Making green wood work in existing coal plants

By Anna Austin | April 14, 2011

One of the challenges of cofiring biomass with coal at existing plants is effectively dealing with high moisture content in wood.

If a biomass power plant is built from scratch, wood with 50 percent moisture content can be used with no problems, said Tom Kimmerer, senior scientist at Moore Ventures LLC. “However, if you’re cofiring in an existing plant, high moisture in wood causes all kinds of problems,” he said.

Coal has relatively low moisture content. “At harvest time, woody biomass typically has moisture content of about 50 percent,” Kimmerer said, and there are three main problems that this can cause.

The first is in the supply chain. “If you’re going to transport green wood, meaning you’re grinding the material out in the woods and transferring it to the plant at 50 percent moisture, you’re delivering half of a truck of water,” Kimmerer said. “So there’s inefficiency in energy delivery.”

The next issue involves the physical handling of the biomass fuel. “When you cofire biomass with coal, moisture causes problems such as clumping, and that’s true whether you’re actually mixing it with coal or combusting it in a separate burner,” Kimmerer said. “It’s difficult to handle and it sticks to things.”

Lastly, the sudden flash over of water in the wood to water vapor causes back pressure on the fans that move air to oxygenate the fire. “If you knew that you’d be burning material with 50 percent moisture back when the plant was being built, you would oversize the fan,” Kimmerer said. “Unfortunately, that’s not the case in the existing plant.”

Measuring the moisture content of every truckload coming into the plant is one solution to the aforementioned problems. “You don’t want to pay for water, and you know whether further treatment of the biomass is required,” he said.

High moisture wood has to be dried somehow, Kimmerer said. “You can install dryers at the power plant, but then you waste energy,” he said. “The key is to use the material when it isn’t wet at all, and there is a drying method that doesn’t involve wasting energy, and it can be done at very low cost.”

Find out what that method is and how it can be done during Kimmerer’s presentation Conquering the Biomass and Coal Co-Combustion Challenge at the International Biomass Conference & Expo, which will be held May 2-5 in St. Louis. 

Joining Kimmerer on the panel will be Michael Goerndt, University of Missouri-Columbia postdoctoral fellow; Edward Harris, Metso process/product manager; Mark D’Agostini, research associate Air Products and Chemicals Inc.; and Phil Hirsch, CEO of E.A.R.T.H. Corp.

For more information or to register for the event, go to