Regenerative Thermal Oxidation of Airborne N, N-Dimethylformamide and Its Associated Nitrogen Oxides Formation Characteristics
Abstract In this study, a two-bed electrically heated regenerative thermal oxidizer (RTO) was used to test the thermal destruction and oxides of nitrogen (NOx) formation characteristics in burning airstreams that contain either N, N-dimethylformamide or dimethylformamide (DMF) mixed with methyl ethyl ketone (MEK). The RTO contained two 0.152 m × 0.14 m × 1 m (L × W times] H) beds, both packed with gravel particles with an average diameter of approximately 0.0111 m and a height of up to 1 m with a void fraction of 0.42 in the packed section. The thermal recovery efficiency (TRE) and the gas pressure drop over the beds were also studied. Experimental results reveal that, with a valve shifting time (t s) of 1.5 min, a superficial gas velocity (U g) of 0.39 m/sec (evaluated at an influent air temperature of around 30 °C) and preset maximum destruction temperatures (T S) of 750–950 °C, no NOx was present in the effluent gas from the RTO when it was loaded with DMF-free air. When only DMF was present in the influent air, the average destruction efficiencies exceeded 96%, and increased with the influent DMF concentration from 300 to 750 mg/N∙m3. The “NOx-N formation/DMF-N destruction” mass ratios were in the range 0.76–1.05, and decreased as the influent DMF concentration increased within the experimental range. When both DMF and MEK were present in the influent gas, the NOx formation ratio was almost the same and the DMF destruction efficiency increased with the influent MEK/DMF ratio from 150/300 to 4500/300 (mg/mg) and in the preset temperature range. The NOx formation ratios were in the range 0.75–0.96. The TRE decreased as U g increased but was invariant with T s. The Ergun equation was found to suffice in the estimation of the pressure drop when the gas flowed over the packing beds.
Reverse flow reactors as sustainable devices for performing exothermic reactions: Applications and engineering aspects
Abstract Reverse flow reactors are fixed-bed reactors combining in a single intensified device chemical reaction and regenerative heat transfer (energy is stored in a bed as sensible heat). To accomplish this goal, reverse flow reactors are operated in a forced unsteady state created by periodically changing the flow direction. The most important applications of reverse flow reactor are reviewed: oxidation of hydrocarbons and sulphur dioxide, and selective catalytic reduction of nitrogen oxides. These applications involve exothermic reactions. However, recent developments have also made possible it possible the use of reverse flow reactors with endothermic reactions, such methane steam reforming. The modelling of reverse flow reactors is addressed based on the models available for fixed-bed reactors. Practical considerations regarding reverse flow reactors are considered: the selection of the type of bed, the issues of the use of lab-scale devices at dynamic conditions, the assessment of autothermal operation and heat extraction and the integrated adsorption concept. The latter is an innovative concept based on the periodical adsorption in the bed of some of the reactants, products or other feed compounds. This mass regeneration can be combined with the heat regeneration capabilities of reverse flow reactors to increase the degree of process intensification.
Factors Affecting NOx Formation from Nitrogen-Containing Fuels
Abstract Data from a turbulent diffusion flame burner indicate that the summation of fixed-nitrogen species (NO + HCN + NHi) is a significant measure of a flame's emission potential. Raising the flame temperature by preheating of the inlet air causes a reduction in the summation of fixed-nitrogen species in rich flames. This reduction occurs over only a narrow range of equivalence ratios and only for flame conditions with low concentrations of unburned hydrocarbons at burner exhaust. Therefore, the fuel and air must be intensely mixed at the entrance of the burner. Flames resulting in soot formation heavy enough to turn the flame bright yellow yield high concentrations of unburned hydrocarbons and HCN. For rich flames this results in a high summation of fixed-nitrogen species, because HCN is the dominant nitrogen containing species of these flames. The distribution of fixed-nitrogen species leaving the burner is independent of the form of the nitrogen in the fuel; the reduction of fixed-nitrogen species with increased air preheat occurs at fuel nitrogen concentrations as high as 5%.