BIOENERGY PRODUCTION

Energy is a central issue in agriculture. The very reason for the agricultural industry’s existence is to supply energy. It does this by making use of solar power to convert energy into biomass, which in turn supplies energy to human beings and animals in the form of nutrition. The ability of the agricultural industry to supply energy in the form of biomass is greatly enhanced by the use of mineral fertilizers in agriculture, representing further efficiency in the use of solar energy.

On the consumption side, food production accounts for 15% of total energy consumption in Europe, the remaining 85% being used by industry, traffic, private households and public services. Of this 15%, just 5% is consumed by agriculture, and this figure includes the energy used to produce mineral fertilizers.
In fact, since the early 1900s, the efficiency of energy use in the production of nitrogen fertilizers has improved so dramatically that modern fertilizer factories are now close to the theoretical minimum of energy consumption.

 

Efficiency of energy use is also a central issue on farms. Grain yields increase as more mineral nitrogen is applied, but there is an economic optimum which makes the most efficient use of the farmer’s resources.

 

When using 170kg nitrogen on 1ha of land, wheat yields are approximately 8.2 tonnes a hectare, compared with 4.7 tonnes a hectare without nitrogen fertilizer. Expressed in terms of energy, this equates to 126 gigajoules of solar energy captured on every hectare in the form of biomass when nitrogen is applied at the optimum rate, compared with 71 gigajoules a hectare without nitrogen fertilizer.

Energy efficiency in biodiesel production

 

The biomass-bound energy needs to be transformed into a usable form of energy, such as heat or liquid biofuels. This transformation, however, is not 100% efficient. For example, one ton of rapeseed contains 24 GJ of total energy (primary energy), and the amount of energy in form of biodiesel that can be produced from one ton of oilseed rape is lower than the total energy content. About 380 liters of biodiesel can be produced from one ton of oilseed rape. One liter biodiesel contains 32.7 MJ of energy. Hence one ton of oilseed rape allows for the production of 12.4 GJ of energy in the form of 380 liters of biodiesel. Again the use of mineral fertilizer increases the energy efficiency of biodiesel production. The graph, N Fertilizer inputs increase the energy efficiency of biodiesel production,below shows results from a field trial growing oilseed rape in Germany. The figure shows negative values for energy input (10 GJ/ha for production, transport and application of N fertilizer and 7 GJ/ha for all other activities, e.g. tractor use). The positive values give the energy output in terms of biodiesel that can be produced from the harvested oilseed rape. At optimum N supply (around 220 kg N/ha), the ratio between total energy input and "biodiesel-energy" output is about 1:4, i.e. fertilizers help to produce 4 times more “biodiesel-energy” than what is being required to produce, transport and apply the fertilizers. An extra energy input of 10 GJ/ha for N fertilizer results in an additional biodiesel production of 39.2 GJ/ha. 

Fertilizers greatly increase the positive energy balance of arable production	»
N Fertilizer inputs increase the energy efficiency of biodiesel production	»

Conclusions

- The energy and CO² balance of arable production is generally positive
- If arable crops are used for energy purposes, fossil fuel reserves last longer and CO² emissions are reduced
- Although biomass will have an important role in future energy provision, its use will be restricted by

 

• limitations of available land and water
• rotational and regulatory constraints (e.g. a maximum 30% oilseed rape in rotation)

 

 

• all available land should be used to produce energy crops (in Europe, set-aside is 4 million of ha.)
• imports of biofuels and raw material (e.g. oilseeds) need to be increased
• the production efficiency on the available land must be optimised by using mineral fertilizer

 

For more information, download the EFMA publication Producing Bioenergy and Making the Best of European Land