Later Stages Of Combustion Research Articles

Coal Blending with Petroleum Coke in a Pulverized-Fuel Power Plant

The current work investigates the performance of petroleum coke (PC) as a blended fuel under pulverized-fuel combustion conditions. Three full-scale combustion experiments were carried out: a pure Carboniferous, high volatile bituminous coal and two blends of this coal with different proportions of PC. The samples studied included feed fuels and blends, fly ashes, chars taken at different positions of the combustion chamber, and chars prepared in a drop tube reactor to test the performance of the individual fuels. The addition of PC led to a substantial increase in the unburned carbon of the fly ashes. The petrographic analysis of the granulometric fractions of the fuels revealed that this increase cannot be attributed to an enrichment in coke of the coarser fractions, as reported in the literature. On the contrary, the finer fraction contained slightly more coke than the raw blend. The petrographic analysis of the chars collected with the suction probe and the fly ashes showed that the two blended fuels were strongly enriched in PC-derived material, indicating a poorer combustibility compared to the high volatile bituminous coal. It is concluded that the reactivity of the blends in the later stages of combustion is related with the contents of PC-derived chars and the burnout itself.

Coupling thermal deactivation with oxidation for predicting the combustion of a solid fuel

A thermal annealing model has been coupled with a detailed single particle model for char oxidation to predict the burning of solid fuels in combustion facilities. The combined model is tested against reported laminar flow reactor burning profiles of several coal chars at the intermediate stages of combustion and for a single coal char also at the later stages of combustion. The model shows for all coals a transition from combustion controlled by oxygen bulk diffusion at low char conversions to kinetically controlled combustion at high conversions. The impact of annealing on char reactivity is strongest at the early stages of combustion, and when the simultaneous annealing of the char is not accounted for, shorter burning times are obtained. In many cases, the experimental burning profiles of the coal chars can only be explained by taking into account the presence of fragments being released at the periphery of the particle. This type of fragmentation appears to be the prime means of particle shrinking and is most accentuated at higher oxygen concentrations in the reactor. The model was able to predict reasonably well the unburned carbon fraction of a single coal char at the latest stages of combustion (up to ∼98%). The model relies on reasonable estimates of the temperature and oxygen profiles in the reactor, as well as the char density, particle size, porosity and initial reactivity (e.g., at room temperature) of the non-annealed fuel. The latter is particularly important for determining burnout at the final stages of combustion. This can be determined by simply combining the char’s reactivity (extracted from a single TGA experiment of the fuel involving pyrolysis at high temperature and subsequent char oxidation), with the proposed annealing model.

Visualization of Dust Explosion Under Microgravity Conditions

The objective of this work was to apply visualization methods to the experimental study of cornstarch dust-air mixture combustion in a closed vessel volume under microgravity conditions. A dispersion system with a small scale of turbulence was used in the experiments. A gas igniter initiated combustion of the dust-air mixture in the central or lop pan of the vessel. Flame propagation through the quiescent mixture was recorded by a high-speed video camera. Experiments showed a very irregular flame front and irregular distribution of the regions with local reactions of re-burning behind the flame front, at a later stage of combustion. Heat transfer from the hot combustion products to the walls is shown to have an important role in the combustion development. The maximum pressure and maximum rate of pressure rise were higher for flame propagation from the vessel center than for flame developed from the top part of the vessel. The reason for smaller increase of the rate of pressure rise, for the flame developed from the top of the vessel, in comparison with that developed from the vessel center, was much faster increase of the contact surface of the combustion gases with the vessel walls. It was found that in dust flames only small part of heat was released at the flame front, the remaining part being released far behind it.

Ash Liberation from Included Minerals during Combustion of Pulverized Coal: The Relationship with Char Structure and Burnout

In this study, the float fraction (<specific gravity of 2.0) of a size cut (63−90 μm) bituminous coal was combusted in a drop tube furnace (DTF) at a gas temperature of 1300 °C under an atmosphere of air, to investigate the ash liberation at five coal burnoff levels (35.5%, 54.3%, 70.1%, 87.1%, and 95.6%). The data indicated that char structure determines the ash liberation at different burnoff levels. Fragmentation of porous char was found to be the determinative mechanism for formation of fine ash during the early and middle stages of char combustion, while coalescence of included mineral matter determines the coarse ash formed in the later stages of combustion. The investigation confirmed that the char morphology and structure play a key role in determining char fragmentation, char burnout history, and the ash liberation during combustion.

Experimental Study on Smoke and NOx Reduction by High Turbulent Two Stage Combustion in Diesel Engines. Trials of Variety Types of Piston Configurations, EGR Combination, and Fumigation.

The authors previously reported significant reductions of particulate emissions by generating strong turbulence during combustion process. Extending this, it was attempted to reduce NOx, particulate, and fuel consumption simultaneously by two-stage combustion ; forming a fuel rich mixture at the initial combustion stage to prevent NOx formation, and inducing strong turbulence in the combustion chamber at the later stage of combustion to oxidize the particulate. The purpose of this study is to examine the further potential and extent of the two-stage combustion for the emission control. The paper shows experimental results for variety trials of piston configurations, EGR-combinations, and fumigation technology together with the two-satge combustion system. The result was not successful for realizing apparent improvement in emission and fuel consumption over wide range of operating conditions, although simultaneous reduction in NOx and smoke was possible at limited conditions with the two-stage combustion. The paper provides information for discussing points to be investigated in the next stage and options for future combustion control technology.

Simultaneous Reductions in Diesel NOx and Smoke Emissions with Metallic Water Solution Directly Injected into the Combustion Chamber.

The effects of several metallic salt water solutions on NOx and smoke reductions in a divided-chamber diesel engine were examined. The solutions were directly injected into the divided chamber independent of the fuel injection. Significant reductions in NOx and smoke were simultaneously achieved with alkali metallic solutions, especially with sodium salt solution. The optimum solution injection timing in terms of NOx and smoke reductions was just prior to the fuel injection. The NOx and smoke decreased with increasing amount of sodium salt solution, but excessive amounts of the solution resulted in saturation in the reductions of both emissions and the deposition of sodium compounds on the combustion chamber wall. The sodium salt solution reduced dry soot significantly, while total particulate matter increased with increase in the water-soluble fraction mainly composed of the sodium sulfate. The in-chamber gas sampling showed that NOx in the chamber was more reduced with sodium salt solution than with water, especially in later stages of combustion.

Simultaneous Reduction of NOx and Smoke of Diesel Engines without Sacrificing Thermal Efficiency.

This paper describes various technologies for compliance with diesel engine emission regulations up to the year 2000. It is anticipated that future diesel emission standards will be met by a combination of systems, including such innovations as high-pressure injection, injection rate control, strong turbulence in the later stages of combustion, cooled EGR (Exhaust Gas Recirculation), SCR (Selective Catalytic Reduction) and particulate trap filters. For further reduction of NOx, the application of water may be required.

Experimental Study on Fuel-Air Mixture Formation and Ignition Process in Diesel Combustion.

The purpose of this study is to investigate fuel-air mixture formation during the ignition delay period and to clarify its effect on the initial combustion process. Thermal decomposition of fuel injected into hot compressed air or nitrogen was studied by detecting gaseous hydrocarbons, with a rapid compression machine and a total gas sampling device. The results show that the ambient temperature is a very important factor affecting the fuel decomposition process. If the temperature is sufficiently high, a considerable amount of injected fuel can be gasified even in the delay period when much oxygen is not yet supplied into the spray. When fuel is injected into very low-temperature air, the ignitability deteriorates because an adequate mixture to ignite exists in only the limited area in the combustion space. At very high temperature, residual fuel in the later stage of combustion is severely cracked into unburned gas and soot, because of the insufficient mixing until ignition.

Multicomponent spray computations in a modified centerbody combustor

The flow properties of the near-wake region of a ducted, bluff-body combustor are presented, based upon finite-difference computations involving turbulent, reacting flows with polydisperse and multicomponent sprays. The analysis advances the state-of-the-art for the representation of the liquid-phase and for the coupling between the phases. The gas phase representation is existing state-of-the-art. Both droplet vaporization and turbulent mixing rates are found to be rate-controlling in the primary reaction region, and the mixing rate is identified to be the rate-controlling process in the later stages of combustion. Under the inflow conditions leading to an equivalence ratio of one based on the primary air/fuel mass flow rates, the primary reaction region is found to be fuel-rich. Enhanced energy conversion is observed to result from improved mixing; the effects of fuel composition are found to be negligible. Near the wake-extinction limit of the combustor the effects of fuel composition are found to be very significant.

An Analysis of Diesel Flame by Picture Processing

A picture processing technique is applied to study the flame motion in a direct-injection diesel engine. From the high-speed photographs, two-dimensional cross-correlation coefficients of flame luminosity between two successive frames are obtained. The location and value of the maximum coefficients give us the mean and fluctuating velocities of the flame, respectively. The results show that the fluctuating velocity obtained in a wide region is almost independent of measuring position and size in the middle and later stages of combustion, indicating that the turbulent flow field is almost uniform and isotropic over the space. Furthermore, the effects of the swirl and chamber geometry on the flame behavior are discussed in detail.

An analysis of diesel flame by picture processing.

A picture processing technique is applied to study the flame motion in a direct-injection diesel engine. From the high-speed photographs, two-dimensional cross-correlation coefficients of flame luminosity between two successive frames are obtained. The location and value of the maximum coefficients give us the mean and fluctuating velocities of the flame, respectively. The results show that the fluctuating velocity obtained in a wide region is almost independent of measuring position and size in the middle and later stages of combustion, indicating that the turbulent flow field is almost uniform and isotropic over the space. Furthermore, the effects of the swirl and chamber geometry on the flame behavior are discussed in detail.

A Study on Combustion in Direct-Injection Diesel Engines : 1st Report, In the Case of Shallow-Dosh Chamber

To reveal the mixture formation and combustion in a shallow-dish type engine with a multi-hole nozzle, a high-speed photographic study has been made. Comparisons of the photographs taken under various conditions have let to the following results. Under no swirl condition, various kinds of gas motion are produced in the flame in the piston cavity soon after ignition. They govern the subsequent state of combustion, finally affecting the air-utilization in the later stages of combustion, and they are dependent on the diameter of the nozzle-hole as well on the direction of injection. When a swirl is employed, the radial extension of the spray is considerably reduced with its intensity, thereby the subsequent spread of the flame being suppressed. Unless a sufficient penetration of the spray is assured, air-utilization is poor in the peripheral space, leading to the so-called overswirling.

Role of hydroxyl and hydroperoxyl radicals in slow combustion of methane

The combustion of methane has been re-investigated in a static system, using a boric-acid-coated reaction vessel. An estimate is made of the relative importance and concentration of OH and HO2 radicals in the oxidation of methane and the intermediate products formaldehyde and carbon monoxide. The ratio of rate constants of the reactions HO2+HCHO →H2O2+HCO (6) HO2+CO →OH+CO2(10) is k6/k10= 280. Both hydrogen peroxide and formaldehyde are probably involved in branching reactions while reaction (10) causes inhibition in the later stages of combustion.