Recovery of energy and materials from waste

| Hans Wolkers

Flash pyrolysis, a mild heat treatment lasting just a few seconds, is becoming an important recycling tool. The University of Twente is leading in this promising technology that contributes to the circular economy, since a wide variety of waste streams can be well recycled. Organic waste streams, paper sludge, car tires, and in the near future maybe even composite materials can be effectively separated into energy and valuable reusable materials.

Photo by: Hans Wolkers

The laboratory of Department of Thermal and Fluid Engineering is stacked with impressive looking equipment. Shiny metal tubes, colorful wiring and countless meters and switches challenge the senses. In this science fiction-like world, Gerrit Brem, Professor of Energy Technology, and his team perform small and medium scale tests separating waste materials into valuable components. Brem holds a glass bottle with a sticky pitch-black fluid. ‘This oil is one of the major and most valuable products resulting from flash pyrolysis of agricultural and woody waste streams, like straw, wood, and leaves,’ he says. ‘With our technologies we are able to produce these high-quality biomass-based oils for use in gas turbines and engines.’

Valuable products

Flash pyrolysis is a low-temperature separation technology used since the 1980-‘s. In contrast to combustion, where temperatures reach between 800 and 1000 °C and only ash and heat remain, pyrolysis occurs at only 500 °C and without oxygen being present. As a result, the material doesn’t burn, but is separated into different fractions due to the exposure to heat. It is a relatively cost-effective and mild thermal separation method, resulting in energy and valuable components that can be reused. For example, flash pyrolysis of wood, where small wood fragments are exposed to heat for just seconds, results in about 75 percent oil vapor, 15 percent gas and 15 percent carbon (char).

Over the years, Brem’s team has developed and optimized the pyrolysis of woody waste streams. First the biomass is chopped into small particles, just a few millimeters in size. These particles are blown along the walls of a hot cyclone reactor of 500 °C . ‘We can’t use air to blow the particles inside the reactor chamber, because the oxygen present would burn the biomass,’ Brem explains. ‘Instead, we reuse the gas formed during the pyrolysis process.’ Sand is used as a heat carrier, resulting in a fast heat transfer. Within seconds, the materials are pyrolyzed. The resulting oil vapor is quickly cooled down to room temperature and collected at the bottom of the reactor. The gas formed is used to blow in new biomass fragments into the reactor, while the char contains energy and can be used to heat the pyrolysis reactor, resulting in an energy neutral operation.

Better oil quality

Although the newly developed process is sustainable and cost-effective, there is room for improvement. The resulting oil is quite acidic and its consistency changes over time: it thickens due to polymerization. In addition, the caloric value is relatively low because of the presence of oxygen, originating from cellulose, an important wood component. ‘It is possible to increase the oil quality during its formation, in the reactor,’ Brem says. ‘We can achieve that by adding catalysts, such as sodium- or potassium carbonate. This results in the removal of oxygen containing components and a much better oil quality.’ Although this better-quality oil has similar properties as fossil oil, the yield is relatively low. Therefore, the team focuses on developing a milder deoxygenation and hydrogenation method that is a compromise between oil yield and oil quality.    

Close cycles

After the success of wood waste recycling, Brem and his team further developed the pyrolysis method for application to other waste streams. The processing of paper sludge using pyrolysis proves to be another promising application. This large waste stream of the paper industry consists of equal amounts of short-fibred cellulose and minerals in water. Until recently, there was no sustainable method to treat this waste stream and dumping it on landfills was common practice. But applying flash pyrolysis on paper sludge results in an efficient and cost-effective thermal separation: the cellulose in the sludge is converted into reusable oil, while the minerals remain. The oil can be used to fuel the paper industry machinery, or to dry the paper sludge before pyrolysis. The minerals fall down and can be collected at the bottom of the reactor.

'With his patented method we have managed to close the paper cycle'

‘These minerals still contain a small fraction of char, that can be removed by mild combustion, resulting in pure minerals that are perfectly suitable for reuse,’ Brem says. ‘With his patented method we have managed to close the paper cycle.’ The waste recycling company Alucha in Arnhem has bought the patent and has successfully scaled-up the process in a working pilot plant in a mobile container with a capacity of 100 kg sludge per hour. The company is currently developing a large demonstration facility for the paper industry. 

Enormous waste

The application of Brem’s pyrolysis techniques to other waste streams also proved effective. The team’s research has shown that flash pyrolysis also may solve the problem of the enormous amounts of old car tires. Worldwide, more than 800 million tires are wasted every year. Most of these are incinerated, resulting in environmental issues and an enormous waste of materials. ‘We are currently developing a fast pyrolysis technique, where car tires are fully recycled into fuels and high quality carbon black,’ says PhD researcher and Brem’s team member Balan Ramani. ‘Carbon black is the most valuable compound resulting from pyrolyzed car tires.’ He shows a jar filled with pitch black grains: carbon black recovered from car tires. The black, grainy substance is used as pigment, but it also makes car tires more resistant towards wear and tear as well as UV radiation. Because of its high value, the new flash pyrolysis process is aimed at producing the highest quality carbon black possible that can be reused in new tires. The other resulting components are oil, that can be used as fuel, and chemicals like benzene, toluene, and xylene, important for the chemical industry.

'We are currently developing a fast pyrolysis technique, where car tires are fully recycled'


Brem and his team are also working on the thermal separation and recycling using pyrolysis of modern composite materials. Many consumer as well as industrial products contain carbon- and glass-based composite materials. For example, wind turbine blades, car parts and boats contain a lot of these materials. They often contain plastics as a base, reinforced with a matrix of carbon- or glass-fibers. Recycling proves to be very challenging, because mechanical recovery results in inferior materials, not suitable for reuse. ‘With increasing applications of these composite materials, it will be essential to develop technologies for recycling, to save materials and reduce costs,’Brem states. ‘We are currently exploring new pyrolysis techniques to recover these fibers from waste streams. The aim is to collect high-quality fibers and fuels that can be reused, hence closing energy and material cycles.’

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