Calcium looping

Calcium looping is based on the reversible reaction between CaO and C02 to form calcium carbonate. 

Calcium looping consists of two fluidised bed reactors, namely carbonator and calciner. In the... 

Figure 17.6 illustrates a gasification process integrated with the calcium looping process. Once the water gas mixture is formed at the exit of the gasifier, calcium oxide fines are injected into the fuel gas stream. As the fuel gas flows past the WGS catalyst, the WGS reaction takes place and forms additional C02. The injected CaO sorbent particles react with C02 and H2S in the gas stream, thereby allowing further catalytic WGS reaction to occur. The reactions involved in the calcium looping scheme are... 

Conceptual flowsheet depicting integration of various units in the calcium looping processes for H2 generation in typical coal-gasifier facility. 

We would also like to stress the importance of additional requirements related to the physical properties such as the mechanical strength of potential CO2 sorbents. The calcium looping process will most likely be operated in a circulating fluidized bed reactor set-up. This requires the development of attrition-resistant materials. However, the mechanical strength or attrition resistance of new developed CO2 sorbents is rarely assessed and, thus, requires more attention if the materials are to be relevant for practical applications. 

Manovic V, Wu Y, He I, Anthony EJ (2012) Spray water reactivation/peUetization of spent CaO-based sorbent from calcium looping cycles. Environ Sd Technol 46 12720-12725... 

Blarney, J., Anthony, E., Wang, J. and Fennell, P. (2010) The calcium looping cycle for large-scale CO2 capture. Progress in Energy and Combustion Science, 36, 260-279. 

Additional phosphonic acid is derived from by-product streams. In the manufacture of acid chlorides from carboxyUc acids and PCl, phosphonic acid or pyrophosphonic acid is produced, frequentiy with copious quantities of yellow polymeric LOOP. Such mixtures slowly evolve phosphine, particularly on heating, and formerly were a disposal problem. However, purification of this cmde mixture affords commercial phosphonic acid. By-product acid is also derived from the precipitate of calcium salts in the manufacture of phosphinic acid. As a consequence of the treatments of the salt with sulfuric acid, carbonate is Hberated as CO2 and phosphonic acid goes into solution. 

Parvalbumin is a muscle protein with a single polypeptide chain of 109 amino acids. Its function is uncertain, but calcium binding to this protein probably plays a role in muscle relaxation. The helix-loop-helix motif appears three times in this structure, in two of the cases there is a calcium-binding site. Figure 2.13 shows this motif which is called an EF hand because the fifth and sixth helices from the amino terminus in the structure of parvalbumin, which were labeled E and F, are the parts of the structure that were originally used to illustrate calcium binding by this motif. Despite this trivial origin, the name has remained in the literature. 

Figure 2.12 Two a helices that are connected by a short loop region in a specific geometric arrangement constitute a helix-turn-helix motif. Two such motifs are shown the DNA-binding motif (a), which is further discussed in Chapter 8, and the calcium-binding motif (b), which is present in many proteins whose function is regulated by calcium.

The loop region between the two a helices binds the calcium atom. Carboxyl side chains from Asp and Glu, main-chain C =0 and H2O form the ligands to the metal atom (see Figure 2.13b). Thus both the specific main-chain conformation of the loop and specific side chains are required to provide the function of this motif. The helix-loop-helix motif provides a scaffold that holds the calcium ligands in the proper position to bind and release calcium. 

Calcium-binding residues are brown, and residues that form the hydrophobic core of the motif are light green. The helix-loop-helix region shown underneath is colored as in Figure 2.13. 

Figure S.28 Schematic diagrams of the two-sheet P helix. Three complete coils of the helix are shown in (a). The two parallel P sheets ate colored gieen and red, the loop regions that connect the P strands ate yellow, (b) Each stmctuial unit Is composed of 18 residues forming a P-loop-P-loop structure. Each loop region contains six residues of sequence Gly-Gly-X-Gly-X-Asp where X is any residue. Calcium Ions are bound to both loop regions. (Adapted from F. Jumak et al., Ciirr. Opin. Struct. Biol. 4 802-806, 1994.)...

The basic structural unit of these two-sheet p helix structures contains 18 amino acids, three in each p strand and six in each loop. A specific amino acid sequence pattern identifies this unit namely a double repeat of a nine-residue consensus sequence Gly-Gly-X-Gly-X-Asp-X-U-X where X is any amino acid and U is large, hydrophobic and frequently leucine. The first six residues form the loop and the last three form a p strand with the side chain of U involved in the hydrophobic packing of the two p sheets. The loops are stabilized by calcium ions which bind to the Asp residue (Figure S.28). This sequence pattern can be used to search for possible two-sheet p structures in databases of amino acid sequences of proteins of unknown structure. 

Figure 12.7 Ribbon diagram of one subunit of potin from Rhodobacter capsulatus viewed from witbin tbe plane of tbe membrane. Sixteen p strands form an antiparallel p barrel tbat traverses tbe membrane. Tbe loops at tbe top of tbe picture are extracellular whereas tbe short turns at tbe bottom face the periplasm. The long loop between p strands 5 and 6 (red) constricts the channel of the barrel. Two calcium atoms are shown as orange circles. (Adapted from S.W. Cowan, Curr. Opin. Struct. Biol. 3 501-507, 1993.)...

Fi re 12.8 Schematic diagram of the trimerlc porin molecule viewed from the extracellular space. Blue regions illustrate the walls of the three porin barrels, the loop regions that constrict the channel are red and the calcium atoms are orange. 

Nonrepetitive but well-defined structures of this type form many important features of enzyme active sites. In some cases, a particular arrangement of coil structure providing a specific type of functional site recurs in several functionally related proteins. The peptide loop that binds iron-sulfur clusters in both ferredoxin and high potential iron protein is one example. Another is the central loop portion of the E—F hand structure that binds a calcium ion in several calcium-binding proteins, including calmodulin, carp parvalbumin, troponin C, and the intestinal calcium-binding protein. This loop, shown in Figure 6.26, connects two short a-helices. The calcium ion nestles into the pocket formed by this structure. 

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