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Fernandez J.R.,CSIC - National Coal Institute | Abanades J.C.,CSIC - National Coal Institute | Murillo R.,CSIC - Institute of Carbochemistry
Applied Energy | Year: 2014

A dynamic model was developed to describe the oxidation of Cu in a large-scale Cu/CuO chemical looping process performed in adiabatic fixed-bed reactors at high pressure. An ideal plug flow pattern without axial dispersion or radial gradients and with negligible intra-particle concentrations and temperature gradients on the scale of millimeters were assumed. Cu oxidation is favoured at high pressure and therefore fast reaction rates and total oxygen conversion were achieved, even with low contents of oxygen in the feed (around 4-6%). Short breakthrough periods were achieved, which is highly favorable in operations carried out in alternative fixed-bed reactors. In order to maximize energy efficiency, the oxidation needs to be carried out at the highest allowable temperature, but CuO tends to decompose and agglomerate at relatively low temperatures (over 1223K). Also the high exothermicity of Cu oxidation can generate hot spots in the reaction front. The use of a large recycle of nitrogen (previously cooled down) so that it mixes with regulates the temperature in the reaction front. At these conditions, the gas-solid heat exchange front advances faster than the reaction front and the oxidized bed is finally left at a lower temperature (as the cooled N2 recycle), which is insufficient to initiate the subsequent reduction of CuO to Cu. Therefore, an additional stage is introduced to carry out a gas-solid heat exchange between the hot N2 rich recycled gas and the oxidized bed. The bed is then ready for the next reaction step that involves the exothermic reduction of CuO. Operating parameters, such as the recirculation ratio (content of O2 in the feed) and the proportion of Cu in the solid bed, which have a substantial effect on Cu oxidation and CO2 capture efficiency, were also evaluated. Recirculation ratios higher than 0.75 and inlet gas temperatures of around 423K limit the maximum temperature to reasonable values (generally below 1200K). A trade-off between the O2 content in the feed (4-6%) and the amount of Cu in the bed (20-33%) leads to high energy efficiencies in CLC processes and minimal CaCO3 calcination in the case of the Ca-Cu looping process. © 2013 Elsevier Ltd. Source

Alonso M.,CSIC - National Coal Institute | Criado Y.A.,CSIC - National Coal Institute | Abanades J.C.,CSIC - National Coal Institute | Grasa G.,CSIC - Institute of Carbochemistry
Fuel | Year: 2014

Calcium looping CO2 capture systems use CaO as a reversible sorbent of CO2. Therefore, the evolution of the CO2 carrying capacity of CaO-materials at increasing number of carbonation- calcination needs to be determined to assess sorbent performance. Thermogravimetric analyzers (TGA) are commonly used for this purpose, by simulating around a small batch of material the average cyclic conditions expected in the real system. Many variables have been reported to influence the results and we review in this paper the main observations and trends, which can at times be conflicting when diffusional effects are not ruled out from the experiments. Furthermore, in a selected number of tests on a typical limestone using four different TG equipment, we have detected that some design characteristics of the TGA apparatus can strongly affect the determination of the CO2 carrying capacities of the material. In particular, we note that the decay in CO2 carrying capacity is accelerated as the power density of the TGA oven increases. This effect is most pronounced in the first calcination cycle, and it seems to be linked to an additional shrinking of the particles taking place in the TG apparatus with the highest heating rates. The use of larger sample masses and/or larger particle sizes tends to reduce the error in the determination of CO2 carrying capacity curves at the expense of departing from differential conditions that are required to obtain kinetic information on the sample. ©2013 Elsevier Ltd. All rights reserved. Source

Fonts I.,Centro Universitario Of La Defensa Of Zaragoza | Fonts I.,Aragon Institute of Engineering Research | Gea G.,Aragon Institute of Engineering Research | Azuara M.,Aragon Institute of Engineering Research | And 2 more authors.
Renewable and Sustainable Energy Reviews | Year: 2012

The high output of sewage sludge, which is increasing during recent years, and the limitations of the existing means of disposing sewage sludge highlight the need to find alternative routes to manage this waste. Biomass and residues like sewage sludge are the only renewable energy sources that can provide C and H, thus it is interesting to process them by means of treatments that enable to obtain chemically valuable products like fuels and not only heat and power; pyrolysis can be one of these treatments. The main objective of this review is to provide an account of the state of the art of sewage sludge pyrolysis for liquid production, which is under study during recent years. This process yields around 50 wt% (daf) of liquid. Typically, this liquid is heterogeneous and it usually separates into two or three phases. Some of these organic phases have very high gross heating values, even similar to those of petroleum-based fuels. The only industrial sewage sludge pyrolysis plant operated to date is currently closed due to some technical challenges and problems of economic viability. © 2012 Elsevier Ltd. All rights reserved. Source

Arias B.,CSIC - National Coal Institute | Grasa G.S.,CSIC - Institute of Carbochemistry | Abanades J.C.,CSIC - National Coal Institute
Chemical Engineering Journal | Year: 2010

It is well known that the solid sorbents used in calcium looping CO2 capture systems experience a reduction in carrying capacity with the number of cycles. Several sorbent reactivation schemes have been proposed as means of overcoming this deactivation process. This work analyzes the integration of a reactivation process in a Ca-looping cycle by means of a hydration reactor. The mass balances involved in this three-reactor systems must then be solved in order to evaluate the effect of the different variables on the average activity of the sorbent. The positive impact of reactivation by hydration (i.e. average increase in activity of the sorbent arriving at the carbonator) is discussed in conjunction with the negative impacts on the overall operation of the system (e.g. steam consumption, etc.) and on the large reactivation reactors. Two different scenarios employing different degrees of hydration have been evaluated. The results obtained show that steam is used more efficiently when only a small fraction of the circulating solids is hydrated to a high degree. Moreover, the performance of the sorbent reactivation step is better when low make up flows of limestone are used. © 2010 Elsevier B.V. Source

Arias B.,CSIC - National Coal Institute | Grasa G.S.,CSIC - Institute of Carbochemistry | Alonso M.,CSIC - National Coal Institute | Abanades J.C.,CSIC - National Coal Institute
Energy and Environmental Science | Year: 2012

This paper presents a novel sorbent regeneration technique for post-combustion calcium looping CO2 capture systems. The advantage of this technique is that it can drastically reduce the consumption of limestone in the plant without affecting its efficiency and without the need for additional reagents. The method is based on the re-carbonation of carbonated particles circulating from the carbonator using pure CO2 obtained from the gas stream generated in the calciner. The aim is to maintain the CO2 carrying capacity of the sorbent close to optimum values for CaL post-combustion systems (around 0.2). This is achieved by placing a small regeneration reactor between the carbonator and the calciner. This reactor increases slightly the conversion of CaO to carbonate so that it exceeds the so-called maximum CO2 carrying capacity of the sorbent. This increase compensates for the loss of CO2 carrying capacity that the solids undergo in the next calcination-carbonation cycle. Two series of experiments carried out in a thermogravimetric analyzer over 100 cycles of carbonation-recarbonation-calcination show that the inclusion of this recarbonation step is responsible for an increase in the residual CO2 carrying capacity from 0.07 to 0.16. A conceptual design of the resulting capture system shows that a limestone make-up flow designed specifically for a CO2 capture system can approach zero, when the solid sorbents purged from the CaL system are re-used to desulfurize the flue gas in the existing power plant. This journal is © 2012 The Royal Society of Chemistry. Source

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