High yield from green mass
In the tropics and subtropics, Napier grass is widespread. The other names are King grass, royal grass, Sudanese grass, perennial elephant grass. We did batch tests of Napier grass from India, Brazil, Indonesia, Kenya, the Philippines. The variety of the Napier grass is not so important for biogas. It's more important for agriculture. What is more important for biogas is the harvesting age.
After CO2 removal or the upgrading of biogas, the resulting end-gas is an analog of natural gas. It's called biomethane, compressed biogas (CBG) or bio-compressed natural gas (bio-CNG). Though CBG and bio-CNG are commonly used terms in India, in Europe the word "biomethane" is more common. All words are correct. CBG consists of 98-99% methane, 1-2% carbon oxide, 0,2% oxygen, 0,2% nitrogen. Sulphur level is 0 ppm if good milty-stage purification is used.
A young Napier grass gives little biogas because it has a lot of moisture. And the old Napier grass gives also little because a lot of lignin. For farmers of course it is feasible to sell the young green Naier grass because it's juicy and they are selling water to the biogas plant owners. The young 50-day Napier grass is cheap but it gives 2 times less biogas than the older 100-day Napier grass.
Between the young green Napier grass and old brown Napier straw there is a stage of green-yellowish Napier grass. So between 18% solid in the young and 80% solid in the old Napier grass there is an optimum harvested Napier grass with 27-35% solid. You just need to harvest Napier grass when it's still greenesh-yellow or yellowish-green. The optimum harvesting age is 100-120 days. But be careful, don't wait until the Napier grass becomes brown.
Before the biogas plant it is recommended to put Napier grass into a silage storage for several reasons. These reccomendations are general not only for our technology but for all kinds of biogas technologies.
(1) Fresh Napier grass has a lot of air. And because of the air Napier grass can abruptly go afloat in the reactors. It's not only difficult to mix the floating grass, but it is also dangerous. Floating grass may destroy the roof and clog the gas pipes. During silaging grass is pressed by vehilcles as tractors and air is removed.
(2) During silaging acidation and hydrolisis processes start and biogas yeilds are increased 5-10%. Grass is pre-fermented with the lacto-acid bacteria. The minimum silaging time is 35 days.
(3) Silage storage is also needed during the raining seasons when harvesting is not possible. Reactors must be fed constantly independently from the weather.
Feedstock preparation
High-load reactor (HLR) is the best biogas technology for Napier grass. HLR reactor is 3 times smaller and cheaper than the conventional CSTR. For 10 tpd bioCNG a single HLR reactor 4000 m3 is enough. At the same time, HLR is more relaible and consumes less electric energy for own needs than CSTR. Zorg Biogas implemented the world's largest project with HLR 26MW electric power or 110 tpd bioCNG. This project uses sorghum and maize. The same technology is suitable for Napier grass or sugar cane.
The smallest offered model of HLR is 5 tpd bioCNG or 1,2 MW electric power. Technically it's possible to make smaller plants like 500 kW or 2 tpd bioCNG. But we consider it's not viable. For2 tpd or 10 tpd bioCNG there will be 1 reactor anyway.
For 2,3MW electric power plant or 10 tpd bioCNG it is required 120 tonnes Napier grass with 33% total solid per day. With 27% solid 146 tonnes Napier grass per day is required. And 500 acres of agri-land is required to grow enough Napier grass for the 10 tpd bioCNG plant.
Proposal for a plant 5 tonnes Bio-CNG per day using Napier grass
Proposal for a plant 10 tonnes Bio-CNG per day using Napier grass
Proposal for a plant 20 tonnes Bio-CNG per day using Napier grass
Proposal for a Napier grass-based biogas plant 9.2MW electic power (40 tpd bioCNG equivalent)
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