Temperature Measurement for Soot Blowing
Optimization
Introduction
In a mill, the recovery boiler is used to recover inorganic chemicals for reuse in the
pulping process and to supply heat for steam generation. The control objectives for the
recovery boiler are to optimize energy efficiency, minimize air emissions, and reduce the
variability of steam production.
By optimizing the soot blowing operation, mills will reduce high pressure steam use by
maximizing heat transfer in the superheaters and will reduce the variability of quality in
the steam produced.
Soot Blowing Overview
As the spent cooking liquor (black liquor) is burned in the furnace, the inorganics
fall to the hearth as molten smelt and the organics combust. The heat produced is used to
evaporate water in the water tubes and in the upper and lower boiler drums. The resulting
flue gas is channeled for use in the superheaters. The steam produced in the upper (steam)
drum is sent to be superheated. There are multiple superheaters for each recovery boiler.
The flue gas is cooled slightly as it rises past the water filled tubes in the furnace
wall, and by the time it has reached the superheaters it has cooled below the fusion
temperature (800 oC or lower). Solids are still deposited on the superheater tubes but are
easily removed by means of a soot blowing operation.
The Process
Challenge
Soot blowing is accomplished with the use of high pressure steam. This is typically a
sequenced operation, timed to ensure that deposits don’t significantly reduce heat
transfer from the flue gas to the steam, thereby reducing energy efficiency. High pressure
steam is wasted if soot blowing is done more often than necessary.
To find the right balance between optimum heat transfer and minimum high pressure steam
use, highly sensitive and accurate temperature and differential temperature measurements
are required. By measuring the differential temperature between the superheater inlet and
outlet, controllers detect when heat transfer is significantly affected and start the soot
blowing process.
The
Rosemount Solution
The Rosemount Model 3244MV provides a high performance, reliable and cost effective
measurement for soot blowing control. This single device, using HART based communications,
can transmit superheater inlet and outlet temperatures and the differential. This saves on
instrumentation costs since only one transmitter is needed for all three measurements.
Installation, wiring, and inventory costs are all reduced by using a single transmitter.
Engineering time in the DCS is saved since the differential calculation is performed by
the Model 3244MV.
Even more important are the energy savings realized from the performance of the Model
3244MV. It is the only differential temperature device that can accept calibrated
RTD Callendar-Van Dusen constants and generate a special custom curve to match specific
sensors. Commonly called sensor matching, this feature provides the highest
accuracy and performance for a temperature device.
The Results
Optimization of the soot blowing process can reduce high pressure steam use 20%. Assuming
a conventional boiler with 600 tpd of black liquor feed, using 50,000 lbs/hr of steam for
soot blowing, a mill can realize savings surpassing $255,000 per year (assuming 49 weeks
per year of operation) .
This savings estimate is based on the performance of the Model 3244MV, which provides
0.9 0F accuracy for the differential measurement
at nominal superheater temperatures and ranges. Compare that to 6.0 0F accuracy for other temperature devices that do not provide sensor
matching. Also consider that these savings are in addition to the equally important
benefits of reducing variability in heat transfer, which in turn will reduce the
variability in the superheated steam produced.
Rosemount provides a complete line of temperature measurement sensors and transmitters
to fit almost all pulp and paper applications.
For more information, link to the Model
3144/3244MV product page.
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Emerson Process Management,
Rosemount Division, 8200 Market Blvd.
Chanhassen, MN USA 55317
North America: 1-800-999-9307 International:
952-906-8888 Fax: 952-949-7001
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