HYDROGEN PRODUCTION THROUGH SORPTION ENHANCED REFORMING H.T.J. Reijers, D.F. Roskam-Bakker, J.W. Dijkstra, R.P. de Smidt, A. de Groot, R.W. van den Brink Address: Energy research Centre of the Netherlands, ECN, P.O. Box 1 Postal code: 1755 ZG, city: Petten, Country: The Netherlands Phone:+31(0)224-564588 e-mail: reijers@ecn.nl Keywords: Hydrogen, Sorption Enhanced Reforming, Modelling, Adsorbents, Catalysis, Methane steam reforming Introduction Introduction of hydrogen as an energy carrier offers an opportunity to reduce the CO 2 emission from diffuse sources, like vehicles and newly built residential districts. In the long term, it is expected that a hydrogen infrastructure will contribute to CO 2 reduction. In the short term, hydrogen will likely play a role where the application, especially fuel cells, asks for hydrogen. These applications include the transport sector and small-scale combined heat and power. On-site hydrogen production on a gas station or in a residential district requires an average hydrogen production rate between 1000 and 4000 Nm³/hour. At the moment, hydrogen is produced industrially in large-scale steam-reformers at rates in the order of 100,000 Nm³/hour and at high pressures (20 40 bar) and high temperatures (800 950 ºC). To withstand these extreme conditions, expensive materials are required. Besides, a considerable amount of export steam is produced, which cannot be used in the small-scale hydrogen energy systems mentioned before. So there is a need for hydrogen production units operating at milder conditions, while maintaining a high system efficiency. One of the technologies currently investigated at ECN for this purpose is sorption enhanced reforming (SER). Here the methane steam reforming process is conducted in the presence of a CO 2 sorbent. By removing reaction product CO 2, the equilibrium is shifted to the product side, yielding a relatively pure hydrogen stream. The system is operated periodically in two modes: an sorption cycle during which natural gas and steam are fed to the SER reactor, and a desorption cycle in which the sorbent is regenerated. The CO 2 that is released during regeneration could possibly be used for CO 2 sequestration. The CO 2 sorbent should fulfill the following requirements: high CO 2 uptake, rapid kinetics, chemical stability at high H 2 O concentrations and low costs. A material which satisfies these requirements is hydrotalcite of general formula A 2x B 2 (OH) 4x+4 CO 3 nh 2 O. Here A is a divalent, B a trivalent metal ion. In hydrotalcite, these ions are arranged in layers having a brucite (Mg(OH) 2 ) structure, in which part of the divalent ions is replaced by trivalent ions. The excess positive charge of these layers is compensated by the CO 3 2- ions in the intermediate layers. After heat treatment the hydrotalcite decomposes to a mixture of metal oxides offering a plurality of adsorption sites for CO 2.
Systems studies Apart from experiments, systems studies have been performed for a hydrogen production system with integrated steam-reformer and CO 2 sorbent, including steam generation, hydrogen purification and heat integration (Figure 1). The system consists of 2 reactors, one of which is in adsorption mode, the other in desorption or purge mode. The purge is performed using steam so that a pure CO 2 stream can be obtained for CO 2 sequestration. A model has been developed in which the overall system efficiency is calculated. The heat management in the system is addressed using a pinch analysis approach. The first results are based on the experimental data obtained by Air Products 1. They indicate that the efficiencies in the pressure and temperature ranges of interest are too low to be competitive with small-scale SR (steam-reforming) with PSA (pressure swing adsorption) for hydrogen purification (Table 1) 2. Tail gas naar to burner brander CH 4 + H 2 O Reformer PSA H 2 product CO 2 Figure 1 Base configuration for systems studies Table 1 System data for conventional steam-reforming and SER small-scale SR SER T (ºC) 800 500 p (bar) 10 1,7 CH 4 conversion (%) 84 66 efficiency (LHV) 70 39 steam/ch 4 3 6 steam/h 2 1,1 2,3 H 2 purity (%) 99,999 89 fraction CO 2 adsorbed 0 50 Careful analysis has shown that the efficiency can be improved by: decreasing the amount of steam and by increasing the CH 4 conversion. For that reason, 2 paths are followed to increase the efficiency: changing the operating conditions or sorbent materials such that the desorption kinetics is improved so that less steam is needed for regeneration, and developing alternative system configurations which increase the overall CH 4 conversion. For the future, we want to investigate the above-mentioned two routes the improve the efficiency. Preliminary results are shown below.
1,6 1,4 1,2 CO 2 (ml/min) 1,0 0,8 0,6 0,4 0,2 1st cycle 5th cycle 10th cycle 20th cycle 30th cycle 40th cycle blank 0,0 0 5 10 15 20 25 30 35 40 4,0 3,5 CO 2 (ml/min) 3,0 2,5 2,0 1,5 1st cycle 5th cycle 10th cycle 20th cycle 30th cycle 40th cycle blank 1,0 0,5 0,0 0 5 10 15 20 25 30 35 40 Time (min) Figure 2 Adsorption (above) and desorpion (below) curves of htc at 400 ºC and 1 atm Experimental Lab-scale experiments have been performed to determine the CO 2 breakthrough of a fixed bed reactor using various hydrotalcite (htc) samples. was obtained from SASOL (PURAL MG70), Zn-Al htc was home-made by coprecipitation 3. The htc samples were calcined at 400 ºC for 4 hours. To increase the adsorption, the samples were impregnated with 22 wt% K 2 CO 3. A tube reactor (diameter: 16 mm) was loaded with 3 g htc sample (particle size: 0,212 0,450 mm, bed height: 30 mm). The sample was heated up to the operation temperature (400 or 500 ºC) and a CO 2 -containing gas was allowed to flow past the bed (5% CO 2 /29% H 2 O/66% N 2 at 30 ml/min). After 75 minutes, gas conditions were changed to a CO 2 -free atmosphere (29% H 2 O/71% N 2 at 100 ml/min) to regenerate the bed. Again after 75 minutes, a new adsorption cycle was begun. A typical experiment consisted of 20 adsorption/desorption cycles. A blank experiment, i.e. an experiment with a non-co 2 adsorbing material of the same morphology as the used hydrotalcite, was done in order to correct for instrumental effects.
Desorption percentage (%) 130 120 110 100 90 80 70 21 st cycle ` 40 th cycle 30 th cycle 500 Celcius Zn-Al htc without K 2 CO 3 60 50 40 0 100 200 300 400 500 600 700 800 900 1000 Cumulative purge flow (mol purge gas/mol adsorbed CO 2 ) Figure 3 Comparison of CO 2 desorption fraction of various htc samples Results and discussion Figure 2a and b show a typical set of adsorption and desorption curves respectively. It is seen that the more cycles have been performed, the closer both the adsorption and desorption curves come together. From the 20 th cycle onwards, changes in the curves are minor. The CO 2 loading capacity varies from 0.27 (1 st cycle) to 0.22 mmol/g (40 th cycle). Figure 3 shows the desorption fraction vs. cumulative purge flow for various htc samples and conditions. In all cases, the samples are impregnated with K 2 CO 3, data of the 5 th cycle is used and the operation temperature is 400 ºC, unless otherwise indicated. To make a comparison between the results possible, the cumulative purge flow is expressed per mol of adsorbed CO 2. The fact that the desorption percentage obtains values above 100% indicates that the CO 2 of which uncalcined htc consists, has not completely been removed by calcination. From this figure, it follows that the purge flow for 100% desorption decreases in the order Mg-Al without K 2 CO 3, Zn-Al htc, Mg-Al at 500 ºC, Mg-Al 40 th cycle and Mg-Al. It is seen that the amount of purge gas needed for 100% desorption differs by more than a factor 5 for the sample with the largest amount ( without K 2 CO 3 ) and the smallest amount (, ). Conclusions 1. CO 2 can be removed effectively by using htc as adsorbent. The amount adsorbed decreases in the first cycles and becomes constant after the 20th cycle. 2. Of the tested htc samples and applied test conditions follows that impregnated with K 2 CO 3 at 400 ºC is the preferred adsorbent. 3. Systems studies have shown that in order to increase the efficiency, the amount of steam for regeneration must be reduced, and the overall CH 4 must be increased.
References 1 J.R. Hufton, S.J. Weigel, W.F. Waldron, M.B. Rao, S. Nataraj, S. Sircar, T.R. Gaffney: Sorption Enhanced Reaction Process for the Production of Hydrogen, Final Report 2000. Air Products and Chemicals Inc., DE-FC36-95G010059, Allentown, USA. 2 D.B. Myers, G.D. Ariff, B.D. James, J.S. Lettow, C.E. Thomas and R.C. Kuhn: Cost and Performance Comparison of Stationary Fueling Appliances, Task 2 Report, April 2002. Directed Technology Inc., 703/2431-3383, Arlington, USA. 3 K. Schulze: Ni/Mg/Al Catalysts derived form hydrotalcite-type precursors for the partial oxidation of propane, PhD Thesis, Gerhard-Mercator University, Duisburg, July 2001.