Saturday, July 23, 2016

Continuous Emission Monitoring Systems

Custom CEMS
Custom CEMS by CEMTEK
CEMS stands for "continuous emission monitoring systems" and are used to monitor the flue gas exiting to the atmosphere from a boiler, a furnace, or oven.

CEMS are installed by commercial and industrial plants to ensure compliance with the Environmental Protection Agency's requirement limiting the volume of harmful gasses (such as CO2) into the air.

CEMS take samples of the flue gas, and then measures, acquires data, stores records and produces reports of the gas emissions. CEMS may also provide additional information such as measuring and reporting the gas flow, its opacity and the gas moisture content.

CEMS usually have the same primary components, which are:
  • a sampling probe; 
  • a filter; 
  • a sampling line;
  • a process to condition the sample gas;
  • a calibration gas;
  • gas analyzers set to the gases being monitored.
The most common gases measured are:
  • carbon dioxide
  • carbon monoxide
  • airborne particulate
  • sulfur dioxide
  • volatile organics
  • mercury
  • nitrogen oxides
  • hydrogen chloride
  • oxygen
The US Environmental Protection Agency requires a data acquisition system and handling process to collect and report the data, which CEMS provides. CEMS must operate and provide data continuously in order to assure governmental compliance and meet record keeping requirements.

For further information on CEMS, contact:

Arjay Automation
1178 E. Cliff Road
Burnsville, MN 55337-1577
Phone (800) 761-1749
www.arjaynet.com

Wednesday, July 20, 2016

pH Measurement: Power Plant Sulphur Dioxide Scrubber Application

pH Control
pH Control
(Yokogawa)
Analytical measurement and control of pH within a system is necessary for many processes. Common applications include food processing, wastewater treatment, pulp & paper production, HVAC, chemical industries, and power generation.

To maintain the desired pH level in a solution, a sensor is used to measure the pH value. If the pH is not at the desired set point, a reagent is applied to the solution. When a high alkaline level is detected in the solution, an acid is added to decrease the pH level. When a low alkaline level is detected in the solution, a base is added to increase the pH level. In both cases the corrective ingredients are called reagents.

Accurately applying the correct amount of reagent to an acid or base solution can be challenging due to the logarithmic characteristics a pH reaction in a solution. Implementing a closed-loop control system maintains the pH level within a certain range and minimizes the degree to which the solution becomes acidic or alkaline.


Real World Application - Power Plant Sulphur Dioxide Scrubber Systems (courtesy of Yokogawa)

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.

power plant scrubber
Typical sulphur dioxide scrubber pH control system.
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.

For more information, contact:
Arjay Automation
1178 E. Cliff Road
Burnsville, MN 55337-1577
Phone (800) 761-1749
www.arjaynet.com

Tuesday, July 19, 2016

pH and ORP Learning eBook by Yokogawa

ph ORP controller
pH ORP controller
(Yokogawa)
Measuring pH/ORP is very common, but taking true measurements and correct interpretation of the results is not self-evident. Certain effects can potentially cause problems if not taken into consideration.

The purpose of the book provided below (courtesy of Yokogawa Electric Corporation) is to provide a comprehensive understanding of pH/ORP measurement and how to achieve reliable results. Basic information on the principles of measuring pH/ORP, the construction of the sensing elements and their basic use in process applications are provided.

A part of achieving accurate and reliable pH/ORP measurements requires suf cient and correct maintenance and storage conditions. Prevention of common errors during maintenance and storage, as well as consistent detection of loop failures is important. This book describes how these can be avoided and how failures can be detected.

This book is accompanied with a frequently asked question and answer section as well as an appendix that includes helpful information like a Chemical Compatibility Table and a Liquid-Application-Data-Sheet, which can be used to describe the user’s application.


Form ore information on pH/ORP instrumentation and control, contact:

Arjay Automation
1178 E. Cliff Road
Burnsville, MN 55337-1577
Phone (800) 761-1749
Fax (612) 861-4292

Thursday, July 14, 2016

Applications for Mass Flow Meters in Water & Wastewater Treatment Plants (WWTP)

Reprinted with permission from Eldridge Products
Inline Thermal Mass Flow Meter
Inline Thermal Mass Flow Meter
(courtesy of Eldridge Products)

The treatment of water and wastewater is a critical element of municipal responsibility. Increased public and private awareness of water quality, availability, and cost is a driving force behind the demands for better efficiency and economy in these processes. Whether local needs call for new facilities or improvements to existing facilities, thermal flow meters can help meet these demands and at the same time eliminate the undesirable system pressure drops and high maintenance costs associated with the older technology of differential flow meters and rotary load meters.

Aeration Basins

A common use of thermal flow meters at WWTP facilities is to measure the air (or oxygen) flow required for the secondary treatment of the activated sludge process when air and “seed” sludge are added to the wastewater to facilitate decomposition. To stimulate the growth of aerobic bacteria and other organisms that are present in the sewage, air is pumped at a pre-determined rate into large aeration tanks where the wastewater and sludge are mixed. The rate of the air flow must be carefully monitored and adjusted, as necessary, throughout the tanks and throughout the overall process for optimal efficiency. Adding either too much or too little air can have a very noticeable negative impact on this important step of the treatment process, so a well-balanced and properly distributed air/oxygen supply in an aeration system is a critical element in any effective wastewater treatment plant. Providing accurate measurement of the air flow is often the primary application for thermal flow meters at treatment plants.

After additional steps, such as settling and re-circulating, the sludge is subjected to anaerobic treatment where the sludge is placed in digesters (oxygen-free tanks) and heated for a number of days to stimulate the growth of anaerobic bacteria. This digestion process is required to convert as much of the sludge as possible into water and a mixture of carbon dioxide and methane gas called digester gas or biogas — and this presents another excellent and increasingly critical opportunity to incorporate thermal flow meters into the plant operations.

Potential points of measurement in a WWTP air system are:
  • (A) Blower inlet air
  • (B) Total air usage
  • (C) Distribution pipes
  • Typical thermal mass flowmeter applications
    Typical thermal mass flowmeter applications
  • (D) Aeration basins
Digester Gas / Energy Production

Water and wastewater treatment processes, such aeration and pumping, are energy-intensive. (Energy costs are commonly the second leading expense of WWTP facilities behind only facility staffing.) However, the digester gas from the anaerobic process typically contains 60–80% methane gas. Rather than allowing the gas to escape into the atmosphere — with its own set of environmental problems and restrictions! — the methane can be captured for use as an energy source to drive turbines that produce electricity or to directly drive other plant equipment. The gas can also be used in boilers to provide heat for the facility’s buildings. All of this helps reduce the need for purchasing natural gas from another source.

A typical digester system will contain the captured gas in a storage tank that acts as a buffer to balance fluctuations in the production of gas in the digesters. Production is usually greater in summer than in winter which is often the opposite of the facilities pattern of usage. As supplies dwindle, natural gas must supplement the captured digester gas. And it is here that the accurate measurement of both digester gas and natural gas will have a critical impact in the cost-effective operation of the treatment facility. Closely monitoring the use of both gases as it is distributed through the treatment facility provides the information needed for efficient operation and for the reporting of the cost savings derived from the capture and use of the digester gas itself.

Thermal Technology

Constant temperature thermal mass flow meters (such as those produced by Eldridge Products, Inc. of Marina, CA), operate on the principle of thermal dispersion or heat loss from a heated Resistance Temperature Detector (RTD) to the flowing gas. Two active RTD sensors are operated in a balanced state. One acts as a temperature sensor reference; the other is the active heated sensor. Heat loss to the flowing fluid tends to unbalance the heated flow sensor and it is forced back into balance by the electronics. With this method of operating the constant temperature sensor, only the skin temperature is affected by the fluid flow heat loss. This allows the sensor core temperature to be maintained and produces a very fast response to fluid velocity and temperature changes. Additionally, because the power is applied as needed, the technology has a wide operating range of flow and temperature. The heated sensor maintains an index of overheat above the environmental temperature sensed by the unheated element. The effects of variations in density are virtually eliminated by molecular heat transfer and sensor temperature corrections.
Specifying the Requirements

A number of factors must be considered when selecting and specifying any instrumentation and this is true for thermal mass flow meters to be used in WWTP systems. To specify the best configuration, you must determine:

What are the flow measurement conditions, such as the minimum and maximum flow rates to be measured, the process pipe size, and the gas temperature and line pressure?
All flow meters have minimum and maximum flow limits for any given pipe size, and temperature and pressure ranges for the physical construction. Assuring that the flow meter meets these basic requirements is the first step in specifying the proper mass flow meter. These parameters will determine the calibration scale and the expected accuracy, as well as help to identify potential issues with the overall installation.

Where will the flow meter be installed and what is the piping configuration upstream and downstream of that location?
The flow readings will be their most accurate where the gas flow profile in the pipe is uniform and consistent so that the sensor output at the point of measurement is truly representative of the overall flow through the pipe. All instrument manufacturers have recommended straight run requirements for the installation of their meters. These recommendations are offered to help end-users determine suitable locations for the flow meters, but it is important to recognize that these are only guidelines and not guarantees of optimal positioning.

Is there moisture present at the point of measurement?
By its nature, most digester gas is considered to be “wet”. Simply stated, thermal mass flow meters will not read accurately* if water droplets come into contact with the sensor RTDs. Although there are successful strategies for minimizing the potential for problems, installing the flow meter at a location where the gas is dry is strongly advised.
* The heat loss to a liquid such as water droplets is so much greater than the heat loss to a dry gas that the meter’s flow signal will typically rise to a higher value producing an error that will remain until the heated RTD is dry again. Although Eldridge Products’ flow meters will be affected by these droplets, they will not cause damage to the flow sensor. Non-condensing water vapor in the gas is acceptable.

For more information on the proper application of thermal mass flow meters, contact :

Arjay Automation
1178 E. Cliff Road
Burnsville, MN 55337-1577
Phone (800) 761-1749
Fax (612) 861-4292