Sustainability Through Napier Grass: A Renewable Feedstock for CBG Production

For large-scale operations, such as those spanning several acres, efficient machinery is essential to optimize productivity. Key equipment includes tractors, harrows, cultivators, spray pumps, rotovators, weed rotovators, and trolleys.

October 29, 2024. By News Bureau

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CEID Consultants and Engineering

Velmurugan

Renewable energy sources

Agricultural waste

Compressed Biogas

Sustainable Energy

The recent shift towards environmental concerns has intensified the hunt for sustainable and renewable energy sources. While materials like agricultural waste and forest leftovers show great potential for generating biogas, another renewable alternative standing out in recent times is Napier grass (Pennisetum purpureum), often referred to as "elephant grass." It is a perennial plant that grows rapidly in tropical and subtropical regions. Its high biomass yield, low maintenance requirements, and ability to grow on marginal lands make it a prime candidate for biogas production, particularly compressed biogas (CBG). This fast-growing grass can be harvested multiple times a year, providing a continuous and sustainable source of raw material.

Here are some of the reasons why Napier grass stands out as a sustainable solution:

High Biomass Yield and Carbon Sequestration for Sustainable Energy
Napier grass is one of the most efficient feedstocks for bioenergy due to its high biomass yield, producing 100-250 tonnes per acre annually (varies on Napier type, soil & climate conditions). This output ensures a steady supply for CBG (compressed biogas) plants, making it ideal for large-scale biogas production. Additionally, as a C4 plant, Napier grass efficiently absorbs CO2 during growth, contributing to carbon sequestration and forming a closed carbon loop that offsets emissions, supporting clean energy generation.

Versatility and Resilience in Challenging Environments
Napier grass is a versatile crop that can grow in various climatic conditions, including drought-prone and nutrient-poor soils. Its adaptability makes it suitable for regions facing climate uncertainty and agricultural limitations. This resilience reduces the need for chemical fertilizers and irrigation, lowering its environmental impact compared to traditional crops. This versatility is particularly important as many areas globally are experiencing shifting climate patterns that make conventional agriculture more difficult. Napier grass can be cultivated in these regions, providing a renewable energy source where other crops might fail. It also serves as a sustainable option for land that would otherwise remain unproductive, thus increasing its overall economic and environmental value.

CEID Consultants have leveraged Napier grass as a core feedstock in several of their upcoming CBG projects under the SATAT (Sustainable Alternative Towards Affordable Transportation) initiative. The anaerobic digestion of Napier grass produces high-quality biogas, which can be purified and compressed to generate CBG, a renewable substitute for fossil-based CNG. The process involves pre-treatment, digestion, and post-digestion processes designed to maximize gas yields while minimizing environmental impact. Our focus is not only on producing biogas but also on ensuring that by-products such as digestate can be repurposed as organic fertilizers, contributing to the circular economy. To truly harness the potential of Napier grass as a renewable feedstock, large-scale cultivation is essential.

According to Velmurugan, Executive Director (Research and Development) at CEID Consultants & Engineering Pvt Ltd, Napier grass holds immense potential due to its high biogas yield, which is directly tied to its solid content. For example, if Napier grass has 22% TS, it means that 220 kg of solids are present in 1 ton of fresh Napier grass. If the VS is 80%, this means 80% of the total solids can potentially be converted into biogas. In our example, if there are 220 kg of total solids, 80% of that is volatile solids, that is 176 kilograms of volatile solids.

The biogas yield from volatile solids can be estimated using the biogas yield per kg of VS. The typical yield varies by feedstock, but for Napier grass, it is approximately 0.40 to 0.45 m³ of biogas per kg of VS. So, from 1 ton of fresh Napier grass, you can expect approximately 70-80 m³ of biogas.

Machinery Used to Process Napier Grass:
For large-scale operations, such as those spanning several acres, efficient machinery is essential to optimize productivity. Key equipment includes tractors, harrows, cultivators, spray pumps, rotovators, weed rotovators, and trolleys. For example, for efficient harvesting of 1000 acres of Napier grass, it is recommended to harvest Napier in 5:0 spacing method and use a harvesting machine paired with a tractor, capable of processing a minimum of 300 kg of Napier grass per minute. To maintain optimal operations, at least five such harvesting machine sets are advised.

The efficiency and frequency of these harvesting operations are influenced by the cutting interval, which plays a crucial role in maintaining high nutrient content and dry matter retention in the grass. The first cut should occur approximately three months after planting, followed by intervals of 50–60 days. The harvested Napier grass should be shredded to a maximum size of 2-5 mm before being fed into digesters for anaerobic digestion, yielding approximately 70-80 cubic meters of biogas per ton of Napier grass.

Maintenance is a key aspect of managing large-scale operations. The lifespan of Napier grass ranges from 5-7 years with proper management, after which replanting becomes necessary. We recommend that Napier grass seedlings must have visible eyes and recommend planting a density of 2-3 bud sticks. Operational expenses also include fuel, labor, and maintenance costs, such as blade changes and greasing equipment at frequent intervals, to ensure maximum efficiency and minimal downtime.

The Future of Napier Grass in CBG
As the demand for sustainable and eco-friendly solutions grows, Napier grass presents a viable option for CBG production. Organizations like CEID Consultants are playing a crucial role in supporting the cultivation and aggregation of Napier grass at the scale required to meet the needs of biogas plants. Mr Velmurugan said - “At CEID, we have invested in acquiring and cultivating land for Napier grass production, with over 2000 acres dedicated to this sustainable resource at multiple locations. This strategic move ensures a steady supply of feedstock for our upcoming CBG plants. We are also actively working with local communities to promote the cultivation of Napier grass, offering farmers a new revenue stream while simultaneously ensuring that our CBG plants are supplied with high-quality, locally sourced biomass.”

- Velmurugan, Executive Director (Research and Development) at CEID Consultants & Engineering Pvt Ltd