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

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.

Global warming from greenhouse gases had caused environmental impact especially from the use of fossil fuel in heating and the electricity sector. Using of biogas was an alternative energy that reduced the amount of greenhouse gases. Napier grass is an attractive feedstock for biogas & bioethanol production. Napier grass is a perennial species so helps in soil conservation and it has become a priority source of biomass for alternative energy production in Thailand, as the National Energy Policy Council has raised the target of power production from Napier Grass to 3,000 MW under the 10-year alternative energy development plan (2012-2021) for Thailand. Hybrid Napier can be harvested up to 5 to 6 times per year. In addition to use as animal feed. It is the energy crop of the future because the grass is photo-emissions from the extraction of organic fuels such as alcohols, methane and pyrolytic oils. Napier grass is specifically stored for the production of biofuels and bio-based products, primarily due to its high cellulose content (34.2–40%), high yields per unit area, drought tolerance and forms a complete crop due to a good water use efficiency. Biogas is an economical alternative source of energy that will reduce the share of fossil fuels used in the transportation sector and productive industry, reducing environmental concerns caused by widespread use of these resources (eg, climate change and global warming) and the energy source will increase, besides the safety, and power supply will also be improved. Nowadays, biomass is responsible for 10% of primary energy consumption worldwide and can be used as a promising feedstock for biofuel production.

Feasibility of Biogas Production from Napier Grass Napier grass contains 30.9% total carbohydrates, 27% protein, lipid 14.8%, total ash 18.2%, fiber 9.1% (dry weight). Its organic compositions are an ideal feedstock for biogas production. The fresh grass was harvested after 5-week plantation, grounded in 0.5 to 1 mm diameter size before fermentation in mesophilic batch reactors, with varying solid concentrations of 5 – 15%. The optimum condition for the biogas production was found at 5% solid concentration. Methane content, yield and production rate were 53%, 122.4 mL CH4/g TVS remove, 4.8 mL/hr at the optimum condition. Economic analysis of biogas obtained from the experiment with liquid petroleum gas (LPG) with benefit/cost ratio (B/C ratio) greater than 1 suggested that the Napier grass is considered as a potential energy crop. Napier Grass is considered to be the fresh energy crop to be used in power production, with great potential. The specific species that suits best as energy crop is Pakchong 1 (Super Napier) , harvestable 5-6 times a year and yield as much as 70-80 tons/year/rai, almost seven times more than other grasses. Its nutrition structure fits for microbe to grow and generate methane gas. Napier grass can generate bio gas generation at 6,860-7,840 cubic meters/rai/year that can produce Compressed Bio Gas with heating value of 14-18 MJ/kg, replacing usage of NGV by 3,118-3,563 Kg./year. There are many high yielding Bajra Napier hybrids viz., APBN-1, Super Napier, CO3, CO5, BNH3, BNH10, IGFRI-5, IGFRI-10, IGFRI-16, NB-21 and PNB-233, are recommended to the dairy farmers in India. At present scenario, the Super Napier showed promising and remained at the top for high forage production per unit area and time. It produced the highest green forage yield of 300 to 400 tons/ha in 6 to 7 cuttings in one year and thereafter 2- and 3-year, green forage yield is 250 to 300 tons/ha/year. The first cutting will be taken after 50-60 days of planting and the subsequent cuttings at 45-50 days interval. After reaping and fermenting the age of 60-day old fresh Napier grass, the anaerobic digestion process will result in bio gas. Napier grass can generate bio gas generation at 6,860-7,840 cubic meters/rai/year that can produce Compressed Bio Gas with heating value of 14-18 MJ/kg, replacing usage of NGV by 3,118-3,563 Kg. /Year. One metric ton of Napier Grass can generate 90 cubic meters of bio gas, convertible in to 170 KW/day electricity. Study of Napier Grass Harvesting Age Influencing on Biogas Production Biogas production of Napier grass is influenced due to variety/hybrids of Napier grass, soil type and climatic conditions, nutrient management, cutting interval and harvesting age. Here, we will discuss on harvesting age. Harvesting age of grass is a key consideration factor in biogas plant operation form grass bio methanation. Study of methane (CH4) production from Napier grass at various cutting intervals. Napier grass at the cutting interval of 60 days gave the highest CH4 yield of all various cutting intervals. For further information on forage production technology on Napier Grass or other forage crops or on nutrient management on any field crops, please contact to me. Dr.P.C. Patel, or Mob No. +91-9909022116


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