CO(2)-free hydrogen can be produced from coal gasification power plants by pre-combustion decarbonisation and carbon dioxide capture. Potassium carbonate promoted hydrotalcite-based and alumina-based materials are cheap and excellent materials for high-temperature (300-500 degrees C) adsorption of CO(2), and particularly promising in the sorption-enhanced water gas shift (SEWGS) reaction. Alkaline promotion significantly improves CO(2) reversible sorption capacity at 300-500 degrees C for both materials. Hydrotalcites and promoted hydrotalcites, promoted magnesium oxide and promoted gamma-alumina were investigated by in situ analytical methods (IR spectroscopy, sorption experiments, X-ray diffraction) to identify structural and surface rearrangements. All experimental results show that potassium ions actually strongly interact with aluminium oxide centres in the aluminium-containing materials. This study unambiguously shows that potassium promotion of aluminium oxide centres in hydrotalcite generates basic sites which reversibly adsorb CO(2) at 400 degrees C.

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This study reports on the application of chemical looping combustion (CLC) in pressurized packed bed reactors using syngas as a fuel. High pressure operation of CLC in packed bed has a different set of challenges in terms of material properties, cycle and reactor design compared to fluidized bed operation. However, high pressure operation allows the use of inherently more efficient power cycles than low pressure fluidized bed solutions. This paper quantifies the challenges in high pressure operation and introduces a novel reactor concept with which those challenges can be addressed. Continuous cyclic operation of a packed bed CLC system is simulated in a 1D numerical reactor model. Importantly, it is demonstrated that the temperature profiles that can occur in a packed bed reactor as a result of the different process steps do not accumulate, and have a negligible effect on the overall performance of the system. Moreover, it has been shown that an even higher energy efficiency can be achieved by feeding the syngas from the opposite direction during the reduction step (i.e. countercurrent operation). Unfortunately, in this configuration mode, more severe temperature fluctuations occur in the reactor exhaust, which is disadvantageous for the operation of a downstream gas turbine. Finally, a novel reactor configuration is introduced in which the desired temperature rise for obtained hot pressured air suitable for a gas turbine is obtained by carrying out the process with two packed bed reactor in series (twostage CLC). This is shown to be a good alternative to the single bed configuration, and has the added advantage of decreasing the demands on both the oxygen carrier and the reactor materials and design specification.

A novel route for precombustion decarbonization is the sorption-enhanced water−gas shift (SEWGS) process. In this process carbon dioxide is removed from a synthesis gas at elevated temperature by adsorption. Simultaneously, carbon monoxide is converted to carbon dioxide by the water−gas shift reaction. The periodic adsorption and desorption of carbon dioxide is induced by a pressure swing cycle, and the cyclic capacity can be amplified by purging with steam. From previous studies is it known that for SEWGS applications, hydrotalcite-based materials are particularly attractive as sorbent, and commercial high-temperature shift catalysts can be used for the conversion of carbon monoxide. Tablets of a potassium promoted hydrotalcite-based material are characterized in both breakthrough and cyclic experiments in a 2 m tall fixed-bed reactor. When exposed to a mixture of carbon dioxide, steam, and nitrogen at 400 °C, the material shows a breakthrough capacity of 1.4 mmol/g. The sharp adsorption front is accompanied by an exotherm that travels along the bed. Even after breakthrough carbon dioxide continues to be taken up by the bed albeit at a much lower rate. It is shown that the total capacity of this material can exceed 10 mmol/g, which has not been reported before. Desorption curves indicate efficiencies of removing additional carbon dioxide by purging with low-pressure, superheated steam at various flow rates. During cyclic operation for more than 1400 adsorption and desorption cycles, the carbon dioxide slip is very low and remains stable which indicates that carbon recoveries well above 90% can be obtained. The sorbent shows a stable cyclic capacity of 0.66 mmol/g. In subsequent experiments the material was mixed with tablets of promoted iron−chromium shift catalyst and exposed to a mixture of carbon dioxide, carbon monoxide, steam, hydrogen, and nitrogen. It is demonstrated that carbon monoxide conversion can be enhanced to 100% in the presence of a carbon dioxide sorbent. At breakthrough, carbon monoxide and carbon dioxide simultaneously appear at the end of the bed. During more than 300 cycles of adsorption/reaction and desorption, the capture rate, and carbon monoxide conversion are confirmed to be stable. Two different cycle types are investigated: one cycle with a CO2 rinse step and one cycle with a steam rinse step. The performance of both SEWGS cycles are discussed. These experimental results will allow optimization of process conditions and cycle parameters, and especially the reduction of steam consumption needed for sorbent regeneration. The results will provide the basis for scale-up to a pilot unit, which will demonstrate precombustion decarbonization in fossil-fuels-based power generation or hydrogen production.

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Sorption-enhanced water-gas shift (SEWGS): H2 from syngas in single unit operation. SEWGS cycle design based on recently published CO 2 and H 2 O interaction with K-HTC. Adsorption of steam during rinse enhances CO 2 product purity. Cycle consumes significantly less steam than previously reported cycle designs.

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