Some conceptual thinking on hydrocarbons
+ General concepts
+ A look at fuel synthesis
+ A further look at biomass
+ Why direct methane decomposition is a bad idea
In the figure below a representation is given on hydrocarbon conceptual
- The processes involving biomass (1,2, and 3) start from the centre
of the figure.
In the case of hydrothermal upgrading a combination of H2O
and CO2 is released from the biomass under
almost supercritical (with respect to water) conditions.
- After gasification of oil, and more so in case of gasification of coal,
the CO - H2 ratio is insuffiecient to use
the produced syngas directly in a Fischer and Tropsch synthesis process.
- Only in case of partial oxidation of NG this CO - H2
ratio is (almost) sufficient.
- In methane reforming the highest CO - H2
ratio is obtained. However, this process requires heat.
Synthesis of fuels via gasification
The syngas can, amongst other possibilities, be employed to produce
(high quality) fuel using the Fischer & Tropsch process. In this
process a CO - H2 ratio of 2 is required,
so if other feedstocks than NG are used a significant part of the CO has to be "shifted
away". (CO + H2O
In the table below the overview in this respect is given:
Feedstock C/H ratio CO/H2 ratio C lost C use
(in syngas) (in shift) (in end product)
Biomass 1 : 2 (+ 1 O) 1 : 1/1.5 25 % 35 % (*)
Coal 1 : 0.7 2 : 1 50 % 50 %
Oil 1 : 1.4 1 : 1 35 % 65 %
Natural Gas 1 : 3.5/4 1 : 1.8/2 < 10 % > 90 %
(*) In biomass gasification the combustion value of the material makes it neccesary to generate
extra CO2 to keep the process autothermic.
A further look at biomass
In the table below a comparison between three process routes from biomass to fuel
Process C use
(in end product)
a. Hydrothermal upgrading 65 %
b. Low Temp gasification + Fischer & Tropsch 50 %
c. Higher T gasification + Fischer & Tropsch 35 %
a. Hydrothermal upgrading:
In this process effectively one third of the oxygen in the carbonhydrate mass is removed as
water, while two thirds are removed as carbon dioxide so the overall reaction roughly reads:
+ x H2O + x CO2
This leads to net carbon utilisation rate of roughly 65%.
b. Low Temp gasification + Fischer & Tropsch:
In a low T process biomass is converted to a 1:1 mixture of methane and carbondioxide. This
process can be fermentation or lower T gasification. The methane can be converted to
fuel in a Fischer & Tropsch process on a 1 to 1 basis. So now the net carbon utilisation rate
is roughly 50%. For large industrial application the relatively slow reaction rates in the
gasification/fermentation step can be a significant drawback.
c. Higher T gasification + Fischer & Tropsch:
In a direct gasification biomass is converted into a mixture of CO, CO2,
H2 and H2O. As (CO and
H2O) and (CO2 and
H2) are interlinked, and for a Fischer & Tropsch process a
CO / H2 ratio of 1 : 2 is required the overall carbon utilisation
rate of this process route will be about 35%.
Why direct methane decomposion is a bad idea
Time and again proposals are made to decompose methane to carbon and hydrogen. The idea:
Don't burn the carbon, no carbondioxide. From an
energetics point of view this is not a good idea.
A process based on direct decomposition of methane has to be a high pressure process (reactor
This implies that the temperature at which it runs must be in the order of 1500 °C. A lower
temperature would lead to incomplete decomposition, and more bothering, formation of appreciable
amounts of (poly)aromatic hydrocarbons.
When looking at energetics the following picture emerges:
The heat of combustion of methane is 192 kcal/mol, for hydrogen it is 58 kcal/mol and for
carbon it is 90 kcal/mol.
The heat required to decompose methane into carbon and hydrogen is 22 kcal/mol, while the
heat required to bring it at 1500 °C is about 25 kcal/mol.
Going from the assumption that the heat to decompose methane is utilised with a 100% efficiency
(highly optimistic) and no heat recorery within the process (somewhat conservative) the
utilisable heating value left is:
2 ´ 58 = 116 kcal/mol, 60% of the initial heating value.
The amount of energy generated with respect to the energy "invested" is:
116 / (22+25) ´ 100 » 250%
This is disappointingly low as the net heating value left is only
(116 - (22+25)) / 192 ´ 100 » 35% of the initial heating value.
If the power required in the process (plasma arc for heating?) is generated using fossil
fuels, the conclusion is: This is the wrong way!