Power to Gas (PtG) processes have appeared in the last years as a long-term solution for renewable electricity surplus storage through methane production. These promising techniques will play a significant role in the future energy storage scenario since it addresses two crucial issues: electrical grid stability in scenarios with high share of renewable sources and decarbonisation of high energy density fuels for transportation. There are a large number of pathways for the transformation of energy from renewable sources into gaseous or liquid fuels through the combination with residual carbon dioxide. The high energy density of these synthetic fuels allows a share of the original renewable energy to be transported and stored in the long-term. The first objective of this review is to thoroughly gather and classify all these energy storage techniques to define in a clear manner the framework which includes the Power to Gas technologies. Once the boundaries of these PtG processes have been evidenced, the second objective of the work is to detail worldwide existing projects which deal with this technology. Basic information such as main objectives, location and launching date is presented together with a qualitative description of the plant, technical data, funding source/budget and project partners. A timeline has been built for every project to be able of tracking the evolution of research lines of different companies and institutions.

The Calcium Looping (CaL) technology, based on the multicyclic carbonation/calcination of CaO in gassolid fluidized bed reactors at high temperature, has emerged in the last years as a potentially low cost technology for CO 2 capture. In this manuscript a critical review is made on the important roles of energy integration and sorbent behavior in the process efficiency. Firstly, the strategies proposed to reduce the energy demand by internal integration are discussed as well as process modifications aimed at optimizing the overall efficiency by means of external integration. The most important benefit of the high temperature CaL cycles is the possibility of using high temperature streams that could reduce significantly the energy penalty associated to CO 2 capture. The application of the CaL technology in precombustion capture systems and energy integration, and the coupling of the CaL technology with other industrial processes are also described. In particular, the CaL technology has a significant potential to be a feasible CO 2 capture system for cement plants. A precise knowledge of the multicyclic CO 2 capture behavior of the sorbent at the CaL conditions to be expected in practice is of great relevance in order to predict a realistic efficiency from process simulations. The second part of this manuscript will be devoted to this issue. Particular emphasis is put on the behavior of natural limestone and dolomite, which would be the only practical choices for the technology to meet its main goal of reducing CO 2 capture costs. Under CaL calcination conditions for CO 2 capture (necessarily implying high CO 2 concentration in the calciner), dolomite seems to be a better alternative to limestone as CaO precursor. The proposed techniques of recarbonation and thermal/mechanical pretreatment to reactivate the sorbent and accelerate calcination will be the final subjects of this review.

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Lately, an outstanding research interest for CO2 capture sorption/desorption looping systems is the improvement of sorbents reactivity and durability. In particular, in calcium-looping cycles the control of sintering processes in the sorbent by thermal pretreatments, doped limestone, or dolomite have deserved excellent works and have shown good experimental results. Also, synthetic sorbents have been tested and demonstrate a lasting capture capacity. Nevertheless, in most cases this long-term conversion enhancement increases the cost of the sorbent and, thus, the system operation and CO2 capture cost. Therefore, any comparison among sorbents will be accurate if both chemical and economical considerations are taken into account in the assessment. In this work, a common basis for sorbent comparison is presented. The integration of the sorbent cost and its chemical and mechanical performance have been studied for different options. The energetic and economical characteristics of several high temperature sorbents have been checked in a CO2 looping cycle applied to an existing coal-fired electrical generation power plant. The aim is to compare the cost of avoided CO2 as a function of average conversion of solid population and cost for different sorbents. Despite excellent conversion results, the unit cost of the sorbent is crucial to maintain the CO2 looping concept as economically attractive. High cost sorbents, even if their residual activity remains at a high level, will be preferred to operate in systems fed with inert-free fuels, that is, natural gas, instead of applications operating with coal. Both their low cost and long-term performance make thermally pretreated limestones competitive sorbents for carbonation/calcination cycles.

Calcium looping is an emerging technology for CO2 capture that makes use of the calcium oxide as a sorbent. One of its main issues is the significant energy consumption in the calciner, where the regeneration of the sorbent takes place. Nevertheless, as a high temperature looping technology, the surplus heat flows may be used to reduce the energy needs in this reactor. The addition of a cyclonic preheater similar to those used in the cement industry is proposed in this work. A calcium looping system was modeled and simulated to assess the advantages and disadvantages of the inclusion of a cyclonic preheater. Despite the negative effect on the maximum average capture capacity of the sorbent, a reduction on the coal and oxygen consumptions and on the extra CO2 generated in the calciner is obtained.

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