Why Natural Gas?
Wood and BioMass
Wet wood = about 4,000 BTUs/Pound
Dry wood = about 7,000 BTUs/Pound
6.3 lbs dry wood = 1 Boiler Horse Power
11.6 lbs wet wood = 1 Boiler Horse Power
Hog Fuel = wet wood waste
When dry wood is burned in perfect combustion, it has a theoretical BTU value of about 9,000 BTUs per pound. However, wood is almost always wet and existing technologies do not come near perfect combustion. Therefore, a more realistic value is 4,000 – 6,000 BTUs per pound.
Hog Fuel is a general term used for a piece of machinery and the wood waste that comes out of it. A ‘hog’ is a grinder, de-barker or otherwise pre-processor of green wood logs. Hogs are used by wood product manufacturers to prepare logs for further processing, and by forester operations for clearing an area of wood waste. In some cases, entire trees are ‘hogged’ roots and all. Hog Fuel is therefore very wet, full of dirt and other debris, and has an undetermined BTU value.
Dry Wood Waste
Dry wood waste comes from a variety of sources including dimension lumber re-saw mills (sawing of kiln-dried lumber), wood furniture manufacturing, plywood and OSB manufacturing, and construction site refuse. Dry wood may also include resins and other process ‘contaminates’, which could increase or otherwise change its BTU content.
On an industrial scale, wood is burnt similar to older coal-fired boilers. Large wood waste and hog fuel is generally burnt on a grate, like a stoker furnace. Sawdust and ground small, dry wood waste is generally burnt in a pulverizer or fluidized bed method. This is also known as ‘cyclone’ combustion or some similar term used to distinguish a suspended combustion in a lot of air, as opposed to a stationary grate combustion.
When burnt in a boiler, it takes about 6.3 lbs of dry wood or 11.6 lbs of wet wood, to produce 1 Boiler Horse Power (34.5 lbs/hour of steam OUTPUT). This is highly variable based on the actual moisture content of the wood, the wood species and the boiler efficiency.
Source: Bob Fegan 3/02