What Are the Problems with Fossil Fuels?
Coal, Gas and Petroleum
Common environmental issues include contribution to climate change via greenhouse gasses, atmospheric sulphur dioxide (SO2) which is also known as acid rain; Smog and airborne particulates which contribute to lung diseases; nitrous oxide (N2O) which contributes to ozone formation at low altitudes; carbon monoxide (CO) and heavy metals. Aside from environmental issues, one of the biggest issues is the fact that fossil fuels are not renewable, they are finite.
While Natural Gas is considered the cleanest of the fossil fuels there are still many associated issues. One of the main concerns is ‘fracking’ where new deep drilling technologies break up rock formations leaving toxic fluids available to pollute groundwater.
The actual extraction of these types of fuels is as problematic and environmentally damaging as their actual consumption. Some risks associated with the production of these fuels includes: oil spills; heavy metal contamination and habitat disturbance.
Wood as Fuel
Wood is a renewable fuel; but like fossil fuels, one that we use by burning.
When wood is bone-dry, which means it has been dried top a constant weight at 105 degrees Centigrade all types of firewood, weight for weight, have about the same calorific value (i.e they give off the same amount of heat). Softwoods may have slightly higher calorific values than hardwoods such as Eucalypts due to the presence of oils and resins, and, on average, a higher lignin content.
Firewood is ideally burnt at with air dry moisture content. This is approximately 12-18%, compare to a moisture content of around 50% for green wood. When green wood is burned much of the energy is used to evaporate the high water content. In comparison green wood has about 40% of the heating value of dry wood. It is also much harder to ignite. To get best results from green wood it should be air dried first.
Typical calorific values for air dry wood are:
Hardwoods 17.2 megajoule per kg (7400 BTU/lb)
Softwoods 17.9 megajoule per kg (7700 BTU/lb)
1 megajoule = 947.81 BTU's
Black Coal 30.8 megajoule per kg
Fuel Oil 46.3 megajoule per kg
The temperature at which a fire burns is related to how quickly it burns, and this will vary from timber to timber. Because wood is a poor conductor, it is difficult to ignite unless its surface to volume ration is large. This is why kindling, paper fire-starters, etc, are required. At first, as wood burns, moisture is vaporised and the wood burns slowly. As moisture is lost and the temperature of burning increases to above 280 degrees Centigrade combustible gases are produced, heat transfer increases rapidly and burning becomes self sustaining. Above 500 degrees C. charcoal produced in earlier burning stages is consumed by a glowing a glowing and almost non-flaming type of combustion. 70% of the heat energy in the combustion of wood comes from the burning of the gases released from the wood, while the remainder comes from the combustion of charcoal.
Wood that forms glowing coals radiates more heat into the room than quickly burning wood, which usually gives off heat as hot gases which quickly move up the chimney, and hence are lost. Dense timbers give the best coals, making them again better than lighter ones for open fires. Slow combustion heaters were developed to slow up the rate of burning of softwood timbers, as these are generally poor, in terms of open fire performance.
Woods of higher density (e.g. grey box, red ironbark) are usually considered as better firewood for open fires than softwoods, such as radiata pine (Pinus radiata). Dry box timber has a density of about 2.25 times that of radiata pine, so even though pine has a slightly higher calorific value than box, around twice as much pine by volume is required to give the same amount of heat. The rate at which timber burns is inversely related to density, so to maintain a pine fire would take double the amount of stoking to produce the same amount of heat as box. Density, and burning properties of wood can differ even within the same species, depending on such things as site quality, tree provenance, dominance, health and age. Heartwood is usually much denser than sapwood or bark, so wood from larger trees with a higher percentage of heartwood gives a higher overall density than does smaller diameter or decayed wood.
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