92
Water for Energy and Fuel Production
which was obtained through a Cassegrain optical configuration that made use of a
hyperbolic reflector at the top of the solar tower to redirect the sunlight collected by
a heliostat field to a receiver located at the ground level. The reactor had two cavities
in series. The upper one absorbed the solar radiation and contained a small aperture to
gather concentrated solar radiation. The lower cavity contained carbonaceous materials on the top of a steam injector. An emitter plate separated the two cavities.
A 3D compound parabolic concentrator (CPC) was incorporated in the aperture
of the reactor, further augmenting the incident solar flux before passing it through
a quartz window in the upper cavity. The emitter plate acted as a transmitter of the
radiation to the lower cavity, thus avoiding the direct contact between the quartz window and the reactants and products. This set-up also provided uniform temperature
in the lower cavity and a constant supply of radiant heat through the upper cavity that
can act as energy storage, which was needed due to intermittent supply of radiant
heat. This type of batch, two-cavity solar reactor, has been successfully used for the
carbothermal reduction of ZnO and the detoxification of solid waste. The reactor can
be operated with a wide variety of particle sizes, and as the reaction proceeds, both
the particle size and the packed bed reactor volume decrease. The detailed dimensions and the operation of this type of reactor are given by Piatkowski and Steinfeld
[195]. Piatkowski et al. [197] and Perkins et al. [196] also showed an effective use
of such a reactor to produce syngas from coal, biomass, and other carbonaceous
feedstock. Z’Graggen [198] and Z’Graggen et al. [202] produced hydrogen from
petroleum coke using solar gasification process.
The solar energy is also used as the heat carrier for the pressurized coal gasification process. In this process, finely powdered coal is fed by a specially designed
injection system. The oxidizing and fluidizing agent is a superheated steam. The heat
required for the endothermic gasification reaction is introduced by means of a tubular heat exchanger assembly immersed in the fluidized bed. The technical feasibility
of a solar power tower and pressurized gasifier integration has been demonstrated in
a small pilot plant [194,199,202]. Solar energy has also been used to gasify biomass
in different types of reactors [196,203,204,206,208,209].
4.7.3 SolAr reForming
The high temperatures required for solar reforming effectively limit the nature
of solar energy collector [192,194,199–201,207–214] (Yeheskel et al., 2012, pers.
comm.). The bulk energy production, whether in closed-loop or open-loop configurations, probably must be carried out on a large scale to compete with fossil fuels and
probably requires the tower (central receiver) solar technology. Solar reforming can
be carried out using different processes such as direct and indirect, each requiring
different type of reformer configuration [192].
4.7.3.1 asteriX: solar steam reforming of methane
Advanced Steam Reforming of Methane in Heat Exchange (ASTERIx) experiment, an earlier joint Spanish–German project, examined steam reforming of methane using solar-generated high-temperature process heat by an indirectly heated
reformer [192,194,199]. The specific objectives of the ASTERIx experiments were
Water for Energy and Fuel Production
which was obtained through a Cassegrain optical configuration that made use of a
hyperbolic reflector at the top of the solar tower to redirect the sunlight collected by
a heliostat field to a receiver located at the ground level. The reactor had two cavities
in series. The upper one absorbed the solar radiation and contained a small aperture to
gather concentrated solar radiation. The lower cavity contained carbonaceous materials on the top of a steam injector. An emitter plate separated the two cavities.
A 3D compound parabolic concentrator (CPC) was incorporated in the aperture
of the reactor, further augmenting the incident solar flux before passing it through
a quartz window in the upper cavity. The emitter plate acted as a transmitter of the
radiation to the lower cavity, thus avoiding the direct contact between the quartz window and the reactants and products. This set-up also provided uniform temperature
in the lower cavity and a constant supply of radiant heat through the upper cavity that
can act as energy storage, which was needed due to intermittent supply of radiant
heat. This type of batch, two-cavity solar reactor, has been successfully used for the
carbothermal reduction of ZnO and the detoxification of solid waste. The reactor can
be operated with a wide variety of particle sizes, and as the reaction proceeds, both
the particle size and the packed bed reactor volume decrease. The detailed dimensions and the operation of this type of reactor are given by Piatkowski and Steinfeld
[195]. Piatkowski et al. [197] and Perkins et al. [196] also showed an effective use
of such a reactor to produce syngas from coal, biomass, and other carbonaceous
feedstock. Z’Graggen [198] and Z’Graggen et al. [202] produced hydrogen from
petroleum coke using solar gasification process.
The solar energy is also used as the heat carrier for the pressurized coal gasification process. In this process, finely powdered coal is fed by a specially designed
injection system. The oxidizing and fluidizing agent is a superheated steam. The heat
required for the endothermic gasification reaction is introduced by means of a tubular heat exchanger assembly immersed in the fluidized bed. The technical feasibility
of a solar power tower and pressurized gasifier integration has been demonstrated in
a small pilot plant [194,199,202]. Solar energy has also been used to gasify biomass
in different types of reactors [196,203,204,206,208,209].
4.7.3 SolAr reForming
The high temperatures required for solar reforming effectively limit the nature
of solar energy collector [192,194,199–201,207–214] (Yeheskel et al., 2012, pers.
comm.). The bulk energy production, whether in closed-loop or open-loop configurations, probably must be carried out on a large scale to compete with fossil fuels and
probably requires the tower (central receiver) solar technology. Solar reforming can
be carried out using different processes such as direct and indirect, each requiring
different type of reformer configuration [192].
4.7.3.1 asteriX: solar steam reforming of methane
Advanced Steam Reforming of Methane in Heat Exchange (ASTERIx) experiment, an earlier joint Spanish–German project, examined steam reforming of methane using solar-generated high-temperature process heat by an indirectly heated
reformer [192,194,199]. The specific objectives of the ASTERIx experiments were