Abstract:
Hybrid thermal oxidizer systems and methods for combusting waste gas and heating utility oil using an efficient transfer of heat from fuel gas. The hybrid thermal oxidizer includes a combustion chamber, a gas preheater and a quench chamber positioned between the combustion chamber and the gas preheater. The combustion chamber burns impurities in the waste gas to produce an exhaust gas. The gas preheater preheats the waste gas before burning impurities in the combustion chamber. And, the quench chamber controls a temperature of the exhaust gas before preheating the waste gas.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a Divisional Application of and claims priority to U.S. application Ser. No. 13/790,781, titled “Hybrid Thermal Oxidizer Systems And Methods”, filed on Mar. 8, 2013, which is incorporated herein by reference in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Not applicable. 
       FIELD OF THE INVENTION 
       [0003]    The present invention generally relates to hybrid thermal oxidizer systems and methods. More particularly, the invention relates to a hybrid thermal oxidizer for combusting waste gas and heating utility oil using an efficient transfer of heat from fuel gas. 
       BACKGROUND OF THE INVENTION 
       [0004]    In facilities that process liquefied natural gas (“LNG”), the natural gas is typically cleaned of impurities and cooled thus, removing a fair amount of energy to bring it to a liquid state. In this state, it is easy to transport in large quantities. Before bringing the gas to a liquid state, the impurities are removed from the raw gas. These impurities are burned in a conventional thermal oxidizer to break them down to CO 2 , H 2 O and nitrogen, for example. Based on the impurities, the thermal oxidizer needs to operate at elevated temperatures to minimize emissions. When a thermal oxidizer operates at a high temperature, the fuel gas leaves the unit at very high temperatures thus, wasting heat. 
         [0005]    Referring now to  FIG. 1 , a conventional thermal oxidizer  100  is illustrated for use in an LNG facility. A fuel gas stream  101  enters a burner  102  at the same time a combustion air stream  104  enters the burner  102 . The burner  102  combusts the fuel gas stream  101  and the combustion air stream  104  in a combustion chamber  106 . Impurities from a waste gas  107  enter the combustion chamber  106  through inlet opening  108  at about 122° F. and are burned with the fuel gas stream  101  and the combustion air stream  104  to break them down into an exhaust gas comprising CO2, H2O and nitrogen, for example. Based on the type of impurities in the waste gas  107 , the combustion chamber  106  needs to operate at an elevated temperature to minimize emissions in the exhaust gas. Emission requirements often require operating a conventional thermal oxidizer at much higher temperatures to obtain a 99.99% Destruction and Removal Efficiency (“DRE”). DRE is defined as the percentage of molecules of a compound removed or destroyed in the thermal oxidizer related to the number of molecules that entered the system. The operating temperature of a thermal oxidizer therefore, varies depending upon the impurities in the waste gas. If, for example, benzene, toluene, ethyl-benzene and xylenes (collectively referred to as “BTEX”) are present, then the combustion chamber  106  needs to operate at about 1742° F. with a residence time of 1.5 to 2 seconds for 99.99% DRE. Residence time is defined as the time of exposure of waste gas in the combustion chamber  106 . The combustion air stream  104  entering the burner  102  may be regulated with a valve  112  so that if the temperature in the combustion chamber  106  drops below or goes above a predetermined value such as, for example, about 1742° F. when detected by a temperature sensor  110 , the flow of combustion air stream  104  into the burner  102  may be increased or decreased using the valve  112 . Likewise the fuel gas stream  101  entering the burner  102  may be regulated with a valve  103  so that if the temperature in the combustion chamber  106  drops below or goes above a predetermined value such as, for example, about 1742° F. when detected by the temperature sensor  110 , the flow of fuel gas stream  101  into the burner  102  may be increased or decreased using the valve  103 . In order to maintain the combustion air stream  104  ahead of the fuel gas stream  101  for safety reasons, the combustion air stream  104  entering the burner  102  may be regulated with the valve  112  so that if the oxygen in the combustion chamber  106  drops below a predetermined value such as, for example, about 2% when detected by an oxygen sensor  111 , the flow of the combustion air stream  104  into the burner  102  may be increased using the valve  112 . The exhaust gas from the combustion chamber  106  with impurities enters the fuel gas duct  113  before entering the exhaust stack  114  and exiting the top of exhaust stack  114  through an opening  116  into the atmosphere at about 1742° F. The exhaust gas exiting the conventional thermal oxidizer illustrated in  FIG. 1  therefore, wastes a significant amount of heat. 
         [0006]    Referring now to  FIG. 2 , a conventional fired heater  200  is illustrated for use in an LNG facility. Utility oil is used in the LNG facility to heat the feed gas, to heat gas turbine fuel and to remove carbon dioxide from the feed gas. The utility oil must be separately heated in a hot oil heater also referred to as a fired heater. A combustion air stream  202  and a fuel gas stream  204  enter a burner  206  at the same time. As a result, the combustion air stream  202  and the fuel gas stream  204  are heated by the burner  206  in a radiant section  208 . The radiant section  208  includes vertical coiled tubing  210 . A convection section  212  includes horizontal tubing (not shown). A utility oil stream  214  may be heated by directing the utility oil stream  214  through an inlet opening  216 , through the horizontal tubing, through the vertical coiled tubing  210  and out an outlet opening  218  as a preheated utility oil stream  220 . The utility oil is thus, heated from about 260° F. to about 475° F. as heat from the combustion of the combustion air stream  202  and the fuel gas stream  204  in the radiant section  208  and in the convection section  212  passes around the vertical coiled tubing  210  and the horizontal tubing as it rises through the fired heater  200  and exits through an exhaust stack  216  into the atmosphere at about 400° F. 
         [0007]    Both a conventional thermal oxidizer and fired heater are significant pollutant emitting equipment in any LNG facility. With EPA regulations becoming more stringent, end users, EPA companies and heater/burner vendors face a constant challenge to improve processes and equipment design to reduce pollutant emissions. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention therefore, meets the above needs and overcomes one or more deficiencies in the prior art by providing systems and methods for combusting waste gas and heating utility oil using an efficient transfer of heat from fuel gas in a hybrid thermal oxidizer. 
         [0009]    In one embodiment, the present invention includes a method for processing a hazardous waste gas, which comprises: i) burning impurities in the waste gas to produce exhaust gas; ii) controlling a temperature of the exhaust gas before preheating the waste gas; and iii) preheating the waste gas before burning the impurities using heat transferred from the exhaust gas preheater. 
         [0010]    Additional aspects, advantages and embodiments of the invention will become apparent to those skilled in the art from the following description of the various embodiments and related drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention is described below with references to the accompanying drawings, in which like elements are referenced with like numerals, wherein: 
           [0012]      FIG. 1  illustrates a conventional thermal oxidizer used in an LNG facility. 
           [0013]      FIG. 2  illustrates a conventional fired heater used in an LNG facility. 
           [0014]      FIG. 3  illustrates one embodiment of a hybrid thermal oxidizer for use in an LNG facility. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    The subject matter of the present invention is described with specificity, however, the description itself is not intended to limit the scope of the invention. The subject matter thus, might also be embodied in other ways, to include different steps or combinations of steps similar to the ones described herein, in conjunction with other present or future technologies. Moreover, although the term “step” may be used herein to describe different elements of methods employed, the term should not be interpreted as implying any particular order among or between various steps herein disclosed unless otherwise expressly limited by the description to a particular order. While the following description refers to the oil and gas industry, the systems and methods of the present invention are not limited thereto and may be applied in other industries to achieve similar results. 
         [0016]    Referring now to  FIG. 3 , one embodiment of a hybrid thermal oxidizer is illustrated for use in an LNG facility. A fuel gas stream  301  enters the burner  302  at the same time a combustion air stream  304  enters the burner  302 . The burner  302  combusts the fuel gas stream  301  and the combustion air stream  304  in a combustion chamber  306 . Impurities from a preheated waste gas stream  307  enter the combustion chamber  306  through inlet opening  308  and are burned with the combustion air stream  304  and the fuel gas stream  301  at about 1742° F. to break them down into an exhaust gas in the same manner as described in reference to  FIG. 1 . The preheated waste gas stream  307 , however, enters the combustion chamber  306  at a much higher temperature of about 900° F. than the waste gas stream entering a conventional thermal oxidizer. In this manner, less fuel gas stream  301  is required to burn and break down the impurities in the preheated waste gas stream  307  through combustion. The combustion air stream  304  entering the burner  302  may be regulated with a valve  312  so that if the temperature in the combustion chamber  306  drops below or goes above a predetermined value such as, for example, about 1742° F. when detected by a temperature sensor  310 , the flow of combustion air stream  304  into the burner  302  may be increased or decreased using the valve  312 . Likewise, the fuel gas stream  301  entering the burner  302  may be regulated with a valve  303  so that if the temperature in the combustion chamber  306  drops below or goes above a predetermined value such as, for example, about 1742° F. when detected by the temperature sensor  310 , the flow of fuel gas stream  301  into the burner  302  may be increased or decreased using the valve  303 . In order to maintain the combustion air stream  304  ahead of the fuel gas stream  301  for safety reasons, the combustion air stream  304  entering the burner  302  may be regulated with the valve  312  so that if the oxygen in the combustion chamber  306  drops below a predetermined value such as, for example, about 2% when detected by an oxygen sensor  311 , the flow of the combustion air stream  304  into the burner  302  may be increased using the value  312 . 
         [0017]    A waste gas stream  314  enters a gas preheater  318  through inlet opening  316  where it passes through a coiled tubing and exits the gas preheater  318  through outlet opening  320  as the preheated waste gas stream  307  at about 900° F. The waste gas stream  314  may enter the gas preheater  318  at a temperature of about 122° F. The waste gas stream  314  should not be heated above a predetermined auto ignition temperature of the hydrocarbons in the waste gas stream  314  when the hydrocarbons in the waste gas stream  314  are more than 50% of a lower explosion limit. A lower explosion limit is the concentration of a gas or vapor in air capable of producing a flash fire in the presence of an ignition source. 
         [0018]    A quench chamber  322  is positioned between the combustion chamber  306  and the gas preheater  318  to control the temperature of the exhaust gas exiting the combustion chamber  306  before it enters the gas preheater  318 . A quench air stream  324  enters the quench chamber  322  through inlet opening  326 , which is controlled and regulated by a quench air valve  328  and a temperature sensor  321  to maintain a predetermined temperature in the quench chamber  322  of about 1400° F. In this manner, the temperature of the exhaust gas from the combustion chamber  306  can be controlled to about 1400° F. before passing through to the gas preheater  318 . Controlling the temperature of the exhaust gas before it enters the gas preheater  318  is necessary in order to avoid damaging the gas preheater  318 . If, for example, the waste gas stream  314  entering the gas preheater  318  is interrupted for a while due to unexpected reasons, then the exhaust gas from the combustion chamber  306  may be controlled to a temperature of about 1400° F. in the quench chamber  322  before it passes through the gas preheater  318  at about the same temperature without damaging the coiled tubing therein. Otherwise, the exhaust gas exiting the combustion chamber  306  at about 1742° F. would directly enter the gas preheater  318  at about the same temperature and most likely damage the coiled tubing therein because the gas preheater  318  cannot handle such an elevated temperature due to high thermal expansion stresses. If, however, the waste gas stream  314  entering the gas preheater  318  is consistently uninterrupted at about 74,132 lbs/hr, then exhaust gas exiting the combustion chamber  306  at about 1742° F. is cooled in the quench chamber  322  to about 1400° F. and loses some of its heat in the gas preheater  318 , to the waste gas stream  314  passing therethrough. The exhaust gas exits the gas preheater  318  at about 1097° F. 
         [0019]    The exhaust gas exiting the gas preheater  318  enters a waste heat recovery module  330 . A utility oil stream  332  enters an upper portion of the waste heat recovery module  330  through inlet opening  334 , passes through a coiled tubing therein and exits the waste heat recovery module  330  through outlet opening  336 . The utility oil stream  332  is used in a separate process for the LNG facility and, in this manner, is heated to about 475° F. using heat from the exhaust gas exiting the gas preheater  318  at about 1097° F. The heat from the exhaust gas in the waste heat recovery module  330  therefore, passes around the coiled tubing containing the utility oil stream  332 , which exits outlet opening  336  as a preheated utility oil stream  338 . 
         [0020]    Heat from the exhaust gas passing through the hybrid thermal oxidizer  300  is therefore, used to efficiently produce a preheated waste gas stream  307  and a preheated utility oil stream  338 . The exhaust gas exits exhaust stack  340  through an opening  341  into the atmosphere at about 424° F. or less. In order to control the temperature in the waste heat recovery module  330 , a valve  342  and a temperature sensor  331  are used to regulate exhaust gas through outlet opening  344  thus, bypassing the waste heat recovery module  330  and entering exhaust stack  340  through inlet opening  346  at a temperature of about 1097° F. Regulation of the valve  342  therefore, controls the temperature of the preheated utility oil stream  338  to about 475° F. The temperature in the waste heat recovery module  330  may also be indirectly regulated by valve  303 . If, for example, the utility oil temperature falls below about 475° F., even after full closure of valve  342 , the fuel gas stream  301  may be increased through the valve  303  to increase the utility oil temperature to about 475° F. 
       EXAMPLE 
       [0021]    In the example below, table 1 summarizes the cost of using a regular Thermal Oxidizer (Regular TO x ) and a fired heater. Table 2 summarizes the savings associated with using a Hybrid Thermal Oxidizer (Hybrid TO x ) according to the present invention. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 (Regular TO x + 
               
               
                   
                 Regular TO x   
                 Fired Heater 
                 Fired Heater) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Equipment 
                 $1,340,000 
                 $985,000 
                 $2,325,000 
               
               
                 Cost (+fuel skid) ($) 
               
               
                 Fuel Cost ($/yr) 
                 $1,401,600 
                 $1,236,900 
                 $2,638,500 
               
               
                 NO x  Emissions 
                 25.580 
                 10.820 
                 36.400 
               
               
                 (lbs/MM Btu/yr) 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Regular TO x + 
                   
                   
               
               
                   
                 (Fired Heater) 
                 Hybrid TO x   
                 Savings 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Equipment 
                 $2,325,000 
                 $2,200,000 
                 $125,000 
               
               
                 Cost (+fuel skid) ($) 
               
               
                 Fuel Cost ($/yr) 
                 $2,638,500 
                 $1,401,600 
                 $1,236,900 
               
               
                 NO x  Emissions 
                 36.400 
                 25.580 
                 10.820 
               
               
                 (lbs/MM Btu/yr) 
               
               
                   
               
             
          
         
       
     
         [0022]    In table 1, the fired heater fuel cost assumptions are 85% thermal efficiency for a 30 MM Btu/hr heater with a fuel usage of about 35.3 MM Btu/hr. The fuel cost is estimated at $4/MM Btu (no inflation/fluctuation considered), which results in about $1,236,900 per year. The Regular TO x  fuel cost assumptions include a 40 MM Btu/hr Thermal Oxidizer with a fuel usage of about 40 MM Btu/hr. The fuel cost is estimated at $4/MM Btu (no inflation/fluctuation considered), which results in about $1,401,600 per year. 
         [0023]    In table 2, the Hybrid TOx fuel cost assumes that no additional fuel consumption is required to heat the hot oil when the Hybrid TO x  is operating under normal conditions to burn a waste gas stream. 
         [0024]    In addition to the fuel cost savings, the Hybrid TO x  also produces fewer noxious emissions (“NO x  Emissions”). In table 1, the NO x  Emissions for a conventional fired heater assume: 
         [0000]    NO x  emitted by a 30 MM Btu/hr heater, lbs/MM Btu/hr 0.035
 
Efficiency of the heater=85%
 
NO x  emissions eliminated, lbs/MM Btu/yr=0.035*35.29*8,760=10,820
 
In table 1, the NOx Emissions for a Regular TO x  assume:
 
NO x  emitted by a 40 MM Btu/hr TO x , lbs/MM Btu/hr=0.073
 
NOx emissions, lbs/MM Btu/yr=0.073*40*8,760=25,580
 
         [0025]    In addition to the significant and substantial cost savings and environmental impact by reducing noxious emissions by approximately 10,820 lbs/yr, eliminating the use of a separate fired heater will provide cost savings by eliminating the maintenance and operational costs associated with a fired heater. Moreover, construction costs and space are reduced by eliminating the requirement of a separate fired heater. 
         [0026]    While the present invention has been described in connection with presently preferred embodiments, it will be understood by those skilled in the art that it is not intended to limit the invention to those embodiments. It is therefore, contemplated that various alternative embodiments and modifications may be made to the disclosed embodiments without departing from the spirit and scope of the invention defined by the appended claims and equivalents thereof.