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Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming
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Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming Apichaya Yahom a , Jonathan Powell b , Varong Pavarajarn a , Patiwat Onbhuddha c , Sumittra Charojrochkul c , Suttichai Assabumrungrat a,∗ a
Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand Newcastle University International – Singapore, School of Chemical Engineering and Advanced Materials, Faculty of Science, Agriculture and Engineering, Newcastle University, NE1 7RU, United Kingdom c National Metal and Materials Technology Center (MTEC), 114 Paholyothin Road, Klong 1, Klongluang, Pathumthani 12120, Thailand b
a b s t r a c t The production of hydrogen from methane via two chemical looping reforming (CLR) processes was simulated and thermodynamically analysed, one process being the conventional CLR process, the other being a CO2 sorption enhanced process. The aim of the work was to identify suitable operating conditions for obtaining an optimum hydrogen gas purity and yield, whilst operating auto-thermally, at atmospheric pressure and with no carbon formation. In both simulations, the reactors were simulated using the Gibbs minimisation technique. NiO was used as the oxygen storing species, whilst CaO was used as the CO2 adsorbent. For conventional CLR, within the range of conditions tested, the optimum reactor operating conditions are a temperature of 800 ◦ C, a H2 O/CH4 ratio of 3, and a NiO/CH4 ratio of 1 resulting in an approximate hydrogen production yield of 2.5 mol of H2 per mole of CH4 and an approximate hydrogen purity of 75%. However, with the application of in situ CO2 adsorption, a hydrogen purity > 90% and a yield within the region of 3 mol of H2 per mole of CH4 , can be achieved with a NiO/CH4 ratio ≈ 1, a CaO/CH4 ratio ≥ 1, a H2 O/CH4 ratio ≥ 2 and a temperature between 500 ◦ C and 600 ◦ C. The results indicate that the implementation of in situ CO2 adsorption could potentially bring about significant improvements in both yield and purity of hydrogen. © 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Chemical looping; NiO; CO2 sorbent; CaO; Simulation
1.
Introduction
Hydrogen is an important feedstock for many chemical and petrochemical industries. It is used to produce various chemicals such as ammonia, methanol and hydrochloric acid. Hydrogen has also received much attention as an energy carrier for use in a hydrogen network such that in some applications it could replace hydrocarbon fuels leading to improved
local air quality. When hydrogen is produced using renewable energy sources, its use as an energy carrier also leads to reduced demand on depleting fossil fuel resources and a reduction in net carbon dioxide emissions, a gas that contributes towards climate change. In circumstances where the hydrogen gas is produced from fossil fuels, it is also possible to capture the emitted carbon dioxide, although the development and implementation of carbon capture technology at the
∗
Corresponding author. Tel.: +66 22 186868; fax: +66 22 186877. E-mail addresses: [email protected], [email protected] (S. Assabumrungrat). http://dx.doi.org/10.1016/j.cherd.2014.04.002 0263-8762/© 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Yahom, A., et al., Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.04.002
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Fig. 1 – Schematic diagram of the chemical looping reforming process. industrial scale still has some way to go, in particular due to its associated carbon footprint and cost. Established technologies include pre-combustion, post-combustion and oxy-fuel combustion techniques. Chemical looping combustion (CLC) provides the possibility to combust fossil fuels whilst inherently allowing carbon dioxide to be captured as a separate gas (Ishida and Jin, 1996). Fig. 1 is a schematic representation of the chemical looping process, which uses a metal oxide as an oxidising species in replacement of oxygen gas and so eliminates the need for an air separation unit or other gas separation units which have large energy demands. Previous studies have considered the use of the steam in CLC as an alternative means of oxidising the oxygen carrier, whilst also producing hydrogen gas (Cleeton et al., 2009; Gupta et al., 2007; Rydén and Arjmand, 2012; Svoboda et al., 2008). Iron has received particular attention, and chemical looping processes have been proposed where the steam is used as an oxidant in the air/oxygen reactor, with the iron oxygen carrier being cycled between Fe3 O4 , in the oxidised state, to FeO, in the reduced state. Ryden and Lyngfelt proposed a process which incorporates steam reforming into chemical looping combustion (SRICLC) (Rydén and Lyngfelt, 2006), for the production of syngas as well as the generation of heat. The steam reformation (SR) takes place in a separate stream to the CLC cycle, with heat from the exothermic oxidation reaction being used to sustain the endothermic SR reaction. Separate steam reformation and water gas shift reactor units are also used. The application of chemical looping to steam reforming has also received much interest and development in recent years (Dupont et al., 2008; Pröll et al., 2010; Rydén et al., 2006). Work carried out by Dupont et al. involved using a single reactor with the feed cycling between a fuel/steam stream and an air stream. As such, at the beginning of each cycle, there is a dead time in the hydrogen production due to oxidation by the metal oxide. In the reactor, various reactions take place, including steam reformation, the water gas shift reaction, partial oxidation and oxidation of the fuel. Under favourable conditions, this process can be operated auto-thermally. The adsorption of CO2 gas or removal of hydrogen from the steam reforming reactor promotes the formation of hydrogen, resulting in higher production rates and higher hydrogen gas purities and improved conversion of the hydrocarbon fuel (Chen et al., 2008; Dupont et al., 2008; Hufton et al., 1999). The CO2 sorbent can be circulated with the oxygen carrier material, such that the CO2 adsorbed in the reduction reactor, can then be desorbed in a separate reactor.
In selecting an oxygen carrier, various factors are considered including oxidation and reduction reactivity, tendency to undergo agglomeration and attrition, oxygen transport capacity, the extent to which the metal oxide will be reduced, as well as toxicity and cost (Najera et al., 2011). Various materials have been studied for use as oxygen carriers in CLC and CLSR, in particular, period four transition metals (Abad et al., 2007; Adánez et al., 2004a; Cho et al., 2004) such as iron, copper, nickel, cobalt and manganese, dispersed on inert substrates such as alumina. Iron, copper and nickel have exhibited good oxidation and reduction reaction rates (Abad et al., 2007; Adánez et al., 2004b; Cho et al., 2004), however copper and iron show a greater tendency to agglomerate. Furthermore, iron has a relatively low oxygen transport capacity, with thermodynamic restrictions preventing Fe2 O3 from being reduced to Fe in the fuel reactor, although iron does benefit from being a low cost material. Copper oxide has a high oxygen transport capacity, and is able to reduce to copper in the fuel reactor and therefore able to combust a greater quantity of fuel. In addition, it is cheaper than nickel and cobalt and has less environmental impact. However, due to its low sintering temperature, it has a tendency to defluidize and degrade more readily, and cannot be made by mechanical mixing and wet impregnation, as it is easily agglomerated. Various authors have investigated pure copper oxide in thermogravimetric analyses, during reactions with fuel gases and directly with coal (Adanez et al., 2012; Cao et al., 2006; de Diego et al., 2004; Siriwardane et al., 2009; Tian et al., 2008). The results showed copper oxide to have a high reactivity even at low temperatures, but the oxidation reaction rate of pure copper oxide decreased significantly with an increasing number of cycles. The reaction of the metal oxide with its support, such as alumina, may result in the formation of an inert phase, that reduces the reduction and oxidation reactivity. For NiO supported on alumina, this issue is less pronounced when compared to CuO and other metal oxides (Mattisson et al., 2003). NiO also acts as an effective catalyst for the steam reformation of methane (Matsumura and Nakamori, 2004; Münster and Grabke, 1981) making it particularly interesting for use as an oxygen carrier in the CLSR process. Multiple reactions take place in the steam reforming reactor of a CLR process. These include partial and complete oxidation of the fuel, the water gas shift reaction, steam reformation and cracking. The overall reaction can in general be written as follows: MO(s) + NiO(s) + fuel + H2 O → (MCO3 (s)) + Ni(s)) + (C(s), CO, CO2 , H2 O) + H2 ,
H > 0
(R1)
When the fuel is methane and the oxygen carrier is NiO, (R1) can be written in terms of the following separate reactions
CH4 + 4NiO → CO2 + 2H2 O + 4Ni, H298 = 174.9 (kJ/mol) (R2) CH4 + NiO → CO + 2H2 + Ni,
H298 = 208.6 (kJ/mol)
(R3)
CH4 + 2NiO → CO2 + 2H2 + 2Ni, H298 = 169.9 (kJ/mol) (R4) CH4 + H2 O CO + 3H2 ,
H298 = 206.1 (kJ/mol)
(R5)
Please cite this article in press as: Yahom, A., et al., Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.04.002
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CH4 + CO2 2CO + 3H2 , H2 O + CO CO2 + H2 ,
H298 = 357.8 (kJ/mol) H298 = −41.2 (kJ/mol)
(R6) (R7)
Ni + 1/2O NiO, H298 = −244.3 (kJ/mol)
(R8)
Ni + H2 O NiO + H2 ,
(R9)
H400 = −21.95 (kJ/mol)
If using CaO as the CO2 sorbent, the sorption reaction is as follows: CaO + CO2 CaCO3 ,
H298 = −178.8 (kJ/mol)
(R10)
This paper reports the findings from simulations of conventional looping steam reforming as well as sorption enhanced chemical looping reforming (SECLR), using methane as the fuel, NiO as the oxygen carrier and CaO as the CO2 adsorbent. The effects of the steam/methane ratio and the NiO/methane ratio on the purity of hydrogen in the product stream, the hydrogen gas yield, energy balance and the propensity for carbon formation are considered. Results from the SECLR simulation are compared to experimental results provided by Rydén and Ramos (2012) for validation of the simulation.
2.
Methods
2.1.
Simulation
hydrogen as a major product, some fractions of CH4 , unreacted steam, CO2 , CO, and Ni solid reduced from NiO. All of the products are transported to the cyclone, CYCLONE1, for separation of the Ni solid from the gases. The H2 -rich gas stream passes through heat exchanger, HX1, where it cools to 150 ◦ C and exchanges heat with the fresh reactant stream, CH4 + H2O, which increases in temperature, becoming stream REACTANT1. The Ni solid is transferred to the air reactor for oxidation with air forming NiO. The air reactor is assumed to operate under adiabatic conditions, such that much of the heat of oxidation is transferred to the NiO as sensible heat. The heat of reaction for the oxidation of Ni into NiO causes an increase in temperature of the NiO particles to a temperature typically greater than 1000 ◦ C. The NiO and nitrogen gas are separated using a cyclone, CYCLONE2. The gas stream, N2 , passes through a heat exchanger where it exchanges heat with the reactant stream, REACTANT1, which is cooled to 150 ◦ C. The REACTANT1 stream is subsequently heated to the reactor temperature, becoming stream REACTANT2, which is fed to the reactor along with the NiO exiting CYCLONE2. The sensible heat of the NiO provides energy required for the endothermic reactions in the fuel reactor.
2.3.
Both a conventional chemical looping process and a sorption enhanced chemical looping process were simulated, with the aim of finding appropriate operating conditions for the production of high purity hydrogen gas with low or zero energy process requirements and minimal carbon formation. The simulations were carried out using Aspen Plus programme, using the Gibbs minimisation method, where the reactions are not specified by the user, but the reactants, products and temperature are. The Aspen Plus simulations were setup with a stream class of MIXCIPSD, to allow for the separation between solids and gases. Various property methods were used and compared, including Peng–Robinson, NRTL and SOLIDS property methods. They all gave the same results. The simulations were run with varying NiO/CH4 and H2 O/CH4 ratios, as well as varying temperatures, with the objective being to find the optimum hydrogen yields and purities, with near zero heat duty and minimal or no carbon formation. For simulations of the CLC and the SECLR, the reactor fluid feed stream, unless otherwise stated, contains 1 mol of CH4 and 2 mol of H2 O at 35 ◦ C and atmospheric pressure, is preheated and introduced to the fuel reactor under various isothermal conditions (500, 600, 700, and 800 ◦ C) at atmospheric pressure.
2.2.
CLR process
Fig. 2 shows the diagram of the CLR process simulated using the Aspen Plus programme. The system consists of a fuel reactor, FUEL-REA, an air reactor, AIR-REA, cyclones, CYCLONE1 AND CYCLONE2, as well as two heat exchangers, HX1 and HX2. The reactant stream, CH4 + H2O – containing 1 mol of CH4 for every 2 mol of H2 O, at 35 ◦ C and atmospheric pressure – is preheated via heat exchangers HX1 and HX2, and then fed into the reactor under isothermal conditions at 1 bar pressure to react with NiO solid. The product stream contains
3
SECLR process
A SECLR system (Fig. 3) was also simulated using the Aspen Plus programme. The simulated system consists of a fuel reactor, FUEL-REA, a calcination reactor, CALCINA, for release of adsorbed CO2 , an air reactor, AIR-REA, cyclones, CYCLONE1 and CYCLONE2, as well as heat exchangers, HX1, HX2 and HX3. The reactant stream, CH4 + H2O, containing 1 mol of CH4 for every 2 mol of H2 O, at 35 ◦ C at atmospheric pressure, is preheated via heat exchangers HX1, HX2 and HX3 and then fed into the reactor under isothermal conditions and a pressure of 1 bar. The major product is hydrogen, but other species in the product stream include CH4 , H2 O, CO, CaCO3 , and Ni. All of the products are transported to a cyclone, CYCLONE1, to separate the Ni and CaCO3 solids from the gases. The H2 -rich gas stream passes through heat exchanger, HX1, where it cools to 150 ◦ C and exchanges heat with the fresh reactant stream, CH4 + H2O, which increases in temperature, becoming stream REACTANT1. The separated solids, stream NI + CACO3, are transferred to the calcination reactor, CALCINA, which is operated at 880 ◦ C and 1 bar, where CaCO3 is calcined into CaO releasing CO2 . The CO2 gas is subsequently separated from the solids using cyclone CYCLONE2. The CO2 is passed through a heat exchanger, HX2, where it exchanges heat with stream REACTANT1 and cools to 150 ◦ C. The REACTANT1 stream increases in temperature becoming stream REACTANT2. The Ni and CaO in stream NI + CAO, are then transferred to the air reactor, AIR-REA, where the Ni reacts with air under adiabatic conditions, forming NiO. The heat of oxidation results in an increase in temperature of the CaO and NiO due to sensible heat effects, with their temperature typically being over 1000 ◦ C. The gas and solid stream, N2 + SOLID, from reactor AIRREA, is fed to the cyclone, CYCLONE3, where the NiO and CaO are then separated from the nitrogen gas. The solids are subsequently fed to the fuel reactor as stream NIO–CAO. The hot N2 , stream N2, is passed through heat exchanger 3, HX3, exchanging heat with stream REACTANT2 and cooling to
Please cite this article in press as: Yahom, A., et al., Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.04.002
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Fig. 2 – Diagram of chemical looping reforming in Aspen Plus program. 150 ◦ C. REACTANT2 stream is heated to the reactor temperature, to which it is then fed.
2.4.
Energy balance
Energy balances were carried out on each unit. The air reactors and the cyclones are considered to be adiabatic. There is also no shaft work done across the boundaries of these units, but the Ni oxidises, therefore heat is generated. The energy ˙ h, balance for the air reactors simply becomes HAR = m i l i where HAR is the heat of oxidation of the Ni, where i denotes ˙ is the mass flow the stream or a component in a stream, m rate and h is the specific enthalpy. For the heat exchangers, it is considered that heat is only transferred between streams and not to the external environ ment. The energy balance therefore becomes 0 = mh. i i i
The fuel reactor is considered to be diathermal and so the ˙ h − HFR , where Q is the energy balance is given as Q = m i l i heat duty of the reactor and HFR represents the enthalpy of reaction, considering all reactions as one overall reaction (refer to (R1)). One of the objectives of this study is to find optimal operating conditions where Q = 0.
2.5.
Carbon activity
With regards to the effect of operating conditions on carbon formation, this is considered by calculating the activity of pure carbon in the reactor at equilibrium and standard conditions. Under such conditions, if at chemical equilibrium, the carbon would have an activity of one. A calculation of its activity therefore indicates the displacement of the carbon forming
Fig. 3 – Diagram of chemical looping reforming with CO2 sorbent in Aspen Plus program. Please cite this article in press as: Yahom, A., et al., Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.04.002
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Table 1 – Simulation from this study and experimental results from Ryan and Rodus, for equivalent SECLR processes under the same operating conditions. Simulation
Reference
Simulation deviation
mol/hr 0.030 1.328 2.811 0.011 0.008 % 98.7 97.0
% −16.7 −6.25 −6.01 −9.09 −12.5
H2 purity CH4 conversion
mol/hr 0.025 1.245 2.642 0.010 0.007 % 98.44 97.46
CO2
mol/hr 0.900
mol/hr 0.950
Fuel reactor product gas CH4 H2 O H2 CO2 CO
Calciner product gas
reactions from equilibrium and therefore the propensity for carbon formation or consumption. It is assumed that in the CLR of methane, carbon formation only occurs according to the following four equilibrium reactions: 2CO CO2 + C
(C1)
CH4 2H2 + C
(C2)
CO + H2 H2 O + C
(C3)
CO2 + 2H2 2H2 O + C
(C4)
For each of these equilibrium reactions, a corresponding equilibrium constant, K can be written. These equations were written in terms of partial pressures of the corresponding gases and the activity of the solid carbon. As shown in Eqs. (1)–(4), these equations can be rearranged to solve for the carbon activity when the equilibrium constant and partial pressures are known. aC1 = KC1
aC2 = KC2
aC3 = KC3
aC4 = KC4
p2CO
(1)
pCO2 pCH4 p2H
(2)
2
pCO pH2 pH2 O pCO p2H
2O
p2H
(3)
(4)
2O
The term a refers to the carbon activity, K is the equilibrium constant, whilst the subscripts C1, C2, C3 and C4 refer to the equilibrium reaction to which the equation refers to. The partial pressure of the various gaseous components is denoted by p. When a > 1, the chemical reaction is not at equilibrium with a driving force for the formation of carbon. If a = 1, the chemical reaction is at equilibrium with the solid carbon, and if a < 1, a carbon formation is thermodynamically not preferred and any carbon in the system is consumed by this reaction.
2.6.
Simulation validation
The SECLR process simulated in this study is based on that developed by Ryden and Ramos (Rydén and Ramos, 2012), and was run with operating conditions taken from this same study.
−0.30 0.52 −5.26
Methane is fed into the reactor at 1 mol per 2.2 mol of steam, where the steam is at 312 ◦ C, and the fuel reactor has a temperature of 580 ◦ C. The calcination reactor was simulated to be at 880 ◦ C and a pressure of 1 bar.
3.
Results and discussion
CLR and SECLR of methane were simulated using Aspen Plus program. The particular interest of this study was the effect of operating conditions such as the H2 O/CH4 ratio and NiO/CH4 ratio on the hydrogen production yield, hydrogen purity, overall energy balance, and propensity for carbon formation.
3.1.
Simulation validation
The simulations were validated by running the SECLR simulation under the same conditions used by Ryden and Ramos (Rydén and Ramos, 2012) and then comparing the composition of the reactor product gas stream. Table 1 shows the experimental results from Ryden and Rodus and the results from this study obtained from the validation of the SECLR simulation. The results show that the hydrogen purity deviates from the experimental results by a negligible amount of −0.30%, whilst the hydrogen yield deviates by −6.01%, which is acceptable for the purpose of this study, which is to compare results from two simulations and identify the range of operating conditions that provide an optimum performance. The CH4 conversion values also match very closely. The large deviations in the CH4 , CO2 and CO flow rates can be accounted for by the small flow rates involved.
3.2.
Conventional chemical looping reforming
3.2.1.
Hydrogen production yield and purity
Fig. 4 shows the change in hydrogen production yield and purity with the H2 O/CH4 ratio at varying temperatures. As expected, the data from the simulation shows an increase in hydrogen production with increasing an amount of steam in the feed, shifting the equilibrium compositions of the steam reformation reaction (R5) and water gas shift reaction (R7) to the right. Likewise, an increase in temperature leads to the equilibrium position of the endothermic reaction, (R5), shifting to the right, as such the hydrogen purity and yield increase with increasing temperature. However, this trend of increasing yield with increasing temperature approaches a limit when the maximum temperature 800 ◦ C is approached, with the hydrogen yield and purities obtained at 700 ◦ C approaching those at 800 ◦ C with increasing steam content. At a H2 O/CH4
Please cite this article in press as: Yahom, A., et al., Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.04.002
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Fig. 4 – Moles of hydrogen produced per mole of methane fed and hydrogen dry mole fraction at various ratios of H2 O/CH4 at different temperatures for chemical looping reforming (NiO/CH4 = 1). ratio of 3, a higher hydrogen production yield is achieved at 700 ◦ C than at 800 ◦ C. The maximum hydrogen production yield of 2.5 moles of H2 per mole of CH4 , achieved at 700 ◦ C and a H2 O/CH4 ratio of 3, is less than the stoichiometric quantity of 3 that would be expected from reactions (R5) and (R6) alone. This limiting hydrogen production yield of 2.5 is a result of a competing reaction, such as the oxidation of Ni by steam. Fig. 4 shows that within the range of H2 O/CH4 ratios simulated, the highest dry hydrogen purities are obtained at 500 ◦ C, 600 ◦ C, 700 ◦ C, and 800 ◦ C, a H2 O/CH4 ratio of 3 and a NiO/CH4 ratio of 1, are 61.25%, 70.78%, 72.06%, and 71.56%, respectively. Likewise, the highest hydrogen production yields under these same conditions are 1.58, 2.42, 2.58 and 2.52 mol of H2 per mole of CH4 . Referring to Fig. 5, the hydrogen production yield and purity apparently decrease with increasing NiO/CH4 ratio as would be expected, with the oxidation reaction (R2) becoming more dominant with an increasing amount of NiO. At low NiO contents and high temperatures, higher hydrogen production
Fig. 6 – Net energy demand for the chemical looping reforming process at various H2 O/CH4 ratios (NiO/CH4 = 1) and various NiO/CH4 ratios (steam/methane = 2). yields are achieved due to the increasing dominance of reaction (R5). At the high temperatures, an upper limit of 3 moles of H2 per mole of CH4 is achieved due to competing shifts in equilibrium i.e. reactions (R5) and (R7). The hydrogen yield is particularly sensitive to temperature at low NiO/CH4 ratios, due to the equilibrium composition of reaction (R5), being shifted further to the left with decreasing temperature. Fig. 5 shows that within the range of NiO/CH4 ratios simulated, with a constant H2 O/CH4 ratio of 2, the highest dry hydrogen purities obtainable at 500 ◦ C, 600 ◦ C, 700 ◦ C, and 800 ◦ C, are 61.2%, 70.8%, 72.1%, and 71.6%, respectively, with a NiO/CH4 ratio of 0. Likewise, the highest hydrogen production yields under these same conditions are 1.58, 2.42, 2.58 and 2.52 mol of H2 per mole of CH4 .
3.2.2.
3.2.3.
Fig. 5 – Moles of hydrogen produced per mole of methane fed into the reactor and the hydrogen mole fraction at various ratios of NiO/CH4 and different temperatures via chemical looping reforming (H2 O/CH4 = 2).
Energy balance
Fig. 6 shows the change in the energy balance of the simulated chemical looping process, either with changing amount of steam and constant amount of NiO, or with various amount of NiO and constant amount of steam. The data shows that with increasing amount of steam and increasing temperature, the energy balance becomes more positive approaching a limiting value of zero, partly due to a decrease in enthalpy of the stream exiting the reactor but also due to an increase in flow rate of steam into the reactor. Likewise, with increasing amount of NiO, the energy balance decreases, which is due to the increasing importance of the exothermic oxidation reactions. Referring to Fig. 6, one can see that a net energy balance of zero can be achieved with a NiO/CH4 ratio and a H2 O/CH4 ratio of approximately 1 and 2, respectively.
Carbon activity
Fig. 7 shows the activity of carbon formed by reactions (C1)–(C4) for varying H2 O/CH4 ratios (NiO/CH4 ratio of 1) as well as varying NiO/CH4 ratios (H2 O/CH4 ratio of 2). The carbon activity is used here as a measure of the thermodynamic propensity for carbon formation. The data shows that the carbon activity is particularly sensitive to the H2 O/CH4 ratio with values below 1 leading to thermodynamically unfavourable conditions for carbon formation. With decreasing amounts of steam, carbon forming reactions (C1) and (C2) become increasingly crucial. However, with an excess of steam (H2 O/CH4 ratio
Please cite this article in press as: Yahom, A., et al., Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.04.002
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Fig. 7 – Activity of carbon formed by reactions (C1)–(C4) in the chemical looping reforming process at 500 ◦ C and 800 ◦ C for (A) various nickel oxide/methane ratios (H2 O/CH4 = 2) as well as (B) various H2 O/CH4 ratios (NiO/CH4 = 1). of 2), as the amount of NiO decreases and the degree of partial oxidation increases, the formation of carbon via reactions (C1) and (C3), is hindered by the water gas shift reaction, converting CO to CO2 . Due to an excess of steam, the equilibria of reactions (C2) and (C4), are shifted further to the left, thus reducing the formation of carbon via these reactions.
3.3.
Sorption enhanced chemical looping reforming
The effects of the H2 O/CH4 ratio and NiO/CH4 ratio on hydrogen production, hydrogen purity, net energy balance and carbon activity were determined to find an appropriate condition for hydrogen production via SECLR.
3.3.1.
Hydrogen production yield and purity
Fig. 8 shows simulation results representing the change in hydrogen production yield and purity with varying H2 O/CH4 ratios (NiO/CH4 ratio of 1) and varying temperatures. Contrary to the results obtained for the simulated CLR process – where the highest hydrogen production yields were obtained
Fig. 8 – Moles of hydrogen per mole of methane and the dry hydrogen mole fraction at various ratios of H2 O/CH4 and different temperatures for the sorption enhanced chemical looping reforming process (NiO/CH4 = 1 and CaO/CH4 = 1).
7
Fig. 9 – Moles of hydrogen per mole of methane and the dry hydrogen mole fraction at various NiO/CH4 ratios and different temperatures for the sorption enhanced chemical looping reforming process (H2 O/CH4 = 2 and CaO/CH4 = 1).
at the highest operating temperatures – in the presence of CO2 adsorption, the highest production yields and purities are obtained at 500 ◦ C and 600 ◦ C. This is a result of the equilibrium reaction (R9), which involves the exothermic adsorption of CO2 by CaO. The equilibrium composition of this reaction shifts to the right at lower temperatures, which in turn promotes the formation of hydrogen via reactions (R5), (R6) and (R7). Referring to Fig. 8, at H2 O/CH4 ratios lower than one, reactions (R2)–(R4) play a more important role in the formation of hydrogen. This is indicated by the higher hydrogen yields at higher temperatures, in which case the hydrogen production is dominated by either partial oxidation reactions ((R3) and (R4)), which produce hydrogen directly, or by the complete oxidation reaction (R2), where the water product can be used to produce hydrogen via reactions (R5) and (R6). Fig. 8 shows that within the range of H2 O/CH4 ratios simulated, with a constant NiO/CH4 ratio of 1, and CaO/CH4 ratio of 1, the highest dry hydrogen purities obtainable at 500 ◦ C, 600 ◦ C, 700 ◦ C, and 800 ◦ C, are 99.5%, 98.5%, 84.5%, and 71.5%, respectively, with a H2 O/CH4 ratio of 3. Likewise the highest hydrogen production yields under these same conditions are 2.95, 2.94, 2.76 and 2.52 moles of H2 per mole of CH4 . Fig. 9 shows the change in hydrogen production yield and purity with varying NiO/CH4 ratio (H2 O/CH4 ratio of 2) and varying temperatures. As would be expected in light of the equation for the exothermic reaction (R9), for high NiO contents, much higher hydrogen purities are obtained at lower temperatures due to the equilibrium composition of this reaction, being shifted further to the right at lower temperatures. At lower NiO/CH4 ratios, the variation in hydrogen purity and yield with temperature is much smaller due to the promotion of the steam reformation reaction (R5) at higher temperatures, balanced by the promotion of reactions (R5) and (R7) at lower temperatures, where adsorption of CO2 is thermodynamically favoured. Fig. 9 shows that within the range of NiO/CH4 ratios simulated, with a constant H2 O/CH4 ratio of 2 and a CaO/CH4 ratio of 1, the highest dry hydrogen purities obtainable at 500 ◦ C, 600 ◦ C, 700 ◦ C, and 800 ◦ C, are 99.9%, 98.1%, 78.4%, and 44.8%, respectively. Likewise, the highest hydrogen production yields
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Fig. 10 – Amount of hydrogen produced per mole of methane feed at various ratios of calcium oxide/methane and different temperature via sorbent enhanced chemical looping reforming (net energy requirement = 0, H2 O/CH4 ratio is constant). under these same conditions are 2.94, 3.00, 3.07 and 3.19 mol of H2 per mole of CH4 . The NiO/CH4 ratios corresponding to these results range between 0 and 0.25. Fig. 10 shows the hydrogen yield and purities obtained at various CaO/CH4 ratios and temperatures. The chart shows that for high calcium oxide contents, the optimum hydrogen yield is achieved at 600 ◦ C, whereas for the highest hydrogen purity, the operating temperature should be 500 ◦ C. However at lower calcium oxide contents, the partial oxidation reactions play a more important role in the production of hydrogen – as such the highest hydrogen yields and hydrogen purities are achieved at a 700 ◦ C.
3.3.2.
Energy balance
Fig. 11 shows the change in energy balance of the simulated sorption enhanced chemical looping process, both in terms of varying H2 O/CH4 ratio (NiO/CH4 ratio = 1) and in terms of varying NiO/CH4 ratios (H2 O/CH4 ratio = 2). Similar results are obtained with those obtained from the CLR process. Referring to Fig. 11, a net energy balance of zero can be achieved at
Fig. 11 – Net energy demand for the sorption enhanced chemical looping reforming with various steam/methane ratios (NiO/CH4 = 1) and various NiO/CH4 ratios (H2 O/CH4 = 1) for CaO/CH4 = 1.
Fig. 12 – Activity of carbon formed by reactions (C1)–(C4) in the sorbent enhanced chemical looping reforming process at 500 ◦ C and 800 ◦ C and (A) various NiO/CH4 ratios (H2 O/CH4 = 2) as well as (B) various H2 O/CH4 ratios (NiO/CH4 = 1) for CaO/CH4 = 1. 600 ◦ C, with a NiO/CH4 ratio and a H2 O/CH4 ratio of approximately 0.75 and 2, respectively.
3.3.3.
Carbon activity
Fig. 11 shows the activity of carbon formed by reactions (C1)–(C4), at various H2 O/CH4 ratios (NiO/CH4 ratio of 1) and various NiO/CH4 ratios (H2 O/CH4 ratio of 2) for the SECLR process. Over the full range of conditions simulated, the carbon activity remains below 0.6 indicating that under these conditions there is no thermodynamic drive for the formation of carbon which would otherwise lead to a deactivation of the catalyst. As with the results obtained for the CLR simulations, the carbon activities are the greatest at low H2 O/CH4 ratios with the carbon activity being particularly sensitive to this parameter (see Fig. 12).
3.3.4.
Comparison of optimum operating conditions
Both CLR and SECLR were simulated assuming Gibbs equilibrium conditions. The objective is to determine the optimum range of operating conditions that provide a near zero energy balance and without the formation of carbon leading to the deactivation of carbon. As a result, the chemical looping process approximates the optimum reactor operating conditions to be at 800 ◦ C, a H2 O/CH4 ratio of 3 and a NiO/CH4 ratio of 1, leading to an approximate hydrogen production yield of 2.5 moles of H2 per mole of CH4 and an approximate hydrogen purity of 75%. In comparison, the simulation results show that with the application of in situ CO2 adsorption, hydrogen purities approaching 100% can be achieved with a NiO/CH4 ratio ≥ 1, a CO/CH4 ratio ≥ 1, a H2 O/CH4 ratio ≥ 2 and a temperature between 500 ◦ C and 600 ◦ C. For optimum hydrogen yields, the same conditions apply except for the NiO/CH4 ratio for which any increase above unity would result in a decrease in hydrogen yield. Under optimum conditions, a hydrogen purity > 90% and a yield within the region of 3 moles of H2 per mole of CH4 can be achieved, indicating that the implementation of in situ CO2 adsorption could potentially bring about significant improvements in both yield and purity of hydrogen. Other than the clear improvements in hydrogen production yield and purity that can be achieved under Gibbs equilibrium
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conditions, another significant advantage of the sorption enhanced process is the lower range in optimum operating temperatures. At temperatures within the range of 500 ◦ C and 600 ◦ C, the sintering problem of the catalyst will be significantly suppressed, therefore reducing its degradation. The dramatic reduction in the carbon activity across the full range of simulated conditions, also indicates that deactivation of the catalyst due to the formation carbon, is essentially eliminated. It should be noted that in this study the gas product compositions were calculated using the Gibbs minimisation method. In real operation, the selection of metal oxides is an important issue as they influence both the reaction selectivity and reactivity which directly determine the reaction performance.
4.
Conclusions
A comparison between CLR with SECLR simulations operated using the Gibbs minimisation method, show that in situ CO2 adsorption leads to a significant increase in hydrogen yield and purity, coupled with a drop in the operating temperature required to achieve these higher yields and purities. This drop in operating temperature offers the added advantage of improved stability of the catalyst including a decrease in the impact of sintering on the catalyst performance. The calculated carbon activities for these simulated processes also show that, with in situ CO2 adsorption (CaO/CH4 = 1), there is no thermodynamic drive for the formation of carbon, therefore limiting the deactivation of the catalyst due to carbon formation.
Acknowledgement The authors gratefully acknowledge the support from the Ratchadaphiseksomphot Endowment Fund from Chulalongkorn University (RES560530168-EN).
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Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming Apichaya Yahom a , Jonathan Powell b , Varong Pavarajarn a , Patiwat Onbhuddha c , Sumittra Charojrochkul c , Suttichai Assabumrungrat a,∗ a
Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand Newcastle University International – Singapore, School of Chemical Engineering and Advanced Materials, Faculty of Science, Agriculture and Engineering, Newcastle University, NE1 7RU, United Kingdom c National Metal and Materials Technology Center (MTEC), 114 Paholyothin Road, Klong 1, Klongluang, Pathumthani 12120, Thailand b
a b s t r a c t The production of hydrogen from methane via two chemical looping reforming (CLR) processes was simulated and thermodynamically analysed, one process being the conventional CLR process, the other being a CO2 sorption enhanced process. The aim of the work was to identify suitable operating conditions for obtaining an optimum hydrogen gas purity and yield, whilst operating auto-thermally, at atmospheric pressure and with no carbon formation. In both simulations, the reactors were simulated using the Gibbs minimisation technique. NiO was used as the oxygen storing species, whilst CaO was used as the CO2 adsorbent. For conventional CLR, within the range of conditions tested, the optimum reactor operating conditions are a temperature of 800 ◦ C, a H2 O/CH4 ratio of 3, and a NiO/CH4 ratio of 1 resulting in an approximate hydrogen production yield of 2.5 mol of H2 per mole of CH4 and an approximate hydrogen purity of 75%. However, with the application of in situ CO2 adsorption, a hydrogen purity > 90% and a yield within the region of 3 mol of H2 per mole of CH4 , can be achieved with a NiO/CH4 ratio ≈ 1, a CaO/CH4 ratio ≥ 1, a H2 O/CH4 ratio ≥ 2 and a temperature between 500 ◦ C and 600 ◦ C. The results indicate that the implementation of in situ CO2 adsorption could potentially bring about significant improvements in both yield and purity of hydrogen. © 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Chemical looping; NiO; CO2 sorbent; CaO; Simulation
1.
Introduction
Hydrogen is an important feedstock for many chemical and petrochemical industries. It is used to produce various chemicals such as ammonia, methanol and hydrochloric acid. Hydrogen has also received much attention as an energy carrier for use in a hydrogen network such that in some applications it could replace hydrocarbon fuels leading to improved
local air quality. When hydrogen is produced using renewable energy sources, its use as an energy carrier also leads to reduced demand on depleting fossil fuel resources and a reduction in net carbon dioxide emissions, a gas that contributes towards climate change. In circumstances where the hydrogen gas is produced from fossil fuels, it is also possible to capture the emitted carbon dioxide, although the development and implementation of carbon capture technology at the
∗
Corresponding author. Tel.: +66 22 186868; fax: +66 22 186877. E-mail addresses: [email protected], [email protected] (S. Assabumrungrat). http://dx.doi.org/10.1016/j.cherd.2014.04.002 0263-8762/© 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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Fig. 1 – Schematic diagram of the chemical looping reforming process. industrial scale still has some way to go, in particular due to its associated carbon footprint and cost. Established technologies include pre-combustion, post-combustion and oxy-fuel combustion techniques. Chemical looping combustion (CLC) provides the possibility to combust fossil fuels whilst inherently allowing carbon dioxide to be captured as a separate gas (Ishida and Jin, 1996). Fig. 1 is a schematic representation of the chemical looping process, which uses a metal oxide as an oxidising species in replacement of oxygen gas and so eliminates the need for an air separation unit or other gas separation units which have large energy demands. Previous studies have considered the use of the steam in CLC as an alternative means of oxidising the oxygen carrier, whilst also producing hydrogen gas (Cleeton et al., 2009; Gupta et al., 2007; Rydén and Arjmand, 2012; Svoboda et al., 2008). Iron has received particular attention, and chemical looping processes have been proposed where the steam is used as an oxidant in the air/oxygen reactor, with the iron oxygen carrier being cycled between Fe3 O4 , in the oxidised state, to FeO, in the reduced state. Ryden and Lyngfelt proposed a process which incorporates steam reforming into chemical looping combustion (SRICLC) (Rydén and Lyngfelt, 2006), for the production of syngas as well as the generation of heat. The steam reformation (SR) takes place in a separate stream to the CLC cycle, with heat from the exothermic oxidation reaction being used to sustain the endothermic SR reaction. Separate steam reformation and water gas shift reactor units are also used. The application of chemical looping to steam reforming has also received much interest and development in recent years (Dupont et al., 2008; Pröll et al., 2010; Rydén et al., 2006). Work carried out by Dupont et al. involved using a single reactor with the feed cycling between a fuel/steam stream and an air stream. As such, at the beginning of each cycle, there is a dead time in the hydrogen production due to oxidation by the metal oxide. In the reactor, various reactions take place, including steam reformation, the water gas shift reaction, partial oxidation and oxidation of the fuel. Under favourable conditions, this process can be operated auto-thermally. The adsorption of CO2 gas or removal of hydrogen from the steam reforming reactor promotes the formation of hydrogen, resulting in higher production rates and higher hydrogen gas purities and improved conversion of the hydrocarbon fuel (Chen et al., 2008; Dupont et al., 2008; Hufton et al., 1999). The CO2 sorbent can be circulated with the oxygen carrier material, such that the CO2 adsorbed in the reduction reactor, can then be desorbed in a separate reactor.
In selecting an oxygen carrier, various factors are considered including oxidation and reduction reactivity, tendency to undergo agglomeration and attrition, oxygen transport capacity, the extent to which the metal oxide will be reduced, as well as toxicity and cost (Najera et al., 2011). Various materials have been studied for use as oxygen carriers in CLC and CLSR, in particular, period four transition metals (Abad et al., 2007; Adánez et al., 2004a; Cho et al., 2004) such as iron, copper, nickel, cobalt and manganese, dispersed on inert substrates such as alumina. Iron, copper and nickel have exhibited good oxidation and reduction reaction rates (Abad et al., 2007; Adánez et al., 2004b; Cho et al., 2004), however copper and iron show a greater tendency to agglomerate. Furthermore, iron has a relatively low oxygen transport capacity, with thermodynamic restrictions preventing Fe2 O3 from being reduced to Fe in the fuel reactor, although iron does benefit from being a low cost material. Copper oxide has a high oxygen transport capacity, and is able to reduce to copper in the fuel reactor and therefore able to combust a greater quantity of fuel. In addition, it is cheaper than nickel and cobalt and has less environmental impact. However, due to its low sintering temperature, it has a tendency to defluidize and degrade more readily, and cannot be made by mechanical mixing and wet impregnation, as it is easily agglomerated. Various authors have investigated pure copper oxide in thermogravimetric analyses, during reactions with fuel gases and directly with coal (Adanez et al., 2012; Cao et al., 2006; de Diego et al., 2004; Siriwardane et al., 2009; Tian et al., 2008). The results showed copper oxide to have a high reactivity even at low temperatures, but the oxidation reaction rate of pure copper oxide decreased significantly with an increasing number of cycles. The reaction of the metal oxide with its support, such as alumina, may result in the formation of an inert phase, that reduces the reduction and oxidation reactivity. For NiO supported on alumina, this issue is less pronounced when compared to CuO and other metal oxides (Mattisson et al., 2003). NiO also acts as an effective catalyst for the steam reformation of methane (Matsumura and Nakamori, 2004; Münster and Grabke, 1981) making it particularly interesting for use as an oxygen carrier in the CLSR process. Multiple reactions take place in the steam reforming reactor of a CLR process. These include partial and complete oxidation of the fuel, the water gas shift reaction, steam reformation and cracking. The overall reaction can in general be written as follows: MO(s) + NiO(s) + fuel + H2 O → (MCO3 (s)) + Ni(s)) + (C(s), CO, CO2 , H2 O) + H2 ,
H > 0
(R1)
When the fuel is methane and the oxygen carrier is NiO, (R1) can be written in terms of the following separate reactions
CH4 + 4NiO → CO2 + 2H2 O + 4Ni, H298 = 174.9 (kJ/mol) (R2) CH4 + NiO → CO + 2H2 + Ni,
H298 = 208.6 (kJ/mol)
(R3)
CH4 + 2NiO → CO2 + 2H2 + 2Ni, H298 = 169.9 (kJ/mol) (R4) CH4 + H2 O CO + 3H2 ,
H298 = 206.1 (kJ/mol)
(R5)
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CH4 + CO2 2CO + 3H2 , H2 O + CO CO2 + H2 ,
H298 = 357.8 (kJ/mol) H298 = −41.2 (kJ/mol)
(R6) (R7)
Ni + 1/2O NiO, H298 = −244.3 (kJ/mol)
(R8)
Ni + H2 O NiO + H2 ,
(R9)
H400 = −21.95 (kJ/mol)
If using CaO as the CO2 sorbent, the sorption reaction is as follows: CaO + CO2 CaCO3 ,
H298 = −178.8 (kJ/mol)
(R10)
This paper reports the findings from simulations of conventional looping steam reforming as well as sorption enhanced chemical looping reforming (SECLR), using methane as the fuel, NiO as the oxygen carrier and CaO as the CO2 adsorbent. The effects of the steam/methane ratio and the NiO/methane ratio on the purity of hydrogen in the product stream, the hydrogen gas yield, energy balance and the propensity for carbon formation are considered. Results from the SECLR simulation are compared to experimental results provided by Rydén and Ramos (2012) for validation of the simulation.
2.
Methods
2.1.
Simulation
hydrogen as a major product, some fractions of CH4 , unreacted steam, CO2 , CO, and Ni solid reduced from NiO. All of the products are transported to the cyclone, CYCLONE1, for separation of the Ni solid from the gases. The H2 -rich gas stream passes through heat exchanger, HX1, where it cools to 150 ◦ C and exchanges heat with the fresh reactant stream, CH4 + H2O, which increases in temperature, becoming stream REACTANT1. The Ni solid is transferred to the air reactor for oxidation with air forming NiO. The air reactor is assumed to operate under adiabatic conditions, such that much of the heat of oxidation is transferred to the NiO as sensible heat. The heat of reaction for the oxidation of Ni into NiO causes an increase in temperature of the NiO particles to a temperature typically greater than 1000 ◦ C. The NiO and nitrogen gas are separated using a cyclone, CYCLONE2. The gas stream, N2 , passes through a heat exchanger where it exchanges heat with the reactant stream, REACTANT1, which is cooled to 150 ◦ C. The REACTANT1 stream is subsequently heated to the reactor temperature, becoming stream REACTANT2, which is fed to the reactor along with the NiO exiting CYCLONE2. The sensible heat of the NiO provides energy required for the endothermic reactions in the fuel reactor.
2.3.
Both a conventional chemical looping process and a sorption enhanced chemical looping process were simulated, with the aim of finding appropriate operating conditions for the production of high purity hydrogen gas with low or zero energy process requirements and minimal carbon formation. The simulations were carried out using Aspen Plus programme, using the Gibbs minimisation method, where the reactions are not specified by the user, but the reactants, products and temperature are. The Aspen Plus simulations were setup with a stream class of MIXCIPSD, to allow for the separation between solids and gases. Various property methods were used and compared, including Peng–Robinson, NRTL and SOLIDS property methods. They all gave the same results. The simulations were run with varying NiO/CH4 and H2 O/CH4 ratios, as well as varying temperatures, with the objective being to find the optimum hydrogen yields and purities, with near zero heat duty and minimal or no carbon formation. For simulations of the CLC and the SECLR, the reactor fluid feed stream, unless otherwise stated, contains 1 mol of CH4 and 2 mol of H2 O at 35 ◦ C and atmospheric pressure, is preheated and introduced to the fuel reactor under various isothermal conditions (500, 600, 700, and 800 ◦ C) at atmospheric pressure.
2.2.
CLR process
Fig. 2 shows the diagram of the CLR process simulated using the Aspen Plus programme. The system consists of a fuel reactor, FUEL-REA, an air reactor, AIR-REA, cyclones, CYCLONE1 AND CYCLONE2, as well as two heat exchangers, HX1 and HX2. The reactant stream, CH4 + H2O – containing 1 mol of CH4 for every 2 mol of H2 O, at 35 ◦ C and atmospheric pressure – is preheated via heat exchangers HX1 and HX2, and then fed into the reactor under isothermal conditions at 1 bar pressure to react with NiO solid. The product stream contains
3
SECLR process
A SECLR system (Fig. 3) was also simulated using the Aspen Plus programme. The simulated system consists of a fuel reactor, FUEL-REA, a calcination reactor, CALCINA, for release of adsorbed CO2 , an air reactor, AIR-REA, cyclones, CYCLONE1 and CYCLONE2, as well as heat exchangers, HX1, HX2 and HX3. The reactant stream, CH4 + H2O, containing 1 mol of CH4 for every 2 mol of H2 O, at 35 ◦ C at atmospheric pressure, is preheated via heat exchangers HX1, HX2 and HX3 and then fed into the reactor under isothermal conditions and a pressure of 1 bar. The major product is hydrogen, but other species in the product stream include CH4 , H2 O, CO, CaCO3 , and Ni. All of the products are transported to a cyclone, CYCLONE1, to separate the Ni and CaCO3 solids from the gases. The H2 -rich gas stream passes through heat exchanger, HX1, where it cools to 150 ◦ C and exchanges heat with the fresh reactant stream, CH4 + H2O, which increases in temperature, becoming stream REACTANT1. The separated solids, stream NI + CACO3, are transferred to the calcination reactor, CALCINA, which is operated at 880 ◦ C and 1 bar, where CaCO3 is calcined into CaO releasing CO2 . The CO2 gas is subsequently separated from the solids using cyclone CYCLONE2. The CO2 is passed through a heat exchanger, HX2, where it exchanges heat with stream REACTANT1 and cools to 150 ◦ C. The REACTANT1 stream increases in temperature becoming stream REACTANT2. The Ni and CaO in stream NI + CAO, are then transferred to the air reactor, AIR-REA, where the Ni reacts with air under adiabatic conditions, forming NiO. The heat of oxidation results in an increase in temperature of the CaO and NiO due to sensible heat effects, with their temperature typically being over 1000 ◦ C. The gas and solid stream, N2 + SOLID, from reactor AIRREA, is fed to the cyclone, CYCLONE3, where the NiO and CaO are then separated from the nitrogen gas. The solids are subsequently fed to the fuel reactor as stream NIO–CAO. The hot N2 , stream N2, is passed through heat exchanger 3, HX3, exchanging heat with stream REACTANT2 and cooling to
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Fig. 2 – Diagram of chemical looping reforming in Aspen Plus program. 150 ◦ C. REACTANT2 stream is heated to the reactor temperature, to which it is then fed.
2.4.
Energy balance
Energy balances were carried out on each unit. The air reactors and the cyclones are considered to be adiabatic. There is also no shaft work done across the boundaries of these units, but the Ni oxidises, therefore heat is generated. The energy ˙ h, balance for the air reactors simply becomes HAR = m i l i where HAR is the heat of oxidation of the Ni, where i denotes ˙ is the mass flow the stream or a component in a stream, m rate and h is the specific enthalpy. For the heat exchangers, it is considered that heat is only transferred between streams and not to the external environ ment. The energy balance therefore becomes 0 = mh. i i i
The fuel reactor is considered to be diathermal and so the ˙ h − HFR , where Q is the energy balance is given as Q = m i l i heat duty of the reactor and HFR represents the enthalpy of reaction, considering all reactions as one overall reaction (refer to (R1)). One of the objectives of this study is to find optimal operating conditions where Q = 0.
2.5.
Carbon activity
With regards to the effect of operating conditions on carbon formation, this is considered by calculating the activity of pure carbon in the reactor at equilibrium and standard conditions. Under such conditions, if at chemical equilibrium, the carbon would have an activity of one. A calculation of its activity therefore indicates the displacement of the carbon forming
Fig. 3 – Diagram of chemical looping reforming with CO2 sorbent in Aspen Plus program. Please cite this article in press as: Yahom, A., et al., Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.04.002
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Table 1 – Simulation from this study and experimental results from Ryan and Rodus, for equivalent SECLR processes under the same operating conditions. Simulation
Reference
Simulation deviation
mol/hr 0.030 1.328 2.811 0.011 0.008 % 98.7 97.0
% −16.7 −6.25 −6.01 −9.09 −12.5
H2 purity CH4 conversion
mol/hr 0.025 1.245 2.642 0.010 0.007 % 98.44 97.46
CO2
mol/hr 0.900
mol/hr 0.950
Fuel reactor product gas CH4 H2 O H2 CO2 CO
Calciner product gas
reactions from equilibrium and therefore the propensity for carbon formation or consumption. It is assumed that in the CLR of methane, carbon formation only occurs according to the following four equilibrium reactions: 2CO CO2 + C
(C1)
CH4 2H2 + C
(C2)
CO + H2 H2 O + C
(C3)
CO2 + 2H2 2H2 O + C
(C4)
For each of these equilibrium reactions, a corresponding equilibrium constant, K can be written. These equations were written in terms of partial pressures of the corresponding gases and the activity of the solid carbon. As shown in Eqs. (1)–(4), these equations can be rearranged to solve for the carbon activity when the equilibrium constant and partial pressures are known. aC1 = KC1
aC2 = KC2
aC3 = KC3
aC4 = KC4
p2CO
(1)
pCO2 pCH4 p2H
(2)
2
pCO pH2 pH2 O pCO p2H
2O
p2H
(3)
(4)
2O
The term a refers to the carbon activity, K is the equilibrium constant, whilst the subscripts C1, C2, C3 and C4 refer to the equilibrium reaction to which the equation refers to. The partial pressure of the various gaseous components is denoted by p. When a > 1, the chemical reaction is not at equilibrium with a driving force for the formation of carbon. If a = 1, the chemical reaction is at equilibrium with the solid carbon, and if a < 1, a carbon formation is thermodynamically not preferred and any carbon in the system is consumed by this reaction.
2.6.
Simulation validation
The SECLR process simulated in this study is based on that developed by Ryden and Ramos (Rydén and Ramos, 2012), and was run with operating conditions taken from this same study.
−0.30 0.52 −5.26
Methane is fed into the reactor at 1 mol per 2.2 mol of steam, where the steam is at 312 ◦ C, and the fuel reactor has a temperature of 580 ◦ C. The calcination reactor was simulated to be at 880 ◦ C and a pressure of 1 bar.
3.
Results and discussion
CLR and SECLR of methane were simulated using Aspen Plus program. The particular interest of this study was the effect of operating conditions such as the H2 O/CH4 ratio and NiO/CH4 ratio on the hydrogen production yield, hydrogen purity, overall energy balance, and propensity for carbon formation.
3.1.
Simulation validation
The simulations were validated by running the SECLR simulation under the same conditions used by Ryden and Ramos (Rydén and Ramos, 2012) and then comparing the composition of the reactor product gas stream. Table 1 shows the experimental results from Ryden and Rodus and the results from this study obtained from the validation of the SECLR simulation. The results show that the hydrogen purity deviates from the experimental results by a negligible amount of −0.30%, whilst the hydrogen yield deviates by −6.01%, which is acceptable for the purpose of this study, which is to compare results from two simulations and identify the range of operating conditions that provide an optimum performance. The CH4 conversion values also match very closely. The large deviations in the CH4 , CO2 and CO flow rates can be accounted for by the small flow rates involved.
3.2.
Conventional chemical looping reforming
3.2.1.
Hydrogen production yield and purity
Fig. 4 shows the change in hydrogen production yield and purity with the H2 O/CH4 ratio at varying temperatures. As expected, the data from the simulation shows an increase in hydrogen production with increasing an amount of steam in the feed, shifting the equilibrium compositions of the steam reformation reaction (R5) and water gas shift reaction (R7) to the right. Likewise, an increase in temperature leads to the equilibrium position of the endothermic reaction, (R5), shifting to the right, as such the hydrogen purity and yield increase with increasing temperature. However, this trend of increasing yield with increasing temperature approaches a limit when the maximum temperature 800 ◦ C is approached, with the hydrogen yield and purities obtained at 700 ◦ C approaching those at 800 ◦ C with increasing steam content. At a H2 O/CH4
Please cite this article in press as: Yahom, A., et al., Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.04.002
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Fig. 4 – Moles of hydrogen produced per mole of methane fed and hydrogen dry mole fraction at various ratios of H2 O/CH4 at different temperatures for chemical looping reforming (NiO/CH4 = 1). ratio of 3, a higher hydrogen production yield is achieved at 700 ◦ C than at 800 ◦ C. The maximum hydrogen production yield of 2.5 moles of H2 per mole of CH4 , achieved at 700 ◦ C and a H2 O/CH4 ratio of 3, is less than the stoichiometric quantity of 3 that would be expected from reactions (R5) and (R6) alone. This limiting hydrogen production yield of 2.5 is a result of a competing reaction, such as the oxidation of Ni by steam. Fig. 4 shows that within the range of H2 O/CH4 ratios simulated, the highest dry hydrogen purities are obtained at 500 ◦ C, 600 ◦ C, 700 ◦ C, and 800 ◦ C, a H2 O/CH4 ratio of 3 and a NiO/CH4 ratio of 1, are 61.25%, 70.78%, 72.06%, and 71.56%, respectively. Likewise, the highest hydrogen production yields under these same conditions are 1.58, 2.42, 2.58 and 2.52 mol of H2 per mole of CH4 . Referring to Fig. 5, the hydrogen production yield and purity apparently decrease with increasing NiO/CH4 ratio as would be expected, with the oxidation reaction (R2) becoming more dominant with an increasing amount of NiO. At low NiO contents and high temperatures, higher hydrogen production
Fig. 6 – Net energy demand for the chemical looping reforming process at various H2 O/CH4 ratios (NiO/CH4 = 1) and various NiO/CH4 ratios (steam/methane = 2). yields are achieved due to the increasing dominance of reaction (R5). At the high temperatures, an upper limit of 3 moles of H2 per mole of CH4 is achieved due to competing shifts in equilibrium i.e. reactions (R5) and (R7). The hydrogen yield is particularly sensitive to temperature at low NiO/CH4 ratios, due to the equilibrium composition of reaction (R5), being shifted further to the left with decreasing temperature. Fig. 5 shows that within the range of NiO/CH4 ratios simulated, with a constant H2 O/CH4 ratio of 2, the highest dry hydrogen purities obtainable at 500 ◦ C, 600 ◦ C, 700 ◦ C, and 800 ◦ C, are 61.2%, 70.8%, 72.1%, and 71.6%, respectively, with a NiO/CH4 ratio of 0. Likewise, the highest hydrogen production yields under these same conditions are 1.58, 2.42, 2.58 and 2.52 mol of H2 per mole of CH4 .
3.2.2.
3.2.3.
Fig. 5 – Moles of hydrogen produced per mole of methane fed into the reactor and the hydrogen mole fraction at various ratios of NiO/CH4 and different temperatures via chemical looping reforming (H2 O/CH4 = 2).
Energy balance
Fig. 6 shows the change in the energy balance of the simulated chemical looping process, either with changing amount of steam and constant amount of NiO, or with various amount of NiO and constant amount of steam. The data shows that with increasing amount of steam and increasing temperature, the energy balance becomes more positive approaching a limiting value of zero, partly due to a decrease in enthalpy of the stream exiting the reactor but also due to an increase in flow rate of steam into the reactor. Likewise, with increasing amount of NiO, the energy balance decreases, which is due to the increasing importance of the exothermic oxidation reactions. Referring to Fig. 6, one can see that a net energy balance of zero can be achieved with a NiO/CH4 ratio and a H2 O/CH4 ratio of approximately 1 and 2, respectively.
Carbon activity
Fig. 7 shows the activity of carbon formed by reactions (C1)–(C4) for varying H2 O/CH4 ratios (NiO/CH4 ratio of 1) as well as varying NiO/CH4 ratios (H2 O/CH4 ratio of 2). The carbon activity is used here as a measure of the thermodynamic propensity for carbon formation. The data shows that the carbon activity is particularly sensitive to the H2 O/CH4 ratio with values below 1 leading to thermodynamically unfavourable conditions for carbon formation. With decreasing amounts of steam, carbon forming reactions (C1) and (C2) become increasingly crucial. However, with an excess of steam (H2 O/CH4 ratio
Please cite this article in press as: Yahom, A., et al., Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.04.002
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Fig. 7 – Activity of carbon formed by reactions (C1)–(C4) in the chemical looping reforming process at 500 ◦ C and 800 ◦ C for (A) various nickel oxide/methane ratios (H2 O/CH4 = 2) as well as (B) various H2 O/CH4 ratios (NiO/CH4 = 1). of 2), as the amount of NiO decreases and the degree of partial oxidation increases, the formation of carbon via reactions (C1) and (C3), is hindered by the water gas shift reaction, converting CO to CO2 . Due to an excess of steam, the equilibria of reactions (C2) and (C4), are shifted further to the left, thus reducing the formation of carbon via these reactions.
3.3.
Sorption enhanced chemical looping reforming
The effects of the H2 O/CH4 ratio and NiO/CH4 ratio on hydrogen production, hydrogen purity, net energy balance and carbon activity were determined to find an appropriate condition for hydrogen production via SECLR.
3.3.1.
Hydrogen production yield and purity
Fig. 8 shows simulation results representing the change in hydrogen production yield and purity with varying H2 O/CH4 ratios (NiO/CH4 ratio of 1) and varying temperatures. Contrary to the results obtained for the simulated CLR process – where the highest hydrogen production yields were obtained
Fig. 8 – Moles of hydrogen per mole of methane and the dry hydrogen mole fraction at various ratios of H2 O/CH4 and different temperatures for the sorption enhanced chemical looping reforming process (NiO/CH4 = 1 and CaO/CH4 = 1).
7
Fig. 9 – Moles of hydrogen per mole of methane and the dry hydrogen mole fraction at various NiO/CH4 ratios and different temperatures for the sorption enhanced chemical looping reforming process (H2 O/CH4 = 2 and CaO/CH4 = 1).
at the highest operating temperatures – in the presence of CO2 adsorption, the highest production yields and purities are obtained at 500 ◦ C and 600 ◦ C. This is a result of the equilibrium reaction (R9), which involves the exothermic adsorption of CO2 by CaO. The equilibrium composition of this reaction shifts to the right at lower temperatures, which in turn promotes the formation of hydrogen via reactions (R5), (R6) and (R7). Referring to Fig. 8, at H2 O/CH4 ratios lower than one, reactions (R2)–(R4) play a more important role in the formation of hydrogen. This is indicated by the higher hydrogen yields at higher temperatures, in which case the hydrogen production is dominated by either partial oxidation reactions ((R3) and (R4)), which produce hydrogen directly, or by the complete oxidation reaction (R2), where the water product can be used to produce hydrogen via reactions (R5) and (R6). Fig. 8 shows that within the range of H2 O/CH4 ratios simulated, with a constant NiO/CH4 ratio of 1, and CaO/CH4 ratio of 1, the highest dry hydrogen purities obtainable at 500 ◦ C, 600 ◦ C, 700 ◦ C, and 800 ◦ C, are 99.5%, 98.5%, 84.5%, and 71.5%, respectively, with a H2 O/CH4 ratio of 3. Likewise the highest hydrogen production yields under these same conditions are 2.95, 2.94, 2.76 and 2.52 moles of H2 per mole of CH4 . Fig. 9 shows the change in hydrogen production yield and purity with varying NiO/CH4 ratio (H2 O/CH4 ratio of 2) and varying temperatures. As would be expected in light of the equation for the exothermic reaction (R9), for high NiO contents, much higher hydrogen purities are obtained at lower temperatures due to the equilibrium composition of this reaction, being shifted further to the right at lower temperatures. At lower NiO/CH4 ratios, the variation in hydrogen purity and yield with temperature is much smaller due to the promotion of the steam reformation reaction (R5) at higher temperatures, balanced by the promotion of reactions (R5) and (R7) at lower temperatures, where adsorption of CO2 is thermodynamically favoured. Fig. 9 shows that within the range of NiO/CH4 ratios simulated, with a constant H2 O/CH4 ratio of 2 and a CaO/CH4 ratio of 1, the highest dry hydrogen purities obtainable at 500 ◦ C, 600 ◦ C, 700 ◦ C, and 800 ◦ C, are 99.9%, 98.1%, 78.4%, and 44.8%, respectively. Likewise, the highest hydrogen production yields
Please cite this article in press as: Yahom, A., et al., Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.04.002
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Fig. 10 – Amount of hydrogen produced per mole of methane feed at various ratios of calcium oxide/methane and different temperature via sorbent enhanced chemical looping reforming (net energy requirement = 0, H2 O/CH4 ratio is constant). under these same conditions are 2.94, 3.00, 3.07 and 3.19 mol of H2 per mole of CH4 . The NiO/CH4 ratios corresponding to these results range between 0 and 0.25. Fig. 10 shows the hydrogen yield and purities obtained at various CaO/CH4 ratios and temperatures. The chart shows that for high calcium oxide contents, the optimum hydrogen yield is achieved at 600 ◦ C, whereas for the highest hydrogen purity, the operating temperature should be 500 ◦ C. However at lower calcium oxide contents, the partial oxidation reactions play a more important role in the production of hydrogen – as such the highest hydrogen yields and hydrogen purities are achieved at a 700 ◦ C.
3.3.2.
Energy balance
Fig. 11 shows the change in energy balance of the simulated sorption enhanced chemical looping process, both in terms of varying H2 O/CH4 ratio (NiO/CH4 ratio = 1) and in terms of varying NiO/CH4 ratios (H2 O/CH4 ratio = 2). Similar results are obtained with those obtained from the CLR process. Referring to Fig. 11, a net energy balance of zero can be achieved at
Fig. 11 – Net energy demand for the sorption enhanced chemical looping reforming with various steam/methane ratios (NiO/CH4 = 1) and various NiO/CH4 ratios (H2 O/CH4 = 1) for CaO/CH4 = 1.
Fig. 12 – Activity of carbon formed by reactions (C1)–(C4) in the sorbent enhanced chemical looping reforming process at 500 ◦ C and 800 ◦ C and (A) various NiO/CH4 ratios (H2 O/CH4 = 2) as well as (B) various H2 O/CH4 ratios (NiO/CH4 = 1) for CaO/CH4 = 1. 600 ◦ C, with a NiO/CH4 ratio and a H2 O/CH4 ratio of approximately 0.75 and 2, respectively.
3.3.3.
Carbon activity
Fig. 11 shows the activity of carbon formed by reactions (C1)–(C4), at various H2 O/CH4 ratios (NiO/CH4 ratio of 1) and various NiO/CH4 ratios (H2 O/CH4 ratio of 2) for the SECLR process. Over the full range of conditions simulated, the carbon activity remains below 0.6 indicating that under these conditions there is no thermodynamic drive for the formation of carbon which would otherwise lead to a deactivation of the catalyst. As with the results obtained for the CLR simulations, the carbon activities are the greatest at low H2 O/CH4 ratios with the carbon activity being particularly sensitive to this parameter (see Fig. 12).
3.3.4.
Comparison of optimum operating conditions
Both CLR and SECLR were simulated assuming Gibbs equilibrium conditions. The objective is to determine the optimum range of operating conditions that provide a near zero energy balance and without the formation of carbon leading to the deactivation of carbon. As a result, the chemical looping process approximates the optimum reactor operating conditions to be at 800 ◦ C, a H2 O/CH4 ratio of 3 and a NiO/CH4 ratio of 1, leading to an approximate hydrogen production yield of 2.5 moles of H2 per mole of CH4 and an approximate hydrogen purity of 75%. In comparison, the simulation results show that with the application of in situ CO2 adsorption, hydrogen purities approaching 100% can be achieved with a NiO/CH4 ratio ≥ 1, a CO/CH4 ratio ≥ 1, a H2 O/CH4 ratio ≥ 2 and a temperature between 500 ◦ C and 600 ◦ C. For optimum hydrogen yields, the same conditions apply except for the NiO/CH4 ratio for which any increase above unity would result in a decrease in hydrogen yield. Under optimum conditions, a hydrogen purity > 90% and a yield within the region of 3 moles of H2 per mole of CH4 can be achieved, indicating that the implementation of in situ CO2 adsorption could potentially bring about significant improvements in both yield and purity of hydrogen. Other than the clear improvements in hydrogen production yield and purity that can be achieved under Gibbs equilibrium
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conditions, another significant advantage of the sorption enhanced process is the lower range in optimum operating temperatures. At temperatures within the range of 500 ◦ C and 600 ◦ C, the sintering problem of the catalyst will be significantly suppressed, therefore reducing its degradation. The dramatic reduction in the carbon activity across the full range of simulated conditions, also indicates that deactivation of the catalyst due to the formation carbon, is essentially eliminated. It should be noted that in this study the gas product compositions were calculated using the Gibbs minimisation method. In real operation, the selection of metal oxides is an important issue as they influence both the reaction selectivity and reactivity which directly determine the reaction performance.
4.
Conclusions
A comparison between CLR with SECLR simulations operated using the Gibbs minimisation method, show that in situ CO2 adsorption leads to a significant increase in hydrogen yield and purity, coupled with a drop in the operating temperature required to achieve these higher yields and purities. This drop in operating temperature offers the added advantage of improved stability of the catalyst including a decrease in the impact of sintering on the catalyst performance. The calculated carbon activities for these simulated processes also show that, with in situ CO2 adsorption (CaO/CH4 = 1), there is no thermodynamic drive for the formation of carbon, therefore limiting the deactivation of the catalyst due to carbon formation.
Acknowledgement The authors gratefully acknowledge the support from the Ratchadaphiseksomphot Endowment Fund from Chulalongkorn University (RES560530168-EN).
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Please cite this article in press as: Yahom, A., et al., Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.04.002