Sulfur Dioxide Scrubber: pH Control

다운로드 (162 KB)

Introduction

Power plant boiler houses designed to burn coal or high sulfur oil are required by Federal and State pollution regulations to "scrub" (remove) sulfur dioxide from flue gasses to meet emission limits. SO2 in flue gasses is known to be harmful to the environment, as it is one contributor to the formation of acid rain. pH control is critical for the proper functioning of the scrubber system. Flue gas desulfurization (FGD) technology, is commonly referred to as a scrubber, is proved and effective method for removing sulfur dioxide (SO2) emissions from the exhaust of coal-fired power plants.

Scrubber System

The basic principle of a sulfur dioxide scrubber system is the removal of SO2 by using its chemical characteristics to combine with water. In some cases, parallel rotating rods create a series of short throat Venturi openings. A series of low pressure, large orifice spray nozzles direct the scrubbing solution into the system. "Scrubbing liquor" is introduced into the system with the flue gas stream. Depending on the design of the scrubber, the gas can flow either concurrent (with) or counter-current (against) the scrubbing liquor. The high velocity turbulence caused by the Venturi openings ensures maximum gas to liquid contact. It is here that the droplets absorb the SO2 as well as impacting and dropping particulates out of the stream. The scrubbed gas is then sent through a demister or re-heater to prevent condensation and exhausted to atmosphere.

The scrubbing liquor can be bubbled through a slurry or either lime, Ca(OH)2, or limestone, CaCO3 and water. Either lime or limestone will combine with the sulfite ions from the flue gas to form gypsum, CaSO3. The SO2 that is captured in a scrubber combines with the lime or limestone to form a number of byproducts. A primary byproduct is calcium sulfate, commonly known as gypsum. Spent scrubbing liquids are sent to clarifier where the insoluble gypsum is removed and the water is returned to the scrubber system.

The addition of lime or limestone to scrubbing solution is controlled by monitoring the pH of the solution. Lime slurries are generally alkaline with a control point near a pH of 12 while limestone slurries are more neutral.

Sulfur Dioxide Scrubber

pH Control

A pH measurement is one of the testing methods used to monitor continuous blowdown and replenishment. The SO2 within the scrubbing gases can be controlled by maintaining the level of caustic scrubbing chemicals that are commonly used. pH is a critical factor for proper operation of a scrubber. It is also difficult to measure due to 2-15% solids and tendencies towards scaling, coating and plugging.

CaSO4 concentration decreases slightly as pH decreases. Furthermore, because the concentration of oxygen dissolved in the slurry is constant, the formation of sulfate depends only on the concentration of SO3. The precipitation of CaSO4 increases as pH decreases, thus CaSO4 is apt to form scale at a lower pH. Hard scale formation can be controlled by keeping the pH high.

The solubility of CaSO3 increases greatly as pH decreases or conversely CaSO3 forms a precipitate as pH increases. If pH is too high, "soft pluggage" occurs. Soft pluggage is due to formation of calcium sulfite precipitates which appear as large leaf like masses. Obviously maintenance of equipment that has soft pluggage is easier than with equipment that has hard scale. In many cases where soft pluggage has occurred, it can be melted off simply by lowering the pH (increasing solubility).

It is obvious that a potential dilemma exists, operation at too low pH promotes the formation of hard scale and operation at too high of a pH promotes the formation of soft pluggage. Only through experience can the proper pH range be determined. Typically limestone is added to achieve the desired level of SO2 removal based on the sulfur content of the coal, the boiler load and the monitored SO2 concentration of the flue gas, while maintaining the pH in the reaction tank at 5.5 to 6.0 pH. The pH sensor can be located in the re-circulating tank or the re-circulating line.

Conductivity Control

Conductivity one of the most common testing methods used to monitor the concentration of scrubbing chemicals and by-products. As the concentration of the scrubbing chemical is depleted, its contribution to the total conductivity value will also decrease. However, occurring at the same time, the contribution to conductivity from the by-products is increasing. Therefore, a measurable decrease in conductivity is detected as the scrubbing solution is depleted.

Difficulties can arise, however, when more than one gas is being scrubbed. Depending upon the relative proportions of the gases, the by-products formed will differ, leading to variations in the conductivity background. Although a conductivity measurement can be difficult or impossible, it may still provide a useful alarm point to alert the operator to check a grab sample. In scrubbers where the scrubbing chemical concentration is maintained by continual replenishment and blowdown, conductivity can be used to initiate blowdown to prevent high dissolved solids build-up.

In continual replacement scrubbers, conductivity can be used to initiate blowdown to prevent high dissolved solids build up. Torodial or Inductive conductivity is the best form of measurement to use in this application and the sensor should be located where it will be exposed to a representative sample.

Product Recommendations

pH Measurement System

Transmitter
FLXA21 2-wire pH/ORP measurement system
PH450G 4-wire pH/ORP measurement system Sensor

Option 1: FU20/FU24 pH/ORP Combination electrode
Option 2: FF20 Flow-thru assembly with individual measure, reference and temperature electrodes
i.e. SM21-AG4, SR20-AP24 and SM60-T1; SC21C- AGC55 and SM60-T1
Option 3: PR10 Retractable with combination electrode Option 4: PH8EFP with PH8HS3 Holder

Conductivity Measurement System

Transmitter
FLXA21 2-wire Inductive Conductivity Analyzer
ISC450G 4-wire Inductive Conductivity analyzer

Sensor
ISC40 Inductivity Conductivity electrode with various installation options available (insertion, flow through, retractable)

업종

Related Products & Solutions

2-Wire Transmitter/Analyzer FLXA202/21

FLEXA ™ 시리즈 분석기는 산업용 설비의 연속 온라인 측정에 사용됩니다. 단일 또는 이중 센서 측정 옵션이 있어 가장 유연한 2 선식 분석기입니다.

4-Wire pH/ORP Analyzer PH450

PH450 pH와 ORP 분석계인 PH450은 pH와 온도, ORP(Redox) 측정을 결합한 다변형 분석계로, mA 전류 출력 2개, 독립적인 SPDT 접점 출력 및 HART 등 다양한 출력기능을 통해 활용할 수 있습니다. 

All-in-One pH/ORP Sensor Series FU20/FU24/PH20

PH20, FU20, FU24는 올인원 타입의 pH 와 ORP 센서로서 Yokogawa가 "Simple is best" 이라는 모토를 센서 기술에 어떻게 적용하는 지를 보여줍니다.

Flow/NPT Fittings FF20/FS20

Yokogawa는 설치 및 유지보수 시간을 줄이고 결과적으로 운영 비용을 절감하는 설계에 특히 중점을 두어 전체 범위의 피팅을 생산하는데 상당한 설계 및 개발 시간을 투자했습니다.

Hot-Tap Holder PR10

온라인 측정은 특히 일상적인 유지보수가 필요할 때 추가적인 문제를 야기하는 경우가 많습니다. PR10은 공정을 중단하거나 종료하지 않고 센서를 제거해야 하는 어플리케이션에 이상적입니다. 특별한 도구가 없이 PR10은 최대 5 bar (72 psi)의 압력의 공정에서 안전하게 탈거할 수 있습니다. PR10을 사용하면 PG13.5 연결부가 있는 용존 산소 센서를 retractable assembly에 장착할 수 있습니다.

Industrial pH/ORP Electrodes

pH 측정 루프의 핵심은 전극 시스템입니다. Yokogawa는 이 핵심이 가장 가혹한 조건에서도 발휘될 수 있도록 광범위한 전극을 설계했습니다.

pH and ORP Analyzers

pH와 ORP 미터, 분석계 및 전송기는 수질/제품 품질을 보장하고 유출물 배출량, 배치 중화, 펄프 스톡, 세정기, 냉각 타워, 화학, 폐수 처리 및 기타 여러 어플리케이션의 연속 공정 모니터링에 사용합니다.

pH and ORP Sensors

pH 전극 및 센서는 pH 측정의 감지 부분입니다. Retractable, flow-though, 침적 및 직접 삽입을 포함한 다양한 설치 옵션이 있습니다.

적절한 pH 전극 / 센서 선택은 최적의 측정 결과를 위해 중요합니다.

×

Have Questions?

Contact a Yokogawa Expert to learn how we can help you solve your challenges.

Top