Abstract:
Methods and systems for a zero discharge waste water treatment system are provided. The system includes a filtration train including filter media having successively smaller diameter filtration elements, a reverse osmosis apparatus including a pump and a membrane coupled in flow communication with said filtration train, a vapor compressor coupled in flow communication with said reverse osmosis apparatus, and a spray dryer coupled in flow communication with said vapor compressor, said spray dryer configured to separate moisture in a brine solution from particulate suspended in the brine solution.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to integrated gasification combined-cycle (IGCC) power generation systems, and more specifically to methods and systems for integrated water treatment of IGCC process water. 
         [0002]    At least some known IGCC power plants generate large amounts of waste water and therefore require large amounts of make-up water. Siting versatility dictates using as little make-up water as possible and government regulations tend to require less waste water discharge off site. Initially designing a plant for zero wastewater discharge garners community acceptance and streamlines the permitting process. Recycling wastewater greatly decreases the amount of makeup water that must be purchased from the local utility and eliminates the local control and costs of sewer disposal. Wastewater recycling also allows a greater freedom in selecting a site for an industrial plant because there are fewer concerns about adequate water supply. In many cases, poor quality water can be used for make-up since it is upgraded in-house. For example, at several zero discharge sites, secondary sewage effluent or wastewater from other industrial sites is used as make-up. 
         [0003]    Clean water laws such as the National Pollution Discharge Elimination System (NPDES) and the implementation of similar “zero liquid discharge” regulations at the local level are spurring treating highly saturated brine wastewaters such as cooling tower blowdown, which had previously been dumped into rivers. These wastewaters, saturated with calcium sulfate and silica, are difficult to evaporate because they are already at the scaling point. Current zero discharge waste water treating systems use a vapor recompression system, a forced circulation evaporator, and a spray dryer in series to treat process waste water. Frequent vaporizer system cleaning is required due to calcium and silica scaling. In addition the system has a high capital cost and power consumption since the chloride content in the gasification water system water is limited to 3500 parts per million (ppm). Higher chloride content in the gasification water system causes low pH values and corrosion since chloride recycled to the gasifier produces hydrochloric (HCl) acid. 
         [0004]    Purified ammonium chloride recovered from wastewater can be sold as a byproduct to the fertilizer industry. The ammonium component is soluble and promotes plant growth. The chloride component is also soluble and provides soil chemistry balancing for low chloride soils. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    In one embodiment, a system for a zero discharge waste water treatment system includes a filtration train including filter media having successively smaller diameter filtration elements, a reverse osmosis apparatus including a pump and a membrane coupled in flow communication with the filtration train, a vapor compressor coupled in flow communication with the reverse osmosis apparatus, and a spray dryer coupled in flow communication with the vapor compressor, the spray dryer configured to separate moisture in a brine solution from particulate suspended in the brine solution. 
         [0006]    In another embodiment, a method of treating waste liquids from a process includes generating a third waste stream including a precipitate by combining a first waste stream and a second waste stream, filtering the third waste stream such that the precipitate is substantially removed from the third waste stream, filtering silica from at least a portion of the filtered waste stream using an ultrafiltration membrane, filtering the substantially silica free waste stream using a nanofiltration membrane such that substantially all formates and remaining calcium are removed from the permeate and the formate is concentrated in the retentate, and pressurizing the nanofiltered permeate on a high pressure side of a reverse osmosis membrane to generate permeate on the low pressure side that is substantially free of chlorides and to generate retentate including a relatively high concentration of chlorides. 
         [0007]    In yet another embodiment, an integrated gasification combined-cycle (IGCC) power generation system includes a gasifier including a blowdown system configured to remove blowdown water with a relatively high concentration of impurities from the gasifier wherein the relatively high concentration of impurities includes at least one of about 3000 parts per million (ppm) chlorides, about 1000 ppm formate, and calcium at about saturation concentration, a condensate stripper configured to separate ammonia entrained in a gasifier process condensate stream from the condensate, and a waste treatment system configured to process the blowdown water and the stripped ammonia into waste streams that are at least one of recycled back into the gasifier and collected as a solid waste such that the waste stream produces substantially zero liquid discharge. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic diagram of an exemplary integrated gasification combined-cycle (IGCC) power generation system; and 
           [0009]      FIG. 2  is a schematic view of a zero discharge waste water treatment system in accordance with an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]      FIG. 1  is a schematic diagram of an exemplary integrated gasification combined-cycle (IGCC) power generation system  50 . IGCC system  50  generally includes a main air compressor  52 , an air separation unit  54  coupled in flow communication to compressor  52 , a gasifier  56  coupled in flow communication to air separation unit  54 , a gas turbine engine  10 , coupled in flow communication to gasifier  56 , and a steam turbine  58 . In operation, compressor  52  compresses ambient air. The compressed air is channeled to air separation unit  54 . In some embodiments, in addition or alternative to compressor  52 , compressed air from gas turbine engine compressor  12  is supplied to air separation unit  54 . Air separation unit  54  uses the compressed air to generate oxygen for use by gasifier  56 . More specifically, air separation unit  54  separates the compressed air into separate flows of oxygen and a gas by-product, sometimes referred to as a “process gas”. 
         [0011]    The process gas generated by air separation unit  54  includes nitrogen and will be referred to herein as “nitrogen process gas”. The nitrogen process gas may also include other gases such as, but not limited to, oxygen and/or argon. For example, in some embodiments, the nitrogen process gas includes between about 95% and about 100% nitrogen. The oxygen flow is channeled to gasifier  56  for use in generating partially combusted gases, referred to herein as “syngas” for use by gas turbine engine  10  as fuel, as described below in more detail. In some known IGCC systems  50 , at least some of the nitrogen process gas flow, a by-product of air separation unit  54 , is vented to the atmosphere. Moreover, in some known IGCC systems  50 , some of the nitrogen process gas flow is injected into a combustion zone (not shown) within gas turbine engine combustor  14  to facilitate controlling emissions of engine  10 , and more specifically to facilitate reducing the combustion temperature and reducing nitrous oxide emissions from engine  10 . IGCC system  50  may include a compressor  60  for compressing the nitrogen process gas flow before being injected into the combustion zone. 
         [0012]    Gasifier  56  converts a mixture of fuel, the oxygen supplied by air separation unit  54 , steam, and/or limestone into an output of syngas for use by gas turbine engine  10  as fuel. Although gasifier  56  may use any fuel, in some known IGCC systems  50 , gasifier  56  uses coal, petroleum coke, residual oil, oil emulsions, tar sands, and/or other similar fuels. In some known IGCC systems  50 , the syngas generated by gasifier  56  includes carbon dioxide. The syngas generated by gasifier  52  may be cleaned in a clean-up device  62  before being channeled to gas turbine engine combustor  14  for combustion thereof. Carbon dioxide may be separated from the syngas during clean-up and, in some known IGCC systems  50 , vented to the atmosphere. Gasifier blowdown connection is coupled to a waste treatment system (not shown in  FIG. 1 ). The power output from gas turbine engine  10  drives a generator  64  that supplies electrical power to a power grid (not shown). Exhaust gas from gas turbine engine  10  is supplied to a heat recovery steam generator  66  that generates steam for driving steam turbine  58 . Power generated by steam turbine  58  drives an electrical generator  68  that provides electrical power to the power grid. In some known IGCC systems  50 , steam from heat recovery steam generator  66  is supplied to gasifier  52  for generating the syngas. 
         [0013]    In the exemplary embodiment, IGCC system  50  includes a syngas condensate stripper  76  configured to receive condensate from a stream of syngas discharged from gasifier  56 . The condensate typically includes a quantity of ammonia dissolved in the condensate. At least a portion of the dissolved ammonia is formed in gasifier  56  from a combination nitrogen gas and hydrogen in gasifier  56 . To remove the dissolved ammonia from the condensate the condensate in raised to a temperature sufficient to induce boiling in the condensate. The stripped ammonia is discharged from stripper  76  and channeled to a waste treatment system (not shown in  FIG. 1 ). In an alternative embodiment, the stripped ammonia is returned to gasifier  56  at a pressure higher than that of the gasifier, to be decomposed in the relatively high temperature region of the gasifier proximate nozzle tip  72 . The ammonia is injected such that the flow of ammonia in the vicinity of the high temperature area proximate nozzle tip  72  facilitates cooling nozzle tip  72 . 
         [0014]      FIG. 2  is a schematic view of a zero discharge waste water treatment system  200  in accordance with an exemplary embodiment of the present invention. Waste water treatment system  200  a waste receiving subsystem  202 , a filtration train  204 , a reverse osmosis sub-system  206 , a vapor recompression subsystem  208 , a spray dryer  210 , and a baghouse  212  coupled together in at least partial serial flow communication. 
         [0015]    Process blowdown water  214  containing chloride, formate, and saturated in calcium and reflux water  216  from a process condensate stripper  218  are mixed in a precipitator vessel  220 . In the exemplary embodiment, the blowdown water includes about 3,000 parts per million (ppm) chlorides, about 1,000 ppm formate, and calcium at a substantially saturated concentration. The reflux water includes a relatively high concentration of ammonium carbonate of about 10% by weight. The ammonium carbonate precipitates calcium from blowdown water  214 . The precipitated calcium is pumped with precipitator effluent  222  to a sand filter  224 . The precipitated calcium is filtered out of precipitator effluent  222  and a sand filter effluent  226  having less than about  50  ppm calcium is channeled to an ultrafiltration unit  228 . Sand filter  224  is backflushed to a backflush receiving tank  230  using VR condensate  232 . The calcium precipitate is then pumped to a gasification settling system, which concentrates the solid calcium precipitate and recycles it to the coal slurry feed, ultimately allowing it to be discharged as part of the discharge slag stream. 
         [0016]    Sand filter effluent  226  is channeled to ultrafiltration unit  228 , where a separation process using membranes with a pore size sized to reject molecules with molecular weight greater than about 1000 Daltons. Such pore size is large enough that salts and sugar molecules are capable of passing through the membrane into a permeate  234  ultrafiltration unit  228 , however silica is removed from a portion of the filtered water using the ultrafiltration membrane. The ultrafiltration membranes may be configured in a hollow fiber, spiral wound, flat, sheet, tubular and ceramic. A retentate  236  of ultrafiltration unit  228  is returned to the gasification water system (not shown) as softened recycle grey water. 
         [0017]    The silica free ultrafiltration permeate  234  is pumped to a nanofiltration system  238  which removes substantially all the formates and the remaining calcium from a nanofiltration system permeate  240  and concentrates the formate in a nanofiltration system retentate  242 . The high formate retenate is recycled to the gasifier for destruction. Nanofiltration permeate  240  is channeled to a reverse osmosis subsystem  244  which extracts purified water as a reverse osmosis permeate  246  and concentrates chloride in a reverse osmosis retentate  248 . The relatively high chloride and relatively low calcium reverse osmosis retentate  248  is channeled to vapor recompression system  208 . 
         [0018]    Reverse osmosis retentate  248  is pumped through a heat exchanger  250  that raises the temperature to the boiling point and then through a deaerator  252 , which removes non-condensable gases such as carbon dioxide and oxygen from the heat exchanger effluent  254 . Hot deaerated feed  256  is channeled to a sump  258  of an evaporator  260 , where it combines with a recirculating brine slurry  262 . Slurry  262  is pumped to the top of a bundle of heat transfer tubes  264 , where it falls by gravity in a thin film down the inside of tubes  264 . As slurry  262  falls, a small portion evaporates and the remaining falls into sump  258  to be recirculated. The vapor travels down tubes  264  with the brine, and is drawn up through a plurality of mist eliminators  266  and into a suction of a vapor compressor  268 . Compressed vapor  270  flows to the outside of heat transfer tubes  264 , where latent heat of compressed vapor  270  is transferred to the cooler brine slurry  262  falling inside of tubes  264 . As vapor  270  gives up heat, it condenses as distilled water. The distillate  272  is pumped back through heat exchanger  250 , where it transfers sensible heat to the incoming reverse osmosis retentate  248 . A small amount of the brine slurry is continuously released from the evaporator to control density. Typically 95% of the reverse osmosis retentate  248  feed is converted to distillate having, for example, less than about 10 ppm total dissolved solids, for reuse in IGCC system  50 . 
         [0019]    A portion of slurry  262  is channeled to spray dryer sub-system  210 . Slurry  262  enters a spray dryer vessel  274  through an atomizing wheel  276  spinning at a high speed, which sprays slurry  262  into a hot, gas-fired chamber  278 . Water in slurry  262  instantly evaporates from the slurry droplets allowing solids entrained in slurry  262  to collect in a bin portion  280 . From bin portion  280  the solids are pneumatically conveyed into baghouse  212  to be collected and used as a fertilizer, chemical feedstock or discarded as a waste. 
         [0020]    The above-described filtering elements, such as filtration train  204 , sand filter  224 , ultrafiltration unit  228 , and/or nanofiltration system  238 , can be cleaned online using either vibration or spare streams with a backflow/purge of the offline screen to remove solids that have been filtered out of a fluid stream. 
         [0021]    The exemplary embodiment permits integration of the IGCC process water system with the waste water treating system such that membrane filtration is used upstream of the vapor recompression and spray drying system. The use of the ultrafiltration, nanofiltration, and reverse osmosis membrane filtration reduces the rate of feed into the vaporizer permitting a smaller vaporizer size, improves vaporizer availability due to upstream removal of calcium, silica, and suspended solids, reduces salt production because formates are removed from the waste water and destroyed in the gasifier. The use of membrane filtration also eliminates the need for forced circulation in the evaporator due to a lower concentration of calcium, silica, and solids in the feed permits the falling film type evaporator to operate at higher concentration ratios eliminating the need for a forced circulation evaporator and because softened water is returned to the gasifier for use in, for example, the syngas scrubber and radiant cooler calcium deposition in such vessels and enables the use of a spray quench. The purified water from the vapor recompression system and RO membrane is used as cooling tower makeup or as feed water to the boiler water treating system. 
         [0022]    Exemplary embodiments of IGCC zero discharge waste treatment systems are described above in detail. The waste treatment system components illustrated are not limited to the specific embodiments described herein, but rather, components of each system may be utilized independently and separately from other components described herein. For example, the waste treatment system components described above may also be used in combination with different IGCC system components. 
         [0023]    The above-described IGCC zero discharge waste treatment system is cost-effective and highly reliable. In the exemplary embodiment, the IGCC zero discharge waste treatment system includes a membrane filtration train having subsequently smaller diameter porosity such that successively smaller molecules are removed from the waste stream. Retentate from the filtration train is returned to the gasifier for destruction and recycling and permeate from the final filtration stage is channeled to a vapor recompression system and spray dryer for recovery of the remaining water and collection of solids. As a result, the use of upstream membrane filters facilitates selection of a smaller capacity vapor compression subsystem and greater recovery of waste water for recycling and less waste discharge in the integrated gasification combined-cycle (IGCC) power generation system in a cost-effective and reliable manner. 
         [0024]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.