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Some conceptual thinking on hydrocarbons

Contents:

+ 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 thinking.

CHO Scheme

  • 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   ¾®  H2 + CO2)

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 is given:


    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:

(C3H6O3)x   ¾®   (C2H4)x + 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 size). 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!


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