Wednesday, February 11, 2009

Biomass Gasification.

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.

Related Reading.

The figure of 500 EJ in 2005 from:

There is another link at: (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.


André Sautou said...

Hi, Chris,

From what source did you get that world energy use in 2005 was 500 EJ or 5 x 10^20 J ? Does this figure include biomass ?

[Many WEB pages display energy curves originating from Schilling & Al. (1977), IEA (2002), Observatoire de l'Energie (1997). From these data we get that world energy production in 2005 was about 10.5 Gtoe (billion tonne of oil equivalent) by adding oil, coal, gas, nuclear and renewables (including hydraulic.
Acknowledging that 1 Gtoe = 42 GJ, this amount equals 4.4 x 10^20 J. Same order of magnitude.]



André Sautou said...

Rectification to previous comment : 1 toe = 42 GJ
(1 tonne of oil equivalent approximately equals 42 billion joule).

energybalance said...

Hi Andre',

it's O.K. I knew what you meant! Thanks for your comment and I have added my sources under "Related reading" which might help other readers too.



André Sautou said...

Thank you, Chris, for having added those interesting links.

Allow me to add a remark that might interest your readers : the annual amount of used energy measures a quantity of power.

Converted in SI unit (W, or J/s) the quantity 5 x 10^20 joule/year equals about 1.6 x 10^13 watt, equivalent to the power delivered by 10,000 third-generation EPR nuclear reactors working together (1600 MW per unit ; not yet built, Finland being due to commission the first one in 2011, France the second one in 2012).

Much regards.


energybalance said...

Thanks Andre',

yes, 5 x 10^20 J/yr/(3600 s/h x 8760 h/yr) = 1.6 x 10^13 W. As you say that is 10,000 1.6 GW plants or 16,000 standard 1 GW stations.

If all of that energy had to be provided in the form of electricity the problem would be vast (it is vast in any case!), but we lose 2/3 of the thermal energy when we make electricity, and so using the heat directly when possible is the more efficient strategy.

i.e. a 1 GW "electric" nuclear plant produces around 3 GW of thermal energy but we only get back about 36%, via the Carnot cycle, in the form of electricity.

Maybe the "new" 1.6 GW plants are more efficient?

Much regards,


André Sautou said...

Yes, EDF (Electricité de France) and AREVA expect performance improvements with third generation EPR (more efficient fuel use and higher turbine yield, thanks to higher temperatures and vapor pressure). One of the target goal is to generate 22% more electricity with the same amount of nuclear fuel. Another one is to increase the life span (around 60 years).

Sources :

It is important, of course, to make a difference between the thermal energy generated from nuclear fission and the delivered energy (electric, about 2.3 smaller if we assume a yield around 40 to 45 %). Your remark is quite appropriate.



energybalance said...

Hi Andre',

those new power stations sound very impressive, and their design seems to address the important (but often overlooked) aspect of energy-efficiency, i.e. if we use the energy we have better, the supply-demand "gap" is less threatening.