ACTUAL PROBLEMS OF ORGANIZING OF COMBUSTION USING PERMEABLE MEDIUMS. V.К. Baev

Size: px

Start display at page:

Download "ACTUAL PROBLEMS OF ORGANIZING OF COMBUSTION USING PERMEABLE MEDIUMS. V.К. Baev"

James Cobb
5 years ago
Views:

1 International Conference on Methods of Aerophysical Research, ICMAR 008 ACTUAL PROBLEMS OF ORGANIZING OF COMBUSTION USING PERMEABLE MEDIUMS V.К. Baev S.A.Christianovich Institute of Theoretical and Applied Mechanic 60090, Novossibirsk, Russia. Introduction. The global task of organizing of combustion is chemical transformation of fuel (combustible + oxidant) with energy production in thermal, mechanical, radiant and electrical form and/or obtaining required chemical compound of products with maximal effectiveness, i.e. with degree of transformation into target product in relation to thermodynamically possible energy and/or composition. The main characteristic of combustion processes in energetic and thermodynamic objects is substantial difference in energy release and total chemical transformation velocity. The matter is not only in different velocities of appropriate chemical reactions, but in necessity to mix at molecular level and possibility to achieve chemical transformation at substantially lower temperature level, than at energy release phase. Therefore, practically the methods of combustion organizing are a compromise solution between these two stages and often such stages are realized in different devices by their specific ways. An internal-combustion engine with a system of (catalytic) aftercombustion and deleterious substances neutralizing. The contradiction between requirements of maximum possible temperature to obtain high thermodynamical efficiency of thermal machine resulted by incompleteness of chemical transformations due to dissociation or creation of deleterious substances (nitrogen oxides, for example) is principally important. Moreover, as obtaining extremely high temperatures during combustion of organic fuels in air is possible only in case of significant preparative heating of the fuel in the system of thermal regeneration or in result of high compression degree, fuel mixture with acidifier is followed by inflammation up to the end of mixing, which can results by carbon black formation and a new problem of its after combustion. Nevertheless smoke black presence as an intermediate product being a radiating element sometimes can be useful and effective. All aspects mentioned above should be inevitably considered while organizing combustion in innovative fuel-consuming plants; it s obvious that in formulating the fundamental research in combustion field it is obviously needed to aim to general problems appearing in thermodynamically and ecologically efficient fuel-consuming plants, which design is dictated by actual in modern conditions problems. One of such a problem is using for energy supply low-powered but efficient and ecofriendly thermo-power plants, which are capable to operate with different kinds of fuel, including low-grade and organic waste. Potentially such plants can be gas turbine plants working by model cycles (GTPMC) with elements of direct transformation of heat into electricity and systems of purification from exhaust gases. Conditions to organize combustion in GTPMC. Thermodynamical attractiveness of model cycles explains by the fact that their thermal (ideal) effectiveness is equal to the Karno cycle and doesn t depend on the degree of compression in machines. V.K. Baev, 008

2 Mini-symposium Ideally, compression and expansion happens isothermally, but heat regeneration for GTP happens with constant pressure for every side. Isothermal processes are needed for high degree regeneration of heat in the cycle, but if the compression degrees are low and relatively high heating in generator can be used adiabatic compression and expansion processes without significant lost of thermodynamic effectiveness. Practical attractiveness of GTP functioning in such cycles working is that single-stage centrifugal compressor and turbine can be used, which production is large-scale, especially for pressurization system in diesel engines. In case of small compression degree an approximate value for effectiveness in the installation with real elements is as follows η = ηт θ η Т Р ηт + Т θ т 0 where η Т is adiabatic turbine effectiveness η is adiabatic compressor effectiveness θ is ratio of temperature in front of the turbine to the temperature at the input to GTP Т 0. Т is temperature difference in the regenerator Р is pressure at the input Т is decreased pressure the turbine is pressure increase in the compressor χ m=, where χ is adiabat index χ Ratio characterizes relative pressure loss in regenerator. A simple and exemplar ratio follows from () corresponding to the zero balance of powers (η=0), by which we can appreciate relative influence of elements imperfection. η Т η θ = () Т For small machines η and η Т 0,7 and if there are not any hydraulic losses in regeneration system θ>, and to obtain η 0, θ= 4 is needed. This corresponds to practically maximum allowed temperature of gases in front of the turbine. The increasing of > is possible in case of additional thermal compression in pulsating combustion chamber and () shows how effective it is. As long as the question is about the installations with small degrees of compression (extension), so the temperature change in the turbine and, consequently, in the combustion chamber will not be high, that should correspond to high (some units) air excess coefficients. Additional considerations about conditions combustion organization in such installations follow from analysis of conditions thermal capacity (of batch operation) is quite probable. In this case it is preferably to have the burner unit with symmetric configuration functioning in case of change of medium moving direction. Thus, the conditions of combustion organization include: high initial (air) temperatures; large air excesses periodical flow direction change. ()

3 International Conference on Methods of Aerophysical Research, ICMAR 008 Influence of fuel type is usually determinant to chose the way of process organization. If the question is about producing multifuel equipment, so it can be based on simple consideration, as far as the combustion always happens in gas phase, the processes of gasification of solid and liquid fuels should take place in the installation itself and as the most difficult case we should base it on the organizing combustion of hard fuel and in case of it change we should use inert permeable medium. Principal considerations about organization of solid fuel combustion. The approximate theory of sublimate carbon particles combustion [] is used to analyze the problem and to create rational schemes of combustion organizing orienting to such plants as GTP. The following considerations of principals result from the theory.. The velocity of mass loss of carbon particle is determined by the velocity of carbon transition to gas-phase state, which is determined by the surface temperature and the gradient of gas phase concentration of gas phase.. The combustion near the particle surface not only compensate the heat input for sublimation, but it creates as well high concentration gradient, which results at temperatures about 00 о K and velocity difference is higher, the mass losses while gasification and combustion are about two degrees, whereas when the temperatures are higher than 000 о К they are comparable.. If the temperatures are о the value r kg sec k s const 0, () DU m sec m kg where k s is mass burning rate, ; m sec m D is diffusion coefficient, ; sec r is particle radius, m; m u is particle ambient velocity,. sec It follows from (), that the limit combustion modes near the particle surface exist, as far as the u flame stabilization condition is τ r = const (τ r is typical combustion time). r 4. The particle ambient velocity U can be determined by different factors, among them gravity force, centrifugal acceleration, turbulent pulsations. 5. Regardless the way of increasing the particle ambient velocity and decreasing its size, the combustion of particles group will be performed as gaseous fuel with sources of gas-phase carbon determined by the gasification velocity. 6. Gas-phased carbon entry into the medium with reduced temperature should result by its condensation, forming smoke particles, possibility of their coagulation and etc. 7. Radiation exchange between particles and ambient medium can significantly influence the process.. 8. Relatively high coefficient of air excess in GTP lets certain loose in organizing combustion processes, as far as it doesn t require high perfection of mixing in case of stoichiometrical composition of fuel/oxidant. The consideration so f principals mentioned above and qualitative calculation estimates allowed elaborating and testing the schemes of combustion organization represented further. Therewith the experimental information of combustion in permeable capsules was considered, represented in [].

4 Mini-symposium Scheme Burning grid. The main idea of the scheme is forming the stream of oxidant (air) through the fuel layer with depth of some sizes of gross grained particles (5 0mm), moving (for example under gravity force) in lateral direction in proportion to its burning out (fig. ). The layer is formed by enclosing permeable high-temperature walls (). The boring system is placed upstream, for example gas-fired burner (4). A heat-resisting filter-afterburner is placed downstream (). 5 The permeable walls are not only forming 4 (б) walls, but radiation screen as well. Besides, their sickness can be selected as sufficiently Fif. Concept scheme of the process burning grid large, as it is shown in the Fig., where (b) is ceramic honeycomb, which plays the role of heat accumulator. Its heat-retaining features can be used while lighting up and/or as elements of discontinuous regenerator, that can be easily done by reason of symmetry of construction. It should be remarked that the presence of such elements allows burning up of small particles. The scheme described was tested during the work with charcoal. The photo of the working model is showed in the Fig. мвт Q F м 4 r = 0 n 0 - Fig. Working model,0 0, 0,5 u=м/се experiment; combustion; gasification (Q F conditionally, in dependence of fuel consumption), α - coefficient of air excess. - Hgrate-fired furnaceh with the charcoal. Fig. Thermal charge during layer burning The Fig. shows the calculation characteristics for combustion of coal layer in the atmosphere, which corresponds to the experiment conditions. The experimental points are marked there as well. The model work was tested as well while burning wood dust and supplying the mixture of air with fine grinding charcoal to the charcoal layer. The complex of the results obtained proves generally the rightness of the area of focus chosen to organize combustion in conditions oriented to GTP, functioning with different kinds of fuel. In addition to that it is possible to mention at least two facts stimulating further perfection of such system. First, this is necessity to clean filtering elements periodically and continually, second, there are difficulties while organizing combustion of small particles layers, as long as the layer absolute sizes are small and sufficiently high velocity of its motion is required, either the hydraulic resistance of the layer with depth increased, but decreasing the velocity of oxidant changes combustion into gasification α 4

5 International Conference on Methods of Aerophysical Research, ICMAR 008 The difficulties can be overcome in the scheme represented below, which is practically the development of the idea of burning grid. Scheme of combustion organizing with localized and/or periodical blowing/exhaustion. The scheme is shown in the Fig. 4. Here the filtering element purifies not only from solid particles, but from grate grass, and, if necessary from catalytic afterburner; the supply channels can for heat regenerator. To clean the filter periodical change of stream direction is periodically done. Herewith the heat from the combustion chamber can be transferred by radiant or convection streams along the plant working medium, along the object heated. Besides the radiation can radiate directly from the fuel burning, if the chamber walls are transparent in appropriate wave length diapason. The Fig. 4 shows possible lines of flow. Along them the gas parameters, their composition can change by significantly various ways. Q air Burn pr. Pr. gas Fig. 4 Scheme of process organization with localized combustion zone solid fuel zone of intens. reactions. filtering element Therefore different variants can be obtained according the geometrical and thermophysical characteristics of filtering element. Thus, a part of air can not take part in fuel oxidation, but can simply be an coolant of the fire-grate. Localization of the zone of intensive burning near the filtering element is the main idea of this scheme. For this reason the absolute sizes of burning zone shouldn t be large and should represent some typical sizes of pieces of solid fuel. The form of filtering element, the methods of reverse of the oxidant flow, heat dissipation and regeneration can be very different and the structural scheme very various. It is certainly possible the use of gaseous and liquid fuels in case of change of solid fuel for inert header or without it. Some experimental testimonials of working capacity of the scheme described of combustion organization are represented below. The Fig. 5 represents the photo of charcoal burning near the cylindrical surface of filtering element, where the air blowing is done along one halve and the exhaust of combustion products is done along another (end view). The Fig. 6 represents the photos of the chamber with the same cylindrical element with different position of fuel level in proportion to its burning out without supplementary supply. In this 5

Mini-symposium case weak dependence of heating power (i.e. velocity of charcoal burning) of the position of fuel level in sufficiently large limits is deduced from experiments. Fig.

6 Combustion in chamber with cylindrical filtering element Besides, the heat amount transmitted from the combustion chamber to the ambiance was 50 70% from heat liberation, and summary coefficient of

6 Mini-symposium case weak dependence of heating power (i.e. velocity of charcoal burning) of the position of fuel level in sufficiently large limits is deduced from experiments. Fig. 5 Charcoal combustion near cylindrical surface with injection-exhaust Fig. 6 Combustion in chamber with cylindrical filtering element Besides, the heat amount transmitted from the combustion chamber to the ambiance was 50 70% from heat liberation, and summary coefficient of air excess was α 5 7 with temperature о C in the combustion chamber. Conclusion The keynote of the mentioned above was studying of questions of combustion organization as applied to the problem of design of gas turbine plants with low compression degree, which work with different kinds of fuel. This concretizes to a certain degree the study, but the area of actual problems where the decisions tested and proposed can be found is certainly much larger. It is possible to name such as use of associated gas on the oil field, of wood waste, creating economically profitable radiators to produce biological product in simulated conditions. Therefore the present study is generally illustrative from the point of view of declaring actual fundamental in combustion area using permeable mediums. However, the exemplifications represented seem to be sufficient to call actual the following:. Studying stream flows in well permeable mediums, especially in case of combustion.. Combustion in small sized channels of linear and space configuration in non diabatic conditions.. Precipitation and burning out of fine carbon in porous mediums. 4. Radiating capacity of diffusion flames in case of oxidant high temperatures. 5. Conditions of wave processes formation during combustion in porous mediums. 6. Combustion in porous rotors. The research is done with financial support under the program «Energosberejenie SB RAS - 006, 007». REFERENCES. Baev V.K. Approximate theory of sublimate combustion of carbon particles and some experimental results of combustion in permeable capsules //Doc. dig. VI All-Russian conference «Hard fuel combustion». 8- of November 006, Novosibirsk, p

DESIGN AND SIMULATION OF A TRAPPED-VORTEX COMBUSTION CHAMBER FOR GAS TURBINE FED BY SYNGAS

DESIGN AND SIMULATION OF A TRAPPED-VORTEX COMBUSTION CHAMBER FOR GAS TURBINE FED BY SYNGAS A. Di Nardo, G. Calchetti, C. Mongiello antonio.dinardo@enea.it via Anguillarese 301-00123 Roma Abstract The trapped