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Wood Burners for the MicroGrid

If you walk outside and encounter a vast and impenetrable forest, then probably you could be burning wood. Wood burning stoves can be quite effective in supplying household heating needs. For many of us it is a past-time that keeps us busy during summer and winter months, while clearing our lands of excess dead fall.

The designs of wood burners are many, and their performance will depend heavily on the airflow characteristics and type of wood. The primary goal is good combustion efficiency, achieved through the proper air and fuel combination. The secondary goal should be maximizing the useful heat obtained from the furnace before exhaust gases are expelled to the atmosphere.

The primary component of wood that participates in combustion is cellulose, a polysaccharide molecule whose chemical formula is often listed as (C6H10O5)n. Other shorter chained polysaccharide variants such as hemicellulose and lignin are also found in wood, and contribute to combustion.

With the many species of trees in existence, and the many combinations and variations of cellulose that are possible, it is difficult to generalize when it comes to wood combustion. We could expect each species to vary, and some more than others.

We can investigate the combustion of pure cellulose, however, as a theoretical starting point. The stoichiometric combustion reaction we come up with is: C6H10O5 + 6O2 --> 6CO2 + 5H2O. When doing the math, the theoretical mass air to mass fuel ratio is approximately 4.6 to 1. Compared to gasoline (approx 15:1) and coal (approx 12:1), this is quite low and can be attributed to the amount of available oxygen trapped in the fuel.

Combustion is never perfect, though, even in controlled environments. Lignins and hemicelluloses exist in the fuel in varying percentages and change the burn characteristics for each type. In this case it is better to generalize and work within the realm of experimental data and practical experience.

At 0% moisture content the heating value of wood can be nearly 8,200 BTU/lb.

At 25% moisture content, a more realistic value, one could assume ~7,000 BTU/lb higher heating value for wood.

At 50% moisture, we’re down to 4,000 - 5,000 BTU/lb as we’re losing more and more energy dispelling moisture from the fuel. Most of us have experienced these difficulties when starting a campfire from a moderately wet pile of wood.

Good quality wood which has been cut and dry stored for about one year probably falls in the range of 15 to 25% moisture. Kiln dried wood is as low as 7%.

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