Biomass Production
One of the most important limitations of biomass as a widespread fuel is the space required to grow it. Bridgwater (2002) estimates that “a sustainable crop of 10 dry t/ha/y of woody biomass can be produced in Northern Europe rising to perhaps 15 or maybe 20 t/ha/y in Southern Europe.” Assuming that 1000 dry t/y will generate 150-300 kW depending on conversion efficiency then an area of the order of magnitude of 100 km2 or more would be required to support a 30^40 MW power station.
It is unlikely that all transportation fuels and organic chemicals can in the future be produced from biomass. This would call for an immense acreage of biomass plantations of a few million square kilometers. This seems unrealistic for various reasons:
• The sheer size of such projects.
• It is far from certain that large-scale plantations of more or less conventional crops such as Eucalyptus, miscanthus, euphorbia, and other species are so green as the advocates of these plantations claim it to be. The large-scale biomass production in Minas Gerais in Brazil for making charcoal to be used in blast furnaces is a well-known environmental problem. Fertilizers, herbicides, and pesticides will be required; ash may have to be recycled in order to recycle alkali, phosphorus, and so on. Irrigation may be a problem, and so may soil erosion. Moreover, monocultures are very sensitive to pests. Last but not least, such developments are often difficult to reconcile with the wish for more biodiversity.
• Biomass is easy to transport when it comes to gathering firewood for nearby domestic use. However, it is expensive to transport in large quantities over any appreciable distance. The energy density of biomass is one-tenth that of liquid hydrocarbons, and further, it is a solid that cannot be pumped. In order to make wood and other biomass more amenable to transport, it can be chipped, but then the energy density will decrease even further. As a result, the energy required to get bulk biomass to usually distant markets is appreciable and constitutes a sizeable percentage of the fuel market it wants to replace.
On the other hand, there are circumstances where large quantities of waste biomass arise as a result of some other economic activity such as forestry, papermaking, and sugar. Under these circumstances most of the costs associated with the collection of the biomass and its transport to a central location is borne by the main product. Thus in these areas somewhat larger biomass gasifiers (500-1000 t/d corresponding 80-160 MWth) can and have been built. However, none of these are of a capacity to be compared with world-scale plants based on fossil fuel feedstocks of 200-1000 MWth.
