3.1 Thermochemical Conversion Technologies
65
suitable for large size feedstock and have a compact design. However, high mechanical abrasion of biochar was commonly observed in these reactors. The bubbling
fluidized bed can be heated by recycled gas, hot inert gas, fire tubes or autothermally
via partial combustion. These types of reactors have high heat transfer rates, limited
char abrasion and very good solid mixing. They have a simple configuration, and the
residence time of the solids and vapors can be controlled by the fluidizing gas flow
rate, whereas fine particle size (<2 mm) of the feedstock is generally required. On the
other hand, Circulating Fluidized Bed (CFB) reactors are heated through in-process
combustion/gasification of char. These reactors are advantageous for their high heat
transfer rates, however, erosion and abrasion of biomass and biochar could be severe,
leading to much higher char production than that in other types of reactors. In addition, CFB pyrolysis reactors are more complex in design but allow larger particle
sizes up to 6 mm. Another type of reactor for biomass pyrolysis is entrained flow
reactors that have relatively lower heat transfer rates, limited gas/solid mixing and a
particle size limit of less than 2 mm [11].
3.1.2.1 Fast Pyrolysis
Fast pyrolysis occurs at a high heating rate (10–1000 °C/s), very short retention
time (in the order of seconds) and rapid cooling of the pyrolysis vapors. Due to the
high heating rate, biomass reaches to the final temperature before it is completely
decomposed. The aim of fast pyrolysis is to maximize the bio-oil yield up to 77–
80% [25, 26]. The use of catalysts could increase the performance of the pyrolysis
process and the characteristics of the produced bio-oil. Catalysts can decrease the
oxygen content of bio-oil, hence reduce the oil acidity, and increase the oil stability
by preventing polymerization and condensation reactions. They can also reduce the
nitrogen content of bio-oils. Catalysts could be either mixed with the biomass and
placed inside the reactor or they could be placed as a catalyst bed in the downstream
of the biomass pyrolysis zone. The pyrolysis vapors pass through the catalyst bed
where upgrading (catalytic cracking) takes place. The bio-oils produced from the
catalytic pyrolysis were reported to have lower acid contents, higher aromatics and
higher heating values compared to the ones produced without catalysts [26].
Fast pyrolysis technologies have been initially commercialized by several companies worldwide, among which the dominant ones are Ensyn (Canada), Dynamotive
(Canada) and Renewable Oil Corporation (Australia). There are also a number of
competitive projects in China [35]. Till 2015, there was no commercial pyrolysis oil
production plant in Europe in operation, until the Empyro pyrolysis oil production
unit was started in the Netherlands in 2015. The plant produces bio-oil as well as
electricity and steam. The production capacity is planned to gradually increase to
more than 20 million liters of pyrolysis oil per year. The process is a flash pyrolysis
based on the Biomass Technology Group (BTG) and designed based on the experience gained by BTG through the construction of a 50 ton/day pyrolysis demo plant in
Malaysia [36]. Both Ensyn (Canada) and Dynamotive (Canada) ceased operation of
65
suitable for large size feedstock and have a compact design. However, high mechanical abrasion of biochar was commonly observed in these reactors. The bubbling
fluidized bed can be heated by recycled gas, hot inert gas, fire tubes or autothermally
via partial combustion. These types of reactors have high heat transfer rates, limited
char abrasion and very good solid mixing. They have a simple configuration, and the
residence time of the solids and vapors can be controlled by the fluidizing gas flow
rate, whereas fine particle size (<2 mm) of the feedstock is generally required. On the
other hand, Circulating Fluidized Bed (CFB) reactors are heated through in-process
combustion/gasification of char. These reactors are advantageous for their high heat
transfer rates, however, erosion and abrasion of biomass and biochar could be severe,
leading to much higher char production than that in other types of reactors. In addition, CFB pyrolysis reactors are more complex in design but allow larger particle
sizes up to 6 mm. Another type of reactor for biomass pyrolysis is entrained flow
reactors that have relatively lower heat transfer rates, limited gas/solid mixing and a
particle size limit of less than 2 mm [11].
3.1.2.1 Fast Pyrolysis
Fast pyrolysis occurs at a high heating rate (10–1000 °C/s), very short retention
time (in the order of seconds) and rapid cooling of the pyrolysis vapors. Due to the
high heating rate, biomass reaches to the final temperature before it is completely
decomposed. The aim of fast pyrolysis is to maximize the bio-oil yield up to 77–
80% [25, 26]. The use of catalysts could increase the performance of the pyrolysis
process and the characteristics of the produced bio-oil. Catalysts can decrease the
oxygen content of bio-oil, hence reduce the oil acidity, and increase the oil stability
by preventing polymerization and condensation reactions. They can also reduce the
nitrogen content of bio-oils. Catalysts could be either mixed with the biomass and
placed inside the reactor or they could be placed as a catalyst bed in the downstream
of the biomass pyrolysis zone. The pyrolysis vapors pass through the catalyst bed
where upgrading (catalytic cracking) takes place. The bio-oils produced from the
catalytic pyrolysis were reported to have lower acid contents, higher aromatics and
higher heating values compared to the ones produced without catalysts [26].
Fast pyrolysis technologies have been initially commercialized by several companies worldwide, among which the dominant ones are Ensyn (Canada), Dynamotive
(Canada) and Renewable Oil Corporation (Australia). There are also a number of
competitive projects in China [35]. Till 2015, there was no commercial pyrolysis oil
production plant in Europe in operation, until the Empyro pyrolysis oil production
unit was started in the Netherlands in 2015. The plant produces bio-oil as well as
electricity and steam. The production capacity is planned to gradually increase to
more than 20 million liters of pyrolysis oil per year. The process is a flash pyrolysis
based on the Biomass Technology Group (BTG) and designed based on the experience gained by BTG through the construction of a 50 ton/day pyrolysis demo plant in
Malaysia [36]. Both Ensyn (Canada) and Dynamotive (Canada) ceased operation of