Thermodynamic Investigation of Carbon Deposition and Sulfur Evolution in Chemical Looping Combustion with Syngas

Abstract

Chemical looping combustion (CLC) with syngas, a synthesized gas mixture of CO, H 2 , CO 2 , H 2 O(g), N 2 , and H 2 S, was investigated using thermodynamic simulation, with focus on carbon deposition and sulfur evolution in CLC. Five metal oxides, such as NiO, CuO, Fe 2 O 3 , Mn 3 O 4 , and CoO, were selected as oxygen carriers for CLC application. Different influencing factors on the formation of carbon deposits were investigated, including pressure, fuel reactor (FR) temperature, oxygen excess number Φ (denoting the availability of lattice oxygen in the oxygen carrier to the fuel), and fuel gas composition. Higher temperature and larger oxygen excess number Φ inhibited the formation of carbon deposits while the pressurized condition caused the opposite. The increase of H 2 O(g) and CO 2 fraction in syngas reduced carbon deposition while, in contrast, a larger H 2 S occurrence in syngas led to more carbon deposits to be formed. A sensitivity analysis to the different factors revealed that carbon deposition was mainly determined by the FR temperature and the oxygen carriers provided while other factors played a minor role. In addition, the predominant C-bearing species and their distributions at different temperatures were thermodynamically investigated. At low FR temperature and oxygen-deficient condition (i.e., oxygen excess number Φ < 1), the predominant carbon species as solid deposits were mainly elemental carbon or carbonates for NiO, CuO, Fe 2 O 3 , and CoO while MnC 2 and MnCO 3 were the main species for Mn 3 O 4 . In terms of the evolution of sulfur in CLC with syngas containing a basic composition of CO, N 2 , H 2 , and H 2 S, the low pressure, high temperature, and adequate lattice oxygen would make more sulfur species form in the gas phase. After that, CO 2 and H 2 O(g) were introduced into the syngas, and they were found to possibly serve as additional oxidizers to convert H 2 S into SO 2 . The oxidation function of CO 2 was slightly stronger than that of steam. Again, the evolution and distribution of various sulfur species was studied. For four metal oxides (NiO, Fe 2 O 3 , Mn 3 O 4 , and CoO), the most possible solid sulfur compounds were Ni 3 S 2 , Fe 0.84 S, MnSO 4 , and Co 0.89 S, respectively. But for CuO, at Φ < 1, Cu 2 S was the main solid sulfur compound while at Φ > 1 CuSO 4 and Cu 2 SO 4 dominated.

Performance of a highly reactive impregnated Fe2O3/Al2O3 oxygen carrier with CH4 and H2S in a 500Wth CLC unit

“…Moreover, other factors like kinetics of sulfur compounds can affect the sulfur gas product distribution in the CLC system predicted by thermodynamics. Finally, it is important to note that the conclusions found by Jerndal et al [3] and Wang et al [18] Up to now, the effect of sulfur on the performance of Fe-based oxygen carriers in continuous CLC units has not been studied yet. In this sense, more tests in continuous facilities are needed to gain an adequate understanding of the behavior and usefulness of these particles for this kind of combustion technology.…”

“…Jerndal et al [3] and Wang et al [18] concluded that there was no risk of iron sulfide or iron sulfate formation under usual CLC process conditions in terms of temperature, concentration of sulfur-containing gases or oxygen excess. However, previous works in our research group [19,20] found that the fate of sulfur in a CLC process could not be predicted based only with thermodynamic analyses.…”

Syngas combustion in a chemical-looping combustion system using an impregnated Ni-based oxygen carrier

“…Nickel materials have received more attention due to its higher reactivity and thermal stability. Wang et al [27] studied the carbon formation and the effect of the presence of sulphur in the syngas, and they found that the pressurized conditions led to more solid carbon and sulphur species deposited on the OC. Also, some experimental data at pressurized conditions can be found.…”

Fabrication of Fe2O3/TiO2 Oxygen Carriers for Chemical Looping Combustion and Hydrogen Generation

“…The carbon deposited oxygen carrier particles can contribute to producing CO as described Eq. (6) in the H 2 generation process (Ishida et al, 1998;Jerndal et al, 2006;Wang et al, 2008), lowering the purity of H 2 .…”

“…The combustion of syngas was noticeable for experiment conducted at 400°C, and reached 100% for experiments conducted at temperatures higher than 500°C. Based on the thermodynamic analysis by previous researchers (Wang et al, 2008;Li et al, 2010), CO and H 2 can be completely converted to CO 2 and H 2 O through the exothermic reaction with Fe 2 O 3 (or Fe 2 TiO 5 ) at temperatures ranging from 200 to 1200°C. However, the reaction rates for experiments conducted at 400°C (or lower) might be too slow to reach equilibrium conversion.…”