Recent Technologies in Capture of CO2

Fluidized beds particular features drive this technology as an appropriate candidate technology to apply oxy-fuel combustion, producing so a highly concentrated CO₂ flue gas stream to be processed and stored. Still, there are several issues differencing the conventional combustion to that with O₂/CO₂ mixtures. This chapter examines the main issues involved in oxy-fuel combustion in fluidized beds through the experimental results obtained in the CIRCE oxy-fuel bubbling fluidized bed.

The fluidization velocity during oxy-firing is in general, below the air-firing case. This is caused by the higher O₂ concentration in the oxidant stream, together with the higher gas density when substituting air-N₂ by CO₂. The lower fluidization velocity affects in opposite ways the combustion and pollutant formation: it increases the residence time of particles in bed, whereas poorer mixing of fuel particles disadvantages reactions. Higher O₂ at inlet obliges to increase the fuel input to maintain proper fluid-dynamics conditions. This is adequate for the combustion efficiency and also, for the in-furnace SO₂ capture. Unlike in conventional combustion, SO₂ capture optimum temperature is higher than 850 ºC, because of the influence of high CO₂ partial pressure.

NO_x emissions showed no significant differences with air-firing case, if concentration is expressed per unit of energy. This is due again to the lower flow of flue gases per thermal fuel input. Plant heat balance of large oxy-fuel fluidized bed boilers will change considerably in oxy-fuel case. Boilers will be more compact and thus, additional heat transfer surface will be essential.

During the utilization of CO₂/N₂ gas separation membranes for postcombustion capture, the most important problem is how to create the driving force efficiently because the feed flue gas has only ambient pressure and a relatively low CO₂ content. Multi-stage systems are necessary using feasible membranes in order to fulfill the separation target, required by the following pipeline transport, and limited by the storage capacity. The whole work was divided into two steps: energy consumption analysis and capture cost analysis.

This book chapter describes mass and energy balances for single-stage and multi-stage membrane systems used in coal-fired power plant. After the recirculation of flue gas and variation of the feed gas compressor and vacuum pump on the permeate side, two concepts were developed and optimized to achieve minimum energy consumption. In order to evaluate different membrane capture concepts, a comparison with chemical absorption process was carried out, considering different degrees of CO₂ separation. Furthermore, a cost model was developed to make further analysis of the optimized concept in view of the tradeoff balance between material and energy consumption. The correlation between the membrane parameters (selectivity, permeability) and capture cost was investigated.

This chapter reviews the thermal integration for minimum external energy consumption of CO₂ capture process using amines. Post combustion capture of CO₂ through the use of amines is a well established technique; the stand alone process is highly energy-intensive since the recovery of the amine solution is achieved through the use of a separation process where heating and cooling are required. Energy integration of the hot flue gas coming from a power station plant from where CO₂ is absorbed can serve the purpose of providing the heating and cooling needs of the process. Heat recovery through steam raising is considered for heating, cooling and for the production of power for the operation of pumps and compressors. The results show that the needs of the largest energy user of the process can be fully met by heat integration.

The purpose of this work is to evaluate the minimum energy consumption for solvent regeneration and maximum CO₂ absorption with 600 t/hr flue gas flow simulated by Aspen Plus™ of CO₂ capture process, using Monoethanolamine (MEA) at 30 weight%. The parameters studied were: 1) energy consumption at reboiler of stripper, 2) absorption separation efficiency, 3) flow ratio (L/G) in order to find the load on turbulence regimen in absorption process, and 4) absorption and stripper column diameters at different flue gas flows. This work contributes structured packing study in separation columns, like: ININ 18, Sulzer BX and Mellapak 250Y, and the advancement in CO₂ capture technology. Hydrodynamic and mass transfer models were used to evaluate pressure drops and height of mass transfer equivalent unit, per each packing, from experimental data of CO₂ absorption column and predict up to 600 ton/h flue gas flow by Aspen Plus™. The results showed that Sulzer BX has the highest volumetric mass transfer coefficient values and the lowest height of mass transfer equivalent unit, with 3.76s-1 and 0.316m, respectively, and the most absorption efficiency of 89.17% in comparison with respect to the other two packings and 120MW reboiler energy.

This technology makes use of the idea that lime may be reused in a cyclic process to remove CO₂ from a mixture of gases where carbonate is calcined to generate a pure stream of CO₂ ready for sequestration. Flue gas from an existing power plant is introduced in the carbonator where the CO₂ reacts with CaO to form CaCO₃. This process must occur at elevated temperatures (600-650 ºC depending on CO₂ partial pressure). Removal rates around 80-90% seem to be a reasonable target for this technology.

The formed calcium carbonate is circulated to a different reactor where sorbent regeneration takes place. CaCO₃ is calcined and produces a concentrated stream of CO₂ suitable for capture and compression. Calcination step is highly energy demanding and it will likely occur at temperatures above 920 ºC. Heat requirements for sorbent calcination are covered by oxyfuel combustion in the second reactor itself. Once regenerated, the sorbent is returned to the carbonator to begin a new sorption cycle.

Because of the elevated temperatures, the entire cycle might be integrated in a steam cycle, reducing energy penalties of the capture system by several percentage points. The cost of natural sorbents for these cycles is significantly low, reducing operation costs.

This chapter examines the energy penalties of the Ca-looping CO₂ capture system, different types of sorbents and their performance subjected to repeated cycles of carbonation and calcination, the CO₂ capture efficiency and the possibility of integration of Ca-looping and power plants to reduce energetic penalties.

The purpose of this chapter is to discuss some structural characteristics and properties of three regular packing materials: metallic, polymeric and ceramic; in order to select the best one to capture CO₂ in an absorption column. The study was conducted by making the following tests: geometric physical properties such as wetted area and porosity; mechanical properties like, stress, hardness, modulus and compression resistance, structure and microstructure morphologic, chemical composition, rate of corrosion in electrochemical cell in medium of 1N of H₂SO₄ and Monoethanolamine (MEA) at 30% in aqueous solution by using standard procedures of the American Society of Testing Materials (ASTM) and own developed procedures for the equipment used. The structures of materials were also evaluated by X-ray diffraction and the surface of the material by scanning electron microscopy. It was concluded that metallic material is suitable for CO₂ gas treatment because it was presented with the lower etching 1N of H₂SO₄₂.

Comparison of rate-based models (RBM) and equilibrium models (EQM) for absorption of CO₂ in ammonia, along with details on selecting process equipment, are presented in the first part of the Chapter. Differences in temperature and concentration profiles along a 10-stage packed column were found with the two models. In EQM, liquid temperature matches the inlet gas temperature at the fourth stage and remains constant until the bottom of the column. Oppositely, three temperature profiles were obtained in RBM: interface, gas, and liquid streams; temperatures in the EQM were almost 10 °C higher than in the RBM. For intermediate stages, the EQM predicted higher mol fraction of CO₂ than RBM.

A non-equilibrium stage, rate-based model is developed in the second part of the chapter. Simultaneous mass and energy transfer through the gas-liquid interface, as well as the hydrodynamics of packed columns with the corresponding correlations to calculate pressure drop, liquid hold up and mass transfer coefficients, are included in the model which is based on absorption segments. Simulation of the absorption of CO₂, from a mixture with ammonia indicated more than 80% of absorption of CO₂; complete absorption of SO₂ and NO₂ present in the stream could also be accomplished. It was necessary to increase ammonia concentration in the lean solution and implement an ammonia washer in order to avoid ammonia slip. These results were used for sizing a packed column for the treatment of a gas stream with a composition similar to that expected from a wet cement kiln.

In several studies related to atmospheric emission of anthropogenic CO₂, it has been established that the resulting greenhouse effect is a direct factor in the climate change observed around the world. These climate changes are a consequence of the influence of the greenhouse effect on the atmospheric thermodynamic state. The importance has been recognized of measuring the atmospheric CO₂ contamination with precision. Also, for a proper disposal of this contaminant, there have been developed some efficient physicochemical procedures for either CO₂ dissociation or recycling. Ion accelerators provide a suite of techniques, collectively referred to as IBA, offering excellent options for the analysis of this kind of contamination.