Entire plant is used, with high moisture content (whole plant harvest)

The characteristic that the entire plant is used with high moisture content describes a defining attribute of fresh-cut energy crops like Napier grass, sweet sorghum, and whole-plant maize used as biogas feedstock. Unlike grain harvesting, where only the seed head is collected, energy-crop harvesting takes the full above-ground biomass — leaves, stems, and developing flower heads — leaving only the basal stubble (10–15 cm above ground) for regrowth. At harvest, this whole-plant material contains 70–80% moisture — water is the majority component by mass.

The high moisture content has four operational consequences that Indian CBG operators must design around. First, transport economics: hauling fresh Napier grass beyond 30–50 km costs more per tonne of dry matter than the biomass is worth, because trucks are filled with water. This drives the captive-plantation model where energy crops are grown within 5–15 km of the digester. Second, storage challenges: fresh biomass cannot be stockpiled — it ferments aerobically within 24–48 hours, losing dry matter to CO2 release and developing temperatures that risk fire. Year-round supply requires ensiling, which converts moisture from a liability to a process input. Third, energy density: at 75% moisture, the gross calorific value is roughly 4–5 MJ per kg, against 14–16 MJ per kg for the same biomass at 15% moisture (air-dried).

Fourth, and most importantly for biogas, the moisture is useful. Wet anaerobic digestion (the dominant Indian technology) requires 88–92% water in the digester slurry. Each tonne of fresh Napier grass at 25% TS contributes 0.75 tonnes of usable water, reducing the freshwater dilution demand of the plant. By contrast, paddy straw or wheat straw at 10–15% moisture requires 4–6 tonnes of freshwater per tonne of biomass to achieve digester slurry consistency, which adds significantly to operating cost and digestate volume. This is one reason CBG plants increasingly blend high-moisture fresh crops with low-moisture residues — the wet stream provides the water that the dry stream needs, halving net freshwater consumption and reducing digestate dewatering load. The whole-plant, high-moisture model integrates cropping, water management, and digester design into a single optimisation problem.