Abstract:
A steam mill system for transport of powdered activated carbon (PAC) from a storage silo to a steam powered jet mill to produce milled sorbent for use in a coal-fired power plant flue gas for mercury control. More specifically, the present disclosure relates to a cyclone separator arranged in a PAC transport line prior to a steam powered jet mill for separation of PAC according to particle size for the purpose of achieving reductions in associated compressed air usage, operating costs and capital costs.

Description:
FIELD OF THE INVENTION 
       [0001]    The present disclosure relates to a steam mill system useful for transport of powdered activated carbon (PAC) from a storage silo to a steam powered jet mill for use of PAC milled therein in mercury control applications. More specifically, the present disclosure relates to a cyclone separator arranged in a PAC transport line prior to a steam powered jet mill for the purpose of achieving system reductions in associated compressed air usage, operating costs and capital costs. 
       BACKGROUND OF THE INVENTION 
       [0002]    The use of activated carbon for the adsorption of mercury vapor has been successfully demonstrated in various applications. The utilization factor for activated carbon in adsorption of mercury vapor is limited by the relatively large particle size and low surface area of the activated carbon, which limits mercury adsorption. Using activated carbon with a mean particle size of about 5 microns with a top size of about 10 microns improves mercury capture efficiency, but storage, handling, transport and dispersion of particles of such size is extremely difficult. As a result, the use of activated carbon for mercury capture in coal-fired power plants is costly. In coal-fired power plant mercury control applications, adsorption utilization of the activated carbon is quite low with a minimum mole ratio of carbon to mercury of 10,000 to 1. Hence, methods and/or systems that reduce costs associated with the use of activated carbon for mercury capture are needed. 
       SUMMARY OF THE INVENTION 
       [0003]    An object of the present disclosure is to provide a system and a method for mercury capture from a coal-fired power plant flue gas comprising mercury using a carbonaceous sorbent, such as activated carbon (AC) or powdered activated carbon (PAC), that reduces associated compressed air usage, operating costs and capital costs. 
         [0004]    The subject system is a steam mill system that transports a carbonaceous sorbent, such as AC or PAC from a storage silo to a steam powered jet mill using a compressed air powered eductor, or more preferably, a lower pressure eductor and blower combination. In the subject system, a cyclone separator is arranged in a transport line immediately prior to the steam powered jet mill to separate the carbonaceous sorbent from the transport air. The cyclone separator separates carbonaceous sorbent particles from the transport air that are equal to or greater in size than the nominal mill outlet particle size. These relatively larger separated carbonaceous sorbent particles are injected directly into the steam powered jet mill by means of a steam powered eductor arranged on the steam powered jet mill. In the steam powered jet mill, these relatively larger separated carbonaceous sorbent particles are milled to a desired particle size and emerge as milled carbonaceous sorbent particulates. The milled carbonaceous sorbent particulates are then injected directly into flue gas produced in a coal-fired boiler. Carbonaceous sorbent particles in the transport air stream that are smaller than the desired size are carried through the cyclone separator with the transport air and bypass the steam powered jet mill as a bypass air stream with entrained smaller carbonaceous sorbent particles therein. This bypass air stream with entrained smaller carbonaceous sorbent particles is also injected by a steam injector directly into flue gas produced in the coal-fired boiler. As such, the carbonaceous sorbent flow capability of the overall system is increased. Additionally, the cyclone separator and the steam injector are operated to process the carbonaceous sorbent at the same flow rate as that of the carbonaceous sorbent supplied from the storage silo. Hence, no carbonaceous sorbent storage is required at the location of the steam powered jet mill. An additional feature of the subject system that improves safety is that the cyclone separator operates to relieve backpressure to a connected transport line in the case of an obstruction or pressure increase in an air line or steam line. 
         [0005]    The subject disclosure is accordingly directed to a system for processing carbonaceous sorbent comprising a sorbent silo, a cyclone separator for separating transport air and small sorbent particles from the larger sorbent particulates, a steam powered jet mill for milling larger sorbent particulates to produce milled sorbent, an air line for transport of smaller sorbent particles for injection of the smaller sorbent particles into a boiler flue gas containing mercury for mercury adsorption by the smaller sorbent particles, and a steam line for transport of milled sorbent for injection of milled sorbent into a boiler flue gas containing mercury for mercury adsorption by the milled sorbent. The smaller sorbent particles are transported in an air line at a pressure of about 5 to about 15 psig, or preferably at a pressure of about 1.5 to about 6 psig, supplied by a compressed air powered primary transport eductor. The smaller sorbent particles so transported are about 3 to about 18 microns in size. The milled sorbent particulates are transported in a steam line at a pressure of about 5 to about 15 psig, or preferably at a pressure of about 1.5 to about 6 psig, supplied by a steam powered eductor. The milled sorbent particulates so transported are about 3 to about 18 microns in size. Prior to milling, the larger sorbent particulates are about 9 to about 30 microns in size. In this described system, the cyclone separator releases pressure into a transport line upon an obstruction in the transport of smaller sorbent particles or milled sorbent. Also in the subject system, an injector for injection of milled sorbent is operated at an operation rate the same as an operation rate of the cyclone separator thus avoiding need for storage of carbonaceous sorbent at the site of milling. 
         [0006]    The subject disclosure is likewise directed to a method for processing carbonaceous sorbent comprising providing a sorbent silo for supply of a carbonaceous sorbent to a cyclone separator for separation the carbonaceous sorbent into larger sorbent particulates and smaller sorbent particles, using a steam powered jet mill for milling larger sorbent particulates to produce milled sorbent, transporting smaller sorbent particles to injection into a boiler flue gas comprising mercury for mercury adsorption by the smaller sorbent particles, and transporting milled sorbent to injection into a boiler flue gas comprising mercury for mercury adsorption by the milled sorbent. According to the subject method, the smaller sorbent particles are transported in an air line at a pressure of about 5 to about 15 psig, or preferably at a pressure of about 1.5 to about 6 psig, supplied by a compressed air powered primary transport eductor. The smaller sorbent particulates so transported are about 3 to about 18 microns in size. The milled sorbent particulates are transported in a steam line at a pressure of about 5 to about 15 psig, or preferably at a pressure of about 1.5 to about 6 psig, supplied by a steam powered eductor. The milled sorbent particulates so transported are about 3 to about 18 microns in size. Prior to milling, the larger sorbent particulates are about 9 to about 30 microns in size. In this described method, the cyclone separator releases pressure into an injection duct upon an obstruction in the transport of smaller sorbent particles or milled sorbent. Also in the subject method, an injector for injection of milled sorbent is operated at an operation rate the same as an operation rate of the cyclone separator thus avoiding need for storage of carbonaceous sorbent at the site of milling. 
         [0007]    Other features and advantages of the subject system and method will become apparent from the following drawings and specification. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The present system and method is further described and its numerous features and advantages made apparent to those skilled in the art by reference to the accompanying drawing in which: 
           [0009]      FIG. 1  is a schematic diagram of a carbonaceous sorbent processing system according to the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Illustrated in  FIG. 1 , is a carbonaceous sorbent processing system  12  useful for processing a carbonaceous sorbent  14 , such as activated carbon (AC) or powdered activated carbon (PAC) for use in a coal-fired power plant  10  for mercury capture applications. 
         [0011]    The subject carbonaceous sorbent processing system  12  includes a steam powered mill system  16  that transports a carbonaceous sorbent  14 , such as AC or PAC from a storage silo  18  to a steam powered jet mill  20  using a compressed air powered eductor  22 . The eductor  22  provides a pressure of about 15 psig, or more preferably, a lower pressure eductor  22  and blower  46  combination provide a combined pressure of about 3 psig to the steam powered mill system  16 . The lower pressure eductor  22  and blower  46  combination is preferred in the subject steam powered mill system  16  in order to further reduce system operation costs and capital costs. 
         [0012]    In the subject steam powered mill system  16 , blower  46  is fluidly connected to eductor  22  of storage silo  18  by an air line  48 . Transport air (TA) from blower  46  flows through air line  48  to eductor  22  of storage silo  18  to transport carbonaceous sorbent  14  from storage silo  18  to a fluidly connected cyclone separator  28 . As such, cyclone separator  28  is arranged in a transport line  30  fluidly connecting cyclone separator  28  to eductor  22  of storage silo  18 . Cyclone separator  28  is arranged immediately prior to and fluidly connected to steam powered jet mill  20  by means of a steam powered eductor  32  arranged therebetween within the subject steam powered mill system  16 . Cyclone separator  28  separates the carbonaceous sorbent  14  from the transport air TA. As such, the cyclone separator  28  separates carbonaceous sorbent  14  particulates from the transport air TA that are equal to or greater in size than the nominal mill outlet particle size. The nominal mill outlet particle size is about 5 microns (D50). These relatively larger separated carbonaceous sorbent particulates  50 , about 9 to about 30 microns in size (nominally 18 microns), are injected directly into the steam powered jet mill  20  by means of the steam powered eductor  32  arranged on and fluidly connected to the steam powered jet mill  20 . In the steam powered jet mill  20 , these relatively larger separated carbonaceous sorbent particulates  50  are milled utilizing steam as a grind material to a desired particle size to emerge as milled carbonaceous sorbent  34  particulates about 3 to about 18 microns in size (nominally 5 microns). The desired particle size of the milled carbonaceous sorbent  34  particulates is 3 to 18 microns. The milled carbonaceous sorbent  34  particulates are then transported through steam line  44 , which fluidly connects steam powered jet mill  20  with boiler  36  upstream of air preheater  52 . As such, milled carbonaceous sorbent  34  particulates are injected by steam injector  54  directly into flue gas (FG) comprising mercury produced in coal-fired boiler  36  of coal-fired power plant  10  for mercury capture. 
         [0013]    Carbonaceous sorbent  14  particles in the transport air TA stream that are smaller than the desired size of 18 microns are carried through the cyclone separator  28  with the transport air TA bypassing the steam powered jet mill  20  as smaller sorbent particles  15 , about 3 to about 18 microns in size (nominally 5 microns), entrained in a bypass air (BA) stream. Bypass air BA stream with entrained smaller sorbent particles  15  is transported through air line  42 . Air line  42  fluidly connects cyclone separator  28  with boiler  36  upstream of air preheater  52 . The smaller sorbent particles  15  entrained in the bypass air BA are then also injected by a steam powered injector  38  directly into flue gas FG comprising mercury produced in the coal-fired boiler  36 . Since smaller sorbent particles  15  and excess transport air TA bypass steam powered jet mill  20 , steam powered jet mill  20  has more capacity for milling the larger separated carbonaceous sorbent particulates  50  to produce milled carbonaceous sorbent  34  particulates. Also, because the cyclone separator  28  and steam injectors  38  and  54  are operated to process smaller sorbent particles  15  and carbonaceous sorbent  34  at the same flow rate as the carbonaceous sorbent  14  is fed from the storage silo  18 , no additional carbonaceous sorbent storage is required. With no such carbonaceous sorbent storage requirement, both capital and operation expenses are reduced. As an added safety feature of the subject steam powered mill system  16 , the cyclone separator  28  relieves backpressure to transport line  30  in the case of an obstruction or pressure increase in air line  42  or steam line  44 . 
         [0014]    All steam powered components of the subject steam powered mill system  16  are powered through a steam source  56 . Steam source  56  is powered by and/or is supplied steam from fluidly connected coal-fired power plant  10 . Steam source  56  and coal-fired power plant  10  are fluidly connected by transport line  62 . Steam from steam source  56  is supplied to steam powered eductor  32  through fluidly connected steam line  58 . Likewise, steam from steam source  56  is supplied to steam powered jet mill  20  through fluidly connected steam line  60 . 
         [0015]    A method for processing carbonaceous sorbent  14  using the subject steam powered mill system  16  of the subject carbonaceous sorbent processing system  12  is accomplished by providing a sorbent silo  18  for supply of a carbonaceous sorbent  14  to a cyclone separator  28  for separation the carbonaceous sorbent  14  into larger separated carbonaceous sorbent particulates  50 , about 9 to about 30 microns in size, and smaller sorbent particles  15 , about 3 to about 18 microns in size. A steam powered jet mill  20  is then used for milling the larger separated carbonaceous sorbent particulates  50  to produce milled sorbent  34 , about 3 to about 18 microns in size. The smaller sorbent particles  15  are entrained in bypass air BA and transported thereby to a steam powered injector  38  for injection into boiler  36  flue gas FG comprising mercury for mercury adsorption by the smaller sorbent particles  15 . Additionally, milled sorbent  34  is transported to steam powered injector  54  for injection into boiler  36  flue gas FG comprising mercury for mercury adsorption by the milled sorbent  34 . According to the subject method, the smaller sorbent particles  15  are transported in air line  42  at a pressure of about 5 to about 15 psig, or preferably at a pressure of about 1.5 to about 6 psig, supplied by eductor  22  and optional blower  46 . As mentioned previously, if eductor  22  is used in combination with blower  46 , a lower pressure eductor  22  may be used lowering both operation costs and capital costs associated therewith. The milled sorbent particulates  34  are transported in steam line  44  at a pressure of about 5 to about 15 psig, or preferably at a pressure of about 1.5 to about 6 psig, supplied by a steam powered eductor  32 . As such, the milled carbonaceous sorbent  34  particulates transported through steam line  44 , are injected by steam injector  54  directly into flue gas (FG) comprising mercury produced in coal-fired boiler  36  of coal-fired power plant  10  for mercury capture. 
         [0016]    According to this method, the cyclone separator  28  releases pressure into transport line  30  upon an obstruction in the transport of smaller sorbent particles  15  or milled sorbent  34 . Another feature of the subject method is that an injector  54  for injection of milled sorbent  34  is operated at an operation rate the same as an operation rate of the cyclone separator  28  thus avoiding need for storage of the larger separated carbonaceous sorbent particulates  50  or milled sorbent  34  at the site of milling. 
         [0017]    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.