My main driver of doing a research project has been the idea of increasing the value of waste due to the numerous worldwide challenges to manage waste production from waste/water treatment, and the impacts of climate change due to the excess use of fossil fuels. So, how to make the waste more valuable? Anaerobic Digestion process (Figure 1) has been one of the best approaches as waste-to-energy production system. Although, biogas is the main product from this bioprocess, there is residual waste, called digestate, which can be further converted into more valuable products. Thermal technologies, such as pyrolysis, is one of the most potential means to be implemented for this conversion. 

Why did I apply BRISK2? 

My project has been based on theoretical research where the literature has played an important role in understanding the principles of thermal decomposition of biomass in pyrolysis. Kinetic reactions and correlations have analysed to develop a model to predict how lignocellulose is decomposed in pyrolysis, this approach is based on approach previously reported in literature. However, pyrolysis products composition of the model cannot be validated due to the lack of detailed information of digestate composition and pyrolysis products from digestate. Thus, experimental work is crucial to understand pyrolysis behaviour of digestate as well as the analysis of pyrolysis product composition in order to obtain key data to adjust and validate the kinetic model. 

BRISK2 has a range of specialised institutes with great experience to help students with their research to have better results. I applied for a fast pyrolysis facility in VTT, the Technical Research Centre of Finland because it has a well developed system, with great operation and with a high pyrolysis product recovery, which is favourable for my project to obtain significant data and information of fast pyrolysis of digestate. This, as a result, will support a comparison I am performing between bio-oil products from other two thermal processing at the university: slow and microwave pyrolysis 

 

VTT experiments  

My first proposal was just about getting to know fast pyrolysis as thermal processing, understand a bit more how this technology works due to greater amount of bio-oil produced. However, everything turned into the possibility of working with my own samples. So, crop digestate was sent from the UK to Finland to be processed in fast pyrolysis, which made my experience more valuable because this digestate is generated from a large scale in the industry in the UK.  

Previous VTT visit, the crop digestate was collected and dried at 105oC for 24 hrs to be sent to Finland. Due to some requirements for the feedstock to reach higher efficiency in the reactor, such as water content lower than 10%, and the particle size less than 1 mm, the digestate had to go through a pre-treatment in VTT. This was changed from raw material into pellets (Picture 1), then grinded, and finally sieved to obtain particle sizes between 0.5 and 0.9 mm to get the final feedstock for the system (Picture 2). 

Four experiments were performed at different temperatures: 460oC, 480oC, 520oC and 560oC. In order to get consistent values for a reliable overall mass balance, each run had a continuous feeding for 3 hours, resulted in a flow rate of around 800g of digestate per hour. Water content was measured just before every run (Picture 3) to quantify and differentiate the water produced during the reactions from the water in the feedstock.  

The main parameters in each experiment are shown in Table 1; where the feed rate was calculated at the end of the experiment by the difference between the initial feed and the final crop digestate remained divided by the time the experiment was run. 

The four experiments were successful with an average of 92% of products recovery. These results are shown in Table 2, where pyrolytic is the sum of the organics and water produced.  

An example of the products recovered from the four fractions of fast pyrolysis system is shown in Picture 4. Picture 5 is the char produced at 520oC. Bio-oil is mostly collected in the first two fractions, where it is noticeable two phases due to some water in. Some results are still being processed, such as bio-oil composition to compare with the other two pyrolysis, and to make further adjustments to the kinetic model. 

 The general diagram that can represent the system of fast pyrolysis of crop digestate is shown in Figure 2. 

Acknowledgments 

There is a huge effort behind what it shown in this report, and involved very important people. Thank you to… 

John Robinson, who is my main supervisor at University of Nottingham and who has supported my project including this great opportunity of collaborative work with other institutes. 

Sanna Tuomi, who was one of the main contacts in VTT to make this happen. She was essential during the whole process, including to get safe to Finland. 

Christian Lindfors (Picture 6), the expert in fast pyrolysis in VTT, and a crucial expert in this visit, not only about the entire preparation of what it was needed to run the system, but the constant communication during and after the experiments. 

Elmeri Pienihäkkinen (Picture 7) and Joona Lahtinen (Picture 8) are the engineers in charge of running the rig, who explained every detail of fast pyrolysis process in order to know how this works. They also solved challenging circumstances during the experiments that helped to get great inputs about this technology. 

VTT staff in general, because they made me feel so welcome and part of the institution. 

 

Finally, BRISK2, which is such a great organisation, with great vision and support to promote research collaboration with open and diverse opportunities to share knowledge. It gave me one of the best experiences in my career.