POWER July 2016 - 25

ENVIRONMENTAL ISSUES
1. A continuous emissions monitoring
system. Courtesy: EPRI
efficiency on a continuous basis, which
would be needed to complete the heat rate
calculation. One method of accurately determining
boiler efficiency, the heat loss method,
requires installation and maintenance of
instrumentation to sample gas concentration
and temperature at the air heater inlet and
outlet. However, adequate sampling grids are
expensive to install and difficult to maintain.
CEMS, which are capable of measuring
the concentration of CO2 in the stack gas
and calculating the flow rate of the exhaust
gas, have the potential to offer an alternative
method for a real-time (continuous) unit heat
rate calculation without additional instruments
or labor-intensive processes.
The issue with CEMS, though, is that the
accuracy required for a meaningful heat rate
value is much tighter than current CEMS
practices. With typical uncertainties greater
than 5%, CEMS could not be used to identify
a heat rate change of 2% to 4%, which will
potentially be required by the Clean Power
Plan (assuming it survives the current legal
challenges).
Understanding the Uncertainty of
CEMS Heat Rate Measurements
Over the last several years, researchers at
the Electric Power Research Institute (EPRI)
have conducted several studies to identify the
technology gaps and to propose and evaluate
actions required to use CEMS for heat rate
monitoring. These studies have sought to
better understand the current uncertainty of
CEMS measurements and the drivers of the
differences in uncertainty between CEMS
and a precision boiler efficiency test.
In 2014, EPRI and a member utility conducted
a series of precision performance tests
July 2016 | POWER
at the member's 350-MW coal-fired unit to
investigate how accurately the actual boiler
heat input could be determined when quality,
nonbiased CEMS data are available. First, a
series of boiler efficiency test runs were undertaken
to calculate boiler efficiency and net
unit heat rate by methods based on ASME's
Performance Test Codes (PTC) 4 and 46.
Then, the values calculated from the test results
were compared to those obtained from
the CEMS. The tests were conducted at full,
medium, and minimum load-approximately
the same unit load levels utilized for the most
recent RATA testing at this unit.
The results were further refined through
the calculation of uncertainty of the boiler
efficiency and heat rate calculated by each
method. These uncertainty calculations were
performed by methods described in ASME
PTC 19.1, Test Uncertainty.
For the full-load test, gross heat rate calculated
by the CEMS method was slightly
higher than with the PTC 46 method. For the
medium-load test, the heat rate was almost
equal for the CEMS and PTC 46 methods.
With the minimum-load test, the heat rate for
the CEMS method was much lower than for
the PTC 46 method. The reasons behind the
non-predictable differences in heat rate were
not identified.
The uncertainty analysis showed that the
surement uncertainty was the measurement
of stack gas flow rate.
Best Practices Guideline
To address this issue, in 2015, EPRI developed
a best practices guide to improve the
accuracy of the measurement of stack gas
flow rate.
The project team sought to evaluate available
information on the performance of CO2
CEMS and continuous flow monitors, and
to identify the uncertainty and biases of
measurement system parameters with the
greatest effect on measurement data quality.
Then, using the knowledge gained regarding
the uncertainty and biases of the system
parameters, the project aimed to recommend
enhancements to the operation, calibration,
maintenance, and auditing of these systems
that would extend the usefulness of the
CEMS and flow monitors to provide a primary
means of unit heat rate determination.
In support of this effort, EPRI developed
a multi-faceted approach to identify the state
of current industry best practices as well as
a methodology for achieving reduced uncertainty
in the measurements. The approach
involved conducting an open literature
search pertaining to CEMS operation; contacting
end-users for supplemental information
regarding CEMS operation, calibration,
The key contributor to the high measurement
uncertainty was the measurement of
stack gas flow rate.
uncertainty of the boiler efficiency value
using the CEMS method was greater than
5%, compared to an uncertainty of 0.29%
using the ASME PTC 4 test method. Improvements
in the accuracy of the CEMS
could greatly reduce the uncertainty, but the
uncertainty of boiler efficiency determined
by this method is dependent on both the accuracy
of the CEMS measurements and the
instrumentation used to measure the heat
output of the boiler.
While this plant site operated and maintained
its CEMS beyond the minimum EPA
requirements, the large differences in results
and the uncertainties of the results strengthened
the notion that CEMS could not be used
to monitor or report heat rate with confidence.
The key contributor to the high meawww.powermag.com
tune-ups,
and maintenance practices; determining
industry best practices related to
pre-RATA flow monitor calibrations, reference
methods used, and pre-RATA adjustments;
summarizing the effects of different
reference method practices on measurement
uncertainty; and establishing target uncertainties
and identifying whether the instrumentation
can achieve these.
A model was developed to estimate the
uncertainty contributions for the heat rate
calculation. The model allows estimation
of the uncertainties of the various measurements
used to calculate a unit heat rate-as
well as hourly heat input, including flue gas
flow rate, flue gas CO2 concentration, the
carbon-based fuel factor, and the power output
(Figure 2). The model then propagated
25
http://www.powermag.com
POWER July 2016

Table of Contents for the Digital Edition of POWER July 2016
