A close look at Carbon Capture and Storage Technologies
Carbon capture and storage (CCS) involves capture, transport, injection and containment of CO2 in geological structures located deep in the earth’s crust, salt caverns, or unmineable coal beds. CCS technologies can be applied to processes with large-scale emissions, including coal and gas-fired power generation, natural gas processing and fertiliser production, as well as in manufacture of industrial materials such as iron and steel, cement, pulp and paper etc.
The CCS technology system has four components, namely:
- injection, and
Capture is separation of CO2 from an effluent stream and its compression into a liquid or supercritical state. In most cases, the resulting CO2 concentration is >99%, though lower concentrations can be acceptable. Transport consists of the conveyance of CO2 from its source to the storage reservoir. CO2 is dried and usually compressed before being transported to storage. Transporting large quantities of CO2 is most economically achieved with a pipeline. Injection consists of depositing of CO2 into the storage reservoir. The potential reservoirs include the deep-ocean, ocean sediments, or mineralisation (conversion of CO2 to minerals). Once the CO2 is injected, the storage site is to be monitored to show that the CO2 remains in the reservoir and to make sure that the sequestration operation is effective, meaning that almost all the CO2 stays out of the atmosphere for centuries. CO2 is neither toxic nor flammable, hence, it poses only a minimal environmental, health and safety risk.
CO2 Capture Technologies
CO2 can be captured from large stationary emission sources, such as natural gas production facilities, fossil fuel-fired power stations, iron and steel plants, cement plants and some chemical plants. The first component of CCS, i.e., CO2 capture, is technology dependent and is the most expensive step.
Major separation technologies for capturing CO2 presently are (i) using a liquid solvent to absorb the CO2 (absorption), (ii) using solid materials to attract the CO2 to the surface, where it becomes separated from other gases (adsorption), and (iii) using membranes to separate the CO2 from the other gases.
The main competing technologies for CO2 capture from fossil fuel usage are (i) post-combustion capture from flue gas of combustion-based plants, (ii) pre-combustion capture from syngas in gasification-based plants, and (iii) oxy combustion through the direct combustion of fuel with oxygen.
Post-Combustion Capture Technologies
Post-combustion capture can be considered a form of flue-gas clean-up. The process is added to the back end of the plant, after the other pollution control systems.
There are over 50 post-combustion CO2 capture concepts under development which can be grouped into
(i) chemical absorption,
(iv) biological, and
(v) others. There are considerable developments with respect to the first two groups.
Chemical absorption process: It involves one or more reversible chemical reactions between the CO2 and an aqueous solution of an absorbent, such as mono-ethanol-amine (MEA)-based solvent, and high-performance amines (activated methyldiethanolamine, aMDEA) etc. Upon heating the product, the bond between the absorbent and CO2 can be broken, yielding a stream enriched in CO2. Amine-based processes are being used commercially for the removal of acid gas impurities (CO2 and H2S) from process gas streams. It is, hence, a proven and well-known technology. A typical flowsheet of chemical absorption process for CO2 recovery from flue gas is shown in Fig 1.
Fig 1 Typical flowsheet of chemical absorption process for CO2 recovery from flue gas
Physical absorption: For physical absorption, CO2 is physically absorbed in a solvent, according to Henry’s Law. The absorption capacity of organic or inorganic solvents for CO2 increases with increasing pressure and with decreasing temperatures. Absorption of CO2 occurs at high partial pressures of CO2 and low temperatures. The solvents are then regenerated by either heating or pressure reduction.
Solid physical adsorption: An adsorption process consists of two major steps namely (i) adsorption, and (ii) desorption. The technical feasibility of a process is dictated by the adsorption step, while the desorption-step controls its economic viability. Adsorption requires a strong affinity between an adsorbent and the component to be removed from a gas mixture (in this case, CO2). However, the stronger the affinity, the more difficult it is to desorb the CO2 and the higher the energy consumed in regenerating the adsorbent for reuse in the next cycle. Hence, the desorption step has to be very carefully balanced against the adsorption step for the overall process to be successful.
Adsorption processes are quite attractive for the CO2 capture mechanism. The separation can be carried out by pressure swing adsorption (PSA) (Fig 2), vacuum-pressure swing adsorption (VPSA), temperature swing adsorption (TSA), pressure-temperature swing adsorption (PTSA), or electric swing adsorption (ESA) processes. The beds of the installation are filled with solid adsorbents. The selectivity depends on difference in adsorption equilibrium or adsorption rates and on the effectiveness (concentration and recovery) and has significance on the cycle configuration, adsorption time, pressure of adsorption and desorption, temperature during the process as well as the kind of applied adsorbent.
Fig 2 Pressure swing adsorption process
The main advantage of physical adsorption over chemical absorption is its simple and energy-efficient operation and regeneration, which can be achieved with a pressure swing or temperature swing cycle (a swing in pressure or temperature as the process goes through an absorption-desorption cycle in order to achieve separation).
Pre-combustion Capture Technologies
Pre-combustion capture technologies involve removing pollutants and CO2 in the upstream treatment of fossil fuels prior to their combustion for the recovery of heat (via steam), or the production of electric power or H2.
Pre-combustion strives by decarbonising the process stream rich in CO2 before combustion of the remaining H2-rich fuel. To achieve decarbonisation of hydrocarbon fuels, they are first converted into a syngas through the gasification of a fuel with O2 (or air). The syngas is a mixture of CO (carbon monoxide), H2, CO2 and water, depending on the conversion process and the fuel and other components.
The syngas is an intermediate product, which can then be converted to produce (i) H2, (ii) integrated electric power, using the water-gas shift reaction, or (iii) poly generation where a range of energy products can be there, including power, heat, H2 and synfuels and other chemicals. The process involved with each of these end-energy products is described below.
Production of H2 by methane reforming: The most widely used method today for producing H2 is by catalytic steam reforming of methane (CH4). The reforming reaction of converting CH4 and H2O to CO and H2 is endothermic. The reaction is carried out over a Ni (nickel) catalyst at a high temperature in a direct-fired furnace fuelled by CH4. The syngas is, in turn, passed through a catalytic water-shift converter, where the CO is reacted exothermically with steam to produce H2 and a CO2 by-product. These by-products are then removed from the system.
Coal gasification: The gasification technologies can produce a gas stream, which is high in CO2 and at moderate pressure. The feed coal is gasified in O2 (or air) to produce a syngas. The syngas is cooled to 200?C in syngas coolers generating high-temperature and low-temperature steams. It is then shifted further in a low-temperature water gas shift reactor. The water gas shift reactor is a catalytic reactor where the CO is reacted with steam to produce more H2 and CO2. The gas is then cooled to 35?C in preparation for acid gas removal. A PSA unit can be used to separate 85% of the H2 from the S-free syngas. The H2 leaves at around 60kg/sq cm and high purity (greater than 99.99%). The CO2 can be scrubbed from the syngas downstream of the sulphur capture system.
The three main types of coal gasifiers are (i) moving bed, (ii) fluidised bed, and (iii) entrained flow. However, most gasifiers considered for CO2 capture are currently based on entrained-flow gasifiers.
Poly-generation: Syngas is a good building block, as it can be used to produce a wide range of energy products. The greatest flexibility offered is poly-generation, in which ‘syngas’ can produce steam, electric power, H2 and chemicals (such as methanol, Fischer-Tropsch liquids) in a single plant complex.
A number of different separation technologies, including solvent, adsorbent and membrane technologies, can be applied to separate CO2 from the products of gasification.
Absorption: The conventional technology is physical absorption in a two-stage process which removes H2S and then captures CO2. However, the gas needs to be cooled after the water gas shift reaction and then reheated before generating power.
Adsorption: Adsorbents can be used to separate CO2 from post-combustion flue gas streams downstream of the water gas shift reaction. Both temperature swing adsorption (TSA) and vacuum/pressure swing adsorption (VSA/PSA) can be used to recover the CO2 from the adsorbent.
The CO2 is at low pressure when recovered via VSA/PSA and needs to be compressed for storage.
Membranes: Advanced membrane-based gas separation systems are currently being developed to combine the gas shift reaction and H2 separation in one step. The membrane-based systems employ a water gas shift H2 separation membrane reactor (HSMR) to shift the syngas and extract the H2. The maximum temperature of around 475? C ensures fast chemical kinetics and good water gas shift equilibrium performance is obtained by continuous removal of the H2 product.
Oxy-fuel combustion represents an emerging novel approach to near zero-emission and cleaner fossil fuel combustion. It is accomplished by burning the fuel in pure O2 instead of air. By eliminating N2 (nitrogen) in the combustion process, the exhaust of the flue gas stream is mainly composed of water and CO2, without any N2. High purity CO2 can be recovered by condensation of water.
A primary benefit of oxy-fuel combustion is the very high-purity CO2 stream which is produced during combustion. After trace contaminants are removed, this CO2 stream is more easily purified and removed than post-combustion capture.