RU2777092C1 - Method for storage of fly ash of brown coals - Google Patents

Abstract

FIELD: ash and slag waste recycling.

SUBSTANCE: invention relates to the field of recycling of ash and slag waste (ASW) from coal combustion at thermal power plants (TPP), in particular fly ash of brown coal after electrostatic precipitators of large TPP boilers, for use in agriculture. To store fly ash of brown coal, the content of calcium oxide in which is 20-50%, before loading fly ash into storage pits dug in the ground, the bottom and walls of the storage pit are wetted by spraying dispersed water at the rate of 2-4 l/m² of irrigated area. Upon completion of the loading, the fly ash is moistened by spraying dispersed water jets in the amount of 2-4 l/m² of the fly ash surface.

EFFECT: simplification of fly ash storage technology, its timely delivery to the places of use, improvement of the quality of fly ash as a mineral additive to soils, the ability to store for a long time, at least up to a year.

3 cl, 5 dwg

Description

The invention relates to the field of disposal of ash and slag waste (ZOSCH) from coal combustion at thermal power plants (CHP), in particular fly ash from brown coal after electrostatic precipitators of large CHP boilers, for use in agriculture.

“In the processes of layered or flaring coal combustion at thermal power plants (TPPs), gaseous products and solid ash and slag wastes (ASW) are formed - fly ash and slag. The annual volume of ASW in Russia is about 30.4 million tons (2015 data), while according to the most optimistic forecasts, about 4.2 million tons of ASW were shipped to consumers. The rest of the ash and slag waste is stored in ash dumps and landfills" (Dis. Fomenko N.A. "The use of oxidized brown coal to improve the environmental safety of ash and slag waste disposal", Moscow, 2019).

There are up to 30 different macro- and microelements in the CHP fly ash, many of which are vital for plants; therefore, coal fly ash is used as microfertilizers. Such top dressing enriches poor lands with microelements. In addition, brown coal fly ash can be used for soil deoxidation (patent No. 2757060 dated 03/05/2021).

Fly ash from coals burned at thermal power plants is produced in large quantities in Siberia in the winter season. In summer - 5-6 times less. While the use of ash in agriculture is intensive in the warm season, in connection with which it is important to create storage facilities near the places where fly ash is used - agricultural crop areas.

Fly ash from brown coal of the Siberian region is characterized by a high content of calcium oxide of 24-45% (Dis. Fomenko N.A. “Use of oxidized brown coal to improve the environmental safety of ash and slag waste disposal”, Moscow, 2019).

In particular, fly ash from brown coal of the Borodino open pit, burned at CHPP-5 in Novosibirsk, contains about 30% calcium oxide (Quality certificate for fly ash dated September 27, 2018).

At the thermal power plant, excess ash is removed to settling ponds, often by wet transfer of 2-3 km. At the same time, its useful properties, primarily due to the dissolution of the most valuable component of calcium oxide (CaO) in water, are completely lost.

The objective of the invention is to create a method for storing brown coal fly ash that is simpler and more affordable for use in agriculture and allows you to save its useful properties.

The technical result of using the method is the simplification of the storage technology of fly ash, the implementation of its timely delivery to the places of use, the improvement of the quality of fly ash as a mineral additive to soils , the ability to store for a long time, at least up to a year.

The problem is solved by the proposed method, which consists in the fact that brown coal fly ash is loaded at any time of the year into pits dug in the ground - storages, without expensive concreting of the walls and creating special covers.

This method is based on our three-year studies of the behavior of ash in summer and winter, under snow and in rain, when dumping both into concrete and earthen pits and simply onto the soil in different modes and temperatures of poured ash.

The main property of brown coal fly ash, which makes it possible to implement our idea, is the presence of CaO in the amount of 20-50%. It is this component that gives the ash properties when interacting with external, soil or snow-rain moisture to produce a process that we call film (crust) self-concreting.

In FIG. 1 shows a diagram of a storage pit, where 1 is earth, 2 is fly ash, 3 is a self-concreting crust.

The method is carried out as follows: A storage pit, preferably rectangular, is dug out in the soil by an excavator. The dimensions of the pit - storage are determined by the volume of fly ash to be stored. With the size of an average pit for one ash truck (20-25 tons), it is 10 meters long by 2-3 meters wide and a meter deep. The numerical model of our development makes it possible to vary these parameters within a wide range, for example, by 5 ash trucks at once.

We note an important property of earthen storages. The lower boundary will be in the zone of positive temperatures for almost all regions of Siberia. This means that, as our calculations and the numerous known experimental data on heat transfer in Siberian soils show, the bulk of the ash will not freeze, which will additionally preserve its positive qualities - high dispersion, small particle size in the conglomerate, not watering CaO to Ca(OH) 2.

On the other hand, if you need to start other reactions, you can add, for example, a few percent of water to the pit to bind dust particles, which will dramatically reduce dust when removing ash from the storage pit.

Usually, falling asleep in a pit of one machine - 20 tons, occurs quickly, in 20-30 minutes maximum, this allows us to consider (and this is experimentally confirmed) that the ash does not have time to capture atmospheric moisture in its main volume.

Moisture from the surface can be absorbed to an ash depth of about 1.5 cm. But at the same time, the formation of a concreting layer is triggered (binding derivatives of CaO are formed, as in the manufacture of branded concrete). After binding - up to a day, the layer ceases to practically pass water and air into the ash.

Experiments were carried out to determine the rate of moisture penetration and the growth of a self-concreting crust at the stage of moisture seepage from the soil.

Ash about 1 kg was poured into a cone and the changes were photographed. When moisture seeped through, the ash changed color to darker, which made it possible to measure the progress of the moisture limit in the ash in photometry programs. It was shown that at an early stage, moisture rises along the side faces of the embankment by 3 mm per hour, then by 1 mm, and in 12 hours the boundary advances completely when a thickness of 9–15 mm is reached. At the same time, the sound frequency inside the concreted crust was measured by the impact sound Fourier analysis method, and according to this characteristic, we determined the course of the process and its end.

Figures 2, 3, 4, 5 show photographs of an ash sample that was poured into the soil a month ago, forest chernozem, and then cleared of the soil. The shape of the filling cone is preserved. Then the hardness of the self-concreting shell (Fig. 3, 4) was measured by the conical needle method and the frequency analysis method. These data are comparable to real concrete crusts 10 mm thick.

On FIG. 5 shows the determination of the wall thicknesses of the self-concreting shell. The lower part is cleared of soil, the upper part of ash. According to field measurements of pH and ionic conductivity, the ash inside the cone did not change the calcining properties.

We propose to use such an experiment before choosing a place where large ash pits will be organized.

The measurements were carried out in the summer, when the soil was warm enough and normally moistened.

Depending on the weather conditions, after loading the fly ash into the storage pit, it is possible to use forced spraying by spraying finely dispersed water jets over the surface of the ash (2-4 l / m ² until the thickness of the moistened layer reaches about 10-15 mm), or in stages as it is filled storage pits.

Typical times - the time of unloading 20 tons of ash is about 20 minutes. The hardening time of the crust when spraying with water is 5-10 minutes. If without spraying - 12-24 hours (due to absorption of atmospheric moisture).

When the calcium oxide content in the fly ash is less than 20%, in order to form a self-concreting crust, it is recommended to spray with a dispersed water jet with the addition of liquid glass in it with a concentration of 2-5% of the volume of sprayed water.

Before loading fly ash into the storage pit, it is recommended to first determine the rate of moisture penetration from the ground into the ash mass. In practice, this is easily done by pouring a test heap of ash (0.5 kg) onto the soil from the pit. Water begins to be sucked into the ash, with a darker front clearly visible. If the rate of advance of the water front from the soil into the ash is more than 1-2 mm/hour, then this is sufficient for the processes of self-concreting of highly calcined ash.

If the rate of moisture penetration from the ground into the ash mass is less than indicated, then it is necessary to additionally wet the bottom and walls of the pit by spraying dispersed water at the rate of 2-4 l / m ^{2 of} the irrigated surface to accelerate the formation of a concreting crust.

As a rule, when filling the fly ash into the storage pit in the summer, there is enough moisture in it to suck the ash and form a crust about 1 cm thick.

In pits subjected to "freezing", i.e. dug 5 or more hours before filling the fly ash, two options are possible: if the ash has a temperature of 30 ° C or more, the storage pit does not require any additional processing, because the heated array of fly ash will ensure the thawing of the adjacent soil and the penetration of moisture from formation of a self-concreting crust.

If the fly ash has cooled down after shipment from the CHP bunkers, then before loading the fly ash into the storage pit, the bottom and walls of the pit are wetted by spraying dispersed water at the rate of 2-4 l / m ^{2 of} the irrigated surface, or sprinkled with snow in the amount equivalent to the required amount of water in winter. The ground with a depth of more than half a meter gives a flow of heat into the ash, and after 2-3 days the temperature reaches plus 5 ° C, the soil water thaws at the boundaries of the walls of the pit, or the snow thrown into the pit melts, and the process of self-concreting continues.

To create denser crusts, as well as in the case of less than 20% calcium oxide in the fly ash, it is necessary to use a 2-5% aqueous solution of liquid glass to irrigate the bottom and walls of the storage pit at the rate of 2-4 l / m ^{2 of} the irrigated surface.

Experimental measurements of the thickness of the self-concreting crust showed that in all modes a crust is formed with a thickness of 0.5–1.5 cm.

The crust performs the following functions:

does not allow moisture and carbon dioxide (CO2) atmospheric (about 400 ppm) and soil CO2 (5-10% of the atmosphere can reach the concentration of soil CO2), which allows you to stop the formation of chalk (CaCO3) from calcium hydroxide (Ca(OH) 2)

experience shows that the ash under the crust remains practically unchanged in the mode of full self-concreting for at least a year.

As certain properties of the ash mature or in case of production need in the fields and piles, the pit can be emptied by an excavator and taken out to the fields, or to litter herds.

In winter, there may not be free water in the field, in which case snow can be used to avoid bringing water in when it gets cold. At temperatures below 15-20°C, spraying dispersed water will cause it to quickly freeze on the walls of the pit, while the ash is not yet loaded (in 10-15 minutes). Therefore, it is better to use snow. Snow is poured at the bottom immediately, on the surface of the walls of the pit as the level of ash in the pit rises. At the end, it is desirable to pour a layer of snow on the surface of the ash in the pit to create a crust. Otherwise, you will have to wait up to 10 days for the suction of moisture from the dry atmospheric winter air.

We carried out winter experiments in 2019-2021 with masses of 2-3 kg of fly ash from various selections from CHPP-5 (up to 15 samples). Two modes were tested - without spraying with water, and with spraying with dispersed water.

With spraying with positive temperature water and cold ash, the formation of a crust took place in 10 minutes maximum. After that, the crust acquired the properties of impermeability to gases and water that we needed.

Without spraying with water, moisture came from cold air, dry in winter, and the crusting process could take up to 2 days. But water vapor did not penetrate deeper than 10-15 mm, which made it possible to leave the property of ash inside the main volume at the same initial level, and store it, including spring snowmelt, on this crust. The water from melting simply flowed down the sides, so we recommend making the initial filling of ash with a small slide (2-5 degrees slope from the center to the edges of the filling).

Claims (3)

1. A method for storing brown coal fly ash, which consists in using brown coal fly ash, the content of calcium oxide in which is 20-50%, while before loading fly ash into storage pits dug in the ground, produce wetting the bottom and walls of the storage pit by spraying dispersed water at the rate of 2-4 l/m ^{2 of} the irrigated surface, upon completion of loading, the fly ash is moistened by spraying dispersed water jets in the amount of 2-4 l/m ² of the fly ash surface. 2. The method according to claim 1, characterized in that to moisten fly ash in winter, snow is used in equivalent terms for a mass of water of 2-4 liters per m ² , taking into account the density of the snow used. 3. The method according to claim 1, characterized in that in order to obtain a denser self-concreting crust, additional spraying of a dispersed aqueous solution of liquid glass with a concentration of 2-5% is carried out. at the rate of 2-4 l / m ^{2 of} the irrigated surface.

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