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.
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.
Gasification and chemical-looping combustion experiments with a lignite (Hambach) char are reported, using an electrically heated fluidized bed in a 25 mm diameter tube at about 1073 K. The fluidizing gas was CO2 in nitrogen, with either batch or continuous feed of char into a bed of sand or particles of Fe2O3. The experiments were also modeled using the two-phase theory of fluidization; a well-mixed particulate phase is assumed, and the bubble flow is augmented by gasification products. This was combined with Langmuir−Hinshelwood (L−H) kinetics of gasification deduced from the experiments on gasification, with CO2, of this char in sand. These L−H kinetics are complex; the rate constants are different for two ranges of partial pressure of CO2: (i) from 0 to 0.05 bar and (ii) from 0.05 to 0.9 bar. The theory gives good predictions of (i) off-gas concentrations of CO and CO2 and (ii) accumulation of carbon in the bed. Combustion also occurred when the char was fed into a bed of Fe2O3 particles fluidized by nitrogen, giving the appearance of a solid−solid reaction, i.e., char oxidized by Fe2O3. The occurrence of a solid−solid reaction is however very unlikely, and it is believed that the reaction actually occurs via gaseous intermediates, CO and CO2, and is triggered by small amounts of oxygen in the char or air entrained with the char. This hypothesis is well-supported by the theoretical model. The results are particularly relevant for the chemical-looping combustion of char in Fe2O3.
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