Perpetual Next, a clean tech company founded in 2019 in Netherlands, develops and operates production facilities using state-of-the-art conversion technologies to convert regional low-grade organic waste streams into renewable biobased products like biocoal, biochar, and green gas. The company’s mission is to make cities and communities inclusive, resilient, and sustainable, and it contributes to this goal by supplying sustainable products.
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Challenges: biomass
Biomass is organic material that is renewable and comes from plants and animals. To avoid carbon dioxide emissions from fossil fuel use, the use of biomass fuels for transportation and for electricity generation is growing in many developed countries. 57 EJ (exajoules) of energy was produced from biomass in 2019, according to IEA, compared to 190 EJ from crude oil, 168 EJ from coal, 144 EJ from natural gas, 30 EJ from nuclear, 15 EJ from hydro, and 13 EJ from wind, solar and geothermal combined.
Torrefaction reactor uses organic materials to produce biocoal that can be utilized to replace fossil fuel. In principle, torrefaction is a pyrolysis process of biomass that occurs at atmospheric pressure and in absence of oxygen other than the oxygen contained in the biomass. The biocoal is characterized by high energy density, homogeneity, hydrophobicity, no biologic activity, and improved grindability, making it not only a product coming from a sustainable source, such as renewable wood supplies, but also an environmentally friendly one, often eliminating CO2 emissions when directly replacing natural coal.
Stamproy Green developed a torrefaction reactor plant by modifying a standard commercial dryer. How this reactor works is depicted in the figure below.
Stamproy Green’s torrefaction reactor has a perforated plate that heats the wood chip biomass that has been introduced via a screw conveyor. At the distal end of the perforated plate is a cooling screw conveyor that transports outgoing biomass. Biomass fills the entire space between the screw conveyor and the conduits. In this manner, biomass itself seals the reactor.
The torrefaction process begins with the introduction of heated gas at 500 °C under the perforated plate. The wood chips are torrefied by passing heated gas through holes in the plate and then through the wood chip biomass. The evolved syngas has a temperature of 250 °C. The biocoal produced exits the screw conveyor.
The efficiency of the torrefaction process is defined by the ratio of the energy output in the form of biocoal to the energy input in the form of biomass. The net energy lost is in the form of evolved syngas, which is lost as radiated or exhausted heat.
In order to utilize waste heat, 250 °C syngas is piped into a heat exchanger via conduits and a manifold. Heat exchanger is fed by the 800°C exhaust from the burner fueled by the evolved syngas. The superhot exhaust transfers thermal energy to the 250 °C syngas, producing the 500°C heat introduced into the reactor for convective heating of the biomass.
Stamproy Green’s torrefaction reactor has significant safety issues.
In the presence of oxygen, syngas is extremely flammable and could potentially explode. Any leak involves flammable hot syngas looking for oxygen and also results in the release of deadly carbon monoxide. Leakage risk in vibrating torrefaction reactors is caused by the inherent vibration conveying design. which loosens the couplings between the vibrating reaction chamber and the fixed conduits used to transport hot syngas.
Stamproy Green’s torrefaction reactor has 15 ducting conduits attached to its sides. At each of these large pipes or conduits is a gland with the flexible joints between the fixed conduits and the vibrating reactor that are designed to prevent oxygen from leaking into the reactor and toxic gas from escaping from the reactor. However, it has been discovered that these glands fail frequently, rendering such torrefaction systems unsafe, as air/syngas leaks are not only difficult to prevent, but also difficult to detect.
This heat exchanger also poses a safety risk.
Because the output of the burner is approximately 800 °C, a highly dangerous situation exists outside the reactor as a result of the superheated flue gas produced by this burner. This superheated gas is heat exchanged with recirculated syngas from the reactor to produce injected recirculated syngas as high as 500 °C, requiring a special heat exchanger that can cope with high differential temperatures of 800 °C on one side and 300 °C on the other side. The energy balance between the two sources is difficult to regulate and non linear in nature.
In addition to the safety concerns, the torrefaction reactor has an average thermal efficiency between 50% and 60%. The thermal runaway is the cause of the insufficient torrefaction process.
When wood is torrefied at temperatures between 250 °C and 300 °C, about 40% of the thermal energy within the wood is volatile. Managing the process so that only 20% of the heat should be utilized, as anything more would be wasted. If the remaining 20 percent of the torrefaction heat isn’t absorbed, process heat tends to escape. This is because the process requires that all volatile components of the wood escape at 340 °C. This in turn makes most torrefaction processes very inefficient.
Currently, in addition to slowing down the biomass feed stock supply, which can take up to 30 minutes to be effective, the entire plant must be shut down to stop thermal runaway. Production-wise, this is unacceptable. Consequently, it is crucial to be able to control thermal runaway by incorporating a process element that removes energy from the system.
This book covers the suitability of different methods for conversion of different types of biomass. (see on Amazon)
Perpetual Next Technology
Perpetual Next torrefaction reactor
Perpetual Next designed torrefaction reactors that are safer and more thermally efficient. The figure below depicts the reactor.
The reactor has a single pipe to transport all of the evolving syngas to a cooler for tar removal. Using one pipe instead of 15 conduits in Stamproy Green’s torrefaction reactor can significantly reduce risk concerns.
The output of the cooler is coupled to a gas engine. The gas engine can operate with syngas at high efficiency. These gas engines are readily available and are ideally suited for use in this system because the output gas temperature is precisely that required to heat the reactor. The gas engine can also be coupled with an electrical generator to generate the electrical energy to power the vibrating motors and other plant equipment, as well as to sell any excess electricity to the grid.
In operation one does not initially have syngas. To start up the reactor one must first ignite and idle the gas engine utilizing very little natural gas. Once heated, the reactor is actively rendered inert with steam and inert nitrogen to prevent combustion. Biomass is introduced slowly until syngas is produced. The process then gradually ramps up as the engine power increases, allowing for an increase in the biomass input.
The output of the gas engine (primarily nitrogen and carbon dioxide) is non-volatile and substantially inert, although some oxygen remains. This inert gas is completely safe for employees and reduces the risk of explosion. This gas engine output exits at approximately 500°C which is then injected through conduit into the bottom of the reactor.
Perpetual Next’s torrefaction reactor utilizes a solid, unperforated plate to torrefy biomass in order to separate the biomass from the gas used to heat the plate below. The hot inert output gas from the gas engine impinges upon the lower surface of the plate, heating it to 500 °C. The biomass is roasted in a manner similar to that of cooking biomass in a frying pan placed on a plate.
The fact that the plate is sealed to the ends of the reactor chamber, thereby separating the chamber into a lower heating chamber and an upper process chamber, will also be appreciated. As a result, while inert gas from the gas engine is used to heat the plate, the only portion of the reactor where flammable gas exists is above the plate, and with the utilization of only one conduit the vibration-related leakage is minimized.
In addition to its safe design, the torrefaction reactor of Perpetual Next can achieve an overall thermal efficiency of up to 80%.
20% volatile syngas energy is used to fuel the gas engine that generates heat for the torrefaction process, while the remaining 20% is converted into electrical energy, instead of contributing to thermal runaway.
In addition, the thermal runaway is prevented by introducing cold air through damper assembly into the bottom heating chamber of the reactor. The air damper system is utilized to stabilize the amount of volatiles being produced in order to stabilize the temperature of the gas applied to the bottom of the plate.
Perpetual Next biocoal or biochar
The conventional torrefaction process typically produces biocoal with an energy density between 20 and 21 GJ/ton.
The energy density is primarily determined by the temperature and the residence time of the biomass in the process chamber, with a longer residence time or higher temperatures producing biocoal with a higher energy density. However, as the energy density increases, so does the quantity of tar produced and contained in the syngas. This necessitates the removal of greater quantities of tar from the syngas, because tar in the syngas could spontaneously condense and contaminate the gas engine, and because the polycyclic aromatic hydrocarbons due to the use of tar have been linked to various forms of cancer may be released.
In order to produce biocoal with high energy density, Perpetual Next’s modified torrefaction reactor is designed to maintain the temperature of the process chamber above the condensation temperature of tar. Consequently, tar remains in the syngas and burned by the oxidizer to supply heat to the heating chamber and thus the process chamber. Therefore, the tar does not require treatment separate from the torrefaction apparatus, but rather provides energy to run the torrefaction apparatus.
As illustrated in above Figure, biomass is fed into the dryer, where it is dried with the warm gas from the oxidizer. The dried biomass is fed into the process chamber of the reactor through the inlet airlock. Airlocks allow biomass to enter and exit the process chamber while preventing air exchange between the process chamber and the ambient atmosphere.
In the process chamber, biomass is heated to temperature between 300 °C and 400 °C so that it becomes torrefied and, over time, transforms into biocoal while releasing syngas. The biomass resides in the process chamber for between 15 and 30 minutes. The temperature of the process chamber is between 400 °C and 500 °C.
Once the biomass has transformed into the predetermined grade of biocoal, which occurs after the predetermined residence time inside the process chamber, the biocoal is discharged through the outlet airlock. The resultant biocoal is hydrophobic and therefore can be easily stored without additional weather protection. The biocoal then is used for a variety of applications, including energy production, as an additive in steel production, and as a permanent carbon storage.
The energy density of the biocoal is determined by the residence time of torrefying dried biomass in the process chamber and the resulting final temperature of the biomass. For example, for a process chamber at a temperature of about 500°C, a 15 minutes residence time provides biocoal with an energy density of about 22 GJ/ton. The energy density increases as the residence time increases at the same temperature. For a 30-minute residence time, the energy density may be about 28 GJ/ton. This also implies that the amount of tar released from a specific volume of torrefying biomass increases with time.
During torrefaction, the syngas containing tar, H2, CH4, and CO is separated from the dried biomass and passes through the conduit. The syngas in the process chamber and in the conduit is maintained at a temperature higher than the condensation temperature of tar to prevent its condensation, which occurs below 400 °C. Thus, tar remains in the gas phase.
The temperature is maintained by preventing temperature exchange with the environment of the process chamber and the conduit, by combusting a small amount of the tar, or by supplying heated air from the oxidizer into the process chamber and/or the conduit to combust the tar. The material used to construct the process chamber absorbs and dampens temperature fluctuations. This is particularly advantageous for biomass of varying quality that releases syngas of varying components during torrefaction.
The syngas produced in the process chamber is transported to the oxidizer through the conduit. The conduit may have a fan to generate a pressure inside the process chamber that is lower than the ambient pressure. Consequently, if the process chamber has a leak, little syngas will escape into the environment.
The oxidizer burns the syngas at a temperature above 750 °C. The oxidizer air feed is configured to control the thermal energy generated by the oxidizer so that the process chamber is heated to a temperature between 400 °C and 500 °C.
The temperature of the heating chamber is maintained at no more than 600°C. By limiting the temperature of the process chamber and/or the heating chamber, less expensive materials can be used to construct the entire torrefaction apparatus, resulting in a more cost-effective design.
Perpetual Next Products
The HTT4 Reactor is the first commercially available High Temperature Torrefaction reactor. The reactor can be integrated smoothly into a variety of plant layouts. Utilizing local, underutilized waste or by-product feedstocks, it converts the waste’s physical form into a carbon neutral/dense product with multiple applications. Thus, customers can produce their own high-quality renewable carbon from local organic waste. The output of one single torrefaction line can produce up to 20,000 tonne per annum, depending on the type of feedstock and product required.
The picture below shows construction of the Baltania torrefaction plant in Estonia continues in 2020.
Perpetual Next Funding
Perpetual Next has raised a total of $832M in funding over 4 rounds, including three Seed rounds, and a Series A round. Their latest funding was raised on Dec 31, 2021 from a Series A round.
Perpetual Next Investors
Perpetual Next is funded by Momentum Capital.
Perpetual Next Founder
Martijn van Rheenen is co-founder.
Perpetual Next CEO
Niels Wage is CEO.
