Solar-derived fuels

By employing solar energy, solar hydrogen and CO2, solar hydrocarbons can be synthesized. In this way solar hydrocarbons can play the role of a

20.16 image516
(a) Partitioning of the SSPS-CRS heliostat field at the PSA (Roeb et al., 2011); (b) focus of solar radiation on the dual HYDROSOL-II reactor (Konstandopoulos and Lorentzou, 2010).

renewable energy carrier since they utilize solar energy and consume waste CO2.

A very well-known technology that could be applied for the conversion of solar energy, hydrogen and carbon monoxide (e. g., from the solar decom­position of CO2) into solar hydrocarbons is Fischer-Tropsch synthesis. The term Fischer-Tropsch is applied in a rather large variety of chemical pro­cesses used for the production of synthetic hydrocarbons (e. g., paraffins,

image517

Chain growth parameter, a

1 2 4 8 ~

Degree of polymerization, D = 1/(1-a)

20.17 Product yield in Fischer-Tropsch synthesis (Perry, 2008).

olefins and alcohols, while depending on the reaction conditions or the catalyst used, other compounds may be produced) from synthesis gas (hydrogen and carbon monoxide) (Perry, 2008). Some of the most common reactions that might take place in a Fischer-Tropsch system are: Boudouard (Eq. [20.8]), water-gas shift (Eq. [20.13]), methanation (Eq. [20.14]) and reactions for the production of heavier hydrocarbons (Eq. [20.15]) (Opdal, 2006). The first two reactions (methanation and Boudouard) are considered undesirable, while the latter one (Eq. [20.15]) consists of the chain building reaction. In Fig. 20.17, the product yield of the Fischer-Tropsch processes is shown.

CO + H2O ^ CO2 + H2

AH0 = 41 kJ/mol

[20.13]

CO + 3H2 ^ CH4 + H2O

AH298K = -247 kJ

[20.14]

nCO + 2nH2 ^ CnH2n + nH2O

[20.15]

The demonstration of the Fischer-Tropsch (F-T) processes at commercial scale dates back to around 1935, when Ruhrchemie A. G. was formed by a group of Ruhr companies that had in common the main objective of con­structing the first commercial plant (Hall and Hsensel, 1945). In fact, the Ruhrchemie company at the time was the holder of the exclusive rights over the F-T process and the first plant was also used for the further research and development of the technology. Later on (until 1945) Ruhrche – mie A. G. was the owner of eight additional Fischer-Tropsch plants all built within Germany.

Currently one of the largest operators of F-T plants converting gas and coal into liquid fuels, and also a leading fuel provider in South Africa, is Sasol (Sasol, 2011). Another leading company in the field of F-T plants, is PetroSA company, which is also located in South Africa and operates a semi-commercial unit (http://www. petrosa. co. za/; Njobeni, 2011). Besides these two leading companies, there are several others that utilize F-T pro­cesses, thus proving the maturity of the technology on an industrial scale as well as the potential for further commercialization.

Where the solar fuel process has begun with the decomposition of a hydrocarbon, the production of syngas mixtures is a natural consequence. If production of Fischer-Tropsch liquids was to be pursued following pro­duction of pure hydrogen, a source of CO would also be needed. Separating CO2 from the atmosphere is difficult because of the very low concentration levels.

If CO2 emissions were not treated as chemical waste but rather as a raw material for the formation of energy-rich products, CO2 could be incorpo­rated in a cyclic operation, where after its production from carbonaceous sources, it would be reused as a storage medium of the solar energy that is abundant, renewable and freely dispensable. Zeman and Keith (2008) studied the issue of obtaining carbon-neutral hydrocarbons as a viable alternative to hydrogen or conventional biofuels, and investigated the eco­nomics of such an approach based on hydrogen generation from coal/fossil fuels in combination with carbon capture and storage (CSS) technology and CO2 sourced from biomass or air capture and arrived at the conclusion that: ‘the lack of a clear technological “winner” warrants equal attention and funding on all potential solutions’ (Zeman and Keith, 2008). This is not unexpected, since it is clear that in any such scheme where, on the one hand, CO2 is treated as waste that needs to be disposed of and, on the other hand, as a raw material that is being collected (worst as it may be in a very diluted form), one part of the process fights the other. In addition, hydrogen from fossil coal and similar sources can never be sufficient to turn the process economics around. Put simply, to synthesize hydrocarbon fuels we need a source of hydrogen and a source of carbon independent of each other.

Another alternative to synthesizing a fuel from pure hydrogen is to convert it to ammonia. This can be done using standard Haber Bosch ammonia synthesis (Eq. [20.16])

H2 + N2 ^ NH3 [20.16]

Ammonia liquefies at modest pressures and can be transported and handled using similar equipment to LPG. Operation of gas turbines and internal combustion engines using ammonia as a fuel has been successfully demonstrated and is the subject of ongoing work (Dunn et al., 2012).

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