Thermodynamic Analysis of Syngas Production and Sulfur Capturing

Subscriber access provided by University of South Dakota

Process Systems Engineering

Thermodynamic Analysis of Syngas Production and Sulfur Capturing from a Mixture of Methane and Hydrogen Sulfide using a Solar Thermochemical Redox Cycle Abhishek Singh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02484 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Thermodynamic Analysis of Syngas Production and Sulfur Capturing from a Mixture of Methane and Hydrogen Sulfide using a Solar Thermochemical Redox Cycle Abhishek Singh*† Department of Physics, Colorado School of Mines, 1500 Illinois Street, Golden, CO-80401, USA

ABSTRACT

Syngas production and sulfur capturing from a mixture of methane and H2S via a novel two-step solar thermochemical cycle based on metal oxide/metal sulfide redox reactions is thermodynamically analyzed. Fe2O3/FeS is used as a model metal oxide/metal sulfide pair for this study. During the reduction step, Fe2O3 is reduced to FeS using a mixture of CH4 and H2S. In this process, syngas (CO + H2) is produced in the gas phase. In the oxidation step, FeS is oxidized using air to obtain Fe2O3 in the solid phase and SO2 and unreacted N2 in the gas phase. The produced SO2 can be used to generate sulfuric acid. Favorable operating conditions for the redox cycle are determined using an open system, thermodynamic model. For the reduction step,

ACS Paragon Plus Environment

1

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

T = 1200 K at 1 bar pressure provides complete conversion of Fe2O3 to FeS. At these conditions, gas phase products contain mainly H2 and CO. Carbon formation is also predicted by the model between 580 and 1150 K temperatures. Thermodynamic analysis shows complete conversion of FeS to Fe2O3 during oxidation step at T ≥ 600 K and 1 bar pressure. Effect of higher system pressures over the Fe2O3 reduction process is also determined using the model. With the increase in system pressure, carbon formation decreases. At 10 bar system pressure, no carbon formation is predicted by the thermodynamic model. A thermodynamic process model is also developed to assess the energetic feasibility of the complete process. Concentrated solar power can be used to provide the necessary energy for the endothermic Fe2O3 reduction process. Effect of concentration ratio and heat recuperation from exhaust gases over solar to fuel energy efficiency is studied. Energy efficiencies of the complete process at various oxidation temperatures are also determined using the thermodynamic process model. An overall system efficiency of 46.9 % can be achieved for heat exchanger effectiveness of 0.9 and concentration ratio of 1000 when reduction and oxidation reactions are performed at 1200 K temperature.

1. INTRODUCTION Hydrogen sulfide is an abundantly available source of hydrogen and sulfur1. It naturally exists in crude oil and natural gas wells2. H2S is also an important by-product of fossil fuel processing industries such as coal gasification, oil refinery or any other desulfurization of petroleum stocks processes in which sulfur content is transformed to gaseous hydrogen sulfide3. It is a colorless, toxic, corrosive and flammable gas and considered as a pollutant and poses hazardous effects on both human health and environment4.


2

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The current industrial process for sulfur capturing from hydrogen sulfide is the Claus process5. In this process, hydrogen is converted to a low-quality steam thereby wasting hydrogen. Previously various processes were proposed to extract hydrogen from H2S while capturing sulfur. These processes include thermolysis and thermocatalytic methods, photocatalytic methods, multi-step thermochemical methods. A comprehensive review of these processes is given by Zaman and Chakma6. The focus of these studies2 was on the decomposition of the pure H2S stream but H2S is released along with other components such as CH4, CO2, hydrocarbons, N2, and H2O. Separation of H2S from other components incurs further energy penalty and cost. Efforts have been carried out to extract hydrogen from H2S without separating it out from the mixture. One of the studied processes for hydrogen sulfide treatment and hydrogen production is to reform hydrogen sulfide with methane7. In this process, hydrogen is produced along with carbon sulfide (CS2). CS2 can be condensed out from the mixture. Waterman and Van Vlodrop8 and Erekson9 experimentally investigated this process and showed the feasibility of this process. Karan and Behle10 investigated the reaction between CH4 and H2S in a quartz tubular flow reactor at high temperatures. Their focus was CS2 production using this process. They concluded that the reaction between methane and sulfur was quite rapid and H2S conversion can be increased in the presence of methane. Huang et al.11 also performed a thermodynamic analysis of the H2S reformation of methane. They identified the conditions in which carbon formation can be minimized to zero by increasing the H2S content in the gas mixture. This concentration ratio requirement limits the applicability of this process because usually methane concentration is much higher than the H2S concentration in the mixture11. Megalofonous and Pappayanakos12 also investigated the thermodynamic equilibrium composition of H2S and CH4 reaction products. They also performed an experimental study of this process in the temperature range of 713 to


3


Page 4 of 30

860 oC using MoS2 catalyst for accelerating the reaction. Various other researchers also developed catalysts for acceleration of this reaction13-15. Alternatively, H2S and CH4 mixture can be converted to syngas while capturing sulfur in this process. The Syngas is a valuable product that can be used for the production of methanol, dimethyl ether, and ammonia16. It can also be fed to Fischer-Tropsch process to produce traditional fuel such as gasoline17. One of the leading processes for syngas production is the conversion of methane to syngas. There are various pathways for methane to syngas conversion processes such as natural gas oxyforming18, including steam reforming19, dry reforming20 with CO2, and partial oxidation21. Another process is to split water and CO2 to generate syngas22-24. In this process, water is used as a source of hydrogen. In the present study, a novel method for syngas production and sulfur capturing using 2 thermochemical non-catalytic steps is presented. In the first high temperature, endothermic step, a higher valance metal oxide is made to react with a mixture of methane and hydrogen sulfide. In this step, metal oxide converts to metal sulfide and produces syngas. In the second step (exothermic), metal sulfide is oxidized using air. In this step, metal sulfide converts back to metal oxide and produces SO2 along with unreacted N2. The produced SO2 and nitrogen mixture can be fed to the contact process to form sulfuric acid5. In the commercial contact process, air is introduced in the process to convert SO2 to SO3 therefore N2 separation from SO2 is not required. However additional air must be supplied for SO3 production. The overall reaction steps can be represented by (endothermic step) MxOy + y CH4 + x H2S ↔ x MS + y CO + (2y+x) H2 ,

Δh > 0

(1)

and (exothermic step)


4

Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


x MS + 2.38(2x+y) (0.21 O2 + 0.79 N2) ↔ MxOy + x SO2 + 1.88(2x+y) N2, Δh < 0

(2)

In this process, no separation of H2S from methane is required. SO2 produced can be used to form sulfuric acid using commercial contact process. There is also no requirement of a catalyst to carry out this process. Carbon formation due to methane cracking can be reduced or eliminated due to the availability of oxygen released from metal oxide during the reduction process. This released oxygen can react with carbon and produce carbon monoxide. Although the metal oxide reduction process is endothermic, metal sulfide oxidation in air is highly exothermic and with proper thermal energy management, the overall energy requirement of the complete process can be reduced. In the present study, Fe2O3/FeS system is used to carry out the process analysis. Previously, Iron oxide based reaction schemes were exploited by various researchers for water splitting process25. Nakamura26 first proposed Fe3O4/FeO based system for water splitting. In this process, Fe3O4 is thermally reduced at high temperature and low pressure to FeO. For an operating pressure of 10-3 bar, 1450 oC temperature is required for the iron oxide thermal reduction reaction27. Hightemperature, low pressure, thermal reduction of iron oxide has limitations in terms of high reradiation losses, energy penalty for pressure reduction, high-temperature reactor material, and insulation losses16. Due to limitations posed by thermal reduction, many researchers used carbothermal reduction using methane or CO for the reduction of operating temperature for the iron oxide reduction step25,28,29. In the current work, a mixture of CH4 and H2S is used for the reduction of Fe2O3. A thermodynamic feasibility study using an open system, thermodynamic model is performed to find the favorable operating conditions for the redox cycle. Effect of pressure over Fe2O3 to FeS


5


Page 6 of 30

conversion and carbon formation in the reduction step is also determined. A thermodynamic process model is developed to carry out the energy balance of the redox cycle and to determine the energy requirement of the complete process. Figure 1 illustrates the proposed cycle using Fe2O3/FeS system.

Figure 1 Schematic of the proposed Fe2O3/FeS cycle for syngas production and sulfur recovery in the form of SO2 (For Tred ≥ 1200 K and Toxi ≥ 600 K). Generated SO2 can be used to produce sulfuric acid using the contact process.


6

Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2. THERMODYNAMIC MODEL 2.1 Open System Thermodynamic Model To determine the operating conditions for Fe2O3 reduction step and FeS oxidation step, an open system, quasi-steady state, thermodynamic equilibrium model developed by Singh et al.30 is used. The species balance for a gas-solid reaction is expressed as, 𝑛̇ i,gas = 𝑛̇ (𝑦i,gas,in − 𝑦i,gas,eq (𝐧gas , 𝐧solid , 𝑇, 𝑃))

(3)

𝑦i,solid = 𝑦i,solid,eq (𝐧gas , 𝐧solid , 𝑇, 𝑃) where 𝐧gas = (𝑛1,gas , 𝑛2,gas , … . . , 𝑛n,gas ) 𝐧solid = (𝑛1,solid , 𝑛2,solid , … . . , 𝑛m,solid ) 𝑛i,gas

𝑦i,gas = ∑𝑛

(4)

𝑛i,solid

1=1 𝑛i,gas

, 𝑦i,solid = ∑𝑛

1=1 𝑛i,solid

Constant pressure and temperature and ideal gas behavior are assumed for the gas-solid reaction. Species in the solid phase are assumed to be incompressible. The solid solution is ideal, and all solids are assumed to be in one phase. First, closed system thermodynamic equilibrium calculations are performed using HSC software31 to determine the most probable product species for each step. All the product species with mole fractions less than 10-5 have been omitted from the analysis. Species considered in the thermodynamic equilibrium model for the complete cycle are H2O, H2, O2, N2, CO, CO2, CH4, SO2, COS, H2S, C, Fe, FeO, Fe3O4, Fe2O3, Fe3C, FeCO3, FeS, FeS2, Fe2S3, and FeSO4. The Gibbs free energy minimization principle is used to determine the equilibrium compositions for the open system Fe2O3 reduction and FeS oxidation processes 𝑦𝑖,gas⁄solid,𝑒𝑞 (𝒏gas , 𝒏solid , 𝑇, 𝑝) = 𝐚𝐫𝐠 𝐦𝐢𝐧 𝐺(𝒏gas , 𝒏solid , 𝑇, 𝑝)

(5)

𝐧gas ,𝐧solid


7


Page 8 of 30

where arg min is used to determine the constrained minimization of the Gibbs free energy. The Gibbs free energy is calculated assuming gaseous and solid phases in close contact with each other, 𝐺 = ∑𝑛𝑖=1 𝐺i,gas + ∑𝑚 𝑖=1 𝐺i,solid 𝐺i,gas = 𝑛𝑖 𝑔̅𝑖0 + 𝑛𝑖 𝑅𝑇 ln(

𝑦𝑖 𝑃 ⁄𝑃 ) ref

𝐺i,solid = 𝑛𝑖 𝑔̅𝑖0 + 𝑛𝑖 𝑅𝑇 ln 𝑦𝑖

(6) (7) (8)

where 𝑔̅𝑖0 is the reference Gibbs function of species i evaluated (kJ kmol-1), yi is the species i mole fraction, P is total system pressure (N m-2), and Pref is a reference pressure (N m-2). To satisfy the elemental balance of the system, number of moles of all considered species are conserved. HSC 7.0 database31 is used to obtain the reference values for enthalpy, entropy, and the temperature dependent specific heat. Usually, gas-solid reactions are limited by a growing diffusion layer of a solid product over the surface of a solid reactant. In the proposed process, the Fe2O3 reduction may be limited by the growing diffusion layer of FeS. As the present analysis solely considers thermodynamics of the process, limitations due to growing diffusion layer are neglected. 2.2 Thermodynamic Process Model To determine the overall process efficiency of the proposed cycle, a thermodynamic process model is developed. This model utilizes the operating conditions and reaction yields predicted by the open system, thermodynamic model (explained in section 2.1). Figure 2 shows the schematic of a process layout for the Fe2O3 reduction and FeS oxidation cycle. The system consists of 2 reactors for reduction and oxidation processes. Two heat exchangers for the gas phase heat recuperation are also shown in the schematic. FeS and Fe2O3 from the reduction and oxidation


8

Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reactors respectively are cycled between the reactors. Various inlet and outlet temperatures and species are labeled in the process layout. A gas separation unit for extraction of unreacted H2S and CH4 is also shown in the layout.

Figure 2 Process layout of the proposed redox cycle. Block arrows represent energy flows and solid arrows represent mass flows. Concentrated solar power provides the process heat (𝑄̇solar ) for the endothermic reduction process. Heat from high-temperature reduction reaction products is used to preheat the incoming gas phase reactants using a heat exchanger (HX1). Preheated gaseous reactants are fed to the reduction reactor, where they are further heated to the reduction temperature using 𝑄̇solar . Concentrated solar power also provides the necessary heat for the reduction reaction and heating Fe2O3 from Toxi to Tred. Exhaust gases from the reduction reactor may also contain some unreacted reactant gases (CH4 and H2S). These gases can be extracted from the exhaust gases


9


Page 10 of 30

using membrane separation and recycled to the reduction reactor. Incoming air for the oxidation process is also preheated by using the oxidation reaction products (SO2 and N2) exiting the oxidation reactor. HX2 is used for the preheating of the oxidation reactants. Fully mixed flow is assumed for both the reactors. Due to this assumption, solid and the gas phase should have the same temperature at the end of the oxidation/reduction process32. After Fe2O3 reduction, the produced FeS in the solid phase at the reduction temperature is transported from the reduction reactor to the oxidation reactor. If required, heat produced due to the exothermic oxidation reaction is used to heat the solid and gas phase reactants of the reaction during the oxidation step. Excess heat from the oxidation reaction is rejected to the atmosphere to carry out the oxidation process at temperature Toxi. After completion of the oxidation reaction, Fe2O3 from the oxidation reactor is transported from the oxidation reactor to the reduction reactor. It is assumed that there is no heat loss during the transport of solid products from the reduction reactor to the oxidation reactor and vice versa. Reactants for both reduction and oxidation processes are supplied to the system at an ambient temperature (T1=T4=Tamb). System pressure and its surroundings are assumed to be 1 bar. The energy balance of the system is given by 𝑄̇solar = 𝑄̇CH4+H2S,𝑇2 →𝑇red + 𝑄̇red,rxn + 𝑄̇Fe2 O3 ,𝑇Oxi →𝑇red + 𝑄̇rerad + 𝑄̇conv,loss + 𝑄̇oxi,process

(9)

Where 𝑄̇solar is the energy supplied by the concentrating solar power to provide sensible heating to the reactants and also to carry out the endothermic reduction reaction, 𝑄̇CH4 +H2 S,𝑇2 →𝑇red is the energy required to heat the preheated reduction process inlet gases to the reduction temperature, 𝑄̇red,rxn


10

Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


is the reaction energy required during Fe2O3 reduction, 𝑄̇Fe2 O3,𝑇Oxi →𝑇red is the energy required to heat Fe2O3 from oxidation to the reduction temperature, Heat losses due to re-radiation and convection to the ambient from the reduction reactor are given by 𝑄̇rerad and 𝑄̇conv,loss terms. 𝑄̇oxi,process term accounts for the energy required to carry out the oxidation process. 𝑄̇oxi,process = 𝑄̇O2+N2 ,𝑇6 →𝑇red − 𝑄̇oxi,rxn − 𝑄̇FeS,𝑇red→𝑇oxi

(10)

If 𝑄̇O2+N2 ,𝑇6 →𝑇red ≤ 𝑄̇oxi,rxn + 𝑄̇FeS,𝑇red→𝑇oxi , 𝑄̇oxi,process= 0 Where 𝑄̇O2 +N2 ,𝑇6 →𝑇red is the energy needed to heat the preheated inlet gases for the oxidation reaction to the oxidation temperature, 𝑄̇oxi,rxn is the energy released by the exothermic oxidation of FeS, and 𝑄̇FeS,𝑇red→𝑇oxi is the energy that can be extracted from FeS by cooling it from the reduction to the oxidation temperature. Energy balance at the heat exchangers is used to calculate the temperatures of gases leaving the heat exchangers. For the reduction process, CH4 and H2S are preheated using the reduction reaction effluents. The amount of heat exchange is given by 𝑄̇HX,red = 𝑛̇ CH4 (ℎCH4 ,𝑇2 − ℎCH4,𝑇1 ) + 𝑛̇ H2S (ℎH2S,𝑇2 − ℎH2S,𝑇1 ) 𝜖 min ( ) 𝑛̇ CO (ℎCO,𝑇3 − ℎCO,𝑇red ) + 𝑛̇ CH4 (ℎCH4 ,𝑇3 − ℎCH4,𝑇red ) + 𝑛̇ H2 (ℎH2 ,𝑇3 − ℎH2 ,𝑇red ) + 𝑛̇ H2S (ℎH2S,𝑇3 − ℎH2 S,𝑇red )

(11) Where 𝜖 is the heat exchanger effectiveness. T2 and T3 are calculated by using below equations 𝑄̇HX,red = 𝑛̇ CH4 (ℎCH4 ,𝑇2 − ℎCH4 ,𝑇1 ) + 𝑛̇ H2 S (ℎH2 S,𝑇2 − ℎH2 S,𝑇1 )

(12)

𝑄̇HX,red = 𝑛̇ CO (ℎCO,𝑇3 − ℎCO,𝑇red ) + 𝑛̇ CH4 (ℎCH4 ,𝑇3 − ℎCH4 ,𝑇red ) + 𝑛̇ H2 (ℎH2 ,𝑇3 − ℎH2 ,𝑇red ) + (13)

𝑛̇ H2 S (ℎH2 S,𝑇3 − ℎH2 S,𝑇red )


11


Page 12 of 30

Same process is used to obtain the 𝑄̇HX,oxi , T5, and T6 for the oxidation side of the heat exchanger. Solar receiver is assumed to be effectively black thereby no solar radiation absorption losses occur in the receiver. Re-radiation losses from the solar receiver are given by16 4

𝜎𝑇 𝑄̇rerad = CG 𝑟𝑒𝑑 𝑄̇solar

(14)

input

Convective losses from the receiver to the ambient are treated as a fraction of the absorbed solar energy33. 𝑄̇conv,loss = 𝐹(𝑄̇solar − 𝑄̇rerad )

(15)

Where 𝜎 is the Stefan Boltzmann constant (W m-2 K-4), Tred is the reduction temperature (K), C is the concentration ratio (-), Ginput is the direct normal irradiance (W m-2), and F is the heat loss factor (-). The minimum energy for extraction of a gas from a mixture of gases is given by27 1

(16)

𝐸Sep,min,i = 𝑅𝑇 ln(𝑦 ) 𝑖

Where R is the universal gas constant (J mol-1 K-1) and 𝑦𝑖 is the mole fraction of the gas in the mixture. This is the minimum energy required and if the extraction process involves mechanical or electrical work, a corresponding efficiency (𝜂w ) must also be taken in to account. For the current analysis, 𝜂w = 0.1 is assumed27. It is assumed that two separate membranes might be required for the separation of CH4 and H2S and the total minimum energy required for this process is given by 𝑄̇Sep,min,total = (𝑛̇ CH4 𝐸Sep,min,CH4 + 𝑛̇ H2 S 𝐸Sep,min,H2 S )/𝜂w

(17)

The solar to fuel efficiency is calculated by

𝜂=

𝑛̇ CO𝐻𝐻𝑉CO + 𝑛̇ H2 𝐻𝐻𝑉H2 −𝑛̇ CH4 𝐻𝐻𝑉CH4 𝑄̇solar + 𝑄̇Sep,min,total

(18)


12

Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Where numerator is calculated by using the higher heating values of CH4 (888.3 kJ mol-1), CO (282 kJ mol-1) and H2 (286 kJ mol-1) and their production yields. Due to environmental regulations, use of H2S as a fuel is not allowed. Therefore, the higher heating value of H2S is not considered for the efficiency calculations. 3. RESULTS AND DISCUSSION The open system, thermodynamic equilibrium model is used to determine the product species composition for the iron oxide reduction step. In this step, the incremental amount of CH4 and H2S is introduced in the reactor having a stoichiometric amount of Fe2O3 according to the reaction shown in Figure 1. The cumulative molar ratio of CH4 to H2S employed is equal to 3:2 (stoichiometric amount of both the gas phase input species). The gaseous products formed during this reaction are continuously taken out from the reactor due to which equilibrium of the reaction shifts to the product side according to the Le Chatelier’s principal. Solid phase product species remain in the reactor until the completion of the process.


13


Page 14 of 30

Figure 3 Variation of solid composition with Figure 4 Variation of cumulative gas phase temperature for the Fe2O3 reduction using H2S composition with temperature for the Fe2O3 and CH4.

reduction using H2S and CH4.

Figure 3 shows the variation of the solid phase composition in the reactor after the completion of the process for different operating temperatures. Pressure is assumed to be 1 bar for the reduction process. FeS is the only specie formed at T > 1150 K. Carbon formation due to the cracking of methane happens between 580 and 1150 K temperatures. Maximum carbon is formed around 880 K temperature. At lower temperatures, Fe2O3 converts to mainly FeS2 and Fe2S3 but at higher temperatures production of these species is not predicted by the thermodynamic equilibrium model. Figure 4 shows the cumulative gas phase composition at various operating temperatures. At 1200 K temperature, the formation of mainly H2 and CO is predicted by the thermodynamic model. Formation of H2O decreases with the increase of reaction temperature. At higher temperatures, carbon formation decreases due to the availability of more oxygen from Fe2O3 reduction reaction. For this process, T ≥ 1200 K at 1 bar operating pressure is recommended. Figure 5 shows the variation of the solid phase composition with temperature for the iron sulfide (FeS) oxidation process. Air having 0.21 and 0.79 molar fractions of oxygen and nitrogen respectively is used as an oxidizer in this process. Trace gases such as CO2, Argon etc present in the air are neglected in this analysis. T ≥ 600 K is predicted to be thermodynamically favorable for FeS to Fe2O3 conversion. This temperature range is close to the experimental results of FeS oxidation in air34. At lower temperatures, FeS reacts with oxygen in the air and forms FeSO4 which is also experimentally determined by Kennedy and Sturman 34. Figure 6 shows the formation of SO2 at various operating temperatures. The model predicts completion of SO2


14

Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


formation for temperatures ≥ 600 K. For FeS oxidation process, T ≥ 600 K at 1 bar pressure is recommended.

Figure 5 Variation of the solid composition

Figure 6 Variation of SO2 production with

with temperature for the iron sulfide (FeS)

temperature for the iron sulfide (FeS)

oxidation in air.

oxidation in air.

Higher system pressures are preferred in industrial applications35 therefore the effect of pressure on Fe2O3 reduction is also investigated. The total system pressure of 1, 5, and 10 bar are considered for the analysis. Figure 7 shows the fractions of solid products formed at different considered system pressures and temperatures. Carbon formation decreases considerably when total system pressure is increased from 1 to 5 bar. At higher pressures, the thermodynamic equilibrium of Fe2O3 reduction reaction is shifted to the left in such a way to relieve the pressure in accordance with Le Chatelier’s principle. Due to this effect, conversion of Fe2O3 to FeS completed at higher temperatures for higher pressures. No carbon formation is predicted by the thermodynamic equilibrium model at 10 bar pressure. The decrease in carbon formation at high


15


Page 16 of 30

pressures enables this process to be carried out at lower temperatures as compared to the same at 1 bar pressure. For 10 bar operating pressure, T > 900 K is suitable to carry out iron oxide reduction process.

Figure 7 Produced solid fraction for the iron oxide reduction step at 1, 5, and 10 bar pressures. Figure 8 shows the variation of Qred,rxn and Qoxi,rxn of the proposed cycle. All the reactants are assumed to be supplied at ambient temperature for these calculations. For the oxidation reaction, slope change in the temperature range 500-600 K is due to variable composition of SO2 in the reaction exhaust. At 600 K, Qoxi,rxn= -852 kJ mol-1 of Fe2O3 for this process. After 600 K, enthalpy change increases monotonically due to the constant composition of the products. For the reduction process, Qred,rxn = 960 kJ mol-1 of Fe2O3 at 1200 K. The changes in the slope of the curve are mainly due to methane cracking in the temperature range of 600-1150 K. At lower temperatures, the process is exothermic due to incomplete reduction of Fe2O3.


16

Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8 Variation of reactions energies with reaction temperatures during reduction and oxidation processes. Reactants are assumed to be supplied at ambient temperature. After determining the favorable operating conditions of the proposed process, the thermodynamic process model developed in section 2.2 is used to determine the solar to fuel efficiency of the process. For this analysis, the reduction reaction is assumed to be at 1200 K and oxidation reaction temperature is varied between 600 and 1200 K. Although 1200 K is close to the melting point of FeS (1467 K), experimentally it has been shown that the reaction kinetics of FeS oxidation in air is not significantly affected by the high temperatures36. Impact of oxidation temperature over efficiency is assessed in this study. Effect of concentration ratio and heat exchanger effectiveness (𝜖) over efficiency is also reported. All the heat exchangers are assumed to have the same effectiveness. For this analysis, 1 kW solar energy input is assumed. Results may be scaled to arbitrary input power. Heat loss factor of 0.2 based on other analysis33,37 is assumed to calculate the convective losses from the reactor. Solar to fuel energy efficiency is calculated using equation 18.


17


Figure 9 Variation of solar to fuel energy

Figure 10 Variation of solar to fuel

efficiency of the complete process with

efficiency with oxidation temperature. Heat

concentration ratio. Heat recuperation is not

exchangers effectiveness (𝜖) is also varied. C

considered. Operating temperatures of 600

= 1000 and Tred = 1200 K are used for these

and 1200 K for the oxidation and reduction

calculations.

Page 18 of 30

reactions respectively are considered. Figure 9 shows the solar to fuel energy efficiency variation of the complete process with concentration ratio. For this calculation, heat recuperation from the reduction and oxidation reactor exhaust gases are not considered. With the increase of the concentration ratio, the overall efficiency of the process increases. For C > 1000, the increase in efficiency is relatively small. Only 1.5 % increase in efficiency is predicted when C is increased from 1000 to 2000 but the corresponding cost of the increase in concentration ratio is much higher due to the increase in the corresponding solar field size. The concentration ratio of 1000 is also available in the commercial central receiver concentrating solar power plants38. Since there is not much benefit in using concentration ratio higher than 1000, for further study C = 1000 is used.


18

Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For the considered operating conditions, 𝑄oxi,process is found to be zero which in turn makes the oxidation process energetically self-sustaining and does not require any energy input. Figure 10 shows the variation of the solar to fuel efficiency with the oxidation temperature. The solar to fuel efficiency increases with the increase of oxidation temperature. This is due to the decrease in energy required to heat Fe2O3 from the oxidation temperature to the reduction temperature. As expected, an increase in heat exchanger effectiveness (𝜖) also increases the overall efficiency of the process. A solar to fuel efficiency of 40.5 % can be achieved for Toxidation = 600 K, 𝜖 = 0.9, and C = 1000. At 1200 K oxidation temperature, the solar to fuel efficiency increases to 46.9 %. The proposed process has advantages as compared to the H2S methane reforming process. The H2S methane reforming process produces a significant amount of carbon which in turn deactivates the reforming catalyst. In the proposed process, carbon formation can be avoided by operating at relatively higher temperatures (≥ 1150 K) or higher pressures (≥ 10 bar) and it does not require any catalyst. The proposed process works at a higher CH4/H2S ratio which is more suitable for the naturally occurring mixture of CH4 and H2S as compared to lower CH4/H2S ratio required by the H2S methane reforming process. POTENTIAL PRACTICAL PROBLEMS AND LIMITATIONS The carbon formation or coking is one of the biggest challenges in the practical implementation of the proposed process. The carbon formation may reduce the reactor construction material lifetime because of harmful carburization of the reactor construction material. Unreacted carbon can produce CO/CO2 in the exhaust gases during the oxidation process. Unwanted carbonaceous gases in the oxidation exhaust gases will require separation which will incur further energy and economic penalty to the overall process. As shown in the previous section, either high


19


Page 20 of 30

temperature or high-pressure process can eliminate the coking. But increase in operating temperature or pressure may require expensive reactor and piping material thereby increasing the overall cost of the plant. At high operating temperatures, re-radiation losses from the cavity reactor will also increase which in turn increases the heliostat field size thereby increasing the overall cost of the plant. While selecting the operating conditions of the proposed process, aforementioned factors should also be considered. In general, gas-solid reactions are limited by the growing diffusion layer of the solid product at the surface of the solid reactant. In this case, FeS diffusion layer growth over the surface of Fe2O3 may hinder the gas-solid reaction which in turn may reduce the syngas production and decreases the cycle lifetime of the process. An in-depth experimental study is necessary for the determination of the reaction kinetics limitations of the proposed process. Since the proposed process utilizes sulfur-containing gases, corrosion of reactor and piping material due to the presence of sulfur is another limitation of the process. Any leakage of H2S and SO2 to the environment is also a potential hazard involved in the implementation of this process to the industrial scale. For any large-scale implementation of the proposed process, special precaution should be taken to avoid any leakage of sulfur-containing gases in the environment. CONCLUSION A thermodynamic feasibility analysis of a novel approach for syngas production and sulfur capturing from a mixture of CH4 and H2S using Fe2O3/FeS pair has been performed. An open system, thermodynamic equilibrium model was used to determine the suitable operating conditions for the Fe2O3 reduction and FeS oxidation steps. For the iron oxide reduction process,


20

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1200 K temperature at 1 bar pressure provided complete conversion of Fe2O3 to FeS and gas phase product stream contained mainly H2 and CO. Carbon formation between 580 and 1150 K temperatures was predicted by the thermodynamic model. At higher temperatures, carbon formation was not predicted by the model. Iron sulfide oxidation to Fe2O3 in the air was favored at T > 600 K at 1 bar pressure. The product gas stream of the oxidation reaction contained mainly SO2 and N2. Effect of high pressure over reduction reaction was also assessed using the thermodynamic model. High operating pressure reduces the formation of carbon. At 10 bar system pressure, no carbon formation was predicted by the model. A thermodynamic process model has also been developed to perform the energy efficiency analysis of the proposed cycle. The concentrated solar energy was employed to satisfy the energy requirement of the endothermic reduction step. For the analysis, reduction temperature was assumed to be at 1200 K and the oxidation temperature was varied between 600 and 1200 K at 1 bar pressure. Heat recuperation from the high-temperature gas products was employed in the process analysis. Effect of concentration ratio over the solar to fuel efficiency was also determined using the model. It is determined that an increase of concentration ratio from 1000 to 2000 increases efficiency by only 1.5 % thereby a concentration ratio of 1000 is recommended for the proposed cycle. For heat exchanger effectiveness of 0.9 and Toxi = Tred = 1200 K, solar to fuel efficiency of 46.9 % can be achieved. AUTHOR INFORMATION Corresponding Author *Email: [email protected]


21


Page 22 of 30

Present Addresses † The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, 771 Ferst Dr., Atlanta, GA 30332, USA. NOMENCLATURE C

-

Concentration ratio (-)

cp

-

Specific heat capacity (kJ kmol-1 K-1)

ESep

-

Energy required to extract one gas specie from a gas mixture (kJ mol-1)

F

-

Heat loss factor (-)

G

-

Gibbs free energy (kJ)

Ginput

-

Direct normal irradiance (W m-2)

𝑔̅𝑖0

-

Reference Gibbs function of species i evaluated (kJ kmol-1)

h

-

Enthalpy (kJ kmol-1)

HHV

-

Higher heating value (kJ mol-1)

M

-

Molar mass (kg kmol-1)

ni

-

Number of moles for a species i (kmol)

𝑛̇ 𝑖

-

Molar flow rate (kmol s-1)

P

-

Total system pressure (N m-2)


22

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pref

-

Reference pressure (N m-2)

𝑄̇

-

Power (W)

R

-

Ideal gas constant (kJ kmol-1 K-1)

T

-

Temperature (K)

yi

-

Mole fraction (-)

Greek Letters 𝜂

-

Solar to fuel efficiency (-)

𝜂w

-

Electrical to mechanical work efficiency (-)

𝜖

-

Heat exchanger effectiveness (-)

𝜎

-

Stefan Boltzmann constant (W m-2 K-4)

amb

-

Ambient

conv

-

Convective

i

-

Species

oxi

-

Oxidation

red

-

Reduction

rxn

-

Reaction

Subscripts


23


rerad

-

Re-radiation

sen

-

Sensible

Page 24 of 30

Abbreviations HX

-

Heat Exchanger

REFERENCES (1) Adewale, R.; Salem, D. J.; Berrouk, A. S.; Dara, S. Simulation of hydrogen production from thermal decomposition of hydrogen sulfide in sulfur recovery units. J. Clean. Prod. 2016, 112, 4815. (2) El-Melih, A. M.; Iovine, L.; Al Shoaibi, A.; Gupta, A. K. Production of hydrogen from hydrogen sulfide in presence of methane. Int. J. Hydrogen Energy 2017, 42, 4764. (3) Li, Y.; Guo, Q.; Dai, Z.; Dong, Y.; Yu, G.; Wang, F. Study of oxidation for gas mixture of H2S and CH4 in a non-premixed flame under oxygen deficient condition. Appl. Therm. Eng. 2017, 117, 659. (4) Reverberi, A. P.; Klemeš, J. J.; Varbanov, P. S.; Fabiano, B. A review on hydrogen production from hydrogen sulphide by chemical and photochemical methods. J. Clean. Prod. 2016, 136, 72. (5) Moeller, W.; Winkler, K. The double contact process for sulfuric acid production. J. Air Pol. Cont. Assoc. 1968, 18, 324.


24

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6) Zaman, J.; Chakma, A. Production of hydrogen and sulfur from hydrogen sulfide. Fuel Process. Technol. 1995, 41, 159. (7) Sadooghi, P.; Rauch, R. Experimental and modeling study of hydrogen production from catalytic steam reforming of methane mixture with hydrogen sulfide. Int. J. Hydrogen Energy 2015, 40, 10418. (8) Waterman, H. I.; van Vlodrop, C. The preparation of carbon disulphide from methane and hydrogen sulphide. J. Soc. Chem. Ind. 1939, 58, 109. (9) Erekson, E. J.; Miao, F. Q. Gasoline from Natural Gas by Sulfur Processing. Quarterly progress report January 1994--March 1994, final version. DOE/PC/92114-T3; ON: DE94017371; TRN: 94:007788, DOE Contract Number AC22-93PC92114; USDOE, Washington, DC, USA and Institute of Gas Technology, Chicago, IL, USA, 1994. (10) Karan, K.; Behie, L. A. CS2 formation in the Claus reaction furnace: a kinetic study of methane-sulfur and methane-hydrogen sulfide reactions. Ind. Eng. Chem. Res. 2004, 43, 3304. (11) Huang, C.; T-Raissi, A. Thermodynamic analyses of hydrogen production from subquality natural gas: Part I: Pyrolysis and autothermal pyrolysis. J. Power Sources 2007, 163, 645. (12) Megalofonos, S. K.; Papayannakos, N. G. Hydrogen production from natural gas and hydrogen sulphide. Int. J. Hydrogen Energy 1991, 16, 319. (13) Guldal, N. O.; Figen, H. E.; Baykara, S. Z. New catalysts for hydrogen production from H2S: preliminary results. Int. J. Hydrogen Energy 2015, 40, 7452.


25


Page 26 of 30

(14) Martínez-Salazar, A. L.; Melo-Banda, J. A.; Reyes de la Torre, A. I.; Salazar-Cerda, Y.; Coronel-García, M. A.; Martínez, B. P.; Martínez-Sifuentes, V. H.; Domínguez-Esquivel, J. M.; Rodrigo, R. S. Hydrogen production by methane reforming with H2S using Mo, Cr/ZrO2–SBA15 and Mo, Cr/ZrO2–La2O3 catalysts. Int. J. Hydrogen Energy 2015, 40, 17272. (15) Reshetenko, T. V.; Khairulin, S. R.; Ismagilov, Z. R.; Kuznetsov, V. V. Study of the reaction of high-temperature H2S decomposition on metal oxides (γ-Al2O3, α-Fe2O3, V2O5). Int. J. Hydrogen Energy 2002, 27, 387. (16) Krenzke, P. T.; Davidson, J. H. Thermodynamic analysis of syngas production via the solar thermochemical cerium oxide redox cycle with methane-driven reduction. Energy Fuels 2014, 28, 4088. (17) Klausner, J. F.; Li, L.; Singh, A.; Yeung, N. A.; Mei, R.; Hahn, D.; Petrasch, J. The role of heat transfer in sunlight to fuel conversion using high temperature solar thermochemical reactors. In Proceedings of the 15th International Heat Transfer Conference, IHTC-15 August 10-15, 2014, Kyoto, Japan. 2014. (18) York, A. P.; Xiao, T. C.; Green, M. L.; Claridge, J. B. Methane oxyforming for synthesis gas production. Cat. Rev. 2007, 49, 511. (19) Rostrup-Nielsen, J. R. Production of synthesis gas. Catal. Today 1993, 18, 305. (20) Havran, V.; Duduković, M. P.; Lo, C. S. Conversion of methane and carbon dioxide to higher value products. Ind. Eng. Chem. Res. 2011, 50, 7089.


26

Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(21) Enger, B. C.; Lødeng, R.; Holmen, A. A review of catalytic partial oxidation of methane to synthesis gas with emphasis on reaction mechanisms over transition metal catalysts. Appl. Catal., A 2008, 346, 1. (22) Li, L.; Chen, C.; Singh, A.; Rahmatian, N.; AuYeung, N.; Randhir, K.; Mei, R.; Klausner, J. F.; Hahn, D. W.; Petrasch, J. A transient heat transfer model for high temperature solar thermochemical reactors. Int. J. Hydrogen Energy 2016, 41, 2307. (23) Singh, A.; Lapp, J.; Grobbel, J.; Brendelberger, S.; Reinhold, J. P.; Olivera, L.; Ermanoski, I.; Siegel, N. P.; McDaniel, A.; Roeb, M.; Sattler, C. Design of a pilot scale directly irradiated, high temperature, and low pressure moving particle cavity chamber for metal oxide reduction. Solar Energy 2017, 157, 365. (24) Ermanoski, I.; Grobbel, J.; Singh, A.; Lapp, J.; Brendelberger, S.; Roeb, M.; Sattler, C.; Whaley, J.; McDaniel, A.; Siegel, N. P. Design and construction of a cascading pressure reactor prototype for solar-thermochemical hydrogen production. In AIP Conference Proceedings 1734, 120001(2016); AIP Publishing, 2016. (25) Steinfeld, A. Solar thermochemical production of hydrogen––a review. Solar energy 2005, 78, 603. (26) Nakamura, T. Hydrogen production from water utilizing solar heat at high temperatures. Solar Energy 1977, 19, 467. (27) Singh, A. K.; AuYeung, N. J.; Randhir, K.; Mishra, R.; Allen, K. M.; Petrasch, J.; Klausner, J. F. Thermal reduction of iron oxide under reduced pressure and implications on thermal conversion efficiency for solar thermochemical fuel production. Ind. Eng. Chem. Res. 2015, 54, 6793.


27


Page 28 of 30

(28) Singh, A. K. Multi-scale, multi physics modeling of thermo-chemical iron/iron oxide cycle for fuel production. Ph.D. Dissertation, University of Florida, Gainesville, FL, 2013. (29) Klausner, J. F.; Hahn, D. W.; Petrasch, J.; Mei, R.; Mehdizadeh, A. M.; Barde, A.; Allen, K.; Rahmatian, N.; Stehle, R. C.; Bobek, S. M.; Al-Raqom, F.; Greek, B.; Li, L.; Singh, A.; Takagi, M. Novel Magnetically Fluidized Bed Reactor Development for the Looping Process: Coal to Hydrogen Production R&D. DOE NETL Final Report, DOE Contract Number DE-FE0001321; USDOE, Washington, DC, USA and University of Florida, Gainesville, FL, USA, 2010. (30) Singh, A.; Al-Raqom, F.; Klausner, J.; Petrasch, J. Production of hydrogen via an Iron/Iron oxide looping cycle: Thermodynamic modeling and experimental validation. Int. J. Hydrogen Energy 2012, 37, 7442. (31) Roine, A. HSC Chemistry 7; Outotec, 2009. (32) Singh, A.; Al-Raqom, F.; Klausner, J.; Petrasch, J. Hydrogen production via the iron/iron oxide looping cycle. In ASME 2011 5th International Conference on Energy Sustainability, Parts A, B, and C, Washington, DC, USA, August 7–10, 2011, 2011. (33) Lapp, J.; Davidson, J. H.; Lipiński, W. Efficiency of two-step solar thermochemical non-stoichiometric redox cycles with heat recovery. Energy 2012, 37, 591. (34) Kennedy, T.; Sturman, B. T. The oxidation of iron (II) sulphide. J. Therm. Anal. Calorim. 1975, 8, 329.


28

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(35) Gálvez, M. E.; Halmann, M.; Steinfeld, A. Ammonia production via a two-step Al2O3/AlN thermochemical cycle. 1. Thermodynamic, environmental, and economic analyses. Ind. Eng. Chem. Res. 2007, 46, 2042. (36) Yang, M.; Chen, X.; Wang, Y.; Yue, X. Thermal oxidation kinetics analysis of ferrous sulfide under different heating rates. Acta Technica 2016, 61,19. (37) Bader, R.; Venstrom, L. J.; Davidson, J. H.; Lipiński, W. Thermodynamic analysis of isothermal redox cycling of ceria for solar fuel production. Energy Fuels 2013, 27, 5533. (38) Alexopoulos, S.; Hoffschmidt, B. Solar tower power plant in Germany and future perspectives of the development of the technology in Greece and Cyprus. Renew. Energy 2010, 35, 1352.


29


Page 30 of 30

FOR TABLE OF CONTENTS ONLY


30

Thermodynamic Analysis of Syngas Production and Sulfur Capturing

Recommend Documents