While biomass energy is prominent in Europe, co-firing wood with coal in a coal boiler is finally starting to get some attention in the U.S. A recent article in The New York Times addressed some of these efforts by power companies to include more renewables in their portfolio by feeding some wood into their coal boilers. Wood is considered carbon-neutral, since it absorbed carbon while growing and can be replanted after harvesting.
I applaud the power companies for trying wood out. They are doing the best they can with what is on the market at this time – white pellets, waste wood chunks and sawdust. Predictably, though, they are experiencing problems. All these problems would go away if torrefied biomass pellets or briquettes were commercially available to them. So let me specifically address the issues raised in the NY Times article:
- On feeding: “Small amounts of wood can be mixed in with coal and added to existing equipment that pulverizes coal into powder, which is then burned, but that limits co-firing to about 5 percent of fuel, and some companies say that their pulverizing equipment cannot handle the wood.” Raw wood, in any form, is fibrous. It does not grind easily like coal. Therefore, only very small amounts can be fed into a coal boiler. Retrofitting feeding systems to handle raw wood is very expensive. In Europe, some coal-fired power plants have invested $100 million for each 300MW in electric production and built a separate feeding system for white pellets. Even then, since white pellets contain only 75 to 80% energy density of coal, a coal furnace will tend to burn at a low temperature, reducing its thermal efficiency.
- On storage: “…coal-fired power plants are not used to fuel that can rot or grow fungus.” Torrefied wood has been roasted at a high temperature, and is water resistant, so it does not rot. It can be stored outside without cover, just like coal with no ill effects.
- On size: “The larger size of wood compared to coal is also an issue. A pound of wood can produce only about two-thirds as much heat as a pound of coal, and it is a lot bigger. So to produce the same amount of energy, companies must enlarge their fuel- handling systems.” Torrefied wood is energy dense. For example, pound for pound, our TorrB® briquettes contain more energy than coal – approximately 20% more. This is because they are more energy dense than raw wood pellets, and they contain less moisture than western coal, which typically has 30% moisture.
- On availability: “The material is difficult to get in any quantity and any predictable form.” According to a study done by Oak Ridge National Laboratory for the USDA in 2005, there are 1.3 billion dry tons of biomass available on a sustainable annual basis. Of this, approximately 770 million dry tons are currently not utilized. In Oregon, our analysis shows there are 6 to 10 million bone dry tons economically available.
- On predictability: If we take the slash piles left behind after logging of forest restoration efforts, we will process it into energy dense, water resistant briquettes of uniform size that can be fed with coal in any ratio directly into a coal-fired powder boiler without modifications needed.
Here’s another prediction: once torrefaction is commercialized, it will run circles around white pellets. Some large companies have designed their own successful torrefaction processes. The sticking point for commercialization has been how to densify the torrefied biomass into pellets or briquettes that are water resistant and sturdy. HM3 Energy has demonstrated densification of torrefied biomass on commercial equipment which has been modified using our proprietary technology. We produced very sturdy and water resistant briquettes. So, now that the last barrier to commercialization of torrefaction has been surpassed, things should get very interesting.