Renewable Energy

Napier grass: A game-changer in bio-CNG production

With an energy output-to-input ratio of approximately 25:1, it emerges as one of the most promising energy crop

By Rahul Jain
Published: Wednesday 10 May 2023
Napier grass: A game-changer in bio-CNG production
Photo: iStock Photo: iStock

The use of lignocellulosic materials like forest leftovers, agricultural waste and energy grasses, among others, shows great potential for generating bioenergy. These resources are widely available worldwide and also address concerns about food shortages that were associated with first-generation biofuels produced from edible materials.

Napier grass, also known as elephant grass, is a productive and versatile forage grass native to Africa and southeast Asia. Due to its high yield, it is widely used as feed for livestock and in bioenergy applications. 

While it may be a relatively new energy crop in India, Thai farmers have been cultivating it for over 30 years, with more than 130 varieties. This fast-growing perennial grass can reach a height of 10-15 feet and can be harvested 5-6 times annually. 

The first harvest occurs four months after planting, followed by subsequent harvests every two months for up to seven years. Napier grass is categorised as lignocellulosic biomass, with its carbohydrate composition typically consisting of 35-39 per cent cellulose, 19-23 per cent xylan and 15-19 per cent lignin on a dry mass basis. 

With an energy output-to-input ratio of approximately 25:1, it emerges as one of the most promising energy crops for the creation of cost-effective and efficient bioenergy systems.

In India, the reported annual production yield of Napier grass ranges from 150-200 tonnes per acre per year, which is significantly higher (25-35 tonnes per hectare) compared to other energy grasses like miscanthus and switchgrass. 

However, there are specific varieties that have shown even higher yields. Mahendra Thakur, a microbiologist and farmer, achieved biomass productivity of 350-400 tonnes per acre per year by cultivating a hybrid variety called Super Napier in Maharashtra's Gondia district.

Due to its significant cellulose and xylan content, Napier grass holds promise as a viable source for biogas production. When its structure undergoes hydrolysis, it breaks down into monomeric sugars that can be utilised as substrates for microbial activity. It exhibits numerous favourable attributes as an energy crop, including a short growth cycle, a relatively high methane content and a high level of water use efficiency. 

Additionally, Napier grass promises a high content of easily digestible organic matter, along with high yields and the ability to withstand drought conditions. These qualities make it an excellent feedstock for anaerobic digestion processes. 

Furthermore, the use of Napier grass as a feedstock can help address the issue of uncertain feedstock procurement, as continuous and reliable supply to biogas plants is often reliant on external parties.

Traditionally, the yield of biogas from Napier grass has ranged from 90-110 cubic metres per tonne (equivalent to 38-46 kilograms of compressed biogas, CBG), Sachin Kumar, a senior scientist at the Sardar Swaran Singh National Institute of Bio-Energy (SSS-NIBE) in Kapurthala, Punjab, told Down To Earth.

However, through their research at SSS-NIBE, Kumar and his team have pioneered a new technology with the potential to achieve biogas yields of up to 150 cubic metres per tonne (equivalent to 63 kilograms of CBG). 

Notably, this technology claims to have a hydraulic retention time (HRT) of 15 to 20 days. “This enhanced yield can be achieved with a reduced land requirement of 120 acres for a five tonnes per day CBG plant, as opposed to the previous demand for a large 200-acre area,” Kumar said.

Similarly, PC Patel, a former senior research scientist at Anand Agricultural University in Gujarat, conducted pilot-scale experiments to analyse hybrid Napier grass as feedstock for CBG production. The tests were conducted in collaboration with Biofics Ltd, a CBG plant installation company based in Surat. 

Through these experiments, it was discovered that with a 24-hour microbial pretreatment, 100 kgs of the grass yielded 6 kgs of biogas, which had a 62.3 per methane content. Alternatively, when the same feedstock underwent a 24-hour pretreatment with 1 per cent NaOH, a slightly higher yield of 6.5 kg of biogas with 61.5 per cent methane content was obtained. 

At present, the utilisation of Napier grass in CBG plants in India is still in its nascent stages, with no operational plants solely focused on Napier grass. However, several ventures are underway. Notably, ZVS International is embarking on a project in the Junagadh District of Gujarat, where they are installing a 10 TPD capacity CBG production facility utilising Napier grass. 

Additionally, in the Gondia district of Maharashtra, two CBG plants with capacities of 5 TPD and 2 TPD are currently in progress, showcasing an enthusiasm to harness the potential of Napier grass. 

The primary challenge in effectively utilising Napier grass lies in its resistance to enzymatic and microbial hydrolysis. Consequently, pretreatment of the lignocellulosic biomass is imperative to enhance digestibility and maximise biogas production. 

The core objectives of lignocellulosic biomass pretreatment involve augmenting the available surface area, reducing cellulose crystallinity and modifying or eliminating hemicellulose and lignin components. 

Another crucial factor to consider is the operational intricacies of a CBG plant exclusively reliant on Napier grass. Numerous laboratory-scale studies have demonstrated that co-digestion, involving a combination of Napier grass with cow dung or food waste, results in higher yields compared to using Napier grass alone.

Moreover, it is essential to note that Napier grass is a warm-season grass and undergoes dormancy during the winter months. To ensure an uninterrupted supply of feedstock throughout the year for the CBG plant, it is vital to have alternative feedstock options available during the winter season. 

PC Patel emphasised the significance of incorporating maize crops as the ideal alternative feedstock for the winter season when planning a Napier grass-based CBG plant. This strategic approach guarantees a continuous and sustainable feedstock supply, mitigating any potential disruptions during the dormant phase of Napier grass.

To forge ahead with the advancement of a bioenergy system centered on Napier grass, it is imperative to guarantee its sustainability by effectively tackling the conundrum of food versus fuel. This entails implementing careful land-use strategies, the advocacy of high-yield agricultural methodologies and the deliberate prioritisation of marginal or degraded lands for the cultivation of Napier grass, thus mitigating the encroachment upon arable land traditionally dedicated to food production.

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