Modeling the Contributions of Volatile and Char-Bound Nitrogen to the Formation of NO_<i>x</i> Species in Iron Ore Rotary Kilns

Given that more stringent NO_<i>x</i> emission limits are expected in the near future, several industrial processes are in need of NO_<i>x</i> mitigation measures. The Grate-Kiln process, applied in the iron ore industry, is one such process. NO_<i>x</i> formation is inherently high in the process, and due to the combustion conditions, several standard mitigation strategies are impractical. Alternative solutions are thus needed. The current paper aims at developing a model capable of describing the NO formation under conditions relevant in iron ore rotary kilns and to identify governing parameters that may be modified for mitigation purposes. The developed model uses detailed reaction modeling for the homogeneous combustion chemistry combined with simpler modeling with apparent kinetics for the heterogeneous chemistry. The main findings are that thermal NO is of low significance and that the NO formation during char combustion is the main contributor to the high NO_<i>x</i> emissions. Attempting to control the partitioning between the volatile nitrogen and the char-bound nitrogen is suggested as a mitigation strategy, since the combustion of char is challenging to control compared to the combustion of volatiles

Influence of Operating Conditions on SO₃ Formation during Air and Oxy-Fuel Combustion

Because SO₃ participates in both high- and low-temperature corrosion processes, there is a general concern about the SO₃ formation under oxy-fuel fired conditions. This work has the aim to evaluate the influence of combustion parameters on the formation of SO₃. Experiments were conducted in oxy-fuel and air-fired experiments with propane as fuel and injection of SO₂ in the oxidizer. The SO₃ concentration was measured with a controlled condensation method at the furnace outlet as well as in the flame. The experiments show that the gas-phase is an important contributor to SO₃ formation and that the SO₃ formation is strong during burnout of the fuel. In oxy-fuel combustion with wet flue-gas recycle (FGR), more SO₃ was formed than during dry FGR at similar temperature conditions, which indicates that H₂O enhances SO₃ formation. The experiments also show that the SO₃ formation rises with an increase in furnace temperature. Because temperature and residence time in the furnace increases with reduced FGR ratio, the FGR ratio directly influences the SO₃ formation in oxy-fuel combustion. This was obvious during the experiments, and the SO₃ concentration rose with a reduced FGR ratio

Modeling the Nitrogen and Sulfur Chemistry in Pressurized Flue Gas Systems

A rate-based model is developed to elucidate the chemistry behind the simultaneous absorption of NO_<i>x</i> and SO_<i>x</i> under pressurized conditions (pressures up to 30 bar) that are applicable to the flue gases obtained from CO₂ capture systems. The studied flue gas conditions are relevant to oxy-fuel and chemical-looping combustion systems. The kinetics of the reactions implemented in the model is based on a thorough review of the literature. The chemistry of nitrogen, sulfur, and N–S interactions are evaluated in detail, and the most important reaction pathways are discussed. The effects of pH, pressure, and flue-gas composition on the liquid-phase chemistry are also examined and discussed. Simulations that use existing kinetic data reveal that the pH level has a strong influence on the reaction pathway that is followed and the types of products that are formed in the liquid phase. In addition, the pressure level and the presence of NO_<i>x</i> significantly affect the removal of SO₂ from the flue gas

Characteristic scale of response for <i>Elater ferrugineus</i> using two models.

<p>i) Pooled density of trees within the groups <i>Quercus</i>, Noble 1 and Noble 2, and ii) density of <i>Quercus</i> only. The grey line indicates p<0.05, corresponding to Wald value 3.9.</p

Maps of predicted occurrence of <i>Elater ferrugineus.</i>

<p>The maps show >25%, >50%, >75% and >90% probability of occurrence in the study area (<b>a,b,c,d</b>) and in the county of Östergötland (<b>e,f</b>) and the trap capture. Empty traps are presented by crosses (×) while occupied traps are marked with open circles (Ο) which size is proportional to the number of individuals caught. The first column (<b>a,c,e</b>) shows predictions based on models including the pooled density of trees within the groups <i>Quercus</i>, Noble 1 and Noble 2, while the second column (<b>b,d,f</b>) shows predictions based on models including the density of <i>Quercus</i> only. (<b>a,b</b>) shows trap captures in systematically placed traps, (<b>c,d</b>) the strategically sampled validation data and (<b>e,f</b>) the validation data set sampled in the entire Östergötland. In each map, the predictions are based on two models, one for each characteristic scales of response (blue tones represent a smaller scale: 433 m (pooled density of <i>Quercus</i>, Noble 1 and Noble 2) and 327 m (density of <i>Quercus</i> only), while orange tones represent prediction at larger scale: 4051 m (<i>Quercus</i>, Noble 1 and Noble 2) and 4658 m (<i>Quercus</i>).</p

The relationships between occurrence of <i>Elater ferrugineus</i> and density of trees in different hollow classes.

<p>The relationships are expressed as Wald-values from 63 simple binomial GLMs. The explanatory variable ‘tree density’ is measured at 31 different spatial scales and includs trees from three different tree hollow classes: hollow trees ≥1 m dbh, hollow trees <1 m dbh and non-hollow trees. All models showed positive relationship between probability of occurrence and tree density. The grey line indicates p<0.05, corresponding to Wald value 3.9.</p

Categorization of trees in the study.

<p>Deciduous trees in the study area (49.4 km × 49.4 km) categorized according to the six tree groups and three tree hollow groups: ‘hollow’, which were then divided into two groups according to the diameter at breast height (dbh), and ‘non-hollow’ >0.70 m dbh.</p

Tree density required for 25%, 50% 75% and 90% probability of occurrence of <i>Elater ferrugineus</i>, for two candidate models and for two characteristic scales of response.

<p>Tree density required for 25%, 50% 75% and 90% probability of occurrence of <i>Elater ferrugineus</i>, for two candidate models and for two characteristic scales of response.</p

Trap locations in the county of Östergötland.

<p>(<b>a</b>) Trap location for systematically placed traps. Dots (•) represent occurrence of <i>E. ferrugineus</i> and crosses (×) represent non-occurrences. Thin lines delimit municipalities. (<b>b</b>) Trap locations of the two validation data sets. Filled circles (•) represent the strategic data set, sampled within the present study area and open circles (○) represent the Östergötland data set. Distributions of deciduous trees are marked in grey.</p

Correlation between the densities of tree groups that explained the occurrence of <i>Elater ferrugineus</i>.

<p>Correlation between the densities of tree groups that explained the occurrence of <i>Elater ferrugineus</i>.</p