The capacity of calcined limestone to react repeatedly with CO2, according to CaO(cr) + CO2(g) = CaCO3(cr) (eq I), and also its regeneration in the reverse reaction have been studied in a small, electrically heated fluidized bed of sand, for five different limestones. The forward step of eq I is a promising way of removing CO2 from the exhaust of, for example, a coal-fired power station, ready for sequestration or as part of a scheme to generate H2 using an enhanced water−gas shift reaction. The reverse step regenerates the sorbent. The uptake of CO2 by CaO, produced by calcining limestone, was measured using a bed of sand fluidized by N2 at ∼1023 K. For each experiment, a small quantity of limestone particles was added to the hot sand, whereupon the limestone calcined to produce CaO. Calcination was completed in ∼500 s for particles of a mean diameter of ∼600 μm. Next, CO2 was added to the fluidizing nitrogen to carbonate the CaO for ∼500 s. Measurements of [CO2] in the off-gases enabled the rates of calcination and the subsequent carbonation to be measured as functions of time. Many successive cycles of calcination and carbonation were studied. The forward step of reaction I is shown to exhibit an apparent final conversion, which decreases with the number of cycles of reaction; the final conversion fits well to a correlation from the literature. The reverse (calcination) reaction always proceeded to completion. Particles of limestone, removed from the reactor after several cycles, in either their partially carbonated or fully calcined state, were studied using X-ray diffraction, gas adsorption analysis, mercury porosimetry, and scanning electron microscopy. It was found that the carrying capacity of CaO for CO2 on the nth cycle of carbonation was roughly proportional to the voidage inside pores narrower than ∼150 nm in the calcined CaO before carbonation began. Thus, morphological changes, including reduction in the volume of pores narrower than 150 nm within a calcined limestone, were found to be responsible for much of the fall in conversion of reaction I with increasing numbers of cycles. The rate of attrition of the particles of limestone in a fluidized bed, while cycling between the calcined and carbonated states, was also studied. It was found that most limestones lost less than 10% of their mass due to attrition over the course of a typical experiment, lasting ∼8 h.

No abstract

No abstract

No abstract

A chemical looping process, which uses a packed bed of the various oxides of iron, has been formulated to produce separate, pure streams of H 2 and CO 2 from syngas. The process has the following stages: (1) Reduction of Fe 2 O 3 to Fe 0.947 O in the syngas from gasifying coal or biomass. This stage generates pure CO 2 , once the water has been condensed. (2) Subsequent oxidation of Fe 0.947 O to Fe 3 O 4 using steam, to simultaneously produce H 2 . (3) Further oxidation of Fe 3 O 4 to Fe 2 O 3 using air to return the oxide to step 1. Step 1 was studied here using a mixture of CO + CO 2 + N 2 as the feed to a packed bed of iron oxide particles, while measuring the concentrations of CO and CO 2 in the off-gas; step 2 was investigated by passing steam in N 2 through the packed bed and measuring the quantity of H 2 produced. The third step simply involved passing air through the bed. Reduction to Fe, rather than Fe 0.947 O, in step 1 gave low levels of H 2 in step 2 after 10 cycles of reduction and oxidation and led to the deposition of carbon at lower temperature. Step 3, i.e. reoxidizing the particles in air to Fe 2 O 3 , led to no deterioration of the hydrogen yield in step 2 and benefited the process by (i) increasing the heat produced in each redox cycle and (ii) preventing the slip of CO from the bed in step 1. The proposed process is exothermic overall and very usefully generates separate streams of very pure H 2 and CO 2 without complicated separation units.

Coprecipitation and hydrolysis of CaO have been employed to produce Ca-based synthetic sorbents suitable for capturing CO 2 in a fluidized bed. Their composition, CO 2 uptake, volume in small pores (2-200 nm) and resistance to attrition were measured and compared to those of limestone and dolomite. Sorbents produced by hydrolysis showed the highest uptake and resistance to attrition. After 20 cycles of carbonation and calcination, two sorbents exceeded the uptake of both limestone and dolomite, when subjected to the same regimes of reaction. A sorbent's uptake of CO 2 was shown to be determined by the volume in pores narrower than ∼200 nm.On a eu recoursà la co-précicipation età l'hydrolyse de CaO pour produire des sorbants convenantà la capture de CO2 dans un lit fluidisé. Leur composition, le retrait de CO 2 , le volume dans les petits pores (2-200 nm) et la résistanceà l'attrition ontété mesurés et comparésà ceux du calcaire et de la dolomite. Les sorbants produits par hydrolyse montre l'absorption et la résistanceà l'attrition les plusélevées. Après 20 cycles de carbonisation et calcination, deux sorbants dépassent la capacité de retrait du calcaire et de la dolomite, lorsqu'on applique les mêmes régimes de réaction. On montre que le retrait de CO 2 par un sorbant est déterminé par le volume des pores plusétroit que 200 nm.

No abstract

No abstract