
1. Introduction For their abilities to achieve high carbon conversion efficiencies, to produce tar free syngas, to reach high production capacities and to produce syngas with higher concentrations of H2 and CO [1,2] entrained flow gasifiers are attractive for chemicals production purposes. Entrained flow gasifiers mostly operate at high temperatures (1200e1500 C). The ash melts at these temperatures and flows out of the gasifier chamber as slag [3]. Since finely crushed coal particles (below 100 mm) are used in entrained flow gasifiers [4], short residence time gives satisfactory carbon conversion value in the gasifier. According to fuel feeding technology, entrained flow gasifiers are divided into two categories as dry-fed and slurry-fed [5]. In slurry-fed gasifiers, the fuel is mixed with some amount of water (around 40%) and fed to the gasifier as slurry (fuel þ water) by a slurry pump. Since feeding dry (solid) powder into high pressure chambers brings various difficulties, slurry-fed gasifiers are preffered for higher operating pressures. On the other hand, for slurry feeding, the energy penalty arising from water usage has to be considered, since significant amount of energy is required for the evaporation of the water [6,7]. Due to the energy penalty of slurry-fed gasifiers, it can be claimed that dry-fed gasifiers would be much more efficient for low rank coals such as lignite [8]. However, dry feeding also has its own disadvantage. It uses an inert gas, generally nitrogen, to blanket and transport pulverized fuel to the gasifier. This may cause a dilution of the produced syngas and a decrease in the lower heating value of the syngas produced. But the negative impact brought by the dilution effect, is expected to decrease as the capacity of the gasifier is increased. Even though entrained flow gasifiers are quite commonly used in commercial scale, parametric studies based on important parameters such as oxygen/fuel ratio [9], heating value, volatile matter and moisture content [10] which strongly affect the gas composition and carbon
* Corresponding author. Tel.: þ90 262 677 27 37; fax: þ90 262 641 23 09. E-mail address: [email protected] (A. Ünlü). http://dx.doi.org/10.1016/j.joei.2015.09.001 1743-9671/© 2015 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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conversion efficiency must be studied for each new fuel. Experimental data obtained from laboratory and pilot scaled gasifiers are still required for each type of fuel for modeling [11] design, and feasibility studies. Especially pilot scale testing (1e5 MWth), of coal plays an important role in improving the process understanding, validation, application of fundamental experimental and gasification technology development E.g. Roberts et al. compared the results of the laboratory and pilot scale entrained flow gasification tests and led a way to the refinement of the laboratory test procedures, measuring the properties such as slag viscosity and char reactivity [12]. Yun et al. performed pilot scale gasification testing of nine coals and obtained a “coal properties guideline” of ash melting temperature range and coal reactivity for IGCC plants [13]. Guo et al. investigated the effects of oxygen/carbon ratio and steam/carbon ratio to the gasification performance of the coal along with the operational performances of the instruments in a 30e45 t/day pilot scale pressurized entrained flow gasifier [14] and Shinada et al. investigated reliability and operating experience of a 200 t/d pilot entrained flow IGCC plant for scaling purposes [15]. Determination of optimum operation conditions and learning more about the transient behavior of the system for evaluation of low rank coals are crucial en route to ongoing R&D studies [16]. The data in the literature mostly generated from laboratory scaled gasifiers. Usually the effects of equivalence ratio (ER), gasification temperature, residence time, particle dimensions and different types of fuel on the measured gas composition and carbon conversion can be found in these studies. Generally, high residence time and gasification temperature increase the performance of the system but altering these parameters is not an easy task in pilot scale gasifiers due to the system dimensions and autothermal nature of the pilot gasifiers [17]. Another important parameter is the ash composition of the particular fuel studied. It has strong impact on the performance of entrained flow gasifier in two ways. It affects the fuel suitability, which is dependent on the ash flow temperature [18] and, it affects the H2/CO ratio of the produced syngas due to the catalytic effect of some minerals (Fe, Zn, Ni) for the water-gas shift reaction [19]. For example, for methanol production the so called M module (or stoichiometric ratio), which is defined as the (H2eCO2)/(CO þ CO2) mole fraction ratio, must be adjusted to a value just above 2 [20] by the water-gas shift reactor down-stream. The size of this reactor is thus affected by the original H2/CO ratio. Among all of these parameters equivalence ratio (ER) has major impact on gasification performance. When oxygen flow decreases, the concentrations of CO and H2 increase and carbon conversion decreases [21]. Reduction in ER also leads to a decrease in the reactor temperature. It is known that effective slag flow is very important and working temperature of the reactor must be adjusted at least 100 C above the ash flow temperature for continuous operation [22,23]. The entrained flow gasifier used in this study is located at the Turkish Coal Enterprise's (TKI) R&D facilities in Tunçbilek/Kütahya region of Turkey. The experiments were performed as a part of the “Optimization of Tunçbilek Gasification Systems” project, which is a joint project of TKI and TUBITAK Marmara Research Center Energy Institute. Currently, a continuation project between the same parties, namely the “Tunçbilek Coal to Methanol” project, is under progress where the gasifier is going to be integrated with a more through gas-cleaning process (COS hydrolysis and H2S removal), subsequently connected to gas conditioning (water-gas shift reactor and carbon dioxide removal system), syngas to methanol and distillation units. Presently, the gas-cleaning process is under construction, and the subsequent processes are under detailed design phase. The whole coal-to-methanol pilot-facility is expected to be operational at the second quarter of 2017. Although, gasification of coal is not a new concept, literature data about pilot scale gasification of Turkish lignites is very scarce. To the best of our knowledge, the pilot scale entrained flow gasifier used in this study is the only operational one in Turkey. Therefore, the main motivation of this study was to produce literature data for the entrained flow gasification of various Turkish lignites and to screen out the best candidates for further processing, i.e. for methanol production in the continuation project. 2. Pilot experiments A photograph of the pilot scale entrained flow gasification system can be seen in Fig. 1. The Tunçbilek gasifier is a dry-fed, autothermal pilot scale (6 ton/day) entrained flow type reactor operated at around atmospheric pressures. It is a cylindrical reactor with an inside diameter of 1 m and a height of 2.5 m. The vertically oriented gasifier has a top-down flow design with an integrated quencher (another 2.5 m of height) at the bottom. The pilot scale experimental procedure is as follows (see Fig. 2): at the start-up phase the solid fuel (lignite) was crushed (down to <100 mm) and dried (aimed to reach a moisture content around 10%) by the coal preparation facility. The gasifier was initially heated up to
Fig. 1. A photograph of the pilot scale entrained flow gasification system.
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Fig. 2. Schematic drawing of the pilot scale system.
around 600 C by an oil burner. After that point, oxygen and coal were started to be fed into the gasifier at proportions appropriate for combustion. At around 1100 C the oil burner was turned off and taken out of the gasifier. Finally, the coal to oxygen ratio was adjusted gradually to the desired value and the reactor was gradually switched from combustion to gasification mode. The fuel feeding rate was adjusted by using a screw feeder. Since pulverized coal and pure oxygen become very reactive when mixed, nitrogen was used as the carrier gas inside the fuel spray feeder. Oxygen and coal were only allowed to come into contact just before entering the reactor. The gasification reactions took place in the upper chamber of the reactor, and the produced gas was then led to the lower chamber where quenching occurs. In the quench chamber, water was sprayed into the syngas, which leads to a temperature drop to around 400 C. Later on, the syngas was further cooled down to 70 C by using a shell & tube heat exchanger. A spray tower located after the heat exchanger was used to remove remaining particulates from the gas stream. A mist eliminator located right after the spray tower was used to separate the mist in the gas stream. Finally, the syngas was directed to the flare through an ID fan, where it was combusted before releasing to the atmosphere. The maximum operating temperature for this particular reactor is around 1600 C, which is actually limited by the maximum operating temperature of the refractory lining. It should also be noted that the minimum operating temperature for any lignite should be 100 C above its ash flow temperature, since otherwise operating problems arise due to unmelted and partially softened ash. Thus, the operating temperatures were set accordingly. The syngas composition measurements were done by on-line analyzers. CO and CO2 were measured by SIEMENS ULTRAMAT 23 infrared analyzer system, H2 was measured by using SIEMENS CALOMAT 67 analyzer system and O2 was measured by using SIEMENS OXYMAT 61 analyzer system. _ from mines in various provinces of Turkey; Tekirdag /Saray mine, Çanakkale/ The lignites were provided by Turkish Coal Enterprises (TKI) € Çan mine, Orhaneli/Gümüs¸pınar mine, Tunçbilek/Omerler mine and Soma/Imbat coal washery. Table 1 lists the proximate analysis, ultimate analysis, heating values and ash flow temperatures of the lignites used. For the proximate analysis ASTM D-5142-09, for the ultimate analysis ASTM-D-5373-08 and for the determination of heating value ASTM-D- 5865-10 standard methods were used.
3. Results and discussion With respect to their Lower Heating Values (LHV), the lignites used in this study could be sorted as follows: Saray < Çan < Orhaneli < Tunçbilek < Soma. Coals with higher LHV values tend to have lower ash contents. When sorted according to ash content, the lignites turned out to be in the following order: Saray > Çan > Tunçbilek > Orhaneli > Soma. Another important parameter for entrained flow gasification is the ash flow temperature. The ash flow temperatures determined under reducing environment were in the following order: Saray < Soma < Orhaneli < Tunçbilek < Çan. In Fig. 3, the relation between equivalence ratio (ER), the reactor temperature value and operation time were given for all lignites. For each lignite, the ash flow temperature was also indicated on the corresponding plot. The last points on the plots were taken to represent the steady state values, after which the reactor temperatures are expected to stay constant with respect to time. Please cite this article in press as: A. Ünlü, et al., Pilot scale entrained flow gasification of Turkish lignites, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.09.001
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Table 1 LHV, ash flow, ultimate and proximate analysis of fuels tested. Fuel specifications
Çan lignite
Orhaneli lignite
Tunçbilek lignite
Soma lignite
Original based Dry based Moisture (wt. %) 14.35 e Ash (wt. %) 47.41 55.35 Volatile matter (wt. %) 26.27 30.67 Fixed carbon (wt. %) 11.97 13.97
Original based Dry based 9.90 e 34.63 34.62 31.81 35.31 27.09 30.10
Original based 21.85 10.41 38.57 29.17
Dry based e 13.32 49.35 37.32
Original based Dry based 8.20 e 19.91 21.69 31.79 34.63 40.01 43.68
Original based Dry based 7.05 e 11.38 12.24 36.54 39.31 45.03 48.44
C (wt. %) H (wt. %) N (wt. %) O (wt. %) S (wt. %) LHV (kcal/kg) Ash flow temperaturea
26.85 2.26 0.51 5.41 3.21 2196 1241 C
38.40 2.95 0.81 8.34 4.93 3444 >1500 C
53.05 4.14 0.86 8.01 1.75 4326 1302 C
67.88 5.3 0.99 10.26 2.25 5535
61.53 4.1 3.07 1.8 1.36 5391 1329 C
65.93 3.95 1.3 9.3 1.09 5658 1288 C
Highlightsb
Low LHV High amount of ash Low ash flow temperature
a b
Saray lignite
31.35 2.64 0.60 6.32 3.75 2593
42.63 3.28 0.91 9.26 5.48 3822
67.03 4.47 3.35 1.97 1.49 5872
Medium LHV High LHV High LHV Medium amount of ash Low amount of ash Low amount of ash Very high ash flow High amount of moisture High ash flow temperature Medium ash flow temperature temperature
70.93 4.25 1.4 10.01 1.17 6087
High LHV Low amount of ash Medium ash flow temperature
Measurements are made under reducing atmosphere. Low, medium and high terms used in this table shows relative values of a specification to each other and must not be understood globally.
For Saray lignite, due to its low lower heating value, an operating temperature 100 C above ash flow temperature could not be achieved. The high ash flow temperature of Çan lignite could not be surpassed, although a high value for the operating temperature (around 1440 C) was achieved. Due to operation below ash flow temperatures, these two lignites caused operational problems such as partial blockage of the reactor throat, problems for ash disposal system due to partial agglomeration of softened but not melted large ash pieces, fly ash build-up in heat exchanger tubes (Fig. 4). For the reasons listed above, these two lignites, namely Saray and Çan lignites, were found to be inappropriate for the pilot scale entrained flow gasifier. On the other hand, for the Tunçbilek and Soma lignites, the operation temperatures have well reached values that were at least 100 C higher than the corresponding ash flow temperatures. As a result, from operational point of view, these two lignites turned out to be suitable for the pilot scale entrained flow gasifier used. It should be noted that the coal-drying equipment was designed to reduce the moisture content of the pulverized coal on a single-pass basis. Therefore, Orhaneli lignite, which had high original moisture content, could not be dried down to the desired values before gasification with only single-pass drying. This cast a shadow on the performance of the Orhaneli lignite by decreasing the gasifier efficiency. With an appropriate drying process (that can decrease the moisture content to around 10%), most probably a much better gasification performance would have been achieved for the Orhaneli lignite. The compositions of the syngas for each lignite can be seen in Fig. 5. For an efficient gasification process, it is expected to have relatively higher CO and H2, and lower CO2 values in the syngas produced. With 60.1% CO and 20.5% H2 the Tunçbilek lignite and with 62.4% CO and 24.2% H2 the Soma lignite produced preferable syngases when compared with the other three lignites (see Table 2). Furthermore, the CO2 concentrations for Tunçbilek (19.4%) and Soma (13.4%) were much lower than the rest of the lignites (31.9%, 37.7% and 45.8% for Orhaneli, Çan and Saray, respectively). Therefore, for Tunçbilek and Soma lignites lower capacity CO2 separation units can be designed and total installation costs can be decreased. The actual gas-composition values, LHV of the syngases produced and cold gas efficiencies are listed in Table 2. Lower heating value of the syngas (LHVgas) was calculated by the following equation [24]
LHVgas ¼
283 CO þ 241:83 H2 100 22:4
And the cold gas efficiency of the syngas (hcold
hcold gas ¼
gas)
was calculated using the following formula [25]
Qgas LHVgas 1000 100 mcoal LHVcoal 4:18
The were calculated based on the syngas compositions including nitrogen. The cold gas efficiencies calculated for Tunçbilek and Soma lignites, as 64% and 72% respectively, were promising. The LHVgas values produced from Tunçbilek and Soma lignites were 9.8 MJ/Nm3, 10.5 MJ/Nm3 respectively and these values were suitable for subsequent processing technologies. With a 41% cold gas efficiency and a syngas lower heating value of 8.3 MJ/Nm3, the true potential of the Orhaneli lignite could not be realized. The main reason of this poor performance was wasting of the energy for the evaporation of moisture in the fuel. Although Saray lignite had a reasonable cold gas efficiency value of 41% and a syngas lower heating value of 6.6 MJ/Nm3, it did not seem to be cut out for entrained flow gasification technology. A similar performance issue could also be noticed for the gasification of Çan lignite, which has reached to 42% cold gas efficiency and 7.6 MJ/Nm3 syngas lower heating value. Saray and Çan lignites also caused ash melting related issues (as mentioned above). The gasification temperatures of Saray and Çan lignites could have been increased above the ash flow temperature to overcome the ash melting related blockage problems, if these coals were gasified at higher ER values i.e. close to the combustion zone. However, such an increase in the ER will lead to significant loss of lower heating value of syngas and therefore instead of gasification, direct combustion turns out to be the appealing alternative technology for Çan and Saray lignites.
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Fig. 3. Reactor temperature and ER values with respect to time for each lignite.
4. Conclusions Lignite type coals are located at the lowest boundary of the suitable fuel scope of entrained flow gasifiers, and, in order to advance and spread this technology, to utilize lignites throughout the world, further R&D work is crucial. An experimental investigation of five different Turkish lignites has been conducted using a pilot entrained flow gasifier. Results obtained from the lignite samples that have lower heating values, ranging from 2196 kcal/kg to 5658 kcal/kg, have reassured that an operation temperature above 100 C of ash flow temperature is crucial to achieve a smooth operation. Gasification results of the Tunçbilek and Soma lignites showed that lignites that have ash flow Please cite this article in press as: A. Ünlü, et al., Pilot scale entrained flow gasification of Turkish lignites, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.09.001
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Fig. 4. Partially blocked reactor throat (left) and fly ash build-up in shell & tube heat exchanger (right).
Fig. 5. Obtained syngas (nitrogen free) composition for each lignite.
temperature below 1300 C, LHV above 5000 kcal/kg, moisture below 15% and ash below 10% can be gasified without any significant problems. The moisture content of Orhaneli lignite could not be reduced to around 10% and thus the cold gas efficiency was far from the desired value. For this reason, in order to prevent feeding problems and to avoid energy penalties arising from evaporation of moisture during gasification, the lignites with moisture content higher than 15% needs to be dried further when dry-fed type fuel feeding system is used. This also indicates that slurry-fed type entrained flow gasification process is not a suitable choice for lignite type coals, which have lower heating values. Lastly, gasification of the Saray and Çan lignites in the entrained flow gasifier verified that lignites, having ash above
Table 2 Summary of experimental results for lignites tested. Condition
Saray lignite
Çan lignite
Orhaneli lignite
Tunçbilek lignite
Soma lignite
Temperature ( C) ER () a Syngas LHVmax (MJ/Nm3) b Syngas LHVgas (MJ/Nm3) mcoal (kg/h) Qgas (Nm3/h) hcold gas (%) As produced gas compositions CO (vol. %) H2 (vol. %) CO2 (vol. %) N2 (vol. %) Nitrogen free gas compositions CO (vol. %) H2 (vol. %) CO2 (vol. %)
1217 C 0.50 6.6 5.7 478 314 41
1437 C 0.53 7.6 6.6 381 351 42
1469 C 0.53 8.3 6.1 318 390 41
1518 C 0.51 9.8 9.3 280 430 64
1465 C 0.47 10.5 10.0 242 410 72
35.6 11.4 39.9 13.1
43.2 10.4 32.5 13.9
36.1 14.3 23.6 26.0
57.2 19.5 18.5 4.8
59.7 23.1 12.8 4.4
41.0 13.2 45.8
50.1 12.2 37.7
48.8 19.3 31.9
60.1 20.5 19.4
62.4 24.2 13.4
a b
Values calculated from nitrogen free gas composition values. Values calculated from as produced gas composition values (including fuel purge nitrogen).
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30% require higher ER operation in order to achieve required temperatures and this significantly drops the LHVgas and the efficiency, therefore Saray and Çan lignites are not suitable for entrained flow gasification and must be utilized by other means such as fluidized bed gasification or direct combustion methods. Acknowledgment _ for the financial support in the “Optimization of Tunçbilek Authors would like to acknowledge The Turkish Coal Enterprises (TKI) Gasification Systems” project in which this study was carried out. Nomenclature
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Please cite this article in press as: A. Ünlü, et al., Pilot scale entrained flow gasification of Turkish lignites, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.09.001