What are the advantages of biomass pyrolysis?
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What are the advantages of biomass pyrolysis?


Pyrolysis is the thermal decomposition of biomass that occurs in the absence of oxygen. It is the basic chemical reaction that precedes the combustion and gasification processes and occurs naturally within the first two seconds. The products of biomass pyrolysis include biochar, bio-oil and gases, including methane, hydrogen, carbon monoxide and carbon dioxide. Depending on the thermal environment and final temperature, pyrolysis will produce biochar primarily at low temperatures below 450 0C, where heating is slow, and at high temperatures above 800 0C, where gases are primarily produced, where heating is rapid. At moderate temperatures and relatively high heating rates, the main product is bio-oil.

Pyrolysis can be performed at relatively small scales and in remote areas, thereby increasing the energy density of the biomass resource and reducing transportation and handling costs. Heat transfer is a key area in pyrolysis, as the process is heat absorbing and must provide sufficient heat transfer surfaces to meet the process heat requirements. Pyrolysis provides a flexible and attractive way to convert solid biomass into easily stored and transportable liquids that can be successfully used to produce heat, electricity and chemicals.


Three types of biomass pyrolysis:
(1) Slow pyrolysis

Mainly used for charcoal burning, biomass is pyrolyzed at a very low heating rate and at temperatures below 400°C for a long time (hours ~ days) to obtain a maximum coke yield of 35% (mass fraction), a process also known as charring of biomass.

(2) Conventional pyrolysis

The biomass feedstock is put into a conventional pyrolysis unit and pyrolyzed at less than 500°C, lower heating rate (10~100K/min) and 0.5~5s residence time of pyrolysis products, which can make the same proportion of gas, liquid and solid products (20%~25% biochar and 10%~20% bio-oil by weight of feedstock can be obtained).

(3) Rapid pyrolysis

Fast pyrolysis is to put finely ground biomass raw materials into the fast pyrolysis device, biomass at atmospheric pressure, ultra-high heating rate (1000~10000K/s), ultra-short product residence time (0.5~2s), moderate pyrolysis temperature (500~650℃) instantaneous gasification, and then rapid condensation into liquid, can obtain the maximum liquid yield, the product of bio-oil can generally reach The heat required for the rapid pyrolysis process is partly generated by the pyrolysis gas as the heat source.


Pyrolysis has economic and environmental advantages:

(1) Utilization of renewable resources through carbon neutral pathways – environmental potential

(2) Utilization of waste materials, such as wood processing waste (bark, sawdust, deforestation, etc.), agricultural residues (straw, fertilizer, etc.) – economic potential

(3) Self-sustainable energy – economic potential

(4) Conversion of low energy in biomass into high energy density liquid fuels – environmental and economic potential

(5) Potential for production of chemicals from biological resources – environmental and economic potential


Utilization of pyrolysis products:

Biomass pyrolysis products mainly consist of bio-oil, non-condensable gas and charcoal. Bio-oil is mainly used in combustion for heating, electricity production, fuel oil and chemical production. Although the calorific value of bio-oil is only about half that of fossil fuel oil and the large moisture content makes its application more difficult, the advantages of liquid products for combustion applications are easy handling, transportation and storage, as well as facilitating the utilization of existing fuel oil installations. The main problem of bio-oil application for combustion is the high viscosity of biomass oil and its susceptibility to deterioration. The advantage of producing a liquid product is that the production of this liquid product can be separated from the production of electricity, so it is feasible to set up peaking power stations with pyrolysis units or to send the liquid product directly to a central power station to generate electricity. Bio-oil contains many chemicals and products, and hundreds of substances have been identified in the liquid product of pyrolysis, so there is a great deal of interest in recovering compounds or congeners from it, and the potential value of some specific compounds, even if recovered in small amounts, is much higher than the value of recovered oil. In conclusion, the application of bio-oil as a fuel in boilers, turbines and diesel engines is relatively successful experimentally, and its refining and purification and extraction of chemical substances are not yet mature in terms of research and do not have economic feasibility.


Biomass pyrolysis process considerations:

In addition to the pyrolyzer itself, any pyrolysis unit requires several components.

(1) Production of bio-oil from the pyrolyzer. The bio-oil produced is refined and fractionated to have potential as a transportation fuel. Some can also be used to make specialty chemicals, especially cyclic structured compounds that can be used in adhesives. The gases produced contain combustible components, so these are used to generate heat. Biochar is also produced. Biochar can be used as a soil conditioner to improve soil quality and can even be used as a carbon material as a catalyst carrier or activated carbon. The process also results in a mineral material called ash. Usually, the ash must be included.

(2) The next unit to consider is the separation unit. The carbon is a solid and is therefore usually separated using a cyclone or baghouse. It can be used as a catalyst for further decomposition into gas because of the minerals inherent in charcoal and the fact that carbon catalyzes the gasification reaction. The liquid and the gas must also be separated. Usually, the liquid and gas must be cooled to separate the condensable liquid from the non-condensable gas. The liquid is then fractionated and likely further processed to improve the stability of the liquid. Sometimes, the liquid fraction may become clogged due to heavier components. Non-condensable gases need to be cleared of any trace liquid and can be reused if needed.

(3) The next consideration is the heat source for the equipment. The hot flue gas is used to dry the feed material. Since the flue gases contain combustible gases, they can be partially burned to provide heat. Any remaining coke will be burned as the primary heat supply. Also, the biomass can be partially burned as another major heat source.

(4) Another important process to consider is the mode of heat transfer. Most of them are indirect, through metal walls and tube and shell units. Direct heat transfer is associated with char and biomass combustion. In a fluidized bed unit, the carrier (usually sand) brings in heat because the carrier is heated externally and recycled to provide heat to the pyrolyzer.