M.U. Alzueta

Theoretical study of the influence of mixing in the SNCR process. Comparison with pilot scale data

Abstract A theoretical study of the influence of mixing on the selective non-catalytic reduction (SNCR) process has been performed. The study includes the use of a detailed kinetic reaction mechanism together with a simple approach for mixing based on the “maximum mixedness model” proposed by Zwietering (1959) . Two different configurations for that simple mixing approach have been considered and discussed, i.e. the “direct approach” which implies entrainment of the jet flow (containing the SNCR reduction agent) into a bulk flow (containing the products of the main combustion zone); and the “reverse approach” which represents entrainment of the bulk flow into the jet stream. The main features of both approaches applied to the SNCR process are analyzed and discussed. Furthermore, comparison of the results obtained with the present approaches with different experimental pilot scale results is performed.

Experimental and Kinetic Study of the Interaction of a Commercial Soot with NO at High Temperature

An experimental and kinetic study at high temperature was made of the interaction of NO with Printex-U, a commercial carbon black, considered as diesel soot model compound. Foremost, a characterization of this soot was performed by using different techniques such as elemental analysis, BET surface area analysis, SEM, TEM, and XRD. Two series of experiments were carried out. The first one took into account the inlet NO concentration influence (from 200 to 2000 ppm) at a given temperature of 1273 K. In the second one, the temperature influence (from 1173 to 1373 K) at a given NO concentration of 2000 ppm was considered. From the experimental results, a strong effect of the inlet NO concentration and temperature was observed. A kinetic model, the shrinking core model with decreasing size particle and chemical reaction control, was successfully applied, obtaining a fractional reaction order of 0.26 with respect to NO and activation energy of 110 kJ/mol.

Acetylene soot reaction with NO in the presence of CO.

The heterogeneous reaction of soot with NO can be considered as a means of reduction of the emissions of both pollutants from combustion systems. In this paper, the influence of the presence of CO in the soot-NO reaction is studied. Soot was obtained by pyrolysis at 1373 K of 5000 ppmv acetylene in nitrogen. The study of the influence of CO on the soot-NO reaction was performed in experiments fixing NO concentration at 900 ppmv and introducing different CO concentrations among 0 and 9900 ppmv. An increase in both the carbon consumption rate and NO reduction by acetylene soot was observed as the concentration of CO increases. These results can be explained by the oxide-stripping reaction, CO+C(f)(O)-->CO(2)+C(f). The direct reaction of CO with NO catalyzed by the carbon surface, CO+NO-->CO(2)+1/2N(2) may not be considered in this case the dominant process due to the absence of mineral impurities in the acetylene soot. The influence of CO in the acetylene soot-NO reaction was also tested in the presence of oxygen (250-5000 ppmv). In these conditions and for relatively high CO/O(2) ratios, CO seems to also contribute to NO reduction by the previous oxide-stripping reaction.

EFFECT OF RECIRCULATION GASES ON SOOT FORMED FROM ETHYLENE PYROLYSIS

Flue gas recirculation (FGR) is an effective technology both to control NOx emissions during combustion processes and to control temperature and make-up for the volume of the missing N2 during oxy-fuel combustion processes. In this article, a study of the individual role of the main products that are expected to form part of the recycled flue gas (CO, H2, H2O, and CO2) on soot and gas products formed during the thermal decomposition of ethylene-additive gas mixtures in the 975–1475 K temperature range is reported. Experimental results obtained in a quartz flow reactor are examined with the main objective of assessing the effectiveness of each gas additive in suppressing or boosting soot formation. Additionally, experimental data have been interpreted in terms of a literature detailed gas-phase kinetic model to analyze the evolution of gas products and get a better understanding of the gas-phase processes involving the thermal decomposition of the ethylene-additive gas mixtures, although soot formation reactions are not included in such mechanism.

An Approach to the Analysis of Mixing in Reactive Systems

A simple mixing approach for use in reactive systems is considered. This mixing approach has so far been applied to reburning and selective non-catalytic reduction (SNCR) processes, two well-known techniques to reduce NO x emissions. Detailed kinetic models together with the mixing approaches, based on the work of Zwietering, are used to simulate both the chemistry and mixing of the reactants. Two different configurations for the mixing approach have been considered: the so-called direct and reverse approach. The study includes a comparison between different experimental results obtained in pilot installations and theoretical results calculated with the present approach.

Formation of Polycyclic Aromatic Hydrocarbons in the Pyrolysis of 2-Methylfuran at Different Reaction Temperatures

ABSTRACT The formation of soot and 16 priority polycyclic aromatic hydrocarbons (PAH) has been studied during the pyrolysis of 2-methylfuran (2-MF), with a constant 2-MF inlet concentration at different reaction temperatures (975–1475 K) in a tubular flow reactor installation. Light gases and soot were measured at the reactor outlet. The concentration of the target PAH was determined according to their distribution in different phases (gas phase, adsorbed on soot, and stuck on reactor walls). This work revealed that PAH were mainly adsorbed on a soot surface. Only naphthalene was present in major amounts in the gas phase. The carcinogenic equivalence sum (KE) related to PAH concentration was also evaluated, and the highest value was found in the PAH collected from the soot and reactor walls at 1175 K. The results showed that, while more soot is formed by increasing the temperature, the PAH yield exhibits a maximum in the temperature range considered.

Experimental study and modeling of the influence of the inlet no concentration in the natural gas reburning process

Publisher Summary This chapter discusses the experimental study and modeling of the influence of the inlet NO concentration in the natural gas reburning process. Significant NO and NO 2 (NOx) amount is produced during coal combustion. Among the different methods, reburning with natural gas is an attractive technique that can be applied to existing or new coal boilers, with few modifications and low investment costs. The reburning process divides the boiler into three zones. In the primary zone, the main combustion of coal is produced and the NO x and other typical combustion products are generated. Downstream, natural gas is injected into the so-called “reburning zone” to create a reducing environment where the NO x are broken down by the action of hydrocarbon radicals. The (NO)p value influences the final concentration of NO and other nitrogenous species.

Interaction of Soot–SO2: Experimental and Kinetic Analysis

ABSTRACT This study aims to evaluate the capability of SO2 to interact with soot and to determine the kinetics of this reaction under conditions of interest for combustion. The conditions of the soot reactivity experiments were: 1% SO2 with nitrogen to balance, around 10 mg of soot, and different reaction temperatures for each run: 1275, 1325, 1375, 1425, and 1475 K. Results demonstrate that SO2 does interact with soot. The evaluation of the soot reactivity has been based on the calculation of the time for the complete conversion of carbon through the employment of the Shrinking Core Model equations for decreasing size particle with chemical reaction control. The reactivity of soot with SO2 increased by a factor of about 3 when increasing the reaction temperature of the test from 1275 K to 1475 K. Kinetics in terms of Arrhenius parameters showed that the activation energy of the interaction of soot with SO2 was around 82 kJ/mol.

NOx EMISSION REDUCTION IN BIOMASS COMBUSTION SYSTEMS

ABSTRACT Significant amounts of NOx can be generated from different biomass combustion systems. This work focuses on the study of natural gas reburning, at temperatures lower than 1200 °C, as a NOx reduction method. The influence of the natural gas amount on the NO, HCN and TFN concentrations has been analyzed at temperatures of 1000 and 1100 °C, for a given O2 concentration. Also, the nitrogenous species concentrations at the outlet of the reburning zone are determined when the amounts of natural gas and oxygen are varied simultaneously, in order to consider the effect of the dilution.