The Pith: A Critical Factor in Napier Grass Biogas Production

For several years, we have been immersed in the world of compressed biogas feedstocks, with napier grass being one of the key focus. Our understanding is still in its nascent stages, yet we have consistently observed a unique phenomenon across all varieties: a spongy, white substance forms on the lower-middle section of the stem, growing more pronounced as the plant matures.

This recurring observation prompted us to engage in detailed discussions with numerous napier grass and biomethanation experts. We also conducted our own background research and concluded that now is the ideal time to present these preliminary findings to the industry for your information and refinement.

Let's delve into the details.

Napier Grass Pith: Anatomy, Age, and Health

The spongy, white substance found in a napier grass stem is the pith. It is the central parenchymatous tissue and a crucial indicator of the plant's structural anatomy, maturity, and biochemical composition. Understanding the pith is essential for optimizing napier grass as a feedstock for anaerobic digestion.

Age and Maturity

As napier grass matures, the stem thickens and the pith tissue expands, becoming more fibrous. In young, immature grass (e.g., 40-60 days old), the stems are tender, and the pith is small and moist. In contrast, mature grass (e.g., 75+ days) has a prominent, drier, and more lignified pith.

Biochemical Composition

The pith is rich in structural carbohydrates, primarily cellulose and hemicellulose, but also contains a significant amount of lignin. The concentration of these components changes with age:

• Lignin Content: Lignin, a complex polymer, increases significantly as the grass matures. This makes the stem more rigid and less digestible. The "woody" feel of an older napier grass stem is a direct result of this increased lignification, with the pith being a key site for this process.

Varietal Differences in Pith Formation

The rate and extent of pith formation can vary between different napier grass varieties. This is because cultivars are bred for different characteristics like stem thickness, leaf-to-stem ratio, and maturity rate.

• Faster Pith Formation: Varieties bred for high biomass yield, such as Super Napier, often have more rapid pith formation. These types prioritize vigorous stem growth to maximize total dry matter production, leading to thicker stems and a larger, more pronounced pith at an earlier age.
• Slower Pith Formation: Varieties favored for their forage quality, which may have a higher leaf-to-stem ratio and a slower maturation rate, typically have a less developed pith at the same age. This is often an advantage for animal feed, where high digestibility is desired.

For biogas production, where high biomass is paramount, varieties with more rapid pith formation are generally preferred, despite the challenges associated with the material's properties in the digester.

Impact on Anaerobic Digestion

The pith’s unique characteristics have a significant impact on the anaerobic digestion process. Its fibrous, low-density, and highly lignified nature can lead to several operational problems.

1. Scum Layer Formation

The fibrous nature of the pith contributes heavily to the formation of a thick, persistent scum layer at the top of the digester. These fibrous particles float due to trapped biogas and their natural buoyancy. This scum layer is a major operational issue as it:

• Hinders the transfer of heat and gas.
• Reduces the effective volume of the digester.
• Prevents proper contact between the substrate and the microbial community, leading to inefficient digestion and lower biogas yields.

2. Foaming

The presence of finely chopped fibrous material can also contribute to a persistent foam layer in the digester. Foaming can cause digester overflow, block pipes, and lead to safety hazards.

3. Wettability and Settling

The fibrous nature of the pith makes the napier grass substrate less wettable than other feedstocks. The fibers tend to clump together and float rather than mix with the liquid phase of the digester. This also negatively affects the settling properties of the digester content, as a significant portion of the solid biomass remains suspended in the liquid or floats to the top, disrupting proper mixing.

Wayforward

The white pith in napier grass is a normal feature that indicates the plant's maturity and structural development. While its presence is a sign of a healthy, high-yield plant, its fibrous nature and high lignin content present significant challenges for anaerobic digestion. These properties reduce biochemical efficiency and create mechanical problems like scum and foam formation. Therefore, understanding the pith is crucial for managing the feedstock and ensuring a successful and efficient biogas production process. Proper harvesting age and effective pre-treatment are essential to mitigate these issues.