FARGO, N.D. — Gasification furnaces extract twice the heat from a cord of wood as do conventional outdoor water furnaces, says a furnace manufacturer.
Brian Martin of Portage and Main Outdoor Water Furnaces said the firebox in a conventional outdoor water furnace is used to burn wood and is the source of heat for the liquid exchange system. Approximately half the energy goes up the chimney as smoke and particles or coat the chimney as creosote. It’s not highly efficient.
Gasification wood furnaces, on the other hand, use the firebox to cook the wood, releasing energy-packed gases that are blown into a secondary combustion chamber for the real fire. The wood-burning chamber is not the prime source of heat in such a system, which operates in the 85 percent efficiency range.
The gasification concept isn’t new. The first units were built in Europe in 1839, and the first gasification powered vehicle was built in 1901.
Approximately 700,000 gasification-powered vehicles operated in Europe by the end of the Second World War. They were popular because they circumvented the need for petroleum fuel. Each carried a complete gasification plant on board.
The exhaust vapours fueled a regular internal combustion engine.
Today, the process is used for heat and electrical generation.
Martin said North America is finally catching on to the efficiencies and environmental benefits of extracting more energy from gases rather than from solid fuel sources.
“The primary wood firebox is thickly lined with refractor brick,” he said .
“Refractor brick retains massive amounts of heat and pushes the temperature in the primary firebox up to 1,300 degrees F and higher. You need that high of a temperature to unleash the gases. Conventional wood furnaces can’t maintain those kinds of temperatures.”
Portage and Main uses pre-cast, heat-treated, stainless steel reinforced refractory brick. The bricks are the same size and shape as firebricks commonly used in fireboxes, but they are built to a higher standard to handle higher temperatures.
Martin said wood cooks in the first firebox, which is pressurized with a small fan so the smoke, unburned material and fresh air are pushed below into the second firebox, called the reaction chamber.
Temperatures in the reaction chamber exceed 1,000 C, about double the typical sustainable temperature of a conventional firebox. The gasification process creates more British thermal units (BTUs) than a conventional furnace because it consumes all the smoke, creosote and other particles.
The lack of creosote buildup means the liquid heat exchanger tubes, which serve as the exhaust tubes, can be located directly in the second firebox. Martin said tubes located in the firebox of a conventional outdoor furnace cake up with creosote and become ineffective.
“The reaction chamber is also heavily lined with refractor brick,” he said.
“That’s how we boost the temperature up beyond 2,000 degrees. At that temperature, we ignite the smoke and all the unburned material blown down from the top firebox.
“And that’s how you extract the most BTUs from your fuel. If you put a piece of steel into the reaction chamber, it turns cherry red in three minutes. White hot in four to six minutes. Heat within the reaction chamber is intense. You could do a little blacksmithing if you wanted.”
The exhaust contains almost no polluting elements because of the near-total combustion. From the reaction chamber, exhaust flows up a meter through a half-dozen vertical heat exchange tubes. They split into 10 heat exhaust tubes as they exit the reaction chamber and enter the water jacket.
The intense heat from the reaction chamber transfers to the liquid. The tubes make three horizontal passes through the water jacket, with each pass measuring five feet front to back. Nearly all the heat has been extracted from the tubes by the third pass.
“By the time the exhaust leaves the chimney to the atmosphere, it’s nothing more than warm air. It’s gone from 2,000 F (1,000 C) down to 240 F (115 C), ” he said.
“There’s no smoke. No visible signs of pollutants. And you can put your hand up to the chimney. We’ve captured just about every BTU that could possibly be extracted from a unit of wood. Plus, we’ve burned every unit of unburned fuel that would otherwise go into the atmosphere as pollutants in a conventional outdoor furnace.”
The exhaust tubes have long, twisty steel bars running down the middle.
“The swirl in the bars spins the exhaust around so it has better contact with the walls of the tube,” Martin said.
“Without the twisted bars, exhaust takes the course of least resistance, which is down the middle of the exchange tube. That would waste heat. By forcing the exhaust into contact with the walls, we extract more heat.”
The bars slide out of the heat extraction tubes for periodic cleaning. The front and back of the furnace open completely for full access to all components.
The high tech heat control system is computerized because there’s always the chance of overheating liquid when exhaust at 1,000 C and higher runs through heat exchange tubes.
The Portage and Main solution is to regulate the fan that blows air into the primary firebox. The fan shuts down when water in the system approaches 80 C, thus slowing the combustion process in both fireboxes.
The fan starts up again when it cools to 75 C , feeding more fresh air and oxygen to the fireboxes. Water temperature is maintained between 75 C nd 85 C.
There is an automatic shutoff if the water level drops.
Portage and Main also manufactures coal burning gasification, outdoor water furnaces.
“Coal is the most plentiful fuel source on the planet and the cost to the farmer is about one sixth the cost of petroleum based fuels,” Martin said.
The company, located in Piney, Man., manufactures a variety of wood and coal burning outdoor water furnaces.
The Economizer IDM 100 puts out 140,000 BTUs per hour. It has a 70-gallon water capacity and takes logs up to 18 inches.
At the top end, the Optimizer 450 puts out 600,000 BTUs per hour. It has a 2,800 litre capacity and takes logs up to 38 inches.
Prices on wood burning furnaces are $8,500 to $11,500.