Tok CHP

Report
Alaska’s Boreal Forest
The vast spruce, aspen, and birch
forests of the eastern interior of Alaska
offer unparalleled wildlife habitat and
natural beauty. Unfortunately, wildfire
is an unavoidable element in this
ecosystem. Small communities can be
devastated by the economic impacts
of even relatively small burns.
The community of Tok, after years of
fighting to obtain fuel reduction
funding from the state and federal
governments, has chosen a more proactive solution by building the nation’s
first public school owned and operated
combined heat and power generation
system (CHP) fueled entirely by locally
produced biomass. The community is
reducing the danger from wildfire by
creating firebreaks around the area,
and using the downed timber as a
valuable fuel for the school’s boiler.
Turning a Liability into
Opportunity and Energy
Large swathes of timber are being
cut in the surrounding forests to act as
firebreaks, and the timber is decked
and allowed to dry for up to three
years. The soil is macerated to
promote rapid growth of aspens,
willows, and other deciduous trees,
which are substantially less prone to
fires and provide increased wildlife
forage and shelter.
All cutting is regulated by the State
Division of Forestry. Profits from
biomass sales are used for land
preparation and to cover cutting costs,
with huge savings as compared to
wildfire suppression efforts.
Cutting is calculated to be
sustainable on a 25 to 50 year rotation
cycle, with both the community and
local wildlife benefitting from the
enhanced forest conditions.
A Valuable Byproduct
The dried timber and brush is
chipped with a rotary chipping trailer
purchased by the community and
leased to local loggers, sawmills and
vendors. The school buys chips at
competitive rates, which are delivered
by trailer to the boiler yard.
Construction of a large storage shed is
planned for this summer.
A Solution
The process of turning raw chips into
electricity and heat takes place in
AGSD”s new Biomass boiler facility.
This 60’ x 90’ building is located
adjacent to the school.
Outside Bulk Storage
Chips are stockpiled behind the
plant, and loaded into inside storage
bins as needed
An overview
This view of the boiler facility, taken
from the southwest corner, shows the
exhaust stack on the left, the glycol
circulation pipes leaving the building
and heading underground to the
school (center), and the large fuel bin
loading doors(behind truck on right).
Inside Fuel Storage
The building has two fifty-ton
capacity storage bins, providing
enough fuel to run the plant for up to
two weeks, depending on the steam
load of the boiler.
Traveling Bin Auger
Below the bins is a traveling auger
system which pulls a steady stream of
chips from the bottom of the bins onto
a conveyor belt .
Conveyor System
The conveyor system transports
chips up to a metering bin slightly
above furnace level. When the
metering bin is filled, the conveyor
automatically stops until enough chips
have been fed into the furnace to
restart the process.
This feed system runs continuously,
independent of all other furnace and
boiler processes.
Feeding the Furnace
Chips are forced out of the metering
bin and into the furnace through a
series of screw augers. The rate of feed
is varied by the process controller as it
tries to maintain a constant boiler
pressure despite varying steam
demand and fuel quality.
Main Components
The heart and lungs of the system
consist of a 150 HP Hurst boiler sitting
atop a Messersmith furnace which can
consume up to eight tons of wood
chips per day under full energy load.
Balanced Draft
Air flow through the system is
maintained by a balance of forced
draft fans blowing above and below
the grates into the furnace, and an
induced draft fan sucking gases out of
the furnace and up the stack via an
electrostatic precipitator (ESP).
Induced Draft Fan
The system controller always
regulates air flow to maintain a slightly
negative furnace pressure. This ID fan
is controlled by a variable frequency
drive which actually changes the
motor speed.
Forced Draft Fans
The six forced air fans blowing into
the furnace –four below the grates,
and two above-run at full speed
constantly; air flow is regulated by
louvers (which are adjusted by the
controller.) The blue boxes in the
upper right of this view are two of the
louver actuators.
Electrostatic Particulate
Precipitator Keeps Exhaust
Clean
Exhaust gases are passed through a
chamber filled with plates charged
with a high voltage. Particulates
passing by wires near the entrance
receive an opposite charge, and
subsequently are attracted to the
plates. Hammers on top of the
chamber periodically strike the plates,
dislodging the particles of fly ash
which are funneled down into
containers below.
A Simple Potassium
Fertilizer
The collected fly ash will eventually
be used as a fertilizer for the
greenhouse.
Fine bark dust and inorganic matter
collected under the conveyor belts will
be used as soil amendments in the
raised beds.
There is Always Some
Waste…
The only unused byproduct of the
process is the inorganic slag which
must be raked off the furnace grates
daily. Composed mainly of rocks and
silicates contained in the Spruce
needles, this harmless refuse can be
used as fill, or disposed of in any
landfill.
Process Controller
The brain of the entire system
consists of an automatic process
controller. The controller uses a
predetermined mathematical
algorithm to attempt to maintain
steam pressure at any chosen level
(set-point) by varying the fuel feed
rate and air flow through the furnace.
A Wealth of Information
The controller constantly monitors two
main variables: deviation from set-point
(or differential), and rate of pressure
change. If either variable gets too far
below zero, the fire rate (fuel feed and air
supply) will be increased accordingly. If
either variable gets too high, the fire rate
will be reduced or even stopped
temporarily by placing the furnace in pilot
duty until the pressure drops sufficiently.
In this view, the controller screen shows
that desired pressure was exceeded
(currently 1 pound above the 150 psi setpoint) , and the system has been in pilot
for the last minute. The rate of change
(-5.7 psi/minute) is very large, indicating
that the steam demand is quite high. The
relatively high stack temperature
indicates that the furnace is working hard
to maintain system pressure under this
load.
The bin auger is currently not
running(no current draw) because the
metering bin is full, but has been
travelling to the right.
Steam Turbine
As water is boiled and steam
generated, the pressure flows through
insulated piping to the turbine. Much
of the heat energy is utilized in
spinning this turbine and, in turn, the
connected electrical generator.
Electrical generation
The 208 volt, 125 kW synchronous
AC generator provides three-phase
electricity to the school building,
satisfying most of its power needs.
Power and Control
The turbine and generator are
controlled and monitored by this
panel. Most switches for steam feed,
electrical synchronization , connection
to the grid, and emergency shutdown
are at the bottom, while monitoring
of voltage, current, frequency, power
factor, plant and buss loads, and
related information are provided by
the gages above.
Back-up
Monitoring and Recording
A digital logging power meter allows
for a precise and accurate running
tally of power produced, as well as real
time back-up monitoring of the data
shown on the analog meters. The
meter is currently showing an energy
output of 50 kW (fairly high for the
system at present).
A Successful Energy
Producer
As of early March, 2014, this system
had produced nearly 125.5 megawatthours (125,500 kWh) of electricity
worth over $69,000. While not a huge
savings, the waste heat is a valuable
by-product, and replaces the oil-fired
boiler heat that used to be necessary
for space and water heating in the
school.
And, having local control over school
power production is very desirable in
this age of rising fossil fuel costs.
Steam Condensers
Of course, not all of the heat and
energy in the steam are exhausted in
driving the turbine. To maximize the
pressure differential across the
turbine, the post-turbine steam
pressure must be reduced as much as
possible. Four large condenser
radiators are used to accomplish this
task.
These radiators are located inside
the building, which gives the added
advantage of pre-heating (and
thawing) the chips stored in the
adjacent storage bins. If temperatures
get too high, thermostatically
controlled louvers open to vent the
heat outdoors. These vents are on
opposite walls of the building; two of
the three in the north bank are open
in this view (behind radiators at right).
Capture of Still More
Energy
Before the low-pressure steam is
returned to the boiler, some of the
remaining heat energy is removed by
this heat exchanger which circulates
glycol from the school’s heating system
past the steam, cooling and
condensing it before it is stored in the
tank below. The glycol leaves the
building at about 200 degrees F, while
the steam is cooled to 220 degrees
(now a liquid under 5 PSI pressure)
before temporary storage and further
processing.
Maintaining Circulation
This pair of pumps circulate the
heated glycol solution to the air
handling equipment in the school, as
well as to yet another heat exchanger
that supplies domestic hot water for
rest rooms, showers, kitchen use, etc.
The large electrical panel on the right
contains programmable controllers
that maintain glycol temperature (by
varying steam flow through the heat
exchanger) and vary the motor speed
to these pumps, depending on school
heat demand.
Into the School
The well insulated pipes run
underground before emerging just
outside the school’s mechanical room.
(The third pipe in this view contains
the water line providing makeup boiler
water, the fire sprinkler system pipe to
the boiler plant, and a small glycol
heating loop to keep the other two
pipes from freezing.)
Tie-in
This pair of fuel oil-fired
conventional boilers now serve as
back-up for the biomass system, which
ties into the old circulation system
behind them. The 600 gallon oil-fired
water heater also just behind them (a
large brown object barely seen in this
view) now serves mainly as a storage
tank for the wood-heated water.
Deaerator
The cooled and condensed steam
(water) is reheated to remove any
potentially corrosive dissolved gases
(mainly oxygen and carbon dioxide) in
this deaerator tank. Any treatment
chemicals needed to prevent tube
scaling or corrosion are injected just
prior to the water being returned to
the boiler.
Venting
Any gases released by the deaerator
tank are vented outside the building.
Injection Pumps
The small yellow box is the chemical
injection pump, which operates in
conjunction with the two feed-water
injection pumps behind it. These
thirteen stage, 7.5 HP pumps are
capable of producing up to 385 PSI to
overcome boiler steam pressure and
re-introduce make-up water into the
system as needed.
(This system is designed to operate at
less than one-half that pressure,
however.)
Back-up Safety
Maintaining the proper boiler water
level is one of the most important
requirements of operating any boiler.
Should the level drop below the heat
transfer tubes while the furnace is still
producing heat, the danger of an
explosion becomes very real.
To assure that water is available at all
times, the AGSD system has a separate
water source (a well in the produce
processing building) and a backup
emergency generator and automatic
transfer switch. Should utility power
be interrupted, the controller would
disconnect the turbine generator from
the grid and shut down the furnace
immediately; the emergency generator
would come on-line in less than a
minute. This assures that electrical
power for the feed water pumps is
always available for a safe, orderly
shutdown of the boiler system.
More Uses for waste Heat
A new addition to AGSD’s biomass
system is the greenhouse-produce
processing building. Both are heated
by a separate glycol circulation loop,
and temperatures of approximately 70
degrees were maintained inside the
greenhouse last winter, even during
the coldest ambient temperatures.
The first crops are being planted now
(mid March, 2014), and hopefully yearround production can be achieved.
Fresh site-grown vegetables and
greens will be a welcome addition to
the school lunch program.
Room to Grow
A series of raised and heated beds
within the 30’ by 90’ building should
provide a good environment for a
variety of plants, as well as a natural
biology and horticulture lab for science
classes.
The Future is Bright
If future funding can be secured,
plans include a heat loop to the
Hockey arena adjacent to the school,
to replace even more expensive oilfired heating equipment.
As the biomass system is refined and
improved upon (and energy-saving
measures such as LED lighting and
motor upgrades are introduced),
hopefully even more of AGSD’s energy
needs can be supplied by green, clean,
renewable biomass fuel.

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