Carbon dioxide capture is the first step in Carbon Capture and Sequestration processes. Several methods of carbon capture are in use on a semi-commercial basis. These can be described as Amine Capture, Ammonia Capture, and Water/Alkaline Capture. The Ammonia Capture process is the carbon capture process which relates to this invention.
Ammonia Capture Process
In the ammonia capture process, a concentrated solution of ammonia in water, either cooled or at ambient or higher temperature, is contacted with a gas stream containing carbon dioxide, such as power plant flue gas, cement kiln gas or even possibly air. Carbon dioxide reacts with the ammonium based ions in the water and ammonia solution producing in effect a mixture of ammonia, ammonium carbonate, ammonium bicarbonate, and ammonium carbamate in water. In this discussion we will use the term ammonium carbonates to refer to all species formed during the reaction of ammonia with carbon dioxide. Given sufficient ammonia added to the solution, eventually a very high concentration of ammonium carbonate species can be reached and either solid crystals or a very concentrated solution is produced. The crystals of ammonium carbonates along with the concentrated solution can be decomposed under mild conditions, releasing ammonia and carbon dioxide as gases. Ammonia can be separated from the CO2 using a variety of means, including cold condensing surfaces which liquefy the ammonia. This separation allows the carbon dioxide to pass through the system as a gas and be compressed into a liquid form for later sequestration or use. The liquid ammonia is now available for recycle to capture additional carbon dioxide. No desalination is accomplished in this standalone process.
Typical examples of this approach include the ECO2 process from Powerspan or the Alstrom CAP—Chilled Ammonia Process. The presentation entitled “ECO2 Technology—Basin Electric Power Cooperative's 120 MWe CCS Demonstration”, Alix et al, MIT Carbon Sequestration Forum IX, 2008 provides a very detailed overview of the ammonia based carbon capture process and economics. Also the presentation “Chilled Ammonium Process (CAP) for Post Combustion CO2 Capture,” Gal et al, 2nd Annual Carbon Capture and Transportation Workshop, California, March 2006 provides details of the chilled ammonia process and economics.
Forward Osmosis Process
An entirely different process called “forward osmosis” is currently being developed to desalinate saline and contaminated waters. In this forward osmosis process a “draw solution” is used to create an osmotic pressure differential and the water to be desalinated is “drawn” through an osmotic membrane into the draw solution. In osmotic membranes the water passes preferentially through the membrane over salts dissolved in the water, resulting in a desalination. The water is then separated from the draw solution as purified or desalinated water and the draw solution is reused. The water to be desalinated, as is amply described in the references, may range from seawater, to oil or gas produced waters, to industrial and municipal wastewaters. The common feature of these waters to be desalinated is that they all contain dissolved salts above the level at which the water can be used for any particular purpose such as potable water, agricultural irrigation water, or cooling tower makeup. “Forward osmosis: Principles, applications, and recent developments,” Elimelech et al, Journal of Membrane Science 281 (2006) 70-87 provides a basic review and detail discussion of the process and its applications.
In one variation of this forward osmosis technique the “draw solution” is based on ammonium bicarbonate (in this application I treat the term ammonium carbonate solution as a mixture of ammonia, ammonium, carbonate, bicarbonate, carbamate, and CO2 species as will be readily apparent to anyone skilled in the art). Ammonium carbonate in high concentration exhibits a very high osmotic pressure and when separated from seawater by an osmotic membrane, water (but not salts) permeates the membrane and flows into the draw solution. The draw solution is now somewhat diluted. To recover the water, the ammonia and carbon dioxide needs to be recovered from the draw solution. This is typically accomplished by heating the solution, causing the ammonia and carbon dioxide to vaporize from the solution where they can be recovered and re-dissolved in water to create more draw solution. Of course, in a large scale setting this will be done on a continuous basis. The paper “A novel ammonia-carbon dioxide forward (direct) osmosis desalination process,” J. R. McCutcheon et al./Desalination 174 (2005) 1-11 describes this system in detail. U.S. Pat. Nos. 6,391,205 and 7,560,029 as well as US Patent Application No. 20050145568 describes similar processes.
Osmotic Power
Another yet entirely different yet related process called Osmotic Power, or direct osmosis or pressure retarded osmosis or salinity gradient osmosis is also currently being developed and used to generate osmotic power. In this process, fresh water is contacted through a semi-permeable membrane against a more concentrated solution. Water flows from the freshwater into the more concentrated water. If the concentrated water side is constrained in volume, a pressure develops which can ultimately equal the osmotic pressure differential between the two solutions. Typically this process is applicable to areas where fresh water rivers empty into the sea. The osmotic power process is currently in large scale prototype development, primarily in Europe. The Norwegian company StatKraft is the current leader in the process. Background information can be found in references such as Stein Erik Skilhagen—Osmotic Power presentation March 2008 at Wirec 2008 or in “Salinity Power Plants May be the Next Eco-Power Generating Tech,” by Kit Eaton, Feb. 26, 2009 in FastCompany (www.fastcompany.com).
Osmotic Power Heat Engines are also described by Elimelech et al in “A novel ammonia-carbon dioxide osmotic heat engine for power generation,” Journal of Membrane Science 305 (2007) 13-19. In these applications, power is produced through a combination of forward osmosis of high purity distilled water into a concentrate of ammonium carbonates. This produces a pressurized, but now diluted ammonium carbonate stream. The pressure energy is recovered via a turbine or work exchanger device and the diluted draw solution is reconstituted using heat in the typical manner of forward osmosis. No net desalinated water is in the process and the recovered water from the draw solution is recycled back to the process.