Using Wood as Fuel
Humans have used wood as fuel for millennia. It has been used for cooking, heating, and more recently power production. I thought this topic would be timely given that we are experiencing our first real cold snap of 2021. In fact, I’ve had our wood stove burning all weekend – enjoying the fire inside while looking out our frosty windows.
Many people believe that wood is an old-fashioned fuel. It’s great for smores on campfires but ends there. What surprises people is how much wood is used as fuel. Globally, 51% of all wood used is for fuel. In developing regions, like Africa, 90% of wood used is for fuel. Closer to home the U.S., about 8% of wood used is for fuel.
One major advantage of wood fuel is that it is carbon neutral. Carbon is released when wood is burned, but it is also sequestered in our forests at the same time. The U.S. Environmental Protection Agency treats wood biomass from managed forests as carbon neutral - something that is realized every day in Wisconsin given our efforts at sustainable forest management.
Wood pellets are a form of wood fuel that have received a lot of attention in recent years. You find both hardwood pellets, made from broad leaf trees, and softwood pellets, made from needled trees. Pellets are typically made from a wood byproduct such as ground waste wood from a hardwood flooring plant. The cut off ends, sawdust, and planner shavings are ground up and a roller forces the raw material through a die under intense pressure. This process produces a densified wood pellet similar in diameter to a drinking straw, which break off in lengths less than ½ inch long. Both hardwood pellets and softwood pellets are very hard since they are densified in the die. A typical pellet weighs in at around 40 lbs. per cubic foot versus 10-25 lbs. per cubic foot for split firewood at the same moisture content.
It’s important to understand how moisture content impacts wood fuel. Pellets, for example, have a moisture content range of 4 to 8%. Split firewood can range in moisture content from 20 to 70% depending upon how well it is seasoned. If you’ve ever tried building a campfire with wet wood, you know the drill. It’s hard to get started, burns poorly, and produces a lot of smoke.
Wood pellets have to be made with dry wood residue. When the material is densified as it is forced through the die, the moisture in the residue is heated under the intense pressure. Once the pellet moves out of the die, the heated water turns to steam bursting the pellet into powder. This is avoided by using dry residue. Some manufacturers buy pulpwood roundwood, chip the roundwood, and dry the chips prior to producing the pellets. This strategy is used when dry residue is not readily available, and the market conditions are strong for pellets.
Pound for pound, do you get more heat from oak firewood or from hardwood pellets? Again, this depends upon the moisture content. If the pellets are drier, and they usually are, you will get more heat measured in BTUs per pound. This all has to do with how the residual water interacts during combustion. Water in the wood needs to be boiled off, called the latent heat of evaporation, before temperatures can increase enough to release and ignite the combustible gasses. The higher the moisture content in wood, the more heat energy is consumed in this drying process.
Wood pellets for heat is one thing, but wood pellets for food, now that is exciting! A number of grill manufacturers produce wood pellet grills and smokers to bring flavor into our lives. There are very specific pellet blends available for grilling and smoking meats including maple, hickory, apple, pecan, cherry and more. In most cases maple wood makes up the bulk of these pellet blends and the specialty woods are added for flavor.
My coworker makes smoked salmon and home cured smoked bacon on his pellet smoker with some very tasty results. I am always happy to sample his latest experiments. I hope you can see that wood fuel is still prominent today and will continue to play a role in our energy and culinary needs.