Principles and Advantages of Pyrolysis

The waste pyrolysis plant in operation

The Waste Pyrolysis Plant in Operation

Today, solid waste is increasingly flooding the planet. The good news is that we already have many mature technologies for effective waste management. Pyrolysis is one of the most effective solutions.

So, what is pyrolysis? What factors affect it? What are the advantages of pyrolysis? What are the common practical applications of this technique? We will answer them one by one.

Overview and Basic Principles of Pyrolysis

Definition of pyrolysis: Pyrolysis is the process of using thermal instability of organic matter to decompose under the conditions of high temperature, oxygen-free, or hypoxia. In this process, organic matter undergoes chemical decomposition to obtain gaseous, liquid, or solid flammable substances.

The pyrolysis reaction process can be expressed using this simple equation:

Organic solid waste ----->  flammable gas + organic liquid + solid residue.

Schematic illustration of pyrolysis of organic waste macromolecules

Schematic illustration of Pyrolysis of Organic Waste Macromolecules

Pyrolysis technology was first applied to the treatment of municipal solid waste and can be traced back to the early 1970s. Treated waste can be converted into fuel and chemical products that are easy to store and transport.

With the development of modern industry, pyrolysis technology has become more environmentally friendly, safer, and more efficient. It can process biomass, municipal solid waste, waste plastic, waste rubber (tire), industrial waste, agricultural waste, and other organic solid waste with a certain energy.

The 5 Main Factors Affecting Pyrolysis

The process of pyrolysis reaction is very complicated. The properties of the final product are related to many factors, such as reaction temperature, heating rate, reaction duration, raw material properties, reactor type, and so on. Pyrolysis techniques are often named after these factors.

Reaction Temperature

Temperature> 1000 ℃: The heating method is almost always direct heating, and the final product is usually gaseous.

Temperature = 600 ℃ -700 ℃: It is mainly used for pyrolysis of wastes with relatively single components, such as waste tires and waste plastics. The final product is usually a liquid oil.

Temperature <600 ℃: It is mainly used for the treatment of biomass and agricultural and forestry waste. The final product is usually charcoal and a small amount of combustible gas.

Proportion of pyrolysis final product (different reaction temperature)

The Proportion of Pyrolysis Final Product (different reaction temperature)

Heating Rate

Affect the production ratio of pyrolysis products. Through the combination of reaction temperature and heating rate, the formation ratio of each component in the pyrolysis product can be controlled.

Low temperature-low heating rate: After the organic matter decomposes, it recombines into a thermally stable solid, which is difficult to decompose again, and the solid content increases.

High temperature-high heating rate: organic matter will undergo comprehensive pyrolysis, and low molecular organic matter and gas composition will increase.

Reaction Duration

This factor refers to how long the waste material stays in the reactor. It determines the decomposition conversion rate of waste raw materials. To make full use of the organic materials in the raw materials and remove the volatiles as much as possible, the residence time of the raw materials in the reaction kettle should be extended.

With a long residence time, pyrolysis will be sufficient, but the amount of waste treated will be small. In contrast, with short residence time, pyrolysis will not be thorough, but the amount of waste treated will be large.

Waste Material Properties

The properties of material usually refer to its organic content, moisture content, and size. They also have significant effects on the pyrolysis process.

Organic ingredients account for a large proportion of raw materials, and the recyclability of raw materials is excellent. The final product has high calorific value, less residue, and high value.

The low water content of the raw material will consume less fuel for the dry raw material. It can reach the working temperature of pyrolysis faster, and shorten the duration of the pyrolysis reaction.

The small particle size of the raw material is conducive to heat transfer and can make the pyrolysis reaction more complete. Save heating fuels.

Reactor Type

The reaction kettle is the place where the pyrolysis reaction takes place. Different reaction kettles work differently and have different capabilities for treating waste.

A common type of reaction kettle is a horizontal rotary reactor. This type of reactor can evenly heat the raw materials in the furnace through rotation and recover the organic components in waste as much as possible. But horizontal rotary reactors are usually used in batch pyrolysis plants, which treat waste in batches, so the daily processing capacity is often around 20 metric tons.

After many years of research, MoreGreen has produced the latest fully continuous pyrolysis equipment. This equipment can work 24 hours a day and continuously process waste, so the daily processing capacity is much higher than that of horizontal rotary pyrolysis equipment. And this completely continuous pyrolysis equipment can automatically feed and automatically remove waste residue, which can greatly save labor.

Advantages of Pyrolysis Technology

There are currently three main approaches to treating municipal solid waste: landfill, incineration, and pyrolysis.

Among them, incineration and pyrolysis treat wastes at high temperatures and directly use energy substances contained in wastes.

Therefore, we will mainly compare incineration and pyrolysis to explain the advantages of pyrolysis in waste recovery.

Technical Aspects

Item Incineration Pyrolysis
Amount of air injected Need enough oxygen. Large diameter pipes Anaerobic / hypoxic. Small pipe diameter
Equipment volume Large size Small size
Reaction type Oxidation under aerobic conditions Reduction reaction under oxygen-free conditions
Equipment structure Open structure Closed structure
Secondary pollution Dioxin / Heavy metal No dioxin / Heavy metals are fixed by residues

Economic Aspects

Item Incineration Pyrolysis
Investment scale High cost Low cost
Maintenance cost High cost / More labor required Low cost / Only need 2-3 labor
Working Environment Harsh working environment Good working environment
Area Large footprint Small footprint

Other Aspects

1. Pyrolysis technology can convert solid waste into storage energy, mainly fuel gas, fuel oil, and carbon black.

2. Due to anaerobic or anoxic decomposition, the emission of harmful substances, especially exhaust gas, is small. Pyrolysis produces less NOx, SOx, and HCl, and can effectively reduce the pollution to the atmospheric environment after filtering system and activated carbon treatment.

3. Most of the harmful components such as sulfur and heavy metals in waste raw materials are fixed in carbon black.

4. The residue of the pyrolysis reaction does not contain corrupt organic matter and can prevent the elution of metal substances.

5. Capable of processing garbage waste that is not suitable for incineration and landfill.

Practical Application of Pyrolysis Technology

Pyrolysis technology is most commonly used in the treatment of municipal solid waste, plastic, waste tires, or waste rubber.

Pyrolysis of Waste Plastics

Take PE and PP as examples. Waste PE and PP polymers can be pyrolyzed at high temperatures, and the final products of pyrolysis also vary with temperature.

At 800 ° C, the pyrolysis products are mostly ethylene, propylene, and methane.

Between 400 ° C and 500 ° C, the pyrolysis products include bio-oil, gas, and solid residue. Among them, gas accounts for 20% -40%, liquids 35% -70%, and residues 10% -30%.

Below 400 ° C, more high-boiling compounds are produced.

The plastic pyrolysis process

The plastic pyrolysis process

Pyrolysis of Waste Tires (waste rubber)

When the temperature inside the reaction kettle is lower than 200 ° C, the rubber is relatively stable, and it is difficult to cause pyrolysis reaction.

Between 200 ° C and 300 ° C, small molecular weight substances in the rubber are pyrolyzed, and the residue becomes insoluble substances. Rubber will pyrolyze some chemicals with immense molecular weight. Therefore, the oil at this time is dark and sticky.

Above 300 ° C, the decomposition rate of rubber is significantly accelerated, and the molecular weight of the pyrolyzed chemical substances is small. The bio-oil obtained at this time has good flowability, and the color gradually becomes transparent.

Pyrolysis of Urban Garbage

Because the composition of urban garbage(organic waste) is involved, its pyrolysis process is also very complicated. This process can be summarized as follows:

Municipal solid waste -----> gas (H2, CXHY, CO, CO2, NH3, H2S, HCN, H2O, SO2, etc.) + organic liquid (tar, aromatics, kerosene, alcohol, etc.) + carbon black.

Although the product of daily trash pyrolysis is complex, high-calorie fuels can still be obtained in the end for use in power plants and other places. And the final solid ash can be used for paving and brick making.

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