What is Pyrolysis of Biomass?

Waste biomass is a renewable raw material. Pyrolysis is one of the important technologies for recycling waste biomass. Valuable bio-oil, biochar, and pyrolysis gas can be obtained by pyrolysis of biomass under hypoxia condition.

What is Pyrolysis of Biomass?

Biomass

In a broad sense, biomass includes all animals, plants, microorganisms, and their wastes, such as crops and crop waste, wood and wood waste, and animal manure.

The biomass commonly used for pyrolysis on the market is mainly lignocellulosic biomass, that is, forest residues and agriculture residues, such as straws, wood chips, etc. These materials are the fibrous structural parts of plants. They are largely made of cellulose, hemicellulose and lignin that are difficult to be broken down into chemical composition by nature. As a result, it is a challenge to recycle this carbon-rich waste resource.

Pyrolysis

Pyrolysis is the oxygen-free thermochemical conversion of organic materials. Pyrolysis of biomass is the thermal decomposition of biomass occurring in the absence of oxygen. It is usually performed at 300°C – 600°C or above. The high-temperature of pyrolysis of biomass causes the cellulose, hemicellulose and some lignin in the biomass to break down into smaller molecules in gaseous form. Upon cooling, these gases condense into a liquid state and become bio-oil. And the remainder of the original biomass (mainly the remaining lignin) is left as solid biochar and non-condensable syngas.

Comparison of Fast Pyrolysis and Slow Pyrolysis

Pyrolysis can be divided into two subgroups: slow pyrolysis and fast pyrolysis. The key differences are the maximum reaction temperature and heating rate. The different reaction temperatures and heating rates are principal factors to influence the morphology of the main pyrolysis products. The table below presents a comparison between the features of slow and fast pyrolysis. Usually, slow pyrolysis favors solid char production while fast pyrolysis favors liquid bio-oil production.

Features	Slow Pyrolysis	Fast Pyrolysis
Temperature	300℃-600 ℃	600℃-1000℃
Residence time	Heat up of the sample within hours or days.	Heat up of the sample within seconds (1–2 s) or minutes (1-10min) and the rapid condensation of products within seconds.
Heating rates	0.1°C/s-80°C/s	100°C/s–1000°C/s
Aeration	Oxygen-free or limited	Oxygen-free
Particle size	<2mm	<1mm
Main product	Solid biochar	Liquid bio-oil
Advatages	High yield of biochar. Ability to accept a wide range of particle size of biomass.	Higher yield of bio-oil. Simple and fast process. Very efficient energy conversion.
Disdvatages	Further treatment of gases is needed due to high CO. Possibility of requiring additional energy.	Low stability of bio-oil. Prefer biomass with low moisture content(<10%). Limited commercial experience.

Byproducts of Pyrolysis of Biomass and Their Applications

Because there is no oxygen present in pyrolysis, combustion does not occur. Instead, pyrolysis of biomass produces three products: liquid bio-oil, solid biochar, and biogas. The proportion of these products depends on biomass composition and process parameters.

Wide applications of biochar

Biochar is the main product of slow pyrolysis (300℃-600 ℃). It is a black carbonaceous material generated during pyrolytic reaction at a high-temperature and low-oxygen environment. It is a stable carbon-rich residue left after volatile gases and liquids have been driven off during pyrolysis. Here are several common uses of biochar:

Soil Amendment: Biochar is used as a soil conditioner to improve soil fertility, water retention, and nutrient availability.

Carbon Sequestration: It serves as a method for carbon sequestration. This helps to mitigate greenhouse gas emissions and combat climate change.

Efficient fuel: It can serve as an efficient industrial fuel in steel factories, cement factories, coal-fired power plants, etc. It is also widely used as fuel in cooking and home heating, barbecues, and survival in the wild.

Biochar animal feed: Added into livestock feed, it can reduce methane gas emissions from ruminants by regulating the digestive capacity of livestock, which is beneficial to the environment.

Great potential of bio-oil

Bio-oil is the main product of fast pyrolysis (600℃-1000℃). It is a dark brown liquid. It is a mixture of complex hydrocarbons with large amounts of oxygen and water. Its calorific value is 15.0 ~ 16.0 MJ/kg, which is 50-70% of petroleum-based fuel. The following explains the potential application areas of bio-oil.

Industrial fuel: Compared to gaseous products, bio-oil has a higher energy density and is more easily stored or transported. Therefore, it is considered a promising alternative feedstock for petroleum-based fuel. Its potential applications include its use as fuel for heating and power generation of boilers, engines, and turbines.

Chemicals Manufacture: Bio-oil contains a variety of valuable compounds. Therefore, the chemical extraction or reaction of bio-oil can be used to produce specialty chemicals. For example, it can be used to produce preservatives, tobacco liquids, resin precursors, fertilizers and additives in the pharmaceutical industry, and flavorings in the food industry.

Biogas

Because the biogas produced by pyrolysis is not stable enough, it is difficult to store. Therefore, the biogas is generally directly recycled and utilized to provide heat for the biochar pyrolysis equipment, reducing fuel costs.

Common Biomass Raw Materials for Pyrolysis

A variety of biomass materials can be used in the pyrolysis process. Compared with biomass in a broad sense, lignocellulosic biomass in a narrow sense is more widely used in pyrolysis.

Forest biomass

Wood waste from logging or pruning of trees
These wastes usually include branches, bark, tree tops, leaves, etc. They are the parts that have no direct use value in logging or pruning activities.
Wood scraps waste during wood processing
These wastes usually include wood chips, sawdust, wood shavings, etc. They are usually a by-product of wood processing or a substandard product.

Agriculture biomass

Straws: They are mainly straws of various grains, such as wheat straw and corn straw and so on.
Grain residue: This refers to residue produced after harvest of crops, such as corn cobs, rice husks, etc.
Vegetable and fruit residue: This means waste parts produced in the vegetable and fruit industry, such as skins, fruit pits, etc.

Biomass Pyrolysis Process

The process of pyrolysis of biomass typically consists of the following stages:

Drying: Before pyrolysis begins, the biomass is dried to reduce its moisture content. Excess moisture can lower the efficiency of the pyrolysis process and lead to incomplete decomposition.
Heating: The dried biomass is then heated to a certain temperature in a pyrolysis reactor. The temperature range can vary depending on the desired products and the type of biomass.
Pyrolysis: With the increasing temperature and the absence of oxygen, the biomass undergoes thermal decomposition and breaks down into compounds with lower molecular weight. Thus produce three main products.
Condensation: After the decomposition, the bio-oil and biogas are cooled, causing the liquid bio-oil.

Benefits of Biomass of Pyrolysis

Biomass pyrolysis is a valuable and promising technology with economic and environmental benefits. Specific information is as follows.

Economic benefits

Create a revenue stream for the business: It can convert organic waste into valuable biochar and bio-oil. With low-cost waste biomass raw materials and widespread use of biochar and bio-oil, enterprises engaged in biomass pyrolysis can achieve a high rate of return by selling these energy products.
Increase resident income: Biomass pyrolysis project creates jobs both in the construction phase and in ongoing operations. This can stimulate the local economy. Moreover, recycling large amounts of waste biomass resources helps to increase farmers’ income.

Environmental Benefits

Sustainable waste treatment solution: It reduces the amount of biomass waste that needs to be incinerated or landfilled. This minimizes methane emissions from landfills and contamination of the land. By converting waste into useful products, pyrolysis contributes to a more sustainable waste management system.
Clean production: The gas produced by biomass pyrolysis plant can be directly recycled. This reduces the use of fossil fuels and greenhouse gas emissions. The wastewater generated in the pyrolysis process can be directly recycled for heating, reducing environmental pollution.
Carbon sequestration: The product biochar breaks down very slowly, which means it can lock carbon in the soil for longer periods, helping to mitigate climate change.
Renewable energy: Biomass pyrolysis converts waste biomass into valuable biochar and bio-oil. These renewable energy products can replace fossil fuels and reduce air pollution caused by fossil fuels.

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