World energy use in 2005 was 500 EJ or 5 x 10^20 J. Assuming that a calorific value of 15 GJ/tonne could be recovered, to provide that amount from biomass would require 33.3 Gt (billion tonnes) of biomass. Sums are often done assuming a given mass of "residue" from crops, but it is necessary for the good health of soil to return some of that chaff to the ground to preserve its organic carbon content, otherwise it becomes stripped and increased in mineral form, thus needing artificial fertilizers forever, or for as long as they can be provided.
Even at a yield of 10 tonnes/hectare of biomass residue, we need 3.33 x 10^9 hectares or 3.33 x 10^7 km^2 of land to produce it on, which at 33 million km^2 is over twice the area of arable land on earth (15 million km^2) and more than one fifth of the total land area of around 150 million km^2 (30% of the total 500 million km^2 of the surface of this blue planet). Clearly to provide all our energy from biomass is a very tall order, and it is obvious that we cannot simply substitute biomass in matching amount for fossil mass, as supplies of oil, gas and coal begin to wane. Since however, we will not need to convert overnight from fossil mass to biomass, and energy conservation will be forced on us by a simple lack of resources, biomass offers the potential to provide a significant proportion of the final energy bill, once we have made efforts to use less energy overall. Certainly it can make a significant contribution to the transitional period from the high energy status quo to a future civilization based on a more efficient use of energy and which furthermore is generated from renewable resources like biomass.
Most biomass is simply burned to provide heat, and this can be done more efficiently in CHP (Combined Heating and Power) systems particularly in small-scale units. However, we need a more adaptable form of energy which is best provided in the form of liquid and gaseous fuels. In the latter aspect, synthesis gas or "syngas" is especially flexible, since not only can it be piped and burned directly, but also converted to methanol, other alcohols including ethanol and synthetic diesel using Fischer-Tropsch catalysis.
The simplest firm of gasification is done by pyrolysis, which usually involves heating biomass, e.g. wood, in a restricted supply (or the absence of) air. Thus, the cellulose, hemicellulose and lignin is decomposed to a mixture of solid (char), liquids (bio-oil) tar and a mixture of gases, mainly CO2, H2, CO and methane. The relative amounts of the different phases can be changed according to the temperature of the pyrolysis, the contact time with the heated zone, the pressure and the amount of oxygen present either in the diluted form of air or in some applications pure oxygen is used, but providing this adds-in its own contribution to the overall energy budget.
In terms of gasification, at temperatures >1000 degrees C, and short contact times of less than a second around 70% or more of the initial charge of biomass is converted to gas. There are gasifiers that work at lower temperatures say 400 degrees C and use more air, but provide a gas with a low thermal content of maybe 6 GJ/tonne which is around one fifth that of coal-gas (27 GJ/tonne) and about a tenth that of natural gas (methane, 55.7 GJ/tonne).
During WWII, cars and tractors were run using on-board wood-gasifiers, to cope with the fuel shortages in Europe, petrol and diesel being reserved for the military. The unit was called Gazogene. Full EROEI analyses are necessary to evaluate such gasification strategies, it is generally assumed that (as in making biochar by pyrolysis) the external heat source will come from biomass too. The beauty of using air/oxygen is that the gasification reaction becomes self-sustaining, i.e. the material effectively "burns" albeit in a controlled manner.
In addition to using biomass taken from fields, there is the option to use the technology to convert land-fill waste into useful fuel, as well as directly gasifying wet-biomass including algae which saves energy in drying the material prior to use as is normally necessary. In terms of converting algae to fuel, it may prove more efficacious to gasify the total mass directly rather than choosing a high-oil yielding variety, extracting the oil from it and then transesterifying that into biodiesel. The syngas could be used directly as a fuel or converted instead to synthetic diesel using FT rather than biodiesel. NB the calorific value of biodiesel is around 36 GJ/tonne compared with syn-diesel at 44 GJ/tonne, which is the same as for normal diesel.
The focus on biomass is of course that it is renewable, ideally carbon-neutral (on the grounds that the carbon content of the plant was taken from the air originally through photosynthesis), and is hence a better bet than fossil fuels which are being exhausted continually from their finite reserve and which contribute CO2 to the atmosphere.
I shall post more on this subject as I think more about it, but these are just some initial impressions.
The figure of 500 EJ in 2005 from: http://en.wikipedia.org/wiki/World_energy_resources_and_consumption
There is another link at: www.sfpa.sk/dok/PDFI/MZeman.pdf (on slide number 6) that mentions 10,878 Mtoe which x 42GJ/t = 4.6 x 1020 J for 2005, and is thus also in the same ball-park.