Wood can be exploited as a renewable resource if fast growing species are used. In the UK willow is used (popular was also used once). The plants are grown close together then once a sufficient root system is established they are coppiced. Then all new growths when of the correct length are harvested and chipped. Once the willow is dried and then used. This is often used on low quality land (marginal) or set aside, or areas where biological control is being encouraged.
Biomass to Gasohol
Fermentation of sugar by yeasts converts the energy in sugar biomass to ethanol, which can then be used as a fuel. Gasohol is made of 80-90% unleaded petrol and 10-20% ethanol and is used in cars (It can be used in pure form, but petrol is added to stop people drinking it). The sugar is obtained from sugar cane and sugar beet. This form has been relatively unsuccessful.
Sugar cane is grown in tropical areas, where as sugar beat is grown in temperate countries. The sugary sap is extracted and then treated to make molasses, which is then stored until it is needed for fermenting. Ethanol is a much cleaner fuel and produces much less pollution, but it is much more expensive to produce. Especially as the farmers can also sell the crop to be made into refined sugar. This production is unlikely ever to work due to politics and the oil industry. In the end it can not be considered as a real alternative ,as 80-90% of it is made from a fossil fuel. Once the sugar is extracted the fibrous residue (bagasse) is dried and then burnt as a fuel to ferment the malasses.
1) Growing and cropping of sugar.
2) Extraction of sugars by crushing and washing cane.
3) Crystallization out of sucrose (for sale) leaving the syrup of glucose and fructose (molases)
4) Fermentation of molasses to yield dilute alcohol.
5) Distillation of dilute alcohol to give pure ethanol, using bagasse as a power source.
A more promising is oilseed rape which has been used to power buses in certain areas of the UK as part of a trial and in Italy, it is added to diesel and can be added to make up 30% of the mixture (rpae methyl ester, RME). It produces fewer sooty particles and no sulphur dioxide. Also coconut oil in Philippines, palm and castor oil, Brazil and sunflower oil in South Africa.
Domestic waste
Disposal of domestic and industrial waste has become a large problem around the world. In the UK about 300kg of waste is produced per person per year. Dumping the waste causes many problems e.g. leakage of pollution and using up land. It might be possible to burn the fuel for energy (cellchip in the UK in London does this but people have to separate some of the waste first. However, all the houses that take part close to the plant reciev free hot water.)
Some landfill sites have been constructed to collect the gas (methane) produced from the anaerobic conditions for use.
Agricultural waste (Biogas)
Biogas can convert the energy in biomass into biogas, a gaseous fuel that consists mainly of methane, by fermentation. Typically biogas is made of:
65% methane, 35% CO2, with tracea of ammonia, hydrogen sulphide and water vapour.
usually dung (slurry) from farm animals is used in the fermentation process so that waste products are turned into something useful, as the leftover sludge can be used as a fertiliser.
The digestion process occurs in 3 stages:
1) Aerobic bacteria convert lipids and carbohydrates in the biomass into sugars, fatty acids, amino acids and glycerol by hydrolysis (0-10 days, acids cause pH to fall).
2) Acetogenic bacteria convert sugars and other products of stage 1 into short-chain fatty acids e.g. acetic acid = acetogenesis (stages 2+3, 10-45 days acids used up, pH increases, methane produced.).
3) Anaerobic bacteria convert the fatty acids into methane = methogenesis. Conditions must be anaerobic as the bacteria are only active when there is no oxygen present. They are called obligate anaerobes. Temperatures must be kept between 30-40o because the bacteria are sensitive to temperature changes.
The reactions take place in a digester. It must be
i) Strong and large enough to hold large volumes of liquid and withstand pressure build up.
ii) Gas-tight and allow aerobic conditions to be maintained.
iii) Have an inlet for loading material, an outlet for the gas and a way of unloading the residue.
iv) be buried in the ground to help withstand pressure and act as an insulator.
Often several digesters are used together to maintain a continuous supply of gas. It provides a useful way of getting rid of animal waste. (eutrophication)
