Chemical storage of concentrated solar power

Solar energy is one of the most important sources among renewables to reach the global energy requirements without the concomitant production of greenhouses gases. Concentrating Solar Power (CSP) systems have been recognized as a promising technology thanks to the easy integration with Thermal Energy Storage (TES) devices, which allow to overcome the intrinsic unsteady nature of the solar energy. Gas–solid Fluidized Bed (FB) systems can play a key role when employed as solar receiver/reactor: the high thermal conductivity and diffusivity in such reactors can help in absorbing the concentrated solar energy ensuring low heat losses and in equalizing the heat received avoiding thermo mechanical stresses to both reactor walls and reactive materials. The work performed in this PhD Thesis was devoted to the investigation of gas–solid FBs as solar receiver/reactor in CSP systems. In particular, in the present PhD Thesis an integration between solar energy and conventional fossil fuels sources has been proposed. An integrated scheme of a continuous Calcium Looping (CaL) process for CO2 capture from combustion flue gases of a power plant, based on FB technology and sustained by a CSP, was developed. Storage of the excess incident solar power during the daytime as calcined sorbent, which is eventually utilized in the CaL loop during the nighttime, was proposed to overcome the inherently unsteadiness of CSP systems. The potentialities of a similar integrated scheme were assessed by means of model computations to estimate the main features, performance, storage and energy requirements of the process. Model computations of a base case suggested that in order to couple a CSP with a 100 MWth combustion plant, the use of two storage vessels of nearly 2000 m3 is required, together with the building of an heliostat field of 0.26 sqkm. The heat recovery in the carbonator is of crucial importance and allows to largely increase the overall thermal throughput of the power plant. It was also highlighted that larger inlet Ca/C molar ratio could result into a higher CO2 capture efficiency but at the expense of a larger energy requirement for the heliostat field. In order to study the practical feasibility of the process, directly irradiated FB systems were considered, as the direct heating configuration permits to obtain the high operating temperatures required to perform the calcination reaction. Initial studies were targeted at the comprehension of the heat transfer phenomena in directly irradiated FBs, with the aim of establishing the efficiency of the solar FB receiver. Experimental investigations were performed by mapping through a thermal infrared camera the surface of a fluidized bed subjected to a concentrated solar simulated radiation. Experimental results suggested that collection of incident radiation changes from a surface-receiver to a volumetric receiver paradigm as the gas superficial velocity is increased from fixed to bubbling fluidized bed conditions: this also results in a significant reduction of peak temperature and temperature fluctuations at the focal point of the receiver. A strong reduction of bed surface temperature has been obtained by exploring an uneven fluidization strategy and, in turn, by tailoring the hydrodynamics of the bed through injecting a chains of bubbles from a nozzle located in the proximity of the radiation focal point. A compartmental heat transfer model which provides a simple, though accurate, equation for predicting in-bed dispersion of radiative power and extent of temperature non-uniformity was also developed. Finally, a directly irradiated gas–solid FB reactor suitable for the investigation of thermochemical heat storage processes has been designed and operated to study the solar CaL process. Several calcination carbonation reactions were performed on an Italian Ca based sorbent (Massicci), and its CO2 capture capacity was evaluated over repeated cycling. The operation of the directly irradiated FB reactor resulted into a continuous fluctuation of the bed surface temperature, with a mean over temperature value of over 100 °C. The CO2 capture capacity of the investigated limestone showed a first initial decreasing trend followed by a stabilization at lower values, with a residual CO2 capture capacity of about 6%. According to the obtained results, the solar CaL process appears to be a practicable way toward the integration of conventional fossil fuels and energy sources, with the aim of both taking advantage of the solar energy and reducing CO2 emissions

Thermal behaviour of fluidized beds directly irradiated by a concentrated solar radiation

Directly-irradiated fluidized bed reactors are very promising in the context of concentrated solar power applications as they can be operated at process temperatures high enough to perform thermochemical storage with high energy density. The present study aims at experimentally investigating the direct interaction between a concentrated simulated solar radiation and a fluidized bed by measuring the time-resolved bed surface temperature with an infrared camera under different fluidization gas velocities. The effect of a localized generation of bubbles was investigated too, by injecting a chain of bubbles through a nozzle located just at the centre of the concentrated solar beam. The obtained results encourage the localized generation of bubbles, just at the larger value of the impinging radiative heat flux, as a strategy to reduce the overheating of the bed surface and, as a consequence, the energy losses related to fluidizing gas and radiative re-emission

Technoeconomic analysis of a fixed bed system for single/two–stage chemical looping combustion

Chemical looping combustion (CLC) is a promising carbon capture technology allowing integration with high-efficiency Brayton cycles for energy production and yielding a concentrated CO2 stream without requiring air separation units. Recently, dynamically operated fixed bed reactors have been proposed and investigated for CLC. This study deals with the technoeconomic assessment of a CLC process performed in packed beds. Following a previously published work on the topic, two different configurations are considered: one relying on a single oxygen carrier (Cu/CuO based) and the other on two in–series oxygen carriers (Cu/CuO based first, Ni/NiO based later). For both configurations, relevant process schemes are devised to obtain continuous power generation. Despite slightly larger capital costs, two-stage CLC performs better in terms of efficiency, levelized cost of electricity, and avoided CO2 costs. Fuel price and high–temperature valves costs are identified as the main variables influencing the economic performance. The use of two in–parallel packed bed reactors (2.0 m length, 0.7 m internal diameter) enables a power output of 386 kWe, a net electric efficiency of 37.2%, a levelized cost of electricity of 91 € MWhe −1, and avoided CO2 costs of 55 € tonCO2 −1 with respect to a reference pulverized coal power plant

Preliminary Assessment of Copper/cerium Mixed Oxides for Thermochemical Energy Storage Applications

An extensive exploitation of renewable energies is required to face climate change, and reduce the dependence on fossil fuels. Thermochemical energy storage (TCES) systems are bound to play a major role in the next years to overcome the intrinsic variability of many renewable sources, thus increasing their dispatchability. Within this field, reversible gas-solid chemical reactions are among the most promising systems thanks to high theoretical values of energy storage density, and virtual unlimited time scales of energy storage. Copper oxide is an interesting candidate for TCES, but suffers from particle sintering/agglomeration at the high process temperature required by the system. In this work, several Cu:Ce mixed oxides have been synthesized and tested in a thermogravimetric analyzer under process parameters relevant to TCES applications, with the aim of preventing copper oxide sintering/agglomeration. Morphology and chemical composition of the synthesized and reacted samples have been scrutinized by means of XRD and SEM-EDS analyses

Modeling gasification of waste-derived fuels in a rotary kiln converter operated with oxygen staging

Thermal conversion of waste-derived fuels is gaining a clear role in the general frame of the circular economy as one pathway to close the recycle loop when a material or chemical recycle is impossible or economically unfeasible. Sewage sludge derived from the treatment of urban wastewaters is currently facing rapidly increasing production volumes and severe restrictions of the conventional disposal options: thermal conversion stems out as the most viable strategy, entailing large reduction of the sludge volume and thermal destruction of the toxic organic constituents. In the frame of thermochemical processing of waste-derived fuels, pyrolysis/gasification presents several advantages over the direct waste-to-energy combustion path, mostly related to the generation of syngas and condensable species which can be easily transported, burned or even exploited in gas-to-liquid fuel or chemical processes. The present study addresses the development of a process for oxy-pyrolysis of sewage sludge in a rotary kiln converter. The aim of the process is the production of syngas from devolatilization of a waste-derived fuel, with oxygen playing the role of promoting autothermal operation of the pyrolyzer by controlled oxidation of volatile compounds. The specific concern of the study is the assessment of the effectiveness of staged oxygen feeding, as opposed to localized feeding at the reactor inlet, as a tool to selectively promote desired secondary reactions occurring in gas phase, like partial oxidation of tars. The converter consists of a rotary kiln in which the oxidizer is fed at multiple coordinates along the reactor axis, so as to obtain a reactant contacting pattern resembling that of a Zwietering reactor. The reactor is modelled at steady state using a 1.5D frame. Material and energy balances are set up considering a semi-lumped kinetic mechanism that was purposely developed to represent the complex chemical pathways of the solid fuel, of the gaseous compounds, of different tar components and of soot. Model results are analyzed with a focus on the effect of axial staging of the oxidizer on the quality of the produced gas and on the performance of the reactor

An experimental characterization of Calcium Looping integrated with concentrated solar power

Carbon Capture and Sequestration (CCS) and renewable energy sources are both essential to mitigate the CO2 emissions in the near future. Calcium Looping (CaL) is an important post-combustion carbon capture technology that has reached the maturity of the pilot plant stage. On the other side Concentrated Solar Power (CSP) is a fastgrowing renewable technology in which solar energy, concentrated up to several MW m−2, can be used to produce electricity or to drive an endothermic chemical reaction. The integration between a CSP system and a CaL cycle, in order to use a renewable source to supply the energy required by the calciner, would strongly improve the performance of the CaL process by overcoming some of its main drawbacks. However, the role that highly concentrated radiation can have on the sorbent properties in the CaL cycle is still matter of investigation. In this study, the CaL-CSP integrated process is experimentally investigated through the use of a directly irradiated Fluidized Bed (FB) reactor. Simulated concentrated solar radiation featured a peak flux on the FB surface of approximately 3 MW m−2 and a total power of about 3 kWth. Several calcination and carbonation tests have been performed on samples of a commercial Italian limestone, in order to establish the evolution of the sorbent capacity of CO2 capture at increasing number of cycles. The properties of the limestone samples were further investigated by means of microstructural characterization. The comparison between results obtained with and without the use of the solar concentrated flux to thermally sustain calcination provides useful information on the potential of solar driven CaL and on the measure to overcome some of its potential limitations

Chemical storage of concentrated solar power

Chemical storage of concentrated solar powe

Characterization of limestone calcination-carbonation for thermochemical energy storage applications

Concentrating Solar Power (CSP) systems represent a growing market to exploit solar energy thanks to the easy integration with energy storage systems. The thermochemical storage (TCES) technology relies on reversible chemical reactions to store the solar energy in the form of chemical bonds. Limestone calcination/carbonation is an appealing reaction for TCES. This cycle has been widely studied in the Calcium Looping (CaL) process for Carbon Capture and Sequestration/Use (CCS/U), within which the calcination is usually carried out in a CO2-rich environment at temperature of 940–950 °C. When the CaL cycle is meant for TCES, the energy required by the calciner is supplied by CSP and the whole system has to work in a closed loop, as the CO2 released during the calcination is required for the subsequent carbonation. Therefore, the operating conditions recall the ones of the CCS/U CaL. Our idea is to perform a CaL-TCES cycle working in an open loop configuration by coupling the system with a CO2 emitting industry. In this way, the calcination can be accomplished under air atmosphere at lower temperature, thus reducing the sintering phenomena and preserving the material reactivity. In this work, the open loop CaL-TCES cycle has been experimentally investigated using a Fluidized Bed (FB) reactor directly heated by a solar simulator (3 MW m–2 peak flux, 3 kWth total power). Several looping cycles have been carried out on a commercial limestone sample to estimate the sorbent reactivity over cycling. The properties of regenerated sorbents have been investigated by chemical physical analyses. The results obtained have been eventually compared with those obtained under CCS/U CaL calcination conditions to scrutinize the potential advantages of working in an open loop configuration

A model of integrated calcium looping for CO2 capture and concentrated solar power

Calcium Looping (CaL) is a promising post-combustion CO2 capture and storage technique. Thermal input to the calciner, which is needed to sustain the endothermicity of sorbent regeneration, is usually accomplished via oxy-combustion of an auxiliary fuel. The idea behind the present study is to couple CaL with a Concentrated Solar Power (CSP) system, so that all the thermal energy required in the calciner is supplied by a renewable source. The integration of a CaL cycle with a CSP system offers several potential technical, economical and environmental advantages, but must cope with the inherently unsteady nature of incident solar power. The cyclic character of incident solar power as compared with steady CaL operation could be managed in different ways. In the present study a simple scheme of integrated CaL-CSP process is suggested, based on storage of the excess incident power during the daytime as calcined sorbent, which is eventually utilized in the CaL loop during the nighttime. A preliminary assessment of the performance of this integrated scheme is accomplished by means of model computations. The model is based on a population balance model on sorbent particles, which takes into account the cyclic operation of the system. The parameters of the solar field and the influence of the main operating parameters (sorbent residence time, sorbent/CO2 inlet molar ratio, fluidization velocity) on carbonation degree and efficiency, on sorbent loss by elutriation, on thermal power demand at the calciner and on thermal power produced in the carbonator have been assessed