Abstract:
Ultra cleaning of combustion gas to near zero concentration of residual contaminants followed by the capture of CO 2  is provided. The high removal efficiency of residual contaminants is accomplished by direct contact cooling and scrubbing of the gas with cold water. The temperature of the combustion gas is reduced to 0-20 degrees Celsius to achieve maximum condensation and gas cleaning effect. The CO 2  is captured from the cooled and clean flue gas in a CO 2  absorber ( 134 ) utilizing an ammoniated solution or slurry in the NH 3 —CO 2 H 2 O system. The absorber operates at 0-20 degrees Celsius. Regeneration is accomplished by elevating the pressure and temperature of the CO 2 -rich solution from the absorber. The CO 2  vapor pressure is high and a pressurized CO 2  stream, with low concentration of NH 3  and water vapor is generated. The high pressure CO 2  stream is cooled and washed to recover the ammonia and moisture from the gas.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to systems and methods for ultra cleaning of combustion gas followed by the capture and regeneration of CO 2 .  
       BACKGROUND  
       [0002]     Most of the energy used in the world today is derived from the combustion of carbon and hydrogen containing fuels such as coal, oil and natural gas. In addition to carbon and hydrogen, these fuels contain oxygen, moisture and contaminants such as ash, sulfur, nitrogen compounds, chlorine, mercury and other trace elements. Awareness to the damaging effects of the contaminants released during combustion triggers the enforcement of ever more stringent limits on emissions from power plants, refineries and other industrial processes. There is an increased pressure on operators of such plants to achieve near zero emission of contaminants and to reduce CO 2  emission.  
         [0003]     The art teaches various processes and technologies designed to reduce the emission of contaminants from combustion gases. Baghouses, electrostatic precipitators and wet scrubbers are typically used to capture particulate matter, various chemical processes are used to reduce sulfur oxides, HCl and HF emissions, combustion modifications and NO x  reduction processes are used to reduce NO x  emission and processes are being developed to capture mercury and other trace elements from combustion gas.  
         [0004]     Significant progress has been made in the last 20-30 years and plants today are a lot cleaner and safer to the environment than in the past. However, there are growing indications that even small concentration of particulate matter and especially the very fine, less than 2.5 micron size particles (PM2.5), sulfur oxides, acid mist and mercury are harmful to human health and need to be controlled.  
         [0005]     Controlling the residual emission is still a challenge and with existing technologies the cost of capturing the last few percents of harmful contaminants is very high.  
         [0006]     In addition, in the last few years, there is a growing concern related to the accumulation of CO 2 , a greenhouse gas, in the atmosphere. The accelerated increase of CO 2  concentration in the atmosphere is attributed to the growing use of fuels, such as coal, oil and gas, which release billions of tons of CO 2  to the atmosphere every year.  
         [0007]     Reduction in CO 2  emission can be achieved by improving efficiency of energy utilization, by switching to lower carbon concentration fuels and by using alternative, CO 2  neutral, energy sources. However, short of a major breakthrough in energy technology, CO 2  emitting fuels will continue to be the main source of energy in the foreseeable future. Consequently, a low cost low energy consuming process for capturing and sequestering CO 2  is needed to reverse the trend of global warming.  
         [0008]     State of the art technologies for capturing CO 2  are not suitable for operation with dirty, low pressure, low CO 2  concentration, and oxygen containing combustion gases. Available commercial technologies for CO 2  capture are energy intensive and high cost. If applied they would impose a heavy toll on the cost of energy utilization.  
         [0009]     An applicable process currently available for post combustion CO 2  capture is the amine process using Mono-Ethanol-Amine (MEA) or similar amines to react with CO 2 . The MEA process is capable of achieving high capture efficiency and of generating a concentrated CO 2  stream for sequestration. However, the process has several drawbacks including: 
        The MEA reagent is expensive and degrades in oxygen and CO 2  environment.     The MEA is corrosive and can be used only in a relatively diluted form.     The reaction of MEA with CO 2  is highly exothermic.     Regeneration is energy intensive.     The process is a large consumer of heat and auxiliary power.        
 
         [0015]     The cost of the amine process and system is very high and the net power output of a power plant equipped with amine system to capture CO 2  is greatly reduced.  
         [0016]     To achieve clean burning of fuels with near zero emission, including the emission of CO 2 , there is a need for a low cost low energy process that: 
        Captures residual contaminants     Captures CO 2  and releases it in a concentrated and high pressure form for sequestration.        
 
         [0019]     Accordingly, it would be considered an advance in the art to develop new systems and methods to overcome the current problems and shortcomings.  
       SUMMARY OF THE INVENTION  
       [0020]     The present invention is an integrated method and system to efficiently and cost effectively reduce the emission of residuals, such as SO 2 , SO 3 , HCl, HF and particulate matter including PM2.5, from combustion gas, downstream of conventional air pollution control systems, to near zero levels. Further, the system of the current invention reduces CO 2  emission by capturing and delivering it to sequestration in a concentrated form and at high pressure. It is the objective of this invention that the process would be relatively uncomplicated, would utilize low cost reagent, would generate no additional waste streams and most importantly, would be a low cost and low energy consumer.  
         [0021]     The present invention is a wet method and system whereby the saturated combustion gas, downstream of conventional air pollution control equipment and system, is cooled to well below its ambient saturation temperature. The cooling is achieved by direct contact with cold water in dedicated vessels. The direct contact between the gas and the liquid, combining with massive condensation of moisture from the saturated gas, is a very efficient wet scrubber. Optionally, alkaline materials such as sodium or ammonium carbonate can be added to the direct contact cooler to enhance the capture of the acidic species in the gas. The direct cooling to low temperature can be achieved in one or more cooling stages. Continuous bleed from the direct contact cooler, prevents the accumulation of the captured contaminants in the direct contact coolers.  
         [0022]     In a preferred embodiment, the chilled water will be generated in cooling towers with additional cooling, to low temperature in the range of 0-20, or even 0-10, degrees Celsius, by efficient mechanical vapor compression where the water itself is used as the refrigerant.  
         [0023]     In accordance with the current invention, cooling of the gas substantially reduces its moisture content. The cooled and low moisture gas has relatively low volume and relatively high CO 2  concentration thus making the efficient capture of CO 2  easier and lower cost.  
         [0024]     The invention further involves the mass transfer and the reaction of gaseous CO 2  from the combustion gas with CO 2 -lean ammoniated solution to form CO 2 -rich ammoniated solution. According to the current invention, the absorption reaction occurs in a CO 2  absorber operating at about atmospheric pressure and at low temperature preferably in the temperature range of 0-20, or even 0-10, degrees Celsius. The low temperature enhances mass transfer of CO 2  to the solution while substantially reducing the vapor pressure of ammonia and preventing its evaporation into the gas stream. One or more stages of CO 2  absorption can be used depending on the capture efficiency requirements.  
         [0025]     Further, in accordance with the current invention, the pressure of the CO 2 -rich solution from the CO 2  absorber is elevated by high-pressure pump to the range of 30-2000 psi and it is heated to the temperature range of 50-200 degrees Celsius and preferably to the temperature range of 100-150 degrees Celsius. Under the conditions above the CO 2  separates from the solution and evolves as a relatively clean and high-pressure gas stream. The high pressure CO 2  gas stream contains low concentration of ammonia and water vapor, which can be recovered by cold washing of the CO 2  gas steam.  
         [0026]     The regeneration reaction is endothermic. However, the heat of reaction is low and the overall heat consumption of the process is relatively low. Moreover, the high-pressure regeneration minimizes the evaporation of ammonia and water thus minimizing the energy consumed in the process. Also, low-grade heat can be used for the regeneration of the CO 2  to further reduce the impact of the CO 2  capture on the overall efficiency of the plant. The CO 2 -lean solution used in the absorber to capture the CO 2  contains NH 3 /CO 2  mole ratio in the range of 1.5-4.0 and preferably in the range of 1.5-3.0. The CO 2 -rich solution sent for regeneration contains NH 3 /CO 2  mole ratio in the range of 1.0-2.0 and preferably in the range of 1.0-1.5.  
         [0027]     The present invention has the advantage of high efficiency low cost capture of residual contaminants from the combustion gas followed by high efficiency low cost capture and regeneration of CO 2 . Low temperature absorption and high-pressure regeneration are critical to successful operation of the process and system. The simple, low cost and efficient system has notable advantage over other cleaning and CO 2  capturing processes and it is a real breakthrough in achieving the objective of near zero emission of contaminants. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]     The above and other advantages of this invention will become more apparent from the following description taken in conjunction with the accompanying drawings, in which:  
         [0029]      FIG. 1  is a schematic representation of the integrated system to capture residual contaminants and CO 2  from combustion gas downstream of conventional air pollution control systems. The system includes gas cleaning, CO 2  absorption and CO 2  regeneration.  
         [0030]      FIG. 2  is a schematic of the subsystems for the cooling of the gas and for deep cleaning of residual contaminants.  
         [0031]      FIG. 3  is a schematic of the CO 2  capture and regeneration subsystems. It includes CO 2  absorber which operates at low temperature and CO 2  regenerator which operates at moderate temperature and at high pressure. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]     In accordance with the present invention, a process and system to remove most contaminants, including CO 2 , from gas streams is provided. These gases are typically resulting from the combustion or gasification of coal, liquid fuels, gaseous fuels and organic waste materials. The contaminants include residual of e.g. SO 2 , SO 3 , HCl, HF, CO 2 , particulate matter including PM2.5, mercury and other volatile matter. The high removal efficiency of the contaminants is achieved by saturation and efficient cooling of the gas to below its adiabatic saturation temperature and preferably to as low as 0-20, or even 0-10, degrees Celsius. Fine particles and acid mist are nucleation sites for the condensation of water. Thus, practically all fine particles and acid mist are removed from the gas stream. The low temperature creates an environment of low vapor pressure of SO 2 , SO 3 , HCl, HF, Mercury and other volatile matter, which condense into the cold water as well.  
         [0033]     The cooling of the flue gas enables the efficient capture of CO 2  in CO 2 -lean ammoniated solution or slurry. Absorption of the CO 2  is achieved at low temperature preferably at as low as 0-20 degrees Celsius or at as low as 0-10 degrees Celsius. The absorbent is regenerated by elevating the temperature of the solution or slurry to the range of 50-200 degrees Celsius and to pressures in the range of 30-2000 psig. The low temperature of absorption and the high pressure of regeneration result in high CO 2  capture efficiency, low energy consumption and low loss of ammonia through evaporation.  
         [0034]     The CO 2  absorption takes place in the aqueous NH 3 —CO 2 —H 2 O system where the ammonia can be in the form of ammonium ion, NH 4   + , or in the form of dissolved molecular NH 3 . The CO 2  can be in the form of carbonate, CO 3   = , bicarbonate, HCO 3   −  or in the form of dissolved molecular CO 2 . The capacity of the solution to absorb CO 2  and the form in which the species are present depends on the ammonia concentration, on the NH 3 /CO 2  mole ratio and on the temperature and pressure.  
         [0035]     High NH 3 /CO 2  mole ratio increases the vapor pressure of ammonia and results in ammonia losses through evaporation. Low NH 3 /CO 2  ratio increases the vapor pressure of CO 2  and decreases its capture efficiency. Thus, the optimal NH 3 /CO 2  mole ratio for absorption is in the range of 1.0-4.0 and preferably in the range of 1.5 to 3.0. High temperature increases the vapor pressure of both ammonia and CO 2 . As a result, the absorber should operate at the lowest practical temperature and preferably in the 0-20 degrees Celsius temperature range or even in the 0-10 degrees Celsius temperature range.  
         [0036]     At high concentration and lower temperature the solubility limits may be reached and solids particles precipitate. These solids particles are typically in the form of ammonium carbonate (NH 4 ) 2 CO 3  for high NH 3 /CO 2  ratio and ammonium bicarbonate NH 4 HCO 3  for low NH 3 /CO 2  ratio.  
         [0037]     The mass transfer and absorption reactions for concentrated low temperature slurries are the following: 
 
CO 2 ( g )-→CO 2 ( aq ) 
 
CO 2 ( aq )+H 2 O-→H + +HCO 3   − 
 
(NH 4 ) 2 CO 3 ( s )-→2NH 4   + +CO 3   = 
 
H + +CO 3   = -→HCO 3   − 
 
HCO 3   − +NH 4   + -→NH 4 HCO 3 ( s ) 
 
         [0038]     Where CO 2  captured from the gas converts ammonium carbonate to ammonium bicarbonate. The reactions above are reversible and CO 2  is stripped from the liquid phase at elevated temperature.  
         [0039]     Depending on the operating conditions, side undesired reactions may occur such as: 
 
NH 4   + +CO 3   = -→NH 3 ( g )+HCO 3   − 
 
NH 4   + +HCO 3   − -→NH 3 ( g )+CO 2 ( g )+H 2 O 
 
         [0040]     Causing emission of NH 3  into the gas phase. Lower temperature and lower NH 3 /CO 2  ratio in the absorber suppresses these undesired reactions. However, during the regeneration and at elevated temperature, gaseous ammonia is formed. To prevent ammonia from escaping from the liquid phase (and for other reasons) the regenerator is deigned to operate under elevated pressure and under conditions where the solubility of ammonia in the solution is very high and the emission of gaseous ammonia is very low.  
         [0041]      FIG. 1  is a schematic representation of the integrated process, which includes cleaning and cooling of the gas, CO 2  absorption into CO 2 -lean ammoniated solution and CO 2  regeneration from the CO 2 -rich solution. Stream  102  is a gas stream from combustion or industrial process containing residual contaminants, CO 2  and inert gas species. The CO 2  concentration of the gas is typically 10-15% for coal combustion and 3-4% for natural gas combustion. Subsystem  130  represents a series of conventional air pollution control processes which, depending on the source of the gas may include particulate collectors, NO x  and SO 2  control, acid mist capturing device and more. The contaminants collected in the system are removed in stream  112 . Stream  104 , downstream of the conventional cleaning devices, contains residual contaminants not collected by the conventional systems. It is typically water saturated and in the temperature range of 40-70 degrees Celsius. Subsystem  132  is a series of one or more Direct Contact Coolers (DCC), where cold water generated in cooling towers and chillers (not shown) is used to wash and scrub the gas, capture its residual contaminants and lower its moisture content. Stream  114 , is a bleed from subsystem  132  designed to purge all the residual contaminants captured.  
         [0042]     Stream  106  is a cooled gas suitable for CO 2  capture in the CO 2  absorber. Subsystem  134  represents the CO 2  absorber and may comprise of a series of absorber stages, depending on the removal efficiency required and the operating conditions of the plant. The clean gas with low CO 2  concentration, stream  108 , is released to the atmosphere. Stream  124  is a cooled CO 2 -lean ammoniated solution from the regenerator, subsystem  136 , which is used as the absorbent to capture the CO 2  in the absorber. The resultant stream  120  is a CO 2 -rich ammoniated solution sent for regeneration.  
         [0043]     The regenerator, subsystem  136 , operates at high pressure and elevated temperature and may be a single or a series of regeneration reactors. The pressure of the ammoniated solution fed to the regenerator is elevated using high pressure pump, pump  138 , to yield stream  122  which is CO 2 -rich and at high pressure. Typically, the pressure of stream  122  is in the range of 50-2500 psi, higher than the regenerator pressure to prevent premature evaporation of CO 2 . Heat is provided to the regenerator by heating stream  126  in heater  140 . The high pressure and high temperature in the regenerator cause the release of high-pressure gaseous CO 2 , stream  110 . The high-pressure regeneration has major cost and energy advantage. Low quality thermal energy is used to generate the high pressure CO 2  stream instead of high-value electric power.  
         [0044]      FIG. 2  is a schematic representation of the cooling and cleaning subsystems, which may optionally include waste heat recovery, heat exchanger  240 , for utilization of the residual heat in the gas. The residual heat in stream  202  can be extracted in heat exchanger  240  by transferring of the heat to a cooling medium streams  220  and  222 . The heat can then be used downstream for CO 2  regeneration.  
         [0045]     Vessel  242  is a wet direct contact scrubber used to adiabatically cool and saturate the gas. If the gas contains high concentration of acid species, such as gas from coal or oil fired power plants, then reactor  242  is used for flue gas desulfurization. Acid absorbing reagent, such as limestone, stream  226 , is added to vessel  242  and the product, such as gypsum, stream  224 , is withdrawn. Make up water, stream  227 , is added to vessel  242  from the Direct Contact Cooler (DCC)  244 . The make up stream contains all the contaminants collected in the direct contact coolers. These contaminants are removed from the system with the discharge stream  224 . Gas stream  202  in coal fired boiler is typically at temperature in the range of 100-200 degrees Celsius, gas stream  204  is typically at temperature range of 80-100 degrees Celsius and gas stream  206  is typically water-saturated and at temperature range of 40-70 degrees Celsius.  
         [0046]     Two stages of direct contact cooling and cleaning, vessels  244  and  246 , are shown in  FIG. 2 . The actual number of direct contact coolers may be higher and it depends on optimization between capital cost, energy efficiency and cleaning efficiency requirements.  
         [0047]     Gas stream  206  is cooled in DCC  244  to just above the cooling water temperature of stream  230 . The temperature of the cooling water, stream  230 , depends on the ambient conditions and on the operation and process conditions of Cooling Tower  250 . Cooling Tower  250  can be of the wet type with temperature slightly below or slightly above ambient temperature, or the dry type with temperature above ambient temperature. Ambient air, Stream  212  provides the heat sink for the system and the heat is rejected in Stream  214 , which absorbs the heat from water stream  228 . The resultant cooled water stream  230 , is typically at temperature range of 25-40 degrees Celsius and the resultant cooled combustion gas stream from DCC  244  is at about 1-3 degrees Celsius higher temperature. Alkaline materials such as ammonium or sodium carbonate can be added to DCC  244  to neutralize the acidic species captured. The alkaline materials can be added in makeup water, stream  225 .  
         [0048]     The cleaner and lower temperature, Stream  208  flows to DCC  246 , which is similar to DCC  244  except for the fact that colder water, stream  234 , is used for cooling. Stream  234  is a chilled water stream cooled by Chiller  248 , which is preferably a mechanical vapor compression machine with water as its refrigerant. Heat from Chiller  248  is rejected via stream  236  to Cooling Tower  250  with returning stream  238 . Cooling water stream  234  can be as cold as 0-3 degrees Celsius or higher resulting in combustion gas temperature, stream  210 , exiting DCC  246  being at 0-10 degrees Celsius temperature or few degrees higher. The heat absorbed from the gas stream is removed from DCC  246  via water stream  232 . More condensation occurs in DCC  246  and further capture of contaminants. These contaminants are bled from the system to vessel  242 . (Bleed stream is not shown).  
         [0049]     Gas Stream  210 , the product of the cooling and cleaning subsystem shown in  FIG. 2 , is at low temperature; it contains low moisture and practically has no particulate matter, acidic or volatile species.  
         [0050]      FIG. 3  is a schematic representation of the CO 2  capture and regeneration subsystem. Stream  302  is a clean and cooled gas stream, similar to stream  210  in  FIG. 2 . It flows into the CO 2  absorber  350 , where the CO 2  is absorbed by a cooled CO 2 -lean ammoniated solution or slurry, Stream  324  containing NH 3 /CO 2  mole ratio in the range of 1.5-4.0 and preferably 1.5-3.0. Depending on the absorber design and the number of absorption stages used, more than 90% of the CO 2  in Stream  302  can be captured to yield a cold and CO 2  depleted gas stream  304 . Residual ammonia in stream  304  can be washed in vessel  356  by cold water or by cold and slightly acidic solution, stream  338 . Stream  338  is cooled in heat exchanger  368 . As a result of the cooling, cleaning and CO 2  capture, the gas stream discharged from the system, Stream  306 , contains mainly nitrogen, oxygen and low concentration of CO 2  and H 2 O.  
         [0051]     Stream  324  is a CO 2 -lean stream from the regenerator, which is cooled in the regenerative heat exchanger  354  and further by chilled water in heat exchanger  362 . It captures CO 2  in absorber  350  and discharges from the absorber, Stream  312 , as a CO 2 -rich stream with NH 3 /CO 2  mole ratio in the range of 1.0-2.0 and preferably with NH 3 /CO 2  mole ratio in the range of 1.0-1.5. In a preferred embodiment, stream  312  contains high concentration of dissolved and suspended ammonium bicarbonate. A portion of stream  312  is optionally recycled back to the absorber while the balance, Stream  314 , is pressurized in high pressure pump  360  to yield high pressure ammoniated solution stream  316 . Stream  316  is heated in regenerative heat exchanger  354 , by exchanging heat with the hot and CO 2 -lean stream from the regenerator, stream  322 , which is a portion of stream  320  discharged at the bottom of regenerator  352 .  
         [0052]     The CO 2 -rich stream from the regenerative heat exchanger  354 , stream  318 , can be further heated with waste heat from the boiler or from other sources. It flows into the regenerator  352 , which has one or more stages of regeneration. More heat is provided to the regenerator from heat exchanger  364 , which heats stream  330 . The heat provided to the system from the various sources, elevates the regenerator temperature to 50-150 degrees Celsius or higher, depending on the desired pressure of the CO 2  stream  308  and subject to cost optimization consideration. The higher the temperature the higher will be the pressure of the CO 2  that evolves from the solution, stream  308 . The higher the pressure the lower will be the ammonia and water vapor content of stream  308 . To generate low temperature and highly concentrated CO 2  stream, stream  308  is washed and cooled in direct contact vessel  358  with cold water, stream  336  from heat exchanger  366 . Excess water and NH 3  captured in vessel  358 , stream  332 , flows back to regenerator  352  while the balance, stream  334 , is cooled and recycled to the wash chamber, vessel  358 .  
         [0053]     The present invention has now been described in accordance with several exemplary embodiments, which are intended to be illustrative in all aspects, rather than restrictive. Thus, the present invention is capable of many variations in detailed implementation, which may be derived from the description contained herein by a person of ordinary skill in the art. All such variations and other variations are considered to be within the scope and spirit of the present invention as defined by the following claims and their legal equivalents.