Pyrolysis process: types and necessary equipment

Pros and cons of burning

Garbage pyrolysis is a procedure for neutralizing waste by exposing it to high temperatures. It involves burning hazardous objects using specialized installations.

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To carry out this task, certain conditions must be met: limiting the access of oxygen to the sealed chamber, as well as creating a high temperature regime inside. During this process, products are broken down at the molecular level. The result is simpler compounds that are completely safe for nature.

Advantages and disadvantages of the technology:

• decomposition products are not released into the environment, which avoids the spread of harmful substances;
• complete safety for human health;
• raw materials are available to everyone and are inexpensive, they can be found almost anywhere where people live;
• landfills have a large amount of resources to burn;
• it is possible to process those types of waste deposits that are not amenable to other methods of destruction;
• no hazardous substances are formed during the decomposition process;
• pyrolysis products can be buried without harm to nature;
• reducing the volume of processed material several times;
• heavy and hazardous metals are completely converted into ash, which eliminates the possibility of their further spread.

• high cost of equipment and installation work;
• complexity of system management;
• a large number of employees are required to control the process.

All the disadvantages of pyrolysis are associated with the resources necessary for its implementation. Large costs are required to ensure continuous recycling, which not all enterprises can afford.

Dry pyrolysis method

The main principle of the method is the careful conservation of irreplaceable natural resources.

With this method:

• receive fuel;
• neutralize recyclable materials;
• produce chemicals used in industry.

Work is carried out in the temperature range:

• low;
• average;
• tall.

The process occurring at temperatures from 450 to 550 degrees is called low-temperature. In this case, a large amount of charcoal is obtained, as well as a certain amount of pyrolysis gas.

Also, the output is resins, which are subsequently used for the production of rubber. Coke is used as fuel for domestic and industrial needs.

If the temperature is set to 800 degrees, then this will be medium-temperature pyrolysis. With this option, a much larger amount of gas is released, less coke and liquid resins are released than with the low-temperature method.

Watch the video: Installation of continuous waste pyrolysis

What is the technology

It consists of performing the following steps:

1. Materials for combustion are placed in a high-temperature chamber where they remain for a long time. This is a stage that requires large energy expenditures.
2. Direct pyrolysis. The most unstable compounds melt at a temperature of 300 degrees. As a result, carbon dioxide, carbon and other gases are released. Next, the process of burning solid waste and turning it into ash continues.
3. At the final stage, products are obtained that are buried or disposed of in another way.

The output may be different materials depending on what components were originally used. The resulting products are of several types:

• liquid - oily or plain water;
• picarbon;

• gaseous substance.

They are completely harmless. They can be buried or used in secondary production of products depending on the need.

Pyrolysis technology

At the end of the process the following products are obtained:

• a mixture of gases (flammable and non-flammable), it is called pyrolysis gas, synthesis gas, pyrolytic gas;
• pyrolytic oil, which subsequently serves as fuel for furnaces or material for further processing;
• water;
• picarbon (coal is a solid residue containing carbon).

Pyrolysis of any type takes place in 4 stages:

1. Drying raw materials in a drying chamber.
2. Actually pyrolysis (distillation).
3. Combustion of solid components.
4. Obtaining pyrolytic oil, gas and carbonaceous residue.

Watch the video: Pyrolysis installation. All about it

Types of pyrolysis

There are several different options for the procedure depending on the methods used for processing primary products. The table below provides descriptions.

Each of the options is actively used in practice and has its own scope. Technologies for their implementation are constantly evolving.

Composition of pyrolysis products when used

Various raw materials

Pyrolysis of hydrocarbon raw materials is carried out at temperatures of 800-900°C in the presence of a diluent - water vapor in tube furnaces. The contribution of secondary reactions decreases at low pressure (dilution with water vapor) and short contact times (from fractions of a second to several seconds).

Modern high-performance ethylene plants (EP-300 and EP-450) with a capacity of 300 and 450 thousand tons of ethylene per year, respectively, use powerful pyrolysis furnaces, specially designed for conditions of intense high-temperature heating (up to 870-920°C), with a residence time of raw materials in reaction coils within 0.01 - 0.1 s.

They are characterized by a vertical arrangement of radiant coil pipes in the form of a single-row screen with double-sided irradiation by flameless combustion panel burners (or with torch burners with a flat flame). The passage through the radiant coil pipes is organized in the form of several (from 4 to 12) parallel flows (sections). Each section consists of several heat-resistant pipes (from 3 to 12) with a length of 6 to 16 m and a diameter of 75-150 mm. The capacity of one pyrolysis furnace reaches up to 50 thousand tons of ethylene per year. A diagram of one of the modern pyrolysis furnaces is shown in Fig. 1.10.

Rice. 1.10. Diagram of a pyrolysis furnace.

1 - body; 2 - panel burners; 3 - radiant chambers; 4 — pyrotubes; 5 - convection chamber.

The next most important pyrolysis apparatus is a quenching apparatus, which is designed for rapid cooling of pyrolysis products. Modern quenching-evaporation units (QEA) are gas-tube waste heat boilers. As a result of the high linear speed of pyrolysis products moving through the pipes, settling of solid particles on their walls is prevented, the heat transfer coefficient increases and rapid (in 0.015 - 0.03 s) cooling of the pyrolysis gas to 350 - 400 ° C is achieved. Due to this heat, high-pressure steam (11-13 MPa) is generated from the water condensate entering the ZIA, which is separated in the steam collector, overheated to 450 ° C in one of the sections of the pyrolysis furnace and then used as a diluent for raw materials during pyrolysis or for driving turbochargers.

The schematic flow diagram of the EP-300 pyrolysis installation is shown in Fig. 1.11.

Rice. 1.11. Schematic diagram of a gasoline pyrolysis installation.

Ι - raw materials; II - condensate; 111 - pyrolysis gases; IV - gasoline fraction (n.c. - 150 C); V - light resin; VI - heavy resin; VII- water vapor.

The plant's raw materials are the 62 - 180°C fraction of straight-run gasoline and the 62 - 140°C fraction of catalytic reforming raffinate gasoline. Pyrolysis of ethane and propane is also provided.

After heating in heat exchangers, gasoline is supplied to nine parallel-operating tube furnaces (one is shown in the diagram), and the ethane-propane fraction is supplied to the furnace (P). At the exit from the convection chamber, water vapor is introduced into the raw material in an amount of 50% by weight. for gasoline and 30% for ethane-propane. The temperature of the pyrogas at the outlet of the furnace coils is 810 - 840 ° C, the reaction duration is 0.3 - 0.6 s. The pyrolysis products then enter the pipes of the ZIA quenching apparatus.

Pyrogas cooled to 400°C is sent to the bottom of the K-1 washing column, where, upon contact with the cooled fraction of 150 - 250°C, it is cooled to 180°C and washed from solid carbon particles (coke). Heavy condensate from the bottom of K-1 is fed into column K-2 for rectification. Gases and vapors rising from the bottom of K-1 pass through a semi-blind plate and undergo rectification on the upper plates of the column.

Condensate from the K-1 cube and the semi-blind plate enters the K-2 column. The pyrogas leaving the top of the K-1 column is cooled in a water cooler to 30°C and enters the gas separator C-1. Light condensate is supplied for irrigation at the top of K-1 and for rectification in K-2. The pyrogas removed from the top of the C-1 is supplied to the HFC.

Pyrobenzoine (fraction n.k. -150°C) is removed from the K-2 cube. light resin (150 - 250°C) and heavy resin (> 250°C). Some of the light resin circulates as an absorbent in K-1.

Separation of pyrolysis gases

The composition of pyrolysis products depends on the type of raw material and installation mode. In any case, it contains dozens of components, of which it is necessary to isolate one (ethylene or propylene) with a high degree of purity, at least 99.9%. The following sequence of separation of pyrolysis products is used.

1. Isolation of the gas fraction. Pyrolysis products after the furnace and ZIA go through a heavy fraction column (HFC) installation, where heavy hydrocarbons including C4 are separated. Hydrocarbons C1-C4, H2, H2O, H2S, CO, CO2 remain in the gas.

2. Preparation of pyrogas for separation. It is carried out in 4 stages: compression, purification from acidic compounds, gas drying, purification from acetylene compounds.

· Compression is carried out using turbochargers, up to a pressure of about 3.7 MPa;

· Purification from acidic compounds (H2S and CO2). The cleaning method is chosen depending on the sulfur content in the raw material. With a high sulfur content in the raw material (0.1-0.6%), the pyrogas is first purified with a solution of monoethanolamine and then with an alkali solution: 2HOCH2-CH2NH2 + H2S à (HOCH2-CH2NH3)2S (for CO2 the same is true with only one MEA molecule) . If the sulfur content is less than 0.1%, they are limited to 2-stage alkaline cleaning. If the sulfur content is more than 0.6%, hydrotreating must be used;

· Gas drying is carried out using zeolites.

· Purification from acetylene compounds is carried out by selective hydrogenation on heterogeneous catalysts that do not affect olefins. Palladium and nickel-cobalt chromite catalysts are used, operating at a temperature of 150-230C.

After drying and purification, the pyrogas is separated into: H2 + CH4 – methane-hydrogen fraction; ethane + ethylene – EEF, C3H6 + C3H8 – PPF. Next, if necessary, separate each of the fractions. Absorption and rectification are used for separation.

IMF allocation

IMF can be isolated using the absorption-rectification method (ARM) or the low-temperature rectification method (LTR).

AWP . The essence of the method is that C2-C4 hydrocarbons are extracted from a gas mixture by absorption, which are then separated by rectification. The top of the absorption-distillation column works as an absorber, and the bottom as a stripping column. Fraction C4 is used as an absorbent.

NTR . There are two types of the method, differing in pressure: at low pressure (0.1-0.2 MPa) and at high pressure (3-4 MPa). The differences are in the temperature and number of plates. Operation at high pressure places high demands on the stability of the composition. Most often, separation is carried out at high pressure.

Advantages and disadvantages of IMF allocation methods.

The advantages of NTR are lower consumption of steam and water and greater purity of ethylene; the disadvantages are the complexity of compressor equipment, lower temperatures and high requirements for the stability of gas composition.

The advantages of automated workplaces include moderately low temperatures and comparative ease of operation; The disadvantages are increased energy consumption, costs for the absorbent and the need for its separation.

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Comparative characteristics of types of pyrolysis

All the described methods have their advantages and disadvantages. The features of each of them are shown in the table below.

Pyrolysis technique

The process of wood decomposition occurs in devices for reproducing thermal reactions - retorts. They differ:

1) Operating principle:

• Continuous.
• Semi-continuous.
• Periodic

2) Heating method:

• Interior.
• External.

Semi-continuous cycle devices are actively used in industry.

Wood is placed in them in small portions at intervals.

What waste can be recycled

Pyrolysis is a procedure that has a special procedure. It requires preliminary study. Otherwise, problems with the law may arise.

Expert opinion

Alexandra Yu.

When thinking about environmental issues, it is important not only to sort, but also to reduce consumption.

There are organic and inorganic waste. Only the first ones are allowed to be processed in this way.

The following types of waste deposits are classified as organic:

• Compostable. These are cardboard, sawdust, branches, plastic and leather products, various products such as wires, etc.
• Non-compostable. These are substances with high viscosity or a creamy structure, clothing, leather and rubber products, earth with flammable components in the composition.

Read also

• 

  Working project “waste into income”. Feasibility study of disposal of rubber goods and carbon dioxide

• 

  Waste recycling plant, from the “Waste to Income” project series

• 

Pyrolysis plant

The arrangement and technological processes at such a plant have a number of features and also require compliance with certain rules. Must be present:

• drying components;
• grinding and processing of incoming raw materials on a belt;
• liquid purification process;

• formation of oily liquids based on pyrolysis;
• receiving exchange products.

Expert opinion

Alexandra Yu.

When thinking about environmental issues, it is important not only to sort, but also to reduce consumption.

This processing scheme fully complies with safety standards, providing a high-quality purified product. The operation of a plant requires a large number of different devices and means.

Equipment

The pyrolysis unit ensures the decomposition of organic waste at the molecular level. This generates energy that can be used in further stages of production.

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The design is based on a reactor. In the upper compartments, incoming materials are dried at a temperature of no more than 200 degrees. In the following containers the temperature can reach values ​​of 1,200 units. After this, the remains are compacted.

During combustion, carbon is released, which allows you to maintain the same temperature without the oxidation process. In the last department, solid residues are cooled, compressed and sent for disposal.

There are several types of installations used depending on the application:

• for processing plastics and tires;
• for the destruction of wood processing industry products;
• for pyrolysis of food and other types of organic waste.

Secondary products

During the processing process the following is obtained:

• oil;
• gas for thermal and electrical industries;

• fuel for diesel engines;
• charcoal.

These products are used in many areas, being an excellent source of energy. But in practice they don’t always work out. It all depends on the conditions created in the combustion chambers.

Pyrolysis boiler diagram

Before starting assembly work, you should prepare: draw up a detailed diagram of the fuel equipment. Only an experienced heating engineer can do this on his own, so to simplify this task, you can use a ready-made diagram that can be slightly modified.

Below is a diagram of a 40 kW unit. Changes made to the circuit should not affect the size of the internal chamber. During its test run, it is necessary to determine the efficiency: this is not difficult to do if you pay attention to the nature of the smoke coming from the chimney. If there is no smell of carbon monoxide, then everything is in order. Such a check must be performed at different loadings of the pyrolysis chamber.

An example of a drawing of a pyrolysis boiler

Legend:

A – boiler circuit controller; B – door for loading fuel; C – ash pan cover; D — blower fan; E – temperature fuse sensor coupling with a radius of half an inch; F – connection for emergency line; G – boiler circuit supply line; H – cold water supply to the protective heat exchanger; K – hot water supply to the protective heat exchanger; L – return line of the boiler circuit; M – expansion tank and drainage pipe.

Water is not always used as a coolant: air often performs this function. It is supplied in a heated state through pipes, and back along the floor, by gravity. This is convenient because there is no risk of the system defrosting. In addition, there is no need to monitor the tightness of pipes.