Sierra Instruments, Inc.

Choosing flow monitors for CEMS applications

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Courtesy of Sierra Instruments, Inc.

As a result of the 1990 Clean Air Act Amendments1, electric power plants must reduce their emissions of sulfur dioxide and nitrous oxides—the primary causes of acid rain. The U.S. Environmental Protection Agency (EPA), in its proposed rules2, designates the 111 Phase I fossil-fueled plants ^roughly 268 boiler units) that must have continuous emissions monitoring systems (CEMS) installed, certified and operational by November 15,1993. The EPA also designates the 662 Phase II plants (roughly 2,200 boiler units) that have a January 1, 1995 deadline.

Under the rules, each plant is assigned an SO2 and NOx mass emissions allowance in tons per year. One allowance authorizes the emission of one ton (2000 lbrn) of SO2 per year. Plants that exceed their allowances are fined $2000 for each excess ton per year.

Plants that release less than their annual allowance receive one credit per ton of SO2. Through an emissions credit trading market, utilities can sell or trade their credits to plants exceeding their allowances. At this writing, the estimated market value of each credit is $300 to $500.

Plant operators must monitor S02 emissions continuously, recording a data point at least every 15 minutes. Each plant must report the hourly average of SO2 emissions, and compliance with the annual allowance is determined from the sum of the hourly mass emissions over a year.

To monitor S02 mass emissions dwldt (lbmVh), the operator must monitor both the S02 concentration C (lbm/scf) and the mass flow rate Q (scf/h) of the stack gas, because:

dw/dt = CQ

This means that a flow monitor must now be included in systems that monitor dw/dt. Called continuous emissions monitoring systems (CEMS), these systems consist of:

  • A sampling probe,
  • Gas analyzers for S02 and NOx,
  • Opacity meter and flow monitor,
  • Data acquisition and handling system.

The accuracy and reliability of the CEMS is far more critical than conventional emissions instrumentation packages because this system becomes something of a money meter—a meter that determines either the amount of a fine or, hopefully, savings in the bank.

The following will analyze the performance of differential pressure, ultrasonic, and thermal flow monitors in three typical continuous emissions monitoring locations, all of which are characterized by nonuniformities in velocity profile and, in some instances, temperature. To help the plant operator choose the right flow monitor for a given CEMS application, an analysis is presented of the errors in accuracy of the three flow monitors when deployed side by side at the three CEMS locations.

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