Biochar production technology overview

In recent years, biochar production become a hot topic on the market. Generally speaking, biochar production refers to the process of pyrolysis of biomass energy raw materials. Many biomass materials including wood logs, sawdust, bamboo chips, crop stalks, rice husks are reused in a green way. Except for environmental objectives, biomass to biochar is also a good way to turn waste into wealth, More and more people want to start a business of biochar production. how much do you know about biochar production? here is an overview of biochar production technology. Let’s learn the latest technology in biochar production together.

Biochar is a kind of functional material with high carbon content, rich pore structure, and strong adsorption capacity, which is made by pyrolysis and carbonization of biomass under hypoxia conditions. How was biochar discovered? The answer is “black soil”! It was the study of the black soil of the Amazon that got scientists interested in biochar. Hundreds of years ago, Amazon Indians discovered that mixing biochar and organic matter into their soil created a rich black soil that allowed crops to thrive. The raw materials for the preparation of biochar are widely sourced, mainly including crop straw, forestry waste, livestock manure, domestic waste, and other waste biomass, of which straw accounts for the highest proportion.

Every year will produce a lot of agricultural waste, most of which can not be fully used. Nowadays, many researchers have developed ideas to make biochar from waste biomass through various technical means to promote the sustainable use of resources and the sustainable development of ecosystems.

The traditional method of biochar production is to pile up the biomass, cover it with a thin layer of soil, ignite the smoke at the seal, or heat the biomass in the form of a biochar kiln, so that it is burned and pyrolysis in an anoxic environment to form biochar (commonly known as “smothered charcoal “). This method has disadvantages such as poor production conditions, long cycles, and serious environmental pollution. Large-scale biochar production will lead to a series of problems such as deforestation, intensified greenhouse effect, and air pollution. Therefore, more and more scholars are committed to the study of new biochar production methods with high feasibility, and the most widely used method is thermal pyrolysis (pyrolysis or high-temperature pyrolysis).

Pyrolysis biochar production

The principle of biochar pyrolysis is that under the conditions of oxygen restriction and high temperature, the chemical bonds of macromolecules of biomass organic polymers are cut off by heat energy, so that they are quickly broken into low molecular short chain substances, and solid, liquid and gas products can be obtained through pyrolysis.

Biochar pyrolysis process

The biochar pyrolysis process is mainly divided into the following stages:

1. Drying stage: At the initial stage, the external heat source begins to heat up, the temperature inside the biochar kiln is low, the pyrolysis rate is very slow, and the water in the material begins to evaporate, but the chemical composition is unchanged. This stage is the endothermic reaction stage, which mainly relies on the outside world to provide heat to ensure that the temperature rises.
2. Pre-carbonization stage: The temperature in the biochar kiln gradually rises, the thermal decomposition reaction changes significantly, and the chemical composition of the material gradually changes, especially the unstable components (such as hemicellulose, etc.) decompose into CO, CO2, and a small amount of acetic acid and other substances. This stage is still the endothermic reaction stage.
3. Carbonization stage: The temperature in the biochar kiln continues to rise, the thermal decomposition of the material is rapidly carried out, and a large number of decomposition products are produced. This stage is the main stage of pyrolysis, which releases a large amount of heat energy and produces liquids such as acetic acid, wood tar, and methanol (separated when cooled), as well as gases such as CO2, CO, CH, and H2, and increases the content of combustible components.
4. Calcination stage: At this stage, the charcoal is calcined. The C-containing chemical bond (C-H, C-0 bond) is further cracked, the remaining volatile substances in the charcoal are discharged, and the fixed carbon content is increased. The boundaries of the four reaction stages in the pyrolysis process of biochar are difficult to demarcate, and each stage often crosses.

Biochar production technology

According to the different biochar pyrolysis technology, the current biochar production method. It can be divided into carbonization technology, liquefaction technology, gasification technology, microwave pyrolysis technology, etc.

Carbonization technology

Carbonization technology is the most commonly used technology for biochar production. The main process is to put the chopped or formed biomass raw materials in the biochar kiln and heat them to burn under the condition of limited oxygen, which causes the internal decomposition of biochar, biological oil and gas.

According to the different heating temperature, heating rate, and reaction residence times, carbonization technology can be divided into four kinds of process types: slow pyrolysis, medium pyrolysis, fast pyrolysis, and flash pyrolysis. Among them, the low heating rate of 5~7 C/min is used for slow pyrolysis, and the reaction rate is 1~2d at medium and low temperature of 400~650 C. The final carbon formation rate is about 35%, the proportion of bio-oil is about 30%, and the proportion of gas is about 35%. The medium rate of pyrolysis is 300C/min, and the reaction is 10-20s under medium and low temperatures of 400-550 C. The final carbon formation rate is about 20%, the proportion of bio-oil is about 50%, and the proportion of gas is about 30%. The rapid pyrolysis rate is 1000 C/s, and the reaction rate is 1~2s at 400~550 C. The final carbon formation rate is about 12%, the proportion of bio-oil is about 75%, and the proportion of gas is about 13%. Flash pyrolysis uses a rapid heating rate of 1 0009C/s, and the reaction time is less than 1s under the high-temperature condition of 1050~1 300 C, and the final carbon formation rate is 10%~25%, the proportion of bio-oil is 50%~75%, and the proportion of gas is 10%~ 30%.

As the heating temperature and rate change, the ratio of bio-oil, biochar and gas in the product will also change. Therefore, in order to obtain more biochar, the reaction conditions should be controlled at a lower temperature level and a slower heating rate, but too low temperature will lead to incomplete carbonization, carbonization time is too long, and the carbon formation rate is reduced. By comprehensive comparison, the proportion of biochar produced by slow pyrolysis process is the highest, and the particle size requirements of raw materials are not strict.

Hydrothermal carbonization technology

Hydrothermal carbonization technology, also known as wet pyrolysis technology, is essentially a slow pyrolysis and a pretreatment process that significantly enhances the performance of biomass. It is the process of biomass carbonization in which the biomass is heated in a closed reaction vessel at high temperature and high pressure for more than 1 h with aqueous solution as the medium. After hydrothermal carbonization of biomass, insoluble solid products, and soluble organic by-products are produced, among which the former is a carbon microsphere with microstructure and contains a large number of hydrophilic functional groups on the surface. The latter is aldehydes, organic acids and other soluble organic matter.

Hydrothermal carbonization is a free radical reaction, including the competitive reaction process of depolymerization of macromolecules into small molecules and repolymerization of small molecular fragments into large molecules, including hydrolysis, dehydration, decarboxylation, polycondensation, aromatization, and other steps, accompanied by deoxygenation and dehydrogenation. Hydrothermal carbonization is a typical exothermic reaction, which mainly reduces the content of H and 0 in raw materials through dehydration and decarboxylation.

The hydrothermal carbonization process is mainly divided into the following stages:

1. Hydrolysis. The low molecular polymer produced in the initial stage is hydrolyzed to monomer, and the pH value of the reaction system decreases.
2. Dehydration. Dehydration in hydrothermal carbonization involves both chemical reactions and physical processes that remove water from the biomass matrix without changing its chemical composition. The monomers were dehydrated and decomposed into different soluble products.
3. Decarboxylation. Some carboxyl groups are eliminated in the hydrothermal carbonization process.
4. Polycondensation. An easily polymerizable unsaturated compound is produced by eliminating hydroxyl and carboxyl groups, and a soluble polymer is formed by polymerization or condensation triggered by intermolecular dehydration or aldol condensation.
5. Aromatization. The polymer undergoes aromatization to form the final product.

Compared with carbonization technology, hydrothermal carbonization technology has the following advantages:

• The reaction conditions are mild and the energy consumption is low;
• The raw material is unlimited, and because the reaction is carried out in an aqueous solution, the biomass does not need to be dried, which is easy to deal with the biomass with a higher moisture content (such as sludge), and helps to retain the surface functional group;
• The equipment is easy to operate and easy to master;
• The carbon fixation efficiency and carbonization yield are high, and the application prospect is broad.

Gasification technology

Gasification technology refers to the process of converting biomass feedstock into gas through thermochemical process under high temperature control conditions. The by-products in the gasification process are called biochar and biomass extract. According to whether the gasification agent is used, the gasification technology can be divided into two process types: using gasification agent and not using gasification agent.

Gasification agent – generally contains air, 02, H20, H2 and compound gas, etc. The gasification technology using gasifier needs to go through four stages: drying, pyrolysis, oxidation combustion and gasification:

1. The biomass raw materials are heated and dried after entering the gasification reactor;
2. With the increase of temperature, volatiles are gradually precipitated, and biomass raw materials are pyrolyzed at high temperature;
3. The pyrolysis product and the gasification agent undergo oxidation reaction and combustion in the oxidation zone;
4. The heat energy released by combustion is used to maintain the drying, pyrolysis and reduction reaction of raw materials, and finally generate a mixture of gases (including CO, CH4, H2,), and the yield of biochar is generally about 10%.

Dry distillation gasification is a gasification technology that does not use gasification agents, and the process is relatively simple. It is a process in which biochar, wood vinegar, wood tar, and gas are obtained by pyrolysis and gasification of biomass under the condition of limited oxygen or completely no oxygen, and the yield of biochar is generally 28% to 30%. In addition to dry distillation gasification, other gasification technologies generate less biochar, which is mainly used to produce combustible gas for gasification gas supply and power generation.

Microwave pyrolysis technology

Microwave pyrolysis technology is a new technology that uses microwave heating of biomass (temperature 400~500C) under the condition of limited oxygen, so that the biomass raw materials can be cracked into biochar in a certain time.

Microwave is an electromagnetic wave with a wavelength between 1 mm and 100 cm and a corresponding frequency of 300 MHz to 300 CHz. Microwave heating is a heating method that relies on the object to absorb microwave energy and convert it into heat energy, so that the whole body heats up at the same time, which is completely different from the traditional heating method. The principle of microwave heating is to increase the temperature of the material through the “internal friction heat” generated by the high-frequency reciprocating movement of dipole molecules inside the heated material, and the material can be heated inside and outside at the same time without any heat conduction process
And heating, heating speed is fast and uniform example.

Compared with conventional pyrolysis technology, microwave pyrolysis can directly penetrate the material and enter its interior, heat inside and outside the material evenly, heating time is short, and the secondary reaction of volatile components can be reduced, and the properties of biochar can be improved. Low energy consumption, high controllability, high energy efficiency, strong economy, but also selective heating. In the process of microwave pyrolysis, some moisture will participate in the reaction, which can enhance the microwave absorption capacity of biomass. Based on the above characteristics, the use of microwave pyrolysis instead of conventional pyrolysis to treat biomass waste has gradually developed into a research hotspot.

Outlook of biochar production technology

The energy consumption, carbon formation rate and physical and chemical properties of biochar are also different due to the different raw materials and technologies used for biochar production. Therefore, according to the raw materials and the requirements of biochar products, it is particularly important to choose the appropriate biochar production method. The results show that the yield of biochar prepared by slow pyrolysis, hydrothermal carbonization and microwave pyrolysis can reach more than 30%, and the yield of hydrothermal carbonization and microwave pyrolysis can reach 50%. The yield of biochar prepared by medium speed, fast speed, flash pyrolysis and gasification is lower than 30%. With the progress of science and technology, laser, plasma and other advanced technologies have gradually been applied to the production of biochar, due to its expensive equipment, high technical requirements for reaction conditions and equipment operators, has not yet been large-scale application, is still in its infancy.

At present, the technical system of biochar production by biomass carbonization is still not perfect, and further in-depth research is still needed from the following aspects:

1. The types and quantities of biomass raw materials in different regions are not the same, so it is necessary to maximize the use of local main biomass raw materials to produce biochar according to local conditions and reduce the waste of resources;
2. According to the nature of raw materials and the purpose of use of biomass, select the appropriate technology to prepare biochar;
3. Fully consider the economy of biochar production, optimize the reaction device, and reduce energy loss;
4. Establish relevant industry standards for the biochar industry to achieve standardized biochar production.

With the continuous development and improvement of biochar production technology, the application of biochar in various fields will be more mature, and more and more waste biomass will turn waste into treasure to promote the sustainable development of low-carbon economy.