- Email: [email protected]
Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals Maan Al-Zareer*, Ibrahim Dincer, Marc A. Rosen Clean Energy Research Laboratory, Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario, L1H 7K4, Canada
highlights The effect on syngas of co-gasification of petcoke and coal parameters is investigated. Effect is examined of various co-gasification parameters on the hydrogen production rate. Effect is investigated of various co-gasification parameters on the energy efficiency. Mixtures require higher steam and lower oxygen to achieve maximum energy efficiency.
article info
abstract
Article history:
This paper investigates the effects of various gasification parameters on the composition of
Received 21 July 2019
the syngas produced from the co-gasification of petcoke and coal for improved applica-
Received in revised form
tions. Two types of coal and one type of petcoke are considered to form four different
10 October 2019
mixtures for study in order to analyze the system operating conditions and assess the
Accepted 14 October 2019
system performance. The gasifier is modeled and simulated in Aspen Plus based on Gibbs
Available online xxx
free energy minimization approach of an entrained flow gasifier with oxygen being the gasification oxidant generated by a cryogenic air separation unit. The energy efficiency
Keywords:
assessment of the four feed mixture shows that the maximum energy efficiency of the
Energy efficiency
gasification is improved for higher grade coals when co-gasifying them with petcoke in
Gasification
contrast to the lower grade coal. The four mixtures require higher steam and lower oxygen
Coal
to achieve the maximum energy efficiency compared to the gasification of coal only, which
Petcoke
appears to be true for mixtures of both considered coal types.
Hydrogen
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Syngas
Introduction One of the byproducts of petroleum conversion processes is petcoke, also referred to petroleum coke. As the demands on petroleum increase, installations with deep petroleum
conversion processes are expected to rise in number, leading to increases in petcoke production [1]. Petcoke contains high carbon content and high calorific value, and is usually combusted for power generation. The combustion process of petcoke for electricity or thermal energy production is usually characterized with low efficiency
* Corresponding author. E-mail addresses: [email protected] (M. Al-Zareer), [email protected] (I. Dincer), [email protected] (M.A. Rosen). https://doi.org/10.1016/j.ijhydene.2019.10.108 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
2
international journal of hydrogen energy xxx (xxxx) xxx
and the release with pollutants especially for petcoke with high sulfur content. However, based on its properties such as the high carbon content it is an attractive option to be considered for gasification for hydrogen, syngas and power production [2,3]. However, the low reactivity of petcoke plus having low ash content resulted in limitations on petcoke gasification processes [4e6]. To overcome the limitations of the low petcoke reactivity, increasing the temperature of the gasification is an effective solution as presented in Ren et al. [7]. Ren et al. [7] found that increasing the petcoke gasification temperature from 1200 C to 1600 C increases the gasification rate up to 25 times, which is still lower than the gasification rate of coal. Achieving high petcoke gasification conversion rates requires high gasification temperatures and more residence time, which both are not proportional with the process economics. On contrary, coal is the most suitable feed for gasification plants for power and hydrogen production, and the coal gasification reactivity increases as the rank of the coal gets higher. The coal reactivity increases as the volatile matter content in coal increases, however the low heating value and the low concentration in coal and water slurry reduces the gasification reactivity [8]. However, recent studies showed that gasifying low rank coal is more energy efficient than gasifying high grade coals [9]. The promising results of the low rank coal gasification in terms of energy efficiency of the process have led researchers to investigate the gasification of waste tires due to having a similar proximate analysis [10]. Biomass, similar to petcoke, can be used as a feedstock to gasification power and hydrogen production plant. Biomass has a high carbon content, however it is still lower than coal. In fact the carbon in biomass is neutral, since the amount of carbon that the gasification or combustion of biomass will release in the air is equal to what was captured by the pant during the photosynthesis process [11]. Similar to petcoke, biomass generally has lower calorific value than coals. This has caused some researchers to refer to a biomass fed gasification plants for power production as unrealistic substitutes for coal gasification based power production systems [12]. It was suggested by Yan et al. [12] and Pinto et al. [13] to cogasify coal and biomass so that the coal consumption will compensate the negatives of the biomass such as low calorific value of biomass and utilize the advantages of biomass in power production such as the high carbon content. Yan et al. [12] co-gasified coal and biomass for the purpose of power production where the co-gasification was analyzed. Coal and biomass co-gasification utilizes coal and biomass in a more efficient and cleaner coal and biomass power production through the integrating the system with a carbon capture unit. Similar to the treatment biomass received in [12,13], petcoke received the same treatment in [14] where the concept of using it in a gasifier with coal (co-gasifications) so that the high temperature requirements and the low ash content of petcoke are solved. Ren et al. [14] proposed gasifying petcoke together with low grade coal, so that the low reactivity of petcoke is improved and the co-gasification also improves the gasification properties of low rank coal, which offers a more efficient way of utilizing the large amounts of available low rank coal in the world. Investigating the behavior of cogasifying petcoke and coal especially low rank coal is an essential step for the development of the technology. Number of researchers considered the co-gasification of coal and
petcoke, where they found that some synergetic effects could occur through the gasification of petcoke and coal [15,16]. A co-gasification of petcoke and coal at high temperature was studied by Ren et al. [14], where they focused on identifying which components in coal has a favorable effect on the cogasification at high temperature. Ren et al. [14] considered the combinations of four coal types with variable ranks and two types of petcoke. They found that the petcoke gasification reactivity improved greatly through co-gasification with coal, an effect that was also noticed at higher temperatures. In terms of the synergistic effects it was found that the composition of the minerals in the coal are more related than the rank of coal [14]. Based on the promising results of the Ren et al. [14] and other researchers [15,16] on the co-gasification of coal and petcoke, however the literature lacked the study of the effect of the gasification agent and the gasification oxidant on the composition of the syngas produced from various mixtures of coal and petcoke. The proposed study follows the recommendations of Ren et al. [14] on the selection of the petcoke and coal mixtures fractions and feed them into a an entertained flow gasifier. The proposed system is modeled on Aspen Plus, two various types of mixtures are investigated under different flow rates of oxygen (gasification oxidant) and steam, which is the gasification agent and then the syngas composition of these mixtures is compared to that of coal alone. The two mixtures consist of the same petcoke type in both and each mixture has a different coal type with different grade and quality. Furthermore, this paper investigates the effect of adding petcoke to the coal mixture compared to using coal only. The novelty of this paper is how it uniquely develops a cogasification system and investigates the effects of the coal type and petcoke type forming the input mixture, the amount of gasification oxidant, and the amount of the gasification agent fed to the gasifier. In addition, the paper considers the effects of the various gasification parameters on the energy efficiency of the gasification process. Consequently, the paper aims to a conclude, based on the present analysis, on the potentials of co-gasification of coal and petcoke for improved practical applications and for providing enhanced efficiency, cost and environmental characteristics.
System development and description The petcoke and coal mixture is fed to an entrained flow gasifier, which receives its gasification oxidant from a cryogenic air separation unit (CASU) when oxygen is the oxidant or directly compressed air from the environment compressed when air is the oxidant. The CASU controls the oxygen flow rate by adjusting the amount of air that enters its compressor, which in turn changes the amount of energy consumed by the unit. The gasification agent considered in this study is steam. Although in the work done by Ren et al. [14] they used CO2 as the gasification agent, this paper will consider similar operating conditions to that of only coal fed integrated gasification combined cycle (IGCC) systems, which also will show how these mixtures will operate under an IGCC gasifier conditions. We proposed earlier in [10] two Aspen Plus models to simulate the gasification process of coal, the first model was
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
international journal of hydrogen energy xxx (xxxx) xxx
a kinetics based model (the kinetics were derived from experimental results) and the second model was based on the minimization approach of Gibbs free energy. The comparisons between these two models in [9,10,17] showed that the simulating the gasification process by depending on the Gibbs free energy minimization approach resulted in a maximum difference with the experiment data of 6.5% less hydrogen produced. The low difference obtained by using the Gibbs free energy model with the experimental results, and the fact that the Gibbs energy model predicted lower values than the experimental results and with flexibility that the Gibbs free energy the model provides regarding the type of coal to be gasified, resulted in using the Gibbs free energy based model to simulate the co-gasification of petcoke and coal. The petcoke gasification and treatment in modeling and simulation is similar to coal, and based on that the chemical reactions of the gasification of the coal and petcoke mixture can be written as follows. The first step in the gasification can be considered as the decomposing of coal into char and volatile matter including tar (represented as C6 H6 ) and moisture as follows: Coal/ðH2 OþCOþH2 þCO2 þCH4 þC6 H6 þH2 SþN2 Þ þ Char
(1)
where volatile matter is presented by the chemical species appearing between the parentheses. The chemical balance of the pyrolysis reaction meaning the amount of each product depends on the coal and petcoke chemical composition. The gasification of Illinois #6 for example happens at a temperature around 1050 C [18], which is the temperature required in order for the pyrolysis reaction to take place. Note that this temperature is specific for Illinois #6 coal and each coal has its own gasifier temperature. Regarding the petcoke and coal mixture gasification temperature, it was taken as 1400 C based on the recommendation of Ren et al. [14], where they found at a high temperature of 1400 C the gasification reactivity of petcoke improved greatly even at a temperature higher than ash fusion temperature. XLT is a type of coal that is considered in this article that is at a lower quality than the Illinois #6 coal. In this article XLT coal, Illinois #6 coal and YS petcoke composition is presented in Table 1, which is taken from Ren et al. [14] are used to assess the performance of the co-gasification of coal and petcoke in an IGCC system for hydrogen and power production purposes. YS is the type of petcoke considered in this article to be fed with coal into the gasifier, which is selected since it is one of the petcoke types that is the closest to coal. After the pyrolysis reaction occurs the volatile matter is combusted, and the balanced chemical reactions presenting the volatile matter combustion are as follows: C6 H6 þ 7:5O2 /3H2 O þ 6CO2
(2)
H2 þ 0:5O2 /H2 O
(3)
CO þ 0:5O2 /CO2
(4)
CH4 þ 2O2 /CO2 þ 2H2 O
(5)
3
The char is decomposed simultaneously as follows: Char / C þ H2 þ O2 þ N2 þ S þ ASH
(6)
The products of the volatile combustion reactions (Eqs. (2)e(5)) and the char decomposition reaction (Eq. (6)) reacts with the available oxygen, steam and other chemical species as follows: C þ O2 /CO2
(7)
C þ 0:5O2 /CO
(8)
C þ CO2 /2CO
(9)
C þ 2H2 /CH4
(10)
S þ H2 /H2 S
(11)
C þ H2 O/CO þ H2
(12)
CH4 þ 2H2 O/CO2 þ 4H2
(13)
CO þ H2 O/CO2 þ H2
(14)
A detailed description of the gasification model based on the Gibbs free energy can be found in our previous work, where it was also presented that the Gibbs free energy based model showed a close proximate results to the kinetics based model and experimental results which are available elsewhere [9,10,17]. The main operating parameters of the considered gasifier and the CASU are presented in Table 2. However, in the proposed Gibbs free energy gasification based model there is two Ryield reactors (decomposes coal or petcoke based on its ultimate analysis as presented in Table 1), one for the coal part of the mixture and the other is for the petcoke. Note that the nitrogen does not appear in any of the above equations since the gasifier considered in the IGCC system is an oxygen blown gasifier to reduce the amount of thermal energy required to increase the temperature of the inert nitrogen in air, which means higher temperatures can be reached with having the gasification oxidant being oxygen with high purity. Note that there will be nitrogen based products in the syngas due to the small elemental presence of nitrogen in the composition of the gasifier feed; however these products make up a very small part of the synthesis gas. CASU separates the air two main components nitrogen and oxygen to produce oxygen that is then delivered to the gasifier. As shown in Fig. 1 and Fig. 2 the air entering the CASU is compressed to the distillation column operating pressure. The high pressure air is cooled by the produced oxygen and nitrogen dropping the compressed air temperature to 146 C. Then the cold air is throttled before entering the distillation column, where the oxygen is separated from the rest of the air components. Table 3 present the all the cycle pressures including distillation column. The general overall system assumptions and the specific assumptions made in the modeling, simulation and analyses of the proposed system are summarized in Table 4.
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
4
international journal of hydrogen energy xxx (xxxx) xxx
Table 1 e Chemical composition of the coal and the petcoke types used as the feed stock to the IGCC system. XLT (coal) [14]
Proximate analysis
Moisture Fixed carbon Volatile matter Ash Ultimate analysis C H N S O Ash LHVa (MJ/kg)
a
YS (petcoke) [14]
Illinois #6 (coal) [9]
Wet basis (wt.%)
Dry basis (wt.%)
Wet basis (wt.%)
Dry basis (wt.%)
Wet basis (wt.%)
Dry basis (wt.%)
9.09 42.55 38.96 9.40 62.93 3.92 1.03 1.00 12.64 9.40
0.00 46.8 42.9 10.34 69.22 4.31 1.13 1.09 13.91 10.34 28.07
9.09 82.85 7.26 0.80 80.17 2.81 0.91 1.23 4.99 0.80
0.00 91.13 7.99 0.88 88.19 3.09 1.00 1.35 5.49 0.88 29.99
9.09 52.74 24.05 14.12 67.55 5.70 0.65 1.61 1.20 14.17
0.00 58.21 26.46 15.53 73.90 6.24 0.71 1.77 1.32 15.53 33.93
Calculated using Aspen Plus.
Results and discussion The effects of the gasification parameters and the effect of the adding a petcoke to coal as mixture feed to the gasifier are investigated in this section based on the syngas composition including the hydrogen, and carbon monoxide flow rate ratio to the feed mixture flow rate on mass basis. The results are generated by the Aspen Plus model based on the Gibbs free energy minimization approach.
Syngas composition of XLT coal and XLT and YS petcoke mixture Firstly, the coal XLT is considered at various oxygen and steam flow rates. The chemical composition of the XLT coal is presented in Table 1, where the composition is presented in terms of the proximate and ultimate analyses in dry and wet basis. The XLT coal is first dried and then mixed with 10% distilled water producing the analysis on wet basis. The hydrogen content in the syngas for unit coal flow for 6 different values of steam to coal ratio (mass basis) and variation of the air to coal ratio entering the CASU from 0 to 10. The air to coal ratio was initially varied from 0 to 100 and then it was found that most of the changes occurred in the air to coal ratio range from 0 to 10. Note that it is the air to coal ratio
Table 2 e Main operating parameters of the considered gasifier and cryogenic air separation unit. Unit
Main parameters
Gasifier Unit coal feed rate is used (mass basis) Operating pressure of the gasifier is 24.3 bar [9] Gasification agent type is steam fed to the gasifier at 420 C Gasification oxidant type is oxygen fed to the gasified at 490 C Gasification temperature ¼ 1400 C [14] CASU Oxygen pressure exiting the CASU is 8.0 atm Nitrogen pressure exiting the CASU is reduced from 8.0 atm to 1.1 atm by passing it through gas turbine Condenser refrigerator COP is 1.0 [9]
and that air goes to the CASU as mentioned earlier to produce oxygen, which is the gasification oxidant. As shown in Fig. 3a for the case of only coal is the gasification fuel, as the steam to the feed mixture ratio increases the hydrogen content in the syngas increases. When the steam to coal ratio increased from 0 to 5 the maximum hydrogen in the syngas to the coal ratio increased from 0.045 to 0.145, which both were around air to feed mixture ratio of 1.5 and 2.0 respectively. It is observed that when the steam to coal ratio increases, the increase in the maximum hydrogen in the syngas become smaller. An interesting phenomenon occurs at higher air to coal ratio, around 7, where the arrangement of the hydrogen content in the syngas shifts, and at that point all stream flow rates produces the same hydrogen content in the syngas. The shift in the arrangement of the hydrogen content in the syngas because at low steam flow rates there are more carbon monoxide available that did not react with water to produce hydrogen and carbon dioxide, which means that as the oxygen flow increase the more oxygen is available to react with the carbon monoxide. Since carbon monoxide is more favorably to reacts with oxygen than hydrogen, the produce hydrogen is not consumed by combustion since more carbon monoxide is available for the oxygen to react with, which translate to higher hydrogen content in the syngas than cases with higher steam flow rates. Regarding the variation of the carbon monoxide in the syngas resulting from gasifying only coal with variation of steam and air to coal ratio is presented in Fig. 4a. The reaction of carbon monoxide flow rate in the syngas is opposite to that of the hydrogen, where low steam flow rates corresponds to higher carbon monoxide. A maximum carbon monoxide to coal ratio in the syngas when XLT coal is fed alone to the gasifier corresponds to 1.45 when no steam is fed to the gasifier and an air to coal ratio of nearly 3.0. From Fig. 4a, it is noticed that the carbon monoxide concentration decreases gradually as air to the gasifier increases, which justify the earlier explanation of the hydrogen flow rate in the syngas at higher air to coal ratio. The changes that occur to the hydrogen and carbon monoxide curves when 50% of the feed is changed from XLT coal to YS petcoke (YS petcoke proximate and ultimate are presented
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 1 e Schematic diagram of the oxygen blown gasifier, and the cryogenic air separation unit (CASU) producing the required oxygen to operate the gasifier.
in Table 1) are shown in Figs. 3b and 4b for hydrogen and carbon monoxide in the syngas respectively. As shown in Figs. a and b, replacing 50% of the XLT coal with YS petcoke resulted in slightly reducing the overall maximum hydrogen content in the syngas at the same steam to coal ratio. Another effect of replacing 50% of the XLT by YS is that the point at which maximum hydrogen content in the syngas is that now it requires more air to the CASU to feed mixture ratio, and it increased from 2.0 to 2.2 air to feed mixture. The introduction of the petcoke reduced the total amount of hydrogen in the syngas per unit mixture, since more coal is required to be burned to provide the required thermal energy in order to decompose the coal and the syngas, which is also noticed in the increase in required amount of oxygen (air to CASU to mixture ratio). More coal is combusted it reduce the amount of hydrogen since more oxygen is available for the hydrogen to react with and the amount of carbon monoxide reduced (at the same air to mixture ratio) due to the increase in the required amount of coal to be combusted. Fig. 4a and b and shows that more air is required to achieve higher carbon monoxide in the syngas. It is also noticed that the ait to the CASU to feed mixture ratio at which nearly all steam to feed mixture ratio results in the same hydrogen content increased as shown in Fig. 3a and b and, since the addition of the petcoke increased the content of carbon monoxide in the syngas need more oxygen.
Syngas composition of Illinois #6 coal and Illinois #6 and YS petcoke mixture Investigating the effect of using a higher grade coal on the syngas composition is done by varying the same gasification parameters as done earlier, namely the air to the CASU and the steam to feed mixture ratio, and the higher grade coal is Illinois #6 coal, where its composition is presented in Table 1. The higher grade coal has higher carbon content, higher ash content and lower volatile matter. The resulting syngas hydrogen and carbon monoxide content is shown in Figs. 5 and 6. Fig. 5a shows the hydrogen content in the syngas exiting the gasifier when Illinois #6 coal is the only fuel feed to the gasifier. Comparing Illinois #6 coal hydrogen content in the syngas at the same steam flow rates with XLT Fig. 3a, Illinois #6 having higher carbon content than XLT has pushed the point of maximum hydrogen content to a higher air to CASU (oxygen to the gasifier). The use of Illinois #6 coal produced slightly lower hydrogen content in the syngas compared to XLT coal at the same steam flow rate. But, the syngas produced from Illinois #6 coal had a much higher carbon monoxide content in the syngas than XLT syngas at the same steam flow rate. Investigating the effect of using a higher grade coal (Illinois #6) and YS petcoke is done by feeding the gasifier with a mixture of the Illinois #6 and YS petcoke made of 50% of each
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 2 e Aspen Plus flow sheet of the cryogenic air separation unit and the Gibbs free energy minimization approach of the gasification process.
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
7
international journal of hydrogen energy xxx (xxxx) xxx
Table 3 e Detailed description of the blocks for the gasification system as shown in Fig. 2. Block Aspen namea Plus block type A1 A2
A3
A4
A5
A6
A7
A8
A9
C1
G
P1
a
Function
Compressor Compress O2 entering the gasifier Heat Cools the compressed exchanger air by using the produced cold nitrogen and oxygen streams Valve Causes a sudden decrease in the pressure of the low temperature air Valve Decrease the pressure of the nitrogen flow exiting the distillation column Dstwu Separate air main distillation components to column produce high purity oxygen
Valve
Reduce the pressure of the oxygen exiting the distillation column Compressor Increase the pressure of the air entering the CASU Turbine Produce work from the N2 produced by the cryogenic air separation unit Heat Recover thermal exchange energy from the hot nitrogen exhausted from the turbine B12 Ryield Decomposes coal reactor based on the ultimate analysis of coal Rgibbs Carries out the reactor gasification process together with the heat requirement and the decomposition results of the Ryield reactors to produce the syngas Ryield Decomposes petcoke reactor based on the ultimate analysis of coal
Parameters
Table 4 e The general overall system assumptions and the specific assumptions for specific components and subsystem. Component of the system
Exit pressure is 24.3 bar Pressure losses are neglected
Exit pressure is 5.1 bar
Exit pressure 1.5 bar
Column pressure is 5.1 bar Uses WinnUnderwoodGilliland method to separate O2 from the air with a purity of more than 95%. Light key is nitrogen with recovery in distillate set to 0.99 Heavy key is oxygen with recovery in distillate set to 0.01 Exit pressure 1.5 bar
Exit pressure is 8.1 bar
Exit pressure is 1.1 bar
Exit pressure is 1.1 bar
Operating pressure is 24.3 bar Operating pressure is 24.3 bar
Assumptions
Steady state operation mode of the overall system Kinetic and potential energy changes are neglected Material property sources used in the simulation and analyses of the performance of the system are: 1984 NBS/NRC steam table correlations for water thermodynamics propertiesa Correlations by the internal association for properties of steam (IAPS) are used for the transport properties of water is utilizeda Coal and petcoke: Gross calorific value: ASTM standard D5865-07a Heat of combustion and standard heat of formation: Boie correlation Heat capacity: Kirov correlation (Property name in Aspen is Hcoalgen) Mass density: Equations published by [19] (Property name in Aspen is Dcoaligt) The method above requires the correct composition of coal and petcoke to be entered as follows: proximate analysis in on mass and wet basis, ultimate analysis in on mass and dry basis, and sulfuric analysis on wet and mass basis. Redlich Kwong Soave (RKS) cubic equation of state that utilizes the Boston Mathias alpha function are used for the remaining material properties Heat exchangers and Combustion chambers: combustion chambers Complete combustion Heat exchangers: Heat losses in the steam generator and the re-heater are assumed to be 10% of the total heat exchange Heat losses in all other heat exchangers are 10% of the total heat exchange (unless otherwise stated) Pressure drops are neglected Turbines Adiabatic Isentropic efficiency is indicated for each component in Table 3 General overall system
a
Both can be found in the Aspen Plus property selection method Steamnbs.
Operating pressure is 24.3 bar
Refer to Fig. 2 for the location of each component in the system (Aspen Plus flow sheet).
wet sample. The samples used in the simulation are wetted with 10% of distilled water. Similar to behavior was noticed with the hydrogen syngas content in XLT and YS petcoke feed mixture, the hydrogen content in the syngas of the Illinois #6
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
8
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 3 e Variation of the hydrogen content in the syngas exiting the gasifier with air to the CASU to feed mixture ratio and steam to feed mixture ratio for the case when the feed mixture coal is XLT.
decreased by replacing 50% with YS petcoke. However, the decrease of syngas hydrogen content for Illinois #6 coal was smaller than the drop for XLT coal compared to the case where 50% of the coal was replaced with YS petcoke as shown in Fig. 5a and b. The slight change in the maximum hydrogen content in Illinois #6 syngas when the petcoke was introduced is also reflected in the small increase in the required air so that all the steam flow rates to the gasification produce the same
hydrogen content in the syngas. The slight changes in the hydrogen syngas content of Illinois #6 compared to XLT is due to the smaller volatile matter content difference between Illinois #6 and YS to that between XLT and YS. Regarding the carbon monoxide content in the syngas of Illinois #6 it increased when the petcoke was introduced to the gasifier feed mixture, since the total carbon content in the mixture increased.
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
international journal of hydrogen energy xxx (xxxx) xxx
9
Fig. 4 e Variation of the carbon monoxide content in the syngas exiting the gasifier with the ratio of air to feed mixture and the ratio of steam to feed mixture for the case when the feed mixture coal is XLT.
Conditions of maximum syngas composition species In this section, the gasification conditions leading to the maximum hydrogen content in the syngas are summarized. In addition, the gasification conditions leading to a maximum carbon monoxide content in the syngas for all four feed mixtures to the gasifier are summarized in Table 5. Table 6 presents
those that achieves maximum hydrogen content in the syngas for the four coal and petcoke mixtures considered in this paper.
Energy efficiency of gasification process To investigate the performance of the gasification system including the gasifier and the CASU, the energy efficiency was calculated for all the cases considered earlier for finding the
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
10
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 5 e Variation of the hydrogen content in the syngas exiting the gasifier with the ratio of to feed mixture and the ratio of steam to feed mixture for the case when the feed mixture coal is Illinois #6.
composition of the syngas. The energy efficiency of the gasification process was calculated as follows:
petcoke and it presented in Table 1, f is the fraction of coal or _ is the work rate, and h is the petcoke in the feed mixture, W
_ syngas LHVsyngas m h¼ _ net;CASU Þ þ ðm _ steam ðhin ho ÞÞ _ FM LHVcoal þ f petcoke m _ FM LHVpetcoke þ ðW f coal m
_ refers to the mass flow rate, LHV is the lower heating Here, m value, which is calculated for each of the coals and for the
(15)
specific enthalpy. The subscripts FM refers to the feed mixture, in for the any stream entering the gasifier and o refer
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
international journal of hydrogen energy xxx (xxxx) xxx
11
Fig. 6 e Variation of the carbon monoxide content in the syngas exiting the gasifier with the ratio of air to feed mixture and the ratio of steam to feed mixture for the case when the feed mixture coal is Illinois #6. to the ambient conditions. Furthermore, the LHV of syngas varies with the composition of the syngas, which can be calculated as follows: LHVsyngas ¼
X n_ i LHVi
(16)
i
Here, n_ i is the gas constitute i mole flow rate and LHVi is the gas constitute lower heating value, where i is a part of the syngas (gas mixture). The LHV of the syngas is calculated for
each syngas composition, which varies with the variation of the feed mixture and the gasification operation parameters. Fig. 7 shows the energy efficiency of the gasification process for the case when XLT is the coal in the feed mixture, where Fig. 7a shows the variation of the energy efficiency of the gasification process with the air to the CASU to feed mixture ratio and steam to feed mixture ratio when the feed mixture is 100% XLT coal. Fig. 7b shows the variation of the energy efficiency when the feed mixture is 50% XLT coal and
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
12
international journal of hydrogen energy xxx (xxxx) xxx
Table 5 e The gasification parameters leading to a maximum carbon monoxide content in the syngas for the different feed mixtures considered. Feed mixture
Air to the CASU to feed mixture ratio (mass basis)
Steam to feed mixture ratio (mass basis)
100% XLT coal
2.7 2.4 3.0 3.3 3.6 3.8 3.2 2.8 3.4 3.7 3.9 4.2 3.5 4.1 4.6 4.9 5.1 5.3 3.5 3.7 4.2 4.5 4.8 5.0
0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0
50% XLT þ 50% YS
100% Illinois #6
50% Illinois #6 þ 50% YS
Syngas composition (mass basis) CO to feed CO2 to feed H2 to feed H2 O to feed mixture ratio mixture ratio mixture ratio mixture ratio 1.48 1.14 0.87 0.68 0.54 0.43 1.62 1.24 0.94 0.73 0.58 0.46 1.55 1.11 0.85 0.66 0.52 0.42 1.66 1.22 0.93 0.72 0.57 0.46
0.06 0.53 0.96 1.26 1.49 1.66 0.01 0.61 1.08 1.41 1.65 1.83 0.04 0.73 1.14 1.43 1.65 1.82 0.00 0.72 1.18 1.50 1.73 1.91
0.05 0.08 0.08 0.09 0.09 0.09 0.04 0.08 0.08 0.08 0.09 0.09 0.06 0.08 0.08 0.09 0.09 0.09 0.04 0.07 0.08 0.08 0.09 0.09
0.01 0.71 1.70 2.66 3.64 4.63 0.01 0.65 1.62 2.57 3.52 4.53 0.03 0.90 1.87 2.83 3.79 4.78 0.00 0.76 1.70 2.65 3.64 4.61
Table 6 e The gasification parameters leading to a maximum hydrogen content in the syngas for the different feed mixtures considered. Feed mixture
Air to the CASU to feed mixture ratio (mass basis)
Steam to feed mixture ratio (mass basis)
100% XLT coal
3.1 2.2 2.2 2.3 2.3 2.4 3.1 2.2 2.2 2.3 2.3 2.4 3.5 3.3 3.3 3.4 3.5 3.5 3.5 3.0 3.0 3.1 3.1 3.2
0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0
50% XLT þ 50% YS
100% Illinois #6
50% Illinois #6 þ 50% YS
a
Syngas composition (mass basis) CO to feed CO2 to feed H2 to feed H2 O to feed mixture ratio mixture ratio mixture ratio mixture ratio 1.44 1.14 0.83 0.62 0.46 0.36 1.60 1.18 0.80 0.58 0.41 0.323 1.55 1.03 0.69 0.51 0.38 0.28 1.66 1.15 0.78 0.57 0.41 0.32
0.08 0.54 1.02 1.35 1.60 1.76 0.00 0.68 1.25 1.60 1.84 1.99 0.04 0.82 1.32 1.62 1.82 1.96 0.00 0.79 1.35 1.69 1.92 2.07
0.04 0.09 0.11 0.12 0.13 0.137a 0.04 0.09 0.12 0.13 0.14 0.142a 0.06 0.10 0.12 0.13 0.14 0.142a 0.04 0.09 0.11 0.13 0.14 0.141a
0.09 0.65 1.46 2.35 3.25 4.21 0.00 0.48 1.25 2.13 3.04 4.00 0.02 0.66 1.47 2.37 3.31 4.26 0.00 0.55 1.33 2.22 3.13 4.09
Shows maximum value for each case.
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
international journal of hydrogen energy xxx (xxxx) xxx
13
Fig. 7 e Variation of the gasification energy efficiency with the ratio of air to feed mixture ratio and the ratio of steam to feed mixture ratio for the case when the feed mixture coal is XLT (a) the feed mixture is 100% XLT coal (b) the feed mixture is 50% XLT coal and 50% YS petcoke.
50% YS petcoke. Fig. 7 shows the effect of the co-gasification compared to coal gasification, as shown the maximum energy efficiency decreased when 50% of the feed mixture was replaced with petcoke. For each steam mass flow rate to feed mixture ratios of o and 1, achieving the maximum energy efficiency for that specific steam ratio requires higher oxygen mass flow rate, where the oxygen supply remained
unchanged for higher mass flow rates. Which shows that at higher steam to feed ratios the required oxygen to achieve a maximum efficiency remains the same when half of the feed mixture comes for petcoke. It can be concluded that replacing 50% of the feed mixture by petcoke when the coal is XLT the steam required to achieve maximum energy efficiency increases while the required oxygen decreases.
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
14
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 8 e Variation of the gasification energy efficiency with the ratio of air to feed mixture ratio and the ratio of steam to feed mixture for the case when the feed mixture coal is Illinois #6 (a) the feed mixture is 100% Illinois #6 coal (b) the feed mixture is 50% Illinois #6 coal and 50% YS petcoke.
Fig. 8 shows the effect of coal grade on the behavior of the energy efficiency of the gasification process, where the coal in the feed mixture is Illinois #6. The higher grade coal consumed higher oxygen at the same steam mass flow rate compared to the lower grade coal to achieve the maximum energy efficiency corresponding to each steam to feed mixture ratio. However, the maximum achievable energy efficiency in the
specified ranges of air and steam to feed mixture ratios decreased when using the higher grade coal. Replacing 50% of Illinois #6 coal in the feed mixture with petcoke increases the maximum achievable energy efficiency for the specified ranges of the varied parameters, which is the opposite to what the lower grade coal experienced. The increase in the efficiency of the higher grade coal when co-gasified with petcoke
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
international journal of hydrogen energy xxx (xxxx) xxx
is the result of the energy efficiency definition which is based on the LHV that compares the efficiency of the gasification to the efficiency of combustion and gasifying lower grade coal is more efficient than higher grade coals compared to their combustion. Similar to the XLT coal (low grade coal) the more steam is required to achieve the maximum energy efficiency with lower amount of oxygen.
Conclusions The mixture of petcoke and coal as a fuel feed to the gasifier is investigated in this paper in terms of the syngas composition mainly hydrogen and carbon monoxide, under the variation of the gasification agent and oxidant. The oxygen fed to the gasifier is separated from nitrogen and other gases in the air through the use of cryogenic air separation unit. A set of optimum conditions is considered where the maximum carbon monoxide in the feed mixture syngas and maximum hydrogen content in the syngas are included for comparative analysis and assessment. Also, the conditions are presented in terms of air to cryogenic air separation unit, the steam flow rate to the gasifier and the composition of the feed mixture. The analysis performed on this gasification system is based on Gibbs free energy minimization approach. It is concluded that the co-gasification of petcoke and coal produces syngas with a composition similar to the syngas produced by gasifying the coal only, and the gasification operating conditions are slightly different. Further research is merited to investigate the performance of the co-gasification in integrated gasification combined cycle for power and hydrogen production. The energy efficiency analysis of the four feed mixture showed that the energy maximum energy efficiency of the gasification is improved for higher grade coals when co-gasifying them with petcoke in contrast to the lower grade coal. The petcoke and coal mixtures requires more steam and lower oxygen to achieve the maximum energy efficiency compared to the gasifying coal only, which was true for both coal types. Further analysis is required to investigate the exergetic behavior of this integrated gasification combined cycle where two cases can be considered as coal gasification and coal and petcoke cogasification. Finally, it is recommended to consider a higher gasification operating temperature, around 1600 C when using petcoke gasification only, based on the present study using the proposed model.
Acknowledgement The authors acknowledge the support provided by the Natural Sciences and Engineering Research Council of Canada.
Nomenclature f h LHV _ m _ W
coal or petcoke fraction in the feed mixture Specific enthalpy (kJ/kg) Lower heating value (kJ/kg) Mass flow rate (kg/s) Work rate (kW)
15
Greek letters h Energy efficiency Subscripts FM Feed mixture net Net result CASU Cryogenic air separation unit Acronyms CASU Cryogenic air separation unit SFMR Steam to feed mixture ratio
references
[1] Wang J, Anthony EJ, Abanades JC. Clean and efficient use of petroleum coke for combustion and power generation. Fuel 2004;83:1341e8. https://doi.org/10.1016/ j.fuel.2004.01.002. [2] Murthy BN, Sawarkar AN, Deshmukh NA, Mathew T, Joshi JB. Petroleum coke gasification: a review. Can J Chem Eng 2014;92:441e68. https://doi.org/10.1002/cjce.21908. [3] Jayaraman K, Gokalp I. Gasification characteristics of petcoke and coal blended petcoke using thermogravimetry and mass spectrometry analysis. Appl Therm Eng 2015;80:10e9. https://doi.org/10.1016/j.applthermaleng.2015.01.026. [4] Wu Y, Huang S, Wu S, Gao J. Investigations on CS2-solube fractions and gasification reactivity of liquid-phase carbonization cokes. Energy Fuels 2010;24:5596e601. https:// doi.org/10.1021/ef100910t. [5] Wu Y, Wang J, Wu S, Huang S, Gao J. Potassium-catalyzed steam gasification of petroleum coke for H2 production: reactivity, selectivity and gas release. Fuel Process Technol 2011;92:523e30. https://doi.org/10.1016/j.fuproc.2010.11.007. [6] Li Y, Yang H, Hu J, Wang X, Chen H. Effect of catalysts on the reactivity and structure evolution of char in petroleum coke steam gasification. Fuel 2014;117:1174e80. https://doi.org/ 10.1016/j.fuel.2013.08.066. [7] Ren L, Yang J, Gao F, Yan J. Laboratory study on gasification reactivity of coals and petcokes in CO2/steam at high temperatures. Energy Fuels 2013;27:5054e68. https://doi.org/ 10.1021/ef400373y. [8] Boylu F, Dincer H, Atesok G. Effect of coal particle size distribution, volume fraction and rank on the rheology of coal-water slurries. Fuel Process Technol 2004;85:241e50. https://doi.org/10.1016/S0378-3820(03)00198-X. [9] Al-Zareer M, Dincer I, Rosen MA. Modeling and performance assessment of a new integrated gasification combined cycle with a water gas shift membrane reactor for hydrogen production. Comput Chem Eng 2016. https://doi.org/10.1016/ j.compchemeng.2016.11.024. [10] Hasan A, Dincer I. Assessment of an integrated gasification combined cycle using waste tires for hydrogen and fresh water production. Int J Hydrogen Energy 2019;44:19730e41. https://doi.org/10.1016/ j.ijhydene.2019.05.075. [11] Oreggioni GD, Brandani S, Luberti M, Baykan Y, Friedrich D, Ahn H. CO2 capture from syngas by an adsorption process at a biomass gasification CHP plant: its comparison with amine-based CO2 capture. Int J Greenh Gas Control 2015;35:71e81. https://doi.org/10.1016/j.ijggc.2015.01.008. [12] Yan L, Yue G, He B. Exergy analysis of a coal/biomass cohydrogasification based chemical looping power generation system. Energy 2015;93:1778e87. https://doi.org/10.1016/ j.energy.2015.10.042. RN, Carolino C, Miranda M. Hot treatment and [13] Pinto F, Andre upgrading of syngas obtained by co-gasification of coal and
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
16
international journal of hydrogen energy xxx (xxxx) xxx
wastes. Fuel Process Technol 2014;126:19e29. https://doi.org/ 10.1016/j.fuproc.2014.04.016. [14] Ren L, Wei R, Gao Y. Co-gasification reactivity of petcoke and coal at high temperature. Fuel 2017;190:245e52. https:// doi.org/10.1016/j.fuel.2016.11.020. [15] Edreis EMA, Luo G, Li A, Xu C, Yao H. Synergistic effects and kinetics thermal behaviour of petroleum coke/biomass blends during H2O co-gasification. Energy Convers Manag 2014;79:355e66. https://doi.org/10.1016/ j.enconman.2013.12.043. [16] Shen CH, Chen WH, Hsu HW, Sheu JY, Hsieh TH. Cogasification performance of coal and petroleum coke blends
in a pilot-scale pressurized entrained-flow gasifier. Int J Energy Res 2012;36:499e508. https://doi.org/10.1002/er.1821. [17] Al-Zareer M, Dincer I, Rosen MA. Development and analysis of an integrated system with direct splitting of hydrogen sulfide for hydrogen production. Int J Hydrogen Energy 2016;41:20036e62. https://doi.org/10.1016/ j.ijhydene.2016.08.217. [18] Wen CY, Chaung TZ. Entrainment coal gasification modeling. Ind Eng Chem Process Des Dev 1979;18:684e95. https://doi.org/10.1021/i260072a020. [19] Technology I of G. Coal conversion systems technical data book. Springfield, VA: NTIS; 1978.
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals Maan Al-Zareer*, Ibrahim Dincer, Marc A. Rosen Clean Energy Research Laboratory, Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario, L1H 7K4, Canada
highlights The effect on syngas of co-gasification of petcoke and coal parameters is investigated. Effect is examined of various co-gasification parameters on the hydrogen production rate. Effect is investigated of various co-gasification parameters on the energy efficiency. Mixtures require higher steam and lower oxygen to achieve maximum energy efficiency.
article info
abstract
Article history:
This paper investigates the effects of various gasification parameters on the composition of
Received 21 July 2019
the syngas produced from the co-gasification of petcoke and coal for improved applica-
Received in revised form
tions. Two types of coal and one type of petcoke are considered to form four different
10 October 2019
mixtures for study in order to analyze the system operating conditions and assess the
Accepted 14 October 2019
system performance. The gasifier is modeled and simulated in Aspen Plus based on Gibbs
Available online xxx
free energy minimization approach of an entrained flow gasifier with oxygen being the gasification oxidant generated by a cryogenic air separation unit. The energy efficiency
Keywords:
assessment of the four feed mixture shows that the maximum energy efficiency of the
Energy efficiency
gasification is improved for higher grade coals when co-gasifying them with petcoke in
Gasification
contrast to the lower grade coal. The four mixtures require higher steam and lower oxygen
Coal
to achieve the maximum energy efficiency compared to the gasification of coal only, which
Petcoke
appears to be true for mixtures of both considered coal types.
Hydrogen
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Syngas
Introduction One of the byproducts of petroleum conversion processes is petcoke, also referred to petroleum coke. As the demands on petroleum increase, installations with deep petroleum
conversion processes are expected to rise in number, leading to increases in petcoke production [1]. Petcoke contains high carbon content and high calorific value, and is usually combusted for power generation. The combustion process of petcoke for electricity or thermal energy production is usually characterized with low efficiency
* Corresponding author. E-mail addresses: [email protected] (M. Al-Zareer), [email protected] (I. Dincer), [email protected] (M.A. Rosen). https://doi.org/10.1016/j.ijhydene.2019.10.108 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
2
international journal of hydrogen energy xxx (xxxx) xxx
and the release with pollutants especially for petcoke with high sulfur content. However, based on its properties such as the high carbon content it is an attractive option to be considered for gasification for hydrogen, syngas and power production [2,3]. However, the low reactivity of petcoke plus having low ash content resulted in limitations on petcoke gasification processes [4e6]. To overcome the limitations of the low petcoke reactivity, increasing the temperature of the gasification is an effective solution as presented in Ren et al. [7]. Ren et al. [7] found that increasing the petcoke gasification temperature from 1200 C to 1600 C increases the gasification rate up to 25 times, which is still lower than the gasification rate of coal. Achieving high petcoke gasification conversion rates requires high gasification temperatures and more residence time, which both are not proportional with the process economics. On contrary, coal is the most suitable feed for gasification plants for power and hydrogen production, and the coal gasification reactivity increases as the rank of the coal gets higher. The coal reactivity increases as the volatile matter content in coal increases, however the low heating value and the low concentration in coal and water slurry reduces the gasification reactivity [8]. However, recent studies showed that gasifying low rank coal is more energy efficient than gasifying high grade coals [9]. The promising results of the low rank coal gasification in terms of energy efficiency of the process have led researchers to investigate the gasification of waste tires due to having a similar proximate analysis [10]. Biomass, similar to petcoke, can be used as a feedstock to gasification power and hydrogen production plant. Biomass has a high carbon content, however it is still lower than coal. In fact the carbon in biomass is neutral, since the amount of carbon that the gasification or combustion of biomass will release in the air is equal to what was captured by the pant during the photosynthesis process [11]. Similar to petcoke, biomass generally has lower calorific value than coals. This has caused some researchers to refer to a biomass fed gasification plants for power production as unrealistic substitutes for coal gasification based power production systems [12]. It was suggested by Yan et al. [12] and Pinto et al. [13] to cogasify coal and biomass so that the coal consumption will compensate the negatives of the biomass such as low calorific value of biomass and utilize the advantages of biomass in power production such as the high carbon content. Yan et al. [12] co-gasified coal and biomass for the purpose of power production where the co-gasification was analyzed. Coal and biomass co-gasification utilizes coal and biomass in a more efficient and cleaner coal and biomass power production through the integrating the system with a carbon capture unit. Similar to the treatment biomass received in [12,13], petcoke received the same treatment in [14] where the concept of using it in a gasifier with coal (co-gasifications) so that the high temperature requirements and the low ash content of petcoke are solved. Ren et al. [14] proposed gasifying petcoke together with low grade coal, so that the low reactivity of petcoke is improved and the co-gasification also improves the gasification properties of low rank coal, which offers a more efficient way of utilizing the large amounts of available low rank coal in the world. Investigating the behavior of cogasifying petcoke and coal especially low rank coal is an essential step for the development of the technology. Number of researchers considered the co-gasification of coal and
petcoke, where they found that some synergetic effects could occur through the gasification of petcoke and coal [15,16]. A co-gasification of petcoke and coal at high temperature was studied by Ren et al. [14], where they focused on identifying which components in coal has a favorable effect on the cogasification at high temperature. Ren et al. [14] considered the combinations of four coal types with variable ranks and two types of petcoke. They found that the petcoke gasification reactivity improved greatly through co-gasification with coal, an effect that was also noticed at higher temperatures. In terms of the synergistic effects it was found that the composition of the minerals in the coal are more related than the rank of coal [14]. Based on the promising results of the Ren et al. [14] and other researchers [15,16] on the co-gasification of coal and petcoke, however the literature lacked the study of the effect of the gasification agent and the gasification oxidant on the composition of the syngas produced from various mixtures of coal and petcoke. The proposed study follows the recommendations of Ren et al. [14] on the selection of the petcoke and coal mixtures fractions and feed them into a an entertained flow gasifier. The proposed system is modeled on Aspen Plus, two various types of mixtures are investigated under different flow rates of oxygen (gasification oxidant) and steam, which is the gasification agent and then the syngas composition of these mixtures is compared to that of coal alone. The two mixtures consist of the same petcoke type in both and each mixture has a different coal type with different grade and quality. Furthermore, this paper investigates the effect of adding petcoke to the coal mixture compared to using coal only. The novelty of this paper is how it uniquely develops a cogasification system and investigates the effects of the coal type and petcoke type forming the input mixture, the amount of gasification oxidant, and the amount of the gasification agent fed to the gasifier. In addition, the paper considers the effects of the various gasification parameters on the energy efficiency of the gasification process. Consequently, the paper aims to a conclude, based on the present analysis, on the potentials of co-gasification of coal and petcoke for improved practical applications and for providing enhanced efficiency, cost and environmental characteristics.
System development and description The petcoke and coal mixture is fed to an entrained flow gasifier, which receives its gasification oxidant from a cryogenic air separation unit (CASU) when oxygen is the oxidant or directly compressed air from the environment compressed when air is the oxidant. The CASU controls the oxygen flow rate by adjusting the amount of air that enters its compressor, which in turn changes the amount of energy consumed by the unit. The gasification agent considered in this study is steam. Although in the work done by Ren et al. [14] they used CO2 as the gasification agent, this paper will consider similar operating conditions to that of only coal fed integrated gasification combined cycle (IGCC) systems, which also will show how these mixtures will operate under an IGCC gasifier conditions. We proposed earlier in [10] two Aspen Plus models to simulate the gasification process of coal, the first model was
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
international journal of hydrogen energy xxx (xxxx) xxx
a kinetics based model (the kinetics were derived from experimental results) and the second model was based on the minimization approach of Gibbs free energy. The comparisons between these two models in [9,10,17] showed that the simulating the gasification process by depending on the Gibbs free energy minimization approach resulted in a maximum difference with the experiment data of 6.5% less hydrogen produced. The low difference obtained by using the Gibbs free energy model with the experimental results, and the fact that the Gibbs energy model predicted lower values than the experimental results and with flexibility that the Gibbs free energy the model provides regarding the type of coal to be gasified, resulted in using the Gibbs free energy based model to simulate the co-gasification of petcoke and coal. The petcoke gasification and treatment in modeling and simulation is similar to coal, and based on that the chemical reactions of the gasification of the coal and petcoke mixture can be written as follows. The first step in the gasification can be considered as the decomposing of coal into char and volatile matter including tar (represented as C6 H6 ) and moisture as follows: Coal/ðH2 OþCOþH2 þCO2 þCH4 þC6 H6 þH2 SþN2 Þ þ Char
(1)
where volatile matter is presented by the chemical species appearing between the parentheses. The chemical balance of the pyrolysis reaction meaning the amount of each product depends on the coal and petcoke chemical composition. The gasification of Illinois #6 for example happens at a temperature around 1050 C [18], which is the temperature required in order for the pyrolysis reaction to take place. Note that this temperature is specific for Illinois #6 coal and each coal has its own gasifier temperature. Regarding the petcoke and coal mixture gasification temperature, it was taken as 1400 C based on the recommendation of Ren et al. [14], where they found at a high temperature of 1400 C the gasification reactivity of petcoke improved greatly even at a temperature higher than ash fusion temperature. XLT is a type of coal that is considered in this article that is at a lower quality than the Illinois #6 coal. In this article XLT coal, Illinois #6 coal and YS petcoke composition is presented in Table 1, which is taken from Ren et al. [14] are used to assess the performance of the co-gasification of coal and petcoke in an IGCC system for hydrogen and power production purposes. YS is the type of petcoke considered in this article to be fed with coal into the gasifier, which is selected since it is one of the petcoke types that is the closest to coal. After the pyrolysis reaction occurs the volatile matter is combusted, and the balanced chemical reactions presenting the volatile matter combustion are as follows: C6 H6 þ 7:5O2 /3H2 O þ 6CO2
(2)
H2 þ 0:5O2 /H2 O
(3)
CO þ 0:5O2 /CO2
(4)
CH4 þ 2O2 /CO2 þ 2H2 O
(5)
3
The char is decomposed simultaneously as follows: Char / C þ H2 þ O2 þ N2 þ S þ ASH
(6)
The products of the volatile combustion reactions (Eqs. (2)e(5)) and the char decomposition reaction (Eq. (6)) reacts with the available oxygen, steam and other chemical species as follows: C þ O2 /CO2
(7)
C þ 0:5O2 /CO
(8)
C þ CO2 /2CO
(9)
C þ 2H2 /CH4
(10)
S þ H2 /H2 S
(11)
C þ H2 O/CO þ H2
(12)
CH4 þ 2H2 O/CO2 þ 4H2
(13)
CO þ H2 O/CO2 þ H2
(14)
A detailed description of the gasification model based on the Gibbs free energy can be found in our previous work, where it was also presented that the Gibbs free energy based model showed a close proximate results to the kinetics based model and experimental results which are available elsewhere [9,10,17]. The main operating parameters of the considered gasifier and the CASU are presented in Table 2. However, in the proposed Gibbs free energy gasification based model there is two Ryield reactors (decomposes coal or petcoke based on its ultimate analysis as presented in Table 1), one for the coal part of the mixture and the other is for the petcoke. Note that the nitrogen does not appear in any of the above equations since the gasifier considered in the IGCC system is an oxygen blown gasifier to reduce the amount of thermal energy required to increase the temperature of the inert nitrogen in air, which means higher temperatures can be reached with having the gasification oxidant being oxygen with high purity. Note that there will be nitrogen based products in the syngas due to the small elemental presence of nitrogen in the composition of the gasifier feed; however these products make up a very small part of the synthesis gas. CASU separates the air two main components nitrogen and oxygen to produce oxygen that is then delivered to the gasifier. As shown in Fig. 1 and Fig. 2 the air entering the CASU is compressed to the distillation column operating pressure. The high pressure air is cooled by the produced oxygen and nitrogen dropping the compressed air temperature to 146 C. Then the cold air is throttled before entering the distillation column, where the oxygen is separated from the rest of the air components. Table 3 present the all the cycle pressures including distillation column. The general overall system assumptions and the specific assumptions made in the modeling, simulation and analyses of the proposed system are summarized in Table 4.
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
4
international journal of hydrogen energy xxx (xxxx) xxx
Table 1 e Chemical composition of the coal and the petcoke types used as the feed stock to the IGCC system. XLT (coal) [14]
Proximate analysis
Moisture Fixed carbon Volatile matter Ash Ultimate analysis C H N S O Ash LHVa (MJ/kg)
a
YS (petcoke) [14]
Illinois #6 (coal) [9]
Wet basis (wt.%)
Dry basis (wt.%)
Wet basis (wt.%)
Dry basis (wt.%)
Wet basis (wt.%)
Dry basis (wt.%)
9.09 42.55 38.96 9.40 62.93 3.92 1.03 1.00 12.64 9.40
0.00 46.8 42.9 10.34 69.22 4.31 1.13 1.09 13.91 10.34 28.07
9.09 82.85 7.26 0.80 80.17 2.81 0.91 1.23 4.99 0.80
0.00 91.13 7.99 0.88 88.19 3.09 1.00 1.35 5.49 0.88 29.99
9.09 52.74 24.05 14.12 67.55 5.70 0.65 1.61 1.20 14.17
0.00 58.21 26.46 15.53 73.90 6.24 0.71 1.77 1.32 15.53 33.93
Calculated using Aspen Plus.
Results and discussion The effects of the gasification parameters and the effect of the adding a petcoke to coal as mixture feed to the gasifier are investigated in this section based on the syngas composition including the hydrogen, and carbon monoxide flow rate ratio to the feed mixture flow rate on mass basis. The results are generated by the Aspen Plus model based on the Gibbs free energy minimization approach.
Syngas composition of XLT coal and XLT and YS petcoke mixture Firstly, the coal XLT is considered at various oxygen and steam flow rates. The chemical composition of the XLT coal is presented in Table 1, where the composition is presented in terms of the proximate and ultimate analyses in dry and wet basis. The XLT coal is first dried and then mixed with 10% distilled water producing the analysis on wet basis. The hydrogen content in the syngas for unit coal flow for 6 different values of steam to coal ratio (mass basis) and variation of the air to coal ratio entering the CASU from 0 to 10. The air to coal ratio was initially varied from 0 to 100 and then it was found that most of the changes occurred in the air to coal ratio range from 0 to 10. Note that it is the air to coal ratio
Table 2 e Main operating parameters of the considered gasifier and cryogenic air separation unit. Unit
Main parameters
Gasifier Unit coal feed rate is used (mass basis) Operating pressure of the gasifier is 24.3 bar [9] Gasification agent type is steam fed to the gasifier at 420 C Gasification oxidant type is oxygen fed to the gasified at 490 C Gasification temperature ¼ 1400 C [14] CASU Oxygen pressure exiting the CASU is 8.0 atm Nitrogen pressure exiting the CASU is reduced from 8.0 atm to 1.1 atm by passing it through gas turbine Condenser refrigerator COP is 1.0 [9]
and that air goes to the CASU as mentioned earlier to produce oxygen, which is the gasification oxidant. As shown in Fig. 3a for the case of only coal is the gasification fuel, as the steam to the feed mixture ratio increases the hydrogen content in the syngas increases. When the steam to coal ratio increased from 0 to 5 the maximum hydrogen in the syngas to the coal ratio increased from 0.045 to 0.145, which both were around air to feed mixture ratio of 1.5 and 2.0 respectively. It is observed that when the steam to coal ratio increases, the increase in the maximum hydrogen in the syngas become smaller. An interesting phenomenon occurs at higher air to coal ratio, around 7, where the arrangement of the hydrogen content in the syngas shifts, and at that point all stream flow rates produces the same hydrogen content in the syngas. The shift in the arrangement of the hydrogen content in the syngas because at low steam flow rates there are more carbon monoxide available that did not react with water to produce hydrogen and carbon dioxide, which means that as the oxygen flow increase the more oxygen is available to react with the carbon monoxide. Since carbon monoxide is more favorably to reacts with oxygen than hydrogen, the produce hydrogen is not consumed by combustion since more carbon monoxide is available for the oxygen to react with, which translate to higher hydrogen content in the syngas than cases with higher steam flow rates. Regarding the variation of the carbon monoxide in the syngas resulting from gasifying only coal with variation of steam and air to coal ratio is presented in Fig. 4a. The reaction of carbon monoxide flow rate in the syngas is opposite to that of the hydrogen, where low steam flow rates corresponds to higher carbon monoxide. A maximum carbon monoxide to coal ratio in the syngas when XLT coal is fed alone to the gasifier corresponds to 1.45 when no steam is fed to the gasifier and an air to coal ratio of nearly 3.0. From Fig. 4a, it is noticed that the carbon monoxide concentration decreases gradually as air to the gasifier increases, which justify the earlier explanation of the hydrogen flow rate in the syngas at higher air to coal ratio. The changes that occur to the hydrogen and carbon monoxide curves when 50% of the feed is changed from XLT coal to YS petcoke (YS petcoke proximate and ultimate are presented
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 1 e Schematic diagram of the oxygen blown gasifier, and the cryogenic air separation unit (CASU) producing the required oxygen to operate the gasifier.
in Table 1) are shown in Figs. 3b and 4b for hydrogen and carbon monoxide in the syngas respectively. As shown in Figs. a and b, replacing 50% of the XLT coal with YS petcoke resulted in slightly reducing the overall maximum hydrogen content in the syngas at the same steam to coal ratio. Another effect of replacing 50% of the XLT by YS is that the point at which maximum hydrogen content in the syngas is that now it requires more air to the CASU to feed mixture ratio, and it increased from 2.0 to 2.2 air to feed mixture. The introduction of the petcoke reduced the total amount of hydrogen in the syngas per unit mixture, since more coal is required to be burned to provide the required thermal energy in order to decompose the coal and the syngas, which is also noticed in the increase in required amount of oxygen (air to CASU to mixture ratio). More coal is combusted it reduce the amount of hydrogen since more oxygen is available for the hydrogen to react with and the amount of carbon monoxide reduced (at the same air to mixture ratio) due to the increase in the required amount of coal to be combusted. Fig. 4a and b and shows that more air is required to achieve higher carbon monoxide in the syngas. It is also noticed that the ait to the CASU to feed mixture ratio at which nearly all steam to feed mixture ratio results in the same hydrogen content increased as shown in Fig. 3a and b and, since the addition of the petcoke increased the content of carbon monoxide in the syngas need more oxygen.
Syngas composition of Illinois #6 coal and Illinois #6 and YS petcoke mixture Investigating the effect of using a higher grade coal on the syngas composition is done by varying the same gasification parameters as done earlier, namely the air to the CASU and the steam to feed mixture ratio, and the higher grade coal is Illinois #6 coal, where its composition is presented in Table 1. The higher grade coal has higher carbon content, higher ash content and lower volatile matter. The resulting syngas hydrogen and carbon monoxide content is shown in Figs. 5 and 6. Fig. 5a shows the hydrogen content in the syngas exiting the gasifier when Illinois #6 coal is the only fuel feed to the gasifier. Comparing Illinois #6 coal hydrogen content in the syngas at the same steam flow rates with XLT Fig. 3a, Illinois #6 having higher carbon content than XLT has pushed the point of maximum hydrogen content to a higher air to CASU (oxygen to the gasifier). The use of Illinois #6 coal produced slightly lower hydrogen content in the syngas compared to XLT coal at the same steam flow rate. But, the syngas produced from Illinois #6 coal had a much higher carbon monoxide content in the syngas than XLT syngas at the same steam flow rate. Investigating the effect of using a higher grade coal (Illinois #6) and YS petcoke is done by feeding the gasifier with a mixture of the Illinois #6 and YS petcoke made of 50% of each
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 2 e Aspen Plus flow sheet of the cryogenic air separation unit and the Gibbs free energy minimization approach of the gasification process.
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
7
international journal of hydrogen energy xxx (xxxx) xxx
Table 3 e Detailed description of the blocks for the gasification system as shown in Fig. 2. Block Aspen namea Plus block type A1 A2
A3
A4
A5
A6
A7
A8
A9
C1
G
P1
a
Function
Compressor Compress O2 entering the gasifier Heat Cools the compressed exchanger air by using the produced cold nitrogen and oxygen streams Valve Causes a sudden decrease in the pressure of the low temperature air Valve Decrease the pressure of the nitrogen flow exiting the distillation column Dstwu Separate air main distillation components to column produce high purity oxygen
Valve
Reduce the pressure of the oxygen exiting the distillation column Compressor Increase the pressure of the air entering the CASU Turbine Produce work from the N2 produced by the cryogenic air separation unit Heat Recover thermal exchange energy from the hot nitrogen exhausted from the turbine B12 Ryield Decomposes coal reactor based on the ultimate analysis of coal Rgibbs Carries out the reactor gasification process together with the heat requirement and the decomposition results of the Ryield reactors to produce the syngas Ryield Decomposes petcoke reactor based on the ultimate analysis of coal
Parameters
Table 4 e The general overall system assumptions and the specific assumptions for specific components and subsystem. Component of the system
Exit pressure is 24.3 bar Pressure losses are neglected
Exit pressure is 5.1 bar
Exit pressure 1.5 bar
Column pressure is 5.1 bar Uses WinnUnderwoodGilliland method to separate O2 from the air with a purity of more than 95%. Light key is nitrogen with recovery in distillate set to 0.99 Heavy key is oxygen with recovery in distillate set to 0.01 Exit pressure 1.5 bar
Exit pressure is 8.1 bar
Exit pressure is 1.1 bar
Exit pressure is 1.1 bar
Operating pressure is 24.3 bar Operating pressure is 24.3 bar
Assumptions
Steady state operation mode of the overall system Kinetic and potential energy changes are neglected Material property sources used in the simulation and analyses of the performance of the system are: 1984 NBS/NRC steam table correlations for water thermodynamics propertiesa Correlations by the internal association for properties of steam (IAPS) are used for the transport properties of water is utilizeda Coal and petcoke: Gross calorific value: ASTM standard D5865-07a Heat of combustion and standard heat of formation: Boie correlation Heat capacity: Kirov correlation (Property name in Aspen is Hcoalgen) Mass density: Equations published by [19] (Property name in Aspen is Dcoaligt) The method above requires the correct composition of coal and petcoke to be entered as follows: proximate analysis in on mass and wet basis, ultimate analysis in on mass and dry basis, and sulfuric analysis on wet and mass basis. Redlich Kwong Soave (RKS) cubic equation of state that utilizes the Boston Mathias alpha function are used for the remaining material properties Heat exchangers and Combustion chambers: combustion chambers Complete combustion Heat exchangers: Heat losses in the steam generator and the re-heater are assumed to be 10% of the total heat exchange Heat losses in all other heat exchangers are 10% of the total heat exchange (unless otherwise stated) Pressure drops are neglected Turbines Adiabatic Isentropic efficiency is indicated for each component in Table 3 General overall system
a
Both can be found in the Aspen Plus property selection method Steamnbs.
Operating pressure is 24.3 bar
Refer to Fig. 2 for the location of each component in the system (Aspen Plus flow sheet).
wet sample. The samples used in the simulation are wetted with 10% of distilled water. Similar to behavior was noticed with the hydrogen syngas content in XLT and YS petcoke feed mixture, the hydrogen content in the syngas of the Illinois #6
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
8
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 3 e Variation of the hydrogen content in the syngas exiting the gasifier with air to the CASU to feed mixture ratio and steam to feed mixture ratio for the case when the feed mixture coal is XLT.
decreased by replacing 50% with YS petcoke. However, the decrease of syngas hydrogen content for Illinois #6 coal was smaller than the drop for XLT coal compared to the case where 50% of the coal was replaced with YS petcoke as shown in Fig. 5a and b. The slight change in the maximum hydrogen content in Illinois #6 syngas when the petcoke was introduced is also reflected in the small increase in the required air so that all the steam flow rates to the gasification produce the same
hydrogen content in the syngas. The slight changes in the hydrogen syngas content of Illinois #6 compared to XLT is due to the smaller volatile matter content difference between Illinois #6 and YS to that between XLT and YS. Regarding the carbon monoxide content in the syngas of Illinois #6 it increased when the petcoke was introduced to the gasifier feed mixture, since the total carbon content in the mixture increased.
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
international journal of hydrogen energy xxx (xxxx) xxx
9
Fig. 4 e Variation of the carbon monoxide content in the syngas exiting the gasifier with the ratio of air to feed mixture and the ratio of steam to feed mixture for the case when the feed mixture coal is XLT.
Conditions of maximum syngas composition species In this section, the gasification conditions leading to the maximum hydrogen content in the syngas are summarized. In addition, the gasification conditions leading to a maximum carbon monoxide content in the syngas for all four feed mixtures to the gasifier are summarized in Table 5. Table 6 presents
those that achieves maximum hydrogen content in the syngas for the four coal and petcoke mixtures considered in this paper.
Energy efficiency of gasification process To investigate the performance of the gasification system including the gasifier and the CASU, the energy efficiency was calculated for all the cases considered earlier for finding the
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
10
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 5 e Variation of the hydrogen content in the syngas exiting the gasifier with the ratio of to feed mixture and the ratio of steam to feed mixture for the case when the feed mixture coal is Illinois #6.
composition of the syngas. The energy efficiency of the gasification process was calculated as follows:
petcoke and it presented in Table 1, f is the fraction of coal or _ is the work rate, and h is the petcoke in the feed mixture, W
_ syngas LHVsyngas m h¼ _ net;CASU Þ þ ðm _ steam ðhin ho ÞÞ _ FM LHVcoal þ f petcoke m _ FM LHVpetcoke þ ðW f coal m
_ refers to the mass flow rate, LHV is the lower heating Here, m value, which is calculated for each of the coals and for the
(15)
specific enthalpy. The subscripts FM refers to the feed mixture, in for the any stream entering the gasifier and o refer
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
international journal of hydrogen energy xxx (xxxx) xxx
11
Fig. 6 e Variation of the carbon monoxide content in the syngas exiting the gasifier with the ratio of air to feed mixture and the ratio of steam to feed mixture for the case when the feed mixture coal is Illinois #6. to the ambient conditions. Furthermore, the LHV of syngas varies with the composition of the syngas, which can be calculated as follows: LHVsyngas ¼
X n_ i LHVi
(16)
i
Here, n_ i is the gas constitute i mole flow rate and LHVi is the gas constitute lower heating value, where i is a part of the syngas (gas mixture). The LHV of the syngas is calculated for
each syngas composition, which varies with the variation of the feed mixture and the gasification operation parameters. Fig. 7 shows the energy efficiency of the gasification process for the case when XLT is the coal in the feed mixture, where Fig. 7a shows the variation of the energy efficiency of the gasification process with the air to the CASU to feed mixture ratio and steam to feed mixture ratio when the feed mixture is 100% XLT coal. Fig. 7b shows the variation of the energy efficiency when the feed mixture is 50% XLT coal and
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
12
international journal of hydrogen energy xxx (xxxx) xxx
Table 5 e The gasification parameters leading to a maximum carbon monoxide content in the syngas for the different feed mixtures considered. Feed mixture
Air to the CASU to feed mixture ratio (mass basis)
Steam to feed mixture ratio (mass basis)
100% XLT coal
2.7 2.4 3.0 3.3 3.6 3.8 3.2 2.8 3.4 3.7 3.9 4.2 3.5 4.1 4.6 4.9 5.1 5.3 3.5 3.7 4.2 4.5 4.8 5.0
0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0
50% XLT þ 50% YS
100% Illinois #6
50% Illinois #6 þ 50% YS
Syngas composition (mass basis) CO to feed CO2 to feed H2 to feed H2 O to feed mixture ratio mixture ratio mixture ratio mixture ratio 1.48 1.14 0.87 0.68 0.54 0.43 1.62 1.24 0.94 0.73 0.58 0.46 1.55 1.11 0.85 0.66 0.52 0.42 1.66 1.22 0.93 0.72 0.57 0.46
0.06 0.53 0.96 1.26 1.49 1.66 0.01 0.61 1.08 1.41 1.65 1.83 0.04 0.73 1.14 1.43 1.65 1.82 0.00 0.72 1.18 1.50 1.73 1.91
0.05 0.08 0.08 0.09 0.09 0.09 0.04 0.08 0.08 0.08 0.09 0.09 0.06 0.08 0.08 0.09 0.09 0.09 0.04 0.07 0.08 0.08 0.09 0.09
0.01 0.71 1.70 2.66 3.64 4.63 0.01 0.65 1.62 2.57 3.52 4.53 0.03 0.90 1.87 2.83 3.79 4.78 0.00 0.76 1.70 2.65 3.64 4.61
Table 6 e The gasification parameters leading to a maximum hydrogen content in the syngas for the different feed mixtures considered. Feed mixture
Air to the CASU to feed mixture ratio (mass basis)
Steam to feed mixture ratio (mass basis)
100% XLT coal
3.1 2.2 2.2 2.3 2.3 2.4 3.1 2.2 2.2 2.3 2.3 2.4 3.5 3.3 3.3 3.4 3.5 3.5 3.5 3.0 3.0 3.1 3.1 3.2
0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0
50% XLT þ 50% YS
100% Illinois #6
50% Illinois #6 þ 50% YS
a
Syngas composition (mass basis) CO to feed CO2 to feed H2 to feed H2 O to feed mixture ratio mixture ratio mixture ratio mixture ratio 1.44 1.14 0.83 0.62 0.46 0.36 1.60 1.18 0.80 0.58 0.41 0.323 1.55 1.03 0.69 0.51 0.38 0.28 1.66 1.15 0.78 0.57 0.41 0.32
0.08 0.54 1.02 1.35 1.60 1.76 0.00 0.68 1.25 1.60 1.84 1.99 0.04 0.82 1.32 1.62 1.82 1.96 0.00 0.79 1.35 1.69 1.92 2.07
0.04 0.09 0.11 0.12 0.13 0.137a 0.04 0.09 0.12 0.13 0.14 0.142a 0.06 0.10 0.12 0.13 0.14 0.142a 0.04 0.09 0.11 0.13 0.14 0.141a
0.09 0.65 1.46 2.35 3.25 4.21 0.00 0.48 1.25 2.13 3.04 4.00 0.02 0.66 1.47 2.37 3.31 4.26 0.00 0.55 1.33 2.22 3.13 4.09
Shows maximum value for each case.
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
international journal of hydrogen energy xxx (xxxx) xxx
13
Fig. 7 e Variation of the gasification energy efficiency with the ratio of air to feed mixture ratio and the ratio of steam to feed mixture ratio for the case when the feed mixture coal is XLT (a) the feed mixture is 100% XLT coal (b) the feed mixture is 50% XLT coal and 50% YS petcoke.
50% YS petcoke. Fig. 7 shows the effect of the co-gasification compared to coal gasification, as shown the maximum energy efficiency decreased when 50% of the feed mixture was replaced with petcoke. For each steam mass flow rate to feed mixture ratios of o and 1, achieving the maximum energy efficiency for that specific steam ratio requires higher oxygen mass flow rate, where the oxygen supply remained
unchanged for higher mass flow rates. Which shows that at higher steam to feed ratios the required oxygen to achieve a maximum efficiency remains the same when half of the feed mixture comes for petcoke. It can be concluded that replacing 50% of the feed mixture by petcoke when the coal is XLT the steam required to achieve maximum energy efficiency increases while the required oxygen decreases.
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
14
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 8 e Variation of the gasification energy efficiency with the ratio of air to feed mixture ratio and the ratio of steam to feed mixture for the case when the feed mixture coal is Illinois #6 (a) the feed mixture is 100% Illinois #6 coal (b) the feed mixture is 50% Illinois #6 coal and 50% YS petcoke.
Fig. 8 shows the effect of coal grade on the behavior of the energy efficiency of the gasification process, where the coal in the feed mixture is Illinois #6. The higher grade coal consumed higher oxygen at the same steam mass flow rate compared to the lower grade coal to achieve the maximum energy efficiency corresponding to each steam to feed mixture ratio. However, the maximum achievable energy efficiency in the
specified ranges of air and steam to feed mixture ratios decreased when using the higher grade coal. Replacing 50% of Illinois #6 coal in the feed mixture with petcoke increases the maximum achievable energy efficiency for the specified ranges of the varied parameters, which is the opposite to what the lower grade coal experienced. The increase in the efficiency of the higher grade coal when co-gasified with petcoke
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
international journal of hydrogen energy xxx (xxxx) xxx
is the result of the energy efficiency definition which is based on the LHV that compares the efficiency of the gasification to the efficiency of combustion and gasifying lower grade coal is more efficient than higher grade coals compared to their combustion. Similar to the XLT coal (low grade coal) the more steam is required to achieve the maximum energy efficiency with lower amount of oxygen.
Conclusions The mixture of petcoke and coal as a fuel feed to the gasifier is investigated in this paper in terms of the syngas composition mainly hydrogen and carbon monoxide, under the variation of the gasification agent and oxidant. The oxygen fed to the gasifier is separated from nitrogen and other gases in the air through the use of cryogenic air separation unit. A set of optimum conditions is considered where the maximum carbon monoxide in the feed mixture syngas and maximum hydrogen content in the syngas are included for comparative analysis and assessment. Also, the conditions are presented in terms of air to cryogenic air separation unit, the steam flow rate to the gasifier and the composition of the feed mixture. The analysis performed on this gasification system is based on Gibbs free energy minimization approach. It is concluded that the co-gasification of petcoke and coal produces syngas with a composition similar to the syngas produced by gasifying the coal only, and the gasification operating conditions are slightly different. Further research is merited to investigate the performance of the co-gasification in integrated gasification combined cycle for power and hydrogen production. The energy efficiency analysis of the four feed mixture showed that the energy maximum energy efficiency of the gasification is improved for higher grade coals when co-gasifying them with petcoke in contrast to the lower grade coal. The petcoke and coal mixtures requires more steam and lower oxygen to achieve the maximum energy efficiency compared to the gasifying coal only, which was true for both coal types. Further analysis is required to investigate the exergetic behavior of this integrated gasification combined cycle where two cases can be considered as coal gasification and coal and petcoke cogasification. Finally, it is recommended to consider a higher gasification operating temperature, around 1600 C when using petcoke gasification only, based on the present study using the proposed model.
Acknowledgement The authors acknowledge the support provided by the Natural Sciences and Engineering Research Council of Canada.
Nomenclature f h LHV _ m _ W
coal or petcoke fraction in the feed mixture Specific enthalpy (kJ/kg) Lower heating value (kJ/kg) Mass flow rate (kg/s) Work rate (kW)
15
Greek letters h Energy efficiency Subscripts FM Feed mixture net Net result CASU Cryogenic air separation unit Acronyms CASU Cryogenic air separation unit SFMR Steam to feed mixture ratio
references
[1] Wang J, Anthony EJ, Abanades JC. Clean and efficient use of petroleum coke for combustion and power generation. Fuel 2004;83:1341e8. https://doi.org/10.1016/ j.fuel.2004.01.002. [2] Murthy BN, Sawarkar AN, Deshmukh NA, Mathew T, Joshi JB. Petroleum coke gasification: a review. Can J Chem Eng 2014;92:441e68. https://doi.org/10.1002/cjce.21908. [3] Jayaraman K, Gokalp I. Gasification characteristics of petcoke and coal blended petcoke using thermogravimetry and mass spectrometry analysis. Appl Therm Eng 2015;80:10e9. https://doi.org/10.1016/j.applthermaleng.2015.01.026. [4] Wu Y, Huang S, Wu S, Gao J. Investigations on CS2-solube fractions and gasification reactivity of liquid-phase carbonization cokes. Energy Fuels 2010;24:5596e601. https:// doi.org/10.1021/ef100910t. [5] Wu Y, Wang J, Wu S, Huang S, Gao J. Potassium-catalyzed steam gasification of petroleum coke for H2 production: reactivity, selectivity and gas release. Fuel Process Technol 2011;92:523e30. https://doi.org/10.1016/j.fuproc.2010.11.007. [6] Li Y, Yang H, Hu J, Wang X, Chen H. Effect of catalysts on the reactivity and structure evolution of char in petroleum coke steam gasification. Fuel 2014;117:1174e80. https://doi.org/ 10.1016/j.fuel.2013.08.066. [7] Ren L, Yang J, Gao F, Yan J. Laboratory study on gasification reactivity of coals and petcokes in CO2/steam at high temperatures. Energy Fuels 2013;27:5054e68. https://doi.org/ 10.1021/ef400373y. [8] Boylu F, Dincer H, Atesok G. Effect of coal particle size distribution, volume fraction and rank on the rheology of coal-water slurries. Fuel Process Technol 2004;85:241e50. https://doi.org/10.1016/S0378-3820(03)00198-X. [9] Al-Zareer M, Dincer I, Rosen MA. Modeling and performance assessment of a new integrated gasification combined cycle with a water gas shift membrane reactor for hydrogen production. Comput Chem Eng 2016. https://doi.org/10.1016/ j.compchemeng.2016.11.024. [10] Hasan A, Dincer I. Assessment of an integrated gasification combined cycle using waste tires for hydrogen and fresh water production. Int J Hydrogen Energy 2019;44:19730e41. https://doi.org/10.1016/ j.ijhydene.2019.05.075. [11] Oreggioni GD, Brandani S, Luberti M, Baykan Y, Friedrich D, Ahn H. CO2 capture from syngas by an adsorption process at a biomass gasification CHP plant: its comparison with amine-based CO2 capture. Int J Greenh Gas Control 2015;35:71e81. https://doi.org/10.1016/j.ijggc.2015.01.008. [12] Yan L, Yue G, He B. Exergy analysis of a coal/biomass cohydrogasification based chemical looping power generation system. Energy 2015;93:1778e87. https://doi.org/10.1016/ j.energy.2015.10.042. RN, Carolino C, Miranda M. Hot treatment and [13] Pinto F, Andre upgrading of syngas obtained by co-gasification of coal and
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108
16
international journal of hydrogen energy xxx (xxxx) xxx
wastes. Fuel Process Technol 2014;126:19e29. https://doi.org/ 10.1016/j.fuproc.2014.04.016. [14] Ren L, Wei R, Gao Y. Co-gasification reactivity of petcoke and coal at high temperature. Fuel 2017;190:245e52. https:// doi.org/10.1016/j.fuel.2016.11.020. [15] Edreis EMA, Luo G, Li A, Xu C, Yao H. Synergistic effects and kinetics thermal behaviour of petroleum coke/biomass blends during H2O co-gasification. Energy Convers Manag 2014;79:355e66. https://doi.org/10.1016/ j.enconman.2013.12.043. [16] Shen CH, Chen WH, Hsu HW, Sheu JY, Hsieh TH. Cogasification performance of coal and petroleum coke blends
in a pilot-scale pressurized entrained-flow gasifier. Int J Energy Res 2012;36:499e508. https://doi.org/10.1002/er.1821. [17] Al-Zareer M, Dincer I, Rosen MA. Development and analysis of an integrated system with direct splitting of hydrogen sulfide for hydrogen production. Int J Hydrogen Energy 2016;41:20036e62. https://doi.org/10.1016/ j.ijhydene.2016.08.217. [18] Wen CY, Chaung TZ. Entrainment coal gasification modeling. Ind Eng Chem Process Des Dev 1979;18:684e95. https://doi.org/10.1021/i260072a020. [19] Technology I of G. Coal conversion systems technical data book. Springfield, VA: NTIS; 1978.
Please cite this article as: Al-Zareer M et al., Production of hydrogen-rich syngas from novel processes for gasification of petroleum cokes and coals, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.108