Wood Gasification: Smart Alternative?

June 30, 2010

The pros and cons of turning wood into energy

Interest in renewable energy is heating up, and building owners are exploring an old technology with a proven track record: wood gasification.

Over the past 15 years, this environmentally friendly, energy-efficient solution has grown in popularity, and is especially effective in environments with a steady heating demand in winter months.

How it Works
There are five components to a wood gasification system:

  1. Storage. Solutions vary depending on the fuel chosen (wood chips or wood pellets), system location, and system size. Wood chips are less expensive to purchase, but require larger storage bunkers and more extensive systems to transport them into the gasifier.Although almost twice as expensive per BTU as chips, wood pellets occupy one-third of the volume and are more dense and uniform; therefore, they burn more efficiently. Delivery and storage can be handled like liquids. Pellets can even be blown into interior storage containers through flexible tubing.

  2. Auger System. Fuel is automatically fed from the storage container to the gasifier through a screw-type auger.

  3. Gasifier. Fuel is metered into the gasifier as needed to achieve the proper burn rate to meet heating loads, and gasified through partial combustion in a limited oxygen environment at temperatures in excess of 2,000 degrees F. to produce wood gas.

  4. Boiler. A standard hot water tube boiler is fueled by the gas to produce hot water and fed through traditional means into a distribution system.

  5. Exhaust. Flue gas is routed from the boiler into a heat recovery unit and then into a cyclone that spins the exhaust to separate remaining solids. The amount of waste generated from a 25-ton load of green hardwood chips can fit in a 25-gallon trashcan, and serves as excellent fertilizer.

Optimal Installation
Having the space necessary for delivery and storage is key, giving rural settings an advantage. Pellets are a logical choice for tighter sites.

Access to steady, local supplies of fuel also contributes to success.

Wood gasifiers realize highest efficiency when running at full load. Because the systems take longer to start up and are more difficult to modulate than oil and natural gas-fired boilers, a back-up system is often installed to provide heat more efficiently in the spring and fall months, when loads vary or are light.

Show Me the Savings
This year, Hartford Central School District in upstate New York became the first New York State public school to utilize wood gasification as a primary heating source. In recent heating seasons, the district spent more than $110,000 per year for fuel oil. Utilizing wood chips, fuel costs were close to one-third that amount.

The WILD Center in Tupper Lake, NY, has had similar success. Pellets are the fuel of choice for a new gasification system at this museum. Paired with a solar thermal system that supplements supply water and generates domestic hot water, the system provides heat for the 54,000-square-foot building. In its start-up year, the cost for wood pellets represented a 40-percent savings as compared to the propane previously used.

Get to Carbon-Neutral
Unlike fossil fuels, fuels for wood gasifiers are produced from the waste of activities like timber harvesting or sawmill operation. The carbon released from burning wood is already in the carbon cycle and would decompose and wind up in the atmosphere anyway. In addition, an institutional-scale wood gasifier system is clean and efficient, discharging less than 10 percent of the particulate emissions of the average residential wood stove. B

Kevin Schaefer is director of engineering for CSArch Architecture (www.csarchpc.com).

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