Research Article | Open Access
Chemical Looping Combustion of Methane: A Technology Development View
Methane is a reliable and an abundantly available energy source occurring in nature as natural gas, biogas, landfill gas, and so forth. Clean energy generation using methane can be accomplished by using chemical looping combustion. This theoretical study for chemical looping combustion of methane was done to consider some key technology development points to help the process engineer choose the right oxygen carrier and process conditions. Combined maximum product (H2O + CO2) generation, weight of the oxygen carrier, net enthalpy of CLC process, byproduct formation, CO2 emission from the air reactor, and net energy obtainable per unit weight (gram) of oxygen carrier in chemical looping combustion can be important parameters for CLC operation. Carbon formed in the fuel reactor was oxidised in the air reactor and that increased the net energy obtainable from the CLC process but resulted in CO2 emission from the air reactor. Use of CaSO4 as oxygen carrier generated maximum energy (−5.3657 kJ, 800°C) per gram of oxygen carrier used in the CLC process and was found to be the best oxygen carrier for methane CLC. Such a model study can be useful to identify the potential oxygen carriers for different fuel CLC systems.
Energy demand and thus energy generation are ever increasing in many parts of the world. Carbon-based fossil fuels are the main source of energy for combustion reactions. However, the major product of energy generation using fossil fuels by combustion is CO2, and growing CO2 pollution has become a matter of serious concern amongst developed as well as developing countries. CO2 is mainly emitted from energy sector that uses coal, oil, and natural gas for combustion to generate energy for different purposes using air as an oxygen source. Although CO2 capture and sequestration is being projected as a potential option to control GHG emissions, CO2 capture technologies in-vogue are beset with several limitations including cost and energy penalty [1, 2]. Hence, these flue gases are directly vented to the atmosphere without CO2 separation and became responsible for environmental impacts of energy generation from fossil fuels, for example, global warming and climate change phenomena [3–5]. In 2010, CO2 emissions have already increased to 389.0 ppm and the burning of fossil fuels is one of the main causes for the same as reported by World Meteorological Organization . This has promoted the development of clean energy technologies as a major research area worldwide. Chemical looping combustion (CLC) is an emerging clean energy technology to generate energy using abundantly available fuels and also provides sequestration ready CO2 stream [7, 8]. CLC uses a solid oxygen carrier (OC) to oxidize the carbon and hydrogen in the fossil fuel to CO2 and H2O in an endothermic fuel reactor. The reduced OC is regenerated by air oxidation in the exothermic air reactor [9, 10] in the next step. Both reactors are interconnected and operate simultaneously. The energy released in the CLC system is considered to be of similar magnitude as direct combustion but with a crucial advantage of having separate CO2 stream [11, 12]. The pure CO2 steam can be captured/sequestered easily, thereby reducing CO2 emissions to atmosphere .
Development of chemical looping combustion technology requires the relatively cheap and long term availability of fuel and oxygen carrier. Fossil fuel such as methane is abundantly available as natural gas in many countries [14–21]. It is also available as biogas [22–25], landfill gas [26–31], gas hydrates [32–35], and coal bed methane  and can be generated from other organic wastes [37–40], and it is a major source of energy by combustion worldwide . The selection of a suitable oxygen carrier (OC) for methane CLC is one the most important aspects for successful technology development study. Apart from the low-cost and abundant availability criteria, the OC must have suitable reactivity with the fuel, easy reducibility, and regenerability under CLC conditions in addition to some other essential properties such as high melting point, attrition resistance, mechanical stability, negligible loses during operation, negligible toxicity, and environmental impacts. The suitability of the oxygen carrier for a fuel can be determined by trial experiments and also by theoretical studies. Some theoretical (thermodynamic) studies of chemical looping combustion have already been reported [42–44], while experimental research studies of chemical looping combustion using methane as fuel with different oxygen carriers have also been published. Mattisson et al.  have proposed kinetic data of a promising oxygen carrier consisting NiO/NiAl2O4 from experiments in a small fluidized-bed batch reactor using methane as fuel. Corbella and Palacios  have experimentally tested titania-supported iron oxides with different iron loadings in a fixed-bed quartz reactor in multicycle tests in a fixed-bed reactor for methane CLC. Johansson et al.  have experimentally studied manganese oxides on pure zirconia and zirconia stabilized by CaO, MgO, or CeO2 as oxygen carrier in a laboratory fluidized-bed reactor of quartz for CLC of methane. Rydén et al.  have experimentally investigated red iron oxide, brown iron oxide and iron oxide scales and NiO-based particles as oxygen carrier in a batch fluidized-bed reactor in CLC of methane. Forero et al.  have experimentally evaluated Cu-based oxygen carrier in a continuous CLC plant operating with methane and also H2S as fuel. Kuusik et al.  have studied the NiO-based oxygen carriers under oxidising and reducing conditions using ceramic fluidized-bed furnace at high temperatures in CLC of methane. Zafar et al.  have proposed reduction and oxidation kinetics of Mn3O4/Mg = ZrO2 oxygen carrier particles in CLC of methane. Abad et al.  have studied CLC of methane with CuO impregnated on alumina and developed mathematical model for fluidized bed. Song et al.  have studied the chemical-looping combustion of methane with calcium sulfate as a novel oxygen carrier in a laboratory scale fixed-bed reactor.
These studies are helpful to understand the kinetics/reactivity in CLC of methane with different oxygen carriers. However, a theoretical study about CLC with a focus on some important parameters for technology development view is not yet published in the literature to the best of our knowledge. The present study therefore aimed to determine theoretical aspects that can help the technology development criteria for CLC of methane. This study considers the key technology development issues in CLC of methane to evaluate suitability of the oxygen carriers based on other important process parameters such as net desired product generation, net energy obtainable from the system, byproduct generation, fuel conversion, relative weight of the oxygen carriers, effect of pressure and amount of oxygen carrier, and air requirement in oxidation reactor, which are vital parameters to be considered for CLC process design and its practical exploitation.
Technology development requires the use of low-cost, easily available oxygen carrier that can be tested rigorously and repeatedly during experimentation and must have low weight to enable fluidization and transport at low energy consumption in CLC. A detailed theoretical study is essential before experimentation to reduce the efforts of trial experimentation, which are often quite tedious and with several variables significantly affecting the results. Hence, this theoretical study was limited to low-cost, easily available oxygen carriers, preferably simple inorganic compounds consisting of common oxides and sulphates to enable swift experimentation and put technoeconomic potential and commercialization aspects on fast track. Selection of low-cost and easily available oxygen carriers has been discussed in detail in some studies [54, 55]. Based on a similar analysis, the following oxygen carriers were selected for the present study:
The stoichiometric chemical reactions of the oxygen carrier reduction by methane in the fuel reactor as well as the oxygen carrier regeneration by oxygen (air) in the oxidation reactor are presented in Table 1.
The main reactions for CLC of methane are where Me is a divalent metal.
Some side reactions leading to byproducts may also occur in the fuel reactor and are listed below:
The fuel reactor product stream has a complex composition. Hence, thermodynamic equilibrium calculations were used to obtain the composition of the product stream of the fuel reactor. These calculations generally use Gibbs free energy minimization algorithms [56–59], simultaneous solution of nonlinear reaction equations using MATLAB programs, or equilibrium reactor modules of commercial software like Design II, HYSYS, or Aspen Plus. The simultaneous solution of nonlinear reaction equations based on equilibrium constants might sometimes become stiff and nonsolvable. An alternate procedure to calculate equilibrium that takes account of chemical species only (not chemical reactions) based on minimization of the total Gibbs energy is shown by the expression: It shows that all irreversible processes occurring at some constant (temperature) and (pressure) proceed in a direction to the equilibrium state that has the lowest total Gibbs energy attainable at the given and . The method is based on the set of species and is better than that of independent reactions among the species, as the number and nature of chemical reaction equations are not always known perfectly in a new system. HSC Chemistry version 5.1  has been used for this thermodynamic equilibrium study. It uses the Gibbs free minimization algorithm to find the equilibrium compositions using species and not chemical reaction equations. This software is popular and user friendly . Detailed description about calculation of thermodynamic equilibrium using HSC Chemistry is already discussed [62–65]. The species such as CH4(g), CO2(g), H2O(g), CO(g), H2(g), and C (solid) are considered in this study. Oxygen carriers such as (solid) NiO [66–68], CuO [69, 70], CoO [71, 72], CaSO4 [73–76], and Fe2O3 [77, 78] have been successfully used in chemical-looping studies. Hence, only these oxygen carriers (including hydroxides and carbonates, all solids) along with Na2SO4 and Mn2O3 were considered in this study. Table 2 shows the species (with reduced oxygen carrier species) used for each oxygen carrier.
The input species are methane and oxygen carrier at the particular temperature-pressure condition. No other product-byproduct formations are considered in this study. One mole of methane feed is considered in all the calculations and stoichiometric quantities of oxygen carrier have been used (defined by (2) and (3)). The equilibrium study is limited only to the gas-solid reaction in the fuel reactor. It is assumed that the air reactor conversions are complete due to extremely fast oxidation reactions. The material and energy balance calculations were performed using the inbuilt databases in the HSC Chemistry software package. It is also assumed that the CLC fuel and air reactor operate at the same temperature. The reaction products are assumed to be in thermodynamic equilibrium in the exit of the CLC fuel reactor. The operating temperature range for this study was 600 to 1200°C at 1 bar pressure especially considering methane as fuel. The equilibrium composition data generated by the equilibrium composition module was further used to generate the enthalpy data using the reaction equations module of HSC Chemistry package, for example, input of (CH4(g) + 4NiO) to the equilibrium composition module produced, (0.1554H2(g) + 1.8448H2O(g) + 0.0505CO(g) + 0.9459CO2(g) + 3.7869Ni + 0.2131NiO + 0.0036C) at 700°C. These compositions were compiled in the reaction equation as This reaction was input to the reaction equations module of HSC Chemistry software, which calculated the reaction enthalpy () as 145.69 kJ at 700°C. The data analysis and use of this data for technology development aspects have been discussed in next section.
3. Results and Discussion
3.1. Desired Product Yield
The most desired products in CLC of methane are CO2 and H2O. CLC process conditions to maximise their formation in the CLC fuel reactor are of vital interest for successful technology development. As a preliminary step, the individual CO2 and H2O formation in the CLC fuel reactor was studied and discussed in detail in the next section.
3.1.1. H2O Yield
The maximum possible H2O yield in the CLC fuel reactor for 1 mole methane input is 2 moles of H2O. Figure 1 shows the variation of H2O yield (moles) for CLC of methane fuel reactor within the temperature range from 600 to 1200°C at 1 bar pressure. It was observed that the H2O yield for selected oxygen carriers showed a mixed trend; that is, for some oxygen carriers, the H2O formation increased with increase in temperature, while in other cases, the H2O yield decreased with increase in temperature. From the figure, it was seen that the moles of H2O slightly decreased from 1.85 to 1.83 moles with increase in temperature from 600°C to 1200°C for NiO. The same trend was observed for CoO; that is, it decreased from 1.705 to 1.62 moles. But the H2O yield increased from 1.80 to 1.89 moles up to 850°C then it decreased to 1.81 moles (1200°C) for CaSO4, while the H2O yield increased with increase in temperature (i.e., 1.16 at 600°C to 1.64 at 1200°C) for Na2SO4. It was also observed that the H2O yield first decreased from 0.83 to 0.78 moles up to 750°C and then increased to 1.03 moles for Fe2O3. However, the H2O yield remained almost constant at all temperatures (that was the maximum moles of H2O obtainable) for CuO when compared to all other oxygen carriers (e.g., 1.99 moles at 600°C and 1.99 moles at 1200°C). But the H2O yield decreased from 0.87 to 0.29 moles with increase in temperature from 600 to 1200°C for Mn2O3. The minimum moles of H2O produced were 0.29 moles for Mn2O3 at 1200°C, while the maximum H2O moles produced were 2.00 for CuO at 600°C. The ranking of oxygen carriers with respect to increasing order of moles of H2O produced from methane was found to be as follows: CuO (1.99, 600°C) > CaSO4 (1.89, 850°C) > NiO (1.86, 600°C) > CoO (1.70, 600°C) > Na2SO4 (1.64, 1200°C) > Fe2O3 (1.03, 1200°C) > Mn2O3 (0.87, 600°C).
3.1.2. CO2 Yield
Similarly, the effect of temperature and oxygen carrier on CO2 formation in the CLC fuel reactor was studied. The net CO2 obtainable in the CLC fuel reactor was 1 mole per mole of methane feed. The trends in the CO2 formation with selected oxygen carriers are shown in Figure 2. It was observed that the CO2 yield also showed mixed trends (similar to H2O) for different oxygen carriers with increase in temperature from 600 to 1200°C. The maximum CO2 yield in CLC fuel reactor was 1.00 mole for its reaction with CuO at 600°C, while the minimum CO2 yield of 0.06 moles for Mn2O3 was calculated at 1200°C. The selectivity of the oxygen carriers based on maximum CO2 yield (moles) was analysed and was found to be as follows: CuO (1.00, 600°C) > NiO (0.96, 600°C) > CaSO4 (0.95, 900°C) > CoO (0.88, 600°C) > Na2SO4 (0.71, 1050°C) > Fe2O3 (0.38, 800°C) > Mn2O3 (0.21, 700°C).
3.1.3. (H2O + CO2) Yield
It was clearly seen that the ranking of the oxygen carriers with their suitable operating temperatures was not the same for both major products. Hence, selection of the optimum process conditions was done by using a different parameter. In this case, the combined desired product (H2O and CO2) yield from the CLC fuel reactor is considered as a better parameter than individual yields. The maximum yield of 3 moles (1 mole CO2 + 2 moles H2O) can be obtained from one mole methane in the CLC fuel reactor. The combined yields of H2O and CO2 were plotted in Figure 3 for selected oxygen carriers in the considered temperature range. It was observed that similar to the individual H2O and CO2 yields, the oxygen carriers showed different trends for combined yield (CO2 + H2O) with increase in CLC fuel reactor temperature. It was observed that the (CO2 + H2O) yield for NiO decreased from 2.81 to 2.65 moles with increase in temperature from 600 to 1200°C as reported earlier . Similar trend was observed for Mn2O3 and CoO, where the (CO2 + H2O) yield decreased from 2.59 to 2.25 moles for CoO and from 1.06 moles to 0.35 moles for Mn2O3 with increase in temperature from 600 to 1200°C. The (CO2 + H2O) yield increased from 1.45 to 2.34 moles up to 1100°C, and then it remained almost constant till 1200°C for Na2SO4. The (CO2 + H2O) yield increased with increase in temperature, that is, from 1.04 moles (600°C) to 1.33 moles (1200°C) for Fe2O3, while for CaSO4, the (CO2 + H2O) yield increased from 2.64 to 2.84 moles up to 850°C and then decreased to 2.65 moles at 1200°C. In case of CuO, the (CO2 + H2O) yield remained almost constant at all temperatures, that is, ~3.00 moles. A maximum (CO2 + H2O) yield of 3.00 moles was obtained with CuO at almost all temperatures, while the minimum (CO2 + H2O) yield of 0.35 moles was observed for Mn2O3 at 1200°C. The ranking of oxygen carriers based on the maximum (CO2 + H2O) yield was seen as follows: CuO (2.99, 600°C) > CaSO4 (2.84, 850°C) > NiO (2.81, 600°C) > CoO (2.59, 600°C) > Na2SO4 (2.34, 1100°C) > Fe2O3 (1.33, 1200°C) > Mn2O3 (1.06, 600°C). This combined product criterion for selection of oxygen carriers and process temperature was useful for process operation.
3.2. Undesired Product Formation
Some undesired products are also formed in the CLC fuel reactor due to side reactions and thermodynamic limitations under the CLC conditions. An estimation of the nature and amount of the byproducts is important for technology development and hence was studied in detail.
3.2.1. Syngas (H2 + CO) Formation
Syngas (H2 + CO) is one of the undesired products of the CLC process. The data of syngas formation in the CLC fuel reactor with selected oxygen carriers within the temperature range from 600 to 1200°C was analyzed and is shown in Figure 4. From the figure, it was observed that the syngas formation increased with increase in process temperature from 600 to 1200°C for all oxygen carriers except for Fe2O3, where it was slightly decreased at higher temperature. The syngas formation (moles) increased from 1.13 to 2.64 (Mn2O3), 0.35 to 0.75 (CoO), 0.17 to 0.35 (NiO), 0.16 to 0.49 (Na2SO4), 0.05 to 0.29 (CaSO4), and 0.00 to 0.01 moles (CuO) with increase in temperature from 600 to 1200°C. But it was also seen that the syngas formation increased from 1.16 to 1.75 moles up to 900°C and then slightly decreased to 1.67 moles at 1200°C in the case of Fe2O3. CuO produced negligible syngas when compared to all other oxygen carriers at all temperatures. The minimum syngas moles produced were negligible for CuO, while the maximum syngas of 2.64 moles was produced by Mn2O3 at 1200°C in this study. The ranking of the oxygen carriers based on minimum syngas production (moles) in the CLC fuel reactor was observed as follows: CuO (0.00) > CaSO4 (0.05) > Na2SO4 (0.16) > NiO (0.17) > CoO (0.35) > Mn2O3 (1.13) > Fe2O3 (1.16) at 600°C.
3.2.2. Carbon Formation
Carbon formation is undesirable in the CLC process as coking may reduce the activity of the solid oxygen carriers. Figure 5 shows the carbon formation in the CLC fuel reactor for methane fuel within the temperature range of 600°C to 1200°C at 1 bar pressure. From the figure, it was observed that the carbon formation was higher at lower temperatures (till ~700°C) due to carbon formation side reactions (5), (6), and (7). It was also seen that the carbon formation decreased with increase in temperature from 600 to 1200°C for all oxygen carriers except for CuO (as found earlier ) and CaSO4 as it was almost zero at all temperatures. The carbon formation steeply decreased from 0.68 at 600°C to 0.010 at 1200°C for Mn2O3, while it decreased from 0.63 to 0.001 moles for Fe2O3. The carbon formation slightly decreased with increase in temperature, that is, from 0.06 moles at 600°C to 0.00 at 1200°C for CoO. The carbon formation also decreased from 0.013 moles to 0.00 (NiO), from 0.003 moles to 0.00 (Na2SO4) within 600 to 1200°C. Fe2O3 and Mn2O3 produced higher carbon in comparison with the rest of the oxygen carriers at the same temperatures. The maximum carbon yield was 0.68 moles with Mn2O3 at 600°C. This solid carbon can be separated from gas stream but cannot be separated from the oxygen carrier, and it is carried to the air reactor where it is oxidised to CO2. This combustion generates energy but also contaminates the N2 stream from the air reactor with some CO2 and increases the air requirement in CLC air reactor. It was seen that all oxygen carriers gave their maximum carbon yield at 600°C. The selectivity of the oxygen carrier as per the decreasing yield of carbon (in moles) is observed as follows: Mn2O3 (0.677) < Fe2O3 (0.625) < CoO (0.06) < NiO (0.013) < Na2SO4 (0.003) < CaSO4 (0.0002) < CuO (0.00) at 600°C.
3.2.3. SO2 and H2S Formation
The possibility of SO2, COS, and H2S formation in the CLC fuel reactor while using sulphates as oxygen carriers was also considered in this study, as it was reported by other researchers [79, 80]. Both SO2 and H2S are highly undesirable for the process. The results showed that SO2 and H2S formation takes place in the CLC fuel reactor.
Figure 6 shows the H2S and SO2 formation for the 3 sulphate-based oxygen carriers within the temperature range from 600 to 1200°C at 1 bar pressure per mole methane used in the feed. It was observed that the H2S formation decreased with increase in temperature for all sulphates. The H2S formation (moles) decreased from 0.16 to 0.06 for CaSO4 and from 0.69 to 0.08 moles for Na2SO4 with increase in temperature from 600 to 1200°C. The maximum H2S yield was found to be 0.69 moles at 600°C for Na2SO4, while the minimum H2S yield was found to be 0.06 moles at 1200°C for CaSO4. Figure 6 also showed that the SO2 formation increased with increase in temperature for all sulphates. The SO2 formation increased from 0.00 to 0.1 moles for CaSO4 and from 0.00 to 0.02 moles for Na2SO4 with increase in temperature from 600 to 1200°C. The maximum SO2 yield was found to be 0.1 moles at 1200°C for CaSO4, while zero SO2 formation was observed at lower temperatures till 850°C for Na2SO4 and till 650°C for CaSO4. Hence, according to the data analysis, the choice of better oxygen carrier amongst sulphates was CaSO4 > Na2SO4. H2S or SO2 formation reduces the sulphate to its oxide (e.g., CaO and Na2O), which can be regenerated by using an acid but may require additional processing cost. Commercial systems to trap SO2 as well as H2S are available; however, deterioration of sulphate-based oxygen carrier is always a matter of greater concern. Depending on the cost and availability of the sulphate, smart choice of sulphate oxygen carrier can be made.
3.3. Reactant Conversions
A study of conversion of the reactants is also important for CLC technology development and hence studied in detail and discussed in the following section.
3.3.1. CH4 Conversion
Methane conversion in the CLC fuel reactor is one of the important aspects for choosing the oxygen carrier and operating temperature range, as the reactivity of oxygen carriers with methane may be different. Figure 7 depicts the trends in CH4 conversion with increase in temperature from 600 to 1200°C at 1 bar pressure for the selected oxygen carriers. It was observed from Figure 7 that the CH4 conversion was almost 100% for all oxygen carriers except for Fe2O3 and Mn2O3, where lower methane conversion was seen at lower temperatures. The CH4 conversion increased from 94.22% to 100% for Fe2O3, while it was increased from 95.46% to 100% for Mn2O3 (also experimentally demonstrated earlier ), with increase in CLC fuel reactor temperature from 600 to 1200°C. Complete conversion of CH4 was achieved for all oxygen carriers in the conditions considered in this study, as these oxygen carriers gave almost 100% methane conversion above 800°C only.
3.3.2. Oxygen Carrier Reduction
The reduction of oxygen carrier in the fuel reactor is a key reaction responsible for oxygen availability in CLC reaction and is controlled by the thermodynamic limitations. A study of oxygen carrier conversions in the fuel reactor is also important for calculating the air requirement for regeneration of the oxygen carrier. Hence, all possible reduced states of the oxygen carrier were considered in this study. It was observed that oxygen carriers like NiO, CuO, and CoO were reduced to their respective metals in the fuel reactor. However, oxygen carriers like Fe2O3 and Mn2O3 were reduced to different reduced states. Similarly, many other species were also formed with the sulfate oxygen carriers in the CLC fuel reactor. The species formed in the CLC fuel reactor of Fe2O3 and CaSO4 were discussed in detail.
3.3.3. Reduced Species of Fe2O3
Figure 8(a) shows the different reduced species of Fe2O3 for CLC of methane in the temperature range from 600 to 1200°C at 1 bar pressure. From the figure, it was observed that Fe2O3 was almost reduced completely into its different reduced species like Fe, FeO, and Fe3O4. The moles of FeO and Fe produced increased with increase in temperature; that is, for FeO, it increased from 0.99 to 1.31 moles and for Fe, it increased from 0.44 to 1.08 moles, while the moles of Fe3O4 produced decreased with increase in temperature; that is, it decreased from 0.41 to 0.09 moles, while negligible amount of Fe2O3 was observed remaining at all temperatures.
3.3.4. Reduced Species of CaSO4
Figure 8(b) shows the different reduced species of CaSO4 for CLC of methane process in the temperature range from 600 to 1200°C at 1 bar pressure. Reduction behaviour of sulphur containing oxygen carrier is even more important due to the possibility of formation of sulphur-based compounds. From Figure 8(b), it was observed that CaSO4 almost reduced completely into its different species like CaS, CaO, CaCO3, Ca(OH)2, COS, H2S, and SO2. The moles of SO2 and CaO produced were increased with increase in temperature; that is, it increased from 0.00 to 0.1 moles for SO2 and increased from 0.00 to 0.15 moles for CaO, while moles of CaCO3 produced decreased with increase in temperature; that is, it decreased from 0.15 moles to its lowest value. Moles of H2S produced decreased till 950°C, and then slightly increased with increase in temperature; that is, it decreased from 0.16 to 0.05 moles and then increased to 0.06 moles while reverse trend was observed for CaS, that is, moles of CaS produced increased till 850°C and then slightly decreased with increase in temperature, for example, it increased from 0.83 to 0.93 moles and then decreased to 0.84 moles. CaS being the most desirable reduced state, the CLC fuel reactor temperature can be limited to 950°C when CaSO4 is used as oxygen carrier. Negligible amount of COS and Ca(OH)2 formation was observed with increase in temperature.
It was observed that the sulfate oxygen carriers suffer losses due to conversion to the oxide and carbonate in the CLC fuel reactor. The oxide and carbonate cannot be separated from the solid stream in continuous operation. Hence, a small amount of sulfate oxygen carrier needs to be added to the process continuously to keep the amount of active oxygen carrier intact. This will also require a continuous small purge of the spent oxygen carrier from the system to avoid overloading of the system. This purged oxygen carrier can be reacted with dilute H2SO4, filtered, and dried to convert the oxide and carbonate back to the sulfate for reuse in the system. This modification will be governed by the throughput of the CLC plant, operating cost, and cost of the oxygen carrier.
3.4. Process Energy
The CLC system consists of two reactors in which energy and oxygen carrier transfer occurs between the exothermic (air) reactor and the endothermic (fuel) reactor. It is generally considered that the magnitude of energy obtained in CLC is same as the energy of combustion of the fuel. However, the energy obtainable in CLC depends on the conversion of fuel to different products at different temperatures. As seen in the earlier sections, some other byproducts are also formed in the CLC fuel reactor apart from CO2 and H2O. This affects the net energy obtainable from the CLC system for the different oxygen carriers. The study of fuel and air reactor enthalpy trends within the selected temperature range from 600 to 1200°C was essential to calculate the optimum conditions and is discussed in the next section.
3.4.1. Enthalpy of the Fuel Reactor
Methane undergoes oxidation in the fuel reactor generating primarily CO2 and H2O, and the oxygen carrier is reduced to its lower oxidation state. The enthalpy of this reaction is plotted against temperature in Figure 9 for different oxygen carriers, based on the equilibrium compositions obtained at those conditions. The CLC fuel reactor enthalpy showed different trends for different oxygen carriers. It was observed that the fuel reactor enthalpy increased with increase in temperature from 600 to 1200°C for CoO (135.15 kJ to 156.22 kJ), while it increased till ~1100°C and then became almost constant with further increase in temperature for Fe2O3, Mn2O3, and CaSO4; that is, it increased from 150.09 kJ to 263.04 kJ and remained almost constant (Fe2O3), increased from 30.66 kJ to 124.41 kJ and remained almost constant (Mn2O3), and also increased from 137.42 kJ to 161.80 kJ (from 600 to 1150°C) and remained almost constant for CaSO4. However, the reaction enthalpy decreased with increase in temperature in the case of NiO, where it decreased from 146.33 kJ (600°C) to 139.61 kJ (1200°C). The reaction enthalpy showed some increase-decrease trend with increase in temperature for CuO and Na2SO4 as shown in the figure. The ranking of oxygen carriers based on the minimum fuel reactor enthalpy is as follows: Fe2O3 (263.04 kJ, 1100°C) < Na2SO4 (194.90 kJ, 1200°C) < CaSO4 (161.80 kJ, 1150°C) < CoO (156.22 kJ, 1200°C) < NiO (146.33 kJ, 600°C) < Mn2O3 (124.41 kJ, 1100°C) < CuO (−161.48 kJ, 1100°C). However, it was observed that some of the oxygen carriers did not get completely converted to the desired reduced state, and hence there was some loss of oxygen carrier material. Table 3 shows the conversion of the oxygen carrier to its lower oxidation states, some of which can be regenerated by acid reaction. However, this loss was found to be negligible, and assuming that the oxygen carriers are very cheap, the CLC process did not seem to suffer any major cost issues on this point.
3.4.2. Enthalpy of the Air Reactor
The reduced oxygen carrier along with solid carbon is continuously circulated back to the air reactor for regeneration. Air is passed through the air reactor, and the reduced oxygen carrier and carbon get oxidized completely in the air reactor. This carbon oxidation to CO2 in the air reactor increases the exothermicity of the air reactor as shown below: Figure 10 shows the variation of air reactor enthalpy within the temperature range from 600 to 1200°C at 1 bar pressure. It was observed that exothermicity of oxidation increased with increase in air reactor temperature for Na2SO4; that is, it increased from −268.51 to −787.29 kJ for Na2SO4, while the exothermicity decreased with increase in temperature for NiO, CoO, and Mn2O3; it decreased from −903.24 kJ to −852.05 kJ for NiO, decreased from −845.94 kJ to −762.33 kJ for CoO, and decreased from −513.18 kJ to −240.21 kJ for Mn2O3 with increase in air reactor temperature from 600 to 1200°C. A different trend was observed for CaSO4, where the exothermicity of oxidation first increased and then decreased with increase in temperature; that is, it increased from −794.68 kJ (600°C) to −877.44 kJ (800°C) and then decreased to −784.08 kJ (1200°C), while mixed trend was observed for CuO. An enthalpy decrease-increase-decrease trend was observed for Fe2O3 as seen in the figure. The selectivity of oxygen carriers according to the maximum exothermicity of the reaction is as follows: NiO (−903.24 kJ, 600°C) > CaSO4 (−877.44 kJ, 800°C) > CoO (−845.94 kJ, 600°C) > Na2SO4 (−787.29 kJ, 1200°C) > CuO (−640.17 kJ, 1100°C) > Fe2O3 (−629.23 kJ, 1200°C) > Mn2O3 (−513.18 kJ, 600°C).
3.4.3. Net Process Heat
The net energy required for the CLC process was calculated by summing up the energies of the exothermic air reactor and endothermic fuel reactor for identical oxygen carrier and carbon processing for 1 mole methane feed. The energy generated due to the exothermic air reactor is utilized for reduction of the oxygen carrier in the fuel reactor. The carbon formed in the fuel reactor and its oxidation to CO2 in the air reactor provided additional energy.
Figure 11 shows that net process heat was obtained by using different oxygen carriers within the temperature range from 600 to 1200°C at 1 bar pressure. It was observed that the net process exothermicity decreased with increase in temperature for NiO, Mn2O3, and CoO; that is, it decreased from −756.91 kJ to −712.44 kJ for NiO, decreased from −482.52 kJ to −116.99 kJ for Mn2O3, and decreased from −710.79 kJ to −606.11 kJ for CoO, while net process heat increased with increase in temperature for Na2SO4; that is, it increased from −164.35 kJ to −592.38 kJ for Na2SO4. The net exothermicity first increased and then decreased with increase in temperature; that is, it increased from −657.26 kJ (600°C) to −730.48 kJ (800°C) and then decreased to −623.75 kJ (1200°C) for CaSO4, while the exothermicity first decreased and then increased with increase in temperature for Fe2O3; that is, it decreased from −464.15 kJ (600°C) to −346.2 kJ (900°C) and then increased to −366.46 kJ (1200°C). It was observed that the net process exothermicity remained almost constant for CuO. The oxygen carriers were ranked according to the increase in the net process exothermicity (kJ), and the trend was observed as follows: CuO (−801.68 kJ, 1150°C) > NiO (−756.91 kJ, 600°C) > CaSO4 (−730.48 kJ, 800°C) > CoO (−710.79 kJ, 600°C) > Na2SO4 (−592.38 kJ, 1200°C) > Mn2O3 (−482.52 kJ, 600°C) > Fe2O3 (−464.15 kJ, 600°C). These values were compared with the value of energy of methane combustion. It was observed that the CLC did not exactly give the complete energy that can be obtained from 1 mole methane by air oxidation. This is due to the byproduct formation in the fuel reactor. Although carbon formed in the CLC fuel reactor is oxidized in the air reactor, the syngas and unconverted methane escape in the CLC fuel reactor product stream mainly causing the loss in fuel and subsequent energy. This loss can be reduced by adding excess of oxygen carrier in the fuel reactor. However, this large additional amount of oxygen carrier for small conversion in the fuel reactor thereby increases the process operating cost. Hence, adding a secondary fuel reactor using some more oxygen carriers to oxidise the H2, CO, and CH4 completely in the second stage seems to be a better option.
3.5. Weight of Oxygen Carrier
The CLC system involves circulation of the hot oxygen carrier from the air reactor to fuel reactor for reduction. The reduced oxygen carrier is then sent back to the air reactor for oxidation. This continuous recirculation of the solid oxygen carrier requires considerable energy. Hence, an estimate of weight of the oxygen carrier is important for the process engineer to help choose the right oxygen carrier. Table 4 shows the oxygen carrier requirement per mole methane used in the CLC process. It was observed that 4 moles of CuO, NiO, and CoO were required per mole methane used in the CLC process; hence, their weight is much higher than the sulfate oxygen carriers. The CuO, NiO, and CoO oxygen carriers were the heaviest, followed by Fe2O3 and Mn2O3. The Ca and Na sulfates were found to have lowest weight for use in this study. The favorable oxygen carrier was the lowest amount of the required oxygen carrier (in weight), and the ranking based on this criterion was found to be CaSO4 > Na2SO4 > Mn2O3 > Fe2O3 > NiO > CoO > CuO. However, the trend of net energy obtainable with stoichiometric amount of oxygen carrier was found to be different. Hence, this criterion was compared on the basis of maximum net energy obtained per gram of the oxygen carrier (Table 4) and the ranking based on this criterion was observed to follow the sequence: CaSO4 (−5.3657 kJ, 800°C) > Na2SO4 (−4.1705 kJ, 1200°C) > NiO (−2.5334 kJ, 600°C) > CuO (−2.5196 kJ, 1150°C) > CoO (−2.3714 kJ, 600°C) > Mn2O3 (−2.2981 kJ, 600°C) > Fe2O3 (−2.1855 kJ, 600°C).
3.6. Air Requirement
The CLC of methane can be accomplished by using a variety of oxygen carriers. The reduction of oxygen carriers is different to different potentials depending on CLC conditions. The regeneration of the reduced oxygen carrier as well as carbon oxidation reaction requires air for regeneration. The (stiochometric) requirement of air moles in the air reactor per mole of methane used in CLC was calculated to understand the operating costs related to air input. Figure 12 shows the total air moles required for different oxygen carriers in CLC of methane process in the temperature range from 600°C to 1200°C. The oxygen carrier and carbon were oxidised by air in the air reactor. It was observed that total moles of air used decreased with increase in temperature for NiO, Mn2O3, and CoO; it decreased from 9.11 to 8.69 moles for NiO, decreased from 6.40 to 3.24 moles for Mn2O3, and decreased from 8.68 to 7.73 moles for CoO, while the total air moles used remained almost constant at 9.50 moles for CuO and it increased with increase in temperature for Na2SO4, that is, from 2.56 to 7.55 moles for Na2SO4, while for Fe2O3, it first decreased from 6.21 (600°C) to 5.35 moles (850°C) and then increased to 5.55 moles (1200°C). The total air moles used first increased till 850°C and then slightly decreased with increase in temperature for CaSO4; that is, it increased from 7.93 to 8.81 moles and then slightly decreased to 8.04 moles. Selectivity of the oxygen carriers according to the lowest amount of air moles used in the process was as follows: Na2SO4 (2.56 moles, 600°C) > Mn2O3 (3.24 moles, 1200°C) > Fe2O3 (5.35 moles, 850°C) > CoO (7.73 moles, 1200°C) > CaSO4 (7.93 moles, 600°C) > NiO (8.69 moles, 1200°C) > CuO (9.49 moles, 1200°C).
3.7. Effect of Pressure and Amount of Oxygen Carrier on Methane CLC Process
Pressure is an important process parameter. According to Le Chatelier’s principle, the effect of increase in process pressure on the reaction in the CLC fuel reactor seems negative; that is, the H2O and CO2 yield decreased with increase in pressure at constant temperature for all oxygen carriers. The amount of oxygen carrier in the CLC system is based on the stoichiometric reaction. However, formation of syngas is seen due to thermodynamic limitations. An increase in the amount of oxygen carrier (greater than the stoichiometric amount) can enhance the conversion of fuel selectively to CO2 and H2O. However, this increase in the oxygen carrier quantity can put load on the solid conveying system of CLC. Table 5 shows the increase in the CO2 yield by using different amounts (S—stoichiometric amount, 2S and 3S) of the oxygen carrier (NiO) at different temperatures. It can be seen that increase in the amount of oxygen carrier increases the solid input to the system in a huge quantity, but the CO2 yield increases only meagrely. Hence, stoichiometric amount of the oxygen carrier is the optimum quantity to be used in the CLC system.
Properties of oxygen carrier significantly affect the overall performance of CLC process. The oxygen carriers selected for this study have been reported by researchers as potential oxygen carriers under experimental reaction and regeneration trials. The results obtained in this study inferred valuable information for selection of oxygen carrier for methane CLC. Table 6 shows the preferable oxygen carriers for the different criteria. From Table 6, it is observed that CuO and NiO showed better results among the oxide-based oxygen carriers. CuO and NiO have good desired product yield with negligible undesired product formation. Net process heat per gram of oxygen carrier is found to be maximum for NiO, followed by CuO. Among the sulphate-based oxygen carriers, CaSO4 showed better results under most of the conditions considered. CLC operation involves the fluidization of oxygen carrier particles, and the energy required for solid oxygen carrier transport is a critical parameter for process operation. Hence, it is desirable to obtain maximum energy in CLC using lowest weight (amount) of oxygen carrier. Although the performance of oxygen carriers like NiO and CaSO4 is similar, CaSO4 produces higher net process heat in methane CLC per gram of oxygen carrier used in the process.
A theoretical study of CLC of methane process was done for some important criteria related to technology development using different low-cost, easily available inorganic solids as oxygen carriers and some interesting results were obtained. While oxygen carrier plays very crucial role in technoeconomic feasibility of CLC process, the choice of the oxygen carrier in CLC for a particular fuel is affected by several factors. The combined desired product yield (H2O + CO2) is a better criterion than individual products yield to obtain optimum condition in CLC. Carbon formation in the fuel reactor eventually forms CO2 in the air reactor, and hence the oxygen carrier/process condition resulting in negligible carbon formation in fuel reactor must be chosen. The CLC process should be operated at the desired condition to get maximum net obtainable energy. The oxygen carrier can result in several reduced species. The oxygen carrier that delivers the maximum net energy per unit weight in the CLC process should be preferred for the reasons discussed. The oxygen carrier requiring lowest continuous air input in the air reactor is also preferable. Oxides such as Fe2O3 and Mn2O3 showed multiple reduced states, while CuO, CoO, and NiO had only one reduced state. H2S, SO2, oxide, and carbonate formation were observed in sulfates as oxygen carriers; however, in this case, the operating weight of oxygen carrier as well as operating air requirement was less as compared to the oxide-based oxygen carriers. The combined desired product yield (CO2 + H2O) was maximum for CuO and CaSO4. It was observed that CuO resulted in zero carbon formation over the operating temperature range, while CaSO4 had almost negligible carbon formation. The net energy obtained per gram of the oxygen carrier and CH4 conversion was found to be maximum for CaSO4. The reduced species of CaSO4 like CaS can be regenerated by air oxidation, while CaO can be reacted with dil. H2SO4 to form CaSO4. Use of CaSO4 results in slight formation of H2S and SO2. The formation of syngas is found to be negligible for CaSO4. Even though CuO shows better results for few cases, CaSO4 is more preferable because of the limitation of the operating temperature range for CuO as it could not be reoxidized to its original state due to sintering, which has been studied by some researchers [82, 83]. This also explained some discrepancy in graphs observed for CuO at higher temperatures. Overall, considering the complexity of CLC operations with large number of variables as well as significantly different behaviours of various oxygen carriers at different temperatures, it is essential to understand their thermodynamic properties and theoretical estimations of various parameters. These findings can also be extended to other similar materials and fuels for CLC.
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