CO2 Control Methods. CO2 Emissions vrs Efficiency
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CO2 Control Methods
CO2 Control MethodsCO2 Emissions vrs Efficiency
Overview: CO2 Reuse, Capture and Storage
CO2 Control MethodsPre-combustion capturePost-combustion captureOxy-fuel Combustion
Pre-combustion CaptureThe oil, gas and chemical industries have been separating CO2 from gas streams for decades to meet the required downstream product requirementsThe term pre-combustion capture recently used in the context of gasification based power plants, Those plants are designed to convert the gas produced from gasification ( syngas) to hydrogen and CO2 and to remove the CO2 from the syngas stream prior to the combustion of the hydrogen rich gas in the gas turbine. Pre-combustion Capture TechnologyThe ultimate capture of the CO2 is accomplished under pressure by an acid gas removal process of absorption in a solvent followed by regenrative stripping of the rich solvent to relase the CO2 which with supsequent compression can be sent to sequestration or supplied for enhanced oil recovery.Pre-combustion Capture Technology
Global CCS InstitutePost Combustion CapturePost-combustion capture (PCC) refers to the separation of CO2 from flue gas derived from combusting fossil fuels in air.The combustion results (for coal) in a flue gas mixture consisting of N2, CO2, H2O, O2, and a host of compounds such as SOx, NOx, and heavy metals amongst others. Some of these are removed using existing technologies such as selective catalytic reduction (SCR), electrostatic precipitation (ESP), and flue-gas desulphurization (FGD). A PCC process then aims to selectively separate CO2 from the remaining gas mixture.
After capture, CO2 can be compressed and stored underground, used in some other processes such as enhanced oil recovery (EOR), or used in some other capacity that does not result in its emission into the atmosphere. A Typical PCC Process
Specific CO2 Control MethodsAbsorption (Lithium Hydroxide, Molecular Sieves, Solid Phase Amines, Silver Oxide)AdsorptionMineralizationBiological Methods
PCC AbsorptionThe most common example of a PCC absorption process is 30 wt% aqueous monoethanolamine (MEA) which has been used commercially capturing up to 1000 tonne/day of CO2. Current estimates of capture with MEA followed by compression for underground storage increases the cost of electricity by 60-90%. These relatively high values result from the relatively large quantity of energy needed to regenerate the solvent. Therefore, much of the current research in absorption-based PCC is focus on development of new solvents that reduce the regeneration energy.Some early-stage research is also being conducted in more novel chemistries involving ionic liquids, phase separation solvents, and siloxane oligomers.
12AdsorptionAdsorption refers to uptake of CO2 molecules onto the surface another material for example, adhering CO2 molecules onto the surfaces of a solid sorbent such as 13X zeolites. A claimed advantage of adsorption is that the regeneration energy should be lower relative to solvents since the heat capacity of the solid sorbent is lower than aqueous solvents. However, such claims based on singular rationale are often insufficient, and a careful analysis requires consideration of multiple effects such as heat capacity, working capacity, and heats of reaction. These analyses are not straightforward, do not often point to one clear technology choice over another, and is the subject of early-stage research to help guide development of new capture materials. Potential disadvantages for adsorbents include particle attrition, handling of large volumes of sorbent and thermal management of large-scale adsorber vessels.Biological RemovalBiological removal of CO2from an exhaust stream is possible by passing the stack emissions through an algae or bacterial solution in sunlight. Removal rates of 80% for CO2and 86% for NOXhave been reported, resulting in the production of 130,000 litres/ha/yr of biodiesel (Greenfuels 2004) with residues utilized as animal feed. Other unconventional biological approaches to CCS or fuel production have been reported (Greenshift, 2005; Patrinos, 2006). Another possibility is the capture of CO2from air. Studies claim costs less than 75 US$/tCO2and energy requirements of a minimum of 30% using a recovery cycle with Ca(OH)2as a sorbent. However, no experimental data on the complete process are yet available to demonstrate the concept, its energy use and engineering costs.Membrane technology in carbon dioxide removal
Membrane technology in carbon dioxide removal
Membranes can separate CO2 from flue gas by selectively permeating it through the membrane material. If CO2 has a higher permeability (permeability, defined as the product of solubility and diffusivity, in the membrane relative to other species in the flue gas, then CO2 will selectively permeate the membrane. In some cases, chemical agents that selectively react with CO2 are also added to the membrane to increase the membranes selectivity for CO2. CO2 transports a membrane only if its partial pressure is higher on one side of the membrane relative to the other side. This partial pressure gradient can be obtained by pressurizing the flue gas on one side of the membrane, applying a vacuum on the other side of the membrane, or both.Mineralization
The mineral carbonation (MC), a process of converting CO2 into stable minerals (mineralization)Current Status
Oxy-fuel Combustion Pure O2 instead of air is used in combustion. CO2 in emission is easier to captureBecause a high concentration of nitrogen in the flue gas can make CO2 capture unattractiveThe use of pure oxygen in the combustion process instead of air eliminates the presence of nitrogen in the flue gas, but combustion with pure oxygen results in very high temperatures
Oxy-fuel Combustion Oxygen at greater than 95% purity and recycled flue gas are used for fuel combustion, producing a gas that is mainly CO2 and water
Captured CO2 can be transfered through pipelines after pressurizing with comprressor Another alternative would be to liquidfy CO2 under appropriate T and P, and then transfer with tankers. This method would be more costly though.
TransferPhase Diagram for CO2
CARBON STORAGE and SEQUESTERATIONGeological StorageOil-Natural Gas AreasDeep Saline FormationsOcean StorageMineral Carbonation (Industrial fixation CO2 into inorganic carbonates)GEOLOGICAL STORAGE
STORAGE Injection of CO2 in suitable geological reservoirs could lead to permanent storage of CO2. Geological storage is the most mature of the storage methods, with a number of commercial projects in operation. Ocean storage, however, is in the research phase and will not retain CO2 permanently as the CO2 will re-equilibrate with the atmosphere over the course of several centuries. Industrial fixation through the formation of mineral carbonates requires a large amount of energy and costs are high. Significant technological breakthroughs will be needed before deployment can be considered.CO2 StorageStorage of CO2can be achieved in deep saline formations, oil and gas reservoirs and deep unminable coal seams using injection and monitoring techniques similar to those utilized by the oil and gas industry. Of the different types of potential storage formations, storage in coal formations is the least developed. If injected into suitable saline formations or into oil and gas fields at depths below 800 m, various physical and geochemical trapping mechanisms prevent the CO2from migrating to the surface. Projects in all kinds of reservoirs are planned.
Storage capacity in oil and gas fields, saline formations and coal beds is uncertain. The IPCC (IPCC, 2005) reported 675 to 900 GtCO2for the relatively well-characterized gas and oil fields, more than 1000 GtCO2(possibly up to an order of magnitude higher) for saline formations, and up to 200 GtCO2for coal beds. Bradshaw et al. (2006) highlighted the incomparability of localized storage-capacity data that use different assumptions and methodologies. They also criticized any top-down estimate of storage capacity not based on a detailed site characterization and a clear methodology, and emphasized the value of conservative estimates. In the literature, however, specific estimates were based on top-down data and varied beyond the range cited in the IPCC (2005). For instance, a potential of >4000 GtCO2was reported for saline formations in North America alone (Dooley et al., 2005) and between 560 and 1170 GtCO2for injection in oil and gas fields (Plouchart et al., 2006). Agreement on a common methodology for storage capacity estimates on the country- and region-level is needed to give a more reliable estimate of storage capacities.
Before the option of ocean injection can be deployed, significant research is needed into its potential biological impacts to clarify the nature and scope of environmental consequences, especially in the longer term (IPCC, 2005). Concerns surrounding geological storage include the risk of seismic activity causing a rapid release of CO2and the impact of old and poorly sealed well bores on the storage integrity of depleted oil and gas fields. Risks in CO2transportation include rupture or leaking of pipelines, possibly leading to the accumulation of a dangerous level of CO2in the air. Dry CO2is not corrosive to pipelines even if it contains contaminants, but it becomes corrosive when moisture is present. Any moisture therefore needs to be removed to prevent corrosion and avoid the high cost of constructing pipes made from corrosion-resistant material. Transport of CO2by ship is feasible under specific conditions, but is currently carried out only on a small scale due to limited demand (IPCC, 2005).Clarification of the nature and scope of long-term environmental consequences of ocean storage requires further research (IPCC, 2005)