Inorganic Chemistry in Combustion of Alternative Fuels

Inorganic chemistry in alternative fuels chemistry has vital implications in waste-to-energy (WTE) plants and biomass-fired plants. During thermochemistry conversion, the main releases of inorganic chemistry in the gas phase are alkali metals (mainly potassium), chlorine, and sulfur species that can form compounds (mainly KCl, KOH, and K₂SO₄) both in the gas phase and as aerosols. It can involve a high risk of high-temperature corrosion (HTC) and fouling problems in plants. Also, it may affect the emission of regulated compounds. Thus, it is an attractive to study the inorganic chemistry and the interactions with itself or/and regulated compounds gas (CO, NOx, SOx).

The influence of KCl on the moist oxidation of CO was investigated in this work. Experiments were performed in the absence of O₂ (gasification), as well as under reducing and fuel-lean conditions in a laminar flow quartz reactor at temperatures ranging from 873 to 1473 K, and for different quartz reactors in the experiments: clean (unused), used (exposed to CO oxidation), and dirty (exposed to KCl). The results indicated that the observed KCl presence inhibited the CO conversion to CO₂ under all conditions. Under gasification and reducing conditions, the reactor surface state significantly affects CO oxidation since the reactive surface catalyzes free radical recombination. Loss of hydrogen atoms on the wall, enhanced by exposure to KCl, strongly inhibits CO oxidation, with gas-phase inhibition playing a minor role. Under fuel-lean conditions, the surface state does not affect CO oxidation, and the observed inhibition can be attributed to gas-phase reactions. The chain-terminating step KO₂ + OH ⇄ KOH + O₂ is the key reaction for the accuracy of the prediction. Thus, the rate constant is calculated based on dipole-dipole interaction to capture the results. According to model analysis, the model predicted the effect of the inhibition of KCl on CO oxidation under oxidizing conditions satisfactorily.

To further investigate moist CO oxidation, the coupling between CO oxidation and sulfation of main alkali compounds (KCl or KOH) with SO₂ was studied. The experiment was conducted in a laboratory-scale setup with a quartz reactor at atmospheric pressure, and temperatures ranging from 873 to 1523 K. It was interpreted by a detailed chemical kinetic model for the K/S/Cl chemistry. The results showed that the presence of SO₂ significantly reduced the inhibiting effect of KCl and KOH on CO oxidation compared to the inhibition of CO oxidation with KCl and KOH addition. The sulfation degree of K-species (KCl and KOH) with SO₂ was evaluated by EDX analysis of collected particles on a filter downstream of the reactor and by FTIR analysis with consumption of SO₂ and the formation of HCl. The results indicated that the degree of KCl sulfation with SO₂ was small, in accord with the slight inhibition of CO conversion by KCl- SO₂ addition. For KOH presence, the EDX results showed full sulfation, since potassium silicates may be formed by KOH on the quartz surface of the reactor. A chemical kinetic model had been updated to validate the experimental results and the thermodynamic properties of some K-species were re-calculated. The prediction of the CO conversion behavior in the K-S-Cl species presence by the model yielded satisfactory results, while the model overpredicted the sulfation degree of KCl, as well as HCl and SO₂ released from KCl- SO₂.

To reduce corrosion and ash deposition in waste and biomass-fire power plants, the mitigation of highly corrosive KCl to less corrosive K₂SO₄ via alkali sulfation reaction in the boiler is an option. This study investigates the sulfation of  gaseous KCl with the addition of H₂SO₄ as a novel concept of the sulfur recirculation technique. An experiment was performed in a laminar flow quartz reactor at atmospheric pressure and variable temperatures ranging from 873 to 1523 K. In addition, to provide a better and deeper understanding of the hidden reaction mechanism in this work, the prediction with a detailed kinetic model for the K/S/Cl chemistry in the gas phase and condensed phase was investigated. The sulfation degree was evaluated by analysis of the elemental compositions (Cl, S, and K) of the collected particle samples on a filter downstream of the reactor. Moreover, HCl and SO₂ concentrations in the outlet were measured. The results indicate that the degree of KCl sulfation depends significantly on the reaction temperature and the reactant concentration ratio. KCl is sulfated efficiently by H₂SO₄ at the temperature range of 1073-1273 K, especially at a high proportion ofH₂SO₄/KCl, while the efficiency of sulfation drops sharply above 1300 K. According to the kinetic  analysis, initially, the H2SO4 is decomposed to SO₃ which then reacts directly with KCl converting it to gaseous K₂SO₄ through the  ntermediates KSO₃Cl and KHSO₄ (KCl→ KSO₃Cl→ KHSO₄ → K₂SO₄). During cooling, gaseous K₂SO₄ is converted completely through the homogenous nucleation to condensed K₂SO₄. The prediction with the kinetic model yielded satisfactory results, providing the temperature window for the sulfation process. The consequence is that both the validation of the kinetic model and the experimental characterization support the potential evaluation of the sulfur recirculation process.