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PROJECT OVERVIEW
In 1991, Inproheat converted a tangentially fired 450,000 lb/hr 625 psig CE hog
fuel boiler in a pulp mill to natural gas. Before the conversion, boiler burned
Bunker-C oil along with bark and other wood residue. For an integrated mill
such as Harmac, wood waste is an important energy source and provides a
considerable contribution to total energy requirements. In a hog fuel boiler,
natural gas or some other fuel is used to sustain load when moisture of the
wood is high or when steam demand exceeds the capacity of the boiler fired on wood
alone. Heating value of wood waste decreases sharply as moisture (which can run
as high as 63% by weight) increases. At Nanaimo, the hog fuel is derived from
sea borne logs and thus has a relatively high moisture level. The boiler
typically runs 80% to 85% on hog fuel with the remaining percentage provided by
gas or oil burners. If the hog fuel supply is interrupted, gas or oil burners are
capable of sustaining the entire boiler load. Steam from the boiler drives a
steam turbine for electricity generation and provides heat for various pulping
processes at the mill. The boiler had a history of unstable combustion and was
subject to secondary combustion "puffs" caused by delayed ignition of
combustible gases generated at the grate level due to a sudden increase in hog
fuel moisture content. The gases wouldn't ignite until they flowed upward about
15 to 20 feet in the furnace and reached the vicinity of the oil burner. Once
they ignited, they produced a large, heavy furnace pressurization of "puff".
The objective of the conversion was to add natural gas firing capability (lower
cost and cleaner fuel compared to Bunker C oil) and to stabilize boiler
operation and improve boiler control.
THE CHALLENGE
To eliminate the "puffs" and stabilize operations, an ignition source closer to
the grate was needed. That way, the gases would ignite sooner and at a point at
which the air supply was adequate. In addition, the air distribution throughout
the boiler needed improvement. These objectives plus the addition of natural
gas firing with fuel oil standby capability became very challenging due to a very
tight boiler design and already crowded area surrounding it. A special
consideration had to be given to the materials of construction of the burners,
which were subject to accelerated corrosion in the presence of chlorides
originating from the sea salt entrapped in the hog fuel.
The higher temperatures and lower luminosity of natural gas products of
combustion created a concern of excessive temperatures over the boiler
superheater section.
THE SOLUTION
The boiler conversion was a joint project. Inproheat Industries teamed up with
Coen Company of Burlingame, Ca and Coen Burners Canada of Montreal. Coen
Burlingame carried out an elaborate computer modeling of combustion air velocities
in the windbox and gas velocities and temperatures throughout the boiler and
prepared the preliminary burner design. Coen Canada worked on fabrication
details and fabricated final burner components. Inproheat designed and supplied
the gas headers and local fuel train racks as well as modifications to the
existing burner control system. The installation and startup were supervised by
both Coen and Inproheat.
A total of sixteen overfire gas burners and eight new oil burners
were provided. The computer modeling led to a development of venturi inserts
and flow distributor plates which were added to the existing windbox.
To address the concern about high superheater temperatures the gas
burners were set to fire slightly sub-stochiometrically and utilized a portion of
the under-fire combustion air rising from the grates. This method produces a
radiant, yellow gas flame which is completely oxidized and yet allows the
furnace to absorb the amount of radiant energy similar to an oil flame.
To control high temperature salt corrosion, special
corrosion-resistant alloys were used in the high temperature windbox discharge
zones.
To stabilize combustion and eliminate the "puffs", Coen and Inproheat
designed a special 30 MM BTU/hr gas hearth burner that could fire downward at
about a 30 degree angle and sideways at a 22 degree angle. The burner produces a
long sharp flame which reaches almost 20 feet and sweeps over the grate
providing an ignition source at grate level.
THE RESULT
Natural gas hog fuel boiler conversion stabilized combustion and eliminated any
secondary combustion, reduced acid dew-point corrosion of boiler components and
improved air distribution through special computer modeling. When the gas
conversion components were put to the test, a near perfect air balance to all
burners was achieved.
The superheater temperature rise was insignificant thanks to the
setup of the gas burners.
The utilization of special corrosion-resistant alloys prevented
deterioration of burner components. Also, because natural gas does not contain
sulfur, as oil does, cold-end acid dew-point corrosion was naturally
eliminated.
After a few months of operation, a problem was discovered with the
chloride corrosion of 316 stainless steel gas flexible hoses. Chlorides would
enter the hoses through the gas burner nozzles while the gas burners were off
line. The problem was solved by replacing stainless steel hoses with special
Teflon-lined flexible hoses.
The gas conversion was put to the test when the main hog fuel
conveyor failed during boiler startup and was out of service for several days.
The burners worked as they were supposed to and even exceeded performance
expectations for turndown and control. Operators were able to get enough
turndown on all burners to avoid taking them individually off line.
Along the stabilized boiler operation, the hearth burner made it
easier to bring the grate fire back into service after grate cleaning. With
high moisture hog fuel, it took in the past a considerable amount of time to
re-establish load on the grate. With the hearth burner the time required
dropped to 15 minutes. According to Fred Williamson, Chief Power Engineer at
Harmac: "...The burner has stabilized operation to the point that there are
virtually no combustion puffs. Before, we really didn't dare to push the
boiler. Now because the combustion process is secure, we can operate the boiler
on a more automatic basis... The results of the conversion have been
excellent. It allowed us to automate and put the boiler under computer control. Our
only problem is that the system operates so well, it's tempting for the
operators to keep running on gas more than necessary..."
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