Measuring biomass and chemical energy
In order to estimate how much organic matter has been built up by a plant (or a number of plants), it is necessary to take a sample, (weigh it to get the fresh weight
), then drive off the water. It is normal to place it for several hours in an oven set at 105 °C.
The sample is then weighed and returned to the oven again. This is repeated 'to constant mass'. This gives the dry mass.
Obviously the plant material is killed by this process.
It is usual to express this 'dry biomass' of plant tissue in terms of the unit area occupied by the plants, and the time scale involved. This means that the process is repeated, perhaps annually or at different stages of the growing season.
Why is the oven set at 105 °C?
>This is (just above) the boiling point of water. Higher temperatures could cause the dried plant material to burn.
What does 'drying to constant mass' mean?
>Checking that all the water has been driven off.
Mass of carbon in biomass
Organic compounds all contain the element carbon, in chemical combination with varying amounts of other elements - mostly hydrogen and oxygen, and (about 16%) nitrogen in proteins.
In theory, this could be related back to the carbon taken in as carbon dioxide
in the light independent stage of photosynthesis, but it is not possible to directly measure the mass of carbon (only) in biomass. However the energy it contains can be measured - see below
Using calorimetry to measure chemical energy
The chemical energy in dry biomass can be measured by burning it and calculating the amount of heat energy that is released.
1 calorie is the amount of heat needed to raise the temperature of 1 g of water by 1 °C.
1 kilocalorie (kcal) is the the amount of heat needed to raise the temperature of 1000 g of water by 1 °C,
or to raise the temperature of 100 g of water by 10 °C ....
It is more usual to express energy in terms of Joules and Kilojoules (kJ). 4.18 joules = 1 calorie; 4.18 kJ = 1 kcal
1 joule is the amount of heat needed to raise the temperature of 0.24 g (1/4.18) of water by 1 °C.
1 kilojoule (kJ) is the the amount of heat needed to raise the temperature of 240 g of water by 1 °C,
or to raise the temperature of 24 g of water by 10 °C ....
A bomb calorimeter
Mouseover for schematic diagram
It is very inefficient attempting to measure heat by simply burning it under a container of water.
The standard equipment used in this is a bomb calorimete
The biomass sample is burned in an atmosphere of oxygen inside a strong metal container, surrounded by an insulated water jacket.
As the name implies, the combustion process is quite powerful, and it is started by supplying electrical power to a heating element.
The heat absorbed can be calculated after measuring the temperature rise.
This needs careful calibration using known compounds.
A similar process is used to calculate the energy values for food items.
The value obtained for chemical energy from dry biomass of plants can be compared with the value of light energy (from the sun) over the growing period, and the efficiency of transfer of the products of the photosynthetic process can thus be calculated.
Anyone who has stood in front of a burning log fire wll be aware of the energy content of biomass!
And anyone who has tried to light one knows the importance of the dryness of the wood!
Biomass in power generation
Although much of the UK's electrical power continues to be generated from fossil fuels (oil 7.8%, gas 40.2%, coal 8.6%), in recent years there has been an increased contribution from wind (10.6%) and solar, via photovoltaic cells (2.8%) and so-called Bio-energy
(8.4%). Nuclear power plants provide 20.1% and Hydroelectric plants supply 1.5%.
Some of the bio-energy comes from methane gas from sewage and landfill sites, but it may also be provided by plant crops specially grown for the process. Some carbohydrate-rich crops may be used to feed fermentation plants which produce ethanol for burning or road fuel, but there is concern that these could be better used in providing food for animal or human use.
Less controversial is the cultivation of fast-growing plant crops which can be burnt in power stations. There is a long history of cultivation of certain tree species, often coppiced on a 12-18 year cycle for fencing purposes or to produce charcoal, but short-rotation coppicing
of willow and poplar on a yearly cycle has produced fuel for power generation.
Other crops have been developed specifically for use in power generation. Miscanthus
- also colloquially known as Elephant Grass - is being increasingly grown for this purpose.
Miscanthus being harvested
Both fossil fuels and bio-energy crops have a biological origin.
Why are fossil fuels being phased out?
> They release large amounts of carbon dioxide - from carbon in plant/animal remains held underground for millions of years, and the extra CO2 in the atmosphere is a greenhouse gas which causes global warming and climate change.
Why are bio-energy crops seen to be less of a problem?
> They are seen as carbon-neutral in that they absorb the same amount of CO2 for photosynthesis as they give out when they are burned. BUT fossil fuels are probably used in the harvesting and drying process as well as for delivery to the power stations.