Abstract:
A process is provided for the removal of impurities from the exhaust gases being emitted from a power generation plant. A multi-stage temperature based separation system is being utilized. The removal of undesirable compounds/elements is accomplished by directing the exhaust gases from the power plant through a first stage cooling unit to lower the temperature of the exhaust gases to a first predetermined level that is below the boiling point of one or more of the undesired compounds/elements, diverting the resulting liquid to storage, directing the gas to a second stage cooling unit to further lower the temperature of the exhaust gases to a second predetermined level that is below the boiling point of several more of the undesired compounds/elements. The resulting exhaust gas is a very clean syngas that may be used in various commercial applications.

Description:
TECHNICAL FIELD 
       [0001]    The subject design relates generally to a process of separating impurities from exhaust gases being emitted from a power plant and more specifically relates to a process of using a temperature based system to separate the impurities from the exhaust gases. 
       BACKGROUND 
       [0002]    A common problem with power plant exhaust gas processing is dealing with emissions and impurities. Most industries find that dealing with this problem is very costly as well as complex. An example would be coal fired plants and their emission of sulfur dioxide (SO 2 ), nitrogen oxides (NO 2 ), carbon dioxide (CO 2 ) and other potentially harmful gases. Due to the high cost of cleaning up their missions, many coal fired plants choose to shut down. Likewise, even when using plasma fired plants, gases, such as, hydrogen chloride (HCl), carbon dioxide (CO 2 ), hydrogen sulfide (H 2 S), and carbonyl sulfide (COS) need to be removed in order to produce a more clean synthesis gas (syngas). There have been many different arrangements that attempt to remove detrimental exhaust gas compositions but most of them are only partially effective in removing most if not all of the detrimental exhaust gas compositions. This many times is based on the extreme costs of effective types of exhaust gas clean-up systems. The subject design serves as a possible solution to at least the removal of the above noted detrimental exhaust gases. 
       SUMMARY OF THE INVENTION 
       [0003]    According to the present design, a process for clean-up of emitted gases from various types of power plants is provided. The subject process includes various operational steps. The steps generally include directing the gases from a power plant to a first stage cooling unit wherein the temperature of the gases is lowered to a predetermined level resulting in some components of the gases converting to their liquid state since the temperature within the first stage cooling unit is below their respective boiling points. Following the first stage cooling step, the solution of gas and liquid is pumped from the first stage cooling unit into a first gas-liquid separator. The liquid is directed to a first storage unit and the gas is directed to a second stage cooling unit wherein the temperature therein is at a much lower level resulting in other components in the gas converting to their liquid state since the temperature within the second stage cooling unit is below their respective boiling points. Following the second stage cooling step, the solution of gas and liquid is pumped from the second stage cooling unit into a second gas-liquid separator. The liquid within the second gas-liquid separator is directed to a second storage unit and the cleaned gas is directed downstream for use in known commercial applications. 
         [0004]    Other objects, features, and advantages of the subject concept will become more apparent from the following detailed description of the preferred embodiment and certain modifications thereof when taken together with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  diagrammatically illustrates a process flow and system diagram of an aspect of the subject invention; 
           [0006]      FIG. 2  graphically represents temperature verses a mass percent of impurities present; and 
           [0007]      FIG. 3  graphically represents the liquid nitrogen (LN 2 ) required in the process system for various temperatures. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    Referring to  FIG. 1  of the drawings, a power generating system  10  is disclosed. The power generating system  10  includes a power plant  12 , a first stage cooling unit  16 , a second stage cooling unit  18 , a first gas-liquid separator  20 , a second gas-liquid separator  22 , first and second liquid storage units  24 , 26 , and a downstream commercial application  30 . The power generating system  10  also includes a temperature controlled liquid nitrogen generator  32 , and a temperature regulator  34  operative to control the respective temperatures within the first and second stage cooling units  16 , 18 . An optional water gas shift arrangement  36  may be disposed between the power plant  12  and the first stage cooling unit  16 . Likewise, an optional cold methanol solvent solution  38  may be disposed between the second gas-liquid separator and the downstream commercial application  30 . 
         [0009]    The power plant  12  may be a plasma fired power plant that generally emits syngas that has some unwanted impurities therein, such as, for example, hydrogen chloride (HCl), carbon dioxide (CO 2 ), hydrogen sulfide (H 2 S), and carbonyl sulfide (COS) as well as other impurities. Additionally, other gases, such as, hydrogen (H 2 ), carbon monoxide (CO), methane (CH 4 ), and nitrogen (N 2 ), are in the syngas coming from the plasma power plant. Additionally, the power plant  12  may be a fossil fuel fired power plant that generally emits some potentially harmful gases, such as, for example, nitric oxides (NO x /NO 2 ), sulfur oxides (SO x /SO 2 ), and carbon dioxides (CO 2 ). Some of these harmful gases may require known wet and dry scrubbers within the power plant as well as known absorbers in order to aid in the removal of these gases. Furthermore, in order to aid in the proportional relationship between the hydrogen and the carbon monoxide in the exhaust gases, the water gas shift arrangement  36  is added in a conduit  39  that connects the power plant  12  to the first stage cooling unit  16 . Whether, the power plant  12  is fired by plasma torches or fossil fuels, the subject cleaning system is useful in removing many undesirable gases from the exhaust gases of the power plant  12 . 
         [0010]    The first and second stage cooling units  16 , 18  are substantially the same and only the first stage cooling unit  16  will be discussed in detail. Like elements have like element numbers. The first stage cooling unit  16  has a first inlet port  40 , a first outlet port  42 , a second inlet port  44 , a second outlet port  46  and defines a cavity  50  therein. A spiral tube  52  is disposed within the cavity  50  and interconnects the first inlet port  40  and the first outlet port  42 . Even though the spiral tube  52  is shown and described as a spiral tube, it is recognized that the shape of the tube  52  could be changed without departing from the essence of the subject invention. 
         [0011]    The first and second gas-liquid separators  24 , 26  are substantially the same and only the first gas-liquid separator  24  will be discussed in detail. Like elements have like element numbers. The first gas-liquid separator  24  has an inlet port  54 , an outlet port  56  and a discharge port  58 . A conduit  59  is connected between the outlet port  56  of the first gas-liquid separator  20  and the first inlet of the second stage cooling unit  18 . A first pump  60  is disposed in a conduit  62  that is connected between the first outlet port  42  of the first stage cooling unit  16  and the inlet port  54  of the first gas-liquid separator  20 . A second pump  62  is disposed in a conduit  64  that is connected between the first outlet port  42  of the second stage cooling unit  18  and the inlet port  54  of the second gas-liquid separator  22 . The first liquid storage unit  24  is connected to the outlet port  58  of the first gas-liquid separator  20  and the second liquid storage unit  26  is connected to the second gas-liquid separator  22 . 
         [0012]    The liquid nitrogen generator  32  is operatively connected to the temperature regulator  34 . The temperature regulator  34  is operative to control the temperature of the liquid nitrogen being delivered therefrom. A first liquid conduit  68  is connected between the temperature regulator  34  and the second inlet port  44  of the first stage cooling unit  16  and delivers liquid nitrogen, at a first predetermined temperature level, from the temperature regulator  34  to the cavity  50  of the first stage cooling unit  16 . A second liquid conduit  70  is connected between the temperature regulator  34  and the second inlet port  44  of the second stage cooling unit  18  and delivers liquid nitrogen, at a second predetermined temperature level, from the temperature regulator to the cavity  50  of the second stage cooling unit  18 . A first gas return conduit  72  is connected between the second outlet port  46  of the first stage cooling unit  16  and the liquid nitrogen generator  32  and operative to return nitrogen gas from the cavity  50  of the first stage cooling unit  16  to the liquid nitrogen generator  32 . A second gas return conduit  74  is connected between the second outlet port  46  of the second stage cooling unit  18  and the liquid nitrogen generator  32  and operative to return nitrogen gas from the cavity  50  of the second stage cooling unit  18  to the liquid nitrogen generator  32 . 
         [0013]    A conduit  74  is connected between the outlet port  56  of the second gas-liquid separator  22  and the downstream commercial application  30 . As needed, the cold methanol solvent solution  38  may be disposed in the conduit  74 . 
         [0014]    A simulation of the process was performed using Aspen HYSYS. The assumptions made were that no reaction occurs within the apparatus which would induce endothermic or exothermic tendencies within the gas and that the separation is based solely on the differences in boiling points. The results were as expected where the first stage, at about 10 degrees Celsius (C), resulted in the removal of liquid water and liquid nitrogen oxide. Likewise, in the second stage, at about −150 C, removal of liquid carbon dioxide (CO 2 ), liquid hydrogen sulfide (H,S), liquid carbonyl sulfide (COS), liquid hydrochloric acid (HCl), and liquid sulfur dioxide (SO 2 ) were removed. 
         [0015]    Referring to  FIG. 2 , the graph disclosed therein has a horizontal scale representing temperature and a vertical scale representing the mass percent of impurities present. Depending on the boiling points of the various impurities, they convert from a gas to a liquid and the liquid can be removed. 
         [0016]    Referring to  FIG. 3 , the graph disclosed therein has a horizontal scale representing temperature and a vertical scale representing the mass flow (tons/day) of liquid nitrogen required to obtain the desired temperature. The simulation concluded that about 25 tons/day of liquid nitrogen would be needed in order to bring the temperature of a 15 ton/day syngas to the target temperature of —150 C. As set forth before, this temperature is where most of the impurities are converted to a liquid and removed. The result is a very clean syngas that can be utilized in various commercial processes. 
       INDUSTRIAL APPLICABILITY 
       [0017]    The subject process for exhaust gas clean-up provides a simple, safe, and cost effective process to provide a clean syngas for use in various commercial applications. 
         [0018]    During operation of the subject process, the exhaust gases from the power plant  12  is directed to the first stage cooling unit  16  and passes through the spiraled tube  52  disposed in the cavity  50  therein. Since the liquid nitrogen being directed into the cavity  50  is at about 10 degrees C., the temperature of the gases being directed through the spiraled tube  52  is lowered to about 10 degrees C. The level of the liquid nitrogen in the cavity  50  varies but is generally maintained at the midpoint as illustrated in  FIG. 1 . The portion of the liquid nitrogen that converts back to a gas is returned to the liquid gas generator  32  through the first return gas conduit  72  to be used to produce more liquid nitrogen. 
         [0019]    The solution of liquids and gases are withdrawn from the first outlet  42  by the first pump  60  and delivered to the first gas-liquid separator  20 . Within the first gas-liquid separator  20 , the separated liquids are diverted to the first storage unit  24  and the separated gases are passed through conduit  59  to the first inlet port  40  of the second stage cooling unit  18 . 
         [0020]    Within the second stage cooling unit  18 , the gases from the first inlet port  40  thereof are directed through the spiral tube  52  to the first outlet port  42  thereof. Since the liquid nitrogen being directed into the cavity  50  is about −150 degrees C., the temperature of the gases being directed through the spiraled tube  52  is lowered to about −150 degrees C. The portion of the liquid nitrogen within the cavity  50  of the second stage cooling unit  18  that converts back to a gas is returned to the liquid gas generator  32  through the second return gas conduit  74  to be used to produce more liquid nitrogen. 
         [0021]    The solution of liquids and gases are withdrawn from the first outlet  42  of the second stage cooling unit  18  by the second pump  64  and delivered to the second gas-liquid separator  22 . Within the second gas-liquid separator  22 , the separated liquids are diverted to the second storage unit  26  and the separated gases are passed through the conduit  76  to the downstream commercial application(s)  30 . 
         [0022]    It may be desirable to utilize the cold methanol solvent solution  38  in the conduit  76  to dissolve any methane (CH 4 ) contained therein. 
         [0023]    The liquids contained in the respective first and second storage units  24 / 26  may be further separated and commercially used in other commercial applications as desired. 
         [0024]    The subject process for the clean-up of exhaust gases provides a simple, safe, and cost effective process for removing detrimental compounds/elements from the gases being emitted from a power plant. 
         [0025]    Other embodiments as well as certain variations and modifications of the embodiment herein shown and described will obviously occur to those skilled in the art upon becoming familiar with the underlying concept. It is to be understood, therefore, that the subject design, as claimed, may be practiced otherwise than as specifically set forth above.