right304927000In a 21st century, the biggest threat earth is facing, the climate change in the form of global warming. As reported by IPCC, in the instrumental record of the global surface temperature, the last three decades (1983-2012) are the warmest year since 1850 1. In the last 50 years, the major contribution to global warming is the human activities such as excessive use of fossil fuel and coal. As a consequence of these activities the gases emitted to atmosphere is the so-called greenhouse gases (GHGs). The GHGs mainly consist up of CH4, CO2, N2O, chloroflouro carbons (CFC’s), H2O and SF6 2. The highest contribution in GHGs is from CO2 (78%) 3. In the key world energy statistics 2018 by International Energy Agency (IEA) reported that, in last 4 decades, the Total Fuel Consumption (TFC) is amplified by 48% and resulted in CO2 emission increase by 47%. figure 1 shows the CO2 emissions and fuel share of TFC 4.
The increase in energy demand, depletion in fossil fuel reserves and environmental threats in the form of global warming, pull researchers to work on alternative and eco-friendlier energy sources 5. The researches have done on fuels like biodiesel, CNG, LPG and hydrogen (H2). The H2 is marked as most suitable alternative and said to be the fuel of the future. As a fuel H2 is clean in its nature as it does not emit harmful products upon burning. The only byproduct is water vapors 6,7. H2 can also be used as energy carrier for both domestic and industrial purposes 8. Largely, H2 is consumed in petroleum refining and to produce fertilizers. The H2 produces higher amount of energy on mass basis than any other fuel such as gasoline, coal and methane (CH4) 9.
Currently, H2 is being produced from different methods such as reforming, gasification, electrolysis and pyrolysis 10. At present, 90% of world’s H2 is produced from fossil fuels such as crude oil, Natural gas (NG) and coal. Among these sources, NG is most important source to produce H2 11, 12.
The processes available to produce H2 from NG are (1) Steam methane reforming (SMR), (2) Auto-thermal reforming (ATR) and (3) Partial oxidation (POx). The low price and commercially castoff process to produce H2 is steam methane reforming (SMR). Almost 50 % of world’s total H2 is produced from SMR 9, 13. The SMR process has two main steps, the first sept (R1) includes conversion of CH4 to H2 at moderate pressure (20-35 atm) and high temperature (800-1000 ?C). The second step (R2) includes water gas shift (WGS) reaction 14.
CH4(g) + H2O(g) ? CO(g) +3H2(g) ?H298 K = +206 kJmol-1 (R1)
CO(g) + H2O(g) ? CO2(g) + H2(g) ?H298 K= -41 KJmol-1 (R2)
The overall SMR reaction is:
CH4 (g) + 2H2O(g) ? CO2(g) + 4H2(g) ?H298 K = +165 kJmol-1 (R3)
As clear from (R3), the overall SMR reaction is highly endothermic (+165 kJmol-1) and required heat from outside. The efficiency of SMR process is depend upon catalyst, or on oxygen carrier material (OTM). The properties of OTM, such as high reactivity with CH4, high selectivity and stability are play important role in the selection of OTM 9. The common OTM has reactivity order with CH4 is NiO ; CuO ; Mn2O3 ; Fe2O3 15. Commonly metallic nickel is used in SMR as a catalyst and nickel-based oxides are treated as most favorable OTM. With time SMR process become expensive and less effective. Due to very high temperature, life period of the furnace tube is reduced from 11.4 to 2 years in SMR process 16. To overcome this high temperature, issue a new technique was introduced in 2000 called chemical looping reforming (CLR) 17. The process of transporting oxygen is discussed as chemical looping. In SMR reactions metal is reduced, to start new cycle of chemical looping metal should be oxidized. During this process OTM and two reactors are used. Two reactors are fuel reactor (FR) and air reactor (AR). The reaction taking place in them for chemical looping steam methane reforming(CL-SMR) are given below 9.
Direct partial oxidation with metal oxide:
CH4(g) + NiO(s) ? CO(g) +2H2(g) + Ni(s) ?H1200K = 213 kJ mol-1 (R4)
CO(g) + NiO(s) ? CO2(g) +Ni(s) ?H1200K= -48 kJ mol-1 (R5)
H2(g) + NiO(s) ? H2O(g) +Ni(s) ?H1200K = -13 kJ mol-1 (R6)
The WGS and reforming reactions:
CH4(g) + H2O(g) ? CO(g) + 3H2(g) ?H1200K = 228 kJ mol-1 (R7)
CO(g) + H2O(g) ? CO2(g) + H2(g) ?H1200K = -33 kJ mol-1 (R8)
The oxidation of metal:
Ni(s) + 1/2O2(g) ? NiO(s) ?H1200K = -468 kJ mol-1 (R9)
The overall reaction of CL- SMR is:
CH4(g) + H2O(g) + 1/2O2(g) ? CO2(g) + H2(g) (R10)
The advantages of CL-SMR over SMR are (1) for the reforming reaction no external head is required; (2) As no external combustion is involved so no CO2 emission externally; (3) less amount of catalyst and steam is required per unit of fuel feed; (4) No big concerns for sulfur contaminants and no thermal NOx is formed 18, 19.
As 90% of H2 fuel is produced by fossil fuels resulting in the emission of GHGs that disturb our environment. Researchers go for developing a new method to produce clean H2 fuel 20. A method that gain attention is sorption-enhanced steam methane reforming (SE-SMR) as of its hybrid nature of capturing CO2 and reforming. CO2 is captured by means of sorbent along with catalyst in this process. In this process reaction equilibrium is shifted to product side and H2 yield is increased (;90%). The other advantage of SE-SMR is lower reaction temperature (732-873 ?C) then SMR process. The CO2 capturing reaction (R11) in SE-SMR, when CaO is used as sorbent is mention below 21.
CaO(s) + CO2(g) ? CaCO3(s) ?H298 K = -178 kJmol-1 (R11)
The key disadvantage of SE-SMR is sorbent regeneration required high temperature conditions. To overcome this temperature condition and make process more eco-friendly SE-SMR is coupled with CLR. The process is called sorption-enhanced chemical looping steam reforming (SE-CLSR). In this process the heat produced by carbonation of CaO is used by reforming reaction. In the regeneration step, heat required for the CaCO3 decomposition into CaO and CO2 (R12) is provided by the oxidization of the reduced metal (R9) 22.
CaCO3(s) ? CaO(s) + CO2(g) ?H298 K = 178 kJmol-1 (R12)
In this process 95% pure H2 is produced in reformer reactor and total reformer reactor operates under thermos-natural conditions 23. In SE-SMR and SE-CLSR one of the most important part is sorbent selection. The selection criteria for selection are stability, high CO2 capturing capacity, inert for catalyst, high sorption rate and easy regeneration. Researchers performed many experiment for selection of the sorbent. The most highlighted sorbents are CaO, Li2ZrO3, KLiZrO3, Li4SiO4, MgO and Na2ZrO3. Among these CaO is the most promising sorbent for SE-CLSR process 6, 24, 25.
Development of the two-dimensional mathematical model of SE-CLSR process using NiO/Al2O3 as catalyst and CaO as sorbent in an adiabatic packed bed reactor.
To examine the effect of different operational conditions of pressure, temperature, gas mass flow velocity (Gs) and steam to carbon ratio(S/C) on the performances of SE-CLSR process.
Comparison of modelling results by an independent equilibrium-based software i.e. chemical equilibrium with applications (CEA) in terms of H2 purity, H2 yield (wt. % of CH4) and CH4 conversion.
According to US energy department 50% of global H2 is produced by NG and the main SMR process used for this production is 75% efficient. That’s why not ending research is carried out to improve this process 8.
At first in 18th century, Akers et al. 26 presented the first kinetic study of SMR for H2 production over Ni catalyst and temperature range of 609-911K. they presented that the reaction rate of H2O and CH4 was first ordered. The product of this reaction was CO2 and CO. The formation rate of CO2 is far higher than CO 27. Ross et al. 28 worked on the kinetic of SMR over Ni/Al2O3, temperature array of 773-953K and pressure array of 0-10 torr. They presented that rate decisive step of SMR is adsorption of CH4 on catalyst surface and CH4 compete with H2O for catalyst’s active site. Van Hook et al. 29 had huge amount of work on the kinetic study of SMR over temperature array of 533-1200K, catalyst activities (200,000-folds) and pressure array of 0.01-50 atm.
Wide-ranging work had been done in 19th century for the improvement of the SMR process technology. One of the most promising improvement is the concept of chemical looping technology (CL). In early 1950s, Lewis et al. 30, 31 presented the concept of chemical looping for CO2 and syngas manufacture over copper and iron based OTM from carbon-based fuels. They also presented the concept of solid circulation in two interconnected fluidized bed reactor (FBR). This concept is still used in chemical looping combustion (CLC). Later, Richter et al. 32 establish the principle of CLC. They increased the efficiency of power plant by using metal oxides (CuO and NiO) as OTM in interconnected FBR system.
Mattisson et al. 33 was first to introduce the concept of CLR. The product of CLR is H2 instead of heat, as CLR works on the same principle of CLC. Later Mattisson et al. 34 observed the reactivity of CH4 with various metal oxides such as NiO, CuO, Fe2O3 and Mn2O3 supported on SiO3 base in laboratory. Their research work also explains oxygen fraction (not more than 0.3 of total oxygen) in steam to maintain high temperature and CH4 conversion. Zafar et al. 35 experimented different metal oxides (NiO, CuO, Fe2O3 and Mn2O3) on SiO2 and MgAl2O4 support in laboratory for FBR. They examined that MgAl2O4 has high reactivity than SiO2 during redox reactions.
Ryden et al. 36, 37 has done a remarkable work on H2 production. They studied pressurized and atmospheric CLR processes. They found that pressurized process is 5% more efficient in reducing energy requirement for H2 compression than atmospheric process. They also experimented Fe2O3/MgAl2O3 as OTM with NiO as traces on it. They observed 1% addition of NiO on OTM increases selectivity and reactivity of desired reaction. Johansson et al. 38 observed CLR process in continuous fashion, using two different Ni-based OTM, having different support NiAl2O3 and MgAl2O4. They found that using NiO/MgAl2O3 favors higher fuel(CH4) conversion and less propensity for carbon formation. Diego et al. 39 has also examined different supports such as ?-Al2O3, ?-Al2O3, ?-Al2O3 for NiO. They determined that OTM braced on ?-Al2O3 has high reactivity then other during reduction reactions. They also examined that the decrease in carbon deposition during reduction reaction with the increase in temperature and steam to carbon ratio (H2O/CH4). Ryden et al. 40, 41, 42 tried various supports in 500W continuous unit such as ?-Al2O3, ?-Al2O3, ZrO2-MgO and MgAl2O4 for Ni-based OTM. They reached the complete conversion of CH4 and high selectivity of H2 and CO in all units.
Diego et al. 39 tested the 900W unit CLR process over NiO/?-Al2O3 and NiO/ ?-Al2O3. They examined the process by varying different operational variables such as H2O/CH4 ratio (0- 0.5), FR temperature (800-900 ?C) and solid circulation rate. Proll et al. 18 tested 140kW pilot plant using CLR process over NiO/NiAL2O4+MgO. They investigated the process at temperature array of 750 – 900 ?C. All the work stated above is done on atmospheric pressure. Ortiz et al. 43 tested pressurized (upto 10 bar) CLR process in 900W unit with same OTMs formerly studied at atmospheric pressure by Diego et al. 39. The results were same as stated by Diego et al. 39.
In all these studies, it is observed that Ni-based OTM with Al2O3 or MgAl2O3 support has high reactivity, longer life and most suitable for CLR processes 2. As production of H2 is higher with CLR process but the issue of CO2 emissions is not addressed. To address this issue researchers, move towards another process called sorption-enhanced steam methane reforming (SE-SMR).
A lot of theoretical work is done on SE-SMR in literature. Rostrop 44first presented the idea of hydrocarbon conversion in the prsences of steam and Ca-basedsorbent. Han et al. 45 studied the synchronized process of H2 production and in suit CO2 removal using dolomite as sorbent in fixed bed reactor. They also analyzed the effect of CO2 removal on temperature, pressure, synthesis gas composition and space velocity of gas stream in SMR process. Ding et al. 27 examined the SE-SMR over Ni-based catalyst along with hydrotalcite adsorbent. They stated that the overall CH4 conversion is increased with in-suit CO2 capturing due to equilibrium shift. Dou et al. 46 examined SE-SR of glycerol in continues FBR using dolomite for CO2 capture at atmospheric pressure and temperature array of 400-700 ?C. They recommended the optimum temperature of ~500 ?C for attaining maximum H2 purity. Silva et al. 47 studied the effect of temperature and H2O/CH4 ratio in convention stem reforming (SR) and SE-SR process of oxygenated hydrocarbon over CaO sorbent. They found that H2O/CH4 ratio was different for different hydrocarbons, but the optimum temperature was ~973 K to attain high purity of H2 in conventional SR. While in SE-SR relatively high purity of H2 (;97%) was attained at relativity low temperature array of 723-873 K, with trace amount of CH4, CO and CO2. Chanburanasiri et al. 48 established the multifunctional catalyst for SE-SMR and studied that possibilities to improve process. They proposed that the use of multifunctional catalyst eliminate the need for catalyst support. Dou et al. 49 studied the SE-SR of glycerol over Ni-based catalyst and CaO sorbent in temperature array of 500- 600 ?C with synchronized regeneration of sorbent and catalyst. The founded that continuous reaction and regeneration of catalyst/sorbent did not affect their activity at 500 ?C and 600 ?C, the attainted H2 purity is 93.9% and 96.1% respectively. Wang et al. 50 also studied SE-SR of glycerol with different composition of catalyst (Ni-based) and sorbent (CaO). Their result suggested that the catalyst/sorbent composition of NiO 41.21 wt%, Al2O3 28.02 wt% and CaO 30.77 wt% showed high capacity and sorption rate for CO2. Radfarnia et al. 51 studied SE-SMR with Al-stabilized CaO-nickel catalyst as NiO support. Their found that 25 wt% of NiO over Al-stabilized CaO-nickel catalyst support give best results. Xu et al. 52 examined the SE-SMR over Ni/CaO-Ca5Al6O14 powder catalyst. Their work stated that catalyst display stability with high CaO efficiency. The catalytic stability and activity did not display much deterioration even after 20 cycles at steam to carbon ratio (S/C) of 2 and 923 K.
Later different studied shows that the regeneration of sorbent in SE process required high amount of heat. To address this issue researchers coupled the sorption-enhanced process with CL H2 production process, most promising of them is Sorption-enhanced chemical looping steam reforming (SE-CLSR).
Ryden et al. 23 first proposed that innovative process for H2 production by using three interconnected fluidized bed reactor (1) reforming reactor (2) calcination reactor and (3) air reactor. They observed that overall reformer reactor is operated under thermo-neutral condition and achieved 95%+ pure H2. The heat for the renewal of sorbent is provided by the oxidization of OTM. Pimenidou et al. 53 examined the SE-CLSR process by using waste cooking oils fuel in packed bed reactor. They used 18 wt% NiO supported on ?-Al2O3 and dolomite as CO2 sorbent. They found batter conversion of fuel then without using sorbent.
At present when computation analysis is become an essential part of researches. Modelling and simulation serves as most powerful tool to analyze the different process on various operating condition to find best optimum condition. In literature many mathematical models and simulations are available for SMR, CLR, SE process on several operating conditions and catalyst compositions. Xu et al. 54 established the model and determined the rate equation for SMR process over NiO/MgAl2O3 catalyst which still serves as the base model for research work. Halabi et al. 54 established the model 1-D model of adiabatic FBR for analyzing the performances of auto thermal reforming (ATR) process. Zhou et al. 55 developed the 1-D model of PFR for CLC process using NiO as catalyst. They analyze the reduction behavior using isothermal and isobaric process conditions. Ghous et al. 56 developed the 2-D model heterogenous model of SMR process. They supposed the complete mixing of the species and on carbon deposition. They used equation of mass and energy transfer in solid, gas and within pellets. J. Morgado et al. 57 developed the model for FBR to compare gas switching reforming (GSR) and CLR process. This model presents the state of the art closure for bubbling, turbulent and fast fluidization regime. Simulation were used to examine the degree of OTM utilization as an important process variable. Singhal et al. 58 projected the multiscale model for CLR using packed bed reactor. They offered the comparison of reactive flows on two different scales (1) 1-D packed bed model and (2) particle resolved direct numerical simulation. Fernandez et al. 59 developed the dynamic pseudo-heterogenous model for SE-SMR in FBR over CaO as sorbent in adiabatic conditions. They proposed the conditions of temperature (923 K), pressure (3.5 MPa), S/C (5) and space velocity (SV) (3.5 km/m2) that formed 95% pure H2 with 85% CH4 conversion. Diglio et al. 60 developed the 1-D model for numerical investigation of SE-SMR in network of FBRs under isothermal conditions. S. Z. Abbas et al. 24 developed the model for SE-SMR in packed bed reactor to produce H2. The simulate the model at different operating condition and predicted the behaviors of operating conditions on H2 yield and purity.
Ryden et al. 23 first developed the model for SE-CLSR in fluidized bed reactor taking NiO as catalyst and CaO as sorbent by using ASPEN PLUS. They simulate the model on various operating conditions and proposed the best one. Most recently. S. Z. Abbas et al. 3 developed the 1-D model of SE-CLSR of methane in an adiabatic packed bed reactor. They performed the thermodynamic analysis of the process. Their results show that the SE-CLSR process gives significantly higher purity and yield of H2 among any other SMR process.
Their work should provide the base for this research work. As this work is aiming to develop the 2-D heterogenous model of SE-CLSR of methane over nickel-based catalyst and CaO as sorbent and simulate the model for different operating conditions of temperature, pressure, S/C, and gas velocity.
right37084000The methodology adopteded for research work is as follow:
Propose the optimum condition for SE-CLSR process over Ni/CaO-Al2O3 for both laboratory and industrial scale process.
This work will be a very helpful addition in the ongoing work of SE-CLSR process. Later, this work will be submitted in an international journal of hydrogen energy with the title “Two dimensional mathematical model of Sorption-enhanced chemical looping steam reforming of methane over 18 wt % Ni/?-Al2O3 in an adiabatic packed bed reactor” for the publication.
038045400The research follow the schedule: