Perpetual Next (€720M to turn organic waste into biocoal, biochar, and biogas)

Perpetual Next, a Dutch cleantech company founded in 2019, develops and operates production facilities using state-of-the-art torrefaction technologies to convert regional low-grade organic waste streams into renewable biobased products like biocoal, biochar, and biogas. The company's mission is to make cities and communities inclusive, resilient, and sustainable, and it contributes to this goal by supplying sustainable products.

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
Stamproy Green’s torrefaction reactor .

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 feedstock 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.

Perpetual Next Technology

Perpetual Next develops a torrefaction reactor to convert regional low-grade organic waste streams into renewable biobased products like biocoal, biochar, and biogas. The torrefaction reactor is designed to be safer and more thermally efficient, achieving an overall thermal efficiency of up to 80%. The biogas can be upgraded into green gas, methanol, and hydrogen through gas processing.

Perpetual Next torrefaction reactor

Perpetual Next designed torrefaction reactors that are safer and more thermally efficient. The figure below depicts the reactor.

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