Peter Glarborg

Fuel nitrogen conversion in solid fuel fired systems

Abstract Understanding of the chemical and physical processes that govern formation and destruction of nitrogen oxides (NO x ) in combustion of solid fuels continues to be a challenge. Even though this area has been the subject of extensive research over the last three decades, there are still unresolved issues that may limit the potential of primary measures for NO x control. In most solid fuel fired systems oxidation of fuel-bound nitrogen constitutes the dominating source of nitrogen oxides. The present paper reviews some fundamental aspects of fuel nitrogen conversion in these systems, emphasizing mostly combustion of coal since most previous work deal with this fuel. However, also results on biomass combustion is discussed. Homogeneous and heterogeneous pathways in fuel NO formation and destruction are discussed and the effect of fuel characteristics, devolatilization conditions and combustion mode on the oxidation selectivity towards NO and N 2 is evaluated. Results indicate that even under idealized conditions, such as a laminar pulverized-fuel flame, the governing mechanisms for fuel nitrogen conversion are not completely understood. Light gases, tar, char and soot may all be important vehicles for fuel-N conversion, with their relative importance depending on fuel rank and reaction conditions. Oxygen availability and fuel-nitrogen level are major parameters determining the oxidation selectivity of fuel-N towards NO and N 2 , but also the ability of char and soot to reduce NO is potentially important. The impact of fuel/oxidizer mixing pattern on NO formation appears to be less important in solid-fuel flames than in homogeneous flames.

Kinetic Modeling of Hydrocarbon/Nitric Oxide Interactions in a Flow Reactor

Abstract The reduction of nitric oxide by reaction with C1 and C2 hydrocarbons under reducing conditions in a flow reactor has been analyzed in terms of a detailed chemical kinetic model. The experimental data were partly adopted from previous work and partly obtained in the present study; they cover the temperature range 800–1500 K and the reburn fuels CH4, C2H2, C2H4, C2H6, and natural gas. Modeling predictions indicate that, under the conditions investigated, HCCO + NO and CH3 + NO are the reactions most important in reducing NO. The HCCO + NO reaction is the dominant reaction when using natural gas or C2 hydrocarbons as reburn fuels. This reaction leads partly to HCNO, which is recycled to NO, and partly to HCN, which is converted to N2 or NO. When methane or natural gas are used as reburn fuel, the CH3 + NO reaction contributes significantly to remove NO. Modeling predictions are in reasonably good agreement with the experimental observations for the fuels investigated, even though the NO reduction potential is overpredicted for methane and underpredicted for ethane. Modeling predictions for NO are very sensitive to the formation of HCCO and to the product branching ratio for the HCCO + NO reaction. Furthermore, the present analysis indicates that more work is needed on critical steps in the hydrocarbon oxidation scheme.

Kinetic modeling and sensitivity analysis of nitrogen oxide formation in well-stirred reactors☆

Abstract We have modeled the experimental data of Bartok et al. and Duterque et al. on methane combustion in stirred reactors. A method to calculate the first-order sensitivities of the mole fractions and temperature with respect to the rate constants is discussed and applied to nitric oxide production. We have thus been able to evaluate the nitrogen chemistry in the presence of hydrocarbons under stirred conditions. We find the extended Zeldovich mechanism to be the major source of NO under lean conditions, while the prompt-NO formation is dominant under fuel-rich conditions. The important features of the model under fuel-rich conditions are the following: 1. 1. The reaction CH + N2 ⇄ HCN + N is the only important initiating step in the prompt-NO formation. 2. 2. The CH concentration is established through the sequence CH 3 + ⇄ CH 2 + HX , CH 2 + ⇄ CH + HX , CH 2 + X ⇄ C + OH , where X is H or OH. 3. 3. Nitric oxide is recycled back to CN and HCN through reactions with C, CH, and CH2. This results in the exhaust of significant quantities of HCN from the reactor.

Impact of SO2 and NO on CO oxidation under post-flame conditions

An experimental and theoretical study on the effect of SO2 on moist CO oxidation with and without NO present has been carried out. The experiments were performed in an isothermal quartz flow reactor at atmospheric pressure in the temperature range 800–1300 K. Inlet concentrations of SO2 ranged from 0 to 1800 ppmv, while the NO ranged between 0, 100, or 1500 ppm. SO2 inhibits CO oxidation under the conditions investigated, shifting the fast oxidation regime 20–40 K towards higher temperatures at 1500 ppm SO2. The inhibition is most pronounced at high O atom levels. The experimental data supported by model analysis suggest that SO2 primarily reacts with O atoms forming SO3, which is subsequently consumed mainly by reaction with O and HO2. Addition of NO significantly diminishes the effect of SO2. Since NO is usually present in combustion flue gases, the impact of SO2 on CO burnout in most practical systems is projected to be small. The H/S/O thermochemistry and reaction subset has been revised based on recent experimental and theoretical results, and a chemical kinetic model has been established. The model provides a reasonable overall description of the effect of SO2 and NO on moist CO oxidation, while the SO3/SO2 ratio is well predicted over the range of conditions investigated. In order to enhance model performance further, rate constants for a number of SO2 and SO3 reactions need to be determined with higher accuracy.

Influence of process parameters on nitrogen oxide formation in pulverized coal burners

This paper describes the influence of burner operating conditions, burner geometry and fuel parameters on the formation of nitrogen oxide during combustion of pulverized coal. Main attention has been paid to combustion test facilities with self-sustaining flames, while extensions have been made to full-scale boilers and furnace modeling. Since coal combustion and flame aerodynamics have been reviewed earlier, these phenomena are only treated briefly.

Experimental and kinetic modeling study of the oxidation of benzene

The oxidation of benzene under flow-reactor conditions has been studied experimentally and in terms of a detailed chemical kinetic model. The experiments were performed under plug-flow conditions, at excess air ratios ranging from close to stoichiometric to very lean. The temperature range was 900–1450 K and the residence time of the order of 150 ms. The radical pool was perturbed by means of varying the concentration of water vapor and by adding NO. Furthermore, a few experiments were conducted on pyrolysis and oxidation of phenol. Benzene oxidation is initiated at ∼1000 K; the temperature for complete oxidation depends on stoichiometry, ranging from 1100 K (very lean conditions) to 1300 K (close to stoichiometric). The water vapor level and the presence of NO have only a minor impact on the temperature regime for oxidation. The proposed chemical kinetic model was validated by comparison with the present experimental data as well as flow reactor and mixed reactor data from literature. The model provides a reasonably good description of the overall oxidation behavior of benzene over the range of conditions investigated. However, before details of the oxidation behavior can be predicted satisfactorily, a number of kinetic issues need to be resolved. These include product channels and rates for the reactions of phenyl and cyclopentadienyl with molecular oxygen as well as reaction chemistry for the oxygenated cyclic compounds formed as intermediates in the oxidation process.

Inhibition and sensitization of fuel oxidation by SO2

Abstract An experimental and theoretical study of the interaction of SO 2 with the radical pool under combustion conditions has been carried out. Experiments on moist CO oxidation were conducted in an isothermal quartz flow reactor at 1 atm; temperature ranged from 800 to 1,500 K and stoichiometries from fuel-rich to very lean. In addition, literature data on sulfur species concentration profiles and H atom decay in fuel-rich H 2 /O 2 flames doped with SO 2 were analyzed. The results show that under flow-reactor conditions SO 2 may inhibit or promote oxidation of fuel, depending on conditions. In a narrow range of operating conditions close to stoichiometric SO 2 promotes oxidation through the sequence: SO 2 + H ⇌ SO + OH, SO + O 2 ⇌ SO 2 + O. Inhibition of oxidation by removal of radicals can be explained in terms of the SO 2 +O+M reaction, even under fuel-rich conditions. From the shift in temperature for the onset of CO oxidation because of SO 2 addition under reducing conditions an upper limit of 3.0 × 10 14 cm 6 mol −2 s −1 at 1,060 K can be estimated for the rate constant of H + SO 2 + N 2 ⇌ HOSO + N 2 . This value is consistent with a significant barrier to reaction as proposed theoretically, but an order of magnitude lower than indicated by both ab initio calculations (Marshall and co-workers) and reaction rates derived from flames. However, we find that data on H atom decay in flames doped with SO 2 are not suitable for deriving rate constants because of uncertainty in important side reactions involving SO. Furthermore, we propose that the enhanced H atom decay observed in these flames may be attributed to recombination of H atoms with SO and S 2 species, rather than to a mechanism initiated by the H + SO 2 + M reaction.

Low temperature oxidation of methane: the influence of nitrogen oxides

An experimental investigation of methane oxidation in the presence of NO and NO2 has been made in an isothermal plug-flow reactor at 750-1250K. The temperature for on-set of oxidation was lowered by 250 K in the presence of NO or NO2 at residence times of 200 ms. At shorter residence times (140 ms) this enhancement effect is reduced for NO but maintained for NO2. Furthermore two temperature regimes of oxidation separated by an intermediate regime where only little oxidation lakes place exist at residence times of 140 ms, if NO is the only nitrogen oxide initially present. The results were explained by the competition between three reaction paths from CH3 to CH2O. A direct high temperature path (A), a two-step NO2 enhanced low temperature path (B) and a slow three step NO enhanced path (C), which may produce NO2 to activate path B. The negative temperature coefricient behaviour was explained by a competition between paths (A) and (C). A previously published reaction set was modified to take these reaction patterns into account. The processes discussed here presumably occur to some extent in the exhaust of natural gas engines, but conditions may be modified, either to ensure an enhanced activity to oxidize methane in the exhaust, or alternatively to decrease the activity 10 reduce the production of unwanted intermediates such as formaldehyde.

Nitrogen chemistry during burnout in fuel-staged combustion

Abstract A parametric study of the chemistry of the burnout zone in reburning has been performed in laboratory plug flow reactors in the temperature range 800–1350 K. Inlet mole fractions of NO, NH3, HCN, CO, and O2 were varied, together with different temperatures and residence times to simulate reaction conditions in practical systems. Under lean conditions, a minimum in NO emission exists as a function of temperature. Both HCN and NH3 can act as either NO reductants or as sources for NO by oxidation. Reactions and selectivities for HCN and NH3 are controlled by the radical pool produced by fuel (CO) oxidation. As increasing amounts of CO were added, temperatures for both ignition and the minimum in NO became lower. At 2% CO, 4% O2, and 100 ms residence time, the minimum in NO was found at approximately 1000 K. At low temperatures, significant amounts of N2O were measured in the reactor outlet. This is attributed to N2O formation by HCN/NO reactions and to the slow decomposition of N2O at these temperatures. Large reductions in NO were seen under fuel-rich conditions and at high temperatures. The observed NO reduction was very much dependent on the inlet mole fraction of O2. Detailed chemical kinetic modeling of the experiments showed reasonable predictions for overall fuel-lean conditions, but the model failed to predict experimental results under fuel-rich conditions. The present results provide guidelines for optimizing the conditions for the burnout process of reburning, as well as other processes for NOx reduction by staged combustion. The results also provide a test basis for verifying kinetic models for nitrogen chemistry at low temperatures (800–1350 K).

Low temperature interactions between hydrocarbons and nitric oxide: An experimental study☆

Abstract Gas reburning is a NOx control technique that can be applied in different combustion systems. In it, gas is injected into the furnace downstream of the primary burners to form a substoichiometric zone followed by the injection of burnout air. The variables that influence the chemistry in the reducing zone (temperature between 900 and 1450 K, reburn fuel type, stoichiometry, NO concentration and residence time), which is mainly controlled by the interactions between hydrocarbon radicals and NO, have been studied in a laboratory scale flow reactor. A significant NO reduction efficiency can be obtained with proper operating conditions. The NO destruction is closely coupled to the hydrocarbon oxidation regime, which differs depending on the hydrocarbon fuel considered, and NO is to a significant extent converted to HCN. The concentration of hydrocarbons and oxygen, and the temperature are important parameters affecting the results obtained, whereas the inlet NO concentration and the residence time only have a minor effect. The practical implications of the present results for gas reburning are discussed.