03182 Fragmentation of small coal particles under fluidized-bed combustor conditions

03182 Fragmentation of small coal particles under fluidized-bed combustor conditions

09 Combustion (burners, combustion systems) model allows determination of (a) the concentration of generated pollutants, (b) the conditions for in...

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09

Combustion

(burners, combustion

systems)

model allows determination of (a) the concentration of generated pollutants, (b) the conditions for initiation and termination of each product formation, (c) the effects of temperature variation, (d) the effects of air:fuel ratio and (e) the effects of secondary-air injection. The results from the model compared well with experimental data obtained in the field.

98103182 Fragmentation bed combustor conditions

of small coal particles under fluidized-

Stanmore, B. R. et al. Symp. (Int.) Cornbust., [Proc.l, 1996, 26, (2), 32693275. Rapid radiant heating under nitrogen and/or 5% oxygen was carried out on particles of four coals screened to mean diameters (d,,) of 1.5 and 2.5 mm. After a few seconds, the product particles were collected and the number distributions of size for the products were compared to the original distributions. Limited breakage was experiences by the 1.5 mm particles with some dust and production of a small amount of d,,/2 fragments. The amount of breakage was independent of volatile matter content, but greater with higher vitrinite materials. The behaviour was consistent with the predictions of a model based on thermally induced stress. The manner of breakage of 2.5 mm particles of a very high VM coal and an anthracite were contradictory, with only some particles of the former breaking, but most of the latter. Increasing the pressure to 1.0 MPa led to less breakage with the high VM coal, suggesting that pressure from the volatiles contributes to breakage. Under 1.0 MPa, the degree of fragmentation increased for the anthracite, which is consistent with the operation of the thermal stress model. In this case, the volatile matter content determines the influence of oxygen on breakage.

98103183 coal-water

Fundamental experiment on the combustion mixture and modelling of the process

of

Takeno, K. et al. Symp. (I&.) Combust., [Proc.]. 1996, 26, (2), 3223-3230 The combustion process of coal-water mixtures has been the subject of a fundamental study. The surface and centre temperatures and the weight change of a single droplet under combustion were measured in detail, where the water content, properties of coal, initial diameter of droplet and external heat flux were parameters. An IR intensive laser was used in the experiment as the external heat source to simulate the conditions around the droplets atomized in an actual furnace. The reaction rates in the processes of water vaporization, devolatilization and char combustion were formulated, based on the data obtained. For the devolatilization stage, the activation energy derived by assuming a first-order reaction was almost the same as that for the mother coal, but the apparent frequency factor was much smaller than that for the mother coal by 11140-1150. In addition, the apparent frequency factor was much smaller for the stage of char combustion. The specific surface area and the micropores in the coal particle were much reduced after undergoing processes of water vaporization and devolatilization. It is possible that the pore would be masked when the water trapped in the pore was escaping from it with the agglomeration of coal particles proceeding. According to the data obtained and the combustion model proposed here, computer simulation codes for the combustion of a single droplet and for the combustion process in a furnace were written. The calculated results for the temperature variation and the concentrations of oxygen and unburned carbon in the exhaust gas agreed well with the experimental data obtained using a one-dimensional test furnace. This confirms the validity of the combustion model and reaction data.

98103184 Fundamental study on N20 formationldecomposition characteristics by means of low-temperature pulverized coal combustion Naruse, I. et al. Symp. (Int.) Cornbust., [Proc.], 1996, 26, (2), 3213-3221. A one-dimensional elec. heated laminar drop furnace was used to study N20 formation/decomposition characteristics and its mechanisms in pulverized coal combustion, especially at low temperature. Both the sampled burning particles and combustion gas were analysed and the mass balance of nitrogen was calculated to investigate the behaviour of nitrogen compounds along the furnace axis. In addition, the effects of the combustion efficiency and the ratio of fixed carbon to volatile matter content (fuel ratio) on NzO formation/decomposition characteristics were elucidated experimentally. As a result, coals that evolve more HCN than NH, produce higher N20 concentration In the downstream region beyond the point where volatile matter combustion is complete, N20 concentration increases, but NO concentration decreases gradually. In this region, the increase in NZO may be caused not only by the reactions between NO and carbon in char, but also by the direct heterogeneous reaction between nitrogen in char and NO. For the coals with low fuel ratio, NzO conversion decreases because of high flame temperature surrounding the coal particles caused by strong volatile matter combustion. The decomposition reaction of NZO by hydrogenradicals produced by the oxidation reaction of CO by OH radicals also contributes near the particle surface. Thereafter, the combustion atmosphere and temperature surrounding the coal particles affects N20 formation/decomposition characteristics. The concentration of exit NZO increases with decreasing combustion efficiency and decreasing fuel ratio. Nine different types of coal were tested and show that the exit NZO concentration has a good correlation with the combustion efficiency and the fuel ratio.

294

Fuel and Energy Abstracts

July 1998

98lO3185 Gas-liquid bed reactors

interfacial

areas

in three-phase

fixed

Larachi, F. et al. Chemical Engineering and Processing, 1997, 36, (6), 497504. Based on the theory of liquid emulsions, a simple model is proposed. It aims to develop correlations suitable for estimating gas-liquid interfacial areas for the bubble flow regime in three-phase fixed bed reactors operated with gas and liquid flowing concurrently upflow. The model is also employed for trickle-bed reactors for estimating the increase in gas-liquid interfacial areas when these reactors are operated under high pressure conditions. Developed correlations were discovered to account satisfactorily for most of the data of gas-liquid interfacial areas available in the literature for concurrent upflow three-phase fixed bed reactors and for trickle-bed reactors operated at high pressure conditions.

98/03188 High-precision calorimetry py of combustion of methane

to determine

the enthal-

Dale, A. et al. PTB-Ber. ThEx Phy.sicaI Tech. Bundesonst.. 1997, 55-67. The combustion enthalpy of CH4 at 25’C was determined with a previously presented high-precision, constant-pressure gas-burning calorimeter. Combustion enthalpy is the most important property used in the determination of the calorific value of natural gas. A value of 890.61 kJ/mol with an uncertainty of 0.21 kJ/mol was obtained which is in good agreement with the value of 890.63 kJ/mol proposed in the draft IS0 6976.

Impact of temperature and fuel-nitrogen combustion with coal pyrolysis gas

98103187

fuel-staged

content on

1996, 26. (2), 2231-2239. Greul, U. et al. Symp. (Inr.) Combust.. [Proc.]. The process of fuel splitting and staging was investigated at an experimental facility of the Stuttgart University with a view to lower NO, emissions. Raw coal was introduced into either an electrically heated, entrained flow reactor or a fluidized bed reactor where it is split into pyrolysis gas and residual char. In a following staged combustion. the remaining char or raw coal is used as primary fuel and the pyrolysis gas as rehurn fuel. The test facility allows a systematic study to evaluate independently the effect ot different parameters on NO, reduction. Pyrolysis experiments, combustion studies. and calculations have been performed to explain the effective NO, reduction with coal pyrolysis gas. To compare the results and show the advantage of this process, investigations with fuel staging were carried out. Reburning with coal pyrolysis gas increases the optimum air/fuel ratio from 0.75 to 0.9, yielding the same or even lower NO, emissions. The main parameters are stoichiometry and residence time in the fuel-rich secondary zone. The results also show an increase of the reduction efficiency with increasing reburning temperatures in the reburn zone. The tar components in the reburn fuel and the fuel-nitrogen content are very important parameters in achieving excellent results when using coal pyrolysis gas. These appear to have a positive effect on the reduction of NO, emissions. To explain this effect, the model of the perfectly stirred reactor (PSR) was employed. The modelling results agree well with the experimenral investigations.

98/03188 hydrogen

An induction parameter combustion simulations

model

for shock-induced

Combclstlon attd Flame. 199X. 113, (112). 106-l IX. Clifford, L. J. et al. For the simulation of shock-induced combustion, an induction parameter model has been constructed. It incorporates the repro-modelling approach for the description of the energy release phase. Only explicit, algebraic functions are applied by the model for the description of the chemical kinetics. These functions parameterize a set of data calculated from homogeneous combustion simulations using a complete and detailed reaction mechanism. A model has thus been created for the simulation of shock-induced combustion of hydrogen in an argon atmosphere. Very close approximation of the results of the full chemistry is achieved by the parameterized model, but the algebraic functions can be computed in a fraction of the time of the full chemistry solution. The model has been employed in one- and two-dimensional reactive flow simulations and the results were found to simulate experimental results well.

98103189 tomography

Industrial

combustion

control

using

UV emission

Leipertz, A. et al. Symp. (Int.) Combust., [Proc.], 1996, 26. (2), 2869-287.5. Based on UV emission spectroscopy measurements in combination with tomographic evaluation procedures, a new strategy for monitoring industrial combustion and control is presented. In contrast to usual closed-loop control for low pollutant combustion on the basis of flue gas composition measurements, this new strategy prevents the production of combustion NO, by an early detection of NO, generating flame radicals in a two-dimensional plane of the combustion system. The paper presents preliminary results for the application of this new control strategy. Tht tomographic sensor system for the control loop has been tested in various combustion systems. In an incineration station, a strong correlation of high temperature and high NO, flue gas emission with the appearance of NH and CN radiative emission bands were found. The radial intensity distribution was calculated under brown coal firing conditions in a tomography test with two single UV sensors. Along a line perpendicular to the connecting line of the two sensors, the determination of the temperature found by comparing the spectra with Planck’s function agreed well with suction thermocouple measurements. Using the same burner with propane fuel, the two-dimensional CH distribution in the longitudinal