Fast Pyrolysis of Biomass Residues in a Twin-screw Mixing Reactor

A procedure for thermochemical conversion of biomass residues is presented that aims at maximizing the yield of liquid products (fast pyrolysis). It is based on a technology proven on an industrial scale and especially suitable for treating a straw type of biomass.

The overall goal of this procedure is to provide a technical solution to produce biofuels or biofuel intermediates from biomass residues, such as straw, with fast pyrolysis process conditions. The method applied in our fast pyrolysis process contributes to a more efficient use of biomass residues by a compact reactor and a customized product recovery concept. The main advantage of this technology is that the size of the equipment is reduced compared to state of the art fluidized bed technology because no fluidizing gas is needed.

Another key element to account for the difficulty to treat ash rich material, like wheat straw, is a fractionated product separation in order to obtain defined and stable products. Demonstrating the procedure will be Daniel Richter, the engineer responsible for the test rig. To begin this procedure, activate the pyrolysis and condensation system by starting the auxiliary nitrogen supply and the pyrolysis gas fan.

Then, regulate the fan by opening the fans menu in the process control and adjusting its nominal volumetric flow such that the pressure in the reactor is three to eight millibars above ambient pressure. Next, fill the bio-oil cycle with an appropriate amount of ethylene glycol as the starting medium for the quenching system to allow safe operation of the pump and homogenizer. Record the weight of this starting medium.

Then, fill the aqueous condensate cycle with an appropriate amount of water as the starting material to allow safe operation of the pump. Record the weight of this starting material. Heat up the system, including the heat carrier heater, and all auxiliary heaters by opening their menus in the process control and entering their desired values.

Following this start the cooling cycle for the heat exchangers in both condensation cycles by switching on the cooler. Start the pumps of both condensation cycles by opening their menus in the process control and clicking on activate. Use the same menus to adjust the mass flow to provide enough cooling power.

Now switch on the electrostatic precipitator. After both condensation cycles have run for 10 to 20 minutes check the nozzles of the quenching system for blocking and remove any blockage present. Start the heat carrier loop by opening the menu of the bucket elevator and the heat carrier feeding screw in the process control and clicking on activate.

Set the heat carrier temperature to a value above the desired reactor temperature in order to allow a smoother startup by accounting for the heat requirements with a pyrolysis reaction. After the system has reached the set temperatures, start feeding biomass by filling the biomass storage with the desired feed stock. Subsequently, open the lock hopper, and start the biomass feeding screw by clicking on activate in their menus in the process control.

Slowly increase the feed rate every five to 10 minutes in order to prevent excessive pressure fluctuations. Record the amount of biomass fed in order to account for balancing and take appropriate samples. Check for the desired reactor temperature and regulate the heating of the heat carrier loop accordingly.

Following this regulate the fan by adjusting its nominal volumetric flow to keep the desired reactor pressure. Then, check for blocking in the nozzles of the quenching system. Observe the pressure drop across the cyclones and the quenching system in order to detect excessive scaling early enough.

Clean the tubes cross section with a rod to remove excess scaling during operation, especially at the point of the first temperature drop of the pyrolysis vapors. Seal the rod with a gasket to prevent intake of air into the quenching system. Then, install a ball valve at the inlet point of the rod to further decrease air leakage if the cleaning is not in operation.

Next, monitor the condensation temperatures of both condensation cycles. Remove condensate from the cycles as soon as 80%of the maximum allowable filling level has been reached. To stop the experiment, turn off the biomass feed, and regulate the fan to keep the desired reactor pressure.

After allowing the system to run for another 30 to 40 minutes, turn off the heating of the heat carrier loop. Then, turn off the condensation cycle pumps and electrostatic precipitator. Empty both condensate cycles, and record the weight of each condensate.

Subtract the amount of starting material before setting up the balances. After allowing the containers for char collection to cool down to room temperature, weigh the amount of char. Finally, clean the bio-oil cycle with fresh ethylene glycol in the aqueous condensate cycle with a one to one mixture of water and ethanol.

On an as received basis, the solid's yield is in the range from 14 to 25%by weight for the investigated feed stocks, and increases with their ash content. Total condensate yields range from 53 to 66%by weight, whereas gas yields are relatively similar for all three biomasses. In this study, organic oil yields increase, with decreasing ash contents of the feed stocks and yields of reaction water are in a comparatively narrow range from 12 to 14%by weight.

It becomes evident from the elemental carbon balances that the larger part of carbon is recovered in the bio-oil and that higher ash content of the feed stock does not necessarily contribute to more formation of organic char fraction. A mass fraction of only about three to 4%of carbon is recovered in the aqueous condensate. Once mastered this technology can be realized in industrial scale.

KIT runs a pilot plant with 500 kilograms per hour feed capacity for several years as a next step to achieve this goal. Don't forget that working with pyrolysis in general requires precautions to avoid explosive atmospheres and exposure to products. Therefore, only well-educated staff should perform this procedure.