WO2016074190A1

WO2016074190A1 - Treatment of flue gas desulfurization wastewater

Info

Publication number: WO2016074190A1
Application number: PCT/CN2014/090987
Authority: WO
Inventors: Cheng Yang; Minjia ZHAO; Qingwei CHU
Original assignee: Dow Global Technologies Llc
Priority date: 2014-11-13
Filing date: 2014-11-13
Publication date: 2016-05-19
Also published as: CN107073394A

Abstract

A method for removing sulfur oxides from flue gas (10) comprising the steps of: i) contacting the flue gas (10) with a slurry (14) comprising a calcium reactant to form a wastewater (28) including an insoluble calcium compound and soluble calcium ions; ii) pre-treating the wastewater (28) to remove at least a portion of the insoluble calcium compounds; iii) passing the resulting pre-treated wastewater (16) across a nanofiltration (NF) membrane (30); and iv) reusing at least a portion of the NF reject stream (32) from step iii) in step i).

Description

TREATMENT OF FLUE GAS DESULFURIZATION WASTEWATER

FIELD

The invention is directed toward methods for conducting flue gas desulfurization including the treatment of flue gas desulfurization wastewater.

INTRODUCTION

Most coal-burning power plants make use of some type of flue gas desulfurization (FGD) . One common embodiment utilizes a wet scrubber and an alkaline reagent (e.g. limestone， lime， etc. ) to adsorb sulfur dioxide from flue gas generated from boilers. The alkaline reactant and sulfur dioxide react to form insoluble calcium compounds (e.g. calcium sulfite， calcium sulfate， etc. ) which are then separated (via clarifier-thickeners， hydroclones， etc. ) from the FGD wastewater ( “blowdown” ) . The remaining FGD wastewater has variable quantities of suspended solids and metals along with a high amount of chloride ions. The high chloride content of the FGD wastewater limits its re-use in the scrubber. In addition to being corrosive， the high chloride content of the wastewater reduces desulfurization efficiency. As a consequence， the majority of the FGD wastewater must be discharged in order to maintain the chloride ion concentration in the wet scrubber at an acceptable level (e.g. less than 15,000 ppm) . Prior to discharge， wastewater may be subject to primary and secondary treatment including sedimentation， chemical precipitation (e.g. coagulation， flocculation) ， biological treatment and evaporation. Given the cost associated with treating and discharging FGD wastewater， new treatment methodologies are desired.

SUMMARY

The present invention includes a method for reusing soluble calcium reactants present in the FGD wastewater so that it can be re-used as “makeup” in the wet scrubber. The invention reduces -calcium reactant consumption and wastewater disposal demands. In a preferred embodiment， the invention includes a method for removing sulfur oxides from flue gas comprising the steps of： i) contacting the flue gas containing sulfur oxides with a slurry including a calcium reactant to form a wastewater including an insoluble calcium compound， soluble divalent ions including calcium and sulphate， and monovalent ions including chloride ions； ii) pre-treating the wastewater to remove at least a portion of the insoluble calcium compounds； iii) passing the resulting pre-treated wastewater across a nanofiltration (NF) membrane to produce： a) a NF reject stream including an enriched concentration of divalent ions including calcium and sulphate and a concentration of monovalent ions including chloride， and b) a NF permeate stream including monovalent ions including chloride and a reduced concentration of divalent ions including calcium and sulphate； and iv) reusing at least a portion of the NF reject stream from step iii) in step i) .

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a flow chart illustrating an embodiment of the invention.

DETAILED DESCRIPTION

With reference to Figure 1， the invention includes a method for flue gas desulfurization (FGD) involving the removing sulfur oxides from flue gas generated from boilers， e.g. as used in coal-burning power plants. In a preferred embodiment， the method involves steps (i-iv) including step (i) ： introducing a flue gas (10) containing sulfur oxides into a wet scrubber (12) and contacting the flue gas with a slurry (14) comprising a calcium reactant， (e.g. lime， limestone， etc. ) to form a wastewater (16) including an insoluble calcium compound， (e.g. calcium sulfite， calcium sulfate， etc. ) ， heavy metals (e.g. mercury， arsenic， chromium， etc. ) ， soluble divalent ions including calcium and sulfate， and monovalent ions including chloride ions.

The wastewater (16) from step i) is subsequently pre-treated (18) in step (ii) to remove at least a portion of the insoluble calcium compounds. Pre-treatment may involve one or more of sedimentation (e.g. settling ponds (20) ) ， centrifugal separation (e.g. hydroclone) ， chemical precipitation (22) and micro or ultrafiltration (24) . The insoluble calcium compounds may be recovered and disposed (26) of or used in various applications， (e.g. as a source of gypsum used in production of wall board) . Pre-treatment may also remove at least a portion of heavy metals present in the wastewater. This may be accomplished by increasing the pH (e.g. addition of alkali such as lime) and the addition of chemical precipitants such as sulfides (e.g. trimercapto-triazine， sodium sulfide， etc. ) . The pre-treatment may also include magnesium precipitation (e.g. lime softening at elevated pH) ， silica removal (e.g. magnesium oxide addition) and other scaling factor removal that is necessary. Pre-treatment preferably also includes passing the wastewater through a micro or ultrafiltration membrane (24) which further reduces suspended solids present in the wastewater. The micro or ultrafiltration membrane (24) may include a wide variety of based modules (e.g. spiral wound， hollow fiber， capillary， flat disks， and tubular membrane modules or “elements” ) . Representative semi-permeable membranes include those made from： various ceramics， metals， celluloses， polysulfones， polyether sulfones， polyvinylidene fluoride， polyamides， polyacrylonitrile， polyolefins， etc. The average pore size of the membranes utilized may be selected so as to preferentially remove suspended solids， e.g. average pore sizes in the microfiltration range (i.e. 0.1 to 5 micron) . In a preferred embodiment， the average pore size of the membrane is in ultrafiltration range， (i.e. 0.01 to 0.10 micron) . Representative examples of suitable membranes include UF membrane modules SFx 2660 and SFx 2680 (x ＝ P， R， D， etc. ) available from The Dow Chemical Company.

The resulting pre-treated wastewater (28) is further treated in step (iii) by passing the pre-treated wastewater across a nanofiltration (NF) membrane (30) to produce： (a) a NF reject stream (32) comprising an emiched concentration of soluble divalent ions including calcium and sulphate ( .e.g. preferably at least 10％， 25％， and even 50％more calcium ions than present in the wastewater) along with a concentration of monovalent ions including chloride， and (b) a NF permeate stream (34) comprising monovalent ions including chloride and a reduced concentration of divalent ions including calcium and sulphate. Importantly， the ratio of soluble divalent ions to monovalent ions is greater than that of the wastewater (and NF permeate stream) . The nanofiltration membrane may be a spiral wound element， disk tube or hollow fiber module.

As part of a step (iv) ， at least a portion of the NF reject stream from step iii) is reused in step i).That is， the NF reject stream is recycled to the wet scrubber as make-up water. The enriched calcium ion content of the NF reject stream (as compared with the wastewater) improves adsorption of sulfur oxides in the flue gas and reduce the consumption of fresh lime or limestone. In addition， the enriched sulphate ion in the reused NF reject stream is reacted and precipitated in the wet scrubber (12) to reduce the fouling risk of the post-treatment apparatus after NF. As an optional step not shown， the NF permeate stream may be post-treated by passing it across a reverse osmosis (RO) membrane to produce： i) a RO reject stream including an enriched concentration of monovalent ions， and ii) a RO permeate stream including an reduced concentration of monovalent ions. The RO reject stream may be further treated by evaporation or electrolysis to further reduce discharge from the operation. At least a portion of the RO permeate stream may be reused in step i) . The RO permeate stream may also be used in other applications.

RO membranes are relatively impermeable to virtually all dissolved salts and reject more than about 95％of salts having monovalent ions such as sodium chloride. RO membranes also reject more than about 95％of inorganic molecules as well as organic molecules with molecular weights greater than approximately 100 Daltons. NF membranes are more permeable than RO membranes and reject less than about 95％of salts having monovalent ions while rejecting more than about 50％ (and often more than 90％) of salts having divalent ions -depending upon the species of divalent ion. NF membranes also typically reject particles in the nanometer range as well as organic molecules having molecular weights greater than approximately 200 to 500 Daltons.

The flue gas may include nitrogen oxides that can be converted to molecular nitrogen (N₂) as part of a separate step that is independent from the subject method. That is， the step of denitrifization is separate from that of desulfurization. For example， at least a portion of the nitrogen oxides may be removed from the flue gas used in step i) either prior to or after the subject method. Nitrogen oxide removal may be accomplished by bringing the nitrogen oxide into contact with ammonia or urea， or a scrub liquid including a metal ion chelate. Such metal ion chelates are preferably not present in the subject method.

Claims

A method for removing sulfur oxides from flue gas comprising the steps of：

i) contacting the flue gas with a slurry comprising a calcium reactant to form a wastewater comprising an insoluble calcium compound， soluble calcium ions， and monovalent ions including chloride ions；

ii) pre-treating the wastewater to remove at least a portion of the insoluble calcium compounds；

iii) passing the resulting pre-treated wastewater across a nanofiltration (NF) membrane to produce：

a) a NF reject stream comprising an enriched concentration calcium ions， and

b) a NF permeate stream comprising a reduced concentration of calcium ions； and

iv) reusing at least a portion of the NF reject stream from step iii) in step i) .
The method of claim 1 wherein the reject stream comprises at least 10％ more calcium ions than the wastewater.
The method of claim 1 wherein the step of pre-treating the wastewater comprises at least one or more of： sedimentation， chemical precipitation and ultrafiltration.
The method of claim 1 wherein the NF permeate stream is post-treated by passing across a reverse osmosis (RO) membrane to produce：

a RO reject stream comprising an enriched concentration of monovalent ions， and

a RO permeate stream comprising an reduced concentration of monovalent ions； and subjecting the RO reject stream to evaporation or electrolysis.
The method of claim 4 wherein at least a portion of the RO permeate stream is reused in step i) .
The method of claim 1 wherein the flue gas comprises nitrogen oxides， and wherein at least a portion of the nitrogen oxides are removed from the flue gas as part of a separate step.
The method of claim 1 wherein the flue gas comprises nitrogen oxides， and wherein at least a portion of the nitrogen oxides are removed from the flue gas prior to step i) .