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 pro-active 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 ComponentsThe 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 DraftAir 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 set-point) , 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 megawatt-hours (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 PumpsThe 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 year-round 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 oil-fired 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.