(439c) Sorption Enhanced Reaction Process for Electricity Production and CO2 Capture

INTRODUCTON

There is a growing awareness that future energy production must be coupled with reduced greenhouse gas emissions. However, fossil fuels will remain the most important energy source for the first half of this century. As a consequence, a whole range of new technologies have been suggested to reduce the emission of the CO₂ produced from the burning of fossil fuels. This CO₂ can be captured prior to combustion or with end-of-pipe technologies from the flue gas of the energy production unit. A promising pre-combustion route is the sorption-enhanced reaction process (SERP) for reforming of methane (Reijers et al., 2003). The reactions taking place in the reformer are:

                                                                                                  (1)

                                                                                                    (2)

for steam reforming of methane (SRM) only, and in addition

                                                                                                  (3)

                                                                                               (4)

                                                                                              (5)

for autothermal reforming of methane (ATRM). When the reforming or shift catalyst is mixed with a sorbent, as in SERP, the CO₂ produced in the above reactions is simultaneously absorbed, so that the hydrogen production rate is enhanced. The reforming process can now be performed at significantly lower temperatures than conventional reforming while retaining the same overall conversion of CH₄. SERP is a batch process necessitating the regeneration of the sorbent saturated with CO₂. The preferred regeneration gas or purge gas is steam. Compared with other purge gases (air, nitrogen or methane), steam can be easily separated from the purge stream making the CO₂ suitable for sequestration. A simplified scheme of a SERP/SRM system is shown in Figure 1. Both reactors are filled with a mixture of steam reforming catalyst and CO₂ sorbent. A CH₄/H₂O mixture is fed to reactor 1, purge gas to reactor 2. After the sorbent of reactor 1 is saturated with CO₂, the gas flows to both reactors are interchanged and CO₂ is desorbed from the bed of reactor 1, whereas steam reforming is performed in reactor 2. To improve the CO₂ desorption, the temperature may be increased or the pressure may be decreased with respect to the temperature and pressure during adsorption. In comparison to conventional SRM, shift reactors are no longer needed in a SERP/SRM system.

Figure 1  Schematic representation of a SERP/SRM  system for electricity production

EXPERIMENTAL

To test the suitability of the above concept, we have performed lab-scale experiments using a cayalyst/sorbent mixture. A commercial hydrotalcite (Pural MG70, an aluminum magnesium hydroxide carbonate of formula Mg_5.4Al₂(OH)_14.8CO₃.4H₂O, supplied by SASOL) was used as CO₂ adsorbent. The hydrotalcite as received was calcined in air, and was subsequently impregnated with 22 wt% K₂CO₃. The impregnated hydrotalcite, hereafter htc-ECN, was compacted and a sieve fraction of particles was obtained with sizes between 0.212 and 0.425 mm. A similar particle size fraction of low-temperature steam reforming catalyst was obtained. The sample, a mixture of 3.0 g htc-ECN and 1.5 g catalyst, was placed in a reactor tube and subjected to a series of reaction and regeneration cycles at 400 ºC and 1 atm.

RESULTS AND DISCUSSION

Figure 2 shows the results of a proof-of-principle 100 cycle SERP experiment. The chosen length of the reaction step corresponds with complete loading of the adsorbent. More than 95% conversion was obtained during 100 repetitive cycles. This is an enhancement of almost a factor 2 with respect to the process without adsorbent (equilibrium conversion: 53%). The adsorbent did not show any capacity decay. Two striking observations were made in the above SERP experiment:

1.      The breakthrough of CH₄ starts almost at the beginning of the reaction step whereas no breakthrough of CO₂ is observed during this step. The CH₄ breakthrough can be postponed by adding more catalyst: when 1.5 g extra catalyst is added, no CH₄ breakthrough is observed during the reaction step.

2.      The desorption rate of CO₂ during regenation is initially rapid, but slows down considerably making a long desorption period necessary for complete CO₂ removal.

The former implies that the sample is lacking of catalytic activity. This is remarkable since from activity measurements follows that a two orders of magnitude lower w/F ratio, where w is the catalyst weight and F the CH₄ flow, would be sufficient to achieve the equilibrium conversion under the conditions of the above SERP experiment (53%). Experiments where SERP was applied to water-gas shift (reaction 2) only, using a mixture of high temperature shift catalyst and htc-ECN, showed that a much smaller amount of extra catalyst is needed to obtain nearly complete conversion of CO to CO₂. A possible explanation is that the steam reforming catalyst deactivates due to poisoning of potassium which moves from the impregnated sorbent to the catalyst. More experiments are required to confirm this.

Figure 2  SERP demonstrated for steam reforming of CH₄ at 400 ºC and 1 atm, 3 cycles shown; adsorption conditions: 25 ml/min, 2.9% CH₄, 17.5% H₂O and 79.5% N₂, 10 minutes; desorption conditions: 100 ml/min, 29% H₂O, 71% N₂, 75 minutes; sample: 1.5 g catalyst, 3.0  htc-ECN

Another important issue is the reduction of the mol/mol ratio of purge gas to adsorbed CO₂, hereafter S/CO₂, since this ratio determines the systems efficiency to a large degree (see below). In the above experiments, this ratio is in the range 400 - 500. To reduce the S/CO₂ ratio, various parameters were varied:

1.      Mg was replaced by other metals (Zn and Cu) to make the htc less basic (Cavani et al., 1991).

2.      The Mg-Al ratio was decreased from 2.7 to 0.5 using MgAl₂(OH)₆CO₃.2H₂O (Di Cosimo et al., 1998).

3.      Different K₂CO₃ loadings were investigated (11, 33 and 44 wt%)

4.      Other promotors were tried (Na₂CO₃, LiNO₃, KNO₃, CsNO₃).

5.      Different purge flows were tried.

6.      Different regeneration step lengths were used.

The use of different divalent metals or K₂CO₃ loadings did not change the S/CO₂ ratio. Decreasing the Mg-Al ratio increases the basic site density. Indeed, the adsorption capacity was increased by 30%, but again the S/CO₂ ratio did not change significantly. Also, using Na₂CO₃ as promotor instead of K₂CO₃, hardly affected S/CO₂. Nitrate salts had no promoting effect at all on the CO₂ adsorption capacity of htc and their S/CO₂ was similar as the non-promoted htc. Only by decreasing the purge flow or the regeneration step length, a S/CO₂ reduction could be obtained. By lowering the purge flow from 100 to 30 ml/min and the length of the regeneration step from 75 to 10 min, (S/CO₂)_c came into the range 60 - 80. In doing so, also the CO₂ adsorption capacity decreased by approximately a factor 2 since the adsorbent bed was but partly desorbed. Thus a larger reactor vessel would be required resulting in higher capital cost.

SYSTEMS STUDIES

Systems studies are being performed to select one or two promising SERP concepts for the reforming of methane. The investigated variants include:

1.      ATRM vs. SRM,

2.      O₂ blown vs. air blown ATRM, and

3.      sorbents which are regenerated by pressure swing vs. sorbents which are regenerated by temperature swing.

The first results show that the efficiencies of these variants are similar (around 50% LHV). The CO₂ capture ratio (ratio of captured CO₂ and produced CO₂) is larger for ATRM (90%) than SRM (51% -65%). By using part of the produced H₂ instead of CH₄ for firing the reformer, the CO₂ capture ratio can be increased at the expense of a reduced efficiency. Further, the efficiency is very sensitive to the S/CO₂ ratio. For acceptable efficiency, this ratio should be in the same range as the steam/methane ratio of the feed gas (3 - 6).

In the future, the effects of purge flow and length of regeneration step will be examined more closely. It will be investigated if SERP allows complete integration of all reactions (1-5) in a single reactor. This would make the system less complex. Systems studies will be extended with an economic analysis of the selected concepts.

CONCLUSIONS

1.      The principle of SERP has been demonstrated for steam reforming of methane in a 100 cycle lab-scale experiment.

2.      Under the conditions of SERP (combination with adsorbent, almost complete CH₄ conversion at a relatively low temperature), the activity of the steam reforming catalyst is considerably lower than expected.

3.      The S/CO₂ ratio can be reduced most effectively by decreasing the purge flow or the length of the regeneration step at the expense of a reduced CO₂ working capacity. For an acceptable system efficiency, it should be reduced at least one more order of magnitude.

4.      The efficiencies of the investigated system variants are quite similar. The CO₂ capture ratio depends on the system variant.

REFERENCES

F. Cavani, F. Trifiro and A. Vaccari, Hydrotalcite-type anionic clays: preparation, properties and applications, Catalysis Today, (1991), 173-301.

J.I. Di Cosimo, V. K. Diez, M. Xu, E. Iglesia and C. R. Apesteguia, Structure and surface and catalytic properties of Mg-Al basic oxides, Journal of Catalysis, (1998), 499-510.

H.Th.J. Reijers, D. F. Roskam-Bakker, J. W. Dijkstra, R. Smit, A. de Groot and R.W.van den Brink, Hydrogen production through sorption-enhanced reforming from Proceedings of the 1^st European Hydrogen Energy Conference, 2-5 September 2003, Grenoble.