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Kinetic characteristics of in-situ char-steam gasification following pyrolysis of a demineralized coal
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 3 ( 2 0 1 8 ) 1 0 9 9 1 e1 1 0 0 1
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Kinetic characteristics of in-situ char-steam gasification following pyrolysis of a demineralized coal Yijun Zhao a, Wenda Zhang a, Pengxiang Wang a,*, Peng Liu a, Guang Zeng a, Shaozeng Sun a, Shu Zhang b,** a b
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China
article info
abstract
Article history:
This study aims to examine the char-steam reactions in-situ, following the pyrolysis
Received 1 December 2017
process of a demineralized coal in a micro fluidized bed reactor, with particular focuses on
Received in revised form
gas release and its kinetics characteristics. The main experimental variables were tem-
28 March 2018
peratures (925 C1075 C) and steam concentrations (15%e35% H2O), and the combination
Accepted 30 April 2018
of pyrolysis and subsequent gasification in one experiment was achieved switching the
Available online 25 May 2018
atmosphere from pure argon to steam and argon mixture. The results indicate that when temperature was higher than 975 C, the absolute carbon conversion rate during the char
Keywords:
gasification could easily reach 100%. When temperature was 1025 C and 1075 C, the
Demineralized coal
carbon conversion rate changed little with steam concentration increasing from 25% to
Char
35%. The activation energy calculated from shrinking core model and random pore model
Steam
was all between 186 and 194 kJ/mol, and the fitting accuracy of shrinking core model was
Gasification
higher than that of the random pore model in this study. The char reactivity from dem-
Kinetics
ineralized coal pyrolysis gradually worsened with decreasing temperature and steam partial pressure. The range of reaction order of steam gasification was 0.49e0.61. Compared to raw coal, the progress of water gas shift reaction (CO þ H2O 4 CO2 þ H2) was hindered during the steam gasification of char obtained from the demineralized coal pyrolysis. Meanwhile, the gas content from the char gasification after the demineralized coal pyrolysis showed a low sensitivity to the change in temperature. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction As the coal fired power plant is the biggest CO2 emission source, it is urgent to develop a new coal-based power
generation technology for improving the efficiency of energy conversion and thus reducing the CO2 emission. In recent years, many researchers have proposed a new generation of oxygen enriched combustion technology with the steam as diluent. Seepana S et al. [1] proposed that the hydrocarbon
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Zhao), [email protected] (W. Zhang), [email protected] (P. Wang), [email protected] (P. Liu), [email protected] (G. Zeng), [email protected] (S. Sun), [email protected] (S. Zhang). https://doi.org/10.1016/j.ijhydene.2018.04.240 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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fuel could burn in an O2/H2O atmosphere boiler to enhance heat transfer with the working medium of water indirectly flowing through the water wall. Andserson et al. [2] with CES company proposed that water or steam could be injected into the combustion zone of gas-oxygen combustion at high temperature and high pressure to obtain high temperature steam directly. Sun et al. [3] reported that the demineralized coal could burn in an O2/H2O atmosphere to produce high temperature and high pressure gas (CO2 and H2O) that could then drive turbine to generate electricity. The direct combustion using demineralized coal under high concentrations of steam is a breakthrough compared to the traditional indirect heating transfer to the water inside the wall of boiler. The direct conversion from the chemical energy in the fuel to the working medium (steam) was definitely more efficient and desirable. There is a great difference in the reaction rate between gasification and traditional combustion. Hecht et al. [4] indicated that kH2O/kO2 ¼ 1.1 1034.5 107 at 800 C, which means that the gasification rate was extremely slow and even negligible compared to the combustion rate. Also the consumption rate of volatile in high concentration H2O environment will go down with igniting delay [5]. However, in the presence of oxygen, the gasification reaction induced by high concentration of H2O can promote carbon consumption to a certain degree. The results of experiment and theory calculation by Hecht et al. [4] showed that the endothermic gasification would reduce the surface temperature of char particle but increase the overall consumption rate of carbon probably due to the co-effects between char-steam and char-oxygen reactions. For demineralized coal Oxy-Steam Combustion System, the gasification of demineralized coal in steam is the rate-limiting reactions and is the basis of knowing the reactivity of demineralized coal under the O2/H2O. The reaction characteristics of demineralized coal are different from raw coal. Uzun's [6] results show that the reactivity of demineralized coal char is slightly reduced because of removing ash content. Kong [7] has reported that the steam catalytic gasification rate of demineralized coal is lower than that of raw coal. In addition, the steam concentrations were always changing in different reaction/combustion zone. Therefore it is practically necessary to study the kinetics of gasification between char and steam at different steam partial pressures and different temperatures. The traditional research method of char-gas reaction kinetics is to prepare the char in advance of the ex-situ gasification or combustion of char in a separate experiment. The research of Sun et al. [8] indicated that the reactivity of pyrolysis char experiencing the cooling and reheating process after the ex-situ steam gasification was lower than that in situ steam gasification. Fang et al. [9] also suggested that the reactivity of char was apparently inhibited by the cooling and reheating up process. In a practical gasifier or boiler, it is always the hot char that reacts in-situ with steam or oxygen instantly after being formed from the decomposition of coal particles. Based on the above reasons, Sun et al. [8] developed a micro fluidized bed experimental system with a fast and stable atmosphere switching device, to achieve the coupling of pyrolysis and subsequent in-situ gasification or combustion in two successive stages within a single experiment. In other
words, the hot char derived from the pyrolysis stage was directly gasified in situ by immediately supplying steam into the reactor. The reaction of demineralized coal under O2/H2O atmosphere is the key step of energy conversion inside the demineralized coal oxy steam combustion system. The fundamental understanding on kinetic characteristics of demineralized coal reaction under O2/H2O atmosphere is thus very significant for the improvement of fuel conversion efficiency and the optimization of combustion systems. As a first step, this paper focuses on studying the kinetic characteristics of in-situ steam gasification of char from the demineralized coal in the micro fluidized bed experimental system, to obtain insights on the effects of reaction temperatures and steam partial pressures on the in-situ char-steam reactions.
Experimental Experimental material The demineralized coal of Zhundong was prepared by threestage acid washing. Zhundong raw coal with particle size below 125 mm was put into 6 mol/L HCl solution. After continuous stir in water bath at 60 C, the coal sample was filtered out and washed by ultrapure water. Then the filtered coal sample was put into HF solution with concentration of 40%. Following stir in the water bath, the coal sample was also filtered out and washed. Finally, the coal sample was moved into 6 mol/L HCl solution, which was filtered out and washed after stirring in the water bath. The coal sample was continuously washed by the pure water until no precipitate could be observed when dripping the AgNO3 solution into the collected acid solution (filtrate). The leached coal powder was dried before being stored for experiments. The proximate and ultimate analytical data of Zhundong raw coal (ZD-RAW) and its demineralized coal (ZD-AW) are presented in Table 1, it can be seen that the ash content of demineralized coal was reduced to as low as 0.2%, and the nearly ash free “pure coal” was hence acquired.
Experimental system The main components of experimental system are one micro fluidized bed reactor, one radiation heating furnace, one process mass spectrometer, one steam generator, and one fast gas changeover device at high temperature atmosphere. The schematic diagram of experimental system is shown in Fig. 1, and details are given elsewhere [8,10]. During the experiment, the heating furnace was first heated to 1075 C. After the target temperature was stabilized, 20 mg pulverized coal particles in the feed tube were injected into the hot fluidized bed by pulsed argon. The coal particles in the fluidized zone were heated rapidly (>1034 K/s) owing to its excellent heat and mass transfer characteristics, and thus the hot in-situ char could be produced by the rapid pyrolysis in seconds. Ten seconds later, the gasification temperature in the furnace was adjusted to the setting temperatures (1075 C、1025 C、975 C、925 C). After the temperature became stable, the four way ball valve was rotated to switch
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Table 1 e Proximate and ultimate analytical data of Zhundong raw coal and its demineralized coal. Material name
Proximate analysis (%)
Ultimate analysis (%)
Mad
Vad
Aad
FCad
Cdaf
Hdaf
Ndaf
Sdaf
Odaf
7.02 1.81
27.59 29.84
4.05 0.20
61.34 68.15
80.72 80.31
4.66 4.04
2.04 1.95
0.20 0.18
12.38 13.51
ZD-RAW ZD-AW
ad-air dried basis; daf-dry ash free basis.
the atmosphere from the inert argon to the prescribed steam concentration (15% H2O, 25% H2O, or 35% H2O) to commence the gasification reaction. The combination of pyrolysis and subsequent gasification in one experiment was achieved by the atmosphere changeover in between, thus avoiding the negative impact on the char reactivity from the cooling and reheating process in a traditional analysis method. Owing to the moderate reactivity of demineralized coal, the carbon conversion in the studied range of gasification temperature and steam concentration was not 100% completed sometimes. The absolute carbon conversion rate was required in this paper to calculate the kinetic parameters. At the end of the gasification reaction, the steam atmosphere was switched to 21%O2/79%Ar, and the remaining carbon was thus burned out to obtain the absolute carbon conversion rate in the gasification process, i.e., the actual conversion rate.
the H2O concentration in the fluidizing air, %; CH2(t þ td), CCH4(t þ td), CCO(t þ td) and CCO2(t þ td) are the concentration of H2, CH4, CO and CO2 at the sampling points of mass spectrometer at the time t þ td, respectively, %;td is the flow time from the reactor outlet to the mass spectrometer sampling point, related to the fluidizing air flow of QF, s. The fluidization gas flow selected by gasification experiment was 1.5 NL/min, combined with the length of gas sampling tube, the td is calculated to be 6.5s. Carbon conversion rate X and gasification reactivity are calculated as follows:
mi ¼
Qm ðtÞ ¼
ð1 lÞ QF 1 CH2 ðt þ td Þ þ CCH4 ðt þ td Þ þ CCO ðt þ td Þ þ CCO2 ðt þ td Þ (1)
where QF is the fluidizing air flow, NL/min; Qm(t) is the total gas flow (dry gas) after gasification reaction at time t, NL/min; l is
Fig. 1 e Schematic diagram of micro fluidized bed experimental system [6,8].
Zi
Qm ðtÞ CCH4 ðtÞ þ CCO ðtÞ þ CCO2 ðtÞ dt
(2)
0
Xi ¼
mi m0
(3)
Ri ¼
dXi dt
(4)
Data processing method According to the Ar flow conservation calculation, the total gas production (dry gas) after gasification is calculated as follows
MC vm
where mi is the accumulated mass of the carbon released by the gasification reaction until the moment i, g; MC is the molar mass, 12.017 g/mol; vm is the molar volume of gas under the standard conditions, 22.414 L/mol; CCH4, CCO and CCO2 are the volume concentrations of CH4, CO and CO2 in the tail gas at time i, respectively, %; m0 is the total mass of the carbon in the pulverized coal, which is calculated by the amount of gas released, g. If the carbon element is not completely consumed during the gasification process, the reaction atmosphere will be switched to air for burning out the remaining carbon. And the total carbon in the solid sample was thus calculated according to the gases containing carbon in the whole process; Xi is the absolute conversion rate of carbon at moment i, %; Ri is the gasification reactivity of gas at the moment i, s1. The general dynamic equation of Arrhenius equation is used to describe the reaction process, the model is expressed as follows [11]: GðXÞ ¼ KðTÞ t
(5)
Ea KðTÞ ¼ Aexp RT
(6)
where G(X) is the model function; T is the temperature; K(T) is rate constant, s1; A is the pre-exponential factor, s1; Ea is activation energy; R is the ideal gas constant, 8.314 J mol1 K1. Shrinking core model (SCM) [12,13] and random pore model (RPM) [14,15] are generally used to describe the char steam gasification process. The shrinking model assumes that the rate of gas diffusion is far less than the rate of chemical reaction. The initial stage of the reaction is carried out on the
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outer surface of the particle. And an unreacted core has been formed inside. As the reaction continues, the unreacted core is shrinking. The random pore model considers that solid particles are made up of many different sizes and diameters pores. The diameter of these pores is distributed according to a certain regularity, and the distribution function of the random pores can be introduced to describe them. The integral forms of the two models can be expressed as follows: 1
=
1 ð1 XÞ 3 ¼ KSCM t
(7)
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi i 2h 1 jlnð1 XÞ 1 ¼ KRPM t j
(8)
Here KSCM和KRPM are the rate constant of shrinking core model and random pore model, respectively, s1; j is the pore structure parameter of random pore model, which can be calculated as follows: j¼
2 2lnð1 Xmax Þ þ 1
(9)
here Xmax is the absolute conversion rate corresponding to the fast conversion rate of Rmax.
Results and discussion Characteristics of char by in situ steam gasification from demineralized coal pyrolysis Carbon conversion rate Carbon conversion rate trend as time changes is shown in Fig. 2. When gasification temperatures were 1075 C and 1025 C, the carbon in the char could be consumed completely. The carbon conversion rate was higher than 90% when the temperature reduced to 975 C with about 1000s holding time. The carbon consumption during gasification only accounted for 75% of total carbon in coal when the temperature decreased to 925 C. It can be seen that the carbon conversion rate increased very obviously upon to the temperature increased from 925 to 975 C. It took 100s for the carbon
Fig. 2 e Carbon conversion rate of char steam gasification as a function of time (ZD-AW).
conversion rate to reach 50% under the condition of 1075 C and 25% H2O, while 600 s was required to reach 50% carbon conversion under the condition of 925 C and 25% H2O. As the char-steam gasification was an endothermic process, the increase of gasification temperature was a favorable condition for enhancing the gasification reaction. The effect of steam concentration on the carbon conversion rate was affected by the gasification temperature. At 925 C, the carbon conversion rate apparently increased with increasing steam concentration increased. The increase in steam concentration gradually became less effective for increasing the carbon conversion as the gasification temperature increased. It can be seen from the Fig. 2 that when the steam concentration is in the range of 15%e25%, the rate of char-steam gasification increased obviously with increasing steam concentration. When the steam concentration was higher than 25%, the further increase of steam concentration has no obvious effect on promoting charsteam gasification. The increase in steam concentration from 25% to 35% could see very little difference in carbon conversion at 1075 C, implying that the char-steam reactions was mainly chemical reaction control at 1075 C, compared to the diffusion-controlled reaction mechanism at low temperatures.
Parameter calculation of gasification reaction kinetics The in situ steam gasification process of char following the demineralized coal pyrolysis was simulated by the shrinking core model (SCM) and random pore model (RPM). The detailed fitting results are shown in Figs. 3 and 4 respectively. The correlation coefficient R2 is always higher than 0.99, which is satisfactory. In order to eliminate the effect of reaction atmosphere fluctuation on the experimental results at the initial stage and final stage of the gasification reaction, the carbon conversion rate between 20% and 70% was used to calculate the kinetic parameters. The KSCM and KRPM were calculated according to the definitions of shrinking core model and random pore model as introduced in Experimental section. As shown in Figs. 3 and 4, with increasing steam concentration, the slope of the straight line increases, i.e., the gasification rate increases. With known values of KSCM and KRPM, the pre-exponential factor and activation energy could be obtained based on Arrhenius law in expression (6), and all the correlation coefficient R2 of fitting results were also greater than 0.99 (as shown in Fig. 5). The results of preexponential factor A and activation energy E are listed in Table 2, it can be found that the difference in the activation energies from the two models was minor, both varying between 186 and 194 kJ/mol, while the steam concentration had nearly zero effect on the activation energy. However, the calculation results of shrinking core model show that the preexponential factor A increased with increasing steam concentration. The pre exponential factor was closely related to the quantity of active sites [16]. After the increase of steam concentration, the probability of the effective adsorption of steam molecules on the surface active sites of char increased, and thus more active carbon structures were involved in the chemical reactions, leading to the increase of the preexponential factor A. Clearly, there was a certain deviation between the carbon conversion rates curves calculated by the two models (SCM
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Fig. 3 e The experimental data of char steam gasification fitted by shrinking core model (ZD-AW).
Fig. 4 e The experimental data of char steam gasification fitted by random pore model (ZD-AW).
and RPM) and the experimental data. The reliability of the results was affected by the deviation. Therefore, the two models were further validated as follows: According to the integral forms of the two models, the relation of the carbon conversion rate with time can be deduced:
X ¼ 1 ð1 Κ SCМ tÞ3
(10)
X ¼ 1 exp½ KRPM tð1 þ KRPM tJ=4Þ
(11)
here the gasification reaction rate constants KSCM and KRPM and the pore structure parameter j were already obtained,
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Table 3 e The Fitting deviations of SCM and RPM. H2O
Fig. 5 e Kinetics fitting results of char-steam gasification. thus the carbon conversion rate Xcal,i under the corresponding time can be calculated accordingly. The quality of the fitting was assessed by comparing the deviations between calculated value Xcal,i and experimental values Xexp,i. The deviation between experimental curve and calculated curve is calculated by the expression as follows: "
N X 2 DEV Xð%Þ ¼ 100 Xexp;i Xcal;i
, #1=2 , N max Xexp
(12)
i¼1
here N is the number of the experimental data, max Xexp is the maximum carbon conversion rate among the experimental data. The fitting deviations of SCM and RPM models under different cases are listed in the Table 3. It can be seen that fitting deviations of the two models increased as the gasification temperature decreased. The fitting deviation of SCM from 1.40% under 1075 Cand 15% H2O increased to 5.61% under 925 Cand 15% H2O, and the fitting deviation of RPM changed from 3.67% to 12.18% under the same conditions. Another obvious phenomenon is that the fitting deviation of shrinking core model was smaller than that of random pore model, which indicates that the shrinking core model may better reflect the char gasification in steam. The char particle was derived from demineralized coal and its inherent mineral matters (catalytic species) could be ignored. The reactions between steam and char containing little catalytic metallic species are non-catalytic reactions and thus non-selective. The non-selective gasification reactions of chars tend to consume the char from the external surface and thus shrinking the char gradually. The shrinking core model could thus describe the char-steam reactions appropriately in this
SCM
RPM
T ( C)
15%
25%
35%
15%
25%
35%
1075 1025 975 925
1.40 1.61 5.41 5.61
1.35 1.49 6.22 4.35
1.77 4.81 8.22 6.79
3.67 6.88 11.00 12.18
6.12 6.69 11.44 10.26
7.27 10.55 13.24 11.93
study although the char particles are not really in spherical shapes. So the fitting deviation of shrinking core model is smaller. The pore structure parameter of random pore model was determined based on the maximum gasification reaction rate. According to expression (3) and (4), the trend of reactivity of char-steam gasification at different carbon conversion rate are shown in Fig. 6. It shows that the reactivity under low temperature (925 C and 975 C) and low steam concentration (15%) became worsen, resulting in the burr phenomenon of the curve. The waved curve means that multiple maximum conversions would be determined, leading to the large errors for Xmax, Rmax and thus j. This might be the main reason for the lower fitting accuracy of random pore model than that of shrinking pore model. During the actual char gasification process, the steam concentration changed continuously. The value of reaction order n represented the sensitivity of gasification rate to the steam concentration. Therefore, the value of n played an important role in the accurate simulation and prediction of gasification process, and the value of n is generally considered to be between 0 and 1. When steam concentration was 15%, 25% and 35% in the experiments, the char in situ gasification kinetics of the demineralized coal were studied. The following expression was established based on the experimental data and calculation results under the three steam concentrations [17]. KðTÞ ¼ bPnH2 O
(13) 1
here K(T) is apparent rate constant of gasification, s ; b is constant; PH2O is the partial pressure of steam, atm; n is the reaction order. On both sides of the logarithmic equation, the linear slope of lnKlnP is the reaction order n, and the detailed fitting results are shown in the Fig. 7. The fitting results of n are listed in the Table 4, the value of n obtained by the two models under the same conditions had no obvious difference, which were both in the range of 0.44e0.56. It shows that the gasification temperature had no obvious effect on the reaction order. The experimental result of Fermoso et al. [17] shows that the range of the reaction order of steam gasification was 0.49e0.61, which is very close to the results from this study.
Table 2 e Kinetics parameters of char-steam gasification. Samples
ZD-AW
Concentration
15% 25% 35%
SCM
RPM
Activation energy kJ/mol
Pre- exponential factor (s1)
R2
Activation energy kJ/mol
Pre- exponential factor (s1)
R2
192.0 189.6 189.6
5.90 104 6.20 104 7.25 104
0.996 0.999 0.998
193.9 193.2 186.4
1.61 105 2.00 105 1.23 105
0.997 0.999 0.997
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Fig. 6 e The reactivity of char-steam gasification at different carbon conversion rate.
Comparison between characteristics of raw coal and demineralized coal gasified through in-situ char steam gasification Zhundong Coal is a kind of low rank lignite with high moisture content, low ash content and good reaction characteristics. The alkali and alkaline earth elements in Zhundong coal play an important role in the thermochemical conversion process. The ash content in demineralized Zhundong coal was reduced to 0.2% (as shown in Table 1), and the metal elements that can promote the gas-solid reaction were basically eliminated, therefore the reactivity of demineralized coal should be different from the raw coal. The study of carbon conversion rate of in situ char-steam reaction after the pyrolysis of demineralized coal under different temperatures and steam concentrations has been elaborated. The same method was
applied to Zhundong raw coal under four temperatures (1075, 1025, 975 and 925 C) and one steam concentration (25%).
Comparison of gas phase composition in products and carbon conversion rate The volume fraction of gasification products of demineralized coal and raw coal under 25% H2O condition are listed in the Table 5. The relative volume fraction of each component in the gasification products was calculated by the following expression: ZX Qi ðXÞ ¼
Qm ðtÞ Ci ðtÞ
(14)
0
4i ðXÞ ¼
Qi ðXÞ 100% QH2 ðXÞ þ QCO ðXÞ þ QCH4 ðXÞ þ QCO2 ðXÞ
(15)
Here Qm(t) is the total gas flow after gasification reaction at moment t, NL/min; Ci(t) is the component concentration of H2, CH4, CO and CO2 at moment t, %; Qi(X) is the production of component i at carbon conversion rate of X, NL/min; 4i(X) is the volume fraction of component i in the gasification products. The sequence of four main gas products contents during the steam gasification of char after the demineralized coal pyrolysis is shown as below: H2>CO > CO2>CH4. CO and H2
Table 4 e Reaction order n of char-steam gasification. Model
Fig. 7 e Calculation of reaction order of char steam gasification.
T ( C) 1075 1025 975 925
SCM shrinking core model
RPM random pore model
0.540 0.451 0.556 0.541
0.513 0.437 0.598 0.561
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Table 5 e Gas products of 25% H2O gasification of chars from demineralized coal/raw coal pyrolysis. Cases
925 C-25%H2O
975 C-25%H2O
1025 C-25%H2O
1075 C-25%H2O
Demineralized coal ZD-AW
Raw coal ZD-RAW
X (%)
4CO (%)
4CO2 (%)
4CH4 (%)
4H2 (%)
X (%)
4CO (%)
4CO2 (%)
4CH4 (%)
4H2 (%)
30 40 50 60 70 30 40 50 60 70 30 40 50 60 70 30 40 50 60 70
24.05 24.29 24.36 24.31 24.28 22.90 23.09 23.20 23.26 23.26 22.91 22.86 22.88 22.93 22.98 22.45 21.95 21.71 21.68 21.76
11.80 11.76 11.90 12.17 12.42 12.41 12.11 11.91 11.83 11.88 12.95 12.54 12.20 11.93 11.76 13.86 13.52 13.19 12.87 12.52
1.58 1.53 1.48 1.43 1.33 1.96 1.96 1.95 1.93 1.92 1.91 1.88 1.86 1.84 1.82 2.26 2.20 2.14 2.09 2.03
62.56 62.42 62.26 62.09 61.97 62.74 62.84 62.94 62.98 62.95 62.23 62.72 63.06 63.30 63.45 61.43 62.33 62.96 63.37 63.69
30 40 50 60 70 30 40 50 60 70 30 40 50 60 70 30 40 50 60 70
12.42 12.28 12.12 11.98 11.88 18.18 17.90 17.52 17.04 16.56 27.22 26.51 25.86 25.18 24.29 41.11 40.21 39.25 37.98 36.68
22.36 22.57 22.77 22.97 23.17 18.76 19.00 19.27 19.59 19.92 14.49 14.61 14.84 15.17 15.62 8.66 8.84 9.10 9.50 9.98
2.47 2.50 2.52 2.54 2.55 2.13 2.16 2.19 2.23 2.26 1.71 1.73 1.76 1.79 1.83 1.22 1.24 1.26 1.29 1.33
62.74 62.65 62.59 62.52 62.40 60.93 60.93 61.01 61.14 61.27 56.57 57.15 57.55 57.87 58.25 49.01 49.71 50.39 51.23 52.01
were the main products, and the production of H2 was the maxiumum which was in the range of 61%e64%. The production of CH4 was extreme low which was in the range of 1.3%e2.3%. For raw coal, with the change of reaction temperature, the sequence of the contents of H2 and CH4 remained unchanged. But the content of CO and CO2 changed alternately, the content of CO increased rapidly while CO2 decreased slowly. The content of CO2 was higher before 975 C, and the content of CO increased rapidly after 975 C. The main reactions of char-steam gasification occurred as follows [18]: R1: C þ H2 O4CO þ H2
DH ¼ þ135:0 kJ=mol
R2: CO þ H2 O4CO2 þH2 R3: CH4 þH2 O/CO þ 3H2
DH ¼ 41:0 kJ=mol DH ¼ þ206:0 kJ=mol
R4: Cn Hm þ2nH2 O4ð2n þ m=2ÞH2 þnCO2 The content of H2 in gas products of char gasification from the demineralized coal pyrolysis was much higher than that of CO, CH4 and CO2, which was greatly contributed by R1 and R2. H2 can also be produced from the R3, R4 and other reactions, and thus the content of H2 was higher than that of any other gas species. Through the comparison between the experimental results of two samples, it can be found that CO/ CO2 >1 in the steam gasification of char from the demineralized coal, while the CO/CO2 <1 when using raw coal pyrolysis as feed at temperatures of 925 C and 975 C. It indicates that the steam gas reaction (water-gas shift reaction) as R2 in the gasification of pyrolysis char from the demineralized coal was inhibited, and the CO failed to be converted into CO2. This is because the water-gas shift reaction requires additional catalysis as a driving force to take place [19,20]. The alkali and alkaline earth metal elements in raw coal played an important catalytic role in the actual gasification process, and promoted
the water gas reaction. The mineral content in the demineralized coal has been reduced to 0.2%. There was no catalytic metals involved in the gasification reaction, and hence the consumption rate of CO decreased. When the temperature increased, the reaction rate of endothermic gasification reaction R1 increased, meanwhile the exothermic reaction R2 tended to move in the reverse reaction direction, accelerating the transformation of CO2 to CO. From the data in Table 5, when gasifying the raw coal pyrolysis char at 925 C, CO/CO2 ¼ 0.5, and CO/H2 ¼ 0.2. With the gasification temperature increasing, the contents of H2 and CO2 in the syngas decreased continuously, and H2 and CO2 were converted to CO by the reverse reaction of R2. In the case of demineralized coal, owing to the weak water gas reaction R2, the content of each product was less sensitive to the change in temperature. Meanwhile, reaction R3 was occurring because of high concentration of H2 for demineralized coal, which resulted in the increase of CH4 slightly. The gas volume fraction of the products in different gasification stages (X ¼ 30e70%) was investigated by using the advantages of on-line detection of the process mass spectrometer. It can be seen that in the same experimental conditions, the steam gasification of the demineralized coal char was a smooth process, the contents of four products of CO, CO2, CH4 and H2 had no obvious change at different reaction stages. The same phenomenon was observed in the steam gasification of the raw coal pyrolysis char at temperatures of 925 and 975 C. However, when the temperature was higher than 975 C, with the increasing carbon conversion X, the content of CO decreased slightly, with corresponding increase in the content of H2 and CO2. This is probably because the relative content of ash residue in char at the late stage increased significantly compared to that at the initial stage. A part of alkali metal catalyst was released out of interior carbon matrix during the gasification process, and the amount of carbon oxygen complexes on the surface of carbon layer
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Fig. 8 e Hydrogen release trend comparison during the char-steam gasification.
Fig. 9 e Carbon conversion rate trend during the charsteam gasification (25%H2O).
increased, or CO directly adsorbed on the active sites and then converted into CO2 [21]. Therefore the relative content of CO in the product decreased, while CO2 and H2 increased. Hydrogen release trend (on-line analysis) in the in-situ char gasification process between demineralized coal and raw coal are compared in Fig. 8. Under the same experimental conditions, compared with the demineralized coal, the hydrogen generation rate of raw coal was faster and the curve for raw coal was relatively smoother. It also indicates that the gasification rate of raw coal pyrolysis char was faster, and there was no obvious “burr” phenomenon in the curve of raw coal. As shown in Fig. 9, the absolute conversion rate of char gasification from the raw coal was more than 90% even at the lowest temperature of 925 C, and the carbon can be converted completely in the process of gasification at other three temperatures. But the absolute conversion rate of char
Fig. 10 e The fitting of steam gasification process of raw coal pyrolysis char.
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Conclusions The demineralized coal leached by HClHFHCl was used as the main raw material in this study. Study on in-situ gasification characteristics of char obtained from the pyrolysis under the conditions of 925e1075 C and 15%e35% H2O was conducted in a micro fluidized bed experimental system. Based on the discussion above, the main conclusions are as follows.
Fig. 11 e The fitting of steam gasification kinetics of raw coal pyrolysis char.
gasification from the demineralized coal was less than 80% at 925 C. The carbon conversion rate of steam gasification of raw coal char at 975 C was almost the same as that of demineralized coal char at 1075 C. The gap between conversion rates of raw coal pyrolysis char and demineralized coal pyrolysis char was more obvious below 1000 C. It took only 180s for the conversion rate of raw coal char to reach 50% at the temperature of 925 C, but it took around 600s for that of demineralized coal pyrolysis char. With the temperature increasing, the gap of the conversion rate decreased gradually.
Comparison of gasification kinetics parameters Similarly, the shrinking core model and random pore model were used to fit the gasification process of raw coal pyrolysis char. The fitting results are shown in Fig. 10, and the correlation coefficients R2 are greater than 0.99. The logarithm function relationship between lnK and 1/T (Fig. 11) were created at both sides of the Arrhenius equation (6), the slope of the straight line was -E/R and the intercept was lnA. The activation energy E obtained by the shrinking core model and random pore model were both 160.2 kJ/mol, which reduced by nearly 30 kJ/mol compared with the activation energy of steam gasification of the demineralized coal pyrolysis char. The activation energy is the energy level required by a molecule to be activated for initiating chemical reactions. The activation energy of steam gasification of the raw coal pyrolysis char is smaller than that of the demineralized coal pyrolysis char, which results in the faster gasification rate. The main reason for the decreasing activation energy is that the alkali and alkaline earth metal elements (AAEMs) in raw coal played a catalytic role in the gasification. During the gasification process, the active metal is inserted into carbon structure and forms MOC (M is metal element) structure [22], which is catalytic active intermediates. The active intermediates can accelerate the reaction through the interaction between the oxidation state (M2O2C) and the reduced state (M2OC).
(1) When temperature was higher than 975 C, the absolute carbon conversion rate during gasification of char obtained from the demineralized coal pyrolysis reached 100%. The steam concentration had little obvious effect on the carbon conversion rate at high reaction temperatures. When temperature was 1025 C and 1075 C, the carbon conversion rate no more increased if the steam concentration was higher than 25%. (2) The activation energy calculated from shrinking core model and random pore model was located between 186 and 194 kJ/mol. And the reactivity of char from the demineralized coal pyrolysis worsened under the low temperature (925, 975 C) and low concentration of water vapor (15%). The calculation accuracy of pore structure j for the random pore model was also negatively affected by the fluctuation of char reactivity, resulting in that the fitting accuracy of shrinking core model was better than the random pore model. The range of reaction order of steam gasification was 0.49e0.61. (3) Compared to raw coal, the progress of water gas shiftS reaction (CO þ H2O 4 CO2 þ H2) was hindered by the lack of inherent minerals during the steam gasification of demineralized coal char, thus increasing the content of CO in the gas product. Meanwhile, the sensitivity of each gas product to temperature was low during the insitu char gasification of the demineralized coal.
Acknowledgments The authors acknowledge the financial support for this research provided by the National Natural Science Foundation of China (Grant No. 51536002).
references
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Kinetic characteristics of in-situ char-steam gasification following pyrolysis of a demineralized coal Yijun Zhao a, Wenda Zhang a, Pengxiang Wang a,*, Peng Liu a, Guang Zeng a, Shaozeng Sun a, Shu Zhang b,** a b
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China
article info
abstract
Article history:
This study aims to examine the char-steam reactions in-situ, following the pyrolysis
Received 1 December 2017
process of a demineralized coal in a micro fluidized bed reactor, with particular focuses on
Received in revised form
gas release and its kinetics characteristics. The main experimental variables were tem-
28 March 2018
peratures (925 C1075 C) and steam concentrations (15%e35% H2O), and the combination
Accepted 30 April 2018
of pyrolysis and subsequent gasification in one experiment was achieved switching the
Available online 25 May 2018
atmosphere from pure argon to steam and argon mixture. The results indicate that when temperature was higher than 975 C, the absolute carbon conversion rate during the char
Keywords:
gasification could easily reach 100%. When temperature was 1025 C and 1075 C, the
Demineralized coal
carbon conversion rate changed little with steam concentration increasing from 25% to
Char
35%. The activation energy calculated from shrinking core model and random pore model
Steam
was all between 186 and 194 kJ/mol, and the fitting accuracy of shrinking core model was
Gasification
higher than that of the random pore model in this study. The char reactivity from dem-
Kinetics
ineralized coal pyrolysis gradually worsened with decreasing temperature and steam partial pressure. The range of reaction order of steam gasification was 0.49e0.61. Compared to raw coal, the progress of water gas shift reaction (CO þ H2O 4 CO2 þ H2) was hindered during the steam gasification of char obtained from the demineralized coal pyrolysis. Meanwhile, the gas content from the char gasification after the demineralized coal pyrolysis showed a low sensitivity to the change in temperature. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction As the coal fired power plant is the biggest CO2 emission source, it is urgent to develop a new coal-based power
generation technology for improving the efficiency of energy conversion and thus reducing the CO2 emission. In recent years, many researchers have proposed a new generation of oxygen enriched combustion technology with the steam as diluent. Seepana S et al. [1] proposed that the hydrocarbon
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Zhao), [email protected] (W. Zhang), [email protected] (P. Wang), [email protected] (P. Liu), [email protected] (G. Zeng), [email protected] (S. Sun), [email protected] (S. Zhang). https://doi.org/10.1016/j.ijhydene.2018.04.240 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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fuel could burn in an O2/H2O atmosphere boiler to enhance heat transfer with the working medium of water indirectly flowing through the water wall. Andserson et al. [2] with CES company proposed that water or steam could be injected into the combustion zone of gas-oxygen combustion at high temperature and high pressure to obtain high temperature steam directly. Sun et al. [3] reported that the demineralized coal could burn in an O2/H2O atmosphere to produce high temperature and high pressure gas (CO2 and H2O) that could then drive turbine to generate electricity. The direct combustion using demineralized coal under high concentrations of steam is a breakthrough compared to the traditional indirect heating transfer to the water inside the wall of boiler. The direct conversion from the chemical energy in the fuel to the working medium (steam) was definitely more efficient and desirable. There is a great difference in the reaction rate between gasification and traditional combustion. Hecht et al. [4] indicated that kH2O/kO2 ¼ 1.1 1034.5 107 at 800 C, which means that the gasification rate was extremely slow and even negligible compared to the combustion rate. Also the consumption rate of volatile in high concentration H2O environment will go down with igniting delay [5]. However, in the presence of oxygen, the gasification reaction induced by high concentration of H2O can promote carbon consumption to a certain degree. The results of experiment and theory calculation by Hecht et al. [4] showed that the endothermic gasification would reduce the surface temperature of char particle but increase the overall consumption rate of carbon probably due to the co-effects between char-steam and char-oxygen reactions. For demineralized coal Oxy-Steam Combustion System, the gasification of demineralized coal in steam is the rate-limiting reactions and is the basis of knowing the reactivity of demineralized coal under the O2/H2O. The reaction characteristics of demineralized coal are different from raw coal. Uzun's [6] results show that the reactivity of demineralized coal char is slightly reduced because of removing ash content. Kong [7] has reported that the steam catalytic gasification rate of demineralized coal is lower than that of raw coal. In addition, the steam concentrations were always changing in different reaction/combustion zone. Therefore it is practically necessary to study the kinetics of gasification between char and steam at different steam partial pressures and different temperatures. The traditional research method of char-gas reaction kinetics is to prepare the char in advance of the ex-situ gasification or combustion of char in a separate experiment. The research of Sun et al. [8] indicated that the reactivity of pyrolysis char experiencing the cooling and reheating process after the ex-situ steam gasification was lower than that in situ steam gasification. Fang et al. [9] also suggested that the reactivity of char was apparently inhibited by the cooling and reheating up process. In a practical gasifier or boiler, it is always the hot char that reacts in-situ with steam or oxygen instantly after being formed from the decomposition of coal particles. Based on the above reasons, Sun et al. [8] developed a micro fluidized bed experimental system with a fast and stable atmosphere switching device, to achieve the coupling of pyrolysis and subsequent in-situ gasification or combustion in two successive stages within a single experiment. In other
words, the hot char derived from the pyrolysis stage was directly gasified in situ by immediately supplying steam into the reactor. The reaction of demineralized coal under O2/H2O atmosphere is the key step of energy conversion inside the demineralized coal oxy steam combustion system. The fundamental understanding on kinetic characteristics of demineralized coal reaction under O2/H2O atmosphere is thus very significant for the improvement of fuel conversion efficiency and the optimization of combustion systems. As a first step, this paper focuses on studying the kinetic characteristics of in-situ steam gasification of char from the demineralized coal in the micro fluidized bed experimental system, to obtain insights on the effects of reaction temperatures and steam partial pressures on the in-situ char-steam reactions.
Experimental Experimental material The demineralized coal of Zhundong was prepared by threestage acid washing. Zhundong raw coal with particle size below 125 mm was put into 6 mol/L HCl solution. After continuous stir in water bath at 60 C, the coal sample was filtered out and washed by ultrapure water. Then the filtered coal sample was put into HF solution with concentration of 40%. Following stir in the water bath, the coal sample was also filtered out and washed. Finally, the coal sample was moved into 6 mol/L HCl solution, which was filtered out and washed after stirring in the water bath. The coal sample was continuously washed by the pure water until no precipitate could be observed when dripping the AgNO3 solution into the collected acid solution (filtrate). The leached coal powder was dried before being stored for experiments. The proximate and ultimate analytical data of Zhundong raw coal (ZD-RAW) and its demineralized coal (ZD-AW) are presented in Table 1, it can be seen that the ash content of demineralized coal was reduced to as low as 0.2%, and the nearly ash free “pure coal” was hence acquired.
Experimental system The main components of experimental system are one micro fluidized bed reactor, one radiation heating furnace, one process mass spectrometer, one steam generator, and one fast gas changeover device at high temperature atmosphere. The schematic diagram of experimental system is shown in Fig. 1, and details are given elsewhere [8,10]. During the experiment, the heating furnace was first heated to 1075 C. After the target temperature was stabilized, 20 mg pulverized coal particles in the feed tube were injected into the hot fluidized bed by pulsed argon. The coal particles in the fluidized zone were heated rapidly (>1034 K/s) owing to its excellent heat and mass transfer characteristics, and thus the hot in-situ char could be produced by the rapid pyrolysis in seconds. Ten seconds later, the gasification temperature in the furnace was adjusted to the setting temperatures (1075 C、1025 C、975 C、925 C). After the temperature became stable, the four way ball valve was rotated to switch
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Table 1 e Proximate and ultimate analytical data of Zhundong raw coal and its demineralized coal. Material name
Proximate analysis (%)
Ultimate analysis (%)
Mad
Vad
Aad
FCad
Cdaf
Hdaf
Ndaf
Sdaf
Odaf
7.02 1.81
27.59 29.84
4.05 0.20
61.34 68.15
80.72 80.31
4.66 4.04
2.04 1.95
0.20 0.18
12.38 13.51
ZD-RAW ZD-AW
ad-air dried basis; daf-dry ash free basis.
the atmosphere from the inert argon to the prescribed steam concentration (15% H2O, 25% H2O, or 35% H2O) to commence the gasification reaction. The combination of pyrolysis and subsequent gasification in one experiment was achieved by the atmosphere changeover in between, thus avoiding the negative impact on the char reactivity from the cooling and reheating process in a traditional analysis method. Owing to the moderate reactivity of demineralized coal, the carbon conversion in the studied range of gasification temperature and steam concentration was not 100% completed sometimes. The absolute carbon conversion rate was required in this paper to calculate the kinetic parameters. At the end of the gasification reaction, the steam atmosphere was switched to 21%O2/79%Ar, and the remaining carbon was thus burned out to obtain the absolute carbon conversion rate in the gasification process, i.e., the actual conversion rate.
the H2O concentration in the fluidizing air, %; CH2(t þ td), CCH4(t þ td), CCO(t þ td) and CCO2(t þ td) are the concentration of H2, CH4, CO and CO2 at the sampling points of mass spectrometer at the time t þ td, respectively, %;td is the flow time from the reactor outlet to the mass spectrometer sampling point, related to the fluidizing air flow of QF, s. The fluidization gas flow selected by gasification experiment was 1.5 NL/min, combined with the length of gas sampling tube, the td is calculated to be 6.5s. Carbon conversion rate X and gasification reactivity are calculated as follows:
mi ¼
Qm ðtÞ ¼
ð1 lÞ QF 1 CH2 ðt þ td Þ þ CCH4 ðt þ td Þ þ CCO ðt þ td Þ þ CCO2 ðt þ td Þ (1)
where QF is the fluidizing air flow, NL/min; Qm(t) is the total gas flow (dry gas) after gasification reaction at time t, NL/min; l is
Fig. 1 e Schematic diagram of micro fluidized bed experimental system [6,8].
Zi
Qm ðtÞ CCH4 ðtÞ þ CCO ðtÞ þ CCO2 ðtÞ dt
(2)
0
Xi ¼
mi m0
(3)
Ri ¼
dXi dt
(4)
Data processing method According to the Ar flow conservation calculation, the total gas production (dry gas) after gasification is calculated as follows
MC vm
where mi is the accumulated mass of the carbon released by the gasification reaction until the moment i, g; MC is the molar mass, 12.017 g/mol; vm is the molar volume of gas under the standard conditions, 22.414 L/mol; CCH4, CCO and CCO2 are the volume concentrations of CH4, CO and CO2 in the tail gas at time i, respectively, %; m0 is the total mass of the carbon in the pulverized coal, which is calculated by the amount of gas released, g. If the carbon element is not completely consumed during the gasification process, the reaction atmosphere will be switched to air for burning out the remaining carbon. And the total carbon in the solid sample was thus calculated according to the gases containing carbon in the whole process; Xi is the absolute conversion rate of carbon at moment i, %; Ri is the gasification reactivity of gas at the moment i, s1. The general dynamic equation of Arrhenius equation is used to describe the reaction process, the model is expressed as follows [11]: GðXÞ ¼ KðTÞ t
(5)
Ea KðTÞ ¼ Aexp RT
(6)
where G(X) is the model function; T is the temperature; K(T) is rate constant, s1; A is the pre-exponential factor, s1; Ea is activation energy; R is the ideal gas constant, 8.314 J mol1 K1. Shrinking core model (SCM) [12,13] and random pore model (RPM) [14,15] are generally used to describe the char steam gasification process. The shrinking model assumes that the rate of gas diffusion is far less than the rate of chemical reaction. The initial stage of the reaction is carried out on the
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outer surface of the particle. And an unreacted core has been formed inside. As the reaction continues, the unreacted core is shrinking. The random pore model considers that solid particles are made up of many different sizes and diameters pores. The diameter of these pores is distributed according to a certain regularity, and the distribution function of the random pores can be introduced to describe them. The integral forms of the two models can be expressed as follows: 1
=
1 ð1 XÞ 3 ¼ KSCM t
(7)
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi i 2h 1 jlnð1 XÞ 1 ¼ KRPM t j
(8)
Here KSCM和KRPM are the rate constant of shrinking core model and random pore model, respectively, s1; j is the pore structure parameter of random pore model, which can be calculated as follows: j¼
2 2lnð1 Xmax Þ þ 1
(9)
here Xmax is the absolute conversion rate corresponding to the fast conversion rate of Rmax.
Results and discussion Characteristics of char by in situ steam gasification from demineralized coal pyrolysis Carbon conversion rate Carbon conversion rate trend as time changes is shown in Fig. 2. When gasification temperatures were 1075 C and 1025 C, the carbon in the char could be consumed completely. The carbon conversion rate was higher than 90% when the temperature reduced to 975 C with about 1000s holding time. The carbon consumption during gasification only accounted for 75% of total carbon in coal when the temperature decreased to 925 C. It can be seen that the carbon conversion rate increased very obviously upon to the temperature increased from 925 to 975 C. It took 100s for the carbon
Fig. 2 e Carbon conversion rate of char steam gasification as a function of time (ZD-AW).
conversion rate to reach 50% under the condition of 1075 C and 25% H2O, while 600 s was required to reach 50% carbon conversion under the condition of 925 C and 25% H2O. As the char-steam gasification was an endothermic process, the increase of gasification temperature was a favorable condition for enhancing the gasification reaction. The effect of steam concentration on the carbon conversion rate was affected by the gasification temperature. At 925 C, the carbon conversion rate apparently increased with increasing steam concentration increased. The increase in steam concentration gradually became less effective for increasing the carbon conversion as the gasification temperature increased. It can be seen from the Fig. 2 that when the steam concentration is in the range of 15%e25%, the rate of char-steam gasification increased obviously with increasing steam concentration. When the steam concentration was higher than 25%, the further increase of steam concentration has no obvious effect on promoting charsteam gasification. The increase in steam concentration from 25% to 35% could see very little difference in carbon conversion at 1075 C, implying that the char-steam reactions was mainly chemical reaction control at 1075 C, compared to the diffusion-controlled reaction mechanism at low temperatures.
Parameter calculation of gasification reaction kinetics The in situ steam gasification process of char following the demineralized coal pyrolysis was simulated by the shrinking core model (SCM) and random pore model (RPM). The detailed fitting results are shown in Figs. 3 and 4 respectively. The correlation coefficient R2 is always higher than 0.99, which is satisfactory. In order to eliminate the effect of reaction atmosphere fluctuation on the experimental results at the initial stage and final stage of the gasification reaction, the carbon conversion rate between 20% and 70% was used to calculate the kinetic parameters. The KSCM and KRPM were calculated according to the definitions of shrinking core model and random pore model as introduced in Experimental section. As shown in Figs. 3 and 4, with increasing steam concentration, the slope of the straight line increases, i.e., the gasification rate increases. With known values of KSCM and KRPM, the pre-exponential factor and activation energy could be obtained based on Arrhenius law in expression (6), and all the correlation coefficient R2 of fitting results were also greater than 0.99 (as shown in Fig. 5). The results of preexponential factor A and activation energy E are listed in Table 2, it can be found that the difference in the activation energies from the two models was minor, both varying between 186 and 194 kJ/mol, while the steam concentration had nearly zero effect on the activation energy. However, the calculation results of shrinking core model show that the preexponential factor A increased with increasing steam concentration. The pre exponential factor was closely related to the quantity of active sites [16]. After the increase of steam concentration, the probability of the effective adsorption of steam molecules on the surface active sites of char increased, and thus more active carbon structures were involved in the chemical reactions, leading to the increase of the preexponential factor A. Clearly, there was a certain deviation between the carbon conversion rates curves calculated by the two models (SCM
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Fig. 3 e The experimental data of char steam gasification fitted by shrinking core model (ZD-AW).
Fig. 4 e The experimental data of char steam gasification fitted by random pore model (ZD-AW).
and RPM) and the experimental data. The reliability of the results was affected by the deviation. Therefore, the two models were further validated as follows: According to the integral forms of the two models, the relation of the carbon conversion rate with time can be deduced:
X ¼ 1 ð1 Κ SCМ tÞ3
(10)
X ¼ 1 exp½ KRPM tð1 þ KRPM tJ=4Þ
(11)
here the gasification reaction rate constants KSCM and KRPM and the pore structure parameter j were already obtained,
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Table 3 e The Fitting deviations of SCM and RPM. H2O
Fig. 5 e Kinetics fitting results of char-steam gasification. thus the carbon conversion rate Xcal,i under the corresponding time can be calculated accordingly. The quality of the fitting was assessed by comparing the deviations between calculated value Xcal,i and experimental values Xexp,i. The deviation between experimental curve and calculated curve is calculated by the expression as follows: "
N X 2 DEV Xð%Þ ¼ 100 Xexp;i Xcal;i
, #1=2 , N max Xexp
(12)
i¼1
here N is the number of the experimental data, max Xexp is the maximum carbon conversion rate among the experimental data. The fitting deviations of SCM and RPM models under different cases are listed in the Table 3. It can be seen that fitting deviations of the two models increased as the gasification temperature decreased. The fitting deviation of SCM from 1.40% under 1075 Cand 15% H2O increased to 5.61% under 925 Cand 15% H2O, and the fitting deviation of RPM changed from 3.67% to 12.18% under the same conditions. Another obvious phenomenon is that the fitting deviation of shrinking core model was smaller than that of random pore model, which indicates that the shrinking core model may better reflect the char gasification in steam. The char particle was derived from demineralized coal and its inherent mineral matters (catalytic species) could be ignored. The reactions between steam and char containing little catalytic metallic species are non-catalytic reactions and thus non-selective. The non-selective gasification reactions of chars tend to consume the char from the external surface and thus shrinking the char gradually. The shrinking core model could thus describe the char-steam reactions appropriately in this
SCM
RPM
T ( C)
15%
25%
35%
15%
25%
35%
1075 1025 975 925
1.40 1.61 5.41 5.61
1.35 1.49 6.22 4.35
1.77 4.81 8.22 6.79
3.67 6.88 11.00 12.18
6.12 6.69 11.44 10.26
7.27 10.55 13.24 11.93
study although the char particles are not really in spherical shapes. So the fitting deviation of shrinking core model is smaller. The pore structure parameter of random pore model was determined based on the maximum gasification reaction rate. According to expression (3) and (4), the trend of reactivity of char-steam gasification at different carbon conversion rate are shown in Fig. 6. It shows that the reactivity under low temperature (925 C and 975 C) and low steam concentration (15%) became worsen, resulting in the burr phenomenon of the curve. The waved curve means that multiple maximum conversions would be determined, leading to the large errors for Xmax, Rmax and thus j. This might be the main reason for the lower fitting accuracy of random pore model than that of shrinking pore model. During the actual char gasification process, the steam concentration changed continuously. The value of reaction order n represented the sensitivity of gasification rate to the steam concentration. Therefore, the value of n played an important role in the accurate simulation and prediction of gasification process, and the value of n is generally considered to be between 0 and 1. When steam concentration was 15%, 25% and 35% in the experiments, the char in situ gasification kinetics of the demineralized coal were studied. The following expression was established based on the experimental data and calculation results under the three steam concentrations [17]. KðTÞ ¼ bPnH2 O
(13) 1
here K(T) is apparent rate constant of gasification, s ; b is constant; PH2O is the partial pressure of steam, atm; n is the reaction order. On both sides of the logarithmic equation, the linear slope of lnKlnP is the reaction order n, and the detailed fitting results are shown in the Fig. 7. The fitting results of n are listed in the Table 4, the value of n obtained by the two models under the same conditions had no obvious difference, which were both in the range of 0.44e0.56. It shows that the gasification temperature had no obvious effect on the reaction order. The experimental result of Fermoso et al. [17] shows that the range of the reaction order of steam gasification was 0.49e0.61, which is very close to the results from this study.
Table 2 e Kinetics parameters of char-steam gasification. Samples
ZD-AW
Concentration
15% 25% 35%
SCM
RPM
Activation energy kJ/mol
Pre- exponential factor (s1)
R2
Activation energy kJ/mol
Pre- exponential factor (s1)
R2
192.0 189.6 189.6
5.90 104 6.20 104 7.25 104
0.996 0.999 0.998
193.9 193.2 186.4
1.61 105 2.00 105 1.23 105
0.997 0.999 0.997
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Fig. 6 e The reactivity of char-steam gasification at different carbon conversion rate.
Comparison between characteristics of raw coal and demineralized coal gasified through in-situ char steam gasification Zhundong Coal is a kind of low rank lignite with high moisture content, low ash content and good reaction characteristics. The alkali and alkaline earth elements in Zhundong coal play an important role in the thermochemical conversion process. The ash content in demineralized Zhundong coal was reduced to 0.2% (as shown in Table 1), and the metal elements that can promote the gas-solid reaction were basically eliminated, therefore the reactivity of demineralized coal should be different from the raw coal. The study of carbon conversion rate of in situ char-steam reaction after the pyrolysis of demineralized coal under different temperatures and steam concentrations has been elaborated. The same method was
applied to Zhundong raw coal under four temperatures (1075, 1025, 975 and 925 C) and one steam concentration (25%).
Comparison of gas phase composition in products and carbon conversion rate The volume fraction of gasification products of demineralized coal and raw coal under 25% H2O condition are listed in the Table 5. The relative volume fraction of each component in the gasification products was calculated by the following expression: ZX Qi ðXÞ ¼
Qm ðtÞ Ci ðtÞ
(14)
0
4i ðXÞ ¼
Qi ðXÞ 100% QH2 ðXÞ þ QCO ðXÞ þ QCH4 ðXÞ þ QCO2 ðXÞ
(15)
Here Qm(t) is the total gas flow after gasification reaction at moment t, NL/min; Ci(t) is the component concentration of H2, CH4, CO and CO2 at moment t, %; Qi(X) is the production of component i at carbon conversion rate of X, NL/min; 4i(X) is the volume fraction of component i in the gasification products. The sequence of four main gas products contents during the steam gasification of char after the demineralized coal pyrolysis is shown as below: H2>CO > CO2>CH4. CO and H2
Table 4 e Reaction order n of char-steam gasification. Model
Fig. 7 e Calculation of reaction order of char steam gasification.
T ( C) 1075 1025 975 925
SCM shrinking core model
RPM random pore model
0.540 0.451 0.556 0.541
0.513 0.437 0.598 0.561
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Table 5 e Gas products of 25% H2O gasification of chars from demineralized coal/raw coal pyrolysis. Cases
925 C-25%H2O
975 C-25%H2O
1025 C-25%H2O
1075 C-25%H2O
Demineralized coal ZD-AW
Raw coal ZD-RAW
X (%)
4CO (%)
4CO2 (%)
4CH4 (%)
4H2 (%)
X (%)
4CO (%)
4CO2 (%)
4CH4 (%)
4H2 (%)
30 40 50 60 70 30 40 50 60 70 30 40 50 60 70 30 40 50 60 70
24.05 24.29 24.36 24.31 24.28 22.90 23.09 23.20 23.26 23.26 22.91 22.86 22.88 22.93 22.98 22.45 21.95 21.71 21.68 21.76
11.80 11.76 11.90 12.17 12.42 12.41 12.11 11.91 11.83 11.88 12.95 12.54 12.20 11.93 11.76 13.86 13.52 13.19 12.87 12.52
1.58 1.53 1.48 1.43 1.33 1.96 1.96 1.95 1.93 1.92 1.91 1.88 1.86 1.84 1.82 2.26 2.20 2.14 2.09 2.03
62.56 62.42 62.26 62.09 61.97 62.74 62.84 62.94 62.98 62.95 62.23 62.72 63.06 63.30 63.45 61.43 62.33 62.96 63.37 63.69
30 40 50 60 70 30 40 50 60 70 30 40 50 60 70 30 40 50 60 70
12.42 12.28 12.12 11.98 11.88 18.18 17.90 17.52 17.04 16.56 27.22 26.51 25.86 25.18 24.29 41.11 40.21 39.25 37.98 36.68
22.36 22.57 22.77 22.97 23.17 18.76 19.00 19.27 19.59 19.92 14.49 14.61 14.84 15.17 15.62 8.66 8.84 9.10 9.50 9.98
2.47 2.50 2.52 2.54 2.55 2.13 2.16 2.19 2.23 2.26 1.71 1.73 1.76 1.79 1.83 1.22 1.24 1.26 1.29 1.33
62.74 62.65 62.59 62.52 62.40 60.93 60.93 61.01 61.14 61.27 56.57 57.15 57.55 57.87 58.25 49.01 49.71 50.39 51.23 52.01
were the main products, and the production of H2 was the maxiumum which was in the range of 61%e64%. The production of CH4 was extreme low which was in the range of 1.3%e2.3%. For raw coal, with the change of reaction temperature, the sequence of the contents of H2 and CH4 remained unchanged. But the content of CO and CO2 changed alternately, the content of CO increased rapidly while CO2 decreased slowly. The content of CO2 was higher before 975 C, and the content of CO increased rapidly after 975 C. The main reactions of char-steam gasification occurred as follows [18]: R1: C þ H2 O4CO þ H2
DH ¼ þ135:0 kJ=mol
R2: CO þ H2 O4CO2 þH2 R3: CH4 þH2 O/CO þ 3H2
DH ¼ 41:0 kJ=mol DH ¼ þ206:0 kJ=mol
R4: Cn Hm þ2nH2 O4ð2n þ m=2ÞH2 þnCO2 The content of H2 in gas products of char gasification from the demineralized coal pyrolysis was much higher than that of CO, CH4 and CO2, which was greatly contributed by R1 and R2. H2 can also be produced from the R3, R4 and other reactions, and thus the content of H2 was higher than that of any other gas species. Through the comparison between the experimental results of two samples, it can be found that CO/ CO2 >1 in the steam gasification of char from the demineralized coal, while the CO/CO2 <1 when using raw coal pyrolysis as feed at temperatures of 925 C and 975 C. It indicates that the steam gas reaction (water-gas shift reaction) as R2 in the gasification of pyrolysis char from the demineralized coal was inhibited, and the CO failed to be converted into CO2. This is because the water-gas shift reaction requires additional catalysis as a driving force to take place [19,20]. The alkali and alkaline earth metal elements in raw coal played an important catalytic role in the actual gasification process, and promoted
the water gas reaction. The mineral content in the demineralized coal has been reduced to 0.2%. There was no catalytic metals involved in the gasification reaction, and hence the consumption rate of CO decreased. When the temperature increased, the reaction rate of endothermic gasification reaction R1 increased, meanwhile the exothermic reaction R2 tended to move in the reverse reaction direction, accelerating the transformation of CO2 to CO. From the data in Table 5, when gasifying the raw coal pyrolysis char at 925 C, CO/CO2 ¼ 0.5, and CO/H2 ¼ 0.2. With the gasification temperature increasing, the contents of H2 and CO2 in the syngas decreased continuously, and H2 and CO2 were converted to CO by the reverse reaction of R2. In the case of demineralized coal, owing to the weak water gas reaction R2, the content of each product was less sensitive to the change in temperature. Meanwhile, reaction R3 was occurring because of high concentration of H2 for demineralized coal, which resulted in the increase of CH4 slightly. The gas volume fraction of the products in different gasification stages (X ¼ 30e70%) was investigated by using the advantages of on-line detection of the process mass spectrometer. It can be seen that in the same experimental conditions, the steam gasification of the demineralized coal char was a smooth process, the contents of four products of CO, CO2, CH4 and H2 had no obvious change at different reaction stages. The same phenomenon was observed in the steam gasification of the raw coal pyrolysis char at temperatures of 925 and 975 C. However, when the temperature was higher than 975 C, with the increasing carbon conversion X, the content of CO decreased slightly, with corresponding increase in the content of H2 and CO2. This is probably because the relative content of ash residue in char at the late stage increased significantly compared to that at the initial stage. A part of alkali metal catalyst was released out of interior carbon matrix during the gasification process, and the amount of carbon oxygen complexes on the surface of carbon layer
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10999
Fig. 8 e Hydrogen release trend comparison during the char-steam gasification.
Fig. 9 e Carbon conversion rate trend during the charsteam gasification (25%H2O).
increased, or CO directly adsorbed on the active sites and then converted into CO2 [21]. Therefore the relative content of CO in the product decreased, while CO2 and H2 increased. Hydrogen release trend (on-line analysis) in the in-situ char gasification process between demineralized coal and raw coal are compared in Fig. 8. Under the same experimental conditions, compared with the demineralized coal, the hydrogen generation rate of raw coal was faster and the curve for raw coal was relatively smoother. It also indicates that the gasification rate of raw coal pyrolysis char was faster, and there was no obvious “burr” phenomenon in the curve of raw coal. As shown in Fig. 9, the absolute conversion rate of char gasification from the raw coal was more than 90% even at the lowest temperature of 925 C, and the carbon can be converted completely in the process of gasification at other three temperatures. But the absolute conversion rate of char
Fig. 10 e The fitting of steam gasification process of raw coal pyrolysis char.
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Conclusions The demineralized coal leached by HClHFHCl was used as the main raw material in this study. Study on in-situ gasification characteristics of char obtained from the pyrolysis under the conditions of 925e1075 C and 15%e35% H2O was conducted in a micro fluidized bed experimental system. Based on the discussion above, the main conclusions are as follows.
Fig. 11 e The fitting of steam gasification kinetics of raw coal pyrolysis char.
gasification from the demineralized coal was less than 80% at 925 C. The carbon conversion rate of steam gasification of raw coal char at 975 C was almost the same as that of demineralized coal char at 1075 C. The gap between conversion rates of raw coal pyrolysis char and demineralized coal pyrolysis char was more obvious below 1000 C. It took only 180s for the conversion rate of raw coal char to reach 50% at the temperature of 925 C, but it took around 600s for that of demineralized coal pyrolysis char. With the temperature increasing, the gap of the conversion rate decreased gradually.
Comparison of gasification kinetics parameters Similarly, the shrinking core model and random pore model were used to fit the gasification process of raw coal pyrolysis char. The fitting results are shown in Fig. 10, and the correlation coefficients R2 are greater than 0.99. The logarithm function relationship between lnK and 1/T (Fig. 11) were created at both sides of the Arrhenius equation (6), the slope of the straight line was -E/R and the intercept was lnA. The activation energy E obtained by the shrinking core model and random pore model were both 160.2 kJ/mol, which reduced by nearly 30 kJ/mol compared with the activation energy of steam gasification of the demineralized coal pyrolysis char. The activation energy is the energy level required by a molecule to be activated for initiating chemical reactions. The activation energy of steam gasification of the raw coal pyrolysis char is smaller than that of the demineralized coal pyrolysis char, which results in the faster gasification rate. The main reason for the decreasing activation energy is that the alkali and alkaline earth metal elements (AAEMs) in raw coal played a catalytic role in the gasification. During the gasification process, the active metal is inserted into carbon structure and forms MOC (M is metal element) structure [22], which is catalytic active intermediates. The active intermediates can accelerate the reaction through the interaction between the oxidation state (M2O2C) and the reduced state (M2OC).
(1) When temperature was higher than 975 C, the absolute carbon conversion rate during gasification of char obtained from the demineralized coal pyrolysis reached 100%. The steam concentration had little obvious effect on the carbon conversion rate at high reaction temperatures. When temperature was 1025 C and 1075 C, the carbon conversion rate no more increased if the steam concentration was higher than 25%. (2) The activation energy calculated from shrinking core model and random pore model was located between 186 and 194 kJ/mol. And the reactivity of char from the demineralized coal pyrolysis worsened under the low temperature (925, 975 C) and low concentration of water vapor (15%). The calculation accuracy of pore structure j for the random pore model was also negatively affected by the fluctuation of char reactivity, resulting in that the fitting accuracy of shrinking core model was better than the random pore model. The range of reaction order of steam gasification was 0.49e0.61. (3) Compared to raw coal, the progress of water gas shiftS reaction (CO þ H2O 4 CO2 þ H2) was hindered by the lack of inherent minerals during the steam gasification of demineralized coal char, thus increasing the content of CO in the gas product. Meanwhile, the sensitivity of each gas product to temperature was low during the insitu char gasification of the demineralized coal.
Acknowledgments The authors acknowledge the financial support for this research provided by the National Natural Science Foundation of China (Grant No. 51536002).
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