How Pyrolysis Works
Biochar Update 4
For those of you who are new biochar is a material that can capture carbon and increase plant yield. My goal is to learn about the how biochar is benefitable and how it is produced. This article covers on a high level the process of pyrolysis which is used to make biochar.
TLDR: Process of Pyrolysis
Biomass (input to make biochar) storage and drying. This is where were the moisture (or water levels) are decreased to a certain threshold to enable the most effcient burning
The biomass is transported to the pyrolysis reactor through a bucket feeder
Biomass heated in the Pyrolysis Reactor to remove volatile organic compounds which has been shown cause detrimental effects on leaves, flowering, seed production, protein content, plant metabolism. This is important because we are placing biochar (the output of the pyrolysis process) into plants
Biomass output, in this case biochar is cooled to the temperature of ambient air. When biochar is put into soil at the same temperature it was heated at (300 to 1000°C), you boil the water in the soil into evaporation — water has a 100°C boiling point.
Step 1: Biomass storage and drying
Why Drying is Important?
The reason you can’t have wet biomass is that Wood that is too wet to burn can struggle to catch fire, produce more smoke, release less heat
The Environmental Protection Agency (EPA) states that firewood with a moisture content within the 15 to 20% moisture range burns the most efficiently on fires. Moisture refers to the amount of water content within a material. Water is made up of two elements, hydrogen, and oxygen. Hydrogen is flammable, but oxygen is not. When both hydrogen and oxygen are combined they have different chemical properties. This is because it is made of hydrogen, which has been fully oxidized and can’t react with oxygen any further (because they are sharing hydrogens’ spare electrons). Remember oxidation means losing electrons. In this case, the oxygen is making the hydrogen lose electrons. This results in a slightly negative pole (for oxygen) and a slightly positive pole (hydrogen).
Good reason to reduce biomass as well. Through a drying process, moisture can be largely removed from biomass. The result is a reduction in the weight of the biomass. This leads to a reduction of the processing costs, as well as of the costs for storage and transport.
Step 2: Transported to the pyrolysis reactor through a bucket feeder
The raw biomass is pre-dried inside the bucket feeder through hot and dry air flowing in the opposite direction as the biomass (pink arrows in the image below). This decreases moisture by a few % which helps the pyrolysis process in the later steps occur faster.
Step 3: Biochar Heated in the Pyrolysis Reactor.
In the pyrolysis reactor, there are 4 different stages required to create biochar.
Stage 1: All the Moisture in the biomass has to evaporate to remove Volatile Organic Compounds.
The evaporation happens through a process of destructive distillation, which is when a solid is broken down in a closed system to remove volatile organic compounds (VOC). VOCs are hydrocarbons composed primarily of carbon and hydrogen atoms. Many of these compounds are volatile and can easily vaporize into the atmosphere at room temperature and atmospheric pressure.
VOCs have been shown to cause detrimental effects on leaves, flowering, seed production, protein content, plant metabolism. Since VOCs vaporize at room temperature — around 68 to 77°F (20 to 25°C) and ideal soil temperatures for planting most plants are between 65 to 75 degrees F. (18–24 C.). This suggests that VOC content will evaporate in plants when in soil. When in soil, they dissolve into water. This suggests that when it is room temperature (which is about the ideal temperature for plants) VOC will vaporize and take the water content with them. Drying out the soil, limiting available water for plants
Stage 2: The biomass is degasified
Degasification is a process to remove dissolved gases in liquids. The gases are the VOCs. This stage is when VOC removal is the most efficient. After mixing with humid air from the 1st stage of the pyrolysis reactor the reminder of the gases is can be used for energy (process).
This waste heat (or process heat) created from this stage can power the next batch of biochar or it can reduce the energy required to activate the process.
Stage 3: Carbonization
Dried and degasified biomass (from stages 1 and 2) is treated with high temperatures. The process results in a quick concentration of carbon and the disappearance of the fibrous structure, improving grindability. The Hardgrove Grindability Index (HGI) represents a material’s resistance to being crushed into smaller pieces.
The smaller the HGI, the harder and less grindable the raw material. The higher the grindability the lower the energy requirement, which is correlated to lower cost.
Depending on the temperatures inside the reactor and the duration of the carbonization process the ready material can have a calorific value. Calorific Value is the amount of heat energy present in the material (biochar).
The material has a high calorific value when the heat inside the water vapor in the pyrolysis reactor can be recovered. The material has a low calorific value when the heat inside the water vapor in the pyrolysis reactor can not be recovered. The more heat inside the water vapor we can recover the less energy we need to power our next batch of pyrolysis.
Specific Calorific Values (CV) of the pyrolysis process is between 21 to 29MJ/kg. Materials that have a similar CV to the pyrolysis process. My assumption is they would be effectively broken down into biochar and produce a good amount of heat vapor to power the next batch of pyrolysis. Some of the materials include
PVC — a type of plastic that is not recyclable has 35 MJ/kg CV
Medical waste has a CV of 19 MJ/kg
Due to high carbon and hydrogen contents, the dried food and market waste resulted in a gross calorific value of 23.25 MJ/kg
Dried food and market waste resulted in a gross calorific value of 23.25 MJ/kg
Wood has a Calorific Value of 21.2 to 21.70 (MJ/kg)
Manure has a CV of 19.77 MJ/kg
Note: Normal SI units for CV is J/kg. Mega joules have 1 million times more heat energy than SI units. For Context, The highest CV is 141.80MJ/kg for hydrogen.
Stage 4: The cooling process
Stage 4: The cooling process
The biochar we got from the 3 prior stages was being heated to anywhere from 300 to 1000 degrees. In this stage, the biochar is cooled to the temperature of ambient air. This happens through cool air flowing horizontally as the biochar is passing through. The reason you don’t want high-temperature biochar in soil is that it will increase the heat of the soil. Increased heat increases the speed of water movement until it has too much kinetic energy to stay in liquid form. The water boiling temperature is 100 degrees.
Note: Depending on the type of biomass and the moisture content when put as feedstock this can take from several seconds to more than a dozen seconds. Efficiency is due to the higher temperature in the upper part of the reactor.