Coal Combustion Conditions Research Articles

Formation Mechanism of Thermal NO<SUB>x</SUB> in Pulverized Coal Combustion

The effects of various basic factors of combustion conditions and coal properties on thermal NOx formation behaviors in pulverized coal combustion have been experimentally clarified, and a theoretical analysis for the flame structural change around a coal particle has been performed, including full chemical kinetics of prompt NOx formation. Thermal NOx concentrations much higher than those predicted by the extended Zeldovich mechanism have been experimentally observed even in the usual pulverized coal combustion conditions. The contribution of thermal NOx to the total NOx rapidly increases with the increase of flame temperature and oxygen-fuel stoichiometric ratio, especially for highly volatile coals. Both the large amount of thermal NOx formation and the effects of various factors on it have been well explained by considering the prompt NOx formation in the flame zone around each coal particle through HCN and NH formed by the reactions between N2 in air and hydrocarbons in the evoluted volatile matter.

Formation and destruction of N 2O in pulverized fuel combustion environments between 750 and 970 °C

Formation and destruction of N 2O were investigated under dilute pulverized coal combustion conditions in an entrained-flow reactor. The effects of temperature, atmosphere (oxidizing and reducing) and reaction environment (homogeneous and heterogeneous) were studied. N 2O was measured on-line by FT-i.r. spectrometry. No N 2O evolution occurred above 950 °C. At oxygen concentrations below 1.5%, in the presence of CO, the N 2O already formed was rapidly destroyed above 900 °C and fairly rapidly even at 830 °C. At high oxygen concentrations, destruction occurred more slowly, and not at all below 900 °C. Heterogeneous reactions did not noticeably increase the destruction rate of N 2O in a dilute fuel-air suspension. Low O N ratio of coal favoured N 2O formation. The results should assist the design of low-temperature combustors that generate only low levels of N 2O.

Deposition of bituminous coal ash on an isolated heat exchanger tube: Effects of coal properties on deposit growth

Fouling deposits were collected on a steam-cooled tube at the exit of a pilot scale furnace during combustion of bituminous coal. The objective of the work is to identify the coal properties and combustion conditions with which one may anticipate fouling and slagging of superheaters in electric utility boilers. Because of the high fusion temperatures of the ashes investigated, deposition on a time scale practical for the pilot scale experiments was observed only at furnace exit gas temperatures above 1700 K (2600°F), higher than the temperatures expected in the convective section of a boiler. A method is proposed for analyzing such measurements to obtain quantitative descriptions of deposit growth under less severe conditions. The model utilizes ash melting temperatures and slag viscosities to account for the variation of particle sticking probabilities with gas and surface temperatures. These parameters are sufficient to reproduce the strong influence of iron during the early stages of deposit growth. As deposit thickness increases, the correlation of deposit loading with iron content disappears. An empirical shedding frequency was introduced to simulate the average loading at long times. The model provides a quantitative description of deposit growth on tubes, and a means for comparison of fouling propensities of different ashes under various conditions. Comparisons with field data are needed to determine whether the proposed model and parameters derived from pilot scale measurements at high temperatures can provide useful estimates of deposit growth on heat exchangers in boilers.

Study on the formation mechanism of thermal NOx in pulverized coal combustion.

Effects of various basic factors of combustion conditions and coal properties on thermal NOx formation behaviors in pulverized coal combustion have been experimentally clarified, and a theoretical analysis for the flame structural change around a coal particle has been performed including full chemical kinetics of prompt NOx formation. Thermal NOx concentrations much higher than that predicted by the extended Zeldovich mechanism have been experimentally observed even in the usual pulverized coal combustion conditions. The contribution of thermal NOx to the total NOx rapidly increases with the increases of flame temperature and oxygen-fuel stoichiometric ratio especially for high volatile coals. Both the large amount of thermal NOx formation and effects of various factors on it have been well explained by considering the prompt NOx formation in the flame zone around each particle through HCN and NH formed by the reactions between N2 in air and hydrocarbons in the evoluted volatile matter.

On the role of macromolecular configuration in rapid coal devolatilization

The role of macromolecular configuration in rapid coal devolatilization is investigated with two reaction models which differ only in their means to relate intermediates and reactant species. The conventional model invokes stoichiometric coefficients, while the depolymerization model invokes probability expressions which depend on the extents of pyrolytic degradation and char formation. For each model, correlations of transient weight loss data, parametric sensitivities, and extrapolations to pulverized coal combustion conditions are given. These results show that the quantitative impact of macromolecular configuration can be masked by uncertainties in the chemical rate coefficients for the pyrolysis reactions. However, in far-ranging extrapolations, only predictions from the macromolecular depolymerization model compare favourably with the available data. This model also provides a mechanistic basis for yield enhancement by faster heating: namely, the concurrent formation of stable char links, as the coal macromolecule dissociates, inhibits the subsequent formation of tar precursors to an extent which depends on the thermal history.

Predicting devolatilization at typical coal combustion conditions with the Distributed-Energy Chain Model

Predictions from the Distributed-Energy Chain Model (DISCHAIN) are compared with transient volatiles yields for heating rates between 10 3 and 10 5K/s and ultimate temperatures between 1000 and 2100K. All model parameters were assigned by correlating transient volatiles yields for much lower heating rates and temperatures, and no further adjustments have been made. Predicted reaction time scales and yields agree quantitatively with the behavior in an atmospheric entrained-flow pyrolysis study, an atmospheric entrained-flow study of the initial stages of coal combustion, and a fuel-rich, one-dimensional coal flame.

The distributed-energy chain model for rapid coal devolatilization kinetics. Part II: Transient weight loss correlations

The Distributed-Energy Chain Model (DISCHAIN) is formulated in a companion paper [1]. In this paper, numerical predictions are compared to the transient weight loss from a bituminous coal in vacuum during heatup and throughout isothermal pyrolysis for two heating rates (10 2, 10 3K/s) at temperatures between 700 and 1300K. Based on seven independent reaction rate parameters, the model predicts transient yields for unreacted coal, gas, tar, and char. Except during heatup at the lower heating rate, the model correlations are in good quantitative agreement with the measured weight loss, and agree with the observed yield enhancement for greater heating rates within the experimental error. This study also includes data correlations from the Distributed Activation Energy Model, to identify the most important model parameters, and model predictions from DISCHAIN for typical pulverized coal combustion conditions, to further illustrate the extent that transient devolatilization on short time scales is determined by transient thermal histories.

The sulphidation of alloys used in a fluidized bed combustor

AbstractThe results of an investigation of the sulphidation behaviour of lncoloy BOOH and AISI 310 stainless steel in coal combustion conditions at temperatures between 650°C and 900°C are presented. Laboratory tests were carried out in controlled environments in the presence of solids representative of fluidized bed combustor operation. The morphology and composition of scale formed during sulphidation was examined and compared with that from tube samples removed from test rigs operated by the National Coal Board. The results have shown that at 900°C a gas mixture corresponding to the equilibrium conditions close to the coexistence point for CaS04, CaO and CaS on the phase stability diagram is sufficient to induce sulphidation. However, at lower temperatures an interaction between the deposit and the protective oxide is necessary and in some cases the presence of typical contaminants found in coals is an added requirement. The morphology of the corrosion products found in practice can be reproduced under...

Studies on the Unburned Carbon Content in Flyash from Pulverized-coal Combustion : 1st Report, Prediction of the Influences of Some Factors

The influences of coal properties and combustion conditions on the unburned carbon content in flyash from the modern pulverized-coal-fired boiler were predicted numerically through a one-dimensional model, in which the profiles of temperature and air-fuel ratio in the furnace were predetermined. The main factors inestigated are the percentage of two stage combustion, furnace length, heat liberation rate in the furnace, exhaust gas recirculation and fuel properties. Predicted results give useful information to reduce unburned carbon in flyash.

The effects of CaCl 2 on limestone sulfation in fluidized-bed coal combustion

In an investigation into the effects of CaCl 2 on the sulfation of limestones in a laboratory furnace simulating fluidized-bed coal combustion conditions, small additions of CaCl 2 (<1 mole%) to the limestone prior to calcination were found to increase the extent of sulfation by changing the limestone pore structure. These changes were effected by trace amounts of liquid in the system in a synthetic SO 2/O 2 flue gas at 850°C. At much higher concentrations of CaCl 2, large amounts of a liquid phase are produced, containing a substantial quantity of dissolved CaO and leading to greatly enhanced sulfation when exposed to SO 2/O 2. The use of CaCl 2 additive in fluidized-bed combustion would reduce the quantity of limestones required to meet air pollution standards for SO 2 and also reduce the quantity of solid waste generated.

On the fate of fuel nitrogen during coal char combustion

AbstractThe physical and chemical characteristics that influence the conversion of fuel nitrogen to nitrogen oxides during coal char combustion were theoretically examined by using a simplified model in which nitric oxide is an intermediate product between fuel nitrogen and N2.It was found that diffusion‐reaction interactions were important in determining the selectivity of the char particle toward nitric oxide production. At low temperature fluidized bed combustion conditions, pore size is important, and low conversion of fuel nitrogen to nitric oxide is favored by long narrow pores. Under high temperature, pulverized coal combustion conditions, the model provided insight into mechanisms of nitric oxide formation and predicted the observed weak temperature dependence of fuel nitrogen conversion, as well as a significant effect of particle size.