Energy 36 (2011) 5450e5459
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Study and development of a high temperature process of multi-reformation of CH4 with CO2 for remediation of greenhouse gas Chunguang Zhou*, Lan Zhang, Artur Swiderski, Weihong Yang, Wlodzimierz Blasiak School of Industrial Engineering and Management, Division of Energy and Furnace Technology, Royal Institute of Technology, 100 44 Stockholm, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 April 2011 Received in revised form 13 July 2011 Accepted 26 July 2011
A new carbon capture and recycle (CCR) system based on multi-reforming of CH4 with CO2 is proposed in this study. The aim was to develop a novel method to remediate greenhouse gases (CO2) using a high temperature (over 1173 K) process of reforming CH4 and/or O2, and/or H2O without catalysts. Using this novel method, the reactants are individually preheated to over 1173 K using a ceramic honeycomb heat exchanger, and then these high temperature streams enter the reactor to start the reforming reactions. Both thermodynamic and experimental studies were carried out on this novel method. Thermodynamic equilibrium models were built for four types of reforming, including dry reforming, bi-reforming, autothermal reforming, and tri-reforming. Only dry reforming was experimentally tested. The feasibility of this novel technology was proven by simulated and experimental results. High temperatures significantly promoted the multi-reforming process while avoiding the problem of catalyst deactivation. The experimental results on the direct system also showed that potential improvements in the efficiency of the novel technology could be achieved by optimizing the reforming reactants. Therefore, a continuous system was proposed. Moreover, the power source for the application of CCR systems was also discussed. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: CO2 emission CCS Dry reforming
1. Introduction The heavy exploitation and utilization of coal and other fossil fuels in the 20th century has resulted in global warming. Abnormal climate change phenomena resulting from greenhouse gases (GHG) emissions (CO2, CH4, N2O, etc) have been emerging since the 1990s [1]. The global energy system will likely remain fossil fuel dominated in the next few decades, especially in new and developing economies [2,3]. There has been increasing awareness of global warming in recent decades and a consensus that action must be taken to lower the levels of the main greenhouse gas, CO2, has been reached [4]. According to the estimation of the Intergovernmental Panel on Climate Change (IPCC), to stabilize atmospheric levels of greenhouse gases below 490 ppm CO2 equivalents while taking into account regional implications and equity issues, by 2050, developed countries should achieve an approximately 80e95% reduction in CO2 emission levels relative to the levels in 2000 [5]. The European Commission established a target of limiting the average earth surface temperature increase to 2 C and proposed that the EU * Corresponding author. Present address: Brinellvägen 23, 100 44 Stockholm, Sweden. Tel.: þ46 8 790 8402; fax: þ46 8 207681. E-mail address:
[email protected] (C. Zhou). 0360-5442/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.07.045
reduce GHG emissions relative to 1990 levels by 20% in 2020 and by 60% in 2050 [5,6]. Due to a relatively large potential for renewable energy sources in Sweden, the Swedish government has proposed to achieve a 35% emissions reduction by 2020 [7]. In the last few decades, the Swedish government has implemented a range of polices and clean energy technologies to combat climate change. It is well-known that energy structure adjustments and energy utilization efficiency improvements can reduce CO2 emissions. However, the contribution is insignificant and makes it difficult to meet the targets. Sweden had already reached a relatively low level of emissions intensity by the early 1990s, and it has a high share of hydro and nuclear power production in installed electric power capacity [8].The production increase is the main driver for the increased CO2 emissions [9]. Alternatively, carbon capture and storage (CCS) has been recognized as a key mitigation option that could reduce CO2 emissions significantly while allowing for the use of coal to meet pressing energy needs [10e13]. There are two parts of the CCS process. First, CO2 is captured using different technologies. Second, CO2 is sequestered in geological media including depleted oil and gas reservoirs, unminable coal seams, and deep saline porous formations [14,15]. However, CCS has not been scaled up for worldwide commercial use due to its high costs and leakage risk [16]. The security of sequestration largely depends on site characteristics and management. The potential for CO2
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leakage, the impact on the quality and movement of groundwater, and conflicts of interest with respect to the use of aquifers should also be considered [17]. Thus, the simultaneous establishment of carbon management security and lower CO2 emissions is an important global energy objective for modern societies. If technologies that use CO2 as a reagent and convert it efficiently could be developed, CO2 could be recycled, thus significantly reducing CO2 emissions. Therefore, the concept of carbon capture and recycle (CCR) is proposed and discussed in this paper. To realize CCR, captured CO2 must be efficiently converted using certain technologies. In this paper, CO2 reforming of CH4 was investigated using a primary energy source that had no CO2 emissions to provide the entire energy demand. The produced synthesis gas could be reused for a variety of downstream processes. The carbon capture and recycle (CCR) was able to recycle carbon and transform energy without CO2 emission. CCR is expected to solve CO2 emissions issues; therefore, the feasibility of CCR based on CO2 reforming of CH4 was discussed thermodynamically and experimentally. Moreover, the application of CCR was also discussed.
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than dry reforming alone [25]. Auto-thermal reforming, which combines dry reforming and partial oxidation, provides partial heat for endothermic reactions and increases the H2/CO ratio [26e31]. Moreover, tri-reforming, in which both O2 and steam are added, improves the H2/CO ratio and provides partial heat for endothermic reactions [32e34]. In this paper, the thermodynamic equilibrium of all four types of reforming processes are discussed. The possible reactions involved in the reforming processes are as follows: Reforming Reaction:
Methane CO2 reforming CH4 þ CO2 42CO þ 2H2 ; DH298K ¼ 247 KJ=mol; (1) Methane steam reforming CH4 þ H2 O4CO þ 3H2 ; DH298K ¼ 206 KJ=mol; (2) Oxidations:
2. CCR system based on CO2 reforming of CH4 2.1. Multi-reforming CH4 with CO2 Compared with other organic synthesis processes for CCR, including the syntheses of urea and the Electrochemical KolbeSchmitt process, CO2 reforming of CH4 is of great interest due to the natural abundance, relatively inexpensive price of feeding, and significant mitigation in CO2 emissions. CO2 reforming of CH4 is mostly used for products of synthesis gases. Through reforming with CO2, CH4 can be converted to valuable synthesis gas, primarily a mixture of H2 and CO, which can be utilized in a variety of downstream processes, such as methanol synthesis, FischereTropsch (FeT) synthesis, and several other carbonylation and hydrogenation processes [18e20]. Thus, in this paper the performance of a CCR based on CO2 reforming of CH4 is assessed. The process of CO2 reforming of CH4, which is usually called dry reforming, has a high selectivity to CO and offers a way to fix CO2 for CO2-emitting processes. However, it is a highly endothermic reaction that requires extra heat [21e23]. In addition, the H2/CO ratio for dry reforming is 1:1, while the H2/CO ratio of syngas for high level hydrocarbons via Fischer-Tropsch synthesis is expected to be approximately 2:1 [24]. Three other types of reforming technologies (bi-reforming, auto-thermal reforming, and tri-reforming) have been based on this basic type of reforming and were studied and compared with dry reforming of CH4 via thermodynamic analysis, as shown in Fig. 1. Bi-reforming, which combines dry reforming and steam reforming, could achieve a higher H2/CO ratio
Fig. 1. Four types of reforming technologies.
Partial oxidation CH4 þ 1=2O2 4CO þ 2H2 ; DH298K ¼ 36 KJ=mol;
(3)
Total oxidation CH4 þ 2O2 4CO2 þ 2H2 O; DH298K ¼ 802 KJ=mol;
(4)
Side reactions:
Water gas shift ðWGSÞ CO þ H2 O4CO2 þ H2 ; DH298K ¼ 41 KJ=mol;
(5)
Methanation reaction CO2 þ 4H2 4CH4 þ 2H2 O; DH298 K ¼ 165 KJ=mol;
(6)
Moreover, experimental studies on CO2 reforming of CH4 were also carried out to confirm the results of thermodynamic analysis. Due to the high temperature required for the endothermic reactions in the reforming process, a catalyst is usually used to help the reforming process proceed at a relatively low temperature. Catalytic reforming was investigated as early as 1888, and many experimental studies have been carried out [30,35e42] in the last one hundred years. However, catalyst deactivation caused by carbon deposition on the surface and metal sintering are still the most critical barriers to the transfer of the catalytic dry reforming process from laboratory scale to industrial scale projects [30]. If the reforming reaction could be carried out at a high temperature, catalyst would be unnecessary [43]. To address this need, a novel method for high temperature air/steam combustion (HiTAC) without catalysts was developed to remediate greenhouse gases. HiTAC equipped with a ceramic honeycomb heat exchanger was first used in the process of industrial combustion, for example in the iron and steel industries. In HiTAC, combustion air is preheated to 1073 K via the heat exchanger system. As a result, combustion can be carried out at a high temperature (over 1473 K). Thus, HiTAC could provide a high enough temperature for the reforming reaction. The heat exchanger system also brings other benefits to HiTAC, such as a nearly 30% energy savings and a 25% reduction in facility size [44]. Therefore, after applying the ceramic honeycomb heat exchanger to the CO2 reforming process for CH4, improved reforming results or energy savings can be expected.
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process. By changing the reaction conditions, such as pressure, temperature, catalyst, and H2/CO ratio, different carbonylation and hydrogenations are produced. During the usage of the products, a portion of the CO2 corresponding with the energy released would be emitted. In a CCR system, the strong endothermic processes of CO2 reduction, as well as the CO2 capture and synthesis reaction, require extra heat. The total energy input of these three processes should be larger than the energy output of the usage process. Thus, a steady and powerful energy source is the essential requirement for a CCR. 3. Methodologies 3.1. Thermodynamic analysis
Fig. 2. The principle of CCR system based on dry reforming.
2.2. Concept of CCR system A CCR system could achieve cyclic use of carbon and could reduce CO2 emissions significantly. The structure of a CCR system based on CO2 reforming of CH4 is shown in Fig. 2. The whole system consists of four elemental processes: synthesis of synthesis gas, synthesis gas usage, CO2 capture, and CO2 conversion. A steady CO2 emission source, such as a large coal power plant in which CO2 is captured and separated via effective technologies, is necessary. In the CO2 reduction process, captured CO2 reacts with CH4 resulting in synthesis gases (H2 and CO), which are provided to the next
The minimization of total Gibbs free energy is a commonly used and suitable method for calculating the equilibrium compositions of any reacting system [45]. The method of thermodynamic analysis by minimization of Gibbs free energy has been reported in many studies [26,27,46]. The minimization function of total Gibbs free energy for a reaction system of N compounds containing Ni moles of species i can be expressed as: N1 X i¼1
0 Ni @G0i;T
1 yi Fi P X pj aij A þ Nc G0c;T ¼ 0 þ RT ln þ 0 P j
(7)
where, G0i;T is the standard Gibbs free energy, Ni is the number of moles of species i, R is the molar gas constant, T is the temperature of system, yi is the gas phase mole fraction, Fi is the fugacity
Table 1 Reactants and theoretical products for each reforming process for the multi-reforming of CH4 with CO2. Reforming type
Input Reactants
Products
Dry reforming Bi-reforming Auto-thermal reforming Tri-reforming
CH4:CO2 ¼ 1:1 CH4:CO2:H2O ¼ 2:1:1 CH4:CO2:O2 2:1:0.5 CH4:CO2:H2O:O2¼ 3:1:1:0.5
C, C, C, C,
Fig. 3. The scheme of the high temperature multi-reforming facility in KTH.
CO, CO, CO, CO,
CO2, CO2, CO2, CO2,
CH4, CH4, CH4, CH4,
H2, H2, H2, H2,
H2O, H2O, H2O, H2O,
C2H2, C2H4, C2H6 C2H2, C2H4, C2H6 O2, C2H2, C2H4, C2H6 O2, C2H2, C2H4, C2H6
C. Zhou et al. / Energy 36 (2011) 5450e5459 Table 2 Running conditions of the experiment. Cases Number
CH4 input, Nm3/h
CO2 Input, Nm3/h
Pressure, atm
Temperature, K
1 2 3
0.5 0.5 0.5
0.5 0.5 0.5
1 1 1
1153 1169 1213
coefficient of species i, pj is the Largrange multiplier, and aij is the number of atoms of the jth element present in each molecule of species i. A thermodynamic equilibrium calculation employing Gibbsenergy minimization for each type of reforming process was further modeled in the Aspen Plus program, which is capable of single phase or multi-phase multi-component equilibrium modeling. Each reforming process was modeled separately. The processes have different reactants; therefore, different products were expected in the overall reactions. In the models, the stoichiometric ratio of the reactants was investigated. The settings for each reforming process are presented in Table 1. The reforming temperature ranged from 300 K to 2000 K, and the pressure ranged from 0.5 to 3 atm. The calculations were computed using the Soave-Redlich-Kwong method. The proportions of reactants and products, as well as the reforming temperatures, were clarified and compared.
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Several parameters, such as CH4 conversion, CO2 conversion, H2 yield, and H2/CO, were introduced to evaluate the performance of multi-reforming. The higher the conversion of reactants the better result; therefore, the conversion ratio of CH4 and CO2 were defined as:
CH4 conversion% ¼ 100 FCH4 ; in FCH4 ; out FCH4 ; in
(8)
CO2 conversion% ¼ 100 FCO2 ; in FCO2 ; out FCO2 ; in
(9)
H2 yield% ¼ 100 FH2 ; out = 2 FCH4 ; in þ FH2 O; in
(10)
H2 =CO ¼ FH2 ; out =FCO; out
(11)
where Fin is the molar flow rate input and Fout is the molar flow rate output. The H2/CO ratio was another important parameter. According to Fischer-Tropsch synthesis, a ratio less than 3 is required but a value around 2 is preferred for methanol. Therefore, the H2/CO ratio is a useful parameter for evaluating the optimal reforming conditions, and it is often related to the input reactants.
3.2. Experimental methods The experimental demonstrations of the thermodynamic analysis results were carried out in a high temperature multi-reforming
Fig. 4. The main species concentrations of the four types of reforming plotted versus temperature. For the thermodynamic equilibrium modeling, the reactants were input at the stoichiometric ratio, 1 atm of pressure, and a temperature between 300 K and 2100 K: a) dry reforming; b) bi-reforming; c) auto-thermal reforming; d) tri-reforming.
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Fig. 5. The main parameters of the four types of reforming processes plotted versus temperature. For thermodynamic equilibrium modeling, the reactants were input at the stoichiometric ratio, 1 atm of pressure, and a temperature between 300 K and 2100 K: a) CH4 conversion; b) CO2 conversion; c) H2 yield; d) H2/CO ratio.
facility, which was built in Kungliga Tekniska Högskolan (KTH), Sweden. Fig. 3 shows the scheme of this facility. One ceramic honeycomb heat exchanger was installed in the high temperature multi-reforming facility and was used to accumulate the heat from combustion to preheat the multi-reforming reactants to a high temperature. The combustion of fuel gas was carried out in the first combustion chamber; the hot flue gases then left this chamber, passed through the ceramic honeycomb heat exchanger, which was heated by the flue gases, and flowed through the second combustion chamber toward the facility outlet. During the reforming process, the gas burner was shut off. Both CO2 and CH4 were fed into one mixing tube at ambient temperature and pressure. This mixture of gases then flowed through the first chamber and the heated ceramic honeycomb heat exchanger. After passing through the hot honeycomb, this mixture of gases was heated to a temperature in the range of 1100 Ke1473 K. This temperature was measured by a thermocouple outside the honeycomb and was recorded as “T1”. The reforming reactions were carried out in the honeycomb as the temperature increased and were completed in the second combustion chamber. A microgas chromatograph was used to test the produced gas collected at the facility outlet. There was an observation hole made of glass in
the second combustion chamber through which the reaction could be recorded by digital camera. All of the dry reforming processes under three different reaction temperatures were carried out at a stoichiometric CH4/CO2 ratio at ambient pressure. No catalyst was used during the reaction. The measured temperature, T1, is referred to as the reaction temperature. With regard to the in-depth reactions in the second chamber, we assume that the chemical equilibriums of all the reforming reactions were achieved. Table 2 shows the 3 running conditions. 4. Results and discussions 4.1. Discussion of the simulated results According to the thermodynamic equilibrium models of the multi-reforming of CH4 with CO2 technology, the theoretical performance of four reforming processes at a high temperature were investigated. The molar fractions of the main species are plotted versus the reforming temperature in Fig. 4. According to the Gibbs free energy change for the reaction, steam reforming is not favorable below 900 K and CO2 reforming will not proceed below 925 K. However, the conversion can proceed
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Fig. 6. Effect of pressure on dry reforming: a) CH4 conversion; b) H2/CO ratio; c) CO2 conversion from 400 K to 1000 K; d) CO2 conversion from 1000 K to 1400 K.
at temperatures ranging from 500 K to 1100 K because of side reactions, such as WGS and the methanation reaction, which play a key role in the reforming process. Although there were different reactant inputs in the four reforming processes, the conversion phenomena were similar. Only the intermediate H2O performed differently. In dry reforming, H2O was formed significantly between 600 K and 1100 K and reached a maximum at 860 K; methanation reactions reversed above 860 K and methane steam reforming was favored above 900 K. In auto-thermal reforming, total CH4 oxidation was carried out even at low temperatures. Therefore, H2O was detected starting at 300 K until the temperature rose to 1100 K, at which point almost all H2O was converted to H2 through steam reforming. In bi-reforming and tri-reforming, H2O is one reactant input. It is consumed by steam reforming and almost disappears after 1100 K. From the mole fraction plots, it can be seen that all four reforming processes have high conversion ratios above 1100 K. However, it is clear that there are different H2/CO ratios among the four reforming processes. To further compare the conversion of the reactants and the distribution of the products for the four multi-reforming processes, CH4 conversion, CO2 conversion, the H2/CO ratio, and the H2 yield are plotted versus temperature (Fig. 5). As shown in Fig. 5 (a) and Fig. 5 (b), the conversions increased with increasing temperature, and most of the reactants were consumed in the 700 Ke1100 K temperature range. When the temperature was greater than 1100 K, 90% of the CH4 was converted in all four types of reforming. A higher conversion, 99%, was achieved above 1300 K. However, the four types of reforming performed similarly in terms of CH4 conversion, but performed significantly differently in CO2
conversion under 1300 K. Dry reforming maintained the highest CO2 conversion ratio. For all the other types of reforming, the phenomenon of negative CO2 conversion could be seen at temperatures below 825 K. This is because the WGS reaction proceeds, while the methanation reaction is inhibited as a result of steam injection. At 1100 K, CO2 conversion rates for dry reforming, bi-reforming, auto-thermal reforming, and tri-reforming were 96%, 94%, 94%, and 92%, respectively. When the temperature was greater than 1300 K, more than 99% of CO2 was converted for all four types of reforming. Thus, high temperature multi-reforming could theoretically proceed at a high conversion ratio. If the temperature were over 1300 K, no residual CH4 and CO2 would be expected. For all reactions, the H2 yield was related to H2O and to the hydrocarbon concentrations in product. In addition, H2 yield depended on the CH4 conversion ratio. The hydrocarbons produced occupied only a very small mole fraction. The reactants were mostly converted above 1100 K, and the H2O concentration decreased to a low value at 1100 K. Hence, as shown in the plot, all four types of reforming had similar H2 yields of over 90% above 1100 K and 99% above 1300 K. This means that at a high temperature, most hydrogen atoms in the compounds were converted to H2. The H2/CO ratio is one important parameter for syngas. As shown in Fig. 5 (d), all four types of reforming had a constant H2/CO ratio under 2 above 1000 K. Among them, tri-reforming had the best expected H2/CO ratio, 1.75, while dry reforming only had a 0.99 ratio. To achieve a higher CO2 and CH4 conversion rate, the effect of pressure on dry reforming and bi-reforming was also investigated via thermodynamic equilibrium analysis at varied temperatures
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600e1300 K. The same tendency was observed for the H2/CO ratio at temperatures between 400 K and 1300 K, as shown in Fig. 6 (b). The effect of pressure on the H2/CO ratio and the CO2 conversion rate for bi-reforming are plotted in Fig. 7. The low pressure increased the H2/CO ratio. For the CO2 conversion, the effect of pressure within the 750e1400 K temperature range was completely opposite that below 750 K. That is because the high pressure improved the inhibited methanation reactions that proceeded at the low temperature.
4.2. Discussion on experiments results
Fig. 7. Effect of pressure on bi-reforming: a) H2/CO; b) CO2 conversion.
between 400 K and 1400 K. The CH4 and CO2 conversion rates and the H2/CO ratios for dry reforming are shown in Fig. 6. The figure indicates that both CH4 and CO2 conversion were significantly suppressed by increasing pressure within the temperature range of
To investigate the feasibility of the high temperature multireforming technology, the conversion ratio and product concentrations were discussed and compared with simulated data under the same conditions. From Fig. 8, it is clear that the CH4 conversion, which was a little bit better than determined in the simulated results, was equal to 1 in all three runs. Thus, almost all CH4 was consumed. The average mole fraction of H2 was 42%, which was lower than the modeled value of 49% at the same temperature. These two phenomena are the result of the extra input of CO2 from the combustion flue gas. In the experiments, the reforming reactions were carried out after combustion of natural gas; therefore, there could be a small excess quantity of O2, CO2, and steam left inside the facility. There was not enough CH4 to convert all of the CO2. In the products tested, because CO2 could be left over, and less CO and H2 were produced. The high conversion of CH4 and the high H2 concentration provided evidence to support the high temperature multi-reforming technology. Regarding the H2 and CO flow rate, the H2 produced increased from 0.56 Nm3/h to 0.74 Nm3/h with increasing temperature rose from 1153 K to 1213 K. In the simulated results, the H2 flow rate was maintained above 0.94 Nm3/h. On the other hand, the CO produced was much less than that suggested by the model. However, the CO flow rate increased sharply as the temperature rose. At 1153 K and 1169 K, the CO flow rate was only 0.16 Nm3/h and 0.33 Nm3/h, respectively. At 1213 K, the CO flow rate reached 0.68 Nm3/h; at this temperature the flow rate was expected to be 0.98 Nm3/h. Although the H2 and CO flow rates were influenced by the extra CO2
Fig. 8. Comparison of CH4 conversion and H2 concentration for the thermodynamic equilibrium models and the high temperature dry reforming experiment. Temperatures ranged from 300 K to 2100 K; the reactants were input at the stoichiometric ratio and 1 atm of pressure.
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Fig. 9. Bar chart of the comparison of H2 and CO flow rates for the high temperature dry reforming experiment and the thermodynamic equilibrium models; the reactants were input at the stoichiometric ratio and 1 atm of pressure.
input from the combustion flue gas, the experiments at high temperatures, such as 1213 K, showed good potential for usage of this high temperature multi-reforming technology. Interestingly, relatively higher values of hydrocarbons were produced than were expected from the simulated results. As shown in Fig. 10, although hydrocarbons represent only a very small proportion of the total products, they were detected at dramatically higher levels in the experiment than were suggested by the models. C2H2, C2H4, and C2H6 were studied, and C2H2 decreased from 0.63% to 0.24% and C2H6 decreased from 1.53% to 0.78% as the temperature increased. C2H4 was extremely high at 1153 K but was undetected at 1169 K and 1213 K. The formation of these hydrocarbons has been explained in the literature [47]. There was little O2 left inside the facility; therefore, after CH4 input there was an environment containing an ultra-rich CH4eO2 mixture. Thus, hydrocarbons were formed during the reaction of CH4 and O2. The
hydrocarbons could be further converted to useful compounds that might lead to new ideas for CH4 reforming with CO2 (Fig. 9). According to the experimental results, high temperature enhances the multi-reforming of CH4 with CO2 in the absence of catalysts. This technology is feasible and could be optimized by evaluating the temperature and input reactants ratio. 5. Proposed concept of the CCR system From the results of simulation and experimentation we conclude that it is possible to achieve good performance results through high temperature multi-reforming of CH4 with CO2 without catalysts. However, the whole system requires a powerful energy source for its highly endothermic reactions. A high temperature gas reactor type nuclear reactor could be a very suitable energy source for CCR because high temperature outputs of up
Fig. 10. Bar chart of the comparison of hydrocarbon concentrations for the high temperature dry reforming experiment and the thermodynamic equilibrium models; the reactants were input at the stoichiometric ratio and 1 atm of pressure.
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Fig. 11. Schematic diagram of CCR system driven by HTGR.
to 950 C can be achieved with non-carbon emissions and because a sufficient amount of nuclear fuel is available to meet a country’s demand [48]. As discussed above, 950 C is high enough to achieve high CO2 and CH4 conversion. According to the latest data, Sweden has 10 operating nuclear power reactors that provide over 40% of its electricity [49]. Thus, the nuclear reactors in Sweden could provide enough energy demand for a CCR. The application of a CCR based on dry reforming driven by HTGR is proposed in Fig. 11. To reduce the heat loss of the whole system, the sensible heat carried by the hot synthesis (a mixture of CO and H2) should be accumulated and used to preheat the CO2 and CH4. Wind energy is also a good power source and its use is rapidly growing [50]. Sweden has good potential wind resources. In 2010, the total installed capacity of wind energy reached 2163 MW; this represents a 35% rate increase since 2009. According to the Swedish wind energy association, the wind energy potential in Sweden is estimated to be around 540 TWh/year [51]. Thus, the electricity generated could also provide part of the energy demand for the reforming. Sweden also has relatively rich biomass resources compared to the rest of Europe [52]. The use of biomass resources could result in carbon neutrality [53]. Thus, biomass could also be a good energy source for CCR system. A schematic diagram is shown in Fig. 12. According to the experimental studies on the direct heating system, which have been discussed above, HiTAC could be used to provide the energy demand for the reforming reactions. Therefore, a continuous setup driven by HiTAC of fuel gas from biomass gasification was proposed here. As shown in Fig. 13, and different
Fig. 12. Schematic diagram of CCR system driven by biomass.
Fig. 13. Principle layout of the proposed method of remediation of CO2 using CH4 reforming (1: reactor; 2: heat exchanger; 3: reforming tube; 4: regenerative burners system; 5: insulated combustion chamber).
from the small direct heating facility used above, the indirect heating approach is used for this continuous setup. The heating system primarily consists of a pair of regenerative burners, an insulated combustion chamber and a reforming tube. Firstly, the fuel gas provided from the biomass gasification plant was fired by a pair of regenerative burner systems. Due to the ceramic honeycomb equipped in the system, HiTAC could be achieved in the isolated combustion chamber. When it was running, the isolated combustion chamber was heated to 1200 K and the exhaust gas temperature after the regenerative burner system was dropped to 120 C. Thus, very high energy utilization efficiencies could be reached. The CO2 and CH4 were mixed in a reactor and then heated in a heat exchanger by the hot syngas from the reforming tube. After the mixture was heated to a certain temperature, the heated mixture of CO2 and CH4 entered the reforming tube, which was located inside of the combustion chamber. 6. Conclusion The feasibility of a novel technology for high temperature multireforming of CH4 with CO2 was demonstrated through equilibrium modeling and experimental studies. The three primary conclusions were the following: 1. Thermodynamic equilibrium models demonstrated that at a high temperature (over 1300 K), over 98% reactant conversions and H2 yields would be achieved by all four types of multi-reforming processes. In terms of the H2/CO ratio, among all types of stoichiometric reforming, tri-reforming had the best H2/CO ratio, close to 2 for methanol formation and close to 1 for dry reforming. The pressure also had a significant effect on reactant conversion and product yields. At a high temperature, a low pressure should be maintained to achieve better performance results. 2. According to the experimental results, if the temperature is sufficiently high then the multi-reforming process will be stable, achieve high conversion, and have good product outputs. If a high enough reacting temperature can be reached, a catalyst is not necessary and coke formation is no longer a problem. Lower CO2 conversion and more hydrocarbons were detected than were expected based on thermodynamic equilibrium modeling. This may be attributed to the small quantity
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of excess O2, produced CO2, and steam left inside the facility after combustion. 3. Instead of the direct heating setup, a continuous indirect heating system was also presented. Nuclear, wind and biomass energy could be powerful energy sources for this system in Sweden.
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