what is biofuels?
what is pyrolysis??
El hombre ha devastado grandes extensiones de vegetación natural para satisfacer sus necesidades energeticas
la extensa demanda por parte de los humanos para obtener energía ha generado una perdida de madera en cadena.
Wood chips,wood pellets and grasses plays a role as fuel.
What is Biomass?
Wood as a biofuel energy source
The use of wood as fuel for cooking and heating has a long history. In developing countries, 50-90% of fuel used for such purposes comes from either wood or other plant biomass. However, wood is increasingly providing a fuel for electricity generation.
The burning of wood is considered to be carbon neutral as it does not release more CO2 into the atmosphere than if the wood were to decompose naturally, although this CO2 is released in a very short space of time.
Woodlands can be managed sustainably to allow appropriate harvesting and replanting. This also provides some local employment and creates a pleasant and enjoyable place to visit. Woodlands also perform a useful function by temporarily storing rainwater, thereby preventing excessive water from entering streams and rivers too quickly.
Wood burning also provides an outlet for wood residues from forestry activities which would otherwise end up in landfill sites. There is concern, however, that there will not be enough material to sustain increasing demand for woodfuels.
The process of producing heat and electricity from wood is complex.
First, the wood has to be dried, then it undergoes pyrolysis (heating in the absence of oxygen) to produce gases. The gases are then purified and burnt to generate electricity.
The ash created during pyrolysis contains nutrients which potentially could provide a plant fertiliser, though it is possible the ash may contain contaminants from the soils in which the trees had originally been growing.
Potential sources of woodfuel are many and include early thinnings from commercial plantations, the residues from timber harvesting and arboricultural activities, coppicing and sawmills.
In addition, wood pellets made from highly compressed waste sawdust are gaining in popularity as domestic fuels in the USA and various Scandinavian countries.
Some wood harvesting systems use the whole tree for chipping, while others utilise only the stem wood. In ‘short rotation forestry’, fast-growing trees are cultivated and grown until they reach what is considered an economically optimum size; the time this takes varies depending on the tree species.
The trees are then either harvested or coppiced. Harvesting involves completely cutting down the trees, possibly removing the roots, and then replanting with saplings. In coppicing, the young stems are cut back to encourage a number of new stems to grow from the ‘coppice stool’. This can help increase levels of carbon dioxide uptake (carbon sequestration) as the coppice stool re-grows.
Willow (Salix species) and poplar (Populus species) are fast-growing trees, and because they can be densely planted, they give a high yield of wood in a relatively small area. Stems can be coppiced every three to five years and the coppice stools remain productive (produce new stems) for up to 30 years before the root stocks need to be replenished.
Establishing willow or poplar plantations has other benefits, such as creating a method of diversification for agricultural land use, increasing biodiversity and providing shelter or screens both for wildlife and against pollution.
Willow, in particular, is very good at taking pollutants, including excess nitrate from fertilisers, out of the soil (a process known as phyto-remediation) but this can lead to a high risk of contaminants in the wood.
Grasses as a biofuel energy source
There is increasing interest in using grasses as a biofuel source. Grasses grow quickly, produce a large amount of biomass per unit growing area and leave only small amounts of residue when they are burned.
Typically, grasses are burned to produce heat and steam to power turbines in conventional power plants. For example, the largest power station in the UK, Drax, currently burns 300,000 tonnes of Miscanthus x giganteus annually, alongside its usual coal supplies
Grasses which grow year on year are called perennials. Harvesting of perennial grasses for biofuel can take place anytime from late November to April. The moisture content of the leaves is highest in November meaning that the grass cannot be baled and needs to be chopped into smaller pieces which dry more easily.
As time passes from November to April the grass loses leaves, thus reducing the harvestable biomass, but harvesting in April means it can be baled which costs less than chopping the grass up during harvest.
Many burners used to generate heat or electricity from wood fuel can also combust bales of grass or straw. Various studies have revealed that it becomes uneconomic to transport grasses for burning as biofuels more than about 80 kilometres.