Environmental Engineering Reference
In-Depth Information
the stored fuel. Therefore, it must be produced and consumed locally, as energy consumption
associated with transportation over long distances might even exceed that of the fuel itself.
This means that biomass power generating units are relatively small compared to conven-
tional plant, (relying on local supply chains for feedstock) and possess the characteristics of
small embedded generating units.
There are three basic thermochemical conversion technologies that use solid biomass as a
primary fuel for the production of electricity, namely direct combustion, gasifi cation and
pyrolysis. In addition, the use of liquid biomass (such as sewage sludge) for the production
of methane via anaerobic digestion is increasingly common.
Electricity production using solid biomass fuels is still a developing industry and as a
consequence is not competitive on price with electricity from fossil fuels without some kind
of government fi scal or policy support. However, it is competitive with nuclear power and
possibly new-build clean coal power stations, but not with modern gas fi red power stations
within the current regulatory and economic climate. With the correct support, as currently in
the UK, co-fi ring of coal with biomass is commercially attractive. In the longer term, grid-
connected biomass generation (using the full range of possible technologies) may become
competitive; the greatest potential is for small scale embedded generation using gasifi cation,
pyrolysis or high speed steam engine based plant. In the short term, small scale (100-500 kWe)
dedicated plants for use on farms or by rural industry has the greatest potential. In the medium
term when increased demand for electricity could be causing the grid to become overloaded
and unreliable, then larger (1-20 MWe) embedded biomass generation plant providing end-
of-grid support may become an attractive alternative to reinforcing the grid.
2.8.2 Resource Sustainability
Biomass has become less important as countries have industrialized and now accounts for
less than 3% of energy in the developed world. In contrast developing countries remain highly
reliant on wood and other natural biomass with over 30% of their energy needs being sup-
plied from these sources. Of course with growing populations this supply is not sustainable.
Ironically, the industrialized countries need to make increased use of biomass, and the less
developed regions limit their use of this resource to a sustainable level.
One of the key features of biomass is that the energy expended in growing it, i.e. planting,
watering, use of chemicals and pesticides to enhance yield, harvesting, drying etc., is not
negligible. Specifi cally for ethanol production used as fuel for transport, the refi neries them-
selves are fi red by fossil fuels to ferment the crop and to purify ethanol from the product of
fermentation. A US Department of Agriculture report found that the energy from corn bio-
ethanol was a mere 8% in excess of the input production energy and a recent paper in
Science
[15] found that the energy ratio was net-positive when the energy savings from 'co-products'
for cattle feed were included. Efforts are now being made to produce bioethanol from cellu-
losic crops and not from fermentation. This promises to produce twice the amount of ethanol
per hectare of crop.
The scene is a lot brighter if biomass is used to generate electricity, especially in CHP
plants in small decentralized power stations. The benefi ts are compounded if the crops are
grown organically, if possible, and used locally. The choice of crop is also vital in the effec-
tiveness of CO
2
mitigation. Table 2.3 compares the ratio of energy out to energy in for a
