Challenges in Using Particulate Materials As Heat Transfer Fluid or Reactant in Solar-Chemical Processes

Kurzfassung

Concentrating solar technology can replace fossil fuels not only in electricity generation, but also via generation of fuels and commodities in the industrial and transportation sectors. To advance such solar applications, new technologies are needed: new receivers in which absorbs solar radiation, heat transfer media, high-temperature heat exchangers and storage. Particles made of ceramics or metals have proven in recent years to be an interesting alternative to the previous liquid heat transfer media. They can be used both in receivers, where they directly absorb solar radiation, and as storage materials. In some cases even the particles themselves represent the material to be treated and converted to the desired product like in the case of solar lime or cement production (Moumin et al 2019, Meier et al 2006). A major advantage is that they can be used for temperatures over 1000 °C. At the same time, they are often cheaper than liquid heat transfer media and non-toxic. Various solar particle receivers have already been successfully tested. However, working with particles at high temperatures and under high solar concentrations comes along with a number of challenges like particles degradation and attrition, erosion of particles ducts, heat transfer and temperature homogeneity in the particle bed, radiation absorptivity of the particles, flowability of particles. The present contribution will deal with two challenges specifically. One is related to the transport of hot particles, the second one deals with directly heated reactors which require a quartz window which if often prone to failure because of deposition of material on its surface. Handling the particles is still a major challenge, because the hot particles have to be transported further from the solar receiver to the next process components, such as the storage tank or the heat exchanger. A project, carried out at DLR, deals with the transport of particles for solar thermal and solar thermochemical applications. Within the project a high temperature transport and lock system for particles at temperatures up to 1000 °C was develop and qualified. For this purpose, a protype setup was developed to demonstrate the controlled handling and low-heat-loss transport of particles. The particles are heated in a closed rotary receiver by concentrated radiation from a solar simulator and then automatically conveyed through the system. The system transports the particles to a storage tank from where the particles are returned to the receiver, thus closing the particle cycle. For some of the key components the use of innovative fibre-enforced materials was investigated, which can withstand the high temperatures. They turned out chemically inert and suitable for frequent transfer of particles. A quartz window is often the bottleneck of directly irradiated receiver-reactors due to the deposition of material on its surface, which causes overheating and failure of the window. Several systems analysed in the past to overcome the window problem have been developed based on aerodynamic protection methods such as air curtains or vortex flows (e.g. E. Koepf 2015, A. Chinnici 2016). However, the high amount of gas required by these systems considerably reduces the efficiency of the reactor, and in several cases the window could not be completely protected from particle deposition. We intend to present an unexplored method to avoid the window problem in a solar reactor. The novel system developed uses electrostatic precipitation of particles (ESP) to prevent the migration of solid particles to the window. The application of ESP in a solar reactor is analysed in depth. An experimental setup was developed to demonstrate and proof the performance of the system at representative conditions. The precipitator showed its best performance using positive corona discharge. Compared to comparison with negative discharge this mode is characterised by lower current densities, enabling a wider operation range with more stable corona discharge, and lower energy consumption. The window protection system turned out to be able to keep the window free from particle contamination when using particles of limestone and cement raw meal.