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The evaluation of a process for clean syngas based on lump coal pressurized gasification
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
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The evaluation of a process for clean syngas based on lump coal pressurized gasification Zhaoyi Shi a,1, Shuguang Shen a,*,1, Tianjin Li a, Yujuan Sun a, Weiwei Shan a, Yonghui Bai b, Qinggeng Zhang c, Fan Li b a
College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China State Key Laboratory Breeding Base of Coal Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China c Saiding Engineering Co., Ltd., Taiyuan 030024, China b
article info
abstract
Article history:
Raw Gas generated from LCPG (lump coal pressurized gasifier) contains quantities of or-
Received 31 October 2016
ganics including methane so that it is not suitable as syngas. A process was proposed to
Received in revised form
solve this problem, and Aspen Plus was used to discuss its possibility that Raw Gas was
9 February 2017
reformed to clean syngas through introducing oxygen on the principle of thermodynamic
Accepted 10 February 2017
equilibrium. The simulation results reveal that almost all organics are completely con-
Available online xxx
verted to simple substances such as hydrogen and carbon monoxide whether or not to preheat. Raw Syngas and wastewater no longer contain various organic pollutants elimi-
Keywords:
nated difficultly besides very small amounts of ammonia and hydrogen sulfide, which
Lump coal gasification
certainly simplifies the purification and post-treatment processes. A Raw Syngas with H2/
Clean syngas
CO ratio of 2.0 was attained under the operating condition of 2.35 Mpa and 918 C. Such H2/
Reforming
CO ratio is higher than those of coal gas derived from existing gasification technologies.
Wastewater
Meanwhile, the total yield and content of the effective gas (CO þ H2) have been increased to
Aspen Plus
1109 m3/t and 72.0% respectively in the preheating process, and GE (gasification efficiency) rises to 84.8%. The OC (oxygen consumption), CC (coal consumption) and SC (steam consumption) have been reduced by 47.3%, 50.7% and 49.4% respectively compared with LCPG. In addition, the feasibility of this process was further verified with kinetic simulation. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Coal gasification has become one of the key technologies for high efficiency and clean utilization of coal. Its development goals are to use solid fuel including inferior coal to continuously and efficiently produce coal gas as fuel gas, reducing gas, or feedstock gas for chemical synthesis, hydrogen fuel cell and
synthetic natural gas [1]. Therefore, coal gasification is deservedly regarded as the footstone of current coal chemical industry. There are numerous gasification technologies that are commercially available, in which these gasifiers can be mainly categorized into three main types based on reactor beds including fixed bed, fluidized bed, and entrained-flow bed [2]. These generic types of gasifiers differ mainly in their
* Corresponding author. College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China. E-mail address: [email protected] (S. Shen). 1 The two authors contributed equally to this work and should be considered co-first authors. http://dx.doi.org/10.1016/j.ijhydene.2017.02.078 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
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Nomenclature: Raw Coal a feed stock of LCPG Raw Gas the exit gas of LCPG, which is comprised of Coal Gas, steam and tar vapor Coal Gas a mixed gas containing H2, CO, CO2, CH4, CnHm, N2, NH3 and H2S Raw Syngas a mixed gas containing H2, CO, CO2, and small quantities of CH4, N2, NH3 and H2S
operating conditions such as fluid dynamics, coal particle size, temperature range and heating rate. Therefore, the composition of Raw Gas derived from different gasifiers is greatly different. Specially, the yield and composition of liquid oil and wastewater vary widely. LCPG belongs to fixed bed gasifier, which has been attracted to more attention and applied widely for relatively lower cost and mature technology in China. According to incomplete statistics, nowadays, there are 168 sets of LCPGs in China, which are used in the production of city gas, natural gas, methanol, dimethyl ether, ammonia and synthetic oil. Their corresponding production is 1.5 billion m3/a, 11.3 billion m3/a, 1.56 million t/a, 1.05 million t/a, 900 thousand t/a, and 160 thousand t/a, respectively [3e6]. Its superiority mainly shows as follows: (1) Raw Gas countercurrent contacts with lump coal, so that the ash content in Raw Gas is much less than those of fluidized bed gasifiers and entrained-flow bed gasifiers; (2) the upward hot flow of Raw Gas contributes to drying and pyrolysis of coal, and the recuperation of sensible heat can decrease temperature of Raw Gas and improve GE [7]; (3) the preparation and transport of feed coal is simpler, and less investment is needed than other types of gasifiers with the same scale. Raw gas from LCPG includes approximately 10% (v/v) CH4, which is a distinct advantage for synthesis of nature gas because of obviously reducing the load of methanation unit [8]. However, extra reforming unit is indispensable for the conversion of CH4 to CO and H2 considering producing syngas [9]. Furthermore, the component of Raw Gas is considerably complex, such as light hydrocarbons, naphtha, phenols, hydrogen sulfide and ammonia that will increase the difficulty in purification and separation units. For example, the presence of tar not only decreases GE, but also easily leads to pipe blockage due to temperature dropping [10,11]. Provided the tar component enters into downstream equipment, the safety operation of gas turbine and compressor would be seriously affected. What's more, traditional treatment processes containing pretreatment-biological treatment-advanced treatment have been generally adopted for LCPG wastewater [12e14]. Stripping, extraction and distillation have become major means for the pretreatment process to remove phenols, hydrogen sulfide and ammonia. Then, anaerobic and/or aerobic biological technologies are presented for the biological treatment process with the purpose of eliminating residual organics. Finally, due to a small quantity of nitrogenous substance and refractory organics, the COD and chromaticity of effluent could not meet the discharge standards. So advanced treatments are still indispensible to reduce the negative impact on the
receiving ecosystem, such as physical methods (coagulation precipitation, adsorption and membrane separation) and advanced oxidation process (Fenton oxidation and ozone oxidation). Obviously, the removal of CH4 and tar, and wastewater treatment in LCPG technology is so complicated. According to the current technology level, there seem to be no choice but to use other gasification technologies. However, we expect to change this situation. A new process that can generate clean syngas based on LCPG was proposed in order to solve the above problems. In this process, Raw Gas, without cooling and separation, from gasifier exit was directly introduced into a high-temperature cracking and reforming reactor together with oxygen, and then complex interactions occur among Coal Gas, tar, stream and oxygen under high temperature. The feasibility of this process was discussed with thermodynamics and kinetics using Aspen Plus software (11.1).
Models and methods The reforming process For acquiring clean coal gas which is especially suitable as syngas, the original idea was presented in Fig. 1. In accordance with different gasification pressures and coal ranks, the temperature of raw gas from LCPG is between 280 and 500 C. Meanwhile, some substances in raw gas including tar, phenols, and light hydrocarbons, are introduced into a hightemperature cracking and reforming reactor, and interact with O2, H2O and CO2. And then large quantities of simple molecular substances such as H2 and CO are generated. The high-temperature cracking and reforming reactor is similar as multiinjector reactor, such as multiinjector water-coal slurry
Fig. 1 e The reforming process schematic.
Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
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gasifier with an operating condition of 2.45 Mpa and 1200e1400 C [15,16].
Description of modules For studying the feasibility and reforming effect of this process shown as Fig. 1, the simulation process was set up as Fig. 2 using Aspen Plus software. In the simulation process shown as Fig. 2, Raw Gas and O2 was introduced into an R-GIBBS (Gibbs-reactor). Adjusting O2 volume flow, the temperature of R-GIBBS was controlled at 800e1400 C. Under the high temperature, tar, light hydrocarbons and CH4 was reformed to CO and H2. The descriptions of blocks and streams used in Fig. 2 are as follows: R-GIBBS: a block that represents a Gibbs-reactor, in which the minimization of Gibbs free energy was applied to calculate the thermodynamic equilibrium at a given pressure. A series of chemical reactions, such as pyrolysis, combustion and reforming, occur in the reactor at the temperature range of 800e1400 C. COOLER: a block that cools the exit gas of R-GIBBS to room temperature. SEP: a block that separates wastewater from the exit gas of the R-GIBBS to acquire Raw Syngas. R-G (Raw Gas): the exit gas of LCPG, containing Coal Gas (H2, CO, CO2, CH4, CnHm, N2, NH3 and H2S), gaseous tar and steam, etc. OXY (O2): a stream that was introduced into R-GIBBS at room temperature, its pressure is equal to that of R-GIBBS. W-W (wastewater): a stream that mainly contains water and tiny pollutants such as H2S, NH3, HCN, COS, etc. R-SYN (Raw Syngas): mainly contains H2, CO, CO2, and small quantities of pollutants such as H2S, NH3, HCN, COS, etc. H-LOSS: a negative heat flow that stands for total heat loss to surroundings at the R-GIBBS, accounting for about 4.5% of combustion heat of Raw Coal. In order to determine the influence of these operation conditions on the process, variations in the following five parameters were studied:
1. OC (oxygen consumption) is defined as the oxygen demand to generate per unit of effective gas, including the part which is introduced into LCPG as a gasifying agent (145 m3/ t coal) [1]. OC ¼ ðOXY þ 145Þ=ðCO þ H2 Þ
m3 =m3
(1)
2. CC (coal consumption) is defined as the coal demand to generate per unit of effective gas: CC ¼ 1000=ðCO þ H2 Þ
kg=m3
(2)
3. SC (steam consumption) is defined as the steam demand to generate per unit of effective gas, only standing for the part which is introduced into LCPG as a gasifying agent (1050 kg/ t coal) [1]: SC ¼ 1050=ðCO þ H2 Þ
kg=m3
(3)
4. H2/CO ratio is defined as the molar ratio of H2 to CO, which is used to evaluate the suitability of syngas for methanol synthesis [17]. 5. GE (Gasification Efficiency): the combustion heat ratio of Coal Gas to Raw Coal in conventional coal gasification industry. In the reforming process, it stands for the combustion heat ratio of Raw Syngas to Raw Coal, showing the energy conversion efficiency [18,19].
Assumptions of various parameters The exit gas (Raw Gas) of LCPG had a temperature of 315 C. When per ton lignite was gasified, Raw Gas generated from LCPG contains Coal Gas (1106.0 m3/h), steam (825.0 kg/h) and tar (69.4 kg/h) [1]. In addition, tar contains hundreds of kinds of composition. The component of tar was represented by some main substances with relatively higher content in this simulation. An assumption of tar component was as follows: aliphatic hydrocarbon, unsaturated aliphatic hydrocarbon, aromatic hydrocarbon, sulfur-containing heterocyclic compounds, oxygen-containing heterocyclic compounds, nitrogen-containing heterocyclic compounds and phenols
Fig. 2 e The simulation flowsheet. Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
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was represented by C20H42 (N-eicosane), C12H12 (dimethylnaphthalene), C14H10 (phenanthrene), C12H8S (dibenzothiophene), C12H8O (dibenzofuran), C11H11N (dimethyl-quinoline), C8H10O (xylenol), respectively [20]. Their detailed content was listed in Table 1. The minimization of Gibbs free energy was used to evaluate the process feasibility in Aspen Plus. In previous research, this method was applied widely. For instance, Chan S.H. applied the minimization of Gibbs free energy into autothermal fuel reforming of fuel cells [21]. Seo Y.-S. investigated three different types of reforming process with the aid of thermodynamic analysis using Aspen Plus [22]. Turpeinen E. evaluated the feasibility of hydrogen production using alternative feedstock, such as coke oven gas, refinery gas and biogas by Aspen Plus software [23]. Stephen G. Gopaul compares the syngas production capabilities of biomass from the dry reforming of methane using the ASPEN Plus in terms of H2 and CO yield [24]. Accurate simulation results were gained in these researches. The possible reactions in this system were shown as Table 2. It should be noted that these reactions do not involve detailed reaction mechanism. Nevertheless, in accordance with Hess's law and material balance law, these chemical equations are convenient for the calculation of thermodynamic equilibrium. This reactor is characterized by high temperature and high pressure. And in the reactor, the main substances includes CH4, H2, CO, CO2, H2O, O2, light hydrocarbons, tar, NH3, etc. Therefore, the Peng-Robinson equation of state with BostonMathias modifications (PR-BM) was selected as the state equation [25,26]. In this simulating system, using PengRobinson equation of state will obtain more accurate simulating results.
Results for calculations There is a one-to-one relationship between O2 flow rate and RGIBBS temperature under a certain pressure, so the sensitivity analysis of O2 flow rate was conducted on the R-GIBBS under a pressure of 2.5 Mpa. In accordance with Fig. 2, a series of endothermic and exothermic reactions occurred after Raw Gas along with O2 was introduced into R-GIBBS, and the products such as CO and H2 were generated. The simulation results are presented as Table 3.
The influence on pollutants As shown in Table 3b, with the increase of O2 flow, the temperature increases gradually due to exothermic reactions, and the concentration of NH3 and HCN in Raw Syngas (Table 3a) always tends to decrease so that the concentration of N2 tends to increase. Compared with Coal Gas, the concentration of NH3 and HCN declines markedly. It can be presumed that Reaction 30 and 31 in Table 2 occur, which contributes to the
elimination of NH3 and HCN. And the concentration of COS also declines markedly, which results from the conversion of COS to H2S. It is worth mentioning that tar disappears thoroughly in Raw Syngas via this process, which greatly reduces the burden on the purification unit. As shown in Table 3b, the increasing temperature contributes to the cracking and reforming of organic pollutants to effective gas (CO þ H2), and wastewater no longer contains hydrocarbons and other organics. NH3, HCN, H2S and COS are the main remaining pollutants in wastewater. In accordance with the integrated wastewater discharge standard from coal industry (GB 20426-2006), NH3 concentration in wastewater meets the second grade standards in a temperature range 1014e1196 C, and meets the first grade standards (<15 mg/L) at an equilibrium temperature of 1285e1374 C. The cyanide content in wastewater generated under all operating conditions is much lower (several orders of magnitude) than that of GB 20426-2006 (<0.5 mg/L). The sulfide concentration ranging from 2.52 to 3.05 mg/L is slightly higher than the standard (<1 mg/L). Because sulfide exists primarily in the form of H2S, the wastewater could meet the standard after eliminating H2S by some simple processes. Compared with the wastewater (the content of oil, phenols, NH3-N, and cyanide is 1500e5500 mg/L, 3500e9000 mg/L, 10e400 mg/L, and 50e200 mg/L respectively [27,28]) derived from LCPG, this process shows an obvious advantage on the elimination of pollutants.
The influence on yield and component of gas phase It can be seen from Table 3 that the volume flow of H2 also increases to a highest value at an O2 volume flow of 238.2 m3/h and a temperature about 853.0 C, and then it decreases gradually. The increase of H2 concentration is attributed to the partial oxidization and reforming of organics, which can be demonstrated by the decrease of CH4, C2H4 and C6H6 in Table 3a together with the disappearance of tar in Table 3b. It is worth mentioning that the concentration of CH4 in gaseous phase decreases to less than 1.0% due to the reforming reactions of CH4 (No. 16 and 24 in Table 2), which is similar to other simulation results [24,29]. And with the increase of O2 input, the yield of H2O always increases due to the combustion of H2. After introducing O2, the yield and concentration of CO and CO2 increase simultaneously compared with Coal Gas. With the continuous increasing of O2 flow, the volume fraction of CO2 and H2 decline constantly, and that of CO increase constantly in the R-GIBBS outlet. Therefore, we speculated that the increase of temperature affected the endothermic reverse shift reaction (No. 27 in Table 2), shifting the equilibrium toward the product side. Actually, there are two factors influencing on the endothermic reverse shift reaction. One is the promotion of increasing temperature on the endothermic
Table 1 e The composition of Coal Gas and tar. Coal Gas (Vol.%)
H2 32.3
Tar (wt.%)
CO
CO2
CH4
17.2
31.9
15.1
C2H4 0.56
C6H6
NH3
N2
0.14
0.83
1.48
H2S 0.49
C20H42
C12H12
C14H10
C12H8S
C12H8O
C11H11N
C8H10O
36.5
6.6
24.5
0.2
0.6
3.5
28.1
Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
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Table 2 e The possible reactions in the R-GIBBS. Reaction
△H* (KJ/mol)
C20H42 þ 10O2 / 20CO þ 21H2 C12H12 þ 6O2 / 12CO þ 6H2 C14H10 þ 7O2 / 14CO þ 5H2 C12H8S þ 6O2 / 12CO þ 3H2 þ H2S C11H11N þ 5.5O2 / 11CO þ 4H2 þ NH3 C12H8O þ 5.5O2 / 12CO þ 4H2 C8H10O þ 3.5O2 / 8CO þ 5H2 CH4 þ 0.5O2 / 2H2 þ CO C20H42 þ 20H2O / 20CO þ 41H2 C12H12 þ 12H2O / 12CO þ 18H2 C14H10 þ 14H2O / 14CO þ 19H2 C12H8S þ 12H2O / 12CO þ 15H2 þ H2S C11H11N þ 11H2O / 11CO þ 15H2 þ NH3 C12H8O þ 11H2O / 12CO þ 15H2 C8H10O þ 7H2O / 8CO þ 12H2 CH4 þ H2O 4 CO þ 3H2 C20H42 þ 20CO2 / 40CO þ 21H2 C12H12 þ 12CO2 / 24CO þ 6H2 C14H10 þ 14CO2 / 28CO þ 5H2 C12H8S þ 12CO2 / 24CO þ 3H2 þ H2S C11H11N þ 11CO2 / 22CO þ 4H2 þ NH3 C12H8O þ 11CO2 / 23CO þ 4H2 C8H10O þ 7CO2 / 15CO þ 5H2 CH4 þ CO2 4 2CO þ 2H2 2H2 þ O2 4 2H2O 2CO þ O2 4 2CO2 H2 þ CO2 4 H2O þ CO H2S þ CO 4 COS þ H2 H2S þ O2 4 SO2þH2 2NH3 4 N2 þ 3H2 2HCN þ 1.5O2 4 N2 þ H2O þ 2CO
1687.7 1318.2 1749.6 1627.1 1262.6 1380.4 715.3 22.6 3286.9 1666.5 1732.6 1357.7 1473.3 1355.6 1025.9 250.3 3961.9 2071.6 2205.1 1762.7 1844.7 1726.9 1262.0 259.9 497.2 564.9 34.0 3.3 271.6 111.0 739.2
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Here, △H* were calculated and obtained at 2.5 Mpa and 800 C.
reaction; another is the inhibition of decreasing H2 concentration and increasing H2O flow on the reverse shift reaction. When the temperature exceeds 1105 C, the equivalent impacts effect of two factors on the reverse shift reaction contributes to the constant molar fraction of CO and CO2 at RGIBBS outlet. Moreover, as a small quantity of O2 is introduced, the yield of H2 rises markedly because of the partial oxidation and reforming of hydrocarbon compared with that of Coal Gas, which results in the increase of H2/CO ratio. However, with
the increase of O2 flow, H2/CO ratio decreases gradually, and reaches 2.0 when the O2 volume flow is 249.5 m3/h and the temperature approaches to 886.4 C. As also shown in Table 3b, it is observed that the yield of effective gas (CO þ H2) reaches a peak value of 951.6 m3/t (coal) at 886.4 C, increasing by 73.8% compared with Coal Gas.
The influence on main consumption indexes GE rises in some extent at low O2 flow, and then it falls sharply with the increase of O2 flow. The OC rises with the increase of
Table 3a e The results of gas phase after reforming as Fig. 1. No.
Coal Gas 1 2 3 4 5 6 7 8 9
The composition and yield of Raw Syngas (Vol.% or m3/h)
Gas molar fraction in R-GIBBS outlet (%) H2
CO
CO2
H2
VH2
CO
VCO CO2 VCO2
e 25.4 25.3 24.8 23.9 21.8 19.7 17.8 16.0 14.4
e 10.6 11.8 12.3 12.8 13.4 13.8 13.9 14.0 13.9
e 19.0 18.2 17.8 17.5 16.9 16.5 16.3 16.3 16.4
32.3 43.9 44.3 43.9 43.1 40.9 38.5 36.1 33.8 31.4
357.2 635.9 645.9 635.1 615.3 562.1 508.3 458.5 412.8 370.8
17.2 18.4 20.7 21.9 23.0 25.1 26.9 28.3 29.4 30.2
190.2 266.9 301.5 316.8 328.1 345.0 354.9 359.6 360.0 356.7
31.9 33.0 31.9 31.6 31.5 31.6 32.2 33.1 34.3 35.8
352.8 477.6 465.4 457.1 449.1 434.4 424.9 420.2 419.8 423.1
CH4 15.1 2.4Eþ0 8.9E-1 4.3E-1 1.8E-1 3.1E-2 5.7E-3 1.3E-3 3.2E-4 8.9E-5
C2H4 C6H6 0.56 0 0 0 0 0 0 0 0 0
0.14 0 0 0 0 0 0 0 0 0
NH3
HCN
N2
H2S
COS
8.3E-1 4.8E-3 3.6E-3 2.9E-3 2.2E-3 1.3E-3 7.7E-4 4.7E-4 3.1E-4 2.0E-4
2.0E-3 2.4E-5 2.6E-5 2.6E-5 2.5E-5 2.2E-5 1.9E-5 1.6E-5 1.4E-5 1.2E-5
1.48 1.32 1.32 1.33 1.35 1.40 1.46 1.52 1.59 1.63
0.49 0.34 0.33 0.33 0.34 0.35 0.36 0.38 0.40 0.40
3.2E-1 6.7E-3 7.2E-3 7.7E-3 8.3E-3 9.7E-3 1.1E-2 1.3E-2 1.5E-2 1.7E-2
Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
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Table 3b e The results of various indexes after reforming as Fig. 1. No. O2 (m3/h) T ( C) H2O (kg/h) Raw Syngas (m3/h) H2/CO ratio Total tar (kg/h) The main pollutants in wastewater (mg/L)
NH3 HCN H 2S COS tar
Effective gas (m3/t) OC (m3/m3) CC (kg/m3) SC (kg/m3) GE (%)
LCPG
1
2
3
4
5
6
7
8
9
e 315 825 1108.6 1.72 69.4 300 [27] 170 [28] 215 [27] e 8000 [27] 547.4 0.26 1.83 1.92 73.0
215.5 803.7 842.9 1447.0 2.38 0 167.6 2.5E-5 2.54 5.9E-3 0 902.8 0.40 1.11 1.16 76.5
238.2 853 870.6 1457.1 2.14 0 125.2 2.7E-5 2.52 6.4E-3 0 947.4 0.40 1.06 1.11 74.6
249.5 886.4 889.6 1446.6 2.01 0 101.0 2.6E-5 2.54 6.8E-3 0 951.6 0.41 1.05 1.10 73.3
260.9 925.8 911.5 1426.3 1.88 0 78.2 2.5E-5 2.57 7.4E-3 0 943.4 0.43 1.06 1.11 71.8
283.5 1014 957.6 1372.8 1.63 0 45.1 2.3E-5 2.66 8.7E-3 0 907.0 0.47 1.10 1.16 68.5
306.2 1105.3 1001.2 1318.7 1.43 0 26.7 1.9E-5 2.76 1.0E-2 0 863.1 0.52 1.16 1.22 65.1
328.9 1196.2 1041 1268.7 1.28 0 16.6 1.7E-5 2.86 1.2E-2 0 818.1 0.58 1.22 1.28 61.6
351.6 1285.9 1077.5 1222.8 1.15 0 10.6 1.4E-5 2.95 1.3E-2 0 772.8 0.64 1.29 1.36 58.2
374.3 1374.3 1111.1 1180.5 1.04 0 7.1 1.2E-5 3.05 1.5E-2 0 727.5 0.71 1.37 1.44 54.7
O2 flow. At the temperature about 886.4 C, CC and SC can decline to a minimum of 1.05 kg/m3 from 1.83 kg/m3 (LCPG), and a minimum of 1.10 kg/m3 from 1.92 kg/m3 (LCPG), decreasing by 42.5% and 42.4% respectively. All in all, this goal for clean syngas based on LCPG can be achieved substantially by the process. However, there still are some problems as follows: (1) the concentration of CO2 in Raw Syngas is relatively high; (2) the OC is rather high although both CC and SC have been reduced to a large extent; and (3) the improvement of GE is also restricted. Thus, we deduce that the main reason resulting in these problems is the irrational utilization of energy in the system.
The improved process To resolve the above-mentioned irrational utilization of energy, Raw Gas and oxygen were preheated by the exit gas of RGIBBS to specified temperature, which may greatly decrease the consumption of oxygen and hydrogen. The preheating process was set up as Fig. 3. The oxygen was preheated to 180 C (an absolutely safe temperature used widely as
gasifying agent in fixed bed and entrained-flow bed gasification technology [30]), the preheating temperature of Raw Gas varied with the temperature of R-GIBBS.
The comparison between the preheating and non-preheating processes Compared with the non-preheating process, the preheating process definitely changes thermodynamic equilibrium and leads to the variation of component of Raw Syngas. The results of simulation are illustrated in Figs. 4e7. As shown as Fig. 4A, in addition to the steam (825 kg/h) generated from LCPG, the water at R-GIBBS outlet comes mainly from (1) the reforming reaction of hydrocarbons with CO2 and O2, (2) the combustion reaction of H2, and (3) the reverse watergas shift reaction. In the non-preheating process, the mass flow rate of water is higher than that generated from LCPG in the whole temperature range. However, in the preheating process, the mass flow rate of water is higher than that generated from LCPG only above 960 C. In other words, the preheating of Raw Gas and O2 affects the above-mentioned reaction equilibrium and the gaseous composition in the R-GIBBS. Compared with
Fig. 3 e The simulation flowsheet with preheating system. Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
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Fig. 4 e Results for flow rate of water (A) and pollutant content in water (B) under the preheating process (solid line) and nonpreheating process (short dot line).
Fig. 5 e Results for gas volume percentage (A) and volume flow (B) of Raw Syngas under the preheating process (solid line) and non-preheating process (short dot line).
LCPG, the right amount of oxygen not only does not make the wastewater increase, but reduces it to some extent in the preheating process reversely. In according with Fig. 4B, in the wastewater derived from the preheating process, the variation trends of NH3 and H2S are the same as those of the two components in Raw Syngas (explained later). The main pollutant in the wastewater no longer contains any unmanageable organic pollutants such as tar and phenols whether or not to preheat, which greatly reduces the difficulty degree of wastewater treatment. In according with the Fig. 5, both the volume percentage and volume flow of H2, CO and CO2 present similar variation trends with the increase of temperature whether or not to preheat. Both the volume percentage and volume flow of H2 and CO in the preheating process have been increased, and the increase amplitudes become more obvious with the increase of temperature. The dominant reason leading to this phenomenon is that less amount of O2 is required, less amount of H2 is consumed and less amount of H2O is generated compared with the non-preheating process, which causes that the equilibrium of reverse shift reaction (No. 27 in Table 2) shifts to the right. Therefore, the concentration and flow rate of CO2 decreases markedly with the increase of temperature in the preheating process. Apparently, the preheating not only decreases the flow rate of wastewater, but also increases the concentration and yield of effective gas. In addition, Qian Yu also acquired a similar result of significant decline in CO2 emission in a simulative integrated process of coke-oven gas tri-reforming and coal gasification to methanol [31].
As shown in Fig. 6, the volume percentage of CH4 and NH3 is higher relatively in the preheating process, which primarily is contributed to the higher H2 content described in Fig. 5. Moreover, the volume percentage of CH4, NH3 and HCN can be markedly decreased to an ignorable degree, which would facilitate subsequent synthesis steps. There is a interesting phenomenon: the concentration of H2S and COS in preheating process is lower than that in non-preheating process. The
Fig. 6 e Results for content of pollutants in Raw Syngas under the preheating process (solid line) and nonpreheating process (short dot line).
Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
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Fig. 7 e Results for consumption index (A), H2/CO ratio (B) and GE (C) under the preheating process (solid line) and nonpreheating process (short dot line).
result leading to this phenomenon is that the increasing total volume of Raw Syngas plays a dominant role as denominator. As Fig. 7B depicts, H2/CO ratio decreases continuously with the increase of temperature due to the increasing CO concentration. However, the non-preheating system has a higher consumption quantity of H2. As a result, H2/CO ratio is higher in the preheating process, which is helpful to reduce the stress of gas shift unit for methanol synthesis. It is worth mentioning that a Raw Syngas with H2/CO ratio of 2.0 can be attained, such H2/CO ratio is higher than that of coal gas derived from existing gasification technologies: about 0.54 of BGL, about 1.66 of TG, about 0.98 of GE, about 0.79 of E-Gas, about 0.51 of Shell gasification technologies [32]. As shown in Table 3b, the OC, CC and SC of LCPG is 0.26 m3/m3, 1.83 kg/m3 and 1.92 kg/m3, respectively. Just as Fig. 7A, the OC in the nonpreheating process increases to a large extent, and is much
higher than that of LCPG under high temperature. Because of the rational energy utilization, the increase amplitude of OC in the preheating process is rather finite. Moreover, the CC and SC present an obvious descending trend at the whole temperature range in the preheating process. When the temperature exceeds 925 C, the CC and SC is 0.90 kg/m3 and 0.94 kg/ m3, reducing by 50.8% and 51.0% respectively compared with that of LCPG. As Fig. 6C depicts, GE of the preheating process is much higher than those of LCPG and the non-preheating process, only having a slight decrease with the increasing of temperature. The reasons are that the effective utilization of the high temperature gas from R-GIBBS exit was achieved through the preheating system, and almost all organics including tar are translated to effective gas. In other words, nearly 78e85% of chemical energy in coal is transferred into Raw Syngas.
Fig. 8 e Results for concentration of key components (H2: A1 and A2, CO: B1 and B2, CO2: C1 and C2, CH4: D1 and D2, HN3: E1 and E2) in Raw Syngas under the effect of pressure and temperature.
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Fig. 9 e Flow rate of H2O and main pollutants in liquid phase under the effect of pressure and temperature.
The influence of pressure and temperature In accordance with the comparison between preheating and non-preheating process, various results and parameters attained from preheating process are filled with huge superiority. Actually, the simulation study of the effect of pressure on this preheating process is indispensible. For the preheating process, the results relating to co-effect of pressure and
temperature are shown in Figs. 8e10. These figures show the 3D surfaces and the contour plots obtained for these parameters as a function of temperature and pressure. As shown in Fig. 8, the effects of pressure on CO and CO2 can be neglected. However, pressure has a significant impact on H2, CH4 and NH3, which can be attributed to methanation reaction (reverse reaction of No. 16 in Table 2) and ammoniaformation reaction (reverse reaction of No. 30 in Table 2) with
Fig. 10 e Results for various indexes (OC: A1 and A2, CC: B1 and B2, SC: C1 and C2, H2/CO ratio: D1 and D2, GE: E1 and E2) under the effect of pressure and temperature. Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
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Fig. 12 e Results for kinetic simulation of combustionreforming of methane and water-gas shift reaction.
Fig. 11 e The combination of CH4 content (Fig. 8D2) and H2/ CO ratio (Fig. 10D2).
the increasing pressure in lower temperature. A similar result (the effects of pressure and temperature on conversion of CH4 and H2O) was also gained in an equilibrium prediction of CO2 reforming of coke oven gas [26]. As mentioned earlier, the variation of H2O is mainly influenced by water-gas shift reaction and combustion reaction of H2. This negligible influence caused by pressure leads to an essentially constant flow rate of H2O. With respect to NH3 and H2S, their variation tends are attributed to the influence of pressure on chemical reaction equilibrium involving H2, which is similar as those of CH4 and NH3 in Raw Syngas. In accordance with Fig. 10, there is no obvious variation with respect to the effects of pressure on OC, CC and SC with the increasing pressure in higher temperature. Only in lower temperature, they increase greatly due to the obvious changes of H2 and CO shown in Fig. 8. And the similar variation trends of H2 and CO make H2/CO ratio and GE constant with the increasing pressure in the whole temperature range. The H2/CO ratio of 2.0 and CH4 content less than 0.3% is regard as a basic condition for syngas gas [33]. In accordance with Fig. 11, the operating conditions of 2.35 Mpa and 918 C just meet this basic condition in view of the effect of pressure on reaction rate. Under the most optimal operating conditions, obtained results and the comparison with previous reports are listed in Table 4. The preheating process is characterized by smaller OC and appropriate H2/CO ratio,
which means higher GE and the removal of water-gas shift reactor. As a result, this preheating process will become more appropriate for methanol synthesis.
Model validations As previously described, the simulation results show the process is feasible in accordance of thermodynamic equilibrium. Nevertheless, dynamics factors may play a dominating role in restricting the validity of the model. It is extremely difficult to set up a set of dynamical models because of the lacking of integrated dynamical data and the complexity of reactions in the R-GIBBS. In spite of this, the model validations can be evaluated by the combination of the combustionreforming of methane and entrained-flow or fluidized bed gasification technologies being applied widely in industries. It is well known that the thermal degradation of CH4 is identified as the slowest reaction for all organics. And, the production of ethylene and propylene only needs a residence time of 0.4e1.0 s from raw diesel (C10-22) at thermal cracking temperature of 760e860 C [38,39]. Therefore, it is reasonable that CH4 was used to represent all organics for kinetic simulation with the aid of a series of kinetic data [40,41] under 2.35 Mpa and 918 C due to such short residence time of 0.4e1.0 s. It can been seen obviously from Fig. 12 that the oxidation reaction of CH4 is completed in a remarkably short period of time, and the reforming reaction of CH4 and watergas shift reaction reach equilibrium within about 0.25 s. Furthermore, a large number of HTW (fluidized bed) and
Table 4 e The comparison between this paper and previous reports [34e37].
Content in Raw Syngas (vol.%)
Index
H2/CO ratio GE (%)
H2 CO CO2 OC (m3/m3) CC (kg/m3) SC (kg/m3)
This process
LCPG
HTW
HT-L
Shell
Texaco
Amplitude to LCPG (%)
48.0 24.0 25.5 0.137 0.902 0.972 2.0 84.8
32.3 17.2 31.9 0.260 1.83 1.92 1.88 73.0
35.3 51.9 8.9 0.256 0.674 0.330 0.68 74.0
20.4 71.2 3.2 0.310 0.590 e 0.29 84.0
26.7 63.3 1.5 0.320 0.520 e 0.42 82.5
34.8 41.6 10.8 0.430 0.610 e 0.84 69.0
þ48.6 þ39.5 20.1 47.3 50.7 49.4 þ6.38 þ16.2
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TEXACO (entrained-flow bed) gasification technology have been widely operated on a commercial basis. There occur various heterogeneous and homogeneous reactions within a residence time less than 10 s [15,42] in these gasifiers, such as water evaporation, coal drying, coal pyrolysis, combustion, and gasification reactions. However, all reactions (only the pyrolysis and gasification of tar vapor and light hydrocarbon) in the entire R-GIBBS are homogeneous. Therefore, it is determined that the reactions listed in Table 2 are easy to reach equilibrium via keeping a certain residence time within the designed R-GIBBS. So we can safely draw the conclusion that the designed process is effective with respect to thermodynamics and kinetics.
Conclusions Through the reforming of Raw Gas from LCPG by introducing oxygen, the organics including tar, phenols and light hydrocarbon are reformed to clean syngas. In the Raw Syngas, the highest NH3 concentration merely is 0.008%, and the CH4 concentration can be reduced to less than 0.3%, so that the reforming or separating unit of methane can be removed. There are only small amounts of NH3 and H2S in wastewater, which greatly simplifies the process of wastewater treatment. Compared with the non-preheating process and LCPG, the yield and volume percentage of effective gas, and GE have all been increased more obviously in the preheating process. Their corresponding value is 1109 m3/t (coal), 72.0%, and 84.8% respectively under the operating condition of 2.35Mpa and 918 C. And OC, CC and SC decrease by 47.3%, 50.7% and 49.4% respectively in the preheating process compared with LCPG. Furthermore, the kinetic analysis demonstrates that the process is feasible.
Acknowledgements The authors gratefully acknowledge the financial support from the National High Technology Research and Development Program of China (863 Program, No. 2015AA050503).
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The evaluation of a process for clean syngas based on lump coal pressurized gasification Zhaoyi Shi a,1, Shuguang Shen a,*,1, Tianjin Li a, Yujuan Sun a, Weiwei Shan a, Yonghui Bai b, Qinggeng Zhang c, Fan Li b a
College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China State Key Laboratory Breeding Base of Coal Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China c Saiding Engineering Co., Ltd., Taiyuan 030024, China b
article info
abstract
Article history:
Raw Gas generated from LCPG (lump coal pressurized gasifier) contains quantities of or-
Received 31 October 2016
ganics including methane so that it is not suitable as syngas. A process was proposed to
Received in revised form
solve this problem, and Aspen Plus was used to discuss its possibility that Raw Gas was
9 February 2017
reformed to clean syngas through introducing oxygen on the principle of thermodynamic
Accepted 10 February 2017
equilibrium. The simulation results reveal that almost all organics are completely con-
Available online xxx
verted to simple substances such as hydrogen and carbon monoxide whether or not to preheat. Raw Syngas and wastewater no longer contain various organic pollutants elimi-
Keywords:
nated difficultly besides very small amounts of ammonia and hydrogen sulfide, which
Lump coal gasification
certainly simplifies the purification and post-treatment processes. A Raw Syngas with H2/
Clean syngas
CO ratio of 2.0 was attained under the operating condition of 2.35 Mpa and 918 C. Such H2/
Reforming
CO ratio is higher than those of coal gas derived from existing gasification technologies.
Wastewater
Meanwhile, the total yield and content of the effective gas (CO þ H2) have been increased to
Aspen Plus
1109 m3/t and 72.0% respectively in the preheating process, and GE (gasification efficiency) rises to 84.8%. The OC (oxygen consumption), CC (coal consumption) and SC (steam consumption) have been reduced by 47.3%, 50.7% and 49.4% respectively compared with LCPG. In addition, the feasibility of this process was further verified with kinetic simulation. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Coal gasification has become one of the key technologies for high efficiency and clean utilization of coal. Its development goals are to use solid fuel including inferior coal to continuously and efficiently produce coal gas as fuel gas, reducing gas, or feedstock gas for chemical synthesis, hydrogen fuel cell and
synthetic natural gas [1]. Therefore, coal gasification is deservedly regarded as the footstone of current coal chemical industry. There are numerous gasification technologies that are commercially available, in which these gasifiers can be mainly categorized into three main types based on reactor beds including fixed bed, fluidized bed, and entrained-flow bed [2]. These generic types of gasifiers differ mainly in their
* Corresponding author. College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China. E-mail address: [email protected] (S. Shen). 1 The two authors contributed equally to this work and should be considered co-first authors. http://dx.doi.org/10.1016/j.ijhydene.2017.02.078 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
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Nomenclature: Raw Coal a feed stock of LCPG Raw Gas the exit gas of LCPG, which is comprised of Coal Gas, steam and tar vapor Coal Gas a mixed gas containing H2, CO, CO2, CH4, CnHm, N2, NH3 and H2S Raw Syngas a mixed gas containing H2, CO, CO2, and small quantities of CH4, N2, NH3 and H2S
operating conditions such as fluid dynamics, coal particle size, temperature range and heating rate. Therefore, the composition of Raw Gas derived from different gasifiers is greatly different. Specially, the yield and composition of liquid oil and wastewater vary widely. LCPG belongs to fixed bed gasifier, which has been attracted to more attention and applied widely for relatively lower cost and mature technology in China. According to incomplete statistics, nowadays, there are 168 sets of LCPGs in China, which are used in the production of city gas, natural gas, methanol, dimethyl ether, ammonia and synthetic oil. Their corresponding production is 1.5 billion m3/a, 11.3 billion m3/a, 1.56 million t/a, 1.05 million t/a, 900 thousand t/a, and 160 thousand t/a, respectively [3e6]. Its superiority mainly shows as follows: (1) Raw Gas countercurrent contacts with lump coal, so that the ash content in Raw Gas is much less than those of fluidized bed gasifiers and entrained-flow bed gasifiers; (2) the upward hot flow of Raw Gas contributes to drying and pyrolysis of coal, and the recuperation of sensible heat can decrease temperature of Raw Gas and improve GE [7]; (3) the preparation and transport of feed coal is simpler, and less investment is needed than other types of gasifiers with the same scale. Raw gas from LCPG includes approximately 10% (v/v) CH4, which is a distinct advantage for synthesis of nature gas because of obviously reducing the load of methanation unit [8]. However, extra reforming unit is indispensable for the conversion of CH4 to CO and H2 considering producing syngas [9]. Furthermore, the component of Raw Gas is considerably complex, such as light hydrocarbons, naphtha, phenols, hydrogen sulfide and ammonia that will increase the difficulty in purification and separation units. For example, the presence of tar not only decreases GE, but also easily leads to pipe blockage due to temperature dropping [10,11]. Provided the tar component enters into downstream equipment, the safety operation of gas turbine and compressor would be seriously affected. What's more, traditional treatment processes containing pretreatment-biological treatment-advanced treatment have been generally adopted for LCPG wastewater [12e14]. Stripping, extraction and distillation have become major means for the pretreatment process to remove phenols, hydrogen sulfide and ammonia. Then, anaerobic and/or aerobic biological technologies are presented for the biological treatment process with the purpose of eliminating residual organics. Finally, due to a small quantity of nitrogenous substance and refractory organics, the COD and chromaticity of effluent could not meet the discharge standards. So advanced treatments are still indispensible to reduce the negative impact on the
receiving ecosystem, such as physical methods (coagulation precipitation, adsorption and membrane separation) and advanced oxidation process (Fenton oxidation and ozone oxidation). Obviously, the removal of CH4 and tar, and wastewater treatment in LCPG technology is so complicated. According to the current technology level, there seem to be no choice but to use other gasification technologies. However, we expect to change this situation. A new process that can generate clean syngas based on LCPG was proposed in order to solve the above problems. In this process, Raw Gas, without cooling and separation, from gasifier exit was directly introduced into a high-temperature cracking and reforming reactor together with oxygen, and then complex interactions occur among Coal Gas, tar, stream and oxygen under high temperature. The feasibility of this process was discussed with thermodynamics and kinetics using Aspen Plus software (11.1).
Models and methods The reforming process For acquiring clean coal gas which is especially suitable as syngas, the original idea was presented in Fig. 1. In accordance with different gasification pressures and coal ranks, the temperature of raw gas from LCPG is between 280 and 500 C. Meanwhile, some substances in raw gas including tar, phenols, and light hydrocarbons, are introduced into a hightemperature cracking and reforming reactor, and interact with O2, H2O and CO2. And then large quantities of simple molecular substances such as H2 and CO are generated. The high-temperature cracking and reforming reactor is similar as multiinjector reactor, such as multiinjector water-coal slurry
Fig. 1 e The reforming process schematic.
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gasifier with an operating condition of 2.45 Mpa and 1200e1400 C [15,16].
Description of modules For studying the feasibility and reforming effect of this process shown as Fig. 1, the simulation process was set up as Fig. 2 using Aspen Plus software. In the simulation process shown as Fig. 2, Raw Gas and O2 was introduced into an R-GIBBS (Gibbs-reactor). Adjusting O2 volume flow, the temperature of R-GIBBS was controlled at 800e1400 C. Under the high temperature, tar, light hydrocarbons and CH4 was reformed to CO and H2. The descriptions of blocks and streams used in Fig. 2 are as follows: R-GIBBS: a block that represents a Gibbs-reactor, in which the minimization of Gibbs free energy was applied to calculate the thermodynamic equilibrium at a given pressure. A series of chemical reactions, such as pyrolysis, combustion and reforming, occur in the reactor at the temperature range of 800e1400 C. COOLER: a block that cools the exit gas of R-GIBBS to room temperature. SEP: a block that separates wastewater from the exit gas of the R-GIBBS to acquire Raw Syngas. R-G (Raw Gas): the exit gas of LCPG, containing Coal Gas (H2, CO, CO2, CH4, CnHm, N2, NH3 and H2S), gaseous tar and steam, etc. OXY (O2): a stream that was introduced into R-GIBBS at room temperature, its pressure is equal to that of R-GIBBS. W-W (wastewater): a stream that mainly contains water and tiny pollutants such as H2S, NH3, HCN, COS, etc. R-SYN (Raw Syngas): mainly contains H2, CO, CO2, and small quantities of pollutants such as H2S, NH3, HCN, COS, etc. H-LOSS: a negative heat flow that stands for total heat loss to surroundings at the R-GIBBS, accounting for about 4.5% of combustion heat of Raw Coal. In order to determine the influence of these operation conditions on the process, variations in the following five parameters were studied:
1. OC (oxygen consumption) is defined as the oxygen demand to generate per unit of effective gas, including the part which is introduced into LCPG as a gasifying agent (145 m3/ t coal) [1]. OC ¼ ðOXY þ 145Þ=ðCO þ H2 Þ
m3 =m3
(1)
2. CC (coal consumption) is defined as the coal demand to generate per unit of effective gas: CC ¼ 1000=ðCO þ H2 Þ
kg=m3
(2)
3. SC (steam consumption) is defined as the steam demand to generate per unit of effective gas, only standing for the part which is introduced into LCPG as a gasifying agent (1050 kg/ t coal) [1]: SC ¼ 1050=ðCO þ H2 Þ
kg=m3
(3)
4. H2/CO ratio is defined as the molar ratio of H2 to CO, which is used to evaluate the suitability of syngas for methanol synthesis [17]. 5. GE (Gasification Efficiency): the combustion heat ratio of Coal Gas to Raw Coal in conventional coal gasification industry. In the reforming process, it stands for the combustion heat ratio of Raw Syngas to Raw Coal, showing the energy conversion efficiency [18,19].
Assumptions of various parameters The exit gas (Raw Gas) of LCPG had a temperature of 315 C. When per ton lignite was gasified, Raw Gas generated from LCPG contains Coal Gas (1106.0 m3/h), steam (825.0 kg/h) and tar (69.4 kg/h) [1]. In addition, tar contains hundreds of kinds of composition. The component of tar was represented by some main substances with relatively higher content in this simulation. An assumption of tar component was as follows: aliphatic hydrocarbon, unsaturated aliphatic hydrocarbon, aromatic hydrocarbon, sulfur-containing heterocyclic compounds, oxygen-containing heterocyclic compounds, nitrogen-containing heterocyclic compounds and phenols
Fig. 2 e The simulation flowsheet. Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
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was represented by C20H42 (N-eicosane), C12H12 (dimethylnaphthalene), C14H10 (phenanthrene), C12H8S (dibenzothiophene), C12H8O (dibenzofuran), C11H11N (dimethyl-quinoline), C8H10O (xylenol), respectively [20]. Their detailed content was listed in Table 1. The minimization of Gibbs free energy was used to evaluate the process feasibility in Aspen Plus. In previous research, this method was applied widely. For instance, Chan S.H. applied the minimization of Gibbs free energy into autothermal fuel reforming of fuel cells [21]. Seo Y.-S. investigated three different types of reforming process with the aid of thermodynamic analysis using Aspen Plus [22]. Turpeinen E. evaluated the feasibility of hydrogen production using alternative feedstock, such as coke oven gas, refinery gas and biogas by Aspen Plus software [23]. Stephen G. Gopaul compares the syngas production capabilities of biomass from the dry reforming of methane using the ASPEN Plus in terms of H2 and CO yield [24]. Accurate simulation results were gained in these researches. The possible reactions in this system were shown as Table 2. It should be noted that these reactions do not involve detailed reaction mechanism. Nevertheless, in accordance with Hess's law and material balance law, these chemical equations are convenient for the calculation of thermodynamic equilibrium. This reactor is characterized by high temperature and high pressure. And in the reactor, the main substances includes CH4, H2, CO, CO2, H2O, O2, light hydrocarbons, tar, NH3, etc. Therefore, the Peng-Robinson equation of state with BostonMathias modifications (PR-BM) was selected as the state equation [25,26]. In this simulating system, using PengRobinson equation of state will obtain more accurate simulating results.
Results for calculations There is a one-to-one relationship between O2 flow rate and RGIBBS temperature under a certain pressure, so the sensitivity analysis of O2 flow rate was conducted on the R-GIBBS under a pressure of 2.5 Mpa. In accordance with Fig. 2, a series of endothermic and exothermic reactions occurred after Raw Gas along with O2 was introduced into R-GIBBS, and the products such as CO and H2 were generated. The simulation results are presented as Table 3.
The influence on pollutants As shown in Table 3b, with the increase of O2 flow, the temperature increases gradually due to exothermic reactions, and the concentration of NH3 and HCN in Raw Syngas (Table 3a) always tends to decrease so that the concentration of N2 tends to increase. Compared with Coal Gas, the concentration of NH3 and HCN declines markedly. It can be presumed that Reaction 30 and 31 in Table 2 occur, which contributes to the
elimination of NH3 and HCN. And the concentration of COS also declines markedly, which results from the conversion of COS to H2S. It is worth mentioning that tar disappears thoroughly in Raw Syngas via this process, which greatly reduces the burden on the purification unit. As shown in Table 3b, the increasing temperature contributes to the cracking and reforming of organic pollutants to effective gas (CO þ H2), and wastewater no longer contains hydrocarbons and other organics. NH3, HCN, H2S and COS are the main remaining pollutants in wastewater. In accordance with the integrated wastewater discharge standard from coal industry (GB 20426-2006), NH3 concentration in wastewater meets the second grade standards in a temperature range 1014e1196 C, and meets the first grade standards (<15 mg/L) at an equilibrium temperature of 1285e1374 C. The cyanide content in wastewater generated under all operating conditions is much lower (several orders of magnitude) than that of GB 20426-2006 (<0.5 mg/L). The sulfide concentration ranging from 2.52 to 3.05 mg/L is slightly higher than the standard (<1 mg/L). Because sulfide exists primarily in the form of H2S, the wastewater could meet the standard after eliminating H2S by some simple processes. Compared with the wastewater (the content of oil, phenols, NH3-N, and cyanide is 1500e5500 mg/L, 3500e9000 mg/L, 10e400 mg/L, and 50e200 mg/L respectively [27,28]) derived from LCPG, this process shows an obvious advantage on the elimination of pollutants.
The influence on yield and component of gas phase It can be seen from Table 3 that the volume flow of H2 also increases to a highest value at an O2 volume flow of 238.2 m3/h and a temperature about 853.0 C, and then it decreases gradually. The increase of H2 concentration is attributed to the partial oxidization and reforming of organics, which can be demonstrated by the decrease of CH4, C2H4 and C6H6 in Table 3a together with the disappearance of tar in Table 3b. It is worth mentioning that the concentration of CH4 in gaseous phase decreases to less than 1.0% due to the reforming reactions of CH4 (No. 16 and 24 in Table 2), which is similar to other simulation results [24,29]. And with the increase of O2 input, the yield of H2O always increases due to the combustion of H2. After introducing O2, the yield and concentration of CO and CO2 increase simultaneously compared with Coal Gas. With the continuous increasing of O2 flow, the volume fraction of CO2 and H2 decline constantly, and that of CO increase constantly in the R-GIBBS outlet. Therefore, we speculated that the increase of temperature affected the endothermic reverse shift reaction (No. 27 in Table 2), shifting the equilibrium toward the product side. Actually, there are two factors influencing on the endothermic reverse shift reaction. One is the promotion of increasing temperature on the endothermic
Table 1 e The composition of Coal Gas and tar. Coal Gas (Vol.%)
H2 32.3
Tar (wt.%)
CO
CO2
CH4
17.2
31.9
15.1
C2H4 0.56
C6H6
NH3
N2
0.14
0.83
1.48
H2S 0.49
C20H42
C12H12
C14H10
C12H8S
C12H8O
C11H11N
C8H10O
36.5
6.6
24.5
0.2
0.6
3.5
28.1
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Table 2 e The possible reactions in the R-GIBBS. Reaction
△H* (KJ/mol)
C20H42 þ 10O2 / 20CO þ 21H2 C12H12 þ 6O2 / 12CO þ 6H2 C14H10 þ 7O2 / 14CO þ 5H2 C12H8S þ 6O2 / 12CO þ 3H2 þ H2S C11H11N þ 5.5O2 / 11CO þ 4H2 þ NH3 C12H8O þ 5.5O2 / 12CO þ 4H2 C8H10O þ 3.5O2 / 8CO þ 5H2 CH4 þ 0.5O2 / 2H2 þ CO C20H42 þ 20H2O / 20CO þ 41H2 C12H12 þ 12H2O / 12CO þ 18H2 C14H10 þ 14H2O / 14CO þ 19H2 C12H8S þ 12H2O / 12CO þ 15H2 þ H2S C11H11N þ 11H2O / 11CO þ 15H2 þ NH3 C12H8O þ 11H2O / 12CO þ 15H2 C8H10O þ 7H2O / 8CO þ 12H2 CH4 þ H2O 4 CO þ 3H2 C20H42 þ 20CO2 / 40CO þ 21H2 C12H12 þ 12CO2 / 24CO þ 6H2 C14H10 þ 14CO2 / 28CO þ 5H2 C12H8S þ 12CO2 / 24CO þ 3H2 þ H2S C11H11N þ 11CO2 / 22CO þ 4H2 þ NH3 C12H8O þ 11CO2 / 23CO þ 4H2 C8H10O þ 7CO2 / 15CO þ 5H2 CH4 þ CO2 4 2CO þ 2H2 2H2 þ O2 4 2H2O 2CO þ O2 4 2CO2 H2 þ CO2 4 H2O þ CO H2S þ CO 4 COS þ H2 H2S þ O2 4 SO2þH2 2NH3 4 N2 þ 3H2 2HCN þ 1.5O2 4 N2 þ H2O þ 2CO
1687.7 1318.2 1749.6 1627.1 1262.6 1380.4 715.3 22.6 3286.9 1666.5 1732.6 1357.7 1473.3 1355.6 1025.9 250.3 3961.9 2071.6 2205.1 1762.7 1844.7 1726.9 1262.0 259.9 497.2 564.9 34.0 3.3 271.6 111.0 739.2
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Here, △H* were calculated and obtained at 2.5 Mpa and 800 C.
reaction; another is the inhibition of decreasing H2 concentration and increasing H2O flow on the reverse shift reaction. When the temperature exceeds 1105 C, the equivalent impacts effect of two factors on the reverse shift reaction contributes to the constant molar fraction of CO and CO2 at RGIBBS outlet. Moreover, as a small quantity of O2 is introduced, the yield of H2 rises markedly because of the partial oxidation and reforming of hydrocarbon compared with that of Coal Gas, which results in the increase of H2/CO ratio. However, with
the increase of O2 flow, H2/CO ratio decreases gradually, and reaches 2.0 when the O2 volume flow is 249.5 m3/h and the temperature approaches to 886.4 C. As also shown in Table 3b, it is observed that the yield of effective gas (CO þ H2) reaches a peak value of 951.6 m3/t (coal) at 886.4 C, increasing by 73.8% compared with Coal Gas.
The influence on main consumption indexes GE rises in some extent at low O2 flow, and then it falls sharply with the increase of O2 flow. The OC rises with the increase of
Table 3a e The results of gas phase after reforming as Fig. 1. No.
Coal Gas 1 2 3 4 5 6 7 8 9
The composition and yield of Raw Syngas (Vol.% or m3/h)
Gas molar fraction in R-GIBBS outlet (%) H2
CO
CO2
H2
VH2
CO
VCO CO2 VCO2
e 25.4 25.3 24.8 23.9 21.8 19.7 17.8 16.0 14.4
e 10.6 11.8 12.3 12.8 13.4 13.8 13.9 14.0 13.9
e 19.0 18.2 17.8 17.5 16.9 16.5 16.3 16.3 16.4
32.3 43.9 44.3 43.9 43.1 40.9 38.5 36.1 33.8 31.4
357.2 635.9 645.9 635.1 615.3 562.1 508.3 458.5 412.8 370.8
17.2 18.4 20.7 21.9 23.0 25.1 26.9 28.3 29.4 30.2
190.2 266.9 301.5 316.8 328.1 345.0 354.9 359.6 360.0 356.7
31.9 33.0 31.9 31.6 31.5 31.6 32.2 33.1 34.3 35.8
352.8 477.6 465.4 457.1 449.1 434.4 424.9 420.2 419.8 423.1
CH4 15.1 2.4Eþ0 8.9E-1 4.3E-1 1.8E-1 3.1E-2 5.7E-3 1.3E-3 3.2E-4 8.9E-5
C2H4 C6H6 0.56 0 0 0 0 0 0 0 0 0
0.14 0 0 0 0 0 0 0 0 0
NH3
HCN
N2
H2S
COS
8.3E-1 4.8E-3 3.6E-3 2.9E-3 2.2E-3 1.3E-3 7.7E-4 4.7E-4 3.1E-4 2.0E-4
2.0E-3 2.4E-5 2.6E-5 2.6E-5 2.5E-5 2.2E-5 1.9E-5 1.6E-5 1.4E-5 1.2E-5
1.48 1.32 1.32 1.33 1.35 1.40 1.46 1.52 1.59 1.63
0.49 0.34 0.33 0.33 0.34 0.35 0.36 0.38 0.40 0.40
3.2E-1 6.7E-3 7.2E-3 7.7E-3 8.3E-3 9.7E-3 1.1E-2 1.3E-2 1.5E-2 1.7E-2
Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
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Table 3b e The results of various indexes after reforming as Fig. 1. No. O2 (m3/h) T ( C) H2O (kg/h) Raw Syngas (m3/h) H2/CO ratio Total tar (kg/h) The main pollutants in wastewater (mg/L)
NH3 HCN H 2S COS tar
Effective gas (m3/t) OC (m3/m3) CC (kg/m3) SC (kg/m3) GE (%)
LCPG
1
2
3
4
5
6
7
8
9
e 315 825 1108.6 1.72 69.4 300 [27] 170 [28] 215 [27] e 8000 [27] 547.4 0.26 1.83 1.92 73.0
215.5 803.7 842.9 1447.0 2.38 0 167.6 2.5E-5 2.54 5.9E-3 0 902.8 0.40 1.11 1.16 76.5
238.2 853 870.6 1457.1 2.14 0 125.2 2.7E-5 2.52 6.4E-3 0 947.4 0.40 1.06 1.11 74.6
249.5 886.4 889.6 1446.6 2.01 0 101.0 2.6E-5 2.54 6.8E-3 0 951.6 0.41 1.05 1.10 73.3
260.9 925.8 911.5 1426.3 1.88 0 78.2 2.5E-5 2.57 7.4E-3 0 943.4 0.43 1.06 1.11 71.8
283.5 1014 957.6 1372.8 1.63 0 45.1 2.3E-5 2.66 8.7E-3 0 907.0 0.47 1.10 1.16 68.5
306.2 1105.3 1001.2 1318.7 1.43 0 26.7 1.9E-5 2.76 1.0E-2 0 863.1 0.52 1.16 1.22 65.1
328.9 1196.2 1041 1268.7 1.28 0 16.6 1.7E-5 2.86 1.2E-2 0 818.1 0.58 1.22 1.28 61.6
351.6 1285.9 1077.5 1222.8 1.15 0 10.6 1.4E-5 2.95 1.3E-2 0 772.8 0.64 1.29 1.36 58.2
374.3 1374.3 1111.1 1180.5 1.04 0 7.1 1.2E-5 3.05 1.5E-2 0 727.5 0.71 1.37 1.44 54.7
O2 flow. At the temperature about 886.4 C, CC and SC can decline to a minimum of 1.05 kg/m3 from 1.83 kg/m3 (LCPG), and a minimum of 1.10 kg/m3 from 1.92 kg/m3 (LCPG), decreasing by 42.5% and 42.4% respectively. All in all, this goal for clean syngas based on LCPG can be achieved substantially by the process. However, there still are some problems as follows: (1) the concentration of CO2 in Raw Syngas is relatively high; (2) the OC is rather high although both CC and SC have been reduced to a large extent; and (3) the improvement of GE is also restricted. Thus, we deduce that the main reason resulting in these problems is the irrational utilization of energy in the system.
The improved process To resolve the above-mentioned irrational utilization of energy, Raw Gas and oxygen were preheated by the exit gas of RGIBBS to specified temperature, which may greatly decrease the consumption of oxygen and hydrogen. The preheating process was set up as Fig. 3. The oxygen was preheated to 180 C (an absolutely safe temperature used widely as
gasifying agent in fixed bed and entrained-flow bed gasification technology [30]), the preheating temperature of Raw Gas varied with the temperature of R-GIBBS.
The comparison between the preheating and non-preheating processes Compared with the non-preheating process, the preheating process definitely changes thermodynamic equilibrium and leads to the variation of component of Raw Syngas. The results of simulation are illustrated in Figs. 4e7. As shown as Fig. 4A, in addition to the steam (825 kg/h) generated from LCPG, the water at R-GIBBS outlet comes mainly from (1) the reforming reaction of hydrocarbons with CO2 and O2, (2) the combustion reaction of H2, and (3) the reverse watergas shift reaction. In the non-preheating process, the mass flow rate of water is higher than that generated from LCPG in the whole temperature range. However, in the preheating process, the mass flow rate of water is higher than that generated from LCPG only above 960 C. In other words, the preheating of Raw Gas and O2 affects the above-mentioned reaction equilibrium and the gaseous composition in the R-GIBBS. Compared with
Fig. 3 e The simulation flowsheet with preheating system. Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
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Fig. 4 e Results for flow rate of water (A) and pollutant content in water (B) under the preheating process (solid line) and nonpreheating process (short dot line).
Fig. 5 e Results for gas volume percentage (A) and volume flow (B) of Raw Syngas under the preheating process (solid line) and non-preheating process (short dot line).
LCPG, the right amount of oxygen not only does not make the wastewater increase, but reduces it to some extent in the preheating process reversely. In according with Fig. 4B, in the wastewater derived from the preheating process, the variation trends of NH3 and H2S are the same as those of the two components in Raw Syngas (explained later). The main pollutant in the wastewater no longer contains any unmanageable organic pollutants such as tar and phenols whether or not to preheat, which greatly reduces the difficulty degree of wastewater treatment. In according with the Fig. 5, both the volume percentage and volume flow of H2, CO and CO2 present similar variation trends with the increase of temperature whether or not to preheat. Both the volume percentage and volume flow of H2 and CO in the preheating process have been increased, and the increase amplitudes become more obvious with the increase of temperature. The dominant reason leading to this phenomenon is that less amount of O2 is required, less amount of H2 is consumed and less amount of H2O is generated compared with the non-preheating process, which causes that the equilibrium of reverse shift reaction (No. 27 in Table 2) shifts to the right. Therefore, the concentration and flow rate of CO2 decreases markedly with the increase of temperature in the preheating process. Apparently, the preheating not only decreases the flow rate of wastewater, but also increases the concentration and yield of effective gas. In addition, Qian Yu also acquired a similar result of significant decline in CO2 emission in a simulative integrated process of coke-oven gas tri-reforming and coal gasification to methanol [31].
As shown in Fig. 6, the volume percentage of CH4 and NH3 is higher relatively in the preheating process, which primarily is contributed to the higher H2 content described in Fig. 5. Moreover, the volume percentage of CH4, NH3 and HCN can be markedly decreased to an ignorable degree, which would facilitate subsequent synthesis steps. There is a interesting phenomenon: the concentration of H2S and COS in preheating process is lower than that in non-preheating process. The
Fig. 6 e Results for content of pollutants in Raw Syngas under the preheating process (solid line) and nonpreheating process (short dot line).
Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
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Fig. 7 e Results for consumption index (A), H2/CO ratio (B) and GE (C) under the preheating process (solid line) and nonpreheating process (short dot line).
result leading to this phenomenon is that the increasing total volume of Raw Syngas plays a dominant role as denominator. As Fig. 7B depicts, H2/CO ratio decreases continuously with the increase of temperature due to the increasing CO concentration. However, the non-preheating system has a higher consumption quantity of H2. As a result, H2/CO ratio is higher in the preheating process, which is helpful to reduce the stress of gas shift unit for methanol synthesis. It is worth mentioning that a Raw Syngas with H2/CO ratio of 2.0 can be attained, such H2/CO ratio is higher than that of coal gas derived from existing gasification technologies: about 0.54 of BGL, about 1.66 of TG, about 0.98 of GE, about 0.79 of E-Gas, about 0.51 of Shell gasification technologies [32]. As shown in Table 3b, the OC, CC and SC of LCPG is 0.26 m3/m3, 1.83 kg/m3 and 1.92 kg/m3, respectively. Just as Fig. 7A, the OC in the nonpreheating process increases to a large extent, and is much
higher than that of LCPG under high temperature. Because of the rational energy utilization, the increase amplitude of OC in the preheating process is rather finite. Moreover, the CC and SC present an obvious descending trend at the whole temperature range in the preheating process. When the temperature exceeds 925 C, the CC and SC is 0.90 kg/m3 and 0.94 kg/ m3, reducing by 50.8% and 51.0% respectively compared with that of LCPG. As Fig. 6C depicts, GE of the preheating process is much higher than those of LCPG and the non-preheating process, only having a slight decrease with the increasing of temperature. The reasons are that the effective utilization of the high temperature gas from R-GIBBS exit was achieved through the preheating system, and almost all organics including tar are translated to effective gas. In other words, nearly 78e85% of chemical energy in coal is transferred into Raw Syngas.
Fig. 8 e Results for concentration of key components (H2: A1 and A2, CO: B1 and B2, CO2: C1 and C2, CH4: D1 and D2, HN3: E1 and E2) in Raw Syngas under the effect of pressure and temperature.
Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
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Fig. 9 e Flow rate of H2O and main pollutants in liquid phase under the effect of pressure and temperature.
The influence of pressure and temperature In accordance with the comparison between preheating and non-preheating process, various results and parameters attained from preheating process are filled with huge superiority. Actually, the simulation study of the effect of pressure on this preheating process is indispensible. For the preheating process, the results relating to co-effect of pressure and
temperature are shown in Figs. 8e10. These figures show the 3D surfaces and the contour plots obtained for these parameters as a function of temperature and pressure. As shown in Fig. 8, the effects of pressure on CO and CO2 can be neglected. However, pressure has a significant impact on H2, CH4 and NH3, which can be attributed to methanation reaction (reverse reaction of No. 16 in Table 2) and ammoniaformation reaction (reverse reaction of No. 30 in Table 2) with
Fig. 10 e Results for various indexes (OC: A1 and A2, CC: B1 and B2, SC: C1 and C2, H2/CO ratio: D1 and D2, GE: E1 and E2) under the effect of pressure and temperature. Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
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Fig. 12 e Results for kinetic simulation of combustionreforming of methane and water-gas shift reaction.
Fig. 11 e The combination of CH4 content (Fig. 8D2) and H2/ CO ratio (Fig. 10D2).
the increasing pressure in lower temperature. A similar result (the effects of pressure and temperature on conversion of CH4 and H2O) was also gained in an equilibrium prediction of CO2 reforming of coke oven gas [26]. As mentioned earlier, the variation of H2O is mainly influenced by water-gas shift reaction and combustion reaction of H2. This negligible influence caused by pressure leads to an essentially constant flow rate of H2O. With respect to NH3 and H2S, their variation tends are attributed to the influence of pressure on chemical reaction equilibrium involving H2, which is similar as those of CH4 and NH3 in Raw Syngas. In accordance with Fig. 10, there is no obvious variation with respect to the effects of pressure on OC, CC and SC with the increasing pressure in higher temperature. Only in lower temperature, they increase greatly due to the obvious changes of H2 and CO shown in Fig. 8. And the similar variation trends of H2 and CO make H2/CO ratio and GE constant with the increasing pressure in the whole temperature range. The H2/CO ratio of 2.0 and CH4 content less than 0.3% is regard as a basic condition for syngas gas [33]. In accordance with Fig. 11, the operating conditions of 2.35 Mpa and 918 C just meet this basic condition in view of the effect of pressure on reaction rate. Under the most optimal operating conditions, obtained results and the comparison with previous reports are listed in Table 4. The preheating process is characterized by smaller OC and appropriate H2/CO ratio,
which means higher GE and the removal of water-gas shift reactor. As a result, this preheating process will become more appropriate for methanol synthesis.
Model validations As previously described, the simulation results show the process is feasible in accordance of thermodynamic equilibrium. Nevertheless, dynamics factors may play a dominating role in restricting the validity of the model. It is extremely difficult to set up a set of dynamical models because of the lacking of integrated dynamical data and the complexity of reactions in the R-GIBBS. In spite of this, the model validations can be evaluated by the combination of the combustionreforming of methane and entrained-flow or fluidized bed gasification technologies being applied widely in industries. It is well known that the thermal degradation of CH4 is identified as the slowest reaction for all organics. And, the production of ethylene and propylene only needs a residence time of 0.4e1.0 s from raw diesel (C10-22) at thermal cracking temperature of 760e860 C [38,39]. Therefore, it is reasonable that CH4 was used to represent all organics for kinetic simulation with the aid of a series of kinetic data [40,41] under 2.35 Mpa and 918 C due to such short residence time of 0.4e1.0 s. It can been seen obviously from Fig. 12 that the oxidation reaction of CH4 is completed in a remarkably short period of time, and the reforming reaction of CH4 and watergas shift reaction reach equilibrium within about 0.25 s. Furthermore, a large number of HTW (fluidized bed) and
Table 4 e The comparison between this paper and previous reports [34e37].
Content in Raw Syngas (vol.%)
Index
H2/CO ratio GE (%)
H2 CO CO2 OC (m3/m3) CC (kg/m3) SC (kg/m3)
This process
LCPG
HTW
HT-L
Shell
Texaco
Amplitude to LCPG (%)
48.0 24.0 25.5 0.137 0.902 0.972 2.0 84.8
32.3 17.2 31.9 0.260 1.83 1.92 1.88 73.0
35.3 51.9 8.9 0.256 0.674 0.330 0.68 74.0
20.4 71.2 3.2 0.310 0.590 e 0.29 84.0
26.7 63.3 1.5 0.320 0.520 e 0.42 82.5
34.8 41.6 10.8 0.430 0.610 e 0.84 69.0
þ48.6 þ39.5 20.1 47.3 50.7 49.4 þ6.38 þ16.2
Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
TEXACO (entrained-flow bed) gasification technology have been widely operated on a commercial basis. There occur various heterogeneous and homogeneous reactions within a residence time less than 10 s [15,42] in these gasifiers, such as water evaporation, coal drying, coal pyrolysis, combustion, and gasification reactions. However, all reactions (only the pyrolysis and gasification of tar vapor and light hydrocarbon) in the entire R-GIBBS are homogeneous. Therefore, it is determined that the reactions listed in Table 2 are easy to reach equilibrium via keeping a certain residence time within the designed R-GIBBS. So we can safely draw the conclusion that the designed process is effective with respect to thermodynamics and kinetics.
Conclusions Through the reforming of Raw Gas from LCPG by introducing oxygen, the organics including tar, phenols and light hydrocarbon are reformed to clean syngas. In the Raw Syngas, the highest NH3 concentration merely is 0.008%, and the CH4 concentration can be reduced to less than 0.3%, so that the reforming or separating unit of methane can be removed. There are only small amounts of NH3 and H2S in wastewater, which greatly simplifies the process of wastewater treatment. Compared with the non-preheating process and LCPG, the yield and volume percentage of effective gas, and GE have all been increased more obviously in the preheating process. Their corresponding value is 1109 m3/t (coal), 72.0%, and 84.8% respectively under the operating condition of 2.35Mpa and 918 C. And OC, CC and SC decrease by 47.3%, 50.7% and 49.4% respectively in the preheating process compared with LCPG. Furthermore, the kinetic analysis demonstrates that the process is feasible.
Acknowledgements The authors gratefully acknowledge the financial support from the National High Technology Research and Development Program of China (863 Program, No. 2015AA050503).
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Please cite this article in press as: Shi Z, et al., The evaluation of a process for clean syngas based on lump coal pressurized gasification, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.078