S173 Plants and People - Notes from Study Period 4 - Part 2 of 2

In part 2 of these notes for SP4, I'll be noting about wood as an energy source, grass as an energy source, biofuels and some of the issues surrounding the use of biofuels.

Wood as an energy source
Burning wood is considered as carbon neutral as it does not release more CO2 into the atmosphere than if it was to decompose naturally. Woodlands can be managed through techniques such as coppicing, whereby the stems of fast growing trees, such as willow and poplar species, are cut from the close to the base - the coppice stool. Many more stems then grow due to the lower levels of auxin which are grown towards the tip of stems and prevent side shoots from growing - meaning that the plant will concentrate all its growth on the main stem. The lower level of auxin allows these side shoots to grow. As coppice trees are normally grown in dense groups, these stems grow straight and long in the struggle for light.

Woodlands can temporarily hold rainwater to prevent excessive water entering streams and rivers too quickly. The burning of wood provides an outlet for wood residues that would otherwise enter landfill sites.

The process of producing heat and electricity from woodfuel is basically as follows:
1) Wood is dried
2) It is then heated in the absence of oxygen (pyrolysis) to produce gases.
3) Gases are then purified and burnt to produce electricity.

Grass as an energy source
Grass grows quickly. produce large amounts of biomass per unit of growing area and leave only small amounts of residue when burned. Grasses generally grow by underground rhyzomes, spreading quickly and generate new shoots readily. Harvesting of perennial grasses  for biofuels can happen anytime between late November to April in the northern hemisphere. As harvesting gets closer to April the grass loses leaves, reducing the harvestable biomass - but it has less water content and can be baled, reducing costs. Growing multiple grass species together allows maximal interception of light and growth at different rates. As perennials they are considered carbon neutral and can be sown on degraded agricultural land - so there is no competition against land for food crops.

The largest UK powerstation, Drax, currently burns 300,000 tonnes of Miscanthus x giganteus annually.

Biofuels
Biodiesel is produced from oil-seed crops, such as oilseed rape, sunflower oil, palm oil, and soybean oil and make up 5% of the overall blend of petroleum in the UK. Oilseed rape is the main oil crop, producing around 1,300 litres of biodiesel per hectare planted.
Biodiesel is produced by:
1) waste or newly produced plant oil is heated to 50C
2) processed by adding mixture of sodium or potassium and methanol
3) this becomes unwashed biodiesel and glycerol, which is seperated
4) the unwashed biodiesel is washed and allowed to dry. It is then ready for use.

Bioethanol is currently used in countries such as Brazil, France, USA, UK, argentina and South Africa. It has been particularly widespread in Brazil at the expense of large tracts of natural vegetation including rainforest. Estimations of one-third of car fuel in Brazil is entirely bioethanol with two-third a mixture of bioethanol and petrol. Bioethanol is produced by:
1) converting statch to sugar for plant material containing starch - or extracting sugar from plants containing high levels of sugar, such as sugar can and sugar beet.
2) the sugar is fermented whereby microbes use the sugar as a food source
3) this becomes bioethanol and after separation from  other components by heating is added to petrol.

Biogas is mainly a mixture of methane and carbon dioxide produced via the breakdown of organic materials in the absence of oxygen. This includes municipal, food and animal waste, sewage, and biomass crops. A great example of the usage of biogas is the biogas created at the sewage treatment works being used to generate electricity which powers the works. Production of biogas from food waste reduces the amount of waste that reaches landfill as any waste remaining after the gas is produced can be used as agricultural fertiliser.
Sweden is currently the world leader in the use of biogas, having the first biogas-powered train which runs the 100 km route from Linkoeping to Vaestervik.


Issues surrounding the use of biofuels

Cost of producing biodiesel and bioethanol is double that of conventional fossil fuels. It may become more competative as the price of crude oil rises.

It is possible that using land for fuel crops rather than food crops rises the price of food.

Crops for biofuels require less in the way of nutrients than food crops.

Due to the growing, harvesting, and processing costs (pre-treatment, processing for energy release, removal of residual waste) research in to crops that can deal with low maintenance, low nutrient, fast growth, and poor soil.

The use of biofuels only releases CO2 into the atmosphere that was recently taken out. Rather than adding to current CO2 with that of CO2 locked away in fossil fuels for millions of years. It is therefore seen as carbon neutral. Biofuels do release nitrous oxide, which is a greenhouse gas 240 times more powerful than CO2, in larger quantities that fossil fuels. Biofuels also generate CO2 in the vehicles used to grow and transport the crop.


I hope this is helpful. If there's anything that you want to see in these note, please leave a comment :)