Capture

 

Inputs for the amine system absorber and regenerator in PC and NGCC plants are entered on the Capture parameter screen.

The absorber is the vessel where the flue gas makes contact with the MEA-based sorbent, and some of the CO2 from the flue gas is dissolved in the sorbent. The column may be plate-type or a packed one. Most of the CO2 absorbers are packed columns using some kind of polymer- based packing to provide large interfacial area. The following parameters apply to the absorber:

•Sorbent Concentration: (Not shown for Cansolv.) The solvent used for CO2 absorption is a mixture of monoethanolamine (MEA) with water. MEA is a highly corrosive liquid, especially in the presence of oxygen and carbon dioxide, and hence needs to be diluted. Today the commercially available MEA-based technology supplied by Fluor Daniel uses 30% w/w MEA solvent with the help of some corrosion inhibitors. Other suppliers, who do not use this inhibitor, prefer to use lower MEA concentrations in the range of 15%-20% by weight.

•Lean CO2 Loading: Ideally, the solvent will be completely regenerated on application of heat in the regenerator section. Actually, even on applying heat, not all the MEA molecules are freed from CO2. So, the regenerated (or lean) solvent contains some "left-over" CO2. The level of lean solvent CO2 loading mainly depends upon the initial CO2 loading in the solvent and the amount of regeneration heat supplied, or alternatively, the regeneration heat requirement depends on the allowable level of lean sorbent loading.

•Sorbent Losses (excluding acid gasses): (Not shown for Cansolv.) MEA is a reactive solvent. In spite of dilution with water and use of inhibitors, a small quantity of MEA is lost through various unwanted reactions, mainly the polymerization reaction (to form long-chained compounds) and the oxidation reaction forming organic acids and liberating ammonia. It is assumed that 50% of this MEA loss is due to polymerization and the remaining 50% of the MEA loss is due to oxidation to acids.

•Sorbent Recovered: (Not shown for Cansolv.) This is the amount of sorbent regenerated by caustic added to the reclaimer.

•Liquid to Gas Ratio: (Not shown for Cansolv.) The liquid to gas ration is the ratio of total molar flow rate of the liquid (MEA sorbent plus water) to the total molar flow rate of flue gas being treated in the absorber.

•Ammonia Generation: (Not shown for Cansolv.) The oxidation of MEA to organic acids (oxalic, formic, etc.) also leads to formation of NH3. Each mole of MEA lost in oxidation, liberates a mole of ammonia (NH3).

•Gas Phase Pressure Drop: (Not shown for Cansolv.) This is the pressure drop that the flue gas has to overcome as it passes through a very tall absorber column, countercurrent to the sorbent flow.

•ID Fan Efficiency: The cooled flue gas is pressurized using a flue gas blower before it enters the absorber. This is the efficiency of the fan/blower to convert electrical power input into mechanical work output.

•Makeup Water for Wash Section: This is the amount of makeup water required by the wash section, expressed as a percent of the weight of the raw flue gas.

•Activated Carbon Used: (Not shown for Cansolv.) This is the amount of activated carbon in the sorbent circuit to help remove the polymeric sorbent compounds.

The regenerator is the column where the weak intermediate compound (carbamate) formed between the MEA-based sorbent and dissolved CO2 is broken down with the application of heat and CO2 gets separated from the sorbent to leave reusable sorbent behind. In case of unhindered amines like MEA, the carbamate formed is stable and it takes large amount of energy to dissociate. It also consists of a flash separator where CO2 is separated from most of the moisture and evaporated sorbent, to give a fairly rich CO2 stream. The following parameters apply to the regenerator:

•Regeneration Heat Requirement: This is the total amount of heat energy required in the reboiler for sorbent regeneration.

•Regenerator Steam Heat Content: The regeneration heat is provided in the form of LP steam extracted from the steam turbine (in case of coal-fired power plants and combined- cycle gas plants), through the reboiler (a heat exchanger). In case of simple cycle natural gas fired power plants, a heat recovery unit may be required. This is the enthalpy or heat content of the steam used for solvent regeneration.

•Heat-to-Electricity Efficiency: (Only shown when an auxiliary boiler is not used.) This is the efficiency of converting low pressure steam to electricity. The value reflects the loss of electricity to the base plant when the LP steam is used for regenerator heat.

•Solvent Pumping Head: The solvent has to flow through the absorber column (generally through packed media) countercurrent to the flue gas flowing upwards. So, some pressure loss is encountered in the absorber column and sufficient solvent head has to be provided to overcome these pressure losses. Solvent circulation pumps are used to provide the pressure head.

•Pump Efficiency: This is the efficiency of the solvent circulation pumps to convert electrical power input into mechanical power output.

•Percent Solids in Reclaimer Waste: (Not shown for Cansolv.) This is the amount of solids typically present in the reclaimer waste.

•Capture System Cooling Duty: This is the total amount of cooling water normalized by CO2 product.

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