Improved sorbent for the sorption-enhanced water-gas shift process

AbstractFor the sorption-enhanced water-gas shift (SEWGS) process, a new sorbent material has been developed. The sorbent is a potassium-carbonate promoted hydrotalcite-based material. The material has been tested under realistic process conditions in an experimental rig of 2 m length. The cyclic capacity of the material is 27% higher than the cyclic capacity of the reference sorbent, which was used in CACHET, a previous R&D project. Moreover, 36% less steam is required for its regeneration. The sorbent pellets also have a 65% higher crush strength than the reference sorbent. Contrary to the reference material, the novel material does not form notable amounts of MgCO3 under relevant operating conditions. Due to the absence of this slow CO2 uptake process, the sorbent remains mechanically stable, the cyclic steady state is reached rapidly, CO2 slip in the product gas is reduced, and steam requirements are lowered. It is demonstrated that the sorbent remains mechanically stable during operation of at least 1200 adsorption–desorption cycles. With this new, higher density material, carbon capture levels exceeding 95% can be obtained more efficiently and vessels will be smaller

Performance of sorption-enhanced water-gas shift as a pre-combustion CO2 capture technology

AbstractThe sorption-enhanced water-gas shift (SEWGS) process is a promising technology for pre-combustion decarbonisation. It is well suited for decarbonising syngas produced from natural-gas and coal based fuels in combined-cycle power production schemes. Higher capture rates could be obtained by SEWGS at lower efficiency penalties and at lower costs than by absorption. In the SEWGS process, multiple reactor vessels are packed with mixtures of CO2 sorption pellets and water-gas shift catalyst pellets. The technology is developed using potassium promoted hydrotalcite-based materials as the CO2 sorbent.In a first series of experiments, the performance of this material is investigated under typical SEWGS process conditions. The sorbent was loaded in 2 m and 6 m tall fixed-bed reactor vessels. Breakthrough capacities of 1.3–1.4 mmol/g are reported. After breakthrough the sorbent continues to take up CO2, albeit at a much lower rate. Total sorption capacities exceeding 8 mmol/g are observed. This capacity is attributed to the formation of MgCO3 in the bulk of the sorbent material and requires moderate to high partial pressures of CO2 and steam. The stability of the sorbent material during cyclic operation was demonstrated for more than 4,000 adsorption and desorption cycles. A stable and low slip of CO2 was established, corresponding to a carbon capture ratio of well above 90%.After investigation of the relevant sorbent characteristics, the reactor was loaded with sorbent and catalyst material in order to provide a proof-of-principle of the SEWGS technology and establish the performance and stability of sorbent and catalyst material. When the reactor was fed with a gas mixture that simulated a syngas typically produced by auto-thermal reforming of natural gas, it was demonstrated that carbon monoxide conversion can be enhanced from 55% in absence of a sorbent to 100% in the presence of a sorbent. Neither the change of gas composition nor the mixing of sorbent with catalyst did significantly impact CO2 breakthrough capacity. In a cyclic duration test, the carbon capture rate and carbon monoxide conversion were confirmed to be above 98% without excessive steam demand, and reasonably stable for at least 500 cycles. The experimental data will be used for modelling, cycle optimization, and scale-up to a pilot unit

Simulation of a catalytic converter of automotive exhaust gas under dynamic conditions

An adiabatic, one-dimensional model of a catalytic converter of automotive exhaust gas was used to simulate the behavior during cyclic feeding and warming-up. Both the oxidn. of CO, C2H2 and C2H4, and the redn. of NO are considered. Accumulation of mass in the bulk gas phase, in the pores of the washcoat and on the catalyst surface is accounted for, as is accumulation of energy in both the gas and the solid phase. Exhaust gas components are converted in a fixed sequence and the light-off temp. of individual components is rather irrelevant for the behavior of a real exhaust gas because of mutual interactions in a mixt. Forced concn. cycling below the light-off temp. of CO, C2H2 and C2H4 can reduce the emissions of the individual components, but the optimal feed temp. is not the same for each component

Isotherm model for high-temperature, high-pressure adsorption of CO2 and H2O on K-promoted hydrotalcite

Sorption-enhancedwater-gas shift (SEWGS) combines thewater–gas shift reactionwith in situ adsorption of CO2 on potassium-promoted hydrotalcite (K-HTC) and thereby allows production of hot, high pressure H2 fromsyngas in a single process. SEWGS is a cyclic process, that comprises high pressure adsorption and rinse, pressure equalisation, and lowpressure purge. In order to design theSEWGS process, the equilibria and kinetics of adsorptionmust beknownfor the entire pressure range.Here, amulticomponent adsorptionisothermis presented for CO2 and H2O on K-HTC at 400 C and 0.5–24 bar partial pressure, that has been derived from integrated experimentally determined breakthrough curves with special attention being given to the high pressure interaction. The experimental results can be well described by assuming that the isothermconsists of a lowpartial pressure surface adsorption part and a high partial pressure nanopore adsorption part. Surface adsorption occurs at specific and different sites for CO2 or H2O. In contrast, the nanopore adsorption mechanism is competitive and explains the interaction observed in the capacity data at partial pressures over 5 bar. Based on the characteristics of the sorbent particles, a linear driving force relation has been derived for sorption kinetics. Adsorption isotherm and linear driving force kinetics have been included in a reactor model. Model predictions are in agreement with breakthrough as well as regeneration experiment

CAESAR: Development of a SEWGS model for IGCC

AbstractSorption-enhanced water-gas-shift (SEWGS) is a promising new technology for improving the efficiency of pre-combustion power production with carbon capture. It combines removal of CO2 from syngas using a packed bed of sorbent at high temperature (350 °C–550 °C) with conversion of CO and H2O to CO2 and H2O through the water-gas-shift reaction. Previous studies on SEWGS have focused on natural gas derived syngas, but no work has been completed to date on alternative feedstocks such as coal. Therefore, experimental, modelling and process simulation work has been completed on a new sorbent material within the CAESAR project to address this question. Key to the success of SEWGS is the amount of steam that must be used for purging and rinsing. Modelling results show that similar amounts of steam are required to operate a SEWGS unit fed using syngas derived from either coal or natural gas. For both cases a target of 2 moles of steam supplied per mole of carbon in the feed appears achievable