Some of the major challenges---both technical and economic---of the Zn/ZnO two-step thermochemical hydrogen production cycle are investigated in this study. Technically, complete hydrolysis of Zn in the hydrogen production step remains a major barrier to implementation, and much attention has been given to Zn nano-scale reacting aerosols as a solution. Smaller particles favor faster reaction kinetics, and because they can be entrained and reacted in a gas flow, a continuous controllable process is possible. However, success of this continuous process depends on achieving high particle yields and high conversions in the aerosol, neither of which have yet been achieved in laboratory reactors.
The ability of a new reactor concept based on transverse jet fluid dynamics to control the flow field and rapidly cool the Zn vapor is investigated. In the transverse jet reactor, evaporated Zn entrained in an Ar carrier gas issues vertically into the horizontal tubular reactor through which cooler H2O and Ar flow. Particles are formed in the presence of steam at ~450 K. The objective of controlling the flow field is to keep Zn away from the walls, thereby reducing particle deposition in the reactor and increasing particle yields on the filter. A computational fluid dynamics (CFD) model indicates that the trajectory of the jet can be controlled so that the majority of the Zn mass is directed down the center of the reactor, not near the reactor walls. Furthermore, the model shows that quench rates of 2x10^4 K/s are achieved and reactants are well mixed. Experimentally, maximum particle yields of 93% of the mass entering the reactor are obtained.
Hydrolysis experiments are conducted in the transverse jet reactor at 418 K, 573 K, 603 K, and 713 K to assess the mechanisms of particle growth and hydrolysis. Experiments are conducted with and without steam to assess the effect of the reacting gas on particle morphology. SEM images of particles collected on a filter downstream from the reaction zone indicate that particle growth is dominated by condensation, resulting in hexagonal particles generally with lengths across their hexagonal face of 300 nm to 1micron in experiments with stream, and 1 to 3 micron in experiments without steam. Furthermore, the SEM images indicate that in hydrolysis experiments, ZnO forms on the surface of particles early on, protecting them from re-evaporation.
Particle yield on the filter, Y, is defined as the fraction of the total mass entering the reactor that is collected on a filter placed downstream of the reaction zone. Overall conversion, X, is measured by monitoring the H2 content of the effluent gas throughout experiments with a gas chromatograph. Conversion of aerosol particles, Z, is the ZnO content (by mole) of particles collected from the downstream filter; it is measured by x-ray diffractometry with the internal standard calibration method. At all temperatures, particle yield remains high---generally 70 to 80% in hydrolysis experiments---and particle deposition on the walls of the reaction zone is eliminated for temperatures of 573 K and above. However, the conversion in the aerosol is <7% and decreases with reaction zone temperature. The overall conversion ranges from 11% at 418 K to 49% at 713 K. The higher overall conversion than conversion in the aerosol is attributed to heterogeneous Zn vapor hydrolysis. Visual observation proves heterogeneous hydrolysis occurs on the reactor walls; it is inferred that the heterogeneous Zn vapor reaction also occurs on the surface of aerosol particles. In this study, high particle yields are achieved for the first time---an important step forward for the continuous aerosol process. However, complete conversion of the aerosol particles remains a major challenge.
In an economic and policy study of the Zn/ZnO cycle, the time frame for economic viability is assessed through the use of experience curves under minimal input, mid-range, and aggressive incentive policy scenarios. For the technology to become cost competitive, incentive policies that lead to early implementation of solar hydrogen plants will be necessary to allow the experience effect to draw down the price. Under such policies, a learning curve analysis suggests that hydrogen produced via the Zn/ZnO cycle could become economically viable between 2032 and 2069, depending on how aggressively the policies encourage the emerging technology. Thus, if the technical challenges are resolved, the Zn/ZnO cycle has the potential to be economically viable by mid-century if incentive policies--such as direct financial support, purchase guarantees, low interest rate loans, and tax breaks--are used to support initial projects.