Fuel Nitrogen Release Research Articles

Effect of Inherent Minerals on the Release of Fuel-Nitrogen during Coal Pyrolysis

The formation of NO x precursors (mainly HCN and NH 3 ) from the pyrolysis of Pingshuo coal with or without minerals was studied in a quartz fixed bed reactor. The coal sample in the reactor was heated in two kinds of fashion: at a slow heating rate by temperature-programmed and at a fast heating rate than constant temperature. The experimental results revealed that demineralization by acid washing drastically decreases nitrogen conversion to NH 3 at a fast heating rate and at a slow heating rate, but for HCN, its formation is different at fast heating and slow heating rates. The yield of HCN from demineralized coal is slightly lower than that from raw coal at a fast heating rate and it is the reverse at a slow heating rate. On the other hand, the pyrolysis experiments data of demineralized coal and raw coal added with ash from Pingshuo coal combustion also indicated that minerals in coal play an important role in gaseous-N formation, but the reaction mechanism is different for raw coal and demineralized coal.

Release of fuel-nitrogen during the gasification of Shenmu coal in O 2

Formation of NH 3, HCN, NO and NO 2 from the gasification of a Chinese coal (Shenmu) in a fluidised-bed/fixed-bed reactor in 4.1% O 2 in argon was investigated. The presence of O 2 even at 673 or 773 K could lead to the gradual gasification of char to form HCN and NH 3. The yields of NH 3 and HCN showed maxima at about 773 K, amounting to 40% of coal-N. In the presence of O 2, the H radicals required for the formation of NH 3 and HCN may be generated from the reactions in the gas phase and within the solid particles. The yields of NO x increased from 773 to 973 K. A non-negligible proportion (∼5%) of coal-N was converted into NO 2 at 773 K through homogeneous and heterogeneous reactions. The yield of NO 2 diminished with increasing temperature. At 1173 K, the reactions between char-N and H 2O (or O/H-containing radicals) formed from the oxidation of char and volatiles led to the formation of NH 3 but not HCN. This route of NH 3 formation is believed to be an important pathway for the formation of NH 3 in an air-blown gasifier.

A study of combustion behavior of pulverized coal in high-temperature air

High-temperature air combustion is a promising technology to increase the usage of combustion energy and to improve combustion efficiency. This technology has been mainly developed for gaseous fuels, and recently application of this technology to solid fuels like pulverized coal has also become of interest. For the development of high-temperature air combustion technology for pulverized coal, it is important to experimentally investigate the combustion behavior of pulverized coal in high-temperature air. In this study, high-temperature air is applied to a pulverized coal burner to investigate the effect of air temperature on ignition, coal burnout, and NO x emission. Pulverized coal is introduced into a cylindrical furnace of 1 m diameter and 3 m height using a water-cooled stainless nozzle of 15 mm diameter. Combustion air is preheated using a heat exchanger with a gas burner and electrical furnace. The temperatures of the combustion air are set to 623 or 1073 K in order to compare the effect of air temperature. It is observed that ignition delay decreases as the air temperature increases, which is due to the more rapid devolatilization caused by higher particle heating rates. It is possible to form a stable flame even for low-volatile coals like anthracite. The difference in measured peak flame temperatures between 623 and 1073 K air is about 100 K, which is smaller than expected. Coal burnout is improved in the 1073 K air condition, which seems to be due to the increase of porosity of the particle. NO x concentration decreases for higher temperature due to enhancement of the reduction zone by rapid devolatilization of coal, as the volatile and fuel nitrogen release is enhanced in high-temperature air.

Laboratory Studies on Devolatilization and Char Oxidation under PFBC Conditions. 2. Fuel Nitrogen Conversion to Nitrogen Oxides

Fuel nitrogen release and its conversion to nitrogen oxides in pressurized fluidized bed combustion have been recently studied in a laboratory-scale batch reactor, which can be controlled at well-defined operating conditions. The conversion to nitrogen oxides during devolatilization and char oxidation was investigated under the base operating conditions (Tb = 850 °C, p = 11 bar, O2 = 3% v/v, Vf = 0.24 m/s) with 10 different fuels, from high-volatile peat to relatively low-volatile coke. The effects of other parameters, such as bed temperature, oxygen concentration in the fluidizing gas, pressure, fluidizing velocity, and particle size, on the fuel-N behavior were compared in detail with three of the fuels: lignite, Illinois No. 6 coal, and Kiveton Park coal. A special focus of this study was the influence of pressure which was divided into two parts: oxygen partial pressure and total pressure. Results showed that fuel nitrogen conversion to NOx and N2O was dependent on fuel type and operating parameters...

Initial stages of the devolatilization of pulverized-coal in a turbulent jet

The initial stages of the devolatilization of three United Kingdom pulverized coals have been studied in a coal-laden turbulent air jet emerging into the combustion gas environment produced by a methane-air flat flame burner. The temperature range provided by these flames was between 1670 and 1870 K and the corresponding particle heating rate was about 10 5 K/s. Data are reported for major gas species concentrations, particle weight losses, and nitrogen released, which quantify the effects of gas temperature, particle size, and coal type. Overall, the results show that the rate of devolatilization increases with temperature, significantly decreases with particle size and that the composition of the volatiles released is a strong function of the structure of the parent coal. The fuel nitrogen release is correlated to the total volatile yield. The formation of fuel-NO depends on the extent of the nitrogen release and the coal particle combustion mechanism. The turbulence intensity has an important effect on the extent of devolatilization; hence the total weight losses detected in the present experiment exceed those found under nonturbulent conditions.

Including heterogeneous combustion in first-order and distributed activation energies models of coal nitrogen release

Models of fuel-nitrogen release in pulverized-coal flames are developed to account for 1. (1) the occurrence of parallel heterogeneous and homogeneous coal-N removal and 2. (2) the release of coal-N by numerous different first-order reactions characterized by a normal distribution of activation energies. These models are fitted to previously reported flat-flame, coal-dust burner data. Including heterogeneous combustion effects in a model of coal-N release during the early, largely devolatilization, stage of the flames results in a first-order coal-N devolatilization rate constant with an activation energy that is significantly, but not substantially, higher than the activation energy that is determined neglecting the effects of heterogeneous combustion. Allowance is made for coal devolatilization in excess of that predicted by the ASTM proximate analysis test. A distributed activation energies model is fitted to the combined coal-N release data for particular coals burnt at two different stoichiometries. The mean and standard deviation of the activation temperature for nitrogen devolatilization including the effects of heterogeneous combustion are 8200 ± 600 K for the bituminous coal studied and 5100 ± 2300 K for a subbituminous coal.

Release and reaction of fuel-nitrogen in a high-pressure entrained-coal gasifier

Effects of pressure, flame type and coal feed rate on fuel-nitrogen release and nitrogen pollutant formation were examined in a laboratory scale, entrained-coal gasifier. A Utah, high-volatile bituminous coal was used. With a water-quenched probe, gas-particulate samples were collected for oxygen-coal mass ratios from 0.6 to 1.1, pressures of 1, 4.9 and 10.4 atm and coal feed rates of 25 and 35 kg h −1. Two injector types were utilized; one produced a diffusion flame, the other a premixed flame. Fuel-nitrogen release from the coal showed little dependence on oxygen-coal ratio, pressure or coal feed rate. Values at the gasifier exit averaged 83% for the diffusion flame and 92% for the premixed flame. Fuel-nitrogen release, mostly during devolatilization, exceeded fuel-carbon release by ≈ 10% for the premixed flame and ≈ 30% for the diffusion flame, depending on oxygen-coal mass ratio. Over 50% of the released fuel-nitrogen formed N 2, with significant amounts of NH 3 and HCN, and smaller amounts of NO. Increased pressure at constant mass feed rates caused sharp decreases in effluent NO concentrations (to near zero) for both flame types which was explained by a combination of increased residence time and increased homogeneous NO decay rate. Elevated pressure also increased the effluent NH 3 and decreased HCN concentrations for the diffusion flame whereas the more complete mixing of the premixed flame resulted in lower NH 3 and HCN levels, and higher N 2 levels. In general, nitrogen species concentrations were not largely affected by coal feed rate, though increased coal feed rate decreased NH 3 levels somewhat.

Prediction of nitrogen oxide formation in turbulent coal flames

A model of nitrogen pollutant formation and destruction in pulverized coal reactors, whose foundations were presented recently, is briefly outlined and then evaluated by comparisons of predictions with measurements. The model incorporates effects of turbulence on the NO reactions and subsequent predictions show these effects to be important. Thermal and prompt NO were neglected. The model predicts: 1) An initial decrease followed by a gradual increase in NO emissions with increased swirl number as observed for the particular cases modeled; 2) an increase in NO emissions with increased stoichiometric ratio as observed; at higher values, the model underpredicts NO concentrations, partly because of higher thermal NO contributions which are not modeled, and 3) the observed increase in NO with a decrease in particle size and with increase in moisture percentage. Predicted NO emissions for the four test variables differed by an absolute average of 9% from measured values for 24 comparisons. Comparisons with local NO and HCN concentrations from pulverized-coal flames show some agreement except in regions immediately following ignition where oxygen depletion and devolatilization are overpredicted. Predicted NO effluent emissions for six profile cases differ by an absolute average of 24% from measured values. Results suggest that fuel nitrogen release during devolatilization and gas phase reactions of HCN and oxygen control NO formation, while fuel nitrogen conversion to HCN may be near quantitative and rapid. According to the predictions, NO reduction is dominated by HCN-NO reactions and not by the char-NO reactions, while turbulence has an important impact on the gaseous reactions.

Unresolved problems in SO x, NO x, soot control in combustion

Combustion and pollution have been somewhat synonymous topics since mankind's discoveryof fire. As a result of a concerted, worldwide concern over the past fifteen or so years, for what is being emitted to the environment via the smokestack and the tailpipe, great strides have been made toward burning fuels cleanly. In line with this concern, the Combustion Institute has been devoting sessions to air pollution since the Twelfth Symposium in 1968. A great deal of progress, both in an understanding of the technical issues and the implementation of this knowledge, has taken place in this period. This paper attempts to keynote where we are today and what are the principal unanswered questions in the three key areas of SO x , NO x , and soot control in combustion. Combustion-pollution technology is probably on its firmest grounds in the area of SO x , onslightly less firm ground in the area of NO x control, and has farthest to go in soot control. Major attention in SO x technology is directed to fuel-rich combustion and the application of SO x kinetics to staged combustion, interaction of SO x with NO x control mechanisms, and a renewed interest in limestone capture of sulfur. Principal concerns of NO x control address issues of fuel nitrogen release, the nature of fuel nitrogen species, interactions of nitrogen-containing species in fuel-rich combustion, radical overshoot in rich flames, and the conversion of NO to NO 2 in gas turbines and gas burners. Problem areas in soot control are brought forth by consideration of six principal areas which make up a general mechanism for soot formation. Also addressed in the soot discussions are the role of additives, the relation of polycyclic aromatic hydrocarbon formation to soot, and the generation of soot in diesel combustion. The problem areas brought forth by our concerns for clean combustion have had a positive,synergistic effect in advancing our understanding of the combustion process.