Abstract:
A system for removing gaseous pollutants, such as mercury from flue gases of a solid-fueled furnace ( 17 ), includes a sorbent mill ( 34 ) that receives superheated steam and a sorbent ( 28 ), such as activated carbon, in a partially agglomerated state, that processes the sorbent ( 28 ) by de-agglomerating and comminuting the sorbent ( 28 ). An educator ( 35 ) transports the processed sorbent ( 28 ) to a distributor ( 36 ) that injects it into the flue gases at a contact location having a temperature between 500° F. and 900° F., whereupon the sorbent ( 28 ) adsorbs mercury from the flue gas. A particle collection device ( 24 ) removes the processed sorbent ( 28 ) having adsorbed mercury, from the flue gas.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from U.S. Provisional Patent Application Ser. No. 61/538,475, filed Sep. 23, 2011. The present application is also related to U.S. Pat. No. 7,780,765 issued Aug. 24, 2010 that was a continuation of U.S. patent application Ser. No. 10/961,697, filed Oct. 8, 2004, which is a continuation-in-part of U.S. patent application Ser. No. 10/453,140, filed Jun. 3, 2003, now U.S. Pat. No. 6,848,374, all of which are incorporated by reference as if they were set forth in their entirety herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to apparatus and a method for removing gaseous pollutants from the flue gas of solid fuel fired steam generators and more particularly to apparatus and a method for removing elemental mercury and mercury compounds from the flue gases from coal fired boilers. 
     The use of activated carbon and carbonaceous particles for the adsorption of pollutant gases, such as mercury vapor has been successfully demonstrated in various applications, such as municipal waste incineration. However, there are significant differences in the concentration of mercury from waste incinerators compared to coal-fired power plants with the concentration from the coal-fired power plants being anywhere from 10 to 100 times lower. Also, the mercury from waste incinerators is usually in the form of mercury chloride whereas a larger percentage of mercury from coal-fired power plants is usually in the form of elemental mercury. Both of these differences make it more difficult to remove the mercury from the flue gas from a coal-fired power plant. 
     The efficiency of the sorbent is limited by its surface area to mass ratio. A relatively large particle has a low available surface area/mass ratio that limits the adsorption of pollutant gas. Using a carbonaceous sorbent with mean particle size of about 5 microns with a maximum size of about 10 microns would improve adsorption efficiency, but storage, handling, transport and dispersion of these small particles is extremely difficult. 
     In conventional methods, the sorbent particles are injected in the flue gas duct upstream of particulate removal device such as baghouses and electrostatic precipitators and downstream of air heaters. The particle removal devices then collect the sorbent with the adsorbed the pollutant gases. 
     U.S. Pat. No. 7,780,765 issued Aug. 24, 2010 describes the injection of activated carbon into flue gas by using injected compressed air. This reduces mercury emissions in flue gases, but can become quite costly due to the need for equipment to provide the compressed air and the power the equipment uses. 
     It is also known in the art that vapor phase mercury in the flue gas emerging from the high temperature boiler is in the form of elemental mercury. Oxidation of elemental mercury to oxidized mercury (Hg 2+ ) is beneficial to mercury control since it can be removed more easily by carbonaceous sorbent. Similarly, combination of elemental mercury with halogens, results in a compound that has greater affinity for the sorbent. 
     Currently, there is a need for a system that efficiently and economically removes gaseous pollutants, such as elemental mercury and mercury compounds from combustion flue gases of solid fueled boilers. 
     SUMMARY OF THE INVENTION 
     The present invention may be embodied as a method for removing gaseous pollutants from flue gases generated by solid fuel fired boiler, the method comprising: 
     providing a carbonaceous sorbent in powdered form potentially having some agglomeration; 
     extracting superheated steam from the boiler; 
     processing the superheated steam to provide dry, high quality superheated steam of the proper pressure, temperature that is below a predetermined moisture content; 
     providing the processed steam to a sorbent mill to process the sorbent to deagglomerate it and to break the particles of the powdered sorbent into a greater number of smaller particles; 
     injecting the processed sorbent into contact with the flue gas to adsorb mercury and mercury compounds; and 
     removing the sorbent having mercury adsorbed thereon from the flue gas. 
     The present invention may also be embodied as a pollution removal device for more efficiently removing pollutant gases from flue gases of a solid-fueled steam generator comprising: 
     a sorbent source for providing carbonaceous sorbent in powdered form; 
     a steam tap line for extracting superheated steam from said boiler  18 ; 
     a steam processing device coupled to the steam tap line for receiving the superheated steam from the steam tap line and for processing the steam to provide steam of a desired pressure, temperature and moisture content; 
     a sorbent mill coupled to the steam processing device and the sorbent source, adapted to receive sorbent from the sorbent source, and employ the processed steam to process the sorbent by de-agglomerating and comminuting particles in the sorbent; 
     a distributor coupled to the sorbent mill adapted to receive the sorbent and to inject the sorbent into said flue gases of the boiler thereby adsorbing the gaseous pollutant; and 
     a particulate collection device adapted to collect the solid particles in the flue gas and remove them from the gases. 
     An object of the present invention is to provide a more efficient method of removing gaseous pollutants from flue gases. 
     Another object of the present invention is to provide a more efficient method of removing mercury and mercury compounds from flue gases. 
     Another object of the present invention to provide a more cost effective means of removing gaseous pollutants from flue gases. 
     It is another object of the present invention to provide a more cost effective costly means of removing mercury from flue gases. 
     Other objects and advantages of the invention will become apparent from the drawings and specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of a first embodiment of a system in accordance with the present invention for removing gaseous pollutants from the flue gases created by a solid fueled boiler; and 
         FIG. 2  is a more detailed, enlarged view of one example of a steam processing device of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows an embodiment of the present invention incorporated into a solid fuel fired steam generator or boiler  18 . This may be part of a coal-fired power plant. 
     The raw coal  14  is fed to at least one pulverizer/crusher, each referred to as a mill  16  where the raw coal is reduced to desired particulate size. Ambient air is provided to an air preheater  22  that preheats the air. The preheated air is provided as primary air to the mills  16  that carries the solid fuel particles that were pulverized in mills  16 , to the furnace  17  of boiler  18 , where the fuel particles are burned to boil water into steam. 
     Air preheater  22  also provides secondary air directly to furnace  17 . 
     The temperature of the flue gases leaving the furnace  17  ranges from 1400 to 2200° F. 
     The steam created in the furnace  17  is provided to superheater  19 . The hot flue gases are also provided to superheater  19 . The superheater transfers heat from the flue gases to the steam creating superheated steam in headers typically at 600 psig that is provided to a steam turbine  20 . The flue gases exit the superheater  20  with a temperature that is approximately 600 to 800° F. 
     The steam turbine  20  does various work, such as turning a generator to create electricity. The superheated steam that returns after running through at least one stage of steam turbine  20  is provided to a cold reheat inlet  23  of a reheater  21 . The reheater  21  receives the flue gas and transfers the heat to the steam being returned from the steam turbine  20  to reheat it and returns the reheated steam to another stage of the steam turbine  20 . 
     Some of the superheated steam returning from the turbine is routed, as one example, to a steam processing device  100 , which will be discussed in more detail below. 
     The flue gases are then routed through the air preheater  22 . Heat from the flue gases is transferred to the atmospheric inlet air that will be used as the primary and secondary air in the mills  16  and the furnace  17 . 
     Flue gases exit the air preheater  22  with a temperature range from 220 to 370° F. and enter a particle separation device  24 . The particle separation device  24  may be an electrostatic precipitator (ESP), a fabric filter  24  or other known device for collecting solid particulates entrained in a gas. The particle separation device  24  collects the solid particulates and provides them to a disposal system  38  for disposal. 
     A carbonaceous sorbent  28  in powdered form, such as activated carbon particles, or other carbon particles is stored in a silo  30 . The sorbent  28  in silo  30  typically clumps together because very small particles thereof tend to stick to each other and agglomerate. 
     Accordingly, the sorbent  28  is fed by a feeder  32  to an eductor  74  that provides the sorbent  28  to a sorbent mill  34 . Since this eductor is typically a significant distance from the sorbent mill  34 , it is best to use air as a transport medium to blow the sorbent  28  to the sorbent mill  34 . There is a risk of dropping the temperature to a level where condensation occurs if steam were used as the transport medium in this part of the system. 
     Superheated steam is provided from the cold reheat inlet  23  of the reheater  21 . In this example, other sources of superheated steam are also available from boiler  18 , including blending of streams of steam. This passes through the steam tap  101  to the steam processing device  100  that processes the superheated steam to reduce the pressure and temperature of the superheated steam, and to remove any residual condensation. The steam processing device  100  may also recover some heat energy for use elsewhere in the plant. 
     The processed steam is provided to the sorbent mill  34 . The sorbent mill  34  breaks up the clumps in the sorbent  28  and de-agglomerates it. The sorbent mill  34  also operates to comminute the sorbent particles into a greater number of particles having a smaller size. The smaller size increases the available surface/mass ratio, allowing faster reaction time, while the greater number of particles causes a greater dispersion in the flue gases and increases the chances that the particles will physically contact the mercury gases, increasing the efficiency of the system. 
     Once the sorbent  34  is deagglomerated and comminuted, it is provided to a steam eductor  35  and/or piping to receive the processed steam from steam processing device  100  as the motive force. The steam eductor  35  and/or piping then sends the processed sorbent  28  to a distributor  36 . 
     One such type of sorbent mill  34  compatible with the present invention is a jet mill. In prior art systems, air was compressed and forced into the separation device to cause clumps of agglomerated particles to be broken up. It required significant auxiliary plant energy to compress air to the point required by the separation device. 
     The present invention does not use compressed air as the energy source for de-agglomeration and comminuting the activated carbon particles. It uses superheated steam available in the power plant or facility. The logic of using steam is contrary to accepted logic, since steam typically has moisture and moisture causes agglomeration of powders. 
     However, by keeping the temperature above the condensation point, the moisture does not come out of its gaseous form, and does not create liquid droplets. In fact, superheated steam actually removes moisture from a powdered sorbent, reducing agglomeration. 
     Unfortunately, the pressure of the superheated steam in the headers of the system is on the order of 550 psig, or more. This is much higher than the pressure that can be used by the sorbent mill  34 , typically about 100 psig. If the pressure of the superheated steam is too high, the separation device  34  can become damaged or non-functional. The cold reheat inlet  23  is available as one potential source for superheated steam. The cold reheat inlet can have superheated steam at a pressure of about 600 psig and a temperature of 635 Deg. F. Therefore, the steam must be modified to reduce the pressure and temperature. 
     During start-ups and shut-downs, the steam tap will have a temperature below that of the condensation temperature. As a result, moisture forms in the steam tap  101 . As indicated above, moisture causes agglomeration, which reduces the efficiency of the system and should be avoided. Therefore, the superheated steam must be processed to result in the proper pressure/temperature while also removing moisture or condensation. 
       FIG. 2  shows a more detailed diagram of the steam processing device  100  of  FIG. 1 . Steam processing device  100  employs a moisture reduction unit  110  that may include water traps and/or driers that remove condensation from the superheated steam. 
     The superheated steam is then provided to a temperature reduction unit  120  that may include water sprayers and/or heat exchangers to de-superheat the steam provided to it. The temperature of the steam may be reduced by spraying small amounts of water into the steam. This amount must be accurately calculated and metered so that it will all remain in the vapor phase and not cause condensation throughout its use in the system. 
     The superheated steam is provided to the pressure reduction unit  130  that may include pressure reduction valves that can be operated by an external controller. 
     There are sensors  140  that measure at least one of the pressure and temperature in one or more locations in the steam processing device  100 . These sensors  140  feed their measurements to a controller  150 , which may be part of the steam processing device  100 , or external to it. 
     Controller  150  functions, if necessary to read sensor input from the Moisture reduction unite  110  to determine if there is condensation that should be removed. Controller  150  then actuates water trap valves and other equipment such as driers to remove the condensate. The controller  150  may also read valve settings, flow rates, total accumulated water removed, etc. to make its calculations and actuate parts of temperature reduction unit  120 . 
     The controller  150  reads sensors  140  to determine the temperature and pressure, mass flow rate and other necessary parameters of the superheated steam at the location of the temperature reduction unit  120 , to calculate the proper amount of water to spray into the superheated steam. It also operates to actuate valves within temperature reduction unit  120  to dispense the proper amount of water calculated if de-superheat spray is required. 
     Controller  150  also receives input from the sensors  140  indicating the temperature, pressure, mass flow rate and other necessary parameters of the superheated steam at the location of the pressure reduction unit  130 , to calculate the proper valve opening for pressure reduction valves within the pressure reduction unit  130 . 
     In one embodiment, the sensors  140  and controller  150  interactively perform their duties to provide superheated steam of a pressure of approximately 100 psig, temperature of 550 Deg. F. with minimal moisture. 
     Controller  150  may receive input and data from other processors of the system and/or operator input. 
     The processed steam is provided to the sorbent mill  34 . Superheated steam has superheated water in vapor form. This has significantly more enthalpy than compressed air used in prior art devices. 
     The inherent energy in the superheated steam is expended during the deagglomeration and comminution of the sorbent  28 . This results in finer grinding and at significantly reduced operating energy costs. The finer grinding allows more particles to be dispersed in the same volume. This reduces the spacing between particles and increases the probability that a sorbent particle comes in contact with the pollutant gases. 
     The sorbent mill  34  may be a particle-particle separator or a jet mill, where high-pressure, superheated steam is the energy source. 
     Sorbent mill  34  performs three functions: particle-particle deagglomeration; particle size reduction; and classification of particles into a) fine particles to use and/or b) coarse particles to return to the silo  30  or retention or return for further milling (particle size reduction). 
     Some of the larger particles are comminuted by device  34 . The resulting carbonaceous sorbent has a particle size distribution of carbonaceous sorbent of d 50 &lt;15 microns, where d 50  represents 50% of the particles by mass in the entire distribution in the carbonaceous sorbent  28 . 
     The target particle size distribution is d 50 &lt;15 microns, preferably d 50 &lt;8 microns and most preferably d 50 &lt;4 micron. 
     The static pressure of air leaving the separation device  34  is typically above atmospheric pressure. A steam eductor  35  connected to sorbent mill  34  moves the processed sorbent  28  to the distributor  36 . A pressure of about 1-5 psig is preferred. 
     Distributor  36  having multiple injection lances  37  that inject the processed sorbent  28  into the flue gases, preferably between the reheater  21  and the air heater  22 . This causes the sorbent  28  to be disbursed throughout the flue gases that come in physical contact with, and adsorbs gaseous pollutants, such as elemental mercury and mercury compounds in the flue gas. Dioxins and furans may also be adsorbed by the sorbent  28 , as well as certain other hazardous elements and compounds such as HCl, selenium, arsenic, antimony, beryllium, cadmium, cobalt, lead, manganese, nickel, and others. 
     The sorbent  28  may also be injected into the flue gas stream  12  between the boiler  18  and the convective pass/superheater  20 , between the convective pass/superheater  20  and the air preheater  22 , or between the air preheater  22  and the ESP/fabric filter  24 . 
     Thus, the system for removing elemental mercury or mercury compounds handles carbonaceous sorbent  28 . 
     In an alternative embodiment, a portion of the coal pulverized in the pulverizer  16  is extracted at a location  70  from the pulverizer as sorbent  28 . Preferably between 10 to 1000 lb/hr of coal (about 0.01 to 1.0 percent of total coal feed to boiler), more preferably between 50 and 500 lb/hr, and most preferably between 100 and 200 lb/hr is extracted at the location  70 . A blower  72  may be required to provide the necessary motive force for moving the extracted sorbent solids  28 . 
     As an alternative embodiment, superheated steam is sourced for tap  101  and used directly in a sorbent mill  34  without steam processing  100 . Heat energy recovered may or may not be extracted with this arrangement. 
     The extracted sorbent solids  28  are subjected to one or more processes. The sorbent solids  28  may be sprayed with a solution  29  to deposit a halogen on the surface of the sorbent particles  28 . The solution  29  is chosen from potassium iodide, iodine dissolved in potassium iodide, alkali halides (e.g. NaCl), and halide salts (e.g. CaCl 2 ), or halogen acids (e.g. HCl, Hl, HBr, HF) dissolved in water. A typical additive amount is expected to have a halogen concentration in the sorbent  28  of about 0 to 10 percent by weight. 
     Additives  29  may also be added by injecting them into the sorbent mill  34  and mixing them with the carbonaceous sorbent  28  and heating them to a temperature that will volatilize the additive locally but distribute it by subsequent adsorption on the carbon. It is preferred that the temperature to which the carbonaceous material and additive are heated is above 400 Deg. ° F. and most preferably above 500 Deg. F, to ensure that it will be stable when injected in the flue gas at those temperatures. An example of additive  29  that can be incorporated in the above fashion is iodine or bromine. These additives  29 , when added to the sorbent mill  34  are more efficiently incorporated into the processed sorbent  28 , and will have greater efficiency in adsorbing gaseous pollutants. 
     The steam tap  101  explained in the above embodiment extracts superheated steam from the cold reheat inlet header  23  typically at 635 Deg. F. and a pressure of 600 psig. In other embodiments of the present invention, the steam tap  101  may extract superheated steam from other locations within the boiler  18 . For example, the steam tap  101  could come off of: 
     1) the low temperature superheater inlet header. This steam is typically at 650-850 Deg. F. and 2600-3000 psig; 
     2) the superheater de-superheater outlet header at 650-850 Deg. F. and 2600-3600 psig; 
     3) the superheater outlet header at 900-1150 Deg. F. and 2500-3600 psig; 
     4) the reheater outlet header at 900-1150 Deg. F. and 500-700 psig; 
     5) the wall sootblower stations. The temperature and pressure of these vary. All of these will require differing amounts of pressure and temperature reduction that can be interactively applied by the present invention. 
     Several advantages of the present invention above prior art devices include the ability to grind the sorbent  28  to a finer powder to reduce moisture in the processed sorbent ( 28 ). The reduced moisture thereby reduces corrosion within the equipment of the system. Also, it was noted that by using superheated steam instead of compressed air, there was less static electricity in the sorbent. This reduced the magnetic attraction of the particles and their tendency to magnetically agglomerate in the flue gases. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.