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ISSN Online: 2377-424X

ISBN Print: 1-56032-797-9

International Heat Transfer Conference 11
August, 23-28, 1998, Kyongju, Korea

FUNDAMENTAL INVESTIGATIONS OF SULPHURIC ACID DECOMPOSITION IN VOLUMETRIC RECEIVERS

Get access (open in a dialog) DOI: 10.1615/IHTC11.100
pages 57-62

Résumé

The thermochemical sulphur-iodine-cylce, consisting of a closed loop sequence of three chemical reaction steps, offers a method of hydrogen production with a high thermal efficiency. The overall reaction path of this process is the decomposition of water into hydrogen and oxygen. In a first step, the so-called Bunsen reaction, H20, combined with S02 and l2, is reacted to sulphuric acid and HI. At medium temperatures HI is splitted into iodine and the product gas hydrogen. In order to close the loop, the sulphuric acid needs to be decomposed into sulphur dioxide, oxygen and water, which requires a temperature level above 1000 K. This part of the process can be carried our in the central receiver system of a solar tower. In volumetric receivers the incident radiation is absorbed by the fluid directly as well as by a ceramic multicavity structure redistributing the heat into the fluid by forced convection and heat radiation.
3, H20 and the ambient air enters the absorber matrix. In order to investigate the dissociation of the sulphur trioxide, the flow and the exchange phenomena in one of the channels have been modeled. Numerical calcluations were done with an extended version of the CFD code KIVA II. Finite difference approximations to the governing equations for the description of the flow phenomena were solved. The heat exchange by radiation is considered with the help of the discrete transfer method. Absorption of radiation by gaseous components is taken into account by using spectrally averaged absorption coefficients, the Mie theory describes the behaviour of the acid droplets.
According to the description of the processes in the solar reactor, the evaporation and dissociation calculation is seperately investigated in two spatial domains.
In the front region the sulphuric acid is introduced at an axial injection point into the carrier air, forming a spray cone. The resulting droplet diameters are distributed around the Sauter mean radius following a gamma function. Geometry, position and orientation of the injection cone have been varied.
In compliance with the chemical equilibrium the evaporated sulphuric acid is decomposed into free water and sulphur trioxid. The following reaction of the trioxide in the second area of calculation is limited by the reaction velocity and is described by a kinetical formulation.
Calculations have been carried out for a prototype reactor with 0.4 m opening diameter. Numerical reasons demand the liquid sulphuric acid to be injected in the center of the receiver. A complete evaporation and a uniform distribution can be achieved with a mean droplet diameter below 40 µm.
The realization of the volumetric effect is strongly dependent on the selected parameters and can be achieved with small incident angles of the concentrated radiation and small diameters of the absorber channels. To achieve a significant decomposition of S03 a satisfactory receiver and hence reaction length and a sufficiently high temperature have to be provided. The maximum conversion rate is mainly limited by the necesary residence time at high temperatures and is therefore strongly dependent on the carrier air stream, the heat flux density of the irradiated energy and the length of the receiver.