- Email: [email protected]
Simulation analysis of hybrid coal gasification according to various conditions in entrained-flow gasifier
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Simulation analysis of hybrid coal gasification according to various conditions in entrained-flow gasifier Dong-Ha Jang a, Sang-Phil Yoon a, Hyung-Taek Kim a,*, Young-Chan Choi b, Chan Lee c a
Division of Energy Systems Research, Ajou University, Suwon, Republic of Korea Clean Fuel Department, High Efficiency and Clean Energy Research Division, Korea Institute of Energy Research (KIER), 71-2, Jang-dong, Yuseong, Daejeon 305-343, Republic of Korea c Division of Mechanical Engineering, Suwon University, Suwon, Republic of Korea b
article info
abstract
Article history:
Today, as more and more power is required globally, securing a sufficient supply of energy
Received 30 April 2014
is very important. Coal is one of the energy resources that may be useful in solving the
Received in revised form
energy crisis. Currently, we have to reduce greenhouse gas emissions. Coal gasification is
27 August 2014
one of the methods that use coal and at the same time solve the climate problem. This
Accepted 10 September 2014
technology produces synthesis gas, which is composed of hydrogen, methane, and carbon
Available online 24 December 2014
monoxide, from coal through procedures of drying, pyrolysis, and gasification. In this
Keywords:
with water to make a coal slurry. Hybrid coal is a fuel combining coal and sugarcane
Hybrid coal
derived from bioliquid. Transportation of the coal slurry, which is related to slurry vis-
Coal gasification
cosity, was considered as an aspect of energy efficiency. Viscosity experiments were per-
Simulation
formed on the KIDECO coal and hybrid coal by Korea Institute of Energy Research (HCK
Aspen Plus
coal) using a viscometer at 40, 50, and 60 wt% of water content. Simulation analysis was
study, KIDECO coal (HCK raw coal) and hybrid coal (biomass/coal ¼ 10 wt%) were mixed
performed by the Aspen Plus program. In this study, the entrained-flow gasifier is the proposed basic model for simulating gasification of HCK raw coal and HCK coal. The carbon conversion and the cold gas efficiency were also calculated to analyze the energy efficiency. Sensitivity analysis was performed according to various conditions such as slurry water content, oxygen ratio, operating temperature, and operating pressure to investigate the process characteristics after verifying the basic model. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Coal gasification is an important technology for use of clean coal today. Many countries such as China, the United States,
and countries in Europe have been operating gasification plants. The Gasification Technologies Conference reports that synthesis gas produced 70,000 MWth in 2010. Currently, energy production from synthesis gas has increased to
* Corresponding author. E-mail addresses: [email protected] (D.-H. Jang), [email protected] (H.-T. Kim). http://dx.doi.org/10.1016/j.ijhydene.2014.09.176 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
2163
Fig. 1 e Schematic diagram of the preparation procedure of HCK coal (Source: Korea Institute of Energy Research).
approximately 100,000 MWth. This is mainly caused by increase in power consumption in the United States and chemical increase in China [1]. In this study, KIDECO bituminous coal from Indonesia was selected for the gasification technology. The biggest advantage of the KIDECO coal is its low sulfur content. This means that the flue gas desulfurization (FGD) process for the removal of SOx is not needed. In addition, KIDECO coal has low ash levels. These advantages play an important role to achieve a clean gasification process and for cost reduction [2]. The two-in-one coal upgrading technology using biomass fuel was developed and manufactured by the Korea Institute of Energy Research (KIER). The hydrophilic surface of the coal is coated with carbon derived from biomass materials, and this procedure can modify the properties of the coal surface from hydrophilic to hydrophobic. This will prevent the re-adsorption of water to the drying coal. Thus, hybrid coal, which mixes the carbon contents of raw coal and biomass, can have a high calorific value. In this study, the hybrid coal produced was used as the fuel to perform the simulation experiments on with the purpose of improving the efficiency of coal gasification. The coal that is produced through a pretreatment process is called hybrid coal by KIER (HCK coal). The process steps for making HCK coal are as follows. First, a paste of raw coal and a solution derived from biomass is made. Second, the paste is fed into the carbonization furnace where drying and carbonization of the biomass-derived materials are carried out simultaneously. From this, the hybrid coal is obtained. Third, a water/alcohol, water/surfactant, or water/alcohol/surfactant dispersion medium may be selected and mixed with the hybrid coal. Subsequently, high density slurry from hybrid coal is produced. Fig. 1, provided by KIER, shows a conceptual diagram of the manufacturing process of HCK coal [3]. Prior to the simulation, the coal gasification type is discussed below. This study used wet gasification. Wet gasification involves coal being transported as slurry mixed with water. The entrained-flow gasifier was selected in this research as the gasification reactor. The main advantages of the entrained-flow gasifier are its application to all types of coal and the formation of clean gas without tar. Further, ash from the device is collected as an inert slag or glass material. Oxygen consumption increases when using coal water slurry
(CWS) or coal that contains high moisture and ash in the entrained-flow gasifier [4]. GE Energy is a representative manufacturer of the entrained-flow gasifier. The basic model for the simulations in this study is based on the GE Energy gasifier. The reason for this is because this type of gasifier uses CWS feed and has features such as refractory-lined reactor, cooling with radiant cooler or quench, and slagging, and can use various fuels such as bituminous coal, pet coke, and blends of pet coke and lowrank coals [5]. A schematic presentation of the GE Energy gasifier is shown in Fig. 2. Briefly, coal slurry mixed with water and oxygen as an oxidizing agent is fed to the top of the reactor. The reactor temperature is approximately 1320e1430 C; therefore, combustion of the coal slurry and oxygen occurs very fast at high temperature. The gasification reaction occurs with volatile matter and steam as the coal slurry moves downward. High-pressure steam and synthesis gas are produced because the GE Energy gasifier has a radiant syngas cooler from the middle to the bottom of the reactor.
Fig. 2 e GE energy gasifier (entrained bed).
2164
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
Table 1 e Proximate and ultimate analysis of KIDECO raw coal and HCK coal.
Proximate analysis (wt%)
Ultimate analysis (wt%)
M V.M F.C Ash C H O N S Ash
HHV (kcal/kg) LHV (kcal/kg)
KIDECO raw coal
Hybrid coal by KIER
25.39 33.71 34.46 6.44 66.7 4.75 20.89 1.18 0.04 6.44 5,970 5,560
1.16 42.98 48.78 7.08 70.9 4.51 16.11 1.36 0.04 7.08 6,290 6,040
Modeling and simulation
exchange reactor was used as the slurry mixing device with a maximum capacity of 10 L. Next, simulation of the HCK coal gasification process was conducted with reference to the report “Tampa Electric Integrated Gasification Combined-Cycle Project” since it was operated with a GE Energy gasifier. This project was carried out by the Clean Coal Technology Program by the US Department of Energy’s (DOE) National Energy Technology Laboratory (NETL) in 1985. The Texaco entrained-flow gasifier successfully operated more than 18,000 h and a demonstration unit was set up in Tampa, Florida, in 1996. Table 2 presents the input values with reference to the above-mentioned report. Also, the synthesis gas results are shown in Table 3 [6]. Proximate and ultimate analysis of Pittsburgh No. 8 seam coal is absent in the Tampa project report. Thus, this information was found in other NETL literature. It is indicated in Table 4 [7].
B. Simulation methods
A. Initial conditions Before the simulation, we discuss the coal used in this study. KIDECO coal was selected as the raw coal for the experiments. From KIDECO coal, HCK coal could be produced through a pretreatment process with sugar cane biomass such as molasses. The size of the coal samples was 75 mm. Proximate and ultimate analysis of the KIDECO coal and the HCK coal were performed to determine the basic physical properties, and these results are shown in Table 1. HCK coal has low moisture content, and high volatile matter and fixed carbon content when compared to KIDECO raw coal. The important properties of KIDECO coal are its very low contents of ash and sulfur. HCK coal was developed by KIER and can be helpful in removing FGD devices in combustion and gasification processes. In order to determine the suitable composition of the coal slurry, i.e., water content, for the gasification, experiments on slurry transportation are needed. For this purpose, viscosity measurements were carried out on mixtures of HCK coal and water. The possibility of slurry transportation was determined by these viscosity experiments. The procedure for the slurry experiments is shown in Fig. 3. Brookfield’s Programmable Digital Viscometer was used to measure the viscosity. An ion
For the simulation of the entrained-flow gasification model, it is necessary to understand the gasification reaction. The coal gasification reaction is influenced by various conditions such as the coal type, oxidizing agent, gasifier type, and operating conditions. However, the gasification reaction generally occurs in the following steps. Initially, the coal is fed into the reactor, and the coal is heated rapidly. The moisture is removed from the coal in the drying step. Next, volatile matter is released and solid particles are melted during devolatilization and pyrolysis. The final step is gasification of the coal char. The char reacts with the oxidizer, e.g., oxygen, and steam. In this study, the simulation procedure followed these general steps listed above, and the details of the procedure are discussed below. The gasification model simulations were carried out under steady state conditions.
Drying The GE Energy’s entrained-flow gasification is a slurry type. Coal is injected into the reactor as a slurry mixed with water. This coal water slurry is called CWS. In the first step, drying occurs in the reactor at high temperature and pressure. The reaction can be expressed as follows [8]. Drying : Coal slurry/Coal þ H2 OðgÞ
Fig. 3 e Slurry mixing and viscosity measurement.
(1)
2165
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
Table 2 e Input summary e Tampa project report. Input conditions
Selected conditions
Reactor type Typical coal Coal (tons/day) Oxygen (tons/day) Coal/water slurry (%) Operating temperature ( C) Operating pressure (atm) Carbon conversion (%)
Entrained bed type Pittsburgh No. 8 seam 2,200 2,171 60 ~ 70 1320 ~ 1430 30 95
Table 4 e Proximate and ultimate analysis of Pittsburgh No. 8 seam coal.
Proximate analysis (wt%)
Ultimate analysis (wt%)
The CWS contains a large quantity of water, and heat converts the water to steam. HHV (kcal/kg) LHV (kcal/kg)
Pyrolysis and devolatilization Pyrolysis releases volatile matter from the coal. Char and ash remain in the solid phase. This can be expressed by the following reaction [8]. Pyrolysis : Coal/Volatiles þ Char þ Ash
Moisture V.M F.C Ash C H O N S Cl Moisture Ash
(2)
Usually, devolatilization takes place when the coal is heated at 350e850 C. Depending on the reaction rate, reactions are divided into slow and fast. Pyrolysis and gasification reactions take place simultaneously because they operate at high temperature in the entrained-flow gasifier [4]. Volatile matter consists of polymerization components such as C, H, O, N, S, and ash. The decomposition results of pyrolysis are carbon monoxide, carbon dioxide, hydrogen, methane, hydrogen sulfide, nitrogen, and tar [9]. The devolatilization components from coal pyrolysis react with oxygen as the oxidant. The reaction rate of the volatilization compounds is 0.5e2 s according to the temperature, which is very fast. This allows the equilibrium configuration. The Gibbs function can be adopted as an equilibrium model for the devolatilization phenomenon from coal as DG ¼ 0 [10]. Gibbs free energy (G) is the extractable energy in a thermodynamic system when constant pressure and temperature conditions are maintained. Gibbs free energy is also the chemical potential that is minimized at constant pressure and temperature. The change in Gibbs free energy (DG) is proportional to the entire entropy (S). That is, DG ¼ DH TDS. In this case, when DG > 0, then the forward reaction is an involuntary reaction and the reverse reaction is a voluntary reaction. When DG ¼ 0, the forward and reverse reactions are in a state of equilibrium. When DG < 0, the forward reaction is
Table 3 e Composition of cleaned syngas. Constituent
Volume%
Carbon monoxide Hydrogen Carbon dioxide Methane Water Nitrogen Argon Hydrogen sulfide Carbonyl sulfide Ammonia
42.7 38.3 14.4 0.1 0.3 3.3 0.9 200 ppmv 10 ppmv 0.0 ppmv
As-received
Dry
2.63 35.82 52.38 9.17 73.15 4.97 6.22 1.46 2.36 0.04 2.63 9.17 7,292 7,032
0.00 36.79 53.79 9.42 75.13 5.10 6.39 1.50 2.42 0.04 0.00 9.42 7,488 7,227
spontaneous. Gibbs free energy also has standard reaction values of entropy and enthalpy The standard Gibbs free energy values are the values at a constant temperature of 298 K. Thus, Gibbs free energy changes at different temperatures. Gibbs free energy at a given temperature can be obtained from the following equation [11]. DG0 ¼
X
ni G0product
DG ¼ DG0 þ RT ln Q
X
ni G0reactant
(3) (4)
In this study, simulation of pyrolysis and devolatilization in the entrained-flow gasifier was conducted on the basis of the theory of Gibbs free energy. The gasification simulation was performed with the Aspen Plus program, which is based on the thermodynamic theory, and Gibbs reaction model was used. One thing to note here is that some volatile matter and other components remain in char. However, in this model carbon and other components remained in the char and only volatile matter was considered by the Gibbs reaction model. Thus, after devolatilization, volatile matter generates chemical compounds that contain C, H, O, N, and S.
Gasification For the gasification, in a few exceptional cases, it is impossible to model the reaction as a thermodynamic equilibrium model. One of these cases is the moving bed gasifier that uses coal with oxygen and steam in reverse flow. In addition, biomass gasification and catalytic gasification also cannot be interpreted as thermodynamic equilibriums [4]. In this study, however, the gasification type is the entrained-flow gasification. This means that the coal reacts with oxygen and steam at high temperature. Each reaction such as drying, pyrolysis, and gasification occurs very fast. Hence, the equilibrium model can be adopted in this research. The entrained-flow gasifier is not an exceptional case and is reported in the literature. Thus, Gibbs free energy model was implemented for the gasification phenomenon such as pyrolysis. Coal char was produced from the coal slurry after drying and devolatilization. Char contains a large amount of fixed carbon. The solid phase changes to gaseous phase during
2166
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
Fig. 4 e Schematic presentation of entrained-flow gasification process with Aspen Plus.
combustion. The equations of the gasification reaction are discussed below. For the entrained-flow coal gasification, 11 major reaction equations were selected from numerous studies. These equations are presented as follows from (5) to (15) [12]. C þ 0:5O2 /CO C þ CO2 /2CO C þ H2 O/CO þ H2 C þ 2H2 /CH4
DH0298 DH0298
¼ 123 kJ mol
¼ þ159:7 kJ mol
Reactions (5)e(8) are heterogeneous reactions of char particles. These reactions produce CO, H2, and CH4. H2 þ 0:5O2 /H2 O
DH0298 ¼ 242 kJ mol
(9)
CO þ 0:5O2 /CO2
DH0298 ¼ 283 kJ mol
(10)
DH0298 ¼ 40:9 kJ mol
(11)
(5) CO þ H2 O/CO2 þ H2 (6)
DH0298 ¼ þ118:5 kJ mol
(7)
DH0298 ¼ 88:4 kJ mol
(8)
CH4 þ H2 O/CO þ 3H2
DH0298 ¼ þ206 kJ mol
(12)
Reactions (9)e(12) are homogeneous reactions generated from the char particles.
Fig. 5 e Schematic presentation of HCK coal gasification process with Aspen Plus.
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
S þ H2 /H2 S N2 þ 3H2 /2NH3 COS þ H2 O/CO2 þ H2 S
DH0298 ¼ þ79:9 kJ mol DH0298 ¼ 40:9 kJ mol DH0298 ¼ 40:9 kJ mol
2167
(13) (14) (15)
Reactions (13)e(14) are reactions of hydrogen and organic materials such as sulfur and nitrogen. Reaction (15) produces CO2 and H2S from carbonyl sulfide (COS) and H2O. Based on the input values, the entrained-flow coal gasification modeling was performed using the Aspen Plus program. Reaction models for the pyrolysis and the gasification were separated as RGIBBS models [13]. A schematic representation of the Aspen Plus model is shown in Fig. 4. The entire process was separated into three blocks: coal input, gas input, and coal gasification processes. The coal is first pulverized to 75 mm size particles. Then the coal slurry is made with water for injection. In the gasifier, the coal is converted to synthesis gas and slag through drying, pyrolysis and devolatilization, and gasification reactions. The coal gasification through combustion with an oxidizing agent is fast, and thus the equilibrium model can be implemented. The simulation is set for each block, which has built-in functions, using Aspen Plus, following the above procedure.
C. Process modeling for hybrid coal In this study, hybrid coal is the fuel in focus. It is manufactured from KIDECO raw coal and biomass-derived carbon through the method of coating. The simulation of HCK coal gasification was conducted in order to determine the gasification reactivity, quality of the synthesis gas, and operating conditions. A basic model was adopted with reference to the Tampa project report, and simulation of the HCK coal gasification process was performed by applying this basic model. According to KIER, the HCK raw coal is crushed to 75 mm size and then made into a paste. Next, the paste is reformed by biomass-derived carbon from biomass fuel. The raw coal is allowed to stand for 12 h and then dried for 6 h at 105 C. This is expected to enhance the performance by removing inherent moisture in the coal pores. In addition, drying and carbonization of the biomass-derived material is performed at 105e900 C for 0.1e10 h. The biomass-derived material (ethanol in this study) is added to 10 wt% coal. The solvent of the biomass can be water or an organic solvent [3,14]. Manufactured HCK coal has properties similar to general coal. In this study, the model for HCK coal gasification was completed through modification of the basic model. A flow diagram of the HCK coal gasification process is shown in Fig. 5. In the above figure, the production of hybrid coal from KIDECO raw coal and biomass-derived material is added to the process model (see Fig. 5). Coal slurry is manufactured at 40, 50, and 60 wt% of water content. It is slightly different from the 60e70% in the basic model in the Tampa report. This is attributed to the characteristics of the HCK coal; 50 wt% water content is set for the basic model. All other settings such as input conditions, temperature, and pressure are the same as in the Tampa report.)
Fig. 6 e Sensitivity analysis to changes in water slurry.
Sensitivity analysis can be applied to obtain better results after completion of the HCK coal gasification simulation using the basic model. The sensitivity analysis deals with various conditions such as the amount of water in the slurry, oxygen ratio, operating temperature, or operating pressure (see Figs. 6e9).
Results and discussion Viscosity results Viscosity measurements of the HCK coal slurry were performed with the experimental apparatus shown above in Fig. 3. The obtained results are shown in Table 5.
Fig. 7 e Sensitivity analysis to changes in oxygen ratio.
2168
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
Table 5 e Results of slurry viscosity experiments. Slurry concentration (%) 40 50 55 60
Fig. 8 e Sensitivity analysis to changes in operating temperature.
Without surfactant (viscosity cP)
Sikament NN (viscosity cP)
HCK
KIDECO raw
HCK
KIDECO raw
75 320 650 1350
e 7.5 30 530
e 28 157 580
e 6 9 140
From the results of the viscosity experiments, the potential for slurry transportation was checked with reference to various documents. The possible viscosities of coal feed are presented as 500e1000 cp or less than 2000 cp at 25 C and shear rate 100 s1. According to the above conditions, the slurry concentrations of 55 and 60% are best suited for coal slurry gasification [15,16]. Regarding suitable slurry viscosities with the use of other surfactants, continued research is needed. Thus, with reference to the above, the coal water slurry was considered at 40, 50, and 60 wt% of water content.
Basic model Laboratory experiments were conducted with 500 ml beakers at 20 C ambient atmosphere. The coal slurry viscosity can be reduced with a surfactant in order to improve the slurry mobility. However, the slurry transportation is inhibited when viscosity is too low. Thus, sometimes coal slurry is manufactured without a surfactant.
Carbon conversion ð%Þ ¼
The entrained-flow gasification model as the basic model was completed with the input conditions shown in Tables 2e4. The simulation results of the basic model were compared to the Tampa report results. The comparison of the synthesis gas composition is shown in Table 6. The carbon conversion was calculated from the simulation results by using the following basic equation [4].
Carbon in gasification residue ½kmol=h 246:946 ½kmol=h 100 ¼ 1 100 ¼ 95:12ð%Þ 1 Carbon in feedstock ½kmol=h 5064:573 ½kmol=h
The calculated carbon conversion is 95.12%. This result is nearly identical to the 95% in the reference report. After completing the basic model with reference to the Tampa project report, simulation results of the HCK coal gasification were obtained by applying the same conditions.
Table 6 e Comparison of report vs. simulation results e composition of synthesis gas. Constituent
Fig. 9 e Sensitivity analysis to changes in operating pressure.
Carbon monoxide Hydrogen Carbon dioxide Methane Water Nitrogen Argon Hydrogen sulfide Carbonyl sulfide Ammonia
Volume% (reference)
Volume% (simulation)
42.7
42.3
38.3 14.4 0.1 0.3 3.3 0.9 200 ppmv 10 ppmv 0.0 ppmv
38.86 14.02 Trace Trace 3.26 0.83 6829 ppmv 329 ppmv 44 ppmv
2169
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
Table 7 e Comparison of simulation results of basic model and HCK model e composition of syngas. Constituent
Volume% (basic model)
Volume% (HCK model)
42.3 38.86 14.02 Trace Trace 3.26 0.83 6829 ppmv 329 ppmv 44 ppmv
43.36 34.38 17.66 Trace Trace 3.64 0.94 130 ppmv 7 ppmv 38 ppmv
Carbon monoxide Hydrogen Carbon dioxide Methane Water Nitrogen Argon Hydrogen sulfide Carbonyl sulfide Ammonia
The results of the HCK model were compared to the results of the basic model, which are shown in Table 7. The very low yields of H2S and COS in the synthesis gas of HCK coal are important results. These values match the values of H2S (200 ppmv) and COS (10 ppmv) in Table 3. From this, it is concluded that the FGD process is not required.) The yields of CO and CO2 in the synthesis gas of HCK coal are slightly increased, whereas the yield of H2 is slightly decreased. The carbon conversion was calculated from the simulation results of the HCK coal.
Carbon conversion ð%Þ ¼
Heating value in product gas ðkcal=hrÞ Heating value in feedstock ðkcal=hrÞ
Cold gas efficiencyð%Þ ¼
100 ¼
437;216;883ðkcal=hrÞ ¼ 74:77ð%Þ 584;771;258ðkcal=hrÞ Heating value in product gas ðkcal=hrÞ Heating value in feedstock ðkcal=hrÞ
Cold gas efficiencyð%Þ ¼
100 ¼
372;882;608ðkcal=hrÞ ¼ 74:24ð%Þ 502;277;929ðkcal=hrÞ
The cold gas efficiencies of the basic model and the HCK model were calculated to 74.77% and 74.24%, respectively. These results indicate that HCK coal, despite low fixed carbon and low calorific value, when coated with biomass-derived materials, can have almost the same efficiency when compared to the Pittsburgh No. 8 seam coal used in the Tampa project. This is a consideration in the decision whether to use lignite in the HCK coal gasification process.
Sensitivity analysis The simulation results above were verified through sensitivity analysis. This analysis was performed to predict suitable
Carbon in gasification residue ½kmol=h 229:972 ½kmol=h 100 ¼ 1 100 ¼ 95:26ð%Þ 1 Carbon in feedstock ½kmol=h 4848:515 ½kmol=h
When compared to the carbon conversion of the Tampa report, the carbon conversion of HCK coal is higher (95.26%). In addition, the cold gas efficiency of the basic model and the HCK model were calculated to identify further characteristics of the synthesis gas [4].
operating conditions when using HCK coal. For this purpose, the yields of CO, H2, CO2, and CH4 in the synthesis gas were analyzed under several conditions. Furthermore, the findings from the sensitivity analysis will help to determine suitable operating conditions for downstream processes also.
Table 8 e Sensitivity analysis to changes in water slurry. Mixing water (wt% of coal/water slurry) 40 50 60
CO (vol%)
H2 (vol%)
CO2 (vol%)
CH4 (vol%)
Carbon conversion (%)
Cold gas efficiency (%)
46.86 45.44 44.13
35.42 36.04 36.62
17.72 18.51 19.25
0.00 0.00 0.00
91.18 95.26 99.36
74.38 74.24 74.11
Table 9 e Sensitivity analysis to changes in oxygen ratio. Oxygen ratio (Basic model ¼ 1) 0.5 1 1.5
CO (vol%)
H2 (vol%)
CO2 (vol%)
CH4 (vol%)
Carbon conversion (%)
Cold gas efficiency (%)
46.69 45.44 37.77
46.55 36.04 23.56
6.74 18.51 38.67
0.02 0.00 0.00
75.20 95.26 100
99.80 74.24 47.18
2170
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
Table 10 e Sensitivity analysis to changes in operating temperature. Operating temperature ( C) 1320 1330 1340 1350 1360 1370 1380 1390 1400 1410 1420 1430
CO (vol%)
H2 (vol%)
CO2 (vol%)
CH4 (vol%)
Carbon conversion (%)
Cold gas efficiency (%)
45.44 45.60 45.76 45.91 46.06 46.21 46.36 46.50 46.65 46.79 46.92 47.06
36.04 35.97 35.90 35.84 35.77 35.71 35.64 35.58 35.52 35.45 35.39 35.33
18.51 18.42 18.34 18.25 18.16 18.08 18.00 17.92 17.84 17.76 17.68 17.61
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
95.26 96.81 98.46 100 100 100 100 100 100 100 100 100
74.24 74.26 74.27 74.29 74.30 74.32 74.33 74.35 74.36 74.37 74.39 74.40
First, sensitivity analysis was performed for the different slurry compositions of 40, 50, and 60 wt% of water content (see Table 8). The results of the sensitivity analysis show a slight decrease in the yields of CO and CH4 and a slight increase in the yields of H2 and CO2. It can be assumed that the rates of reaction Equations (7) and (11) during the gasification are enhanced by water. It is determined that for the transportation of the coal slurry, it is beneficial to provide more mixing water. In addition, an increasing carbon conversion and a decreasing cold gas efficiency are confirmed. This is caused by the carbonesteam reaction and the wateregas shift reaction. The former increases the carbon conversion and H2, and the latter converts the CO gas to CO2 gas. The results show that the cold gas efficiency decreases from a yield reduction of CO gas, which has a high heating value. From these observations, it can be inferred that more slurry water improves the chemical fuel or power generation. Secondly, the yield of synthesis gas related to the oxygen ratio was analyzed (see Table 9).
The amount of oxygen was determined the most important factor under various conditions. The investigated oxygen ratios were 0.5, 1, and 1.5. The results show that CO2 gas increases sharply for oxygen ratio above 1. Conversely, CO and H2 gases decrease. This verifies that less oxygen abruptly changes the combustion reaction to a gasification reaction. The combustion reaction is very fast in the entrained-flow gasifier, which has a very high temperature. Thus, the combustion reaction takes place rapidly when increasing the amount of oxygen, and this leads to increased carbon conversion and CO2 gas. In addition, the cold gas efficiency has a tendency to decrease because of the reductions of CO, H2, and CH4 that have high heating values. In conclusion, the amount of oxygen needs to be maintained at a ratio of 1 in order to produce more CO and H2 gas. Thirdly, the sensitivity to changes in the operating temperature of the reactor was analyzed (see Table 10). The sensitivity analysis was conducted at 1320e1430 C according to the Tampa report. Results show a slight increase in CO gas following increasing temperature. The
Table 11 e Sensitivity analysis to changes in operating pressure. Operating Pressure (bar) 1 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
CO (vol%)
H2 (vol%)
CO2 (vol%)
CH4 (vol%)
Carbon conversion (%)
Cold gas efficiency (%)
45.43 45.44 45.44 45.45 45.45 45.45 45.45 45.45 45.46 45.46 45.46 45.46 45.46 45.45 45.45 45.45 45.45 45.45 45.44 45.44
36.05 36.05 36.04 36.03 36.03 36.02 36.01 36.00 35.99 35.97 35.96 35.95 35.93 35.92 35.90 35.88 35.86 35.84 35.82 35.80
18.51 18.51 18.51 18.51 18.52 18.52 18.52 18.53 18.53 18.54 18.55 18.55 18.56 18.57 18.58 18.59 18.60 18.61 18.62 18.64
0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.02 0.02 0.03 0.04 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12
100 98.47 95.35 93.48 92.19 91.20 90.42 89.77 89.21 88.72 88.28 87.88 87.51 87.17 86.86 86.56 86.27 86.00 85.74 85.49
74.25 74.24 74.24 74.23 74.23 74.22 74.22 74.21 74.20 74.20 74.19 74.18 74.17 74.16 74.15 74.14 74.13 74.12 74.10 74.10
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
basic model has already a high operating temperature and carbon conversion. Therefore, increasing the operating temperature does not cause any significant changes. In theory, the carbon conversion is 100% when there is sufficient oxygen and steam to react with solid carbon. When the carbon conversion is calculated, carbon present in ash and fine particles is usually not considered because of the very small amount. However, ash and fine particles were considered in the simulation model. Furthermore, increased cold gas efficiency will not have a great advantage when considering the energy loss due to increased operating temperature. Finally, the sensitivity to changes in the operating pressure of the reactor was analyzed (see Table 11). The pressure of the reactor does not influence the yield of synthesis gas. The advantage of high pressure synthesis gas is that it saves the compression energy in downstream processes. Thus, the results of the sensitivity analysis following operating pressure confirmed that only subtle changes in the composition of the synthesis gas could be seen according to the pressure. In addition, the results indicate a reduction in the carbon conversion and cold gas efficiency with increasing pressure. Therefore, operating pressure should be considered as an attentive condition depending on the desired end products and downstream facilities. From the above results, it can be determined that the amount of oxygen and the temperature are the most important factors in the entrained-flow gasifier using HCK coal.
Acknowledgment This work was conducted under the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER) (B3-8417).
Nomenclature HCK DG DG0 ni R Q
hybrid coal by Korea Institute of Energy Research change in Gibbs free energy standard change of reaction in Gibbs free energy number of particles composing the ith chemical component gas constant reaction quotient
Conclusions 1. In this study, hybrid coal made from KIDECO bituminous coal and biomass-derived carbon were considered for the modeling of HCK coal gasification. An entrained-flow gasifier was used based on the results of the Tampa report. In addition, viscosity experiments for wet coal gasification were performed. 2. Slurry viscosity measurements determined the appropriate water content in the coal feed. Next, a computer simulation was completed for the basic model of the entrained-flow gasification that refers to the Tampa
2171
project. The simulation model could thus be verified by previous experimental results. 3. The HCK coal gasification simulation was completed with the basic model. From the simulation results, the characteristics of the synthesis gas, carbon conversion, and cold gas efficiency were revealed. After confirmation of the above results, sensitivity analysis was performed to search for important factors that influence the gasification process. 4. The sensitivity analysis showed that the most important factor of HCK coal gasification in the entrained-flow gasifier is the amount of oxygen. The water content of the slurry causes only a slight change in the yield of synthesis gas. Also the reactor temperature and pressure have little effect on the yield of synthesis gas in the HCK model. 5. Finally, it was concluded that gasification of HCK coal, which is manufactured from coal with low heating value, has a sufficient efficiency in the entrained-flow gasifier. The operating conditions can be predicted primarily through the amount of water in the slurry preparation and the amount of oxygen as oxidant.
references
[1] Higman Chris, Consulting Higman, GmbH. State of the gasification industry e the updated worldwide gasification database. In: Gasification technologies conference Colorado Springs; 2013. [2] KIDECO. Why KIDECO Coal, http://www.KIDECO.com/coal/ whycoal.asp. [3] Lee Dong-Wook, Jong-SooBae Young-Joo Lee, Park Se-Joon, Hong Jai-Chang, Lee Byoung-Hwa, Jeon Chung-Hwan, et al. Two-in one fuel combining sugar cane with low rank coal and its CO2 reduction effects in pulverized-coal power plants. Environ Sci Technol 2013;47(3):1704e10. [4] Higman Christopher, Burgt Maarten van der. Gasification. Elsevier; 2008. [5] Tennant Jenny B. Overview of DOE’s gasification program. National Energy Technology Laboratory; 2010. p. 34. [6] U.S. Department of Energy and Tampa Electric Company. The Tampa electric integrated gasification combined-cycle project, an update. 2011. [7] U.S. Department of Energy. Quality guidelines for energy system studies e detailed coal specifications. 2012. p. 38e9. [8] Qian Feng, Kong Xiangdong, Cheng Hui, Du Wenli, Zhong Weimin. Development of a kinetic model for industrial entrained flow coal gasifiers. Ind Eng Chem Res 2013;52:1819e28. [9] Rakesh Govind, Jogen Shah. Modeling and simulation of an entrained flow coal gasifier. AlChE 1984;30(1):79e92. [10] Debrand S, Hahn OJ. Gas composition calculationfor Thein Situ gasificationof Thin Seamsand the Approachto modeling, vol. 23(1). New York, NY: Am ChemSoc; 1978. p. 240e51. [11] Wikipedia. Gibbs free energy, http://en.wikipedia.org/wiki/ Gibbs_free_energy. [12] Williams A, Pourkashanian M, Jones JM, Skorupska N. Combustion and gasification of coal. Taylor & Francis Inc; 1999. [13] ASPEN plus V7.1 manual. ASPENTech; 2009.
2172
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
[14] Jong-SooBae Dong-Wook Lee, Lee Young-Joo, Park Se-Joon, Park Ju-Hyoung, Hong Jai-Chang, Kim Joeng-Geun, et al. Improvement in coal content of coal-water slurry using hybrid coal impregnated with molasses. Powder Technol 2014;254:72e7. [15] Serant F, Agapov K, Kuzmin,Yu A, Ovchinnikov V, Pugach L. Development and application of promising technologies
forfiring coal-water fuels. NovosibirskRussian Federation; 2008. p. 5. [16] Ramachandran Prakash, Tsai Ching-Yi, Schancha Gary W. An evaluation of coal water slurry fuel burners and technology. Construction Engineering Research Laboratories of US Army Corp of Engineers; 1992. p. 26.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Simulation analysis of hybrid coal gasification according to various conditions in entrained-flow gasifier Dong-Ha Jang a, Sang-Phil Yoon a, Hyung-Taek Kim a,*, Young-Chan Choi b, Chan Lee c a
Division of Energy Systems Research, Ajou University, Suwon, Republic of Korea Clean Fuel Department, High Efficiency and Clean Energy Research Division, Korea Institute of Energy Research (KIER), 71-2, Jang-dong, Yuseong, Daejeon 305-343, Republic of Korea c Division of Mechanical Engineering, Suwon University, Suwon, Republic of Korea b
article info
abstract
Article history:
Today, as more and more power is required globally, securing a sufficient supply of energy
Received 30 April 2014
is very important. Coal is one of the energy resources that may be useful in solving the
Received in revised form
energy crisis. Currently, we have to reduce greenhouse gas emissions. Coal gasification is
27 August 2014
one of the methods that use coal and at the same time solve the climate problem. This
Accepted 10 September 2014
technology produces synthesis gas, which is composed of hydrogen, methane, and carbon
Available online 24 December 2014
monoxide, from coal through procedures of drying, pyrolysis, and gasification. In this
Keywords:
with water to make a coal slurry. Hybrid coal is a fuel combining coal and sugarcane
Hybrid coal
derived from bioliquid. Transportation of the coal slurry, which is related to slurry vis-
Coal gasification
cosity, was considered as an aspect of energy efficiency. Viscosity experiments were per-
Simulation
formed on the KIDECO coal and hybrid coal by Korea Institute of Energy Research (HCK
Aspen Plus
coal) using a viscometer at 40, 50, and 60 wt% of water content. Simulation analysis was
study, KIDECO coal (HCK raw coal) and hybrid coal (biomass/coal ¼ 10 wt%) were mixed
performed by the Aspen Plus program. In this study, the entrained-flow gasifier is the proposed basic model for simulating gasification of HCK raw coal and HCK coal. The carbon conversion and the cold gas efficiency were also calculated to analyze the energy efficiency. Sensitivity analysis was performed according to various conditions such as slurry water content, oxygen ratio, operating temperature, and operating pressure to investigate the process characteristics after verifying the basic model. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Coal gasification is an important technology for use of clean coal today. Many countries such as China, the United States,
and countries in Europe have been operating gasification plants. The Gasification Technologies Conference reports that synthesis gas produced 70,000 MWth in 2010. Currently, energy production from synthesis gas has increased to
* Corresponding author. E-mail addresses: [email protected] (D.-H. Jang), [email protected] (H.-T. Kim). http://dx.doi.org/10.1016/j.ijhydene.2014.09.176 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
2163
Fig. 1 e Schematic diagram of the preparation procedure of HCK coal (Source: Korea Institute of Energy Research).
approximately 100,000 MWth. This is mainly caused by increase in power consumption in the United States and chemical increase in China [1]. In this study, KIDECO bituminous coal from Indonesia was selected for the gasification technology. The biggest advantage of the KIDECO coal is its low sulfur content. This means that the flue gas desulfurization (FGD) process for the removal of SOx is not needed. In addition, KIDECO coal has low ash levels. These advantages play an important role to achieve a clean gasification process and for cost reduction [2]. The two-in-one coal upgrading technology using biomass fuel was developed and manufactured by the Korea Institute of Energy Research (KIER). The hydrophilic surface of the coal is coated with carbon derived from biomass materials, and this procedure can modify the properties of the coal surface from hydrophilic to hydrophobic. This will prevent the re-adsorption of water to the drying coal. Thus, hybrid coal, which mixes the carbon contents of raw coal and biomass, can have a high calorific value. In this study, the hybrid coal produced was used as the fuel to perform the simulation experiments on with the purpose of improving the efficiency of coal gasification. The coal that is produced through a pretreatment process is called hybrid coal by KIER (HCK coal). The process steps for making HCK coal are as follows. First, a paste of raw coal and a solution derived from biomass is made. Second, the paste is fed into the carbonization furnace where drying and carbonization of the biomass-derived materials are carried out simultaneously. From this, the hybrid coal is obtained. Third, a water/alcohol, water/surfactant, or water/alcohol/surfactant dispersion medium may be selected and mixed with the hybrid coal. Subsequently, high density slurry from hybrid coal is produced. Fig. 1, provided by KIER, shows a conceptual diagram of the manufacturing process of HCK coal [3]. Prior to the simulation, the coal gasification type is discussed below. This study used wet gasification. Wet gasification involves coal being transported as slurry mixed with water. The entrained-flow gasifier was selected in this research as the gasification reactor. The main advantages of the entrained-flow gasifier are its application to all types of coal and the formation of clean gas without tar. Further, ash from the device is collected as an inert slag or glass material. Oxygen consumption increases when using coal water slurry
(CWS) or coal that contains high moisture and ash in the entrained-flow gasifier [4]. GE Energy is a representative manufacturer of the entrained-flow gasifier. The basic model for the simulations in this study is based on the GE Energy gasifier. The reason for this is because this type of gasifier uses CWS feed and has features such as refractory-lined reactor, cooling with radiant cooler or quench, and slagging, and can use various fuels such as bituminous coal, pet coke, and blends of pet coke and lowrank coals [5]. A schematic presentation of the GE Energy gasifier is shown in Fig. 2. Briefly, coal slurry mixed with water and oxygen as an oxidizing agent is fed to the top of the reactor. The reactor temperature is approximately 1320e1430 C; therefore, combustion of the coal slurry and oxygen occurs very fast at high temperature. The gasification reaction occurs with volatile matter and steam as the coal slurry moves downward. High-pressure steam and synthesis gas are produced because the GE Energy gasifier has a radiant syngas cooler from the middle to the bottom of the reactor.
Fig. 2 e GE energy gasifier (entrained bed).
2164
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
Table 1 e Proximate and ultimate analysis of KIDECO raw coal and HCK coal.
Proximate analysis (wt%)
Ultimate analysis (wt%)
M V.M F.C Ash C H O N S Ash
HHV (kcal/kg) LHV (kcal/kg)
KIDECO raw coal
Hybrid coal by KIER
25.39 33.71 34.46 6.44 66.7 4.75 20.89 1.18 0.04 6.44 5,970 5,560
1.16 42.98 48.78 7.08 70.9 4.51 16.11 1.36 0.04 7.08 6,290 6,040
Modeling and simulation
exchange reactor was used as the slurry mixing device with a maximum capacity of 10 L. Next, simulation of the HCK coal gasification process was conducted with reference to the report “Tampa Electric Integrated Gasification Combined-Cycle Project” since it was operated with a GE Energy gasifier. This project was carried out by the Clean Coal Technology Program by the US Department of Energy’s (DOE) National Energy Technology Laboratory (NETL) in 1985. The Texaco entrained-flow gasifier successfully operated more than 18,000 h and a demonstration unit was set up in Tampa, Florida, in 1996. Table 2 presents the input values with reference to the above-mentioned report. Also, the synthesis gas results are shown in Table 3 [6]. Proximate and ultimate analysis of Pittsburgh No. 8 seam coal is absent in the Tampa project report. Thus, this information was found in other NETL literature. It is indicated in Table 4 [7].
B. Simulation methods
A. Initial conditions Before the simulation, we discuss the coal used in this study. KIDECO coal was selected as the raw coal for the experiments. From KIDECO coal, HCK coal could be produced through a pretreatment process with sugar cane biomass such as molasses. The size of the coal samples was 75 mm. Proximate and ultimate analysis of the KIDECO coal and the HCK coal were performed to determine the basic physical properties, and these results are shown in Table 1. HCK coal has low moisture content, and high volatile matter and fixed carbon content when compared to KIDECO raw coal. The important properties of KIDECO coal are its very low contents of ash and sulfur. HCK coal was developed by KIER and can be helpful in removing FGD devices in combustion and gasification processes. In order to determine the suitable composition of the coal slurry, i.e., water content, for the gasification, experiments on slurry transportation are needed. For this purpose, viscosity measurements were carried out on mixtures of HCK coal and water. The possibility of slurry transportation was determined by these viscosity experiments. The procedure for the slurry experiments is shown in Fig. 3. Brookfield’s Programmable Digital Viscometer was used to measure the viscosity. An ion
For the simulation of the entrained-flow gasification model, it is necessary to understand the gasification reaction. The coal gasification reaction is influenced by various conditions such as the coal type, oxidizing agent, gasifier type, and operating conditions. However, the gasification reaction generally occurs in the following steps. Initially, the coal is fed into the reactor, and the coal is heated rapidly. The moisture is removed from the coal in the drying step. Next, volatile matter is released and solid particles are melted during devolatilization and pyrolysis. The final step is gasification of the coal char. The char reacts with the oxidizer, e.g., oxygen, and steam. In this study, the simulation procedure followed these general steps listed above, and the details of the procedure are discussed below. The gasification model simulations were carried out under steady state conditions.
Drying The GE Energy’s entrained-flow gasification is a slurry type. Coal is injected into the reactor as a slurry mixed with water. This coal water slurry is called CWS. In the first step, drying occurs in the reactor at high temperature and pressure. The reaction can be expressed as follows [8]. Drying : Coal slurry/Coal þ H2 OðgÞ
Fig. 3 e Slurry mixing and viscosity measurement.
(1)
2165
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
Table 2 e Input summary e Tampa project report. Input conditions
Selected conditions
Reactor type Typical coal Coal (tons/day) Oxygen (tons/day) Coal/water slurry (%) Operating temperature ( C) Operating pressure (atm) Carbon conversion (%)
Entrained bed type Pittsburgh No. 8 seam 2,200 2,171 60 ~ 70 1320 ~ 1430 30 95
Table 4 e Proximate and ultimate analysis of Pittsburgh No. 8 seam coal.
Proximate analysis (wt%)
Ultimate analysis (wt%)
The CWS contains a large quantity of water, and heat converts the water to steam. HHV (kcal/kg) LHV (kcal/kg)
Pyrolysis and devolatilization Pyrolysis releases volatile matter from the coal. Char and ash remain in the solid phase. This can be expressed by the following reaction [8]. Pyrolysis : Coal/Volatiles þ Char þ Ash
Moisture V.M F.C Ash C H O N S Cl Moisture Ash
(2)
Usually, devolatilization takes place when the coal is heated at 350e850 C. Depending on the reaction rate, reactions are divided into slow and fast. Pyrolysis and gasification reactions take place simultaneously because they operate at high temperature in the entrained-flow gasifier [4]. Volatile matter consists of polymerization components such as C, H, O, N, S, and ash. The decomposition results of pyrolysis are carbon monoxide, carbon dioxide, hydrogen, methane, hydrogen sulfide, nitrogen, and tar [9]. The devolatilization components from coal pyrolysis react with oxygen as the oxidant. The reaction rate of the volatilization compounds is 0.5e2 s according to the temperature, which is very fast. This allows the equilibrium configuration. The Gibbs function can be adopted as an equilibrium model for the devolatilization phenomenon from coal as DG ¼ 0 [10]. Gibbs free energy (G) is the extractable energy in a thermodynamic system when constant pressure and temperature conditions are maintained. Gibbs free energy is also the chemical potential that is minimized at constant pressure and temperature. The change in Gibbs free energy (DG) is proportional to the entire entropy (S). That is, DG ¼ DH TDS. In this case, when DG > 0, then the forward reaction is an involuntary reaction and the reverse reaction is a voluntary reaction. When DG ¼ 0, the forward and reverse reactions are in a state of equilibrium. When DG < 0, the forward reaction is
Table 3 e Composition of cleaned syngas. Constituent
Volume%
Carbon monoxide Hydrogen Carbon dioxide Methane Water Nitrogen Argon Hydrogen sulfide Carbonyl sulfide Ammonia
42.7 38.3 14.4 0.1 0.3 3.3 0.9 200 ppmv 10 ppmv 0.0 ppmv
As-received
Dry
2.63 35.82 52.38 9.17 73.15 4.97 6.22 1.46 2.36 0.04 2.63 9.17 7,292 7,032
0.00 36.79 53.79 9.42 75.13 5.10 6.39 1.50 2.42 0.04 0.00 9.42 7,488 7,227
spontaneous. Gibbs free energy also has standard reaction values of entropy and enthalpy The standard Gibbs free energy values are the values at a constant temperature of 298 K. Thus, Gibbs free energy changes at different temperatures. Gibbs free energy at a given temperature can be obtained from the following equation [11]. DG0 ¼
X
ni G0product
DG ¼ DG0 þ RT ln Q
X
ni G0reactant
(3) (4)
In this study, simulation of pyrolysis and devolatilization in the entrained-flow gasifier was conducted on the basis of the theory of Gibbs free energy. The gasification simulation was performed with the Aspen Plus program, which is based on the thermodynamic theory, and Gibbs reaction model was used. One thing to note here is that some volatile matter and other components remain in char. However, in this model carbon and other components remained in the char and only volatile matter was considered by the Gibbs reaction model. Thus, after devolatilization, volatile matter generates chemical compounds that contain C, H, O, N, and S.
Gasification For the gasification, in a few exceptional cases, it is impossible to model the reaction as a thermodynamic equilibrium model. One of these cases is the moving bed gasifier that uses coal with oxygen and steam in reverse flow. In addition, biomass gasification and catalytic gasification also cannot be interpreted as thermodynamic equilibriums [4]. In this study, however, the gasification type is the entrained-flow gasification. This means that the coal reacts with oxygen and steam at high temperature. Each reaction such as drying, pyrolysis, and gasification occurs very fast. Hence, the equilibrium model can be adopted in this research. The entrained-flow gasifier is not an exceptional case and is reported in the literature. Thus, Gibbs free energy model was implemented for the gasification phenomenon such as pyrolysis. Coal char was produced from the coal slurry after drying and devolatilization. Char contains a large amount of fixed carbon. The solid phase changes to gaseous phase during
2166
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
Fig. 4 e Schematic presentation of entrained-flow gasification process with Aspen Plus.
combustion. The equations of the gasification reaction are discussed below. For the entrained-flow coal gasification, 11 major reaction equations were selected from numerous studies. These equations are presented as follows from (5) to (15) [12]. C þ 0:5O2 /CO C þ CO2 /2CO C þ H2 O/CO þ H2 C þ 2H2 /CH4
DH0298 DH0298
¼ 123 kJ mol
¼ þ159:7 kJ mol
Reactions (5)e(8) are heterogeneous reactions of char particles. These reactions produce CO, H2, and CH4. H2 þ 0:5O2 /H2 O
DH0298 ¼ 242 kJ mol
(9)
CO þ 0:5O2 /CO2
DH0298 ¼ 283 kJ mol
(10)
DH0298 ¼ 40:9 kJ mol
(11)
(5) CO þ H2 O/CO2 þ H2 (6)
DH0298 ¼ þ118:5 kJ mol
(7)
DH0298 ¼ 88:4 kJ mol
(8)
CH4 þ H2 O/CO þ 3H2
DH0298 ¼ þ206 kJ mol
(12)
Reactions (9)e(12) are homogeneous reactions generated from the char particles.
Fig. 5 e Schematic presentation of HCK coal gasification process with Aspen Plus.
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
S þ H2 /H2 S N2 þ 3H2 /2NH3 COS þ H2 O/CO2 þ H2 S
DH0298 ¼ þ79:9 kJ mol DH0298 ¼ 40:9 kJ mol DH0298 ¼ 40:9 kJ mol
2167
(13) (14) (15)
Reactions (13)e(14) are reactions of hydrogen and organic materials such as sulfur and nitrogen. Reaction (15) produces CO2 and H2S from carbonyl sulfide (COS) and H2O. Based on the input values, the entrained-flow coal gasification modeling was performed using the Aspen Plus program. Reaction models for the pyrolysis and the gasification were separated as RGIBBS models [13]. A schematic representation of the Aspen Plus model is shown in Fig. 4. The entire process was separated into three blocks: coal input, gas input, and coal gasification processes. The coal is first pulverized to 75 mm size particles. Then the coal slurry is made with water for injection. In the gasifier, the coal is converted to synthesis gas and slag through drying, pyrolysis and devolatilization, and gasification reactions. The coal gasification through combustion with an oxidizing agent is fast, and thus the equilibrium model can be implemented. The simulation is set for each block, which has built-in functions, using Aspen Plus, following the above procedure.
C. Process modeling for hybrid coal In this study, hybrid coal is the fuel in focus. It is manufactured from KIDECO raw coal and biomass-derived carbon through the method of coating. The simulation of HCK coal gasification was conducted in order to determine the gasification reactivity, quality of the synthesis gas, and operating conditions. A basic model was adopted with reference to the Tampa project report, and simulation of the HCK coal gasification process was performed by applying this basic model. According to KIER, the HCK raw coal is crushed to 75 mm size and then made into a paste. Next, the paste is reformed by biomass-derived carbon from biomass fuel. The raw coal is allowed to stand for 12 h and then dried for 6 h at 105 C. This is expected to enhance the performance by removing inherent moisture in the coal pores. In addition, drying and carbonization of the biomass-derived material is performed at 105e900 C for 0.1e10 h. The biomass-derived material (ethanol in this study) is added to 10 wt% coal. The solvent of the biomass can be water or an organic solvent [3,14]. Manufactured HCK coal has properties similar to general coal. In this study, the model for HCK coal gasification was completed through modification of the basic model. A flow diagram of the HCK coal gasification process is shown in Fig. 5. In the above figure, the production of hybrid coal from KIDECO raw coal and biomass-derived material is added to the process model (see Fig. 5). Coal slurry is manufactured at 40, 50, and 60 wt% of water content. It is slightly different from the 60e70% in the basic model in the Tampa report. This is attributed to the characteristics of the HCK coal; 50 wt% water content is set for the basic model. All other settings such as input conditions, temperature, and pressure are the same as in the Tampa report.)
Fig. 6 e Sensitivity analysis to changes in water slurry.
Sensitivity analysis can be applied to obtain better results after completion of the HCK coal gasification simulation using the basic model. The sensitivity analysis deals with various conditions such as the amount of water in the slurry, oxygen ratio, operating temperature, or operating pressure (see Figs. 6e9).
Results and discussion Viscosity results Viscosity measurements of the HCK coal slurry were performed with the experimental apparatus shown above in Fig. 3. The obtained results are shown in Table 5.
Fig. 7 e Sensitivity analysis to changes in oxygen ratio.
2168
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
Table 5 e Results of slurry viscosity experiments. Slurry concentration (%) 40 50 55 60
Fig. 8 e Sensitivity analysis to changes in operating temperature.
Without surfactant (viscosity cP)
Sikament NN (viscosity cP)
HCK
KIDECO raw
HCK
KIDECO raw
75 320 650 1350
e 7.5 30 530
e 28 157 580
e 6 9 140
From the results of the viscosity experiments, the potential for slurry transportation was checked with reference to various documents. The possible viscosities of coal feed are presented as 500e1000 cp or less than 2000 cp at 25 C and shear rate 100 s1. According to the above conditions, the slurry concentrations of 55 and 60% are best suited for coal slurry gasification [15,16]. Regarding suitable slurry viscosities with the use of other surfactants, continued research is needed. Thus, with reference to the above, the coal water slurry was considered at 40, 50, and 60 wt% of water content.
Basic model Laboratory experiments were conducted with 500 ml beakers at 20 C ambient atmosphere. The coal slurry viscosity can be reduced with a surfactant in order to improve the slurry mobility. However, the slurry transportation is inhibited when viscosity is too low. Thus, sometimes coal slurry is manufactured without a surfactant.
Carbon conversion ð%Þ ¼
The entrained-flow gasification model as the basic model was completed with the input conditions shown in Tables 2e4. The simulation results of the basic model were compared to the Tampa report results. The comparison of the synthesis gas composition is shown in Table 6. The carbon conversion was calculated from the simulation results by using the following basic equation [4].
Carbon in gasification residue ½kmol=h 246:946 ½kmol=h 100 ¼ 1 100 ¼ 95:12ð%Þ 1 Carbon in feedstock ½kmol=h 5064:573 ½kmol=h
The calculated carbon conversion is 95.12%. This result is nearly identical to the 95% in the reference report. After completing the basic model with reference to the Tampa project report, simulation results of the HCK coal gasification were obtained by applying the same conditions.
Table 6 e Comparison of report vs. simulation results e composition of synthesis gas. Constituent
Fig. 9 e Sensitivity analysis to changes in operating pressure.
Carbon monoxide Hydrogen Carbon dioxide Methane Water Nitrogen Argon Hydrogen sulfide Carbonyl sulfide Ammonia
Volume% (reference)
Volume% (simulation)
42.7
42.3
38.3 14.4 0.1 0.3 3.3 0.9 200 ppmv 10 ppmv 0.0 ppmv
38.86 14.02 Trace Trace 3.26 0.83 6829 ppmv 329 ppmv 44 ppmv
2169
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
Table 7 e Comparison of simulation results of basic model and HCK model e composition of syngas. Constituent
Volume% (basic model)
Volume% (HCK model)
42.3 38.86 14.02 Trace Trace 3.26 0.83 6829 ppmv 329 ppmv 44 ppmv
43.36 34.38 17.66 Trace Trace 3.64 0.94 130 ppmv 7 ppmv 38 ppmv
Carbon monoxide Hydrogen Carbon dioxide Methane Water Nitrogen Argon Hydrogen sulfide Carbonyl sulfide Ammonia
The results of the HCK model were compared to the results of the basic model, which are shown in Table 7. The very low yields of H2S and COS in the synthesis gas of HCK coal are important results. These values match the values of H2S (200 ppmv) and COS (10 ppmv) in Table 3. From this, it is concluded that the FGD process is not required.) The yields of CO and CO2 in the synthesis gas of HCK coal are slightly increased, whereas the yield of H2 is slightly decreased. The carbon conversion was calculated from the simulation results of the HCK coal.
Carbon conversion ð%Þ ¼
Heating value in product gas ðkcal=hrÞ Heating value in feedstock ðkcal=hrÞ
Cold gas efficiencyð%Þ ¼
100 ¼
437;216;883ðkcal=hrÞ ¼ 74:77ð%Þ 584;771;258ðkcal=hrÞ Heating value in product gas ðkcal=hrÞ Heating value in feedstock ðkcal=hrÞ
Cold gas efficiencyð%Þ ¼
100 ¼
372;882;608ðkcal=hrÞ ¼ 74:24ð%Þ 502;277;929ðkcal=hrÞ
The cold gas efficiencies of the basic model and the HCK model were calculated to 74.77% and 74.24%, respectively. These results indicate that HCK coal, despite low fixed carbon and low calorific value, when coated with biomass-derived materials, can have almost the same efficiency when compared to the Pittsburgh No. 8 seam coal used in the Tampa project. This is a consideration in the decision whether to use lignite in the HCK coal gasification process.
Sensitivity analysis The simulation results above were verified through sensitivity analysis. This analysis was performed to predict suitable
Carbon in gasification residue ½kmol=h 229:972 ½kmol=h 100 ¼ 1 100 ¼ 95:26ð%Þ 1 Carbon in feedstock ½kmol=h 4848:515 ½kmol=h
When compared to the carbon conversion of the Tampa report, the carbon conversion of HCK coal is higher (95.26%). In addition, the cold gas efficiency of the basic model and the HCK model were calculated to identify further characteristics of the synthesis gas [4].
operating conditions when using HCK coal. For this purpose, the yields of CO, H2, CO2, and CH4 in the synthesis gas were analyzed under several conditions. Furthermore, the findings from the sensitivity analysis will help to determine suitable operating conditions for downstream processes also.
Table 8 e Sensitivity analysis to changes in water slurry. Mixing water (wt% of coal/water slurry) 40 50 60
CO (vol%)
H2 (vol%)
CO2 (vol%)
CH4 (vol%)
Carbon conversion (%)
Cold gas efficiency (%)
46.86 45.44 44.13
35.42 36.04 36.62
17.72 18.51 19.25
0.00 0.00 0.00
91.18 95.26 99.36
74.38 74.24 74.11
Table 9 e Sensitivity analysis to changes in oxygen ratio. Oxygen ratio (Basic model ¼ 1) 0.5 1 1.5
CO (vol%)
H2 (vol%)
CO2 (vol%)
CH4 (vol%)
Carbon conversion (%)
Cold gas efficiency (%)
46.69 45.44 37.77
46.55 36.04 23.56
6.74 18.51 38.67
0.02 0.00 0.00
75.20 95.26 100
99.80 74.24 47.18
2170
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
Table 10 e Sensitivity analysis to changes in operating temperature. Operating temperature ( C) 1320 1330 1340 1350 1360 1370 1380 1390 1400 1410 1420 1430
CO (vol%)
H2 (vol%)
CO2 (vol%)
CH4 (vol%)
Carbon conversion (%)
Cold gas efficiency (%)
45.44 45.60 45.76 45.91 46.06 46.21 46.36 46.50 46.65 46.79 46.92 47.06
36.04 35.97 35.90 35.84 35.77 35.71 35.64 35.58 35.52 35.45 35.39 35.33
18.51 18.42 18.34 18.25 18.16 18.08 18.00 17.92 17.84 17.76 17.68 17.61
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
95.26 96.81 98.46 100 100 100 100 100 100 100 100 100
74.24 74.26 74.27 74.29 74.30 74.32 74.33 74.35 74.36 74.37 74.39 74.40
First, sensitivity analysis was performed for the different slurry compositions of 40, 50, and 60 wt% of water content (see Table 8). The results of the sensitivity analysis show a slight decrease in the yields of CO and CH4 and a slight increase in the yields of H2 and CO2. It can be assumed that the rates of reaction Equations (7) and (11) during the gasification are enhanced by water. It is determined that for the transportation of the coal slurry, it is beneficial to provide more mixing water. In addition, an increasing carbon conversion and a decreasing cold gas efficiency are confirmed. This is caused by the carbonesteam reaction and the wateregas shift reaction. The former increases the carbon conversion and H2, and the latter converts the CO gas to CO2 gas. The results show that the cold gas efficiency decreases from a yield reduction of CO gas, which has a high heating value. From these observations, it can be inferred that more slurry water improves the chemical fuel or power generation. Secondly, the yield of synthesis gas related to the oxygen ratio was analyzed (see Table 9).
The amount of oxygen was determined the most important factor under various conditions. The investigated oxygen ratios were 0.5, 1, and 1.5. The results show that CO2 gas increases sharply for oxygen ratio above 1. Conversely, CO and H2 gases decrease. This verifies that less oxygen abruptly changes the combustion reaction to a gasification reaction. The combustion reaction is very fast in the entrained-flow gasifier, which has a very high temperature. Thus, the combustion reaction takes place rapidly when increasing the amount of oxygen, and this leads to increased carbon conversion and CO2 gas. In addition, the cold gas efficiency has a tendency to decrease because of the reductions of CO, H2, and CH4 that have high heating values. In conclusion, the amount of oxygen needs to be maintained at a ratio of 1 in order to produce more CO and H2 gas. Thirdly, the sensitivity to changes in the operating temperature of the reactor was analyzed (see Table 10). The sensitivity analysis was conducted at 1320e1430 C according to the Tampa report. Results show a slight increase in CO gas following increasing temperature. The
Table 11 e Sensitivity analysis to changes in operating pressure. Operating Pressure (bar) 1 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
CO (vol%)
H2 (vol%)
CO2 (vol%)
CH4 (vol%)
Carbon conversion (%)
Cold gas efficiency (%)
45.43 45.44 45.44 45.45 45.45 45.45 45.45 45.45 45.46 45.46 45.46 45.46 45.46 45.45 45.45 45.45 45.45 45.45 45.44 45.44
36.05 36.05 36.04 36.03 36.03 36.02 36.01 36.00 35.99 35.97 35.96 35.95 35.93 35.92 35.90 35.88 35.86 35.84 35.82 35.80
18.51 18.51 18.51 18.51 18.52 18.52 18.52 18.53 18.53 18.54 18.55 18.55 18.56 18.57 18.58 18.59 18.60 18.61 18.62 18.64
0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.02 0.02 0.03 0.04 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12
100 98.47 95.35 93.48 92.19 91.20 90.42 89.77 89.21 88.72 88.28 87.88 87.51 87.17 86.86 86.56 86.27 86.00 85.74 85.49
74.25 74.24 74.24 74.23 74.23 74.22 74.22 74.21 74.20 74.20 74.19 74.18 74.17 74.16 74.15 74.14 74.13 74.12 74.10 74.10
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
basic model has already a high operating temperature and carbon conversion. Therefore, increasing the operating temperature does not cause any significant changes. In theory, the carbon conversion is 100% when there is sufficient oxygen and steam to react with solid carbon. When the carbon conversion is calculated, carbon present in ash and fine particles is usually not considered because of the very small amount. However, ash and fine particles were considered in the simulation model. Furthermore, increased cold gas efficiency will not have a great advantage when considering the energy loss due to increased operating temperature. Finally, the sensitivity to changes in the operating pressure of the reactor was analyzed (see Table 11). The pressure of the reactor does not influence the yield of synthesis gas. The advantage of high pressure synthesis gas is that it saves the compression energy in downstream processes. Thus, the results of the sensitivity analysis following operating pressure confirmed that only subtle changes in the composition of the synthesis gas could be seen according to the pressure. In addition, the results indicate a reduction in the carbon conversion and cold gas efficiency with increasing pressure. Therefore, operating pressure should be considered as an attentive condition depending on the desired end products and downstream facilities. From the above results, it can be determined that the amount of oxygen and the temperature are the most important factors in the entrained-flow gasifier using HCK coal.
Acknowledgment This work was conducted under the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER) (B3-8417).
Nomenclature HCK DG DG0 ni R Q
hybrid coal by Korea Institute of Energy Research change in Gibbs free energy standard change of reaction in Gibbs free energy number of particles composing the ith chemical component gas constant reaction quotient
Conclusions 1. In this study, hybrid coal made from KIDECO bituminous coal and biomass-derived carbon were considered for the modeling of HCK coal gasification. An entrained-flow gasifier was used based on the results of the Tampa report. In addition, viscosity experiments for wet coal gasification were performed. 2. Slurry viscosity measurements determined the appropriate water content in the coal feed. Next, a computer simulation was completed for the basic model of the entrained-flow gasification that refers to the Tampa
2171
project. The simulation model could thus be verified by previous experimental results. 3. The HCK coal gasification simulation was completed with the basic model. From the simulation results, the characteristics of the synthesis gas, carbon conversion, and cold gas efficiency were revealed. After confirmation of the above results, sensitivity analysis was performed to search for important factors that influence the gasification process. 4. The sensitivity analysis showed that the most important factor of HCK coal gasification in the entrained-flow gasifier is the amount of oxygen. The water content of the slurry causes only a slight change in the yield of synthesis gas. Also the reactor temperature and pressure have little effect on the yield of synthesis gas in the HCK model. 5. Finally, it was concluded that gasification of HCK coal, which is manufactured from coal with low heating value, has a sufficient efficiency in the entrained-flow gasifier. The operating conditions can be predicted primarily through the amount of water in the slurry preparation and the amount of oxygen as oxidant.
references
[1] Higman Chris, Consulting Higman, GmbH. State of the gasification industry e the updated worldwide gasification database. In: Gasification technologies conference Colorado Springs; 2013. [2] KIDECO. Why KIDECO Coal, http://www.KIDECO.com/coal/ whycoal.asp. [3] Lee Dong-Wook, Jong-SooBae Young-Joo Lee, Park Se-Joon, Hong Jai-Chang, Lee Byoung-Hwa, Jeon Chung-Hwan, et al. Two-in one fuel combining sugar cane with low rank coal and its CO2 reduction effects in pulverized-coal power plants. Environ Sci Technol 2013;47(3):1704e10. [4] Higman Christopher, Burgt Maarten van der. Gasification. Elsevier; 2008. [5] Tennant Jenny B. Overview of DOE’s gasification program. National Energy Technology Laboratory; 2010. p. 34. [6] U.S. Department of Energy and Tampa Electric Company. The Tampa electric integrated gasification combined-cycle project, an update. 2011. [7] U.S. Department of Energy. Quality guidelines for energy system studies e detailed coal specifications. 2012. p. 38e9. [8] Qian Feng, Kong Xiangdong, Cheng Hui, Du Wenli, Zhong Weimin. Development of a kinetic model for industrial entrained flow coal gasifiers. Ind Eng Chem Res 2013;52:1819e28. [9] Rakesh Govind, Jogen Shah. Modeling and simulation of an entrained flow coal gasifier. AlChE 1984;30(1):79e92. [10] Debrand S, Hahn OJ. Gas composition calculationfor Thein Situ gasificationof Thin Seamsand the Approachto modeling, vol. 23(1). New York, NY: Am ChemSoc; 1978. p. 240e51. [11] Wikipedia. Gibbs free energy, http://en.wikipedia.org/wiki/ Gibbs_free_energy. [12] Williams A, Pourkashanian M, Jones JM, Skorupska N. Combustion and gasification of coal. Taylor & Francis Inc; 1999. [13] ASPEN plus V7.1 manual. ASPENTech; 2009.
2172
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 4 0 ( 2 0 1 5 ) 2 1 6 2 e2 1 7 2
[14] Jong-SooBae Dong-Wook Lee, Lee Young-Joo, Park Se-Joon, Park Ju-Hyoung, Hong Jai-Chang, Kim Joeng-Geun, et al. Improvement in coal content of coal-water slurry using hybrid coal impregnated with molasses. Powder Technol 2014;254:72e7. [15] Serant F, Agapov K, Kuzmin,Yu A, Ovchinnikov V, Pugach L. Development and application of promising technologies
forfiring coal-water fuels. NovosibirskRussian Federation; 2008. p. 5. [16] Ramachandran Prakash, Tsai Ching-Yi, Schancha Gary W. An evaluation of coal water slurry fuel burners and technology. Construction Engineering Research Laboratories of US Army Corp of Engineers; 1992. p. 26.