4.1: Wood

4.1 Wood

History of Burning Wood

Wood has been used as a source of energy for thousands of years (the first known use of fire was determined when archeologists made discoveries of humans living 400,000 years ago), and wood was the obvious source to make fire. In the Americas, in 1637, the people of Boston suffered from the scarcity of wood. It became America’s first energy crisis after less than one century of settlement. During the late 1700s, Benjamin Franklin invented a cast iron stove for indoor use. It held heat in the room after the fire burned out. However, it had a design flaw in that it had no way to pull in air, so fires went out quickly. So David R. Rittenhouse added a chimney and exhaust pipe to improve upon it.

Burning Wood

First, we will look at where energy is stored in materials, starting with the methane molecule. The combustion of methane is exothermic (releases heat as the reaction proceeds), but the reaction must be initiated before it will sustain itself with the continued availability of methane and oxygen. The formula below shows the reaction in a stoichiometric format:

CH4+4O₂→CO₂+H₂O (plus heat!)

Figure 4.1 shows the same reactants and products, but with the bonds before reaction and after reaction, on a molecular/atomic level. The number of atoms in each molecule doesn't change, but how they are arranged and connected does. The only real change is how the atoms are linked - these are the chemical bonds. Since ENERGY comes out of a burning system, then it must mean that more energy is stored in 4 C-H bonds and 2 O-O bonds than in 4 H-O and two C-O bonds. The ENERGY released during chemical combustion comes from ENERGY stored in chemical bonds of fuel & oxygen.

Figure 4.1: 1 Methane and 2 oxygen reaction showing bond connections before and after the combustion reaction.

Click here for a text description of what is occurring in the reaction

Reactants: Methane will react with two oxygen molecules. All of the four hydrogens in methane are connected to a single carbon atom by 4 single bonds. The oxygen molecules are each two oxygen atoms connected by a double bond.

During combustion, the atoms rearrange and form new bonds.

Products: The carbon atom connects to 2 oxygen atoms with a double bond between the carbon and each oxygen to produce one carbon dioxide molecule. Additionally, each of the other remaining oxygen atoms forms 2 single bonds to 2 of the remaining hydrogen atoms to form a water molecule. The net products of the reaction are 1 CO2 molecule and 2H2O molecules.

We now know the reaction chemistry of methane combustion, but wood is a much more complex material than methane. Wood contains up to 50% water. Water in the wood will reduce the heating value of the wood, and if the wood is very wet, it will lead to a smoky fire. The main components of wood (we will cover this in more depth in a later lesson) are cellulose (what paper is made from) and lignin (the part of a tree that makes it have a sturdy structure). In order to start a fire, you typically must ignite a material that burns easily to begin heating the wood (this can be newspaper or a “fire starter”). The components begin to decompose from the heat (therefore we are not technically “burning” yet), which produces vapors and char. The vapors are called “volatiles” and the char is composed of carbon and ash. The volatiles are what actually begins to burn, producing a flame. The carbon-rich char produces glowing embers or “coals,” which are needed to keep the fire sustained. Wood does not typically contain sulfur, so no sulfur oxides (or SOx) are produced.

There can be problems with burning wood. The smoke comes from particulates that did not burn or only partially burned that can pollute the atmosphere, and typically come from resins in the trees. It isn’t an issue when one or two people are burning wood, but when thousands of people burn wood in fireplaces. In State College, Pennsylvania, in the winter, one can see smoke in the air from fireplaces. Wood fires in fireplaces can also deposit soot and creosote in the chimneys, which if not cleaned periodically, can ignite. Burning wood (or really most things) will produce an ash material (minerals in wood and coal that react with air under combustion conditions); the ash must be disposed of. Wood smoke also contains a variety of chemicals that can be carcinogenic.

Now let’s begin discussing different biomass sources, how we measure different properties of different biomasses, and how to determine the atomic composition of biomass.