(469d) Hydrogen Production by Sulphur Iodine Cycle Fed by Solar Energy: Realization of a Laboratory Plant and Possible Spin-off On the Industrial Field

In a desirable future in which energy could be not related to greenhouse emissions of gases into the atmosphere and without consuming resources owned from few countries, with its resulting strategic use, the hydrogen produced from water using renewable energy sources seems the best solution.

In this framework, ENEA has started a study of the thermolysis through thermochemical cycles powered by solar energy. The TEPSI project, in this regard, has as main objective, the construction of a plant for the production of 10 NL/h of hydrogen by sulfur-iodine cycle, which starting from water through a closed cycle of reactions that generate and consume iodine, hydriodic acid, sulfuric acid and sulfur dioxide, produces hydrogen and oxygen at a temperature compatible with current solar collectors technology and with a higher efficiency then the traditional methods of water thermolysis. A massive hydrogen production by this researched innovative methods will presumably be achieved by 2030, when all the equipments, built by appropriate materials, will be tested at pilot scale and when the solar technology has reached maturity levels allowing a suitable reduction of its impact on total production costs, which currently is about 2/3, but for that date is expected to fall by half. In the short term, however, it is possible using the knowledge learned in this study for purposes related to the industrial sector, such as the treatment of sulfur compounds from industrial processes. This method, patented by ENEA as part of this project, allows this result, at a cost-effective, without catalyst and with a contemporary production of hydrogen and sulfuric acid.

This research work concerns also the tests of the equipments related to this cycle, executed in the ENEA laboratories, with particular emphasis on the ones concerning the hydriodic section. These equipments, indeed, consume the most part of energy and, if bad designed, can invalidate the efficiency of the all cycle.

In the ENEA laboratory plant schema, the light phase of the Bunsen reaction (2H2O + I2 + SO2 ?³ H2SO4 + 2HI ) composed of H2SO4 and H2O is sent to an H2SO4 decomposer, after concentration in a distillation column, to generate SO2, H2O and O2, which return to Bunsen reactor after a washing step to take away the unreacted species. The heavy phase, instead, is composed by a mixture containing HI, H2O, traces of H2SO4 and a lot amount of I2 (84-92 %wt). This phase, after purification from the H2SO4, is sent to a distillation column able to produce an azeotropic mixture of HI-H2O (57 %wt of HI) as distillate and I2 (95 %wt) as residue, which is sent to Bunsen reactor. By the carried out tests, the correct working of the column depends on the content of HI in I2 free basis. If it is too low (< 55%wt) the column will produce a mixture containing a great amount of water and the separation of I2 is not total, so different tests have been executed in order to find the best composition of the heavy Bunsen phase. The HI azeotropic mixture is sent to decomposer that works at 500 ¢XC and produces hydrogen and iodine with a conversion of about 22 %. The vapor from this equipment is condensed to separate hydrogen, but the liquid phase has an HI content of ~50%wt in iodine free basis and a content of iodine of ~11%wt. The I2 purification of this stream is executed by the described column, to which is sent as lateral stream after to have taken away the water in excess in another packing column. This water obtained as distillate has resulted with very low iodine and HI content (< 0.5 %wt) and it is sent to Bunsen. The residue for the I2 purification column is composed by a mixture of 14 %wt and 56.5 %wt of HI in iodine free basis.

This work analyses also the possible spin-off in the industrial field concerning the sulfuric compounds and the advantages in economical terms that are possible with the use of this cycle, using only tested equipments.