Evolution of wood boilers

March 10, 2016
The true first gasification boilers in their present form appeared after the introduction of electric draft inducers in the early 1980s. This combined with two completely separated primary and secondary combustion chambers set the stage for gasification combustion temperatures of 2,200ºF. These high temperatures are necessary to facilitate the complete and clean burning of wood gases.

Almost 40 years ago when my company Stadler Corporation introduced one of the first residential European wood boilers to New England, they were elementary in design and in their infancy of wood burning know-how.

The boiler feeds a three-zone hydronic radiant floor system in a New England home.

Today's boilers are true marvels of wood burning technology. Nowadays combustion efficiencies exceed 90% with clean, zero visible smoke discharge at the flue stack. They have built in microprocessors that control combustion air intake to optimize combustion and emission levels, boiler return water temperature; manage buffer storage tank loading and unloading, as well as system consumer valves. 

In the early years of wood boiler designs, a single combustion chamber with horizontal top flue gas fire tube heat exchangers would result in poor combustion efficiencies, smoky flue discharge and excessive creosote condensation on the boiler plate, which inhibited heat transfer.

The only form of control was a non-electric thermostatic immersion type boiler temperature control, which opened and closed a combustion air intake damper. Most of the time when there were small BTU load requirements on the boiler, this damper would be in the closed position, which caused the wood burning in the combustion chamber  to smolder, creating high levels of creosote buildup on the boiler plate heat exchanger surfaces. This was especially the case if the boiler was oversized for the system.

An improved version of these wood boilers were single combustion chambers with bottom discharge that forced flue gasses down over a hot coal bed before entering a vertical secondary heat exchanger. This resulted in more complete combustion and higher efficiencies — still a natural draft unit and highly dependent on chimney draft conditions.

The true first gasification boilers in their present form appeared after the introduction of electric draft inducers in the early 1980s. This combined with two completely separated primary and secondary combustion chambers set the stage for gasification combustion temperatures of 2,200°F. These high temperatures are necessary to facilitate the complete and clean burning of wood gases.

Pre-combustion takes place at the bottom of the primary wood storage chamber, generating incompletely burned wood gasses, which are forced via a powerful suction or blower type draft inducer (suction type inducers at the exhaust stack are preferred, creating negative pressure throughout the boiler passages, making boiler operation safer) into the secondary high temperature chamber, which is lined with refractory fire bricks to accommodate the necessary 2,200°F. This turns wood gasses into almost invisible natural gas like flames, guaranteeing complete and clean combustion.

Wood gasification at its finest; fuel used is sawdust briquettes and cord wood.

This process totally incinerates dirty wood gasses as they would normally exit the boiler and instead reuses these super heated and clean gasses as they are forced back through the main heat exchanger with the help of the draft inducer, resulting in significantly lower flue gas temperatures in the 250°F to 350°F range. Vertical fire tubes with hanging gas turbulator baffles are the most common and efficient means to transfer heat to the boiler water.   

Advantages of wood boiler technology 

Incomplete "dirty" wood smoke combustion has finally come to an end with the development of gasification combustion, resulting into a myriad of advantages and possibilities in wood boiler technology, such as:

  • Clean, smoke free and non-polluting combustion.
  • Dramatically increased combustion efficiencies of up to 92%.
  • Ash production reduced dramatically down to 1.5% by weight.
  • Creosote condensation on heat exchanger surfaces has been eliminated.
  • Versatile wood product usage:  cord wood, pressed wood briquettes and pellets, and woodchips using both automated and hand feeding methods.

Rules to follow

In spite of the miracles of wood gasification technology, there are however some important rules to be followed when designing and operating a trouble free and safe wood boiler system.

  • Always use thermal storage buffer tanks (sizing rule of thumb: combustion chamber volume in gallons x 12 = minimum tank capacity in gal.)
  • Burn wood at 20 percent moisture content or below only (split and seasoned for two years, only 25 percent of its BTU value is wasted), wood with higher moisture content dramatically reduces the BTU value of your wood.  At 35 percent moisture content, 50 percent of its calorific value is used to drive the water out of it in form of steam during combustion. This number goes up to 80 percent loss when burning green, freshly cut wood.
  • Assure a minimum boiler return temperature of 158°F (70°C) at all times by means of a mixing valve and boiler circulator to avoid thermal shock and to reduce creosote condensation in the primary combustion chamber.
  • Always use non-electric thermal safety drain valves in conjunction with internal boiler copper coils for overheat dumping, even during power failures. Cold water pressure to the valve must be assured at all times. In European wood boilers this safety feature is standard equipment, required by law.

To simplify a complex system watch this neat animated combustion and piping schematic video on the Kob wood-fired boiler:

The key to operate any wood boiler under ideal conditions is always at full load output, i.e., load it up and let it rip! Your boiler will love you for it. This is only possible with properly sized buffer tanks. This can be done with a single, double or triple tank arrangement depending on the volume of storage needed and the configuration of the boiler room. Again, a safe formula to use is the volume of the combustion chamber in gallons x 12 = minimum total buffer tank storage capacity.

After the boiler's wood load is fully burned out and the buffer tanks have absorbed all excess BTUs generated during the combustion cycle, which were not immediately needed by the consumers, the system will then draw off the stored up heat as needed from the tanks during low load conditions.

Thus the boiler runs under ideal full load conditions without overheating, dramatically decreasing stoking cycles. Also a slightly over sized boiler is not an issue anymore by utilizing buffer tanks.

A great national & natural energy resource

The average heating value of wood at a moisture content of 20 percent is 6,500 BTU/per pound. There are small variations among various wood species. High density hardwood of course means you can pack more BTUs into your fire box. The BTU value per pound compared to soft wood is just about the same.

Twenty-five pounds of seasoned firewood is equivalent to one gallon of heating oil which translates into 200 gallons or 277 therms of natural gas and 330 gallons of propane for each cord of seasoned hardwood (20 percent moisture content).

Boiler control performs many functions, and ensures reliable operation.

It's a great and abundant renewable green heat source, totally CO2 neutral with zero sulfur-dioxide emission. And as they say, it will heat you twice. Once handling it and again burning it.

Control strategies, basic functions

During boiler cold start: Draft inducer turns on. Boiler return three-way mixing valve goes into full bypass position and boiler recirculation pump turns on until boiler temperature  has reached 158°F to protect boiler from internal condensation and thermal shock, assuring full and constant flow through the boiler at all times.

During heating cycle: Boiler return valve is gradually opening to the system supply and consumer pumps are activated. Once system supply water temperatures have reached set points, the buffer tank  three-way valve starts slowly modulating to the tanks open position, loading them gradually with the boiler's excess output capacities. At this point the boiler pump is charging the tanks and feeding the consumers, while the buffer three-way valve is now taking over, giving minimum boiler return temperature always priority.  The temperature of 158°F is considered safe to prevent acidic boiler plate corrosion, especially when burning oak. The boiler return valve stays in the fully open position.

As the boiler temperature keeps rising, the buffer tank valve is gradually modulating to the 100 percent open position to fully charge the tanks.

After combustion cycle is completed: By the time the buffers are fully charged, the boiler temperature is gradually dropping until the wood load is burning out. The buffer valve is now fully closing, isolating the tanks from the system, while fully utilizing the residual heat of the boiler.

As soon as the flue gas temperature has dropped below 194°F and a residual oxygen level of above 16 percent at the exhaust Lambda sensor has been reached, the draft inducer and boiler pump are shutting off. The boiler return valve goes into a 100 percent by-pass position to isolate the boiler from the rest of the system. With loaded buffer tanks, the buffer valve is now fully opening (by-pass closed), making hundreds of thousands of BTU's available to being released to the heating water to supply the system consumer valves and their circulators according to their heat load conditions as needed. The flow direction through the buffers is now reversing during the unloading phase, while the system circulators are drawing on the tanks.

After buffer tanks depletion: Once the buffer tanks are temperature depleted, the buffer valve goes into the fully closed position, isolating the tanks from the rest of the system. A back up boiler will be enabled if necessary or desired. During the entire tank loading and unloading process, the heating circuits and the boiler circuit are constant flow temperature modulated, i.e., controlled via mixing valves (temperature control), whereas the buffer tanks are being controlled via flow modulation (volume control ) at all times.

Buffer tank valve: The buffer control valve is playing a key role in the system control sequence. It is a modulating mixing valve, controlling the proper water return temperature back to the boiler during the tank loading phase, while controlling flow rates through the tanks based on boiler output and instantaneous heating requirements at the consumer valves, a constant balancing act during the tank loading phase.

During the unloading phase, the valve stays in the fully open position with the by-pass closed. Once the tanks are depleted, it closes fully, isolating the tanks from the system. An installed check valve in its by-pass prevents system return water from short cycling into the system supply.

Auxiliary control functions: There are many other auxiliary functions the boiler control is performing.  From combustion air intake to optimize clean combustion, to individual consumer circuit control of multiple mixing valves and circulators, DHW control and display of all pertinent parameters such as flue gas temperature and O2 levels, top, middle and bottom temperature of buffer tanks as well as boiler and system supply temperatures.  If the boiler should ever overheat, the draft inducer shuts down and the non-electric thermal safety valve cools the boiler down safely.

Joe Fiedrich is a former hydronic heating columnist for CONTRACTOR. He was the president of Stadler Corp., which is now part of Viega North America, and he is the inventor of the Viega Climate Panel and Uponor Quik Trak. He can be reached at [email protected]. Also, visit his company Hydronics Manufacturing.

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