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Re: C4 Grasses (Was Re: ..and energy from biomass)

The grass in question is of the genus Miscanthus, which is most commonly
seen tieing dunes together on the sea shore. It is likley to yield about
60 MT/ha of dry biomass once the basic agronomy and varietal selection is
complete and under European conditions. The largest cost of biomass power
is the establishment of the crop and its subsequent handling. Trees take a
long time to grow (even coppiced). This implies a long pay-out time for 
the farmer and high ultimate profit requirements. They also do considerable 
damage to farm land by destroying drains and, if the land it to revert to 
crops for strategic reasons, presenting a major hurdle in respect of stumping 
and rooting. It is costly to cut and handle trees and farmers need equipment 
that they do not have. Biomass power from trees would be expensive.

Annual crops have to grow from seed; and this implies a long period of low
interception of the sunlight; and thus low conversion efficiency. What is 
required is a perennial crop which can be cut on a 1-4 year cycle, offering
fast establishment (compared to trees) and automatic regeneration. Plants
such as Miscanthus and bamboo (on which data is scant) offer considerable
opportunities; for they can stand on the land until needed, the upper parts 
can be chemically killed (paraquat) so that they dessicate whilst standing 
and they can be cut and chipped or baled with convertional farm equipment.

Biomass is a poor source of energy, which is why we stopped using it when we
could afford to do so. In order to upgrade biomass, the best route is to
burn it effectively and to deliver the energy as electricity. Routes through
chemical purification (ethanol, methanol, RME etc) look hideously uneconomic.
The way to do this is, it would seem, is to establish many small-ish rural
power stations. These consist of a high-pressure gasifier and an aero-derived
turbine, possibly with a combined cycle tail end or - problematic - with 
steam injection. A modified RB211, for example, would deliver about 30 MW
from a partly-manned station with the footprint of a conventional farmyard.
Farmers would bring the crop in from a 10 km catchment area in which 50%
of the land was down to Miscanthus (etc).The plant would also burn 
conventional residues, such as straw. 

Power could be sold from the fifth station (allowing for the experience curve) 
at 3.8 p/kWh in order for the project to make 8% real. Power would be sold to 
the 33 kV reticulation system, making for low attachment costs. The UK 
Government study on renewables (1990?) showed that biomass so treated 
represented far the largest (70% or so) of the renewable potential identified
under 8 p/unit.(I quote without looking it up, so don't quote me!)

Advantages:    no CO2, real rural employment, security of supply, third 
               world-appropriate solution.
Disadvantages: Costs more than CCGT and =~ clean coal. Many small plant to
               run. Noise (said to be managable). Intrusion (a design
               problem, probably fixable). Some NOx, fixable with NH3 
               injection. Extensive monoculture: business as usual.

The C3/C4 debate is a non-sequitur. Soem plants fix CO2 by one route which
requires them to open their staomat (pores in the leaves) when there is
sunlight. They lose water by doing this. Other open their stomata at night
and fix the CO2 chemically into malate, a 4 carbon compound, which is then
used as the substrate for light-driven reactions, which occur with the
stomata shut. There are intermediary stratgegies; and C4 plants may open
their stomata when there is light if water is not in short supply. C4 plants
often grow fast but are not intrinsically rapid growers: cactuses, orchids
and liverworts, for example, do not impress by their speed.

Try Power from Plants:  Walt Paterson; Chatham House, 10 St James's Sq 
                        London SW1Y 4LE
Biomass Energy:         Shell Briefing Service, Shell International, 
                        Shell Centre, London SE1 7NA
 
_________________________________________________

  Oliver Sparrow
  ohgs@chatham.demon.co.uk