Evaluation of Biomass Wood Pellets for Power Generation Applications (Flexible Funding 2022)

Dr Nejat Rahmanian at the University of Bradford was awarded funding in the UKCCSRC’s Flexible Funding 2022 call to look at the “Evaluation of Biomass Wood Pellets for Power Generation Applications”.

In the ever-evolving landscape of renewable energy in the UK, biomass has surged as a major player, contributing 12.9% of the total energy production in 2021. With the nation’s ambitious net-zero emissions target for 2050, biomass energy is set to constitute 10% of the energy mix. Bioenergy with Carbon Capture and Storage (BECCS) will play a vital role in emissions reduction. Wood pellets are central to this transition, crucial for co-firing and co-generation, but their proper handling is essential for efficiency and safety in energy production. This integration reflects the UK’s commitment to sustainable, eco-friendly power generation.

Figure 1 – Bioenergy with Carbon Capture and Storage (BECCS). Source: earth.org

It is important to note that while the utilisation of wood pellets for biomass energy has gained momentum, there still exists a critical gap in understanding the many different aspects of the nature of these pellets. This includes their physical, mechanical and chemical properties, particularly when used in power generation applications. This project aimed to address these knowledge gaps by characterising biomass wood pellets used as a feedstock in UK power plants.

This project strives to shed light on the physical, mechanical and chemical properties of 12 biomass wood pellets samples used in the UK (Figure 2). Various properties were examined, including density, durability, moisture, particle size distribution and strength. The core goal was to establish a comprehensive understanding of how these properties interrelate and influence energy release and emissions, particularly the generation of CO2.

Figure 2 – 12 Biomass wood pellet samples

The pellet density was found to vary significantly among samples, with Sample G at 979.06 kg/m3 and Sample E at 1150.07 kg/m3. While there is no specific density standard, ENplus standards for pellet size were met. Density is influenced by factors such as wood type, moisture, particle size, binders and compression pressure used in manufacturing process. Bulk density also varies, with Sample J at 628.20 kg/m3 and Sample E at 589.90 kg/m3, falling mostly within the ENplus standard of 600-750 kg/m3. Wood type, moisture content and manufacturing processes impact bulk density. High bulk density is preferable for cost effective transportation and storage.

Mechanical durability, a key property, varied among samples. While meeting the ENplus standard of ≥ 98%, Sample G had the lowest mechanical durability at 98.73%, and sample E had the highest at 99.96%. Mechanical durability is vital for pellets to withstand transportation, storage and handling. Low durability can lead to issues such as reduced combustion efficiency and higher emissions, as well as an increased risk of fire explosions during handling. Moisture content analysis revealed that all samples fell into the M10 category with relative moisture less than 10%.

The particle size distribution for wood pellet samples A to L was generally uniform, with slight deviations in C and E, likely due to increased moisture and durability. The maximum force needed to break the pellets varied significantly, with Pellet K being the strongest at 0.835 kN, and Sample G the weakest at 0.451 kN. These variations are influenced by factors like density, durability and moisture. Additionally, thermal treatment can alter pellet properties, affecting strength, grindability, HHV and particle size.

The HHV of wood pellets represents the heat released during complete combustion. It fluctuates based on wood type, moisture content and the production process. Woods composition, including cellulose, lignin, protein and starch impacts HHV. Carbon and hydrogen content, being energy rich elements, also influence HHV. Sample K displayed the highest HHV at 22.37 MJ/kg, while Sample C had the lowest at 16.71 MJ/kg. Gas release analysis indicated that the amount of oxygen used significantly affected CO2 emissions. The specific burning conditions and wood type contributed to variations in carbon emissions.

In conclusion, this project has shed some light on the intricate world of wood pellets and their critical role in the sustainable biomass energy landscape of the UK. It was found that various properties influence their performance, from density and mechanical strength to heating values and CO2 emissions. These findings provide a comprehensive foundation for understanding the complexities of wood pellets and offer valuable insights for enhancing the efficiency and sustainability of biomass energy utilisation in the UK’s ever-evolving energy landscape. As the demand for sustainable sources continues to grow, a deeper understanding of wood pellets is pivotal in ensuring a greener, more eco-conscious future.

Read more on Nejat’s Flexible Funding 2022 project page.