• Energy Research

The use of membrane gas absorption for carbon dioxide production from flue gases is discussed with special reference to the combined supply of heat and carbon dioxide to greenhouses. Novel absorption liquids are introduced which show an improved performance in terms of system stability and mass transfer compared to monoethanolamine when used in combination with commercially available, inexpensive polyolefin membranes. It is shown that the combined supply of heat and carbon dioxide from cogeneration plants to greenhouses will lead to significant primary energy savings. Carbon dioxide can be produced by the membrane gas absorption process and delivered to greenhouses at lower cost than current supply methods. © 1997 Elsevier Science Ltd.

Abstract It is projected that net zero carbon emissions are required early in the second half of this century to maintain global temperature increase to below 2 °C. To achieve this, the CO2 capture rates in CO2 Capture and Storage (CCS) chains would need to be higher than most current design values, ranging from 85% to 90%. In this study, results from previous studies were assessed to explore the potential to increase CO2 capture rates across the range of capture routes for power stations including post-/pre-combustion capture and oxyfuel. Next, using the most advanced technology for CO2-capture from power stations, i.e. liquid absorbent based post-combustion CO2-capture, the techno-economic performance of a capture process at higher capture rates (>90%) has been assessed. The work has focused on an initial standard design using a 30 wt% Monoethanolamine (MEA) solution and investigated the impact of increasing capture rates on the technical performance, in particular the specific reboiler duty. It was then extended to a number of cases studies, also allowing for process design modifications that aimed to minimise reboiler duty. The overall results are compared to the standard design in terms of process performance, capital costs and operational costs. Additionally, the use of biomass in combination with CCS by co-combustion was incorporated and the impact of fuel prices was assessed.

Abstract Using a tubular ceramic membrane as the transport membrane condenser for simultaneous water and heat recovery from gaseous streams is experimentally investigated in the current study. The effects of several important operational parameters (e.g. gas flow rate, coolant flow rate, transmembrane pressure and inlet gas temperature) on the process performance in terms of mass and heat transfer across the membrane are systematically studied. It is found that mass and heat transfer rates can be enhanced by increasing the gas flow rate, coolant water flow rate and the temperature of the inlet gas stream. To improve the water and heat recovery, a low gas flow rate but a high coolant flow rate should be maintained. Increasing the transmembrane pressure difference decreases the mass and heat transfer mainly due to the reduced inlet gas humidity, enthalpy and flow rate. However, water and heat recovery does not change significantly with the change in transmembrane pressure. 20–60% water recovery and 33–85% heat recovery are achievable when using cold water as the coolant. The mass transfer mechanism in membrane condensation is complex and needs further exploration. These findings offer significant implications in using transport membrane condensers for water and heat recovery from gas streams with high moisture.

The energy penalty associated with solvent based capture of CO2 from power station flue gases can be reduced by incorporating flow sheet modifications to the standard process. Fifteen process flow sheet modifications for chemical based CO2 absorption processes are reviewed, with a particular focus on the patent literature. The proposed flow sheet modifications identify potentially moderate to large improvements in the energy performance of the chemical absorption process. Most process modifications suggested in the patent literature report very little if any supporting experimental evidence. Where supporting data does exist it tends to be based on process modelling results. Moreover, earlier patents tend to focus on the gas processing industry and it is not immediately clear whether the same benefits can be extended to CO2 capture from near atmospheric pressure flue gases. It is clear from the survey that there is considerable scope for achieving improved process performance through process flow sheet modifications. However further process modelling and, in particular, experimental work focused on post-combustion CO2 capture is needed to map the technical potential for improvements.

In this study, the process of carbon dioxide (CO2) capture directly from ambient air in a conventional monoethanolamine (MEA) absorption process was simulated and optimized using a rate-based model in Aspen Plus. The process aimed to capture a specific amount (148.25 Nm3/h) of CO2 from the air, which was determined by a potential application aiming to produce synthetic methane from the output of a 2.7 MW electrolyser (593 Nm3/h H2). We investigated the technical performance of the process by conducting a sensitivity analysis around different parameters such as air humidity, capture rate defined as a ratio of moles of CO2 captured during the process to the total mole of CO2 in the feed stream, CO2 loading of lean and rich absorption liquids and reboiler temperature, and evaluated the energy consumption and overall cost in this system. In order to meet the design requirement for standard packed columns, the rich absorption liquid was circulated to the top of the absorber. A capture rate of 50% was selected in this process as a baseline. At higher capture rates, the required energy per ton of captured CO2 increases due to a higher steam stripping rate, required in the desorber, and at lower capture rates, the size of equipment, in particular, absorber and blowers increases due to the need for processing a significantly larger volume of air at the given CO2 production volume. At the base case scenario, a reboiler duty of 10.7 GJ/tCO2 and an electrical energy requirement of 1.4 MWh/tCO2 were obtained. The absorber diameter and height obtained were 10.4 and 4.4 m, respectively. The desorber is found to be relatively small at 0.54 m in diameter and 3.0 m in height. A wash water section installed at top of the absorber decreased the MEA loss to 0.28 kg/ton CO2. However, this increased capital cost by around 60% resulting in CO2 capture costs of $1,691 per ton CO2 for the MEA base scenario. Based on the techno-economic analysis, assuming a non-volatile absorbent rather than MEA thereby avoiding a wash water section, and using an absorption column built from cheaper materials, the estimated cost per ton of CO2 produced was reduced to $676/tCO2. The overall cost range was between $273 and $1,227 per ton of CO2 depending on different economic parameters such as electricity ($20–$200/MWh) and heat price ($2–$20/GJ), plant life (15–25 years) and capital expenditure (±30%). In order to reduce the cost further, the use of innovative cheap gas-liquid contactors that operate at lower liquid to gas ratios is crucial.

Abstract Amine-based CO2 capture is considered the most mature technology for industrial application in coal-fired power stations, but its large energy requirement represents a major barrier to commercial deployment. Here, we introduce a novel approach involving a CO2 regenerative amine-based battery (CRAB), which harvests the chemical energy from amine-based CO2 capture through a metal-mediated electrochemical process. The CRAB process uses the dual ability of amines (i.e. ammonia) to reversibly react with CO2 and complex with metal ions (i.e. copper) to convert the CO2 reaction enthalpy into electrical energy. To determine how the CRAB process harvests energy from CO2 capture, we established a validated chemical model for CRAB system which was used to predict the electrode potentials, and conceive a CRAB cycle that links CO2 absorption/desorption with the electrochemical process. Modelling results indicate that CRAB could at best produce 8.4 kJe/mol CO2 from a copper/ammonia based energy harvesting system, while optimised CRAB experimentally discharged 6.5 kJe/mol CO2 of electrical energy with a maximum power density of 32 W/m2. When CRAB is coupled with an advanced ammonia process, the experimentally achieved energy output could reduce capture energy requirement to 0.177 MW h/tonne CO2 (including CO2 compression to 150 bar), with a high thermodynamic efficiency of CO2 capture of 62.1% (relative to thermodynamic minimum work of 0.11 MW h/tonne CO2). Our results demonstrate the technical feasibility of the CRAB system to harvest electrical energy from CO2 capture process, providing a pathway to significantly reduce capture energy requirement.

The application of membrane technology in carbon dioxide removal was investigated for a number of different gasstreams. These gasstreams have been defined in the framework of the Dutch national programme on carbon dioxide removal and are typical of streams occurring in present and future electricity generation plants based on fossil fuels (natural gas and coal). They are either mixtures of mainly nitrogen, water and carbon dioxide or mixtures of mainly hydrogen, water and carbon dioxide. The membrane based carbon dioxide removal unit needs to meet certain requirements regarding product gas recovery and purity. Two different types of membrane operations have been considered i.e. carbon dioxide removal based on gas-absorption membranes making use of e.g. conventional amine technology and carbon dioxide removal based on gas separation membranes. Gas separation membranes appeared to be limited by their low selectivities for the given gas mixtures. In contrast, gas absorption membranes appeared to be more promising as they combine the advantages of absorption technology (high selectivity) and membrane technology (compactness of equipment).

AbstractAmino acid salts are effective promoters to improve CO2 absorption in NH3–based solutions, but at the expense of an increase in NH3 vapour loss. To address this issue, we proposed the neutralisation of amino acids using NH3 instead of KOH and investigated the effect of neutralisation methods on NH3 vapour loss and mass transfer coefficient of CO2 in amino acids/NH3 mixtures at 15oC. It has been found that NH3 neutralized amino acids solutions can enhance KG of CO2 in the NH3 solution and suppress NH3 loss at the same time. Among three amino acids investigated, taurine is most suitable for the NH3 neutralisation. We developed the chemical equilibrium model for amino acid–NH3–CO2–H2O and used the model to predict the species profiles in the mixture and explain the experimental results.