RU2750102C1 - Method for super high frequency ignition of wood-type fuel (wood residue, firewood) with high moisture content - Google Patents

Abstract

FIELD: combustion processes.

SUBSTANCE: invention describes a method of super high frequency ignition of wood-type fuel with a high moisture content, characterized in that, prior to being exposed to super high frequencies, solid wood fuel is crushed to 3 to 10 cm, stripped of metal inclusions and moistened to a moisture content of 15 to 70%.

EFFECT: fast and efficient heating followed by ignition of the fuel to a temperature of 250 to 300°C, reduced duration of ignition, heating of the inner layers of the fuel to a temperature of 300 to 350°C, harmful emissions of CO and NO_x reduced by 5 to 12%, increased capacity of the boiler, decomposition of complex hydrocarbons, possibility of adjusting the capacity setup modes of boiler units under different conditions of wet fuel without treatment.

1 cl, 2 tbl, 1 ex

Description

The invention relates to the technology of ignition (arson) in power boilers of solid wood fuel with a high moisture content. The method consists in short-term microwave exposure to wet solid fuel in a high-gradient microwave field with the introduction of a directed microwave radiation flow into the furnace. Solid fuel in the form of lumps, granules or in loose form is placed in the furnace of the boiler unit and is exposed to the microwave field. The materials are ignited before deep cracks appear on the fuel surface and the fuel temperature rises to the formation of hot coals. The end of the stage of the ignition process is the appearance of a continuous flame of activation volatile hydrocarbons leaving the fuel.

Installation of a microwave generator is possible in existing boiler plants, without significant reconstruction of the boiler. The method is based on the selective effect of an electromagnetic field on dipole water molecules, which, absorbing microwave radiation, acquire oscillatory motion, continuously orienting themselves in the direction of the changing field. Non-polar molecules are polarized due to the displacement of their charge, as a result of which the frequency of their collisions increases. These processes are the cause of molecular friction, which leads to heat generation and heating of the dielectric material from the inside.

The microwave frequency of the electromagnetic field is 2450 MHz or 2.45 GHz. Microwaves act on water molecules in an object, causing them to rotate millions of times per second, creating molecular friction, which in turn heats the object.

The propagation speed of microwaves depends on the medium or material in which they move. These values are characterized by the absolute refractive index of the medium, which is equal to the ratio of the velocities from electromagnetic waves in vacuum to their phase velocity in the medium:

where ξ, μ - electrical and magnetic permeability of the medium;

C is the speed of propagation of waves in vacuum;

V is the speed of propagation of waves in the medium.

Knowing that electromagnetic waves propagate at the speed of light C equal to 300,000 km / s, we conclude that the wavelength L of microwave radiation of a given frequency in air will be 12.25 cm.

From the reference data, we determine the wavelength intervals for wood at various moisture values:

1) Absolutely dry ranges from 4.53 to 8.58 cm;

2) Wood with a moisture content of 20% from 2.61 to 2.9 cm;

3) Wood with a moisture content of 45% from 1.92 to 2.0 cm.

Thus, to create the effect of "locking" microwaves inside the irradiated object, it is necessary to ensure its dimensions are not less than the wavelength for the given medium. In this case, internal reflection of waves from the surface layer will occur, which will lead to resonant oscillations and the formation of "standing waves" inside the body. The amplitude of these vibrations will be in the center of the body. This will increase the heat generation inside the body. Therefore, microwave heating does not occur from the edges of the irradiated object, if the microwaves were absorbed into the surface, but from the inside.

EFFECT: fast and efficient heating with subsequent ignition of fuel to a temperature of 250-300 ° C; reducing the duration of ignition (from 2.5 to 10 minutes) tenfold when compared with the traditional method and without the use of additional energy sources; warming up the inner layers of the fuel to a temperature of 300-350 ° С; reduction of harmful emissions of CO and ΝΟ _x by 5-12%; increasing the boiler power while smoothing the batch fuel loading cycles (from 3 to 7%); decomposition of complex hydrocarbons into simpler and more flammable ones in the depths of the fuel pieces before the start of combustion and, as a consequence, a decrease in the loss of mechanical underburning; the possibility of regulating the modes of setting up the capacities of the boiler units at different states of wet fuel without preliminary processing.

The invention relates to the technology of ignition (ignition) of solid wood fuel (wood waste, firewood) with a high moisture content and can be used in a wide range of industries (roasting, smelting, pyrometallurgy, etc.), utilities (waste incineration, boiler etc.), energy (various types of internal combustion engines, thermal power plants, etc.), etc. for getting work and / or getting energy.

From the prior art there is a method of microwave gradient activation of coal fuel according to the patent RU 2458107 C2, 11/10/2010 and a method of microwave gradient activation of coal fuel using a protective film, RU 2514826 C1, 01.10.2012.

A known method of burning coal fuel with additional ignition of a jet of coal dust in boilers when passing this jet through plasma arcs formed by a plasmatron with electrodes. As a result, the particles of coal dust are heated and, with further movement inside a large power boiler, burn faster (Zhukov M.F. et al. Plasma oil-free kindling of boilers and stabilization of combustion of a pulverized coal torch. - Novosibirsk: Nauka, 1995. - 304 p .; Imankulov E. R. et al. Plasma ignition and stabilization of the torch combustion of the Donetsk ASh // Teploenergetika. - 1990. - No. 1. - P. 51-53). Disadvantages of this method: high electricity consumption by the plasmatron, sometimes up to 10% of the electricity generated by the CHP boiler, contamination of the coal dust jet by particles of the plasmatron electrode material, rapid wear of the plasmatron electrodes, the method cannot be applied to low-power (less than 0.1 MW) energy boilers, since plasmatrons are complex and expensive technical devices and they can only pay off when working at large power plants and pulverized coal boilers with a capacity of more than 30 MW.

A known method of burning coal fuel in the form of coal dust in high-frequency microwave plasma (Burov V.F., Strizhko Yu.V. Microwave plasmatron with a free-floating plasmoid // Combustion and Plasma Chemistry, - vol. 4, No. 2, 2007, C 103-109; Burov V.F., Strizhko Yu.V. Microwave plasmatron with a free-floating plasmoid // Collected reports of the VI All-Russian conference "Combustion of solid fuel" November 8-10, 2006, Novosibirsk: IT SB RAS, 2006 ; Burov V.F., Strizhko Yu.V. Microwave plasmatron: for ignition of coal dust we use a freely floating plasmoid. Equipment. Development. Technologies, No. 2 (02), 2007, pp. 45-48; RF Patent No. 2328095, application filing date: 23.06.2006). In this method, the microwave plasma formed by the electrodeless plasmatron is also ignited at the base of the jet of coal powder entering the boiler furnace, however, a plasma-type discharge is created due to the ionization of the gas carrying the coal powder. Disadvantages of this method: low, less than 10-15%, efficiency, because almost all the energy of the microwave discharge is spent on heating the plasma-forming gas and maintaining the gas discharge, and the particles of coal fuel absorb only a small fraction of the energy expended (less than 10-15%); The microwave plasma at the base of the jet is very unstable and special complex structures with gas flows are required to implement the method.

The closest in technical essence to the claimed microwave ignition of wood fuel (wood waste, firewood) with high moisture content is "Method of microwave gradient activation of coal fuel" (RF patent No. 2458107, filing date: 10.11.2010). In this method of activating coal fuel, including microwave action on coal fuel, microwave gradient activation is performed in a high-gradient microwave field in the mode of controlling the rate of increase of the microwave field until a deep fracture appears in a piece of coal, which does not lead to its complete destruction.

The invention relates to a technology for preparing coal fuel of different grades for combustion in power boilers of different types, but has its own disadvantages:

1) With microwave-gradient internal activation, microwave heating of the internal volume of the coal lump occurs, the formation and heating of water vapor, which leads to a sharp formation of cracks in the coal lump from the center to its very surface. Volatile hydrocarbons, both light and heavy, begin to evolve through the cracks, light hydrocarbons predominantly ignite above the coal surface, and heavy hydrocarbons predominantly form aerosol from hydrocarbons in the atmosphere above the coal surface, which does not allow reducing the ignition temperature of the fuel to 500-600 ° C and to increase the completeness of fuel combustion up to 98% in terms of the content of hydrocarbons in the primary coal. And also for this reason, when high temperature thresholds are reached, microexplosions are possible in convective heating surfaces and gas ducts with solid particles of unburned fuel carried away from the boiler unit's furnace;

2) There is no information about the possibility of its application to other types of solid fuels;

3) Due to the low moisture content from 5 to 10% and the low dielectric constant of coal fuel from 1.2 to 1.81, microwave exposure is too energy-intensive with a relatively low efficiency;

4) There is no information about the effect of "locking" waves inside the fuel.

A short-term microwave effect on moist wood fuel in a high-gradient microwave field with the introduction of a directed microwave radiation flow into the furnace is proposed. Solid fuel in the form of lumps, granules or in loose form (3-10 cm in size to ensure partial "blocking" of waves inside the fuel) is placed in the furnace of the boiler unit and is exposed to the microwave field. The materials are ignited before deep cracks appear on the fuel surface and the fuel temperature rises to the formation of hot coals. The end of the stage of the ignition process is the appearance of a continuous flame of activation volatile hydrocarbons coming out of the fuel. The process is repeated with new loading of a portion of fuel. Protection against microwave radiation is achieved by creating protective screens on the surface of the boiler furnace.

The impact on the fuel and the regulation of the reaction rate is carried out by supplying microwave radiation with a magnetron to a portion of wet fuel. The impact on the environmental characteristics of the boiler, namely on the concentration of CO, NO _x , water vapor in the flue gases and the formation of soot, is produced by changing the power and duration of microwave radiation using flue gas analysis.

Example

The previously described conclusions were obtained from a number of experiments carried out with various types of fuel at different values of their moisture content and fraction, the results of which are shown in Tables 1 and 2.

The duration of microwave exposure to fuel to reduce harmful impurities in flue gases is selected empirically. The start of the ignition process is controlled by light and temperature sensors, and possibly in manual mode through the viewing windows.

Claims (1)

A method of microwave ignition of a wood fuel with a high moisture content, characterized in that before the microwave exposure, solid wood fuel is crushed to 3-10 cm, metal inclusions are removed from it and moistened to a moisture content of 15 to 70%.