Patent Application
20120145000
The combustion of fossil fuels like coal or natural gas is commonly used to provide heat required for many different industrial processes including electricity generation, hydrogen production, steel making etc. The combustion process involves burning of carbon containing species in the presence of oxygen to produce heat, carbon dioxide, water along with other pollutants like SOx, NOx, mercury etc. It is well known that carbon dioxide is a greenhouse gas causing climate change and many government regulations are underway to prevent carbon dioxide emissions.
Carbon Capture and Sequestration (CCS) is one of the most promising routes to capture carbon dioxide and sequester in under ground geologic formations or use for enhanced oil recovery application etc. However, there is limitation on the amount of gas that can be stored under ground and hence pure carbon dioxide is preferred for storage in order to maximize the utility of the storage space. Power plants are considered to be one of the biggest source for carbon dioxide emission. Conventional air fired boilers use air as an oxidant for the combustion process and produces flue gas with between 10% and 16% carbon dioxide content. Amine solvents are well known to be used for capturing carbon dioxide from the post combustion process. However, the amount of regeneration energy needed for solvent regeneration is very high and decreases the efficiency of overall capture process.
Oxy-combustion is one of the most promising technologies to capture carbon dioxide from power plants. Oxy-combustion involves the use of essentially pure oxygen (>89% purity) instead of air for the combustion process, thereby concentrating the amount of carbon dioxide in the flue gas to more than about 70% to about 80% content. Nitrogen is essentially eliminated from the process since it does not participate in the combustion process and dilutes the amount of carbon dioxide in flue gas in the air fired boiler. The use of pure oxygen can increase the flame temperature significantly hence carbon dioxide is usually recycled back to the boiler in order to maintain similar flame characteristics as the air fired boiler.
Flue gas from oxy-fired boiler contains more than about 70% to about 80% (dry basis) carbon dioxide with other compounds including but not limited to nitrogen, oxygen, argon, SOx, NOx, mercury, water vapor etc. The amount of carbon dioxide will depend on air infiltration, oxygen purity and coal composition. New power plants can be designed to minimize the air infiltration inside the boiler. Flue gas can be further treated to remove all the impurities and produce pure carbon dioxide for capture in a carbon dioxide Compression and Purification Unit (CPU).
The CPU system generally consists of low pressure impurity removal, compression, high pressure impurity removal followed by optional partial condensation and distillation. Moisture in the flue gas can react with SOx, NOx and CO2 at high pressure to form sulfuric acid, nitric acid or carbonic acid along with other compounds. It can also cause corrosion problem inside the compressor, freeze at cold conditions inside the cold box etc. Flue gas drying is very critical in order to avoid corrosion, unwanted reactions or freezing at cold conditions.
Thermal swing adsorption systems (TSA) using adsorbent at high pressure have commonly been employed to remove moisture from flue gas. However, moisture removal at high pressure can lead to corrosion at the upstream process and also require acid condensate handling procedures. Flue gas drying at low pressure can solve the corrosion problem with equipments. However traditional means of drying, such as fixed adsorption beds, are not economical because of reduced adsorption loading capacity.
An apparatus and method for drying a moist gas is provided. The apparatus includes a revolving desiccant rotor with at least an adsorption sector and a regeneration chamber; the regeneration chamber comprising at least a first dry gas sector, a hot regeneration sector, and a second dry gas sector, and optionally a cold regeneration sector. The method includes at least an adsorption sector and a regeneration chamber; the regeneration chamber comprising at least a first dry gas sector, a hot regeneration sector, and a second dry gas sector, and optionally a cold regeneration sector This method includes the steps of; contacting a moist gas stream with the desiccant in the adsorption sector, thereby producing a dry gas stream; contacting a first dry gas stream with the desiccant in the first dry gas sector, thereby producing a first wet gas stream; contacting a hot partially wet gas stream with the desiccant in the hot regeneration sector, thereby producing a warm wet gas stream; contacting a dry regeneration gas stream with the desiccant in the second dry gas sector, thereby producing a wet regeneration gas stream, and contacting a regeneration purge gas stream with the desiccant in the cold regeneration sector, thereby producing a warm purge gas stream.
a and 4b illustrates a more detailed layout of the rotating desiccant in accordance with one embodiment of the present invention.
Illustrative embodiments of the invention are described below. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer's specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.
The present invention utilizes a revolving desiccant rotor at low pressure to remove moisture from a wet gas (such as a flue gas). A desiccant rotor is a continuous drying process with a portion of the rotor in an adsorption mode and a portion of wheel in a regeneration mode concurrently. Flue gas drying can be accomplished in a single stage at low pressure or multiple stages in combination with high pressure drying. For a single stage drying using desiccant rotor, the flue gas drying is accomplished using at least one rotating desiccant wheel in order to avoid moisture condensation in the downstream processes. In a multiple stage flue gas drying solution, the desiccant rotor can be used in combination with high pressure fixed bed or high pressure liquid carbon dioxide scrubbing column or any other high pressure drying technique. The first stage drying using the desiccant rotor is to ensure that the dried flue gas is well below saturation at the operating condition of units in between the two drying units in order to avoid moisture condensation. The second stage drying is to remove remaining moisture at high pressure using high pressure drying techniques and avoid moisture condensation in the downstream processes.
Carbon dioxide capture using oxy-combustion and CPU is well known in the art, as represented in
The CPU process, as represented in
Referring now to
A portion of the rotor is in the adsorption mode (Sector 1) and a portion of the rotor in the regeneration mode (Sector 2 and Sector 3). Regeneration could consist of several stages including, but not limited to, hot regeneration and cold regeneration etc. The rotor may consist of an adsorption sector and a regeneration chamber. In its most basic form, the regeneration chamber may consist of a hot regeneration sector (Sector 2) and optionally a cold regeneration sector (Sector 3). As the rotating desiccant wheel enters the regeneration phase, a dry hot gas is introduced into the First Dry Gas Sector, in order to preheat the adsorbent and prepare it for desorption. A hot partially wet gas stream then enters the Hot Regeneration Sector, which further heats the adsorbent and begins the regeneration. Then a hot dry gas is introduced into the Second Dry Gas to complete the regeneration process
The rotor section in the adsorption mode could vary from 40% to 80% preferable from 45% to 65% and more preferable from 50% to 60%. The rotor section in hot regeneration mode could vary from 10% to 50% preferable from 15% to 35% and more preferably from 20% to 30%. The rotor section in cold regeneration mode could vary from 10% to 50% preferable from 15% to 35% and more preferably from 20% to 30%. Cold regeneration step may be optional depending on the desired moisture content in the process output gas.
In one embodiment of the present invention, a moist gas stream (for example, a flue gas) 301 enters the adsorption sector (Sector 1), comes in contact with dry, regenerated adsorbent and exits as dry gas stream 302. Simultaneously, a hot partially wet gas stream 303 enters the hot regeneration sector (Sector 2), comes in contact with an adsorbent to be regenerated, and exits as warm wet gas stream 304. And also simultaneously, a regeneration purge gas stream 305 may enter the cold regeneration sector (Sector 3), comes in contact with partially regenerated adsorbent, and exits as a warm purge gas stream 306.
The flow of process gas and regeneration gas could be co-current or counter-current, preferably co-current for process gas and cold regeneration gas and counter-current with hot regeneration gas. The hot regeneration gas could be hot flue gas directly from the boiler or heated non-condensable gas from cold box or heated nitrogen from ASU or heated dry process gas (flue gas). As indicated in
In another embodiment of the present invention, a moist gas stream (for example, a flue gas) 401 enters the adsorption sector (Sector 1), comes in contact with dry, regenerated adsorbent and exits as dry gas stream 402. Simultaneously, a first dry gas stream 407 enters the first dry gas sector of the hot regeneration sector, comes in contact with an adsorbent to be regenerated, and exits as a first wet gas stream 408. Simultaneously, a hot partially wet gas stream 403 enters the hot regeneration sector (Sector 2), comes in contact with an adsorbent to be regenerated, and exits as warm wet gas stream 404. And also simultaneously, a regeneration purge gas stream 405 may enter the cold regeneration sector (Sector 3), comes in contact with partially regenerated adsorbent, and exits as a warm purge gas stream 406.
The cold regeneration gas could be a slip stream from dry process gas from the desiccant rotor or non-condensable gas from cold box or nitrogen from ASU. The wet process gas is dried in the rotor during adsorption mode and the outlet dry process gas is further sent to CPU. The hot regeneration gas is used to desorb moisture from the adsorbent where the temperature of regeneration gas decreases. The hot regeneration gas from the outlet of the rotor can be either recycled back to CPU for processing or recycle back to boiler in case flue gas is used for hot regeneration gas. The cold regeneration gas is used to cool down the adsorbent before further adsorption in order to increase the adsorption capacity. The cold regeneration gas from the outlet of the rotor can be either recycled back to CPU for processing or recycle back to boiler in case flue gas is used for cold regeneration gas.
Regeneration purge gas stream 507 then passes through the revolving desiccant rotor 504 in accordance with the above discussions and exits as warm purge gas stream 508. At least a portion 509 of warm purge gas stream 508 is directed to heater 510, where it exits as hot partially wet gas stream 511. Hot partially wet gas stream 511 then passes through the revolving desiccant rotor 504 in accordance with the above discussions and exits as warm wet gas stream 512. Other configurations are possible, and would not require undue experimentation to the skilled artisan. For example, heater 510 may be directly incorporated in the revolving desiccant rotor 504.
The desiccant rotor may be incorporated into a conventional CPU system in a number of ways depending on the specific need, and none of which would require undue experimentation for the skilled artisan. In one embodiment, as indicated in
A moist gas stream (for example, a flue gas stream) 601 enters a Low Pressure (LP) cooler and polisher 602. The output from the LP cooler and polisher 602 has the pressure boosted by booster fan 603, and cooled in first heat exchanger 605. The resulting moist gas stream is then introduced to desiccant rotor 606, as discussed and described above. The resulting dry gas stream is then compressed in compressor 608 and cooled in second heat exchanger 609. The resulting stream is then admitted into the cold box 610, thereby producing incondensable gas stream 611 and CO2 stream 612.
As indicated in
A moist gas stream (for example, a flue gas stream) 701 enters a Low Pressure (LP) cooler and polisher 702. The output from the LP cooler and polisher 702 has the pressure boosted by booster fan 703, and cooled in first heat exchanger 705. The resulting moist gas stream is then introduced to desiccant rotor 706, as discussed and described above. The resulting dry gas stream is then compressed in compressor 708 and cooled in second heat exchanger 709. The dry, cooled and compressed stream is then introduced into a High Pressure (HP) dryer 713, for example of the fixed bed design. The resulting stream is then admitted into the cold box 710, thereby producing incondensable gas stream 711 and CO2 stream 712.
As indicated in
A moist gas stream (for example, a flue gas stream) 801 enters a Low Pressure (LP) cooler and polisher 802. The output from the LP cooler and polisher 802 has the pressure boosted by booster fan 803, and cooled in first heat exchanger 805. The resulting moist gas stream is then introduced to desiccant rotor 806, as discussed and described above. The resulting dry gas stream is then compressed in compressor 808 and cooled in second heat exchanger 809. The dry, cooled and compressed stream is then introduced into a High Pressure (HP) carbon dioxide scrubber 813, which produces a waste water stream 814. The resulting stream is then admitted into the cold box 810, thereby producing incondensable gas stream 811 and CO2 stream 812.
